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Comprehensive Guide on Polypropylene (PP)

Polypropylene (PP) is a tough, rigid, and crystalline thermoplastic. It is made from propene (or propylene) monomer. This linear hydrocarbon resin is the lightest polymer among all commodity plastics. PP comes either as homopolymer or as copolymer and can be greatly boosted with additives.



PP has become a material of choice, especially when you are looking for a polymer with superior strength (e.g., vs Polyamide) in engineering applications or simply looking for cost advantage in blow molding bottles (vs. PET).

Overview

What Does Polypropylene Mean?

What Does Polypropylene Mean?

Polypropylene is a type of polyolefin which is slightly harder than polyethylene. It is a commodity plastic with low density and high heat resistance. Its chemical formula is (C3H6)n. It finds application in packaging, automotive, consumer good, medical, cast films, etc.
Molecular Structure of Polypropylene
Molecular Structure of Polypropylene


Facts to Know Polypropylene was first polymerized by German chemist named Karl Rehn and an Italian chemist named Giulio Natta to a crystalline isotactic polymer in 1954. This discovery soon led to a large-scale production of polypropylene starting in 1957 by the Italian firm Montecatini.

Syndiotactic polypropylene was also first synthesized by Natta and his coworkers.

Depending how it is produced and formulated, polypropylene 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?

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 Reaction of Propene Monomer to Polypropylene Polymer




Upon polymerization, PP can form three basic chain structures depending 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
Basic Chain Structures of PP




Types of Polypropylene & How to Select Them?

Types of Polypropylene & How to Select Them?

Homopolymers and Copolymers are the two major types of polypropylene available in the market.

  • Polypropylene 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 family is further divided into random copolymers and block copolymers produced by polymerizing of propene and ethane:

    1. Polypropylene Random Copolymer is produced by polymerizing together ethene and propene. It features Ethene units, usually up to 6% by mass, incorporated randomly in the polypropylene chains. These polymers are flexible and optically clear making them suitable for applications requiring transparency and for products requiring an excellent appearance.

    2. While in Polypropylene Block Copolymer, ethene content is larger (between 5 and 15%). It has co-monomer units arranged in regular pattern (or blocks). The regular pattern hence makes thermoplastic tougher and less brittle than the random co-polymer. These polymers are suitable for applications requiring high strength, such as industrial usages.

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Polypropylene, Impact Copolymer – Propylene Homopolymer containing a co-mixed Propylene Random Copolymer phase which has an ethylene content of 45-65% is referred to PP impact copolymer. It is useful in parts which require good impact resistance. Impact copolymers are mainly used in packaging, houseware, film, and pipe applications, as well as in the automotive and electrical segments.

Property

TPO

Impact

Coa

Homob

Talc

GFc

LFRTd

SRPPe

Density, g/cm3

0.9-1

0.9

0.9

0.9

0.97-1.25

0.97-1.25

1.2

0.8-0.9

Shore hardness, D

10-99

45-55

70-80

70-83

75-85

70-88

Rockwell hardness, M

10-45

Stress at yield, MPa

11-28

20-35

35-40

22-28

19-70

Elongation at break, %

450-850

20-700

200-600

15-600

20-30

2-30

2

Tensile modulus, GPa

0.4-1

1-1.2

1.1-1.6

1.5-3.5

1-10

4-8

4-14

Notched impact strength ASTM D256, J/m

110-NB

110-NB

60-500

20-60

30-200

38-160

HDT A(1.8 MPa), °C

46-57

50-60

50-60

56-75

50-140

160

Minimum service temperature, °C

-40 to -20

-40 to -20

-20 to -10

-20 to -10

-20 to -5

-30 to -5

UL94 fire rating

HB

HB

HB

HB

HB

HB

HB

HB

Coa - Copolymer
Homob - Homopolymer
GFc - Glass Filled
LFRTd - Long Fiber Reinforced Thermoplastic
SRPPe - Self Reinforced PolyPropylene
TPO - Thermoplastic PolyOlefin
NB - No Break
Main characteristic examples of various PP derivatives

Expanded Polypropylene – It is a closed-cell bead foam with ultra-low density. EPP is used to produce three-dimensional polymer foam products. EPP bead foam has higher strength to weight ratio, excellent impact resistance, thermal insulation, and chemical and water resistance. EPP is used in various applications ranging from automobiles to packaging, from construction products to consumer goods and more.

