International Journal of Adhesion and Adhesives

Published by Elsevier
Print ISSN: 0143-7496
Publications
The aim of the present work is directed towards the manufacturing and characteristics of weld-bonded commercial aluminum sheets. Dynamic response (natural frequency and damping), nugget size, and tensile shear strength were tested and reported. The present work demonstrates a tremendous improvement in tensile shear strength associated with higher damping capacity for weld-bonded joints. However, the natural frequency seems to be independent on shear strength as well as joining technique (spot welding or weld bonding).
 
The effects of 13 pre-treatments were examined to determine their effect on the surface region of homopolymer polypropylene. Five of the pre-treatments were examined in detail due to excellent joint strengths. They were: corona discharge, flame, fluorination, low-pressure vacuum plasma and atmospheric plasma. The pre-treatments were examined using X-ray photoelectron spectroscopy (XPS), angle resolved XPS (AR-XPS) and atomic force microscopy (AFM), to determine surface chemistry and topography. Of the 13 pre-treatments examined, it was found that the first five all showed the highest surface chemical modification of the pre-treatments studied. It was identified that the surface chemistry, concentration depth and topography varied widely across the five pre-treatments. However, all have been shown to have similar bond strengths with polyurethane adhesives, indicating that a number of significant factors were responsible for bond strength. It is surmised that the depth of the functional group concentration is the determinant joint strength parameter and not the O : C ratio or surface roughness.
 
A 99% 15N-labelled polymeric methylenebis(phenylisocyanate) (pMDI) resin was used to make a series of wood- 15N-pMDI composites cured as a function of temperature and time. Bondline chemistry was monitored using 15N CP/MAS NMR. Urea linkages were identified as the major chemical moieties at low cure temperatures, while biurets were predominant at higher temperatures. Bondline chemistry did not change with time when curing at 120°C. However, cure chemistry at 185°C did change with time, from mostly biuret to urea. This transition involved the thermal decomposition of biurets, producing urea and isocyanate. It is believed that urethane formation has been detected under the conditions that cleave biurets. This suggests that urethanes result from the thermal liberation of isocyanate via biuret cleavage. The 15N technique has proven valuable for the elucidation of this complex cure chemistry.
 
The formation and growth of fibrils during debonding seems to be crucial to the peel strength and the tack of polymers to be used as pressure- sensitive adhesives. The objective of this work is to study the influence of a number of parameters such as contact force and time, surface roughness, temperature, and rate of separation on fibrilation, and to establish relations between the nucleation and growth of fibrils and the mechanical behaviour as well as the molecular structure of the adhesives. Measurements of a large number of model polymers lead to the conclusion that the average mass between entanglements, Me, is an important factor governing fibrilation. Polymers with a high Me above about 1×104 g/mol show fibrilation whilst materials with an entanglement length below this limit debond by an apparently homogeneous deformation with lower-energy dissipation.
 
The aim of this study is to analyze by finite element method the single- and double-sided composite patch repairs designed to reduce the concentration of the stresses at circular notches and cracks. The results show that there is a considerable reduction in the asymptotic value of the stress-intensity factors and the normal stresses at the crack tip. The use of a double-sided patch suppresses the bending effect due to the eccentricity of the patch on one side only and reduces the shear stresses in adhesive.
 
The evolutionary structural optimisation method (EVOLVE) was employed to optimise the shape of adhesive fillets found in tabs of tensile test specimens in which the adherends were titanium and T800 unidirectional tape bonded with a generic paste adhesive. Typically, the selection of an adequate joint requires a number of design and analysis cycles which rely heavily on the trial-and-error method. The EVOLVE method relies for the cases presented, on an iterative finite element analysis and progressive removal of elements which takes the guess work out of the design loop. The objectives in both cases are to minimise the maximum stresses in the joints. The EVOLVE code was found to be effective in reducing the maximum principal stress in the adhesive for all models studied. The reduction in maximum principal stress achieved with respect to the baseline model (CFRP upper and middle adherends with ) ranged between 48 and 64%. The peak peel stresses in the central CFRP adherend and in the adhesive were reduced by up to 66%.
 
A growing number of structural applications and repairs require strong adhesively bonded joints between composite parts. The viability of the finite elements method (FEM) for the design and analysis of such joints has been shown by several researchers (Composites 13(1982) 29, mechanics and mechanisms of damage in composites and multi-materials, ESIS11, Mechanical Engineering Publications, London, 1991, Structural adhesive joints in engineering, Elsevier Applied science Publishers, Amsterdam, 1984). Most of the proposed plane stress models for adhesively bonded joints have traditionally relied on effective lamina properties for the composite adherends because one of the primary goals was the study of adhesive stresses and failure criteria (J. Adhes. 42 (1993)). Such work has also been usually carried out on mainframe computers. However, with the rapid developments in computing, commercial FE codes now exists for PC applications. These present new possibilities, namely the construction of models which are closer in their approximation to the structures they represent. Within this context, a new 2D plane stress approach has been developed which allows ply-by-ply modelling of composite adherends in a joint. It is a development of the traditional 2D plane stress model used by several authors such as Siener (Compos. Mater.: Testing Des. 10 (1992) 444) in investigating of composite repair joints.
 
