(164k) Polymers: Classification, Polymerization, Structure, Properties, Processing Techniques, Polymer Technology, Reinforcement and Applications

Polymeric materials are used in many forms such as plastics, fibers, rubber, paints, coatings or adhesives and also as molded and fabricated articles implies that there must be a variety of ways in which the compounded resins can be processed and converted into finished products.

The individual small molecules from which the polymer is formed are known as monomers and the process by which the monomer molecules are linked to form a big polymer molecule is called polymerization. Polymers can have different chemical structures, physical properties, mechanical behavior, thermal characteristics, optical properties, heat capacity, NMR, dielectric constant etc., and can be classified in different ways, as (i) Natural and synthetic polymers- depending on their origin. Those isolated from natural polymers such as cotton, silk, wool and rubber. Cellophane, cellulose rayon, leather is, in fact, chemical modifications of natural polymers. Polymer synthesized from low molecular weight compound are called synthetic polymers e.g., PE, PP, PS, PC, nylon and terylene. (ii) Organic and inorganic polymers- a polymer whose backbone chain is made of carbon atoms is termed an organic polymer and some atoms attached to the side valancies of the backbone carbon atoms may be hydrogen, oxygen, nitrogen, etc. The molecules of inorganic polymers generally contain no carbon atoms in their chain backbone as glass and silicon rubber. (iii) Thermoplastic and thermo-setting polymers- Polymers may soften on heating, reshaping, retaining the shape and stiffen on cooling and can be repeated several times are termed as thermoplastics. It is shaped into hard, tough and strong utility articles by the use of PE, PP, PVC, PS, PMMA, etc. Some polymers, on the other hand undergo some chemical change on heating and convert themselves into an infusible mass, are called thermo-setting polymers e.g., yolk of egg. (iv) Elastomers- when polymers vulcanized into rubbery products exhibiting good strength and elongation, are termed as elastomers e.g., natural rubber, synthetic rubber, silicon rubber. (v) Fibers- polymers drawn into long filament- like materials, whose length is at least 100 times its diameter, are called fibers e.g., nylon and terylene. (vi) Resins- Polymers used as adhesives, coatings, potting compounds, sealants, etc., in a liquid form e.g., epoxy adhesives, polysulphide sealants.

John Wesley Hyatt (New York) gave the concept that a material which resembled ivory and was capable of being molded in different shapes by applying heat which he called celluloid. Carothers (1929) gave the terms addition and condensation polymerization. Further, have been modified by H.F. Mark (1950) as chain and step polymerization. Chain polymerization consists of 3 major steps, namely, initiation, propagation and termination and the process can be brought about by a free radical, ionic, ring- opening, or coordination mechanism. Compounds containing reactive double bonds can, therefore, undergo chain polymerization e.g., vinyl compounds, allyl compounds, olefins, and dienes. In step polymerization, the polymer build- up proceeds through a reaction between functional groups of the monomers e.g., poly-condensation, poly-addition. In poly-condensation e.g., ethylene glycol and adipic acid forms polyesters , self poly-condensation reaction of amino enanthic acid to form polyenanthoamide, PET can be formed by dimethyl terephthalate and ethylene glycol, Poly-addition polymerization is brought by the migration of atoms from one monomer molecule to another, or to the intermediate product e.g., PS, can be polymerized by perchloric acid. It requires higher activation energy as compared to the free- radical or ionic process- the polymer growth is rather gradual and is brought about step by step, as against the rapid chain growth in chain polymerization. In poly-addition reaction between reactive functional groups, no elimination of simple molecules is involved e.g., polyurethane polymer is formed from a diisocyanate and diol by a polyaddition reaction, in which, the hydrogen from the hydroxyl group of the diol migrates and adds on to the nitrogen of the isocyanate group. Polyureas of very high molecular weight can be formed by poly-addition polymerization with polyisocyanates and polyamines. Monomers having a ring structure can be opened and polymerized by ring-opening polymerization e.g., oxiranes can be formed by polymerization of ethylene oxide by sodium methoxide, nylon 6 can be formed by caprolactum.

