As windpower has become an important part of the world’s overall energy production, wind turbine manufacturers have sought out tough, durable adhesives to join the large, complex composite structures in wind turbine blades and nacelles.
Structural adhesives are specifically formulated to join a variety of composite materials – thermosets and thermoplastics, as well as metal – with assembly methods that are faster, less labor intensive, lighter weight, and more economical than traditional mechanical joining methods. Another benefit from structural adhesives is that the assembly joints are typically cleaner, more aerodynamic, and more visually pleasing than mechanically-fastened joints. Many manufacturers and installers have turned to these specialized adhesives for the fabrication and repair of wind turbine components.
Structural adhesives for wind turbine applications represent a technical evolution from adhesive formulations developed for other harsh environmental and dynamically-stressed conditions, such as marine structures and vessels constructed from composite materials. There are specific characteristics that many wind turbine manufacturers look for in selecting a structural adhesive for composites used in wind energy installations:
• Many manufacturers select methyl methacrylate adhesive (MMA) formulations for their wind turbine blades. Unlike epoxy adhesives, MMAs require virtually no surface preparation, and no sanding of laminate surfaces. In addition to significantly reducing labor costs during assembly, MMAs eliminate a major source of environmental dust during the manufacturing process.
• Because MMAs cure at room temperature, they do not require post-cure steps that involve an oven. Given the sheer size of many components used to make wind turbine blades, this eliminates both time and energy cost in the assembly process.
• Depending on blade design, the ability to use a large bead of adhesive on a slanted or vertical surface without sagging or slumping – and with virtually no movement prior to curing – can be critical to the assembly process. Because of their unique properties, MMAs can be applied in joints as thick as 40mm to 50mm, with a wide tolerance for thickness variations in the composite material being joined.
• MMAs achieve “green” strength (the ability to have a bond strong enough to move the bonded component before final cure) much more quickly than other adhesive technologies. This speeds the assembly process and reduces the number of molds required – often a major expense in composite manufacturing.
• An important advantage of MMAs is their “10:1” mixture of ingredients, which enables their cure time to be adjusted to compensate for the ambient temperature of the manufacturing environment, thus making for a wide operational window for manufacturing. Therefore, if the factory is hot (which speeds up the cure time), it can be adjusted to cure more slowly. The reverse is true for a cold factory environment (where the manufacturer can adjust the adhesive mixture for a faster cure). It is critical to note that the strength and durability of an MMA bond is not affected by this adjustment of the cure time.
• The overall advantage of MMAs is the strength and durability of the final bond, as well as its resistance to fatigue and the dynamic loading, shock loading and thermal stresses without losing strength.
• The adhesive supplier should also have a comprehensive test facility to perform the required testing for structural joint strength evaluation, temperature and humidity controlled chambers to test different environmental conditions, as well as chemical immersion and salt fog test chambers for joint durability testing.
Article by Manny Tesfaye, Director of Global Technical Services at SCIGRIP
Filed Under: Turbines