Originally, plastic resins were made from vegetable matter including cellulose from cotton, furfural from oat hulls, oils from seeds and various starch derivatives. Bakelite (phenol formaldehyde resin), one of the first plastics made from synthetic components, was developed by Belgian born chemist Leo Backeland in New York in 1907. Bakelite is made through an elimination reaction of phenol with formaldehyde. It was originally lused for its electrical non-conductivity and heat-resistant properties in electrical insulators, radio and telephone casings. As it has a pleasing appearance, it was also used to make consumer products such as jewelry. Today, however, most plastics are made from petrochemicals including natural gas.
Polymer Synthesis: How Plastics are Made
Plastics are organic materials that contain such elements as carbon (C), hydrogen (H), nitrogen (N) chlorine (Cl) and sulfur (S). They are typically made from raw materials such as oil, natural gas and coal. The first step in making plastics is the polymerization of the raw material, called a monomer. Hydrocarbons are heated in a “cracking phase.” In this process, in the presence of a catalyst larger molecules are broken down into smaller ones such as ethylene (ethane) C2H4, propylene (propane) C3H6, butane C4H6 and other hydrocarbons. The yield of ethylene produced is controlled by the cracking temperature and can be more than 30% at a temperature of 850°C. Styrene and vinylchloride can be produced in subsequent reactions.
The two main types of polymerization are addition and condensation reactions. These processes can occur in the gaseous, liquid and sometimes in a solid phase of the monomer.
In a condensation reaction, two molecules combine with the loss of a smaller molecule such as water or an alcohol or acid. In this type of reaction, monomer one and monomer two both have hydrogen (H) and hydroxyl groups (OH). When they come together with a catalyst one monomer loses a hydrogen atom while the other loses the hydroxyl group. The hydrogen and the hydroxyl group combine together to make water (H2). The electrons that remain form a covalent bond between the monomers, forming a long chain of copolymers.
The resultant monomers can then be bonded into chemical chains called polymers. Different polymers are created by chains of different monomers each with individual properties and characteristics. The variability allows for plastics that can be shaped into products that meet application requirements such as heat tolerance, chemical resistance, strength, and more.
In an addition reaction, electrons with a double bond are rearranged within the monomer to form single bonds with other monomers. The addition reaction below between an ethane molecule and a chlorine molecule shows the formation of a polymer.
H H Cl Cl
\ / │ │
C = C + Cl – Cl → H ― C ― C ― H
/ \ │ │
H H H H
Here the double bond between the carbon atoms becomes a single bond with chlorine atoms are added to each end.
After the base polymer is formed, chemical additives can then be included to improve certain characteristics. These include antioxidants to protect the polymer from degradation from ozone or oxygen, flame retardants, antistatic additives, and lubricants for greater polymer flexibility. Additionally, plasticizers to improve flexibility, ultraviolet stabilizers to prevent degradation from the UV rays of the sun and pigments to add color are included. Composites can be made by adding glass, carbon and other fiber to the resins. Plastics can be compounded that can withstand strong acids, bases and alkalis, retard fire in home furnishings, contain lubricants for bearings or withstand the tropical sun.
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