WO2010056213A1

WO2010056213A1 - Adhesive with the addition of liquid wood and the process of its preparation

Info

Publication number: WO2010056213A1
Application number: PCT/SI2009/000023
Authority: WO
Inventors: Vesna Tisler; Sergej Medved; Matjaz Kunaver
Original assignee: Kemijski Institut
Priority date: 2008-11-17
Filing date: 2009-05-29
Publication date: 2010-05-20
Also published as: DE112009002726T5; AT513075A1; AT513075B1; SI22912A

Abstract

The topic of the invention is an adhesive with the addition of liquid wood - that is liquefied lignocellulosic materials and/or wood, as a component of adhesives for the mass production of products made of bonded wood with a lower content of formaldehyde and the process of its preparation. The invention also allows the curing of the adhesive at lower temperatures thus resulting in energy savings. The adhesive is characterized by containing a melamine, melamine-formaldehyde, melamine-urea-formaldehyde or urea-formaldehyde adhesive or a combination of these adhesives and liquefied wood, that is added up to 50 % (w/w), where the liquefied wood is prepared from materials containing cellulose.

Description

ADHESIVE WITH THE ADDITION OF LIQUID WOOD AND THE PROCESS OF ITS PREPARATION

The topic of the invention is an adhesive with the addition of liquid wood - that is, liquefied lignocellulosic materials and/or wood, as a component of adhesives used in the mass production of products made from bonded wood, with a lower content of formaldehyde, and the process of its preparation. The invention enables the curing of the adhesive at a lower temperature resulting in savings in energy.

State of the art

Products made from bonded wood are used in everyday life. One of the typical and most commonly used products is particleboard, which is used in the furniture industry for the production of the majority of furniture elements. Particleboard is made in a continuous process with the use of wood particles and various adhesives. Wood particles are first impregnated with the adhesive, after which the obtained mass is compressed in a press at 180 °C or above so that curing of the adhesive occurs.

Existing adhesives that are used in the mass production of particleboards and fiberboards are mainly urea-formaldehyde adhesives. Other adhesives, such as e.g. melamine-formaldehyde adhesives, epoxide adhesives, and polyurethane adhesives, have a number of weaknesses that limit their use or do not allow the production of user-friendly products, especially at costs suitable for mass production.

Adhesives synthesized on the basis of urea and formaldehyde emit formaldehyde upon bonding. This is true for furniture, particleboards and other products made from wood and similar materials for which the named adhesives are used. Formaldehyde evaporates from such materials for many days or even weeks. That is why we can sense a specific odor of formaldehyde upon entering a space with new furniture. The high content of formaldehyde is the reason why products bonded with these adhesives are environmentally problematic and harmful to health, although, despite these problems, their use is widespread due to their relatively low price. On the other hand, melamine-formaldehyde adhesives are more environmentally and user friendly since they contain less formaldehyde in the final products. However, their prices are higher and consequently their use is smaller.

Due to the harmful nature of formaldehyde, which is most problematic in widely used products such as particleboards, there are relatively strict limitations concerning the content of formaldehyde in adhesives and products. Standards for particular wood-board composites, where the use of such adhesives is greatest, also define the quality of these products with regard to free formaldehyde emissions. In wood-board composites that are considered to be environmentally and user friendly the concentrations of formaldehyde should not exceed values defined for emission class El. Particleboards and fiberboards that are most commonly used in the production of furniture should not emit more than 8 mg formaldehyde / 100 g board. The standards that define the emission class El are: SIST EN 312 or SIST EN 622-1, SIST EN 622- 2, SIST EN 622-3, SIST EN 622-4, and SIST EN 622-5. In member states of the EU only boards that can be classified as emission class El should be used.

Epoxide adhesives and polyurethane adhesives do not possess the same weakness, but their use is problematic since they are two-component adhesives and their use has time limitations. When the user mixes both components, the adhesive must be used within several hours otherwise the adhesive will cure and, in the worst-case scenario, damage equipment. The price of such adhesives is at least two times higher than that of formaldehyde based adhesives. The process of curing a urea-formaldehyde adhesive requires raised temperatures, most commonly above 180 ⁰C, so that the energy use for the heating of press plates is high. Any lowering of the temperature has a negative impact on the mechanical properties of the products. There is therefore a constant problem in terms of the development of low-cost and environmentally friendly adhesives with low contents of toxic compounds such as formaldehyde. This problem is particularly apparent with adhesives that are used in the bonding of wood and cellulose materials and in the production of products such as particleboard, fiberboard, veneer board, and other products made from bonded wood, paper etc. — products with which consumers are in constant contact.

