A substance that can bond wood to wood or wood to other materials through surface glue. The adhesive joint surface is much larger than the nail joint surface and the tenon joint surface, so the stress is distributed evenly. Adhesives can be used to make artificial boards and glued wood products, thereby making full use of wood and promoting the industrial production of artificial boards.
A brief history
China has a long history of using glue to bond wood. "Century of the Emperors" records that King Zhou Zhao, the fourth king of the Western Zhou Dynasty (1066 BC - 770 BC), visited the south. "The Chu people were evil and used glue boats to enter the king. The king's boat reached the middle stream, and the glue boat was released... ". Qu Yuan (about 340 BC - 278 BC) mentioned this matter in "Tianwen". "Zhou Li Kao Gong Ji" talks about skin glue or bone glue, and also talks about isin glue. The brushes found in the Warring States Period Tombs in Zuojiagang Mountain, Changsha, had rabbit arrow hairs wrapped with raw silk, and were painted on the outside. The raw lacquer was used as an adhesive. In 4000 BC, asphalt was used to glue ivory on Babylonian statues, and casein glue was used in ancient Egypt during the Pharaonic era. In 1754, the United Kingdom issued the first patent for the manufacture of isinglass, and in 1814, the United States issued a patent for bone glue. Phenolic resin became a commercial product in 1919, urea-formaldehyde resin began to be used in 1938, and the patent for melamine adhesive was obtained in the same year. Wood adhesives developed greatly during and after World War II.
Classification
There is no completely unified classification method for wood adhesives. The following are commonly used: ① According to the source of the main ingredients, they are divided into two categories: natural and synthetic. ② According to the chemical structure and performance of the main components, they are divided into thermosetting adhesives, thermoplastic adhesives and rubber adhesives. ③ According to the bonding strength and anti-aging performance, they are divided into: structural adhesives such as resorcinol resin glue, phenolic resin glue, melamine resin glue, isocyanate glue, epoxy resin glue, etc.; non-structural adhesives such as urea-formaldehyde resin glue, polyvinyl acetate Emulsion, protein glue, etc. ④ According to the physical state of the adhesive, it is divided into liquid, solid, powder and film. ⑤ According to water resistance, it is divided into: advanced water-resistant glue such as resorcin resin glue, phenolic resin glue, epoxy resin glue, etc.; water-resistant glue such as urea-formaldehyde resin glue, cross-linked polyvinyl acetate emulsion, blood glue, etc.; non-water-resistant glue Such as polyvinyl acetate emulsion, soy protein glue, etc. ⑥According to the bonding process, it can be divided into pressure-sensitive type, rewetting type, hot-melt type and reactive type.
The influence of adhesive on bonding
The performance of the main component of the adhesive determines the bonding effect. The excellent properties of resorcin resin adhesives such as high bonding strength, water resistance, and aging resistance are all determined by the properties of resorcin resin. Soy protein glue is not strong and water-resistant, but it is odorless, which is the characteristic of soy protein. The proportion of each component in the adhesive must be appropriate. If the resin content is too low, the bonding degree of the glue will be low; glue that is all resin without fillers is neither economical nor necessarily the highest strength. The adhesive must be able to soak into the wood and spread to achieve a good bonding; if the wettability is too high, a large amount of glue will penetrate into the wood, resulting in a lack of glue on the bonding surface and a large amount of glue consumption. The wettability is closely related to the viscosity of the glue. If the viscosity is high, the wettability is poor. Both strong acids and alkalis destroy wood, so the adhesive should have a suitable pH. Adhesives with too short active periods can easily cause gel or pre-curing, leading to poor bonding; curing speed is too slow, which wastes time. In addition, the curing conditions and degree of curing of the adhesive directly affect the bonding quality.
Selection of adhesive
Generally, the following factors should be considered comprehensively. Whether the bonding object is between wood, or between wood and other materials, and what kind of tree it is; whether the bonding form is plane bonding, end connection, or other forms; the usage status of the bonded product, whether it is a long-term load, whether it is used indoors or outdoors, and whether No special requirements. In addition, the equipment and process conditions of bonding, the cost of bonding, the performance of the adhesive (see the table on the previous page), etc., must be carefully considered.
Outlook
The development of society will inevitably require the development of wood adhesives with multiple vectors, high quality, no (low) toxicity, low price, and easy to use. It can be expected that new situations will emerge in the following two aspects: first, conduct in-depth research on existing adhesives, clarify the mechanism, and make breakthrough progress in modification; second, develop new adhesives, especially those using renewable resources as raw materials. Tannin, lignin, starch, etc. In recent years, tannin gum has been used in industrial production in Australia, South Africa and other countries.
The scale of trials of glue-free artificial panels is getting larger and larger, and industrial production is expected in the near future. Research on making adhesives using polysaccharides (starches) as raw materials has progressed and is promising.