Showing papers in "Polymer Composites in 1989"

Interfacial Shear Strength Studies Using the Single-Filament-Composite Test. I: Experiments on Graphite Fibers in Epoxy

Abstract: The failure of the interface in a carbon fiber-epoxy system was studied for six different epoxy blends using the single-filament-composite technique. The blends were formulated to yield a wide range of stiffnesses, and their effect on interfacial failure was examined. Specimens were made from Hercules IM6-G carbon fiber and the different blends of epoxy, and then strained to obtain a distribution of fiber fragment lengths. Birefringence patterns near the fiber breaks were observed and recorded. Some of the specimens were strained until they failed and the resulting fracture surfaces were observed under a scanning electron microscope to determine fracture patterns and the existence of debonding. The fragment length distributions were interpreted using a Monte-Carlo simulation of a Poisson/Weibull model for fiber strength and flaw occurrence. The results were used to calculate an effective interfacial shear strength. From this analysis we conclude that one cannot accurately predict the interfacial properties of a composite based solely upon conventional single fiber and bulk matrix properties. Local matrix properties and fiber/matrix interactions, on a microscale, play a key role in composite strength.

Future prospects for wood cellulose as reinforcement in organic polymer composites

Abstract: Wood cellulose, a versatile and renewable natural resource, has potential for use as a reinforcement for synthetic organic polymers. During the past 80 years a number of materials using the reinforcing properties of wood cellulose have found major markets. Forms of wood cellulose proposed as reinforcements include: wood fibers, cellulose fibers, microfibillar, and microcrystalline cellulose. Recent attention has been given to them as fillers/reinforcements in thermoplastics and elastomers. Most cellulosic composites derive their existence from their comparatively low materials cost and the filling rather than reinforcing properties of cellulose. However, cellulose chains have a potential stiffness much higher than glass and in the same range as superstiff aramid fibers. This paper examines the state of the art of combining wood cellulose with synthetic organic polymers to from composites and considers new ways for better using cellulose's reinforcing potential.

Interfacial shear strength studies using the single-filament-composite test. Part II: A probability model and Monte Carlo simulation

Abstract: A Monte Carlo simulation is developed to provide a foundation for interpreting experimental data from the single-filament-composite test. The main focus is on developing an improved procedure for arriving at a realistic value for the shear strength of the fiber-matrix interface, but it is also shown how the test may be used to characterize the strength distribution of fibers at a length scale much shorter than is achievable in standard tension tests. The simulation is based on the widely used Poisson/Weibull probability model for fiber failure that characterizes the strength in terms of the random flaws distributed along the fiber. The primary mechanical model for stress buildup at the fiber end is the same as that assumed by most authors and assumes a constant interfacial shear stress in this shear transfer zone reminiscent of a yielding plastic matrix. We also, however, consider a bilinear model that allows for a zone of debonding with a constant shear stress lower than in the primary “plastic” zone. Simulation results are cast in terms of nondimensional variables and tabulated to allow for wide applicability. Sample size and confidence interval issues are also discussed.

Tensile properties of injection molded long fiber thermoplastic composites

Abstract: This paper deals with the mechanical performances of a new class of injection molded long fiber composites based on PP and PBT matrices. Effects of material parameters such as fiber concentration, breakage, orientation, and matrix composition are analyzed. The critical fiber length, l, of the PP long fiber composite, evaluated from the pull-out length of the tensile fracture surface, was found to be much higher than those previously reported. Tensile strength calculated from the measured ll and fiber length distribution in the molded samples was found to be in agreement with the measured values. From this work it is concluded that higher mechanical performances of the long fiber reinforced thermoplastics will be attained by the injection molding process to further reduce fiber breakage.

Some studies on glass fiber‐reinforced polypropylene. Part I: Reduction in fiber length during processing

Abstract: The reduction in fiber length during extrusion and injection molding of two commercial glass fiber-reinforced polypropylene products containing 30 percent by weight of glass fibers was studied. The first product had very small fibers of average length around 0.5 mm and also contained a coupling agent. The second product contained relatively longer glass fibers of 9 mm length and no coupling agent. In both cases, fiber attrition occurs predominantly at the solid-melt interface in the meiting zone of the extruder. However, in the short fiber granules, the maximum of the length distribution, which for the initial sample is around 0.5 mm, moved to shorter fiber lengths along the screw channels further from the hopper. In the long fiber granules, a bimodal length distribution was obtained in the intermediate channels; the first maximum was around the original length of 9 mm and the second centered around 0.5 mm. Thus, the forces at the solid-melt interface result in fiber breakage to lengths which are predominantly around 0.5 mm. The fiber attrition was observed to be more severe in injection molding apparently because of higher shear rates and also because the fibers had to pass through narrow channels. The measured distributions of fiber length along the screw channels for the two products are presented, and the possible mechanisms of fiber breakage are discussed. The mechanical properties of samples containing different fiber length distributions and the effects of fiber length and interfacial adhesion on properties are presented and discussed in Part II.

