Resins resol casting

The importance of the nature of the catalyst on the hardening reaction must also be stressed. Strong acids will sufficiently catalyse a resol to cure thin films at room temperature, but as the pH rises there will be a reduction in activity which passes through a minimum at about pH 7. Under alkaline conditions the rate of reaction is related to the type of catalyst and to its concentration. The effect of pH value on the gelling time of a casting resin (phenol-formaldehyde ratio 1 2.25) is shown in Figure 23.15. 

Processing of phenol-aldehyde oligomers into various articles is based on a polycondensation reaction which leads to solidification of the material at temperatures below 200°C and pressures exceeding 10 MPa. The process is accompanied by volatile product formation. However, phenol-formaldehyde resins of the resol type can be cast without additional pressure and heat. The raw molding reactants contain different organic and mineral fillers and other additives in addition to the basic resin. 

Castings are usually used for decorative articles, and bowling balls. The resole resin containing diluents, iilleis, and pigments are cast into open molds and maintained there during curing [1]. 

The structure of a casting resin resembles that of a resol (Section 12.4.1) but a larger number of methylol groups is present owing to the greater amount of formaldehyde used in the reaction. 

As indicated in Section 12.4.4, casting resins have a resol-type structure but contain a greater proportion of methylol groups. It may therefore be anticipated (cf.. Section 12.5.1) that an appreciable number of ether links are produced when a casting resin is cured. Further, since castings are normally cured at relatively low temperatures these ether links are likely to persist in the final product. It may be noted that it is possible to prepare castings which are colourless, whereas resols cured at about 150°C are dark coloured. This observation is in accordance with the suggestion that it is the thermal decomposition of ether links into quinone methides which leads to colour formation in cured resols. 

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