Effect of fiber orientation angle on thermal degradation and ablative properties of short‐fiber reinforced EPDM/NBR rubber composites

A series of short aramid fibers reinforced ethylene‐ propylene‐diene terpolymer (EPDM)/acrylonitrile‐butadiene rubber (NBR) composites with designed fiber orientation angle (β) between the short fibers orientation and the ablative surface have been prepared in this work. The influence of β on the thermal degradation and ablation resistance properties of the composites under the oxyacetylene flame is carefully discussed. With the increasing of β, the porosity of the char layer is reduced, whereas its hardness is improved. As a result, the pyrolysis rate (R_p) and the carbonization rate (R_c) are accelerated since the thermal conductivity of the char layer is enlarged after the composites were carbonized. The acceleration of the thermal degradation results in an increasing mass loss during the pyrolysis reaction. However, the erosion rate (R_L), mainly caused by the mechanical ablation of the high speed heat flux, is reduced gradually as β increases due to the improved hardness of the char layer. Dominated by the two kinds of ablation methods oppositely as β increases, the mass loss rates (R_m) decreases firstly and then increases gradually. POLYM. COMPOS., 31:1223–1231, 2010. © 2009 Society of Plastics Engineers     

An excellent ablative composite based on PBO reinforced EPDM

Short poly(p-phenylene-2,6-benzobisoxazole) (PBO) fibers were first used to reinforce ethylene–propylene–diene terpolymer (EPDM) as thermal insulation materials. The effects of PBO fiber length and content on the mechanical and ablative properties of the composites were investigated in detail. Comparing with the severe breakage occurred in short aramid fibers as fillers, only some necking deformation is observed in PBO fibers filled EPDM after processed. After ablated by oxyacetylene flame, the carbonized PBO fibers still keep solid fibrous structure instead of hollow one of carbonized aramid fibers in the char layer. As a result, the PBO fibers/composites show significantly higher tensile strength and ablation resistant abilities than the aramid fibers/composites. Moreover, with the length and content of PBO fibers increasing, both the tensile strength and the ablation resistance of the composites increase gradually though the break elongation reduces sharply. Considering the properties requirement of thermal insulator, PBO fibers with 3.42–5.56 wt% in content and 3–4 mm in length are preferred. The mass loss rate and the erosion rate as low as 0.05 g s⁻¹ and 0.10 mm s⁻¹ are observed in the optimal samples, respectively, which is evidently lower than that of the best aramid fibers/EPDM-based insulations reported so far.     

Ablation behavior of inorganic particle-filled polybenzoxazine composite coating irradiated by high-intensity continuous laser

Further utilization of aircraft structural materials is threatened by the fact that high-intensity continuous lasers are widely used in the field of military defense. To protect the aircraft structure from laser damage, ammonium polyphosphate filled polybenzoxazine composite coatings were prepared on the substrate. The anti-laser ablation characteristics of the coatings were investigated. Results showed that the addition of the inorganic filler improved the anti-laser ablation performance of polybenzoxazine. The back-surface temperature of substrates covered with the composite coatings was more 50% lower than that in the case of a pure polybenzoxazine coating after laser ablation. Further, the residue of the composite coating could be vertically divided into three distinct regions, with the dense surface char layer and the porous pyrolysis layer acting as shielding layers for the laser beam and preventing any heat-related transformations from occurring. The addition of the inorganic particles improved the surface reflectivity of the coatings resulting in much more laser energy dissipation. The decreased pyrolysis rate ensured that the pneumatic cooling effect of pyrolysis gas was more lasting and stable, owing to which the composite coatings could act as effective thermal protection layer for longer. These results confirmed that the inorganic filler modified polybenzoxazine coating exhibits excellent anti-laser properties and are suitable for protecting structural materials from laser-related damage.     

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.     

Toughening study of fire‐retardant high‐impact polystyrene

Fire‐retardant high‐impact polystyrene (HIPS) was modified by melt blending with varying amounts of three types of tougheners. The effects of the tougheners on the properties of the fire‐retardant HIPS were studied by mechanical, combustion tests, and thermogravimetric analysis. The morphologies of fracture surfaces and char layers were characterized through scanning electron microscopy. The results show that the impact properties of styrene–butadiene–styrene (SBS)‐containing composites were better than those of ethylene–propylene–diene monomer (EPDM)‐containing or ethylene–vinyl acetate copolymer (EVA)‐containing composites. The tensile strength and flexural modulus of the fire‐retardant HIPS decreased evidently with the addition of tougheners. It is found that the compatibility between SBS copolymer and HIPS matrix was best among the three types of tougheners. The addition of SBS had little influence on the thermal property, residue, flammability, and morphology of char layer of the fire‐retardant HIPS, but the addition of EPDM rubber or EVA brought adverse influence on the residue, flammability, and morphology of char layer of the fire‐retardant HIPS, especially for EPDM. Copyright © 2009 John Wiley & Sons, Ltd.     

