Influence of the interfacial reaction on interfacial adhesion in polyarylacetylene resin–silica glass composites

The influence of interfacial reaction on interfacial adhesion in silica glass/polyarylacetylene resin composites was studied. In order to achieve chemical reaction at the interface, vinyltrimethoxysilane was grafted onto silica glass surface to react with polyarylacetylene resin. The reaction between polyarylacetylene resin and vinyltrimethoxysilane was proved based on the model reaction between phenylacetylene and vinyltrimethoxysilane. At the same time, the modified silica glass surface characteristics were evaluated by contact-angle measurements and surface energy determination. The interfacial adhesion in silica glass/polyarylacetylene resin composites was evaluated by shear strength testing and fracture morphology analysis. It was concluded that polyarylacetylene resin reacted with vinyltrimethoxysilane. Furthermore, due to the reaction between polyarylacetylene resin and vinyltrimethoxysilane at the interface, the interfacial adhesion in composites was significantly increased. The improvement in interfacial adhesion was solely attributed to the interfacial reaction.     

Synergistic Toughening of Epoxy–Copper Interface Using a Thiol-Based Coupling Layer

A novel concept of tuning the fracture properties of the interface through the treatment process of the coupling layer according to the cohesive critical strain energy release rate of the epoxy is proposed for optimizing the joint strength between epoxy and copper substrate. In most coupling agent application recipes, the treatment condition design has omitted the influence of the fracture properties of the corresponding adhesive. Conceivably, excessive strengthening of the adhesive–substrate interface may not lead to optimal interfacial strength. Synergistic toughening of the interface takes place when there is simultaneous interfacial debonding and failure of adhesive under a comparable critical stress state. Under critical applied load, energy is concurrently dissipated through the fracture of the interface, the fracture in the adhesive, and possible non-reversible failure processes such as shear yielding or micro-cracking of the adhesive. These combined energy dissipation processes result in extensive energy absorption around the crack tip. The adhesive joint, therefore, becomes more crack resistant. In this study, the interfacial adhesion promotion concept with synergistic toughening was demonstrated using three different epoxy systems bonded to copper substrates modified by a thiol-based coupling layer. The coupling layer was formed by treating the copper substrate with a thiol-based coupling agent. Critical strain energy release rate of the treated tapered double cantilever beam samples in different treatment conditions was measured for each of the epoxy systems. From the failure path analysis, mixed interfacial and cohesive failure was observed. This observation indicated that extensive energy dissipation occurs around the crack tip that results in synergistic toughening of the interface. This work shows the significance of matching the fracture property of the coupling layer with the adhesive. Up to 2.3 times improvement in the critical strain energy release rate was achieved with optimized thiol treatment compared to non-optimized treatment.     

A Probe on the Failure Mechanism in Rubber-Modified Epoxy Blends: Morphological and Acoustic Emission Analysis

In this work, diglycidyl ether of bisphenol A based epoxy resin (DGEBA) was modified with varying amounts of two liquid rubbers: carboxyl terminated copolymer of butadiene and acrylonitrile (CTBN); and a hydroxyl terminated polybutadiene (HTPB), using an anhydride hardener. The ultimate aim of this study was to investigate the failure mechanism operating in the rubber-modified epoxies and to evaluate this by correlating these results with the miscibility and interfacial adhesion between the components and the morphology of the cured network. Some of the mechanical and fracture properties, which are associated with the two-phase particulate morphology, were investigated. The visoelastic behavior of modified epoxies was also analyzed and variations in the shift of T _g values in toughened epoxies were explained. The samples were carefully analyzed by an acoustic emission technique to investigate the failure mechanism operating in them. From the response of force and number of acoustic events as well as from the amplitude of acoustic events, we were able to explain the failure mechanisms in the elastomer incorporated epoxy resins supplemented by morphological evidence.     

26—THE RÔLE OF WATER IN THE SETTING OF WOOL: A STUDY OF SETTING AT TEMPERATURES ABOVE 100°C: PART I: DEGREE OF SET

Permanent setting of wool fabrics can be achieved by setting in steam autoclaves at temperatures above 100°C. An account is given in this paper of experiments performed on a laboratory scale and on full-scale equipment in which fabrics were set in rolls. One of the main problems in this kind of process is to ensure an even setting throughout the roll. Temperature and moisture regain are rate-determining factors in steam-setting, so a study was made of the temperature and moisture-regain changes in the different parts of the roll during heating up and subsequent steaming. The degree of set also depends on the steaming time and acidity of the cloth, but these factors are more easily controlled.

The investigation shows that the steam-setting of wool-containing fabrics imparts a very high degree of permanent set and that reproducible and even setting may be achieved, provided that all treatment variables are kept under close control. Of all the parameters studied, the moisture regain seems to be the most important.     

Nanocomposites of Polyaniline as Conductive Adhesives

Electrically conductive adhesives are emerging as the best possible alternative to traditional tin/lead soldering. In this work polyaniline (PANI) as powder and as nano-fibres was introduced as filler into epoxy/anhydride matrix to produce isotropic conductive adhesives (ICAs). PANI nano-fibres show uniform dispersion and thus percolation threshold is low. Uniformity and smaller size of nano-fibres help in formation of a strong epoxy network with least hindrance from filler phase. This results in better impact performance of these ICAs. SEM observations show an improved diffusion of nano-PANI into the epoxy matrix. Overall, the properties obtained with PANI nano-filler show a significant improvement when compared to PANI powder of macroscopic dimensions.     

