Epoxies are versatile polymer systems that are “go-to materials” for electrical, electronic, and microelectronic systems, especially in applications where outstanding electrical insulation properties are needed. Their wide usage is due to their excellent adhesion to a wide variety of substrates, superb chemical and heat resistance, and long-term durability. They are serviceable for bonding, sealing, coating, and encapsulating/potting applications.
The primary focus of this article is twofold; the first is to discuss the electrical insulation properties as they pertain to epoxies. The other is to delve into the variation of these properties, based on the chemistry of the system (especially the role of the curing agent) as well as the operating conditions of the application.
Prior to curing, an epoxy consists of a resin and curing agent, which when mixed, polymerize and form a cured matrix. There are many different types of epoxy resins and curing agents. When combined, they create distinct cross-linking patterns resulting in different attributes of the polymerized system. The choice of the curing agent depends not only on the electrical insulation values desired, but also on other parameters such as operational temperatures, chemical resistance, and physical strength requirements, among others. Another consideration in selecting the hardener is to assess its processing capabilities and constraints. We will start by discussing some of the fundamental electrical insulation properties, i.e. dielectric constant, dissipation factor, dielectric strength, and volume resistivity. We will then correlate these values in terms of processing to the ultimate properties obtained with various groups of curing agents, including aliphatic amines, polyamides, cycloaliphatic amines, aromatic amines, anhydrides, lewis acids, and imidazoles.
Also known as relative permittivity, the dielectric constant indicates the ability of a material to store electrical energy in response to an electric field. It is a dimensionless number defined as the ratio of the permittivity of a material relative to that of a vacuum, where permittivity is a measure of the electrical energy stored as a result of an applied voltage. Generally, a low value (2-5) is desirable for epoxies and other materials intended for use as electrical insulators, although in certain applications, a mid-level dielectric constant (6-12) is required.
The standard test method for measuring the dielectric constant of a solid electrical insulating material is ASTM D150. It involves placing a sample of the material between two capacitor plates and measuring the resulting capacitance — the ability to store an electrical charge. This is then compared to the capacitance of the same plates with air or a vacuum between them. The resulting ratio is the dielectric constant of the material.
For a cured epoxy system, the dielectric constant varies with temperature, frequency, and filler. For instance, a particular system may have a dielectric constant that increases with temperature (3.46 at 23 °C, 3.55 at 100 °C, and 4.24 at 150 °C) for a 60- Hz application, but fluctuates with temperature (3.28 at 23 °C, 2.99 at 100 °C, and 3.87 at 150 °C) for a 1-KHz application. In general, but not always, the dielectric constant increases with higher temperatures and decreases with higher frequencies. Essentially, epoxies lose some of their insulation capabilities at higher temperatures, but exhibit better insulation properties for higher frequencies. The addition of mineral filler particles increases the dielectric constant of a particular epoxy system slightly, while metallic fillers will have a more notable impact.
The dissipation factor (DF) is a measure of power loss in a material subjected to an alternating electric field. According to the standard ASTM D150, the DF is the ratio of the power dissipated to the power applied. (An additional standard, ASTM D2520, is recommended for characterizing DF at microwave frequencies.) A lower DF is desirable in order to reduce the heating of the material and minimize the impact on the surrounding circuit. Dissipation factor can be a very useful measure of other characteristics of a material, such as degree of cure, voids, moisture content, and contamination. Over time, a significant change in DF can occur when the operational conditions are too severe for the cured system.
The DF is typically 0.003 to 0.030 at 1 KHz, and up to 0.050 at 1 MHz. At ambient temperatures, DF (in most cases) increases as the frequency gets higher. As temperature rises, the effect on DF varies greatly depending on the operating frequency and the specific chemistry. For example, at 1 KHz, the dissipation factor of a particular system falls from roughly 0.02 to less than 0.01 as the temperature increases from ambient to 125 °C, at which point the DF rises dramatically, nearly reaching 0.8. For the same system operating at 8.5 × 109 Hz, the DF rises gently from 0.02 and then levels off below 0.05 as temperature increases.
The overall effect of mineral fillers is to somewhat increase the DF, although the degree of change is highly dependent on temperature and frequency. For metallic fillers, the DF increases greatly.
Another significant criterion in assessing isolation properties of an epoxy is the dielectric strength, which is often expressed in volts/mil (1 mil = 0.001 inch). This is defined as the maximum voltage that can be applied across a sample of the material without causing dielectric breakdown. The resistance of the material in dielectric breakdown decreases rapidly and it becomes electrically conductive.
ASTM D149 is the standard test used to determine theoretical dielectric strength. The test method consists of placing a sample of the material between two electrodes in water or oil and applying a voltage across the electrodes. The voltage is then increased at a uniform rate from zero until the point at which the material exhibits burn-through punctures or begins to decompose. The resulting breakdown voltage is divided by the sample thickness to derive the intrinsic dielectric strength. Higher values indicate better electrical insulation characteristics.
In practice, dielectric strength is highly dependent on the thickness of the material, with thinner samples having higher values per unit thickness. For example, the dielectric strength values for epoxy systems could be as high as 2,000 volts/mil for a 0.010" sample, gradually reducing to about 425-475 volts/mil for a 0.125" specimen. Thicker sections tend to retain this dielectric strength value of about 425-475 volts/mil at ambient temperatures. Thus, one of the major factors in assessing an epoxy’s dielectric strength is a very precise elucidation of the test method used as it relates to the thickness of the cured epoxy. Dielectric strength generally decreases as operating temperature or frequency increases. Since dielectric strength is application dependent, it is important to validate epoxies for their dielectric strength for specific uses, especially for those involving high currents.
Most non-conductive mineral fillers have little effect on the epoxy’s dielectric strength, and metallic fillers decrease the dielectric strength depending on the nature of the filler and filler loading.