Showing papers in "Journal of Adhesion Science and Technology in 2002"

Engineered natural fiber reinforced polypropylene composites: influence of surface modifications and novel powder impregnation processing

Abstract: Environmentally-benign powder impregnation processing is adopted in fabricating bio-composites from chopped natural fibers and powder polypropylene. The surface modifications of natural fibers involving alkali treatment and treatment with maleated polyolefins are found to be very effective in improving fiber-matrix adhesion. Bast fiber (kenaf) based composites showed best tensile and modulus properties while leaf fiber (henequen) based biocomposites showed best impact behavior. Through a blend of surface-treated bast and leaf fibers, 'engineered natural fibers' (ENFs) can be designed and such ENFs on reinforcement with a polymer matrix result in biocomposites with superior physico-mechanical properties.

Use of the combined Lifshitz–van der Waals and Lewis acid–base approaches in determining the apolar and polar contributions to surface and interfacial tensions and free energies

Abstract: Recently, a number of authors have been rearranging the various combinations and permutations of the different apolar and polar liquids with which contact angles can be measured on polar surfaces and in so doing have arrived at bizarre results. The rational order and procedures to be followed in the determination of the apolar and polar surface tension properties of polar materials, according to the van Oss-Chaudhury-Good components and parameters of the approach, are reiterated.

Surface modification of textile fibers for improvement of adhesion to polymeric matrices: a review

Abstract: Surface treatments have long been utilized to modify the chemical and physical structures of the surface layers of textile fibers, thus improving the properties of fibers in many applications. This review discusses the feasibility and characteristics of different methods of surface modification of polymeric textile fibers, focusing on tailoring fiber-matrix bond strength in fiber-reinforced composite materials. The influence of various treatments on the chemical and mechanical properties of different fibers is discussed. Some very recent developments in surface modification of textile fibers are highlighted.

Structural characterization of bis-[triethoxysilylpropyl]tetrasulfide and bis-[trimethoxysilylpropyl]amine silanes by Fourier-transform infrared spectroscopy and electrochemical impedance spectroscopy

Abstract: Bis-[triethoxysilylpropyl]tetrasulfide (or bis-sulfur silane) and bis-[trimethoxysilylpropyl] amine (or bis-amino silane) were deposited on 2024-T3 aluminum alloy (AA 2024-T3). The structures of the films were characterized using Fourier-transform infrared spectroscopy (FTIR) and electrochemical impedance spectroscopy (EIS) techniques. The results showed that: (1) The silane structures were affected significantly by the hydrolysis time of the silane solutions. A minimum hydrolysis time is required to obtain a crosslinked silane film. (2) Hydrolysis progressed more readily and faster in the bis-amino silane system than in the bis-sulfur silane system, probably due to the catalytic action of the amine of the bis-amino silane. (3) Both silane systems experienced significant crosslinking upon curing at 100°C, during which denser interfacial layers were formed via crosslinking in the interfacial regions. The interfacial layer contributes to corrosion protection of metals by silanes. (4) A new phase was observe...

On the interpretation of contact angle hysteresis

Abstract: The determination of solid surface free energy is still an open problem. The method proposed by van Oss and coworkers gives scattered values for apolar Lifshitz-van der Waals and polar (Lewis acid-base) electron-donor and electron-acceptor components for the investigated solid. The values of the components depend on the kind of three probe liquids used for their determination. In this paper a new alternative approach employing contact angle hysteresis is offered. It is based on three measurable parameters: advancing and receding contact angles (hysteresis of the contact angle) and the liquid surface tension. The equation obtained allows calculation of total surface free energy for the investigated solid. The equation is tested using some literature values, as well as advancing and receding contact angles measured for six probe liquids on microscope glass slides and poly(methyl methacrylate) PMMA, plates. It was found that for the tested solids thus calculated total surface free energy depended, to some ex...

Role of surface roughness in capillary adhesion

Abstract: The adhesion forces between a smooth spherical particle and flat surfaces of alumina, silver, and titanium-coated Si wafers were measured with an atomic force microscope (AFM) under various humidity conditions. The results showed that there is a discrepancy between the experimentally observed and the theoretically predicted values of capillary adhesion forces. The reason for the discrepancy is explained considering the relative humidity of the surrounding atmosphere and the surface roughness profiles of the contacting surfaces. Two geometrical configurations to define the contact region profile are suggested. The equations to calculate the capillary adhesion forces were modified using the new geometries. The discrepancy was largely eliminated using the modified equations.

