Preparation method of starch-polyvinyl alcohol-based room temperature curing wood adhesive for plywood

Abstract

The invention discloses a preparation method of a starch-polyvinyl alcohol-based room temperature curing wood adhesive for plywood, and belongs to the technical field of wood adhesives. At present, wood adhesives mainly come from petrochemical resources and mostly need hot-pressing curing, the invention provides the preparation method of the wood adhesive with starch and polyvinyl alcohol as main materials, and heat-free bonding can be achieved. The preparation method comprises the following steps: firstly, reacting corn starch with isocyanate to obtain carbamic acid esterified starch, then adding a polyvinyl alcohol aqueous solution, uniformly stirring to obtain a starch-polyvinyl alcohol-based room-temperature curing wood adhesive, uniformly coating a wood veneer with the starch-polyvinyl alcohol-based room-temperature curing wood adhesive, forming an odd-layer plate blank according to the principle that the fiber directions of adjacent layers are vertical, and then carrying out cold press molding to obtain the wood veneer. The starch-polyvinyl alcohol-based room temperature curing plywood can be obtained. The prepared starch-polyvinyl alcohol-based room temperature curing plywood has the advantages of being easy to prepare, high in bonding strength, good in water resistance, free of formaldehyde, free of hot pressing and the like, and has a good market popularization prospect.

Description

Technical Field

[0001] The present invention relates to a preparation method of a starch-polyvinyl alcohol-based room-temperature curing wood adhesive for plywood, belonging to the technical field of wood adhesives. Background Art

[0002] At present, the wood adhesives sold on the market are mainly synthetic resins based on petrochemical products. The use of such adhesives will release formaldehyde and volatile organic compounds (VOCs) into the environment. It is worth noting that the International Agency for Research on Cancer has classified formaldehyde as a known carcinogen. Therefore, the development and application of adhesives based on renewable resources not only reduce the environmental burden but also conform to the current global trend of pursuing sustainable development. Biomass resources are of great significance in replacing traditional petroleum-based products and in the application in the wood industry.

[0003] Starch is an abundant natural polymer with the advantages of being renewable and widely sourced. It is also the cheapest carbohydrate in the industry and can be effectively used in bio-based adhesives. However, numerous hydroxyl groups in starch molecules will simultaneously promote the formation of hydrogen bonds between starch and water molecules, resulting in insufficient water resistance of unmodified starch as an adhesive. Polyvinyl alcohol is a long-chain polymer with the characteristics of being degradable, having a multi-hydroxyl structure, and excellent compatibility. Starch-polyvinyl alcohol-based adhesives have good compatibility, but their water resistance and bonding strength are relatively poor, and the curing temperature for preparing plywood is high, which is not conducive to industrial promotion. Summary of the Invention

[0004] To solve the deficiencies of the prior art, the purpose of the present invention is to provide a preparation method of a starch-polyvinyl alcohol-based room-temperature curing wood adhesive for plywood.

[0005] To achieve the above objectives, the present invention adopts the following technical solutions:

[0006] A preparation method of a starch-polyvinyl alcohol-based room-temperature curing wood adhesive for plywood, the structural schematic diagram is as Figure 1 shown;

[0007] The preparation method of the above adhesive includes the following steps:

[0008] (1) Mix corn starch with isocyanate and stir at a speed of 11000 rpm for 30 min at room temperature to obtain a carbamylated starch prefabricated material;

[0009] (2) Mix polyvinyl alcohol with distilled water and stir at a speed of 150 rpm for 3 h at 90 - 100 °C to obtain an aqueous solution of polyvinyl alcohol;

[0010] (3) The prefabricated material prepared in step (1) and the polyvinyl alcohol aqueous solution prepared in step (2) are fully mixed and stirred to obtain a starch-polyvinyl alcohol-based room temperature curing wood adhesive.

[0011] Furthermore, the isocyanate in the above step (1) is one or more of diphenylmethane diisocyanate, polymethylene polyphenyl polyisocyanate, toluene diisocyanate, hexamethylene diisocyanate, and isophorone diisocyanate.

[0012] Furthermore, the concentration of the polyvinyl alcohol aqueous solution in the above step (2) is 10 wt %.

[0013] Furthermore, when the adhesive is used to prepare plywood, the starch-polyvinyl alcohol-based room temperature curing wood adhesive is double-sidedly coated and evenly applied on the surface of the wood veneer, with an application amount of 200 to 250 g / m 2 , according to the principle that the wood grains of two adjacent layers of veneer are perpendicular, three layers of slabs are formed, and starch-polyvinyl alcohol-based room temperature curing plywood is obtained by cold pressing.

