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How Is Plastic Made?

Overview.

Plastic can either be ‘synthetic’ or ‘biobased’. Synthetic plastics are derived from crude oil, natural gas or coal. Whilst biobased plastics come from renewable products such as carbohydrates, starch, vegetable fats and oils, bacteria and other biological substances.

The vast majority of plastic in use today is synthetic because of the ease of manufacturing methods involved in the processing of crude oil. However, the growing demand for limited oil-reserves is driving a need for newer plastics from renewable resources such as waste biomass or animal-waste products from the industry.

Which was the first human made plastic?

Meso American cultures (Olmec, Maya, Aztecs, 1500 BCE) used natural latex and rubber to make containers and clothes water-resistant.

Alexander Parkes (UK, 1856) patented the first man-made bioplastic, called Parkesine, made from cellulose nitrate. Parkesine was a hard, flexible and transparent plastic. John Wesley Hyatt (US, 1860s) made a fortune with Parkes’ invention. The Hyatt brothers improved plastic’s malleability of cellulose nitrate by adding camphor and renamed the plastic as Celluloid. The aim was to produce billiard balls, which until then were made from ivory. The invention is considered as the earliest example of man-made bioplastic by many.

The first truly synthetic plastic was Bakelite made from phenol and formaldehyde resin. Leo Baekeland (Belgium, 1906) invented Bakelite that was coined as a ‘National Historic Chemical Landmark as it completely revolutionized every industry present in modern life. It has the property of high resistance to electricity, heat, and chemicals. It has non-conducting properties, which is extremely essential when designing electronic devices such as radio and telephone casings.

What is the main ingredient in plastic?

The main ingredient in most plastic material is a derivative from crude oil and natural gas.

There are many different types of plastics – clear, cloudy, solid colour, flexible, rigid, soft, etc.

Plastic products are often a polymer resin which is then then mixed with a blend of additives . The additives are important as each of them are used to provide plastic with targeted optimum properties such as toughness, flexibility, elasticity, colour or to make them safer and hygienic to use for a particular application .

What type of plastic a product is made from can be sometimes be identified by looking at the number at the bottom of plastic containers.

Plastic derivation steps.

Most of the plastic in use today is derived by the following steps:

1. Extraction of raw materials. (largely crude oil and natural gas, but also coal) – these are a complex mixture of thousands of compounds that then need to be processed.

2. Refining process. transforms crude oil into different petroleum products – these are converted to yield useful chemicals including “monomers” (a molecule that is the basic building blocks of polymers). In the refining process, crude oil is heated in a furnace, which is then sent to the distillation unit, where heavy crude oil separates into lighter components called fractions. One of these, called naphtha, is the crucial compound to make a large amount of plastic. However, there are other means, such as using gas.

3. Polymerisation. is a process in the petroleum industry where light olefin gases (gasoline) such as ethylene, propylene, butylene (i.e., monomers) are converted into higher molecular weight hydrocarbons (polymers). This happens when monomers are chemically bonded into chains.

There are two different mechanisms for polymerisation:

▪️Addition polymerization. The addition polymerisation reaction is when one monomer connects to the next one (dimer) and dimer to the next one (trimer) and so on. This is achieved by introducing a catalyst, typically a peroxide. This process is known as chain growth polymers – as it adds one monomer unit at a time. Common examples of addition polymers are polyethylene, polystyrene and polyvinyl chloride.

▪️Condensation polymerization. Condensation polymerisation includes joining two or more different monomers, by the removal of small molecules such as water. It also requires a catalyst for the reaction to occur between adjacent monomers. This is known as step growth, because you may for example add an existing chain to another chain.  Common examples of condensation polymers are polyester and nylon.

4. Compounding/Processing. In compounding, various blends of materials are melt blended (mixed by melting) to make formulations for plastics. Generally, an extruder of some type is used for this purpose which is followed by pelletising the mixture. Extrusion or a different moulding process then transforms these pellets into a finished or semi-finished product. Compounding often occurs on a twin-screw extruder where the pellets are then processed into plastic objects of unique design, various size, shape, colour with accurate properties according to the predetermined conditions set in the processing machine.

How is synthetic plastic created from crude oil?

Synthetic plastic comes from petrochemicals. When the source of oil beneath the surface of the Earth is identified, holes are drilled through the rocks in the ground to extract oil.

Extraction of oil Oil is pumped from underground to the surface where tankers are used to transport the oil to the shore. Oil drilling can also take place under the ocean using support from platforms. Different size pumps can produce between 5 – 40 litres of oil per stroke.

Refining of oil – Oil is pumped through a pipeline that can be thousands of miles long and transported to an oil refiner . Spillage of oil from the pipeline during transfer can have both immediate and long-term environmental consequences but safety measures are in place to prevent and minimise this risk.

