
Polyester is a category of
polymer
A polymer () is a chemical substance, substance or material that consists of very large molecules, or macromolecules, that are constituted by many repeat unit, repeating subunits derived from one or more species of monomers. Due to their br ...
s that contain one or two
ester
In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...
linkages in every repeat unit of their main chain.
As a specific
material
A material is a matter, substance or mixture of substances that constitutes an Physical object, object. Materials can be pure or impure, living or non-living matter. Materials can be classified on the basis of their physical property, physical ...
, it most commonly refers to a type called
polyethylene terephthalate (PET). Polyesters include some naturally occurring chemicals, such as those found in
plant
Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
s and
insect
Insects (from Latin ') are Hexapoda, hexapod invertebrates of the class (biology), class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body (Insect morphology#Head, head, ...
s. Natural polyesters and a few synthetic ones are
biodegradable, but most synthetic polyesters are not. Synthetic polyesters are used extensively in clothing.
Polyester fibers are sometimes spun together with natural fibers to produce a
cloth
Textile is an umbrella term that includes various fiber-based materials, including fibers, yarns, filaments, threads, and different types of fabric. At first, the word "textiles" only referred to woven fabrics. However, weaving is n ...
with blended properties.
Cotton
Cotton (), first recorded in ancient India, is a soft, fluffy staple fiber that grows in a boll, or protective case, around the seeds of the cotton plants of the genus '' Gossypium'' in the mallow family Malvaceae. The fiber is almost pure ...
-polyester blends can be strong, wrinkle- and tear-resistant, and reduce shrinking.
Synthetic fiber
Synthetic fibers or synthetic fibres (in British English; see spelling differences) are fibers made by humans through chemical synthesis, as opposed to natural fibers that are directly derived from living organisms, such as plants like cott ...
s using polyester have high water, wind, and environmental resistance compared to plant-derived fibers. They are less
fire-resistant 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.
Types

Polyesters can contain one ester linkage per repeat unit of the polymer, as in
polyhydroxyalkanoates like
polylactic acid, or they may have two ester linkages per repeat unit, as in
polyethylene terephthalate (PET).
Polyesters are one of the most economically important classes of polymers, driven especially by PET, which is counted among the commodity plastics; in 2019 around 30.5 million metric tons were produced worldwide.
There is a great variety of structures and properties in the polyester family, based on the varying nature of the R group (see first figure with blue ester group).
Natural
Polyesters occurring in nature include the
cutin
Cutin is one of two waxy polymers that are the main components of the plant cuticle, which covers all aerial surfaces of plants, the other being cutan. It is an insoluble substance with waterproof quality. Cutin also harbors cuticular waxes, ...
component of
plant cuticle
A plant cuticle is a protecting film covering the outermost skin layer (epidermis) of leaves, young shoots and other aerial plant organs (aerial here meaning all plant parts not embedded in soil or other substrate) that have no '' periderm''. The ...
s, which consists of
omega hydroxy acids and their derivatives, interlinked via
ester
In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...
bonds, forming polyester polymers of indeterminate size. Polyesters are also produced by
bees in the genus ''
Colletes'', which secrete a
cellophane-like polyester lining for their underground brood cells
earning them the nickname "polyester bees".
Synthetic
The family of synthetic polyesters comprises
* Linear aliphatic high molecular weight polyesters (''M
n'' >10,000) are low-melting (m. p. 40 – 80 °C) semicrystalline polymers and exhibit relatively poor mechanical properties. Their inherent degradability, resulting from their hydrolytic instability, makes them suitable for applications where a possible environmental impact is a concern, e.g. packaging, disposable items or agricultural mulch films or in biomedical and pharmaceutical applications.
* Aliphatic linear low-molar-mass (''M
n'' < 10,000) hydroxy-terminated polyesters are used as macromonomers for the production of polyurethanes.
