CLAY/POLYMER NANOCOMPOSITES FOR PRESSURE ...

CLAY/POLYMER NANOCOMPOSITES FOR PRESSURE SENSITIVE ADHESIVES Liza Lofton, senior research chemist, Rohm and Haas Company, Spring House, PA Abstract Composites of polymer and nanoclay can be synthesized, where the tremendous surface area of the clay can significantly influence polymer properties at much lower weight loadings than with traditional fillers. In recent years there has been much work with engineering plastics systems that amply demonstrate polymer property changes induced by modification with nanoclay. Comparatively little effort, however, has been focused on nanocomposites with film forming polymers. I will describe some of our work in synthesizing clay/polymer nanocomposites for pressure sensitive adhesive applications. Preliminary work suggests that these clay nanocomposites may improve the balance of PSA properties, and maybe applicable to many pressure sensitive applications. In our systems we have good evidence of polymer – nanoclay interaction in wet dispersion samples as well as in clear films. Introduction Nanotechnology refers to the research and development of materials that contain structures or features that have at least one length scale ranging from molecular to approximately 100 nm, and which exhibit improved or novel properties that are the direct result of their small sizeThese novel properties result from the tremendous amount of surface area that can result between phases and/or confinement effects1-3 Recently, Rohm and Haas has been investigating unique polymer/clay nanocomposites based on pressure sensitive adhesives. These nanocomposites display unusual cohesive strength and high temperature resistance properties without overly compromising the pressure sensitive nature of the latex. A variety of synthetic techniques have been developed which result in stable latexes with clay in or adhered to the polymer particle. The emulsion polymer/clay nanocomposites have different, and advantageous properties relative to the emulsion polymers alone, and can be produced at a commercially viable cost. After a survey of important emulsion polymer nanocomposites, this talk will address our recent work to synthesize and characterize polymer/clay nanocomposites, as well as to test them for pressure sensitive adhesive properties. Emulsion polymer suppliers, as well as many other material and product suppliers, have been practicing nanotechnology for decades. The current state of the art emulsion polymer expertise has produced a variety of high value additives, binders and pressure sensitive adhesives that rely on nanotechnology. Common small sized emulsion polymers can be described as nanotechnology. Multi phased emulsion polymers can have nanosized features and unambiguously represent nanotechnology. Figure 1 shows a number of such products developed in our research facilities. The kinetic and thermodynamic factors that control morphology development are becoming well known.4-7 Synthesis chemists have utilized these factors to control particle morphology, resulting in nanostructures made with hard and soft composites that have hollow cores (opaque polymer, OP), 8-9 multiple lobes (ML), 10 controlled shells (soluble shell polymers, SSP), 11-12 high aspect ratio polymers (HARP), 13 and a variety of other two or three phase emulsion polymers. Polymer/Clay Nanocomposites

