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What is Polypropylene?

Polypropylene, or polypropene, is a form of polypropylene. Polypropylene plastic is a thermoplastic polymer that can be used for a range of purposes. It is made from monomer propylene via a chain-growth polymerization process. Polypropylene plastic is a crystalline thermoplastic that is commonly used in consumer items such as packaging trays, household goods, battery cases, and medical devices. Polypropylene is abbreviated as PP.

Structure of Polypropylene

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Polypropylene is a thermoplastic made from propene or propylene monomer that is durable, rigid, and crystalline. It's a hydrocarbon resin with a linear structure. Polypropylene's chemical formula is (C3H6)n. Polypropylene plastic is one of the most affordable plastics on the market today.

Properties of Polypropylene

Physical Properties of Polypropylene

The molecular weight and molecular weight distribution, crystallinity, form and proportion of comonomer (if used), and isotacticity all influence the properties of polypropylene. The methyl groups in isotactic polypropylene, for example, are oriented on one side of the carbon backbone. This structure produces more crystallinity and a stiffer material that is less susceptible to creep than both atactic polypropylene and polyethylene.

  • Polypropylene is similar to polyethylene in many ways, especially in terms of solution behaviour and electrical properties.

  • While the chemical resistance decreases, the methyl group increases mechanical properties and thermal resistance.

  • The polypropylene density is between 0.895 and 0.92 g/cm3. As a result, PP is the lowest-density product plastic. Molding parts with lower weight and more parts of a given mass of plastic can be made with lower density. Unlike polyethene, the density of crystalline and amorphous regions differs just slightly. The density of polyethylene, on the other hand, can vary significantly.

  • PP's Young's modulus ranges from 1300 to 1800 N/mm2.

  • When copolymerized with ethylene, polypropylene is usually tough and versatile. This helps polypropylene to compete with materials like acrylonitrile butadiene styrene as an engineering plastic (ABS).

  • Polypropylene has a high fatigue resistance.

  • Since polypropylene has a number of melting points, the melting point is determined by using a differential scanning calorimetry map to find the highest temperature. The melting point of perfectly isotactic PP is 171 °C (340 °F).

  • The melting point of commercial isotactic PP varies from 160 to 166 °C (320 to 331 °F), depending on the atactic content and crystallinity. The melting point of syndiotactic PP with a crystallinity of 30% is 130 °C (266 °F). Polypropylene becomes brittle at temperatures below 0 degrees Celsius.

  • PP has a significant thermal expansion, but it is slightly less than polyethene.

Chemical Properties of Polypropylene

  • Apart from heavy oxidants, polypropylene is resistant to fats and almost all organic solvents at room temperature. PP containers may be used to store non-oxidizing acids and bases. PP can be dissolved in nonpolar solvents including xylene, tetralin, and decalin at high temperatures. Polypropylene has a lower chemical resistance than polyethylene because of the tertiary carbon atom.

  • The majority of commercial polypropylene is isotactic, with a crystallinity intermediate between low-density polyethene (LDPE) and high-density polyethene (HDPE) (HDPE). At 140°C, isotactic and atactic polypropylene dissolve in p-xylene. When the solution is cooled to 25 °C, the isotactic part precipitates, while the atactic portion stays soluble in p-xylene.

  • The melt flow rate (MFR) or melt flow index (MFI) is a measurement of polypropylene's molecular weight. This metric is used to calculate how quickly molten raw material can flow during the refining process. During the injection or blow-moulding process, polypropylene with a higher MFR can fill the plastic mould more easily. Some physical properties, such as impact strength, can decrease as the melt flow increases.

Types of Polypropylene

There are three general types of polypropylene: 

  • Homopolymer 

  • Random copolymer

  • Block copolymer. 

The co-monomer term is typically used with ethylene. Ethylene-propylene rubber or EPDM when added to polypropylene homopolymer (thermosetting polymer) increases its low-temperature impact strength (LTIS). As randomly polymerized ethylene monomers are applied to a polypropylene homopolymer, the crystallinity of the polymer is reduced, the melting point is lowered, and the polymer becomes more transparent.

