Study on the Shock Sensitivity of the Hydrolysis Products of

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Materials and Interfaces

Study on the Shock Sensitivity of the Hydrolysis Products of Hexachlorodisilane Xiaobing Zhou, Mark A. Wanous, Xianghuai Wang, Donald V. Eldred, and Thomas L. Sanders Jr Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01241 • Publication Date (Web): 10 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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Industrial & Engineering Chemistry Research

Study on the Shock Sensitivity of the Hydrolysis Products of Hexachlorodisilane Xiaobing Zhou,* Mark A. Wanous, Xianghuai Wang, Donald V. Eldred and Thomas L. Sanders Jr The Dow Chemical Company, 2200 West Salzburg Road, Auburn, Michigan 48686, United States of America * Corresponding author phone: (989) 496-6349; email: [email protected]

KEYWORDS. Hexachlorodisilane hydrolysis products; shock sensitivity; heat sensitivity; atomic hydrogen

ABSTRACT: The hydrolysis products of hexachlorodisilane (HCDS) show common heat sensitivity and can become shock sensitive under certain conditions. Study of the shock sensitivity has been difficult due to the unpredictable nature of this phenomenon. We have identified the parameters affecting the shock sensitivity of the materials and developed synthetic methods to consistently prepare the hydrolysis products with a high shock sensitivity. We characterized the composition of the hydrolysis products to be [SiOx(OH)4-2x]m[Si2Oy(OH)6-2y]n(H2O)o where x is 0 – 2, y is 0 – 3, m is less than n and o varies. The hydrogen atoms in the silanol groups or absorbed water are the oxidant and the silicon atoms in the Si-Si bonds are the reductant. When the materials are disturbed by a thermal or mechanical impact, fast redox reactions happen to form molecular hydrogen. A sequence of free radical reactions was proposed to

explain the shock sensitivity and shock induced chemical transformation.

INTRODUCTION

One mysterious phenomenon in silane chemistry is the shock sensitivity of the hydrolysis products of some di or higher silanes. The first public report on the formation of explosive hydrolysis products from hexachlorodisilane (HCDS) and the ethoxylated derivatives of HCDS can be tracked back to the year of 1914 when Martin published that these reactive chlorinated or ethoxylated disilanes reacted with traces of moisture to give white explosive precipitates.1 Martin also found that the hydrolysis products could explode upon a sudden heat treatment.2 Since then till 2014, the shock sensitive phenomenon was occasionally mentioned in the literature.3,4 Shock sensitive hydrolysis products known as “popping gels” can be formed when trichlorosilane direct process residue (TCS DPR) is hydrolyzed in air. HCDS, a byproduct formed in the direct process where elemental silicon is reacted with hydrogen chloride, is an abundant species in the DPR and has been attributed to the formation of the popping gels.4 We have observed that these popping gels have a high shock sensitivity and can be initiated by a mild mechanical impact such as a scratch. In 2014, a fatal explosion happened during the cleaning of a heat exchanger at a Mitsubishi Material Corporation plant where high purity polycrystalline silicon was produced. The explosion resulted in severe life loss and property damage. The hydrolysis products of the residual chloropolysilanes in the heat exchanger were identified as the explosioncausing substances.5 Even though the shock sensitivity of the hydrolysis products was attributed to the reactive Si-Si bonds in the materials, the mechanism of the phenomenon was not sufficiently elucidated. HCDS forms solid hydrolysis products in contact with moisture or liquid water. But not every hydrolysis product is shock sensitive or demonstrates a high shock sensitivity like the popping gels. The hydrolysis products may become shock sensitive under certain circumstances. The unpredictability in the formation of the shock sensitive hydrolysis products has made the study of the shock sensitivity of the materials difficult. However, understanding of the mechanism of the shock sensitivity has become important for safe handling of these hazardous materials in the manufacture of HCDS, as this molecule is being adopted as a universal precursor for chemical vapor deposition or atomic layer deposition of silicon-containing semiconductor or dielectric thin films.6-8 We analyzed the HCDS hydrolysis products and studied the common heat sensitivity of the materials first. Then we developed synthetic methods to prepare the hydrolysis products with consistently high shock sensitivity and investigated the shock sensitive phenomenon accordingly. We proposed a mechanism to explain this phenomenon on the basis of experimental and computational results. 1

