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Q&A: starch plastic tensile strength

by Green Plastics


Akhilesh has written in with this question:

Hello everyone
I have prepared starch-based bio-plastic at home.I want to increase its tensile strength.How can that be done?Please help me

Akhilesh, I have to tell you that you have hit on one of the top research questions now going on in the area of bioplastics. Unfortunately, that means that there is no simple answer for your home bioplastic project, although you do have a few options.

First, I want to make sure that we understand what you are really asking for.  The tensile strength of a strip of plastic is technically the force it can bear, under tension, per unit cross sectional area of the film, without breaking.  The “cross sectional area” is measured as the width times the thickness. If all you want to do is increase the strength of your piece of plastic, the simplest thing to do is make it thicker:  this would allow the piece of plastic to bear a greater load under tension without breaking.  However, it would not increase the tensile strength, which is a technical property of a plastic that doesn’t change with the size and shape of the piece of plastic you are considering.

Tensile strength is a very important property in the plastics industry. It determines what types of applications the plastic can be used for, and in many cases is even highly regulated in certain industries.  For example, all plastics used in automobiles must meet specifications to ensure that the particular part performs satisfactorily, and these include specific values that must be met for tensile strength, as well as other properties like impact strength and flexural modulus.  As a result, coming up with a bioplastic that will have satisfactory tensile strength for various industrial applications is absolutely one of the top business questions on the minds of bioplastics producers.

So what about home-made starch plastic?

When you are working on a home or school project, the easiest way to get a bioplastic with a better tensile strength is to use a different polymer. Starch cast films are simply not that strong, so in order to make them stronger you have to change the chemical composition.  Instead of using starch, you could use agar, which is the biopolymer component found in algae plastic or bioplastic made from seaweed.  Agar films have a much higher tensile strength than starch films, so the more agar in the film, the larger the tensile strength. You can also combine starch with agar to make plastics with intermediate tensile strengths.  By making starch-agar films with increasing amounts to agar, you can get increasing tensile strength from your plastic.  We have an earlier article, HOW TO make algae bioplastic, that describes the specific ingredients and measurements for both pure agar bioplastic and starch-agar blends, as well as information about buying agar.

Larger companies, of course, have other more high-tech options that they can pursue for increasing tensile strength in bioplastics.  This is part of the reason that some companies specialize in producing blends that are part bioplastic and part traditional plastic: they claim that the end result is still more “environmentally friendly” because of the bioplastic element, but the traditinal plastic mixed in can make it stronger or more flexible.  Of course, the end result is not biodegradable, so how “environmentally friendly” this option is may be debatable.

There is also some very current research going on with using cellulose particles (either cellulose fibers or nanoparticles) added to bioplastics to add strength. For example, there is work being done to add cellulose fibers to starch foams to increase the (compressive) strength of starch packaging peanuts. The cellulose does increase the strength but it also increases the density of the peanuts, so there’s a trade-off.  However, these products are made with extrusion, which is not what you are doing with your home project.  You are making a cast film.  In homemade cast films, the addition of cellulose may not work.  Cellulose is completely insoluble in water so the cellulose may not interact with the starch enough to affect the properties; the cellulose might even prevent the formation of the films. However, this is a very promising line of inquiry in the industrial manufacture of bioplastics, because if the plastic can be made stronger using cellulose, the end result will still be 100% biodegradable and made from renewable resources.

Best luck with your bioplastic project!


Remember, if you have successfully made bioplastics using our recipes or suggestions please send photos or video to project@green-plastics.net and we will post it on the website and help give you some publicity!


Comments (1)

  1. Akshay says:

    Hi 🙂

    I am working on a 3D printer and want to come up with a biodegradable cheap material that can be 3D printed out of the extruder of a 3D printer.

    The properties required for that will be melting point between 80 – 150 Celsius, quick freezing time after coming out of the nozzle.

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