Research on high temperature friction properties of PTFE/Fluorosilicone rubber/silicone rubber

Silicone rubber (MVQ) has excellent heat resistance, but poor high temperature friction stability, which limits its application in the field of high temperature sealing. Polytetrafluoroethylene (PTFE) is self-lubricating, but its compatibility with rubber is relatively weak. In order to improve the high-temperature friction property of silicone rubber, fluorosilicone rubber (FVMQ) was used as a compatibilizer, and PTFE was added to MVQ by mechanical blending. The friction and wear properties of PTFE/FVMQ/MVQ composites at different temperatures were studied. The results show that compared with MVQ, the mechanical properties of PTFE/FVMQ/MVQ composites was basically unchanged, the coefficient of friction was hardly affected by temperature, and the amount of wear decreased with increasing temperature. PTFE/FVMQ/MVQ composites showed excellent high-temperature abrasion resistance. The high-temperature wear mode was mainly changed from abrasive wear to adhesive wear. The molten layer formed by high-temperature friction can prevent air from directly contacting the surface rubber, which inhibited rubber surface oxidation reaction process.     

In vitro drug release from silicone rubber–polyacrylamide composite

Composite materials containing drugs were prepared from silicone rubber and hydrogel. Cross linked polyacrylamide (PAAm) hydrogel particles were incorporated into a silicone rubber to enhance the hydrophilicity and drug release capacity of silicone rubber as a matrix. Progesterone and Thymol Blue were used as a hydrophobic and hydrophilic drug model, respectively. Different amounts of polyacrylamide (PAAm) were mixed with the drugs and uncured silicone rubber at room temperature. The composite matrices were formed using a compression molding press and cured by thermal and γ-irradiation curing methods. In vitro drug release behavior of composites and their physical and mechanical properties were investigated. The results indicated that the hydrophilic character of silicone rubber was more pronounced with increasing the amount of PAAm. Also, a significant effect on the drug release profiles was observed. The γ-irradiation curing method improved mechanical properties of composites and affected the drug release profiles. It was found that the amounts of released progesterone from γ-irradiated samples increased in comparison with the thermally cross linked composite since released Thymol Blue was reduced.     

硅橡胶/氟橡胶并用胶的制备及力学性能

硅橡胶和氟橡胶作为国防、航天等重要领域的耐热材料一直被人们青睐,但其有着各自地优缺点且价格昂贵,本文尝试将这两种橡胶制成并用胶以解决氟橡胶不耐低温和加工性差的问题,以期增大其使用温度范围。采用机械共混法制备硅橡胶/氟橡胶并用胶,研究了硅橡胶和氟橡胶的混炼工艺、并用比、共硫化体系和硫化条件对并用胶力学性能的影响。结果表明,当硅橡胶/氟橡胶的质量比为10∶90,共硫化体系为3~#硫化剂/过氧化二异丙苯(DCP),一段硫化温度为170℃、硫化压力为10MPa、硫化时间为30min,二段硫化温度为200℃、硫化时间为6h时,并用胶的力学性能达到最好。     

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.     

Enhancement of thermal conductivity and tensile strength of liquid silicone rubber by three-dimensional alumina network

ABSTRACT

Rapidly increasing demands for higher integration density and stability of electronic devices embrace higher requirements for thermally conductive silicone rubber, which is promisingly used in ultra-thin components. In this work, alumina whiskers (AWs) and alumina flakes (AFs) are used to modify liquid silicone rubber (LSR) by fabricating binary (AFs/LSR) or ternary (AWs/AFs/LSR) composites. The thermal conductivity and mechanical strength of the binary and ternary composites were investigated. Thermal conductivity of the binary AFs/LSR composite (25AFs/LSR) was 0.1990 W m^?1 K^?1, while the thermal conductivity of the ternary AFs/AWs/LSR composite (20AFs/5AWs/LSR) was 0.2655 W m^?1 K^?1. Furthermore, the tensile strength of the ternary AWs/AFs/LSR composites increased by 180.9% as compared with the binary system, increased to 7.81 MPa from 2.78 MPa due to the introduction of 1 wt% AWs. As a reason, a significant synergistic effect of AWs and AFs in the enhancement of both thermal and mechanical properties of the LSR was proved. Furthermore, the dielectric property measurements demonstrated that the ternary composites exhibited a lower dielectric constant and dielectric loss, indicating that the AWs/AFs/LSR composites were qualified to be applied in the field of electronic devices.     

