UV Chemistry and Mitigation of Siloxane

48th International Conference on Environmental Systems ICES-2018-284 8-12 July 2018, Albuquerque, New Mexico UV Chemistry and Mitigation of Siloxane Lance Delzeit1. NASA Ames Research Center, Moffett Field, CA, 94035 and ChaKaria J. Hunter2 KBR Wyle, Moffett Field, CA, 94035 The presence of dimethylsilanediol (DMSD) within the water supply of the International Space Station (ISS) causes issues with the Water Processing Assembly (WPA). It can degrade the WPA catalytic reactor’s effectiveness and cause the early replacement of Multifiltration Beds. The DMSD is believed to be a decomposition product of siloxanes which are condensed into the water system with the humidity condensate. This paper presents the ultraviolet (UV) gas phase photochemistry of siloxanes under both long and short wavelength UV radiation, and high energy electron bombardment. The results of this study showed that the siloxanes reacted into progressively larger and more complex moieties which eventually formed large non-volatile components which included their attachment to various accessible surfaces. Nomenclature Ar = Argon m = meter C = Carbon MF = Multi-filtration ca. = calculated MMST = monomethylsilanetriol Cd = Cadmium nm = nanometer DMSD = dimethylsilanediol O = Oxygen Dn = A cyclic Siloxane with n Si atoms OH = Hydroxyl group GCMS = Gas Chromatograph Mass Spectrometry ppb = parts per billion (by mass) H = Hydrogen atom ppm = parts per million (by mass) Hg = Mercury psi = pounds per square inch HS = Head Space SEM = Scanning Electron Microscopy ID = Inner diameter Si = Silicon ISS = International Space Station TMS = trimethylsilyl IUPAC = International Union of Pure and TOC = Total Organic Carbon Applied Chemistry ul = microliter J = Joules UV = Ultraviolet radiation kJ = kilojoules WPA = Water Processing Assembly Kr = Krypton Xe = Xenon Ln = A linear Siloxane with n Si atoms Zn = Zinc 1 Physical Scientist, Bioengineering Branch, and Mail Stop 239-15. 2 Research Engineer, Bioengineering Branch, and Mail Stop 239-15. I. Introduction HE International Space Station (ISS) has previously had issues with a shorter-than-expected life for its multi- T filtration (MF) beds, as well as it can degrade the Water Processing Assembly (WPA) catalytic reactor’s effectiveness.1 Both of these issues have been traced back to the presence of dimethylsilanediol (DMSD) and more recently monomethylsilanetriol (MMST) within the recycled water. Both DMSD and MMST are degradation products of siloxanes. DMSD can be formed directly from the cleavage of a monomeric unit of a cyclic siloxane or from the hydroxyl-dimethyl-silyl ends of a linear siloxane (Dn or Ln, respectively, where n is the number of Si atoms). MMST is the further decomposition of DMSD where an additional methyl group has been removed and replaced by a hydroxyl group.1,2 Plenty of literature exists describing the behavior of siloxanes. It can be best summarized as an equilibrium chemistry which will produce whichever product is not the dominant species; i.e., experiments starting with large siloxanes will produce smaller siloxanes, and experiments starting with small siloxanes will produce larger siloxanes. The reaction also requires the presence of water; although, in higher concentrations the water itself acts as an inhibitor.3,4,5 This research will look at the effects of short and long wavelength UV on the mitigation of siloxanes. It is believed that UV in the 300-400nm will probably not drive any photo chemistry since the energy of those photons are less than the energy required to break the molecular bonds. Photons around 250 nm would have enough energy to break most of the bonds present in a siloxane, and mercury lamps produce a strong band at 254 nm. Mercury (Hg) lamps also produce a line at 185 nm, which may or may not be blocked depending upon the quality of the emission window. The 185 nm line of a Hg lamp is generally only 3-9% of the intensity of the 254 nm line. II. Experimental Setup and Methodologies A. Gas Phase Experimental Setup The primary gas phase research, and the data presented in Appendix A and B, was conducted within the setup shown in Figure 1. The setup consisted of a GCMS headspace (HS) vial cap modified to hold a UVP Pen- Ray lamp. The HS vial cap was affixed to a fixture which had an ultra-torr fitting welded to it. The ultra-torr/cap fixture was then machined to accept and seal the Pen-Ray lamp. A HS vial screwed into this fixture, producing a sealed reaction chamber. A 1 ul sample of a siloxane (Table 1) was placed into the reaction chamber and processed for 5, 15, or 60 minutes. After processing, the Pen- Ray fixture was removed and a normal HS vial cap was placed on the vial. The sample was then analyzed using HS GCMS. The HS GCMS was performed by heating the vial to 