Preparation, Cure, and Characterization of Cyanate Ester-Epoxy Blends

Cyanate ester resins are often blended with lower cost epoxy monomers in order to modify the cost, toughness, and processing characteristics. There are also several choices of catalysts that can be used to improve processing, namely by reducing the cure temperature. This study was undertaken to illustrate how a designed experiment approach can be used to systematically investigate a wide range of material combinations and illuminate the basic cure behavior of some simple cyanate ester – epoxy blend combinations. Two commercial cyanate ester resin products were obtained. Each was blended with a bisphenol F based epoxy resin at two different levels, and the effect of a hindered amine catalyst at low level was also investigated. This resulted in a 23 factorial experiment. Material characterization included differential scanning calorimetry (DSC) and thermal decomposition via thermogravimetric analysis (TGA). Although the addition of epoxy was expected to lower both the glass transition temperature and thermal stability (compared to pure cyanate), the designed experiment approach provided a good map of how these properties change as a function of epoxy substitution. For example, the amount of epoxy steadily decreased the Tg, TGA onset temperature, and char yield in an almost linear fashion from 0 to 50 wt%. Specifically, Tg was reduced by ~3°C per 1% epoxy, TGA onset temperature was reduced by ~1.2 °C per 1% epoxy, and char yield was reduced by ~0.5 wt% per 1% epoxy

Development of a Simple Lab-Scale Vacuum Assisted Resin Transfer Molding (VARTM) Process

The goal of the current study was to develop and demonstrate a simple and quick lab-scale VARTM process for the purpose of making flat panels for subsequent characterization, for example in new materials development efforts. This process was not intended to be optimized for final production, rather it served as the quickest way to make lab-scale composite panels using VARTM while maintaining all the salient features of typical VARTM processes used in larger scale manufacturing. There is a wide variety of ways to implement VARTM, as well as a diverse list of potential materials and supplies from which to choose. The process we arrived at was implemented on a 60 cm x 90 cm (2 ft. x 3 ft.) aluminum plate, which was mounted to a moveable cart and intended for ambient temperature processing (no heaters). Details of the vacuum system, resin distribution strategy, and bagging procedures will be described herein. The system was tested by making carbon/epoxy composite laminates of approximately 30 cm x 45 cm (1 ft. x 1.5 ft.). These panels were tested for thickness variation and fiber volume fraction. Optical microscopy was also used to evaluate the microstructure, and limited tensile testing was performed. The results indicated that the panels were of reasonable quality with no significant porosity

Preparation, Cure, and Characterization of Cyanate Ester-Epoxy Blends Containing Reactive Flame Retardants

Cyanate ester resins are sometimes mixed with lower cost epoxy monomers to modify cost, toughness, and processing capabilities. Despite the high performance of these thermosetting polymers, flame retardancy remains an issue. This study examined blends of three different commercial cyanate ester monomers (LVT-100, LECy, and XU-71787.02) and diglycidyl ether of bisphenol A (DGEBA) at 50/50 wt% of each type. The blends were successfully reacted with two reactive flame retardants (FR): 9,10-dihydro-9-ox-9-phosphaphenanthrene-10-oxide (DOPO) and poly(m-phenylene methylphosphonate) (PMP) at phosphorus contents ranging from 0 to 3 wt%. The curing behavior of EP/CE blends was investigated using differential scanning calorimetry (DSC). It was found that introducing phosphorus into EP/CE blends lowered both the onset reaction temperature and the glass transition temperature Tg for all blends. TGA data revealed that the addition of PMP and DOPO to EP/CE blends resulted in a linear decrease in the onset decomposition temperatures of LVT and LECy blends, with a maximum drop of 36 °C in the EP/XU/DOPO blend compared to the EP/XU baseline blend. In addition, TGA results revealed that the introduction of PMP into EP/CE blends improved the char yield of the blends by 24%, while the DOPO reduced the char residue of the blend by 24% compared to the baseline EP/CE blends. Incorporating PMP and DOPO as reactive FR into the EP/CE network structure has been successfully investigated

Simulation of Laminated Object Manufacturing (LOM) with Variation of Process Parameters

