Adhesion modification by shear-enhanced localized interfacial segregation in model reaction injection moulded (RIM) polyurethanes

—Polyurethane (PU) adhesives are generally multiphase systems and thus naturally consist of materials or phases of intrinsically different chemical and mechanical properties. We describe here a rheological technique (annular RIM system) which has been used to alter the surface chemistry and mechanical properties, and hence the adhesion of the materials, by imposing a shear strain generated during their fabrication. The results of ATR-IR, WAXS, DMTA, HPLC, friction and adhesion measurements are described to assess the degree of phase segregation developed as a function of the shear strain. In the case of simple PU systems, both the degree of phase segregation and the adhesion were found to increase progressively with the introduction of extended shear strains. Additive [multiphase solid and liquid internal mould release (IMR) systems] doped PUs, on the other hand, exhibited a substantial attenuation in the adhesion following nominal shear strain. It is suggested that the observed effect arises from the viscosity difference between the phases and the stress inhomogeneity generated in the processing channel. The augmentation in the adhesion of simple PU systems is thought to occur because of a pressure-induced transverse diffusion of the more viscous soft segment, resulting in an accentuated viscoelastic loss character at the PU/metal interface. The diminution in the adhesion of the IMR doped PU systems is thought to occur due to a synergistic effect of an increased interfacial concentration of the additive species and, for multiphase solid systems, the ability of the particles to modify the mechanical properties of the polymer in the interface regions.     

In situ infrared spectroscopic and scanning Kelvin probe measurements of water and ion transport at polymer/metal interfaces

Scanning ATR-spectroscopy and scanning Kelvin probe studies are introduced as new analysis techniques for the study of water and ion transport at polymer/metal interfaces. The new approach of scanning ATR spectroscopic analysis of water transport at the adhesive/metal interface is combined with scanning measurements in transmission mode of water transport along adhesive joints and non-scanning ATR-measurements of the water transport in a vertical direction through the adhesive. D₂O was chosen in some cases instead of water due to its excellent traceability in the infrared spectra. A scanning Kelvin probe was chosen for the detection of the transport kinetics of hydrated alkali ions along the adhesive/metal interface based on the local measurement of interfacial electrode potentials.The complimentary FT-IR techniques showed that the interfacial diffusion of water, in the case of epoxy adhesives on iron, is about two orders of magnitude faster that the transport through the adhesive itself. Similar transport kinetics at the interface is also shown by hydrated ions. Moreover, the here presented FT-IR-ATR and SKP results reveal more information on how adhesion promoting organosilane layers and organosilanes as additives act at polymer/metal interfaces in the presence of water incorporated in the interphase zone.     

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.     

Study on interfacial interaction energy of Cu–SAM–epoxy systems in a hot and humid environment

Due to poor adhesion, the interfacial delamination is one of the typical failure modes in electronic packages. In this paper, two kinds of self-assembly monolayers (SAMs), SAMA and SAME, are added to Cu–epoxy interface and the effects of temperature, moisture, and cross-link conversion on the modified interfaces are investigated with molecular dynamics (MD) simulation. The results show that the interfacial interaction energy of the systems with SAMA increases with the increasing temperature, decreasing moisture content, and cross-link conversion. However, the interfacial interaction energy of the systems with SAME decreases with the increasing temperature and moisture content, while it is reluctant to the cross-link conversion. In addition, the simulation reveals that the covalent bonds between SAMA and epoxy enhance the interfacial adhesion of Cu–epoxy. However, the nonbond interactions of SAME and epoxy resin weaken the interfacial adhesion. This paper provides a new method for research and valuation the effects of SAM or other adhesive on interfacial adhesion. MD simulation is an efficient tool in predicting the performances of materials.     

