Douglas H. Berry

Effect of Surface Morphology on Crack Growth at a Sol-Gel Reinforced Epoxy/Aluminum Interface

The Boeing sol-gel conversion coating (Boegel-EPII), derived from an acid-catalyzed aqueous solution of organofunctional silane and zirconium alkoxide precursors, is being used as an adhesion promoter for adhesive bonding and painting applications in the aerospace industry. A unique advantage of the sol-gel process is that strong and durable bonds are produced without the hazardous chemical usage and rinse-water requirements of conventional anodizing or etching processes. In this study, a fracture mechanics method was used to investigate the adhesion properties of sol-gel-reinforced epoxy/aluminum joints. The Hugh Brown asymmetric double cantilever beam (ADCB) wedge test was employed, which allowed the measurements of the critical energy-release rate, subcritical crack-growth kinetics, and threshold energy-release rate on a single sample in a reasonably short period of time. These experiments were carried out with aluminum substrates on which the surface morphology was systematically varied by polishing, sanding, grit-blasting, and chemical etching. X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to identify the locus of failure. The surface morphology of the substrates was characterized with SEM, optical profilometry, and spreading kinetics. The macrorough structures drive the crack to within a thin epoxy layer close to the polymer/metal interface, which enhances the initial strength of the sol-gel-reinforced interface. The microroughness of the substrate is, however, more effective than the macroroughness in enhancing the durability. Lastly, an attempt has been made to correlate the energy-release rate with the fractal dimension for sol-gel-reinforced joints with macrorough substrates.

Fracture Toughness of a Silane Coupled Polymer-Metal Interface: Silane Concentration Effects

ABSTRACT Fracture toughness of joints made from a glassy, 343,000 molecular weight polystyrene block bonded to chromic-sulfuric acid etched or phosphoric acid anodized aluminum are investigated. The fracture tests are performed with a 90-degree peel apparatus under “dry” laboratory conditions and “wet” conditions created by submerging the apparatus in a temperature controlled water bath. The bond strengths are controlled using various concentrations of styrl silane coupling agent added directly into the styrene monomer solution that polymerizes against the aluminum. Ellipsometric measurements on smooth silicon surfaces verify that the thickness of bound polymer is controlled by the silane to polystyrene mole ratio. X-ray photoelectron spectroscopy (XPS) analysis of fractured surfaces indicates that the fracture is near the aluminum surface. Both the wet and dry fracture energy as a function of bound polymer thickness on acid etched aluminum joints resemble quite closely the adhesion literature results obtained by fracturing pairs of fused, immiscible glassy polymers. Reasons for this similarity are discussed.

An improved accelerated weathering protocol to anticipate Florida exposure behavior of coatings

A new accelerated weathering protocol has been developed which closely replicates the performance of automotive and aerospace coating systems exposed in South Florida. IR spectroscopy was used to verify that the chemical composition changes that occurred during accelerated weathering in devices with a glass filter that produced a high fidelity reproduction of sunlight’s UV spectrum matched those that occurred during natural weathering. Gravimetric water absorption measurements were used to tune the volume of water absorption during accelerated weathering to match that which occurred during natural weathering in South Florida. The frequency of water exposure was then scaled to the appropriate UV dose. A variety of coating systems were used to verify the correlation between the physical failures observed in the accelerated weathering protocol and natural weathering in South Florida. The new accelerated weathering protocol correctly reproduced gloss loss, delamination, cracking, blistering, and good performance in a variety of diverse coating systems. For automotive basecoat/clearcoat paint systems, the new weathering protocol shows significant acceleration over both Florida and previous accelerated weathering tests. For monocoat aerospace systems, the new weathering protocol showed less acceleration than for automotive coatings, but was still an improvement over previous accelerated tests and was faster than Florida exposure.

A kinetic approach to study the hydrolytic stability of polymer–metal adhesion

A polystyrene-aluminum joint, the adhesion of which was promoted by a silane coupling agent, was examined by fracturing the interface under water at different loads and temperatures. X-ray photoelectron spectroscopy (XPS) analysis of the fractured surfaces showed that the locus of failure was mainly interfacial at low loads, but it gradually moves away from the interface at higher loads. This movement of the failure locus reflects a transition of the mechanism of interfacial debonding from the hydrolysis of the siloxane bonds to the cleavage of the main polymer chains. A rate-dependent bond failure model qualitatively describes the above process, in which the activation energy of bond dissociation is assumed to be a time-dependent parameter.

Effect of Processing Conditions on Adhesion Performance of a Sol-Gel Reinforced Epoxy/Aluminum Interface

The Boeing sol–gel process (Boegel-EPII) is a surface preparation method for metallic substrates for adhesive bonding and painting applications. This paper describes an investigation into the effect of processing conditions on adhesion strength and durability of a sol–gel reinforced, rubber toughened epoxy/aluminum joint. Using an asymmetric double cantilever beam (ADCB) wedge test, the adhesion performance of the sol–gel reinforced epoxy/aluminum joint in a humid environment was measured as a function of sol–gel processing conditions. The sol–gel drying time, concentration and drying humidity all have an effect on adhesion performance. Prolonged drying led to a decrease in fracture energies. The critical and threshold fracture energies show different trends as sol–gel concentration varies, and better adhesion performance was observed for sol–gel dried at higher humidity compared to lower humidity. The failure modes and mechanisms were studied by XPS and SEM. Analysis of locus of failure revealed that the observed trends for adhesion performance can be explained in terms of interdiffusion of the sol–gel film and epoxy. The diffusion of the epoxy into the sol–gel layer is hypothesized to strongly depend on the degree of condensation of the sol–gel film and is directly affected by the sol–gel processing conditions.

Spectral Power Distributions in Accelerated and Natural Weathering Tests and Their Impact on Aerospace Coating Service Life Prediction

Exterior commercial airplane coating systems serve both decorative and protective functions and must retain gloss and color properties as well as provide corrosion and fluid resistance in severe service environments. Commercial aircraft can make up to eight flights per day and cruise at altitudes as high as 13 km for up to 16 h/day. Ultraviolet (UV) radiation exposure can increase by a factor of four and paint surface temperature can range from as high as 75°C down to −60°C between ground and cruise altitudes. Additionally, moisture levels may vary from 100% relative humidity and rain on the ground to completely dry conditions at cruise. At a minimum, the effects of the changes in UV radiation, temperature, and moisture on photooxidation and hydrolysis must be considered and balanced in accelerated test method development, damage mechanism understanding, modeling for service life prediction of these coating systems, and design of future generation coatings. This chapter presents recent investigations aimed at achieving these goals. The first part compares the sensitivity of damage from overlap of polymeric absorption peaks with UV spectral radiance measured at ground level in Southern Florida, predicted at airplane cruise altitudes of 9.1–12.2 km, and produced by xenon arc lamps with various filters (Extended UV and Daylight specified in SAE J2527 and special daylight specified in ASTM D7869). The second part of the chapter assesses aerospace coatings using exposures in Southern Florida, SAE J2527 with daylight filters, and ASTM D7869 and compares these results to expectation based on in service data.