Investigation of the hydrothermal stability of cross-linked liquid silicone rubber (LSR)

We investigate the hydrothermal stability of cross-linked liquid silicone rubber (LSR) in water at 100 °C up to period of two years. Optical microscopy of cross-sections of the exposed samples reveal that only the outer 100 μm of the surface layer is affected after two years. However, the surface chemistry of the material after prolonged exposure becomes significantly modified, as monitored by X-ray photoelectron spectroscopy (XPS) and attenuated total reflectance Fourier transform infrared (ATR-FTIR), which probes depths of 10 nm and 1 μm, respectively. In addition, changes to the bulk physical properties of the rubber samples, prior to and after the exposure, were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). Micro-hardness analysis showed that surface roughness of the two year exposed sample increased from 60 (IRHD) to 75 (IRHD). Furthermore, the volume change (%) measurement showed a significant decrease in the course of exposure at prolonged time. The results provide the experimental basis for development of LSR materials suitable for numerous technical applications.     

Effects of thinner on RTV silicone rubber nanocomposites reinforced with GR and CNTs

The effects of thinner on rubber specimens with carbon nanotubes (CNTs) and graphitic nanofillers (GR) was studied for robotics applications. Rubber specimens were prepared by dispersing GR, CNTs and thinner in room temperature vulcanized (RTV) silicone rubber through solution mixing. Microscopic studies have confirmed occurrence of swelling in polymer chains due to migration of thinner. It results an increase in topological depth from 40 nm (no thinner) to 120 nm (40 phr of thinner). An elastic modulus of ~4.4 MPa (without thinner) was higher than 2.8 MPa (10 phr of thinner). At 100% strain, the lower dissipation losses of 110% (without thinner) and 70% (40 phr of thinner) were obtained. The resistance increases from 4.6 kΩ (without thinner) to 5.7 kΩ (10 phr of thinner). At 0.4‐mm‐thick elastomer slab, an actuation displacement of 0.81 mm (without thinner) was obtained which increased to 1.1 mm (60 phr of thinner). Thus, the thinner can be useful for easier processing, controlled stiffness, minimizing dissipation losses, increasing the actuation displacement and decreasing the cost of the device. Copyright © 2017 John Wiley & Sons, Ltd.     

Effects of functional graphene oxide on the properties of phenyl silicone rubber composites

Graphene oxide (GO) was treated with two types of surfactants, i.e., silane coupling agent (KH550) and 4,4’-diphenylmethane diisocyanate (MDI), incorporated into phenyl silicone rubber at a low concentration (≤0.2 wt%), and cured by the room temperature vulcanized method. The effects of functional graphene oxide on the dielectric behaviour, thermal conductivity, optical transmittance and mechanical properties of the composites were investigated. The results showed that the particle size changed after modification and that the modified GO dispersed well in the phenyl silicone rubber. The composites with MDI modified GO exhibited better electrical insulation and lower light loss in the ultraviolet–visible region than the composites with KH550 modified GO. However, composites filled with KH550 modified GO present better thermal conductivity.     

A quantitative model for the surface restructuring of repeatedly plasma treated silicone rubber

Surface restructuring in ambient air of medical grade silicone rubber surfaces modified by repeated RF plasma treatments using various discharge gases including oxygen, argon, carbon dioxide and ammonia, was studied quantitatively. From advancing and receding water contact angle data, the fraction of the surface covered by mobile and immobile polar groups, and a characteristic time constant of the restructuring process were calculated. For argon plasma treated surfaces, the fraction of immobile polar groups increased with repeated plasma treatments, but remained relatively constant for samples repeatedly treated by an ammonia plasma. The use of an oxygen plasma only yielded incorporation of mobile polar groups but not of immobile polar groups. The increase in the restructuring time constants of argon and ammonia plasma treated silicone rubber with the number of plasma treatments suggested enhanced crosslinking of the silicone rubber by these plasmas. In contrast, when an oxygen plasma was repeatedly used, the restructuring time constant decreased suggesting chain cleavage by an oxygen plasma. Tentatively, the carbon dioxide plasma treatment of silicone rubber may initially (up to 3–4 repeated treatments) yield chain cleavage, while the occurrence of crosslinking is indicated after more repetitions.     

