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butadiene latex
Colloids and Surfaces A: Physicochem. Eng. Aspects 291 (2006) 38–44
Association between a sodium salt of a linear dodecylbenzene sulphonate and a non-ionic fatty alcohol ethoxylate surfactant during film formation of styrene/butadiene latex Kaj Backfolk a,∗ , Caisa Andersson b , Jouko Peltonen c a
b
Stora Enso Oyj, Imatra Research Centre, FI-55 800 Imatra, Finland Karlstad University, Faculty of Technology and Sciences, Department of Chemical Engineering, SE-651 88 Karlstad, Sweden c Abo ˚ Akademi University, Department of Physical Chemistry, Porthansgatan 3, FI-20 500 Turku, Finland Received 5 September 2006; received in revised form 15 September 2006; accepted 18 September 2006 Available online 23 September 2006
Abstract The influence of a linear anionic dodecylbenzene sulphonate (SDBS) and a non-ionic fatty alcohol ethoxylate (AE) surfactant on the compatibility and surface morphology of a medium carboxylated styrene/butadiene latex film was established. The latex films were prepared onto a mica substrate and annealed at 80 ◦ C for 30 min. The AFM topography and phase contrast images enabled the localization of the surfactants on the latex films. The roughness parameters calculated for the captured images were utilized in order to identify the surface characteristics and changes caused by film ageing and addition of different surfactants. Both surfactants, but especially SDBS, increased the contrast between the latex particles. Addition of a blend of AE and SDBS surfactants to the latex emulsion resulted in the formation of hydrophobic nano-sized aggregates located at latex particle–particle boundaries in the films. The formation of the aggregates is ascribed to the association between the anionic and non-ionic surfactants. The segregated anionic surfactant was randomly distributed on-top of the latex films, whereas the surfactant blend showed a highly ordered, branched texture. © 2006 Elsevier B.V. All rights reserved. Keywords: Latex; Styrene-butadiene; Film formation; SDBS; Sodium dodecylbenzenesulphonate; Fatty alcohol ethoxylate; Surfactant; Roughness analysis
1. Introduction A well progressed and developed polymer inter-diffusion is an important step in the film formation of latex emulsions. Good mechanical, physical and chemical durability is required in many industrial applications were latex emulsions are used. In composite matrices containing minerals like paints or paper coating colours, where the latex primarily acts as a binder, both continuous and discontinuous regions can co-exist in the film. The aim for pigment based formulations like paper coating colors is to develop latex emulsions with optimal film formation properties and ability to spread and wet minerals providing good internal strength to the multi-component coating structure. However, various components in the emulsion may reduce the coalescence and latex polymer inter-diffusion, leading to reduced perfor-
∗
Corresponding author. E-mail address: [email protected] (K. Backfolk).
0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.09.030
mance. The de-stabilization and depletion of latex particles must be counteracted through the stabilization of the latex emulsion with appropriate surfactants [1–4]. The surfactants should, however, not hinder the progress of polymer inter-diffusion after the sintering and coalescence of the latex particles. Steric and electrosteric stabilization of latex emulsions using surfactants has been reported in the literature ([5–7], and references therein), whereas the effect of dual surfactant systems is less established. Surfactant-latex compatibility has been demonstrated to be important for the progress of latex film formation [3,4,6–14]. The migration of surfactants has been related to the degree of inter-diffusion of polymer chains [15,16]. The presence of electrolytes and surfactants in the film that causes disruption of the film structure has been shown to decrease the mechanical properties of the latex films [17,18]. A similar mechanism has been proposed to occur in case of barrier properties measured for latex films regarding gas and water vapour permeability [19]. It has been observed that a non-ionic surfactant promotes film formation of a butyl methacrylate latex whereas an anionic sur-
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factant (e.g. the anionic sodium dodecylbenzene sulfonate) acts as a barrier towards inter-diffusion of the latex particles [20]. The migration of surfactants in latex films has been reported for different systems, including the effect of time and temperature, and several mechanisms have been reported [3,4]. Surfactants are claimed to either phase separate and segregate to the liquid–air interface during the initial phase of liquid phase evaporation, or, in case of entrapped surfactants, migrate through an annealed film with time. Prior to full coalescence of the particles, the excess surfactants may migrate to the air–liquid interface via water channels [3,4]. Zhao and Urban have further shown that high surfactant concentration may be found at the air–film interface rather than at the substrate–film interface [11]. The phase separated and migrated surfactants are usually non-specifically bound to surfaces, supported by the fact that they can be rinsed off with water [13,19,21]. However, to the best of our knowledge, few studies have been published regarding latex films prepared from emulsion comprising post-addition of two different surfactants. Atomic force microscopy (AFM) has been frequently employed to characterize the surface properties of latex films. The first studies involved the packing order of latex particles in cast and spin-coated films [22], including the influence of post-added surfactants [14,23] and annealing [14,22,24]. The enhanced image quality enabled studies on film formation, e.g. investigating the role of capillary pressure on the rate of film formation, film flattening and polymer diffusion [25–27]. It has also been demonstrated that the migration of surfactants can be accelerated by coalescing additives [28]. Recently, the heterogeneity, i.e. the distribution of chemical species on latex films has been studied by analyzing the phase shift of the oscillating cantilever [29,30]. The scope of this study was to determine the role of postaddition of linear anionic and non-ionic surfactants, as single components and as a mixture, on the compatibility and surface morphology of the formed styrene/butadiene (SB) latex films. The topography of the latex films as well as the location of the segregated surfactants on the film surface were analysed using AFM operating in the tapping mode. An isothermal microcalorimeter was utilized to determine the enthalpy of interaction between the surfactants and the latex particles. 2. Experimental 2.1. Materials The styrene/butadiene latex used in this work was a product of Dow Chemical (Switzerland). The medium-carboxylated latex was synthesised with a concentration of the emulsifying agent equal to 1/100 of the normal dosage used in a manufacturing process. Since no surfactants were added after the manufacturing process, we consider the latex emulsion to be substantially free of surfactants, stabilized mainly by the carboxyl surface groups. The glass transition temperature, determined by differential scanning calorimetry (DSC) and the estimated minimum film formation temperature (MFFT) were 7 and 10 ◦ C, respectively. The average particle diameter of the unimodal
