Development and Application of Medium-reactivity Epoxy Infusion Resin System in Large-scale Wind Turbine Blades

In view of the requirement of cost reduction and efficiency increase for large-scale wind turbine blades, a medium-reactivity epoxy infusion resin system was developed, which contained 20% fast curing agent and 80% slow curing agent. The properties of medium-reactivity epoxy infusion resin system were evaluated via physical and chemical analysis, application process ability analysis, resin clear casting mechanical properties analysis, and laminate specimens mechanical properties analysis. The results indicated that the mechanical properties of this medium-reactivity epoxy infusion resin system can meet the requirements of blade design, and the pre-curing time of the resin was reduced from 3.0h to 2.3h. The application process ability of medium-reactivity epoxy infusion resin system in large-scale wind turbine blade was also verified. The blade parts pre-curing time was shortened by 0.5-1 hour, which meets the target of blade cost reduction and efficiency increase.


Introduction
Due to the world economy's considerable dependence on fossil fuels, the globe needs to shift to renewable energies to mitigate the challenge of climate change.Among these different renewable resources, due to its huge potential and constantly improving technological advances, wind energy has been witnessed its rapid growth both onshore and offshore areas due to its huge potential and constantly improving technological advances [1][2], and it is expected to make a significant contribution to the low-carbon energy transition.According to Global Wind Report 2022 released by Global Wind Energy Council (GWEC) [3]. Figure 1 depicts the growth trend of wind power demand in 2020-2030.The price policy of wind power has promoted the rapid development of China's wind power industry, the cumulative installed capacity had increased from 31.07 million kW (2010) to 328 million kW by the end of 2021, and the levelised cost of electricity (LCOE) for onshore wind power had decreased by 54% in 2020 compared to 2010 [4].With the decline of power generation costs, renewable energy subsidies to wind have been cancelled since 2021, and China's 14th Five Year Plan has proposed grid parity with traditional coal-fired power generation [4] .The wind power industry has continuously reduced prices significantly in the past two years, and cost-oriented policies have gone hand in hand with quality-oriented policies, exposing the previously protected wind power industry to increasingly fierce market competition, all of which have laid the foundation for China's low-cost energy transition [5].Wind turbine blade is a key component of wind turbine system, and its production cost accounts for about 20% of the cost of wind turbine, so the cost reduction of wind turbine blade naturally becomes an important direction of wind turbine cost control.DOE wind program has put wind turbine blade manufacturing as the focus of wind turbine cost reduction [6].This requires wind turbine blade manufacturers on the one hand to reduce the price of raw materials, on the other hand to improve the production efficiency of the blade, the comprehensive use of the two in order to maximize the cost reduction and efficiency.
At present, epoxy infusion resin has been widely used in the wind turbine blades.Domestic wind turbine blades generally use slow-reactivity epoxy infusion resin system.In the production of large-scale wind turbine blades, it has the advantages of slow viscosity build-up in the early stage, long pot life, easy to use, etc., to achieve the infusion process of large-scale blades [7], but there are also shortcomings of slow-reactivity and low-efficiency.At present, some manufacturers also try to increase the cure speed and efficiency by increasing the pre-curing temperature, which will easily cause defects such as whitening of the surface of FRP, and have influence on the microscopic interface of composite materials.[8] In addition, many optimization studies on epoxy resin for wind turbine blades are mostly aimed at improving mechanical properties.Ren Liubo [9] characterized the mechanical and thermal properties of the developed vacuum infused epoxy resin system for wind turbine blades.The compatibility of epoxy resin and glass fiber was characterized by the simulation experiment of wind blade root section infusion and the performance test of laminate.The results show that the developed epoxy resin can be used in the preparation of wind turbine blades.Deng Shuanghui [10]compared and analyzed the basic characteristics, viscosity change profiles, exothermal peak, gelation time and mechanical properties of the domestic high-performance epoxy resin system vs. two imported products.These studies are only limited to the mechanical and physical and chemical properties of the material itself, while ignoring the application capability of epoxy resin acting as the main material for wind turbine blades in the production process.This is the key to reducing the production cost of wind turbine blades and improving production efficiency.In addition, most of these studies are limited to the study of small neat resin samples in the laboratory to reflect the improvement effect of the material, but for practical applications performances, it is still need to test and verify on the pilot production of the whole large-scale wind turbine blades.These studies are only limited to the mechanical and physical and chemical properties of the material itself, and ignore the manufacturability of epoxy resin as the main material of wind power blade in the production process, which is the key to reduce the production cost of wind power blade and improve the production efficiency.In addition, most of these studies are limited to the study of small pure resin samples in the laboratory to reflect the improvement effect of the material, and for practical applications, it is also necessary to test and verify the pilot production of the whole large wind turbine blade.
In this paper, a new solution was proposed.A medium-reactivity epoxy infusion resin system in large-scale wind turbine blades was jointly developed by our company and suppliers, the development process of the medium-reactivity epoxy resin system was systematically analyzed, and the performance of medium-reactivity epoxy infusion resin system was comprehensively tested in comparison with conventional slow curing system.The application process ability was verified on the blade components.The results showed that the medium-reactivity epoxy infusion resin system can speed up the curing speed of the blade, improve the production efficiency of the blade and provide a new workable solution for the cost reduction and efficiency increase of wind turbine blade

