Plastics are classified into crystalline resins and amorphous resins.
Plastic, also known as a polymer, is a material composed of molecules linked together in a chain-like fashion that have a specific structure. For example, when many propylene molecules link together, they form polypropylene (PP). The PET resin used in plastic bottles is polyethylene terephthalate, which consists of many ethylene glycol and terephthalic acid molecules linked together.
What is the difference between crystalline and amorphous resins?
Crystalline resins feature molecules arranged in a regular pattern that form crystals upon solidification. The size of these crystals is generally several hundred micrometers, larger than the wavelength of light, making most of them opaque. The white color of nylon (polyamide) is a prime example.
Amorphous resins do not form crystals because the molecular chains in the polymer are irregularly arranged, allowing light to pass through and making them transparent. Polycarbonate resin and acrylic resin used in automobile lamp lens are good examples. The surface is smooth and glossy because there are no crystals, which means there are no lumps.
The PP products you see at 100-yen stores are semitransparent. This is because crystalline and amorphous parts are mixed together. As it is irregular, it is not quite glossy.
However, if the crystal size is made smaller than the wavelength of light, even crystalline resins become transparent. Starlite’s coolant tanks are transparent because they are made with highly crystalline PP. The amount of coolant inside can be clearly seen from the outside.
However, when crystalline resin solidifies from a molten state, significant heat shrinkage occurs, causing sink marks. This results in a slightly uneven surface that is less aesthetically pleasing. It is important to understand the properties of plastic and use the right material for the right application.
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