What is Polypropylene (PP), and What is it Used For?
- Apr 19, 2018 -

Polypropylene (PP) is a thermoplastic “addition polymer” made from the combination of propylene monomers. It is used in a variety of applications to include packaging for consumer products, plastic parts for various industries including the automotive industry, special devices like living hinges, and textiles. Polypropylene was first polymerized in 1951 by a pair of Phillips petroleum scientists named Paul Hogan and Robert Banks and later by Italian and German scientists Natta and Rehn. It became prominent extremely fast, as commercial production began barely three years after Italian chemist, Professor Giulio Natta, first polymerized it. Natta perfected and synthesized the first polypropylene resin in Spain in 1954, and the ability of polypropylene to crystallize created a lot of excitement. By 1957, its popularity had exploded and widespread commercial production began across Europe. Today it is one of the most commonly produced plastics in the world.

CNC Cut Polypropylene Living Hinge Prototype Child Safe Lid

CNC Cut Polypropylene Living Hinge Prototype Child Safe Lid by Creative Mechanisms


According to some reports, the current global demand for the material generates an annual market of about 45 million metric tons and it is estimated that the demand will rise to approximately 62 million metric tons by 2020. The major end users of polypropylene are the packaging industry, which consumes about 30% of the total, followed by the electrical and equipment manufacturing, which uses about 13% each. Household appliances and automotive industries both consume 10% each and construction materials follows with 5% of the market. Other applications together make up the rest of the global polypropylene consumption.

Polypropylene has a relatively slippery surface which can make it a possible substitute for plastics like Acetal (POM) in low friction applications like gears or for use as a contact point for furniture. Perhaps a negative aspect of this quality is that it can be difficult to bond Polypropylene to other surfaces (i.e. it does not adhere well to certain glues that work fine with other plastics and sometimes has to be welded in the event that forming a joint is required). Although polypropylene is slippery at the molecular level, it does have a relatively high coefficient of friction - which is why acetal, nylon, or PTFE would be used instead. Polypropylene also has a low density relative to other common plastics which translates to weight savings for manufacturers and distributors of injection molded Polypropylene parts. It has exceptional resistance at room temperature to organic solvents like fats but is subject to oxidation at higher temperatures (a potential issue during injection molding).

One of the major benefits of Polypropylene is that it can be manufactured (either through CNC orinjection molding, thermoforming, or crimping) into a living hinge. Living hinges are extremely thin pieces of plastic that bend without breaking (even over extreme ranges of motion nearing 360 degrees). They are not particularly useful for structural applications like holding up a heavy door but are exceptionally useful for non load-bearing applications such as the lid on a bottle of ketchup or shampoo. Polypropylene is uniquely adept for living hinges because it does not break when repeatedly bent. One of the other advantages is that polypropylene can be CNC machined to include a living hinge which allows for faster prototype development and is less expensive than other prototyping methods. Creative Mechanisms is unique in our ability to machine living hinges from a single piece of polypropylene. 

Another advantage of Polypropylene is that it can be easily copolymerized (essentially combined into a composite plastic) with other polymers like polyethylene. Copolymerization changes the material properties significantly, allowing for more robust engineering applications than are possible with pure polypropylene (more of a commodity plastic on its own).

The characteristics mentioned above and below mean that polypropylene is used in a variety of applications: dishwasher safe plates, trays, cups, etc, opaque to-go containers, and many toys.

What are the Characteristics of Polypropylene?

Some of the most significant properties of polypropylene are:

  1. Chemical Resistance: Diluted bases and acids don’t react readily with polypropylene, which makes it a good choice for containers of such liquids, such as cleaning agents, first-aid products, and more.  

  2. Elasticity and Toughness: Polypropylene will act with elasticity over a certain range of deflection (like all materials), but it will also experience plastic deformation early on in the deformation process, so it is generally considered a "tough" material. Toughness is an engineering term which is defined as a material's ability to deform (plastically, not elastically) without breaking..

  3. Fatigue Resistance: Polypropylene retains its shape after a lot of torsion, bending, and/or flexing. This property is especially valuable for making living hinges.

  4. Insulation: polypropylene has a very high resistance to electricity and is very useful for electronic components.

  5. Transmissivity: Although Polypropylene can be made transparent, it is normally produced to be naturally opaque in color. Polypropylene can be used for applications where some transfer of light is important or where it is of aesthetic value. If high transmissivity is desired then plastics like Acrylic or Polycarbonate are better choices.

