What is polypropylene?
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. It finds application in packaging, automotive, consumer goods, medical, cast films, etc. Its chemical formula is (C3H6)n.
Molecular Structure of PP
Polypropylene – Manufacturing & Properties
Polypropylene – Manufacturing & Properties
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
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
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.
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 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 dilute 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.
PP – Main Types & Their Comparison
PP – Main Types & Their Comparison
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.
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.
| 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
|
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%. Several suppliers are offering pure PP bio-based grades such as:
How to compare 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 Comparison: PP Impact Copolymer, PP Copolymer and PP Homopolymer
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
Browse the largest online database to select your PP grades
Getting the above parameters right early in the design process will save time. It will also ensure that the selected PP type meets all needs of the specific applications. Make the right polypropylene grade selection from our database.
While polypropylene has an impressive array of properties it also comes with some inherent weaknesses to consider. It’s essential to make you aware of these factors to make informed decisions in selecting the right material for specific needs.
Unraveling PP Formulation Challenges
Unraveling PP Formulation Challenges
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
- 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 the Video Here »
- Poor paint adhesion
However, polypropylene is gradually optimized for its performance by improving its properties using different additives.
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. Are you looking for UV-stabilized grades? Here’s the complete list for you »
Some additives include:
|
|
- 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:
-
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 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. Are you 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 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?
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 a 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?
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 the 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
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.
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 »
Discover other processing methods for PP
Some grades are designed for your specific conversion mode like blow molding, compression molding, thermoforming, etc. Check out PP grades with various conversion modes here »
TIP: To meet specific requirements, try using the “Conversion mode” facet to narrow down your search.
Sustainability Aspect of PP
Sustainability Aspect of PP
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. View recyclable PP grades in our database.
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.
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.
Polypropylene vs. Other Polymers
Polypropylene vs. Other Polymers
PP vs. PE – Key Differences You Should Know
Though PP and PE are similar, they also have some unique characteristics. These features can be optimized depending on their manufacturing process and intended application.
Polypropylene is the second most used plastic after polyethylene. Find below what advantages does it offers over PE:
- PP has crystal-clear transparency than PE.
- PP has excellent mechanical properties. It has high resistance to fatigue, impact, heat, and freezing.
- PP is harder and can be used for mechanical and structural applications.
- 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. It is considered more rigid than PP.
- PE is tough yet light, with good resistance to impact and abrasion.
Quick Wrap of Their Main Differences
| Polypropylene |
Polyethylene |
- Propylene monomers make PP
- It can be produced optically clear
- PP exhibits high resistance to cracking, acids, organic solvents, and electrolytes
- It has a 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 |
Are you still struggling to choose between the two? Let’s have a look at some of their distinct features »
Common Features
PP & PE can be designed to be durable and lightweight. These polymers are used in many applications such as bottles and gloves. They 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.
How does PP challenge polyethylene terephthalate?
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.
Get a comprehensive review about PET before you make the final decision on the material of your choice »
How is nylon different from PP?
Nylon and polypropylene vary in their chemical structures. Both these polymers offer superior strength to the final parts. There are some significant distinctions you should consider when 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 and 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.
Find out more about nylon, if it is more suitable for your needs »
What are the commercially available PP grades?
View a wide range of polypropylene grades available in our database. Analyze the technical data of each product, get technical assistance, or request samples.
