A Reinterpretation of Organic Liquid-Polytetrafluoroethylene Surface Interactions

The wettability of polytetrafluoroethylene (PTFE) by organic liquids is reanalyzed in terms of dispersion-polar interactions across the liquid-solid interface. The analysis provides values of γ_s ^d = 19.6 dyne/cm, and γ_S ^D = 2.0 dyne/cm for the respective dispersion and polar parts of the surface tension γ_s = 21.6 for PTFE. The definition of a polar contribution to the surface tension of PTFE clarifies detailed aspects of the wettability of this polymer by different homologous liquid series. A modified analytical definition for work of adhesion is developed and applied to this discussion.     

Critical Surface Tension of Wetting and Flotation Separation of Hydrophobic Solids

Abstract

Wetting characteristics of a number of minerals including layer-type hydrophobic minerals as well as common sulfides were investigated. For the majority of the minerals, the critical surface tension of wetting, γ_c determined using Zisman's technique was in the range of 40 to 45 mN/m. Surface pressures of water, II_e, on molybdenite and coal samples were determined from adsorption isotherms. The dispersion component of the surface-free energy, γ_s ^d, for molybdenite was estimated to be 113 ± 3 mJ/m² as compared to the γ_s ^d value for graphite, 109 mJ/m². The wettability data of aqueous methanol solutions, presented in the form of adhesion tension diagrams, yielded significantly lower γ_c values. Flotation behavior of common sulfides, which was similar to that of inherently hydrophobic polymers and minerals, was attributed to elemental sulfur formation. The relevance of critical surface tension of wetting to selective flotation and separation of hydrophobic solids is discussed.     

Surface Modification of Fluorinated Polymers by Microwave Plasmas

We developed a new plasma treating method, incorporating the use of microwaves generated by an electronic cooking range. Using this method, polytetrafluorethylene (PTFE) and a copolymer of tetrafluoroethylene and hexafluoropropylene (FEP) were treated. Dialkylphthalates (DAP) were used as the standard liquids of contact angle measurements for evaluation of the wetting properties of plasma treated polymers. The components of surface tension (γ_L) due to the dispersion force (γ^d _L) and the polar force (γ^P _L) of DAP were calculated by Fowkes' equation from the contact angles (θ) on polypropylene. After plasma treatment cos θ of several standard liquids on PTFE and FEP increased. The linear relationship between γ_L(1 + cos θ)/(γ^d _L)^½ and (γ^P _L/γ^P _L)^½ was verified. γ_s and γ^d _s and γ^d _s of the plasma treated PTFE and FEP also increased. From the results of ESCA analysis, it was found that a significant amount of oxygen was introduced to the polymer surface by the plasma treatment. Peel strengths of a pressure sensitive adhesive bonded to PTFE and FEP increased approximately two-to threefold if the plasma treatment was used prior to bonding.     

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”.     

Surface energy characterization of modified carbon blacks and its relationship to composites tearing properties

The effects of three types of chemical treatments, i.e. as polar acidic, polar basic, and nonpolar oxidations, on virgin carbon blacks have been studied in terms of pH, acid-base surface values, specific surface area, X-ray diffraction analysis, and surface free energy. The acidic chemical treatment leads to significant changes in surface and adsorption properties, surface free energy, and microstructures. The increased acidic surface functional groups on carbon blacks result from reaction between the basic carbon and the acidic chemical solution. Also, based on the determination of surface free energy from contact angle measurements, a good correlation between the London dispersive component or apolar (γ^d _s) of surface free energy and specific surface area (S_BET) (or crystalline size S along the c-axis, L_C) is shown in this work. Particularly, it is found that the γ^d _s of the carbon blacks studied is highly correlated with the mechanical tearing test results based on hydrocarbon rubber compound composites.     

Studies on the surface free energy and surface structure of PTFE film treated with low temperature plasma

The surface free energy and surface structure of poly(tetrafluoroethylene) (PTFE) film treated with low temperature plasma in O₂, Ar, He, H₂, NH₃, and CH₄ gases are studied. The contact angles of the samples were measured, and the critical surface tension γ_c (Zisman) and γ_c (max) were determined on the basis of the Zisman's plots. Furthermore, the values of nonpolar dispersion force γ^a_s, dipole force γ^b_s, and hydrogen bonding force γ^c_s to the surface tensions for the plasma-treated samples were evaluated by the extended Fowkes equation. Mainly because of the contribution of polar force, the surface free energy and surface wettability of PTFE film which was treated with H₂, He, NH₃, Ar, and CH₄ for a short time increased greatly. Electron spectroscopy for chemical analysis (ESCA) shows that the reason was the decrease of fluorine and the increase of oxygen or nitrogen polar functional group on the surface of PTFE. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1733–1739, 1997     

