Carbon black is a fine particulate material formed through the incomplete combustion of carbon-based fuels such as coal, petroleum, and biomass. Once released into the atmosphere, it can absorb sunlight, thereby reducing the amount of solar radiation that reaches the Earth's surface. It is a black, powdery substance created by the partial combustion or pyrolysis of hydrocarbons in a gas phase under controlled conditions. Its primary composition is elemental carbon, with small amounts of oxygen, hydrogen, and sulfur present. The particles are generally spherical in shape, ranging in size from 10 to 500 micrometers. Many of these particles tend to aggregate, forming three-dimensional structures that resemble bonds or fibrous networks.
In the rubber industry, carbon black is widely used as a reinforcing agent and filler. It is one of the oldest industrial materials, with extensive applications in rubber, paint, ink, and other sectors. A key property used to classify and identify carbon black is its surface area. This is typically measured using gas or liquid phase adsorption methods. The most commonly used technique is the low-temperature nitrogen adsorption method, also known as the BET (Brunauer-Emmett-Teller) method. Since nitrogen molecules are small enough to penetrate the pores of carbon black, this method provides an accurate measure of the total surface area.
In recent years, macromolecular adsorption techniques, such as the CTAB (Cetyl Trimethyl Ammonium Bromide) method, have been developed. Unlike nitrogen, CTAB molecules are too large to enter the micropores of carbon black, so they only measure the external surface area, which is often referred to as the "smooth" surface. For most rubber-grade carbon blacks, which are non-porous, the results from the BET and CTAB methods are similar. However, for certain pigments, the difference between the two measurements reflects the roughness or porosity of the carbon black.
Another widely used method for measuring surface area is the iodine absorption method, which is valued for its simplicity and speed. However, it is influenced by the degree of oxidation on the carbon black surface. The BET method determines the total surface area (NSA), which includes both the external surface area (STSA) and the internal surface area of micropores. These micropores typically range from 2 to 50 nm in diameter.
For carbon black used in rubber reinforcement, the external surface area is more relevant, as the rubber molecules are too large to enter the micropores. In 1971, the CTAB method was introduced as an alternative way to measure the external surface area by determining the adsorption of CTAB in solution. Since CTAB molecules are larger (about 61.6 angstroms), they cannot access the micropores, making this method suitable for evaluating the surface area that directly interacts with rubber.
However, for some types of carbon black, there may be larger or medium-sized pores, ranging from 20 to 500 angstroms. In such cases, CTAB molecules might be able to enter the pores, but rubber molecules still cannot. Despite this, the CTAB method is not widely accepted due to its manual nature and lower accuracy. Additionally, the storage time of the CTAB reagent significantly affects the test results, requiring periodic preparation and calibration.
To address these challenges, the t-method was developed by J.H. de Boer in 1965. This method calculates the external surface area based on the thickness of the adsorbed nitrogen layer. By plotting the relationship between the adsorption layer thickness and the volume of nitrogen adsorbed, a straight line is obtained, allowing for precise calculation of the external surface area.
Today, the measurement of carbon black's external surface area must follow established standards. One of the most widely used is GB/T 10722-2003, which outlines the nitrogen adsorption method for determining both total and external surface areas of carbon black. This standard ensures consistency and reliability in testing procedures across industries.
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