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EXPERIMENTAL SECTION

Materials. 3A, 4A, 5A and 13X molecular sieves were purchased from Sigma-Aldrich and used either as is or conditioned in a 50% relative humidity chamber at room temperature before use. The Aldrich 3A molecular sieves were equivalent to UOP Type 3A molecular sieves. Three spin traps, N-tert-butyl-α-phenylnitrone (PBN; ≥98%), α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN; 99%) and 5,5dimethylpyrroline-1-oxide (DMPO; ≥97%), were purchased from Aldrich and used without purification. Preparation of HCDS. HCDS was sourced from Dow Trichlorosilane (TCS) Direct Process, a reaction between anhydrous hydrogen chloride and a fluidized bed of elemental silicon powder. An example of this process is found in U.S. Patent 3,148,035 granted to WackerChemie in 1964. In the preparation of TCS a high boiling mixture is separated by distillation which is called direct process residue (DPR) and is comprised of compounds having Si-Si linkage, including HCDS, and Si-O-Si linkage. This DPR may also contain silicon particles, metal salts, and precipitated polysilanes and polysiloxanes. An example DPR composition is found in U.S. Patent 6,013,235 granted to Dow Corning in 2000. The HCDS used in this study was isolated in an 11 liter batch distillation process starting with a DPR composition containing 0.4% TCS, 21.8% silicon tetrachloride, 2.6% hexachlorodisiloxane, 1.7% pentachlorodisilane, 72.9% HCDS and 0.6% other chlorosiloxane compounds. The first distillation fraction was collected between the condenser temperatures 34 oC and 120 oC, under 1 atmosphere pressure, which accounted for 17.5% of the starting mass. The product fraction was collected between 124 ᵒC and 148 ᵒC resulting in a yield of 39.7%. The product fraction containing 95% HCDS, 2% silicon tetrachloride, 2% hexachlorodisiloxane and 1% pentachlorodisiloxane was used in this research. Synthesis of Hydrolysis Products by Adding HCDS to Water. To a 100 ml beaker was filled 20.0 g of de-ionized water. The beaker was cooled in an ice-water bath to 0 oC. Under magnetic agitation, 10.0 g of HCDS was added to the beaker dropwise in 40 minutes with a plastic pipette. A white precipitate was formed. Near the end of the addition, the reaction mixture became a thick slurry. A spatula was used to agitate the reaction mixture. After all the HCDS was added, a viscous slurry with a yellowish tint was formed. The slurry was stirred for 12 hours at room temperature and filtered through a Type C glass frit under vacuum. The wet solid collected on the frit was dried by sandwiching it between two pieces of filter paper first and evacuating it under vacuum (1 torr) for 12 hours next. The product was isolated as a white powder (5.5 g). This white powder was heat sensitive. Its shock sensitivity was not observed in this research. The properties of this material are discussed in the Study on the Heat Sensitivity of HCDS Hydrolysis Products in the Result and Discussion section of this publication. Synthesis of Shock Sensitive Hydrolysis Products of HCDS – Method 1. 3A molecular sieve pellets with 3.2 mm diameter (100 g) were manually sorted out to remove small broken pieces. This material was loaded in a 250 ml wide-neck glass bottle. HCDS (100 ml) was added to cover all the pellets. The bottle was capped with a TEFLON-lined lid, and gently shaken for 5 minutes. Then excess HCDS was removed with a plastic pipette. The complete removal of the excess HCDS is important for forming discrete shock sensitive pellets. Otherwise, pellets may be fused with shock sensitive hydrolysis products formed from the excess HCDS. Separation of the pellets may trigger discharge of the shock sensitive products. The wet pellets were poured onto a fiber glass mat in a 12” x 12” x 0.25” glass dish. The fiber glass mat absorbed residual excess HCDS. The pellets were gently laid out with a spatula to form a single layer without touching each other on the mat. The dish was placed in a fume hood where the temperature was 20 oC, the relative humidity was 50% and the air flow was set at maximum (about 150 feet/minute face velocity). The wet pellets gradually became dry while a white acid fume was slowly released and a thin coating of the hydrolysis products was formed on the pellets in one day. Some pellets may jump out of the dish. Some coated pellets started to show shock sensitivity within a few hours. The procedure was also used to prepare shock sensitive pellets at 1/5 scale where 20 g of 3.2 mm 3A molecular sieve pellets were treated with 20 ml of HCDS prior to hydrolysis. To test the shock sensitivity of the pellets coated with the hydrolysis products, 10 pellets were randomly and gently fetched with forceps each time, and placed between the serrated jaws of tongue-and-groove pliers. A minimal force was applied to the forceps to lift a shock sensitive pellet off the fiber glass mat to avoid unintentional discharge of the pellet. The pliers were quickly squeezed to crush the pellets. If the coating is shock sensitive, a smoke, spark or flame should be observed. The percentage of shock sensitive pellets was calculated based on the number of shock sensitive pellets in 10 tested pellets. Only a very low percentage, estimated to be less than 1%, of pellets had a very high shock sensitivity. These pellets discharged in the fetching process before crushing. Synthesis of Shock Sensitive Hydrolysis Products of HCDS – Method 2. A hydrolysis apparatus shown in Scheme 1 was assembled. The portable humidifier in the humidification chamber was started. Compressed air was flowed through the apparatus till a steady relative humidity reading of 70 – 80% was reached. The humidified air was flowed through a buffer chamber to remove any liquid water droplets. 3A molecular sieve pellets with 3.2 mm diameter (20 g) were manually sorted out to remove small broken pieces. This material was loaded in a wide-neck glass bottle. HCDS (20 ml) was added to cover all the pellets. The bottle was capped with a TEFLON-lined lid, and gently

2


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shaken for 5 minutes. Then excess HCDS was completely removed with a plastic pipette. The wet pellets were poured onto a fiber glass mat in a round-shaped glass dish with a 6” diameter. The pellets were gently laid out to form a single layer without touching each other on the mat. The dish was placed in the hydrolysis chamber for 2 hours. The processed pellets having water droplets on the surfaces were dried and aged in a fume hood at 20% relative humidity. The water droplets evaporated in 1 hour. Up to 70% of the treated pellets became shock sensitive in one day when tested with the shock sensitivity testing method described above. To form a second hydrolysis product coating on molecular sieves, the molecular sieve pellets treated once and dried in fume hood for 1 hour were gently removed from the glass fiber mat and placed in a wide-neck glass bottle. HCDS (20 ml) was added. All the pellets floated on the surface. The bottle was capped with a TEFLON-lined lid and gently shaken for 5 minutes. Then excess HCDS was completely removed with a plastic pipette. The wet pellets were poured onto a fiber glass mat in a glass dish. The pellets were gently laid out to form a single layer on the mat. The dish was placed in the hydrolysis chamber for 2 hours. The processed pellets having water droplets on the surfaces were dried and aged in a fume hood at 20% relative humidity. The water droplets evaporated in 1 hour. When the pellets were coated for the third time by the above-described method, no water droplets were formed on the pellets after hydrolysis. Humidity Sensor