Critical role of particle/polymer interface in photostability of nano-filled polymeric coatings

Nanoparticle-filled polymeric coatings have attracted great interest in recent years because the incorporation of nanofillers can significantly enhance the mechanical, electrical, optical, thermal, and antimicrobial properties of coatings. Due to the small size of the fillers, the volume fraction of the nanoparticle/polymer interfacial area in nano-filled systems is drastically increased, and the interfacial region becomes important in the performance of the nano-filled system. However, techniques used for characterizing nanoparticle/polymer interfaces are limited, and thus, the mechanism by which interfacial properties affect the photostability and the long-term performance of nano-filled polymeric coatings is not well understood. In this study, the role of the nanoparticle/polymer interface on the ultraviolet (UV) stability of a nano-ZnO-filled polyurethane (PU) coating system was investigated. The effects of parameters influencing the particle/polymer interfacial properties, such as size, loading, surface modification of the nanoparticles, on photodegradation of ZnO/PU films were evaluated. The nature of the interfacial regions before and after UV exposures were characterized by atomic force microscopy (AFM)-based techniques. Results have shown that the interfacial properties strongly affect chemical, thermo-mechanical, and morphological properties of the UV-exposed ZnO/PU films. By combining tapping mode AFM and novel electric force microscopy (EFM), the particle/polymer interfacial regions have been successfully detected directly from the surface of the ZnO/PU films. Further, our results indicate that ZnO nanoparticles can function as a photocatalyst or a photostabilizer, depending on the UV exposure conditions. A hypothesis is proposed that the polymers in the vicinity of the ZnO/PU interface are preferentially degraded or protected, depending on whether ZnO nanoparticles act as a photocatalyst or a photostabilizer in the polymers. This study clearly demonstrates that the particle/polymer interface plays a critical role in the photostability of nano-filled polymeric coatings.     

聚氨酯改性环氧树脂胶粘剂的制备及性能研究

由端-NCO基聚氨酯(PU)预聚物与环氧树脂反应,制备了PU接枝改性环氧树脂。着重探讨了PU预聚物的含量、活性稀释剂的含量和异氰酸酯结构等因素,对改性环氧树脂的粘度和粘接性能的影响。实验结果表明,该改性环氧树脂的粘度随着PU预聚物含量的增加而逐渐增大,随着活性稀释剂含量的增加而逐渐降低,而且在相同的条件下,用不同的二异氰酸酯改性环氧树脂的粘度大小次序为:IPDI型>MDI型>TDI型;该改性环氧树脂在PU预聚物含量为20%时,对铝片/铝片的剪切强度最大(7.82 MPa);在PU预聚物含量为10%时,对铁片/铁片的剪切强度最大(11.70 MPa),而且TDI型和IPDI型改性环氧树脂的粘接性能明显好于MDI型改性环氧树脂。     

Adhesion at diamond-metal interfaces: a chemical composition perspective

Diamond films were chemically vapor deposited (CVD) on titanium, tungsten, molybdenum, copper and aluminum oxide substrates. In these studies, the interface formed between diamond and the substrate was exposed by mechanically deforming the metal substrate or diamond film to cause film delamination. The observed degree of adhesion for these interfaces can be ranked in the order: Ti » Al₂O₃ (thin films) > Cu > W » Mo. For highly adherent films, delamination procedures were carried out under controlled conditions in order to preserve the integrity of the interfacial species. The exposed interfaces were characterized by X-ray photoelectron spectroscopy (XPS), scanning Auger microscopy (SAM), scanning electron microscopy (SEM) and Raman microprobe spectroscopy. We find that substantial interfacial reaction layers exist at all interfaces except in the diamond-copper system and are composed of both oxides and carbides of the native substrate. Variations in the relative concentration of these species and the distribution throughout the reaction layer also were observed for the different substrates. We believe that both the chemical composition and morphology of the interface influence the adhesion properties of the diamond coating. Correlated investigations of the interfacial surfaces reveal that fracture of the diamond-metal interface occurs discretely at the diamond nucleation plane or within a reaction layer near the diamond interface. We discuss each of these findings in light of qualitative observations of adhesion and suggest avenues for improving the adhesion of diamond films.     

New method for interfacial evaluation of carbon fiber/thermosetting composites by wetting and electrical resistance measurements

Interfacial properties were evaluated for carbon fiber (CF) with different thermosetting polymeric matrices in composites. CF tow was wet by phenolic or epoxies, and the interfacial adhesion evaluated by electrical resistance changes. The interfaces between two types of CF tows with phenolic resin and three types of epoxies were investigated. The change in electrical resistance was found to depend on the wettability of CF by the polymer resins, with the more obvious resistance changes being associated with better wettability. The electrical resistance changes were measured 20?min after the polymer resin was dropped on the CF tow. To confirm the relationship between changes in resistance and interfacial properties, both interfacial shear stress (IFSS) and interlaminar shear stress (ILSS) were also measured. The results of these mechanical measurements were generally consistent with the electrical resistance measurements in that the materials with high electrical resistance also exhibited high IFSS and ILSS.     

