View source for Polypropylene

{{short description|Thermoplastic polymer}}
{{Chembox
|verifiedrevid  = 47717187
|Name = Polypropylene (PP)
|ImageFile1 = Polypropylen.svg
|ImageSize1 = 100px
|ImageClass1 = skin-invert-image
|ImageFile2 = Polypropylene isotactic mini trp.png
|ImageName2 = Polypropylene isotactic
|ImageClass2 = bg-transparent
|ImageFile3 = Polypropylene syndiotactic mini trp.png
|ImageName3 = Polypropylene syndiotactic
|ImageClass3 = bg-transparent
|IUPACName = Poly(1-methylethylene)
|OtherNames = Polypropylene; Polypropene;<br />Polipropene 25 [USAN]; Propene polymers;<br />Propylene polymers; 1-Propene; [-Ch2-Ch(Ch3)-]n
|Section1 = {{Chembox Identifiers
|CASNo_Ref = {{cascite|correct|CAS}}
|UNII_Ref = {{fdacite|correct|FDA}}
|UNII = 1ASA949VIC
|CASNo = 9003-07-0
|ChemSpiderID_Ref = {{chemspidercite|changed|chemspider}}
|ChemSpiderID = None
}}
|Section2 = {{Chembox Properties
|Formula = (C<sub>3</sub>H<sub>6</sub>)<sub>n</sub>
|Density = 0.855 g/cm<sup>3</sup>, amorphous <br />0.946 g/cm<sup>3</sup>, crystalline
|MeltingPtC = 130 to 171
}}
}}
'''Polypropylene''' ('''PP'''), also known as '''polypropene''', is a [[thermoplastic]] [[polymer]] used in a wide variety of applications. It is produced via [[chain-growth polymerization]] from the [[monomer]] [[Propene|propylene]].

Polypropylene belongs to the group of [[polyolefin]]s and is [[Crystallization of polymers#Degree of crystallinity|partially crystalline]] and [[Chemical polarity#Nonpolar molecules|non-polar]]. Its properties are similar to [[polyethylene]], but it is slightly harder and more heat-resistant. It is a white, mechanically rugged material and has a high chemical resistance.<ref name="Ullmanns">{{Ullmann |doi=10.1002/14356007.o21_o04.pub2|title=Polypropylene |year=2014 |last1=Gahleitner |first1=Markus |last2=Paulik |first2=Christian |pages=1–44 |isbn=9783527306732}}</ref>

Polypropylene is the second-most widely produced [[Commodity plastics|commodity plastic]] (after [[polyethylene]]).

==History==
[[Phillips Petroleum]] chemists [[J. Paul Hogan]] and [[Robert Banks (chemist)|Robert Banks]] first demonstrated the polymerization of propylene in 1951.<ref>{{cite journal |doi=10.1021/cen-v065n010.p030 |title=Discoverers of Polypropylene Share Prize |journal=Chemical & Engineering News |volume=65 |issue=10 |pages=30 |year=1987 |last1=Stinson |first1=Stephen }}</ref> The stereoselective polymerization to the isotactic was discovered by [[Giulio Natta]] and Karl Rehn in March 1954.<ref>{{cite book|url=https://books.google.com/books?id=GjtJfmxvSWgC&pg=PA76|page=76|title=Polymer Pioneers: A Popular History of the Science and Technology of Large Molecules|author=Morris, Peter J. T. |publisher=Chemical Heritage Foundation|year=2005|isbn=978-0-941901-03-1}}</ref> This pioneering discovery led to large-scale commercial production of isotactic polypropylene by the Italian firm [[Montecatini (company)|Montecatini]] from 1957 onwards.<ref>{{cite magazine |url=https://www.newscientist.com/article/mg19426014.900-this-week-50-years-ago.html |title=This week 50 years ago |magazine=[[New Scientist]] |date=28 April 2007 |page=15}}</ref> Syndiotactic polypropylene was also first synthesized by Natta.  Interest in polypropylene development is ongoing to the present.  For example, making polypropylene from bio-based resources is a topic of interest in the 21st century.<ref>{{Cite journal |last1=Wang |first1=Suxi |last2=Muiruri |first2=Joseph Kinyanjui |last3=Soo |first3=Xiang Yun Debbie |last4=Liu |first4=Songlin |last5=Thitsartarn |first5=Warintorn |last6=Tan |first6=Beng Hoon |last7=Suwardi |first7=Ady |last8=Li |first8=Zibiao |last9=Zhu |first9=Qiang |last10=Loh |first10=Xian Jun |date=2023-01-17 |title=Bio-Polypropylene and Polypropylene-based Biocomposites: Solutions for a Sustainable Future |url=https://onlinelibrary.wiley.com/doi/10.1002/asia.202200972 |journal=Chemistry: An Asian Journal |language=en |volume=18 |issue=2 |pages=e202200972 |doi=10.1002/asia.202200972 |pmid=36461701 |s2cid=253825228 |issn=1861-4728|url-access=subscription }}</ref>

==Chemical and physical properties==
[[Image:Polypropene migrograph.png|thumb|[[Micrograph]] of polypropylene]]

Polypropylene is, in small aspects, similar to [[polyethylene]], especially in [[Dissolution (chemistry)#Polymers|solution behavior]] and electrical properties. The [[methyl group]] improves mechanical properties and thermal resistance, although the chemical resistance decreases.<ref name="guide">{{cite book |last1= Tripathi|first1=D. |title= Practical guide to polypropylene|date=2001 |publisher=RAPRA Technology |location=Shrewsbury |isbn=978-1859572825}}</ref>{{rp|19}} The properties of polypropylene depend on the molecular weight and molecular weight distribution, crystallinity, type and proportion of comonomer (if used) and the iso[[tacticity]].<ref name="guide" /> In isotactic polypropylene, for example, the methyl groups are oriented on one side of the carbon backbone. This [[stereochemistry|arrangement]] creates a greater degree of crystallinity and results in a stiffer material that is more resistant to creep than both atactic polypropylene and polyethylene.<ref>{{Cite web|url=http://www.porex.com/technologies/materials/porous-plastics/polypropylene/|archive-url=https://web.archive.org/web/20161114233313/https://www.porex.com/technologies/materials/porous-plastics/polypropylene |archive-date=2016-11-14| title=Polypropylene Plastic Materials & Fibers by Porex|website=www.porex.com|access-date=2016-11-09}}</ref>

===Mechanical properties===
The density of PP is between 0.895 and 0.93&nbsp;g/cm<sup>3</sup>. Therefore, PP is the [[Commodity plastics|commodity plastic]] with the lowest density. With lower density, [[Injection moulding|moldings parts]] with lower weight and more parts of a certain mass of plastic can be produced. Unlike polyethylene, crystalline and amorphous regions differ only slightly in their density. However, the density of polyethylene can significantly change with fillers.<ref name="guide" />{{rp|24}}

The [[Young's modulus]] of PP is between 1300 and 1800&nbsp;N/mm².

