14 April 2011

DISPERSION OF SILICA AND CARBON BLACK IN NR/BR BLENDS


PROJECT REPORT
ON
DISPERSION OF SILICA AND CARBON BLACK IN NR/BR BLENDS


AVINASH. R. PAI


CONTENTS
1) Introduction.
2) Scope and objective.
3) The dispersion of carbon black in rubber.
4) Factors that influence the rate of carbon black dispersion.
(a) morphology of carbon black.
(b) Nature& state of elastomers.
(c) machineries.
(d) Ram pressure.r.
(e) Rotor Speed.
(f) Fill factor.
(g) Coolant temperature.
5) Process of manufacture of tread rubber.
6) Non black fillers.
7) silica and its properties.
8) Comparison of silica & carbon black.
9) Application of silica on tread compounds.
10) Carbon black-silica dual phase filler/Eco filler.
11) Testing.
12) Experiments.
13) Results & discussion.
14) Conclusion.
15) Reference




INTRODUCTION
We all think of quality, We all talk of quality, we all insist on quality. In simple words we may say that “quality means fitness for use”. In any industry say, big or small, quality is the main concern. Every effort will be made to control the quality at every stage of manufacture from procuring raw materials to materials in process to final product. Quality and productivity are the twin-keys of survival and success of any modern day industry. The challenge before a rubber technologist, therefore, is to strike a balance by making “sensible compromises”. These sensible compromises which allow attainment of satisfactory levels on both the fronts are termed as optimization.
The quality of a tire is quantified in terms of its performance, and the ability of a tire company to give quality tires is judged by the consistency with which their tires meet or exceed the expected performance. Essentially three performance parameters govern a tire's function. These are 1) Vehicle mission profile 2) Mechanical properties and performance such as wear resistance and casing durability. 3) Esthetics, comfort and behavioral characteristics such as vehicle steering precision. Of which the most important is wear resistance and casing durability. Tread wear is a function of tread compound, including type and percentage of elastomer, type and percentage of carbon black, type and percentage of oils, degree of dispersion and type and state of cure.
Tire is a rubber based product containing various components with various performance requirements, tread wear, heat build up, resistance to cutting and chipping and fuel economy continue to be important to heavy duly truck tire performance. To meet these requirements, compounds based on various elastomers and blends of elastomers are used. The introduction of cis -1,4-polybutadiene during the late 1950s was probably the single most important development relating to the use of elastomer blends in tires. In blends with NR/BR has enabled significantly improved tread-wear and groove cracking resistance without reduction in resilience. Better oil extension and higher black loadings are additional benefits made possible by development of BR. Thus NR/BR blend compounds are mostly used in truck treads as they help to meet most of the above-mentioned requirements. Among the factors contributing to enhanced performance are the size and continuity of the separate polymer phases and the manner in which black is distributed and dispersed between the two phases.
The mixture of rubber and ingredients used for manufacture of any rubber is called “compound” and the art of making such a compound is called compounding. Compounding is the basic processing step in tire manufacturing. In compounding, type and loading of carbon black is the most important factor by which the compounder can alter and control the properties of his compound. Usually blacks with lowest particle size and high structure gives best properties. The increase in abrasion resistance with small particle size is not without Problems. Small particle black are not easily dispersed in rubber and they also generate more heat during mixing.
In tire industry mixing or compounding is earned out in batch mixing banburies followed by sheeting in two-roll mills. This step is important in the sense that, reinforcing filler like carbonblack is mixed with elastomer at this step; While the rate of filler incorporation governs the economics of compounding, filler dispersion in the final compound controls the quality of the end product made from that compound. Inorder to exert its beneficial influence on the properties of rubber vulcanizates, die carbon black must be sufficiently dispersed there in. The beneficial effects are better product homogeneity, improved physical and mechanical properties, reduction of premature failure, increased product life, consistency of product quality etc. Poor dispersion can give rise to certain detrimental effects. Such as 1) reduced product life 2) poor performance in service 3) poor product appearance 4) poor processing characteristics 5) poor product uniformity 6) raw material wastage and high rejection rate 7) excessive energy usage. Poor dispersion on the other hand results in large agglomerates, if are of size larger than the inherent flaw size of rubber (10 µm), results in poor mechanical properties and may also result in premature failure of the product as the large agglomerates act as failure initiating flaws.
1.2 THE DISPERSION OF CARBON BLACK IN RUBBER

The incorporation of carbon black into rubber vulcanizates generally gives improved strength, extensibility, fatigue-resistance, abrasion resistance etc. However, inorder to exert beneficial influence on the properties of the rubber vulcanizates, the carbon black must be well dispersed there in. In addition, poor dispersion can in itself give rise to certain detrimental effects, example, reduced product life, poor performance in service, poor product appearance, poor processing characteristics, poor product uniformity etc.
The full effect of reinforcing filler will be realized only when its surface is thoroughly "wetted" by the polymer in which the filler is completely dispersed. This thorough wetting requires penetration of the polymers into the occluded void space of the structure aggregate into the polymer matrix. The mixing process can be compartmentalized into four major steps for the sake of easy understanding, though it must be remembered that it is not a water-tight compartment. The four major steps are:
  • Wetting of ingredients (incorporation)
  • Distribution of ingredients
  • Dispersion of agglomerates
  • Reduction of viscosity
When carbon black is being mixed into rubber in conventional equipment first the carbon black agglomerates gets encapsulated by the polymer (wetting). At this stage, the interstices within the agglomerates are still filled with air giving a weak crumbling composite. This step is the incorporation step. Then in next stage, the rubber penetrates into the void space. As the rubber penetrates through the narrow channels between the agglomerates, bound rubber is being formed. The bound rubber cements many primary aggregates together. This is the distribution step. The immobilised layer of polymer arising out of this stage-tends to reduce effective cross-section of the channels through which more rubber must force through before reaching the inner part of the agglomerates. This step is followed by the Dispersion Step. Dispersion is considered as a slow erosion phase in which the aggregates are scrapped on the surface of the agglomerates ie., agglomerates are downsized to aggregates, as a result of stresses produced by the strain flux due to the mixing process.
Shiga and Furuta have described morphological changes of carbon black agglomerates during mixing in terms of an “Onion” model for dispersion. They suggest that carbonblack aggregates are scraped from the surfaces of agglomerates and dispersed into the matrix. The scraping might be caused by the velocity difference�between the agglomerates and matrix. The mechanism appears similar to the erosion mechanism* Shiga and Furuta observed “tails” at two ends kf an that the tails comprised concentrated dispersions of aggregates being scraped off the agglomerates as a result of the stresses produced by the strain flux due to the mixing process.


