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1736. Kumagai, H., H. Denbo, N. Fujii, and T. Kobayashi, “Poly(ethylene terephthalate) decomposition process in oxygen plasma: Emission spectroscopic and surface analysis for oxygen-plasma reaction,” J. Vacuum Science and Technology, A22, 1-7, (2004).

Emission spectroscopy was applied to observe the reaction process of poly (ethylene terephthalate) (PET) in an oxygen (O2) plasma generated by a microwave discharge. As the PET was exposed in the O2 plasma flow, light emitted from the PET surface was monitored. In the diagnosis measurement, several emission peaks assigned to the Hα atomic line at 652 nm, Hβ at 486 nm, OH (2Σ→2Π) transition near 244–343 nm and CO (b3 Σ→a3 Σ) near 283–370 nm were observed and measured at various discharge times. These results indicated that after the plasma etching, the PET sample was decomposed by the oxygen plasma reaction, and then, hydrogen abstraction and carbon oxidation processes. We also observed the time profile of oxygen atom, as the atom-emission intensity at 777 nm was monitored. As Hβ atomic and OH molecule lines appeared in the presence of PET, the O atom intensity was significantly reduced. In the surface analysis on Fourier transform infrared and x-ray photoelectron spectroscopy measurements, it was found that for the PET surface treated by O2 plasma containing excited atomic oxygen species, ester bands were broken and carbonization formed on the PET surface.

1732. Kumagai, H., T. Kusunoki, and T. Kobayashi, “Surface modification of polymers by thermal ozone treatments,” AZojomo J. Materials Online, 3, (Dec 2007).

Surface modification of polyethylene (PE), poly(vinylchloride) (PVC), and polystyrene (PS), was performed by thermal-ozone (O3) treatment to improve their properties. Polymer films were exposed to dried O3 gas with 3026 ppm at different temperatures. DRS-FT-IR and UV-Vis-NIR absorption methods were applied to observe the surface characteristics of polymers treated. Absorption band assigned to CDouble BondO stretching appeared near 1720 cm-1 in films treated with O3 at 65°C, whereas O3 treatment at 25°C showed no appearance of the CDouble BondO band on the surface. In PS, the O3 oxidation proceeded regardless of temperature. Comparison between PE-O3 and PS-O3 systems showed that different processes of the surface modification occured. Furthermore, contact angle measurements indicated that the surface wettability of PE and PS was improved by the thermal-O3 treatment.

1486. Kumar, A., and S. Hartland, “Measurement of contact angles from the shape of a drop on a vertical fiber,” J. Colloid and Interface Science, 136, 455-469, (1990).

Photomicrographs were taken of the organic drops formed on surfaces of vertical cylindrical fibers in water. The organic liquids used were 96% paraffin oil + 4% tetrabromoethane, paraffin oil, and 80% paraffin oil + 20% heptane. The fibers studied consised of plyester, a fluoroethylene-propylene copolymer (FEP), and nylon. The upper and lower contact angles, θt and θb, formed by the drops on the fiber surface were measured as a function of the dimensionless maximum drop radius, N, and length, L, from the projected images. As N increased so did θt, whereas θb only initially increased and then became more or less constant. Furthermore, θb decreased slightly close to the highest investigated values of N for systems involving FEP fiber. For a given system, the difference between the values of θt and θb increased as N increased, confirming that gravity forces affect the drop shape and contact angles. Good agreement is found between the measured values of θt and θb, and those obtained by using the theory for the shape of a drop on a vertical fiber [A. Kumar and S. Hartland, J. Colloid Interface Sci. 124, 67 (1988)].

203. Kumar, D., “Surface characterization of polymer substrates, flexographic printing plates, and dried ink films printed with water-based ink systems,” in Surface Phenomena and Additives in Water-Based Coatings and Printing Technology, Sharma, M.S., ed., 151-162, Plenum Press, Feb 1992.

