RE: [GreenYes] Plastic Lumber - What is it made of?

[Jay Donnaway <jdonnawa@dca.state.ga.us>]

Most projects to make mixed resin plastic lumber have failed because the
resins melt at different temperatures and consequently don't mix or bond
very well.  
	Trex is made of both HDPE and Low Density Polyethylene films
(grocery bags, nonfood stretch wrap) due to their similar characteristics,
and is blended with wood flour as filler/reinforcement.  Pure HDPE is often
used (US Plastic Lumber) because it is the household resin present in the
greatest quantities (curbside-collected milk, juice, detergent bottles), is
easily sorted by color, and it is relatively easy to work with.
Polypropylene behaves similarly and is often blended with HPDE for low-value
applications such as flower pots, irrigation piping, and drainage culverts.
	PVC is very recyclable as a straight resin, and has a good recovery
rate in industrial scrap, but is present in very low quantities among
postconsumer bottles.  PVC has good weathering characteristics, but is very
flexible, so PVC fences are made of large diameter hollow sections rather
than the solid extrusions that are easier with recycled resin.  Besides, PVC
turns into a mix of toxic gas and black tar at the temperatures required to
melt PETE.  PVC products containing plasticizers, such as food wrap, baby
teething rings, etc. are the main plastic of concern for their tendency to
leach into our bodies, and PVC also releases toxic smoke in a building fire
(commercial baseboards and carpet backing).  The other common plastic resins
are practically inert, which doesn't mean wildlife won't choke on the stuff
or become bloated with floating pellets picked up in some mid-ocean gyre.
	Recycled High Impact Polystyrene (not expanded polystyrene) is
suitable for lumber, being rigid and lightweight, but those properties give
it much higher value applications in industrial tubs and trays, wire spools,
etc..

Jay
> -----Original Message-----
> From:	Stephan Pollard [SMTP:sp@cast.uark.edu]
> Sent:	Tuesday, January 22, 2002 5:41 PM
> To:	Green Yes
> Subject:	[GreenYes] Plastic Lumber - What is it made of?
> 
> I'd like to understand plastics and plastic lumber a little better.
> Most information sources on plastic lumber talk about it being made of
> HDPE.  I'd like to know if other plastics (PETE, PVC, PP, PS, Other) can
> be used to make plastic lumber, and if so, what they are, and in what
> combinations.  Why is it that HDPE is always mentioned?  Is it possible
> to co-mingle all plastics to make a plastic lumber?
> 
> --
> Stephan
> 
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