6 Carbon Black Wesley A. Wampler, Thomas F. Carlson, and William R. Jones Sid Richardson Carbon Company, Fort Worth, Texas, U.S.A. I. INTRODUCTION Carbon black is produced by the incomplete combustion of organic sub- stances, probably first noted in ancient times by observing the deposits of a black substance on objects close to a burning material. Its first applications were no doubt as a black pigment, and the first reported use was a colorant in inks by the Chinese and Hindus in the third century A.D. (1). It was not until the early twentieth century when carbon black was first mixed into rubber that its possible usefulness in this area was explored. The fact that carbon black has the ability to significantly improve the physical properties of rubber (often referred to as reinforcement) has provided its largest market today, i.e., the tire industry. Currently about 5 million metric tons of carbon black is used worldwide in tires annually (2). A typical tire contains 30–35% carbon black, and there are normally several grades of carbon black in the tire, depending on the reinforcement requirements of the particular component of the tire. Of course, carbon black is also used in many non-tire rubber applications owing to its ability to reinforce the rubber and to its use as a cost reduction diluent in the compound. Non-tire rubber products currently require about 2 million metric tons of carbon black annually on a worldwide basis (2). This chapter brings the reader up to date on how carbon black is manufactured, how its quality is controlled, how the carbon black character- istics influence rubber properties, and how the different grades of carbon black are classified and used, then finally presents a review of carbon black surface chemistry and how the modification of these surfaces holds substan- tial promise for future developments. 4871-9_Rodgers_Ch06_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 239 Copyright © 2004 by Taylor & Francis II. DEFINITIONS Before beginning there is merit in reviewing some basic definitions in carbon black technology. Although it is not attempted to present a comprehensive list of definitions, several important ones will be given, and the reader is referred to ASTM D 3053 for additional carbon black terminology (3). Carbon black Material consisting essentially of elemental carbon in the form of near-spherical particles coalesced into aggregates of col- loidal size, obtained by incomplete combustion or thermal decom- position of hydrocarbons. Carbon black particle A small spheroidal nondiscrete component of a carbon black aggregate. Particle diameters can range from less than 20 nm in some furnace grades to a few hundred nanometers in ther- mal blacks. Carbon black aggregate A discrete, rigid, colloidal entity of coalesced particles; the smallest dispersible unit of carbon black. Aggregate dimensions measured by the Feret diameter method can range from as small as 100 nm to a few micrometers. Figure 1 shows the distinction between a particle and an aggregate in carbon black. Carbon black agglomerate A cluster of physically bound and en- tangled aggregates. Agglomerates can vary widely in size from less than a micrometer to a few millimeters in the pellet. Figure 1 (Left) Carbon black aggregate as viewed by transmission electron mi- croscopy and (right) a schematic showing the distinction between carbon black par- ticles and the aggregate. (Photograph by David Roberts.) 4871-9_Rodgers_Ch06_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 240 Copyright © 2004 by Taylor & Francis Carbon black pellet A relatively large agglomerate that has been den- sified in spheroidal form to facilitate handling and processing. Pellets range in diameter from tenths of a millimeter to 2–3 mm. Carbon black structure The degree of irregularity and deviation from sphericity of the shape of a carbon black aggregate. It is typically evaluated by absorption measurements that determine the voids between the aggregates and agglomerates and thus indirectly the branching and complexity of shape of the carbon black aggregates and agglomerates. Carbon black specific surface area The available surface area in square meters per unit mass of carbon black in grams. Typically the adsorption of molecules such as iodine or nitrogen is measured and then either the amount adsorbed per unit mass is reported or a specific surface area is calculated based on current adsorption theories. III. THE CARBON BLACK MANUFACTURING PROCESS The carbon black manufacturing process consists of several distinct segments. Each segment is important for ensuring economical production and for meeting customer expectations. 