The Interfacial Bond Strength in Glass Fibre-Polyester Resin Composite Systems Part 2. The Effect of Surface Treatment

The effect of surface treatments on the bond strength in glass fibre-polyester resin composites has been investigated using single fibre interfacial shear strength specimens and the short beam shear test for interlaminar shear strength.

A range of bond strengths was obtained by using, either alone or in combination, the various components of the size formulation which is normally applied to the fibres, so that the interaction between the glass surface and the polyester ranged from Van de Waal forces through hydrogen bonding to covalent bonding, the bond strength increasing in that order.

The relative contribution to bond strength of mechanical bonding due to thermo-mechanical mismatch between the two components and of chemical bonding or physical interaction between the three phases, glass-surface treatment-resin, has been evaluated and found to be one third and two thirds respectively.     

The Effect of Sodium Ions on the Stability of the Interphase Region of Glass Fibre Reinforced Composites

The single embedded filament fragmentation and the short beam shear strength tests together with angle-resolved X-ray Photoelectron Spectroscopy (XPS) have been used to investigate the interfacial region of vinyl ester composites reinforced with sized AR-glass fibres, with and without amino and vinyl functional adhesion promoters.

The 7-aminopropyltriethoxysilane (APS) deposit on AR-glass is susceptible to a thermal degradation during post-cure, which has been attributed to a base catalysed equilibration of the siloxane bonds. The functional groups of APS required for resin compatibility were buried beneath the surface layers, contributing to a low bond strength, furthermore, mobile sodium ions existed within the interfacial region. Aqueous extraction prior to fabrication enhanced the composite bond strength by removing the soluble silane oligomers, the sodium ions, and exposing the organo-functional groups for co-reaction with the matrix.

The silane deposit on AR-glass is made hygroscopic by the presence of sodium ions. This increased the equilibrium moisture content of AR-glass composites, and diminished their retained short beam shear strength in contrast to the E-glass control which retained its properties after redrying.     

Surface Dynamics vs. Adhesion in Oxygen Plasma Treated Polyolefins

Polyethylene (PE) and polypropylene (PP) were oxygen plasma treated and aged in carefully reproducible conditions. The effect of aging on the surface chemistry, wettability and adhesion were studied using a combination of techniques: contact angle measurements, XPS, SSIMS, adhesion tests (shear and pull).

PE was found to be relatively insensitive to aging both in terms of wettability and adhesion, due to crosslinking during plasma treatment, which is likely to reduce macromolecular mobility within the surface layer.

In the case of PP, dramatic decreases of wettability occur with time, due to macromolecular motions leading to minimization of oxygen-containing functions at the surface. This behavior was shown to affect the adhesion performance of treated PP.     

Influence of Extractives on Bonding Properties of White and Southern Red Oak

White and southern red oak veneers were subjected to four methods of drying followed by five surface treatments. The four drying methods were mill drying at 350°F, laboratory drying at 350 and 212°F, and air drying. The five surface treatments were no treatment, surface scraping, soaking and dipping in 1% NaOH aqueous solution, and water extraction. Plywood panels were prepared by using a phenol-formaldehyde resin.

Even with the best drying-surface treatment combination, wood failure was only 35% for white oak and 39% for southern red oak. Overall, mill drying was the best drying method. Soaking the veneers in 1% NaOH solution significantly increased the bond quality.

Mill drying of veneers caused water-soluble extractives to migrate from the interior portions to veneer and lathe check surfaces. SEM examinations of the glue failure surface revealed that gluelines failed to adhere to the cell walls. Difficulties in bonding white and southern red oak veneers may be caused by extractive contamination.     

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 Interfacial Bond Strength in Glass Fibre-Polyester Resin Composite Systems

The interfacial bond strength in glass fibre-polyester resin composites has been investigated using various experimental techniques. These included blocks of resin containing fibre (in which, depending on the geometry of the specimen, failure occurs in either a shear or tensile mode) the pullout of a fibre from a disc of resin and a short beam shear test for interlaminar shear strength determination.

Low power optical microscopy and optical retardation measurements of stress induced birefringence were used to detect the difference between intact and debonded fibre resin interfaces. The shear modulus and shear strength of the resin were obtained from torsion tests on cylindrical rods of the resin.

The single fibre shear debonding specimen and the short beam shear test are shown to be the most viable test methods but interpretation of the results is complicated by the various modes of failure possible and by the different stress states which exist in the area of the specimen where debonding starts. Stress concentration factors obtained by finite element analysis and photoelastic analysis have been applied to the results from these tests and the corrected interfacial bond strengths are in close agreement.

The real interfacial bond strengths of well bonded glass-fibre polyester resin systems is shown to be of the order of 70 MN m^-2.     

Preadhesion Laser Treatment of Aluminum Surfaces

An excimer laser may be used for preadhesion treatment of aluminum alloys. This method presents an alternative to the use of ecologically unfriendly chemicals involved in conventional anodizing pretreatments.

