Polypropylene is the second-most commonly used plastic in the world. Learn more about its properties, benefits, drawbacks, and applications.

Have you ever wondered why your shampoo lid is so resistant to use? Chances are that it’s made from polypropylene (PP), a robust and fatigue-resistant plastic used for many durable goods including housewares, utensils, sportswear, toys, and medical as well as laboratory equipment.

Polypropylene is widely available and cheap, making it the second most used plastic in the world after polyethylene. Its processing versatility and outstanding material properties, as well as recyclability, make the semi-crystalline polymer a favorite of the manufacturing sector, especially injection molding.

3D printing with polypropylene is slowly catching up with traditional manufacturing industries, even though it faces some unique challenges. Let’s take a look.

Material Properties
Let’s begin with the material’s chemical structure and corresponding properties. Polypropylene is a semi-crystalline thermoplastic that belongs to the polyolefin family and is one of the cheapest and most widely used commodity plastics in the world. It is manufactured through the thermal cracking of crude oil light distillate commonly known as naphtha and comes in three chemical structures: atactic, syndiotactic, and isotactic. The difference here lies in the molecular arrangement of the methyl groups with only the latter of the three being manufactured in commercially significant volumes.

Depending on desired properties of the end product, polypropylene’s monomer composition may additionally be altered. The material can exclusively consist of propylene monomers to create what is called a homopolymer or incorporate other monomers like ethylene to create copolymers. There are three main grades of polypropylene polymers currently available on the market. Their applications and costs usually overlap:

• Homopolymers: PPH is the most commonly used general-purpose grade. It is composed of a propylene monomer in a semi-crystalline form. Characterized by being stronger and stiffer than other grades, PPH stands out for its good chemical resistance and weldability. In addition, it has a high strength-to-weight ratio, which makes homopolymers suitable for a wide range of applications, including automobile parts, electrical applications, pipes, textiles, and packaging. Its melting point is at 160-165 °C.
• Block copolymers: The group incorporates 5-15% ethylene and has an improved impact resistance which extends to temperatures below -20°C. It has co-monomer units which are arranged in a regular pattern. This makes block co-polymers tougher and less brittle than random copolymers (see below). By adding impact modifiers, their toughness can be further enhanced. They are great for industrial applications with high strength requirements. Their typical melting point is at 135-159 °C.
• Random copolymers: The polypropylene chain molecule incorporates co-monomer units which are arranged randomly. These polymers typically contain 1-7% ethylene and are selected when a lower melting point, more flexibility, and an optically clear finish are desired.

Polypropylene has become a popular choice for manufacturers because of its low cost and many favorable chemical properties with the most important ones listed below:

• Water resistance:  Polypropylene is highly impermeable. This property is essential for total immersion applications in medical and industrial products.
• Allows for transmissivity: When having a biaxial orientation, high transmissivity can be obtained. Aesthetics or transfer of light requires this property.
• Color variability: Adding colors is easy and does not degrade the properties of the plastic (dying process).
• High melting point: When compared to other plastics available in the market, polypropylene’s high melting point makes it an excellent option for food appliances (containers, kitchen utensils) where high temperatures are required, as well as laboratory appliances.
• Chemical resistance: The chemical and solvent resistance of semi-crystalline polymers is usually much better than that of competing amorphous materials since their densely packed crystalline structures protect them against dissolution.
• Biological resistance: It does not mold or deteriorate by biological factors, such as bacteria or fungi (in the short term).
• Insulation: Given its high resistance to electricity it is commonly used in electronic components.
• Non-toxic: According to the Food and Drug Administration, polypropylene is generally considered safe for food and human contact and does not include the controversial additive Bisphenol A (BPA). However, depending on the mixture used, current research suggests that endocrine-disrupting chemicals found in the plastic could still be at risk of leaching into foods and drinks under certain conditions.
• Recyclable: In theory, polypropylene is both chemically and mechanically recyclable. Concerning 3D printing, this study demonstrates that, unlike other common filaments that are susceptible to scission or crosslinking during the recycling process, recycled polypropylene demonstrates excellent material properties due to its high thermal stability. In reality, however, only between one and four percent of all polypropylene is currently recycled, a problem that will be picked up at a later stage.

As good as all of this sounds, the chemical composition of polypropylene also has a number of drawbacks:

• Highly flammable: This material can burn easily, but there are grades that are more flame resistant.
• UV degradation: It’s prone to damage from sunlight, which degrades the polymer bonds holding it together. Therefore, in its pure form, it is not an ideal choice for long-term outdoor light exposure, yet this can be improved with pigments and other additives.
• Limited high-temperature uses: Though it can handle some heat, after a certain point it gets too hot for polypropylene. This is because it has a high-temperature expansion coefficient.
• Poor resistance to chlorinated solvents and aromatics. 
• Poor bonding properties: This makes it difficult to paint and also affects its adherence to certain glues that are used as a contact point. For instance, to install furniture. However, the adhesive strength can be enhanced, which would allow the use of paints and ink.
• Susceptible to oxidation. Polypropylene is susceptible to thermo-oxidative degradation, which can be remedied to an extent by adding antioxidants to the mixture.

Another important aspect of polypropylene’s physical attributes regards its behavior under mechanical loads. Here, it also boasts a number of favorable properties that make it attractive as a manufacturing material. Polypropylene is known for being a versatile, durable, lightweight, and extremely rugged thermoplastic. Depending on the polymerization process used to create it, different properties and textures can be achieved. Some of its most important characteristics are listed below:

• Tensile strength: Despite being lightweight, the material is capable of withstanding heavy loads.
• Elasticity and toughness: Polypropylene is regarded as a tough material because it can deform without breaking. It’s also very flexible, which makes it easy to mold in different shapes and ensures that it can take significant damage prior to breaking.
• Fatigue resistance: This property is the reason why your shampoo lid is made from polypropylene. It retains its shape after being bent or subjected to torsion. This makes it suitable for living hinges which are usually not achievable with more common 3D printing materials like PLA, ABS, or Nylon.

Source: https://all3dp.com/1/polypropylene-pp-all-you-need-to-know/

Our recycled Polypropylene (PP) granules are 100% made from post consumer and industrial waste material. It’s packed in a 25kg durable brown paper sack with monthly capacity up to 30mt. Kindly contact us to find out more about our recycled Polypropylene granules.

Polypeth Ekspor Indonesia is a supplier and export recycled plastic pellets (HIPS & PP) based in Indonesia. We work with plastic recycling manufacturers with years of experience to provide and ensure high quality & the safety of our recycled plastic pellets. Together we reduce the use of virgin plastic resin and reduce the waste of virgin plastic pollution.

To find out more about Polypeth Ekspor Indonesia’s products and services, speak to the team by clicking here.