A kind of polyurethane adhesive for foaming and preparation method thereof

Abstract

The present invention relates to the technical field of wood adhesives, specifically a foaming polyurethane adhesive prepared from the following raw materials in parts by weight: 40-60 parts diphenylmethane diisocyanate (MDI); 20-40 parts polyphenylmethane polyisocyanate (PMDI); 10-30 parts hexamethylene diisocyanate (HDI); 5-15 parts chain extender; 0.1-0.5 parts catalyst; and 1-5 parts foaming agent. The chain extender has a number average molecular weight (Mn) of ≥1000. The invention also relates to a method for preparing the foaming polyurethane adhesive through prepolymer preparation, component mixing, and gradient curing. This invention systematically addresses the problems of performance imbalance, poor penetration, and curing mismatch in foaming adhesives used in particleboard applications.

Description

Technical Field

[0001] The present invention relates to the technical field of wood adhesives, and in particular to a foaming polyurethane adhesive and a preparation method thereof. Background Art

[0002] Polyurethane adhesives, due to their high bonding strength and foamability, are gradually replacing traditional phenolic resins in the manufacture of wood-based materials such as particleboard. However, existing technologies for foaming polyurethane adhesives still face the following bottlenecks:

[0003] 1. The single isocyanate leads to performance imbalance: Traditional formulas mostly use a single isocyanate (such as MDI or PMDI). The MDI system is too rigid and prone to brittle cracking, and the high cross-linking degree of PMDI leads to poor permeability, making it difficult to adapt to the porous particleboard structure.

[0004] 2. Contradiction in the selection of chain extender molecular weight: Although low molecular weight chain extenders (such as ethylene glycol) can increase the cross-linking density, they will significantly increase the viscosity of the system and hinder the penetration of the adhesive into the pores of the substrate; the application of high molecular weight chain extenders is mostly limited to the field of elastomers, and there is a lack of systematic research on the coordinated regulation of viscosity and strength of foaming adhesives.

[0005] 3. Mismatch between curing process and foaming: Single-component moisture-curing adhesives have poor storage stability, two-component systems have a single curing temperature, and bubbles are prone to merging or collapsing during the foaming process, resulting in uneven strength of the adhesive layer.

[0006] In response to the above problems, although some studies have attempted to compound isocyanates or optimize chain extenders, they mostly focus on a certain parameter, which often leads to the degradation of other properties. For example, reducing viscosity will sacrifice cross-linking density. Therefore, the development of a polyurethane foam adhesive with high bonding strength, excellent permeability and process adaptability has become an urgent need in the industry. Summary of the Invention

[0007] In view of this, the purpose of the present invention is to provide a foaming polyurethane adhesive and a preparation method thereof, which systematically solves the problems of performance imbalance, poor penetration, and curing mismatch of foaming adhesives in particleboard applications through multi-component collaborative design and precise process control.

[0008] In order to achieve the above object, the present invention provides the following technical solutions:

[0009] A foaming polyurethane adhesive is prepared from the following raw materials in parts by weight: 40-60 parts of diphenylmethane diisocyanate (MDI); 20-40 parts of polyphenylmethane polyisocyanate (PMDI); 10-30 parts of hexamethylene diisocyanate (HDI); 5-15 parts of a chain extender; 0.1-0.5 parts of a catalyst; and 1-5 parts of a foaming agent. The chain extender has a number average molecular weight (Mn) of ≥1000.

[0010] Furthermore, the chain extender is at least one of polyether diol and polyester diol, and the number average molecular weight thereof is 1000-3000.

[0011] Furthermore, the catalyst is a compound of an organotin catalyst and an amine catalyst, and the mass ratio of the organotin catalyst to the amine catalyst is 1:0.5-2.

[0012] Furthermore, the organotin catalyst is dibutyltin dilaurate (DBTDL), and the amine catalyst is 1,4-diazabicyclo[2.2.2]octane (DABCO).

[0013] Furthermore, the foaming agent is at least one of water and a physical foaming agent, and the physical foaming agent is cyclopentane or HFC-245fa.

