Thermosetting resins cross-linked material

Synthetic polymers can be classified as thermo-plasts, which soften under heat and can be reversibly melted, and thermosets, which by the action of heat or chemical substance undergo chemical reaction and form insoluble materials that cannot be melted. Mixtures of molecules of relatively low molar mass (hundreds to thousands) that are able to react mutually or with other compounds and form cross-linked materials are often called synthetic resins. That is, synthetic resins such as epoxy or polyester or phenol-formaldehyde resins are thermosets. The term oligomer refers to a polymer molecule with relatively low molar mass (roughly below 10 000 g mol whose properties vary significantly with the removal of one or a few of the units. 

Two-Stage Resins. The ratio of formaldehyde to phenol is low enough to prevent the thermosetting reaction from occurring during manufacture of the resin. At this point the resin is termed novolac resin. Subsequently, hexamethylenetetramine is incorporated into the material to act as a source of chemical cross-links during the molding operation (and conversion to the thermoset or cured state). 

The commonly used resins in the manufacture of decorative and industrial laminates ate thermosetting materials. Thermosets ate polymers that form cross-linked networks during processing. These three-dimensional molecules ate of essentially infinite size. Theoretically, the entire cured piece could be one giant molecule. The types of thermosets commonly used in laminates ate phenoHcs, amino resins (melamines), polyesters, and epoxies. 

The thermoplastic or thermoset nature of the resin in the colorant—resin matrix is also important. For thermoplastics, the polymerisation reaction is completed, the materials are processed at or close to their melting points, and scrap may be reground and remolded, eg, polyethylene, propjiene, poly(vinyl chloride), acetal resins (qv), acryhcs, ABS, nylons, ceUulosics, and polystyrene (see Olefin polymers Vinyl polymers Acrylic ester polymers Polyamides Cellulose ESTERS Styrene polymers). In the case of thermoset resins, the chemical reaction is only partially complete when the colorants are added and is concluded when the resin is molded. The result is a nonmeltable cross-linked resin that caimot be reworked, eg, epoxy resins (qv), urea—formaldehyde, melamine—formaldehyde, phenoHcs, and thermoset polyesters (qv) (see Amino resins and plastics Phenolic resins). 

The thermosetting materials are said to be initially linear but are cross-linked by heating in air to a temperature of at least 345°C. It is claimed that they have a useful working range up to 315°C. The materials may be used in compression mouldings powders, as the binder resin in glass cloth laminates and as the polymer base in heat-resistant metal coatings. 

Of the various amino-resins that have been prepared, the urea-formaldehyde (U-F) resins are by far the most important commercially. Like the phenolic resins, they are, in the finished product, cross-linked (thermoset) insoluble, infusible materials. For application, a low molecular weight product or resin is first produced and this is then cross-linked only at the end of the fabrication process. 

Nowadays the major thermosetting resins used in conjunction with glass fibre reinforcement are unsaturated polyester resins and to a lesser extent epoxy resins. The most important advantages which these materials can offer are that they do not liberate volatiles during cross-linking and they can be moulded using low pressures at room temperature. Table 3.1 shows typical properties of fibre reinforced epoxy. 

Rubbers and thermosetting plastics The conventionally covalently cross-linked rubbers and plastics cannot dissolve without chemical change. They will, however, swell in solvents of similar solubility parameter, the degree of swelling decreasing with increase in cross-link density. The solution properties of the thermoelastomers which are two-phase materials are much more complex and dependent on whether or not the rubber phase and the resin domains are dissolved by the solvent. 

Thermosetting materials, which have a rigid, cross-linked structure for example, the polyester and epoxy resins. 

---