Improved adhesion of silicone rubber to polyurethane by surface grafting

Polyurethane (PU) has widespread applications in implantable devices because of its excellent mechanical and biocompatible properties, whereas weak biostability limits its long‐term implantation. The introduction of silicone rubber (SR) onto the PU surface is an effective method for improving the biostability of PU, but the adhesion of these two polymers is unsatisfactory. In this study, the surface modification of PU via grafting through the introduction of vinyl and Si H groups onto the PU surface was attempted to improve the adhesion of PU to SR. Fourier transform infrared, energy‐dispersive X‐ray spectroscopy, and X‐ray photoelectron spectroscopy were employed to investigate the graft reaction on the PU surface. The interfacial and surface morphology was characterized with scanning electron microscopy. Different PU/SR interfaces after oscillation and shear were compared as well. The results indicated that the PU surface was activated by diisocyanate, which generated free isocyanate groups for the further grafting of vinyl and Si H groups. When addition‐type, room‐temperature‐vulcanized SR was poured onto the PU surface, the vinyl and Si H groups on the PU surface underwent an addition reaction, which improved the adhesion of PU and SR by connecting them with chemical bonds. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Atomistic Investigation on the Effects of Thermal Loading on Interfacial Adhesion of Dissimilar Materials

Heat transfer has a large effect on the adhesion and the corresponding failure at material interfaces. When a system becomes extremely small, the conventional finite element method is not capable of accurately capturing all the information, and precise modeling of interfacial properties is essential. In this paper, molecular dynamics (MD) simulations are used to investigate the effects of heat transfer on the adhesion properties of material interfaces. For Al–W and Cr–W interfaces, the interfacial strengths are calculated by MD simulations and are compared with the critical loads obtained from scratch tests. Both the results of MD simulations and experiments show that the interfacial strength of an Al–W interface is larger than that for a Cr–W interface; and furthermore, the Cr–W interface is more sensitive to thermal loading than the Al–W interface. In this work we concluded that the proposed MD model can be used to estimate interfacial adhesion under the effects of heat transfer.     

Investigations on the adhesion and interfacial properties of polyurethane foam/thermoplastic materials

The adhesion and interfacial properties of polyurethane (PU) foams with thermoplastic (TP) materials were investigated using different techniques. The adhesion performance of PU foam with TP materials was evaluated using the peel test method, and the adhesion durability was checked after different climate treatments. X‐ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and contact angle measurements were used to study the surface and interface morphology of PU foam and TP material system. Three types of PU foam samples which differ in their composition and also five commercially available TP blends systems, based on poly(carbonate), poly(styrene‐co‐maleic anhydride), poly(acrylonitrile‐butadiene‐styrene), and silicone acrylate rubber have been used. The slow reacting foam shows the best adhesion properties with all the TP materials. The climate treatments strongly effected the PU foam adhesion durability with poly(carbonate) containing TP materials (70–80% loss in adhesion), but nearly no effect with poly(styrene‐co‐maleic anhydride). The samples with lowered adhesion could be separated by peeling without visible foam residues on the TP surface. AFM, XPS, and surface tension studies have shown that the surface properties of the TP material are still governed by the PU foam. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 479–488, 2007     

Strength of Epoxy/Glass Interfaces after Hygrothermal Aging

The stability of epoxy/glass interfaces subjected to hygrothermal aging was assessed using a fracture-mechanics approach. An epoxy system consisting of diglycidyl ether of bisphenol F cured with 2-ethyl-4-methyl-imidazole was bonded to borosilicate glass adherends that were treated with various types of adhesion promoters to provide a variety of interfaces. Adhesive strength was measured under dry, as-processed conditions and as a function of exposure time to an 85°C/85% relative humidity (RH) environment. As expected, the strain-energy-release rate, G_c, dropped significantly with aging time for the bare epoxy/glass interface. The drop in G_c is assumed to be due to a loss of interfacial forces. The use of two silane-based adhesion promoters, 3-aminopropyltriethoxysilane (APS) and 2-(3,4-epoxycyclohexyl)ethyltrimethoxysilane (ECH) resulted in improved adhesive strength both before and after hygrothermal aging. The improvement in adhesive strength can be explained by the introduction of chemical bonds at the interface. The drop in G_c is assumed to be due to a loss of interfacial forces and hydrolysis of siloxane bonds. In addition to the use of organosilane-based adhesion promoters, a series of polyhydroxyaminoethers (PHAE) thermoplastic adhesive resins was also investigated as potential adhesion promoters. It was found that 2% PHAE in Dowanol® PM, a hydroxyl-group-containing solvent, was the best system for the PHAE-based adhesion promoters. Interestingly, both the acetic acid concentration in the solvent and maleic anhydride content in the PHAE resin were shown to affect the adhesive strength.     

