Scope

This International Standard specifies a method for the determination of the nitrogen surface area (NSA) of carbon blacks and other rubber compounding ingredients, like silicas and zinc oxides, based on the Brunauer, Emmett and Teller (BET) theory of gas adsorption using a multipoint determination as well as the determination of the statistical thickness surface area (STSA), otherwise known as the external surface area. STSA, however, is not applicable to silica and zinc oxide.

The method can also be used for verifying “single-point” procedures described in ISO 4652 and other standards.

Normative references

The following documents, in whole or in part, are normatively referenced in this document and are indispensable for its application. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document (including any amendments) applies.

• ISO 4652:2012, Rubber compounding ingredients — Carbon black — Determination of specific surface area by nitrogen adsorption methods — Single-point procedures

• ISO 5794-1:2010, Rubber compounding ingredients — Silica, precipitated, hydrated — Part 1: Non-rubber tests

Principle

A sample of carbon black, silica, zinc oxide, etc., is placed in a cell of known volume and degassed. Known volume of nitrogen gas is dosed into the cell that is kept at the temperature of liquid nitrogen. The pressure in the cell decreases to the equilibrium due to the adsorption by the sample. The adsorbed amount at the equilibrium is derived from the difference between the amount of the dosed gas and that of remainder in the cell. The pressure is measured together with the temperature in the system, and the volume of the system is measured with a gas that does not adsorb such as helium before the test. NSA is determined by the BET analysis on the adsorption amount obtained, and STSA is determined by t-plot analysis.

Apparatus

1 Automatic volumetric adsorption measurement apparatus, consisting of a pressure gauge or transducer, a calibration volume, a Dewar flask and all other accessories required for the analysis.

The pressure gauge or transducer shall have the ±0,25 % accuracy of full scale at the range from 0 kPa to 133 kPa.

Calibration volume is where the volume of the nitrogen to be adsorbed is measured and shall have a known internal volume with a valve or stopcock. It shall have been calibrated by its manufacturer prior to testing.

Sample port volume is where the adsorption to samples takes place, and the pressure at adsorption equilibrium described in Clause 3 is the pressure of this part. A connector connects the apparatus with an adsorption cell.

NOTE Commercially, there are two types of the apparatus with different calibration method for the volume of the adsorption cell; those which a calibration factor is applied to the volume of the blank cell determined prior to testing, and those which the volume of the cell with a sample is directly measured during testing (with or without parallel measurement of the volume of another blank cell). Either type is suitable.

2 Adsorption cell (hereinafter called “cell”), made of glass and capable of maintaining a vacuum at a pressure 2,7 Pa or below when connected to the adsorption apparatus.

A stopper or valve needs to be attached in order to avoid adsorption after drying.

3 Analytical balance, with 0,1 mg sensitivity.

4 Degassing station and heating mantle, capable of maintaining the temperatures in Table 1 together with vacuum suction or purge gas flow in order to perform degassing procedure specified in Clause 7.

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