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.     

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.     

Effects of Extraction on Wettability and Gluability of Apitong (Dipterocarpus Grandiflorus Blanco)

An investigation was made of the effects of extraction and various chemicals applied on veneer surface on the wettability and gluing properties of apitong, Dipterocarpus grandiflorus Blanco, using urea formaldehyde resin. Wettability was determined by measuring contact angles with distilled water.

It was found that extraction with methanol-benzene greatly improved the wettability and gluability of apitong veneer. Likewise, surface treatment with methanol-benzene significantly increased the wettability of the veneer as well as the dry and wet shear strengths of the resulting bond. Treatment with sodium hydroxide increased both wettability and dry shear, but decreased the wet shear strength of the bond. Acetone did not have a significant effect on both wettability and dry shear, but decreased wet shear strength. On the other hand, ether had adverse effects on the wettability and gluability of apitong veneer.

A positive linear correlation was found between wettability and gluability of apitong veneer.     

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.     

The Adhesively Bonded Aluminium Joint: The Effect of Pretreatment on Durability

The interface in aluminium bonded structures can be revealed by ultramicrotomy and subsequently studied by transmission electron microscopy. By these means, the more usual surface pretreatments encountered, have been characterised in depth.

A similar examination has been effected following exposure of bonded joints (floating roller peel specimens) to 85% relative humidity at 70°C. Although a drop in peel performance is noted over the exposure time, interfacial examination reveals little damage to the adhesive or adherend. Possible mechanisms for bond strength reduction are discussed: subtle undermining of the alumina film and disruption of physico-chemical bonds across the interface. Both are initiated by moisture reaching the alumina film, either passing along the interface itself or travelling through the adhesive matrix. Also considered are the effects of surface pretreatment and “oxide” penetration, by the adhesive, on durability.

The effect of priming the adherend surface prior to bonding, using a heavily strontium chromate filled adhesive primer, is mentioned and its possible influence on durability is briefly discussed.     

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.     

Non-destructive Inspection of Adhesively-bonded Joints

The objective of any system of non-destructively examining an adhesive joint must be to obtain a direct correlation between the strength of the joint and some mechanical, physical or chemical parameter which can readily be measured without causing damage. Faults or defects are defined as anything which adversely affect the short or long term strength of a joint. There are two basic areas for examination, the cohesive strength of the polymeric adhesive, and the adhesive strength of the bond between polymer and substrate.

Adhesive strength is very difficult to measure since it is an interfacial phenomenon involving a very thin layer of material, thin even in comparison with bond-line dimensions. Effectively, it would be necessary to assess intermolecular forces and this is not readily possible with existing techniques. This aspect of quality control is usually reduced to assessing the nature of the adherend surfaces prior to bonding.

The cohesive strength of the adhesive is really the only parameter which can be estimated with any degree of confidence, and it is this which features most on destructive tests of bonded joints.

In this paper, defects including porosity, surface un-bonds, zero-volume unbonds, poor cure and so on are discussed, together with the various methods currently used (and some new methods) for physical non-destructive testing.     

Elementary Mechanisms in the Interaction of Organic Molecules with Mineral Surfaces

Model systems are used to reproduce the actual situations encountered during the interaction of organic molecules with mineral solids under controlled experimental conditions.

The role of local electrical fields produced by surface heterogeneities and the influence of the electronic properties of the functional groups of the organic molecule and of the surface are demonstrated and discussed.

This study enables modelling of the basic mechanisms in the organic molecule-mineral solid bond and of the polymerization process of the organic film.

The results given here may be used to produce theoretical models of predictive nature in the field of adhesion and structural bonding.     

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.     

Structural Adhesive Bonds to Primers Electrodeposited on Steel

Adhesive bond strength and durability were investigated for steel substrates which had been cathodically electroprimed before bonding. Lap shear and torsional impact strengths of two model epoxy adhesives were evaluated. Very poor strengths and durability were found for one adhesive, which was cured with a mixture of three amine curing agents. Scanning electron microscopy and analysis of primer susceptibility to interaction with the curing agents suggested that, for the high concentrations of curing agent in the amine-cured adhesive, chemical and physical degradation of the primer occurred during cure at elevated temperature.

For the second adhesive, which was cured with a single imidazole catalyst, excellent strength and durability were obtained, with no evidence of primer degradation. Surprisingly, for this adhesive, strengths to primed steel were up to 88% higher than to cleaned (i.e., degreased) bare steel. The concurrent improvements in environmental durability over bare steel, as assessed by water immersion and salt spray accelerated exposures, were attributed to the more favourable surface energetics of the adhesive/primer interface.     