The Interfacial Bond Strength in Glass Fibre-Polyester Resin Composite Systems

The interfacial bond strength in glass fibre-polyester resin composites has been investigated using various experimental techniques. These included blocks of resin containing fibre (in which, depending on the geometry of the specimen, failure occurs in either a shear or tensile mode) the pullout of a fibre from a disc of resin and a short beam shear test for interlaminar shear strength determination.

Low power optical microscopy and optical retardation measurements of stress induced birefringence were used to detect the difference between intact and debonded fibre resin interfaces. The shear modulus and shear strength of the resin were obtained from torsion tests on cylindrical rods of the resin.

The single fibre shear debonding specimen and the short beam shear test are shown to be the most viable test methods but interpretation of the results is complicated by the various modes of failure possible and by the different stress states which exist in the area of the specimen where debonding starts. Stress concentration factors obtained by finite element analysis and photoelastic analysis have been applied to the results from these tests and the corrected interfacial bond strengths are in close agreement.

The real interfacial bond strengths of well bonded glass-fibre polyester resin systems is shown to be of the order of 70 MN m^-2.     

The Interfacial Bond Strength in Glass Fibre-Polyester Resin Composite Systems

The interfacial bond strength in glass fibre-polyester resin composites has been investigated using various experimental techniques. These included blocks of resin containing fibre (in which, depending on the geometry of the specimen, failure occurs in either a shear or tensile mode) the pullout of a fibre from a disc of resin and a short beam shear test for interlaminar shear strength determination.

Low power optical microscopy and optical retardation measurements of stress induced birefringence were used to detect the difference between intact and debonded fibre resin interfaces. The shear modulus and shear strength of the resin were obtained from torsion tests on cylindrical rods of the resin.

The single fibre shear debonding specimen and the short beam shear test are shown to be the most viable test methods but interpretation of the results is complicated by the various modes of failure possible and by the different stress states which exist in the area of the specimen where debonding starts. Stress concentration factors obtained by finite element analysis and photoelastic analysis have been applied to the results from these tests and the corrected interfacial bond strengths are in close agreement.

The real interfacial bond strengths of well bonded glass-fibre polyester resin systems is shown to be of the order of 70 MN m^?2.     

High improvement of interfacial and mechanical properties in reinforced PP composite by grafting polyethyleneimine modified carboxylic multi-walled carbon nanotubes on short carbon fiber

The properties of carbon fiber reinforced polymer composites (CFRPs) will benefit greatly from improving interfacial performance. In this study, the interfacial properties of the PEI-CNT-CF/PP composite was improved by coating polyethyleneimine (PEI) modified carboxylic multi-walled carbon nanotubes (CNTs) in aqueous solution (PEI-CNT) onto the surface of the CF (PEI-CNT-CF) to form a network structure. The network formation changed the chemical characteristics and compatibility of CF surface by introducing amine (imine) groups, and could induce transcrystallization (TC) at interface of composite. These positive factors led to a 24.6% increasement in the interfacial shear strength (IFSS) of PEI-CNT-CF/PP, and further resulted in 16.2% and 5.3% improvement in tensile and flexural strength, respectively. SEM images of the fracture surface demonstrated a significant improvement in the interfacial adhesion between PEI-CNT-CF and PP resin. These results indicated that the PEI-CNT was a great choice to strengthen the interface of CF/PP system.     