1. Reaction 2. Filtration/separation 3. Pelletizing 4. Drying Each segment could be discussed in exhaustive detail, but the purpose here is to furnish a short description that allows a working knowledge of how carbon black is produced and how the manufacturing process can affect customer applications. Figure 2 shows the furnace process schematically. A. Reaction There are two main production processes for rubber grade carbon black: the furnace process and the thermal process. However, the furnace process is by far the more dominant process today. 1. Furnace Process There are two broad categories within the furnace carbon blacks: tread and carcass. The processes for manufacturing the two are very similar in most 4871-9_Rodgers_Ch06_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 241 Copyright © 2004 by Taylor & Francis respects, the main differences being that carcass carbon black (used mainly in tire carcasses, sidewalls, and other semireinforcing applications) is made at lower temperatures, lower reaction velocities, and with longer residence times than tread carbon blacks. Tread blacks are used in tire treads and in areas where higher levels of reinforcement are needed. Because of these differences in reaction kinetics, carcass carbon blacks are lower in specific surface area than tread blacks. Carbon black is formed very quickly and at very high temperatures typically generated from the combustion of natural gas with air but with insufficient oxygen to reach the stoichiometric ratio and corresponding temperature. The reaction occurs in refractory-lined vessels that are required to sufficiently contain the high temperature reactor gas stream. The refractory lining presents a problem because of constant erosion at high velocities. The erosion contributes to contamination of the carbon product, which is not good for any customer product application. The erosion of refractory can also significantly change the cross-sectional area of the ‘‘ choke’’ in tread grade furnace reactors, affecting several carbon black properties, most significantly surface area, structure, and tint levels. The ‘‘choke’’ is a narrowing section of the furnace reactor (on tread but not carcass reactors) that is necessary to attain the velocities required to produce the high levels of surface area desired. Figure 2 Schematic of the furnace carbon black process. 4871-9_Rodgers_Ch06_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 242 Copyright © 2004 by Taylor & Francis Velocities can approach supersonic levels at the choke and temperatures approach 3400jF (1870jC). In the first stage of the process, hydrocarbon fuels are used to generate temperatures via combustion that create an exothermic reaction with temper- atures ranging from 2400jF (1315jC) to 3400jF (1870jC). This high temperature is necessary to supply the energy required to ‘‘crack’’ or ‘‘split’’ the carbon–hydrogen bond of the raw material feedstock. The specific surface area of carbon black, which is probably the most important quality param- eter, is directly proportional to the reaction temperature. This means that because more fuel is used to attain higher reaction temperatures for the higher surface area carbon blacks there is a resulting higher production cost. An endothermic reaction (‘‘cracking’’) proceeds concurrently with the exothermic reaction. A hydrocarbon (feedstock) is injected into the reactor for the production of carbon black at elevated pressures and temperatures. High feedstock injection pressures and temperatures are necessary to attain good economics and minimize coke formation. Coke is formed from rapid cooling of the oil droplets or from oil droplet impingement on the reactor refractory walls. This coke is sometimes referred to in the industry as ‘‘grit’’ or ‘‘sieve residue’’ (because of the way it is tested), but these terms also include the refractory in the product due to erosion (see above) and any other process contaminants that are not beneficial to customer applications. The process gas stream velocity is very high at the point of feedstock injection, so relatively high pressures are needed to get the feedstock into the reaction stream and away from the refractory walls. The hydrocarbon feedstock is usually an aromatic oil, but it could also be natural gas, ethylene cracker residual bottoms, or coal tar distillate. This feedstock is injected into the reaction gas stream when temperatures of that stream are greater than 2500jF (1370jC). However, excess oxygen is still present in the stream. Thus a portion of the feedstock burns, with the remaining excess oxygen raising temperatures even higher, while concurrently the remainder of the feedstock is reacting endothermically (the HUC bond is destroyed, resulting in free hydrogen and carbon). Reaction times range from about 0.3 sec to 1 sec before the reaction is ‘‘quenched.’’ Quenching is normally done by injecting a stream of water in sufficient quantity to drop the process stream temperature to less than 1500jF (815jC) or lower (i.e., dropping below ‘‘cracking’’ temperatures). The process gas stream is further cooled through the use of gas–gas or gas–liquid heat exchangers. These heat exchangers return heat to the process by elevating the temperature for process air, feedstock, or water (producing steam), thereby helping to improve the overall energy efficiency of the plant. Carbon black manufacturing is a very capital- and energy-intensive process, making it inherently important to maximize energy recovery or reduce energy use in all segments of the process. 4871-9_Rodgers_Ch06_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 243 Copyright © 2004 by Taylor & Francis By far the majority of the feedstock used by North American producers is the heavy residual oil extracted from the bottom of catalytic crackers in oil refineries. European and Asian manufacturers use a combination of ethylene cracker bottoms, coal tar distillates, and the same catalytic cracker bottoms that are used by the North American producers. 2. Thermal Process The thermal process is similar to the furnace process except for the following main areas. 1. The thermal process is cyclical, whereas the furnace process is continuous. 2. In the thermal process carbon black forms in the absence of oxygen. 3. Carbon black formed in the thermal process is much lower in surface area and structure than carbon black made in the furnace process. 4. The process gas formed as the hydrocarbon splits in the thermal process is almost pure hydrogen, which requires special handling processes and procedures, whereas the process gas formed in the furnace process is mostly N 2 and H 2 O, with smaller amounts of CO, H 2 ,CO 2 ,C 2 H 2 ,andCH 4 . The feedstock for thermal black can be natural gas or catalytic cracker bottoms. Thermal carbon blacks are not as reinforcing as furnace black, can have lower levels of hydrocarbon residuals on the surface, and are lower in tint or blackness. There are some areas where these properties are beneficial, but by far the vast majority of carbon black (>90%) production in the world is uses the furnace process. As a side note, the thermal process was developed in the United Kingdom in the early 1900s as a method to produce hydrogen gas for use in cities to augment or replace coal burning. Carbon black was a secondary product in this H 2 -producing process. 3. Reactor Conditions Versus Properties Carbon black has two primary properties (surface area and structure) that are important to the majority of end users and are controlled predominantly in the reaction area. Specific surface area is manipulated by controlling reaction temperature, reaction time, and reaction velocity. Structure (or branching) is manipulated by increasing or decreasing the amount of turbulence at the point of feedstock injection in the reaction forming zone or by the addition of 4871-9_Rodgers_Ch06_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 244 Copyright © 2004 by Taylor & Francis metallic salts (potassium salts being by far the most prevalent) to prevent the formation of carbon black particulate structure. B. Filtration/Separation Carbon black is formed in a reactor with less oxygen present than would be required for complete combustion, resulting in many species of gas compo- nents in the process gas stream. Gas species present include H 2 O, N 2 , CO, H 2 , CO 2 ,CH 4 ,C 2 H 2 , and trace amounts of other compounds such as SO 2 and H 2 S. The carbon black formed in the reaction section must be separated from these gaseous components. This is accomplished through the use of various types of commercially available cloth filter bags. At this stage of the process the carbon black is in a ‘‘loose’’ or ‘‘fluffy’’ state at about 500jF (260jC). The surface area of the carbon black being very high (25–150 m 2 /g), the loose product is unmanageable for most customers. Carbon black in this state is extremely light, and a few grams can easily obscure most of the light in a 4000 ft 3 room. The gas, often referred to as tail gas, does contain combustible