Effects of the interfacial stress-induced crystallization on the matrix crystalline morphology and the mechanical properties of glass fiber-reinforced high density polyethylene composites

In the melt-mixing process of high density polyethylene (HDPE) and glass fibers (GF), four types of composites with various interfacial bond strength were obtained by adding maleic low molecular weight polyethylene (MPEW) or maleic anhydride (MAH) and initiator, etc. The mechanical properties of these composites and their dependence on the matrix crystalline morphology were investigated by scanning electron microscopy, small-angle light scattering, differential scanning calorimetry, wide-angle X-ray diffraction, and a material universal mechanical testing machine. The occurrence of the interfacial transition regions made of extended-chain crystals around glass fibers was found to be the result of crystallization effects induced by the interfacial stress. The interfacial stress was mainly produced from the matrix shrinkage in the specimen molding process. Under high interfacial bond strength conditions, the forming process of the extended-chain crystals was found to both relax the interfacial stress and at the same time, enhance the interfacial phase modulus and improve the mechanical properties of the composites. Under a 30% glass fiber content condition, the extended-chain crystals formed along the normal direction of glass fiber surfaces connected with each other, fully filled the matrix, and led to a significant increase in the Charpy impact strength of the composites. By contrast, under weak interfacial adhesion, the interfacial stress was released by a dewetting process of the interfacial phase and by the formation of interfacial cracks. Consequently, the interfacial stress did not influence the growth of the spherulites in the matrix and at the same time, the Charpy impact strength of the composite was lower.     

The effects of surface roughness and bond thickness on the fatigue life of adhesively bonded tubular single lap joints

Since the surface roughness of adherends greatly affects the strength of adhesively bonded joints, the effect of surface roughness on the fatigue life of adhesively bonded tubular single lap joints was investigated analytically and experimentally by a fatigue torsion test. The stiffness of the interfacial layer between the adherends and the adhesive was modelled as a normal statistical distribution function of the surface roughness of the adherends. From the investigation, it was found that the optimum surface roughness of the adherends for the fatigue strength of tubular single lap joints was dependent on the bond thickness and applied load.     

Effects of moisture and temperature ageing on reliability of interfacial adhesion with black copper oxide substrate

The reliability of adhesion performance of bare Cu, as-deposited and surface-hardened black oxide coatings on Cu substrates was studied. The interfacial adhesion with a polyimide adhesive tape and an epoxy moulding compound was measured using the button shear and tape peel tests after hygrothermal ageing in an autoclave, high temperature ageing and thermal cycles. Moisture adsorption and desorption studies at different aging times suggested that the black oxide coating was effective in reducing the moisture adsorption. The bond strengths for all substrates remained almost unchanged after thermal ageing at 150°C for 8 h. Thermal cycling between ?50°C and 150°C for 500 cycles reduced by about 20% the button shear strength of the as-deposited black oxide substrate, but it did change much the bonding performance of the bare Cu substrate. Hygrothermal ageing at 121°C/100% RH in an autoclave was most detrimental to adhesion performance because of the combined effect of elevated temperature and high humidity. The reduction in button shear strength after the initial ageing for 48 h was 50–67%, depending on the type of coating. In all accelerated ageing tests, the residual interfacial bond strengths were consistently much higher for the black-oxide-coated substrates than the bare Cu surface, confirming a higher reliability of black oxide coating. Fracture surfaces analysis of tape-peeled bare copper substrates after 500 cycles of thermal loading revealed a transition in failure mechanism from interfacial to cohesive failure. In contrast, the failure mechanism remained unchanged for black-oxide-coated substrates. The observations made from the button shear and tape peel tests were generally different because of the different fracture modes involved.     

Estimation of dynamic adhesion from static test results in the model cord–RFL–rubber system

Generally, nylon and polyester cords are used to reinforce rubber compounds. These composites are used in many sectors, such as tire and belt manufacturing. To increase adhesion performance a resorcinol–formaldehyde–latex (RFL) adhesive is applied on the cord, which bonds chemically to both cord and rubber and, thus, it improves both the thermodynamic work of adhesion and the loss function at the cord/rubber interface. Adhesion strength between the cord and rubber determines the performance of the system. So to study the performance of the cord–rubber system, adhesion strength must be evaluated. Cord–rubber adhesion strength can be evaluated in static and dynamic modes. The H-Pull (H-adhesion) test method is a static and relatively simple method that is usually employed to control raw material quality. Fatigue test is one of dynamic adhesion test methods that are used to determine the performance of cord–rubber interface. Some important factors such as cyclic stress and heat buildup are involved in this test procedure. To investigate the accuracy of the H-Pull test results, the cord–rubber samples were prepared using poly(ethylene terephthalate) (PET) cord and NR/SBR rubber. Then H-adhesion was determined at elevated temperatures. The adhesion strength was also evaluated in dynamic (fatigue) mode at different temperatures. Authors have proposed an equation to estimate dynamic adhesion from H-Pull test results.     

Peel testing of adhesively bonded thin metallic plates with control of substrate plastic straining and of debonding mode mixity

A new peel test method has been developed for allowing debonding of adhesively bonded, thin metallic substrates. The rolling of the substrates on rollers allows control of the plastic dissipation during substrate bending and prevents dissipation associated with substrate unbending. The measured debonding toughness is shown to be independent of substrate thickness when proper account is taken of the work required for straining the adhesive layer. Mechanical analysis shows that control of plastic dissipation in substrates remains effective, as long as the ratio of the plate thickness to the roller radius is large enough to prevent wound-up. Mode mixity during debonding can be conveniently modified by changing the radii of the rollers. This possibility of altering mode mixity allows a better control of the occurrence of the cohesive, or near-interfacial debonding mechanisms. Evidence is given for the fact that the debonding mechanism is not governed by the minimum in the debonding toughness. A significant increase of debonding toughness with increasing mode mixity is observed only when the substrate roughness is high.