Elastomer Technology Development Society, New Delhi CAUSE OF FAILURE OF RUBBER PRODUCTS - TYRE &...

Elastomer Technology Development Society, New Delhi

CAUSE OF FAILURE OF RUBBER

PRODUCTS - TYRE & NON-TYRE CAUSE OF FAILURE OF RUBBER

PRODUCTS - TYRE & NON-TYRE

Dr. S. N. Chakravarty *

President

Elastomer Technology Development Society

© Copyright by the Author

Elastomer Technology Development Society, New Delhi 2

Usage of Rubber Products

Rubber products, both Tyre & Non-Tyre, are used in indoor & outdoor application under various conditions.

Some are expected to withstand atmospheric conditions (Oxygen, Ozone, Sunlight & UV), others under dynamic condition (flex fatigue, typical example is Tyre which goes thru million cycle of compression & deflection), still some other products need to withstand high temperature and / or in contact to oil & solvent (e.g. automotive components under the hood application).


Failed Product


Failed Tyre


Failed Rubber Seal


While developing & manufacturing a rubber product all these factors are kept in mind while formulating the compound & curing of the products.

Let us see what are different factors which could be the cause of failure of rubber products during usage.


Application / Service related factors

To develop a rubber product most important information required are its application / usage pattern. To find out whether the product is

a) subjected to weather conditions – Oxygen / Ozone / UV / Sun Light / UV etc. b) Temperature of usage e) Dynamic application (Flex Fatigue ) d) Abrasion / Wear condition e) Contact with Oil & Solvents, Chemicals, Acid & Alkali etc.


Specification

To develop a product to meet application requirement, basic specification given is followed. It is supposed to give all requirement of properties (e.g. TS, M, SH, Sp. Gr., Abrasion, Flex, Ozone & Oil resistance etc) & retention of these properties over the usage period, judged by carrying out ageing at an elevated temperature.


Material - Polymer / Elastomer & Ingredients.

Depending on requirement from the Product during usage base Elastomer in chosen e.g.

a) For normal applications, temp. limited to 70º / 80ºC & no Ozone & Oil resistance etc

requirement, NR / SBR / BR rubbers may satisfy.

b) In case of higher temp. limit for usage – to go for CR, NBR, CPE, EPDM, HNBR, Silicone &

ultimately FKM .

c) In case of product coming in contact with Fats & Oils, Solvent etc. choice may be NBR, HNBR,

Arcylate & FKM depending on application combination.


d) For Ozone resistance the choice would be CR, CPE, EPDM, Silicone, FKM depending on severity

of application.

e) In case of high temp. & Oil / Solvent contact as well as Ozone resistance requirement, choice is

limited to FKM.

f) For both high & very low temp. application Silicone rubber may be the choice.

g) For electrical insulation & resistance property requirement, ECO may be preferred.


h) Retention of gas / air ( impermeability) property is met by Butyl / Halo butyl rubber. ( Automotive

inner tube & inner liner application).

i) For high chemical resistance choice are CSM, FKM etc.

Wrong choice of base polymer for specific application often is the vital cause of product

failure.


Choice of Polymer as per Temp.

Silicone

- 50 0 50 100 150 200Temperature, º C

SBRIR

PolyacrylateCR

H = NBR NBR ( Carboxylated )

NBRFluorosilicone

FluorophosphazeneFluorocarbon

Ethylene PropyleneEthylene Acrylic

ECO


Compounding

Compounding formulation play important role in a product’s performance & life expectancy.

Vital are the reinforcement of the product to achieve desired level of physical properties (fillers like Carbon Black gives best protection against deterioration, also Silica / Siliane Coupling agent system provides high ageing resistance) and

Curing system (S & Accelerators / Activator choice & phr level in case of diene rubber or other curing agents for special SR).


E .V .System

Efficient vulcanizing (EV) systems are defined as those in which no sulphur or sulphur donor is

used for crosslinking purposes. Such vulcanizates are normally associated with a high

proportion of monosulphidic and dissulphidic crosslinks in the network.

EV system gives vulcanizate with exceptionally good resistance both to reversion and to heat

ageing. They are of particular interest for


-Manufacture of thick articles to avoid uneven cure (e.g. in injection moulding)

- Resistance to heat ageing required beyond the capability of antioxidants.

-Alternative to expensive non-discolouring antioxidants.

Both high accelerator / low sulphur system and the sulphur donor system give good processing safety and excellent resistance to reversion and

to ageing.


Semi-Ev System ( Partial replacement of Sulphur )

Good retention of vulcanizate properties during ageing. With mainly monosulphidic or disulphidic

crosslinks these are more resistant to oxidative and thermal degradation than conventional vulcanizates

which have a high proportion of polysulphidic cross-links. The efficient systems have low initial

fatigue life, but are very stable and show no significant change on ageing.

