A one-step granulation method for an optical cable polyethylene sheath material

Abstract

The application relates to the technical field of high polymer material processing, and discloses a one-step granulation method for a polyethylene sheath material of an optical cable, which comprises the following steps: pre-mixing recycled high-pressure / linear low-density polyethylene film material, recycled low-pressure high-density polyethylene sheet material and an enhanced synergistic additive master batch; performing low-temperature pretreatment at 90-110 DEG C, so that low-melting-point components in the master batch are melted, and the two recycled materials with different physical forms are bonded into uniform agglomerates; feeding the agglomerates into an extruder, and performing melt extrusion granulation at 180-220 DEG C, during which interface reactivity compatibilizers in the master batch play a role. The application solves the problems of uneven feeding and poor compatibility caused by the fact that the traditional one-step method is difficult to process recycled materials with great form difference, can stably obtain a sheath material with excellent mechanical properties and environmental stress cracking resistance, and the whole process is continuous, efficient and low in cost.

Description

Technical Field

[0001] This invention relates to the field of polymer material processing technology, and in particular to a one-step granulation method for polyethylene sheathing material for optical cables. Background Technology

[0002] Polyethylene (PE) is widely used as the sheathing material for optical cables due to its excellent electrical insulation, chemical resistance, and processing properties. With the development of the plastics industry, a large amount of waste polyethylene products, such as packaging film materials (mainly high-pressure / linear low-density polyethylene, LDPE / LLDPE) and rigid container sheets (mainly low-pressure high-density polyethylene, HDPE), have placed enormous pressure on the environment. Therefore, recycling and reusing these products has significant economic and environmental implications.

[0003] Blending these two types of recycled polyethylene, which differ in origin and form, and then extruding and granulating them in a one-step process to prepare sheathing material is an ideal way to achieve their high-value utilization. However, in current technological practices, this process faces severe technical challenges. First, the two recycled materials differ significantly in their physical forms. Recycled film material is typically lightweight and fluffy with extremely low bulk density, while recycled sheet material is dense and heavy granules with a much higher bulk density. When these two materials are simply physically mixed, segregation and stratification are highly likely to occur under the influence of gravity in the hopper and disturbances during the conveying process. This inhomogeneity directly leads to drastic fluctuations in the material composition and mass flow rate at the extruder feed port, resulting in unstable internal pressure and uneven output within the extruder, ultimately leading to highly inconsistent product performance and making stable industrial production difficult.

[0004] Secondly, even if the problem of macroscopic mixing unevenness is overcome, the incompatibility of the two types of polyethylene at the molecular level remains a fundamental obstacle. High-pressure / linear low-density polyethylene and low-pressure high-density polyethylene differ in molecular chain structure, crystallinity, and melt rheology, belonging to a thermodynamically incompatible system. Conventional melt blending fails to form a strong interfacial bond between them, and the resulting blends typically exhibit a coarse "sea-island" structure, with the two-phase interface being the weak point of the system. This directly leads to a severe deterioration in the toughness of the material, especially its elongation at break and resistance to environmental stress cracking (ESCR), far from meeting the stringent requirements for long-term reliability of optical cable sheathing materials. In addition, the aforementioned problems of uneven feeding and poor compatibility also make it difficult for functional additives such as carbon black and antioxidants to achieve uniform dispersion in the melt, further affecting the weather resistance and service life of the product.

[0005] Therefore, how to simultaneously solve the two major challenges of macroscopic physical mixing uniformity and microscopic molecular interface compatibility of recycled polyethylene raw materials in a continuous and efficient one-step process is a technical bottleneck that urgently needs to be addressed in this field. Summary of the Invention

[0006] The technical problem solved by this invention is to provide a one-step granulation method that can effectively solve the problems of uneven feeding caused by the huge difference in the physical form (loose film material and dense sheet material) of recycled high / low density polyethylene, as well as the poor compatibility of the two components during melt blending, thereby achieving stable production and obtaining high-performance fiber optic cable polyethylene sheath material.

[0007] To address the above problems, the present invention provides the following technical solution:

[0008] This invention provides a one-step granulation method for polyethylene sheathing material of optical cables, comprising the following steps:

[0009] a) The recycled high-pressure / linear low-density polyethylene film material, the recycled low-pressure high-density polyethylene sheet material, and a reinforcing synergistic additive masterbatch are premixed to obtain a mixture material;

[0010] b) The mixture is subjected to low-temperature pretreatment at a temperature of 90-110°C to melt the low-melting-point components in the enhanced synergistic additive masterbatch, and the solid recycled high-pressure / linear low-density polyethylene film material is bonded to the recycled low-pressure high-density polyethylene sheet material to form a uniform agglomerate mixture.

[0011] c) The agglomerate mixture is fed into an extruder and melt-extruded and granulated at a melting zone temperature of 180-220°C to obtain the polyethylene sheath material for the optical cable.

