Study on recycling wind turbine blades into reinforcement for filaments used in 3D printing

Heavy applications like power production through wind energy requires light weight but strong material like composites with customizable properties. When the lifetime of the wind turbine blades ends, the parts are dumped in landfills and results in environmental pollution. Natural fibers are great option for improving the biodegradability of the conventional plastic which also ends up in the landfills. Fused Filament Fabrication (FFF) is chosen to combine the benefits of both materials, as the technique is highly customizable and sustainable. Wind turbine blade wastes are recycled using mechanical grinding. Recycled Fiber glass (FG) material is tested for contamination with Fourier Transform Infrared Spectroscopy (FTIR). Wood fiber (WF) is also added in order to improve biodegradability of the materials. Filaments are produced using a single screw extruder with various combinations of 9 wt% fiber content and recycled pellets. Tensile test shows comparable performance of reinforced filaments with recycled Polypropylene (PP) filaments. 6% WF + 3% FG sample withstood up to 380 MPa Young’s modulus.


Introduction
In the energy crisis of the 21st century, renewable energy sources are the only option to produce more and cleaner energy.Among the various renewable energy production methods, wind energy is the cheapest and easiest to harvest electricity [1].Over the time the wind energy extraction is optimized into wind turbines.The concept of the wind turbines is to produce electricity using the wind to turn the turbine blades which operates dynamo to produce alternating current.Wind turbines are well suited for developing countries that have more windy areas because of its cost of running and maintenance.But the capital cost for a wind turbine is high.In order to increase the productivity of the wind turbine, it has to be installed in groups called wind farms.Wind turbines nowadays produce 2-3 MW energy on average [2].Wind turbines are made from various materials from metal to composites.The stem of the wind turbines is made from steel and concrete, since the stem won't be moved over the time and the conventional materials keeps the cost low.But the blades are made from the composite materials, in order to produce tailored properties high strength to weight ratio, low drag coefficient.The wind turbine blades are primarily made from Fiberglass (FG) and Epoxy resin [3][4][5].The Wind turbine blades (WTB) are extremely tough but they have a maximum lifespan of 25 years.In reality the WTB will be in operating conditions for about 15-20 years, due to various reasons for failure [6].After their life time the blades cannot be used for any material reuse purposes, since the shape and composition of the blades prevent these use cases.So, the blades get dumped in landfills for the lack of recycling methods [7].There are some innovative methods that are proposed for recycling the blades but no method is implemented in large scale.Some studies propose non-conventional designs reusing the WTB for structural purposes after their end of life.These studies show promising applications like furniture and roofing, but require remodeling the blades.Remodeling consumes same amount of time and work as new construction, so material recycling is required to implement WTB recycling in large-scale [8].Methods like Pyrolysis will increase the use case of the waste blades but the environmental impact of the methods is restricting the extensive implementation of the method.The optimizations in pyrolysis improve the emission control and the efficiency of energy recovery [9,10].Another method of using the recycled materials as alternatives for construction materials is explored and it is found that the composition of the composites increases the process complexity.So, the segmentation of the blade is proposed as a technique to use different parts of the blade to test its properties for using as structural elements [11].Mechanical grinding is a successful technique to recycle the FG from waste blades, since it reduces the wastes into powder form to be used as reinforcement in other thermoplastics in order to improve the performance of the materials.The study where the FG is used to reinforce the polypropylene material indicates the improvements in mechanical strength and the participation of FG in the improvement [12][13][14].Also using the FG reinforcements in Additive manufacturing is explored extensively by many researchers.Since, the additive manufacturing improves the efficiency and ease of the recycling process.A study performed using Fused deposition modelling (FDM) to reuse the recycled FG from WTB wastes as reinforcements for conventional materials.The study shows the performance of the reinforced filaments for FDM in terms of improvements in tensile strength [15].It also shows the possibilities of mechanical grinding in recycling of the WTB.The production of FG reinforced filaments opens wide range of applications for WTB recycling.The filament production using recycled material as reinforcements is explored well by researchers [16][17][18][19][20].The natural fibers such as wood fibers, starch fibers etc., as reinforcements helps to increase the biodegradability of the composite materials.Natural fiber reinforced filaments prove the possibilities but it has one major problem that is strength reduction [21,22].Since, most natural fibers are hydrophilic and all the thermoplastics are hydrophobic.So, the bonding is not complete which leaves much space inside the material that causes strength failures [23][24][25][26].By using FG as reinforcement for the natural fiber additions to the thermoplastics and fabricating the material in the 3D printable filament form, the recycling efficiency and applications of the WTB wastes increases.This study explores the methods of recycling the FG from WTB wastes and using it as reinforcement for the recycled plastic-natural fiber composites.

