Carbon black volatile content

A convenient method for assessing the extent of surface oxidation is the measurement of volatile content. This standard method measures the weight loss of the evolved gases on heating up to 950°C in an inert atmosphere. The composition of these gases consists of three principal components hydrogen, carbon monoxide, and carbon dioxide. The volatile content of normal furnace blacks is under 1.5%, and the volatile content of oxidized special grades is 2.0 to 9.5%. 

Aromaticity is the most important property of a carbon black feedstock. It is generally measured by the Bureau of Mines Correlation Index (BMCI) and is an indication of the carbon-to-hydrogen ratio. The sulfur content is limited to reduce corrosion, loss of yield, and sulfur in the product. It may be limited in certain locations for environmental reasons. The boiling range must be low enough so that it will be completely volatilized under furnace time—temperature conditions. Alkane insolubles or asphaltenes must be kept below critical levels in order to maintain product quaUty. Excessive asphaltene content results in a loss of reinforcement and poor treadwear in tire appHcations. 

The surface oxides are destroyed at high temperatures. Due to this fact the weight loss at 950 °C ( volatiles ) is a rough indication of the oxygen content of a carbon black. 

The content of volatiles is often used as a measure of the degree of oxidation of carbon blacks. This is defined as the weight loss on heating at 950 °C (DIN 53552). Typical values for the weight fraction of volatiles are 1-3% for furnace blacks, 4-8% for gas blacks, and up to 25% for oxidized gas blacks. 

Blacks used to produce conductive and antistatic plastics are chiefly high-structure furnace blacks with relatively fine particles and low contents of volatile constituents. Black concentrations are around 10-40% for conductive systems. Antistatic plastics (e.g., cable sheathing and floor coverings) contain 4-15% carbon black. 

The furnace black process is capable of producing a chemically pure, fine-particle carbon black with low volatile content, 1-2%, and pH ranging from 6 to 10, which is suitable for most plastics end uses. This process allows precise control of a carbon black s particle size and shape (or morphology), which ensures uniform color and physical properties in plastics applications (Fig. 11.1). 

The chemical nature of the surface of carbon black is crucial to its applications-related behavior and in the first instance is a function of the manufacturing process. In addition to physically adsorbed organic substances, chemically combined surface oxygen is present on the surface, which is formed upon pyrolysis in the presence of sufficient oxygen and accounts for the acidity of gas blacks. Furnace blacks produced in oxygen-poor conditions with feedstocks with low sulfur contents have an excess of basic metal oxides on their surfaces and have neutral to alkaline properties. The content of physically and chemically bonded species is known as the volatile content, since it can be removed by heating to 950°C in the absence of air. 

Heating loss is used to determine moisture content in carbon black. The drying is performed at 125°C for 30 min. Under these conditions moisture is removed but some other volatile materials may also be lost. The automatic equipment such as drying balances is also used (note that carbon black does not absorb infrared rapidly therefore, other sources of heat are normally used). This method gives precise readings because it avoids errors due to reabsorption of moisture. 

The outgassing at room temperature will remove only the most loosely bound water from the carbon surface. The blacks were then heated to 100-110C for prolonged periods of time beyond the point where the contact potential difference (CPD) ceased to change in time. The CPD was followed during cooling back to room temperature. The CPD samples prepared in this manner should be in approximately the same state with respect to water content and other loosely bound volatiles as the carbon black samples used in the titration and EM experiments. 

The pH of the carbon blacks is used to indicate their relative acidity or basicity, because the properties of these materials (i.e., adhesion and charge injection in carbon-polymer composites, electrochemical behavior) depend sensitively on the surface chemical and electronic sfructure [174, 222, 223]. This parameter is usually related to the amount of chemisorbed oxygen present on the surface, and to the very small amount of water soluble salts present in the material. The pH of carbon blacks is neutral to slightly alkaline for most grades, due to the low amount of chemisorbed species. To enhance certain properties, carbon blacks can be oxidised, with a subsequent increase in the amount of chemisorbed oxygen groups. These carbon blacks, therefore, show an acidic pH values, and relationships have been reported between the pH of carbon blacks and their content of volatile matter [223]. 

A typical application of TGA is its use in compositional analysis. For example, a particular polyethylene part contained carbon black and a mineral filler. The electrical properties were important in the use of this product and could be affected by the carbon black content. TGA was used to determine the carbon black content and mineral-filler content for various lots, which were considered either acceptable or unacceptable. The samples were heated in nitrogen to volatilize the PE, leaving carbon black and a mineral-filler residue. The carbon content was then determined by switching to an... 

Plastics waste can also serve as a source of chemical raw materials. The potential possibilities are considerable, here, since about 25%-30% of plastics consumed are thrown away as waste each year. The following process has proved to be useful hydrolyzable plastics are first hydrolyzed to their monomers below about 200° C the monomers are fractionally distilled off. Then, the poly(vinyl chloride) in the mixture is dehalogenated to poly(olefins) at about 350° C. The residues are then pyrolyzed at about 600-800° C in a sand-fluidized bed. The product fractions are very dependent on the composition of the pyrolyzed material. Generally, however, up to 40% fractions of the economically desirable aromatics are obtained by this high-temperature pyrolysis, and, indeed, when additional steam is blown into the system to reduce carbon char formation. Alternatively, what is known as a low-temperature pyrolysis can be carried out at about 400° C in poly(ethylene) wax as reaction medium. In this case, readily volatile oils of high olefin content are obtained together with waxes and carbon black. 

With regard to purity, carbon blacks consist of almost 100% pure carbon, with a semigraphitic structure, but the particle smfaces have oxygen-rich reactive functional groups such as phenolic, ketone, quinone, lactone, hydroxyl and carboxyl. Their concentration is proportional to the volatile content , which is a positive factor in UV stabilisation. 

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