POLYMERS AND POLYMERISATION

Polymers are composed of a large number of repeating units (small molecules) called monomers. A polymer is, therefore, made up of thousands of monomers joined together to form a large molecule of colloidal dimension, called macromolecule. The properties of polymers are intricately related to the structural elements of the material. Most polymers are organic in origin and are based on hydrocarbons, i.e., they are composed of hydrogen and carbon. Moreover, the intramolecular bonds are covalent. Each carbon has four electrons that may participate in covalent bonding, whereas every hydrogen atom has only one bonding electron. A single covalent bond exist when each of the two bonding atoms contribute one electron, as shown in Fig. for methane. Double and triple bonds exist between two carbon atoms involve the sharing of two and three pairs of electrons, respectively. Figure shows the structural formula for ethylene (C2H4).

          The process of linking together of monomers is called polymerisation. The need to start with the process of polymerisation lies on the necessity of breaking the double bond (C=C) of the monomer. It utilizes the valence of the partially filled outer shell of the carbon atom (carbon has a valence of 4) to join smaller units together to form larger chain of molecules. Oxygen, sulphur, silicon, or nitrogen can be used to replace the carbon atom. This requires considerable energy. Polymerisation mechanisms may be of the following two types:

(1) Addition Polymerisation.

(2) Condensation Polymerisation.

VARIOUS TYPES OF POLYMERISATION

VARIOUS TYPES OF POLYMERISATION (ADDITION POLYMERISATION & CONDENSATION POLYMERISATION)

1.Addition polymerisation:

          This polymerisation process is of simplest form. In this polymerisation process, a large unit molecule, the monomer, are chemically added to another monomer to form a large chain, the polymer (referring to many parts), which has a number of repeated units, mers. Mers are the smallest units recognizable in the chain. The degree of polymerisation is the number of repeating units that have identical structures within the chain formed by the polymer. Addition polymerisation involves only one type of mer. Such polymerisation takes place by three steps namely. (i) Initiation  (ii) Chain propagation  (iii) Termination.

Example: Addition polymerisation of ethylene.

          Once the polymerisation process is started, it does not continue indefinitely since it is impossible to link all the monomers in a plastic one long continuous chain. The polymerisation is terminated by a collision between the active ends of two chains or by addition of a terminator, such as free radicals from catalyst.

Copolymerisation:

          It is another type of addition polymerisation.

          In copolymerisation, more than one molecule makes up the mer. Acrylonitride-butadine-styrene (ABS) is an example of a copolymer. Figure shows the copolymerisation process for ABS polymers.

          Many monomers will not polymerise with themselves but will copolymerise with other compounds. Copolymerisation has been applied extensively, in the artificial rubber.

2. Condensation Polymerisation:

          Condensation polymerization involves the chemical reaction of two or more unlike monomers to form a new molecule. This chemical reaction produces a condensate or nonpolymerisable by-product, usually water. A catalyst id often required to start and maintain the reaction. It can also be used to control the reaction rate. The process of condensation polymerisation is shown in Fig.

          With some exceptions, polymers made in chain reactions often contain only carbon atoms in the main chain (homochain polymers), whereas polymers made in step reactions may have other atoms, originating in the monomer functional groups, as part of the chain (heterochain polymers).

Difference between addition polymerization and condensation polymerization

Addition polymerization	Condensation polymerization
The addition polymerization means that  two monomers react with each other and  no other small molecules are generated.  The best example is polymerization of  ethylene.	The condensation polymerization, as a  contrast, normally involves the generation  of small molecule products.
It requires two like molecules.	It requires two unlike molecules.
Kinetic long linear chain reaction.	Intermolecular reaction.
Very fast reaction 10-2 – 10-6sec	Slow reaction takes hours and days to  complete.
No by total.	By product it produced.
Polymer produced thermoplastic.	Chemo setting plastic produced.
Example: PVC, Teflon, Poly ethylene.	Example: Bakelite, Silicon, GRP, Polystyrene

PLASTICS

Definition of Plastic:

A plastic (from the Greek "plastikos," meaning moldable) can be broadly defined as any non-metallic synthetic material made from a wide range of organic polymers such as polyethylene, PVC, nylon, etc., that can be moulded into shape while soft and then set into a rigid or slightly elastic form.

-- Most plastics are of organic nature composed of hydrogen, oxygen, carbon and nitrogen.

-- The synthetic plastic development dates from 1900 when Dr. Beekland announced the production of Phenol formaldehyde (Bakelite). Since then several plastics have been developed.

-- The basic raw materials used in the manufacturing of plastic come from natural resources such as oil, gas and coal (fossil fuels).

