Mapping of adhesion forces on soil minerals in air and water by atomic force spectroscopy (AFS)

The adhesion force between an atomic force microscope (AFM) tip and sample surfaces, mica and quartz substrates, was measured in air and water. The force curves show that the adhesion has a strong dependence on both the surface roughness and the environmental conditions surrounding the sample. The variability of the adhesion force was examined in a series of measurements taken at the same point, as well as at different places on the sample surface. The adhesion maps obtained from the distribution of the measured forces indicated regions contaminated by either organic compounds or adsorbed water. Using simple mathematical expressions we could quantitatively predict the adhesion force behavior in both air and water. The experimental results are in good agreement with theoretical calculations, where the adhesion forces in air and water were mostly associated with capillary and van der Waals forces, respectively. A small long-range repulsive force is also observed in water due to the overlapping electrical double-layers formed on both the tip and sample surfaces.     

Enhancing the Adhesion Retention by Controlling the Structure of the Adhesion Interphase Between Rubber Compound and Metal. Part I. Effect of Cobalt Salt

The adhesion between cobalt salt-containing rubber compounds and brass-plated steel cords was studied to understand the role of cobalt salt as an adhesion promoter. An improvement in adhesion was shown with the low loading of cobalt salt in the range 0.5–1 phr, while a significant decline in adhesion was observed with high loading at 2 phr and a long aging time of 15 days under humidity aging. The adhesion interphase between the rubber compound and the brass-plated steel cord studied using Auger electron spectroscopy (AES) showed stable depth profile by cobalt salt incorporation at low loading, resulting in the enhancement of the adhesion retention. For the high loading of cobalt salt, adhesion interphase grew excessively, especially ZnO layer formed under humidity aging, which resulted in reduced adhesion.     

Enhancing the Adhesion Retention by Controlling the Structure of the Adhesion Interphase between Rubber Compound and Metal. Part II. Effect of Zinc Borate

The adhesion between zinc borate-containing rubber compounds and brass-plated steel cords was studied to understand the role of zinc borate as an adhesion promoter. No improvement in adhesion after cure was shown with loading of zinc borate in the range 0.5–2 phr, while enhancement of adhesion retention after humidity aging was observed with loading of zinc borate in the range 0.5–2 phr. The adhesion interphase between the brass-plated steel cord and the rubber compound subjected to humidity aging treatment showed, using AES, a stabilized depth profile by a moderate loading of zinc borate, resulting in enhancement of adhesion retention. For the high loading of zinc borate, copper sulfide layer and zinc oxide layer grew excessively in the adhesion interphase under humidity aging.     

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.     

The Influence of Chemical Pre-treatment and Magnesium Surface Enrichment on Bonding of Succinic Acid Molecules to Aluminium Alloy

In the present study a combined Fourier Transform Infrared Reflection–Absorption Spectroscopy (FT-IR–RAS) and Auger electron spectroscopy (AES) analysis was performed to investigate the bonding of organic coatings to oxides formed on Al–Mg based alloys. The interfacial bonding of a model molecule, succinic acid, mimicking the functional groups used in practical coatings was studied in detail on various differently pre-treated aluminium alloy (AA5182, 4.8 wt% magnesium) surfaces. Besides these differently pre-treated alloy surfaces, also pure magnesium and aluminium were studied for reference. The surface and near-surface composition of the alloys was studied by Auger electron spectroscopy, ion-milling away the surface oxides to obtain a depth profile. The results show that after heat treatment, the oxides are as much enriched in magnesium as aluminium although the bulk concentration of magnesium is only 4.8 wt%. FT-IR–RAS shows that bonding of the succinic acid molecules takes place in a range of configurations, varying between bridging bidentates and unidentates. The relative portion of unidentates increases with the magnesium surface concentration. This is attributed to the difference in bond lengths of Mg–O and Al–O, forcing the carboxylic acid groups to stretch or to attach to only one oxide group (unidentate) instead of two (bidentate). Also the mixed OH/CO₃ surface after Mg enrichment at the surface has less room for bonding with two groups, thus affecting the bonding configuration in a similar way.     

