Chapter 2
Polymers

Polymers are materials that are widely used in many important emerging technologies. They have a number of beneficial functions with excellent mechanical properties. Due to their chemical inertness and optical, barrier and mechanical properties, they are suitable for use in a variety of products and have a wide range of applications [1]. Polymers are long-chain molecules with properties different from those of metallic and ceramic materials. They are light, strong, easily molded and durable.

Cost, performance and durability are major product requirements. Therefore, polymers have the advantage of economic cost with great potential for part integration and play a very important role in our lives. Quantities of polymer materials are used in many conventional applications. High production can be achieved with polymers.

2.1 Macromolecules

Polymers are built up by linking together monomers in large numbers. Monomers are molecules with functional groups or elements which react with each other to form large molecules. The degree of polymerization determines the size of a polymer. The polymer size is the total number of structural units which includes the end groups and is also related to both chain length and molecular weight.

Polymers are macromolecules which are constructed by the covalent linking of simple molecular repeated units. The structure is implied in the phrase "poly," meaning many, and "mer," designating the nature of the repeating unit [2-4]. Polymers are modified to provide specific compatibility, durability or properties dictated by their expected use in industry and there has been expanded use of polymer blends and composites in industrial applications. Polymers from synthetic resources are mainly non-biodegradable materials. Polymers, the molecules of plastic, are predominantly derived from fossil-based feedstocks. They are remarkably lightweight, durable, protective, conductive, and self-healing, which are just a few of their many attributes.

Polymers play a vital role in human activities by contributing to the sophistication and comfort of society. They are insoluble materials [5] whose properties are modified by using fillers and fibers to suit the high strength/high modulus requirements.

2.2 Types of Polymers

Polymers are categorized as thermoplastics and thermosets. The category is based on the characteristics of the recycling nature of the end products. Polymers have attracted attention because their physical properties differ from the starting monomer material. Figure 2.1 illustrates thermoplastic polymers versus thermoset polymers.

Figure 2.1 Thermoplastic polymers versus thermoset polymers.

Basically the difference between thermoplastic and thermoset materials (Table 2.1) is that thermoplastic materials are independent chains which will flow with heat whereas thermoset materials are chemically bonded crosslinked chains which will not flow with heat. Moreover, thermoplastics are amenable to recycling. Thermosets, on the other hand, cannot be recycled. Thermoplastics can be relatively easy to process. Thermosetting polymers crosslink between separate polymer chains.

Table 2.1 Thermoplastic polymers versus thermoset polymers.

End use needs to be considered in terms of processing either thermoplastics or thermosets. Polymeric materials according to blends and composites fall into two categories, namely:

• Thermoplastic polymers - recycling ability
• Thermoset polymers - non-recycling ability but rigid

2.2.1 Thermoplastic Polymers

Thermoplastic polymers are linear chain polymers and are solid at room temperature. They may be melted and softened at an elevated temperature; and may be re-melted several times. They are simple chain which may differ in molecular weight, degree of branching with long or short chain, stereo structure or composition. Molecular weight distribution depends on polymerization kinetics, process conditions and mechanism. The polymerization must be carefully controlled and monitored to obtain the desired characteristics and processing ability.

Thermoplastic invariably requires a much higher processing temperature and has the following advantages:

• It is easy to process - It is highly viscous at its processing temperatures.
• It is less troublesome to recycle - Recycling of thermoplastic involves reversible physical change by heating the polymer above its processing temperature for shaping it and cools it to room temperature to obtain the desired recycled product.
• It is economical to refabricate.
• It even undergoes significant reduction in properties. However, it retains an acceptable fraction of properties from its original material [6].

Thermoplastic polymeric materials are thermal and electrical non-conductors. Thermoplastic polymers are used in a wide range of everyday applications such as in clothing, housing materials, medical applications, appliances, automotive and aerospace parts, and in communications.

2.2.2 Thermoset Polymers

Thermoset is a physical mixture consisting of base resin, reinforcement material, catalysts, colorants and lubricants subjected to a specific combination of heat, pressure and time. It undergoes an irreversible reaction referred to as polymerization. Upon complete polymerization, thermosets become infusible solids and will not soften when heated.

The polymerization of thermoset can be attained either by the initial application of heat or by the use of chemical accelerator. Upon processing and molding, these thermoset polymers become crosslinked and therefore remolding by heating will not be possible. Heating of thermosets in three-dimensional network must be broken either by cleavage of crosslinks or through the carbon-carbon linkage of the chain backbone, and finally into charred product.

In thermoset, polymer involves prepolymer upon curing whose molecules are capable of entering, through reactive groups, into further polymerization, thereby contributing more than one structural unit to at least one type of chain of the final polymer. It is not reliable or possible for recycling due to the crosslinking nature of the polymer.

2.3 Polymerization

Polymerization is the process in which the smaller monomers are linked together to form large molecules called polymers. The molecular weight of the polymer is a simple multiple of the molar mass of the original monomer. Many polymers involve two or more types of monomers, known as copolymerization. Figure 2.2 illustrates the conversion of simple monomer into polymer.

Figure 2.2 Schematic representation of the polymerization process.

2.4 Polymerization Techniques

Polymers coincide with the molecular long-chain hypothesis, their strategic importance becoming evident during the Second World War through the innovation of artificial rubber. This led to the establishment of highly productive applied polymer research groups in industry and academia [7]. The first attempt at polymer synthesis with modification was made in the middle of the nineteenth century. It could be said that celluloid was the first synthetic, or at least partly synthetic, plastic [8].

Polymerization techniques have been used to synthesize polymer at a large scale for commercial applications. Polymerization processes, such as bulk, suspension, solution and emulsion techniques, are used for conversion of monomers to polymers. Polymerization charge normally consists of monomer, monomer-soluble initiator, and a chain-transfer agent. Monomers combine in the field of polymerization, which allows accessing many advanced technical polymers.

The simplest direct method is bulk polymerization. The heat of reaction is very high, making it difficult to control polymerization, and normally high molecular weight polymer is formed. Heat transfer problems due to reactions are commonly encountered with bulk polymerization. It is also known as mass polymerization [9].

In suspension polymerization, the monomer in droplets is suspended in an inert, non-solvent liquid (almost water). Monomers used in suspension polymerization often have considerably different water solubilities. Relatively nonpolar monomers, such as styrene and butyl acrylate, tend not to be water insoluble, while polar monomers, such as vinyl acetate and methyl acrylate, are highly water soluble. Heat removal is facilitated by keeping the dimension of the reaction mass small. Heat can easily be soaked up by removal from the low-viscosity and inert suspension medium. Particle size, suspending agents, and agitation generally do not affect polymerization rate in batch polymerization [10, 11]. Suspension polymerization produces polymers, such as polyvinylchloride (PVC), in the form of powder with a complex morphology. The properties of the products depend on the composition of the PVC compound and on its processing conditions.

Suspension polymerization describes the process in which monomer(s) is relatively insoluble in water. With steric stabilizer, water is...