Investigation on fracture performance of aqueous polymer isocyanates in terms of energy release rate

In order to investigate the fracture performance of aqueous polymer isocyanates (APIs), an analytical expression for the energy release rate of adhesively-bonded three-point bending specimens has been derived from linear elastic fracture mechanics (LEFM) and the energy release rates of nine API samples are evaluated. Experimental results show that a proper loading of polymeric diphenylmethane-4,4′-diisocyanate (p-MDI) as cross-linker can improve the resistance of aqueous polymer latex adhesive layer to cracking. Excess cross-linker cannot maintain such an effect of strengthening and may decrease considerably the energy release rate of API polymer. When the loading of polyisocyanate is less than the critical value, the polymer cross-linked by aqueous emulsified p-MDI has a higher resistance to cracking. In contrast, polymers modified by p-MDI blended with an organic solvent have much better performance.     

Effects of Polystyrene Block Content on Morphology and Adhesion Property of Polystyrene Block Copolymer

Effects of polystyrene block content on adhesion property and phase structure of polystyrene block copolymers were investigated. Polystyrene-block-polyisoprene-block-polystyrene triblock and polystyrene-block-polyisoprene diblock copolymers with different polystyrene block contents in the range from 13 to 35 wt% were used. In the case of the low polystyrene block content (below 16 wt%), a sea-island structure was observed: near-spherical polystyrene domains having a mean diameter of about 20 nm were dispersed in polyisoprene matrix. The phase structure changed from a sea-island structure to a cylindrical structure with an increase of polystyrene block content (over 18 wt%). Peel strength decreased with an increase of polystyrene block content and the pure triblock copolymers had lower peel strength than their blends with the diblock copolymers. Pulse nuclear magnetic resonance studies indicated that molecular mobility of polyisoprene phase decreased with an increase of polystyrene block content, and the molecular mobility was lower in the pure triblock than in the blend. Thus, the peel strength was found to be related to molecular mobility. The adhesion strength of the block copolymer depended on the molecular mobility: high molecular mobility can promote interfacial adhesion.     

Influence of chlorinated paraffin on properties of silica filled polychloroprene

Abstract

Roles of the two widely used chlorinated paraffins, namely Cereclor S45 and Cereclor 48, in properties of silica filled polychloroprene were investigated. The results reveal that chlorinated paraffin significantly improves the processability. The improvement is more pronounced for Cereclor S45 due to its lower viscosity. However, the addition of chlorinated paraffin has a negative effect on cure. Owing to the reduction of crosslink density and the plasticising effect of chlorinated paraffin, several mechanical properties are impaired, e.g. tensile strength, modulus, hardness, abrasion resistance as well as rebound resilience. However, the deterioration of both tensile strength and modulus is not pronounced at low loading of chlorinated paraffin (≤8·0 parts per hundred of rubber, phr). Although chlorinated paraffin has little influence on the relative tensile strength, the relative 100% modulus is affected to a greater extent due to the combination of post curing retardation and evaporation of chlorinated paraffin during thermal aging.     

Sodium montmorillonite intercalated with poly(ethylene glycol): a modifier of reactive hot-melt polyurethane adhesive

We have examined the effects of a sodium montmorillonite (Na-MMT) intercalated with poly(ethylene glycol) (PEG) on the properties of a reactive hot melt adhesive (RHA) based on polyurethane pre-polymer with terminal isocyanate groups. The bond strengths of the RHA, from the initial to final time period after application, were enhanced and the set time was reduced by the addition of 1–3 phr Na-MMT intercalated with PEG (Na-MMT/PEG). The modifier increased the viscosity and a pseudo-solid-like behavior at low shear rates when subjected to a dynamic shear stress. Further, both the tensile modulus and tensile strength of cured RHA films were improved by the reinforcing effect of Na-MMT. The X-ray diffraction pattern of RHA showed that the gallery of Na-MMT was intercalated with PEG segment.     

Inspection of composite and metal adhesive bonds with an electrochemical sensor

An in-situ sensor, based on Electrochemical Impedance Spectroscopy (EIS), has been used to monitor the health of adhesive bonds constructed from various combinations of aluminum, graphite/epoxy, glass/epoxy, glass/polyester, and glass/vinylester composites and exposed to high humidity and temperature conditions. Modeling of the EIS data as an electric circuit demonstrated that several circuit parameters of the impedance spectra were sensitive to bond performance, as determined by wedge tests and lap shear tests. Moisture absorption by the adhesive and composite was calculated from the circuit capacitance, which was also a function of bonded area and bondline thickness (bondline + composite thickness for glass composites). Material differences, including saturation level of moisture, rate of absorption, and bondline stability, were readily seen among the various materials sets. The sensor electrodes are attached to opposite sides of a bond after fabrication, i.e. they are not embedded. Thus, they are suitable for monitoring existing bonded structures. They have the potential to identify bondlines that are in the early stages of degradation, prior to significant loss of bond strength. As an input to a condition-based maintenance system, they would identify weakening bondlines and allow preventative action to be scheduled and performed.     