Preparation and characterization of a homobifunctional silicone rubber membrane grafted with acrylic acid via plasma-induced graft copolymerization

Polyacrylic acid (PAA) was grafted onto the surface of silicone rubber membrane (SR) by plasma-induced graft copolymerization (PIP). Ar-plasma was used to activate the surface of SR. Also, a determination was made of the influences of plasma treatment power, pressure, time, grafted copolymerization reaction time, and monomer concentration on polymerization yield. The surface properties of SR were measured by ATR-FTIR, ESCA, and SIMS. In those analyses, the elemental composition and different carbon bindings on the surface of SR were examined by ESCA with the amount of O1s/C1s being approximately 0.7 at 60 s by Ar-plasma treatment (60 W, 200 mtorr). The peroxide group introduced on SR was measured via 1,1-diphenyl-2-picryhydrazyl (DPPH). The optimum amount of peroxide groups was 6.85 × 10⁻⁸ mol/cm² at 60 s of Ar-plasma treatment. The peroxide group (33D peak) was introduced onto the surface of SR by negative spectra of SIMS analysis after SR treatment by Ar-plasma. An increase was obtained in grafted polymerization yield with a region of 5 to 50% (v/v) of acrylic acid aqueous solution. Both sites of polyacrylic acid were detected after staining by toluidine blue O. That is, a homobifunctional membrane was developed via this surface modification method. © 1996 John Wiley & Sons, Inc.     

Effect of the platinum catalyst content on the tracking and erosion resistance of addition-cure liquid silicone rubber

An addition-cure liquid silicone rubber (ALSR) with ureido-containing MQ silicone resin (U-MTQ) was prepared by hydrosilylation with platinum catalyst (Pt). ALSR samples were characterized by thermogravimetric analysis (TGA) and thermogravimetry coupled with infrared spectroscopy (TG-FTIR), and the char layers of ALSR samples after inclined plate tracking and erosion test (IPT) were characterized by X-ray photoelectron spectroscopy (XPS). The effects of Pt content (5–100 ppm) on the curing characteristics, mechanical properties and tracking and erosion resistance of ALSR were investigated. It was found that the cure rate increased with increase of Pt content. When the Pt content was 10 ppm or 15 ppm, the ALSR had excellent tracking and erosion resistance, all specimens could pass the 4.5 kV level in IPT, and the erosion rate was only 0.13% and 0.15%, respectively. ALSR samples possessed optimum mechanical properties and thermal stability when the Pt content was 15 ppm. TG-FTIR results showed that ALSR generated less carbonyl compounds (CH₂O) and cyclic oligomers but more methane (CH₄) when the Pt content was 15 ppm. This demonstrated that there were lower levels of the oxidation of methyl groups and the main chain unzipping degradation of ALSR, and more homolysis of Si-CH₃ during the degradation. The XPS results also showed that the macromolecular radicals would further cross-link to form a firm Si-C ceramic barrier to protect ALSR samples under Pt catalysis and high temperature, thus the tracking and erosion resistance of ALSR could be improved.     

Investigation of photo‐oxidative effect on morphology and degradation of mechanical and physical properties of nano CaCO3 silicone rubber composites

Photo‐oxidative degradation of treated and untreated nano CaCO₃: silicone rubber composite was studied under accelerated UV irradiation (≥290 nm) at different time intervals. Prolonged exposure to UV leads to a progressive decrease in mechanical and physical properties along with the change in behavior of filler‐matrix interaction. This was due to decrease in cross‐linking density with increase in mobility of rubber chains. Meanwhile, synthesized nano CaCO₃ was modified with stearic acid for uniform dispersion in rubber matrix. The increase in carbonyl (>CO), hydroxyl (? OH), CO₂, and alkene functional groups on the UV exposed surface of treated and untreated nano CaCO₃: silicone rubber composites at different time intervals was studied using Fourier transform infrared (FTIR) spectroscopy. The change in morphological behavior of filler‐matrix interaction after UV exposure was studied using SEM. Overall, the study showed that the treated nano CaCO₃: silicone composites were affected more by UV exposure than untreated nano CaCO₃: silicone composites and pristine composite after UV exposure. This effect was due to peeling of stearic acid from the surface of CaCO₃, which makes the rubber chains slippery and thus separation of filler and rubber chains takes place with initiation of fast‐degradation. Copyright © 2011 John Wiley & Sons, Ltd.     