* hyperbranched polyesters are used as rheology modifiers in thermoplastics or as crosslinkers in coatings due to their particularly low viscosity, good solubility and high functionality
* Aliphatic–aromatic polyesters, including
poly(ethylene terephthalate) (PET) and poly(butylene terephthalate) (PBT), poly(hexamethylene terephthalate)(PHT), poly(propylene terephthalate)
(PTT, Sorona), etc. are high-melting semicrystalline materials (m. p. 160–280 °C) that and have benefited from engineering thermoplastics, fibers and films.
* Wholly aromatic linear copolyesters present superior mechanical properties and heat resistance and are used in a number of high-performance applications.
*
Unsaturated polyesters are produced from multifunctional alcohols and unsaturated dibasic acids and are cross-linked thereafter; they are used as matrices in composite materials. Alkyd resins are made from polyfunctional alcohols and fatty acids and are used widely in the coating and composite industries as they can be cross-linked in the presence of oxygen. Also
rubber
Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds.
Types of polyisoprene ...
-like polyesters exist, called thermoplastic polyester elastomers (ester TPEs).
Unsaturated polyesters (UPR) are thermosetting
resin
A resin is a solid or highly viscous liquid that can be converted into a polymer. Resins may be biological or synthetic in origin, but are typically harvested from plants. Resins are mixtures of organic compounds, predominantly terpenes. Commo ...
s. They are used in the liquid state as
casting
Casting is a manufacturing process in which a liquid material is usually poured into a mold, which contains a hollow cavity of the desired shape, and then allowed to solidify. The solidified part is also known as a casting, which is ejected or ...
materials, in
sheet molding compounds, as
fiberglass
Fiberglass (American English) or fibreglass (English in the Commonwealth of Nations, Commonwealth English) is a common type of fibre-reinforced plastic, fiber-reinforced plastic using glass fiber. The fibers may be randomly arranged, flattened i ...
laminating resins and in non-metallic auto-body fillers. They are also used as the
thermoset polymer matrix in
pre-pregs. Fiberglass-reinforced unsaturated polyesters find wide application in bodies of yachts and as body parts of cars.
Depending on the chemical structure, polyester can be a
thermoplastic
A thermoplastic, or thermosoftening plastic, is any plastic polymer material that becomes pliable or moldable at a certain elevated temperature and solidifies upon cooling.
Most thermoplastics have a high molecular weight. The polymer chains as ...
or
thermoset
In materials science, a thermosetting polymer, often called a thermoset, is a polymer that is obtained by irreversibly hardening (" curing") a soft solid or viscous liquid prepolymer (resin). Curing is induced by heat or suitable radiation and ...
. There are also
polyester resins cured by hardeners; however, the most common polyesters are thermoplastics. The OH group is reacted with an
Isocyanate functional compound in a 2 component system producing coatings which may optionally be pigmented. Polyesters as thermoplastics may change shape after the application of heat. While combustible at high temperatures, polyesters tend to shrink away from flames and self-extinguish upon ignition. Polyester fibers have high
tenacity and
E-modulus as well as low water absorption and minimal
shrinkage in comparison with other industrial fibers.
Increasing the aromatic parts of polyesters increases their
glass transition temperature
The glass–liquid transition, or glass transition, is the gradual and reversible transition in amorphous materials (or in amorphous regions within semicrystalline materials) from a hard and relatively brittle "glassy" state into a viscous or rub ...
, melting temperature,
thermostability, chemical stability, and solvent resistance.
Polyesters can also be
telechelic oligomers like the polycaprolactone diol (PCL) and the polyethylene adipate diol (PEA). They are then used as
prepolymers.
Aliphatic ''vs.'' aromatic polymers
Thermally stable polymers, which generally have a high proportion of
aromatic structures, are also called
high-performance plastics. This application-oriented classification compares such polymers with
engineering plastic
Engineering plastics are a group of plastic materials that have better mechanical or thermal properties than the more widely used commodity plastics (such as polystyrene, polyvinyl chloride, polypropylene and polyethylene).
