Preparation and properties of hollow glass microsphere-filled epoxy-matrix composites

Hollow glass microsphere (HGM)–filled epoxy composites, with filler content ranging from 0 to 51.3 vol.%, were prepared in order to modify the dielectric properties of the epoxy. The results showed that the dielectric constant (D_k) and dielectric loss (D_f) of the composites decreased simultaneously with increasing HGM content, which was critical for the provision of superior high-frequency device performance. Other properties of the composite, such as the coefficient of thermal expansion (CTE) and the glass transition temperature (T_g), were also improved. The improvement in these properties was related to strong interaction between the HGM and epoxy, which was indicated by the formation of an interphase between the HGM and epoxy-matrix. It was unsatisfactory in this study that the thermal conductivity of the composites also decreased with HGM content. In order to obtain relatively high thermal conductivity and a low dielectric constant simultaneously, this paper suggests further adding other filler.     

Tailoring of thermal and dielectric properties of LDPE-matrix composites by the volume fraction,density, and surface modification of hollow glass microsphere filler

Hollow glass microsphere (HGM) filled low-density polyethylene (LDPE) composites were prepared, and the effects of density, content, and surface modification of HGM on the thermal and dielectric properties of the composites were investigated. It is found that the thermal conductivity of the composites decreases with increasing HGM content or decreasing HGM density. At the same HGM content and density, the composites filled with suitable amount of silane coupling agent (KH570) modified HGM exhibit higher thermal conductivity than unmodified-HGM filled composites. The dielectric constant at 1 MHz of the composites also decreases with increasing HGM content or decreasing HGM density, but their dielectric loss increases with increasing HGM content or increasing HGM density. By modifying the surface of HGM with suitable amount of KH570, the dielectric constant and loss at 1 MHz of the composites can be decreased at the same time. The results of microwave dielectric properties of the composites indicate that the dielectric constant decreases with increasing HGM content or decreasing HGM density, the quality factor (Q × f) decreases with increasing HGM content or increasing HGM density, but both dielectric constant and quality factor are slightly affected by the surface modification of HGM. Due to lower intrinsic thermal conductivity and dielectric constant but higher dielectric loss of HGM than LDPE, the thermal conductivity and dielectric properties of the composites can be controlled with adding HGM and varying its volume fraction. The surface modification of HGM improves the interface contact between HGM and LDPE in the composites, which is confirmed by the SEM observation, and thus the heat conduction and dielectric properties at low frequency are improved. Based on calculated thermal conductivity and dielectric constant of HGM, the experimental trends of thermal conductivity and dielectric constant at 1 MHz of the composites are analyzed by using different models, including typical models for particles-filled composites and special models developed for hollow microsphere filled composites. The results from suitable models show close correlation with the experimental values.     

Investigation of thermal conductivity and dielectric properties of LDPE-matrix composites filled with hybrid filler of hollow glass microspheres and nitride particles

The hybrid filler of hollow glass microspheres (HGM) and nitride particles was filled into low-density polyethylene (LDPE) matrix via powder mixing and then hot pressing technology to obtain the composites with higher thermal conductivity as well as lower dielectric constant (D_k) and loss (D_f). The effects of surface modification of nitride particles and HGMs as well as volume ratio between them on the thermal conductivity and dielectric properties at 1 MHz of the composites were first investigated. The results indicate that the surface modification of the filler has a beneficial effect on thermal conductivity and dielectric properties of the composites due to the good interfacial adhesion between the filler and matrix. An optimal volume ratio of nitride particles to HGMs of 1:1 is determined on the basis of overall performance of the composites. The thermal conductivity as well as dielectric properties at 1 MHz and microwave frequency of the composites made from surface-modified fillers with the optimal nitride to HGM volume ratio were investigated as a function of the total volume fraction of hybrid filler. It is found that the thermal conductivity increases with filler volume fraction, and it is mainly related to the type of nitride particle other than HGM. The D_k values at 1 MHz and microwave frequency show an increasing trend with filler volume fraction and depend largely on the types of both nitride particles and HGMs. The D_f values at 1 MHz or quality factor (Q × f) at microwave frequency show an increasing or decreasing trend with filler volume fraction and also depend on the types of both nitride particle and HGM. Finally, optimal type of HGM and nitride particles as well as corresponding thermal conductivity and dielectric properties is obtained. SEM observations show that the hybrid filler particles are agglomerated around the LDPE matrix particles, and within the agglomerates the smaller-sized nitride particles in the hybrid filler can easily form thermally conductive networks to make the composites with high thermal conductivity. At the same time, the increase of the value D_k of the composites is restricted due to the presence of HGMs.     

