Classification of polymerization inhibitors and retarders

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A polymerization inhibitor is a substance that can rapidly react with free radicals, thereby slowing down or suppressing unwanted chemical reactions, and is used to extend the shelf life of certain monomers and resins. Also known as polymerization inhibitors, it includes chain stoppers and retarders. Polymerization inhibitors can prevent the progression of polymerization, resulting in an induction period during the polymerization process (a time interval during which the polymerization rate is zero). The length of this induction period is proportional to the amount of inhibitor present; once the inhibitor is depleted, the induction period ends and polymerization proceeds at the normal rate as if no inhibitor were present. A retarder is an additive used to adjust the exothermic properties of the resin in order to meet process requirements; it does not cause an induction period and merely reduces the polymerization rate. Aside from many types that are identical to inhibitors, the most effective retarder is α-**ethylene. In fact, polymerization inhibitors, retarders, and stabilizers are all substances that inhibit the cross-linking and curing reaction of resins. Their mechanism of action is to absorb and eliminate the free radicals that can trigger such cross-linking and curing; they are, in other words, free radical inhibitors. In a given resin, those with a stronger ability to eliminate free radicals exhibit polymerization-inhibiting effects ； In another type of resin, the effect that merely reduces the activity of free radicals is known as retardation. A mixture of unsaturated polyester resin and crosslinking monomers can undergo self-polymerization even in the absence of an initiator in the resin, resulting in a loss of its utility; in such cases, a polymerization inhibitor or retarder can be added. Classification of polymerization inhibitors (1) Classification by their activity under different temperature conditions: ① Inhibitors with fixed activity react rapidly with free radicals throughout the normal storage and curing temperature ranges; they are consumed in these reactions, thereby creating an induction period. The polymerization inhibitors that are consumed no longer possess any activity, such as hydroquinone. ②Thermosensitive polymerization inhibitors are inhibitors at normal storage temperatures; they decompose when heated and become ineffective, such as tert-butylhydroquinone. ③Dual-action inhibitors exert an inhibitory effect at lower temperatures and a catalytic effect at higher temperatures; examples include oxygen, organic copper salts, quaternary ammonium salts, sulfur, etc. (2) Classification according to their polymerization inhibition mechanism: ① In the absence of oxygen, in the presence of benzoin without oxygen, it reacts directly with free radicals to form a semiquinone intermediate, which then reacts with another free radical to form a stable compound. ②In the presence of oxygen, hydroquinone and its derivatives react with oxygen to form peroxyl radicals; these peroxyl radicals then react with hydroquinone to create radical complexes, which in turn react with another peroxyl radical to form stable compounds. (3) According to the composition of the inhibitor: ① Single-type: Only one type of inhibitor is required to achieve an inhibitory effect. Such as hydroquinone, paraquinone, etc. ②Composite type refers to the combined use of two or more polymerization inhibitors. To ensure the storage stability of resins and pre-impregnated materials at room temperature, and to enable adjustment of their shelf life without affecting the final curing rate of the products, a composite inhibitor is used in the manufacturing process. The above effect can be achieved by using hydroquinone and tert-butylhydroquinone along with trace amounts of copper salts.