Clay/Polymer Nanocomposites for Pressure-Sensitive Adhesives

ASI presents the winner of the Pressure Sensitive Tape Council's 2004 Carl A. Dahlquist Award for Best Technical Paper.

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 size. These novel properties result from the tremendous amount of surface area that can occur between phases and/or confinement effects.^1-3 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 that 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 article 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 used 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, such as 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 m²/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 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 "tortuous path" for materials passing through the composite.

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 (see 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 SEM²² (see 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 affect 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 T_g = 0

Going from run method A-D (see 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 T_g 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 (see Figure 7). Micrographs show that 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 (T_g = 0

Emulsion Polymer/Clay Nanocomposites as PSAs

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 is shown 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 tack and peel to the control with an order of magnitude increase in shear resistance. The tackified samples show increased tackifier efficiency in the clay modified adhesive (i.e., 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 PSAs, 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 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.

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 for 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.

This article is based on the Carl A. Dahlquist Award paper presented at Tech XXVII in Orlando. PSTC's Tech XXVIII will take place May 4-6, 2005, in Baltimore.

For more information on polymer/clay nanocomposites, contact Rohm and Haas Co., 100 Independence Mall West, Philadelphia, PA 19106-2399; phone (215) 592-3000; fax (215) 592-3377; or visit http://www.rohmhaas.com .

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