Environment-friendly hot melt adhesive and preparation method thereof

Abstract

The invention discloses an environment-friendly hot melt adhesive and a preparation method thereof, and relates to the field of hot melt adhesive preparation.The preparation method comprises the following steps that preheated castor oil-based polyhydric alcohol a with the light component content being 1.5%-3.0% is subjected to multiple distillation treatment, and castor oil-based polyhydric alcohol b with the light component content being smaller than or equal to 0.1% is obtained; carrying out a prepolymerization reaction on castor oil-based polyol b, an organic bismuth catalyst and liquefied MDI to obtain a polyurethane prepolymer, and carrying out melt blending and devolatilization on the polyurethane prepolymer, hydrogenated rosin glyceride and a composite antioxidant through a co-rotating twin-screw extruder to obtain a melt; and injecting an anti-hydrolysis stabilizer into the melt, homogenizing and melting, and then carrying out water-cooling solidification and granulation to obtain the environment-friendly hot melt adhesive particles. Under the synergistic effect of all the steps, the aging resistance of a hot melt adhesive product is improved while the good initial bonding performance is kept, the storage life and the service life are effectively prolonged, and therefore the core problems that the number of processing defects is large, performance attenuation is fast and the storage life is short are solved.

Description

Technical Field

[0001] This invention relates to the field of hot melt adhesive preparation, and more particularly to an environmentally friendly hot melt adhesive and its preparation method. Background Technology

[0002] Environmentally friendly hot melt adhesives are based on environmentally friendly materials. They are applied by heating and melting, and then cooled and cured to achieve bonding. They do not contain organic solvents, have extremely low VOC emissions, and meet environmental standards. They are suitable for packaging, furniture, automotive interiors and other fields. Compared with traditional adhesives, they are safer and more energy-efficient, but their high-temperature resistance is slightly weaker, so they need to be selected according to the specific scenario.

[0003] To reduce reliance on petroleum-based materials, environmentally friendly hot melt adhesives commonly use bio-based polyols such as castor oil as raw materials. However, these naturally derived raw materials inevitably retain low-molecular-weight light components such as monoglycerides, diglycerides, and free acids during the chemical modification process. These light components volatilize when heated during the high-temperature melting process of hot melt adhesives, resulting in persistent air bubbles and surface pinholes within the adhesive, severely affecting coating uniformity and final bonding and sealing performance. Even more challenging is that these residual small molecules gradually migrate to the surface and slowly volatilize during the storage of hot melt adhesive products. This process not only causes changes in the adhesive composition, making it sticky or brittle, but also irreversibly weakens its cohesive strength and durability, leading to accelerated performance degradation and a significantly shortened effective shelf life.

[0004] Therefore, an environmentally friendly hot melt adhesive and its preparation method are proposed to solve the problems of residual light components in bio-based polyols, which easily generate bubbles and pinholes during processing, and the rapid degradation of the properties of the formed colloid and the shortened shelf life. Summary of the Invention

[0005] The purpose of this invention is to provide an environmentally friendly hot melt adhesive and its preparation method, which solves the problems of residual light components in bio-based polyols, easy generation of bubbles and pinholes during processing, rapid degradation of the properties of the formed colloid, and shortened shelf life.

[0006] To achieve this objective, the present invention adopts the following technical solution: A method for preparing an environmentally friendly hot melt adhesive, the method comprising the following steps: Step S1: Castor oil-based polyol a, which has a light component content of 1.5-3.0% after preheating, is subjected to multiple distillations to obtain castor oil-based polyol b, which has a light component content of ≤0.1%. Step S2: Castor oil-based polyol b is prepolymerized with organic bismuth catalyst and liquefied MDI to obtain polyurethane prepolymer. Then, the polyurethane prepolymer, hydrogenated rosin glycerol ester and composite antioxidant are melt-blended and devolatilized through a co-rotating twin-screw extruder to obtain melt. Step S3: After injecting an anti-hydrolysis stabilizer into the melt and homogenizing it, the melt is water-cooled for solidification and granulation to obtain environmentally friendly hot melt adhesive granules.

[0007] Step S1 specifically includes the following steps: Step S11: After preheating the castor oil-based polyol a to 90-100℃, it is fed into a short-path molecular distillation apparatus a and subjected to a first distillation at 120-140℃ and a vacuum degree ≤10Pa. After the first distillation is completed, the condensed light phase distillate a and the unevaporated heavy phase product a are obtained. Step S12: Input the heavy phase product a into a short-path molecular distillation apparatus b, and perform redistillation at 130-150℃ and a vacuum degree ≤5Pa to obtain light phase distillate b and heavy phase product b; the heavy phase product b is castor oil-based polyol b.

[0008] In step S11, the mass of the heavy phase product a is 92-96% of the mass of castor oil-based polyol a, and the remainder is light phase distillate a; In step S12, the mass of the heavy phase product b is 98-99.5% of the mass of the heavy phase product a, and the remainder is light phase distillate b.

[0009] The polyurethane prepolymer is obtained according to the following steps: Castor oil-based polyol b is heated to a first temperature and maintained, then liquefied MDI and organic bismuth catalyst are added and stirred to react, while controlling the NCO (isocyanate) index during the stirring reaction. During the stirring process, the actual viscosity of the material is monitored by an online rotational viscometer. Heating is stopped when the actual viscosity reaches the preset viscosity, thus obtaining a polyurethane prepolymer.

[0010] The first temperature is 78-82℃, the mass ratio of castor oil-based polyol b, liquefied MDI, and organic bismuth catalyst is 100:(18-22):(0.01-0.05), the NCO index is 0.98-1.05, and the preset viscosity is 15000-40000 mPa. s.

[0011] The melt is obtained according to the following steps: The polyurethane prepolymer is fed into the main feed port of a co-rotating twin-screw extruder, and hydrogenated rosin glycerol ester and composite antioxidant are added to the melting section of the extruder. The mixture is then melt-mixed at a third temperature and devolatilized in the vacuum section of the co-rotating twin-screw extruder to obtain the melt.

[0012] The third temperature is 140-160℃, the absolute pressure of the vacuum section is ≤2kPa, and the mass ratio of the polyurethane prepolymer, hydrogenated rosin glycerol ester and composite antioxidant is 100:(40-60):(0.5-1.0); wherein, the composite antioxidant includes primary antioxidant 1010 and secondary antioxidant 168, and the mass ratio of primary antioxidant 1010 to secondary antioxidant 168 is 1:(1-2).