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latex was found to be 0.14 m as determined by a light scattering technique. The latex emulsion was diluted with 0.01 M NaCl prepared from distilled Millipore water and sodium chloride (Pro-analysis, >99.5%, Merck) to a concentration of 1 wt% after which pH was adjusted with NaOH to 8.5. The surfactants used were an anionic linear dodecyl benzene sulphonate sodium salt (SDBS) (MARANIL A 25, Henkel GmbH, Germany) and a non-ionic fatty alcohol ethoxylate surfactant (AE) (Lutensol TO89, BASF, Germany), with the general formula RO(CH2 CHO)8 H where R denotes i-C13 H27 . Degree of ethoxylation was ca. 8 and a cloud point in water was ca. 60 ◦ C (DIN 53917) as reported by the supplier. Critical micelle concentration (cmc) for SDBS in water is 1.2 × 10−3 M at 60 ◦ C and for AE surfactant 3.2 × 10−5 M, 2.7 × 10−5 M, and 2.0 × 10−5 M measured at 15, 25 and 40 ◦ C, respectively [47]. The cmc is thus two orders of magnitude smaller for the AE surfactant than for the anionic SDBS surfactant. The samples for analysis of film formation were prepared by diluting the concentrated latex emulsion into solution containing the respective surfactants at room temperature (21 ± 3 ◦ C). The concentration of the surfactant was 1 wt% based on the dry weight of latex, while it was 2 wt% in the case of the sample comprising both anionic and non-ionic surfactants. The equilibrium adsorption time after addition was >24 h. All chemicals were used without further purification. 2.2. Methods The topography of the latex films was analyzed by using a Nanoscope IIIa atomic force microscope (AFM) (Digital Instruments Inc., Santa Barbara, CA, USA) operated in tapping mode. Silicon cantilevers with a resonance frequency within 250–320 kHZ were used for imaging. The topography and phase contrast images were obtained using a low (20 nm) tapping amplitude in order to gather information about the hydrophilic/hydrophobic character of the latex films [21,31–35]. The latex films were prepared on cleaved mica and dried in oven at 80 ◦ C for 30 min. The scanning probe image processor (SPIP, Image Metrology, Denmark) software was used for the roughness analysis of the images. Before the roughness analysis, the images were processed to remove any possible slope. Four different roughness parameters are utilized in this work. The root mean square (RMS) roughness Sq equals the standard deviation of height and is the most frequently reported roughness parameter. The 10-point-height roughness parameter Sz gives information about the maximal height differences, the value is an average of five highest local maxima and five lowest local minima. Skewness (Ssk ) is a useful parameter giving a measure for the height distribution asymmetry. Negative skewness values refer to a surface dominated by valleys (occasionally referred to as ‘surface pores’), whereas a surface with positive skewness is dominated by local summits. A surface with Ssk = 0 corresponds to a Gaussian height distribution. The roughness parameter Ssc gives a measure for the mean local surface curvature. The enthalpy change accompanying the adsorption of a surfactant onto latex particles was measured using an isothermal titration calorimeter (TAM 2277, Thermometric AB, Sweden)
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operating at 25 ◦ C. Small amount of 1 wt% surfactant stock solution was injected stepwise into a reaction vessel containing 1 wt% latex suspension prepared as described above. A portion of the homogeneous latex suspension (3 g) was introduced into the calorimetric vessel of volume 4 cm3 and stirred at 120 rpm. The amplifier range was 100 W and 30 injections were performed during the experiments. The enthalpies of interaction, Hr , associated with the subsequent adsorption steps, and thereby corresponding to a change in the adsorbed phase during its formation, were calculated from the experimentally measured enthalpy changes according to: Hr = Hobs − Hdil,(l)
(1)
where Hobs denotes the observed (experimental) enthalpy and Hdil,(l) is the enthalpy of dilution. The dilution experiments were performed by injecting small amounts of the stock solution into a 0.01 M NaCl aqueous solution in order to obtain a value for the enthalpy of dilution, Hdil,(l) . Low enthalpic effects were obtained when the polymer was diluted. The cloud point for 1 wt% surfactant solutions was determined using a turbimeter (HACH 2000 DR) as a function of temperature. The solutions were heated stepwise up to 75 ◦ C prior to analyses. 3. Results and discussion 3.1. Surfactant–latex interaction and surfactant–surfactant compatibility The surfactant–latex interaction was studied by measuring the enthalpy of interaction (Fig. 1). The curves are corrected for the heat of dilution according to Eq. (1). The heat of dilution of the SDBS solution was endothermic whereas the corresponding curve for AE was exothermic. However, an endothermic reaction was found between the non-ionic AE and the latex, which levelled out close to zero at a dose level of 0.3 g/g. In the case of the anionic SDBS surfactant, an exothermic enthalpy equal to zero was measured for an addition level close to 0.1 g/g. Since the curves were corrected for the heat of dilution the observed values correspond to adsorption and interaction occurring at the interfaces. The result therefore indicates a higher level of adsorption and apparently a different conformation at the initial phase of
Fig. 1. Enthalpy of interaction measured at 25 ◦ C when injecting (䊉) SDBS surfactant and (+) AE surfactant into the styrene/butadiene latex emulsion (1 wt%).
Fig. 2. Cloud point determined for 1 wt% surfactant solutions: (䊉) 1 wt% SDBS solution, (+) 1 wt% AE solution, and () 1 wt% SDBS and 1 wt% AE solution.