COLLOIDAL CARBON

(DISPERSED AGGREGATES)

Schematic representation of the rupture of carbonblack agglomerates and the dispersion of fragments,

Mc. Kelvery proposed the following dispersibility factor
K = 6 U Re µ / c
where Re - agglomerate radius
µ - Matrix viscosity
- Strain rate
C - Inter aggregate cohesive force
Large agglomerates can give rise to poor mechanical properties. It is possible that the large agglomerates act as failure initiative flaws. Agglomerates larger than the inherent flow size for the rubber would be responsible for decrease in mechanical properties such as ultimate tensile strength, energy to break, tearing energy, fatigue resistance etc. The inherent flow size for natural rubber is of the order, 10µm.
Dispersion will not reach effective levels if it had not been preceded by incorporation and distribution. Unmixed pockets of carbon black usually result not from inherent problems with agglomerates, but from poor wetting and consequent inadequate distribution. Some dispersion, example breakup of pelletized fillers, occurs prior to and along with incorporation. Thus, a problem with dispersion can sometimes interfere with incorporation and distribution, although the reverse is more common.
An effect of incorporating of the carbon black into the rubber is to increase its viscosity and thus its mixing torque. The effect of dispersing the carbon black after it is incorporated is to reduce viscosity and thus the mixing torque. Depending on the relative rate of increase in mixing torque due to incorporation and the relative rate of decrease in mixing torque due to dispersion of carbonblack, a second power peak will or will not be observed.

1.3 FACTORS THAT INFLUENCE THE RATE OF CARBONBLACK DISPERSION
1.3.1 Morphology of Carbon Black
The morphology of the carbon black plays the most important role in determining the rate of incorporation of black into the polymer and its subsequent distribution and dispersion. The factors that plays a major role are
1. Particle size
2. Structure of the aggregates
Dispersion would be promoted by large aggregate size, high strain rate and high viscosity while high cohesive forces that are associated with high carbon black surface area, structure, hard pellets would be detrimental. During distribution stage, if considerable rubber black interaction occurs, subsequent dispersion is rendered more difficult. For this reason low structure, high surface area carbon blacks are more difficult to disperse; their small void space and dense packing leads to large local black cone and their large area provides ample opportunity for early interaction with polymer. At the same time such blacks are quite rapidly incorporated, it is only the subsequent dispersion into individual aggregate that is rendered difficult. High structure blacks have irregular particles packing less tightly with decreased number of inter aggregate contact and are more slowly incorporated, but more easily attain eventually a satisfactory degree of dispersion.
1.3.2 Nature and State of Elastomers
The case of wetting of carbon black by the elastomers depends on the type of elastomers used, the affinity of the individual elastomers towards carbon, the polarity difference (solubility parameter) between the elastomers used in the blend, molecular weight of polymers used, crystallinity, level of unsatruration etc.
BR and SBR are considered to have higher affinity for carbon than NR. The order of affinity follows the pattern BR>SBR>NR.
1.3.3 Machinery
Banbury in the most popular internal mixer used in rubber processing industry for mixing. Mixing being the first step in processing. The quality of the mix obtained at this stage plays a very important role in determining the final quality. The various banbury related parameters are discussed below.




a. Ram Pressure

The major purpose for application of pressure to the ram is to drive the raw materials into the mixing chamber and to prevent their upward exit during mixing, thus the force applied to the ram to ensure that materials charged into the mixer engage rapidly with the rotors and also be sufficient to present subsequent upthrust of the batch from displacing it upwards producing a stagnant region similar to that resulting from excessively high fill factor. The upthrust is strongly dependent on fill factor, and the ram force require high compressed air line pressure and large pneumatic cylinders to facilitate working at high fill factors. Increasing the pressure beyond this point is often expected to increase the mixing speed or improve the mixing. In reality however the opposite occurs. Too high a pressure can impede the rotor action needed for extensive mixing.
b. Rotor Speed
Nearly all the banbury mixers are now equipped with variable speed rotors but they are seldom used to vary the rotor speed during the mix. Mixing is done at optimum speed for a particular batch. The requirement for distributive mixing and dispersive mixing are conflicting with respect to rotor speed. The rate of distributive mixing is a function of rotor speed, proceeding rapidly as speed is increased; but to retain a high viscosity in the rubber for dispersive mixing it is desirable to run the mixer slowly to minimize the rise in temperature. Lower rotor speeds give advantages in the form of better properties due to improved dispersion but at expense of larger mixing times. However low rotor speeds by-improving filler dispersion, may enable the requirement for a second dispersive mixing to be avoided.
c. Fill Factor
Fill factor defines the proportion of the mixer chamber volume occupied by the finished mix, that is, it is the ratio of the batch volume to the actual volume of the mixing chamber. For getting good dispersion results the optimum fill factor should be used. This is same with the compound viscosity also. In the rubber industry, usually a fill factor ranging from 0.65 - 0.85 is used. When the fill factor is too high a part of the batch may escape the mixing there by resulting in non-homogeneity and non-uniformity of the mix. If the fill factor is too low, it results in the formation of voids in the rubber mass behind the rotors wings or nogs.
d. Coolant Temperature
During mixing, a lot of heat is generated. So internal mixers are provided with cooling system inside the rotors, chambers and the doors. When the mixed compound is cooled much it reduces the friction between the compound and the inner surface of the chamber there by causing slippage and so the torque of mixing gets poorly transferred into the compound. Hence it prolongs the mixing time. If it is not cooled sufficiently it may cause rapid temperature rise and there oy affecting the compound properties as well as smooth completion of the mixing cycle. Also sudden temperature increase may cause rapid compound viscosity reduction and the energy spent on the compound is poorly transferred into the bulk. In short, we can prolong or cut short the mixing time by regulating the coolant temperature as per our requirement. Usually chilled water is used for this purpose.

