(Nanowerk Spotlight) Plastics permeate every facet of modern life, from the packaging that protects our food to the components in our smartphones. This ubiquity, however, comes at a steep environmental cost. The production of plastics contributes significantly to greenhouse gas emissions, while plastic waste chokes our oceans and landscapes. As global plastic consumption continues to rise, projected to double by 2040, the urgency to address these environmental challenges has never been greater.
Scientists and engineers have long grappled with the paradox of plastics: how to maintain their utility while mitigating their environmental impact. One promising avenue of research has focused on enhancing the properties of plastics to reduce the amount needed for various applications. This approach not only conserves resources but also potentially decreases the overall carbon footprint of plastic products.
Carbon nanotubes (CNTs), cylindrical molecules composed of rolled-up sheets of carbon atoms, have emerged as a potential game-changer in this field. CNTs possess extraordinary strength, electrical conductivity, and thermal properties. These characteristics make them ideal candidates for reinforcing plastics, potentially allowing for the creation of stronger, lighter materials that require less raw material to achieve the same performance.
However, the widespread adoption of CNTs in plastic composites has been hampered by two significant obstacles: high production costs and the energy-intensive nature of conventional CNT manufacturing processes. Ironically, the production of these nanomaterials often carries its own substantial carbon footprint, seemingly at odds with the goal of creating more environmentally friendly plastics.
“Recent advancements in carbon capture and utilization technologies have opened up new possibilities for addressing these challenges,” Stuart Licht, a professor of Chemistry at George Washington University, tells Nanowerk. “Our group and others have developed innovative methods to convert carbon dioxide (CO2) – the very greenhouse gas at the heart of climate change concerns – into valuable carbon nanomaterials, including CNTs. This approach not only provides a potential pathway for reducing atmospheric CO2 levels but also offers a more economical and environmentally friendly method for producing CNTs.”
The convergence of these technologies – carbon capture, CNT synthesis, and polymer science – has set the stage for a potential breakthrough in creating stronger, more sustainable plastics with a reduced environmental impact. Against this backdrop, Licht and his group recently published a study demonstrating the use of CNTs derived from CO2 to enhance the properties of various plastics, including both thermoset epoxies and thermoplastics.
Their work, published in RSC Sustainability (“Polymer composites with carbon nanotubes made from CO2“), explores the potential of these CO2-derived CNTs to strengthen plastics while simultaneously addressing environmental concerns, potentially marking a significant step forward in the development of next-generation, eco-friendly materials.
The CO2 to graphene nanocarbon material process (carbon nanotube example). (A) CO2 is removed directly from air or flue gas (without preconcentration). (B) CO2 is electrolyzed in molten carbonate. (C) The transition metal nucleated mechanism of electrolytic CO2 transformation to CNT at the electrolysis cathode. (D) A pulled 1700 cm2 cathode with deposited carbonogel (CNTs retaining interstitial electrolyte) subsequent to 18 hours electrolysis at 0.6 A cm−2 in 770 °C Li2CO3. (E) SEM of carbanogel subsequent to excess electrolyte removal & (F) TGA of CNT product. (click on image to enlarge)
The team utilized a process they previously developed – molten carbonate electrolysis – to convert CO2 into CNTs. In this method, CO2 is dissolved in a molten carbonate salt and subjected to electrolysis, resulting in the formation of carbon nanomaterials at the cathode. The product of this process, termed a “carbanogel,” consists of intertwined CNTs retaining some of the electrolyte within their structure. This carbanogel can be further refined to isolate the CNTs for use as additives in plastic composites.
“We then investigated the effects of adding these CO2-derived CNTs to several types of epoxy resins and thermoplastics,” Licht explains. “We prepared composite samples with varying concentrations of CNTs and tested their mechanical properties, focusing particularly on tensile strength – a measure of how much force a material can withstand before breaking when stretched.”