Experimental method
Medium-reactivity epoxy infusion resin system was prepared by mixing fast-reactivity and slow-reactivity epoxy infusion resin systems at different ratios, and they were characterized comprehensively and compared with those of pure slow-reactivity epoxy infusion resin system.The characterization method was as follows: Exothermic peak curve is tested according to DIN 16945, which refers to that in the laboratory environment, after the epoxy resin matrix is added to the curing agent in proportion to a total weight of 100g, the change of exothermic temperature at the center point of the mixture is recorded with a digital recorder, and the highest temperature reached during this test is the exothermic peak temperature.
Mixture viscosity is tested in accordance with the standard ISO3219; The glass transition temperature (Tg) and curing speed were conducted by Tg growth under different curing times, and measured in accordance with the standard ISO11357-2.
Viscosity growth curve test equipment is rheometer, the test standard is ISO3219; The test standards for tensile and bending property of the resin clearcast were ISO 527-2 and ISO 178, respectively.
The preparation method of composite materials: evenly apply an appropriate amount of release agent on the vacuum infusion table.After the solvent of the release agent is fully volatilized, the glass fabric is laid according to the structural requirements, auxiliary materials such as peel ply, flow media net, flow media hose and vacuum film are laid up and and the whole package were sealed in vacumm bag and vacuumed.Mix an appropriate amount of epoxy resin matrix and curing agent, and infuse after 10 minutes of defoaming.When the glass fabric was fully impregnated by resin, stop infusion.Pre-cure the parts at 50℃ for 4 h and post-cure at 75℃ for 6 h.
Composite material sample test: E8-UD1250 (epoxy type silane sizing) and E8-BIAX1200 (epoxy type silane sizing) fabrics were impregnated and cured with medium-reactivity epoxy infusion resin system and slow-reactivity epoxy infusion resin system respectively.The mechanical properties of tensile, compressive and shear were tested according to ISO527, ISO14126 and ASTM D7078 respectively.

