The Difference Between Polypropylene Types
Jan 16, 2024| What is Polypropylene?
Polypropylene is produced from propene (or propylene) monomer. It is a linear hydrocarbon resin. The chemical formula of polypropylene is (C3H6)n. PP is among the cheapest plastics available today, and It has the lowest density among commodity plastics. Upon polymerization, PP can form three basic chain structures depending on the position of the methyl groups:
Atactic (aPP). Irregular methyl group (CH3) arrangement
Isotactic (iPP). Methyl groups (CH3) arranged on one side of the carbon chain
Syndiotactic (sPP). Alternating methyl group (CH3) arrangement
PP belongs to the polyolefin family of polymers and is one of the top three most-used polymers today. Polypropylene has applications-both as a plastic and as a fiber-in the automotive industry, industrial applications, consumer goods, and the furniture market.
Different Types of Polypropylene
Homopolymers and copolymers are the two main types of polypropylene available in the market.
Propylene homopolymer is the most widely utilized general-purpose grade. It contains only propylene monomer in a semi-crystalline solid form. Main applications include packaging, textiles, healthcare, pipes, automotive, and electrical applications.
Polypropylene copolymer is divided into random copolymers and block copolymers produced by polymerizing propane and ethane:
Propylene random copolymer is produced by polymerizing together ethene and propene. It features ethene units, usually up to 6% by mass, incorporated randomly in the polypropylene chains. These polymers are flexible and optically clear, making them suitable for applications requiring transparency and for products requiring an excellent appearance.
Propylene block copolymer contains a higher ethene content (between 5 and 15%). It has co-monomer units arranged in a regular pattern (or blocks). The regular pattern makes the thermoplastic tougher and less brittle than the random co-polymer. These polymers are suitable for applications requiring high strength, such as industrial usage.
Another type of polypropylene is impact copolymer. A propylene homopolymer containing a co-mixed propylene random copolymer phase which has an ethylene content of 45-65% is referred to PP impact copolymer. Impact copolymers are mainly used in packaging, houseware, film, and pipe applications, as well as in the automotive and electrical segments.
Polypropylene suppliers include A. Schulman, Borealis, ExxonMobil Chemical, LyondellBasell, SABIC, and RTP Company.
Polypropylene Homopolymer vs. Polypropylene Copolymer
Propylene homopolymer has a high strength-to-weight ratio and is stiffer and stronger than the copolymer. These properties combined with good chemical resistance and weldability make it a material of choice in many corrosion-resistant structures.
Polypropylene copolymer is a bit softer but has better impact strength. It's tougher and more durable than propylene homopolymer. It tends to have better stress crack resistance and lower temperature toughness than homopolymer at the expense of a small reduction in other properties.
PP Homopolymer and PP Copolymer Applications
The applications are nearly identical because of their extensively shared properties. As a result, the choice between these two materials is often made based on non-technical criteria.
Keeping information about the properties of a thermoplastic beforehand is always beneficial. This helps in selecting the right thermoplastic for an application. It also assists in evaluating whether the end-use requirement would be fulfilled or not. Here are some key properties and benefits of polypropylene:
Melting point of polypropylene. The melting point of polypropylene occurs at a range.
Homopolymer: 160-165°C
Copolymer: 135-159°C
Density of polypropylene. PP is one of the lightest polymers among all commodity plastics. This feature makes it a suitable option for lightweight/weight-saving applications.
Homopolymer: 0.904-0.908 g/cm3
Random copolymer: 0.904-0.908 g/cm3
Impact copolymer: 0.898-0.900 g/cm3
Polypropylene chemical resistance
Excellent resistance to diluted and concentrated acids, alcohols, and bases
Good resistance to aldehydes, esters, aliphatic hydrocarbons, and ketones
Limited resistance to aromatic and halogenated hydrocarbons and oxidizing agents
Other values
Flammability: Polypropylene is a highly flammable material
PP retains mechanical and electrical properties at elevated temperatures, in humid conditions, and when submerged in water. It is a water-repellent plastic
PP has good resistance to environmental stress and cracking
It is sensitive to microbial attacks (bacteria, mold, etc.)
It exhibits good resistance to steam sterilization
Polymer additives like clarifiers, flame retardants, glass fibers, minerals, conductive fillers, lubricants, pigments, and many other additives can further improve PP's physical and/or mechanical properties. For example, PP has poor resistance to UV, hence light stabilization with hindered amines enhances the service life as compared to unmodified polypropylene.