Robert T. Foister

Structure/property relationships for a series of crosslinked aromatic/aliphatic epoxy mixtures

Nine thermoset networks consisting of an aromatic epoxy novolac and an aliphatic epoxy, crosslinked with an imidazole catalyst, were investigated with the aim of correlating macroscopic behaviour and structure as network composition varied. The networks had properties ranging from the two extremes of the brittle, highly crosslinked aromatic, to the rubbery, less crosslinked aliphatic epoxy. The relationships between crosslink density, a measure of network microstructure, and both primary (Tg) and secondary (Tβ) thermal transitions were investigated. These properties were also correlated with tensile toughness (stress/strain behaviour), strength and modulus. It was found that Tg varied linearly with crosslink density whereas Tβ depended on the chemical composition of the network. Although no dependence of the room temperature modulus on crosslink density was found, both tensile strength and toughness were shown to change with crosslink density. Tensile toughness also exhibited a dependence on the temperature and frequency of the β-transition.

The kinetics of displacement wetting in liquid/liquid/solid systems

Abstract Spontaneous displacement wetting, where a drop of one liquid displaces a second immiscible liquid from a smooth solid surface, has been investigated experimentally. Relationships between dynamic advancing contact angle and displacement velocity were established for systems of widely ranging viscosity ratio. The experiments were interpreted by extending a previously established correlation for liquid/vapor systems, as well as via a comparison with the hydrodynamic theory of Cox (J. Fluid Mech. 168, 169 (1986)). To compare experimental data with the hydrodynamic theory, a parameter which was unknown a priori—the ratio of slip length to macroscopic length scale—had to be estimated. Consequently, the relationships between displacement kinetics and material properties of the liquids (viscosities, interfacial tension) and the solid (equilibrium contact angle) were shown to depend on the microscopic dynamics, which were in turn characterized by a degree of uncertainty. Nonetheless, by estimating the ratios of slip lengths to macroscopic length scales, it was possible to obtain a reasonable theoretical description for experimental systems of widely ranging viscosity ratio.

Strength Loss Mechanisms for Adhesive Bonds to Electroplated Zinc and Cold Rolled Steel Substrates Subjected to Moist Environments

Abstract Mechanisms of strength toss which affect the durability of epoxy adhesive bonds in moist environments were investigated for electroplated zinc and cold rolled steel substrates. Activation energies for adhesion loss, formation of corrosion product on the substrate surface, and moisture diffusion in the adhesive were determined experimentally. For cold rolled steel substrates, the activation energy for adhesion loss was identical, within experimental error, to the measured activation energy for moisture diffusion in the adhesive. Both of these values were substantially less (=40%) than the activation energy for formation of corrosion product. This confirms the previous results of Gledhill and Kinloch (J. Adhesion 6, 315 (1974)), who attributed strength loss to thermodynamic instability of the adhesive/substrate interface due to the presence of moisture. In contrast, for electroplated zinc substrates, activation energies for adhesion loss and corrosion product formation were essentially equal, and w...

Adhesive Bonding to Galvanized Steel: II. Substrate Chemistry, Morphology and Bond Failure Analysis

Abstract Galvanized substrate morphology, oxide layer chemistry, bond failure modes, failure loci, and bondline corrosion have been investigated for adhesive bonds to galvanized steel. Significant differences in surface morphology were observed between the relatively smooth surfaces of “hot-dipped” substrates and the considerably rougher texture of “electroplated” substrates. The hot-dipped substrates were also chemically heterogeneous, with significant amounts of Al, Mg, Ca, and Pb, in addition to Zn, constituting the surface layer. For electroplated substrates, on the other hand, Zn was the major constituent. It was concluded that, for a given adhesive, low strengths and poor bond durability generally correlated with the minimum surface roughness and maximum chemical heterogeneity of the hot-dipped substrates. Higher strengths, and better durability, on the other hand, were observed for electroplated substrates, which showed the greater roughness, as well as chemically the more uniform surface. Signific...

Three-phase boundary expansion in thin liquid films

Abstract Early stages in the spontaneous displacement of one liquid from a smooth solid surface by another immiscible liquid, arising in drop coalescence with a planar solid surface, have been investigated experimentally. The kinetics of three-phase boundary (TPB) motion in thin liquid films has been analyzed by relating TPB velocity ( U ) to the interfacial tension ( γ 12 ) between the displacing (1) and displaced (2) liquids, the bulk viscosities ( μ 1 , μ 2 ), and the viscosity ratio ( λ = μ 2 μ 1 ). Using data on systems where TPB expansion occurred from a single isolated rupture point and whose viscosity ratios varied over six orders of magnitude, it was found that U ∼ μ 1 −0.45 μ 2 −0.55 γ 12 × exp [ −8.1r R 0 ] , where r is the instantaneous radius of the TPB, and R 0 is the initial radius of drop 1, the displacing phase. Rupture of the liquid 2 film coupled with subsequent TPB motion was also studied to delineate mechanisms responsible for trapping of residual droplets of the displaced phase. Two modes of trapping were observed. “Single thread” trapping arose when two expanding TPB regions converged along a line, while a second mode resulted from simultaneous rupture of the liquid 2 film in a circular pattern about the “dimple” formed at the base of the liquid 1 drop. Finally, in some cases film rupture and TPB motion were preceded by the formation of a metastable “black” film. Consequently, expansion after rupture was characterized by a moving thin film region just ahead of the TPB.

