A Technical Guide to Cure Inhibition

Cure Inhibition with Addition Silicones

Addition cure silicones will bond to most substrates when heated to >120⁰C (hydrolisation); however certain products on the surface of the substrate could inhibit the cure of the silicone or poison it, resulting in a non-cure.

Addition cure silicone chemistry is based on a platinum to drive the curing reaction and can occur in 1K and 2K systems. The level of platinum vary in different materials, the lower the level of platinum is the more easily the material can be poisoned or inhibited by certain chemicals, materials and elements.

There are varying degrees of cure inhibition ranging from complete non-cure to a slimy or sticky finish. Once the platinum has been deactivated or poisoned, the affected area of silicone may never reach full cure and is characterised by a runny, sticky or gummy appearance localised at the interface between the silicone and the offending substrate.

How to Avoid Silicone Poisoning

  • Apply a barrier coat
  • Always clean your surfaces before applying silicone
  • Avoid curing the silicone when in contact with poisonous substances. Where possible, cure the silicone in advance or remove the cause of the problem
  • If unable to avoid the inhibitive source, use a suitable primer to prevent any effects from the poisonous material
  • Ensure any solvents or cleaning agents used contain low or no levels of sulphur. If this in unavoidable, wipe away any residue left by the cleaner prior to applying the silicone

Substances to Avoid

Below is a list of chemicals, materials and elements which may cause inhibition to addition curing silicones:

  • Acrylonitrile butadiene rubber
  • Masking tapes
  • Polybutadiene rubber (BR)
  • Alkenes
  • Natural rubber
  • Polychloroprene rubber (CR)
  • Amines
  • Neoprene
  • Polyisoprene (IR)
  • Amine containing materials
  • Nitrile rubber (NBR)
  • Polysulphide
  • Arsenic compounds
  • Onion
  • Polyurethanes
  • Ethylene propylenediene rubber (EPDM) 
  • Organometallic compounds
  • Polyvinylchloride (PVC)
  • Amide cured epoxy adhesive
  • Organotin
  • Silicone rubber containing organotin catalyst
  • Amine cured epoxy adhesives
  • Paper tapes
  • Solder flux residues
  • Ultra violet cured epoxy adhesive
  • Peroxide cured silicone rubber
  • Styrene butadiene rubber (SBR)
  • Garlic
  • Phosphate compounds
  • Sulphur
  • Isoprene isobutylene rubber (IIR)
  • Phosphite compounds
  • Unsaturated hydrocarbons Benzene
  • Latex
  • Phosphorus

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