A PLA / PCL-based biodegradable waterborne polyurethane adhesive and a preparation method thereof

Abstract

The application relates to the technical field of water-based polyurethane materials, and provides a PLA / PCL-based biodegradable water-based polyurethane adhesive and a preparation method thereof. The water-based polyurethane adhesive comprises a PLA-based polyol, a PCL-based polyol, a polyisocyanate, a hydrophilic chain extender, a small-molecule chain extender, a neutralizing agent and water; wherein after the water-based polyurethane adhesive is formed into a film, a first crystal region composed of PCL segments and a second crystal region composed of PLA segments are formed, and the thermal transition temperature of the first crystal region is lower than that of the second crystal region. According to the application, the PLA segments and the PCL segments are introduced into the water-based polyurethane molecular chain in cooperation, the different crystallization behaviors of the two are utilized to construct a double-crystal-region regulation structure, so that the early-stage bonding establishment, the later-stage cohesive strength, the heat resistance and the creep resistance of the adhesive layer are considered, and the water dispersibility and the biodegradability of the system are maintained.

Description

Technical Field

[0001] This invention relates to the field of waterborne polyurethane materials technology, and more specifically, to a PLA / PCL-based biodegradable waterborne polyurethane adhesive and its preparation method. Background Technology

[0002] Waterborne polyurethane adhesives have been widely used in packaging, textiles, wood, and film lamination due to their advantages such as low volatile organic compound content, good construction safety, and wide applicability to various substrates. With increasingly stringent environmental requirements and growing demand for sustainable materials, incorporating biodegradable segments into waterborne polyurethane systems to prepare biodegradable waterborne polyurethane adhesives that combine adhesive properties with environmental friendliness is gradually becoming an important development direction in this field.

[0003] In existing technologies, polylactic acid (PLA) materials are characterized by their wide availability, good biodegradability, and high rigidity, while polycaprolactone (PCL) materials offer advantages such as good flexibility, strong crystallinity, and ease of processing. Therefore, some technologies incorporate PLA or PCL segments into polyurethane systems to improve the environmental properties and mechanical properties of adhesives. However, in practical applications, introducing PLA segments alone often results in adhesive layers exhibiting high rigidity and brittleness, with limited flexibility and initial adhesion after film formation. While introducing PCL segments alone can improve flexibility and film formation, the resulting adhesive layers often suffer from insufficient later-stage cohesive strength, heat resistance, and creep resistance.

[0004] Furthermore, even when two types of biodegradable segments are used simultaneously in existing technologies, they are mostly simple combinations or conventional copolymer designs, focusing more on biodegradability or general mechanical property improvements, while lacking targeted control over the role of differences in the crystallization behavior of different segments in the adhesive layer formation process. Especially in waterborne polyurethane systems, adhesives not only need to meet the requirements of dispersion stability and film formation, but also need to consider both early bond establishment and later stability during actual use. If early bond establishment is slow, it will be detrimental to initial positioning and processing efficiency; if the cohesive strength of the adhesive layer is insufficient in the later stage, it is prone to deformation, creep, or adhesive failure under heat or load conditions.

[0005] Therefore, it is still necessary to provide a biodegradable waterborne polyurethane adhesive that, while maintaining good water dispersibility and biodegradability, can effectively coordinate the influence of different degradable segments on the adhesive layer structure and performance, thereby solving the problem that existing biodegradable waterborne polyurethane adhesives cannot simultaneously achieve good early bond establishment speed, late cohesive strength, and heat resistance and creep resistance. Summary of the Invention

[0006] This invention aims to provide a PLA / PCL-based biodegradable waterborne polyurethane adhesive and its preparation method. By synergistically introducing PLA and PCL segments into the waterborne polyurethane molecular chain, and utilizing their different crystallization behaviors to construct a bicrystalline region-controlled structure, the adhesive layer can achieve good early adhesion establishment, late cohesive strength, and heat resistance and creep resistance, while also maintaining the system's water dispersibility and biodegradability.

[0007] To address the aforementioned problems, this invention provides a PLA / PCL-based biodegradable waterborne polyurethane adhesive, comprising: PLA-based polyol, PCL-based polyol, polyisocyanate, hydrophilic chain extender, small molecule chain extender, neutralizer, and water; wherein, after film formation, the waterborne polyurethane adhesive forms a first crystalline region composed of PCL segments and a second crystalline region composed of PLA segments, and the thermal transition temperature of the first crystalline region is lower than that of the second crystalline region.

[0008] In the above technical solution, PLA-based polyols include hydroxyl-terminated polylactic acid diols with a number average molecular weight of 500-5000.

[0009] In the above technical solution, the PCL-based polyol includes hydroxyl-terminated polycaprolactone diol with a number average molecular weight of 500-5000.

[0010] In the above technical solution, the mass ratio of PLA-based polyol to PCL-based polyol is 1:(0.25~4).

[0011] In the above technical solution, the polyisocyanate includes at least one of aliphatic diisocyanate and alicyclic diisocyanate; the hydrophilic chain extender includes at least one of dimethylolpropionic acid, dimethylolbutyric acid, and sulfonic acid-containing diol; the neutralizing agent includes at least one of triethylamine, N-methyldiethanolamine, and ammonia; and the small molecule chain extender includes at least one of 1,4-butanediol, ethylene glycol, hexanediol, ethylenediamine, hydrazine, and hydrazine hydrate.