Effects of cellulose fibers in polypropylene composites

Abstract: A systematic study of the effect of surface pretreatment of cellulosic fibers and the processing time and temperature on the mechanical properties of the cellulose-containing polypropylene was undertaken. Using non-treated fibers, the elastic modulus increased gradually with the cellulose content, typically doubling its value at 30 phr fiber content. Treatment of fibers with coupling agent improves significantly the interfacial adhesion and therefore the mechanical properties of composite. Scanning electron micrographs reveal that the shear stress is sufficiently high to break and delaminate the cellulosic fibers. Addition of maleic anhydride modified polypropylene also improves the properties of resulting composites.

Interfacial shear strength and failure modes of interphase-modified graphite-epoxy composites

Abstract: Electrocopolymerization of acrylonitrile/methyl acrylate/acrylic acid, acrylonitrile/methylacrylate, and glycidyl-acrylate/acrylonitrile interlayers onto Hercules AS4 graphite fibers was used to improve simultaneously the effective interfacial shear strength, τe, and the fracture toughness of graphite-epoxy composite materials. With a single-fiber fracture test, τe for these coated fibers (embedded in a diglycidyl ether of bisphenol A - 4,4′-methylenedianiline matrix) was determined at various temperatures and under various hygrothermal treatments. At room temperature, the coated samples showed slightly improved shear strength over the uncoated sample. At elevated temperatures, a plot of τe vs. temperature for the uncoated sample showed two distinctive regions: an interface-controlled plateau region at low temperatures, and a matrix-controlled region at high (>80°C) temperatures. Only one region, which was controlled by the matrix and the interlayer, was observed for the GA/AN coated sample. The τe values determined were slightly higher than the shear strength of the bulk matrix, possibly because of stronger matrix properties at the interface. Optical micrographs of the coated sample tested at temperatures less than 100°C showed no matrix crack perpendicular to the fiber axis, indicating that the interlayer has effectively blunted the crack tip and restricted its propagation. The effect of moisture in the sample was to reduce τe for the coated sample. Upon dehydration, the strength was partially recovered. The treatments did not affect the uncoated sample, however. The fragmentation length data were fitted well by both Gaussian and Weibull distributions.

Fracture of ultrafine calcium carbonate/polypropylene composites

Abstract: The strength and fracture properties of a polypropylene filled with ultrafine calcium carbonate (0.07 micron) have been studied in the composition range of 0 to 40 percent by volume. Untreated and surface treated (with stearic acid and a titanate coupling agent) grades have been considered. The untreated filler caused a decrease of toughness, whereas a maximum, at about 10 percent, was observed for the treated filler. The fracture energy was analyzed in terms of the crack-pinning model. Due to the very small size of particles, the pinning contribution proved to be negligible. 30 references.

Some studies on glass fiber‐reinforced polypropylene. Part II: Mechanical properties and their dependence on fiber length, interfacial adhesion, and fiber dispersion

Abstract: Two commercial grades of glass fiber-reinforced polypropylene granules, one containing short fibers of average length around 0.5 mm with a coupling agent and the other containing relatively much longer fibers mostly around 9 mm, but no coupling agent, were injection-molded into dumbbells and tested in tension between −43 and 90°C. There is considerable fiber attrition during injection molding; the fiber lengths are reduced to average values of 0.4 to 0.8 mm for thses two samples. Also during injection molding of the test sample, partial molecular alignment of the matrix (polypropylene) occurs which supplements the reinforcement of the matrix due to the aligned glass fibers (30 percent by weight) present in the composite sample. The stiffness and strength of these samples do not reflect the effects of fiber lengths since most of the fibers are of very small length in the molded specimen and also since the sample with longer fibers has a non-uniform distribution of fibers. While the interfacial shear strength does not appear to play a significant role in determining stiffness, it turns out to be extremely important in controlling strength, particularly at the higher test temperatures. The room temperature impact strength is high for the sample containing relatively longer fibers of average length around 0.8 mm in which fiber dispersion is non-uniform and fiber agglomerates are present. Acoustic emission data shows that debonding and fiber pull-out are the main contributors to sample toughness; this observation is supported by scanning electron micrographs of the fracture surfaces.

