[Jim Vandiver]

Jim Vandiver asked the Naked Scientists:
   
Plastics Identification:
 
I'd like to know how to identify different types of plastic and wonder if it's a suitable topic for a Kitchen Science segment. 

Some of our plastic debris can be recyled. It's identified by numbers 1 and 2 inside a triangle. Some plastics bear no identification at all, such as the clear plastic envelope covering many new products, or my lawn cart that's all plastic except for the axle. The cart is a big item for recycling, but the pieces of plastic packaging do accumulate over time.
 
Secondly I have plastic products I want to repair using an adhesive, such as that cart, or a broken piece of my car's interior which also lacks identifying marks. Many adhesives do not adhere to all types of plastic or weld it together with much strength. Some adhesives used for this work are expensive, so experimenting with them quickly exceeds the cost of the item being repaired.  
 
Until the last year I found little internet help about plastics identification for recycling or repair work. Now there are mutliple sites, like  http://www.urethanesupply.com/identify.php, which help, but the academic site, http://www.biologyjunction.com/crime_scene_investigation.htm , in its Activity # 1 on Identifying Plastics seems more useful. However, it doesn't seem to identify the type of oil used to separate type 4 from type 5 plastics.
 
Other sites, such as http://www.modernplastics.com/how_to_identify_plastics.htm , add burning or heating tests that produce smell, flame and smoke characteristics, molten appearance, and type of smoke.  This can be messy and I suspect a little toxic. The hazards are probably why these procedures aren't used on the academic web site's experiment.
 
Please consider a Kitchen Science experiment with an elegant way to do these tests?
 
Thanks.
 
Jim Vandiver,
Smithfield, VA

What do you think?

[Atomic-S]

This sounds like a tough assignment to me. Some ideas come to mind which are not simple. Such as, subjecting the plastics to nonsimple organic test reagents and measuring the rate at which they dissolve off in a known solvent. Using radiation to identify the relative abundances of elements present. If we are to limit ourselves to relatively simple procedures, our options appear quite limited. One idea: the hot needle. For this to work anything clos to well, the temperature should be capable of accurate determination. The idea is to accurately measure the temperature at which the plastic starts to soften significantly. The same process could also yield an odor clue. Another idea is to subject the plastic to an array of solvents and observe how it reacts to each. Another idea is to test its thermal conductivity, which would require some kind of an accurate instrument for this purpose. In all likelihood, a good identification might require using all these methods and others together inasmuch as any of of them may be rather indeterminate.  Better than any of them may be infrared spectroscopy, which might disclose differences in molecular structure; but of course, that is a bit difficult to do with kitchen equipment. Other things that could be measured are coefficients of friction and hardness. What about the tendency of plastics when rubbed by a dissimilar material to generate eletrostatic charge? All these processes offer possibilities but you have to have some way to carry out the measurements fairly accurately to be of any use, and again, you will probably have to carry out several tests in order to get a positive indentification, inasmuch as any one of them will likely be unable to fully separate the types of plastic.