A nonlinear 3D finite element model and a theoretical parameter study in relation to glued-in rods for timber structures are presented. A strain-softening crack band model was used to characterise the behaviour of the adhesive layer between the rod and the wood. The model is general in the sense that it bridges the gap between the theory of an ideal plastic bondline and the theory of linear elastic fracture mechanics. Two parameter studies were made. One in relation to fracture energy and geometrical parameters and the second in relation to loading conditions. The results show that the fracture energy is of major importance for the pull-out load capacity, that the present model can be used to predict such phenomenon like the size effect and that the loading in pull–compression results in lower load-bearing capacities than the loading in pull–pull.
 
Structural, heat-resistant thermoplastic adhesives were evaluated in single lap bonds. The amorphous thermoplastics tested were polyphenylquinoxaline, glass filled Ultem polyetherimide, unfilled Ultem polyetherimide, and Victerex polyethersulfone. The adherend was chromic acid anodized Ti-6Al-4V, tested unprimed and primed with Lica 44 titanate. Initial bond strengths were similar for all adhesives. In general, Lica 44 titanate primer did not affect bond strength. Bond strength was not influenced by 170°C ageing, but 232°C ageing did decrease bond strength when polyphenylquinoxaline was the adhesive. Failure occurred primarily in the adhesive fillet and propagated into the unprimed or primed anodic oxide/polymer interphase.
 
Characterization of adhesive bonded lap joints of C/C-SiC composite and Ti-6Al-4V alloy under varying conditions were examined. The effect of glueline thickness, length, etching time, temperature, exposure time and strain rate were studied after adhesive bonding. The surface morphology was characterized by the scanning electron microscope. Results showed that the C/C-SiC adherence was sensitive to strain rate and temperature than the Ti-6Al-4V adherence, due to poor oxidizing resistance and high brittleness.
 
AA6060-T6 aluminium extrusions have been subjected to various surface treatments before bonding with an epoxy based adhesive and subsequent exposure to chloride and humid atmosphere (82% RH, 40°C) for 50 days. Single lap joints given an alkaline etch pre-treatment before bonding suffered a 7% reduction in strength as a result of environmental exposure. Desmutting in nitric acid after a similar alkaline etch pre-treatment to remove the Mg rich hydroxide film resulted in still higher loss in strength. Application of an experimental phosphate–permanganate conversion coating to the alkaline etched substrate also had no beneficial effect. Examination of the bonded surfaces after lap shear testing revealed the presence of filiform corrosion filaments, providing paths for rapid ingress of moisture to the substrate/adhesive interface. The best results were obtained by AC anodising in hot sulphuric acid to a film thickness of about 0.2 μm.
 
Extruded AA6060 aluminium has been subjected to various surface pre-treatments before application of a one component structural epoxy adhesive, XD4600. The durability of joints has been assessed by wedge adhesion and filiform corrosion (FFC) testing of panels coated with adhesive. A chromate-free Ti–Zr-based pre-treatment provided improved durability relative to an alkaline etch and deoxidation pre-treatment, but was clearly inferior to chromating in terms of adhesion and FFC resistance. Excessive Ti–Zr oxide deposition occurred in the vicinity of intermetallic α-Al(Fe,Mn)Si particles during pre-treatment, which significantly reduced the performance of bonded joints during wedge test. The intermetallics furthermore played a crucial role in promoting FFC, as demonstrated by complete FFC immunity of an AA6060 model analogue alloy (AlMg0.5Si0.4) free from α-Al(Fe,Mn)Si particles. The Ti–Zr-based pre-treatments provided limited protection against FFC on AA6060, apparently due to a limited effect of the Ti–Zr oxide conversion coating in reducing the cathodic activity of the intermetallic particles. In search for chromate-free pre-treatments for painted or adhesive bonded aluminium, inhibition of the cathodic activity on the Fe containing particles is suggested as an important factor to prevent FFC.
 
In this work the effects of both the substrate surface condition and the adhesive properties on single-lap aluminium joint resistance were analysed. The aluminium sheets were mechanically treated with two abrasive surfaces evaluating the induced roughness; four different resins were used in adhesion tests. Moreover, wettability tests were performed in order to evaluate the effect of the above-mentioned parameters on the substrate/adhesive interaction. A design of experiments was defined in order to quantify the effect of the considered factors and their correlation.
 
Abrasive articles consists of cutting particle—very often from electrocorundum, filler—inorganic compound, e.g. potassium fluoroborate or cryolite, binder—novolak resin and wetting agent—resol. The most important stages during manufacturing of grinding tools are: the coverage of the abrasive by wetting agent and proper hardening. The fillers play a very important role during the work of the grinding tools—they collect the heat and prevent the melting of resin. The surface properties of the components of grinding tools influence the interactions between them and the properties of the final product.Surface properties of different kinds of the fillers were examined by means of inverse gas chromatography (IGC). Dispersive component of the surface free energy at relative humidities: 30%, 60% and 90% and at various temperatures: 278, 283, 288 and 293 K was determined. It provided the data on stability of the fillers during their storage and/or weathering. The influence of the type of the filler on the hardening process by differential scanning calorimetry (DSC) was also studied.Principal component analysis (PCA) was applied to search the influence of the temperature and humidity onto the values for various fillers. This analysis showed that changes of the surface properties of fillers are significant at higher humidity close to 90%. Furthermore, PCA demonstrated that the type of filler does not influence meaningfully the hardening process.This approach has shown usefulness of IGC and calorimetric methods in studying the properties of abrasive articles as well as PCA in the interpretation of physicochemical characteristics of abrasive materials.
 
