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W.J. van Ooij501E ERC

Phone 556-3194Fax 556-3773

Email: [email protected] , [email protected]: http://www.eng.uc.edu/~wvanooij

20-MTSC-642-001

Spring 2002MW 2:00-3:15

422D RIEVESCHL

RUBBER SCIENCE AND ENGINEERING

Prerequisites :

Basic knowledge of organic chemistryKnowledge of mechanical properties of materials (e.g., NAPOM)

CHAPTER 6PROPERTIES OF VULCANIZATES


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PROPIEDADES DE LOS VULCANIZADOS [1] INTRODUCTION [1]

What are the properties of the vulcanizates and how arethey tested?

VulcanizateProperties

The major application is automobile tires; each application

has its own set of additional, specific test methods

Applications of theVulcanizates

How do we form and cure the mixtures into strong elasticmaterials?

Vulcanization

How do we mix these ingredients into the elastomers?Processing

The most important additive to the elastomer and the leastunderstood

Fillers

What do we mix into the elastomers and why?Compounding

Which ones exist and what are their basic properties?Elastomers

So far we have discussed:


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PROPIEDADES DE LOS VULCANIZADOS [2] TEST METHODS

Some important tests for rubber properties done in industry:

1. Tests of Raw Materials: the material is mixed with other compounding ingredientsusing a standard formulation and standard mixing procedures; standard sheets arethen vulcanized from the standard compound; its properties are then tested; specs

are given in various ASTM methods (see Long, page 174); we will not discuss themhere (note: before a supplier is approved, the rubber company has to do all this work)

2. Tests for Processibility: ASTM D1646 and D2084 are used; see VULCANIZATION [5]and [6] for the latter (rheometer); D1646 is shown on the next page

3. Tests of Rubber Vulcanizates: comprise >70% of all ASTM rubber test methods;the methods are summarized in ASTM D297; there are more than 200 test methods

Tensile tests stress-strain curves

Stiffness including hardness, indentationElastic recovery compression set, tension setDurability aging, liquid, ozone, lightFatigue test dynamic flexing (cracking)Abrasion tests tread wearTemperature effects high and low T, with and without oxygen

Tear resistance with or without nick or cutElectrical tests electrical conductivity, static charging


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PROPIEDADES DE LOS VULCANIZADOS [3]

Mooney viscosimeter; ASTM D1646

The Mooney viscosity scale is arbitrary and basedon one rate of shear

Processing is more difficult and requires moreenergy if the Mooney viscosity increases

Sensitive to the condition of the apparatus,temperature of the rubber, and slippage

This test does not provide the modulus or the timerequired for vulcanization (t 90); it only measures thefirst part of the rheometer curve

TEST FOR PROCESSIBILITY [1]

Processibility equipment can also measure die swell= % increase in diameter or area after extrusion

Is caused by relaxation after elongation


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PROPIEDADES DE LOS VULCANIZADOS [4] TEST FOR PROCESSIBILITY [2]

Mooney scorch curve

Notice the meaning of T5 and T 35


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PROPIEDADES DE LOS VULCANIZADOS [6] TEST OF VULCANIZATES [2]

Hardness

Resistance to indentation

Important property

Expressed as number depending oninstrument used

Shore durometers are most widely used Their scale is from 0 to 100

Shore A is for soft rubber

Shore D for harder products

Hardness testers


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PROPERTIES OF VULCANIZATES [7] TEST OF VULCANIZATES [3]

Tear ResistanceDie C tear sample

Other tear samples

Ease of tearing of rubber when nicked or cutvaries considerably

Depends on: rate of tearing, crystallization ofrubber (NR), amount and type of filler, shapeof sample and others

Results between tests do not agree

No tests correlates well with field service

Resistance is expressed as N/m (lbs/in)

Sensitive to state of cure and often shows a

sharp maximum

Undercure give smooth curves, tighter curegives knotty tear especially with black/NR

cut


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NBS Abrader

Gives relative abrasion resistance comparedwith a standard compound

Widely use for shoe soles

Drum rotates at 45 rpm

No. of revs to wear compound 2.5 mmis measured (ASTM D1630)

The pico abrader is used for tread wear of tires(ASTM D2228)

