Initiator Systems for Unsaturated Polyester Resins
The basic principles of free radical initiators, their important parameters and new developments as they relate to the selection of effective initiator systems for curing unsaturated polyester resins are reviewed.
Peroxide and azo type initiators continue to be the most popular initiators for the fibre reinforced plastics (FRP) industry. Certain peroxides can be decomposed by specific promoters to provide good cures at ambient temperatures. The mechanism of activation and particularly the detrimental effects of excess promoter are described. The results of studies relating the structure of ketone peroxides to their reactivity are reviewed.
The kinetics of thermal decomposition of initiators are related to important processing parameters such as shelf-life, rate of cure and temperature of cure. For easy reference, specific initiators used for curing in the 100°–160°C temperature range are tabulated along with their 10 h t1/2 temperatures. The advantages of peroxyketals over popular peroxyesters, and their sensitivity to acidic fillers are discussed. The advantages of peroxide blends in terms of fast cure systems and especially the long shelf-life of blends containing unique azo compounds are described. Results of studies directed towards the reduction of residual styrene in cured resin are reviewed.
In addition to azo and peroxide compounds, other sources of free radicals are reviewed including novel carbon-carbon initiator and photoinitiator systems based on both UV and visible light.
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