[0012] By adopting the above technical solution, this invention achieves functional synergy between the two stages of low-temperature physical locking and high-temperature chemical compatibilization in a continuous process flow through zoned temperature control. Due to the adoption of this method, this invention can fundamentally solve the core technical challenges in one-step granulation of recycled polyethylene mixtures. Its specific technical mechanism is reflected in:

[0013] 1. Low-temperature pretreatment stage (low-temperature physical locking):

[0014] In step b), within the temperature range of 90–110°C, the low-melting-point binder (such as ethylene-vinyl acetate copolymer) in the reinforced synergistic additive masterbatch preferentially melts to form a viscous melt, while the main raw material, recycled high / low density polyethylene, remains solid at this time.

[0015] Under stirring, the melt acts as an in-situ generated hot melt adhesive, firmly bonding loose film materials and dense sheet materials with huge differences in physical morphology and bulk density together to form agglomerates with uniform physical morphology, particle size and bulk density.

[0016] This process eliminates the possibility of material segregation due to gravity or vibration during silo and conveying, ensuring the high uniformity and stability of the material entering the extruder feed port in terms of macroscopic composition.

[0017] 2. High-temperature melt extrusion stage (high-temperature chemical compatibilization):

[0018] Within the temperature range of 180–220°C in step c), after the homogeneous agglomerates treated in step b) enter the extruder, all polymer components are completely melted.

[0019] Due to the high stability of the feed, the melting pressure and material shearing process inside the extruder also tend to be stable.

[0020] Under this high-temperature environment, the active functional groups (maleic anhydride groups) of the interfacial reactive compatibilizer (such as maleic anhydride-grafted polyethylene) in the enhanced synergistic additive masterbatch are activated.

[0021] The compatibilizer molecular chain can effectively migrate to the interface between incompatible recycled high-density polyethylene and recycled low-density polyethylene, significantly reducing the interfacial tension between the two phases through chemical reaction or intermolecular forces, and forming an effective interfacial bonding layer.

[0022] This is equivalent to building a molecular-level bridge between two incompatible polymers, which greatly improves the compatibility and interfacial bonding strength of the blend system.

[0023] Therefore, through the synergistic effect of the above two stages, the present invention first ensures the uniformity and stability of the material composition on a macroscopic level, creating the preconditions for effective capacity expansion at the microscopic level; then, at the microscopic level, it solves the problem of inherent material incompatibility through interfacial reactions.

[0024] Ultimately, this invention enables the stable production of high-performance products with excellent mechanical properties, high toughness, resistance to environmental stress cracking, and long-term thermal stability, all of which meet the requirements for use in optical cable sheathing materials, thus realizing the high-value utilization of low-value recycled polyethylene.

[0025] As a preferred embodiment of the present invention, the enhanced synergistic additive masterbatch is made from the following components:

[0026] Ethylene-vinyl acetate copolymer as a low-melting-point adhesive carrier, maleic anhydride-grafted polyethylene as an interfacial reactive compatibilizer, and functional additives, interfacial wetting enhancers, and reactive anhydride scavengers.

[0027] By adopting the above technical solution, this invention designs the component functions of the enhanced synergistic additive masterbatch, enabling it to actively and stepwise play different core roles in different temperature zones of the one-step granulation process. The synergistic mechanism is as follows:

[0028] The low-melting-point adhesive carrier (ethylene-vinyl acetate copolymer) is the core component for achieving low-temperature physical locking. Its melting point is significantly lower than that of the main polyethylene raw material, ensuring selective melting during the low-temperature pretreatment stage of 90-110℃, thus acting as an in-situ adhesive without affecting the solid state of the main raw material.

[0029] Interfacial reactive compatibilizers (maleic anhydride-grafted polyethylene) are the core functional component for achieving high-temperature chemical compatibilization. Activated during the high-temperature melt extrusion stage, they anchor at the phase interface of recycled high / low-density polyethylene through their grafted polar functional groups (maleic anhydride), forming a stable interfacial layer and thus resolving the incompatibility issue between the two polyethylene molecular chains.

[0030] Functional additives (such as carbon black, antioxidants, and light stabilizers) are prepared in the masterbatch with high concentration and high dispersion in advance, which ensures their uniform distribution in the final product. This avoids the agglomeration and uneven dispersion problems that may be caused by directly adding powder additives in a one-step process, thereby ensuring the product's color, weather resistance, and long service life.

[0031] Interface wetting enhancers (such as low molecular weight polyethylene wax) can reduce the local viscosity of the interface region during the high-temperature melting stage, promote the spreading and penetration of interfacial reactive compatibilizers on the phase interface, and improve their compatibilization efficiency.

[0032] The function of reactive anhydride scavengers (such as copolymers containing epoxy groups) is to protect the activity of interfacial reactive compatibilizers. Acidic impurities that may be present in the recycled feedstock can degrade the maleic anhydride functional groups of the compatibilizer, rendering it ineffective. The functional groups (such as epoxy groups) of the scavenger preferentially react with these acidic impurities, thereby ensuring that the interfacial reactive compatibilizer can maximize its compatibilizing effect.

