Sustainability of polymer composites and its critical role in revolutionising wind power for green future

2.1. Mechanical recycling

This method involves the use of crushed waste composites as raw material for secondary raw materials. Thus, in the context of wind turbine blade this involves pre-cutting and crushing. This recycling method is classified into two basic methods/process, the first method involves breaking up the waste into particles of fine powder while the second method is to shred/crush the waste and the recycled material obtained by the two methods are used as fillers/reinforcements or as raw materials that go into cements/concrete. Few researchers used these techniques for instance, Palmer et al. [9] tried a closed-loop recycling on thermoset composites where in, the material was grinded and later used the recycled GFRP as an ideal replacement for pristine composites. Rotor blades with glass fibre reinforced thermosetting resin was recycled through the mechanical method and used in cement production [10]. On a whole mechanical recycling method is economical and simple and is widely used in recycling of fibre reinforced polymer composites. But on the downside the final product obtained through this method is of low value due to the material especially the fibre being damaged during the recovery process and the process’s inability to deliver longer fibres [2].

2.2. Thermal recycling

This method is further classified into three types namely, pyrolysis, fluidized bed recycling process and microwave pyrolysis. The first type essentially involves recovering fibres by decomposing the resin into organic small molecules using inert gas heat. Thus, clean fibres can be obtained by pyrolysis and oxidation and could be subsequently used as reinforcements and can compete with pristine new fibres [11]. Carbon fibres were found to be suitable for this process vis-à-vis the pristine quality of the fibres obtained after pyrolysis but Thomason et al. (2014) found that the strength of the glass fibre after pyrolysis was reduced significantly and there was limited strength re-generation from acid and silane post-treatments of the fibres. Fluidised bed method involves using air as fluidising gas in a fluidised bed reactor to decompose the matrix material through high temperature air heat flow and the heat thus generated recycles the fibre material. This method too has been found to degrade the glass fibre strength by about 50 % [12]. The last documented method under this category is the microwave pyrolysis which decomposes the resin in the composite material by microwave radiation in the microwave cavity. The resin could also be internally heated by absorbing microwave energy from the fibre which then decompose the resin faster. This method was found to be suitable for carbon fabric composites [13, 14].

2.3. Chemical recycling

This method involves the use of chemicals to chemically modify or decompose to make waste into materials that can be recycled. This method when used to recycle thermoset matrix composites is found to be harder and more expensive but the recycling per se is better vis-à-vis the mechanical method. Supercritical fluid method and solvolysis method are the two important methods under this recycling method. The first method refers to a special state in which the temperature and pressure of the fluid exceeds that of its critical temperature and pressure. At this stage, the fluid could decompose/degrade polymer waste, and water or alcohol is used as the decomposition medium [15]. In the context of this method Oliveux et al. recycled GFRP using subcritical water and it was seen that washing of the recycled GF contaminated by residual organic substance was necessary [16]. This method is advantageous on the virtue of it being a clean and less polluting method, in addition to its ability to reproduce clean and good performance fibres. But this method requires high temperature and high pressure and thus adding to the cost of equipments that can withstand such conditions. The solvolysis method involves de-polymerisation of the polymer using the chemical properties of the solvent under heating conditions. This method is good at obtaining carbon fibres from composites [17-19], the strength of the fibre was found to be good. This method has its drawback in the form of using toxic solvents and is expensive.

3.1. Thermoplastic as a suitable re-placement for thermoset resin

A wind turbine blade was manufactured using Elium as the resin by National Wind Technology Centre (NWTC) at National Renewable Energy Laboratory (NREL) in Denver, USA by R. E. Murray et al (2021). This blade was compared structurally (both static and fatigue) with an epoxy blade manufactured by TPI Composites. The thermoplastic blade was able to polymerise at room temperature and there was no requirement for a pre-heated or post-cure equipment thus reducing the manufacturing complexity and hence the costs. It was observed that the thermoplastic blade had a 3-11 % more displacement under the same static-load and this was attributed to differences in the fibreglass weight between the two blades and the adhesive used, though the thermoplastic blade was more flexible than its epoxy counterpart.

Fatigue test on both blades revealed less than 0.5 % variation in stiffness over a million cycles of loading, with no large scale observable degradation in structural or material properties degradation. Both the blade types had natural frequencies within 2.5 % in both the flatwise and edgewise directions while the thermoplastic blade had 5 to 7 times more structural damping. With these structural performances, which is on par with epoxy blades, Elium could serve as an ideal replacement for epoxy thus making recyclability of blade material a real possibility and hence making wind energy more sustainable part of the energy basket [29].

3.2. Recycling thermoplastic

The investigation carried out by D. S. Cousins et al. (2019). highlights the feasibility of using Elium since it can facilitate the recycling of wind turbine parts on a larger scale. In his study four recycling techniques were implemented namely, thermal decomposition of the polymer matrix, mechanical grinding, thermoforming, and dissolution. While thermal decomposition forms a process classified under the thermal recycling method, mechanical grinding and dissolution are classified under the broad category of mechanical and chemical recycling methods, respectively. Thermoforming involves heating thermoplastic composite sheets above the glass transition temperature of the polymer so that the shape can be altered in mould. Based on these four methods the research team weighed in the pro and cons of these methods vis-à-vis the recycled material.

Thermal decomposition (pyrolysis) required less energy to decompose the polymer matrix from the composites when compared to other recovery techniques. But the loss of the high-embedded-energy polymer could be counted as a disadvantage when this method is considered. Thermoplastic resin has an edge over its thermoset counterparts because the resin can be recovered by dissolution. In addition to this the glass fibre rovings recovered from the composite by this method retained its tensile strength with a reduction of only 12 %, while the mechanical property reduction was significant when pyrolysis (thermal decomposition) was employed to recover the fabric. Thermoforming enabled the straightening of a spar cap thus demonstrating the ability of thermoplastic composites ability to be downcycled into other products. An economic analysis on the feasibility of recycling thermoplastic composites and the recovery of the constituent materials revealed that they can replace the virgin materials within the supply chain. And a meaningful reduction in labour costs, obtaining a minimal knockdown on the polymer, and maximising the glass fibre for resale can be realised [22].

Thus, the possibility of up-scaling established recycling methods is a real time possibility. Adopting these methods, gradual replacement of thermosets and further validating its feasibility could pave the way for a fully integrated industrial cycle beginning with blade structure fabrication, installing them, recycling them and further reusing them in the same blade structure. Finally, an attempt towards green economy and an industry that is sustainable could be realised in the context of wind turbine sector.