Long dry glue time formaldehyde-free adhesive for plywood and preparation method and application thereof

Abstract

This invention discloses a formaldehyde-free adhesive for plywood with a long drying time, its preparation method, and its application. The preparation method includes the following steps: 1) Preparing modified isocyanate: Castor oil and polyisobutylene are stirred and mixed at 60-90°C to obtain a mixture, preferably with a stirring time of 0.5-2 hours; a polyfunctional isocyanate is added to the mixture, and after reacting for 2-4 hours, the mixture is cooled to obtain modified isocyanate; 2) A mixture of water, flour, and a composite water-retaining agent is mixed with the modified isocyanate to obtain the adhesive. The above method is used to provide a formaldehyde-free adhesive for plywood with a long drying time.

Description

Technical Field

[0001] This invention relates to a formaldehyde-free adhesive, and more particularly to a formaldehyde-free adhesive for plywood with a long drying time, its preparation method, and its application. Background Technology

[0002] Plywood is an important component of the engineered wood products industry. Currently, the adhesives used in domestic plywood production are mainly urea-formaldehyde resin adhesives (including modified ones) and phenolic resin adhesives. The most prominent problem with these adhesives is that they contain free formaldehyde, which releases into plywood during use, posing a significant health risk. With increasing public awareness of environmental protection and health, there is growing concern about formaldehyde release from engineered wood products. Therefore, a formaldehyde-free adhesive for plywood is needed.

[0003] Patent CN104962218A discloses a modified VAE emulsion formaldehyde-free adhesive and its preparation method. The adhesive contains the following raw materials: 85-95 parts VAE emulsion, 5-15 parts rosin ester emulsion, 3-5 parts isocyanate, and 6-10 parts light calcium carbonate. The adhesive uses VAE emulsion as the main component. VAE emulsion is characterized by its fast drying speed, meaning that the adhesive dries easily after the veneer is coated. When factories use this method to produce plywood, the board blank needs to be fed into the pre-pressing process as soon as possible. Otherwise, the adhesive on the veneer surface will dry, leading to poor formability of the pre-pressed board blank, making it prone to openings or blistering, and failing to meet the requirements for subsequent hot pressing. Even if the board blank is forced into the hot press, the performance of the resulting board at the openings or blistering areas will still be difficult to meet the standard requirements.

[0004] Patent CN106965269A discloses a formaldehyde-free flame-retardant plywood and its preparation method. The adhesive components used include magnesium oxide, magnesium chloride, polyvinyl alcohol, and minor components (foaming agent, dispersant, modifier, etc.). The polyvinyl alcohol used in this adhesive formulation is also prone to drying into a film, which affects the pre-pressing effect and subsequent board making. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention proposes a formaldehyde-free adhesive for plywood with a long drying time, its preparation method, and its application. In the preparation method of the formaldehyde-free adhesive of this invention, castor oil and polyisobutylene are first mixed, and then a polyfunctional isocyanate is added to react, resulting in a modified isocyanate with good system compatibility and meeting the requirements for both initial tack and permanent adhesion. Simultaneously, a mixture of polyvinylpyrrolidone and bisphenol A polyoxyethylene ether in a specific ratio is selected as a composite water-retaining agent, enabling the adhesive in the compound formulation to achieve strong water absorption and retention properties. Maintaining a moist state for a long time not only significantly extends the drying time but also allows for effective penetration into the veneer during pre-pressing, forming adhesive nails and improving the bonding effect.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing a formaldehyde-free adhesive for plywood with a long drying time includes the following steps:

[0008] 1) Preparation of modified isocyanates

[0009] Castor oil and polyisobutylene are stirred and mixed at 60-90°C to obtain a mixture, preferably with a stirring time of 0.5-2 h; polyfunctional isocyanate is added to the above mixture, and after reacting for 2-4 h, the mixture is cooled to obtain modified isocyanate.

[0010] 2) The mixture of water, flour, and composite water-retaining agent is then mixed with modified isocyanate to obtain the adhesive.

[0011] This invention modifies isocyanate with a mixture of castor oil and polyisobutylene, resulting in a system with good compatibility and strong adhesion, simultaneously meeting the requirements for initial and permanent adhesion. Furthermore, the addition of a highly absorbent and moisture-retaining composite water-retaining agent significantly extends the drying time of the adhesive, ensuring the performance requirements of the boards are met even under extended operating times. Additionally, the adhesive incorporates flour containing protein and active starch to prevent excessive penetration of the adhesive components into the wood and to prevent insufficient adhesive application under pressure due to short aging times and high resin fluidity. The synergistic combination of these components achieves excellent adhesion and a long drying time, guaranteeing the superior performance of the adhesive.