In contrast to emulsion polymer nanostructures, or to conventional composites, polymer/clay nanocomposites are an important class of the emerging new nanocomposites. These materials have demonstrated significantly enhanced properties in a number of areas, frequently delivering a superior balance of previously antagonistic properties.14 The first such application was the development of nylon/montmorillonite nanocomposites by Toyota for under-the-hood applications in automobiles.15 Such nanocomposites display an improved balance of toughness, i.e., increased modulus and tensile strength without embrittlement. Barrier properties, thermal stability and flame retardancy are also improved.14 Figure 2 illustrates how polymer/clay nanocomposites differ from conventional composites.16 If a filler and polymer are brought together, for example by polymerizing a mixture of monomer and filler, an appropriately dispersed micron scale filler will usually remain approximately that size in the resulting composite (see the “conventional composite” in Figure 2). To increase stiffness or tensile strength in such composites requires on the order of 20 – 40 % filler. With these levels of filler, other properties are often degraded, yielding undesirable outcomes such as embrittlement, lack of elongation and elimination of pressure sensitive character. While many products successfully balance these antagonistic properties in conventional composites, nanocomposites can yield property balances “outside the box”. Some layered clays can be dispersed to yield nano-scale plates. For smectite clays such as montmorillonite, these layers can be as thin as 0.9 nanometers. Figure 2 illustrates two ways that polymer can access the surface of all or most of these plates in such clays. In intercalated nanocomposites, the polymer enters the gallery between the layers of clay, the clay layers maintain their registration and the increase in spacing between plates can be seen by such techniques as x-ray diffraction. In exfoliated nanocomposites, individual clay plates become dispersed in the polymer. Huge amounts of surface area are created between the polymer and the clay. For montmorillonite, surface areas in excess of 700 meter 2 /gram have been reported. Polymer chain conformation and mobility are changed at this interface. In fact, so many of the polymer chains interact with the clay surface, at levels of only 2 – 5% clay solids on polymer solids, that bulk properties are influenced. Thus, the nanocomposite, in contrast to conventional composites, requires much lower levels of the discontinuous phase. It is more appropriate to think of the nano material as an additive than to think of it as a filler. Giannelis and coworkers3 have simulated polymer chain behavior between clay plates that demonstrates that chains close to the clay interface have lower free volume than the bulk polymer and those away from the clay interface have higher free volume than the bulk polymer. This may begin to explain how nanocomposites can deliver an unusual balance of properties, such as increased toughness with longer elongation. Barrier properties result from the plate-like nature of the clay. Their high aspect ratio (typically 100 – 500 for montmorillonite) creates a “torturous path” for materials passing through the composite. In order to disperse these types of clay in polymer, the clay surface is typically rendered hydrophobic to make it compatible with the polymer. This is achieved by replacing the cations in the exchange layers between plates with alkylammonium surfactants as shown in figure 3. 14-16 Emulsion Polymer/Clay Nanocomposites The properties of toughness, hard but flexible, improved barrier and other properties analogous to those seen in polymer/clay nanocomposites over the last 10 - 14 years would be desirable in soft, film forming latexes. However, the ability of the latex system to form a film cannot be compromised, nor in the case of adhesives, the pressure sensitive character of the resulting film. We report here our investigations into this area of possible use.