The wettability and adhesion of the coating and printing films on the polymer substrates depend on the surface properties of the formulation ingredients and polymer surface. In addition, the transfer of ink from flexographic printing plates to substrates depends on the surface properties of the printing plate, water-based ink and polymer substrate. Among several surface properties such as surface composition, surface roughness, surface tension/surface energies and surface defects, the surface energies: polar, nonpolar and total energies of the dried coating films, flexographic printing plates and polymer substrates were determined by measuring contact angles of water and methylene iodide. These results were used to understand the ink transfer from printing plates to substrates during flexographic printing process, and ink spreading, wetting as well as ink adhesion behavior of the coatings and inks on the polymer substrates. The data indicates that for good ink transfer and adhesion to occur, the surface energy of the water-based ink should be lower than that of the printing plates and substrates.

202. Kumar, D., and S.N. Srisastava, “Wettability and surface energies of polymer substrates,” in Surface Phenomena and Fine Particles in Water-Based Coatings and Printing Technology, Sharma, M.S., and F.J. Micale, eds., 299-308, Plenum Press, Jun 1991.

The coating and printing of polymer films with water-based formulations are relatively difficult as compared to solvent-based formulations. The surface tension of water is higher than that of the solvents. In addition, the surface energy of polymer surfaces is in the range of 25–40 ergs/cm2. In order to understand the wetting and spreading behavior of coating materials, the polar and non-polar surface energies were evaluated by measuring contact angle of water and methylene iodide on various non-porous substrates. These results were utilized to explain the spreading, wettability and adhesion phenomena in order to understand the interactions between water-based coating/printing materials and non-porous substrates.

2699. Kumar, S., “Liquid transfer in printing processes,” Converting Quarterly, 7, 74-80, (Jul 2017).

2958. Kumara, S., B. Ma, Y.V. Seryuk, S.M. Gubanski, et al, “Surface charge decay on HTV silicone rubber: effect of material treatment by corona discharge,” IEEE Transactions on Dielectrics and Electrical Insulation, 19, 2189-2195, (Dec 2012).

Surface charge decay on thick flat samples of high temperature vulcanized silicone rubber is studied prior and after ac and dc corona pre-treatments. It is found that the charge decay rate on the material exposed to ac corona becomes much higher and sensitive to moisture content in the surrounding air. These features are associated with an increased surface conductivity and formation of a silica-like layer on the polymeric surface, both resulting from ac corona treatment. In contrast, characteristics of the charge decay on the material exposed to dc corona are found to be similar to that measured on untreated samples.

1434. Kunz, M., “Surface modification of polymer substrates for improved adhesion of UV-cured systems,” in European Coatings Conference: Adhesion and Performance Enhancement, 115-128, Vincentz Verlag, Sep 2001.

1723. Kunz, M., and M. Bauer, “Superior adhesion with 'smart priming' - New surface modification technology,” RadTech Report, 27-32, (Nov 2000).

1437. Kunz. M., and M. Bauer, “Adhesion to plastic,” Farbe und Lack, 107, 54-62, (2001).

Polyolefins and fluoropolymers have a major drawback: they don't adhere to polymer surfaces properly. If these substrates do have to be coated, however, there are three different approaches to improving adhesion. One is to change the coating, another is to modify the polymer surface, and the third is to apply some kind of adhesion promoter. One successful process which could solve this problem comprises a combination of a plasma pre-treatment and an additional thin acrylated photoinitiator coating. The result is that the purely physical bond gives rise to a covalent chemical bond between the polymer surface and the coating. Further advantages offered by this process are discussed in this article.

2546. Kurdi, J., H. Ardelean, P. Marcus, P. Jonnard, and F. Arefi-Khonsari, “Adhesion properties of aluminum-metallized/ammonia plasma-treated polypropylene: Spectroscopic analysis (XPS, EXES) of the aluminum/polypropylene interface,” Applied Surface Science, 189, 119-128, (Apr 2002).