1.3.4 MIXING SEQUENCE
Sequence of Material Input
There are three ways of mixing rubber in the internal mixer, namely the so called conventional method, the early oil addition and the upside down mix method. Many variations of these three methods are also used to suit the special characteristics of the individual formulation and the machinery used for mixing. In case of tire compounds using substantial quantities of low particle size carbon black, dispersive mixing is usually the rate determining step of mixing. The material input sequence should therefore be used to maximize the forces acting on filler agglomerates. The maximization is achieved by with holding oils, waxes and fatty acids from the early stages of mixing cycle and charging the mixer with only the rubber and the bulk filler, in addition to any particulate additives like ZnO. These all should be charged into the mixer at the same time. Unless mastication of a natural rubber is required. When adequate dispersive mixing has been achieved or its efficiency has been reduced by temperature rise in rubber, the oil and other viscosity reducing ingredients can be added. The mixing cycle can be terminated when the oils, waxes and any other ingredients withheld to minimize their residence time at an elevated temperature are adequately distributed.
When elastomer blends are used, it appears that the sequence of blending and carbon black addition is of utmost importance for the distribution of the black in the blend which intern, largely determine the physical properties of valcanizates. It is noticed that a banbury mixed master batch cut back with the same or with a second elastomer is characterized by a low modulus, hardness, abrasion resistance and flexometer heat generation and by a higher elongation and rebound value than when the black is added to the elastomer blend.
1.3.5 NUMBER OF STAGES
The number of stages in which the mixing is done influences the mixing of the compound. For compounds containing large quantities of reinforcing fillers, three stage mixing sequence are commonly used. They are the master mixing stage, master remilling stage and the final mixing stage of the master with the curatives. Each of the stage is carried out in the internal mixer. However if the temperature is efficiently controlled in the master mixing stage satisfactorily level of carbon dispersion can be attained even with a two stage mixing procedure.
Other factors that extend their influence in determining the carbon dispersion of the final mix are maturation time given between successive stages of mixing, number of cuts given in the mill etc. Usually a minimum of four hours is recommended as maturation period between successive stages of mixing.
1.2. Process of manufacture of tread rubber
The process of manufacture of tread rubber consists of the following operation.
a) Manufacture of tread rubber
b) Preparation of cushion gum
c) Baking of tread with cushion gum
a) Manufacture of tread rubber
The important steps involved in the production of conventional and pre-cured tread rubbers are mastication, mixing, pre-warming and extrusion. In the case of pre-cured tread, additional steps involved after extrusion are blank preparation, moulding. buffing, inspection and packing.
First the rubber is masticated by mechanical process or with the help of pepticer to reduce the viscosity of the rubber to a point, where the compounding ingredients can be added without difficulty. Excessive mastication results in improper dispersion of Carbon black and low vulcanizates properties, so proper time should be given for proper dispersion of carbon black in rubber mix. Mastication involves the rupture of primary bonds in rubber molecular under the stress set up.
After mastication various compounding ingredients are added to the rubber. The aim of mixing is to make an intimate mixture of a homogeneous mass from the individual ingredients. The homogeneity should not only be in the sense of uniform distribution but also in the sense of uniform dispersion. After through mixing, the compound is sheeted out and kept for maturation for about 24 hrs. Maturation helps to reduce the variation in properties from batch to batch and within each stock.
After maturation, the batch is pre-warmed before feeding into the extruder because if the rubber is cold, then the residence time in the extruder will be more and the extrusion process in such a case will not be continues. After extrusion, the batch is earned over conveyers and is cooled so as to eliminate the automatic curing of tread since the tread coming out of the die has got high temperature. Then moisture is removed with a jet of compressed air. After this cushion gum is applied and then wound into rollers and is packed.
For the production of precured tread, the material coming out of the extruder is cut to the exact length as to serve as blanks for feeding the hydraulic press. The blanks are loaded on to the press platens heated to specific temperature and fitted with moulds having the desired tread design. Then the press platens are closed under pressure for a definite period and vulcanization of rubber compound takes place. After vulcanization, the platens are opened and the product is taken out and then it is inspected for any visual defects, buffed and packed with polythene sheet.
b) Preparation of cushion gum<+font>
The function of cushion gum is to impart proper adhesion of tread material with the tyre carcass. Cushion gum is prepared by calendaring, so as to get thin layer of compound. The important property requirement for a cushion gum compound is very good tack and non-blooming characteristics. They are mainly prepared from NR. The cure rate of the cushion gum compound should be adjusted so that it is slightly faster than that of the tread compound.
c) Backing of tread with cushion gum
In the case of conventional process, the tread is supplied after applying a base layer of cushion gum to improve the tack, permitting a better adhesion of tread to the tyre. Usually the cushion gum is calendared to have a thickness of 0.5 to 1 mm. While applying cushion gum, proper care has to be given to avoid the entrapment of air.
For precured treads, cushion gum is applied along with the tread on separate roils in polyethylene sheets.
From the processing point of view, the desired characteristics of tread compounds are
v Long storage life
v Good tack
v Non-blooming tendency
v Low nerve and good dimensional stability


