Method of evaluating the properties of plastic

5-1.Physical properties

Specific gravity

Specific gravity is the ratio of the mass of the material and the mass of water of equivalent volume at equivalent temperature. The range of specific gravity for various plastics is 0.83-2.1. The lightest one is polymethyl pentane with specific gravity of 0.83, and the heaviest one is PTFE (tetraflouride ethylene) having specific gravity of 2.1. Its key feature is that with average specific gravity 1.1, it is almost 1/7 times lighter than steel.

Hygroscopic properties

Keeping a plastic material immersed in water for long time, or keeping it in highly humid environment, result in increase in weight due to water (moisture) absorption. Although this differs some what depending on the material also, usually, higher the temperature, higher is this tendency. Such property is called water absorbability or hygroscopicity, and absorption rate is defined as the gain in weight per unit of surface area or unit weight of sample after a certain period of time, and it is expressed in percentage (%).

5-2.Optical properties

Transparency

One part of the light incident on the object is reflected on the surface of the object, one part is absorbed in the body, and the rest is transmitted light. Transparency represents the degree of the size of transmitted light, and it comprises of parallel light transmittance and total light transmittance. In ASTM D-1003 and D1746-1970, for the transparency of plastic, transmission rate and scattering rate corresponding to the total light transmittance is used. Plastic with high transmission rate includes acrylic resin (93%) and polycarbonate (89%), and their molds are extremely beautiful and they are used in optical components such as lenses and disks.

Refractive index

Measurement of refractive index is described in ASTM D542-1950 using Abbe's refractometer method. Refractive index of plastics is usually seen in the range of 1.338 (fluorocarbon resin) - 1.586 (polycarbonate). For crystals it is 1.55, for corundum it is 1.768, and for diamond it is 2.417.

5-3.Thermal properties

Thermal conductivity

Thermal conductivity is the ratio between heat flowing in perpendicular direction per unit of time through unit area of isothermal surface inside the object and temperature gradient in that direction. Thermal conductivity of plastics is generally low compared to metals, and it ranges from polypropylene, which has the lowest thermal conductivity of 1.3 × 10-4cal/s/cm2 / (℃ / cm) to high-density polyethylene, which has the highest thermal conductivity of 12.4 × 10-4cal / s / cm2 / (℃ / cm ). This is considerably small compared to quartz having thermal conductivity of 33.1 × 10-4cal/s/cm2 / (℃ / cm), common porcelain having thermal conductivity of 30-41 × 10-4, and copper with thermal conductivity of 0.941cal/s/cm2 / (℃ / cm ).

Specific heat

Specific heat, specific heat capacity to be more precise, is a measure of the heat capacity of a substance, is expressed in calories required to raise temperature of unit mass of material by one unit, and (cal / ℃-g) is used in actual practice. For plastics, it is within narrow range of the minimum of 0.22 (cal / ℃-g) for trifluoride resins to the maximum of 0.55 (cal / ℃-g) for ionomer resins, which is higher than 0.185 (cal / ℃-g) of glass and 0.17 (cal / ℃-g) of ferrite.

Coefficient of thermal expansion

Although the coefficient of thermal expansion shows the extent to which the volume of material increases due to increase in temperature, there is β that shows the coefficient of volume expansion and α that shows the coefficient of linear expansion. For a perfect isotropic solid, β = 3α. Coefficient of linear expansion in plastics shows the percentage change in length for 1℃ change in temperature, and it ranges from phenolic resin having the lowest coefficient of linear expansion of (2.5-6.0 × 10-5 ℃ -1), PPO (5.5 × 10-5 ℃ -1) to low-density polyethylene having the highest value of (10.0 ~ 20.0 × 10-5 ℃ -1).

Heat transfer point

At two places, phase of plastic changes due to change in temperature. In other words, they are glass transition point and melting point. The former is prominently seen in non-crystalline plastic. At temperatures above glass transition point, segmental movement inside the molecular chain becomes possible, transformation of molecules become remarkably free, molecules transition from glass state to rubber state, molecules extend significantly, and resistance to transformation declines significantly. The later is prominently seen in crystalline plastic. At temperatures above melting point, crystals disappear. Once the temperature falls after melt is formed, volume reduces drastically due to formation of crystals and solid, and it hardens overall.

Heat deflection temperature

Heat deflection temperature refers to temperature when the tensile deformation of the center of specimen reaches 0.2mm, when a constant bending load (0.45MPa or 1.82MPa) is applied to the center of specimen and temperature is increased at constant speed. It is one of the ballpark figures for short term heat resistance and it is also called heat deformation temperature.

