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, P
0,
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.
Contact Us : www.indiarubberdirectory.com C/o Team Web Power, No.52, First Floor, Anna Nagar Plaza, C-47, IInd Avenue, Anna
Nagar, Chennai 600040. India.
Phone:- +91-44-42170137 Mobile:- +91-9444001705