Preparation method of stone paper high filling stable master batch by grading coating and vacuum devolatilization

Abstract

This invention discloses a method for preparing high-filled stable masterbatch for stone paper using a combination of graded coating and vacuum devolatilization. The method involves separating compounded calcium carbonate powder into a first-stage calcium carbonate powder with a larger particle size and a second-stage calcium carbonate powder with a smaller particle size. These are then subjected to primary anchoring coating based on differences in particle size and specific surface area. The mixture is then combined with polyolefin resin, a melt-grade compatibility coating agent, a segmented lubricating flow stabilizer, and an antioxidant stabilizer before being fed into a twin-screw extruder. The process includes melt mixing, enhanced dispersion, melt-grade compatibility coating, vacuum devolatilization, downstream flow stabilization, secondary homogenization, extrusion cooling, and pelletizing to obtain the high-filled stable masterbatch for stone paper. This method improves the dispersion uniformity of calcium carbonate in the polyolefin system, reduces the moisture content and volatile matter in the masterbatch, improves melt pressure stability and pelletizing consistency, and solves the problems of easy agglomeration, easy gas release, large fluctuations in feeding and extrusion, and unstable subsequent processing quality in existing high-filled masterbatches for stone paper.

Description

Technical Field

[0001] This invention relates to the field of polymer composite materials and plastic masterbatch preparation technology, specifically a method for preparing stone paper high-filled stable masterbatch through graded coating and vacuum devolatilization synergy. Background Technology

[0002] Stone paper is an environmentally friendly sheet or film material made primarily of inorganic mineral powders such as calcium carbonate, with polyolefin resin as the continuous phase carrier. It has applications in packaging paper, printing materials, and composite paper. In the production of stone paper, calcium carbonate powder, polyolefin resin, and additives are typically prepared into a high-filler masterbatch, which is then used in subsequent extrusion, calendering, or casting processes. The dispersion uniformity, moisture content, volatile matter content, and melt flow stability of the masterbatch directly affect the surface smoothness, thickness consistency, and continuous processing stability of the formed stone paper.

[0003] Existing high-filler masterbatches for stone paper are mostly prepared by directly mixing calcium carbonate powder with polyolefin resin, coupling agents, lubricants, and other raw materials, followed by melt extrusion granulation. Because the calcium carbonate content in stone paper masterbatches is high, the powder particles are in close contact. Single-sized calcium carbonate particles are prone to localized accumulation and agglomeration under high-filling conditions, leading to clumps, white spots, or hard particles within the masterbatch. In particular, fine-sized calcium carbonate has a larger specific surface area and stronger hygroscopicity and agglomeration tendency. If the same coating agent dosage is used as for coarse-sized powder, insufficient coating of fine powder and excessive additives in coarse powder can easily occur, thus affecting the dispersion stability of calcium carbonate in polyolefin resin.

[0004] Furthermore, highly filled premixes typically require temporary storage and continuous feeding before entering the extruder. Due to the high calcium carbonate content, the premix has a high bulk density. If the material accumulation height or temporary storage weight in the hopper is too large, the bottom material is easily compressed by the upper layer, resulting in compaction, arching, or bridging. This leads to unstable material flow into the twin-screw extruder, causing torque and die head pressure fluctuations during the melt mixing process. Even with subsequent mixing and granulation processes, it is difficult to completely eliminate the impact of uneven front-end feeding on masterbatch quality.

[0005] Meanwhile, residual moisture on the surface of calcium carbonate powder, low-molecular-weight components in coupling agents and lubricants, and gases entrained during the mixing process may all be released during melt extrusion or subsequent stone paper processing, causing defects such as micropores, bubbles, and odors inside the masterbatch, as well as pinholes, bulges, and pitting on the sheet surface. Even with conventional venting or vacuum devolatilization in existing processes, there is often a lack of process arrangements that coordinate with the powder coating state and the steady-state recovery of the melt after devolatilization, which can easily lead to problems such as unreasonable devolatilization timing, insufficient melt pressure recovery after devolatilization, and unstable die head discharge.

[0006] Therefore, existing methods for preparing high-filler masterbatch for stone paper still suffer from problems such as unreasonable powder particle size distribution, mismatch between coarse and fine powder coating strength, insufficient stability of premixed feed, high volatile residue, and unstable melt flow state after devolatilization. These issues lead to white spots, pitting, bubbles, band breaks, and thickness fluctuations in the final masterbatch during subsequent processing of stone paper sheets, films, or packaging materials. In view of these problems, a method for preparing high-filler steady-state masterbatch for stone paper that combines graded coating and vacuum devolatilization is proposed. Summary of the Invention

[0007] The purpose of this invention is to provide a method for preparing high-filling stable masterbatch for stone paper by synergistic graded coating and vacuum devolatilization, so as to solve the problems mentioned in the background art.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a method for preparing a high-filled stable masterbatch for stone paper using a combination of graded coating and vacuum devolatilization, characterized in that, based on a total mass of 100 parts of raw materials for preparing the high-filled stable masterbatch for stone paper, the raw materials include 75 to 86 parts of compounded calcium carbonate powder, 9 to 18 parts of polyolefin resin, 0.6 to 2.0 parts of primary anchoring coating agent, 1.2 to 4.5 parts of melt-grade compatibility coating agent, 0.8 to 2.5 parts of segmented lubricating flow stabilizer, and 0.1 to 0.5 parts of antioxidant stabilizer, and the sum of the amounts of each raw material component is 100 parts. The preparation method includes the following steps: S1. The compound calcium carbonate powder is divided into a first calcium carbonate powder and a second calcium carbonate powder, wherein the particle size of the first calcium carbonate powder is larger than that of the second calcium carbonate powder. S2. Add a primary anchoring coating agent to the first calcium carbonate powder and the second calcium carbonate powder respectively for surface coating treatment, so that the primary anchoring coating agent adheres to the particle surface of the first calcium carbonate powder and the second calcium carbonate powder to form a primary anchoring coating layer, and the amount of primary anchoring coating agent per unit mass of the second calcium carbonate powder is higher than the amount of primary anchoring coating agent per unit mass of the first calcium carbonate powder. S3. The first calcium carbonate powder after forming the first anchoring coating layer, the second calcium carbonate powder after forming the first anchoring coating layer, polyolefin resin, melt-grade compatibility coating agent, antioxidant stabilizer and part of segmented lubricating and flow-stabilizing agent are mixed to obtain a high-filler premix. S4. The high-filler premix is ​​fed into a twin-screw extruder for melt mixing, so that the polyolefin resin melts and coats the first calcium carbonate powder and the second calcium carbonate powder, while the melt-grade compatibility coating agent forms a melt-grade compatibility coating layer on the outside of the first-level anchoring coating layer. S5. After the melt-level compatibility coating layer is formed, the high-filled melt is subjected to vacuum devolatilization treatment to remove moisture, low-molecular-weight volatiles, residual treatment agents and entrained gases from the high-filled melt. S6. Add the remaining segmented lubricating and flow-stabilizing agent to the vacuum devolatilized high-filled melt and perform secondary homogenization treatment on the high-filled melt to keep the high-filled melt in a continuous flow state before entering the extruder die head. S7. The highly filled melt after secondary homogenization is extruded, cooled and pelletized to obtain high-filled stable masterbatch for stone paper.

[0009] Preferably, the first calcium carbonate powder is heavy calcium carbonate with a D50 particle size of 2.5 μm to 6 μm, and the second calcium carbonate powder is one or more of light calcium carbonate, refined heavy calcium carbonate, or spherical calcium carbonate with a D50 particle size of 0.4 μm to 1.5 μm. The first calcium carbonate powder accounts for 65% to 85% of the total mass of the compound calcium carbonate powder, and the second calcium carbonate powder is the remainder of the compound calcium carbonate powder excluding the first calcium carbonate powder, accounting for 15% to 35% of the total mass of the compound calcium carbonate powder. The sum of the mass percentages of the first calcium carbonate powder and the second calcium carbonate powder is 100%, and the D50 particle size of the second calcium carbonate powder is smaller than that of the first calcium carbonate powder.

[0010] Preferably, the first calcium carbonate powder and the second calcium carbonate powder undergo drying and particle size classification before surface coating treatment. The particle size classification includes one or more of sieving, air classifying, or cyclone classifying. The moisture content of the first calcium carbonate powder and the second calcium carbonate powder after drying is not higher than 0.3%. The high-filler premix enters a buffer silo before entering the twin-screw extruder. The stacking height of the high-filler premix in the buffer silo is controlled to be 0.4m to 1.8m, and the temporary storage volume is controlled to be 0.08m³. 3 up to 0.8m 3 The temporary storage weight of the high-fill premixed material in a single buffer silo is controlled to be between 50 kg and 600 kg, and the ratio of the temporary storage weight to the temporary storage volume is controlled to be 0.5 t / m³. 3 Up to 0.9t / m 3 This reduces the compaction of the high-fill premix at the bottom of the buffer silo.

[0011] Preferably, the primary anchoring coating agent includes one or more of titanate coupling agents, aluminate coupling agents, silane coupling agents, stearic acid, oleic acid, or calcium stearate. The primary anchoring coating agent is added to the first calcium carbonate powder and the second calcium carbonate powder respectively, and the amount of primary anchoring coating agent per unit mass of the second calcium carbonate powder is 1.3 to 3 times the amount of primary anchoring coating agent per unit mass of the first calcium carbonate powder, so that the second calcium carbonate powder obtains a surface coating strength higher than that of the first calcium carbonate powder.

[0012] Preferably, based on 100 parts of the total mass of raw materials used to prepare the high-filled stable masterbatch for stone paper, the raw materials used to prepare the high-filled stable masterbatch for stone paper further include 0.1 to 0.8 parts of a devolatilization stabilization auxiliary agent, and the sum of the amounts of all raw material components, including the devolatilization stabilization auxiliary agent, is 100 parts. The devolatilization stabilization auxiliary agent includes one or more of active magnesium oxide, hydrotalcite, microporous silicate, or molecular sieve. The devolatilization stabilization auxiliary agent is fed into the twin-screw extruder together with the high-filled premix, or is added to the high-filled melt before it enters the vacuum devolatilization treatment, for adsorbing or fixing residual moisture, acidic low molecular weight substances, and volatile small molecule components in the high-filled melt.

[0013] Preferably, the polyolefin resin includes one or more of high-density polyethylene, linear low-density polyethylene, low-density polyethylene, or polypropylene, and the melt-grade compatibility coating agent includes one or more of maleic anhydride-grafted polyethylene, maleic anhydride-grafted polypropylene, or glycidyl methacrylate-grafted polyolefin. The melt-grade compatibility coating agent is distributed between the primary anchoring coating layer and the molten polyolefin resin during the melt mixing process of the twin-screw extruder, and forms the melt-grade compatibility coating layer on the outside of the primary anchoring coating layer for connecting the calcium carbonate particles and the continuous polyolefin phase.

[0014] Preferably, the segmented lubricating and flow-stabilizing agent comprises a front-stage lubricating component and a rear-stage flow-stabilizing component. The front-stage lubricating component is added after the surface coating treatment of the first calcium carbonate powder and the second calcium carbonate powder and before the high-filler premix enters the twin-screw extruder. The front-stage lubricating component accounts for 30% to 60% of the total mass of the segmented lubricating and flow-stabilizing agent. The rear-stage flow-stabilizing component is the remainder of the segmented lubricating and flow-stabilizing agent excluding the front-stage lubricating component, and accounts for 40% to 70% of the total mass of the segmented lubricating and flow-stabilizing agent. The total mass percentage of the flow stabilizing components is 100%. The front-end lubricating component is used to reduce the agglomeration degree between calcium carbonate particles in the high-filled premix. The rear-end flow stabilizing component is added after the high-filled melt has undergone vacuum devouring treatment to improve the flow continuity and pressure stability of the high-filled melt after vacuum devouring. The front-end lubricating component includes one or more of zinc stearate, calcium stearate, ethylene bis-stearamide, or polyethylene wax. The rear-end flow stabilizing component includes one or more of oxidized polyethylene wax, polyethylene wax, or ethylene bis-stearamide.

[0015] Preferably, the twin-screw extruder is sequentially configured with a melt mixing zone, a reinforced dispersion zone, a vacuum devolatilization zone, and a secondary homogenization zone along the material conveying direction. The melt mixing zone is used to melt the polyolefin resin and initially encapsulate the first calcium carbonate powder and the second calcium carbonate powder after forming a primary anchoring coating layer. The reinforced dispersion zone is used to form the melt-level compatible coating layer outside the primary anchoring coating layer with the melt-level compatible coating agent. The vacuum devolatilization zone is located after the reinforced dispersion zone and before the secondary homogenization zone. The vacuum degree of the vacuum devolatilization zone is -0.06MPa to -0.095MPa, and the devolatilization temperature is 160℃ to 210℃. It is used to remove moisture, low-molecular-weight volatiles, residual treatment agents, and entrained gases from the highly filled melt after the melt-level compatible coating layer is formed.

[0016] Preferably, the secondary homogenization zone is located after the vacuum devolatilization zone. The secondary homogenization zone is equipped with one or more of the following: low-shear threaded element, weak mixing threaded element, or pressure-stabilizing conveying threaded element. After the downstream flow stabilizing component is added to the high-filled melt after vacuum devolatilization, it is mixed with the high-filled melt in the secondary homogenization zone under low-shear homogenization, so that the high-filled melt maintains a continuous flow state and stable melt pressure before entering the extruder die.

[0017] Preferably, the calcium carbonate content in the prepared stone paper high-filled stable masterbatch is 75% to 86%, the volatile matter content is not higher than 0.5%, the moisture content is not higher than 0.3%, the particle size of the stone paper high-filled stable masterbatch is 2mm to 5mm, and the stone paper high-filled stable masterbatch is used for extrusion, calendering or casting processing of stone paper sheets, stone paper films, stone paper packaging paper or stone paper composite materials.