Polypropylene is normally tough and flexible, especially when [[copolymer]]ized with [[ethylene]]. This allows polypropylene to be used as an [[engineering plastic]], competing with materials such as [[acrylonitrile butadiene styrene]] (ABS). Polypropylene is reasonably economical.{{Citation needed|date=June 2018}}

Polypropylene has good resistance to [[fatigue (material)|fatigue]].<ref name="Maier" />{{rp|3070}}

===Thermal properties===
The melting point of polypropylene occurs in a range, so the melting point is determined by finding the highest temperature of a [[differential scanning calorimetry]] chart. Perfectly isotactic PP has a melting point of {{convert|171|C|abbr=on}}. Commercial isotactic PP has a melting point that ranges from {{convert|160|to|166|C|F|abbr=on}}, depending on [[atactic]] material and crystallinity. [[Syndiotactic]] PP with a crystallinity of 30% has a melting point of {{convert|130|C|abbr=on}}.<ref name="Maier">{{Cite book|last1 = Maier|first1 = Clive|last2 = Calafut|first2 = Teresa|title = Polypropylene: the definitive user's guide and databook|page = 14|publisher = William Andrew|year = 1998|url = https://books.google.com/books?id=AWaSJd9Non8C&pg=PA14|isbn = 978-1-884207-58-7}}</ref> Below 0&nbsp;°C, PP becomes brittle.<ref name=Kaiser2011p247>{{cite book |last1=Kaiser |first1=Wolfgang |title=Kunststoffchemie für Ingenieure von der Synthese bis zur Anwendung |trans-title=Plastics chemistry for engineers from synthesis to application |language=de |year=2011 |publisher=Hanser |location=München |isbn=978-3-446-43047-1 |edition=3rd |page=247 }}</ref>

The thermal expansion of PP is significant, but somewhat less than that of polyethylene. <ref name=Kaiser2011p247/>

===Chemical properties===
Propylene molecules prefer to join together "head-to-tail", giving a chain with methyl groups on every other carbon, but some randomness occurs.<ref>{{cite web |title=10.4: Synthesis of Polymers |url=https://chem.libretexts.org/Courses/University_of_Connecticut/Organic_Chemistry_-_Textbook_for_Chem_2443/10%3A__Carbenes_and_Free-Radicals/10.04%3A_Synthesis_of__Polymers |website=LibreTexts|date=5 July 2021 }}</ref>

Polypropylene at room temperature is resistant to fats and almost all organic [[solvent]]s, apart from strong oxidants. [[Oxidizing acid|Non-oxidizing acids]] and [[base (chemistry)|bases]] can be stored in [[container]]s made of PP. At elevated temperature, PP can be dissolved in nonpolar solvents such as [[xylene]], [[tetralin]] and [[decalin]]. Due to the tertiary carbon atom, PP is chemically less resistant than PE (see [[Markovnikov rule]]).<ref>{{cite book |last1=Nuyken |first1=von Sebastian |last2=Koltzenburg |first2=Michael |last3=Maskos |first3=Oskar |title=Polymere: Synthese, Eigenschaften und Anwendungen |trans-title=Polymers: synthesis, properties and applications |language=de |year=2013 |publisher=Springer |isbn=978-3-642-34772-6 |edition=1st }}{{page needed|date=July 2018}}</ref>

Most commercial polypropylene is [[isotactic]] and has an intermediate level of [[crystal]]linity between that of [[low-density polyethylene]] (LDPE) and [[high-density polyethylene]] (HDPE). Isotactic & atactic polypropylene is soluble in ''p''-xylene at 140&nbsp;°C. Isotactic precipitates when the solution is cooled to 25&nbsp;°C and atactic portion remains soluble in ''p''-xylene.

The melt flow rate (MFR) or [[melt flow index]] (MFI) is a measure of molecular weight of polypropylene. The measure helps to determine how easily the molten raw material will flow during processing. Polypropylene with higher MFR will fill the plastic mold more easily during the injection or blow-molding production process. As the melt flow increases, however, some physical properties, like impact strength, will decrease.

There are three general types of polypropylene: [[homopolymer]], random copolymer, and [[block copolymer]]. The [[comonomer]] is typically used with [[ethylene]]. Ethylene-propylene rubber or [[EPDM]] added to polypropylene homopolymer increases its low temperature impact strength. Randomly polymerized ethylene monomer added to polypropylene homopolymer decreases the polymer crystallinity, lowers the melting point and makes the polymer more transparent.

==== Molecular structure – tacticity ====
[[File:Polypropylene tacticity en.svg|class=skin-invert-image]]

<small>Polypropylene can be categorized as atactic polypropylene (aPP), syndiotactic polypropylene (sPP) and isotactic polypropylene (iPP). In case of atactic polypropylene, the methyl group (-CH<sub>3</sub>) is randomly aligned, alternating (alternating) for syndiotactic polypropylene and evenly for isotactic polypropylene. This has an impact on the crystallinity (amorphous or semi-crystalline) and the thermal properties (expressed as [[glass transition point]] T<sub>g</sub> and [[melting point]] T<sub>m</sub>).</small>

The term [[tacticity]] describes for polypropylene how the methyl group is oriented in the polymer chain. Commercial polypropylene is usually isotactic. This article therefore always refers to isotactic polypropylene, unless stated otherwise. The tacticity is usually indicated in percent, using the isotactic index (according to DIN 16774). The index is measured by determining the fraction of the polymer insoluble in boiling [[heptane]]. Commercially available polypropylenes usually have an isotactic index between 85 and 95%. The tacticity affects the polymer's [[Physical property|physical properties]]. As the [[methyl group]] is in isotactic propylene consistently located at the same side, it forces the macromolecule in a [[Helix|helical shape]], as also found in [[starch]]. An isotactic structure leads to a [[semi-crystalline polymer]]. The higher the isotonicity(the isotactic fraction), the greater the crystallinity, and thus also the softening point, rigidity, e-modulus and hardness.<ref name="guide" />{{rp|22}}

Atactic polypropylene, on the other hand, lacks any regularity, which prevents it from crystallization, thereby creating an [[Amorphous solid|amorphous]] material.