[0012] In the above technical solution, the solid content of the waterborne polyurethane adhesive is 25~45wt%, and the average particle size of the emulsion is 30~300nm.

[0013] This invention also provides a method for preparing a PLA / PCL-based biodegradable waterborne polyurethane adhesive, the method comprising the following steps: S100. After dehydrating PLA-based polyol and PCL-based polyol, they are subjected to a prepolymerization reaction with polyisocyanate to obtain a prepolymer. S200. Add a hydrophilic chain extender to the prepolymer to obtain an ionic prepolymer; S300. After neutralizing the ionic prepolymer, it is dispersed in water to obtain an aqueous polyurethane dispersion. S400: Add a small molecule chain extender to the waterborne polyurethane dispersion to carry out a chain extension reaction, and obtain a waterborne polyurethane adhesive.

[0014] In the above technical solution, in S100, the dehydration temperature of PLA-based polyol and PCL-based polyol is 80~120℃, and the dehydration time is 0.5~3h; the prepolymerization reaction temperature is 60~90℃, and the reaction time is 1~6h.

[0015] In the above technical solution, in S300, the neutralization reaction temperature is 30~60℃ and the neutralization time is 10~60min.

[0016] In the above technical solution, in S400, the chain extension reaction temperature is 20~50℃ and the chain extension reaction time is 0.5~4h.

[0017] Beneficial effects (1) This invention introduces PLA and PCL segments into the waterborne polyurethane molecular chain in a coordinated manner, and utilizes the differences in their crystallization behavior and thermal transformation characteristics to construct a bicrystalline region control structure that plays a role in stages after the adhesive layer is formed; wherein, the ordered structure of the lower thermal transformation region is conducive to the establishment of the cohesive network in the early stage of film formation, and the ordered structure of the higher thermal transformation region is conducive to providing stronger structural constraints and rigid support in the later stage, so that the adhesive layer can take into account both the early bonding speed and the later use stability. (2) This invention does not simply use PLA and PCL components in a physical manner, but introduces PLA and PCL segments into the same waterborne polyurethane skeleton, and combines hydrophilic chain extension, neutralization dispersion and post-chain extension processes to form a stable waterborne polyurethane dispersion; thereby, it is beneficial to obtain a more suitable emulsion particle size and better storage stability, and further promotes the full merging of emulsion particles and the orderly rearrangement of chain segments during film formation. (3) In this invention, the PCL segments, PLA segments and the hard segment micro-regions formed by the post-chain extension can work together; among them, the PCL segments are beneficial to improving the flexibility of the adhesive layer and the ability to form film and establish adhesion in the early stage; the PLA segments are beneficial to improving the cohesive strength, heat resistance and dimensional stability in the later stage; and the hard segment micro-regions formed by the post-chain extension further enhance the overall cohesive force and structural stability of the adhesive layer. Detailed Implementation

[0018] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, a detailed description of specific embodiments of the present invention will be provided below.

[0019] Unless otherwise specified, all reagents and raw materials used in this invention are commercially available. Experimental methods in the following examples that do not specify particular conditions should be performed according to conventional methods and conditions, or as selected in the product instructions.

[0020] This invention provides a PLA / PCL-based biodegradable waterborne polyurethane adhesive and its preparation method, addressing the challenge of existing biodegradable waterborne polyurethane adhesives in simultaneously maintaining good water dispersibility and biodegradability while also ensuring fast initial bond establishment, high cohesive strength, and good heat resistance and creep resistance. The waterborne polyurethane adhesive of this invention comprises: PLA-based polyol, PCL-based polyol, polyisocyanate, hydrophilic chain extender, small molecule chain extender, neutralizing agent, and water.

[0021] In this invention, PLA-based polyols include hydroxyl-terminated polylactic acid diols. Introducing PLA-based polyols can, on the one hand, introduce degradable and highly rigid polyester segments into the waterborne polyurethane system, thereby improving the rigidity of the adhesive layer, its later cohesive strength, and its heat resistance. On the other hand, PLA segments can form a second crystalline region with a relatively high thermal transition temperature during film formation. This second crystalline region can still exist as a structural constraint point within the adhesive layer at higher operating temperatures, exerting a stronger constraint on the movement of polyurethane molecular chains. This helps to suppress segment slippage, viscous flow deformation, and structural relaxation of the adhesive layer under heated or continuously loaded conditions, thereby improving the adhesive layer's tack, creep resistance, and dimensional stability.

[0022] In this invention, the PCL-based polyol includes hydroxyl-terminated polycaprolactone diol. Introducing the PCL-based polyol introduces biodegradable polyester segments with good flexibility and crystallinity into the system, thereby improving the problems of brittle adhesive layers, hard films, and insufficient flexibility that may occur when only PLA segments are introduced. Simultaneously, PCL segments are more likely to form first crystalline regions with relatively low thermal transition temperatures during film formation. These first crystalline regions can preferentially form in lower temperature ranges or earlier film formation stages, essentially establishing a batch of physical crosslinking points within the adhesive layer that can function earlier. This facilitates the formation of the cohesive network within the adhesive layer and promotes early adhesion establishment. Furthermore, the ordered structure formed by the PCL segments can also improve the segment regularity within the system to a certain extent, providing favorable conditions for the subsequent formation of a more stable ordered structure by the PLA segments, thus promoting the occurrence of later structural reinforcement processes.