The Unified Lewis Acid - Base Approach to Adhesion and Solvation at the Liquid-Polymer Interface

We present our unified Lewis acid–base approach to adhesion and solvation at the liquid-polymer interface. This approach is to complement the original methodologies proposed by Fowkes and by van Oss, Chaudhury and Good (VCG). Intermolecular interactions are primarily dominated by dispersion, d, hydrogen bonding, h, and secondarily affected by orientation, o, and induction, i. Generally, the polarization component, p, represents both i and o interactions. Fowkes suggested that the acid–base component, γ^ab, of the surface tension should consist of both h and p interactions. However, VCG proposed that the acid–base components, γ^ab, result solely from hydrogen bonding, γ^h, that is equivalent to 2(γ⁺ γ^?)^1/2, where γ⁺ and γ^? are the two hydrogen bonding parameters. VCG defined γ^LW as the Lifshitz-van der Waals component consisting of d, o and i contributions, thus, surface tension, γ, equals γ^ab(VCG)+γ^LW. Both Fowkes and VCG assumed that the polar interactions for a liquid on a low energy surface are negligible.

Now, we assume otherwise, and we treat the specific acid-base interaction to be hydrogen bonding. In addition, we also take into account the nonspecific polarization, p, interaction in terms of the equilibrium spreading pressure, π_e, resulting from the adsorption of a liquid vapor on the polymer surface. Thus, our unified approach uses the dispersion component, γ^d, of Fowkes, the hydrogen bonding, h, of VCG and the polarization, p, in terms of π_e. The difference between the initial (theoretical) and equilibrium (experimental) surface tensions is π_e, and others have observed that π_e on some polymers is substantial. The determination of several initial surface tensions of polymers by considering the effect of polarization is discussed.

In the Appendix, we shall illustrate that this polar component, π_e, is equivalent to the LESR polarity-dipolarity parameter, π^e, (represented by the same symbol but in different context) for the solvatochromic treatment. Furthermore, the surface tension components, π^d, γ⁺, γ^? and π_e, are now somewhat comparable with the four parameters in the original Taft-Kamlet relationship, δ, α, B, and π^e. Thus, our proposed unified approach may finally help elucidate the long-debated Lewis acid–base theories pertaining to adhesion and solvation of polymers.     

Contact angle,wetting, and adhesion: a critical review

The theory of the contact angle of pure liquids on solids, and of the determination of the surface free energy of solids, γ_s, is reviewed. The basis for the three components γ^LW _s, γ⊕_s, and γ?_s is developed, and an algebraic expression for these properties in terms of measured contact angles is presented. The inadequacy of the 'two-liquid' methodology (which yields a parameter, 'γ^p') is demonstrated. Attention is given to contact angle hysteresis and to the film pressure, π_e. Some recommendations are made with regard to contact angle measurements. A new treatment of hydrophilicity, and of the scale of hydrophobic/hydrophilic behavior, is proposed. It is shown that there are two kinds of hydrophilic behavior, one due to Lewis basicity (electron-donating or proton-accepting structures) and the other due to Lewis acidity (electron-accepting or proton-donating structures). The properties γ^? and γ^⊕ are the quantitative measures of these types of behavior and they are structurally independent of each other. A triangular diagram, with γ^LW at the hydrophobic corner, and γ^⊕ and γ^? at the two hydrophillic corners, is suggested.     

Acid-base effects in polymer adhesion at metal surfaces

The Lewis acid-Lewis base properties of various polymers have been determined by measuring the contributions γ_s ⁺ and γ_S ^- to the solid surface free energy using the contact angle approach of van Oss, Chaudhury, and Good. A new linear method to solve for γ_S ⁺ and γ_S ^- 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.     