Two tungsten carbide knives are rubbed overthe surface and the weight loss is measured

sample

abrasive paper on rubber-covered drum

weights

In the tire industry actual road tests need to beperformed on a test track

Abrasion Resistance


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Elastic Recovery

Equipment for compression set measurement

using method B

Compression set is the amount (in %) by whicha standard test piece fails to return to itsoriginal thickness after having been subjected

to a standard compressive load or deflectionfor a fixed period of time

ASTM D395 method A = constant load ASTM D395 method B = constant deflection

Aging is included in the test, typically 22 hrsat 70C or 70 hrs at 100C, depending on thetype of rubber

There are many other tests for creep, tension

set and stress relaxation


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Durability [1]

Equipment for heat aging ASTM D865

Dumbbell samples are heated in air in testtubes

At 70C or higher (up to 120C for unsaturated

rubbers)

The difference in ultimate elongation beforeand after heating is the criterion for durability

All rubbers deteriorate but rate depends ontime, temperature and composition

There are many other accelerated heat agingtests

One variation is ASTM D454, aging at 127C at550 kPa pressure (air bomb); very drastic;rubber deteriorates in hours

Heat aging [1]


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Durability [2]

Heat aging [2]

Rubber samples are heated at 70C in 2 MPaoxygen (some rubbers at 80C)

Samples are heated in individual vessels to

avoid cross contamination

Heating time is 1-5 days

Equipment for oxygen bomb agingASTM D572

Several ASTM tests exist for testing resistance toozone; ASTM D149 is widely used

Severe surface cracking can occur at 0.00001%O3 in the air; antiozonants can prevent it

Ozone attack depends on rubber strain, so tape-

red or bent loop samples are used


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Durability [3]

Comparison between natural aging andaccelerated aging for an NR tread compoundis shown here

Note the accelerating factor on the horizontal axis

Conclusion: the ASTM D865 test (page 10) givesthe best correlation and can be used to predictservice life


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Resilience, Heat Buildup and Temperature Effects

Goodyear and Lpke resiliometers

Resilience is ratio of returned to impressed energy

Heat buildup is the amount of impressed energy that isabsorbed

A pendulum is released from a catch at a point marked

100 and hits a rubber specimen held on an anvil

The pendulum has a standard weight and dimensions

The scale at the point of rebound is less than 100; thisis the energy recovered; the difference is the percentof energy loss

Tg

Rebound vs. T


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Equipment for heat buildup and low T

The Goodrich flexometer for heat buildupis ASTM D623

The sample is positioned between heatedanvils

The top one is connected to an adjustableeccentric driven at 1800 rpm

Samples are loaded between 0.7 and 2.1 MPa The stroke is between 4.4 and 9.4 mm

Under the lower load, high quality elastomerswill reach an equilibrium T after 25 min

Under the higher load and stroke, all but the

highest quality rubber will blow out

The blow-out time is a measure of theresistance to failure

The brittle point (ASTM D746) is that tempera-

ture at which a specimen becomes brittle andwill shatter upon bending or impact

Elastomers are far superior to plastics in low-temperature performance

They can be compounded for low-T perfor-

mance

The brittle point can be as low as 100C(silicones)


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Flex Resistance or Fatigue Testing [2]

Samples are 25 x 6.5 x 150 mm with a trans-verse groove

The grips approach each other from 19 to 76mm; cracking occurs in the groove which

simulates a groove in a tire tread

Flexing speed is 300 cycles per min.

After a number of cycles, the specimens arerated by comparison with a set of standards(see insert) grade 0 to 10

Test can be continued until all samples becomecracked; number of flexures is then recorded

Test can be done at non-ambient temperatureand in gases other than air

DeMattia flex machine


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Ross flex machine

Used mainly for soling materials

Test pieces are 25 x 150 mm

They are flexed to an angle of 90at 100 flexings/min.

