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Encapsulants


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Cure Inhibition

Dow Corning brand addition cure gels and elastomers cure rapidly and uniformly at widely different temperatures, in thick sections, and under conditions of air exposure or in total confinement. They can be cured in modules or parts made of a variety of materials and in contact with most common electronics materials.

There are certain situations, however, where the cure reaction cannot proceed normally. These conditions occur when materials called inhibitors are present. In this inhibited area (usually less than 0.02 inches thick) the elastomer remains in its liquid state even though the cure schedule has been completed. This liquid material will remain liquid regardless of any subsequent attempts to convert it to a hard, dry mass.

Inhibition can be overcome in some instances. Pre-baking the unit at the highest tolerable temperature can help remove volatile chemicals causing the inhibition. In many applications, the use of a primer can act as a barrier coating against these inhibiting materials. In other cases, the use of higher or longer cure temperatures may be enough to overcome mild inhibition.

Causes of Inhibition

Inhibition is caused by the contamination of addition cure materials with trace quantities of certain types of chemicals. The catalyst is a chemical containing platinum atoms, which are not only affected by working-time control agents, but can also be poisoned by contaminants. These chemicals interfere with the cure reaction and thus prevent conversion of material to a solid. Extremely small quantities of inhibitors or contaminants may be sufficient to produce this effect. Fortunately, only a small number of material types can cause inhibition.  However, unlike a working-time control agent, once a contaminant attaches to the platinum, any negative effects on the cure are permanent. 

Certain materials, chemicals, curing agents and plasticizers can inhibit the cure of addition cure adhesives. Most notable of these include:

  • Organotin and other organometallic compounds
  • Silicone rubber containing organotin catalyst
  • Sulfur, polysulfones or other sulfur-containing materials
  • Amines, urethanes or amine-containing materials
  • Unsaturated hydrocarbon plasticizers
  • Some solder flux residues

 

If a substrate or material is questionable with respect to potentially causing inhibition of cure, it is recommended that a small-scale compatibility test be run to ascertain suitability in a given application. A portion of the part containing the suspect material can be placed in a clean dish.  The dish is then processed by the desired cure method.  A control dish without the part can be compared to the dish with the suspect part.  The presence of liquid or uncured product at the interface between the questionable substrate and the cured gel indicates incompatibility and inhibition of cure.

 

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< back to Encapsulants Home Page  
 
  1. Gel and Encapsulant Processing - General


  2. Gel and Encapsulant Processing - General


  3. Substrate Preparation


  4. Material Preparation


  5. Applying Gel or Encapsulant


  6. Curing


  7. Cure Inhibition


  8. Where Does Inhibition Occur


  9. Repairing Gels or Encapsulants


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