Electrical properties of hollow glass particle filled vinyl ester matrix syntactic foams

Low dielectric constant materials play a key role in modern electronics. In this regard, hollow particle reinforced polymer matrix composites called syntactic foams may be useful due to their low and tailored dielectric constant. In the current study, vinyl ester matrix/glass hollow particle syntactic foams are analyzed to understand the effect of hollow particle wall thickness and volume fraction on the dielectric constant of syntactic foams. The dielectric constant is found to decrease with increase in the hollow particle volume fraction and decrease in the wall thickness. Theoretical estimates are obtained for the dielectric constant of syntactic foams. Parametric studies are conducted using the theoretical model. It is found that a wide range of syntactic foam compositions can be tailored to have the same dielectric constant, which provides possibility of independently tailoring density and other properties based on the requirement of the application.     

Thermal degradation of the mode I interlaminar fracture properties of stitched glass fibre/vinyl ester composites

Changes to the Mode I interlaminar fracture toughness, GIc, and fracture mechanisms of stitched and unstitched fibreglass/vinyl ester composites were investigated after exposure to elevated temperatures. The fibreglass was stitched through the thickness with Kevlar®-49 thread in two orientations with two stitch densities, and then resin transfer moulded with a cold-curing vinyl ester resin. After curing at room temperature (20°C) for several weeks, the composites were heated to between 100 and 300°C for 1 h or at 175°C for times ranging from 0.25–100 h. The GIc values, which were measured using the double cantilever beam method, of stitched composites in the cold-cured condition were between 1.5 and 2.3 times higher than the unstitched composite. It was observed with scanning electron microscopy that this toughening occurred by deflection of the crack tip at the stitches, by the ability of the stitches to remain intact for a short distance (7–15 mm) behind the crack front, and by partial pull-out of broken stitches. The interlaminar fracture toughness of the unstitched composite increased slightly following heating, despite a possible breakdown of the chemical structure of the vinyl ester between 150 and 300°C. In contrast, the interlaminar toughness of the stitched composites was degraded significantly by heating, and this was probably caused by thermal deterioration of the Kevlar® stitches. This study reveals that the elevated-temperature post-curing of stitched composites will reduce the effectiveness of Kevlar® stitching in raising the Mode I interlaminar fracture toughness. © 1998 Chapman & Hall     

Viscoelastic properties of hollow glass particle filled vinyl ester matrix syntactic foams: effect of temperature and loading frequency

Viscoelastic properties of hollow particle-reinforced composites called syntactic foams are studied using a dynamic mechanical analyzer. Glass hollow particles of three different wall thicknesses are incorporated in the volume fraction range of 0.3–0.6 in vinyl ester resin matrix to fabricate twelve compositions of syntactic foams. Storage modulus, loss modulus, and glass transition temperature are measured and related to the microstructural parameters of syntactic foams. In the first step, a temperature sweep from ?75 to 195 °C is applied at a fixed loading frequency of 1 Hz to obtain temperature dependent properties of syntactic foams. In the next step, selected four compositions of syntactic foams are studied for combined effect of temperature and loading frequency. A frequency sweep is applied in the range 1–100 Hz and the temperature is varied in the range 30–140 °C. Time–temperature superposition (TTS) principle is used to generate master curves for storage modulus over a wide frequency range. The room temperature loss modulus and maximum damping parameter, Tanδ, are found to have a linear relationship with the syntactic foam density. Increasing volume fraction of particles helps in improving the retention of storage modulus at high temperature in syntactic foams. Cole–Cole plot and William–Landel–Ferry equation are used to interpret the trends obtained from TTS. The correlations developed between the viscoelastic properties and material parameters help in tailoring the properties of syntactic foams as per requirements of an application.     

Thermal expansion of an E-glass/vinyl ester composite from 4 to 293 K

We have measured the thermal expansion of the three principal orthogonal directions of an E-glass/vinyl ester structural composite from liquid helium temperature, 4.2 K, to room temperature, 293 K. The linear thermal expansion at 4.2 K ranged from −0.23 to −0.71%, referenced to zero expansion at 293 K. We fitted the linear thermal expansion data from 4.2 to 293 K with a cubic polynomial for each of the three principal orthogonal directions.     

Alumina-coated hollow glass spheres/alumina composites

Coating of alumina onto the surface of hollow glass spheres was accomplished by controlled heterogeneous precipitation from aqueous solutions. The processing conditions were optimized to yield thin and uniform precursor coatings. After calcination, converting the precursor to alumina, the alumina-coated hollow glass spheres formed free-flowing powders that were used to produce glass/alumina composites with up to 35 vol% of controlled and well dispersed closed porosity. The dielectric constants and the flexural strengths of such porous composites were determined as a function of porosity. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effects of fibre coating (size) on properties of glass fibre/vinyl ester composites

Effects of fibre coating (size) on transverse cracking has been investigated. Two glass fibre/vinyl ester model composites were studied, denoted CA and NoCA and based on different size compositions. Various single fibre tests were not able to quantify the interfacial failure of CA as the interface never failed. The CA size consisted of a film former and a methacrylsilane-coupling agent whereas the NoCA size did not contain any coupling agent. The study reveals limitations with single fibre composite tests for fibre/matrix combinations with high interfacial toughness. Cross-ply laminates based on NoCA demonstrated significant inferior transverse cracking toughness as compared with CA laminates. Composites based on commercially sized glass fibre were also investigated and they performed almost as poorly as the NoCA material, demonstrating large potential for improvement in commercial composites. Results further indicate that the remarkable transverse cracking toughness of the CA material stems partly from strong fibre/matrix adhesion but also from high ductility of the matrix region close to the fibre surface.     

