Depolymerization type polyurethane hot melt adhesive as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of hot melt adhesives, and particularly relates to a depolymerizable polyurethane hot melt adhesive as well as a preparation method and application thereof, and the depolymerizable polyurethane hot melt adhesive comprises the following raw materials in parts by mass: 30-70 parts of polymer polyol; 1-15 parts of a chain extender containing a dihydroxy thioester structure; 10 to 30 parts of polyisocyanate; and 0.05-1 part of an auxiliary agent. The polyurethane hot melt adhesive disclosed by the invention not only has excellent multi-substrate bonding performance and environmental aging stability, but also can realize controllable degradation under the condition of not needing high temperature or special equipment, so that the convenience of reworking repair and green circulation is remarkably improved, and the problem that electronic parts are difficult to recycle is solved.

Description

Technical Field

[0001] This invention relates to a depolymerizable polyurethane hot melt adhesive, its preparation method and application, belonging to the field of hot melt adhesive technology. Background Technology

[0002] Polyurethane hot melt adhesives (PURs), as an important category of structural adhesives, combine the initial rapid positioning characteristics of hot melt adhesives after application with the high bond strength, heat resistance, and durability brought about by the cross-linked structure formed after moisture curing. Therefore, they are widely used in various fields such as consumer electronics, automotive manufacturing, packaging industry, and building materials. Especially in precision electronic assembly scenarios such as mobile phone frames, earphone assemblies, and camera modules, polyurethane hot melt adhesives have become a commonly used encapsulation and fixing material due to their excellent flexibility, long-term thermal stability, and good adhesion to various substrates such as metals, glass, and plastics. However, traditional PURs form a highly cross-linked three-dimensional thermosetting network after moisture curing. This structure is difficult to dissolve or degrade under normal conditions, resulting in limitations in component repair, material disassembly, and recycling. This makes it difficult to meet the current electronics industry's requirements for maintainability, disassembly, and recyclability under the principles of "green manufacturing" and a circular economy.

[0003] To address this challenge, researchers in this field have begun to explore the introduction of dynamic covalent chemistry (DCC) structures into PUR systems, such as Diels-Alder bonds, transesterification bonds, and thiol-olefin click chemistry structures, to achieve controlled degradation and fracture of materials under specific conditions such as heat, light, or chemical stimulation. However, these systems are mostly used in foams, gels, or self-healing coatings where high mechanical properties are not required. When used in structural adhesives requiring high mechanical strength, high temperature resistance, and adaptability to large-scale production lines, they often suffer from insufficient crosslinking efficiency, poor thermal stability, poor storage stability, or difficulties in industrializing raw materials. Therefore, developing a polyurethane hot melt adhesive that combines excellent structural bonding performance with controllable depolymerization capability has become an urgent technical problem to be solved in this field. Summary of the Invention

[0004] To address the aforementioned technical problems in the prior art, this invention provides a depolymerizable polyurethane hot melt adhesive based on a dihydroxy thioester chain-extended structure, its preparation method, and its application. This polyurethane hot melt adhesive possesses both excellent structural bonding strength and controllable depolymerization, making it particularly suitable for detachable assembly and environmentally friendly recycling scenarios.

[0005] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: One objective of this invention is to provide a depolymerizable polyurethane hot melt adhesive, comprising the following raw materials by weight: 30-70 parts of polymeric polyol; 1-15 parts of chain extender containing dihydroxythioester structure; 10-30 parts of polyisocyanate; 0.05-1 part of auxiliaries.