[0013] Step S3 specifically includes the following steps: Step S31: After the melt is fed into the homogenization section in a co-rotating twin-screw extruder, an anti-hydrolysis stabilizer is injected to homogenize and melt, resulting in a homogeneous melt. Step S32: The homogeneous melt is extruded at a fourth temperature and then water-cooled, solidified, and granulated to obtain environmentally friendly hot melt adhesive granules.

[0014] In step S31, the mass ratio of the melt to the hydrolysis stabilizer is 100:(0.3-0.5), and the hydrolysis stabilizer is polycarbodiimide; In step S32, the fourth temperature is 160-180℃.

[0015] An environmentally friendly hot melt adhesive, wherein the environmentally friendly hot melt adhesive is prepared by the preparation method described above.

[0016] Compared with the prior art, the present invention has the following beneficial effects: This invention provides an environmentally friendly hot melt adhesive and its preparation method. By subjecting a raw material containing 1.5-3.0% light components to multiple distillations, castor oil-based polyol b with a light component content ≤0.1% can be stably obtained. This eliminates the material basis for the formation of bubbles and pinholes due to the volatilization of light components during subsequent high-temperature processing, thereby ensuring the uniformity and integrity of the coating. Subsequently, castor oil-based polyol b is used for prepolymerization, and the reaction endpoint is precisely controlled by online viscosity monitoring. Combined with subsequent melt blending and deep devolatilization under high vacuum conditions, a molecular weight of [missing information] is synergistically constructed. The narrower distribution and purer internal structure of the colloidal matrix not only reinforces the effect of eliminating processing defects, but also reduces the possibility of changes in colloidal composition and spontaneous performance degradation caused by small molecule migration or residue. Finally, the introduction of anti-hydrolysis stabilizers before molding chemically inhibits the tendency of polymers to hydrolyze and degrade under storage conditions. The synergistic effect of the above methods enables the final hot melt adhesive product to maintain good initial bonding performance while improving its aging resistance, effectively extending its storage and service life, thereby solving the core problems of many processing defects, rapid performance degradation and short shelf life. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] The structures, proportions, sizes, etc., shown in the accompanying drawings of this specification are only for the purpose of assisting those skilled in the art in understanding and reading the content disclosed in the specification, and are not intended to limit the conditions under which the present invention can be implemented. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportions, or adjustments to the size, without affecting the effects and objectives that the present invention can produce, should still fall within the scope of the technical content disclosed in the present invention.

[0019] Figure 1 This is a flowchart of the preparation method in this invention. Detailed Implementation

[0020] To make the objectives, features, and advantages of this invention more apparent and understandable, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the embodiments described below are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0021] In the description of this invention, it should be understood that the terms "upper," "lower," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. It should be noted that when a component is considered to be "connected" to another component, it can be directly connected to the other component or there may be a component positioned centrally in the connection.

[0022] Example 1: Please see Figure 1 This embodiment describes a method for preparing an environmentally friendly hot melt adhesive, which includes the following steps: Step S1: Castor oil-based polyol a, which has a light component content of 1.5-3.0% after preheating, is subjected to multiple distillations to obtain castor oil-based polyol b, which has a light component content of ≤0.1%. Step S2: Castor oil-based polyol b is prepolymerized with organic bismuth catalyst and liquefied MDI (diphenylmethane diisocyanate) to obtain polyurethane prepolymer. Then, the polyurethane prepolymer, hydrogenated rosin glycerol ester and composite antioxidant are melt-blended and devolatilized through a co-rotating twin-screw extruder to obtain melt. Step S3: After injecting an anti-hydrolysis stabilizer into the melt and homogenizing it, the melt is water-cooled for solidification and granulation to obtain environmentally friendly hot melt adhesive granules.

[0023] Specifically, in step S1, castor oil-based polyol a, which has a light component content of 1.5-3.0% after preheating, is subjected to multiple distillations to obtain castor oil-based polyol b, which has a light component content of ≤0.1%. Step S1 specifically includes the following steps: Step S11: After preheating the castor oil-based polyol a to 90-100℃, it is fed into a short-path molecular distillation apparatus a and subjected to a first distillation at 120-140℃ and a vacuum degree ≤10Pa. After the first distillation is completed, the condensed light phase distillate a and the unevaporated heavy phase product a are obtained. In step S11, the mass of the heavy phase product a is 92-96% of the mass of castor oil-based polyol a, and the rest is light phase distillate a; Step S12: Input the heavy phase product a into a short-path molecular distillation apparatus b, and perform redistillation at 130-150℃ and a vacuum degree ≤5Pa to obtain light phase distillate b and heavy phase product b; the heavy phase product b is castor oil-based polyol b.

[0024] In step S12, the mass of the heavy phase product b is 98-99.5% of the mass of the heavy phase product a, and the remainder is the light phase distillate b.

[0025] It should be noted that step S1 is based on the difference in volatility of different components in the mixture, achieving selective separation by heating under high vacuum conditions. The material to be selectively separated is castor oil-based polyol a, with an initial light component content of 1.5-3.0%. These light components mainly include monoglycerides, diglycerides, and free castor oil acid, which are substances with low molecular weight and relatively weak intermolecular forces. Specifically, when castor oil-based polyol a is preheated to 90-100℃, its viscosity decreases and its fluidity increases, creating the necessary conditions for the subsequent formation of a uniform film. After the preheated castor oil-based polyol a enters the short-path molecular distillation apparatus a for initial distillation, a liquid film of <1mm is formed on the evaporation surface under the action of a rotating scraper. A phase transition occurs at a heating temperature of 120-140℃ and a high vacuum environment of ≤10Pa. Under high vacuum conditions, the boiling point of the light components decreases significantly, thus allowing for separation at a temperature of 120-150℃. Sufficient saturated vapor pressure is obtained at the specified temperature, and under the enormous mass transfer driving force provided by high vacuum, light component molecules overcome the surface tension of the liquid film and the resistance to liquid-phase diffusion, selectively evaporating from castor oil-based polyol a. Simultaneously, because the distance between the evaporation surface and the condensation surface is less than the mean free path of the light component molecules, the escaping light component vapor molecules are rapidly captured and condensed with almost no gas-phase collisions or backmixing, thus achieving efficient, low-temperature separation from the high-boiling-point main component, yielding light phase distillate a and heavy phase product a. Meanwhile, heavy phase product a is subsequently subjected to redistillation in a short-path molecular still b. Under conditions of 130-150℃ and a vacuum degree ≤5Pa, the residual trace light components in heavy phase product a are further deeply removed, ultimately yielding heavy phase product b, the refined castor oil-based polyol b, with a light component content reduced to ≤0.1%. This method, through a two-stage series and progressively enhanced approach, achieves efficient physical stripping of trace, high-boiling-point impurities.

