Investigation of the relationship between statistical characteristics of substrate surface roughness and the strength of adhesive joints

The relationship between statistical characteristics of butadiene styrene rubber (BSR) surface roughness and shear strength of adhesive joints has been investigated. The assumption of stationary normal distribution of coordinates of surface points was made to determine the statistical characteristics of surface roughness. The profile length above the selected level l ₁ (u) was introduced as a new surface roughness parameter to characterize adhesive penetration depth. The validity of simulated l ₁ (u) value was verified experimentally. A good correlation between experimental and calculated results was found. A relationship between adhesive penetration depth and the bonding pressure during adhesive joint preparation was also obtained. The dependences among lap shear joint strength, bonding pressure and roughness characteristic l ₁ (u) were determined.     

Effects of preheating treatment on thermal property and adhesion performance of soy protein isolates

The objective of this research was to study the effects of preheating treatment and thermal-setting temperature on the thermal properties and adhesion performance of esterified and cross-linked soy protein isolates. Preheating treatment was achieved by heating a soy protein isolate suspension (5% solid) for 20 min at 60, 80, 110 or 130°C. Thermal-setting temperatures of 130, 160, 190 and 220°C were achieved by adjusting the temperature of the hot press. Differential scanning calorimetry and sodium dodecyl sulfate-polyacrylamide gel electrophoresis were used to determine changes in the protein structure before and after preheating treatment. Preheating treatment had a significant effect on protein structure and adhesion performance. Adhesion strength of control and esterified soy protein isolates reached maximum at 80°C preheating temperature. Severe preheating (over 110°C) caused complete denaturation of proteins and loss of their native structure and was, therefore, detrimental to adhesion performance. Thermal-setting temperature also had a significant effect on protein structure and wet strength of the soy protein isolates. Wet adhesion strength of unmodified, esterified, and cross-linked soy protein isolates increased by 170%, 128% and 80%, respectively, as the thermal-setting temperature increased from 130 to 220°C.     

Microstructure,adhesion strength and failure path at a polymer/roughened metal interface

Metals and polymers are extensively used in microelectronics packaging where they are joined together. Since both the yield and reliability of packages are strongly affected by the interfacial adhesion between polymers and metals, extensive studies have been performed in order to improve the resistance to debonding of many resulting interfaces. In the present work, the interfacial fracture energy of representative polymer/metal interfaces commonly encountered in micoroelectronics packaging was characterized. A copper-based alloy leadframe was used as the metal and an epoxy molding compound (EMC) was used as the polymer. The leadframe surfaces were roughened by chemical oxidation in a hot alkaline solution and molded with the EMC. In general, roughening of metal surfaces enhances their adhesion to polymers by mechanical interlocking, yet often produces a cohesive failure in the polymer. Sandwiched double-cantilever beam (SDCB) specimens were employed to measure the adhesion strength in terms of interfacial fracture energy. After the adhesion test, the microstructures of metal surfaces before molding with the EMC were correlated to the adhesion strength, and the fracture surfaces were analyzed using various techniques to determine the failure path.     

Thermoplastic film adhesives based on phenol-functional acrylic copolymers: synthesis,mechanical and adhesion properties

Acrylic polymers possessing varying proportions of pendant phenol groups were synthesized by the free radical copolymerization of N-(4-hydroxyphenyl) maleimide (HPM) with butyl acrylate (BuA) and acrylonitrile (AN) and characterized. These thermoplastics form excellent films and their mechanical and adhesion properties were evaluated as a function of the phenol content. Enhancing the HPM content increased both the tensile strength and the modulus but decreased the elongation. A nominal increase in the phenol content was found to be conducive for improving the adhesion properties of the films. At higher concentrations, the adhesion properties showed a decreasing trend due to the embrittlement caused by the rigid maleimide groups. The adhesion property at 50°C increased linearly with the HPM content due to an increased T _g, whereas a reverse trend was observed for the adhesion property measured at-196°C, due to the dominance of the embrittlement effect. The reduced flow characteristics of the high HPM-loaded systems led to a diminished honeycomb flat-wise tensile strength. Enhancing the HPM concentration in the chain promoted the adhesion properties for the vulcanization bonding of nitrile rubber to aluminium. Addition of silica filler marginally improved the lap shear strength (LSS) for the metal-metal system, but was detrimental for rubber-metal bonding; a reverse trend was observed for the carbon-filled system. The diminished performance for metal-metal bonding by carbon could be attributed to the weakening of the interphase, whereas the enhanced rubber-metal bonding could be due to possible reinforcement of the rubber phase by carbon. The fillers generally improved the high temperature adhesion. However, they impaired the flow properties of the resin and, thereby, adversely affected the flat-wise tensile strength in both cases.     

