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

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.


Introduction
Hirth and colleagues 1 have recently investigated the distribution of organic material (binder and mineral oil) on stone wool bres. The work follows up previous publications by the authors: Wohlleben et al., 2017 2 and Sauer et al., 2021. 3 The starting point for their publications is the authors' view that hazard assessment of man-made vitreous bres (MMVF) is solely based on biodurability measurements of naked bres (i.e. without binder). Similar to the previous discussion, 4 we would like to bring attention to the fact that in vitro acellular biodurability tests either on bres with or without binder are not relevant for the hazard assessment and regulations on mineral wool bres. Actually, MMVF hazard assessment includes investigation of bre biopersistence via in vivo animal studies with typically nasal inhalation or intratracheal installations of bres produced without binder 5,6 (Note Q of the European Regulation (EC) No. 1272/2008 (CLP) (EC2008)) and epidemiological studies on workers, where the impact of bres produced with binder is studied, both recognised at international and European level. [7][8][9][10][11][12][13] Despite this, the papers 1-3 attempted to nd differences in the in vitro acellular behaviour of bres with and without binder, using binder removal techniques that modify bre chemistry, 14,15 wettability and thus likely solubility. 16,17 The paper 1 explored the distribution of binder (presumably phenolurea-formaldehyde, PUF) on stone wool bres and tried to nd a correlation between dissolution rate of stone wool measured in a simulated lung uid (phagolysosomal simulant uid, PSF) and the amount and thickness of organic material on the bre surface. The article 1 reports the use of surface sensitive techniques, such as X-ray photoelectron spectroscopy (XPS), time-of-ight secondary ion mass spectroscopy (ToF-SIMS) mapping and modelled XPS data with QUASES soware. 18 The general ndings of the paper about organic matter obtained with XPS and QUASES XPS modelling are in line with previously published results on stone wool samples. 19,20 However, we would like to stress several points concerning the assumptions and hypotheses in the publication, 1 analytical techniques limitations in spatial resolutions and interpretation of results and as well present the newest results using QUASES XPS and ToF-SIMS on stone wool samples with PUF binder.

Discussion
The assumptions on biopersistence assessment Hirth et al. 1 state that bre biodurability is currently assessed on "naked" bres (i.e. produced without binder) because there is an assumption that bres produced with organic matter (binder) would not have a completely coating of the bre, and that this would rather be localised in the areas where bres enter into contact and thus leaving large fraction of the bre surface uncoated. It has to be mentioned that for biopersistence tests, bres produced without binder are traditionally used also for other reasons. In in vivo studies, 5,6 bres without binder are recommended because aerosol sizing, bre diameter measurements and sterilization of the test material are impaired by the presence of binder. † Binder presence also causes bres to agglomerate, which may result in suffocation of the animals aer intratracheal instillation. As in real inhalation scenario, respirable bres are present as single bres, while larger agglomerates are not able to reach the alveolar region of the lung, this should also be avoided in the in vivo tests by using bres without binder. † However, in an earlier in vivo study it is shown that stone wool bres produced with and without binder perform similarly (Wagner et al., 1982, 21 Experiment 1 for stone wool bres injected intrapleurally). Fibre safety is also largely explored by epidemiological studies 7-13 at manufacturing sites, where no adverse effects of stone wool bres as produced, meaning possibly with binder, 31 are found on workers. Epidemiology is the rst type of studies that IARC 22 is using to investigate carcinogenicity of substances, including stone wool bres, followed by in vivo investigations. Thus, today, bres' biosolubility in vitro (acellular and cellular) is not the key indicator to assess the stone wool bres hazard assessment.