[0018] Compared with the prior art, the beneficial effects of the present invention are as follows: The present invention separates calcium carbonate powder into first calcium carbonate powder with a larger particle size and second calcium carbonate powder with a smaller particle size, and allocates a primary anchoring coating agent according to the particle size difference, so that the coarse and fine powders form a more stable filling and coating structure in the high-filling system, reducing the problems of fine powder agglomeration, excessive coarse powder additives, and uneven powder dispersion; at the same time, by controlling the stacking height, temporary storage volume, and temporary storage weight through the buffer silo, the risk of compaction, bridging, and feeding fluctuations of the high-filling premix before feeding is reduced; during the melt mixing process, the melt-level compatible coating agent further... This method improves the bonding stability between calcium carbonate particles and the continuous polyolefin phase, and performs vacuum devolatilization after the coating layer is formed to remove moisture, low-molecular-weight volatiles, and entrained gases, reducing bubbles and micropores inside the masterbatch. After vacuum devolatilization, a downstream flow stabilizer is added and secondary homogenization is performed to maintain a stable flow state of the melt before entering the die head. This results in a final masterbatch with uniform dispersion, low volatile content, low moisture content, and good processing stability. This solves the problems of easy agglomeration, easy compaction and bridging, high volatile residue, large melt pressure fluctuations, and unstable subsequent stone paper processing quality in existing technologies for high-filled stone paper masterbatches. Attached Figure Description

[0019] Figure 1 This is a flowchart of the method for preparing high-filling stable masterbatch for stone paper according to the present invention; Figure 2 This is a schematic diagram of the functional sections of the extrusion process of the present invention; Figure 3 This is a comparison image of the appearance of Example 1 and traditional masterbatch particles; Figure 4 The image shows a comparison of the appearance of the masterbatch particles from Example 1 and those without vacuum devolatilization. Detailed Implementation

[0020] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] This invention provides a technical solution: a method for preparing high-filler stable masterbatch for stone paper using a combination of graded coating and vacuum devolatilization. This method is mainly used to prepare high-filler masterbatch suitable for processing stone paper sheets, films, packaging paper, and composite materials. The high-filler stable masterbatch for stone paper uses compounded calcium carbonate powder as the main filler component, polyolefin resin as the continuous phase carrier, and is combined with a primary anchoring coating agent, a melt-grade compatibility coating agent, a segmented lubricating flow stabilizer, and an antioxidant stabilizer to form a composite system suitable for extrusion granulation. Based on a total raw material mass of 100 parts for preparing high-filled stable masterbatch for stone paper, the composition includes 75 to 86 parts of compounded calcium carbonate powder, 9 to 18 parts of polyolefin resin, 0.6 to 2.0 parts of primary anchoring coating agent, 1.2 to 4.5 parts of melt-grade compatibility coating agent, 0.8 to 2.5 parts of segmented lubricating flow stabilizer, and 0.1 to 0.5 parts of antioxidant stabilizer. The proportions of each raw material component are adjusted according to the actual calcium carbonate particle size, resin melt index, target masterbatch hardness, and subsequent stone paper processing method, while maintaining a total volume of 100 parts. This ensures that while maintaining a high inorganic filler content in the masterbatch, it also maintains basic resin coating continuity and extrusion processing stability.

[0022] In the preparation process, the compound calcium carbonate powder is first divided into first calcium carbonate powder and second calcium carbonate powder, with the first calcium carbonate powder having a larger particle size than the second calcium carbonate powder. The first calcium carbonate powder mainly serves as the main filler skeleton in the high-filler system, providing the inorganic filler ratio, rigidity, and cost advantage of the masterbatch. The second calcium carbonate powder, with its smaller particle size, can be distributed in the interparticle gaps formed by the first calcium carbonate powder, thereby improving the packing density of the compound calcium carbonate powder and reducing the large pores and local blank areas formed by single-size powders under high-filler conditions. Through the combination of coarse and fine particle sizes, the compound calcium carbonate powder can form a relatively stable particle size distribution before entering the resin system, which is beneficial for improving the uniformity of powder dispersion and reducing the probability of local agglomeration and particle hardening during subsequent melt mixing.

[0023] Subsequently, a primary anchoring coating agent was added to both the first and second calcium carbonate powders for surface coating treatment. Because the second calcium carbonate powder has a smaller particle size and a larger specific surface area, the surface area requiring treatment per unit mass of powder is significantly larger than that of the first calcium carbonate powder. If the same amount of coating agent is used for both powders, the second calcium carbonate powder may be insufficiently coated and remain agglomerated, while the first calcium carbonate powder may have excessive coating agent, leading to an increase in free surface additives. Therefore, in this embodiment, the amount of primary anchoring coating agent per unit mass of the second calcium carbonate powder is higher than that per unit mass of the first calcium carbonate powder. This ensures that the primary anchoring coating agent prioritizes the surface treatment needs of the fine-particle-size powder while preventing the formation of an excessively thick free lubricating layer on the surface of the coarse-particle-size powder. After the primary anchoring coating agent adheres to the surface of the first and second calcium carbonate powder particles, it forms a primary anchoring coating layer. This primary anchoring coating layer can reduce the polarity and hygroscopicity of the calcium carbonate powder surface, improve the compatibility between calcium carbonate particles and polyolefin resin, and reduce secondary agglomeration of powder during the premixing, conveying and melt mixing stages.

[0024] After primary anchoring coating is completed, the first calcium carbonate powder forming the primary anchoring coating layer, the second calcium carbonate powder forming the primary anchoring coating layer, polyolefin resin, melt-grade compatibility coating agent, antioxidant stabilizer, and a portion of segmented lubricating and flow-stabilizing agent are mixed to obtain a high-filler premix. In this system, the polyolefin resin serves as the continuous phase basis of the masterbatch, encapsulating the calcium carbonate particles in the subsequent molten state. This allows a high proportion of inorganic powders to be linked by the resin phase, forming a granulatable and reprocessable masterbatch structure. The melt-grade compatibility coating agent is used to further improve the connection between the primary anchoring coating layer and the polyolefin continuous phase during the melt mixing stage, ensuring that the calcium carbonate particle surface is not solely bonded to the resin through ordinary mechanical mixing. The antioxidant stabilizer is used to reduce the risk of resin thermal oxidation during high-shear, high-temperature mixing of the high-filler system, minimizing masterbatch yellowing, decreased melt performance, or increased low-molecular-weight degradation products. Some of the segmented lubricating and flow-stabilizing agents are added during the premixing stage, mainly to reduce the dry friction between the coated powder and resin particles, so that the high-filler premix maintains a good loose state and flowability before entering the extruder.

[0025] The highly filled premixed material is then fed into a twin-screw extruder for melt mixing. Under the conveying and shearing action of the twin-screw extruder, the polyolefin resin gradually melts and begins to coat the first and second calcium carbonate powders. As the mixing strength increases, a melt-grade compatibility coating layer is formed on the outside of the primary anchoring coating layer, creating an interfacial transition structure on the surface of the calcium carbonate particles that sequentially includes calcium carbonate particles, the primary anchoring coating layer, the melt-grade compatibility coating layer, and the continuous polyolefin phase from the inside out. This structure reduces the problems of exposed powder, resin phase breakage, and direct contact between inorganic particles inside the highly filled masterbatch, allowing the calcium carbonate particles to be more stably dispersed in the continuous polyolefin phase within the masterbatch, thereby reducing white spots, hard spots, and clumps after the final masterbatch pellets are cut.

[0026] After the melt-level compatibility coating is formed, the highly filled melt undergoes vacuum devolatilization. This vacuum devolatilization is not performed at the initial stage when the powder enters the extruder, but rather after the melt-level compatibility coating has been largely formed. This ensures that the calcium carbonate particles are already in a relatively stable resin-coated state, allowing moisture, low-molecular-weight volatiles, residual treatment agents, and entrained gases from the highly filled melt to be expelled. This avoids premature extraction before the powder is stably coated by the resin phase, which could lead to additive loss or gas-induced disturbance in the powder. It also reduces the possibility of volatiles remaining inside the masterbatch, forming micropores, bubbles, and odors. The reduced gas content inside the highly filled melt after vacuum devolatilization makes it less prone to continuous outgassing, bulging, pinholes, and surface pitting on sheets during subsequent extrusion pelletizing and reuse in stone paper processing.

[0027] After vacuum devolatilization, the remaining segmented lubricating and flow-stabilizing agent is added to the highly filled melt, followed by a secondary homogenization treatment. Because the internal moisture and low-molecular-weight volatiles are removed after vacuum devolatilization, the local flow and pressure states of the melt may change. If it directly enters the die head for extrusion, it can easily cause discharge fluctuations or local melt discontinuities. In this embodiment, the remaining segmented lubricating and flow-stabilizing agent is added after vacuum devolatilization, allowing this component to primarily serve to stabilize the melt pressure and restore flow after devolatilization. The secondary homogenization treatment then ensures thorough mixing of the lubricating and flow-stabilizing component with the highly filled melt, maintaining continuous flow before entering the extruder die head. This treatment reduces die head pressure fluctuations, minimizing issues such as sheet breakage, inconsistent discharge speed, and unstable pellet size.

[0028] Finally, the highly filled melt after secondary homogenization is extruded through an extruder die and then cooled, drawn, and pelletized to obtain a high-filled stable masterbatch for stone paper. In this masterbatch, calcium carbonate powder undergoes differentiated primary coating based on a blend of coarse and fine particle sizes, and is further continuously bonded to polyolefins through a melt-level compatibility coating layer. This results in a more uniform powder distribution within the masterbatch, lower volatile matter and entrained gas content, and more stable particle morphology after pelletizing. When used in the processing of stone paper sheets, films, or packaging materials, this masterbatch can reduce problems such as white spots, pitting, bubbles, bulges, band breaks, and thickness fluctuations that are common in ordinary high-filled masterbatches, improving the continuous processing stability and surface quality consistency during stone paper forming. Specifically, the first calcium carbonate powder uses heavy calcium carbonate with a D50 particle size of 2.5μm to 6μm. Heavy calcium carbonate has a stable source, high whiteness, and low cost, making it suitable as the main inorganic filler material in high-filler masterbatch for stone paper. The first calcium carbonate powder acts as the main filler skeleton in the compound calcium carbonate powder. Its relatively large particle size allows it to form a stable particle packing base in a high-filler system, enabling the polyolefin resin to distribute along the gaps between the first calcium carbonate powder particles after melting, and connecting adjacent particles into a continuous, processable melt structure. The D50 particle size of the first calcium carbonate powder is controlled within the range of 2.5μm to 6μm, which balances filler content, dispersion difficulty, and stone paper surface smoothness. When the particle size of the first calcium carbonate powder is too small, it is easy for its particle size range to become too close to that of the second calcium carbonate powder, weakening the coarse-fine gradation effect and making the particle skeleton in the high-filler system less obvious. When the particle size of the first calcium carbonate powder is too large, hard particle points are easily formed inside the masterbatch, which are likely to manifest as white spots, pits, protrusions or local stress concentrations during subsequent calendering, casting or extrusion sheeting processes.

[0029] The second calcium carbonate powder uses one or more of the following: light calcium carbonate, refined heavy calcium carbonate, or near-spherical calcium carbonate, with a D50 particle size of 0.4 μm to 1.5 μm. The particle size of the second calcium carbonate powder is smaller than that of the first calcium carbonate powder, and it is mainly used to fill the gaps between the particles formed by the first calcium carbonate powder, allowing the compounded calcium carbonate powder to form a denser packing structure under high-filling conditions. The second calcium carbonate powder can reduce local voids and resin-rich areas that are prone to occur in single coarse-particle-size calcium carbonate systems, resulting in a more balanced distribution between the powder and resin, while also improving the surface smoothness of the subsequent stone paper. The D50 particle size of the second calcium carbonate powder is controlled within the range of 0.4 μm to 1.5 μm, ensuring gap-filling ability while avoiding excessive increase in system viscosity. When the particle size of the second calcium carbonate powder is less than 0.4 μm, the specific surface area increases significantly, and the oil absorption, water absorption and agglomeration tendency all increase. This increases the consumption of coating agents and compatibilizers, which can easily lead to an increase in mixing torque and difficulty in dispersion. When the particle size of the second calcium carbonate powder is greater than 1.5 μm, its ability to fill the gaps between the first calcium carbonate powder decreases, and the densification and surface refinement effects brought about by the combination of coarse and fine particle sizes are weakened.

[0030] The first calcium carbonate powder accounts for 65% to 85% of the total mass of the compounded calcium carbonate powder, while the second calcium carbonate powder, as the remainder excluding the first calcium carbonate powder, accounts for 15% to 35% of the total mass of the compounded calcium carbonate powder, with the sum of the two mass percentages being 100%. A higher proportion of the first calcium carbonate powder ensures that the high-filler masterbatch has a stable main filling skeleton and lower raw material costs. Controlling the proportion of the second calcium carbonate powder within the range of 15% to 35% allows for filling the gaps between coarse particles without significantly increasing melt viscosity or coating difficulty. If the proportion of the second calcium carbonate powder is too low, the gap-filling effect of the fine powder is insufficient, and the compounded calcium carbonate powder remains close to a single coarse-particle-size filling system. If the proportion of the second calcium carbonate powder is too high, the total specific surface area of ​​the fine powder increases significantly, easily leading to increased demand for coating agents, increased melt viscosity, increased twin-screw mixing load, and potentially affecting the stability of subsequent extrusion granulation.

[0031] In one specific embodiment, the first calcium carbonate powder is heavy calcium carbonate with a D50 particle size of approximately 4 μm, and the second calcium carbonate powder is light calcium carbonate or refined heavy calcium carbonate with a D50 particle size of approximately 0.8 μm. The first calcium carbonate powder accounts for approximately 75% of the total mass of the compound calcium carbonate powder, and the second calcium carbonate powder accounts for approximately 25% of the total mass of the compound calcium carbonate powder. At this ratio, the first calcium carbonate powder can form a stable filler matrix, and the second calcium carbonate powder can enter the gaps between the first calcium carbonate powder particles, resulting in good packing uniformity of the compound calcium carbonate powder under high-filling conditions. When used to prepare masterbatch, the polyolefin resin can form continuous connections between coarse and fine calcium carbonate particles, reducing problems such as localized powder concentration, localized resin loss, and particle hardening.

[0032] To further improve the gap-filling effect of the second calcium carbonate powder, spherical calcium carbonate or refined heavy calcium carbonate after classification can be selected. Spherical calcium carbonate has good flowability and filling adaptability, reducing mechanical interlocking and frictional resistance between particles when mixed with the first calcium carbonate powder, allowing the compounded powder to maintain a good loose state during high-speed mixing, buffer storage, and twin-screw feeding. Refined heavy calcium carbonate can provide smaller particle size components while maintaining the cost advantage and whiteness of heavy calcium carbonate, making it suitable for use in stone paper masterbatches with high cost control requirements. Light calcium carbonate has a finer particle size and higher surface activity, making it suitable for masterbatch systems with high requirements for paper surface smoothness, opacity, and apparent uniformity.

[0033] By controlling the particle size and ratio of the first and second calcium carbonate powders, the compounded calcium carbonate powder is no longer a simple filler of single-size powders, but rather forms a gradation structure combining a coarse-size main framework with fine-size interstitial filling. This gradation structure provides the basis for subsequent differentiated primary anchoring coating: the first calcium carbonate powder has a relatively small surface area, requiring a lower amount of coating agent; the second calcium carbonate powder has a relatively large surface area, requiring a higher amount of coating agent. Therefore, when allocating the primary anchoring coating agent according to particle size differences, the surface treatment requirements of different powders can be more accurately matched, reducing the problems of insufficient coating of fine powder and excessive coarse powder additives. This results in a more uniform powder dispersion, lower particle agglomeration, and more stable subsequent processing performance in the final high-filled stable masterbatch for stone paper.

