Re: RE- Re: Polyethylene

Subject:  Re: RE- Re: Polyethylene
  Date:   Sun, 15 Jun 1997 17:48:22 -0400
  From:  "Daryl P. Dacko" <mycrump-at-cris-dot-com>
    To:   Tesla List <tesla-at-pupman-dot-com>

At 08:46 AM 6/15/97 -0500, you wrote:

>>         Polyethylene cannot be melted in air:  It burns (starts to
>>         char, actually) before it reaches its melting point.
>>                 This is a generalized rule of thumb.  Some
>>                 of the very low molecular-weight polyethylenes
>>                 do melt at a low enough temperature - i.e.
>>                 before combustion.
>>         The melting (and for that matter, welding) of polyethylene
>>         is done under a nitrogen atmosphere or blanket for this
>>         reason.

<big snip>

>I've been of the impression that polyethylene was a thermo-setting 
>plastic and as such required a raised teperature to crosslink the 
>>         Then go have your head examined.
>I'ts all relative Robert, if one can't find useable polyethylene then 
>it is not so crazy to try and make it, besides if one did find an 
>easy way to recycle polyethelene milk jugs into H.V. capacitors
>it would be of great assistance to the environent.

Polyethylene is a thermoplastic, and will melt, and it is possible to 
recycle it.

Polyethylene and polyproplyene don't really 'melt', they (above a 
certain critical temperature) just get softer and softer. Usually 
you can't get them hot enough to flow by themselves without a fair
amount of degradation of their propertys.

The crosslinking you heard about is used to stiffen the polymer and
give it propertys that it wouldn't have otherwise. If it were carryed
out too far, the polymer wouldn't melt, true enought, just like 
thermoset plastics, but just a little helps... 

A way you =could= recycle it would be to use what's called compression
moulding. You make a frame the same thickness as the sheet of plastic
you want to make, lets say 18 inches square and 90 mills thick. You
cut out a hole in the center to leave a border arount the outside..

                       | |                               | |
                       | |                               | |  
                       | |                               | |  
                       | |                               | |
                       | |                               | | 
                       | |                               | |
      Maybe 20"        | | <--------- 18 " ------------> | |
      square on        | |                               | |
      the outside      | |                               | |
                       | |                               | |
                       | |                               | |
                       | |                               | |
                       | |                               | |

You get a couple pieces of iron plate 24 inches square and about
and inch or so thick and mount heaters on them, or put them in an
old oven. 

Lay a piece of alunimum foil on the iron plate, then the frame and 
then heap up a 'bunch' of sniped up pieces of your favorate plastic
milk jugs in the frame. It usually takes about three or four times
an much as you'd think will fill the frame. Then lay down another
piece of foil and then the other (heavy) iron plate. Heat and apply
pressure. The plastic will slowly flow between the frame and plates
and any excess will ooze out, leaving (if the temperature and 
pressure were right) a nice bubble free mould. Cool and peel off
the foil and pop the plate out.

We use this method every day where we work to prepare test samples 
of many diffrent types of plastic, so it's tryed and true.

You'd have to experiment with what temperature to use, and whether
the iron plates would provide enough pressure by their own weight.
The presses we use apply 5 tons of force to the mould, but I doubt
that that much pressure is needed for low density polyethylene.

Worst come to worst, you could build some sort of stand and use a
car jack to apply pressure...

Just a thought,