[0023] It is understood that this invention does not simply combine PLA and PCL segments, but rather utilizes their differences in crystallization behavior and thermal transition characteristics to design a division of labor. Specifically, the PCL segments are primarily responsible for the lower thermal transition region and the establishment of the early physical cross-linking network, while the PLA segments are primarily responsible for the higher thermal transition region and the later rigidity enhancement. By simultaneously constructing these two types of crystalline regions with different thermal behaviors within the same adhesive layer, a bicrystalline region structure that functions in stages is formed within the adhesive layer.

[0024] Specifically, the first crystalline region preferentially participates in the early cohesive network construction of the film, which is beneficial to improving the early adhesion establishment speed; the second crystalline region provides stronger rigid support and more stable structural constraints for the adhesive layer in the later stage, which is beneficial to improving the later cohesive strength, heat resistance and creep resistance. Thus, the present invention can better balance the relationship between early adhesion establishment and later use stability, avoiding the problems of high brittleness and narrow film formation window of the single PLA system, and insufficient rigid support and weak heat resistance and creep resistance in the later stage of the single PCL system.

[0025] Preferably, the mass ratio of PLA-based polyol to PCL-based polyol is 1:(0.25~4). Within this range, the system has sufficient PCL segments to ensure the flexibility, film-forming properties, and formation of the first crystalline region of the adhesive layer, as well as sufficient PLA segments to ensure the construction of the second crystalline region and the subsequent heat resistance and creep resistance support. If the PLA segment content is too low, the high-thermal-transition zone structure in the system is insufficient, and the rigid support capacity of the adhesive layer is weak in the later stage, which easily leads to insufficient cohesive strength, tack, and heat resistance and creep resistance in the later stage. If the PLA segment content is too high, the flexibility of the system decreases, the adhesive layer is prone to brittleness, and the film-forming window becomes narrower, which is not conducive to the application and overall performance balance of the adhesive.

[0026] Preferably, the number-average molecular weights of both PLA-based and PCL-based polyols are 500-5000. If the molecular weight is too low, the corresponding soft segments will be too short, and the structural characteristics and functional division of the PLA and PCL segments will not be obvious enough, weakening their ability to form effective crystalline regions. This results in insufficient contribution to the adjustment of the adhesive layer's flexibility and the enhancement of its staged structure, making the system more likely to be dominated by hard segments, which is detrimental to the establishment of a bicrystalline region structure and performance balance. If the molecular weight is too high, the viscosity of the prepolymer system will increase significantly, which is not conducive to the control of the reaction process and subsequent water dispersion, easily increasing the difficulty of emulsion preparation and potentially leading to decreased emulsion stability and poorer workability. At the same time, excessively long soft segments may also weaken the later-stage cohesive strength of the adhesive layer and the structural constraint ability in the high-temperature region.

[0027] In this invention, the polyisocyanate includes at least one of aliphatic diisocyanate and alicyclic diisocyanate. Using this type of polyisocyanate is beneficial to improving the yellowing resistance, weather resistance and system stability of the obtained adhesive. At the same time, it is beneficial to form a more balanced polyurethane network structure with the above-mentioned PLA and PCL segments, avoiding adverse effects on the system's flexibility, film-forming properties and bicrystalline region control effect due to the isocyanate structure being too rigid or too reactive.

[0028] In this invention, the hydrophilic chain extender includes at least one of dimethylolpropionic acid, dimethylolbutyric acid, and a sulfonic acid-containing diol. This type of hydrophilic chain extender can introduce ionizable hydrophilic groups into the polyurethane backbone, forming stable ionic centers after neutralization, thereby endowing the prepolymer with good water dispersibility. The neutralizing agent includes at least one of triethylamine, N-methyldiethanolamine, and ammonia water. Its function is to convert the acidic groups introduced by the hydrophilic chain extender into their corresponding ionic forms, thereby enhancing the self-dispersibility of the prepolymer in water and promoting the formation of a waterborne polyurethane dispersion with suitable particle size and good stability. By using the hydrophilic chain extender and the neutralizing agent in combination, both good water-based properties of the system can be ensured, while reducing problems such as migration, precipitation, and decreased water resistance caused by added surfactants.

[0029] In this invention, the small molecule chain extender includes at least one of 1,4-butanediol, ethylene glycol, hexanediol, ethylenediamine, hydrazine, and hydrazine hydrate. The small molecule chain extender can further increase the molecular weight of polyurethane and construct suitable hard segment structures, thereby enhancing the cohesive strength and mechanical properties of the adhesive layer. Alcohol-based chain extenders are advantageous in increasing molecular weight while maintaining system flexibility; amine or hydrazine-based chain extenders are more conducive to forming structural units with strong hydrogen bonding, thereby enhancing hard segment association and improving the later-stage cohesive strength, heat resistance, and creep resistance of the adhesive layer.

