Recycling Glass Fiber from Polyurethane Composite by Pyrolysis Strategy with High Mechanical Properties

With the rapid advancement of the wind power industry, the recycling of retired wind turbine blades and the regeneration of glass fibers have become urgent environmental and economic issues. In this article, a two-step pyrolysis strategy was put forward, in which the relationship between the pyrolysis parameters and the properties of the recycled glass fibers, including the surface morphology, defects, and mechanical properties were demonstrated. We found that pyrolyzing the composites at 500°C under high-temperature water vapor atmosphere to recover the glass fibers, and oxidizing at 450°C to remove the residual carbon of fibers is the optimal choice. In this way, not only can the fibers be recovered from the waste composites, but also the mechanical properties of the fibers can be retained while removing the residual carbon on the surface, which provides a guarantee for subsequent high-value reuse of waste fan blade composite materials.


Introduction
With the rapid advancement of the wind power industry, the recycling of retired wind turbine blades and the reasonable disposal of waste components have become urgent environmental and economic issues [1][2][3][4].Considering the trend of large-scale and lightweight development of wind turbine blades, the manufacturing materials for blades have evolved from the original linen cloth covered with wooden boards to steel, aluminum alloys, to fiber-reinforced composite materials.Because the resin matrix used is generally thermoset polymer and resin, the three-dimensional cross-linking network structure makes it difficult to degrade naturally.Once cross-linked, it is difficult to melt or reshape.In addition, the strong bonding between the fiber and the resin matrix makes it difficult to extract fiber with low costs [5,6].Given the rapid increase of waste glass fiber reinforced composites in fan blades, the inability of natural degradation, and the difficulty of high-value utilization, pyrolysis recovery technology, as a relatively mature mainstream composite material recovery technology, has become the main fan blade composite material recovery method using world widely because of its advantages of scaled, cleanness, and efficiency.The pyrolysis strategy of composites such as fan blades can decompose the organic matter (resin matrix) into pyrolysis oil and/or gas at a certain temperature under anaerobic conditions, thereby extracting recycled glass fibers.Besides, there may be residual carbon existence on the surface of the recovered fibers [7][8][9][10].
According to Ucomposite's analysis [11] on the Carbon footprint and environmental impact life-cycle assessment (LCA) of recycled fiber materials, the environmental impact of producing 1 ton of recycled glass fiber products is about 60 kg CO2 equivalent, of which 49% is from the transportation of raw materials, 23% is from the heating production process of fuel oil, and the CO2 emission of producing 1 ton of raw glass fiber is about 1.9 tons.In addition, the emissions of SO2, NOx, and particles are approximately 0.02 kg, 0.25 kg, and 26 ppm, respectively, mainly comings from the transportation process.The power consumption in the production process is only 60 kWh.It can be seen that the thermal decomposition and recovery treatment of fan blades is a clean production process with low energy consumption and high material utilization rate, good environmental protection, and occupying important ecological value.The ReFiber company [12] successfully recovered composite from fan blades through gasification and pyrolysis in a rotary furnace under 500℃ oxygen-free environment.The recovered glass fiber had a thermal conductivity of 0.041 W/(m K), which was close to commercial insulators (0.037 W/(m K)).The prepared polymer material exhibited excellent sound absorption ability between 100 and 6400 Hz: a material with a thickness of 30 mm had an average sound absorption coefficient of 0.8, which was comparable to commercially available sound insulation boards [13].Replacing traditional aluminum-based and mineral wool soundproofing materials with recycled glass fiber as the core of the sound-absorbing material, the composites was only 30 kg/m and have been successfully used in some roads in Denmark.In addition to recycled fibers, pyrolysis oil or pyrolysis gas can also be generated during the thermal recovery process of composite materials for fan blades, with a calorific value (30-40 MJ/kg) [14] equivalent to that of diesel resin.The continuous composites recycling and processing equipment has been developed and manufactured by the group of Wenjiang Ding, possessing an annual processing capacity of 1500 tons with high-value retention of the fibers.By tailoring the air flow and designing the energy pathway, the energy released by resin decomposing can be returned to the internal pyrolysis system through a cyclic heat-utilization system, achieving low energy consumption operation of the recycling equipment, with a unit energy consumption of only 0.5 kWh/kg or so.In addition, the resin cracking ratio and fiber recovery ratio were both higher than 95%, and the tensile strength retention rate of the recycled glass fiber was higher than 92%.
This study mainly focuses on the clean and efficient pyrolysis recovery technology of fan blade composite materials.High-temperature steam is used as the heat transfer medium and protective atmosphere, while the effects of pyrolysis temperature, oxidation temperature, and treatment time were studied, to demonstrate the relationship between the pyrolysis parameters and the properties of the recycled glass fibers, including the microstructure, surface, and mechanical performances.By optimizing the recycling process parameters and exploring the feasibility of high-performance glass fiber recycling, it is expected to promote the efficient regeneration and high-value reuse of waste fan blade composite materials.