Sugarcane bagasse cellulose/HDPE composites obtained by extrusion

Natural fibers used in this study were both pre-treated and modified residues from sugarcane bagasse. Polymer of high density polyethylene (HDPE) was employed as matrix in to composites, which were produced by mixing high density polyethylene with cellulose (10%) and Cell/ZrO₂·nH₂O (10%), using an extruder and hydraulic press. Tensile tests showed that the Cell/ZrO₂·nH₂O (10%)/HDPE composites present better tensile strength than cellulose (10%)/HDPE composites. Cellulose agglomerations were responsible for poor adhesion between fiber and matrix in cellulose (10%)/HDPE composites. HDPE/natural fibers composites showed also lower tensile strength in comparison to the polymer. The increase in Young’s modulus is associated to fibers reinforcement. SEM analysis showed that the cellulose fibers insertion in the matrix caused an increase of defects, which were reduced when modified cellulose fibers were used.     

Preparation and microwave absorbing performance of TiO<Subscript>2</Subscript>/ NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> /hollow glass microsphere composite with core–shell structure

Microwave absorbing TiO₂/NiFe₂O₄/HGM composite with core–shell structure was prepared via a facile two-step method. The obtained composite was then investigated by SEM, TEM, XRD, XPS, VSM and a vector network analyzer. The results indicated that HGM was completely coated by NiFe₂O₄ nanospheres after the hydrothermal reaction, and TiO₂/NiFe₂O₄/ HGM composite with core–shell structure was also successfully synthesized. The composite exhibited excellent magnetic performance and microwave absorption capacity. Measurement of VSM suggested that the saturated magnetization values (Ms) of TiO₂/NiFe₂O₄/HGM was 27.79 emu.g^??1. When the frequency was 10.3 GHz, the R _L value of TiO₂/NiFe₂O₄/HGM composite with the thickness of 2.6 mm could reach up to ?20 dB. The obtained composite exhibited excellent microwave absorbing properties, which could be used as a promising EM wave absorber.     

On a hybrid method to characterize the mechanical behavior of thin hollow glass microspheres

Hollow glass microspheres possess niche markets and future research areas due to their high strength-to-density ratio and low thermal conductivity. For example, these qualities make them a viable alternative insulation medium. For this study, Scotchlite^™ k1 hollow glass microspheres (HGMs) manufactured by 3M^™ were used; however, one of the challenges in investigating their mechanical properties was due to the limited availability of published data. Most importantly, the thickness (or the range of thicknesses) of produced thin hollow microspheres is not measured by the manufacturer, so the authors needed to develop a hybrid characterization method that depended on both experiments and simulations to estimate the HGMs mechanical properties. The experimental method utilizes a nano-indenter with a spherical sapphire tip of 500 μm in diameter. Each HGM was uniaxialy compressed under the indenter to yield the stiffness, diameter, and the force-displacement at fracture of each HGM tested. ABAQUS^™ was used to numerically model an HGM of mean diameter and stiffness to obtain a relation between the mechanical and geometrical properties of each HGM to its thickness, and investigate the development of stresses on an HGM during its uniaxial compression. The data show large scatter in the measured stiffness of each HGM as a function of diameter. The authors conjecture this is due to a large fluctuation of the microsphere thickness. Finally, the work necessary to deform an HGM was successfully correlated to its radius-to-thickness ratio. This gives us a unique insight on the force-displacement behavior against the geometrical features of HGMs.     