Materials and Methods
The study explores the mechanical grinding method to recycle the FG from the WTB, since the mechanical grinding is the cost effective and simple method among other methods.The process flow of the study methodology is represented in a flow chart in figure 1.The damaged/scrap WTB is either dumped in nearby landfill or shipped back to scrapyards where they stay for while then ends up in the landfills.The recovered FG is used as reinforcement for the conventional plastic pellets.In order to improve the biodegradability of the materials, natural fiber (wood fiber-WF) is blended with pellets.The extrusion method creates the 3D printable filaments, which can be used to print any desired products easily in a FDM printer.

Materials
2.1.1.Fiber glass.The FG material is obtained from a mid-size wind turbine whose blades are burnt in a lightning strike in a wind farm located in Tirunelveli, Tamilnadu.The damaged WTB from landfills are collected and the mechanical grinding is done using a grinding wheel.The waste WTB contains various impurities like dirt, plant matter, paint, etc.In order to avoid the segregation and cleaning processes along with chemical treatments, mechanical grinding method is used to collect the FG from the WTB parts directly.The ground powder is collected and sieved to remove irregular particles.
Composite with various strength and compositions used throughout the blade's structure.So, samples from different parts of the blade like Beam, Sheet and Resin are obtained and tested.The samples from Beam and Sheet are denoted as FG-B and FG-S respectively.The sample collected is shown in figure 2. Fiber glass powder is dried at 60 0 C for 2 hours before mixed with pellets for extrusion.FG additives are chosen in weight percentage (wt %) 0, 3, 6, 9.The 0 % FG indicates the 9 % presence of WF, 3 % FG with 6 % WF, 6 % FG with 3 % WF and 9 % with 0 % WF.Thus, the wt % of additives always stays 9 % in total.Two different mixtures of additives and pellets sample is shown in figure 3. The 9 % threshold is set for the extrusion machine, since the two different material mixing might clog the extrusion and the single step melting of the pellets will be easier with less percentage of additives.

Wood fiber.
The natural fiber to be added with plastic pellets is selected as wood fiber from Burma teak (Tectona grandis).The teak timber is one of the most preferred for its great strength and longevity.It is commonly used by many local carpenters for manufacturing good quality furniture like cot, chair, table, etc.Since the timber can only be processed by conventional manufacturing techniques, the wastage created from manufacturing is excessive.Also, the high density if the timber allows intricate wood works to be carved on it, which creates high amount of fine saw dust all over the workshop.The wood wastes are later collected and thrown in landfills.Some cases might use the saw dust for fertilizers and other uses them creating engineered wood.But the fine dust is usually left in landfills since the retrieval of the improper disposal is nearly impossible.So, it will be a suitable material for the reinforcement in the composite material.Studies show the compatibility of the wood fiber as reinforcement using several wood fibers [27,28].The existence of commercial wood/PLA filament for 3D printing indicates the significance of the wood fiber in composites.The saw dust from the timber is collected and the powder is screened and cleaned with filters.The saw dust is in fine powder from, so filters are enough to control the particle size.The saw dust is dried at 100 0 C for 2 hours before extrusion.Polypropylene (PP) is a non-toxic thermoplastic which is used in food packaging and medical industry.The bio compatibility of the PP helps in the disposal of the material after its life cycle.However, the recycling ability of the material is well studied by researchers [12,23,29].So, the recycled PP is very effective in the composite field as matrix material for various fiber reinforcements and it is a very good choice for this study.Recycled PP pellets are obtained from a recycling plant (Ambika Plastics near Chennai, Tamilnadu).The recycled pellets were dyed in black colour from the recycling plant.It is a common occurrence in the recycling industries, because the pure colour cannot be obtained after recycling due to contaminations.The diameter of the pellets' cross section varies from 0.8 mm to 3 mm.Length of the pellets varies from 3 mm to 6 mm.The pellets are dried at 60 0 C for 2 hours to remove any moisture content.Since the plastic is recycled, the operating temperature of the extruder should be decreased to 200 0 C due to the thermal degradation of the plastic.

