Every material categorized as plastic is fundamentally composed of polymers, which are the large, chain-like molecules providing the material’s structure. The polymer acts as the raw chemical ingredient that is then engineered into the final material we recognize as plastic. Understanding this connection requires looking closely at the core chemical building block and how it is transformed into a functional product.
Defining the Core Building Block What is a Polymer
A polymer is a substance made up of very large molecules, or macromolecules, created by linking together many smaller, identical repeating units called monomers. This linking process, known as polymerization, forms long molecular chains that are covalently bonded, giving the substance its unique physical properties. For example, polyethylene, a common plastic, is formed by linking thousands of ethylene monomers into a single, extended chain.
The length of these molecular chains controls the material’s final characteristics. Longer chains typically result in materials with greater mechanical strength, increased viscosity, and higher thermal stability. Controlling the average number of monomer units added is a primary goal in polymer chemistry to tailor the material for specific uses. The significant size of these macromolecules results in properties like toughness, high elasticity, and a tendency to form semi-crystalline structures.
Defining the Resultant Material What is Plastic
Plastic is a material science term describing an engineered product that is moldable, durable, and highly versatile. While the polymer provides the structural backbone, the substance is not fully considered a plastic until it exhibits these material properties and is formulated for practical use. The transformation from a raw polymer to a functional plastic involves the incorporation of various additives.
These additives are compounds mixed in during manufacturing to modify the polymer’s base characteristics. Stabilizers are added to protect the polymer from degradation caused by heat, UV light, or oxidation, extending the product’s lifespan. Plasticizers increase the material’s flexibility and reduce brittleness, making materials like PVC soft and easier to shape. Other substances, such as colorants, fillers, and flame retardants, are also included to achieve specific performance and aesthetic requirements.
The Essential Link How Polymers Form Plastics
A plastic is simply a polymer formulated and processed to exhibit specific thermal and mechanical properties. The polymers used to make plastic must be synthetic and structured to allow them to be melted, molded, and set into a fixed shape. This moldability is a direct result of the long molecular chains being able to slide past one another when heated, a property known as viscoelasticity.
Plastics are broadly classified based on how they react to heat, falling into two categories: thermoplastics and thermosets. Thermoplastics, like polyethylene and nylon, can be melted and reshaped repeatedly because their molecular chains are linear and held together by weak bonds. Thermoset plastics, such as epoxy resins, undergo an irreversible curing process that forms strong, permanent cross-links between their polymer chains. Once cured, these thermosets cannot be melted or reshaped, offering high-temperature resistance and structural integrity.
Polymers That Are Not Plastics
Although all plastics are polymers, the reverse is not true; many substances fit the chemical definition of a polymer but are not considered plastics. The term polymer is a broad chemical classification that includes both synthetic and natural long-chain molecules. Natural polymers, or biopolymers, are produced by living organisms and include some of the most abundant organic compounds on Earth.
For example, cellulose, the main component of wood and cotton, is a natural polymer made of linked glucose units, but it is not classified as a plastic. Proteins, wool, silk, and DNA are also complex natural polymers. These materials are not categorized as plastic because they lack the specific engineered properties and processing requirements of synthetic plastic materials.