the titration experiment. It is also likely that the AE–latex interaction was stronger than that of the SDBS–latex system because of the lack of charged anionic groups providing electrostatic repulsion. Both surfactants may furthermore exhibit multilayer adsorption on the surface [5]. The cloud point temperature of the 1% surfactant solutions was established in order to determine their individual behaviour upon heating as well as their compatibility. The non-ionic AE surfactant solution displayed a cloud point temperature at 55 ◦ C (Fig. 2), which is lower than the drying temperature used to cure the latex films. The SDBS and the SDBS-AE solutions, however, did not yield any cloud point. This indicates the formation of mixed micelles between the anionic and the non-ionic surfactant, a well established phenomenon [5,36]. The formation of such co-micelles in the aqueous solution prevents the insolubilization of the AE surfactant occurring at elevated temperatures, i.e. over the measured temperature range which is close to the conditions attained during annealing of the latex films. 3.2. The influence of surfactants on latex film morphology Typical topographic 3D images of a surfactant-free film imaged directly after annealing and after storage in room temperature for 3 days are shown in Fig. 3. In both images a locally ordered lattice of latex particles is resolved. The weak interparticle interfaces, however, indicate that film formation has started. The ageing smoothed the surface which could be quantitatively demonstrated by the roughness analysis of the freshly imaged and aged samples. The RMS roughness decreased from 8.0 to 1.9 nm and the 10-point-height roughness from 45.5 to 11.6 nm, respectively [37]. Also the height asymmetry changed from a surface dominated by valleys (“surface pores”) to an almost Gaussian-like surface of symmetric height distribution as demonstrated by the skewness values of −0.68 and 0.32 [37,38]. Flattening of a latex film has been reported to take place as a function of annealing time [26,27]. It was also found out that such flattening is not possible without intraparticle polymer chain movement [27]. Therefore, we conclude that ageing caused flattening as a result of intraparticle polymer chain diffusion, but
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Fig. 3. AFM topography images of the latex films captured directly after annealing (left, <24 h), and after ageing in room temperature (right, >72 h) (scan size 2 m × 2 m and tapping amplitude 20 nm). The dark-light height scale 30 nm (left) and 14 nm (right).
Fig. 4. AFM topographic images of the latex films with (a) no surfactant, (b) 1 wt% SBDS surfactant, and (c) 1 wt% non-ionic AE surfactant. The image size is 2 m × 2 m and the dark-light vertical scale is 30 nm. The images were measured by using a small tapping amplitude (20 nm).
Fig. 5. AFM phase contrast images captured of the latex films with (a) no surfactant, (b) with 1% SBDS surfactant, and (c) with 1% non-ionic AE surfactant. The image size is 2 m × 2 m and the dark-light vertical scale is 20◦ . The images were measured by using a small tapping amplitude (20 nm).
Fig. 6. AFM topography images captured for the latex film with 1 wt% SBDS surfactant and 1 wt% of the non-ionic fatty alcohol surfactant. The image size is 1 m × 1 m (left), 2 m × 2 m (middle) and 5 m × 5 m (right) and the dark-light vertical scale is 14 nm (left), 30 nm (middle) and 50 nm (right). The images were measured by using a small tapping amplitude (20 nm).
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Fig. 7. AFM phase contrast images captured for the latex film with 1 wt% SBDS surfactant and 1 wt% of the non-ionic AE surfactant. The image size is 1 m × 1 m (left), 2 m × 2 m (middle) and 5 m × 5 m (right) and the dark-light vertical scale is 30◦ (left), 75◦ (middle) and 80◦ (right). The images were measured by using a small tapping amplitude (20 nm).
the remaining inter-particle interfaces indicate that little if any inter-particle diffusion had taken place. The influence of ageing time and temperature on film formation and degree of inter-diffusion is well established [19]. Since the main focus in our work was to study the effect of surfactant–latex and surfactant–surfactant compatibility on film morphology, AFM imaging was carried out for films aged at room temperature for 48 ± 24 h after annealing (Figs. 4–8), i.e. at a stage when it has been demonstrated that the latex surface can be easily identified through its ordered lattice structure (Fig. 3). Fig. 4 shows AFM topographic images captured for the styrene/butadiene latex free from surfactants (a) and films including the anionic SDBS (b) and non-ionic AE surfactant (c), respectively. The topographical features of the reference sample without any surfactants were already described. The AE surfactant caused quite marginal changes whereas the film including SBDS lacked an ordered structure. Similar differences in surface structure between non-ionic and anionic surfactants have been reported [17]. The values of the roughness parameters tabulated in Table 1 specify the differences in more details. Clearly, the addition of both surfactants resulted in smoother films, the SBDS–latex film being slightly smoother than the AE–latex film. The addition of surfactants also affected the height distribution asymmetry. The pure latex surface was found to be dominated by
valleys as indicated by the negative skewness (Ssk ) value. Addition of SDBS made the surface close to Gaussian (symmetric height distribution with balance of summits and valleys, skewness close to zero) [38]. The addition of AE surfactant changed the height asymmetry even more, generating a surface dominated by local summits (positive skewness). Quite interestingly, the inverse of the mean curvature Ssc gives a measure for the mean radius of curvature of the latex particles. The change of skewness refers to a “surface pore” filling phenomenon, obviously as a result of migration and segregation of the surfactants to such pores. In tapping mode imaging, the amplitude of the oscillating cantilever is a key parameter which defines the tip-sample interaction dominating during imaging. By choosing a relatively low tapping amplitude tip-sample adhesive forces dominate the interaction. The contrast in the phase image is then proportional to local differences in hydrophilicity/hydrophobicity and hence yields local chemical information about the surface [21,31–35]. The phase images presented in Fig. 5 have been measured by using a small tapping amplitude of 20 nm. The phase image for the reference latex implies that the surface is chemically quite homogenous, the weak contrast and hardly visible boundaries between individual latex particles indicate that film formation has proceeded quite far. Addition of 1% of the non-ionic AE
Fig. 8. AFM topographic (left) and phase contrast (right) images for the latex films with 1 wt% SBDS surfactant and 1 wt% of the non-ionic AE surfactant. The image size is 50 m × 50 m and the dark-light height scale 80 nm (topograph) and 20◦ (phase).