[0006] The core concept of this invention lies in introducing a thioester structure as a dynamic covalent bond into the main chain of polyurethane. The thioester bond possesses both hydrolytic and thiol exchange properties, making it a kinetically controllable bond with advantages such as mild conditions, rapid response, structural stability, and tunable reactivity. This invention selects a thioester monomer with two hydroxyl terminal groups, allowing it to participate in the reaction during the chain extension stage of polyurethane and become part of the polymer network. This design enables the final polyurethane hot melt adhesive to maintain excellent initial tack and high bond strength after moisture curing, while also allowing selective bond breaking and main chain deconstruction under specific external stimuli (such as alkaline environments or environments containing thiol compounds). This achieves the integration of processability, reworkability, and biodegradability within the same adhesive system, providing a highly promising material solution for green manufacturing and circular economy transformation in fields such as consumer electronics.

[0007] Furthermore, the chain extender containing the dihydroxy thioester structure is di(2-hydroxyethyl) thiodipropionate or its derivative, whose molecular structure contains two hydroxyl terminal groups and a degradable thioester bond in the middle. This thioester bond can be selectively broken in an alkaline environment or in the presence of a thiol compound, thereby achieving the controllable deconstruction of the polyurethane backbone.

[0008] Furthermore, the chain extender containing the dihydroxythioester structure is di(2-hydroxyethyl) thiodipropionate.

[0009] Furthermore, the polymer polyol is selected from at least one of polyether polyol, polyester polyol, or polycarbonate polyol.

[0010] Furthermore, the polyether polyol is polypropylene glycol or polytetrahydrofuran ether glycol, preferably PPG2000, PTMG2000, or PPG1000; the polyester polyol is an aliphatic linear copolyester polyol, preferably Dynacoll 7380 (crystalline polyester), Dynacoll 7255 (amorphous polyester), or Capa 2403.

[0011] Furthermore, the polyisocyanate is selected from at least one of diphenylmethane diisocyanate (MDI), isophorone diisocyanate (IPDI), or hexamethylene diisocyanate trimer (HDI trimer).

[0012] Furthermore, the additive is selected from at least one of a catalyst (such as dibutyltin dilaurate), an antioxidant (such as antioxidant 1010), and a wetting and leveling agent (such as BYK-407) to improve the reaction rate, thermal oxidation stability, and construction leveling performance.

[0013] A second objective of this invention is to provide a method for preparing the above-mentioned depolymerizable polyurethane hot melt adhesive, comprising the following steps: S1, vacuum dehydration treatment of polymer polyols; S2. Cool the dehydrated polymer polyol, add polyisocyanate, and react to generate a prepolymer. S3. Add a chain extender containing a dihydroxy thioester structure to the prepolymer and continue the chain extension reaction; S4. Add additives to the reaction system, mix evenly, and obtain the depolymerizable polyurethane hot melt adhesive after degassing.

[0014] Furthermore, in step S1, the polymer polyol is vacuum dehydrated at 120°C for 2–3 hours.

[0015] Furthermore, in step S2, the dehydrated polymer polyol is cooled to 85–95°C; after adding polyisocyanate, the reaction is carried out for 1–2 hours.

[0016] Furthermore, in step S3, the chain extender containing the dihydroxysulfate structure is pre-dried before being added; the temperature of the chain extension reaction is controlled at 85-95°C; the amount of the chain extender containing the dihydroxysulfate structure added is controlled at 1-15 parts, preferably 2.5%-13% of the total mass of the system, so as to effectively adjust the crosslinking density and depolymerization rate of the final colloid. The resulting product can be stored stably at room temperature and is cured by absorbing moisture from the air after construction. After curing, the adhesive layer has good bonding strength, flexibility and aging resistance, and can achieve rapid and complete depolymerization under specific chemical stimulation.

[0017] A third objective of this invention is to provide an application of the above-mentioned depolymerizable polyurethane hot melt adhesive, mainly used in adhesive products.

[0018] The beneficial effects of this invention are as follows: I. This invention achieves a crosslinking effect by rationally combining polymer polyols and isocyanates and introducing chain extenders containing dihydroxysulfides to participate in crosslinking, so that the cured colloid has structural bonding strength, flexibility and high temperature and humidity aging resistance comparable to traditional PUR.