Only limited work on emulsion polymer/clay nanocomposites has been reported.17-20 Commercially available montmorillonite clays,21 hydrophobically modified as described above, are too hydrophobic to transport through water and result in batch coagulation when introduced to an emulsion polymerization. However, sodium montmorillonite (NaMMT) disperses in water and can be introduced cleanly into an emulsion polymerization. When introduced into the early stages of an emulsion polymerization, NaMMT causes a notable increase in viscosity relative to a reaction without it. As more polymer is produced, there is an abrupt decrease in viscosity. Freeze fracture SEM 22 (Figure 4) shows that before the drop in viscosity, the structure is dominated by a “house of cards” arrangement of the NaMMT. When the viscosity drops, the latex has disrupted the NaMMT network. Several modes of addition of NaMMT and emulsion polymerization adjuncts have been developed. Figure 5 shows how these synthesis variations effect the viscosity of the resulting latex as well as the tensile strength and elongation of films cast from the latex (relative to a latex without NaMMT). This work was done with Tg = 0 degree Centigrade acrylic latex, and similar results were obtained with 2 and 5 % NaMMT (magnitude of differences greater at 5%). Going from run method A – D (figure 5) we see the viscosity of the resulting latex decreases, the tensile strength increases and surprisingly little change in elongation. These results can be interpreted as increasing the degree of interaction between the clay and the latex. Method D appears to have the most interaction. This composite also displays the simultaneous property improvements of increased tensile strength with increased elongation. All latexes in this series showed no change in Tg or minimum film formation temperature compared to a control made without NaMMT. Figure 6 shows how dramatic the increase in tensile strength is for these nanocomposites, with just 2% added NaMMT. The use of NaMMT to prepare emulsion polymer/clay nanocomposites relies on ionic and ion – dipole interactions to associate the clay with the latex polymer. Freeze fracture SEM of these composites (in the wet state, i.e., before film formation) shows that much of the MMT is not encapsulated in particles, but associated with the surface of the latex particles (figure 7). Micrographs show most (but not all) of the NaMMT is exfoliated to individual plates. Some emulsion polymer particles appear to have plates of clay protruding from them, while others have clay plates associated with their surface. The flexibility of these approximately 1 nm thick plates can also be seen. While freeze fracture SEM of a blend of NaMMT with latex looks similar to a latex where the NaMMT was present during the polymerization, the physical property enhancements of nanocomposites are greatest for the sample where the clay is placed in the emulsion polymerization. An NMR examination of the wet latex yields some insight into the degree of polymer/clay interaction before film formation. Without any further treatment, the emulsion polymer/clay nanocomposites were examined by proton NMR in a T2 relaxation time experiment. The curves showing the signal degradation related to T2 are shown in figure 8. The faster the signal degradation, the shorter the T2 time. The shorter T2 time results from a more rigid polymer. The control (Tg = 0 deg C, acrylic latex, no NaMMT) had the longest T2 relaxation time, indicating its polymer chains were less rigid than the samples with NaMMT. Emulsion polymers prepared with NaMMT in the reaction had polymer chains that behaved more rigidly than the control, or than the control with NaMMT blended into it. Emulsion Polymer/Clay Nanocomposites as Pressure Sensitive Adhesives Most of the polymer/clay nanocomposites that were synthesized for pressure sensitive adhesive testing were made using synthetic method D, as these composites demonstrated the highest interaction between the clay and the polymer. A variety of compositions and molecular weights were synthesized, which

were stable and exhibited performance enhancements relative to controls made without the addition of clay. A commercially available, tape adhesive was used as a control for process variations, as well as for applications testing. The clay modified adhesives consistently demonstrated large increases in shear resistance and in shear adhesion failure temperature (SAFT) measured in a ramping oven when compared to the unmodified control. Representative data can be see in figure 9. Additionally, 2% clay modified adhesives were further formulated with a commercially available aqueous tackifier, at a level of 70% polymer solids/30% tackifier solids, and compared to the control with and without the same type and level of tackifier. Typically a traditional tackifier would be expected to increase tack and peel, while decreasing shear resistance of the adhesive. In many end use applications, this reduction in shear is undesirable. The data in figure 10 shows that the clay modified adhesive has similar tack and peel to the control with an order of magnitude increase in shear resistance. The tackified samples show an increased tackifier efficiency in the clay modified adhesive (ie tack and peel increase more than in the case of the tackified control).The shear does decrease, but still retains more than double the resistance of the control polymer. This data suggests that a lower level of tackifier could be used in the clay modified adhesives to reach the same level of tack and peel as in the control, which is likely to further increase the shear. These application results demonstrate a superior balance of previously antagonistic properties for pressure sensitive adhesives, where tack, peel and shear were all improved relative to the controls. In terms of the model for the origin of properties, 2% exfoliated clay theoretically has enough surface area to interact with about 60% of the polymer chains, confining them to a lower free volume, and more rigid behavior at the clay surface. This may account for the increase in cohesive strength of the adhesive. The other 40% of the polymer chains reside in a higher free volume state than do the bulk, unmodified polymer chains. These chains have less constricted motion, and exhibit more rubbery behavior than in the unmodified polymer. This allows one to produce clay modified adhesive formulations with simultaneously better tack, peel and shear properties, than in non clay controls. Tackified, clay modified adhesives were also run on our pilot coater on BOPP film, and subsequently converted into tapes, in order to assess some of the practical aspects of PSA processing. When properly formulated, the clay modification did not create any coating issues. Upon knife slitting into tape rolls, the tackified, clay modified adhesives did not show any adhesive build up on the slitter blades, whereas the tackified controls did have build up. Conclusion Emulsion polymer/clay nanocomposites were successfully prepared for use as pressure sensitive adhesives. These nanocomposites display an unusual balance of high temperature SAFT and shear properties, without overly compromising tack and peel of the adhesive. They also demonstrated an excellent tackifier response, improving tack and peel relative to the control, with better retention of shear properties. Formulated nanocomposite adhesives were successfully pilot coated without runnability issues. We also witnessed improved convertibility of these adhesive systems based on a lack of adhesive build up on the slitter blades. References 1) Polymer Nanocomposites: Synthesis, Characterization, and Modeling (ACS Symposium Series, No 804) by Richard A. Vaia (Editor), Ramanan Krishnamoorti (Editor) American Chemical Society 2002

