Fibre, fibre burning bright

A European research team has developed novel strategies for the rapid trace element analysis of metals in polyamide synthetic fibres by graphite furnace atomic absorption spectrometry and inductively coupled plasma mass spectrometry. Their method allows the accurate determination for quality control of polyamide products containing titanium dioxide as an optical brightener.

Isabel De Schrijver, Maite Aramendía and Frank Vanhaecke of Ghent University together with Ann Dumoulin of University College West-Flanders, Belgium, and Martín Resano of the University of Zaragoza, Spain explain in a forthcoming issue of JAAS that titanium dioxide is added to polyamide to help impart whiteness, brightness and high opacity to products, such as synthetic textiles. The worldwide annual demand for polyamide is almost 6 million tonnes.

The brightener is commonly added to polyamides, such as polyhexamethylene adipamide and polycaproamide, simply by combining titanium dioxide with the polymer. By virtue of its UV absorbency the titanium dioxide additive also protects synthetic fibres from sunlight. Other metal salts are also added to improve dyeing properties as well as increasing heat and light stability.

For example, one type of titanium dioxide used as a "delustrant" in fibres is coated with silicon dioxide, aluminium oxide and manganese oxide. The glassy surface provided by the silicon dioxide prevents the organic polymer from coming into direct contact with the catalytically active titanium dioxide surface, which makes the fibres more robust in outdoor use. The aluminium oxide helps keep the titanium dioxide particles apart preventing flocculation. The manganese oxide coating also reduces photocatalytic reactions with the polymer giving them better light fastness and stability.

Unfortunately, the degradation of the titanium dioxide particles and their additive layers can cause leaching of titanium, aluminium, manganese and silicon into during fibre preparation. This obviously reduces the quality of the final product, clogs up the equipment and also represents a waste water issue and reduces the cost-effectiveness of the manufacturing process overall. These additional trace elements require an extra step in the quality control of polyamide products and so a fast and reliable determination method for all these elements is needed.

The researchers point out that common acid digestion techniques are labour-intensive, time-consuming and also risk sample contamination, so they have developed an alternative dissolution procedure that uses formic acid to dissolve the polyamide, although this meant that formic acid has to be added to the calibration standards. They tested their approach on samples donated by BASF Antwerpen NV (Belgium).

The team could then use graphite furnace atomic absorption spectrometry (GFAAS) and inductively coupled plasma mass spectrometry (ICPMS) to determine inorganic content. Direct solid sampling methods were also applied with GFAAS and electrothermal vaporization (ETV)-ICPMS, which proved as accurate with improved detection limits. GFAAS requires sequential determination of the analytes whereas ETV-ICPMS has the advantage of simultaneous determination.

Whichever method is employed in quality control analysis it has to be able to fulfil the rapid sample throughout demands of the industry, the researchers add, it also has to be able to cope with silicon and aluminium analysis which are notoriously difficult.

The performance of the mass spectrometry method makes it well-suited to quality control analysis, the researchers conclude. It has the advantage of multi-elemental capabilities and a wider linear dynamic range than the atomic absorption approach. However, automated solid sampling devices are available commercially and GFAAS instrumentation is available at lower cost than MS equipment and these advantages could offset the benefits of MS, they say.