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Encapsulants


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Curing

 The addition cure reaction occurs between the base polymer and the cross-linker. No reaction by product is liberated during the reaction. This polymerization mechanism will not revert or depolymerize even under conditions of elevated temperature and confinement.  These types of addition-cured gels and encapsulants utilize a platinum-catalyzed addition reaction mechanism for crosslinking or curing.  Typically, these materials are supplied as Part A/Part B in 1:1 mix products or Base/Curing Agent combinations for those requiring 10:1 mix ratios.  In a two-part formulation, part A (or the base) typically contains a catalyst while part B (or the curing agent) contains the crosslinker.
When the two are mixed, the addition reaction can proceed to cure the material.  For one part materials a working time control agent is also included to prevent reaction until the encapsulant is heated for curing.  This technology can also be used to control cure time in two part materials. Fillers are added to impart strength, heat stability and other properties.  Pigments or dyes can add color to some products.  Finally, some of the products contain special additives to enhance adhesion.  Some of the materials will begin to cure at room temperature.   Others require heat for cure.  (Check the data sheet if you are unsure which type you are using.)  In both cases, higher temperatures will drive the curing reaction faster.
In the initial stages of crosslinking or curing there is little observable change in the material.  The viscosity first changes linearly because the first few bonds essentially increase the length of the polymer chain; little building of the cured network has occurred.  In the later stages of cure, when the most observable changes occur, the viscosity changes dramatically with time because bonds created are the final network interconnects. Since so much of the observable change happens at the end of the cure cycle, a small change in conditions can appear to cause a big change in cure time.

Cure time for Dow Corning gels and encapsulants varies depending on the material of choice. Changes in temperature can dramatically alter cure times. The temperature of the material and the temperature of the part being potted will impact the cure time of addition cure materials.   An increase as little as 10°C can cut the cure time in half.  Conversely, a decrease of 10°C can double the cure time.  When heat curing, relatively massive parts will require additional time in the oven to reach the desired temperature. To speed the heat cure process, it may be desirable to preheat the part being potted. That will reduce the overall time it takes to process the part.

Room Temperature Curing

A room temperature cure process can be used for many of the standard gels and encapsulants. After being mixed and applied, the material is simply allowed to cure at ambient room conditions. Most products will require several hours to cure or need to be left undisturbed overnight. Room temperature curing processes are more appropriate for low to medium volume production applications. There are a few products that are designed for rapid room temperature processing.

Heat Curing

For any of the heat curing encapsulants, heating will accelerate the cure rate. For the Standard Encapsulants, heat acceleration occurs at any temperature above room temperature. For Self-Priming Encapsulants, cure and the development of adhesion are not achieved until the material is heated above 100°C. Higher temperatures will result in faster cure. Limitations on cure speed are generally dependent on the temperatures that the unit and components are able to withstand. Heat curing can be done in a batch or conveyor oven.

Product data sheets should be referenced for specific cure details.

 

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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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