MIGRATION OF SURFACTANTS IN PRESSURE-SENSITIVE ADHESIVE FILMS AND THE IMPACT ON PERFORMANCE PROPERTIES Steve Severtson, Associate Professor, University of Minnesota, St. Paul, MN Helen Xu, Postdoctoral Associate, University of Minnesota, St. Paul, MN Jiguang Zhang, Postdoctoral Associate, University of Minnesota, St. Paul, MN Larry Gwin, Senior Research Associate, Franklin International, Columbus, OH Carl Houtman, Research Chemical Engineer, USDA, Forest Products Laboratory, Madison, WI Abstract Results are reviewed from a study on the distribution of surfactants in pressure-sensitive adhesive (PSA) films subsequent to the coating process. The PSAs are water-based acrylics synthesized with n-butyl acrylate, vinyl acetate and methacrylic acid and two commercially available surfactants, disodium (nonylphenoxypolyethoxy)ethyl sulfosuccinate (anionic) and nonylphenoxypoly(ethyleneoxy) ethanol (nonionic). The ratio of these surfactants was varied, while the total surfactant mass content was held constant. Surfactant distributions in adhesive films were investigated using atomic force microscopy (AFM) and confocal Raman microscopy (CRM). AFM images demonstrate the tendency of the anionic surfactant to aggregates at film surfaces inhibiting latex particle coalescence, while CRM shows its enrichment near film interfacial regions. Results indicate that surfactant distributions are governed in large part by surfactant compatibility with the polymer phase. Also discussed is the influence of surfactant content and distribution on the performance of PSA films. Introduction Surfactant species play a crucial role in generation of water-borne pressure-sensitive adhesive (PSA), providing loci for particle nucleation, colloidal stability for generated latexes and aid in coating of latex films. Of non-water components in a PSA formulation, surfactant can be as much as 10 % of the mass. Once adhesive films are cast, the surfactants have served their principal purpose, but are retained, and modify performance of dried films. The presence of amphiphilic species not only affects aggressiveness and adhesion of PSA products, but also behaviors during aging, recycling and biodegradation. Interest in mitigating impacts of surfactant species has prompted extensive research on their fate in polymer films over the last two decades.1-5 While mixtures of both anionic and nonionic surfactants are often used in commercial products, anionic surfactants typically dominate the mixture. Depending on compatibility between surfactant and latex particles, surfactant may partially dissolve into the adhesive polymer, remain at interfaces between particles and/or phase-separate from the polymeric solution and migrate to film interfaces.6 It is believed that the main factors influencing their out-of-plane distribution include chemical nature of the latex system, aging time, film formation environments (temperature and relative humidity) and structures of the surfactants.7 Several researchers have reported higher surfactant concentrations at film interfaces and propose that enrichment hinders or delays particle coalescence in these regions resulting in weaker films.8, 9

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Fourier transform infrared spectroscopy (FTIR),6 attenuated total reflection (ATR) FTIR,6, 7, 10-16 infrared microscopy,10 X-ray photoelectron spectroscopy,17 nuclear magnetic resonance,10, 18 and Rutherford backscattering spectroscopy (RBS)19, 20 have all been applied to collect out-of-plane distribution data on surfactant species in adhesive films. However, these techniques either cannot provide layer by layer depth profiling information or only allow for analysis within a few microns of the surface. In fact, most studies published previously examining surfactant-polymer compatibility and mobility of anionic and nonionic surfactants in latex films are based on the results from ATR-FTIR spectra obtained on film-air and film-substrate interfaces. Confocal Raman microscopy (CRM) combines a high resolution confocal microscope with a sensitive Raman spectroscopy system. By tuning the plane of focus of the confocal microscope in a stepwise fashion, volume elements at increasing depths into a film can be isolated and a Raman spectrum recorded showing differences in chemical composition. This technique has been shown to be effective in the depth profiling of thin films, coatings, membranes and composites.21, 22 In this paper, CRM is applied to a series of emulsion-type adhesives, based on a commercial, label-grade, water-based acrylic PSA. The adhesives use both anionic and nonionic nonylphenol ethoxylate emulsifiers. By switching the dominant surfactant from anionic to nonionic, information is obtained on the tendency of these different species to migrate. Atomic force microscopy (AFM) was used for imaging both the film-air and film-substrate interfaces to further characterize the migration process and show the impact of migration on the coalescence of latex particles. This combination of quantitative and qualitative high resolution techniques is shown to be a powerful method for characterizing the movement of surfactants in water-based PSA films and their impact on structure. Water-based Latexes and Adhesive Films

