Polypropylene (PP) - Types, Properties, Uses & Structure

What is polypropylene?

Polypropylene (PP) is a type of polyolefin that is slightly harder than polyethylene. It is a commodity plastic with low density and high heat resistance. Its chemical formula is (C₃H₆)n.

Molecular Structure of PP

It finds application in packaging, automotive, consumer good, medical, cast films, etc. Depending on how it is produced and formulated, PP can be:

hard or soft,
opaque or transparent,
light or heavy,
insulating or conductive,
neat or reinforced with cheap mineral fillers, short or long glass fibers, natural fibers or even self-reinforced.

How is PP produced?

It is made from the polymerization of propene monomer. There are two main syntheses to produce polypropylene:

Ziegler-Natta polymerization or
Metallocene catalysis polymerization

Polymerization of propene to polypropylene

Upon polymerization, PP can form three basic chain structures. They depend 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

Basic Chain Structures of PP

What are the properties of PP?

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

Melting Point - The melting point of PP occurs at a range.
- Homopolymer: 160 - 165°C
- Copolymer: 135 - 159°C

Density - PP is one of the lightest polymers among all commodity plastics. This feature makes it a 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

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

Flammability: PP is a highly flammable material.

PP retains mechanical & electrical properties at elevated temperatures. This occurs in humid conditions and when submerged in water. It is a water-repellent plastic.

PP has good resistance to environmental stress cracking.

It is sensitive to microbial attacks, such as bacteria and mold.

It exhibits good resistance to steam sterilization.

Learn more about all polypropylene properties and their values - ranging from mechanical and electrical to chemical properties; and make the right selection for your application.

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. Main applications 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.

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

While in 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 thermoplastic tougher and less brittle than the random copolymer. These polymers are suitable for applications requiring high strength, such as industrial usage.

Polypropylene, Impact Copolymer

Propylene Homopolymer 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:

higher strength-to-weight ratio,
excellent impact resistance,
thermal insulation,
chemical and water 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 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. 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%. There are several suppliers offering pure PP bio-based grades such as:

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

How to decide between PP homopolymer vs. PP copolymer?

PP Homopolymer	PP Copolymer
High strength to weight ratio and stiffer & stronger than copolymer Good chemical resistance and weldability Good processability Good impact resistance Good stiffness Food contact acceptable Suitable for corrosion resistant structures	Bit softer but has better impact strength; tougher and more durable than homopolymer Better stress crack resistance and low temperature toughness High processability High impact resistance High toughness Not preferable for food contact applications

The potential applications for 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.

How to compare between the main PP types?

Property	PP, Impact Copolymer	PP Copolymer	PP Homopolymer
Density, g/cm³	0.9	0.9	0.9
Shore hardness, D	45-55	70-80	70-83
Stress at yield, MPa	11-28	20-35	35-40
Elongation at break, %	20-700	200-600	15-600
Tensile modulus, GPa	0.4-1	1-1.2	1.1-1.6
Notched impact strength ASTM D256, J/m	110-No Break	60-500	20-60
HDT A(1.8 MPa), °C	46-57	50-60	50-60
Minimum service temperature, °C	-40 to -20	-20 to -10	-20 to -10
UL94 fire rating	HB	HB	HB
Get more information about polymer properties here »

Detailed Property Comparision: PP Impact Copolymer, PP Copolymer and PP Homopolymer

What are the disadvantages of PP?

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 Here »
Poor paint adhesion

However, polypropylene is gradually optimized for its performance by improving its properties using different additives.

How additives help improve PP properties?

The addition of polymer additives can overcome the above-listed disadvantages. 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. Looking for UV stabilized grade? Here’s the complete list for you »

Some additives include:

Clarifiers
Minerals

Conductive fillers
Lubricants

Flame retardants
Pigments, and others

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:

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

monomaterial 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 applications scope now »

Natural fiber-reinforced polypropylene options are an interesting step toward 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. Looking for grades reinforced with Biofillers? Here’s the complete list for you »

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.

How to compare between PP derivatives?

Property	Thermoplastic Polyolefin	Talc Filled PP	Glass Filled PP	Long Fiber Reinforced Thermoplastic	Self-Reinforced PP
Density, g/cm³	0.9-1	0.97-1.25	0.97-1.25	1.2	0.8-0.9
Shore hardness, D	10-99	75-85	70-88	-	-
Rockwell hardness, M	-	10-45	-	-	-
Stress at yield, MPa	-	22-28	19-70	-	-
Elongation at break, %	450-850	20-30	2-30	2	-
Tensile modulus, GPa	-	1.5-3.5	1-10	4-8	4-14
Notched impact strength ASTM D256, J/m	110-No Break	30-200	38-160	-	-
HDT A(1.8 MPa), °C	-	56-75	50-140	160	-
Minimum service temperature, °C	-40 to -20	-20 to -5	-30 to -5	-	-
UL94 fire rating	HB	HB	HB	HB	HB
Get more information about polymer properties here »

What are the forms of PP films?

PP film is among the leading materials today. Used for flexible packaging as well as industrial applications. Two important forms of polypropylene films include:

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 wide range of temperatures
Have excellent gloss, high transparency
Can be glossy, clear, opaque, matte or metalized
Efficient barrier against oxygen and moisture

How to process PP?

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

Injection Molding with PP

The processing conditions for PP injection molding include:

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%. It depends on:
- processing conditions,
- rheology of the polymer, and
- thickness of the final piece

How to Reduce PP Cycle Time (+ Avoid Shrinkage & Warpage)

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

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

View all PP Grades Suitable for Injection Molding »

Extrusion with PP

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

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

View all PP Grades Suitable for Extrusion »

3D Printing with PP

As a tough, fatigue-resistant and durable polymer, PP is ideal for low-strength applications. It is currently difficult to use PP for 3D Printing processes due to its:

semi-crystalline structure and
heavy warping

Several manufacturers have optimized PP properties or even created blends with improved toughness. This makes it suitable for 3D Printing applications. Hence, it is recommended to thoroughly refer to the documentation provided by the supplier for printing temperature, printing bed, etc.

