Comment on "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" by Hirth et al., 2021, RSC Adv., 11, 39545, DOI: 10.1039/d1ra06251d

Abstract

The article mentioned in the title of this comment paper reports on an investigation of the organic binder presence and distribution on stone wool fibres with surface sensitive techniques (X-ray photoelectron spectroscopy (XPS), QUASES XPS modelling, time-of-flight secondary ion mass spectrometry (ToF-SIMS) mapping) and attempts to correlate the results with fibre performance in in vitro acellular biosolubility tests. However, the study has assumptions, hypothesis and results that do not take into account the recognised science and regulations on biopersistence of stone wool fibres, limitations of the utilized surface sensitive techniques and modelling approach and it contains a contradiction with biosolubility experiments. In this comment article, we discuss these points, propose improved QUASES XPS modelling and present recent ToF-SIMS mapping results that reflect biosolubility behaviour of the stone wool fibres.

Fig. 1. ToF-SIMS imaging results of F3 stone wool fibres (3.6 wt% PUF binder, 0.1 wt% mineral oil) from Barly et al., 2019: (a-a1) Al⁺; (b-b1) overlay of Al⁺ (red) and C₇H₇⁺ (green); (c-c1) overlay of Al⁺ (red) and C₃H₇⁺ (green) and (d-d1) overlay Al⁺ (red) and total intensity (green). The images (a–d) are obtained with Bi₁⁺ ion source and other settings similar to those in Okhrimenko et al., 2022. The images (a1–d1) are obtained with Bi₃⁺⁺ ion source similar to Hirth et al., 2021.

Fig. 2. XPS spectra QUASES fitting using model: (a) with 80% of the surface covered with 7.1 nm organic layer and 20% of the uncoated surface; (b) uniform organic layer of 4.0 nm thick (Okhrimenko et al., 2018).