Rubber Recycling

Rubber Recycling

S. N. Chakravarty* 

KPS Consultants & Impex Pvt. Ltd.,

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

E-mail : kpspltd@gmail.com

Today’s industry and manufacturing process are guided by 4 Rs.

       REDUCE            -   Materials, Resources & Wastage

       REUSE               -   Process waste 

       RECYCLE          -   Up cycle 

       RECOVER         -   Polymers / Rubbers from used / scrap products

Introduction

Discovery of vulcanization by Charles Goodyear in USA and Thomas Hankock in UK established the base of rubber industry and there was an increased demand for rubber.  Initially this demand was met by Natural Rubber and subsequently different types of Synthetic Rubber were introduced and widely used in rubber industry.  However, the price of virgin rubber and it’s fluctuation due to many factors stimulated the interest of inventors and attracted their attention towards the waste vulcanized rubber generated during production of rubber goods.

This waste / scrap rubber had to be de-vulcanized to get to a plastic state so that it can be further processed. Reclaiming is essentially de-polymerization of vulcanized rubber. The sulfur used for vulcanization remains in the product. Reclaimed rubber is primarily made from natural rubber based products.

China and India are the largest manufacturer and consumer of reclaimed rubber. India produces nearly 130,000 MT of reclaim rubber per annum which is also fully consumed.

According to statistics, the production of reclaimed rubber in China increased from 1.1 million ton in 2002 to 2.5 million ton in 2009, accounting for 73% of total recycled rubber products and 81% of total reclaimed rubber worldwide. According to statistics, there are more than 600 companies that produce rubber powder in China, with a total annual production capacity of 5 million ton.

 *President – Elastomer Technology Development Society, India

With the growth of rubber industry and increase in rubber consumption, the demand for reclaimed rubber also increased during past decades.  Reclaimed  rubber offers two prong advantage, namely it reduces the cost of the product and in certain cases gives technical advantage like in calandering & extrusion process.  Of course the proportion of reclaimed rubber in the compound has certain limitation with respect to physical properties and other behaviour.

World Tire production

World total tire sales in 2010 was USD 152 billion. Considering approximately 20% growth in 2 years it is estimated for 2012 at USD 180 billion.

Production of tire of major country are –   China 865 million tires ( USD 79.3 billion ) in 2012, India 124 million tires in 2012 – 13, USA domestic production dropped from 218.4 to 160.3 million tires during 2004 – 2008 ( international economic depression). Considering 20% growth in 2010 if works out to be approx 192 million tires. Japan produced 160 million tires in 2012. European tires production in 2010 was 4.50 million ton (+ 26%, 2008 / 2009 were depressed economic years). European  tire replacement market sales in 2007 was Truck / Bus – 11.6 million, Passenger / LT – 28 million, Motor Cycle 1.6 million.

Annual scrap tire generation

Scrap tires were used for land filling for quite some time but due to detrimental effect on soil restriction to this was introduced. Scrap tires are used as cheap fuel, mainly by the Cement industry. Scrap tires are used as economical alternative to coal as fuel in cement kilns , pulp and paper mills and industrial and utility boilers.

Sector wise used tyre usage in EU in 2010 was

Reclaim rubber                      40%

Energy recovery ( Fuel )       38%

Reuse                                     10%

Retrading                               8%

Landfill / others                      4%     

What is Reclaim Rubber ?

Reclaimed or Re-generated rubber is the product resulting from the treatment of ground vulcanized scrap rubber tires, tubes and miscellaneous waste rubber articles by the application of heat and chemical agents, followed by intense mechanical working. The regenerated rubber has almost the original plasticity of virgin rubber, permitting the same to be compounded, processed and re-vulcanized or reused.

In reclaiming of scrap vulcanized rubber, the scrap is normally ground and is then treated with the application of heat, chemical peptizers and is then intensely worked upon mechanically to partially de-vulcanize (break the bonds between rubber polymer chains) the rubber component. This partially de-vulcanized product is commonly called reclaimed or re-generated rubber and can be compounded, processed and re-vulcanized much like virgin rubber. The availability and processing characteristics of suitable scrap sources has dictated the types of reclaim produced. Reclaimed rubber has become widely accepted as a raw material which possesses processing and economic characteristics that are of great value in the compounding of natural and synthetic rubber compounds.