==== Crystal structure of polypropylene ====
Isotactic polypropylene has a high [[Crystallization of polymers#Degree of crystallinity|degree of crystallinity]], in industrial products 30–60%. Syndiotactic polypropylene is slightly less crystalline, atactic PP is [[Amorphous solid|amorphous]] (not crystalline).<ref name="Domininghaus">{{Cite book|title=Kunststoffe : Eigenschaften und Anwendungen|last=Hans.|first=Domininghaus|date=2011|publisher=Springer Berlin|isbn=9783642161728|edition=8., bearb. Aufl|location=Berlin|oclc=706947259}}</ref>{{rp|251}}

===== Isotactic polypropylene (iPP) =====
Is polypropylene can exist in various crystalline modifications which differ by the molecular arrangement of the polymer chains. The crystalline modifications are categorized into the α-, β- and γ-modification as well as mesomorphic (smectic) forms.<ref>{{cite journal |doi=10.1002/macp.1964.020750113 |title=Crystalline forms of isotactic polypropylene |year=1964 |last1=Jones |first1=A. Turner |journal=Die Marco Chain |volume=75 |issue=1 |pages=134–58 |last2=Wood |first2=Jean M |last3=Beckett |first3=D. R. }}</ref> The α-modifications is predominant in iPP. Such crystals are built from lamellae in the form of folded chains. A characteristic anomaly is that the lame are arranged in the so-called "cross-hatched" structure.<ref>{{cite book |first1=G. |last1=Fischer |title=Deformations- und Versagensmechanismen von isotaktischem Polyp(i-PP) obrhalb dr Glasübergangstemperatur |trans-title=Deformations- und Versagensmechanismen von isotaktischem Polypropylen (i-PP) oberhalb der Glasübergangstemperatur |language=de |type=PhD Thesis |publisher=Universität Stuttgart |year=1988 |oclc=441127075 }}{{page needed|date=July 2018}}</ref> The melting point of α-crystalline regions is given as 185<ref name="melting behavior">{{cite journal |doi=10.1002/pol.1975.180130713 |title=Quantitative structural characterization of the melting behavior of isotactic polypropylene |journal=Journal of Polymer Science: Polymer Physics Edition |volume=13 |issue=7 |pages=1417–46 |year=1975 |last1=Samuels |first1=Robert J |bibcode=1975JPoSB..13.1417S }}</ref><ref>{{cite journal |doi=10.1016/0032-3861(86)90130-8 |title=Melting behaviour of isotactic polypropylene isothermally crystallized from the melt |journal=Polymer |volume=27 |issue=5 |pages=721–7 |year=1986 |last1=Yadav |first1=Y.S |last2=Jain |first2=P.C }}</ref> to 220&nbsp;°C,<ref name="melting behavior" /><ref name="ReferenceA">{{cite journal |doi=10.1002/pen.760070412 |title=Morphological transformations of polypropylene related to its melting and recrystallization behavior |journal=Polymer Engineering and Science |volume=7 |issue=4 |pages=309–16 |year=1967 |last1=Cox |first1=W. W |last2=Duswalt |first2=A. A }}</ref> the density as 0.936 to 0.946 g·cm<sup>−3</sup>.<ref>{{cite journal |doi=10.1016/0032-3861(84)90076-4 |title=On the lamellar morphology of isotactic polypropylene spherulites |journal=Polymer |volume=25 |issue=7 |pages=935–46 |year=1984 |last1=Bassett |first1=D.C |last2=Olley |first2=R.H }}</ref><ref>{{cite book |doi=10.1002/0471532053.bra025 |chapter=Physical Constants of Poly(propylene) |title=The Wiley Database of Polymer Properties |year=2003 |last1=Bai |first1=Feng |last2=Li |first2=Fuming |last3=Calhoun |first3=Bret H |last4=Quirk |first4=Roderic P |last5=Cheng |first5=Stephen Z. D |isbn=978-0-471-53205-7 }}</ref> The β-modification is in comparison somewhat less ordered, as a result of which it forms faster<ref name="Guan-yi Shi 1993">{{cite journal |doi=10.1002/macp.1993.021940123 |title=Melting behavior and crystalline order of ß-crystalline phase poly(propylene) |year=1993 |last1=Shi |first1=Guan-yi |journal=Die Makromolekulare Chemie |volume=194 |issue=1 |pages=269–77 |last2=Zhang |first2=Xiao-Dong |last3=Cao |first3=You-Hong |last4=Hong |first4=Jie }}</ref><ref>{{cite journal |doi=10.1002/macp.1995.021960125 |title=A stereochemical and statistical analysis of metallocene-promoted polymerization |journal=Macromolecular Chemistry and Physics |volume=196 |issue=1 |pages=353–67 |year=1995 |last1=Farina |first1=Mario |last2=Di Silvestro |first2=Giuseppe |last3=Terragni |first3=Alberto }}</ref> and has a lower melting point of 170 to 200&nbsp;°C.<ref name="melting behavior" /><ref name="Varga 1992">{{cite journal |doi=10.1007/BF00540671 |title=Supermolecular structure of isotactic polypropylene |journal=Journal of Materials Science |volume=27 |issue=10 |pages=2557–79 |year=1992 |last1=Varga |first1=J |bibcode=1992JMatS..27.2557V |s2cid=137665080 }}</ref><ref>{{cite journal |doi=10.1002/pol.1977.180150405 |title=Studies on the α and β forms of isotactic polypropylene by crystallization in a temperature gradient |journal=Journal of Polymer Science: Polymer Physics Edition |volume=15 |issue=4 |pages=641–56 |year=1977 |last1=Lovinger |first1=Andrew J |last2=Chua |first2=Jaime O |last3=Gryte |first3=Carl C |bibcode=1977JPoSB..15..641L }}</ref><ref name="ReferenceA"/> The formation of the β-modification can be promoted by nucleating agents, suitable temperatures and shear stress.<ref name="Guan-yi Shi 1993" /><ref>{{cite journal |doi=10.1016/0032-3861(68)90006-2 |title=Morphology of polypropylene crystallized from the melt |journal=Polymer |volume=9 |pages=23–40 |year=1968 |last1=Binsbergen |first1=F.L |last2=De Lange |first2=B.G.M }}</ref> The γ-modification is hardly formed under the conditions used in industry and is poorly understood. The [[mesophase|mesomorphic]] modification, however, occurs often in industrial processing, since the plastic is usually cooled quickly. The degree of order of the mesomorphic phase ranges between the crystalline and the amorphous phase, its density is with 0.916 g·cm<sup>−3</sup> comparatively. The mesomorphic phase is considered as cause for the transparency in rapidly cooled films (due to low order and small crystallites).<ref name="Domininghaus" />

