Polypropylene: How to select the right grade?

What are the types of PP?

The major types of polypropylene available in the market are as follows:

 

Polypropylene homopolymer

PP homopolymer is the most widely utilized general-purpose grade. It contains only propylene monomer in a semi-crystalline solid form. The main applications of PP homopolymer include packaging, textiles, healthcare, pipes, automotive, and electrical applications. 

 

Polypropylene copolymer

This family is produced by polymerizing propene and ethane. It is further divided into random copolymers and block copolymers.
 

1. PP random copolymer is produced by polymerizing ethene and propene together. It usually features ethene units up to 6% by mass. These units are randomly incorporated into the polypropylene chains. These polymers are flexible and optically clear. This makes them suitable for applications requiring transparency. Also, suitable for products requiring an excellent appearance.
    
2. While in the PP block copolymer, ethene content is larger. It ranges between 5 and 15%. It has co-monomer units arranged in regular patterns or blocks. The regular pattern makes the thermoplastic tougher and less brittle than the random copolymer. These polymers are suitable for applications requiring high strength, such as industrial usage.

The potential applications of PP homopolymer and PP copolymer are nearly identical. Because of their extensively shared properties, the choice between these two materials is often made based on non-technical criteria. Some of the non-technical criteria are described in the table below.
 

Polypropylene, impact copolymer

PP impact copolymer contains a co-mixed PP random copolymer phase. It has an ethylene content of 45-65%. It is useful in parts that require good impact resistance. Impact copolymers are mainly used in packaging, houseware, film, and pipe applications. They are also used in the automotive and electrical segments.
 

Expanded polypropylene

EPP is a closed-cell bead foam with ultra-low density. It produces three-dimensional polymer foam products. EPP bead foam has the following features: 
 

• higher strength-to-weight ratio,
• excellent impact resistance,
• thermal insulation,
• water and chemical resistance

EPP is used in automobiles, packaging, construction products, consumer goods, and more.

 

Polypropylene terpolymer

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

 

Polypropylene, high melt strength

It is a long-chain-branched material. It combines both high melt strength and extensibility in the melt phase. The key features of PP HMS grades include:
 

• wide mechanical property range,
• high heat stability, and
• good chemical resistance

HMS PP is widely used to produce soft, low-density foams for food packaging applications. It is also used in the automotive and construction industries.

 

Bio-based polypropylene

It is a bio-based version of PP. Its monomer propylene is derived from renewable feedstocks. The bio-based content can vary anywhere between 30-100%. Several suppliers are offering pure PP bio-based grades, such as: 
 

• PolyFibra® & Trifilon BioLite®
• PP/PE blend - Terralene® PP 2509 and
• In biocomposite form, like Terratek®, etc.
   

Comparing the main types of PP

Each type of PP offers unique advantages depending on the required balance of stiffness, impact strength, and processing ease. The following table highlights how their core properties differ to guide your material selection.

Selecting the right polypropylene type

Finding the perfect PP grade matching your requirements is always critical. But before making the optimal material selection, you should be aware of:
 

• Material properties that the end product should display
• Manufacturing method as per the intended application, and
• Cost requirements

Get your hands on our advanced Master Catalog, which has new and powerful filters that will make your product selection faster & precise than before. Start your selection journey now!

Getting the above parameters right early in the design process will save time. It will also ensure that the selected PP type meets all the needs of the specific applications. 

Still unsure how to find the right grades quickly and efficiently in our Master Catalog? Watch this video where we guide you to step-by-step explore, filter, and compare polypropylene grades on our platform:

While various types of polypropylene have an impressive array of properties, it also comes with some inherent challenges to consider. It is essential to make you aware of these factors to make informed decisions in selecting the right material for specific needs. 

  

Embracing some features allows for strategic design choices and fosters a continuous pursuit of advancements in polypropylene technology.

 

Factors to consider by PP manufacturers

You must consider certain aspects that may be perceived as challenges. Some of them are:
 

• Poor resistance to UV, impact, and scratches
• Embrittlement 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.
• Poor paint adhesion

However, PP is gradually optimized for its performance by improving its properties using different additives. Let's understand the role of additives in improving PP properties in detail.

 

How do additives help improve PP properties?

You can overcome the above-listed challenges by using polymer additives. They can also improve PP's physical and/or mechanical properties. 

For example, PP has poor resistance to UV. The addition of hindered amines to PP provides light stabilization. This enhances the service life as compared to unmodified PP.

Some more additives that are used to improve the properties of PP are mentioned below:
 

Further, fillers and reinforcers achieve significant properties related to processing and end-use application. Check out filled or reinforced options to choose the grade of your choice:
 

• Glass-fiber reinforced PP grades
• Mineral-filled PP grades
• Calcium carbonate-filled PP grades
• Carbon-fiber reinforced PP grades

Moreover, self-reinforced polypropylene composites benefit from several general advantages such as:
 

• mono-material concept,
• low density,
• good mechanical properties,
• high impact resistance, and
• weight savings

Thanks to the combination of low densities and good mechanical properties. It results in up to 50% potential weight savings over glass-reinforced parts. It is easy to recycle. 

There have been significant developments seen for self-reinforced PP. These advances bridge the gap between isotropic polymers and glass-reinforced materials. The self-reinforced PP offers a unique combination of processing and performance features. Learn more about the benefits and application scope now »

Natural fiber-reinforced polypropylene options are an interesting step towards cheap, sustainable composites. Low densities lead to noticeable cost savings and weight savings. This is up to 27% over glass fiber or talc-reinforced polypropylene.

The use of new additives, polymerization processes, and blending solutions significantly increases PP performance. Hence, today, PP is less seen as a low-cost solution but much more as a high-performance material. It competes with traditional engineering plastics and metals. 

 

Comparison of PP-based compounds 

Detailed comparison of polypropylene-based compounds

Now that we have explored how to enhance PP properties, let’s shift focus to its processing methods and the kind of final products they help produce.

Polypropylene can be processed by virtually all processing methods. The most typical processing methods include:

 

Injection molding with PP

The processing conditions for PP injection molding include:
 

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

 
Expanded polypropylene (EPP) may be molded in a specialized process. EPP is an ideal material for the injection molding process. It is majorly used for batch and continuous production. 

Select 9880+ PP grades suitable for injection molding from our master catalog »

 

Extrusion with PP

PP can be extruded into tubes, blow and cast films, cables, etc. The processing conditions for PP extrusion include:
 

1. Melt temperature: 200-300°C
2. Compression ratio: 3:1
3. Cylinder temperatures: 180-205°C
4. Pre-drying: No, 3 hours at 105-110°C (221-230°F) for regrind

Select 2480+ commercial PP grades from our platform suitable for extrusion »
 

Polypropylene film is among the leading materials today. It is used for flexible packaging as well as industrial applications. Depending on the extrusion technique, two main polypropylene film types are produced. These are described below:

 

Cast polypropylene film (CPP)

CPP is widely known for its versatility. 
 

• Super resistance to tears & puncture
• Greater transparency
• Better heat resistance at high temperatures
• Excellent moisture and atmospheric barriers
• High permeability to water vapor

Biaxially oriented polypropylene film (BOPP)

BOPP is stretched in both transversal and longitudinal directions, producing molecular chain orientation in two directions. 
 

• Orientation increases tensile strength and stiffness
• Good puncture and flex crack resistance over a wide range of temperatures
• Have excellent gloss, high transparency
• Can be glossy, clear, opaque, matte, or metalized
• Efficient barrier against oxygen and moisture