Constituents of plastics

The principal constituents of plastics are:

1. Resins or Binders
2. Plasticizers
3. Fillers
4. Lubricants
5. Stabilisers
6. Catalysts or Accelerators
7. Pigments or Dyes

(i) Resins or Binders: The resins are the basic binding materials in plastics, which holds the other constituents of the plastics. The resign determines the moulding method. Cellulose material or synthetic polymers are used as resigns. These are of two types they are thermo plastics and thermo setting. These two types of resigns differ in thermal nature so they require different treatment methods.

(ii) Plasticizers: The chemical compound that improves the plasticity and flexibility of resign is called as plastisizers. Camphor increases the surface hardness, trybutyl phathalate increases the toughness. Plastcizer should be chemically inert, non-volatile and non-toxic.

Commonly used plasticizers include non-drying vegetable oils, camphor, esters of oliec, aluminium stearates, and trybutyl phathalate.

(iii) Fillers: These material added to resin to enhance performance, mechanical properties and reduce manufacturing costs. These additives are used to make polyester resin more chemical and corrosion resistant, act as a fire retardant, enhance shrink-resistance and be thermally stable, as well as capable of withstanding weather conditions. These are powder, fibrous and laminated fillers.

(iv) Lubricants: Lubricants makes the moulding of the plastic easier giving a flawless and glossy finished product. Lubricants also prevent the plastic material from sticking to the moulding equipment.

Common lubricants are Mosallic soaps, Stearates, oils and waxes.

(v) Stabilizers: The thermal stability of the resin is enhanced by the addition of a stabilizer.

Transparent moulding components such as stearic acid salts of lead, Cadmium and Barium also used as stabilizers.

(vi) Catalysts: Catalyst accelerates the chemical reaction. These compounds also act as hardeners and accelerators. These are required only in the case of thermo setting resigns in order to accelerate the polymerization reaction and form highly cross linked finished product. Hydrogen peroxide, copper, lead and zink is used as catalyst.

(vii) Pigments: Colourants such as pigments and organic dyes are added to give required color to the finished product.

Classification Of Plastics

Plastics can be classified into:

1. Thermoplastics and Thermosets
2. Amorphous Thermoplastics and Crystalline Thermoplastics
3. Commodity Plastics and Engineering Plastics.

Thermoplastic materials:

          Thermoplastic materials are those which soften on the application of heat and subsequently melt, with or without pressure and require cooling to be set to a shape.

          Thermoplastics can be reversely melted by heating and solidified by cooling in limited number of cycles without affecting the mechanical properties. On increasing the number of recycling of thermoplastics may result in color degradation, there by affecting their appearance and properties. In the molten state, they are liquids, and in the mushy state they are glassy or partially crystalline. The molecules are joined end-to-end into a series of long chains, each chain being independent of the other. Above the melting temperature, all crystalline structure disappears and the long chain becomes randomly scattered.

          The molecular structure of thermoplastic (figure 1) has an influence on the chemical resistance and resistance against environmental effects like UV radiation. The properties may also vary from optical transparency to opaque, depending on the molecular structure. The important properties of the thermoplastics are high strength and toughness, better hardness, chemical resistance, durability, self lubrication, transparency and water proofing.

          With the application of heat, thermoplastics softens and it can be moulded into desired shapes because they are highly plastic. Some thermoplastics can be joined with the application of heat and pressure. There are several techniques available for the joining of thermoplastics such as mechanical fastening, fusion bonding, hot gas welding, solvent bonding, ultrasonic welding, induction welding and dielectric welding.

      The different types of thermoplastic are: Acrylonitrile Butadiene styrene (ABS), Acetals, Acrylics, Cellulosics, Fluorocarbons, Polyamides, Polycarbonates, Polythylene (PE), Polypropylenes (PP), Polystyrenes, Polyvinyl chloride (PVC), Liquid crystalline polymers (LCP), Polyphenylene sulphide (PPS) and Vinyls.

Figure 1 Molecular structure of thermoplastics

Applications:

Thermoplastics can be used to manufacture the dashboards and car trims, toys, phones, handles, electrical products, bearings, gears, rope, hinges and catches, glass frames, cables, hoses, sheet and windows etc.,

Thermosets

          The property of material becoming permanently hard and rigid after cooling when heated above the melting temperature is called thermosets. The solidification process of plastics is known as curing. The transformation from the liquid state to the solid state is irreversible process, further heating of thermosets result only in the chemical decomposition. It means that the thermosets can't be recycled. During curing, the small molecules are chemically linked together to form complex inter connected network structures (Figure 2). This cross-linking prevents the slippage of individual chains. Therefore, the mechanical properties (tensile strength, compressive strength and hardness) are not temperature dependent, as compared to thermoplastics. Hence, thermosets are generally stronger than the thermoplastics.

