Note: This is the second article of a four-part series covering plastics in wind energy (1) trends, (2) material advances, (3) process technologies and (4) applications.
Wind energy provides significant growth opportunities for advanced composite resin and reinforcement materials. Wind energy developments hold much promise to help meet the growing global energy demand, and innovations in plastics and composites will play a key role. The long-term growth of wind energy requires technical innovation to make wind more competitive with other forms of energy.
Blade manufacturers are seeking ways to improve productivity by reducing cycle time and cutting costs. Robotic lay-up, enhanced finishing techniques, two-piece or segmented blades, and on-site manufacturing are potential tools to trim labor and logistics costs. Also, resin and prepreg suppliers are looking to develop materials that cure faster at lower temperatures.
Let's take a look at some promising new plastics developments in wind energy.
Promising thermoplastic material development for wind turbine blades is taking place. Wind turbine blades are usually made of glass fiber-reinforced epoxy, but it is a time-consuming process. While thermoplastic wind blades can be made faster with injection molding, only small blades can be made, and hot pressing techniques also limit product size.
For industrial-sized turbine blades — dozens of meters long — you need vacuum infusion. But the viscosity of melted thermoplastics is just too high to be able to force the material into every nook, cranny and glass fiber of those giant blades.
An additional drawback is that the material requires a higher processing temperature, which increases cost. To solve these disadvantages, manufacturers and researchers are trying innovative approaches using easy-flow materials, like the cyclic form of polybutylene terephthalate (PBT) and polymerized lactam (APLC-12).
The DUWind research program at the Delft University of Technology in the Netherlands has developed a process to produce thermoplastics composite wind blades using anionic polyamide 6 (APA 6), a reactive thermoplastic that processes like a thermoset. APA 6 is not put into the mold as a polymer, but rather as a reactive mixture with the monomer (ε-caprolactam) polymerizing into the high-molecular-weight thermoplastic polyamide 6 (PA 6).
DUWind/Delft University
Lab-produced 3.3-foot-long thermoplastic composite blade section demonstrator.
APA 6 is processed as a low-viscosity liquid that undergoes a slightly exothermic temperature-activated reaction. The material has a broad processing window and can be used in a thermoset infusion process similar to those familiar to wind-blade manufacturers.
With a viscosity of 10 megapascals (one-tenth the viscosity of epoxy), the unreacted mixture will flow into the mold much faster than epoxy, and it will also cure to form the polymer much faster. Because the viscosity of unreacted APA-6 is 10 times lower than that of epoxy, it does not have problems with wet-out and compaction in thick laminates. Researchers have infused APA-6 laminates to 50 millimeters (approximately 2 inches) thick with good results.
A special chemical-sizing agent is used on the glass fiber, with the reactive thermoplastic forms chemical bonds with the reinforcing fibers. This yields a better fiber-to-matrix bond. In normal thermoplastics, the polymer solidifies around the glass fibers, and the only bond with the glass is caused by shrinkage. So there's only mechanical locking around the fibers.
With the new sizing, the material not only becomes stronger, but the tighter chemical bonds also form stronger barriers for moisture. So the finished product takes up less water, and even when saturated the mechanical properties stay on a par with epoxy.
APA-6 compares well with epoxy in static properties for woven glass-fiber laminates even without optimized fiber sizing. It also outperforms melt-processed PA-6 (nylon) in dynamic fatigue, while matching its toughness, and it achieves a higher interfacial bond strength, which improves fatigue strength.
Continuing, high-quality polyethylene terephthalate (PET) structural foam has been launched into the wind energy market. New Kerdyn PET-based structural foam from BASF is high-performance foam supplied in the form of panels that can be used inside rotor blades to provide additional stability. With its ability to withstand high temperatures and its good chemical resistance, Kerdyn is extremely well-suited for use in composites.
Glass fiber-reinforced composite structures as shown in the above microscope image are composed of Kerdyn PET foam and the Baxxodur epoxy system. They enable larger, highly robust rotor blades and help make wind energy more efficient.
Product advantages include:
- Good mechanical properties (compression/shear)
- Wide process compatibility thanks to high temperature and chemical resistance
- Suitable for infusion, prepreg and resin transfer molding (RTM) technology, as well as for hybrid production processes
- Compatible with various resin systems
- Light and cost-efficient composite structure
In order to ensure sufficient stability, the structural foam has to give under the constant strain caused by gusts of wind. Rotor blades on a wind turbine are by no means rigid — to withstand forces, the right degree of flexibility is essential. Kerdyn properties enable it to withstand both static and dynamic loads.
For vacuum infusion of ever-larger rotor blades, BASF has developed the Baxxodur Epoxy System 5100, consisting of Baxxores ER 5100 resin and Baxxodur EC 5120 hardener. This new low-viscosity system not only results in fast and complete impregnation of the fibers, but also offers a considerably longer processing time than standard systems.
Finally, looking at thermoset advances, a new wind-blade protection coating is commercial. Awareness is growing in the wind industry about the severe impact that erosion on the leading edge of wind blades can have on turbine output.
Rotating up to 250 mph, wind blades are faced with erosion caused by rain, hail, salt spray and other debris, particularly offshore or in desert locations. Over time, these blades suffer pitting, gouging and delamination.
Erosion can affect aerodynamic efficiency and lead to an annual energy production loss of 20 percent or more, according to studies by 3M in collaboration with the University of Illinois. This eventually will cause costly wind turbine downtime. Wind-blade protection coating W4600 recently introduced by 3M, a two component polyurethane, provides 2-3 times longer blade protection than existing conventional coatings.
The material is an elastic two-component polyurethane coating that provides excellent erosion protection properties to help prevent and repair leading edge erosion on wind blades. Blade repair and protection can help provide significant annual energy production improvements, reduce costly downtime and protect the integrity of the blade.
Designed as a single-layer application in original equipment manufacturer (OEM) facilities, it is easily applied via brush or casting. The small increase in cost to apply leading-edge protection during manufacturing will be worthwhile throughout the life of a given wind farm.
3M also produces "wind protection tapes" constructed from tough, abrasion-resistant polyurethane elastomer, which shields leading edges and surfaces from pitting, wear and water ingression.