Raw Materials
Wasted fan blade composite material, 2000*1000*10 mm, was provided by Fengyang Aiers Light Alloy Precision Forming Co., Ltd., the main components of composite materials include polyurethane resin and glass fiber.

Synthesis of recycled glass fiber
The recycle of glass fiber by pyrolysis from fan blade composites was carried out in the self-developed composite material continuous high-temperature pyrolysis recovery equipment.High-temperature steam is used as the heat transfer and protection medium, when the cracking furnace and decarbonization zone of the recycling equipment is heated to specified temperature, the composite material is fed into the equipment at a certain rate.After the sample processing is completed and naturally cooled, recycled glass fiber rGF is obtained and named rGF-X-Y.Among them, X is the resin cracking temperature, and Y is the decarbonization temperature.

Characterization
Field-emission Scanning electron microscopy (FE-SEM, Hitachi S-4800) was used to characterize the morphology of the recycled glass fiber samples, and the acceleration voltage was 10.0 kV.Before shooting, the sample needs to be ultrasonically dispersed in ethanol and dripped onto conductive adhesive, followed by a 60-second gold spray treatment to increase the conductivity.The crystal structure of recycled glass fiber samples was characterized by a multi-functional powder Diffractometer (XRD, Rigaku D).Before testing, the glass fiber should be ground into a powder with a particle size of 200 mesh and compacted with glass flakes.The specific testing conditions are: Cu K is used α Target, λ= 0.15418 nm, with a tube voltage of 35 kV and a tube current of 200 mA on the scanning surface.The scanning range is 5-90°, the scanning speed is 5°/min, and the step length is 0.02 seconds.The graphitized structure of recycled glass fiber samples was characterized using a dispersion-type micro-Raman spectrometer (RAMAN, Senterra R200-L).The specific test conditions were: Ar ion laser, voltage 10 kV, power 10 mW, wavelength 532 nm, spot size 2 mm, irradiation time 10 s, and test wavenumber range from 500 to 3000 cm-1.Use a universal testing machine (BTC-T1-FR020 TN.A50, Zwick) to test the tensile properties of recycled glass fibers according to ASTM D-3379 standard.Among them, place the fiber monofilament in the middle of the sample lining slot, with a length of 25 ± 0.5 mm.Use adhesive to firmly bond the sample with the sample lining.When clamping the sample liner, make the sample coaxial with the loading axis, set the cross-head movement speed range of the stretching machine to 5 mm • min-1, and pre-apply a tension of 0 N. Test 30 samples from each sample, and calculate the average value to obtain the fiber mechanical properties.

