Plastic is a broad category of synthetic or semi-synthetic materials composed of very long molecular chains, which chemists call polymers. The defining characteristic of plastic is its malleability, meaning it can be molded, extruded, or pressed into a vast array of solid forms. This ability to be shaped, combined with properties like low weight, durability, and resistance to chemicals, has made polymers ubiquitous in everything from medical devices to packaging.
Essential Feedstocks and Monomers
The process of creating plastic begins with the extraction of raw materials, primarily fossil fuels such as crude oil and natural gas. Over 99% of plastic is currently derived from these hydrocarbon sources. These raw feedstocks must undergo significant refinement and processing to become the chemical building blocks required for plastic synthesis.
Refining involves breaking down the large, complex hydrocarbon molecules into smaller, simpler compounds. This process, often called cracking, yields small molecular units known as monomers. Examples of these foundational monomers include ethylene and propylene, which are gases at room temperature. These simple molecules are the repeating units that will eventually be linked together to form the polymer chains that define plastic materials.
Converting Monomers Through Polymerization
The actual creation of plastic occurs through a chemical reaction called polymerization, where thousands of individual monomers are joined to form polymer chains. This linking process transforms the simple gaseous or liquid monomers into a solid, granular material known as plastic resin or polymer pellets. The specific method of polymerization dictates the final properties of the plastic material.
One common method is addition polymerization, which involves monomers adding to one another to form a growing chain. This reaction happens without the release of any smaller byproduct molecules. Polyethylene and polystyrene are frequently produced using this mechanism.
The second major method is condensation polymerization, which involves a reaction between two different functional groups on monomers. As the monomers link together, a small molecule, typically water or an alcohol, is eliminated as a byproduct. Polyesters and nylons are common examples of plastics formed by this step-growth process.
Shaping the Raw Resin into Products
Once the plastic resin is formed into pellets, it is ready to be physically manipulated into usable finished products through various manufacturing techniques. These processes involve melting the solid resin and forcing it into a specific shape, which is then cooled and solidified. The choice of technique depends entirely on the geometry and volume of the final part.
Injection molding is used for producing complex, solid shapes with high precision, such as bottle caps, electronic housings, and automotive parts. Molten plastic is injected at high pressure into a closed, temperature-controlled metal mold cavity, where it quickly cools and hardens into the desired form.
For continuous shapes, manufacturers rely on extrusion, a process where the melted plastic is forced through a die to create a uniform profile. The result is a continuous length of material, which can then be cut to size for items like pipes, window frames, or plastic films and sheeting.
Blow molding is specifically designed for creating hollow objects, most notably plastic bottles and containers. This technique involves forming a plastic tube, called a parison, clamping it inside a mold, and then inflating it with compressed air to push the material against the mold walls.
Distinguishing Thermoplastics from Thermosets
Polymers are broadly separated into two categories based on their response to heat: thermoplastics and thermosets. Thermoplastics are the more common type, characterized by linear or branched polymer chains that are not chemically bonded to each other.
Because their chains are held together by weak intermolecular forces, thermoplastics can be repeatedly melted by heating and then cooled to solidify without significant degradation. This reversible characteristic is what makes materials like polyethylene and polypropylene suitable for recycling. In contrast, thermoset plastics undergo an irreversible chemical change during their initial curing process.
This curing forms a dense, three-dimensional network of permanent covalent bonds, known as crosslinks, between the polymer chains. Once set, thermosets such as epoxy resins or Bakelite cannot be melted or reshaped without causing the material to degrade. This structure gives them superior heat resistance and dimensional stability, but it prevents them from being traditionally recycled.