Polypropylene is classified as a “thermoplastic” (as opposed to “thermoset”) material which has to do with the way the plastic responds to heat. Thermoplastic materials become liquid at their melting point (roughly 130 degrees Celsius in the case of polypropylene). A major useful attribute about thermoplastics is that they can be heated to their melting point, cooled, and reheated again without significant degradation. Instead of burning, thermoplastics like polypropylene liquefy, which allows them to be easily injection molded and then subsequently recycled. By contrast, thermoset plastics can only be heated once (typically during the injection molding process). The first heating causes thermoset materials to set (similar to a 2-part epoxy) resulting in a chemical change that cannot be reversed. If you tried to heat a thermoset plastic to a high temperature a second time it would simply burn. This characteristic makes thermoset materials poor candidates for recycling.

Find The Right Plastic For Your Prototype Part

Why is Polypropylene used so often?

Polypropylene is used in both household and industrial applications. Its unique properties and ability to adapt to various fabrication techniques make it stand out as an invaluable material for a wide range of uses. Another invaluable characteristic is polypropylene’s ability to function as both a plastic material and as a fiber (like those promotional tote bags that are given away at events, races, etc). Polypropylene’s unique ability to be manufactured through different methods and into different applications meant it soon started to challenge many of the old alternative materials, notably in the packaging, fiber, and injection molding industries. Its growth has been sustained over the years and it remains a major player in the plastic industry worldwide.

At Creative Mechanisms, we have used polypropylene in a number of applications across a range of industries. Perhaps the most interesting example includes our ability to CNC machine polypropylene to include a living hinge for prototype living hinge development. Polypropylene is a very flexible, soft material with a relatively low melting point. These factors have prevented most people from being able to properly machine the material. It gums up. It doesn’t cut clean. It starts to melt from the heat of the CNC cutter. It typically needs to be scraped smooth to get anything close to a finished surface. But we have been able to solve this problem which allows us to create novel prototype living hinges out of polypropylene. Take a look at the video below:

Video Thumbnail

What Are The Different Types of Polypropylene?

There are two main types of polypropylene available: homopolymers and copolymers. The copolymers are further divided into block copolymers and random copolymers. Each category fits certain applications better than the others. Polypropylene is often called the “steel” of the plastic industry because of the various ways in which it can be modified or customized to best serve a particular purpose. This is usually achieved by introducing special additives to it or by manufacturing it in a very particular way. This adaptability is a vital property.

Homopolymer polypropylene is a general-purpose grade. You can think of this like the default state of the polypropylene material. Block copolymer polypropylene has co-monomer units arranged in blocks (that is, in a regular pattern) and contain anywhere between 5% to 15% ethylene. Ethylene improves certain properties, like impact resistance while other additives enhance other properties. Random copolymer polypropylene – as opposed to block copolymer polypropylene – has the co-monomer units arranged in irregular or random patterns along the polypropylene molecule. They are usually incorporated with anywhere between 1% to 7% ethylene and are selected for applications where a more malleable, clearer product is desired.

How is Polypropylene made?

Polypropylene, like other plastics, typically starts with the distillation of hydrocarbon fuels into lighter groups called “fractions” some of which are combined with other catalysts to produce plastics (typically via polymerization or polycondensation).

Polypropylene for Prototype Development on CNC Machines, 3D Printers, & Injection Molding Machines:

3D Printing Polypropylene:

Polypropylene is not readily available in filament form for 3D printing.

CNC Machining Polypropylene:

Polypropylene is widely used as sheet stock for CNC machine manufacturing. When we prototype a small number of polypropylene parts we typically CNC machine them. Polypropylene has gained a reputation as a material that cannot be machined. This is because it has a low annealing temperature, which means that it starts to deform under heat. Because it is a very soft material in general, it requires an extremely high skill level to be cut with precision. Creative Mechanisms has been successful in doing so. Our teams can use a CNC machine and cut the polypropylene cleanly and with extremely great detail. In addition, we are able to create living hinges with polypropylene that have thickness as little as .010 inches. Making living hinges is a difficult endeavor on its own, which makes using a difficult material like polypropylene even more impressive.

Injection Molding Polypropylene:

Polypropylene is a very useful plastic for injection molding and is typically available for this purpose in the form of pellets. Polypropylene is easy to mold despite its semi-crystalline nature, and it flows very well because of its low melt viscosity. This property significantly enhances the rate at which you can fill up a mold with the material. Shrinkage in polypropylene is about 1-2% but can vary based on a number of factors, including holding pressure, holding time, melt temperature, mold wall thickness, mold temperature, and the percentage and type of additives.

Other:

In addition to the conventional plastic applications, polypropylene also lends itself well to fiber applications. This gives it an even wider range of uses that go beyond just injection molding. Those include ropes, carpets, upholstery, clothing, and the like.

Polypropylene Fiber Applications: Rope

Image From AnimatedKnots.com

What are the Advantages of Polypropylene?