The purpose of this work was to investigate the influence of a low-pressure, low-frequency ammonia plasma treatment on the wettability of polypropylene (PP) thin films and its consequences on the adhesion properties of such treated films to thermally evaporated aluminium coatings. The wettability was determined by contact angle measurements while the adherence was evaluated by a U-Peel test especially suited to thin flexible substrates with thin metallic layers. Furthermore, an image processing system was used to measure the percentage of the peeled-off metal. Measurements carried out on NH3 plasma-treated PP films revealed a sharp increase in the wettability and in the adhesion properties for treatment times as short as 1 s. Electron-induced X-ray emission spectroscopy and X-ray photoelectron spectroscopy showed the formation of new chemical bonds at Al/NH3 plasma-treated PP film interfaces. The new types of bonds have been characterized by well-defined chemical states (C–NHx, CO–NH, Al–N–C) in the N 1s (and C 1s) spectra. The interfacial complexes Al–N–C and Al–N–CO are formed by the NH3 plasma treatment which creates at the PP surface active sites (N(C–NHx) and N(CO–NH)) which react with the evaporated aluminium atoms. These interfacial bonds play an important role in the enhancement of the metal/polymer adhesion.

2167. Kurihara, Y., H. Ohata, M. Kawaguchi, S. Yamazaki, and K. Kimura, “Improvement of adhesion between liquid crystalline polyester films by plasma treatment,” J. Adhesion Science and Technology, 22, 1985-2002, (2008).

Surface modification of thermotropic liquid crystalline aromatic polyester (LCP) films was carried out by low-pressure plasma treatment to improve the initial adhesion as well as the long-term adhesion reliability, a measure of durability between the LCP films used as substrates for printed circuit boards. Plasma irradiation was carried out in various plasma gases with different plasma modes such as reactive-ion-etching, and direct-plasma (DP) with pressures ranging from 6.7 Pa to 26.6 Pa. The introduction of polar groups on the film surface such as phenolic hydroxyl groups and carboxyl groups enhanced the initial adhesion by increased chemical interaction. However, if the concentration of polar groups became too high, the longterm adhesion reliability estimated by the pressure cooker test was degraded due to the acceleration of the penetration of water molecules into the interface. A large surface roughness was also effective in preventing the decrease in the long-term adhesion reliability. However, too much increase in surface roughness decreases the long-term adhesion reliability. The DP-treatment in the O2 atmosphere at a gas pressure of 6.7 Pa was found to be the best plasma condition for both the initial adhesion as well as the long-term adhesion reliability between the LCP films.

2324. Kusabiraki, M., “Surface modification of polytetrafluoroethylene by discharges,” J. Applied Physics, Part 1, 29, 2809-2814, (1990).

A triode glow discharge system was used for the plasma treatment of polytetrafluoroethylene (PTFE) films and the formation of plasma polymerized hexamethyldisiloxane (PPHMDS) films on PTFE films. The nitrogen plasma increased the surface tension of the PTFE films to about 40 dyn/cm by applying an rf voltage to the substrate electrode. The contact angle of water on the PPHMDS films with the rf voltage was changed to 40°∼90° by corona discharge exposure for 30 s at 6 kV. This reduction is due to the decarbonization and the oxidation of PPHMDS films.

1397. Kusano, Y., “Atmospheric pressure plasma processing for polymer adhesion: A review,” J. Adhesion, 90, 755-777, (2014).

Atmospheric pressure plasma processing has attracted significant interests over decades due to its usefulness and a variety of applications. Adhesion improvement of polymer surfaces is among the most important applications of atmospheric pressure plasma treatment. Reflecting recent significant development of the atmospheric pressure plasma processing, this work presents its fundamental aspects, applications, and characterization techniques relevant to adhesion.

1353. Kusano, Y., M. Yoshikawa, I. Tanuma, Y. Fukuura, K. Naito, et al, “Surface treatment of fluoropolymer members and preparation of composite products therefrom,” U.S. Patent 5425832, Jun 1995.

3013. Kusano, Y., S. Teodoru, and C.M. Hansen, “The physical and chemical properties of plasma treated ultra-high-molecular-weight polyethylene fibers,” Surface and Coatings Technology, 205, 2793-2798, (Jan 2011).

