High-frequency welding of glass–fibre-reinforced polypropylene with a thermoplastic adhesive layer: Effects of ceramic type and long-term exposure on lap shear strength

In this study, high-frequency (HF) welding of glass–fibre-reinforced polypropylene (GF/PP) with thermoplastic adhesive layers consisting of zinc oxide (ZnO), anatase-type titanium oxide or silicon carbide was investigated. Effects of the ceramic type and content on the dielectric and temperature characteristics of these adhesive layers were evaluated experimentally, and the ratio of the dielectric loss tangent to the relative dielectric permittivity (tanδ/ε′), which was the index of the HF heating efficiency, exhibited different tendencies by these parameters. This value increased rapidly with increasing temperature even at 10 vol% ZnO, suggesting that the heating of the adhesive layer may be accelerated by combining temperature rise with a small amount of ZnO. During the HF welding process, the ZnO-containing adhesive layer bound to GF/PP in the shortest time (18 s) with high bond strength (~14 MPa). The effects of temperature (50 °C) and moisture (80% relative humidity) on the mechanical strength of the HF-welded specimens after a long-term exposure were also examined.     

Mechanical characterization of a high elongation and high toughness epoxy adhesive

In this paper, a new epoxy adhesive has been mechanically characterized. The adhesive combines the properties of an epoxy adhesive and typical polyurethane (PU) adhesive, such as high elongation and high toughness. Experimental tests were performed to measure the tensile properties, shear properties, thermal properties and fracture properties. The tensile test shows high tensile strength and high elongation. The single lap joint (SLJ) test shows that the failure load is proportional to the overlap length for hard steel adherends. For the SLJs with mild steel adherends, the failure occurred due to adherend yielding. Impact tests were conducted using SLJ specimens and the results are consistent with the SLJ tested under static conditions. The Tg was obtained using a Dynamic Mechanical Analysis (DMA) type of test. The toughness in mode I was determined using the Double Cantilever Beam (DCB) test and the toughness in mode II using End Notched Flexure (ENF) test.     

Development of a two-component structural adhesive for bonding of metals and polymeric composites

To minimize the structure distortion and potential de-bonding in adhesive bonding of dissimilar materials (e.g., metals and polymeric composites), a two-component (2 K) low temperature cure modified adhesive consisting of 93.5 wt% commercial Henkel 5089 adhesive, 2.5 wt% N-(2-Hydroxyethyl) ethylenediamine (AEEA) and 4.0 wt% 2-ethyl-4-methylimidazole (2,4-EMI) was formulated. Experimental results showed that the use of the modified adhesive lowered the curing temperature from recommended 177 °C (for 20 min) for Henkel 5089 to 100 °C (for 20 min) or 120 °C (for 10 min) for AA6061-AA6061 joint, and 120 °C (for 20 min) or 130 °C (for 10 min) for AA6061-C_f/PA6 (Nylon 6) and C_f/PA6-C_f/PA6 joints, respectively, due to the faster curing reaction caused by the combined addition of AEEA and 2,4-EMI. It took 5, 3, and 2 days to cure the adhesive-bonded AA6061-AA6061, AA6061-C_f/PA6, and C_f/PA6-C_f/PA6 joints made with the modified adhesive and cured at ambient temperature, respectively. In addition, the modified adhesive had sound working life (5 h) at ambient temperature. The static strengths of all adhesive-bonded AA6061-AA6061, AA6061-C_f/PA6, and C_f/PA6-C_f/PA6 joints with the modified adhesive were hardly affected by thermal exposure cycle (i.e., exposure to 82 °C for 30 min). These results indicated that the modified adhesive possesses the promising characteristics for joining of similar and dissimilar materials.     

The preparation and application of a new formaldehyde-free adhesive for plywood

In this study, we developed a new formaldehyde-free adhesive prepared by in-situ chlorinating graft copolymerization for plywood. The main ingredients of this adhesive include maleic anhydride (MAH) and high density polyethylene (HDPE) that is MAH grafted onto HDPE (PE-cg-MAH). The reaction between this adhesive and veneer, the optimum formulation to bond veneer and the optimum hot-press conditions to prepare the plywood were investigated. A boiling water test was employed to evaluate the strength and water resistance of plywood bonded with this adhesive. The results showed that the properties of the resulting plywood using PE-cg-MAH as an adhesive can meet the standard of Type I plywood and the optimum hot-press conditions were 160-165 °C and 5 min. When the chlorine contents of PE-cg-MAH was about 3% (wt%), the plywood panels had a higher shear strength after boiling water test above the hot-press conditions.     

