Grantee Research Project Results

Evaluation of Carbon Nanofibers. Carbon nanofibers (CNFs) belong to a new class of materials that have exceptional mechanical and electrical properties. These properties of CNFs are being explored in a variety of ways by imparting functionalities in various intermediate and final value-added consumer products. Applications include the use of CNFs as polymer additives for high strength polymer nanocomposites, use of CNFs in carbon-lithium batteries, start capacitors for electronic devices and electrically conducting polymers. In this study, vapor grown carbon nanofiber (VGCNF) synthesis from hydrocarbons were considered. VGCNFs are produced by high temperature catalytic pyrolysis of hydrocarbons in the presence of a transition metal acting as the catalyst. This research resulted in the first life cycle inventory data of this product (Khanna et al., 2008). It was compiled with data reported in the open literature.

 

Life Cycle Energy Analysis. Traditional engineering approaches tend to focus on the process level details without caring about other steps in a product’s life cycle. However, focusing attention on one process without sufficient attention to others in a product’s supply chain can lead to the unintended environmental tradeoff of one problem for another. On the other hand, the use of a holistic, life cycle approach can help avert this problem. This was the underlying theme of this study. A Life Cycle Energy Analysis is done first at the process or equipment scale, the boundaries are then expanded to account for the other steps or inputs of the life cycle of these materials. Here, “life cycle energy requirements” primarily refer to the cumulative fossil energy requirements for the synthesis of CNFs and does not include the contribution of ecosystem goods and services. Thus, it quantifies the non- renewable energy requirements along the supply chain of these nanoparticles.

 

 

Figure 1. Lice cycle energy of analysis of carbon nanofibes CNFs (a) effect of cycle time (production cycle time ranges
from 1 hours to a continous operation for 300 days), (b) energy distribution along life cycle phases (Khanna et al., 2008)

 

Figure 1 presents a direct comparison of the life cycle energy requirements for CNFs based on different feedstocks with those of traditional materials namely aluminum, steel, and polypropylene. Figure 1 (a) reveals that the life cycle of CNFs is energy intensive, with the life cycle energy requirements ranging from 2,872 MJ/kg for benzene feedstock to around 10,925 MJ/kg for methane. In comparison, the life cycle energy requirements for aluminum, steel, and polypropylene are 218, 30, and 119 MJ/kg. Figure 1 (b) shows the breakdown of energy requirements along the CNF life cycle phases. On a per mass basis, the life cycle energy requirements for producing CNFs is 13-50 times that required for producing traditional materials (i.e., aluminum, steel, and polypropylene).

 

Figure 2. Vapor grown carbon nanofibers - life cycle assessment (VGCNF LCA) (a) global warming potential - 
midpoint indicator (b) DALYs - damage indicator.

 

 

Environmental LCA of CNFs.   The next step was to perform an Environmental Life Cycle Assessment of nanoparticle synthesis. A “cradle-to-gate” Process LCA of CNF synthesis has thus been completed, and midpoint and endpoint impact assessment methods suggest that VGCNFs may impose a higher environmental burden than traditional materials per kilogram of product. This is reflected from the higher impact of CNFs in most environmental impact categories. Two base cases are evaluated for CNF synthesis, one with methane and the other with ethylene as the feedstock. Both cases are considered to have hydrogen as the carrier gas in accordance with the current industrial schemes. Figure 2 shows the result of midpoint and impact assessment methods.  Figure 2 (a) indicates higher global warming potential (GWP) for both methane and ethylene based CNFs when compared with aluminum, steel, and polypropylene on an equal mass basis. Similar trends are observed for other midpoint impact categories. EcoIndicator 99 methodology was used to obtain damage indicators. Under EcoIndicator, only three kinds of environmental damages are weighed, namely damage to human health, damage to ecosystem quality and damage to resources. Figure 2 (b) indicates the higher impact of CNFs in the category of damage to human health, which is expressed in terms of DALYs (Disability Adjusted Life Years) per kilogram of product. DALYs incorporate damage to human health in the form of Years of life lost (YLL) and years of lives disabled (YLD) as a result of emissions of substances along the supply chain of a product. Similar trends are observed for the category of damage to ecosystems and damage to resource categories. It is important to reiterate that human and ecosystem impact of CNFs is not accounted due to lack of information about the human and ecotoxicological impact of these engineered nanoparticles. Detailed knowledge and quantifiable data about the fate, transport, and mechanism of damage of CNFs is not available. Thus, the impact numbers presented here reflect only the material and energy use during the synthesis of CNFs.

 

CNFs are compared with traditional materials on an equal mass basis to quantify the life cycle energy intensity and environmental burden. The results of the study so far indicate significantly higher life cycle energy requirements and higher environmental impact of CNFs as compared to traditional materials like aluminum, steel and polypropylene. Because the comparisons are performed on an equal mass basis, these results cannot be generalized for CNF-based nanoproducts and quantity of their use would decide their cradle-to-grave impact. Studying such products was the next phase of this project.

