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.     

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.     

A Test Method for Accelerated Humidity Conditioning and Estimation of Adhesive Bond Durability

The ability to determine the durability of adhesive bonds remains an elusive task, especially when the service environment involves exposure to diluents such as water. Moisture continues to be of major concern for many adhesive bond systems for a number of reasons including:

1) many adhesives are hydrophilic, picking up significant amounts of moisture over time;

2) most adhesives and some adherends allow moisture permeation, eventually reaching the adhesive/adherend interface;

3) the high surface energies of metallic and certain other substrates result in moisture migrating to the adherend surfaces and displacing the adhesive from the substrates, and possibly oxidizing the adherend, etc., and

4) absorbed moisture induces swelling stresses which can reduce the bond strength.

Recognition of this susceptibility to moisture has led to extensive studies aimed at evaluating the effects of moisture, developing an understanding of the responsible mechanisms, and predicting the performance of adhesive bonds subjected to humid environments. While some studies have focused on the effect of humidity on neat adhesive samples, most studies have recognized the significance of the adhesive/adherend interactions, and have evaluated strength of actual bonded joints. Unfortunately, the time required for typical bonded geometries to reach moisture equilibrium can be quite long. Single lap joints (SLJ) and double cantilever beam (DCB) specimens with a width of 25mm may take several years to equilibrate, depending on the temperature and adhesive. Such lengthy conditioning times hamper the development of improved adhesives, and may delay the acceptance of these adhesives because of the time required to certify them. Methods to accelerate the conditioning of test specimens would be of significant benefit to adhesive formulators and users.     

The Effect of Modification of an Epoxy Resin Adhesive with ATBN on Peel Strength

The effects of rubber content, rate of peel and temperature on peel strength of ATBN modified DGEBA based epoxy resin adhesives have been investigated. The fracture surfaces of peel test specimens and the distribution of rubber particles in cured bulk epoxy resin have been observed with SEM and TEM, respectively. The mechanical properties of bulk rubber modified epoxy resin have been also measured. The peel strengths increased with increasing rubber content, peel rate, and decreasing temperature. The peel strengths were superposed as a function of rate and temperature. Plots of the shift factors against temperature gave two straight lines, which followed an Arrhenius relationship. The region of temperature below the intersection of the two straight lines, temperature somewhat lower than T_g of epoxy adhesive, gave markedly high peel strengths and a stick-slip failure due to plastic deformation of the adhesive, and a number of micro holes produced by the rupture of rubber micro particles on the fracture surface. The region of temperature above the intersection gave lower peel strengths and an apparent interfacial failure with ductile fracture of the adhesive, and larger, shallow holes or no holes. From these results, the marked increase of peel strength was concluded to be mainly attributed to the plastic or viscoelastic deformation of epoxy matrix, the strong bond at the interface between rubber particles and epoxy matrix, and the dilation and rupture of a number of rubber particles.     

Morphology and Viscoelastic Behavior of Styrene-Diene Block Copolymers in Pressure Sensitive Adhesives

Linear (SDS) and radial (SD)_x block copolymers of styrene (S) and dienes (D=butadiene or isoprene), varying in composition and molecular weight, were formulated as pressure sensitive adhesives. The morphology of these compositions was determined by electron microscopy of ultra-thin sections and dynamic viscoelastic measurements were made at 35 Hz between -90° and + 140°C or higher. Pressure sensitive tack and holding power were determined and interpreted in terms of morphological and rheological properties.

A high degree of tack resulted only when the tackifying resin was compatible with the polydiene segments of the block polymer and incompatible with the polystyrene segments, provided also that the polydiene-tackifier phase was the continuum with the polystyrene phase forming spherical domains. All effective tackifying resins raised the glass transition temperature (T_g) of the rubbery phase, but plasticized the polymer at temperatures well above T_g Polystyrene domain connectivity was found to lead to diminished tack in block polymers containing more than 30% styrene, a result of decreased creep compliance on the time scale of the bonding process and failure to achieve full contact with the substrate. For adhesives not limited by contact, tack increased with the loss modulus of the adhesive on the time scale of the debonding process. Holding power (shear resistance) increased with polymer styrene content and molecular weight, the polystyrene domain structure effectively inhibiting viscous flow at temperatures sufficiently below T_g of the styrene blocks.     

