View source for Polyester

{{short description|Category of polymers, in which the monomers are joined together by ester links}}
{{For|the 1981 motion picture|Polyester (film)}}
[[File:Definition polyester ester group.svg|thumb|Ester group <span style="color:blue;">'''(blue)''' </span> which defines polyesters. This diagram shows just one ester linkage per repeat unit.]]
'''Polyester''' is a category of [[polymer]]s that contain one or two [[ester]] linkages in every repeat unit of their main chain.<ref name=":0">{{cite book | vauthors = Köpnick H, Schmidt M, Brügging W, Rüter J, Kaminsky W | chapter = Polyesters | title =Ullmann's Encyclopedia of Industrial Chemistry | date = June 2000 | publisher = Wiley-VCH Verlag GmbH & Co. KGaA | location = Weinheim, Germany }}</ref> As a specific [[material]], it most commonly refers to a type called [[polyethylene terephthalate]] (PET). Polyesters include some naturally occurring chemicals, such as those found in [[plant]]s and [[insect]]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]] with blended properties. [[Cotton]]-polyester blends can be strong, wrinkle- and tear-resistant, and reduce shrinking. [[Synthetic fiber]]s using polyester have high water, wind, and environmental resistance compared to plant-derived fibers. They are less [[Fireproofing|fire-resistant]] and can melt when ignited.<ref>{{Cite book|title = Home Comforts: The Art and Science of Keeping House|url = https://books.google.com/books?id=xfB99Kf38MwC|publisher = Simon and Schuster|date = 17 May 2005|isbn = 9780743272865|first = Cheryl|last = Mendelson | name-list-style = vanc }}</ref>

Liquid crystalline polyesters are among the first industrially used [[liquid crystal polymer]]s. 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.<ref>{{Cite web|url=https://www.gordonengland.co.uk/abradable.htm|title=Thermal Spray Abradable Coatings|website=www.gordonengland.co.uk|access-date=2018-12-12}}</ref>

==Types==
[[File:Polyestershirt.jpg|thumb|A polyester shirt]]
[[File:Polyester Shirt, close-up.jpg|thumb|Close-up of a polyester shirt]]
[[File:SEMexample.jpg|thumb|[[Scanning electron microscope|SEM]] picture of a bend in a high-surface area '''polyester''' [[fiber]] with a seven-lobed cross section]]
[[File:Drop of water on water-resistant textile (100% polyester).jpg|thumb|A drop of water on a water resistant polyester]]
Polyesters can contain one ester linkage per repeat unit of the polymer, as in [[polyhydroxyalkanoate]]s 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.<ref name=":1" /> 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).<ref name=":0" />

===Natural===
Polyesters occurring in nature include the [[cutin]] component of [[plant cuticle]]s, which consists of [[omega hydroxy acid]]s and their derivatives, interlinked via [[ester]] bonds, forming polyester polymers of indeterminate size. Polyesters are also produced by [[bee]]s in the genus ''[[Colletes]]'', which secrete a [[cellophane]]-like polyester lining for their underground brood cells<ref name=hef>{{cite journal | url=https://www.science.org/doi/abs/10.1126/science.204.4391.415 | doi=10.1126/science.204.4391.415 | title=Natural Polyesters: Dufour's Gland Macrocyclic Lactones Form Blood Cell Laminesters in Colletes Bees | year=1979 | last1=Hefetz | first1=Abraham | last2=Fales | first2=Henry M. | last3=Batra | first3=Suzanne W. T. | journal=Science | volume=204 | issue=4391 | pages=415–417 | pmid=17758016 | bibcode=1979Sci...204..415H | s2cid=41342994| url-access=subscription }}</ref> earning them the nickname "polyester bees".<ref>{{cite magazine | last1=Eveleth | first1=R. | first2=D. | last2=Chachra | url=http://www.scientificamerican.com/article.cfm?id=from-pollen-to-polyester | title=Can Bees Make Tupperware? | magazine=Scientific American | date=19 December 2011}}</ref>