[0033] Preferably, by weight, the composition of the enhanced synergistic additive masterbatch is: 40-60 parts of the ethylene-vinyl acetate copolymer; 10-20 parts of the maleic anhydride-grafted polyethylene; and 20-45 parts of the functional additive group.

[0034] By adopting the above technical solution, the components can achieve the best synergistic effect within this ratio range. The amount of ethylene-vinyl acetate copolymer ensures sufficient adhesive strength to form stable agglomerates during the low-temperature pretreatment stage; the amount of maleic anhydride-grafted polyethylene ensures sufficient active sites for interfacial compatibilization during the high-temperature stage; and the content of functional additives balances the performance requirements of the final product with the processing flowability of the masterbatch.

[0035] Preferably, by weight, the raw material composition of the polyethylene sheath material for optical cables of the present invention is as follows: 50-70 parts of recycled high-pressure / linear low-density polyethylene film; 25-45 parts of recycled low-pressure high-density polyethylene sheet; and 3-8 parts of reinforcing synergistic additive masterbatch.

[0036] By adopting the above technical solution, this ratio range can effectively balance the rigidity and flexibility of the final sheath material, while ensuring that the amount of reinforcing synergistic additive masterbatch added is sufficient to achieve macroscopic physical locking and microscopic interface enrichment of the entire system, thereby obtaining a sheath material product with optimal comprehensive mechanical properties.

[0037] Preferably, the preparation method of the enhanced synergistic additive masterbatch is as follows:

[0038] The low-melting-point adhesive carrier, the interfacial reactive compatibilizer, the functional additive group, the interfacial wetting enhancer, and the reactive anhydride scavenger are premixed and then melt-blended and extruded into granules using a twin-screw extruder.

[0039] By adopting the above technical solution and utilizing the powerful shearing and dispersing mixing capabilities of the co-rotating twin-screw extruder, it is possible to ensure that high-content functional additives (especially carbon black) achieve extremely high dispersion levels in the ethylene-vinyl acetate copolymer carrier, thus preparing a highly uniform reinforcing synergistic additive masterbatch, which is the performance basis for its subsequent performance of various functions.

[0040] Preferably, the average residence time of the material in the low-temperature pretreatment is 3 to 8 minutes.

[0041] By adopting the above technical solution, this dwell time range represents the optimal balance between effective pre-bonding and ensuring production efficiency. Too short a time results in insufficient heat transfer and poor bonding; too long a time reduces equipment capacity and increases energy consumption.

[0042] Preferably, the extruder is a single-screw extruder, and a vacuum exhaust step is included in the extrusion process.

[0043] By adopting the above technical solution, since the problem of macro-mixing has been solved in the low-temperature pretreatment step before the extruder, the subsequent melt conveying and compatibilization reaction steps no longer need to rely on expensive and complex twin-screw extruders. Instead, a simpler and lower-cost single-screw extruder can be used, lowering the equipment threshold for industrial production. Simultaneously, the vacuum degassing step effectively removes moisture and low-molecular-weight volatiles entrained in the recycled raw materials, preventing them from forming bubbles or pores in the final product, thus ensuring the density and appearance quality of the sheathing material particles.

[0044] In summary, the present invention has at least one of the following beneficial technical effects:

[0045] 1. This invention solves the problem of uneven feeding caused by the significant differences in the physical morphology of recycled polyethylene raw materials. By setting a low-temperature pretreatment step before the extruder, the low-melting-point components in the reinforcing synergistic additive masterbatch are melted at 90-110℃, which in situ bonds the loose film material and dense sheet material with vastly different physical morphologies to form a uniform agglomerate. This fundamentally eliminates material segregation, ensures high stability in the feeding process, and provides a prerequisite for subsequent stable melt extrusion and uniform product performance.

[0046] 2. This invention significantly improves the overall performance of the final sheath material, particularly its toughness and long-term reliability. During the high-temperature melt extrusion stage, thanks to stable feeding, the interfacial reactive compatibilizer in the reinforcing synergistic additive masterbatch can efficiently migrate to the phase interface in a uniform melt environment, significantly reducing the interfacial tension between the recycled high / low density polyethylene phases. This forms a robust interfacial bonding layer, resulting in a final product that simultaneously possesses high tensile strength, excellent elongation at break, and outstanding resistance to environmental stress cracking.

[0047] 3. This invention simplifies the process and reduces production costs. Through an innovative design of low-temperature physical locking and high-temperature chemical compatibilization, this invention addresses the complex mixing and compatibilization issues step-by-step, reducing reliance on the mixing performance of the main extrusion equipment. Therefore, this method allows the main granulation process to be completed using a simpler, lower-cost single-screw extruder, reducing equipment investment and simplifying process control, resulting in significant economic benefits and industrial application value. Detailed Implementation

[0048] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the scope of protection of the invention.