Study of interfaces of high‐performance glass fibers and DGEBA‐based epoxy resins using single‐fiber‐composite test

Abstract: The single-fiber-composite (SFC) technique was used to study the interfacial behavior between two flexible blends of diglycidylether of bisphenol A (DGEBA)-based epoxy and polyglycol epoxide and three glass fibers. Dog-bone-shaped SFC specimens were made and strained to obtain a distribution of fragment lengths. The fibers were tension-tested at two different gauge lengths. The fragment length distributions, the fiber strength data, and a Monte Carlo simulation of a Poisson/Weibull model for fiber strength and flaws were used to obtain the effective interfacial shear strength values. The results show that the interface does not fail. Instead, penny-shaped transverse cracks appear at every fiber break and grow as the specimen is strained. The interfacial shear strength values are many times higher than the yield shear strength values of bulk epoxy obtained from the tension test.

Mat formation and unstable flows of highly filled suspensions in capillaries and continuous processors

Abstract: Suspensions, which are filled at loading levels approaching their maximum packing fractions, present special processing challenges. The flow and deformation behavior of such highly filled suspensions are dominated by strong slip effects. Furthermore, during pressure driven flows of these suspensions, the polymer matrix can filter out, giving rise to demixing and unstable flows. In this study, the capillary flow and extrusion of highly filled suspensions of ammonium sulfate and other salt fillers in a poly(butadiene acrylonitrile acrylic acid), PBAN matrix were investigated. The experimentally observed instabilities were elucidated in conjunction with a proposed mechanism, involving slip and filtration rates. This mechanism suggested greater affinity to unstable flows with relatively lower matrix shear viscosity, greater particle size of the filler, and greater convergence ratio; all of which were experimentally verified.

Liquid flow in molds with prelocated fiber mats

Abstract: The mechanism associated with mold filling in the manufacture of structural RIM (SRIM) and resin transfer molding (RTM) composites is studied by means of flow visualization and pressure drop measurements. To facilitate this study, an acrylic mold with a variable cavity was constructed and the flow patterns of nonreactive fluid flowing through various layers, types, and combinations of preplaced glass fiber reinforcement mats were photographed for both evacuated and nonevacuated molds. The pressure drops in the flow through a single type of reinforcement (e.g., a continuous strand random fiber mat) and also a combination of reinforcement types (e.g., a stitched bidirectional mat in combination with a random fiber mat) were recorded at various flow rates to simulate high-speed feeding processes (e.g., SRIM) and low-speed feeding processes (e.g., RTM). By changing the amount of reinforcement placed into the mold, the permeabilities of the different types and combinations of glass fiber mats were obtained as a function of porosity. It is shown that partially evacuating the mold cavity decreases the size of bubbles or voids in the liquid, but ultimately increases the maximum pressure during filling. The results also show that glass fiber mats exhibit anisotropic permeabilities with the thickness permeability, Kz, being extremely important and often the determining factor in the pressure generated in the mold during filling.

The prediction of flow and orientation behavior of short fiber reinforced melts in simple flow systems

Abstract: Fiber interactions are modeled as randomizing forces over the rotation of fibers in closed orbits in simple shear. The resulting Fokker-Planck type convection-diffusion equation in orientation space is solved using a finite difference technique. The model predictions for simple shear flow demonstrate the intraction between the structural changes and the bulk rheological properties

Stamping of thermoplastic matrix composites

Abstract: The recent growth in the use of polymer composites has resulted in the need for higher productivity manufacturing processes than are currently used in most thermoset composite production One of the ways to provide increased productivity in composite manufacturing is to use thermoplastic matrix sheet composites In this investigation the part forming characteristics of several thermopolastic composites were investigated Glass fiber reinforced composites based on polypropylene, nylon 12, poly(butylene terephthalate), poly(ethylene terephthalate), poly(phenylene sulfide), and poly(etheretherketone) were examined The processing conditions required for solid-state stamping of these composites were determined A temperature process window for stamping each of these composites was determined, and found to correlate with the melting and re-crystallization peaks from differential scanning calorimetry (DSC) scans