Effect of Surface Roughness on the Adhesive Strength of the Heat-Resistant Adhesive RTV88

Heat-resistant adhesive RTV88 is a hyper-elastic material and so far there have been little research on using RTV88 in adhesive joints. In this study, the effect of surface roughness on the adhesive strength of RTV88 was examined. Aluminum adherends were first sandblasted in order to generate rough surfaces, and then tensile–shear tests on Al/RTV88 single lap joints were performed. The shear strength was shown to be influenced by surface roughness. Peel failure was dominant when the surface roughness was at a low level. However, cohesive failure was the major type of failure when the surface roughness was at a high level. Effective area, peel failure area, and cohesive failure area were introduced to explain the effects of surface roughness on the adhesive strength. An empirical relation for the failure force was proposed, based on these parameters. Tensile tests of the RTV88 bonding was performed in order to obtain the necessary data. Finally, the empirical relation for the failure force was verified by tensile–shear test results.     

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.     

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 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.     

The Effect of Relative Humidity on Particle Adhesion and Removal

The removal of small particles is vital for contamination-free manufacturing. In humid environments liquid can condense between the particle and substrate and give rise to a very large capillary force, which increases the total force of adhesion. The removal and adhesion forces of polystyrene latex (PSL) particles and pigmented coating chips were measured on silicon, polyethylene terephthalate, metallized and polyester coating substrates as a function of humidity. The results indicate that the capillary force is significant at a relative humidity above 50% and dominates at a relative humidity above 70%. At relative humidity below 45%, the electrostatic force becomes significant. The adhesion forces varied depending on the particles and substrates used, but the trend of high adhesion at high and low relative humidity was observed for all PSL particles/substrate systems. The pigmented coating chips/substrate system however, exhibited high adhesion at high relative humidity and low adhesion at low relative humidity.     

The Effect of Relative Humidity on Particle Adhesion and Removal

The removal of small particles is vital for contamination-free manufacturing. In humid environments liquid can condense between the particle and substrate and give rise to a very large capillary force, which increases the total force of adhesion. The removal and adhesion forces of polystyrene latex (PSL) particles and pigmented coating chips were measured on silicon, polyethylene terephthalate, metallized and polyester coating substrates as a function of humidity. The results indicate that the capillary force is significant at a relative humidity above 50% and dominates at a relative humidity above 70%. At relative humidity below 45%, the electrostatic force becomes significant. The adhesion forces varied depending on the particles and substrates used, but the trend of high adhesion at high and low relative humidity was observed for all PSL particles/substrate systems. The pigmented coating chips/substrate system however, exhibited high adhesion at high relative humidity and low adhesion at low relative humidity.     

Impact of the Charge-Regulated Nature of the Bacterial Cell Surface on the Activity of Adhered Cells

Since the first reported observations by Zobell in 1943, it has been recognized that the metabolic activity of adhered bacteria can differ from that of their planktonic counterparts. Many studies have been performed and the overwhelming evidence is that bacterial adhesion to surfaces can result in changes to cellular metabolic activity and that the changes are a function of the surface properties of both the bacterial and adhering surfaces. However, the mechanism that results in these observations has remained elusive. The authors have approached this problem by focusing on cellular bioenergetics, which describes how bacteria obtain, store and use energy, and how adhesion can affect this process. In a series of experimental and numerical studies, the authors developed a hypothesis linking cellular bioenergetics to the physicochemical charge-regulation effect, which causes variations in surface electrostatic properties as a surface containing acid/base functional groups (e.g., the bacterial cell surface) approaches another surface. The purpose of this paper is to synthesize these prior studies and provide a cohesive presentation of the hypothesis. If this hypothesis is ultimately shown to be true, it will provide a fundamental basis for the engineered design of surface materials and coatings that can enhance or inhibit bacterial activity and colonization, depending on the requirements of the system at hand.     