Incomplete information on composition of test material and organic matter
No details are provided in the paper regarding composition of the stone wool bres, unlike in previous publications by the authors. 2,3 The lack of information about bre composition makes it difficult to follow the dependence of the bre dissolution rate on the inorganic composition of the bres, which authors concluded to be the main factor.
The authors 1 state that they expect phenolic resin to be commonly used as a binder based on their nding of traces of nitrogen but no further information on the organic binder chemical composition is provided. In the paper the binder appears to be treated as a classic organic molecule without further differentiation of other binder components (such as oil, coupling agent etc.). We further note that providing SEM images (a standard technique for the study of microstructure on bre surfaces) of the stone wool bres would have been benecial and would have enabled the distinction between micrometre size areas with binder and the rest of the surface.

Resolution of ToF-SIMS and XPS results
We would like to highlight that the used low ToF-SIMS mapping resolution could give the impression that the signal coming from the bre surface is dominated by carbon from oil and binder (as the oil and binder are on the top of the bres). We do not think that this is sufficient documentation in the paper to conclude, that bres are coated almost at 100%. In another recent study, 16 yet with different ToF-SIMS resolution (300 × 300 mm; 128 × 128 pixels), ion source (Bi 1 + ) and binder applied to the bres (sugar-based binder, SBB), it was possible to observe a signal from the bre substrate itself (Al + ), indicating that binder does not completely coat the bre surface. In Fig. 1 we present recent ToF-SIMS imaging results on stone wool bres with PUF binder from Barly et al., 2019 23 (F3 sample, 3.6 wt% PUF binder, 0.1 wt% mineral oil) performed with the same ToF-SIMS settings as in Okhrimenko et al., 2022. 16 The signal from bre substrate (Al + , Fig. 1a, a1) is dominating over the signal from organic layer originated likely from PUF binder (C 7 H 7 + ) and oil (C 3 H 7 + ) in many areas on the bre surface ( Fig. 1b, b1, c and c1), indicating that PUF binder and mineral oil coverage is neither uniform nor complete.
It is important to consider the inuence of the ToF-SIMS mapping spatial resolution and other settings on the conclusions about binder distribution on the bres. This would help to understand why bres with different binders (PUF and SBB) and bres without any binder perform similar in dissolution tests as found in. 16,23 We also note that the journal number and year for the work by Barly  For the XPS results, no survey (wide-scan) spectra of the studied stone wool samples are presented. This does not allow to check the presence of additional chemical elements in the different stone wool samples that were compared. Complete survey spectra would have enabled the authors and the readers to get a rst qualitative view of comparison between the samples. Moreover, it would be benecial for the readers if it was acknowledged that both methods, XPS and ToF-SIMS, are extremely sensitive towards contaminations by adventitious carbon, which can originate from bre storage and handling, as well as from apparatus in situ, and interfere with the performed analysis, reducing its representativity.

