PROPERTIES OF PLASTICS

The plastics exhibit different characteristics than metal.

• LOWER DENSITIES. All plastics have low densities generally in the range of 0.85 to 2.5 g / sq.cm. These figures can be
• extended upwards ( up to 0.01 g/sq.cm) by using foaming additives or
• downwards by using filled polymers.( up to 3.5 g / sq.cm.).

In comparison, density of aluminium is 2.7 g/sq.cm. and density of stainless steel is 7.9 g/sq.cm.

• TOUGHNESS : Some polymers are extreamly tough and virtually indestructible by mechanical treatment. Others are less tough and others are fragile.

 
• RESILIENCE : Plastics show some of the behaviour associated with rubbers in accomodating large strains without fracture and in recovering their original shape and dimensions when the stress is removed.

 
• VIBRATION DAMPING : The quietness of operation of plastics gear trains depends on inherently high degradation of mechanical energy to heat. Metal wire milk bottle crates rattle during transportation and handling whereas plastics crate does not produce irritating noise.

 
• RESISTANCE TO FATIGUE : Some plastics apper to perform remarkably satisfactorily in situations involving dynamic stresses or strains.

 
• LOW COEFFICIENT OF FRICTION : Plastics / plastics and plastics / metal combinations have low coefficient of friction and can often perform unlubricated without fear of seizing.

 
• CORROSION RESISTANCE : Plastics materials are chemical resistant. The degree of resistance is given in the table. Strong acids may cause some attack leading to discolouration and possible embrittlement. Some organic solvents, to which metals are generally inert, may cause swelling, deterioration of properties and eventually dissolution. The degreee of attack is dependent on nature of plastics, and of environment, temperature.

 
• Some polymer absorb moisture and expand or lose moisture and shrink depending on relative humidity of atmosphere.

 
• MIGRATION OF COLOUR pigment takes place in plastics.

 
• INTEGRATED DESIGN : The easy flow characteristics and properties offered allow the design and manufacture of polyfunctional complicated shapes with out the need for assembly.

 
• STRENGTH AND SURFACE HARDNESS : The general level of conventional tensile strength is not high, by metal standards, being in the range of 5000 - 10000 lb / sq.inch. Most thermoplastics can be scratched by pencils of 9H to HB, and hardest plastics - ACRYLIC - is comparable in hardness with aluminium. Abrasion resistance depends on exact condition of use, and ranges from excellant to poor. Nylon gears are known to outwear meshing metal gears.

 
• STRESS-STRAIN characteristic of metal is consistent and is not much influenced by temperature variations (except at very high temperature). Hence, modules of elasticity for metal is constant. It is also not influenced by rate of loading. Whereas, stress-strain curve for polymers (plastics) vary substantially with rate of loading and temperature. Hence, the modules of elasticity is not constant in plastics.

 
• MODULES OF ELASTICITY --KG/CM}

• GLASS	680,000 (varies)
  WOOD	100,000 "
  CONCRETE	200,000.
  ALUMINIUM	680,000.
  COPPER	1100,000.
  STEEL	2000,000.
  POLYETHYLENE	2500
  POLYESTERS	31,000
  ACRYLIC	27,500
  TEFLON	117
  NYLONE	17,200
  POLYCARBONATE	20,000
  POLYPROPYLENE	14,000
  POLYSTYRENE	35,000
  RPVC	27,500
  ABS	30,000

  Modules of elasticity improves with the reinforcement in the polymer.

• The STRENGTH in plastics is more in the direction of orientation. Oriented filament (HDPE, PP, NYLON ) is stronger in tension than a bar or sheet of the same polymer. The blown film is stronger in longitudinal direction than transverse direction.

 
• THERMAL INSULATION : Plastics are good insulators, their conductivity much lower than that of metals.

 
• THERMAL EXPANSION : The co-efficient of thermal expansion in plastic is higher (3 to 10 times) than that in metal. It influences converting operation in plastics and also in service condition. Due allowance should be made at the design stage.

 
• THERMAL CONDUCTIVITY: Plastics are poor conductor of heat. Plastics are good insulators, their conductivity much lower than that of metals.

 
• TEMPERATURE RESISTANCE : Mechanical properties of many thermoplastics markedly affected by temperature. Continous use at elevated temperatures may also cause deterioration of plastics material with consequent loss of properties. However some polymers are servicable even at 150 deg. C.

 
• GLASS TRANSITION TEMPERATURE :

Specific volume v/s temperature - characteristics of polymer melt provides

• Tm= melting temperature and
• Tg= glass transition temperature.

While cooling the melt, the specific volume of the melt sharply drops at a temperature which is termed as Tm.

While cooling non-crystalline polymer melt there is no sharp drop in specific volume and the melt becomes highly viscous and it appears like solid. Since the glass behaves in this manner the temperature at which the specific volume curve changes its slop is called Tg- glass transition temperature.

Polymer becomes :

• hard, stiff and brittle below Tg
• highly viscous but solid at Tg
• rubbery, flexible and softer above Tg


• HEAT DEFLECTION TEMPERATURE : The heat deflection temperature of a plastic is useful for assessing load bearing capacity at an elevated temperature. The sample is mounted on supports 4" apart and loaded as a beam. Abending stress of either 66psi or 264 psi is appliedat the center of the span. The test is conducted in a bath of oil, with temperature increased at a constant rate of 2 ⁰ C per minute, The heat deflection temperature is the temperature at which the sample attains a deflection of 0.010in.

 
• UV RESISTANCE AND OUTDOOR WEATHERING : This can be improved markedly by special additives by suitable formulation.

 
• FLAMABILITY: Most plastics materials burn to a greater or lesser extent, although some are self -extinguishing and many can be formulated to be more flame resistant

 
• CREEP is a slow and continuous increase in deformation under a static load and is a permanent deformation. Creep failure in plastic occurs at stress well below the failure stress given by tensile test. Hence, the allowable stress in plastic should be lower than the creep stress. Environment temperature should also be considered while determining the allowable stress.

  With the knowledge of characteristics and strength of material the plastic components are successfully designed, manufactured and used in automobile, textile, clock, electrical switch gears, instruments, computers, telecommunication equipments, home appliances, medical instruments and equipment's etc.

• Stress oriented area of moulded part may relieve stress and WARP. This can be controlled by part design and processing conditions.

 
• ELECTRICAL PROPERTIES : All plastics are electrical insulators, some are outstanding.

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Click here to see Mechanical Properties of Plastics

Application of Plastics

Things to know - Polymers

Injection Moulding Process

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