Plasma treatment of sisal fibres and its effects on tensile strength and interfacial bonding

Abstract: Argon- and air-plasma treatments have been used to modify the surface of sisal fibres. The Taguchi method of experimental design with three factors and three levels is used to optimise the treatment parameters in relation to fibre strength. The effects of plasma treatment on interfacial bonding between sisal fibres and polypropylene are evaluated by means of a single fibre pull-out test. The optimum treatment parameters have been found to be the shortest plasma treatment time, medium power level and medium chamber pressure. Under optimal treatment, the interfacial shear strength of air-plasma treated fibres is higher than that of the argon-plasma treated fibres. Scanning electron microscopy analyses show that the overall roughness of the plasma treated fibre surface increases with treatment time. The Ar-plasma treated fibre surface reveals obvious corrugations whereas cracking is apparent on the air-plasma treated fibre surface.

Indirect estimation of fiber/polymer bond strength and interfacial friction from maximum load values recorded in the microbond and pull-out tests. Part I: local bond strength

Abstract: The techniques aimed at adhesion strength measurement between reinforcing fibers and polymer matrices (the pull-out and microbond tests) involve the measurement of the force, F max, required to pull out a fiber whose end is embedded in the matrix. Then, this maximum force value is used to calculate such interfacial parameters as the apparent bond strength, τapp, and the local interfacial shear strength (IFSS), τd. However, it has been demonstrated that the F max value is influenced by interfacial friction in already debonded regions, and, therefore, these parameters are not purely 'adhesional' but depend, in an intricate way, on interfacial adhesion and friction. In the last few years, several techniques for separate determination of adhesion and friction in micromechanical tests have been developed, but their experimental realization is rather complicated, because they require an accurate value of the external load at the moment of crack initiation. We have developed a new technique which uses the relati...

The effect of atmospheric pressure helium plasma treatment on the surface and mechanical properties of ultrahigh-modulus polyethylene fibers

Abstract: Ultrahigh-modulus polyethylene fibers were treated with atmospheric pressure He plasma on a capacitively coupled device at a frequency of 7.5 kHz and a He partial vapor pressure of 3.43 × 103 Pa. The fibers were treated for 0, 1, and 2 min. Microscopic analysis showed that the surfaces of the fibers treated with He plasma were etched and that the 2-min He plasma-treated group had rougher surfaces than the 1-min He plasma-treated group. XPS analysis showed a 200% increase in the oxygen content and a 200% increase in the concentration of C—O bonds (from 11.4% to 31%) and the appearance of C=O bonds (from 0% to 7.6%) on the surface of plasma-treated fibers for the 2-min He plasma-treated group. In the microbond test, the 2-min He plasma-treated group had a 100% increase of interfacial shear strength over that of the control group, while the 1-min He plasma-treated group did not show a significant difference from the control group. The 2-min He plasma-treated group also showed a 14% higher single-fiber tensil...

Capillarity at the nanoscale: an AFM view

Abstract: We have used atomic force microscopy (AFM) to image liquid droplets on solid substrates. The technique is applied to determine the contact line tension. Compared to conventional optical contact angle measurements, the AFM extends the range of accessible drop sizes by three orders of magnitude. We analyze the global shape of the droplets and the local profiles in the vicinity of the contact line. These two approaches show that the optical measurement overestimates the line tension by approximately four orders of magnitude.

Comparison of the Lifshitz–van der Waals/acid–base and contact angle hysteresis approaches for determination of solid surface free energy

Abstract: Total surface free energy, γS TOT, for several solids (glass, PMMA, duralumin, steel and cadmium) was calculated from the surface free energy components: apolar Lifshitz–van der Waals, γS LW, and acid–base electron–donor, γS -, and electron–acceptor, γS +. Using van Oss and coworkers' approach (Lifshitz–van der Waals/acid–base (LWAB) approach), the components were determined from advancing contact angles of the following probe liquids: water, glycerol, formamide, diiodomethane, ethylene glycol, 1-bromonaphthalene and dimethyl sulfoxide. Moreover, receding contact angles were also measured for the probe liquids, and then applying the contact angle hysteresis (CAH) approach very recently proposed by Chibowski, the total surface free energy for these solids was calculated. Although the thus determined total surface free energy for a particular solid was expected to depend on the combination of three probe liquids used (LWAB approach), as well as on the kind of the liquid used (CAH approach), surprisingly the...