Modification of montmorillonite by novel geminal benzimidazolium surfactant and its use for the preparation of polymer organoclay nanocomposites

A new benzimidazolium derivative, the benzimidazolium-N,N′-hexadecane-2-hydroxy-ethyl bromide (Bz) featuring two geminal hexadecyl hydrophobic buttress has been synthesized and used for the functionalization of sodium montmorillonite (MMT-Na) via cationic exchange process. The resulting benzimidazolium-modified MMT (MMT-Bz) exhibits a large d-spacing of 3 nm between silicate layers and shows a high thermal stability compared to the commonly used clay modified alkyl ammonium salts (cloisite 20A and cloisite 20B). MMT-Bz was incorporated in high density polyethylene (HDPE) matrix via melt mixing method to produce HDPE/MMT-Bz nanocomposites. The microstructure and the morphology of these nanocomposites were studied by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The dispersion state of the organoclay within HDPE was monitored by UV–Vis spectroscopy and melt rheology. A more homogeneous dispersion or a greater content of the MMT-Bz in the matrix produced stronger solid-like and non-terminal behavior in the nanocomposites. Tensile properties and thermal stability were evaluated and discussed on the basis of the amount of clay incorporated within the nanocomposites. The intercalated structure in the nanocomposites, resulting from both the better dispersion/distribution of clay nano-platelets and their strong interaction with the polymer chains, provides the driving force to significantly enhance the HDPE properties.     

空心玻璃微珠/硬质聚氨酯泡沫复合材料的制备及性能

以空心玻璃微珠(HGM)为添加剂,采用一步法全水发泡制备了一系列HGM/硬质聚氨酯泡沫(RPUF)复合材料。通过SEM、TG、极限氧指数(LOI)和水平燃烧,研究了HGM/RPUF复合材料的泡孔结构、炭层形貌、热稳定性及阻燃性能。采用万能材料试验机测试了HGM/RPUF复合材料的压缩强度和压缩弹性模量。采用热重-傅里叶红外光谱(TG-FTIR)研究了HGM/RPUF复合材料燃烧过程中的气相产物。研究表明,HGM有成核剂作用,可以缩小HGM/RPUF复合材料泡孔孔径。HGM在燃烧过程中迁移到炭层表面,促进形成致密厚实的炭层。当加入5.4wt% HGM时,HGM/RPUF复合材料的压缩强度及压缩弹性模量分别提高至0.14 MPa和4.53 MPa,相对RPUF,分别提高了37.30%和67.16%。同时发现,HGM能明显抑制HGM/RPUF复合材料在燃烧过程中CO的释放,有效提高了其火灾安全性。      

Thermal conductivity of boron nitride reinforced polyethylene composites

The thermal conductivity of boron nitride (BN) particulates reinforced high density polyethylene (HDPE) composites was investigated under a special dispersion state of BN particles in HDPE, i.e., BN particles surrounding HDPE particles. The effects of BN content, particle size of HDPE and temperature on the thermal conductivity of the composites were discussed. The results indicate that the special dispersion of BN in matrix provides the composites with high thermal conductivity; moreover, the thermal conductivity of composites is higher for the larger size HDPE than for the smaller size one. The thermal conductivity increases with increasing filler content, and significantly deviates the predictions from the theoretic models. It is found also that the combined use of BN particles and alumina short fiber obtains higher thermal conductivity of composites compared to the BN particles used alone.     

Preparation of core-shell structured hollow glass microspheres/BaFe<Subscript>12</Subscript>O<Subscript>19</Subscript>/Ag composites with excellent microwave absorbing properties

Hollow glass microspheres/barium ferrite (HGM/BaFe₁₂O₁₉) was first prepared via co-precipitation reaction, which was then performed to fabricate the HGM/BaFe₁₂O₁₉/Ag composites by chemical plating method. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and vibrating sample magnetometry (VSM) were utilized to characterize the structures, morphologies and properties of the resultant composites. Results showed that a homogeneous and complete BaFe₁₂O₁₉ shell was coated on the surface of the HGM, and HGM/BaFe₁₂O₁₉ composites were also fully covered with Ag particles. The conductivity of the HGM/BaFe₁₂O₁₉/Ag composites was 1.24?×?10² S/cm, whereas the saturation magnetizations of the composites was reduced to 12.76 emu/g. The microwave absorption properties of the HGMs/BaFe₁₂O₁₉/Ag composites were significantly improved compared with those of HGMs/BaFe₁₂O₁₉ composites and BaFe₁₂O₁₉ particles. The reflection loss (R) showed that the bandwidth of reflection loss of HGM/BaFe₁₂O₁₉/Ag less than ?10 dB (90% absorption) was 2.1 GHz (from 10.3 to 12.4 GHz), herein, the minimum loss value was ?19.7 dB at 12.4 GHz.     