Stress Analysis of Adhesively Bonded Electroprimed Steel Lap Shear Joints

Abstract Structural applications for adhesive bonding have been increasing in recent years due to improvements in the types of adhesives available and in improved knowledge of bonding procedures. Consequently, there exists a demand for precise numerical modeling of adhesive joint behavior, particularly along bondline interfaces where low surface energy adhesives contact high surface energy metallic oxides. The purpose of the present study is to determine the effect of electrodeposited organic paint primer (ELPO) on the stress and strain distributions within an adhesively bonded single-lap-shear joint. Initial experimental studies have shown that bonding to ELPO-primed steel adherends has enhanced strength and durability characteristics compared to conventional bonds to unprimed steel surfaces. Recent studies based on finite element analysis of varied single-lap-shear joint moduli and thicknesses, and subsequent testing of joints with two different adhesive moduli, have indicated the mechanisms involved in...

Dynamic surface properties due to amine migration and chemical reaction in primary amine/epoxide systems

Abstract We have studied the dynamic surface properties of four different primary amines in a low-molecular-weight epoxide resin by measuring dynamic spreading pressures. These spreading pressures directly manifest surface activity of the primary amines, which include three members of the dialkylaminopropylamine family (dimethyl, diethyl, and dibutyl), and the coupling agent γ-aminopropyltrimethoxysilane. Each of these systems shows a characteristic maximum in spreading pressure with time. Details of the transport processes (diffusion, chemical reaction, and adsorption) responsible for this time-dependence are illustrated in the context of a previously developed theory. The three dialkylaminopropylamines, for example, clearly show the effect of the reaction rate/diffusion rate ratio, or Damkohler number, on the dynamic spreading pressure. Likewise, by varying the bulk concentration of γ-aminopropyltrimethoxysilane, we have been able to demonstrate that saturation of the solution/vapor interface occurs for a bulk concentration of roughly 0.5 mass%. Further experiments using related silane compounds also suggest that the primary amine moiety is preferentially expelled from the solution phase, and is therefore responsible, in large part, for the observed surface activities.

The influence of diffusion and chemical reaction on adsorption kinetics in binary surfactant systems

Abstract A macroscopic transport theory of adsorption kinetics in binary surfactant systems has been developed, taking into account the influence of diffusion as well as bulk and surface chemical reaction. The chemical kinetics involve reaction of a surfactant (solute 1) with either the host liquid or another species present in large excess to form a second surfactant (solute 2), which is also reactive. When the chemical rate equations are “pseudo-first-order” in bulk and surface concentration of surfactant, and there are no energy barriers to adsorption, the transport equations describing subsurface dynamics can be solved analytically. However, when we assume a binary Langmuir-type adsorption isotherm relating surface and subsurface concentrations, we generate a set of coupled, non-linear integral equations which must be solved numerically. These solutions show that, because diffusion increases the surface concentration while chemical reaction ultimately decreases the total amount of surfactant in the system, the surface concentration exhibits a maximum value with time. The position of the maximum, the value of the surface concentration at maximum, as well as the shape of the surface concentration/time curve, are all shown to be sensitive, in varying degrees, to changes in diffusion, chemical reaction, and adsorption parameters. For example, increases in the dimensionless Damkohler number, which is a ratio of chemical reaction to diffusion rates, decrease both the time to maximum, as well as the surface concentration at maximum. The applicability and relevance of this work to situations of technological interest, such as the migration of reactive adhesion promoting species in polymeric resins, is also briefly discussed.

Structural Adhesive Bonds to Primers Electrodeposited on Steel

Abstract Adhesive bond strength and durability were investigated for steel substrates which had been cathodically electroprimed before bonding. Lap shear and torsional impact strengths of two model epoxy adhesives were evaluated. Very poor strengths and durability were found for one adhesive, which was cured with a mixture of three amine curing agents. Scanning electron microscopy and analysis of primer susceptibility to interaction with the curing agents suggested that, for the high concentrations of curing agent in the amine-cured adhesive, chemical and physical degradation of the primer occurred during cure at elevated temperature. For the second adhesive, which was cured with a single imidazole catalyst, excellent strength and durability were obtained, with no evidence of primer degradation. Surprisingly, for this adhesive, strengths to primed steel were up to 88% higher than to cleaned (i.e., degreased) bare steel. The concurrent improvements in environmental durability over bare steel, as assessed b...