Effect of pyrolysis temperature on the physicochemical properties of glass fibers
The crystal forms of recycled glass fibers treated at different pyrolysis temperatures were analyzed by X-ray diffraction.It can be seen from Figure 1 that when the cracking temperature is 450℃, the crystal form of the recovered glass fiber is weak.The diffraction peaks appearing near 26° correspond to quartz phase SiO2 (No.82-1574) and amorphous carbon produced by resin cracking, while the weak diffraction peaks appearing at 46° and 71° are diffraction peaks of related metal oxide impurities (CaO, Al2O3, Na2O, MgO).As the cracking temperature increases, the glass fiber undergoes a phase reconstruction process, with oxide impurities evaporated and O-Si-O bonds gradually breaking and restructuring.The quartz phase SiO2 crystal form becomes much clear, and amorphous carbon undergoes graphitization transformation.With the increase of pyrolysis temperature, the intensification of molecular Thermal motion will lead to a large number of poor crystallinity and amorphous crystalline phase to stable phase structure changes.However, when the cracking temperature is too high, the link structure in the glass fiber will decompose, which will hurt its mechanical properties.Figure 2 shows the microstructure of recycled glass fibers treated at different cracking temperatures under scanning electron microscopy.It can be seen that the surface morphology and size structure of the fibers undergo significant changes at different cracking temperatures.Figure 2(a) shows the SEM image of recycled glass fibers treated at a cracking temperature of 450 ℃.It can be seen that the polyurethane resin undergoes cracking, but the resin products with incomplete cracking will form amorphous carbon and adhere to the fiber surface.As the cracking temperature increases to 500 ℃, shown in Figure 2(b), resin has almost completely cracked, and a small amount of amorphous carbon appears on the surface of the fibers.However, when the cracking temperature rises to 550 ℃, as shown in Figure 2(c), although some residual carbon still adheres to the surface of the fiber, its fiber diameter gradually expands to about 20 um.This is because raising the heat treatment temperature will lead to an increase in the thermal expansion coefficient of glass fibers.Taking into account the changes in fiber phase morphology and the removal of impurities, the fibers will gradually deform during the expansion process, thereby affecting their mechanical properties.Excessive temperatures can even lead to fiber breakage and destruction.Therefore, the appropriate pyrolysis temperature is crucial for recovery and obtaining highperformance glass fibers.

Effect of decarbonization temperature on the physical and chemical properties of glass fibers
From the discussion in the previous section, it can be seen that when the cracking temperature reaches 500 ℃, the polyurethane resin matrix in the composite material of the fan blade will decompose, and its products include small molecule carbon compounds, amorphous carbon, and residual carbon.The presence of these cracking products will affect the cleanliness of recycled glass fibers, thereby adversely affecting the interface between fibers/matrix in the subsequent construction of glass fiber composite materials.Therefore, oxidation is necessary to remove residual carbon on the fiber surface to obtain clean glass fibers.However, excessive oxidation temperature for removing residual carbon can affect the crystal form and structure of fibers while removing residual carbon, thereby affecting their mechanical properties.Therefore, in this section, it is necessary to explore the oxidation temperature and seek the optimal process conditions.Figure 3 shows the microstructure of glass fibers treated at different oxidation temperatures under scanning electron microscopy after being subjected to 500℃ cracking temperature treatment.When the oxidation temperature is 400℃, as shown in Figure 3(a), there are many residual substances on the surface of glass fibers, resulting in poor surface smoothness and roughness.While the oxidation temperature is raised to 450℃ in Figure 3(b), only a small amount of impurities adhere to the fiber surface, and the residual carbon content on the fiber surface significantly decreases.However, when the oxidation temperature continues to rise to 500 ℃, as shown in Figure 3(c), although the surface cleanliness of the fiber is greatly improved and impurities such as residual carbon are hardly visible, small defects appear on its surface.This happens because of the reaction between silicon element in the glass fiber and oxygen in the oxygen environment, forming quartz phase SiO2 oxide through bond breaking and recombination; In addition, a small number of metal impurities on the surface of the fiber undergo oxidation reactions, which combine to form metal oxides, resulting in changes in the microscopic morphology of the glass fiber.Therefore, it is necessary to characterize and analyze the surface structure and defect state of the fiber through Raman spectroscopy to understand the structural evolution behavior of glass fiber and its surface residual carbon during the oxidation and removal of residual carbon.Figure 4 shows the Raman spectra of recycled glass fibers obtained under various oxidation temperature conditions.Normally, characteristic peaks near 1320 /cm, called D peak, represents the defective degree of carbon; while the peak locating at 1580 /cm(G peak) represents its graphitization structure.The ratio of D peak to G peak reflects the degree of graphitization of the material.It can be seen that all three glass fiber samples exhibit obvious characteristic peaks near 1320 /cm, and there are almost no corresponding peaks near 1580 /cm, indicating that the residual carbon on the surface of the glass fiber is a high defect structure.In addition, as the oxidation temperature increases, the height and area of the D peak gradually increase, indicating an enhanced in-plane stretching vibration of sp2 carbon atoms, leading to an increase in defect severity.On the one hand, it represents that residual carbon exists in a more defective state, which is easy to be oxidized and removed; on the other hand, it also implies that the defect structure of glass fiber will gradually increase, which may have an adverse impact on its mechanical properties.