[0030] Furthermore, the solid content of the waterborne polyurethane adhesive is 25-45 wt%, and the average particle size of the emulsion is 30-300 nm. Controlling the solid content to 25-45 wt% avoids the problems of excessive moisture, prolonged drying time, and low film-forming efficiency caused by too low a solid content, while avoiding the problems of excessive viscosity, increased dispersion difficulty, poor storage stability, and decreased leveling properties caused by too high a solid content. This balances the stability of the preparation process, application performance, and film-forming efficiency. Controlling the average particle size of the emulsion to 30-300 nm improves the dispersion stability and interfacial wetting ability of the emulsion, allowing the emulsion particles to spread more evenly on the substrate surface after coating and to more easily aggregate during drying, forming a continuous, dense, and uniform adhesive layer. If the particle size is too large, particle aggregation and film uniformity deteriorate, which is detrimental to the continuity of the bonding interface and the final adhesive layer performance.

[0031] In summary, this invention synergistically introduces PLA-based polyols and PCL-based polyols into an aqueous polyurethane system, utilizing their differences in flexibility, rigidity, crystallization behavior, and thermal transition characteristics. After the adhesive layer is formed, a bicrystalline structure consisting of a first crystalline region and a second crystalline region is constructed. This allows the first crystalline region, with its lower thermal transition region, to preferentially participate in the early cohesive network establishment, while the second crystalline region, with its higher thermal transition region, provides stronger structural constraints and rigid support in the later stages. Thus, while maintaining good water dispersibility and biodegradability, a balance is achieved between the early adhesion establishment speed, later cohesive strength, heat resistance, and creep resistance of the adhesive layer.

[0032] In specific operation, the preparation method of the waterborne polyurethane adhesive of the present invention includes the following steps: S100. After dehydrating PLA-based polyol and PCL-based polyol, they are subjected to a prepolymerization reaction with polyisocyanate to obtain a prepolymer. S200. Add a hydrophilic chain extender to the prepolymer to obtain an ionic prepolymer; S300. After neutralizing the ionic prepolymer, it is dispersed in water to obtain an aqueous polyurethane dispersion. S400: Add a small molecule chain extender to the waterborne polyurethane dispersion to carry out a chain extension reaction, and obtain a waterborne polyurethane adhesive.

[0033] In S100, PLA-based and PCL-based polyols are first dehydrated. This dehydration primarily removes free water from the system, preventing side reactions between water and isocyanate groups. Excess water in the system will react with the isocyanate groups, reducing the effective utilization rate of isocyanate and potentially causing gas release, byproduct formation, and deviations from the expected prepolymer composition. This negatively impacts subsequent reaction control, emulsion stability, and adhesive layer performance.

[0034] After dehydration, PLA-based polyols and PCL-based polyols are prepolymerized with polyisocyanates. This process serves to first construct a polyurethane prepolymer with active end groups, allowing for further modification, while simultaneously introducing PLA and PCL segments into the same polyurethane backbone. Because PLA segments possess relatively high rigidity, and PCL segments exhibit better flexibility and are more prone to forming ordered structures in lower thermal transition regions, prepolymerization embeds both into the polyurethane backbone. This is a prerequisite for the formation of bicrystalline regions to regulate the structure during subsequent dispersion, chain extension, and film formation. Only when both types of degradable segments coexist within the same polyurethane backbone can the subsequent film formation process more effectively achieve the desired structural effect: PCL segments preferentially participate in early network establishment, while PLA segments further enhance rigidity in later stages.

[0035] Furthermore, in S100, the dehydration temperature of PLA-based polyols and PCL-based polyols is 80~120℃, and the time is 0.5~3h. If the dehydration temperature is too low or the dehydration time is too short, the water removal will be insufficient, and the residual water is prone to side reactions with the isocyanate groups in the subsequent process, which is not conducive to the smooth progress of the prepolymerization reaction. Conversely, if the dehydration temperature is too high or the dehydration time is too long, the PLA-based polyols and PCL-based polyols may be subjected to excessive heat history, resulting in increased system viscosity, color changes, or even unnecessary thermal degradation, which is not conducive to the stability of raw materials and subsequent process control.

[0036] The prepolymerization temperature is 60–90°C. This range is beneficial for ensuring a sufficient reaction rate between the isocyanate groups and hydroxyl groups, thus achieving a more complete prepolymer formation. Simultaneously, it avoids excessively high reaction temperatures that could lead to increased side reactions, excessively rapid increases in system viscosity, or overly drastic phase separation. Controlling the reaction time to 1–6 hours helps ensure a complete prepolymerization reaction while avoiding decreased process efficiency and system stability fluctuations caused by excessively long reaction times.

[0037] In S200, a hydrophilic chain extender is added to the prepolymer. Its function is to further react with the isocyanate groups in the prepolymer, introducing ionizable hydrophilic groups into the polyurethane backbone, thereby forming an ionic prepolymer with an internal emulsification center. Hydrophilic chain extenders such as dimethylolpropionic acid, dimethylolbutyric acid, or sulfonic acid-containing diols can covalently integrate into the polyurethane chain segments, resulting in neutralizable carboxyl groups and / or intrinsically ionic groups in the backbone.

[0038] Introducing a hydrophilic chain extender via S200 facilitates the formation of stable ionic centers on the molecular chain after subsequent neutralization, giving the prepolymer better self-dispersibility without excessive reliance on external surfactants. Compared to simply relying on external surfactants, this internal emulsification method is more beneficial for improving emulsion stability and reducing surfactant migration, precipitation, and potential adverse effects on the water resistance and long-term stability of the adhesive layer.

