Linear Siloxanes Research Articles

Methylsiloxane occurrence and distribution in free-range poultry eggs near a rural industrial park: Indicators of potential risks to birds

The ecological harm from methylsiloxanes has drawn worldwide attention. This study investigated three cyclic (D4–D6) and four linear siloxanes (L7–L10) in the eggs of free-range poultry collected near a rural industrial park in China and found total concentrations in the range of 19.2–1204 (median, 268) ng/g dry weight. Higher concentrations of methylsiloxanes were observed in chicken eggs than duck eggs. Cyclic siloxanes represented a median of 62.2% of the total methylsiloxane concentrations. A source assessment indicated that local soils and outdoor dust were more important sources of egg methylsiloxanes than poultry food. The partitioning of methylsiloxanes between egg yolk and egg albumen was investigated, and preferential distributions of the chemicals in the yolk were observed. This study confirmed that methylsiloxanes were highly prevalent in the study poultry eggs. The results suggested that the potential risks to some wild birds inhabiting this area should be of concern, as their physiologies and feeding ecologies are similar to those of the studied poultry, although available ecotoxicological data of the chemicals to birds remains scarce. Additional research is needed to characterize the accumulation of methylsiloxanes in different bird species and its associated adverse effects on their offspring.

Methyl siloxanes in soils from a large silicone-manufacturing site, China: concentrations, distributions and potential human exposure.

Methyl siloxanes are widely found in the environment, but little is known about the distributions of these chemicals in soils especially in areas where they are manufactured. We determined the concentrations of four cyclic (D3-D6) and 13 linear methyl siloxanes (L4-L16) in the soils from a siloxane-manufacturing site in China; the total concentrations of these 17 siloxanes (TSi) in the soils were 17.1-3,191 (median, 134) ng/g. We did not find extremely high concentrations of siloxanes in the soils. The median concentrations of total cyclic siloxanes (TCSi) were approximately sevenfold higher than those of total linear congeners. Hexamethylcyclotrisiloxane and octamethylcyclotetrasiloxane contributed a median of 59.7% and 20.3% of the TSi concentrations, respectively. Higher concentrations of soil TCSi were found in the silicone-manufacturing area relative to the other study areas. Source analysis indicated that industrial activities contributed substantially to soil siloxanes, in addition to the contribution of the siloxane emissions from specific consumer products. We calculated that the median values of daily TSi intakes through soil ingestion were 0.021 and 0.138ng/kg-body weight/day for adults and children, respectively, under high exposure scenarios. Although our estimated daily intakes of the chemicals from soils were low, more research is required to improve our understanding of the health risks posed to humans exposed to siloxanes through other pathways.

Study of the composition of low-molecular compounds in the oligomethylhydridesiloxanes

The presence of reactive hydrogen-silyl (SiH) bonds in oligomethylhydrosiloxanes provided their widespread use in the building, paper, textile and rubber industries. Low molecular weight fractions of methylhydrosiloxanes, both cyclic and linear, negatively affect the performance of oligomer-based materials. The main goal of the present study is detection and identification of these fractions in the commercial product 136-157M, as well as in the co-oligomers synthesized from it. A method of gas chromatography-mass spectrometry was used in the study. First, we proved that an increase in the temperature of the column and evaporator from 60°C to 300°C does not change the composition of the low molecular weight fractions. Consequently, no depolymerization processes related to the formation of low-molecular-weight compounds occur during the analysis. To perform a qualitative analysis, the components of low-molecular-weight fraction were separated on a capillary column and then identified using a mass detector with an ion trap. It is shown that electron ionization triggers fragmentation of molecules and the formation of positively charged ions. As for methylhydrosiloxanes, cations with a molecular weight of [M – 15] + arising from the rupture of the carbon-silyl bond are formed first. Moreover, when such a rupture occurs in cyclosiloxanes with a total number of units of more than four, further dissociation processes can occur with the detachment of the neutral fragment (methylsilane) and the formation of bicyclosiloxane cations. Since the set of positive ions is individual for each of the components and depends on [M] +• , a component-wise description of the entire low molecular weight fraction becomes possible with a high degree of the reliability. It is sown that for the considered commercial sample it can be described by two homologous series — cyclic where m ≥ 4, and linear siloxanes where n ≥ 1

β-Hydrosilylation of Allylic Derivatives with Cyclic and Linear Siloxanes in the Presence of Platinum Catalysts

The aim of this work is to study the influence of the nature and position of substitution at allyl benzene on the regioselectivity of the hydrosilylation reaction. We studied hydrosilylation of the various allyl derivatives with cyclic siloxane, D4H, and linears hydrosilylating agents, 1,1,3,3-tetramethyldisiloxane and MDHM, in the presence of platinum-based catalysts (Karstedt catalyst, platinum black). We also show the effects of the amount of catalyst on the conversion. Moreover, the influence on the temperature on catalytic activity and selectivity of the hydrosilylation reaction was explained. All new compounds were characterized by 1H, 13C, 29Si NMR and HRMS as well.