Engineering plastic ...
s and
commodity plastics. The continuous service temperature of high-performance plastics is generally stated as being higher than 150 °C, whereas engineering plastics (such as polyamide or polycarbonate) are often defined as thermoplastics that retain their properties above 100 °C. Commodity plastics (such as polyethylene or polypropylene) have in this respect even greater limitations, but they are manufactured in great amounts at low cost.
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
polyimides,
polysulfones,
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, New Jersey, US, 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. 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, decreasing 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.
Uses and applications
Fabrics
woven or
knitted from polyester thread or yarn are used extensively in apparel and home furnishings, from shirts and pants to jackets and hats, bed sheets, blankets, upholstered furniture and computer mouse mats. Industrial polyester fibers, yarns and ropes are used in car tire reinforcements,
fabrics for conveyor belts, safety belts, coated fabrics and plastic reinforcements with high-energy absorption. Polyester fiber is used as cushioning and insulating material in pillows, comforters, stuffed animals and characters, and upholstery padding. Polyester fabrics are highly stain-resistant since polyester is a hydrophobic material, making it hard to absorb liquids. The only class of dyes which can be used to alter the color of polyester fabric are what are known as
disperse dyes.
Polyesters are also used to make bottles, films,
tarpaulin,
sails (
Dacron), canoes,
liquid crystal display
A liquid-crystal display (LCD) is a flat-panel display or other Electro-optic modulator, electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers to display information. Liq ...
s,
holograms,
filters,
dielectric
In electromagnetism, a dielectric (or dielectric medium) is an Insulator (electricity), electrical insulator that can be Polarisability, polarised by an applied electric field. When a dielectric material is placed in an electric field, electric ...
film for
capacitor
In electrical engineering, a capacitor is a device that stores electrical energy by accumulating electric charges on two closely spaced surfaces that are insulated from each other. The capacitor was originally known as the condenser, a term st ...
s,
film insulation for
wire
file:Sample cross-section of high tension power (pylon) line.jpg, Overhead power cabling. The conductor consists of seven strands of steel (centre, high tensile strength), surrounded by four outer layers of aluminium (high conductivity). Sample d ...
and
insulating tapes. Polyesters are widely used as a finish on high-quality wood products such as
guitar
The guitar is a stringed musical instrument that is usually fretted (with Fretless guitar, some exceptions) and typically has six or Twelve-string guitar, twelve strings. It is usually held flat against the player's body and played by strumming ...
s,
piano
A piano is a keyboard instrument that produces sound when its keys are depressed, activating an Action (music), action mechanism where hammers strike String (music), strings. Modern pianos have a row of 88 black and white keys, tuned to a c ...
s, 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.
It can be used for fashionable dresses, but it is most admired for its ability to resist wrinkling and shrinking while washing the product. Its toughness makes it a frequent choice for children's wear. Polyester is often blended with other fibres like cotton to get the desirable properties of both materials.
Cured polyesters can be sanded and polished to a high-gloss, durable finish.
Production
Polyester is typically produced through a process known as polymerization. For polyethylene terephthalate (PET), the production process involves the chemical reaction between two primary raw materials: purified terephthalic acid (PTA) or dimethyl terephthalate (DMT) and monoethylene glycol (MEG).
The production process includes the following steps:
# Polycondensation Reaction: The reaction between PTA or DMT and MEG creates polyester polymer chains through a process called polycondensation. This reaction takes place at high temperatures and involves the removal of water or methanol byproducts.
# Extrusion: Once the polymerization is complete, the molten polyester is extruded into long strands. These strands are then cooled and cut into small pellets or chips.
# Spinning: To form fibers, these polyester chips are melted and extruded through spinnerets, forming fine strands of polyester filament. These filaments can be processed further to create continuous fibers, which are then woven into textiles.
# Recycling: The production of polyester has evolved to include the recycling of PET, especially from post-consumer plastic bottles. Recycled PE
(rPET)is increasingly being used in textile production, reducing the environmental impact of polyester manufacturing.
