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What’s the Catch? Reeling in the Opportunities of Recycled Fishing Nets

recycling fishing nets

Recently, statistics have indicated that the maritime environment is polluted by a certain type of plastic:  More than 46% of the waste in maritime litter is represented by fishing nets [1]. This is suggested to be due to bad waste management including recovery, sorting and disposal, producing a delay on the relief of the maritime environment. [2][3] Consequently, inclusiveness of all the involved stakeholders is required to improve the process of waste manipulation.

According to Our World in Data, even higher percentage of the plastic mass in the Great Pacific Garbage Patch is made of plastic lines, ropes and fishing nets.

Nowadays, efforts have been made to reconsider what for many years has been called ‘waste’ (something that does not have any further function) as a ‘raw material’. Plastix A/S contributes to this purpose by recovering fishing gear and improves the material properties to allow the fishing gear to become an option to produce new products (Fig.1).

The interesting issue is, how can the material be modified to be suitable for re-introduction to the manufacturing process? Which modifications need to be made in order to make the recycled material perform like a virgin material?

Tropical reef damaged with old rope underwater.
Most lost fishing gear is made of non-biodegradable plastics that may sink to the sea floor or drift around in currents. It may remain unnoticed until it shows up on coral reefs, beaches and in other coastal habitats.

Initial steps for pioneering the recovery of fishing nets

Fishing nets recovered by Plastix are mainly made from HDPE (High density polyethylene). Its high-density is a consequence of the molecular arrangement of the polymer chains as well as its molecular weight. Due to the linearity of the molecule chains, it is easy for the polymer to arrange neatly and produce a well-arranged microstructure [5][6] As a consequence, the polymer has better mechanical properties compared to the other commercial thermoplastics, as there is a greater cohesive strength between the close-packed molecules, making HDPE ideal for the fabrication of fishing nets.

Plastix is a manufacturer of Green Plastics
Plastix is a manufacturer of ‘Green Plastics’. Plastix converts used fibres, primarily fishnets, trawls and ropes from the maritime industry that would previously have ended up in the ocean or on landfill, into high-grade and virgin-like Green Plastic raw materials. Plastix’ production output, our products, is branded OceanIX rHDPE and OceanIX rPPC, NordIX rHDPE and NordIX rPPC. Their virgin-like quality makes them applicable to most plastic products.

In order to produce fishing nets, the polymer goes through a cold-drawing process in which the polymer is stretched, forcing the chains to re-arrange. HDPE consist on two phases, the crystal phase (ordered chains) and the amorphous phase (disordered chains) while dawning, the crystals within the polymer are broken and forced to slip pass each other, liberating the amorphous phase and allowing the polymer chains to disentangle, increasing the order of the system.

Apart from this, modification of the orientation of the chains parallel to the draw direction also occurs. The result is a highly ordered polymer with a high amount of interaction between the chains, increasing the forces that keep the polymer packed together. This is known as strain hardening. [7]

For the fishing market, this microstructural order is desired as it increases the strength of the polymer. However, this same order complicates the recycling of the fishing nets. The highly ordered chains restrict the chain mobility and in consequence increase the viscosity of the polymer, making it difficult to re-process the polymer. The purpose is then to use mechanical blending as the mechanism that disturbs the order of the chains to reduce the viscosity of the recycled HDPE (rHDPE/ Oceanix).

An estimated 640,000 tonnes of fishing gear are dumped in the ocean each year, polluting the sea and fatally trapping marine life.
An estimated 640,000 tonnes of fishing gear are dumped in the ocean each year, polluting the sea and fatally trapping marine life.

It is expected that the processability of the material will improve if it is compounded with a lower viscosity HDPE polymer. Knowing that it is highly probable that a polymer will be compatible, improving the interaction at the interphase of the two polymers, when blended with the same type of polymer (e.i. HDPE/HDPE, PP/PP, etc.) [8], this type of blend is chosen as the starting point for research. On the other hand, the convenient mechanical properties of the recycled material would preferably be preserved, as different conditions must be considered when performing the compounding.

Resultant behaviour of the compounded blend

Polymer blends can be either miscible or immiscible. The properties are affected depending on the nature of the blend. For miscible blends the following expression is defined:

P= P1x1+ P2x2 + Ix1x2

Where P represent the property to evaluate, x represents the volume fraction polymer species in the blend, and I is the interaction factor which may take positive, negative or zero values. The following is a graphical representation:

If I is positive, then the property is synergist. If the I value is negative, the property is non-synergistic. If the I value is zero, the property is additive. E.g. The mechanical properties of the recycled material are desired to be maintained as well as the easy processability of the virgin material. The hypothesis is that by blending, the final product will at least show additive properties regarding viscosity, in which this is better than the initial recycled polymer. [5]

If the blend is not miscible, this does not mean it is not compatible. Miscibility is when referred to the molecular-thermodynamically level. On the other hand, compatibility is an immiscible blend that still shows good macroscopically uniform physical properties caused by sufficiently strong interactions between the component polymers. This concept is important easier to solve the problem when the correct classification is given.

