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Biodegradable Plastics
Name: Natalia
Status: student
Grade: 6-8
Location: N/A
Question:
What is the buzz about biodegradable plastics? I am
learning about starch bioplastics. My plan is to try and add some
type of biopolymer to make the starch plastic water resistant and
strong. This is because starch is not waterproof or strong. If I
have done my research correctly, starch is blended with other
biopolymers due to its low cost. When it is blended with these other
chemicals it ends up being more expensive. Price is the number one
factor against bioplastics. So, what is really popular about this
topic? What biopolymer do you suggest would complete the task that I
am attempting?
Replies:
The main interest in 'biodegradable' plastics is that they break down into
simpler components sooner than traditional plastics. Usually, this means the
chemistry of the polymers is such that they degrade into smaller molecules
either chemically or by organisms. Biopolymers, however, are a different
thing. The term biopolymer typically refers to polymers that come from
biological sources. They are not necessarily bio-degradable. For instance
you can make polyesters from soybean oil, and these polyesters are not
biodegradable. The interest in biopolymers stems more from using renewable
resources to make the plastics rather than petroleum. Conversely,
biodegradable polymers can be made from petroleum. In other words,
biopolymers are not necessarily biodegradable, and biodegradable polymers
are not necessarily bio-based.
I am assuming that you are creating your "starch-plastic" by rinsing and
acid-treating potato or other starch. You can then press the resultant
starch into semi-hard shapes. There are several reasons why your homemade
starch-plastic is not as strong or waterproof as you like. One reason is the
nature of starch -- it absorbs water. Another reasons is that probably your
starch has a rough surface. A rough surface means it has higher surface
area, which means water can get into it more quickly. Last, the individual
starch molecules have not been 'stretched', and therefore do not make as
strong a matrix as if they had been stretched and entangled.
There are lots of things you can do. One option is to add a little vegetable
oil to the dry starch -- the oil will repel water. Another is to heat the
plastic (melt it) to get a smooth surface. The high temperatures are a
safety risk and I would not do this unless you have proper safety equipment
and equipment to melt it -- this is not a home-based experiment. If you want
to add a bio-polymer, I might suggest PLA (polylactic acid) which is also
made from starch. PLA is very popular these days. However, you would have to
melt it to get it fully incorporated (it comes in pea-sized pellets
commonly). PLA is water resistant, although it is not very strong. Adding
other polymers starts to be a problem, because the way they need to be
processed requires heat (to melt and mix them), and also a particular kind
of mixing that stretches the molecules to make them strong. In industry,
they use equipment called 'extruders', but these are not common in normal
school science labs. Last, when you press the starch, make sure you use a
very smooth surface. The smoother the surface, the longer it will take for
water to infiltrate it (although the water will still get in quite readily).
To increase the compressive strength, you could also add sand or even gravel
to your polymer. The mixture of polymer and hard components will increase
its compressive strength -- but it will increase its weight, and will not
improve other kinds of strength like tensile (stretching) strength or
toughness.
Hope this helps,
Burr Zimmerman
Ni Natalia,
Interest in biodegradable plastics is clearly driven simply because they can
degrade over time in the environment. There is a lot more involved here, than
trying to "add some type of biopolymer to make the starch plastic water
resistant and strong". For one thing, you, as a pretty bright grade 6-8 student,
unfortunately do not have the facilities to "blend" plastics. As they say, "it
just ain't that simple!"
The simplest plastics that can be biodegradable, are made from starch as a
starting point. But you must understand that this does not mean just stirring
a bunch of starch into a vat of plastic resin! Instead, cornstarch is chemically
modified into a new type of plastic resin, in the same way that ethylene gas is
used as a starting point for polyethylene plastic bags (yet there is no ethylene
gas in plastic bags!). This type of resin must NOT be totally waterproof, because
if it were, it would not biodegrade. Typically warmth and moisture cause this
type of plastic to swell and allow moisture to seep into it to begin the
degradation process. Starch is not used only because it is cheap. It happens to
be needed as a starting point to make this type of biodegradable plastic. Starch
is in fact more costly than many traditional plastic "feedstocks". For example,
ethylene gas (the starting point for ordinary polyethylene plastic) is way
cheaper than starch, but of course, the resulting polyethylene is not
biodegradable.
Another class of biodegradable plastics are not starch based, but are made from
plant-based feedstock. An example is polyhydroxyalkanoate (PHA) biopolymer. But
although this uses plant material as a feedstock, the manufacturing process still
uses much more energy than the manufacturing process used in traditional plastics.
So these plastics may be environmentally friendly once they are left to degrade,
but the increased energy needed for their manufacturing process can result in
much more fossil fuels needed (burned to make heat), than ordinary plastics that
are actually made from oil-based chemicals.
You are indeed correct that price is one problem with bioplastics. This is a
direct result of the increased complexity of their manufacture, the often
higher cost of basic feedstock, and their increased requirement for energy
in manufacturing.
Regards,
Bob Wilson
You have raised a thorny bunch of problems!! First, you need to assess "the buzz"
about biodegradable plastics. Second, cellulose is a high molecular weight of
starch, which is a high molecular weight polymers of sugars. Third, cellulose is
pretty stable. Trees and plants are composed largely of cellulose. I do not know
of any non-biological process for decomposing cellulose that is energy efficient.
Now there is a question of "blending" polymers. In general polymers of different
types do not mix. It is like trying to mix water and oil.
You have CORRECTLY identified the problem "PRICE / PERFORMANCE". How to blend
polymers that have the desired properties at a reasonable cost. Adding a
biopolymer to cellulose you recognize is blending one biopolymer to another
bio polymer. In its "native state -- i.e. wood"
cellulose is pretty strong and durable, in the short term. But in the long term
it is "food" for microbes, so it "rots". This has led to the addition of some
pretty powerful "preservatives".
Finding an effective substitute or additive would be a technological breakthrough.
But it is a case of "many have been tested, but none have been found." Your
project is not simple or easy.
A lot of failures can be found.
Possibly you could organize the methods and materials that have been tried and
failed, because I do not think that you will find a method/material that has not
been tried unsuccessfully.
Vince Calder
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Update: June 2012
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