[0026] It should also be noted that an online near-infrared spectrometer is installed on the outlet flow path of the heavy phase product b of the short-path molecular distiller b to monitor the composition of the heavy phase product b in real time. When the intensity of the characteristic absorption peak representing light components such as monoglycerides and diglycerides is continuously lower than the preset threshold, the system determines that the heavy phase product b is a qualified castor oil-based polyol b and automatically switches the valve to transport it to a dedicated storage tank. If the monitored value is higher than the threshold, it is automatically returned to the raw material tank for reprocessing.

[0027] It is known that step S1, through the sequential preliminary distillation and redistillation processes, constitutes a gradient-enhanced separation of the material. This process reliably and stably reduces the content of light components in castor oil-based polyol a, which is complex in composition and contains various high-boiling-point impurities, from the initial 1.5-3.0% to ≤0.1%, achieving deep purification of the material. This reduces the concentration of small molecule interfering substances in the subsequent polymerization reaction system, thereby eliminating the main unstable factors at the source. This provides a highly pure and constant-composition starting material for subsequent steps, laying the foundation for producing a high-performance, highly stable final product.

[0028] Understandably, deep purification of castor oil-based polyol a is a prerequisite for addressing issues such as bubble and pinhole formation during processing, as well as rapid degradation of colloidal properties and shortened shelf life. Specifically, during processing, hot melt adhesives undergo high-temperature melting. If the colloid contains a certain amount of light components, these substances will rapidly vaporize upon heating, forming tiny bubbles that are difficult to expel within the viscous colloid, or creating disruptive pinhole defects on the surface during coating and molding. By pre-removing light components to ≤0.1%, the amount of volatile substances at subsequent processing temperatures becomes smaller, fundamentally reducing the likelihood of bubble and pinhole formation. Internal driving force and material source: During storage and application, the light component small molecules remaining in the colloid will slowly migrate and diffuse due to the concentration gradient. The migration of light component small molecules will lead to changes in the local composition of the colloid, which will cause surface stickiness or a decrease in cohesive strength. The volatilization of light component small molecules will directly cause the loss of effective components and structural defects in the colloid. The above method removes these migratable and volatile small molecules, reducing their content to ≤1%, which effectively improves the chemical and physical structural stability of the final colloidal product, thereby effectively delaying the performance degradation process and substantially extending the effective storage period and service life of the product.

[0029] It is worth noting that the above method employs two short-path molecular distillations to treat castor oil-based polyol a, rather than ordinary single-stage distillation or other separation methods, based on the characteristics of the material. Specifically, castor oil-based polyol and its light components (such as glycerides) are high-boiling-point, heat-sensitive substances. If atmospheric or ordinary vacuum distillation is used, the required operating temperature may cause side reactions such as thermal decomposition, oxidation, or polymerization of the material, introducing new impurities or deteriorating the quality of the raw material. Short-path molecular distillation technology operates under extremely high vacuum conditions of ≤10 Pa and ≤5 Pa, which can significantly lower the boiling point of the material, thereby achieving efficient separation at a relatively mild temperature of 120-150℃, suitable for the purification of heat-sensitive materials. The use of two distillations is based on the fact that single-stage distillation cannot simultaneously achieve high processing efficiency and high separation purity. The preliminary distillation process, under a pressure of ≤10 Pa and a temperature of 120-140℃, is responsible for... The primary separation process involves removing most of the light components from castor oil-based polyol a, which bears the main separation load. A second distillation, performed at ≤5 Pa pressure and 130-150 °C, further treats the product of the initial distillation, i.e., the heavy phase product a, removing residual, less volatile trace impurities, thereby reducing the light component content of castor oil-based polyol b to ≤0.1%. In this process, the heavy phase product a constitutes 92-96% of the feed, indicating the effectiveness of the two distillation processes. Too low a proportion results in excessive yield loss, while too high a proportion leads to insufficient purification. Furthermore, the temperature and vacuum levels during the two distillation processes are crucial for precise control of the separation driving force. Temperature provides energy, while vacuum lowers the boiling point and increases the mass transfer driving force. The combination of these two factors directly determines the rate and completeness of light component molecules escaping from the liquid film. By coordinating these parameters, deep removal of light components is achieved while ensuring the thermal stability of the feedstock.

[0030] Specifically, in step S2, castor oil-based polyol b is prepolymerized with organic bismuth catalyst and liquefied MDI to obtain polyurethane prepolymer. Then, the polyurethane prepolymer, hydrogenated rosin glycerol ester and composite antioxidant are melt-blended and devolatilized through a co-rotating twin-screw extruder to obtain melt. The polyurethane prepolymer is obtained according to the following steps: Castor oil-based polyol b is heated to a first temperature and maintained, then liquefied MDI and organic bismuth catalyst are added and stirred to react, while controlling the NCO index during the stirring reaction. During the stirring process, the actual viscosity of the material is monitored by an online rotational viscometer. Heating is stopped when the actual viscosity reaches the preset viscosity to obtain polyurethane prepolymer.

[0031] The initial temperature is 78-82℃, the mass ratio of castor oil-based polyol b, liquefied MDI, and organic bismuth catalyst is 100:(18-22):(0.01-0.05), the NCO index is 0.98-1.05, and the preset viscosity is 15000-40000 mPa. s.

[0032] It should be noted that in this step, the material system undergoes a chemical process of transforming a low molecular weight monomer into a medium molecular weight polymer through a stepwise addition polymerization reaction. The starting materials are deeply purified castor oil-based polyol b, liquefied MDI, and an organobismuth catalyst. This process involves the nucleophilic addition reaction between the active hydrogen atoms, specifically the hydroxyl groups, in the polyol molecule and the isocyanate groups in the diisocyanate molecule under the catalysis of the organobismuth catalyst. The organobismuth catalyst activates the carbon-nitrogen double bond in the isocyanate group through coordination, making it more susceptible to attack by the lone pair electrons on the oxygen atom of the polyol hydroxyl group, thereby forming a carbamate bond. As the reaction proceeds, carbamate bonds are continuously formed, further enhancing the reaction between the castor oil-based polyol b and the liquefied MDI. MDI molecules link together, causing the molecular chains to grow gradually. In the early stages of the reaction, the material is a low-viscosity liquid with good flowability. As the degree of polymerization increases, i.e., the molecular chain length and molecular weight increase, the entanglement of the polymer chains within the system intensifies, and the intermolecular forces strengthen, leading to a continuous and steady increase in the viscosity of the entire system. Ultimately, a high-viscosity viscous liquid, i.e., polyurethane prepolymer, is formed. In addition, the entire reaction is carried out under continuous stirring to ensure uniform heat transfer and sufficient contact between reactant molecules, thereby ensuring the homogeneity of the reaction process and avoiding local over-reaction or gelation. At the same time, the reaction endpoint is determined by real-time monitoring of the system viscosity using an online rotational viscometer to ensure the effectiveness of the polyurethane prepolymer.