1.3. Non-black fillers
A wide variety of peculate filters are used in the rubber industry to improve the physical properties of rubber compound. A general division of filler is based on the effect of the filler on Tensile Strength, breaking strength, elongation at break, modulus and tear strength of the cure compound.
Among the non-black fillers, the highest hardness is provided by rod-shaped or plats like particles in contrast to the spherical particles of similar diameter. This is because; these shaped particles can achieve parallel orientation during processing. Of the spherical particulate fillers, precipitated silica, surface treated clays and calcium silicates produce high hardness and high modulus compounds. Fine particled silica gives the utmost in reinforcement in rubber of the non-black fillers. Non-black fillers do not attain so much popularity like Carbon black because
1 High reinforcing character of mineral filler is achieved only after treatment. Such treated filler is presently costlier than Carbon black.
2 There is a general tendency to use mineral filler as extenders to reduce cost.
3. Non-black fillers being inorganic in origin, tend to find lack of compatibility with organic origin elastomer. In most cases coupling agents are added for better filler-rubber interaction, which is costly.
1.4. Silica and its properties
Silica is of two types
a) Ground silica or naturally occurring such as crystalline and diatomaceous earth
b) Synthetic or processed silica such as precipitated silica, pyrogenic or fumed silica which are obtained after further processing of ground silica.
Crystalline silica in an inert, abrasive large particle, obtained from grinding sand or quartz. Diatomaceous earth or dolomite is obtained from sedimentary rock and contain upto 30% of organic matter and inorganic impurities such as sand, clay and soluble salts. Precipitated and pyrogenic silica are high surface area, fine-particle reinforcing filler. Figure below shows the structured of layered silica.
In 1976 Wagner reviewed the use of precipitated silica in rubber showing that unique properties were provided including (1) Tear, flex, abrasion and heat resistance (2) Hardness, stiffness and modulus (3) Adhesion (4) Low heat build-up (5) High resilience and (6) neutral colour. Combination “of surface treatment and compound processing were required to obtain some of these benefits. He further indicated that the silica physical properties which affected rubber performance were (1) Metal oxide content (2) Silanol content (3) Adsorbed water (4) Particle agglomeration (5) Structure (6) Ultimate particle size which is related to surface area. Increasing silica surface area, beneficially increase compound scorch protection. tensile and tear strength, ilex fatigue life and aged elongation. Increasing surface area adversely increases compound viscosity; heat build-up and cure time and decrease abrasion resistance.
Charactenstics of silica
1. Addition of silica in rubber tend to increase viscosity more rapidly than most fillers
2. Most frequently used accelerator systems are severely deactivated by silica filler due to their high surface and the adsorption on the filler surface due to porosity. As a result with loading of around 15 phr and above 1-2 phr of DEG or PEG are added to reduce the accelerator requirement
3. Compound viscosity and cure rate are dependent on the absorbed moisture of silica. Moisture in silica behaves as a psudo-plasticizer and cure activator
4. The early addition of silica along with Zno results in lower mooney viscosity, greater extrusion swell and lower modulus
5. Tear strength increases as the surface area of silica increases
6. Reinforcement with silica is enhanced with coupling agent and the most effective coupling agent is silane coupling agent which acts as a bonding bridge between silica and rubber
7. The moisture, which is driven off at 105 °C, must be carefully controlled in silica for rubber use. If it is reduced below 3% adequate dispersion in difficult to obtain
8. Silica is hydrophilic and is incompatible with non-polar rubber, whereas there is an affinity between carbon black and non-polar rubber
1.5 Comparison of silica and carbon black
Although rubber grade silica and carbon blacks are available in various particle size, there major difference is in the surface chemistry. Compared with carbon black whose surface area consists of a certain portion of unorganised carbon, but mainly graphitic basel planes with some functional group, mostly oxygen containing groups located on the edges and crystal defects, the silica surface consists of siloxane and silanol groups that are more polar and considerably more active chemically”. The relatively non-polar surface of carbon black is very compactable with the hydrocarbon polymers, while silica is less compactable with general-purpose polymers and gives much lower cohesive bonding force. Moreover, a higher population of silanol on the silica surface would lead to a strong H-bonding between silica aggregate and a stronger filler network in comparisons with its carbon black counterpart. Moisture too gets attaches to the silica surface through H-bonding. This strong filler network can give a rigid uncured compound that is difficult to process in extrusion and forming operation. In addition, the lower filler-polymer interaction of silica also results in a lower level of bound rubber in the compound.
Figure 4. Surface chemistry of silica particle.
For most of the precipitated silica used in the rubber industry the surface concentration of silanol group varies from 4-7 nm2. The Silanol (-Si-OH) groups are acidic in nature and reactive. Silanols show similarities to carboxylic acid groups in their reactions with amines, alcohols and metal ions. At elevated temperature, the silanol groups on the surface of silica will react with a number of chemical groups present in rubber compounds. Water absorbed on the surface of filler particle reduces the reactivity of silanols. During hot mixing, some of the absorbed water is removed, leaving a very reactive filler surface. If DEG or PEG is present in the recipe, it can replace the volatilised water and reduce the reactivity of filler surface. Figures 4 and 5 shows the surface chemistry of silica and carbon black
1.6. Surface modifications of silica
The introduction of silica in partial replacement of carbon black can cause cure retardation and this effect increase with silica surface area. Surface modification of silica is one of the most effective approaches of changing surface characteristics to meet application requirement. Two frequently practiced approaches used in the rubber industry are
a) Physical modification by adsorption of some chemicals on the filler surface
When certain chemicals are added to a silica compound, they may be strongly adsorbed on the surface via dispersive interaction, polar interaction, H-bonding and acid- basic interaction. Eg: Glycols, glycerols, secondary amines etc. Generally the polar or basic groups of these materials are directed towards the silica surface and the less polar or alkaline groups towards the polar matrix thereby increasing the affinity with the hydrocarbon polymer. Consequently the filler networking of silica can be substantially depressed resulting in better dispersion in the polymer matrix. Lower viscosity of the compound and lower hardness of the vulcanizate. With respect to the dynamic properties, this modification would result in a lower dynamic modulus.
b) Chemical modification
Two types of chemicals are used for surface modification
i) Grafts of chemical group on the filler surface to change the surface characteristics.
ii) Grafts that may react with the Polymer.