[0014] At the same time, the present invention also provides a method for preparing the above-mentioned polyurethane adhesive for foaming, comprising the following steps:

[0015] (1) Preparation of prepolymer: Mix MDI, PMDI and HDI, heat to 60-80°C and react for 1-3 hours, add chain extender and continue to react until the isocyanate group (NCO) content of the prepolymer is 8-12%;

[0016] (2) Component mixing: The prepolymer obtained in step (1) is used as component A and mixed with component B containing a catalyst and a foaming agent in a mass ratio of 2 to 5:1;

[0017] (3) Gradient curing: Apply the mixed adhesive to the substrate and cure it at 30-50°C for 0.5-1.5 hours, 60-80°C for 1-3 hours, and 90-110°C for 0.5-1.5 hours.

[0018] By adopting the above technical solution, the present invention will have the following beneficial effects:

[0019] The synergistic combination of MDI, PMDI, and HDI balances the rigid and flexible segments, preventing brittle fracture in the adhesive layer while enhancing penetration into porous substrates (such as particleboard), achieving a balance of strength and toughness. A chain extender with a number-average molecular weight ≥1000 is used to optimize the balance between crosslink density and viscosity, significantly improving the fluidity of the adhesive while maintaining adhesive layer strength, allowing it to fully penetrate the substrate pores. A gradient curing process employs staged temperature control (low-temperature foaming → medium-temperature chain extension → high-temperature strengthening) to ensure full vaporization of the foaming agent to form uniform pores. Simultaneously, chain extension and deep crosslinking are accomplished in stages, preventing pore collapse and internal stress concentration, and improving the structural stability of the adhesive layer.

[0020] Compared with the existing technology, the present invention systematically solves the problems of performance imbalance, poor permeability and curing mismatch in traditional foaming adhesives in particleboard applications through the design of isocyanate compounding system, molecular weight control of chain extender and innovation of gradient curing process. DETAILED DESCRIPTION

[0021] The technical solutions of the present invention are further described in detail below through specific examples. In the present invention, unless otherwise specified, the raw materials and equipment used can be purchased from the market or commonly used in the field. The methods and equipment in the following examples are conventional methods and equipment in the field unless otherwise specified.

[0022] Example 1

[0023] A foaming polyurethane adhesive is prepared from the following raw materials in parts by weight: 50 parts of MDI; 30 parts of PMDI; 20 parts of HDI; 10 parts of polyether diol (Mn=2000) as a chain extender; 0.3 parts of a catalyst prepared by compounding dibutyltin dilaurate (DBTDL) and diazabicyclo[2.2.2]octane (DABCO) in a mass ratio of 1:1; and a blowing agent consisting of 2 parts of water and 1 part of cyclopentane.

[0024] The preparation method of the above-mentioned polyurethane adhesive for foaming is:

[0025] (1) Preparation of prepolymer: MDI, PMDI and HDI were mixed and heated to 70°C for 2 hours; polyether diol was added and the reaction was continued until the NCO% was 10% as monitored by real-time titration;

[0026] (2) Component mixing: the prepolymer obtained in step (1) is used as component A and mixed with component B containing a catalyst and a foaming agent in a mass ratio of 3:1;

[0027] (3) Gradient curing: Apply the mixed adhesive to the substrate and cure it at 40°C for 1 hour, 70°C for 2 hours, and 100°C for 1 hour.

[0028] Example 2

[0029] A polyurethane adhesive for foaming is prepared from the following raw materials in parts by weight: 40 parts of MDI; 20 parts of PMDI; 10 parts of HDI; 5 parts of polyester diol (Mn=1000) as a chain extender; 0.1 part of a catalyst prepared by compounding dibutyltin dilaurate (DBTDL) and diazabicyclo[2.2.2]octane (DABCO) in a mass ratio of 1:0.5; and 1 part of water as a blowing agent.

