A Comparison of the Work of Adhesion Obtained from Wetting and Vapor Adsorption Measurements

Some of the limitations to determining experimental values of the work of adhesion are discussed. Wetting measurements appear to provide the most direct means of assessing the work of adhesion for a solid-liquid system, but they require the formation of a finite contact angle by the liquid against the solid of interest, and the need for independent knowledge of the equilibrium spreading pressure of the liquid's vapor on the solid further limits their applicability. Vapor adsorption measurement using the technique of inverse gas chromatoraphy (IGC) provides a promising alternative means of determining the work of adhesion not subject to these limitations. The measurements are, furthermore, amenable to solids which are difficult to use with wetting measurements, e.g., those which are porous or granular. An attempt is made here to compare values of the work of adhesion determined using both wetting and vapor adsorption measurements. Good agreement is attained between the two methods for diiodomethane in contact with poly (vinyl chloride), poly (methyl methacrylate), and chemi-thermo-mechanical wood pulp fibers, suggesting that the technique of IGC is particularly well-suited for rapid determination of the work of adhesion.     

Characterization of epoxy resin surface energetics

This study has characterized the energetics of both the liquid state and the solid state of two commercially available epoxy resins: a DGEBA- and a TGMDA-based epoxy system. The surface properties of the liquid epoxies were evaluated by wetting measurements using a dynamic contact angle analysis (DCA). The Lifshitz-van der Waals components of the surface tension were found to be similar for both epoxy systems, while the acid-base components were found to be slightly different. Two different techniques were used to characterize the cured epoxy surface properties: wetting measurements and vapor adsorption measurements by means of inverse gas chromatography (IGC). The Lifshitz-van der Waals components of the surface energy were observed to be nearly the same for both epoxies, confirming that both resins have the same potential for non-specific interactions, in both liquid and solid states. Evaluations of the acid-base components of the work of adhesion by DCA and the Gibbs free energy change by IGC suggest that both cured epoxies show non-negligible specific interactions with both acidic and basic probes. However, computations of the accepticity and donicity parameters showed that both cured epoxies are predominantly basic, but also possess non-negligible acidity. It is likely that the presence of water on the solid surface contributes to the acidic character of the cured epoxies. The temperature dependence of the liquid surface tension for both epoxy systems was investigated. The same temperature dependence was observed: the surface tension decreased with temperature, following a linear regression. Corrections for viscous-drag effects on the liquid surface tension measurements were also made.     

Reversible and Irreversible Adhesion of Polymers: Possibilities for Measurement and Calculation

Comparative analysis of existing direct and indirect techniques for estimation of the work of adhesion (W_A) between a polymer and a solid surface - inverse gas chromatography (IGC), wetting, direct adhesion forces measurement - has been carried out. The work of adhesion was calculated from experimental data obtained using different techniques for identical polymer/solid systems. The relationship between the work of adhesion and the bond strength was analyzed, including possible W_A estimations from destructive micromechanical tests. For non-polar polymers, whose adhesion is due to dispersion interaction only, all techniques are in good agreement with each other. However, the estimates of work of adhesion obtained by different techniques considerably differ for polar polymers. The reason for this obviously consists in deficiency of theoretical knowledge about non-dispersion interactions at interfaces. Each of the considered approaches has its own advantages and shortcomings. The problems concerning the estimation of non-dispersion component of the work of adhesion can be solved only by comprehensive use of several different techniques.     

Reversible and Irreversible Adhesion of Polymers: Possibilities for Measurement and Calculation

Comparative analysis of existing direct and indirect techniques for estimation of the work of adhesion (W_A ) between a polymer and a solid surface - inverse gas chromatography (IGC), wetting, direct adhesion forces measurement - has been carried out. The work of adhesion was calculated from experimental data obtained using different techniques for identical polymer/solid systems. The relationship between the work of adhesion and the bond strength was analyzed, including possible W_A estimations from destructive micromechanical tests. For non-polar polymers, whose adhesion is due to dispersion interaction only, all techniques are in good agreement with each other. However, the estimates of work of adhesion obtained by different techniques considerably differ for polar polymers. The reason for this obviously consists in deficiency of theoretical knowledge about non-dispersion interactions at interfaces. Each of the considered approaches has its own advantages and shortcomings. The problems concerning the estimation of non-dispersion component of the work of adhesion can be solved only by comprehensive use of several different techniques.     

