Manufacture of Conductive Carbon Black
Conductive carbon black grades can be obtained via both manufacturing process principles, the oxygen-free thermal decomposition and the thermal-oxidative decomposition of hydrocarbons (Kiihner and Voll 1993; Taylor 1997; Wang et al. 2004). The creation of highly branched aggregates of small primary carbon black particles and the stabilization of these structures are key features in the manufacturing process. Their achievement in the oil-furnace process usually is accompanied by a significant reduction of the process yield. Conductive furnace blacks are produced by co-injecting preheated oil feedstock and air in a closed flow reactor creating a turbulent flame. The most important process stages influencing the carbon black properties are the atomization and vaporization of the oil droplets in the reactor, the partial combustion and pyrolysis of the atomized feedstock in the turbulent flame, and the secondary reactions between the carbon black particles and the components of the reactor off-gas. The carbon black structure formed in the reactor is largely dependent on the formation and concentration of solid primary particles being determined by the concentration of feedstock vapor and reactor temperature (Kuhner and Voll 1993). Low feedstock/air ratios increase the concentration of feedstock vapor and reactor temperature and therefore cause small primary particle sizes and large carbon black structures. To stop the particle growth and moreover to avoid carbon black loss occurring at these temperatures due to the Boudouard and water-gas reaction with CO2 and H2O gas, respectively, being both present in the reactor, a cooling step and subsequently the separation from the tail gas are performed. These stages are followed by the reaction of the carbon black with oxygen upon the first contact with air and a densification and eventual pelletization step.
The production process of acetylene black is based on the exothermic decomposition of acetylene to carbon black and hydrogen occurring above 800 °C in the absence of oxygen. Once the reaction is started, the acetylene decomposition reaction autogenously provides the energy required for the cracking of acetylene to carbon followed by the synthesis of the carbon black:
The high synthesis temperatures above 2000 °C being typical for the acetylene black production give rise to a relatively high graphitization degree of the primary particles that show low surface area. The high-surface-area gasification blacks are by-products from the production of synthesis gas based on the incomplete combustion of hydrocarbons. Hereby, the preheated oil feedstock reacts with air and vapor at relatively high temperatures to form synthesis gas (H2/CO) and a carbon black by-product that is separated by filters or extraction. The process has no flexibility in producing different grades or changing the carbon black properties as the carbon black production is not the main purpose.
Acetylene black and conductive furnace black were the only conductive carbon black grades available on the market until the mid-1970s when the gasification black products like Ketjenblack EC300J and Philblack XE-2 (later name: Printex XE-2), known as extra-conductive carbon black and both being by-products from the Shell gasification process, became available. In the early 1990s, the low-surface-area, high structure ENSACO® carbon black grades were introduced together with the extraconductive high-surface-area ENSACO® 350 carbon black being all manufactured out of a specifically designed process for conductive carbon black by IMERYS (IMERYS Belgium, ex-MMM Carbon).
The development of new conductive carbon additives for polymers has been ongoing since more than three decades. The major effort has been made to decrease the required carbon black concentration with no deterioration of the level of conductivity of the carbon black-filled polymer compound. The family of the
extra-conductive carbon blacks is the result of this effort. Oxidative aftertreatment of those extra-conductive carbon blacks made another step in this direction. Aftertreated extra-conductive carbon blacks, also called ultra-conductive carbon blacks, generate in some polymers sufficiently high conductivities at concentrations below 2 wt%. Other development directions have been:
1. High purity: low sulfur, low grit, and low metal content.
This trend was mainly pushed by the power cable industry. With increasing voltage, higher purity for conductor shields has been demanded. However, purity also has been beneficial for other numerous plastic applications, e.g., plastic films and reduced degradation of sensitive plastics (like alkaline catalyzed hydrolyzation).
- 2. Stability of the conductivity, i.e., resistance to shear occurring in the extrusion process to prepare conductive polymer compounds.
- 3. Very high conductivities for specific applications.
- 4. Reduced viscosity for conductive compounds.
- 5. Reduced or increased positive temperature coefficient (PTC) effect.