Morphological changes in surfaces of unsaturated polyester resins (isophtalic polyester resin and orthophtalic polyester resin) exposed to ultraviolet (UV) radiation were studied using several microscopic techniques. Changes in the surface roughness and the thickness of each resin layer were also evaluated quantitatively. The experimental results suggested the advantage of the isophtalic polyester resin on UV resistance. Although both resins have equivalent chemical structures except for the ester bonding sites and equivalent average crosslink density, the unit of drop-out from the surface after UV-induced degradation and the roughness of fracture surfaces were larger in the orthophtalic polyester resins than those in the isophtalic polyester resins. These results lead to an assumption that the difference in the UV resistance of both resins relates to the aggregated structure of microgels formed in the curing process of the resins.

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