[0033] It is known that this step, by adjusting the ratio of castor oil-based polyol b, liquefied MDI, and organic bismuth catalyst, and the NCO index during the reaction process, under the influence of the first temperature, and by real-time monitoring of the reaction endpoint using an online rotational viscometer, can achieve precise and repeatable control over the molecular weight and distribution of the generated polyurethane prepolymer. The NCO index and material ratio determine whether the polymer chain growth tends towards a linear structure or may produce branching, thus affecting the overall structure of the prepolymer. By controlling the viscosity endpoint, the average molecular weight of the prepolymer is effectively limited to a suitable range, thereby ensuring that the polymer intermediate, i.e., the polyurethane prepolymer, which is the basis for the final hot melt adhesive performance, has highly consistent and predictable molecular parameters, thus laying the foundation for obtaining hot melt adhesive products with uniform performance and small batch-to-batch differences.

[0034] Understandably, this step, controlling the molecular weight and structure of the polyurethane prepolymer, is crucial for addressing colloidal performance issues. Although step S1 significantly removes foreign small-molecule light components, the stability of the hot melt adhesive itself still depends on the molecular weight distribution of its main polymer, the polyurethane prepolymer. Inaccurate polymerization control can lead to excessive unreacted short-chain oligomers (low molecular weight fractions) or excessively high molecular weight fractions in the prepolymer, resulting in an overly broad molecular weight distribution. During subsequent high-temperature processing, these low-molecular-weight fractions exhibit relatively high volatility or migration, still posing a potential risk of microbubbles or pinholes. More importantly, a polymer system with a broad molecular weight distribution... Molecules of the same chain length may exhibit inconsistent crystallization behavior, phase separation tendencies, and compatibility with tackifying resins during the curing process. This inherent heterogeneity can lead to unpredictable changes in the mechanical properties of the colloid during long-term storage or use, manifesting as a decrease in cohesive strength or adhesive performance. Under the aforementioned conditions, a prepolymer with a narrow molecular weight distribution and controllable structure is generated, effectively reducing the content of low molecular weight segments and improving the homogeneity of the polymer system. This homogeneity indicates that the colloid is more stable in composition when heated, has a lower volatility tendency, and exhibits a more regular network structure after curing, resulting in stronger resistance to stress and aging. This, in conjunction with the purification effect of step S1, further consolidates the effect of resolving processing defects and extending storage stability from the perspective of the polymer itself.

[0035] It is worth noting that in this step, the mass ratio of castor oil-based polyol b to liquefied MDI and the final NCO index jointly determine the stoichiometric balance of the polymer chain. An NCO index of 0.98-1.05 tends to generate prepolymers with hydroxyl-terminated or near-linear structures, which is beneficial to the thermoplasticity and reprocessability of the final hot melt adhesive. The 0.01-0.05% organic bismuth catalyst is intended to provide sufficient catalytic activity while avoiding unnecessary subsequent reactions or catalytic degradation of the product during storage due to excessive catalyst residue. At the same time, the first temperature of 78-82℃ ensures a reasonable reaction rate while minimizing possible side reactions, such as the self-polymerization of isocyanate groups or excessive reaction with trace impurities (such as moisture), thereby ensuring the purity and stability of the prepolymer structure. In addition, using an online rotational viscometer to monitor the actual viscosity to the preset viscosity as the control signal for the reaction endpoint enables real-time, indirect but effective monitoring of the degree of microscopic molecular growth from changes in macroscopic physical quantities, which is more accurate and adaptable than simply relying on a fixed reaction time.

[0036] The melt is obtained through the following steps: The polyurethane prepolymer is fed into the main feed port of a co-rotating twin-screw extruder, and hydrogenated rosin glycerol ester and composite antioxidant are added to the melting section of the extruder. The mixture is then melt-mixed at a third temperature and devolatilized in the vacuum section of the co-rotating twin-screw extruder to obtain the melt.

[0037] The third temperature is 140-160℃, the absolute pressure of the vacuum section is ≤2kPa, and the mass ratio of polyurethane prepolymer, hydrogenated rosin glycerol ester and composite antioxidant is 100:(40-60):(0.5-1.0); wherein, the composite antioxidant includes primary antioxidant 1010 and secondary antioxidant 168, and the mass ratio of primary antioxidant 1010 to secondary antioxidant 168 is 1:(1-2).

[0038] It should be noted that in this step, the material system undergoes a physicochemical process from multi-component solids and high-viscosity prepolymers to a single homogeneous, pure polymer melt through high-temperature melting, intense shear mixing, and vacuum devolatilization. This step utilizes the thermal energy, shear energy, and vacuum environment provided by the co-rotating twin-screw extruder to sequentially complete the melting, dispersion, homogenization, and small molecule removal of the polyurethane prepolymer, hydrogenated rosin glycerol ester, and composite antioxidant. When these materials enter the extruder barrel from the main feed port and the side feeder, at a third temperature of 140-160°C, the solid hydrogenated rosin glycerol ester and composite antioxidant particles first absorb heat and melt into liquid. Subsequently, in the high-intensity shear, stretching, and kneading flow field generated by the intermeshing rotation of the twin screws, the molten hydrogenated rosin glycerol ester and liquid antioxidant are continuously segmented and stretched into fine fluid micro-elements, which, through the combined action of convection and diffusion, gradually penetrate and uniformly disperse into the continuous phase of the polyurethane prepolymer matrix. In this process, the initial phase interfaces between the components are broken, achieving close contact and compatibility at the molecular level. Ultimately, a heterogeneous or multiphase homogeneous mixture system with highly uniform component distribution is formed. Subsequently, this homogeneous mixture is transported to the high-vacuum devolatilization section. Under a strong vacuum environment with an absolute pressure ≤2kPa, the melt is thinned by the screw element, and the surface is continuously renewed. Any trace volatile substances dissolved or encapsulated in the melt, such as residual unreacted monomers, adsorbed moisture, and trace amounts of volatile components that may be brought from the raw materials, have their partial pressure rapidly reduced. This provides sufficient driving force to overcome the melt viscosity resistance and surface tension, allowing them to escape from the melt body and be removed by the vacuum system. During this process, the physical state of the material gradually changes from a clearly separable viscous prepolymer and solid powder / particles at the inlet to a high-temperature viscous melt with uniform color and texture. Furthermore, its volume may shrink slightly due to the removal of small molecules, improving the density and purity of the melt.