[0034] Specifically, the first and second calcium carbonate powders undergo drying and particle size classification before surface coating. Drying can be performed using hot air drying, vacuum drying, fluidized bed drying, or powder drying equipment with stirring function to sufficiently reduce the free moisture in both powders. Because calcium carbonate powder has a certain degree of hygroscopicity, especially the smaller-sized second calcium carbonate powder with a larger specific surface area, it more easily absorbs moisture from the air. If the powder moisture content is too high, during subsequent high-speed mixing, primary anchoring coating, and twin-screw melt mixing, moisture can easily hinder the contact between the primary anchoring coating agent and the calcium carbonate particle surface, and cause water vapor release during melt extrusion, leading to bubbles, micropores, and localized looseness within the masterbatch. Therefore, controlling the moisture content of both the first and second calcium carbonate powders below 0.3% after drying improves the stability of subsequent coating and reduces the load on the vacuum devolatilization zone, allowing the vacuum devolatilization process to primarily remove residual moisture, low-molecular-weight volatiles, and entrained gases.

[0035] Particle size classification can be performed using one or more of the following methods: sieving, air classifying, or cyclone classification. Sieving is suitable for removing coarse particles, hard agglomerates, or foreign particles mixed in with the powder. Air classifying is suitable for refining and separating powders according to aerodynamic particle size. Cyclone classification is suitable for separating and recovering larger particles in continuous production processes. Through particle size classification, the first and second calcium carbonate powders can maintain stable particle size ranges, avoiding excessive coarse particles in the first calcium carbonate powder that would affect the surface quality of the stone paper, and also avoiding large agglomerates in the second calcium carbonate powder that would weaken the gap-filling effect of the fine powder. After particle size classification, the powder particle size distribution is more concentrated, resulting in a more consistent surface treatment effect when entering the primary anchoring and coating stage, reducing uneven coating caused by local coarse particles or powder agglomerates.

[0036] In one specific embodiment, the first calcium carbonate powder is first processed by a hot air drying device, with the drying temperature controlled between 90°C and 120°C. After drying, large particles with abnormal particle size are removed by sieving or air classification. The second calcium carbonate powder is first subjected to continuous drying at a lower temperature, with the drying temperature controlled between 80°C and 110°C, and then agglomerated particles are removed by air classification, so that the second calcium carbonate powder maintains good fineness and flowability. After drying, the first and second calcium carbonate powders should avoid prolonged exposure to high humidity air. They can be fed into a high-speed mixing device through a closed conveying pipeline, screw conveyor, pneumatic conveying pipeline, or temporary storage container with a sealed cover to reduce the reabsorption of moisture by the powder.

[0037] After primary anchoring coating and premixing, the high-filler premix enters a buffer hopper before entering the twin-screw extruder. The buffer hopper receives the high-filler premix output from the upstream high-speed mixing or premixing equipment and continuously supplies it to the twin-screw extruder's feeding mechanism. Because the high-filler premix contains a high proportion of compounded calcium carbonate powder, its overall bulk density is significantly higher than that of ordinary polyolefin resin particles. When the high-filler premix accumulates in the hopper, it is prone to compaction due to its own weight, especially the material near the bottom outlet. Prolonged exposure to the pressure from the upper column can lead to reduced porosity, localized arching, bridging, channel contraction, and instantaneous flow fluctuations. These issues directly affect the feeding uniformity of the twin-screw extruder, causing inconsistent material levels in the melt mixing zone, resulting in melt pressure fluctuations, mixing torque fluctuations, and unstable subsequent vacuum devolatilization.

[0038] To mitigate the aforementioned impacts, the stacking height of the high-fill premixed material in the buffer silo is controlled to be between 0.4m and 1.8m, and the temporary storage volume is controlled to be 0.08m³. 3 up to 0.8m 3The temporary storage weight of high-filled premix in a single buffer hopper is controlled between 50 kg and 600 kg. Stacking height control limits the height of the material column formed within the hopper, reducing the static pressure on the bottom material from the upper layer. Temporary storage volume control prevents the hopper from storing too much high-filled premix at once, causing the material to remain in the hopper for too long, leading to further compaction and moisture absorption. Temporary storage weight control accommodates variations in the bulk density of different batches of high-filled premix, avoiding excessive weight control while only controlling volume. By jointly limiting stacking height, temporary storage volume, and temporary storage weight, the high-filled premix can maintain a relatively loose stacking state within the buffer hopper and enter the twin-screw extruder in a continuous and stable manner.

[0039] When the material accumulation height in the buffer silo is less than 0.4m, the silo's buffering effect on the feed between the front-end premixing equipment and the rear-end twin-screw extruder is insufficient, easily leading to discontinuous feeding of the twin-screw extruder due to short-term fluctuations in front-end discharge. When the accumulation height is greater than 1.8m, the pressure at the bottom of the highly filled premix increases significantly, and compaction and interlocking easily occur between powder and resin particles, increasing the discharge resistance at the bottom of the silo. Temporary storage volume is less than 0.08m³. 3 At times, the buffer silo cannot provide a stable supply margin; the temporary storage volume is higher than 0.8m³. 3 At this time, the high-filler premix has a large residence time in the silo, which is not conducive to controlling the pressure of the material column and the residence time of the material. When the temporary storage weight is less than 50 kg, the buffering effect in continuous production is insufficient; when the temporary storage weight is more than 600 kg, for high-filler premix systems with high powder content, the risk of compaction and bridging at the bottom of the silo increases significantly.

[0040] The buffer silo can be a conical silo (wider at the top, narrower at the bottom), a silo with an arch-breaking structure, or a silo with a low-speed stirring-assisted discharge structure. The silo outlet can be connected to a loss-in-weight feeder, a screw feeder, or a forced feeding mechanism to allow the highly filled premix to enter the twin-screw extruder at a set feeding rate. To further prevent the formation of a compacted layer at the bottom of the silo, the inner wall of the buffer silo can be polished or coated with an anti-adhesion coating. The cone angle of the silo can be adjusted according to the material flowability. A low-amplitude vibrator, a slow-speed stirring paddle, or a flexible arch-breaking component can also be installed at the bottom of the silo to maintain material flow without significant shear heating. For highly filled premixes containing a high proportion of calcium carbonate powder, a loss-in-weight feeder combined with a buffer silo can be preferred to adjust the feed rate in real time, further reducing the impact of powder bulk density changes on extrusion stability.

[0041] In actual production, the buffer silo is not simply used for temporary material storage, but rather serves as a front-end steady-state feeding unit in the high-filler masterbatch preparation process. The front end ensures stable powder moisture content and particle size through drying and particle size classification. The middle section uses the buffer silo to limit the stacking height, temporary storage volume, and temporary storage weight, reducing bottom compaction and discharge fluctuations. Subsequently, twin-screw melt mixing, vacuum devolatilization, and secondary homogenization achieve melt stabilization. Therefore, the high-filler premix already possesses a relatively stable powder and feeding state before entering the twin-screw extruder, reducing feed load fluctuations in the subsequent melt mixing zone. This allows the primary anchoring coating layer and the melt-compatible coating layer to form under more stable material flow conditions, thereby improving the batch consistency and processing stability of the final high-filler steady-state masterbatch for stone paper.

[0042] Specifically, the primary anchoring coating agent is used to form an initial interface treatment layer on the particle surfaces of the first and second calcium carbonate powders, transforming the calcium carbonate particles from a hydrophilic, easily agglomerated state to a state more suitable for contact with polyolefin resin melts. The primary anchoring coating agent may include one or more of titanate coupling agents, aluminate coupling agents, silane coupling agents, stearic acid, oleic acid, or calcium stearate. Titanate and aluminate coupling agents can improve the interfacial affinity between the calcium carbonate particle surface and the polyolefin system, making them suitable for surface activation treatment of heavy and light calcium carbonate; silane coupling agents can enhance the interfacial bonding ability of the calcium carbonate particle surface, making them suitable for use with the second calcium carbonate powder, which has a finer particle size and higher surface activity; stearic acid, oleic acid, and calcium stearate can form a hydrophobic lubricating layer on the calcium carbonate particle surface, reducing friction and agglomeration between powders, resulting in better looseness of the powder during high-speed mixing, silo storage, and extrusion feeding.

[0043] The primary anchoring coating agent is added separately to the first and second calcium carbonate powders, rather than being completely mixed and coated all at once. The reason for separate coating is that the first and second calcium carbonate powders have different particle sizes, specific surface areas, surface energies, adsorption capacities, and agglomeration tendencies. The first calcium carbonate powder has a larger particle size, resulting in a relatively smaller total surface area per unit mass, thus requiring a lower amount of surface treatment agent. The second calcium carbonate powder has a smaller particle size, resulting in a larger total surface area per unit mass, making it more prone to agglomeration due to surface energy interaction. It also more readily adsorbs moisture and low-molecular-weight additives, thus requiring a higher unit mass of primary anchoring coating agent for surface treatment. If the first and second calcium carbonate powders were coated with the same amount of coating agent per unit mass, the second calcium carbonate powder would be insufficiently coated, continuing to enter the melt mixing zone as fine powder agglomerates. Simultaneously, excessive free coating agent might remain on the surface of the first calcium carbonate powder, leading to localized enrichment, slippage, precipitation, or melt pressure fluctuations during subsequent melt mixing.

[0044] Therefore, in this embodiment, the amount of primary anchoring coating agent per unit mass of the second calcium carbonate powder is 1.3 to 3 times that of the first calcium carbonate powder per unit mass. This multiple range allows the second calcium carbonate powder to obtain more sufficient surface coating strength, while preventing the formation of an excessively thick low-molecular-weight layer on the surface of the fine powder due to excessive coating agent. When the multiple is less than 1.3 times, the difference in specific surface area between the second calcium carbonate powder and the first calcium carbonate powder is difficult to be effectively compensated, and there may still be insufficiently coated areas on the surface of the fine powder, which can easily lead to the formation of fine powder agglomeration points later. When the multiple is greater than 3 times, the surface of the second calcium carbonate powder is prone to adsorbing excessive primary anchoring coating agent, which may form free additives during melt mixing, affecting the stability of the continuous coating of polyolefin relative to calcium carbonate particles, and may cause stickiness, precipitation, or increased odor on the surface of the masterbatch.

[0045] In one specific embodiment, the amount of primary anchoring coating agent used in the first calcium carbonate powder can be controlled to be 0.4% to 1.2% of the mass of the first calcium carbonate powder, and the amount of primary anchoring coating agent used in the second calcium carbonate powder can be controlled to be 0.8% to 2.4% of the mass of the second calcium carbonate powder, with the amount of primary anchoring coating agent per unit mass of the second calcium carbonate powder being higher than that of the first calcium carbonate powder. The above amounts can be further adjusted according to the particle size, oil absorption value, moisture content, and surface activity of the second calcium carbonate powder. For example, when the second calcium carbonate powder uses light calcium carbonate with a D50 particle size close to 0.4 μm, a higher amount of primary anchoring coating agent can be selected due to its higher specific surface area and oil absorption; when the second calcium carbonate powder uses refined heavy calcium carbonate with a D50 particle size close to 1.5 μm, a lower amount of primary anchoring coating agent can be selected to avoid excessive additives.

[0046] In another specific implementation, the primary anchoring coating agent can be allocated according to its total amount. Specifically, a higher proportion of the primary anchoring coating agent is preferentially used for the second calcium carbonate powder, with the remainder used for the first calcium carbonate powder. For example, when the first calcium carbonate powder accounts for 75% of the total mass of the compound calcium carbonate powder and the second calcium carbonate powder accounts for 25%, the primary anchoring coating agent can be allocated not simply according to the powder mass ratio, but rather to ensure that the second calcium carbonate powder receives a higher amount of coating agent than its mass percentage, thereby compensating for the larger surface treatment requirements of the fine-particle-size powder. This allocation method avoids the problem in traditional uniform coating where "a large proportion of coarse powder consumes too much additive, while a small proportion of fine powder results in insufficient surface treatment," making the coating agent distribution more closely match the actual interface treatment needs of calcium carbonate powders with different particle sizes.

[0047] The primary anchoring coating can be completed separately in a high-speed mixing device. After the first and second calcium carbonate powders enter the high-speed mixing device, they can be preheated and stirred to raise the powder temperature to a suitable range for the primary anchoring coating agent to spread. Then, the primary anchoring coating agent is sprayed in batches or continuously. When the primary anchoring coating agent is in a liquid or fusible state, it can be added by spraying, atomizing, or metering pump dripping. When the primary anchoring coating agent is in powder or granular form, it can be premixed with a small amount of powder before being added to the high-speed mixing device. During high-speed mixing, the powder surface is continuously renewed under centrifugal, shearing, and tumbling action. The primary anchoring coating agent gradually spreads and adheres to the surface of the calcium carbonate particles, forming a continuous or semi-continuous primary anchoring coating layer. To prevent excessive concentration of coating agent in certain areas, the primary anchoring coating agent can be added in two or three batches, maintaining a certain mixing time after each addition to allow the coating agent to fully diffuse to the powder surface before adding the next batch.

[0048] For the first calcium carbonate powder, the coating treatment can focus on reducing the surface polarity of the powder and improving its basic compatibility with polyolefin resins, enabling the first calcium carbonate powder to stably exist as the main skeleton in highly filled systems. For the second calcium carbonate powder, in addition to reducing surface polarity, the coating treatment also needs to focus on inhibiting secondary agglomeration between fine powders, allowing the second calcium carbonate powder to enter the gaps between the first calcium carbonate powders in a more dispersed state, and be further coated by the polyolefin melt and melt-grade compatibility coating agent during subsequent melt mixing. Thus, although both the first and second calcium carbonate powders form a primary anchoring coating layer, their coating strength, coating agent unit dosage, and coating purpose differ, allowing them to better adapt to coarse and fine particle size blending systems.

[0049] After the primary anchoring coating is completed, the first and second calcium carbonate powders can be temporarily stored separately, or they can be remixed according to a set ratio to form a differentiated coated compound calcium carbonate powder. The differentiated coated compound calcium carbonate powder should maintain a loose powder state in appearance, without obvious wet lumps, hard masses, or large areas of agglomeration. If necessary, low-intensity sieving or agglomeration treatment can be performed after coating to disperse soft lumps formed due to localized pressure or electrostatic adsorption, ensuring stable flowability of the powder entering the subsequent premixing stage. This treatment does not damage the primary anchoring coating layer on the particle surface; it is only used to improve the macroscopic flow state of the powder and the continuity of feeding.

[0050] Through the aforementioned differentiated primary anchoring coating, the second calcium carbonate powder achieves a higher surface coating strength than the first calcium carbonate powder, thereby reducing the agglomeration of fine-particle-size powder in the high-filler system. The first calcium carbonate powder, however, does not generate significant free additives due to excessive coating agent. This method ensures that the compounded calcium carbonate powder has a good interfacial pretreatment foundation before entering the twin-screw extruder, providing a stable surface for the subsequent formation of the melt-level compatible coating layer. In the final high-filler stable masterbatch for stone paper, calcium carbonate particles are more uniformly distributed within the polyolefin continuous phase, reducing white spots, clumps, and hard particles within the masterbatch. Consequently, surface pitting, band breaks, thickness fluctuations, and localized bulging during subsequent stone paper processing are also reduced.

