Ask A Scientist Materials Science Archive

name Abigail
status student
grade 9-12
location WI

Question - Dear Sir or madam,
I am in debate, and this year's topic is "Resolved: That the United States
should change its energy policy to substantially reduce its dependence on
foreign oil."  Each team argues for the resolution with a plan that they
have written in one round, and in the next argues against another team's
plan to change the energy policy.  One popular plan that my partner and I
have come against is thermodepolymerization.  We are having trouble
finding evidence that TDP has disadvantages, will not work on a large
scale, etc.  We have thought of several potential problems with it, but
have not been able to back up our questions with evidence.  Our Regional
Tournament is coming up, and we still have very little evidence against
TDP.  Despite hours of research, I haven't been able to discover whether
or not it costs more, whether or not only biodegradeable wastes can be
used, what constitutes "organic wastes" and whether or not we have enough
of them, if the private sector is widely willing to invest in TDP, what
grade of oil it produces, what refining process it has to go through, if
it has safety issues (especially in the production stage), and if it
causes environmental damage, etc.

I really appreciate your taking time to answer questions for kids and
teens--I really, really value your answers and advice.   If you have any
web sites or links that you could recommend, I would be quite grateful.
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Regarding "thermal depolymerization" [TDP]. There is a large literature
"out there" on TDP. The problem in finding the literature is one of
"jargon". TDP, in the jargon of polymer chemistry, is called "unzipping",
so if you do a Google search on the terms "unzipping polymers" or
"unzipping copolymers" you will enter the literature jungle on the
subject. There are a number of considerations:
1. Ease of de-polymerization, i.e. how hot and under what conditions will 
a polymer "unzip". Styrene, alphamethyl styrene, and methacrylates are vinyl
polymers that will thermally "unzip" and produce the starting monomers at
a relatively low temperature. Other vinyl polymers are more or less
difficult to "unzip". The issue is one of energy input / cost vs. monomer
yield, purification, cost, and reprocessing for reuse. You will have to do
some model calculations involving energy cost, product separation and
purification for re-use. In general oxygen would have to be excluded,
otherwise oxidation will occur.
     2. Condensation polymers, such as polyesters, cellulose polymers,
polyamides, and bio-polymers require a different set of conditions
(hydrolysis or reaction with water) is required to unzip these polymers
because you have to reverse the general reaction: A--H + B--OH ---->
--A--B-- + H2O. These reactions produce monomers A--H and B--OH which then
have to be separated and purified for re-use. There are also issues of other
materials present that could be re-cycled on the one hand, or disposed of,
on the other. Here high temperature and pressure, especially the steam
pressure, are major issues. Biopolymers are readily depolymerized on an
analytical scale -- this is the basis for DNA "fingerprinting". However,
whether this can be done on a manufacturing scale is problematic.
     In either case the point of vulnerability is whether, even at the
present record cost of petroleum, does the cost of recycling compare
favorably with the cost of virgin monomer. I suspect it does not, but I have
no data to support that.

Vince Calder
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