[0014] Furthermore, when the adhesive is used to prepare plywood, the wood veneer can be one of poplar, eucalyptus, basswood, paulownia rubber wood, oak olive, white sycamore, and crabapple wood.

[0015] Furthermore, when the adhesive is used to prepare plywood, the cold pressing is performed by using a press to pressurize the plywood at room temperature for 2 to 12 hours at a pressure of 2 to 3 MPa.

[0016] Furthermore, when the adhesive is used to prepare plywood, the plywood needs to be placed in an environment for 24 hours after being pressed to reduce internal stress.

[0017] The advantages of the present invention are:

[0018] (1) The adhesive prepared by the present invention is prepared by directly blending unmodified starch with a cross-linking agent to prepare a starch-polyvinyl alcohol-based composite adhesive, and the preparation method is simple and fast.

[0019] (2) The adhesive prepared by the present invention does not contain formaldehyde and the raw materials are degradable, which conforms to the green concept of sustainable development.

[0020] (3) When the plywood pressed by the adhesive prepared by the present invention is prepared by cold pressing in a room temperature curing manner. Under room temperature conditions, the adhesive is pressed with a press at 2 to 3 MPa for 2 to 12 hours, and the bonding strength can reach the requirements of Class II plywood in GB / T9846-2015. It has the advantages of high strength, good water resistance and low energy consumption. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of the structure of a starch-polyvinyl alcohol based room temperature curing wood adhesive for plywood.

[0022] Figure 2 This is the infrared spectrum of starch mixed with isocyanate.

[0023] Figure 3 This is the X-ray diffraction pattern of starch mixed with isocyanate.

[0024] Figure 4 This is a scanning electron microscopy image of the adhesive curing cross section.

[0025] Figure 5 The graph shows the bonding strength of different starch-isocyanate addition amounts.

[0026] Figure 6 The bonding strength diagram for different cold pressing times. DETAILED DESCRIPTION

[0027] In order to make the contents of the present invention easier to understand, the technical solution of the present invention is further described below in conjunction with specific implementation methods, but the present invention is not limited thereto.

[0028] Example 1

[0029] (1) 100 parts of corn starch and 10 parts of isocyanate were mixed, and stirred at room temperature at a speed of 11000 rpm for 30 min to obtain a starch preform;

[0030] (2) mixing 40 parts of polyvinyl alcohol with 360 parts of distilled water, and stirring at 90-100° C. and 150 rpm for 3 h to obtain a polyvinyl alcohol aqueous solution;

[0031] (3) The prefabricated material prepared in step (1) and the polyvinyl alcohol aqueous solution prepared in step (2) were fully mixed and stirred, and the sizing amount was 220 g / m 2 It is evenly applied on the wood veneer, and three-layer slabs are formed according to the principle that the fiber directions of adjacent layers are perpendicular. Plywood is obtained by cold pressing at room temperature and 2.5MPa for 4 hours.

[0032] In Example 1, the infrared spectrum of starch and isocyanate after mixing is as follows: Figure 2 As shown in the figure, the isocyanate group (-NCO) appears at 2260-2280cm after starch is mixed with isocyanate. -1 The absorption peaks of benzene ring and its derivatives at 1570cm -1 The absorption peak near , indicating the successful access of isocyanate. The X-ray diffraction pattern of starch mixed with isocyanate is shown in Figure 3As shown, the addition of isocyanate increases the crystallinity of the starch powder, indicating that the amorphous regions of the starch are destroyed. The cured cross-section of the adhesive was scanned by electron microscopy. After curing, the hard shell-like structure formed by polyvinyl alcohol tightly wraps the starch granules through the linkage of isocyanate, providing water resistance and mechanical strength for the adhesive.

[0033] Example 2

[0034] As shown in Example 1, the difference is the different addition amounts of starch-isocyanate.

[0035] (1) 40 parts of corn starch and 4 parts of isocyanate were mixed and stirred at a speed of 11000 rpm for 30 min at room temperature to obtain a starch prefabricated material;

[0036] (2) 36 parts of polyvinyl alcohol and 324 parts of distilled water were mixed and stirred at a speed of 150 rpm for 3 h at 90-100 °C to obtain an aqueous polyvinyl alcohol solution;

[0037] (3) The prefabricated material prepared in step (1) and the aqueous polyvinyl alcohol solution prepared in step (2) were fully mixed and stirred, and uniformly coated on the wood veneer at a sizing amount of 220 g / m 2 According to the principle that the fiber directions of adjacent layers are perpendicular, a three-layer plywood blank was formed, and cold-pressed at room temperature and 2.5 MPa for 4 hours to obtain plywood.

