Polypropylene (PP) Plastic — Complete Guide

What is Polypropylene?

Polypropylene is a tough and rigid, crystalline thermoplastic 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.

Molecular Structure of Polypropylene

PP belongs to polyolefin family of polymers and is one of the top 3 most used polymers today. Polypropylene has applications, both as a plastic and as a fibre, in:

• Automotive Industry
• Industrial Applications
• Consumer Goods, and
• Furniture Market

It has the lowest density among commodity plastics.

Continue reading or click to go on specific section of the page:

• What are the various types of polypropylene available today?
• Find Out Main Differences Between PP Homo vs. PP Copolymer
• Properties of Polypropylene: Physical, Chemical…
• Disadvantages of Polypropylene
• What is PP used for?
• Comparing Polypropylene with Polyethylene
• How is PP Made?
• Polypropylene Processing — Everything You Need to Know about It!
• Is PP toxic? How to recycle PP?

Different Types of Polypropylene

Homopolymers and Copolymers are the two main basic 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 family is further divided into random copolymers and block copolymers produced by polymerizing of propene and ethane:

1. 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 of applications requiring transparency and for products requiring an excellent appearance.
2. While in Propylene 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.

Some of the polypropylene suppliers include: A. Schulman, Borealis, ExxonMobil Chemical, LyondellBasell, SABIC, RTP Company.

» View all PP commercial grades and suppliers in Omnexus Plastics Database

This plastic database is available to all, free of charge. You can filter down your options by property (mechanical, electrical…), applications, conversion mode and many more dimensions.

Polypropylene Homopolymer vs. Polypropylene Copolymer

Propylene Homopolymer has a high strength to weight ratio and is stiffer & stronger than 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, is tougher and more durable than Propylene Homopolymer. It tends to have better stress crack resistance and low temperature toughness than homopolymer at the expense of quite small reductions in other properties.

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.

Properties of Polypropylene

Keeping information about the properties of a thermoplastic beforehand is always beneficial. This helps in selecting the right thermoplastic for a particular 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.

• Homopolymer: 160–165°C
• Copolymer: 135–159°C

1. Density of Polypropylene — PP is one of the lightest polymer among all commodity plastics. This feature which makes it 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

1. Polypropylene Chemical Resistance

• Excellent resistance to diluted and concentrated acids, alcohols and bases
• Good resistance to aldehydes, esters, aliphatic hydrocarbons, ketones
• Limited resistance to aromatic and halogenated hydrocarbons and oxidizing agents

1. Flammability: Polypropylene is a high flammable material
2. PP retains mechanical & electrical properties at elevated temperatures, in humid conditions and when submersed in water. It is a water-repellent plastic
3. PP has good resistance to environmental stress cracking
4. It is sensitive to microbial attacks (bacteria, mold…)
5. It exhibits good resistance to steam sterilization

To learn more about all polypropylene properties and their values — ranging from mechanical to electrical to chemical properties, to make right selection for your application.

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:

Search PP Grades Based on
Their Reinforcement Method

PP has poor resistance to UV, hence light stabilizationwith hindered amines 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.

Thanks to a significant increase of its performances (new additives and fillers, but also new polymerization processes, and blending solutions), PP is less seen as a low cost solution, but much more as a high performance solution, competing with traditional engineering plastics, and sometimes even metal (e.g: long glass fiber 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

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.

1. 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).
2. Rigid Packaging: PP is blow molded to produce crates, bottles, and pots. PP thin walled containers are commonly used for food packaging.
3. Consumer Goods: Polypropylene is used in several household products and consumer goods applications including translucent parts, housewares, furniture, appliances, luggage, toys etc.
4. 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.
5. 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.
6. 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.
7. 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.

Polypropylene vs Polyethylene — Comparison

Polyethylene and Polypropylene are very similar as far as physical properties.

PolypropylenePolyethylene

• Monomer of polypropylene is propylene
• It can be produced optically clear
• It is lighter in weight
• PP exhibits a high resistance to cracking, acids, organic solvents and electrolytes
• It has high melting point and good dielectric properties
• PP is non-toxic
• It is stiffer and resistant to chemicals and organic solvents compared to polyethylene
• PP is more rigid than polyethylene
• Monomer of polyethylene is ethylene
• Polyethylene can only be made translucent like a milk jug
• Its physical properties allow it to stand up better in cold temperatures, particularly when using it as signs
• It is a good electrical insulator
• PE offers good tracking resistance
• Polyethylene is sturdy as compared to Polypropylene

» View all PP Commercial grades » View all PE Commercial Grades

How is PP Made?

Who Invented Polypropylene? Polypropylene was first polymerized by German chemist named Karl Rehn and an Italian chemist named Giulio Natta to a crystalline isotactic polymer in 1954. This discovery soon led to a large-scale production of polypropylene starting in 1957 by the Italian firm Montecatini.

Syndiotactic polypropylene was also first synthesized by Natta and his coworkers.

These days, polypropylene is made from polymerization of propene monomer (an unsaturated organic compound — chemical formula C3H6) by:

• Ziegler-Natta polymerization, or
• Metallocene catalysis polymerization

Structure of PP Monomer
C3H6Ziegler-Natta Polymerization

Or Metallocene Catalysis

Structure of Polypropylene
(C3H6)n

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

Polypropylene Processing Conditions

Polypropylene can be processed by virtually all processing methods. Most typical processing methods include: Injection Molding, Extrusion Blow Molding and General Purpose Extrusion.

1. Injection Molding

• Melt temperature: 200–300°C
• Mold temperature: 10–80°C
• Drying is not necessary if stored properly
• High mold temperature will improve brilliance and appearance of the part
• Mold shrinkage lies between 1.5 and 3%, depending on processing conditions, rheology of the polymer and thickness of the final piece

1. Watch free video tutorial to improve PP cycle time and limit part shrinkage

1. Extrusion (tubes, blow and cast films, cables, etc.)

• Melt temperature: 200–300°C
• Compression Ratio: 3:1
• Cylinder Temperatures: 180–205°C
• Pre-Drying: No, 3 hours at 105–110°C (221–230°F) for regrind

1. Blow molding
2. Compression molding
3. Rotational molding
4. Injection blow molding
5. Extrusion Blow Molding
6. Injection stretch blow molding
7. General Purpose Extrusion

Expanded Polypropylene (EPP) may be molded in a specialized process. Being an ideal material for injection molding process, it is majorly used for batch and continuous production.

Polypropylene Recycling and Toxicity

All plastics have a ‘Resin Identification Code/ Plastic Recycling Code’ based on the type of resin used. PP’s resin identification code is 5.

PP is 100% recyclable. Automobile battery cases, signal lights, battery cables, brooms, brushes, ice scrapers, etc. are few examples which can be made from recycled polypropylene (rPP).

The PP recycling process mainly includes melting of waste plastic to 250°C to get rid of contaminants followed by removal of residual molecules under vacuum and solidification at nearly 140°C. This recycled PP can be blended with virgin PP at a rate upto 50%. The main challenge in PP recycling is related to its amount consumed — currently nearly 1% PP bottles are recycled as compared to 98% recycling rate of PET & HDPE bottles together.

The use of PP is considered safe because it does not have any remarkable effect from an occupational health and safety point of view, in terms of chemical toxicity.