A uniform and smooth transfer of stresses across the polymer matrix/fiber interface is enhanced when adhesion between the matrix and fiber surface is optimized. In the absence of covalent bonds matching the Hansen solubility (cohesion) parameters (HSP) of the fiber surface with the HSP of a matrix polymer assures maximum physical adhesion to transfer loads uniformly. Plasma treatment of ultra-high-molecular-weight (UHMWPE) fibers is shown to significantly increase the amount of oxygen in the surface. There are two distinct types of surfaces in both the plasma treated and the untreated UHMWPE fibers. One type is typical of polyethylene (PE) polymers while the other is characteristic of the oxygenated surface at much higher values of HSP. The oxygenated surface of the plasma treated fibers has the HSP δD, δP, and δH equal to 16.5, 15.3, and 8.2, compared to the pure PE surface with HSP at 18.0, 1.2, and 1.4, all in MPa½. The dispersion parameter has been lowered somewhat by the plasma treatment, while the polar and parameters are much higher. The HSP methodology predicts enhanced adhesion is possible by skillful use of anhydride and nitrile functional groups in matrix or tie polymers to promote compatibility in the system.

2395. Kusano, Y., T. Inagaki, M. Yoshikawa, S. Akiyama, and K. Naitoh, “Corona discharge surface treating method,” U.S. Patent 5466424, Nov 1995.

1006. Kusano, Y., T. Noguchi, M. Yoshikawa, N. Kato, and K. Naito, “Effect of discharge treatment on vulcanised rubber surfaces,” in IRC '95 Kobe International Rubber Conference Proceedings, 432-435, Japan Society of Rubber Industry, 1995.

3014. Kusano, Y., and R. Kusano, “Critical assessment of the correlation between surface tension components and Hansen solubility parameters,” Colloids and Surfaces A: Physicochemical and Engineering Aspects, 677, Part B, (Nov 2023).

Surface or interfacial phenomena, including wetting, adsorption, adhesion, and dissolution, are of significant interest for daily life as well as for industrial and engineering applications. Surface tension and the Hansen solubility parameter (HSP) both represent similar physical characteristics related to these phenomena. It is therefore interesting to study the relation between them, and in the present work, reported empirical relations between surface tension and HSP are critically investigated. There exists an approximately proportional relation between total surface tension and HSP, although the coefficient obtained in the present work is much smaller than the commonly reported ones. The result is supported by an estimation of the coefficient using a simple physical model. On the other hand, finding correlations between the partial components of surface tension and HSP appears to be difficult as they are measured differently. The uses of databases from which measurements are taken must also be taken into question. As an example, the surface tension components of diiodomethane are investigated, and the validity of the reported values are called into question.

204. Kutsch, W.P., “Hot stamping applications and critical surface tension in the plastic industry,” in SPE Decorating Div. RETEC 1993, Society of Plastics Engineers, Oct 1993.

205. Kuusipalo, J., and A. Savolainen, “Ozone, generated at corona treater, as an adhesion promoter in extrusion coating,” in 1994 Polymers, Laminations and Coatings Conference Proceedings, 325-333, TAPPI Press, Aug 1994 (also in TAPPI J., Vol. 77, p.162-166 (Dec 1994)).

The trials documented in this paper were run on a pilot coextrusion coating line at the Tampere University of Technology's Institute of Paper Converting in Finland. The study was conducted to test the ozonization system that was installed in our pilot line to treat the polymer melt in extrusion coating. The effect of the ozone, generated at the corona treater, on adhesion was studied. Ozone was first captured and transported to the nip with separate pipes. It was then led to an air knife near the air gap and blown against the polymer melt. The measured adhesion showed the usefulness of this technique. The following parameters were varied to determine the effectiveness of the ozone: substrate, line speed, coating weight, and melt temperature. Results indicated that thicker coating weights and higher melt temperatures improved adhesion values. The corona-generated ozone clearly improved adhesion compared to corona-treated or untreated samples.

1868. Kuusipalo, J., and A. Savolainen, “Adhesion phenomena in (co) extrusion coating of paper and paperboard,” J. Adhesion Science and Technology, 11, 1119-1135, (1997).

