MadSci Network: Chemistry

Re: How is carbon used in making plastics

Date: Thu Mar 30 19:16:49 2000
Posted By: Narayan Variankaval, Grad student, Polymers/Textile and Fiber Engineering, Georgia Tech
Area of science: Chemistry
ID: 953436142.Ch

Making plastics directly from carbon can actually turn out to be a very 
expensive and laborious process.  This is because carbon by itself is very 
stable.  One has to go to high temperatures and/or pressure to make it 
reactive.  Once it reacts however a lot of useful compounds can be made 
from it including plastics (but at a high cost). 
	For example acetylene can be produced from carbon as follows.  
Coke (which is nothing but carbon) is heated with limestone in an electric 
furnace to form calcium carbide.

CaO + 3C = CaC2 + CO

This calcium carbide that is formed reacts with water to form calcium 
hydroxide and acetylene.   

There are other ways of producing acetylene from carbon. One such 
important process is using an electric arc furnace.  

Now we have synthesized acetylene.  How would you convert acetylene to a 
plastic?  You can polymerize it to polyacetylene which can be called a 
plastic.  However even this procedure is difficult.  You have to use 
compounds called initiators to initiate the polymerization process.  Some 
examples of these initiators are triethyl aluminium (Al(C2H5)3) and Ti(O-I-
C4H9)4. (see reference 1).  Polyacetylene when doped (mixed) with iodine 
is used as a polymeric semiconductor.  As you can see it a complicated and 
expensive process to convert carbon to a plastic.  In addition you have to 
use additional reagent/chemicals to produce the plastic.  

The most important step in the above process is the first step wherein you 
produce acetylene from carbon.  Once you produce acetylene you can make a 
host other compounds from it such as aldehydes, ethylene etc.  These 
compounds can be polymerized to give other plastics such as polyethylene.  

For example acetylene can be hydrogenated to produce ethylene in the 
presence of a catalyst such as Platinum, Palladium or Nickel.  Ethylene 
can be polymerized by heating it to above 160-180C and at high pressures 
of 120-300 megapascals.  You can find details of this process in 
references 2 and 3.  This particular plastic is called low density 
polyethylene. You can also make high density polyethylene and the process 
is also given in reference 3.  

During World War I,  the synthesis of vinyl acetylene by Julius Nieuwland 
led to the development in 1932 of the synthetic rubber, neoprene, by 
DuPont. Its annual output reached 120,000 tons by 1960. 

In Germany after World War I, butadiene made from acetylene was the basis 
of a rubber substitute that made the country self-sufficient in rubber.  
One can thus come a long way with starting from carbon to acetylene to 

Before I end one last example.  Acetylene can be made to react with water 
to produce vinyl alcohol which when polymerized yields polyvinyl alcohol.  
This plastic is one of the substances used to dye the Chicago river green 
during St. Patrick's.  A green dye is dispersed in PVA and released into 
the river.  The plastic itself is harmless and dissolves in the water.  

(1) Ito H, Shirakawa H., Ikeda S., J. Polym. Sci. Poly. Chem. Ed., V12, 
11, 1974
(2) Doak K. W., Low density polyethylene (High Pressure), pp386-429 
in "Encyclopedia of Polymer Science and Engineering", Vol6, 2nd ed., H. F. 
Mark, N.M. Bikales, C.G. Overberger, G. Menges, Eds., Wiley-Interscience 
New York, 1986.
(3) G.Odian, Principles of Polymerization, pp 303-306, John Wiley and 
Sons , NY, 1991

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