[0019] Second, this invention innovatively introduces thioester bonds as dynamic covalent units into the polyurethane backbone. These bonds are stable under normal use conditions, but can break rapidly under alkaline or mercapto-containing media, thus achieving controllable depolymerization of the adhesive layer at the end of its lifespan or during maintenance.

[0020] Third, the polyurethane hot melt adhesive of the present invention not only has excellent multi-substrate bonding performance and environmental aging stability, but also can achieve controlled degradation without high temperature or special equipment, which significantly improves the convenience of rework repair and green recycling; and solves the technical problem of difficult recycling of electronic components.

[0021] Fourth, the raw materials selected in this invention are all industrial products with controllable costs; the preparation process is compatible with existing PUR production lines and requires no special equipment; the product is stable to store and the construction method is consistent with traditional PUR, making it easy to promote and use in existing production processes. Detailed Implementation

[0022] The principles and features of the present invention are described below. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0023] Example 1 Polypropylene glycol PPG2000 (60 parts) was added to a reactor and dehydrated for 2 hours at 120°C and -0.09 MPa vacuum. After devastation, the vacuum was released, the temperature was lowered to 90°C, and 4,4'-MDI (16 parts) was added. The prepolymer was prepared by reacting at 120°C for 1 hour. Di(2-hydroxyethyl) thiodipropionate (10 parts) was slowly added, and the reaction was continued for 1 hour. Antioxidant 1010 (0.1 parts) and organotin catalyst DBTDL (0.05 parts) were then added. After degassing, the mixture was filled while hot.

[0024] In the depolymerization test, the polyurethane hot melt adhesive of this embodiment showed a depolymerization rate of >95% after treatment at 60°C for 48 hours in a 1 mol / L NaOH aqueous solution, and the residue could be directly wiped off.

[0025] Example 2 Polypropylene glycol PPG2000 (60 parts) was added to a reactor and dehydrated for 2 hours at 120°C and -0.09MPa vacuum. After that, the vacuum was released, the temperature was lowered to 90°C, 4,4'-MDI (16 parts) was added, and the reaction was carried out for 1 hour to prepare a prepolymer. Di(2-hydroxyethyl) thiodipropionate (3 parts) was slowly added, and the reaction was continued for 1 hour. Antioxidant 1010 (0.1 parts) and organotin catalyst DBTDL (0.05 parts) were then added. After degassing, the mixture was filled while hot.

[0026] In the depolymerization test, the polyurethane hot melt adhesive of this embodiment showed a debonding rate of >75% in a 1 mol / L NaOH aqueous solution at 60°C for 48 hours, and the residue could be wiped off. Extending the treatment time to 72 hours increased the debonding rate to 90%.

[0027] Example 3 Add Dynacoll 7380 (35 parts) + Dynacoll 7255 (15 parts) + PPG1000 (20 parts) to a reactor. After dehydration for 2 hours at 120℃ and -0.09MPa vacuum, release the vacuum, cool to 90℃, add HDI trimer (20 parts), and react for 1 hour to obtain a prepolymer. Add di(2-hydroxyethyl) thiodipropionate (6 parts) for chain extension, and continue the reaction for 1 hour. Then add antioxidant 1010 (0.1 parts) and organotin catalyst DBTDL (0.05 parts), degas, and fill while hot.

[0028] In the depolymerization test, the polyurethane hot melt adhesive of this embodiment showed a depolymerization rate of approximately 88% after treatment in a 1 mol / L NaOH aqueous solution at 60°C for 48 hours. The residue could be wiped off with mechanical assistance.

[0029] Comparative Example 1 (Conventional PUR without thioester structure) Unlike Example 1, this comparative example uses PPG2000 (60 parts) and 4,4'-MDI (20 parts) to react directly without adding di(2-hydroxyethyl) thiodipropionate. The rest is the same as in Example 1 and will not be repeated here. A conventional NCO-terminated hot-melt PUR is prepared.

