1. Compounding Ingredients and Formulation Construction
1.3 Fillers
1.3.3 Non-Black Fillers
1.3.2.2.4 Hysteresis Compounding
A novel approach to compounding for equal hysteresis can be taken by using the follow- ing equation:
(phr black/phr total)2 × N2SA = Φγ′ factor
The Φγ′ factor of the existing compound and the compound containing the new carbon black of different surface area is solved and then compared.
Table 1.12 will give an idea about the requirements of different carbon blacks in the dif- ferent components of tires.
Control of quality during the production of carbon black is mandatory to get the actual performance in rubber compound. Various quality control tests of carbon black are shown in Table 1.13.
In the last few years, research into changing the basic properties of carbon black has taken place. Modification of the carbon black surface is one such research activity. This modification is generally being done through process modification. Some of the post- process modifications are oxidation, plasma treatment, and polymer grafting. There are also a few in-process modifications that were studied by several workers who had devel- oped blacks called inversion black, carbon-silica dual phase fillers.
TABLE 1.12
Use of Different Carbon Blacks in Tire Components Tire Component
Component Property
Requirement Required Carbon Black Passenger tread compound Treadwear, dry and wet traction,
low hysteresis, resistance to cutting and chipping
N220, N234, N299, N339, N351
Light truck tread compound Same as passenger tire Same as passenger tire Truck/OTR tread compound Less heat generation, resistance to
chipping and chunking N110, N121, N134, N220, N231, N330, N339 Carcass compound Good adhesion, green strength,
resistance to tear and flex fatigue, good calendering properties
N330, N326, N351, N660
Belt-skim compound Good adhesion to the textile or steel cord, resistance to tear and flex fatigue, low hysteresis, relatively high modulus and retention of properties after prolonged aging
N330, N326, N351, N550
Sidewall compound Resistance to flexing, cutting, and weathering and good extrusion properties
N330, N326, N351, N550, N650
Inner-liner compound for tubeless
tire Air retention, resistance flex and
aging, compound green strength, good calendering characteristics
N550, N650, N660
Bead insulation/bead filler
compound High modulus and hardness, low
die swell, good tack, and adequate adhesion to the wire both in green and cured conditions
N326, N550, N660
TABLE 1.13
Quality Control Tests of Carbon Black and Its Criticality
Test Parameters Critical Use Significance
Iodine adsorption number Required for specification Surface area Nitrogen surface area Required for critical application Total surface area Oil absorption number Required for specification Structure
Compressed oil number Required for critical application Compressed structure STSA* Required for critical application External surface area CTAB surface area Required for critical application External surface area Pellet hardness Required for specification Pellet strength
Fines content Required for specification Dust level, handling issue Heat loss Required for specification Moisture level
Sieve residue Required for specification Presence of contaminants Ash content Required for critical application Presence of inorganic materials Tinting strength Required for critical application Fineness of black
*Statistical Thickness Surface Area
Among the various non-black fillers, silica plays a dominant role. The nature, synthesis, properties, and applications of various fillers are briefly mentioned here.
1.3.3.1 Silica Fillers
• Silicas are highly active light-colored fillers.
• Chemically these are made from silicic acids.
• Silica can be manufactured by two processes:
• Solution or precipitated process giving rise to precipitated grades
• Pyrogenic process giving rise to fumed or pyrogenic grade
• For the rubber industry use, mostly precipitated types are used since pyrogenic silica is too active and expensive.
1.3.3.1.1 Precipitated Silicas
• Alkalisilicate solutions are acidified under controlled conditions.
• The precipitated silicic acid (silica) is washed out and dried.
• The activity of the silica fillers depends on the condition of preparation; the prod- ucts with highest activity are pure silicic acids with large surface area.
• These silicas are silicon dioxide containing 10 to 14% water, with particle size in the range of 10 to 40 nm.
1.3.3.1.2 Pyrogenic or Fumed Silicas
• Silicon tetrachloride is reacted at high temperature with water.
SiCl4 + 2H2O → SiO2 + 4HCl
• The reaction products are quenched immediately after coming out of the burner.
• Fumed or pyrogenic silica thus produced contains less than 2% combined water and particle size smaller than precipitated grades.
1.3.3.1.3 Characteristics of Silica Fillers
• Silica is an amorphous material having a tetrahedral structure of silicon and oxygen.
• Particle size ranges from 1 to 40 nm, and surface area ranges from 20 to 300 m2/g.
• Silica is hygroscopic and hence requires dry storage conditions.
• Surface silanol concentration (-Si-O-H) influences the degree of surface hydration.
• Surface activity is controlled by hydroxyl groups on the surface of the silica. This surface activity has an effect on peroxide curing but has no significant effect on sulfur curing.
• All precipitated silicas contain a certain amount of moisture since the time of man- ufacture. This moisture content appreciably influences the processing and vulca- nizing properties of rubber compounds.
All of the above parameters are also true for silicate fillers like calcium silicate, aluminum silicate, magnesium silicate, and sodium aluminum silicates.
1.3.3.1.4 Effect of Silica Fillers on Rubber Compound 1.3.3.1.4.1 Processing Properties
• With increasing surface area, the incorporation of fillers in a rubber compound becomes difficult, and compound viscosity increases considerably.
