Types and methods of identification of plastics

You can take a small piece of plastic with tweezers and observe its burning characteristics after being ignited.

Polyethylene: burns easily, continues to burn after leaving fire, flame is yellow at top and blue at bottom, with melt drops and smell of burning paraffin.

Polypropylene: burns easily, continues to burn after leaving fire, flame is yellow at top, blue at bottom, a small amount of black smoke, drops of molten metal and smell of kerosene.

PVC: hard to burn, goes out immediately after leaving fire, flame is yellow at top and green at bottom, with white smoke, surface is softened and has a sharp sour smell.

Polystyrene: burns easily, continues to burn after leaving fire, orange flame, thick black smoke, carbon wisps, softening, foaming and a special smell of styrene monomer.

Plexiglas: burns easily, continues to burn after leaving fire, light blue flame, white top, softening, foaming, strong smell of flowers, fruits and rotten vegetables.

Below is a simple method for identifying plastic:

1. External plastic identification

By observing appearance of plastics, we can first identify main categories of plastic products: thermoplastics, thermosets, or elastomers. There are generally two types of thermoplastics: crystalline and amorphous. Crystalline plastics are translucent, have a milky or opaque appearance, are transparent only in thin layers, and vary in hardness from soft to keratinous. Amorphous is usually colorless, completely transparent when no additives are added, and hardness is higher than that of horn rubber (additives such as plasticizers are often added at this time). Thermoset plastics usually contain fillers and come in opaque and transparent without fillers. The elastomer feels like rubber and has a certain elongation.

2. Identification of plastics when heated

The heating characteristics of above three types of plastics also differ, which can be determined by heating. Thermoplastics soften when heated, melt easily and become transparent when melted, can often be drawn from melt, and are usually easily welded when heated. Thermosetting plastics are heated until material undergoes chemical decomposition, retaining its original hardness without softening, and its size is relatively stable, and it carbonizes at decomposition temperature. When heated, elastomer does not flow to chemical decomposition temperature, but material decomposes and carbonizes at decomposition temperature.

3. Identification of plastics in solvent processing

Thermoplastics swell in solvents, but are generally insoluble in cold solvents. Some thermoplastics dissolve in hot solvents, such as xylene-soluble polyethylene. Thermosetting plastics are insoluble in solvents and generally do not swell or only swell.Slightly, elastomers are insoluble in solvents, but usually swell.

4. Determination of plastic density

Different types of plastics have different densities. The density measurement method can be used to identify plastics, but in this case, foam products must be separated because density of foamed plastics is not actual density of material. In real industry, there are also plastics that use different densities to sort plastics.

5. Identification of plastic by pyrolysis

The identification method in pyrolysis test is to heat plastic in pyrolysis tube to pyrolysis temperature, and then use litmus paper or pH indicator paper to check pH value of evolved gas for identification.

6. Identification of plastic by burning

The fire test identification method is to use a small fire to burn a sample of plastic, observe flammability of plastic in and out of fire, and droplet shape and smell of melted plastic after flame is extinguished. turned off to determine type of plastic.

7. Color identification of plastics

Some plastics can be identified by different indicators, dissolve or suspend a few milligrams of sample in 2 ml of hot acetic anhydride, after cooling, add 3 drops of 50% sulfuric acid (from equal volumes of water and concentrated sulfuric acid). , Observe color reaction immediately, observe color of sample after sample is left for 10 minutes, then heat sample to 100 degrees in a water bath and observe color of sample.

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