by Jim Greig, Global Sales and Marketing Manager Electronic Materials, LORD Corporation
This is the question that most people consider first when planning to pot an electrical or electronic device. The answer depends on the specific application and the requirements of the device in its intended working surroundings. Typically most epoxy, urethane or silicone systems provide the environmental and electrical protection needed at ambient conditions. The real differentiation comes into play when you evaluate the operating conditions of your device and the specific additional requirements you want from your potting compound besides ambient protection.
The physical and electrical properties of the resin systems do vary within a chemistry family such as epoxy, urethane and silicone. A range of hardness, chemical resistance, adhesion and electrical insulation can be achieved in any of these groups. Normally, there are some differences in the handling properties and cured properties of the materials that can be used to give an overview of each potting compound family.
In typical formulations, epoxy compounds have better adhesion, chemical resistance and high temperature physical properties. Epoxy systems are usually more rigid and have a higher modulus and tensile strength. They work well for a wide range of applications because of their adhesion to a wide range of substrates without the use of primers. Most epoxies have excellent moisture resistance so they can be used in outdoor applications. Further, their ability to provide electrical insulation makes them ideal for potting transformers and switches.
The epoxy systems are usually divided into three categories: 1) room-temperature cure two-component materials (these are usually amine curatives), 2) heat-curing two-component materials (these are usually either anhydride or aromatic amine curatives), and 3) heat-curing one-component materials (these are usually catalytic curatives that disassociate or solubilize at higher temperatures).
In general, the urethanes have better flexibility, elongation and abrasion resistance. These properties give urethanes an advantage in devices where there are large stresses due to mismatch of component composition or dimensions. Many urethanes have very low Tgs so they can easily protect components in the normal consumer electronics operating range of -40C to 105C. Some urethanes can be used down to -70C and some up to 130C.
Urethane formulations can be adjusted easily to have very fast gel times at room temperature or very slow gel times with very low exotherms. They can be formulated to a range of hardness from soft gels to high Shore D.
The urethane chemistry is usually a two-component system with one side being a polyol and the other side being an isocyanate. The isocyanate is sensitive to reaction with moisture in the air, so it is recommended to protect any unused material by putting a layer of nitrogen gas in the container to purge any air out of the container.
The silicone potting compounds are similar to the urethanes in flexibility and elongation. But, unlike the urethanes, the standard silicones do not have a high enough strength to withstand abrasion. There are high-strength silicone potting compounds, but they usually are too high in viscosity for most applications.
Silicones offer the widest range of operating temperatures for a potting compound. Specialty silicone polymers can stay soft to below -100C and some formulated silicone materials can handle 200C operating temperatures. The silicone material stays soft throughout these temperature ranges and provides protection for components with the minimum amount of stress.
Silicones are the safest of these three types of potting compounds from a health and environmental aspect. They are usually easy to mix two-component systems with convenient mix ratios. One-component heat cure systems are also produced.
Chemically, there are two types of silicone systems used for potting compounds. One is the vinyl polymer-based material that is platinum catalyzed. This platinum can be neutralized by sulfur, amine and tin salts, leaving a potting compound with areas of cured material where the contamination is located. The other silicone chemistry is the condensation type, which uses tin salts as catalysts. These formulations usually offer better adhesion but are not good for potting very large masses due to unevenness of cure.
Following is a table of generic uncured and cured properties for epoxy, urethane and silicone potting compounds.
|Cost of Material||Medium||Low to Medium||High|
|Ease of Handling||Good||Good||Excellent|
|Uncured Moisture Sensitivity||Low to Medium||High||Low|
|Speed of Cure||Slow to Fast||Slow to Fast||Slow to Fast|
|Exotherm||Low to High||Medium||Low|
|Shrinkage||Low to High||Low to Medium||Low|
|Hardness (stiffness)||Medium to Hard||Soft to Medium||Soft|
|Expansion Rate||Low to Medium||Medium to High||High|
|Tensile Strength||Medium to High||Medium||Low|
|Tear Strength||High||Medium to High||Low|
|Elongation||Low||Medium to High||High|
|Component Stress||Poor to Fair||Good||Excellent|
|Thermal Shock/ Thermal Cycling||Fair to Good||Good||Good|
|High Temperature Operation||Good||Poor||Excellent|