To increase electrical conductivity and reduce number of processes, this paper aims to fabricate in situ photocurable conductive adhesives with Ag nano-particles in the absence of polymeric protector. The mixture of epoxy–acrylic resin and reactive monomer, as well as AgNO3 in ethylene glycol, is irradiated by UV light to form silver nano-particles without capping agent and was followed by adding photoinitiator to produce photocurable adhesive with silver nano-particles. In particular, as measured by transmission electron microscopy, the diameter of Ag nano-particle is 30–50 nm for 1 M and 80–90 nm for 2 and 3 M of AgNO3, in ethylene glycol. Furthermore, taking 1:1 weight ratio of photosensitive mixture to 3 M of AgNO3 in ethylene glycol as an example, the surface electrical resistivity of photocurable conductive adhesive may be reduced to 8.803×106 (Ω/□) after exposing to 900 mJ/cm2.
 
This paper reports the results of wedge testing adhesive-bonded joints made from dry, and water-immersed and dried, carbon fibre-reinforced plastic (CFRP) laminates based on Ciba Fibredux 914. Five groups of specimens were tested. Those bonded with Hysol EA 9394 adhesive were tested on dry, and immersed and dried, CFRP adherends using mechanical abrasion as the surface preparation. Hysol EA 9390 repair resin was tested on abraded dry samples and on dry, and immersed and dried, peel ply surfaces. Only the initially dry specimens that were also dried before bonding and given the mechanical abrasion treatment gave cohesive failure (now considered to be matrix failure) after 1388 days' immersion. All the peel ply surfaces — whether initially dry, or immersed and then dried — gave interfacial failure. Unfortunately, in the case of the immersed and dried specimens, the two lowest temperature drying cycles were both given to the specimens which were prepared by mechanical abrasion and the two highest temperature drying cycles were given to the peel ply samples. All the peel ply samples gave interfacial failure but the immersed and dried peel ply surfaces gave the same result, in terms of fracture energy and durability, as the initially dry ones, indicating that the higher temperature drying cycles were adequate. One strange and unexpected result was that the fracture energy obtained with Hysol EA 9394 on the immersed and dried surfaces was higher than on the dry surfaces by a large amount. This suggests that, for this particular adhesive, a small amount of moisture actually improves the cure and the fracture toughness, even though a cohesive failure was not obtained. In view of the extensive use of peel plies in industry it would seem that further research is required to achieve more durable bonds. From a repair point of view, where mechanical abrasion is the most common method of surface preparation, it was encouraging to find that in some cases repair bonds could actually be more durable than those made by the manufacturer if peel plies are used.
 
The in-situ bonding of pultruded fibre-reinforced plastic rods into timber structural members is a commonly used technique for making timber-to-timber connections and for the strengthening and repair of timber structures. Ideally, the adhesive should be thixotropic, shear thinning, room temperature cure, environmentally stable, solvent-free and applied without pressure. This study investigates the moisture absorption characteristics of three adhesives, specially formulated for bonded-in timber connections, where two of the adhesives are modified with nano- or micro-particles. The three adhesives are denoted as CB10TSS (standard adhesive), Albipox (standard adhesive with CTBN rubber additions) and Timberset (standard adhesive filled with ceramic particles). The aim of the additions is to improve the environmental stability of the standard adhesive as well as enhancing mechanical properties and raising the glass transition temperature. The effect of high temperatures and high humidity on the properties of the three adhesives was determined following conditioning at different combinations of temperature and relative humidity (20, 30 and 50 °C/95% RH) and soaking in water at 20 °C. In all cases the moisture uptake for the rubber-modified adhesive was less than for the standard adhesive, but the ceramic particle-filled adhesive exhibited the lowest moisture uptake overall. Exposure to humid environments at temperatures lower than Tg resulted in water uptake characterized as Fickian, which had only a modest effect on properties. However, exposure to humid environments at temperatures higher than Tg resulted in non-Fickian uptake of water and significant changes to the diffusion and permeability coefficients.
 
Anodising parameters for pretreatment of the AA6060-T6 aluminium substrates
List of infrared frequencies
The relative infrared intensities of the O-H str and Al-O bend bands on anodised substrates.
Reflection-absorption FT-IR spectrum of an AA6060-T6 surface that was boiled in water for 64 min.
The environmental durability of anodised aluminium alloy AA6060-T6 bonded with a one-component epoxy adhesive has been investigated using the wedge test at 40°C/96% RH. DC and AC anodising in phosphoric and sulphuric acid solutions have been compared. Substrates that were AC anodised in hot phosphoric and sulphuric acid solutions performed very well, almost as good as the well-established Forest Product Laboratory+DC anodising in phosphoric acid pretreatment. The anodic oxide films were studied by reflection–absorption FT-IR spectroscopy. It was shown that the anodic films released water during curing of the adhesive at 180°C. The observations suggested that a transformation from the hydroxide, Al(OH)x, to the oxide, Al2O3, state took place. Hot water sealing of the oxides produced in sulphuric acid significantly reduced the durability.
 