 

Life Cycle Energetic Evaluation of Carbon Nanofiber Polymer Composites. PNCs have enhanced mechanical properties, high strength-to-weight ratios, and are capable of offering specific functionalities such as desired level of electrical conductivity. These combinations of properties are making PNCs as one of the fastest growing plastic segments and an attractive alternative to conventional materials like steel and aluminum. In this work, a cradle-to-gate life cycle study of CNF based PNCs was carried out based on previous work on the CNF life cycle. The boundaries of the CNF life cycle were expanded to include the production of PNC and the end product manufacture. A typical life cycle for nanocomposites consists of alternatives at each processing stage based on the choice of feedstock and the final product manufacture. Polypropylene based CNF nanocomposites were considered as the example. Once the CNFs are dispersed in the polymer, these reinforced materials can be either extruded or injection-molded to obtain a variety of end products.

 

In this project, a cradle-to-gate energetic life cycle assessment of CNF reinforced polypropylene was performed and compared with steel. The functional unit for comparison was the equal stiffness of different components. Five different cases are investigated with varying loading ratios of the CNF in the polypropylene matrix. Two cases with both CNFs and glass fibers were also studied. As depicted in Figure 3, it was observed that for equal stiffness of the components, on a cradle-to-gate basis, CNF-reinforced polypropylene composites are 2-10 times more energy intensive as compared to steel. It was further concluded that the product use phase might govern whether the high upstream energy can be offset during the use phase to realize any life cycle energy savings.

 

Finally, the use of CNF-reinforced nanocomposites in body panels of light-duty vehicles was evaluated. The life cycle of a midsize automobile with CNF reinforced nanocomposite body panels was evaluated and compared relative to conventional steel panels. The results are shown in Figure 3 (b). It was concluded that the use of polymer nanocomposites in automotive body panels results in marginal life cycle energy savings. Besides, polymer nanocomposites based body panels with higher CNF loading ratios appear to be almost at par with steel on a total life cycle basis and do not offer any additional life cycle energy savings. Further, use of CNFs with other additives like glass fibers might be more promising for automotive applications in the near term with net energy savings of around 8 percent relative to steel.

 

Figure 3. (a) Life cycle comparison of carbon nanofiber (CNF) reinforced polymer composits (a) material comparison 
for equal component stiffness design (b) comparison of CNF polymer composites for automotive body panels 
(Khanna and Bakshi, 2008).

 

 

The results obtained have a high degree of uncertainty owing to several reasons. Some of these are: huge life cycle energy requirements for the production of CNFs, possible achievable weight reductions of polymer nanocomposite-based components, and the end-of-life issues specific to polymer nanocomposites that can be significant. Besides, PNCs might be difficult to recycle, reuse, and/or dispose of compared to conventional materials like steel and aluminum. Current work is in progress to address these uncertainty issues and evaluate other thermoset-based nanocomposite materials.

 

Energetic and Environmental Evaluation of Titanium Dioxide Nanoparticles. Typical rutile phase pigment grade TiO₂  particles are on the order of 250 nm to 1 µm in diameter. However, recent demand for smaller particles for use in specialty applications has opened up a market for 20-50 nm anatase phase particles. The Altair hydrochloride process uses a spray hydrolysis step, which allows for further flexibility in the phase and size distribution of the produced particles. The current project used the best available information to investigate the environmental impacts, as well as energy and exergy consumption in the Altair TiO₂ hydrochloride process at both the process and life cycle scales. The goal was to gain insight into the strengths and weaknesses of each approach for identifying opportunities for improvement.

 

The process scale analysis consists of nine main process units and about 25 major flows. Energy and material inputs to each unit were calculated from process information available in the literature from Altair. Any missing data were estimated from these publications or from other sources considering similar units. The life cycle energy and material requirements were calculated using SimaPro and the input data identified in the process scale analysis.

 

Figure 4. Per unit mass comparison of gross energy requirements for the hydrochloride process
with other building materials (Grubb and Bakshi, 2009).

 

The results of the LCA were dominated by fossil fuel use at the life cycle scale, and by the carbon dioxide produced by the combustion of fossil fuels in the local process. The gross energy requirement for the production of 1 kg of nanoparticle product was compared on an equal mass basis with carbon nanofibers, as well as several other more traditional materials. As shown in Figure 4, it was found that titanium dioxide’s energy intensity was several orders of magnitude less than CNFs and more on the order of steel or aluminum per unit mass.

 

First law energy results were found to be misleading when compared to the exergy analysis results due to issues of energy quality and the exclusion of materials, which are paramount to this process. The exergy analysis shown by the Grassman diagram in Figure 5 reveals that the greatest losses are in both the hydrolysis units and the distillation unit. This is due to the destructive use of resources: methane combustion for the hydrolyses, and degradation of high pressure steam for the distillation.

 

At the life cycle scale, it has been shown that the material inputs to the process are just as exergy intensive to produce as the energy-based inputs. This is in contrast to both the first law energy analysis and the LCA, both of which fail to express the value of natural resources other than fossil fuels. At the same time, the energy-based inputs deliver more useful work to the process than their material-based counterparts. For this case study, it appears that the material inputs see most of their processing upstream of the nanomanufacturing process, while the energy inputs are used destructively during manufacture of our product. It would be interesting to see if this trend is observable in other material intensive manufacturing processes.

In summary, this part of the research presented a multi-scale analysis of a manufacturing process producing titanium dioxide nanoparticles. A traditional LCA, first law energy analysis, and second law exergy analysis were performed at both the local process scale and at the broader life cycle scale. The main contribution of this work is to reveal the different insights provided by each method, especially in the context of an emerging technology where data are relatively scarce.

 

 

Figure 5. Grassmann diagram indicting the magnitude of all energy flows withing the process.
Units are all MJ of exergy/kg of product.