Dumbbell-Shaped, Graft and Bottlebrush Polymers with All-Siloxane Nature: Synthetic Methodology, Thermal, and Rheological Behavior.

A methodology for synthesizing a wide range of dumbbell-shaped, graft and bottlebrush polymers with all-siloxane nature (without carbosilane linkers) is suggested. These macroarchitectures are synthesized from SiOH-containing compounds-silanol (Et3 SiOH) and siloxanol dendrons of the first and second generations, with various peripheral substituents (Me or Et)-and from linear siloxanes comprising terminal and internal SiH groups by the Piers-Rubinsztajn reaction. Products and key building blocks are obtained in yields up to 95%. These polymers are heat and frost-resistant siloxanes. As it turns out, the product physical properties are determined not only by the macromolecular structure, the linear chain length, the size and frequency of branched pendant, but also by the type of peripheral substituents-Me or Et-in the pendant. Thus, the viscosity of the graft polymers with branched pendant groups comprising peripheral Me-groups is more than ≈3-5 fold lower than that of analogous polymers with peripheral Et-groups.

Methyl siloxanes in road dust from a large silicone manufacturing site in China: implications of human exposure.

Methyl siloxanes are becoming increasingly prevalent in the environment because of their extensive use in various consumer products. Little is known about the distribution of these chemicals around factories or their presence in road dust. We conducted a survey of four cyclic (D3-D6) and 13 linear (L4-L16) siloxanes in road dusts around a manufacturing site and found a total siloxane concentration range of 47.3-3.16 × 103ng/g (median 372). The predominant siloxanes in the road dust samples were D3, D4, D5, and D6, with median contributions of 65.3%, 17.9%, 6.36%, and 2.16% of the total siloxane concentrations, respectively. Our reported concentrations were comparable to those found in household dusts in previous studies. No high concentrations of siloxanes were observed in the road dust samples from the manufacturing site, which suggests that siloxanes in outdoor dusts are likely affected by many factors. Under a high exposure scenario, the daily intakes of total siloxanes via road dust ingestion at the 95th percentile were 2.13 and 0.313ng/kg-bw/day for children and adults, respectively. Although the estimated exposure to siloxanes in outdoor dust for humans was low, more studies on the distributions and risks of siloxanes released from manufacturing sites are needed.

Part I: Molecular weight characterization of linear polydimethyl siloxanes by secondary ion mass spectrometry

A correlation of SIMS ion intensities to the molecular weight of linear, trimethyl‐terminated polydimethyl siloxane (PDMS) has been developed by normalizing PDMS endgroup‐related signal to backbone‐related signal for a set of well‐characterized polymers with narrow molecular weight distributions. These initial experiments on thick PDMS films illustrate that PDMS molecular weight can be estimated using SIMS. A second paper describes the challenges in analyzing thin PDMS films on metal and polymer substrates. PDMS molecular weight determination is important to understanding surface‐related performance of materials using silicone additives across many different platforms including coefficient of friction, slip, mar, mold release, and nonstick properties as well as providing more specific characterization of surface contamination.

Spatial variations of linear and cyclic volatile methyl siloxanes in a river basin and their air-water exchange

Linear and cyclic volatile methylsiloxanes (VMS) are Si and O containing organic compounds that are increasingly subject to scientific studies in recent years. They are used as solvents and coating materials in personal care products, cleaning materials and industrial applications. In this study, it was aimed to investigate the spatial variations of VMS levels in river water and ambient air in Kucuk Menderes Basin and their air-water exchange patterns. Grab river water samples and passive ambient air samples collected from 10 sites throughout Kucuk Menderes Basin located in Izmir region in Turkey, and they were analyzed for VMS. The concentrations of ΣVMS in river water and ambient air ranged from 48.4 to 148 ng L−1 and from 41.7 to 433 ng m−3, respectively. Among the VMS compounds, the predominant compounds were found to be D5 and D3 in ambient air and river water, respectively. In general, concentrations of all VMS congeners increased towards downstream of the river for both water and ambient air samples and there were substantial fluctuations between the sampling points. These fluctuations may be due to wastewater discharges at some sites (increase), followed by loss by volatilization along the river (decrease). VMS concentrations measured in air and river water samples were used to estimate the net air-water exchange fluxes. In all cases, fluxes were negative, ranging between −159 (L4) and −11100 (D3) ng m−2 day, indicating volatilization from water is a source for atmospheric VMS.

