This article was published in the January/February 2012 issue of our sister publication Reinforced Plastics magazine. To apply to receive your free copy of each issue of Reinforced Plastics please complete the subscription form or contact us for further information.
Standard utility-scale wind turbines, designed to produce 1-3 MW of electricity, are fitted with three blades of 30 to 50 m (100-165 ft) in length. Blade production is labour intensive. Outer laminated shells are supported by a spar cap or main spar, serving as the central spine of the blade. Reinforcements are typically 70-75% E-glass by weight, infused with epoxy or unsaturated polyester resin. Blades are commonly moulded in halves, then joined using an epoxy adhesive.
Glass fibre reinforcements are typically laid in the mould as dry stacks by hand. Balsa, structural foam and engineered three-dimensional materials are used as cores in blade construction to support the outer blade shells. Epoxy prepreg materials of E-glass, carbon fibre and hybrids are used by some manufacturers, especially in the production of longer blades.
Tonnes of composites in the sky
The world’s largest wind turbine producer is Denmark’s Vestas Wind Systems. One 80 m long blade bolted to the hub of the company’s 7 MW offshore wind turbine weighs 35 tons. The three-blade assembly measures 164 m (538 ft) in diameter and puts a lot of fibre reinforced plastic (FRP) composites up in the air.
Vestas uses carbon composites to produce the blades, enabling them to be slimmer, stiffer and lighter than fibreglass. That reduces the load on the turbine and makes it more efficient.
Spain-based Gamesa, another global market leader, also uses carbon fibre/epoxy prepreg materials in its most advanced turbines. Gamesa’s 4.5 MW turbine, called the most powerful wind turbine for the onshore market, towers 120 m (394 ft) up in the air. It is powered by unique Innoblade blades, measuring 62.5 m (205 ft) long. They are segmented, enabling them to be assembled in the field.
The largest manufacturer of blades for wind turbines in the world is Denmark-based LM Wind Power. Operating 12 production facilities on three continents, the company recently produced its record-longest – 73.5 m (271 ft) long blades for the growing European offshore wind market using E-glass and polyester resin in a vacuum assisted resin transfer moulding (VARTM) resin infusion process. The blades are designed to be mounted on a 6 MW wind turbine manufactured by Alstom of France.
LM Wind Power has been working with glass fibre and polyester to produce blades for wind turbines since 1978, “stretching the capabilities” of the materials to achieve the best possible balance between price and performance, the company relates. LM involves its suppliers closely in the production processes in order to optimise resin and reinforcements for vacuum infusion.
The result, claims the manufacturer, is a very strong, uniform laminate with no air bubbles and rapid hardening, which has reduced production time by several hours. With reinforcing fibres, the focus is on tensile strength and the overall weave and structure of glass fibre mats, as well as the chemical sizings on the fibre. Fibre mats used by the company are easier to handle and are less expensive than prepregs, relates the manufacturer.
“Epoxy is three times the price of polyester and requires a considerably more expensive production process, including heating of the moulds," LM observes. "We’ve decided to focus instead on optimising cheaper technology to reduce the overall price of wind energy while maintaining the same good performance properties.”
One manufacturer of polyester resin systems is Switzerland-based DSM Composite Resins. Its Synolite polyester resins are said to offer performance improvements over other traditional unsaturated polyester and epoxy resins. Synolite 1790-G-3 is well-suited for vacuum infusion of wind turbine blades, exhibiting improved wet-out of glass reinforcements and enabling faster production cycles and room-temperature curing, says DSM. The resin is also said to demonstrate very low exothermic heat development in thick laminates for lower mechanical stress in the blade and longer mould life.
Epoxy resin systems
Dow Formulated Systems has developed Airstone epoxy resins and hardeners with reduced peak exotherm during curing for wind blade production. Airstone 780E infusion resin is said to provide extended pot life, fast curing and shortened cycle times. Fabricators are able to control resin curing properties, optimising cycle times without compromising the strength and integrity of the blade structure, says Dow, which expects to make Airstone infusion systems available commercially in 2012.
Momentive, formed in 2010 from the merger of Momentive Performance Chemicals and Hexion Specialty Chemicals, offers Epikote and Epikure epoxy systems for wind turbine composites. They offer ease of processing, excellent fracture toughness, good fibre impregnation properties and outstanding thermal stability, the company relates. Momentive epoxy systems are available in formulations tailored to a variety of processes, including RTM and VARTM resin infusion, hand lay-up and prepregging.
Sika Deutschland GmbH offers Biresin CR83 epoxy resin for wind blade production. The resin is said to be well-suited to this application due to its low viscosity, good wetting characteristics and variable pot life. In trials, the resin infused a fibre lay-up in half the time of a typical competitive infusion system and provided 100% wet-out of the fibres, Sika relates. Three hardeners of different reactivity are available to vary the pot life.
