Plastics Analysis

In this lab, students examine and tabulate several physical and chemical properties of recyclable plastics. Then, using their observations, they identify two unknown plastics. I give them small, uncolored, rectangular samples (about 8 x 30 mm) of the six known plastics (#1, PETE; #2, HDPE; #3, vinyl; #4, LDPE; #5, PP, and #6, PS). They test these for density and halogen and perform an ignition analysis, looking for properties such as the ability to burn when removed from the flame, color of the flame, and appearance and acidity of the smoke. Then, or concomitantly, they examine the same properties of two colored unknown samples. Identification is fairly unequivocal as long as the data they collect are correct.

Known Plastic Samples: For the known plastics, I gave them uncolored samples. PETE came from colorless plastic soda bottles, though there are many other possible sources, as PETE is slowly replacing vinyl in much packaging. HDPE was from a plastic milk bottle. Vinyl is still easy to find as the tough, stiff wrapper for various items hanging on hooks in store displays. LDPE is used in those reclosable lids for coffee and shortening cans. PP is translucent and stiff; look for it in various grocery-store containers and small bric-a-brac containers in department stores. PS came from clear carryout food trays. Since the known samples are uncolored, I cut various types of notches to distinguish them from one another during the tests.

Unknown Plastic Samples: Colored PETE was from plastic soda bottles, microwavable TV dinner trays, newer bottles of NO-AD sunscreen, and some bottles of over-the-counter cough and cold remedies. HDPE comes in so many colors you could make a rainbow: shampoo, detergent, and cleaning products provide all you could imagine. Colored vinyl is getting harder to find as it is being replaced in lots of consumer products by PETE; try leftover bits of vinyl siding from friendly contractors. PP samples came from prescription pill bottles ("orange"), some yogurt cartons (white), plastic lids on glass jars of stuff like instant iced tea mix, and brightly-colored plasticware in department stores. Colored PS is found in odds and ends of packaging for stuff (Christmas cheese gift sets, watercolor paints, watches, etc. etc.). I didn't give them LDPE for some years because it is difficult to distinguish from PP, except by the fairly subjective criterion of texture: LDPE is softer and "waxy", whereas PP is stiff and almost brittle feeling. Colored LDPE is used in lids for cans of salted nuts.

Density: Density is tested in 18 x 150 mm test tubes half full of either water, 50% ethanol in water, or 10% NaCl in water. This test is quick and easy, and is a good check for the instructor on all samples. All eight samples can be tested at once, as long as students are careful to be sure there are no air bubbles adhering to the samples and the tube is shaken to be sure pieces are free to sink without resting on another piece. This test is done first because it is nondestructive. Students are told that the density of water is 1.00, that of 50% ethanol is 0.94, and that of 10% NaCl is 1.08. They are asked to place all their samples in appropriate density categories (less than 0.94, between 0.94 and 1.00, between 1.00 and 1.08, and greater than 1.08). At this point, if you are using LDPE as an unknown, students should compare the textures of their least dense unknowns (LDPE and PP both float in 50% ethanol) with the known samples of LDPE and PP.

Halogen: This is the Beilstein copper wire test. A length of copper wire is heated until the flame is colorless. Then a small sample of plastic is taken up onto the hot wire. When the sample is subsequently burned in the flame, a bright green flame indicates the presence of halogen. In the case of our six plastics, this of course means chlorine. I have always wondered if the presence of halogen in other components of the plastic, such as dyes, might give a false positive, but it's never happened.

Ignition: This is obviously the last test. Samples are heated to burning in the Bunsen burner. Several observations may be made: does the plastic burn outside of the flame; what color is its flame; does it give off smoke, and if so, what is its quality (sooty, white, etc.). But the most important and discriminating test is whether the fumes from the burning plastic turn wet blue litmus paper pink. It is important to have the litmus paper quite wet so it doesn't burn up, and placed directly in the flame or smoke. This should be done in hoods because of toxic fumes, especially from the vinyl, and ideally over something like a sheet of aluminum foil to catch flaming drops falling to the benchtop. Students (and instructors?) find this test both stressful and exhilarating.

The density test is sufficient to distinguish all the plastics except PETE and vinyl, which are easily distinguished by the copper wire and litmus tests, and LDPE and PP, which with care may be distinguished by texture.

Students are asked to identify their two unknowns and to give reasons for their identifications. They are then asked to explain why Styrofoam differs so much in density from PS, to identify the likely acid being detected by litmus paper in burning vinyl, and to describe the six plastics in terms of whether they are addition or condensation polymers and the structures of their monomers and minimal repeat units.

 

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Revised 8/31/06