NEMA grades G10 and FR4 Glass-Cloth Reinforced Epoxy - natural color is typically a yellowish to light green. The most versatile all-around laminate grades are continuous glass woven fabric impregnated with an epoxy resin binder. Epoxy resins are among the most versatile and widely used plastics in the electronics field, primarily because water absorption is virtually nil, rendering it an outstanding insulator. Beyond its electrical insulating properties, epoxy resins exhibit great dimensional stability (shrinkage is usually less than 1 percent) and superior adhesive properties. G10/FR4 has extremely high mechanical strength, good dielectric loss properties, and good electric strength properties, both wet and dry. The main difference between NEMA Grades G10 and FR4 is that FR4 is a fire retardant grade of G10. Therefore, FR4 can be safely substituted where G10 is called out, while G10 can never be substituted where FR4 is called for.