Due to the decline in crude oil deposits, and the resulting higher costs for raw materials for the production of adhesives, there have been some attempts to use renewable resources for adhesives. One of the renewable resources is also liquefied wood. Various authors have tried to modify adhesives with the use of liquefied wood, for example, by adding liquefied wood to polyurethane adhesive or by making epoxides of liquefied wood and adding it into phenol- formaldehyde adhesives. These attempts have not succeeded in significantly removing the weaknesses of these adhesives. Phenol-formaldehyde adhesives with the addition of liquefied wood still contain toxic phenol and formaldehyde, while in epoxy adhesives based on liquefied wood the epoxidation introduces an additional step in the synthesis making the adhesives more expensive. Like epoxy adhesives, polyurethane adhesives are two-component adhesives and contain toxic diisocyanates. These adhesives are also expensive and have a limited use time. This reduces their usefulness and thus these adhesives are not used in mass production. Polyurethane adhesives have similar weaknesses.

Literature Examples

Liquefaction of ground bamboo using phenol, the preparation of a phenol-formaldehyde adhesive and its use in preparing plywood boards (Fu, Shenyuan, Ma Lingfei, Li Wenzhu,

Cheng Shuna, Front. For. China (2006) 2: 219-224).

Preparation and characterization of adhesives on the basis of liquefied wood and isocyanates

(Shin-ichiro Tohmura, Gai-yun Li, Te-fu Qin, J. Appl. Polym. Sci. (2005) 98: 791-795).

Use of liquefied wood for the preparation of an adhesive (Hajime Kishi, Akira Fujita, Hikaru

Miyazaki, Satshi Matsuda, Atsushi Murakami, J. Appl. Polym. Sci. (2006) 102: 2285-2292).

Preparation of liquefied bark with phenol, preparation of a resol adhesive, preparation of particleboard (Wen-Jau Lee, Cheng-Tzu Liu, J. Appl. Polym. Sci. (2003) 87: 1837-1841).

The problem that has thus far not been suitably solved is the preparation of a low cost adhesive, possibly on the basis of renewable resources, with a low content of formaldehyde or other toxic compounds that will be used for the mass production of products such as particleboard, fiberboard, veneerboard, and other products from bonded wood. In addition the adhesive should cure at a lower temperature than currently known adhesives, since a high setting temperature represents a large part of production costs.

The task and goal of the invention is the preparation of a low cost adhesive, possibly on the basis of renewable resources, with a low content of formaldehyde or other toxic compounds that will be used for mass production of products such as particleboard, fiberboard, veneerboard, and other products from bonded wood. In addition, the adhesive should cure at a lower temperature than currently known adhesives. The invention solves the task through independent claims. The invention presents a method by which renewable resources such as wood/ lignocellulosic materials can be used for the preparation of an adhesive that has a lower content of formaldehyde in the final products and that cures at lower temperatures than existing adhesives.

Explanation of used terms

Liquified wood: a liquid product of a reaction of polyfunctional alcohols with lignocellulosic materials, i.e. materials that contain cellulose in excess of 40 % dry matter, hemicellulose and/or lignin, and are classified as renewable resources.

Wood liquefaction: the preparation of liquefied wood that takes place in heated inert reactor containers with the use of a stirrer. In the process of preparing liquefied wood one of the input materials is finely ground wood, but similar materials can also be used, such as cellulose, cotton, paper, starch or wood remains after the production of wood components such as lignin, tannin, and furfural. The main input materials used in the production of liquefied wood in addition to the above mentioned wood and similar materials are glycerol, ethylene glycol, diethylene glycol, triethylene glycol, propylene glycol, or other polyfunctional alcohols, and concentrated sulfuric (VI) acid (98 %) or para-toluenesulfonic acid. An example of a laboratory synthesis of liquefied wood:

A 1000 mL three-neck glass reactor container with ground glass joints is filled with 100 g wood and 300 g glycerol. 9 g concentrated sulfuric (VI) acid is added. The mixture is heated for 1.5 hours at 160 °C while being constantly stirred.

The product of the synthesis is liquefied wood, which is a highly viscous liquid, dark brown in color.

