of unsaturated polyester-, vinylester- and acrylic resins with organic peroxides
Unsaturated polyester resins are cured by free radicals which are formed when organic peroxides decompose. The decomposition initiates a reaction by which unsaturated polyester molecules polymerize with styrene forming a three dimensional network (Duromer or thermoset). Organic peroxides decompose into free radicals either by exposure to heat or in combination with appropriate accelerators.
Accelerators used in practice are mainly organic cobalt salts and tertiary amines. However, these accelerators activate only certain types of organic peroxides. Contrary to this activation, for example if inhibition is required to increase storage life of an activated resin, the use of so called inhibitors is recommended. Inhibitors are chemical compounds, which prevent the polymerization process of monomers or other reactive compounds. Suitable compounds are Quinones or Phenolic compounds.
Amine or cobalt activated curing (Cold curing)
Accelerator activated curing is called cold curing. The most important cold curing systems are Methylethylketone, cyclohexanone or acetylacetone peroxides in combination with organic cobalt salts and Dibenzoyl peroxide in combination with tertiary amines.
Curing without accelerator (Hot Curing)
Curing without accelerator, so called hot curing, requires external support of heat. Minimum kick-off temperatures above 50 °C and typically temperatures inbetween 120 and 160 °C for SMC/BMC hot moulding are required to achieve a good degree of curing within a short period of time.
Organic peroxides used for hot curing are peresters such as tert-Butyl peroxybenzoate, tert-Butyl peroxy-2-ethylhexanoate or perketals such as1,1-Di(tert-butylperoxy)cyclohexane. If a very low kick-off temperature is required (50 - 60 °C), more active initiators such as Di-(4-tert.-butylcyclohexyl)-peroxydicarbonat and Methylisobutylketonperoxide are used.
Very often, combinations of organic peroxides or ready-to-use mixtures are utilized to obtain an efficient curing process and a very good degree of curing.
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