Modification of epoxy resins for improvement of adhesion: a critical review

Modification of epoxy resins for improvement of adhesion has been the subject of intense research throughout the world. Unlike for thermoplastics, physical blending is not successful for improvement of bond strength and impact strength of epoxy resins. The bond strength of an epoxy resin can be improved only by chemical modification with a suitable flexible modifier. Such chemical modification may either plasticize the epoxy matrix or lead to a two-phase microstructure. Both methods of chemical modifications are discussed critically in the present review.     

Adhesion strength of sodium dodecyl sulfate-modified soy protein to fiberboard

Thermal and rheological properties of sodium dodecyl sulfate (SDS)-modified soy protein isolate (SPI) adhesives were studied using differential scanning calorimetry (DSC) and rheometry. The ordered structure of native SPI was denatured as SDS concentration increased, and thermal stability of native SPI decreased at high SDS concentration. The enthalpy of SPI denaturation decreased significantly with increasing SDS concentration. Apparent viscosity of the SPI adhesives increased as SDS concentration increased. The SPI adhesives modified by high concentrations of SDS exhibited characteristics of a Newtonian-type flow. The SDS-modified SPI adhesives were applied to fiberboard, and effects of SDS concentration, press conditions, and assembly time on bond strength were investigated. Shear strength of the SPI adhesives increased with SDS concentration, reaching its maximum value at 3 wt% of SDS, and then decreased significantly. The shear strength increased as press time and/or press temperature increased. High press temperature (100 °C) and long press time (5 min) are needed to achieve relatively good adhesion properties. The shear strength also increased as assembly time increased. The shear strength of the SDS-modified SPI adhesives decreased after soaking in water for 24 h.     

Effect of the microstructure of copper oxide on the adhesion behavior of epoxy/copper leadframe joints

The thermal oxidation of copper leadframe was carried out at 175°C and the adhesion behavior of the epoxy/copper leadframe joint was analyzed by investigating the microstructure changes of copper oxide with the thermal oxidation time of copper. The peel strength increased sharply at an early stage of oxidation (~20 min) followed by a slight increase. After further oxidation (120 min), the peel strength showed a slight decrease. The contact angles of water and diiodomethane decreased sharply at an early stage of oxidation with negligible change afterwards. As the oxidation time increased, X-ray photoelectron spectroscopy (XPS) results revealed that the chemical composition of copper oxide had changed (Cu/Cu₂O → Cu₂O → CuO); this change improved the wettability of the copper surface, which affected the peel strength. Increase of the surface roughness of copper oxide, investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM), causes the epoxy resin and copper oxide to undergo mechanical interlocking, which increases the peel strength. Failure analysis by SEM and XPS indicated that failure was largely in the copper oxide, and the amount of copper oxide on the peeled epoxy increased as the oxidation time increased, due to the weak mechanical strength of the oxide layer. However, a small portion of the epoxy resin was also fractured during the failure process, regardless of the oxidation time. Consequently, fracture proceeded mainly in the copper oxide close to the epoxy resin/copper oxide interface.     