Shrink-resistance and wetting properties of keratin fibres treated by glow discharge

Abstract: The influence of the gas type, air or nitrogen, and the treatment time in an RF glow discharge treatment on the shrink resistance properties of knitted wool fabric and wetting properties of keratin fibres were studied. Wetting properties were determined by means of contact angle measurements on single keratin fibres. This method allows measuring accurately the influence of the plasma gas type and treatment time on fibre hydrophilicity, and its modification with the time elapsed after plasma treatment. The modification of the surface properties should be taken into account, especially, when a biopolymer after-treatment is applied to achieve wool shrink resistance. Surface chemical changes were studied by means of XPS. Topographical changes in the wool fibre surface were observed by scanning electron microscopy (SEM) and surface damage was evaluated by means of the Herbig sac formation. Both air and nitrogen plasma treatments impart shrink resistance to wool fabric and hydrophilic properties to the keratin ...

Surface modification and degradation of poly(lactic acid) films by Ar-plasma

Abstract: Surface modification of poly(lactic acid) (PLA) film surface by Ar-plasma was investigated by contact angle measurements and XPS in order to answer the following two questions. (1) Could the Ar-plasma modify the PLA film surfaces? (2) What chemical reactions occurred on the film surfaces during the Ar-plasma treatment? The Ar-plasma treatment did not lead to hydrophilic modification of the PLA film surface, but to degradation reactions of the PLA film. Poor modification may be due to instability of the carbon radicals formed from C—O bond scission in the PLA chains by the Ar-plasma.

Atmospheric pressure helium + oxygen plasma treatment of ultrahigh modulus polyethylene fibers

Abstract: Ultrahigh modulus polyethylene fibers were treated with atmospheric pressure helium + oxygen plasma in a capacitively coupled device at a frequency of 7.5 kHz. The fibers were treated for 0, 0.5, 1, 1.5, and 2 min. The surfaces of the fibers treated with He + O2 plasma were etched and micro-cracks were formed. XPS analysis showed a 65ndash213% increase in oxygen content on the surfaces of all plasma-treated fibers, except for the 1.5 min group. An increase in the concentration of C—O and the appearance of C=O bonds on the surfaces of plasma-treated fibers were observed. In the micro-bond test, He + O2 plasma-treated groups had a 65–104% increase in interfacial shear strength over that of the control. The tensile strength of the fibers was either unchanged or decreased by 10–13% by the plasma treatments.

Direct measurements of the adhesion between a glass particle and a glass surface in a humid atmosphere

Abstract: An atomic force microscope has been used to measure the adhesion between individual silica-glass particles and a glass substrate in the presence of water vapour It was found that the adhesion between the surfaces was not significantly altered, when compared with the dry case, at relative water vapour pressures of less than 06 Above this level of water vapour, the magnitude of the adhesion between the surfaces increased by approximately an order of magnitude The transition of behaviour at a relative water vapour pressure of 06 corresponds to the formation of a capillary annulus having a critical Kelvin radius of approximately 2 nm These findings are in good agreement with previous research data for the interaction of mica surfaces in water vapour Force-distance data recorded as the particle and surface are separated from one another indicate the presence of a capillary neck between the surfaces The form of these force-distance data indicates that the separation occurs under non-equilibrium conditions that more closely resemble the expected interaction under constant volume conditions (for the capillary neck) The results of this study also indicate the important role of equilibration time for the surfaces when not in contact Successive force scans with only short equilibration times when the surfaces are separated result in the development of larger than expected capillary interaction forces The results are relevant to the interactions between particles in a powder bed under flow

Adhesion improvement of electroless copper to a polyimide film substrate by combining surface microroughening and imide ring cleavage

