Unsaturated polyester resins and cross-linking monomers will also slowly polymerize in the absence of any initiator, thus rendering the resin unusable. Positive monomers, like styrene, often add some anionic polymerization or retardation agents to the unsaturated polyester resin to prevent the polymerization of the resin during storage and transportation and have certain stability. Generally, 0.01%~0.05% polymerization inhibitor is added to the synthesis of the resin. Extend the storage period of molding compound by one week or even 2 months.
A polymerization inhibitor is a substance that can react with chain free radicals to form non-free radical substances or low-activity free radicals that can no longer be initiated, reducing the polymerization rate to zero. The obvious feature is the induction period. The retarder is a substance that can react with free radicals to reduce the polymerization rate and degree of polymerization. The role of polymerization inhibitors and retarders is only different in degree, and there is no qualitative difference. Many substances have both anionic polymerization and slow polymerization. For example, nitrosotoluene not only produces an induction period, but also reduces the polymerization rate after the induction period. Inhibitors and retarders are as important as initiators in unsaturated polyester resins.
In fact, the stability of unsaturated polyester resins can be divided into three types.
The stability of the polymerization process is used to prevent unnecessary products, cross-linking, discoloration or other side reactions during polymerization or dilution.
The stability of resin products plays the role of polymerization inhibition at low temperature, but at a certain molding temperature, the polymerization inhibition can be removed to quickly cure the resin, which requires a certain storage period of polyester resin.
Stability of resin curing Under the combined action of initiator, accelerator and polymerization inhibitor, although the resin can obtain the specified gel time, exothermic peak time and other parameters, it is inevitable that the parameters will drift with the extension of the placement time. , A specific stabilizer required to prevent this change in gel properties.