Polyethylene terephthalate, the polyester with the greatest market share, is a synthetic polymer made of
purified terephthalic acid (PTA) or its dimethyl ester
dimethyl terephthalate (DMT) and
monoethylene glycol (MEG). With 18% market share of all plastic materials produced, it ranges third after
polyethylene
Polyethylene or polythene (abbreviated PE; IUPAC name polyethene or poly(methylene)) is the most commonly produced plastic. It is a polymer, primarily used for packaging (plastic bags, plastic films, geomembranes and containers including bott ...
(33.5%) and
polypropylene
Polypropylene (PP), also known as polypropene, is a thermoplastic polymer used in a wide variety of applications. It is produced via chain-growth polymerization from the monomer Propene, propylene.
Polypropylene belongs to the group of polyolefin ...
(19.5%) and is counted as
commodity plastic.
There are several reasons for the importance of polyethylene terephthalate:
*The relatively easy accessible raw materials PTA or DMT and MEG
*The very well understood and described simple chemical process of its synthesis
*The low toxicity level of all raw materials and side products during production and processing
*The possibility to produce PET in a closed loop at low emissions to the environment
*The outstanding mechanical and chemical properties
*The recyclability
*The wide variety of intermediate and final products.
In the following table, the estimated world polyester production is shown. Main applications are
textile
Textile is an Hyponymy and hypernymy, umbrella term that includes various Fiber, fiber-based materials, including fibers, yarns, Staple (textiles)#Filament fiber, filaments, Thread (yarn), threads, and different types of #Fabric, fabric. ...
polyester, bottle polyester resin, film polyester mainly for
packaging
Packaging is the science, art and technology of enclosing or protecting products for distribution, storage, sale, and use. Packaging also refers to the process of designing, evaluating, and producing packages. Packaging can be described as a coo ...
and specialty polyesters for engineering plastics.
Polyester processing
After the first stage of polymer production in the melt phase, the product stream divides into two different application areas which are mainly
textile
Textile is an Hyponymy and hypernymy, umbrella term that includes various Fiber, fiber-based materials, including fibers, yarns, Staple (textiles)#Filament fiber, filaments, Thread (yarn), threads, and different types of #Fabric, fabric. ...
applications and packaging applications. In the following table, the main applications of textile and packaging of polyester are listed.
Abbreviations:
;PSF: Polyester-staple fiber
;POY: Partially oriented yarn
;DTY: Drawn textured yarn
;FDY: Fully drawn yarn
;CSD: Carbonated soft drink
;A-PET: Amorphous polyethylene terephthalate film
;BO-PET: Biaxial-oriented polyethylene terephthalate film
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
In microeconomics, management and international political economy, vertical integration, also referred to as vertical consolidation, is an arrangement in which the supply chain of a company is integrated and owned by that company. Usually each ...
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.
Synthesis
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
OOCR'COOsub>nROH + 2n H
2O.
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.
:

Azeotrope esterification is a classical method for condensation. The water formed by the reaction of
alcohol
Alcohol may refer to:
Common uses
* Alcohol (chemistry), a class of compounds
* Ethanol, one of several alcohols, commonly known as alcohol in everyday life
** Alcohol (drug), intoxicant found in alcoholic beverages
** Alcoholic beverage, an alco ...
and a
carboxylic acid
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...
is continually removed by
azeotropic distillation
In chemistry, azeotropic distillation is any of a range of techniques used to break an azeotrope in distillation. In chemical engineering, ''azeotropic distillation'' usually refers to the specific technique of adding another component to genera ...
. When melting points of the monomers are sufficiently low, a polyester can be formed via direct esterification while removing the reaction water via vacuum.