[0038] In Example 2, the performance of the room-temperature-cured plywood prepared can be obtained from Figure 5 As can be seen from the figure, the dry shear strength of the plywood is 1.4456 MPa, and the wet shear strength is 0.7433 MPa.

[0039] Example 3

[0040] As shown in Example 1, the difference is the different addition amounts of starch-isocyanate.

[0041] (1) 90 parts of corn starch and 9 parts of isocyanate were mixed and stirred at a speed of 11000 rpm for 30 min at room temperature to obtain a starch prefabricated material;

[0042] (2) 21 parts of polyvinyl alcohol and 189 parts of distilled water were mixed and stirred at a speed of 150 rpm for 3 h at 90-100 °C to obtain an aqueous polyvinyl alcohol solution;

[0043] (3) The prefabricated material prepared in step (1) and the aqueous polyvinyl alcohol solution prepared in step (2) were fully mixed and stirred, and uniformly coated on the wood veneer at a sizing amount of 220 g / m 2 According to the principle that the fiber directions of adjacent layers are perpendicular, a three-layer plywood blank was formed, and cold-pressed at room temperature and 2.5 MPa for 4 hours to obtain plywood.

[0044] In Example 3, the properties of the prepared room temperature cured plywood can be seen from Figure 5 It can be seen from the figure that the dry shear strength of plywood is 0.9760MPa and the wet shear strength is 0.5821MPa.

[0045] Example 4

[0046] As shown in Example 1, the difference is that the cold pressing time is different.

[0047] (1) 100 parts of corn starch and 10 parts of isocyanate were mixed, and stirred at room temperature at a speed of 11000 rpm for 30 min to obtain a starch preform;

[0048] (2) mixing 40 parts of polyvinyl alcohol with 360 parts of distilled water, and stirring at 90-100° C. and 150 rpm for 3 h to obtain a polyvinyl alcohol aqueous solution;

[0049] (3) The prefabricated material prepared in step (1) and the polyvinyl alcohol aqueous solution prepared in step (2) were fully mixed and stirred, and the sizing amount was 220 g / m 2 It is evenly applied on the wood veneer, and three-layer slabs are formed according to the principle that the fiber directions of adjacent layers are perpendicular. Plywood is obtained by cold pressing at room temperature and 2.5MPa for 6 hours.

[0050] In Example 4, the properties of the prepared room temperature cured plywood can be seen from Figure 6 It can be seen from the figure that the dry shear strength of plywood is 1.9207MPa and the wet shear strength is 1.1110MPa.

[0051] Example 5

[0052] As shown in Example 1, the difference is that the cold pressing time is different.

[0053] (1) 100 parts of corn starch and 10 parts of isocyanate were mixed, and stirred at room temperature at a speed of 11000 rpm for 30 min to obtain a starch preform;

[0054] (2) mixing 40 parts of polyvinyl alcohol with 360 parts of distilled water, and stirring at 90-100° C. and 150 rpm for 3 h to obtain a polyvinyl alcohol aqueous solution;

[0055] (3) The prefabricated material prepared in step (1) and the polyvinyl alcohol aqueous solution prepared in step (2) were fully mixed and stirred, and the sizing amount was 220 g / m 2 It is evenly applied on the wood veneer, and three-layer slabs are formed according to the principle that the fiber directions of adjacent layers are perpendicular. Plywood is obtained by cold pressing at room temperature and 2.5MPa for 8 hours.

[0056] In Example 5, the properties of the prepared room temperature cured plywood can be seen from Figure 6 It can be seen from the figure that the dry shear strength of plywood is 1.7830MPa and the wet shear strength is 0.9726MPa.

[0057] Example 6

[0058] As shown in Example 1, the difference is that the cold pressing time is different.

[0059] (1) 100 parts of corn starch and 10 parts of isocyanate were mixed, and stirred at room temperature at a speed of 11000 rpm for 30 min to obtain a starch preform;

[0060] (2) mixing 40 parts of polyvinyl alcohol with 360 parts of distilled water, and stirring at 90-100° C. and 150 rpm for 3 h to obtain a polyvinyl alcohol aqueous solution;

[0061] (3) The prefabricated material prepared in step (1) and the polyvinyl alcohol aqueous solution prepared in step (2) were fully mixed and stirred, and the sizing amount was 220 g / m 2 It is evenly applied on the wood veneer, and three-layer slabs are formed according to the principle that the fiber directions of adjacent layers are perpendicular. Plywood is obtained by cold pressing at room temperature and 2.5MPa for 10 hours.