[0039] It is understood that the above steps achieve highly uniform and stable mixing of polyurethane prepolymer, hydrogenated rosin glycerol ester (tackifying resin), and composite antioxidant in the molten state, ensuring effective dispersion of functional additives in the matrix. Simultaneously, the melt undergoes efficient and thorough devolatilization treatment, maximizing the removal of various residual trace small-molecule volatiles from the system. This prepares a homogeneous, pure polymer melt precursor with preliminary heat oxidation protection (from the composite antioxidant) for the subsequent stabilization and granulation steps. The quality of this precursor directly determines the compositional consistency, appearance purity, and inherent processing stability of the final hot melt adhesive particles, serving as the material basis for ensuring excellent and repeatable application performance and long-term stability of the product.

[0040] Understandably, the homogenization and deep devolatilization techniques described above play a crucial role in consolidating and ultimately solving the problems of processing bubbles, pinholes, and shortened shelf life. Although step S1 has deeply purified the raw materials and step S2 has synthesized a prepolymer with controllable molecular weight, if perfect compatibility and uniform dispersion of each component cannot be achieved in the final melt mixing stage, enrichment zones of hydrogenated rosin glycerol esters or antioxidants will form locally. This inhomogeneity will lead to uneven coating due to differences in local thermal history or rheological behavior during processing heating, and may even indirectly cause defects. More importantly, although the main reaction of the prepolymerization reaction in step S2 is basically completed, trace amounts of free isocyanate monomers, catalysts, or trace small molecules generated by side reactions may still remain in the system. At the same time, the added solid hydrogenated rosin glycerol esters and other materials may also adsorb trace amounts of... Moisture, though present in trace amounts, is a volatile component that, if not effectively removed, becomes a direct source of bubbles during processing. This method utilizes the strong vacuum environment of vacuum devolatilization to provide a significant escape force for these trace molecules. Combined with the efficient surface renewal effect of the twin-screw extruder on the melt, these trace molecules can be effectively stripped from the high-viscosity melt and extracted from the system, thus eliminating the internal volatile substance source that generates bubbles and pinholes at the final melt stage. In addition, the uniformly dispersed composite antioxidant begins to function in the melt stage, effectively inhibiting the thermo-oxidative aging tendency of the melt during high-temperature processing by quenching free radicals and decomposing hydrogen peroxide. This establishes an anti-aging barrier for the product from the very beginning of manufacturing, playing a positive role in delaying the performance degradation of the product during subsequent storage and use, and is key to extending the shelf life.

[0041] It is worth noting that the use of a co-rotating twin-screw extruder coupled with high-vacuum devolatilization in the above-mentioned method is based on comprehensive considerations of mixing quality, devolatilization efficiency, and process continuity. Specifically, the co-rotating twin-screw extruder provides the possibility of continuous production, and its powerful shearing and kneading capabilities are particularly effective for achieving rapid and uniform blending between high-viscosity polyurethane prepolymers and solid tackifying resins. Its self-cleaning properties also help reduce material residues and ensure batch-to-batch cleanliness and consistency. High-vacuum devolatilization, for removing trace amounts of low-molecular-weight substances dissolved in high-viscosity polymer melts, has a removal rate limited by the diffusion rate of small molecules in the melt and the mass transfer rate at the melt-gas interface. High vacuum (absolute pressure ≤ 2 kPa) can maximize the mass transfer driving force, thereby overcoming the mass transfer resistance caused by the high-viscosity medium and achieving deep devolatilization. In addition, during this process, the third temperature of 140-160℃ ensures complete melting of hydrogenated rosin glycerol esters and a decrease in system viscosity. The optimal temperature range ensures effective screw conveying and mixing while preventing excessively high temperatures that could lead to polymer thermal degradation or premature antioxidant depletion. A mass ratio of 100:(40-60) of polyurethane prepolymer to hydrogenated rosin glycerol ester determines the basic performance framework of the final hot melt adhesive. This ratio of hydrogenated rosin glycerol ester (tackifying resin) effectively improves the wettability and initial tack of the adhesive on various substrates without affecting its cohesive strength. The total amount and corresponding ratio of the composite antioxidant are based on their different mechanisms of action in inhibiting oxidative chain reactions: the antioxidant captures free radicals and decomposes hydrogen peroxide, thus achieving a balance between processing thermal stability and long-term thermo-oxidative stability. Adding the composite antioxidant at this step ensures its protective effect throughout subsequent granulation and the product lifecycle. Under these conditions, a melt with uniform composition, purity, and preliminary stability is prepared through a combination of efficient mechanical energy, thermal energy input, and a vacuum environment. This is a crucial step connecting polymer synthesis and final product molding.

[0042] Specifically, in step S3, after injecting an anti-hydrolysis stabilizer into the melt and homogenizing it, the melt is water-cooled for solidification and granulation to obtain environmentally friendly hot melt adhesive particles.

[0043] Step S3 specifically includes the following steps: Step S31: After the melt is fed into the homogenization section in a co-rotating twin-screw extruder, an anti-hydrolysis stabilizer is injected to homogenize and melt, resulting in a homogeneous melt. In step S31, the mass ratio of melt to hydrolysis stabilizer is 100:(0.3-0.5). Step S32: The homogeneous melt is extruded at a fourth temperature and then water-cooled, solidified, and granulated to obtain environmentally friendly hot melt adhesive granules.

[0044] In step S32, the fourth temperature is 160-180℃.

[0045] It should be noted that in step S3, the material system undergoes a physical change process from homogenization and melting to stabilization and final solidification. This step is divided into two stages: homogenization and dispersion, and rapid phase change. Specifically, in the homogenization section of the co-rotating twin-screw extruder, the hydrolysis-resistant stabilizer, acting as polycarbodiimide, is injected into the flowing melt. Under the shearing and mixing action of the extruder screw, the hydrolysis-resistant stabilizer is forcibly dispersed in the form of tiny droplets or particles, and gradually diffuses into the entire polymer matrix as the melt flows and deforms, achieving uniform distribution. During this process, the melt maintains a suitable viscosity at a fourth temperature of 160-180°C, ensuring both sufficient fluidity for mixing and adequate shear strength. Stress is applied to break up agglomerates; subsequently, the homogeneous melt that has undergone stabilization is conveyed to an extrusion die and extruded into strips at a temperature of 160-180℃. The extruded high-temperature melt strips immediately enter a water-cooling environment, where they undergo rapid cooling and solidification. The water-cooling process effectively inhibits the slow side reactions between polymer molecular chains, especially the active groups that may remain at the ends of the polyurethane prepolymer, and forces the melt to rapidly transform from a viscoelastic state to a glassy state or a solid state with a fixed crystalline region. Water cooling not only removes a large amount of heat, but its constant low-temperature environment also ensures the consistency and high efficiency of the curing process. Finally, the solid strips are cut into uniformly sized, environmentally friendly hot melt adhesive granules by a rotary pelletizer, completing the morphological transformation from a continuous melt to a discrete solid product.

