GreenYes Archives

[GreenYes Home] - [Thread Index] - [Date Index]
[Date Prev] - [Date Next] - [Thread Prev] - [Thread Next]


[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.




[GreenYes Home] - [Date Index] - [Thread Index]
[Date Prev] - [Date Next] - [Thread Prev] - [Thread Next]