Polypropylene Applications

Discovered in 1954, polypropylene (PP) is the lightest type of commodity plastic with a density of between 0.895 and 0.92 g/cm³.

PP has excellent mechanical strength and chemical resistance, and can be processed through many converting methods such as injection molding and extrusion. PP material’s high temperature resistance makes it particularly suitable for items such as bottles, carboys, funnels, instrument jars, pails and trays that have to be sterilized frequently for use in clinical environments.

PP 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. PP properties can vary according to process conditions, copolymer components, molecular weight and molecular weight distribution.

Polypropylene applications

1. Fibers and fabrics

Fibers are produced by various kinds of extrusion processes, and they include slit film or slit tape. Here, 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.

Slit film is a wide web extruded film. One major application of slit film is in carpet backings. Nowadays, more carpet backings are produced from PP rather than from natural jute fibers. Some reasons include jute fibers damaging quicker than PP fibers in high-moisture weather; high humidity causes a higher absorption of water inviting mould attacks. Other slit film applications are twine, woven fabrics for feed and fertilizer sacks, sand bags and bulk container bags, tarpaulins, mats, screens for erosion prevention and geotextiles to stabilize soil beds. More conventional fibers than slit-film fibers are known as continuous filament fibers and they result from extrusion. Staple fibers are short fibers ranging from less than an inch to a little less than a foot in length depending on the application ¹.

Nonwoven fabrics are the most common single fiber application for PP usage. There are three types of nonwoven fabrics: thermo-bonded, spun-bonded and melt-blown. The fabrics of each differ from another in properties and appearance. For example, spun-bonded fabrics are strong whereas melt-blown fabrics are soft. However, these types of fabrics are often used in combinations of two types together. The fibers formed in the melt-blowing process are very fine and allow for the production of lightweight uniform fabrics that are soft and not strong. Fabrics from fine melt-blown fibers are utilized in medical applications because they allow the passage of water vapor but prevent the penetration of liquid water and aqueous solutions.

Monofilaments are produced by extruding PP through a plate containing many small holes and then quenched into a water bath that cools the fabrics. Twisting bundles of monofilament together gives us applications like rope, twine and fishing nets that are strong and moisture resistant; making them ideal in marine applications, Figure 1 ¹.

Figure 1. Monofilaments extrusion process

2. Strapping

Strapping is similar to slit film but fabrics here are thicker, approximately in the order of 20 mils. Fibers are produced either from a direct extrusion or from slit sheets. Applications include securing large packages, boxes or to hold stacks together. The most important property of strapping fibers is strength where, sometimes, fabrics can replace steel ¹.

3. Film

An extrusion process of PP produces films. The film is less than 10 mils thick and its uses embrace food products, tobacco and clothing. There are two broad classes of films: cast films and oriented films ¹. Cast films are manufactured by depositing a layer of liquid plastic onto a surface and stabilizing this form by allowing melt to cool or by evaporation of solvent. Film thickness is usually in the range of 1-4 mils. An important feature of cast films is softness. Both homo-polymers and random copolymers are used in cast films. Cast films are converted into products like bags, pages, sheet protectors, tapes and pressure-sensitive labels ¹.

Bi-axially oriented polypropylene film (BOPP) is another film type that is produced by extruding the plastic through a circular die followed by expansion cooling. Two methods are widely used for producing BOPP films: tenter process (film thickness 0.5-2.5 mils) and tubular process (film thickness 0.25-2 mils). BOPP films have excellent clarity and gloss properties. They are printable when using some additional surface treatment technology. The main applications for BOPP films are in flexible packaging where the major use is in snack food packaging. BOPP film provides resistance to moisture vapor keeping snacks crisp, fresh tasting and also provides a heat-sealable layer. BOPP films are also used in packaging of bakery products and many adhesive tapes. An opaque film is a special kind of BOPP which is used in packaging products such as candy, chocolate bars, soaps and labels on soft drink bottles ¹.