The latter are frequently called coupling agent or bifunctional coupling agent as they provide chemical linkages between the filler surface and polymer molecule. The former is referred as mo no functional even though no chemical reaction with the polymer takes place with these grafts. Of this, bifunctional coupling agent is commonly used in tyre industry. The bifunctional chemicals are a group of chemicals, which are able to establish molecular bridges at the interface between the polymer matrix and filler surface. This coupling agent enhances the degree of polymer - filler interaction by reacting with silanol, hence imparts improved performance properties to the filled materials. The most important coupling agent for inorganic filler modifications, silica in particular, is the group of bifunctional organosilanes with the general formula as
X(3-m) RmSi(CH2)nY
X - Hydrolysable group such as halogen, alkoxy or acetoxyl group
Y - Functional group which itself is able to react chemically with polymer either directly or through other chemicals. For Y groups important silane coupling agent include amino, epoxy, acrylate, vinyl and sulphur containing groups such as mercapto, thiocyanate and polysulphide.
The bifunctional silane coupling agent most often contain three (m=0) X groups and the functional group X is generally in the Y position (n=3).
Silane modified silica forms a lesser amount of filler networks to be broken and re-formed and thus the compound consumes less energy than the carbon black filled compounds. Best-cut growth resistance is obtained with relatively low silane modification and the maximum abrasion resistance is achieved with higher coupling agent modification. Silica without silane modification or modification at low levels can give enhanced tear and cut growth resistance. Adhesive bond strength is usually improved with silica and may be primarily due to improved tear and cut growth at the adhesive interface. Either with or without silane modifications silica generally gives improved ageing resistance.
The main functions of coupling agents are :
1. To modify the filler surface to reduce the filler-filler interaction
2. Introducing covalent bonds between filler surface and polymer chain to strength polymer-filler interaction
3. Generating higher bond rubber content to prevent filler flocculation
4. To modify the filler surface to improve wetting and dispersion and reduces the tendency to tie up ingredients of cure system.
The commonly used silane coupling agent in sulphur cured compound filled with non-black filler are A-189 mercapto silane (from OSI), Si 264 thiocyanatosilane and Si-69 tetrasulfide silane (from Degussa).
The methoxy or ethoxy group react during mixing with the silanol groups on the surface of silica to give a strong bond. Alcohol is released as the by-product of the reaction. The sulphur containing groups of each structure react with rubber molecules during mixing and vulcanization to give mono, di and polysulphidic covalent bonds and hence reduce filler-filler network. The final silane coupling bond is as shown below.
The use of the combination of bis[3-triethoxy silyl propyl] - tetra sulphide (TESPT) commonly known as Si-69 and silica to reduce the rolling resistance of truck tyre treads was first reported by S.Wolff in 1986. It was reported by him that rolling resistance was reduced by as much as 30%, wet traction remained virtually unchanged and tread wear index decreased only 5%, when a silane modified precipitated silica was used to entirely replace N220 black in a NR truck tread. Below shown is the structure of si-69.
It is well known that in the absence of silane coupling agent, silica perturbs the sulphur / accelerator cure mechanism, resulting in increased scorch time, slower cure rate and decreased crosslink density. Wolff showed that in the presence of TESPT, the compound Mooney viscosity and rheometer minimum torque of silica containing compounds are reduced. In addition cure characteristics are normalised, resulting in an increased rheometer delta torque, reduced elongation, increased high strain modulus and reduced heat build-up. Bayers demonstrated that a desirable balance of passenger tread properties was obtained when a silane-coupling agent at moderate level was combined with increased accelerator. Bice et.aL, used a composite model truck tread formulation to show that increasing the accelerator level as either silica loading is increased, diminished the effect of the silane coupling agent upon cure related compound physical properties. In that work TESPT levels of 6,8 and 10 % wt. of silica was used. The silica-silane tread compounds demand additional mixing time and temperature to achieve their improved performance.
The uniqueness of TESPT is that, it can be considered part of the cure system, since it contributes some additional sulphur to the compound and therefore should also be treated as a co-curing agent when sulphur cure system are used. It is also the largest molecule and thus requires more time and temp during mixing for adequate reaction with silica filler.
Ever since the modification of silica surface, a lot of work has been going to improve the surface of Carbon black. Even then, the improvement of dynamic properties, hysterisis in particular of carbon filled rubber compounds by coupling reaction was not as appreciable as the case of silica-filled vulcanizate. The difference in effectiveness of the coupling modification between Carbon black and silica may be associated with its micro structure. Silica is an amorphous material and the silanols are randomly distributed on the silica surface, so that coupling agent are spread uniformly over the surface, which lead to better surface coverage. In the case of carbon black, the functional groups are located only on the edges of the graphitic basel plane of the crystalities, so that coupling agent grafts are located only on the edges of the graphitic basel plane which result in poor surface coverage. The higher concentration of the reactive functional group and their random distribution over the surface would be a key advantage of silica over carbon black for coupling reaction.
1.7 Application of silica on tread compounds
The concept of using highly dispersable silica as the sole filler, together with a silane-coupling agent, for the tread compound of low rolling resistance tyre was patented by Michelin in 1991. Silica was able to replace upto 100% of carbon-black in shoe sole compound, but its use in tyre compounds had been limited to two types of compounds namely, OFF THE ROAD tread compounds to improve chipping and chucking resistance and TEXTILE AND STEEL CORD BONDING COMPOUND for enhancing adhesive between the cord surface and rubber material. Even in these compounds silica is blended at a low loading. The reason that it cannot completely replace carbon black as the main filler in tyre compounds especially in tread compound is that, besides its poor cure characteristics and poor processability, it imparts very low failure properties to the tilled rubber due to weak polymer-filler interaction and strong filler-filler interaction.
Use of precipitated silica in the treads of large tyre has been reported to improve both appearance and abrasion resistance on bad roads. “Wolff showed that use of precipitated silica coupled with the bifunctional silane bis[3-triethoxysilyl propyl] tetra sulphide improved the abrasion resistance of earth mover tyre tread compound to level equivalent to that of conventional compound. Adjusting the compound curatives was successful in optimising precipitated silica containing off the road tread compound physical properties. Davier and Lionnet showed that the silane activation of precipitated silica benefited the modulus and abrasion resistance values of NR off-the road tread compound. Increasing the primary sulphenamide level and using a lower sulphur level also obtained very good overall performance. Walker reported that partial replacement of NR with SBR and carbon black with precipitated silica and use of semi EV cure system enhanced performance of a NR based off-the road tread compounds in lab as well as in field.
