Theoretically, PC is formed from the reaction of bis-phenol A and carbonic acid. The structures of these two monomers are given in Figure 6.2.

Commercially, different routes are used, but the PC polymer of the structure shown in Figure 6.3 is the result.

Polycarbonate performance properties include:

Applications include glazing, safety shields, lenses, casings and housings, light fittings, kitchenware (microwaveable), medical apparatus (sterilizable), and CDs (the discs).

PBT is a semi-crystalline, white or off-white polyester similar in both composition and properties to PET. It has somewhat lower strength and stiffness than PET, is a little softer but has higher impact strength and similar chemical resistance. As it crystallizes more rapidly than PET, it tends to be preferred for industrial scale molding. Its structure is shown in Figure 6.4.

PBT performance properties include:

PET polyester is the most common thermoplastic polyester and is often called just “polyester”. This often causes confusion with the other polyesters in this chapter. PET exists both as an amorphous (transparent) and as a semi-crystalline (opaque and white) thermoplastic material. The semi-crystalline PET has good strength, ductility, stiffness, and hardness. The amorphous PET has better ductility but less stiffness and hardness.

It absorbs very little water. Its structure is shown in Figure 6.5.

PET has good barrier properties against oxygen and carbon dioxide. Therefore, it is utilized in bottles for mineral water. Other applications include food trays for oven use, roasting bags, audio/video tapes as well as mechanical components.

Liquid crystalline polymers (LCP) are a relatively unique class of partially crystalline aromatic polyesters based on 4-hydroxybenzoic acid and related monomers shown in Figure 6.6. Liquid crystal polymers are capable of forming regions of highly ordered structure while in the liquid phase. However, the degree of order is somewhat less than that of a regular solid crystal. Typically, LCPs have outstanding mechanical properties at high temperatures, excellent chemical resistance, inherent flame retardancy and good weatherability. Liquid crystal polymers come in a variety of forms from sinterable high temperature to injection moldable compounds.

LCPs are exceptionally inert. They resist stress cracking in the presence of most chemicals at elevated temperatures, including aromatic or halogenated hydrocarbons, strong acids, bases, ketones, and other aggressive industrial substances. Hydrolytic stability in boiling water is excellent. Environments that deteriorate these polymers are high-temperature steam, concentrated sulfuric acid, and boiling caustic materials.

As an example, the structure of Ticona Vectra® A950 LCP is shown in Figure 6.7.

Polycyclohexylene-dimethylene terephthalate (PCT) is a high-temperature polyester that possesses the chemical resistance, processability, and dimensional stability of polyesters PET and PBT. However, the aliphatic cyclic ring shown in Figure 6.8 imparts added heat resistance. This puts it between the common polyesters and the LCP polyesters described in the previous section. At this time only DuPont makes this plastic under the trade name Thermx®.

This material has found use in automotive, electrical, and housewares applications.

Amorphous polyphthalate carbonate copolymer (PPC) is another high-temperature PC. It provides excellent impact resistance, optical clarity, and abrasion resistance. The plastic offers UV protection as well. It is lightweight, impact-resistant, and can be reused after multiple exposures to sterilization. Its structure is shown in Figure 6.9.

Polytrimethylene terephthalate (PTT) is a semi-crystalline polyester polymer that has many of the same property advantages as PBT and PET. However, compared to PBT, compounds composed of PTT exhibit better tensile strengths, flexural strengths, and stiffness. They also have excellent flow and surface finish. PTT can also be more cost-effective than PBT. PTT may have more uniform shrinkage and better dimensional stability in some applications. PTT, like PBT, has excellent resistance to a broad range of chemicals at room temperature, including aliphatic hydrocarbons, gasoline, carbon tetrachloride, perchloroethylene, oils, fats, alcohols, glycols, esters, ethers, and dilute acids and bases. Strong bases may attack PTT and many polyester resins.

The two monomer units used in producing this polymer are 1,3-propanediol and terephthalic acid and its structure is shown in Figure 6.10.

There are numerous polyester blends and alloys. Often the different polyesters are blended.