Know Your Plastics Series: Polypropylene (PP)

When looking up plastic material, have you ever come across the name Polypro? Polypropene? Or Molpen?

Well, they all refer to the same thing – Polypropylene (PP).

Polypropylene containers

Polypropylene (PP) is a white, semi-crystalline, mechanically rugged thermoplastic polymer produced from the propene monomer called propylene. Ranked the second most widely produced commodity plastic after polyethylene, Polypropylene has similar properties and attributes with polyethylene as members of the polyolefin family.

The very first preparation of Polypropylene was an accident.

J. Paul Hogan and Robert Banks were trying to derive gasoline from propylene gas at Phillips Petroleum in 1951 when they stumbled on the polymer and its potential. In 1954, Giulio Natta and Karl Rehr had developed the highly crystalline isotactic polymer for the Montecatini Corporation, which was perfected and mass-produced for commercial and consumer use by 1957.

Widely known as the “Steel” of plastics for its adaptability, Polypropylene can be modified and customized in different ways to serve varying purposes. These modifications/customizations require either applying different fabrication styles or using special additives resulting in the types of polypropylene.

The different types of Polypropylene (PP) available are:

Homopolymer polypropylene is the typical, general-purpose grade polypropylene produced. As a homopolymer, it contains a single long chain of carbon-to-carbon atoms called the propylene monomer. It is in textiles, pipes, packaging, healthcare, electrical and automotive applications.

Copolymer polypropylene is a result of polymerizing two monomers. The most popular co-monomer for propylene is ethylene, also known as ethene. Polypropylene copolymer is divided into:

- Block copolymer polypropylene: As the name implies, it contains co-monomer units of propene and ethylene arranged in a regular block pattern. Ethylene exponentially improves properties like impact resistance and can be combined with other additives such as elastomers to improve bending. A block copolymer polypropylene can contain anywhere between 5% to 15% ethylene at intervals.

This block pattern makes the thermoplastic tougher and less brittle which is suitable for high strength functions in industrial applications.

- Random copolymer polypropylene: Has an irregular structural pattern. Its co-monomer units contain between 1% to 7% ethylene, randomly arranged. Adding a randomly polymerized ethylene monomer to a polypropylene homopolymer reduces crystallinity. Thereby decreases the melting point and making it more transparent for applications that need clarity and more malleability.

Polypropylene Terpolymer is a randomly arranged propylene copolymerization with two other copolymers, ethylene and butane. This fusion improves the transparency on the homopolymer version and reduces the overall crystalline uniformity what makes this three-monomer polymer an ideal sealing film alternative.

Expanded Polypropylene (EPP) is an ultra-low density, closed-cell bead foam used to produce 3D polymer foam products such as consumer goods, packaging, automotive and construction materials. The usefulness of EPP in these applications is due to its excellent thermal insulation, resistance to water, chemicals and impact, and higher strength to weight ratio.

High Melt Strength Polypropylene (HMS-PP) is typically a soft, low-density foam form. High Melt Strength Polypropylene is a long chain, branched material used in food packaging, automotive and construction industries. HMS-PP is known for the following features: its high heat stability, good chemical resistance and wide mechanical property range.

Impact Copolymer Polypropylene - When a propylene homopolymer contains a co-mixed Propylene Random Copolymer with an ethylene content of 45%-60%, it is called a Polypropylene impact copolymer or PP impact copolymer. Impact copolymers are necessary for their excellent impact resistance for housewares, pipes, packaging, film, electrical and automotive applications.

Properties of Polypropylene

As a thermoplastic, Polypropylene can be melted, cooled and reheated again without significant degradation or burning.
It has a low melt-viscosity or melt flow rate/index which allows it to flow well during processing.
It is lightweight.
It has a melting point range of 130⁰ – 171⁰Celsius (266⁰F - 340⁰F).
It is a soft material.
It is semi-crystalline which means, it remains sold until it absorbs a given quantity of heat before it rapidly changes.
It can be repeatedly bent or flexed without breaking. Also known as fatigue resistance, this property also speaks to the high elasticity of Polypropylene.
Toughness: This is an engineering term that refers to the ability of a material to deform plastically without breaking early on in the deformation process. Toughness is very different from elasticity.
The resistance of Polypropylene to electricity is very high, so it is excellent in electrical components.
Polypropylene has excellent resistance to diluted acids, bases and organic solvents (for instance, fats) when at room temperature. This characteristic makes PP containers a good choice for first-aid products and cleaning agents.
Oxidizes at high temperatures
Relatively high friction coefficient.
Relatively low density compared to other commodity plastic.
It has a natural opaque colour but, it can also be translucent or transparent. This property is excellent for aesthetic value in applications where some transfer of light is required (transmissivity).

