New Generation of Wind Blades

The wind industry is continuously seeking ways to reduce the levelized cost of energy (LCOE). One approach is to increase the energy captured by the windmill. That energy depends on the swept area of the wind rotor blade and thus fundamentally on the blade length. On the other hand, as the blade length increases, its weight, installation cost and tip deflection increase which limits the progress. The key to balancing these counteracting trends is to lower the weight and increase the stiffness. The spar cap is the main load carrying structure in the blade and plays a key role in this balance. Figure 1 is a pictorial representation of the key elements that constitutes a wind blade. The optimization of the stiffness-to-weight ratio is accomplished by the use of carbon fiber in the spar cap. The higher modulus and lower density of Carbon fiber offers a unique stiffness-to-weight ratio when compared with glass fiber. Given the cost/performance advantages, carbon fiber is finding increased use in new generation of wind blades. 

Stiffeners for wind blades have been transitioning from epoxy glass fiber infused spar tubes toward carbon fiber epoxy pre-preg spar caps. This evolution is given schematically in Figure 2. Carbon fiber pre-pregs competed with the early-applied glass fiber infused spar tubes. A study lead by Wetzel Engineering Inc. reported 4X times higher stiffness-to-weight ratio of carbon pre-preg compared with infused E-glass fiber at composite level. This evaluated in a wind blade of 57m length represents a 27% weight reduction ^[1]. However, the most recent focus of the wind power industry is posed on applying carbon fiber Pultrusion laminates as alternative to pre-pregs as constituents of the spar caps in the new generation of their wind blades.

Pultrusion offers clear advantages over pre-pregs such as higher fiber content and better fiber alignment, resulting in superior mechanical performance. Pultrusion is a continuous automated process, enabling more consistent quality of the parts, higher productivity and higher volume manufacturing. The pultruded profiles can be made in any length and wound at the end of the Pultrusion line with the help of a coiler unit, up to the length required by the end user. The pultruded carbon profile is fully cured and ready to use upon arrival to the blade manufacturing facility! The coils can be uncoiled, parts cut to the desired length, precisely staked along the length of the blade adjacent to the other structural materials. A single infusion step bonds the carbon profile together with all the other blade structural elements. Figure 3 is a pictorial representation of the mentioned steps required for manufacturing spar caps by Pultrusion.

Process simplification represents then an additional advantage over pre-pregs, that must be kept refrigerated during the supply and storage; undergo a costly and time-consuming step in the vacuum cure process requiring a dedicated mold. The elimination of the vacuum cure-manufacturing step leads to a 25% capital cost reduction when using Pultrusion based on DowAksa internal estimations.

As mentioned in the previous section, Pultrusion is a continuous process to produce high fiber content composites of constant cross-section and a classical line layout is represented in Figure 4. The process relies on a puller/clamping system to pull several hundred tows of high strength fibers from the creel towards an impregnation bath, where the thermoset resin infiltrates into the bundle of filaments. The wet fibers are squeezed by a specially designed entry plate and then introduced into a heated steel die where the resin cures and solidifies detaching from the die wall. The cross-section of the profile is defined by the die geometry. The material exiting the die is gripped by the pulling system, which continually pull it out. In a final stage, a cutting saw station cut the profile to the desired length. 

Unlike epoxy or vinyl ester resins, which can be processed using a classical open bath pultrusion technology, polyurethanes require closed injection pultrusion. This is due to the fact that the polyol and isocyanate start to react immediately after being mixed together even at ambient temperature. A standard polyurethane mixing and injection machine must be connected to a specially modified pultrusion injection box. A suitable injection box design requires substantial know-how to permit proper infiltration of the polymer trough hundreds of tows containing thousands of filaments in a short period of time (residence time in the injection box). Figure 5 is a generic pictorial representation comparing the conventional “open bath” pultrusion lay out and the close injection system required for processing polyurethanes. 

Critical performance requirements for closed injection Pultrusion are low initial viscosity of the reactive mixture and no substantial polymerization at ambient temperature. On the contrary, it requires a very fast polymerization rate in the Pultrusion die to reach a high degree of curing. This concept is referred to in the industry as “snap cure” or “hockey stick” reactivity profile. While Polyurethane glass fiber closed injection has been practiced during the last 10 years, carbon fiber polyurethane Pultrusion have the added complexity of a lower carbon fiber filament diameter. A glass fiber filament diameter is on the order of 20-25µm, while a carbon fiber diameter is on the range of 5 to 7µm. This makes carbon fiber tow, which can contain 24K or 50K of these filaments very difficult to impregnate due to the lower intra-tow porosity (Darcy’s law), a fact that has limited Polyurethane carbon Pultrusion proliferation. Dow’s newly-developed low viscosity polyurethane system VORAFORCE™ TP1270EU/1300 was designed with these critical requirements in mind. In fact, a proper blend of polyols and the use of a low viscosity isocyanate achieved the required low initial viscosity. A systematic study on diverse catalyst and mold release agents and their interaction permitted Dow Scientists to optimize the reactivity profile of the Polyurethane system to enable carbon fiber Pultrusion.

