Polyester is a category of that contain the ester functional group in every repeat unit of their main chain. As a specific material, it most commonly refers to a type called polyethylene terephthalate (PET). Polyesters include naturally occurring chemicals, such as in the cutin of , as well as synthetics such as polybutyrate. Natural polyesters and a few synthetic ones are biodegradable, but most synthetic polyesters are not. The material is used extensively in clothing.
Polyester fibers are sometimes spun together with natural fibers to produce a cloth with blended properties. Cotton-polyester blends can be strong, wrinkle- and tear-resistant, and reduce shrinking. using polyester have high water, wind and environmental resistance compared to plant-derived fibers. They are less Fireproofing and can melt when ignited.
Liquid crystalline polyesters are among the first industrially used liquid crystal polymers. They are used for their mechanical properties and heat-resistance. These traits are also important in their application as an abradable seal in jet engines.
Natural polyesters could have played a significant role in the origins of life. Long heterogeneous polyester chains and membraneless structures are known to easily form in a one-pot reaction without catalyst under simple prebiotic conditions.
Depending on the chemical structure, polyester can be a thermoplastic or thermoset. There are also cured by hardeners; however, the most common polyesters are thermoplastics.
Increasing the aromatic parts of polyesters increases their glass transition temperature, melting temperature, thermostability, chemical stability...
Polyesters can also be telechelic like the polycaprolactone diol (PCL) and the polyethylene adipate diol (PEA). They are then used as .
Poly(ester imides) contain an aromatic imide group in the repeat unit, the imide-based polymers have a high proportion of aromatic structures in the main chain and belong to the class of thermally stable polymers. Such polymers contain structures that impart high melting temperatures, resistance to oxidative degradation and stability to radiation and chemical reagents. Among the thermally stable polymers with commercial relevance are Polyimide, Polysulfone, polyetherketones, and polybenzimidazoles. Of these, polyimides are most widely applied. P. E. Cassidy, T. M. Aminabhavi and V. S. Reddy, in Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley & Sons, Inc., Hoboken, NJ, USA, 2000. The polymers’ structures result also in poor processing characteristics, in particular a high melting point and low solubility. The named properties are in particular based on a high percentage of aromatic carbons in the polymer backbone which produces a certain stiffness.T. Whelan, Polymer Technology Dictionary, Springer Netherlands, Dordrecht, 1994. Approaches for an improvement of processability include the incorporation of flexible spacers into the backbone, the attachment of stable pendent groups or the incorporation of non-symmetrical structures. Flexible spacers include, for example, ether or hexaﬂuoroisopropylidene, carbonyl or aliphatic groups like isopropylidene; these groups allow bond rotation between aromatic rings. Less symmetrical structures, for example based on meta- or ortho-linked monomers introduce structural disorder and thereby decrease the crystallinity.
The generally poor processability of aromatic polymers (for example, a high melting point and a low solubility) also limits the available options for synthesis and may require strong electron-donating co-solvents like HFIP or TFA for analysis (e. g. 1H NMR spectroscopy) which themselves can introduce further practical limitations.
Polyesters are also used to make bottles, films, tarpaulin, sails (Dacron), canoes, liquid crystal displays, , filters, dielectric film for , film insulation for wire and . Polyesters are widely used as a finish on high-quality wood products such as , and vehicle/yacht interiors. Thixotropic properties of spray-applicable polyesters make them ideal for use on open-grain timbers, as they can quickly fill wood grain, with a high-build film thickness per coat. Cured polyesters can be sanded and polished to a high-gloss, durable finish.
There are several reasons for the importance of polyethylene terephthalate:
In the following table, the estimated world polyester production is shown. Main applications are textile polyester, bottle polyester resin, film polyester mainly for packaging and specialty polyesters for engineering plastics. According to this table, the world's total polyester production might exceed 50 million tons per annum before the year 2010.
|+ World polyester production by year ! Product type ! 2002 (million tonnes/year) ! 2008 (million tonnes/year)|
|+ Textile and packaging polyester application list (melt or pellet) ! Textile ! Packaging|
|Staple fiber (PSF)||Bottles for CSD, water, beer, juice, detergents, etc.|
|Filaments POY, DTY, FDY||A-PET film|
|Technical yarn and tire cord||Thermoforming|
|Non-woven and spunbond||biaxial-oriented film (BO-PET)|
A comparable small market segment (much less than 1 million tonnes/year) of polyester is used to produce engineering plastics and masterbatch.
