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Influence of the viscosity and the substrate on the surface hydrophobicity of polyurethane coatings
Applied Surface Science 253 (2006) 805–809 www.elsevier.com/locate/apsusc
Influence of the viscosity and the substrate on the surface hydrophobicity of polyurethane coatings M. Meincken *, A. Klash, S. Seboa, R.D. Sanderson Department of Chemistry and Polymer Science, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa Received 4 August 2005; received in revised form 10 January 2006; accepted 10 January 2006 Available online 14 February 2006
Abstract Tailor-made polyurethane (PU) dispersions were synthesized as coatings for paperboard for dry food packaging. For this purpose a low moisture-vapor transmission rate and a high surface hydrophobicity are desirable characteristics, which are both met by PU. However, it was found that the surface hydrophobicity of water-borne PU dispersions depends strongly on the viscosity of the dispersion. This dependency was studied by static contact angle measurements (SCA) as well as a novel technique using digital pulsed-force mode atomic force microscopy (DPFM-AFM). Comparison of the results validated that DPFM-AFM is a valuable tool to characterize the surface hydrophilicity. Both techniques confirmed that the surface hydrophobicity varies with the viscosity and that an optimum viscosity for the PU coating with a maximum surface hydrophobicity can consequently be determined. It was found that both lower as well as higher viscosities led to a less hydrophobic surface. # 2006 Elsevier B.V. All rights reserved. PACS: 68.37.Ps; 81.16.Dn; 82.35.Lr Keywords: Polyurethanes; Coatings; Surface hydrophobicity; Atomic force microscopy (AFM); Static contact angle (SCA)
1. Introduction The development of conventional solvent-borne systems into environmentally friendly coatings has been studied extensively over the last 25 years. Solvent-borne systems have been widely replaced by solid or powder coatings and by waterborne coatings, both of which are preferred due to their compatibility with conventional coating systems. Water-borne polyurethane systems combine low volatility with the good film formation properties of solvent-borne systems and the inherent favorable characteristics of the urethanes. Despite this, polyurethanes (PU) are not commonly used for paper coatings because of their high cost, especially that of the isocyanate component. In the past, up-scaling polyurethane production exhibited many problems, such as the high reactivity of the isocyanate-group towards impurities like water. These difficulties have, however, been largely overcome and can be controlled. Other than the high cost, PU has almost no other disadvantages. It can, for example, be tailor-made to enhance
* Corresponding author. Tel.: +27 21 8083298; fax: +27 21 8083603. E-mail address: [email protected] (M. Meincken). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.01.014
certain desired properties. These include a very low moisturevapor transmission rate, good control of the particle size, excellent film formation properties, wax compatibility, thermal and chemical stability, recyclability of the coated paperboard and a high chemical and abrasion resistance. Furthermore, they are non-toxic and environmentally friendly. Since polyurethanes can be tailor-made to suit specific demands, they could prove an extremely versatile coating material for paperboard for packaging purposes. Polyurethanes are also known as isocyanate polymers. They are synthesized by the combination of a polyol, which has at least two active hydroxyl groups per molecule, with a diisocyanate or a polymeric isocyanate in the presence of a catalyst. Since many different diisocyanates and a variety of polyols can be used to obtain polyurethanes, a wide range of materials can be synthesized, including fibers, soft and hard elastomers, coatings and adhesives, flexible and rigid foams, thermoplastic and thermosetting plastics. The characteristic feature of polyurethanes is their chain structure, which consists of alternating hard and soft segments. The hard segments contain polar urethane groups, which can form hydrogen bonds and crystalline structures. The soft segments are flexible and non-polar [1,2]. The physical and
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M. Meincken et al. / Applied Surface Science 253 (2006) 805–809
Fig. 1. Self-assembly mechanism of PU molecules on a substrate.