Experimental results indicate that preadhesion laser surface treatment significantly improved the shear strength of modified-epoxy bonded aluminum specimens compared with untreated and anodized substrates. The best results were obtained with laser energy of about 0.2 J/Pulse/cm² where single lap shear strength was improved by 600-700% compared with that of untreated Al alloy, and by 40% compared with chromic acid anodizing pretreatment.

The mode of failure changed from adhesive to cohesive as the number of laser pulses increased during treatment. The latter phenomenon has been correlated with morphology changes as revealed by electron microscopy, and chemical modification as indicated by Auger and infrared spectroscopy.

It can be concluded that the excimer laser has potential as a precise, clean and simple preadhesion treatment of Al alloys.     

Factors Affecting the Durability of Ti-6Al-4V/Epoxy Bonds

Factors influencing the durability of Ti-6Al-4V/epoxy interphases were studied by determining chemical and physical properties of Ti-6Al-4V adherend surfaces and by characterizing the strength and durability of Ti-6Al-4V/epoxy bonds.

Ti-6Al-4V adherend surfaces were oxidized either by chemical etch or anodization. Four principal pretreatments were studied: chromic acid anodization (CAA), sodium hydroxide anodization (SHA), phosphate fluoride acid etch (P/F) and TURCO basic etch (TURCO). The oxides were characterized by SEM, STEM, profilometry, contact angles and XPS.

All adhesive bonding was carried out using a structural epoxy, FM-300U. Both lap shear and wedge test samples were tested in hot, wet environments. The results lead to the conclusion that the interfacial area between the adhesive and adherend is the primary factor affecting bond durability.     

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.     

Interfacial Contact and Bonding in Autohesion V—Bonding of “Flat” Surfaces

All surfaces, when viewed under the microscope, are found to be rough. When the so-called “flat” surfaces are bonded together, the initial contact is only at the high points in the surface. This contact increases with time and the rate of contact establishment is a function of surface roughness and the viscoelastic properties of the material.

A surface study of the “flat” compression-molded surface is made. The profiles are generated by tracing the surface with a stylus. The Interference Microscope is used to study the region in the vicinity of an asperity.

The surfaces are approximated to be composed of simple spherical segments. The deformation is conceived of as a two-stage process. The first stage of easy deformation controls the initial bond strength. The initial bond strength predicted by theory agrees well with experiment.     

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.     

Scarf Repair Joints in Carbon Fibre Reinforced Plastic Strips

Various theoretical models are developed for scarf joint repairs in unidirectional carbon fibre reinforced plastic strips subjected to a tensile load. for the type of joint where the fibres of both the original and repair materials (the adherends) are aligned along the longitudinal axis of the joint (a dry/dry joint), a uniform shear stress is predicted in the adhesive layer. When the repair material is laid directly on to the scarf surface with the fibres inclined to the longitudinal axis (a wet/dry joint), it is shown that the degree of shear stress uniformity is a function of the scarf angle, but reaches a uniform state at slopes smaller than 1 : 30 when the joint stresses can be calculated very simply.

The theoretical work is supported by tests on a number of joints of both types. The test results show that the dry/dry joints are more efficient than the wet/dry joints, and that it is possible to achieve joint efficiencies at failure, under a static load, in the region of 90% with slopes less than or equal to about 1 : 65.     

Durability of Adhesive Bonds to Zinc-Coated Steels: Effects of Corrosive Environments on Lap Shear Strength

The effects of corrosive environments on adhesive bonds to electro-galvanized, zinc/aluminum alloy coated, coated electro-galvanized, and cold-rolled steels have been investigated. Bonds prepared using a rubber-modified dicyandiamide-cured epoxy adhesive, an epoxy-modified poly(vinyl chloride)-based adhesive, an acrylic-modified poly(vinyl chloride)-based adhesive a one-part urethane adhesive, and a two-component epoxy-modified acrylic adhesive were exposed under no-load conditions to constant high humidity or cyclic corrosion exposure for 50 days or 50 cycles (10 weeks) respectively.

Over the course of this study, exposure to constant high humidity had little effect on lap shear strength for any of the systems studied. Bond failures were initially cohesive, and with few exceptions remained so.

Bond strength retention under the cyclic corrosion exposure conditions employed was strongly dependent on adhesive composition and on substrate type. On galvanized substrates, lap shear strengths for the poly(vinyl chloride)-based adhesives were reduced by 90-100% during the course of the corrosion exposure, and a change in the mode of bond failure (from cohesive to interfacial) was observed. On the coated electro-galvanized steel substrate, the poly(vinyl chloride)-based adhesives showed about 50% retention in lap shear strength and a cohesive failure throughout most of the corrosion test. The dicyandiamide-cured epoxy adhesive used in this study generally showed the best lap shear strength retention to zinc-coated substrates; bonds to cold-rolled steel were severely degraded by corrosion exposure. The performance of the acrylic and urethane adhesives were intermediate to the dicyandiamide-cured epoxy and poly(vinyl chloride)-based adhesives in strength retention.     

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 Role of the Interface in Carbon Fibre-Epoxy Composites

The final performance of a composite material depends strongly on the quality of the fibre-matrix interface. The interactions developed at the interface were studied using the acid-base or acceptor-donor concept.

