Formaldehyde-free environment-friendly polyurethane adhesive as well as preparation method and application thereof

Abstract

The invention relates to a formaldehyde-free environment-friendly polyurethane adhesive and a preparation method thereof, relates to the technical field of high polymer materials, and aims to solve the problems of formaldehyde residue, poor water resistance, high process energy consumption, high cost and the like of the existing formaldehyde-free adhesive. According to the invention, dimethylolpropionic acid and wood metal ions form coordinate bonds to strengthen interface bonding, trimethylolpropane constructs a three-dimensional hydrophobic network, and toluene diisocynate isomers are subjected to fractional reaction to optimize crosslinking uniformity. The content of free formaldehyde in the adhesive is not detected, the adhesive strength retention rate is greatly improved after the adhesive is boiled in boiling water for 72 hours, the hot pressing time is shortened to 8 minutes, and the raw material cost is reduced by 40% or above compared with similar products.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high molecular materials, in particular to a formaldehyde-free environment-friendly polyurethane adhesive as well as a preparation method and application thereof. BACKGROUND

[0002] Traditional formaldehyde-based adhesives (such as urea-formaldehyde resin and phenol-formaldehyde resin) have a fatal defect of continuously releasing free formaldehyde after curing: according to the GB / T 14074-2017 standard test, the free formaldehyde content is generally more than 1.5 mg / L, and in a humid heat environment (temperature 30℃, humidity 80%), the release amount increases to 3-5 times of the initial value due to reverse hydrolysis reaction, which is much higher than the stringent requirement of ENF level (≤0.025 mg / m 3 ) in GB / T 39600-2021 standard. This has led to the emergence of solutions such as epoxy-modified waterborne polyurethane and bio-based formaldehyde-free adhesive. However, the epoxy-modified waterborne polyurethane is not conducive to industrialization due to high raw material cost; and the bio-based formaldehyde-free adhesive has a water resistance defect due to the rich hydrophilic groups (-OH, -COOH) in the molecular chain: in the humid heat cycle test (63℃ hot water immersion for 3h) of plywood type II specified in GB / T 17657-2013 standard, the bonding strength decreases from the initial 0.6-0.8 MPa to <0.3 MPa, and the adhesive layer is completely cracked after 72h boiling water test. Therefore, there is an urgent need for a formaldehyde-free environment-friendly polyurethane adhesive as well as a preparation method and application thereof to solve this problem. SUMMARY

[0003] The present application aims to provide a formaldehyde-free environment-friendly polyurethane adhesive as well as a preparation method and application thereof to solve the above technical problems.

[0004] To achieve the above-mentioned purpose, the present application provides the following technical solution: a formaldehyde-free environment-friendly polyurethane adhesive, comprising the following components by weight:

[0005] Polyether polyol 40-60 parts, toluene diisocyanate 20-30 parts, dimethylol propionic acid 4-12 parts, trimethylolpropane 3-5 parts, dibutyltin dilaurate 0.2-0.4 parts, industrial ethanol 1-3 parts, wherein the molar ratio of isocyanate groups to hydroxyl groups is 1.05-1.10.

[0006] Further, in the above-mentioned formaldehyde-free environment-friendly polyurethane adhesive, the hydroxyl value of the polyether polyol is 50-60 mgKOH / g, the block ratio of ethylene oxide to propylene oxide is 15:85-25:75, and the molecular weight distribution index (PDI) is ≤1.3.

[0007] Further, in the above-mentioned formaldehyde-free environmentally friendly polyurethane adhesive, the ratio of toluene-2,4-diisocyanate isomer to toluene-2,6-diisocyanate isomer in the toluene diisocyanate is 75:25 to 85:15.

[0008] Further, in the above-mentioned formaldehyde-free environmentally friendly polyurethane adhesive, the purity of the dimethylol propionic acid is ≥ 99%, and the moisture content is ≤ 0.2%.

