Willard D. Bascom

The adhesion of ice to lubricated surfaces

Abstract The adhesion of ice to lubricated steel has been shown to vary over a wide range depending upon the polar organic additive present in the lubricating oil. The adhesion is least for systems showing contact angles of 170° or larger through water drops on steel submerged in the oil solutions. The extremely low adhesions sometimes observed in such systems arise because water does not readily displace the final thin film of bulk oil from the metal surface to establish true water/solid contact. High water contact angles are associated with strong adsorption of the polar additive at the oil/metal interface and with relatively low adsorption in the oil/water interface.

Effect of temperature on the adhesive fracture behavior of an elastomer-epoxy resin

Abstract The bulk and adhesive fracture behavior of a diglycidyl ether bisphenol-A epoxy modified with 15% carboxy-terminated butadiene acrylonitrile was determined as a function of temperature. The bulk fracture toughness increased sharply near the resin Tg in a manner similar to unmodified epoxy resins. The adhesive fracture energy exhibited a maximum with respect to bond thickness and this maximum broadened and shifted to larger bond thicknesses with increasing temperature. These results are discussed in terms of the size and stress condition of the crack tip deformation zone.

The fracture of an epoxy polymer containing elastomeric modifiers

The fracture energies of elastomer-modified epoxy polymers have been determined over a range of strain rates from 10−2 to 103 sec−1. The modifiers included a liquid carboxyterminated butadiene acrylonitrile and a solid rubber. They were used alone and also in combination. In all cases, the modifiers increased the toughness of the base resin by orders of magnitude and one combination of liquid and solid rubber increased toughness by 60 times. There was a general decrease in fracture energy with increasing strain rate but even during impact testing the modified epoxys were 10 to 20 times tougher than the base polymer. Scanning electron microscopy revealed that, when combined with the liquid rubber, the solid rubber induced a localized shear yielding.

Water at the Interface

ABSTRACT The subject of moisture-induced failure in glass-resin composite materials is reviewed. Emphasis is given to the fundamental surface chemistry and fracture mechanics involved. These disciplines offer substantial information about water adsorption on polar solids, including glass, and about the fracture of solids. However, this information is shown to be inadequate in describing the mechanisms involved in the failure of glass-resin materials. Some insight into these mechanisms is possible from various studies of the moisture-induced failure of bulk glass and adhesive/adherend systems. Also, some light is shed on moisture-induced failure by considering the empirical methods used to improve composite wet-strength, especially the use of silane adhesion promoters. Recent research on the silanes suggests that their action may involve a modification of the resin in the vicinity of the glass/resin interface.

Ice Adhesion to Hydrophilic and Hydrophobic Surfaces

Abstract The adhesional shear strength has been determined for ice formed against polished steel, monolayers adsorbed on steel and thin plastic coatings painted on metal surfaces. The adsorbed monolayers reduced the shear strength to about one-third of that for ice on clean steel. The monolayers also had the effect of changing the character of the breaks from clearly cohesional to apparently adhesional failure. The shear strength from the plastic coatings ranged from values equal to that of ice against clean steel to values 70 to 80% lower. The reduction in shear strength did not correlate with the water contact angle on the coatings but was usually found to be due either to air entrapment at the ice/coating interface or to cohesive failure of the coating itself. The ice separated from the various substrates was examined microscopically by forming plastic replicas of the ice surface. These studies helped determine the mechanism of failure and, since one of the replicating solutions was also an ice etchant...

Apparent interfacial failure in mixed-mode adhesive fracture

Aluminium-epoxy adhesive specimens constructed with the bond at 45‡ to the direction of loading appear to fail very close to the interface. The actual locus of failure was investigated by14C labelling of the epoxy polymer and also by Auger spectroscopy profile analysis. Both techniques indicated a residual film of polymer a few hundred angstroms thick on the aluminium surface. The fracture energy of these specimens was determined and found to be affected by the surface roughness of the aluminium. The mixed-mode fracture energy (GI,II) C45° of these specimens in the absence of any surface roughness effect (polished surfaces) was 140 J m−2 compared to 136 J m−2 for the same polymer in simple opening-modeGI C adhesive fracture. The “interfacial” failure and the effect of surface finish on fracture are discussed in terms of the applied stress directing the failure toward the interface but the approach of the crack to the boundary being limited by the size of the crack tip deformation zone.