Development of medium-reactivity curing agent
Slow-reactivity epoxy infusion resin system is widely used in the industry, the product has a long pot life, but at the same time the product curing speed is slow.Fast-reactivity epoxy infusion resin system is suitable for small parts production or component repair, the product has the characteristics of rapid curing, but its operation time is relatively short, so it is not suitable for medium and large parts (such as spar cap and shell) infusion in the wind turbine blades.
The development of medium-reactivity curing agent is to mix fast-reactivity curing agent and slow-reactivity curing agent in a certain proportion, and select the curing agent with appropriate pot life (the window of process time) through infusion test and exothermic peak test.The pot life of curing agents at different fast-reactivity curing agent proportions mixed with resin is shown in table 3. The test method is the exothermic peak test of the mixture of resin and curing agent (100 g, mixing ratio, epoxy resin: curing agent = 100:32).According to the peak temperature and peak time, combined with the process time requirements of the blade production, the following five types of large component tests were conducted on the two medium-reactivity epoxy infusion resin systems with the proportion of rapid curing agent accounting for 30% and 20%, respectively, to verify the influence of medium-reactivity infusion epoxy resin system on the fiber wetting effect, infusion speed, exothermic peak temperature and pre-curing time: (1) To simulate the lay-up of the bolt embedded type blade root section, the above two resin systems were used to produce the root section parts with a length of 120 cm and width of 10 bolt bushings.Both of these two resin systems could finish the embedded root section parts infusion in 180 min, and the exothermic peak temperature appeared when heating at 50 ℃ for 90-100 min, and no PVC wedge was burned to collapse after curing.
(2) To simulate the lay-up of the drilling hole type blade root section, 123 layers of fibre fabric (including 87 layers of 1500 g/m 2 triaxial fabric) with 170cm length and 100cm width root structural parts were infused.These two parts could be impregnated thoroughly after 180 min of infusion, and the exothermic peak temperature appeared in 10~20 min after heating at 40 ℃.
(3) Simulated the lay-up of balsa sandwich structure, the core material thickness is 2 inches, 15 layers of fabrics were laid on top and bottom side，the panel size is 110cm *110cm.Both of these two resin systems have good infusion quality, and the exothermic peak temperature of the part surface does not exceed 85℃, which is lower than the maximum use temperature of the mold (Tg value 110℃).The total infusion time of medium-reactivity curing agent with the proportion of fast-reactivity curing agent accounting for 20% is about 20-25 min shorter than that of containing 30% fast-reactivity curing agent.
(4) Simulate the lay-up of PVC sandwich structure, the core material thickness is 30 mm, 15 layers of fabric were laid on top and bottom side, the panel size is 70cm * 70 cm.Both of these two resin systems have good infusion quality, the exothermic peak temperature and infusion time are basically consistent with Balsa structural parts.
(5) To simulate the lay-up of spar cap structure of one type wind turbine blades, both of the two resin systems can well infused the 54 layers of glass fibre fabrics (1250 g/m 2 ) panels at 250 cm length and 60 cm width size.The total infusion time of the medium-reactivity curing agent with the proportion of 20% rapid curing agent is about 30 min shorter than that of 30% fast-reactivity curing agent, and the peak exothermic temperature was similar, which did not exceed 85℃.The time of exothermic peak was 57 min and 114 min, respectively.According to the experimental results of above five kinds of experimental schemes, it is concluded that the medium-reactivity epoxy infusion resin system with fast curing agent accounting for 20% is more suitable for wind turbine blades production.

Performance characterization of medium-reactivity curing agent
3.2.1 Viscosity.The performance of the medium-reactivity epoxy infusion resin system with 20% fast curing agent was comprehensively characterized and compared with the slow-reactivity epoxy infusion resin system.
The mixed viscosity of medium-reactivity epoxy infusion resin system and slow-reactivity epoxy infusion resin system at 25 ℃ is shown in table 4. It can be seen from table 4 that the mixed viscosity of epoxy medium-reactivity infusion resin at 25 ℃ is about 250 mPa•s, while that of slow-reactivity epoxy infusion resin system is about 235 mPa•s.The mixed viscosity of epoxy medium-reactivity curing system is 15 mPa•s higher than the slow-reactivity epoxy infusion resin system.

Medium-reactivity epoxy infusion resin system
Slow-reactivity epoxy infusion resin system

Mixing viscosity mPa•s 250 235
The change of mixed viscosity of medium-reactivity and slow-reactivity epoxy infusion resin 15~40 ℃ was tested by rheometer.The results are shown in figure 2. With the gradual increase of temperature, the viscosity of epoxy resin curing system decreased gradually.

Figure 2. Relation between mixture viscosity and temperature
Although the mixed viscosity of medium-reactivity epoxy infusion resin system is 15 mPa•s higher than that of slow-reactivity epoxy infusion resin system, it is also stays in the normal requirement of epoxy resin viscosity of 200-300 mPa•s, which means that theoretically the infusion time required by medium-reactivity epoxy resin system is slightly longer than that of slow-reactivity epoxy resin system.

3.2.2
The pot life.The pot life is the maximum workable time starting from the mixing of resin and curing agent until the resin process becomes unusable, and can be characterized by the exothermic temperature test curve.This test uses medium-reactivity epoxy infusion resin system and slow-reactivity epoxy infusion resin system for comparison test.100g resin and curing agent were mixed in a ratio of 100:32, and the temperature change of the test mixture was recorded using a digital recorder, while the time when the mixture reached to 50 ℃ (when the temperature was lower than 50 ℃, the highest exothermal temperature was reported) was defined as the pot life.The exothermic peak curve of epoxy medium-reactivity and slow-reactivity epoxy infusion resin system at 23 ℃ can be seen from figure 3, the pot life of epoxy medium-reactivity infusion resin at 23 ℃ is about 290 min, the pot life of the slow-reactivity epoxy infusion resin system at 23 ℃ is about 570 min.