The role of adhesion in the mechanical properties of filled polymer composites

Filled polymer composites have been prepared in which the energetics of the filler surfaces was systematically varied in order to investigate the dependence of the mechanical properties of the composite on the interfacial strength as predicted by the thermodynamic work of adhesion at the filler-matrix interface. A high-purity silica filler was used, treated with three different organofunctional silane coupling agents (two alkylsilanes and an aminosilane) to varying degrees from zero to complete coverage. The surface energetics of the modified fillers was characterized using both inverse gas chromatography (IGC) and dynamic contact angle analysis (DCA). While the surface energy assessments from IGC were higher than those obtained with wetting measurements, as expected, the trends with fractional coverage of silane were the same for each method, and were used to evaluate the thermodynamic work of adhesion. Highly filled polymer composites were prepared by dispersing the variously treated silica fillers into the amorphous thermoplastic matrix polymers: poly(methyl methacrylate) and poly(vinyl butyral). Specimens of the composites were tested mechanically to give the yield stress. The poly(methyl methacrylate) composites all failed cohesively in the matrix, unaffected by any of the filler surface treatments. The poly(vinyl butyral) composites, however, all displayed purely interfacial failure, with the yield stress strongly dependent on the type and extent of the filler surface treatment. While all three silanes were found to decrease the filler surface energy, and consequently the thermodynamic work of adhesion, with higher surface coverage, corresponding decreases in the yield stress were found only for the alkylsilanes. For the aminosilane, the measured yield stress was found to increase with surface coverage and therefore to decrease with the work of adhesion. The difference in behavior between the two types of coupling agent is explained in terms of acid-base effects.     

An Improved Procedure for Determining the Work of Adhesion for Polymer-Solid Contact

Force-balance experiments in conjunction with the Johnson-Kendall-Roberts (JKR) theory have been used to determine the work of adhesion between solid systems. It has been shown that deficiencies in understanding the deformation behavior can lead to erroneous results. A modified procedure for determining the work of adhesion by force-balance experiments and JKR theory using normal displacement behavior has been introduced to address these deficiencies. This procedure involves improved experiental and data analysis protocols and has been applied to give more precise work of adhesion values for PDMS poly(dimethylsiloxane)-PDMS, PDMS-F(fluorinated silicon) and PDMS-Si (silicon) systems. The work of adhesion determinations are consistently less than those estimated by contact angle measurements.     

Work of adhesion of water to a Teflon surface and stability of the system Teflon-air bubble-water

Measurements of water drops beginning to slide down on a vertical Teflon plate and the detachment force of air bubbles of the same volume from the face surface of cylindrical Teflon segments (rods) of various diameters in water were made. Simultaneously, the radius of the contact plane between the air bubble and segment was measured at the moment of detachment, as well as the radius of the small air bubble contact plane which was left after disruption of the Teflon-large air bubble-water system. On the basis of the results obtained, the work of adhesion of water to a Teflon surface was determined using equations known from the literature as well as the equation derived in this paper. Agreement between the values of the work of adhesion calculated from these different equations was obtained. It was found that measurements of the air bubble detachment force from a solid surface may be useful for determination of the work of adhesion of a liquid to a solid surface, at least for a solid for which vapor adsorption may be neglected.     

An overview of the basic aspects of polymer adhesion. Part I: Fundamentals

Adhesion between two substrates is a complex phenomenon which at present is still not well understood. The important existing adhesion models (electrical, diffusion, thermodynamic adsorption, chemical, etc.) are reviewed in order to try to explain their mechanisms. Thermodynamic adsorption is now believed to be one of the most importnat mechanisms by which adhesion is achieved. Difusion and wetting are kinetic means in attaining good adsorption of a polymer at the interface. In the case of this model (thermodynamic adsorption), the notion of surface energy is developed and the importance of this property in the understanding of adhesion phenomena is emphasized. The methods of determining the surface characteristics of low and high energy solids are presented. The role played by acid-base interactions in adhesion is also mentioned.     