===== Syndiotactic polypropylene (sPP) =====
Syndiotactic polypropylene was discovered much later than isotactic PP and could only be prepared by using [[metallocene catalysts]]. Syndiotactic PP has a lower melting point, with 161 to 186&nbsp;°C, depending on the degree of tacticity.<ref name="DerosaAuriemma2006">{{cite journal |last1=De Rosa |first1=C |last2=Auriemma |first2=F |year=2006 |title=Structure and physical properties of syndiotactic polypropylene: A highly crystalline thermoplastic elastomer |journal=Progress in Polymer Science |volume=31 |issue=2 |pages=145–237 |doi=10.1016/j.progpolymsci.2005.11.002}}</ref><ref>{{cite book |doi=10.1021/bk-1992-0496.ch008 |chapter=Structure and Morphology of Highly Stereoregular Syndiotactic Polypropylene Produced by Homogeneous Catalysts |title=Catalysis in Polymer Synthesis |volume=496 |pages=104–20 |series=ACS Symposium Series |year=1992 |last1=Galambos |first1=Adam |last2=Wolkowicz |first2=Michael |last3=Zeigler |first3=Robert |isbn=978-0-8412-2456-8 }}</ref><ref>{{cite journal |doi=10.1016/0032-3861(94)90978-4 |title=Crystallization, melting and morphology of syndiotactic polypropylene fractions: 1. Thermodynamic properties, overall crystallization and melting |journal=Polymer |volume=35 |issue=9 |pages=1884–95 |year=1994 |last1=Rodriguez-Arnold |first1=Jonahira |last2=Zhang |first2=Anqiu |last3=Cheng |first3=Stephen Z.D |last4=Lovinger |first4=Andrew J |last5=Hsieh |first5=Eric T |last6=Chu |first6=Peter |last7=Johnson |first7=Tim W |last8=Honnell |first8=Kevin G |last9=Geerts |first9=Rolf G |last10=Palackal |first10=Syriac J |last11=Hawley |first11=Gil R |last12=Welch |first12=M.Bruce }}</ref>

===== Atactic polypropylene (aPP) =====
Atactic polypropylene is amorphous and has therefore no crystal structure. Due to its lack of crystallinity, it is readily soluble even at moderate temperatures, which allows to separate it as by-product from isotactic polypropylene by [[Extraction (chemistry)|extraction]]. However, the aPP obtained this way is not completely amorphous but can still contain 15% crystalline parts. Atactic polypropylene can also be produced selectively using metallocene catalysts, atactic polypropylene produced this way has a considerably higher molecular weight.<ref name="Domininghaus" />

Atactic polypropylene has lower density, melting point and softening temperature than the crystalline types and is tacky and rubber-like at room temperature. It is a colorless, cloudy material and can be used between −15 and +120&nbsp;°C. Atactic polypropylene is used as a sealant, as an [[Insulating materials|insulating material]] for [[automobiles]] and as an additive to [[bitumen]].<ref>{{Cite book |title=Kunststoffchemie für Ingenieure: von der Synthese bis zur Anwendung |trans-title=Plastics chemistry for engineers: from synthesis to application |language=de |last=Wolfgang |first=Kaiser |date=2007 |publisher=Hanser |isbn=978-3-446-41325-2 |edition=2nd |location=München |oclc=213395068 |page=251 }}</ref>

====Copolymers====
Polypropylene [[copolymer]]s are in use as well. A particularly important one is '''polypropylene random copolymer''' ('''PPR''' or '''PP-R'''), a [[random copolymer]] with [[polyethylene]] used for [[plastic pipework]].

====PP-RCT====
'''Polypropylene random crystallinity temperature''' ('''PP-RCT'''), also used for [[plastic pipework]], is a new form of this plastic. It achieves higher strength at high temperature by β-[[crystallization of polymers|crystallization]].<ref>{{cite web|url=https://www.fv-plast.cz/novinka-novinky-v-produkci-fv-plast---od-pp-r-k-pp-rct|archive-url=https://web.archive.org/web/20191130205233/https://www.fv-plast.cz/novinka-novinky-v-produkci-fv-plast---od-pp-r-k-pp-rct|archive-date=2019-11-30|title=Novinky v produkci FV Plast - od PP-R k PP-RCT|access-date=2019-11-30|publisher=FV - Plast, a.s., Czech Republic}}</ref>

====Degradation====
[[File:Failedrope1.jpg|thumb|Effect of UV exposure on polypropylene rope]]
Polypropylene is liable to chain degradation from exposure to temperatures above 100&nbsp;°C. Oxidation usually occurs at the [[tertiary carbon]] centers leading to chain breaking via reaction with [[oxygen]]. In external applications, degradation is evidenced by cracks and [[crazing]]. It may be protected by the use of various [[polymer stabilizers]], including UV-absorbing additives and [[anti-oxidant]]s such as phosphites (e.g. [[tris(2,4-di-tert-butylphenyl)phosphite]]) and hindered phenols, which prevent [[polymer degradation]].<ref name=Ullmanns/>

Microbial communities isolated from soil samples mixed with starch have been shown to be capable of degrading polypropylene.<ref>{{cite journal |pmid=8285678 |pmc=182519 |year=1993 |last1=Cacciari |first1=I |title=Isotactic polypropylene biodegradation by a microbial community: Physicochemical characterization of metabolites produced |journal=Applied and Environmental Microbiology |volume=59 |issue=11 |pages=3695–700 |last2=Quatrini |first2=P |last3=Zirletta |first3=G |last4=Mincione |first4=E |last5=Vinciguerra |first5=V |last6=Lupattelli |first6=P |last7=Giovannozzi Sermanni |first7=G |doi=10.1128/AEM.59.11.3695-3700.1993 |bibcode=1993ApEnM..59.3695C }}</ref>
Polypropylene has been reported to degrade while in the human body as implantable mesh devices. The degraded material forms a tree bark-like layer at the surface of mesh fibers.<ref name="Iakovlev et al 2017">{{cite journal |doi=10.1002/jbm.b.33502 |pmid=26315946 |title=Degradation of polypropylene in vivo: A microscopic analysis of meshes explanted from patients |journal=Journal of Biomedical Materials Research Part B: Applied Biomaterials |volume=105 |issue=2 |pages=237–48 |year=2017 |last1=Iakovlev |first1=Vladimir V |last2=Guelcher |first2=Scott A |last3=Bendavid |first3=Robert }}</ref>

===Optical properties===
PP can be made [[translucent]] when uncolored but is not as readily made transparent as [[polystyrene]], [[acrylic glass|acrylic]], or certain other plastics. It is often [[Opacity (optics)|opaque]] or colored using pigments.

