Properties of Common Plastics

Physical Properties of Plastics

Specific Gravity (Density)

The specific gravity of a plastic is the ratio of the weight of a specimen to the weight of an equal volume of water at a specified temperature, expressed in g/cm³. It is commonly determined using the liquid buoyancy method.

Water Absorption  

Water absorption of plastics refers to the amount of water absorbed by a specimen of specified dimensions after immersion in distilled water at a temperature of (25±2)°C for 24 hours. This absorption affects the specimen's dimensions and shape. When expressed by weight, the water absorption rate is typically indicated as a percentage.  

Air Permeability

Air permeability refers to the volume (cm³) of gas permeating one square meter of plastic film of a given thickness under one atmosphere of pressure within 24 hours. However, the permeation rate is influenced by factors such as film thickness, area, time, temperature, and pressure differential.

Moisture Permeability

Moisture permeability indicates the passage of water vapor through plastic film. Its fundamental principle and definition are identical to those of air permeability.

Transparency

Transmittance is the ratio of light flux transmitted through an object to the light flux incident upon it. The ratio of scattered light in the direction of incident light to total transmitted light is termed haze or opacity. Haze typically occurs in translucent materials exhibiting diffuse transmission of incident light.

Tensile Strength

Tensile strength refers to the maximum load applied along the longitudinal axis of a specimen at specified test temperature, humidity, and tensile speed, measured until specimen failure.

Compressive Strength

Compressive strength is the load applied to a specimen until fracture (for brittle materials) or yield (for non-brittle materials).

Flexural Strength

Flexural strength is the load applied to a specimen at two support points, causing deformation or fracture.

Impact Strength

Impact strength refers to the joules per unit area absorbed when a specimen fractures under impact. For plastics with high impact strength, a notch of specified dimensions is often cut into the specimen center to reduce the joules required for fracture. Different test methods may be used for different specimens: drop-weight impact test, high-speed tensile impact test.

Coefficient of Friction

The coefficient of friction is the ratio of frictional force to normal force. It is determined by applying a normal force to a specimen and measuring the ratio of kinetic to static friction during rigid motion.

Abrasion

Abrasion refers to the mechanical degradation process where particles continuously separate from the friction surface during friction, causing gradual dimensional changes in friction components. It is also termed wear or erosion.

Hardness

Plastic hardness denotes a material's resistance to indentation by a hard object. Common scales include Rockwell and Shore hardness. Shore hardness measures the depth of indentation by a probe under specified pressure and time. Shore indenters are categorized into two types: A and D. The applied load weights are 1.0 kg and 5.0 kg, with a pressing time of 15 seconds. Type A is suitable for soft plastics, while Type D is suitable for semi-rigid plastics. When using Type A, if the measured value exceeds 95% of the scale range, switch to Type D. If Type D measures over 95% of the scale range, switch to Rockwell indentation.

Fatigue Strength

Fatigue strength refers to the strength at which plastic fails under a static destructive force with minor alternating cycles. Fatigue loads originate from tension, compression, bending, torsion, impact, etc.

Creep

Creep refers to the characteristic deformation exhibited by plastics over time under sustained external forces at specific temperature and humidity conditions. This deformation increases with load application and decreases with load removal, gradually recovering thereafter. Sources of creep include tensile creep, compressive creep, and flexural creep.

Plastics Concepts Explained! A Primer for Newcomers to the Plastics Industry