2) Handbook of Organic-Inorganic Hybrid Materials and Nanocomposites, Vols. 1 and 2 Hari Singh Nalwa (Editor) American Scientific Publishers; March 24, 2003. 3) Giannelis EP, Krishnamoorti R, Manias E Polymer-silicate nanocomposites: Model systems for confined polymers and polymer brushes. Adv Polym Sci 138: 107-147 1999 4) Sundberg, D.C.; Cassasa; A.; Pantazopulos, J.; Muscato, M.; Kronberg, B.; Berg, J. Morphology Development of Polymeric Microparticles in Aqueous Dispersions I. Thermodynamic Considerations. Journal of Applied Polymer Science, 1990, 41, 1425. 5) Dimonie, V.; Daniels, E.; Shaffer, O.; El-Aasser, M. Controle of Particle Morphology. In Emulsion Polymerization and Emulsion Polymers, Lovell, P.A.; El-Aasser, M.S., Eds.: John Wiley and Sons Ltd.: New York, 1997, 293-326. 6) Krywko, W.P.; McAuley, K.B.; Cunningham, M.F. Mathmatical Modeling of Particle Morphology Development Induces by Radical Concentration Gradients in Seeded Styrene Homopolymerization. Polymer Reaction Engineering 2002, 10, 135-161. 7) Stubbs, J.M.; Karlsson, O.J.; Sundberg, E.J.; Durant, Y.G.; Johnson, J-E.; Sundburg, D.C. Nonequilibrium Particle Morphology Development in Seeded Emulsion Polymerization. 1: Penetration of Monomer and Radicals as a Function of Monomer Feed Rate During Second Stage Polymerization. Colloids and Surfaces A: Physiochemical and Engineering Aspects 1999, 153, 255-270. 8) Kowalski, A. et.al. US Patent 4,427,836A 9) Harren R.E. Elements Of A Successful Research-Project - The Development of an Opaque Polymer J Coating Technol 55 (707): 79-81 1983. 10) Blankenship, R.M. et.al. US Patent 5,030,666 A 11) Brown, A.B. et.al. US Patent 4,916,171 12) Lorah, D.P. US Patent 4,876,313 13) Chiou, S.-J. et.al. US Patent 5,369,163 A 14) Ray, Suprakas Sinha; Okamoto, Masami.Polymer/Layered Silicate Nanocomposites: A Review from Preparation to Processing. Progress in Polymer Science (2003), 28 (11), 1539-1641. 15) Kawasumi, Masaya et.al. US Patent 4810734 A 16) Kornmann, X, Berglund, L.A., Giannelis, E.P. and Sterte, J. Nanocomposites Based on Montmorillonite and Unsaturated Polyester Polymer Engineering and Science 38 (8); 1351 – 1358. 17) Aguilar-Solis, Carlos; Xu, Yijin; Brittain, William J. PVC nanocomposites via emulsion and suspension polymerization. Abstracts of Papers, 224th ACS National Meeting, Boston, MA, United States, August 18-22, 2002 (2002), 18) Wang, Dongyan; Zhu, Jin; Yao, Qiang; Wilkie, Charles A. A Comparison of Various Methods for the Preparation of Polystyrene and Poly(methyl methacrylate) Clay Nanocomposites. Chemistry of Materials (2002), 14(9), 3837-3843. 19) Kim, Bo-Hyun; Jung, Jae-Hoon; Hong, Seung-Hoon; Joo, Jinsoo; Epstein, Arthur J.; Mizoguchi, Kenji; Kim, Ji W.; Choi, Hyoung J. Nanocomposite of Polyaniline and Na+-Montmorillonite Clay. Macromolecules (2002), 35(4), 1419-1423. 20) Huang, Xinyu; Brittain, William J.. Synthesis and Characterization of PMMA Nanocomposites by Suspension and Emulsion Polymerization. Macromolecules (2001), 34(10), 3255-3260. 21) Southern Clay Products, Inc. Gonzales, TX http://www.nanoclay.com/ 22) Sutanto, Erwin; Ma, Yue; Davis, H. T.; Scriven, L. E.. Cryogenic scanning electron microscopy of early stages of film formation in drying latex coatings. ACS Symposium Series (2001), 790 (Film Formation in Coatings), 174-192.