==Production==
Polypropylene is produced by the [[chain-growth polymerization]] of [[propene]]:

[[File:Polymerization of polypropylene - en.svg|class=skin-invert-image|frameless|467x467px]]

The industrial production processes can be grouped into gas phase polymerization, [[bulk polymerization]] and [[slurry]] polymerization. All state-of-the-art processes use either gas-phase or bulk reactor systems.<ref name="Ullmanns" />

*In gas-phase and slurry-reactors, the polymer is formed around heterogeneous catalyst particles. The gas-phase polymerization is carried out in a [[fluidized bed reactor]], [[propene]] is passed over a bed containing the [[Heterogeneous catalysis|heterogeneous (solid) catalyst]] and the formed polymer is separated as a fine powder and then converted into [[Pelletizing|pellets]]. Unreacted gas is recycled and fed back into the reactor.
*In bulk polymerization, liquid propene acts as a solvent to prevent the precipitation of the polymer. The polymerization proceeds at 60 to 80&nbsp;°C and 30–40&nbsp;atm are applied to keep the propene in the liquid state. For the bulk polymerization, typically [[Chemical reactor#CSTR (continuous stirred-tank reactor)|loop reactor]]s are applied. The bulk polymerization is limited to a maximum of 5% ethene as comonomer due to a limited solubility of the polymer in the liquid propene.
*In the slurry polymerization, typically C4–C6 alkanes ([[butane]], [[pentane]] or [[hexane]]) are utilized as [[Chemically inert|inert]] diluent to [[Suspension (chemistry)|suspend]] the growing polymer particles. Propene is introduced into the mixture as a gas.

===Catalysts===
The properties of PP are strongly affected by its [[tacticity]], the orientation of the [[methyl group]]s ({{chem|CH|3}}) relative to the methyl groups in neighboring monomer units. A [[Ziegler–Natta catalyst]] is able to restrict linking of monomer molecules to a specific orientation, either isotactic, when all methyl groups are positioned at the same side with respect to the backbone of the polymer chain, or syndiotactic, when the positions of the methyl groups alternate.

Commercially available isotactic polypropylene is made with two types of Ziegler-Natta catalysts. The first group of the catalysts encompasses solid (mostly supported) catalysts and certain types of soluble [[metallocene]] catalysts. Such isotactic macromolecules coil into a [[Helix|helical]] shape; these helices then line up next to one another to form the crystals that give commercial isotactic polypropylene many of its desirable properties.

Modern supported Ziegler-Natta catalysts developed for the polymerization of propylene and other [[1-alkene]]s to isotactic polymers usually use {{chem|TiCl|4}} as an active ingredient and {{chem|MgCl|2}} as a support.<ref>{{cite book|author=Kissin, Y. V. |title=Alkene Polymerization Reactions with Transition Metal Catalysts|url=https://books.google.com/books?id=JjhZiJEY5FsC&pg=PA207|year=2008|publisher=Elsevier|isbn=978-0-444-53215-2|pages=207–}}</ref><ref>{{cite book|author1=Hoff, Ray |author2=Mathers, Robert T. |name-list-style=amp |title=Handbook of Transition Metal Polymerization Catalysts|url=https://books.google.com/books?id=5_uqKL9VUmQC&pg=PA158|year= 2010|publisher=John Wiley & Sons|isbn=978-0-470-13798-7|pages=158–}}</ref><ref>{{cite book |last=Moore |first=E. P. |year=1996 |title=Polypropylene Handbook. Polymerization, Characterization, Properties, Processing, Applications |publisher=Hanser Publishers |location=New York |isbn=1569902089}}{{page needed|date=July 2018}}</ref> The catalysts also contain organic modifiers, either aromatic acid esters and diesters or ethers. These catalysts are activated with special co-catalysts containing an [[organoaluminium]] compound such as Al(C<sub>2</sub>H<sub>5</sub>)<sub>3</sub> and the second type of a modifier. The catalysts are differentiated depending on the procedure used for fashioning catalyst particles from MgCl<sub>2</sub> and depending on the type of organic modifiers employed during catalyst preparation and use in polymerization reactions. Two most important technological characteristics of all the supported catalysts are high productivity and a high fraction of the crystalline isotactic polymer they produce at 70–80&nbsp;°C under standard polymerization conditions. Commercial synthesis of isotactic polypropylene is usually carried out either in the medium of liquid propylene or in gas-phase reactors.

[[Image:Syndiotactic polypropene.png|thumb|right|A ball-and-stick model of syndiotactic polypropylene]]

Commercial synthesis of syndiotactic polypropylene is carried out with the use of a special class of metallocene catalysts. They employ bridged bis-metallocene complexes of the type bridge-(Cp<sub>1</sub>)(Cp<sub>2</sub>)ZrCl<sub>2</sub> where the first Cp ligand is the [[cyclopentadienyl]] group, the second Cp ligand is the fluorenyl group, and the bridge between the two Cp ligands is -CH<sub>2</sub>-CH<sub>2</sub>-, >SiMe<sub>2</sub>, or >SiPh<sub>2</sub>.<ref>{{cite book |editor-last1=Benedikt |editor-first1=G. M. |editor-last2=Goodall |editor-first2=B. L. |year=1998 |title=Metallocene Catalyzed Polymers |publisher=ChemTech Publishing |location=Toronto |isbn=978-1-884207-59-4}}{{page needed|date=July 2018}}</ref> These complexes are converted to polymerization catalysts by activating them with a special organoaluminium co-catalyst, [[methylaluminoxane]] (MAO).<ref>{{cite book |editor-last1=Sinn |editor-first1=H. |editor-last2=Kaminsky |editor-first2=W. |editor-last3=Höker |editor-first3=H. |year=1995 |title=Alumoxanes, Macromol. Symp. 97 |publisher=Huttig & Wepf |location=Heidelberg}}{{page needed|date=July 2018}}</ref>

Atactic polypropylene is an amorphous rubbery material. It can be produced commercially either with a special type of supported Ziegler-Natta catalyst or with some metallocene catalysts.