Vent to Fume Hood

Air

HCDSTreated Pellets Portable Humidifier Humidification Chamber

Buffer Chamber

Hydrolysis Chamber

Scheme 1. Sketch of the apparatus used to hydrolyze HCDS-treated molecular sieve pellets in Method 2 Safety Protocol for Hydrolyzing HCDS and Handling Hydrolysis Products. The hydrolysis of HCDS is fast and exothermic. It forms solid hydrolysis products and HCl. The solid hydrolysis products are heat sensitive and can become shock sensitive under certain conditions discussed in this publication. HCl is toxic by inhalation and causes skin burns or eye damage. To mitigate these hazards, hydrolysis of HCDS was pursued in a fume hood in a controlled manner. Other flammable materials were cleared out of and HCl was efficiently vented through a fume hood. The hydrolysis temperature was controlled near or below the ambient temperature. A warning sign showing the main hazard - formation of shock sensitive materials - was placed in the experimental area. Personal protection equipment (PPE) required for the hydrolysis included disposable nitrile gloves, safety glasses and fire-resistance lab coat. Additional PPE, including face shield and fire-resistant gloves (worn over nitrile gloves), was used in the handling of the hydrolysis products. Only a minimal amount of hydrolysis products meeting research needs was made. Wastes containing the hydrolysis products were disposed of by submerging them in a polydimethylsiloxane (PDMS) fluid with 30,000 centistoke viscosity by at least 2-inch depth. Detection and Quantification of Hydrogen after Initiation of Shock Sensitive Hydrolysis Products on 3.2 mm 3A Molecular Sieve Pellets. Fifteen shock sensitive pellets prepared by Method 1 and having a weight of 1.015 g were gently loaded in a 38.5 ml glass Schlenk tube. The tube was then vacuum degassed for three times and back-filled with 1 atm (760 torr) research grade nitrogen. With the tube being closed, it was vigorously shaken for 2 minutes. During the mechanical impact, several small sparks were observed, a “hiss” sound was heard indicating an outgassing, and a mild temperature rise was noticed. The headspace of this tube was sampled for calibrated Gas Chromatography-Thermal Conductivity Detector (GC-TCD) analysis to detect and quantify hydrogen, and Gas Chromatography-Mass Spectrometry (GC-MS) analysis to characterize other possible gaseous species by injecting 0.5 ml of headspace gas in the GC-TCD or GC-MS instruments. The HP 6890 GC in the GC-TCD analysis was equipped with a 5A molecular sieve column (30 m x 0.32 mm i.d. with 25 µm film thickness). Injector and TCD were held at 140 oC and 150 oC, and column was held isothermal at 40 oC for the analysis. Quantitative calibration was performed with external H2 standards prepared by adding 5, 10, 20, 100, and 200 µl of H2 to 5 headspace vials and sampling the standards at 30 oC. The Shimadzu QP2010 GC-MS was equipped with a Restek XTI-5 column (30 m x 0.25 mm i.d. with 0.5 µm film thickness). The GCMS parameters were: split injection with approximately 10 ml total flow and 40:1 split ratio, injector temperature at 200 oC, column 40 oC (2 minutes) with ramp at 10 oC/minute to 200 oC (1 minute), source temperature at 200 oC and scan 15-500 m/z in 0-19 minutes.

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Spin Trapping Experiment. The 5 or 10 mM solutions of spin traps were prepared by dissolving PBN in toluene and POBN or DMPO in de-ionized water. All spin trap solutions were deoxygenated by bubbling nitrogen for 5 minutes prior to use. Forty shock sensitive pellets prepared by Method 2 were loaded into a 30 ml glass vial. Then a deoxygenated spin trap solution was added to just cover the shock sensitive pellets. The headspace of the vial was deoxygenated with nitrogen for 2 minutes. The vial was capped, and vigorously shaken for 5 seconds to initiate the shock sensitive pellets. After the initiation, the liquid phase was quickly transferred into an EPR tube designed for organic or aqueous sampling, and the headspace of the sample tube was briefly deaerated with nitrogen. EPR spectra were collected usually in 2 minutes after the initiation of shock sensitive pellets. Original uncoated molecular sieves, spin trap solutions and solvents were analyzed as the blank samples to verify the signals of shock sensitive pellets. Electron Paramagnetic Resonance (EPR) spectra were collected at room temperature on a Varian E-122 X-band spectrometer located at Illinois EPR Research Center (IERC) at Urbana, Illinois, USA. The EPR settings were as follows: The microwave frequency was about 9.143 GHz for samples in organic solvents and 9.54 GHz for samples in aqueous solvents; The modulation amplitude was 1.0 G and the microwave power was set to 10 dB attenuation; The magnetic fields were calibrated with a Varian NMR Gauss meter and the microwave frequency was measured with an EIP frequency meter. Other Instruments. Fourier Transform-Infrared (FT-IR) spectra were recorded at a resolution of 4 cm-1 on a Nicolet 5SXB FT-IR spectrometer with air as the background. Solid samples were ground with dry KBr and compressed into clear disks. DSC data were collected on a TA 2920 MDSC® instrument. Approximately 2 to 4 mg of material was placed in an open Al pan and ramped to 500 °C at 10 °C/minute in air. Solid state 29Si Nuclear Magnetic Resonance (NMR) spectra were acquired on a Varian Inova NMR spectrometer with a 29Si operational frequency of 80 MHz. Samples were packed in a 7 mm OD ZrO2 rotor and spun at 4 kHz. Direct polarization was utilized for excitation. Thermogravimetric Analysis-Mass Spectrometry (TGA-MS) data were collected on a Perkin-Elmer Pyris 1 TGA interfaced to a Pfeiffer Thermostar GS301 MS through a heated silica capillary at Impact Analytical, Midland, Michigan, USA. The calibration of the TGA instrument was verified with a standard reference material of calcium oxalate monohydrate. Computation. The ab initio calculation on the model molecules was performed using the Amsterdam Density Functional (ADF) program, 2016 version.9-11 The Generalized Gradient Approximation (GGA) function was chosen to be BP86,12,13 the basis set was triple-zeta and one polarization function (TZP) with none frozen cores option was employed.14 The frequencies used in Gibbs free energy computation were calculated with numerical differentiation of gradients method.15 The numerical integration grid was chosen to be good Becke quality.16,17 The Gibbs free energy calculations were based on the optimized structures where the T of 298.15 K was used. For biradicals, the triplet structure was optimized prior to the Gibbs free energy calculation.18-20

RESULTS AND DISCUSSION

Study on the Heat Sensitivity of HCDS Hydrolysis Products. HCDS reacts with water or moisture through the hydrolysis and condensation reactions 1 – 3 to form a crosslinked amorphous solid product.

(1)

(2)

(3) The morphology of the solid varies depending upon hydrolytic conditions. White granular solids are formed when HCDS is added to liquid water under agitation. White or transparent solid coatings are formed when HCDS is exposed to air on substrates such as glass, metals, wood or molecular sieves. When fully hydrolyzed at the ambient temperature, the hydrolysis products made under different conditions have almost identical IR absorptions. Figure 1. Bottom is the absorbance mode FT-IR spectrum of a hydrolysis product made by adding HCDS to liquid water. This spectrum is representative for other hydrolysis products of HCDS made under different conditions (see the Supporting Information for the FT-IR spectra of some other hydrolysis products). The primary features of this FT-IR spectrum are con-

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1120 cm-1 1069 cm-1 Si-O-H at 898 cm-1 Si-H at 882 cm-1 Si-O-Si at 805 cm-1

sistent with those of precipitated silica made by acidifying water glass.21 The IR bands at 1120 (shoulder), 1069 and 450 cm-1 can be assigned to Si-O-Si bonding, the bands at 3233 and 898 cm-1 to Si-O-H bonding, and the bands at 3424 and 1626 cm-1 to water. But the HCDS hydrolysis products have some different IR features from precipitated silica. The hydrolysis products of HCDS have two weak IR signals at 742 and 614 cm-1 and an even weaker signal at 2264 cm-1. The peaks at 614 and 2264 cm-1 can be assigned to Si-Cl bonds and Si-H bonds respectively.22 The peak at 742 cm-1 cannot be assigned at this time.