Figure:2  Molecular structure of thermosets

The joining of thermosets by thermal process like ultrasonic welding, laser welding and gas welding is not possible, but mechanical fastening and adhesive bonding may be used for low strength applications.

         The different types of thermosets are Alkyds, Allylics, Amine, Bakelite, Epoxy, Phenolic (PF), Polyester, Silicone, Polyurethane (PUR) and Vinyl Easter.

Applications:

Thermosets are commonly used for high temperature applications. Some of the common products are electrical equipments, motor brush holders, printed circuit boards, circuit breakers, encapsulation, kitchen utensils, handles and knobs, and spectacle lenses.

Thermoplastic materials

Various TYPES OF THERMO PLASTICS 

( CELLULOSE DERIVATIVES & SYNTHETIC RESINS)

Thermo plastics

These kinds of plastics have long separate and large molecules arranged side by side. The various types of thermoplastics are discussed below.

Cellulose derivatives

(i) Cellulose acetate:

          It is obtained by treating the cellulose with acetic acid. It was first prepared in 1865. It is inserted and compressed in the mould for better stability and for obtaining high mechanical strength. It has the tendency to absorb moisture and is less in weight. It is used for manufacturing photographic films, buttons, radio panels, sheets, tubes, toys etc.,

(ii) Ethyl cellulose:

          It is the most lightest of all cellulose derivatives. It has very good electrical properties, chemical resistance, surface hardness and strength. Generally used for making jigs, fixtures, nozzles and moulded articles.

(iii) Cellulose acetate – butyrate:

          It is obtained by treating cellulose with acetic acid and butoric acid. It has a good stability against light and heat and moisture absorption tendency. Cellulose acetate can also be used for injection moulding. It is used for making radio cabinets, pipes, steering wheels, handles and coating.

(iv) Cellophane:

          It is generally available in extruded form. Cellophane is a thin, transparent sheet made of regenerated cellulose. It has an attractive appearence and good resistance to moisture, solvents and fire. Generally used for making curtains and packing materials.

(v) Cellulose propionate:

          It has low tendency for moisture absorption. Cellulose propionate can withstand temperatures up to  and can be easily moulded. Used for making fountain pens, telephones and flashlight cases.

Synthetic resins

The following are the synthetic resins.

(i) Polyethylenes or polythene (C2H4)n

Polyethylene (abbreviated PE) or polyethene is the most common plastic. These are commercially prepared in America in 1940's. Many kinds of polyethylene are known, with most having the chemical formula (C2H4)n. polyethylene are obtainable as various liquids, gums and tough flexible solids suitable for moulding.

          It has very high resistance to acids, alkalises and the solvents can also be made flexible, tough and good insulators. It has low water absorption. They are wax like in appearance, translucent, odorless and one of the lightest plastics. these are low in cost. These are flexible over a wide range of temperatures. 

Uses:

1. Polythene film is one of the most lightweight and durable packaging mediums available.
2. Plastic packaging makes an important contribution to reducing food spoilage rates by moisture proofing.
3. Polythene ducting is compatible with most fans, heaters, air conditioners, air handling units etc.,
4. Pipes and tanks for water storage.
5. As insulation in submarine cables and radar lines.
6. Lining for lagoons to avoid seepage of polluted water into the underground.
7. It is used for making fabrics, trays, for corrosion resistant coatings.

Disadvantages:

1. Polythene is not biodegradable, and if dumped in the soil causes harm to the plant life, as the toxic substances of polythene get blocked among the soil particles.
2. Polythene threatens the life in the water bodies. The chemicals in polythene affects the survival of flora and fauna of the aquatic and marine Eco-systems.
3. Polythene is also likely to clog the drains causing problems in the water flow of the pipes. The pipe blockages would cause flooding and the free flow of water is disturbed.
4. Polythene is harmful for animals if swallowed. It solidifies inside the abdominal cavity which ultimately becomes lethal to the animal.
5. In most households poly bags are used to preserve food items. It has been found out, the colorful poly bags contains lead and cadmium which are toxic and cause adverse effects to human health.
6. If polythene is burnt in open air hydrogen cyanide which is carcinogenic (cancer causing) is released.
7. Hydrogen cyanide causes environmental pollution and health hazards.

Two types of polyethylene are manufactured depending upon the condition of polymerisation.

              (a) High density polyethylene (HDPE)

              (b) Low density polyethylene (LDPE)

HDPE | High Density Polyethyelene (HDPE)

          A linear polymer, High Density Polyethylene (HDPE) is prepared from ethylene by a catalytic process. The absence of branching results in a more closely packed structure with a higher density and somewhat higher chemical resistance than LDPE. High density polyethylene is also somewhat harder and more opaque and it can withstand rather higher temperatures (120° Celsius for short periods, 110° Celsius continuously). High density polyethylene lends itself particularly well to blow moulding. It has specific gravity 0.96.

