TWI Frequently asked questions
Here are five simple tests which should help you identify the main industrially used thermoplastics. Please take appropriate precautions when doing burn tests and using sharp tools.
Use the tests in conjunction with the table of observations below.
| Test no. | Action |
|---|
| 1. | Look at the sample. Is it transparent, translucent or opaque? |
| 2. | Feel the sample. Does it bend? Can it be scratched? What does the surface feel like? |
| 3. | Cut the sample with a sharp knife. Does it cut easily? Are the edges smooth or jagged? Does it crumble or flake? |
| 4. | Subject the sample to a float test. Does it float or sink? (Note: not applicable to expanded foam materials. Water should be around room temperature). |
| 5. | Try to burn a small piece of the sample. What is the size and colour of the flame? Do molten drips fall from the sample and continue to burn? Does the sample self-extinguish? Is there any odour when the flame hasbeen extinguished? |
| Material | Test no. | Observation |
|---|
Low density
polyethylene
(LDPE) | 1 | Transparent only as thin film, translucent in thicker sections |
| 2 | Fairly flexible; soft, 'waxy' feel, easily scratched |
| 3 | Easily and smoothly cut |
| 4 | Floats |
| 5 | Not self-extinguishing; molten droplets which usually go out on reaching bench or floor; blue flame with yellow tip and little smoke, smell of burning candle/paraffin when flame is extinguished |
| ||
High density
polyethylene
(HDPE) | 1 | Transparent only as thin film, translucent in thicker sections |
| 2 | Fairly stiff and hard, can be scratched by fingernail |
| 3 | Easily cut with smooth edges |
| 4 | Floats |
| 5 | Not self-extinguishing; molten droplets which usually go out on reaching bench or floor; blue flame with yellow tip and little smoke, smell of burning candle/paraffin when the flame is extinguished |
| ||
Polypropylene
(PP) | 1 | Transparent only as thin film, translucent in thicker sections |
| 2 | Stiff; hard, can be scratched by fingernail |
| 3 | Easily cut, fairly smooth edges, when cut with chisel leaves white mark |
| 4 | Floats |
| 5 | Not self-extinguishing; molten droplets which usually go out on reaching bench or floor; flame mainly yellow with a trace of clear blue at the bottom; smell of burning candle/diesel when flame is extinguished. |
| ||
Polyvinyl Chloride, Unplasticised
(uPVC) | 1 | Transparent (unless fillers or pigments have been added) |
| 2 | Stiff; hard |
| 3 | Fairly easy to cut, smooth edges |
| 4 | Sinks |
| 5 | Burns with difficulty, self-extinguishing; yellow flame, blue-green at bottom edges; unpleasant, acrid odour of hydrochloric acid. |