Crushing and energy absorption performance of different geometrical shapes of small-scale glass/polyester composite tubes under quasi-static loading conditions

This paper presents the quasi-static crushing performance of nine different geometrical shapes of small-scale composite tubes. The idea is to understand the effect of geometry, dimension and triggering mechanism on the progressive deformation of small-scale composite tubes. Different geometrical shapes of the composite tubes have been manufactured by hand lay-up technique using uni-directional E-glass fabric (with single and double plies) and polyester resin. Dedicated quasi-static tests (144 tests) have been conducted for all nine geometrical shapes with different t/D (thickness–diameter) ratios and two triggering profiles (45° chamfering and tulip pattern with an included angle of 90°). From this unique study, it was found that the crushing characteristics and the corresponding energy absorption of the special geometrical shapes are better than the standard geometrical shapes such as square and hexagonal cross sections. Furthermore, the tulip triggering attributed to a lower peak crush load followed by a steady mean crush load compared to the 45° chamfering triggering profile which resulted into a higher energy absorption in most of the geometrical shapes of the composite tubes.     

Percolation based enhancement in effective thermal conductivity of HDPE/LBSMO composites

Thermal conductivity of composites with electrically conducting La_0·7Ba_0·15Sr_0·15MnO₃ (LBSMO) filler of nanometric grain size in HDPE matrix is investigated. Volume fraction of LBSMO fillers was varied between 0 and 0·30. SEM photographs of the composites show the presence of clusters and percolative paths, particularly for composites prepared with higher filler volume fractions. The effective thermal conductivity of the composites displays significant enhancement with increasing filler content in HDPE. A maximum enhancement of ~65% compared to that for pure HDPE has been observed for composite with 0·30 volume fraction of LBSMO filler. Most of the models those are generally used to predict the properties of two phase mixtures, has been found either to under/overestimate the measured effective thermal conductivity of the composites. We confirm that the observed rapid increase in the effective thermal conductivity of HDPE/LBSMO composite over the studied range of filler volume fraction (viz. 0?C0·30), is predicted very well, considering the effect of percolation as proposed by Zhang et al (2009).     

水热法制备高反射率TiO2壳层中空玻璃微球

以钛酸四丁酯、盐酸、去离子水、中空玻璃微球(HGM)为原料,乙醇为溶剂,采用水热合成法制备了高反射率的锐钛矿型TiO₂壳层HGM(HGM@TiO₂)。采用SEM、EDS、FTIR、XRD、UV-VIS-NIR、导热系数仪研究了钛酸四丁酯用量对微球的表面形貌、表面化学成分、物相结构、反射性能、导热系数的影响。结果表明:锐钛矿型TiO₂成功包覆于HGM表面,包覆形貌完整且均匀,并且包覆层厚度随着钛源用量的增加而变厚;与原始HGM相比,HGM@TiO₂的导热系数有小幅上升,最大上升幅度仅为0.007 W/(m·K),证明TiO₂的包覆对HGM的隔热性能的影响不大;包覆后的HGM的光谱反射率在可见光波段和近红外波段的反射率得到大幅提升,最大提升幅度为13%,HGM@TiO₂的最高反射率达到90%以上。      

Effect of delamination failure in crashworthiness analysis of hybrid composite box structures