Protocols for the Measurement of Adhesive Fracture Toughness by Peel Tests

This article reports on the work of the European Structural Integrity Society Technical Committee 4 (ESIS TC4) and its activities in the development of test protocols for peel fracture. Thirteen laboratories have been working on peel test methods in ESIS TC4 since 1997 and their activities are ongoing.

The aim of the work is to develop robust and credible test methods for the determination of adhesive fracture toughness by peel tests. Several geometric configurations have been used, namely, multi-angle fixed arm peel, T-peel, and roller assisted peel in the form of a mandrel test.

The starting point of their work is an established analysis of a peel method that is often developed from a global energy approach. The adopted analysis is combined with an experimental approach in order to resolve ambiguities in the determination of adhesive fracture toughness (G_A). The test methods involve the measurement of peel strength in order to calculate the total input energy for peel (G) and the calculation of the plastic bending energy (G_P) during peel. The latter is often obtained from a measurement of the tensile behaviour of the peel arm. Adhesive fracture toughness is then G - G_P.

Four ESIS TC4 projects are described. The first relates to fixed arm peel whilst the second and third involve both fixed arm and T-peel. The fourth project combines mandrel peel and fixed arm peel. Each project uses different types of polymeric adhesives in the form of quite different laminate systems. The selection of the laminate system enables all characteristics of laminate property to be embraced, for example, thin and thick adhesive layers, polymeric, and metallic peel arms and a range of flexibility in the laminates.

The development of the enabling science required to establish the test protocols is described and software for conducting all calculations is referenced.     

Influence of Fabrication Residual Thermal Stresses on Rubber-toughened Adhesive Tubular Single Lap Steel-Steel Joints under Tensile Load

The tensile load bearing capability of adhesively-bonded tubular single lap joints which is calculated under the assumption of linear mechanical adhesive properties is usually much less than the experimentally-determined because the majority of the load transfer of adhesively-bonded joints is accomplished by the nonlinear behavior of rubber-toughened epoxy adhesives. Also, as the adhesive thickness increases, the calculated tensile load bearing capability with the linear mechanical adhesive properties increases, while, on the contrary, the experimentally-determined tensile load bearing capability decreases.

In this paper, the stress analysis of adhesively-bonded tubular single lap steel-steel joints under tensile load was performed taking into account the nonlinear mechanical properties and fabrication residual thermal stresses of the adhesive. The nonlinear tensile properties of the adhesive were approximated by an exponential equation which was represented by the initial tensile modulus and ultimate tensile strength of the adhesive.

Using the results of stress analysis, the failure criterion for the adhesively-bonded tubular single lap steel-steel joints under tensile load was developed, which can be used to predict the load-bearing capability of the joint. From the failure criterion, it was found that the fracture of the adhesively-bonded joint was much influenced by the fabrication residual thermal stresses.     

AN APPLICATION OF GLASS TRANSITION TEMPERATURE TO EXPLAIN RICE KERNEL FISSURE OCCURRENCE DURING THE DRYING PROCESS *

Fissure formation during rice drying is a major cause of rice milling quality reduction. This work has applied principles of polymer science in studying thermal and hygroscopic properties of rice kernels, particularly the glass transition temperature (T_g). This data was used to develop a hypothesis that explains the occurrence of rice kernel fissuring as a result of drying. The drying process was mapped onto a state diagram to illustrate the changes in state that a kernel could incur through drying and tempering operations. An experiment was designed to validate the hypothesis in which the effect of the T_g on rice drying and tempering in terms of milling quality was determined. Results showed that drying air temperatures up to 60°C and high moisture removal rates could be used without reducing the milling quality, as long as sufficient tempering was allowed at a temperature above the T_g of the rice.     

Polyimide Adhesives and Aluminum

This study examines the effect of thioether sulfur in the polyimide backbone, polyimide T_g, and adherend surface pre-treatment on aluminum bond strengths as determined with both peel and wedge specimens. Surface pretreatment and T_g had more of an effect on peel strength than the presence of sulfur in the polyimide backbone. NaOH etching and comparatively low T_g polyimides combined to produce the highest peel strengths. Together, these factors combined the removal of surface oxide from the adherend with a flexible polyimide which could better relieve stress during testing. Little difference was observed between the peel strengths of sulfur and non-sulfur containing polyimides, and no oxidation of sulfur was observed in the peel samples. NaOH etching also caused both wedge and peel specimens to fail more within the polyimide than in the oxide layer of the adherend. Thus, the NaOH etch appeared to increase interfacial adhesion between the aluminum and the polyimide. The low T_g polyimides performed better than the high T_g polyimides in the wedge test, with the polyimide derived from 4,4'-bis(3,4-dicarboxyphenoxy) diphenyl sulfide dianhydride and 4,4'-diaminodiphenyl ether (BDSDA/ODA) performing the best. This observation could be due to a metal-sulfur interaction since oxidized sulfur was surprisingly observed on the failed surfaces of these bonds regardless of the environment or surface pretreatment. A metal component in the aluminum alloy rather the aluminum is believed to promote the sulfur oxidation.     

Linking Cure Process to Adhesive Bulk Strength by Differential Thermal Analysis

A first order kinetic reaction model is utilized to model the cure process of epoxy adhesive Metlbond 1113. Actual states of full cure corresponding to a spectrum of cure temperature-time schedules are identified using the DTA method. Comparison of these experimental results with the corresponding theoretical predictions reveals that the first order kinetic reaction model provides accurate prediction of full cure when low temperature-long time schedules in close proximity to T_g are used. These cure schedules also result in the highest adhesive bulk tensile strength.     

HEAT TRANSFER IN TRICKLE-BED REACTORS

The mechanism of radial heat transfer in two-phase flow through packed beds is examined. A model with 2 parameters: an effective radial thermal conductivity in the bed, k_e, and a heat transfer coefficient, h_w, at the wall, give a satisfactory interpretation of the radial temperature profile.

k_e was expressed in terms of a stagnant contribution, due to the heat conduction through the solid and the fluid in the void space, and a radial mixing contribution of the gas and liquid phases, due to the radial component of the velocity of both fluids. The radial mixing contribution of the liquid ( k_e)_L was compared with radial mass dispersion data, and a satisfactory agreement was obtained.

Moreover, ( k_e)was much higher than the gas mixing and the stagnant contributions.

Correlations for hw and k_e)_L have been proposed in accordance with the hydrodynamic regimes of the two-phase flow.     

Transient Thermal Deformations of the Interphase in Polymer Composites

Heterodyne micro-interferometry was utilized to measure out-of-plane transient displacements in the interphase due to thermal cycling. In-situ measurements were made on single carbon fiber/epoxy samples with interphases of varying glass transition temperature. Interphase properties were tailored such that one set of samples had fibers which were coated with a low T_g resin, another set had a higher T_g coating, and in the third set the fibers were uncoated. The interferometric data demonstrated that interphase T_g has a significant effect on the rate and magnitude of the thermal deformations at the fiber/matrix interface. The presence of a low T_g interphase caused an increase in the magnitude of the thermal displacements due to a local softening of the matrix and increase in coefficient of thermal expansion. In addition, the rate at which the displacements increase was also higher due to the reduction in T_g. Samples with untreated fibers (no tailored interphase) behaved as if a low T_g interphase had formed. Experimental displacement profiles were also compared with finite element predictions to assess the behavior of the tailored interphases.     

The Effect of Temperature on the Strength of Adhesively-Bonded Composite-Aluminium Joints

Different materials have different coefficients of thermal expansion, which is a measure of the change in length for a given change in temperature. When different materials are combined structurally, as in a bonded joint, a temperature change leads to stresses being set up. These stresses are present even in an unloaded joint which has been cured at say 150°C and cooled to room temperature. Further stresses result from operations at even lower temperatures.

In addition to temperature-induced stresses, account also has to be taken of changes in adhesive properties. Low temperatures cause the adhesive to become more brittle (reduced strain to failure), while high temperatures cause the adhesive to become more ductile, but make it less strong and more liable to creep.

Theoretical predictions are made of the strength of a series of aluminium/CFRP joints using three different adhesives at 20°C and 55°C. Various failure criteria are used to show good correlation with experimental results.     

Relationship between Viscoelastic and Peeling Properties of Model Adhesives. Part 1. Cohesive Fracture

The viscoelastic and peeling properties of polybutadiene/tackifying resin compatible blends have been studied in detail. Viscoelastic properties have been described through the variations of the complex shear modulus, G*(ω), as a function of frequency, ω and peeling properties through the variations of peeling force (F) as a function of peeling rate (V).

After showing the objective character of the peeling curves obtained, the variations of the peeling force and peeling geometry have been studied as a function of volume fraction of the tackifying resin.

In this first paper, the analysis is focused on the first domain of the peeling curves, i.e. the cohesive fracture region. In this region, the peeling properties have been related to the viscoelastic properties in the terminal region of relaxation. It is shown that the longest relaxation time, τ_o, is a reducing parameter of the peeling curves, so a peeling master curve-which is independent of temperature, resin volume fraction and polymer molecular weight-may be defined. Furthermore, the variations of the test geometry as a function of peeling rate have been investigated: the variations of the radius of curvature of the aluminium foil have been analyzed with respect to the viscoelastic behavior of the adhesive, which in fact governs the test geometry.

A detailed analysis of all these features leads to a model which allows one to calculate the peeling curves in the cohesive domain from the adhesive formulation.     

CONJUGATE HEAT TRANSFER WITH TURBULENT FLOW IN A CIRCULAR TUBE

A steady heat transfer problem has been analyzed as a conjugate problem with turbulent flow in a circular tube. The three kinds of thermal boundary conditions considered here are specified as constant temperature, constant heat flux and constant heat transfer coefficient at the outer surface of the wall.

From the results of numerical calculation for Prandtl numbers in the range 0.01 ≤ Pr ≤ 10 and for Reynolds numbers in the range 10⁴ ≤ Re ≤ 10⁵, it was confirmed that the dimensionless parameter R_c could have significant effects on the heat transfer and the temperature field in the fluid adjacent to the wall.     

Gaps between Theory and Practice of Adhesion

Diagnosis of many future adhesion problems will be based on current knowledge, reported at this symposium. But, prognosis permitting a decisive choice of materials and of methods in a specific situation, is not always possible. Considering the broad range of disciplines involved, a systems approach is proposed to reach a logical overlap between theory and practice. Four systems will be discussed in some detail.

The established concepts of adhesive strength in mechanical systems should be amended by a consideration of fracture energies in terms of a macroscopic, continuum model, followed by a discussion of microscopic and molecular structures. Interface systems are common to both adhesion science and tribology. Recent work on the nature of static friction and of boundary lubrication has contributed to the understanding of interface adhesion. Among the operational variables evaluated in production systems the 'adhesionable' properties of adherends are badly in need of closer examination. In biological systems the strength and toughness of calciferous tissues should be compared with the corresponding properties of interlocking adhesives.     

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.     

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 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.     

Behaviour of Urethane Adhesives on Rubber Surfaces

The paper presents the results of chemical investigations into some of the bonding problems of the footwear industry and pays particular attention to the importance of the substrate surfaces.

The importance of the presence of metal soaps on rubber surfaces and their detrimental effect on adhesion is pointed out together with the concomitant beneficial effects of solvent soap dispersing treatments on subsequent adhesion.

The special needs of the footwear industry for a single adhesive system capable of adhering strongly to a wide range of substrates led to the requirement of improving the bond of urethane adhesives to rubber surfaces. The role of free isocyanates in promoting this bond is outlined.

The practical advantages of halogenating rubber surfaces in conferring excellent adhesion properties on moulded rubber surfaces when used with solvent urethane adhesives are pointed out. The observed phenomena associated with halogenation are discussed qualitatively in relation to proposed theories of adhesion.