From the FAQ, valid questions for Chemistry.SE include:
- Questions asking for explanation of a chemistry concept
- Questions relating to observed chemical phenomena
- Questions about experimental techniques and technology
- Questions about nomenclature, standards, etcetera pertaining to
A "Chemical Request" question could fall under one of the first three bullets if
- the request includes some pedagogical component
- what's a good material to explain concept X
- the request includes some rationale for the request
- I've observed that material X has such and such properties, are there similar materials that are [cheaper, less toxic, ...]
- the request includes some application/background information
- I'm doing a science fair project dealing with X and am looking for materials that would help me explore the following hypothesis...
The question that raised this meta question does not in my opinion fit any of these criteria and is probably not suitable for Chemistry.SE as written. If there is some way that the question can be expanded by providing an application, purpose, rationale or some other component that ultimately increases the value of the question/answer to a broad range of visitors then it could be allowed.
Based on these criteria, I would think that the thermos question comes close to meeting the requirements, although it would be more beneficial if the OP would provide a reason for the project, which would allow someone to provide a different perspective. The explosive plastic question is, as suggested by the OP, absurd in the absence of any type of reasoning for the project.