Sustainable emerging materials: polymers and nanomaterials
In our time, we are used to satisfy our worldly needs and search for comfort by simply buying products at grocery stores, malls, or shopping centers. Nowadays we can access pretty much any type of product at affordable prices even if is coming from remote locations in the world. However, there is an extra price to pay: unprecedented amounts of non-renewable materials are depleted every single day. Most of these products are derived from petroleum, a non-renewable fossil fuel. To complicate things even further, the processing of petroleum is accompanied by the consumption of large amounts of energy and the generation of undesirable contaminants. Moreover, once products are no longer useful, they are discarded without the slightest chance of being recycled or reused. This unprecedented mass consumption of non-renewable natural resources has been exacerbated by the increasing number of communities around the world in search for better living standards, which translates into a greater access to goods. If we want to survive as an ever growing species on planet earth, a much more sustainable good chain supply will be needed. As of today, much of the discarded goods are generally taken to landfills or incinerated. Landfills are places where all sorts of residues are dumped and accumulated. The biggest concern with landfills is that they eventually reach a saturation point. The non-renewable fossil fuels are not only the main source for good manufacturing, they are also present in various stages of this supply chain. For instance, vehicles to transport the goods are generally powered with fossil fuels. As goods are discarded, these precious resources are just flushed away. Even the very extraction of fossil fuels poses difficulties that are not necessarily considered in the final price of goods. In the case of petroleum, for instance, the extra price is paid by ecosystems being contaminated with the effluents of drilling activities that are full of highly toxic and recalcitrant compounds. Additionally, when petroleum distillates (refined fractions, e.g., gasoline) are combusted, they release considerable amounts of Carbon dioxide, a greenhouse gas capable of promoting a negative climate change.
All these issues in resource and energy use have propelled the emerging field of sustainability. This approach pretends to comprehensively understand the supply chain of a particular good to target stages where a much better use of resources can be considered while maintaining profitability and low environmental impact. There are different approaches to sustainability, which are pursued either independently or combined. One of these approaches is the Life Cycle Assessment (LCA) (Hendrickson, Lave, and Matthews; Horne, Grant, and Verghese). In such a scheme, inputs, outputs and environmental impacts are analyzed for a particular good or product. LCA was conceived as a four stages analysis that includes goal and scope definition, inventory analysis, impact assessment, an interpretation. In the goal and scope phase, the purpose of the analysis is delineated, which is accompanied by a definition of the system to be analyzed; the functional unit; and the data quality. During the inventory analysis phase, energy consumption and potentially environmentally dangerous process streams are precisely quantified. The impact assessment phase is mainly directed towards the classification, characterization, normalization, and valuation of the identified streams. This step is critical to the analysis as it allows quantitative impact estimation. In this regard, according to LCA impacts of most concern are: consumption of non-renewable resources, climate change, ozone reduction, acidification, eutrophication, formation of photochemical compounds, human toxicity, and aquatic toxicity. The last phase of LCA is interpretation, in which the identified impacts are linked to particular life cycle stages or “hot spots”. Prior to postulating the final conclusions of the analysis, a sensitivity analysis must be conducted where the impact of data variability is assessed (Hendrickson, Lave, and Matthews; Horne, Grant, and Verghese).
LCA is not the only emerging approach for sustainability assessment. The field of Chemistry has proposed similar ideas through an approach called green chemistry. As opposed to LCA that is mainly based on a macroscopic process analyses, green chemistry presents some chemical rules in an engineering framework. The notions of green chemistry are condensed in twelve principles: prevention, atom economy, less hazardous chemical syntheses, design of safer chemicals, safer solvents and auxiliaries, design for energy efficiency, use of renewable feedstocks, reduction of derivatives, use of selective catalytic reagents, design for degradation, real-time analysis for pollution prevention, and inherently safer chemistry for accident prevention (Doble and Kumar; Sharma and Mudhoo).