辐照法制备无卤阻燃HDPE/EPDM/硅橡胶共混物

以氢氧化镁和红磷为阻燃剂,硅橡胶、三元乙丙橡胶(EPDM)为共混材料,高密度聚乙烯(HDPE)为基体制备了阻燃共混物,并对其进行电子束辐照,考察了阻燃共混物的力学性能、阻燃性能和微观形貌。结果表明:EPDM的加入明显改善了阻燃共混物的力学性能;硅橡胶的加入使得无机阻燃剂粒子在聚合物基体中的分散性提高,阻燃共混物的氧指(数OI)提高;辐照交联后,阻燃共混物的拉伸强度增强,OI明显提高,燃烧后的炭层更加致密。     

Thermal insulation behaviour of Ethylene propylene diene monomer rubber/kevlar fiber based hybrid composites containing Nanosilica for solid rocket motor insulation

The current research discusses the properties of an elastomeric heat-shielding material, based on nano-silica (NS) filled ethylene propylene diene monomer (EPDM) rubber/Kevlar fiber (KF) hybrid composites. The developed elastomeric insulating material consists of an aromatic polyamide fiber (KF) and silica nanoparticles. An in-depth analysis of mechanical properties, density, coefficient of thermal expansion, thermal conductivity, thermogravimetric analysis, and heat release rate of the insulating materials -was performed. TEM micrograph represents an excellent distribution of nanoparticles in the EPDM matrix. The improvement in the mechanical and the flame retardancy of the NS filled EPDM/KF hybrid composite insulations is based on the fiber/matrix adhesion. Maleic anhydride grafting confers polarity to the nonpolar rubber matrix. The char residues of the insulations inspected by scanning electron microscopy and energy dispersive spectroscopy are depicting a rigid and rough surface by the optimal composites, which can aid in better insulation. The optimal formulation of the hybrid composites exhibited a 220% enhancement in char residue with improved thermal stability and mechanical properties.     

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.     

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.     

Improvement in thermal conductivity and mechanical properties of ethylene‐propylene–diene monomer rubber by expanded graphite

Thermally conductive fillers are usually employed in the preparation of rubber composites to enhance thermal conductivity. In this work, ethylene‐propylene‐diene monomer rubber (EPDM)/expanded graphite (EG) and EPDM/graphite composites with up to 100 phr filler loading were prepared. Compared to EPDM/graphite compounds with the same filler loading, stronger filler network was demonstrated for EPDM/EG compounds. Thermal conductivity and mechanical properties of EPDM/graphite and EPDM/EG composites were compared and systematically investigated as a function of the filler loading. The thermal conductivity of both EPDM/graphite and EPDM/EG composites increased with increasing volume fraction of fillers, and could be well fitted by Geometric Mean Model. The thermal conductivity as high as 0.910 W · m⁻¹ · K⁻¹ was achieved for the EPDM/EG composite with 25.8 vol% EG, which was ∼4.5 times that of unfilled EPDM. Compared to EPDM/graphite composites, EPDM/EG composites exhibited much more significant improvement in thermal conductivity and mechanical properties, which could be well correlated with the better filler‐matrix interfacial compatibility and denser structure in EPDM/EG composites, as revealed in the SEM images of tensile fracture surfaces. POLYM. COMPOS., 38:870–876, 2017. © 2015 Society of Plastics Engineers     

Sepiolite hybridized commercial fillers,and their effects on curing process,mechanical properties,thermal stability,and flammability of ethylene propylene diene monomer rubber composites

