Page 140 - Handbook of Plastics Technologies

THERMOSETS

3.10 CHAPTER 3

FIGURE 3.6 Phenol-formaldehyde chemistry.

groups. These are very reactive. They can condense with each other, with the ortho- or
parahydrogens on other phenol molecules, or with active hydrogens in cellulose or other
materials.
Since the condensation evolves volatiles and heat, it must be controlled to give useful
products. The reaction is controlled by monomer ratio, pH, and temperature. It is generally
run in several separate successive stages. First, it goes to low-molecular-weight “A-stage”
resin, which is soluble and fusible. Then, it is compounded with fillers and additives and
reacted further to moderate-molecular-weight, somewhat cross-linked “B-stage” resin,
which is hard and less soluble but still fusible. Finally, the resin is formed into the shape of
the desired product and thermally cured into fully cross-linked thermoset “C-stage” resin,
which is rigid, insoluble, and infusible.

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