Dissolution of Stone Wool Fibers with Phenol-urea-formaldehyde Binder in a Synthetic Lung Fluid

Abstract

Mineral wool products, composed of stone wool fibers and organic binder, are used in many construction applications. Among all their beneficial properties, the most important requirement is safety for human health, such as when fibers are inhaled. For determining long-term toxicity, biosolubility and biopersistence studies in vitro and in vivo are essential. In vitro fiber dissolution rate, which depends on the medium, fiber composition, and the surface available for dissolution, is a key parameter in determining biopersistence of the material in vivo. We investigated how organic binder (phenol-ureaformaldehyde), which can partially shield fiber surfaces from the solution, influences fiber dissolution kinetics in synthetic lung fluid (modified Gamble's solution) at pH 4.5 and temperature 37 °C, in vitro. Dissolution experiments were made in batch and continuous flow using stone wool fibers with typical insulation product binder amounts (0−6 wt %), applied by the standard industrial process. Dissolution rates were determined from element concentrations in the reacted solution, and changes in fiber surface composition and morphology were monitored. Stone wool fiber dissolution was close to stoichiometric and was similar, whether or not the material contained binder. The high dissolution rate (508 ng of fiber/cm 2 /h) is explained by Al and Fe complexing agents, that is, citrate and tartrate, in the synthetic lung fluid. The organic binder mainly forms micrometersized discrete droplets on the fiber surfaces rather than a homogeneous thick coating. During in vitro tests, fibers with organic binder preferentially dissolved in the areas free of binder, forming cavities, whereas the untreated fibers dissolved homogeneously. Propagation of cavities undermined the binder droplets, leading to complete fiber dissolution. Thus, presence of organic binder on stone wool fibers, produced by the standard industrial process, had no measurable effect on dissolution rate in synthetic lung fluid containing Al and Fe complexing agents.

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

“…a robust dissolution test but should simulate physiological conditions. The near-complete coating by the binder is one of the relevant factors, 9,19 and is supported by the re-analysis of MMVF surfaces by XPS aer dissolution testing: Barly et al found that, aer dissolution testing, the surfaces assimilated from any initial content of binder to a C/Si ratio of 7-10 (compared to a value (1 measured on the MMVF without binder), 8 which we interpret as indication of a universal organic-inorganic composite gel formation that is triggered by the binder, but is not directly scaling with binder content, consistent with our ndings.…”

Transformation of Glass Fiber Waste into Mesoporous Zeolite-Like Nanomaterials with Efficient Adsorption of Methylene Blue

“…In this study, the chemical composition of GFW is an aluminosilicate glass system, similar to Ca-Mg aluminosilicate stone wool fiber, but with less alkali content. Its surface layer has significant dissolution rates with sodium-based lung fluid and preferential leaching of matrix at the low pH of 4.5 [29]. Therefore, this study proposed transforming a valuable waste from the glass fibers into mesoporous zeolite-like nanomaterial (MZN) to adsorb methylene blue in an aqueous solution.…”

Simulated biological fluids – a systematic review of their biological relevance and use in relation to inhalation toxicology of particles and fibres

“…Many other examples of this phenomenon can be found in the literature (Hedberg et al 2010;Rozalen et al 2013;Boisa et al 2014;Utembe et al 2015;Barly et al 2019;Zhong et al 2019). In fact, an excellent correlation was shown between ALF and a sodium citrate buffer, suggesting that no other components of ALF contribute significantly to the extraction efficiency (Mukhtar et al 2015).…”

“…MgHCit and MgH 2 Cit þ ) form, peaking at pH 3-4 and falling by pH 6 (Rozalen et al 2013). Barly et al (2019) also attributed high stone wool fibre dissolution in acidic-modified Gamble's solution to the complexation effects of citrate and tartrate, namely on Fe and Al, and determined that dissolution rates were significantly faster than in solutions of similar pH where no complexing agents were present. Likewise, Steenberg et al (2001) found that citric acid greatly increased dissolution of stone wool fibres, which was not observed with the addition of acetic acid.…”

“…Therefore, the effect of citric acid, or indeed other complexing agents, should not be treated as biologically irrelevant and instead should be evaluated to ensure the extent of complexation matches that occurring within the lung, particularly when multiple complexing agents are present. In particular, consideration should be given as to whether citric acid at high levels is the best simulant for biologically relevant complexation, particularly due to the discussed concerns around aggressive dissolution (Morgensen 1984), non-linear or selective effects (Morgensen 1984;Hedberg et al 2011;Barly et al 2019), and how these impact predictivity and robustness; the level of predictivity that have been observed is considered later.…”