02 September 2010

IMPACT ON RCL CRACK WITH PBD SOURCE AND COMPOUND VISCOSITY


IMPACT ON RCL CRACK WITH PBD SOURCE AND COMPOUND VISCOSITY


By
GEORGE JOSEPH



SCOPE OF PRESENT STUDY

To survive in the competent market,the tyre must be durable.The rectification of tyre defects must be done to increase tyre durability.



One of the major defects observed in tyres is RCL crack which occurs in the sidewall region.The main reason for RCL crack is the improper viscosity level of sidewall compound which results in an incorrect flow of rubber during molding.
The main objective of the present work is to check and reduce the no.of RCL cracks in the factory by fixing a correct viscosity level for the sidewall compound and by using Poly Butadiene(PBD),which is the major polymer for making sidewall compound,from different suppliers.
Introduction

Pneumatic tires are manufactured according to relatively standardized processes and machinery, in around 450 tire factories in the world. Over 1 billion tires are manufactured annually, making the tire industry the majority consumer of natural rubber. Tire factories start with bulk raw materials such as rubber, carbon black, and chemicals and produce numerous specialized components that are assembled and cured. Tyre cost is one of the major components and the measures taken to control the cost therefore can contribute significantly to the profitability. (1) tyre durability; (2) tyre retreadability and (3) tyre mileage to derive the best from the tyre. Usually these three parameters can be clubbed to form an index of measurement of tyre performance known as ‘cost per kilometer (cpkm).


A Pneumatic Tyre is a toroidal shaped flexible high performance composite membrane capable of containing air or fluid under pressure when mounted on a suitable rim


Parts of a Bias Tyre

Tread

The desirable characteristics are very good abrasion resistance with lowest heat development, good resistance to cuts and bruises and better handling characteristics like no skidding.

Separate cap and base compounds are used to achieve very good abrasion resistance at optimum heat development. The gauges of the tread are decided by the Non skid depth (NSD) and the under tread (UT). The NSD represents the wearing portion of the tyre – linked to tyre mileage and tread shuffling. The tread pattern is characterized by the geometrical shape of the grooves, lugs, voids and sipes .It is generally compounded from NR and / or blend of SBR ( Resistance to abrasion ) , PBD(provide good rebound property) ,mixed with carbon black ,oils , curative ingredients and other chemicals

Sidewall

To protect the carcass from external injuries and moisture (the moisture can hrdrolyse and damage the nylon carcass or rust a steel tyre cord. The most exposed portion of the tyre. The desirable characteristics are very good flex resistance, resistance to cracks, very good ageing characteristics – Resistance to oxidants and ozone. Normally made of synthetic rubbers (PBD) with good weathering characteristics. Sidewall is fabricated in a form of rubber layer of appropriate thickness by extrusion process

Plies
Deciding factor is the load carrying capacity and the tyre inflation pressure. The number of layers is decided based on the inflation pressure, bias angle, optimum modulus gradation and the constructional constraints like single bead or twin bead construction etc. from inner plies to outer plies to the breaker and then to the tread there shall be gradual reduction in the modulus to reduce interface strain and hence to avoid tyre failures This gradual reduction in the modulus necessitates for different EPI’s for inner and outer plies, lower denier and lower EPI for breaker.
The coating of rubber on the plies is to absorb the shear strain between the layers and to prevent cutting between the cords. The number of plies are selected based on the carcass safety factor calculation or the single cord tension calculation. The deciding factors are the contained volume of the tyre, inflation pressure and the carcass construction
Breakers
These are short plies EPI (end per inch) cut an angle and are positioned between the tyre casing and tread to strengthen carcass against impact. They also provide cushioning effect and increase the modulus of tread area. They are also made by rubberizing dipped fabric on calendar and cutting them by cutting machine.
Bead
Holds the tyre on to the rim, consists of bead wire High tensile steel wire copper or bronze coated to bond rubber to steel and to prevent rusting of bead wires. The number of beads and bead turns and strands is decided based on the bead safety factor calculation, decided by the tyre gauges, dimensions, the inflation pressure and processing and assembling constraints.
Bead Wrap
It is the rubber or rubberized thin cross woven fabric strip wrapped around the insulated bead to bind them firmly.


Bead Apex


The apex is a triangular extruded profile that mates against the bead. The apex provides a cushion between the rigid bead and the flexible inner liner and body ply assembly. Alternatively called "filler"

Chaffer

To prevent chaffing of the plies against rim. and prevent the penetration of moisture and dirt into the tyre.