A Bonded Plate Having Orthotropic Inclusion in the Adhesive Layer under In-Plane Shear Loading

In the present paper, the shear buckling analyses of adhesively bonded plates including an orthotropic material were performed. The adherends were selected as isotropic homogenous materials and a single adhesive was used in the overlap region. In the adhesive layer, an orthotropic material was located as an inclusion and its shape was assumed with an elliptical geometry. Three different orthotropic inclusions were selected: glass/epoxy, graphite/epoxy, and boron/epoxy. The effects of the inclusion shape and properties on the shear buckling were investigated via finite element analyses. The finite element analyses were verified according to an analytical solution having a full homogeneous isotropic adhesive layer. The analysis results showed that the maximum buckling loads were obtained when using the boron/epoxy inclusion compared to the glass/epoxy and graphite/epoxy inclusions. In addition, the possibility of delamination in the vicinity of the inclusion was examined. By this means, the influences of the delamination shape and magnitude on the critical buckling load were investigated. The greatest effect was obtained when the delamination shape was close to a circle and its magnitude was the largest.     

High conductivity of isotropic conductive adhesives filled with silver nanowires

As an alternative to lead-bearing solder, isotropic conductive adhesives (ICA) have been utilized for many years in microelectronic packaging. In this study, silver nitrate (AgNO₃) as precursor, N-N-dimethylformamide (DMF) as solvent and reducing agent, preparing silver (Ag) nanowires in the nanoporous templates formed by the controlled hydrolysis and condensation of butyl titanate (Ti(OC₄H₉)₄). The Ag nanowires were characterized by X-ray diffraction and transmission electron microscopy. An isotropic conductive adhesive (ICA) has been developed by adding Ag nanowires as conductive filler. Bulk resistivity and shear strength of the ICA are measured and compared with those of conventional ICA filled with micrometer-sized Ag particles (about 1 μm) and nanometer-sized Ag particles (about 100 nm). It is found that the ICA filled with lower content of Ag nanowires exhibits lower bulk resistivity and higher shear strength than ICA filled with micrometer-sized Ag particles and nanometer-sized Ag particles. Possible conductive mechanisms of the ICA are discussed.     

Experimental study of the influence of adhesive reinforcement in lap joints for composite structures subjected to mechanical loads

The paper deals with experimental investigations on reinforcing the adhesive in single lap joints subjected to mechanical loads such as tensile, bending, impact and fatigue. The adhesive used for bonding was an epoxy reinforced with unidirectional and chopped glass fibres as well as micro-glass powder. The adherends were glass reinforced composite laminates. The bonding surfaces were prepared before joining. In the case of unidirectional fibres in the adhesive region, the fibre orientations considered were 0°, 45° and 90°. The volume fraction of fibres in the adhesive layer in all the cases was 30%. The volume fractions of micro-glass powder were 20%, 30% and 40%. The tensile, bending, impact and fatigue tests on the prepared specimens were conducted according to ASTM standards. The results show that except the 90° unidirectional orientation, reinforcing the adhesive with glass fibres or powder increases the joint strength. The use of volume fraction of 30% of micro-glass powder gave the best performance in the above loading conditions. The fatigue life increased by 125%, the ultimate joint strength in tension increased by 72%, the bending ultimate joint strength increased by 112% and the impact joint strength increased by 63%. The microstructure of the debonded area was examined and three modes of failure could be observed namely cohesive failure, light fibre-tear failure and thin layer cohesive failure.     

Effect of a Self-Etching Adhesive System Containing a Novel Antimicrobial Polymer (QAMP) on Inhibiting Carious Lesions by Evaluating the Mechanical Properties of the Resin–Dentin Interface

The objective of this study was to assess the in vitro effect of a quaternary ammonium methacrylate polymer (QAMP) incorporated into a self-etching adhesive system on inhibiting caries by evaluating the mechanical properties of the adhesive interface. Twenty-four human third molars were distributed into: Clearfil? SE Bond containing 5% QAMP (experimental group), Clearfil? Protect Bond (positive control), and Clearfil? SE Bond (negative control). Teeth of each group were divided according to the method for producing artificial caries lesions: pH-cycling or microbiological assay. All samples were sectioned and polished in order to obtain hardness (H) and Young's modulus (E) values by nanoindentation test in the hybrid layer and dentin. Data were analyzed by ANOVA and Games Howell's post hoc test (α = 0.05). Regarding the hybrid layer, Clearfil? SE Bond containing QAMP demonstrated H and E values statistically higher than Clearfil? SE Bond in both pH-cycling and microbiological experiments. Considering dentin, Clearfil? SE Bond containing QAMP showed H and E values statistically higher than Clearfil? Protect Bond and Clearfil? SE Bond in the pH-cycling method and then Clearfil? SE Bond concerning the microbiological method. In general, Clearfil? SE Bond containing QAMP provided better mechanical properties for the resin–dentin interface after cariogenic challenges.     