[0046] It is known that step S3 achieves the final functional modification of the melt and the standardization of the physical form of the product without interfering with the previously established homogeneous system. The addition of the anti-hydrolysis stabilizer is placed in the homogenization stage, which ensures that it is effectively dispersed in the already highly homogeneous matrix, avoiding excessive heat history or affecting the early devolatilization effect due to premature addition. The fourth temperature of 160-180℃ ensures that the melt has sufficient fluidity to extrude smoothly and achieve the final homogenization of the anti-hydrolysis stabilizer, while avoiding the possibility of polymer thermal degradation or failure of the anti-hydrolysis stabilizer itself due to excessive temperature. Water-cooled granulation provides a fast, uniform and controllable cooling path. The above methods endow the final product with the ability to resist long-term aging factors and provide a stable and easy-to-use physical form.

[0047] Understandably, step S3 achieves stabilization and standardized shaping technology. Although steps S1 and S2 have significantly eliminated the internal factors leading to processing bubbles and performance degradation from the perspectives of raw material purification and polymer structure control, polyurethane materials themselves still face the inherent risk of chain breakage through hydrolysis during long-term storage, especially in humid and hot environments. This hydrolysis reaction, triggered by environmental moisture, breaks the urethane bonds that form the basis of the colloid's cohesive strength, leading to a decrease in molecular weight. Consequently, the colloid gradually becomes brittle, and the adhesive strength slowly declines. This is also a factor contributing to the rapid performance degradation and shortened shelf life of the colloid itself. Water is captured by anti-hydrolysis stabilizers. The acidic carboxylic acid terminus generated by the hydrolysis reaction blocks its autocatalytic cycle, thereby inhibiting the spread of hydrolysis degradation at the chemical level. At the same time, the water-cooled granulation process forms uniformly sized and regularly shaped granules through rapid solidification. This regular physical form not only facilitates packaging, transportation, and metering, but more importantly, it ensures that each product melts at the same rate and releases a colloid with consistent performance during subsequent use. This avoids problems such as uneven heating and different melting rates caused by irregular block or sheet materials. From the product form level, it guarantees the stability and reproducibility of the final application performance, thus fully supporting the overall goal of extending the product's effective shelf life and maintaining stable performance.

[0048] It is worth noting that in step S3, injecting the anti-hydrolysis stabilizer in the homogenization section of the twin-screw extruder, rather than in the earlier melting section, ensures that the main components, such as hydrogenated rosin glycerol ester and composite antioxidant, have been fully mixed and devolatilized with the polyurethane prepolymer in the melting section, and the system has become homogeneous and stable. Injecting the anti-hydrolysis stabilizer at this time allows it to focus on its dispersion function, avoiding interference with the preceding vacuum devolatilization process (for example, if added too early, its trace volatiles may increase the devolatilization load). It also reduces the unnecessary heat exposure time it may experience in the high-temperature melting section, which is beneficial for maintaining its chemical stability. At the same time, the homogenization and extrusion temperatures are uniformly controlled at 160-180°C. Within the fourth temperature range of 0℃, excessively low temperatures can prevent excessively high melt viscosity, excessive extrusion pressure, and difficulty in dispersing the anti-hydrolysis stabilizer. Excessively high temperatures increase the risk of thermal oxidation, overburdening the composite antioxidant and causing polymer degradation or failure of the anti-hydrolysis stabilizer. Furthermore, a melt-to-anti-hydrolysis stabilizer mass ratio of 100:(0.3-0.5) provides sufficient protection while avoiding compatibility issues, increased costs, or potential impacts on the basic mechanical properties of the colloid caused by excessive addition. In addition, in water-cooled granulation, the rapid heat exchange capacity of the cooling water enables rapid solidification of the melt strip from the surface to the interior, resulting in a dense particle structure that is beneficial for storage stability.

[0049] The following is a comparison of various data between the environmentally friendly hot melt adhesive in Example 1 and the traditional environmentally friendly hot melt adhesive (control group). Please refer to Table 1 for details: Table 1 As shown in Table 1, Example 1 reduced the content of light components to ≤0.1% through two-stage molecular distillation of the raw materials, which is far lower than the 1.8% of the control group. This eliminated the material basis for the generation of bubbles and pinholes during processing from the source. Therefore, its high-temperature coating appearance is smooth and defect-free, while the control group shows obvious bubbles and pinholes. Furthermore, the lower content of light components and the controllable polymerization process work together to make the viscosity change rate of Example 1 only 5-8% after 72 hours of heat aging at 180°C, showing excellent thermal stability, while the change rate of the control group reached 25%. This directly slows down the rate of decline of colloidal properties. In addition, the above-mentioned characteristics of Example 1 enable it to retain ≥95% of shear strength after 7 days of accelerated aging at 85°C, indicating a significantly extended storage period, while the retention rate of the control group is only about 70%, and the performance declines rapidly. Finally, Example 1 solves the above problems while increasing the shear strength to 9.0-10.5MPa, and its overall performance is superior to that of the control group.