Prediction accuracy of thermodynamic properties using PC-SAFT for high-temperature organic Rankine cycle with siloxanes

High-temperature organic Rankine cycle (ORC) with a heat source temperature over 250 °C can be used as the bottom cycle to form a combined cycle with a heat engine such as gas turbine or internal combustion engine. The accurate calculation of thermo-physical properties of the working fluid is of great significance for the performance prediction of an ORC system. PC-SAFT Equation of State (EoS) based on statistical theory can be used to estimate the thermo-physical properties of different working fluids and even to develop new working fluids. However, its accuracy needs to be investigated comprehensively. In this study, the prediction accuracy for the thermal properties based on PC-SAFT for linear and cyclic siloxanes is evaluated and the influence of the uncertainties on the performance of a high-temperature ORC is estimated. First, a program based on the PC-SAFT is developed to compute the properties of 8 siloxanes (MM, MDM, MD2M, MD3M, MD4M, D4, D5, D6). Then, the parameters of the EoS are fitted. Next, the accuracies of PC-SAFT along with the saturation lines and in the superheated regions are determined and compared with the results of Peng-Robinson (PR) EoS. Finally, the ORC performance is estimated using PC-SAFT. The results indicate that the accuracy of PC-SAFT is better than PR EoS. Only the deviation for the compressibility factor in the superheated region is slightly higher than PR. When PC-SAFT is used for the performance prediction of ORC system, the prediction accuracy increases apparently with the increase of molecular chain length. The deviations of the thermal efficiency by PC-SAFT relative to that of Refprop are basically less than 8% for the siloxanes except MM.

More bullets for PISTOL: linear and cyclic siloxane reporter probes for quantitative 1H MR oximetry

Tissue oximetry can assist in diagnosis and prognosis of many diseases and enable personalized therapy. Previously, we reported the ability of hexamethyldisiloxane (HMDSO) for accurate measurements of tissue oxygen tension (pO2) using Proton Imaging of Siloxanes to map Tissue Oxygenation Levels (PISTOL) magnetic resonance imaging. Here we report the feasibility of several commercially available linear and cyclic siloxanes (molecular weight 162–410 g/mol) as PISTOL-based oxygen reporters by characterizing their calibration constants. Further, field and temperature dependence of pO2 calibration curves of HMDSO, octamethyltrisiloxane (OMTSO) and polydimethylsiloxane (PDMSO) were also studied. The spin-lattice relaxation rate R1 of all siloxanes studied here exhibited a linear relationship with oxygenation (R1 = A′ + B′*pO2) at all temperatures and field strengths evaluated here. The sensitivity index η( = B′/A′) decreased with increasing molecular weight with values ranged from 4.7 × 10−3–11.6 × 10−3 torr−1 at 4.7 T. No substantial change in the anoxic relaxation rate and a slight decrease in pO2 sensitivity was observed at higher magnetic fields of 7 T and 9.4 T for HMDSO and OMTSO. Temperature dependence of calibration curves for HMDSO, OMTSO and PDMSO was small and simulated errors in pO2 measurement were 1–2 torr/°C. In summary, we have demonstrated the feasibility of various linear and cyclic siloxanes as pO2-reporters for PISTOL-based oximetry.

Machine-learning-assisted screening of pure-silica zeolites for effective removal of linear siloxanes and derivatives

By a two-step computational process, namely Grand Canonical Monte Carlo (GCMC) simulations and machine learning (ML), we screened 50 959 hypothetical pure-silica zeolites and identified 230 preeminent zeolites with excellent adsorption performances.

Correction: Machine-learning-assisted screening of pure-silica zeolites for effective removal of linear siloxanes and derivatives

Correction for ‘Machine-learning-assisted screening of pure-silica zeolites for effective removal of linear siloxanes and derivatives’ by Shiru Lin et al., J. Mater. Chem. A, 2020, 8, 3228–3237.