[0051] Specifically, the raw materials for preparing high-filled stable masterbatch for stone paper may also include a devolatilization stabilization aid. Taking 100 parts by total mass of raw materials for preparing high-filled stable masterbatch for stone paper as an example, the amount of devolatilization stabilization aid added is 0.1 to 0.8 parts, and the sum of the amounts of all raw material components, including the devolatilization stabilization aid, is 100 parts. The devolatilization stabilization aid is mainly used to cooperate with subsequent vacuum devolatilization treatment to adsorb or fix residual moisture, acidic low-molecular-weight substances, and volatile small-molecule components that are difficult to completely remove from the high-filled melt, thereby reducing micropores, bubbles, and odor problems inside the masterbatch. Since the proportion of compounded calcium carbonate powder in high-filled stone paper masterbatch is relatively high, the powder surface easily carries a small amount of adsorbed moisture. A small amount of low-molecular-weight volatile components may also exist in the primary anchoring coating agent, lubricant, and compatibility coating agent. Sometimes, relying solely on a single vacuum devolatilization step is insufficient to completely eliminate the influence of these components on the stability of the masterbatch. Therefore, a small amount of devolatilization stabilization aid can further improve the devolatilization effect and the stability of the finished masterbatch.

[0052] Deviation stabilization aids may include one or more of activated magnesium oxide, hydrotalcite, microporous silicates, or molecular sieves. Activated magnesium oxide has certain adsorption and acid neutralization capabilities, enabling it to fix small amounts of acidic low-molecular-weight substances in highly filled melts, making it suitable for systems containing anhydride-grafted melt-grade compatibility coating agents. Hydrotalcite has a layered structure and certain acid adsorption capacity, enabling it to adsorb or stabilize trace amounts of acidic substances generated during resin processing, reducing the impact of small acid molecules on the thermal stability and odor of polyolefin resins. Microporous silicates and molecular sieves have microporous adsorption structures, allowing them to adsorb small amounts of moisture and low-molecular-weight volatiles, making them suitable for production conditions with large fluctuations in powder moisture content or high ambient humidity. The above-mentioned devolatilization stabilization aids can be used alone or in combination according to the raw material humidity, compatibilizer type, vacuum devolatilization efficiency, and target masterbatch odor control requirements.

[0053] The amount of devolatilization stabilizer added is controlled within the range of 0.1 to 0.8 parts. When the amount added is less than 0.1 parts, its adsorption or fixation effect on residual moisture and low-molecular-weight volatiles is not significant, making it difficult to demonstrate an auxiliary effect on vacuum devolatilization. When the amount added is greater than 0.8 parts, the devolatilization stabilizer itself, as an inorganic fine component, may increase the ash content and local particle count of the system. If the dispersion is insufficient, it may also form fine particle points in the masterbatch or subsequent stone paper sheets. Therefore, the devolatilization stabilizer is not used as a main filler component, but rather added in small amounts as a functional auxiliary component to cooperate with vacuum devolatilization and secondary homogenization to improve the internal stability of the highly filled melt.

[0054] Deviation stabilizing agents can be fed into the twin-screw extruder along with the high-filled premix, or they can be added to the high-filled melt before it enters the vacuum devolatilization zone. When the devolatilizing agent is fed into the twin-screw extruder along with the high-filled premix, it can gradually disperse in the high-filled system within the melt mixing zone and the enhanced dispersion zone, and fully contact residual moisture and low-molecular-weight volatile components before the high-filled melt enters the vacuum devolatilization zone. This addition method is suitable for production scenarios where the powder moisture content is relatively stable and the devolatilizing agent particle size is fine and easily dispersed. When the devolatilizing agent is added before the high-filled melt enters the vacuum devolatilization zone, it can concentrate its adsorption or fixation effect near the devolatilization zone, reducing over-dispersion, abrasion, or functional loss caused by excessive residence time in the high-shear zone at the front end. This addition method is suitable for production scenarios with high requirements for volatile matter control or where it is desirable to improve the adsorption efficiency at the front end of the vacuum devolatilization zone.

[0055] In one specific embodiment, the devolatilization stabilizer can be pre-dispersed with a small amount of polyolefin resin, primary anchored coated compounded calcium carbonate powder, or pre-lubricating components before being added to the high-filler premix. Pre-dispersion avoids localized concentration of the devolatilization stabilizer due to its small dosage, resulting in a more uniform distribution within the high-filler premix. For molecular sieve or microporous silicate devolatilization stabilizers, low-temperature drying can be performed before addition to activate their adsorption channels, preventing them from carrying moisture into the high-filler melt. For active magnesium oxide or hydrotalcite devolatilization stabilizers, their particle size can be controlled to be close to or slightly larger than that of the second calcium carbonate powder to reduce the possibility of forming independent coarse particles in the masterbatch.

[0056] In another specific embodiment, the devolatilization stabilization aid can be configured as a compound system, such as the combination of activated magnesium oxide and hydrotalcite, or the combination of microporous silicate and molecular sieve. When activated magnesium oxide is combined with hydrotalcite, it can simultaneously address the fixation of acidic low-molecular-weight compounds and assist in resin thermal stability; when microporous silicate is combined with molecular sieve, it can enhance the adsorption of moisture and low-molecular-weight volatile components. The above compounding methods can be adjusted according to the source of raw materials and the target performance. It is not required that every devolatilization stabilization aid be used simultaneously, as long as it can provide adsorption or fixation functions before the highly filled melt enters the vacuum devolatilization stage.

[0057] A synergistic relationship exists between the devolatilization stabilization aid and the vacuum devolatilization process. Vacuum devolatilization primarily removes moisture, low-molecular-weight volatiles, residual treatment agents, and entrained gases from the highly filled melt using a negative pressure environment. The devolatilization stabilization aid, on the other hand, adsorbs or fixes some of the residual small-molecule components that are difficult to completely remove under negative pressure, preventing them from existing in a free state within the highly filled melt. Especially in cases where the viscosity of the highly filled system is high and the gas diffusion path within the melt is long, the devolatilization stabilization aid can provide local adsorption sites within the melt, reducing the continued release of volatiles during subsequent secondary homogenization, extrusion pelletizing, or reprocessing of stone paper sheets. Thus, vacuum devolatilization is responsible for active removal, while the devolatilization stabilization aid is responsible for assisted adsorption and fixation; both work together to reduce the residual volatile components within the masterbatch.

[0058] It is important to note that the use of devolatilization stabilization additives should not disrupt the main structure of the original compounded calcium carbonate powder, polyolefin resin, and interfacial coating system. The particle size, dosage, and placement of the devolatilization stabilization additive should be matched to the melt-mixing state of the highly filled masterbatch, ensuring uniform dispersion within the polyolefin continuous phase and the gaps between calcium carbonate particles, rather than existing as independent large particles or localized agglomerates. To guarantee its dispersion effect, the devolatilization stabilization additive should pass through an enhanced dispersion zone or at least a melt mixing section before vacuum devolatilization, allowing it to fully contact the highly filled melt. If added before the vacuum devolatilization zone, a secondary homogenization zone is still required after the vacuum devolatilization zone to further stabilize the melt state and avoid fluctuations in the output caused by localized addition.

[0059] By adding a small amount of devolatilization stabilizing agent, the high-filled melt can initially adsorb or fix residual moisture and low-molecular-weight volatile components before vacuum devolatilization. During vacuum devolatilization, the discharge of volatiles is more stable, and the micropores and bubbles inside the high-filled melt after devolatilization are reduced. The resulting high-filled stable masterbatch for stone paper is less prone to problems such as internal voids, surface pinholes, strong odor, and increased volatile release after storage after pelletizing. When this masterbatch is reused in stone paper calendering, casting, or extrusion processes, it can reduce sheet bulging, bubbles, pinholes, pitting, and thickness fluctuations caused by continuous degassing, resulting in better continuous processing stability and surface quality consistency for the stone paper material.

[0060] Specifically, polyolefin resin serves as a continuous phase carrier in the high-filler stable masterbatch of stone paper. During the melt mixing process, it encapsulates the first and second calcium carbonate powders, allowing a high proportion of inorganic powders to be linked through the resin phase to form an extrudable, coolable pelletizing, and reprocessable masterbatch structure. The polyolefin resin can include one or more of high-density polyethylene, linear low-density polyethylene, low-density polyethylene, or polypropylene. High-density polyethylene possesses good rigidity, heat resistance, and processing stability, making it suitable for improving the structural support capacity of stone paper masterbatches. Linear low-density polyethylene and low-density polyethylene offer good flexibility and melt flow adaptability, improving the ductility of high-filler systems during extrusion, calendering, or casting, and reducing the risk of brittleness due to high powder content during subsequent processing of the high-filler masterbatch. Polypropylene has high rigidity and heat resistance, making it suitable for processing scenarios requiring high stiffness, heat distortion stability, and molding strength in stone paper sheets.

[0061] In one specific embodiment, the polyolefin resin can be a blend of a high melt flow rate resin and a low melt flow rate resin. The high melt flow rate resin can more quickly wet the surface of calcium carbonate particles in the early stages of melt mixing, improving the resin's penetration into the gaps between powder particles and reducing the probability of exposed powder and the formation of localized dry powder clumps. The low melt flow rate resin can provide a certain melt strength in the melt, ensuring good continuity of the highly filled melt during vacuum devolatilization, secondary homogenization, and die extrusion, preventing melt breakage or unstable discharge due to insufficient adhesion. By blending these two types of polyolefin resins, a balance can be achieved between powder wetting ability and melt strength, allowing the masterbatch to achieve a high calcium carbonate filling content while maintaining the melt stability required for subsequent processing.

[0062] Melt-grade compatibility coating agents are used to further improve the bonding relationship between the primary anchoring coating layer and the polyolefin continuous phase during the melt mixing process in a twin-screw extruder. Melt-grade compatibility coating agents can include one or more of maleic anhydride-grafted polyethylene, maleic anhydride-grafted polypropylene, or glycidyl methacrylate-grafted polyolefin. Maleic anhydride-grafted polyethylene is suitable for use in combination with polyethylene continuous phases, forming a transitional bond between the calcium carbonate particle surface and the polyethylene melt; maleic anhydride-grafted polypropylene is suitable for use in combination with polypropylene continuous phases, improving the interfacial bonding between calcium carbonate particles and resin in polypropylene systems; glycidyl methacrylate-grafted polyolefin has strong interfacial reactivity and polar interaction capabilities, making it suitable for use in systems requiring improved interfacial bond strength or high mechanical stability of the subsequent sheet.

[0063] The melt-grade compatibility coating agent is not simply added to the system as a regular compatibility agent. Instead, it is distributed between the primary anchoring coating layer and the molten polyolefin resin during the melt mixing process, forming a melt-grade compatibility coating layer on the outside of the primary anchoring coating layer. The primary anchoring coating layer mainly acts on the surface of calcium carbonate particles, transforming them from a hydrophilic, easily agglomerated state to a state more easily wetted by the resin. The melt-grade compatibility coating layer is further located on the outside of the primary anchoring coating layer, serving to form a resin phase transition layer between the calcium carbonate particles and the continuous polyolefin phase. Through this interfacial structure, a coating relationship is formed between the calcium carbonate particles, the primary anchoring coating layer, the melt-grade compatibility coating layer, and the continuous polyolefin phase, gradually transitioning from an inorganic to an organic phase. This reduces the problem of direct phase separation between inorganic powder and the resin phase in highly filled systems.

[0064] In the melt mixing zone of a twin-screw extruder, the polyolefin resin is first heated and melted to form a continuous flowing phase. The first and second calcium carbonate powders in the highly filled premix are gradually wetted by the molten resin under the action of screw conveying, shearing, and compression. As the material enters the enhanced dispersion zone, the melt-grade compatibility coating agent is dispersed to the vicinity of the calcium carbonate particle surface under shearing. Its grafted segments or polar groups can interact with the primary anchoring coating layer or the surface of the calcium carbonate particles. Simultaneously, its polyolefin segments can entangle or be compatible with the molten polyolefin resin, thereby forming a melt-grade compatibility coating layer outside the primary anchoring coating layer. This melt-grade compatibility coating layer can reduce the re-aggregation of calcium carbonate particles under high shear conditions and also improve the resin's ability to retain calcium carbonate particles.

[0065] For polyethylene systems, high-density polyethylene (HDPE) and linear low-density polyethylene (LDPE) blends are preferred as polyolefin resins, and maleic anhydride-grafted polyethylene (MIGPE) is preferred as a melt-grade compatibility coating agent. HDPE provides rigidity and molding support, LLDPE improves melt elongation and fracture resistance during traction, and MIGPE forms an interfacial transition layer between the calcium carbonate particles and the continuous polyethylene phase. This system is suitable for stone paper packaging films, composite paper materials, or stone paper materials requiring high flexibility.

[0066] For polypropylene systems, the polyolefin resin can be polypropylene or a blend of polypropylene and a small amount of polyethylene. The melt-grade compatibility coating agent can be maleic anhydride-grafted polypropylene. Polypropylene can improve the stiffness and heat resistance of the masterbatch in subsequent sheet preparation. Maleic anhydride-grafted polypropylene can improve the interfacial bonding between the continuous polypropylene phase and calcium carbonate particles, making the high-filler system less prone to powder detachment, localized white spots, and sheet brittleness during calendering or casting. For systems requiring further improvement in interfacial bonding strength, a small amount of glycidyl methacrylate-grafted polyolefin can be added to give the melt-grade compatibility coating layer even stronger interfacial bonding capabilities.

[0067] The amount of melt-grade compatibility coating agent added is controlled within the range of 1.2 to 4.5 parts as specified in the claims. When the amount added is too low, the melt-grade compatibility coating layer is not sufficiently formed, and there may still be insufficient local bonding between the calcium carbonate particles after primary anchoring coating and the polyolefin continuous phase, leading to the easy formation of exposed points or agglomeration points in the subsequent masterbatch. When the amount added is too high, the proportion of compatibility coating agent in the system is too large, which may increase melt viscosity or change the flow characteristics of the polyolefin continuous phase, while increasing production costs. Therefore, controlling the melt-grade compatibility coating agent within the range of 1.2 to 4.5 parts can balance interfacial coating effect, melt flow stability, and cost control.