Inverse gas chromatography study of poly(dimethyl siloxane)—liquid crystal mixtures

Inverse gas chromatographic (IGC) measurements are reported for 17 hydrocarbon probes on a low molar mass hexyloxycyanobiphenyl liquid crystal (LC) mixed with linear poly(dimethyl siloxane) (PDMS). The retention properties are compared with a side-chain liquid crystalline bearing the same mesogen. The results show that the retention of the LC polymer is significantly different from a mixture with the same composition. DSC and IGC values for the transition temperatures indicate that PDMS does not blend with HCB although some interactions, characterised by a Flory-Huggins interaction parameter, were seen and some effect on the liquid crystalline behaviour was noted. The usefulness and limitations of the IGC technique for characterising this type of polymer-LC system are discussed.     

Estimation of the dispersive component of surface energy of polymer-grafted lignocellulosic fibers with inverse gas chromatography

Surface energy has traditionally been obtained through contact angle measurements. Hygroscopic, chemically heterogeneous and short fibers, such as wood fibers, may present difficulties in obtaining meaningful results. Inverse gas chromatography (IGC) is an alternative technique which can circumvent this problem. Indeed, IGC is particularly well-suited to measurements with porous, short fiber substrates, and data can be acquired as a function of composition or temperature. Measurements are generally done with a solvent, called 'probe', at near infinite dilution in an inert carrier gas. Little solvent-solvent interaction is involved, which is different from classical wetting measurements. A comparison is made between dispersive components of the surface energy as obtained through contact angle analysis and IGC. The variation of the dispersive component of the surface energy of wood fibers grafted with poly(methyl methacrylate), as a function of the degree of grafting, as obtained with IGC is presented and compared with results of photoelectron spectroscopy and specific surface measurements.     

Estimation of the work of adhesion by means of inverse gas chromatography for polymer complex systems

The usefulness of inverse gas chromatography (IGC) in the examination of complex polymer systems is presented. IGC method was applied for studying resin-bonded abrasive articles. Resin-bonded abrasive articles consist of: abrasive, wetting agent (resol), binder (novolac resin) and filler. IGC technique enabled to characterize one of the most important parameter influencing the quality of final product: adhesion between cross-linked resins and abrasive grains that decide on spalling of the abrasive grains from the binder. The magnitude of adhesion between resin and abrasive grains was expressed by the value of the work of adhesion. Presented in this article the way of estimation of the work of adhesion is quick, simple and can find application e.g. in industry of abrasive articles. The main advantage of this method is the possibility of controlling the quality of raw materials and their influence on the quality of the final product without necessity of manufacturing of trial product.     

Acid–base characterization of wood and selected thermoplastics

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 increased and decreased their acidic and basic properties, respectively. This is presumably due to reorientation of functional groups and oxidation at the wood extractives–air interface. In the vOCG model, considerably higher base/acid ratios were obtained when using probe liquid parameters according to van Oss compared to those obtained by using liquid parameters according to Della Volpe and Siboni. Based on both the vOCG and the CQC models it was shown that nylon 6,6 had greater acid and base parameters than PVC. Additionally, the CQC model seems to be a promising tool to determine the acid–base characteristics of materials. The IGC analysis showed that nylon 6 had greater acid and base parameters than both wood and PVC which implies a strong ability to enter into acid–base interactions. The results also suggested that an increase in the basic character of wood could have the potential to improve its bonding with both PVC and nylon 6.     

Understanding the Affinity between Components of Wood–Plastic Composites from a Surface Energy Perspective

