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

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 were both significantly higher than that for moisture diffusion. Consequently, it was concluded that corrosion of the electroplated zinc layer was responsible for bond strength loss. Formation of corrosion product in the bond was not, therefore, a post-failure phenomenon as was the case for cold rolled steel.     

A Comparison of Linear with Nonlinear Viscoelastic Solutions for Shear Stress Concentration in Double Lap Joints

The correspondence principle based on the Maxwell model and a nonlinear viscoelastic solution involving an iterative scheme are used to describe the time dependent variation of the adhesive maximum shear stress in adhesively bonded double lap joints. The results indicate that if the correspondence principle is applied, the use of Maxwell chain is necessary to approximate the continuous change in the relaxation time and to coincide with the results calculated using the nonlinear viscoelastic theory.     

The Microstructure of Solvent-Welding of PMMA

The microstructure of solvent-welding of PMMA has been investigated. The morphologies of shear, tensile, tear and cleavage fracture surfaces are observed. Effects of solubility parameter, elevated temperature, and cosolvent on the mechanical strength are studied. The solvents used are methanol, ethanol and DMF. The feature sizes of shear, tensile and tear fracture surfaces and the angle of the cleavage curve at the interface of PMMA can be employed to determine the mechanical strength qualitatively. Constituents of shear, tensile and tear fracture surfaces are tongues, long and narrow hackles, and equiaxed hackles, respectively. PMMA treated at high temperature showed enhanced mechanical strength. PMMA treated with 95% by volume ethanol yielded the best quality of adhesion. The solvent affected zone and the residual solvent are also discussed.     

A Method for the Stress Analysis of Lap Joints

A theory is presented for the adhesive stresses in single and double lap joints under tensile loading, while subjected to thermal stress. The formulation includes the effects of bending, shearing, stretching and hygrothermal deformation in both the adherend and adhesive. All boundary conditions, including shear stress free surfaces, are satisfied. The method is general and therefore applicable to a range of material properties and joint configurations including metal-to-metal, metal-to-CFRP or CFRP-to-CFRP. The solution is numerical and is based on an equilibrium finite element approach. Through the use of an iterative procedure, the solution has been extended to cater for non-linear adhesive materials.     

Viscoelastic and Processing Effects on the Fiber-Matrix Interphase Strength. Part I. The Effects of Loading Rate,Test Temperature,Fiber Sizing and Global Strain Level

The effects of loading rate, fiber sizing, test temperature and global strain level on the adhesion strength between carbon fibers and a thermosetting epoxy (Epon 815) are studied using the single fiber fragmentation test procedure. Analytical methodology describing the viscoelastic behavior observed is also presented. The possibility of rate-temperature-interphase thickness superposition for the interfacial strength function is illustrated based on the analytical models discussed. Experimental data are discussed using Weibull statistics and also presented in the form of percent relative frequency histograms for the fiber fragments in a collective fashion. The use of histograms allows for interpretation of the skewness in the data population.     

Off-Optimum Cure of Aerospace Epoxy Adhesives

Four epoxy film adhesives used in aircraft manufacture and repair have been examined to establish the effect of deviation from the cure cycle specified by the manufacturer. In addition to the variation of the cure cycle, two surface preparations of the aluminium adherends (chromic acid etch or grit blast followed by silane treatment) were evaluated. Thermal analysis was used to examine the cure envelope of the adhesive, and its extent of cure and glass transition temperature. The adhesive properties were assessed by shear strength (in both single lap joints and in Iosipescu configuration), durability (Boeing wedge test) and chemical resistance to selected aggressive fluids. The sensitivity of the performance of a particular adhesive to offoptimum cure conditions depends on its composition and needs to be determined, not predicted.     