Contamination characteristics of siloxanes in coastal sediment collected from industrialized bays in South Korea

Siloxanes have been used as chemical additives in various products since the 1940s. They are known to have potentially toxic effects, to be environmentally persistent, and to be bioaccumulative. Previous studies have reported high levels of siloxanes in various environmental matrices. In this study, 6 cyclic siloxanes (D4–D9) and 13 linear siloxanes (L3–L15) in coastal sediment collected from southeastern bays adjacent to industrial zones in South Korea (Busan, Ulsan, Jinhae, and Gwangyang) were analyzed. The contamination levels and spatial distribution of siloxanes in the coastal sediment samples were investigated, with the hazard quotients (HQs) for siloxanes evaluated using Monte Carlo simulation. Across all samples, the total concentration (Σ19) of siloxanes was in the range of 11.6–3877 (mean: 305; median: 133) ng/g dry weight (dw). The highest average concentration of Σ19 siloxanes was found in Busan (mean: 580; median: 233 ng/g dw), followed by Ulsan (mean: 316; median: 209 ng/g dw), Jinhae (mean: 266; median: 125 ng/g dw), and Gwangyang (mean: 33; median: 27 ng/g dw), all of which are suggested to be affected by both industrial and domestic activities. The highest contributions were from D5 (18%) and D6 (34%), followed by D9 (7.3%) and L11 (5.8%). The HQs for siloxanes were less than 1, indicating that there was no risk to benthic organisms in the study areas; however, further monitoring of various environmental matrices is required to fully assess the potential ecological risks.

Adsorption of linear and cyclic siloxanes on activated carbons for biogas purification: Sorbents regenerability

Two commercial activated carbons, namely STIX and AP4, were selected to investigate their ability for siloxane adsorption and further thermal regeneration in relation with their different physico-chemical properties. In the frame of biogas purification, the studied siloxanes were L2, L3, L4 (linear molecules) and D4, D5 (cyclic molecules). The maximum capacity of adsorption was estimated by gas chromatography and gravimetric methods. AP4 presents better adsorptive properties comparing with STIX irrespective of the type of siloxane. Thermal regeneration after siloxanes adsorption was followed up to 400 °C by both gas chromatography and in situ DRIFTS. Except L2, the siloxane polymerization proceeds on the AC containing alkali metals (K, Na) as revealed by the observed release of their decomposition products during the thermodesorption treatment. The same sites are responsible for the cleavage of SiO bonds in linear molecules and formation of L2 as primary product above 100 °C. The presence of such sites, possibly strong basic sites, is detrimental to the regenerability of the spent adsorbent and to the adsorption which could be limited by partial blockage of the AC porosity. Interestingly the regenerability of the alkali-free AC depends only on the volatility of the siloxane.

Linear and cyclic siloxanes functionalized with polar groups by thiol-ene addition: Synthesis, characterization and exploring some material behaviour

1,3-divinyltetramethyldisiloxane (V2), 1,3,5-trivinyl-1,3,5-trimethylcyclotrisiloxane (V3), and 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane (V4) are used as hydrophobic substrates to attach carboxyl groups by thermal or photochemical activated thiol-ene addition of 3-mercaptopropionic acid (R1), thioglycolic acid (R2) and thioacetic acid (R3). The reactions occurrence is monitored by IR following the disappearance of specific absorption bands for -S-H and –CH=CH2 bonds. At the end, the structure of the compounds and thioetherification degree of vinyl groups are determined by NMR. Co-existence in the structure of the highly hydrophobic methyl group and the carboxyl or carbonyl group gives the compounds formed an amphiphilic character, as indicated by the calculated hydrophilic-lipophilic balance, making them capable of self-assembling in solution, as the results of the dynamic light scattering (DLS) and nanoparticle tracking analysis (NTA) measurements also show. Under normal temperature conditions, these compounds are in the liquid state. The DSC measurements in the range (−150) – (+150) reveal only a glass transition in the negative field and no other thermal transitions or decomposition processes. This behavior could make these compounds containing polar groups useful as solvent-free liquid electrolytes. Dielectric spectra reveal an increase in both dielectric permittivity and conductivity of the thiol-ene addition products as compared with starting vinyl-siloxanes. The increase is even more significant after doping with lithium salt, reaching conductivity values of order 10−4 S/cm.

