Wetting Dynamics of an Oil-accommodating Adhesive

Oil-accommodating adhesives are widely used industrially to avoid the necessity of degreasing metals before bonding. We have undertaken a study of the behaviour of drops of an uncured model epoxy adhesive on both degreased and oiled, galvanized, steel sheet during temperature increase. Contact angle, θ, vs. time, t, behaviour is significantly different for the two cases. On a degreased substrate, wetting rate initially increases with temperature, therefore time, due to decreasing viscosity, and then becomes reduced as equilibrium is approached. For oiled substrates, there is a central pseudo-plateau in the θ vs. t curve, corresponding to a modified capillary balance, itself caused by significant absorption of the oil by the adhesive. This behaviour is modelled semi-quantitatively. In the case of spreading on a degreased substrate, a simplified model is proposed to explain dynamic effects related to reduced viscosity during temperature rise.     

New and Improved Tests for Adhesion

Three new methods are discussed for measuring the work G_a, required to detach unit area of an adhering material from a substrate. The first is a simple modification of the Outwater double-torsion test for long rectangular plates, bonded together. This method is suitable for evaluating aluminum-epoxy bonds, for example, or the transverse strength of fibrous composites. The second is a pull-off test for long strips adhering to a rigid surface. It seems suitable for adhesive tapes and laminates. The third is a reconsideration of the “blister” test for films and coatings, in which a circular debond at the interface is made to grow by internal pressure. The relation obtained between pull-off force F for a strip, or blow-off pressure P for a layer, takes the unusual form:

F⁴ (or P⁴) ∞ KG³_a

where K is the tensile stiffness of the detaching layer. This dependence arises from the non-linear (cubic) relation between load or pressure and deflection in these configurations. Nevertheless, the product Fθ, where θ is the angle of detachment of a strip, or Py, where y is the height of a “blister”, give direct measures of the strength of adhesion G_a, independent of the stiffness of the adhering material and of the extent of detachment.     

A Fracture Mechanics Approach to Characterizing Cyclic Debonding of Varied Thickness Adhesive Joints to Electroprimed Steel Surfaces

Applications of adhesive bonding for automotive structures 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 design techniques to assess the influence of bondline thickness on adhesive joint strength. One design approach currently being used is based on limiting shear stresses in the adhesive while designing to eliminate peel stresses. Another design approach is based on fracture mechanics and accounts for shear and peel stresses and both static and fatigue modes of failure. The present study applies fracture mechanics to investigate the mixed-mode response of cracked-lap-shear (CLS) joints bonded with unprimed and electroprimed steel surfaces. Three bondline thicknesses equal to 0.254, 0.813, and 1.27 mm were evaluated for unprimed and primed bondlines. For the experimental portion of the study, debond growth rates (da/dN) were measured using a remote imaging system over a range of applied cyclic loads. Corresponding changes in the strain release rates (ΔG) were calculated, through finite element analyses, as a function of debond length and applied load level. The computations for ΔG applied a finite element formulation to determine both the peel component, ΔG_i, and the shear component, ΔG_ii. When computed ΔG values were plotted against the measured debond growth rates, da/dN, the results showed a power law relationship which characterizes the debond behavior of a given material system and bondline thickness.     

Semi-Structural Hot Melt Adhesives Based on Crosslinkable Functionalized Polyolefins

A structural or semi-structural adhesive is usually applied to the substrates as monomers, oligomers, or melts of polymers with reactive groups and is then polymerized or crosslinked in situ in the joint between the substrates. We have been studying a number of crosslinked functionalized polyolefins blended with tackifier used as semi-structural adhesives for bonding to oily galvanized steel surfaces. The functions of takifier, surface properties of adhesive and substrate, geometry effects of lap joints, adhesive T_β, chain end defects, network chain length, and cure kinetics of these systems will be discussed. Our experimental results indicate that lap shear strengths of galvanized steel joints depend on adhesive storage modulus to the power of roughly 1/2. A rough estimate of the fracture energy of the adhesive bond, G_a could be obtained from this relation. Although some estimated G_a values are too low while the others are too high, they seem to be in rough accord with the degree of interfacial bonding and the locus of failure of the lap shear bonds.     