Different Manufacturing Processes

Reclaimed rubber can be manufactured by different methods of which the most important is the Digester process. The raw material for reclaiming is scrap rubber in a wide variety of forms, mainly tires. The first stage, in all processes, is the cracking and grinding of the scrap rubber to reduce it to a crumb or powder passing through a 20-30 mesh screen

1.         Digester process

2.         Pan process

3.         High pressure steam process

4.         Banbury process

5.         Reclaimator process

Digester Process

Most of the reclaimed rubber produced today is by this process. Tires are cut into smaller pieces after removal of beads of the tire. These pieces are put to the cracker mill (heavy corrugated steel rolls revolving at different speeds thus applying tearing and grinding action on the tire pieces).  Ground scrap is passed through vibrating sieve to remove fine material from the larger pieces which are fed back to the cracker.  This scrap is passed through magnetic separator to remove magnetic metal particles so that no damage to further machinery takes place. 

Reclaiming agent and process oil are added to the weighed quantity of the scrap and put into the digester ( steam heated Autoclave ). High pressure steam and digestion period causes scrap rubber to de-vulcanize.  After digestion, the steam pressure is brought down and water is added / spread to wash the rubber free from digester liquor and compost fiber. This is then conveyed to de-watering press to squeeze out water and then to pass through hot air dryer to reduce the moisture content.  This dried reclaimed rubber is put on refining mill to homogenize the material and disperse it thoroughly by passing through closed tight nip of the refining mill.

Some part of hard improperly de-vulcanized particles remain in the rubber and these are removed which is known as “tailings“. This is then sold as a low price reclaimed rubber used for cheaper molded products.

Fine rubber crumb (20 to 30 mesh) free of fiber and steel is loaded into a digester along with water, chemical reclaiming agents, processing oils, and other additives. The digester is a cylindrical jacketed pressure vessel fitted with a horizontal agitator, and steam or heated thermic fluid can be supplied to both interior and jacket, thus enabling a uniform temperature to be maintained throughout the mass. The contents of the digester are then heated to about             190 ⁰C to 200 ºC at 20 to 22 Kg / cm² pressure and maintained at this temperature for some 4 - 5 hours with continuous mechanical working of the material by means of an agitator. After the process the material is dumped and this is followed by milling, straining and refining in the conventional manner to obtain reclaim rubber in sheet form.

Pan Process

This is a simple reclaiming process. In this case fiber / fabrics are removed from the ground scrap and then reclaiming agents along with oil are added into this finely ground scrap by passing repeatedly through cracker and grinder mills.  This finely ground rubber powder is blended thoroughly with reclaiming agents and process oils  which is then placed on open pans which are put on trolleys at different layers and pushed into an horizontal vulcanizer / autoclave.  High pressure steam ( 14 kg / cm² )  is used for different time as per requirement.

After the process pans are removed and the material is passed through milling operations This is followed by straining and refining in the conventional manner.  Often small amount of China Clay is sprinkled on the mill to avoid sticking of degraded rubber.  Also instead of mills only mixer – extruder type of machine may be used for continuous blending operation.  Straining and finishing of milled reclaimed rubber may be required.

Not only NR and SBR scraps but also IIR (Butyl rubber), CR (Polychloroprene rubber), and NBR (Acrylonitrile Butadiene rubber) compounds are reclaimed by this method. 

The pan process was widely used in the early stages of the reclaim industry before the advent of the wet digester. Its use is now limited to specialty types such as light colored natural rubber reclaim

High pressure steam Process

This process is similar to the above with respect to usage of fiber free coarsely ground scrap mixed with reclaiming agent and process oil etc.  Only difference is the usage of high pressure steam ( 55 to 70 kg / cm² ) in the specially designed autoclave for short period of time ( 10 minutes ). Pressure is reduced suddenly and the reclaimed mass is blown into a cyclone collector where it disintegrates. After drying the mass is processed and refined as described above.

 

Banbury Process

Ground scrap is mixed with reclaiming agent, process oil and small proportion of carbon black which is fed into the banbury having high rpm rotors and high pressure ram.  Intense mixing action increases the temperature substantially       ( above 250°C ) and the de-vulcanization takes place within about 15 minutes. The mass is cooled and discharged on the refiner mill which is further processed as described above.

Reclaimator process

In this process finely ground scrap after removal of the fabric / fiber, is mixed continuously with the reclaiming agent and process oil and conveyed to the Reclaimator.  Reclaimator is a cylindrical unit similar to extruder, having screw for conveying the mass working on it by pressing against the barrel wall. At the clearing end of the unit the clearance between screw and wall can be adjusted.  The process temperature is around 200 °C and the reclaimed rubber level is controlled by the transit time ( to about 4 minutes ). The mass temperature is reduced during discharge and this devulcanized material is further process – refined sheeted etc. as per standard method.

