Storage Stability Prior to Processing

Stabilization of the polymeric and monomeric components is required to avoid premature radical polymer formation of UP resin dissolved in styrene by heat, light, or metallic initiators. Insufficient stabilization causes an increase in viscosity that makes further processing impossible. Quinones and phenols in concentrations 500 ppm are proven stabilizers for unsaturated polyester. The most important examples are hydroquinone, p-benzoquinone, tert-butyl-brenzcatechol and 2,6-di-tert-butyl-p-cresol [574]. For styrene, stabilization with 25 ppm p-tert-butyl-brenz-catechol is required [575]. Slight yellowing caused by the stabilizers is possible appropriate brighteners are recommended for optically demanding applications. 

The storage stability of pre-condensates based on phenol-formaldehyde usually poses no problem. This is not the case with amino-resin pre-condensates. Technically significant are only the reaction products of urea, urea derivatives, and melamine with formaldehyde. The hydroxyl alkyl amines formed in an addition reaction 

Dependence of the durability of impregnating resins on water dilutability [577] 

---