Squeegee

Protects the tube from ply cords and also protects the carcass from the moisture in the air contained in the tube. Usually made of higher green strength compounds.


Belt


Belts are calendared sheets consisting of a layer of rubber, a layer of closely-spaced steel cords, and a second layer of rubber. The steel cords are oriented radially in radial tire construction, and at opposing angles in bias tire construction. Belts give the tire strength and dent resistance while allowing it to remain flexible. Passenger tires are usually made with two or three belts.


Tyre sidewall
Principal task of compounding is concerned with securing and acceptable balance between the demands arising from these 3 considerations namely “properties”, “price” & “processing”.

The sidewall is the part of the tire that bridges between the tread and bead. The sidewall is reinforced with rubber and fabric plies that provide for strength and flexibility. The sidewall transmits the torque applied by the drive axle to the tread in order to create traction. The sidewall, in conjunction with the air inflation, also supports the load of the vehicle. Sidewalls are molded with manufacturer-specific detail; government mandated warning labels, and other consumer information

Sidewall is formulated to have excellent flex fatigue resistance for crack initiation and growth and minimum deterioration in properties on ageing. It should also have good flex life and must have adequate strength and toughness to resist road shocks and service abuse. Compounding involves synthetic base using SBR, PBD,NR blend.

Adequate carbon black reinforcement is employed and high doses of powerful antioxidant/ antiozonents are used in the compound to resist ageing and to improve flex life.

RCL CRACK

Rim Centre Line(RCL) crack is one of the major defects in the tyre which occurs in the sidewall region.


Raw Materials Used.

Ø Raw rubber : Rss- 4, PBD
Ø Activator : Zno & stearic acid
Ø Antidegradants : 6PPD & TMQ
Ø Retarder : N-cyclohexyl thio phthalimide
Ø Fillers : HAF
Ø Accelerators : TBBS, MBS,DPG,TMTM
Ø Peptiseres : Pentachloro thio phenol
Ø Tackifers : PF resin
Ø Vulcanizing agent : Soluble & Insoluble Sulphur
Ø Processing aid : Aromatic oil

RAW RUBBER
The selection of the rubber for each part of the tyre is a complicated matter in which the blending of one or more rubbers is common practice to achive the compromise so necessary , because no one rubber is ideally satisfactory in every respect .
Natural Rubber (NR)

Rubber's stress-strain behavior exhibits the Mullins effect, the Payne effect, and is often modeled as hyperelastic. Owing to the presence of a double bond in each repeat unit, natural rubber is sensitive to ozone cracking. Latex is a natural polymer of isoprene (most often cis-1,4-polyisoprene) - with a molecular weight of 100,000 to 1,000,000. Typically, a small percent (up to 5% of dry mass) of other materials, such as proteins, fatty acids, resins and inorganic materials (salts) are found in natural rubber. Polyisoprene is also created synthetically, producing what is sometimes referred to as "synthetic natural rubber". For the mechanical strength requirements of tires, NR is better than SBR or BR and offers excellent tread chipping/chunking resistance with low tire running temperatures and improved rolling resistance. Although more difficult to mix and process, it is very good in downstream processes such as extruding, calendering, and tire building. Although good in chip/chunk-type wear and in severe stress wear, it is usually faster wearing in nominal passenger and truck over-the-road tires. It offers better ice traction and excellent exibility in extreme low temperature service (ie, less than 40ºC). It is by far the preferred rubber for wire belt and textile ply/breaker coat compounds. It also provide good tear strength, abrasion flex resistance nd low heat buildup

Polybutadiene (BR)

Polybutadiene is produced by solution polymerisation, and one important feature governing the performance of the resultant polymer is the cis 1,4, and 1, 2 vinyl contents. High cis 1,4 polymers (>90%) have a Tg around –90 °C, and hence exhibit excellent low temperature flexibility .They also exhibit excellent resilience and abrasion resistance; unfortunately the high resilience gives poor wet grip in tyre treads, and hence this rubber finds limited use as the sole base for such compounds. As the cis 1,4 content decreases, and 1,2 vinyl content increases, the low temperature properties, abrasion resistance and resilience become inferior.
The polymerisation of butadiene results in a polymer with a narrow molecular weight distribution which can be difficult to break down during mixing and milling, have low inherent tack, and the inherent elasticity of the polymer gives poor extrudability. Peptisers can be used to facilitate breakdown and hence aid processing. Due to the unsaturation present in the main chain, protection is required against oxygen, UV and ozone. Oil resistance is poor and the polymer is not resistant to aromatic, aliphatic and halogenated hydrocarbons. Polybutadiene based compounds can be cured by sulphur. Less sulphur and a higher level of accelerators are required when compared to NR. Compounds based on this polymer only give optimum properties at high filler and oil loadings.