Water-based PSAs were synthesized at Franklin International (Columbus, OH) from the monomers n-butyl acrylate, vinyl acetate and methacrylic acid and combinations of an anionic and nonionic nonylphenolethoxylates (NPEs), specifically disodium (nonylphenoxypolyethoxy)ethyl sulfosuccinate and nonylphenoxypoly(ethyleneoxy) ethanol, respectively. Their molar mass distributions and formulas were gauged using liquid chromatography-mass spectroscopy (LC-MS). Properties of these surfactants are listed below in Table 1. Three PSA systems are reported on here, which were synthesized using similar approaches and conditions. The first PSA contains 100 wt.% anionic surfactant: the second PSA is produced with an emulsifier combination of 50 wt.% anionic and 50 wt.% nonionic surfactant: and the third PSA contains 90 wt.% nonionic and 10 wt.% anionic surfactant. The solids content of produced latexes were 60-63 wt.%. Properties were characterized using techniques described elsewhere and are summarized in Table 2.23-25 For CRM and AFM analysis, drawdown films (1 mil) cast on release liner were tested. An obvious concern with this system are differences in molecular weights, gel contents and glass transition temperatures, which may impact surfactant distribution.26 However, it will be shown that the differences in enrichment are large enough to draw several generalizations with regard to surfactant distribution behavior. In addition, since the films are generated and characterized at temperatures significantly higher than their glass transition temperatures, the impact of these differences are likely mitigated. (This system served as a starting point for the development of the described characterization approach.)

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Table 1. Chemical and Physical Properties of Surfactants

* values reported by the suppliers

Table 2. Properties of PSAs Imaging PSA Film Surfaces AFM images were obtained with a PicoPlus PicoSPM (Agilent Technology, Foster City, CA) system in an environmental chamber with controlled temperature and relative humidity (RH). Intermittent contact or AC mode AFM was utilized. The amplitude of the cantilever is maintained constant during the scanning and the displacement of the piezo scanner through the feedback circuit to maintain this amplitude is recorded as a height image. The phase lag between the cantilever response and the driving oscillation signal is recorded as a phase image. Viscoelastic properties of the surface may also be evaluated from AFM phase images. All tapping mode AFM images presented in this paper were obtained employing integrated silicon cantilevers with tip radii of curvature 5-10 nm. The spring constant of the cantilever was manufacturer-specified in the range of 30-60 N/m and the measured resonant frequency was within 10% of 300 KHz. Here, all PSA films were examined at ambient conditions.

Properties Anionic Surfactant Nonionic Surfactant

Molecular Formula C15H23(OCH2CH2)nC4H3O7Na2S n!11

C15H23(OCH2CH2)nOH n!9

Molar Mass (g/mol.) Mn = 905.26 Mw = 935.84

Mn = 597.54 Mw = 608.32

Critical Micelle Concentration* (% by weight) 1.5"10-2 2.1"10-4

Measurement PSA 1 PSA 2 PSA 3

Anionic: Nonionic wt.% 100:0 50:50 10:90 Latex Viscosity (cps) 88 90 750 Mean Particle Size (nm) 450 690 1250

pH 4.39 4.28 4.32

Molar Mass (g/mol.) Mn =25000

Mw =160000 PDI =6.4

Mn =29841 Mw = 260180

PDI =8.7

Mn =27500 Mw =209000

PDI =7.6 Glass Transition Temperature (#C) -20 -34 -24 Peel Adhesion (lbs.) 2.0 2.0 2.1 Loop Tack (lbs.) 2.6 2.3 2.0

