Polypropylene: How to select the right grade?

What is polypropylene plastic?

Polypropylene (PP) plastic is a type of polyolefin that is slightly harder than polyethylene. PP is a commodity plastic with low density and high heat resistance. It finds application in packagingautomotiveconsumer goodsmedicalcast filmsetc. Its chemical formula is (C₃H₆)_n. 
 

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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 packagingtextileshealthcarepipesautomotiveand 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. Alsosuitable for products requiring an excellent appearance.
    
2. While in the PP block copolymerethene 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 strengthsuch as industrial usage.

The potential applications of PP homopolymer and PP copolymer are nearly identical. Because of their extensively shared propertiesthe 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.
 

Polypropyleneimpact 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 packaginghousewarefilmand pipe applications. They are also used in the automotive and electrical segments.
 

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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 automobilespackagingconstruction productsconsumer goodsand 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. Alsothe incorporation of co-monomers reduces crystalline uniformity in the polymer. This makes it suitable for sealing film applications.

 

Polypropylenehigh 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 stabilityand
• good chemical resistance

HMS PP is widely used to produce softlow-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 gradessuch as: 
 

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

Comparing the main types of PP

Each type of PP offers unique advantages depending on the required balance of stiffnessimpact strengthand 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 selectionyou should be aware of:
 

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

Get your hands on our advanced Master Catalogwhich 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. Explore our platform with new features that will enable you to compare grades & search graphically for a seamless sample ordering experience - Select from a range of 14310+ polypropylene grades. 
 

While various types of polypropylene have an impressive array of propertiesit 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 UVimpactand scratches
• Embrittlement below -20°C
• Low upper service temperature90 - 120°C
• Attacked by highly oxidizing acidsswell 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

HoweverPP 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 examplePP 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:
 

Furtherfillers 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

Moreoverself-reinforced polypropylene composites benefit from several general advantages such as:
 

• mono-material concept,
• low density,
• good mechanical properties,
• high impact resistanceand
• 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 cheapsustainable 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 additivespolymerization processesand blending solutions significantly increases PP performance. HencetodayPP is less seen as a low-cost solution but much more as a high-performance material. It competes with traditional engineering plastics and metals. 

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Comparison of PP-based compounds 

Detailed comparison of polypropylene-based compounds

Now that we have explored how to enhance PP propertieslet’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 polymerand
   • thickness of the final piece
      

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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 tubesblow and cast filmscablesetc. 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: No3 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 techniquetwo 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 directionsproducing 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 glosshigh transparency
• Can be glossyclearopaquematteor metalized
• Efficient barrier against oxygen and moisture 
   

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. Select 95+ recyclable PP grades listed in our master catalog.

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PP recycling process and the main challenge

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%. 

The main challenge in PP recycling is its low rate of recycling. It is due to the difficulties in collection and sorting of PP. Also the cost of rPP can sometime be higher that virgin polypropylene³.

 

Uses of recycled polypropylene (rPP) and safety profile

A few applications of rPP include: 
 

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 viewin terms of chemical toxicity.

Up nextwe’ll compare polypropylene with other polymers to highlight their similarities and differences so you can confidently choose the right one from our platform.

PP vs. PE - Key differences you should know

Though PP and PE are similarthey 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 it offers over PE:
 

• PP has crystal-clear transparency than PE.
• PP has excellent mechanical properties. It has high resistance to fatigueimpactheatand 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 lowmediumand high-density grades.
• High-density polyethylene is known for its high strength-to-density ratio. It is considered more rigid than PP.
• PE is tough yet lightwith good resistance to impact and abrasion.

Quick wrap of their main differences

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 applicationsPP has emerged as a strong competitor to PET. Compared to PETthe key features of PP include:
 

• Less expensive,
• Lighter in weight,
• More resistant to the high temperatures of hot fillingand
• 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 statePP is less clear than PET. Alsothe gas barrier properties of PP are not as high as PETand 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 deficitssuch 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 timesand sometimes also improves clarity as well.

• Barrier layers (EVOH sandwichcoatingsetc.) 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 viscosityand it offers strength and elasticity. 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 flexibilityi.e.easy bending rather than breaking.

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

Find out more about nylonif it is more suitable for your needs »

Having reviewed how PP differs from other polymerswe will now learn its end applications.

Polypropylene is widely used in various applications. Howeverchoosing from a wide range of options can be difficult. Each PP type offers a range of benefitsfrom good chemical resistance to weldability. Todayit stands at the junction between inexpensive commodity plastics and more or less performing engineering plastics. 

Our Master Catalog with over 14310+ PP grades can ease your material selection with its smart comparison and sample ordering features. Some of the popular applications showcasing PP's versatility are listed below.

Packaging industry

Good barrier propertieshigh strengthgood surface finishand low cost make polypropylene ideal for several packaging applications. Polypropylene performs well in blow-molded and sheet thermoformed products for foodpersonal carehealthmedical and labwarehousehold chemicalsand beauty aid products.
 

• Flexible packaging: PP films' excellent optical clarity and low moisture-vapor transmission make them suitable for use in food packaging. Other markets include shrink-film overwrapelectronic industry filmsgraphic artsdisposable diaper tabs and closuresetc. PP grades are used to produce orientedbi-orientedand cast films and foils.
• Rigid packaging: PP is blow-molded to produce cratesbottlesand pots. PP thin-walled containers are commonly used for food packaging.
   

Consumer goods

PP is used in several household products and consumer goods applicationslike translucent partshousewaresfurnitureappliancesluggagetoysetc.

Automotive industry

Due to its low costoutstanding mechanical propertiesand moldabilityPP is used in automotive parts. Used in battery cases and traysbumpersfender linersinterior triminstrument panelsand door trims. Other features include low coefficient of linear thermal expansionspecific gravityhigh chemical resistancegood weatherabilityprocessabilityand impact/stiffness balance.
 

Fibers and fabrics

A large volume of PP is utilized in the market segment known as fibers and fabrics. PP fiber is used in raffia/slit-filmtapestrappingbulk continuous filamentstaple fibersspun bondand continuous filament. PP rope and twine are very strong and moisture-resistantmaking them suitable for marine uses.

Medical industry

PP is used in various medical applications due to its high chemical and bacterial resistance. The medical-grade PP exhibits good resistance to steam sterilization. Disposable syringes are the most common medical application of PP. Other uses include medical vialsdiagnostic devicespetri dishesintravenous bottlesspecimen bottlesfood trayspanspill containersetc.
 

Industrial uses

PP sheets are widely used in the industrial sector to produce acidchemical tankssheetspipesReturnable Transport Packaging (RTP)etc. This is enabled by propertieslike high tensile strengthresistance to high temperaturesand corrosion resistance.

TIP: Click on the market/application name to access technical data of suitable commercial grades and request a sample of the one best suited to your needs.