  1. Polypropylene is readily available and relatively inexpensive.

  2. Polypropylene has high flexural strength due to its semi-crystalline nature.

  3. Polypropylene has a relatively slippery surface.

  4. Polypropylene is very resistant to absorbing moisture.

  5. Polypropylene has good chemical resistance over a wide range of bases and acids.

  6. Polypropylene possesses good fatigue resistance.

  7. Polypropylene has good impact strength.

  8. Polypropylene is a good electrical insulator.

What are the Disadvantages of Polypropylene?

  1. Polypropylene has a high thermal expansion coefficient which limits its high temperature applications.

  2. Polypropylene is susceptible to UV degradation.

  3. Polypropylene has poor resistance to chlorinated solvents and aromatics.

  4. Polypropylene is known to be difficult to paint as it has poor bonding properties.

  5. Polypropylene is highly flammable.

  6. Polypropylene is susceptible to oxidation.

Despite its shortcomings, polypropylene is a great material overall. It has a unique blend of qualities that aren’t found in any other material which makes it an ideal choice for many projects.

What are the properties of Polypropylene?

Property

Value

Technical Name

Polypropylene (PP)

Chemical Formula

Polypropylene Molecular Composition   (C3H6)n

Resin Identification Code (Used For Recycling)

Polypropylene (PP) Resin Identification Code 5 (For Plastic Recycling Purposes)

Melt Temperature

130°C (266°F)

Typical Injection Mold Temperature

32 - 66 °C (90 - 150 °F) ***

Heat Deflection Temperature (HDT)

100 °C (212 °F) at 0.46 MPa (66 PSI) **

Tensile Strength

32 MPa (4700 PSI) ***

Flexural Strength

41 MPa (6000 PSI) ***

Specific Gravity

0.91

Shrink Rate

1.5 - 2.0 % (.015 - .02 in/in) ***

*At standard state (at 25 °C (77 °F), 100 kPa)  ** Source data  *** Source data


Topics: Plastics

Rita Bermudez Esquerra

 

2016/6/13 上午5:51:19

Interested in receiveing news about Creative Mechanisms

Reply to Rita Bermudez Esquerra

Veronica Lazarus

 

2016/8/25 下午10:27:30

We are a global foundation We will like to know more about possible use of Propylene of varies thickness in building poor family houses and community that can stand up too the sere elements like floods, winds like hurricane and severe heat. How safe is it. Can it become a health hazard if it bleeds into water or streams. Please copy your response to all three emails above.

Reply to Veronica Lazarus

Anand talc

 

2016/12/17 下午5:24:35

Awesome! It is great to see. Thanks for sharing

Reply to Anand talc

Kava Crosson-Elturan

 

2016/12/23 上午11:19:07

PP is readily available for 3D printing in Europe at least. I did find it harder to source in the US though. It has pretty severe shrinkage and bonding issues in the context of 3D printing. Packing tape (which is also PP) on the bed and a flexible support arrangement (i.e. raft) can lead to acceptable prints, but I've found that the shrinkage factor is about 4% and also not volumetrically uniform. For parts which have loose tolerances it is great. Much of the dimensional tolerance issue is overcome by the low stiffness - you can "make it fit".

Reply to Kava Crosson-Elturan

Michael Creighton

 

2017/1/7 上午12:24:11

Kava, thanks for your comment and the information you provided. When we say that PP is not available for 3D Printing, we are referring to the highly precise professional and production-quality machines that we use for developing complex products. None of the major 3D printer manufacturers offer actual PP as a material, likely due to the issues you outlined.

Reply to Michael Creighton

Jim Shields

 

2017/2/3 上午10:08:18

We are building a micro-scale plastic recycling system for our school, and hope to recycle discarded consumer products like plastic bottles and containers. Is polypropylene readily recycled? Can we use discarded materials alone, or will we have to add virgin PP? How much does it degrade when recycled? Thanks for any information or ideas you can share.

Reply to Jim Shields

Michael Creighton

 

2017/2/7 上午2:57:24

Jim, PP can be recycled. Typically, in a manufacturing setting, you can see up to 20-25% regrind mixed with virgin material (if the product specifications allow for the use of regrind). If using 100% regrind, you should expect to see significant degradation after a few cycles.

Reply to Michael Creighton

Zachary Reyez

 

2017/4/28 下午7:12:13

Hi Jim, I want to know more about your schools recycling system. Do you have a website or perhaps a blog? I am currently on a search for information about the process of recycling PP#5. Where does the plastic go to be melted and who buys the new resource? Since the recycle center in my city does not take PP#5 I am curious who wants this stuff? How do I find them?

Reply to Zachary Reyez

Sandra Reese

 

2017/3/28 上午2:00:02

Is Polypropylene FHR 13T25A BPA free

Reply to Sandra Reese

Michael Creighton

 

2017/3/30 下午9:17:31

Sandra, BPA is typically only found in some Polycarbonate and PVC plastics.