==Manufacturing from polypropylene==
Melting process of polypropylene can be achieved via [[extrusion]] and [[Molding (process)|molding]]. Common extrusion methods include production of melt-blown and spun-bond fibers to form long rolls for future conversion into a wide range of useful products, such as face masks, filters, diapers and wipes.

The most common shaping technique is [[injection molding]], which is used for parts such as cups, cutlery, vials, caps, containers, housewares, and automotive parts such as batteries. The related techniques of [[blow molding]] and [[injection-stretch blow molding]] are also used, which involve both extrusion and molding.

The large number of end-use applications for polypropylene are often possible because of the ability to tailor grades with specific molecular properties and additives during its manufacture. For example, [[antistatic]] additives can be added to help polypropylene surfaces resist dust and dirt. Many physical finishing techniques can also be used on polypropylene, such as [[machining]]. [[Surface finishing|Surface treatments]] can be applied to polypropylene parts in order to promote adhesion of printing ink and paints.

Expanded Polypropylene (EPP) has been produced through both solid and melt state processing. EPP is manufactured using melt processing with either chemical or physical blowing agents. Expansion of PP in solid state, due to its highly crystalline structure, has not been successful. In this regard, two novel strategies were developed for expansion of PP. It was observed that PP can be expanded to make EPP through controlling its crystalline structure or through blending with other polymers.<ref>{{cite journal |doi=10.1002/pen.10664 |title=Effect of the crystallinity and morphology on the microcellular foam structure of semicrystalline polymers |journal=Polymer Engineering & Science |volume=36 |issue=21 |pages=2645–62 |year=1996 |last1=Doroudiani |first1=Saeed |last2=Park |first2=Chul B |last3=Kortschot |first3=Mark T |doi-access=free }}</ref><ref>{{cite journal |doi=10.1002/pen.10289 |title=Processing and characterization of microcellular foamed high-density polyethylene/isotactic polypropylene blends |journal=Polymer Engineering & Science |volume=38 |issue=7 |pages=1205–15 |year=1998 |last1=Doroudiani |first1=Saeed |last2=Park |first2=Chul B |last3=Kortschot |first3=Mark T }}</ref>

===Biaxially oriented polypropylene (BOPP)===
{{anchor|BOPP}}

When polypropylene film is extruded and stretched in both the machine direction and across machine direction it is called ''biaxially oriented polypropylene''. Two methods are widely used for producing BOPP films, namely, a bi-directional stenter process or a double-bubble blown film extrusion process. <ref>{{cite web |url=https://www.euroshore.com.my/polypropylene-applications/ |title=Polypropylene Applications |date=25 May 2021 |publisher=Euroshore |access-date=2021-05-25}}</ref> Biaxial orientation increases strength and clarity.<ref>{{cite web |url=http://www.granwell.com/bopp/ |title=Biaxially Oriented Polypropylene Films |publisher=Granwell |access-date=2012-05-31}}</ref> BOPP is widely used as a packaging material for packaging products such as snack foods, fresh produce and confectionery. It is easy to coat, print and laminate to give the required appearance and properties for use as a packaging material. This process is normally called [[Converter (industry)|converting]]. It is normally produced in large rolls which are slit on [[Roll slitting|slitting machines]] into smaller rolls for use on packaging machines. BOPP is also used for stickers and labels<ref>{{Cite web|last=Neagu|first=Cristina|date=2020-02-05|title=What is BOPP?|url=https://www.stickermule.com/blog/what-is-bopp|access-date=2021-06-25|website=Sticker Mule}}{{rs?|date=July 2024}}</ref> in addition to OPP.
It is non-reactive, which makes BOPP suitable for safe use in the pharmaceutical and food industry. It is one of the most important commercial polyolefin films. BOPP films are available in different thicknesses and widths. They are transparent and flexible.

==Applications==
<!-- (e.g., [[rope]]s, thermal underwear and carpets)
loudspeakers, 
[[polymer banknote]]s
Polypropylene has a relatively slippery "low energy surface" that means that many common glues will not form adequate joints. Joining of polypropylene is often done using welding processes.
-->
[[Image:Mint box polypropylene lid.JPG|thumb|right|Polypropylene lid of a [[Tic Tac]] box, with a [[living hinge]] and the [[resin identification code]] under its flap]]

As polypropylene is resistant to fatigue, most plastic [[living hinge]]s, such as those on flip-top bottles, are made from this material. However, it is important to ensure that chain molecules are oriented across the hinge to maximise strength.

Polypropylene is used in the manufacturing of [[piping]] systems, both ones concerned with high purity and ones designed for strength and rigidity (e.g., those intended for use in potable plumbing, [[hydronic heating]] and cooling, and [[reclaimed water]]).<ref>{{citation | title=Specification for Pressure-rated Polypropylene (PP) Piping Systems | date=2017 | publisher=ASTM International | location=West Conshohocken, PA | doi=10.1520/f2389-17a}}</ref> This material is often chosen for its resistance to corrosion and chemical leaching, its resilience against most forms of physical damage, including impact and freezing, its environmental benefits, and its ability to be joined by [[heat fusion]] rather than gluing.<ref>[http://contractormag.com/news/green-pipe-mine-2345 Green pipe helps miners remove the black] Contractor Magazine, 10 January 2010</ref><ref>[http://www.achrnews.com/Articles/Feature_Article/BNP_GUID_9-5-2006_A_10000000000000690490 Contractor Retrofits His Business]. the News/ 2 November 2009.</ref><ref>[http://www.esmagazine.com/Articles/Case_In_Point/BNP_GUID_9-5-2006_A_10000000000000691766 What to do when the piping replacement needs a replacement?] Engineered Systems. 1 November 2009.</ref>

[[Image:Red Polypropylene Chair with Stainless Steel Structure.JPG|thumb|left|upright|A polypropylene chair]]

