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Co ri ht Jose h Greene 2001
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Carbon Black
Professor Joe Greene
CSU, CHICO
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Carbon Black Reference: Rubber Technology, Chapter 3
Phenomenon of carbon black reinforcement was discovered in early
1900s Physical and chemical attachments are capable of giving reinforcement effects
by increasing the tensile strength and modulus of the rubbery phase
Carbon black and vulcanization generates a 3-D network
Carbon black
Range of physical and chemical attributes
Particle size, surface area, structure, surface activity
Gas-furnace blacks: Thermal black process: 3% of current carbon black
Initially made using gas as the source of carbon and the fuel source
Carbon black had small particles and were acidic
Worked well with natural rubber
Large amounts of air pollution was generated and expensive
Oil furnace black (1943) is the current manufacturing method: 97% of black
Low grade petroleum feedstock was cheaper, less polluting, and flexible process
Higher structure and more alkaline than gas furnace (channel) blacks
Improved significantly the properties of SBR polymers
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Carbon Black Manufacture Manufacture and Morphology
Typical oil furnace reactor, Figure 3.1
Refractory lined tube that can be horizontal or vertical.
Feedstock oil, natural gas, or other fuel, and air are preheated and injectedinto the combustion zone at specific rates for the carbon black
Burning generates a very hot, turbulent atmosphere for cracking the feedstockoil.
90% of the feedstock is based on refinery heavy bottom oils.
Chemical reactions to convert the aromatic feedstock to elemental carbon arenot well understood and complex
Collision of particles in a liquid-like state produces aggregates of sphericalparticles fused together in a random grape-cluster configuration, Fig 3.2
The carbon is formed in aggregates with a distribution of sizes
Water quench is used to rapidly reduce the temperature and terminate thereaction.
The smoke exiting the reactor is a mixture of carbon black aggregates,combustion gases, and moist air.
The smoke preheats the feedstock and air, and generates steam for plant use.
Fluffy black and gases (tail gas) are separated by filtration, and the loose blackis pulverized to a 325 mesh and then pelletized
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Carbon Black Manufacture Manufacture and Morphology
Wet-pelleting process is used
A rotating, pin-studded shaft mixes the loose black with water and binder toproduce small beads or pellets.
Wet pellets are fed into a rotary drier heated by combustion of thetailgas from the earlier step in the process.
Steam that is generated is removed and replaced with air that oxidizes thecarbon black, which influences the chemical properties of the carbon blackand, in turn, the cure rate and properties of the vulcanizates.
The pelleted black is screened for uniformity and passed overmagnetic separators to remove metallic contamination that mayhave gotten in the product stream.
Finished product is packaged and shipped
Furnace black categories Reinforcing: hard, tread. Have a smaller particle size and lower yields and
more expensive than semi-reinforcing.
Semi-reinforcing: soft, carcass.
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Carbon Black Properties Physical and Chemical Properties
Particle size can be measured by electron micrographs, Figure 3.3
Average diameter is 19 to 95 nm (nanometers or 10-9 m)
Particles are measured manually or with image analysis software
Particle size can be measured by tint strength test (ASTM D3265) Carbon black sample is mixed with zinc oxide and a soybean oil epoxide to
produce a black or gray paste.
Paste is spread to produce a suitable surface for measuring the reflectance ofthe mixture with a photoelectric reflectance meter.
Reflectance is compared to the reflectance of paste containing theIndustry Tint Reference Black (ITRB) prepared in the same manner.
Tint test is affected by the structure as well as the particle size of the
black.
For a given particle size, the higher structure blacks have a lowertinting strengths.
Average particle size can be estimated from statistical equationsthat relate tint strength and structure to particle size as measured
from electron micrographs.
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Carbon Black Surface Area Surface Area
Very important in carbon black because it defines how much
surface is available for interactions with other materials present ina rubber compound.
Small particle-size black will have higher surface area, but the texture ornature of the surface area can also influence the surface area.
BET method (ASTM D3037) to determine surface area
Adsorption of a gas, usually nitrogen, on the surface. Surface area can be measured from electron micrographs
Standard rubber grade black (nitrogen surface area of less than 130 m2/g)are nonporous
Non-specialty furnace blacks give good inverse correlation between nitrogen
surface area and the particle size measurements. Specialty furnace blacks require a devolatilization step to remove residual
oils present on the surface of the blacks
Volume of void space between aggregates per unit weight ofcarbon black increases with the number of particles per aggregate
Non-spherical particles pack differently from spheres
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Carbon Black Chemical Properties Chemical Properties
Chemical nature of a carbon black is variable
Evidence for the presence on the surface of at least four oxygen containinggroups, carboxyl, phenol, quinone, and lactone.
Elastomers are polar in nature, neoprene or nitrile rubber Will react more strongly with fillers with dipoles, OH, COOH, or Cl
Chemical surface groups affect the rate of cure with manyvulcanization systems
Physical adsorption activity of the filler surface is of much greater overallimportance for the mechanical properties of the general-purpose rubbersthan the chemical nature.
Oxygen content influences the cure rate Increased oxygen gives longer scorch period, a slower rate of cure, and a
lower modulus at optimum cure.
Amount of oxidation during the pellet drying operation can affect the curerate and modulus of rubber compounds.