178# Shear Adhesion (min.) 300 200 960 Contact Angle with Water (degree) 106 110 112

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Characterization of Surfactant Distributions Films were characterized using an alpha 300R confocal Raman microscope equipped with a UHTS200 spectrometer and a DV401 CCD detector from WITec (Ulm, Germany). The ratio of the Raman absorbencies at 1612 and 1735 cm-1, assigned to an aromatic C-C, on-ring stretch mode for the surfactant and carbonyl stretch mode of the polymer, respectively, are used to quantify the surfactant concentration across the film thickness. Calibration was accomplished through the addition of known amounts of pure surfactant to pure latex (synthesized by soap free emulsion polymerization method). At least five locations were randomly selected on each film and Raman measurements were done with 0.5-5 $m increments across the film thickness. The data shown in the figures are the average values obtained from at least five Raman measurements and the error bar represents the standard deviation. More focused analyses were often carried out within approximately 5 $m of film interfaces. It is found that these concentrations versus depth data are well fit by an exponential decay model of the form,

C(z) = C0+A exp(-%z) (1) where C(z) is surfactant concentration at a depth z from the interface, C0 is the bulk phase surfactant concentration, and A and % are fitting constants indicating the maximum enrichment of surfactant and rate of decay, respectively. Also used is a modified version of Eq. 1 of the form,

C(z)/C0 = 1 + ! exp(-%z) (2) where ! is equal to A/C0, which we have defined ! as the enrichment factor. This dimensionless parameter indicates the surface concentration of a surfactant relative to the bulk. Enrichment Behavior of Nonionic and Anionic NPEs The concentration profiles of surfactants in PSA 1, PSA 2 and PSA 3 drawn-down films are shown in Fig. 1, respectively. It should be noted that concentrations are given in mmoles of surfactant per gram of adhesive film. This unit was chosen because the surfactants are quantified via their phenyl ring, but are of different molecular weights. As discussed above, the same mass of surfactant was used in all 3 PSAs, which was used to calculate the overall concentration in mmoles/g. Also, the draw-down PSA films were analyzed from the film-air interface, i.e., 0% corresponds to the film-air interface and 100% to the film-substrate (release liner) interface. PSA 1 and PSA 2 show an enrichment of the surfactant at both the film-air and film-substrate interfaces, with greater amounts of surfactant being found at the substrate side of the films. For PSA 3 (Fig. 1c) little enrichment is found at the film-air interface and none is apparent at the film-substrate interface. In all cases, the surfactant concentration in the bulk film is relatively constant showing less variation. The bulk phase concentration is slightly lower than the values calculated based on addition, which is indicated with the dotted line. It appears that the enrichment decreases with a reduction in the concentrations of anionic surfactant, i.e., from PSA 1 to PSA 3. This result indicates a greater tendency of anionic surfactant to locate at film interfaces. Table 3 summarizes an analysis of the interfaces and a fit of these data with Eqs. 1 and 2. These provide for a more quantitative analysis, which indicates that the distribution curves have different shapes at the film-air and film-substrate interfaces.

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Table 3. Parameters from Fit with Exponential Decay Model

Fitting Parameters PSA 1 PSA 2 PSA 3

C0 1.84 1.34 1.41

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& 1.28 1.42 - Figure 2 shows AFM phase images of drawdown films for the 3 PSAs, which were obtained under ambient conditions (45% relative humidity). Images were recorded at the film-air interface. Dark regions with spherical geometry can clearly be seen. These circular regions have diameters that are consistent with the particle size distribution obtained using dynamic light-scattering carried out on the latex emulsion (Table 2). Near spherical geometry and a well-spaced distribution indicate that the coalescence of latex beads has not occurred, nor has particle compaction. The second phase (bright contrast corresponding to less energy dissipative regions) is believed to be primarily surfactant. The combination of an abundant presence of surfactants between and on the surface of latex beads and lack of particle coalescence indicates a likely connection. The amount of surfactant aggregated at this interface is significantly larger for PSA 1 (Fig. 2a), followed by PSA 2 (Fig. 2b), and little surfactant is apparent at the interface of PSA 3 (Fig. 2c). This is consistent with the CRM results reviewed above. It is also consistent with the contact angle data for the films, which indicates that contact angle with water increases from PSA 1 to PSA 3.27