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Table 1 Roughness parameters calculated for the samples [37]
Sq (std) (nm) Ssk (std) Sz (std) (nm) Ssc (std) (1/nm)
SB latex
SB with anionic SDBS surfactant
SB with non-ionic AE surfactant
SB with anionic and nonionic surfactant
8.0 (3.2) −0.68 (0.26) 45.5 (15.0) 0.000041 (0.000002)
3.0 (0.1) −0.10 (0.07) 21.7 (0.7) 0.00020 (0.00005)
3.9 (1.3) 0.51 (0.4) 23.2 (5.8) 0.00012 (0.00008)
2.1 (0.4) 0.20 (0.40) 17.1 (2.6) 0.00015 (0.00004)
surfactant into the film did not influence the structure very much. The phase contrast is still quite homogeneous with no clear material differences indicating that the surfactant remains associated to the latex particles or is located in the interior of the film. The surface of the SDBS–latex film appeared clearly most heterogeneous and the phase contrast indicates a phase-separated system. Hydrophilic material appears as dark whereas the more hydrophobic component gives a lighter contrast. Assuming that the spherical structures still correspond to latex particles, the phase located at the grain boundaries between the particles is logically interpreted to correspond to the surfactant component. The excess amount of surfactant which has not adsorbed onto the particles precipitates upon drying and forms locally continuous areas. A latex emulsion comprising 1% of both surfactants was made in order to establish the surfactant–surfactant compatibility and the influence on film morphology. Topographic images of a typical sample with different magnifications are shown in Fig. 6. Individual latex particles can be identified in the images. The grain boundaries between them, however, are quite diffuse. Obviously the structure is not the same as observed for the reference sample of pure latex. According to the roughness parameters (Table 1), this sample was the smoothest among the studied samples. A closer look at the images, especially those with highest resolution, reveals the existence of very small particles or aggregates on the surface. The contrast differences in the phase images (Fig. 7) corresponding to exactly the same spots as the topographs of Fig. 6 evidently demonstrate the existence, size and spatial distribution of small aggregates. The aggregates are primarily located between the latex particles. Similarly accumulated surfactants on the film surface have been reported by others [10,13]. At sufficiently high concentration it is likely that these aggregates form free surfactant micelles [10,39], either consisting of a single surfactant type or representing AE–SDBS co-micelles. The light contrast in the phase images measured by using low tapping amplitude reveals the hydrophobic character of the small aggregates, in contrast to the clearly more hydrophilic latex particles. Since no other contrast is visible in the phase images, it is reasonable to conclude that SDBS–AE aggregates have been formed. The surfactants are not phase separated from the latex phase in the same way as was observed for the SDBS–latex system, indicating that the formed aggregates have some affinity for the latex surface, presumably by hydrophobic interactions. Topographic and phase contrast images with large scan size were captured in order to further study the distribution of the hydrophobic moieties on the films. The distribution of aggregates (Fig. 8) shows a branched, tree-like texture, resembling
a fractal geometry. However, no detailed fractal analysis was carried out. A relatively quick systematic on-plane distribution of the aggregates may have resulted from evaporation, whereas vertical migration of surfactants and other species, as shown in Fig. 5, was observed when using a single-component, especially anionic surfactant. Such an effect has also been demonstrated by others [8–13,15,17,19]. Surfactant segregation and phase separation occurred upon addition of an anionic surfactant. Both types of surfactant molecules are expected to adsorb with the hydrophobic tail onto the latex particle and the hydrophilic head protruding into the aqueous phase [40], i.e. adsorption is entropy-driven. Adsorption of both types of surfactant but especially the anionic one will thus increase the hydrophilic character of the latex particles [41–46]. The higher enthalpy of interaction measured between the non-ionic surfactant and the latex indicates greater adsorption and higher affinity of this surfactant compared to the anionic surfactant (corrected for heat of dilution). This can be partly addressed to the more hydrophobic character of the non-ionic surfactant, but may also be due to the anionic groups in SDBS providing electrostatic repulsion against latex. However, the phase images show that the studied surfactants modify the interfacial chemistry of the latex particles differently. The low contrast differences in the non-ionic fatty alcohol surfactant and the styrene/butadiene latex system suggest that the compatibility is higher than for the SDBS–latex system, a finding also supported by literature [17] and our data in Fig. 1. Also the film formation had proceeded further in the former case. The observations reported by Hayazaki et al. [20] for a nonionic and a sodium dodecylbenzene sulfonate surfactant on butyl methacrylate latex support our findings that differences in latex film formation depend on the type of surfactant used in the system. The latex films comprising both anionic and non-ionic surfactants display a different behaviour than noted for the single surfactant systems. The surfactants appeared compatible with each other at low concentrations and at medium temperatures as concluded from the turbidity measurements. Nevertheless, simultaneous post-addition of both surfactants was observed to generate aggregates when drying the latex film. According to the high resolution AFM phase contrast images not only spherical but also rod-like aggregates appeared on the surface, preferably located in the meniscus between the latex particles. Since it can be assumed from the turbidity measurements that incompatibility is not a problem in the emulsion phase when the surfactant concentration is low, it is likely that the aggregates are formed when their relative concentrations increased and the liquid phase temperature was high in the final liquid evaporation stage. This
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interpretation is supported by the arrangement of the aggregates. The hydrophobic character of the aggregates indicates the trend of the surfactants to form structures with minimized surface free energy [5]. The surfactants were found to affect the material properties rather than the surface roughness which was found to change only marginally. The introduction of surfactants was found to generate height asymmetry, most probably as a result of filling the voids between the latex particles. 4. Conclusions The post addition of ionic and non-ionic surfactants to a styrene/butadiene latex emulsion resulted in latex films with different material characteristics compared to the reference film which was substantially free from surfactants. A set of roughness parameters were utilized to quantify the topographical changes as a result of aging, and differences between the films of different composition. The association observed in the aqueous phase and the characteristic properties of the surfactants are suggested to be responsible for the different morphologies of the latex films. The anionic surfactant showed surface segregation upon drying and caused disruption in the latex film. The non-ionic surfactant promoted the latex film formation ascribed to less contrast between individual latex particles in the AFM images. The observed differences could be explained by the lower adsorption of the SDBS surfactant due to electrostatic repulsion, i.e. a higher amount of non-adsorbed material which precipitates upon drying. The simultaneous addition of both surfactants to the latex emulsion resulted in a latex film comprising hydrophobic moieties at the film–air interface. These small aggregates were mainly located at the latex particle–particle interfacial region. The formation of such aggregates is ascribed to the association of the anionic and non-ionic surfactants resulting, during the final stage of evaporation, in the formation of hydrophobic regions surrounding the latex particles. Acknowledgement Dow Chemicals is kindly thanked for supplying the latex. References [1] P.M. McGenity, P.A.C. Gane, J.C. Husband, M.S. Engley, Tappi Proceedings—Tappi Coating Conference, 1992, p. 133. ˚ Akademi University, 2000. [2] P. Dahlvik, Doctoral Thesis, Abo [3] H. Warson, C.A. Finch, Applications of syntethic resin lattices Fundamental Chemistry of Lattices and Applications in Adhesives, vol. 1, John Wiley & Sons Ltd., 2001. [4] H. Warson, C.A. Finch, Applications of synthetic resin lattices Latices in Surface Coatings: Emulsion Paints, vol. 2, John Wiley & Sons Ltd., 2001. [5] B. J¨onsson, B. Lindman, K. Holmerg, B. Kronberg, Surfactants and Polymers in Aqueous Solutions, John Wiley & Sons Ltd., 1998.