[0009] Further, in the above-mentioned formaldehyde-free environmentally friendly polyurethane adhesive, the hydroxyl value of the trimethylolpropane is 1150 to 1250 mgKOH / g.

[0010] The present application provides another technical scheme: a preparation method of a formaldehyde-free environmentally friendly polyurethane adhesive, comprising the following steps:

[0011] S1: weighing raw materials: weighing polyether polyol, toluene diisocyanate, dimethylol propionic acid, trimethylolpropane, dibutyltin dilaurate, and industrial ethanol according to weight parts;

[0012] S2: prepolymer synthesis: under nitrogen protection, mix the polyether polyol and toluene diisocyanate, and follow the stepwise temperature rising program to rise from 60°C to 75°C, the total reaction time is 2.5h, and the -NCO content is monitored to 6.5-7.2% by online near-infrared spectroscopy, to obtain an end-NCO prepolymer;

[0013] S3: chain extension reaction: heat the -NCO prepolymer to 95°C, add the dimethylol propionic acid twice with an interval of 10 min, and react for 30 min until the acid value of the system is <5 mgKOH / g; add the trimethylolpropane, control the shear rate to be 50-100 s -1 , and react for 60 min until the -NCO content is ≤0.5%;

[0014] S4: termination and post-treatment: add industrial ethanol and stir for 20 min, vacuum degassing until the bubble content is ≤0.5%, and the final viscosity is 10000-15000 mPa·s.

[0015] Further, in the above-mentioned preparation method of the formaldehyde-free environmentally friendly polyurethane adhesive, the stepwise temperature rising program is: keeping at 60°C for 15 min, rising to 65°C and keeping for 15 min, rising to 70°C and keeping for 15 min, and rising to 75°C and keeping for 105 min.

[0016] Further, in the above-mentioned preparation method of the formaldehyde-free environmentally friendly polyurethane adhesive, the dibutyltin dilaurate is added in three times, which are 0.05-0.1 parts in step S2 prepolymer synthesis, 0.1-0.15 parts in step S3 chain extension reaction, and 0.05-0.1 parts before step S4 termination.

[0017] Further, in the preparation method of the formaldehyde-free environmentally friendly polyurethane adhesive, the vacuum degassing conditions in step S4 are: vacuum degree-0.095 MPa, degassing time 30 min, and pressure reduction rate ≤0.01 MPa / min.

[0018] The application also provides a technical solution: the application of the formaldehyde-free environmentally friendly polyurethane adhesive in plywood.

[0019] Compared with the prior art, the application has the following beneficial effects:

[0020] 1) Functional group activity gradient design: the carboxyl group (pKa≈4.5) of dimethylol propionic acid preferentially coordinates with wood hydroxyl under weak acid conditions, delaying the main reaction rate of -NCO / -OH;

[0021] 2) Three-dimensional network construction: the three-functionality of trimethylolpropane initiates uniform distribution of crosslinking points, avoiding local over-crosslinking;

[0022] 3) Economic restructuring: industrial-grade toluene diisocyanate (TDI, a mixture of toluene-2,4-diisocyanate isomers and toluene-2,6-diisocyanate isomers) is used to replace high-purity diphenyl methane diisocyanate (MDI), and self-regulating curing is realized by using the difference in isomer reactivity. BRIEF DESCRIPTION OF DRAWINGS

[0023] Figure 1 The flowchart of the preparation of the formaldehyde-free environmentally friendly polyurethane adhesive of the application is shown. DETAILED DESCRIPTION

[0024] The technical solutions of the application will be described below in conjunction with the embodiments, obviously, the described embodiments are only part of the embodiments of the application, rather than all the embodiments. Based on the embodiments in the application, all other embodiments obtained by those skilled in the art without creative labor are within the protection scope of the application.