Acknowledgements I would like to thank my coworkers Debra Kline, William Finch, Chris Lester, Robert Slone and Dennis Lorah for their synthesis and characterization efforts, as well as their insights and technical expertise on this project. I would also like to thank Allen Arnwine for assisting me in the applications testing, and for his coating and slitting work. Kebedah Beshah, from our Spring House research site, performed the NMR studies presented here. Professor Skip Scriven and coworkers from the University of Minnesota provided the freeze fracture SEM images presented here. Finally, thanks to professor Emmanuel Giannelis of Cornell University for valuable discussions with our team.

Figure 1 – Some Emulsion Polymer Nanotechnology Examples

Figure 2: Polymer/Clay Nanocomposite Basics

Figure 3 –Clay Modification

Figure 4 – Freeze Fracture SEM during the emulsion polymerization

Figure 5 – Synthesis Variations

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Figure 6 – Mechanical Test Results

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Figure 7 -- Freeze Fracture SEM of Emulsion Polymer / Clay Nanocomposites

Figure 8 – NMR Test Results

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Figure 9 – Shear and SAFT Resistance Shear and SAFT of Polymer/Clay Nanocomposites peel

tack shear 0.5x1" SAFT 1x1" SAFT 0.5x1"

sample SS, N/in SS, N 6.7 9.4 control 2% clay 4.3 6.7 4% clay 3.5 4.5

SS, hours 3C 64 C 225 M

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adhesive transfer coated to OPP, 0.8 mils SS - stainless steel, N - newtons, AFB - adhesive failure from backing C - coheisve failure, M - mixed failure shear weights are 1 kg

Figure 10 – Tackified Polymer/Clay Nanocomposites

Tackified Polymer/Clay Nanocomposites 20 min peel 24 hr peel tack sample control control+tackfier 2% clay 2% clay+tackifier

SS, N/in 5.2 8.5 5.1 11.2

SS, N/in 8.6 11.7 10 13.3

adhesive transfer coated to OPP, 0.8 mils C - coheisve failure, adhesive coat weight = shear weights are 1 kg

shear 1x1"

SS, N SS, hours 6.1 22 C 11.3 13.6 C 5.7 >250 16.3 52.3 C

Home Page Name:Liza Lofton Title:senior research chemist Building Products Research Rohm and Haas Company Spring House, PA Bldg./Room 3c-147 Phone: 641-7196 Current Area(s) of Expertise: construction adhesives dry powder for cement modifiers

Previous Experience: formaldehyde-free non woven binder, wood glue, wet laminating adhesive, PSAs, epoxy acid crosslinked maintenance coatings, and asphalt emulsions

Education: MIT PhD pchem

Personal Interests: children, sports, reading