Editing Polypropylene (section)

==Recycling==
{{More information|Plastic recycling}}
Most polypropylene recycling uses [[Plastic recycling#Mechanical_recycling|mechanical recycling]], as for polyethylene: the material is heated to soften or melt it, and mechanically formed it into new products.<ref name="t634">{{cite journal |last1=Li |first1=Houqian |last2=Aguirre-Villegas |first2=Horacio A. |last3=Allen |first3=Robert D. |last4=Bai |first4=Xianglan |last5=Benson |first5=Craig H. |display-authors=etal |title=Expanding plastics recycling technologies: chemical aspects, technology status and challenges |journal=Green Chemistry |volume=24 |issue=23 |date=2022 |issn=1463-9262 |doi=10.1039/D2GC02588D |pages=8899–9002|doi-access=free }}</ref> As of 2015, less than 1% of polypropylene generated was recycled.<ref name="Thiounn 2020">{{cite journal |last1=Thiounn |first1=Timmy |last2=Smith |first2=Rhett C. |title=Advances and approaches for chemical recycling of plastic waste |journal=Journal of Polymer Science |volume=58 |issue=10 |date=2020 |issn=2642-4150 |doi=10.1002/pol.20190261 |pages=1347–1364}}</ref> Heating degrades the carbon backbone more severely than for polyethylene, breaking it into smaller organic molecules, because the methyl side group of PP is susceptible to [[Thermal degradation of polymers|thermo-oxidative]] and [[Photo-oxidation of polymers|photo-oxidative]] degradation.<ref name="Thiounn 2020" />

Polypropylene has the number "5" as its [[resin identification code]]:<ref>[http://www.wasteonline.org.uk/resources/InformationSheets/Plastics.htm Plastics recycling information sheet] {{webarchive|url=https://web.archive.org/web/20100722003753/http://www.wasteonline.org.uk/resources/InformationSheets/Plastics.htm |date=2010-07-22 }}, Waste Online</ref> [[File:Symbol_Resin_Code_5_PP.svg|class=skin-invert-image|50px]]