[0039] Meanwhile, S200 in this invention is not only related to whether the system can achieve stable waterborne application, but also to whether the subsequent emulsion particle size and particle size distribution are within a suitable range. A waterborne polyurethane dispersion with moderate particle size and relatively uniform distribution is more conducive to the full aggregation of emulsion particles and the orderly rearrangement of chain segments during the subsequent film formation process, thereby providing a more stable microenvironment for the formation of the first crystalline region of PCL segments and the formation of the second crystalline region of PLA segments.

[0040] In S300, the ionic prepolymer is first neutralized before being added to water for dispersion. This process converts the acidic groups, such as carboxyl groups, introduced in S200 into their corresponding ionic salt forms, allowing the prepolymer to form a stable aqueous dispersion upon addition of water through electrostatic repulsion and interfacial stabilization. Specifically, the neutralized ionic groups are distributed along the polyurethane chain segments, giving the polymer particles a certain charge in the aqueous phase, thus reducing the tendency for particle aggregation and improving emulsion stability. If water is added directly without sufficient neutralization, the hydrophilic centers in the system are not effectively ionized, resulting in insufficient self-dispersibility of the prepolymer. This can easily lead to inadequate dispersion, larger particle size, wider particle size distribution, or even localized aggregation, which is detrimental to obtaining a stable and uniform aqueous polyurethane dispersion.

[0041] In this invention, the stability and particle size distribution of the dispersion obtained by S300 have a significant impact on the final performance. If the emulsion particle size is too large or the particle size distribution is too wide, it will weaken the wetting and spreading ability on the substrate surface and reduce the uniformity of particle aggregation during the drying process, ultimately affecting the formation quality of the continuous film. Conversely, a dispersion with a moderate particle size and a relatively uniform distribution is more conducive to the subsequent chain segment migration and ordering process during film formation, thus making it more favorable for PCL segments to preferentially form the first crystalline region and PLA segments to further form the second crystalline region.

[0042] Furthermore, in S300, the neutralization reaction temperature is 30~60℃, and the neutralization time is 10~60min. If the neutralization temperature is too high, the risk of adverse reactions of residual active groups in the system increases, and may increase the instability of the subsequent dispersion process; if the neutralization temperature is too low or the neutralization time is too short, the neutralization is insufficient, the number of ion centers in the system is insufficient, which can easily lead to larger dispersed particle size and poorer emulsion stability.

[0043] In S400, a small-molecule chain extender is added after the formation of the aqueous polyurethane dispersion to initiate a chain extension reaction. Its role is to further increase the molecular weight of the polyurethane, construct or strengthen hard segment microdomains, enhance the cohesiveness and mechanical properties of the adhesive layer, and improve the structural stability after film formation. The small-molecule chain extender can continue to react with residual active groups in the dispersion, causing the polyurethane molecular chains to grow further. For highly reactive diamine or hydrazine chain extenders, the chain extension effect is more pronounced in the aqueous dispersion system, which is beneficial for the rapid formation of structural units with strong hydrogen bonding. For diol chain extenders, it is beneficial for regulating molecular weight growth and hard segment structure in a relatively mild manner. Through the above post-chain extension process, not only can the later-stage cohesive strength of the adhesive layer be improved, but also the heat resistance and creep resistance of the adhesive layer can be enhanced.

[0044] In this invention, S400, in addition to the control of the PLA and PCL dual-crystal regions, further strengthens the cohesive network of the adhesive layer in the later stages through the construction of hard segment microregions and the enhancement of interparticle connections. Thus, the first crystal region is mainly responsible for the initial network establishment, the second crystal region is mainly responsible for the rigid support in the high-temperature range, and the hard segment microregions are mainly responsible for improving the overall cohesive force and structural stability. The three work synergistically, so that the adhesive layer can take into account the initial bonding speed, the later cohesive strength, and the heat resistance and creep resistance.

[0045] Furthermore, in S400, the chain extension reaction temperature is 20~50℃. If the chain extension temperature is too high, it can easily lead to excessively rapid local reactions, particle aggregation, or an increase in side reactions, thereby affecting the stability of the dispersion. A milder chain extension temperature is beneficial for controlling the chain extension rate, improving system stability, and avoiding excessive abrupt changes in the gel structure. Controlling the chain extension time to 0.5~4h is beneficial for ensuring that the chain extension reaction proceeds fully while avoiding excessively long processing time or structural imbalance.

[0046] Furthermore, after the adhesive is coated and formed into a film, it undergoes a curing treatment at 40-80°C. This curing treatment, on the one hand, promotes further aggregation and densification of emulsion particles and facilitates the migration and evaporation of residual moisture, thereby improving the continuity and uniformity of the film layer; on the other hand, it promotes further rearrangement and ordering of PLA and PCL segments after film formation, thereby promoting the formation and stabilization of the first and second crystalline regions. Through the above curing process, the dual-crystal region control structure described in this invention can be further strengthened, allowing the first crystalline region with a lower thermal transition zone to better participate in the maintenance of the early physical crosslinking network, and enabling the second crystalline region with a higher thermal transition zone to provide more stable structural constraints and rigid support in the later stages, thereby further improving the overall adhesive performance, heat resistance, and creep resistance of the adhesive layer.

