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Plastic bottles made from PET are widely used for [[soft drink]]s, both still and [[carbonation|sparkling]]. For beverages that are degraded by oxygen, such as beer, a multilayer structure is used. PET sandwiches an additional [[polyvinyl alcohol]] (PVOH) or [[polyamide]] (PA) layer to further reduce its oxygen permeability.
Non-oriented PET sheet can be [[thermoforming|thermoformed]] to make packaging trays and [[blister packs]].<ref>{{Citation|last=Pasbrig|first=Erwin|title=Cover film for blister packs|date=29 March 2007|url=
PET is permeable to oxygen and carbon dioxide and this imposes shelf life limitations of contents packaged in PET.<ref>{{Cite book |last1=Ashurst |first1=P. |url=https://books.google.com/books?id=FQykAgAAQBAJ&pg=PA104 |title=Soft Drink and Fruit Juice Problems Solved |last2=Hargitt |first2=R. |date=2009-08-26 |publisher=Elsevier |isbn=978-1-84569-706-8 |language=en}}</ref>{{Rp|page=104}}
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In the second step, the preforms are heated rapidly and then inflated against a two-part mold to form them into the final shape of the bottle. Preforms (uninflated bottles) are now also used as robust and unique containers themselves; besides novelty candy, some [[Red Cross]] chapters distribute them as part of the [[Vial of Life]] program to homeowners to store medical history for emergency responders.
The two-step process lends itself to third party production remote from the user site. The preforms can be transported and stored by the thousand in a much smaller space than would finished containers, for the second stage to be carried out on the user site on a 'just in time' basis.
In one-step machines, the entire process from raw material to finished container is conducted within one machine, making it especially suitable for molding non-standard shapes (custom molding), including jars, flat oval, flask shapes, etc. Its greatest merit is the reduction in space, product handling and energy, and far higher visual quality than can be achieved by the two-step system.{{Citation needed|date=March 2013}}
===Degradation===
PET is subject to [[polymer degradation|degradation]] during processing. If the moisture level is too high, [[hydrolysis]] will reduce the [[Molar mass|molecular weight]] by chain [[Bond cleavage|scission]], resulting in brittleness. If the [[residence time]] and/or melt temperature (temperature at melting) are too high, then [[thermal degradation]] or thermooxidative degradation will occur resulting in discoloration and reduced molecular weight, as well as the formation of [[acetaldehyde]], and the formation "gel" or "fish-eye" formations through [[cross-link]]ing. Mitigation measures include [[copolymer]]isation with other
====Acetaldehyde====
[[Acetaldehyde]], which can form by degradation of PET after mishandling of the material, is a colorless, volatile substance with a fruity smell. Although it forms naturally in some fruit, it can cause an off-taste in bottled water. As well as high temperatures (PET decomposes above 300 °C or 570 °F) and long barrel residence times, high pressures and high extruder speeds (which cause shear raising the temperature), can also contribute to the production of acetaldehyde. [[Photo-oxidation of polymers|Photo-oxidation]] can also cause the gradual formation acetaldehyde over the object's lifespan. This proceeds via a Type II [[Norrish reaction]].<ref name="Wiles&DayIII">{{cite journal |last1=Day |first1=M. |last2=Wiles |first2=D. M. |title=Photochemical degradation of poly(ethylene terephthalate). III. Determination of decomposition products and reaction mechanism |journal=Journal of Applied Polymer Science |date=January 1972 |volume=16 |issue=1 |pages=203–215 BHET|doi=10.1002/app.1972.070160118}}</ref>
[[File:Poly(ethylene_terephthalate)_-_Type_II_Norrish_to_acetaldehyde.png|600px|center]]
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=== Shed microfibres ===
Clothing sheds microfibres in use, during washing and machine drying. Plastic litter slowly forms small particles. Microplastics which are present on the bottom of the river or seabed can be ingested by small marine life, thus entering the food chain. As PET has a higher density than water, a significant amount of PET microparticles may be precipitated in sewage treatment plants. PET microfibers generated by apparel wear, washing or machine drying can become airborne, and be dispersed into fields, where they are ingested by livestock or plants and end up in the human food supply. [[Scientific Advice Mechanism|SAPEA]] have declared that such particles 'do not pose a widespread risk'.<ref>{{cite web |title=SAPEA report: Evidence on microplastics does not yet point to widespread risk - ALLEA |url=https://allea.org/sapea-report-microplastics/ |access-date=5 March 2022}}</ref>
PET is known to degrade when exposed to sunlight and oxygen.<ref>{{cite journal |last1=Chamas |first1=Ali |last2=Moon |first2=Hyunjin |last3=Zheng |first3=Jiajia |last4=Qiu |first4=Yang |last5=Tabassum |first5=Tarnuma |last6=Jang |first6=Jun Hee |last7=Abu-Omar |first7=Mahdi |last8=Scott |first8=Susannah L. |last9=Suh |first9=Sangwon |date=9 March 2020 |title=Degradation Rates of Plastics in the Environment |journal=ACS Sustainable Chemistry & Engineering |volume=8 |issue=9 |pages=3494–3511 |doi=10.1021/acssuschemeng.9b06635 |s2cid=212404939 |doi-access=free}}</ref> As of 2016, scarce information exists regarding the life-time of the synthetic polymers in the environment.<ref>{{cite journal |last1=Ioakeimidis |first1=C. |last2=Fotopoulou |first2=K. N. |last3=Karapanagioti |first3=H. K. |last4=Geraga |first4=M. |last5=Zeri |first5=C. |last6=Papathanassiou |first6=E. |last7=Galgani |first7=F. |last8=Papatheodorou |first8=G. |date=22 March 2016 |title=The degradation potential of PET bottles in the marine environment: An ATR-FTIR based approach |journal=Scientific Reports |volume=6 |issue=1 |pages=23501 |bibcode=2016NatSR...623501I |doi=10.1038/srep23501 |pmc=4802224 |pmid=27000994}}</ref>
==Polyester recycling==
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[[Category:Thermoplastics]]
[[Category:Transparent materials]]
[[Category:Food packaging| ]]
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