A reclaimator process is used for the continuous reclaiming of whole tire scrap. Fine rubber crumb (30 mesh) free from fiber and steel is mixed with various reclaiming agents and processing oils is subjected to high temperatures with intense mechanical working in a modified extruder for the partial de-vulcanization of the rubber scrap. Depending on the specification of the finished products, fillers may be added to the reclaimed product before further processing. The material from above process is then milled, strained and refined as dictated by the specification of the finished product before being sheeted or extruded into the finished form.

The reclaimator machine, from which the process takes its name, is screw extrusion type with a hopper at one end into which the crumb, previously mixed with oil and chemicals, is fed at a predetermined rate. It generates heat of de-polymerization by mechanical working with the finely ground rubber crumb under pressure, and discharges the mass as partially de-vulcanized rubber. Temperatures in the machine are controlled by alternating oil and water jackets. The discharged material is then milled, strained and refined in the conventional manner to obtain reclaim rubber in sheet form.

Merits & Demerits of each process

The Pan process is an outdated technology. The pan process requires higher process time and hence provides lower productivity, product quality is not uniform, involves higher labour requirement etc. Still Pan process is used only by the small scale manufacturers or specialized synthetic rubber reclaim manufacturers.

Uses of the Reclaimator process is also limited to high capacity plant. The main reasons are

• Suitable mainly for high output operation
• High cost of plant & machinery
• Continuous process – difficult to control various process parameters (residence time of material inside the extruder is 2 to 4 minutes) – higher possibility of wastage.
• More suitable for reclaiming of synthetic rubbers that show hardening during longer recycling period like SBR.

Wet Digester technology is well established and widely used throughout the world especially in China, India and S.E. Asia. In a wet digester process crumb rubber is de-vulcanized at high temperature and pressure along with mechanical working. This gives high level of de-vulcanization and hence produces quality reclaim rubber. The machinery cost is not high making it the preferred process for the production of reclaim rubber. The only drawback of the wet digester process is that it is a batch process.

Manufacturing Process

Crumb Rubber

Processing of scrap tires to make rubber crumb is one of the most common and useful disposal steps followed in the rubber industry. Crumb rubber is widely used in different applications, both in rubber & non-rubber usage. Crumb rubber is the name given to material derived by reducing scrap tyre or other rubber into uniform granules with the inherent reinforcing materials such as steel and fiber removed along with any other type of inert contaminants such as dust, glass, or rock.

Tire Bead Separator

Tire Shredder

Crumb rubber manufacturing machinery is designed to convert whole scrap tires into clean and size reduced particles, usually 20 to 30 mesh size. The first step in the process involves removal or extraction of steel bead wire from the whole scrap tire. The next step is shredding of the whole scrap tire into smaller pieces (say around 60 mm wide pieces). The shredded pieces are then fed by means of a conveyor into a Cracker Mill to reduce size of the pieces further (say around 20 or 30 mm wide pieces). The cracked rubber pieces are fed by means of a conveyor into a Breaker Mill or Granulator to produce rubber powder of 20 to 30 mesh size. Crumb rubber is typically produced through an ambient grinding process. Ambient describes the temperature of the rubber as it is being size reduced. Typically, the material enters the cracker mill at "ambient" or room temperature. The temperature of the rubber will rise significantly during the process due to the friction generated as the material is being "torn apart." Granulator’s size reduction process by means of a cutting and shearing action. Product size is controlled by a screen within the machine. Screens can be changed to vary end product size.

                                                                              

Cracker Mill

Cracker mills - primary, secondary or finishing mills - all are very similar and operate on basically the same principle. The mills have two large rotating rollers with serrations & cuts in one or both of them. The roll configurations are what make them different. These rollers operate face-to-face in close tolerance at different speed in opposite direction. Product size is controlled by the clearance between the rollers. Cracker mills are low speed machines and the rubber is usually passed through 2-3 mills to achieve various particle size reductions and further liberate the steel and fiber components.

Magnetic Metal & Nylon Fiber Separator

The permanent magnetic drum is major cost saving non-electric separator. It is widely used for removing steel pieces etc. from the powder material processed in bulk form. It consists of a stationery permanent magnetic assembly around which the drum shell revolves. The assembly has a uniform magnetic field across the entire drum. It holds ferrous particles to the revolving shell and the non magnetic material falls freely from the shell. Magnetic flow sweepers are used in combination with magnetic drum to remove any particles on the flow. The Nylon fiber separator uses high pressure air flow to separate nylon fabric fluff from the powder rubber by inertial action. The separated scrap nylon fluff is conveyed by high pressure air draft though pipes and collected in bins.                         

