www.maida.com
201 S. Mallory Street sales@maida.com
Hampton, Virginia 23663
Phone: (757) 723-0785
DEVELOPMENT COMPANY
Manufacturers of Ceramic Capacitors, Varistors and Thermistors
Established 1947
www.maida.com
02/06/2019
Phenolic vs. Epoxy Coating
This white paper is designed to compare the advantages versus the disadvantages of using a
phenolic coating instead of the typical epoxy coating of Metal Oxide Varistors (MOV’s). It
should be noted that Maida utilizes a phenolic distinctly different from most varistor
manufacturers in the industry. Maida uses a true phenolic while most others use a phenolic-
epoxy hybrid.
Definitions:
Metal Oxide Varistor (MOV): Typically a zinc oxide ceramic based electronic component
which has a non-linear voltage-current characteristic. A MOV is used in applications that require
protection against transients. These transients can be created by: lightning – conducted or
induced, by the switching of inductive loads (transformers, relays, or coils), by Electromagnetic
Pulses (EMP), and ESD.
Epoxy: A basic component and cured end product of epoxy resins. Used as an electrical insulator
for electronic components.
Phenolic: A basic component of phenolic resins. Used as an electrical insulator for electronic
components.
A MOV is typically conformal coated with an insulating material (see Figure 1). The insulating
material is typically an epoxy or phenolic material. A MOV will operate and perform its
designed function without the need for conformal coatings. The typical purpose of such coatings
allow for reduced creepage and clearance distances required, on end product Surge Protective
Devices (SPD’s) for example, by numerous safety standards – such as UL840.
.
Figure 1
www.maida.com
201 S. Mallory Street sales@maida.com
Hampton, Virginia 23663
Phone: (757) 723-0785
DEVELOPMENT COMPANY
Manufacturers of Ceramic Capacitors, Varistors and Thermistors
Established 1947
www.maida.com
There are advantages for using either epoxy or phenolic coatings. The use of either coating is
dependant on the end application:
Epoxy Advantages:
High Dielectric Withstand
Moderate to High Moisture Resistivity
Moderate to High Chemical Resistivity
High abrasion and wear resistance
Epoxy Disadvantages:
Less resistant to burning
Phenolic Advantages:
High Resistance to Flame
Moderate to High Thermal Conductivity (useful for Thermal Cut-Off devices)
Moderate Moisture Resistivity
Moderate Chemical Resistivity
Phenolic Disadvantages:
Low to Moderate Dielectric Withstand
Higher Cost
As listed above the main disadvantage for using an epoxy coating is that is burns under flame
test and will burn during out-of-specification overvoltage conditions if appropriate disconnects
are not installed in the application. The burnt epoxy can result in debris on printed circuit boards
causing additional complications in the circuit. The main disadvantage for using a phenolic
coating is its lower dielectric withstand voltage. Maida’s phenolic coating requires much thicker
coating, relative to the epoxy, to obtain the required dielectric withstand ratings.
The most important characteristic of the phenolic coating used by the Maida Development
Company is the high resistance to flame. As seen in Figure 2 the image on the left is the phenolic
coated varistor while the image on the right is the epoxy coated varistor. This is the result of a
Flame Test, which is the application of a propane torch blue flame for 60 seconds. The flames
continued to burn on the standard MOV even after the propane torch was removed. There were
no flames on the phenolic varistor even under the propane torch. Similar results are obtained
during limited current abnormal overvoltage testing (UL1449 3
rd
Edition).