In extrusion coating, the inadequate adhesion between the polymer coating and the fiber-based paper substrate (paper and paperboard) is both a common and a constant problem. The lack of adhesion between the printing ink, or glue, and the polymer coating is another area where adhesion improvement is needed. The common means of improving adhesion are flame, corona, and ozone treatments. A modem extrusion coating line is equipped with both a pretreatment and a post-treatment unit. From the work presented here, the following observations were made. The higher the applied corona power and the thicker the coating, the higher the surface energy and polarity of the low density polyethylene (PE-LD) surface. When a high corona power was applied to the coating, only the polar component of the surface energy was increased. The surface energy decreased sharply as a function of aging, but remained more or less constant after about 2 weeks' storage time. The contact angles of water on paper correlated well with the oxygen contents (determined by ESCA) and with the applied corona power. The polarities of both paper and paperboard increased as a function of the applied corona power. Corona pretreatment of paper and paperboard improved their adhesion to PE-LD remarkably. The adhesion of the polypropylene (PP) homopolymer is based more on mechanical interlocking than on interfacial bonding. On the other hand, the oxidizing pretreatments of the paper substrates significantly promoted the adhesion of the PP copolymer.

2754. Kuusipalo, J., and A. Savolainen, “Adhesion in extrusion coating with polypropylene,” in 1993 Polymers, Coatings and Laminations Conference Proceedings, 469-478, TAPPI Press, Aug 1993.

716. Kuusipalo, J.T., and A.V. Savolainen, “Adhesion phenomena in (co)extrusion coating of paper and paperboard,” Presented at First International Congress on Adhesion Science and Technology, Oct 1995.

206. Kuznetsov, A.Y., V.A. Bagryansky, and A.K. Petrov, “Adhesion properties of glow-discharge plasma treated polyethylene surfaces,” J. Applied Polymer Science, 47, 1175-1184, (1993).

A geometric method proposed by Kaelble and Moacanin for analyzing energetic characteristics of solid surfaces is discussed. It is shown that a number of characteristics can be presented in a visual geometric form. The method is applied to the analysis of adhesion in a three-component system. Results of the analysis, concerned with the problem of creation of biocompatible materials, support the Andrade hypothesis that materials with zero interfacial tension of their water–solid interface show maximum biocompatibility. Data are obtained on wetting of polyethylene by different liquids before and after glow discharge plasma treatment. The analysis of these data in terms of geometric method shows the plasma treatment increased the polar component of the polymer surface energy to change the surface adhesiveness toward probable enhancement of polymer biocompatibility. That is consistent with available data on glow-discharge-treatment enhancement of biocompatibility for a number of polymers. © 1993 John Wiley & Sons, Inc.
https://onlinelibrary.wiley.com/doi/abs/10.1002/app.1993.070470705

548. Kuznetsov, A.Y., V.A. Bagryansky, and A.K. Petrov, “Adhesion properties of glow-discharge-plasma-treated polyethylene surface,” J. Applied Polymer Science, 47, 1175-1184, (Feb 1993).

1018. Kuzuya, M., S. Kondo, M. Sugito, and T. Yamashiro, “Peroxy radical formation from plasma-induced surface radicals of polyethylene as studied by electron spin resonance,” Macromolecules, 31, 3230-3234, (May 1998).

The nature of peroxy radical formation from plasma-induced surface radicals of polyethylene (PE), both low-density polyethylene (LDPE) and high-density polyethylene (HDPE), was studied by electron spin resonance with the aid of systematic computer simulations. It was found that peroxy radical formation varies with the structure of component radicals of plasma-irradiated PE, both LDPE and HDPE:  Among three plasma-induced radicals of PE, dangling bond sites (DBS) undergo an instant conversion into the corresponding peroxy radicals in contact with oxygen, while the midchain alkyl radical is of very low reactivity with oxygen in both LDPE and HDPE. Computer simulations disclosed that ESR spectra of peroxy radicals are similar to each other in LDPE and HDPE, both being composed of two types of spectra, a partial >em>g-averaging anisotropic spectrum and a nearly isotropic single line spectrum due to different molecular motional freedom at the trapping sites of peroxy radicals.

1019. Kuzuya, M., T. Yamashiro, S. Kondo, M. Sugito, and M. Mouri, “Plasma-induced surface radicals of low-density polyethylene studied by electron spin resonance,” Macromolecules, 31, 3225-3229, (May 1998).