===Synthetic===
The family of synthetic polyesters comprises<ref name=":0" />

* Linear aliphatic high molecular weight polyesters (''M<sub>n</sub>'' >10,000) are low-melting (m. p.&nbsp;40 – 80&nbsp;°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<ref>{{cite journal | vauthors = Kong X, Qi H, Curtis JM | title = Synthesis and characterization of high-molecular weight aliphatic polyesters from monomers derived from renewable resources. | journal = Journal of Applied Polymer Science | date = August 2014 | volume = 131 | issue = 15 | pages = 40579–40586 | doi = 10.1002/app.40579 }}</ref>⁠ or in biomedical and pharmaceutical applications.<ref name="Park_2012">{{cite journal | vauthors = Park HS, Seo JA, Lee HY, Kim HW, Wall IB, Gong MS, Knowles JC | title = Synthesis of elastic biodegradable polyesters of ethylene glycol and butylene glycol from sebacic acid | journal = Acta Biomaterialia | volume = 8 | issue = 8 | pages = 2911–8 | date = August 2012 | pmid = 22522011 | doi = 10.1016/j.actbio.2012.04.026 }}</ref>
* Aliphatic linear low-molar-mass (''M<sub>n</sub>'' < 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<ref>{{cite journal | vauthors = Gurunathan T, Mohanty S, Nayak SK | title = Hyperbranched polymers for coating applications: a review. | journal = Polymer-Plastics Technology and Engineering. | date = January 2016 | volume = 55 | issue = 1 | pages = 92–117 | doi = 10.1080/03602559.2015.1021482 | s2cid = 100936296 }}</ref> due to their particularly low viscosity, good solubility and high functionality<ref>{{cite journal | vauthors = Testud B, Pintori D, Grau E, Taton D, Cramail H | title = Hyperbranched polyesters by polycondensation of fatty acid-based AB n-type monomers. | journal = Green Chemistry | date = 2017 | volume = 19 | issue = 1 | pages = 259–69 | doi = 10.1039/C6GC02294D | arxiv = 1911.07737 | s2cid = 102450135}}</ref>
* Aliphatic–aromatic polyesters, including [[Polyethylene terephthalate|poly(ethylene terephthalate) (PET)]] and poly(butylene terephthalate) (PBT), poly(hexamethylene terephthalate)(PHT), poly(propylene terephthalate) [[Sorona|(PTT, Sorona)]], etc. are high-melting semicrystalline materials (m. p.&nbsp;160–280&nbsp;°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.
* [[Polyester resin#Unsaturated polyester|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]]-like polyesters exist, called thermoplastic polyester elastomers (ester TPEs). [[Polyester resin|Unsaturated polyesters]] (UPR) are thermosetting [[resin]]s. They are used in the liquid state as [[casting]] materials, in [[sheet molding compound]]s, as [[fiberglass]] laminating resins and in non-metallic auto-body fillers. They are also used as the [[thermoset polymer matrix]] in [[pre-preg]]s. 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]] or [[thermoset]]. There are also [[polyester resin]]s cured by hardeners; however, the most common polyesters are thermoplastics.<ref>{{Cite book|url=https://books.google.com/books?id=Lqk5QgGoWFkC|title=Plastic product material and process selection handbook| vauthors = Rosato DV, Rosato DV, Rosato MV  |publisher=Elsevier|year=2004|isbn=978-1-85617-431-2|page=85}}</ref> 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 (textile strength)|tenacity]] and [[young's modulus|E-modulus]] as well as low water absorption and minimal [[Shrinkage (fabric)|shrinkage]] in comparison with other industrial fibers.

Increasing the aromatic parts of polyesters increases their [[glass transition temperature]], melting temperature, [[thermostability]], chemical stability, and solvent resistance.

Polyesters can also be [[telechelic]] [[oligomer]]s like the polycaprolactone diol (PCL) and the polyethylene adipate diol (PEA). They are then used as [[prepolymer]]s.