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Association between a sodium salt of a linear dodecylbenzene sulphonate and a non-ionic fatty alcohol ethoxylate surfactant during film formation of styrene/butadiene latex Kaj Backfolk a,∗ , Caisa Andersson b , Jouko Peltonen c a
b
Stora Enso Oyj, Imatra Research Centre, FI-55 800 Imatra, Finland Karlstad University, Faculty of Technology and Sciences, Department of Chemical Engineering, SE-651 88 Karlstad, Sweden c Abo ˚ Akademi University, Department of Physical Chemistry, Porthansgatan 3, FI-20 500 Turku, Finland Received 5 September 2006; received in revised form 15 September 2006; accepted 18 September 2006 Available online 23 September 2006
Abstract The influence of a linear anionic dodecylbenzene sulphonate (SDBS) and a non-ionic fatty alcohol ethoxylate (AE) surfactant on the compatibility and surface morphology of a medium carboxylated styrene/butadiene latex film was established. The latex films were prepared onto a mica substrate and annealed at 80 ◦ C for 30 min. The AFM topography and phase contrast images enabled the localization of the surfactants on the latex films. The roughness parameters calculated for the captured images were utilized in order to identify the surface characteristics and changes caused by film ageing and addition of different surfactants. Both surfactants, but especially SDBS, increased the contrast between the latex particles. Addition of a blend of AE and SDBS surfactants to the latex emulsion resulted in the formation of hydrophobic nano-sized aggregates located at latex particle–particle boundaries in the films. The formation of the aggregates is ascribed to the association between the anionic and non-ionic surfactants. The segregated anionic surfactant was randomly distributed on-top of the latex films, whereas the surfactant blend showed a highly ordered, branched texture. © 2006 Elsevier B.V. All rights reserved. Keywords: Latex; Styrene-butadiene; Film formation; SDBS; Sodium dodecylbenzenesulphonate; Fatty alcohol ethoxylate; Surfactant; Roughness analysis
1. Introduction A well progressed and developed polymer inter-diffusion is an important step in the film formation of latex emulsions. Good mechanical, physical and chemical durability is required in many industrial applications were latex emulsions are used. In composite matrices containing minerals like paints or paper coating colours, where the latex primarily acts as a binder, both continuous and discontinuous regions can co-exist in the film. The aim for pigment based formulations like paper coating colors is to develop latex emulsions with optimal film formation properties and ability to spread and wet minerals providing good internal strength to the multi-component coating structure. However, various components in the emulsion may reduce the coalescence and latex polymer inter-diffusion, leading to reduced perfor-
∗
Corresponding author. E-mail address: [email protected] (K. Backfolk).
0927-7757/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2006.09.030
mance. The de-stabilization and depletion of latex particles must be counteracted through the stabilization of the latex emulsion with appropriate surfactants [1–4]. The surfactants should, however, not hinder the progress of polymer inter-diffusion after the sintering and coalescence of the latex particles. Steric and electrosteric stabilization of latex emulsions using surfactants has been reported in the literature ([5–7], and references therein), whereas the effect of dual surfactant systems is less established. Surfactant-latex compatibility has been demonstrated to be important for the progress of latex film formation [3,4,6–14]. The migration of surfactants has been related to the degree of inter-diffusion of polymer chains [15,16]. The presence of electrolytes and surfactants in the film that causes disruption of the film structure has been shown to decrease the mechanical properties of the latex films [17,18]. A similar mechanism has been proposed to occur in case of barrier properties measured for latex films regarding gas and water vapour permeability [19]. It has been observed that a non-ionic surfactant promotes film formation of a butyl methacrylate latex whereas an anionic sur-
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factant (e.g. the anionic sodium dodecylbenzene sulfonate) acts as a barrier towards inter-diffusion of the latex particles [20]. The migration of surfactants in latex films has been reported for different systems, including the effect of time and temperature, and several mechanisms have been reported [3,4]. Surfactants are claimed to either phase separate and segregate to the liquid–air interface during the initial phase of liquid phase evaporation, or, in case of entrapped surfactants, migrate through an annealed film with time. Prior to full coalescence of the particles, the excess surfactants may migrate to the air–liquid interface via water channels [3,4]. Zhao and Urban have further shown that high surfactant concentration may be found at the air–film interface rather than at the substrate–film interface [11]. The phase separated and migrated surfactants are usually non-specifically bound to surfaces, supported by the fact that they can be rinsed off with water [13,19,21]. However, to the best of our knowledge, few studies have been published regarding latex films prepared from emulsion comprising post-addition of two different surfactants. Atomic force microscopy (AFM) has been frequently employed to characterize the surface properties of latex films. The first studies involved the packing order of latex particles in cast and spin-coated films [22], including the influence of post-added surfactants [14,23] and annealing [14,22,24]. The enhanced image quality enabled studies on film formation, e.g. investigating the role of capillary pressure on the rate of film formation, film flattening and polymer diffusion [25–27]. It has also been demonstrated that the migration of surfactants can be accelerated by coalescing additives [28]. Recently, the heterogeneity, i.e. the distribution of chemical species on latex films has been studied by analyzing the phase shift of the oscillating cantilever [29,30]. The scope of this study was to determine the role of postaddition of linear anionic and non-ionic surfactants, as single components and as a mixture, on the compatibility and surface morphology of the formed styrene/butadiene (SB) latex films. The topography of the latex films as well as the location of the segregated surfactants on the film surface were analysed using AFM operating in the tapping mode. An isothermal microcalorimeter was utilized to determine the enthalpy of interaction between the surfactants and the latex particles. 2. Experimental 2.1. Materials The styrene/butadiene latex used in this work was a product of Dow Chemical (Switzerland). The medium-carboxylated latex was synthesised with a concentration of the emulsifying agent equal to 1/100 of the normal dosage used in a manufacturing process. Since no surfactants were added after the manufacturing process, we consider the latex emulsion to be substantially free of surfactants, stabilized mainly by the carboxyl surface groups. The glass transition temperature, determined by differential scanning calorimetry (DSC) and the estimated minimum film formation temperature (MFFT) were 7 and 10 ◦ C, respectively. The average particle diameter of the unimodal
39