Plasma-induced low-density polyethylene (LDPE) radicals were studied in detail by electron spin resonance (ESR) by its comparison with ESR of high-density polyethylene (HDPE). The observed ESR spectra of plasma-irradiated LDPE are largely different in pattern from those of HDPE. The systematic computer simulation disclosed that such observed spectra consist of three kinds of radicals, midchain alkyl radical (1), allylic radical (2) as discrete radical species, and a large amount of dangling bond sites (DBS) (3) at an intra- and intersegmental cross-linked region. All these component radicals are essentially identical to those of HDPE. One of the most special features unique to plasma-irradiated LDPE, however, is the fact that thermally stable DBS (3) is a major component radical instead of a midchain alkyl radical in HDPE. This can be ascribed to the difference in polymer morphology between LDPE and HDPE:  branched structure with a large amount of amorphous region for LDPE and linear structure with a large amount of crystalline region for HDPE. Since one of the characteristics of plasma irradiation is the fact that it is surface-limited, LDPE would undergo the radical formation preferentially on the surface-branched structural moiety followed by facile cross-link reactions resulting in the formation of DBS. Thus, the nature of radical formation of PE was found to be affected by the polymer morphology in a very sensitive manner.

1698. Kwok, D.Y., “The usefulness of the Lifshitz-van der Waals/acid-base approach for surface tension components and interfacial tensions,” Colloids and Surfaces A: Physicochemical and Engineering Aspects, 156, 191-200, (1999).

The Lifshitz–van der Waals/acid-base approach proposed by van Oss et al. is found to yield inconsistent solid surface tensions and components from contact angles, for fluorocarbon, polystyrene, and poly(methyl methacrylate) solid surfaces. It is also shown that the approach cannot predict the correct interfacial tensions of all liquid–liquid pairs in question: the predicted interfacial tensions range from 34% lower to 112% higher than the experimental values. Thus, the usefulness of the approach for surface tension components and interfacial tensions is open to question. The liquid surface tension components postulated since 1986 are also summarized.

1316. Kwok, D.Y., A. Leung, A. Li, C.N.C. Lam, R. Wu, and A.W. Neumann, “Low-rate dynamic contact angles on poly(n-butyl methacrylate) and the determination of solid surface tensions,” Colloid and Polymer Science, 276, 459-469, (1998).

Low-rate dynamic contact angles of 22 liquids on a poly(n-butyl methacrylate) (PnBMA) polymer are measured by an automated axisymmetric drop shape analysis-profile (ADSA-P). It is found that 16 liquids yielded non-constant contact angles, and/or dissolved the polymer on contact. From the experimental contact angles of the remaining 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. γlv cos θ depends only on γlv 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 [34–37, 45–47]. The solid–vapor surface tension calculated from the equation-of-state approach for solid-liquid interfacial tensions [14] is found to be 28.8 mJ/m2, with a 95% confidence limit of ±0.5 mJ/m2, from the experimental contact angles of the 6 liquids.

1314. Kwok, D.Y., A. Leung, C.N.C. Lam, A. Li, R. Wu, and A.W. Neumann, “Low-rate dynamic contact angles on poly(methyl methacrylate) and the determination of solid surface tensions,” J. Colloid and Interface Science, 206, 44-51, (1998).

Low-rate dynamic contact angles of nine liquids on a poly(methyl methacrylate) (PMMA) polymer are measured by an automated axisymmetric drop shape analysis—profile (ADSA-P). It is found that two liquids dissolved the polymer on contact. From the experimental contact angles of the other seven polar and nonpolar 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., γcos θ depends only on γfor a given solid surface). The dependence of γcos θ on γis explicitly illustrated by replacing the solid surface from the PMMA to other methacrylate polymers: such a procedure shifts the curves in a very regular manner. Thus, because of Young's equation, γsldepends only on γand γ. This contact angle pattern is in harmony with those from other inert and noninert (polar and nonpolar) surfaces. The solid–vapor surface tension of PMMA calculated from the equation of state approach for solid–liquid interfacial tensions is found to be 38.5 mJ/m2, with a 95% confidence limit of ±0.5 mJ/m2from the experimental contact angles of the seven liquids.

1320. Kwok, D.Y., A. Li, and A.W. Neumann, “Low-rate dynamic contact angles on poly(methyl methacrylate/ethyl methacrylate, 30/70) and the determination of solid surface tensions,” J. Polymer Science Part B: Polymer Physics, 37, 2039-2051, (1999).