IGC study of filler–filler and filler–rubber interactions in silica‐filled compounds

A modification of the existing methods for evaluating the dispersive and specific components of surface free energy (γ_d and γ_s, respectively) was made to investigate filler–rubber and filler–filler interactions by inverse gas chromatography. Four silicas as fillers and various probes that mimic elastomers were employed in this study. It was shown that the pretreatment of silicas with helium could increase γ_d and decrease γ_s. Modification of the silica surface with silane could enhance the dispersive interaction and weaken the specific interaction. The temperature dependence of the interfacial interaction was also investigated, and it was found that lower temperatures favored filler–rubber interactions and mixing efficiency. Tests on different sizes of agglomerates demonstrated the existence of a filler–rubber and filler–filler network. It was also found that γ_d played a role in agglomeration or filler–filler interaction. Our study showed that the larger the specific surface area was, the stronger the dispersive and specific interactions were. The effectiveness of various fillers and elastomer probes was also compared. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 2517–2530, 2001     

Low-rate dynamic contact angles on polystyrene and the determination of solid surface tensions

Low-rate dynamic contact angles of 13 liquids on a polystyrene polymer are measured by an automated axisymmetric drop shape analysis – profile (ADSA-P). It is found that 7 liquids yielded non-constant contact angles, and/or dissolved the polymer on contact. From the experimental contact angles of the other 6 liquids, it is found that the liquid-vapor surface tension times cosine of the contact angle changes smoothly with the liquid-vapor surface tension, i.e. γ_lvcosθ depends only on γ_lv for a given solid surface (or solid surface tension). This contact angle pattern is in harmony with those from other inert and non-inert (polar and non-polar) surfaces (7–13, 24–26). The solid-vapor surface tension calculated from the equation-of-state approach for solid-liquid interfacial tensions (33) is found to be 29.8 mJ/m², with a 95% confidence limit of ±0.5 mJ/m² from the experimental contact angles of 6 liquids.     

Interfacial Bonding and Environmental Stability of Polymer Matrix Composites

Recently developed adsorption-interdiffusion (A-I) theory of adhesion is employed to isolate the (London) dispersion γ_i,j ^d and (Keesom) polar γ_i,j ^p components of the excess interfacial free energy γ_i,j=γ_i,j ^d+γ_i,j ^p at the fiber-matrix interface in polymer matrix composites. For adsorption bonded interfaces the theory defines a new method of mapping the surface energy effects of an immersion phase upon the Griffith fracture energy γ_G. The stability of interfacial bonding between graphite fiber-epoxy matrix is defined in terms of the theoretical model and experimentally evaluated by accelerated aging studies which monitor changes in fracture energy for crack propagation perpendicular to the fiber axis. Applications of the model to control fiber surface treatments and select matrix components for optimized bond strength and environmental resistance is discussed.     

Calendar of events

This review reports the successful synthesis of novel oligomeric silanes having end-capped fluoroalkyl groups. Glass surface was effectively modified by these oligomeric silanes. In particular, oligomeric silanes were more reactive and effective in the surface fluoroalkylation than monomeric silanes. From contact angle measurements, surface free energies were reduced to 15–20 and 1–3 mJ/m² for the dispersive and the polar components, respectively, and the surfaces were shown to be both highly water- and oil-repellent. Modified glass surface was analyzed using XPS. A linear correlation was observed between the dispersive component of surface free energy γ_S ^d and the area ratio of the F1s peak to the Si2p peak. The structure of the siloxane layer on the modified glass surface is discussed in terms of a network interphase model.     

New fluorinated polysiloxanes containing an ester function in the spacer—II. Surface tension studies

The surface tensions of fluorinated polysiloxanes prepared by hydrosilylation of unsaturated perfluoroalkyl esters derived from undecylenic acid [CH₂?CH? (CH₂)₈? COO? CH₂? CH₂? R_F, with R_F = C₆F₁₃, C₈F₁₇, and C₈F₁₇? (CH₂)₁₀COO? CH₂? CH₂? CH?CH₂] by methylhydrodimethylsiloxane copolymers of various Si? H contents have been measured. The critical surface tensions, γ_c, and the solid surface tensions, γ^D_s, were deduced from n-alkane and water contact angle data. They decrease as the perfluoroalkyl graft content of the copolymers increases. Some of them, which are in the range of the lowest surface tension fluoro polymers known, are observed when the fluorinated segments are self-organized at the interface, i.e. when the polymers are mesomorphous or crystalline at room temperature.     

Surface tension parameters of ice obtained from contact angle data and from positive and negative particle adhesion to advancing freezing fronts

From contact angle data obtained on flat ice surfaces with a number of liquids, combined with data on particle and macromolecule adhesion or non-adhesion to advancing freezing fronts, the apolar (Lifshitz-van der Waals or LW) and polar (Lewis acid-base or AB) surface tension (γ) components and parameters have been determined. At 0°C these are γ^LW _iee = 26.9 and γ^AB _ice = 39.6 mJ/m². The latter consists of an electron-acceptor (γ^⊕) and an electron-donor (γ^?) parameter: γ^⊕ = 14 and γ^? = 28 mJ/m².     

