Highly Aromatic Linear Polyesters

It has already been shown (e.g. Chapters 20 and 21) that the insertion of a p-phenylene into the main chain of a linear polymer increased the chain stiffness and raised the heat distortion temperature. In many instances it also improved the resistance to thermal degradation. One of the first polymers to exploit this concept commercially was poly(ethylene terephthalate) but it was developed more with the polycarbonates, polysulphone, poly(phenylene sulphides) and aromatic polyketones. 

During the period of development of these materials work proceeded on heat-resistant polyesters. It was found, for example, that reaction of resorcinol with terephthalyl chloride gave a polymer that showed no signs of melting below 

The polyester made by reacting hydroquinone with terephthalic acid also melted above 500°C. That from bis-phenol A and 4,4 -(2,2-butylidene)dibenzoic acid is said to be stable in nitrogen to above 400°C. 

The homopolymer is difficult to fabricate and has been shaped by hammering (like a metal), impact moulding and pressure sintering at 420°C and 35 MPa pressure. The copolymers are somewhat easier to fabricate. The difficulty in fabrication has severely limited the development of these polymers. 

With a somewhat lower level of heat resistance but with many properties that make them of interest as engineering materials alongside the polycarbonates, polysulphones, poly(phenylene sulphides) and polyketones are the so-called polyarylates which are defined as polyester from bis-phenols and dicarboxylic acids. 

Tiible 25.10 Typical properties of commercial polyhydroxybenzoate and related copol3miers 

---

Polyarylates are highly aromatic linear polyesters with high values of (up to 194°C has been quoted) and which are self-extinguishing. 

Most LC polymers in the market are linear, highly aromatic thermotropic polyesters. They combine chemical stability with chain rigidity and retain dimensional stability (remain as glasses) up to 200°C or 300°C and are also very chemically stable (resistant to oxidation). These types of PLCs, such as the commercial Vectra and Xydar, are usually processed in the melt state with conventional fabrication techniques such as extrusion and molding (see Chapter 14). A major advantage of such polymers is that they can be melt processed and form extranely precise molded structures that do not shrink on cooling. They have been used in molded parts for microelectronics and in mounting brackets for optical communications. 

Polycarbonates form a rather specialised class of linear polyesters, since they are formed from a diol, usually an aromatic diol, with a derivative of carbonic acid. The commercially useful products also differ from other types of polyester in that they are generally non-crystalline, melt-processable polymers of high 7J, possessing very high optical clarity and toughness. 

The saturated polyesters that find conunercial applications are mostly linear, except for some specially prepared branched polymers used in the preparation of polyurethanes. The linear polyesters became commercially important materials early in this century and still find many uses in industry. The earliest studies reported condensations of ethylene, trimethylene, hexamethylene, and de-camethylene glycols with malonic, succinic, adipic, sebacic, and orthophthalic acids. Later studies showed that such condensations yield high molecular weight compounds. Nevertheless, these polyesters exhibit poor hydrolytic stability and are generally low-melting. Subsequently, however, it was found that aromatic dicarboxylic acids yield polymers with high melting points, and poly(ethylene terephthalate), which melts at 265 C, is now an important commercial material. 

BASF has introduced their aliphatic-aromatic copolyesters (AAC) product under the name Ecoflex . This material is widely used to produce compostable packaging and films. According to the BASF s corporate website, annual production of Ecoflex has risen to 60,000 MT to keep up with the demand for biodegradable plastics, which is growing at a rate of 20% per year. At the same time, BASF also produces a blend of polyester and PLA — a product called Ecovio . This high-melt-strength polyester—PLA can be directly processed on conventionally blown film lines without the incorporation of additives. Moreover, Ecovio has extraordinary puncture-and tear-resistance and weldability. Another company, Eastman, has also produced AAC, with the tradename Eastar Bio . Eastar Bio has a highly linear structure, while Ecoflex contains... 

A variation of the aromatic polyester structure was utilized by Hawker et al. when they described hyperbranched poly(ethylene glycol)s and investigated their use as polyelectrolyte media [76]. The highly branched structure implies that no crystallization can occur. Linear poly(ethylene) glycols usually crystallize, which has a detrimental effect on their use as polyelectrolyte media. 

BASF s Ecoflex and Novamont s Eastar Bio Ecoflex are aromatic-aliphatic co-polyesters based on butanediol, adipic acid, and terephthalic acid. BASF s products contain long-chain branching while Eastar Bio is highly linear in structure. 

---