The preparation of phenolic resin is affected by many factors, among which the molar ratio of raw materials, the kind and amount of catalyst, the reaction temperature, and the feeding method all have a great influence on the reaction speed, product structure and quality of the phenolic resin. First, the effect of the molar ratio of phenol to formaldehyde The molar ratio of phenol to formaldehyde affects the reaction process and the molecular structure. In the acidic catalytic reaction, when the molar ratio of formaldehyde is smaller than phenol, sufficient hydroxymethyl groups cannot be formed and the condensation reaction proceeds. A certain degree will stop. In the basic catalytic reaction, when the number of moles of formaldehyde is smaller than that of phenol, some of the phenol is present in the resin in a free state and the reaction is incomplete. From the point of view of the ideal structure of the phenolic resin, the number of moles of the thermosetting resin phenol should be slightly smaller than the number of moles of formaldehyde.
The molar ratio of phenol to formaldehyde reacts mainly to o-methylphenol and p-hydroxymethylphenol, of which the content of p-methylolphenol is mostly. When the molar ratio of phenol to formaldehyde is 1:2 or more, dihydric phenol and trihydric phenol are mainly produced.
The molar ratio of phenol to formaldehyde varies with the average molecular weight of the resins. The higher the molar ratio, the greater the average molecular weight of the resin.
The relationship between the molar ratio of phenol and formaldehyde and the average molecular weight of the resin
Normal > molar ratio of phenol to formaldehyde
Normal > 1:1.1
Normal > 1:1.2
Normal > 1:1.3
Normal > 1:1.4
Normal > 1:1.5
Normal > 1:1.6
Normal > 1:1.7
Normal > resin average molecular weight
Normal align=center> 228
Normal > 256
Normal > 291
Normal > 334
Normal > 371
Normal > 437
Normal > 638
For different uses of phenolic resins, different molar ratios of phenol and formaldehyde should be controlled. The best resin for plywood is 1: (1.4~1.5), the yield is less than free phenol; the phenolic resin used for impregnation should have a molar ratio of 1 : (1.1 ~ 1.3), the average molecular weight of the resin, the second permeability is good as a water-resistant phenolic resin to increase the average relative molecular mass of some free phenol to reduce the molar ratio is generally about 1:2.0.
The molar ratio of phenol to formaldehyde also affects the resin's reaction rate and curing time. The greater the molar ratio is, the higher the amount of formaldehyde is, the faster the reaction rate of the resin is. The curing time is shortened and the viscosity is decreased. The storage stability is deteriorated. The effect of phenol and formaldehyde on the physicochemical properties of the resin is shown in the table.
Normal > molar ratio of phenol to formaldehyde
Normal > solid content /%
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