[0049] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0050] Recycled high-pressure / linear low-density polyethylene film material (PE-A) is a mixture of commercially recycled, sorted, crushed, washed, and dried agricultural or packaging polyethylene films. It is in the form of irregularly shaped, loose, crushed material with a melt mass flow rate (MFR, tested according to GB / T 3682-2018 standard at 190°C and 2.16 kg load) of 1.5 g / 10 min.

[0051] Recycled low-pressure high-density polyethylene sheet (PE-B) refers to commercially recycled, sorted, crushed, washed, and dried fragments of rigid HDPE bottles or drums. It is in the form of irregular, dense sheet material with a melt mass flow rate (MFR, tested at 190℃ and 5.0kg load according to GB / T 3682-2018 standard) of 4.0g / 10min.

[0052] Ethylene-vinyl acetate copolymer (EVA) is a random copolymer obtained by high-pressure free radical polymerization of ethylene monomer and vinyl acetate monomer. The vinyl acetate (VA) used has a mass content of 28% and a melt flow rate (MFR, 190℃, 2.16kg) of 20g / 10min.

[0053] Maleic anhydride-grafted polyethylene (PE-g-MAH) is a graft copolymer prepared by melt grafting using linear low-density polyethylene (LLDPE) as the matrix resin. The grafting rate of the maleic anhydride (MAH) used was 1.2% (mass percentage), and the melt flow rate (MFR, 190℃, 2.16kg) was 5.0g / 10min.

[0054] Carbon black N550 is a general-purpose reinforcing carbon black produced by furnace process, with a DBP (dibutyl phthalate) absorbance of 121 cm⁻¹. 3 / 100g.

[0055] Polyethylene wax, CAS No.: 9002-88-4. It is low molecular weight polyethylene, with a number average molecular weight (Mn) of 2000 g / mol, a Brookfield viscosity of 250 mPa·s at 140℃, and a melting point of 105℃.

[0056] Ethylene-glycidyl methacrylate copolymer (E-GMA) is a random copolymer obtained by free radical polymerization of ethylene monomer and glycidyl methacrylate (GMA) monomer. The grade of glycidyl methacrylate (GMA) used has a mass content of 8% and a melt flow rate (MFR, 190℃, 2.16kg) of 3.0g / 10min.

[0057] Linear low-density polyethylene (LLDPE), used as the carrier resin for preparing the masterbatch in Comparative Example 2, has a melt flow rate (MFR, 190°C, 2.16 kg) of 20 g / 10 min.

[0058] Preparation Example 1:

[0059] This preparation example provides a method for preparing enhanced synergistic additive masterbatch C-1, including the following steps:

[0060] Add 50 parts by weight of ethylene-vinyl acetate copolymer (EVA), 15 parts by weight of maleic anhydride grafted polyethylene (PE-g-MAH), 28 parts by weight of carbon black N550, 2 parts by weight of primary antioxidant 1010, 2 parts by weight of secondary antioxidant 168, 2 parts by weight of light stabilizer HALS-944, 2 parts by weight of polyethylene wax, and 1 part by weight of ethylene-glycidyl methacrylate copolymer (E-GMA) to a high-speed and low-speed mixer, stir at high speed for 5 minutes at room temperature, and then switch to low speed for 3 minutes to premix all components evenly.

[0061] Subsequently, the mixed material is fed into a co-rotating twin-screw extruder with a length-to-diameter ratio of 40:1 via a loss-in-weight feeder. The main screw speed is set to 350 rpm, and the temperatures of each section from the feeding zone to the die head are set sequentially as follows: 110℃, 140℃, 160℃, 180℃, and 170℃.

[0062] After being cooled and dried in a water bath, the extruded melt strips are cut into cylindrical particles with a length of 2-4 mm by a pelletizer to obtain masterbatch C-1.

[0063] Preparation Example 2:

[0064] This preparation example provides a method for preparing the enhanced synergistic additive masterbatch C-2, including the following steps:

[0065] 48 parts by weight of ethylene-vinyl acetate copolymer (EVA), 10 parts by weight of maleic anhydride grafted polyethylene (PE-g-MAH), 35 parts by weight of carbon black N550, 3 parts by weight of primary antioxidant 1010, 3 parts by weight of secondary antioxidant 168 and 1 part by weight of polyethylene wax were added to a high-speed and low-speed mixer and stirred at high speed for 8 minutes at room temperature, and then stirred at low speed for 4 minutes to make the components premixed evenly.

[0066] Subsequently, the mixed material is fed into a co-rotating twin-screw extruder with a length-to-diameter ratio of 40:1 through a loss-in-weight feeder. The main screw speed is set to 400 rpm, and the temperatures of each section from the feeding zone to the die head are set sequentially as follows: 120℃, 150℃, 170℃, 190℃, and 180℃.