Distillation of crude oil and production of petrochemicals Crude oil is a mixture of hundreds of hydrocarbons that also contains some solids and some gaseous hydrocarbons dissolved in it from the alkane family (mainly it is CH4 and C2H6, but it can be C3H8 or C4H10). Crude oil is first heated into a furnace then the resultant mixture is fed as a vapour to the fractional distillation tower. The fractional distillation column separates the mixture into different compartments called fractions. There exists a temperature gradient in the distillation tower where the top is cooler than the base. The mixture of liquid and vapour fractions gets separated in the tower depending on their weight and boiling point (boiling point is the temperature at which the liquid phase changes into gaseous). When the vapours evaporate and meet a liquid fraction whose temperature is below the boiling point of vapor, it partly condenses. These vapours of evaporating crude oil condense at different temperature in the tower. Vapours (gases) of the lightest fractions (gasoline and petroleum gas), flow to the top of the tower, intermediate weight liquid fractions (kerosene and diesel oil distillates), lingers in the middle, heavier liquids (called gas oils) separate lower down, while the heaviest fractions (solids) with the highest boiling points remain at the base of the tower. Each fraction in the column contains hydrocarbons with a similar number of carbon atoms, smaller molecules are towards the top and longer molecules nearer the bottom of the column . In this way, petroleum is decomposed into petroleum gas, gasoline, paraffin (kerosene), naphtha, light oil, heavy oil, etc.

After the distillation step, the obtained long chain hydrocarbons are converted into hydrocarbons that can then be turned into many important chemicals which we use for the preparation of a wide range of products applicable from plastic to pharmaceuticals.

Cracking of hydrocarbon is the main process that breaks down the mixture of complex hydrocarbons into simpler low relative molecular mass alkenes/alkanes (plus by-products) by the means of high temperature and pressure.

Cracking can be performed into two ways: Steam cracking and catalytic cracking.

Steam cracKing uses high temperature and pressure to break the hydrocarbons long chains without a catalyst, whilst catalytic cracking adds a catalyst which allows the process to occur at lower temperatures and pressures.

The raw material used by the petrochemical industry is mainly naphtha and natural gas from oil refining operation in the petrochemical feedstock. Steam cracking uses the feedstocks from hydrocarbons mixture from various fractions such as reactant gases (ethane, propane or butane) from natural gas, or liquids (naphtha or gas oil).

(Naphtha is a mixture of C5 to C10 hydrocarbons obtained from the distillation of crude oil).For example, decane hydrocarbon is cracked down into products such as propylene and heptane where the former is then used to make poly(propylene).

Raw materials molecules are converted into monomers such as ethylene, propylene, and butene and others. All these monomers comprise double bonds so that the carbon atoms can subsequently react to form polymers.

Polymerisation hydrocarbon monomers are then linked together by chemical polymerisation mechanism to produce polymers. Polymerisation process generates thick, viscous substances as resins, which are employed to make a plastic product. If we look at a case of ethylene monomer here; ethylene is a gaseous hydrocarbon. When it is subjected to heat, pressure and a certain catalyst, it joins together into long, repeating carbon chains. These joined molecules (polymer) is a plastic resin known as polyethylene (PE).

Production of PE based plastic –poly(ethylene) is processed in a factory to make plastic pellets. The pellets are poured into a reactor, melted into a thick liquid to cast into a mould. The liquid cools down to harden into a solid plastic and produce a finished product. Processing of polymer also includes the addition of plasticizers, dyes and flame-retardant chemicals.

Types of polymerisation.

Synthetic plastic is made by a reaction known as polymerisation, which can be performed in two different ways:

▪️Addition polymerisation: Synthesis includes adding together monomers in a long chain. One monomer connects to the next and so on, when a catalyst is introduced, in a process known as chain growth polymers, adding one monomer unit at a time. Some addition polymerisation reactions are considered to create no side-products and the reaction can be performed in the vapour phase (i.e. gas phase) dispersed in a liquid. Examples: polyethylene, polypropylene, polyvinyl chloride and polystyrene.

  ▪️Condensation polymerisation: In this case, two monomers combine to form a dimer (two units) by releasing a byproduct. Dimers can then join to form tetramers (four units) and so on. These byproducts are necessary to be removed for the success of the reaction. The most common byproduct is water, which is treated and disposed of easily. Byproducts can also be valuable raw materials that are recycled back into the feedstream.Examples: Nylon (polyamide), polyester and polyurethane.

How is plastic created from naphtha?

Plastic is often created from naphtha. Ethylene and propylene, for example, are the main raw material for oil-based plastic coming from Naphtha.

What is Naphtha?

There are different types of naphtha. It is a term used to describe a group of volatile mixtures of liquid hydrocarbons, obtained by the distillation of crude oil. It is a mixture of C5 to C10 hydrocarbons.

Naphtha is decomposed thermally at high temperature (~800 °C) in a steam cracker in presence of water vapor where it splits into light hydrocarbons known as major intermediaries. These are olefins and aromatics. Among the olefins, there is C2 (ethylene), C3 (propylene), C4 (butane and butadiene). The aromatics consist of benzene, toluene and xylene. These small molecules are linked together by into long molecular chains called polymers. When a polymer comes out of the chemical factory they it is still not in the form of plastic – they are in the form of granules or powders (or liquids). Before they can become an everyday use plastic they need to undergo a series of transformations. They are kneaded, heated, melted, and cooled into objects of various shape, size colour with precise properties according to the processing tubes.

For instance, for polymerisation of ethylene into polyethylene (PE), initiators are added to start the chain reaction, only after the formation of PE, it is sent for processing by addition of some chemicals (antioxidants and stabilisers). After which an extruder convertsn PE into strings, thereafter grinders convert it into PE pellets. Factories then melt them into final products.

Reference :

Payal Baheti, “How Is Plastic Made? A Simple Step-By-Step Explanation”، www.bpf.co.uk

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