Posted by
Oude Yi on Tuesday, December 22, 2009 3:43:03 AM
characteristics
the term 'plastics' is used to describe a wide variety of resins or polymers
with different characteristics and uses.
polymers are long chains of molecules, a group of many units, taking its
name from the greek 'poly' (meaning 'many') and 'meros'
(meaning 'parts' or 'units').
while all plastics are polymers, not all polymers are plastic.
for the discussion of recycling, an understanding of two basic types of
polymers is helpful:
* thermoplastic polymers can be heated and formed,
then heated and formed again and again. the shape of the polymer
molecules are generally linear or slightly branched. this means that the
molecules can flow under pressure when heated above their melting point.
* thermoset polymers undergo a chemical change when they are heated,
creating a three-dimensional network. after they are heated and formed,
these molecules cannot be re-heated and re-formed.
comparing these types, thermoplastics are much easier to adapt to PET bottle recycling.
plastic from a ‘blow mold’ (the neck of the bottle is narrower than the body)
has a slightly different structure from the exact same plastic used in an
‘injection mold’ (where the opening is the widest part of the product).
plastic identification / recycling code
when working with plastics there is often a need to identify which
particular plastic material has been used for a given product.
most consumers recognize the types of plastics by the numerical coding
system created by the society of the plastics industry in the late 1980s.
there are seven different types of plastic resins that are commonly used to
package household products. the identification codes listed below can be
found on the bottom of most plastic packaging.
#1- polyethylene terephthalate (PET)
soda & water containers, some waterproof packaging, tennis balls.
#2 - high-density polyethylene (PE)
milk, detergent & oil bottles. toys and plastic bags.
#3 - vinyl / polyvinyl chloride (PVC)
food wrap, vegetable oil bottles, blister packages.
#4 - low-density polyethylene
many plastic bags. shrink wrap, garment bags.
#5 - polypropylene
refrigerated containers, some bags, most bottle tops,
some carpets, some food wrap, chairs (back/seats).
#6 - polystyrene
throwaway utensils, meat packing, protective packing.
#7 - other. usually layered or mixed plastic.
no PET bottle recycling - must be landfilled.
recycling
although all plastic containers bear the chasing arrows symbol with a
number in the middle, suggesting that all such products are recyclable,
it is only 1s and 2s that can be.
there is no market for bottles numbered 3 through 7.
most of the products which are manufactured from what is
recycled, can't be recycled a second time.
so, what you set out at your curb is only one generation away
from a landfill!
PET bottle recycling is similar to the polyethylenes.
bottles may be color sorted and are ground up and washed.
unlike polyethylene, PET sinks in the wash water while the plastic caps
and labels are floated off. the clean flake is dried and often repelletized.
PVC bottles are hard to tell apart from PET bottles,
but one stray PVC bottle in a melt of 10,000 PET bottles can ruin the entire batch.
it's understandable why purchasers of recycled plastics want to make sure that
the plastic is sorted properly.
equipment to sort plastics is being developed, but currently most recyclers are
still sorting plastics by hand (and in the third world)
that's a hard and ugly work, it's expensive and time consuming.
plastics also are bulky and cumbersome to collect.
in short, they take up a lot of space in recycling trucks.
from:odemade