 | | 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.
 | 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. |
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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