In order to produce the polyester melt with a high efficiency, high-output processing steps like staple fiber (50–300 tonnes/day per spinning line) or POY /FDY (up to 600 tonnes/day split into about 10 spinning machines) are meanwhile more and more vertically integrated direct processes. This means the polymer melt is directly converted into the textile fibers or filaments without the common step of pelletizing. We are talking about full vertical integration when polyester is produced at one site starting from crude oil or distillation products in the chain oil → benzene → PX → PTA → PET melt → fiber/filament or bottle-grade resin. Such integrated processes are meanwhile established in more or less interrupted processes at one production site. Eastman Chemicals were the first to introduce the idea of closing the chain from PX to PET resin with their so-called INTEGREX process. The capacity of such vertically integrated production sites is >1000 tonnes/day and can easily reach 2500 tonnes/day.
Besides the above-mentioned large processing units to produce staple fiber or yarns, there are ten thousands of small and very small processing plants, so that one can estimate that polyester is processed and recycled in more than 10 000 plants around the globe. This is without counting all the companies involved in the supply industry, beginning with engineering and processing machines and ending with special additives, stabilizers and colors. This is a gigantic industry complex and it is still growing by 4–8% per year, depending on the world region.
Polyesters can be obtained by a wide range of reactions of which the most important are the reaction of acids and alcohols, alcoholysis and or acidolysis of low-molecular weight esters or the alcoholysis of acyl chlorides. The following figure gives an overview over such typical polycondensation reactions for polyester production. Furthermore, polyesters are accessible via ring-opening polymerization.
Direct bulk polyesteriﬁcation at high temperatures (150 – 290 °C) is well-suited and used on the industrial scale for the production of aliphatic polyesters, unsaturated polyesters and aromatic–aliphatic polyesters. Monomers containing Phenol group or tertiary hydroxyl groups exhibit a low reactivity with and cannot be polymerized via direct acid alcohol-based polyesterification. In the case of PET production, however, the direct process has several advantages, in particular a higher reaction rate, a higher attainable molecular weight, the release of water instead of methanol and lower storage costs of the acid when compared to the ester due to the lower weight.
The term transesteriﬁcation is typically used to describe hydroxy–ester, carboxy–ester, and ester–ester exchange reactions. The hydroxy–ester exchange reaction possesses the highest rate of reaction and is used for the production of numerous aromatic–aliphatic and wholly aromatic polyesters. The transesterification based synthesis is particularly useful for when high melting and poorly soluble dicarboxylic acids are used. In addition, alcohols as condensation product are more volatile and thereby easier to remove than water.
The high-temperature melt synthesis between bisphenol diacetates and aromatic dicarboxylic acids or in reverse between bisphenols and aromatic dicarboxylic acid diphenyl esters (carried out at 220 to 320 °C upon the release of acetic acid) is, besides the acyl chloride based synthesis, the preferred route to wholly aromatic polyesters.
The reaction between diacyl chlorides and alcohols or phenolic compounds has been widely applied to polyester synthesis and has been subject of numerous reviews and book chapters. The reaction is carried out at lower temperatures than the equilibrium methods; possible types are the high-temperature solution condensation, amine catalysed and interfacial reactions. In addition, the use of activating agents is counted as non-equilibrium method. The equilibrium constants for the acyl chloride-based condensation yielding yielding arylates and polyarylates are very high indeed and are reported to be 4.3 × 103 and 4.7 × 103, respectively. This reaction is thus often referred to as a ‘non-equilibrium’ polyesterification. Even though the acyl chloride based synthesis is also subject of reports in the patent literature, it is unlikely that the reaction is utilized on the production scale.
