Thursday, November 21, 2013

Characteristics of Thermoplastics

Thermoplastics (also referred to as thermo-engineering plastics) are high molecular weight polymers, which become liquid upon heating and glassy solids on cooling. Bonding between molecules of a polymer could be of varying strength, thus resulting in different type of thermoplastics (such as polythenes that have weak vander-wall bonds, or stronger dipole-dipole hydrogen bonds in nylon, or very strong stacked aromatic ring bonds in polystyrene). Thermoplastics are mostly addition polymers (meaning that they are formed by an addition reaction where smaller molecules combine together to form a larger molecule) and are therefore chemically inert, non-biodegradable and difficult to recycle.

Characteristics of Thermoplastics

Molecular Arrangement of Thermoplastics

Thermoplastics can be broadly classified as either being amorphous or semi-crystalline, based on their molecular arrangement.

Amorphous thermoplastics are characterized by their high brittleness and stiffness nature. They are also clear solids in their normal state. However, they can undergo a transformation of their molecular arrangement at high temperatures and on application of stress. Examples of amorphous thermoplastics are polyamideimide, polyethersulphone, polyetherimide, polyarylate, polysulphone, amorphous polyamide, polymethylmethacrylate, polyvinylchloride, acrylonitrile butadiene styrene and polystyrene.

Semi-crystalline solids, on the other hand, only partially exhibit such characterstics, given their partially amorphous state. Examples of semi-crystalline are polyetheretherketone, polytetrafluoroethylene, polyamide 6.6, polyamide 11, polyphenylene sulphide, polyethylene terephthalate, high and low density polyethylene, polyoxymethylene and polypropylene.

Physical properties of Thermoplastics

Specific gravity of various thermoplastics typically varies from about 0.92 for polyybutylene to 1.72 for poly-vinylidene fluoride.

The tensile yield strength (defined as the engineering stress that causes irreversible deformation) of thermoplastics however has a wide variance, with as low as 2.8 (*10^3 psi) for cross-linked polythene, to as high as 8.0 (*10^3 psi) for polyvinyl chlorides.

Thermal conductivity of these polymers can vary from 1.0 to 3.2 (Btu/ (h ft^2 C)).

Specific heat is in the range of 0.22 to 0.55 (kcal/kg C).

Characteristics of Specific Thermoplastics

Acrylonitrile butadiene styrene is a strong thermoplastic that is also resistive to most bases and acids. However, chlorinated hydrocarbons can still corrode or damage this polymer. It is usable only about 71C temperature, and is used in drainage and vent pipes.

Polybutylene and polyethylene (normal as well as cross-linked varieties) are flexible polymers that find their most common application in pressurized water systems. However, while polybutylene doesn’t get affected by extreme water temperatures, polyethylene cannot be used for hot water.

Polypropylene is a light-weight polymer, and can be used till about 82C temperature, while also being highly resistive to most acids, bases and solvents. It’s primary use in laboratory plumbing.

Polyvinyl chloride is also a strong and highly resistive plastic, with the maximum usable temperature being 60C. This finds applications in piping but should be avoided in hot water applications. A variant polymer, chlorinated polyvinyl chloride, can however be used for higher water temperatures (upto 82C).

Polyvinylidene fluoride is an extremely strong and tough thermoplastic, which is also resistant and non-abrasive. It can be used up to temperatures 138C, and its most common application is in laboratory plumbing.


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