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I'm looking for information about heaters that can withstand some harsh environments (misc. chemicals), does anyone have any advise?

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For every complex question there is a simple answer, and it is wrong.

Depending on what types of chemicals your heater will be in contact with would drive the type of heater you should use in your application.  Etched flexible Teflon heaters and/or Teflon coated tubular heaters are the most impervious to most harsh chemicals. The type of  Teflon heater to select depends on your application's performance criteria.    Etched Polyimide heaters are also quite chemical resistant but polyimides do not react well to hydroxides.  Other heater options to consider would be silicone rubber blanket heaters that are coated with a gum rubber which seals the woven fibers.  Neoprene heaters are another option to consider in the heater industry.   Cenelec approved etched mica heater encased in metal are approved for harsh (explosion-proof) environments.

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It is not the answer that enlightens, but the question.

I agree with what Buck45 said, including about silicone rubber based heaters.  I'll just add that if your dealing with chronic exposure to strong acids or bases, you need to be cautious with silicone rubber....even if it is just vapors.  Silicone handles hydrochloric acid alright, but hydrofluoric is very hard on it.  Different silicone compounds handle different acids or bases better than others as well.You may want to get in contact with your heater vendor and run the info by them to find out suitability before proceeding in a particular direction.

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'Experience is a hard teacher because she gives the test first, the lesson afterward.' --Vernon Law

Polyimide heaters generally do resist chemicals well, but if your application involves immersion I would highly recommend a Teflon heater.  Even a relatively weak acid solution soak over a period of a couple days can attack a polyimide-insulated heater to the point that the layers will separate on their own.  Also any involvement of potassium hydroxide will take out just about anything but a Teflon heater.

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"A witty saying proves nothing."
-Voltaire

I am interesting to know more about the etched flexible Teflon Heater that you recommend. 

Where can I buy the product? How thick is it? I am curious that how one can make Teflon stick with other mateirls to make the flexible heater?

I am also interested in any documents/sites that can help me to understand about the etched flexible Teflon Heater, and it difference comapre to other types of flexible heaters such as Polyimdies, Silicone Rubber types.

Thank you in advance.

Mars 

Minco is the only heater manufacturer that I know of that can make Teflon etched foil heaters.  I am pretty sure it is a trade secret how they stick pure teflon to anything.  The heaters are usually thicker than a Kapton heater (.020"-.100" depending on application) and while they are still flexible, they are stiffer than say, a rubber heater.  The heaters usually need to be clamped in place.  The temperature limit is around 260°C.  I would have to refer you to Minco for more information or a quote. 

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"A witty saying proves nothing."
-Voltaire

Silicone is vapor-permeable, 5 to 10 times more than other rubbers and teflon, more still than kapton.
HCL, H2O, and O2 vapors, for example, will drift thru it in days or less, making the metal foil basically naked to slow corrosion in a solution that has acid vapors.