components (H 2 , CO, CH 4 ), but the heat content is very low because of the high quantities of nitrogen and water present, 45–75 Btu/ft 3 (1676–2794 kJ/ m 3 ). Natural gas, by comparison, averages around 950–1000 Btu/ft 3 . Even though the heat content is quite low, most carbon black manufacturers have developed technology that allows combustion of this process gas to supply heat to the process or to generate steam and/or electricity. This energy recovery is essential to maintain energy efficiency and meet environmental compliance requirements. After separation the carbon black is conveyed (pneumatically or mechanically) to the next segment of the process, where it is pelleted and dried for ease of shipment and handling by the customers. C. Pelletizing Most customers need carbon black delivered in bulk quantities in a form that is easy to convey and also easy to disperse into their compound (rubber, plastic, ink, paint, etc). To get the loose carbon black into a pelleted form that meets these needs, the carbon black producers are obliged to use mechanical pin mixers, chemical pelleting aids (such as molasses or lignosulfonate), water, and equipment of high capital and continuous operating costs. Because carbon black is formed from a hydrocarbon raw material (which does not mix naturally with water) and has high surface area and structure, large amounts of water are needed to form the pellets, normally with a pelleting aid added to facilitate ‘‘wetting.’’ Water content of the product leaving the pelleting area 4871-9_Rodgers_Ch06_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 245 Copyright © 2004 by Taylor & Francis ranges from 35% to 65% by weight. Water is used extensively in the carbon black process—about five times more water than feedstock. Customers expect to receive uniform pellets capable of withstanding the rigors of being shipped hundreds to thousands of miles but not so hard as to impede incorporation with a minimum of mixing energy and time. It is also highly desirable to minimize the unpelleted carbon black (or minimize pellet breakdown) so as to mitigate customer concerns about fugitive carbon black in their plants. D. Drying The wet pellets, having a high concentration of water, are not a desirable final product form. Therefore, carbon black producers are obliged to use large amounts of energy (with significant capital investment) to drive the water from the wet pellet. It is necessary to lower the moisture content from approximately 50% by weight as it leaves the pelletizer to less than 1% for shipment to customers. Most producers use the process gas, sometimes called tail gas, separated from the carbon black in the filtration section of the process to supply the fuel needed to dry the wet pellets. Although this is an inexpensive fuel, the capital involved to collect, direct, and support combustion of this low Btu gas is relatively high. After drying, the pellets are conveyed to bulk storage tanks for packaging into bags (ranging from 50 to 2000 lb), bulk trucks (45,000 lb), or railcars (100,000 lb). A small number of customers prefer the final product in different forms for one reason or another. But the wet pelleted furnace type products dominate the industry in terms of volume. Other forms of final product are 1. Dry pellets. Using a rotating drum and recycling some carbon black pellets, the loose carbon black is rolled into pellets via me- chanical tumbling action. Dry pellets are softer than the wet pellets and are used in applications where the product must disperse in a vehicle with lower energy than wet pellets. 2. Powder carbon black. The carbon black can be directly packaged before going through he pelleting and drying stage. Typically the customers for this kind of product are looking for carbon black that is very easy to disperse uniformly with minimum energy. Freight costs and packaging costs are naturally higher than for wet pelleted carbon black because of the lower density. A process that has virtually disappeared because of environmental concerns is the channel black process in which natural gas is burned and 4871-9_Rodgers_Ch06_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 246 Copyright © 2004 by Taylor & Francis the resulting carbon black is collected on channel irons that are continuously scraped to obtain the product. It is a highly inefficient process that releases much of the carbon black to the environment. Due to the highly oxidative environment in which the carbon black is produced it has a high oxygen content (3–5%), which results in slow curing characteristics