Polypropylene is the most important material among polyolefins. The great properties of PP such as low density, high melting temperature and chemical inertness with low cost making PP optimum for long-life applications. PP is a highly versatile material meaning that diversity in structural designs and mechanical properties are achievable. PP applications include, but not limited to, fabrics, films, bottles, sheets and automotive products. This polymer is used in almost 30% of automotive parts.
Keywords: Polypropylene, Thermoplastic, Properties, Polymer, Applications.Polypropylene is a downstream petrochemical product that is derived from the olefin monomer propylene. The polymer is produced through a process of monomer connection called addition polymerization. In this process, heat, high-energy radiation and an initiator or a catalyst are added to combine monomers together. Thus, propylene molecules are polymerized into very long polymer molecules or chains. There are four different routes to enhance the polymerization of any polymer: solution polymerization, suspension polymerization, bulk polymerization and gas-phase polymerization. However, polypropylene properties vary according to process conditions, copolymer components, molecular weight and molecular weight distribution. Polypropylene is a vinyl polymer in which every carbon atom is attached to a methyl group and can be expressed as shown in Figure 1.
Plastics are categorized into four main groups: thermoplastics, elastomers, thermosets and polymer compounds. Macromolecular structures distinguish the class of any plastic material as well as its physical properties. Elastomers and thermosets have soft and hard elasticity, respectively; and their resins cannot be melted for recycling purposes. However, thermoplastics are either amorphous or semi-crystalline.
In order to understand the different types of polypropylene, one needs to dig in and understand thermoplastics categories with a different point of view. Thermoplastics are categorized into two groups: commodity thermoplastics, this group covers the major plastics such as polyethylene, polypropylene, polystyrene and polyvinyl chloride. Engineering thermoplastics is the second thermoplastics group and products in this class are related to electrical and mechanical engineering applications. Using these plastics may replace other materials like metals and load-bearing components. These include polysulfone, nylons, polycarbonates, acetal and ABS terpolymers. There are two different types of polypropylene. First, polypropylene containing only propylene monomer in a semi-crystalline solid form; which is called a homo-polymer PP (HPP). Second, polypropylene containing ethylene as a co-monomer in the PP chains and this is referred to as a random copolymer (RCP). Homo-polymer PP (HPP) is the most widely used polypropylene material in industry. HPP is a two-phase system contains both crystalline and non-crystalline regions. The non-crystalline (amorphous) regions have both isotactic PP and atactic PP. The isotactic PP in the amorphous regions is crystallizable and crystallizes slowly over time. In other words, HPP consists of only one propylene unit along the chain with mostly isotactic propylene units and that give us a crystalline structure to the polymer. Therefore, HPP exhibits a high level of stiffness at room temperature and a high melting point but lower transparency as well as diminished impact strength. Random copolymers (RCP) are ethylene/propylene copolymers that are produced in a single reactor by copolymerizing propylene and small amounts of ethylene (usually 7% or lower). Ethylene disrupts the regular structure of polypropylene and results in a reduction of crystalline uniformity in the polymer. The relation between ethylene and crystalline thickness is inversely proportional which means that as the ethylene content increases the crystalline thickness gradually decreases and resulting in reaching lower melting point. Co-polymers usually have slightly better impact properties, decreased melting point and enhanced flexibility. Advantages and disadvantages of PP, in general, are illustrated in Table 1.
Semi-crystalline iPP structures characterize with several crystal modifications including monoclinic (𝛼), trigonal (β) and orthorhombic (γ) forms. Optical transparency in semi-crystalline polymers depends on crystallinity, morphology and surface properties. The lower the transparency is the more haze we get. Hence, reduced crystallinity increases the clarity of the product. iPP polymers are extruded, recycled, several times and cooled upon extrusion to see the effect of recycling on opacity. It is found that opacity decreases on increasing recycling steps and cooling rate. However, increasing the amount of 𝛼 phase and increasing their average size of the spherulites increases opacity. β-modification of isotactic polypropylene (β-iPP) is another commercial grade with unique features. For example, toughness and impact strength of β-iPP significantly exceed those of 𝛼-iPP. The development of β-phase is completed by crystallization in a temperature gradient or in a strained melt. Yet, β-modification cannot substitute or cancel the use of 𝛼-modification since each grade has its own superior properties. Results proved that iPP grades have excellent mechanical properties and appropriate optical characteristics. Optical characteristics are extremely important for food packaging applications. Commercial grades are available in a variety of molecular weight distributions, and co-monomer types as well as contents and additives. Enhanced physical properties allow polypropylene to be the core material in most demanding applications such as films, fibers, tapes, sheets, thermoforming, injections, and blow molding. The glass fibers and phase-transition temperatures for common semi-crystalline thermoplastic resins are illustrated in Tables 2, respectively.