Anisotropic conductive films (ACFs) consist of conducting particles and adhesive polymer resins in a film type and have been widely used for the flat panel display module to be high-resolution, light weight, thin profile and low power consumption in forms of out lead bonding (OLB), flex to printed circuit board bonding (PCB), chip-on-glass (COG) and chip-on-film (COF) in last decades. As the interconnection pitch between driver IC and flex is decreasing, ACF materials have been evolved to meet the fine pitch capability, low-temperature curing and strong adhesion requirements. Multi-layered ACF structures such as double and triple-layered ACFs were developed for the same reason. Flip chip technology has been well-known as one of the solutions to meet today's semiconductor packaging needs of miniaturization of package size as well as reduction in interconnection distance, resulting in high electrical performance. Especially, flip chip assembly using anisotropic conductive adhesives (ACAs) has been gaining much attention for its simple and lead-free processing as well as cost-effective packaging method. High mechanical reliability, good electrical performance at high-frequency range and effective thermal conductivity for high current density are the required properties for the ACF material for wide use in the flip chip application.In this paper, an overview on the principles, recent development and applications of ACF materials for flat panel displays and semiconductor packaging applications, with focus on the fine pitch capability, low-temperature bonding process, electrical/mechanical/thermal performance and wafer level package using ACFs are described.
 
In a research programme to develop an addition polyimide adhesive that is easily processed and exhibits good retention of strength after long term ageing, three different acetylene-terminated polyimide adhesives systems were prepared and characterized. The most promising system, a polyimidesulphone, was selected for a long term ageing study. The results indicate that this polyimidesulphone has improved strength retention over LARC-13 after ageing for extended time at high temperatures. In addition, it exhibited surprising adhesive strength for a short time at 593°C. The synthesis and characterization of these adhesives are discussed.
 
This paper presents the thermal and mechanical contribution of anisotropic conductive films (ACFs) to the electrical conduction establishment of ACF joint. The conduction mechanism of ACF joint strongly depends on the thermal and mechanical properties of ACF. Therefore, it is important to understand the relationship of thermal and mechanical properties of ACF in a bonding process with the electrical conduction establishment. In this study, ACF flip chip process was fully designed based on the material characterization and in situ process monitoring. Moreover, the effect of degree of cure on the ACF conduction establishment was investigated in a bonding process window. The important mechanical mechanism of ACF conduction for good bonding quality is the joint clamping force due to curing and cooling-down processes of ACFs. The build-up behavior of z-axis shrinkage stress in ACF joint during curing and cooling-down processes of ACF materials was experimentally investigated with thermo-mechanical measurement of ACF. These results reveal that shrinkage stress in ACF joint developed during bonding process is the important parameter to establish the electrical conduction of interconnects using ACF material.
 
Pressure sensitive adhesives (PSAs) were produced using latexes synthesized via a starved seeded semi-batch emulsion polymerization process with butyl acrylate (BA), methyl methacrylate (MMA) and sometimes additional monomers, 2-hydroxy ethyl methacrylate (HEMA) and/or acrylic acid (AA). For the BA/MMA comonomer latexes, the amount of cross-linker (allyl methacrylate), monomer emulsion and initiator solution feeding times, and the BA/MMA weight ratio were manipulated to vary the polymer properties. The performance of PSA films cast from these latexes was evaluated by tack, peel strength and shear strength. The effect of polymer properties on PSA performance was related to the viscoelastic properties of the PSAs. For BA/MMA latexes, it was not possible to greatly improve the peel strength even at great sacrifice of shear strength for PSAs with gel contents of ∼60 wt% or higher. This was because the ratio of the loss modulus at debonding frequency to the storage modulus at bonding frequency did not vary significantly for the conditions studied. The addition of HEMA provided a significant influence on latex polymer properties as well as PSA performance for both high (>60 wt%) and low (∼20 wt%) gel contents. For some cases, tack, peel strength and shear strength were simultaneously and greatly improved by the addition of HEMA. Adding both AA and HEMA while decreasing the amount of emulsifier, also resulted in a PSA with much better performance (i.e., higher tack, peel strength and shear strength) than a BA/MMA PSA with similar gel content (∼60 wt%).
 
Dual-curable adhesives were prepared using various epoxy acrylate oligomers, a reactive diluent, photoinitiators, a thermal-curing agent and a filler. The UV- and thermal-curing behaviors of the dual-curable adhesives were investigated using photo-differential scanning calorimetry (photo-DSC), Fourier transform infrared-attenuated total reflection (FTIR-ATR) spectroscopy, and the determination of the gel fraction, pendulum hardness and adhesion strength.The reaction rate and extent of UV curing were found to be strongly dependent on the concentration of CC bonds in the epoxy acrylate oligomers. The FTIR-ATR absorption peak areas representing the relative concentration of CC bonds in the epoxy acrylate oligomers and trifunctional monomer decreased with increase in UV dose because of photopolymerization. When the dual-curable adhesives were irradiated with UV light, the gel fraction increased with increase in CC bond contents in the epoxy acrylate oligomers. Also, after thermal curing, the gel fraction was highly enhanced due to the cross-linking reaction of the unreacted glycidyl groups in epoxy acrylate oligomers induced by the thermal-curing agent. This cross-linked structure of the dual-curable adhesives affects the pendulum hardness and adhesion strength.
 
The present paper examines the reduction of fixture time of a commercially available structural acrylic adhesive while maintaining its ultimate mechanical properties. Experimental work was performed by varying methods of addition and quantities of a promoter pair (saccharin and N,N-dimethyl-p-toluidine (DMpT)) with copper as the bonding substrate. Fixture time and ultimate strength were measured by lap-shear testing at both short and long intervals, while extent of cure was followed simultaneously by differential scanning calorimetry (DSC). Choices of optimal ratios of promoters and addition mode were found to yield substantial reductions in set time—down to 2.5 min, while retaining the desired mechanical performance.
 