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.     

The Effect of Oily Finish Components on the Adhesion Between Aramid Fibers and Rubber

The use of aramid fibers as a reinforcing material in both tires and mechanical rubber goods, such as hoses, belts, etc., is growing. In these dynamic applications, the adhesion between fiber and rubber is critical. This can be optimized by activating the aramid with an epoxy formulation, followed by RFL (Resorcinol Formaldehyde Latex) treatment. In the past, various combinations of analytical techniques have been used to study the relationship between the fiber surface treatment, the resulting microscopic interphase structure and the macroscopic rubber properties. The fundamental knowledge acquired from these past studies has been exploited here to investigate the effect of oily finish components on the aramid–rubber adhesion. For this purpose, aramid yarn has been treated with various combinations of an adhesion improving (epoxy–amine) component and a processability improving (oily) component. Contrary to general belief, the oily components do not directly reduce the SPAF (Strap Peel Adhesion Force) to rubber, rather show some positive effect. Furthermore, there is a relative broad 'safe' oil range, i.e., fluctuations in the amount of oil will not directly lead to adhesion problems. This is in line with earlier observations, but this study using appropriate analytical techniques provides quantitative confirmation and additional understanding of the fundamental principles behind these effects.     

Promotion effect of a chlorotriazine derivative on the adhesion between rubber compounds and a brass-plated steel cord

The adhesion between chlorotriazine derivative (TZ)-loaded rubber compounds and a brass-plated steel cord was studied to understand how TZ acted as an adhesion promoter. With the loading of the rubber compound with TZ, the cure rate became slow, but changes in the physical properties were not significant. An improvement in adhesion was obvious at a low TZ loading ranging from 0.5 to 2.0 phr. An adverse effect was observed with high loadings up to 8 phr and aging treatments for 15 days. Since the concentration of sulfur in the interphase of the rubber compound/ brass film adhesion samples after humidity aging was high for the rubber compound with high loadings of TZ, the acceleration of sulfide formation by TZ loading was confirmed. Lack of oxygen in the interphase indicated the conversion of zinc oxide to zinc sulfide in the rubber compound containing TZ. The control of zinc oxide formation in the adhesion interphase by TZ is suggested to be the reason for the adhesion promotion by TZ.     

The critical humidity effect in the adhesion of epoxy to glass: role of hydrogen bonding

Adhesion between an epoxy [diglycidyl ether of bisphenol F (DGEBF) cured with diethylene triamine] and glass was lowered abruptly when the epoxy was equilibrated with air whose relative humidity (RH) exceeded a critical value of approximately 70% RH. The critical humidity marking the onset of adhesion loss was also associated with a sudden increase of water uptake by the epoxy. In earlier work, it was shown that this 'transition' was not due to capillary condensation, osmotic cell formation or a decrease in the T _g of the material. Instead, it was speculated that the critical humidity effect was due to the trapping of water by hydroxyl groups which become available as inter-chain hydrogen-bonded structures are broken. To verify the above hypothesis, two model compounds were synthesized. One closely mimicked the cured DGEBF resin and the other had all of its hydroxyl groups replaced by hydrogen. Comparison of the water sorption isotherms of these two model compounds clearly suggested that hydroxyl groups played a key role in the critical humidity effect. Using molecular simulation software, hydrogen bonding between the various polar sites of the hydroxylated model compound was also studied. In the dry state or at low water concentrations, simulations predicted the formation of hydrogen bonds between polar sites. These hydrogen bonds always involved one or more hydroxyl groups. At higher water concentrations, molecular simulations showed that water tended to displace the hydrogen bond network of the epoxy, and in the process, water-mediated 'bridges' between polar groups were formed. The large decrease in entropy associated with the formation of such macrocyclic conformers is thought to be offset by the decreased enthalpy of condensation of water made possible by multiple hydrogen bonding. This suggests that the critical humidity effect might be an 'order-disorder' transition associated with the formation of ring structures closed by hydrogen-bonded water linkages between polar groups. The first-order energetics of this type of transition is consistent with the abrupt nature of the critical humidity effect.     