[0067] After being cooled and dried in a water bath, the extruded melt strips are cut into cylindrical particles with a length of 2-4 mm by a pelletizer to obtain masterbatch C-2.

[0068] Preparation Example 3:

[0069] This preparation example provides a method for preparing the enhanced synergistic additive masterbatch C-3, including the following steps:

[0070] 55 parts by weight of ethylene-vinyl acetate copolymer (EVA), 20 parts by weight of maleic anhydride grafted polyethylene (PE-g-MAH), 20 parts by weight of carbon black N550, 1.5 parts by weight of primary antioxidant 1010, 1.5 parts by weight of secondary antioxidant 168, 1 part by weight of light stabilizer HALS-944, and 1 part by weight of ethylene-glycidyl methacrylate copolymer (E-GMA) were added to a high-speed and low-speed mixer and stirred at high speed for 4 minutes at room temperature, then switched to low speed for 2 minutes to ensure that all components were premixed evenly.

[0071] Subsequently, the mixed material is fed into a co-rotating twin-screw extruder with a length-to-diameter ratio of 40:1 via a loss-in-weight feeder. The main screw speed is set to 300 rpm, and the temperatures of each section from the feeding zone to the die head are set sequentially as follows: 110℃, 135℃, 155℃, 175℃, and 170℃.

[0072] After being cooled and dried in a water bath, the extruded melt strips are cut into cylindrical particles with a length of 2-4 mm by a pelletizer to obtain masterbatch C-3.

[0073] Example 1:

[0074] This embodiment provides a one-step granulation method for polyethylene sheathing material of optical cables, including the following steps:

[0075] 60 parts by weight of recycled high-pressure / linear low-density polyethylene film material (PE-A), 35 parts by weight of recycled low-pressure high-density polyethylene sheet material (PE-B), and 5 parts by weight of masterbatch C-1 prepared in Preparation Example 1 were added to a V-type mixer and mixed at low speed at room temperature for 10 minutes to achieve initial uniform distribution.

[0076] The above mixture is fed into a sealed silo with jacket heating and internal stirring functions. The material temperature inside the silo is set to be maintained at 100°C and the average residence time of the material is 5 minutes, so as to form a uniform agglomerate mixture.

[0077] The agglomerate mixture was stably fed into a single-screw extruder with a length-to-diameter ratio of 33:1 via a forced feeding system. The main screw speed was set to 200 rpm, and the temperatures of each section from the feeding section to the die head were set sequentially as follows: 160℃, 180℃, 200℃, and 200℃. Vacuum exhaust was installed in the middle of the extruder, and the vacuum level was maintained at -0.09 MPa.

[0078] After the melt is extruded through the die head, it is granulated through a strip-drawing water-cooled pelletizing system to obtain the final sheath material pellets.

[0079] Example 2:

[0080] This embodiment provides a one-step granulation method for polyethylene sheathing material of optical cables, including the following steps:

[0081] 70 parts by weight of recycled high-pressure / linear low-density polyethylene film material (PE-A), 25 parts by weight of recycled low-pressure high-density polyethylene sheet material (PE-B), and 5 parts by weight of masterbatch C-2 prepared in Preparation Example 2 were added to a V-type mixer and mixed at low speed at room temperature for 10 minutes to achieve initial uniform distribution.

[0082] The above mixture is fed into a sealed silo with jacketed heating and internal stirring functions. The material temperature inside the silo is set to 90°C and the average residence time of the material is 7 minutes to form a uniform agglomerate mixture.

[0083] The agglomerate mixture was stably fed into a single-screw extruder with a length-to-diameter ratio of 33:1 via a forced feeding system. The main screw speed was set to 150 rpm, and the temperatures of each section from the feeding section to the die head were set sequentially as follows: 150℃, 175℃, 195℃, and 195℃. Vacuum exhaust was installed in the middle of the extruder, and the vacuum degree was maintained at -0.09 MPa.

[0084] After the melt is extruded through the die head, it is granulated through a strip-drawing water-cooled pelletizing system to obtain the final sheath material pellets.

[0085] Example 3:

[0086] This embodiment provides a one-step granulation method for polyethylene sheathing material of optical cables, including the following steps:

[0087] 50 parts by weight of recycled high-pressure / linear low-density polyethylene film material (PE-A), 42 parts by weight of recycled low-pressure high-density polyethylene sheet material (PE-B), and 8 parts by weight of masterbatch C-3 prepared in Preparation Example 3 were added to a V-type mixer and mixed at low speed at room temperature for 10 minutes to achieve initial uniform distribution.

[0088] The above mixture is fed into a sealed silo with jacket heating and internal stirring functions. The material temperature inside the silo is set to be maintained at 110°C and the average residence time of the material is 4 minutes, so as to form a uniform agglomerate mixture.