Use of precipitated silica in NR and SBR agricultural tyre tread compounds increased resistance to chipping and chunking. Wolff performed tyre testing of carbon black filled NR treads containing at least 10 phr precipitated silica without use of a coupling agent and obtained good abrasion resistance.
Precipitated silica used in combination with carbon black improves the performance of a truck tyre tread compound. Chakravarthy and coworkers has found that use of 30 phr precipitated silica and a mercapto silane coupling agent at 1% as a direct replacement for carbon black increased the resistance to cutting and chipping. Higher levels of silica could be used without a significant sacrifice in heat build-up and tread wear by using the mercapto silane coupling agent.
Ahmed and Schacfer showed that the rolling resistance of a passenger tyre tread of styrene -butadiene / butadiene rubber was reduced about 25% without a substantial loss in wet or dry traction, by using up to 36 phr of precipitated silica and a mercapto silane coupling agent at 3% of the silica level in a 72 phr total filler system. Wolff reported that with the bis[3-triethoxy silyl propyl] tetra sulphide coupling agent a compound with 20 phr precipitated silica and 40 phr N339 carbon black gave 9% lower rolling resistance with negligible changes in tread wear and wet traction
1.8. Carbon Silica Duel Phase Filler CSDP Filler or Ecoblack Filler
After the patent by Michelin in 1991 for using highly dispersible silica as the sole filler together with silane coupling agent for getting low rolling resistance, carbon black manufacturer's developed various concepts to match the performance of silica with new type of carbon blacks. One interesting approach is the joint combination of mineral oil and silica compound in a modified furnace process, which result in particles having silica domain dispersed in carbon phase.
The CSDP filler or Ecoblack filler is a non-standard carbon black and is developed by M/s. Cobat corporation. This new filler consists of a silica phase distributed in the carbon phase with the level of this silica content being one of the variables in making different product. The performance of this Ecoblack filler is very close to that of the new grades of highly dispensable silica. The use of this new CSDP filler requires same different compounding and processing technique to optimise the performance.
When added to hydrocarbon rubber, this filler is characterised by higher filler-polymer interaction in relation to a physical blend of carbon black and silica and lower filler-filler interaction in comparison with either conventional carbon black or silica having comparable surface area. In CSDP filler there would be less hydrogen-bonding, the main cause of the higher filler-filler interaction between silica aggregates and between the silica domains on neighbouring aggregates, since their average interaggregate distance would be greater. Here since filler-filler interaction is lower, less amount of coupling agent is required to being the same level of hysterisis as compared to silica compound. Unfortunately these silicas shows some disadvantages in processing like high compound viscosities, storage hardening and extrusion difficulties like pressure and temperature, scorch and poor surface quality, especially at the edges of the trend
2,3 TESTING
2.3.1 Characterisation of Compounds
The compounds can be characterised by the following tests.
A) Mooney Viscosity Test
The mooney test, which is used as a routine test to asses the processability of raw stock and compounds, was carried out for all the compounds in accordance with ASTM D 1646-81. The equipment used was MONSANTO Co. USA make model MV- 2000. Here the sample is sheared by a rotating disc in a shallow cylindrical cavity as it cures. The rotor speed is 2 rpm. The surfaces of the disc and of dies which form the cavity are serrated in a grid pattern to grip the rubber mechanically. The optimum test specimen consists of two pieces which will fill the cavity completely, one sample is placed above the rotor and the other beneath it.
The specifications of the testing was,
Temperature of the die cavity 100°c
Rotor size Large
Pre-heating time 1 minute
Set time 4 minutes
Total test time : 5 minutes
The results can be expressed as ML (1 + 4) 100°C.a typical mooney chart is shown below.
The results were obtained as a plot of time verses viscosity in mooney units. Actually torque required to move the rotor is measuring there. Viscosity at 5 minutes was taken for analysis. Mooney viscosity gives some idea on the extrusion properties of the compound. But it is not simulating to the actual shear rate in processing operations.
b. Rheometer Properties
Monsanto Oscillating Disc Rheometer (ODR- 2000)
The cure characteristics of the compounds were determined using ODR 2000 as per ASTM D-2084. The cure rate, state of cure and processing characteristics can be understood from this test. Here the sample is subjected to a constant amplitude of shearing as it cures. The torque required to oscillate the rotor which is embedded in the sample confined to the die cavity under pressure and controlled temperature is measured. As the curing proceeds, the torque required to shear the rubber increases and a curve of torque verses time is obtained. A typical rheograph is shown below
The parameters obtained from the rheograph are:
Minimum Torque (Mj)
It is a measure of viscosity of the stock at the test temperature. The rheometer minimum torque is proportional to mooney viscosity when both the tests are conducted at the same temperature
Maximum Torque
It is a measure of stiffness or modulus at the test temperature. It is also an effective measure of changes in tensile modulus and cross link density.
Induction Time (Ts2)
At normal rhenometer temperature the induction time is a measure of the time available for mould flow. It is also a measure of processability similar to mooney scorch.
Optimum Cure Time (Tc 90)
It corresponds to the achievement of 90% of maximum cure. It is calculated as, Time for 90% of maximum cure, Tc 90 = (M L- MH) x 0.9 + ML
Cure Rate Index
Cure rate index corresponds to the rate of cure. It is calculated from the Ts2 and T90 values as,
Cure rate index = 100 / T90- Ts2
2,3.2 Physical Test Methods
Sampling, Sample Preparation and Moulding
Sample is taken approximately from the centre of the banbury compound. It is then kept for 24 hour maturation. After maturation, the sample is cured in a single cavity two piece mould
Curing temperature : 150°c
Curing time : 25 rnin
Curing pressure : 11/2 tonne
5 test samples from different parts of cured sample is tested and average value is calculated.
Moulding
The final compounds were aged for 24 hours before moulding. The compounds were sheeted out at the required thickness using laboratory mill. The sample is then cured in a single cavity two piece mould. Moulding is done in a steam heated pneumatic press at 150°C at a pressure of 1 ½ tonne for 25±5 minutes. The moulded piece were kept for 24 hours maturation.
a. Tensile Stress-Strain Properties
The stress strain values were measured as per ASTM D-412. Dump bell
samples were stumped from cured sheet parallel to the grain direction using dump bell die (C-type). The cross head speed was 500 mm per minute. The thickness of the specimen was measured and feed to the machine so that the following parameters were obtained.

