Polyester

Thermally stable polymers, which have a high proportion of aromatic structures, are also called high-performance plastics; this application-oriented classification compares such polymers with engineering plastics and commodity plastics. The continuous service temperature of high-performance plastics is generally stated as being higher than 150 °C,[13] whereas engineering plastics (such as polyamide or polycarbonate) are often defined as thermoplastics that retain their properties above 100 °C.[14]⁠ 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.[15] 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.[16]⁠ 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.[15]⁠ 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.[7]

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.

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 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. According to this table, the world's total polyester production might exceed 50 million tons per annum before the year 2010.

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.

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 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.

In this classical method, an alcohol and a carboxylic acid react to form a carboxylic ester. To assemble a polymer, the water formed by the reaction must be continually removed by azeotrope 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.

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 phenolic or tertiary hydroxyl groups exhibit a low reactivity with carboxylic acids and cannot be polymerized via direct acid alcohol-based polyesterification.[7]⁠ 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.[1]

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 (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.[7] 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.[18]

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.[7]

In acylation, the acid begins as an acid 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.[7]⁠[19][20][21] 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.[22] The method is limited by the acid dichlorides’ high cost, its sensitivity to hydrolysis and the occurrence of side reactions.[23]

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:[21]

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).[24]

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.[21]

The procedure is used for the production of polyarylates (polyesters based on bisphenols), polyamides, polycarbonates, 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.[7] Water could be in some cases replaced by an immiscible organic solvent (e. g. in the adiponitrile/carbon tetrachloride system).[21] The procedure is of little use in the production of polyesters based on aliphatic diols which have higher pK_a values than phenols and therefore do not form alcoholate ions in aqueous solutions.[7] 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.[21] However, it is known that tertiary amines can cause side-reactions such as the formation of ketenes and ketene dimers.⁠[25]

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

Silyl acetate method

Aliphatic polyesters can be assembled from lactones under very mild conditions, catalyzed anionically, cationically, metallorganically or enzyme-based[26][27] 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.[28][29]

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.[7]

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:[7]

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.[7]

Polyesterifications are grouped by some authors[7][19] 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 (H₂O) against the acid-based (-C(O)OH) and alcohol-based (-OH) monomers is described by the equilibrium constant K_C.

${\displaystyle K_{C}={\frac {[...-C(O)O-...][H_{2}O]}{[-C(O)OH][-OH]}}}$

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.[20]

${\displaystyle DP_{n}~=~{\sqrt[{2}]{K_{C}}}+1}$

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.[20] The condensation product is therefore removed at reduced pressure and high temperatures (150–320 °C, depending on the monomers) to prevent the back reaction.[9] 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 ≥ 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.

Unsaturated polyesters are thermosetting resins. They are generally copolymers prepared by polymerizing one or more diol with saturated and unsaturated dicarboxylic acids (maleic acid, fumaric acid...) or their anhydrides. The double bond of unsaturated polyesters reacts with a vinyl monomer, usually styrene, resulting in a 3-D cross-linked structure. This structure acts as a thermoset. The exothermic cross-linking reaction is initiated through a catalyst, usually an organic peroxide such as methyl ethyl ketone peroxide or benzoyl peroxide.

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 pollution.[35][36][37]

Polyester is a synthetic petroleum-based fibre, and is therefore a non-renewable carbon-intensive resource.[38] Nearly 70 million barrels of oil are used each year to make polyester around the world, which is now the most commonly used fiber in making clothes. However, polyester takes more than 200 years to decompose.[39]