Polymers in plastics
The core reason plastics exhibit such diverse properties lies in their polymers. Polymers are the building blocks of plastics, possessing unique molecular structures and performance characteristics. Polymers, also known as high-molecular compounds, are macromolecular chains composed of numerous identical or different monomers linked by covalent bonds. Their molecular weights typically range from tens of thousands to hundreds of thousands, or even higher. This long chain structure imparts polymers with excellent plasticity, allowing them to be processed into various shapes under certain temperatures and pressures while maintaining their shape after cooling. For example, polyethylene, a polymer formed from the polymerization of ethylene monomers, has a soft and resilient molecular chain, giving polyethylene plastics excellent flexibility and impact resistance, making it widely used in the production of films, containers, and other products.
The molecular structure of polymers determines the basic properties of plastics. There are three main types: linear, branched, and cross-linked. Polymers with linear structures have straight molecular chains, weak intermolecular forces, and good elasticity and plasticity. They melt easily when heated and solidify after cooling, such as polyethylene and polypropylene. These plastics generally have good processing properties and can be repeatedly heated and molded. Polymers with branched molecular chains have branches. Their properties are similar to those of linear structures, but the presence of the branches prevents the molecules from being tightly packed together, resulting in a relatively low density, such as low-density polyethylene. Polymers with cross-linked structures have molecular chains connected by chemical bonds, forming a three-dimensional network structure. These polymers have high strength, heat resistance, and solvent resistance, but they do not melt when heated and can only be molded in one go, such as phenolic plastics and epoxy resins.
The aggregate structure of polymers also has a significant impact on the properties of plastics. These structures are primarily categorized into crystalline and amorphous states. Crystalline polymers have molecular chains arranged regularly in space, forming a crystal structure that exhibits high strength, hardness, and a melting point. Crystalline plastics such as polyethylene and polypropylene, for example, exhibit excellent mechanical properties in their plastic parts. Amorphous polymers have disordered molecular chains and no fixed melting point, resulting in good transparency and flexibility. Amorphous plastics such as polyvinyl chloride and polystyrene are commonly used to make transparent products and decorative plastic parts. The higher the crystallinity of a polymer, the greater its density and improved mechanical properties, but also reduces its transparency. Conversely, the lower the crystallinity, the higher the plastic’s transparency but also its relatively low strength.
The molecular weight and distribution of polymers are also key factors influencing plastic performance. Generally speaking, a higher molecular weight improves the polymer’s mechanical properties, such as tensile strength and impact strength. However, excessively high molecular weight can also reduce the plastic’s fluidity, increasing processing difficulties. Molecular weight distribution refers to the ratio of molecules of different molecular weights within a polymer. Polymers with a narrow molecular weight distribution exhibit more uniform and stable properties, and better fluidity and moldability during processing. Polymers with a wider molecular weight distribution, on the other hand, exhibit significant performance variations, potentially leading to uneven performance in the plastic part. For example, when producing precision plastic parts, polymers with a narrow molecular weight distribution are often selected to ensure consistent performance across all parts of the part.
Polymers age under the influence of external factors, a significant factor affecting the lifespan of plastics. Aging primarily occurs when polymer chains break or crosslink under the influence of factors such as light, heat, oxygen, and mechanical stress, leading to a gradual deterioration in the plastic’s performance, such as hardening, brittleness, cracking, and discoloration. To slow polymer aging, stabilizers such as antioxidants and light stabilizers are often added to plastics. These additives inhibit or slow down the oxidation and degradation reactions of the polymer. For example, in plastic products used outdoors, the addition of an appropriate amount of light stabilizer absorbs ultraviolet light, protecting the polymer chains from damage and extending the plastic’s lifespan. Furthermore, carefully selecting the type and structure of the polymer can improve the plastic’s aging resistance and meet the requirements of different usage environments.