Polypropylene Can be Categorized as 

  • Atactic polypropylene, represented as PP-at.

  • Syndiotactic polypropylene, represented as PP-st. 

  • Isotactic polypropylene, represented as PP-it. 

The methyl group (-CH3) is uniformly aligned in atactic polypropylene, alternating for syndiotactic polypropylene and evenly for isotactic polypropylene. This affects the crystallinity (amorphous or semi-crystalline) and thermal properties of the material.

Atactic polypropylene, on the other hand, is amorphous and lacks any regularity, making it difficult to crystallise.

Crystal Structure of Various Polypropylene

  • Crystallinity is high in isotactic polypropylene, which is formed in industrial products. Approximately 30%–60%.

  • Syndiotactic polypropylene is less crystalline, whereas atactic polypropylene is amorphous (not crystalline).

  • Isotactic polypropylene comes in a variety of crystalline forms, each with its own molecular arrangement of polymer chains. In iPP, the -modification is the most common. Lamellae in the shape of folded chains are used to create these crystals. The lamellae are arranged in a "cross-hatched" pattern, which is a distinctive feature.

  • Since atactic polypropylene is amorphous, it lacks a crystal structure. It is readily soluble even at moderate temperatures due to its lack of crystallinity, allowing it to be extracted as a by-product from isotactic polypropylene.

Decomposition of Polypropylene

Effect of UV Exposure on Polypropylene Material

Polypropylene material when exposed to temperatures above 100 °C, its chain will degrade. Oxidation normally happens at the tertiary carbon centres, causing the chain to split due to an oxygen reaction. Cracks and crazing are signs of deterioration in external applications. Various polymer stabilisers, such as UV-absorbing additives and anti-oxidants like phosphites (e.g. tris(2,4-di-tert-butyl phenyl)phosphite) and hindered phenols, can help to prevent polymer degradation.

Effect of Microbes

Polypropylene degradation has been demonstrated by microbial communities isolated from soil samples mixed with starch. As implantable mesh products, polypropylene has been known to degrade in the human body. At the surface of mesh fibres, the damaged material forms a tree bark-like coating.

Synthesis of Polypropylene

The polymer is built around heterogeneous catalyst particles in gas-phase and slurry reactors. Propene is passed over a bed containing the heterogeneous (solid) catalyst during the gas-phase polymerization, and the resulting polymer is separated as a fine powder and then converted into pellets. Gas that has not been reacted is recycled and pumped back into the reactor.

Liquid propene is used as a solvent in bulk polymerization to keep the polymer from precipitating. The polymerization takes place at 60–80 °C, with a pressure of 30–40 atm used to hold the propene liquid. Loop reactors are often used for bulk polymerization. Because of the polymer's reduced solubility in liquid propene, bulk polymerization is limited to a maximum of 5% ethene as a commoner.

In slurry polymerization, inert diluents such as C4–C6 alkanes (butane, pentane, or hexane) are used to suspend the expanding polymer particles. Propene (three carbon alkane chains) is introduced into the mixture as a gas form.

The tacticity of Polypropylene, or the orientation of the methyl groups (CH3) relative to the methyl groups in neighbouring monomer units, has a significant impact on its properties. The tacticity of polypropylene can be regulated by selecting the right catalyst.

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Uses of Polypropylene

  • Many plastic living hinges, such as those on flip-top bottles, are made of polypropylene because it is fatigue resistant.

  • Polypropylene is used in the production of piping systems, both those requiring high purity and those requiring strength and rigidity. 

  • This material is often chosen for its resistance to corrosion and chemical leaching, as well as its resistance to most types of physical damage, including impact and freezing, as well as its environmental benefits and ability to be joined by heat fusion rather than glueing.

  • It's used in the production of polypropylene chairs.

  • Polypropylene can be used to make a variety of medical and laboratory products because it can withstand the heat of an autoclave.

