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A Definite Guide to Polypropylene (PP)

The Definitive Guide to Polypropylene (PP)

Polypropylene (PP) is a rigid and crystalline thermoplastic used widely in everyday objects like packaging trays, household products, battery cases, medical devices, etc. Explore this comprehensive guide and learn everything you need to know about this widely used thermoplastic.

Overview

What is Polypropylene and What It's Used for?

What is Polypropylene and What It's Used for?

Polypropylene is a tough, rigid and crystalline thermoplastic produced from propene (or propylene) monomer. It is a linear hydrocarbon resin. The chemical formula of polypropylene is (C3H6)n. PP is among the cheapest plastics available today.


Molecular Structure of Polypropylene

Molecular Structure of Polypropylene


PP belongs to polyolefin family of polymers and is one of the top three widely used polymers today. Polypropylene has applications both as a plastic and a fiber in:

  • Automotive Industry 
  • Industrial Applications
  • Consumer Goods, and
  • Furniture Market

It has the lowest density among commodity plastics.

Facts to KnowPolypropylene 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.


How to Produce Polypropylene?

How to Produce Polypropylene?

These days, polypropylene is made from polymerization of propene monomer (an unsaturated organic compound - chemical formula C3H6) by:

  • Ziegler-Natta polymerization or
  • Metallocene catalysis polymerization

Structure of PP Monomer
Structure of PP Monomer
C3H6
Ziegler-Natta Polymerization
Arrow
Or Metallocene Catalysis
Structure of Polypropylene
Structure of Polypropylene
(C3H6)n


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

Types of Polypropylene


Types of Polypropylene & their Benefits

Types of Polypropylene & their Benefits

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 of 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.

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.

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.

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

This is because of their extensively shared properties. As a result, the choice between these two materials is often made based on non-technical criteria.


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
  • Poor paint adhesion


How additives help improve PP properties?

How additives help improve PP properties?

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. For example:

Search PP Grades Based on
Their Reinforcement Method


Search PP Grades Based on Their Reinforcement Method

PP has poor resistance to UV, hence additives like hindered amines provide light stabilization and enhances the service life as compared to unmodified polypropylene.

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.

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


PP vs. PE – Selecting the Suitable Polymer

PP vs. PE – Selecting the Suitable Polymer

Though Polyethylene and Polypropylene are similar in physical properties but here are key points to consider to select the polymer suitable to your needs.

Polypropylene Polyethylene
  • Monomer of polypropylene is propylene
  • It can be produced optically clear
  • It is lighter in weight
  • PP exhibits a high resistance to cracking, acids, organic solvents and electrolytes
  • It has high melting point and good dielectric properties 
  • PP is non-toxic
  • It is stiffer and resistant to chemicals and organic solvents compared to polyethylene
  • PP is more rigid than polyethylene
  • Monomer of polyethylene is ethylene
  • Polyethylene can only be made translucent like a milk jug
  • Its physical properties allow it to stand up better in cold temperatures, particularly when using it as signs
  • It is a good electrical insulator
  • PE offers good tracking resistance
  • Polyethylene is sturdy as compared to Polypropylene
   » View all PP Commercial Grades    » View all PE Commercial Grades

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

  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


(Credit: FormFutura)


Is PP toxic? How to recycle PP?

Is PP toxic? 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.

Key Applications

Key Properties

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