1. Copyright Joseph Greene 2001 1 Carbon Black Professor Joe Greene CSU, CHICO

2. Copyright Joseph Greene 2001 2 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

3. Copyright Joseph Greene 2001 3 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 injected into the combustion zone at specific rates for the carbon black –Burning generates a very hot, turbulent atmosphere for cracking the feedstock oil. –90% of the feedstock is based on refinery heavy bottom oils. –Chemical reactions to convert the aromatic feedstock to elemental carbon are not well understood and complex –Collision of particles in a liquid-like state produces aggregates of spherical particles 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 the reaction. –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 black is pulverized to a 325 mesh and then pelletized

4. Copyright Joseph Greene 2001 4 Carbon Black Manufacture Manufacture and Morphology –Wet-pelleting process is used A rotating, pin-studded shaft mixes the loose black with water and binder to produce small beads or pellets. –Wet pellets are fed into a rotary drier heated by combustion of the tailgas from the earlier step in the process. Steam that is generated is removed and replaced with air that oxidizes the carbon black, which influences the chemical properties of the carbon black and, in turn, the cure rate and properties of the vulcanizates. –The pelleted black is screened for uniformity and passed over magnetic separators to remove metallic contamination that may have 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.

5. Copyright Joseph Greene 2001 5 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 of the mixture with a photoelectric reflectance meter. –Reflectance is compared to the reflectance of paste containing the Industry 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 lower tinting strengths. »Average particle size can be estimated from statistical equations that relate tint strength and structure to particle size as measured from electron micrographs.

6. Copyright Joseph Greene 2001 6 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 in a rubber compound. Small particle-size black will have higher surface area, but the texture or nature 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 of carbon black increases with the number of particles per aggregate Non-spherical particles pack differently from spheres

7. Copyright Joseph Greene 2001 7 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 containing groups, 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 many vulcanization systems Physical adsorption activity of the filler surface is of much greater overall importance for the mechanical properties of the general-purpose rubbers than 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 cure rate and modulus of rubber compounds. –Carbon blacks are generally electrically conductive because of the highly conjugated bonding scheme in crystalline regions

8. Copyright Joseph Greene 2001 8 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

9. Copyright Joseph Greene 2001 9 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 in surface area

10. Copyright Joseph Greene 2001 10 Carbon Black Properties Three compound recipes based upon different polymers enable the observation of changes in carbon black effects from one polymer to another –Table 3.3 –Figures 4 through 12 Mechanical properties for Different concentrations (loading levels) of carbon black

11. Copyright Joseph Greene 2001 11 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 surface available for attachment to the polymer result in the rubber compound to be more 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 N 2 surface area) had the highest viscosity, modulus at 200% elongation, and hardness; and the least amount of extrudate swell. –The higher the N 2 surface area the higher the viscosity, modulus at 200% elongation, and hardness; and the lower amount of extrudate swell. –The N660 carbon black (lowest N 2 surface area and lowest void volume) had the lowest viscosity, modulus at 200% elongation, and hardness; and the most amount of extrudate swell.

12. Copyright Joseph Greene 2001 12 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 and increased 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 small improvements in abrasion, regardless of carbon black loadings Higher surface area HAF blacks (N339 and N356) contribute better improvements in abrasion, depending on carbon black loadings. Higher structure N356 black reach maximum abrasion resistance at lower loadings than N339, but N339 ultimately gives higher abrasion resistance

13. Copyright Joseph Greene 2001 13 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, then decreases after that. Structure causes a shift in the strength curve to the left (lower limiting value for 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 a maximum tensile strength is attained. Higher surface area HAF blacks give improved tensile strength compared to GPF blacks, but not significantly difference due to structure. NR compound has inherently higher tensile strength in the unfilled natural rubber 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.