1. Compounding Ingredients and Formulation Construction
1.3 Fillers
1.3.2 Compounding with Carbon Black
At optimum loading the effects of carbon black particle size and structure on rubber com- pound processing and properties are shown in Tables 1.5 and 1.6.
The effect of increased loading is as follows:
On processing properties:
• Increase in viscosity of a compound
• Decrease in die swell, better surface finish
• Extrusion rate goes through an optimum
• Decrease in calender shrinkage and better surface finish
• Increase in green strength and decrease in green tack
• Decrease in scorch safety unless the filler is a retarding type On vulcanizate properties:
• Tensile strength, tear strength, and abrasion resistance go through an optimum
• Elongation at break, resilience, and volume swell in fluids decrease
• Hardness, modulus, and compression set increase
Carbon black changes the hardness of compounds, which is measured by Shore A durom- eter. Often compounds are designed by following the increase or decrease of hardness.
Tables 1.7 and 1.8 give the base durometer hardnesses for various rubbers and changes of hardness by the addition of fillers or softeners.
1.3.2.2 Equal Hardness Approach
Table 1.9 can be used to handle any change in carbon black while maintaining the hardness.
To determine the approximate loading of N660 for use as a replacement for MT at equal hardness, multiply the MT loading by 0.60.
To determine the approximate loading of N762 as a replacement for FEF at equal hard- ness, multiply the FEF loading by 1.22.
TABLE 1.4
Summary of Typical Properties of Carbon Black Industry
Name
ASTM No.
Iodine No.(mg/g)
Nitrogen (mg/g)
CTAB (mg/g)
DBP (cc/100 g)
CDBP (cc/g)
SAF N110 145 143 126 113 98
ISAF LS N210 118 120 113 78 75
ISAF N220 121 119 111 114 100
ISAF LM N231 121 117 108 92 86
ISAF HS N234 120 126 119 125 100
HAF LS N326 82 84 83 72 69
HAF N330 82 83 83 102 88
HAF HS N339 90 96 95 120 101
HAF HS N347 90 90 88 124 100
FEF N550 43 42 42 121 88
GPF HS N650 36 38 38 122 87
GPF N660 36 35 35 90 75
SRF LS N762 27 28 27 65 57
SRF HS N765 31 31 33 115 86
SRF HM N774 29 29 29 72 62
TABLE 1.5
Effect of Carbon Black Particle Size and Structure on Compound Processing
Processing Properties Decreasing Particle Size Increasing Structure
Loading capacity Decreases Decreases
Incorporation time Increases Increases
Oil extension potential Little Increases
Dispersibility Decreases Increases
Mill bagging Increases Increases
Viscosity Increases Increases
Scorch time Decreases Decreases
Extrusion shrinkage Decreases Decreases
Extrusion rate Decreases Little
Extrusion smoothness Increases Increases
TABLE 1.6
Effect of Carbon Black Particle Size and Structure on Vulcanizate Properties
Vulcanizate Properties Decreasing Particle Size Increasing Structure
Rate of cure Decreases Little
Tensile strength Increases Decreases
Modulus Increases to maximum then
decreases Increases
Hardness Increases Increases
Elongation Decreases to minimum then
increases Decreases
Abrasion resistance Increases Increases
Tear resistance Increases Little
Cut-growth resistance Increases Decreases
Flex resistance Increases Decreases
Resilience Decreases Little
Heat buildup Increases Increases slightly
Compression set Little Little
Electrical conductivity Increases Little
TABLE 1.7
Base Hardness (Shore A) of Rubbers
For 100 Parts of Polymer Base Durometer
Polychloroprene and nitrile rubber 44
Natural rubber and cold polymerization SBR 40
Hot polymerization SBR 37
Butyl rubber 35
25 parts oil extended cold SBR 31
37.5 parts oil extended cold SBR 26
TABLE 1.8
Effect of Fillers and Softeners on Hardness Fillers and Softeners Durometer Change FEF, HAF, channel blacks +1/2 part of loading
ISAF black +1/2 part of loading + 2
SAF black +1/2 part of loading + 4
SRF black +1/3 part of loading
Thermal blacks and hard clay +1/4 part of loading Whiting (in natural rubber) +1/7 part of loading Factice and mineral rubber –1/5 part of loading Most liquid softener –1/2 part of loading
1.3.2.2.1 Unit Replacement Factor
This type of change involves changing the loading of a given grade of black and main- taining the hardness by adjusting oil. It is good to remember that for N330, 1 phr of oil is needed for each phr of black (see Table 1.10). For example, the unit replacement factor for N660 is 1.74; this means 0.74 parts of oil must be added for each additional part of N660 in order to maintain compound hardness.
1.3.2.2.2 Varying Hardness Approach
Table 1.11 can be used as a guide for designing new compounds, switching blacks, and/or varying hardness.
1.3.2.2.3 Modulus Compounding
Modulus can be varied by changing the carbon black structure and carbon black/oil loading:
• As a guideline, the addition of 1 phr of carbon black will raise the modulus by 0.14 to 0.28 MPa, depending on the structure of the black and the polymer system.
• As a guideline, the addition of 1 phr of oil will lower the modulus by 0.21 to 0.28 MPa, depending on the structure of the black and the polymer system.
TABLE 1.9
Hardness Conversion Factors for Replacing Carbon Blacks Replacement Carbon Black
Hardness Conversion Factors for Replacing the Following Carbon Blacks Industry
Type ASTM No. MT SRF GPF
GPF-HS/
FEF HAF
SRF N762 0.665 1.00 1.10 1.22 1.50
SRF N774 0.665 1.00 1.10 1.22 1.50
GPF N660 0.60 0.90 1.00 1.11 1.36
GPFHS N650 0.545 0.82 0.90 1.00 1.23
FEF N550 0.545 0.82 0.90 1.00 1.23
HAF N330 0.445 0.67 0.74 0.82 1.00
TABLE 1.10
Unit Replacement Factor
Carbon Black (or Other Filler) Black Type
ASTM No.
Oil Requirement
Unit Replacement Factor
SRF N762 0.667 1.667
SRF N774 0.667 1.667
GPF N660 0.74 1.74
GPFHS N650 0.80 1.80
FEF N550 0.80 1.80
HAF N330 1.00 2.00
MT N990 0.435 1.435
Whiting — 0.435 1.435
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.