THERMOSETS Thermosetting plastics are low-molecular-weight monomers and oligomers with multi- ple reactive functional groups, which can be poured, melted, or squeezed into the shape we want and then solidified again by chemical reactions forming multiple primary covalent bonds that cross-link them into three-dimensional molecules of almost infinite molecular weight. These are irreversible chemical processes that cannot be repeated. They account for 15 percent of the plastics industry, they include a great variety of chemical reactions and conversion processes, and they go into a very broad range of final products. Thus, there is a great difference between thermoplastics and thermosets, both in terms of materials chemistry and applications, and in terms of the mechanical processes used to produce finished products.

MATERIALS AND APPLICATIONS The major thermosetting plastics, in order of decreasing market volume, are polyurethanes, phenol-formaldehyde, urea-formaldehyde, and polyesters. More specialized thermosets include melamine formaldehyde, furans, “vinyl esters,” allyls, epoxy resins, silicones, and polyimides. While they may sometimes compete with each other and with thermoplastics, for the most part, each of them has unique properties and fills unique markets and applications

Polyurethanes Polyurethanes are the leading family of thermosetting plastics. Of the 100 or so families of commercial plastics, they are the most versatile, finding use in rigid plastics, flexible plastics, elastomers, rigid foams, flexible foams, fibers, coatings, and adhesives. They offer unique qualities in processability, strength, abrasion resistance, energy absorption, adhesion, recyclability, and resistance to oxygen, ozone, gasoline, and motor oil. Thus, they find major use in appliances, autos, building, furniture, industrial equipment, packaging, textiles, and many other fields. Their versatility comes from the range of liquid monomers and oligomers that can be mixed, poured, polymerized, and cured in a minute or so at room temperature.

Polyurethane Products

 Formaldehyde Copolymers Formaldehyde reacts readily with several types of active hydrogen monomers (phenol, urea, and melamine) to form highly cross-linked thermoset plastics. They form a family in their fundamental chemistry, and they form complementary families in terms of materials properties, markets, and practical applications.  Phenol-Formaldehyde Phenol-formaldehyde resins were the first commercial synthetic plastics. Since their invention in 1908, they have grown and matured into the second most important family of thermoset plastics, with a U.S. market volume of 4 billion lb/yr

 Urea-Formaldehyde. Urea-formaldehyde resins are one of the oldest families of commercial plastics; with a U.S. market volume of 3 billion lb/yr, they are the third largest thermosetting resin. Urea and melamine have similar polymer chemistry, so they are often discussed together as “amino resins;” but their markets and applications are quite different and are best studied separately.  Melamine-Formaldehyde. Melamine-formaldehyde and urea-formaldehyde have similar polymerization chemistry, so they are often referred to as “amino resins.” However, they differ in properties, applications, economics, and market volume, so they are best studied independently. Melamine offers superior resistance to heat, weather, and moisture, but it is more expensive than urea, so it is used only when its superior performance is required. The U.S. market volume is about 350 million lb/yr

Unsaturated Polyesters. Unsaturated polyesters are the fourth largest family of thermosetting plastics, with a U.S. market volume of 2 billion lb/yr. They are often called thermosetting polyesters or alkyds. In commercial use for 60 yr and now fairly mature, they are the largest class of reinforced plastics (Table 3.16), popularly used in building panels, chemical equipment, boats, cars, buses, trains, and planes.

Epoxy Resins Epoxy resins enjoy a combination of fast, easy cure, high adhesion to many surfaces, and heat and chemical resistance, which leads to a U.S. market of 600 million lb/yr with a wide range of uses in plastics, coatings, and adhesives. The name “epoxy resins” is applied loosely both to epoxy monomers and prepolymers, and also to the cured thermoset final products

Cross-Linking of Thermoplastics Thermoplastics are generally stable linear molecules, but we do occasionally cross-link them to improve specific processes or properties. Some of them contain reactive groups that can be cross-linked directly. Others we modify so as to make them accessible to cross- linking.

Quite a number of typical improvements can be made by cross-linking: Foam processing Modulus Strength Creep-resistance Adhesion Abrasion-resistance Dimensional stability Heat deflection temperature Heat-shrink film and tubing Hot strength Dimensional stability Flame retardance Solvent resistance Water resistance Gelation Age resistance (Cage effect)

PROCESSES There are a great variety of processes for manufacturing thermoset plastic products. Some of them are modifications of conventional thermoplastic processes, but most of them are uniquely designed for handling the simultaneous shaping and cross- linking that are involved. They may be classified as molding processes, reinforced plastics processes, pouring processes, and powder processes.

Molding Processes Liquid B-stage resin is held in a closed cavity and heated to crosslink it to a rigid solid product. This is done in a variety of ways Compression Molding. The original and classic method of producing thermoset plastic products is by compression molding. A two-part steel mold is made with a cavity representing the shape of the desired product. The cavity surface is chrome-plated to give a smooth corrosion- resistant finish. The mold is mounted in a vertical compression press with two horizontal platens. The mold half with the deeper cavity is mounted on the lower platen; the other mold half is mounted on the upper platen. The mold is heated (143 to 232°C), originally by steam and more recently by electricity

The thermosetting resin is measured into the lower mold cavity, either by weight or by volume, or preferably as a cold- pressed preformed pellet. The press is closed to heat and compress the resin. If the cure reaction releases water or other volatile by-products, the press is opened briefly to release the gases and then closed again at full pressure (1,000 to 12,000 psi). Molding pressure is maintained until thermosetting cure is complete (1/2 to 5 min). Then, the press is opened, and the molded product is ejected by the help of “knock-out pins.” After that, the cycle is repeated.

For faster, easier, more uniform melt flow, the performs can be preheated to 82 to 138°C by microwave or infrared heaters. This reduces abrasion of the mold and produces higher- quality molded parts. Originally, the molding cycle was carried out manually. With progress, most molders converted to semiautomatic operation: the operator loads the resin into the mold and takes the product out of the mold, but he activates the process by simply pressing a button, and the entire molding cycle proceeds automatically. More recently, many molders have converted to fully automatic processes wherein loading, molding, and removing the molded product are all done automatically on a preset cycle

Transfer Molding. Transfer molding is intermediate between compression and injection molding. Whereas the compression mold has just one cavity for the finished product, the transfer mold also contains a preliminary auxiliary cavity. The resin is loaded into the auxiliary cavity (“pot”), preferably as a preheated perform, where it is heated to melt processing temperature. Then, a plunger forces the molten resin into the final mold cavity, where it cures to the finished product. This eliminates the need to “breathe” out gases. It operates at lower pressure and therefore only needs a lighter weight mold. It is more fluid, so there is less mold wear. It gives a faster cycle and a more uniform product, to very close tolerances. There is less flash, so less post-molding finishing is needed

It is particularly useful when making complicated parts, thin walls, working with fragile inserts, and encapsulation. The original process used a single ram, both to close the mold and to transfer the resin from the pot to the mold cavity; this was a manual operation. The preferred process at present uses one ram to close the mold and another auxiliary ram to transfer the resin from the pot to the mold cavity; this is semiautomatic. A third process, which is used occasionally, uses a screw preplasticator to warm the resin and feed it to the transfer pot; this is completely automatic.