[0068] Melt-grade compatibility coating agents can be added together with polyolefin resin during the high-filler premix formation stage, or they can be added through the front-side feed port of a twin-screw extruder. If added during the premixing stage, the melt-grade compatibility coating agent can pre-contact with the polyolefin resin particles and the coated compounded calcium carbonate powder, suitable for conventional continuous production. If added through the front-side feed port, the melt-grade compatibility coating agent can enter the system after the polyolefin resin begins to melt, reducing uneven distribution caused by localized powder adsorption during the dry powder conveying stage. The specific addition method can be selected based on the twin-screw extruder structure, resin form, and compatibility coating agent form.

[0069] By combining polyolefin resin and melt-grade compatibility coating agent, the compounded calcium carbonate powder no longer relies solely on primary anchoring coating agent for surface treatment. Instead, a resin phase transition coating structure is further formed during the melt mixing stage. This structure improves the stable dispersion of calcium carbonate particles in the continuous polyolefin phase, ensuring a relatively complete interfacial coating foundation for the highly filled melt before vacuum devolatilization. Because the calcium carbonate particles are well encapsulated by the resin phase and compatibility coating layer, powder disturbance or interfacial peeling is less likely to occur during vacuum devolatilization. A second homogenization after devolatilization yields a more stable melt state. The resulting highly filled stable masterbatch for stone paper exhibits better powder-resin interfacial bonding, reducing issues such as powder shedding, surface white spots, localized pitting, and strength fluctuations in subsequently processed stone paper sheets or films.

[0070] Specifically, the segmented lubricant and flow stabilizer consists of a pre-lubricating component and a post-flow stabilizing component. The pre-lubricating component is mainly added after the first and second calcium carbonate powders have undergone surface coating treatment and before the highly filled premix enters the twin-screw extruder. The post-flow stabilizing component is mainly added after the highly filled melt has undergone vacuum devolatilization treatment. By dividing the lubricant and flow stabilizer into two addition stages, the same type of lubricant and flow stabilizing system can play different roles in different process sections: the pre-lubricating component focuses on reducing dry friction between powder, resin particles, and coated calcium carbonate particles, reducing agglomeration, arching, and local compaction of the highly filled premix during mixing, temporary storage, and feeding; the post-flow stabilizing component focuses on restoring the flow continuity and pressure stability of the highly filled melt after vacuum devolatilization, reducing pressure fluctuations, sheet breakage, and uneven discharge that may occur when the devolatilized melt directly enters the die head.

[0071] The pre-lubricating components may include one or more of zinc stearate, calcium stearate, ethylene bis-stearamide, or polyethylene wax. Zinc stearate and calcium stearate have good surface lubrication properties for powders, reducing frictional resistance between calcium carbonate particles and between calcium carbonate particles and polyolefin resin particles, thus maintaining good flowability of the high-filler premix in high-speed mixing equipment, buffer silos, and feeding mechanisms. Ethylene bis-stearamide has both internal and external lubrication properties, improving the premixed dispersion state of the high-filler premix and reducing local shear resistance in the initial stage of subsequent melt mixing. Polyethylene wax has good compatibility with polyolefin systems and can improve the flow state between premix particles without significantly disrupting the continuous polyolefin phase, making it suitable for systems with a high powder content, such as high-filler masterbatch for stone paper.

[0072] The initial lubricant component accounts for 30% to 60% of the total mass of the segmented lubricant and flow stabilizer. When this proportion is below 30%, the powder of the highly filled premix is ​​insufficiently lubricated before entering the twin-screw extruder, which can easily lead to problems such as mutual adsorption between calcium carbonate particles, decreased premix looseness, and unstable discharge from the buffer hopper. When this proportion is above 60%, too much lubricant component is added at the front end, which may cause an excessively thick free lubricant layer on the powder surface in the early stage of melt mixing, affecting the wetting of the calcium carbonate particle surface by the polyolefin resin and the formation of the melt-level compatibility coating layer. It may also lead to the enrichment of local low-molecular-weight components before subsequent vacuum devolatilization. Therefore, controlling the initial lubricant component within the range of 30% to 60% can balance the flowability of the premix powder and the stability of subsequent melt coating.

[0073] The downstream flow stabilizer can include one or more of oxidized polyethylene wax, polyethylene wax, or ethylene bis-stearamide. Oxidized polyethylene wax has certain polarity and good high-temperature stability, which can improve the flow transition state of the resin phase around calcium carbonate particles in highly filled melts and reduce the resistance of the melt at the barrel, screw, and die head. Polyethylene wax has good compatibility with the continuous polyolefin phase and is suitable for improving the flow continuity of the melt after vacuum devolatilization. Ethylene bis-stearamide can play an auxiliary lubricating and dispersing role in highly filled melts, allowing the melt with locally increased viscosity after devolatilization to regain a more uniform flow state. The downstream flow stabilizer should preferably be a low-volatility lubricating material with good processing temperature resistance to avoid generating more low-molecular-weight volatiles again when added after vacuum devolatilization, which would affect the devolatilization effect.

[0074] The downstream flow stabilizer component accounts for 40% to 70% of the total mass of the segmented lubricating flow stabilizer, and as the remainder excluding the upstream lubricating component, its mass percentage is 100% of the upstream lubricating component. When the downstream flow stabilizer component proportion is less than 40%, the highly filled melt after vacuum devolatilization lacks sufficient flow stabilization and correction, easily leading to insufficient melt flow recovery in the secondary homogenization zone. When the downstream flow stabilizer component proportion is greater than 70%, the upstream lubricating component proportion decreases accordingly, and the looseness and feeding stability of the highly filled premix before entering the twin-screw extruder may be insufficient. Therefore, a balance needs to be maintained between the upstream lubricating component and the downstream flow stabilizer component to ensure both the flowability of the powder at the front end and the stability of the melt at the rear end.

[0075] In one specific embodiment, when the total amount of segmented lubricant and flow stabilizer is 1.5 parts, 0.6 parts can be used as the front-end lubricant component and 0.9 parts as the rear-end flow stabilizer component. The front-end lubricant component can be a compound of calcium stearate and polyethylene wax, used to reduce dry friction between calcium carbonate particles during the premixing stage; the rear-end flow stabilizer component can be a compound of oxidized polyethylene wax and ethylene bis-stearamide, used to improve the pressure-stabilizing and conveying capacity of the highly filled melt after vacuum devolatilization. This setup avoids all lubricants undergoing a complete high-shear and vacuum devolatilization process after being added all at the front end, reducing the risk of local enrichment, volatilization loss, or impact on the formation of a melt-level compatibility coating of low-molecular-weight lubricant components at the front end.

[0076] In another specific implementation, when the discharge stability of the front buffer silo is poor or the proportion of second calcium carbonate powder is high, the proportion of the front-end lubricating component can be appropriately increased, making it account for 50% to 60% of the total mass of the segmented lubricating and flow-stabilizing agent, to enhance the fluidity of the high-filled premix during temporary storage, conveying, and feeding stages. When the front-end feeding stability is good, but the melt pressure fluctuation is significant after vacuum devolatilization, the proportion of the rear-end flow-stabilizing component can be appropriately increased, making it account for 60% to 70% of the total mass of the segmented lubricating and flow-stabilizing agent, to enhance the pressure stabilization and continuous flow capability of the melt after devolatilization. The above proportion adjustments should still ensure that the sum of the mass percentages of the front-end lubricating component and the rear-end flow-stabilizing component is 100%, to avoid problems such as double counting of the front and rear dosages or incomplete total calculations.

[0077] When adding the pre-lubricating component, it can be premixed with the coated first calcium carbonate powder, coated second calcium carbonate powder, polyolefin resin, melt-grade compatibility coating agent, and antioxidant stabilizer after the first-stage anchoring coating is completed. At this stage, the pre-lubricating component is distributed between the powder particles and resin particles, improving the macroscopic flowability of the highly filled premix and preventing the material from forming significant agglomeration and compaction layers after entering the buffer silo. For production conditions with high powder content or high ambient humidity, the pre-lubricating component can be added in two stages: one part during the remixing of the coated powder, and the other part after the addition of the polyolefin resin, to improve the uniformity of contact between the powder and resin particles.

[0078] When adding the downstream stabilizing component, it is preferably placed after the vacuum devolatilization zone and before the secondary homogenization zone. It can be added through the side feed port, melt feed port, liquid metering pump, or melt additive injection port of the twin-screw extruder. If the downstream stabilizing component is a solid particle or powder, it can be metered and added through the side feed port; if it is a low-melting-point waxy material, it can be preheated and melted before being added to the high-filled melt via a metering pump. After addition, the downstream stabilizing component should not enter the die head directly, but should pass through the secondary homogenization zone to ensure thorough mixing with the high-filled melt after vacuum devolatilization. This prevents uneven die head output, die orifice accumulation, or sticky particle surfaces caused by excessively high local lubricant concentrations.

[0079] Because vacuum devolatilization removes moisture, low-molecular-weight volatiles, and entrained gases from highly filled melts, the original gases and small-molecule components inside the melt may change local viscosity and flow resistance after removal. If no flow stabilization correction is performed after devolatilization, the highly filled melt is prone to pressure fluctuations before entering the die head, especially when the calcium carbonate content reaches 75% to 86%, where the interaction between powders and the continuity of the resin phase are more easily affected by changes in the flow state. In this embodiment, by adding a downstream flow stabilizing component after devolatilization and cooperating with low-shear homogenization in a secondary homogenization zone, the highly filled melt can regain a stable and continuous flow state after the volatiles are removed.

[0080] The segmented lubricant and flow stabilizer can also synergize with the primary anchoring coating agent and the melt-compatible coating agent. The primary anchoring coating agent mainly improves the surface condition of calcium carbonate particles, the melt-compatible coating agent mainly forms a resin phase transition coating layer, and the segmented lubricant and flow stabilizer mainly regulates the flow behavior in both the powder and melt stages. The front-end lubricant component prevents the coated powder from re-agglomerating before entering the twin-screw extruder, and the back-end flow stabilizer component prevents the melt from losing continuous flowability due to volatile matter discharge and local viscosity changes after devolatilization. With the combination of these three components, the preparation process of high-filler masterbatch can be improved simultaneously in terms of powder dispersion, interfacial bonding, and melt stability.

[0081] By using the segmented lubricant and flow stabilizer described above, this embodiment avoids the problems of front-end additive enrichment, vacuum devolatilization loss, and insufficient flow stabilization at the back end caused by the one-time addition of lubricant in traditional processes. The resulting high-filled stable masterbatch for stone paper produces more continuous strips and more stable particle size during pelletizing, with less noticeable stickiness or precipitation on the particle surface. When subsequently used for processing stone paper sheets, films, or packaging materials, melt pressure fluctuations are reduced, sheet thickness consistency is improved, and white spots, pitting, strip breaks, bubbles, and bulging are reduced, thereby improving the continuous production stability of high-filled stone paper materials.

[0082] Specifically, the twin-screw extruder is sequentially configured along the material conveying direction with a melt mixing zone, a reinforced dispersion zone, a vacuum devolatilization zone, and a secondary homogenization zone. This zoning is not simply based on the heating section of a conventional extruder, but rather on functional configurations tailored to the characteristics of high-filler stable masterbatch for stone paper, including a high proportion of powder, stringent interface treatment requirements, difficulty in removing volatiles, and the need for the melt to regain a stable flow state after devolatilization. After entering the twin-screw extruder, the high-filler premix first undergoes polyolefin resin melting and initial powder wetting in the melt mixing zone. Then, in the reinforced dispersion zone, calcium carbonate particles are further dispersed, and a melt-level compatibility coating layer is formed. Next, moisture, low-molecular-weight volatiles, residual treatment agents, and entrained gases are removed in the vacuum devolatilization zone. Finally, it enters the secondary homogenization zone for low-shear homogenization and pressure-stabilized conveying. This sequential arrangement ensures that the coating, devolatilization, and steady-state recovery processes are sequentially connected, preventing premature devolatilization of the high-filler system before full coating and avoiding fluctuations in output caused by the melt entering the die head without homogenization after devolatilization.

[0083] The melt-mixing zone, located at the front of the twin-screw extruder, is used to gradually melt the polyolefin resin and initially coat the first and second calcium carbonate powders after the formation of the primary anchoring coating. When the high-filler premix first enters the melt-mixing zone, the polyolefin resin is still in a granular or semi-molten state, and the proportion of calcium carbonate powder is relatively high. If excessive shearing is applied immediately, it can easily lead to the compaction of powder agglomerates or excessive local temperature rise. Therefore, the melt-mixing zone preferentially uses conveyor screw elements, weak shear mixing elements, or screw combinations with gradual compression to allow the polyolefin resin to gradually soften and melt during a stable temperature rise, penetrating into the gaps between the compounded calcium carbonate powders. At this time, the primary anchoring coating can reduce the surface polarity of the calcium carbonate particles, making it easier for the molten resin to wet the powder surface, providing a foundation for subsequent enhanced dispersion and the formation of melt-level compatible coatings.

[0084] The temperature of the melt mixing zone can be adjusted according to the type of polyolefin resin. When the polyolefin resin is mainly polyethylene, the temperature of the melt mixing zone can be controlled between 150℃ and 190℃; when the proportion of polypropylene in the polyolefin resin is high, the temperature of the melt mixing zone can be appropriately increased to 170℃ to 200℃. If the temperature is too low, the polyolefin resin will not melt sufficiently and will not be able to effectively encapsulate the calcium carbonate particles, easily forming dry powder clumps and exposed powder points; if the temperature is too high, the thermal oxidation of the resin and the generation of low-molecular-weight volatiles will increase, which is not conducive to subsequent odor control and masterbatch color stability. Therefore, the temperature of the melt mixing zone should be avoided while ensuring that the resin is fully melted, so that the high-filled premix forms a high-filled melt with initial continuity.

[0085] The enhanced dispersion zone, located after the melt mixing zone, is used to allow the melt-grade compatibility coating agent to form a melt-grade compatibility coating layer outside the primary anchoring coating layer. After the melt mixing zone, the polyolefin resin has formed a basically continuous melt phase, and the calcium carbonate particles are initially wetted. However, some fine-particle-size second calcium carbonate powder may still exist in the form of small agglomerates, and there may also be areas of tight contact and insufficient resin penetration between some first calcium carbonate powder particles. The enhanced dispersion zone further breaks up these powder agglomerates through shearing, stretching, and folding mixing, allowing the melt-grade compatibility coating agent to disperse near the surface of the calcium carbonate particles and form a transition coating layer between the primary anchoring coating layer and the continuous polyolefin phase. This process enables the calcium carbonate particles to no longer rely solely on the primary anchoring coating layer for contact with the resin phase, but instead form a more stable interfacial bond with the continuous polyolefin phase through the melt-grade compatibility coating layer.