Key Properties
Key Properties
|
| Property |
POLYPROPYLENE |
| Chemical Resistance |
| Acetone @ 100%, 20°C |
Satisfactory |
| Ammonium hydroxide @ 30%, 20°C |
Satisfactory |
| Ammonium hydroxide @ diluted, 20°C |
Satisfactory |
| Aromatic hydrocarbons @ 20°C |
Non Satisfactory |
| Aromatic hydrocarbons @ hot conditions |
Non Satisfactory |
| Benzene @ 100%, 20°C |
Limited |
| Butylacetate @ 100%, 60°C |
Non Satisfactory |
| Butylacetate @ 100%, 20°C |
Limited |
| Chlorinated solvents @ 20°C |
Non Satisfactory |
| Chloroform @ 20°C |
Limited |
| Dioctylphtalate @ 100%, 20°C |
Satisfactory |
| Dioctylphtalate @ 100%, 60°C |
Limited |
| Ethanol @ 96%, 20°C |
Satisfactory |
| Ethyleneglycol (Ethane diol) @ 100%, 100°C |
Satisfactory |
| Ethyleneglycol (Ethane diol) @ 100%, 20°C |
Satisfactory |
| Ethyleneglycol (Ethane diol) @ 100%, 50°C |
Satisfactory |
| Glycerol @ 100%, 20°C |
Satisfactory |
| Hydrogen peroxide @ 30%, 60°C |
Limited |
| Kerosene @ 20°C |
Limited |
| Methanol @ 100%, 20°C |
Satisfactory |
| Methylethyl ketone @ 100%, 20°C |
Satisfactory |
| Mineral oil @ 20°C |
Satisfactory |
| Phenol @ 20°C |
Satisfactory |
| Silicone oil @ 20°C |
Satisfactory |
| Sodium hydroxide @ <40%, 60°C |
Satisfactory |
| Sodium hydroxide @ 10%, 20°C |
Satisfactory |
| Sodium hydroxide @ 10%, 60°C |
Satisfactory |
| Sodium hypochlorite @ 20%, 20°C |
Satisfactory |
| Strong acids @ concentrated, 20°C |
Satisfactory |
| Toluene @ 20°C |
Limited |
| Toluene @ 60°C |
Non Satisfactory |
| Xylene @ 20°C |
Non Satisfactory |
| Electrical |
| Arc Resistance, sec |
135 - 180 |
| Dielectric Constant |
2.3 |
| Dielectric Strength, kV/mm |
20 - 28 |
| Dissipation Factor x 10-4 |
3 - 5 |
| Volume Resistivity x 1015, Ohm.cm |
16 - 18 |
| Mechanical |
| Elongation at Break, % |
150 - 600 |
| Flexural Modulus (Stiffness), Gpa |
1.2 - 1.6 |
| Hardness Rockwell M |
1 - 30 |
| Hardness Shore D |
70 - 83 |
| Tensile Strength at Break, MPa |
20 - 40 |
| Tensile Strength at Yield, MPa |
35 - 40 |
| Toughness at Low Temperature, J/m |
27 - 107 |
| Toughness, J/m |
20 - 60 |
| Young's Modulus, GPa |
1.1 - 1.6 |
| Optical |
| Gloss, % |
75 - 90 |
| Haze, % |
11 |
| Transparency (Visible Light Transmission), % |
85 - 90 |
| Physical |
| Density, g/cm3 |
0.9 - 0.91 |
| Gamma Radiation Resistance |
Poor |
| Glass Transition Temperature, °C |
-10 |
| Shrinkage, % |
1 - 3 |
| Sterilization Resistance (Repeated) |
Poor |
| UV Light Resistance |
Fair |
| Water Absorption 24 hours, % |
0.01 - 0.1 |
| Service Temperature |
| Ductile / Brittle Transition Temperature, °C |
-20 - -10 |
| HDT @0.46 Mpa (67 psi), °C |
100 - 120 |
| HDT @1.8 Mpa (264 psi), °C |
50 - 60 |
| Max Continuous Service Temperature, °C |
100 - 130 |
| Min Continuous Service Temperature, °C |
-20 - -10 |
| Thermal |
| Coefficient of Linear Thermal Expansion x 10-5, /°C |
6 - 17 |
| Fire Resistance (LOI) |
17 - 18 |
| Flammability, UL94 |
HB |
| Thermal Insulation, W/m.K |
0.15 - 0.21 |