in rubber. IV. CONTROLLING THE QUALITY OF CARBON BLACK To control the quality of carbon black during production it must be tested for the characteristic properties that can be related to its performance in rubber. Before discussing carbon black characterization and the various quality control tests, it is worthwhile to point out that the carbon black industry has done numerous things to standardize and improve the product received by customers. Examples of this would be the establishment of industry-wide target properties for each grade of carbon black (4), standard practices for calculation of process indices from process control data (5), standard methods for sampling packaged and bulk shipments (6,7), standard practices for reducing and blending samples (8), standardized test methods for every quality parameter and establishment of standard reference blacks with accepted values to ensure uniformity of test data from any lab (9), and a laboratory proficiency program that cross-checks data between over 60 labs worldwide on a semiannual basis. It is only appropriate that a more detailed discussion of the character- ization properties used for quality control purposes is now undertaken in some detail. Table 1 briefly summarizes the quality control tests, what they measure, and how they should be used. A. Specific Surface Area The specific surface area is by definition the available surface area in square meters per unit mass of carbon black in grams. This parameter is evaluated through the use of adsorption measurements. In the absence of significant microporosity, which includes almost all rubber grade carbon blacks, the measure of specific surface area exhibits an inverse correlation with the size of the carbon black particles (10). In theory the calculation of the amount of surface in square meters is Sðm 2 Þ¼W m NA=M ð1Þ where S is the surface area, W m is the weight of the adsorbate monolayer (g), N is Avogadro’s number (6.023 Â 10 23 mol ÀI ), A is the cross-sectional area of 4871-9_Rodgers_Ch06_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 247 Copyright © 2004 by Taylor & Francis adsorbate (m 2 ), and M is the molecular weight of the adsorbate (g/mol). Thus the specific surface area, in square meters per gram, can be determined by dividing S by the mass of the unknown sample. However, because of the energetically heterogeneous surface of carbon black (11), no molecules adsorb in a monolayer, and even theories that account for multilayer adsorption assume an energetically homogeneous surface (12). Nonetheless, adsorption tests still provide the best available technique for quality control of carbon black specific surface area, and the most widely used is the adsorption of iodine from aqueous solution. Other methods are also used to assess this property, and each will subsequently be reviewed. Regardless of the tech- nique, it is clear that this is a property that greatly influences the final properties of compounds that contain the carbon black. Increasing only the specific surface area of the carbon black used in a rubber compound will typically increase such attributes as the compound’s blackness, stiffness, hysteresis, and wear resistance. The iodine number test is a well-defined procedure (13) in which a sample of carbon black is added to a 0.0473 N solution of iodine, whereupon Table 1 A Brief Summary of the Quality Control Tests for Carbon Black, What They Measure, and How They Should Be Employed Test Measures Use a Oil or DBP absorption No. Structure A Compressed DBP or Oil No. Structure after compression B Compressed volume index Relative structure level B Iodine adsorption No. Surface area A Nitrogen surface area Total surface area B STSA External surface area B CTAB surface area External surface area B Tinting strength Fineness/color B Pellet hardness Strength of pellets A Fines content Dustiness level A Pour density Bulk density B Mass strength Resistance to packing C Pellet size distribution Pellet sizes C Toluene discoloration Extractables C Ash content Inorganics from water B Heating loss Moisture A Sieve residue Contaminants A Natural rubber mix 300% modulus, tensile strength B a A = typical specification property; B = specified only if application is critical to this measurement; C = needs to be used only for process control. 4871-9_Rodgers_Ch06_R2_052404 MD: RODGERS, JOB: 03286, PAGE: 248 Copyright © 2004 by Taylor & Francis [...]