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INTRODUCTION TO INDUSTRIAL LAMINATES
In 1906 Dr. Leo Bakeland experimented with the polymerization of phenolic resins. He found that by adding formaldehyde and heat, a chemical cross linkage took place; thermoset plastics were born. Soon after, it was discovered that cotton cloth and paper materials could be impregnated with this same mixture, semicured and then made into a stack of sheets or wrapped around a mandrel and subsequently put into a hydraulic press where heat and pressure could be applied. Full polymerization took place rendering hard, dense, reinforced thermoset plastics which today are known as industrial laminates.
Simply put, thermoset plastics can be compared to cement; once cured they're set, hence the name thermoset. Thermoplastics can be compared more to wax inasmuch as they can be remelted and reshaped upon the reapplication of heat. However, one key resulting difference is that thermoplastics lack the rigidity of thermoset laminates, as the tensile modulus of the following materials indicate:
- Graphite Epoxy (thermoset plastic) 40,000,000 psi
- Steel (metal) 30,000,000 psi
- Aluminum (metal) 10,000,000 psi
- Glass Epoxy (thermoset plastic) 5,800,000 psi
- Polycarbonate (thermoplastic) 450,000 psi
Thermoset Plastic Industrial Laminates
Thermoset plastic industrial laminates are uniformly dense and structurally strong materials that will not soften appreciably under the reapplication of heat. They are extremely durable plastics that are lightweight and moisture resistant. They are thermoset plastic resin impregnated reinforcing substrate materials that are cured under heat and pressure to form solid shapes having high mechanical and insulating properties. Industrial laminates are available in sheet, rod, tube and angle. Since these laminates are comprised of a combination of materials, they are also referred to as composites.
Thermoset plastic industrial laminates typically have a layered construction with no fewer than two components:
The first is a reinforcing substrate such as woven glass cloth, random glass mat, glass filaments, woven canvas cotton fabric, paper, woven aramid fabric, random mat aramid, woven graphite fabric, random mat graphite and others. The second is a thermoset plastic resin binder that serves to adhere the layers of reinforcing substrates to each other to form a solid unit. Resin binders include epoxies, melamines, phenolics, polyesters, silicones and others.
Unlike other groups of plastic materials, thermoset plastic industrial laminates have their own standards which are published by NEMA (National Electrical Manufacturers Association). In concert with member manufacturers, NEMA standards are set and "minimum values" published. The most commonly used NEMA thermoset laminate grades are as follows:
Epoxy Grades
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 superior adhesive properties and great dimensional stability -- shrinkage is usually less than 1 percent.) 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 for, while G10 can never be substituted where FR4 is called for. (G10 certifies to Mil-I-24768/2 GEE; FR4 certifies to Mil--24768/27 GEE-F.)
NEMA grades G11 and FR5 Glass-Cloth Reinforced Epoxy -- natural color is typically yellow green to amber. This grade is similar to G10/FR4 with the addition of a higher operating temperature and some improved mechanical strength at elevated temperatures. The main difference between NEMA Grades G11 and FR5 is that FR5 is a fire retardant grade of G11. Therefore, FR5 can be safely substituted when G11 is called for, while G11 can never be substituted where FR5 is called for. (G11 certifies to Mil-I-24768/3 GEB; FR5 certifies to Mil-I-24768/28 GEB-F.)
Melamine Grades
NEMA grades G5 and G9 Glass-Cloth Reinforced Melamine -- natural color is typically a grayish brown. These grades are composed of a continuous glass woven cloth base impregnated with a melamine resin binder. (Melamine resins, once cured, are the hardest, most rigid and abrasion resistant of the standard resins employed in the production of industrial laminates. However, prolonged exposure to high temperature can adversely affect its mechanical and electrical strength properties. Arc resistance, however, may remain unaffected despite excessive thermal exposure.) Melamines are the hardest of laminates, exhibiting good dimensional stability and arc resistance. It's also caustic resistant. A key difference between NEMA Grades G5 and G9 are that G9 is more resistant to the elements of the environment. Thus, G9 can be safely substituted where G5 is called for. (G5 certifies to Mil-I-24768/8 GMG; G9 certifies to Mil-I-24768/1 GME.)
Silicone Grade
NEMA grade G7 Glass-Cloth Reinforced Silicone -- natural color is typically cream to white. Composed of a continuous glass woven clothe base impregnated with a silicone resin binder, this grade has excellent heat and arc resistance. (Silicone thermosetting resins are among the best of all polymer materials in resistance to temperature. Hence, silicone is broadly used for high temperature electronic applications requiring low electrical losses. Silicone isn't as strong as epoxies and phenolics upon aging at lower temperatures but is stronger upon aging over 400° F.) G7 has extremely good dielectric loss properties under dry conditions and good electrical properties under humid conditions, although the percentage of change is high (G7 certifies to Mil-I-24768/17 GSG.)