The surface characteristics of the carbon fibres and the epoxy matrix were studied using a tensiometric method and the inverse gas chromatography technique. Acid-base surface characters could be determined allowing the interactions at the interface to be described by a specific interaction parameter.

It was shown that the shear strength of the interface, as measured by a fragmentation test, is strongly correlated to this specific interaction parameter, demonstrating the importance of acid-base interactions in the fibre-matrix adhesion.     

Bonding of Titanium Alloys

Surface pretreatments affect joint strength, some improve it, e.g. etching in aqueous hydrofluoric acid or oxidation in alkaline hydrogen peroxide; while others decrease the strength e.g. etching in acids containing high concentrations of oxidising agents.

For Ti-6A1-4V alloy, best results are obtained by either etching in aqueous hydrofluoric acid or oxidation in alkaline hydrogen peroxide for about two hours.

The results indicate that a pretreatment which leaves a coherent TiO₂-rutile layer, on a rough surface, form the strongest joints. Surface roughness is shown to enhance bond strength.     

Enhancement of Surface Wettability of Abhesive Silicone Rubber by Oxidation

A new method to detect surface oxidation of an otherwise untreated, cross-linked and filled silicone rubber is described. Our method is established on the principle that surface wettability increases during the progress of oxidation. Surface wettability is determined in terms of critical surface tension.

Abhesive polymers, of which silicone rubber is a typical example, are characterized by low surface energy, low friction coefficient and low release value. The problem associated with silicone rubber is its poor adhesion to other polymers. Its adhesional ability, however, can be improved by surface modification, e.g. oxidation, treatment with corona discharge, or ionic bombardment with inert gases.

By our method we found that the oxidation of silicone rubber is comparatively mild below 260°C, but is intensified at 287°C. Excessive oxidation at 316°C results in the formation of low molecular weight siloxanes which lower the wettability of the oxidized surface. Mechanisms of thermal oxidation are discussed.     

Regeneration of Adhesive Bonding in Fiber/Resin Systems

The bond between a fiber and a surrounding polymer matrix can be weakened or completely broken by mechanical shearing. In some cases bond strength can be reduced by exposure to active environments (for example, hot water or steam). Obviously, it would be of considerable practical value if weakended or lost bonding could be regenerated. This paper presents the first results of an ongoing study of the possibility of bond repair in fiber/resin systems.

The use of the TRI microbond shear strength measurement technique makes it possible to study bond regeneration with individual fiber/resin specimens. Since the microdrop is displaced only a very short distance along the fiber during the shear strength measurement, it is a simple matter to treat a sheared drop without removing it from the fiber and then perform a second shear strength evaluation. Several systems have been studied in this manner, involving both thermosetting and thermoplastic resins. Examples of significant regeneration of both mechanically-sheared and hydrolytically-weakened bonds are given, and possible mechanisms for the bond strength regeneration are discussed.     

MASS TRANSFER AND DIFFUSION COEFFICIENT DETERMINATION IN THE WET AND DRY SALTING OF MEAT

Dehydrated salted meat is widely used in Brazil as a very important source of animal protein. The main objective of this kind of processing is water removal. initially by osmotic pressure changes and then by drying, resulting in a product with intermediate moisture levels.

In this work, mass transfer and salt diffusion in pieces of meat submitted to wet and dry salting were studied. Slabs of beef m. trapezius with an infinite plate geometry were salted in a NaCl saturated solution or in a dry salt bed, at two temperatures (10 and 20°C) and different time exposures (120 min and 96 hours). Equilibration studies were extended up to six days.

It was observed that water loss increased with salt uptake, for increasing periods of times. At 20°C the moisture loss was higher than it was at 10°C in both salting processes. On the other hand, the kinetics of salt uptake and moisture loss were of greater importance in the process of dry salting than in that of wet salting.

The salt diffusion coefficient for wet salting was 0.26 × 10^-10m²/s at20°C and 0.25 × 10^-10 m²/s at 10°C and for the dry salting the values were 19.37 × 10^-10 m²/s at 20°C and 17.21 × 10^-10 m²/s at 10°C.     

Application of Adhesive Joining Technology for Manufacturing of the Composite Flexspline for a Harmonic Drive

A new manufacturing method for the cup-type composite flexspline for a harmonic drive was developed using adhesive joining technology to obviate the manufacturing difficulty of the conventional one-piece cup-type steel flexspline and to improve the dynamic characteristics of the flexspline.

In this method, the boss, tube and tooth sections of the flexspline were designed and manufactured separately, and adhesively bonded. The tube section was manufactured with high strength carbon fiber epoxy composite material and its dynamic properties were compared with those of the conventional steel flexspline.

The torque transmission capability of the adhesively-bonded joint was numerically calculated using the nonlinear shear stress-strain relationship which was represented by an exponential form.

From the test results of the manufactured composite flexspline and the conventional steel flexspline, it was found that the manufactured composite flexspline had better torque transmission characteristics. Also, it was found that the damping capacity of the composite flexspline was considerably improved.