The wind industry is interested in composites because it allows manufacturing large yet light structures such as the nacelle housing and long blades. Demand in the U.S. composites market is expected to grow at 6.6% annually, reaching $10.3 billion by 2019.
“High-performance composites consist of carbon, Kevlar, or glass fibers impregnated with a resin and cured into a shape using various molding methods,” says Mac Larsen, Director of Application Development at Graco Inc. “Because of its lighter weight and greater durability, carbon fiber is slowly earning wider use over glass fiber for use in blades, especially as wind turbines grow in size. Lighter blades also allow lighter turbine and tower components, so their cost savings justify the higher price of carbon.”
One widely used resin-fiber intermediate – a “prepreg” for pre-impregnated fiber sheets – is formed when a thermoset heat-curable resin is combined with carbon fiber and partially cured in a continuous film impregnation process. “Prepregs are made under factory-controlled conditions to ensure a tightly managed carbon fiber-to-resin ratio for consistent quality. As demand for high-performance composites grows, traditional prepreg processes must also keep up by transforming from basic batch operations into more efficient and higher-capacity automated processes,” says Larsen.
The prepreg resin that is combined with carbon fiber is a formulation of different resins, modifiers, and curatives, which are mixed together and then poured or pumped onto a filmer or coating machine by hand or with the assistance of a transfer pump. “The mixed resin formulation, called a premix, can either be frozen to suspend its reactivity for filming at a later time or it can be delivered to the filming process while still hot.
To ensure the highest quality prepreg product with the correct carbon fiber-to-resin ratio, the resin premixes are created in limited volumes multiple times daily and delivered to the prepreg filming process,” he says.
As the resin mix is coated onto a base release paper, a continuous supply of dry carbon-fiber strands are pressed into the resin film, producing prepreg. This product continues down the coating line where it’s cooled and cut into sheets or rolled up. Once the carbon fiber and resin mix cools, the resin solidifies and enters what is known as a “B-stage,” an intermediate stage where the resin is partially reacted yet stable for a short period.
Many resins used in high-performance composites have molecular make-ups that result in a thick or semi-solid product at room temperature. “That makes it necessary to heat and mix the composites into proper formulations before processing into prepreg intermediates. One idea for increasing capacity is to simply speed up the resin pre-heating to keep up with the demand of larger scale, prepreg processes,” he adds.
The benefits of faster heating include higher quality end products, improved prepreg-production logistics, and reduced resin waste, which is common with lengthy heating and reheating cycles. “Point-of-use resin heating is a continuous process that offers a high-volume, cost-saving alternative to traditional bulk-heating methods. Heating only what’s needed when needed is the primary goal of point-of-use resin heating, saving time and money,” he say.