[0012] In a preferred embodiment of the present invention, the amounts of raw materials used in the preparation method, by weight, are as follows:

[0013]

[0014] As a preferred embodiment of the present invention, the polyfunctional isocyanate is selected from any adhesive aromatic polyisocyanate, aliphatic polyisocyanate, alicyclic isocyanate, preferably one or more of toluene diisocyanate, diphenylmethane diisocyanate and polymethylene polyphenyl polyisocyanate.

[0015] In a preferred embodiment of the present invention, the molecular weight of the polyisobutylene is 680-2450.

[0016] In a preferred embodiment of the present invention, the composite water-retaining agent is a mixture of polyvinylpyrrolidone and bisphenol A polyoxyethylene ether, with a mass ratio of 10:(1-2).

[0017] In a preferred embodiment of the present invention, the average molecular weight of the polyvinylpyrrolidone is ≥8000 and the average molecular weight of the bisphenol A polyoxyethylene ether is ≥492.

[0018] The present invention also provides a formaldehyde-free adhesive for plywood with a long drying time, prepared according to the method described above.

[0019] This invention also provides the application of the formaldehyde-free adhesive with a long drying time for plywood as described above in the preparation of plywood. A specific application method is, for example, producing plywood according to the following method:

[0020] (1) Apply the adhesive described above to the surface of the veneer; for example, use a four-roller glue coater to apply the adhesive evenly to the surface of the veneer by double-sided glue application.

[0021] (2) Arrange the glued veneers according to the odd-numbered layer principle for assembly, and pre-press the veneers after they are laid out;

[0022] (3) The slab is hot-pressed to obtain the plywood;

[0023] Preferably, the veneer is made of materials including but not limited to poplar, eucalyptus, pine, birch, etc., and preferably has a moisture content of 7-40 wt%.

[0024] Preferably, the amount of adhesive applied to each side of the veneer is 150-250 g / m². 2 ;

[0025] Preferably, the preform is assembled within 2 hours after the adhesive is applied; pre-compression is carried out within 4 hours after the preform is assembled, with a pre-compression pressure of 0.7-1.0 MPa and a pre-compression time of 0.5-6 hours.

[0026] Preferably, the hot pressing temperature is 110-160℃, the hot pressing pressure is 0.1-1.0MPa, and the hot pressing factor is 1-4min / mm.

[0027] Those skilled in the art will understand that the hot pressing is carried out in a hot press: the slab is heated and pressurized, and mass and heat transfer are carried out by water vapor convection inside the slab, so that the adhesive is cured and a strong bond is formed.

[0028] The beneficial effects of this invention are as follows:

[0029] (1) A formaldehyde-free adhesive is provided, which has low cost, moderate viscosity and fluidity, and is particularly suitable for the production of plywood by roller coating process, and the resulting plywood has high strength.

[0030] (2) By premixing castor oil and polyisobutylene and then reacting it with isocyanate, the compatibility problem between polyisobutylene and isocyanate is solved, and a formaldehyde-free adhesive with stable quality can be obtained.

[0031] (3) This invention utilizes the permanent adhesiveness of polyisobutylene, combined with a composite water-retaining agent composed of polyvinylpyrrolidone and bisphenol A polyoxyethylene ether, to significantly extend the drying time of formaldehyde-free adhesives, which can be adapted to the existing plywood production process. Detailed Implementation

[0032] The present invention will be further illustrated below with specific embodiments. These embodiments are merely illustrative and do not limit the scope of the invention.

[0033] The main raw material information involved in the following embodiments of the present invention is as follows:

[0034] Polymethylene polyphenyl polyisocyanate (PMDI): Functionality approximately 2.7, viscosity at 25°C 180-250 cP, -NCO content 30.5-32%, Wanhua Chemical Group Co., Ltd.

[0035] Polyvinylpyrrolidone A, average molecular weight 8000, Beijing Innocare Technology Co., Ltd.;

[0036] Polyvinylpyrrolidone B, average molecular weight 58,000, Beijing Innocare Technology Co., Ltd.

