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Rubber Engineering |
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
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(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 20ēC. 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.
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Introduction:
Tolerance stack-up, testing bias, rubber compounds, the rubber test
laboratory
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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.
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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.
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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.
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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).
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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.
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Styrene Butadiene
Rubber (SBR): Uses, advantages and disadvantages, Delta Mooney test,
test recipes, Raw Mooney viscosity targets, and % bound styrene.
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EPDM: Uses,
advantages and disadvantages, test recipes, and % ENB or DCPD in EPDM.
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NBR and HNBR: Uses,
advantages and disadvantages, and % unsaturation of HNBR (2 methods).
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Butyl Rubber: Uses,
advantages and disadvantages, and bromine content in BIIR.
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Polybutadiene,
Polychloroprene and Polyisoprene: Uses, advantages and
disadvantages, and test recipes.
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Carbon Black:
Particle size, surface area, primary and secondary structures, DBP
and CTAB absorption, iodine #, and compressed DBP.
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Rubber
Engineering
