In discussing plastics, three terms- plastics, polymers, and resins- are often used interchangeably. Plastic is a term that has a technical and chemical viewpoint. According to Paul DeGarmo, from a technical perspective, it is applied to a group of engineered materials characterized by molecules that are built up by the joining of smaller molecules called polymers (194). Usually, polymer is used in the more general sense to encompass large molecular-chain hydrogen-carbon materials, such as rubbers, plastics, and wood (Clauser 204). Chemists view plastic as organic substances containing hydrogen, oxygen, carbon, and nitrogen. Also as any synthetic organic material that can be molded under heat and pressure into a shape that is retained after the heat and pressures are removed. Practically these materials are natural or synthetic resins, or their compounds, that can be molded, extruded, cast, or used as thin films or coatings. Their advantages include low density, low tooling costs, and design versatility. In short, a plastic material is defined by the Society of the Plastics Industry as " any one of a large and varied group of materials consisting wholly or in part of combinations of carbon with oxygen, hydrogen, nitrogen, and other organic and inorganic elements which, while solid in the finished state, at some stage through the application, either singly or together, of heat and pressure."(204)

Plastic materials are predominantly synthetic materials. In the last thirty years, plastics have enjoyed a growth that has been unequaled by any other group of materials. It can be stated that plastic products meet the following criteria: their functional performance meets use requirements, they lend themselves to aesthetic treatments at comparatively low cost, and finally, the finished product is cost competitive (Dym 1). Some of the specific properties that are determining factors when considering the use of plastics are weight, thermal and electrical insulation, coefficient of friction, chemical resistance, corrosion resistance, variety of optical properties, formability, and low energy content.

1. Weight

The average weight of most plastic materials is roughly one-eighth that of steel.

In the automotive industry, lower weight means more miles per gallon of gasoline, therefore the utilization of plastics is increasing with every model-year. Lower weight is beneficial in shipping and handling, and as a safety factor to humans.

2. Thermal and electrical insulation

The value of heat insulation is fully appreciated in the use of plastic drinking

cups and of plastic handles on cooking utensils, electric irons, and other devices where heat can cause discomfort and burning. In electrical devices plastic material’s application is extended to provide not only voltage insulation where needed, but also the housing that would protect the user against accidental grounding.

3. Coefficient of friction

Many plastic materials inherently have a low coefficient of friction. Other plastic

plastic materials can incorporate this property by compounding a suitable ingredient into the base material. It is an important feature for moving parts, which provide for self- lubrication.

4. Chemical resistance

Chemical resistance is another characteristic that is inherent in plastic materials.

The range of this resistance varies among materials.

5. Corrosion resistance

Many plastics perform well in hostile, corrosive environments. A number of

notably resistance to acid corrosion.

6. Variety of optical properties

Many plastics have an almost unlimited color range and the color goes

throughout, not just on the surface. Both transparent and opaque materials exist.

7. Formability

Objects can frequently be produced from plastics in a single operation. Raw

material can be converted to final shape through such processes as casting, extrusion, and molding. Relatively low temperatures are required for the forming of plastics.

8. Low energy content.

As we can see, compared to metals, polymers are generally characterized by low density, low strength and stiffness, low electrical and thermal conductivity, good resistance to chemicals, and high coefficient of thermal expansion (see table 1). However, the useful temperature range for most polymers is generally low – up to 350°C (660°F) and they are not dimensionally stable in service, over a period of time, as metals are.

It is helpful to have an understanding of the molecular structure of plastics. Many are based on paraffin-type, in which carbon and hydrogen combine in the relationship C_nH_2n+2. Theoretically, the atoms can link together indefinitely to form very large molecules, extending as in Figure 1.

H H H

H Methane H H Ethane

Th bonds between the various atoms are all single pairs of covalent electrons. Carbon and hydrogen can also form molecules in which the carbon atoms are held together by double or triple covalent bonds. These molecules do not have the maximum possible number of hydrogen atoms, so they are important in the polymerization process, where small molecules link to form large ones with the same constituent atoms.

In each of the organic compounds, four electron pairs surround each carbon atom and one electron pair is shared with each hydrogen atom (DeGarmo 194). Other atoms or structures can be substituted for carbon and hydrogen. Chlorine or fluorine can take the place of hydrogen and oxygen. Silicon, sulfur, or nitrogen can take the place of carbon.

Monomers in polymers can be linked in repeating units to make longer and larger molecules by a chemical reaction known as the polymerization reaction. Although there are many variations, the polymerization process takes place by either an addition or condensation mechanism. Figure 2 illustrates polymerization by addition. This is where a number of basic units, called monomers, are added together to form a large molecule (polymer) in which there is a repeated unit called a mer. In addition polymerization, also known as chain-growth or chain-reaction polymerization, bonding takes place without reaction by-products. It is called chain-reaction because of the high rate at which long molecules from simultaneously, usually within a few seconds. This rate is much higher than that for condensation polymerization.

Activators or catalysts initiate and terminate the chain. Thus, the amount of activator relative to the amount of monomer determines the average molecular weight or length of the chain (196). The average number of mers in the polymer, known as the degree of polymerization, ranges from 75 to 750 for most commercial plastics.