[0047] In summary, this invention, through steps S100 to S400, first constructs a polyurethane skeleton containing PLA and PCL segments, then introduces ionizable hydrophilic centers to achieve stable water dispersion, subsequently strengthens the hard segment microregions and cohesive network through post-chain extension, and finally forms a bicrystalline structure composed of a first crystalline region and a second crystalline region during film formation and curing, thereby achieving a balance between water dispersibility, biodegradability, early adhesion establishment speed, late cohesive strength, and heat resistance and creep resistance. Example 1

[0048] This embodiment provides a PLA / PCL-based biodegradable waterborne polyurethane adhesive and its preparation method. The preparation method includes the following steps: S100. Add 20 parts of hydroxyl-terminated polylactic acid diol (number average molecular weight of about 1000) and 40 parts of hydroxyl-terminated polycaprolactone diol (number average molecular weight of about 2000) to a reactor and dehydrate under vacuum at 105°C for 1.5 h. After dehydration, cool down to 80°C, add 22 parts of isophorone diisocyanate, and keep the reaction at this temperature for 3 h to obtain the prepolymer. S200. Add 6 parts of dimethylolpropionic acid to the prepolymer and continue the reaction at 75°C for 1.5 h to obtain an ionic prepolymer. S300, cool the system to 45℃, add 4.5 parts of triethylamine for neutralization for 25 min; then, under high-speed stirring, slowly add 170 parts of deionized water to the system within 25 min for dispersion to obtain an aqueous polyurethane dispersion; S400, cool the system to 30℃, add 3 parts of 1,4-butanediol, and continue the chain extension reaction for 1 hour to obtain waterborne polyurethane adhesive. Example 2

[0049] This embodiment provides a PLA / PCL-based biodegradable waterborne polyurethane adhesive and its preparation method. The preparation method includes the following steps: S100. Add 25 parts of hydroxyl-terminated polylactic acid diol (number average molecular weight approximately 2000) and 25 parts of hydroxyl-terminated polycaprolactone diol (number average molecular weight approximately 2000) to a reaction vessel and dehydrate under vacuum at 100°C for 1 hour. After dehydration, cool to 75°C and add 23 parts of 4,4'-dicyclohexylmethane diisocyanate. Keep the reaction at this temperature for 4 hours to obtain the prepolymer. S200. Add 5.5 parts of dimethylolpropionic acid to the prepolymer and continue the reaction at 70°C for 1 hour to obtain an ionic prepolymer. S300, cool the system to 40℃, add 4 parts of triethylamine for neutralization for 20 min; then, under high-speed stirring, slowly add 160 parts of deionized water to the system within 25 min for dispersion to obtain an aqueous polyurethane dispersion; S400, cool the system to 25°C, add 1.8 parts of ethylenediamine, and continue the chain extension reaction for 1 hour to obtain waterborne polyurethane adhesive. Example 3

[0050] This embodiment provides a PLA / PCL-based biodegradable waterborne polyurethane adhesive and its preparation method. The preparation method includes the following steps: S100. 35 parts of hydroxyl-terminated polylactic acid diol (number average molecular weight approximately 3000) and 20 parts of hydroxyl-terminated polycaprolactone diol (number average molecular weight approximately 1000) were vacuum dehydrated at 110°C for 2 hours; after cooling to 85°C, 24 parts of a mixture of isophorone diisocyanate and hexamethylene diisocyanate in a mass ratio of 1:1 were added, and the mixture was reacted for 2.5 hours to obtain the prepolymer. S200. Add 6 parts of dimethylolbutyric acid to the prepolymer and continue the reaction at 75°C for 1.5 h to obtain an ionic prepolymer. S300, cool the system to 50℃, add 4.2 parts of triethylamine to neutralize for 30 min; then, under high-speed stirring, slowly add 175 parts of deionized water to the system within 25 min for dispersion to obtain an aqueous polyurethane dispersion; S400, 2.5 parts of hexanediol were added at 35℃ for chain extension, and the reaction was carried out for 2 hours to obtain waterborne polyurethane adhesive.

[0051] Comparative Example 1 This comparative example provides a PLA-based biodegradable waterborne polyurethane adhesive and its preparation method. The preparation method is the same as that shown in Example 2, except that in S100, hydroxyl-terminated polycaprolactone diol is not added, that is, 50 parts of hydroxyl-terminated polylactic acid diol are added.

[0052] Comparative Example 2 This comparative example provides a PCL-based biodegradable waterborne polyurethane adhesive and its preparation method. The preparation method is the same as that shown in Example 2, except that in S100, terminal hydroxyl polylactic acid diol is not added, that is, 50 parts of terminal hydroxyl polycaprolactone diol are added.

[0053] Comparative Example 3 This comparative example provides a PLA / PCL-based biodegradable waterborne polyurethane adhesive and its preparation method. The preparation method is the same as that shown in Example 1, except that in S100, the mass ratio of PLA-based polyol to PCL-based polyol is 1:6, that is, 10 parts of terminal hydroxyl polylactic acid diol (number average molecular weight of about 1000) and 60 parts of terminal hydroxyl polycaprolactone diol (number average molecular weight of about 2000) are added.

[0054] Comparative Example 4 This comparative example provides a PLA / PCL-based biodegradable waterborne polyurethane adhesive and its preparation method. The preparation method is the same as that shown in Example 1, except that in S100, the mass ratio of PLA-based polyol to PCL-based polyol is 1:0.17, that is, 60 parts of terminal hydroxyl polylactic acid diol (number average molecular weight of about 2000) and 10 parts of terminal hydroxyl polycaprolactone diol (number average molecular weight of about 2000) are added.

