Analytical pyrolysis of poly(dimethylsiloxane) and poly(oxyethylene) siloxane copolymers. Application to the analysis of sewage sludges

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Highlights

  • Pyrolysis of PEG dimethicones evolved both cyclic and linear dimethyl siloxanes.

  • Pyrolysis of PEG dimethicones produced clusters of ethylene oxide derivatives.

  • Mass spectra of distinctive pyrolysis products characterised by m/z 133.

  • Figures of merit of Py-GC–MS for PEG-12 dimethicone analysis resulted satisfactory.

  • Good recovery of PEG-12 dimethicone from Py-GC–MS of sewage sludge extracts.

Abstract

Dimethicone (poly(dimethylsiloxane), PDMS) and its copolymers bearing a poly(ethylene glycol) (PEG) side chain (PEG-12 dimethicone and PEG-8 dimethicone) are utilised as ingredients in personal care products. These materials along with bis-PEG-18 methyl ether dimethyl silane were analysed by Py-GC–MS. The pyrolysates of dimethicone was dominated by cyclic dimethyl siloxanes from D3 to over D10. Besides Dn, the pyrolysates of PEG-dimethicone were featured by the presence of linear dimethyl siloxanes Ln. Hydroxylated siloxanes were also tentatively identified. Pyrograms were also characterised by PEG oligomers with different terminal groups (hydroxyl, ethyl, ethenyl). The molecular structure of pyrolysis products from the combined structural units of PDMS and PEG could not be assigned. Internal standard calibration protocols using pyrolysis products of PEG-12 dimethicone (L6, L8, L9, and triethyleneglycol monoethyl ether) exhibited satisfactory linearity (R2 > 0.992) in the 0.75−58 μg range and repeatability (RSD < 15 %). Three samples of dried sewage sludge from a municipal water treatment plant were extracted with tetrahydrofuran and the extracts analysed by Py-GC–MS. The pyrograms were characterised by intense signals due to the lipid matrix (hydrocarbons, fatty acids, steranes, sterenes, sterols). Dn and Ln were identified indicative for the presence of siloxanes at levels around 60−290 μg gdw−1, while pyrolytic markers of PEG were not revealed. The recovery of PEG-12 dimethicone (17 μg) in spiked sample was 76 %. Galaxolide, galaxolide lactone and triclosan were identified in the pyrograms. The results of this study evidenced the potential of Py-GC–MS as a screening tool for the determination of cosmetic ingredients in sewage sludge.

Introduction

Polysiloxanes are inorganic polymers consisting of a backbone of alternating silicon and oxygen atoms, although the term silicones is often used to designate polysiloxanes with predominant dimethylsiloxane units. Silicones are uniquely man-made materials that depending on their molecular weight and substituents can appear in different forms and properties. Therefore, they have found applications in a variety of sectors such as agriculture, transportation, construction materials, electronics, energy, healthcare, industrial processes, personal care products and so forth [[1], [2], [3]]. The worldwide production of silicones is several million tons per year; the personal care production presented the third-largest application (industrial processes 35 %, construction materials 26 % and personal care products 17 %) [3]. Geographically, the largest silicon market is Europe and the categories products are toiletries, skin care, hair care, fragrances and make-up [3]. The predominant silicone involved in personal care products is poly(dimethylsiloxane) (PDMS), commercially known as dimethicone according to the International Nomenclature of Cosmetic Ingredients (INCI). Besides the homopolymer PDMS, copolymers bearing poly(oxyalkylated) substituents, especially poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG) derivatives are widely used in personal care products [4]. They were originally called dimethicone copolyols and the name has been commercially replaced by PEG (or PPG) dimethicone and bis-PEG (or PPG) dimethicone [5]. Depending on the location of the poly(oxyalkylated) substituents, they are classified in three primary configurations: end-capped polysiloxanes, alkoxy-polysiloxane/polysiloxane copolymers and combinations of the latter [6].

In this study, attention was payed to PEG-dimethicones because these copolymers are widely used in pharmaceutical and personal care products [2], principally as emulsifiers [4] thanks to the presence of the water-soluble group (PEG) that makes dimethicones surface active and water-soluble, and because of their low toxicity [7]. Given their large use, it is expected that these products could enter wastewater treatment plants (WWTP). Thus, the knowledge on the occurrence of these polymers in treated wastewaters and sewage sludge is important in order to identify the existence of potential source of contamination in the environment.

