Phenolic resin is the first synthetic resin. In 1872, A.von Bayer found that phenol and formaldehyde can form resin under the catalysis of acid. In 1883, A.Michael reported that alkali can also catalyze resins. 1907, Baekeland obtained the patent of hot pressing process, which promoted the application of phenolic resin. Phenolic resin 1909 is used as wood coatings. Made of 19 10 phenolic paper laminate. 19 15 had a patent for phenolic fiberboard. Since then, the theoretical research and practical application of phenolic resin have been greatly developed.
reaction mechanism
Due to the different pH values of catalyst and reaction system, the reaction between phenol and formaldehyde has the following types:
Strong acid catalysis
Formaldehyde aqueous solution is an equilibrium system, which contains the condensate of formaldehyde monomer and formaldehyde hydrate ethylene glycol. In acidic solution, methylglycol generates hydroxymethylene cation in the first step, and then reacts with phenol to generate hydroxymethylphenol, and the reaction is slow. Under acidic conditions, the generated hydroxymethylphenol quickly reacts with another phenol molecule, and condensation reaction begins, and the molecular chain is gradually elongated into linear resin, so the phenolic reaction catalyzed by strong acid is quite intense. The equation is as follows:
Metal ion catalysis
In a certain pH range (pH 4 ~ 7), when divalent metal salts are used as catalysts, such as acetate of calcium, magnesium and zinc, phenol is excessive. This kind of resin is characterized by fast curing speed and difficult control of reaction heat during production. The equation is as follows:
Alkali catalysis
The molar ratio of formaldehyde to phenol is 1.5 ~ 3.0, and most of them are greater than 1.5. Using sodium hydroxide as catalyst, first-class phenolic resin (first-class phenol for short) can be produced. The recognized reaction process is:
Under the conditions of lower than 60℃ and higher pH value, the condensation reaction of hydroxymethylphenol is slow, and the resin is usually formed at 60 ~ 100℃.
Ammonia catalysis
Ammonia catalyzes the first-order phenol, which is yellow, indicating that it has an imino group (-—ch = n—-) and has a large molecular weight, which is obviously different from other alkali-catalyzed reactants. Ammonia-catalyzed resins are known to contain structures similar to dibenzylamine and triphenylamine.
Development of resin preparation
Emulsion phenolic resin was applied in 1970s. It uses water as the medium and has no pollution; The molecular weight is much larger than that of water-soluble resin, and its performance is good. There are usually two preparation methods: one is to generate phenolic resin under the catalysis of alkali, and then suspend the resin in water with protective colloid under the condition of near neutrality to further improve the molecular weight. From 65438 to 0975, China has been used to produce plywood cement formwork. The second is to concentrate the resin to a predetermined molecular weight range under alkaline conditions, add emulsifier, and make it into emulsion through strong mechanical stirring. The emulsion is also used for nylon dry net treatment on paper machines in paper mills. Because the problem of reaction heat is solved, high ortho-novolak resin can be produced.
Resin curing
The curing of phenolic resin is a process from soluble liquid to insoluble solid. So far, people are not very clear about this very complicated curing process. Methylphenol can be cured by heating (under alkaline conditions) or adding acid. It is generally believed that there are two stages of curing. First, methylene ether 8967 is formed, then the ether bond is broken and the degree of crosslinking is increased. There are different opinions on how to break the ether bond. The preferred viewpoint is splitting into methylene quinone and then crosslinking. The curing mechanism of cresol under acidic conditions is similar to the formation of linear phenolic aldehyde, and the reaction speed is very fast, and a large amount of heat release in turn promotes curing.
Phenolic linear resin does not have enough hydroxymethyl, so it is a thermoplastic resin. It can only be cured by adding components containing methylene, such as hexamethylenetetramine. Its mechanism is not completely clear, and it is inferred that the cured resin contains imine groups only from its yellow color.
The cured phenolic resin is amorphous, its strength is lower than the theoretical calculation value, and the micrograph shows the existence of microbubbles. Some researchers think that there is not much cross-linking, and some infer that the cured phenolic resin has two phases, a skeleton formed by high molecular compounds and low molecular compounds filled in the skeleton.
App application
Phenolic resin has high adhesive strength, excellent water resistance, durability and good heat resistance, and is generally used as structural adhesive or outdoor adhesive. The first-order phenol catalyzed by caustic soda is often used in plywood, and the glued board is water-resistant and weather-resistant, such as aviation plywood for gliders. In order to increase the strength and water resistance of fiberboard, alkali-catalyzed phenolic resin was added as waterproof agent and deposited on the fiber by using its precipitation characteristics under the action of aluminum salt and acid. When particleboard is used outdoors or as a structural material, phenolic resin adhesive is needed. Phenolic resin is seldom used in particleboard production because of its slow curing speed, strict requirement on moisture content of shavings and high price. Rotary-cut veneer impregnated with phenolic resin (alcohol-soluble or water-soluble) can be pressed into laminated wood-plastic composites, which can be used to manufacture mechanical parts, wear-resistant materials, textile shuttles and so on. There are two types of paper laminates: industrial board and decorative board. Industrial laminates are made of impregnated paper or impregnated tape by hot pressing, and are used as electrical insulation materials or wear-resistant materials. The decorative board is made of kraft paper impregnated with phenolic resin as the bottom layer and hot pressed with decorative paper. In addition, phenolic resin can also be used for coating, molding, grinding materials and manufacturing phenolic fibers.
Phenolic resins commonly used in China wood industry are shown in the following table.