[0037] Bisphenol A polyoxyethylene ether A, average molecular weight 492, Shanghai Titan Technology Co., Ltd.

[0038] Bisphenol A polyoxyethylene ether B, average molecular weight 580, Shanghai Titan Technology Co., Ltd.

[0039] Bisphenol A polyoxyethylene ether C, average molecular weight 668, Shanghai Titan Technology Co., Ltd.

[0040] Polyisobutylene PB450, average molecular weight 450, Daelim, South Korea;

[0041] Polyisobutylene PB680, average molecular weight 680, Daelim, South Korea;

[0042] Polyisobutylene PB1300, average molecular weight 1300, Daelim, South Korea;

[0043] Polyisobutylene PB2400, average molecular weight 2450, Daelim, South Korea;

[0044] Castor oil, hydroxyl value 150-165 mgKOH / g, Guangzhou Baosen Chemical Co., Ltd.;

[0045] Flour, commercially available industrial flour;

[0046] Eucalyptus veneer, moisture content 15%, thickness 1.7mm.

[0047] The main testing methods used in the following embodiments of the present invention are as follows:

[0048] 1. Evaluation of storage stability of modified isocyanate: After sampling the modified isocyanate, it was placed in ovens at 5℃ and 45℃ for 60 days respectively, and the presence of stratification was observed. The results are shown in Table 2.

[0049] 2. Evaluation method for slab formability: Observe the slab pre-pressed under the operating conditions of the application example, and record whether it is tightly closed, and whether there are openings or bubbles; then drop the slab vertically from a height of 1.5m and observe whether there are openings. Formability is calibrated numerically, and the meaning of the values ​​is shown in Table 1. The larger the value, the better the formability of the slab under a longer operating time, that is, the longer the drying time.

[0050] Table 1. Meaning of Numerical Calibration

[0051]

[0052]

[0053] 3. Adhesion performance of the boards: The obtained boards were subjected to Class II adhesive strength test in accordance with GB / T 17657-2013 Test Methods for Physical and Chemical Properties of Wood-based Panels and Decorative Wood-based Panels.

[0054] 4. Formaldehyde emission test: The desiccator method is used for testing.

[0055] Example 1:

[0056] The adhesive is prepared according to the following method:

[0057] (1) Heat 10 kg of polyisobutylene PB680 to 75°C; while stirring, add 10 kg of castor oil to the polyisobutylene, maintain the temperature at 75°C and continue stirring for 1 h; add 30 kg of PMDI to the above mixture, react at 75°C for 3 h and then cool to room temperature to obtain modified isocyanate.

[0058] (2) Mix 100kg of water, 75kg of flour, and 10kg of composite water-retaining agent (8.35kg of polyvinylpyrrolidone A and 1.65kg of bisphenol A polyoxyethylene ether A) evenly, and then mix them evenly with the above modified isocyanate to obtain a formaldehyde-free adhesive.

[0059] Example 2:

[0060] The adhesive is prepared according to the following method:

[0061] (1) Heat 7 kg of polyisobutylene PB1300 to 60°C; while stirring, add 7 kg of castor oil to the polyisobutylene, maintain the temperature at 60°C and continue stirring for 2 h; add 26 kg of PMDI to the above mixture, raise the temperature to 80°C and react for 3 h, then lower to room temperature to obtain modified isocyanate.

[0062] (2) Mix 100kg of water, 80kg of flour, and 8kg of composite water-retaining agent (7.25kg of polyvinylpyrrolidone B and 0.75kg of bisphenol A polyoxyethylene ether C) evenly; then mix evenly with the above modified isocyanate to obtain a formaldehyde-free adhesive.

[0063] Example 3:

[0064] The adhesive is prepared according to the following method:

[0065] (1) Heat 8 kg of polyisobutylene PB2400 to 90°C; while stirring, add 9.5 kg of castor oil to the polyisobutylene, maintain the temperature at 90°C and continue stirring for 0.5 h; add 27 kg of PMDI to the above mixture, cool to 75°C and react for 4 h, then cool to room temperature to obtain modified isocyanate;

[0066] (2) Mix 100kg of water, 78kg of flour, and 9kg of composite water-retaining agent (7.8kg of polyvinylpyrrolidone B and 1.2kg of bisphenol A polyoxyethylene ether B) evenly; then mix evenly with the above modified isocyanate to obtain a formaldehyde-free adhesive.

