Let's say I have several pieces of trash and each is made with a different type of material like:

  • Plastic bottle : High Density Polyethylene (HDPE)
  • Cereal box : Paperboard & Epichlorohydrin
  • Shrink Wrap : Polyolefin
  • Kitchen utensil : Melamine-resin
  • Textiles : Urea-formaldehyde
  • Crown Cork Bottle Cap : Metal and Plastisol lining
  • Rock
  • Plasterboard
  • Coffee cup : Ceramic and Glaze (silica + metal oxides)

My objective is to build a system complete with sensors that automatically determines the material (chemical makeup) of each object. Are there material sensors currently on the market that can identify all of the materials that make up these objects?


2 Answers 2


You can identify many polymers without any clever sensor technologies at all but by following a number of simple physical tests. Sensor technology might help for some of the very fine distinctions, but if you can make a broad categorisation first with simple physical tests then that is what you should do.

And, of course, most plastic opponents are actually labelled with symbols to aid in recycling so you can often just look to identify the chemical nature of the polymer. For reference the symbols are shown below (V identifies chlorine-containing polymers like PVC):

polymer recycling symbols

The steps required to identify polymers, if you don't have a label, involve some simple physical tests. First identify whether the polymer is elastic or hard. Then identify whether it is a thermoplastic or a thermoset (by using a hot object like a soldering iron to see whether it remelts). Then a variety of tests can be applied: does it float in water or not? Does it burn and if so how does it behave and what does the flame look like?

One flowchart for this is available here as a pdf. Another simpler one is here also as a pdf but sufficient to distinguish the major classes of polymer. There are even interactive versions of such flowcharts such as this one (flash).

Included below is another version originally produced by the major, but now defunct, British chemical company ICI:

ICI polymer flowchart

  • $\begingroup$ Sorry. I realize I did not make my original question clear enough but I just edited it adding more information that should clarify it. In brief I am looking to create an automated material sensor system that can identify the chemical make up of any object that might be thrown out as trash (basically any object in existence). Are there material sensors available that one can purchase that can achieve this? $\endgroup$
    – max
    Jan 12, 2014 at 12:09
  • $\begingroup$ @max You may be being overambitious. the reason people are encouraged to separate their recycling into different groups is that a universal identification/separation system is not remotely cost-effective. If you only need to separate plastics or plastics plus paper, the techniques described above can work at a reasonable cost. Many actual recycling plants also use some human-driven separation which is also often cheaper than any available automated process. $\endgroup$
    – matt_black
    Jan 12, 2014 at 19:33
  • $\begingroup$ I understand but an automatic material sensing system is a requirement for a technology I am designing. I just need to know if such a system can be purchased or if the necessary sensors to identify all the objects in my question actually exist and can be purchased. Thanks. $\endgroup$
    – max
    Jan 12, 2014 at 21:25

Well, it seems the question has emerged to How to design a polymer recycling plant? ;)

Rocks and pieces of plasterboard can be separated from the rest of material by streams of compressed air along or across perforated sheets of metal in a conveyor belt: they'll keep sitting there - like a rock ;)

Plastics should be milled down to flakes then. The mixture can be submitted to a swim-and-sink step: PE whould swim on water, PS and PVC will sink.

Whether it is technically useful to repeat this step using NaCl solutions of different concentration to achieve further separation is beyond my knowledge.

IR spectroscopy will provide a lot of information on the type of polymer measured. Aren't InGaAs line cameras/sensors used in this context? Pattern matching with stored spectra might be used to control streams of air to blow identified material off a conveyor belt.

I'm sorry that I can't provide more precise information, but I'm not an engineer. However, I wouldn't be astonished if full-fledged setups are commerically available nowadays.

  • $\begingroup$ I edited my question. Sorry for being rather vague before. $\endgroup$
    – max
    Jan 12, 2014 at 12:09
  • $\begingroup$ Haha. Yes a polymer recycling plant indeed! Thank you for you answer. I will look into some of the keywords you mentioned. Oh and where would be the best place online see if full-fledged material identification setups are commercially available? I am completely new to the realm of chemistry. $\endgroup$
    – max
    Jan 12, 2014 at 21:22

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