One end is clamped, the other canslide between rollers during flexing

Before the test a 2.5 mm cut is madewith an awl at the point of maximum flex

In the tests the cut growth is measured

in 2.5 mm increments


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Compared here are black-filled rubbers vs. extension ratio or strain energy

The latter is calculated from the stress-strain curve for a prefatigued sampleand integrating the area under the curve up to the extension used in the test

Comparison at equal strain energy eliminates differences due to differences inmodulus

The degree of mixing has a strong influence on the fatigue properties



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Weather Resistance [1]

Effect of weathering on inadequately protected rubbers

sponge

stress

edbybendingaro

undamandril

Outdoor exposure of stressed rubber strips on the roof of a plant or test farms is widely done

There are several ASTM tests, e.g., D518 and D1171

Rubber inherently has a very poor weathering resistance, but is protected by additives

The factors that contribute to the degradation are: heat, oxygen, moisture, light (200-250 nm)and O 3 at 1 ppm or higher levels

Similar to paint and other polymers, there are accelerated tests for rubber compounds


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Weather-o-meter for rubber

In this chamber rubber samples are testedfor discoloration by exposing them to astandard source of UV light under control-led conditions

ASTM D1148, 925, 750 describe theconditions

This test is performed with white or lightcolored products


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Outdoor dynamic testing using the Goodrich

flex machine

The component of the weathering conditionsresponsible for the cracking under stressed orstrained conditions is ozone, O3

The severity of the attack is modulus dependent

and a direct function of the ozone concentra-tion

The strain at which maximum ozone damageoccurs is lower for rubbers that crystallizeupon stretching, such as polyisoprene

Ozone cracking does not occur in saturatedpolymers, it is an attack of the double bond

The extent of damage increases with degree ofmain chain unsaturation

This sensitivity is one of the reasons why NRis not used in treads or sidewalls

There are several ASTM tests, e.g., D1149

The ozone concentration in such tests is typi-cally 50-100 ppm


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Rapid cycles of loading/unloading occurs intires, belts, shocks mounts, diaphragms, tanktrack pads, and many others

The question of how elastomers respond has been extensively studied in the last decade

Cyclic stresses are modeled as a sine curve,i.e., stress builds up smoothly and declinessmoothly in both direction s

The two extremes in dynamic responses areelastic vs. viscous materials

A steel spring is completely elastic: the springdeforms in complete synchronization with aforce so that the deflection is maximum whenthe force is at its maximum

As it is forced to deflect, the spring stores100% of the energy expended through theapplication of the force to compress it

The ratio of the deformation to the force is a constant (the spring constant)

A viscous damper is a cylinder filled with a fluidof high viscosity; a piston can move in thecylinder but the liquid can flow through the

clearance between the cylinder and the wall When a force is applied to the piston it will

move, causing the fluid to flow through theclearance

The flow lags behind the force and the viscous

flow of the liquid translates all the energy of thepistons motion to heat due to internal friction:0% energy is stored

Dynamic Properties [1]


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Force diagram for rubber specimen

damping force

(viscous)

elastic force

complex force(rubber)

phase or loss angle, delta

deflection

Rubber is neither purely elastic nor purely viscous in its dynamic behavior: some energy is storedand some is dissipated

It has a complex dynamic spring rate K* and a complex modulus E*


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Mechanical model of dynamicresponse of rubber

The complex modulus E* is the vector sum of the two: E is the storage modulus (100% is stored);the E is the loss modulus (100% is lost, 0% is stored); the angle between the two is d

For rubber in shear, G and G are used

Both E* and d depend on the compound formulation The degree of storage can be measured in the resilience test (see properties [14])

The loss factor or tan d = E/E is measured more accurately in a Dynamic Mechanical Analyzeras a function of T: it measures the tiny time delay between the force applied and the strainbeing reached; that plus the complex spring rate gives E, E and tan d

NR has a loss factor of 0.08-0.14; very damped polymers can get to 0.3 or higher; a higher factormeans: more energy is converted to heat when the rubber is repeatedly strained

Relationship between complex,storage and loss moduli

loss; heat


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Thus, higher levels of strain and faster cycling of the stress on the rubber will increase theheat that is liberated

We arrive at the important conclusion that for ANY rubber compound, past some point,the amount of strain and the frequency of the dynamic input will combine to produce heat

faster than can be conducted away, and heat buildup in the rubber (which is a poor thermalconductor!) can go out of control, possibly resulting in catastrophic failure

Since the belt region of a steel-belted tire undergoes the highest shear, such a tire will blow outby complete belt separation (BLB mode)

The chances of experiencing such an event increase with the speed (higher frequency of

dynamic input), and with the degree of underinflation (higher strain) Two other factors are the outside temperature (conduction!) and vehicle load (higher strain)

Important advice: keep the tires inflated, obey the speed limits and do not overload the vehicle!

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