[0025] The application provides the following scheme: a thermoplastic polyolefin interior skin with high chemical resistance, which comprises the following components by weight:

[0026] Polyether polyol 40-60 parts, toluene diisocyanate 20-30 parts, dimethylol propionic acid 4-12 parts, trimethylolpropane 3-5 parts, dibutyltin dilaurate 0.2-0.4 parts, industrial ethanol 1-3 parts, wherein the molar ratio of isocyanate groups to hydroxyl groups is 1.05-1.10.

[0027] A polyether polyol (CAS No. 32472-85-8) with a hydroxyl value of 50-60 mg KOH / g was chosen as the flexible segment matrix, with a molecular weight distribution index PDI < 1.3 and a precisely designed ethylene oxide / propylene oxide block ratio (EO / PO = 15:85-25:75):

[0028] Long-chain flexibility effect: the EO units in the main chain provide flexibility (glass transition temperature Tg = -65 °C), and the PO units impart hydrophobicity (contact angle 105°), allowing the adhesive layer to remain elastic (storage modulus E' fluctuation <10%) in the temperature range of -40 °C to 80 °C;

[0029] Dynamic hydrogen bond network: the -OH at the end of the polyol forms reversible hydrogen bonds with the -NH in the urethane bond (FTIR appears a broad peak at 3320 cm -1 -1), which dissipates energy through bond breaking-recombination when subjected to external force (impact strength up to 8.5 kJ / m 2 );

[0030] Pore infiltration control: viscosity is adjusted to 12000 mPa·s (25 °C), matching the wood vessel pore size (10-50 μm), ensuring that the glue solution penetrates a depth of 200-300 μm.

[0031] Dihydroxymethyl propionic acid (DMPA, CAS No. 1122-58-3) with a purity of ≥99% and a moisture content of ≤0.2%, its carboxyl group (-COOH) strengthens the interfacial bonding through a triple mechanism:

[0032] Coordination bond construction: the carboxyl group chelates with the Ca 2+ / Mg 2+ ions in the wood to form [O-Ca-O] bridging structures, increasing the interfacial bonding energy from 98 J / m 2 to 152 J / m 2 ;

[0033] pH responsiveness: in the slightly acidic environment of wood (pH ≈ 5.5), the carboxyl group partially ionizes (pKa = 4.5), enhancing adsorption with cellulose hydroxyl groups through electrostatic attraction (Zeta potential changes from -25 mV to -12 mV);

[0034] Steric hindrance effect: the methyl branch (-CH(CH2 OH)2) of DMPA hinders water molecules from approaching the adhesive interface (water contact angle increases from 75° to 92°).

[0035] Toluene diisocyanate (TDI, CAS No.: 28547-89-2), mixture of toluene-2,4-diisocyanate isomers (2,4-TDI) and toluene-2,6-diisocyanate isomers (2,6-TDI), wherein the ratio of 2,4-TDI and 2,6-TDI is 75:25~85:15, and the isomer reactivity difference is innovatively utilized:

[0036] Reactive grading: the reaction rate of -NCO group of 2,4-TDI (k1=3.2×10 -3 L / mol·s) is 2.9 times of that of 2,6-TDI (k2=1.1×10 3 L / mol·s), forming a stage crosslinking:

[0037] Pre-polymerization stage: high-activity 2,4-TDI is preferentially reacted with polyols to construct the main chain skeleton;

[0038] Chain extension stage: low-activity 2,6-TDI is reacted with trimethylolpropane to form a terminal crosslinking point;

[0039] Network uniformity: by controlling the NCO / OH molar ratio to be 1.08, the crosslinking point spacing is shortened from 15 nm of the traditional system to 9 nm, and the diffusion of water molecules is significantly inhibited (water absorption rate <2.5%).

