PHYSICAL TESTING OF RUBBER VULCANIZED

Physical testing of the rubber vulcanized is necessary to trace out the short falls in processing methods, to control and maintain the quality of the products and to undertake research and development work. The quality of the finished products depends not only on the quality of the starting materials but also on the correct operation of the various processing steps. Processing errors committed during the manufacture can seriously affect the properties of the final product. For example, too much milling of the rubber in the mixing mill or in the internal mixer can give a product of low strength. Errors made in weighing or the omission of any one of the ingredients during mixing can give variations in properties of the product depending on the ingredient which has been omitted. The various physical tests performed on the vulcanized can give an indication of the steps that have gone wrong during processing. To assess the quality and to maintain uniformity in quality of the products regular testing of the vulcanized is a must. Now a days quality standards for the products are framed by the consumers, manufacturer or by the Government bodies. The manufacturer has to assure himself that the product that he has made, meets the limits imposed by the specifications. Laboratory tests and actual performance tests help the manufacturer to assess and maintain the quality of the products he makes. For the research and development work, testing of the vulcanized is done to understand the behavior, nature and effect on the properties of the compounding ingredients. As a result of the evaluation of such test results, new polymers or compounding ingredients which may be less costly or having better properties may be evolved.

Even though it is true that the basic polymer properties have a g profound influence on the actual service life of a product, it also depends on processes involved in the fabrication of the product. In certain products like tyres, hoses, V. Belts etc. The design of the product also equally affects the final performance. Hence the tests that are done on the products, to evaluate their service life, should include the basic tests and accelerated performance tests. The important basic tests done on vulcanized are the Stress train tests, ageing tests, hardness tests, low temperature tests, tear tests, resilience tests, electrical tests etc. The tests which are related to the performance tests are abrasion tests, flexing tests, compression tests etc. Even though most of the accelerated performance tests are done under conditions which are almost equal to the condition in which the product is expected to be put in use, the results of the accelerated performance tests done under laboratory conditions and the performance in the actual service field the product do not correlate well. But the laboratory tests help to get a comparative performance data of different compounds and designs when they are used under identical conditions. In all the tests conduced, the procedure followed and the testing machines used should be of the some standard, if reproducible results in inter laboratory testing are to obtained.

1. Tensile Test

By the tensile testing of a rubber vulcanized three promaters it; the tensile strength, elongation at break and modulus at a particular elongation of the sample are obtained at a time. Tensile strength is defined as the force per unit area of original cross section of the sample, required to stretch the original cross section of the sample, required to stretch the rubber test piece to its breaking point. Modulus is the tensile stress 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. There are different types of machines used can be either in dumb bell shaped or ring shaped. The dumb bell shaped test pieces are commonly used. The test pieces are cut from the vulcanized test sheets with the help of a die in such a way that mill grain is along the direction of the length of test pieces. The test piece is then clamped in the testing machine and stretched under constant rate. From the stress strain graph obtained the tensile strength modulus and elongation at break can be calculated knowing the original thickness and width of the test piece. The average value of four of five test values of the same sample is taken as the actual value. The tensile test results can be used to evaluate the strength of the vulcanized and the degree of cure of the vulcanized. The tensile testing machine can also be used to find out the tension set i.e.; the extension remaining after a specimen has been stretched and allowed to retract in a specified manner, of the sample.

2. Tear Tests

Tear strength is defined as the force per unit thickness required to cause a nick out in a rubber when it is stretched, under constant rate, in a direction substantially perpendicular to the plane of the cut. The tear test can be performed using the tensile testing machine itself. There are different types of test pieces used for conducting the tear tests. Since the tear strength is susceptible of the nick cut, tests performed using the test pieces with a right angle nick gives better reproducibility of test results. Tear tests give an indication of the behavior of the vulcanized in tear initiation and tear propagation.

3. Hardness Tests

Hardness test involves the measurement of the depth of penetration of an indenter of specified dimensions under the application of a load neither by a dead weight or by a spring. The indentation hardness is a measure of the elastic modulus of the material under conditions of small strain. There are different types of instruments used for measuring the hardness. Some of the most popular ones are the shore A Durometer, the Rex Gauge, Wallancve Hardness Meter, the international Rubber Hardness Tester etc. Hardness is an important property to the compounded since its specification imposes limits upon the type and quantity of certain compounding ingredients like fillers, plasticizers etc. in a particular compound.

4. Rebound Resilience Test

In rubber, resiliency may be defined as the ration of the returned to the impressed energy i.e.., resilience is a measure of the ability of the rubber vulcaunizates to return the energy used to deform it. Various testing machines like the Dunlop Tripsometer, Yourself Oscilograph, Luke, Impact Resiliometer etc. are used for determing the rebound resilience. In the Luke resiliometer, the testing is done as follows. The test piece is placed in position in the equipment and ten conditioning impacts are given in quick succession. The initial angle of strike is set at O1 (Usually 15°) and the rebound angle O2 is noted after impact. Rebound resilience is calculated from the above data using the equation.

Rebound Resilience =(1-COSO2) X 100
________________
(1- COSO1)

5. Low Temperature Properties

The low temperature performance of the rubber vulcanized is usually estimated by determining the freeze point and brittleness temperature of the Vulcenizates. The freeze point is defined as the temperature at which the modulus is ten tines its value at 20șC. It is determined by measuring the torque required to produce an angular deflection at various low temperatures and calculating the rigidity modulus. Brittleness temperature is the temperature, estimated setistically, 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 the development of cracks. Since rubber products become hard and loose their elasticity at very low temperature of the vulcanized gives an indication of the low temperature serviceability of the products.