Many plastic items for medical or laboratory use can be made from polypropylene because it can withstand the heat in an [[autoclave]]. Its heat resistance also enables it to be used as the manufacturing material of consumer-grade [[kettle]]s{{Citation needed|date=February 2012|reason=A PP kettle could not be used on a stovetop}}. Food containers made from it will not melt in the dishwasher, and do not melt during industrial hot filling processes. For this reason, most plastic tubs for dairy products are polypropylene sealed with aluminum foil (both heat-resistant materials). After the product has cooled, the tubs are often given lids made of a less heat-resistant material, such as LDPE or polystyrene. Such containers provide a good hands-on example of the difference in modulus, since the rubbery (softer, more flexible) feeling of LDPE with respect to polypropylene of the same thickness is readily apparent. Rugged, translucent, reusable [[plastic container]]s made in a wide variety of shapes and sizes for consumers from various companies such as [[Rubbermaid]] and [[Sterilite]] are commonly made of polypropylene, although the lids are often made of somewhat more flexible LDPE so they can snap onto the container to close it. Polypropylene can also be made into disposable bottles to contain liquid, powdered, or similar consumer products, although HDPE and [[polyethylene terephthalate]] are commonly also used to make bottles. Plastic pails, car batteries, wastebaskets, pharmacy prescription bottles, cooler containers, dishes and pitchers are often made of polypropylene or HDPE, both of which commonly have rather similar appearance, feel, and properties at ambient temperature. An abundance of medical devices are made from PP.<ref>{{Cite journal|last1=Collinet|first1=Pierre|last2=Belot|first2=Franck|last3=Debodinance|first3=Philippe|last4=Duc|first4=Edouard Ha|last5=Lucot|first5=Jean-Philippe|last6=Cosson|first6=Michel|s2cid=2648056|date=2006-08-01|title=Transvaginal mesh technique for pelvic organ prolapse repair: mesh exposure management and risk factors |journal=International Urogynecology Journal|language=en|volume=17|issue=4|pages=315–320|doi=10.1007/s00192-005-0003-8|pmid=16228121|issn=0937-3462}}</ref>

[[File:PolypropyleneItemsForLaboratoryUse.jpg|thumb|Polypropylene items for laboratory use. Blue and orange closures are not made of polypropylene.]]

A common application for polypropylene is as biaxially oriented polypropylene (BOPP). These BOPP sheets are used to make a wide variety of materials including clear [[Plastic bag|bags]]. When polypropylene is biaxially oriented, it becomes crystal clear and serves as an excellent packaging material for artistic and retail products.

Polypropylene, highly colorfast, is widely used in manufacturing carpets, rugs and mats to be used at home.<ref>[http://www.fibersource.com/f-tutor/olefin.htm Rug fibers] {{Webarchive|url=https://web.archive.org/web/20100405073442/http://www.fibersource.com/f-tutor/olefin.htm |date=2010-04-05 }}. Fibersource.com. Retrieved on 2012-05-31.</ref>

Polypropylene is widely used in ropes, distinctive because they are light enough to float in water.<ref>[http://www.contractorsrope.com/braided-polypropylene-rope.html Braided Polypropylene Rope is Inexpensive and it Floats]. contractorrope.com. Retrieved on 2013-02-28.</ref> For equal mass and construction, polypropylene rope is similar in strength to polyester rope. Polypropylene costs less than most other synthetic fibers.

Polypropylene is also used as an alternative to [[polyvinyl chloride]] (PVC) as insulation for electrical cables for [[Low Smoke Zero Halogen|LSZH]] cable in low-ventilation environments, primarily tunnels. This is because it emits less smoke and no toxic halogens, which may lead to production of acid in high-temperature conditions.

Polypropylene is also used in particular roofing membranes as the waterproofing top layer of single-ply systems as opposed to modified-bit systems.

Polypropylene is most commonly used for plastic moldings, wherein it is injected into a mold while molten, forming complex shapes at relatively low cost and high volume; examples include bottle tops, bottles, and fittings.

It can also be produced in sheet form, widely used for the production of stationery folders, packaging, and storage boxes. The wide color range, durability, low cost, and resistance to dirt make it ideal as a protective cover for papers and other materials. It is used in [[Rubik's Cube]] stickers because of these characteristics.

The availability of sheet polypropylene has provided an opportunity for the use of the material by designers. The light-weight, durable, and colorful plastic makes an ideal medium for the creation of light shades, and a number of designs have been developed using interlocking sections to create elaborate designs.

Polypropylene fibres are used as a [[concrete]] additive to increase strength and reduce cracking and [[spalling]].<ref>{{cite journal|doi=10.14359/4439 |title= Properties of Polypropylene Fiber Reinforced Concrete|year=1993 |author1=Bayasi, Ziad |author2=Zeng, Jack |name-list-style=amp |journal=ACI Materials Journal|volume=90 |issue=6 |pages=605–610}}</ref> In some areas susceptible to earthquakes (e.g., California), PP fibers are added with soils to improve the soil's strength and damping when constructing the foundation of structures such as buildings, bridges, etc.<ref>{{cite journal|doi=10.1080/17486025.2015.1029013 |title=Effect of fibre inclusion on dynamic properties of clay |year=2015 |author1=Amir-Faryar, Behzad |author2=Aggour, M. Sherif |s2cid=128478509 |name-list-style=amp |journal=Geomechanics and Geoengineering|volume=11 |issue=2 |pages=1–10}}</ref>

===Clothing===
[[File:Textielmuseum-cabinet-03.jpg|thumb|Various polypropylene yarns and textiles]]
Polypropylene is a major polymer used in [[nonwovens]], with over 50% used{{Citation needed|date=March 2009}} for diapers or sanitary products where it is treated to absorb water (hydrophilic) rather than naturally repelling water (hydrophobic). Other non-woven uses include filters for air, gas, and liquids in which the fibers can be formed into sheets or webs that can be pleated to form cartridges or layers that filter in various efficiencies in the 0.5 to 30 [[micrometre]] range. Such applications occur in houses as water filters or in air-conditioning-type filters. The high surface-area and naturally oleophilic polypropylene nonwovens are ideal absorbers of oil spills with the familiar{{citation needed|date=June 2015}} floating barriers near oil spills on rivers.

