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[GreenYes] FW: [GreenYes] Re: Fwd: Plastic Bags - how much oil is wasted in producing them?




Subject: [GreenYes] Re: Fwd: Plastic Bags - how much oil is wasted in
producing them?


Correction----------------------I should have been more careful in posting.

This should have read "Pound for pound, plastics contain as much energy as
petroleum or natural gas, so this represents the loss of almost 4 MILLION
tons of MATERIAL CONTAINING energy each year. (Not 4 billion tons)

When I get a chance to do so, I'll do the calculations on the energy content
of 4 million tons of plastic bags.

On the question of what are plastic bags made from? The answer is
Polyethylene which means that they're made from polymerized ethylene.

Ethylene is gasoline.

See below. Source:
http://pubs.acs.org/cen/whatstuff/stuff/8238plasticbags.html

Plastics are composed of polymers--large molecules consisting of repeating
units called monomers. In the case of plastic bags, the repeating units are
ethylene, or ethene. When ethylene molecules are polymerized to form
polyethylene, they form long chains of carbon atoms in which each carbon
also is bonded to two hydrogen atoms.

Many kinds of polyethylene can be made from ethylene. Plastic bags typically
are made from one of three basic types: high-density polyethylene (HDPE),
low-density polyethylene (LDPE), or linear low-density polyethylene (LLDPE).
Those thick, glossy shopping bags from the mall are LLDPE, while grocery
bags are HDPE, and garment bags from the dry cleaner are LDPE. The major
difference between these three materials is the degree of branching of the
polymer chain. HDPE and LLDPE are composed of linear, unbranched chains,
while LDPE chains are branched.

Branching can influence a number of physical properties including tensile
strength and crystallinity. The more branched a molecule is, the lower is
its tensile strength and crystallinity. That's why garment bags from the dry
cleaner are so weak and flimsy. They are made from highly branched LDPE.

Another difference among these types of plastics is the method in which they
are made. HDPE and LLDPE are made by Ziegler-Natta vinyl polymerization, a
method that uses a transition-metal catalyst, like TiCl3, to initiate
polymerization. Karl Ziegler and Giulio Natta received the 1963 Nobel Prize
in Chemistry for developing this method.

For a long time, Ziegler-Natta polymerization was the only way to produce
linear unbranched polyethylene. Recently, a new way of producing
polyethylene has become available: metallocene-catalyzed polymerization.
Like the Ziegler-Natta method, the metallocene method uses metal complexes
as catalysts and is often used to make novel copolymers.

LDPE, the branched polyethylene, is made by a different process called
free-radical polymerization. The oldest of the three methods, it uses an
initiator molecule like benzoyl peroxide instead of a metal complex.
The polymerization starts when the benzoyl peroxide molecules break apart
into free radicals. The unpaired electrons of the free radicals attack
ethylene's carbon-carbon double bond, forming new free radicals.
These then react with other ethylene molecules and so on, forming a chain of
single-bonded carbon atoms that grows until the reaction terminates, often
when two free radicals join together.

What else do plastic bags contain? According to William F. Carroll Jr., a
polymer chemist at Occidental Chemical Corp. and president-elect of the
American Chemical Society, "With the possible exception of a little
lubricant to help in extrusion, plastic bags are pretty much just the native
polymer." However, different pigments may be added to produce colored bags.



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