mechanical properties of polyurethanes depend on factors such as their molecular weight, their composition and length of the soft and hard segments, and the presence of anomalous linkages such as branching and crosslinking. Polyurethane coatings on a polar surface should ideally be highly orientated [3], with all the polar, hydrophilic sections attached to the substrate, and their hydrophobic sections pointing to the surface, as displayed in Fig. 1. The degree of this orientation depends strongly on the ionic content and the viscosity of the PU emulsion upon coating [4,5], which influences the mobility of the molecules and therefore their ability to self-assemble. Excessively low mobility as well as too high mobility leads to poor molecule orientation. In order to self-assemble, the polymer chains need suffi cient time and space to orientate their polar and non-polar segments on the substrate during the coating process. In order to achieve optimum self-assembly, the mobility of the polar and the non-polar segments of the PU emulsion particles must be controlled. This can be done by measuring, and subsequently adjusting the ionic content and the viscosity of the PU emulsion. In this study the ionic content was kept at a constant value of 5.1% dimethylproponic acid (DMPA), while the viscosity was varied in order to determine the optimum value for selfassembly by digital pulsed-force mode atomic force microscopy (DPFM-AFM) and static contact angle (SCA) measurements. Polyurethane dispersions with viscosities ranging from 100 to 400 mPa s were analyzed by both techniques and their results compared. Paperboard proved to be too rough a substrate for AFM analysis, with a surface roughness ranging from 0.5 to 3 mm. Therefore, the PU films were also coated on silicon and on freshly cleaved, atomically flat mica. Measurement of the hydrophobicity as a function of the viscosity by both techniques showed the same trend for different substrates, which indicates that the orientation of the PU chains is independent of the substrate used, as long as the substrate is polar. Coatings on unpolar surfaces, such as polyethylene, dewetted the substrate surface and did not form a smooth film. The hydrophobicity of molecules can be distinguished by AFM with an additional digital pulsed-force mode (DPFM) controller, which allows the determination of the adhesion and elastic properties of the sample at each scan point [6,7]. Images obtained in this way result in a surface ‘‘map’’, showing different adhesive forces between the tip and the sample on the scanned area as different colours. Therefore, it is possible to distinguish the polar and the non-polar parts of molecules on the surface, and to determine an average adhesion value for the sample surface, which is equivalent to the surface hydrophilicity [8]. A silicon tip was used to scan the surface of the films investigated in this study. Untreated silicon has a native
oxide layer with hydroxyl bonds. These OH groups are adsorption sites for water molecules and the surface is therefore hydrophilic. A conventional silicon tip with SiO2 groups at the surface will show a higher adhesive force on a hydrophilic than on a hydrophobic surface [7,9]. The image of the adhesion force therefore represents the hydrophilicity of the sample, where lighter parts represent more hydrophilic and darker parts represent more hydrophobic compounds. Okabe et al. [10] used the same technique to discriminate the functional groups of self-assembled sample surfaces, by using chemically modified tips. Sato et al. [11] determined the distribution of adhesive force values on self-assembled sample surfaces. 2. Experimental 2.1. Polymer preparation Polyurethane dispersions were prepared with a 70 wt% soft segment content to form a thin thermoplastic coating for paperboard. The soft PU segments were prepared from a linear polyester-based polyol as a precursor, which was synthesized from four monomers: adipic acid (AA), neopentyl glycol (NPG), 1,4-cyclohexane dicarboxylic acid (CHDCA) and 2phosphonobutane 1,2,4-tricarboxylic acid (PBTCA). The polyester was based on AA and NPG as the primary monomers, while CHDCA and PBTCA formed the secondary monomers. A maximum of 15% CHDCA and 10% PBTCA was used for the polyol synthesis. CHDCA was added in order to increase the hydrophobicity of the polyol, while the PBTCA was added to further enhance, e.g. the barrier properties of the coating. The polyol was dried and degassed overnight at 80 8C in a vacuum oven. For the synthesis of the hard segments, the reactor was charged with toluene diisocyanate (TDI) and dimethyl proponic acid (DMPA), placed in a water bath, and purged with N2-gas. NMP was added to the reaction mixture at room temperature followed 20 min later by ethylene glycol (EG) chain extender. After another 20 min the soft component was added to the reaction mixture and kept at a temperature of 55–65 8C for 90 min. The pendent acid groups of the DMPA were afterwards neutralized with tri-ethylamine and the PU subsequently dispersed in water. The pH of the dispersion was adjusted to 8. 2.2. Viscosity PU dispersions were diluted with distilled water until the desired viscosities of 100, 150, 200, 250, 300, 350 and 400 mPa s were reached. The viscosities of the dispersions were determined at ambient temperature with a Brookfield LVT viscometer.