I.
Tensile Strength
This is defined as force per unit area of original cross sectional area required to rupture the sample.
Maximum Torque
It is a measure of stiffness or modulus at the test temperature. It is also an effective measure of changes in tensile modulus and cross link density.
Induction Time (Ts2)
At normal rhenometer temperature the induction time is a measure of the time available for mould flow. It is also a measure of processabihty similar to mooney scorch.
Optimum Cure Time (Tc 90)
It corresponds to the achievement of 90% of maximum cure. It is calculated as, Time for 90% of maximum cure, Tc 90= (ML-MH ) x 0.9 + ML
Cure Rate Index
Cure rate index corresponds to the rate of cure. It is calculated from the Ts2 and T90 values as,
Cure rate index = 100 / T90- Ts2
2.3.2 Physical Test Methods
Sampling, Sample Preparation and Moulding
Sample is taken approximately from the centre of the banbury compound. It is then kept for 24 hour maturation. After maturation, the sample is cured in a single cavity two piece mould.
Load at break
Tensile strength = (KG/cm2)
Initial cross sectional are
II. 300% Modulus
Modulus is the stress at a particular strain. It is calculated as follows
Load at 300% elongation
300% modulus (KG/cm2)
Initial cross sectional area
III. Elongation at Break
Elongation describes the ability of the rubber to stretch without breaking. Elongation at break is calculated as
Length al break- Initial length
Elongation at break (%) = x 100
Initial length
b. Tear Strength
Tear strength was measured as per ASTM D-624 with unnecked 90° angular specimen using Instron universal Tester. ASTM die-c was used for cutting samples. Cross sectional speed was 500 mm per minute. The result is expressed in kg/cm. The gauge of the specimen were measured and fed to the instrument.
Load required to tear the specimen
Tear strength - (Kg/cm)
Specimen thickness
c. Hardness
Hardness is the modulus at low strains. Hardness of the vulcanizate was measured with the help of Shore - A Durometer as per ASTM D-2240, The instrument consists of a calibrated spring to provide the indenting force. The Load imposed by the spring varies with the indentation. Reading was taken after 10 seconds of the indentation when firm contact had been established with the specimen and the mean value of three measurements is reported. Minimum thickness of the specimen for the last test is 4 mm.
2.3.3 Dispersion Analysis
The carbon black dispersion is measured in a carbon dispergrader (model 1000 NT) manufactured by OPTIGRADE AB, Sweden. This is an instrument for control of filler distribution and the presence of large agglomerates in all types of black rubber. This is a test equipment, which determines the degree of dispersion of carbon black in the rubber compounds by means of a split field microscopic technique as per ASTM 2663-88 method B. It rates the test piece against a set of transparencies or electronically stored standard (G scale). The equipments presents sample characteristics in a matrix where the X value represents filler distribution and Y value represents the presence of large agglomerates on a scale of 1-10.
X value: Classification of filler Distribution
For most products, wearing properties and the need for increased process homogeneity are reasons enough for regular dispersion testing. The X value is based on an image comparison with a set of ten reference pictures. A rating of 1 represents poor dispersion while a rating of 10 represents excellent dispersion.
Y value: Agglomerate Count
For some applications, the absence of large agglomerates might be the main concern. The presence of large agglomerates might cause surface defects and / or fatigue problems. The Y-value is based on size and number of surface irregularities with a diameter above 23 µm. A rating of 1 represents the practical maximum number of large agglomerates. While a rating of 10 represents the total absence of agglomerates above 23 µm.
The equipment makes use of the fact that in a compound in which ingredients are well dispersed, light is reflected from a freshly cut surface, revealing a smooth, unblemished texture. The presence of improperly dispersed ingredients is shown by irregularities which usually take the form of a circular, convex bumps or convex pock marks on the surface, and their presence indicates a less than perfect dispersion of the compounding ingredients. The size and frequencies of these irregularities may be used to judge the degree to which the compound falls short of optimum dispersion. A set of 10 standards based on size and frequency of these irregularities has been established to which numerical ratings has been assigned.
Below is a optigrader 1000NT used for the analysis of dispersion of both silica & carbon black..
Fig: An optigrader 1000NT
EXPERIMENTALS
Studies were done on compounds by
1) Varying the NR:BR ratio .
2) Varying the RSS grade used.
3) Varying the oil injection temperature.
4) Using TBBS instead of MBS.
Studies were mainly done on dispersion of both silica & carbon black by conducting the above trials. How ever other parameters like viscosity of master, remill & final were also studied using a mooney viscometer. All the Rheo properties were examined with a mosanto rheometer. Effect of oil injection temperature was studied at three different temperatures at 140 ,145 ,150ºc. This was done in ban#4 with examining all other parameters.
An attempt to improve the dispersion of silica was done by varying the accelerator used. i.e using same quantity of TBBS instead of MBS.
For study 4 compounds namely A,B,C,D were studied.
A
NR:BR(55:45)
B
NR:BR(50:50)
C
NR:BR(50:50) RSS4
D
NR:BR(50:50) RSS4+RSS5
FORMULATIONS USED
INGREDIENTS
A
B
NR
55
50
BR
45
50
STEARIC ACID
4.5
4.5
ZnO
4.5
4.5
PCTS(PEPTIZER)
0.09
0.09
SASOLWAX
3
3
6PPD
4.5
4.5
SILICA
2
2
N220
30
30
AROMATIC OIL
7.4
7.4
SULPHER
1.48
1.48
CTP
0.20
0.20
MBS
0.969
0.969
In the case of compounds C & D, the only change in the formulation is the usage of RSS 4 & RSS 5,for checking the dispersion of carbon black & silica and also analyzing their physical properties.

