Polymers

Plastics are polymers.Polymers are long-chain molecules and are formed by polymerization process, linking and cross linking a particular building block (monomer, a unit cell).The term polymer means many units repeated many times in a chainlike structure. Most monomers are organic materials, atoms are joined in covalent bonds (electron-sharing) with other atoms such as oxygen, nitrogen, hydrogen, sulfur, chlorine etc.There are two major classifications of polymers:

Thermoplastics

As the temperature is raised above the melting point, the secondary bonds weaken, making it easier to form the plastic into any desired shape. When polymer is cooled, it returns to its original strength and hardness. The process is reversible. Polymers that show this behavior are known as thermoplastics.

Thermosetting Plastics (thermosets)

Thermosetting plastics are cured into permanent shape. Cannot be re-melted to the flowable state that existed before curing, continued heating for a long time leads to degradation or decomposition. This curing (cross-linked) reaction is irreversible.  Thermosets generally have better mechanical, thermal and chemical properties. They also have better electrical resistance and dimensional stability than do thermoplastics.

Polymer’s Structures 

Bonding – monomers are linked together by covalent bonds, forming a polymer chain (primary bonds). The polymer chains are held together by secondary bonds. The strength of polymers comes in part from the length of polymer chains. The longer the chain, the stronger the polymer. More energy is needed to overcome the secondary bonds.

Linear polymers

A sequential structure resulting in thermoplastics like nylon, acrylic, polyethylene. A linear polymer may contain some branched and cross-linked chains resulting in change in properties.

Branched polymers

Side branch chains are attached to the main chain which interferes with the relative movement of the molecular chains. This results in an increase in strength, deformation resistance and stress cracking resistance. Lower density than linear chain polymers.

Cross-linked polymers

Three dimensional structure, adjacent chains are linked by covalent bonds. Polymers with cross-linked chains are called thermosetting plastics (thermosets), epoxy and Silicones.

Network polymers

A three dimensional network of three or more covalent bonds. Thermoplastic polymers that have been already formed could be cross-linked to obtain higher strength. Polymers are exposed to high-energy radiation.

Types of Polymers:

  • PVC (Poly-Vinyl Chloride)
  • Polystyrene/ABS (Acrylonitrile Butadiene-Styrene)
  • Polyethylene
  • PTA (Purified Terephthalic Acid)
  • Polypropylene
  • Polyesters
  • Nylon
  • Bio-Degradable Plastics
  • Synthetic Rubber
  • Latex

Features & Benefits

The use of polymers reflects the unique performance advantages potentially realizable from these materials. While each application poses different opportunities and challenges, certain general features and  benefits are as follows:

  • High strength and stiffness/weight ratio
  • Corrosion-resistance
  • Manufacturing flexibility
  • Variety of properties
  • Flexibility
  • Chemical stability
  • Low cost

Applications

Plastics

The two main types of plastics are thermoplastics and thermosets. Thermoplastics soften on heating and harden on cooling while thermosets, on heating, flow and cross-link to form rigid material which does not soften on future heating. Thermoplastics account for the majority of commercial usage.

Among the most important and versatile of the hundreds of commercial plastics is polyethylene. Polyethylene is used in a wide variety of applications because, based on its structure, it can be produced in many different forms. The first type to be commercially exploited was called low density polyethylene (LDPE) or branched polyethylene. This polymer is characterized by a large degree of branching, forcing the molecules to be packed rather loosely forming a low density material. LDPE is soft and pliable and has applications ranging from plastic bags, containers, textiles, and electrical insulation, to coatings for packaging materials.

Another form of polyethylene is high density polyethylene (HDPE) or linear polyethylene. This form demonstrates little or no branching, enabling the molecules to be tightly packed. HDPE is much more rigid than branched polyethylene and is used in applications where rigidity is important. Major uses of HDPE are plastic tubing, bottles, and bottle caps.

Other forms of this material include high and ultra-high molecular weight polyethylenes. HMW and UHMW, as they are known. These are used in applications where extremely tough and resilient materials are needed.

Fibers

Man-made fibers include materials such as nylon, polyester, rayon, and acrylic. The combination of strength, weight, and durability have made these materials very important in modern industry.

Synthetic polymers have been developed that posess desirable characteristics, such as a high softening point to allow for ironing, high tensile strength, adequate stiffness, and desirable fabric qualities. These polymers are then formed into fibers with various characteristics.

From textiles to bullet-proof vests, fibers have become very important in modern life

Processing Polymers(Injection Molding, Extrusion, Spinning)

Once a polymer with the right properties is produced, it must be manipulated into some useful shape or object. Various methods are used in industry to do this. Injection molding and extrusion are widely used to process plastics while spinning is the process used to produce fibers.

One of the most widely used forms of plastic processing is injection molding. Basically, a plastic is heated above its glass transition temperature (enough so that it will flow) and then is forced under high pressure to fill the contents of a mold. The molten plastic in usually "squeezed" into the mold by a ram or a reciprocating screw. The plastic is allowed to cool and is then removed from the mold in its final form. The advantage of injection molding is speed; this process can be performed many times each second.

Extrusion is similar to injection molding except that the plastic is forced through a die rather than into a mold. However, the disadvantage of extrusion is that the objects made must have the same cross-sectional shape. Plastic tubing and hose is produced in this manner.

The process of producing fibers is called spinning. There are three main types of spinning: melt, dry, and wet. Melt spinning is used for polymers that can be melted easily. Dry spinning involves dissolving the polymer into a solution that can be evaporated. Wet spinning is used when the solvent cannot be evaporated and must be removed by chemical means.

Elastomers

Rubber is the most important of all elastomers. There are two types of Rubber:

Natural rubber is the one that is a polymer whose repeating unit is isoprene. This material, obtained from the bark of the rubber tree, has been used by humans for many centuries.

The other one is a synthetic rubber variety called styrene-butadiene rubber (SBR). Initial attempts to produce synthetic rubber revolved around isoprene because of its presence in natural rubber.