Applications of Polypropylene

Polypropylene Applications

The most popular applications of Polypropylene include:

Plastic living hinges as in flip-flop bottles
Fabrics and textiles
Ropes
Dishwasher-safe food containers, trays, cups and housewares
Diapers and sanitary products
Luggage
Toys
Wastebaskets
Packaging (shrink-film wrap, electronic film, crates)
· Storage boxes
Opaque to-go containers
Disposable bottles (liquid or powdered consumer products)
Engineering plastic
Waterproof top-layer for roofing
Automotive applications (battery cases, interior trims, bumpers, fender liners)
Loudspeaker drive units
Insulation for electric cables
Filters for air conditioning and water
Piping systems (Potable plumbing, reclaimed water and water-based heating and cooling)
Laboratory equipment (a reference to items used in an autoclave).
Medical applications (Pill bottles, disposable syringes, medical vials, specimen bottles)

Advantages of Polypropylene

The unique properties and adaptability to different methods of fabrication make Polypropylene an invaluable material for the following reasons:
It can function as both a fibre (ropes, promotional tote bags and carpets) and a plastic (sheet and pellets) material.
The semi-crystalline nature of Polypropylene provides a high flexural strength that makes it excellent in the manufacture of living hinges. These are the very thin yet extremely flexible pieces of plastic that bend to extreme angles without breaking as in the lid of a shampoo or sauce bottle.
The lower density of polypropylene ensures a lighter weight, making it a top choice for manufacturers and distributors of injection-moulded parts.
The surface of polypropylene tends to be slippery. This characteristic is a good contact point for furniture and in low friction gear applications.
Using the Computerized Numerical Control (CNC) method, polypropylene form a living hinge for faster and cheaper development of prototypes.
As a fibre, Polypropylene is used in drywall joint compounds to increase flexibility and dimensional stability, to reduce shrinkage and cracking when it dries and as reinforcement.
It has a relatively low melting point.
It is resistant to absorbing moisture.
Polypropylene is easily combined with other polymers, like polyethylene, into a composite plastic which allows for more robust applications than pure polyethylene can provide.
In low ventilation environments, PP emits very little smoke and no toxic halogens, making it a suitable alternative for the insulation of electrical cables.
The transmissive nature of Polypropylene makes it a suitable replacement for acrylic or Polycarbonate in low-level light transfer applications.
Polypropylene ropes are as strong as polyester ropes but have the added advantage of being light enough to float in water.
Though naturally without colour, pigments can give the desired colour.
When dyeing PP, no water is needed. This process is called dope-dyed and, it poses a low to moderate hazard to the environment.
It is readily available, relatively inexpensive and cost-effective.
It is excellent for producing cold-weather base layers of long sleeve shirts and long underwear known as polypro.
Its low cost, durability and resistance to dirt make polypropylene an ideal protective cover for paper and other materials.
The foam version of polypropylene, known as Expanded Polypropylene (EPP), has very low stiffness, which allows it to resume shape after impact. This ability to absorb impact is vital in radio-controlled vehicles and model air crafts.
In areas susceptible to earthquakes, Polypropylene is used as a concrete additive when constructing the foundation of buildings and bridges to improve soil strength and reduce cracking

Disadvantages of Polypropylene

It has a low annealing temperature that causes it to deform under heat.
Its low melting point causes it to melt when using cutting methods that release heat (CNC ), making it sticky. It has a reputation for not being machine-able.
Its use in high-temperature applications is limited because polypropylene has a high thermal expansion coefficient.
It is highly flammable.
Contact with metals depreciates its heat-ageing stability.
The softness of PP makes cutting difficult. For a finished look. PP material must be scraped or roughened up a bit.
Its slippery nature prevents polypropylene from bonding to other surfaces. It is either welded or joined by heat fusion in such cases. For example, when forming a joint.
It is a challenge to paint.
Polypropylene's high friction coefficient makes it ineffective in numerous applications where materials like PTFE, acetal and nylon thrive.
It is not used in structural applications like load-bearing functions, for instance, the hinges of a heavy door.
It has a poor resistance to chlorinated solvents and aromatics that causes swelling.
It is susceptible to oxidation.
It is susceptible to UV degradation.
It requires special training and skill for efficient handling.

Polypropylene vs Polyethylene

PE vs PP

The fact that polypropylene and polyethylene materials share many similarities is well-known in the plastic and fibre industry. But like fraternal twins, there are some differences:

Polypropylene

Its monomer is propylene.

Its appearance is opaque, a trans-lucid white.

It is more rigid than polyethylene.

It is more flexible

It has a higher melting point.

It does not melt below 160⁰C.

It has a higher resistance to chemicals, organic solvents, acids, electrolytes and cracking.

Polyethylene

Its monomer is ethylene.

Its natural appearance is translucent or colourless.

It is more sturdy than polypropylene.

It is more elastic

It is a better electrical insulator.

It melts around 100⁰C.

It withstands cold temperatures better, particularly as signposts.

Polypropylene and Sustainability

In terms of chemical toxicity, Polypropylene is a safe material because it does appear to have a significant effect on occupational health and safety and is not traceable to cancer in humans.

All PP materials are considered 100% recyclable and can even be combined with virgin fibre and plastic up to 50%. Polypropylene is one of the plastic types that is rated BPA free and, the resin identification code is 5. Recycled PP is used to produce brushes, brooms, battery cables and cases and signal lights.

Polypropylene Resin Identification Code

The major problem with recycling PP is a general problem faced in the recycling industry – the rate of consumption is much higher than that of recycling.

SOURCE POINTS

· https://www.creativemechanisms.com/blog/all-about-polypropylene-pp-plastic

· https://en.wikipedia.org/wiki/Polypropylene#:~:text=Polypropylene%20(PP)%2C%20also%20known,partially%20crystalline%20and%20non%2Dpolar.

· https://omnexus.specialchem.com/selection-guide/polypropylene-pp-plastic

· https://sewport.com/fabrics-directory/polypropylene-fabric

· https://www.healthline.com/health/is-polypropylene-safe#about-polypropylene

· https://www.spg-pack.com/blog/en/5-differences-between-polypropylene-and-polyethylene-food-packaging/

· https://www.machinedesign.com/community/article/21837192/whats-the-difference-between-polypropylene-types