These two frameworks can be used to evaluate whether a new technology is a suitable alternative for obtaining sustainable products. The booming of nanotechnology has provided novel products that supposedly exhibit increased sustainability when compared with the conventionally produced counterparts. This claim has spurred considerable controversy in the scientific community around the world. In light of the core principles of LCA analysis, i.e., the environmental impacts; there are several advantages and drawbacks. Nanoscale materials are generally designed to be active at extremely low concentrations, so their production requires a lot less consumption of non-renewables. Accordingly, their production can be done in small vessels with a considerable reduction in energy usage for heating, mixing, pumping, and downstream purification. The emission of undesired substances to air, soil and water can be substantially reduced as well. Despite these advantages, the production of these materials poses a significant environmental threat. This is mainly due to the unknown effects of these tiny materials when they are delivered in water streams, soil or air. Thus far, research in this field is quite limited and the environmental fate and transport of these materials remains poorly understood. For instance, we don’t know if the silver nanoparticles added to soaps and shampoos to increase detergency are to going to end up in our food chain with negative effects due to bioaccumulation. We don’t really know if nanofertilizers are going to runoff from crops soil and reach superficial waters where algae or other aquatic life forms are able to take them up. Carbon nanotubes used in the treatment of cancer can be excreted by the afflicted patient via urine and therefore potent pharmacological compounds may be unexpectedly triggering a major equilibrium change in naturally balanced aquatic ecosystems. All nanomaterials exhibit a tremendous adsorptive ability towards various chemical compounds that are available in nature. This tendency might lead to the transport of compounds from one place to the other with devastating consequences. This suggests that even the LCA approach might need to include other impacts as a deeper understanding of what nanomaterials do the environment becomes available. It is very likely that new mathematical models need to be developed to describe the complex and multiparametric relationships of nanomaterials with various natural matrices. Thus far, it is impossible to truly quantify the real sustainable potential of nanomaterials. If the green chemistry principles are applied, the scenario is not that different. Benefits can be mostly seen in the production of materials that are inherently more active and require much less energy and reagent consumption. Thus far, however, there is no real proof for processes that incorporate safer solvents or chemicals or designed to be degradable.
If we consider polymers, both the LCA and green chemistry analysis will lead to similar conclusions. Most polymers are derived from petroleum and consequently their production involves several environmental impacts. Perhaps one main advantage is that most of them are designed to be recyclable. Unfortunately, our solid waste management system makes recycling a not very profitable activity. Additionally, during the recycling process, polymers tend to lose some of their physical properties by thermally-induced mechanisms. This inevitably leads to a minimal use of recycled material every time a new product is manufactured. Due to their chemical structure, when polymers are discarded they are not easily degradable and may subsequently reach superficial and ocean waters promoting thereby a negative impact on aquatic ecosystems. This adds an important environmental impact that should be considered when conducting a LCA analysis (Doble and Kumar).
An emerging alternative, scientists have considered the engineering of naturally-derived materials. In this strategy, sustainability is substantially increased due to the reduced dependence on petroleum-derived feedstocks and degradability of the obtained materials. Within this group, polymers extracted from plants have attracted much attention mainly due to their relatively high abundance, and renewable origin. One remarkable example is cellulose, a natural polymer extracted from plants that is being engineered into major commodity products such as films, plastic cases, coatings, separation membranes and flexible displays. These developments, however, remain as lab scale curiosities mainly due to the difficulty in modifying the chemical functionality and consequently some properties of cellulose. For instance, limited durability and resistance have narrowed the number of marketable applications. As a result, considerable effort has been invested towards the search for avenues to efficiently engineer this hard to access chemistry. These initiatives have merged with the booming interest in nanotechnology leading to the appearance of nanostructured cellulose (Gama, Gatenholm, and Klemm). This material offers the opportunity for subtle modulation of physical and chemical properties and therefore adjustment to a particular application. The LCA analysis of nanocellulose will have reduced environmental impacts considering that it comes from a renewable source and that is completely biodegradable. The green chemistry requirements will be satisfied in at least two postulates: use of renewable feedstocks and design for degradation. The major environmental concerns are those associated with the extraction of nanocellulose, which is conducted in a chemically harsh environment and requires excess water. Currently, research groups around the world are working on strategies to approach to a much more sustainable production of nanocellulose. One of the strategies is to replace the solvent for extraction with the so-called green solvents, which are safer and reusable media (Sharma and Mudhoo). Examples of green solvents include ionic liquids and supercritical fluids. Similar analyses will be obtained for other naturally-obtained materials such as chitin and chitosan.
The future for natural polymers is bright considering recent advances in nanotechnology and molecular engineering. We expect that especially designed nanomaterials can be tailored using minimal amounts of energy derived fossil fuels. The materials will be completely degradable and exhibit high specificity.
Works Cited
Doble, M., and A. Kumar. Green Chemistry and Engineering. Elsevier Science, 2010.
Gama, M., P. Gatenholm, and D. Klemm. Bacterial Cellulose: A Sophisticated Multifunctional Material. Taylor & Francis Group, 2012.
Hendrickson, C. T., L. B. Lave, and H. S. Matthews. Environmental Life Cycle Assessment of Goods and Services: An Input-Output Approach. Taylor & Francis, 2012.
Horne, R. E., T. Grant, and K. L. Verghese. Life Cycle Assesment: Principles, Practice and Prospects. CSIRO Pub., 2009.
Sharma, S. K., and A. Mudhoo. Green Chemistry for Environmental Sustainability. Taylor & Francis, 2010.