0.4

Si-O-Si at 450 cm-1

Si-Cl at 614 cm-1

H2O at 1626 cm-1

Si-H at 2264 cm-1

0.6

Si-O-H at 3233 cm-1

0.8

H2O at 3424 cm-1

Si-O-Si

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Hydrolysis product after sudden 380 oC heat treatment in an inert atmosphere (Absorbance offset by +0.35)

0.2

Hydrolysis product after sudden 380 oC heat treatment in air 0.0 -0.2 742

3000

2000

Initial hydrolysis product prepared by adding HCDS to water (Absorbance offset by -0.25)

1000

Wav enumbers (cm-1)

Figure 1. FT-IR spectra of a HCDS hydrolysis product before and after sudden 380 oC heat treatment The relative intensities of these IR bands suggest that the hydrolysis products of HCDS are a crosslinked siloxane containing a high content of silanol groups and absorbed water. It also has some residual Si-Cl groups and a low level of Si-H groups. The Si-H groups can be formed during the hydrolysis through cleavage of Si-Si bonds by HCl (Reaction 4). This reaction is much slower than the hydrolysis and condensation (Reactions 1 – 3), resulting in a low level of Si-H groups in the hydrolysis products. Monitored with FT-IR when the hydrolysis products were aged at the ambient temperature for 1 month, the Si-H content did not change with time.

(4) The existence of Si-Si bonds in the hydrolysis product prepared by adding HCDS to water was verified with solid state 29Si NMR (Figure 2. Bottom). The signals between -50 and -90 ppm and between -95 and -130 ppm can be attributed to Si2Oy(OH)6-2y (y = 0 – 3) and SiOx(OH)4-2x (x = 0 – 2) respectively by comparing these chemical shifts with the literature 29Si NMR chemical shifts of hexamethoxydisilane, permethoxysiloxanes and quaternary siloxanes.23,24 The 29Si NMR chemical shift was reported to be -53 ppm for hexamethoxydisilane, Si2(OMe)6. The 29Si NMR chemical shifts of primary, secondary and tertiary methoxysiloxanes, (MeO)3SiOSiX3, (MeO)2Si(OSiX3)2 and (MeO)Si(OSiX3)3 where X is a substituent, fall in the range from - 85 to -110 ppm. Quaternary siloxanes, Si(OSiX3)4, have a 29Si NMR chemical shift at -110 ppm. The relative intensities of these signals indicate that the content of SiOx(OH)4-2x (x = 0 – 2) is much lower than the content of Si2Oy(OH)6-2y (y = 0 – 3). This suggests that the majority of the Si-Si bonds still remained intact in the hydrolysis product. Only a small portion of the bonds were oxidized in hydrolysis. The assignment of the signals between -50 and -90 ppm to Si2Oy(OH)6-2y (y = 0 – 3) is consistent with the literature solid state 29Si NMR peak assignment for the silicon suboxides prepared by hydrolyzing HCDS with a stoichiometric amount of water in diethyl ether at -75 oC or lower temperatures. A solid state 29Si NMR signal was observed at -71 ppm for the suboxides and assigned to Si2O3.25,26 The hydrolysis products of HCDS can ignite to form a flame and a yellowish solid residue when the materials are exposed to a sudden thermal impact. For instance, the materials ignited when they were placed on a pre-heated 380 oC hotplate in air. The combustion residues are no longer ignitable. One residue was analyzed with solid state 29Si NMR and FT-IR. The solid state 29Si NMR spectrum (Figure 2. Middle) shows that the peaks between -50 and -90 ppm are now much weaker than the peaks between -95 and -130 ppm. Therefore, SiOx(OH)4-2x (x = 0 – 2) became the predominant component. The content of Si2Oy(OH)6-2y (y = 0 – 3) was remarkably reduced, suggesting that most Si-Si bonds were oxidized during the combustion. The FT-IR data (Figure 1. Middle) is consistent with the results derived from the solid state 29Si NMR. The water and silanol contents were significantly reduced as the water peaks at 3424 cm-1 and 1626 cm-1 became much weaker and the silanol peaks at 3233 cm-1 and 898 cm-1 became almost invisible. Almost all the residual Si-Cl bonds were consumed 5



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as well. However, the Si-H content only increased by a small degree after the combustion, as the IR peak at 2264 cm-1 became only slightly more intensified and a new weak peak at 882 cm-1 was observed or resolved. This new peak can be assigned to the bending mode of Si-H bonds.22 Another new IR peak was observed at 805 cm-1 for the combustion residue and can be assigned to Si-O-Si symmetric stretching vibration.21,27 Therefore, the sudden thermal treatment triggered fast oxidation of the Si-Si bonds by Reactions 5 and 6, but didn’t form many more Si-H bonds. Interestingly, when the hydrolysis product was heated gradually up to 380 oC on a hotplate in air, it didn’t ignite. Furthermore, the product formed by this slower thermal treatment is no longer ignitable upon a sudden thermal treatment, i.e., placement on a pre-heated 380 oC hotplate. The product formed by the slower thermal treatment has the FT-IR features identical to Figure 1. Middle, suggesting that the slower thermal treatment also caused oxidation of the Si-Si bonds.

