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GENERAL:

Phenolic resins are the condensation products of phenols and aldehydes. Unsubstituted phenol and formaldehyde are the main raw materials used to produce phenolic resins. Although phenol is a component of coal tar, it is nowadays mainly produced from benzene using the cumene process. Formaldehyde, the second component in the phenolic resin synthesis, is obtained on an industrial scale by catalytic dehydration of methanol. In an electrophilic reaction, phenol reacts with formaldehyde at the ortho and para positions to yield phenolic alcohols. Methylol (-CH2OH) groups are created in this reaction.

Depending on the reaction conditions, the reaction of phenol yields novolacs or resoles.

 

NOVOLAC CHEMISTRY:

Novolacs are formed by the reaction of phenol with formaldehyde in the presence of mineral acid or strong organic acid catalysts when the molar ratio of the components is one to less than one (approx. 1:0.4 – 1:0.9). The threefold functionality of phenol leads to production of a mixture of isomers in the condensation reaction with formaldehyde. Thus, three different binuclear compounds are possible, seven different trinuclear compounds, 28 different quadrinuclear compounds and so forth.

 

Novolacs are generally solid products with a softening point of 40 – 110 °C and a molecular weight of 250 – 900 g/mol. This corresponds to an average linkage of three to nine phenolic rings.

 

Novolacs themselves are not heat sensitive. They are most frequently cured with hexamethylenetetramine (“hexa”), that reacts with the novolac to form methylene and dimethyleneamino groups.

 

As novolac resin cross-link, in combination with a curing agent, they gradually pass through the following morphologic stages:

A-Stage: The original novolak stage, liquid or fusible and soluble

B-Stage: The intermediate resitol stage, infusible, but still capable of being shaped when warm, swells, low mechanical strength

C-Stage: The final resite stage, infusible, insoluble, high mechanical strength, high chemical resistance

 

Novolacs cross-linking reactions result in formation of a high molecular weight resite network. Depending on the type of resin and the curing conditions, a larger or smaller number of defective links is to be expected in the resite network.

 

Novolac resins can be modified to alter and enhance their physical and chemical properties in various ways, for example by

1. Use of phenol derivatives such as alkylphenols and Bisphenol A

2. Reaction with unsaturated compounds as for instance natural oils

3. Reaction with inorganic acids or other inorganic compounds

4. Reaction with other organic compounds

5. Physical modification by addition of materials such as polymers or rubber

6. Curing with epoxy compounds or polyisocyanates (polyethers, polyurethanes)