Formaldehyde-free adhesive with long glue drying time for plywood as well as preparation method and application of formaldehyde-free adhesive

Abstract

The invention discloses a formaldehyde-free adhesive with long glue drying time for plywood as well as a preparation method and application of the formaldehyde-free adhesive. The preparation method comprises the following steps: 1) preparing modified isocyanate: stirring and mixing castor oil and polyisobutene at 60-90 DEG C to obtain a mixture, and preferably stirring for 0.5-2 hours; adding polyfunctional isocyanate into the mixture, reacting for 2-4 hours, and cooling to obtain modified isocyanate; and 2) mixing a mixed component consisting of water, flour and the composite water-retaining agent with the modified isocyanate to prepare the adhesive. The method is used for providing the formaldehyde-free adhesive with long glue drying time for the plywood.

Description

Technical Field

[0001] The invention relates to a formaldehyde-free adhesive, in particular to a formaldehyde-free adhesive with a long curing time for plywood and a preparation method and application thereof. Background Art

[0002] Plywood is an important part of the wood-based panel industry. At present, the adhesives used in domestic plywood production are mainly urea-formaldehyde resin adhesives (including modified ones) and phenolic resin adhesives. The most prominent problem of this type of adhesive is that it contains free formaldehyde. The plywood prepared in this way will release formaldehyde, which is extremely harmful to the human body during use. As people's environmental protection and health awareness have been significantly improved, they have paid close attention to the issue of formaldehyde release in wood-based panels. Therefore, a plywood adhesive without formaldehyde addition is needed.

[0003] Patent CN104962218A discloses a modified VAE emulsion formaldehyde-free adhesive and a preparation method thereof, wherein the adhesive comprises the following raw materials: 85-95 parts of VAE emulsion, 5-15 parts of rosin ester emulsion, 3-5 parts of isocyanate, and 6-10 parts of light calcium carbonate. The adhesive is mainly composed of VAE emulsion, and VAE emulsion has the characteristics of fast drying speed, which means that the veneer is easy to dry after gluing. When the factory adopts this scheme to produce plywood, the slab needs to be sent to pre-press as soon as possible, otherwise the adhesive on the surface of the veneer will dry, which will lead to poor formability of the slab after pre-pressing, easy opening or bubbling, and cannot meet the requirements of subsequent hot pressing into the board. Even if the slab is barely sent to the hot press, the performance of the plate obtained after hot pressing at the opening or bubbling is difficult to meet the standard requirements.

[0004] Patent CN106965269A discloses a formaldehyde-free flame-retardant plywood and a preparation method thereof, wherein the adhesive components used include magnesium oxide, magnesium chloride, polyvinyl alcohol and small materials (foaming agent, diffusant, modifier, etc.). The polyvinyl alcohol used in the adhesive formula is also easy to dry and form a film, thereby affecting the pre-pressing effect and subsequent board making. Summary of the invention

[0005] In order to solve the above technical problems, the present invention proposes a formaldehyde-free adhesive with a long drying time for plywood and its preparation method and application. In the preparation method of the formaldehyde-free adhesive of the present invention, castor oil and polyisobutylene are first mixed, and then multifunctional isocyanate is added for reaction, so as to obtain a modified isocyanate with good system compatibility, and both initial adhesion and permanent adhesion meet the use requirements; at the same time, a mixture of polyvinyl pyrrolidone and bisphenol A polyoxyethylene ether in a specific ratio is selected as a composite water-retaining agent, so that the adhesive under the compound formula can achieve strong water absorption and moisture retention, and maintain a moist state for a long time, which not only greatly prolongs the drying time, but also can effectively penetrate into the veneer during pre-pressing to form glue nails to improve the bonding effect.

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

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

[0008] 1) Preparation of modified isocyanate

[0009] Stirring castor oil and polyisobutylene at 60-90° C. to obtain a mixture, preferably for 0.5-2 h; adding multifunctional isocyanate to the mixture, reacting for 2-4 h and then cooling to obtain modified isocyanate;

[0010] 2) Mixing the mixed components consisting of water, flour and composite water retaining agent with modified isocyanate to prepare the adhesive.