=== Aliphatic ''vs.'' aromatic polymers ===
Thermally stable polymers, which generally have a high proportion of [[Aromaticity|aromatic structures]], are also called [[high-performance plastics]]. This application-oriented classification compares such polymers with [[engineering plastic]]s and [[commodity plastics]]. The continuous service temperature of high-performance plastics is generally stated as being higher than 150&nbsp;°C,<ref>{{Citation|last1=Parker|first1=David|title=Polymers, High-Temperature|date=2012-04-15|url=http://doi.wiley.com/10.1002/14356007.a21_449.pub3|encyclopedia=Ullmann's Encyclopedia of Industrial Chemistry|pages=a21_449.pub3|editor-last=Wiley-VCH Verlag GmbH & Co. KGaA|place=Weinheim, Germany|publisher=Wiley-VCH Verlag GmbH & Co. KGaA|language=en|doi=10.1002/14356007.a21_449.pub4|isbn=978-3-527-30673-2|access-date=2020-12-13|last2=Bussink|first2=Jan|last3=van de Grampel|first3=Hendrik T.|last4=Wheatley|first4=Gary W.|last5=Dorf|first5=Ernst-Ulrich|last6=Ostlinning|first6=Edgar|last7=Reinking|first7=Klaus|last8=Schubert|first8=Frank|last9=Jünger|first9=Oliver|url-access=subscription}}</ref> whereas engineering plastics (such as polyamide or polycarbonate) are often defined as thermoplastics that retain their properties above 100&nbsp;°C.<ref>H.-G. Elias and R. Mülhaupt, in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany, 2015, pp. 1–70.</ref>⁠ 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 [[imide]]s) 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]]s, [[polysulfone]]s, [[polyetherketones]], and [[polybenzimidazoles]]. Of these, polyimides are most widely applied.<ref name=":2">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.</ref> 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.<ref>T. Whelan, Polymer Technology Dictionary, Springer Netherlands, Dordrecht, 1994.</ref>⁠ 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.<ref name=":1"/>

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. [[Proton NMR Spectroscopy|<sup>1</sup>H NMR spectroscopy]]) which themselves can introduce further practical limitations.

== Uses and applications ==
Fabrics [[Woven fabric|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, and therefore has difficulty absorbing liquids. The only class of dyes which can be used to alter the color of polyester fabric are what are known as [[disperse dye]]s.<ref name="Aatcc">{{cite book|chapter-url=https://books.google.com/books?id=FPz0QRwKHsMC&pg=PA21|title=Dyeing Primer| last = Schuler | first = Mattias J. | name-list-style = vanc |publisher=AATCC|year=1981|page=21|chapter=Part 8: Dyeing with disperse dyes|id=GGKEY:SK3T00EYAFR}}</ref>

Polyesters are also used to make bottles, films, [[tarpaulin]], [[sails]]<ref>{{Cite web |title=Dacron sails {{!}} Elvstrøm Sails since 1954 |url=https://elvstromsails.com/sail-technology/sail-materials/woven-sail-material-dacron/ |access-date=2024-09-16 |website=elvstromsails.com |language=en-US}}</ref> ([[Dacron]]), canoes, [[liquid crystal display]]s, [[hologram]]s, [[filter (chemistry)|filter]]s, [[dielectric]] film for [[capacitor]]s, [[Electrical insulation|film insulation]] for [[wire]] and [[insulating tape]]s.  Polyesters are widely used as a finish on high-quality wood products such as [[guitar]]s, [[piano]]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 PET [https://verive.eu/whats-rpet-and-why-do-we-use-it-for-food-packaging/#:~:text=What%20is%20rPET%3F,bottles%20are%20made%20from%20PET. (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]] (33.5%) and [[polypropylene]] (19.5%) and is counted as [[Commodity plastics|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]] polyester, bottle polyester resin, film polyester mainly for [[packaging]] and specialty polyesters for engineering plastics.