  • Its heat resistance also allows it to be used as a consumer-grade kettle manufacturing material. Food containers made of it will not melt in the dishwasher or during hot filling processes in the industry.

  • Polypropylene is a tough, transparent, reusable plastic container that comes in a range of shapes and sizes for customers from companies including Rubbermaid and Sterilite.

  • Although the lids are often made of a more flexible LDPE to allow them to snap onto the container to close it. 

  • Polypropylene can also be used to make disposable bottles for liquid or powdered substances.

  • Polypropylene or HDPE is commonly used in plastic pails, car batteries, wastebaskets, pharmacy drug bottles, cooler containers, pots, and pitchers.

  • Biaxially focused polypropylene is a popular application for polypropylene (BOPP). BOPP sheets are used to create a wide range of products, including transparent bags. Polypropylene becomes crystal clear when biaxially focused, making it an ideal packaging material for creative and retail items.

  • Polypropylene, which is extremely colorfast, is commonly used in the production of domestic carpets, rugs, and mats.

  • Polypropylene is a popular material for ropes because it is light enough to float in water.

  • Polypropylene is also used as an electrical cable insulation alternative to polyvinyl chloride (PVC).

  • Polypropylene is often used as the waterproofing top layer of single-ply systems rather than modified-bit systems in roofing membranes.

  • Polypropylene is most widely used for plastic mouldings, in which it is melted and injected into a mould to form complex shapes at a low cost and high volume.

  • Loudspeaker drive units are usually made of polypropylene.

  • Polypropylene fibres are used in concrete to improve strength and reduce cracking and spalling.

  • Polypropylene fibres are also used as insulation in drywall joint compounds. It can improve the joint compound's durability and dimensional stability while also reducing shrinkage and cracking as it dries.

Expanded Polypropylene (Expanded Polypro)

Expanded polypropylene (EPP) is a type of polypropylene that comes in a foam shape. Due to its low stiffness, EPP has excellent impact characteristics, allowing it to regain its shape after being struck. Hobbyists use EPP extensively in model aircraft and other radio-controlled vehicles. This is due to its ability to withstand impacts, making it an ideal material for beginner and amateur RC aircraft. In hobbyist radio control model aircraft, expanded polypropylene (EPP) foam is used as a structural material. Unlike EPS, which is friable and easily breaks on impact, EPP foam is able to withstand kinetic impacts well without cracking, maintains its original shape, and has memory foam characteristics that enable it to return to its original shape in a short period of time.

Disadvantages of Polypropylene (Polyprop) Material

  • UV effects and scratch resistance is low.

  • Embrittles at temperatures below -20°C.

  • 90-120 degrees celsius is a low upper service temperature.

  • They swell quickly in chlorinated solvents and aromatics when attacked by strongly oxidising acids.

  • Action with metals has a negative impact on heat-aging stability.

  • Dimensional changes after moulding due to crystallinity results.

  • Paint adhesion is low.

Did You Know That?

  • Polypropylene is a material that is fully recyclable.

  • Bacteria, fungi, and other microbes are vulnerable to polypropylene.

  • Polypropylene is a non-toxic material found in nature.

FAQ (Frequently Asked Questions)

Question: What is Polypropylene?

Answer: Polypropylene is a polymer that belongs to the polyolefin family and is one of the most commonly used polymers today. It's a hydrocarbon resin with a linear structure. Polypropylene's chemical formula is (C3H6)n.

Question: Explain the Properties of Polypropylene.

Answer: The properties of polypropylene is given below:

  • Polypropylene's melting point ranges from 160 to 165°C for homopolymer and 135 to 159°C for copolymer.

  • Of all commodity plastics, PP is one of the lightest polymers. 

  • Polypropylene has a density range of 0.898 g/cm3 to 0.908 g/cm3.

  • Polypropylene material is resistant to a wide range of chemicals.

  • Polypropylene is a flammable material.

  • At high temperatures, in humid environments, and when immersed in water, Polypropylene material maintains mechanical and electrical properties. It's made of water-resistant plastic.

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