Swancor of Taiwan has developed an epoxy resin system, SW2511-1, for VARTM processing of wind blades. The resin is said to offer low viscosity, adjustable gel time and low exothermic temperatures, reacting at room temperature without additional pressure. Gel time of 9-10 hrs for SW2511-1 compares to 6-7 hours for traditional epoxy systems, while its exothermic peak temperature of 35-40°C compares to 50-70°C of other epoxy systems, Swancor relates.
Gurit offers a range of composites products for wind blade production, including the PRIME 20LV Epoxy Infusion System. This offers much reduced viscosity and longer working time than traditional epoxy resins, says Gurit, noting that the resin is ideal for infusing very large parts with complex reinforcements in one operation. The PRIME system is said to maintain exceptionally low exotherm characteristics, avoiding premature gellation in thick sections and extended life of mould tools. It is available with a range of hardeners to control working times and cure speeds.
California-based Materia has developed Proxima, a proprietary thermoset resin system as an alternative to epoxy resins in wind blade production. Metathesis catalyst technology was used in the development. Metathesis enables chemical compounds to be synthesised with greater efficiency under less stringent reaction conditions and with reduced byproducts, says the company. Proxima resin is said to provide greater toughness, lower weight and a smaller carbon footprint at a lower cost than epoxy systems used for wind blade production. Its lower viscosity enables faster infusion rates than traditional resins, claims Materia.
Vinyl ester for blades
AOC produces a range of closed mould resins for wind blade composites, including bisphenol A epoxy-based vinyl ester, isophthalic and orthophthalic polyester and general purpose polyester. The resins are formulated to address a range of processing, performance and cost requirements. The general purpose unsaturated polyester, R920-E Series, is a high-value pre-promoted, resin for fast fibre wet-out and infusion speed. High-performance, high elongation vinyl ester resin in the R013-A Series offers the strength of epoxy and the cycle time of polyester, while R015-G Series is a highly reactive vinyl ester resin for ultimate strength and stiffness, providing fast infusion time, notes AOC.
DSM has developed a range of low viscous vinyl ester resins for the production of wind turbine blades. When combined with glass reinforcement with the proper sizing, it is said to provide at least the same mechanical performance as epoxy-based systems. The resin enables fast cycle times and can be cured with normal low active MEK peroxides, notes DSM.
Derakane 601-200 epoxy vinyl ester resin from Ashland Performance Materials is said to improve cycle times by 25% versus epoxy resin systems in infusion processing of wind blades. The resin narrows the performance/price gap between low-priced unsaturated polyester resins and high-priced epoxy resins, says Ashland. Derakane 601-200 cures much faster than straight epoxy and can provide cost savings of 25-30% per blade over epoxy systems, the company claims.
Reichhold offers a pre-promoted bisphenol-epoxy vinyl ester resin for wind blade production in its Dion Impact 9102-75 series. It is formulated to provide reduced viscosity and improved curing for enhanced performance in infusion of large structural reinforced composite wind blades. Also available from Reichhold is Polylite 32850-00 series, an orthophthalic unsaturated polyester resin, pre-promoted and formulated to provided reduced viscosity and improved curing for infusion of large wind blades.
New Bayer PU systems
A new polyurethane (PU)-based composite technology developed by Bayer MaterialScience LLC is said to dramatically improve fatigue and fracture toughness over epoxy-based systems used in the production of longer wind blades. The material incorporates Bayer’s Baytubes carbon nanotube reinforcement technology, which improves the fracture toughness by as much as 48% – double that of epoxy, the company says.
The new composite material has also been shown to exhibit superior processing and handling properties, which can lower total manufacturing costs by as much as 16%, Bayer adds. The development was funded by Bayer, the US Department of Energy and Molded Fiber Glass Companies (MFG) in a public/private sector partnership. Bayer is working to manufacture, test and certify full-scale prototype blades in collaboration with wind industry experts, the company notes.
Bayer and MFG researchers had been comparing the performance characteristics of Bayer’s Baydur resin infusion polyurethane systems with those of epoxy and vinyl ester-based composites for wind blades. While other polyurethane systems are designed for fast throughput and fast gelling/fast demold properties, Bayer’s systems are designed to provide the lower viscosities and longer gel times needed for moulding very large wind blades. Test results show that the new polyurethane-based systems outperformed epoxy and vinyl ester samples in tensile fatigue, interlaminar fracture toughness testing and fatigue crack growth testing.