latex was found to be 0.14 m as determined by a light scattering technique. The latex emulsion was diluted with 0.01 M NaCl prepared from distilled Millipore water and sodium chloride (Pro-analysis, >99.5%, Merck) to a concentration of 1 wt% after which pH was adjusted with NaOH to 8.5. The surfactants used were an anionic linear dodecyl benzene sulphonate sodium salt (SDBS) (MARANIL A 25, Henkel GmbH, Germany) and a non-ionic fatty alcohol ethoxylate surfactant (AE) (Lutensol TO89, BASF, Germany), with the general formula RO(CH2 CHO)8 H where R denotes i-C13 H27 . Degree of ethoxylation was ca. 8 and a cloud point in water was ca. 60 ◦ C (DIN 53917) as reported by the supplier. Critical micelle concentration (cmc) for SDBS in water is 1.2 × 10−3 M at 60 ◦ C and for AE surfactant 3.2 × 10−5 M, 2.7 × 10−5 M, and 2.0 × 10−5 M measured at 15, 25 and 40 ◦ C, respectively [47]. The cmc is thus two orders of magnitude smaller for the AE surfactant than for the anionic SDBS surfactant. The samples for analysis of film formation were prepared by diluting the concentrated latex emulsion into solution containing the respective surfactants at room temperature (21 ± 3 ◦ C). The concentration of the surfactant was 1 wt% based on the dry weight of latex, while it was 2 wt% in the case of the sample comprising both anionic and non-ionic surfactants. The equilibrium adsorption time after addition was >24 h. All chemicals were used without further purification. 2.2. Methods The topography of the latex films was analyzed by using a Nanoscope IIIa atomic force microscope (AFM) (Digital Instruments Inc., Santa Barbara, CA, USA) operated in tapping mode. Silicon cantilevers with a resonance frequency within 250–320 kHZ were used for imaging. The topography and phase contrast images were obtained using a low (20 nm) tapping amplitude in order to gather information about the hydrophilic/hydrophobic character of the latex films [21,31–35]. The latex films were prepared on cleaved mica and dried in oven at 80 ◦ C for 30 min. The scanning probe image processor (SPIP, Image Metrology, Denmark) software was used for the roughness analysis of the images. Before the roughness analysis, the images were processed to remove any possible slope. Four different roughness parameters are utilized in this work. The root mean square (RMS) roughness Sq equals the standard deviation of height and is the most frequently reported roughness parameter. The 10-point-height roughness parameter Sz gives information about the maximal height differences, the value is an average of five highest local maxima and five lowest local minima. Skewness (Ssk ) is a useful parameter giving a measure for the height distribution asymmetry. Negative skewness values refer to a surface dominated by valleys (occasionally referred to as ‘surface pores’), whereas a surface with positive skewness is dominated by local summits. A surface with Ssk = 0 corresponds to a Gaussian height distribution. The roughness parameter Ssc gives a measure for the mean local surface curvature. The enthalpy change accompanying the adsorption of a surfactant onto latex particles was measured using an isothermal titration calorimeter (TAM 2277, Thermometric AB, Sweden)
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operating at 25 ◦ C. Small amount of 1 wt% surfactant stock solution was injected stepwise into a reaction vessel containing 1 wt% latex suspension prepared as described above. A portion of the homogeneous latex suspension (3 g) was introduced into the calorimetric vessel of volume 4 cm3 and stirred at 120 rpm. The amplifier range was 100 W and 30 injections were performed during the experiments. The enthalpies of interaction, Hr , associated with the subsequent adsorption steps, and thereby corresponding to a change in the adsorbed phase during its formation, were calculated from the experimentally measured enthalpy changes according to: Hr = Hobs − Hdil,(l)
(1)
where Hobs denotes the observed (experimental) enthalpy and Hdil,(l) is the enthalpy of dilution. The dilution experiments were performed by injecting small amounts of the stock solution into a 0.01 M NaCl aqueous solution in order to obtain a value for the enthalpy of dilution, Hdil,(l) . Low enthalpic effects were obtained when the polymer was diluted. The cloud point for 1 wt% surfactant solutions was determined using a turbimeter (HACH 2000 DR) as a function of temperature. The solutions were heated stepwise up to 75 ◦ C prior to analyses. 3. Results and discussion 3.1. Surfactant–latex interaction and surfactant–surfactant compatibility The surfactant–latex interaction was studied by measuring the enthalpy of interaction (Fig. 1). The curves are corrected for the heat of dilution according to Eq. (1). The heat of dilution of the SDBS solution was endothermic whereas the corresponding curve for AE was exothermic. However, an endothermic reaction was found between the non-ionic AE and the latex, which levelled out close to zero at a dose level of 0.3 g/g. In the case of the anionic SDBS surfactant, an exothermic enthalpy equal to zero was measured for an addition level close to 0.1 g/g. Since the curves were corrected for the heat of dilution the observed values correspond to adsorption and interaction occurring at the interfaces. The result therefore indicates a higher level of adsorption and apparently a different conformation at the initial phase of
Fig. 1. Enthalpy of interaction measured at 25 ◦ C when injecting (䊉) SDBS surfactant and (+) AE surfactant into the styrene/butadiene latex emulsion (1 wt%).
Fig. 2. Cloud point determined for 1 wt% surfactant solutions: (䊉) 1 wt% SDBS solution, (+) 1 wt% AE solution, and () 1 wt% SDBS and 1 wt% AE solution.
the titration experiment. It is also likely that the AE–latex interaction was stronger than that of the SDBS–latex system because of the lack of charged anionic groups providing electrostatic repulsion. Both surfactants may furthermore exhibit multilayer adsorption on the surface [5]. The cloud point temperature of the 1% surfactant solutions was established in order to determine their individual behaviour upon heating as well as their compatibility. The non-ionic AE surfactant solution displayed a cloud point temperature at 55 ◦ C (Fig. 2), which is lower than the drying temperature used to cure the latex films. The SDBS and the SDBS-AE solutions, however, did not yield any cloud point. This indicates the formation of mixed micelles between the anionic and the non-ionic surfactant, a well established phenomenon [5,36]. The formation of such co-micelles in the aqueous solution prevents the insolubilization of the AE surfactant occurring at elevated temperatures, i.e. over the measured temperature range which is close to the conditions attained during annealing of the latex films. 3.2. The influence of surfactants on latex film morphology Typical topographic 3D images of a surfactant-free film imaged directly after annealing and after storage in room temperature for 3 days are shown in Fig. 3. In both images a locally ordered lattice of latex particles is resolved. The weak interparticle interfaces, however, indicate that film formation has started. The ageing smoothed the surface which could be quantitatively demonstrated by the roughness analysis of the freshly imaged and aged samples. The RMS roughness decreased from 8.0 to 1.9 nm and the 10-point-height roughness from 45.5 to 11.6 nm, respectively [37]. Also the height asymmetry changed from a surface dominated by valleys (“surface pores”) to an almost Gaussian-like surface of symmetric height distribution as demonstrated by the skewness values of −0.68 and 0.32 [37,38]. Flattening of a latex film has been reported to take place as a function of annealing time [26,27]. It was also found out that such flattening is not possible without intraparticle polymer chain movement [27]. Therefore, we conclude that ageing caused flattening as a result of intraparticle polymer chain diffusion, but
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Fig. 3. AFM topography images of the latex films captured directly after annealing (left, <24 h), and after ageing in room temperature (right, >72 h) (scan size 2 m × 2 m and tapping amplitude 20 nm). The dark-light height scale 30 nm (left) and 14 nm (right).
