PHYSICAL TESTING OF RUBBER VULCANIZATES

The tests can be classified into mainly three broad categories such as

1) Quality control tests 2) As a performance requirement specification test

3) Research and Development test.

Organisations producing standards

-International organization 1972 -National Organization -Individual Organization

British-BS 1901 America-ASTM French -NIFT German-DIN 1971 Indian -IS ISO -1972

Quality Control Quality control is mainly concerned with maintaining the quality

of products to set standards. Quality control means control of incoming materials, control of

compounds produced, and the control of manufacturing processes, and guaranteeing the maximum possibility of the final product.

Quality control or assurance schemes utilize physical tests as a most important part of their system.

Standardised test methods are intended for quality control use and majority of testing carried out is for quality assurance purposes.

Quality assurance in wide sense means to consider specifications, the relevance of test methods and the accuracy of test methods.

Testing on Testing Equipment Before actual testing 1) Calibrate the testing equipment using the

prescribed calibration procedure. 2) Various test conditions should be noted

e.g.: temperature, atmospheric pressure, size and type of the test specimen, type of test equipment etc.

3) Testing equipment should be in perfect order and can record or display the test reading accurately.

Procedure for test sample preparation

1. Standard procedure for mixing of the compound. 2. Standard procedure for moulding of the test

sheet/sample. 3. Procedure for cutting of the test sample from the

test sheet. 4. Procedure for test piece preparation from the

finished product. 6. Conditioning of the test piece before actual testing

is carried out. 7. Testing atmosphere and temperature.

1. Hardness Test

Hardness represents the elastic modulus of material under conditions of small strain. Hardness test involve the measurement of the depth of penetration of an indentor of specified dimensions under the application of a load either by a dead weight or by a spring.

Shore 0=for sponge rubber in degree. Shore A=for soft rubber in degree Shore D=for hard rubber in degree Shore A correspond to IRHD but lower

below 300

Universal Testing Machine

2. Tensile stress/strain Tensile strength is defined as the force per unit area

of original cross section of the sample, required to stretch a rubber test piece to its breaking point.

Modulus is the tensile stress strain required to stretch a rubber test piece to a predetermined elongation.

Elongation at break is the maximum elongation, expressed as the percentage of the Original length, prior to the rupture of the sample.

3. Tear Tests

Tear Strength is defined as the force per unit thickness required to cause a nick out in a rubber test piece to extend by the tearing of the rubber when it is stretched, under constant rate, in a direction perpendicular to the plane of the cut.

Universal Testing Machines

4. Set, Creep and Stress Relaxation The deformation that remains in an elastomer after

the removal of an applied stress or strain is “Permanent Set”.

When a constant load is applied to an elastomer the deformation is not constant but increases gradually with time, this behaviour is known as “strain relaxation” or “Creep”.

When an elastomer is subjected to a constant strain, a decrease in stress takes place with time, this behaviour is called “stress relaxation”

The phenomena of permanent set, creep and stress relaxation are the result of physical and chemical changes in elastomer, both of which occur simultaneously.

5. Set Properties

Recovery after removal of an applied stress or strain is the set. It depends on time for which test is conducted and time allowed for recovery.

Set=To-Tr -------X100

To-Ts

To-Original thickness Ts-Compressed thickness Tr-Recovered thickness

6. Accelerated ageing tests The testing is done usually, by keeping the test

samples under the influence of temperature, oxygen or ozone for a specified period and then determining the physical properties like tensile strength or noting the visual appeal.

The fall in properties from the initial value or the change in appearance gives an indication of the resistance of the rubber vulcanizates to that particular factor.

Apparatus like hot air ovens, oxygen bomb, ozone chamber etc. are used for performing the accelerated ageing.

7. Abrasion Tests Abrasion resistance may be defined as the

resistance of the rubber vulcanizate to wearing away by rubbing or impact during service. The principle involved in the test is to rub the test sample against a standard rough surface for a specified time. The loss is expressed in weight or volume per hour.