[0040] Trimethylolpropane (TMP, CAS No.: 77-99-6) has a hydroxyl value of 1150~1250 mgKOH / g, realizing the dimensional upgrading of the crosslinking network:

[0041] Crosslinking density is doubled: the trifunctional structure makes the crosslinking point density increase from 2.1×10 20 sites / cm 3 to 3.5×10 20 sites / cm 3 ;

[0042] Moisture and heat stability is improved: the methylene chain (-CH2-CH(CH2OH)-) of TMP forms a hydrophobic barrier, so that after the adhesive layer is aged at 100℃ / 95%RH (relative humidity) for 1000 hours, the E' attenuation rate is <8% (25% of the comparative system without addition);

[0043] Stress dispersion mechanism: the three-dimensional network forms a "hard segment island (hard block)-soft segment sea (soft block)" structure through microphase separation, so that more energy is consumed for crack propagation (fracture toughness KIC increases from 0.8 MPa·m 1 / 2 to 1.6 MPa·m 1 / 2 ).

[0044] A preparation method of a formaldehyde-free environmentally friendly polyurethane adhesive, comprising the following steps:

[0045] S1 Take / measure raw materials: take polyether polyol, toluene diisocyanate, dimethylol propionic acid, trimethylolpropane, dibutyltin dilaurate, industrial ethanol (Xinwei Chemical's anhydrous ethanol, purity ≥ 99.5%) by weight;

[0046] S2 Prepolymer synthesis: under nitrogen protection, mix polyether polyol with toluene diisocyanate, and add 0.05-0.1 parts of dibutyltin dilaurate, and follow the stepwise temperature program from 60°C to 75°C, the total reaction time is 2.5h, and the NCO content is monitored by online near infrared spectroscopy to 6.5-7.2%, to obtain end-NCO prepolymer, wherein the stepwise temperature program is: 60°C for 15min, heated to 65°C and kept for 15min, heated to 70°C and kept for 15min, heated to 75°C and kept for 105min;

[0047] Temperature-viscosity coordination during the reaction: initial low temperature (60°C) inhibits side reactions (urethane formation <0.5%), and later temperature rise (75°C) accelerates chain growth, so that the prepolymer molecular weight distribution is optimized from PDI=1.5 to 1.2;

[0048] Precise control of NCO content: lock the NCO content of the prepolymer at 6.5-7.2% (CV value <1.5%), leaving active sites for subsequent chain extension;

[0049] S3 Chain extension reaction: heat the -NCO prepolymer to 95°C, add 0.1-0.15 parts of dibutyltin dilaurate, and add dimethylol propionic acid twice with an interval of 10 minutes, and react for 30 minutes until the acid value of the system is <5mgKOH / g; add trimethylolpropane, control the shear rate at 50-100s -1 , and react for 60min until the -NCO content is ≤0.5% (HPLC detection);

[0050] Among them, DMPA preferentially chain extension: first add DMPA for carboxyl end capping (reaction degree >95%), and form a dense anchor layer at the adhesive-wood interface (TOF-SIMS detects carboxylate calcium characteristic fragments m / z=143.02 at the interface);

[0051] TMP bulk crosslinking: then add TMP to trigger three-dimensional crosslinking, and adjust the shear rate (50-100s -1 ) to control the network topology, so that the bulk crosslinking density of the adhesive layer (Mc=2800g / mol) and the interface layer (Mc=4500g / mol) form a gradient distribution, and both strength and toughness are considered;

[0052] S4 termination and post-treatment: 0.05-0.1 parts of dibutyltin dilaurate and 1-3 parts of industrial ethanol were added, and stirred for 20 min to make the residual -NCO content <0.5% (determined by HPLC), and the micropores (pore size <5 μm) formed by the volatilization of ethanol enhanced the air permeability of the adhesive layer, avoiding bubbling during hot pressing;

[0053] Vacuum degassing cooperation: degassing for 30 min at -0.095 MPa, the bubble content decreased from the initial 5.2% to 0.3%, and by controlling the depressurization rate (0.01 MPa / min) to prevent the depolymerization of the prepolymer caused by turbulent flow of the glue solution, the final viscosity was 10000-15000 mPa·s, matching the wood vessel pore size (10-50 μm), ensuring the penetration depth of the glue solution to 200-300 μm.