As other water soluble or liquid polymers, PEG-dimethicones have not attracted the same attention of water insoluble solid polymers, such as microplastics, as potential environmental contaminants. However, water soluble polymers are produced in large quantity and could reach the environment if not efficiently removed by degradation or sorption in WWTP [8]. Moreover, some water-soluble polymers are persistent and should not be ignored just because they do not fall within the category of microplastics [9]. The analytical determination of trace concentrations of polymers in a complex heterogeneous matrix is challenging and requires selective and sensitive techniques as those based on mass spectrometry. In a proof of principle concept, Huppertsberg et al. [8] have identified PEG in wastewater effluents by mass spectrometry with in-source fragmentation technique that convert PEG into distinctive fragment ions (C4H9O2+, C6H13O3+, C8H17O4+ and C10H21O5+).

Analytical pyrolysis (Py) combined with gas chromatography-mass spectrometry (GC–MS) has demonstrated its validity for the determination of microplastics in various matrices [[10], [11], [12]]. Instead, the potential of Py-GC–MS to the analysis of liquid or water-soluble polymers in environmental samples has not been deeply investigated [13].

A fundamental step in developing new analytical methods by Py-GC–MS is the knowledge of the thermal behaviour of the polymers under investigation. Several articles reported on the analysis of PDMS [[14], [15], [16], [17]] and PEG [18,19] by Py-GC–MS, but to the best of our knowledge no articles have been published dealing with Py-GC–MS of their copolymers (virtually between dimethylsiloxane and methylsiloxane bearing a poly(ethylene glycol) side-chain).

The main purposes of this study were (1) to gather information on the molecular composition of pyrolysates of dimethicone and PEG-dimethicones, (2) evaluate the potential of Py-GC–MS for their quali-quantitative analysis in complex organic matrices such as sewage sludge.

Section snippets

Materials

Polydimethylsiloxanes from Dow Corning were kindly provided by Prof. Luca Valgimigli, University of Bologna. Namely, INCI name (commercial name): dimethicone (ACESIL 350), PEG-12 dimethicone (XIAMETER® OFX-0193 Fluid), PEG-8 dimethicone (FANCORSIL® LIM-1), and and bis-PEG-18 methyl ether dimethyl silane (DOWSIL™ 2501 Cosmetic Wax). Dimethicone and copolyols were dissolved in tetrahydrofuran (THF, Sigma Aldrich) to prepare calibration solutions with final concentrations in the 0.02-13 mg mL−1

Pyrolysis products from the siloxane chain

The mass spectra of some methylsiloxanes belonging to different groups (cyclic, linear and with hydroxyl groups) identified in the pyrolysates of the investigated polymers are presented in Fig. 2.

The pyrograms of dimethicone (an example is shown in Fig. 3a) were dominated by cyclic methyl siloxanes (Dn) in agreement with literature data on the pyrolysis of PDMS [21] [24]. It has been reported that the proportion of the various Dn formed upon thermal degradation decreases from D3 to higher

Conclusions

Flash pyrolysis of PEG dimethicones produced cyclic dimethyl siloxanes and ethylene oxide derivatives evolved from the PDMS and PEG chains, respectively. In addition, linear dimethyl siloxanes were produced which were proposed as pyrolytic markers of siloxane copolymers as they were not present in the pyrolysates of the homopolymer. The chemical structure of pyrolysis products indicative of the polyoxyethylene methylsiloxane moiety remained elusive. Even though not specific of PEG dimethicones,

Authorship contributions

Please indicate the specific contributions made by each author (list the authors’ initials followed by their surnames, e.g., Y.L. Cheung). The name of each author must appear at least once in each of the three categories below

Category 1

Conception and design of study: D. Fabbri, A.G. Rombolà.

acquisition of data: I. Coralli.

analysis and/or interpretation of data: D. Fabbri, C. Torri, A.G. Rombolà, Irene Coralli.

Category 2

Drafting the manuscript: D. Fabbri.

revising the manuscript critically for

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgements

All persons who have made substantial contributions to the work reported in the manuscript (e.g., technical help, writing and editing assistance, general support), but who do not meet the criteria for authorship, are named in the Acknowledgements and have given us their written permission to be named. If we have not included an Acknowledgements, then that indicates that we have not received substantial contributions from non-authors

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