Bond failures at the acrylic teeth and denture base resin interface are still a common clinical problem in prosthodontics. The effect of methyl methacrylate (MMA) monomer on the bond strength of three types of denture base resins (Acron MC, Lucitone 550 and QC-20) to two types of acrylic teeth (Biotone and Trilux) was evaluated. Twenty specimens were produced for each denture base resin/acrylic tooth combination and were randomly divided into control (acrylic teeth received no surface treatment) and experimental groups (MMA was applied to the surface of the acrylic teeth for 180 s) and were submitted to shear tests (1 mm/min). Data (MPa) were analyzed using three-way ANOVA/Student's test (α=0.05). MMA increased the bond strength of Lucitone denture base resins and decreased the bond strength of QC-20. No difference was detected for the bond strength of Acron MC base resin after treatment with MMA.
 
Air-activated acrylic adhesives are one-part, rapid room-temperature setting adhesives which cure by redox-initiated polymerisation of (meth)acrylate monomers. Their cure is triggered by ambient atmosphere and results in cured adhesive properties similar to conventional two-part acrylic adhesives. A test method developed using dynamic mechanical thermal analysis to characterise their cure behaviour was subsequently used in the assessment of the effects of ageing on the adhesives. The objective of this study is to investigate if the rate of cure of the adhesives retarded on storage at room temperature over a period of one year. Tubes of adhesive were tested at selected time intervals and the findings are reported and analysed using analysis of variance. It was found that over a 12 month period there was no ageing effect on the cure characteristics of air-activated adhesives.
 
In this study, the synthesis and characterization of acrylic polymer/montmorillonite (MMT) clay nanocomposite pressure sensitive adhesives (PSA) are presented. Different types and amounts of modified and unmodified montmorillonite clays were dispersed in ethyl acrylate (EA)/2-ethylhexyl acrylate (2-EHA) monomer mixture, which was then polymerized using a suspension polymerization technique. Polymerization was monitored in-line using attenuated total reflectance-fourier transform infrared (ATR-FTIR) spectroscopy. The adhesion properties of the synthesized nanocomposite materials were determined using standard measurements of tack, peel and shear strength. Viscoelastic properties of dried adhesive films were analyzed using dynamic mechanical analysis (DMA). The results showed that the kinetics of suspension polymerization was independent of the addition of MMT clays. On the other hand, adhesive properties were strongly influenced by the type and the amount of MMT clay added. While peel strength and tack gradually decreased with higher amount of modified MMT clay, a substantial increase in shear strength was determined with a maximal value at 1 wt% of added MMT clay. Moderate influence on tack, peel and shear strength was observed when the unmodified type of MMT clay was used. DMA analysis showed an increase in storage modulus (G′) for adhesives synthesized with MMT clay addition, but no significant differences were determined between particular types of MMT clays. A decrease in tan δ value for adhesives with 1 wt% of added MMT clay was observed, which also concurs with higher shear strength and implies to the improved cohesion of adhesive.
 
The effects of four surface preparation methods on bonds produced with three acrylic adhesives have been studied and compared with those for a previously tested acrylic adhesive. The comparisons were made using lap shear strengths, and fracture energies obtained from the Boeing wedge test.
 
In an attempt to enrich the world of dentistry through the development of new materials, this study proposes to synthesize and incorporate a monomer containing the N-hydroxysuccinimide (NHS) ester reactive group to a dental adhesive. As such, this study developed a simple method to obtain NHS esters by employing acrylic acid (AA) and NHS in the presence of EDC (N-ethyl-N′-(3-dimethylaminopropyl)carbodiimide), from an aprotic medium. The experimental N-acryloxysuccinimide (NSA) monomer was analyzed by infrared spectroscopy (FTIR) and nuclear magnetic resonance of hydrogen (1H NMR). This monomer was then incorporated within a prepared dental adhesive, in a 5% proportion. The modified adhesive, containing NSA, was applied to the dental surface of bovine teeth to obtain an adhesive/dentin interface in vitro. The characterization of this interface by Raman spectroscopy presented the formation of new amide bonds. Moreover, through scanning electron microscopy (SEM), it became possible to observe an intense penetration of this modified adhesive on the dental surface. Considering the outcome, it can be concluded that the synthesized NSA monomer provided a favorable condition for the dental adhesives to interact chemically with the dentin collagen fibers.
 
The dielectric relaxation spectra measured over a frequency range from 10−1 to 105 Hz and over a temperature range from 245 to 350K are reported for two delayed action heat seal adhesives. Measurements of the peel strength were made over a temperature range from 225 to 313K at two peel rates, 40 and 200 mm min−1. A correlation was observed between the magnitude of the dielectric loss and the peel strength for these adhesives, indicating the role of polymer motion in determining the adhesive strength. The importance of these observations, both in the context of the fundamental understanding of the peel process and in the design of new adhesive systems, is discussed.
 
