Thermal stability and ablation properties of silicone rubber composites

Effects of incorporation of clay and carbon fiber (CF) into a high temperature vulcanized (HTV) silicone rubber, i.e., poly(dimethylsiloxane) (PDMS) containing vinyl groups, on its thermal stability and ablation properties were explored through thermogravimetric analyses (TGA) and oxy‐acetylene torch tests. Natural clay, sodium montmorillonite (MMT), was modified with a silane compound bearing tetra sulfide (TS) groups to prepare MMTS₄: the TS groups may react with the vinyl groups of HTV and enhance the interfacial interaction between the clay and HTV. MMTS₄ layers were better dispersed than MMT layers in the respective composites with exfoliated/intercalated coexisting morphology. According to TGA results and to the insulation index, the HTV/MMTS₄ composite was more thermally stable than HTV/MMT. However, addition of CF to the composites lowered their thermal stability, because of the high thermal conductivity of CF. The time elapsed for the composite specimen, loaded with a constant weight, to break off after the oxy‐acetylene flame bursts onto the surface of the specimen was employed as an index for an integrated assessment of the ablation properties, simultaneously taking into consideration the mechanical strength of the char and the rate of decomposition. The elapsed time increased in the order of: HTV < HTV/CF ≈ HTV/MMTS₄ < HTV/CF/MMTS₄ ≈ HTV/MMT < HTV/CF/MMT. This order was different from the increasing order of the thermal stability determined by TGA results and the insulation index. The decreased degree of crosslinking of the composites with MMTS₄ compared with that of the composite with MMT may be unfavorable for the formation of a mechanically strong char and could lead to early rupture of the HTV/MMTS₄ specimen. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Ablation properties of aluminum silicate ceramic fibers and calcium carbonate filled silicone rubber composites

The ablative performance of aluminum silicate ceramic fiber (ASF) and calcium carbonate (CaCO₃) filled silicone rubber composites prepared through a two‐roll mill was examined. The properties of the composites were investigated by thermogravimetry, thermal conductivity measurements, and oxyacetylene torch testing. After the material was burnt, the structure and composition of the char were analyzed by Fourier transform infrared spectroscopy, X‐ray diffraction, and scanning electron microscopy (SEM). The results of the ablation test showed that the ablation resistance improved greatly in an appropriate filler scope. Combined with SEM, it was proven that a firm, dense, and thermal insulation layer, which formed on the composites surface during the oxyacetylene torch test, was a critical factor in determining the ablation properties. Thermogravimetric analysis revealed that the thermal stability of the composites was enhanced by the incorporation of ASF and CaCO₃. The thermal conductivity measurements showed that the silicone rubber composites had a very low thermal conductivity ranging from 0.206 to 0.442 W m^?1 K^?1; this significantly prevented heat from transferring into the inner matrix at the beginning of the burning process. The proportion of 20/40 phr (ASF/CaCO₃) was optimum for improving the ablation resistance of the silicone rubber composites. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41619.     

A comparison of ablative resistance properties of liquid silicone rubber composites filled with different fibers

Flexible ablative materials play a key role in thermal protection systems to protect space vehicles during hypersonic flight missions. In this work, epoxy resin modified liquid silicone rubber was adopted as elastomeric matrix. Two inorganic fibers (carbon fibers (CF) and quartz fibers (QF)) and two organic fibers (aramid fibers (AF) and poly (p-phenylene benzobisoxazole) fibers (PBO)) were chosen as functional fillers. The ablation resistance and thermal insulation properties were evaluated using oxyacetylene torch test. Microstructure and phase composition of the char layer was fully characterized to investigate the ablation mechanism. SEM observations revealed that the fibers are well dispersed and integrated in the matrix. TGA analysis indicated that the inorganic fibers possess significantly higher thermal stability than organic fibers. In addition, a molten silica film can be formed on the material surface, which can exert a better thermal protection effect on the matrix. Furthermore, the ablation test shows that the linear ablation rate decreases first and then increases with an increase of fiber content. This work provides basic data to guide optimal selection of fibrous ablatives to enhance ablation performance.     