[0030] The preparation method of the above-mentioned polyurethane adhesive for foaming is:

[0031] (1) Preparation of prepolymer: MDI, PMDI and HDI were mixed and heated to 60°C for 1 hour; polyester diol was added and the reaction was continued until the NCO% was 8% as monitored by real-time titration method;

[0032] (2) Component mixing: the prepolymer obtained in step (1) is used as component A and mixed with component B containing a catalyst and a foaming agent in a mass ratio of 2:1;

[0033] (3) Gradient curing: Apply the mixed adhesive to the substrate and cure it at 30°C for 0.5 hour, 60°C for 1 hour, and 90°C for 0.5 hour.

[0034] Example 3

[0035] A polyurethane adhesive for foaming is prepared from the following raw materials in parts by weight: 60 parts of MDI; 40 parts of PMDI; 30 parts of HDI; 15 parts of polyether diol (Mn=3000) as a chain extender; 0.5 parts of a catalyst prepared by compounding dibutyltin dilaurate (DBTDL) and diazabicyclo[2.2.2]octane (DABCO) in a mass ratio of 1:2; and 5 parts of a blowing agent, HFC-245fa.

[0036] The preparation method of the above-mentioned polyurethane adhesive for foaming is:

[0037] (1) Preparation of prepolymer: MDI, PMDI and HDI were mixed and heated to 80°C for 3 hours; polyether diol was added and the reaction was continued until the NCO% was 12% as monitored by real-time titration;

[0038] (2) Component mixing: The prepolymer obtained in step (1) is used as component A and mixed with component B containing a catalyst and a foaming agent in a mass ratio of 5:1;

[0039] (3) Gradient curing: Apply the mixed adhesive to the substrate and cure it at 50°C for 1.5 hours, 80°C for 3 hours, and 110°C for 1.5 hours.

[0040] To clearly verify the synergistic effect of the isocyanate compound system (MDI / PMDI / HDI), high molecular weight chain extender (Mn ≥ 1000), and gradient curing process, the following multiple control experiments were designed to systematically demonstrate the technological breakthrough of this combination through the comparison of key performance indicators.

[0041] 1. Experimental Grouping and Variable Control

[0042]

[0043] 2. Comparison of key performance data

[0044]

[0045] 3. Test Standards

[0046] 1. Bonding strength

[0047] Standard: GB / T 17657-2013 "Test methods for physical and chemical properties of wood-based panels and veneered wood-based panels"

[0048] Equipment: Universal testing machine (such as Instron 5967)

[0049] Parameters: sample size 100 mm × 25 mm × 3 mm, loading rate 5 mm / min, test environment 25±2℃.

[0050] 2. Penetration depth

[0051] Method: After curing, slice the cross section of the adhesive layer and measure the penetration distance under a microscope (randomly select 5 areas and take the average value).

[0052] Equipment: Optical microscope or scanning electron microscope (SEM).

[0053] 3. Elongation at break

[0054] Standard: GB / T 1040-2018 "Determination of tensile properties of plastics"

[0055] Equipment: Universal testing machine

[0056] Parameters: tensile rate 50 mm / min, specimen gauge length 50 mm.

[0057] 4. Crosslink density

[0058] Method: Swelling method (based on Flory-Rehner theory)

[0059] Solvent: Toluene (analytical grade), soak at a constant temperature of 25°C for 48 hours until swelling equilibrium is reached.

[0060] Calculation: Calculate the crosslink density (unit: mol / m³) by the mass change after swelling.

[0061] 5. Viscosity

[0062] Standard: GB / T 2794-2012 "Determination of viscosity of adhesives"

[0063] Equipment: Rotational viscometer (such as Brookfield DV2T)

[0064] Parameters: Rotor model RV7, rotation speed 20 rpm, test temperature 25±0.5℃.

[0065] 6. Cell uniformity

[0066] Methods: The cell structure was observed by scanning electron microscopy (SEM). Five randomly selected areas were measured for diameter and the standard deviation was calculated.

[0067] Equipment: SEM (such as Hitachi SU8010), accelerating voltage 5 kV, magnification 500×.