The Relationship of Glass Transition Temperature to Adhesive Strength

The effects of glass transition temperature (T_g) on mechanical properties have been further demonstrated by the observation of adirect relationship between the T_g of an epoxy adhesive and its lap shear bond strength to metal at elevated test temperatures. An additive (coupling agent) which lowers the T_g from a point near or above the test temperature to below it causes a subsequent decrease in the strength of the system and generally increased cohesive failure. Therefore effects on the T_g of the adhesive are more important than on interfacial properties. The end result is that differential thermal analysis (DTA) can be utilized as a effective screening method for adhesives and additives, and can be a good indicator of maximum use temperature. From these data T_g can also be used to estimate adhesive strength at a given use temperature.     

Analysis of the thermal hysteresis in partially deuterated tryglicine selenate

Dielectric constant data on partially deuterated triglycine selenate (TGSe)_1-x, (DTGSe)_x for x = 0, 0.35, 0.80 show a steady increase of thermal hysteresis ΔT(x) with deuteration, in agreement with previous data by Gesi. In this work we correlate this observed thermal hysteresis with the parameter g(x) = (γ/β)N²μ² obtained from hysteresis loops data, in order to characterize the ferroelectric transition. For x = 0, the transition is close to a tricritical point (g ≌ 1/3) and we see that cooling very slowly through T_c under these quasitricritical conditions results in a very large residual contribution to the dielectric constant at the ferroelectric phase.     

Ferroelectric Pb5-xBix (Ge.Si)3-xBxO11 single crystals as a material for IR radiation detectors

The coupled substitution of Bi³⁺ for Pb₂₊ and B³⁺ for Ge⁴⁺ was successfully achieved in ferroelectric Pb₅Ge₃O₁₁. Large single crystals of optical quality were grown from the melt. Deterioration of the crystal quality could not be observed within a period of two years under ambient conditions. The dielectric permittivity ε_r, electric conductivity p and pyroelectric coefficient γ are investigated as a function of temperature. The room temperature data (εr = 43, γ = 2.4·10^-4Cm^-2 K^-1, p = 7·10⁷ Ω, volume specific heat s = 2·10⁶ Jm^-3K^-1) show the usefulness of Pb_5-xBi_xGe_3-xB_xO₁₁ around x = 0.1 as a material for pyroelectric thermal detectors operating at room temperature. Also the difficulty of producing lead germanate silicate mixed crystals in the size and quality needed for vidicon applications is overcome by additional replacement of some Pb and Ge by Bi and B.     

The Adhesive Fracture Energy of Bonded Thermoplastic Fibre-Composites

The present paper first discusses the problems that occur when thermoplastic-based fibre-composite materials are bonded using structural engineering adhesives, such as epoxy and acrylic adhesives. A double-cantilever beam joint has been employed and it is shown that the value of the adhesive fracture energy, G_c, is very low when a simple abrasion/solvent wipe pretreatment is used for the thermoplastic fibre-composites. This arises from crack growth occurring along the adhesive/composite interface, which is relatively weak when such a pretreatment is employed. Secondly, it is demonstrated how very effective a corona surface pretreatment may be for these materials. Indeed, when such a pretreatment is used, interfacial crack growth is no longer observed but the crack now propagates either cohesively in the adhesive or through the composite substrate; both failure modes lead to relatively high values of G_c, with the former resulting in the highest values of G_c being recorded. Finally, from measuring the fracture properties of the composites and combining these data with a detailed analysis of the stresses in the DCB joint, calculated using a finite element analysis, the reasons for these different loci of failure may be readily understood and predicted.     

Wetting Kinetics and the Strength of Adhesive Joints

The work of adhesion, depending solely on the contact angle and surface tension, is unreliable as a guide to the strength of a joint. The kinetics of joint formation, exemplified by the rate of wetting, is also important and can be measured by rate of change of contact angle, a process for which an empirical equation has previously been suggested. A proposed mechanism introduces a viscosity term into the differential form of this equation.