Preparation and characterization of silicone rubber through the reaction between γ‐chloropropyl and amino groups with siloxane polyamidoamine dendrimers as cross‐linkers

A novel heat‐curable silicone rubber (MCSR/Si‐PAMAM) was prepared by using siloxane polyamidoamine (Si‐PAMAM) dendrimers as cross‐linkers and polysiloxane containing γ‐chloropropyl groups as gums. The chemical cross‐linking occurs through the reaction between Si‐PAMAM dendrimers and polysiloxane containing γ‐chloropropyl groups. The effect of various amounts of cross‐linkers on mechanical properties of MCSR/Si‐PAMAM was discussed in this paper. MCSR/Si‐PAMAM exhibits favorable mechanical properties with a tensile strength of 10.06 MPa and a tear strength of 47.9 kN/m when the molar ratio r of [N‐H]/[CH₂CH₂CH₂Cl] is 1:1. These excellent mechanical properties can be attributed to the formation of concentrative cross‐linking from Si‐PAMAM dendrimers in the cross‐linking networks, along with the introduction of Si–O–Si units in the internal structure of dendrimers. The introduction of Si–O–Si units reduces the steric hindrance of molecular structure, which facilitates the N–H bonds in the interior layers of dendrimers to react with γ‐chloropropyl groups. In addition, thermogravimetric analysis results indicate that MCSR/Si‐PAMAM is thermally stable even at high temperatures in a nitrogen atmosphere. Differential scanning calorimetry analysis reveals that the glass transition peak of MCSR/Si‐PAMAM is not identified in the temperature range −150 to −30°C, only a melting endothermic peak at −40°C.     

Correlation between mechanical and electrical properties for assessing the degradation of ethylene propylene rubber cables used in nuclear power plants

The correlation among the indenter modulus, the elongation-at-break, and the breakdown-voltage strength for the ethylene propylene rubber (EPR) cables subjected to long-term thermal aging and high-temperature water submerging conditions were investigated in this work. The mechanical and electrical limits of EPR insulation material were evaluated by using an indenter modulus, with non-destructive advantage, to monitor the cable condition and to assess its remaining life. Results show that it is plausible to use indenter modulus to monitor the EPR cables for the condition approaching the end of cable service life. Testing parameters on indenter modulus, including indenter diameter, penetration speed, and penetration depth, were studied for monitoring cable degradation condition. Finally, this paper discusses the cable acceptance criteria under thermal and moisture-related aging environments. Results show that the breakdown-voltage strength is a better acceptance criterion for assessing the cable aging condition in a wet environment. While, if the cable is located in a high temperature environment, the elongation-at-break is a better criterion.     

Comparative study on degradation of ethylene-propylene rubber for nuclear cables from gamma and beta irradiation

Considering safety is the priority concern of nuclear power plants, equipment qualification testing of the nuclear components manufactured should be paid special attention to. Thereinto, equivalent conversion (1:1) from the absorbed beta doses to gamma doses is a rule of thumb for irradiation qualification testing of the polymers used as nuclear cables, however whether it is reasonable and applicable to Chinese domestic polymers still requires investigation. In this paper, both gamma and beta irradiation testing with the actual dose rates and total absorbed doses in China Advanced Passive (CAP) series nuclear power plant was performed upon one domestically manufactured ethylene-propylene rubber intended for nuclear cable insulation in China. The mechanical and the electrical properties before and after irradiation were measured to compare the extent and the trend of degradation between the two irradiation types, and related oxidation degradation mechanism especially its attenuation along the thickness was quantitatively addressed.     

Thermal characterization of Al2O3 and ZnO reinforced silicone rubber as thermal pads for heat dissipation purposes

Silicone rubber filled with thermally conductive, but electrically insulating Al₂O₃ or ZnO fillers were investigated to be used as elastomeric thermal pads, a class of thermal interface materials. The effect of Al₂O₃ or ZnO fillers on the thermal conductivity and coefficient of thermal expansion (CTE) of the silicone rubber were investigated, and it was found that with increasing Al₂O₃ or ZnO fillers, the thermal conductivity of the thermal pads increases, while the coefficient of thermal expansion (CTE) decreases. The thermal conductivity results obtained were also analyzed using the Agari model to explain the effect of Al₂O₃ or ZnO fillers on the formation of thermal conductive networks. Thermal gravimetry analysis (TGA) showed that the addition of either Al₂O₃ or ZnO fillers increases the thermal stability of the silicone rubber, while the scanning electron microscope (SEM) showed that at 10 vol.% filler loading percolation threshold has yet to be reached.     

