Redox hardener system

Processing aids are used to directly influence the synthesis process function as reaction controllers. Depending on their chemical state they can function as reaction accelerators (the actual catalysts and starters or initiator substances), crosslinkers and/or hardeners, reaction inhibitors or catalyst deactivators, molecular weight controllers, chain splitters or lengtheners. From a chemical standpoint (structure and method of function) the radical builders, mainly peroxides and azo compounds, are treated separately from the catalysts which are mainly metals, metal oxides, salts (redox systems) and organo-metal compounds. The carrier substances, promoters and deactivators are placed in the catalyst class of substances. 

Structural acrylic adhesives contain monomer, and cure is by a redox system which generates free radicals which cause cure by addition polymerization. Redox systems have two components, one being dissolved in the adhesive and the other in the hardener or catalyst. Once mixed cure is rapid. Examples are cumene hydroperoxide and N,N-dimethyl aniline. Should the peroxide be in the adhesive, then it will slowly decompose to give free radicals, which will cause cure and eventually terminate the shelf life, which is typically 12 months. Chemical reactions obey the Arrhenius relation (O Eq. 18.1). Here k is the rate constant. 

The reaction of peroxyesters, hydroperoxides or peroxides, which are more storage stable than BP, with tertiary amines is generally too slow to give a sufficiently rapid cure for acrylic resins. Composite mixes containing t-butyl perbenzoate, -butyl hydroperoxides or dicumyl peroxide-tertiary amines do not harden for days. Many other redox systems have been suggested for vinyl polymerizations, only a few have been employed in dental resins. Substitution of 2 toluene sulfinic acid or sulfinic acid derivatives for tertiary amines yields colorless products (38-42). Most of these compounds have poor shelf-life. They readily oxidize in air to sulfonic acids which do not activate polymerization. Lauroyl peroxide in conjunction with a metal mercaptide (such as the zinc hexadecyl mercaptide) and... 

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