Carbon blacks are generally electrically conductive because of thehighly conjugated bonding scheme in crystalline regions
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Carbon Black Nomenclature Nomenclature
First digit following the letter indicates the particle size range
Lower numbers for smaller particle-size blacks
Last two digits are arbitrarily assigned by ASTM
Table 3.1
Properties
ASTM D1765
ASTM Old Name
N110 SAF (Super-abrasion furnace)N220 ISAF (Intermediate Super-abrasion furnace)
N330 HAF (High abrasion furnace)
N358 SPF (Super processing furnace)
N660 GPF (General-purpose furnace)
N762 SRF (Semireinforcing furnace)
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Carbon Black Properties Properties
High surface area and high structure carbon blacks are associated with increased
reinforcement Particle size affects abrasion resistance, heat build-up (resilience), tensile strength,
and tear strength.
Structure has more of an effect on modulus, hardness, and extrudate swell.
Four carbon blacks are shown to demonstrate the effects of varying surface area,structure, and black loadings on various compound properties.
Structure Differences
N339 vs N356
N650 vs N660
Both pairs have
Equivalent surface N2 surface area Large differences in structure from
DBP absorption and void volume data
N339 and N356 vs N650 and N660 shows large difference insurface area
Nitrogen
Surface
area m2/g
DBP
Absorption
cc/100g
Void
Volume
cc/100g
Tint
Strength,
%ITRB
N339 98.9 118.4 69.2 108.8
N356 100.1 160.2 77.7 103.2
N650 38.8 123.7 57.4 52.7
N660 39.3 89.7 44.5 59.2
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Carbon Black Properties Three compound recipes based upon different
polymers enable the observation of changes incarbon black effects from one polymer to another
Table 3.3
Figures 4 through 12
Mechanical properties for Different concentrations (loading
levels) of carbon black
EPDM Compound
EPDM 100 phr
Naphthenic oil 12Zinc oxide 5
Stearic acid 1
Processing aid 2
Sulfur 1.5
MBT 0.5
TMTD 3
Carbon black 0 to 80phr
SBR Compound
SBR-1500 100 phr
Aromatic oil 5Zinc oxide 3
Stearic acid 1.5
Sulfur 1.75
CBS 0.85
DPG 0.28
Carbon black 0 to 80phr
NR Compound
Natural Rubber 100 phr
Highly aromatic oil 15
Zinc oxide 5
Stearic acid 2.5
Antioxidant 2
Antiozonant 2
Sulfur 1.5
CBS 1.6Carbon black 0 to 80phr
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Carbon Black Properties Compound Property Group 1
Viscosity, modulus, hardness, extrudate swell
Measures the degree of stiffening that carbon contributes
High structure and an increase in the amount of carbon black surfaceavailable for attachment to the polymer result in the rubber compound to bemore viscous and less elastic
Viscosity, modulus, hardness, extrudate swell, Figures 3.4, 3.5,3.6
Increases with increased amount of carbon black for all three recipes, SBR,EPDM, and NR
The N356 carbon black (highest N2 surface area) had the highest viscosity,modulus at 200% elongation, and hardness; and the least amount of extrudateswell.
The higher the N2 surface area the higher the viscosity, modulus at 200%elongation, and hardness; and the lower amount of extrudate swell.
The N660 carbon black (lowest N2 surface area and lowest void volume) hadthe lowest viscosity, modulus at 200% elongation, and hardness; and the mostamount of extrudate swell.
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Carbon Black Properties Compound Property Group 2
Abrasion resistance, tear strength, and tensile strength
Measures the resistance to failure under several types of stress
Strength related properties enhanced by carbon black surface area andincreased black loading up to a limiting value that is dependent on the
packing characteristics (morphology) of the carbon black aggregates.
High structure and an increase in the amount of carbon black surface available
for attachment to the polymer in the rubber compound. Asblack loading in increased to maximum level, the carbon aggregates are no
longer adequately separated by polymer which weakens the rubber composite
Abrasion Resistance, Figures 3.8a, 3.8b, 3.8c
Abrasion resistance is most affected by surface area and loading
Lower surface area GPF blacks (N650 and N660) contribute smallimprovements in abrasion, regardless of carbon black loadings
Higher surface area HAF blacks (N339 and N356) contribute betterimprovements in abrasion, depending on carbon black loadings.
Higher structure N356 black reach maximum abrasion resistance at lowerloadings than N339, but N339 ultimately gives higher abrasion resistance
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Carbon Black Properties Compound Property Group 2
Tear-strength, Figures 3.9a and 3.9b
As carbon black is increased, the tear strength increases up to a peak, thendecreases after that.
Structure causes a shift in the strength curve to the left (lower limiting valuefor strength because of the effect of higher structure on packing)
Tensile strength, Fig 3.10a
Unfilled EPDM rubber compound has very low tensile strength.
Tensile strength is increased dramatically as carbon black is added until amaximum tensile strength is attained.
Higher surface area HAF blacks give improved tensile strength compared toGPF blacks, but not significantly difference due to structure.
NR compound has inherently higher tensile strength in the unfilled naturalrubber due to its crystallizing ability.
Carbon black causes less of a change in NR
Tensile strength reaches a maximum at relatively low carbon black loadings (2-40 phr) and shows a decreasing tendency as the black loading is increased.