{| class="wikitable"
|+ World polyester production by year
! Product type
! 2002 (million tonnes/year)
! 2008 (million tonnes/year)
|-
| Textile-PET
| 20
| 39
|-
| Resin, bottle/A-PET
| 9
| 16
|-
| Film-PET
| 1.2
| 1.5
|-ьр
| Special polyester
| 1
| 2.5
|-
! Total
! 31.2
! 59
|}

===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]] applications and packaging applications. In the following table, the main applications of textile and packaging of polyester are listed.

{| class="wikitable"
|+ Textile and packaging polyester application list (melt or pellet)
! Textile
! Packaging
|-
| [[Staple (textiles)|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)
|-
| Mono-filament
| Strapping
|}

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 [[horizontal integration|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.

==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)<sub>2</sub> + n R'(COOH)<sub>2</sub> → HO[ROOCR'COO]<sub>n</sub>ROH + 2n H<sub>2</sub>O.

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.

:[[File:Overview polyester formation reaction.svg|frameless|upright=2.5]]

Azeotrope esterification is a classical method for condensation. The water formed by the reaction of [[alcohols|alcohol]] and a [[carboxylic acid]] is continually removed by [[azeotropic distillation]]. When melting points of the monomers are sufficiently low, a polyester can be formed via direct esterification while removing the reaction water via vacuum.

:[[File:Polyester formation via direct esterification.svg|frameless|upright=2.5]]

Direct bulk polyesterification at high temperatures (150 – 290&nbsp;°C) is well-suited and used on the industrial scale for the production of aliphatic, unsaturated, and aromatic–aliphatic polyesters. Monomers containing [[Phenolic acid|phenolic]] or [[tertiary hydroxyl groups]] exhibit a low reactivity with [[carboxylic acid]]s and cannot be polymerized via direct acid alcohol-based polyesterification.<ref name=":1"/>⁠ 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.<ref name=":0"/>

===Alcoholic transesterification===
{{main|Transesterification}}

:[[File:Polyester formation via transesterification.svg|frameless|upright=2.5]]

'''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, <nowiki>R''</nowiki> is a sacrificial unit such as a [[methyl group]] ([[methanol]] 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.<ref name=":1" /> 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.<ref>{{cite book | vauthors = Ravve A | title = Principles of Polymer Chemistry | publisher = Springer | location = New York, New York, NY | date = 2012}}</ref>

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&nbsp;°C upon the release of acetic acid) is, besides the acyl chloride based synthesis, the preferred route to wholly aromatic polyesters.<ref name=":1"/>

=== Acylation ===
In [[acylation]], the acid begins as an [[acyl chloride]], and thus the polycondensation proceeds with emission of [[hydrochloric acid]] (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.<ref name=":1" />⁠<ref name=":4" /><ref name="Duda_2005" /><ref name=":5">{{cite book | vauthors = Pilati F | chapter = Polyesters | title = Comprehensive Polymer Science and Supplements | publisher = Elsevier | date = 1989 | volume = 5 | pages = 275–315}}</ref> 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<sup>3</sup> and 4.7 × 10<sup>3</sup>, 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.<ref>{{cite book | vauthors = Lienert KW | chapter = Poly (ester-imide)s for industrial use | veditors =  Kricheldorf HR | title = Progress in Polyimide Chemistry II | series = Advances in Polymer Science | date = 1999 | volume = 141  | pages = 45–82 | publisher = Springer | location = Berlin, Heidelberg | doi = 10.1007/3-540-49814-1_2 | isbn = 978-3-540-64963-2}}</ref> The method is limited by the acid dichlorides' high cost, its sensitivity to hydrolysis and the occurrence of side reactions.<ref name="pmid25473252">{{cite journal | vauthors = Sokolsky-Papkov M, Langer R, Domb AJ | title = Synthesis of aliphatic polyesters by polycondensation using inorganic acid as catalyst | journal = Polymers for Advanced Technologies | volume = 22 | issue = 5 | pages = 502–511 | date = April 2011 | pmid = 25473252 | pmc = 4249767 | doi = 10.1002/pat.1541}}</ref>