Polypropylene is a lightweight polymer with a density of 0.90 g/cm3 that makes it suitable in many industrial applications.
Fibers are produced by various kinds of extrusion processes. Fibers include slit film or slit tape. The advantages offered by PP include low specific gravity which means greater bulk per given weight, strength, chemical resistance and stain resistance. There are different applications for fibers like slit film, staple fibers, nonwoven fabrics and monofilaments(Fig.2).
An extrusion process of PP produces films. Film is less than 10 mils thick. The film uses embrace food products, tobacco and clothing .
Thermoforming process involves heating of a thermoplastic sheet to its softening point followed by forming of the softened sheet into a desired shape by mechanical means and finally solidification into the desired shape Extrusion process produces a sheet that is greater than 10 mils in thickness and typical thickness is about 40 mils. Sheet width is usually between 2-7 ft. Sheets are used in the production of thermoformed containers for rigid packaging applications.
In this process, granules of polymer are heated until melting. Then, the molten material is injected into a closed mould. The mould normally consists of two halves which are held together under pressure to overcome the force of the melt. After that, the injected material is allowed to cool and solidify in the mould. Further, injection molding includes some appliances and hand tools applications and different medical applications such as disposable syringes.
The basic principle of blow molding process is to produce a hollow object by blowing a thermoplastic with hot air. A heated thermoplastic hollow tube is known as parison is placed inside a closed mould before blowing. The parison takes the shape of the mould, after blowing, and retains the shape upon leaving it. Bottles and jars are the main products of blow molding process.
Polypropylene has a large presence in vehicles. For example, one of the original uses is in battery cases and AC ducts. Since PP is considered as the lightest thermoplastic due to its low density, much of the plastics in new cars are PP because car companies tend to reduce the overall weight of their cars to save some gas expenses for customers. Interior trim like doors, pillars, quarter panels, and consoles are all molded of PP. Weight reduction has been an important factor so PP became a major material for automotive exterior parts (Fig.3).
Polypropylene pipe is suitable for transporting numerous fluids in a wide range of industrial applications, including heated and chilled water for space heating and cooling, chiller condensate, chemical transport, process cooling, and potable water (Fig. 4).
In order to improve the properties of PP pipes, a new product of polypropylene random copolymer with crystallinity modification (PP-RCT) was designed and produced by Borealis with high hydrostatic pressure resistance. The feature brings unique advantages for these pipes, which include:
The PP-RCT mechanical properties was produced by borealis Company, was illustrated in Table.3.
To achieve these desirable features in the pipe, Behein Pardazan Koosha, a factory in Giti Pasand Industrial Group, has produced PP-RCT masterbatch for the first time in Iran which has been used in Azin pipe Company in the production of single-layer and three-layer PP-RCT pipes. The produced PP-RCT pipes, despite their lower thickness, have shown excellent properties. This production received a certification from the Road, Housing and Urban Development Research Center in the Ministry of Roads and Urban development.
The vast number of applications from PP showed that it is the ideal choice among all other polymers to produce flexible, long-lasing, cheap and light plastics for numerous industrial, commercial, medical and personal uses. Branching into a linear polypropylene produces plastics with high modulus and tensile strength, rigidity and excellent heat resistance. However, semi-crystalline isotactic polypropylene (iPP) showed appropriate optical characteristics upon recycling.
By: Marzieh Shams Harandi
Edition by : Samin Saleki
1. Hisham A. Maddah, Polypropylene as a promising plastic: A Review, American Journal of Polymer Science 2016, 6(1): 1-11. 2. Bikiaris, D., & Docoslis, A. Harutun G. Karian, Handbook of polypropylene and polypropylene composites, 2003.