What is polypropylene, and why is it essential? It’s a versatile and durable polymer with many uses in various industries — making it a material of choice in sectors like rail and transit, food and beverage manufacturing, mining and even green energy.
The global demand for polypropylene is consistently rising. The global market volume of polypropylene in 2021 was around 75.6 million metric tons, representing a 2.2% rise over the previous year. Moreover, by 2029, it may increase to 100 million metric tons, a compound annual growth rate of 3.6% from 2022 to 2029.
This flexible and durable plastic is undoubtedly valuable — but what is it? Why is it so favored? When considering polypropylene, consider its properties and applications. This article will give you an overview of polypropylene, its fundamental properties and other essential information.
Table of Contents
What Is Polypropylene?
Polypropylene, also known as polypropene, is a versatile thermoplastic polymer that you can use in various applications. For clarification, thermoplastic, often called thermosoft plastic, is any plastic polymer substance that becomes flexible and moldable at high temperatures and hardens at low temperatures. This capability, combined with polypropylene’s capacity to melt rather than burn when subjected to high temperatures, are two essential characteristics that make it a sought-after choice for manufacturing applications.
J. Paul Hogan and Robert Banks of Phillips Petroleum, an American oil company, initially demonstrated propylene polymerization in 1951. Giulio Natta and Karl Rehn discovered stereoselective polymerization to the isotactic in 1954. This groundbreaking finding paved the way for Montecatini’s Italian business to begin large-scale commercial production of isotactic polypropylene in 1957. Natta was also the first to create syndiotactic polypropylene.
Among all commercial plastics, this linear hydrocarbon resin is the lightest. Manufacturers can considerably enhance PP with particular additives. But what does polypropylene mean? Polypropylene is a polyolefin that is tougher than polyethylene. It is a low-density, high-heat resistance commodity plastic with the chemical formula C3H6. Industries such as packaging, medical, automotive, consumer goods and cast films typically use PP.
PP can be:
- Hard or soft
- Light or heavy
- Opaque or transparent
- Insulating or conductive
- Neat or reinforced with other inexpensive mineral fillers, short or long glass fibers and natural or self-reinforced fibers
The Polypropylene Manufacturing Process
Making polypropylene involves polymerizing propylene gas using the Ziegler-Natta catalyst. The desired polymer grade determines the polymerization parameters, which include pressure, temperature and reactant concentrations. Several manufacturing processes may occur in either a gas-phase or liquid-phase process.
- Gas-phase process: Heterogeneous catalyst particles can make polymers in gas-phase and slurry reactors. In a fluidized bed reactor, temperatures range between 70 and 100 degrees Celsius. Then, transport the propene and hydrogen over a bed containing heterogeneous or solid catalysts. The produced propene transforms into a fine powder. Cyclone separators remove it from unreacted propene and hydrogen. The fine powder then turns into pellets. The unreacted gas gets recycled and reintroduced into the reactor.
- Spheripol process: This polymerization process uses propene, hydrogen and ethylene. During this process, suspend the catalyst and monomer units in an inert solvent, such as light hydrocarbons. Pour the slurry of monomer and catalyst into one or two vertically stirred gas phase reactors, where polymerization occurs. Once polymerization completes, dip tubes release the monomer-polymer powder combination from the reactors, while the unreacted monomer gets recycled back into the stirred gas reactor.
Depending on the location of the methyl groups during polymerization, PP can form one of three fundamental polypropylene structures.
- Atactic polypropylene: An irregular arrangement of methyl groups.
- Isotactic polypropylene: An arrangement of methyl groups on one side of the carbon chain.
- Syndiotactic polypropylene: An alternating arrangement of methyl groups.
Types of Polypropylene
There are two primary types of polypropylene — homopolymer and copolymer. Depending on its usage, you’ll require a specific kind of polypropylene. Some people call PP the “steel” of the plastic industry due to its adaptability to serve specific purposes, generally by adding particular ingredients or producing them uniquely. Here are the critical differences between the two polypropylene types.
- Polypropylene homopolymer: This type is the most widely used general-purpose PP. When polymerizing a single monomer to produce a macromolecule, a homopolymer forms. A homopolymer consists of repeating units of a single monomer type. Examples of homopolymers include polycaprolactam, polyester and polyethylene.