Characterization of the impact properties of three-dimensional glass fabric-reinforced vinyl ester matrix composites

Three-dimensional glass woven fabric-reinforced composites (3-D composites) were fabricated by impregnating vinyl ester resin in a hand lay-up procedure for the honeycomb sandwich structure. Three kinds of 3-D had been investigated in terms of the impact properties as a function of the z-direction fiber length, i.e., 3, 4.5 and 8 mm in core structures. In this work, more precise impact behaviors were studied in the context of differentiating between initiation and propagation energies, and ductility index (DI) along with maximum force and total energy as a useful measure. As a result, the highest impact properties were reached in the case of the specimens with 3 mm length of z-direction fibers, but the lowest DI was observed in the same specimens with 3 mm length of ones, probably due to the effect of buckling. The damage photo analyses after impact tests were also discussed in terms of a hollowing on the front face against the load applied, and a whitening spread in the impact point on the back face of the fabric composites studied.     

Effects of particle size on the thermal expansion behavior of SiCp/Al composites

The coefficients of thermal expansion (CTEs) of 20 vol% SiCp/Al composites fabricated by powder metallurgy process were measured and examined from room temperature to 450 °C. The SiC particles are in three nominal sizes 5, 20 and 56μm. The CTEs of the SiCp/Al composites were shown to be apparently dependent on the particle size. That the larger particle size, the higher CTEs of the composites, is thought to be due to the difference in original thermal residual stresses and matrix plasticity during thermal loading. At low temperature, the experimental CTEs show substantial deviation from the prediction of the elastic analysis derived by Kerner and rule of mixture (ROM), while the Kerner’s model agrees relatively well at high temperatures for the composite with the larger particle size.     

空心陶瓷/铝基复合材料的阻尼减振性能

采用挤压铸造法制备了70%(体积分数)空心陶瓷/6061Al复合材料,采用弯曲共振法测试了复合材料的阻尼性能.结果表明,空心陶瓷的加入明显提高了6061Al合金的阻尼性能,且尺寸减小会进一步提高空心陶瓷复合材料的阻尼性能.采用弯曲共振法测得的含较小粒径的空心陶瓷/6061Al复合材料的阻尼为基体6061Al阻尼的5.7倍,这表明空心陶瓷/6061Al复合材料阻尼与材料内部所含孔洞有密切的关系.结合组织观察,本文对相关的阻尼机理进行了探讨.     

空心陶瓷／铝基复合材料的阻尼减振性能

采用挤压铸造法制备了70%（体积分数）空心陶瓷/6061Al复合材料．采用弯曲共振法测试了复合材抖的阻尼性能，结果表明．空心陶瓷的加科明显提高了6061Al合金的阻尼性能，且尺寸减小会进一步提高空心陶瓷复合材料的阻尼性能。采用弯曲共振法测得的含较小粒径的空心陶瓷/6061Al复合材料的阻尼为基体6061Al阻尼的5.7倍，这表明空心陶瓷／6061Al复合材料阻尼与材料内部所含孔洞有密切的关系。结各组织观察，本文对相关的阻尼机理进行了探讨。     

Effect of nanoclay addition on vibration properties of glass fibre reinforced vinyl ester composites

This paper presents the experimental study of free vibration and damping characteristics of hybrid nanocomposite laminates by reinforcing short fibre chopped strand mat and organically modified montmorillonite clay (0, 1, 3 and 5 wt.%) in the vinyl ester matrix by hand lay-up technique. Theoretical study is also carried out to study the vibration and damping characteristics of hybrid nanocomposites. Dynamic result shows that the second phase nanoscale dispersion in the matrix and E-glass fibre significantly enhances the internal damping of hybrid composites.     