M. Meincken et al. / Applied Surface Science 253 (2006) 805–809
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2.3. Sample preparation for hydrophobicity measurements Paperboard, freshly cleaved mica and silicon were coated with thin PU films with the viscosities indicated above. The paperboard was coated with PU with a laboratory drawdown coating machine from SMC, operated at medium speed. The resulting film thickness was approximately 20 mm. The samples were subsequently dried in an oven at 120–130 8C for 5 min, and then left in air at room temperature for 20 h. One square centimeter of each coated paperboard was cut and stuck to a magnetic sample holder for AFM measurements. Films with a thickness of about 20 mm on silicon and mica were prepared by dropping a small amount of PU dispersion on a 1 cm2 slide of the substrate, which was fixed to the sample holder. The PU was dispersed with a microscope slide into a thin, flat film. The sample was then dried in air at room temperature for at least 24 h. 2.4. Static contact angle measurements SCA analysis was performed by depositing 1 ml droplets of de-ionized water onto the coated substrate surface and taking photos through a magnifying lens using a digital camera. This process was repeated 8–10 times for each sample. The height h and the length l of the droplet were measured and the static contact angle between the droplet and the coated surface calculated using the equation tan(u/2) = 2h/l, where u is the static contact angle in degrees. A higher contact angle value represents a more hydrophobic surface. 2.5. Adhesion measurements with DPFM-AFM All AFM measurements were made with a Veeco Multimode instrument in conjunction with a Witec digital pulsed-force mode controller. In the digital pulsed-force mode [6,7,12,13] the AFM is operated in contact mode, while a sinusoidal modulation is applied to its z-piezo. The modulation frequency is typically between 1 and 10 kHz, which is well below the resonance frequency ( f r) of the cantilever (a force modulation cantilever with f r 70 kHz). In this way force–distance curves are measured at every scan point, by continuously bringing the tip into contact with the surface and subsequently retracting it. Thus, topography and physical surface properties, such as stiffness or adhesion, can be measured simultaneously and with the same spatial resolution. In order to interpret the adhesive force values of each adhesion image, the images were displayed as histograms, as shown in Fig. 2. This histogram shows the relative frequency with which each voltage value occurs within the scanned area. The average value represents the average adhesive force value, which corresponds to the average hydrophilicity of the sample. The standard deviation is a measure of the range of these values and serves as an error bar in our data presentation. Every image was recorded with a scan size of 5 mm 5 mm with a resolution of 256 256 pixels. The value of the adhesive force was therefore determined by averaging over 65,536
Fig. 2. Histogram of the voltage distribution in an adhesion image.
measurements. This presents a good statistical distribution, compared to the few individual force–distance measurements customarily used to describe adhesive forces. Quantitative values of the adhesive force F adh are given by F adh = VadhkS, where Vadh is the average voltage value of the adhesion image, k the spring constant of the cantilever and S is the sensitivity of the photodiode. The spring constant k of the force modulation cantilever was 2.8 N/m [14] and the sensitivity S was determined to be 85.5 nm/V. A higher value of the adhesive force represents a more hydrophilic surface. Since DPFM measurements are highly susceptible to the environment and depend on the humidity, it is not possible to compare the results of measurements performed on different days directly and quantitatively. This results, for example, in different absolute values for the adhesive force. The trends of the adhesive force within one measurements series, however, can be compared in a qualitative way, since every series was measured under the same conditions. 3. Results and discussion SCA measurements on paperboard (Fig. 3a) show an increasing contact angle for viscosities ranging from 100 to 250 mPa s. From there onwards the contact angle decreases again. The optimum surface hydrophobicity, which relates to the highest contact angle, is therefore reached at a viscosity of approximately 250 mPa s. The individual contact angle values measured by SCA on one sample are, however, scattered over a wide range, which results in large error bars. This could be explained by the large surface roughness of several micrometers of the paperboard, which also influences the contact angle. SCA measurements on freshly cleaved, atomically flat mica showed the same results with significantly smaller error bars (Fig. 3b). Again the contact angle is increasing in the viscosity range from 100 to 250 mPa s, where it reaches a maximum, and decreasing for higher viscosities. These results were confirmed by DPFM-AFM measurements. Films with viscosities ranging from 100 to 400 mPa s were coated on paperboard, mica and silicon. Two adhesion images were acquired on each sample, of which each image resulted in an average value for the adhesive force, which relates directly to the hydrophobicity, and an error bar. A larger
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M. Meincken et al. / Applied Surface Science 253 (2006) 805–809
Fig. 3. SCA results on coated paperboard (a) and mica (b) for PU coatings with varying viscosity.