- Polypropylene copolymer: Polymerizing more than one kind of monomer generates a copolymer. As a result, a copolymer always contains two or more types of repeating units created by a process known as condensation polymerization. Examples of copolymers include ethylene-vinyl acetate, high-impact polystyrene and acrylonitrile butadiene styrene.
Unlike homopolymers, copolymers have further classifications depending on the arrangement of the monomers on the main chain. There are two subtypes of copolymers.
- Polypropylene block copolymer: PP-BC comprises comonomer units organized in a regular pattern and contains 5% to 15% ethylene. Ethylene increases specific qualities, like impact resistance, whereas other additions improve others.
- Polypropylene random copolymer: This subtype goes through the process of polymerizing ethene and propene together. In contrast to PP-BC, the comonomer units in PP-RC occur in irregular or random patterns along the polypropylene molecule. They mix about 1% to 7% ethylene for applications requiring a more flexible, transparent result.
Properties of Polypropylene
Knowing the numerous characteristics of a thermoplastic is usually helpful, as this aids in selecting the appropriate thermoplastic for a particular application. Here are some of the most critical properties of polypropylene.
- Melting temperature: With homopolymer, the PP melting point can range from 160 to 165 degrees Celsius or 320 to 329 degrees Fahrenheit and above. Copolymer’s melting point can start at 135 to 159 degrees Celsius or 275 to 318 degrees Fahrenheit and above.
- Chemical resistance: Polypropylene plastic has a higher chemical resistance than polyethylene or “ordinary” plastic. More specifically, PP has exceptional resistance against diluted alcohols, acids and bases and relatively good resistance against esters, aldehydes, ketones and aliphatic hydrocarbons. However, it has limited resistance against aromatic, halogenated hydrocarbons and oxidizing agents.
- Tensile strength: Compared to many other materials, PP has a high tensile strength — around 27 megapascals. This quality allows PP to carry more than it weighs.
- Density: PP’s density is about 900 kilograms per cubic meter. As a result, PP is the lowest-density commodity plastic.
- Elasticity: PP has Young’s Modulus of Elasticity of 1.3 gigapascals. Polypropylene is strong and flexible by nature, especially when copolymerized with ethylene. Polypropylene has a high fatigue resistance.
- Flammability: According to the U.S.-based National Fire Protection Association, PP has a fire hazard classification of four, meaning it’s highly flammable.
- Water absorption: PP is extremely water-resistant, as it absorbs less than 0.01% of its weight in water during a 24-hour soak test. It’s perfect for immersion applications that require protecting materials from chemical exposure.
What Is Polypropylene For?
Because of polypropylene’s characteristics, you can apply it in residential and industrial settings. Its adaptability to diverse manufacturing processes and applications distinguishes it as a primary player in the plastic industry. Another significant property is PP’s capacity to behave as a plastic and a fiber. Here are a few polypropylene uses in various industries.
- Product packaging: PP offers outstanding barrier qualities, making it an excellent choice for packaging applications. It provides strength, water resistance and superior surface quality at a low cost. Polypropylene plastic works well when blow-molded or thermoformed to create packaging shapes for items like food, personal care products and medical and laboratory items. Crates, bottles and pots are examples of blow-molded polypropylene plastic for flexible packaging, whereas items like syringes and reusable food containers are examples of thermoformed polypropylene plastic for more rigid packaging.
- Automotive parts: Due to PP’s excellent mechanical and chemical qualities and moldability, the automotive industry relies on it to produce bumpers, battery boxes and trays, instrument panels, fender liners, door trims and other interior elements.
- Fabrics: The fibers and fabrics industry uses a significant amount of PP. Raffia or slit-film, strapping, tape, staple fibers, continuous bulk filament, spun bond and continuous filaments are all applications for PP fiber. PP rope and twine are robust and moisture-resistant, making them ideal for marine applications.
- Medical supplies: PP’s microbiological, chemical and steam sterilization resistance makes it incredibly useful in medical settings. You can find polypropylene in disposable syringes, pill containers, Petri dishes, medical vials, intravenous bottles, diagnostic devices and food trays.
- Cable protection: Polypropylene is a durable solution to protect cables from operational and environmental stresses like water, chemicals, foot traffic and even rodent predation. Examples of PP cable protection include plastic conduits and cable drag chains. Cable protection is crucial because cables are subject to substantial strain during their lives, which causes them to decay. They usually require replacement if they have deteriorated to the point that they can no longer perform their tasks or are hazardous to use.