Polyester Grades
NEMA grade Glass-Mat Reinforced Polyester -- GPO-1 (tan color), GPO-2 (red color) and GPO-3 (red color). These grades are composed of random mat (non-woven) fiberglass reinforcement held together by polyester resin binder. (Polyesters are versatile resins which handle much like epoxies. Of course, the basic resins are chemically different. It's their physical application forms which make them similar. Despite lower costs, the important disadvantages of polyesters, as compared with epoxies, is lower adhesion to most substrates, higher polymerization shrinkage, a greater tendency to crack during cure or in thermal shock and greater change of electrical properties in a humid environment.) (GPO-2 offers superior arc resistance over GPO-1 while GPO-3 offers both arc and track resistance. (GPO-1 certifies to Mil-I-24768/4 GPO1; GPO-2 certifies to Mil-I-24768/5 GPO2 and GPO3 certifies to Mil-I-24768/6 GPO3.)
Phenolic Paper Grades
NEMA grades X, XX and XXX Paper Reinforced Phenolic -- natural color is typically light tan to brown. These grades are composed of a paper base impregnated with a phenolic resin binder. (Phenolics are the oldest, best-known general purpose thermoset resins. They are among the lowest in cost and easiest to process. Phenolics are quite adequate for a fair percentage of electrical applications. Generally speaking, they are not equivalent to epoxies in resistance to moisture, humidity, dimensional stability, shrinkage and retention of electrical properties in extreme environments.) The paper reinforced phenolic has good electric strength properties with fair mechanical strength and are outstanding for use as template material and/or back-up material. A good rule of thumb in selecting a paper grade is to remember the more X's the better the electrical properties, while the fewer X's represent superior mechanical properties. Paper grades can be modified by adding plasticizers to make them more conductive to part punching resulting in a P suffix added to the X designation, i.e., XP, XXP, XXXP. (Grade X certifies to Mil-I-24768/12 PBM; grade XX to Mil-I-24768/11 PBG; and grade XXX to Mil-I-24768/10 PBE.)
Phenolic Cotton Grades
NEMA grades C, and CE Canvas Cotton-Cloth Reinforced Phenolic -- natural color is typically light tan to brown.
C and CE grades are composed of a continuous cotton woven cloth impregnated with a phenolic resin binder. These grades contain a medium weave canvas and are known primarily for their mechanical properties. These grades are not recommended for primary electrical insulation. (Grade C certifies to Mil-I-13768/16 FBM; grade CE certifies to Mil-I-24768/14 FBG.)
NEMA grades L and LE Linen Cotton-Cloth Reinforced Phenolic -- natural color is typically light tan to brown. This grade is composed of a continuous cotton woven cloth impregnated with a phenolic resin binder. This grade contains a fine weave linen and, like the canvas phenolic, is known for its mechanical properties. The finer weave allows for machining more intricate details than canvas grade, such as gear teeth, etc. These grades are not recommended for primary electrical insulation, but grade LE exhibits superior moisture resistance to grades L, C and CE, and thus is an adequate insulator for a fair number of applications. (Grade L certifies to Mil-I-24768/15 FBI; Grade LE certifies to Mil-I-24768/13 FBE.)
Industrial laminates are used in many industries:
Electronics, Electrical, Machinery, Automotive and Trucking, Metal Finishing, Aviation, Chemical, Home Appliance, Textile.
Of these, the largest markets are electrical and electronics, thermoset industrial laminates provide strong reliable insulators for applications in:
Relays, Bus Bars, Washers, Vanes, Panels, Coils, Motors, Gaskets, Circuit Boards, Switches, Standoffs, Arc Shields, Test Boards, Sockets, Fuses, Generators, Transformers, Condensers and Terminal Strips.
By Michael Stacey, President, Accurate Plastics, Inc., manufacturers of industrial thermoset laminate sheet, rod, tube, angle, and channel.
For more information, contact Accurate Plastics, Inc., 18 Morris Place, Yonkers, NY 10705-1929, 800-445-1884, FAX 304-723-1625
Neither The Plastics Distributor & Fabricator Magazine, nor PMD Publishing, Inc., is responsible for the information or opinions contained in this magazine. All such information and opinions are those of the authors.
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