[0050] The data for the control group in Table 1 are the average values ​​after multiple tests on the traditional environmentally friendly hot melt adhesive. The preparation method of the environmentally friendly hot melt adhesive is as follows: First, commercially available industrial-grade castor oil polyol (light component content of about 1.5-2.5%) and diisocyanate are added to a reactor at a set NCO / OH ratio, a catalyst is added, and the mixture is stirred and reacted at 80-90℃, atmospheric pressure or low vacuum for 1-3 hours to synthesize a polyurethane prepolymer. Subsequently, the prepolymer is transferred to a kneader or internal mixer, heated to 120-140℃, and tackifying resin, paraffin wax, antioxidants, etc. are added according to the formula. The mixture is melt-blended at atmospheric pressure or medium vacuum for 1-2 hours to homogenize the components and remove some volatiles. Finally, the homogenized adhesive is extruded through a hot melt extruder, water-cooled, and pelletized to obtain the finished product. The test data of Example 1 are obtained based on three specific Examples a, b, and c. Example a is as follows: Castor oil-based polyol a, with a light component content of 1.5%, was preheated to 90°C and then fed into a first-stage short-path molecular still. A first distillation was performed at an evaporation temperature of 120°C and a vacuum of 10 Pa, yielding light-phase distillate a and heavy-phase product a, with the heavy-phase product a accounting for 92% of the mass of castor oil-based polyol a. Subsequently, heavy-phase product a was fed into a second-stage short-path molecular still and redistilled at an evaporation temperature of 130°C and a vacuum of 5 Pa, yielding light-phase distillate b and heavy-phase product b (i.e., castor oil-based polyol b), with the heavy-phase product b accounting for 98% of the mass of heavy-phase product a. The light component content of castor oil-based polyol b was determined to be ≤0.1%. Castor oil-based polyol b (mass ratio 100) was heated to 78°C and maintained, then liquefied MDI (mass ratio 18) and an organic bismuth catalyst (mass ratio 0.01) were added, and the reaction was carried out with stirring under conditions of an NCO index of 0.98. Monitoring was conducted using an online rotational viscometer, and the material viscosity was measured to be 15000 mPa. Heating is stopped at time s to obtain a polyurethane prepolymer. This prepolymer is fed to the main feed port of a co-rotating twin-screw extruder, and hydrogenated rosin glycerol ester (40 by mass), primary antioxidant 1010 (0.25 by mass), and secondary antioxidant 168 (0.25 by mass) (total mass ratio 0.5) are added to the extruder's melting section. After melting and mixing at 140°C, the mixture is devolatilized in a vacuum section at an absolute pressure of 1.5 kPa to obtain a melt. This melt is then fed to the extruder's homogenization section, where polycarbodiimide (anti-hydrolysis stabilizer) is injected at a mass ratio of 0.3 and homogenized to obtain a homogeneous melt. Finally, the homogeneous melt is extruded at 160°C, water-cooled for curing, and pelletized to obtain environmentally friendly hot melt adhesive granules.

[0051] The specific test data for Example a are as follows: the light component content of the raw material castor oil-based polyol b is ≤0.1%; after 72 hours of heat aging at 180°C, the change rate of melt viscosity is 5%; when coated at high temperature at 180°C, the surface of the adhesive strip is smooth and no bubbles or pinholes are observed; after accelerated heat aging at 85°C for 7 days, the shear strength retention rate is 95.2%; the shear strength is 9.0 MPa.

[0052] Example a employs a relatively mild distillation temperature and moderate polymerization viscosity. While controlling energy consumption and raw material thermal history, the content of light components is stably reduced from 1.5% to ≤0.1%, thereby cutting off the source of materials for processing bubbles and pinholes. This results in a smooth and defect-free coating appearance. At the same time, its precise viscosity endpoint control ensures uniform molecular weight of the prepolymer. Combined with anti-hydrolysis stabilizers, it reduces the risk of low molecular weight migration and chain segment degradation at the polymer structure level. As a result, a 95.2% aging strength retention rate is achieved at a lower overall cost, effectively solving the problem of rapid performance degradation.

[0053] Example b is as follows: Castor oil-based polyol a, with a light component content of 2.25%, was preheated to 95°C and then fed into a first-stage short-path molecular still. A first distillation was performed at an evaporation temperature of 130°C and a vacuum of 8 Pa, yielding light-phase distillate a and heavy-phase product a, with the heavy-phase product a accounting for 94% of the mass of castor oil-based polyol a. Subsequently, heavy-phase product a was fed into a second-stage short-path molecular still and redistilled at an evaporation temperature of 140°C and a vacuum of 4 Pa, yielding light-phase distillate b and heavy-phase product b (i.e., castor oil-based polyol b), with the heavy-phase product b accounting for 98.75% of the mass of heavy-phase product a. The light component content of castor oil-based polyol b was determined to be ≤0.1%. Castor oil-based polyol b (mass ratio 100) was heated to 80°C and maintained, then liquefied MDI (mass ratio 20) and an organic bismuth catalyst (mass ratio 0.03) were added, and the reaction was carried out with stirring under conditions of an NCO index of 1.02. Monitoring was conducted using an online rotational viscometer, and the material viscosity was measured to be 27500 mPa. Heating is stopped at time s to obtain a polyurethane prepolymer. This prepolymer is fed to the main feed port of a co-rotating twin-screw extruder, and hydrogenated rosin glycerol ester (50 by mass), primary antioxidant 1010 (0.25 by mass), and secondary antioxidant 168 (0.50 by mass) (total mass ratio 0.75) are added to the extruder's melting section. After melting and mixing at 150°C, the mixture is devolatilized in a vacuum section at an absolute pressure of 1.0 kPa to obtain a melt. This melt is then fed to the extruder's homogenization section, where polycarbodiimide (anti-hydrolysis stabilizer) is injected at a mass ratio of 0.4 and homogenized to obtain a homogeneous melt. Finally, the homogeneous melt is extruded at 170°C, water-cooled for curing, and pelletized to obtain environmentally friendly hot melt adhesive granules.

[0054] The specific test data for Example b are as follows: the light component content of the raw material castor oil-based polyol b is ≤0.1%; after heat aging at 180°C for 72 hours, the change rate of melt viscosity is 6.5%; when coated at 180°C, the surface of the adhesive strip is smooth and no bubbles or pinholes are observed; after accelerated heat aging at 85°C for 7 days, the shear strength retention rate is 95.8%; the shear strength is 9.8 MPa.

[0055] Example b achieves optimal matching of raw material purity, polymer chain structure, and additive dispersibility, maximizing the overall performance and storage stability of the product. Its aging strength retention rate of up to 96.5% is the highest among the three specific examples, indicating that this example can most effectively inhibit long-term thermo-oxidative aging and hydrolysis, so that the performance of the colloid hardly declines after long-term storage, solving the problem of shortened shelf life.

[0056] Example c is as follows: Castor oil-based polyol a, with a light component content of 3.0%, was preheated to 100°C and then fed into a first-stage short-path molecular still. A first distillation was performed at an evaporation temperature of 140°C and a vacuum of 5 Pa, yielding light-phase distillate a and heavy-phase product a, with the heavy-phase product a accounting for 96% of the mass of castor oil-based polyol a. Subsequently, heavy-phase product a was fed into a second-stage short-path molecular still and redistilled at an evaporation temperature of 150°C and a vacuum of 2 Pa, yielding light-phase distillate b and heavy-phase product b (i.e., castor oil-based polyol b), with the heavy-phase product b accounting for 99.5% of the mass of heavy-phase product a. The light component content of castor oil-based polyol b was determined to be ≤0.1%. Castor oil-based polyol b (mass ratio 100) was heated to 82°C and maintained, then liquefied MDI (mass ratio 22) and an organic bismuth catalyst (mass ratio 0.05) were added, and the reaction was carried out with stirring under conditions of an NCO index of 1.05. Monitoring was conducted using an online rotational viscometer, and the material viscosity was measured to be 40,000 mPa. Heating is stopped at time s to obtain a polyurethane prepolymer. This prepolymer is fed to the main feed port of a co-rotating twin-screw extruder, and hydrogenated rosin glycerol ester (60 by mass), primary antioxidant 1010 (0.33 by mass), and secondary antioxidant 168 (0.67 by mass) (total mass ratio 1.0) are added to the extruder's melting section. After melting and mixing at 160°C, the mixture is devolatilized in a vacuum section at an absolute pressure of 0.5 kPa to obtain a melt. This melt is then fed to the extruder's homogenization section, where polycarbodiimide (anti-hydrolysis stabilizer) is injected at a mass ratio of 0.5 and homogenized to obtain a homogeneous melt. Finally, the homogeneous melt is extruded at 180°C, water-cooled for curing, and pelletized to obtain environmentally friendly hot melt adhesive granules.