Thermal expansion of composites

A theory is developed for the overall thermal expansion of a composite consisting of either spherical or long cylindrical inclusions of one material in a matrix of another. The strain field of a single inclusion consists of a uniform expansion and a short-range strain field. These two components are related by minimizing the elastic strain energy. To account for a dense array of inclusions, average properties of the mixture are used for the long-range field, but those of the matrix alone for the short-range field. The net dilatation is thus found for inclusions of mismatching volume; hence one finds a differential expression for the thermal expansion in terms of the volume fraction of inclusions, the individual thermal expansivities, the bulk moduli of inclusion and matrix, the shear modulus of the matrix, and, in the case of cylinders, the shear modulus of the inclusions. This expression is integrated over temperature; one accounts for plasticity by letting the shear modulus depend on the temperature and on the accumulated shear strain. A representative numerical example is given.Invited paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

Thermal expansion of composites

Coefficient of thermal expansion (CTE) has been determined for selected composite materials using differential thermal analysis. Variables evaluated were: type of material, with particular emphasis on filler content; annealing; thermal history, with particular attention being payed to the effects of multiple heating and cooling cycles; ageing in wet or dry conditions. Filler content was a major factor involved in controlling CTE, although clearly other factors such as the type of filler, resin and degree of conversion are important. For an inlay material, annealing at 120°C significantly reduced the value of coefficient of thermal expansion and this is most likely due to an increase in conversion of methacrylate groups. The findings of this study confirm those of previous studies regarding the reduction in CTE following an initial heating. This is most likely due to the relief of internal stress. New information reported here relates to the fact that stress release can occur slowly without heating and that rapid stress release can be achieved through water storage at mouth temperature. These results suggest that, clinically, internal stresses induced by polymerization will be dissipated rapidly. A further finding was that long-term water storage causes an increase in CTE, which may reflect changes at the resin-filler interface.     

Thermal expansion coefficient and wear performance of aluminium/SiC composites with bimodal particle distributions

Abstract

High volume fraction metal matrix composites were produced by infiltration of liquid aluminium into preforms made by mixing and packing SiC particulates having two different average diameters (170 and 16 μm). The maximum particle volume fraction (0.74) was attained for a mixture containing 67% of coarse particles. The variation of particle volume fraction with percentage of coarse particles can be reasonably well understood in terms of a simple model. Experimental results for the threshold pressure indicate that it is mainly determined by the local volume fraction of fine particles, a result which is shown to be compatible with the model used to estimate the particle volume fraction. On the other hand, the coefficient of thermal expansion (CTE) is mainly determined by the total particle volume fraction of the composite. Wear performance was evaluated through sliding wear of the composite against a static alumina ball. The results indicate that, in this case, the key parameter is the coarse particle content of the composite.     

Compression creep rupture behavior of a glass/vinyl ester composite laminate subject to fire loading conditions

The growing use of polymer matrix composites in civil infrastructure, marine and military applications provides the impetus for developing mechanical models to describe their response under combined mechanical and fire loading. A viscoelastic stress analysis using classical lamination theory is conducted on an E-glass/vinyl ester composite. The model includes a characterization of the non-linear thermo-viscoelasticity and its inclusion into a compression strength failure criterion for the prediction of laminate failure under combined compressive load and temperature profile simulating fire exposure. By accounting for the viscoelastic non-linearity at T_g, the proposed model yields good predictions for lifetimes of the studied composite ([0/+45/90/−45/0]_S).     

Interfacial equilibria in titanium particle/glass ceramic composites

Titanium particle-reinforced glass ceramic matrix composites were prepared by sintering under an Ar atmosphere titanium particles (Ti_p) and the glass powders 45ZnO-15PbO-40B₂O₃ (mol%) (ZPB) for the ZPBT composites and 48.8SiO₂-48.8CaO-2.4B₂O₃ (SCB) for the SCBT composites. Each composite was characterized by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and differential thermal analysis. In order to modify the interfacial strength of the composites, glass matrices with different amounts of TiO₂ (TZPB and TSCB, 7 and 2.1 mol% TiO₂, respectively) were prepared. TiO₂ allows the formation of a continuous interface between Ti_p and the TSCB matrix and the modification of the interfacial equilibrium between Ti_p and the TZPB matrix.     

Poisson's ratio of hollow particle filled composites

The present study is focused on the experimental measurement of the Poisson's ratio of glass hollow particle filled composites called syntactic foams. The effect of hollow particle wall thickness and volume fraction on the Poisson's ratio of the composites is investigated. Results show that the Poisson's ratio of the composites is lower than that of the neat vinyl ester resin used as the matrix material. Despite being a fundamental elastic constant, a reliable value of the Poisson's ratio of the various types of composites is not readily available. These experimental results can be useful in benchmarking available theoretical models and guiding modeling efforts on functionally graded composites.     

Estimation of thermal conductivity for polypropylene/hollow glass bead composites

The thermal conductivity of hollow glass bead (HGB)-filled polypropylene (PP) composites was estimated using the thermal conductivity equation of inorganic hollow microsphere-filled polymer composites published in the previous paper. The estimations were compared with the measured data of the PP composites filled with two kinds of HGB with different size (the mean diameter was respectively 35 μm and 70 μm). The results showed that the predictions of the thermal conductivity were in good agreement with the measured data except to individual data points. Furthermore, both the estimated and measured thermal conductivity decreased roughly linearly with increasing the HGB volume fraction when the HGB volume fraction was less than 20%; the influence of the particle diameter on the thermal conductivity was insignificant.