Abstract: In order to enhance the adhesion strength of copper metal film to a polyimide (PI) film substrate, a method combining surface microroughness formation and imide ring cleavage was investigated. The results showed that imide rings were cleaved with a KOH treatment while carboxyl and amide groups were formed on the surface of the PI film. The surface micro-roughness did not change with the KOH treatment, and the adhesion strength of the copper metal film to the PI film was slightly improved to 30 g/mm, which could be attributed to the interaction of both carboxyl and amide groups with the copper atoms. When the PI films were successively treated with an alkaline permanganate and a KOH solution, many recesses were formed on the surface in an alkaline permanganate solution, and the size and depth of the recesses increased with alkaline permanganate treatment time. The results of the AFM measurements showed that the average roughness (R) increased from 3.54 to 10.23 nm after combined treatment with alkaline per...

Improvement of wettability and reduction of aging effect by plasma treatment of low-density polyethylene with argon and oxygen mixtures

Abstract: To improve the hydrophilicity and reduce the aging effect, argon and oxygen mixtures were employed in the plasma treatment of low-density polyethylene (LDPE). Argon resulted in producing more oxygen ions and radicals in the plasma than only oxygen and forming cross-linked layers on the LDPE surface. Therefore, the water contact angle on plasma-treated LDPE decreased and the oxygen content measured by X-ray photoelectron spectroscopy (XPS) increased with the increase of argon content. The aging effect was also much reduced with the increase of argon content since argon induced cross-linking.

Influence of chemical treatments on the electrokinetic properties of cellulose fibres

Abstract: Changes in the surface composition of chemically treated cellulose fibres obtained from the sheath of banana plants were investigated using electrokinetic (ζ-potential) measurements. Scanning electron microscopy (SEM) was used to observe changes in the surface morphology of the fibres. Spectroscopic methods were also used to analyse the changes on the cellulose fibre surface. Chemical treatments such as alkali treatment, acetylation, treatment with a triazine coupling agent, various silanes, etc. reduced the hydrophilicity of the fibres. The surface morphology of the fibres showed considerable changes. Chemical treatments reduced the acidity of the already polar cellulose fibre. The high iso-electric point (IEP) of the silane A1100-treated fibres shows that basic groups dominate at these surfaces. The observations are consistent with the values obtained using solvatochromic measurements.

Nanoparticle removal from substrates with pulsed-laser induced plasma and shock waves

Abstract: Removing particles with nanometer scale diameters from substrates is a challenging task with numerous critical applications. A novel removal method for nanoparticles is developed and tested. The technique, which is dry and non-contact, takes advantages of shock wavefronts initiated by plasma formation under a focused laser beam pulse and its interaction with the substrate. Experimental results indicate that silica particles down to 500 nm on silicon wafers can be removed without substrate damage. In the reported experiments, a Q-switched Nd : YAG pulsed laser with a 5-ns pulse width and 360-mJ pulse energy at 1064 nm wavelength is employed as a plasma generation source. It is reported that the traditional dry laser cleaning method based on the rapid thermal expansion under direct laser irradiation often results in surface damage in the nanometer scale due to light diffraction around nanoparticles and/or stress localization in the thermal skin. This occurs when the characteristic dimensions of the particle...

Three-dimensional finite element analysis of single-lap adhesive joints under impact loads

Abstract: This paper deals with the stress wave propagation and stress distribution in single-lap adhesive joints subjected to impact tensile loads with small strain rate. The stress wave propagations and stress distributions in single-lap joints have been analyzed using an elastic three-dimensional finite-element method (DYNA3D). An impact load was applied to the single-lap adhesive joint by dropping a weight. One end of one of the adherends in the single-lap adhesive joint was fixed and the other adherend to which a bar was connected was impacted by the weight. The effects of Young's modulus of the adherends, the overlap length, the adhesive thickness and the adherend thickness on the stress wave propagations and stress distributions at the interfaces have been examined. It was found that the maximum stress occurred near the edge of the interface and that it increased with an increase of Young's modulus of the adherends. It was also seen that the maximum stress increased as the overlap length, the adhesive thickn...