Si

Si

+ 2X

Si

OH

2X

Si

O

Si

+ H2

(5)

Si

Si

+ H 2O

Si

O

Si

+ H2

(6)

SiOx(OH)4-2x (x = 0 – 2) between -95 and -130 ppm Si2Oy(OH)6-2y (y = 0 – 3) between -50 and -90 ppm

Hydrolysis product after sudden 380 oC heat treatment in an inert atmosphere

Hydrolysis product after sudden 380 oC heat treatment in air

Initial hydrolysis product prepared by adding HCDS to water

Figure 2. Solid state 29Si NMR spectra of a HCDS hydrolysis products before and after sudden 380 oC heat treatment The fast oxidation of the Si-Si bonds (Reactions 5 and 6) in the hydrolysis products of HCDS can happen in an inert environment. In an experiment, a flat-bottom flask containing the hydrolysis product was deoxygened by ten cycles of evacuation and nitrogen filling, and then placed on a pre-heated 380 oC hotplate for 5 minutes. Only a tiny spark was observed. A “hiss” sound was heard suggesting fast outgassing during the sudden thermal treatment and a white solid was formed after the sudden thermal treatment. The treated solid was cooled down to room temperature in the inert environment, and analyzed with FT-IR and solid state 29Si NMR. As shown by Figure 1. Top and Figure 2. Top, the treated material has an almost identical composition as the combustion residue formed in air (Figure 1. Middle and Figure 2. Middle). The same compositional change achieved in air and in the absence of air suggests that the fast oxidation of the Si-Si bonds in the hydrolysis products is self sufficient. Oxygen is not required, but can contribute to the oxidation when the oxidation happens at an elevated temperature in air. The self oxidation and outgassing observed in inert environment was further studied with inert TGA-MS. TGA quantified the weight loss of the hydrolysis product and MS characterized the composition of outgas while the hydrolysis product was heated in a helium atmosphere. A hydrolysis product prepared by adding HCDS to water was used in this inert TGA-MS study. TGA detected a 22% weight loss below 400 oC. MS detected molecular hydrogen and water as the primary outgassing species (Figure 3. Top and Middle). H2 was formed

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between 100 and 600 oC with the peak at 230 oC. Water release started from the room temperature and continued up to 600 oC with the peak at 150 oC. Oxygen was also detected at up to 250 oC with the peak at 120 oC (Figure 3. Bottom). No pyrophoric silanes such as SiH4 or Si2H6 were detected in the outgas. Similar thermal outgassing was reported for the silicon suboxides prepared by hydrolyzing HCDS with a stoichiometric amount of water in diethyl ether at -95 oC. H2O and H2 were released out of the materials at 50 – 400 oC.25

H2

H2O

O2

Figure 3. Outgas detected by inert TGA-MS from the HCDS hydrolysis product prepared by adding HCDS to water The hydrolysis product prepared by adding HCDS to water was studied in air with open-pan DSC. An endotherm was observed at lower than 200 oC (Figure 4), which might be caused by the weight loss observed by TGA. An intensive exotherm caused by oxidation of Si-Si bonds was observed from 216 to 504 oC. Quantification of the exotherm was not attempted due to the interference of the weight loss.

Figure 4. Open-pan DSC data of the HCDS hydrolysis product prepared by adding HCDS to water

7



Synthesis of Shock Sensitive Hydrolysis Products of HCDS. The hydrolysis products prepared by adding HCDS to water undergo either fast or slow oxidation depending upon thermal treatment conditions. In spite of this heat sensitivity, these materials, however, are typically not shock sensitive or do not have a shock sensitivity as high as the popping gels. Therefore, they are not an ideal candidate for studying the shock sensitivity. We have developed two methods to make shock sensitive hydrolysis products from HCDS in high yield by using molecular sieves as substrates. The first method involves treating molecular sieves with HCDS and exposing the treated molecular sieves at the ambient temperature (20 oC) to humid air at 50% relative humidity. Four types of molecular sieves in the shape of beads or pellets were assessed by this method. The results are summarized in Table 1. The four types of molecular sieves in two different shapes show different effects on generation of shock sensitivity. Each has a unique shock sensitivity development pattern. The cylindrical pellets work better than the spherical beads. The pellets or beads with a larger diameter or particle size work better than those with a smaller diameter or particle size. The length of the pellets seems to have no noticeable effect on the formation of shock sensitive hydrolysis products. Under the conditions of Method 1, hydrolysis is completed to result in a solid siloxane coating on the substrates in 1 day. The 3.2 mm pellets of 3A, 4A, 5A and 13X molecular sieves gained 44, 45, 39 and 70% weights respectively after exposure to 50% RH air for 7 days. These weight gains can be attributed to the formation of the hydrolysis products of HCDS. The molecular sieves used as is or conditioned at 50% relative humidity worked equally well for forming shock sensitive hydrolysis products. However, the molecular sieves saturated with liquid water before treating with HCDS only formed heat sensitive (not shock sensitive) hydrolysis products. This result is consistent with the observation that the hydrolysis products prepared by adding HCDS to liquid water are typically not shock sensitive or do not have a high shock sensitivity. The hydrolyzed siloxane coating on the molecular sieve pellets starts to show shock sensitivity within a few hours after hydrolysis begins and the percentage of shock sensitive pellets usually reaches a maximum within 1 - 7 days. The shock sensitivity develops faster on 3A and 4A molecular sieve pellets than on 5A and 13X molecular sieve pellets. The development of the shock sensitivity can be accelerated by aging the coated pellets in a 32 oC oven at 20% relative humidity after the hydrolysis is completed. The percentages (yields) of shock sensitive pellets are more consistent on 3A, 5A and 13X molecular sieves than on 4A molecular sieve pellets. Figure 5 shows a shock sensitivity development pattern for the 3.2 mm 3A molecular sieve pellets coated with HCDS hydrolysis product by Method 1. The percentage of shock sensitive pellets increased with time, reached 50% in one day and 70% in two days, and fluctuated between 50% and 70% on the 3rd and 4th days. After the pellets were placed in a 32 oC oven at 20% relative humidity on the 5th day, the percentage of shock sensitive pellets increased to and stayed at 80 - 90% from the 5th to 11th days. 100%

Percentages of Shock Sensitive Pellets

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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90% 90% 85% 85% 85%

90%

80%

80%

70% 64%

70% 58%

60%

50%

50%

50% 40% 1

2

3

4

5

6

7

8

9

10 11

Time (Days)

Figure 5. Development of shock sensitivity of the 3.2 mm 3A molecular sieve pellets treated with HCDS by Method 1. 1 – 4 Days: 20 oC and 50% relative humidity; 5 – 11 Days: 32 oC and 21% relative humidity in an oven In addition to types of substrates, purity of HCDS, humidity of air and flow rate of humid air in hydrolysis also affect generation of shock sensitivity. The percentage of shock sensitive pellets dropped down to about 10% when a concentrated direct process residue of trichlorosilane containing 70% HCDS and 30% chlorosiloxanes and chloro higher silanes was used to treat 3.2 mm 3A molecular sieve pellets. Less than 10% of the treated 3A molecular sieve pellets became shock sensitive when the hydrolysis was pursued in air at less than 30% relative humidity at 20 oC. In such a dry environment, very little hydrolysis products were formed as most HCDS evaporated. Only 10% of the treated 3A molecular sieve pellets developed shock sensitivity after they were first hydrolyzed for one day in static air at 50% relative humidity in a closed vessel and then aged in a fume hood. In this case, HCl by-product was not removed in a timely fashion during the hydrolysis.