[0089] The agglomerate mixture was stably fed into a single-screw extruder with a length-to-diameter ratio of 33:1 via a forced feeding system. The main screw speed was set to 250 rpm, and the temperatures of each section from the feeding section to the die orifice were set sequentially as follows: 165℃, 185℃, 205℃, and 205℃. Vacuum exhaust was installed in the middle of the extruder, and the vacuum level was maintained at -0.09 MPa.

[0090] After the melt is extruded through the die head, it is granulated through a strip-drawing water-cooled pelletizing system to obtain the final sheath material pellets.

[0091] Comparative Example 1:

[0092] Compared to Example 1, the difference is that a pre-prepared masterbatch is not used. Instead, the same amount of functional components (i.e., carbon black, primary antioxidant, secondary antioxidant, light stabilizer, maleic anhydride-grafted polyethylene, polyethylene wax, and ethylene-glycidyl methacrylate copolymer) contained in masterbatch C-1 of Example 1 are directly dry-mixed with recycled high-pressure / linear low-density polyethylene film (PE-A) and recycled low-pressure high-density polyethylene sheet (PE-B) in their own independent powder or granule form. Then, the subsequent low-temperature pretreatment and extrusion granulation steps are performed, and all other process parameters are the same.

[0093] Comparative Example 2:

[0094] Compared to Example 1, the difference lies in the carrier resin used in the preparation of the masterbatch. Conventional linear low-density polyethylene (LLDPE) was used instead of ethylene-vinyl acetate copolymer (EVA) in Example 1 as the main carrier resin, while the types and contents of other additives, the masterbatch preparation process, and the preparation process of the final sheath material were all the same.

[0095] Comparative Example 3:

[0096] Compared to Example 1, the difference lies in the composition of the masterbatch. In preparing masterbatch C-1, the addition of maleic anhydride-grafted polyethylene (PE-g-MAH) was omitted, and its weight fraction was replaced by an equal amount of ethylene-vinyl acetate copolymer (EVA). The remaining components and processes were the same.

[0097] Comparative Example 4:

[0098] The difference from Example 1 lies in the composition of the masterbatch. In preparing masterbatch C-1, the addition of polyethylene wax and ethylene-glycidyl methacrylate copolymer (E-GMA) was omitted, and their total weight parts were replaced by an equal amount of ethylene-vinyl acetate copolymer (EVA). The remaining components and processes were the same.

[0099] Test Example 1:

[0100] The experimental steps are as follows:

[0101] 1) Material bulk density test:

[0102] This test aims to quantify the impact of low-temperature pretreatment steps on the physical morphology of the mixture.

[0103] The testing equipment was a funnel-type loose bulk density meter conforming to GB / T 1636-2008. The agglomerate mixture prepared in Example 1 after low-temperature pretreatment (100°C) and the mixture in Comparative Example 1 that was not pretreated at low temperature and was only subjected to simple physical dry mixing were used as test objects.

[0104] At the start of the experiment, close the lower outlet of the funnel and slowly pour the sample into the funnel from a height of 10-20 mm above it until the funnel is full and overflows. Use a ruler to level the funnel horizontally along its upper edge, removing any excess sample. Note that no vibration or pressure should be applied during this process. Open the lower outlet of the funnel and allow the sample to fall freely into a graduated cylinder of known volume (V) below, until it is full and overflows. Level the graduated cylinder horizontally again along its upper edge. Weigh the mass (m) of the sample in the graduated cylinder. The bulk density (ρ) is calculated using the formula ρ = m / V. Repeat the measurement three times for each material and take the arithmetic mean of the results.

[0105] 2) Mass flow rate fluctuation test during feeding process:

[0106] This test aims to evaluate the direct impact of the physical uniformity of materials on the feeding stability in industrial continuous production.

[0107] The testing equipment was a high-precision loss-in-weight feeder. The pretreated material from Example 1 and the dry mixture from Comparative Example 1 were loaded into the feeder's hopper, respectively. The target mass flow rate of the feeder was set to 50 kg / h, and the feeding program was started. After the feeder stabilized, the instantaneous mass flow rate data output by its controller was recorded, with a data sampling frequency of 1 Hz, continuously for 10 minutes. After the test, all 600 instantaneous flow rate data points were exported, and the standard deviation of the dataset was calculated.

[0108] The test results of the materials of Example 1 and Comparative Example 1 in terms of mixing uniformity and feeding stability are summarized in Table 1.

[0109] Table 1. Results of material mixing and feeding stability tests:

[0110] Test object <![CDATA[Bulk density (g / cm 3 )]]> Standard deviation of feed flow rate (kg / h) Pretreated materials of Example 1 0.53 0.42 Dry mixture of Comparative Example 1 0.28 2.15

[0111] The test results in Table 1 clearly show that, compared to the dry-mixed material in Comparative Example 1, the loose bulk density of the material pretreated at low temperature in Example 1 was 0.28 g / cm³. 3 Significantly increased to 0.53 g / cm³ 3 Furthermore, the standard deviation of the mass flow rate during continuous feeding was significantly reduced from 2.15 kg / h to 0.42 kg / h.