adhesive force relates to a more hydrophilic surface. The optimum surface configuration with a maximum surface hydrophobicity is therefore obtained when the adhesive force between the sample and the AFM tip reaches a minimum. The measurements on paperboard (Fig. 4a) were extremely difficult to obtain and also hard to reproduce due to the high surface roughness. This resulted in a wide distribution of values obtained for the surface hydrophobicity, although these were all acquired using the same sample. Nevertheless, a trend can be observed. The adhesive force decreases between 100 and 150 mPa s. For higher viscosities the adhesive force values are scattered widely but tend to increase in magnitude. The dotted line follows the average between the widely scattered results and serves only to guide the eye, in order to make out a trend.
The results on the coated mica (Fig. 4b) are more uniform and reproducible. In good agreement with the SCA results, the measured adhesive force decreases between 100 and 250 mPa s and increases again for higher viscosities. The most hydrophobic surface was therefore obtained with a PU coating having a viscosity of 250 mPa s. PU on silicon (Fig. 4c) exhibits the same general trend: the adhesive force decreases for viscosities ranging from 100 to 250 mPa s and increases again for higher viscosities. The average values measured for each sample are more spread out than they were for the mica substrate, however. This could be due to the atomic structure of the silicon, which might force the PU chains into a different orientation. This effect is the subject of ongoing investigations.
Fig. 4. Adhesive forces measured on coated paperboard (a), mica (b) and silicon (c) for PU coatings with varying viscosity.
M. Meincken et al. / Applied Surface Science 253 (2006) 805–809
The results show clearly that the self-assembly mechanism of polyurethane chains is affected by the mobility of the hydrophilic and hydrophobic chain segments during the coating process, which is governed by the emulsion viscosity. The viscosity directly influences the mobility of the hydrophilic segments—an increase in viscosity results in an increase in ionic clustering of the polar segments, which therefore leads to a decrease in chain mobility of the polar segments. However, if the viscosity is too low, the coating will show poor selfassembly due to a poor interpenetrating polymer network. Optimum coating properties and an optimum self-assembly were obtained with an emulsion viscosity of 250 mPa s. 4. Conclusions The surface hydrophobicity of polyurethane coatings was measured by means of SCA and DPFM-AFM. The results of both methods agree well, showing that the novel technique of DPFM-AFM is a viable tool to determine the surface hydrophilicity of coatings. It was further shown that this optimum viscosity seems to be independent of the substrate, as long as the substrate was polar. SCA measurements were performed on paperboard and mica, while DPFM-AFM measurements were performed on paperboard, mica and silicon. All results agree in that they indicate that the optimum self-assembly, and therefore the optimum surface hydrophobicity, is obtained at a viscosity of 250 mPa s. To our knowledge, this dependency on viscosity has not been shown before.
