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[greenyes] polystyrene terms


Dear All,

Let me try to straighten out some terms in the recent "It's polystyrene not
styrofoam" discussion.

1. Styrene is the monomer, or building block, from which polystyrene, the
polymer, is made.
2. There are many forms of polystyrene or PS:
- foamed PS (or Dow's Styrofoam™) is the material that carryout cups,
clamshells, meat & produce trays at the grocery store, & some packing peanuts
are made from starting with the PS resin in pellet form; these items used to be
foamed with CFCs & HCFCs, but according to my understanding, industry HAS
phased out use of CFCs and HCFCs, & now uses steam or CO2 to foam the plastic.
- expanded PS, or EPS refers to the puffed up block material packing
shapes made to precisely fit computers, TVs, electronics, etc; maybe some
packing peanuts are made this way too; it's a different process, and it starts with
PS granules, not pellets; I don't remember how they expand the PS, but think
this, too, no longer involves CFCs* or HCFCs*.
- there are myriad other forms and ways of molding PS:
- rigid PS cutlery (knives, forks) is injection molded from PS
pellets with no foaming or expanding involved (but note that cutlery is also made
from polypropylene, or PP)
- clear, rigid disposable drink cups & carryout containers are made
from injection molded PS (but also can be made from PET)
- high impact PS, or HIPS, is often used for making computer
casings, boom boxes, audio cassette cases, & many other semi-durable or durable
items, including some auto interior & other parts (I forget--HIPS may be a
co-polymer that's reinforced with another polymer or rubber, or possibly the strength
comes from a different molecular structure).
- PS crystal - I think this is the term - refers to extruded thin,
clear trays, for instance the trays inside a cookie package that keep the
cookies from getting broken.
- rigid, blow-molded PS bottles - on a rare occasion you'll see a
bottle, like a brown plastic vitamin bottle, made from PS; however, use of PS
for bottles is very limited.
- possibly there are extruded PS applications, tho I can't think of
any examples at the moment; extrusion is a molding process that forces
pellets thru a cylindrical chamber with a high shear screw inside, the friction
melts the plastic, & it comes out the other end kind of like toothpaste from a
toothpaste tube; products are typically long profiles such as lumber, pipe, &
tubing; my guess is that PS is too brittle for this sort of application.
However, at least 1 plastic lumber company does make PS lumber reinforced with
fiberglass.

About polypropylene packing shapes for computers, etc, I've never seen these,
but don't rule out that this could be possible. The main alternative foamed
packing material I've seen for such products has been foamed HDPE (high
density polyethylene) or foamed LDPE (low density polyethylene). This material can
be distinguished from EPS because it'll be a bit squishy, i.e. will give more
if you squeeze it, whereas EPS is more likely to crumble into pieces (if you
squeeze it hard enough).

* Under the Montreal Protocol, industry was obliged to sharply cut back on
production/use of CFCs (chlorofluorocarbons) and HCFCs
(hydrogen-chlorofluorocarbons). It's my understanding that industry really did comply with this.
Given the choice between packaging applications & using CFCs/HCFCs as
coolants/refrigerants (in air conditioners, refrigerators, etc), industry opted for the
higher value use of the diminishing supply. That's why, for instance, methods
were developed & are now widely used to extract freon (CFC) from your auto
air conditioner or discarded refrigerator, clean it up, then recharge it back
into, say, your car AC, or sell it back to mfgrs of appliances that need
coolants.

I'm not a polymer chemist, so this is a layperson's explanation after many
years studying plastics & looking for ways to recycle them. I have not
commented on possible leaching hazards from styrene not because I don't suspect
hazards, but because I don't have the scientific data.

Hope these clarifications help.

Cheers,
Gretchen Brewer
Earth Circle
San Diego, CA
ph 619-282-5132






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