[0011] The present invention uses a mixture of castor oil and polyisobutylene to modify isocyanate, and the system has good compatibility and strong adhesion, and can simultaneously meet the use requirements of initial adhesion and permanent adhesion. The present invention also greatly prolongs the drying time of the adhesive by adding a composite water-retaining agent with strong water absorption and moisture retention, and can still ensure the performance requirements of the board even under a longer operating time. In addition, flour containing ingredients such as protein and active starch is added to the adhesive of the present invention at the same time to prevent the adhesive components from excessively penetrating into the wood, and to prevent the glue shortage phenomenon caused by the glue liquid being squeezed out during pressurization due to the short aging time and high resin fluidity. The coordinated cooperation of the above components achieves the characteristics of excellent bonding performance and long drying time, and ensures the excellent performance of the adhesive.

[0012] As a preferred embodiment of the present invention, in the preparation method, the amounts of raw materials used in parts by weight are respectively:

[0013]

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

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

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

[0017] As a preferred embodiment of the present invention, the average molecular weight of the polyvinyl pyrrolidone 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 with a long curing time for plywood, which is prepared according to the method described above.

[0019] The present invention also provides an application of the formaldehyde-free adhesive with a long curing time for plywood described above in the preparation of plywood. The specific application method is, for example, to produce plywood according to the following method:

[0020] (1) applying the adhesive described above to the surface of the veneer; for example, using a four-roller gluing machine to evenly apply the adhesive to the surface of the veneer by double-sided gluing;

[0021] (2) Arranging the veneers after gluing according to the principle of odd number of layers for assembly, and pre-pressing the slabs after aging;

[0022] (3) hot pressing the slab to obtain the plywood;

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

[0024] Preferably, the glue amount per side of the veneer is 150-250g / m 2 ;

[0025] Preferably, the blanks are assembled within 2 hours after the gluing; the pre-pressing is performed within 4 hours after the gluing and the blanks are assembled, the pre-pressing pressure is 0.7-1.0 MPa, and the pre-pressing time is 0.5-6 hours;

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

[0027] Those skilled in the art will appreciate that the hot pressing is performed in a hot press: the slab is heated and pressurized, and water vapor convection is used inside the slab to transfer mass and heat, so that the adhesive is cured and a strong bond is formed.

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

[0029] (1) Provided is a formaldehyde-free adhesive with low cost, moderate viscosity and fluidity, particularly suitable for making plywood by roller coating process, and the produced board has high strength.

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

[0031] (3) The present invention utilizes the permanent viscosity of polyisobutylene and combines it with a composite water-retaining agent composed of polyvinyl pyrrolidone and bisphenol A polyoxyethylene ether to significantly prolong the drying time of the formaldehyde-free adhesive, thereby adapting to the existing plywood production process. DETAILED DESCRIPTION

[0032] The present invention is further described below by means of specific examples. The examples described in the present invention are only used to illustrate the present invention and do not limit the scope of the present 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 about 2.7, viscosity 180-250 cP at 25°C, -NCO content 30.5-32%, Wanhua Chemical Group Co., Ltd.

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

[0036] Polyvinylpyrrolidone B, average molecular weight 58000, Beijing Inokai 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 test methods used in the following embodiments of the present invention are as follows:

[0048] 1. Storage stability evaluation of modified isocyanate: After taking samples of modified isocyanate, place them in ovens at 5°C and 45°C for 60 days to observe whether there is stratification. The results are shown in Table 2.

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

[0050] Table 1. Meaning of numerical calibration

[0051]

[0052]

[0053] 3. Board bonding performance: According to GBT 17657-2013 Test methods for physical and chemical properties of wood-based panels and veneered wood-based panels, the obtained boards were subjected to Class II bonding strength test.

[0054] 4. Formaldehyde emission test: The test is carried out using the dryer method.