6. Electrical Properties

Rubber Vulcanized can be used as a good insulator in applications like wire and cables. But in certain cases like antistatic mountings it is made conductive by proper compound designs. In applications mentioned above. Properties like electrical resistively, dielectric strength, power factor, dielectric constant are very important. Dielectric strength is a measure of the ability of an 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 insulation ability to store electrical energy. It is the ration of the electrical capacity of a condenser using the elastomer under the test, 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 discharge, 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.

7. Accelerate Ageing Tests.

The natural deterioration of the Vulcanized under the action of heat, light, oxygen, ozone etc. is termed as 'ageing'. The service life of a product is too long to wait for getting information regarding the performance of the product under the influence of the above mentioned agents. It is therefore necessary to test the Product, under conditions which can produce accelerated ageing effects, to get some idea of the service life and performance of the products. Accelerated ageing tests magnify the influence of one or more of the above agents which affect the service life of the products. 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 vulcanized to that particular factor. Apparatus like hot air ovens, oxygen bomb, ozone chamber etc. are used for performing the accelerated ageing.

8. Compression Set

Compression set in rubber may be defined as the amount (percent) by which a standard test piece fails to return to its original thickness after being subjected to a standard compressive load or deflection for a specified period of time. Whether the testing is done under constant stress or strain, in involves compressing of the test specimen between two parellel plates and keeping it in that position for a specified period at a particular temperature. After the specified time, it is taken out and kept at room temperature for half an hour. The thickness of the sample is then measured and compression set calculated. For products like oil seals, gaskets, engine mounts, bridge bearings etc. the set value should be very low. Usually high loading of reinforcing fillers and under curing of the compound give high set values.

9. AbrasionTests

Abrasion resistance may be defined as the resistance of the rubber vulcanized to wearing away by rubbing or impact during service. The principle involved in the test is to rub the test sample against standard rough surface, such the sandpaper for a specified time. The loss due to this rubbing is then calculated and expressed as loss in weight or abraders available in the market, all of them have been found deficient, in one way or another, as a tool for gredicting the service life accurately. But comparative assessments of different compounds can be made with reasonable accuracy Abraders that are more popular now a days are the Du Pont Abrader, Abron Abrader, Good Year Angle Abrader, Pico Abrader etc. Abrasion test gives an indication of the resistance of the compounds to abrasive wearing and is very helpful in developing compounds for tyre treads, shoe soles and heels, conveyor belt covers, rice polishers and roller covers.

10. Flex Resistance

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 vulcanized 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 Flax Machine (used for evaluation of footwear compounds) are commonly used for evaluating the flex resistance of rubber compounds. Resistance to flex cracking is composed of two parts i.e. (1) resistance to crack initiation and (2) resistance to crack growth. In crack growth testing, a crack is initiated purposely by means of a especially shaped tool and the rate of growth of the cut is measured during flexing. In the case of flex resistance, the result is evaluated by comparing with a graded set of standards are resenting increased stages of cracking from 0(no crack) to 10 (complete Cracking).

At the Rubber Research Institute of India, the following tasting equipment are installed.

1. Wallace Rapid Plastimeter
2. Mooney Viscometer
3. Viscurometer
4. Tensile Testing Machine
5. Du Pont abrader
6. Tabular Ageing Oven
7. Ozone Chamber
8. Shor A durometer
9. Shore D Durometer
10. Compression Set Apparatus
11. Flash and Pour Point Appratus
12. Direct Reading Specific Gravity Balance
13. Smoke Point Apparatus
14. Aniline Point Apparatus
15. I.R.Spectrophotometer
16. U.V. Spectrophotometer
17. Rebound resilence tester.

•  Introduction: Tolerance stack-up, testing bias, rubber compounds, the rubber test   laboratory

•  General Test Methods: Stress-strain testing, Williams plastometer, Mooney viscometer  with viscosity, stress relaxation, and scorch test, ODR curemeter, scorch, cure  rate, state of cure, reversion and marching modulus, and Rotorless curemeter.

•  Rotorless Shear Rheometry (RPA): Rheological properties, dynamic modulus, dynamic   viscosity, correlating to polymer MW, MWD, and LCB, measuring shear thinning,  and measuring after-cure dynamic properties.

•  Physical Test Methods to Characterize Vulcanizates: D2000 tests, durometer, air   oven aging, compression set, ozone cracking, volume swell, low temperature properties,   tear resistance, dynamic fatigue, adhesion testing, staining, and Oscillograph.

•  Natural Rubber Testing: Advantages and disadvantages, uses, major producing  countries, the 15 different grades, percent dirt, Wallace rapid plastimeter, P0, PRI,  non-rubber content, moisture, test recipes, and the new rotorless shear rheometry testing   (RPA).

•  Synthetic Rubber Testing: General physical test methods, applications of Mooney   and RPA testing, general chemical methods, organic acids, soaps and total extractables,  water soluble ash, % carbon black in master batch, rubber hydrocarbon content,  % gel, swelling index and dilute solution viscosity, and metallic impurities by AAS.

•  Styrene Butadiene Rubber (SBR): Uses, advantages and disadvantages, Delta  Mooney test, test recipes, Raw Mooney viscosity targets, and % bound styrene.

•  EPDM: Uses, advantages and disadvantages, test recipes, and % ENB or DCPD in EPDM.

•  NBR and HNBR: Uses, advantages and disadvantages, and % unsaturation of HNBR   (2 methods).

•  Butyl Rubber: Uses, advantages and disadvantages, and bromine content in BIIR.

•  Polybutadiene, Polychloroprene and Polyisoprene: Uses, advantages and
   disadvantages,   and test recipes.

•  Carbon Black: Particle size, surface area, primary and secondary structures,       DBP and  CTAB absorption, iodine #, and compressed DBP.