Polypropylene Terpolymer – It is composed by propylene segments joined by monomers ethylene and butane (co-monomer) which appear randomly throughout the polymer chain. PP terpolymer has better transparency than PP homo. Also, the incorporation of co-monomers reduces crystalline uniformity in the polymer making it suitable for sealing film applications.

Polypropylene, High Melt Strength (HMS PP) – It is a long chain branched material, which combines both high melt strength and extensibility in the melt phase. PP HMS grades have a wide mechanical property range, high heat stability, good chemical resistance. HMS PP is widely used to produce soft, low density foams for food packaging applications as well as used in automotive and construction industries.

Bio-based Polypropylene – It is bio-based version of PP where its monomer bio-based 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.

PP Homopolymer vs. Copolymer – How to decide between the two?


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.

Get Access to All Polypropylene Grades Here »
Analyze technical data of each product, get technical assistance or request samples to test them in your application.

Material Properties of Polypropylene

Material Properties of Polypropylene

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:

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

  2. Density of Polypropylene - 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

  3. Polypropylene 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

  4. Flammability: Polypropylene is a highly flammable material

  5. PP retains mechanical & electrical properties at elevated temperatures, in humid conditions and when submersed in water. It is a water-repellent plastic

  6. PP has good resistance to environmental stress cracking

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

  8. 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.

Disadvantages of Polypropylene

  • 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. See how additives help improve PP properties.

How Additives Help Improve PP Properties?

How Additives Help Improve PP Properties?

Commercially available polypropylene grades include un-reinforced to glass reinforced, flame retardant and high melt strength materials for various commodity as well as engineering applications.

Addition of polymer additives like clarifiers, flame retardants, glass fibers, minerals, conductive fillers, lubricants, pigments and many other additives can further improve PP physical and/or mechanical properties overcoming above listed disadvantages. For example, PP has poor resistance to UV, hence additives like hindered amines provide light stabilization and enhances the service life as compared to unmodified polypropylene.

Looking for UV stabilized grade? Here’s the complete list for you »

Further, fillers (clays, talc, calcium carbonate…) and reinforcements (glass fiber, carbon fiber…) are added to achieve significant properties related to processing and end-use application.

Moreover, self-reinforced polypropylene composites benefit from several general advantages such as the monomaterial concept, low density, good or excellent mechanical properties with high impact resistance, weight savings thanks to the combination of low densities and good mechanical properties (up to 50% potential weight savings over equivalent glass reinforced parts) and easy recycling.

There have been significant developments seen for self-reinforced PP to bridge the gap between isotropic polymers and glass-reinforced materials and 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 up to 27% over glass fiber or talc reinforced polypropylene.

Looking for grades reinforced with Biofillers? Here’s the complete list for you »

Check out filled or reinforced options available today to choose the grade of your choice:

Furthermore, development & the use of new additives, latest polymerization processes as well as blending solutions significantly increase polypropylene performance. Hence, today PP is less seen as a low-cost solution, but much more as a high-performance material competing with traditional engineering plastics and sometime event metal (e.g., long glass fiber reinforced PP grades).


Usefulness of Polypropylene Films

Usefulness of Polypropylene 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


Cast polypropylene commonly known as CPP and widely known for its versatility.

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

Biaxially Oriented Polypropylene Film


Biaxially oriented polypropylene film (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 and high transparency can be glossy, clear, opaque, matte or metalized
  • Efficient barrier against oxygen and moisture

Don’t think PP matches your film needs, find out if PE films are more suited for you »


Polypropylene Processing – Everything You Need to Know About It

Polypropylene Processing – Everything You Need to Know About It

Polypropylene can be processed virtually by all processing methods. The most typical processing methods include: Injection Molding, Extrusion, Blow Molding and General-Purpose Extrusion.

