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

812 Devika Tower , 6 Nehru Place

New Delhi 110019

( E-mail: polymcon@gmail.com )

Rubber products, both Tyre & Non-Tyre goods, are used in indoor and 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).

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.

Scrap Tyre Yard

Failed Tyre

Failed Auto Seal

Below are listed possible causes for such failure –

1.         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.

Lecture delivered at Kolkata , Chennai, Ludhiana, Delhi, India

2.         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.

3.                  Material – Polymer / Elastomer & Ingredients.

Depending on requirement from the Product during usage base           Polymer / Elastomer in chosen. For example

a) For normal applications, temperature limited to 70º / 80ºC and no Ozone & Oil resistance etc requirement, NR / SBR / BR rubbers may satisfy.

b) In case of higher temperature limit for usage – one has to go for CR, NBR, CPE, EPDM, HNBR, Silicone & ultimately FKM type of rubbers.

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 temperature & Oil / Solvent contact as well as Ozone resistance requirement, choice is limited to FKM.

f) For both high & very low temperature 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. 

4.                  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 Synthetic Rubber).

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

Crosslininking mechanism which, though not dependent on additions of sulphur nevertheless form effective crosslink sites containing sulphur atoms.

Vulcanization with Sulphur Donors

           

These choice & dosage level have bearing on physico – mechanical properties (SH / TS / M / Tear, Abrasion loss / Wear Characteristics, ageing at a higher temperature, 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.

On ageing, rubber vulcanizate (product), degrades and physical properties drop, ultimately to a point that the product fails. Ageing is primarily oxidative (ozone) degradation; heat/ sunlight/ UV /Copper/ Iron / 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.

Following is the reaction mechanism for the auto-oxidation of pure hydrocarbons in the absence of added initiators or inhibitors.

The oxidation of a rubber molecule is shown below.

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.

5.                  Processing – Dispersion of ingredients & Machinery.

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 and 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.      

6.                  Vulcanizing system & Vulcanization

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

Stages of vulcanization : is followed by checking of physical property like modulus or shear modulus with time of cure.  Five distinct stages are observed.

1.         Incubation Period

2.         Induction stage ( Scorch )

3.         Crosslinking stage

4.         Plateau

5.         Reversion

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 or Vulcanization

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 natural rubber , polyisoprene and butyl rubbers. On prolonged cures, the physical properties of the vulcanizate will start deteriorating. Other synthetic rubbers like SBR, NBR, CR will not generally show reversion.

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

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.  

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 temperature & time also have bearing on vulcanizate      structure and in turn on ageing effect.  

7.                  Product Design - Geometry / Construction / Safety factor.

It has been shown that the design of the product has pronounced          effect on certain application behaviour life 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.   

8.                  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.   

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

Let us now analyze few product example:

Composite Products

1.         Tyre  (Most important)

2.         Conveyor belt

3.         Hose (braiding / reinforcement)

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 the 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.

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 eleminate these.

Vital factor for product performance is the adhesion between rubber and reinforcing cord ( Cotton, Nylon Rayon, Polyester, Aramide, and Steel Cord). Such bond failure will lead to product failure due to separation between rubber compound and 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. All these needed 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.