Activator

* Zno & stearic acid

Activators are processing additives which support vulcanization, in particular the normal sulphur cure. Actually they should be more correctly called cure activators. The most important inorganic activator is zinc oxide.. They can also enhance filler incorporation and dispersion.

Zinc Oxide and Stearic Acid system assumes dual role, namely peptizing function during Mastication and Activating function during Valcanisation. In the absence of an accelerator, the activators zinc oxide and stearic acid are ineffective in increasing the number of cross-links produced of accelerator used. Zinc oxide can, at reasonable addition levels, also assist with reduction of mould shrinkage values. Specific gravity of Zinc oxide is 5.57 & zinc oxide content is 98%. Specific gravity of stearic acid is 0.84-0.86.

Reinforcing agents

* Carbon black

Carbon black Material consisting essentially of elemental carbon in the form of near-spherical particles coalesced into aggregates of colloidal size, obtained by incomplete combustion or thermal decomposition of hydrocarbons constituents. The main heteroatoms incorporated into the carbon structure are hydrogen, oxygen, and sulfur. Thermal blacks typically contain less than 1% of these heteroatoms, and furnace grades less than 2–3%. The physical properties imparted to a given rubber compound by carbon black are dominated by three factors: 1) the loading of the carbon black, 2) the specific surface area of the carbon black, and 3) the structure of the carbon black.

Carbon black is incorporated into rubber through shear forces generated by adding the carbon black to rubber in an internal mixer or open mill. Once carbon black is mixed into rubber, the resulting filled rubber compound is subjected to processes such as calendering, extrusion, and molding before it is cured to make the finished rubber good.
The addition of carbon black increases the viscosity of the compound, and these increases in viscosity can be correlated with increasing loading of the carbon black, with increasing structure of the carbon black used, and, to a lesser extent, with increasing surface area of the carbon black. These increases in viscosity with carbon black additions obviously change the flow characteristics of the filled compound. It is noted that the typical polymer by itself, when made to flow at low shear rates, will exhibit a shear stress proportional to the shear rate (Newtonian flow), whereas the carbon black–filled polymer results in highly non-Newtonian flow.

Accelerators

* TBBS, TMTM, DPG
Accelerators and accelerator systems are chosen on the basis of their ability to control the following processing/performance properties of rubber compounds:
  1. Time delay before vulcanization begins (scorch safety)
  2. Speed of the vulcanization reaction after it is initiated (cure rate)
  3. Extent of the vulcanization after the vulcanization reaction is complete (state of cure)
  4. Other factors such as green stock storage stability, fiber or steel adhesion, and bloom tendency
Compounds classified as primary accelerators usually provide considerable scorch delay, medium-to-fast cure rates, and good modulus development. Compounds classified as secondary accelerators or ultras usually produce scorchy, very fast curing stocks.

Vulcanisation

* Sulphur

Vulcanization is a chemical process where sulfur or other materials form crosslinks in the elastomer and thereby improve the polymer’s mechanical properties. The majority of cure systems in use today involve the generation of sulfur containing cross-links, usually with elemental sulfur in combination with an organic accelerator.

As the cross-link density of the vulcanizate increases (or the molecular weight between cross-links decreases), elastic properties such as tensile and dynamic modulus, tear and tensile strength, resilience, and hardness increase whereas viscous loss properties such as hysteresis decrease. Further increases in cross-link density will produce vulcanizates that tend toward brittle behavior. Thus at higher cross-link densities such properties as hardness and tear and tensile strength plateau or begin to decrease.

CV, conventional vulcanization system; SEV, semi efficient vulcanization system; PV, peroxide vulcanization system.

Specifcation of Soluble Sulfur:-
Specific gravity : 2.05
Solubility in CS : 98% (min)
Ash content : 0.1% (max)
Acidity : 0.1 %(max)

Processing Oils

* Aromatic oil
Petroleum oils are offered to the rubber industry to meet two basic processing and compound requirements: to act as a processing additive, or to act as a rubber extender and softener. The classification depends upon the oil volume added to the rubber compound. As processing additives, the oil addition level is usually no more than 5-10 phr; for additions in excess of this the oils are regarded as extenders. All process oils used by the rubber industry are in practice mixtures of all classes of the three components, the mixtures deriving their classification from the preponderance of the main constituent type.