:

Direct bulk polyesterification at high temperatures (150 – 290 °C) is well-suited and used on the industrial scale for the production of aliphatic, unsaturated, and aromatic–aliphatic polyesters. Monomers containing
phenolic or
tertiary hydroxyl groups exhibit a low reactivity with
carboxylic acid
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an Substituent, R-group. The general formula of a carboxylic acid is often written as or , sometimes as with R referring to an organyl ...
s 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
Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical compound and the simplest aliphatic Alcohol (chemistry), alcohol, with the chemical formula (a methyl group linked to a hydroxyl group, often ab ...
and lower storage costs of the acid when compared to the ester due to the lower weight.
Alcoholic transesterification
:

Transesterification: 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
In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula (whereas normal methane has the formula ). In formulas, the group is often abbreviated a ...
(
methanol
Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical compound and the simplest aliphatic Alcohol (chemistry), alcohol, with the chemical formula (a methyl group linked to a hydroxyl group, often ab ...
is the byproduct of the esterification reaction).
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.
Acylation
In
acylation, the acid begins as an
acyl chloride, and thus the polycondensation proceeds with emission of
hydrochloric acid
Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungency, pungent smell. It is classified as a acid strength, strong acid. It is ...
(HCl) instead of water.
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 × 10
3 and 4.7 × 10
3, 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 method is limited by the acid dichlorides' high cost, its sensitivity to hydrolysis and the occurrence of side reactions.
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
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula . It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly corrosive base (chemistry), ...
solution, the acyl chloride in an organic solvent immiscible with water such as
dichloromethane,
chlorobenzene or
hexane
Hexane () or ''n''-hexane is an organic compound, a straight-chain alkane with six carbon atoms and the molecular formula C6H14.
Hexane is a colorless liquid, odorless when pure, and with a boiling point of approximately . It is widely used as ...
, 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),
polyamide
A polyamide is a polymer with repeating units linked by amide bonds.
Polyamides occur both naturally and artificially. Examples of naturally occurring polyamides are proteins, such as wool and silk. Artificially made polyamides can be made throug ...
s,
polycarbonate
Polycarbonates (PC) are a group of thermoplastic polymers containing carbonate ester, carbonate groups in their chemical structures. Polycarbonates used in engineering are strong, toughness, tough materials, and some grades are optically transp ...
s,
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
Carbon tetrachloride, also known by many other names (such as carbon tet for short and tetrachloromethane, also IUPAC nomenclature of inorganic chemistry, recognised by the IUPAC), is a chemical compound with the chemical formula CCl4. It is a n ...
system).
The procedure is of little use in the production of polyesters based on aliphatic diols which have higher
p''K''a 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, Et
3N) 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
In chemistry, amines (, ) are organic compounds that contain carbon-nitrogen bonds. Amines are formed when one or more hydrogen atoms in ammonia are replaced by alkyl or aryl groups. The nitrogen atom in an amine possesses a lone pair of elec ...
can cause
side-reactions such as the formation of
ketenes and ketene dimers.
Silyl method
In this variant of the HCl method, the carboxylic acid chloride is converted with the
trimethyl silyl ether of the alcohol component and production of trimethyl silyl chloride is obtained
Acetate method (esterification)
:
Ring-opening polymerization
:
Aliphatic polyesters can be assembled from
lactones under very mild conditions, catalyzed
anionically,
cationically,
metallorganically or enzyme-based. A number of catalytic methods for the copolymerization of epoxides with cyclic anhydrides have also recently been shown to provide a wide array of functionalized polyesters, both saturated and unsaturated. Ring-opening polymerization of lactones and lactides is also applied on the industrial scale.
Other methods
Numerous other reactions have been reported for the synthesis of selected polyesters, but are limited to laboratory-scale syntheses using specific conditions, for example using dicarboxylic acid salts and dialkyl halides or reactions between bisketenes and diols.
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.
Thermodynamics of polycondensation reactions
Polyesterifications are grouped by some authors
into two main categories: a) equilibrium polyesterifications (mainly alcohol-acid reaction, alcohol–ester and acid–ester interchange reactions, carried out in bulk at high temperatures), and b) non-equilibrium polyesterifications, using highly reactive monomers (for example acid chlorides or activated carboxylic acids, mostly carried out at lower temperatures in solution).