Thermal Oxidative Degradation and Ageing Performance of Silicone Rubber Filled with Attapulgite

The natural attapulgite (NAPT) was disaggregated by high-pressure homogenization technology combined with extrusion process to prepare the attapulgite with disaggregated rod crystal bundles (DAPT) and large specific surface area of 133.7 m²/g. NAPT and DAPT were incorporated into the silicone rubber to obtain the composite NAPT-SR and DAPT-SR by mechanical blending method, respectively. After thermal oxidative ageing at 300 ℃ for 0.5 h, temperature for the 5% weight loss increased greatly from 385 ℃ of the neat silicone rubber to 396℃ - 399 ℃ with addition of NAPT and DAPT. NAPT and DAPT enhanced the interaction between the filler nanoparticles and rubber matrix thus inhibited the nanoparticle agglomeration. The conservation rate of the side methyl group in NAPT-SR and DAPT-SR was greatly improved after ageing. Therefore, the thermal oxidative degradation and ageing performance of the silicone rubber composites was significantly reinforced. Moreover, DAPT could greatly restrain the growth of nanoparticles after ageing. Therefore, DAPT-SR showed the better retention of tensile strength (40.6%), elongation at break (34.9%) and tear strength (30.1%) compared with the corresponding mechanical properties of the neat silicone rubber (10.6%, 7.4%, and 5.0%) after ageing.     

Effect of nanosilica on thermal oxidative degradation of SBR

The effect of silica content on thermal oxidative stability of styrene–butadiene rubber (SBR)/silica composites has been studied. Morphologies of silica in SBR with different contents are investigated by scanning electron microscopy, which indicates that silica can well disperse in SBR matrix below the content of 40 %, otherwise aggregates or agglomerates will generate. Composites with around 40 % silica content show excellent mechanical properties and retention ratios after aging at 85 °C for 6 days. The values of activation energy (E _a) of pure SBR and its composites are calculated by Kissinger and Flynn–Wall–Ozawa methods based on thermogravimetric (TG) results, which suggests that composite with about 20 % silica has minimum E _a, and composite with 30–40 % silica has maximum E _a. According to TG curves, it is found that silica can suppress the formation of char leading to decline in stability to some extent. On the other side, silica also has positive effect on improving thermal stability of the matrix as filler. Thus, the SBR/silica composites with silica content of 30–40 % can possess both excellent resistance to thermal oxidative degradation and superior mechanical properties.     

Fabrication and characterization of gamma-irradiated recycled (thermoplastic/elastomer) matrix filled with feldspar composites

A composite of waste polyethylene, recycled waste rubber powder and reactive compatibilizing agent maleic anhydride, 60/40/2 mass%, was loaded with increasing contents, up to 20 mass%, of the reinforcing filler, feldspar [K (Al SiO₃O₈)]. The composites were gamma-irradiated at various doses up to 150 kGy. Selected physical, mechanical, and thermal parameters were investigated as functions of radiation dose and filler content. Gamma irradiation led to a significant improvement in the properties for all composites irradiated with 150 kGy. Similarly, the increase in feldspar content provided substantial improvement in properties as a result of development in the interfacial adhesion between the filler particles and composite components. The results were confirmed by examining the fracture surfaces using scanning electron microscopic techniques.     

