Plastics that are relatively IR transparent

You can check the IR transparency windows of polymer films, as well as the interference of additives, by examining their IR spectra that can be found in the literature or experimentally being taken.
You can also check the resistance to solvents and corrosives of various plastics, by searching in the corresponding tables that can be found in the web, e.g.:
http://www.plasticsintl.com/plastics_chemical_resistence_chart.html
http://www.curbellplastics.com/technical-resources/pdf/chemical-resistance-plastics.pdf

Thank you both.

We are going through a number of plastics to see their reaction to the chemistry (short term, long term and with different chemical reactions).

Yes and No.
No, if transparency bellow 1600cm-1 is desired.
Yes, if a frequency window between 2800-1650 cm-1 is adequate.
Besides, PVC is very robust against organic solvents and corrosive chemicals and therefore, it is widely used for WC and sewage piping.

A mere transparency band is not desired for thermography, because the polymer radiates at wevelengths where it isn't transparent, so one would measure the polymer's temperature.

Polyethylene is the common choice to make windows for thermal infrared detectors. Though, 90°C is already much; you'd better check each PE (there are dozens) for stability, and avoid by design mechanical stress on the PE.

Producer datasheets give a deflection temperature. A handbook in German:
Kunststofftabellen, by Bodo Carlowitz

Here a few polymer spectra, including around 1000/cm
http://www.ftir-polymers.com/soon.htm
showing that polypropylene is a candidate as well, and it may resist heat better (again, check exactly which PP). Polymethylpentene is excellent at heat but I haven't seen its spectrum; Tydexoptics gives one but their spectrum for PE is obviously wrong. Ask Mitsui directly?

Thank you Enthalpy for your reply! I will look into this further.

Quote from:

1). PVC IR spectrum has a frequency window around 800 cm-1 ≈ 12.5 µm.
2). If transparency fits, the (corrosives resistant) PVC is suitable for "building a small reactor with corrosive chemicals".

Agreed.  If PVC is indeed transparent at the wavelengths needed.  But how household use translates to corrosive chemicals, (which the OP left too open ended to make much of a determination anyway,) I don't get.

I mean, my poop and concentrated sulfuric acid are the same, in that I don't want them poured on my arm, but for totally different reasons chemically.

The camera has a spectral range of 7.5-13µm, and it is necessary that the plastic transmits all this range or nearly.

The reason is that if the plastic absorbs a part of it, not only will a part of the radiation by the liquid be blocked; the plastic will also radiate at the wavelengths it absorbs (as a consequence of thermo's second law), so the camera would measure the plastic's temperature more than the liquid's temperature.

PVC absorbing badly right in the camera's range (and the linked measure is for a thin sheet), it doesn't fit the needs. PE is good (and known for this use), PP not bad.

A number of corrosive chemicals are used in house cleaning that all finish their useful file in the drainage piping. Such corrosive chemicals are HCl for the WC, KOH and NaOH for the washbasin, ammoniac solutions for the floor and NaClO for disinfection, together with large amounts of surfactants. Consequently, the drainage pipes must be resistant to these corrosive chemicals.
Besides, specific natural products and natural metabolites can destroy various plastics or at least, deteriorate their mechanical properties, under daily contact in the piping curves. As a few indicative examples: polyesters are not resistant to catabolic amines and urea, epoxy resins are not resistant to lactic acid, polyurethanes are not resistant to fats and vegetable oils, polyolefins are not resistant to long term contact with concentrated nonionic surfactants, etc. Thus, low plasticizer content PVC that is one of the most chemicals resistant materials can resist for a long, to all above and can be the choice material for house drainage piping, at a reasonable cost. A thicker modification of lower plasticizer content is used for chemistry, industrial and heavy duty applications.

High density polyethylene (HDPE), polypropylene, cross linked polypropylene and PVC seem to be to most appropriate materials, depending on the resistance to the specific corrosive chemical.

Polyethylene (and most polyolefins) has a number of transparent IR windows. However keep in mind that most commercial plastic containers have a lot of additives in them that may also be absorptive in your region of interest. Also, how much light penetrates will depend on the thickness of the material.

So, it's not necessarily just a matter of identifying a polymer. There are other parameters to worry about. It may be easier in the long run just to do some trial and error.

PVC transparent at 7.5-13µm? These curves speak against
http://www.ftir-polymers.com/soon.htm

Quote from:

Yes and No.
No, if transparency bellow 1600cm-1 is desired.

So no, given that's what the O.P. asked for.

Quote

Besides, PVC is very robust against organic solvents and corrosive chemicals and therefore, it is widely used for WC and sewage piping.

And this is pertinent to this question how?

1). PVC IR spectrum has a frequency window around 800 cm-1 ≈ 12.5 µm.
2). If transparency fits, the (corrosives resistant) PVC is suitable for "building a small reactor with corrosive chemicals".