Introduction to Wind Turbine Blade Pultrusion Process

 

Pultrusion Process: refers to the process in which the yarn is pulled by a traction machine, and then goes through yarn spreading, impregnation, preforming, molding, step-by-step cooling and curing, and cutting to finally obtain a pultruded sheet.

 

Pultrusion process is a continuous production process, and theoretically, it can produce sheets of unlimited length. Specifically, the yarn spreading stage is different in process due to the different ways of rolling carbon fiber yarn and glass fiber: carbon fiber is similar to household needles and threads, and is wound on a bobbin; glass fiber has no bobbin, and the thread is pulled out from the inside to the outside for use. The impregnation stage is different according to the different processes of the matching resin: epoxy or unsaturated resins usually use open impregnation, and the fiber is impregnated and heated and cured when it passes through the open glue groove; polyurethane uses injection impregnation, and the resin is pressed into the impregnation in a fully enclosed environment. Compared with the two, the latter avoids resin volatilization and has less impact on the environment. After the yarn is impregnated, it is preliminarily heated in a small mold before entering the molding mold for heating; after molding, the temperature is gradually reduced and steadily cured while keeping warm, avoiding product defects caused by rapid curing, and finally the pultruded sheet is cut.

 

Infusion Process: also known as vacuum introduction process. At first, the fiber fabric was dipped into the resin by hand-laying, and then attached to the mold layer by layer.

 

Later, the airtight film was used to evacuate the air, and the bubbles or defective stratification between the resin and the cloth layer was reduced after the gas was extracted. The infusion process is as follows: first, after the surface treatment of the molding mold, the fiber fabric is placed on the mold layer by layer according to the cutting area, product area, waste area, etc., and then the core material is filled between the fibers to balance the strength and weight. Then, the air between the fibers is extracted under the action of auxiliary materials such as permeable film cloth, guide net, vacuum belt film, etc., so that negative pressure is formed inside, and then the resin is vacuumed into the infusion.

 

The permeable film cloth is used to provide a rough interface on the surface of the product, which can be used as an infusion port or for later reinforcement of the product; the guide net is used to assist the flow of resin; the vacuum belt film is attached to the mold to form a vacuum environment. The difference between the pultrusion process and the infusion process is that the pultrusion process can be produced continuously and has higher production efficiency; the infusion process needs to complete a product before continuing production, but its product plane structure is more complex.

 

Advantages of Pultrusion: Since the pultrusion process was invented by Americans in the 1950s, its advantages have been continuously improved.

 

1) The Pultrusion Process has a High Utilization Rate of Raw Materials. In theory, yarn and resin are produced continuously, and the yarn and resin are basically fully utilized in the mold; while the infusion process mold has product area, cutting area, and auxiliary material area, and the waste of scraps is inevitable during the flow of resin.

 

2) The Pultrusion Process can be used to Manufacture Profiles with Complex Cross-Sections and has Strong Designability.

Some special-shaped profiles such as I-beams, plastic steel doors and windows, etc. are formed by pultrusion, while infusion, hand lay-up, prepreg and other processes are difficult to complete. Although 3D printing, CNC machine tools and other processes can be completed, the cost is higher than that of pultrusion. In addition, in terms of structural design, the fiber content of specific parts can be adjusted according to the stress characteristics of the profile, which is used for directional reinforcement of the profile, improving the designability.

 

3) The Pultrusion Process has high Production Efficiency and Strong Controllability of Product Quality.

Different industries have different requirements for the production efficiency of pultrusion process. The wind power field is currently about 0.6-0.7m/minute, which is expected to be further improved in the future to control costs. In the pultrusion process, the yarn impregnation, equipment traction, resin viscosity, etc. are more controllable, and the product quality is stable. Even if the quality is poor somewhere, it can be cut and continued to be produced, with very little waste; while the vacuum infusion process is relatively more difficult to control. If there is a problem locally and it cannot be repaired, the entire board will be scrapped and wasteful.

 

4) High Flexibility of Raw Materials.

The matching between different yarns and resins is multi-directional, and different yarns can be impregnated with the same resin for production; while the prepreg process, if different yarns are used, may cause problems such as stratification of the product.

 

Downstream of Pultrusion Products:

Pultrusion products are mainly long strips such as sticks, tubes, flat plates or profiles. Downstream applications include: cable trays, pole brackets, carbon cores of optical fiber high-voltage cables (which have weight advantages over traditional metals), electrical components, etc. in the power industry; pumping rods in the chemical industry, guardrails, grilles, brackets, anti-corrosion products, etc. on offshore platforms; fishing rods, flagpoles, fence handrails, signboards, etc. in life; pultruded materials are used in bridge reinforcement and bridge maintenance in the construction industry.

 

Pultruded materials are used in isolation barriers, signboards, and handrails on overpasses on highways. The most common application is pultruded plates in the wind power market, including pultruded plates of glass fiber and carbon fiber, which are mainly used for the main beams of wind power. Pultruded materials are also used in the roots and webs of wind power blades.