A series of metal lacquers was prepared from epoxy, U/F and M/F resins in various ratios and also epoxy/P/F and epoxy/acrylic mixtures. Stainless steel substrates, coated with these lacquers were then cemented together using polyethylene as an adhesive. It is shown that joint strength correlates better with a high wetting constant (γ/ηL) than with a low contact angle except where this is very low.     

The equivalent pressure of molecular hydrogen in cavities within metals in terms of the overpotential developed during the evolution of hydrogen

In traditional electrochemistry, the pressure in cavities in equilibrium with surfaces on which H₂ is being evolved is (f_H2)_int = exp (−2ηF/RT), (1) where η is the hydrogen overpotential, a negative quantity. The problem of the internal fugacity-f is examined in terms of electrochemical kinetics.

It is shown here that (1) is not generally applicable. Depending upon the mechanism of hydrogen evolution, the situation may be less embrittling, ie (f_H2)_int set up by a given η is less than given by (1). Relations analogous to (1) (different for each mechanism) are tabulated.

The values derived are fugacities and may be very high (10⁶ atm). They are related to the pressure of H₂ (the relevant quantity for embrittlement), but p_H2 < f_H2.     

A Survey of Rheological Properties of One-Component Epoxy Adhesives

Flow characteristics of seven commercially available one-component epoxy adhesive pastes were measured using a controlled shear stress rheometer and a controlled shear rate rheometer over a temperature range from 5°C to 60°C. Combining data obtained from both controlled rate and controlled stress experiments over a wide range of shear rates, we observed Newtonian flow (shear stress proportional to shear rate) at very low shear rates, a plateau “shear thinning” region at intermediate shear rates, and a second region of linear dependence of shear stress on shear rate at high shear rates. The adhesive pastes exhibited a very broad range of rheological behavior. Two flow parameters important to adhesive application technology, the plastic viscosity and the apparent yield stress, were measured for each adhesive. The plastic viscosity ranged from 11.6 to 329.5 Pa. s; the apparent yield stress ranged from 56.2 to 413 Pa. The temperature dependence of the rheological parameters of the epoxy adhesive pastes was also determined. The results are reported as the activation energies, E_η and E_σ, of plastic viscosity and apparent yield stress, respectively. The apparent yield stress of each adhesive paste was much less sensitive to changes in temperature than was the plastic viscosity. This suggests that the processing characteristics are likely to show qualitative as well as quantitative changes with temperature.     

INTERPRETATION OF MICROMIXING EFFECTS ON FAST CONSECUTIVE-COMPETING REACTIONS IN SEMI-BATCH STIRRED TANKS BY A SIMPLE INTERACTION MODEL

A model is proposed for interpreting micromixing experiments in a semi-batch reactor. In these experiments, a fast consecutive-competing reaction system is used A + B → R, R + B → S, B being added either dropwise or as a pulse into an excess of A. A segregation index X_s = 2n_s/n_B0 is measured after completion of the reaction for various locations of the injection point. The macroscopic flow pattern is assumed to be known, essentially characterized by the recirculation time t_c. Micromixing then takes place within the cloud via a mechanism of interaction with the mean environment (IEM model, micromixing time t_m). Experimental results published by Barthole et al. (precipitation of barium sulphate) and Bourne et al. (diazo coupling) are successfully interpreted by this model. The influence of stirring speed, injection volume, concentration of species and mode of injection (pulse or dropwise) are especially well accounted for. This model provides a simple method for predicting the influence of mixing on selectivity in semi-batch reactors.     

The hydrogen-electrode reaction in the isotopically mixed system—I. The relation between forward and backward unidirectional rates and evaluation of the stoichiometric number of the rate-determining step

A general consideration of the possible reaction routes of the hydrogen-electrode reaction leads to the classification of all the conceivable mechanisms into two groups according to the value of the stoichiometric number, ν, of the rate-determining step.

