Everything you need to know about PP plastic

Polypropylene plastic properties result in a range of characteristics that put this plastic in high demand. We’ve put together helpful information to help you understand if this material is right for your application. From the melting point of polypropylene plastic to polypropylene plastic uses, this is your one-stop guide, covering:

• What is polypropylene (PP) plastic?
• What are the characteristics of polypropylene?
• What is PP plastic used for?
• The properties of polypropylene
• Injection molding polypropylene
• 3D printing polypropylene
• Types of polypropylene
• The advantages of polypropylene
• The disadvantages of polypropylene
• How is polypropylene made?
• How to clean polypropylene plastic

What is polypropylene (PP) plastic?

Polypropylene material is a semi-rigid, tough thermoplastic polymer made from the monomer propylene. A versatile, low-cost material, this linear hydrocarbon resin is the lightest polymer of any commodity plastic. Polypropylene plastic products range from everyday items, such as plastic bottles, to industrial applications, which you’ll see below.

PP plastic was first polymerized in 1951 and today, it’s one of the most commonly used plastics in the world. 

What are the characteristics of polypropylene?

It's polypropylene’s characteristics that make this plastic so popular and used across nearly all industries. 

Is polypropylene plastic flexible? Despite its toughness, the answer is yes, especially when copolymerized with polyethylene. This duality is partly what makes the material so versatile. The melting point of polypropylene plastic makes it ideal for applications that need moderate heat resistance. 

PP plastic, like any material, can be modified by the addition of various additives, such as fillers, stabilizers, and pigments. Additives can improve the mechanical, thermal, and chemical properties of the material, as well as its appearance and processing characteristics. That said, these are the basic characteristics of polypropylene:

• Lightweight: Polypropylene’s low density translates to light weight, especially compared to other plastics 
• Durable and strong: High-impact resistance – the material can withstand significant force without breaking 
• Resistant to corrosion and chemicals: Suitable for use in harsh environments
• Excellent moisture resistance: Ideal for use in wet or humid environments
• Low coefficient of friction: Good for applications where reduced friction is needed
• Light transmissivity: Can be manufactured in both transparent and opaque forms (If you need high transmissivity, then acrylic or polycarbonate will serve you better)
• Creep and fatigue resistance: Retains its shape after bending and flexing. Perfect for applications where repeated stress is a concern, such as plastic hinges 
• Insulation: High resistance to electricity even at high temperatures
• Poor UV resistance: Can degrade quickly when exposed to sunlight, making it unsuitable for some outdoor applications. However, additives can correct this problem 
   

What is PP plastic used for?

Uses of polypropylene plastic are wide reaching. Examples of this tough material include:

Automotive parts: Used extensively in the automotive industry for the production of various components, such as interior and exterior trims, bumpers, and instrument panels. Its low density helps to improve fuel efficiency.

Packaging: Like polyethylene terephthalate (PET), PP is commonly used in packaging material for bottles, food containers, hot beverage cups, caps, and packaging systems. It’s also strong and stiff enough to help protect the contents of the package and prevent damage during transportation.

Consumer appliances: Its low cost and ease of processing make it an attractive option for manufacturers.

Medical devices: Excellent plastic for a range of medical applications, such as disposable syringes, petri dishes and even sutures. Its chemical resistance and ability to withstand sterilization make it an ideal material for the medical industry.  

You can learn more in our guides, What plastic is used for medical devices? and What is medical-grade plastic?

Electronics: Excellent resistance to electricity makes it ideal for electronic components. 

Industrial applications: From electrical and equipment manufacturing to machinery, polypropylene is a first-choice material. An example of the material’s variety of uses can be seen below in different industrial polypropylene plastic products.

Polypropylene plastic uses

The properties of polypropylene

Polypropylene’s characteristics – and therefore its popularity – result from its material properties. 

Crystalline regions are highly ordered and tightly packed, while amorphous regions are disordered and loosely packed. Its properties are similar to polyethylene (PE), but it’s somewhat harder and has higher heat resistance. 

You can learn more in our guide, PP vs. PE: which is best for your component? 

Now for a closer look at the material properties:
 

Polypropylene plastic properties 

Polypropylene plastic chemical resistance  

Injection molding polypropylene

The material is especially useful for injection molding. For starters, it has a relatively high melt flow rate (MFR). This is a measure of its flowability, expressed in grams per 10 minutes. The higher the MFR, the more easily the material flows. PP plastic has a relatively high MFR compared to other engineering plastics, which makes it ideal for injection molding. 

Polypropylene plastic properties include a relatively low melt viscosity, which means it has a low resistance to flow. This allows it to easily fill complex molds and produce parts with intricate details. It has a high shear rate, which is the rate at which the material is being sheared or deformed during injection molding. 

The MFR and shear rate are not exact values across the board. They vary depending on the specific type of polypropylene and the processing conditions used during injection molding. 

Wall slip is another consideration. This occurs when the material slides along the mold wall during the injection-molding process, resulting in a thin layer of material that does not fully conform to the mold. Polypropylene has a low wall slip, which means it adheres well to the mold and produces parts with high dimensional accuracy.

Polypropylene is available in the form of pellets. Shrinkage in polypropylene is about 1-2% but can vary based on a number of factors, including holding pressure and mold wall thickness. 

Polypropylene plastic properties can be modified by adding various additives, such as fillers, stabilizers, and pigments. These additives can improve the mechanical, thermal, and chemical properties of the material to achieve the characteristics you’re after – such as UV protection – as well as its processing characteristics.