[0086] The enhanced dispersion zone can utilize a combination of kneading blocks, toothed mixing elements, reverse elements, and conveying elements to form a moderately sheared structure. For highly filled systems, the length of the high-shear section in the enhanced dispersion zone should not be excessively increased, otherwise it will cause excessive melt temperature rise, accelerated resin degradation, or excessive melt pressure. A better approach is to use segmented enhanced dispersion, i.e., setting short-range kneading blocks or toothed mixing elements between several conveying thread sections, allowing the highly filled melt to be dispersed through multiple short-range shearing and conveying recovery cycles. This not only breaks up calcium carbonate agglomerates but also avoids long-term high shearing that could cause powder recompaction or breakage of the continuous resin phase. The temperature of the enhanced dispersion zone can be controlled within the range of 160℃ to 210℃ to maintain the polyolefin resin in a good molten state while ensuring that the melt-grade compatibility coating agent has sufficient fluidity and interfacial migration capability.

[0087] The vacuum devolatilization zone is located after the enhanced dispersion zone and before the secondary homogenization zone. This location is a crucial control point in this embodiment. If the vacuum devolatilization zone is set too early, i.e., before or at the beginning of the melt-mixing zone, the polyolefin resin has not yet fully melted, the melt-level compatibility coating layer has not yet formed, and the calcium carbonate powder is still in a loose or semi-coated state. Vacuum extraction at this time can easily cause powder disturbance, localized loss of additives, or uneven devolatilization. If the vacuum devolatilization zone is set too late, i.e., near the die head, the highly filled melt has already entered a stable extrusion state, and the volatile components do not have enough time to diffuse and exit from the melt. This can easily cause residual gas to enter the die head with the melt and be released during subsequent extrusion pelletizing or stone paper processing. Therefore, setting the vacuum devolatilization zone after the enhanced dispersion zone and before the secondary homogenization zone allows devolatilization to be performed after the melt-level compatibility coating layer has formed and the powder dispersion state is relatively stable, while still retaining sufficient secondary homogenization length to restore the melt state after devolatilization.

[0088] The vacuum level in the vacuum devolatilization zone is controlled between -0.06 MPa and -0.095 MPa, and the devolatilization temperature is controlled between 160°C and 210°C. Below -0.06 MPa, the negative pressure extraction capacity is insufficient, resulting in inadequate removal of moisture, low-molecular-weight volatiles, and entrained gases from the highly filled melt. Above -0.095 MPa, the suction effect is too strong, potentially leading to excessive volatilization of some low-molecular-weight additives, or even melt overturning, material being sucked into the exhaust pipe, or localized foaming instability. Below 160°C, the viscosity of the highly filled melt is high, the diffusion rate of volatile components is slow, and the devolatilization efficiency decreases. Above 210°C, the risk of thermal oxidation of the polyolefin resin increases, and low-molecular-weight components in the primary anchoring coating agent, lubricant, and compatibility coating agent may also increase volatilization, thus increasing the devolatilization load. Therefore, operating the vacuum devolatilization zone within the aforementioned vacuum level and temperature range can balance volatile removal efficiency and the stability of the highly filled melt.

[0089] The vacuum devolatilization zone can be equipped with one or more vacuum exhaust ports, which are connected to a vacuum pump, a condensation and collection device, and a filter. A structure to prevent powder carry-out can be installed above the vacuum exhaust port, such as a baffle, an enlarged exhaust chamber section, a metal filter, or a cyclone gas-solid separation structure, to reduce the risk of fine powder particles in the highly filled melt entering the vacuum pipeline with the airflow. Since the highly filled melt in this embodiment has already passed through a reinforced dispersion zone and formed a melt-level compatibility coating layer, the calcium carbonate particles are well encapsulated by the resin phase and the compatibility coating layer, relatively reducing the risk of powder being carried away during vacuum devolatilization. However, production stability can still be further ensured through the exhaust port structure and negative pressure control. For powder batches with large fluctuations in moisture content, a short-distance pressure-reducing conveying section can also be set before the vacuum devolatilization zone to appropriately reduce the pressure of the highly filled melt before entering the vacuum exhaust port, avoiding sudden pressure release that could cause melt turbulence.

[0090] During vacuum devolatilization, moisture, low-molecular-weight volatiles, residual treatment agents, and entrained gases in the highly filled melt gradually migrate from the melt interior to the melt surface and are discharged through the exhaust port under negative pressure. Moisture mainly originates from residual adsorbed water on the surface of the calcium carbonate powder and trace amounts of moisture introduced from the production environment; low-molecular-weight volatiles may originate from small amounts of low-molecular-weight components in the polyolefin resin, volatile components in the lubricant, or low-boiling-point substances in the processing aid; residual treatment agents mainly originate from free components in the primary anchoring coating agent that are not fully adhered to the powder surface; and entrained gases mainly originate from air introduced during powder mixing, silo storage, and extrusion feeding of the highly filled premix. By performing vacuum devolatilization after the formation of the melt-level compatible coating layer, the content of the aforementioned volatile components can be reduced without significantly damaging the coating interface.

[0091] A secondary homogenization zone is set up after the vacuum devolatilization zone to prevent the devolatilized high-filled melt from directly entering the extruder head. After vacuum devolatilization, the internal gas and low-molecular-weight components of the high-filled melt are reduced, and the local melt volume, viscosity, and pressure states may change. At the same time, the downstream flow-stabilizing components also need to be added and fully dispersed after devolatilization. If extrusion is carried out immediately after devolatilization, sudden changes in melt pressure, discontinuous extrusion, uneven particle size, or local material accumulation in the die head are likely to occur. The secondary homogenization zone performs low-shear mixing and pressure-stabilized conveying of the high-filled melt after the vacuum devolatilization zone, so that the high-filled melt re-establishes a continuous, uniform, and pressure-stable flow state, providing stable conditions for subsequent extrusion, cooling, and pelletizing.

[0092] Through the functional zone configuration of the twin-screw extruder described above, this embodiment forms a continuous processing path of "melt wetting—enhanced dispersion—post-coating devolatilization—post-devolatilization homogenization". The melt mixing zone ensures that the resin is fully melted and initially coats the powder; the enhanced dispersion zone ensures the formation of a melt-level compatible coating layer and reduces powder agglomeration; the vacuum devolatilization zone removes volatile components after the coating layer is formed; and the secondary homogenization zone restores the flow continuity and pressure stability of the melt after devolatilization. This processing path can better adapt to stone paper masterbatch systems with calcium carbonate content as high as 75% to 86%, resulting in final masterbatch with lower volatile content, fewer internal micropores, more stable particle morphology, and better adaptability to subsequent stone paper processing.

[0093] Specifically, the secondary homogenization zone is located after the vacuum devolatilization zone and is used for low-shear homogenization, mixing, and pressure stabilization of the high-filled melt after vacuum devolatilization. After passing through the vacuum devolatilization zone, moisture, low-molecular-weight volatiles, residual treatment agents, and entrained gases are expelled from the high-filled melt, reducing the internal gas content. However, because the expulsion of volatile components alters the local melt volume and flow state, the high-filled melt may experience local viscosity changes, short-term pressure fluctuations, or decreased flow continuity after the devolatilization zone. This is especially true in systems with a calcium carbonate content of 75% to 86%, where the proportion of continuous polyolefin resin is relatively low, making the melt more sensitive to pressure fluctuations and local flow changes. If the high-filled melt after vacuum devolatilization directly enters the extruder head, problems such as inconsistent extrusion thickness, breakage, pressure fluctuations at the extruder head, and inconsistent pellet size can easily occur. Therefore, a secondary homogenization zone is set up after the vacuum devolatilization zone to restore the high-filled melt to a continuous, uniform, and stable flow state before it enters the extruder head.

[0094] The secondary homogenization zone can be equipped with one or more of the following: low-shear threaded elements, weak-mixing threaded elements, or pressure-stabilizing conveying threaded elements. Low-shear threaded elements are primarily used to stably convey the highly filled melt towards the die head without excessively increasing shear heat, allowing the melt pressure after devolatilization to gradually recover. Weak-mixing threaded elements are mainly used to gently mix the downstream stabilizing component and the highly filled melt, ensuring that the downstream stabilizing component is uniformly dispersed within the highly filled melt, preventing further damage to the resin phase or re-agglomeration of calcium carbonate particles due to strong shear. Pressure-stabilizing conveying threaded elements are mainly used to establish a relatively stable melt conveying pressure at the end of the secondary homogenization zone, maintaining a stable feed state when the highly filled melt enters the extruder die head. These threaded elements can be combined and arranged according to the twin-screw extruder specifications, masterbatch filling ratio, resin melt index, and target output rate.

[0095] After the downstream stabilizing component is added to the high-filled melt after vacuum devolatilization, it undergoes low-shear homogenization mixing with the high-filled melt in the secondary homogenization zone. The downstream stabilizing component can be added through a side feed port, melt additive injection port, or metering pump located after the vacuum devolatilization zone on a twin-screw extruder. If the downstream stabilizing component is a solid granular or powdered material, it can be added via side feeding, allowing it to enter the high-filled melt at the beginning of the secondary homogenization zone. If the downstream stabilizing component is a low-melting-point waxy material, it can be heated and melted first, and then injected into the high-filled melt via metering pump. After the downstream stabilizing component is added, it should undergo sufficient low-shear mixing in the secondary homogenization zone to ensure its uniform distribution within the polyolefin continuous phase and the gaps between calcium carbonate particles. This prevents excessively high local stabilizing component content, which could cause die slippage, surface precipitation, or particle stickiness.

[0096] The use of a low-shear method in the secondary homogenization zone is of great significance. The enhanced dispersion zone before vacuum devolatilization is mainly used to break up powder agglomerates and form a melt-level compatible coating layer, exhibiting relatively high shear strength. However, after vacuum devolatilization, the calcium carbonate particles are already sufficiently dispersed, and the melt-level compatible coating layer has formed. Continuing to use excessive shear at this point may disrupt the existing interfacial coating, increasing the risk of thermal oxidation of the polyolefin resin and causing the melt temperature to continue to rise. Using low-shear and weakly mixing threaded elements can maintain the stability of the interfacial coating structure while dispersing the downstream flow-stabilizing components and restoring the melt pressure. This allows the highly filled melt to gradually transition from a locally unstable state after devolatilization to a stable flow state suitable for die extrusion.

[0097] In one specific embodiment, a set of weak mixing threaded elements can be set in the front section of the secondary homogenization zone to initially introduce the downstream stabilizing component into the highly filled melt; a low-shear conveying threaded element is set in the middle section to extend the contact time between the downstream stabilizing component and the highly filled melt; and a pressure-stabilizing conveying threaded element is set in the final section to establish a stable pressure before entering the extruder die. The weak mixing threaded elements can be kneading blocks or dispersing mixing elements with a small staggered angle to achieve moderate agitation and dispersion of the melt; the low-shear conveying threaded elements can be conventional forward conveying thread sections to convey the highly filled melt forward under low shear conditions; and the pressure-stabilizing conveying threaded elements can be conveying thread sections with a relatively stable compression ratio to reduce pressure fluctuations of the melt before entering the die.

[0098] In another specific embodiment, when the calcium carbonate content in the high-filled melt is close to 75%, the continuous resin phase is relatively abundant, and the secondary homogenization zone can be appropriately shortened, mainly serving the functions of dispersing the downstream flow stabilizing components and restoring pressure. When the calcium carbonate content is close to 86%, the proportion of powder in the high-filled melt is high, increasing the melt flow resistance. Therefore, the secondary homogenization zone can be appropriately extended, and the length of the pressure-stabilizing conveying thread element can be increased, allowing the high-filled melt to achieve more thorough low-shear homogenization and pressure stabilization before entering the die head. Thus, the secondary homogenization zone can be adapted according to the masterbatch filling ratio, enabling stable extrusion of melts with different high filling levels.

[0099] The secondary homogenization zone can also be used in conjunction with temperature control to achieve steady-state delivery. The temperature of the secondary homogenization zone can be controlled within the range of 150℃ to 190℃, with the specific temperature adjusted according to the type of polyolefin resin and the viscosity of the high-filled melt. If the temperature is too low, the viscosity of the high-filled melt after vacuum devolatilization increases, making it difficult for the downstream flow stabilizer component to disperse evenly, and the die head pressure is prone to increase. If the temperature is too high, the polyolefin resin and lubricating flow stabilizer component are prone to generating additional low-molecular-weight volatiles, weakening the effect of the preceding vacuum devolatilization. Therefore, the temperature of the secondary homogenization zone should be lower than or not higher than the high temperature state of the reinforced dispersion zone to maintain the flowability of the high-filled melt while reducing resin thermal oxidation and additive volatilization.

[0100] The pressure-stabilizing conveying function of the secondary homogenization zone also improves pelleting stability. If the pressure fluctuates significantly before the high-filled melt enters the extruder die, the diameter of the extruded strips will change, resulting in uneven pellet size after cooling, and some masterbatches may be hollow, broken, or have a rough surface. By using the secondary homogenization zone to homogenize and stabilize the pressure of the high-filled melt after vacuum devolatilization, the melt pressure entering the die remains relatively stable, resulting in more consistent extruded strip diameters and more uniform particle size and appearance of the cooled and pelletized masterbatches. This effect has practical significance for subsequent automatic metering, continuous feeding, and melt processing in stone paper production.

[0101] Through the aforementioned secondary homogenization zone setup, the downstream stabilizing component can fully exert its effect after vacuum devolatilization and form a uniform distribution with the highly filled melt. The secondary homogenization zone neither exerts excessive shear on the melt like the enhanced dispersion zone nor merely serves a simple pushing function like a conventional conveying section. Instead, it undertakes three functions: restoring the melt state after devolatilization, dispersing the downstream stabilizing component, and stabilizing the pressure at the die head. Therefore, the preparation process of high-filled stable masterbatch for stone paper can maintain stable extrusion even after the volatiles are removed by vacuum devolatilization, resulting in masterbatch with more stable particle size, lower internal micropores, and better flowability for subsequent processing.

[0102] Specifically, a high-filled stable masterbatch for stone paper is obtained after extrusion, cooling, and pelletizing. This masterbatch contains 75% to 86% calcium carbonate, no more than 0.5% volatile matter, no more than 0.3% moisture, and has a particle size of 2mm to 5mm. Controlling the calcium carbonate content within the range of 75% to 86% ensures a high inorganic filler ratio, meeting the requirements of reducing polyolefin resin usage, increasing the proportion of inorganic powder, and achieving a paper-like texture in stone paper production. Simultaneously, it retains a sufficient continuous polyolefin phase, enabling the masterbatch to have basic melt flow capability during subsequent extrusion, calendering, or casting processes. If the calcium carbonate content is below 75%, the high-filling characteristics of the masterbatch are insufficient, leading to a lower inorganic filler ratio in subsequent stone paper products, which is detrimental to achieving the necessary stiffness, opacity, and cost advantages of stone paper materials. If the calcium carbonate content is above 86%, the proportion of the continuous polyolefin resin phase is too low, making it easy for calcium carbonate particles to form direct contact, reducing melt continuity, and causing breakage, brittleness, white spots, and pressure fluctuations during subsequent processing.

