After a part has been potted with the liquid potting compound, it is turned into a solid plastic by a chemical reaction. Epoxy, silicone, polyester, acrylics and polyurethane materials are classified as thermoset plastics because after hardening, they cannot be melted as can thermoplastic plastics. Very high heat (600°F) causes these plastics to char and burn but not flow. Several different chemical reactions can cause hardening. Some occur at room temperature and others need to be heated to 200°F to 300°F.
There are two types of systems, a two-part and a one-part. The two part systems have the resin and the hardener separate since, when they are mixed, the hardening reaction starts. For a one-part system the hardener is relatively inactive when mixed with the resin and heat or UV light start the reaction.
A. Two Part Systems
The room temperature curing type requires the resin and hardener to be thoroughly mixed together and in a proper ratio. This is to link every resin with every hardener molecule. Most potting compounds develop adequate properties after 16 hours of hardening. Two-part systems can be harden faster or slower by raising or lowering the temperature when mixed together.
B. One Part Systems
Certain types of hardeners are not active at room temperature with the resin. They can be mixed into the resin and not increase the viscosity for six months. The advantage of this system is that no mixing is required and no ratio measurement is needed. As you would expect, this type of system requires heat or light to harden. The speed of hardening of a one-part system also can be affected by increasing the temperature or the amount of light energy. Selection of a one part potting compound does limit the range of properties that are available to you.
Resin systems are available that harden in 3 minutes to many days. The selection of hardeners and modification of the resin with accelerators determines the resulting hardening time. The resin system hardens by linking chains together in a three dimensional structure and continues until the material becomes it’s designed hardness. At the gel point of the resin system the unit can be moved, and the potted device can have other assemblies performed on it, even though it has not reached the full strength. When dealing with high-speed production, the gel time becomes the critical factor not the cure time. For room cure systems the development of the final properties can happen as the part sits in the shipping box.
Potting compounds harden when a chemical reaction occurs. During the chemical reason heat is given off as the chains connect. The faster the chemical reaction the higher the internal temperature becomes since the potting compound does not dissipate heat readily, see Thermal Conductivity. Also, the higher the temperature the potting compound becomes, the faster the reaction occurs. So it is like a snow ball rolling down a hill, gathering more snow as it travels, making it heavier and making it roll faster. This exothermic temperature can be a help or a hindrance -- a help to harden the polymer faster, a hindrance if the device has heat sensitive components on it which cannot exceed a temperature without damaging it. Exothermic reactions like these can become so hot that the potting compound starts to burn. Normally, these are in very large masses of a gallon or more of material.
When the chemical reaction occurs, the potting compound also shrinks in volume. This volume change is from 1 to 4 % depending if the potting compound has filler, which is non-shrinking, in it or no filler. This shrinkage would cause cracking and other problems if it were not for the way the epoxy and polyurethane polymer hardens. As described in the exotherm, the reaction heats the polymer and does not transmit the heat quickly. Therefore, the outside edges of the polymer can dissipate the heat but in the middle it cannot, which makes the middle react faster and faster until it is hard and all shrinkage has happened. The surrounding polymer is still liquid and flows into the shrunken area so no shrinkage is noticed. This is the case when the potting is open to the air. If it is totally enclosed, the shrinkage will be a problem.The speed of hardening also affects the shrinkage. The faster the gel time, the greater the shrinkage. So, ideally, the best system is one which hardens very slowly and which has no exotherm. However, production speed is also a factor. So a balance of fast gel and low shrinkage requirements is what makes the selection of an ideal potting compound difficult.
When certain hardeners are used the surface of the potting compound can become mottled especially when the humidity is high when the system hardens. To reduce this effect cure the resin system in at a low temperature oven, 100°F. If you want a very shiny surface then change the resin system to one that does not blush. See Surface Appearance
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Last Modified: July 20, 2012