The effect of processing variables on microstructure of injection molded short fiber reinforced polypropylene composites

Abstract: Injection molded composites of polypropylene reinforced with short glass fibers were obtained under a variety of injection molding conditions. The microstructure of the moldings was determined using a variety of experimental techniques, including optical and scanning electron microscopy, differential scanning calorimetry, Fourier transform infra-red spectroscopy, and thermogravimetry. Thus, it was possible to obtain a detailed characterization of the crystallinity, morphology, and orientation distribution in the matrix, in addition to the distribution of fibers and their orientation of the fibers in the composite. The influence of molding conditions on the above microstructural characteristics is summarized in an effort to explain the experimental observations.

Void formation in short-fiber thermoplastic composites

Abstract: Voids, or bubbles, are formed in short-fiber thermoplastic composites because of entrapment of air in the compounding and melt flow processing steps and as a result of uneven shrinkage due to temperature gradients involved in the solidification step by cooling. The experimental results suggest that bubble nucleation takes place at fiber ends and their volume content depends on processing conditions. The shape and distribution of the voids were studied by optical examination of longitudinal sections of extrudates, using standard metallographic polishing technique and reflected light microscopy. The void volume fraction of extrudates was found to increase with increasing extrusion rate, temperature, fiber concentration, and fiber length, and with a decreasing draw ratio. The void content in fiber-glass-reinforced injection moldings is low, ≈1% by volume; however, by heating these moldings, the contained pressurized bubbles expand and significant void volume fractions are observed.

Short‐fiber‐reinforced thermoplastics. Part III: Effect of fiber length on rheological properties and fiber orientation

Abstract: The effect of fiber length on fiber-filled melt viscosity and on shear-induced fiber breakage and orientation development in polystyrene was studied. The incorporation of fibers, increasing their length, results in an increasing viscosity over the entire shear rate range. Significant fiber length reduction was observed even at low shear rates. Determined fiber orientation distributions, as affected by shear rate and fiber length, are discussed. Orientation effects are expressed by a single orientation parameter which is used to account for the average fiber orientation contribution to the relative viscosity. Calculated orientation parameters indicate higher fiber orientation for shorter fibers. Fiber orientation in the flow direction increases with increasing shear rate and results in the expected viscosity decrease.

Low temperature relaxation of DGEBA epoxy resins: A thermally stimulated discharge current (TSDC) study

Abstract: The effect of curing conditions on the low temperature relaxation behavior of catalyst-cured epoxy systems based on digliycidyl ether of bisphenol A (DGEBA) has been characterized by the thermally stimulated discharge current (TSDC) technique. In these chemically relatively simple epoxy systems, five relaxation processes, designated as γ, β, β′, βOH, and β″, have been observed: Their molecular origins are discussed in detail. The results are in agreement with prior suggestions of an inhomogeneous cross-link density morphology.

Properties of carbon/graphite fibers modified by plasma polymerization

Abstract: Carbon fibers and pyrolytic graphite blocks were treated with plasma of acrylonitrile (AN) and styrene (ST) monomers, using an induction-coupled, RF-plasma reactor. Both substrates were stable towards plasma, leading to a deposition of thin, coherent coatings of 400A∼1000A thickness. Both monomers produced surfaces which are substantially more polar (γc=54 dynes/cm for AN and 40 dynes/cm for ST) than the untreated surfaces (γc=32 dynes/cm). ESCA and IR studies indicate that the plasma polymers contain a high concentration of oxygen (12 percent in PPAN and 17.8 percent in PPST), in the form of CO, COOH, COC, and OH groups. Also, treated fibers exhibited slightly higher tensile strengths than the untreated counterparts, suggesting that the plasma coatings effectively heal some of the surface flaws of the fiber. The abundant surface polar groups combined with the improved tensile properties of the plasma treated fibers make them attractive reinforcements for advanced composite materials.