In the present paper the effects of delamination failure of hybrid composite box structures on their crashworthy behaviour will be studied and also their performance will be compared with non-hybrid ones. The combination of twill-weave and unidirectional CFRP composite materials are used to laminate the composite boxes. Delamination study in Mode-I and Mode-II with the same lay-ups was carried out to investigate the effect of delamination crack growth on energy absorption of hybrid composite box structures. The end-loaded split (ELS) and double-cantilever beam (DCB) standard test methods were chosen for delamination studies. In all hybrid composite boxes the lamina bending crushing mode was observed. Regarding the delamination study of hybrid DCB and ELS the variation of the specific energy absorption (SEA) versus summation of G_IC and G_IIC were plotted to combine the effect of Mode-I and Mode-II interlaminar fracture toughness on the SEA. From this relationship it was found the hybrid laminate designs which showed higher fracture toughness in Mode-I and Mode-II delamination tests, will absorb more energy as a hybrid composite box in crushing process. The crushing process of hybrid composite boxes was also simulated by finite element software LS-DYNA and the results were verified with the relevant experimental result.     

Effect of natural stitched composites on the crashworthiness of box structures

In the present paper the effects of stitching on the energy absorption and crashworthy behaviour of composite box structures will be studied. The combination of unidirectional carbon fibre-reinforced polymer (CFRP) and glass fibre-reinforced polymer (GFRP) composite materials are used to laminate the composite boxes. Delamination study in Mode-I with the same lay-up was carried out to investigate the effect of stitching on delamination crack growth on energy absorption of stitched and non-stitched composite box structures. The double cantilever beam (DCB) standard test method was chosen for delamination studies. For non-stitched and stitched composite boxes the lamina bending and brittle fracture crushing modes were observed. It was found that the stitched composite boxes which show higher fracture toughness in Mode-I delamination tests, are not necessarily able to absorb more crushing energy in comparison with non-stitched composite boxes. It was also observed that the position of stitched area can affect the crushing mode and consequently energy absorption capability of composite box structures. The main reason can be related to other mechanisms such as bending, friction and bundle fracture which significantly contribute to energy absorption. The analytical model based on energy balance approach is proposed to estimate the mean crushing force, F_m, in axial crushing of square composite box.     

Reinforcing effects of Kevlar fiber on the mechanical properties of wood-flour/high-density-polyethylene composites

Kevlar fibers (KFs) were used as a reinforcement for wood-flour/high-density-polyethylene composites (WF/HDPE) to improve the mechanical properties of the resulting composites. Addition of a small amount (2–3%) of KF caused an improvement in the tensile, flexural, and impact properties of WF/HDPE. Surface grafting of KF with a mixture of allyl chloride and 3-chloropropyltrimethoxysilane resulted in a further improvement on the mechanical properties of the resulting composites due to the increased interfacial compatibility between KF and HDPE as evidenced by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR). It can accordingly be concluded that the grafted KF can be used as a reinforcement to improve the strength and toughness of WF/HDPE composites.     

Quasi-static penetration resistance behavior of glass fiber reinforced thermoplastic composites

Quasi-static penetration resistance of a composite structure represents the energy dissipating capacity of the structure under transverse loading without dynamic and rate effects. In this paper, a comparative study of the quasi-static penetration resistance behavior of S-2 Glass/SC-15, S-2 Glass/HDPE and E-Glass/HDPE composite systems with varying thicknesses, i.e., 1.4–8.4-mm, is presented using the Quasi-Static Punch Shear Test (QS-PST) methodology developed earlier. The penetration resistance behavior is usually presented by a series of force–displacement graphs at different support conditions, the integral of which is the energy dissipated by the composite during the quasi-static penetration at corresponding support conditions. The penetration energy varies with the diameter of the support span which is usually higher than the punch diameter, and also with the thickness of the composite laminate. During QS-PST experiments, a flat punch of diameter 7.6-mm with a range of support spans 8.89–50.8-mm has been used to obtain varying support span to punch diameter ratios (i.e., SPR = D_S/D_P = 1.16, 1.33, 1.67, 2.00, 2.33, 2.67, etc.). In order to compare the penetration resistance behavior of three different material systems, the S-2 Glass/SC-15, S-2 Glass/HDPE and E-Glass/HDPE composites of identical layer counts are used and the S-2 Glass/SC15 composite system is considered as the baseline. Composite plate specimens are sectioned after the test and then dipped into an ink–alcohol solution to study the damage mechanisms at different SPRs. Non-linear penetration stiffness and an average penetration resistance force are defined to quantify the average penetration resistance of each material. S-2 Glass and E-Glass reinforced HDPE composite material showed lower stiffness, lower peak force, higher deflection, lower damage area, and lower energy dissipation as compared to the baseline. A detailed comparison of results is presented.     