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Figure 2. Atomic force microscopy phase images of film-air interfaces for (a) PSA 1, (b) PSA 2 and (c) PSA 3. (Scan size 2.5 $m " 2.5 $m)

Surfactant Properties Controlling Distribution Behavior The results presented above indicate that enrichment of surfactants studied here at interfaces of PSA film depends primarily on relative percent of anionic surfactant; more anionic surfactant in the blend results in more significant surfactant segregation. Other researchers have reported similar findings for other systems.28 This is consistent with the water-flux mechanism.29 The anionic surfactant is more water soluble and hydrophilic. (This is evident from its properties, but more importantly the fact that it possesses at least one ionized acid functional group.) Thus, these molecules are more apt to be found in the aqueous phase available to be carried to the film-air interfaces during water evaporation. However, it still remains to be determined whether the surfactant fate is a matter of kinetics with drying conditions and film structure pushing the distribution towards a metastable state. Figure 3a shows concentration profiles of nonionic NPE in the PSA film added at levels up to 54 mass% to the PSA 2 latex following dialysis. The concentrations of surfactant added are indicated on the figure. It appears that the nonionic NPE is highly compatible with the latex polymer. Although the re-addition of surfactant subsequent to dialysis could conceivably produce differences in the contact between surfactants and latex particles, these differences would be expected to lessen the degree of interactions. Of course, this is just one type of surfactant and more precisely one specific structure, but others have reported similar conclusions for a variety of nonionic species. For example, Evanson et al. systematically investigated mobility of anionic and nonionic surfactants in ethyl acrylate/methyl acrylic acid latex films using FTIR-ATR. 20 They concluded nonionic surfactant distributed evenly in the bulk film because surfactant enrichment was not found at film-air and film-substrate interfaces. A study on emulsion polymerization of styrene and methacrylic acid carried out with polyoxyethylene nonyl phenyl ether nonionic emulsifier found that nonionic surfactant is incorporated inside latex particles.30 The amount of nonionic emulsifier that partitioned into the latex particle reached as high as 75 wt.% of total surfactant utilized. The lower HLB number of the nonionic surfactant, the greater the amount incorporated inside latex particles.31

PSA 1 PSA 2 PSA 3(a) (b) (c)PSA 1 PSA 2 PSA 3(a) (b) (c)

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Figure 3. Concentration of surfactant measured in PSA following dialysis and the addition of pure a.) nonionic surfactant (added to latex) and b.) anionic surfactant (added to film surface).

Attempts to generate a curve similar to Fig. 3a by adding anionic surfactant to dialyzed PSA 2 latex produced significant variations in surfactant concentration and increasing interfacial enrichment with greater surfactant addition. This led to an alternative approach in which dried anionic surfactant was applied to the film-air interface of films cast from the dialyzed sample. Figure 3b shows a plot of surfactant concentration versus film depth for the anionic surfactant. The interface between the nearly pure surfactant layer and the film can be clearly identified. This sample was allowed to equilibrate for 1 day. It can be seen that the anionic surfactant concentration drops and plateaus at a constant value. Plotted are several repeats of the same measurement. It is evident that the anionic surfactant is much less compatible. Its level in the polymer phase is about 0.022 mmoL./g, which is close to the concentrations used to produce PSA 1 and PSA 2. From this it might be concluded that enrichment is simply a matter of solubility. However, given the complexity of these systems, further study is necessary, but it appears fair to conclude that the compatibility is a key factor governing distribution behavior. Impact of Surfactant Distribution Behavior on PSA Performance From results listed in Table 2, it can be seen that PSA 1 and PSA 2, produced with 100% anionic surfactant and a 50:50 mixture (by mass) of the anionic and nonionic surfactants, respectively, possess similar performance properties, while PSA 3, which contains primarily nonionic surfactant, has a substantially higher shear. However, this is likely not due primarily to the influence of the surfactant given that PSA 3 also possess a higher molar mass and significantly higher gel content, which would be expected to increase the cohesive strength of the PSA.32, 33 Further insights were obtained on this relationship through the use of seeded growth polymerization to generate the PSAs, which is currently being utilized for this project. Under a monomer-starved situation, regardless of the type/amount of surfactant being used or addition of additives, this approach produces final particles that are