The high temperature reaction (100 to > 300&nbsp;°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:<ref name=":5" />

:[[File:Polyester formation via neat acyl chloride.svg|frameless|upright=2.5]]

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).<ref name="pmid25473252"/>

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.<ref name=":5" />

:[[File:Polyester formation via interfacial acyl chloride.svg|frameless|upright=2.5]]

The procedure is used for the production of [[polyarylate]]s (polyesters based on bisphenols), [[polyamide]]s, [[polycarbonate]]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.<ref name=":1" /> Water could be in some cases replaced by an immiscible organic solvent (e. g. in the [[adiponitrile]]/[[carbon tetrachloride]] system).<ref name=":5" /> The procedure is of little use in the production of polyesters based on aliphatic diols which have higher [[PKa|p''K''<sub>a</sub>]] values than phenols and therefore do not form alcoholate ions in aqueous solutions.<ref name=":1" /> 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<sub>3</sub>N) or [[pyridine]] as acid acceptors:

:[[File:Polyester formation via amine acyl chloride.svg|frameless|upright=2.5]]

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.<ref name=":5" /> However, it is known that [[tertiary amines]] can cause [[Side reaction|side-reactions]] such as the formation of [[ketene]]s and ketene dimers.⁠<ref>{{Cite book| vauthors = Kricheldorf HR, Nuyken O, Swift G |url=https://www.worldcat.org/oclc/1156408945 | title = Handbook of Polyermer Synthesis | edition = 2nd |date=2004|publisher=CRC Press |isbn=0-367-57822-0|oclc=1156408945}}</ref>

===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)===
:[[File:Polyester formation via transesterification.svg|frameless|upright=2.5]]

===Ring-opening polymerization===
:[[File:Polyester ring-opening formation.svg|frameless|upright=2.5]]

[[Aliphatic]] polyesters can be assembled from [[lactone]]s under very mild conditions, catalyzed [[anionic]]ally, [[cationic]]ally, [[metallorganic]]ally or enzyme-based.<ref>{{cite journal | vauthors = Varma IK, Albertsson AC, Rajkhowa R, Srivastava RK| title = Enzyme catalyzed synthesis of polyesters. | journal = Progress in Polymer Science | date = October 2005 | volume = 30 | issue = 10 | pages = 949–81 | doi = 10.1016/j.progpolymsci.2005.06.010}}</ref><ref>{{Cite journal | vauthors = Nuyken O, Pask SD | date = April 2013 |title=Ring-Opening Polymerization—An Introductory Review |journal=Polymers|language=en|volume=5|issue=2|pages=361–403|doi=10.3390/polym5020361|issn=2073-4360|doi-access=free}}</ref> 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.<ref>{{cite journal | vauthors = Jérôme C, Lecomte P | title = Recent advances in the synthesis of aliphatic polyesters by ring-opening polymerization | journal = Advanced Drug Delivery Reviews | volume = 60 | issue = 9 | pages = 1056–76 | date = June 2008 | pmid = 18403043 | doi = 10.1016/j.addr.2008.02.008 | hdl = 2268/3723 | url = http://orbi.ulg.ac.be/handle/2268/3723 | hdl-access = free }}</ref><ref name="pmid15584698">{{cite journal | vauthors = Dechy-Cabaret O, Martin-Vaca B, Bourissou D | title = Controlled ring-opening polymerization of lactide and glycolide | journal = Chemical Reviews | volume = 104 | issue = 12 | pages = 6147–76 | date = December 2004 | pmid = 15584698 | doi = 10.1021/cr040002s}}</ref>

===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.<ref name=":1">{{cite book | vauthors = Rogers ME, Long TE | title = Synthetic Methods in Step-Growth Polymers | publisher = John Wiley & Sons, Inc. | location = Hoboken, New Jersey, US | date = 2003}}</ref>

Instead of acyl chlorides, so-called activating agents can be used, such as [[Carbonyldiimidazole|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:<ref name=":1"/>