A previously developed and verified thermal model for Laminated Object Manufacturing (LOM) was used to investigate the effects of various processing parameters on the temperature profile in a LOM part during the build cycle. The mathematical model, based on 3-dimensional transient heat conduction in a rectangular geometry LOM part, allows calculation ofthe transient temperature distribution within the part during the application of a new layer as well as during other periods ofthe LOM build cycle. The parameters roller temperature, roller speed, chamber air temperature, base plate temperature, and laser cutting time were independently varied, and the LOM process response simulated. The results were analyzed in order to gain insight into potential strategies for intelligent process control.Mechanical Engineerin

Comparison of Tensile Properties of Triaxial Braided Carbon Fiber Composites Made from Vacuum Assisted Resin Transfer Molding (VARTM) and Autoclave Molding

Triaxially braided fiber composites are increasingly being used in aerospace, ballistic, and sporting good applications due to their inherent damage tolerance, torsional stability, and cost compared to woven fabrics and unidirectional preforms. There have been numerous publications over the past 15-20 years on the mechanical properties and failure mechanisms of triaxial braided composites. However, most of these have involved panels made with autoclave curing. In the present study, braided carbon fiber composites were made using autoclave curing and vacuum assisted resin transfer molding (VARTM). The goal of the study was to compare the physical and tensile properties of quasi-isotropic panels produced from these two methods while keeping the fiber and matrix materials constant. Material characterizations included density and fiber volume fraction (Vf), tensile modulus and strength in both the 0° and 90° directions, and microstructure via optical microscopy and scanning electron microscopy. The results revealed that the 0° vs. 90° tensile properties of QISO composites are equivalent or very close is most respects regardless of processing technique. The VARTM panels had slightly lower Vf autoclave. However, the tensile properties of the VARTM panels compared favorably with autoclave cured panels when normalized for fiber volume fraction. Overall this study represents a very good side-by-side comparison of braided carbon fiber composites made with two significantly different processes

Development of a Methodology for Characterizing Reaction Kinetics, Rheology, and In-Situ Compaction of Polyimide Prepregs During Cure

PMR-type polyimide prepregs are challenging to fabricate into high quality composites due to volatiles that are generated and must be removed in situ during processing. The current work was conducted to develop accurate, reliable, and practical characterization techniques of the prepreg rheology, volatile generation, and subsequent volatile removal from the prepreg during composite fabrication. Thermal analysis was used to characterize volatile generation, reaction rates, and rheology. A novel approach was used to measure the thickness of the prepreg in situ during vacuum bag/oven processing using a high-temperature LVDT. Experimental results are presented for the commercially available RM-1100 polyimide/carbon prepreg system, including the reaction rate, rheology, and panel thickness results for a series of heating rates and ply counts. The results show the key interrelationships in these coupled phenomena and how that information can be used to select the optimum temperature of pressure application to minimize the final void content

The Effect of Fabric Architecture on the Processing and Properties of Composites Made by Vacuum Assisted Resin Transfer Molding

The goal of this research project was to evaluate and compare the effect of fabric architecture on the processing and properties of composites made by Vacuum Assisted Resin Transfer Molding (VARTM). The fabric architectures investigated included plain weave, satin weave, and warp-knit unidirectional. The fiber types included E-glass and standard modulus carbon fiber. Flat panels were fabricated with a lab scale VARTM process using an epoxy resin system. Fabric plies were cut to 45 cm x 30 cm (18 in. x 12 in.), and the number of plies used depended on the fiber areal weight of each fabric to produce panels of similar final thickness. The speed of resin infusion was recorded by visually monitoring the flow front which was visible through the bag. Fiber volume fraction was evaluated using thickness measurements, and porosity was investigated via optical microscopy. Mechanical testing was performed via tensile and 3-point flexure. The results showed the fabric type had minimal effect on the infusion speed with the exception of the plain weave and satin weave fiberglass. From the mechanical testing results, there are many comparisons made of the modulus, strength, and strain-to-failure results, for example carbon vs. glass, unidirectional vs. woven, tensile vs. flexure. The rule of mixtures was able to predict some but not all of these properties. The results, which are discussed in detail herein, illustrate the main advantage of selecting carbon vs. glass in stiffness driven applications

Failure Mechanism of Woven Roving Fabric/Vinyl Ester Composites in Freeze–Thaw Saline Environment