RESULTS & DISCUSSIONS
MOONEY VISCOSITY VALUES OF MASTER , REMILL & FINAL
ML(1+4)@100°C(MASTER)
SL NO
ML(1+4)@100°C(REMILL)
SL NO
ML(1+4)@100°C(FINAL)
1
A
B
C
D
1
A
B
C
D
1
A
B
C
D
2
75
81
91
82
2
65
69
71
64
2
59
52
64
56
3
81
82
86
88
3
67
68
71
61
3
60
53
62
58
4
82
79
83
82
4
67
70
68
66
4
61
51
62
57
5
82
77
84
81
5
70
69
71
66
5
58
51
65
55
6
81
78
87
83
6
68
68
75
64
6
59
52
65
57
AVG
80.2
79.4
86.2
83.2
AVG
67.4
68.8
71.2
64.2
AVG
59.4
51.8
63.6
56.6
CBD VALUES OF COMPOUDS AT DIFFERENT OIL INJECTION TEMPERATURES
9.7
CBD @ O/I 140°C & DUMP 165°C
CBD @ 145°C O/I & DUMP 165
CBD @ 150°C O/I & DUMP 165



F MODE
G MODE
F MODE
G MODE
F MODE
G MODE

X
Y
X
Y
X
Y
X
Y
X
Y
X
Y

6.1
9.5
8.6
9.5
6.5
9.6
9.3
9.6
7.3
9.8
9.9
9.8

5.9
9.5
8.4
9.5
6.7
9.7
9.3
9.7
7.2
9.7
9.8
9.7

6.3
9.5
8.8
9.5
6.9
9.6
9.8
9.6
7.3
9.7
9.9
9.7

5.5
9.3
8
9.4
6.9
9.7
9.7
9.7
6.9
9.6
9.8
9.6

5.8
9.4
8.3
9.4
6.9
9.7
9.9
9.7
7.2
9.8
9.7

5.6
9.4
8.8
9.4
6.4
9.6
9.1
9.6
7.2
9.7
10
9.7

5.6
9.4
8.1
9.4
6.5
9.7
9.1
9.7
7
9.7
9.8
9.7

6
9.4
8.4
9.4
6.7
9.7
9.5
9.5
7
9.8
9.7
9.8

5.7
9.3
8
9.3
6.8
9.6
9.7
9.6
6.9
9.7
9.8
9.7

5.7
9.4
8.2
9.4
7
9.7
9.8
9.6
7.2
9.8
9.9
9.8

5.6
9.3
8
9.3
6.9
9.7
9.8
9.6
7.3
9.8
9.8
9.8

6.3
9.4
8.9
9.4
6.9
9.6
9.8
9.6
7.4
9.8
9.8
9.8

6.1
9.5
8.7
9.5
6.8
9
9.7
9.6
7
9.8
10
9.8

5.8
9.4
8.3
9.4
6.4
9.4
9
9.4
6.9
9.8
9.8
9.8

5.9
9.5
8.5
9.5
6.7
9.6
9.4
9.7
7.4
9.7
10
9.7

6.3
9.5
8.9
9.5
6.8
9.6
9.6
9.6
6.9
9.8
9.9
9.7

5.8
9.5
8.4
9.5
6.6
9.6
9.4
9.6
7
9.7
9.9
9.8

5.8824
9.4235
8.4294
9.4294
6.744
9.594
9.524
9.612
7.124
9.741
9.859
9.7412

CARBON BLACK DISPERSION VALUES FOR NR:BR RECIPES
CBD FOR 55:45 (NR:BR)RECIPE



CBD VALUES(F MODE)

SL NO
X
AVG
Y
AVG

1
6.8
7.2
7.8
6.4
6.5
6.94
9.6
9.7
9.8
9.6
9.7
9.68

2
6.9
7.2
7.6
5.8
7.3
6.96
9.7
9.7
9.8
9.6
9.8
9.72

3
6.3
5.9
7.3
7.4
7.1
6.8
9.6
9.6
9.8
9.7
9.8
9.7

4
7
6.8
7.3
6.8
7.3
7.04
9.7
9.7
9.8
9.7
9.7
9.72

5
7
7.7
6.3
7.1
6.8
6.98
9.7
9.8
9.6
9.7
9.7
9.7

TOTAL AVG
6.944
9.704

CBD VALUES(G MODE)

SL NO
X
AVG
Y
AVG

1
9.8
10.1
10.5
8.9
9.2
9.7
9.6
9.8
9.8
9.6
9.7
9.7

2
9.6
10.1
10.5
8.4
10.3
9.78
9.7
9.7
9.8
9.6
9.8
9.72

3
8.9
8.2
10.4
10.5
10.3
9.66
9.6
9.6
9.8
9.7
9.8
9.7

4
9.8
9.6
10.1
9.6
10.3
9.88
9.7
9.7
9.8
9.7
9.7
9.72

5
9.9
10.5
8.6
9.9
9.5
9.68
9.7
9.8
9.6
9.7
9.7
9.7

TOTAL AVG
9.74
TOTAL AVG
9.708


CBD FOR 50:50 (NR:BR)RECIPE



CBD VALUES(F MODE)

SL NO
X
AVG
Y
AVG

1
5.9
5.9
6.3
6.3
6.1
6.1
9.2
9.2
9.5
9.5
9.4
9.36

2
6.1
5.5
5.4
6
5.8
5.76
9.4
9.1
9.2
9.4
9.4
9.3

3
6.1
6
6.3
6.4
5.9
6.14
9.4
9.4
9.5
9.5
9.4
9.44

4
5.8
5.3
5.3
5.3
5.3
5.4
9.3
9.3
9.2
9.3
9.3
9.28

5
5.3
5.4
5.4
5.6
5.5
5.44
9.4
9.3
9.5
9.4
9.4
9.4

TOTAL AVG
5.768
TOTAL AVG
9.356

CBD VALUES(G MODE)

SL NO
X
AVG
Y
AVG

1
8.4
9
9
9.1
8.8
8.86
9.3
9.4
9.5
9.4
9.4
9.4

2
8.8
7.8
7.9
8.6
8.2
8.26
9.4
9.1
9.1
9.4
9.4
9.28

3
8.7
8.5
9
9
8.4
8.72
9.4
9.4
9.5
9.5
9.4
9.44

4
8.3
7.6
7.5
6.2
7.7
7.46
9.3
9.2
9.2
9
9.3
9.2

5
7.6
7.7
7.7
8.8
8.8
8.12
9.4
9.3
9.5
9.4
9.4
9.4

TOTAL AVG
8.284
TOTAL AVG
9.344

EFFECT OF MBS & TBBS ON DISPERSION OF SILICA & CARBON BLACKS.
T 718 WITH MBS(530)
F MODE
G MODE
X
Y
X
Y
7.2
9
8.4
9.2
7.4
9.4
8.6
9.1
7.4
9
8.2
9.3
7.3
9.3
8.7
9
7.2
9.1
8.3
9.1
T 718 WITH TBBS(732)
F MODE
G MODE
X
Y
X
Y
7.6
9.1
8.9
9.3
7.4
9.1
9
9.1
7.3
9.3
9
9.1
7.4
9.2
9.2
9.2
7.5
9.1
9.1
9.1
CONCLUSION
CONCLUSION
Dispersion of both silica & carbon blacks are very essential for the superior properties of the vulanizates. With the increase in the NR content , almost all the physical properties like tensile strength, tear strength, modulus,& hardness was found to increase.
This can be attributed to the fact that ,the affinity of carbon black & silica is more with NR than BR. An increased dispersion rating of 6.9 & 9.7 for carbon black & silica was observed in case of 55:45 (NR:BR) compounds. With the change in RSS grade,dispersion was almost similar but an increase in properties was observed in vulcanizates with RSS4 when opposed to those having RSS 5.
Oil injection plays a crutial role in deciding the levels of dispersion for NR:BR compounds this was analysed at three different temperatures 140,145,&150ºc. It was observed that as the oil injection temperature increases the dispesion was found to increase but the cycle time for the batch also increases, so we need to optimise the oil addition temp so as to meet our productivity & profit.
Finally an attempt was made to improve the dispersion of silica in rubber compounds just by changing the primary accelerator used. The usage of TBBS instead of MBS in same levels was found to increase the dispersion of silica only.a dispersion rating of 8.4 was observed with MBS & 9.04 in case of TBBS.






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