[0055] Comparative Example 5 This comparative example provides a PLA / PCL-based biodegradable waterborne polyurethane adhesive and its preparation method. The preparation method includes: preparing PLA-type waterborne polyurethane emulsion A and PCL-type waterborne polyurethane emulsion B from comparative examples 1 and 2; then mixing emulsion A and emulsion B at a mass ratio of 1:1 and stirring at room temperature for 30 minutes to obtain the waterborne polyurethane adhesive.

[0056] Performance testing Solid content test: Take about 1.0g of adhesive emulsion and place it in an aluminum dish that has been pre-weighed. Weigh the initial mass. Place it in an oven at 105℃ and dry for 2 hours. After removing it, place it in a desiccator to cool to room temperature, weigh the mass after drying, and calculate the solid content.

[0057] Emulsion average particle size test: Take the adhesive emulsion, dilute it with deionized water to a suitable concentration, and then use a dynamic light scattering particle size analyzer to measure the average particle size of the emulsion at 25℃. Film preparation: Each adhesive emulsion was uniformly coated on the surface of the polytetrafluoroethylene release plate, placed at room temperature for 24 hours, and then cured in an oven at 60°C for 12 hours. The free film was then peeled off and set aside for later use. Thermal transition test in the bicrystalline region: Take 5~10mg of the above free membrane sample and test it using a differential scanning calorimeter. Under nitrogen protection, perform a heating scan at a heating rate of 10℃ / min and record the thermal transition peak in the second heating curve. The lower temperature thermal transition peak is recorded as the first thermal transition temperature T1, and the higher temperature thermal transition peak is recorded as the second thermal transition temperature T2. 180° peel strength test: The adhesive emulsion was evenly coated on the surface of the PET film, dried at 60°C for 5 min, and then laminated with another PET film. The film was pressed once with a pressure roller. After being placed at room temperature for 0.5 h and 24 h respectively, the film was cut into 15 mm wide samples and subjected to a 180° peel test on an electronic tensile testing machine. The peel strength was recorded. Overlap shear strength test: The adhesive is applied to the surface of the PET substrate to make an overlap sample with an overlap area of ​​25mm×12.5mm. After curing at room temperature for 24h, a tensile test is performed on an electronic tensile testing machine. The maximum breaking load is recorded and converted into the overlap shear strength. Heat resistance and creep resistance test: The above lapped specimens were placed in an environment of 70℃ and a constant load of 1kg was applied. After 1 hour, the relative displacement of the lapped ends was measured and recorded as creep displacement in mm.

[0058] Table 1 Table 2 Table 3 As shown in Table 1, the solid content of the adhesives obtained in Examples 1-3 is within a suitable range, and the average particle size is between 96 and 132 nm. No significant stratification or sedimentation was observed after 30 days at room temperature, indicating that within the PLA-based polyol to PCL-based polyol ratio range defined in this invention, the system has good water dispersibility and storage stability. In contrast, the emulsion particle size of Comparative Examples 1 and 4 is significantly increased, especially Comparative Example 4, whose particle size reaches 224 nm, and significant thickening occurs. This indicates that when the PLA segment ratio is too high, the system rigidity increases and the chain segment mobility decreases, which is not conducive to the stable dispersion of the prepolymer, thus affecting the control of emulsion particle size and storage stability. Although Comparative Example 5 contains both PLA and PCL components, since they are first formed into emulsions separately and then physically mixed, they failed to achieve synergistic construction within the same polyurethane backbone. Therefore, its particle size and storage stability are not as good as those of Examples 1-3.

[0059] As shown in Table 2, the film-forming samples of Examples 1-3 all exhibited two relatively clear thermal transition peaks. The first thermal transition peak T1 in the lower temperature region and the second thermal transition peak T2 in the higher temperature region coexisted, indicating that a first crystalline region composed of PCL segments and a second crystalline region composed of PLA segments were indeed formed in these examples, and the first thermal transition temperature was lower than the second thermal transition temperature. Comparative Example 1 only showed a thermal transition peak in the high-temperature region, and Comparative Example 2 only showed a thermal transition peak in the low-temperature region, indicating that introducing PLA segments alone or PCL segments alone could not form the bicrystalline region structure described in this invention. Although two thermal transition peaks were also observed in Comparative Examples 3 and 4, one peak was significantly weaker, indicating that when the proportions of the two types of segments were unbalanced, the division of labor and synergy in the bicrystalline region structure was insufficient. Although Comparative Example 5 showed two thermal transition peaks, the peak shape was relatively broad, indicating that simple physical blending could not achieve the ordered control effect formed by synergistically introducing the two types of segments into the same polyurethane skeleton.

[0060] As shown in Table 3, the peel strength results at 0.5 h indicate that the early peel strength of Examples 1-3 is significantly higher than that of Comparative Examples 1 and 4. This suggests that in the system of this invention, the first crystalline region in the low-temperature zone formed by the PCL segments is beneficial for preferentially establishing a physical cross-linking network in the early stage of film formation, thereby improving the early adhesion establishment speed. The 0.5 h peel strength of Comparative Examples 2 and 3 is also relatively high, indicating that increasing the proportion of PCL segments does indeed help improve the early adhesion establishment ability. However, combined with the heat resistance and creep resistance results described later, it is clear that if the system lacks sufficient PLA segments to provide structural support in the later high-temperature zone, it is difficult to ensure stability in later use. Therefore, this invention does not simply increase the PCL segment content, but rather achieves a balance between early and late performance through the synergistic design of PLA and PCL segments.