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monodisperse and of similar sizes. Seeded polymerization is an excellent technique for separating various parameters. Initial results indicate that, for the species studied here, the surfactant has little direct impact on shear strength. However, it was found that, in an indirect way, the choice of surfactant can have a significant impact on polymer properties. Our results do indicate peel strength is influenced by the surfactant content and type, which is attributed to the different mobilities within the film and/or different wetting properties of surfactants.34 Also, we previously reported that the presence of surfactant, including emulsifier and wetting agent, plays a significant role in determining the fragmentation of PSA during paper recycling operations and accordingly their impact on the process26 and have found that surfactant content and distribution govern to a great extent how adhesive films interact with moisture, which controls how films age and perform under various environmental conditions. Summary It was demonstrated that confocal Raman microcopy (CRM) can provide a layer-by-layer analysis of surfactant content in the thickness direction of a PSA film. It was also shown that by combining CRM concentration profiling with AFM phase images of adhesive interfaces, a more comprehensive analysis of surfactant migration behavior in PSA film is obtained. Application of this approach to the PSA systems demonstrate the greater tendency of the anionic nonylphenol ethoxylate to concentrate at film interfaces. Analysis of binary mixtures combining pure surfactant and dialyzed latex indicate that surfactant-polymer compatibility govern, to a great extent, the scale of surfactant enrichment that results. Future research will employ model systems with greater control over polymer properties to identify surfactant and polymer properties controlling migration behavior as well as the impact of environmental conditions (i.e., temperature and humidity) on this process. It is also expected that CRM-AFM analysis will contribute to our efforts to better understand the long-term performance of PSA products and in the development of recyclable/biodegradable adhesive materials. Literature Cited

1. Staicu, T.; Micutz, M.; Leca, M. Prog. Org. Coat. 2005, 53, 56-62. 2. Charmeau, J. Y.; Berthet, R.; Gringreau, C.; Holl, Y.; Kientz, E. Int. J. Adhes. Adhes. 1997, 17,

169-176. 3. Tobing, S. D.; Klein, A. J. Appl. Polym. Sci. 2001, 79, 2230-2244. 4. Mulvihill, J.; Toussaint, A.; Wilde, M. D. Prog. Org. Coat. 1997, 30, 127-139. 5. Donkus, L. J. Adhes. Age 1997, 32, 35-37. 6. Kientz, D.; Holl, Y. Colloids Surf. A 1993, 78, 255-270. 7. Holl, Y. Macromol. Symp. 2000, 151, 473-478. 8. Voyutskii, S. S. J. Polym. Sci. 1958, 32, 528-534. 9. Keddie, J. L. Mater. Sci. Eng. 1997, 3, 101-170. 10. Guigner, D.; Fischer, C.; Holl, Y. Langmuir 2001, 17, 6419-6425. 11. Belaroui, F.; Hirn, M. P.; Grohens, Y.; Marie P.; Holl, Y. J. Colloid Interface Sci. 2003, 261, 336-

348. 12. Urban, M. W. Prog. Org. Coat. 1997, 32, 215-229. 13. Zhao, C.L.; Holl, Y.; Pith, T.; Lambla, M. Colloid & Polymer Sci. 1987, 265, 823-829. 14. Tebelius, L. K.; Urban, M. W. J. Appl. Poly. Sci. 1995, 56, 387-395.

