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Tuesday, May 13, 2008

Sulphonated Phenolic Resin Adhesives

The only object of sulphonated is to convert a water insoluble resin to one that is soluble. Sulphonated resins may be prepared by a number of different methods, on of which is to react a novolak with concentrated sulfuric acid. The sulphonated novolaks may be cured at room temperature by reaction with additional formaldehyde, or they may be converted to resols that are water soluble by reacting with formalddhyde under alkaline conditions. In sulphonating a novolak it is important that is should have a sufficiently low molecular weight; otherwise the relatively large molar ratio of sulfuric acid that is required will cure the resin.

A better method is to use a sulphonated phenol, such as phenol para-sulphonic acid or its sodium salt. The whole amount of the phenol need not be sulphonated in order to give a resin with satisfactory water solubility; it therefore follows that it is possible to mix together a sulphonated novolak and a resol.

When sulphonated are made entirely under alkaline conditions, precipitation of the resin does not occur at any stage and so, in the preparation of the resin, the reaction cannot be followed by diluting samples with water. It can, however, be followed, and an end-point established by diluting with a selected organic solvent (perhaps mixed with water), or by measuring viscosity.

The sulphonated resols may be spry-dried to produce powder glue which can be easily re-dissolved in water. Curing can be effected by adding a suitable acid such as dilute hydrochloric acid.