Interfacially modified LDPE/GTR composites with non-polar elastomers: From microstructure to macro-behavior

This paper provides some new insights into the mechanism of interaction and modifications in thermoplastic composites based on low density polyethylene (LDPE), ground tire rubber (GTR) and non-polar elastomer. The composites were prepared using a co-rotating twin-screw extruder at variable LDPE/GTR ratio and constant elastomer content. Two types of commercial elastomer were applied: styrene-butadiene-styrene (SBS) block copolymers (Kraton^®) with different topologies (linear/branched) and partially cross-linked butyl rubbers (Kalar^®) with different Mooney viscosities. Processing characteristics, static mechanical properties (tensile strength, elongation at break, hardness), dynamic mechanical properties, thermal properties and morphology of the resulting thermoplastic composites were investigated. Microstructure analysis shows that modification of LDPE/GTR composites with non-polar elastomers caused encapsulation of GTR particles within the elastomer phase. This phenomenon has significant influence on macro-behavior of thermoplastic composites based on LDPE/GTR blends. The results indicate that SBS copolymer improves interfacial interactions between GTR and LDPE, which enhances mechanical and thermal properties of the composites. On the other hand, cross-linked butyl rubber showed partial compatibility with LDPE and low compatibility with GTR particles.     

The thermal and dielectric properties of high performance cyanate ester resins/microcapsules composites

Novel high performance bisphenol A dicyanate ester (BADCy) resins/poly(urea-formaldehyde) microcapsules filled with epoxy resins (MCEs) composites have been prepared. The effects of different contents of MCEs on the thermal and dielectric properties of cured BADCy were investigated using dynamic mechanical analyzer (DMA), thermalgravimetric analyzer (TGA) and broadband dielectric analyzer. The dielectric properties of BADCy/MCEs treated in hot water and hot air were also discussed. The morphologies of BADCy/MCEs composites were characterized by scanning electron microscopy (SEM). Results indicate that the appropriate content of MCEs can improve or maintain the thermal stability, the low dielectric constant and dielectric loss of cured BADCy mainly owing to higher conversion of cyanate ester (-OCN) groups. After aged in hot water and hot air, respectively, BADCy/MCEs composites with small content of MCEs can retain the low dielectric constant and dielectric loss.     

多苯基芳基类硅化合物作为硅聚合物热稳定剂的研究

本文通过Diels-Alder反应合成了四苯(基)苯基三乙氧基硅烷及四苯(基)苄基三乙氧基硅烷,产率均为96％左右。并作了下列耐热性研究:(1)把两种化合物分别作为硅橡胶混炼时的热稳定剂,所得硅弹性体其热老化稳定性及机械强度均较好。(2)把两种化合物分别作为室温硫化硅橡胶固化时的交联剂,所制得的胶片在350℃老化10小时仍保持弹性。(3)两种化合物分别与H_3BO_3反应,制得耐热性硅硼树脂,是一种优良的耐高温涂料。     

Mechanical and thermal properties of Acacia leucophloea fiber/epoxy composites: Influence of fiber loading and alkali treatment

In this work, the influence of fiber content and alkali treatment on the mechanical and thermal properties of Acacia leucophloea fiber-reinforced epoxy composites was studied. Ten composite samples were fabricated by varying fiber content (5, 10, 15, 20, and 25 wt%); both untreated and treated fiber were soaked in a 5% NaOH solution for 45 min by using hand-layup method. The composite reinforced with 20 wt% treated fiber content exhibited better mechanical properties and thermal properties. Fourier transform infrared analysis, morphological analysis by atomic force microscope, and scanning electron microscope of composites were also performed.     

Carbon nanofibers reinforced soy polyol-based polyurethane nanocomposites

Vapor-grown carbon nanofiber (CNF)-modified soy polyol-based polyurethane (PU) nanocomposites with different hydroxyl value of polyols (OH) were synthesized. The glass transition, thermal stability, mechanical properties, and morphology of the PU nanocomposites were characterized through differential scanning calorimetry, thermogravimetry, universal test machine, and scanning electron microscopy. The addition of CNFs increased the glass transition temperature as well as significantly improved tensile strength and Young’s modulus of PU nanocomposites. Meanwhile, thermal and mechanical properties of PU composites were influenced by the different hydroxyl value of polyols due to those different structures. In particular, in the case of 2 mass% CNF addition in PU derived from soy polyol with the OH number of 164 mg KOH g^?1, 20.8 °C improvement in the glass transition temperature, 115 % increment in tensile strength, and nearly eightfold increase in Young’s modulus were obtained.     