Polypropylene, or 'polypro', has been used for the fabrication of cold-weather base layers, such as long-sleeve shirts or long underwear. Polypropylene is also used in warm-weather clothing, in which it transports sweat away from the skin. Polyester has replaced polypropylene in these applications in the U.S. military, such as in the [[Extended Cold Weather Clothing System|ECWCS]].<ref>[https://web.archive.org/web/20081217143906/http://peosoldier.army.mil/factsheets/SEQ_CIE_ECWCS.pdf Generation III Extended Cold Weather Clothing System (ECWCS)]. PM Soldier Equipment. October 2008</ref> Although polypropylene clothes are not easily flammable, they can melt, which may result in severe burns if the wearer is involved in an explosion or fire of any kind.<ref>[http://permanent.access.gpo.gov/lps11992/2002/fsmnov02.pdf USAF Flying Magazine. Safety. Nov. 2002]. access.gpo.gov</ref> Polypropylene undergarments are known for retaining body odors which are then difficult to remove. The current generation of polyester does not have this disadvantage.<ref>
Ellis, David. [https://web.archive.org/web/20100602092559/http://www.outdoorsnz.org.nz/cms_show_download.php?id=168 Get Real: The true story of performance next to skin fabrics]. outdoorsnz.org.nz
</ref>

===Medical===
Its most common medical use is in the synthetic, nonabsorbable [[surgical suture|suture]] [[Prolene]], manufactured by [[Ethicon Inc.]]

Polypropylene has been used in [[hernia]] and pelvic organ prolapse repair operations to protect the body from new hernias in the same location. A small patch of the material is placed over the spot of the hernia, below the skin, and is painless and rarely, if ever, rejected by the body. However, a polypropylene mesh will erode the tissue surrounding it over the uncertain period from days to years.

A notable application was as a transvaginal mesh, used to treat vaginal prolapse and concurrent urinary incontinence.<ref>{{Cite journal|last1=Collinet|first1=Pierre|last2=Belot|first2=Franck|last3=Debodinance|first3=Philippe|last4=Duc|first4=Edouard Ha|last5=Lucot|first5=Jean-Philippe|last6=Cosson|first6=Michel|s2cid=2648056|date=2006-08-01|title=Transvaginal mesh technique for pelvic organ prolapse repair: mesh exposure management and risk factors|journal=International Urogynecology Journal|language=en|volume=17|issue=4|pages=315–320|doi=10.1007/s00192-005-0003-8|pmid=16228121|issn=0937-3462}}</ref> Due to the above-mentioned propensity for polypropylene mesh to erode the tissue surrounding it, the FDA has issued several warnings on the use of polypropylene mesh medical kits for certain applications in pelvic organ prolapse, specifically when introduced in close proximity to the vaginal wall due to a continued increase in number of mesh-driven tissue erosions reported by patients over the past few years.<ref>[https://web.archive.org/web/20110717092929/http://www.fda.gov/MedicalDevices/Safety/AlertsandNotices/ucm262435.htm UPDATE on Serious Complications Associated with Transvaginal Placement of Surgical Mesh for Pelvic Organ Prolapse: FDA Safety Communication], FDA, July 13, 2011</ref> On 3 January 2012, the FDA ordered 35 manufacturers of these mesh products to study the side effects of these devices.
Due to the outbreak of the [[COVID-19 pandemic]] in 2020, the demand for PP has increased significantly because it is a vital raw material for producing [[melt blowing|meltblown]] fabric, which is in turn the raw material for producing facial masks.

==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]]

==Repairing==
PP objects can be joined with a [[Epoxy resin|two-part epoxy]] glue or using hot-glue guns.<ref>{{cite book|last=Athavale|first=Shrikant P.|title=Hand Book of Printing, Packaging and Lamination: Packaging Technology|url=https://books.google.com/books?id=DjxvDwAAQBAJ&pg=PT224|date=20 September 2018|publisher=Notion Press|isbn=978-1-64429-251-8|page=224}}</ref>

PP can be melted using a [[speed tip welding]] technique. With speed welding, the plastic welder, similar to a soldering iron in appearance and wattage, is fitted with a feed tube for the plastic weld rod. The speed tip heats the rod and the substrate, while at the same time it presses the molten weld rod into position. A bead of softened plastic is laid into the joint and the parts and weld rod fuse. With polypropylene, the melted welding rod must be "mixed" with the semi-melted base material being fabricated or repaired. A speed tip "gun" is essentially a soldering iron with a broad, flat tip that can be used to melt the weld joint and filler material to create a bond.

==Health concerns==
The advocacy organization [[Environmental Working Group]] classifies PP as of low hazard.<ref>[https://www.ewg.org/skindeep/ingredients/705094-POLYPROPYLENE Polypropylene || Skin Deep Cosmetics Database | Environmental Working Group]. ewg.com. Retrieved on 2024-07-02.</ref>{{why|date=June 2019}}
PP is [[Wet processing engineering#Solution dyeing|dope-dyed]]; no [[water]] is used in its dyeing, in contrast with [[cotton]].<ref>Chapagain, A. K. ''et al.'' (September 2005) [http://www.waterfootprint.org/Reports/Report18.pdf The water footprint of cotton consumption]. UNESCO-IHE Delft. Value of Water Research Report Series No. 18. waterfootprint.org</ref> Polypropylene was the most common microplastic fiber found in the [[olfactory bulb]]s in 8 of 15 deceased individuals in a study.<ref>Luís Fernando Amato-Lourenço, PhD1,2; Katia Cristina Dantas, PhD2; Gabriel Ribeiro Júnior, PhD2; et al (September 16, 2024) [https://jamanetwork.com/journals/jamanetworkopen/fullarticle/2823787 Microplastics in the Olfactory Bulb of the Human Brain] JAMA Network Open Original Investigation Environmental Health</ref>

==Combustibility==
Like all organic compounds, polypropylene is combustible.<ref>{{Cite book | doi = 10.1007/978-94-011-4421-6_34| chapter = Fire hazard with polypropylene| title = Polypropylene| volume = 2| pages = 247–253| series = Polymer Science and Technology Series| year = 1999| last1 = Shields| first1 = T. J.| last2 = Zhang| first2 = J.| publisher = Springer, Dordrecht| isbn = 978-94-010-5899-5}}</ref> The [[flash point]] of a typical composition is 260&nbsp;°C; [[autoignition temperature]] is 388&nbsp;°C.<ref name="AC">{{Cite web|url=https://www.acplasticsinc.com/techsheets/Polypropylene_MSDS.pdf|title=A&C Plastics Polypropylene MSDS}}</ref>

==References==
{{reflist|35em}}

==External links==
{{Commons category|Polypropylene}}
*[https://www.pslc.ws/mactest/pp.htm Chain structure of Polypropylene]
*[http://www.bpf.co.uk/plastipedia/polymers/pp.aspx Polypropylene on Plastipedia]
*[https://omnexus.specialchem.com/selection-guide/advanced-guide-on-polypropylene Polypropylene Properties & other information]

{{Plastics}}

{{Authority control}}

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