 

The pultrusion process is widely used in the market, but because the characteristic of the pultrusion process is that the cross-section is regular, it is limited in application in places with curvature or curved surface changes. In the future, pultruded materials that can change the curved surface according to regulations or settings will also be developed.

 

Application of Pultruded Board in Blade Industry:

The main beam of the blade (also called beam cap or beam) is the main load-bearing part or main structural part in the wind turbine blade, similar to the keel of a ship. If the main beam breaks, the entire blade will also break. Initially, the main beam used the vacuum infusion method, that is, laying glass fiber layer by layer on it, and then infusing resin; later, prepreg appeared. Whether it is glass fiber or carbon fiber, the prepreg is laid layer by layer, and the product is formed after the vacuum membrane is pressed.

 

The current process is to pultrude a certain length of pultruded board first, with a common cross-sectional size of 120 mm × 5 mm. During the production process, the pultruded boards are pieced together, and a cloth layer of certain specifications is laid in the middle. Then they are bundled together and put into the blade, and infused together with the blade. After infusion, demolding becomes the structural part inside the blade.

 

Reasons for Offshore and Onshore Wind Power Using Pultrusion Technology:

 

1) The Pultrusion Process Ensures the High Fault Tolerance of the Product.

In the past, whether the infusion process or prepreg was used for production, if there were defects somewhere during the production process, the impact on the main beam would be fatal. For example, if the main beam is 80 meters long and 60 centimeters wide, as long as there are defects somewhere, this main beam will have safety hazards. However, if a problem or defect occurs on a pultruded board during the production process, the pultruded board can be scrapped and replaced with a new one. This means that even if there is a problem with the main beam of the blade during the assembly process, it is enough to replace the problematic pultruded board, and it will not affect other pultruded boards. The cost of scrapping a pultruded board is much lower than that of scrapping the entire main beam.

 

2) The Pultrusion Process Makes the Main Beam Structure Stronger.

Because the fiber content of the pultrusion process, the amount of yarn and resin used can be controlled and adjusted, and the yarn content can reach 70% (volume content), but the yarn content of vacuum infusion can only reach 60% at most. The higher the fiber content, the better the mechanical properties or load-bearing state of the main beam. At present, the length of pultruded plates used in onshore wind power is mostly between 80 meters and 97 meters, such as Shanghai Electric has 85 meters, 89 meters, and 90 meters; Dongfang Electric has 90 meters and 91 meters; Goldwind and Zhongke Yuneng mainly promote blades with a blade shape of 93 meters; Yunda has 92 meters and 97 meters. Among the blades of 80 meters to more than 90 meters, except for the need to use vacuum infusion or prepreg for production due to contract signing, or due to some economic reasons, basically 98% of the blades are pultruded. Including the 99-meter blades of offshore wind power and the 123-meter blades of Goldwind, they also use pultrusion. Blades are available in carbon plates and glass fiber plates, and there is also a certain gap depending on the blade shape.

 

Downstream Carbon Plate Industry’s Planning for Pultruded Materials:

Sinoma Technology, Goldwind Technology and Zhongfu Lianzhong all control some pultruded plate manufacturers or produce pultruded plates independently. For example, Zhongfu Lianzhong’s subsidiary Fengdian Technology has begun to produce pultruded plates and wind turbine blades; Mianyang and Huamei have carried out strategic cooperation; Sany Heavy Industry and Fengdu New Materials have carried out strategic cooperation, etc., all of which indirectly illustrate the importance of pultruded plates to wind turbine blades.

 

Technical Situation of Pultruded Beams:

According to the resin classification, most manufacturers now use epoxy resin technology. At the same time, everyone is actively developing polyurethane pultrusion technology. Compared with epoxy resin, polyurethane resin has a lower price and is more suitable for the future trend of declining installed prices in the wind power industry. However, in a short period of time, polyurethane resin has higher requirements for materials and moisture content in the air.

 

In the next 3 to 4 years, epoxy resin will still be the mainstream.

 

Material manufacturers: Domestic pultrusion manufacturers of glass fiber are mainly Taishan Fiberglass, Chongqing International, Tongxiang Jushi, and foreign OC companies. OC’s yarn products are still used in China.

 

1) In terms of carbon fiber, foreign companies mainly include Zoltek and Toray, and domestic companies mainly include Jilin Guoxing, Jilin Baojing, Zhejiang Jinggong, Jiangsu Hengshen, Weihai Guangwei and other companies. Among them, Guoxing, Hengshen and Guangwei can produce yarn, pultruded plates and some carbon fiber products. Guangwei’s fishing rod products are very popular. Jilin Guoxing has invested a lot of energy and funds in carbon fiber and carbonization of carbon fiber in recent years.