Determination of linear and cyclic volatile methyl siloxanes in biogas and biomethane by solid-phase microextraction and gas chromatography-mass spectrometry

A new method based on solid-phase microextraction (SPME) followed by gas chromatography-mass spectrometry (GC-MS) was developed for the analysis of seven linear (L2 – L5) and cyclic (D3 – D5) volatile methyl siloxanes (VMS) in biogas and biomethane, directly collected into Tedlar® bags (Tedlar SPME) from anaerobic digesters and wastewater treatment plants. The method was employed to monitor VMS content in biomethane produced by biogas upgrading with a pilot-plant membrane unit and provided adequate limits of quantification (< 0.05 mg m−3) to detect trace siloxane impurities. Tedlar SPME was validated against a standard procedure based on indirect sampling of gas streams with sorbent tubes followed by solvent extraction and GC-MS. Method precision (RSD) on total and individual VMS concentrations was lower than 10%, while RSD values of the standard procedure were higher than 20%. Tedlar SPME suitably revealed high VMS levels, expressed as total volatile silicon (> 1 mgSim−3), in wastewater biogas and provided a more efficient sampling of heavier VMS in comparison to the sorbent tubes method. At low values (< 0.1 mgSim−3) typical of wood waste biogas and biomethane, no statistically significant differences were observed between the two methods. Overall, Tedlar SPME simplified the analytical procedure by reducing the procedural steps, avoiding the use of solvents and demonstrated its applicability for testing the quality of biomethane as advanced biofuel.

Thermodynamic Comparison of Gas Turbine and ORC Combined Cycle with Pure and Mixture Working Fluids

AbstractThis paper proposed a gas turbine and organic Rankine cycle (GT-ORC) combined cycle to further improve the energy efficiency of gas turbines. Alkanes, linear siloxanes, and aromatics were s...

Bioaccumulation of Methyl Siloxanes in Common Carp (Cyprinus carpio) and in an Estuarine Food Web in Northeastern China.

Despite the high production volume and widespread use of methyl siloxanes, limited studies have been conducted to investigate the bioconcentration, biomagnification, and trophic magnification potentials of these substances. In the present study, bioconcentration factors (BCFs) of methyl siloxanes were determined with common carp exposed at environmental relevant levels for 32days. BCF of octamethylcyclotetrasiloxane (D4) was estimated as 6197L/Kg, indicating strong bioconcentration potential in the common carp. To assess the food chain transfer of methyl siloxanes, 12 aquatic invertebrates and vertebrates species were collected from a food web in Shuangtaizi estuary in northeastern China and concentrations of methyl siloxanes in these species were determined with gas chromatography mass spectrometry. Trophic magnification factors (TMFs) of decamethylcyclopentasiloxane (D5), dodecamethylcyclohexasiloxane (D6), and linear siloxanes (L7-10) were significantly greater than 1 in one food chain selected, which suggest trophic magnification potentials of these methyl siloxanes. Biomagnification factors of D4-D6 and L7-L10 from planktons to Japanese snapping shrimp were greater than 1, indicating biomagnification potentials of these methyl siloxanes from the prey to predator. This is the first study to investigate the bioaccumulation behaviors of methyl siloxanes by coupling BCF and BMF with TMF.

Bubble-Point Measurements and Modeling of Binary Mixtures of Linear Siloxanes.

The bubble-point pressures of three binary mixtures of linear siloxanes have been measured. The binary mixtures consist of hexamethyldisiloxane (MM) which is mixed with either octamethyltrisiloxane (MDM), decamethyltetrasiloxane (MD2M), and dodecamethylpentasiloxane (MD3M). For each mixture, three compositions were measured where MM was present in approximately 25 mol%, 50 mol%, and 75 mol%. The bubble-point pressures were measured over a temperature range of 270 K to 380 K for all mixtures. Large uncertainties are observed for the lower temperatures (below 320 K) due to non-condensable impurities. A detailed analysis is performed to determine the effect of non-condensable gases on the measured bubble-point pressure data. The newly obtained bubble-point pressure data is used to determine new binary interaction parameters for the multicomponent Helmholtz energy model. The data used for the fitting of the binary interaction parameters are weighted by the relative uncertainty, this ensures that data points with high uncertainty contribute less to the final binary interaction parameter. In this work, a description of the experimental apparatus and measurement procedure is given, as well as the measured bubble-point pressure data and newly obtained binary interaction parameters.

Synthesis and Gas-Transport Properties of Iron- and Zirconium-Containing Polydimethylsiloxanes

Novel membrane materials—three-dimensional polydimethylsiloxane networks with nonaggregated metal atoms (Fe, Zr)—have been synthesized using commercial siloxane rubber SKTN and polyfunctional metallosiloxane as a crosslinking agent. For the resulting composites, the permeability, diffusion, and solubility coefficients for a wide range of gases have been determined. It has been found that the permeability coefficients for most of the gases are close to values previously obtained for linear siloxanes; however, the permeability and Р(С4Н10)/Р(СН4) selectivity for n-butane are significantly higher. It has been shown that the differences in the permeability coefficients are attributed to higher solubility coefficients of gases in the synthesized composites.