Ghost fishing
Ghost fishing: Ghost fishing occurs when fishing gear is lost or abandoned at sea. The gear can continue to catch fish, dolphins, whales, turtles, and other creatures as it drifts through the water and after it becomes snagged on the seabed. When driftnets were used on the High Seas, an estimated 1,000km of ghost nets were released each year into the North Pacific Ocean alone. (WWF)

The obtained results on the compounding of the recycled fishing nets with virgin HDPE showed that another important variable is the blend ratio, as this, in the current case, determines the impact of the desired property. The best overall results are that the 50/50 blend is the most optimal.

The strength properties of the recycled material are maintained and the processability conditions are improved. Nevertheless, these results were obtained at the selected compounding conditions and without any additives (as the HDPE/HDPE blend is already miscible). It is suggested to carry out the same process at different conditions are tested in order to create a robust database in which the prediction of blend referred.

One step at a time- when it comes to materials “impossible is nothing”

The fact that fishing nets come from various resources leads to various questions like: what are the mechanisms resulting in the degradation of the polymer? Are the degradation mechanisms uniform though all the material? How does this degradation affect the final properties? Can the degraded material be optimised by means of blending? Or are better results obtained if other processes are involved? Can other different materials (E.g. PA, PS, PET, PP) be compounded with with rHDPE?

Circular Ocean
In pursuit of innovative and sustainable solutions for marine plastic waste, the Circular Ocean project seeks to inspire enterprises and entrepreneurs to realise the hidden opportunities of discarded fishing nets and ropes in the Northern Periphery & Arctic (NPA) region. (www.circularocean.eu)

It is important to consider that when designing a product, it must exhibit a desired performance at a specific cost. The material and the manufacturing technique, as well as sustainability issues or the post-consumer life, must be considered. Fishing nets are an attractive possibility to enable a resource of HDPE for the industry. To achieve that, the development of a responsible material handling will simplify the biggest obstacles for material engineers when it comes to the recovery of materials. When the knowledge of the use of the material is known, such as its manufacturing to the use and disposal, the option of predicting its behaviour when recycled becomes easier.

Nevertheless, the continuous research of polymer molecular dynamics opens the door for the understanding of the performance limits of polymers and ways to recover them in order to become part of a cycle instead of a linear chain of consumption.

"The idea of a platform where curious, risk-taker minds from around the world collaborate caught my attention. The challenges that we’ve got, requires the teamwork that Matmach provides. As I read and continue reading the articles I am constantly reminded that the impossible is just the next milestone to overcome."
Amanda Salazar del Campo
Amanda Salazar del Campo
Materials Scientist and Engineer

References:

[1] Lebreton, L., Slat, B., Ferrari, F., Sainte-Rose, B., Aitken, J., Marthouse, R., Hajbane, S., Cunsolo, S., Schwarz, A., Levivier, A., Noble, K., Debeljak, P., Maral, H., Schoeneich-Argent, R., Brambini, R. and Reisser, “Evidence that the Great Pacific Garbage Patch is rapidly accumulating plastic”. Scientific Reports, 8(1). March 2018.
[2] European Commission, “New proposal will tackle marine litter and “ghost fishing”.
[3] UN, “FEATURE: UN’s mission to keep plastics out of oceans and marine life”.
[4] Plastix A/S.
[5] M. Stevens, Polymer Chemistry: An Introduction. Oxford University Press, 1999.
[6] A. Peacock, Handbook of Polyethylene: Structures: Properties, and Applications. Plastics Engineering, Taylor & Francis, 2000.
[7] H.Wenbing, Polymer Physics. Springer-Verlag Wien, 2013.
[8] L. Utracki, Polymer Blends Handbook. No. vb. 1 in Polymer Blends Handbook, Kluwer Academic Pub, 2002.

*This article is the work of the guest author shown above. The guest author is solely responsible for the accuracy and the legality of their content. The content of the article and the views expressed therein are solely those of this author and do not reflect the views of Matmatch or of any present or past employers, academic institutions, professional societies, or organizations the author is currently or was previously affiliated with.

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