Crystallization of poly(aryl-ether-ether-ketone) - Effect of thermal history of the melt on crystallization kinetics

Abstract: Differential Scanning Calorimetry (DSC) and optical microscopy were used to investigate the effect of the thermal history of the melt on the crystallization of a commercial sample of poly(aryl-ether-ether-ketone) (PEEK). Heating a film of PEEK at a temperature above the melt temperature for various periods of time changes the nucleation and crystal growth rate upon cooling the sample. Destruction of existing nuclei, formation of new nuclei, chain branching, cross linking, and chemical degradation of the macromolecular chains are all believed to take place at different times and to different extents during the thermal melt processing of the polymer. This study suggests that the thermal history of the melt plays an important role in the crystallization of PEEK samples. 45 refs.

A simple illustration of structure-properties relationships for short fiber-reinforced thermoplastics

Abstract: In this paper, a simple theoretical model for elastic properties of short fiber-reinforced thermoplastic (SFRTP) composite systems is described, which considers the effect of two important structural parameters, viz., fiber aspect ratio distribution and fiber orientation distribution. An experimental technique for producing SFRTP samples featuring a fairly uniform fiber orientation is described, and a broad set of experimental results on the mechanical properties of SFRTP systems based on polystyrene (PS) and polyethylene (PE) resins with several fiber loadings is presented. The use of the simple theoretical model to analyze the experimentally-determined elastic properties is discussed.

Polypropylene/low density polyethylene blend matrices and short glass fibers based composites. I. Mechanical degradation of fibers as a function of processing method

Abstract: The fiber length degradation during compounding (two-roll milling and twin-screw extrusion) of glass fiber and polypropylene (PP)/low density polyethylene (LDPE) blend matrices based composites was investigated. The effect of LDPE percentage and fiber content on fiber length were studied using a semiautomatic image analysis system. Two-roll milling causes a more severe attrition of the fibers than twin-screw extrusion. In the first case, the higher the LDPE percentage in the polymer matrix, the larger the final fiber length. Both methods lead to a broader fiber length distribution as LDPE percentage increases. The effect of fiber content is opposite to that of the LDPE percentage, but in the case of twin-screw extrusion it is less noticeable, During the injection molding of the composites a slight decrease of the final fiber length takes place. This decrease depends on the initial fiber length, the effect being more pronounced for longer fibers.

Synergism on electromagnetic inductance (EMI)‐shielding in metal‐ and ferroelectric‐particle filled polymers

Abstract: In this paper the electromagnetic inductance (EMI)-shielding of metal-and ferroelectric-particle filled polymers is described. The composite, containing only metallic fillers, has a shielding effectiveness comparable to conventional products. The addition of a few percent of ferroelectric material to this composite enhances the shielding behavior by several orders of magnitude—without increasing its electrical conductivity. In this three component composite the shielding effectiveness reaches a maximum close to the ferroelectric/paraelectric transition temperature. The synergetic effect, obtained by the addition of ferroelectrical particles, is related to the high dielectric losses of ferroelectric material close to its transition temperature.

Parameters affecting interlaminar stresses in tapered laminates under static loading conditions

Abstract: Tapered laminates by internal ply drop-offs have been studied. An extensive finite element program has been developed for microcomputers using displacement formulation. A 20 node, three dimensional element was used. Parameters which can influence the strength of the laminate were studied. These parameters include the location of the drop-off in the thickness direction, the degree of resin filling inside the drop-off, the shape of the drop-off, the effect of width of laminate, and the orientation of the lamina where the drop-off was made. Locations of failure based on maximum stress criterion were btained. Crack growth stimulation was also performed by consecutive removal of failed elements. Results show that crack growth is not continuous.

Long-fiber-reinforced thermoplastic matrix composites by slurry deposition

Abstract: A process for producing long-fiber-reinforced thermoplastic matrix composites is described This process, based on papermaking technology, consists of depositing a mixture of a polymeric powder and reinforcing fibers from an aqueous slurry The degree of homogeneity of the mixture and the subsequent ability to handle the dried sheet have been significantly improved by the addition of less than 5% pulp fibers, based on solids concentration, to the slurry The dried sheet is then consolidated into a composite sheet with a continuous matrix phase by the application of heat and then pressure Tensile properties and the degree of reinforcement efficiency of composites produced by this process have been shown to be comparable to those produced by melt impregnation techniques

Delamination shape effects in aramid-epoxy-aluminum (ARALL) laminates with fatigue cracks

Abstract: An aramid reinforced aluminum-epoxy-laminate, ARALL, which contains a fatigue crack and a delamination zone is analyzed. It is assumed that the interlaminar shear forces between the aluminum and aramid/epoxy layers are transmitted along the delamination boundary. The aramid/epoxy layer of the laminate is considered a series of linear springs. The tensile stress in the aramid/epoxy layer and the stress intensity factor in the aluminum layer are found for various experimentally observed delamination shapes. A residual strength criterion based on the maximum tensile stress in the aramid/epoxy layer is applied and the analytical results are correlated with the available experimental data.