On the work of adhesion and peel strength between pressure sensitive adhesives and the polymeric films used in LCD devices

Peel strength, a convenient measure of bond strength in adhesive/adherend systems, is known to be a function of various factors such as the thermodynamic work of adhesion, rate of measurement, thermal history, and temperature. Generally, it is believed that the work of adhesion is primarily involved in the first stage of adhesion through wetting phenomenon and beyond that its role diminishes in that the portion of thermodynamic contribution to actual bond strength is insignificant. In practice, however, we often observe that a suitable surface treatment increases the surface energy of the substrate, which further enhances the bond strength. One practical example is the surface treatment carried out in LCD industry to obtain sufficient bond strength between pressure sensitive adhesives and polymeric films. To further our understanding of the effect of surface treatment, we attempted to establish a possible correlation, if any, between the thermodynamic work of adhesion and peel strength. For this, we carefully measured the contact angles of water and diiodomethane against various polymeric films, and calculated the surface energy and the thermodynamic work of adhesion using the two widely used approaches: Young-Fowkes-Girifalco-Good, and Wu methods. Before establishing a correlation, some general aspects of the above two methods are discussed. The values of the work of adhesion obtained were compared with the measured peel strength. Indeed, we observed a clear correlation between the two quantities: the increase of the work of adhesion led to the increase of peel strength. As a reason for this correlation, we proposed that the increase of surface energy might be associated with the increase of various surface functional groups, which, in turn, contributed to the formation of chemical bonding with the PSA leading to the increase of peel strength.     

The Effects of Roughness on Adhesion Hysteresis

Adhesion hysteresis is defined as the difference between the work needed to separate two surfaces and that originally gained on bringing them together. Adhesion hysteresis is a common phenomenon in most surface/interface interactions. This paper studies the effects of surface roughness on adhesion hysteresis. We assumed that the surface asperity height distribution is Gaussian. Numerical simulations based on Fuller's model showed that adhesion hysteresis depended upon a single dimensionless parameter, the adhesion parameter, which represents the statistical average of a competition between the compressive forces exerted by the higher asperities, which are trying to separate the surfaces, and the adhesion forces of the lower asperities which are trying to hold the surfaces together.     

The effect of surface properties on the adhesion of modified polychloroprene used as adhesive

The adhesion properties of polychloroprene can be improved by addition of such materials as piperylene–styrene co-polymer (PSC), VeoVa-10 polymer, VeoVa-11/methyl methacrylate/2ethylhexyl acrylate co-polymer (VeoVa-11/MMA/2EHA) and poly(vinyl acetate) waste (wPVAc). Here, the relationship between adhesion properties and surface tension of polychloroprene was investigated. Contact angle measurements have been used to study the effects of nature and content of polymeric additives on the adhesion and surface properties of polychloroprene. Low-surface-tension VeoVa-10 polymer has the tendency to migrate to the surface of polychloroprene; thus, adhesion is determined mainly by this additive property. Enrichment of polychloroprene film bottom layer by the additive was observed using high-surface-tension PSC and wPVAc. In this case, the adhesion properties of polychloroprene depend on the interactions at the interface. Adhesion properties of polychloroprene were found to depend not only on compatibility between adhesive components, but also on compatibility between the adherend and the adhesive.     

Influence of block molecular weight on the adhesion properties of segmented polyamides

The effects of molecular weight variations in the hard and soft segments on the adhesion strength of segmented polyamides against aluminium, copper, and steel were investigated using 180° peel strength measurements. It was found that the adhesion strength of the segmented polyamides was largely influenced by block molecular weight variations. The nature of the substrate, the rate of peeling, cooling in different environments, and thermal ageing, etc. had significant effects on the adhesion strength of the joints, whereas variation in the moulding conditions used in these experiments did not have much impact on the strength of the joints. The joint strength increased with a decrease in hard block molecular weight at a constant soft block molecular weight of 1000, or with an increase in soft block molecular weight at a constant hard block molecular weight of 1100.     

Effects of contact time,humidity, and surface roughness on the adhesion hysteresis of polydimethylsiloxane

Dynamic contact mechanics experiments have been performed on small polydimethylsiloxane (PDMS) lenses and several substrates in both ambient air and in dry nitrogen. The experimental results are analyzed with the Johnson-Kendall-Roberts theory. While the theory adequately describes the approach data, it is unable to account for the large hysteresis observed upon retraction. Adhesion hysteresis is shown to scale with the roughness of the substrate, the hydrophilicty of the substrate, the time of contact, and the ambient humidity. The experimental results also demonstrate that this method is sensitive to changes in the surface energy of the substrate. The cumulative adhesion hysteresis is quantified and is shown to be largest for rough, hydrophilic substrates in relatively high humidity and smallest for smooth substrates in dry nitrogen. The origin of the hysteresis is analyzed by considering favorable interfacial bonding resulting from water-mediated bonding between the substrate and oxygen atoms in the PDMS backbone or other polar species on the polymer surface. Capillary forces are also postulated to contribute to the cumulative adhesion hysteresis.     