Peel Adhesion of Solid Films-The Surface and Bulk Effects

The peel strength of rubber and paint films has been measured over a range of peeling velocities using a dead weight method. At low peel rates the peel force is fairly constant but rises rapidly at higher peeling speeds.

Experiments show that the peel strength is a function both of the energy of interfacial bonds which must be broken as peeling proceeds and of bulk energy losses in a viscoelastic peeling material.

The interfacial effect has two components: an equilibrium surface force which accounts for the peel strength at low velocities, and a viscous peeling force which depends on the peeling rate. This viscous interfacial force explains the increase in peel strength of purely elastic films at higher peeling velocities.

The energy loss in the bulk of the peeling film introduces two additional effects: a magnification of the peel strength in steady peeling over a certain velocity range, and a slowing down or stopping of peeling as transient relaxation occurs shortly after the application of the peel force.     

Improved Fracture Toughness of a Titanium Alloy Laminate by Controlled Diffusion Bonding

The use of laminate composites containing a weak interface to increase the fracture toughness of high strength titanium alloys has been studied. Billets were fabricated from Ti-6A1-4V sheet material using a diffusion bonding process. Six billets were fabricated, each billet having an interface with different properties. Results indicate that toughness, as measured by the precracked Charpy test, may be increased when delamination or splitting of the bond occurs.

A simple model to predict the conditions necessary for delamination has been formulated. Correlations between the model and experimental results are made. The model can account for the effect of different base metal and interface material properties and thicknesses. It is seen that a thin, low yield strength interface material with a full strength diffusion bond to a high yield strength, fairly tough base metal leads to optimum composite toughness.     

Hot Melt Adhesive Model: Interfacial Adhesion and Polymer/Tackifier/Wax Interactions

This work continues our study of the hot melt adhesive (HMA) model published earlier [1]. This HMA model was developed based on the pressure sensitive adhesive (PSA) tack model established previously [2]:

P = P₀BD (1)

where P is the adhesive bond strength, P₀ is the interfacial (intrinsic) adhesion term, B is the bonding term and D is the debonding term. The previous paper [1] describes the B and D terms in detail. However, only a brief discussion of the P₀ term was given. The present paper will provide a more in-depth but still rather qualitative study of the P₀ term within the framework of the adhesion model described in Eq. (1). HMAs studied are ethylene/vinyl acetate copolymer (EVA)/tackifier/wax blends. Substrates studied are untreated and corona-discharge-treated polyolefins such as polypropylene (PP) and polyethylene (PE). First, it has been found that the tackifier surface tension could be roughly correlated with one of its thermodynamic parameters: the solubility parameter dispersion component. Secondly, except for EVA/tackifier binary blends, the compatibility of any two of these three components, the EVA polymer, the tackifier and the wax, in a HMA can be estimated from surface tension and X-ray photoelectron spectroscopy (XPS) measurements. Thirdly, based on the study of the EVA/mixed aliphatic-aromatic tackifier/wax model HMA system, it has been observed that the HMA/polyolefin substrate interfacial composition depends on the wax/substrate compatibility. The cause of an inferior peel strength of a HMA containing a high wax content to a polyolefin substrate is possibly due to the formation of a weak boundary layer (WBL) of wax at the interface and/or low dissipative properties of the HMA.

Also, the relationship between EVA/tackifier/wax interactions and HMA peel strength will be discussed. A correlation between the EVA/tackifier compatibility measured by cloud point and viscoelastic experiments to the debonding term, D, in Eq. (1) has been found.     

Effects of environmental exposure on fiber/epoxy interfacial shear strength

The TRI microbond technique for direct determination of fiber/resin interfacial shear strength in composites has been used for investigation the influence of environmental conditions on adhesive bonding in certain systems. The small dimensions involved in the method facilitate uniform exposure and short exposure times. We have observed significant changes in both average shear strength and in shear strength distributions on exposing aramid/epoxy and glass/epoxy microbond assemblies to steam or hot water. Shear strength dropped to a plateau value in both cases, the reduction being more drastic with the glass fiber. Vacuum drying restored shear strength completely in aramid/epoxy microassemblies, even when the surface of the aramid fiber had been chemically modified, but there was only partial regeneration of bond strength with the glass/epoxy system.     