Improving ablation properties of liquid silicone rubber composites by in situ construction of rich-porous char layer

In this article, ammonium polyphosphate (APP) and ammonium pentaborate (APB) were introduced to liquid silicone rubber with an aim of building rich-porous char structure in situ, thereby improving thermal insulation properties. Thermogravimetric analyses indicated that the incorporation of APP greatly increased the char residue of the composites. Oxy-acetylene torch tests showed that the addition of either APP or APB powders effectively enhanced the ablation resistance of the composites, whereas Shore A hardness tests revealed that the APP-containing composites exhibited a higher hardness than APB-filled counterparts. The linear ablation rates of composites with 40 phr APP or APB were reduced by 34.16% and 36.19%, respectively, when compared with the control sample. The maximum back-face temperatures of composites with 40 phr APP or APB was reduced to as low as 73°C. The APP-containing composites exhibited superior ablation resistance, considering both the linear ablation rate and the mechanical properties of char layer. In addition to SiO₂, SiC, and C, B₂O₃ was produced in the APB composites, as characterized by XRD and Raman analysis. Combined with SEM, it was proven that the formation of a firm, continuous, rich-porous and thermal insulation char layer was advantageous to improve the ablation and insulation properties.     

Ablation and thermo-mechanical investigation of polyarylacetylene impregnated heat vulcanising silicone rubber for ultrahigh temperature applications

Hot vulcanised silicone rubber composites filled with polyarylacetylene (PAA) prepolymer or carbon fibre (CFs) are prepared. The residual weight, mechanical properties and ablation property of the composite are tested. The results show that the incorporation of PAA into silicon rubber can obviously increase the residual weight of the resulting composite. The addition of PAA or CFs has improved the hardness of the composites. The tensile stress and elongation at break of the composite are found increased initially and then decreased with the increasing of the concentration of PAA. PAA and CFs can greatly improve the ablation properties of the composites. The ablated surface contains SiO₂, SiC and/or Si, which hinder the oxygen diffusion and the thermal conduction, and consequently prevent the decomposition and oxidation of the composite. The crack is a crucial factor determining the thermal conduction of the ablation layer when exposed to the oxyacetylene torch.     

The curing retardation and mechanism of high temperature vulcanizing silicone rubber filled with superconductive carbon blacks

The curing retardation and mechanism of high‐temperature vulcanizing silicone rubber (HTV SR) filled with superconductive carbon black (CB) BP2000 have been studied experimentally and theoretically. The results show that both rubber matrix and CBs have influences on the peroxide curing of rubber/CBs composites. The retardation does not appear as prominent in nature rubber (NR)/BP2000 composites as in HTV SR/BP2000 composites. Quantum chemistry calculations reveal that the curing retardation of HTV SR/BP2000 composite should not be attributed to the curing reaction dynamics of HTV SR molecules. Fourier transform infrared (FTIR) and X‐ray photoelectron spectroscopy (XPS) analyses of CBs show that the effect of radical scavenging of phenol‐OH groups existing on BP2000 surface is the main reason for the retardation in the peroxide curing reaction. The effect is found to be more effective in HTV SR/BP2000 composite and thus retards its curing. The curing retardation does not appear in silicone rubber (SR)/BP2000 composites vulcanized by condensation reaction, and the resulting vulcanizates have excellent physical properties. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers.     

抗静电氧化锌晶须对热硫化硅橡胶性能的影响

研究了抗静电用氧化锌晶须对热硫化硅橡胶的硫化特性、力学性能及电性能的影响。结果表明：在硅橡胶硫化初期氧化锌晶须能加速硫化反应、在硫化后期延迟硫化反应，可提高其最低扭矩和最高扭矩；硫化胶的硬度增加，拉伸强度、扯断伸长率有所降低，表面电阻率和体积电阻率降低。     