[0068] 7. Moisture and heat resistance retention rate

[0069] Conditions: Treat in a constant temperature and humidity chamber at 70°C and 95%RH for 48 hours, then test the bonding strength after returning to room temperature for 2 hours.

[0070] Calculation: Retention rate = (strength after moist heat treatment / initial strength) × 100%.

[0071] 8. Stress in the adhesive layer

[0072] Method: Three-point bending method (ASTM D1002)

[0073] Equipment: Universal testing machine (e.g., Instron 5967) with a three-point bend fixture. Calculate the mean breaking load and verify with a t-test.

[0074] Parameters: specimen size 80×10×2 mm, universal testing machine loading rate 1 mm / min, span 50 mm.

[0075] 4. Data Analysis and Mechanism Principles

[0076] 1. Synergistic effect of compound isocyanate system (MDI / PMDI / HDI)

[0077] (1) Experimental data: The bonding strength (2.1 MPa) of group B (compounded with isocyanate only) increased by 24% compared with group A (single MDI), and the penetration depth (0.8 mm) increased by 60%.

[0078] (2) Mechanism principle:

[0079] MDI (rigid segment): provides high cross-linking density and gives the adhesive layer basic strength;

[0080] PMDI (high functionality cross-linking): forms a three-dimensional cross-linked network, enhancing moisture and heat resistance (75% moisture and heat resistance retention of group B vs. 62% of group A);

[0081] HDI (flexibility compensation): lowers the glass transition temperature (Tg), improves the flexibility of the molecular chain, and relieves internal stress (elongation at break of group B 70% vs. 50% of group A).

[0082] (3) Conclusion: The composite system balances strength and toughness through the complementarity of rigid and flexible segments. However, due to the high cross-linking density of the low molecular weight chain extender (cross-linking density of group B is 300 mol / m³), the permeability is still insufficient.

[0083] 2. Optimization effect of high molecular weight chain extenders (Mn ≥ 1000)

[0084] (1) Experimental data: The elongation at break (150%) of group C (compound + polymer chain extender) increased by 114% compared with group B (compound only), and the viscosity (1000 mPa·s) decreased by 44% compared with group B (1800 mPa·s).

[0085] (2) Mechanism principle:

[0086] Long-chain dilution effect: The molecular chain length of polyether diol (Mn=2000) reduces the number of crosslinking points per unit volume (crosslinking density of group C is 180 mol / m³ vs. 300 mol / m³ of group B), thus reducing the viscosity of the system.

[0087] Contribution of flexible chain segments: Long chain extenders introduce ether bonds (-O-), enhancing the flexibility of the molecular chain and significantly improving the elongation at break;

[0088] Optimized permeability: The low-viscosity adhesive can more easily penetrate the pores of the particleboard (penetration depth of group C is 1.2 mm vs. 0.8 mm for group B).

[0089] (3) Conclusion: The polymer chain extender significantly improves the fluidity and toughness by reducing the cross-linking density and increasing the flexibility of the molecular chain. However, due to the limitation of the constant temperature curing process (cell diameter of group C is 50-180 μm), the cell uniformity is still not ideal.

[0090] 3. Timing control of gradient curing process

[0091] (1) Experimental data: The bonding strength (3.2 MPa) of group D (compound + chain extender + gradient curing) was 28% higher than that of group C (constant temperature curing), and the uniformity of cell diameter (30-120 μm) was 50% higher.

[0092] (2) Mechanism principle:

[0093] Low temperature stage (30-50℃): Low temperature slows down the volatilization rate of the blowing agent (water / cyclopentane), and CO2 bubble nucleation and growth proceed simultaneously, forming uniform cells. Moreover, the chain extension reaction rate at low temperature is lower than the foaming rate, and the cell structure is stable, thus preventing cell merging.

[0094] Medium temperature stage (60-80℃): Heating accelerates the reaction between chain extender and isocyanate, forming a long-chain flexible network (elongation at break 200% vs. 150% in group C); medium crosslinking density locks the cell morphology and prevents collapse.