:[[File:Polyester formation via reactive reagent.svg|frameless|upright=2.5]]

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.<ref name=":1"/>

=== Thermodynamics of polycondensation reactions ===
Polyesterifications are grouped by some authors<ref name=":1"/><ref name=":4">{{cite journal | vauthors = Vinogradova SV | title = The basic principles of non-equilibrium polycondensation. | journal = Polymer Science USSR | date = January 1977 | volume = 19 | issue = 4 | pages = 769–808 | doi = 10.1016/0032-3950(77)90232-5 }}</ref> 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<sub>2</sub>O) against the acid-based (-C(O)OH) and alcohol-based (-OH) monomers is described by the equilibrium constant ''K<sub>C</sub>''.

:<math chem="">K_C = \frac{[...\ce{-C(O)O -}...][\ce{H2O}]}{[\ce{-C(O)OH}][\ce{-OH}]} </math>

The equilibrium constant of the acid-alcohol based polyesterification is typically ''K<sub>C</sub>'' ≤ 10, what is not high enough to obtain high-molecular weight polymers (''DP<sub>n</sub>'' ≥ 100), as the number average degree of polymerization (''DP<sub>n</sub>'') can be calculated from the equilibrium constant ''K<sub>C</sub>''.<ref name="Duda_2005">{{cite book | vauthors = Duda A, Penczek S | chapter = Mechanisms of Aliphatic Polyester Formation | title = Biopolymers Online | veditors = Doi Y, Steinbüchel A | publisher = Wiley-VCH Verlag GmbH & Co. KGaA | location = Weinheim, Germany | date = 2005 | pages = 371–383 | doi = 10.1002/3527600035.bpol3b12 }}</ref>

:<math>DP_n ~ = ~ \sqrt[2]{K_C} + 1 </math>

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.<ref name="Duda_2005" /> The condensation product is therefore removed at reduced pressure and high temperatures (150–320&nbsp;°C, depending on the monomers) to prevent the back reaction.<ref name="Park_2012" /> 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<sub>C</sub>'' ≥ 10<sup>4</sup> are achieved when using reactive reactants ([[acid chlorides]] or [[acid anhydrides]]) or activating agents like [[Carbonyldiimidazole|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]] began research on large molecules and synthetic fibers. This early research, headed by [[Wallace Carothers]], centered on what became [[nylon]], which was one of the first synthetic fibers.<ref>{{Cite web |url=http://www.madehow.com/Volume-2/Polyester.html |title=How polyester is made - material, manufacture, making, history, used, structure, steps, product, History |publisher=www.madehow.com |access-date=2018-12-04}}</ref> 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 [[United Kingdom of Great Britain and Northern Ireland|Britain]] by the British [[General Electric Company]].<ref>{{Cite journal |doi = 10.1080/19447015108663852 |title=The Development of the Synthetic Fibres |year=1951 |vauthors = Loasby G |journal = Journal of the Textile Institute Proceedings|volume = 42|issue = 8|pages = P411–P441}}</ref> Carothers' project was revived by British scientists [[John Rex Whinfield|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).<ref name=":0"/>

== Biodegradation and environmental concerns ==
{{main| Biodegradation}}
The [[Futuro]] houses were made of [[fibreglass]]-reinforced polyester plastic; polyester-[[polyurethane]], and [[poly(methyl methacrylate)]]. One house was found to be degrading by [[cyanobacteria]] and [[Archaea]].<ref>{{cite journal | vauthors = Cappitelli F, Principi P, Sorlini C | title = Biodeterioration of modern materials in contemporary collections: can biotechnology help? | journal = Trends in Biotechnology | volume = 24 | issue = 8 | pages = 350–4 | date = August 2006 | pmid = 16782219 | doi = 10.1016/j.tibtech.2006.06.001 }}</ref><ref>{{cite journal | vauthors = Rinaldi A | title = Saving a fragile legacy. Biotechnology and microbiology are increasingly used to preserve and restore the world's cultural heritage | journal = EMBO Reports | volume = 7 | issue = 11 | pages = 1075–9 | date = November 2006 | pmid = 17077862 | pmc = 1679785 | doi = 10.1038/sj.embor.7400844 }}</ref>