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15. Amalvy, J. I.; Soria, D. B. Prog. Org. Coat. 1996, 28, 279-283. 16. Evanson, K. W.; Thorstenson, T. A.; Urban, M. W. J. Appl. Poly. Sci. 1991, 42, 2297-2307. 17. Zhao, C. L.; Dobler, F.; Pith, T.; Holl, Y.; Lambla, M. J. Colloid Interface Sci. 1989, 128, 437-

449. 18. Mallégol, J.; Gorce, J. –P.; Dupont, O.; Jeynes, C.; McDonald, P. J.; Keddie, J. L. Langmuir,

2002, 18, 4478-4487. 19. Mallégol, J.; Dupont, O.; Keddie, J. L. Proceedings of the 28th Coating Science and Technology;

Athens: Greece, 2002. 20. Lee, W. P.; Gundabala, V. R.; Akpa, B. S.; Johns, M. L.; Jeynes, C.; Routh, A. F. Langmuir 2006,

22, 5314-5320. 21. Belaroui, F.; Grohens, Y.; Boyer, H.; Holl, Y. Polymer 2000, 41, 7641-7645. 22. Schmidt, U.; Hild, S.; Ibach, W.; Hollricher, O. Macromol. Symp. 2005, 230, 133-143. 23. Guo, J.; Severtson, S. J.; Gwin, L. E. Ind. Eng. Chem. Res. 2007, 46, 2753-2759. 24. Guo, J.; Severtson, S. J.; Kroll, M. S. Ind. Eng. Chem. Res. 2004, 43, 1443-1450. 25. Nowak, M. J.; Severtson, S. J.; Wang, S. P.; Kroll, M. S. Ind. Eng. Chem. Res. 2003, 42, 1681-

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Symposium Series, 790(Film Formation in Coatings), 2001. 27. Guo, J., Severtson, S. J., Gwin, L. E., Houtman, C. J. Ind. Eng. Chem. Res. 2008, 47, 2612-2617. 28. Tzitzinou, A.; Keddie, J. L.; Geurts, J. L.; Mulder, M.; Satguru, R.; Treacher, K. E. ACS

Symposium Series, 790(Film Formation in Coatings), 2001. 29. Kientz, E.; Holl, Y. Colloids Surf. A: Physicochem. Engng. Aspects 1993, 78, 255-270. 30. Okubo, M.; Furukawa, Y.; Shiba, K.; Matoba, T. Colloid Polym. Sci. 2003, 281, 182-186. 31. Okubo, M.; Chaiyasat, A.; Yamada, M.; Suzuki, T.; Kobayashi, H. Colloid Polym. Sci. 2007, 285,

1755-1761. 32. Gonzalez, I.; Leiza, J. R.; Asua, J. M. Macromolecules 2006, 39, 5015-5020. 33. Czech, Z. J. Adhes. Sci. Tech. 2007, 21, 625-635. 34. Zosel, A.; Schuler, B. J. Adhes. 1999, 70, 179-195.

Acknowledgments This research was partially funded by a grant from the Unites States Postal Service, Stamp Acquisition and Distribution, and the United States Department of Energy project numbers DE-FC36-04GO14309.

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TECH 32 Technical Seminar Speaker Migration of Surfactants in Pressure Sensitive Adhesive Films and the Impact on Performance Properties Steven J. Severtson, Ph.D., University of Minnesota

Steve Severtson is an associate professor in the Department of Bioproducts and Biosystems Engineering at the University of Minnesota (Twin Cities Campus) where he teaches courses on the topics of surface and colloid science, materials science and mechanics of composite materials. His current research projects include the study of wetting and adhesion involving soft substrates, characterization of small molecule migration in polymer films and development of environmentally benign coatings and adhesives. Severtson received his B.A. in chemistry from Augsburg College (Minneapolis, MN), M.S.M.E. from Georgia Tech. and Ph.D. from IPST, which is currently IPST at Georgia Tech. Prior to joining the faculty at the University of Minnesota, he spent three years working as a senior chemist for Nalco Chemical Company (Naperville, IL). He can be reached at sever018@umn.edu.

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