[0103] The volatile matter content of the masterbatch should not exceed 0.5%, primarily used to limit the total amount of residual moisture, low-molecular-weight volatiles, residual treatment agents, and entrained gases within the masterbatch. High-filler masterbatches have a higher powder content; if residual moisture and low-molecular-weight additives on the powder surface are not properly controlled, they can easily continue to be released during the remelting and processing of stone paper sheets or films, leading to internal bubbles, surface pinholes, localized bulges, pitting, and odors. Therefore, by controlling the final volatile matter content of the masterbatch below 0.5% through front-end powder drying, differentiated primary anchoring coating, melt-level compatibility coating, vacuum devolatilization, and secondary homogenization after devolatilization, the continuous gas release during subsequent processing can be reduced, resulting in a more stable stone paper processing process.

[0104] The moisture content of the masterbatch should not exceed 0.3%. This moisture content indicator corresponds to the moisture content control in the powder pretreatment stage, but their roles differ. Controlling the moisture content of the first and second calcium carbonate powders at a low level during the powder drying stage improves the primary anchoring and coating effect and reduces moisture release during melt mixing. The final masterbatch moisture content of no more than 0.3% ensures that the finished masterbatch is less prone to bubbles, voids, and sheet surface defects due to excessive internal residual moisture during storage, transportation, and reprocessing. For continuous stone paper processing lines, a lower masterbatch moisture content also reduces the burden of re-drying and improves production stability after material input.

[0105] The particle size of high-filler stable masterbatch for stone paper is controlled between 2mm and 5mm. If the particle size is less than 2mm, the masterbatch particles are too small, easily generating dust during packaging, transportation, and feeding, and may also cause bridging or metering deviations during automatic metering feeding. If the particle size is greater than 5mm, the masterbatch particles are too large, slowing down the melting speed during subsequent melting processing, and easily forming locally incompletely plasticized particles in the extruder or calender, affecting the surface smoothness of the stone paper sheet. Controlling the particle size within the range of 2mm to 5mm balances cooling and pelletizing stability, packaging and transportation convenience, automatic metering feeding accuracy, and remelting and plasticizing efficiency, making the masterbatch more suitable for continuous stone paper production.

[0106] In one specific embodiment, the highly filled melt after secondary homogenization is extruded into strips through an extruder die. The strips are then cooled and shaped using a water-cooling tank or an air-cooled conveyor, and subsequently fed into a pelletizer via a traction device. During cooling, the outer and inner layers of the strips should be cooled uniformly to prevent the surface from hardening while the interior remains soft at a high temperature, which could lead to pellet deformation or agglomeration after pelleting. During pelleting, the cutter speed, traction speed, and extrusion diameter can be adjusted according to the target particle size to maintain the masterbatch particle size stably within the range of 2mm to 5mm. For masterbatches with a high calcium carbonate content, the traction stability and cutter sharpness can be appropriately increased to prevent edge chipping, powder shedding, or breakage of the highly filled strips during pelleting.

[0107] The resulting high-filler stable masterbatch for stone paper can be used for extrusion, calendering, or casting of stone paper sheets, films, packaging paper, or composite materials. When used for stone paper sheet processing, the masterbatch can be added to sheet production equipment along with appropriate amounts of polyolefin resin, processing aids, or other functional fillers. After melt plasticizing, calendering to thickness, and cooling and winding, sheet material is formed. Because the calcium carbonate powder in the masterbatch has undergone coarse and fine particle size blending, differentiated coating, and vacuum devolatilization treatment, it is easier to form a uniform melt after entering the sheet production line, reducing white spots, pits, particle protrusions, and localized bulges on the sheet surface.

[0108] When used in stone paper film processing, the masterbatch can be combined with polyolefin resin for casting or extrusion film formation. Ordinary high-filler masterbatches are prone to localized weak points due to powder agglomeration during film processing, or pinholes and bubbles due to volatile release. The masterbatch prepared in this embodiment has more uniform powder dispersion and lower volatile matter and moisture content, reducing the risk of film breakage, pores, crystal points, and thickness fluctuations during film formation. For stone paper films requiring printing, lamination, or heat sealing, the improved masterbatch stability also reduces surface precipitation and localized powder exposure, making subsequent surface treatments more stable.

[0109] When used in stone paper packaging paper processing, masterbatch can improve the surface smoothness, thickness consistency, and continuous forming stability of the packaging paper. Packaging paper typically needs to possess certain stiffness, opacity, and printability. The compounded calcium carbonate powder in high-filler masterbatch provides an inorganic paper feel and whiteness base, the continuous polyolefin phase ensures forming and toughness, and the melt-compatible coating layer improves the bonding stability between the powder and resin. By controlling the volatile matter and moisture content of the final masterbatch, surface defects caused by secondary gas release from the masterbatch are less likely to occur during packaging paper production, which is beneficial for subsequent printing patterns, lamination, and composite processing.

[0110] When used in stone paper composite processing, masterbatch can be used in conjunction with other resin layers, fiber layers, film layers, or functional coatings. Because the calcium carbonate particles in the masterbatch are relatively stably dispersed, localized particle agglomeration during composite processing is less likely to cause interfacial unevenness, thus improving the bonding stability between composite layers. For stone paper materials requiring hot pressing, lamination, or coating, low-volatile masterbatch can reduce gas release during hot processing, resulting in a smoother composite interface and reducing problems such as blistering, delamination, and localized voids.

[0111] Regarding masterbatch storage, because the calcium carbonate granules undergo primary anchoring coating and melt-compatible coating treatment, free powder and free additives are relatively reduced, making it less prone to significant powder return, stickiness, and precipitation on the surface of the masterbatch granules. The masterbatch has a low moisture content, making it less likely to clump due to moisture absorption or release gas during reprocessing. In actual storage, moisture-proof packaging bags or inner-lined film packaging bags can be used for sealed packaging to avoid prolonged exposure to high humidity environments. For mass production, the masterbatch moisture content, volatile matter, particle size distribution, and appearance can be recorded batch by batch to allow subsequent stone paper production lines to adjust feeding based on the masterbatch's condition.

[0112] Through the control of the above-mentioned finished product indicators, the high-filled stable masterbatch for stone paper exhibits clearly detectable performance at the finished product stage. Calcium carbonate content reflects the high filling degree of the masterbatch, volatile matter content reflects the effect of vacuum devolatilization and its stabilizing effect, moisture content reflects the drying of the powder and the final storage stability of the masterbatch, and particle size reflects the extrusion pelleting stability and subsequent feeding adaptability. These indicators collectively reflect that this preparation method does not merely complete ordinary mixing and granulation, but rather achieves a relatively stable state in terms of composition, interface structure, volatile matter control, and particle morphology of the high-filled masterbatch through graded coating, melt-level compatibility coating, vacuum devolatilization, and secondary homogenization.

[0113] Therefore, the high-filler stable masterbatch for stone paper prepared in this embodiment can reduce common problems of powder agglomeration, white spots, pitting, bubbles, bulging, band breakage, thickness fluctuation and precipitation after storage in the subsequent processing of stone paper sheets, films, packaging paper or composite materials, thereby improving the continuous production stability, surface quality consistency and adaptability of stone paper materials to subsequent composite processing.

[0114] Example 1 Based on a total raw material mass of 100 parts for preparing high-filled stable masterbatch for stone paper, the following components are used: 80 parts of compounded calcium carbonate powder, 14 parts of polyolefin resin, 1.2 parts of primary anchoring coating agent, 2.6 parts of melt-grade compatibility coating agent, 1.4 parts of segmented lubricating flow stabilizer, 0.3 parts of antioxidant stabilizer, and 0.5 parts of devolatilization stabilization aid. In the compounded calcium carbonate powder, the first calcium carbonate powder is heavy calcium carbonate with a D50 particle size of 4.0 μm, with an addition amount of 60 parts; the second calcium carbonate powder is light calcium carbonate with a D50 particle size of 0.8 μm, with an addition amount of 20 parts. The polyolefin resin consists of 8 parts of high-density polyethylene and 6 parts of linear low-density polyethylene. The melt-grade compatibility coating agent is maleic anhydride-grafted polyethylene. The antioxidant stabilizer is a combination of hindered phenolic antioxidant and phosphite antioxidant. The devolatilization stabilization aid is a combination of hydrotalcite and microporous silicate.

[0115] In the primary anchoring coating agent, 0.72 parts are used for the first calcium carbonate powder and 0.48 parts are used for the second calcium carbonate powder. The amount of primary anchoring coating agent per unit mass of the first calcium carbonate powder is 1.2%, and the amount per unit mass of the second calcium carbonate powder is 2.4%. The amount of primary anchoring coating agent per unit mass of the second calcium carbonate powder is twice that of the first calcium carbonate powder. In the segmented lubricating and flow-stabilizing agent, 0.56 parts are used for the initial lubrication component and 0.84 parts are used for the subsequent flow-stabilizing component. The initial lubrication component accounts for 40% of the total mass of the segmented lubricating and flow-stabilizing agent, and the subsequent flow-stabilizing component accounts for 60%.

[0116] The first and second calcium carbonate powders were subjected to hot air drying and air classification, respectively. After drying, the moisture content of the first calcium carbonate powder was 0.18%, and that of the second calcium carbonate powder was 0.21%. Both types of calcium carbonate powders were then fed into a high-speed mixer for primary anchoring coating. After coating, they were mixed with polyolefin resin, melt-grade compatibility coating agent, antioxidant stabilizer, devolatilization stabilization aid, and pre-lubricating components to obtain a high-filler premix. The high-filler premix was then fed into a buffer silo, where the material stacking height was controlled at 1.0 m and the temporary storage volume was controlled at 0.35 m³. 3 The temporary storage weight is controlled at 260kg.

[0117] The highly filled premixed material is fed into a co-rotating twin-screw extruder with a length-to-diameter ratio of 44:1 and a screw diameter of 75 mm. The production rate is controlled at approximately 280 kg / h to 320 kg / h. The temperature in the melt mixing zone is controlled at 165℃ to 180℃, the temperature in the enhanced dispersion zone is controlled at 180℃ to 190℃, the temperature in the vacuum devolatilization zone is controlled at 185℃, and the vacuum degree is controlled at -0.085 MPa. After vacuum devolatilization, a downstream stabilizing component is added, and the temperature in the secondary homogenization zone is controlled at 175℃ to 185℃. After low-shear homogenization, the material is extruded, cooled, and pelletized to obtain highly filled stable masterbatch for stone paper.

[0118] Example 2 Based on a total raw material mass of 100 parts for preparing high-filled stable masterbatch for stone paper, the following components were used: 84 parts of compounded calcium carbonate powder, 10.8 parts of polyolefin resin, 1.4 parts of primary anchoring coating agent, 2.0 parts of melt-grade compatibility coating agent, 1.3 parts of segmented lubricating flow stabilizer, 0.2 parts of antioxidant stabilizer, and 0.3 parts of devolatilization stabilization aid. In the compounded calcium carbonate powder, the first calcium carbonate powder was heavy calcium carbonate with a D50 particle size of 5.0 μm, with an addition amount of 67.2 parts; the second calcium carbonate powder was refined heavy calcium carbonate with a D50 particle size of 1.1 μm, with an addition amount of 16.8 parts. The polyolefin resin used was polypropylene, the melt-grade compatibility coating agent was maleic anhydride-grafted polypropylene, and the devolatilization stabilization aid was hydrotalcite.

[0119] In the primary anchoring coating agent, 0.84 parts are used for the first calcium carbonate powder and 0.56 parts are used for the second calcium carbonate powder. The amount of primary anchoring coating agent per unit mass of the first calcium carbonate powder is 1.25%, and the amount per unit mass of the second calcium carbonate powder is 3.33%. The amount of primary anchoring coating agent per unit mass of the second calcium carbonate powder is approximately 2.67 times that of the first calcium carbonate powder. In the segmented lubricating and flow-stabilizing agent, 0.65 parts are used for the initial lubrication component and 0.65 parts are used for the subsequent flow-stabilizing component, each accounting for 50% of the total mass of the segmented lubricating and flow-stabilizing agent.

[0120] The first and second calcium carbonate powders were dried, sieved, and air-classified, respectively. After drying, the moisture content of the first calcium carbonate powder was 0.20%, and that of the second calcium carbonate powder was 0.24%. After the high-filler premix entered the buffer silo, the material stacking height was controlled at 0.9m, and the temporary storage volume was controlled at 0.30m³. 3The temporary storage weight is controlled at 240 kg. The twin-screw extruder has a screw diameter of 75 mm, and the production rate is controlled at approximately 260 kg / h to 300 kg / h. The temperature in the melt mixing zone is controlled at 175℃ to 190℃, the temperature in the enhanced dispersion zone is controlled at 190℃ to 200℃, the temperature in the vacuum devolatilization zone is controlled at 195℃, and the vacuum degree is controlled at -0.09 MPa. After vacuum devolatilization, a downstream stabilizing component is added, and after secondary homogenization, it is extruded and pelletized to obtain a high-filled stable masterbatch for stone paper with a high calcium carbonate content.

[0121] Example 3 Based on a total raw material mass of 100 parts for preparing high-filled stable masterbatch for stone paper, the following components were used: 78 parts of compounded calcium carbonate powder, 16 parts of polyolefin resin, 1.0 part of primary anchoring coating agent, 3.0 parts of melt-grade compatibility coating agent, 1.5 parts of segmented lubricating flow stabilizer, 0.2 parts of antioxidant stabilizer, and 0.3 parts of devolatilization stabilizing agent. In the compounded calcium carbonate powder, the first calcium carbonate powder was heavy calcium carbonate with a D50 particle size of 3.5 μm, with an addition amount of 54.6 parts; the second calcium carbonate powder was near-spherical calcium carbonate with a D50 particle size of 0.7 μm, with an addition amount of 23.4 parts. The polyolefin resin consisted of 9 parts of high-density polyethylene and 7 parts of low-density polyethylene. The melt-grade compatibility coating agent was a compound of maleic anhydride-grafted polyethylene and glycidyl methacrylate-grafted polyolefin.

[0122] In the primary anchoring coating agent, 0.55 parts are used for the first calcium carbonate powder and 0.45 parts are used for the second calcium carbonate powder. The amount of primary anchoring coating agent per unit mass of the first calcium carbonate powder is approximately 1.01%, and the amount per unit mass of the second calcium carbonate powder is approximately 1.92%. The amount of primary anchoring coating agent per unit mass of the second calcium carbonate powder is approximately 1.9 times that of the first calcium carbonate powder. In the segmented lubricating and flow-stabilizing agent, 0.6 parts are used for the initial lubrication component and 0.9 parts are used for the subsequent flow-stabilizing component. The initial lubrication component accounts for 40%, and the subsequent flow-stabilizing component accounts for 60%.