Nanocomposite adhesives: Mechanical behavior with nanoclay

The major objective for this research was to examine the role of epoxy-clay nanocomposites in the area of epoxy bonding to porous stone (granite) substrates. Two bisphenol A epoxy systems were selected based on the prior work that determined optimal adhesive properties from a larger set of epoxy systems to determine the role of viscosity on the intercalation and exfoliation of the clay tactiods in the epoxy resin. The systems were characterized and mechanically tested at varying levels of intercalated and exfoliated organic clay tactiods. In the first stage of the work, epoxy-clay systems were characterized by wide-angle X-ray diffraction (WAXD) to detect inter-laminar distances of clay layers and to determine if the mixing procedures had indeed dispersed and exfoliated the clay layers sufficiently. The second stage of the work involved examining mechanical properties of the epoxy-nanoclay systems. Fracture behavior was studied using granite stone substrates in notched double lap configuration. Compressing a wedge between the cover plates induced the fracture. Fracture toughness was approximated by the load at fracture. Tensile properties were measured using cast dog bone tensile samples. The better layered silicate nanocomposite performance was seen with the lower viscosity resin. The most noticeable improvements in mechanical properties for the lower viscosity resin system were found to be maximum stress, elastic modulus, and yield stress. Increased toughness and stress whitening at 1% by weight nanoclay loading revealed that the clay can act as a shear-yielding toughening agent in this epoxy system.     

High strain-rate compression and tension behaviour of an epoxy bi-component adhesive

The dynamic behaviour of the bonded joints is influenced by the dynamic mechanical properties of the material of adherends and adhesives. The literature contains plenty of information about the dynamic mechanical properties of many structural materials (especially metals), obtained through different test types. Conversely, the study of the dynamic mechanical properties of the adhesives is not so common. The purpose of this work is to assess the dynamic mechanical behaviour of an epoxy bi-component adhesive for structural bonding. In particular, the study has been focussed on the influence of the strain-rate on the tensile and compressive strength of specimens made of adhesive. The experimental tests have been performed with a hydraulic universal testing machine and a tensile-compression Hopkinson bar. The results of the tests show that the adhesive strength increases substantially by increasing the strain-rate. The Cowper–Symonds and Johnson–Cook models of strain-rate dependence have been used to fit the experimental data with unsatisfactory results, thus also a poly-linear fit has been adopted.     

Standardized methodology for in vitro assessment of bone-to-bone adhesion strength

Advancement of adhesives technology has been limited in orthopedics, which still has a striking reliance on metal hardware to help facilitate fracture healing. Despite an obvious clinical need, bone adhesives are not currently available on the market. Testing the bone adhesion strength and other aspects of the adhesive performance is extremely complex. This paper presents standardized methodology for in vitro assessment of bone-to-bone adhesion strength. Two test configurations (lap shear and butt joint) were used to comparatively assess the adhesion strength of four commercially available adhesive material, poly(methyl methacrylate) cement (CMW), glass-ionomer cement (FUJI), dimethacrylate resin (SB) and cyanoacrylate resin (VB), which were allowed to set under two environmental conditions (air and water). Under dry conditions, both test configurations generally yielded similar measurements of adhesion strength, which was around 1.1, 2.8 and 9.1 MPa for CMW, FUJI and VB, respectively. The dry adhesion strength for SB measured using the butt configuration (2.5 MPa) was 43.2% higher compared to that measured using lap shear (1.4 MPa). In a wet environment, the measured adhesion strength generally decreased and was dependent on the test configuration used. The failure mode of the samples adhered using CMW was adhesive, while that using FUJI, SB and VB was a combination of both adhesive and cohesive, independent of the test configuration and the setting condition. This proposed methodology is comparable to ASTM standards and can be used to study the effect of different biomaterial formulations as well as test parameters on the bone-to-bone adhesion strength, in a reproducible manner.     

