I have always been told to look for innovation in the most common areas. Sometimes developers get mesmerized with the technology and novelty of the developing product and fail to see the most simple and common applications. I believe that this type of assessment could possibly apply to nanotechnology and hot melt or waterborne adhesive in the future.

It seems at first glance that the subjects of nanotechnology and hot melt or waterborne adhesives are helplessly far removed from one another. One deals with materials at a cost of hundreds of euro per gram and aims for niche, high tech applications that can support the cost. At a materials cost that could be 10 times more than micron-scale materials, future applications will have to be targeted where nanotechnology brings maximum value and be able to deliver product in commercial quantities. Thus, one would rationally think that nanomaterials are likely to first find application in products where materials must be engineered for specific high performance characteristics and no conventional materials exist today.

The other product (hot melt and waterborne adhesives) deals with a cost sensitive commodity-type product that nearly everyone uses on a daily basis. Everyone is looking to reduce cost. Although the demand and applications are growing, there is nothing radically new on the horizon that will cause a quantum jump in the demand for these adhesives. However, when we look again at what is going on recently in the field of nanotechnology, perhaps these two areas are better matched than we originally thought.

Nanocomposites that are now commercial are based primarily on thermoplastic materials (primarily for packaging). Thus, their melt processing methods have become well established. It is possible that these processes may be easily adaptable to hot melt adhesive formulation and allow for new products that have properties unlike any that we have seen before. For example, the use of nanotechnology to produce electrically conductive adhesives or adhesives that are barriers to gas have been under development.

Nanoscale iron oxide particles are also being developed by Degussa for additives in adhesive formulations to allow for bonding and debonding on command. The concept is to use an alternating magnetic field applied by mobile generators to heat the nanoparticles in an adhesive layer locally so that the layer can cure in the field. Subsequent heating at a higher energy rating would cause the cured adhesive to soften and debond. This technique could also be used to cure or debond adhesive materials or coatings at specific locations on a substrate.

Melt compounding may be used to produce a concentrated masterbatch with heavily compatibilized nanoparticles; then the master batches are melt blended at the adhesive formulator with pure polymer to produce the final nanocomposite material.

An important variation in producing nanocomposites is an emulsion polymerization process in water. This method is being investigated due to the improvements provided by processing in water instead of solvent, melt, or monomer. These include the low viscosity of the polymer emulsion and safety / environmental compliance through elimination of an organic solvent.

Only limited work on emulsion polymer / clay composites has been reported. The efforts have been primarily based on acrylic, PVC, and polyaniline emulsions. The main application seems to be in cast films that can be used as adhesives.

Commercially available montmorillonite clays are too hydrophobic to transport through water and result in coagulation when introduced to an emulsion polymerization. However, sodium montmorillonite (NaMMT) has been found to disperse easily in water and can be introduced cleanly into an emulsion polymerization process. When introduced into the early stages, NaMMT causes a notable increase in viscosity relative to a reaction without it. As more polymer is produced during the polymerization process, there is an abrupt decrease in viscosity. When this occurs, the latex has disrupted the NaMMT network.

Several methods for addition of the NaMMT and important emulsion polymerization parameters have been developed. These variations effect the viscosity of the resulting latex as well as the tensile strength and elongation of films cast from the latex. In adhesive systems, nanocomposites produced in this manner display an unusual balance of high temperature and shear properties without compromising tack and peel strength of the adhesive. 

Waterborne processing is also being looked at for incorporating nanofillers into polymers in the melt form. Dresden University and the Elf Atochem Research and Development Center (Serquigny, France) have been working on a modified process in which, rather than adding the filler to the melt, it is brought in right at the beginning of the extrusion process in a water slurry, and mixed with a powdered form of polypropylene. Development is still in the early stages, but it is thought that such a process could come within the competence of a good technical compounder.

However, the formulator should keep a watchful eye on the combined developments in nanomaterials and adhesives in general. Nanomaterials will be able to provide products with properties never before realized. These advanced materials are developing rapidly both in the performance properties that they can impart as well as their cost effectiveness.