[0062] In Example 6, the properties of the prepared room temperature cured plywood can be seen from Figure 6 It can be seen from the figure that the dry shear strength of plywood is 1.8301MPa and the wet shear strength is 1.0358MPa.

[0063] Comparative Example 1

[0064] As in Example 1, except that no starch-isocyanate was added.

[0065] 40 parts of polyvinyl alcohol and 360 parts of distilled water were mixed and stirred at 90-100°C at a speed of 150 rpm for 3 hours to obtain a polyvinyl alcohol aqueous solution. The sizing amount was 220 g / m 2 It is evenly applied on the wood veneer, and three-layer slabs are formed according to the principle that the fiber directions of adjacent layers are perpendicular. Plywood is obtained by cold pressing at room temperature and 2.5MPa for 4 hours.

[0066] In Comparative Example 1, the properties of the prepared room temperature cured plywood can be seen from Figure 5 It can be seen from the figure that the dry shear strength of plywood is 0.6807MPa and the wet shear strength is 0.0400MPa.

[0067] The wet bonding strength obtained in the above tests meets the strength requirements (≥0.7 MPa) of Class II national plywood GBT 9846-2015 when the addition amount of starch-isocyanate and the cold pressing time are appropriate. Therefore, the room temperature curable starch-polyvinyl alcohol adhesive prepared by the present invention has a completely qualified bonding strength and excellent performance.

[0068] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the above embodiments do not limit the present invention in any form. Any technical solutions obtained by using equivalent replacements or equivalent transformations fall within the protection scope of the present invention.

Claims

1. A method for preparing a starch-polyvinyl alcohol-based room temperature curing wood adhesive for plywood, characterized in that: Corn starch is reacted with isocyanate to obtain carbamate starch, and then a polyvinyl alcohol aqueous solution is added and stirred to obtain a starch-polyvinyl alcohol-based room temperature curing wood adhesive; the method specifically comprises the following steps: (1) corn starch and isocyanate are mixed, and stirred at room temperature at a rotation speed of 11000 rpm for 30 minutes to obtain a carbamate-treated starch preform; (2) mixing polyvinyl alcohol with distilled water, and stirring at 90-100° C. at a rotation speed of 150 rpm for 3 h to obtain a polyvinyl alcohol aqueous solution; (3) The prefabricated material prepared in step (1) and the polyvinyl alcohol aqueous solution prepared in step (2) are fully mixed and stirred to obtain a starch-polyvinyl alcohol-based room temperature curing wood adhesive. The amount of each material used includes, by weight: 40 to 100 parts of corn starch, 4 to 10 parts of isocyanate, 21 to 40 parts of polyvinyl alcohol, and 189 to 360 parts of distilled water.

2. The method for preparing a starch-polyvinyl alcohol-based room temperature curing wood adhesive for plywood according to claim 1, characterized in that: The isocyanate in step (1) is one or more of diphenylmethane diisocyanate, polymethylene polyphenyl polyisocyanate, toluene diisocyanate, hexamethylene diisocyanate, and isophorone diisocyanate.

3. The method for preparing a starch-polyvinyl alcohol-based room temperature curing wood adhesive for plywood according to claim 1, characterized in that: The concentration of the polyvinyl alcohol aqueous solution in step (2) is 10 wt %.

4. The starch-polyvinyl alcohol-based room temperature curing wood adhesive for plywood according to claim 1, characterized in that: When preparing plywood, the starch-polyvinyl alcohol-based room temperature curing wood adhesive is double-sidedly coated and evenly applied on the surface of the wood veneer, with an application amount of 200-250g / m 2 , according to the principle that the wood grains of two adjacent layers of veneer are perpendicular, three layers of slabs are formed, and starch-polyvinyl alcohol-based room temperature curing plywood is obtained by cold pressing.

5. The starch-polyvinyl alcohol-based room temperature curing wood adhesive for plywood according to claim 4, characterized in that: When preparing plywood, the wood veneer can be one of poplar, eucalyptus, basswood, paulownia, rubber wood, oak olive, white sycamore, and crabapple.

6. The starch-polyvinyl alcohol-based room temperature curing wood adhesive for plywood according to claim 4, characterized in that: When preparing the plywood, the cold pressing is performed by applying a pressure of 2 to 3 MPa to the cold pressing machine for 2 to 12 hours at room temperature.

7. The starch-polyvinyl alcohol-based room temperature curing wood adhesive for plywood according to claim 4, characterized in that: When preparing plywood, the plywood needs to be placed in the environment for 24 hours after being pressed to reduce internal stress.

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