[0061] The 24-hour peel strength and lap shear strength results show that Examples 1-3 all exhibit high late-stage adhesive strength, with Examples 2 and 3 being particularly outstanding. This indicates that in the system of this invention, the higher thermal transition zone structure formed by the PLA segments and the hard segment microregions formed by post-chain extension can provide stronger structural constraints and cohesive support in the later stages of the adhesive layer, thereby improving the later-stage strength and stability of the adhesive layer. In contrast, although Comparative Example 2 has a higher early-stage peel strength, its 24-hour peel strength and lap shear strength are significantly lower, indicating that the later-stage rigidity support of the adhesive layer obtained by relying solely on the PCL segments is insufficient. Although Comparative Example 1 has a certain later-stage strength, its early-stage peel strength is significantly lower, indicating that relying solely on the PLA segments leads to insufficient early-stage film flexibility and initial network establishment ability.

[0062] As shown in Table 3, the creep displacement results at 70℃ are significantly lower in Examples 1-3 than in the comparative examples, especially in Examples 2 and 3, where the creep displacements are only 0.9 mm and 0.8 mm, respectively. This indicates that the adhesive layer of the present invention has good structural stability and creep resistance under heat and load conditions. The creep displacements of Comparative Examples 2 and 3 are significantly larger, indicating that when there are insufficient PLA segments in the system, the structural constraint in the high-temperature zone is insufficient, and the adhesive layer is more prone to segment slippage and viscous flow deformation under thermal load. Although the creep displacements of Comparative Examples 1 and 4 are lower than those of Comparative Examples 2 and 3, their overall performance is still inferior to Examples 1-3 due to their poor early adhesion establishment ability. The creep displacement of Comparative Example 5 is also significantly higher than that of Examples 1-3, further illustrating that simple physical blending cannot replace the structural advantages brought about by the synergistic introduction of PLA and PCL segments into the same polyurethane skeleton. Any person skilled in the art can make various modifications and alterations without departing from the spirit and scope of the present invention; therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A PLA / PCL-based biodegradable waterborne polyurethane adhesive, characterized in that, The waterborne polyurethane adhesive comprises: PLA-based polyol, PCL-based polyol, polyisocyanate, hydrophilic chain extender, small molecule chain extender, neutralizer, and water; The waterborne polyurethane adhesive forms a first crystalline region composed of PCL segments and a second crystalline region composed of PLA segments after film formation, and the thermal transition temperature of the first crystalline region is lower than that of the second crystalline region.

2. The waterborne polyurethane adhesive according to claim 1, characterized in that, The PLA-based polyol includes hydroxyl-terminated polylactic acid diol with a number average molecular weight of 500-5000.

3. The waterborne polyurethane adhesive according to claim 1, characterized in that, The PCL-based polyol includes hydroxyl-terminated polycaprolactone diol with a number-average molecular weight of 500-5000.

4. The waterborne polyurethane adhesive according to claim 1, characterized in that, The mass ratio of the PLA-based polyol to the PCL-based polyol is 1:(0.25~4).

5. The waterborne polyurethane adhesive according to claim 1, characterized in that, The polyisocyanate includes at least one of aliphatic diisocyanate and alicyclic diisocyanate; The hydrophilic chain extender includes at least one of dimethylolpropionic acid, dimethylolbutyric acid, and sulfonic acid diol. The neutralizing agent includes at least one of triethylamine, N-methyldiethanolamine, and ammonia. The small molecule chain extender includes at least one of 1,4-butanediol, ethylene glycol, hexanediol, ethylenediamine, hydrazine, and hydrazine hydrate.

6. The waterborne polyurethane adhesive according to claim 1, characterized in that, The waterborne polyurethane adhesive has a solid content of 25-45 wt% and an average emulsion particle size of 30-300 nm.

7. A method for preparing the waterborne polyurethane adhesive according to any one of claims 1 to 6, characterized in that, Includes the following steps: S100. After dehydrating the PLA-based polyol and the PCL-based polyol, they are subjected to a prepolymerization reaction with the polyisocyanate to obtain a prepolymer. S200. Add the hydrophilic chain extender to the prepolymer to obtain an ionic prepolymer; S300. The ionic prepolymer is neutralized and then dispersed in water to obtain an aqueous polyurethane dispersion. S400. Add a small molecule chain extender to the waterborne polyurethane dispersion to carry out a chain extension reaction, thereby obtaining the waterborne polyurethane adhesive.

8. The preparation method according to claim 7, characterized in that, In S100, The dehydration temperature of the PLA-based polyol and the PCL-based polyol is 80~120℃, and the dehydration time is 0.5~3h. The temperature of the prepolymerization reaction is 60~90℃, and the reaction time is 1~6h.

9. The preparation method according to claim 7, characterized in that, In S300, the neutralization reaction is carried out at a temperature of 30~60°C for a time of 10~60 min.

10. The preparation method according to claim 7, characterized in that, In S400, the chain extension reaction is carried out at a temperature of 20~50℃ for a time of 0.5~4h.

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