The aim of this study was to finalise a 90° peeling test on 60 human teeth. The dentinal surfaces to be stuck were characterised by micro-hardness, wettability, optical microscopy, SEM, AFM, XPS and EDS. The so-called fifth-generation bonding agent (Prime & Bond® 2.1 DeTrey Dentsply) was used to perform the peeling-test. The extremity of a membrane was included in the adhesive which rendered it possible to hold. The other extremity of the membrane was connected to an Instron universal testing machine, on which the peeling system (MTS) was fixed. The cross head speed was set to 0.5 mm s−1.Two types of curves (force versus displacement) were obtained: for bond strengths higher than 5 N (average=7 N) curves were typical of cleavage; for bond strengths lower than 5 N (average=2.8 N) peeling was observed. The obtained curve which showed maximal stress versus strain, indicated the membrane rheology and made it possible to rank the samples with respect to their surface fracture. Correlation between failure and physico-chemical dentine composition was observed.The observation of the fracture surface with an electron microscope in combination with X-ray analysis by energy dispersive spectroscopy showed interfacial failure when cleavage was obtained and cohesive fracture when peeling was obtained. It showed some very heterogeneous fractures, from adhesive resin interfacial fractures to cohesive fractures in the adhesive resin or restorative composite resin. This work demonstrated that dentine/resin adhesive interface was highly complex. The adherence results depended not only on dentine superficial parameters, but also on the physico-chemistry, rheological behaviour of the resin and the composite and on test parameters (e.g. speed, etc.).
 
This paper deals with glass-fiber-reinforced vinylester composite laminates manufactured by resin infusion and bonded with an epoxy adhesive. The effect of joint configuration, adhesive layer thickness, defects, humidity, spew fillet, and adherend stiffness were investigated by means of tension tests. Four joint configurations were considered: two configurations representing real aeronautical applications namely, the joggle lap joints (JLJ) and the L-section joints (LSJ) and two configurations representing the most popular standard joints namely, the single lap joint (SLJ) and the double strap joints (DSJ). The SLJ and DSJ show higher ultimate loads and displacements to failure than the JLJ and LSJ. The failure load and displacement were found to decrease dramatically when the adhesive layer thickness was increased or when the joint was aged in a hot-humid environment. Intentionally unbonded area in the middle of the joined area did not seem to affect the joint resistance. The investigation of the spew fillet and adherend stiffness for the JLJ showed that the spew fillet effect depended on both the adherend stiffness and the adhesive ductility. The fracture mechanisms were found to change dramatically. Finally, the effect of adherend stiffness was investigated for the JLJ configuration with adherend laminates having stacking sequences of (±45)n and (0/90)n. Most of the joints made from (0/90)n laminates failed at the adherend joggle knee and all the joints made from (±45)n laminates failed at the interface at lower load.
 
A recently popular method for retrofitting reinforced concrete (RC) beams is to bond fibre reinforced polymer (FRP) plates to their tensile faces. An important failure mode of such plated beams is the debonding of the FRP plates from the concrete due to high level of stress concentration in the adhesive at the ends of the FRP plate. This paper presents an improved solution for interfacial stresses in a concrete beam bonded with the FRP plate by including the effect of the adherend shear deformations. The analysis is based on the deformation compatibility approach where both the shear and normal stresses are assumed to be invariant across the adhesive layer thickness. In the present theoretical analysis, the adherend shear deformations are taken into account by assuming a parabolic shear stress through the thickness of both the concrete beam and the bonded plate. Numerical results from the present analysis are presented both to demonstrate the advantages of the present solution over existing ones and to illustrate the main characteristics of interfacial stress distributions.
 
As part of the National Center for Manufacturing Sciences contract on adhesive bonding of dissimilar materials, UTRC, as a sub-contractor, is evaluating environmentally acceptable surface preparation techniques for selected metals and composites. The selected processes are automatable and capable of being transferred into the manufacturing arena. One aspect of the programme is the evaluation of plasma-sprayed, microporous, thin coatings as surface pretreatments for both resin-based composites and metals. Mechanical test results are presented which demonstrate the viability of this approach to achieve acceptable joint strengths with steel, titanium and aluminium bonded to several types of resin-matrix composite. The environmental impact, by elimination of organic primer systems and strong acid etching and anodizing solutions, will be discussed.
 
Applications for structural adhesive in a marine environment will generally involve much thicker adherends than used in some other industries. The effect of aging on adhesively bonded joints in a wet environment is particularly important for marine applications. A major concern is the sensitivity of the adhesive to the effects of water. The durability performance of thick-adherend steel lap joints and the bulk adhesive were investigated using accelerated aging techniques (30°C, 100% relative humidity). Various simple geometric factors are shown to affect joint performance. The removal of the spew fillet, the application of a stress and the joint orientation will all significantly influence durability performance. It was found that the water affects both the adhesive and the adhesive/adherend interfacial zone.
 
The effect of the length and depth of a parallel slot as well as the elastic modulus of the adhesive on the stress distribution at the mid-bondline and in the adherend was investigated using the elastic finite element method. The results showed that the peak stress in mid-bondline decreased markedly when there were two of parallel slots located in the outside of the adherend, corresponding to the middle part of the lap zone and the original low stress in this zone of the joint increases. The peak stress decreased at first, and then increased again as the length of the parallel slot was increased. The stress distribution in the mid-bondline at the position corresponding to the parallel slot decreased significantly as the depth of the parallel slot was increased. The high peak stresses caused by the tensile load occurred close to the edge of the parallel slot in the adherend. Almost all the peak values of stresses at the mid-bondline increased when the elastic modulus of the adhesive was increased. The effect of the parallel slot on the peak stress at the mid-bondline with a low elastic modulus adhesive was negligible, but the peak stress decreased markedly for adhesives with a high elastic modulus.
 