8


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Table 1. Molecular sieves evaluated by Method 1 for synthesis of the HCDS shock sensitive hydrolysis products

Substrates

Maximal Percentages of Shock Sensitive Pellets in 7 Days at 20 oC

Characteristics 3.2 mm (diameter) Pellets

90%

1.6 mm (diameter) Pellets

30%


50%

4-8 Mesh beads

10%

8-12 Mesh beads

0%

5A Molecular sieves


60%

13X Molecular sieves


80%

3A Molecular sieves

4A Molecular sieves

The second synthetic method is more robust than the first method. As shown in Scheme 1, the HCDS-treated molecular sieves are exposed to a humidified air flow at 70 – 80% relative humidity in Method 2. Hydrolysis is completed in 2 hours. Since hydrolysis is accelerated by this way, pellets can be coated for two or three times before shock sensitivity is developed. The pellets coated multiple times tend to have more robust shock sensitivity or less susceptibility to environmental change. Figure 6 shows that 70 – 90% of the 3.2 mm 3A molecular sieve pellets treated with HCDS for three times remained shock sensitive in the 7-day aging at 25 oC and 21% relative humidity.

100%

Percentages of Shock Sensitive Pellets

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


90%

95%

95%

95%

80% 70%

90%

89%

6

7

80% 75%

60% 50% 40% 1

2

3

4

5

Time (Days)

Figure 6. Development of shock sensitivity of the 3.2 mm 3A molecular sieve pellets treated with HCDS by Method 2 and aged at 25 oC and 21% relative humidity Therefore, after hydrolysis, a warm dry aging environment helps to maintain shock sensitivity and a cold humid environment tends to reduce shock sensitivity. The change of shock sensitivity is reversible by adjusting environmental conditions. Shock sensitivity has been observed to fluctuate with time in an uncontrolled environment. Figure 7 is an image of the shock sensitive pellets prepared by Method 1 or 2. Since each of the pellets contained a limited amount of hydrolysis products, it forms a limited size of discharge when it is crushed with tongue-and-groove pliers in the shock sensitivity test. Figure 8 shows the typical size of a discharge. Since the hydrolysis products prepared under different conditions have a similar chemical composition, the shock sensitive hydrolysis products made on the molecular sieve pellets should have a similar chemical composition as the popping gels formed from TCS DPR. The 3.2 mm 3A molecular sieve pellets have a low crush strength of 138 kPa (20 psi) according to 9



Page 10 of 17

the UOP technical datasheet 4270-31A4 0805A0C-A4. The shock sensitive hydrolysis products on these pellets discharge before or when the pellets are crushed. This evidence leads us to conclude that the HCDS hydrolysis products coated on the molecular sieve pellets should have a shock sensitivity comparable to the popping gels. These shock sensitive pellets have consistently high shock sensitivity and form controlled size of discharge. They are reliable and safe candidates for studying the shock sensitive phenomenon.

Figure 7. Image of the shock sensitive 3.2 mm 3A molecular sieve pellets prepared by Method 1 or 2 (ruler scale at cm) Study on the Shock Sensitivity of HCDS Hydrolysis Products. When the shock sensitive 3.2 mm 3A molecular sieve pellets were crushed with pliers in air, flames were usually discharged (Figure 8) and an instantaneous popping sound was usually heard. When the same materials were crushed in a nonflammable fluid such as water, a gas was released out of the pellets. The gas bubbles ignited if they reached the surface with air fast (in less than 0.5 seconds) or didn’t ignite if they were retained in water for a long enough time. Therefore, to prevent the ignition of the gas, we crushed the shock sensitive pellets in a viscous nonflammable polydimethylsiloxane (PDMS) fluid. The gas bubbles slowly rose to the surface with air in 30 seconds and burst in another 10 seconds without self ignition (Figure 9). The gas is apparently a product of the shock induced chemical reaction. Characterization of the composition of the gas can provide a hint to the mechanism of the reaction. To do this, the shock sensitive molecular sieve pellets were vigorously shaken in a glass tube that had been strictly deoxygenated and inerted with research grade nitrogen (see the Experimental Section for the procedure). After the mechanical impact, the gas phase in the tube was analyzed with GC equipped with a TCD. This GC instrument was pre-calibrated for detection and quantification of molecular hydrogen. A strong signal at the retention time of 1.120 minutes was observed on the GC which can be assigned to H2. Molecular hydrogen was detected as the predominant species in the gas phase (Figure 10). Only a trace amount of molecular oxygen was detected at the retention time of 1.303 minutes. The gas was further analyzed with GC-MS. No other species such as silane or disilane were detected. As expected, the gas and the solid remains in the tube did not self ignite when they were exposed to air after the analysis.

Figure 8. Image of a shock sensitive pellet discharged by crushing with tongue-and-groove pliers in air

10


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Figure 9. Image of the shock sensitive pellets discharged by crushing with a metal rod in 30,000 centistoke PDMS fluid H2

