Phenolic resins include synthetic thermoset resins such as those obtained by the reaction of phenols with formaldehyde.
Phenolic resins are divided into two different types, novolacs and resoles.
Novolacs are phenol–formaldehyde resins made when the molar ratio of formaldehyde to phenol is less than one. The polymerization is brought to completion using acid–catalysis such as oxalic acid, hydrochloride acid, or sulfonate acid. The phenol units are mainly linked by methylene and/or ether groups.Novolacs are commonly used as photoresists. The molecular weights are in the low thousands, corresponding to about 10–23 phenol units.
There are many Novolacs in the phenolic resin family. TCC makes Novolac resins to client specifications. Contact us by e-mail or telephone for further details.
Hexamethylene tetramine or “hexamine” is a hardener added to cross-link novolac. At a temperature of >90° C, the hexamine forms cross-links to make methylene and dimethylene amino bridges.
Base–catalyzed phenol–formaldehyde resins are made with a formaldehyde to phenol ratio of greater than one (usually around 1.5). These resins are called resoles. Phenol, formaldehyde, water and catalyst are mixed in the desired amount, depending on the resin to be formed, and then heated. The first part of the reaction, around 70° C, forms a thick reddish–brown tacky material, which is rich in hydroxymethyl and benzylic ether groups.
The rate of the base–catalyzed reaction initially increases with pH, and reaches a maximum of about pH = 10. The reactive species is the phenoxide anion (C6H50–) formed by deprotonation of phenol. The negative charge is delocalized over the aromatic ring, which then reacts with the formaldehyde.
Resoles our major polymeric resin materials widely used for cooling and bonding building materials. Exterior plywood oriented strand board (OSB), engineered laminated composite lumber (LCL) are some of the typical applications.