• If substances that are adsorbed by the silicas are added in silica compounds, com- pound viscosity reduces and processing becomes easier. Additive of this kind includes accelerators like Diphenyl guanidine (DBG), Diorthotolyl guanidine (DOTG), Hexamethylene tetramine (HMT); glycols like diethylene and triethylene glycols; amines like triethanolamines, dibutylamines, cyclohexyl, and dicyclo- hexylethyleneamines; etc. These additives not only facilitate processing but also reduce acceleration adsorptions.
• As amounts of additives are normally calculated in terms of rubber, the filler activators are also calculated in terms of filler amount. For example, when Mercaptobenzothiazyl disulfide (MBTS) is used as the main accelerator, hexameth- ylene tetramine, DPG, or DOTG is recommended as additional accelerator with the amount being 4 to 6 phr on 100 phr of silica.
• Silanes are also suitable silica activators. The difference is that while the earlier acti- vator used to block the adsorptive centers on surfaces of the filler, silanes take part in a chemical reaction with the silanol groups of the silica. This influences the filler- rubber interaction markedly, due to which the compound viscosity is reduced and a fairly large amount of silica fillers can be incorporated in the rubber matrix.
1.3.3.1.4.2 Vulcanizate Properties
• There is comparatively high hardness along with low modulus values.
• Tear property improves. The situation is exploited in off-the-road (OTR) tires, where 10 to 15 parts of silica are added along with reinforcing blacks.
• Precipitated silica fillers play an important role in bonding systems, catalyzing the resin formation or resin-rubber interaction.
• Precipitated silicas often give better hot air resistance than carbon blacks.
• Under dynamic conditions, compounds containing precipitated silicas have lower loss factors than carbon blacks, and this result is exploited in tires with a carbon black/silica blend along with a silane activation system, where improved reversion resistance, better retention of tear, and tear propagation resistance are obtained.
• The benefits of reinforcement of silicas/silanes in certain blends of solution SBR and BR are translated into a new generation of tires having considerable lower rolling resistance. These tires are called “green tires.”
1.3.3.2 Other Filler Systems 1.3.3.2.1 China Clay
China clay can be classified into four varieties:
Soft clay (particle size greater than 2 millimicron)
• Used mainly in mechanical goods as semi-reinforcing fillers
• Hardness and tensile strength are greater but resilience is less as compared to calcium carbonate
• Not used in bright-colored articles
Hard clay (particle size less than 2 millimicron)
• Give better hardness, tensile strength, tear and abrasion properties than soft clays
• Compounds have high electrical resistivity
• Used in mechanical goods, hose, flooring, and shoe soling Calcined clay
• Hard clays calcined to remove combined water
• Hardness, tensile strength, and electrical resistivity higher than hard clays
• Used where color or electrical properties are important Treated clay
Three types are available:
− Amine coated
− Silane coated
− Polybutadiene coated
This variety of the clay gives greater reinforcement than untreated grades and finds application in high-grade mechanical goods.
1.3.3.2.2 Ground Chalk or Whiting
• A white powder of particle size below 30 nm
• Used in low-cost compounds
• Gives moderate hardness and fairly high resilience at high loadings with poor tensile strength and tear properties
1.3.3.2.3 Limestone (Calcium Carbonate)
• Particles below 100 mesh
• Used where low cost is the prime consideration
• High loadings can give moderate hardness 1.3.3.2.4 Barytes (BaSO4)
• Used as inert filler to reduce cost
• Purer grades produced by precipitation of barium salts are used in pharmaceuti- cal products
• Used in chemical-resistant compound for tank lining as an inert filler 1.3.3.2.5 Titanium Dioxide
• Finds extensive use in white and other colored products where appearance is important
• Used in white sidewall for passenger tires, hospital accessories, floor tiles, etc.
1.3.3.2.6 Mica Powder
• Natural mica, washed and ground to pass through 200 to 300 mesh
• A filler imparting good resistance to heat and lower permeability to gases
• Finds use in inner tube for tires
1.3.3.2.7 Talc or French Chalk (Magnesium Silicate)
• Used as an inert filler in heat-resistant compounds like gaskets
• Talc also widely used as a lubricant to prevent uncured stocks from sticking to themselves or other surfaces
1.3.3.2.8 Aluminum Hydroxides
This material is used less for its filler effect but more for its ability to split off water at high temperatures as a flame retardant and finds application in cable industries.
1.3.3.2.9 Fibrous Filler 1.3.3.2.9.1 Asbestos
• Asbestos is a naturally occurring siliceous fiber
• Used as short fibers or ground material in flame-resistant or heat-resistant com- pounds, in gaskets, brake shoes, etc.
1.3.3.2.9.2 Cellulose Fiber
• Hardwood fibers chemically surface treated to make them compatible with rubber
• Capable of producing very high modulus vulcanizates at 60 phr loading 1.3.3.2.9.3 Flocks
• Short fibers of cotton, rayon, or nylon
• Increase tensile strength, tear resistance, and abrasion resistance of vulcanizates
• Used in shoe soles and similar products