[0095] High temperature stage (90-110℃): The remaining NCO groups react fully to form a dense cross-linked network (bonding strength 3.2MPa); the high cross-linking density inhibits the penetration of water molecules (moisture and heat resistance retention rate 93% vs 85% of group C).

[0096] (3) Conclusion: Gradient curing achieves simultaneous optimization of cell uniformity, adhesive layer strength, and weather resistance by regulating reaction kinetics in stages.

[0097] 4. The irreplaceable synergy of the three

[0098] (1) The necessity of compounding isocyanate system

[0099] Limitations of Group E (single MDI + gradient curing): The rigid chain segments of single MDI lead to stress concentration in the adhesive layer (1.1 MPa), inhibiting uniform expansion of cells and resulting in poor cell uniformity (70-250 μm);

[0100] Comparative group D (compound + gradient): Through compensation by HDI flexible chain segments, the internal stress was reduced to 0.6 MPa, and the cell uniformity was improved to 30-120 μm.

[0101] Conclusion: Gradient curing alone cannot make up for the rigidity defects of a single isocyanate. The rigidity-flexibility balance of the compound system (MDI / PMDI / HDI) is the basis for optimizing the pore structure.

[0102] (2) Irreplaceability of polymer chain extenders

[0103] Failure of Group F (low molecular weight chain extender + gradient curing): The low molecular weight chain extender (ethylene glycol) resulted in a dense crosslinked network with a high crosslink density (410 mol / m³) and a penetration depth of only 0.6 mm (much lower than the 1.8 mm of Group D).

[0104] Comparative group C (compound + polymer chain extender + constant temperature curing): cross-linking density 180 mol / m³, penetration depth 1.2 mm, but constant temperature curing still resulted in uneven pores (50-180 μm).

[0105] Conclusion: High molecular weight chain extenders (Mn ≥ 1000) reduce crosslinking density through the long chain dilution effect and are the core elements for improving permeability and toughness. Low molecular weight chain extenders cannot replace them even when combined with gradient technology.

[0106] (3) Timing control of gradient curing process

[0107] Bottlenecks of Group C (compound + polymer chain extender + constant temperature curing): Constant temperature curing leads to a mismatch between foaming and cross-linking rates, severe cell merging, and insufficient cell uniformity (50-180 μm).

[0108] Comparative group D (gradient curing): temperature control in stages (low-temperature foaming → medium-temperature chain extension → high-temperature strengthening), the cell uniformity was improved to 30-120 μm, and the bonding strength exceeded 3.2 MPa.

[0109] Conclusion: Gradient curing solves the timing contradiction between foaming and curing by matching reaction kinetics in stages. It is a key process for optimizing the cell structure and adhesive layer performance.

[0110] (4) The indivisibility of the three elements

[0111] Single variable failure verification:

[0112] Group E (no compounding): single MDI + gradient process, performance significantly deteriorated;

[0113] Group F (no polymer chain extender): low molecular weight chain extender + gradient process, cross-linking density and permeability are not improved;

[0114] Group C (non-gradient curing): compound + polymer chain extender, but constant temperature curing, insufficient cell uniformity.

[0115] Comparison of synergistic effect data:

[0116]

[0117] (5) Conclusion: Only by combining isocyanate + polymer chain extender + gradient curing in a coordinated manner can a simultaneous breakthrough in strength, permeability and weather resistance be achieved.

[0118] 5. Statistical significance verification (p-value)

[0119]

[0120] 1. Significance marking

[0121] (P<0.01): The difference is highly significant (99% confidence level), indicated by **;

[0122] (P<0.05): The difference is statistically significant (95% confidence level), indicated by *;

[0123] (P>0.05): Not statistically significant.

[0124] 2. Key Conclusions

[0125] The comprehensive advantages of group D (the present invention): compared with all the comparison groups (groups C, E, and F), there were extremely significant differences in bonding strength, cell uniformity, moisture and heat resistance, and penetration depth (P<0.01), verifying the necessity of the synergy of the three.