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 (H
2O) against the acid-based (-C(O)OH) and alcohol-based (-OH) monomers is described by the equilibrium constant ''K
C''.
:
The equilibrium constant of the acid-alcohol based polyesterification is typically ''K
C'' ≤ 10, what is not high enough to obtain high-molecular weight polymers (''DP
n'' ≥ 100), as the number average degree of polymerization (''DP
n'') can be calculated from the equilibrium constant ''K
C''.
:
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 ''K
C'' ≥ 10
4 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.
History
In 1926, United States–based
DuPont
Dupont, DuPont, Du Pont, duPont, or du Pont may refer to:
People
* Dupont (surname) Dupont, also spelled as DuPont, duPont, Du Pont, or du Pont is a French surname meaning "of the bridge", historically indicating that the holder of the surname re ...
began research on large molecules and synthetic fibers. This early research, headed by
Wallace Carothers, centered on what became
nylon
Nylon is a family of synthetic polymers characterised by amide linkages, typically connecting aliphatic or Polyamide#Classification, semi-aromatic groups.
Nylons are generally brownish in color and can possess a soft texture, with some varieti ...
, which was one of the first synthetic fibers. Carothers was working for DuPont at the time. Carothers' research was incomplete and had not advanced to investigating the polyester formed from mixing ethylene glycol and terephthalic acid. In 1928 polyester was patented in
Britain by the British
General Electric Company
The General Electric Company (GEC) was a major British industrial conglomerate involved in consumer and Arms industry, defence electronics, communications, and engineering.
It was originally founded in 1886 as G. Binswanger and Company as an e ...
. Carothers' project was revived by British scientists
Whinfield and Dickson, who patented
polyethylene terephthalate (PET) or PETE in 1941. Polyethylene terephthalate forms the basis for synthetic fibers like
Dacron, Terylene and polyester. In 1946, DuPont bought all legal rights from Imperial Chemical Industries (ICI).
Biodegradation and environmental concerns
The
Futuro houses were made of
fibreglass-reinforced polyester plastic; polyester-
polyurethane
Polyurethane (; often abbreviated PUR and PU) is a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethane term ...
, and
poly(methyl methacrylate). One house was found to be degrading by
cyanobacteria
Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
and
Archaea
Archaea ( ) is a Domain (biology), domain of organisms. Traditionally, Archaea only included its Prokaryote, prokaryotic members, but this has since been found to be paraphyletic, as eukaryotes are known to have evolved from archaea. Even thou ...
.
Cross-linking
Unsaturated polyesters are
thermosetting polymers. They are generally
copolymers prepared by
polymerizing one or more
diols with saturated and unsaturated
dicarboxylic acids (
maleic acid
Maleic acid or ''cis''-butenedioic acid is an organic compound that is a dicarboxylic acid, a molecule with two carboxyl groups. Its chemical formula is HO2CCH=CHCO2H. Maleic acid is the ''cis'' Cis–trans isomerism, isomer of butenedioic acid, ...
,
fumaric acid, etc.) or their
anhydride
An acid anhydride is a type of chemical compound derived by the removal of water molecules from an acid (chemistry), acid.
In organic chemistry, organic acid anhydrides contain the functional group . Organic acid anhydrides often form when one ...
s. The double bond of unsaturated polyesters reacts with a
vinyl
Vinyl may refer to:
Chemistry
* Polyvinyl chloride (PVC), a particular vinyl polymer
* Vinyl cation, a type of carbocation
* Vinyl group, a broad class of organic molecules in chemistry
* Vinyl polymer, a group of polymers derived from vinyl ...
monomer
A monomer ( ; ''mono-'', "one" + '' -mer'', "part") is a molecule that can react together with other monomer molecules to form a larger polymer chain or two- or three-dimensional network in a process called polymerization.
Classification
Chemis ...