[0030] In the depolymerization test, the polyurethane hot melt adhesive of this comparative example, after being treated in a 1 mol / L NaOH aqueous solution at 60°C for 72 hours, only showed slight surface softening, with the overall structure remaining intact. The adhesive could not be completely peeled off, and there was obvious residue.

[0031] Comparative Example 2 (using highly polar diols for chain extension) Unlike Example 1, this comparative example uses PPG2000 (60 parts) and 4,4'-MDI (16 parts) to react, followed by the addition of dihydroxymethylphenol (10 parts) as a chain extender. The rest is the same as in Example 1 and will not be repeated here.

[0032] In the depolymerization test, the polyurethane hot melt adhesive of this comparative example showed no change in the colloid after being treated in a 1 mol / L NaOH aqueous solution at 60°C for 72 hours. However, severe residual adhesive adhered to the surface, making secondary assembly impossible.

[0033] Comparative Example 3 Polypropylene glycol PPG2000 (60 parts) and 4,4'-MDI (16 parts) were added to a reactor and dehydrated for 2 hours at 120°C and -0.09MPa vacuum. Then, the prepolymer was prepared by reacting at 120°C for 1 hour. The temperature was lowered to 90°C, and di(2-hydroxyethyl) thiodipropionate (0.5 parts) was slowly added. The reaction was continued for 1 hour. Then, antioxidant 1010 (0.1 parts) and organotin catalyst DBTDL (0.05 parts) were added. After degassing, the mixture was filled while hot.

[0034] In the depolymerization test, the polyurethane hot melt adhesive of this comparative example, after being treated in a 1 mol / L NaOH aqueous solution at 60°C for 48 hours, only showed slight surface expansion and softening, with no internal structure disintegration and severe residual adhesive adhesion.

[0035] The following performance tests were performed on the polyurethane hot melt adhesives of the examples and comparative examples: 1. Peel strength after moisture curing: adhesive line width 2mm.

[0036] 2. Tensile strength: The dumbbell plate with a glue line thickness of 0.2mm was tested.

[0037] 3. Hardness: Tested using a Shore A hardness tester.

[0038] 4. Depolymerization test: The hot melt adhesive was treated in a 1 mol / L NaOH aqueous solution at 60℃, and the residual amount of hot melt adhesive was recorded at 48h and 72h respectively.

[0039] 5. Thermal stability: Aging at 85℃ and 85%RH for 500 hours, and record the hot melt adhesive retention rate.

[0040] The performance test results are shown in Table 1.

[0041] Table 1 Performance test results of the examples and comparative examples

[0042] The comparison between the above embodiments and comparative examples shows that the chain extender containing a dihydroxy thioester structure introduced in this invention has a decisive structural function in the polyurethane backbone, and its dosage directly affects the depolymerization efficiency and degumming effect.

[0043] In Examples 1 to 3, the addition of thioester chain extender can form effective "degradable points". After treatment in an alkaline solution or a mercapto-containing environment (such as 1 mol / L NaOH aqueous solution or 5% cystamine aqueous solution) for 48 hours, the hot melt adhesive can achieve an overall depolymerization rate of more than 80%, with excellent degumming efficiency, little residue, and easy subsequent repair or recycling.

[0044] To verify the key influence of the thioester chain extender and its dosage on the performance of this invention, a series of comparative examples were set up. In Comparative Example 1, the formulation contained no chain extender with the dihydroxythioester structure. The results showed that although the colloid after curing in Comparative Example 1 had excellent initial mechanical properties, after being treated in a 1 mol / L NaOH aqueous solution at 60°C for 48 hours, the colloidal structure remained intact, and effective depolymerization could not be achieved. This result proves that the controllable depolymerization performance has a clear dependence on the introduction of the thioester structure.