FTIR.
The fiber glass samples are tested for any contamination or moisture absorption to predict the material behaviour during extrusion and printing.The FTIR results are compared with the virgin fiber glass without any additives from literature, in order to find the type of contamination [30].FTIR helps to find out any organic substance in the sample by calculating their absorbance of the IR waves.
The different wavelength absorptions show different organic bonds in the test material.

Extrusion.
The mixed materials are extruded into filaments using an in-house developed single screw filament extruder machine.The design of the machine is based on the 'Recyclebot' but all the parts are redesigned for improved functionality and metric standards.The machine is built using local available parts and 3D printed parts, which makes the machine highly scalable as shown in figure 4. The additives are mixed with plastic pellets thoroughly and preheated before fed into the extruder, while the heaters are set at 200 0 C along with preheating time of 10 mins for the nozzle to reach proper temperature.No thermal degradation is observed from the extrusion due to the additives.The filaments are extruded properly at the rate of 2 m per minute.

3D Printing.
The reinforced filament then used in a FDM machine (Creality Ender 3 pro) with the CAD model designed for the prints.The print parameters are set in the slicing software according to the base material PP and test specimens.Due to inconsistency in the filament diameter printing process was often interrupted.The filament diameter variation causes the print head stepper motor to slip, few layers were printed but the flow of the filament is cut in the nozzle and resulting in clogging.The print speed slowed down to check the filament flow but the print didn't get affected.The low surface finish of the filament is the main cause for the improper printing.

Tensile Testing.
The fabricated filament is tested directly in the Tensile testing machine (Initial gauge length -100 mm, feed rate -1 mm/s, max load -20 kN) with the help of specially prepared clamps for mounting the filaments without breaking them.This method of testing the filaments is carried out as pilot study for composites before they can be printed on the FDM machines [15,31].A clamp setup was designed for mounting the filaments in the vice.The filaments are tested with longitudinal load along the gauge length for its tensile strength and modulus of elasticity.The test parameters and test results are used to plot stress vs strain graphs and modulus of elasticity.There were 5 samples for each composition tested along with a recycled PP filament that is extruded from the same extruder machine as the samples and a commercial grade PLA filament.

FTIR results
The primary component of the fiber glass is sand and it is superheated to create glass and fiberglass.So, the figure 5 shows prominent peak in samples' graphs as Si-O-Si bond.The peak shows up on the samples from Beam, Sheet and also in the Resin at 1050 cm -1 .Since the epoxy resin is mixed with some fiber glass to create the strength needed in the packing between sheets and beam.The use of epoxy resin in the samples from Beam and Sheets are evident from the peak at 2800 -3050 cm -1 , which indicates the CH3 and CH2 bonds in the sample.The final peak at 3700 cm -1 indicates the O-H bonding, which confirms the presence of the plastic reinforcements in the composite.

Visual inspection
The images from the optical microscopes are taken at 10x zoom level are shown in figure 6.The images show the individual fiber glass particles clearly.The fiber length is averaging in 1.2 µm.And also, from the images the resin particles can be seen clumping together.It is happening because of the thermal degradation of the resin particles, since the resin is a thermosetting polymer and the grinding process heated the particles while shaving them out.The visual inspection shows that without chemical treatment, the resin particles cannot be completely removed from the FG samples.The resin particles may affect the bonding of the FG with the PP matrix, resulting in the poor surface finish and low strength of the filaments.