[0054] Dibutyltin dilaurate (DBTDL, CAS No.: 77-58-7) as a catalyst, its addition strategy breaks through the traditional homogeneous catalysis mode:

[0055] Stepwise addition method: the catalyst is added in three times to make the reaction rate ratio (kprepolymer: kchain extension: ktermination = 1:0.6:0.3) in different stages match the process requirements;

[0056] Local concentration control: realize the gradient distribution of the catalyst through existing microfluidic equipment to avoid local overheating;

[0057] Gel time locking: the final system gel time is stabilized at 25±2 minutes (determined by a rotational viscometer), which is reduced by 80% compared with the traditional process fluctuation range (±10 minutes).

[0058] Example 1

[0059] Select 50 parts of polyether polyol with a hydroxyl value of 56 mgKOH / g as the flexible chain segment matrix, the molecular weight distribution index PDI = 1.2, and the ethylene oxide / propylene oxide block ratio EO / PO = 20 / 80; 4 parts of dimethylol propionic acid; 25 parts of toluene diisocyanate, the ratio of 2,4-TDI and 2,6-TDI is 80 / 20; 5 parts of trimethylolpropane; 0.3 parts of dibutyltin dilaurate; 2 parts of industrial ethanol.

[0060] Prepared according to the following steps:

[0061] S1 weighing / quantitative raw materials: according to the above weight parts, polyether polyol, toluene diisocyanate, dimethylol propionic acid, trimethylolpropane, dibutyltin dilaurate, industrial ethanol were weighed / quantitatively taken;

[0062] Synthesis of S2 prepolymer: In a nitrogen-protected reactor, 50 parts of polyether polyol, 25 parts of toluene diisocyanate, and 0.1 part of dibutyltin dilaurate were added in sequence. The temperature was raised from 60°C to 75°C according to a stepwise heating program for a total reaction time of 2.5 hours. The NCO content was monitored by online near-infrared spectroscopy to 6.5-7.2%, yielding a terminal NCO prepolymer.

[0063] S3 chain extension reaction: heat the -NCO prepolymer to 95°C, add 0.15 parts of dibutyltin dilaurate, and add dimethylolpropionic acid twice (2 parts each time), with an interval of 10 minutes, and react for 30 minutes until the acid value of the system is less than 5mgKOH / g; add 5 parts of trimethylolpropane, and control the shear rate to 50-100s -1 , react for 60 minutes until the NCO content is ≤0.5%;

[0064] S4 termination and post-treatment: Add 0.05 parts of dibutyltin dilaurate and 2 parts of industrial ethanol and stir for 20 minutes to reduce the residual -NCO content to less than 0.3% (HPLC determination). At the same time, the micropores (pore size less than 5 μm) formed by ethanol volatilization enhance the air permeability of the adhesive layer and prevent bubbling during hot pressing;

[0065] Vacuum degassing synergy: After degassing for 30 minutes at -0.095 MPa, the bubble content dropped from the initial 5.2% to 0.3%, and the decomposition of the prepolymer caused by the turbulence of the adhesive was prevented by controlling the pressure reduction rate (0.01 MPa / min). The final viscosity was 12300 mPa·s, which matched the pore size of the wood vessels (10-50 μm), ensuring that the adhesive penetration depth reached 200-300 μm.

[0066] The plywood was prepared using the formaldehyde-free environmentally friendly polyurethane adhesive prepared above according to the parameters in Table 1. The water absorption rate of the plywood was ≤2.5%, and the impact strength was ≥8kJ / m 2 , fracture toughness ≥1.5MPa·m 1 / 2 The preparation of plywood complies with GB / T9846-2015 "Ordinary Plywood".

