Ask A Scientist© Chemistry Archive

 
name       omry
status     other
age        20s

Question - hi,i would like to know if there is a risk in melting
plastic material( in order to make models from the plastic ), by heating
it in a pot, not by burning it (such as - dangerous gas ). also, it would
be great if you can give me another way for shaping plastic materials.
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Yes, there is a possible hazard. Heating some plastics will
cause them to degas residual solvent, and heating others will
cause them to decompose or degrade into what could be toxic
or hazardous gases.

I'm afraid that I can't advise you on other methods to try. You
need to use the method that is recommended by the manufacturer
of the plastic you are working with. It should be easy to
get these recommendations from the manufacturer, as well as
the MSDS (materials safety data sheet) on the plastic.
They are required to give you an MSDS on request by federal law.

best regards,
prof. topper
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Different polymers have different melting points. Many plastics are shaped
by heating them until they are soft, and then molding , pressing, stamping,
or forming in some other way. It is hard to do this at home, however,
because it is necessary to carefully control the temperature. If the
plastics are heated much above their softening points, they can darken and
char. Since plastics are poor conductors of heat, it is difficult to get the
whole sample hot enough to melt without crisping the outside surface. This
is a very significant risk if you try to melt the plastic in a pot.

More controlled, lower temperatures are necessary than you can usually get
over a stove. If the plastic has a melting point less than the boiling
point of water, you could heat it in a water bath or in a double boiler.
Most plastics need a higher temperatures, however. Your best bet, if you
lack a plastic welding setup (which melts the plastic with a stream of
heated nitrogen gas), is to use a hair dryer.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
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Without knowing what you are trying to accomplish, it is hard to give you
the answer you really want or need, so I'll offer some generalizations. (If
you had asked, "Is it difficult to crush stone?", the answer would vary
depending on whether you intended to crush limestone (relatively easy), or
granite (relatively hard)). In melting plastics, there can be a risk of
fire, contact with hot materials or toxic fumes (especially with overheated
or burning PVC or CPVC). Now for shaping plastic materials. Since assorted
shapes of pre-formed materials, from which pieces may be cut (or carved),
are available, I will confine the rest of the reply to melting and molding
(or casting) materials. For thermosetting resins (e.g. Bakelite), melting
and reforming is not generally an option. For the recyclable thermoforming
resins such as polyethylenes, polypropylenes, and perhaps polystyrenes, you
have a better bet. Keep in mind that the companies that regularly work with
these resins do so using controlled heat and "no air". My suggestion is to
exercise plenty of caution,(including, but not necessarily limited to, lots
of ventilation and fresh air, protective gloves and eyewear, proper fire
extinguisher, aid within easy earshot, etc.) and to try working with small
quantities (a few grams) at least at first. Choose a single material and
learn how to handle it before trying something else. Pay close attention
and keep good notes. It's often as important to what doesn't work, and why.
Especially note temperatures where materials soften, liquify and/or start to
smoke or emit fumes. Softened material can be injection-molded, liquified
material can be cast, smoking material may be damaged beyond usefulness.
Avoid heating by flame. While flame btus are cheaper than electric btus,
the electric heat can be made more uniform and easier to control
temperature. Overheating can be easier to avoid too. Help may be available
from industry webpages like www.plastics.org or from supplier websites like
www.huntsman.com for gathering information before you start. Good luck!.
Tim Spry
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THERE IS SUBSTANTIAL RISK IN MELTING/HEATING PLASTICS [ =POLYMERS ] 
WITHOUT A FUME HOOD AND WITHOUT KNOWING EXACTLY WHAT IS IN THE PLASTICS. 
IT IS NOT RECOMMENDED FOR THE REASONS LISTED BELOW AND SOME THAT DON'T 
COME TO MIND:

1.   There are two classes of polymers: thermoplastic and thermosetting. 
The former will soften when heated; the latter will never soften, just 
decompose, when heated. Bakelite is an example of the latter.

2.   Some polymers produce toxic gases when heated. For example, vinyl 
chloride produces HCl gas when heated.

3.    Plastics are not pure materials; they are formulated products. Some 
of the chemicals used to give the plastics their desired mechanical and 
physical properties also form toxic vapors when heated. An example are the 
common phthalate plasticizers.

4.   Some polymers "unzip" when heated evolving monomer vapors. Acrylates 
polymers are notorious for this. Acrylate monomers are toxic.

A safer route would be to use water dispensable gums [ e.g. algenates ] 
like dentists use to form a denture. These harden and can be used to form 
models from plaster of Paris, or self-reacting styrene formulations 
available at hobby stores. Even in this case adequate ventilation is 
necessary because styrene vapor is also toxic.

Vince Calder
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