at 1.120 minutes

O2

at 1.303 minutes

Figure 10. Detection and quantification of molecular hydrogen by calibrated GC-TCD The volume of molecular hydrogen released from the shock sensitive pellets was further quantified using a calibration curve. The moles of H2 were then correlated to the moles of cleaved Si-Si bonds. The amount of hydrolysis product on the molecular sieve pellets was derived from the typical 44% weight gain measured after the formation of the hydrolysis product on the molecular sieve pellets. It was estimated that only 10 – 20% of the Si-Si bonds in the shock sensitive hydrolysis products were consumed in the shock-induced reaction to generate H2. Most Si-Si bonds still remained intact after the shock. This explains that the post-initiation pellets were no longer sensitive to a mechanical shock, but still sensitive to a sudden heating. The pellets still self ignited on a pre-heated 380 oC hotplate surface in air, as a result of the fast oxidation of the remaining Si-Si bonds. The detection of molecular hydrogen after the initiation of the shock sensitive pellets suggests that atomic hydrogen is very likely the initial product of shock-induced reaction. Atomic hydrogen is a highly reactive short-lived species. It can combine to form molecular hydrogen in the absence of oxygen or react with oxygen upon contact with air. Spin trapping technique reported in the literature was attempted for the purpose of trapping atomic hydrogen during the initiation of the shock sensitive pellets. The solutions of three spin traps, N-tertbutyl-α-phenylnitrone (PBN), α-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN) and 5,5-dimethylpyrroline-1-oxide (DMPO) that were reported to form spin adducts with atomic hydrogen,28-30 were used in the spin trapping experiment. Only spin adducts of hydroxyl radicals with these three spin traps were detected with EPR. No spin adducts of atomic hydrogen that should have a NH2 coupling pattern were detected. Atomic hydrogen was not trapped in this spin trapping experiment probably due to inefficient diffusion of atomic hydrogen through the gas-liquid interface to complex with the spin traps and faster competitive combination of atomic hydrogen to form molecular hydrogen. Proposed Mechanism for the Shock Sensitivity of HCDS Hydrolysis Products. We propose that the formation of atomic hydrogen should be an important reaction for elucidating the shock sensitivity of HCDS hydrolysis products. The source of the hydrogen atoms can be either silanols or absorbed water. However, the formation of atomic hydrogen is not the initial reaction. It should be preceded by the cleavage of the Si-Si bonds in the hydrolysis products. The Si-Si bonds can be cleaved via two different pathways. In one pathway, the SiSi bonds can be strained. A strained Si-Si bond can homolytically break upon a mechanical impact to form two silyl radicals (Reaction 7). In the other pathway, some Si-Si bonds can be oxidized by oxygen during hydrolysis in air to form bissilyl peroxides. The weak peroxide bond can break upon a mechanical impact to form siloxy radicals, SiO· (Reaction 8), which undergo radical transfer with Si-Si bonds to form silyl radicals (Reaction 9). Once formed, silyl radicals can react with the silanols or water in the hydrolysis products to form atomic hydrogen (Reactions 10 and 11). In Reactions 10 and 11, the H atoms in silanols or water become reduced and the Si atoms in silyl radicals become oxidized. Therefore, both the oxidant which is the H atoms in silanols or water and the reductant which is the Si atoms in the Si-Si bonds are present in the hydrolysis products. Since atomic hydrogen combines to form molecular hydrogen in the absence of oxygen, H· + H· → H2, the overall effect of Reaction 7, Reactions 11 and 12, and atomic hydrogen self quenching is equivalent to Reactions 5 and 6

11



Page 12 of 17

observed for HCDS hydrolysis products at elevated temperatures. In other words, Reactions 5 and 6 can be triggered by heat or shock. Thus, the heat induced reactions may probably have the same free radical mechanism. Reactions 7 and 8 should have positive enthalpy changes due to homolysis of chemical bonds. The Gibbs free energy changes of these two reactions should be positive as well, since the Gibbs free energy changes of the two model reactions 14 and 15 were calculated to be 244 and 143 kJ/mol respectively. These two model reactions simulate Reactions 7 and 8. Reactions 9 – 11 were estimated to have -475, 372 and -302 kJ/mol enthalpy changes based on the literature bond energy data.31 These large negative enthalpy changes should result in negative Gibbs free energy changes as the entropy changes are expected to be insignificant for these three reactions. This thermodynamic estimation suggests that the formation of atomic hydrogen from silyl radicals should be favored.

(7)

(8)

(9)

(10)

(11) The formation of silyl radicals by energetic homolysis of Si-Si bonds in disilanes, as proposed in Reaction 7, has been reported. For instance, silyl radicals were formed by photolytic homolysis of aryldisilanes and trapped with unsaturated compounds or chloroform.32,33 The strained Si-Si bonds proposed in Reaction 7 can be formed as a result of interaction between the hydrolyzed siloxane coating and substrates or local chemical bond distortion. Stress can be generated when the Si-Cl groups are hydrolyzed (Reaction 1) and the resultant silanols are condensed (Reactions 2 and 3) on molecular sieves. The formation of stress is evidenced by the “jumping” and fragmentation of HCDS-treated molecular sieve pellets during and after hydrolysis. The chemical bonds carrying the stress are not in the terminal Si-OH, Si-H or Si-Cl groups, but in the Si-O-Si and Si-Si backbones in the HCDS hydrolysis products. Compared to the strong Si-O-Si bonding (Bond Dissociative Energy of Si-O bond = 800 kJ/mol),31 the Si-Si bonds are much weaker (Bond Dissociative Energy of Si-Si bond = 325 kJ/mol).31 Therefore, the Si-Si bonds are more susceptible to a distortion under stress. When the stress becomes strong enough, the strained Si-Si bonds can be weakened enough to rupture under a mechanical impact. The homolytic cleavage of the Si-Si bonds releases the strain energy and forms silyl radicals. Stress is sensitive to environmental conditions such as temperature and humidity. The change of environmental conditions may cause the change of stress which subsequently affects the shock sensitivity of the hydrolysis products. Local strained Si-Si bonds in the form of three-membered ring structure, cyclo-Si2O, can possibly be formed by intramolecular condensation between a silanol group and a vicinal Si-Cl group (Reaction 12). Most compounds containing the Si2O ring, namely oxadisilacyclopropanes, are unstable due to ring strain, except some that are stabilized by bulky substituted phenyl groups on the two Si atoms. These stable oxadisilacyclopropanes were made by oxidizing corresponding stabilized disilenes with nitrous oxide (N2O).34 Although oxadisilacyclopropanes haven’t been made by any condensation reactions, we still include Reaction 12 as a possible route towards this strained ring species in this mechanistic discussion because this reaction is an extension of the intermolecular condensation (Reaction 2) and because vicinal SiOH and Si-Cl groups may be prevalent at the early stage of hydrolysis when moisture is the limiting reagent.