Fiber Separator

Rubber Cracker Mill with Magnetic metal separator, Belt Conveyer and Vibrating Sieve

Reclaim Rubber

Following steps are used in the manufacture of crumb & reclaim rubber from whole scrap tire.  

1.      Bead is removed from the tire by De-beading machine, thus removing the steel wire portion from tires. 

2.                  The de-beaded tires are conveyed to a Shredding unit where they are reduced to small pieces (50 to 60 mm).

3.                  These pieces are then conveyed to a Cracker mill to reduce the sizes further (20 to 30 mm).

4.                  These pieces are then conveyed to a Breaker mill to produce fine rubber powder of 20 to 30 mesh size.

5.                  Simultaneously a back-feeding conveyor passes the material through a Metal & Fiber Separator system i.e. through magnets to remove the metal pieces and high pressure air blow to remove the fiber pieces. 

6.                  Simultaneously the material is subjected to Vibratory Sieving separating finer mesh crumb from larger granules. The higher size granules are fed back for further grinding to finer mesh size.  

7.          Such prepared crumb rubber is mixed thoroughly with water, process oil & reclaiming agent and allowed to mature for a while.

8.                  This mixture is fed into a Dynamic De-Vulcanizer and subjected to mechanical working under heat & pressure by steam or other heating medium.

9.                  After a definite period of de-vulcanization, the mass is taken out and fed into a series of mixing mills for homogenization, through mixing and sheeting out.

10.             The material is passed through a Strainer Extruder to eliminate lumps / larger particles / foreign matter etc. and then

11.             Subjected to refining on refining mills followed by sheeting out

12.             It is then packed in bale form for dispatch.

Crumb rubber powder (20 to 30 mesh size) free from fiber and metal is then mixed with process oil (e.g. Pine tar and / or Aromatic oil) 3 - 4 parts and reclaiming agent (like Diaryl Disulphide / Di-Xylene Disulphide) at 1 part level. This is then fed to Dynamic De -vulcanizer or Autoclave heated by Steam / Thermic Fluid etc. to desired temperature (around 200⁰ C) and pressure ( 20 to 22 Kg / cm²) for a definite period of time (around 4 - 5 hours) during which rubber becomes partially de-vulcanized. The material inside the autoclave is continuously mechanically worked upon by means of an agitator (rotating paddle).

Devulcanizer ( Autoclave )

This partially de-vulcanized mass is dumped and then fed onto a Two Roll Mixing Mill followed by preliminary refining on a Kneading Mill (another two roll mixing mill). At this stage often China clay and / or Carbon Black may be added which aid in smoothing the dried stock.

Kneading Mixing Mill

Two Roll Mixing Mill

The mass is then strained using a Strainer extruder to remove any non-magnetic metal or other foreign matter. Finally, this is then subjected to refining using a Refining Mill where the rolls are set with very low nip gap. Any hard, improperly de-vuclanized particles remaining on the rolls are periodically removed to be designated as “tailings” which is used in low priced molded products. Refiner produces paper thin layer which is then collected, multiplied to a thicker sheet and finally made a bale (20-30 Kg), wrapped in polyethylene sheet & put in paper bag.      

        

Strainer Extruder

Strainer Extruder Head

Refining Mill

Typres of Reclaimed Rubber 

There are different types of Reclaimed rubber, primarily based on the rubber scrap source. For example whole tire reclaim ( WTR ) is based on the scrap tire, mainly the tread portion where the filler is carbon black only. This is relatively high grade of reclaim rubber. Other major type of reclaimed rubber is from tubes- based on Natural Rubber (Cycle tubes) and reclaimed rubber from butyl rubber based inner tube and bladder / curing bag.

Now a day’s reclaimed rubber of specialized elastomer like nitrile & silicon are also available in the market. Typical analytical properties of reclaimed rubber is given in the following table.

Reclaim Rubber- Specifications

Properties ( IS 7490 : 1997 )	WTR - R	TRR - N	TRR - I
( Method of Test of RR – IS 6306)
Volatic matter, (%)	1.0	1.0	1.0
Ash (%)	8	7	6
Carbon Black (%)	27±3	25±3	34±3
Acetone Extract (%)	16.0	10.0	10.0
RHC by Difference (%)	50	52	52
Specific Gravity	1.16 ±0.03	1.12 ±0.03	1.13 ±0.03
Tensile Strength (kg / cm²)	8.0	5.0	6.0
Elongation at Break (%)	280	280	450
Hardness at Shore A
Mooney Viscosity ML (1+4 ) @ 100°C.	40±5	40±5	--

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.