Studies on adhesion of polyethylene. Part I. Influence of functionality and phase transfer catalyst

Low-density polyethylene (LDPE) has been oxidized using phase transferred permanganate as an oxidant. The resulting surface modifications have been characterized by different methods and the consequent adhesion promotion has been characterized in terms of contact angle and peel strength measurements. From contact angle measurements using water and formamide liquid drops, reversible work of adhesion, and thence γ^p _s and γ-^d _s, the contributions of polar and dispersion components, respectively, have been calculated. The polar contributions increased in each oxidized LDPE relative to untreated LDPE and the surface energies also increased appreciably. The adhesion strengths between aluminium and untreated LDPE, as well as those between aluminium and oxidized LDPEs, have been examined by peel strength measurements. We found that the adhesion strength increased about 8-28 times in the case of oxidized LDPEs. Maximum strength was observed when benzyl triphenyl phosphonium permanganate was the oxidant. Both the peel force and the thermodynamic work of adhesion increase sharply with an increase in the carboxyl content, the total number of oxo groups, and the combined CO and COOH content. The dependence of these quantities on carbonyl content is either weak or even follows a reverse trend. It is proposed that the adhesion proceeds primarily through hydrogen bonds involving carboxyl group and/or coordinative bond formation between aluminium and epoxy and/or hydroperoxo groups.     

Surface tension versus barrier property for fluorinated surfaces

The barrier property of fluorinated surfaces was compared to the surface tension measurements. A relationship between barrier property and the solvent's surface tension was developed. However, other properties of both the solvent and the polymer contribute to barrier property. The surface tension of the polymer surface was separated into nonpolar, polar, and hydrogen bonding forces. By estimating the % γ of various solvent mixtures, the % γ_S^D was estimated for the polymer surface.     

The Extraction of 137Cs and 89Sr from Waste Simulants Using Pillared Montmorillonite

ABSTRACT

Two samples of a silica-pillared montmorillonite produced using 3-aminopropyltrimethoxy silane and an alumina-pillared montmorillonite were evaluated for the removal of ¹³⁷Cs and ⁸⁹Sr from a simulated nuclear waste solution and a simulated groundwater, and the results were compared to the parent montmorillonite and two zeolites, AW500 (chabazite) and clinoptilolite. The parent and pillared clays were characterized using x-ray powder diffraction and surface area analysis by nitrogen adsorption/desorption studies. The pillared clays exhibited d-spacings of between 173 and 182 A after calcination, and surface areas ranging from 71 to 264 m²g^-1. Both of the silica-pillared clays and the alumina-pillared clay exhibited excellent K_dsfor ¹³⁷Cs from simulated groundwater with values of 23,650, 23,260 and 144,570 mL/g, respectively. These were far better than the K_ds obtained by clinoptilolite and AW500 which had K_ds of only 14,560 and 9650 mL/g, respectively. None of the pillared clays showed a high selectivity for ⁸⁹Sr from groundwater or ^l37Cs from simulated alkaline tank waste. They did, however, show a slight selectivity for ⁸⁹Sr in the simulated Hanford tank wastes, but this is thought to be due to a precipitation mechanism rather than to ion exchange.     

Use of different size grids to control the surface chemistry of plasma‐polymerized acrylic acid films in a hybrid discharge

The surface chemistry of plasma‐polymerized acrylic acid (ppAc) films were controlled in a two‐stage (primary and processing) hybrid radio frequency (RF) discharge by changing the grid wire spacing (d_s). Two regions were defined in terms of d_s with respect to Debye length (λ_d) in the primary chamber at the grid to control the electron temperature (T_e) and surface chemistry of the ppAc films deposited in the processing chamber. A higher T_e (>3 eV) in the processing plasma was possible for d_s > λ_d, whereas decreasing d_s relative to λ_d reduced T_e. X‐ray photoelectron spectroscopy was used to characterize the ppAc films deposited on a glass substrate. The ppAc films surface characterization showed the maximum proportion of carbon atoms as carboxylic/ester [C(?O)OX] functionalities in C1s at the surface of films for the grid with d_s ≈ λ_d. The proportion of carbon atoms as ? [C(?O)OX] and COX in C1s at the surface decreased when d_s decreased relative to λ_d. The proportion of carbon atoms as carbonyl (C?O) at the film surface showed very good stability for all of the d_s values explored in this study. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2219–2224, 2007