The anomalous lowering of the glass transition of an epoxy resin by plasticization with water

The validity of the assertion that 6 to 7 wt% absorbed water lowers the glass transition,Tg, (~250° C) of a highly cross-linked, high temperature epoxy resin (NARMCO 5208) by 100 to 150° C was investigated by a number of thermoanalytical techniques. This plasticization is of significance to the use of organic matrix composition skins on supersonic aircraft which experience a sudden “thermal spike”, and supposedly a sudden loss in modulus. The study was complicated by the loss of water above 100° C during the tests and the decomposition of the polymer at about the temperature of its apparent glass transition. No glass transition could be observed by differential scanning calorimetry and the thermal mechanical analyser gave inconclusive results. The dynamic mechanical analyser gave a clear indication of a glass-like transition at about 250° C which was reversible, reappearing upon cooling and reheating. This transition could not be related to the decomposition of the sample. Absorption of 7 wt% water broadened and lowered the transition by about 50° C. However, after studying the rate of change of the modulus of the resin from the plasticized state to the dry state it was concluded that the absorbed water lowersTg by only 50° C and not the 100 to 150° C claimed by others. Moreover, the mechanism of this apparent glass transition differs from that of a normalTg, and may involve the breaking of hydrogen bonds.

Air Entrapment in the Use of Structural Adhesive Films

Abstract Air entrapment during the preparation of bonds with structural adhesive films was examined by a microscopy study of glass/adhesive/glass specimens. It was observed that initially a thin film of air is trapped between the adhesive and adherend and covers over 50 % of the interfacial area. As the specimen is heat cured this air draws up into bubbles that are eventually displaced into the adhesive layer. Incomplete displacement occurs if the resin does not fully wet the adherend surface (contact angle greater than zero) or the resin does not become sufficiently fluid during the heat cure. The trapped air, whether displaced into the resin or held at the interface, could be eliminated from the bond by starting the cure in vacuum (5 mm Hg) and subsequently releasing the vacuum at the temperature at which the resin is in its most fluid condition. Specimens of bonded aluminum panels were tested in a T-peel configuration. An increase in bond strength of as much as 30% could be realized by complete void re...

Effect of bond angle on mixed-mode adhesive fracture

A maximum in mixed-mode adhesive fracture energy has been observed at bond angles of 45° using scarf-joint test specimens. It is shown here that by reducing the adherend surface roughness from 1.2μm CLA roughness (milled surfaces) to 0.08μm CLA roughness (polished surfaces) the fracture energy becomes a linear function of bond angle (no maximum at 45°) and there is an overall crease in fracture energy at all bond angles. These results are discussed in terms of crack initiation being “focused” into the interfacial region and a pinning of crack-tip shear displacements by the surface roughness of the milled adherends which does not occur for the polished adherends.

The distribution of acetic acid between solvent and soap micelles in benzene solutions

Abstract The distribution of acetic acid between benzene and micelles of sodium dinonylnaphthalene sulfonate, magnesium dinonylnaphthalene sulfonate, and magnesium phenylstearate was examined by measurements of the vapor composition in equilibrium with such systems. In each case the initial increments are more firmly bound than later portions, the amount of strongly bound acid ranging from one to two molecules per cation present. The magnesium salts bind the acid more strongly than the sodium compound. The partition coefficient for acid at higher concentrations is one or two orders of magnitude smaller than that for the initial increments. Its magnitude depends on the nature of the anion in the micelle. Water was found to reduce the initial acid-binding capacity of the magnesium salts. The data indicate that the coordinating properties of the cation are a major factor in the initial firm binding of acetic acid by oil-soluble soap micelles, but that significant amounts of acid are more loosely held by forces which may be attributed to the organic anion present.