Aromatic - major portion ≥35% Ca
Paraffinic - major portion ≥50% Cp
Naphthenic - major portion ≥30-40% Cn
In general terms, oils of an aromatic preponderance give best processability in general purpose rubbers, such as NR and SBR, but are prone to causing stains, can give poor odour and may contribute to poor ageing resistance. There is a growing awareness that continuous contact with aromatic oils can represent a health hazard.

* Paraffinic oils are less effective as processing additives, but also have less effect upon ageing, giving good colour and contact staining behaviour.

* Naphthenic oils give characteristics midway between the other two types. Oils of the three types are offered in a range of viscosities and this will influence their processing character to some extent

* The temperature influenced rate of change of viscosity is much greater for the aromatic oils than for the paraffinics

The small additions of oil to a compound help with filler dispersion by lubricating the polymer molecular chains and thus increasing their mobility. There will also be some ‘wetting out’ of the filler particles which enables them to achieve earlier compatibility with the rubber and improve their distribution and dispersion speed. Process oils are controlled and marketed, within a molecular type, by viscosity which is influenced by the molecular weight and degree of branching of the molecular chains and the degree of cyclisation and aromaticity.

Viscosity Gravity Constant (VGC)

It is now common to use Viscosity Gravity Constant (VGC) as the criterion for choice of a processing oil. The VGC represents the overall average aromaticity of an oil independent of its molecular weight.
Rubber process oils can be classified using the VGC classification:
* Aromatic - VGC <0.950
* Paraffinic - VGC <0.850
* Naphthenic - VGC <0.850 - 0.900

Specifications are:-
Appearance : Dark coloured oil
Specific gravity @ 60°F : 1.005
Aniliene point : 45
Say bolt viscosity : 125

Antioxidants and Other Protecting Systems

* DPPD, 6PPD ,Wax
To extend the service life of vulcanized rubber goods, it is very important to protect them from oxygen, ozone, light, heat, and flex fatigue. Most natural and synthetic rubbers containing unsaturated backbones—natural rubber (NR), styrene butadiene rubber (SBR), polybutadiene rubber (BR), Antioxidants react with oxygen to prevent oxidation of vulcanized rubber and react with free radicals that degrade vulcanized rubber.
There are four principal theories on the mechanisms of antiozonant protection of vulcanized rubber.
* The first is the scavenger theory, which postulates that the antiozonant competes with the rubber for ozone.
* The second theory is that the ozonized antiozonant forms a protective film on the surface of the vulcanized rubber, preventing further attack.
* The third mechanism postulated is that the antiozonants react with elastomer ozonide fragments, relinking them and essentially restoring the polymer chain.
* The fourth theorized mechanism suggests that Criege zwitterions are formed from the ozonide produced.
The various types of antidegradants are described below.
Resistance to
Type common chemical staining oxygen ozon heat flex use name name
Amine DPPD Dimethyl Very Exc- Mild Exce- Good Tread&
Butyl Para strong Sidewall
Phenylene
Liamine
6PPD Dimethyl Very Exc- Good Exc- Good Sidewall
Pentyl Para strong
Phenylene
Diamine
Wax MC Micro No Nil Exc- Nil Nil Tread&
Wax crystalline Sidewall

Paraffinic waxes generally protect better at low temperatures, Due to the slow migration of microcrystalline wax, it provides longer term static protection from ozone cracking. Blends (paraffin waxe, microcrystalline)offer a wider temperature range of protection. Waxes are used for, only static ozone protection. During dynamic flexing the barrier of wax is exposed owing to breaking of the wax film. Therefore, reactive antiozonants are required for dynamic ozone crack resistance.

The amount of wax addition to a rubber compound will depend upon a number of factors:
  1. the type of rubber being protected
  2. the required appearance of the rubber product
  3. the temperature of service for the component 
  4. the type of service - static or dynamic
  5. the effect of filler type
  6. the effect of process oils
  7. the rate of wax migration to rubber surface
  8. the type of wax being used.