[0055] Embodiment 1:

[0056] Prepare the adhesive as follows:

[0057] (1) 10 kg of polyisobutylene PB680 was heated to 75° C.; 10 kg of castor oil was added to the polyisobutylene while stirring, and the temperature was maintained at 75° C. and stirred for 1 hour; 30 kg of PMDI was added to the above mixture, and the mixture was reacted at 75° C. for 3 hours and then cooled to room temperature to obtain modified isocyanate;

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

[0059] Embodiment 2:

[0060] Prepare the adhesive as follows:

[0061] (1) 7 kg of polyisobutylene PB1300 was heated to 60° C.; 7 kg of castor oil was added to the polyisobutylene while stirring, and the temperature was maintained at 60° C. and stirred for 2 h; 26 kg of PMDI was added to the above mixture, and the temperature was raised to 80° C. for reaction for 3 h and then cooled to room temperature to obtain modified isocyanate;

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

[0063] Embodiment 3:

[0064] Prepare the adhesive as follows:

[0065] (1) 8 kg of polyisobutylene PB2400 was heated to 90° C.; 9.5 kg of castor oil was added to the polyisobutylene while stirring, and the temperature was maintained at 90° C. and stirred for 0.5 h; 27 kg of PMDI was added to the above mixture, and the temperature was reduced to 75° C. and reacted for 4 h, and then cooled to room temperature to obtain modified isocyanate;

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

[0067] Embodiment 4:

[0068] Prepare the adhesive as follows:

[0069] (1) 7 kg of polyisobutylene PB2400 was heated to 65° C.; 10 kg of castor oil was added to the polyisobutylene while stirring, and the temperature was maintained at 65° C. and stirred for 1.5 hours; 29 kg of PMDI was added to the above mixture, and the temperature was raised to 80° C. for reaction for 2 hours and then cooled to room temperature to obtain modified isocyanate;

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

[0071] Embodiment 5:

[0072] Prepare the adhesive as follows:

[0073] (1) 8 kg of polyisobutylene PB1300 was heated to 70° C.; 9 kg of castor oil was added to the polyisobutylene while stirring, and the temperature was maintained at 70° C. and stirred for 1 hour; 28 kg of PMDI was added to the above mixture, and the temperature was raised to 80° C. for reaction for 2 hours and then cooled to room temperature to obtain modified isocyanate;

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

[0075] Comparative Example 1:

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

[0077] Comparative Example 2:

[0078] The formaldehyde-free adhesive was prepared in a method substantially the same as that in Example 1, except that the polyisobutylene PB680 in step (1) was replaced by polyisobutylene PB450.

[0079] Comparative Example 3:

[0080] The formaldehyde-free adhesive was prepared in a method substantially the same as that 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 a method substantially the same 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 a method substantially the same as in Example 1, except that the composite water-retaining agent in step (2) was replaced with polyvinyl pyrrolidone A of the same mass.

[0085] Comparative Example 6:

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

[0087] Comparative Example 7:

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

[0089]

Application examples

[0090] The plywood was prepared by using the formaldehyde-free adhesives provided in Examples 1-5 and Comparative Examples 1-7, respectively, in the following manner:

[0091] (a) Use a four-roll gluing machine to evenly apply the adhesive on the surface of the veneer by double-sided gluing, and control the glue application amount on each side to 160g / m 2 ;

[0092] (b) Assembly 2h after gluing: The veneers are assembled according to the principle of odd number of layers, with the fiber directions of adjacent veneers perpendicular to each other, and the number of slab layers is 9;

[0093] (c) Pre-pressing was performed 4 h after assembly, with a pre-pressing pressure of 0.7 MPa and a pre-pressing time of 6 h;

[0094] (d) The pre-pressed slab is sent to a hot press, the hot pressing temperature is 120°C, the hot pressing pressure in the high pressure section is 0.8MPa, the hot pressing pressure in the low pressure section is 0.3MPa, the hot pressing factor is 4min / mm, and the hot pressing time ratio between the high pressure section and the low pressure section is 3:2, thereby obtaining a plywood.