Fig. 4. AFM topographic images of the latex films with (a) no surfactant, (b) 1 wt% SBDS surfactant, and (c) 1 wt% non-ionic AE surfactant. The image size is 2 m × 2 m and the dark-light vertical scale is 30 nm. The images were measured by using a small tapping amplitude (20 nm).
Fig. 5. AFM phase contrast images captured of the latex films with (a) no surfactant, (b) with 1% SBDS surfactant, and (c) with 1% non-ionic AE surfactant. The image size is 2 m × 2 m and the dark-light vertical scale is 20◦ . The images were measured by using a small tapping amplitude (20 nm).
Fig. 6. AFM topography images captured for the latex film with 1 wt% SBDS surfactant and 1 wt% of the non-ionic fatty alcohol surfactant. The image size is 1 m × 1 m (left), 2 m × 2 m (middle) and 5 m × 5 m (right) and the dark-light vertical scale is 14 nm (left), 30 nm (middle) and 50 nm (right). The images were measured by using a small tapping amplitude (20 nm).
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Fig. 7. AFM phase contrast images captured for the latex film with 1 wt% SBDS surfactant and 1 wt% of the non-ionic AE surfactant. The image size is 1 m × 1 m (left), 2 m × 2 m (middle) and 5 m × 5 m (right) and the dark-light vertical scale is 30◦ (left), 75◦ (middle) and 80◦ (right). The images were measured by using a small tapping amplitude (20 nm).
the remaining inter-particle interfaces indicate that little if any inter-particle diffusion had taken place. The influence of ageing time and temperature on film formation and degree of inter-diffusion is well established [19]. Since the main focus in our work was to study the effect of surfactant–latex and surfactant–surfactant compatibility on film morphology, AFM imaging was carried out for films aged at room temperature for 48 ± 24 h after annealing (Figs. 4–8), i.e. at a stage when it has been demonstrated that the latex surface can be easily identified through its ordered lattice structure (Fig. 3). Fig. 4 shows AFM topographic images captured for the styrene/butadiene latex free from surfactants (a) and films including the anionic SDBS (b) and non-ionic AE surfactant (c), respectively. The topographical features of the reference sample without any surfactants were already described. The AE surfactant caused quite marginal changes whereas the film including SBDS lacked an ordered structure. Similar differences in surface structure between non-ionic and anionic surfactants have been reported [17]. The values of the roughness parameters tabulated in Table 1 specify the differences in more details. Clearly, the addition of both surfactants resulted in smoother films, the SBDS–latex film being slightly smoother than the AE–latex film. The addition of surfactants also affected the height distribution asymmetry. The pure latex surface was found to be dominated by
valleys as indicated by the negative skewness (Ssk ) value. Addition of SDBS made the surface close to Gaussian (symmetric height distribution with balance of summits and valleys, skewness close to zero) [38]. The addition of AE surfactant changed the height asymmetry even more, generating a surface dominated by local summits (positive skewness). Quite interestingly, the inverse of the mean curvature Ssc gives a measure for the mean radius of curvature of the latex particles. The change of skewness refers to a “surface pore” filling phenomenon, obviously as a result of migration and segregation of the surfactants to such pores. In tapping mode imaging, the amplitude of the oscillating cantilever is a key parameter which defines the tip-sample interaction dominating during imaging. By choosing a relatively low tapping amplitude tip-sample adhesive forces dominate the interaction. The contrast in the phase image is then proportional to local differences in hydrophilicity/hydrophobicity and hence yields local chemical information about the surface [21,31–35]. The phase images presented in Fig. 5 have been measured by using a small tapping amplitude of 20 nm. The phase image for the reference latex implies that the surface is chemically quite homogenous, the weak contrast and hardly visible boundaries between individual latex particles indicate that film formation has proceeded quite far. Addition of 1% of the non-ionic AE
Fig. 8. AFM topographic (left) and phase contrast (right) images for the latex films with 1 wt% SBDS surfactant and 1 wt% of the non-ionic AE surfactant. The image size is 50 m × 50 m and the dark-light height scale 80 nm (topograph) and 20◦ (phase).