Adhesive for the bonding of wooden, plastic or other metal materials: a liquid polymer mixture that reacts at raised temperature and becomes solid, thus permanently binding the material to which it was previously applied.

The term "melamine, melamine-formaldehyde, melamine-urea-formaldehyde or urea- formaldehyde adhesive" relates to the type of adhesive that is used for the bonding of various materials, primarily wood, paper, textile, and other materials.

The invention is additionally described with examples that in no way limit the invention but present some of the various uses of the invention. Example 1

The viscosity of the liquefied wood is first reduced with the addition of distilled water. A reduction in viscosity is needed for applications using nozzles as well as for improved mixing with a synthetic adhesive and other components of the adhesive mixture such as, for example, the inhibitor, hydrophobic agent, etc. The process of preparing the adhesive is carried out by mixing 50 % (w/w) urea-formaldehyde adhesive or melamine-formaldehyde adhesive with 50 % (w/w) liquefied wood. Any stirrer for the mixing of liquids with viscosities below 20 Pa.s. can be used. The container in which mixing takes place must be made of a material resistant to acids. At room temperature, the obtained mixture is stable for 72 hours. The suitably prepared adhesive mixture, i.e. synthetic adhesive + liquefied wood + additives, is applied through nozzles onto wood particles in a machine for wood particle impregnation. The addition of the adhesive mixture is between 7 and 13 % of the mass of wood particles and depends mainly on the size of the wood particles and the layer of the board. The time of impregnation and mixing is between 2 and 5 minutes. After the finished impregnation, the impregnated wood particles are formed into a wood particle loaf. The wood particle loaf is formed on a metal sheet. The prepared wood particle loaf + metal sheet are placed in a press. The binding or pressing time depends on the type of adhesive, and the thickness and density of the board. For 16 mm thick boards the binding time is 4 minutes. The temperature of the press plates is 180 ⁰C, the pressure is 3 N/mm².

Example 2 - Preparation of boards

Appropriately dried wood particles with a moisture content of 2-4 % are impregnated: the middle layer with 7.5 % of the adhesive mixture and the outer layer with 11.5 % of the adhesive mixture. The adhesive mixture is composed of: adhesive: 20 to 75 % liquefied wood: 0 to 50 % curing agent: 0 to 10 % hydrophobic agent: 7 to 15 % water: 15 to 20 %

The composition of the adhesive depends mostly on the layer in which the wood particles are used, since the adhesive mixture used for wood particles in the outer layer contained no curing agent. The impregnation and mixing time is 5 minutes. After the finished impregnation the wood particles are formed into a wood particle loaf. The wood particle loaf is formed on a 500 x 500 mm metal sheet. The formed wood particle loaf and the metal sheet are placed in a press together. The bonding or pressing time is 4 minutes. The nominal thickness is 16 mm, and the nominal density 0.7 g/cm³. The temperature of the heating plates is set to 180 °C, and the pressure is 3 N/mm².

After finishing the pressing and conditioning, which takes 5 days, tests of some of the mechanical and physical properties of the boards are performed:

Table 1. Mechanical and physical properties of boards prepared with an adhesive that contained the addition of 50 % (w/w) liquefied wood.

Claims

CLAIMS:

1. An adhesive with the addition of liquefied wood for use in the production of wooden products characterized by containing a melamine, melamine-formaldehyde, melamine- urea-formaldehyde or urea-formaldehyde adhesive or a combination of these adhesives, and liquefied wood, that is added up to 50 % (w/w), where the liquefied wood is prepared from materials that contain cellulose.

2. An adhesive according to claim I₃ characterized by that the liquefied wood is prepared from lignocellulosic materials such as plant stems, particularly wood, straw, hay, bark and needles, leaves, fruits, seed shells, cones, paper, starch, cotton, cellulose, wood flour, products and waste from these materials or wood remains after the production of wood components such as lignin, tannin, and furfural.

3. An adhesive according to claims 1 and 2, characterized by that the bonding temperature of this adhesive is below 180 °C.

4. The process of preparing an adhesive based on liquefied wood, characterized by consisting of mixing of a urea-formaldehyde adhesive or melamine-formaldehyde adhesive or melamine-urea-formaldehyde adhesive with liquefied wood, where the viscosity of the liquefied wood was previously appropriately reduced by the addition of distilled water, and a stirrer was used appropriate for stirring liquids with viscosities below 20 Pa.s., and that the container in which the mixture was mixed must be made of a material resistant to acids.