Types of Polypropylene & their Benefits

Homopolymers and Copolymers are the two major types of polypropylene available in the market.

·         Polypropylene 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 family is further divided into random copolymers and block copolymers produced by polymerizing of propene and ethane:

1.      Polypropylene 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 of applications requiring transparency and for products requiring an excellent appearance.

2.      While in Polypropylene Block Copolymer, ethene content is larger (between 5 and 15%). It has co-monomer units arranged in regular pattern (or blocks). The regular pattern hence makes thermoplastic tougher and less brittle than the random co-polymer. These polymers are suitable for applications requiring high strength, such as industrial usages.

Polypropylene, Impact Copolymer – Propylene Homopolymer containing a co-mixed Propylene Random Copolymer phase which has an ethylene content of 45-65% is referred to PP impact copolymer. It is useful in parts which require good impact resistance. Impact copolymers are mainly used in packaging, houseware, film, and pipe applications, as well as in the automotive and electrical segments.

Expanded Polypropylene - It is a closed-cell bead foam with ultra-low density. EPP is used to produce three-dimensional polymer foam products. EPP bead foam has higher strength to weight ratio, excellent impact resistance, thermal insulation, and chemical and water resistance. EPP is used in various applications ranging from automobiles to packaging, from construction products to consumer goods and more.

Polypropylene Terpolymer - It is composed by propylene segments joined by monomers ethylene and butane (co-monomer) which appear randomly throughout the polymer chain. PP terpolymer has better transparency than PP homo. Also, the incorporation of co-monomers reduces crystalline uniformity in the polymer making it suitable for sealing film applications.

Polypropylene, High Melt Strength (HMS PP)– It is a long chain branched material, which combines both high melt strength and extensibility in the melt phase. PP HMS grades have a wide mechanical property range, high heat stability, good chemical resistance. HMS PP is widely used to produce soft, low density foams for food packaging applications as well as used in automotive and construction industries.

PP Homopolymer vs. Copolymer – How to decide between the two?

The potential applications for PP homopolymer and PP copolymer are nearly identical

This is because of their extensively shared properties. As a result, the choice between these two materials is often made based on non-technical criteria.

Interesting Material Properties of Polypropylene

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 if the end use requirement would be fulfilled or not. Here are some key properties and benefits of polypropylene:

1.      Melting Point of Polypropylene - The melting point of polypropylene occurs at a range.

o    Homopolymer: 160 - 165°C

o    Copolymer: 135 - 159°C

2.      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.

o    Homopolymer: 0.904 – 0.908 g/cm³

o    Random Copolymer: 0.904 – 0.908 g/cm³

o    Impact Copolymer: 0.898 – 0.900 g/cm³

3.      Polypropylene Chemical Resistance

o    Excellent resistance to diluted and concentrated acids, alcohols and bases

o    Good resistance to aldehydes, esters, aliphatic hydrocarbons, ketones

o    Limited resistance to aromatic and halogenated hydrocarbons and oxidizing agents

4.      Flammability: Polypropylene is a highly flammable material

5.      PP retains mechanical & electrical properties at elevated temperatures, in humid conditions and when submersed in water. It is a water-repellent plastic

6.      PP has good resistance to environmental stress cracking

7.      It is sensitive to microbial attacks, such as bacteria and mold

8.      It exhibits good resistance to steam sterilization

Learn more about all polypropylene properties and their values - ranging from mechanical and electrical to chemical properties; and make the right selection for your application.

How additives help improve PP properties?

Polymer additives like clarifiers, flame retardants, glass fibers, minerals, conductive fillers, lubricants, pigments and many other additives can further improve PP physical and/or mechanical properties. For example:

PP has poor resistance to UV, hence additives like hindered amines provide light stabilization and enhances the service life as compared to unmodified polypropylene.

Further, fillers (clays, talc, calcium carbonate…) and reinforcements (glass fiber, carbon fiber…) are added to achieve significant properties related to processing and end use application.

Development & the use of new additives, latest polymerization processes as well as blending solutions significantly increase polypropylene performance. Hence, today PP is less seen as a low cost solution, but much more as a high performance material competing with traditional engineering plastics and sometime event metal (e.g. long glass fiber reinforced PP grades).

Disadvantages of Polypropylene

·         Poor resistance to UV, impact and scratches

·         Embrittles below -20°C

·         Low upper service temperature, 90-120°C

·         Attacked by highly oxidizing acids, swell rapidly in chlorinated solvents and aromatics

·         Heat-aging stability is adversely affected by contact with metals

·         Post molding dimensional changes due to crystallinity effects – this can be solved with nucleating agents » Watch video

·         Poor paint adhesion

Key Areas of Applications of Polypropylene
Polypropylene is widely used in various applications due to its good chemical resistance and weldability. Some common uses of polypropylene include:

1.      Packaging Applications: Good barrier properties, high strength, good surface finish and low cost make Polypropylene ideal for several packaging applications.

a.       Flexible Packaging: PP films’ excellent optical clarity and low moisture-vapor transmission make it suitable for use in food packaging. Other markets shrink-film overwrap, electronic industry films, graphic arts applications, disposable diaper tabs and closures, etc. PP Film is available either as Cast Film or bi-axially orientated PP (BOPP).

b.      Rigid Packaging: PP is blow molded to produce crates, bottles, and pots. PP thin walled containers are commonly used for food packaging.

2.      Consumer Goods: Polypropylene is used in several household products and consumer goods applications including translucent parts, housewares, furniture, appliances, luggage, toys etc.

3.      Automotive Applications: Due to its low cost, outstanding mechanical properties and moldability, polypropylene is widely used in automotive parts. Main applications include battery cases and trays, bumpers, fender liners, interior trim, instrumental panels and door trims. Other key features of automotive applications of PP include low coefficient of linear thermal expansion and specific gravity, high chemical resistance and good weatherability, processability and impact/stiffness balance.

4.      Fibers and Fabrics: A large volume of PP utilized in the market segment known as fibers and fabrics. PP fiber is utilized in a host of applications including raffia/slit-film, tape, strapping, bulk continuous filament, staple fibers, spun bond and continuous filament. PP rope and twine are very strong and moisture resistant very suitable for marine applications.

5.      Medical Applications: Polypropylene is used in various medical applications due to high chemical and bacterial resistance. Also, the medical grade PP exhibits good resistance to steam sterilization. Disposable syringes is the most common medical application of polypropylene. Other applications include medical vials, diagnostic devices, petri dishes, intravenous bottles, specimen bottles, food trays, pans, pill containers, etc.

6.      Industrial Applications: Polypropylene sheets are widely used in industrial sector to produce acid and chemical tanks, sheets, pipes, Returnable Transport Packaging (RTP), etc. because of its properties like high tensile strength, resistance to high temperatures and corrosion resistance.