高温硫化硅橡胶改性及老化研究进展

高温硫化硅橡胶由于其优异的高低温性能、耐候性、表面性能等特点而在航天、电力和医疗器械等领域中得到广泛应用。本文主要介绍了最近五年来,通过化学或物理方法对硅橡胶表面性能和低温性能等进行强化方面的相关研究,以及在实现产品功能化方面取得的进展。此外还介绍了在加速老化条件下不同硅橡胶材料的力学、介电性能等变化及其机理,以及在老化研究方法的精确化、可靠化和可视化方面的研究进展,重点关注多因素影响下的材料寿命分析和预测;并特别关注了水在硅橡胶老化过程中的作用及机理研究成果。最后,对高温硫化硅橡胶改性和老化研究的前景进行了展望。未来,极端环境下的硅橡胶功能化研究以及多因素影响下的材料寿命分析和预测将成为重要的发展方向。     

Thermal stability and ablation properties study of aluminum silicate ceramic fiber and acicular wollastonite filled silicone rubber composite

The thermal stability and ablation properties of silicone rubber filled with silica (SiO₂), aluminum silicate ceramic fiber (ASF), and acicular wollastonite (AW) were studied in this article. The morphology, composition, and ablation properties of the composite were analyzed after oxyacetylene torch tests. There were three different ceramic layers found in the ablated composite. In the porous ceramic layer, the rubber was decomposed, producing trimers, tetramers, and SiO₂. ASF and part of AW still remained and formed a dense layer. The SiO₂/SiC filaments in the ceramic layer reduced the permeability of oxygen, improving the ablation properties of the composites. The resultant ceramic layer was the densest, which acted as effective oxygen and heat barriers, and the achieved line ablation rate of the silicone composite were optimum at the proportion of 20 phr/40 phr (ASF/AW). Thermogravimetric analysis (TGA) confirmed that thermal stability of the composites was enhanced by the incorporation of ASF and AW. The formation of the ceramic layer was considered to be responsible for the enhancement of thermal stability and ablation properties. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 39700.     

硅橡胶隔热复合材料的制备及性能

采用空心玻璃微珠（HGB）和膨胀蛭石（EVMT）作为隔热填料,分别与高温硫化硅橡胶（HTV）共混制备HGB/HTV和EVMT/HTV隔热复合材料,并考察了隔热填料的种类和用量对隔热复合材料的隔热性能和力学性能的影响。结果表明,随着HGB和EVMT用量的增加,隔热复合材料的导热性能明显下降。相比于HTV试样,当HGB与HTV的质量比为20/100,EVMT与HTV的质量比为10/100时,隔热复合材料的导热系数分别下降了20.44%和12.34%。HGB和EVMT均能提高HTV隔热复合材料的100%定伸应力和邵尔A硬度,降低其拉伸强度与扯断伸长率。当HGB与HTV的质量比为20/100,EVMT与HTV的质量比为10/100时,隔热复合材料的拉伸强度分别为10.51 MPa和11.59 MPa,邵尔A硬度分别为71.6和59.1。     

Highly branched phenolic resin-grafted silicone rubber copolymer for high efficiency ablation thermal protection coating

Highly branched phenolic resin-grafted silicone rubber (PGS) was synthesized by grafting allyl ether phenolic resin onto silicone rubber with Si─H groups via hydrosilylation. The chemical structure of the copolymer was characterized by Fourier transform infrared spectroscopy, nuclear magnetic resonance, and gel permeation chromatography, and its microphase morphology was observed via scanning electron microscopy (SEM). The mechanical properties and thermal stability of the material were also characterized. Thereafter, an ablation thermal protection coating (ATPC) based on PGS as the polymer matrix was prepared. In comparison with silicone rubber-based ATPC, the PGS-based ATPC showed better thermal stability and a stronger char layer. After oxygen–acetylene testing, the micromorphology and chemical composition of the PGS-based ATPC were characterized via SEM, energy-dispersive spectrometry, and X-ray diffraction. Finally, the PGS- and silicone rubber-based ATPCs were subjected to a heat flow of 370 kW m⁻² for 70 s, the background temperature of the PGS-based ATPC was only 70 °C. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48353.     