Sulphur- Free Vulcanization System for Diene Rubber

Vulcanization with Sulphur Compounds without Free Sulphur


Vulcanization with Sulphur Donors

Crosslininking mechanism which, though not dependent on additions of sulphur nevertheless form effective crosslink sites containing sulphur atoms. These choice & dosage level have bearing on physico – mechanical properties ( SH / TS / M / Tear , Abrasion loss / Wear characteristics, ageing at a higher temp., flex fatigue etc.


Anti - degredants

Choice & dosage level of anti-degredants (antioxidants / antiozonants, M.C. Wax) play important role in product life & performance,

Aminic type of anti-degredants are more effective against heat ageing where as p-plylenediamine types are effective against ozone & flex fatigue resistance.

In case of non-black products bisphenolic & MBI type are used to a level of success.


contd.

On ageing, vulcanizate (product) degrades & physical properties drop, ultimately to a point that the product fails. Ageing is primarily oxidative (ozone) degradation; heat / sunlight / UV / Copper / Iron in ionic form / flexing all have catalytic effect on degradation.

Oxidative ageing breaks down the rubber chain ( unsaturation point ) causing drop in physical properties. Hence, while compounding, one has to protect the product against oxidative and other ageing factors.


Ageing Mechanism • . • Initiation ROOH RO + OH • Ki .• 2 ROOH RO + RO2 + H2 O • • . Kp .• Propagation RO + RH ROOH + R• . fast .• R + O2 RO2

• .• Termination 2 R R – R • . .• R + RO2 RO2 R• Ki• 2 RO2 Non Radical Products


Oxidative Ageing of NR• The oxidation of a rubber molecule is shown below. • • CH3 CH3

• I I .• - CH2 - C = CH – CH2 – CH2 – C = CH – CH2 - + RO2

• CH3 CH3

• I I• - CH3 – C = CH – CH – CH2 – C = CH – CH2 – • • CH3 02 CH3

• I I• - CH2 - C = CH – CH – CH2 – C = CH – CH2 – • I• O O* • CH3 CH3 • I I• - CH2 – C = CH - CH – CH2 – C – CH – CH2 – • I I • O O


NR & SR are attacked by oxygen even at room temperature and the reaction is accelerated by heat, light and the presence of certain metallic

impurities which catalyse the decomposition of the peroxides to form free radicals.

Consequently, the addition of an antioxidant is required to minimize oxidative degradation.

All hydrocarbon polymers undergo scission as a consequence of thermal oxidation.


Processing - Dispersion of ingredients

Processing steps – Mixing, Extrusion, Calendaring, etc. contribute towards incorporation/ distribution & dispersion of large quantity of fillers – both reinforcing & non-reinforcing in a compound, Dispersion of ingredients is important to achieve desired property level and product performance or premature failure.

Improper dispersion of ingredients & presence of grits in a product subjected to flexing will fail prematurely because the stress factor on those points will be different causing failure. Abrasion / wear is also affected.


Dispersion

C-black 250 – 2000 µ m 100 µ m 1µ m

Phases during Mixing of Rubber


Rubber Stage


Vulcanizing systemCuring system and curing process perhaps the most important criteria in a product’s property level and performance.

Physical property level & product performance is based on the crosslink density and nature / type of crosslink formed by vulcanization.

Higher Sulphur level in the compound produces more polysulphidic crosslinks which gives higher TS / M as well flex fatigue but has lower resistance to heat (oxidative) ageing by which polysulphidic crosslink’s breaks down to lower sulphidic crosslink and properties fall.


Stages of Vulcanization 1. Incubation Period 2. Induction stage ( Scorch ) 3. Crosslinking stage 4. Plateau5. Reversion

COURSE OF VULCANIZATION

1 2

3

4

5

TIME

MO

DU

LU

S


Incubation period

During this period , the rubber compound is heated to the curing temperature . Rubber is

a bad conductor of heat.

For thin walled goods, the incubation period is short but in case of thick-walled rubber

goods, the interior of the article may take a fairly long time to get heated to the curing

temperature.


Induction stage

The time interval at the curing temperature at which no measurable crosslinking can be observed is know as the induction period. This period depends on the polymer and the curing system used.

The induction period represents the safety margin in processing the stock and is an indication of the storage life of the stock.

Every compound must have a sufficient induction period so that the compound processes satisfactorily.


Crosslinking / Plateau

Crosslinking

This is the period when crosslinking starts, the rate depending on the cure system, the compound and the temperature of cure.

Plateau

This is the period during which the physical property, after attaining the maximum value, remains constant with continued cure.


Reversion

This occurs in the vulcanization of NR, polyisoprene and butyl rubbers. On prolonged cures, the physical properties of the vulcanizate will start deteriorating. Other SR like SBR, NBR, CR will not generally show reversion..