Characterization of mechanical properties of recycled glass fibers
To obtain recycled glass fibers with high cleanliness and mechanical properties is the core of high-value recycling.Here, single filament tensile strength tests are conducted on glass fibers under various pyrolysis conditions.Due to the loss of sizing agent protection on the surface of recycled glass fibers and the presence of structural defects after oxidation treatment, to reduce the impact of experimental errors on their mechanical properties, it is necessary to conduct at least 5 tests on each group of samples and calculate their tensile properties by taking the average value.From Figure 5, it can be seen that as the cracking temperature increases, the tensile strength of glass fibers shows a significant downward trend, and their elongation also gradually decreases.This phenomenon can be attributed to the fact that the heat treatment temperature during the process of cracking the resin matrix will simultaneously cause phase reconstruction of the glass fiber.During the bond-breaking process, the mechanical strength of oxides with different crystal forms decreases while their brittleness increases, leading to a decrease in their mechanical properties.In addition, comparing glass fibers with different oxidation temperatures, it can be found that their elongation remains unchanged while their tensile strength sharply decreases.This is because oxygen in the air atmosphere will undergo an oxidation reaction with glass fibers at a certain temperature, damaging the structure of glass fibers and leaving many defects, leading to a decrease in their tensile properties.Although a lower cracking temperature can ensure a larger retention rate of tensile properties, and the oxidation temperature will not affect its elongation, under this cracking process condition, there are more residual carbon and amorphous cracking products on the fiber surface.The presence of these cracking products will affect the subsequent surface sizing of glass fibers, as well as the interface and comprehensive mechanical properties of re-manufactured composite materials.Therefore, considering the surface cleanliness and tensile strength performance of recycled glass fibers, as well as the subsequent application of composite materials, it is necessary to conduct a comprehensive design of the cracking temperature and oxidation temperature to determine the optimal process conditions.

Figure 5.
Tensile properties of glass fibers under various cracking and recovery process conditions.
Table 1 shows the mechanical properties of recycled glass fiber obtained through pyrolysis.In this work, tensile strength is measured in the range of 400-828 MPa, which is larger than the values in the table.This may happen because we use low temperature during the whole process.The combination of pyrolysis temperature of 450℃ and decarbonization temperature of 400℃ shows the best strength.However, higher mechanical strength does not equal to better performance, residual carbon may still exist because of not efficient processing time, this reminds researchers to look for the balance between mechanical strength and performance of recycled fibers.

Conclusion
The pyrolysis and recovery process of glass fibers in composite materials for fan blades was studied, and the effects of pyrolysis temperature and oxidation temperature on the crystal structure, defects, surface structure, and tensile properties of glass fibers were studied.By optimizing the pyrolysis and recovery process, recycled glass fibers with high surface cleanliness and mechanical properties were successfully obtained.The design and optimization of the pyrolysis and recycling process for fan blades can achieve efficient recycling and high-value regeneration of glass fibers.In summary, it is critical to balance the temperature at which fibers are recovered from waste with the temperature at which carbon residue is removed from the fiber surface by oxidation.Specifically, 500 ℃ and 450 ℃ were used to crack the composites to recover glass fibers and remove carbon residue, respectively.Too low a temperature will result in insufficient cracking of the resin and incomplete removal of the carbon residue, but too high a temperature will greatly damage the prior properties and will be harmful subsequent reuse.It can effectively and properly treat composite material waste, reduce the use cost of fibers, improve the utilization value of fibers, and provide effective reference for the subsequent application of fiber-reinforced composite materials, with broad application prospects, At the same time, it also conforms to the national doublecarbon policy and the overall layout of economic and social development and ecological civilization construction.

Figure 1 .
Figure 1.X-ray diffraction patterns of glass fibers at different pyrolysis temperatures.

Figure 4 .
Figure 4. Raman spectra of glass fibers at different oxidation temperatures.

Table 1 .
Mechanical properties of recycled glass fiber obtained through pyrolysis.