Ethylene propylene diene monomer rubber (EPDM)-based composites containing sepiolite (sep) hybridized with calcium carbonate (CaCO₃), silica (Sil) or carbon black (CB) were prepared on a two-roll mill. The influence of fillers’ contents on the curing, mechanical, thermal and flammability of the composites was investigated. In comparison with EPDM/sep at 30 parts per hundred rubbers (phr) as a control composite, EPDM/sep/CB composites exhibited an outstanding improvement in tensile strength followed by EPDM/sep/Sil and EPDM/sep/CaCO₃ composites. EPDM/sep/CB displayed the highest thermal stability and also improved flammability resistance. In addition, a higher amount of carbon black gave higher tensile strength. The results were influenced by the ability of CB to disperse well and form protective layers acting as mass transport barriers in the matrix. The field emission scanning electron microscopy analyses proved better dispersion of CB in the matrix. The presence of protective layers on the surface of samples consequently improved the thermal properties of the EPDM composites. The mechanism of formation of char protective layer in hybrid EPDM composites was also investigated based on morphological observations of char residues. According to this work, Sil and CB were able to hybrid with sep, while sep could be a potential substitution of CaCO₃ in the EPDM composites.     

烧蚀材料高成碳树脂的研究进展

耐烧蚀材料在国防工业上有十分重要的应用价值,碳化型烧蚀材料是利用高分子材料在高温碳化吸热量的材料.树脂基烧蚀材料一般要求具有高相对分子质量、高芳基化、高交联密度、高C/O比,以使材料烧蚀后成碳率高.材料的烧蚀率与成碳率成反比关系,树脂的成碳率越高,其耐烧蚀性能越好.材料的成碳率高低由树脂的化学结构决定.目前烧蚀材料的研...     

Effect of borax on the thermal and mechanical properties of ethylene‐propylene‐diene terpolymer rubber‐based heat insulator

The effect of Borax on the mechanical and ablation properties of three different ethylene‐propylene‐diene terpolymer (EPDM) compounds containing 20 phr carbon fiber, 20 phr Kevlar or 10 phr/ 10 phr carbon fiber/ Kevlar was investigated. All formulations contained 30 phr fumed silica powder and 10 phr paraffinic oil. It was found that adding Borax to the composite samples containing carbon fiber or Kevlar fiber or their mixture with an equal ratio can increase the tensile strength, elastic modulus and hardness with a slightly decrease in the elongation at break of the rubber samples. The results of thermogravimetry analysis (TGA) on the various samples showed significant increase in the char yield at 670°C by adding Borax to the rubber compounds. Moreover, ablation resistance of samples was also improved by increasing Borax content. Meanwhile, density and thermal conductivity of the insulator were also reduced up to about 10% when the carbon fiber was replaced with the Borax. The results indicated that composites containing Kevlar have high storage modulus and produce compact and stable char. EPDM rubber composite containing Borax (20 phr), carbon fiber (10 phr), and Kevlar (10 phr) showed thermal and ablative properties comparable with those of the asbestos‐ filled EPDM. The thermal conductivity and ablation rate of the above‐ mentioned sample were 0.287 W/m/K and 0.13 mm/s respectively. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41936.     

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.     

Properties of Paper Sludge Filled Polypropylene (PP)/Ethylene Propylene Diene Terpolymer (EPDM) Composites: The Effect of Silane-Based Coupling Agent

Paper sludge was used as a filler in PP/EPDM composites and 3-aminopropyl triethoxysilane (3-APE) was used in this study as a coupling agent. The effects of filler loading and 3-APE on the mechanical properties, water absorption, morphology, and thermal properties of the composites were investigated. It was found that incorporation of a silane coupling agent (3-APE) increased the stabilization (equilibrium) torque, tensile strength, and Young's modulus but decreased the elongation at break and water absorption. Scanning electron microscopy (SEM) study of the tensile fracture surface of the composites indicated that the presence of 3-APE increased the interfacial interaction between paper sludge and PP/EPDM matrix. The addition of a silane coupling agent also increased the crystallinity of PP and thermal stability of PP/EPDM/PS composites.     