Miscibility and PSA Performance of Acrylic Copolymer and Tackifier Resin Systems

The influence of miscibility of an acrylic PSA and several tackifier resin systems upon PSA performance was investigated. When the acrylic copolymer and the resins were blended in various proportions, three types of mixing state were found: miscible system, partially miscible system and immiscible system. In the case of miscible systems, PSA performance (tack, peel strength and shear resistance) depended upon the viscoelastic properties of the PSA. In the case of completely immiscible systems, the above PSA performance depended primarily upon the viscoelastic properties of a continuous matrix phase, and the separated resin phase acted as a kind of filler. In the case of partially miscible systems, the PSA performance changed discontinuously at the resin concentration where phase separation occurred. It suggests that the phase structure of a PSA greatly influences the PSA's performance.     

The Improvement of Bond Strength Properties and Surface Characteristics of Resinous Woods

Apitong (Dipterocarpus spp.) and Caribbean pine (Pinus caribaea Morelet) contain high amounts of extractives that contribute to poor bonding. To reduce, if not to eliminate, the effects of these extraneous substances, surfaces of small wood blocks were Soxhlet-extracted for 8 hours by different solvents. Wettability of the wood surfaces was then measured by droplet and dynamic methods using water and dilute NaOH as liquids. Tensile shear strengths of extracted wood bonded with aqueous vinyl polymer isocyanate (API), resorcinol formaldehyde (RF) and polyvinyl acetate (PVAc) resin adhesives were also measured. Results revealed that although Caribbean pine had much higher resin content than Apitong, the former had better wettability than the latter. Solvent extraction of the adherend with either hexane or ethanol-benzene (1:2) for 8 hours was not enough to improve its wettability but enough to improve its gluability. However, successive extraction with hexane, methanol and ethanol benzene rendered the wood satisfactorily wettable. Generally, a direct relationship between wettability and bond strength could not be observed. In a separate experiment to improve bonding strengths, test specimens were either overheated or autoclaved for 4 minutes at 125°C during the pressing period. Autoclave treatment was found to be useful in increasing the bond strengths of API, RF, PVAc and urea formaldehyde (UF)-bonded Apitong and Caribbean pine.     

The Effect of Adhesion on the Rheological and Frictional Behavior of a Confined Lubricant Film

We investigated the rheological and frictional behavior of a model system of lubricated, atomically-smooth, solid surfaces at zero and negative external normal load. The measurements were performed with a surface forces apparatus modified for oscillatory shear. For low deflection amplitudes, and negative loads up to the point when the surfaces jumped apart, the confined liquid layer (0.7 ± 0.2 nm perfluorinated heptaglyme) showed a highly elastic behavior independent of load. In the sliding regime at large amplitudes, the behavior was mostly dissipative but also independent of normal load. The force necessary to separate the surfaces was not affected by any sliding conditions. However, the friction force showed a very pronounced decrease as a consequence of sliding at large amplitudes. Thus, for our system, friction and adhesion are decoupled. We propose a mechanism of in-plane rearrangements of the molecules and explain the shear-induced reduction of friction by the formation of shear-bands.     

Surface force arising from adsorbed graphene oxide in alumina suspensions with different shape and size

The effects of graphene oxide (GO) on the yield stress‐pH of α‐Al₂O₃ (alumina) suspensions were investigated. For micron‐sized platelet alumina suspensions, micron‐sized GO additive increased the maximum yield stress by as much as six‐folds. This was attributed to GO‐mediated bridging interactions between the platelet particles. This type of bridging interactions was much less effective with submicron‐sized, spherical, and irregular shape alumina. Adsorption of the anionic GO reflected by the shift of pH of zero zeta potential to a lower pH is particularly high for platelet alumina. The 1.0 dwb % GO concentration added is sufficient to reinforce each platelet particle–particle bond, assisted by a directed GO–platelet interaction configuration. This is, however, not true with submicron‐sized particles as the particle concentration increases sharply with the inverse of the particle diameter to power of 3. Moreover, a GO sheet can adsorb several submicron‐sized particles and this does not produce the right interaction configuration. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3633–3641, 2013