In contrast to polymerization by addition, where all of the component atoms appear in the product molecule, condensation polymerization occurs as reactive molecules combine with one another to produce a polymer plus small, by-product molecules. One characteristic of this reaction is that reaction by-products such as water are condensed out, hence the term condensation. This process is also known as step-growth or step-reaction polymerization because the polymer molecule grows step by step until all of one reactant is consumed. Heat, pressure, and catalysts are often required to drive the reaction. Figure 3 shows an example of a reaction that is condensation polymerization.

Figure 3. The structure of condensation polymers can either be linear chains or a three-dimensional framework in which all atoms are linked by strong, primary bonds. H

H H C H H C H

C == O + C C C C + H₂O

C C C C

Formaldehyde H H H

H Phenol(*2) Phenol Formaldehyde Water

(2 Phenol Molecules)

The molecular structure of a polymer determines its chain which influences a polymer’s mechanical and physical properties (see figure 4). There are four main polymer chains: linear, branched, cross-linked, and networked.

Linear polymers. The chainlike polymer as in figure 2 are called linear polymers because of their linear structure (see figure 4a). A linear molecule is not completely straight in shape. Generally, a polymer consists of more than one type of structure. Thus a linear polymer may contain some branched and cross-linked chains. As a result of branching and cross-linking, the polymer’s properties can change.

Branched polymers. The properties of a polymer depend not only on their arrangement in the molecular structure. In branched polymers, side-branched chains are attached to the main chain during the synthesis of the polymer (see figure 4b). Branching interferes with the relative movement of the molecular chains. As a result, resistance to deformation and stress-crack are affected. Also, the density of branched polymers is lower than that of linear-chain polymers as branches interfere with the packing efficiency of polymer chains. The behavior of branched polymers can be compared to that of linear-chain polymers by making an analogy with a pile of tree branches (branched polymers) and a bundle of straight logs (linear). Notice that it is more difficult to move a branch within the pile of branches than to move a log in its bundle. The three-dimensional entanglements of branches make movements more difficult, a phenomenon akin to increased strength.

Cross-linked polymers. Generally, three-dimensional in structure, cross-linked polymers have adjacent chains linked by covalent bonds (see figure 4c). Polymers with cross-linked chain structure are called thermosets, or thermosetting plastics, such as epoxies, phenolics, and silicones. Cross-linking has a major influence on the properties of polymers (generally imparting hardness, strength, stiffness, brittleness, and better dimensional stability).

Network polymers. Network polymers consist of spatial (three-dimensional) networks of three active covalent bonds (figure 4d). A highly cross-linked polymer is also considered a network polymer. Thermoplastic polymers that have already been formed or shaped can be cross-linked to obtain greater strength by subjecting them to high-energy radiation, such as ultraviolet, x-rays, or electron beams. However, excessive radiation can cause degradation of the polymer.

Polymers such as polycarbonate and polystyrene are generally amorphous; that is, the polymer chains exist without long-range order. The amorphous arrangement of polymer chains is often described as a bowl of spaghetti, or worms in a bucket, all intertwined with each other. However, in some polymers it is possible some crystallinity and thereby modify their characteristics. The crystalline regions in polymers are called crystallites (figure 5).

These crystals are formed when the long molecules arrange themselves in an orderly manner, similar to a fire hose in a cabinet. Thus, it is regarded that a partly crystalline polymer as a two-phase material, one phase being crystalline and the other as amorphous.

The mechanical and physical properties of polymers are greatly influenced by the degree of crystallinity. As it increases, polymers become stiffer, harder, less ductile, more dense, less rubbery, and more resistant to solvents and heat (see figure 6).

Figure 6. Behavior of polymers as a function of temperature and (a) degree of crystallinity and (b) cross-linking. The combining elastic and viscous behavior of polymers is known as viscoelasticity.

Optical properties are also affected by the degree of crystallinity. The reflection of light from the boundaries between the crystalline and amorphous regions in the polymer causes opaqueness. Furthermore, because the index of refraction is proportional to density, the greater the density difference between the amorphous and crystalline phases, the greater is the opaqueness of the polymer. Polymers that are completely amorphous can be transparent, such as acrylics.

Glass-transition temperature

Amorphous polymers do not have a specific melting point, but they undergo a distinct change in their mechanical behavior across a narrow range of temperature. At low temperatures they are hard, rigid, brittle, and glassy and at high temperature are rubbery or leathery. The temperature at which this transition occurs is called the glass-transition temperature, T_g, and is also called the glass point or glass temperature.

To determine T_g, the specific volume of the polymer is measured and plotted it against temperature to find the sharp change in the slope of the curve (see figure 7).

Figure 7. Specific volume of polymers as a function of temperature. Amorphous polymers have a glass-transition temperature, T_g, but do not have a specific melting point, T_m. Partly crystalline polymers contract sharply at their melting points during cooling.

However, in the case of highly cross-linked polymers, the slope of the curve changes generally near T_g, making it difficult to determine T_g for these polymers. The glass-transition temperature varies with different polymers (see table 3). For example, room temperature is above T_g for some polymers and below it for others. Unlike amorphous polymers, partly crystalline polymers have a distinct melting point, T_m (figure 7 and table 3).

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