Simple identification method of common plastics
Quickly and accurately identifying plastic types is crucial in applications such as plastic recycling, product repair, and material selection. Common plastics include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), polyethylene terephthalate (PET), and acrylonitrile-butadiene-styrene (ABS). Each plastic has unique physical and chemical properties, allowing for easy differentiation through simple methods such as visual inspection, combustion tests, and density testing. Mastering these identification methods not only improves work efficiency but also prevents quality issues and safety hazards caused by material misuse.
Appearance inspection is the first step in identifying plastics. Different types of plastics can be initially distinguished by color, transparency, hardness, and surface characteristics. Polyethylene (PE) is typically milky white and translucent, with a smooth, waxy surface. It is soft and easily deformed when pressed. It is commonly found in products like plastic bags and bottles. Polypropylene (PP) is similar in appearance to PE, but is slightly more transparent, slightly harder, and has a glossier surface. It makes a slight “clicking” sound when bent and is often used in plastic buckets and turnover boxes. Polyvinyl chloride (PVC) comes in two types: soft and hard. Soft PVC is slightly elastic and is often used in electrical wiring and raincoats. Hard PVC is harder, has a smooth surface, and comes in a variety of colors. It is often used in pipes and door and window profiles. Polystyrene (PS) is highly transparent but brittle and easily breaks. It makes a crisp sound when dropped and is often used in disposable tableware and toy casings. Polyethylene terephthalate (PET) has good gloss and transparency, is hard, and is impact-resistant. It is often used in beverage bottles and film. Acrylonitrile-butadiene-styrene copolymer (ABS) has a certain glossy surface and is mostly opaque black or white in color. It has a hard and tough texture and is often used in home appliance housings and automotive parts.
Combustion tests are an effective means of identifying plastics. By observing the flame color, smoke characteristics, combustion odor, and residue during combustion, plastic types can be further differentiated. Polyethylene (PE) burns with a blue flame with a yellow tip, burns rapidly, and is accompanied by molten dripping. Its odor is similar to that of burning paraffin, and its residue is white and waxy. Polypropylene (PP) has similar combustion characteristics to PE, but with a brighter flame and faster dripping. It produces a small amount of black smoke and a slightly petroleum-like odor, and its residue is also white. Polyvinyl chloride (PVC) is difficult to burn and extinguishes immediately upon removal from the flame. Its flame is yellow-green with a large amount of black smoke. It emits a pungent odor of hydrogen chloride (similar to hydrochloric acid), and its residue is a black, hard mass. Polystyrene (PS) is highly flammable, with an orange-yellow flame accompanied by thick black smoke. It produces a sweet, pungent odor of styrene monomer, and its residue is a black, brittle, hard mass. Polyethylene terephthalate (PET) burns slowly and yellow, producing black smoke and a pungent odor (similar to burnt paper). The residue is a black, hard, brittle mass. Acrylonitrile-butadiene-styrene copolymer (ABS) burns with a yellow flame accompanied by black smoke. The combustion process produces the pungent odor of acrylonitrile, and the residue is a black, porous, hard mass.
Density testing uses the differences in density of different plastics to identify them. It’s simple and highly accurate. To do this, prepare a cup of water (density approximately 1g/cm³) and a cup of salt water (density approximately 1.1g/cm³). Cut the plastic sample into small pieces and place them in the water to observe whether they float or sink. Polyethylene (PE) and polypropylene (PP) both have densities less than 1g/cm³ and will float in clear water. PP’s density (0.90-0.91g/cm³) is slightly lower than PE’s (0.91-0.96g/cm³). In less concentrated salt water, PP may still float, while PE may sink. Polyvinyl chloride (PVC) has a higher density (1.3-1.4g/cm³) and will sink in clear water. Polystyrene (PS) has a density of approximately 1.05g/cm³ and will slowly sink in clear water but may float in salt water. Polyethylene terephthalate (PET) has a density of approximately 1.38g/cm³ and will clearly sink in clear water. The density of acrylonitrile-butadiene-styrene copolymer (ABS) is approximately 1.04-1.06 g/cm³. It is suspended or slowly sinks in clean water and can be further distinguished from PS by its burning smell.
Hardness and solubility tests can be used as auxiliary identification tools. Hardness tests can be performed by scratching the plastic surface with a fingernail or a coin. Polyethylene (PE) and polypropylene (PP) are relatively low in hardness and easily scratched. Rigid varieties of polyvinyl chloride (PVC) are relatively hard and difficult to scratch. Polystyrene (PS) is brittle and may break if scratched with force. Polyethylene terephthalate (PET) and acrylonitrile-butadiene-styrene copolymer (ABS) are relatively hard and resistant to scratching. Solubility tests use the solubility characteristics of plastics in specific solvents to distinguish them. For example, polystyrene (PS) is soluble in organic solvents such as toluene and xylene, while polyethylene (PE) and polypropylene (PP) are insoluble in most solvents at room temperature. Polyvinyl chloride (PVC) is soluble in tetrahydrofuran, while polyethylene terephthalate (PET) is soluble in a phenol-tetrachloroethane mixture under heating. However, solubility tests require the use of chemical reagents and require safety precautions during operation, making them suitable for laboratory use.
Combining multiple methods can improve the accuracy of plastic identification. In practice, a preliminary judgment can be made through visual observation, followed by further confirmation through combustion and density tests, supplemented by hardness or solubility tests if necessary. For example, if a transparent plastic bottle has a hard and shiny appearance, produces black smoke and a pungent odor when burned, and sinks in clean water, it can be preliminarily identified as PET. If a plastic barrel has a waxy surface, burns with a bright and dripping flame, and floats in clean water, it is likely PE or PP. The two can be distinguished by whether there is a “clicking” sound when bending. Mastering these simple identification methods can help relevant personnel quickly and accurately identify common plastics in the absence of specialized equipment, providing strong support for material recycling, product repair, and quality control.
