Chemistry of inorganic compounds – Carbon or compound thereof – Elemental carbon
Reexamination Certificate
1997-02-26
2002-10-29
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Elemental carbon
C423S449100
Reexamination Certificate
active
06471933
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to ozone—oxidized carbon blacks and the methods of producing them.
BACKGROUND OF THE INVENTION
Carbon black is the accepted generic name for a family of small particle size carbon pigments which are formed in the gas phase by the thermal decomposition of hydrocarbons. Carbon blacks are currently sold in the form of more than 100 different commercial grades which vary in their particle size, aggregate structure, porosity, and surface chemistry.
Historically, carbon blacks have been manufactured by five different major processes: the lampblack process, the impingement process, the acetylene black process, the thermal black process, and the oil furnace process. The process used to manufacture the carbon black is extremely important; indeed, the manufacturing process frequently has more of an impact on the product and its final characteristics than the raw material from which it is made.
The lampblack process is the oldest of all the processes and is now practically obsolete in the U.S. and most other locations. Lampblacks are made by burning aromatic oils such as naphthalene, anthracene, or creosote. Typically, the oil is burned in shallow pans with limited air supply and the carbon black formed is drawn off into settling areas and collected periodically. This is a low temperature operation and the particle size tends to be fairly large (about 70 to about 100 nm average diameter) and highly aggregated to produce a stringy chain structure.
Impingement (or “channel”) process carbon blacks were first produced during the 1870's and reached their peak production in the 1920's and 1930's. In this process, natural gas (typically reinforced with a vapor of hot oil) was burned from slotted lava tips to produce fan—type flames in evenly spaced rows. These were set to impinge on channel irons on which the carbon black was deposited. During exposure to air on the hot channel irons, the carbon black becomes highly oxidized (e.g., 2.5 to 3.5% oxygen content). This high level of oxygen, obtained without resorting to a post treatment, is unique among the different carbon black processes. The combination of the natural gas feedstock and cooling without quench water also provides for a very low inorganic content. Unfortunately, the process is relatively inefficient in its use of increasingly expensive natural gas and it severely pollutes the atmosphere. Thus, it too has fallen into the same category as the lampblack process and is little used in the U.S. Carbon blacks with high oxygen surface functionality are now generally produced by post chemical oxidation of oil furnace products.
In the acetylene black process, carbon black is made from the thermal decomposition of acetylene gas. In one common embodiment, acetylene gas is introduced at the top of a cylindrical reactor which is several meters high. The reactor is preheated to about 800° C. to decompose the acetylene. This is a rather violent exothermic reaction which produces temperatures up to about 3000° C. Proceeding from the entry point of the acetylene gas down into the upper part of the furnace there is a temperature gradient of about 3000° C. Carbon black formation most likely takes place in the 800 to 2000° C. zone, followed by partial graphitization in the higher temperature region. Air enters into the bottom of the reactor and acts to combust the hydrogen that is formed. In addition, the air flow prolongs the residence time of the acetylene black aggregates in the hot zone of the reactor which eliminates any residual hydrocarbons and further unifies the highly graphitic microstructure of the final product. Acetylene black is also produced commercially by the thermal decomposition of acetylene gas in stainless steel retorts which are water cooled. Because of its highly graphitic surface, acetylene black exhibits low reactivity and contains a very low level of oxygen volatile. This type of carbon black is frequently used in conductive applications.
Thermal blacks are manufactured by the decomposition of natural gas or oil. They are made in the absence of air by means of a batch type process which is based on sets of dual furnaces (generators). The generators are lined with an open checker brickwork which is preheated prior to charging them with the gas or oil feedstock. The respective heat and make cycles commonly require a few minutes each. Following the make cycle, there is a one—minute steam purge to remove the carbon black, which is then water quenched, passed through the collection filter, and air conveyed to the beader, dryer, and bulk storage loading tank. Following the purge to remove the carbon black, air is passed through the system and carbon remaining on the walls of the generators is burned off to produce additional heat for the next make cycle. Thermal blacks are uniquely large in size (250 to 500 nm average particle diameter) and low in structure (aggregation) relative to all other types of carbon black. They are typically used in cross—linked polyethylene and in rubber applications requiring very high filler volume fractions, or in costly specialty polymers sensitive to degradation with other carbon black grades.
Most of the carbon black grades available today are made by the oil furnace process, which involves the decomposition of highly aromatic oil feedstock in a heated reactor. This is a very rapid, continuous process in which the oil is converted to carbon black aggregates in a few milliseconds. The feedstock oil from storage tanks is injected into the reactor which is heated continuously with a mixture of fuel (oil or gas) and air. The carbon black is water—quenched immediately after formation and then further cooled as it passes through a heat exchanger and on through the bag filter and into the beaders, dryer, and storage tank. Because of the very high gaseous flow rates, the carbon black aerosol may reach the bag collector in less than a second after being injected as oil into the reactor.
A considerable amount of process water is used to quench, cool, and bead the carbon black during the furnace black process. Rubber grade carbon blacks typically require 6 to 8 pounds of water for every pound of carbon black produced. Very small particle size high color carbon blacks for industrial applications (e.g., Raven 5000) require a process water—to—carbon black ratio of more than 40 to 1. A high percentage of the furnace black production is pelletized (beaded) to provide a more easily handled and dust free form. Carbon blacks utilized in rubber and plastics applications are most typically beaded with water which provides denser, more durable beads that are most suitable for transporting the black as a free—flowing powder through bulk handling systems. Rubber and plastics compounding is typically based on high shear Banbury or roller mill mixing which is ideally suited for wet beaded carbon blacks. However, many industrial applications such as inks and coatings utilize low energy mixing procedures. These applications can only accommodate powder and dry beads which are more easily broken down and dispersed in liquid vehicle systems. Dry beads are formed by slowly tumbling the powdered carbon black in a rotating drum. Agglomerates of carbon black act as nuclei, and growth into beads takes place slowly as concentric layers of black are progressively added.
As stated above, most of the carbon blacks available today are made by the oil furnace process. This invention is preferably applied to carbon blacks originally made by this process, although it may be applied to carbon blacks made by the other processes as well.
It has long been recognized that oxidized carbon blacks (carbon blacks treated so as to have oxygen—containing functional groups at the surface) feature characteristics which are important to specific applications. For example, in paint, ink, toner, and coatings applications, oxidized carbon blacks provide improved wettability and rheology, characteristics which are important in these applications.
In some of these applications, users of oxidized carbo
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