You can learn more in our guide, What is plastic injection molding and how does it work?

3D printing polypropylene

Polypropylene’s semi-crystalline structure makes the material difficult to 3D print, causing parts to warp during cooling. It also has poor bonding properties, which makes it difficult for prints to stick to the print bed. But while bonding polypropylene plastic to another material results in repeated failures, you can experiment with ways to get around that problem. While the material won’t stick to your printing bed, you can try bonding polypropylene plastic by printing directly onto the tape. But looking for methods to circumvent this and other problems will be difficult and cost more – expect to waste a lot of material and time getting it right. 

It’s even possible to create a polypropylene blend to improve toughness, but it might be more cost effective to use plastics suited to 3D printing, such as Acrylonitrile butadiene styrene (ABS) or Polylactic acid (PLA). 

Types of polypropylene

The two primary types, or grades, of PP plastic are homopolymers and copolymers. 
Polypropylene homopolymer is a general-purpose grade, used in healthcare, automotive, packaging, electrical applications, pipes and more. Polypropylene copolymer is divided into random copolymers and block, or impact, copolymers, produced by polymerizing propene and ethane.

Polypropylene homopolymer

Polypropylene homopolymer (PPH) contains only propylene monomer in a semi-crystalline solid form. It has a high strength-to-weight ratio, and is stiffer and stronger than copolymers. These properties combined with good chemical resistance make it ideal for many corrosion-resistant applications. PPH also has a relatively high melting point and good impact resistance, weldability and processability. One of the key advantages of polypropylene homopolymer is its ability to be easily processed through injection molding.

Polypropylene copolymers

Random copolymers are produced by polymerizing a mixture of propylene and another comonomer, typically ethylene, in which the comonomer is randomly incorporated into the polymer chain. This results in a polymer with a lower melting point and improved clarity, flexibility, and impact resistance compared to PPH. This copolymer is commonly used in the production of transparent containers, packaging films, and medical devices.

Block copolymers are produced by blending PPH with an elastomeric polymer, typically ethylene-propylene rubber or butadiene rubber, to create a material with improved toughness, impact resistance, and low-temperature performance compared to PPH. Impact copolymers are commonly used in automotive parts, household appliances, and other applications where high impact strength is required.

Even with the differences between copolymers, we can still make a general comparison with polypropylene homopolymer.

Polypropylene homopolymer vs. Copolymers

The advantages of polypropylene

As you’ve no doubt already seen, PP plastic is versatile with a range of advantages, including:

• High stiffness: This characteristic makes it ideal for applications that require structural integrity, required in applications ranging from consumer appliances to automotive. 
• Chemical resistance: Resists many chemicals, including acids, alkalis, and solvents, making it suitable for use in harsh chemical environments, which is necessary in medical applications. 
• Lightweight: Excellent choice for applications where weight is a concern, such as in automotive and aerospace industries.
• Low-moisture absorption: Polypropylene’s low moisture absorption rate, means it resists water damage. With additives to protect against UV rays, it’s an excellent material for outdoor applications.
• Low cost: Relatively inexpensive to produce, making it a cost-effective option for manufacturers.
• 100% Recyclable: Polypropylene can undergo the recycling process multiple times to produce other products.

The disadvantages of polypropylene

Remember, additives can enhance characteristics to give you the material that fits your needs.

• Low impact strength: PP plastic has good impact resistance, which refers to the energy needed to break the material. But it has low impact resistance, which makes it susceptible to damage from high stress or impact loads.
• Low temperature resistance: Can become brittle at freezing temperatures, limiting its use in certain applications.
• Flammable: PP plastic is flammable and can ignite easily, which may be a concern in some applications.
• Poor UV resistance: Can degrade quickly when exposed to sunlight, so just a reminder, additives and make it ideal for outdoor applications.
• Limited aesthetics: PP plastic is often translucent or opaque and has limited options for color and texture, limiting its aesthetic appeal in some applications.

How is polypropylene made?

Polypropylene is created using slurry, solution, or gas-phase processes. The steps to produce polypropylene are as follows:

1. The propylene monomer is typically produced from petroleum or natural gas through the refining process. 
2. The propylene monomers are subjected to heat and pressure in the presence of a catalyst system. How this works: The propylene monomer is fed into a reactor vessel along with a catalyst, which initiates the polymerization process. The reaction produces long chains of polypropylene molecules that eventually form a solid polymer.
3. This achieves polymerization at relatively low temperature and pressure. The resulting product is translucent but can be easily colored. Alterations in catalyst and production conditions can modify the plastic's properties.
4. The resulting polymer is typically purified by washing it with water to remove any residual catalyst or other impurities.

How to clean polypropylene plastic

PP plastic surfaces are easy to clean. If an area is only a little bit dirty, usually a moistened cloth or sponge will do. For anything dirtier, use water with a gentle cleaner. Do not use liquid cleaning agents that scour or are abrasive – this can damage the surface. A few tips to remember:

• Use cleaning cloths, wipes or synthetic sponges – not cleaning pads made of plastic or steel wool
• Use a gentle liquid soap
• Use a soft brush for crevices 
• If marks are made by pens, felt-tips, or other markers, try to remove before they dry 
• Extremely dirty surfaces that involve paint stains, adhesives or spray paint should be removed by a specialist. 

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