In 1897, A.Speier obtained insoluble matter from the reaction of resorcinol with formaldehyde. In 192 1, resorcinol is used to make resins. 1930 was patented. 1943 resorcinol resin adhesive appeared. Compared with phenol, resorcinol introduced a second hydroxyl group into benzene ring, which greatly improved the reaction activity. A.Pizzi and others compared resorcinol-formaldehyde system and phenol-formaldehyde system with the same reaction conditions (same molar ratio and concentration), and found that the reaction speed of the former was 10 ~ 15 times that of the latter. Resorcinol resin can be cured under alkaline conditions or under neutral conditions at room temperature. It has high bonding strength and good aging resistance, and does not creep under load, so it is mainly used as structural adhesive, and also used for outdoor rapid bonding and high moisture content wood bonding, such as finger bonding of wet wood (moisture content ≤30%). Disadvantages are high price and dark color.
The reaction speed of resorcinol and formaldehyde depends on their molar ratio, reaction concentration, reaction temperature and pH value.
The suitable molar ratio of pure resorcinol resin is resorcinol/formaldehyde = 1/0.5 ~ 0.7. In order to facilitate control, formaldehyde can be gradually added in batches, such as 2500 parts by weight of resorcinol, 5 parts by weight of oxalic acid as catalyst, 500 parts by weight of the first batch of 37% formaldehyde solution and 750 parts by weight of the second batch of formaldehyde, which should be gradually added to prevent boiling. It is sometimes more convenient to use caustic soda instead of oxalic acid, because weak alkalinity is beneficial to resin curing. Pure resorcinol resin has strong reactivity and high cost, and phenol-containing polymers are often used.
Phenol-resorcinol-formaldehyde resin
This is the most commonly used resorcinol resin. Whether phenol, resorcinol, formaldehyde, phenolic resin and resorcinol-formaldehyde resin are mixed alone, the effect is the same. The ratio of phenol to resorcinol is usually 50/50 or even as high as 80/20. However, with the decrease of resorcinol dosage, the properties of the resin also decreased accordingly. Resins with various properties can be obtained by changing the molar ratio and reaction conditions as needed. Its reaction is shown at the bottom left.
Urea-resorcinol-formaldehyde resin
Resorcinol is connected to both ends of urea-formaldehyde resin, and the reaction formula is as follows. The performance of this resin is slightly inferior to that of phenol-resorcinol-formaldehyde resin, which is water-resistant and can be cured at high frequency and can be used in plywood production. This resin has a high formaldehyde/urea ratio. Generally, dimethylol urea is first prepared and then reacted with resorcinol. Most of them are catalyzed by acid, usually p-toluenesulfonic acid or sodium hydroxide.
Tannin-resorcinol-phenol-formaldehyde resin
Tannin is a kind of polyphenol compound, which easily reacts with formaldehyde, and its price is much cheaper than resorcinol, and it often partially replaces resorcinol. This resin has been used as plywood and finger joint adhesive in South Africa and Australia, and most of them use black wattle tannin and 130 100 lignans.
Because resorcinol resin is a linear resin, it needs to be cured by adding curing agent, and the molar ratio of total formaldehyde content to resorcinol is 1 ~ 2: 1, so it can be cured at room temperature. Paraformaldehyde is commonly used. In addition, fillers, such as coconut shell powder and walnut shell powder, are usually added in order to reduce the shrinkage after curing, improve the interstitial property, reduce the cost and increase the solid content. After the curing agent is added, the application period of the adhesive is not long, and it should be prepared when necessary.