22.3 Classification of Polymers

According to end use polymers can be classified as: plastics, synthetic fibers, and rubbers.

Plastics

In general, plastics are subclassified into thermoplastics—polymers that can be resoftened by heat, and thermosets—which cannot be resoftened by heat.

  • Thermoplastics have moderate crystallinity. They can undergo large elongation, but this elongation is not as reversible as it is for elastomers. Examples of thermoplastics are polyethylene and polypropylene.

  • Thermosetting plastics are usually rigid due to high cross-linking between the polymer chains. Examples of this type are phenol fomaldehyde and polyurethanes.

  • With the advancement of technology, the line between thermoplastic and thermosetting plastic is becoming thinner.

  • The thermoplastic polymers—polyethylene and polyvinyl chloride wire coverings and pipe can be converted to thermoset structures by cross-linking their molecules under the influence of high-energy radiation or free radicals released by decomposition of peroxides in the polymer compound.

  • Thermoplastics consist of long-chain molecules often without any branching (e.g.: high-density polyethylene). Even if there is branching (e.g.: low-density polyethylene) the polymer may still be two dimensional; whereas thermosets have elaborately cross-linked three-dimensional structures.

  • Thermoplastics may be used in the five main applications of polymers (plastics, fibers, elastomers, coatings, and adhesives).

  • Thermosets are used for plastics, elastomers (lightly cross-linked), coatings, and adhesives but not fibers. Fibers require unbranched linear molecules that can be suitably oriented by stretching during the spinning and drawing processes in order to achieve high tensile strength.

Synthetic Fibers

  • Synthetic fibers are long-chain polymers characterized by highly crystalline regions resulting mainly from secondary forces (e.g.: hydrogen bonding).

  • They have a much lower elasticity than plastics and elastomers.

  • They have high tensile strength.

Rubbers

  • Synthetic rubbers (elastomers) are high molecular weight polymers with long flexible chains and weak intermolecular forces.

  • They have low crystallinity (highly amorphous) in the unstressed state, segmental mobility, and high reversible elasticity.

  • Elastomers are usually crosslinked to impart strength.

Co-polymers

  • Polymers made with two or more monomers are copolymers. Polymers derived from a single monomer are called homo-polymers.

  • Most step growth polymers are copolymers.

  • LLDPE is actually a copolymer prepared at low temperature and low pressure from a mixture of ethylene and about 10% of a \(\ce{C4}\)–\(\ce{C8}\) olefin.

  • Many copolymers have been developed to combine the best features of each monomer. For example, poly vinyl chloride (called a homopolymer because it is made from a single monomers) is brittle. By copolymerizing vinyl chloride with vinyl acetate, a copolymer is obtained that is flexible

Stress-strain Characteristics of Polymers

The distinction between elastomers, fibers, and plastics is most easily made in terms of the characteristics of tensile stress-strain curves.


Elastomers Plastics Fibers
Young’s modulus (psi) 15–150 1,500–200,000 150,000–1,500,000
Percent Elongation 100–1000 20–100 <10
Crystallinity Low Moderate High
Polymer example Natural rubber Polyethylene Nylon

  • Some chemical species can be used both as fibers and as plastics. The fiber making process involves alignment of polymer molecules in the fiber direction. This increases the tensile strength and stiffness and reduces the elongation at break. Nylon-66 is a fiber and plastic.

  • Not all plastics can form practical fibers, however, because the intermolecular forces or crystallization tendency may be too weak to achieve useful stable fibers. Ordinary polystyrene is an example of such a plastic material, while poly amides, polyesters, and polypropylene are prime examples of polymers that can be used in both areas (fiber and plastic).