[0067] Example 4:

[0068] The adhesive is prepared according to the following method:

[0069] (1) Heat 7 kg of polyisobutylene PB2400 to 65°C; while stirring, add 10 kg of castor oil to the polyisobutylene, maintain the temperature at 65°C and continue stirring for 1.5 h; add 29 kg of PMDI to the above mixture, raise the temperature to 80°C and react for 2 h, then lower to room temperature to obtain modified isocyanate.

[0070] (2) Mix 100kg of water, 80kg of flour, and 8.5kg of composite water-retaining agent (7.5kg of polyvinylpyrrolidone B and 1kg of bisphenol A polyoxyethylene ether A) evenly; then mix evenly with the above modified isocyanate to obtain a formaldehyde-free adhesive.

[0071] Example 5:

[0072] The adhesive is prepared according to the following method:

[0073] (1) Heat 8 kg of polyisobutylene PB1300 to 70°C; while stirring, add 9 kg of castor oil to the polyisobutylene, maintain the temperature at 70°C and continue stirring for 1 h; add 28 kg of PMDI to the above mixture, raise the temperature to 80°C and react for 2 h, then lower to room temperature to obtain modified isocyanate.

[0074] (2) Mix 100kg of water, 80kg of flour, and 9kg of composite water-retaining agent (8kg of polyvinylpyrrolidone A and 1kg of bisphenol A polyoxyethylene ether C) evenly; then mix evenly with the above modified isocyanate to obtain a formaldehyde-free adhesive.

[0075] Comparative Example 1:

[0076] The formaldehyde-free adhesive was prepared in basically the same way as in Example 1, except that in step (1) when preparing the modified isocyanate, polyisobutylene PB680, castor oil and PMDI were mixed at one time and heated to 75°C and stirred continuously for 3 hours.

[0077] Comparative Example 2:

[0078] The formaldehyde-free adhesive was prepared in essentially the same manner as in Example 1, except that polyisobutylene PB680 in step (1) was replaced with polyisobutylene PB450.

[0079] Comparative Example 3:

[0080] The formaldehyde-free adhesive was prepared in essentially the same manner as in Example 1, except that polyisobutylene PB680 was not added when preparing the modified isocyanate in step (1).

[0081] Comparative Example 4:

[0082] The formaldehyde-free adhesive was prepared in essentially the same manner as in Example 1, except that no composite water-retaining agent was added in step (2).

[0083] Comparative Example 5:

[0084] The formaldehyde-free adhesive was prepared in essentially the same manner as in Example 1, except that in step (2), the composite water-retaining agent was replaced with the same mass of polyvinylpyrrolidone A.

[0085] Comparative Example 6:

[0086] The formaldehyde-free adhesive was prepared in essentially the same manner as in Example 1, except that in step (2), the composite water-retaining agent was replaced with the same mass of bisphenol A polyoxyethylene ether A.

[0087] Comparative Example 7:

[0088] The formaldehyde-free adhesive was prepared in essentially the same manner as in Example 1, except that in step (2), the composite water-retaining agent was replaced with a mixture of 7.5 kg of polyvinylpyrrolidone A and 2.5 kg of bisphenol A polyoxyethylene ether A.

[0089]

Application Example

[0090] Plywood was prepared using the formaldehyde-free adhesives provided in Examples 1-5 and Comparative Examples 1-7, respectively, and the methods are as follows:

[0091] (a) Using a four-roller glue applicator, apply the adhesive evenly to the surface of the veneer using a double-sided glue application method, controlling the glue application amount on each side to be 160g / m². 2 ;

[0092] (b) Assembly 2 hours after gluing: Assemble the veneers according to the odd number of layers principle, with the fiber direction of adjacent veneers perpendicular to each other, and the number of veneer layers is 9;

[0093] (c) Pre-pressing is carried out 4 hours after billet assembly. The pre-pressing pressure is 0.7 MPa and the pre-pressing time is 6 hours.

[0094] (d) The pre-pressed slab is fed into a hot press. The hot pressing temperature is 120℃, the hot pressing pressure is 0.8MPa in the high-pressure section and 0.3MPa in the low-pressure section, the hot pressing factor is 4min / mm, and the ratio of hot pressing time in the high-pressure section to that in the low-pressure section is 3:2 to obtain plywood.

[0095] Storage stability tests were conducted on the modified isocyanates obtained during the preparation of formaldehyde-free adhesives in Examples 1-5 and Comparative Examples 1-7, and the test results are shown in Table 2. In addition, the formability of the plywood blanks, the bonding performance of the boards, and the formaldehyde release were tested, and the test results are shown in Table 2.