, usually
styrene, resulting in a 3-D
cross-link
In chemistry and biology, a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
ed structure. This structure acts as a thermoset. The
exothermic cross-linking reaction is initiated through a
catalyst, usually an
organic peroxide such as
methyl ethyl ketone peroxide or
benzoyl peroxide.
Pollution of freshwater and seawater habitats
A team at Plymouth University in the UK spent 12 months analysing what happened when a number of synthetic materials were washed at different temperatures in domestic washing machines, using different combinations of detergents, to quantify the microfibres shed. They found that an average washing load of 6 kg could release an estimated 137,951 fibres from polyester-cotton blend fabric, 496,030 fibres from polyester and 728,789 from acrylic. Those fibers add to the general
microplastics
Microplastics are "synthetic solid particles or polymeric matrices, with regular or irregular shape and with size ranging from 1 μm to 5 mm, of either primary or secondary manufacturing origin, which are insoluble in water." Microplastics a ...
pollution
Pollution is the introduction of contaminants into the natural environment that cause harm. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the component ...
.
Safety
Fertility
Ahmed Shafik was a sexologist who won a
Ig Nobel Prize on his research regarding how polyester can affect the fertility of rats,
dogs,
and men.
Bisphenol A
Bisphenol A (BPA) is a chemical compound primarily used in the manufacturing of various plastics. It is a colourless solid which is Solubility, soluble in most common organic solvents, but has very poor solubility in water. BPA is produced on a ...
which is a
endocrine disrupting chemical may be used in the synthesis of polyester.
Recycling
Recycling of polymers has become very important as the production and use of plastic is continuously rising. Global
plastic waste
Plastic pollution is the accumulation of plastic objects and particles (e.g. plastic bottles, bags and microbeads) in the Earth's environment that adversely affects humans, wildlife and their habitat. Plastics that act as pollutants are cate ...
may almost triple by 2060 if this continues. Plastics can be recycled by various means like mechanical recycling, chemical recycling, etc. Among the recyclable polymers, polyester
PET is one of the most recycled plastics. The ester bond present in polyesters is susceptible to
hydrolysis
Hydrolysis (; ) is any chemical reaction in which a molecule of water breaks one or more chemical bonds. The term is used broadly for substitution reaction, substitution, elimination reaction, elimination, and solvation reactions in which water ...
(acidic or basic conditions),
methanolysis and glycolysis which makes this class of polymers suitable for chemical recycling. Enzymatic/biological recycling of PET can be carried out using different enzymes like
PETase,
cutinase,
esterase,
lipase, etc. PETase has been also reported for enzymatic degradation of other synthetic polyesters (PBT, PHT, Akestra™, etc.) which contains similar aromatic ester bond as that of PET.
See also
*
Epoxy
Epoxy is the family of basic components or Curing (chemistry), cured end products of epoxy Resin, resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide fun ...
*
Glycerine phthalate
*
Microfiber
*
Oligoester
*
Polyamide
A polyamide is a polymer with repeating units linked by amide bonds.
Polyamides occur both naturally and artificially. Examples of naturally occurring polyamides are proteins, such as wool and silk. Artificially made polyamides can be made throug ...
*
Rayon
Rayon, also called viscose and commercialised in some countries as sabra silk or cactus silk, is a semi-synthetic fiber made from natural sources of regenerated cellulose fiber, cellulose, such as wood and related agricultural products. It has t ...
*
Viscose
References
Further reading
*''Textiles'', by Sara Kadolph and Anna Langford. 8th Edition, 1998.
* Spinning Machines https://en.wikipedia.org/wiki/Spinning_(textiles)
External links
Lipase catalyzed polyesterification: Enzyme-Catalyzed Polymerization of End-Functionalized Polymers in a Microreactor
{{Authority control
Carboxylate esters
Packaging materials
Synthetic resins
Synthetic fibers
Thermoplastics
Dielectrics
Airship technology
Thermosetting plastics