Pros and Cons of Polypropylene Materials
Because of its durability, flexibility and other advantages, polypropylene is a popular material for the packaging, automotive, medical and fabric industries. Before using it, consider its upsides and downsides.
Advantages
Polypropylene has several advantages that contribute to its popularity among producers who may put it to use in various applications. See below for PP’s primary benefits.
- High tensile strength: Polypropylene has a high tensile strength, which means it can carry about 27 MPa or 4,000 pounds per square inch, making it an excellent material for large loads.
- Flexibility and malleability: When polypropylene reaches its melting point, it can mold into any desired shape numerous times without deteriorating. PP is malleable, which means it may form into a living hinge or a piece of material that can repeatedly bend without breaking.
- Highly water-resistant: Polypropylene absorbs less than 0.01% of its weight when soaked in water. This quality makes it ideal for objects that require waterproofing.
- Good chemical resistance: Because of its chemical qualities, polypropylene doesn’t react with acids, making it perfect for containers designed to retain acidic solutions such as cleaning chemicals. Since it is also resistant to corrosion and chemical leakage, it’s excellent for pipe systems.
- Excellent electrical insulator: Polypropylene doesn’t conduct electricity effectively. As a result, it’s a leading choice for insulating electrical components such as cables and audio equipment.
Disadvantages
Like any manufactured material, propylene has various downsides that consumers and producers should be aware of before using it.
- High thermal expansion coefficient: Polypropylene has a limit to its high-temperature applications, as a high thermal expansion coefficient means that the material will expand when heated. Thermal expansion is a material’s tendency to alter size, shape and volume as its temperature rises.
- Susceptible to UV degradation: Polypropylene is unsuitable for usage at high altitudes or in areas with strong UV penetration, as it’s vulnerable to UV degradation.
- Poor bonding properties: Polypropylene has poor bonding characteristics, making it challenging to paint.
- Highly flammable: Polypropylene is combustible and will quickly melt when heated. Its flashpoint is only 260 degrees Celsius.
- Prone to oxidation: Polypropylene has restricted usage in high temperatures due to chain breakdown, which can result in oxidation — causing cracks in the PP.
Polypropylene for Cable Protection
If your company relies on cables, dependable cable protection is critical to your safety, productivity, safety and success. Your cables may be subject to environmental and operational stressors, yet they have limited built-in protection to safeguard the tiny wires and sheaths. Durable cable protectors provide high-quality, heavy-duty buffers against impacts, UV rays, high temperatures, chemical exposure, moisture and other stresses. Consider the following polypropylene cable protection options.
Cable Drag Chains
A cable drag chain is a protective covering that separates cables and allows them to move through automated machinery. With many industries embracing automation, drag chains are essential. When automated machinery requires cables or hydraulic hoses to move, the cables need protection. Enclosing the wires or hoses in a carrier keeps them from tangling with each other or the machines.
Many different industries and machines use drag chains. Some sectors include rail and transit, robotics, food and beverage manufacturing, mining, marine and offshore operations and green energy.
Cable drag chain manufacturers only started using polypropylene a couple of decades after its discovery. However, steel and plastic remain the preferred materials for cable carriers because of their flexibility, durability and heat resistance. The insights in our ultimate guide to cable drag chains may help you pick the correct type for your circumstances.
Cable Conduits
Conduits are critical safety measures in residential and commercial structures. Cable conduits are pipes that shield electrical cables and wires from dampness and other potentially destructive elements. Conduits may adapt to a wide range of wire needs, depending on the installation environment and wiring type. Conduits are available in various materials and forms, including plastics like polypropylene, polyvinyl chloride, polyurethane and polyamide.
Manufacturers can design plastics to meet specific environmental requirements and provide several advantages to consumers. PP cable conduits are more lightweight than heavier metallic materials, making them easier to handle. Due to its consistency, PP is also simple to cut or reorganize as your project requirements change.
Additionally, a nonmetallic conduit offers optimal protection against various elements. Many plastic conduits are watertight and protect wires from moisture. They are also resistant to high temperatures and other chemicals.
Contact AerosUSA for Durable Polypropylene Cable Protection
At AerosUSA, we provide a large selection of cable protectors, concealers and accessories. Our unique devices are simple to install and adaptable enough to handle various industrial conditions. They provide improved longevity and protection, and our knowledgeable cable protection experts are always available to answer any questions.
Contact AerosUSA online today for all your cable protection needs.