Adhesively bonded structures are widely used in the aerospace industry. Unfortunately, adhesion is a macroscopic property depending specifically upon numerous sub-microoscopic parameters so that producing a strong and durable bond is a difficult task which needs many fundamental and applied research investigations. In this context, the surface composition of adherends plays an extremely important role in the overall performance of a bonded structure: it is well known that surfaces of ‘as-received’ materials are chemically very different from the bulk composition and are usually unsuitable for adhesive bonding. Consequently, surfaces of such materials hav to be submitted to appropriate chemical or electrochemical prebonding treatments in order to modify both their morphology and chemistry.This paper discusses the surface characterization of aluminium, titanium and their alloys at various prebonding stages. Practical applications of X-ray fluorescence spectroscopy, low-energy electron-induced X-ray spectroscopy and glow discharge optical spectroscopy are described.
 
Carbon fibre composites are being widely considered for many classes of heavily loaded components. A common feature of such components is the need to introduce local or global loads into the composite structure. The use of adhesive bonding rather than mechanical fasteners offers the potential for reduced weight and cost. However, such bonded joints must be shown to behave in a predictable and reliable way. A major aspect of this is to demonstrate that the progress of cracks through the bonds is well understood. The simulation work presented here complements the experimental work presented in Part I. The observed failure processes and their sequence are successfully described and modelled.
 
Adhesive joints with dual adhesives to be used over a wide temperature range (−55 to 200 °C) were studied theoretically in a previous paper. The joint strength predictions have shown that, for identical adherends, the mixed modulus technique is of little benefit. However, for metal/composite joints, there is a real improvement, especially if the difference of coefficients of thermal expansion is high.The objective of the present study is to investigate experimentally if the use of a mixed adhesive joint is advantageous as predicted. The manufacture is particularly complex, and a way to produce such joints has been described. Experiments are then presented for titanium/titanium and titanium/composite double lap joints. It is shown that, for a joint with dissimilar adherends, the combination of two adhesives gives a better performance (increased load capacity) over the temperature range than a high temperature adhesive alone. Mixed adhesive joints were also cycled thermally to prove that they can be used at low temperatures after a stage at high temperatures, and vice versa.
 
Preadhesion treatment of two thermoplastic substrates (Lexan polycarbonate and Ultem polyetherimide) by means of ArF excimer laser has been evaluated by comparing the lap shear strengths of polyurethane-bonded joints with untreated, laser-treated and SiC abraded adherends. Experimental results showed a significant increase in lap shear strength following ArF laser treatment. Different laser conditions were needed to achieve the same level of bond strength improvement for both thermoplastic adherends. Surface analysos of the ArF laser-treated adherends and the fractured polyurethane joints revealed morphological and chemical modifications due to the intense ultra-violet laser irradiation. For best results laser irradiation intensity, dose and operating atmosphere should be further studied.
 
The influence of stress-reduction methods on the strength of adhesively bonded joints composed of brittle adherends was studied. Experimental and numerical investigations were carried out on two types of adherends: (a) fibre reinforced polymers and (b) timber, considering three different stress-reduction methods: (i) adhesive roundings, (ii) chamfering and (iii) adhesive grading. The experiments showed that the increase in strength using these stress-reduction methods is negligible. Numerical analyses showed that although the stress peaks are reduced, these act over a larger volume. A probabilistic strength prediction method is applied, which explains most of the experimental results. The presented work allows for a better insight into the relation between stress reduction and strength increase of adhesively bonded joints, which are greatly affected by the brittleness of the adherends.
 
Carbon fibre composites are being widely considered for many classes of heavily loaded components. A common feature of such components is the need to introduce local or global loads into the composite structure. The use of adhesive bonding rather than mechanical fasteners offers the potential for reduced weight and cost. However, such bonded joints must be shown to behave in a predictable and reliable way. A major aspect of this is to demonstrate that the progress of cracks through the bonds is well understood. The work presented here illustrates one possibility for establishing a measure of control over that crack propagation.
 
This paper describes a finite element analysis of two adhesive bonded joint configurations, namely single lap joint and joggle lap joint, for which an extensive experimental analysis had been made previously. The adherends were assumed to behave as linearly elastic materials, while the adhesive layer was assumed to be nonlinear. The nonlinear geometric deformations of the joint were also taken into account. The numerical results agree well with the experimental results up to 2000 N. Moreover, the distinctive geometric deformations observed experimentally with the joggle lap joint under loading were accurately predicted by the finite element model.
 
In this work, the effect of oxygen plasma on the surface characteristics of polypropylene (PP) and birch was investigated. In addition, studies on the effects of hexamethyldisiloxane (HMDSO) plasma on the surface characteristics of lignocellulosics and on the adhesion properties of PP to the HMDSO-treated wood surface were carried out. Atomic force microscopy (AFM) and contact angle measurements were used to study the changes in the topography and surface free energy of the materials due to plasma treatments. AFM revealed distinct changes in the topography of PP due to oxygen plasma. Nodular structure is formed on the PP surface during the treatment and the size of the nodules increases with the treatment time. The extent of the topographical changes was nearly equal in the case of the two power levels studied. A clear increase in the surface free energy of PP and wood due to oxygen plasma was recorded. AFM studies on filter paper, kraft pulp and birch surfaces treated with HMDSO plasma showed that the plasma polymer follows the features of the substrate without forming an actual film on the surface at the treatment levels used. The substrate surfaces became highly hydrophobic when exposed for 5 min or longer to the plasma. No improvement in the adhesion of PP film to wood was achieved by pre-treating the substrate with HMDSO plasma polymer.
 
