Today’s match-up is Polypropylene vs. PLA. We’ll take a look at what products are made out of the different materials, how they compare in the 3D printing world, and how they stand up to one another in the battle of who’s stronger!
Polypropylene (PP) is a thermoplastic polymer that was discovered in 1954 by Giulio Natta, which was then mass-manufactured as a polymer by the Italian firm Montecatini in 1957. It’s used to make many different plastic products such as: DVD cases, Tic-Tac lids, chairs, sewage floats, etc. Until recently, PP was not available for use in 3D printing applications. The Stratasys Objet is one of the first machines to be able to use PP-like materials.
The Corn Plastic
Polylactic Acid (PLA) is a thermoplastic aliphatic polyester derived from corn starch discovered in 1932 by Wallace Carothers. Dupont then patented Carothers’ process in 1954 but, due to high manufacturing costs at the time, reserved it for medical use only. Technologies in the 1980’s, however, allowed for the process of fermentation of glucose (which turns glucose into lactic acid) to come down in price. Today in the United States, PLA is most always made from corn starch given it is the most abundantly available crop. However, PLA can be made with the fermented glucose of almost any crop, and sources like cassava roots or sugarcane are used in other regions.
In 1987, Cargill began researching PLA production and in 2001 launched NatureWorks PLA technology with help via a joint venture with Dow Chemical Company. PLA has many uses such as biodegradable cups, mulch film, medical implants, and our personal favorite: 3D printing filament. It has become the staple polymer in the FDM-style 3D printing world due to its many physical properties as well as its rapid cooling time.
How does PP compare to PLA in the world of? We’re going to look at 5 different aspects:
- Flexural Modulus – The tendency for a material to bend
- Impact Strength – How much force does it take to break a piece of the material
- Flexural Strength – The material’s ability to resist deformation under load
- Heat Distortion Temperature (HDT) – The temperature at which a sample deforms under a specific load
- Elongation – Flexing the material to see if it can return to its original shape
The graph below shows the different calculations of PP vs. PLA. The closer you are to the center of the pentagon, the lower the number value:
Let’s look at Impact Strength: you’ll notice that PP has a higher amount of impact strength than PLA. So, if you take your 3D printed Tic-Tacs cover and smash it with a hammer, it’ll more likely break if it’s PLA than if it’s PP. Now, that all depends on the design of the product. If you have a solid softball made of PLA, it’s more likely to not break due to the shear mass of the PLA.
The next three tests we’re going to look at – Flexural Modulus, Flexural Strength, and Elongation – have an interesting connection. The reason PP is low on Flexural Modulus and Flexural Strength is due to its ability to be elastic (or elongate). PP is much more flexible then PLA. This is why you can’t print snap-fits or living-hinges very well with PLA; it isn’t elastic enough.
Lastly, when looking at heat distortion, PLA is considerably lower than PP, even though their melt temperatures are both right around 130C-170C. One question that could be asked here is why does MakerBot have their temperature set at 230C when extruding PLA? The reason: because the plastic needs to be fully molten to extrude. When it says the melt temperature is at 130C it means that it will start to deform due to the heat at that temperature.
So, who is still standing after this polymer matchup?
Well, for 3D printing right now it’s PLA because PP isn’t readily available in FDM filament form. You also have to ask yourself: what characteristics do you want? Do you want a material that is stiff and rigid because you’re creating a frame for your RC car or helicopter? If so, PLA is going to be your better choice. Or, do you want something that is more flexible because you want to have a living-hinge in your part design? In that case going with PP is a better option.
All-in-all, PLA is a great plastic to work with because of it’s unique properties, and the same goes for PP.