===Cross-linking===
Unsaturated polyesters are [[thermosetting polymer]]s. They are generally [[copolymer]]s prepared by [[polymerizing]] one or more [[diol]]s with saturated and unsaturated [[dicarboxylic acid]]s ([[maleic acid]], [[fumaric acid]], etc.) or their [[anhydride]]s. The double bond of unsaturated polyesters reacts with a [[vinyl group|vinyl]] [[monomer]], usually [[styrene]], resulting in a 3-D [[cross-link]]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&nbsp;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]] [[plastic pollution|pollution]].<ref>{{cite web | vauthors = O'Connor MC | date = 27 October 2014 | url = https://www.theguardian.com/sustainable-business/2014/oct/27/toxic-plastic-synthetic-microscopic-oceans-microbeads-microfibers-food-chain | title = Inside the lonely fight against the biggest environmental problem you've never heard of | work = The Guardian }}</ref><ref>{{cite web|last1=Williams|first1=Alan | name-list-style = vanc |title=Washing clothes releases thousands of microplastic particles into environment, study shows|date=27 September 2016 |url=https://www.plymouth.ac.uk/news/washing-clothes-releases-thousands-of-microplastic-particles-into-environment-study-shows|publisher=Plymouth University|access-date=9 October 2016}}</ref><ref>{{cite journal | vauthors = Napper IE, Thompson RC | title = Release of synthetic microplastic plastic fibres from domestic washing machines: Effects of fabric type and washing conditions | journal = Marine Pollution Bulletin | volume = 112 | issue = 1–2 | pages = 39–45 | date = November 2016 | pmid = 27686821 | doi = 10.1016/j.marpolbul.2016.09.025 | bibcode = 2016MarPB.112...39N | hdl-access = free | hdl = 10026.1/8163}}</ref>

==Safety==
===Fertility===
[[Ahmed Shafik (sexologist)|Ahmed Shafik]] was a sexologist who won a [[Ig Nobel Prize]] on his research regarding how polyester can affect the fertility of rats,<ref name="Shafik 1993 pp. 375–380">{{cite journal | last=Shafik | first=Ahmed | title=Effect of Different Types of Textiles on Sexual Activity | journal=European Urology | publisher=Elsevier BV | volume=24 | issue=3 | year=1993 | issn=0302-2838 | doi=10.1159/000474332 | pages=375–380| pmid=8262106 }}</ref> dogs,<ref name="Shafik 1993 pp. 367–370">{{cite journal | last=Shafik | first=Ahmed | title=Effect of different types of textile fabric on spermatogenesis: an experimental study | journal=Urological Research | publisher=Springer Science and Business Media LLC | volume=21 | issue=5 | year=1993 | issn=0300-5623 | doi=10.1007/bf00296839 | pages=367–370| pmid=8279095 | s2cid=22096865 }}</ref> and men.<ref name="Shafik 1992 pp. 439–451">{{cite journal | last=Shafik | first=Ahmed | title=Contraceptive efficacy of polyester-induced azoospermia in normal men | journal=Contraception | publisher=Elsevier BV | volume=45 | issue=5 | year=1992 | issn=0010-7824 | doi=10.1016/0010-7824(92)90157-o | pages=439–451| pmid=1623716 }}</ref>

[[Bisphenol A]] which is a [[Endocrine disruptor|endocrine disrupting chemical]] may be used in the synthesis of polyester.<ref name="DAngelo Meccariello 2021 p. 2392">{{cite journal | last1=D'Angelo | first1=Stefania | last2=Meccariello | first2=Rosaria | title=Microplastics: A Threat for Male Fertility | journal=International Journal of Environmental Research and Public Health | publisher=MDPI AG | volume=18 | issue=5 | date=2021-03-01 | issn=1660-4601 | doi=10.3390/ijerph18052392 | page=2392 | pmid=33804513 | pmc=7967748 | doi-access=free }}</ref>