12


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(12) We estimated the energy stored in the strained Si2O ring by calculating the Gibbs free energy change (124 kJ/mol) for the model Reaction 13 where the strained Si-Si bond in the Si2O ring is homolytically cleaved to form a silyl biradical and the Gibbs free energy change (244 kJ/mol) for the model Reaction 14 where an unstrained Si-Si bond is homolytically cleaved to form two silyl radicals. The difference of 120 kJ/mol between these two energy changes can be considered as the strain energy of the Si2O ring. This strain energy is about 37% of typical Si-Si bond dissociative energy. It is less than, but not very different from the literature strain energy, 188 kJ/mol, calculated for cyclo-(H2Si)2O.35

(13)

(14) Bissilyl peroxide is proposed in the other pathway leading to the formation of siloxy radicals by homolysis first and silyl radicals by radical transfer next (Reactions 8 and 9). We cannot exclude the possibility of peroxide formation in HCDS hydrolysis products as molecular oxygen was detected in the heat sensitive and shock sensitive hydrolysis products by TGA-MS (Figure 3. Bottom) and GC-TCD (Figure 10) even after strict inerting. We calculated the Gibbs free energy changes for homolytic cleavages of the O-O bonds in two model peroxide compounds. The calculated Gibbs free energy changes are 143 kJ/mol for the homolysis of the linear compound (HO)3Si-O-O-Si(OH)3 (Reaction 15) and 150 kJ/mol for the homolysis of the five-membered cyclic compound cyclo-(HO)2SiO3Si(OH)2 (Reaction 16). These two energy changes are similar and small, suggesting that the O-O bonds in the two model compounds are similarly weak and the fivemembered ring in the cyclic compound is not strained. Bissilyl peroxides have been reported in the literature to form in the oxidation of disilanes by molecular oxygen.36 Cyclic bissilyl peroxides can be formed from the strained oxadisilacyclopropanes by oxidation of molecular oxygen.37 The O-O bonds in silyl peroxides have low bond dissociation energies, even though these bonds are usually stronger than the O-O bonds in organic peroxides.38 Homolysis of weak O-O bonds is a common degradation pathway for peroxides including silyl peroxides.36 Therefore, linear and cyclic bissilyl peroxides can undergo O-O bond homolysis to form siloxy radicals (Reaction 8) and biradicals. These siloxy radicals or biradicals can oxidize Si-Si bonds to form siloxanes and silyl radicals (Reaction 9). In addition to the favoring thermodynamics, Reaction 9 is proposed also because formation of silyl radicals by radical transfer reactions was reported in the literature between an oxygen-centered silylperoxy radical RSiOO· and a disilane moiety (Reaction 17),36 and in a trisilylsiloxy radical, (Me3Si)3SiO· → (Me3Si)2(Me3SiO)Si·.39 Silysiloxy radicals, SiSiO·, can possibly form in the shock sensitive hydrolysis products and undergo similar intramolecular radical transfer.

(15)

(16)

(17)

13



Page 14 of 17

Silyl radicals without protecting bulky organic groups were reported in the literature to have a high reactivity and a short lifetime.40 Theoretically, the silyl radicals formed in Reactions 7 and 9 can alternatively react with silanols or water through hydrogen abstraction (Equation 18) to form Si-H species and SiO· or HO· radicals. However, this reaction has a positive enthalpy change of 129 kJ/mol when R is Si or 199 kJ/mol when R is H, estimated with the literature bond dissociation energies,31 and very likely a positive Gibbs free energy change as the entropy change is expected to be insignificant. It is, therefore, not thermodynamically favored. If this reaction happened, a significant amount of Si-H bonds should have been formed. This assumption is inconsistent with the FT-IR evidence that the initial and heattreated hydrolysis products all contain a low level of Si-H bonds (Figure 1 and discussion). Therefore, hydrogen abstraction may not happen to a significant degree in the initiation of the shock sensitive hydrolysis products.

(18) Transfer of silyl radicals similar to Reactions 10 and 11 was reported in the pyrolysis of a solution of Hg(SiPh3)2 in HSiPh3 where the Ph3Si· radicals generated from Hg(SiPh3)2 abstract the phenyl groups, instead of the hydrogen atom, in HSiPh3 to form Si(Ph)4.41 Silyl radicals were also proposed to react with methylamines to form silyl amides and atomic hydrogen through radical transfer (Reaction 19).42

(19) Oxidation of a Si-Si bond by a geminal SiOH group (Equation 20) was proposed to explain a highly exothermic, sometimes explosive, self reaction of perhydroxylpolysilanes.43 Like Equation 18, this reaction is unlikely to happen to a significant degree in the initiation of the heat or shock sensitive hydrolysis products of HCDS due to the same reason that the Si-H contents detected with FT-IR are insignificant in the initial and heat-treated hydrolysis products (Figure 1 and discussion).

(20)

CONCLUSIONS

Hydrolysis of HCDS forms a solid product having the composition of [SiOx(OH)4-2x]m[Si2Oy(OH)6-2y]n(H2O)o where x is 0 – 2, y is 0 – 3, m is less than n and o varies. Most Si-Si bonds remain intact in the hydrolysis products. These hydrolysis products show a common heat sensitivity as the materials ignite upon a sudden thermal treatment. The hydrolysis products become shock sensitive under certain favoring conditions, such as, when the materials are formed on substrates in a humid air flow and aged in a dry environment. A high purity of HCDS and a warm aging temperature were found to promote the generation of the shock sensitivity. Hydrolysis products with a high shock sensitivity have been consistently prepared by hydrolyzing HCDS in humidified air on molecular sieves substrates. 3A molecular sieve pellets with 3.2 mm diameter gave the highest yield of shock sensitive products. The hydrolysis products formed on the 3A molecular sieve pellets have a shock sensitivity comparable to the popping gels formed from trichlorosilane direct process residue (TCS DPR). When the heat or shock sensitive HCDS hydrolysis products are discharged by either a thermal or a mechanical impact, fast redox reactions happen to form molecular hydrogen. In the redox reactions, the Si atoms in the Si-Si bonds are oxidized and the H atoms in the silanol groups or absorbed water are reduced. A sequence of free radical reactions starting from the homolytic cleavage of strained Si-Si bonds or weak O-O bonds in bissilyl peroxides is proposed to explain the shock sensitivity and the shock-induced redox reactions. The results of this mechanistic study are applicable to the popping gels and have been leveraged to develop techniques to either preventively or curatively mitigate the hazards of the shock sensitive hydrolysis products in the manufacture of HCDS.

ACKNOWLEDGMENT We thank The Dow Chemical Company for funding this research.

14


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SUPPORTING INFORMATION Six FT-IR spectra recorded on the HCDS hydrolysis products prepared under different conditions.

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Study on the Shock Sensitivity of the Hydrolysis Products of

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