Liquid metal/ceramic interactions in the (Cu,Ag, Au)/ZrB2 systems

Wetting and spreading experiments on ZrB₂ in contact with liquid Cu, Ag and Au have been performed by the sessile drop technique under a vacuum. The wetting and spreading characteristics and the interfacial reactions are discussed as a function of time and of the metal involved. The interfacial morphologies, analysed by optical microscopy, SEM and EDS show the presence of regular interfaces without macroscopic reaction layers. Gold, to a very large extent and copper are shown to give rise to extensive penetration along grain-boundaries, whereas silver neither wets nor penetrates. Interfacial diffusion/dissolution is taken into account and the consequent changes in liquid metal surface tension and wetting behaviours have been evaluated by means of thermodynamic calculations.Moreover, interfacial energetics at the atomistic level has been investigated by means of pseudopotential-based Density Functional Theory (DFT) technique. It is shown how the calculation of the ideal work of separation on the specific transition metal borides-molten metal systems can be used to interpret the wetting behaviour. Moreover, the dependence of the adhesion behaviour on the electronic structure at the interface and on the interface epitaxy and composition is also briefly discussed.     

Ultrasonic modification of aramid fiber–epoxy interface

An ultrasonic irradiation technique is used during the process of fabricating aramid fiber–epoxy resin reinforced composites to improve the interfacial adhesion performance. Under the ultrasonic treatment, the change of the resin viscosity is studied. The results of a microbond test show obvious improvement in the interfacial shear strength after ultrasonic treatment. The mechanical properties of the composites, such as the interlaminar shear strength and tensile strength, are measured. Combined with the SEM results, these show it is the mechanical properties that are improved and the fracture modes are varied from the interface between the fibers and resin to the fibrillation of fibers and resin. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2764–2768, 2001     

Atomistic modeling: interfacial diffusion and adhesion of polycarbonate and silanes

The performance and strength of many composites, hybrid and thin multi-layered material systems are very much dependent upon the mechanical properties of interfaces. However, continuum mechanics approach to the characterization of interfacial properties has had limited success because it is often unable to incorporate the effects of molecular and chemical interactions into the model. There is therefore a need to understand and study the influence of these factors on mechanical properties such as adhesion strength at a more fundamental level. In the present work, the interfaces of two common coupling agents and matrix polymers in composites are studied with atomistic modeling and simulation. The polymer matrix is polycarbonate (PC) and the coupling agents studied are gamma amino-propyl-triethoxysilane (AMPTES) and stearic-propyl-triethoxysilane (SPTES). Two interface models, SPTES‐PC and AMPTES‐PC were built and the work of adhesion was calculated from molecular dynamics (MD) simulation. The separation of the coupling-agents‐matrix interfaces was simulated using MD calculations and the mechanical properties were obtained. It is shown that the higher work of adhesion of the interface is not equal to higher interfacial toughness.     

The effect of electrolyte anions incorporated into anodic oxide films on the experimental transport numbers of ions

The growth of barrier anodic film is considered theoretically with regard to the migration of three ionic carriers: oxygen and metal ions and electrolyte anions. It is shown that the consideration of anion transport leads to the conclusion that the film grows at three interfaces: the metal/oxide and oxide/electrolyte interfaces and the interface between an oxide layer containing electrolyte anions (contaminated layer) and the oxide layer free of them (“pure” layer). The error in the measured transport numbers of metal and oxygen, which is caused by ignoring a contribution of electrolyte anions to the total charge transport, is maximum in the absence of anion motion.     

Study of adhesion failure due to molding compound additives at chip surface in electronic devices

Today the microelectronics market requires devices with failure levels approaching zero. To attain this goal all production processes must be subjected to extreme quality control. Molding is one of the most critical assembly processes in power plastic packages. This is related to the complexity of phenomena which may occur at the interfaces involved in this process. This paper reports an adhesion study of epoxy-phenolic molding compounds to the most relevant surfaces encountered in power devices assembled in plastic packages such as copper oxide-hydroxide, nickel oxide-hydroxide, aluminium oxide-hydroxide, and silicon 'nitride'. The study was carried out by combining delamination (scanning acoustic microscopy) and pull strength data with the interface chemistry studied using ESCA. Different adhesion failure mechanisms were found to be operative in these systems. These mechanisms are related to either the chemical nature and thickness of the inorganic layer or the segregation of various additives such as wax, polyoxyalkylene ethers, and alkylsiloxanes, contained in the molding compound.