Specification of 6PPD:-
Specific Gravity : 1
Melting point : 47°C
Ash content @ 950 °C : 0.1 (max)
Specification of Wax Blend:-
Specific gravity : 0.91
Drop melting point : 64 – 70 °C
Ashy content @ 950 °C : 0.1% (max)

Tackifers

* Phenol Formaldehyde Resins
Both thermoplastic and thermoset phenol formaldehyde resins find application in rubber compounds. The thermoplastic resins act as excellent tackifiers whilst the thermoset resins act as plasticisers during processing then crosslink to give a reinforcement structure to the compound. Suitable resins for many applications can be tailored by variation in the type of phenol and the ratio of formaldehyde. Lower levels of formaldehyde produce tackifying properties whilst the higher formaldehyde levels contribute, in the presence of hexamethylene tetramine during the rubber cure, to a greater degree of resin polymerisation within the rubber network creating a crosslinked structure which contributes to the hardness and abrasion properties of the compound.

The various types of resin have been discussed above. However, the listings for these materials have been categorised to indicate their prime usage in the industry and have been assigned according to the manufacturers classification:

  1. Homogenising resins
  2. Petroleum resins and proprietary blends
  3. Reinforcing/bonding resins
  4. Resins for adhesives
  5. Tackifier and dispersing resins
  6. Vulcanising resins.

mixing
The key words, homogeneous and dispersion, seem to sum up the principal purposes of most mixing operations: the various ingredients must be dispersed evenly so that the product is the same (homogeneous) everywhere. If the mixer has done a perfect job, each ingredient is evenly intermingled with every other and even the smallest sample taken from any part of the product is identical to any other sample.
The four main components of mixing are
1. Incorporation: Incorporation means each individual particle must be thoroughly wetted and form a coherent mass.
2. Distribution: Incorporated material should be distributed all over the medium.
3. Dispersion: In dispersion the ingredients that are in the form of agglomerates must be broken down into fine individual particles.
4. Viscosity reduction: During mixing, considerable reduction in viscosity of rubber occurs, due to effective molecular break down. Viscosity reduction enhances the processability of rubber.
Due to the partly elastic nature and very viscosity of rubber, power intensive, sturdy machinery like mixing mills and internal mixers are necessary.
Mixing is usually done in two ways
1. Rolling mill mixing.
2. Mixing by internal mixers.
1.Rolling mill mixing.
The rolling mill is furnished with two rolls. The rolls have internal water-cooling. Both roll have slightly different peripheral speeds giving a more efficient mixing. The ratio of the roller speeds is 1:1.2. the power rating is around 200kw with a roll of 20 rpm. Typical mixing cycle takes 30 min. and batch size are 60-79 kg.
At mixing, the mix is following one of the rolls and additives are added in between the two rolls. During mixing the gap is slightly opened. After mixing, the rubber strips is cut in slices and fed in between the rolls, a number of times. This is ensure through mixing.
An important difference between rolling mills and batch mixers is that the former gives a good mix while the latter gives a fast mix. Furthermore, technological opportunities in the rolling mill mixing procedure are nil compared to the internal batch mixers. Automatization is also a hurdle in rolling mill mixers.
2.Internal Mixers
The mixing in a banbury takes place in the gap between the rotor tips and the chamber walls. At the start of the cycle when the polymer is normally in large pieces and the powders are still loose, the initial breakdown and mixing takes place in the large space between the rotors and against the door top. As the material softens, flow over the rotor tips starts to take place but until the viscosity of the mix has fallen sufficiently, most of the work is done between rotors, along the rotor helix angles and between rotors and discharge door top. The area just in front of rotor tip is similar to the nip of a mill, and causes internal shear in the material. The mixer should be cooled to remove heat generated by the mechanical energy of mixing
The figure illustrates the equipment used for mixing:
Mixers used in rubber processing: (a) two-roll mill, and (b) internal mixer