Interlaminar Stresses at the Delamination Fronts in Tubular Adhesive Joints with Delaminated Composite Adherends

The use of composite materials has been extensively increasing in the recent decades, mainly due to their high strength and stiffness to weight ratios, as well as their non-corrosive attribute. Adhesive joints are used effectively to join composites to composites or to dissimilar materials. Components made of composites may contain some defects in the form of delaminations that may adversely affect their overall behavior and response when subjected to different loading systems. Interlaminar stresses (including out-of-plane stresses) are caused by the mismatch in material properties, especially in Poisson's ratio and the so-called 'coefficient of mutual influence' (between adjacent layers). The goal of this paper is to evaluate the interlaminar stresses that exist at the delamination fronts in a composite pipe, hosting a small delamination, adhesively bonded to an aluminum pipe. The aim is also to study the effect of various parameters (such as delamination length, depth, fiber orientation angles, and stacking sequence) that influence the performance, using the finite element method. The system is subjected to a torsional moment, which can be considered as a critical loading condition in tubular adhesive joints. Results of the study provide valuable information about the behavior of adhesive joints with delaminated composite adherends, and reveal the nature and distribution of interlaminar stresses along various delaminated fronts under torsional moments.     

Fatigue damage evaluation of adhesively bonded butt joints with a rubber-modified epoxy adhesive

A damage evolution of adhesively bonded butt joints with a rubber-modified adhesive has been investigated under cyclic loading. An isotropic continuum damage model coupled with a kinetic law of damage evolution was applied to the butt joint. To solve the kinetic law, analytic and numerical methods were tried: the former solution was derived with some simplifications and the latter one was derived rigorously without simplications. On comparing the analytic solutions with the numerical ones, it was confirmed that differences in the two solutions were small. Furthermore, the estimated S-N curves based on the analytic equation agreed well with experimental data.     

Matrix-assisted pulsed-laser evaporation of DOPA-modified poly(ethylene glycol) thin films

3,4-Dihydroxyphenyl-L-alanine-modified poly(ethylene glycol) (mPEG-DOPA₃) is a biologically-inspired material that exhibits unique adhesion properties. In this study, mPEG-DOPA₃ thin films were prepared using a novel laser process known as matrix-assisted pulsed-laser evaporation (MAPLE). The films were examined using Fourier transform infrared spectroscopy, atomic force microscopy, profilometry, antifouling studies and cell adhesion studies. The Fourier transform infrared spectroscopy data demonstrated that the main functional groups in the MAPLE-deposited mPEG-DOPA₃ films remained intact. Profilometry and atomic force microscopy studies confirmed that MAPLE provides excellent control over film morphology, as well as film thickness. High resolution patterns of mPEG-DOPA₃ thin films were obtained by masking. MAPLE-deposited mPEG-DOPA₃ thin films demonstrated an absence of cytotoxicity and acceptable antifouling properties against the marine bacterium Cytophaga lytica. MAPLE-deposited mPEG-DOPA₃ thin films potentially have numerous biomedical and marine applications.     

A study of the assembly process and reliability performance of contactless smart cards fabricated using a non-conductive adhesive film

Contactless smart cards are widely used in various applications, such as access control, transit fare payment systems, etc. In order to improve the performance and at the same time to reduce the production costs, research effort on the application of new materials and the development of new processes continues. In this study, the assembly process of contactless smart cards using a film-type non-conductive adhesive (NCA) was investigated and modified taking into account the limited heat resistance of the poly(ethylene terephthalate) substrate used in the tests. Pressure cooker tests were then performed to further evaluate the reliability performance of the adhesive joints in high temperature and high humidity conditions. The assemblies underwent both mechanical and electrical characterizations after the assembly process, as well as during and after the reliability tests. Examination of surface morphologies and cross-sections by scanning electron microscopy and optical microscopy was conducted to establish the failure mechanism.     

The influence of porosity and morphology of hydrated oxide films on epoxy-aluminium bond durability

Clad aluminium alloy was pretreated by immersion in boiling water for times ranging between 30 s and 4 h. The chemical and physical properties of the films produced in the 100°C water were characterized by techniques including X-ray photoelectron spectroscopy (XPS), Rutherford backscattering spectroscopy (RBS), scanning electron microscopy (SEM), and secondary ion mass spectrometry (SIMS). The durability of the bonds formed between the boiling water films and a rubber-toughened epoxy adhesive was assessed in terms of the film properties and fracture analysis of failed wedge specimens. In the early pretreatment stage, bond durability was limited by the fracture of the porous oxide film at the film-metal interface. For immersion times greater than 4 min, a decrease in film porosity and bond durability was observed.