Low-density polyethylene (LDPE)

          LDPE is created by free-radical polymerization. This is obtained by high pressure process. LDPE is defined by a density range of 0.910-0.940 g/cm3, softening temperature 86°c. LDPE has a high degree of short and long chain branching, which means that the chains do not pack into the crystal structure as well. It has, therefore, less strong inter molecular forces as the instantaneous dipole induced dipole attraction is less. This results in a lower tensile strength and increased ductility.

(ii) Polystyrene (PS):

                                   Chemical formula of Polystyrene (PS) is

A high strength plastic is obtained by addition polymerisation in presence of SnCl4 catalyst. It can be toughened by mixing another polymeric material acrylonitrile butadiene to give ABS plastics. It is available in any form and any color and physiologically harmless nature make polystyrene suitable for use in contact with food stuffs. It has good dimensional stabilities and strain resistance. Polystyrene's are easily joined by cementing. It has good electrical properties and negligible water absorption.

Uses:

It is used for making boxes, Lenses, Cases, radio parts, food containers, automobile dash blanks, table ware and insulation parts.

(iii) Acrylic resins (polymethyl methacrylate PMMA):

          They are the polymer of methyl methacrylate having trade name "prespex". PMMA is manufactured by addition polymerisation of methyl methacrylate in presence of a peroxide catalyst. It has a high transparency tendency. It is made in any color with high dielectric properties, good strength, resistance to moisture and very good light transmitting power. This material can be casted, injection moulded, extruded and can be stretched into sheets.

Uses:

It is used for manufacturing tubes, hospital equipment's, light weight garments, plates, lamination, display cases, sanitary ware, helmets and valves.

(iv) PolyVinylechloride (PVC):

          Chemical formula of Polyvinyl  chloride (PVC) is

It is generally called as PVC. This is the cheapest and most widely used plastic. They are flexible and rigid. It has good electrical and weather resistance. These are resistant to water and available in variety of colors. The vinylchlorides are formed from hydrochloric acid. Limestone and natural gas or coal. The forms of vinylchlorides are almost unlimited. It is manufactured by addition polymerisation of vinylchloride. Polymerisation is carried out in the presence of catalyst. PVC can be manufactured in expanded or cellular form. It is available in two forms namely flexible and rigid.

Properties:

• Termite proof.
• Tough and durable.
• Anti corrosive.
• Light weight.
• Maintenance free.
• Superb quality.
• Have a pleasing appearance.
• Chemical and moisture resistant.
• Available in wide ranges of colors.

Disadvantages:

1. Plastic doors are not suitable for entry doors as they are very of light weight.
2. They are not weather proof like wooden or metal doors.
3. They cannot resist the harsh environment conditions.

Uses:

• Cable jackets.
• Rubber substitute.
• Fabric coating.
• Rain water goods.
• Flooring and ceiling panels.
• Gramphone records.
• Pipes and fittings of water service.

(v) Polytetra fluoroethylene (PTFE):

         Chemical formula of Polytetra fluoroethylene (PTFE) is

It is named after Teflon. The mer of PTFE is represented as CF2 = CF2. PTFE is manufactured by addition polymerisation techniques. Polymerisation is carried out under organic peroxide or oxygen as catalyst.

          It has good chemical resistance and can withstand temperatures up to 288°C. It is non-flammable. It cannot be dissolved in any solvent. It has high electrical resistance, low friction and very low adhesion to other substances. It mostly available in the form of sheets, rods and tubes.

Uses:

It is used for making non-stick coating, bearing bushes, piston rings, mouldings in aircraft's, gaskets, electrical insulators, laboratory equipment etc.,

(vi) Nylon (polyamides):

          It is also known as polyamide. It is produced by series of condensation reaction between an amine and organic acids. Nylon is produced from the condensation polymerisation reaction between hexamethylene diamine and adipic acid.

         It has high strength, toughness and elasticity. It can be moulded and extruded into rods. The powder metallurgy methods can also be used for this type of plastics. It is a good insulator and has good wear resistance. They are resistant to most of the solvents and chemicals.

Uses:

Used for making Yarn for making clothes, insulation wires, moulding gears, bearings and combs.

(vii) Poly Methyl methacrylate  (PMMA)(Acrylics):

Chemical formula of Polymethyl methacrylate (PMMA) is

          It's trade name is Lucite and Plexiglass. It is easily formed at temperatures around . It is known for its clear color and high light transmission ability.

Uses:

Used for manufacturing aircraft parts, bowls, contact lenses and various surgical instruments.