* Mixer Size /Type : 11D BANBURY MIXER
* Chamber Volume : 237 Liters
* Number of wings : 2W
* Discharge Door Type : DROP
* Electrical motor : 415 VOLTS 3 PHASE 50 HZ
BASIC OPERATING CYCLE
* Start Ram up
§ Hopper door open
§ Discharge door closed
* Charge Add ingredients
* Close hopper door
* Ram down
* Mix for required period
* Open discharge door
* Close discharge door Raise ram
* Open hopper door Return to start
Extrusion
Sidewall is made by extrusion technique. After final batch, the mixed compounds are then lead to the extruder. The rubber compound is given continuous shape by forcing it through a die. The extrudate is cooled, cut into specified length and stored
Hot feed extruder of 8 inch size - dedicated for sidewall extrusion
Flow characteristics of rubber is necessary for an understanding of extrusion.
1. Text Box: Shear StressRubbers are strongly non-Newtonian; i.e. the rate of shear deformation is not proportional to the shear stress. The shear rate increases more than proportionality to stresses
2. At any given shear stress, the shear rate increases with temperature.
3. The rate of flow under a fixed stress is time dependent
4. Rubbers show more or less elastic recovery when the deforming stress is remove
Mixing
The mixing was carried out in two stages.
1st stage:
This stage include master batch preparation. Here the black and other compounding except curatives, accelerator, retarder were mixed with rubber. The mixing was done in a 11D internal mixer at 150ºC. The capacity of internal mixer is 237 kgs & the rotor speed has been adjusted to 60 rpm.
2 st stage:
A portion of Master batch along with Curing agents ,accelerators, retarder are mixed in a 11D internal mixer mixer at 118ºC. The capacity of internal mixer is 237 kgs & the rotor speed has been adjusted to 40 rpm. The final batch was done after 24 hrs of ageing the stocks were sheeted out in the open mixing mill(33cm x 15cm) having 1:1.4 friction ratio at a nip of 1/8”. Mixing temperature and time were controlled for all the batches.