The previous treatment on the validity, in the isotopically mixed system, of the relation between the forward unidirectional rate of the over-all reaction, V_f, its backward rate, V_b, the overpotential, η, and ν; V_f/V_b = exp (− 2Fη/νRT), is re-investigated rigorously on the basis of general considerations. It is shown, in agreement with the previous conclusion, that this relation holds invariably irrespective of the isotope content and of the magnitude of the isotope effect, provided that V_f and V_b are replaced by the respective sums of contributions of both isotopes, and η by the potential of the test electrode referred to its rest potential in the same isotopically mixed system.

Based on the relation thus verified, a method is advanced that enables us to determine experimentally (a) ν by combining the rate of isotopic exchange with the polarization resistance of the electrode used as the catalyst, without neglecting the isotope effect, and (b) the magnitude of the isotope effect from similar data. For the hydrogen-electrode reaction on nickel in an alkaline solution, ν has been determined to be 1·0 and the deuterium separation factor in the hydrogen ionization reaction to be 1·5.     

Interfacial Interactions in Polymers: The Dependence of the Measured Surface Tension of Solid Polymer on the Surface Tension of Wetting Liquid

Careful measurements of the surface tension of solid polymers, y_s, based on the data on contact angles for wetting liquids with various surface tension, y_L, allows one to establish the functional dependence of y_s = f(y_L). This dependence is divided into three zones: one zone, where there is no dependence of y_s on y_L and two zones where y_s changes linearly with y_L.     

Wettability and Adhesion Characteristics of Plasma-Treated Carbon Fibers

The surface free energy (γ_s) of modified carbon fibers was determined by tensiometry and effects of CF₄-O₂ plasma treatment were evaluated. The treatment with the gas mixture in which oxygen was above 40% accelerated preferentially the oxidation of fiber surfaces and the nondispersive component of the surface free energy, γ^P_S, increased to about three times that of the untreated fiber. On the other hand, the treatment with the gas containing CF₄ above 80% induced fluorination and surface species such as - CF, - CF₂, or - CF₃ were formed. The γ^P_S values decreased to almost zero and the dispersive component became about 18 mJ/m². The calculated work of adhesion between various fibers and the epoxy resin was well correlated with the interfacial shear strength of the composites formed with these materials.     

Accelerated-curing epoxy structural film adhesive for bonding lightweight honeycomb sandwich structures

Accelerating the curing of epoxy/aromatic amine adhesives and improving their toughness are challenges in heat-resistant epoxy structural adhesives. Herein, we report an epoxy/aromatic amine adhesive accelerated curing system with an oxo-centered trinuclear (chromium III) complex, which is toughened using a thermoplastic block copolymer (TPBC). The reaction characteristics, heat resistance, microstructure, and bonding properties of the accelerated epoxy adhesives were analyzed. The reaction peak temperature of the epoxy with 3% catalyst was 113.1°C, which was 113.6°C lower than that of epoxy without catalyst, and the modified epoxy resin demonstrated a potential for rapid curing at medium temperature. The glass transition temperature of the TPBC-toughened epoxy adhesive was 125°C after curing, indicating excellent thermal stability after medium temperature curing. The introduction of the TPBC increased the single-lap shear strength of the epoxy adhesive without reducing its heat resistance. The shear strength at room temperature and 120°C of the modified epoxy adhesive with 50 phr of TPBC was 25.2 and 10.9 MPa, respectively. Moreover, the epoxy film adhesive exhibited outstanding bonding properties when used in the bonding of lightweight honeycomb sandwich structures.     

Hygrothermal Properties of Epoxy Film Adhesives

The hygrothermal response of high performance epoxy film adhesives, in their bulk state, has been characterized over a wide range of temperatures, following exposure to a combination of humidity (95% R.H.) and heat (50°C).

Experimental results have indicated that the testing temperature has a pronounced effect on both tensile modulus and strength of the adhesives, while the effect of moisture content varies with respect to the adhesive type. The moduli of the film adhesives, which have a wide range of glass transition temperatures (T_g), have been related to both moisture level in the adhesive and testing temperature. This has been accomplished by employing a dimensionless temperature, which incorporates the wet and dry T_g and the testing, as well as a reference, temperature. The strength properties have shown a higher degree of scatter using the abovementioned dimensionless temperature.