The Effect of Time and Humidity on Particle Adhesion and Removal

Time and humidity greatly influence particle adhesion and removal in many particlesubstrate systems. The effect of time (aging) and humidity on the adhesion and removal of 22 μm PSL (Polystyrene Latex) particles on polished silicon wafers is investigated. The results show that the effect of time on the adhesion and removal of the 22 μm PSL particles on silicon substrates in high humidity environment is very significant. The removal efficiency of PSL particles significantly decreased after the samples were aged for more than one day in high humidity environment. The combined effect of the van der Waals force and the capillary force tend to accelerate the adhesion-induced deformation process. When capillary force occurs at the particle substrate interface, the removal efficiency decreases quickly by more than 50% within 24 hours. Without the capillary force, the adhesion-induced deformation is negligible within the first 24 hours.     

The Effect of Time and Humidity on Particle Adhesion and Removal

Time and humidity greatly influence particle adhesion and removal in many particlesubstrate systems. The effect of time (aging) and humidity on the adhesion and removal of 22 μm PSL (Polystyrene Latex) particles on polished silicon wafers is investigated. The results show that the effect of time on the adhesion and removal of the 22 μm PSL particles on silicon substrates in high humidity environment is very significant. The removal efficiency of PSL particles significantly decreased after the samples were aged for more than one day in high humidity environment. The combined effect of the van der Waals force and the capillary force tend to accelerate the adhesion-induced deformation process. When capillary force occurs at the particle substrate interface, the removal efficiency decreases quickly by more than 50% within 24 hours. Without the capillary force, the adhesion-induced deformation is negligible within the first 24 hours.     

The effect of polymer surface on the wetting and adhesion of liquid systems

Young's equation describes the wetting phenomenon in terms of the contact angle between a liquid and a solid surface. However, the contact angle is not the only parameter that defines liquid–solid interactions, an additional parameter related to the adhesion between the liquid drop and the solid surface is also of importance in cases where liquid sliding is involved. It is postulated that wetting which is related to the contact angle, and interfacial adhesion, which is related to the sliding angle, are interdependent phenomena and have to be considered simultaneously. A variety of models that relate the sliding angle to the forces developed along the contact periphery between a liquid drop and a solid surface have been proposed in the literature. Here, a modified model is proposed that quantifies the drop-sliding phenomenon, based also on the interfacial adhesion that develops across the contact area of the liquid/solid interface. Consequently, an interfacial adhesion strength parameter can be defined depending on the mass of the drop, the contact angle and the sliding angle. To verify the proposed approach the adhesion strength parameter has been calculated, based on experimental results, for a number of polymer surfaces and has been correlated with their composition and structure. The interaction strength parameter can be calculated for any smooth surface from measurements of the contact and the sliding angles.     

Effect of Press-on Forces on Drug Adhesion in Dry Powder Inhaler Formulations

Press-on forces play a major role in dry powder inhaler (DPI) formulations. In this work, we have quantified the press-on forces on different substrates and drugs under controlled conditions. Externally applied forces significantly affect drug adhesion to surfaces and were found to be independent of the type of substrate and drug. A conservative estimate of press-on forces was made and was found to depend inherently on the size and shape of the drug particle and also on the contact area of the drug and substrate. These press-on forces were positively correlated to the mixing energy. It was found to increase quadratically with an increase in mixing speed. This work is of relevance to the pharmaceutical process of mixing DPI formulations, as it relates dispersion performance to the mixing time, batch size and carrier and drug properties.