Shear bond strength of bis-acryl resin provisional material repaired using a flowable composite

This study investigated the shear bond strength of a bis-acryl composite repaired with a flowable composite after different surface treatments. Sixty standardized cylindrical silicone molds were filled with bis-acryl resin provisional material and then divided into six groups (n = 10 per group) to undergo different surface treatments. After a surface treatment had been performed, the flowable composite was injected directly into the cylinder of each specimen, and the specimens were then cured over a 10-mm-thick glass slide for 20 s. The shear bond strength was determined using a universal testing machine at a crosshead speed of 1.0 mm/min by placing a knife-edged blade immediately adjacent and parallel to the adhesive interface between the repair material (flowable composite) and the bis-acryl resin provisional material. The mean shear bond strengths ranged from 8.98 to 17.14 MPa. The highest mean shear bond strength corresponded to the bonding group (17.14 MPa), whereas the air-particle abrasion group exhibited the lowest mean shear bond strength (8.98 MPa). Surface treatment of bis-acryl resins with bonding appears to be a promising approach for improving repair bond strength, and the bonding group exhibited the highest levels of bond strength.     

The Effectiveness of Silane Adhesion Promoters in the Performance of Polyurethane Adhesives

A model polyurethane (PUr) adhesive has been modified by the addition of various silane adhesion promoters and used to bond PVC, ABS, a polyblend and glass. Inverse gas chromatography (IGC) data showed that epoxy silane substantially increased the surface reactivity of the adhesive while maintaining its amphoteric character. An aminosilane shifted the PUr surface to basicity, while vinyl, mercapto and chlorosilanes promoted surface acidity. Lap-shear data identified the amphoteric epoxy silane as the most successful adhesion promoter in all polymer and glass assemblies, increasing their initial bond strengths and also their residual bond strengths following accelerated aging. Elsewhere, the success of silane additives reflected the strength of interfacial acid/base interactions, the aminosilane being favored for bonding PVC, the others being preferred for the basic ABS and polyblend substrates. Correlations were developed between residual bond strength and initial bond properties of the assemblies and also between these system characteristics and an acid/base interaction parameter. The correlations may be useful as guidelines to the formulation of superior adhesives for bonding with substrates of known acid/base interaction potential.     

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.     

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.     

Effect of Interphase Structure on the Debonding of Polycarbonate from S-2 Glass Fibers

The effect of interphase structure on the debonding of polycarbonate from S-2 glass fibers has been studied. The shear strength, fracture toughness and hydrolyic stability of the interphases were measured in a single fiber composite of a continuous S-2 glass fiber embedded in a polycarbonate matrix. Polycarbonate oligomers were chemically grafted onto the glass fiber surfaces through use of a silicon tetrachloride intermediary and the properties of the resulting interphases were compared with those of two commercial sizings and ozone-cleaned surfaces. Evaluation was accomplished by measuring the stress transmission across the interphase, τ, by carrying the embedded single fiber fragmentation test to saturation and by using computer simulations and a finite element analysis to calculate the strain energy release rate, G, of the observed fiber-matrix debonding accompanying the first fiber fracture. The oligomer-grafted interphase exhibited improved stress transmissibility and toughness, after 24 hours in boiling water. The tenacity of the tightly bound oligomers was confirmed via DRIFT, TGA and GC/MS experiments on Soxhlet-extracted fibers.

The grafting reaction was modeled on a high surface area silica and studied using solid state NMR to determine reasons for the greater stability of the oligomer-treated surfaces. Measurements of chemical shifts and spin-lattice relaxation times indicate that the oligomers are chemically attached to the surfaces, providing for a well bonded, water resistant interphase. Parallel experiments on a monomeric Bisphenol A-primed silica surface provided evidence that chemical bonding was primarily responsible for the greater hydrolytic stability.     

The Anisotropy of the Surface Tension of Polar Liquids: The Case of Liquid Crystals

The contact angle of the liquid crystal (LC) 4-phentyl, 4'-cyano biphenyl (5CB) on glass or polyethylene does not correspond to the value expected from its surface tension measured by the “du Nouy” method (29 × 10^-3 JM^-2). The value deduced from the Young-Dupre law is 40 × 10^-3 JM^-2.

Both these values have already been reported in the literature. Their apparent discrepancy is explained by the LC surface tension anisotropy. We show that 5CB as many other LCs orient perpendicular to the free surface but parallel to glass or polyethylene Thus both values of the surface tension correspond to two different molecular orientations.

The LC oriented perpendicularly has a surface tension of 29 × 10^-3 JM^-2 and 40 × 10^-3 JM when it lies parallel to the surface. We suggest that the anisotropy between the perpendicular and parallel state of LCs also exists between two perpendicular orientations. Furthermore, such anisotropy associated with any polar molecules explains the difference of the interfacial energy of similar polar or non polar (i.e., octanol-octane) compounds and water.