Editing Polypropylene (section)

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

Polypropylene is in little 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.