[0112] The above data verifies the core mechanism of the technical solution of this invention. In Comparative Example 1, recycled membrane material (light and fluffy) and recycled sheet material (heavy and dense) with huge differences in physical morphology and bulk density are prone to segregation under the influence of gravity in the hopper and the disturbance during the conveying process. This uneven material distribution causes the material entering the feeding screw to be sometimes mainly membrane material and sometimes mainly sheet material, resulting in drastic fluctuations in instantaneous feeding volume and mass, manifested as an extremely high standard deviation of feeding flow rate.

[0113] Conversely, the technical solution of Example 1 introduces a low-temperature pretreatment step. During this stage (90–110°C), the low-melting-point binder (EVA) in the synergistic additive masterbatch melts, and under stirring, the originally independent film and sheet materials with vastly different physical properties are bonded together in situ, forming agglomerates with more uniform physical morphology and density. This structural transformation directly leads to a significant increase in its loose bulk density. More importantly, the formation of these agglomerates fundamentally eliminates the preconditions for material segregation, ensuring that each unit volume of material entering the feed screw has nearly identical composition and mass. Therefore, the feeding process exhibits high stability and an extremely low standard deviation in mass flow rate.

[0114] In summary, this test case demonstrates that the in-situ pre-bonding mechanism achieved through composition design and process conditions can effectively solve the problem of uneven feeding caused by differences in physical morphology in the one-step granulation process of recycled materials.

[0115] Test Example 2:

[0116] The experimental steps are as follows:

[0117] The sheathing material granules prepared in Examples 1-3 and Comparative Examples 1-4 were dried in a forced-air drying oven at 80°C for 4 hours. Subsequently, the dried granules were injection molded at 190-210°C using an injection molding machine to form specimens conforming to the requirements of GB / T1040.2-2006 and GB / T 1842-2008 standards for subsequent performance testing. The specimens were conditioned in a standard laboratory environment (temperature 23±2°C, relative humidity 50±5%) for no less than 24 hours.

[0118] Melt mass flow rate (MFR) test: The test was conducted in accordance with GB / T 3682-2018 standard using a melt flow rate tester at 190℃ and under a load of 2.16kg.

[0119] Mechanical property testing: In accordance with GB / T 1040.2-2006 standard, the tensile strength and elongation at break of the specimen were tested using a universal testing machine at a tensile speed of 50 mm / min.

[0120] Environmental stress cracking resistance (ESCR) test: conducted according to GB / T 1842-2008 standard, under constant tensile stress method. The specimens with standard indentations were placed in a 10% Igepal CO-630 surfactant aqueous solution at 50℃, and the time (F50) when 50% of the samples cracked was recorded.

[0121] Thermal aging performance test: In accordance with GB / T 7141-2008 standard, the standard tensile specimens were placed in a forced-ventilation thermal aging chamber at 100℃±2℃ for 240 hours. After removal, they were conditioned for 24 hours under standard laboratory conditions, and their tensile strength and elongation at break were tested again. The residual performance rate was calculated using the formula (performance after aging / performance before aging × 100%).

[0122] The performance test data of the final products obtained from Examples 1-3 and Comparative Examples 1-4 are summarized in Table 2.

[0123] Table 2 shows the performance test results of the final products obtained from Examples 1-3 and Comparative Examples 1-4:

[0124]

[0125] The comprehensive performance data in Table 2 show that the final products of Examples 1-3 are significantly superior to all comparative examples in terms of mechanical properties, resistance to environmental stress cracking, and thermal aging stability.

[0126] Comparative Example 1, which used a direct physical mixing method, resulted in a product with comprehensively deteriorated performance, exhibiting extremely low elongation at break and ESCR values. This is directly related to the severe feed fluctuations observed in Test Example 1. Uneven feeding led to uncontrolled instantaneous component ratios of the material within the extruder, extremely poor dispersion of functional additives, and the inability of high- and low-density polyethylene components to form a homogeneous blend system. Ultimately, the material exhibited typical poor mixing characteristics, and its macroscopic properties failed to meet application requirements.

[0127] Comparative Example 2, using conventional LLDPE as the masterbatch carrier, yielded results inferior to the Example 1 but superior to Comparative Example 1. This confirms the necessity of using a masterbatch pre-dispersion agent, but also reveals the crucial role of selecting low-melting-point EVA as the carrier in this invention. LLDPE has a high melting point and cannot melt during the low-temperature pretreatment stage, thus failing to achieve in-situ pre-bonding of the main material. Material segregation remains, resulting in insufficient macroscopic uniformity of the final product, particularly in toughness (elongation at break) and ESCR performance.

[0128] Comparative Example 3 lacked the interfacial reactive compatibilizer (PE-g-MAH) in its formulation. Although its processing may have been relatively stable due to the adhesive effect of EVA, the elongation at break and ESCR performance of the final product showed a precipitous drop. This proves that the molecular chains of LDPE and HDPE are inherently incompatible. Without the compatibilizing effect of PE-g-MAH at the interface, the interfacial bonding between the two phases is extremely weak, and the material will crack rapidly along the phase interface under stress, exhibiting brittleness.

