Resins used in creating phenolic sheet
Phenolic resins - are proven superior in high temperature applications and particularly useful in composite parts that must comply with smoke emissions, combustion and toxicity requirements. Previously available only in pre-preg form or as injection molding compounds, phenolic resins now come in low viscosity versions that are easier to process and practical for a wide range of applications including those requiring low volumes. When impregnated with fabric, phenolic sheets offer low density, good thermal insulation, outstanding durability and ease of formability to complex contours.
Epoxy resin – is another popular form of thermoset that is used for carbon fibers and electronic applications like printed circuit boards. Epoxies are generally more expensive than polyesters but they have less tendency to shrink and they have higher strength and stiffness at moderate temperatures. Epoxy resins are also resistant to corrosion when exposed to solvents, alkalis and some types of acids. Different formulations of epoxy resins provide the desired properties for phenolic sheets so that they can be used for the desired application.
Melamine resins – have excellent dielectric properties with low molecular weight and are water soluble. These properties make them very useful for laminating and impregnating paper, paperboard, cotton cloth and glass fibers. Recent developments in the composition of melamine resins have virtually eliminated alcohol emissions as well as particulate emissions into the environment during the process of lamination.
Making a choice for substrate
Phenolic laminates make use of different substrates like Kraft paper, mica paper, cotton canvas cloth, cotton linen cloth or glass cloth. The combination of substrate and resin must provide a different set of properties that will be useful for the intended application. Paper is commonly used when cost is a critical factor. Paper retains superior electrical properties but there is usually a noticeable loss in mechanical properties when compared to other NEMA grades of laminates. In the paper reinforced phenolic, the more the number of X’s the better are the electrical properties. When machinability and mechanical strength is important for the application, cotton can address the requirement. The most popular NEMA grades for cotton are CE for the coarse weave fabric and LE for a fine weave fabric.
When using glass fiber as reinforcement, cost is also the deciding factor. Glass fibers are priced according to quantity, filament diameter and other significant factors. Product designers must weigh in the benefits of using the more expensive glass fibers against their appropriateness for the application. In the reinforced phenolic laminate the resin properties dominate and they are enhanced by the glass fibers.