Low-rate dynamic contact angles of 12 liquids on a poly(methyl methacrylate/ethyl methacrylate, 30/70) P(MMA/EMA, 30/70) copolymer were measured by an automated axisymmetric drop shape analysis-profile (ADSA-P). It was found that five liquids yield nonconstant contact angles, and/or dissolve the polymer on contact. From the experimental contact angles of the remaining seven 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., γl|Kv cos θ depends only on γl|Kv for a given solid surface or solid surface tension). This contact angle pattern is in harmony with those from other methacrylate polymer surfaces previously studied.45,50 The solid–vapor surface tension calculated from the equation-of-state approach for solid–liquid interfacial tensions14 is found to be 35.1 mJ/m2, with a 95% confidence limit of ± 0.3 mJ/m2, from the experimental contact angles of the seven liquids. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2039–2051, 1999
https://onlinelibrary.wiley.com/doi/abs/10.1002/(SICI)1099-0488(19990815)37:16%3C2039::AID-POLB8%3E3.0.CO;2-O

1806. Kwok, D.Y., C.J. Budziak, and A.W. Neumann, “Measurement of static and low rate dynamic contact angles by means of an automated capillary rise technique,” J. Colloid and Interface Science, 173, 143-150, (Jul 1995).

Six solid surfaces were compared with respect to their surface quality, by measuring advancing contact angles along the solid surfaces (in the vertical and horizontal directions) at constant immersion rate. It was found that surfaces of mica, dip coated in FC-721, Teflon (FEP) heat pressed against mica, and siliconized glass yield essentially constant advancing contact angles at different locations of the solid surfaces and, thus, are well suited to dynamic contact angle measurements. Static and low rate dynamic contact angles of a number of pure liquids were therefore measured on these solid surfaces. Low rate dynamic contact angles were found to be identical with the static contact angles and independent of the velocity of the three-phase contact line (up to 0.5 mm/min).

1294. Kwok, D.Y., C.N.C. Lam, A. Li, A. Leung, R. Wu, E. Mok, and A.W. Neumann, “Measuring and interpreting contact angles: A complex issue,” Colloids and Surfaces A: Physicochemical and Engineering Aspects, 142, 219-235, (1998).

Low-rate dynamic contact angles of 30 liquids on a FC-725-coated wafer surface were measured by an automated axisymmetric drop shape analysis-profile (ADSA-P). Surprisingly, results indicate that FC-725 behaves differently in some respects from what one would expect for non-polar surfaces: only nine liquids yield essentially constant contact angles whereas the others show slip/stick contact angle behaviour. In the worst case, the contact angle increases from ca 50 to 160° at essentially constant three-phase contact radius. These angles should be disregarded for the interpretation in terms of surface energetics since there is no guarantee that Young's equation is applicable. If one employs a conventional goniometer-sessile drop technique, such contact angle behaviour cannot be easily seen in all cases. These results indicate that the claim from van Oss et al. [Langmuir 4 (1988) 884] that liquids with the same contact angles do not have the same surface tensions is misleading. If the meaningful contact angles are plotted as the liquid–vapour surface tension times cosine of the contact angle versus the liquid–vapour surface tension, that is, γlv cos θ versus γlv, a smooth curve emerges. Thus, intermolecular forces (or surface tension components) do not have an additional and independent effect on the contact angles, in good agreement with the results from other studies on non-polar and polar polymers.

1315. Kwok, D.Y., C.N.C. Lam, A. Li, K. Zhu, R. Wu, and A.W. Neumann, “Low-rate dynamic contact angles on polystyrene and the determination of solid surface tensions,” Polymer Engineering and Science, 38, 1675-1684, (1998).

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. γlvcosθ depends only on γlv 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.

1317. Kwok, D.Y., C.N.C. Lam, A. Li, and A.W. Neumann, “Low-rate dynamic contact angles on poly(methyl methacrylate/n-butyl methacrylate) and the determination of solid surface tensions,” J. Adhesion, 68, 229-255, (1998).