Design of functionally graded joints using a polyurethane-based adhesive with varying amounts of acrylate

Adhesives with graded properties along the bondline are being developed to increase the strength of adhesively bonded joints. Efforts to do this in the past have resulted in mixed results. Two adhesive parameters need to be considered: the geometry of the gradation and the material properties of the adhesive at different gradation levels. In order to consider both of these aspects, a computational model was created to aid in not only the design of adhesive gradations but also judge whether a specific adhesive gradation method will be able to result in strength increases. In this study, the model was introduced and compared with published results. A new adhesive gradation system was created by using a polyurethane-based adhesive with varying amounts of acrylate, and a numerical analysis was performed to determine the potential advantages of the adhesive gradation.     

Numerical and experimental analysis of vibration damping performance of polyurethane adhesive in machine operations

Machining operations inherently cause structural vibrations of the work piece and machine involved, especially during roughening operations. This clearly adversely affects the surface roughness, tolerances and tooling lifetime. These effects are even more prominent in case the stiffness of the work piece to be machined is low and machining vibration excitation frequency range strongly overlaps the range of structural resonance frequencies of the work piece. This is typically the case for lightweight (sheet) metal structures. Additional fixation is costly, time-consuming and moreover it may induce structural loading and deformation which, after finishing, causes an out-of-tolerance work piece.This work presents an alternative approach which applies Polyurethane foam adhesive bonding as a temporary measure to reduce structural vibrations during milling operations of a large welded sheet metal construction.The first part of this work analyses the actual structural vibrations that occur during milling operations. Experimental and numerical modal analysis is carried out in order to determine the excitation and resonant vibration behaviour of the sheet metal structure. A dedicated Finite Element Model (FE – model) is set up which enables the necessary insights in where and how critical vibration levels may be reduced and how the work piece supports can be optimized. The second part of the work discusses the development and application of suitable vibration dampening supports applying Polyurethane foam adhesive. For this purpose, experiments are carried out to determine the vibration dampening performance of different foam layer thicknesses. These involve hammer excitation of dedicated small-scale adhesively bonded samples. FE modelling is applied to optimize the mounting of bonded Polyurethane dampers and to predict the effect on structural vibrations during milling operations. This part extensively outlines the specific experiments involved and the strategy for modelling the viscoelastic foam dampeners in the structural numerical model in a robust but effective way. The third part describes the validation measurement campaign and extensively compares structural vibration levels before and after providing the foam dampening measures. It concludes by showing that critical vibration levels may be reduced by over 58% with the method applied.     

Adhesive joints in glass structures: effects of various materials in the connection,thickness of the adhesive layer,and ageing

This paper reports on an investigation of glued joints in glass load-bearing structures, with reference to the effect of various substrates (glass, steel, stainless steel, aluminium) and their surface treatment (sandblasting for the glass surface) on the adhesion of selected adhesives. The thickness of the adhesive layer and the effect of artificial ageing – a simulation of 5 years of ageing in outdoor central-European conditions – are also discussed. Tensile and shear tests were carried out on three series of specimens with various adhesives and substrates – two series for tensile and shear tests, and one series for shear tests on specimens exposed to ageing. Our results show that sandblasting the glass surface can improve the adhesion, and thus the strength values, of an adhesive joint in cases where, with a smooth glass surface, cohesive failure is not reached. The thickness of the adhesive layer had a significant effect for a semi-rigid acrylate adhesive, where the joint achieved higher strength values with less thickness of the glue. The effect of ageing varied according to the adhesive. The most visible changes were observed for a two-component acrylate adhesive and for methacrylate UV-adhesives. One of the selected glues was marked as unsuitable for load-bearing connections due to significant worsening of its mechanical properties after ageing.     

Use of organosolv lignin in phenol-formaldehyde resins for particleboard production: II. Particleboard production and properties

The objective of this work was to demonstrate the utility of lignin-based resins designed for application as an adhesive in the production of particleboard. Bond qualities of lignin-phenol-formaldehyde resins, phenolated-lignin-formaldehyde resins and commercial phenol-formaldehyde (PF-com) resin were assessed by using an automatic bonding evaluation system, prior to production of particleboards. In order to evaluate the quality of lignin-based resins, particleboards were produced and physical and mechanical properties were investigated. These physical properties included internal bond, modules of rupture and modulus of elasticity. Thickness swell and water absorption properties of particleboards bonded with lignin-based resins were also determined. The lignin-based resins have been reported previously in Part I of this study. The results showed that particleboards bonded with phenolated-lignin formaldehyde resins (up to 30% lignin content) exhibited similar physical and mechanical properties when compared to particleboards bonded with PF-com. The work has indicated that phenolated-lignin formaldehyde resins (up to 30% substitution level) can be used successfully as a wood adhesive for constructing particleboard. The performance of these panels is comparable to those of boards made using PF-com resin.     

