Addition vs Condensation Silicone

Overview: Two Cure Chemistries for Silicone Rubber

Silicone rubber can be cured by two distinct chemical mechanisms: addition cure (platinum-catalyzed) and condensation cure (tin-catalyzed). Both produce a cross-linked silicone elastomer, but they differ significantly in reaction mechanism, by-products, shrinkage, cure speed sensitivity, inhibition behavior, and application suitability. Selecting the correct cure chemistry is a critical upstream decision in silicone product design.

Addition Cure (Platinum Hydrosilylation)

In addition cure, platinum catalyst drives a hydrosilylation reaction between vinyl-functional siloxane polymer chains and methylhydrosiloxane crosslinker. No atoms are lost from the network — the reaction is strictly additive. This has several important consequences:

Near-zero shrinkage: Because no by-product is released, cured addition-cure silicone shrinks <0.1% (versus 0.3–1.0% for condensation cure). This precision is essential for mold-making, optical components, and precision-gasketed assemblies.

No by-product release: Addition cure releases nothing into the surrounding environment. This makes it compatible with copper electrical contacts (condensation-cure acetic acid corrodes copper), optical systems (no fogging), and sensitive biological environments (no amine, alcohol, or acid release).

Clean biocompatibility profile: FDA 21 CFR, USP Class VI, and food-contact compliance is more readily achieved with addition-cure systems because the extractable profile is inherently clean. All LSR and medical-grade HTV uses platinum addition cure.

Inhibition sensitivity: The platinum catalyst can be deactivated by contact with sulfur-containing compounds (latex, polysulfide, sulfur-vulcanized rubber), organotin compounds (condensation-cure silicone residues), nitrogen compounds (polyurethanes, amine-cured epoxies), and certain plasticizers. Mixing tools and substrates contaminated with these inhibitors will prevent cure.

Condensation Cure (Tin-Catalyzed)

Condensation cure involves reaction of silanol end groups with either acetoxy, oxime, or alkoxy functional crosslinkers in the presence of tin catalysts, releasing a small-molecule by-product:

• Acetoxy type: releases acetic acid (corrosive to copper, zinc, marble)
• Alkoxy type: releases methanol or ethanol (mild odor, broad substrate compatibility)
• Oxime type: releases butanone oxime (low odor, suitable for electronics)

Moisture dependency: Condensation cure requires atmospheric moisture to complete, particularly in deep sections. Cure proceeds from the surface inward, limiting practical section thickness.

Higher shrinkage: By-product release and moisture-driven cure produce 0.3–1.0% shrinkage, limiting use in precision mold making.

Lower cost and inhibition-free: Tin catalysts are cheaper than platinum, and condensation cure is tolerant of most organic contamination. Suitable for general sealing, potting, and construction applications where dimensional precision is not critical.

Decision Guide: When to Choose Each

Common Mistakes to Avoid

Using condensation-cure RTV near copper contacts causes corrosion. Using addition-cure RTV in tools contaminated with sulfur or tin causes cure failure. Mixing addition-cure and condensation-cure components in the same workspace without cleaning causes inhibition of the addition-cure system. Always test a small quantity on the actual substrate before production use.

Contact us to verify cure system compatibility for your application and request samples from our supplier network.