Prepreg technology
Prepreg materials from Gurit are available in several lines. The WE91-1 prepreg, which is part of the WE and WT range of prepregs, is a high-flow, diuron-free epoxy prepreg well-suited to the manufacture of thick sections. The prepregs can be cured at temperatures as low as 85°C or with a faster (55 minute) cure at 120°C. Unidirectional (UD) glass prepregs are available in a wide range of E-glass fibre weights. Also available are a range of carbon fibre prepregs, which excel where high mechanical properties are required. In addition, Gurit manufactures SparPreg unidirectional prepreg with glass or carbon fibre for SPRINT-quality blade production.
Hexcel, another global supplier of materials to the wind energy market, produces epoxy/carbon fibre and UD glass prepregs. Carbon prepregs can be a cost-effective option for very large diameter blades, says the manufacturer, since less material is required to achieve the same strength as glass. Hybrid reinforcements of glass and carbon are also available from Hexcel, which operates six prepreg manufacturing facilities worldwide. The prepregs use HexPly M9.6G and M19G epoxy resins. M9G cures in 15-20% less time while exhibiting the same handling and mechanical properties, says Hexcel.
The company also supplies HexPly M9F formulated epoxy resin systems for wind blade production. The resins are designed for low pressure processing and permit a range of temperatures from 85°C up to 150°C. HexPly M11 and M11.5, both diuron-free, adding environmental benefits, have been developed for low-vacuum bagging processes and rapid cure cycles – 3.6 hours at 80°C or 8.7 hours at 75°C.
New reinforcements
A new range of fibre reinforcements engineered to provide optimum performance for the manufacture of wind turbine blades has been introduced by 3B – The Fibreglass Company. Each product is designed for specific resin systems. The first product in the series to be introduced, Advantex SE2020, is a single-end roving specifically engineered for epoxy polymer systems utilised in resin infusion or prepreg processes.
“At 3B, we focus on understanding the needs of wind energy OEMs by working hand in hand with the designers, the weavers and, ultimately, the manufacturers of turbine blades," the company says. "Collaborating with the entire value chain enables us to bring to market new benchmark rovings which further push the limits of glass fibre composite blade designs.”
Compared to conventional materials, the new Advantex SE2020 roving offers better wet-out, providing a more consistent laminate quality and an improved resin matrix adhesion, which delivers higher shear strength and greater inter-fibre strength, adds 3B.
A new foam core system for wind blades, COMPAXX700, was recently introduced by Dow Formulated Systems. The product is the first in a line of new core materials designed to help manufacturers extend blade life through the creation of high-performance sandwich composites. According to Dow, extensive sandwich panel fatigue testing showed COMPAXX 700 exhibits long-term dynamic behaviour and shear strength properties yielding lightweight composites with excellent mechanical strength and fatigue resistance. These properties, coupled with peel strength about three times higher than the historical reference of core material polyvinyl chloride (PVC) 60 kg/m3, “create the intimate core-to-skin bond necessary to achieve blade durability," observes Dow.
The product has a specific roughness and surface grooving that help achieve the excellent bonding performance. In addition, COMPAXX 700 offers a high run-to-run consistency leading to predictable mechanical properties that structural engineers can use to create more precise blades, the company adds.
The new COMPAXX system is now available globally in commercial volumes.
PPG Industries produces Hybon 2026 direct glass fibre roving, said to be well-suited for wind blades, in facilities in the USA, the UK and China. The roving is designed to provide the high mechanical performance required for critical structural designs while offering advantages in blade production. The reinforcement also provides excellent adhesion in multiple resin systems, high tensile strength and fatigue performance needed for demanding wind blade applications, relates PPG.
A fibre-reinforced composite core material for wind blade sandwich construction is available from Webcore Technologies. TYCOR W5 is the latest product in the company’s W line of core materials. E-glass fibre reinforcements are combined with low-density foam in an engineered architecture for vacuum infusion of blades. TYCOR saves about 1000 lbs (454 kg) per blade versus balsa core, says the company, enabling the production of longer blades without switching to costlier reinforcements. One customer realised a 9% reduction in the total bill of materials owing to the combined core and resin savings, adds Webcore.
Automating production
A CNC-controlled Rapid Material Placement System (RMPS) from MAG IAS, based in Kentucky, USA, offers integrated manufacturing capabilities for wind turbine blade production. The RMPS system consists of a gantry system with multi-axis end effectors capable of spraying in-mould coatings, dispensing reinforcement materials and applying adhesives automatically.
The RMPS system features a modular platform with a 10-roll magazine for reinforcing materials that cuts and dispenses plies of multiple widths into the mould. Material placement is rated at 3 m/s (10 ft/s). A pair of articulating powered brushes smoothes out the plies as they are paid out. The system also features laser and vision-based wrinkle detection systems. Two gantry systems adjacent to each other can produce a 45 m blade shell-half in less than two hours, with half the manual labour of conventional methods, the manufacturer relates. The lay-up system is said to be mechanically repeatable to within 2 mm, with application tolerance of ±5 mm.