Polypropylene is suitable for:

Complex models
Prototypes
Small series of components, and
Functional models

View all PP Grades Suitable for 3D Printing »

Other processing methods for PP

Blow molding
Compression molding
Rotational molding
Injection blow molding
Extrusion Blow Molding
Injection stretch blow molding
General-Purpose Extrusion

Some grades are designed for your specific conversion mode like blow molding, compression molding, thermoforming, etc. Check out PP grades with various conversion mode here »

TIP: To meet specific requirements, try using the “Conversion mode” facet to narrow down your search.

Is PP recyclable?

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.

Polypropylene recycling process

The PP recycling process includes:

Melting waste plastic to 250°C to get rid of contaminants.
Removal of residual molecules under vacuum and solidification at approximately 140°C.

This recycled PP can be blended with virgin PP at a rate of 50%.

PP recycling – The main challenge

The main challenge in PP recycling is related to the amount consumed. Currently, 1% of PP bottles are recycled as compared to the 98% recycling rate of PET & HDPE bottles together.

Uses of recycled polypropylene (rPP)

A few applications of rPP include:

Automobile battery cases,
Signal lights,
Battery cables,
Brooms,
Brushes,
Ice scrapers, etc.

Meet the urgent demands for greener polypropylene products (lighter, recyclable, high-performance PCR grades...) with beta nucleation to gain an edge over your competition. Take this exclusive course by Industry Expert Dr. Philip Jacoby.

Beta Nucleation for PP (Cheaper, Lighter & Recyclable)

Is PP safe?

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

How PP differs from polyethylene (PE)?

Both PP and PE are very similar, but they also have characteristics that are unique to each other. These features can be maximized depending on:

how they are made and
which application they are used for.

PP is the second most used plastic after polyethylene. They both can be designed to be durable and lightweight. These polymers are used in many applications such as bottles and gloves. But PP has crystal-clear transparency than PE. PP has excellent mechanical properties. It has high resistance to fatigue, impact, heat and, freezing.

PE is tough yet light, with good resistance to impact and abrasion. PP is harder and can be used for mechanical and structural applications.

They both are highly malleable and have relatively similar impact resistance. This means strength does not have to be a concern when these plastics are in use. Density is another key factor that differentiates PE from PP. Polypropylene’s density is fixed and only varies when it is filled. Polyethylene selection is highly dependent on the varying densities of multiple versions. PE is available in low, medium, and high-density grades. High Density Polyethylene is known for its big strength-to-density ratio. HDPE is considered more rigid than PP.

Here is a quick wrap-up of their main differences:

Polypropylene	Polyethylene
Propylene monomers make PP It can be produced optically clear PP exhibits a high resistance to cracking, acids, organic solvents and electrolytes It has high melting point and good dielectric properties PP is more rigid than polyethylene	Ethylene monomer make PE It can only be made translucent like a milk jug PE stand up better in cold temperatures It is a good electrical insulator and offers good tracking resistance PE is sturdy as compared to PP
PP Grades	PE Grades

If PE is more suited for your needs, find out more about the polymer here »

How PP challenges polyethylene terephthalate (PET)?

In blow-molded bottle applications, PP has emerged as a strong competitor to PET. Compared to PET, the key features of PP include:

Less expensive,
Lighter in weight,
More resistant to the high temperatures of hot filling, and
Less permeable to moisture.

PP bottles can be hot-filled at temperatures up to 100°C. While PET cannot withstand filling temperatures above 76°C.

In its pure state, PP is less clear than PET. Also, the gas barrier properties of PP are not as high as PET, and PP also falls below PET in stiffness. PP has about five times the moisture barrier properties of PET. But PP is about 30 times more permeable than PET to gases such as oxygen and carbon dioxide.

The production cycles of PP bottles have generally been longer than PET bottles. PP also has a narrower range of processing temperatures than PET.

But producers of PP resins and the additives that go into them are making important strides in overcoming these deficits, such as:

Inserting clarifiers into PP enables it to match PET's transparency. Clarified grades of PP have a clarity and gloss comparable to PET.

Nucleators can speed up crystal formation in PP during cooling. This thereby shortens cycle times, and sometimes also improves clarity as well. Explore several nucleated PP grades here »

Barrier layers (EVOH sandwich, coatings, etc.) enable PP to compete on a cost basis with both glass and PET containers in many food and beverage applications while offering good barrier properties.

The above steps make PP competitive with PET and can narrow that cost advantage considerably. But PP still comes out as the more economical packaging choice than PET for many applications.

Learn more about PET before you make final decision on material »

How nylon differs from PP?

Polyamide and polypropylene vary in their individual structures. Both these polymers offer superior strength to the final parts. There are some significant distinctions you should consider choosing any of them.

The key benefit of PP is its low melt viscosity, and it offers strength and elasticity. A low melt viscosity enables materials to be easily used in injection molding. It also opens more opportunities and capabilities.

While Nylon is more heat-resistant than PP. And nylon is more malleable than PP. It can offer designers greater design flexibility i.e., easy bending than breaking.

From the end application perspective, both polymers have poor UV stability performance. They require suitable additives to mitigate UV damage risk.

If Nylon is more suitable for your needs, find out more about the polymer here »

What are the commercially available PP grades?

View a wide range of polypropylene grades available today, analyze technical data of each product, get technical assistance or request samples.

Comprehensive Guide on Polypropylene (PP)