Furthermore, BOPP films are used in electrical applications to store energy. Hydroxylated polypropylene (PP-OH) is an optimum material to increase electric energy density. Dielectric constant (ε) of 4.6 is recorded for PP-OH that contains 4.2 mol% OH. A cross-linked polypropylene (x-PP) has a dielectric value (ε) around 3. The PP network structure decreases loss of electricity and narrows breakdown distribution. Therefore, dielectric constant maintains constant over a wide range of temperatures (–20-100°C) and frequencies (0.1-1MHz). PP-OH is applied in various electrical applications as a reliable thin film energy storage capacitor with high releasing energy density > 7 J/cm3 upon an electric field of E = 600 MV/m. PP-OH dielectric constant and energy density values are better than those of BOPP by two to three times ².

4. Sheet/Thermoforming

The 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. The 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 feet. Sheets are used in the production of thermoformed containers for rigid packaging applications. Differences between PP and polystyrene in producing rigid packages are perceived by noticing that PP resins resist stress cracking in the presence of fatty products whereas polystyrene resins do not. PP is tougher and more rigid than polystyrene and has lower specific gravity than polystyrene, allowing the manufacture of lighter weight containers at the same thickness and shape ¹.

5. Injection moulding

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. The two halves of the mould are then opened. Usually, mould shapes have very complex geometries that provide designers with infinite numbers of design possibilities. Thin-wall injection moulding is used for rigid packaging container applications. The thickness does not usually exceed 25 mils and is often less than that. Rigid packaging containers are used in consumer items such as food storage containers and water bottles. Water bottles have many shapes and can be round, square, flat or tall. Injection moulding is the best choice for producing containers in many shape varieties ¹. Housewares applications include storage systems, toys, sports equipment, paintbrushes and garden furniture ³. Screw caps for bottles and jars are some examples of closures applications produced from PP. Further, injection moulding includes appliances and hand tools applications like coffee makers, can openers, blenders and mixers as well as different medical applications such as disposable syringes ¹.

Figure 2. Injection moulding process.

6. Blow moulding

The basic principle of the blow moulding process is to produce a hollow object by blowing a thermoplastic with hot air. A heated thermoplastic hollow tube 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 the blow moulding process. There are three types of blow moulding: extrusion blow moulding in which produced PP bottles have hot-filling capability and good contact clarity, injection blow moulding to produce relatively small bottles and wide-mouth jars and injection stretch blow moulding to produce bi-axially oriented jars and bottles with greater clarity, strength and barrier properties ¹. Typical applications are water bottles, shampoo bottles and lubricant/pesticides containers.

7. Automotive

PP 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, 0.9 g/mL, much of the plastics in new cars are made of PP because car companies tend to want to reduce the overall weight of their cars to save gas expenses for consumers. Also, most interior trim and several exterior components are made completely of PP or PP compounds. Interior trim like doors, pillars, quarter panels, and consoles are all moulded of PP. Weight reduction has been an important factor so PP became a major material for automotive exterior parts. A special material that is produced from PP is called thermoplastic olefin (TPO) used in car bumpers. TPO is also used in air dams, body side claddings, rocker panels and even grills in some vehicles ¹.

A study in 2005 showed that the global consumption of PP by end use application is primarily for injection moulding and fibers, Figure 3 ⁴.

Figure 3. Global consumption of polypropylene by end use application

PP has many other applications associated with plastics in medical or laboratory tools, plastic tubs, plastic containers, wastebaskets, pharmacy prescription bottles, cooler containers, dishes, pitchers, rugs, insulation for electrical cables, stationery folders, storage boxes, light shades, loudspeaker drive units and water filters or air-conditioning-type filters. Furthermore, PP is used to produce clothes or even products related to clothing like diapers or sanitary products where PP is treated to absorb water (hydrophilic) rather than naturally repelling water (hydrophobic). PP is perfect for fabrication of cold-weather base layers and under-armor clothing. Another interesting PP application is called polypropylene sheet foam, Table 1 ⁵.

Table 1. Applications of polypropylene sheet foam

Sources:

Article based on an original work, “Polypropylene as a Promising Plastic: A Review”, by Hisham Maddah