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References [1] Z.S. Petrovic, J. Ferguson, Prog. Polym. Sci. 16 (1991) 695–836. [2] S. Subramani, Prog. Org. Coat. 48 (2003) 71–79. [3] I. Capek, Nature and properties of ionomer assemblies, Adv. Colloid Interface Sci. 118 (2005) 73–112. [4] A. Dong, G. Hou, D. Sun, Properties of amphoteric polyurethane waterborne dispersions: II. Macromolecular self-assembly behavior, J. Colloid Interface Sci. 266 (2003) 276–281. [5] S. Seboa, Synthesis and characterization of tailored polyurethane coatings, Ph.D. Thesis, University of Stellenbosch, South Africa, 2005. [6] O. Marti, T. Stifter, H. Waschipky, M. Quintus, S. Hild, Scanning probe microscopy of heterogeneous polymers, Colloids Surf. 154 (1999) 65–73. [7] H. Krotil, T. Stifter, H. Waschipky, K. Weishaupt, S. Hild, O. Marti, Pulsed force mode: a new method for the investigation of surface properties, Surf. Interface Anal. 27 (1999) 336–340. [8] M. Meincken, S.P. Roux, E.P. Jacobs, Determination of the hydrophilic character of membranes by pulsed force mode atomic force microscopy, Appl. Surf. Sci. 252 (2005) 1772–1779. [9] S. Jarvis, J. Pethica, Comparison of hydrophobic modifying layers on SiO2 studied with a force-controlled atomic force microscope, Thin Solid Films 273 (1996) 284–288. [10] Y. Okabe, U. Akba, M. Fujihira, Chemical force microscopy of –CH3 and – COOH terminal groups in mixed self-assembled monolayers by pulsed force mode atomic force microscopy, Appl. Surf. Sci. 157 (2000) 398–404. [11] F. Sato, H. Okui, U. Akiba, K. Suga, M. Fujihira, A study of topographic effects on chemical force microscopy using adhesive force mapping, Ultramicroscopy 97 (2003) 303–314. [12] C.D. Frisbie, L.F. Rozsnyai, A. Noy, M.S. Wrighton, C.M. Lieber, Functional group imaging by chemical force microscopy, Science 265 (1994) 2071–2074. [13] S. Akari, D. Horn, H. Keller, Chemical imaging by scanning force microscopy, Adv. Mater. 7 (1995) 549–551. [14] Nanoworld, Productguide, 2002.
Influence of the viscosity and the substrate on the surface hydrophobicity of polyurethane coatings M. Meincken *, A. Klash, S. Seboa, R.D. Sanderson Department of Chemistry and Polymer Science, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa Received 4 August 2005; received in revised form 10 January 2006; accepted 10 January 2006 Available online 14 February 2006
Abstract Tailor-made polyurethane (PU) dispersions were synthesized as coatings for paperboard for dry food packaging. For this purpose a low moisture-vapor transmission rate and a high surface hydrophobicity are desirable characteristics, which are both met by PU. However, it was found that the surface hydrophobicity of water-borne PU dispersions depends strongly on the viscosity of the dispersion. This dependency was studied by static contact angle measurements (SCA) as well as a novel technique using digital pulsed-force mode atomic force microscopy (DPFM-AFM). Comparison of the results validated that DPFM-AFM is a valuable tool to characterize the surface hydrophilicity. Both techniques confirmed that the surface hydrophobicity varies with the viscosity and that an optimum viscosity for the PU coating with a maximum surface hydrophobicity can consequently be determined. It was found that both lower as well as higher viscosities led to a less hydrophobic surface. # 2006 Elsevier B.V. All rights reserved. PACS: 68.37.Ps; 81.16.Dn; 82.35.Lr Keywords: Polyurethanes; Coatings; Surface hydrophobicity; Atomic force microscopy (AFM); Static contact angle (SCA)
1. Introduction The development of conventional solvent-borne systems into environmentally friendly coatings has been studied extensively over the last 25 years. Solvent-borne systems have been widely replaced by solid or powder coatings and by waterborne coatings, both of which are preferred due to their compatibility with conventional coating systems. Water-borne polyurethane systems combine low volatility with the good film formation properties of solvent-borne systems and the inherent favorable characteristics of the urethanes. Despite this, polyurethanes (PU) are not commonly used for paper coatings because of their high cost, especially that of the isocyanate component. In the past, up-scaling polyurethane production exhibited many problems, such as the high reactivity of the isocyanate-group towards impurities like water. These difficulties have, however, been largely overcome and can be controlled. Other than the high cost, PU has almost no other disadvantages. It can, for example, be tailor-made to enhance
* Corresponding author. Tel.: +27 21 8083298; fax: +27 21 8083603. E-mail address: [email protected] (M. Meincken). 0169-4332/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2006.01.014
certain desired properties. These include a very low moisturevapor transmission rate, good control of the particle size, excellent film formation properties, wax compatibility, thermal and chemical stability, recyclability of the coated paperboard and a high chemical and abrasion resistance. Furthermore, they are non-toxic and environmentally friendly. Since polyurethanes can be tailor-made to suit specific demands, they could prove an extremely versatile coating material for paperboard for packaging purposes. Polyurethanes are also known as isocyanate polymers. They are synthesized by the combination of a polyol, which has at least two active hydroxyl groups per molecule, with a diisocyanate or a polymeric isocyanate in the presence of a catalyst. Since many different diisocyanates and a variety of polyols can be used to obtain polyurethanes, a wide range of materials can be synthesized, including fibers, soft and hard elastomers, coatings and adhesives, flexible and rigid foams, thermoplastic and thermosetting plastics. The characteristic feature of polyurethanes is their chain structure, which consists of alternating hard and soft segments. The hard segments contain polar urethane groups, which can form hydrogen bonds and crystalline structures. The soft segments are flexible and non-polar [1,2]. The physical and
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M. Meincken et al. / Applied Surface Science 253 (2006) 805–809
Fig. 1. Self-assembly mechanism of PU molecules on a substrate.