To evaluate surface compatibility in wood-plastic composites (WPCs), the dispersion and acid–base components of surface energy of various thermoplastic resins (matrices) and several wood-based reinforcing materials were determined using inverse gas chromatography (IGC). Polypropylene (PP), nylon 6, poly(ethylene terephthalate) (PET), poly(trimethyl terephthalate) (PTT), high impact polystyrene (HIPS), and styrene maleic anhydride (SMA) were used as thermoplastic resins, while wood flour (hot water extracted and un-extracted), microcrystalline cellulose (MCC) (50 μm and 90 μm), α-cellulose (60 μm), and silicified microcrystalline cellulose (SMCC) (60 μm) were used as reinforcing materials. All matrices and reinforcing components were exposed to low vapor concentrations of apolar (decane, heptane, nonane, octane) and polar (chloroform, ethyl acetate, dichloromethane, acetone, and tetrahydrofuran) probes. Methane and helium were employed as reference and carrier gases, respectively. IGC retention times were used to determine the acid–base component of surface energy of the analyzed materials. The corresponding surface energy, work of adhesion, and work of cohesionwere calculated based on the van Oss–Chaudhury–Good approach (acid–base and Lifshitz–van der Waals interactions). Composite performance was analyzed by measuring tensile and flexural strengths according to ASTM standards. The results indicated that for the same type of filler (assuming similar shape and dimensions), the mechanical properties of the composites increased when the ratio of the work of adhesion to the work of cohesion increased. A similar trend was observed when the thermoplastic resin employed to create the composite possessed an acid–base component of surface energy greater than zero.     

Characterization of surface‐modified poly(ethylene terephthalate) fibres by inverse gas chromatography

The effect of surface cleanliness on the alkaline hydrolysis of poly(ethylene terephthalate) fibres was investigated using inverse gas chromatography (IGC) in conjunction with mass loss measurements and electron microscopy. The sizing agent was removed from the fibre surface by two methods: soxhlet cleaning in acetone and washing in an aqueous solution of a non‐ionic detergent. Alkaline hydrolysis was carried out using two concentrations of aqueous sodium hydroxide, 1% and 10% by mass. The measurement of the specific retention volume of undecane and the heat of adsorption using IGC indicated that the acetone cleaned samples were essentially surface contaminant free, while partial contamination of the surface by the sizing agent remained in the detergent cleaned samples. The presence of sizing agent significantly altered the degree of hydrolysis and the surface topography. The increasing values of the heat of adsorption indicated that significant surface hydrolysis increased the surface crystallinity. © 2000 Society of Chemical Industry     

气固界面吸附团簇分布及相变机制研究

为了深入探讨气固界面吸附过程中团簇分布及其演化规律,结合实验测量和理论分析研究了水蒸气在二氧化硅和石墨表面的吸附特征,获取了不同压比下的团簇分布,确定了吸附相变及润湿转变的临界条件。结果表明,实验测量结果和Zeta吸附理论预测吸附曲线吻合很好,明确了气体分子以团簇形式吸附在固体表面,在低压比区,小分子团簇和零吸附单元占主导地位,随着压比的增加,吸附团簇类型增多,当压比达到某一临界值时,吸附熵达到极大值,界面发生吸附相变。确定了零吸附情况下石墨和二氧化硅表面张力及界面润湿临界条件,润湿压比下,性质均一的大分子团簇聚集,形成类液膜润湿界面。     

Comparison of polarity parameters for glass fibers obtained by inverse gas chromatography and solvatochromism

Glass fibers with different surface properties (differently sized and unsized) have been investigated by means of inverse gas chromatography (IGC) at infinite dilution as well as by UV/Vis spectroscopy of solvatochromic probe dye molecules. Surface acid–base parameters obtained from the specific energies of adsorption of polar probes using IGC were compared with empirical polarity parameters obtained from shifts in the UV/Vis absorption maxima of adsorbed solvatochromic probe dyes. Both methods give useful information on surface characteristics. Solvatochromism seems to be suitable for a quick sizing characterization and, therefore, this method may become a significant surface analytical tool in the field of adhesion compared with the established IGC methodology.     

Inverse gas chromatography, surface properties, and interactions among components of paint formulations

Inverse gas chromatography (IGC) was applied to the surface characterization of polymers and pigments used in the formulation of protective coatings. IGC measurements over a significant temperature range provided surface energy and acid-base interaction parameters for these materials. Two sets of IGC data were obtained: (1) the quantity of vapor used to probe solid surfaces was extremely small, with results describing the properties of the most energetic surface sites; and (2) finite concentrations of vapor probes were used, with results describing the average properties of surface sites. A comparison of the two sets gave information on the heterogeneity of sites on the polymer and pigment surfaces. Heterogeneity parameters were defined for sites interacting through dispersion forces as well as for those able to interact as acids and bases. The present work reinforces the usefulness of surface characterizations by IGC by showing that the stability of pigments dispersed in the polymer vehicles was a function of the acid-base interaction between polymer-pigment pairs. Dept. of Chemical Engineering, Ecole Polytechnique, P.O. Box 6079, Stn. Centre Ville, Montreal, Que. H3C 3A7, Canada. 19-17 Ikeda Nakamachi, Neyagawa, Osaka 572, Japan.     

