Sustainability has an important place in successful commercialization of new technologies. This is especially true in the case of new technologies based on nanomaterials. To understand environmental implications of wide-scale use of such technologies, we should consider the effects of dispersion and aggregation on particle size.
Most nanomaterials exhibit the advanced properties which make them unique and highly valuable only when they are dispersed, that is, when comprising nanoscale size particles. However, this coin also has a flip side: typically, nanoscale particles are more dangerous for humans and the environment compared to the same material comprised of macroscopic size particles. The same mechanisms that give rise to the desired advanced properties may become the source of increased danger when brought into contact with living organisms or the nature. For example, the high aspect ratio of carbon nanotubes (CNT) which leads to increased electrical conductivity in plastic composites may cause carcinogenic behavior, similar to the asbestos particles. This creates a difficult dichotomy: nanoscale particles are needed to power new technologies, but these virtues may give rise to serious problems when exposed to the environment.
On the other hand, by their very nature, nanoscale particles tend to aggregate and form larger particles in order to decrease their enormous surface energy. That means, we have to not only generate nanoscale particles, but also keep them separated from each other and prevent re-aggregation, at least until they are incorporated into the product. This is currently achieved by introducing dispersing agents, such as surfactants, into the nanomaterials when they get dispersed. Surfactants reduce the surface energy and make the system more stable. In some cases, for example, the case of carbon nanotubes, there is an alternative way based on covalent attachment of certain functional groups to CNT walls which greatly facilitate dispersion in selected solvents.
The problem with the first approach is that surfactants typically have to be removed from the product once the nanoscale particles are introduced into the product. The removal is typically done by introducing a post-processing step of washing the product with copious amounts of solvent. For example, a plastic-CNT composite can be fabricated by mixing a plastic powder with an aqueous CNT dispersion and evaporating the water solvent. If the dispersion also contained surfactants, the dried composite would have to be washed multiple times with a lot of clean water. Note that the amount of water used for washing may far exceed the amount of water used as solvent - the latter also can be captured and re-used.
The washing step creates two problems from the perspective of environmental impact. First, it generates large amount of waste relative to the amount of product, for example, it may generate 10 or more grams of waste solvent per each 1 gram of product. Furthermore, the waste typically contains not only the solvent and surfactants but also finely dispersed nanoscale particles carried away with the surfactant. These nanoparticles are in their most environmentally dangerous form, making it a hazardous waste. The manufacturing process then becomes less efficient and environmentally less feasible. Second, the nanoparticles incorporated into the product are safe for the environment as long as the product is intact. However, if they got released from the product for whatever reason (e.g., broken, worn out, or decomposed product), some of them may enter the environment with surfactants still attached to them thereby keeping them dispersed upon the release. This adds yet another potential environmental hazard.
The second approach based on artificially attached functional groups also has a profoundly negative environmental impact. In this case, it is due to the process of CNT functionalization which involves extensive use of boiling strong acids and subsequent disposal of these acids with embedded residual CNT fragments. Again, as with the surfactants, large amounts of highly hazardous acid-filled waste is generated relative to the weight of the product (i.e., the functionalized CNT material) making it a wasteful and environmentally non-feasible process.
The Voxelum technology offers the first sustainable and environmentally friendly alternative to the current approaches. It doesn't involve either functionalization process or use of surfactants or any other additives, thereby eliminating major sources of hazardous waste. It can work with water and alcohol solvents which are among the most forgiving solvents when it comes to safety, handling, or environmental mitigation. These solvents are also extremely versatile and suitable for a large variety of applications. Emergence of such flexible, additive-free dispersion technologies will greatly facilitate commercially viable applications of CNT materials in various industries.