[0123] The first and second calcium carbonate powders are dried to a moisture content of less than 0.3%. When the high-filler premix enters the buffer silo, the stacking height is controlled at 1.1m and the temporary storage volume is controlled at 0.40m³. 3 The temporary storage weight is controlled at 285 kg. The twin-screw extruder has a screw diameter of 75 mm and a production rate of approximately 300 kg / h. The temperature in the melt mixing zone is controlled at 160℃ to 175℃, the temperature in the strengthening and dispersing zone is controlled at 175℃ to 188℃, the temperature in the vacuum devolatilization zone is controlled at 180℃, and the vacuum degree is controlled at -0.08 MPa. After vacuum devolatilization, a downstream stabilizing component is added, and after secondary homogenization, the mixture is extruded, cooled, and pelletized to obtain a highly filled stable masterbatch suitable for processing stone paper film and packaging paper.

[0124] Comparative Example 1 Comparative Example 1 used the same total raw material amount and total calcium carbonate content as Example 1, but only 80 parts of heavy calcium carbonate powder with a D50 particle size of 4.0 μm were used, and no second calcium carbonate powder was added; the primary anchoring coating agent was applied using a one-time uniform coating method; all segmented lubricating and flow stabilizing agents were added during the premixing stage, and no further flow stabilizing components were added after vacuum devolatilization. The remaining extrusion temperature, vacuum devolatilization conditions, and pelletizing method were the same as in Example 1.

[0125] Comparative Example 2 Comparative Example 2 used the same ratio of first calcium carbonate powder to second calcium carbonate powder as Example 1, and also adopted a differentiated primary anchoring coating method. However, vacuum devolatilization was not performed during the twin-screw extrusion process, and the downstream stabilizing component was still added in the downstream stage and subjected to a second homogenization treatment. All other conditions were the same as in Example 1.

[0126] Comparative Example 3 Comparative Example 3 used the same ratio of first calcium carbonate powder to second calcium carbonate powder as Example 1, and also underwent vacuum devolatilization. However, the primary anchoring coating agent was evenly distributed according to the mass ratio of the first calcium carbonate powder to the second calcium carbonate powder, and the amount of primary anchoring coating agent per unit mass of the second calcium carbonate powder was the same as that of the first calcium carbonate powder. All the segmented lubricating and flow-stabilizing agents were added during the premixing stage, and no further downstream flow-stabilizing components were added after vacuum devolatilization. All other conditions were the same as in Example 1.

[0127] Detection methods The moisture content of the masterbatch was tested using a 105℃ drying method, with a sample size of 50g and a drying time of 2 hours. The volatile matter content of the masterbatch was tested using a 190℃ thermogravimetric analysis, with a sample size of 20g and a heating time of 30 minutes. Die head pressure fluctuations were based on pressure records after 30 minutes of normal, stable operation of the twin-screw extruder; the maximum and minimum pressure gauge values ​​were recorded, and the fluctuation range was calculated. Torque fluctuations were based on the torque variation range recorded in the equipment operation log over 30 minutes. Masterbatch particle size was statistically analyzed using random sampling and sieving. Sheet appearance inspection was conducted using the same small-scale calendering line, with a sheet thickness set at 0.18mm and a width of 600mm. Each batch of masterbatch was continuously processed for 2 hours, and the middle section of the continuous sheet was used to statistically analyze white spots, pitting, bubbles, pinholes, and thickness fluctuations.

[0128] Test results

[0129] As can be seen from the above test results, Examples 1 to 3 can produce stone paper high-filled stable masterbatches with a calcium carbonate content of 75% to 86% under pilot-scale conditions in a typical factory. The moisture content of the masterbatches is all below 0.3%, the volatile matter is all below 0.5%, and the particle size is mainly distributed in the range of 2 mm to 5 mm. Due to the higher calcium carbonate content, the die head pressure and torque fluctuations in Example 2 are slightly higher than those in Examples 1 and 3, but are still within the acceptable range for continuous extrusion.

[0130] Comparative Example 1, which did not employ a blend of coarse and fine-grained calcium carbonate and differentiated coating, showed a higher number of white spots and pits on the sheets, indicating that single-size calcium carbonate is more prone to forming localized powder concentrations and hardened particles under high-fill conditions. Comparative Example 2, which did not undergo vacuum devolatilization, showed a significant increase in volatile matter, moisture content, and the number of bubbles and pinholes in the sheets, demonstrating that vacuum devolatilization effectively reduces residual volatiles in high-filled masterbatches. Comparative Example 3, although employing a blend of coarse and fine-grained particle sizes and vacuum devolatilization, did not coat the second calcium carbonate powder at a higher proportion, nor did it add subsequent flow stabilizing components after devolatilization. The pressure fluctuations at the die head, the number of white spots on the sheets, and the thickness fluctuations were still higher than in Example 1, indicating that differentiated primary anchoring coating and post-devolatilization flow stabilization have a synergistic effect on improving the dispersion stability and processing stability of high-filled masterbatches.

[0131] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing highly filled stable masterbatch for stone paper using a combination of graded coating and vacuum devolatilization, characterized in that, Based on a total mass of 100 parts of raw materials for preparing highly filled stable masterbatch for stone paper, the raw materials include 75 to 86 parts of compounded calcium carbonate powder, 9 to 18 parts of polyolefin resin, 0.6 to 2.0 parts of primary anchoring coating agent, 1.2 to 4.5 parts of melt-grade compatibility coating agent, 0.8 to 2.5 parts of segmented lubricating flow stabilizer, and 0.1 to 0.5 parts of antioxidant stabilizer, with the sum of all raw material components being 100 parts. The preparation method includes the following steps: S1. The compound calcium carbonate powder is divided into a first calcium carbonate powder and a second calcium carbonate powder, wherein the particle size of the first calcium carbonate powder is larger than that of the second calcium carbonate powder. S2. Add a primary anchoring coating agent to the first calcium carbonate powder and the second calcium carbonate powder respectively for surface coating treatment, so that the primary anchoring coating agent adheres to the particle surface of the first calcium carbonate powder and the second calcium carbonate powder to form a primary anchoring coating layer, and the amount of primary anchoring coating agent per unit mass of the second calcium carbonate powder is higher than the amount of primary anchoring coating agent per unit mass of the first calcium carbonate powder. S3. The first calcium carbonate powder after forming the first anchoring coating layer, the second calcium carbonate powder after forming the first anchoring coating layer, polyolefin resin, melt-grade compatibility coating agent, antioxidant stabilizer and part of segmented lubricating and flow-stabilizing agent are mixed to obtain a high-filler premix. S4. The high-filler premix is ​​fed into a twin-screw extruder for melt mixing, so that the polyolefin resin melts and coats the first calcium carbonate powder and the second calcium carbonate powder, while the melt-grade compatibility coating agent forms a melt-grade compatibility coating layer on the outside of the first-level anchoring coating layer. S5. After the melt-level compatibility coating layer is formed, the high-filled melt is subjected to vacuum devolatilization treatment to remove moisture, low-molecular-weight volatiles, residual treatment agents and entrained gases from the high-filled melt. S6. Add the remaining segmented lubricating and flow-stabilizing agent to the vacuum devolatilized high-filled melt and perform secondary homogenization treatment on the high-filled melt to keep the high-filled melt in a continuous flow state before entering the extruder die head. S7. The highly filled melt after secondary homogenization is extruded, cooled and pelletized to obtain high-filled stable masterbatch for stone paper.

2. The method for preparing high-filling stable masterbatch for stone paper by synergistic graded coating and vacuum devouring according to claim 1, characterized in that, The first calcium carbonate powder is heavy calcium carbonate with a D50 particle size of 2.5 μm to 6 μm, and the second calcium carbonate powder is one or more of light calcium carbonate, refined heavy calcium carbonate, or spherical calcium carbonate with a D50 particle size of 0.4 μm to 1.5 μm. The first calcium carbonate powder accounts for 65% to 85% of the total mass of the compound calcium carbonate powder, and the second calcium carbonate powder is the remainder of the compound calcium carbonate powder excluding the first calcium carbonate powder, accounting for 15% to 35% of the total mass of the compound calcium carbonate powder. The sum of the mass percentages of the first calcium carbonate powder and the second calcium carbonate powder is 100%, and the D50 particle size of the second calcium carbonate powder is smaller than that of the first calcium carbonate powder.

3. The method for preparing high-filling stable masterbatch for stone paper by synergistic graded coating and vacuum devouring according to claim 1, characterized in that, Before surface coating, the first and second calcium carbonate powders undergo drying and particle size classification, respectively. The particle size classification includes one or more of sieving, air classifying, or cyclone classifying. After drying, the moisture content of both the first and second calcium carbonate powders is no higher than 0.3%. The highly filled premixed material enters a buffer silo before entering the twin-screw extruder. The stacking height of the highly filled premixed material in the buffer silo is controlled to be between 0.4m and 1.8m, and the temporary storage volume is controlled to be 0.08m³. 3 up to 0.8m 3 The temporary storage weight of the high-fill premixed material in a single buffer silo is controlled to be between 50 kg and 600 kg, and the ratio of the temporary storage weight to the temporary storage volume is controlled to be 0.5 t / m³. 3 Up to 0.9t / m 3 This reduces the compaction of the high-fill premix at the bottom of the buffer silo.

4. The method for preparing high-filling stable masterbatch for stone paper by synergistic graded coating and vacuum devouring according to claim 1, characterized in that, The primary anchoring coating agent includes one or more of titanate coupling agents, aluminate coupling agents, silane coupling agents, stearic acid, oleic acid, or calcium stearate. The primary anchoring coating agent is added to the first calcium carbonate powder and the second calcium carbonate powder respectively, and the amount of primary anchoring coating agent per unit mass of the second calcium carbonate powder is 1.3 to 3 times the amount of primary anchoring coating agent per unit mass of the first calcium carbonate powder, so that the second calcium carbonate powder obtains a surface coating strength higher than that of the first calcium carbonate powder.

5. The method for preparing high-filling stable masterbatch for stone paper by synergistic graded coating and vacuum devouring according to claim 1, characterized in that, Based on a total mass of 100 parts of raw materials for preparing the high-filled stable masterbatch of stone paper, the raw materials for preparing the high-filled stable masterbatch of stone paper also include 0.1 to 0.8 parts of devolatilization stabilization auxiliary agent, and the sum of the amounts of all raw material components, including the devolatilization stabilization auxiliary agent, is 100 parts. The devolatilization stabilization auxiliary agent includes one or more of active magnesium oxide, hydrotalcite, microporous silicate, or molecular sieve. The devolatilization stabilization auxiliary agent is fed into the twin-screw extruder together with the high-filled premix, or is added to the high-filled melt before the high-filled melt enters the vacuum devolatilization treatment, for adsorbing or fixing residual moisture, acidic low molecular weight substances, and volatile small molecule components in the high-filled melt.

6. The method for preparing high-filling stable masterbatch for stone paper by synergistic graded coating and vacuum devouring according to claim 1, characterized in that, The polyolefin resin includes one or more of high-density polyethylene, linear low-density polyethylene, low-density polyethylene, or polypropylene. The melt-grade compatibility coating agent includes one or more of maleic anhydride-grafted polyethylene, maleic anhydride-grafted polypropylene, or glycidyl methacrylate-grafted polyolefin. The melt-grade compatibility coating agent is distributed between the primary anchoring coating layer and the molten polyolefin resin during the melt mixing process of the twin-screw extruder, and forms the melt-grade compatibility coating layer on the outside of the primary anchoring coating layer to connect the calcium carbonate particles and the continuous polyolefin phase.

7. The method for preparing high-filling stable masterbatch for stone paper by synergistic graded coating and vacuum devouring according to claim 1, characterized in that, The segmented lubricating and flow-stabilizing agent comprises a front-stage lubricating component and a rear-stage flow-stabilizing component. The front-stage lubricating component is added after the surface coating treatment of the first calcium carbonate powder and the second calcium carbonate powder and before the high-filler premix enters the twin-screw extruder. The front-stage lubricating component accounts for 30% to 60% of the total mass of the segmented lubricating and flow-stabilizing agent. The rear-stage flow-stabilizing component is the remainder of the segmented lubricating and flow-stabilizing agent excluding the front-stage lubricating component, and accounts for 40% to 70% of the total mass of the segmented lubricating and flow-stabilizing agent. The total mass percentage of the flow components is 100%. The front-end lubricating component is used to reduce the agglomeration degree between calcium carbonate particles in the high-filled premix. The back-end flow stabilizing component is added after the high-filled melt has undergone vacuum devouring treatment to improve the flow continuity and pressure stability of the high-filled melt after vacuum devouring. The front-end lubricating component includes one or more of zinc stearate, calcium stearate, ethylene bis-stearamide, or polyethylene wax. The back-end flow stabilizing component includes one or more of oxidized polyethylene wax, polyethylene wax, or ethylene bis-stearamide.

8. The method for preparing high-filling stable masterbatch for stone paper by synergistic graded coating and vacuum devouring according to claim 1, characterized in that, The twin-screw extruder is sequentially configured with a melt mixing zone, a reinforced dispersion zone, a vacuum devolatilization zone, and a secondary homogenization zone along the material conveying direction. The melt mixing zone is used to melt the polyolefin resin and initially encapsulate the first calcium carbonate powder and the second calcium carbonate powder after forming a primary anchoring coating layer. The reinforced dispersion zone is used to form the melt-level compatible coating layer outside the primary anchoring coating layer with the melt-level compatible coating agent. The vacuum devolatilization zone is located after the reinforced dispersion zone and before the secondary homogenization zone. The vacuum degree of the vacuum devolatilization zone is -0.06MPa to -0.095MPa, and the devolatilization temperature is 160℃ to 210℃. It is used to remove moisture, low-molecular-weight volatiles, residual treatment agents, and entrained gases from the highly filled melt after the melt-level compatible coating layer is formed.

9. The method for preparing high-filling stable masterbatch for stone paper by synergistic graded coating and vacuum devouring according to claim 8, characterized in that, The secondary homogenization zone is located after the vacuum devolatilization zone. One or more of the following are provided in the secondary homogenization zone: low-shear threaded element, weak mixing threaded element, or pressure-stabilizing conveying threaded element. After the downstream flow stabilizing component is added to the high-filled melt after vacuum devolatilization, it is mixed with the high-filled melt in the secondary homogenization zone under low-shear homogenization, so that the high-filled melt maintains a continuous flow state and stable melt pressure before entering the extruder die.

10. The method for preparing high-filling stable masterbatch for stone paper by synergistic graded coating and vacuum devouring according to claim 1, characterized in that, Based on the total mass of the raw materials, the calcium carbonate content corresponding to the preparation of the high-filled stable masterbatch for stone paper is 75% to 86%. The volatile matter content of the prepared high-filled stable masterbatch for stone paper is not higher than 0.5%, the moisture content is not higher than 0.3%, the particle size of the high-filled stable masterbatch for stone paper is 2 mm to 5 mm, and the high-filled stable masterbatch for stone paper sheets, stone paper films, stone paper packaging paper or stone paper composite materials is used for extrusion, calendering or casting processing.

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