What are the different types of potting and encapsulating compounds?
When first designing an application where potting or encapsulating is necessary, generally the first question that arises is what kind of material should I use: epoxy, urethane, or silicone? Many times, if all that is needed is protection of the circuitry or to hold in place deep fixtures, any type of material will work. The real answer depends on the specific application and the requirements of the device in its intended working surroundings. Typically, most epoxy, silicone, and urethane potting compounds will provide environmental and electrical protection at room temperature. The unique properties of the application and process are where you begin differentiating among the types of materials. Each family of chemistries has strengths and weaknesses and it is important to know what you need out of the potting material in the design. Among the three chemistries, there are differences in properties such as hardness, chemical resistance, adhesion, flame retardant properties etc. Normally, there are some differences in the handling properties as well the cured properties.
During the design or prototype stage it is helpful to consider production process as well. A fast setting, dual cartridge potting compound may be easy to use in a lab setting or for designs however is it best during mass production? If the material cures too quickly or does not have enough handling time on a production line there can be devastating consequences. There are also several properties to consider when testing potting materials such as conductivity (insulation), flame retardation, temperature etc. Standard potting materials not meant for high temperatures or that do not dissipate heat well may lead you to specialized systems formulated for specific characteristics.
Epoxy Potting Compounds
Epoxy potting compounds generally have better adhesion, high-temperature resistance, and chemical resistance. Epoxy systems have better adhesion to a wide variety of substrates and typically do not need primers. Epoxies also tend to have higher rigidity, modulus, and tensile strength however there are specialized low modulus epoxy potting compounds available as well. EC-1006M-4 and EC-1006M-5 are examples of low modulus, low hardness epoxy potting materials. Epoxies also generally have very good moisture resistance. This makes it an excellent option for outdoor applications. Epoxies have excellent dielectric properties and are used extensively in potting transformers and switches.
There are different types of epoxy potting compounds that are available. 2-part, room temperature epoxy potting materials are epoxy systems that will cure over a period of 24-48 hours at room temperature (25°C). These types of potting compounds are the most abundant in the market and is generally used for much of the applications that cannot or do not want to cure with heat. Generally these are amine or polyamide curing agents. The second option is a 2-part heat cure epoxy potting compound. 2-part heat cure systems require curing at elevated temperatures (usually 80°C and above) to achieve a full cure. Many of these types of epoxy potting compounds can be used at temperatures above 180 °C. These types of epoxy potting compounds are aromatic amine or anhydride curing systems. One-part epoxy potting compounds are used extensively for small mass potting applications. These types of systems are room temperature stable for anywhere from 2 weeks to 6 months. One part epoxy potting compounds have latent curing agents that solubilize at higher temperatures or catalytic curing agents that disassociate at higher temperatures.
Urethane Potting Compounds
Urethane potting compounds generally have better flexibility, elongation, and abrasion resistance. Many devices that require potting are comprised of several substrates including different metals, metal alloys, ceramics, glass, plastics etc. Urethane potting compounds have an advantage in devices where there are large stresses due to a mismatch in component composition. Many urethanes have very low glass transition temperatures (Tg) so they can easily protect components in the normal consumer electronics operating range of -40 °C to 105 °C. Specially formulated urethane potting compounds can be used as low as -70 °C and as high as 135 °C.
Urethane potting compounds can be formulated to have a wide range of physical properties. Urethane materials can be made to react rapidly at room temperature using a wide array of isocyanates and catalysts. Urethanes can also very long pot lives and little exotherm during cure. Most urethanes are soft and flexible with a hardness in the low to mid shore A. However, some urethane systems can be made to be harder measuring in the mid-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.
Silicone Potting Compounds
Silicone potting compounds and encapsulating materials are also soft, flexible, and have better elongation. Silicone materials also provide the widest operating temperatures. Specially formulated silicone potting compounds can operate at below -100°C and most silicone materials can handle 200 °C temperatures. Due to the low modulus of silicone potting materials, they exhibit the least amount of stress on components at these temperature ranges. Most silicone systems are available in convenient mix ratios. Many are 1:1 or 10:1 by weight or volume allowing for easy processing and automation.
There are two main types of silicone potting compounds. One type is a vinyl polymer based, platinum catalyzed silicone systems. Generally, platinum cure silicone systems can be made to be 1:1 ratio and offer excellent performance. The second type of silicone potting compound is a condensation cure system where generally tin is the primary catalyst. These are generally 10:1 systems. Tin cure silicones have better adhesion and but are not advised for potting large masses. Tin catalyzed systems are also not generally able to be accelerated by heat.
What are the costs of potting compounds?
While the costs of any of three types of potting compounds vary among manufacturers and distributors, there are trends that follow. Urethane potting compounds generally tend to be priced on the lower/medium end. Silicones on the other hand generally are more expensive than compared to epoxies and urethanes. Epoxy potting compounds have a wide range of prices, but they generally are cheaper than silicones and more expensive than most urethane potting compounds.