mechanical properties of polyurethanes depend on factors such as their molecular weight, their composition and length of the soft and hard segments, and the presence of anomalous linkages such as branching and crosslinking. Polyurethane coatings on a polar surface should ideally be highly orientated [3], with all the polar, hydrophilic sections attached to the substrate, and their hydrophobic sections pointing to the surface, as displayed in Fig. 1. The degree of this orientation depends strongly on the ionic content and the viscosity of the PU emulsion upon coating [4,5], which influences the mobility of the molecules and therefore their ability to self-assemble. Excessively low mobility as well as too high mobility leads to poor molecule orientation. In order to self-assemble, the polymer chains need suffi cient time and space to orientate their polar and non-polar segments on the substrate during the coating process. In order to achieve optimum self-assembly, the mobility of the polar and the non-polar segments of the PU emulsion particles must be controlled. This can be done by measuring, and subsequently adjusting the ionic content and the viscosity of the PU emulsion. In this study the ionic content was kept at a constant value of 5.1% dimethylproponic acid (DMPA), while the viscosity was varied in order to determine the optimum value for selfassembly by digital pulsed-force mode atomic force microscopy (DPFM-AFM) and static contact angle (SCA) measurements. Polyurethane dispersions with viscosities ranging from 100 to 400 mPa s were analyzed by both techniques and their results compared. Paperboard proved to be too rough a substrate for AFM analysis, with a surface roughness ranging from 0.5 to 3 mm. Therefore, the PU films were also coated on silicon and on freshly cleaved, atomically flat mica. Measurement of the hydrophobicity as a function of the viscosity by both techniques showed the same trend for different substrates, which indicates that the orientation of the PU chains is independent of the substrate used, as long as the substrate is polar. Coatings on unpolar surfaces, such as polyethylene, dewetted the substrate surface and did not form a smooth film. The hydrophobicity of molecules can be distinguished by AFM with an additional digital pulsed-force mode (DPFM) controller, which allows the determination of the adhesion and elastic properties of the sample at each scan point [6,7]. Images obtained in this way result in a surface ‘‘map’’, showing different adhesive forces between the tip and the sample on the scanned area as different colours. Therefore, it is possible to distinguish the polar and the non-polar parts of molecules on the surface, and to determine an average adhesion value for the sample surface, which is equivalent to the surface hydrophilicity [8]. A silicon tip was used to scan the surface of the films investigated in this study. Untreated silicon has a native
oxide layer with hydroxyl bonds. These OH groups are adsorption sites for water molecules and the surface is therefore hydrophilic. A conventional silicon tip with SiO2 groups at the surface will show a higher adhesive force on a hydrophilic than on a hydrophobic surface [7,9]. The image of the adhesion force therefore represents the hydrophilicity of the sample, where lighter parts represent more hydrophilic and darker parts represent more hydrophobic compounds. Okabe et al. [10] used the same technique to discriminate the functional groups of self-assembled sample surfaces, by using chemically modified tips. Sato et al. [11] determined the distribution of adhesive force values on self-assembled sample surfaces. 2. Experimental 2.1. Polymer preparation Polyurethane dispersions were prepared with a 70 wt% soft segment content to form a thin thermoplastic coating for paperboard. The soft PU segments were prepared from a linear polyester-based polyol as a precursor, which was synthesized from four monomers: adipic acid (AA), neopentyl glycol (NPG), 1,4-cyclohexane dicarboxylic acid (CHDCA) and 2phosphonobutane 1,2,4-tricarboxylic acid (PBTCA). The polyester was based on AA and NPG as the primary monomers, while CHDCA and PBTCA formed the secondary monomers. A maximum of 15% CHDCA and 10% PBTCA was used for the polyol synthesis. CHDCA was added in order to increase the hydrophobicity of the polyol, while the PBTCA was added to further enhance, e.g. the barrier properties of the coating. The polyol was dried and degassed overnight at 80 8C in a vacuum oven. For the synthesis of the hard segments, the reactor was charged with toluene diisocyanate (TDI) and dimethyl proponic acid (DMPA), placed in a water bath, and purged with N2-gas. NMP was added to the reaction mixture at room temperature followed 20 min later by ethylene glycol (EG) chain extender. After another 20 min the soft component was added to the reaction mixture and kept at a temperature of 55–65 8C for 90 min. The pendent acid groups of the DMPA were afterwards neutralized with tri-ethylamine and the PU subsequently dispersed in water. The pH of the dispersion was adjusted to 8. 2.2. Viscosity PU dispersions were diluted with distilled water until the desired viscosities of 100, 150, 200, 250, 300, 350 and 400 mPa s were reached. The viscosities of the dispersions were determined at ambient temperature with a Brookfield LVT viscometer.