This experimental study investigates the degradation mechanisms of a glass fiber-reinforced plastic material commonly used in civil engineering applications. A substantial reduction in tensile, shear, and compression properties was observed after 100 days of freeze–thaw cycling in saline environment (-20°C to 20°C). Non-destructive inspection techniques were progressively conducted on unexposed (ambient condition) and exposed (conditioned) specimens. The dynamic mechanical analysis showed permanent decrease in storage modulus that was attributed to physical degradation of the polymer and/or fiber–matrix interface. This indicated the formation of internal cracks inside the exposed glass fiber-reinforced plastic laminate. The 3D X-ray tomography identified preferred damage sites related to intralaminar and interlaminar cracks. The ultrasonic C-scan and optical microscopy showed the nature of the damage and fibers fracture. The thermal cycling events degraded the matrix binding the warp and fill fibers, thus impairing the structural integrity of the cross-ply laminate. The result of this work could benefit a multi-scale durability and damage tolerance model to predict the material state of composite structures under typical service environments

Investigation of Various Techniques for Controlled Void Formation in Fiberglass/Epoxy Composites

The effect of porosity in composite materials has been studied for years due to its deleterious effects on mechanical properties, especially matrix dominated properties. Currently there is an increasing use of composites in infrastructure worldwide, for example bridge components, residential and building structures, marine structures such as piers and docks, and large industrial chemical tanks. Most of these applications use fiberglass composites. Unfortunately, most of the published literature has focused on carbon fiber composites, in which fiber diameter and gas-fiber interactions are different than fiberglass composites. Therefore, the present study was undertaken to revisit the effect of porosity but specifically in fiberglass composites. The goal of this experimental study was to implement and evaluate various methods for creating porosity in fiberglass composites in a controlled manner in terms of obtaining repeatable void content, morphology, and location within the laminate. The various methods included using different amounts of autoclave pressure, adding a small amount of water between prepreg layers, and using dry fabric layers to starve the laminate of resin. Ultrasonic C-scan nondestructive evaluation was used to assess the quality of the cured panels, as well as optical and electron microscopy and void content measurements via resin burn-out. The cured panels were mechanically tested using the short beam shear (SBS) method. The results showed that the water spray method proved to be the best in terms of producing noticeably different levels of porosity, although the panels required drying to remove residual water after cure. The voids from all three techniques were either oval or elongated in-plane between the plies, but they were not uniformly distributed in-plane. The use of C-scan proved to be helpful for characterizing overall uniformity of each panel, although the results could not be used to directly compare void content between panels. The use of SBS testing was successful for evaluating void dominated properties in panels with high void content, although it was not very sensitive to coupons with lower void contents. Several interesting observations are offered in this manuscript of the fracture surface details and their relation to the SBS load deflection curves. Overall, it was found that the failure mechanisms were mixed mode and the voids did not serve as failure initiation sites. However, the voids participated mainly in the horizontal propagation of cracks between layers, presumably making it easier when they were intersected by a crack and reducing SBS strength

Study of an Alternative Process for Oxidizing Vapor Grown Carbon Nanofibers using Electron Beam Accelerators

The use of a high-energy electron beam was explored in this study as an alternative technique for oxidizing vapor grown carbon nanofiber surfaces. The radiation exposures were carried out at three different electron beam facilities with beam energies of 1.5, 3.0 and 4.5 MeV and radiation doses ranging from 1000 to 3500 kGy. XPS analysis showed that oxygen was readily incorporated on the surface: the ratio O1s/C1s increased approximately by a factor of 4 when the carbon nanofibers were irradiated at 3500 kGy. The oxidized nanofibers exhibited better dispersion in a water/methanol solution (50% v/v) than as-received nanofibers. Raman spectroscopy revealed that the ID/IG ratios for most of the samples were statistically unchanged because the damage on the nanofiber surface was highly localized and did not lead to modifications on the bulk carbon nanofiber structure. The samples irradiated at higher dose rate exhibited significantly higher ID/IG ratios. The radiation process introduced defects on the graphene layers leading to a decrease of the decomposition onset temperatures up to 56 °C lower than the non-irradiated samples. Overall the results were repeatable across all facilities, illustrating the robustness of the process