Crystalline vs Amorphous Plastics - The Difference

Crystalline plastics, also known as polymer crystals, are composed of molecules arranged in an orderly manner. The crystallinity, the proportion of the crystalline part, generally ranges from 10% to 80%, making crystalline plastics often referred to as semi-crystalline plastics. This article uses the term crystalline plastics to include both crystalline and semi-crystalline plastics. They have a highly ordered lattice structure, thus exhibiting higher hardness, rigidity, and tensile strength. These plastics are usually opaque, such as polypropylene, polyethylene, and polyamide, with high melting points requiring high temperatures for shaping. Crystalline plastics are typically used in applications requiring high strength and rigidity, such as automotive parts, electrical casings, and household goods.

Amorphous plastics are composed of molecules arranged in a disordered manner, lacking an orderly lattice structure, resulting in lower hardness, rigidity, and tensile strength. These plastics are generally transparent, such as polyvinyl chloride, polystyrene, and polycarbonate. Their melting points are usually lower than those of crystalline plastics, allowing them to be shaped at lower temperatures. Amorphous plastics are often used in applications requiring flexibility and toughness, such as food packaging, cling films, and disposable items.

Crystallinity is an important characteristic of crystalline polymers, affecting the mechanical, optical, thermal, and chemical properties of the material. High crystallinity crystals can only be obtained from small molecule materials, which are generally brittle. Materials maintained below their melting point for a long time can also achieve high crystallinity, though at a generally higher cost and limited to special cases.

Crystalline polymers have stronger intermolecular interactions, which reduce the softening of materials above the glass transition temperature. The elastic modulus only changes significantly at high temperatures (above the melting point). The higher the material's crystallinity, the higher its hardness and thermal stability, but the material becomes more brittle. However, the amorphous regions provide some elasticity and impact resistance to the material. Another characteristic of crystalline polymer materials is the significant anisotropy in mechanical properties, i.e., mechanical properties differ significantly in directions parallel and perpendicular to molecular alignment.

Crystalline polymers are generally opaque due to the scattering of light at the interfaces between crystalline and amorphous regions within the material. These interfaces have very low density, hence materials with low crystallinity have higher transparency than those with high crystallinity. For example, atactic polypropylene is generally an amorphous polymer and is transparent, whereas isotactic polypropylene has a crystallinity of about 50 and is opaque. Crystallinity also affects the dyeing of polymers, with crystalline polymers being more difficult to dye than amorphous polymers because dye molecules can more easily penetrate amorphous polymer materials.