Figure 3. Comparison of exotherm curves under 23℃ between medium-reactivity and slow-reactivity epoxy resin system
From the production experience of large-scale blades, the pot life of medium-reactivity epoxy curing resin can still meet the infusion requirements of wind turbine blade.

Curing speed.
After the epoxy resin is infused, it can start to heat the mold.During the temperature rise, the internal reaction of the matrix resin begins to accelerate, the cross linking density and curing degree continue to increase, and the glass transition temperature also gradually increases.By testing the curing speed of the resin under different curing temperatures and time, the dataset can support defining the conditions for the blade to achieve enough strength.The mixture of epoxy resin and curing agent is casted into a 50mm diameter*1mm (thickness) disk and placed in the oven for curing.The curing rate of the resin was characterized by test the glass transition temperature (Tg) of the samples cured for different curing time at 70 ℃.
The Tg growth curves of medium-reactivity and slow-reactivity epoxy resin systems at 70 ℃ are shown in figure 4. As can be seen from figure 4, the Tg growth rate of the medium-reactivity epoxy infusion resin system is slightly higher than that of the slow-reactivity epoxy infusion resin system.The curing requirement of wind turbine blade shell to has enough strength is that Tg reaches to 60 ℃.As shown in figure 4, the curing time required for slow-reactivity epoxy resin to reach the curing degree is about 3.0h, while the curing time required for medium-reactivity epoxy resin is about 2.3h.The curing speed of the medium-reactivity epoxy infusion resin system is about 0.7h faster than that of the slow-reactivity epoxy infusion resin system.During the operation of wind turbine blades, although fibre is the main load-bearing material of composite materials, the resin matrix performance is also very important, and matrix resin plays the role of bonding, supporting, protecting reinforcement materials and transferring load in composite materials.According to the requirements of the DNV GL specification, the Tg of the resin after curing is generally required to reach higher than 65℃.Table 5 shows the Tg of medium-reactivity epoxy infusion resin system.It can be seen from table 5 that the Tg of medium-reactivity and slow-reactivity epoxy infusion resin system can both meet the requirements.
Table 5 shows the comparison of the clear casting properties of medium-reactivity and slow-reactivity epoxy infusion resin system, showing that the medium-reactivity epoxy infusion resin system has higher tensile strength and modulus, and the bending property is comparable to that of the slow-reactivity infusion resin system, and both the tensile and bending properties can meet the DNV-GL specification requirements.Fibre content of the laminate is controlled according to design requirements, and fibre volume content of unidirectional fabric is generally controlled at 56%, and fibre volume content of biaxial fabric is controlled at about 54%.Tensile, compression and shear properties of medium-reactivity epoxy infusion resin system and slow-reactivity epoxy infusion resin composite material are tested, and the results are shown in table 6 and table 7.As can be seen from the table, medium-reactivity and slow-reactivity epoxy infusion resin system have similar tensile, compression and shear properties.This may be caused by the fact that the basic raw materials used in the medium-reactivity epoxy infusion resin system are similar to those used in the slow-reactivity epoxy infusion resin system.The properties of both composite materials can meet the requirements of wind turbine blades.