Effects of compression cycles and precompression pressure on the repeatability of piezoresistivity for carbon black‐filled silicone rubber composite

We studied the influences of the compression cycles and the precompression pressure on the piezoresistivity of carbon black‐filled silicone rubber composite. The experimental results show that if the load pressure is less than the maximum pressure in the precompression cycle, the repeatability of the piezoresistivity is improved with the increase of the compression cycles. Once the load pressure surpasses the maximum pressure in the precompression cycle, the piezoresistivity of the composite is changed distinctly. The experimental phenomena are explained and described qualitatively by analyzing the changes in the composite resistance under the zero pressure and the slippage of the molecule chain during the compression. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1050–1061, 2008     

Exploring the effect of radiation crosslinking on the physico‐mechanical,dynamic mechanical and dielectric properties of EOC–PDMS blends for cable insulation applications

This work focuses on the effect of electron beam irradiation on the physico‐mechanical, dynamic mechanical and dielectric properties of blends based on ethylene octene copolymer (EOC) and poly dimethyl siloxane (PDMS) rubber. It is found that electron beam irradiation caused considerable improvement in the physico‐mechanical properties; the tensile strength was enhanced by nearly 35% for 70:30 EOC:PDMS blend. Phase morphology of the blends analyzed before irradiation by scanning electron microscopy (SEM) exhibited droplet/matrix morphology with sizes of the PDMS rubber domain varying from 0.55 µm to 0.47 µm as the amount of PDMS rubber decreased from 30 wt% to 10 wt%. This reduction in the PDMS rubber domain has been correlated with the physico‐mechanical properties of the blends. Further, the dynamic mechanical properties and creep behavior of these EOC:PDMS blends before and after irradiation has been studied. It is inferred that the 70:30 blend after radiation crosslinking shows a 17% decrease in the creep compliance, i.e. higher creep resistance compared to neat blends. All the radiation crosslinked blends exhibited lower dielectric constant, lower dielectric loss and higher electrical resistivity as compared to the virgin blends which makes it suitable for cable insulating application. Copyright © 2016 John Wiley & Sons, Ltd.     

Chemical composition of contemporary black printing inks based on infrared spectroscopy: basic information for the characterization and discrimination of artistic prints

This article aims to provide the basic information necessary to afford the discrimination of artistic prints based on chemical information. The presence of original and non-original prints in the art market is a real problem, especially for contemporary artists. Most of the non-original elements can be distinguished on the basis of their stylistics and printing characteristics, but sometimes this information is not enough. To facilitate discrimination in these cases, we propose to add, to the previously indicated aspects, the chemical information of the materials involved in the production of the original series (inks, paper and pencil). Comparison of the composition of any element with that established for the original series will permit the attribution of any doubtful element. This article is focused on the evaluation of this approach by determining the chemical composition of contemporary black printing inks by infrared spectroscopy and by searching, on the basis of the information obtained, for criteria capable to discriminate between prints made of different inks. The success of the approach with this set of inks could easily be extended to the study of colour inks. The study also includes the characterization and discrimination of black pigments because they are the most characteristic component of these inks and because this information can also be useful for other artistic studies. The procedure proposed permits the analysis of inks with minimum deterioration of the artwork. Results indicate the capability of the approach because, following the procedure, it is possible to distinguish between black pigments attending to the presence of hydroxyapatite, silicate and some other specific bands, whereas black inks can be discriminated by paying attention to the inclusion in their composition of such pigments as bone black, Prussian blue, mars brown or other specific bands. Scanning electron microscopy analyses have corroborated the results obtained.     

Application of an efficient strategy based on liquid–liquid extraction,high‐speed counter‐current chromatography,and preparative HPLC for the rapid enrichment,separation, and purification of four anthraquinones from Rheum tanguticum

This study presents an efficient strategy based on liquid–liquid extraction, high‐speed counter‐current chromatography, and preparative HPLC for the rapid enrichment, separation, and purification of four anthraquinones from Rheum tanguticum. A new solvent system composed of petroleum ether/ethyl acetate/water (4:2:1, v/v/v) was developed for the liquid–liquid extraction of the crude extract from R. tanguticum. As a result, emodin, aloe‐emodin, physcion, and chrysophanol were greatly enriched in the organic layer. In addition, an efficient method was successfully established to separate and purify the above anthraquinones by high‐speed counter‐current chromatography and preparative HPLC. This study supplies a new alternative method for the rapid enrichment, separation, and purification of emodin, aloe‐emodin, physcione, and chrysophanol.