A study of γ‐radiation‐crosslinked HDPE/EPDM composites as flame retardants

The dynamic flammability of flame‐retardant composites that consist of high‐density polyethylene (HDPE) and ethylene–propylene–diene rubber (EPDM) and other additives, and can be used as wire‐ and cable‐insulation materials, was studied before and after irradiation. The data for the heat‐release rate (HRR), the time to ignition, the specific extinction area and the concentrations of CO and CO₂ from the burning process of cone colorimeter tests were assessed. By blending HDPE with EPDM, the HRR of HDPE was reduced and the residue char of the composite increased. The HRR of HDPE/EPDM was further reduced and the residue char of HDPE/EPDM was further increased after irradiation. The oxygen index, mechanical properties, and thermal stability of the composites, and the morphology of the char formed in the cone calorimeter test, were also investigated. Copyright © 2004 Society of Chemical Industry     

Ablation behavior of organoclay‐NBR insulator: Modeling and experimental

Nitrile‐based nanocomposite heat insulators are very attractive materials compared with their similar nonelastomeric counterparts, due to their higher deformation bearing capacity in special applications. Modeling of these ablative nanocomposites enables us to determine the exact required thickness of the insulator and temperature distribution across it at predetermined thermal conditions. The complete form of the ablation equation is a transient nonlinear second‐order differential equation with variable temperature‐dependent thermo‐physical properties, which must be determined during thermal degradation. In this work, in addition to experimental investigation of ablative elastomeric nanocomposites based on NBR, the ablation process is modeled analytically with perturbation theory in the Lagrangian coordinate system because of surface recession and moving boundaries. Kirchhoff transformation was used to get rid of the temperature dependence of k in each zonal of virgin and char. The theoretical results were confirmed by experimental data obtained from the oxyacetylene flame test. The results proposed a competitive nitrile‐based nanocomposite heat insulator with superior ablation properties: mass ablation rate 0.014 g s^?1, linear ablation rate 0.012 mm s^?1, and insulating index number 6800 s m^?1, under a standard test with a heat flux of 2500 kW m^?2 for 15 seconds.     

有机硅基耐烧蚀材料的研究

以室温硫化硅橡胶为基材,采用联二脲、芳纶纤维作为配合剂制成了固体火箭发动机用耐烧蚀材料,讨论了联二脲和芳纶对材料烧蚀率和机械性能的影响。结果表明,联二脲、芳纶纤维单独和混合使用都能够降低烧蚀材料的烧蚀率。当以7．5／0．5的比例混合使用时,能够获得最低的烧蚀率（线烧蚀率和质量烧蚀率分别为0．146mm/s和0．153g/s)。由于联二脲、芳纶纤维与室温硫化硅橡胶的相容性较差,材料的机械性能有所下降。     

纳米炭粉改性苯并噁嗪树脂烧蚀性能研究

为进一步提高苯并噁嗪树脂的烧蚀性能,采用纳米炭粉对其进行了改性研究。采用透射电镜(TEM)和场发射电子显微镜(SEI)观察了纳米炭粉在苯并噁嗪树脂的中的分散状态;通过热失重分析研究了纳米炭粉质量分数对苯并噁嗪树脂残炭率的影响并测试了烧蚀性能;同时采用X-射线衍射法(XRD)对炭层结构进行了分析。结果表明,质量分数为10%纳米炭粉的改性苯并噁嗪800℃残炭率可达到63.6%,该体系700℃炭化后的压缩强度为纯树脂的3.8倍。改性后的苯并噁嗪树脂炭化层结构致密,裂纹小,石墨化度与炭结构的有序度大大提升,最终使树脂的耐烧蚀性能与抗热震性获得改善。     

Evaluation of additively manufactured ultraperformance polymers to use as thermal protection systems for spacecraft

In this study, high-temperature thermoplastic polymers were three-dimensional (3D) printed and evaluated for ablation performances for the first time. The purpose of this study is to fabricate, test, and evaluate several ultraperformance thermoplastics, including polyetherimide (PEI), polyether ether ketone (PEEK), and poly(ether ketone ketone) (PEKK) for thermal, flammability, and ablation properties using fused filament fabrication. Among all tested materials, PEKK exhibit the highest char yield while the two PEEKs exhibit the highest decomposition temperature. Although PEI have the lowest onset decomposition temperature, microscale combustion calorimetry results indicate that it has the lowest heat release properties. For the 15 s oxyacetylene test bed (OTB) ablation test, all five materials experienced various amount of intumescent or swelling behavior. Samples after 30 s test experienced greater mass loss among which PEKK and ULTEM 9085 shows the highest char yield. Scanning electron microscopy microstructural analysis of the char and pyrolysis zones reveal highly porous char structures caused by the rapid generation of the volatile decomposition products. To fully exploit the experimental data provided by the OTB, the flowfield generated during aerothermal testing using this heat source was modeled. Computational fluid dynamics analysis using Ansys/Fluent 19.1 code was used to the heat transfer between the ablative surface and the combustion gases generated by the OTB and compared with experimental results.