by Green Plastics
In a comment on one of our articles, student maksonglao asks an interesting question:
Hi! For our Chem Proj, we made bioplastics from corn and potato starch. We used: 1 tsp glycerin, 1 tsp vinegar, 30 ml H2O, and each of these combinations for the polymer
1 tbsp corn, or
1/2 tbsp corn, 1/2 tbsp potato, or
1 tbsp potato
for our set-ups. According to our data, the 50-50 (corn and potato starch) was the best with it ranking 1st for folding endurance and tearing strength (how hard it is to tear).
Maybe I’m mistaken but I think flexibility plays a factor in folding endurance and I have read in your other answers that the more flexible, the weaker the plastic so I don’t know why the one with the most folding endurance is also the strongest plastic.
Can you explain why this is?
This is a good question about the relationship between different properties like folding and tearing. It brings up some very important (and somewhat complicated) issues about how to measure the properties of your home-made bioplastic, especially when you are trying to make comparisons between different types of bioplastic for a project.
The issue you raise reflects the use of “everyday” words to refer to highly technical terms. Most of the home-made plastics that we talk about on this site are thin plastic films cast from polymer solutions. One common way to characterize such films is to measure how the films respond to a stress applied by clamping the ends and stretching. This is called tensile testing. The everyday words “strong” and “strength” in this case refer to “tensile strength” (or “tensile strength at break”, or “ultimate tensile strength”), referring to the maximum stress that can be applied before the film breaks.
The everyday word “flexibility”, in this case, refers to how far the plastic film can be stretched before it breaks; the technical term for that property is “elongation” (or “elongation at break”). It is true that, in these films, the “tensile strength” is inversely proportional to the amount of plasticizer (e.g., glycerol) and the “elongation” is directly proportional to the amount of plasticizer. In other words: the more the plasticizer, the smaller the (tensile) strength and the larger the elongation (“flexibility”). This is something that we’ve repeated many times, and that you remember reading on this site before.
However, besides tensile testing there are many other types of testing for plastics. These include compressive testing, shear testing and others which can give quantitative measurements of things like tensile modulus, compressive strength, shear strength, impact resistance, impact strength, crack-resistance, indentation hardness, abrasion resistance, bearing strength, “creep”, scratch resistance, and others. There are standardized test methods to get quantitative measurements for all of these.
You have used two different types of test (not tensile tests). Your “tearing strength” test has technical analogues of “tear resistance” and “tear propagation resistance” (Notice that if your term “tearing strength” means “how hard it is to tear” the proper name for it would be “tear resistance”, a term which actually exists in polymer science.)
It’s not clear how you evaluated your “folding endurance”. If it was how many times you could fold it, unfold it, refold it, etc., before it broke, it would be somewhat related to the flexural properties (flexural strength, bending modulus or flexural modulus of elasticity).
Although it might sound OK to start making correlations between different pairs of properties, it’s not that straightforward. There are, in fact, four different properties involved that we have talked about so far:
tensile strength(“strength” as usually used on this web site, related to plasticizer amount),
elongation (“flexibility ” as usually used on this web site, also related to plasticizer),
tear resistance (your “tearing strength”),
folding endurance (how many times it can be folded and unfolded before breaking?)
Just because the two tensile properties are related (i.e., as more plasticizer is added, the plastic gets less “strong” but more “flexible”), you can’t extrapolate to the other two properties.
That still leaves your question open, though: How can your result be explained, that the sample with the higher folding endurance also has the higher tear resistance?
If you have found that your results are reproducible, one hypothesis you might make to explain them is the following. As you say, folding involves “flexibility”, but each act of folding and refolding is also a new stress, localized at the fold, which could weaken the film at the fold, and the net result in your particular samples is that the “stronger” sample also has the higher “folding endurance.”
Thank you for the great question! I hope this discussion also motivates other students to try testing the different properties they get when they experiment with making their own bioplastics!
(NOTE: If you want some more reading on this topic, keep in mind that the Green Plastics Book has an entire chapter dedicated to talking about different properties of bioplastic and how to measure them!)