MIXING process are accepting on the basis of following tests

Master batches : Specific Gravity, Moony Viscosity.
Final batches : Specific Gravity, Moony Viscosity, Mooney Scorch
and Rheological properties
Moulding & Testing
1. Moulding & Vulcanisation
The Moulding of test specimens were done a steam heated single daylight press (INDEX PELL) with a platen size of 24” X 24” having 12” round size. The mould were preheated to the vulcanization temperature. After mounding the curd sample were cooled upon removal from the mould to room temperature and conditioned at ambient temperature for 24 hrs before testing.
Following test samples were molded for study
v Tensile and tear sample
CURE TIME
TEMPERATURE
60 min
141.7 °C
2. Testing
1. Mooney Viscosity Measurements (ASTM D 1646-81)
Mooney viscosity is defined as the shearing torque resisting rotation of a cylindrical metal disk (or rotor) embedded in rubber within a cylindrical cavity. The test provides information on processability of the compound.The Mooney scorch time, and Mooney viscosity were also determined by using a Monsanto automatic Mooney Viscometer (MV 2000). Condition is ML(1+1.5) at 135°C. The testing procedure was conducted according to the method described in ASTM D 1646-94.
The Mooney Viscometer was the most common tool used to determine the processing characteristics for a given batch of rubber. Many compounders still use the Mooney to verify viscosity (which is indicative of molecular weight) when obtaining raw polymer stock. This works because a compound’s resistance to being moved by the Mooney’s internal rotor is directly linked to its viscosity.
From the chart of the mooney unit v/s time the scorch time can be easily determined by noting the time where the cure turns upwards. A 5 point raise above taken from the mooney cure are
v MV Minimum viscosity
v t5 Time to scorch at Mv + 5 units
v t35 Time to cure at Mv + 35 units
v t1 Cure index which is t35 – t5
Specifications
Standards ASTM-D1646, ISO-289
Rotor Speed : 2 RPM
Graphic Output Curves generated :
· MOONEY viscosity curve
· Scorch curve
· Stress relaxation curve.
· Temperature curve of the upper and lower die
Sample Volume : Two piece test specimen having a combined - volume of 25 ± 3cm3
Temperature 135 °C
2.Rheometer
Rheometer is a versatile instrument used in rubber industry to study curing characteristics of rubber compounds. Basically, it consists of a bi-conical rotor oscillating at desired arc inside in a heated cavity, filled with a rubber compound. The resistance offered to rotor by the compound is measured in terms of torque v/s time. The Rheometer curve give useful information on compound safety, cure rate, optimum cure etc.. From the
data Tc 90, ML, MH, Ts2 of compounds were found out from the plot of torque Vs time , scorch time (Ts2), time for 90% cure (tc 90) and cure rate can be calculated .
There are two main types of Rheometers currently in use: the ODR and the MDR.
ODR
1 Cure rate
Cure rate = MH - ML
Tc 90 – Ts2
Cure rate is the rate at which stiffness (modulus) develops after the scorch point. During this period, compound changes from a soft plastic to a tough elastic material, owing to the introduction of cross-links.
2 Minimum Torque(ML)
It is a measure of the viscosity of the stock at the test temperature. The Rheometer minimum torque is proportional to the Moony Viscosity when both the tests are conducted at same temperature.
3 Maximum Torque(MH)
It is the measure of the stiffness or shear modulus of the the test specimen at the test temperature. It is also an effective measure of the changes in tensile modulus and cross-link density.
4 Induction Time ( Ts2)
It is the time corresponding to 2 unit rise in torque about ML. It is a measure of the time available for mold flow. At lower temperature Ts2 is a processability measure similar to mooney scorch.
5 Optimum Cure ( Tc 90 )
It is the time to achieve 90% of the maximum torque .
Tc 90 = [( MH – ML ) X 90/100] + ML
Rheometer Cure Curve
Monsanto Moving Die Rheometer (MDR – 2000)
It also measure the cure characteristics of the compound. It differs from ODR 2000 in the following respect. In MDR 2000 there is no separate disc, instead the lower half of the die oscillates at 1.66 hz (100 cpm). The reaction torque measured at the upper die correlates with the degree of vulcanization as a function of cure time. The parameters measured from the rheograph are ;
Ø Minimum Torque ( ML )
Ø Maximum Torque ( MH )
Ø Optimum Cure ( Tc 90 )
Ø Tan∂
MDR holds the sample between a pair of heated dies (metal plates forming a cavity). As one of the dies moves across a small arc, the other die gauges the reaction torque generated in the sample. This again results in a cure curve that can show the optimum cure time for the desired blend of properties.
MDR generates 3 curves S’, S" and Tan Delta.The MDR produces a cure curve labeled S’ which is similar to the ODR curve
S’ is called the storage modules, elastic modulus or in phase modulus. S" is called the loss modulus, viscous modulus, or out-of-phase modulus. All of these terms have been used to mean the same thing. The unit of S’ and S" in the product family instruments are in lbs. These are units of torque. They can also be in dnm. Which are also torque units.
Tan∂ = Loss modulus ( S”)
Storage Modulus ( S’)
The smaller the Tan delta, then the more viscous the material.
Specific Gravity
Specific gravity of the compounds was determined according to IS:3400 and compared with theoretically calculated value.
Physical Properties
Tensile properties (ASTM D 412)
Molding
Cure condition 60 min at 141.7 ºC
The final compounds were for 24 hours before molding. The compounds were sheeted out at the required thickness using laboratory mill. Molding was done in a 24” X 24” steam heated automatic press at 141.70 ºC and 75 Kg /sq.cm pressure. The mould pieces were cooled in water and conditioned at 23_+2 and 50% RH for 24 hours before testing.
Equipment:
INSTRON- 4301 Tensile tester with a crosshead speed of 500 mm/ min (ASTM D 412)
Sample preparation :
The test pieces are punched out from the vulcanized sheets, parallel to the mill grain direction. Dumb bell types are most popular whereas ring types test pieces are also used to a limited extent.
Total length : 15mm
Width of ends : 25+_1mm
Width of the narrow parallel portion : 6+_0.4mm
Length of the narrow parallel portion : 33+_2mm
Test:
The test were carried out at 23+_2ºC dumb bell specimens were placed in the grips of INSTRON universal testing machine. Two grips were also there, one inch apart to indicate elongation. There were fixed to the sample in the narrow region. area corresponding to the thickness was fed to the microprocessor. the rate of separation of grip was 20 inch per min. After testing tensile strength, elongation at break, moduli were noted.
Tensile strength:
It is the tensile load per unit cross sectional area of the original cross section within the narrow parallel portion of the test specimen at the point of breakage of the test specimen.
Tensile strength (psi) = load at break
Initial cross sectional area
Modulus:
It is the stress per unit cross sectional area of the original cross section with in the narrow parallel portion pf the test specimen for a particular elongation.
Eg:
300 % modulus (psi) = load at 300% elongation
Initial cross sectional area
Elongation at break:
It is the elongation within the narrow parallel portion of test specimen at the point of breakage of the test specimen expressed as percentage. Elongation describes the ability of the rubber to stretch without breaking.
Elongation at break = length at break- initial length x100
Initial length
Tear strength (ASTM D 634)
The test were carried out at 23+- 2 ºC dumb bell specimens were placed in the grips of INSTRON universal testing machine. Two grips were also there, one inch apart to indicate elongation. These were fixed to the sample in the narrow region. Area corresponding to the thickness was fed to the microprocessor. The rate of separation of the grip was 20 inch per min. after testing tensile strength, elongation at break, moduli were noted. The result is expressed in Kg/cm or lbs/inch instruments.
Tear strength= load required to tear the specimen / specimen thickness
Shore A Durometer Hardness (ASTM D 2240)
Hardness may be defined as a material's resistance to permanent indentation
Shore A hardness is a measure of resistance to the penetration of an indenter of specified size and shape forced in to the test specimen. The hardness is measured in Shore A Hardness units ranging from zero to hundred.
This test method permits hardness measurements based on either initial indentation or after a specified period of time or both.
Test specimen:
The minimum dimensions of the specimen are 30mm diameter, and 6mm thick, tensile pads may be tested for hardness by sticking two pads of similar hardness.
The surface of the specimen must be flat and as free of surface irregularities possible.
Conditional specimen at 23 ºC + 2ºC for a minimum of 16 hours testing.
Procedure
Durometer, like many other hardness tests, measures the depth of an indentation in the material created by a given force on a standardized presser foot. This depth is dependent on the hardness of the material, its viscoelastic properties, the shape of the presser foot, and the duration of the test. ASTM D2240 durometers allows for a measurement of the initial hardness, or the indentation hardness after a given period of time. The basic test requires applying the force in a consistent manner, without shock measuring the hardness (depth of the indentation). If a timed hardness is desired, force is applied for the required time and then read. The material under test should be a minimum of 6.4 mm (.25 inch) thick.