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2.3. Sample preparation for hydrophobicity measurements Paperboard, freshly cleaved mica and silicon were coated with thin PU films with the viscosities indicated above. The paperboard was coated with PU with a laboratory drawdown coating machine from SMC, operated at medium speed. The resulting film thickness was approximately 20 mm. The samples were subsequently dried in an oven at 120–130 8C for 5 min, and then left in air at room temperature for 20 h. One square centimeter of each coated paperboard was cut and stuck to a magnetic sample holder for AFM measurements. Films with a thickness of about 20 mm on silicon and mica were prepared by dropping a small amount of PU dispersion on a 1 cm2 slide of the substrate, which was fixed to the sample holder. The PU was dispersed with a microscope slide into a thin, flat film. The sample was then dried in air at room temperature for at least 24 h. 2.4. Static contact angle measurements SCA analysis was performed by depositing 1 ml droplets of de-ionized water onto the coated substrate surface and taking photos through a magnifying lens using a digital camera. This process was repeated 8–10 times for each sample. The height h and the length l of the droplet were measured and the static contact angle between the droplet and the coated surface calculated using the equation tan(u/2) = 2h/l, where u is the static contact angle in degrees. A higher contact angle value represents a more hydrophobic surface. 2.5. Adhesion measurements with DPFM-AFM All AFM measurements were made with a Veeco Multimode instrument in conjunction with a Witec digital pulsed-force mode controller. In the digital pulsed-force mode [6,7,12,13] the AFM is operated in contact mode, while a sinusoidal modulation is applied to its z-piezo. The modulation frequency is typically between 1 and 10 kHz, which is well below the resonance frequency ( f r) of the cantilever (a force modulation cantilever with f r 70 kHz). In this way force–distance curves are measured at every scan point, by continuously bringing the tip into contact with the surface and subsequently retracting it. Thus, topography and physical surface properties, such as stiffness or adhesion, can be measured simultaneously and with the same spatial resolution. In order to interpret the adhesive force values of each adhesion image, the images were displayed as histograms, as shown in Fig. 2. This histogram shows the relative frequency with which each voltage value occurs within the scanned area. The average value represents the average adhesive force value, which corresponds to the average hydrophilicity of the sample. The standard deviation is a measure of the range of these values and serves as an error bar in our data presentation. Every image was recorded with a scan size of 5 mm 5 mm with a resolution of 256 256 pixels. The value of the adhesive force was therefore determined by averaging over 65,536
Fig. 2. Histogram of the voltage distribution in an adhesion image.
