Measuring the oil absorption of carbon black. (Process Machinery).

Measurement and analysis of the oil absorption number (OAN) of carbon black and other powders according to ASTM D2414 and D3493 can be accomplished with the Absorptometer "C" drive and data acquisition system. The "C" model, a redesigned and modernized version of the "E" model, has the capability to improve carbon black, silica and other powder testing precision by obtaining and analyzing an absorption curve via customized software. The software is 100% Windows-based. Other improvements include replacement with a more versatile burette, construction of a more easily detachable mixer assembly, and the option for local or remote operation for up to four individual drives. These rugged, compact and accurate systems are said to meet the demanding requirements of research, quality control and production laboratories.

Instrument description

The instrument is comprised of a frequency inverter drive unit for precise torque measurement, a high-precision measuring mixer with special blades, a high-precision burette and 32 bit software for Windows 98, ME, XP and 2000. The drive is housed in a modern stainless steel cabinet. It is capable of generating up to 15,000 mNm of torque. The mixer assembly, previously bolted to the drive coupling, has been fitted with a back stand so that it can be attached to a mounting pad and drive coupling in seconds. Safety contacts on the mixer prevent inadvertent contact of hands or other objects with the rotating mixing blades by automatically shutting down operation. The mounting platform facilitates removal of an existing mixer and replacement by a spare mixer. While one mixer is being cleaned, the other can be running a test. The burette can be programmed to titrate fixed or varying amounts of liquid during an absorption test. The data generated by the instrument are sent to a PC that utilizes a proprietary software program to measure, record and analyze the sample curve.

Measurement principle

Oil absorption measurement is one of the primary techniques used to characterize the physical properties of carbon black. More recently, the introduction of silica as a filler in rubber compounds has spurred the need to understand and characterize silica oil absorption properties as well. The oil absorption number of carbon black and other fillers is directly related to the processing and vulcanization properties of the rubber compounds prepared with these materials.

The Absorptometer measures the torque response of a sample dosed with oil. After a sample of powder has been poured into the mixer, oil is titrated into the mixing chamber at a constant rate. As the oil mixes with and wets the sample, three distinct material phases can be observed in the torque curve, including the free flowing phase, the agglomeration phase and the end phase.

During the free flowing phase, liquid is absorbed into the structure of the carbon black, but does not cause the particles to stick to one another. Upon absorption of sufficient liquid, the particles begin to adhere to each other to form agglomerates. The resistance to mixing continues to increase more rapidly, shown by a sharp increase in torque. As the maximum torque (saturation) is reached, the sample undergoes a state change from a liquid in a solid to a solid in a liquid.

As an instrument validation method, standard reference blacks (SRBs), materials developed by the carbon black industry, are used to verify proper operation of the instrument. The SRB series carbon blacks have a wide range of physical and chemical properties typically seen in the carbon black applications, and have become benchmark materials that the industry uses to ensure that instruments and materials are within calibration.

Data acquisition software program

The user-optimized, modern 32-bit software of the Absorptometer "C" data acquisition system is said to offer numerous advantages for daily laboratory use. Chart recorders are no longer necessary to plot curves that then have to be subjectively interpreted. All data are now sent to a PC where the test data and graphics can be stored, printed and saved for future reference.

The test setup screen can be filled in with the information about the sample, batch number, sample mass, date, etc. In contrast to older models, the instrument and burette no longer must be stopped automatically when the torque limit switch is reached. They can now be run according to the new procedure in ASTM 2414. Certain grades of carbon black fail to give endpoint values because of insufficient torque generation. The preferred method for determining results is to record the torque curve, apply a mathematical fit to the curve and then read the endpoint at 70% of the curve torque maximum. The software applies a third order polynomial fit curve to the measured curve in order to yield more consistent absorption numbers.

The operator can also set specific saving options and instruct the software to evaluate torque values at the desired percentage points. Upon completion of a test, the polynomial fit curve calculates the absorption values that can be printed out with the sample information. A test printout shows test information and the graph of the curve with all appropriate information. Set testing patterns can be saved and retrieved to reduce retyping of information. Additional options include system normalization with standard reference carbon blacks, including normalization with other grades of carbon blacks (for laboratories with their own reference carbon blacks).

The Absorptometer "C" has simplified and modernized the oil absorption measurement process. It is a capable system with the flexibility to work with all powders under different testing speeds and titration rates. The acquisition software improves precision by measuring, recording, analyzing and testing data. Both hardware and software have taken a large step towards helping users better understand and characterize the behavior of their materials.

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