Figure 3: Silicone exhibits the strongest resistance to UV light exposure as shown by this transmission versus wavelength graph.
In an encapsulating gel or thermoset composition, the material’s cured mechanical consistency is particularly sensitive to the final crosslink density. In other words, materials with low-crosslink density (like silicone gels) are greatly impacted by inhibition. Weak cure inhibition on a substrate’s surface causes the gel to appear cured in bulk, but “wet” at the substrate interface. Modest cure inhibition results in a lower final cured durometer (i.e. softer) than expected, while severe cure inhibition can lead to complete cure elimination (the mixed gel’s form exhibits little or no perceptible viscosity increase after the nominal cure time).

Avoid Cure Inhibition

It is imperative to analyze adhesives, plastics, and elastomers for cure inhibition prior to selection for use near or in contact with a silicone compound. This analysis should include materials used in any transfer containers, dispensing hoses, or utensils that come in contact with the silicone components. The following list gives a partial inventory of suspect materials:


  • Amine-cured epoxies
  • Amide-cured epoxies
  • UV-cured epoxies
  • Elastomers
  • Peroxide-cured silicones
  • Organotin-cured silicones
  • Certain grades of addition-cured silicones
  • Chlorinated elastomers
  • Polyvinylchloride (PVC), plasticized
  • Certain grades of THV fluoropolymers
  • Neoprene
  • Buna N (nitrile)
  • Natural rubber
  • Latex


  • Some chlorinated plastics

Other Materials

  • Certain sulfur-containing compounds, especially thiols, sulfides, sulfites, and thioureas
  • Certain tin-containing compounds, especially tin salts, and tin soaps
  • Some mold-release lubricants or other agents
  • High-pH organic materials
  • Wood, leather, and clay
  • Some solder flux
  • Paper tapes and masking tapes
  • Certain grades of vinyl tapes
  • Cellophane tapes

Testing candidate materials that may contact a curing optical gel for compatibility is the most effective way to eliminate “poisons.” It is important to test each material’s particular manufacturer’s grade. For example, some vinyl electrical tapes show no cure inhibition at all, whereas others may exhibit inhibition. Placing fragments of the suspect material into uncured silicone can reveal whether or not an adverse reaction has occurred (after the silicone has undergone the proper cure schedule).

The use of silicones in HBLED encapsulating applications can provide device developers with significant benefits. Understanding the proper use of silicone systems, and, more specifically, the elimination of cure poisons, can ensure the smooth transition from prototype to finished product.

This article was written by Bill Riegler, product director-engineering materials, and Stephen Bruner, marketing director, at NuSil Technology LLC. For more information, contact Mr. Riegler at This email address is being protected from spambots. You need JavaScript enabled to view it..

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