The high temperature reaction (100 to > 300 °C) of an diacyl chloride with an dialcohol yields the polyester and hydrogen chloride. Under these relatively high temperatures the reaction proceeds rapidly without a catalyst:
The conversion of the reaction can be followed by titration of the evolved hydrogen chloride. A wide variety of solvents has been described including chlorinated benzenes (e.g. dichlorobenzene), chlorinated naphthalenes or diphenyls, as well as non-chlorinated aromatics like terphenyls, benzophenones or dibenzylbenzenes. The reaction was also applied successfully to the preparation of highly crystalline and poorly soluble polymers which require high temperatures to be kept in solution (at least until a sufficiently high molecular weight was achieved).
In an interfacial acyl chloride-based reaction, the alcohol (generally in fact a phenol) is dissolved in the form of an alkoxide in an aqueous sodium hydroxide solution, the acyl chloride in an organic solvent immiscible with water such as dichloromethane, chlorobenzene or hexane, the reaction occurs at the interface under high-speed agitation near room temperature.
The procedure is used for the production of polyarylates (polyesters based on bisphenols), , , poly(thiocarbonate)s, and others. Since the molecular weight of the product obtained by a high-temperature synthesis can be seriously limited by side reactions, this problem is circumvented by the mild temperatures of interfacial polycondensation. The procedure is applied to the commercial production of bisphenol-A-based polyarylates like Unitika's U-Polymer. Water could be in some cases replaced by an immiscible organic solvent (e. g. in the adiponitrile/carbon tetrachloride system). The procedure is of little use in the production of polyesters based on aliphatic diols which have higher PKa values than phenols and therefore do not form alcoholate ions in aqueous solutions. The base catalysed reaction of an acyl chloride with an alcohol may also be carried out in one phase using tertiary amines (e. g. triethylamine, Et3N) or pyridine as acid acceptors:
While acyl chloride-based polyesterifications proceed only very slowly at room temperature without a catalyst, the amine accelerates the reaction in several possible ways, although the mechanism is not fully understood. However, it is known that tertiary amines can cause Side reaction such as the formation of and ketene dimers.
Instead of acyl chlorides, so-called activating agents can be used, such as 1,1'-carbonyldiimidazole, dicyclohexylcarbodiimide, or triﬂuoroacetic anhydride. The polycondensation proceeds via the in situ conversion of the carboxylic acid into a more reactive intermediate while the activating agents are consumed. The reaction proceeds, for example, via an intermediate N-acylimidazole which reacts with catalytically acting sodium alkoxide:
The use of activating agents for the production of high-melting aromatic polyesters and polyamides under mild conditions has been subject of intensive academic research since the 1980s, but the reactions have not gained commercial acceptance as similar results can be achieved with cheaper reactants.
The acid-alcohol based polyesterification is one example of an equilibrium reaction. The ratio between the polymer-forming ester group (-C(O)O-) and the condensation product water (H2O) against the acid-based (-C(O)OH) and alcohol-based (-OH) monomers is described by the equilibrium constant KC.
The equilibrium constant of the acid-alcohol based polyesterification is typically KC ≤ 10, what is not high enough to obtain high-molecular weight polymers ( DPn ≥ 100), as the number average degree of polymerization ( DPn) can be calculated from the equilibrium constant KC.
In equilibrium reactions, it is therefore necessary to remove the condensation product continuously and efficiently from the reaction medium in order to drive the equilibrium towards polymer. The condensation product is therefore removed at reduced pressure and high temperatures (150–320 °C, depending on the monomers) to prevent the back reaction. With the progress of the reaction, the concentration of active chain ends is decreasing and the viscosity of the melt or solution increasing. For an increase of the reaction rate, the reaction is carried out at high end group concentration (preferably in the bulk), promoted by the elevated temperatures.
Equilibrium constants of magnitude KC ≥ 104 are achieved when using reactive reactants (acid chlorides or acid anhydrides) or activating agents like 1,1′-carbonyldiimidazole. Using these reactants, molecular weights required for technical applications can be achieved even without active removal of the condensation product.