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hoser4963 (Automotive)
17 Apr 07 20:03
I want to weld some plastic parts and need to identify the plastic. How do you determine the type of plastic?
Thanks
CoryPad (Materials)
17 Apr 07 21:36
The typical method of identifying an unknown polymer is infrared spectrometry (commonly known as FTIR for Fourier Transform InfrRed).  Many independent testing laboratories will conduct this testing for a modest fee.

Regards,

Cory

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Demon3 (Materials)
18 Apr 07 2:28
Yes, FTIR is a very good way. Here is a quick guide to how to identify plastics.

http://www.modernplastics.com/how_to_identify_plastics.htm


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

rconner (Civil/Environmental)
18 Apr 07 17:26
Demon3,
I think the procedure you provided the link to is likely well-meaning and originally from a legitimate source, and I think this verbatim procedure even additionally appears or has been picked up on multiple other websites.  However, the first step/instruction in the second paragraph on this and some other identical web pages appears to me to be either missing some original copy or arguably very poorly written (that could maybe turn some readers off right off the bat?)
I think the writer instead likely intended a first step of procedure something to the effect that you heat the dickens out of a rod, wire or nail or something like that, and then touch it, scribe it on, or try to embed it into the plastic.  If the plastic melts or marks very easily, it is likely "thermoplastic" (let me know if I am not getting at least the gist of this right).  If someone didn't even have any way to precisely gauge temperature (e.g. 500 degrees), I guess they might get at least an idea of the initial category I.D. by heating a steel nail, wire or rod etc. up until it glows to begin with (this is of course generally more than 500 F.), and then maybe letting it cool some to hope to get at least the ~500 degrees F. etc.?
[Of course all appropriate safety precautions for heating to very high temperatures and burning unknown plastics must also be followed, in whatever procedure intended.]   
CoryPad (Materials)
18 Apr 07 23:39
I have seen that procedure using a soldering iron (no need to have glowing metal, that is much higher than necessary).

Regards,

Cory

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patprimmer (Publican)
19 Apr 07 0:36
I agree paragraph two is a bit vague.

I simply have a flame, and cut a sliver of the plastic and grasp it with tweezers and slowly approach the flame with it. If it melts before decomposing, it is thermoplastic, if it decomposes without melting it is thermoset.

Thermosets may leave a sticky residue as a product of decomposition, but that does not mean they are thermoplastic.

If you are not well experienced with the sniff test, samples of known material to use as a reference are quite valuable.

The sniff test was much more conclusive before flame retardants, rubber modifiers and alloys were so common, as the afore mentioned factors can create a confusing smell.

Another very basic test is the floatation test. If a solid piece of the plastic, with no bubbles or voids, floats, it is a polyolefin. If it slowly sinks it is a plastic with an SG of over 1.0 but less than 1.2. If it sinks like a rock, it probably has an SG of over 1.2.

Still another is to compare surface hardness and surface feel.

Some tests with basic equipment are to determine an accurate SG, measure surface hardness, measure melting temperature.

A rough house melt temperature test is to place a small piece of the plastic between different known samples on a cold hotplate then turn on the hotplate and note the sequence of melting.

Regards

eng-tips, by professional engineers for professional engineers
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Demon3 (Materials)
19 Apr 07 7:11
Yes, it is vague with mistakes. I saw that later when I looked in more detail. I know there is a small book explaining it much better but that's the best I could find in 20 seconds using Google. It may well be that there's something more accurate out there available to a more persistant Googler.

I would do FTIR.


There is not any memory with less satisfaction than the memory of some temptation we resisted.
- James Branch Cabell

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