Heterogeneity in the strength of interfacial bonds and the resultant synergism in promoting polyurethane/SBR adhesion strength

Polymer adhesion and its evaluation are very important both from academic and industrial points of view. Adhesion phenomenon depends mainly on the strength of interfacial bonds and the deformability of adhering partners, which act as another energy absorbing term. Using a combination of acid-base interactions and a coupling agent at the interface of styrene-butadiene (SBR)-polyurethane system, a synergistic adhesion promotion was observed. The SBR surface was treated with an acidified aqueous solution of calcium hypochlorite to obtain polar groups, which can interact with the isocyanate groups of the polyurethane system. Aminopropyltriethoxysilane, a coupling agent, was also deposited at the SBR surface, after treating it with the aforementioned solution. The polar groups on the SBR apparently interact with the OH sites of the coupling agent at the surface and push the amino groups toward the polyurethane surface. This interesting finding, synergistic adhesion promotion, was attributed to the tortuous path for the crack growth at the interface, which was created by formation of the interfacial bonds with different strengths (heterogeneous bond strength). Furthermore, to obtain the highest synergistic effect, it seems that certain ratio of bonds with different strengths should be formed. The ratio itself depends on the deformability ratio of the adhering materials.     

Strength and failure of bulky adhesive joints with adhesively-bonded columns

Strength and failure properties of bulky (i.e. with thick substrate) adhesive joints with adhesively-bonded columns (ABCs) subjected to external loads were investigated experimentally and analytically. From the experimental results, it was found that the strengths of the bulky adhesive joints with ABCs increased considerably when they were subjected to external tensile loads or lateral bending loads. And the joint strengths increased with increasing depths of the blind holes. The failure process of the joints with ABCs was simulated by the technique of element birth and death developed in the finite element method (FEM). The conclusions obtained from FEM coincide with those obtained from the experiments.     

The role of interfacial interactions and loss function of model adhesives on their adhesion to glass

The measurement of adhesion and the evaluation of influencing factors are of great scientific and technological importance. There are two distinct viewpoints on adhesion: (i) surface chemistry, and (ii) fracture mechanics. For elucidation of the relative importance of mechanical properties in the bonding of adhesives, the strength of adhesion between model adhesives and glass plates was measured by the wedge cleavage (WC) test method. Copolymers of methyl methacrylate (MMA) with n-butyl acrylate (nBA) and methyl methacrylate with styrene (S) were prepared as model adhesives. The results show that in MMA-nBA copolymers, by increasing the amount of nBA, both the loss function and the adhesion energy of the adhesives increase. However, by increasing the amount of nBA above a certain level, the adhesion strength begins to decrease. In this situation, the cohesive strength of the adhesive dominates the failure mechanism. On the other hand, a decrease in adhesion was expected upon increasing the amount of styrene in the poly(styrene-co-methyl methacrylate) adhesive, because methyl methacrylate, an interactive monomer with glass, is replaced by a non-interacting styrene monomer, while the loss function of the adhesive is almost constant. But our practical adhesion measurement technique was not sensitive enough to detect this adhesion loss.     

Contact Time and Temperature Dependencies of Tack in Polyacrylic Block Copolymer Pressure-Sensitive Adhesives Measured by the Probe Tack Test

The adhesion strength of an adhesive is affected by two factors: the development of interfacial adhesion and the cohesive strength of the adhesive. In order to evaluate the relative contributions of these two factors, the tack of polyacrylic block copolymer-based adhesives was measured using a probe tack test. For this purpose, three model adhesives were prepared: poly(methyl methacrylate)-block-poly(n-butyl acrylate)-block-poly(methyl methacrylate) triblock copolymer (A), a mixture of the triblock and poly(methyl ethacrylate)-block-poly(n-butyl acrylate) diblock copolymer (7/3, w/w) (B), and a mixtureof the triblock and poly(n-butyl acrylate) oligomer (8/2, w/w) (C). The tack measured at room temperature was in the order B ≈ C > A and increased gradually with an increase in the contact time. The temperature dependence of tack showed peak tack values above room temperature, and the peak tack temperature was in the order A > B > C. The storage and loss moduli measured by dynamic mechanical analysis were also in the order A > B > C. The molecular mobility of the poly(n-butyl acrylate) unit in the block copolymer measured by H-pulse NMR was in the order C> B > A. It was concluded from these results that the relative contribution of interfacial adhesion to the tack of the different systems was in the order C > B > A.     