Filament quality
The filament quality is directly affected by two main factors, namely Extrusion pressure and Puller speed.The extrusion pressure is controlled by the extruder motor speed.But the extruder motor speed doesn't take effect immediately like puller motor speed.The extruder motor speed change creates extrusion pressure difference near the nozzle after some time delay.That's why it affects indirectly.The extrusion pressure pushes the filament through the nozzle and the puller motor pulls the filament while it is cooled by cooling fans.The push and pull balance is very important in determining the diameter of the filament.Also, the filament has to be cooled below its glass transition temperature before reaching the puller, otherwise the puller rollers will deform the filament like figure 7(a).While increasing the filament diameter by reducing the puller motor speed or increasing the extrusion pressure, the filament quality started to drop dramatically.The filament became filled with void and brittle after the alteration.When the conditions are reversed and the perfect balance between puller speed and extruder speed is achieved, the diameter of the filament is back to around 1.6 mm.The problem is recreated multiple times to find out the cause and every time the diameter is start to increase the improper cooling and brittle filament is obtained.But the decrease in diameter is happening smoothly at every speed combination.
So, it is found that the diameter inconsistency is due to the improper bonding between the fiber and matrix.The fine powders of FG and WF didn't stick with the PP pellets thoroughly and the particles didn't go through the extrusion chamber in the same speed as the pellets.As a result, the initial extrusion brought low concentration of reinforcement with them.So, the filament quality resembled the pure PP filaments.
After the extrusion built up perfectly, the mixture is obtained perfectly and the filament quality is exact representation of the composite.After the extrusion pressure drops at the end of the extrusion, the remaining reinforcements carried by the remaining pellets.So, the filament is very rough, filled with void and brittle at this point.

Tensile test results
Figure 9 shows the tensile test results of the reinforcement samples and pure samples.The decrease in strength and the plastic deformation shows the improper bonding between the fiber and matrix in the material.But the behaviour of the recycled PP is also very different from the commercial PLA filament despite having no fiber in the material.The main difference between those materials came down to the thickness of the filaments.The commercial filament is thicker (1.75 mm) than the recycled PP filament which meant that the filament might slip on the clamp without providing good tension before the test starts.After the test initiated the filament began to grip.The material started to plastically deform at 0.05%.But the recycled PP filament and reinforced filaments are thinner (around 1.6 mm) so they provided enough tension from the beginning.But the reinforced samples failed after 0.1% strain, the recycled PP filament actually didn't fail even at 215% strain.The test is stopped after 255% elongation.The graph is in 'serrated' pattern which indicates that some particles prevent failure.Which is not desired in the working products.Unwanted elongation might cause strangling or stinging effects in material failure.So, the samples might be weaker in strength performance but the material behaviour is predictable and desirable.Among the samples, the 6% WF+3% FG samples show the best results in terms of strength and the Young's modulus of the material is also higher than the rest of the samples.Figure 10(b) shows that it has modulus of elasticity as 378 MPa which is the closest one to the recycled PP (386 MPa).It is due to the minimal amount of FG present in the material.The WF is reinforced by the FG particles, while the WF particles bond with the matrix it also takes the FG with it.Because the 9% WF samples itself couldn't create good bonding with the matrix.And same can be said for the 9% FG samples.The higher concentration of the FG also didn't help the material.However further studies in this regard is needed.It is found that in reinforcement the natural fiber has to be more than the FG content.

Conclusion
Wind turbine blade waste was effectively used as a reinforcement in making filaments for FDM process Since the thermosetting nature of the epoxy resin affects the bonding of the fiber and matrix materials, it is challenging to determine the true potential of the recycling method.So, further in-depth studies are required to improve the efficiency of the process.6% WF+3% FG sample shows promising tensile strength good surface quality.It also shows that the natural fiber needs to be in higher concentration than the FG reinforcement, which is a critical finding in this study.

Future work and Applications
Double melt method and NaOH treatments for FG and WF will be investigated.Further commercial products will be fabricated as a case study.The applications of these reinforced and recycled filaments in 3D printing are endless, yet the ease of access of the FDM machines makes these filaments suitable for small-scale companies and personal use.Studies shows promising results while making small machine parts and household items [20].However, these recycling studies help to accelerate conventional processes to adapt recycled materials.This study helps to ensure the feasibility of simple mechanical recycling of WTB and the conversion into FDM filaments is possible and useful in personal and industrial level.

Figure 2 .Figure 3 .
Figure 2. Damaged edge of the wind turbine blade

Figure 4 .
Figure 4. Single screw extruder with parts marked

Figure 5 .
Figure 5. Average results of FTIR test from various parts of WTB

Figure 8 (
b) shows the diameter measurement using Vernier calliper.Lower fiber content might improve the situation, since the pure PP filaments can be extruded at any diameter without losing the filament quality.The diameter measurements were made multiple times in a 10 m filament.The diameter distribution of the filaments throughout its length is shown in figure in a form of a histogram chart 8(a).The surface quality of the filament varies throughout the extrusion 3 times distinctively in all the ratios of fiber reinforcements.The initial, mid and final stages of the given mixture vary by significant margin.