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Although the recycling program has made significant progress in reducing plastic waste to landfills, a large amount of plastic is still put into landfills, and a large amount of plastic eventually flows into rivers, lakes, oceans and other waterways, causing destructive effects on the environment. environment. Polylactic acid (PLA) compares very well with traditional plastics made from fossil fuels.
Image source: photokup/Shutterstock.com
In addition to recycling, the latest way to solve this problem is to develop biodegradable plastics, the most popular of which is polylactic acid (PLA). PLA is formed by the condensation of lactic acid. This plastic material is both biodegradable and biocompatible. Due to its low toxicity, PLA is commonly used in agricultural, biomedical, and food packaging applications.
PLA compares very well with traditional plastics made from fossil fuels. Unfortunately, PLA is poor in durability. For many practical purposes, it can be a bit brittle and structurally insufficient. Therefore, the focus of research is to improve the strength and durability of PLA, and a popular method is to add carbon nanomaterials, such as graphene and carbon nanotubes.
When these materials are added, the resulting polymer is superior in structure and function to the corresponding polymer without these additives. This brings prospects for significantly improving the strength and durability of PLA.
For the development of PLA with graphene or carbon nanotube additives, rheological analysis must be a key component of material characterization. The addition of graphene or carbon nanotubes to PLA can affect the rheological behavior of the polymer matrix in many ways, especially by increasing the values of dynamic modulus and viscosity.
As the amount of filler increases, the resulting network structure becomes more and more interconnected. Eventually it will reach a critical point, called the rheological permeability threshold. This depends on the composition of the filling material.
Rheometer. Image source: Image Source Trading Ltd/Shutterstock.com
Generally speaking, rheological behavior can be divided into three different groups. At the "dilution" level, the inclusion of nanomaterials will only lead to short-range interactions. At the "semi-dilution" level, a loose network is formed, and the resultant rheological behavior depends on the interaction between the fillers. At the "concentrated" level, the viscosity and dynamic modulus are extremely high.
Importantly, the rheological behavior of these polymer systems can reveal clues about their composition, because the rheological properties depend on the size, shape, and surface composition of the filling material.
In a recent study, researchers found that PLA with carbon nanotubes or graphene additives has a stronger polymer network, which can be transformed into higher melt strength and more crystalline nucleation sites. These physical properties make the composite matrix better able to withstand bubble growth. It also prevents the collapse and coalescence of bubbles.
The researchers created a foam made from their PLA composite material. They found that these foams have excellent electrical conductivity and compressive strength, and are two similar foams that do not contain carbon nanotubes or graphene additives.
The increase in strength and durability comes from cross-linking within the polymer matrix. This means that the optimization of PLA reinforced by graphene and/or carbon nanotubes depends on the uniform dispersion of these materials in the matrix. This can happen when the polymer is in a liquid state, because melting of the polymer is common in most traditional manufacturing processes.
In addition, additive manufacturing is an innovative technology that seems to be the future of manufacturing. The inclusion of carbon nanomaterials brings many major benefits to the 3D printing of PLA and other polymers. The key issue in the future will be how to manage the decentralized process, the interaction between the materials to be printed, and the quality of the finished product.
Further reading: Plastic alternatives: where are we now?
Due to its ease of use and biodegradability, PLA has been widely used in 3D printing. Although the addition of graphene or carbon nanotubes can bring many benefits, their addition will also have a negative impact on the extrusion of materials in the 3D printing process.
Although there is a lot of research on the rheology of PLA foam containing graphene and carbon nanotubes, more research can help materials scientists better understand the relationship between microstructure and performance and various production processes. Experts suggest that research should focus on the rheological behavior of nanocomposite PLA under various flow conditions common in today's thermoplastic manufacturing processes.
If researchers can gain a deeper understanding of how the microstructure affects the rheology of these materials, it will open the door to countless manufacturing applications. It can also increase the adoption of PLA, which will significantly reduce the amount of plastic that ends up in the ocean and landfills.
Li, S. etc. The synergistic effect of carbon nanotubes and graphene enhances rheological properties and crystallization behavior, thereby improving the foaming behavior of PLA. Journal of Applied Polymer Science. [Online] Available at: https://onlinelibrary.wiley.com/doi/abs/10.1002/app.51874
Sanchez-Rodriguez, C. Graphene oxide and ionic liquid modified extruded PLA nanocomposite. polymer. [Online] Available at: https://www.mdpi.com/2073-4360/13/4/655/pdf
Parker, I'll wait for someone. Melt rheology and mechanical properties of polylactic acid/alkylated graphene oxide nanocomposites. polymer. [Online] Available at: https://www.researchgate.net/publication/346277862_Melt_Rheology_and_Mechanical_Characteristics_of_PolyLactic_Acid
Ivannova, R. etc. Ecological study of polylactic acid nanocomposites with carbon nanotubes and graphene additives as a material characterization tool for 3D printing applications. Applied rheology. [Online] Available at: https://www.degruyter.com/document/doi/10.3933/applrheol-28-54014/pdf
Arrigo, R. etc. Rheological behavior of polymer/carbon nanotube composites: an overview. Material. [Online] Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7344594/
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Brett Smith is an American freelance writer with a bachelor's degree in journalism from Buffalo State University and has worked in a professional laboratory for 8 years.
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