Potting Compound Types
There are six basic types of resins used in potting with those five having categories within them. The thermosetting materials are epoxy, urethane, silicone, acrylic and polyester defined by the chemistry that they use. There are resins that are combinations of these chemistries but we define the type by the chemical unit that cross links the polymers together. The sixth type are hot melt thermoplastic materials made from a base of wax and modified with asphalt and other materials for low cost.
Each of the types have positive and negative factors with them, so the selection of the type is a trade off of the factors. Below is a discussion of each of the types, their categories and the good and bad characteristics.
Strengths - Epoxy resins are very stable materials before and after hardening. They are easy to use and are predicable. They have good high temperature resistance up to 400°F (200°C) and excellent chemical resistance except for some acids. They give excellent strength and excellent adhesion to metals and porous surfaces. The epoxy resins are hardened with many chemical which gives them a full range of hardened properties. This include amines, acids and anhydrides.
Weaknesses - They are notch sensitive, which means that a small crack will easily spread though the whole hardened piece. This cracking problem limits the use in high impact situations unless special techniques are used. Potting of PC boards with SMT components is not recommended because of the Tg of the epoxy it too high. The lowest one I know of is -22C. The bond to some plastics is not very good because the epoxy is too hard and does not "give" so when the plastic bends the bond is easily broken. Epoxy is also a high energy material and some plastics are low energy so they are not "wet" by the epoxy. Lactic acid (milk) is very hard on epoxy and it will penetrate the potting. Also other strong acids will attack the epoxy.
Urethane or Polyurethane are a class of materials that form a urethane bond to make a polymer.
Strengths: Urethanes give a broad range of hardness from ,Shore D of 80 to Shore 000 of 50 (hard rubber to Jell-O). Since they can have Tg below -40°C they are a good selection for SMT PC board potting. Since their gel time can be easily changed with an accelerator without changing the properties, they can easily be modified by the supplier to meet you speed of hardening requirements.
Weaknesses: Resistance to elevated temperatures are limited to 130°C (266°F) continuous use and intermittent use at 150°C (300°F). The chemical resistance is not as good as epoxy but will stand splashes of chemicals but not immersion.
Strengths: Excellent high temperature 200°C and low temperature properties. Have Tg below -40°C so they are a good selection for SMT PC board potting.
Weaknesses: The cost is the largest factor. Care should be taken to use only not acid hardening type so that corrosion does not occur with time. Adhesion to parts can sometimes be a problem and primers may have to be used.
Other potting compounds include:
1. Acrylic potting compounds are UV and heat hardening materials - Strengths: Very fast hardening, adequate chemical resistance, clear in appearance. Weaknesses: high cost, limited thickness of the potting compound unless as secondary heat cure.
2. Polyester materials - Strengths: Low cost and good chemical resistance. Weaknesses: Shrinkage and cracking with temperature cycling. Must be highly filled to stop cracking.
3. Hot melt materials - Strengths: Very low cost materials. Weaknesses: Shrinkage, limited temperature since they melt, no chemical resistance other than resistance to water
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Last Modified: March 28, 2015