An Overview of Polyester and Polyester Dyeing Part-2

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Making polyesters as an example of condensation polymerisation:
In condensation polymerisation, when the monomers join together a small molecule gets lost. That's different from addition polymerisation which produces polymers like poly(ethene) - in that case, nothing is lost when the monomers join together.

A polyester is made by a reaction involving an acid with two -COOH groups, and an alcohol with two -OH groups.

In the common polyester drawn above:

The acid is benzene-1,4-dicarboxylic acid (old name: terephthalic acid).

The alcohol is ethane-1,2-diol (old name: ethylene glycol).

Now imagine lining these up alternately and making esters with each acid group and each alcohol group, losing a molecule of water every time an ester linkage is made.

That would produce the chain shown above (although this time written without separating out the carbon-oxygen double bond - write it whichever way you like).

In addition to pure (homopolymer) PET, PET modified by copolymerization is also available. In some cases, the modified properties of copolymer are more desirable for a particular application. For example, cyclohexane dimethanol (CHDM) can be added to the polymer backbone in place of ethylene glycol. Since this building block is much larger (6 additional carbon atoms) than the ethylene glycol unit it replaces, it does not fit in with the neighboring chains the way an ethylene glycol unit would. This interferes with crystallization and lowers the polymer's melting temperature. In general, such PET is known as PETG or PET-G (Polyethylene terephthalate glycol-modified; Eastman Chemical, SK Chemicals, and Artenius Italia are some PETG manufacturers). PETG is a clear amorphous thermoplastic that can be injection molded or sheet extruded. It can be colored during processing.

Replacing terephthalic acid (right) with isophthalic acid (center) creates a kink in the PET chain, interfering with crystallization and lowering the polymer's melting point.

Another common modifier is isophthalic acid, replacing some of the 1,4-(para-) linked terephthalate units. The 1,2-(ortho-) or 1,3-(meta-) linkage produces an angle in the chain, which also disturbs crystallinity.

Such copolymers are advantageous for certain molding applications, such as thermoforming, which is used for example to make tray or blister packaging from co-PET film, or amorphous PET sheet (A-PET) or PETG sheet. On the other hand, crystallization is important in other applications where mechanical and dimensional stability are important, such as seat belts. For PET bottles, the use of small amounts of isophthalic acid, CHDM, DEG or other comonomers can be useful: if only small amounts of comonomers are used, crystallization is slowed but not prevented entirely. As a result, bottles are obtainable via stretch blow molding ("SBM"), which are both clear and crystalline enough to be an adequate barrier to aromas and even gases, such as carbon dioxide in carbonated beverages.

Manufacturing of polyethylene terephthalate
The reaction takes place in two main stages: a pre-polymerisation stage and the actual polymerisation.

In the first stage, before polymerisation happens, you get a fairly simple ester formed between the acid and two molecules of ethane-1,2-diol.

In the polymerisation stage, this is heated to a temperature of about 260°C and at a low pressure. A catalyst is needed - there are several possibilities including antimony compounds like antimony(III) oxide.

The polyester forms and half of the ethane-1,2-diol is regenerated. This is removed and recycled.

Recycling to the monomers:
Polyethylene terephthalate can be depolymerized to yield the constituent monomers. After purification, the monomers can be used to prepare new polyethylene terephthalate. The ester bonds in polyethylene terephthalate may be cleaved by hydrolysis, or by transesterification. The reactions are simply the reverse of those used in production.

Partial glycolysis:
Partial glycolysis (transesterification with ethylene glycol) converts the rigid polymer into short-chained oligomers that can be melt-filtered at low temperature. Once freed of the impurities, the oligomers can be fed back into the production process for polymerization.

The task consists in feeding 10–25% bottle flakes while maintaining the quality of the bottle pellets that are manufactured on the line. This aim is solved by degrading the PET bottle flakes — already during their first plasticization, which can be carried out in a single- or multi-screw extruder — to an intrinsic viscosity of about 0.30 dℓ/g by adding small quantities of ethylene glycol and by subjecting the low-viscosity melt stream to an efficient filtration directly after plasticization. Furthermore, temperature is brought to the lowest possible limit. In addition, with this way of processing, the possibility of a chemical decomposition of the hydro peroxides is possible by adding a corresponding P-stabilizer directly when plasticizing. The destruction of the hydro peroxide groups is, with other processes, already carried out during the last step of flake treatment for instance by adding H3PO3.]The partially glycolyzed and finely filtered recycled material is continuously fed to the esterification or prepolycondensation reactor, the dosing quantities of the raw materials are being adjusted accordingly. Total glycolysis, methanolysis, and hydrolysis

The treatment of polyester waste through total glycolysis to fully convert the polyester to bis(2-hydroxyethyl) terephthalate (C6H4(CO2CH2CH2OH)2). This compound is purified by vacuum distillation, and is one of the intermediates used in polyester manufacture. The reaction involved is as follows:

[(CO)C6H4(CO2CH2CH2O)]n + n HOCH2CH2OH → n C6H4(CO2CH2CH2OH)2

This recycling route has been executed on an industrial scale in Japan as experimental production.

Similar to total glycolysis, methanolysis converts the polyester to dimethyl terephthalate, which can be filtered and vacuum distilled:

[(CO)C6H4(CO2CH2CH2O)]n + 2n CH3OH → n C6H4(CO2CH3)2

Methanolysis is only rarely carried out in industry today because polyester production based on dimethyl terephthalate has shrunk tremendously, and many dimethyl terephthalate producers have disappeared.

Also as above, polyethylene terephthalate can be hydrolyzed to terephthalic acid and ethylene glycol under high temperature and pressure. The resultant crude terephthalic acid can be purified by recrystallization to yield material suitable for re-polymerization:

[(CO)C6H4(CO2CH2CH2O)]n + 2n H2O → n C6H4(CO2H)2 + n HOCH2CH2OH

This method does not appear to have been commercialized yet.

Hydrolysis of polyesters
Simple esters are easily hydrolysed by reaction with dilute acids or alkalis.Polyesters are attacked readily by alkalis, but much more slowly by dilute acids. Hydrolysis by water alone is so slow as to be completely unimportant. (You wouldn't expect your polyester fleece to fall to pieces if you went out in the rain!)

If you spill dilute alkali on a fabric made from polyester, the ester linkages are broken. Ethane-1,2-diol is formed together with the salt of the carboxylic acid.

Because you produce small molecules rather than the original polymer, the fibres are destroyed, and you end up with a hole!

For example, if you react the polyester with sodium hydroxide solution:

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Mazharul Islam Kiron is a textile consultant and researcher on online business promotion. He is working with one European textile machinery company as a country agent. He is also a contributor of Wikipedia.

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