For a deep-pour epoxy resin called Timber Cast, the addition of 1.5% by weight of CNTs resulted in a 30% increase in tensile strength compared to the pure epoxy. This significant enhancement suggests that the same strength could be achieved using less epoxy material, potentially reducing the polymer’s carbon footprint by nearly a third for strength-related applications.
Even more dramatic results were observed with a thin-coat epoxy called Varathane. Composites containing just 1% CNTs by weight exhibited a remarkable 55% increase in tensile strength. This implies that achieving the same strength as pure Varathane epoxy would require approximately 36% less material when using the CNT-enhanced composite.
A fast-curing epoxy known as Jetset-Metlab also showed substantial improvements. Room temperature-cured samples with 1.0 to 1.5% CNTs demonstrated a 48% increase in tensile strength. Interestingly, this epoxy showed significant strength enhancements even at lower CNT concentrations, with noticeable improvements starting at just 0.5% CNT content.
The researchers also examined how curing conditions affected the performance of the CNT-epoxy composites. When cured at 60 °C instead of room temperature, the Jetset-Metlab epoxy showed further increases in tensile strength, both with and without added CNTs. This highlights the importance of optimizing processing conditions to maximize the benefits of CNT additives.
In addition to tensile strength, the team measured the hardness of the epoxy composites. They found that hardness also increased with CNT content, reaching a maximum in the same concentration range (1 to 1.5% CNTs) where tensile strength peaked. This correlation suggests that the CNTs are enhancing multiple mechanical properties simultaneously.
Statue titled “The world on our shoulders” made with CO2 captured directly from the air. The CO2 is transformed to carbon nanotubes by the C2CNT process. The CNTs are mixed with PLA to make a strong CNT-composite, which is 3D printed forming the statue. (Image: Stuart Licht)
The study also explored the potential of CNT additives in thermoplastics, focusing on polylactic acid (PLA), a biodegradable polymer often used in 3D printing. Preliminary results showed that PLA composites containing 6% CNTs exhibited a 65% increase in tensile strength compared to pure PLA. This finding indicates that the benefits of CO2-derived CNTs extend beyond thermoset epoxies to include thermoplastic materials as well.
“The implications of our findings are quite significant,” Licht points out. “By enhancing the strength and other properties of plastics, CNT additives could allow for the use of less material to achieve the same performance characteristics. This reduction in material use could translate directly into a lower carbon footprint for plastic products. Moreover, the use of CO2 as a feedstock for producing the CNTs adds an additional layer of environmental benefit, potentially turning a greenhouse gas into a valuable material resource.”
From an economic perspective, the Licht suggests that the CO2-to-CNT process could potentially reduce the cost of producing carbon nanomaterials significantly. While conventional chemical vapor deposition methods for producing CNTs can result in costs of around $1 million per tonne, the electrolysis-based approach is estimated to bring costs down to approximately $1000 per tonne in bulk production. However, Licht cautions that this figure is a projection and would require further validation at larger scales.
The potential impact of this technology extends beyond just reducing material use. Enhanced electrical and thermal conductivity in plastics could open up new applications in electronics and heat management. However, these applications are suggested based on the inherent properties of CNTs and may require additional research to confirm feasibility in CNT-enhanced plastics.
Improved mechanical properties could lead to lighter, stronger materials for transportation and construction, potentially reducing fuel consumption and enhancing durability. Yet, it’s important to note that this research is still in its early stages. While the results are promising, further studies will be needed to fully understand the long-term performance and environmental impacts of these CNT-enhanced plastics. Questions remain about the scalability of the CO2-to-CNT process and how these materials might behave during recycling or disposal at the end of their life cycle.
“Despite these uncertainties, our work represents a significant step forward in the quest for more sustainable plastics,” Licht concludes. “By combining carbon capture technology with materials science, we were able to demonstrate a potential pathway to address multiple environmental challenges simultaneously. As this technology continues to develop, it could play a crucial role in reducing the environmental footprint of plastics while maintaining or even enhancing their performance characteristics.”
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