The improvement of thermal stability and adhesion of silicone rubber composites modified by phenolic epoxy resin

The phenolic epoxy resin (F51) was siliconized by KH550 and the product was named as FKS. A hydroxyl-terminated polydimethylsiloxane (HTPDMS) which was modified with FKS was prepared. The siliconization reaction ensured a segment of siloxane on the side chain of F51. FT-IR and ¹H-NMR were employed to confirm the chemical structure of FKS. Morphology observations revealed that the enhancement of mechanical properties of the silicone rubber systems can be attributed to good compatibility between FKS and silicone rubber matrix. Thermogravimetric analysis showed that the residual yield at 800?°C of silicone rubber composites increased significantly when compared with that of neat HTPDMS. The mechanical properties demonstrated that tensile strength and elongation at break of silicone rubber system increased distinctly after modification, especially when 30 phr siliconized F51 were added to the silicone rubber. Shear strength was improved gradually with the addition of FKS. These above observations emphasize the vital effect of FKS on the behavior of modified HTPDMS.     

Dynamic electrical thermal analysis on zinc oxide/epoxy resin nanodielectrics

The dielectric response of ZnO/epoxy resin nanocomposites was studied by means of dynamic electrical thermal analysis in the frequency range of 10^?1 to 10⁷ Hz, and over the temperature range of 30–160 °C, varying the content of the reinforcing phase. Scanning electron microscopy pictures were used for assessing the composites morphology and for examining the particles’ dispersion. The thermal properties of nanocomposites were examined by differential scanning calorimetry in the temperature range of 0–170 °C. Dielectric data were analyzed via dielectric permittivity and electric modulus formalisms. Recorded relaxation phenomena include contributions from both the polymeric matrix and the presence of the reinforcing phase. Processes related to the polymer matrix are attributed to the glass to rubber transition (α-relaxation) of the epoxy resin and local motions of polar side groups of the main polymer chain (β-relaxation). Finally, the slower process appearing at low frequencies and high temperatures, originates from interfacial phenomena due to the accumulation of unbounded charges at the system’s interface.     

Surface modification of calcium sulfate whisker using thiol-ene click reaction and its application in reinforced silicone rubber

The hemihydrate calcium sulfate whisker (HCSW) was modified by γ-(methacryloxy)propyl trimethoxy silane (KH570) and trimethylolpropane tris(3-mercaptopropionate) via wet modification and thiol-ene click reaction, and then the unmodified and modified HCSW were added into α, ω-dihydroxy polysiloxane (DPS) matrix to prepare silicon rubber composites. After the dual-surface modification, the surface of HCSW was transformed to hydrophobic, the hydration of whisker was obviously improved, and the whisker dispersed more evenly in the polymer. The mechanical properties, dynamic mechanical properties, and the medium resistance of the silicone rubber composite were compared. The tensile test shows that the silicone rubber shows better mechanical properties after adding the modified whiskers, among which HCSW-KH570-SH has the most significant reinforcement effect. Moreover, DPS/HCSW-KH570-SH shows the best medium resistance in toluene, gasoline, and water. The addition of modified whiskers can improve the storage modulus of silicone rubber significantly, while DMA and DSC show that the addition of modified whiskers can reduce the glass transition temperature of silicone rubber. The bound rubber indicates that the interface interaction between HCSW-KH570-SH and silicone rubber is the best.     

Thermal behaviour and mechanical properties of novel RTV silicone rubbers using divinyl-hexa[(trimethoxysilyl)ethyl]-POSS as cross-linker

