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Making plastics – from crude oil to plastic pellets
Although originally a natural product, nowadays plastic is mostly synthetic and widely produced. The production starts with extracting oil from the underground with the help of pumps that produce between 5 and 40 liters of oil per stroke, which is then transported to an oil refiner through pipelines. The crud oil is poured into preheater, where it is boiled and sent to a furnace. There, according to its molecular weight, oil is separated into several groups of chemicals — petroleum, gasoline, paraffin, etc.
For plastics, what’s essential is naphtha. Naphta has to be broken down into smaller units in a cracking process to get ethene. There are two types of cracking processes. Stem cracking is done at high temperatures and pressure, which is not required for catalytic cracking, which uses catalyst instead. Ethene is forwarded into a reaction chamber where the process of polymerization links the hydrocarbon monomers together into thick, viscous substances used to make plastics. The resulting product, polythene, is processed into strings in an extruder before being ground into pellets dispatched to factories to be molded into end products.
Plastic processing equipment and transformation of pellets into usable products
Forming usable plastic products, for instance, bottles, hoses, or toys, is executed using various processes: compounding, forming, extrusion and molding. Compounding is the first step – mixing liquids with other ingredients according to the desired recipe in conventional stirred tanks. Other essential plastic processing equipment includes two types of mixers; the workhorse mixer, which applies heat and pressure simultaneously, and the Banbury mixer which reminds of a robust dough mixer with two interrupted spiral rotors.
The next step is forming plastics into various shapes by melting, shaping, and solidifying. Extrusion and molding processes create finished or semi-finished products. During extrusion, the melted polymer is continuously forced through an orifice in an extruder for products such as sheets, tubing, and grocery bags. Various types of molding rely on molds to create the desired shape.
To illustrate, rotational molding produces large, hollow products such as kayaks. On the contrary, blow molding is used for small hollow pieces – bottles or fuel tanks. With compression molding, plastic parts can replace metal in the automotive industry. Finally, the most common type, injection molding, has virtually limitless uses with a large production capacity.
Classification of plastics
Plastics are classified according to their unique properties acquired during production processes. The main classification is based on how plastic responds to heat – it is differentiated between thermoplastics and thermosets. Thermoplastics are more common than thermosets. They can be heated without burning, melted, cooled and reheated without losing their properties. On the other hand, thermosets can be heated only, typically during injection molding, once due to irreversible chemical changes.
Any further heating would result in burning. For this reason, thermosets are not recycling-friendly. Further categorization is between amorphous, also known as shapeless, material and semi-crystalline material. While the former gradually softens when exposed to heat, the latter maintains its shape until a certain temperature point, when it rapidly becomes liquid. Finally, according to their monomer composition, plastics can be classified as homopolymers, made of a single type of monomer, or, on the contrary, copolymers.
Bioplastics, biodegradable plastics and recycled plastics – ecological alternatives?
The environmental effects of plastic are a widely known topic. It takes up to 450 years for a plastic bottle and 50 to 80 years for a plastic cup to decompose. Alternatives to traditional plastics are biodegradable, bio-based and recycled plastics. Biodegradable plastics are made of petrochemicals but degraded by microorganisms. However, it can be safely done under warm conditions; otherwise, it degrades into harmful microplastic.
Bioplastic is sourced from renewable natural materials (corn, sugarcane, cellulose), which are compostable. The downside is that using food sources, as well as that, owing to chemical and mechanical differences, can be challenging to recycle. Finally, recycled plastic does not exploit raw material, but it can be used only to produce lower-grade products. Furthermore, it uses energy, water and emits gases. However, it is important to recycle already existing materials as replacing them with mass production of non-reusable lower-quality products is not the ideal solution either.
Green innovations - plastic-eating waxworms
Considering that bioplastics, biodegradable plastics and recycled plastics have downsides, scientists from Canada’s Brandon University have found a unique possible solution. Namely, they discovered that waxworms could live off polyethylene. Their guts contain bacteria that biodegrade plastics and turn them into glycol. In the lab-controlled environment, 60 waxworms can eat more than 30 square centimeters of a plastic bag in less than a week. While it is unrealistic to expect that waxworms can solve the plastic pollution issue, understanding the symbiosis of waxworms and gut bacteria could lead to the development of better plastic biodegradation systems.