200oC to ensure vaporization of the volatile and semi-volatile siloxanes. A 10 ul sample was then removed from the vial and injected directly onto the GCMS column. The column was a 60 m long, 0.320 ID, PerkinElmer Elite 5 (95% methyl 5% phenyl) column. The GC method was programmed to hold 30oC for 3 minutes, ramp to 300oC at 20oC per minute, and then soak at 300oC for 5 min to ensure removal of all siloxanes from the GC column. The analysis of the GCMS chromatograms are discussed below and presented in Appendix A. The vials were not specially treated, and as such, were filled with room Figure 1. Gas Phase Reaction air which contains oxygen and water vapor. If this process was used to Vial mitigate siloxanes within a space application, oxygen and water would be present within that application. The Pen-Ray Hg lamp reportedly heats to 100oC during operations. Additionally, the GCMS HS sampler heats the HS vial to 200oC to ensure vaporization. To check the possibility of thermal processing, a siloxane sample was place within a HS vial and allowed to thermally process at 200oC for 1 hour. No degradation products were detected due to the thermal processing in the HS vial at 200oC. The GCMS and Headspace methods used are presented in Appendix C. 2 International Conference on Environmental Systems B. Shorthand Nomenclature for Naming the Siloxane A general shorthand for naming linear and cyclic siloxanes already exists. They are described as Ln and Table 1. Siloxanes (shorthand) and Boiling Points Dn, respectively, where n is the number of Si atoms within the molecule. The Si atoms are all tetrahedrally Chemical (Shorthand) Boiling bonded, the backbone Si atoms are connected through an Point oxygen bridge, i.e., Si-O-Si, and any “missing” Tetramethylsilane (4-0) 26 6 coordination to the Si is a methyl group. Trimethylsilanol (3-1) 99 However, for this work, this nomenclature is DMSD (2-2) 122 inadequate to name the plethora of molecules observed MMST (1-3) 200 and the full IUPAC (International Union of Pure and Applied Chemistry) names are too long and tedious. Orthosilicic acid (0-4)) ? Thus, this paper will use the naming convention of replacing long, tedious parts of the IUPAC name with the Tetraethoxysilane (TEOS) 168 Ln and Dn nomenclature, where Ln and Dn are n Si atoms in a linear or cyclic configuration, respectively. For Hexamethyldisiloxane (L2) 101 example, a Dn with two Ln siloxyl groups coming off of Octamethyltrisiloxane (L3) 152 it would be named x,y-Ln-Dn, where x and y will describe the position where the Ln is attached. Additionally, if an Decamethyltetrasiloxane (L4) 195 L2 had two hydroxyl groups attached to it, it would be Dodecamethylpentasiloxane (L5) ? named 1,1-OH-L2 or 1,2-OH-L2 depending on if the hydroxyl groups are both on the same or different Si Hexamethylcyclotrisiloxane (D3) 134 atoms, respectively. This molecule could otherwise be Octamethylcyclotetrasiloxane (D4) 175 named as L2(OH)2 if the number, but not the position, of Decamethylcyclopentasiloxane (D5) 210 the hydroxyls are known. The bi-, tri-, tetra-, as well as Dodecamethylcyclohexasiloxane (D6) 245 bis, tris, and tetrakis nomenclature is dropped. This is because, for example, since the x,y-Ln- and 1,1-OH- nomenclature used in the above examples makes it Methyl-tris(trimethylsiloxy)silane (B4) 60-70 obvious that there are two branches present, bis- or bi- is Tetrakis(trimethylsiloxy)silane (B5) 105 not strictly required. C. GCMS Identification Standards for L2, L3, L4, L5, D3, D4, and D5 can be purchased, but those are only 7 of the ~100 peaks that are produced. For the other 90+ peaks which do not have reference standards, the following conventions were used. The largest peak of the mass spectrum is assumed to be the –CH3 peak, and thus the parent molecule is assumed to be 15 mass units heavier. This assumption is made because it was observed with the siloxane reference compounds. Since siloxanes are made from a limited number of atoms (Si, O, C, and H) with given coordination Si (4), O (2), C (4), and H (1), there is a limited number of combinations which can produce a given mass (parent molecule). For example, masses 15 and 17 could only be CH3 and OH, respectively. The mass of the parent molecule will identify the general class of the molecule, linear or cyclic, but not its exact structure. For example, both the linear and the branched variants of the L4 class contain 4 Si atoms and will have the same mass. Additionally, they will also have very similar mass spectra just as butane and iso-butane have the same mass and very similar mass spectra. However, they will have different retention times within the GCMS, and thus can be individually identified by their retention times; a similar effect happens with siloxanes.

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