Reply to Michael Creighton

Jen Ise

 

2017/6/23 下午2:46:34

We were wondering if there is an adheisive which you can recommend for bonding Polypropylene to other materials? A double sided tape would be ideal.

Reply to Jen Ise

Michael Creighton

 

2017/6/23 下午9:03:23

Jen, PP is very difficult to bond because of its low surface energy. I suggest trying a tape from 3M's line of VHB tapes specifically for low surface energy substrates.

Reply to Michael Creighton

Andrew Jones

 

2018/3/6 上午5:04:37

3M DP8005 adhesives work great with PP and PE materials. Not cheap however but the only thing I have ever found which works.

Reply to Andrew Jones

Jody Spencer

 

2017/8/16 下午5:02:27

Hi Michael, Can you recommend any companies to print in virgin PP a small water device about 5.2" in length and 1.2" Diameter. Thank you

Reply to Jody Spencer

Michael Creighton

 

2017/8/17 下午10:22:53

Jody, to my knowledge, there are no companies that 3D print in actual PP. Some 3D printers can print in PP-like materials, but these materials are not actual thermoplastics, but rather photopolymers. The material properties of PP do not make it a good candidate for the 3D printing process.

Reply to Michael Creighton

mark mcgoveran

 

2017/9/8 上午2:16:08

In some cases in the field various low energy surfaces are "cleaned" with a bit of gasoline on a cotton rag, then glued, I am of teh theory that the low energy surface actuall disolves abit and links on to an aromatic then the glue replaces that, and opinon on this theory?

Reply to mark mcgoveran

Michael Creighton

 

2017/9/8 上午4:50:44

Mark, thanks for the comment. There are several different types of surface treatments that can be done to increase the ability of PP to bond with adhesives. One of those methods is using a liquid primer that is spread across the surface and then evaporates. I am not a chemist, so I cannot say if using gasoline works as you describe in your theory. Perhaps someone else with more knowledge on the subject can comment below.

Reply to Michael Creighton

Frank Dunmore

 

2017/12/13 上午6:14:40

Is there any degradation in terms of lowering of the melting point or increased flammability if polypropylene is heated repeatedly just below its softening/melting temperature. Several brands of toaster (simple 2-slice type) have polypropylene enclosures, that during repeated normal use are heated to temperatures just below pp's softening or melting point (approx. 300 deg F). there have been reports of fire, some claimed to be spontaneous in origin (only from normal use!)

Reply to Frank Dunmore

Michael Creighton

 

2017/12/13 下午9:50:38

Frank, thanks for the question. If the toaster is UL certified, I would expect that they should be using a flame retardant PP material.

Reply to Michael Creighton

FRANKLIN DUNMORE

 

2017/12/14 上午12:59:59

Despite the valid UL listing, the plastic (chemically tested to be pp) is not flame retardant (the plastic ignites easily and burns untill fully consumed). Nevertheless, the plastic is UL 94 HB plastic compliant. Needless to say, the standard is weak.

Reply to FRANKLIN DUNMORE

Alice Lee

 

2018/2/14 上午4:09:52

Essential oils are included in aromatics when you mention polypropylenes do not hold well with aromatics?

Reply to Alice Lee

Michael Creighton

 

2018/2/14 上午5:02:05

Alice, the aromatics we are referencing are aromatic compounds (from organic chemistry) not aromas (scents). Two different things, similar names, often confused.

Reply to Michael Creighton

justine

 

2018/2/24 下午11:47:35

We are researching whether it is possible to use PP in the recycling of textiles, is this already done?

Reply to justine

Michael Creighton

 

2018/3/2 下午11:33:10

Justine, PP is available in textile form. Since PP is a thermoplastic it is therefore recyclable. A quick Google search shows that recycled PP textiles are available.

Reply to Michael Creighton

Lori Westphal

 

2018/3/13 上午10:56:46

Hello from a newbie! I do not have any plastics expertise so please forgive me! 

I have developed a product but it is having manufacturing problems and I haven't been able to find a solution. 

My product requires an EVA masterbatch to be blended with flame retardant PP for injection molding. (This is an electrical product and I am not the manufacturer.:)) 

I know that EVA is polar and PP is non-polar but, the masterbatch I'm trying to use is marketed as compatible with almost all polymers, including a "good" blending rating with PP. 

The EVA only needs to be 5% of the total construction with FR-PP being 95%. However, when the molds come out, almost all of them have surface cracks which makes them un-usable. 

If anybody has any advice on how to blend EVA with FR-PP I would really appreciate it! 

I've read about using PPMA & EVAOH compatibilizers but I really doubt my manufacture would consider using another additive. 

Is this my only option or would there be any other solutions? 

Thanks so much!