Which fraction of stone wool fibre surface remains uncoated by binder? A detailed analysis by time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy

Abstract

Biodurability of man-made vitreous fibres (MMVF) is often measured on naked fibres, i.e. fibres devoid of the phenol-urea-formaldehyde (PUF) binder that is sprayed and baked on the commercial product to reduce dustiness and to provide mechanical strength to fibre mats. This simplification of the hazard assessment relies on the assumption that the binder would not actually coat the entire fibre surface, but would occur only at the touching points where two fibres are glued together. We challenged this assumption by using surface analysis by X-ray photoelectron spectroscopy (XPS) and Time-of-Flight Secondary Ion mass spectrometry (ToF-SIMS). We analysed commercial stone wool MMVF sourced from Denmark, United Kingdom and Germany. XPS as well as ToF-SIMS-mapping combined with gas-cluster-ion-sputtering revealed that all mineral fibres investigated show a complete layer of organics over the surface of the fibres with only a few defects: before sputtering, organic components (PUF binder and oils) uniformly cover the spatial structures; only after sputtering, the inorganic components of the stone wool emerge on the visible surfaces. A preferential localisation of PUF binder on fibre-to-fibre touching points or as droplets was not observable. We finally explored the correlation to dissolution rates, but found that total PUF binder content and the experimentally determined thickness of the PUF binder layer are not sufficient to predict dissolution rates, which instead must consider chemical composition and other properties. In summary, none of the investigated stone wool fibre surfaces were uncoated by the PUF binder.

Fig. 1. Result of the XPS-quantification of the survey spectra given in atom-%. The error bars describe the standard-deviation across the three measurement positions. Note: one measurement position of UK1 showed extremely low carbon amounts and is therefore presented as a separate measurement.

Fig. 2. The buried layer model was used to obtain the film thicknesses from the Si 2s signals with QUASES Analyze 7.04 (IMFP 3.5 nm, angle of emission 45°)- (a) example fit of DK1, (b) resulting coating thicknesses that show considerable spread in correlation to the amounts of carbon and nitrogen that were detected on the fibres respectively.

Fig. 3. The loadings of Factor 1 (upper panel) and Factor 2 (lower panel) of the PCA analysis across the set of MVVF investigated in the range of 0–200 m/z.

Fig. 4. Intensity ratio of the elements calcium, magnesium, aluminium, and silicon on the surface referenced to their respective bulk intensity that was obtained after GCIB-sputtering to remove the coating. The intensities of said elements on the surface are very low for DK1 to DK5 and higher by a factor of 2–3 for UK1 and DE1, but still on a low level. For all materials, only a fraction between 1% and 15% may be considered as uncoated.

Fig. 5. ToF-SIMS-imaging results of the fibres DK1 to DK4 as received (upper line) and after GCIB-sputtering (lower line). Elements shown are (from left to right): Na⁺, Al⁺, total ion intensity, sum of all phenolic signals as stated in the text, C₃H₇⁺ as signal from hydrocarbon, RGB-overlay. The RGB overlays (rightmost panels) allow to check for the colocalization of Al⁺ (red), aromatics (phenol, in green) and C₃H₇⁺ (blue). All images are scaled in such a way, that the signal range is the same for as received and sputtered samples.

Fig. 6. ToF-SIMS-imaging results of the fibres DK5, DE1, DE2, and UK1 as received (upper line) and after GCIB-sputtering (lower line). Elements shown are (from left to right): Na⁺, Al⁺, total ion intensity, sum of all phenolic signals as stated in the text, C₃H₇⁺ as signal from hydrocarbon, RGB-overlay. The RGB overlays (rightmost panels) allow to check for the colocalization of Al⁺ (red), aromatics (phenol, in green) and C₃H₇⁺ (blue). All images are scaled in such a way, that the signal range is the same for as received and sputtered samples. For DE1 no sputter profile was obtained.

Fig. 7. (a): Relation of the binder mass to the dissolution rate. (b): Relation of the overlayer thickness to dissolution rate.