A polyester polymer contains the ester functional group in their main chain. The backbones are formed by the “esterification condensation of polyfunctional alcohols and acids1.”
There are several reasons polyester is important:
- The relatively easy accessible raw materials: Purified terephthalic acid (PTA) or Dimethylterephthalate (DMT) and Mono-ethylene glycol (MEG).
- The simple chemical process of polyester synthesis is very well understood and described
- The possibility to produce PET in a closed loop at low emissions to the environment
- The outstanding mechanical and chemical properties of polyester
- The recyclability
- The wide variety of intermediate and final products made of polyester.
As fibers, polyester is a term often defined as “long-chain polymers chemically composed of at least 85 percent by weight of an ester and a dihydric alcohol and a terephthalic acid”. In other words, it means the linking of several esters within the fibers. Reaction of alcohol with carboxylic acid results in the formation of esters.2
Polyesters include naturally occurring chemicals, such as in the cutin of plant cuticles, as well as synthetics through step-growth polymerization such as polybutyrate. Polybutyrate (polybutyrate adipate terephthalate, or PBAT) is a biodegradable random copolymer, specifically a copolyester of adipic acid, 1,4-butanediol and dimethyl terephthalate. It is generally marketed as a fully biodegradable alternative to low-density polyethylene (LDPE), having many similar properties including flexibility and resilience, allowing it to be used for similar uses such as plastic bags. Most synthetic polyesters are not biodegradable. Many people refer to polyester as a specific material; most commonly called polyethylene terephthalate (PET).
Depending on the chemical structure, polyester can be a thermoplastic or thermoset. There are also polyester resins cured by hardeners; however, the most common polyesters are thermoplastics.
Polyesters are used to make bottles (PET), films (e.g. Mylar®), tarpaulin, canoes, liquid crystal displays, holograms, filters, dielectric film for capacitors, film insulation for wire and insulating tapes. Polyesters are widely used as a finish on high-quality wood products such as pianos, guitars and vehicle/yacht interiors. Thixotropicity, a rather unique, time-dependent shear thinning property of polyesters3, is leveraged in spray-applications for use on open-grain timbers, since they can quickly fill wood grain and pores, with a high-build film thickness per coat.
Most of the backsheets of solar panels are constructed of polyester film, although some of these are slowly being replaced by fluoropolymers.
Synthesis
There are multiple chemical reactions to produce polyesters, such as:
Polycondensation
Synthesis of polyesters is generally achieved by a polycondensation reaction. The general equation for the reaction of a diol with a diacid is:
(n+1) R(OH)2 + n R´(COOH)2 → HO[ROOCR´COO]nROH + 2n H2O
Azeotrope esterification
In this method, an alcohol and a carboxylic acid react to form a carboxylic ester. To produce a polymer, the water formed by the reaction must be continually removed by azeotrope distillation.
Alcoholic transesterification
Transesterification is a process wherein an alcohol-terminated oligomer and an ester-terminated oligomer condense to form an ester linkage, with loss of an alcohol. R and R’ are the two oligomer chains, R” is a sacrificial unit such as a methyl group.
Polyester polyols are used in the manufacture of powder coating resins as well as epoxy polyester hybrid powder coatings. The former provides good mechanical properties, corrosion resistance and overbake stability, while the hybrid powder coatings are especially useful in appliance, shelving, office furniture, fixtures and general industrial applications. Polyesters are by far the dominant chemistry on powder coatings. Increasing the aromatic parts of polyesters increases their glass transition temperature (Tg), melting temperature, thermal stability, chemical stability and solvent resistance.
Unsaturated polyesters are thermosetting resins. They are generally copolymers prepared by polymerizing one or more diol with saturated and unsaturated dicarboxylic acids (maleic acid, fumaric acid, etc.) or their anhydrides. The double bond of unsaturated polyesters reacts with a vinyl monomer, resulting in a 3-D cross-linked structure. This structure acts as a thermoset. The exothermic cross-linking reaction is initiated through a catalyst, usually an organic peroxide such as benzoyl peroxide. They are mostly used in reinforced plastics. These are the most widely used and economical family of resins.
Saturated polyesters refer to that family of polyesters in which the polyester backbones are saturated. They are thus not as reactive as unsaturated polyesters. They consist of low molecular weight liquids used as plasticizers, and as reactants in forming urethane polymers, and linear, high molecular weight thermoplastics such as polyethylene terephthalate (dacron and Mylar). Usual reactants for the saturated polyesters are a glycol and an acid or anhydride.
Polyester fiber is formed through an extruder (spinnerette) to produce very fine threads of PET. Polyester clothing is silky in feel, although it can cause skin irritation for some people. Some polyester is blended with other fabrics to minimize this.
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Thanks for the overview. I was always confused by the fact that polyester can be both thermoplastic and thermoset.
Nathan, thanks. A lot of chemistries can be either thermoplastic or thermoset by the functional groups attached to the main polymer chain.
This is awesome. I am learning a lot about Polyesters. Thanks for sharing.