Scanning electron microscopy of the fracture surfaces have shown a good agreement between the effects of moisture and the mechanical properties. Adhesives which exhibited good moisture resistance, as manifested by the stability in their tensile properties, showed minor changes in their fracture surfaces regardless of moisture conditioning. Distinctively, the effect on strength properties has been correlated with typical moisture-induced fracture mechanisms.     

The Fatigue and Durability Behaviour of Automotive Adhesives. Part I: Fracture Mechanics Tests

A fracture mechanics approach has been successfully used to examine the cyclic fatigue behaviour of adhesively-bonded joints, which consisted of aluminium-alloy or electro-galvanised (EG) steel substrates bonded using toughened-epoxy structural paste-adhesives. The adhesive systems are typical of those being considered for use, or in use, for bonding load-bearing components in the automobile industry. The results were plotted in the form of the rate of crack growth per cycle, da/dN, versus the maximum strain-energy release rate, G_max, applied in the fatigue cycle, using logarithmic axes. Of particular interest was the presence of a threshold value of the strain-energy release rate, G_th, applied in the fatigue cycle, below which fatigue crack growth was not observed to occur. The cyclic fatigue tests conducted in a relatively dry environment of 23°C, and 55%; RH were shown to cause crack propagation at far lower values of G_max compared with the value of the adhesive fracture energies, G_c, which were determined from monotonically-loaded fracture tests. Cyclic fatigue tests were also conducted in a “wet” environment, namely immersion in distilled water at 28 C. The “wet” fatigue tests clearly revealed the further significant effect an aggressive, hostile environment may have upon the mechanical performance of adhesive joints, and highlighted the important influence that the surface pretreatment, used for the substrates prior to bonding, has upon joint durability. The development and standardisation of “wet” fatigue tests may provide the basis for a very effective accelerated-ageing test.     

SURFACE TENSION AND BUOYANCY DRIVEN INSTABILITIES IN A LAYER OF LIQUID TIN HEATED FROM BELOW

The linear theory of Pearson (1958) and Nield (1964) is modified here to study liquid tin and include the finite thermal resistances of the bounding layers of boron nitride, copper and air (∼10^-2 torr) in the experiments of Ginde et al. (1989). The magnitude of the ΔT_c across the layer of liquid tin required for the onset of convection depends on the ratios of the thermal conductivities and thicknesses of the supporting layers of boron nitride and copper to those of the tin.

According to our theory surface tension contributes more than buoyancy to the instability observed experimentally. The critical ΔT_c observed required for the onset of convection in the layer of tin, is up to 25% lower than that predicted, which shows the layer is less stable than the theory indicates. Thus the surface of the tin was uncontaminated, or a significantly larger observed critical ΔT_c would be expected.

The boundary condition on the thermal fluctuations at the base of the supporting layer of copper does not appear to be important in these experiments. However, the thermal resistance of the boron nitride would have to be assumed to be unrealistically large to obtain agreement within experimental error with the theory.     

Transmission and Mechanical Properties of Optical Adhesives

The optical, mechanical and durability performance of selected epoxy, polyester, UV-curable acrylic, cyanoacrylate and silicone adhesives were evaluated and measured for bonding applications of optically transparent glasses in the visible and infra-red regions of the electromagnetic spectra.

From the initially selected adhesives only the UV-curable modified acrylic, two-component silicone and room temperature cured epoxy, were found to be of high performance characteristics, having good transmission properties and enhanced endurance in a combination of heat and humidity and following thermal cycling.

Sodium chloride substrates served as adherends for the transmission characterization of the optical adhesives, due to their high transmission properties in the 0.4-10 m μ spectral range. A modified lap shear specimen was designed for studying the mechanical properties and failure mechanisms of the adhesives and their durability in a humid and not environment. Finally, a two-piece glass doublet was used for investigating the optomechanical characteristics of the optical adhesive following environmental conditioning and thermal shock cycling.

Due to the inherent C-C bond, polymer adhesives are limited in utility, as far as transparency is concerned, close to 3.5 μm and in most of the 8-12 μm spectral range.