Application Verification
Through the comparison of the exothermic test of the epoxy infusion resin system, the results showed that the exothermic peak of the medium-reactivity epoxy infusion resin system was higher than that of the slow-reactivity epoxy infusion resin system.Before using the medium-reactivity epoxy infusion resin system in the wind turbine blades, in order to avoid the potential risk of high temperature during curing, the application process was optimized as follows: (1) The material of the sensitive part is adjusted to the material with better temperature resistance: for example, BBI is changed to PET material, and the core material of the blade root wedge is changed to PVC with better temperature resistance, so as to avoid the risk caused by high temperature; (2) Adjustment of vacuum auxiliary materials: vacuum film with temperature resistance of 150 ℃, flow media hose with temperature resistance of 150 ℃, PVC hose steel reinforced with temperature resistance of 150 ℃ and other vacuum auxiliary materials are adjusted from the details to control.
The shape of the 90m wind turbine blade is shown in figure 5, which adopts a three-web structure.Among them, medium-reactivity epoxy infusion resin system was adopted in the shear web (The yellow section in figure 5) and the shell (The green section in figure 5) on the trial production.The Spar Cap (The purple section in figure 5) is made of pultruded fiber-reinforced polymer (FRP) composite, and this article is not described in detail, if interested please refer to this reviews and research articles.TianQiao Liu [11] presents a comprehensive review on experimental studies investigating the mechanical performance of pultruded FRP composites subjected to long-term environmental effects, including water/moisture, alkaline solutions, acidic solutions, low/high temperatures, ultraviolet radiation, freeze-thaw cycles, wet-dry cycles, and in situ environments.Euan Duernberger [12] indicates modern wind turbine blades are manufactured using a combination of both glass and carbon fibre reinforced polymer (GFRP and CFRP) and possess complex geometries, aerodynamically optimised to simultaneously increase efficiency, and reduce loading on the structure.Whilst the root is optimised for strength, sections further from the hub are designed primarily for aerodynamic efficiency.The trial production results showed that the pre-curing time was shortened by 0.5-1 h.As can be seen from figure 6, the web was produced using the medium-reactivity epoxy infusion resin system, the inner surface of auxiliary materials was removed, and the outer surface after mold release was free of delamination, white, dry yarn, envelope, and different color of core material.The infusion effect was good without abnormal phenomena.

4.1.2
The shell trial production .The medium-reactivity epoxy infusion resin system was used in the shell of 90-m wind turbine blade, injected with an integrated on-line vacuum degassing/mixing machine [13] .Trial run details were show in figure 6.
. When the shell is produced by medium-reactivity epoxy resin system, the curing time of the shell can be shortened by 0.5-1 h.As can be seen from figure 7, after removed the auxiliary materials and blade demould, the surface of shell not showing defects like delamination, white area, dry yarn, dry spot and color change on the core material, etc.The infusion quality is good and there is no abnormal phenomenon been reported.Finally, the weight and torque of the 90-m wind turbine blade meet the design requirements

Benefit calculation
The benefits of using medium-reactivity epoxy infusion resin system can be calculated from cost reduction and production efficiency increase.It comes from two parts, one is the reduction of labor cost after shortening pre-curing time, and the other is the improvement of benefits brought by reducing mold holding time.
(1) Cost reduction benefits: The use of medium-reactivity curing system can save 15-25 working hours per piece blade; (2) Efficiency benefit: the reduction of mold holding time, followed by the benefit on improvement of blade production efficiency.
The total benefit of cost reduction and efficiency improvement is also related with the production quantity of parts/blades

Conclusions
In this paper, in order to improve the production efficiency of wind turbine blades, a medium-reactivity curing epoxy infusion resin system was developed, and its performance was tested and validated on both specimens level and blade trial.Here are the conclusions.(1) The medium-reactivity epoxy infusion resin system with 20% fast curing agent is more suitable for our blades workshop using.(2) The curing speed of the medium-reactivity curing epoxy resin is significantly faster than that of the slow-reactivity curing epoxy resin, which can effectively improve the production efficiency of wind turbine blades.In general, the development of this medium-reactivity curing epoxy resin system provides a new choice for wind turbine blades production to reduce cost and improve efficiency.

Figure 1 .
Figure 1.Growth trend of wind power demand in 2020-2030 (Quote from: Global Wind Report 2022)

Figure 4 .
Figure 4. Tg test curve established after 70℃ curing of epoxy medium-reactivity and slow rate infusion resin system

Figure 5 .
Figure 5.The shape and structure of the 90-m wind turbine blade

Figure 6 .
Figure 6.The trial production of shear web by epoxy medium-reactivity infusion resin.

Figure 7 .
Figure 7.The shell produced by medium-reactivity epoxy resin system

Table 2 .
Specifications of glass fibre fabrics

Table 3 .
Operating time of curing agents with different proportions of high-reactivity curing agent

Table 5 .
Comparison of clear casting properties between medium-reactivity and slow-reactivity epoxy infusion resin system Composite properties.The performance of composite material is the key input of blade design.

Table 6 .
Comparison of properties of medium-reactivity and slow-reactivity epoxy infusion resin system with unidirectional fabric E8-UD1250

Table 7 .
Performance comparison of epoxy medium-reactivity and slow-reactivity infusion resin systems with biaxial fabric E8-BIAX1200