[0096] Table 2. Performance Test Results

[0097]

[0098]

[0099] The test results of Examples 1-5 and Comparative Example 1 show that the products obtained by first mixing castor oil and polyisobutylene evenly and then modifying isocyanate have better storage stability, slab formability, and higher slab strength and pass rate.

[0100] The results of tests in Example 1 and Comparative Example 2 indicate that the molecular weight of the polyisobutylene used should not be lower than 680, otherwise it will cause a decrease in the formability of the slab and the performance of the sheet.

[0101] The results of Example 1 and Comparative Examples 3 and 4 show that the addition of polyisobutylene and the composite water-retaining agent are crucial for maintaining the adhesive's long dry time. In Example 1, after a 6-hour interval between adhesive application and pre-pressing, the adhesive still exhibited good initial tack, ensuring effective slab formation and acceptable board performance. In Comparative Example 3, the system without polyisobutylene, and in Comparative Example 4, the system without the composite water-retaining agent, completely lost their initial tack after 6 hours of adhesive application, resulting in failure to form the slab after pre-pressing and consequently lower board performance.

[0102] The results of Example 1 and Comparative Examples 5, 6 and 7 show that when polyvinylpyrrolidone or bisphenol A polyoxyethylene ether is used alone, or when the ratio of the two is not within the preferred range, the water retention of the adhesive system is insufficient, resulting in poor formability of the slab after pre-pressing. It is necessary to use the two in combination within a specific range to better meet the requirements of the board production process.

[0103] Based on the above examples and comparative results, the adhesive obtained by first mixing castor oil and polyisobutylene evenly, then modifying isocyanate, and further compounding with water, flour, and composite water-retaining agent has a long drying time. After application, the slab still has good formability even after pre-pressing for up to 6 hours, and the strength and pass rate of the slab can meet the standard requirements.

[0104] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements and additions without departing from the method of the present invention, and these improvements and additions should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a formaldehyde-free adhesive with a long drying time for plywood, characterized in that, Includes the following steps: 1) Preparation of modified isocyanate Castor oil and polyisobutylene were stirred and mixed at 60-90℃ to obtain a mixture; a polyfunctional isocyanate was added to the mixture, and the mixture was reacted for 2-4 hours and then cooled to obtain a modified isocyanate. 2) The mixture of water, flour, and composite water-retaining agent is then mixed with modified isocyanate to obtain the adhesive; In the preparation method, the amounts of raw materials used, in parts by weight, are as follows: 7-10 parts castor oil 7-10 parts of polyisobutylene 26-30 parts of multifunctional isocyanate 100 parts water 75-80 portions of flour 8-10 parts of compound water-retaining agent; The composite water-retaining agent is a mixture of polyvinylpyrrolidone and bisphenol A polyoxyethylene ether, with a mass ratio of 10:(1-2).

2. The method for preparing the formaldehyde-free adhesive for plywood with long drying time according to claim 1, characterized in that, In step 1), the stirring time for castor oil and polyisobutylene is 0.5-2 hours.

3. The method for preparing a formaldehyde-free adhesive with a long drying time for plywood according to claim 1 or 2, characterized in that, The polyfunctional isocyanate is selected from aromatic polyisocyanates, aliphatic polyisocyanates, and alicyclic isocyanates.

4. The method for preparing the formaldehyde-free adhesive for plywood with long drying time according to claim 3, characterized in that, The polyfunctional isocyanate is selected from one or more of toluene diisocyanate, diphenylmethane diisocyanate, and polymethylene polyphenyl polyisocyanate.

5. The method for preparing the formaldehyde-free adhesive for plywood with a long drying time according to claim 3, characterized in that, The molecular weight of the polyisobutylene is 680-2450.

6. The method for preparing the formaldehyde-free adhesive for plywood with a long drying time according to claim 1, characterized in that, The average molecular weight of the polyvinylpyrrolidone is ≥8000, and the average molecular weight of the bisphenol A polyoxyethylene ether is ≥492.

7. A formaldehyde-free adhesive for plywood with a long drying time, prepared by the method according to any one of claims 1-6.

8. The use of a formaldehyde-free adhesive with a long drying time for plywood, prepared by the method according to any one of claims 1-6, in the preparation of plywood.

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