Effects of natural-resource-based scavengers on the adhesion properties and formaldehyde emission of engineered flooring

In this study we investigated the effects of using four additives, wheat flour (WF), tannin, rice husk (RH) and charcoal, to melamine-formaldehyde (MF) resin for decorative veneer and base plywood in engineered flooring in order to reduce the formaldehyde emission levels and improve the adhesion properties. We determined the effects of variations in hot-press time, temperature and pressure on the bonding strength and formaldehyde emission. Blends of various MF resin/additive compositions were prepared. To determine and compare the effects of the additives, seven MF resin blends were prepared with the four different additives: four with a wt ratio of 8:2 (MF/WF, MF/tannin, MF/RH and MF/charcoal), and three in the wt ratio of 8:1:1 (MF/WF/tannin, MF/WF/RH and MF/WF/charcoal). The desiccator and perforator methods were used to determine the level of formaldehyde emission. The formaldehyde emission level decreased with all additives, except for RH. At a charcoal addition of only 20%, the formaldehyde emission level was reduced to nearly 0.1 mg/l. Curing of the high WF and tannin content in this adhesive system was well processed, as indicated by the increased lap-shear strength. In the case of WF, the lap shear strength was much lower due to the already high temperature of 130°C. The adhesive layer was broken when exposed to high temperature for extended time. In addition, both WF and tannin showed good mechanical properties. With increasing WF or tannin content, the initial adhesion strength increased. The MF resin samples with 20% added tannin or WF showed both good lap shear and initial adhesion strengths compared to the pure MF resin.     

A generalized stress singularity approach for material failure prediction and its application to adhesive joint strength analysis

The concept of stress is very useful to describe the effect of external loads on structures. However, as a basis for the prediction of failure the concept of stress becomes meaningless when the structure encompasses singularities as a result of discrete stiffness steps or geometric anomalies such as cracks. In this article it is argued that the concept of failure stress is incorrect and should be replaced by a generalized concept based on stress intensity factors and singularity orders. It appears that material failure stress is the critical stress intensity factor for a zero-order singularity stress field. By plotting the critical stress intensity factor as a function of singularity order, the strength of a material can be characterized in a general fashion that integrates tensile strength, fracture toughness and critical singularities in adhesive joints. It is also shown that plasticity does not eliminate the stress singularity in an adhesive joint but changes the order of the singularity due to the induced change in interface corner angle between the dissimilar materials in the joint.     

The interrelationships between electronically conductive adhesive formulations,substrate and filler surface properties,and joint performance. Part I: the effects of adhesive thickness

Electronically conductive adhesives (ECAs) have received a great deal of attention for interconnection applications in recent years. Even though ECAs have excellent potential for being efficient and less costly alternative to solder joining in electronic components, they still possess a number of problems with respect to durability and design to meet specific needs. One of the issues that requires understanding is regarding the optimum adhesive thickness (AT) to be used. This study addresses this issue in relation to the formulations of the conductive adhesives and their interactions with adherend surfaces. For this purpose, two different adherends varying in surface characteristics were utilized along with three different conductive adhesive formulations with varying particle loadings, and shapes and sizes of conductive nickel fillers. Joints were also prepared with two different AT values, to gain insight into the influence of AT on the joint strength, deformation and joint conductivity. As the AT was increased, only a small reduction in failure load and ultimate displacement values were observed with unetched adherends. With etched adherends, however, a small increase in joint stretchability was evident with higher adhesive thickness tested at a lower crosshead speed. When the AT was increased, we also noted a corresponding increase in the initial joint resistance.