[0067] Table 1 Parameters for preparing plywood

[0068] Parameters Setting value Substrate Masson pine veneer (moisture content 8-10%) Structure 5-layer symmetrical structure (2.0 mm per layer) Coating process One side coated, coat weight 150 g / m 2 ]]> Hot-pressing condition 110℃11.2 11118m11 Curing condition 25°C / 50% RH 22 curing for 72 hours

[0069] Example 2

[0070] The same preparation method and parameter control were adopted as in Example 1, except that the amount of dimethylolpropionic acid added was 7 parts, and the amount added twice was 3.5 parts respectively.

[0071] Example 3

[0072] The same preparation method and parameter control as in Example 1 were adopted, except that the amount of dimethylol propionic acid added was 8 parts, and the amount added in two times was 4 parts each time.

[0073] Example 4

[0074] The same preparation method and parameter control as in Example 1 were adopted, except that the amount of dimethylol propionic acid added was 12 parts, and the amount added in two times was 6 parts each time.

[0075] Comparative Example 1

[0076] The same preparation method and parameter control as in Example 1 were adopted, except that no dimethylol propionic acid was added.

[0077] Comparative Example 2

[0078] The aldehyde-free adhesive and plywood were prepared by using the technical solution recorded in Chinese patent CN114539973B, which represents the advanced level of current polyurethane aldehyde-free adhesive, and the cost is about 20,000 yuan / ton. The curing conditions of the plywood are 115℃×10 minutes, and the pressure is 1.0MPa. Among them, the high hydroxyl content (4.2-5.1mmol / g) of cellulose leads to the following chain problems: shortening of the pot life: the viscosity rises from 8000mPa·s to 50000mPa·s in 2 hours (exceeding the upper limit of the coating equipment tolerance); uneven crosslinking: local premature gelation forms an “island structure”, leading to stress concentration cracking in the wet heat test; poor process stability: the residual amount of -NCO fluctuates in the range of ±15%, forcing the production line to frequently adjust the hot pressing parameters.

[0079] The plywood prepared in Examples 1-4 and Comparative Examples 1 was tested for bonding strength and free formaldehyde according to GB / T 17657-2013 “Test methods of physical and chemical properties of wood-based panels and veneered wood-based panels” and GB / T14074-2017 “Acetylacetone spectrophotometric method”. The results are shown in Tables 2 and 3.

[0080] Table 2 Bonding strength test results

[0081]

[0082]

[0083] Table 3 Free formaldehyde test results

[0084] Results Judgment standard Example 1 Not detected (<0.05 mg / L) ENF class (≤ 0.025 mg / m 3 )]]> Example 2 Not detected (<0.05 mg / L) ENF class (≤ 0.025 mg / m 3 )]]> Example 3 Not detected (<0.05 mg / L) ENF class (≤ 0.025 mg / m 3 )]]> Example 4 Not detected (<0.05 mg / L) ENF class (≤ 0.025 mg / m 3 )]]> Comparative Example 1 0.12 mg / L [Level E0 (≤ 5.5 mg / m 3 )]]>

[0085] Analyzing the results in Tables 2 and 3, the bonding strength and retention rate are greatly improved, and the carboxyl group (-COOH) of dimethylol propionic acid reacts with Ca2+ / Mg 2+ The ion chelation forms [O-Ca-O] bridging structure, so that the interface binding energy is greatly improved; in the slightly acidic environment of wood, the carboxyl group is partially ionized, and the adsorption with the hydroxyl group of cellulose is enhanced through electrostatic attraction; the methyl branch of dimethylol propionic acid hinders the approach of water molecules to the adhesive interface, and controls the amount of free formaldehyde to ENF level.

[0086] Comparing examples 1-4 and comparative example 2, the cost of the technical scheme of the application is about 12,000 yuan / ton, that is, the cost can be reduced by about 40%, the hydroxyl group density is reduced by 30-40% (2.8-3.2 mmol / g), and the pot life is increased to more than 3h (the viscosity change rate is less than 10% / h).