Acid-base effects in polymer adhesion at metal surfaces

Abstract: The Lewis acid-Lewis base properties of various polymers have been determined by measuring the contributions γs + and γS - to the solid surface free energy using the contact angle approach of van Oss, Chaudhury, and Good. A new linear method to solve for γS + and γS - is employed in addition to the usual approach which uses three simultaneous equations. The set of liquid surface tension parameters developed by van Oss, Chaudhury, and Good, and the recent set of values developed by Della Volpe and Siboni are both useful in distinguishing between acidic and basic polymers. The adhesion (peel force) of an acidic pressure-sensitive adhesive is greatest on a basic oxide film. In addition, the adhesion (pull-off force) of the basic polymer poly(methyl methacrylate) is greatest for acidic oxide films. Thus, direct experimental evidence is provided as to the importance of Lewis acid-Lewis base effects in the adhesion of polymers on oxide-covered metals.

Effect of surface oxygen content and roughness on interfacial adhesion in carbon fiber–polycarbonate composites

Abstract: The effect of surface chemistry and rugosity on the interfacial adhesion between Bisphenol-A Polycarbonate and a carbon fiber surface subjected to surface treatment to add surface oxygen groups was investigated. The surface oxygen content of PAN based intermediate modulus IM7 carbon fibers was varied by an oxidative surface treatment. The oxygen content of the carbon fiber surface increased from 4 to 22% by changing the degree of surface treatment from 0 to 400% of nominal commercial surface treatment levels. The oxidative surface treatment also causes an increase in surface roughness by creating pores and fissures in the surface by removing carbon from the regions between the graphite crystallites. To decouple the effects of surface roughness and the surface oxides on the interfacial adhesion, the oxidized fiber surface was passivated via hydrogenation at elevated temperature. Thermal hydrogenation removes the oxides on the surface without significantly altering the surface topography. The results of int...

Microparticle adhesion studies by atomic force microscopy

Abstract: Atomic force microscopy (AFM) is one of the most flexible and simple techniques for probing surface interactions. This article reviews AFM studies on particle adhesion. Special attention is paid to the characterization of roughness and its effect on adhesion. This is of importance when comparing the measured adhesion forces to theoretical values, as the contact area is included in the contact mechanics theories. Even though adhesion models for time-independent adhesion are reasonably well developed, it remains difficult to connect the measured values to model predictions, especially because of the unknown value of the true contact area. The true area of contact depends on both the roughness of the probe as well as of the substrate. Our studies on the interactions between smooth silica particles, or rougher toner particles, and silicon substrates as a function of the surface roughness of the latter has shown the utility of AFM for measuring both roughness and particle adhesion.

Antifouling poly(vinylidene fluoride) microporous membranes prepared via plasma-induced surface grafting of poly(ethylene glycol)

Abstract: Poly(vinylidene fluoride) (PVDF) microporous membranes with antifouling property were prepared by argon plasma-induced grafting of poly(ethylene glycol) (PEG) onto the membrane surface, including the pore surfaces. The PVDF membranes were pre-coated with PEG in a PEG/CHCl3 solution. The microstructure and composition of the PEG-grafted PVDF (PEG-g-PVDF) membranes were characterized by attenuated total reflectance (ATR) FTIR and X-ray photoelectron spectroscopy (XPS). A moderate radio-frequency (RF) plasma power and plasma treatment time led to a high concentration of the grafted PEG polymer. The morphology of the modified membranes was studied by scanning electron microscopy (SEM). It was found that the flux of water decreased with increasing surface concentration of the grafted PEG polymer, while the pore size remained almost unchanged. Protein adsorption and protein solution permeation experiments revealed that the PEG-g-PVDF membranes with a PEG graft concentration, defined as the [CO]/[CF2] ratio, abo...

Adhesion Forces Between Individual Gold and Polystyrene Particles

Abstract: Using the particle interaction apparatus, adhesion forces between individual pairs of spherical polystyrene and gold particles (o 2−20 μm) were measured. With the particle interaction apparatus the contact time and load can be adjusted keeping all other parameters constant. Adhesion forces between different pairs of particles varied by a factor of five even for similar particle sizes. Adhesion forces were significantly lower than expected from the JKR or DMT theory. AFM images confirmed that one reason was probably the surface roughness on the nanometer scale. For both materials the adhesion force did not depend on the load (at least up to loads of 1 μN). Thus plastic deformation is probably negligible. The adhesion force increased with contact time. We interpret this increase as being at least partially caused by squeezing out of surface films. By removing the surface film a closer contact between the particle surfaces is established.