[0129] Comparative Example 4 lacked the interface wetting enhancer (PE wax) and the reactive anhydride scavenger (E-GMA). Its performance was significantly lower than Example 1, but still superior to the other comparative examples. This indicates that the absence of PE wax reduces the efficiency and strength of the low-temperature pre-bonding step, while the absence of E-GMA may cause acidic impurities in the system to consume some of the active functional groups of PE-g-MAH, thereby weakening its compatibilizing effect. The presence of these two components significantly improves the stability of the core technical solution and the upper limit of the final performance, especially in terms of long-term resistance to environmental stress cracking and thermal aging.

[0130] The results of Examples 1-3 demonstrate that the present invention can stably produce high-performance products within the required proportions and process ranges. Its high tensile strength and ultra-high elongation at break prove that a uniform and robust blend structure is formed within the system; the ESCR performance exceeding 900 hours and excellent thermal aging residual rate indicate that the material prepared by the present invention fully meets the stringent requirements for long-term reliability of optical cable sheath materials.

[0131] In summary, the technical effect of this invention is not a simple superposition of the effects of a single component, but rather a systematic solution to a series of technical challenges from macroscopic physical mixing to microscopic interfacial compatibility through the organic combination of the special component design of the synergistic additive masterbatch (low-melting-point carrier, compatibilizer, wetting agent, purifying agent) and a two-stage temperature control process (low-temperature pre-bonding, high-temperature melting reaction), thereby realizing the high-value reuse of recycled polyethylene.

[0132] 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 one-step granulation method for polyethylene sheathing material of optical cables, characterized in that, Includes the following steps: a) Premix the recycled high-pressure / linear low-density polyethylene film material, the recycled low-pressure high-density polyethylene sheet material, and the reinforcing synergistic additive masterbatch to obtain a mixture material; The enhanced synergistic additive masterbatch is made from the following components: ethylene-vinyl acetate copolymer as a low-melting-point adhesive carrier; maleic anhydride-grafted polyethylene as an interfacial reactive compatibilizer; functional additive group; low molecular weight polyethylene wax or Fischer-Tropsch wax as an interfacial wetting enhancer; and ethylene-glycidyl methacrylate copolymer as a reactive anhydride scavenger. b) The mixture is subjected to low-temperature pretreatment at 90-110°C to melt the ethylene-vinyl acetate copolymer in the reinforced synergistic additive masterbatch and to bond the solid recycled high-pressure / linear low-density polyethylene film with the recycled low-pressure high-density polyethylene sheet to form a uniform agglomerate mixture. c) The agglomerate mixture is fed into a single-screw extruder and melt-extruded and granulated at a melting zone temperature of 180-220°C. A vacuum exhaust step is set during the extrusion process to obtain the polyethylene sheath material for the optical cable.

2. The one-step granulation method for polyethylene sheath material of optical cables according to claim 1, characterized in that, The preparation method of the enhanced synergistic additive masterbatch is as follows: The ethylene-vinyl acetate copolymer, the maleic anhydride-grafted polyethylene, the functional additive group, the interface wetting enhancer, and the reactive anhydride scavenger are premixed and then melt-blended and extruded into granules using a twin-screw extruder.

3. The one-step granulation method for polyethylene sheath material of optical cables according to claim 1, characterized in that, The composition of the enhanced synergistic additive masterbatch, by weight, is as follows: 40-60 parts of the ethylene-vinyl acetate copolymer; 10-20 parts of the maleic anhydride-grafted polyethylene; The functional additive group consists of 20 to 45 parts; Two parts of the interface wetting enhancer; One part of the reactive anhydride scavenger.

4. The one-step granulation method for polyethylene sheath material of optical cables according to claim 1, characterized in that, By weight, the raw material composition of the polyethylene sheath material for optical cables mentioned in step a) is as follows: 50-70 parts of recycled high-pressure / linear low-density polyethylene film material; 25-45 parts of recycled low-density polyethylene sheet material; 3-8 parts of enhanced synergistic additive masterbatch.

5. The one-step granulation method for polyethylene sheath material of optical cables according to claim 1, characterized in that, The low-melting-point adhesive carrier is an ethylene-vinyl acetate copolymer, wherein the mass content of vinyl acetate is 18-28%.

6. The one-step granulation method for polyethylene sheath material of optical cables according to claim 1, characterized in that, The interfacial reactive compatibilizer is maleic anhydride-grafted polyethylene, with a maleic anhydride grafting rate of 0.8-1.5%.

7. The one-step granulation method for polyethylene sheath material of optical cables according to claim 1, characterized in that, The average residence time of the material in the low-temperature pretreatment described in step b) is 3 to 8 minutes.

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