Essentially, linear high polymers comprising heterocyclic rings, or groups of rings, linked together by one or more covalent bonds. As the search has continued for polymeric materials having useful properties at high temperatures (500°C or 930°F or higher), much attention has been given to heterocyclic polymers. The possibility of forming rigid molecules that can be ordered into anisotropic arrays having exceptional stiffness may become of even greater interest. As a group such polymers are often both mechanically rigid and inherently resistant to thermal degradation.
Some of these polymers form molecules in which the rings are fused together, as shown symbolically in the illustration (ladder polymers), and some form molecules in which fused rings are joined by single bonds (stepladder polymers). Similar considerations hold for simple aromatic systems (for example, linear polymers of benzene), but the heterocyclic systems have been more useful in application.
In practice, the heterocyclic resins have rather high molecular weights and high glass transition temperatures. Some cross-linking may also be introduced during curing. Because of the insolubility and infusibility of the unmodified polymers, processing and fabrication was originally accomplished in stages. First, soluble prepolymers were prepared and fabricated into the final form desired (film, molding, coating, impregnated glass cloth, and so on). In this stage, the heterocyclic rings are not yet closed. Closure of the rings by condensation reactions was then effected by heating, and volatile by-products were eliminated. In contrast, poly(amide-imide) and the unmodified resins can be processed by more conventional techniques.
Major applications for these polymers are as metal-to-metal adhesives and as laminating resins for fibrous composites for structural applications in the aerospace industry. Other applications requiring both strength and resistance to oxidation at elevated temperatures have developed, including valve seats, bearings, and turbine blades. See also: Polymer composite
The basic synthetic reaction to form the prepolymer is the condensation of an aromatic dianhydride with an aromatic diamine. Thus, pyromellitic dianhydride may be added to 4,4′-diaminodiphenyl ether in an anhydrous medium to give a high-molecular-weight poly(amic acid), as shown in reaction (1). Solvents such as N,N-dimethylformamide are suitable media. The ingredients must be extremely pure, and must be present in equal molar amounts. Other dianhydrides and diamines may be used.
The prepolymer solution may then be used to impregnate fiber-glass cloth or other reinforcement, or applied to other substrates. Then, the solvent must be driven off and the rings closed by condensation, usually in stages [reaction (2)]. Frequently, most of the solvent is removed in a precuring stage and the major condensation effected later, at temperatures in the range 180–380°C (360–720°F). A subsequent postcuring step, also at elevated temperatures, may be used. The structure shown is of the stepladder type; ladder structures may be obtained by use of phenyl or fused-ring diamines.
Properties depend on the structures of the ingredients and on the reaction and curing conditions. Fiberglass composites that are made by using typical resins retain considerable strength after they are aged for 100 h at 500–600°C (930–1100°F). Adhesive bonds also show good resistance to aging at high temperatures. Certain polyimides are available as films and as molding powders, and some can be spun into fibers.
By introducing amide groups into a polyimide, some thermal stability is sacrificed, but improved processibility is gained. A typical structure is given in notation (3). Applications have developed in high-performance electrical connectors, engine parts, pumps, valves, and turbines.
The basic reaction for the synthesis of aromatic polybenzimidazoles is the reaction of an aromatic tetramine with an aromatic diacid or diester. Although the details of the intermediate steps are not completely understood, the overall reaction for a typical example, the condensation of 3,3′-diaminobenzidine and the diphenylester of isophthalic acid, is as shown in reaction (4).
Polymerizations may be conducted in the melt, and to varying degrees of reaction. Partially polymerized resins may be dissolved in solvents such as N,N′-dimethylformamide and applied as a solution for laminating and adhesive applications. As with other heterocyclic polymers, final curing is effected at elevated temperatures (up to about 400°C or 750°F) under pressure.
Properties depend on the structures of the ingredients, and on reaction and curing conditions. In general, polybenzimidazoles are somewhat less stable in air at high temperatures than the polyimides. Applications are mainly as laminating and adhesive resins for composites and metals.
Polybenzothiazoles are typically prepared by the reaction of a dimercaptobenzidine with an aromatic diacid, diester, or diacyl chloride, as shown in reaction (5).
A mainly soluble prepolymer is prepared by carrying the reaction forward to only a limited extent. As with the other resins in this family, the solvent is removed and the reaction is completed by heating after application to the substrate desired. Polybenzothiazoles find the same applications as other heterocyclic polymers and are intermediate in stability between polyimides and polybenzimidazoles. See also: Polyester resins; Polyether resins; Polymer; Poly(p-xylylene) resins; Polysulfone resins