But this development is not only about resin design. DowAksa with strong support of Dow focused on the creation of a disruptive closed injection system. The injection box (I-Box) is a fundamental part of the injection system and its design requires substantial know-how being one of the key elements of the novel process. Rheology and kinetics parameters were determined on Dow’s developed resin system and used by Dow’s computational modelers to simulate the fluid dynamics on different injection box designs and the curing kinetics on the Pultrusion die. Computational fluid dynamic models (CFD) and finite element analysis (FEA) were calibrated vs. process data and used for prediction on speeds and cure degree under different conditions ^[2] Figure 6 recreates how Dow and DowAksa engineers and scientists worked on predictive models and process data to design an optimum hardware to meet the stringent impregnation requirements of carbon fibers using a closed injection box system.

The significant challenge that Dow and DowAksa have overcome can be explained in the following practical example. In a pultruded laminate for spar cap containing about 70% of carbon fiber in volume, there are more than 10 million single carbon filaments. These filaments, are pulled continuously trough the injection box where they meet with the Polyurethane resin having a residence time in the Injection Box of typically less than one minute. The impregnation must be perfect; no dry spots are acceptable for the entire length of the coil as these could act as crack initiators. The length of the coils could be in the order of 280-300m and current throughput demand is in the order of several thousands of kilometers per week. Figure 7 shows the several hundred carbon bobbins unwinding from the creel and the carbon tows being directed towards the injection box. Figure 8 shows Scanning Electron Microscopy (SEM) pictures executed on the cross-section of the profiles that confirmed excellent wetting of the fibers. Online non-destructive analysis (NDT) continuously scan the production for dry spot detection, confirming excellent impregnation delivered consistently on millions of meters produced so far. This gives just a flavor of the enormous technical challenge DowAksa and Dow solved with the new technology.

Closed injection Pultrusion of VORAFORCE™ TP1270EU/1300 polyurethane combined with carbon fiber offers the best mechanical properties and productivity balance. Trials in a commercial scale Pultrusion line, under controlled conditions, were executed by Dow and DowAksa for benchmarking purposes. VORAFORCE™ TP1270EU/1300 Polyurethane was compared with best in class Epoxy and Vinyl-ester Pultrusion grades. Content of carbon fiber was fixed at 62% fiber volume fraction for all the runs. Epoxy and Vinyl ester were run at 1X speed in a classical open bath configuration, while VORAFORCE™ TP1270/1300 was run with the closed injection configuration at more than 3X times the speed of the controls. Figure 9 summarize the results in a spider plot. The mechanical properties^[3] where normalized to the results of VORAFORCE™ TP1270/1300, properties tested crosswise to the fiber direction are indicated with a 90-degree symbol, while those tested along the fiber direction are marked as 0-degree.

Properties of Epoxy fall at about 80% of those of VORAFORCE™ 1270/1300 but with the later running 3X times faster on the Pultrusion line. When compared with a commercial grade vinyl-ester control, VORAFORCE™ TP1270EU/1300 clearly demonstrated much superior mechanical properties, especially in crosswise direction where the resin has more impact. In addition, Polyurethane is styrene-free which renders a healthier work environment. Closed injection Pultrusion limits Volatile Organic Compound emissions and contributes to a very clean process in terms of industrial hygiene.

Subsequent field trials confirmed, in good agreement with modelling predictions^[2], VORAFORCE TP1270EU/1300 reaches more that 90% degree of cure at high Pultrusion speeds^[2] demonstrating that cure kinetics of VORAFORCE TP1270/1300 will not be a limiting factor to keep increasing productivity rate, unlocking the historical speed constrain of Epoxies. This positions DowAksa and Dow technology as an extremely competitive solution to satisfy the increasing demand of spar caps for the wind industry.

As a consequence of the Dow and DowAksa collaborative breakthroughs, a new technology was born to help advancing new generation of wind blade design. The obtained product offers the best mechanical performance/productivity ratio and simplifies processing when compared with incumbent technologies and products for the use in wind blade spar cap application.

This novel technology required developments along the process chain, from chemistry to hardware to enable stable processing and a high quality product. The DowAksa pultruded profile passed the stringent product requirements of Vestas Wind Power AS, a key global Windmill manufacturer, obtaining its full qualification in 2019, becoming the world’s first commercially available Polyurethane based composite produced by Pultrusion for wind blade spar cap applications. So far, millions of meters of pultruded profiles has been successfully delivered to Vestas at the highest level of quality, proving the robustness of the newly developed technology. This project serves the increasing demand of wind power installations by providing a reliable and innovative product made by an advanced processing technology to deliver improved turbine efficiency. This newly minted material exemplifies the power of Science and its boundless potential in helping to build a better future.