These five stages are exhibited when sulphur is used as the cross linking agent. When the cure systems other than elemental sulphur are used the reversion stage is generally absent. With sulphur the reversion occurs because of the fact that poysulphidic crosslinkages formed at the earlier stages are rather unstable and break up on continued heating.


Crosslink & Property

Physical property level and product performance is based on the crosslink density and nature / type of crosslink formed by vulcanization.

Higher Sulphur level in the compound produces more polysulphidic cross links which gives higher TS / M as well flex fatigue but has lower resistance to heat (oxidative) aging by which polysulphidic crosslink’s breaks down to lower sulphidic crosslink and properties fall.


contd.

Lower sulphur & higher accelerators (specially sulphenamide types) produces more stable crosslink’s and sulphurless curing gives most stable structure which have very good resistance to heat ageing & retains the property level for longer period of product life.

Similarly cure temp. & time also have bearing on vulcanizate structure and in turn on ageing effect.


Vulcanizate Structure


Product Design - Geometry / Construction / Safety factor.

It has been shown that the design of the product has pronounced effect on certain application behaviour / flex fatigue, wear characteristics, compression set etc. Higher the displacement from the “null” point during flexing of a product (e,g tyre sidewall), more pronounced is the oxidative (ozone) effect on the property, faster crack formation leading to failure.

Geometry & design of products for specific application like Bridge Bearing Pad, Seismic Insulation Pad, and Railway rubber items are important. Sharp contour / curvature cause early failure. While designing a product adequate “Safety Factor” need to be built in.


Product Design


Storage conditions

Storage conditions are important for product usage after longer storing. Ageing effect continues even at room temperature albeit very slow.

Hence it is advised to store the rubber product away from heat, Sunlight & moisture, preferably covered with black poly. Contact with grease, solvent, oil etc. should be strictly avoided.

Also FIFO system and storage stack height / rotation should be followed if stored for longer time to avoid any de-shaping.


Factors playing role in failure of products

Till now we saw that following factors play paramount role in the failure of rubber products.

1. Selection of Elastomer / Blend.2. Reinforcing fillers and Ingredients as well as

their proportion in the compound.3. Curing agents and vulcanization system

(proportion, temperature, time & pressure).4. Reinforcing agent like Cord / Fabric.& Metal /

Steel cord.


Composite Products

1. Tyre ( Most important Example )

2. Conveyor belt

3. Hose ( braiding / reinforcement )


Pneumatic Tyre

Pneumatic tyre is the most prominent rubber product as the sector consume about 50% of total rubber consumption of the country.

Air pumped inside the tyre (body or carcass) carries load of the vehicle. It is air inside the carcass made of rubberized tyre cord – nylon / polyester / steel, is the load bearing part of the tyre. Load bearing capacity increases with increasing air pressure till rupture point (cord strength limit) is reached. Hence, load & inflation pressure are embossed on tyre sidewall as specified in the standard. Tyre designer build in necessary safety factor ( SF ) above this level.


Pneumatic Tyre


Improper choice of tyre cord (denier / strength / twist / etc.) ,design ( not building in required SF etc.) & compounding will lead to premature tyre failure.

Wear characteristics of a tyre will have impact on users (customers) of the tyre. Fast wear, tear , crack development will shorten tyre life. Proper choice of polymer, vulcanizing system and protective agents usage can eliminate these.


Conveyor Belt


Hose


Composite

Vital factor for product performance is adhesion between rubber & reinforcing cord ( cotton, Nylon Rayon, Polyester, Aramide, and Steel Cord ).

Bond failure will lead to product failure due to separation between rubber layer & reinforcing cord / fabric. No adhesion problem is encountered with cotton which can be used for low strength reinforcement.

Adhesion becomes critical with Aramid and Steel Cord.These need pretreatment and special additives in the compound to achieve proper bond strength. Important is bond strength under dynamic condition at an elevated temperature.


Gaskets / Seal / Bush / Car Channels / Engine Mount / Load Bearing Pads – Bridge / Seismic isolation / Railway items.

One property i.e. compression set & compression deflection, play vital role in the function of these items, especially at an elevated temperature and in contact with grease / oil / solvent.


Contact with grease / oil / solvent causes swelling and shrinks on drying , looses sealing property ,causing leakage.

Many of these products are exposed to weather conditions (Oxygen / Ozone / Sunlight / UV light etc.) which will affect compression set. Besides,crack formation takes place which on propagation, ultimately leads to product failure.


Elastomer Technology Development Society C/o. Polym Consultants

812, Devika Tower 6, Nehru Place,New Delhi -110019 (India)

Tel: +91-11 26213885, Fax: +91-11 41616899E-mail: [email protected]

Web: www.elastomer.in

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