[0095] The storage stability of the modified isocyanate obtained in the preparation process of the formaldehyde-free adhesive in Examples 1-5 and Comparative Examples 1-7 was tested, and the test results are shown in Table 2. In addition, the slab formability, board gluing performance, and formaldehyde emission of the plywood 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 product obtained by uniformly mixing castor oil and polyisobutylene and then removing the modified isocyanate has better storage stability and slab formability, and has higher board strength and qualified rate.

[0100] The test results of Example 1 and Comparative Example 2 show that the molecular weight of the polyisobutylene used should not be lower than 680, otherwise the formability of the slab and the performance of the plate will be reduced.

[0101] The results of Example 1, Comparative Example 3 and Comparative Example 4 show that the addition of polyisobutylene and composite water-retaining agent is an important guarantee for the adhesive to maintain a long drying time. In Example 1, the time interval between the application of glue and pre-pressing is 6 hours, and the adhesive still has good initial adhesion, ensuring the effective forming of the slab and the qualified performance of the board; the system without polyisobutylene in Comparative Example 3 and the system without composite water-retaining agent in Comparative Example 4 completely lose the initial adhesion after 6 hours of application, and the slab is not formed after pre-pressing, resulting in low performance of the board.

[0102] The results of Example 1, Comparative Example 5, Comparative Example 6 and Comparative Example 7 show that when polyvinyl pyrrolidone or bisphenol A polyoxyethylene ether is used alone, or the compounding 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. The two need to be compounded within a specific range to better meet the requirements of the board production process.

[0103] The results of the above embodiments and comparative examples show that the adhesive obtained by uniformly mixing castor oil and polyisobutylene and then modifying isocyanate, and further compounding with water, flour, and a composite water-retaining agent has a long drying time. After the adhesive is applied, the pre-pressed slab still has good formability within 6 hours, and the strength and qualified rate of the board can meet the standard requirements.

[0104] The above is only a preferred embodiment of the present invention. It should be pointed out that ordinary technicians in this field can make several improvements and supplements without departing from the method of the present invention. These improvements and supplements should also be regarded as the scope of protection of the present invention.

Claims

1. A method for preparing a formaldehyde-free adhesive with a long curing time for plywood, characterized in that: The following steps are involved: 1) Preparation of modified isocyanate Stirring castor oil and polyisobutylene at 60-90° C. to obtain a mixture, preferably for 0.5-2 h; adding multifunctional isocyanate to the mixture, reacting for 2-4 h and then cooling to obtain modified isocyanate; 2) Mixing the mixed components consisting of water, flour and composite water retaining agent with modified isocyanate to prepare the adhesive.

2. The method for preparing the formaldehyde-free adhesive with long curing time for plywood according to claim 1, characterized in that: In the preparation method, the amounts of raw materials used in parts by weight are respectively:

3. The method for preparing the formaldehyde-free adhesive with long curing time for plywood according to claim 1 or 2, characterized in that: The multifunctional isocyanate is selected from aromatic polyisocyanates, aliphatic polyisocyanates, alicyclic isocyanates, preferably one or more of toluene diisocyanate, diphenylmethane diisocyanate and polymethylene polyphenyl polyisocyanate.

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

5. The method for preparing the formaldehyde-free adhesive with long curing time for plywood according to claim 1, characterized in that: The composite water-retaining agent is a mixture of polyvinyl pyrrolidone and bisphenol A polyoxyethylene ether, and the mass ratio of the two is 10:(1-2).

6. The method for preparing the formaldehyde-free adhesive with long curing time for plywood according to claim 5, characterized in that: The average molecular weight of the polyvinyl pyrrolidone is ≥8000, and the average molecular weight of the bisphenol A polyoxyethylene ether is ≥492.

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

8. Use of the formaldehyde-free adhesive with a long curing time for plywood according to any one of claims 1 to 6 in the preparation of plywood.

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