[0045] Furthermore, in Comparative Example 3, the amount of the chain extender containing the dihydroxythioester structure was reduced to 0.5 parts. The results showed that the initial adhesive strength of the colloid did not decrease significantly, but its controllable depolymerization performance deteriorated significantly. Under the same depolymerization conditions, the adhesive layer only achieved slight surface softening, while the internal cross-linked network structure remained intact, resulting in a significantly reduced depolymerization rate. To achieve partial degradation, the treatment time needed to be extended to over 72 hours or the treatment temperature needed to be increased to over 80°C, and significant colloid residue remained on the substrate surface after treatment, thus negating its advantage of rapid and clean debonding in precision electronic assembly.

[0046] In summary, this invention successfully constructed controllable depolymerization chemical bonds in the molecular backbone of polyurethane hot melt adhesives by introducing a specific amount of a chain extender containing a dihydroxythioester structure. This design enables the adhesive to maintain excellent initial bond strength and durability while achieving efficient and rapid degradation and interfacial debonding under mild alkaline or reducing chemical environments.

[0047] In particular, by optimizing and controlling the dosage of the chain extender, this invention ensures an optimal balance between the final mechanical properties and end-recyclability of the material. This characteristic greatly enhances the application value and prospects of polyurethane hot melt adhesives in advanced manufacturing fields requiring repairable and disassembly-compatible electronic assemblies, green packaging, and environmentally friendly recycling.

[0048] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A depolymerizable polyurethane hot melt adhesive, characterized in that, By weight, it includes the following raw materials: 30-70 parts of polymeric polyol; 1-15 parts of chain extender containing dihydroxythioester structure; 10-30 parts of polyisocyanate; 0.05-1 part of auxiliaries.

2. The depolymerizable polyurethane hot melt adhesive according to claim 1, characterized in that, The chain extender containing a dihydroxythioester structure is di(2-hydroxyethyl) thiodipropionate or a derivative thereof.

3. The depolymerizable polyurethane hot melt adhesive according to claim 2, characterized in that, The chain extender containing a dihydroxythioester structure is di(2-hydroxyethyl) thiodipropionate.

4. The depolymerizable polyurethane hot melt adhesive according to claim 1, characterized in that, The polymer polyol is selected from at least one of polyether polyol, polyester polyol, or polycarbonate polyol.

5. The depolymerizable polyurethane hot melt adhesive according to claim 3, characterized in that, The polyether polyol is polypropylene glycol or polytetrahydrofuran ether glycol; and / or, the polyester polyol is an aliphatic linear copolyester polyol.

6. The depolymerizable polyurethane hot melt adhesive according to claim 1, characterized in that, The polyisocyanate is selected from at least one of diphenylmethane diisocyanate, isophorone diisocyanate, or hexamethylene diisocyanate trimer.

7. The depolymerizable polyurethane hot melt adhesive according to claim 1, characterized in that, The additive is selected from at least one of catalysts, antioxidants, and wetting and leveling agents.

8. A method for preparing a depolymerizable polyurethane hot melt adhesive as described in any one of claims 1 to 7, characterized in that, Includes the following steps: S1, vacuum dehydration treatment of polymer polyols; S2. Cool the dehydrated polymer polyol, add polyisocyanate, and react to generate a prepolymer. S3. Add a chain extender containing a dihydroxy thioester structure to the prepolymer and continue the chain extension reaction; S4. Add additives to the reaction system, mix evenly, and obtain the depolymerizable polyurethane hot melt adhesive after degassing.

9. The method for preparing the depolymerizable polyurethane hot melt adhesive according to claim 8, characterized in that, In step S2, the dehydrated polymer polyol is cooled to 85-95°C; after adding the polyisocyanate, the reaction is carried out for 1-2 hours; in step S3, the chain extender containing the dihydroxythioester structure is pre-dried before being added; the temperature of the chain extension reaction is controlled at 85-95°C.

10. An application of a depolymerizable polyurethane hot melt adhesive as described in any one of claims 1 to 7, characterized in that, Used in adhesive products.

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