Short fiber reinforced thermoplastics. I. Rheological properties of glass fiber reinforced Noryl

Abstract: An experimental study on the flow behavior of glass fiber reinforced Noryl (a commercial poly(phenyleneoxide)/polystyrene blend) using a capillary rheometer is described. The effect of fiber concentration on shear viscosity and die swell was studied at various temperatures. Breakage of glass fibers during flow through the rheometer is discussed; it was found that the average fiber length (about 230 μm) was not significiantly altered, except at the highest shear rate (575 s−1) studied. The incorporation of short fibers into thermoplastic polymer melts increases their viscosity without changing the basic rheological character-shear rate dependency. No discernible viscosity changes were measured by incorporating 10 weight percent fibers, and upon further increase of fiber concentration from 20 to 30 weight percent no appreciable increase in viscosity was noted. It is shown that short glass fibers cause a large reduction in extrudate swell. The presence of voids and fiber orientation contribute to the decrease of the die swell, an effect greater than expected from volumetric considerations alone.

A study of glass fiber–epoxy composite interfaces

Abstract: Fourier transform infrared spectroscopy was utilized to characterize the interfacial bonding in glass-fiber-reinforced composites. The pH of the treating solution significantly affects the interfacial chemical bonding. The natural pH of the solution gives the maximum interfacial bonding. The decrease of the interfacial bonding is due to the side reactions that occur when NaOH or HCl is added to the solution. The temperature of the curing process has little effect on the amount of the interfacial chemical bonding. We attribute this to the reaction rate constant of the interfacial reaction being much larger than that of the epoxy polymerization. From the hygrothermal behavior, we have found the amount of silane needed for hygrothermal condition is different from that for dry condition.

The processing of fiber reinforced thermoplastics using co‐rotating twin screw extruders

Abstract: For short fiber reinforced thermoplastics the effect of fiber length and the fiber/matrix bond on the ultimate properties of the material are well understood. But, how the process conditions under which the composite is made affect the fiber length and the fiber matrix bond has not been so thoroughly reported in the literature. A study of this relationship has been made on a co-rotating twin screw extruder incorporating 30 percent by weight of glass fiber into nylon 66 to make the composite. Experimental results are presented to show how machine variables such as mixing configuration and screw speed affect the material properties and process efficiency. The properties of the composite were assessed by measurement of the fiber length distribution and tensile strength. From the results in the text it is possible to select machine variables to give desired production conditions. Although most of the work concerns the twin screw extruder, some of the consequences of subsequent injection molding are also shown.

Properties and processing characteristics of dielectric‐filled epoxy resins

Abstract: A family of casting composites, epoxy resins with mineral fillers, having a range of electrical properties, are being developed. In such composites, the dielectric constant is controlled primarily by varying the filler material in composition and proportions. The present work reports on the mechanical properties of composites made with two types of filler, an alumina powder (XA3500 from ALCOA) and a BaTiO3/TiO2 ceramic powder (ATD-50 from Ampex). Dependence of mechanical properties on curing agents was also determined. Filler contents from 0 to 40 percent volume were used. Epoxy systems contained single epoxy resin with both amine and anhydride hardeners. Processing of the anhydride-cured systems was easier than that of the amine-based systems because of their lower viscosity and longer gel time of the former. However, the anhydride-cured systems required higher processing temperatures. Curing kinetics and molecular bonding were investigated using a combination of differential scanning calorimetry, dynamic mechanical thermal analysis, and scanning electron microscopy. Activation energies of 11.2 kcal/mole and 12.1 kcal/mole were obtained for the curing of the amine-based and the anhydride-based composites respectively, and a small difference in the glass transition temperature was also observed. These effects can be attributed to the difference in the structure of the curing agents. The epoxy resin cured with NMA is less ductile compared with those cured with MTHPA or MHHPA due to slight chemical modification on the ring structures. This dependence of ductility on curing agent was observed in specimens with different filler contents. Although the presence of the filler materials was found to enhance the mechanical properties of the epoxy, the fracture mode in these materials is still brittle.