Divinyl-hexa[(trimethoxysilyl)ethyl]-POSS (DVPS) as an octavinyl-POSS derivative was first prepared. A series of novel polydimethylsiloxane (PDMS)/DVPS hybrid materials as room temperature vulcanized (RTV) silicone rubber were prepared. The chemical incorporation of novel POSS into hydroxyl-terminated PDMS system by hydrolytic condensation reaction was verified by attenuated total reflection (ATR) infrared spectroscopy. Thermal degradation, thermo-oxidative stability and mechanical properties of these novel RTV silicone rubbers were studied by means of thermogravimetric analysis and tensile testing. The results exhibited significantly enhanced effects on the thermal stabilities and mechanical properties as compared to the PDMS polymer prepared with tetraethoxysilane (TEOS). The observed improvements in thermal properties could be attributed to the effective three-dimensional network structures resulting from the structure of DVPS. The thermal decomposition of the RTV silicone rubbers in nitrogen was also monitored by TGA coupled with real-time FTIR, and the degradation residues were also characterized by FTIR. It was found that the POSS cross-linker facilitated the formation of cross-links in the degradation residues. The striking improvement in mechanical properties could be attributed to the synergistic action of the structure of three-dimensional multi-arm cross-linker (vinyl-POSS derivative), the plasticization of self-cross-linking Vinyl-POSS derivative and perfect distribution of vinyl-POSS derivative.     

Study on mechanical properties of elastomers reinforced by zinc dimethacrylate

In this work, mechanical properties of various elastomers with the representative structural features reinforced by zinc dimethacrylate (ZDMA) were studied. The results showed that there is great difference in reinforcing effects of ZDMA for different elastomers. Strain-stress curves revealed that the tensile-induced crystallization of chains might be the main reason for high strength of ZDMA/rubber composites at room temperature. The saturation and regularity of rubber chains are two essential features to determine the mechanical properties of the composites at room temperature. The kind of ZDMA makes considerable impact on performances of ZDMA/rubber composites, except for ZDMA/hydrogenated nitrile-butadiene rubber (HNBR) composite. The high temperature strengths of ZDMA/rubber composites are not high and are dominated by the polarity and saturation of matrix. Basing on the previous researching, a possible model for microstructure of ZDMA/rubber composites was put forward, which could well interpret the observed phenomenon.     

An investigation of dynamic mechanical, thermal, and electrical properties of housing materials for outdoor polymeric insulators

The present paper reports the results about a study of mechanical, thermal, dynamic mechanical and electrical properties of housing (weather shed) materials for outdoor polymeric insulators. Silicone rubber, ethylene-propylene-diene monomer (EPDM) and alloys of silicon-EPDM are known polymers for use as housing in high voltage insulators. The result of dynamical mechanical measurement shows that the storage modulus of blends enhances with increase EPDM in formulation. It can be seen from the result of TGA measurement that initial thermal degradation of silicone rubber improves by the effect of EPDM in blends. The blends of silicone-EPDM show good breakdown voltage strength compared to silicone rubber. Surface and volume resistance of silicone rubber improve by EPDM content. The mechanical properties of EPDM such as strength, modulus and elongation at break improve by silicone.     

微波处理和溶胶凝胶法共同改性废胶粉及其与天然橡胶复合材料性能的研究

采用微波处理打断废胶粉(WRP)的三维网状结构用来提高WRP在有机溶剂中的溶胀性,然后采用溶胶凝胶法,将微波改性后的WRP浸入正硅酸乙酯中,通过水解反应和缩合反应,在WRP表面原位生成SiO2网络,从而制得改性废胶粉(MWRP).将制得MWRP与天然橡胶(NR)共混,制备了NR/MWRP复合材料,研究了NR/MWRP复合材料的性能.通过热重分析仪、差示扫描量热仪和力学分析表明微波处理最佳时间是20 s.由于微波处理提高了NR与WRP的相容性,原位生成的SiO2粒子起到了补强作用,所以所制备的NR/MWRP复合材料拥有较好的力学性能;随着Si69的加入,抑制了SiO2粒子聚集,提高了SiO2粒子的分散性,从而进一步提高复合材料的力学性能并降低复合材料的Payne效应;在进行频率扫描时,硫化胶的储存模量随频率的增大而增大;硫化胶的温度扫描结果表明,随着温度的升高,复合材料中SiO2粒子聚集程度加剧并且复合材料出现老化的现象.为了提高复合材料的耐老化性能,N,N-间苯撑双马来酰亚胺(BMI)作为一种防老剂加入复合材料中,BMI利用Diels-Aider反应补偿橡胶在老化过程中所损失的交联键并提高NR与WRP的界面相容性,从而提高复合材料的耐老化性能.