Electrical and mechanical properties of electrically conductive adhesives from epoxy,micro-silver flakes,and nano-hexagonal boron nitride particles after humid and thermal aging

In this study, we incorporated micro-silver flakes and nano-hexagonal boron nitride (BN) particles into a matrix resin to prepare electrically conductive adhesives (ECAs). The humid and thermal aging results under a constant relative humidity level of 85% at 85 °C revealed that the aged ECAs containing 3 wt% of nano-hexagonal BN particles had high reliability. The contact resistance was low and the shear strength high. Nano-hexagonal BN particles have a good effect on the reliability of ECAs that can be used to improve the properties of ECAs.     

Studying the effects of chain extenders chemical structures on the adhesion and mechanical properties of a polyurethane adhesive

Chain extenders including diethanol amine (DEA) and 2-cyano-N,N-bis-hydroxyethyl acetamide (CNBA) were utilized to enhance the adhesion and mechanical properties of a polyurethane (PU) adhesive. The adhesion and mechanical properties of the adhesives were studied by peel test, tensile test and dynamic thermal analysis (DMTA) respectively. Results revealed that both chain extenders (DEA and CNBA) increased peel test modulus. The cross-linking density and toughness of the adhesive were significantly increased using chain extenders. Results showed that chain extender without side chain (DEA) could improve the adhesion and mechanical properties of the adhesive greater than chain extender with aliphatic side group (CNBA).     

The preparation and performance of high-temperature adhesives for graphite bonding

Two kinds of high-temperature adhesives (HTAs) were prepared. One was composed of phenol-formaldehyde (PF) resin and boron carbide (PF+B₄C), the other was composed of PF resin, B₄C and fumed silica (PF+B₄C+SiO₂). Graphite materials were bonded by the above adhesives and heat-treated at temperatures ranging from 200 to 1500 °C. The joining strength was tested at room temperature. The results show that the graphite joints exhibit satisfactory bonding strength and that ceramics fillers show a marked property modification effect. The strength of graphite joints bonded by PF+B₄C and PF+B₄C+SiO₂ adhesive and treated at 1500 °C are 9.3 and 17.1 MPa, respectively. The property modification mechanism of ceramics fillers is also discussed in this paper. A strong chemical bonding force is introduced at the bonding interface and the volume shrinkage is restrained, which can be responsible for the good adhesive properties of HTAs for graphite bonding.     

Effect of laminating parameters on the adhesion property of flexible copper clad laminate with adhesive layer

In this study, the effect of two laminating parameters, laminating pressure and holding time, on the adhesion strength of flexible copper clad laminate (FCCL) with an epoxy-type adhesive layer was evaluated. The changes in the adhesion property, fracture surface, morphology, and chemical bonding state were characterized by 90° peel test, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The results showed that the adhesion strength of the FCCL was decreased as the laminating pressure was increased beyond the suitable level. Laminating pressure exerted the greatest influence over the FCCL adhesion strength. On the other hand, the holding time did not significantly affect the peel strength of the FCCL. The fracture of FCCL occurred at the interface between the rolled Cu and the adhesive layer. In addition, the FCCL with high adhesion strength was stable with the variation of adhesion strength after temperature and humidity test.     

Experimental investigation and numerical modelling of the mechanical response of a semi-structural polyurethane adhesive

The use of adhesives in load-carrying structures and components has increased recently, especially in the automotive industry. There has been many studies on structural adhesives, but when it comes to semi-structural adhesives, there is a lack of literature. In this article, a semi-structural two component polyurethane adhesive has been studied experimentally and modelled numerically. It was performed uniaxial tension tests at rates ranging from 10^-3s^-1 to 10^-1s^-1. The tests were monitored by two perpendicular digital cameras and a thermal camera. Similarly, uniaxial compression tests were performed at rates ranging from 10^-3s^-1 to 350s^-1, where a split Hopkinson pressure bar (SHPB) was used for the highest rates. The low-rate tests were recorded with high-resolution digital cameras, while a high-speed camera and a thermal camera were used for the SHPB tests. In addition, it was performed notched tensile tests at a low rate to study failure. These tests also served as a validation case for the numerical simulations. A high-resolution camera was used, such that the local strains in the notch could be captured using digital image correlation. The experiments indicated that the adhesive behaved similar as rubbers. Therefore, the Bergström-Boyce constitutive model was applied in the numerical simulations. The overall prediction of the test results was seen to be satisfactory, but the initial stiffness was too high compared to the response measured from the experiments. An investigation of the numerical results indicated that this mismatch was likely linked to the formulation of the inelastic shear rate.