== Recycling ==
[[Plastic recycling|Recycling of polymers]] has become very important as the production and use of plastic is continuously rising. Global [[plastic waste]] may almost triple by 2060 if this continues.<ref>{{Cite web |title=Global plastic waste set to almost triple by 2060, says OECD |url=https://www.oecd.org/environment/global-plastic-waste-set-to-almost-triple-by-2060.htm |access-date=2022-10-15 |website=www.oecd.org}}</ref> Plastics can be recycled by various means like mechanical recycling, chemical recycling, etc. Among the recyclable polymers, polyester [[PET bottle recycling|PET]] is one of the most recycled plastics.<ref>{{Cite web |title=How to keep a sustainable PET recycling industry in Europe - EPBP - European PET Bottle Platform |url=https://www.epbp.org/ |access-date=2022-10-15 |website=www.epbp.org}}</ref><ref>{{Cite web |title=Which Plastic Can Be Recycled? |url=https://www.plasticsforchange.org/blog/which-plastic-can-be-recycled |access-date=2022-10-15 |website=Plastics For Change |date=20 May 2021 |language=en-US}}</ref> The ester bond present in polyesters is susceptible to [[hydrolysis]] (acidic or basic conditions), [[methanolysis]] and glycolysis which makes this class of polymers suitable for chemical recycling.<ref>{{Cite journal |last1=Ghosal |first1=Krishanu |last2=Nayak |first2=Chinmaya |date=2022-02-21 |title=Recent advances in chemical recycling of polyethylene terephthalate waste into value added products for sustainable coating solutions – hope vs. hype |journal=Materials Advances |language=en |volume=3 |issue=4 |pages=1974–1992 |doi=10.1039/D1MA01112J |s2cid=245886607 |issn=2633-5409|doi-access=free }}</ref> Enzymatic/biological recycling of PET can be carried out using different enzymes like [[PETase]], [[cutinase]], [[esterase]], [[lipase]], etc.<ref>{{Cite journal |last1=Ghosal |first1=Krishanu |last2=Nayak |first2=Chinmaya |date=2022 |title=Recent advances in chemical recycling of polyethylene terephthalate waste into value added products for sustainable coating solutions – hope vs . hype |url=http://xlink.rsc.org/?DOI=D1MA01112J |journal=Materials Advances |language=en |volume=3 |issue=4 |pages=1974–1992 |doi=10.1039/D1MA01112J |s2cid=245886607 |issn=2633-5409|doi-access=free }}</ref> 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.<ref>{{Cite journal |last1=Wagner-Egea |first1=Paula |last2=Tosi |first2=Virginia |last3=Wang |first3=Ping |last4=Grey |first4=Carl |last5=Zhang |first5=Baozhong |last6=Linares-Pastén |first6=Javier A. |date=January 2021 |title=Assessment of IsPETase-Assisted Depolymerization of Terephthalate Aromatic Polyesters and the Effect of the Thioredoxin Fusion Domain |journal=Applied Sciences |language=en |volume=11 |issue=18 |pages=8315 |doi=10.3390/app11188315 |issn=2076-3417|doi-access=free}}</ref>

==See also==
{{div col|colwidth=22em}}
* [[Epoxy]]
* [[Glycerine phthalate]]
* [[Microfiber]]
* [[Oligoester]]
* [[Polyamide]]
* [[Rayon]]
* [[Viscose]]
{{Div col end}}

== References ==
{{reflist}}

== Further reading ==
*''Textiles'', by Sara Kadolph and Anna Langford. 8th Edition, 1998.
* Spinning Machines https://en.wikipedia.org/wiki/Spinning_(textiles)

== External links ==
*[http://pubs.acs.org/doi/abs/10.1021/ma301178k Lipase catalyzed polyesterification: Enzyme-Catalyzed Polymerization of End-Functionalized Polymers in a Microreactor]

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[[Category:Carboxylate esters]]
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