 

2) In terms of epoxy resin, it is mainly Taiwan Shangwei, Shanghai Daosheng Tianhe, Shanghai Huibai and Sichuan Dongshu. At present, it is mainly the pultrusion process of epoxy system. There are few polyurethane manufacturers, mainly Shanghai Covestro. The year before last, Covestro, Shanghai Goldwind and Zhongfu Lianzhong jointly developed a pure polyurethane wind turbine blade with a length of more than 60 meters, which accumulated technology for the future development of polyurethane wind turbine blades.

 

Equipment manufacturers: Nanjing Nortai, Guangdong Renda, Jiangsu Kecheng and Xinchuang are currently the most commonly used. The pultrusion equipment of these four companies is relatively good.

 

Q&A

Q: In the process of converting the injection pressure method to the pultrusion method, what changes will occur in the curing agent?

A: Most of the injection processes use polyamine curing agents, and the curing temperature will be relatively low. The pultrusion process uses anhydride curing agents, which need to be heated and cured, and are more suitable for the pultrusion process.

 

Q: What is the unit consumption of curing agents per GW wind turbine blade?

A: The theoretical consumption of anhydride curing agents per GW wind turbine blade is 400 tons.

 

Q: What is the usage ratio of anhydride curing agents and traditional curing agents? A: Taking a 90-meter wind turbine blade as an example, the weight of the entire blade is between 25-30 tons, and the pultrusion process product is 6-7 tons. Among them, the usage ratio of anhydride curing agents and polyamine (traditional) curing agents is 1:3. Anhydride curing agents are suitable for pultrusion processes as blade load-bearing parts, but high polyamine curing agents are also used in other parts of the blade molding process. As the demand for anhydrides continues to increase with the development of the wind power industry, traditional curing agents will also increase.

 

Q: Is there a supply and demand gap for anhydride curing agents now?

A: There has been no significant increase at present.

 

Q: What is the penetration rate of wind turbine blade pultrusion in 2021?

A: The use of pultrusion technology began as early as 2019-2020, and Zhongfu Lianzhong was the first to use it. The technology matured in 2020-2021. In 2021, most wind turbine blades were also vacuum infused; currently, 90% of domestic blades have used pultrusion technology.

 

Q: Are acid anhydride curing agents also used in other wind power components?

 

A: Maleic anhydride curing agents can also be used in the blade roots and I-beam components of wind turbine blades. In the past, the blades needed to be fixed directly on the surface of the blades by bolting holes, but with the change of technology, it has now been converted to be directly installed in the wind turbine during the formation of wind turbine blades, and the pultruded material fills the gap between the two round rolls. There are also I-beam connectors at the bonding point between the two blades of the wind turbine, and pultruded materials are also used. Taking a 90-meter blade as an example, pultruded materials similar to I-beams can be used at the 3-meter root and 89-meter between the blades. At this stage, I-beams are used less in China and are mainly used abroad. In terms of usage, the use of blade roots and I-beam parts will increase the demand for anhydride curing agents by 10%-15%, and the overall use of anhydride curing agents per GW will increase by 40~60 tons.

 

Q: Suppliers of anhydride curing agent products?

 

A: The main supplier is Puyang Huicheng, and the main product used is methyl tetrahydrophenyl glycoside. In terms of supply process, Puyang Huicheng first sells material curing agents to companies such as Daosheng Tianhe and Shanghai Huibai, and companies such as Daosheng Tianhe and Shanghai Huibai purchase epoxy base materials and add curing agents and other formulas, and then sell them to pultrusion blade companies.

 

Q: What is the current penetration rate of I-shaped pultruded materials?

 

A: I-shaped pultruded bottom materials can only guarantee the improvement of product quality, but are not a necessity. They will gradually become popular in the next one or two years, but it will take two or three years to use them all.

 

Q: What is the approximate domestic blade output in the first half of the year?

 

A: In the first half of the year, Shanghai and Jiangsu were affected by the epidemic, and most of the resin factories were in Shanghai, but generally the output of the wind power industry in the first half of the year was relatively low, and the output in the second half of the year was basically twice that of the first half of the year. At present, the completion rate in the first half of the year is about 85%, so the impact of the epidemic is not particularly large.

 

Q: What is the inventory of blades in the first half of the year?

A: The production of each company in the first half of the year was not particularly saturated, and there was a certain amount of inventory, but not much. The production in the second half of the year can still reach twice the level of the first half of the year.

 

Q: Will the future advancement of blade technology, such as the trend of large-scale blades, increase the amount of curing agent used in blades?

A: Yes. Offshore blades use carbon plate pultrusion plates, but the current offshore wind power consumption is relatively small. Whether it is Goldwind Technology or Zhongfu Lianzhong, most blades only produce two or three per month. In the next two or three years, the installed capacity of offshore wind power will be more, and the amount of anhydride curing agent will continue to rise.

 

Q: Changes in the amount of curing agent used for carbon glass blades and pure glass fiber blades?

A: The process is the same, and there is not much change. The blades can be made longer.

 

Q: As the blades get longer and longer, will the amount of curing agent used per GW increase?

A: The overall demand has increased, but the amount used per GW is basically stable