Low-rate dynamic contact angles of 12 liquids on a poly(methyl methacrylate/n-butyl methacrylate) P(MMA/nBMA) copolymer are measured by an automated axisymmetric drop shape analysis-profile (ADSA-P). It is found that 6 liquids yield non-constant contact angles, and/or dissolve the polymer on contact. From the experimental contact angles of the remaining 6 liquids, it is found that the liquid- vapour surface tension times the cosine of the contact angle changes smoothly with the liquid-vapour surface tension, i.e., γiv cos θ depends only on γiv for a given solid surface (or solid surface tension). This contact angle pattern is in harmony with those from other inert and noninert (polar and non-polar) surfaces [34-42, 51 -53]. The solid-vapour surface tension calculated from the equation-of-state approach for solid -liquid interfacial tensions [14] is found to be 34.4 mJ/m2, with a 95% confidence limit of \pm 0.8mJ/m2, from the experimental contact angles of the 6 liquids.

723. Kwok, D.Y., D. Li, and A.W. Neumann, “Capillary rise at a vertical plate as a contact angle technique,” in Applied Surface Thermodynamics, Neumann, A.W., and J.K. Spelt, eds., 413-440, Marcel Dekker, Jun 1996.

In the vast majority of contact angle studies in the literature, the method used is direct measurement of sessile drops. Recent developments in image analysis and processing have increased the accuracy and reduced the subjectivity considerably (see Chapter 10). Nevertheless, there are certain limitations which leave room for other techniques. One of the limitations of the classical sessile-drop method is that the camera or imaging device will be focused on the largest meridian section, and hence reflect only the contact angles at the point in which the meridian plane intersects the three-phase line. Surface heterogeneity and/or roughness could well cause variations of the contact angle along the three-phase line. An alternate approach is to infer the contact angle from the drop contact diameter (see Chapter 10). But even on very good solid surfaces, on which such effects are absent, there is a systematic problem with this method: contact angles will change as a function of drop size for drops up to approximately 1 cm diameter. While this dependence can indeed be used to determine line tension (see Chapter 4), this and other similar effects interfere with the interpretation of contact angles in terms of surface energetics (see Chapter 3).

1305. Kwok, D.Y., D. Li, and A.W. Neumann, “Fowkes' surface tension components approach revisited,” Colloids and Surfaces A: Physicochemical and Engineering Aspects, 89, 181-191, (1994).

By comparing the number of degrees of freedom obtained from the phase rule for capillary systems, the Fowkes surface tension component approach for interfacial tensions is shown to require more degrees of freedom than are available for a two-component solid—liquid—vapour system. Only in a special case has the Fowkes approach two degrees of freedom: a dispersive liquid on a dispersive solid, suggesting that there are no surface tension components. Experimental results suggest that the Fowkes component approach does not describe physical reality; only the liquid and solid surface tensions, γ1v and γsv, are operative in the two-component solid—liquid—vapour system. Generalization of the Fowkes component approach, of course, will increase the number of independent variables and hence definitely require more degrees of freedom than are available.

The number of degrees of freedom of the equation of state for interfacial tensions is shown to agree with that predicted from the phase rule for capillary systems as well as with experimental results. By using the empirical form of the equation of state, essentially constant solid tensions, γsv, are obtained from a variety of dispersive and non-dispersive liquids for three solid surfaces: fluorocarbon (FC721), Teflon (FEP) and poly(ethylene terephthalate) (PET).

1306. Kwok, D.Y., D. Li, and A.W. Neumann, “Evaluation of the Lifshitz-van der Waals/acid-base approach to determine interfacial tensions,” Langmuir, 10, 1323-1328, (1994).

1312. Kwok, D.Y., L.K. Cheung, C.B. Park, and A.W. Neumann, “Study on the surface tensions of polymer melts using axisymmetric drop shape analysis,” Polymer Engineering and Science, 38, 757-764, (1998).

It is shown that Axisymmetric Drop Shape Analysis (ADSA) is well-suited for the study of polymer melt surface tensions. The technique is not restricted to equilibrium surface (interfacial) tensions; it is also suitable for measuring the time dependence (or kinetics) of surface tension of polymer melts. Results for three polymers, polypropylene, polyethylene, and polystyrene, at temperatures above 170°C are reported. Contrary to the well-known decrease of surface tension in low molecular weight surfactant solutions as a result of equilibration, an increase in the melt surface tension is observed under isothermal conditions.

 

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