Calculation and Interpretation of Surface Free Energy of Wetting of E-Glass by Vapors

The surface free energy of wetting of E-glass by water and benzene vapors was calculated from Bangham and Razouk's free-energy equation. The method of Fu and Bartell for determining specific surface area was used to analyze the microstructure. The surface free energies, which were computed from adsorption isotherms obtained at temperatures from 18° to 20°C, had values from -235 to -254 ergs/cm² and -71.1 to -72.4 ergs/cm² for water and benzene adsorption, respectively, depending on temperature, surface geometry, and sample treatment. The lower affinity of E-glass for benzene, which resulted in a smaller work of adhesion, suggests that active adhesion of hydrocarbons is impossible in the presence of bulk water. Microstructural analysis indicated that the water-modified E-glass surface contains micropores 23 Å in diameter.     

Influence of surface characteristics on the adhesion of Alcaligenes denitrificans to polymeric substrates

The adhesion of Alcaligenes denitrificans to several polymeric materials was investigated. As the nature of the surfaces of the micro-organisms and the substrate materials is an important factor in the adhesion process, characteristics such as the electrokinetic potential and hydrophobicity were also determined and correlated with the capacity of bacterial cells to adhere to solid surfaces. The substrates used were high-density polyethylene (HDPE), polypropylene (PP), poly(vinyl chloride) (PVC), and poly(methyl methacrylate) (PMMA). The electrokinetic potential of the cells and the substrates was determined by measurements of electrophoretic mobility and the hydrophobicity was determined by contact angle measurements. All the substrates studied as well as the bacterial strain have a negative zeta potential, which means that adhesion is not mediated by electrostatic interactions. As far as hydrophobicity is concerned, PP is the most hydrophobic material, PMMA is the least hydrophobic, whereas HDPE and PVC present an intermediate behavior. As bacteria cells are hydrophilic, adhesion is favored to PP; therefore, this substrate material seems to be the one that promotes a stronger adhesion and the development of the most stable biofilm for use as a biomass carrier in denitrifying inverse fluidized bed reactors. This was confirmed by the results of adhesion tests. In this way, adhesion seems to be dominated by hydrophobic interactions.     

Measurement of the Surface Free Energy of Amorphous Cellulose by Alkane Adsorption: A Critical Evaluation of Inverse Gas Chromatography (IGC)

Surface energies of amorphous cellulose “beads” were measured by IGC at different temperatures (50 to 100°C) using n-alkane probes (pentane to undecane). The equation of Schultz and Lavielle was applied which relates the specific retention volume of the gas probe to the dispersive component of the surface energy of the solid and liquid, γ^d_s and γ^d_l, respectively, and a parameter (“a”) which represents the surface area of the gas probe in contact with the solids. At 50°C, γ^d_s was determined to be 71.5 mJ/m², and its temperature dependence was 0.36 mJ m^-2 K^-1. Compared with measurements obtained by contact angle, IGC results were found to yield higher values, and especially a higher temperature dependence, d(γ^d_s)/dT. Various potential explanations for these elevated values were examined. The surface energy, as determined by the Schultz and Lavielle equation, was found to depend mostly on the parameter “a”. Two experimental conditions are known to affect the values of “a”: the solid surface and the temperature. While the surface effect of the parameter “a” was ignored in this study, the dependence of the surface energy upon temperature and probe phase was demonstrated to be significant. Several optional treatments of the parameter “a” were modeled. It was observed that both experimental imprecision, but mostly the fundamental difference between the liquid-solid vs the gas-solid system (and the associated theoretical weakness of the model used), could explain the differences between γ^d_s and d(γ^d_s)/dT measured by contact angle and IGC. It was concluded that the exaggerated temperature dependence of the IGC results is a consequence of limitations inherent in the definition of parameter “a”.