Influence of carbon nanotube-polymeric compatibilizer masterbatches on morphological, thermal, mechanical, and tribological properties of polyethylene

Study was made of the effect of multiwall carbon nanotubes (MWCNTs) and polymeric compatibilizer on thermal, mechanical, and tribological properties of high density polyethylene (HDPE). The composites were prepared by melt mixing in two steps. Carbon nanotubes (CNTs) were melt mixed with maleic anhydride grafted polyethylene (PEgMA) as polymeric compatibilizer to produce a PEgMA-CNT masterbatch containing 20 wt% of CNTs. The masterbatch was then added to HDPE to prepare HDPE nanocomposites with CNT content of 2 or 6 wt%. The unmodified and modified (hydroxyl or amine groups) CNTs had similar effects on the properties of HDPE-PEgMA indicating that only non-covalent interactions were achieved between CNTs and matrix. According to SEM studies, single nanotubes and CNT agglomerates (size up to 1 μm) were present in all nanocomposites regardless of content or modification of CNTs. Addition of CNTs to HDPE-PEgMA increased decomposition temperature, but only slight changes were observed in crystallization temperature, crystallinity, melting temperature, and coefficient of linear thermal expansion (CLTE). Young’s modulus and tensile strength of matrix clearly increased, while elongation at break decreased. Measured values of Young’s moduli of HDPE-PEgMA-CNT composites were between the values of Young’s moduli for longitudinal (E₁₁) and transverse (E₂₂) direction predicted by Mori-Tanaka and Halpin-Tsai composite theories. Addition of CNTs to HDPE-PEgMA did not change the tribological properties of the matrix. Because of its higher crystallinity, PEgMA possessed significantly different properties from HDPE matrix: better mechanical properties, lower friction and wear, and lower CLTE in normal direction. Interestingly, the mechanical and tribological properties and CLTEs of HDPE-PEgMA-CNT composites lie between those of PEgMA and HDPE.     

The optimization of welding parameters for friction stir spot welding of high density polyethylene sheets

Friction stir spot welding parameters affect the weld strength of thermoplastics, such as high density polyethylene (HDPE) sheets. The strength of a friction stir spot weld is usually determined by a lap-shear test. For maximizing the weld strength, the selection of welding parameters is very important. This paper presents an application of Taguchi method to friction stir spot welding strength of HDPE sheets. An orthogonal array, the signal to noise ratio (S/N), and the analysis of variance (ANOVA) are employed to investigate friction stir welding parameter effects on the weld strength. From the ANOVA and the S/N ratio response graphs, the significant parameters and the optimal combination level of welding parameters were obtained. Experimental results confirmed the effectiveness of the method.     

Preparation and microwave absorbing properties of hollow glass microspheres/Fe3O4/Ag composites with core–shell structure

The low-density, conductive and magnetic hollow glass microspheres (HGM)/Fe₃O₄/Ag composites have been successfully synthesized via co-precipitation and chemical plating method. The morphology, composition, microstructure, magnetic and microwave absorbing properties of the composites were investigated based on the analyses of the results using scanning electron microscope, energy dispersive spectroscopy, X-ray diffraction, vibrating sample magnetometer and vector network analyzer. The results showed that the HGM/Fe₃O₄ composites were successfully prepared, and the coating layers on the surface of HGM are compact and continuous. Moreover, the final composites were completely covered with Ag nanoparticles. With the addition of Ag nanoparticles, the saturation magnetization of the HGM/Fe₃O₄ composites reduces from 32.08 to 14.77 emu/g, whereas its conductivity increases to 0.48 S/cm. The reflection loss (R) of HGM/Fe₃O₄/Ag composites is lower than ?10 dB at 8.2–8.7, 9.6–10.8 and 11.4–11.9 GHz, and the minimum loss value is ?19.1 dB at 9.9 GHz.