Tables and Graphs
RCL Crack on the basis of PBR source
1.Comparison of tensile properties.
Compound A
Compound B
Compound C
Kg/Sq.Cm
Kg/Sq.Cm
Kg/Sq.Cm
197.45
192.303
206.95
177.853
183.597
193.745
188.524
184.308
208.502
194.663
162.017
214.447
189.134
141.011
227.099
194.009
182.649
198.44
195.676
173.82
194.753
193.041
192.02
210.809
2.Comparison of tear strength.
Compound A
Compound B
Compound C
Kg/Sq.Cm
Kg/Sq.Cm
Kg/Sq.Cm
70.061
67.612
75.119
80.478
59.713
87.699
770.92
86.65
90.446
84.885
70.063
70.049
83.576
76.483
92.534
82.149
77.182
76.841
74.031
80.126
73.435
79.818
61.165
70.655
3.Comparison of hardness.
Compound A
Compound B
Compound C
57
51.5
57
55
54.5
58.5
55
54
57.5
Comparison of RCL crack by using different sidewall compounds

No.of tyres followed
No.of RCL cracks
Compound A
50
1
Compound B
1200
160
Compound C
1200
180



GRAPHS
RCL crack on the basis of compound viscosity
Viscosity comparison
1.
Compound A
Compound B
Compound C



55
54
61
58.5
56
58
57
57
58

52
59

55
62

57
63

56
62


60


62


62
2.
Compound D
Compound E
Compound F



64
81.2
74
72
82.2
76
72.5
79.8
75

84.4
76

74.9
73

79.4
74

78
74

73.5
82

77
74

76.2
77

78.6
75



Avg. Viscosity&no.of RCL cracks

Avg.Viscosity
No.of RCL cracks
Compound A
56.83
1
Compound B
59.5
200
Compound C
60.7
225
GRAPHS
1.
2.

Avg.viscosity&no.of RCL cracks
Comparison of Results.
1.PHYSICAL PROPERTIES
TENSILE STRENGTH
On comparing with B&C,compound A have most of it’s tensile strength values in the desired limit(190 ±5 kg/sq.cm) for making a sidewall compound.
TEAR STRENGTH
On comparing with other two compounds,compound A have most of it’s tear strength values in the allowable limit.(80±5).
HARDNESS
Compound A has it’s hardness in the desired range (55±2) for making sidewall.
RCL by using different sidewall compounds
By using compound A,the RCL crack was 2%,which was 13.33% for compound B and 15% for compound C.

2.Viscosity
By using compound A ,the avg.viscosity of sidewall compound obtained was 56.83 which comes within the specified limit of viscosity for a sidewall compound.(57±2).The no.of RCL cracks for this viscosity level was 1.
For compound B,the avg viscosity obtained was 59.5 and the no.of RCL cracks for this viscosity level was 200.
By using compound C,the avg viscosity obtained was 60.7 and the no.of RCL cracks were 225.


CONCLUSION
Based on PBR source
From the tests and observations,the no.of RCL cracks were tremendously decreased by using compound A.Out of 50 tyres by compound A,the no.of RCL cracks were only 1.
A mong other two compounds,compound B was better than compound C.
Based on compound viscosity
The no.of RCL cracks were very low in the viscosity level below 60.Thus the present viscosity level(62±2) can be shifted to 57±2.

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