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Table 1 Roughness parameters calculated for the samples [37]
Sq (std) (nm) Ssk (std) Sz (std) (nm) Ssc (std) (1/nm)
SB latex
SB with anionic SDBS surfactant
SB with non-ionic AE surfactant
SB with anionic and nonionic surfactant
8.0 (3.2) −0.68 (0.26) 45.5 (15.0) 0.000041 (0.000002)
3.0 (0.1) −0.10 (0.07) 21.7 (0.7) 0.00020 (0.00005)
3.9 (1.3) 0.51 (0.4) 23.2 (5.8) 0.00012 (0.00008)
2.1 (0.4) 0.20 (0.40) 17.1 (2.6) 0.00015 (0.00004)
surfactant into the film did not influence the structure very much. The phase contrast is still quite homogeneous with no clear material differences indicating that the surfactant remains associated to the latex particles or is located in the interior of the film. The surface of the SDBS–latex film appeared clearly most heterogeneous and the phase contrast indicates a phase-separated system. Hydrophilic material appears as dark whereas the more hydrophobic component gives a lighter contrast. Assuming that the spherical structures still correspond to latex particles, the phase located at the grain boundaries between the particles is logically interpreted to correspond to the surfactant component. The excess amount of surfactant which has not adsorbed onto the particles precipitates upon drying and forms locally continuous areas. A latex emulsion comprising 1% of both surfactants was made in order to establish the surfactant–surfactant compatibility and the influence on film morphology. Topographic images of a typical sample with different magnifications are shown in Fig. 6. Individual latex particles can be identified in the images. The grain boundaries between them, however, are quite diffuse. Obviously the structure is not the same as observed for the reference sample of pure latex. According to the roughness parameters (Table 1), this sample was the smoothest among the studied samples. A closer look at the images, especially those with highest resolution, reveals the existence of very small particles or aggregates on the surface. The contrast differences in the phase images (Fig. 7) corresponding to exactly the same spots as the topographs of Fig. 6 evidently demonstrate the existence, size and spatial distribution of small aggregates. The aggregates are primarily located between the latex particles. Similarly accumulated surfactants on the film surface have been reported by others [10,13]. At sufficiently high concentration it is likely that these aggregates form free surfactant micelles [10,39], either consisting of a single surfactant type or representing AE–SDBS co-micelles. The light contrast in the phase images measured by using low tapping amplitude reveals the hydrophobic character of the small aggregates, in contrast to the clearly more hydrophilic latex particles. Since no other contrast is visible in the phase images, it is reasonable to conclude that SDBS–AE aggregates have been formed. The surfactants are not phase separated from the latex phase in the same way as was observed for the SDBS–latex system, indicating that the formed aggregates have some affinity for the latex surface, presumably by hydrophobic interactions. Topographic and phase contrast images with large scan size were captured in order to further study the distribution of the hydrophobic moieties on the films. The distribution of aggregates (Fig. 8) shows a branched, tree-like texture, resembling
a fractal geometry. However, no detailed fractal analysis was carried out. A relatively quick systematic on-plane distribution of the aggregates may have resulted from evaporation, whereas vertical migration of surfactants and other species, as shown in Fig. 5, was observed when using a single-component, especially anionic surfactant. Such an effect has also been demonstrated by others [8–13,15,17,19]. Surfactant segregation and phase separation occurred upon addition of an anionic surfactant. Both types of surfactant molecules are expected to adsorb with the hydrophobic tail onto the latex particle and the hydrophilic head protruding into the aqueous phase [40], i.e. adsorption is entropy-driven. Adsorption of both types of surfactant but especially the anionic one will thus increase the hydrophilic character of the latex particles [41–46]. The higher enthalpy of interaction measured between the non-ionic surfactant and the latex indicates greater adsorption and higher affinity of this surfactant compared to the anionic surfactant (corrected for heat of dilution). This can be partly addressed to the more hydrophobic character of the non-ionic surfactant, but may also be due to the anionic groups in SDBS providing electrostatic repulsion against latex. However, the phase images show that the studied surfactants modify the interfacial chemistry of the latex particles differently. The low contrast differences in the non-ionic fatty alcohol surfactant and the styrene/butadiene latex system suggest that the compatibility is higher than for the SDBS–latex system, a finding also supported by literature [17] and our data in Fig. 1. Also the film formation had proceeded further in the former case. The observations reported by Hayazaki et al. [20] for a nonionic and a sodium dodecylbenzene sulfonate surfactant on butyl methacrylate latex support our findings that differences in latex film formation depend on the type of surfactant used in the system. The latex films comprising both anionic and non-ionic surfactants display a different behaviour than noted for the single surfactant systems. The surfactants appeared compatible with each other at low concentrations and at medium temperatures as concluded from the turbidity measurements. Nevertheless, simultaneous post-addition of both surfactants was observed to generate aggregates when drying the latex film. According to the high resolution AFM phase contrast images not only spherical but also rod-like aggregates appeared on the surface, preferably located in the meniscus between the latex particles. Since it can be assumed from the turbidity measurements that incompatibility is not a problem in the emulsion phase when the surfactant concentration is low, it is likely that the aggregates are formed when their relative concentrations increased and the liquid phase temperature was high in the final liquid evaporation stage. This
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interpretation is supported by the arrangement of the aggregates. The hydrophobic character of the aggregates indicates the trend of the surfactants to form structures with minimized surface free energy [5]. The surfactants were found to affect the material properties rather than the surface roughness which was found to change only marginally. The introduction of surfactants was found to generate height asymmetry, most probably as a result of filling the voids between the latex particles. 4. Conclusions The post addition of ionic and non-ionic surfactants to a styrene/butadiene latex emulsion resulted in latex films with different material characteristics compared to the reference film which was substantially free from surfactants. A set of roughness parameters were utilized to quantify the topographical changes as a result of aging, and differences between the films of different composition. The association observed in the aqueous phase and the characteristic properties of the surfactants are suggested to be responsible for the different morphologies of the latex films. The anionic surfactant showed surface segregation upon drying and caused disruption in the latex film. The non-ionic surfactant promoted the latex film formation ascribed to less contrast between individual latex particles in the AFM images. The observed differences could be explained by the lower adsorption of the SDBS surfactant due to electrostatic repulsion, i.e. a higher amount of non-adsorbed material which precipitates upon drying. The simultaneous addition of both surfactants to the latex emulsion resulted in a latex film comprising hydrophobic moieties at the film–air interface. These small aggregates were mainly located at the latex particle–particle interfacial region. The formation of such aggregates is ascribed to the association of the anionic and non-ionic surfactants resulting, during the final stage of evaporation, in the formation of hydrophobic regions surrounding the latex particles. Acknowledgement Dow Chemicals is kindly thanked for supplying the latex. References [1] P.M. McGenity, P.A.C. Gane, J.C. Husband, M.S. Engley, Tappi Proceedings—Tappi Coating Conference, 1992, p. 133. ˚ Akademi University, 2000. [2] P. Dahlvik, Doctoral Thesis, Abo [3] H. Warson, C.A. Finch, Applications of syntethic resin lattices Fundamental Chemistry of Lattices and Applications in Adhesives, vol. 1, John Wiley & Sons Ltd., 2001. [4] H. Warson, C.A. Finch, Applications of synthetic resin lattices Latices in Surface Coatings: Emulsion Paints, vol. 2, John Wiley & Sons Ltd., 2001. [5] B. J¨onsson, B. Lindman, K. Holmerg, B. Kronberg, Surfactants and Polymers in Aqueous Solutions, John Wiley & Sons Ltd., 1998.
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