[0057] The specific test data for Example c are as follows: the light component content of the raw material castor oil-based polyol b is ≤0.1%; after heat aging at 180°C for 72 hours, the change rate of melt viscosity is 8%; when coated at 180°C, the surface of the adhesive strip is smooth and no bubbles or pinholes are observed; after accelerated heat aging at 85°C for 7 days, the shear strength retention rate is 96.5%; the shear strength is 10.5 MPa.

[0058] Example c employs the highest distillation vacuum, polymerization viscosity, and stabilizer dosage. Even with raw materials containing up to 3.0% light components, it can achieve deep purification and high-strength polymerization. This solves the problem of unstable product performance in traditional processes when there are large fluctuations in the quality of raw material batches. It demonstrates that this example can still produce a product with a strength of 10.5 MPa and a retention rate of 95.8% even at the most unfavorable starting point, while ensuring no processing defects. This achieves dual assurance of performance and lifespan in demanding application scenarios.

[0059] Example 2: This embodiment describes an environmentally friendly hot melt adhesive, which is prepared using the method described in Example 1.

[0060] The above-described embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing an environmentally friendly hot melt adhesive, characterized in that, The preparation method includes the following steps: Step S1: Castor oil-based polyol a, which has a light component content of 1.5-3.0% after preheating, is subjected to multiple distillations to obtain castor oil-based polyol b, which has a light component content of ≤0.1%. Step S2: Castor oil-based polyol b is prepolymerized with organic bismuth catalyst and liquefied MDI to obtain polyurethane prepolymer. Then, the polyurethane prepolymer, hydrogenated rosin glycerol ester and composite antioxidant are melt-blended and devolatilized through a co-rotating twin-screw extruder to obtain melt. Step S3: After injecting an anti-hydrolysis stabilizer into the melt and homogenizing it, the melt is water-cooled for solidification and granulation to obtain environmentally friendly hot melt adhesive granules.

2. The method for preparing the environmentally friendly hot melt adhesive according to claim 1, characterized in that, Step S1 specifically includes the following steps: Step S11: After preheating the castor oil-based polyol a to 90-100℃, it is fed into a short-path molecular distillation apparatus a and subjected to a first distillation at 120-140℃ and a vacuum degree ≤10Pa. After the first distillation is completed, the condensed light phase distillate a and the unevaporated heavy phase product a are obtained. Step S12: Input the heavy phase product a into a short-path molecular distillation apparatus b, and perform redistillation at 130-150℃ and a vacuum degree ≤5Pa to obtain light phase distillate b and heavy phase product b; the heavy phase product b is castor oil-based polyol b.

3. The method for preparing the environmentally friendly hot melt adhesive according to claim 1, characterized in that, In step S11, the mass of the heavy phase product a is 92-96% of the mass of castor oil-based polyol a, and the remainder is light phase distillate a; In step S12, the mass of the heavy phase product b is 98-99.5% of the mass of the heavy phase product a, and the remainder is light phase distillate b.

4. The method for preparing the environmentally friendly hot melt adhesive according to claim 1, characterized in that, The polyurethane prepolymer is obtained according to the following steps: Castor oil-based polyol b was heated to a first temperature and maintained, then liquefied MDI and organic bismuth catalyst were added and stirred to react, while controlling the NCO index during the stirring reaction. During the stirring process, the actual viscosity of the material is monitored by an online rotational viscometer. Heating is stopped when the actual viscosity reaches the preset viscosity, and a polyurethane prepolymer is obtained.

5. The method for preparing the environmentally friendly hot melt adhesive according to claim 4, characterized in that, The first temperature is 78-82℃, the mass ratio of castor oil-based polyol b, liquefied MDI, and organic bismuth catalyst is 100:(18-22):(0.01-0.05), the NCO index is 0.98-1.05, and the preset viscosity is 15000-40000 mPa. s.

6. The method for preparing the environmentally friendly hot melt adhesive according to claim 5, characterized in that, The melt is obtained according to the following steps: The polyurethane prepolymer is fed into the main feed port of a co-rotating twin-screw extruder, and hydrogenated rosin glycerol ester and composite antioxidant are added to the melting section of the extruder. The mixture is then melt-mixed at a third temperature and devolatilized in the vacuum section of the co-rotating twin-screw extruder to obtain the melt.

7. The method for preparing the environmentally friendly hot melt adhesive according to claim 1, characterized in that, The third temperature is 140-160℃, the absolute pressure of the vacuum section is ≤2kPa, and the mass ratio of the polyurethane prepolymer, hydrogenated rosin glycerol ester and composite antioxidant is 100:(40-60):(0.5-1.0); wherein, the composite antioxidant includes primary antioxidant 1010 and secondary antioxidant 168, and the mass ratio of primary antioxidant 1010 to secondary antioxidant 168 is 1:(1-2).

8. The method for preparing the environmentally friendly hot melt adhesive according to claim 1, characterized in that, Step S3 specifically includes the following steps: Step S31: After the melt is fed into the homogenization section in a co-rotating twin-screw extruder, an anti-hydrolysis stabilizer is injected to homogenize and melt, resulting in a homogeneous melt. Step S32: The homogeneous melt is extruded at a fourth temperature and then water-cooled, solidified, and granulated to obtain environmentally friendly hot melt adhesive granules.

9. The method for preparing the environmentally friendly hot melt adhesive according to claim 1, characterized in that, In step S31, the mass ratio of the melt to the hydrolysis stabilizer is 100:(0.3-0.5), and the hydrolysis stabilizer is polycarbodiimide; In step S32, the fourth temperature is 160-180℃.

10. An environmentally friendly hot melt adhesive, characterized in that, The environmentally friendly hot melt adhesive is prepared using the preparation method described in any one of claims 1-9.

Images