Effect of surface chemistry and morphology of silica on the thermal and mechanical properties of silicone elastomers

In this study, high‐temperature vulcanized silicone rubbers (HTV‐SRs) using fumed silica (FSi), precipitated silica (PSi), and modified precipitated silica (MPSi) as reinforcing fillers were prepared. The effect of morphology and surface chemistry of the silica on the thermal and mechanical properties of the resultant silicone rubbers was investigated using curing rheometer, scanning electron microscopy, mechanical test, and dynamic mechanical analysis. The thermo‐oxidative stability and solvent resistance of the vulcanized silicone rubbers were further evaluated via heat ageing test, extraction, and swelling experiments. It is shown that the mechanical properties (tensile modulus and tensile strength) of the as‐prepared HTV‐SRs are in the order of FSi > PSi > MPSi, which could be attributed to the molecular interaction between the filler and the matrix. FSi has the highest surface area, which enhances the hydrogen bonding interaction between the filler and the silicone matrix; while MPSi, in which part of Si? OH groups have been consumed during modification, shows the weakest interaction among the three. The filler–matrix interaction could also explain the lowest swelling and sol fraction in FSi‐filled HTV‐SR, and the low viscosity and good processibility of PSi‐ and MPSi‐filled HTV‐SR. Furthermore, it is also shown that the MPSi‐filled HTV‐SR exhibits the highest retention of mechanical properties after thermal aging at 250 °C for 24 h, which could be attributed to the lowest acidity of the fillers. The possible mechanism for acid catalyzed hydrolytic chain scission and intramolecular chain backbiting has been proposed. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46646.     

Synthesis of hyperbranched organo‐montmorillonite and its application into high‐temperature vulcanizated silicone rubber systems

N,N‐Di(2‐hydroxyethyl)‐N‐dodecyl‐N‐methyl ammonium chloride was used as intercalation agent to treat Na⁺‐montmorillonite and form a type of organic montmorillonite (OMMT). Hyperbranched OMMT (HOMMT) was prepared by condensation reaction between OMMT and the monomer we synthesized. It was then used in the preparation of high‐temperature vulcanizated silicone rubber (HTV‐SR)/HOMMT nanocomposite. Different types of HTV‐SR/HOMMT nanocomposites were prepared with different amounts of HOMMT and compared with the composites directly incorporated with OMMT. Tensile properties such as tensile strength, elongation at break, permanent distortion, and shore A hardness were researched and compared. A combination of Fourier transform infrared spectroscopy, wide angle X‐ray diffraction, and transmission electron microscopy studies showed that HTV‐SR/HOMMT composites were on the nanometer scale, and the structure of HTV‐SR was not interfered by the presence of HOMMT. Results showed that the tensile properties of HTV‐SR/HOMMT systems were better than that of the HTV‐SR/HOMMT and HTV‐SR. This was probably due to the surface effect of the exfoliated silicate layers and anchor effect of HOMMT in the SR matrix. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

氢氧化铝复合阻燃剂对热硫化硅橡胶性能的影响

研究了氢氧化铝、氢氧化铝／三氧化二锑并用、氢氧化铝／三氧化钼并用对热硫化(HTV)硅橡胶阻燃性能和力学性能的影响；试图在力学性能与阻燃性能之间寻找平衡。结果表明：当100份硅橡胶中氢氧化铝的用量为80份时，硅橡胶的燃烧氧指数(OI值)可达30％，但力学性能却受到严重损害，发烟量为B级；氢氧化铝／三氧化二锑并用时，HTV硅橡胶的性能较理想；当(氢氧化铝 三氧化二锑)用量为50份时，硅橡胶的OI值为31％、拉伸强度为6．3MPa、扯断伸长率为660％、撕裂强度为23．7kN／m、邵尔A硬度为57度，但发烟量为C级；氢氧化铝／三氧化钼并用可使HTV硅橡胶燃烧时的发烟量达到A级，但不能显著提高其阻燃性能，当(氢氧化铝 三氧化钼)用量为96份时，HTV硅橡胶的OI值仅28％，且此时其力学性能受到较大损害。     

