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Strategies for producing recycled carbon fiber/PLA composites

Write in the journal Polymersa team of researchers from the University of the United Arab Emirates has reported the development of a recycled carbon fiber/PLA composite to upcycle waste streams and meet growing commercial demands for both materials.

Study: Recycled carbon fiber/PLA composite. Image Credit: Ihor Matsiievskyi/Shutterstock.com

Sustainable polymers

In recent decades, there has been a growing awareness of environmental damage caused by human activity. Polymer materials have shown wide application in multiple industries due to their favorable mechanical and physicochemical properties, but their manufacture comes with a huge environmental cost due to their accumulation, non-biodegradability and sub- toxic products.

Several strategies have been proposed to address these issues, including manufacturing green plastics by incorporating organic biopolymers, recycling plastic products, and reusing waste streams to produce value-added products such as biofuels, construction and industrial chemicals.

Polylactic acid (PLA) is a commonly used biopolymer, possessing both biodegradability and bioactive properties. This polymer has found wide application in automotive, 3D printing and packaging industries among many commercial sectors. PLA is however limited for many commercial applications by its lack of thermal and mechanical stability.

Chemical structure of polylactide acid.

Chemical structure of polylactide acid. Image credit: Al Zahmi, S et al., Polymers

Several composites composed of PLA and filling materials such as carbon fiber, carbon nanotubes and various organic and inorganic nanomaterials have been developed in recent years to improve the mechanical and thermal properties of this biopolymer. Composites produced from natural fibers and biopolymers such as PLA offer benefits such as biodegradability for a wide range of commercial uses.

The recycling of these composites is however problematic due to their complex compositions. Mechanical recycling strategies have proven to be the most effective method as they are resource efficient. However, these processes can be hindered by the exposure of materials to harmful substances during recycling.

Recycling PLA is a key goal in the green polymer industry because the environmental friendliness of this material can be retained and the polymers can be reused to make multiple commercial products. Additionally, PLA is heavier than other commercial polymers such as HDPE and LDPE, which means it can be easily separated by density.

Carbon fiber reinforced polymers

Carbon fiber reinforced polymers have attracted recent research attention due to their favorable properties and commercial applications. Since the discovery of carbon fiber in the 1960s, production of this material has grown exponentially to meet the demands of industries such as aviation, transportation, construction and biomedicine.

Carbon fiber reinforced polymers have also been researched due to the increasing amount of plastic waste caused by the increase in plastic consumption. The demand for carbon fiber reinforced polymers is expected to increase over the next few decades.

The materials can be easily recycled using 3D printing methods. Scrap prepreg can be incorporated into recycled materials to provide improved physiochemical, thermal and mechanical properties. Several studies have reported the manufacture of viable commercial products from recycled carbon fiber composite polymers.

There are, however, challenges associated with the recycling and reuse of these materials, such as the difficulty of heating, shaping, processing and re-crosslinking thermoset carbon fiber reinforced polymers. Due to the substantial benefits of these materials in terms of durability and environmental friendliness, research has focused on improving the recyclability of carbon fiber reinforced polymers.

Recycling process from virgin carbon fiber to CFRP.

Recycling process from virgin carbon fiber to CFRP. Image credit: Al Zahmi, S et al., Polymers

The study

The article studied two different wastes, polylactic acid and carbon fiber remnants. The main objective is to produce and characterize composite materials to provide solutions to the increase in industrial waste. Several tests were performed to investigate the impact of carbon scrap reinforcements on the properties of processed PLA/carbon fiber composites.

Samples of PLA/carbon fiber composite materials with different percentages of additives were prepared by shredding, grinding, blending, and melt-blending. A universal testing machine was used to characterize the effect of additives on the composite samples. FTIR, XRD and TGA tests were used by the authors to study various properties and structural characteristics of the samples.

Study results

The authors observed different impacts on the performance of the composite depending on the percentage and the type of filler used. For example, a 20% carbon fiber composite has a higher modulus of elasticity than a 20% carbon fiber composite and prepreg sheets. The yield strength was also highest in the 20% carbon fiber composite sample.

Generally speaking, carbon fiber has improved the ultimate strength of PLA composite. Negative effects on the ductility of composite materials using carbon fiber reinforcements have however been observed. Overall, the use of recycled PLA and carbon fiber fillers offers advantages for the manufacture of commercial products in terms of cost, processing efficiency and simple methods of recycling waste streams.

Samples of (a) CFRP, (b) PLA, (c) CF, (d) twin screw extruder

Samples of (a) CFRP, (b) PLA, (vs) HR, (D) twin-screw extruder. Image credit: Al Zahmi, S et al., Polymers

The study demonstrated a novel approach to recycling plastic and carbon fiber waste streams from many industries. The composite materials produced in research can be considered for the manufacture of several commercial products, using different mixing ratios to achieve the desired properties.

Although the economic benefits of this process have not yet been explored, the authors hope that their research will provide a starting point for future research on carbon fiber reinforced polymer composites.

Further reading

Al Zahmi, S et al. (2022) Recycled carbon fiber/PLA composite Polymers 14(11) 2194 [online] mdpi.com. Available at: https://www.mdpi.com/2073-4360/14/11/2194

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