Home » Clay/Polymer Nanocomposites for Pressure-Sensitive Adhesives
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.
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.
Read all about packaging, from formulating packaging adhesives to choosing the best packaging option for your product. This issue also includes the 2020 Raw Materials and Chemicals Overview in this issue, so be sure to check it out!