硅橡胶在电力系统外绝缘中的应用

通过试验数据分别分析了在输变电设备上使用的室温硫化硅橡胶和热硫化硅橡胶（HTV）的运行特性，着重介绍了运行条件对HTV硅橡胶憎水性能的影响、运行HTV硅橡胶的耐漏电起痕及电蚀损性能、机械性能和积污特性。认为因老化引起的憎水性、机械强度、积污性能下降以及材料劣化均影响着其长期工作的可靠性，需改进配方，以满足外绝缘材料长期运行的要求；并比较了3种硅橡胶材料的老化试验方法：斜面法、1000h盐雾法及5000h老化试验方法的试验条件、试验程序、试验现象、试验结果和试验评定的差异。     

Ablation properties of zinc borate and aluminum hypophosphite filled silicone rubber composites

The ablative properties of epoxy modified silicone rubber composites filled with zinc borate (ZB) and aluminum hypophosphite (AHP) were studied. The decomposition of the added fillers and covering connection of residual on the substrate contribute to the improvement of heat insulation. The ablation test shows that the formation of a dense char layer with certain strength is key to improving the ablative properties. The optimal ablative performance is achieved when the concentration of ZB and AHP is 10 and 5 phr, respectively. Under such circumstances, the linear ablation rate is as low as 0.032 mm/s, which is about 61% lower than the pure silicone rubber. Furthermore, the structure and composition of the formed char layer were analyzed using X-ray diffraction analysis, scanning electron microscopy, and Fourier transform infrared spectroscopy.     

氢氧化铝对复合绝缘子用硅橡胶的性能影响研究

研究了氢氧化铝的粒径及填充量对复合绝缘子用热硫化硅橡胶的力学性能、电学性能、阻燃性、憎水性能的影响。实验结果表明：当氢氧化铝粒径为2.4μm时,硅橡胶的力学性能及阻燃性能达到最佳,憎水性及电学性能适中;氢氧化铝填充量为115～125份之间时,硅橡胶综合性能最佳,漏电起痕达到1A4.5,阻燃性能满足FV-0。     

硅橡胶复合材料的制备及摩擦性能

考察了白炭黑、羟基硅油对硅橡胶复合材料摩擦性能的影响,研究了硅橡胶复合材料在干、湿光滑玻璃表面摩擦系数的影响因素及摩擦机理.结果表明,硅橡胶复合材料在光滑玻璃表面的干摩擦系数(fD)与损耗因子(tanδ)/邵尔A硬度(H)、tanδ/剪切储能模量(G')均呈线性正相关,随白炭黑用量的增加,fD降低.当增塑剂用量小于7....     

硅橡胶对金属粘合性能的研究

研究了四乙烯基硅烷和四烯丙基硅烷对提高HTV硅橡胶粘合性能的影响。发现用掺入法的效果比表面处理法的好得多。为提高表面处理法的效果 ,合成了两种新化合物 ,β-三乙烯基乙基三甲氧基硅烷和γ -三烯丙基丙基三甲氧基硅烷。另外 ,从改变聚硅氧烷分子链结构 ,使其对金属获得粘合性出发 ,还合成了两种新型加成型硅橡胶 ,并对其与金属的粘合性能进行了研究     

Improving the tracking and erosion resistance of silicone rubber using Fe2O3 and platinum catalyst

In typical HTV silicone rubber for composite insulators subjected to a 4.5 kV inclined plane (IP) test, alumina trihydrate loading exceeds 40 wt%, which limits the rubber content and weakens the antiaging properties. This study compares two methods for enhancing tracking and erosion resistance while reducing ATH content to meet IP test requirements. Increasing Fe₂O₃ content from 1.5 wt% to 3 wt% promotes the formation of mullite. The formed ceramic layer isolates heat and oxygen and hence facilitates the specimen to pass the IP tests with less content of ATH. By contrast, Pt/NS system cannot suppress the tracking efficiently by forming the Si O C ceramics at high ATH loading. The decomposition of ATH will generate water vapor and lead to holes in the surface. Both moisture and oxygen absorbed have a detrimental effect on the suppression mechanism of the Pt/NS system. Consequently, it is recommended that the content of Fe₂O₃ can be increased slightly to lower the ATH content in the traditional HTV SiR formula used for the composite insulator. Besides, when the content of ATH is relatively high, the use of the Pt/NS system is not recommended.