  1. Injection Molding
    • 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%, depending on processing conditions, rheology of the polymer and thickness of the final piece

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

  2. Extrusion (tubes, blow and cast films, cables, etc.)
    • 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

  3. Blow molding
  4. Compression molding
  5. Rotational molding
  6. Injection blow molding
  7. Extrusion Blow Molding
  8. Injection stretch blow molding
  9. General-Purpose Extrusion

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

3D Printing with Polypropylene


As a tough, fatigue resistant and durable polymer, PP is ideal for low strength applications. Due to its semi-crystalline structure and heavy warping, it is currently difficult to use polypropylene for 3D Printing processes.

Today, several manufacturers have optimized PP properties or even created blends with improved toughness making it suitable for 3D Printing applications. Hence, it is recommended to thoroughly refer to documentation provided by supplier for printing temperature, printing bed, etc., while 3D Printing with polypropylene...View all PP Grades Suitable for 3D Printing »

Polypropylene is suitable for:
  • Complex models
  • Prototypes
  • Small series of components, and
  • Functional models

Key Areas of Applications of Polypropylene

Key Areas of Applications of Polypropylene

Polypropylene is widely used in various applications due to its good chemical resistance and weldability. Today, it is at the point of junction between cheap commodity plastics and more or less performing engineering plastics such as polyamides (PA6, PA66), POM, PBT and transparent resins.

Some of the popular applications showcasing PP versatility are listed below.

  1. Packaging Applications: Good barrier properties, high strength, good surface finish and low cost make Polypropylene ideal for several packaging applications. Polypropylenes perform well in blow molded and sheet thermoformed products for food, personal care, health, medical and labware applications, household chemicals and beauty aid products.

    • Flexible Packaging: PP films’ excellent optical clarity and low moisture-vapor transmission make it suitable for use in food packaging. Other markets shrink-film overwrap, electronic industry films, graphic arts applications, disposable diaper tabs and closures, etc. PP grades are used to produce oriented, bi-oriented and cast films and foils.
    • Rigid Packaging: PP is blow molded to produce crates, bottles, and pots. PP thin-walled containers are commonly used for food packaging.

  2. Consumer Goods: Polypropylene is used in several household products and consumer goods applications including translucent parts, housewares, furniture, appliances, luggage, toys etc.

  3. Automotive Applications: Due to its low cost, outstanding mechanical properties and moldability, polypropylene is widely used in automotive parts. Main applications include battery cases and trays, bumpers, fender liners, interior trim, instrumental panels and door trims. Other key features of automotive applications of PP include low coefficient of linear thermal expansion and specific gravity, high chemical resistance and good weatherability, processability and impact/stiffness balance.

  4. Fibers and Fabrics: A large volume of PP utilized in the market segment known as fibers and fabrics. PP fiber is utilized in a host of applications including raffia/slit-film, tape, strapping, bulk continuous filament, staple fibers, spun bond and continuous filament. PP rope and twine are very strong and moisture resistant very suitable for marine applications.

  5. Medical Applications: Polypropylene is used in various medical applications due to high chemical and bacterial resistance. Also, the medical grade PP exhibits good resistance to steam sterilization. Disposable syringes is the most common medical application of polypropylene. Other applications include medical vials, diagnostic devices, petri dishes, intravenous bottles, specimen bottles, food trays, pans, pill containers, etc.

  6. Industrial Applications: Polypropylene sheets are widely used in industrial sector to produce acid and chemical tanks, sheets, pipes, Returnable Transport Packaging (RTP), etc. because of its properties like high tensile strength, resistance to high temperatures and corrosion resistance.

(Click on application name to access suitable commercial grades and choose the one best suited to your needs)
How to Recycle PP?

How to Recycle PP?

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

PP is 100% recyclable. Automobile battery cases, signal lights, battery cables, brooms, brushes, ice scrapers, etc., are few examples which can be made from recycled polypropylene (rPP).

The PP recycling process mainly includes melting of waste plastic to 250°C to get rid of contaminants followed by removal of residual molecules under vacuum and solidification at nearly 140°C. This recycled PP can be blended with virgin PP at a rate upto 50%. The main challenge in PP recycling is related to its amount consumed – currently nearly 1% PP bottles are recycled as compared to 98% recycling rate of PET & HDPE bottles together.

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

Beta Nucleation for PP (Cheaper, Lighter & Recyclable)


Find Suitable Polypropylene Grades

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

Key Applications

Key Properties

Suppliers

Brands

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