[0126] Limitations of Group F: There was no significant difference between Group F (low molecular weight chain extender + gradient curing) and Group A (traditional formula) (P>0.05), proving that even with the combination of low molecular weight chain extenders and gradient curing, they still could not break through the performance bottleneck.

[0127] VI. Final Conclusion

[0128] Through multiple sets of systematic experiments and statistical analysis, this paper clearly demonstrates the synergistic effect of an isocyanate composite system, a high molecular weight chain extender, and a gradient curing process. The combination of these three significantly outperforms single or two-way combinations in terms of bond strength, cell uniformity, and moisture and heat resistance. By combining the isocyanate composite system to provide a balanced rigidity-flexibility framework, the polymer chain extender to optimize the crosslinking network, and the gradient curing process to coordinate the reaction sequence, this paper addresses the performance imbalance, poor permeability, and curing mismatch issues associated with traditional foaming adhesives in particleboard applications.

[0129] The above general description of the invention and the description of the specific embodiments involved in this application should not be construed as limiting the technical solutions of the invention. Based on the disclosure of this application, those skilled in the art may, without violating the constituent elements of the invention involved, add to, subtract from, or combine the disclosed technical features in the above general description and / or embodiments to form other technical solutions within the scope of protection of this application.

Claims

1. A polyurethane adhesive for foaming, characterized in that: It is prepared from the following raw materials in parts by weight: 40-60 parts of diphenylmethane diisocyanate (MDI); Polyphenylmethane polyisocyanate (PMDI) 20-40 parts; Hexamethylene diisocyanate (HDI) 10-30 parts; 5 to 15 parts of a chain extender, wherein the chain extender is at least one of a polyether diol or a polyester diol, and has a number average molecular weight of 1000 to 3000; 0.1~0.5 parts of catalyst; 1-5 parts of foaming agent; The foaming polyurethane adhesive is obtained by the following preparation method: (1) Preparation of prepolymer: Mix MDI, PMDI and HDI, heat to 60-80°C and react for 1-3 hours, add chain extender and continue to react until the isocyanate group (NCO) content of the prepolymer is 8-12%; (2) Component mixing: The prepolymer obtained in step (1) is used as component A and mixed with component B containing a catalyst and a foaming agent in a mass ratio of 2 to 5:1; (3) Gradient curing: Apply the mixed adhesive to the substrate and cure it at 30-50°C for 0.5-1.5 hours, 60-80°C for 1-3 hours, and 90-110°C for 0.5-1.5 hours.

2. The polyurethane adhesive for foaming according to claim 1, characterized in that: The catalyst is a compound of an organotin catalyst and an amine catalyst, and the mass ratio of the organotin catalyst to the amine catalyst is 1:0.5-2.

3. The polyurethane adhesive for foaming according to claim 2, characterized in that: The organotin catalyst is dibutyltin dilaurate (DBTDL), and the amine catalyst is 1,4-diazabicyclo[2.2.2]octane (DABCO).

4. The polyurethane adhesive for foaming according to claim 1, characterized in that: The foaming agent is at least one of water and a physical foaming agent, and the physical foaming agent is cyclopentane or HFC-245fa.

5. A method for preparing the polyurethane adhesive for foaming according to any one of claims 1 to 4, characterized in that: The following steps are involved: (1) Preparation of prepolymer: Mix MDI, PMDI and HDI, heat to 60-80°C and react for 1-3 hours, add chain extender and continue to react until the isocyanate group (NCO) content of the prepolymer is 8-12%; (2) Component mixing: The prepolymer obtained in step (1) is used as component A and mixed with component B containing a catalyst and a foaming agent in a mass ratio of 2 to 5:1; (3) Gradient curing: Apply the mixed adhesive to the substrate and cure it at 30-50°C for 0.5-1.5 hours, 60-80°C for 1-3 hours, and 90-110°C for 0.5-1.5 hours.

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