5-4.Electrical properties

Electrical resistance

When the current flows at potential difference (V) through an object, R (= V / I) is called electrical resistance or simply resistance. In general, plastic is an insulating body with the volume resistivity of 108Ω-cm and above. However, this value is not constant as it depends on the configuration of object, method of applying the voltage and environment. Therefore, we use three standard types of resistance, viz. insulation resistance, volume resistance and surface resistance for practical purposes. Insulation resistance is the value obtained by dividing DC voltage applied between two electrodes with total current flowing between two electrodes, and it includes both the volume resistance and surface resistance of the specimen. To measure this value, using a 500V DC voltage and constant specimen, measurement is conducted at 20 ℃ and 65% RH. Volume resistance is the value obtained by dividing DC voltage applied between two electrodes with the current passing through a unit volume of the specimen sandwiched between the electrodes. Also, for practical purpose, volume resistance (Ω- cm) is used. In plastics, this ranges from the minimum of 1011Ω-cm for phenolic resins to the maximum of 1018Ω-cm for tetrafluoride. Surface resistance is the value obtained by dividing DC voltage applied between two electrodes the current flowing through the surface layer.

Dielectric breakdown strength

In plastic, which is also an insulator, very high potential difference causes large currents, resulting in the fracture of material. Dielectric breakdown strength is the value obtained by dividing minimum effective voltage (breakdown voltage) at which the specimen breaks under stipulated test conditions with distance between two electrodes ( thickness of the specimen), and generally it is expressed in the units of MV/mm. For plastics, it ranges from the minimum of 11.8 MV/mm for phenolic resin to the maximum of 30 MV/mm for polypropylene.

Permittivity

Permittivity in units of electric field shows the magnitude of electrostatic energy accumulated in unit volume. This value is affected by frequency, moisture, and environment, and in plastics it ranges from the minimum value of 2.1 for tetrafluoride resin to the maximum value of 4.7 for polyamide at 106Hz.

Dielectric tangent

Although adding sinusoidal voltage (E) to a dielectric circuit results in phase difference of 90 ° between E and current (I) in case of an ideal condenser; however, current of tangent phase flows though the actual dielectric body. This tangent of the complement of angle of phase difference is called dielectric tangent. Although this value is affected by applied pressure frequency, environment etc. depending on the material, in case of plastics, against 106Hz, usually the range if from minimum of 0.0001 for polystyrene to maximum of around 0.03 for polyamide 6 and epoxy resin. This value is the reason for dielectric loss in high-frequency circuit, it is a valid property value in case of high-frequency heating of materials.

Arc resistance

Even in plastics with high insulation strength, prolonged use at high voltages can cause deterioration by partial discharge. Resistance to this discharge deterioration is called arc resistance.

5-5.Mechanical properties

Tensile characteristics

Cases of only tensile external force getting applied on material are very few; however, tensile external force create uniform tensile stress on all the parts resulting in simple stress distribution. Moreover, since in many cases material breaks when the tensile stress exceeds a certain limit, tensile characteristics are treated as typical mechanical properties. Tensile strength is a value obtained by dividing maximum tensile load with original minimum cross-sectional area of test specimen, and it is shown in Mpa. Tensile elongation is the distance obtained by subtracting the distance between original reference points from the distance between parallel reference points at the time of fracture of test specimen.

Bending properties

Behavior of material when bending external force is applied on the object is called bending characteristics.

Hardness

Although there is no hard and fas physical definition of hardness, but because it can be easily measured in non-destructive testing, hardness is often used in practice, and it has become a useful measure. Rockwell hardness is a standard for measuring the hardness of plastics. Rockwell hardness HR is used in plastics, steel ball indenter in the indentation load, plus standard first Po, when the relative depth of the recess, further test load P for a certain time (ASTM in the 15s) then put the depth of indentation load back to standard when the plastic h (mm) calculated using the following measures. HR = 130-500h. However, each scale has a Rockwell hardness depending on the size and weight of steel balls, JIS has R scale and M scale, while ASTM gas 5 scales, viz. K / E / L / M / R.

Impact characteristics

Mechanical strength of the material, i.e. static strength that is measured by slowly increasing the load, and strength in case of an impact, in other words toughness and impact strength do not always show the same tendency. Principle to measure the toughness of such material is to measure the energy required to break them by adding impacting weight to the material. Izod impact strength test is the typical test method, where pendulum shape Izod impact testing machine is used and impact bending stroke is blown on a test specimen hanging on cantilever. Izod impact value is obtained by dividing the energy required to break the test specimen with the width of test specimen.

Creep characteristics

Behavior of material when a constant stress is continuously applied in a constant state is called creep. However, it shows the following two characteristics in this case. 1. Deformation increases with age. 2. The failure stress decreases with age. That is, one. even though 1. does not pose a problem because of less deformation when load time is short, if the load is applied for long time, deformation becomes considerably large, and it can not withstand the use especially due to plastic deformation, and 2. refers to material fracture when time of load becomes large even though the stress is below the static strength.

Fatigue characteristics

In the event of repeated stress or strain on an object, behavior of weakening of the material accelerating the fracture is referred to as material fatigue. In other words, when the stress is applied only once, fracture does not occur if this stress is less than static strength. However, even if the stress is considerably lower than static strength, cases of repeated application of stress leading to fracture increases. When using a plastic material under such conditions, unless the stress behavior is clearly known, safety against fracture can not be ensured.

Friction and abrasion characteristics

When two plastic surfaces are in contact with each other, and if there is relative movement between them, friction interferes in such movement, while abrasion refers to detrition/ wearing of material from surface.