Plastic materials have been created using many different kinds of matter over the years. Originally, resins were made from vegetable matter including cellulose from cotton, furfural from oat hulls, oil 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 Baekeland in New York in 1907. Bakelite is made through an elimination reaction of phenol with formaldehyde. It was used for its electrical non-conductivity and heat-resistant properties in electrical insulators, radio and telephone casings. Because of its pleasing appearance, it was also used to make consumer products such as jewelry. Today, however, most plastics are made from petrochemicals including natural gas.
Plastics are organic materials that contain such elements as carbon (C), hydrogen (H), nitrogen (N) chlorine (Cl) and sulfur (S). They are made from raw materials such as oil, natural gas and coal. The first step in making plastics is the polymerization of the raw materials, resulting in a product called a monomer. Hydrocarbons are then 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 vinyl chloride 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 the solid phase of the monomer.
In this type of polymerization, two molecules combine, causing the loss of a smaller molecule such as water, 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, which form 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, etc.
In this type of polymerization, 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 added to each end.
Chemical additives can be mixed into the base polymer to improve certain characteristics. These include 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 often included. Strong composites can be made by adding glass, carbon and other fiber to the resins. Some compounded plastic materials can withstand strong acids, bases and alkalis, retard fire in home furnishings, contain lubricants for bearings and more.
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