CHARACTERISTICS OF PLASTICS

Introduction

Plastic is the general term for a wide range of synthetic or semi-synthetic polymerization products derived from ethylene or coal tar products. Plastic/s as per Webster dictionary include any of a large group of materials of a high molecular weight that usually contain as the essential ingredient a synthetic or semi synthetic organic substance made by polymerization or condensation, or derived from a natural material or any chemical treatment, that are molded, cast, extruded, drawn or laminated under various conditions into objects of all sizes and shapes including films and filaments.

They are composed of organic condensation or addition polymers and may contain other substances to improve performance or economics. There are many natural polymers generally considered to be "plastics". Plastics can be formed into objects or films or fibers. Their name is derived from the fact that many are malleable, having the property of plasticity.
Plastics are polymers where long chains of atoms are bonded to one another. A polymer is a substance composed of molecules with large molecular mass composed of repeating structural units, or monomers, connected by covalent chemical bonds. The word is derived from the Greek, πολυ, polu, "many"; and μέρος, meros, "part". Well known examples of polymers include plastics, DNA and proteins.

While the term "polymer" in popular usage suggests "plastic", polymers comprise a large class of natural and synthetic materials with a variety of properties and purposes. Biopolymers such as proteins and nucleic acids play crucial roles in biological processes. A variety of other natural polymers exist, such as cellulose, which is the main constituent of wood and paper. Typical synthetic polymers are Bakelite, neoprene, nylon, PVC (polyvinyl chloride), polystyrene, polyacrylonitrile and PVB (polyvinyl butyral)

Developments in Polymers

·         People experimented with plastics based on natural polymers for centuries.

·         The first plastic based on a synthetic polymer was made from phenol and formaldehyde, with the first viable and cheap synthesis methods, the product being known as Bakelite.

·         Subsequently poly vinyl chloride, polystyrene, polyethylene (polyethene), polypropylene (polypropene), polyamides (nylons), polyesters, acrylics, silicones, polyurethanes were amongst the many varieties of plastics developed and used commercially.

·         The development of plastics has come from the use of natural materials (e.g., chewing gum, shellac) to the use of chemically modified natural materials (e.g., natural rubber, nitrocellulose, collagen) and finally to completely synthetic molecules (e.g., epoxy, polyvinyl chloride, polyethylene).

·         Recently, corn has been used to make biodegradable containers. Corn can be used to create non-petroleum plastic, which is often compostable.

Types of plastics

·         Thermosets: Thermosets soften and flow when heated but at the same time an irreversible chemical reaction takes place so that once hardening has occurred the material again can not again be softened by heating. If strong heat is applied to thermoset, it will char and decompose bu will not soften and melt. Eg. Urea formaldehyde, Phenol formaldehyde.

·         Thermoplastics: Thermoplastics soften on heating and harden again on cooling, a process which can be repeated any number of times. Eg. Polyethylene, NylonCommon thermoplastics range from 20,000 to 500,000 in molecular weight, while thermosets are assumed to have infinite molecular weight. These chains are made up of many repeating molecular units, known as "repeat units", derived from "monomers"; each polymer chain will have several 1000's of repeat units. The vast majority of plastics are composed of polymers of carbon and hydrogen alone or with oxygen, nitrogen, chlorine or sulfur in the backbone. (Some of commercial products are silicon based.) The backbone is that part of the chain on the main "path" linking a large number of repeat units together and to vary the properties of plastics, both the repeat unit with different molecular groups "hanging" or "pendant" from the backbone. This customization by repeat unit's molecular structure has allowed plastics to become such an indispensable part of twenty first-century life by fine tuning the properties of the polymer.

Urea formaldehyde
It is a thermoset. Urea formaldehyde resins are resistant to solvents but are decomposed by strong acids and attacked by strong alkalis. Their main use in food packaging is for screw cap closures.

Polyacetals

The name polyacetals covers polymers and co-polymers of formaldehyde with compounds such as ethylene oxide. They have excellent load bearing properties and have been used in engineering applications. Chemically they are resistant to weak acids and alkalis but are attacked by strong ones. They have excellent solvent resistance.
Polyacetal container is used for hair lacquer aerosol.