The nature of the monomer, the type of polymerization mechanism chosen, the required physical form of the polymer and the viability of the process for industrial production lead to the various polymerization techniques such as bulk, solution, suspension, emulsion, melt poly-condensation, solution poly-condensation, interfacial condensation, etc. In all these polymerization techniques, the ultimate properties of the polymer such as molecular weight, molecular weight distribution, molecular structure, morphology, glass transition temperature and chemical homogeneity are very much dependent on factors such as (i) effective removal of heat of polymerization, (ii) provision of proper residence time, and (iii) effective mixing to ensure uniform temperature profile and reactant distribution throughout the system.

Polymer processing may be defined as an engineering specialty used to fabricate useful end products from polymeric materials. The calendering process is employed to produce continuous films and sheets. Hollow articles such as balls and dolls are produced by a process called rotational casting. A film casting technique is also used to produce polymeric films. The compression molding process is a widely used to produce articles from thermo-setting materials. The injection molding process is best suited for producing articles made of thermoplastic materials. Containers, soft drink bottles and numerous other hollow articles are produced by blow molding technique. Extrusion is one of the cheapest methods available for producing many common plastic products such as films, filaments, tubes, sheets, pipes, rods, hoses and straps, all in continuous length. Thermoforming is a highly useful process for fabricate three- dimensional articles from plastic sheets such as submarine hulls are made of ABS sheets. Foaming is an ingenious process for producing expanded or spongy materials using materials with cushioning ability, light weight and low thermal conductivity such as PU.

Reinforcing a polymer matrix with high strength fiber material results in the formation of what are called fiber- reinforced plastics (FRP). The main reinforcing fibers (both short- chopped and continuous) are those of glass, graphite, alumina, carbon, boron and beryllia. Reinforcement is produced by suitably bonding a fiber material with a resin matrix and curing the same under heat and pressure. The common resin matrix used in FRP include polyesters, epoxy, phenolic, silicon, melamine, vinyl derivatives and polyimides. There are several methods available for the production of reinforced plastics. Three of the most commonly used techniques are: (i) The hand lay-up technique, (ii) The filament winding technique and (iii) The spray- up technique. A remarkably high strength- to- weight ratio is the main feature attracting spacecraft designers to use reinforced plastics in satellites. Elegance, light weight and corrosion resistance qualifies reinforced plastics for use in boat hulls which are required to rough through marine salt water. The corrosion resistance property of reinforced plastics is fully utilized when they are employed as huge storage tanks for storing acids.

Molecular theories for polymeric liquids include the kinetic theories for polymers and can be divided roughly into two classes: network theories and single- molecule theories:

1. The network theories were originally developed for describing the mechanical properties of rubber. One imagines that the polymer molecules in the rubber are joined chemically during vulcanization. The theories have been extended to describe molten polymers and concentrated solutions by postulating an ever- changing network in which the junction points are temporary, formed by adjacent strands that move together for a while and then gradually pull apart. It is necessary in the theory to make some empirical statements about the rates of formation and rupturing of the junctions.
2. The single- molecule theories were originally designed for describing the polymer molecules in a very dilute solution, where polymer- polymer interactions are infrequent. The molecule is usually represented by means of some kind of “bead spring” model, a series of small spheres connected by linear or nonlinear springs in such a way as to represent the molecular architecture; the bead spring model is then allowed to move about in the solvent, with the beads experiencing a Stokes’ law drag force by the solvent as well as being buffeted about by Brownian motion. Then from the kinetic theory one obtains the “distribution function” for the orientations of the molecules (modeled as bead spring structures); once this function is known, various macroscopic properties can be calculated.

The process of polymerizing a single monomer, is termed as homo polymerization and the polymer thus obtained is termed a homo polymer. In co-polymerization, on the other hand, two or more monomers are polymerized simultaneously and, hence, the polymer is composed of more than one repeat unit e.g., SBR, vinylchloride and vinyl acetate. Co-polymerization can be brought about by many types of polymerization reactions such as free- radical, ionic or poly-condensation polymerization. The majority of polymers have been synthesized under the impectus of requirements for new and improved properties. Composites are being increasingly used in sophisticated ways to obtain special properties. The low temperature tolerance of PP is improved. Blends give us another way for synthesizing new polymer combinations.