measurements. This presents a good statistical distribution, compared to the few individual force–distance measurements customarily used to describe adhesive forces. Quantitative values of the adhesive force F adh are given by F adh = VadhkS, where Vadh is the average voltage value of the adhesion image, k the spring constant of the cantilever and S is the sensitivity of the photodiode. The spring constant k of the force modulation cantilever was 2.8 N/m [14] and the sensitivity S was determined to be 85.5 nm/V. A higher value of the adhesive force represents a more hydrophilic surface. Since DPFM measurements are highly susceptible to the environment and depend on the humidity, it is not possible to compare the results of measurements performed on different days directly and quantitatively. This results, for example, in different absolute values for the adhesive force. The trends of the adhesive force within one measurements series, however, can be compared in a qualitative way, since every series was measured under the same conditions. 3. Results and discussion SCA measurements on paperboard (Fig. 3a) show an increasing contact angle for viscosities ranging from 100 to 250 mPa s. From there onwards the contact angle decreases again. The optimum surface hydrophobicity, which relates to the highest contact angle, is therefore reached at a viscosity of approximately 250 mPa s. The individual contact angle values measured by SCA on one sample are, however, scattered over a wide range, which results in large error bars. This could be explained by the large surface roughness of several micrometers of the paperboard, which also influences the contact angle. SCA measurements on freshly cleaved, atomically flat mica showed the same results with significantly smaller error bars (Fig. 3b). Again the contact angle is increasing in the viscosity range from 100 to 250 mPa s, where it reaches a maximum, and decreasing for higher viscosities. These results were confirmed by DPFM-AFM measurements. Films with viscosities ranging from 100 to 400 mPa s were coated on paperboard, mica and silicon. Two adhesion images were acquired on each sample, of which each image resulted in an average value for the adhesive force, which relates directly to the hydrophobicity, and an error bar. A larger
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M. Meincken et al. / Applied Surface Science 253 (2006) 805–809
Fig. 3. SCA results on coated paperboard (a) and mica (b) for PU coatings with varying viscosity.
adhesive force relates to a more hydrophilic surface. The optimum surface configuration with a maximum surface hydrophobicity is therefore obtained when the adhesive force between the sample and the AFM tip reaches a minimum. The measurements on paperboard (Fig. 4a) were extremely difficult to obtain and also hard to reproduce due to the high surface roughness. This resulted in a wide distribution of values obtained for the surface hydrophobicity, although these were all acquired using the same sample. Nevertheless, a trend can be observed. The adhesive force decreases between 100 and 150 mPa s. For higher viscosities the adhesive force values are scattered widely but tend to increase in magnitude. The dotted line follows the average between the widely scattered results and serves only to guide the eye, in order to make out a trend.
The results on the coated mica (Fig. 4b) are more uniform and reproducible. In good agreement with the SCA results, the measured adhesive force decreases between 100 and 250 mPa s and increases again for higher viscosities. The most hydrophobic surface was therefore obtained with a PU coating having a viscosity of 250 mPa s. PU on silicon (Fig. 4c) exhibits the same general trend: the adhesive force decreases for viscosities ranging from 100 to 250 mPa s and increases again for higher viscosities. The average values measured for each sample are more spread out than they were for the mica substrate, however. This could be due to the atomic structure of the silicon, which might force the PU chains into a different orientation. This effect is the subject of ongoing investigations.
Fig. 4. Adhesive forces measured on coated paperboard (a), mica (b) and silicon (c) for PU coatings with varying viscosity.
M. Meincken et al. / Applied Surface Science 253 (2006) 805–809
The results show clearly that the self-assembly mechanism of polyurethane chains is affected by the mobility of the hydrophilic and hydrophobic chain segments during the coating process, which is governed by the emulsion viscosity. The viscosity directly influences the mobility of the hydrophilic segments—an increase in viscosity results in an increase in ionic clustering of the polar segments, which therefore leads to a decrease in chain mobility of the polar segments. However, if the viscosity is too low, the coating will show poor selfassembly due to a poor interpenetrating polymer network. Optimum coating properties and an optimum self-assembly were obtained with an emulsion viscosity of 250 mPa s. 4. Conclusions The surface hydrophobicity of polyurethane coatings was measured by means of SCA and DPFM-AFM. The results of both methods agree well, showing that the novel technique of DPFM-AFM is a viable tool to determine the surface hydrophilicity of coatings. It was further shown that this optimum viscosity seems to be independent of the substrate, as long as the substrate was polar. SCA measurements were performed on paperboard and mica, while DPFM-AFM measurements were performed on paperboard, mica and silicon. All results agree in that they indicate that the optimum self-assembly, and therefore the optimum surface hydrophobicity, is obtained at a viscosity of 250 mPa s. To our knowledge, this dependency on viscosity has not been shown before.
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