... N3 47 N351 N356 N358 N 375 N539 N550 N582 N630 N642 N650 N660 N683 N754 N762 N765 N 772 N 774 N7 87 N9 07 N908 N990 N991 90 92 90 68 92 84 90 43 43 100 36 36 36 36 35 24 27 31 30 29 30 — — — — Copyright © 2004 by Taylor & Francis 120 130 124 120 154 150 114 111 121 180 78 64 122 90 133 58 65 115 65 72 80 34 34 43 35 99 104 99 95 112 108 96 81 85 114 62 62 84 74 85 57 59 81 59 63 70 — — 37 37 91 96 85 71 ... strength, D 3265 145 160 122 121 1 17 142 151 — 121 121 120 145 108 — 82 82 92 113 113 114 132 104 1 27 135 85 114 92 125 100 124 79 72 102 110 97 97 99 111 89 103 1 17 82 98 86 102 88 104 77 68 88 94 1 27 1 37 126 122 122 143 141 120 119 111 119 122 104 89 78 78 85 115 124 113 114 121 1 37 — 1 07 106 1 07 112 111 97 86 76 75 85 123 123 129 119 125 131 119 115 116 120 123 120 113 1 17 111 104 110 Copyright © 2004... Butadiene Rubber) Ingredient SBR-1500 Natural rubber, SMRL Carbon black Zinc oxide Stearic acid Sulfur TBBS (accelerator) MBTS (accelerator) Total D3192 (NR), phr D3191 (SBR), phr 100.00 100.00 50.00 5.00 3.00 2.50 0.60 161 .75 50.00 3.00 1.00 1 .75 1.00 156 .75 F In -Rubber Tests ASTM has developed two rubber recipes specifically for evaluating carbon black in rubber One formula is for natural rubber (34) and. .. offroad applications C Examples of N300 Series (N2SA = 70 –99) N330—N2SA =78 (Iodine No = 82), OAN = 102 This is one of the most important basic blacks in the industry It provides good economic abrasion resistance with high resilience, easy processing, and relatively good tensile and tear properties It has a wide range of applications in both tires and mechanical rubber goods for high severity applications. .. way as to change the basic properties of rubber compounds in which the carbon was incorporated Instead, these studies attempted to employ adsorbents as means to probe the basic nature and chemistry of this substrate As part of a series on carbon black and compounding published in 19 57, Studebaker (55– 57) reported on the interaction of such gases as H2, NH3, and H2S with channel blacks at elevated temperatures... between carbon black loading and selected rubber properties Figure 7 Relationship of G V and G VV with strain for N23 4- lled SBR (D3191) and unfilled D3191 Copyright © 2004 by Taylor & Francis (0.1%) to 10% strain for a typical carbon black compound and for the corresponding unfilled polymer It is clear that the response is quite different for the carbon black–filled compound and that the filler is the main... its wide applications in tire carcasses, tubes, belts, hose, and many other industrial products The lower structure gives lower modulus and viscosity than, for example, N550 N762—N2SA = 29 (Iodine No = 27) , OAN = 65 Originally known as SRF (semireinforcing furnace) owing to its good mechanical processing efficiency and its ability to be highly loaded in rubber, which make it useful in such applications. .. 80 32 39 36 35 36 25 29 34 32 30 32 9 9 8 8 88 92 83 70 87 78 91 38 39 — 32 — 35 34 34 24 28 32 30 29 32 9 9 8 8 111 112 105 100 106 98 114 — — 67 — — — — — — — — — — — — — — — N134—N2SA = 143 (Iodine No = 142), OAN = 1 27 This high structure N100 series rubber gives higher abrasion resistance than N110 and high tensile strength Well suited for truck and passenger tire treads, especially for heavily loaded... (high abrasion furnace) N326—N2SA =78 (Iodine No = 82), OAN = 72 This is a carbon black with significantly lower structure than N330 It has good reinforcement and processability like N330 but with better tensile and tear properties Used in tires for carcass compounds, belt skim, and steel cord adhesion compounds It also finds uses in mechanical rubber goods for high severity applications N339—N2SA =91 (Iodine... carbon blacks and the carcass grades as semireinforcing carbon blacks The carcass, or soft, blacks, are typically the N500, N600, and N700 series carbon blacks, and some examples are given below There are no N400 series blacks listed in ASTM D 176 5 D Examples of Carcass or Semireinforcing Grades Carcass or semireinforcing grades include N500 (N2SA = 40–49), N600 (N2SA = 33–39), and N700 Series (N2SA . black is also used in many non-tire rubber applications owing to its ability to reinforce the rubber and to its use as a cost reduction diluent in the compound. Non-tire rubber products currently. data (5), standard methods for sampling packaged and bulk shipments (6 ,7) , standard practices for reducing and blending samples (8), standardized test methods for every quality parameter and establishment. transmission electron mi- croscopy and (right) a schematic showing the distinction between carbon black par- ticles and the aggregate. (Photograph by David Roberts.) 4 87 1-9 _Rodgers_Ch06_R2_052404 MD: