The Effect of Moisture Content in Epoxy Film Adhesives on their Performance. III: Bulk Properties

Humidity absorbed by epoxy film adhesives during low temperature storage or exposure to atmosphere may result in reversible changes and irreversible modifications. Vacuum treatment may partially remedy the reversible changes. The consequences of vacuum drying are manifested in enhancement of both the peel and shear properties of bonded joints (Part I and Part II of this series of papers) and the thermal, physical and mechanical properties of the bulk adhesive, characterized in the present study.

Experimental results have shown that the bulk properties of structural epoxy based adhesives are highly correlated with the aging processes caused by water absorption in the prepolymerized adhesive. Applying the vacuum process is harmful to fresh unaged adhesive due to devolatization of low molecular species of the film adhesive.

The characterization of bulk properties for the purpose of following the aging and recovery processes is advantageous, since the bulk is independent of geometrical and interfacial effects which dominate in the case of property evaluation of the adhesive in a bonded joint.     

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.     

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.     

Nonlinear Analysis of theTorque Transmission Capability of Adhesively Bonded Tubular Lap Joints

Calculated torque transmission capability of adhesively bonded tubular lap joints using linear elastic material properties is usually much less than the experimentally-determined one because the majority of the load transfer of the adhesively bonded joints is accomplished by the nonlinear behavior of rubber-toughened epoxy adhesives.

Although the adhesively bonded tubular double lap joint has better torque transmission capability and reliability than the single lap joint, the nonlinear analytic or numerical analysis for the adhesively bonded tubular double lap joint has not been performed because of numerical complications.

An iterative solution that includes the nonlinear shear behavior of the adhesive was derived using the analytic solution. Since the iterative solution can be obtained very quickly due to the simplicity of the algorithm, it is an attractive method of designing adhesively bonded tubular single and double lap joints.     

Room Temperature Curing Epoxy Adhesives for Elevated Temperature Service

Room Temperature curing compositions of epoxy resins with high temperature service capability (95-120°C) were formulated and evaluated. The compositions were based on selected high functionality atomatic epoxy polymers and multicomponent poly amine curing agent systems. Toughening was achieved by addition of a rubbery phase either by prereaction of the epoxy resin with carboxyl terminated (CTBN) or by amine terminated (ATBN) poly butadiene acrylonitrile. The latter elastomeric component served as a part of the poly amine curing agent.

Best results were achieved with an adhesive formulation comprising tetra glycidyl-4-4'-diaminodiphenylmethane (TGDDM) and triglycidyl ether of p-aminophenol with triethylenetetramine and addition of ATBN with a felt carrier.

Lap shear strengths of aluminum/aluminum specimens primed by silane coupling agent in the order of 22 MPa at 25°C and 11 MPa at 120°C with T-Peel strengths of 1.6N/mm at 25°C and 0.52 N/mm at 120°C, were obtained.

The thermal behaviour and transitions, the chemical and mechanical properties, the microstructure and morphology of the selected adhesive formulation were studied, using DSC, Gehman, FTIR, mechanical testing and SEM analysis, respectively.

Experimental results showed that the selected compositions could develop good high temperature (120°C) properties while cured at room temperature. Furthermore, their high temperature performance compares favorably or even exceeds that of commercially available room-temperature-curing adhesive compounds, and are competitive with elevated temperature cured film adhesives.     

Bulk and Interphase Effects in Aged Structural Joints

Although structural adhesives are becoming widespread in numerous applications, one important limitation at present is the long term behaviour of bonded assemblies under conditions of high humidity, especially at elevated temperatures. This study presents a comparison between bulk properties of a structural epoxy resin and its behaviour in a torsional joint consisting of a hollowed-out cylinder bonded to a plate—both substrates being in stainless steel. Exposure to ca. 100% relative humidity at 70°C leads to modification of the bulk properties of the polymer, notably reduction of its elastic modulus. Although this may explain some differences in behaviour of the torsional joint, premature failure is attributed to weaknesses in the interphase zone.

Auger Electron Spectroscopy (AES) has been employed to investigate both unbonded steel surfaces and fracture zones. Although prolonged exposure to water leads to a more extensive degree of (apparently) adhesive failure at the interface polymer/metal, AES has shown the presence of non-negligible quantities of carbon, attributed to residual polymer. Failure would seem to occur, at least partly, in a weak interphase of the polymer, near, but not at, the interface.

Various possible causes are evoked. For dry failure, residual polymer may be due to the topography of the metal surface and/or local modification of the adhesive during cure. In the case of aged joints, in addition there are potential effects due to swelling and differential stresses, secondary bond failure and molecular chain scission within the polymer, all provoked by the presence of water.     

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.     

Effect of Xylene Formaldehyde Resins on Epoxy Resin Adhesive

The effect of some types of xylene formaldehyde on epoxy resin adhesive is studied. Xylene formaldehyde resin or modified xylene formaldehyde resins are mixed into liquid epoxy resin and curing properties of the blends, their adhesive properties and the dispersion state of xylene formaldehyde resin in cured adhesive film are examined. The results obtained are as follows.

1) Generally, by the addition of xylene formaldehyde resins, the degree of curing of blends are decreased, but pot life is prolonged, and tensile shear strength of steel bonds is increased.

2) It is observed that effects of the amount of xylene formaldehyde resins and curing condition on tensile shear strength vary with the kind of xylene formaldehyde resin, because of the difference in chemical structure of xylene formaldehyde resins and their reactivity to epoxy resin.

3) It is found that a limited region of compatibility, between 80 and 100 phr, exists for 100% xylene formaldehyde resin in epoxy resin. It is also found that joint strength is reduced with higher viscosity and molecular weight of 100% xylene formaldehyde resin in the case of 80 phr blends, and that these results have some relation to the dispersion state of xylene formaldehyde resin in epoxy resin, judging from the cured adhesive film observed under a phase contrast microscope.     

Effects of Loading Rate and Temperature on the Mechanical Properties of Structural Adhesives Containing a Carrier

The viscoelastic related properties of four structural adhesives were studied in their bulk form. All four adhesives were based on rubber-toughened epoxy resin with a thermoplastic carrier. Two of the adhesives were commercial film adhesives (120°C curing systems) and the other two were formulated by us from commercially available constituents. The first formulation is a high-temperature-curing system based on cyclo-aliphatic resin and anhydride hardener, toughened with carboxy-terminated butadiene elastomer. The second self-prepared formulation is a special room-temperature-curing adhesive for elevated temperature service, based on a blend of trifunctional and tetrafunctional expoxies cured with triethylene tetramine toughened with amine-terminated butadiene elastomer. The latter formulation was also prepared, in addition to the carrier-containing composition, without the thermoplastic carrier.

As expected for viscoelastic materials, it was found that the yield stress and modulus decreased with temperature. The rate of loading had a pronounced effect on the yield stress which increased with increasing loading rates, and a negligible influence on the modulus. The rate-temperature effects on the yield stress were shown to obey the superposition as described by Eyring's theorem of viscosity. Consequently, the activation energy and activation volume were determined. The high-temperature-curing adhesives comprising a carrier exhibited higher activation energies compared with the room-temperature-curing formulation and other epoxy adhesives cured with aliphatic amines or polyamides reported in the literature.     

A New Method for the Study of Adhesion: Application of Inelastic Electron Tunneling Spectroscopy

Inelastic electron tunneling spectroscopy provides an incredibly sensitive and versatile method of detecting and identifying molecular species adsorbed on the surface of a metal oxide. IETS spectra have been measured on the components of the commercial adhesive, Hercules 3501. This epoxy system consists of two molecular components; diamino diphenyl sulfone (DPS) and tetraglycidycl 4,4' diamino diphenyl methane (DPM). IETS spectra of the individual components and of the epoxy mixture adsorbed on aluminum oxide have been obtained and the vibrational modes and frequencies assigned by comparison with computer calculations and existing infrared optical spectra.

IETS is a useful tool for the study of molecular adhesion to an oxide. It is one of the most important methods available for determining the interface physics and chemistry of adhesive bondlines. Its usefulness is greatly enhanced by the fact that in situ studies can be made.

Spectra have been obtained on the components and mixture of the high performance, two component epoxy Hercules 3501. The vibrational modes were identified by comparison with infrared and Raman data, and with computer calculations using force constant information. Evidence for an aging effect was found in the DPS component.

It is suggested that initial studies using IETS should be applied to adhesives of simple molecular structure before proceeding to more complex systems.     

Application of Macromonomers for Pressure Sensitive Adhesives II. Phase Separation and Adhesive Properties

Pressure sensitive adhesives (PSA) were prepared by copolymerization of butyl acrylate, methyl methacrylate and styrene macromer and the effects of macromer on the adhesive properties and phase separated structures were investigated. The results are as follows:

(1) The cohesion of the PSA increased as the quantity of the styrene macromer increased.

(2) The reason why the cohesion of the PSA increased seemed to be that the polystyrene phase, as the graft chain, was separated from the polybutyl acrylate phase as matrix.

(3) When styrene was used instead of styrene macromer, there were not these effects on the adhesive properties.     

NMR Study of the Epoxy Crosslinking Reactions of N,N-Dimethyl-4-Chlorophenyl Urea

The crosslinking of one-part epoxy adhesives is a complex process involving reactions of dicyandiamide and any of a variety of accelerators with epoxide functional polymers. Variations in adhesive formulation and process affect the final properties of the adhesive bond. The reactions of N,N-dimethyl-4-chlorophenyl urea, an accelerator for one-part, dicyandiamide crosslinked epoxy adhesives has been studied with Carbon-13 NMR in model systems employing phenyl glycidyl ether as the epoxy. A complex reaction mixture was observed whose composition varied with epoxy-accelerator stoichiometry and reaction temperature.

NMR peaks having chemical shifts consistent with 2-N-(4-chlorophenyl)-4-phenoxymethyl oxazolidone, a quaternary amine terminated polyether and epoxy elimination products have been observed in reaction mixtures modeling adhesive formulations. The quaternary amine terminated polyether likely results from condensation of epoxy with the dimethyl amine that is formed from N,N-dimethyl-4-chlorophenyl urea under cure conditions. Of the three products observed, only the quaternary amine terminated polyether would afford crosslinks in the actual adhesive. The other two products would consume epoxide functionality without the concurrent formation of crosslinks. The relative amounts of these three products varied as a function of reaction temperature, suggesting that variations in process conditions may affect final properties of dicyandiamide-crosslinked epoxy adhesives that are accelerated with N,N-dimethyl-4-chlorophenyl urea.     

The Effects of Molecular Weight on the Single Lap Shear Creep and Constant Strain Rate Behavior of Thermoplastic Polyimidesulfone Adhesive

The bonded shear creep and constant strain rate behavior of zero, one, and three percent end capped Thermoplastic Polyimidesulfone adhesive were examined at room and elevated temperatures. End capping was accomplished by the addition of phthalic anhydrides.

The viscoelastic Chase-Goldsmith and elastic nonlinear relations gave a good fit to the experimental stress strain behavior. Ultimate stress levels and the safe levels for creep stresses were found to decrease as molecular weight was reduced.

The primary objective was to determine the effects of molecular weight on the mechanical properties of the adhesive in the bonded form. Viscoelastic and nonlinear elastic constitutive equations were utilized to model the adhesive. Crochet's relation was used to describe the experimental creep failure data. The effects of molecular weight changes on the above mentioned mechanical behavior were assessed.     

Flexibilized Copolyimide Adhesives

Two copolyimides, LARC-STPI and STPI-LARC-2, with flexible backbones were prepared and characterized as adhesives. The processability and adhesive properties were compared to those of a commercially available form of LARC-TPI.

Lap shear specimens were fabricated using adhesive tape prepared from each of the three polymers. Lap shear tests were performed at room temperature, 177°C, and 204°C before and after exposure to water-boil and to thermal aging at 204°C for up to 1000 hours.

The three adhesive systems possess exceptional lap shear strengths at room temperature and elevated temperatures both before and after thermal exposure. LARC-STPI, because of its high glass transition temperature provided high lap shear strengths up to 260°C. After water-boil, LARC-TPI exhibited the highest lap shear strengths at room temperature and 177°C, whereas the LARC-STPI retained a higher percentage of its original strength when tested at 204°C [68% versus 50% (STPI-LARC-2) and 40% (LARC-TPI)].

These flexible thermoplastic copolyimides show considerable potential as adhesives based on this study and because of the ease of preparation with low cost, commercially available materials.     

Effect of Peel Load on Stringiness Phenomena and Peel Speed of Pressure-Sensitive Adhesive Tape

The factors governing interfacial separation in lightly cross-linked polymer adhesives at low pulling rates as demonstrated by their stringiness phenomenon are investigated.

Cohesive failure and adhesive/substrate interfacial separation of uncross-linked polymer adhesives have been adequately explained. However, in lightly cross-linked polymer adhesives, where cohesive failure cannot occur because there is no viscous flow, there are two regions of interfacial separation at low rate and this phemonenon cannot be readily explained by present viscoelastic theories.

Investigation of the stringiness phenomenon of peeling pressure-sensitive adhesive tapes at constant loads shows that two peeling speeds exist for any peeling load up to the vicinity of 200 g/25 mm. Also it is clear that stringiness structure differs greatly at each peeling speed. The stringiness phenomenon of each of these two regions is analyzed using Miyagi's observation apparatus. These two measurements are then reversed and a comparison shows that the two peeling speeds correspond to each steady peeling region.

This field of investigation, when added to the present viscoelastic property studies, should lead to a new peeling adhesive theory which, in turn, may lead to the development of new high peel force pressure-sensitive adhesives.     

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.     

Development of a Failure Model for the Adhesively Bonded Tubular Single Lap Joint

The accurate calculation of the stresses and torque capacities of adhesively bonded joints is not possible without understanding the failure phenomena of the adhesive joints and the nonlinear behavior of the adhesive.

In this paper, an adhesive failure model of the adhesively bonded tubular single lap joint with steel-steel adherends was proposed to predict the torque capacity accurately.

The model incorporated the nonlinear behavior of the adhesive and the different failure modes in which the adhesive failure mode changed from bulk shear failure, via transient failure, to interfacial failure between the adhesive and the adherend, according to the magnitudes of the residual thermally-induced stresses from fabrication.     

Some Effects of Heat Processing on Adhesively Bonded Automotive Steel

During a program to develop new structural adhesives that would meet the processing requirements of the current automotive “high heat” bake cycles, substantial differences in performance were noted between the “standard” cold rolled steel (CRS) of SAE1008 and certain Drawing Quality steels (DQSK).

In parallel tests, certain DQSK test specimens (bonded with 200°C heat cycle) failed consistently in the interfacial region, while the CRS samples failed center of bond.

The surface characteristics of the steels and failed adhesive specimens were examined with ESCA, AUGER, and ISS spectroscopic methods. The metal failure surfaces of the DQSK samples were shown to contain relatively high levels of silicon and oxygen, and smaller amounts of boron, with a lower concentration of iron, as compared to CRS which shows iron surface with minor contaminations.

In subsequent testing, with other samples of DQSK SAE1008, this effect was not observed. These samples did not exhibit the same levels of contamination. It is suggested that certain DQSK processes may involve processing steps that are detrimental to the surface properties of the rolled sheet stock.     

Aging Process of Metal/Epoxy Laminates Investigated with X-ray Photoelectron Microscopy and Spectroscopy

In this article we describe the application of X-ray photoelectron spectroscopy to epoxy/dicyandiamide laminates on zinc galvanized steel which were aged under different environmental conditions involving high humidity and temperatures.

X-ray photoelectron microscopy allows us to identify the distribution of chemical elements with a lateral resolution of 10μm. Areas selected in the microscopy mode were then analyzed in the spectroscopy mode in order to get information on the local chemical composition.

We compared the spectroscopic features of the aged but freshly delaminated surfaces of samples stored under ambient conditions at room temperature with samples exposed to the “Kataplasmann” and the “KWT” test, respectively. Furthermore, a comparison was made with a model sample which was prepared in vacuum and on which the curing process was investigated.

Though there is no substantial loss in the lap-shear strength of the samples, we find drastic spectroscopic changes in the Kataplasma and KWT treated samples compared with the sample kept at room temperature. We conclude that the chemical changes induced by these tests cause an internal interphase boundary between the epoxy/metal interface and the bulk adhesive along which delamination occurs. Comparison with the behavior of the water-vapor-treated model sample gives evidence that hydrolysis is the main reaction in these tests.

The results described here complement our former study.¹     

Physical Considerations in the Development of Pressure-Sensitive Adhesive Sheets

A pressure-sensitive adhesive sheet is a special kind of paper used in non-impact printers which use a heating process to apply toner to paper. As a result, it needs special characteristics that general pressure-sensitive adhesive paper for labels do not require.

One of these characteristics is that the edge of the folded paper used in non-impact printers must not incline after printing. This was done by making the degree of orientation of the fibers in the face stocks and the release liners low.

The other characteristics are that adhesive must not ooze out from the edges during the slitting or guillotining process and that the labels must not come off of the release liner by themselves during the printing process. Ooze characteristics were found to be related to the adhesive coat weight. An adhesive paper with both a high peel strength and lower adhesive coat weight was developed by studying the dynamic viscoelastic properties of adhesives and release layers. The storage modulus of the release layer concerned with the release force was also found to be related to the self-peeling tendency of the labels.

These points were considered during the development of pressure-sensitive adhesive paper used in non-impact printers which use a heating process to apply toner to paper.