Nikhil Verghese

Strength of Epoxy/Glass Interfaces after Hygrothermal Aging

ABSTRACT The stability of epoxy/glass interfaces subjected to hygrothermal aging was assessed using a fracture-mechanics approach. An epoxy system consisting of diglycidyl ether of bisphenol F cured with 2-ethyl-4-methyl-imidazole was bonded to borosilicate glass adherends that were treated with various types of adhesion promoters to provide a variety of interfaces. Adhesive strength was measured under dry, as-processed conditions and as a function of exposure time to an 85°C/85% relative humidity (RH) environment. As expected, the strain-energy-release rate, G c , dropped significantly with aging time for the bare epoxy/glass interface. The drop in G c is assumed to be due to a loss of interfacial forces. The use of two silane-based adhesion promoters, 3-aminopropyltriethoxysilane (APS) and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane (ECH) resulted in improved adhesive strength both before and after hygrothermal aging. The improvement in adhesive strength can be explained by the introduction of chemical bonds at the interface. The drop in G c is assumed to be due to a loss of interfacial forces and hydrolysis of siloxane bonds. In addition to the use of organosilane-based adhesion promoters, a series of polyhydroxyaminoethers (PHAE) thermoplastic adhesive resins was also investigated as potential adhesion promoters. It was found that 2% PHAE in Dowanol® PM, a hydroxyl-group-containing solvent, was the best system for the PHAE-based adhesion promoters. Interestingly, both the acetic acid concentration in the solvent and maleic anhydride content in the PHAE resin were shown to affect the adhesive strength.

An inverse approach to characterize anisotropic thermal conductivities of a dry fibrous preform composite

Fiber-reinforced composites play an important role in enabling the industrial composites industry including automotive, pressure vessels etc. to meet ever increasingly aggressive fuel efficiency requirements. Modeling and analysis is often employed in composite development as a tool to achieve faster and more cost-effective solutions to both product and process designs. Thermal conductivity measurements play a critical role in enhancing the fidelity of these models, especially in scenarios involving non-isothermal processing. Accurate characterizations of anisotropic thermal conductivities that are typical of dry fibrous preforms are especially challenging because of their highly porous and complex structures. In this article, an inverse approach is developed to estimate the anisotropic thermal conductivities of a dry preform made of biaxial [0°/90°] stitched glass-fiber mats based on the measurements of thermal conductivities of cured epoxy-matrix composites. This method is found to not only yield more accurate results than direct preform measurements but also provide the capability to characterize preforms in multiple directions rather than only through-thickness direction. The estimated thermal conductivities are used in preform heating simulation that is validated against experiments.

Estimation of in-plane elastic properties of stitch-bonded, non-crimp fabric composites for engineering applications

Stitch-bonded, non-crimped fabric composites are among the most common forms of structural fiber-reinforced polymer composites. A complex three-dimensional finite element model is usually required for the accurate prediction of the mechanical properties of these composites. The objective of this work was to develop a model to predict the in-plane elastic properties of non-crimped fabric composites (without structural stitching) with no inputs from the experimental characterization of the composites themselves. The motivation for this work was to develop a swift, accurate methodology that would be very beneficial with ever reducing design cycle times (for screening different fabrics) and as these fabrics find new application areas. The modeling approach used only the properties of the dry non-crimped fabric and the resin as inputs. Models were constructed to account for the geometrical aspects of the non-crimped fabric such as yarn width and yarn spacing, which depend on the stitching pattern employed. They included regions of pure matrix between fiber tows as well as between fabrics parallel to the stacking plane. The stitching threads, voids, crimping of the fiber tows and damage or disturbances to the fiber due to the stitching were not modeled. The effective elastic properties of uni-directional, bi-axial and tri-axial non-crimped fabric composites are computed using classical composite lamination theory and finite element analysis. The predicted results are found to be in good agreement with experimental results obtained using composites fabricated by vacuum-assisted resin transfer molding.

PCB drillability: a material science approach to resin development

In the manufacturing process for making printed circuit boards (PCB) it is necessary to drill holes in the base copper clad laminate. This is a crucial step in the case of multi‐layer boards where the holes must be plated with copper to complete the electrical connection between the layers. Drilling is an expensive process as it requires the use of extremely sophisticated equipment. Most often this resides with a handful of companies; namely board shops and drill bit manufacturers. In recent years, with the evolution of high performance resins such as high glass transition (Tg) and decomposition temperature (Td) as well as low dielectric constant (Dk) and the continued embracement of the use of phenolic cured resins compared to dicyandiamide (DICY), issues around drillability have increased. As a part of our efforts we compared the mechanical and thermo‐mechanical properties of three resins. Actual drilling studies were performed on three‐high, copper clad stacks made from these three resins at Megatool in California, in order to confirm our fundamental property correlations. Resin toughness was found to play a crucial role in the final PCB drillability.