# Which is the cheapest plastic that retains its structure at 85 °C or higher temperature?

In an academic project, I need to protect some electronics components from heat (around 85 °C). I am searching for both a cheap plastic film and a cheap plastic to be used in a injection molding process.

As matt_black referred, the main criteria of the selection is "the point where structural integrity is reduced by repeated use at that temperature or the point where the polymer starts to creep".

By now I am using cork, but I would like to use a more robust material.

Those materials should be both thermal and electrical insulators and should not absorb water. They should also be resistant to the external environment: rain, sun (UV radiation), dust, etc.

Which materials are the best option in this case? From what I have searched, these are the main options in terms of thermal requirements:

• PPO/PPE based
• Polypropylene
• Polyurethane
• Vinyls
• Polybutylene
• Acetals
• ABS and SAN
• Polystyrene
• ABS/Polycarbonate alloy
• Acrylics
• Cellulosics
• Polyethylene and copolymers
• When you say "protect from heat" do you mean insulate or something else? Protecting from things happening at that temperature (like water ingress) is different from protecting against reaching that temperature. – matt_black Jun 15 '18 at 10:44
• @matt_black The main function of the film is to thermally insulate, being the no water absorption also an important factor. If used a container it would be a plus to protect it from external environment. – DDDD Jun 15 '18 at 12:07
• It is also worth noting that most solid plastics won't do much insulating from heat (except for short periods) though they will act as a barrier to water or air to some extent. Good insulation requires something else like a foamed material (or cork which you already use) possibly plus a heat reflector. You could use cork coated in foil all sealed inside a plastic film. – matt_black Jun 19 '18 at 9:00
• Would a thermoset plastic be OK (e.g. Bakelite?). – Jon Custer Jun 19 '18 at 17:27

The melting point is not going to give you the answer you want. To properly select a polymer you need to look at the glass transition temperature $(T_\mathrm{g})$. This is the temperature at which a material loses crystallinity and starts to more easily flow. This parameter varies widely with thermal history and internal stress of a polymer material and can be raised by annealing.

Based on UV resistance, I would exclude any olefin or vinyl polymers as well as ABS.

For water resistance, I would exclude cellulose polymers and acetals.

Based on the temperature requirement I would say polystyrene ($T_\mathrm{g} = \pu{100^\circ C}$), acrylics $(T_\mathrm{g} = 85 - \pu{165^\circ C})$, polycarbonate ($T_\mathrm{g} = \pu{135^\circ C}$), or PPE/PPO ($T_\mathrm{g} = \pu{215^\circ C}$) have sufficiently high $T_\mathrm{g}$'s to remain as acceptable for the mentioned specifications.

• In terms of prices, can you give me a hint on the prices (the lowest)? Do you think ABS with a UV stabilizer would be an option? – DDDD Jun 19 '18 at 3:59
• Well, you can get acrylic and polycarbonate at most hardware stores; acrylic being the cheaper of the two, for which, I would go with acrylic then. – A.K. Jun 19 '18 at 4:02
• The Tg of a polymer depends also on its molecular weight. So it doesn't mean a lot of thing to speak only about the Tg. See Kanig ueberreiter's equation. – ParaH2 Jun 19 '18 at 17:42
• I did intend to include molecular weight in my discussion but forgot to include it. – A.K. Jun 20 '18 at 4:14

Googling "common plastic melting point" produces this list for me, and taking a quick look at the result it looks like ordinary polyethylene might do OK (mp 140C) and if you go to nylon (mp 220-270C) you would have a pretty comfortable margin. A well-known issue with nylon, however, is that it tends to absorb water and in damp environments might not keep its dimensions exactly. Only you know whether that affects your application.

• It isn't the melting point you have to worry about: it is the point where the structural integrity is reduced by repeated use at that temperature or the point where the polymer starts to creep. Polyethylene is not usually recommended for use at ~100°C but polypropylene is better (if I remember correctly). – matt_black Jun 15 '18 at 10:41
• @matt_black Is there a table with that info? Thanks – DDDD Jun 15 '18 at 12:10