Different amounts of a polyisocyanate (3–5 wt%) were added to a polychloroprene (PCP) adhesive to determine their influence on the adhesion properties of an ethylene vinyl acetate copolymer containing 12 wt% vinyl acetate (EVA12). EVA12 was treated with sulphuric acid to increase its adhesion, and improved wettability, creation of carbon–oxygen moieties and roughness were achieved. The thermal, rheological and surface properties of the PCP and polyisocyanate mixtures were obtained. The higher the polyisocyanate content in the PCP, the higher the peel strength value of sulphuric acid-treated EVA12 adhesive joints. By increasing the polyisocyanate content in the adhesive, the locus of failure changed from adhesion to mixed (adhesion+cohesion in EVA12). The ageing of the sulphuric acid-treated EVA12/PCP+polyisocyanate adhesive joints produced a decrease in T-peel strength, although always a cohesive failure in the EVA12 was obtained.
 
This paper investigates the use of silane reagents in a non-traditional role as a primer for modifying the chemical characteristics of chromium (Cr) tanned heavy-duty leather (Salz leather). The silanes investigated were 3-aminopropyltriethoxysilane (APES) and 3-(2-aminoethylamino)propyltriethoxysilane (AEAPES), applied as a polymer solution to leather surfaces treated with an isocyanate monomer to create an interpenetrating polymer network (IPN), prior to bonding with the polyurethane adhesive. The assembled strengths of APES and AEAPES samples were measured using the T-peel test against 2-hydroxyethyl methacrylate samples and conventionally treated samples (abrasion, chloroform and surfactant washing), under dry and wet testing conditions. The physical structure of leather surfaces was investigated using scanning electron microscopy (SEM) and the chemical composition by electron dispersive X-ray spectroscopy (EDX) for Cr, Si and Al impurities. The results indicate that APES and AEAPES treatments significantly enhance surface adhesion with respect to abrasion alone and in the latter case by 179% (dry) and 52% under wet conditions. The adhesive joints tested showed cohesive failure indicating that the improvements in adhesion were due to the generation of a consistent network across the interface due to increased physical adsorption of the silane polymer with the leather surface and interaction with the polyurethane adhesive via van der Waals forces and/or hydrogen bonding.
 
Certain small creeping animals and insects generate the necessary adhesion force through the hairs of their feet. In addition to size, the shape of the fibrillar structures dramatically affects adhesion performance. One must develop a mathematical model to represent the relationship between the characteristics of fibers, surface roughness and adhesion quality. This paper provides a mathematical model and shows that wide-end fibrillar systems (WFS) are more compatible with variation in surface roughness. These systems adhere better than conventional systems with simple cylindrical fibers. The optimum geometry of fibers for maximum adhesion is discussed in this article. The obtained results are in good agreement with biological studies and are applicable to guide experimentation to achieve optimum artificial contacts.
 
The polymers PET, PA6, PVDF, HD-PE, and PP are activated by a commercially available plasma jet system at atmospheric pressure to improve adhesive bondability. The adhesion properties of the activated surfaces are evaluated by lap shear tests. The results are correlated with the surface properties that are investigated by XPS, AFM, and contact angle measurements. In addition the influence of operational parameters of the plasma treatment is studied. The activated samples exhibit a substantially increased bonding strength. The improvement can be related to an increase of oxygen concentration, and to changes of the topology of the substrate surface induced by the thermal component of the plasma. The most influential parameters in the plasma treatment are the distance between substrate and nozzle exit and the treatment time.
 
The use of polymer films for technical applications has increased considerably in the last years, since they offer good balanced properties. Polymer films find many applications as individual materials or as laminates with other films, foams, membranes, etc. In these cases it is necessary to improve the low intrinsic surface energy of polymer films to ensure their optimum mechanical performance. In this work, low-pressure glow discharge plasma with different gases is used to improve the adhesive properties of a low-density polyethylene (LDPE) film, to obtain the optimum mechanical response of laminates with polyolefin foam for automotive applications (steering wheels). The results show a remarkable increase in T-peel strength of the adhesive joints. Furthermore, since automotive industry is characterized by high technical requirements, the evaluation of the durability of the adhesive joints (in terms of storage conditions: temperature and relative humidity) shows that the T-peel strength of adhesive joints is subjected to an aging process that slightly decreases their mechanical performance, but does not restrict the use of these laminates in automotive uses.
 
Bacterial adhesion on the surfaces of medical devices, food processing equipment, heat exchangers and ship hulls has been recognized as a widespread problem. Bacterial adhesion mechanism is complex and many factors affect cell adhesion. In this paper, the effect of surface free energy of the coatings on bacterial adhesion was investigated. The metal-polymer composite coatings with various surface free energies were developed by an electroless plating technique. Bacterial adhesion behaviour on these coatings was investigated. Contact angles were obtained using a sessile drop method with a Dataphysics OCA-20 contact angle analyser. According to the contact angle values, the surface energies of the samples and their dispersive and polar components were calculated using van Oss acid–base approach. The experimental results showed that the surface free energy of the coatings had a significant influence on bacterial adhesion. The bacterial adhesion behaviour on the surfaces was explained using the extended DLVO theory.
 
Top-cited authors
L.F.M. da Silva
  • University of Porto
R. D. Adams
  • University of Bristol
Raul Campilho
  • Instituto Superior de Engenharia do Porto
Gary Critchlow
  • Loughborough University
M. D. Banea
  • University of Aveiro