[0087] The above is only a preferred embodiment of the application, but the protection scope of the application is not limited to this, any person skilled in the art can easily think of changes or replacements within the technical range disclosed by the application, which should be covered in the protection scope of the application. Therefore, the protection scope of the application should be limited by the protection scope defined by the claims.

[0088] The parts not described in the application are well-known technologies of the person skilled in the art.

Claims

1. A formaldehyde-free environment-friendly polyurethane adhesive, characterized in that: The components include the following weight parts: polyether polyol 40-60 parts, toluene diisocyanate 20-30 parts, dimethylol propionic acid 4-12 parts, trimethylolpropane 3-5 parts, dibutyltin dilaurate 0.2-0.4 parts, industrial ethanol 1-3 parts, wherein the molar ratio of isocyanate groups to hydroxyl groups is 1.05-1.

10.

2. The formaldehyde-free environment-friendly polyurethane adhesive according to claim 1, characterized in that: The polyether polyol has a hydroxyl value of 50-60 mgKOH / g, a block ratio of ethylene oxide to propylene oxide of 15:85-25:75, and a molecular weight distribution index of ≤1.

3.

3. The formaldehyde-free environment-friendly polyurethane adhesive according to claim 1, characterized in that: The toluene diisocyanate has a ratio of toluene-2,4-diisocyanate isomer to toluene-2,6-diisocyanate isomer of 75:25-85:

15.

4. The formaldehyde-free environment-friendly polyurethane adhesive according to claim 1, characterized in that: The dimethylol propionic acid has a purity of ≥99% and a moisture content of ≤0.2%.

5. The formaldehyde-free environment-friendly polyurethane adhesive according to claim 1, characterized in that: The trimethylolpropane has a hydroxyl value of 1150-1250 mgKOH / g.

6. A process for preparing the formaldehyde-free environment-friendly polyurethane adhesive according to any one of claims 1 to 5, characterized in that, The method comprises the following steps: S1: weighing raw materials: weighing the polyether polyol, toluene diisocyanate, dimethylol propionic acid, trimethylolpropane, dibutyltin dilaurate, and industrial ethanol according to weight parts; S2: pre-polymer synthesis: mixing the polyether polyol and toluene diisocyanate under nitrogen protection, and raising the temperature from 60°C to 75°C according to a stepwise temperature raising procedure, with a total reaction time of 2.5 h, and monitoring the NCO content to 6.5-7.2% by online near-infrared spectroscopy, to obtain an end-NCO pre-polymer; S3 chain extension reaction: the -NCO prepolymer is warmed to 95°C, dimethylol propionic acid is added in two portions, the time interval between the two portions of dimethylol propionic acid is 10 min, and then the reaction is carried out for 30 min until the acid value of the system is <5 mg KOH / g; trimethylolpropane is added, the shear rate is controlled to be 50-100 s -1 , the reaction is carried out for 60 min until the -NCO content is ≤0.5%; S4: termination and post-treatment: adding industrial ethanol and stirring for 20 min, and vacuum degassing to a bubble content of ≤0.5% and a final viscosity of 10000-15000 mPa·s.

7. The method of claim 6, wherein: The stepwise temperature raising procedure is: maintaining 60°C for 15 min, raising the temperature to 65°C and maintaining for 15 min, raising the temperature to 70°C and maintaining for 15 min, and raising the temperature to 75°C and maintaining for 105 min.

8. The method of claim 6, wherein: The dibutyltin dilaurate is added in three gradients, i.e. 0.05-0.1 parts in step S2, 0.1-0.15 parts in step S3, and 0.05-0.1 parts before step S4.

9. The method of claim 6, wherein: The vacuum degassing conditions in step S4 are: vacuum degree -0.095 MPa, degassing time 30 min, and pressure reduction rate ≤0.01 MPa / min.

10. Use of the formaldehyde-free environmentally friendly polyurethane adhesive according to any one of claims 1-5 in plywood.

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