Limitations of the modelling of surface layer thickness
We would like to note that the results of the QUASES XPS modelling to support the hypothesis that binder and mineral oil completely shield the surface of the bres should be interpreted with greatest caution.
QUASES XPS modelling works the best if reference spectra are available, i.e. in this case this would be a spectrum of "naked" bres without organic matter on their surface. In the absence of reference spectra, several models describing experimental XPS spectra are possible. The authors 1 chose to simulate the surface layer in a similar way as in the study by Okhrimenko et al., 2018, 20 i.e. as a uniform carbon layer with thickness 1-3 nm on top of the bres. While the approach of using the background of the Si and Al XPS peaks to determine the binder distribution can be relevant at the considered thicknesses (#10 nm), we note that binder droplets can be thicker (30-50 nm) and they are "blind" spots for QUASES analysis.
With QUASES soware version 7.5, the results from Okhrimenko et al., 2018 20 can be re-evaluated using the automation option. QUASES v.7.5 uses the simplex method to determine the combination of all structure parameters which gives the minimum root mean square, RMS, between the spectrum and the background in the desired energy range. Using the automated structure determination facility that varies the structure until the RMS deviation in the 1270 to 1310 eV energy range reaches a minimum, we observed an improved t of the XPS spectra when the soware applies the model where 20% of the bre surface remained uncoated (Fig. 2a (Fig. 2b, RMS 20.3 × 10 −4 ). This reduction in RMS is substantial but it should also be supplemented by a visual inspection of the spectra: the t in Fig. 2a is seen to be virtually perfect in the full energy range from 1270 to 1295 eV, whereas there are clear deviations in Fig. 2b in this energy range. Any other structural model also gives substantially worse ts to the background.
Presence of the organics-free areas correlates with ToF-SIMS mapping results presented in our work for PUF treated bres and by Okhrimenko et al., 2022 16 for SBB treated bres and also with the similar dissolution behaviour of the bre material irrespective if binder was applied or not 23 and which type of the binder was applied. 16 To sum up, the realization in the paper 1 analyses without reference to neither the substrate nor the binders and without proof of the goodness of the modelling (only one example provided with narrow energy range, 1225-1345 eV) and the fact that their XPS spectrum was recorded with a rather low signal to noise level does not allow to ensure the solidity of the results and the ndings. In addition, we note that information about how the general background is accounted for in the analysed energy region would have been very helpful. In QUASES XPS this is done by subtracting a straight which is tted to the spectrum on the high energy side of the region. However, if peaks are present in this region (which is clearly the case here as seen in Fig. 2), the slope of this line can be uncertain and this adds to the uncertainty of the analysis. We avoid this problem by including in the analysis all peaks on the high energy side in the full energy range.

Fibre dissolution and dissolution rate evaluation
The paper 1 acknowledges that binder thickness is not a predictor for dissolution rates. This is conrmed by Fig. 7a in the paper, 1 showing no correlation between dissolution rates and total binder content determined with thermogravimetry (TGA). It even shows that there is a reverse dependence of the dissolution rate on organic layer thickness in Fig. 7b of the paper 1 (i.e. the higher the thickness of the organic layer, the faster bres dissolve). The results contradict with the authors' 1 hypothesis that the bres are completely coated with the binder. The same authors previously demonstrated that mass loss of bre with binder can reach up to 10% aer 30 days of dissolution in PSF 2 and higher in liquids with citrate (up to 100% within few days 3 ). Taking into account such mass losses during dissolution and no time delay of the dissolution in the beginning of the tests, one can hardly expect any surface shielding effects by binder/organic layer shortly aer beginning the dissolution test. The observed bre (with binder) dissolution can be explained by the fact that the organic layer on the bre surface is incomplete and inhomogeneous in reality and leaves bare surface available for dissolution, as we have just shown with the newest QUASES XPS modelling and ToF-SIMS results presented here for PUF-and recently for SBB-treated bers. 16 Therefore, Hirth et al., 2021 1 results conrmed that binder presence cannot affect the dissolution of the stone wool bres. It was shown that it is the inorganic chemical composition of the bre that is among of the prime factors in in vivo pathogenicity 5,6 and for in vitro cellular 24,25 and acellular 26-28 dissolution rates. Besides that, experimental conditions 16,28-30 (e.g. uid ow rate to sample surface area ratio, uid composition, temperature, pH, dynamic or batch experiment and sample preparation) are crucial for the determination of the dissolution rates in in vitro acellular studies.

Conclusions
In conclusion, we nd that there are several methodological limitations in the article, which might provide an incorrect image of the dissolution and biosolubility of stone wool bres. The conclusions made by Hirth et al., 2021 1 are in contrast with the existing science and regulations on biopersistence of stone wool bres and other MMVF bres. The present authors hope that provided comments, the additional examples of QUASES XPS modelling approach and application of ToF-SIMS mapping technique would support a better understanding of the biosolubility of the stone wool bres, accepted terminology and existing regulations on MMVF biopersistence.

Conflicts of interest
The authors declare following competing nancial interest(s): D. V. O. and M. S. are employees of ROCKWOOL A/S, a company producing stone wool bres. E. P. is employed by Knauf Insulation, a company producing stone and glass wool bres.