DIN ABRADER

DIN ABRADER

8. Flex-Cracking and Cut -Growth Tests

Products like tyres, conveyor belts; shoe soles etc. are subjected to repeated flexing during service. This repeated flexing may gradually lead to failure of the product. This is because repeated flexing of a rubber vulcanizate causes cracks to develop in that part of the surface where tension stress is set up during flexing or if that part contains a crack or cut, causes the crack to extend in the direction perpendicular to the stress. Various machines like De mattia Flexing machine (used for evaluation of tyre tread and side wall compounds) Du Pont Flex machine (used for evaluation of conveyor and transmission belt compounds) and Ross Flex machine (used for evaluation of footwear compounds) are commonly used for evaluating the flex resistance of rubber compounds.

Resistance to flex cracking consists of two parts

ie 1)resistance to crack initiation and 2)resistance to crack growth.

In Crack growth testing, a crack is initiated purposely by means of a specially shaped tool and the rate of growth of the cut is measured during flexing.

9. Heat Build Up This is an important property for tyre tread where fatigue is

caused by repeated cyclic deformation. Subjecting a rubber to repeated deformation cycles results in a change in stiffness and a loss of mechanical strength.

‘Heat build -up ‘type of fatigue test is carried out on an apparatus generally called a Flexometer which operate in compression, shear or a combination of the two.

In Goodrich Flexometer a cylindrical test piece is cycled at 1800 cycles/min with a stroke of 4.45mm for 25 min and the temperature rise is recorded.

Apart from measuring temperature rise ,the static deflection of the test piece ,its dynamic deflection, compression set, and indentation hardness are recorded.

10. Rebound resilience test

In rubber, resiliency may be defined as the ratio of the energy returned to the impressed energy i.e. resilience is a measure of the ability of the rubber vulcanizate to return the energy used to deform it

Rebound Resilience= (1-cosØ) x100 ------------------ (1-cos 45)

11. Low Temperature Properties The low temperature performance of the rubber vulcanizates is

usually estimated by determining the freeze point and brittleness temperature of the vulcanizates.

The freeze point is defined as the temperature at which the modulus is ten times its value at 200c. It is determined by measuring the torque required to produce an angular deflection at various low temperature and calculating the rigidity modulus.

Brittleness temperature is the temperature, estimated statistically at which 50% of the specimens would fail in the specified test. This is measured by giving impact blows to the sample at various low temperatures and observing the samples for development of cracks.

Since rubber products become hard and loose their elasticity at very low temperatures the evaluation of the freeze and brittleness temperature of the vulcanizates gives an indication of the low temperature serviceability of the products.

12. Electrical Properties Rubber vulcanizates can be used as a good insulator in applications like wires and cables. But in certain cases like antistatic mountings it is made conductive by proper compound

designs. In applications mentioned above, properties like electrical resistivity, dielectric strength, power factor, dielectric constant etc. are very important.

Dielectric strength is a measure of the ability of insulation to withstand Voltage. It is the voltage per unit thickness at which electrical breakdown occurs when a potential difference is applied under specific conditions.

The dielectric constant or specific inductive capacity is a measure of the insulations ability to store electrical energy. It is the ratio of the electrical capacity of a condenser using the elastomer under the test, as the dielectric, to the capacity of a similar condenser using air as the dielectric.

The power factor of an insulating material indicates its tendency to generate heat in service. If a capacitor using an elastomer as the dielectric is charge and then immediately discharges, there is an energy loss in the form of heat. If the frequency of charging and discharging is high, the heat generated will be very high. The ratio of this loss, to the energy required to charge the capacitor is known as the power factor.

The surface resistivity of a test piece is determined by measuring the current passing under an applied D.C.potential between two electrodes in intimate contact with the surface under test and separated from one another by a standard distance.