Acid–base characterization of wood and selected thermoplastics

Abstract: The objective of this work was to study the acid–base properties of wood, poly(vinyl chloride) (PVC), nylon 6 and 6,6 by wetting and inverse gas chromatography (IGC) analyses. Information about the acid–base characteristics of these materials should be useful to improve the intermolecular bonding properties in wood-plastic composites. The acid–base properties of pine wood veneers, PVC and nylon 6,6 were determined by contact angle analysis using the work of adhesion (or Fowkes), van Oss–Chaudhury–Good (vOCG) and Chang–Qin–Chen (CQC) approaches. The IGC analysis was performed on maple wood, PVC and nylon 6 particles, and was carried out at infinite dilution using a series of both non-polar and polar acid–base probe gases. The contact angle analysis of the wood veneers using both the work of adhesion and the vOCG approaches showed that the presence of wood extractives was the dominant factor influencing the acid–base properties of the veneers. Particularly, it was shown that aging of non-extracted veneers i...

Friction, adhesion, and deformation: dynamic measurements with the atomic force microscope

Abstract: A summary is given of the author's recent experimental and theoretical work involving the atomic force microscope, with the focus being upon dynamic measurements. Four topics are covered: calibration techniques for the friction force microscope, quantitative measurements of friction and the effect of adhesion, measurement and theory for the deformation and adhesion of viscoelastic particles, and the interaction and adhesion of hydrophobic surfaces due to bridging nanobubbles.

The effect of moisture on the failure locus and fracture energy of an epoxy–steel interface

Abstract: Experimental studies have been undertaken that provide the fracture energy of an epoxy-steel interface that has been exposed to various uptake levels of moisture. The test configuration used for these studies is the mixed mode flexure (MMF) test. The specimens have been exposed to the moist environment as open-faced specimens. The results provided by the test show a steady degradation in the interface fracture energy with increasing moisture content. Surface characterisation techniques such as X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to characterise the nature of the failure surfaces. Both of these analyses show a reduction of adhesive fragments and carbon overlayer thickness on the steel fracture surface with increasing moisture levels. The variation of carbon overlayer thickness with moisture exhibits a similar trend as the fracture energy, indicating that the two may be related.

Morphology and failure in nanocomposites. Part I: Structural and mechanical properties

Abstract: Nanocomposites are expected to exhibit new and improved properties when compared to their microcomposite counterparts. By lowering the particle size to nanodimensions (<100 nm), the special effects in polymer composites appear. In this study we compared the properties of composites filled with micro- and nano-sized calcium carbonate (CaCO3) filler particles in poly (vinyl acetate) (PVAc) matrix. The morphology of the composite was found to be responsible for the composite properties. The filler nanoparticles are dispersed in the matrix in the form of a 'net-like' structure, contrary to microparticles, which are dispersed as 'islands' in the matrix. The other systems investigated in this study were based on polyacrylate (PA) copolymer matrices filled with layered kaolin filler, which is well suited for creating nanocomposites. Mathematical models were used to quantify the interfacial interactions in the composites under investigation. Improved mechanical properties are obtained where there is a strong inte...

Pull-off forces measured between hexadecanethiol self-assembled monolayers in air using an atomic force microscope: analysis of surface free energy

Abstract: The pull-off forces were measured between hexadecanethiol monolayers, self-assembled on gold-coated silicon nitride cantilever tip and silicon wafer, using atomic force microscopy (AFM). The blind tip reconstruction technique was used for determination of the curvature of the AFM tip. The measured pull-off force value remained practically unaffected by a variation of the maximum applied load in a range of 5–80 nN. This result suggests that the use of continuum elastic contact mechanics in the analysis of AFM pull-off force measurements is not as straightforward as usually assumed in the literature reports on similar systems. The surface free energy of hexadecanethiol monolayer on a gold film of γ = 24–27 mJ/m2 was calculated based on the pull-off forces (F), measured using R = 60–80 nm radius tips, and next applying the Derjaguin approximation: F = 4πRγ. These γ-values were found to match the surface free energy value calculated from contact angle data and using Lewis acid–base interfacial free energy theory.