Wind energy is regarded as the number one choice in Europe’s efforts to move towards clean, renewable power and is expected to power industry, business and homes with clean electricity for many years to come. The trend has led to the growth in the size of wind turbines, particularly in offshore wind farms. In what has become a rapid growth phase for the industry, certain technical and quality-related issues have emerged, creating significant challenges which manufacturing and material innovations offer the power to overcome. In the wind industry, the ability to optimise the adhesive application on very large surface areas and tominimise labour intensive assembly processes has been identified as a key factor. Here Nastassja Rothe, Wind Energy Marketing Manager for Europe, from Huntsman Advanced Materials reports.
The trend goes to larger wind turbines. (photos: Huntsman)
The trend in increasing turbine size has created the need for bigger rotor blades to broaden the sweep area and ability to capture more energy. Rotor blades over 50m are now gaining an increasingly larger market share and 60m blades are set to become more commonplace. The development of 70 to 80m blades has started and the 100m is also being considered for offshore applications. The increasing length of offshore wind turbine blades has prompted the more widespread use of advanced composite materials which in turn has led to growing requirements for new materials and manufacturing methods.
The six steps of composite blade production using a resin infusing system (photos: Huntsman)
With larger blades more complex tooling and composite mould designs come, longer production times and higher processing costs. Larger blades also create greater stress on the structural, mechanical and gear components of the turbine. Hence, manufacturing and material innovations are mandatory for making this a cost effective renewable technology.
Araldite adhesives help to streamline structural designs and processes in wind blade manufacturing
Huntsman offers a comprehensive range of Germanischer Lloyd (GL) approved Araldite epoxy resin systems which are said to meet stringent processing and performance requirements for composite mould production and wind blade manufacturing. The formulation of epoxy-based resin systems that can be used to vacuum infuse dry fibres or preforms holds structural benefits for producing large, complex composite parts with less than 1% void content and controllable resin-to-fibre ratio. This leads to increased strength as well as predictable performance and integrity of the finished part.
Vacuum infusion using GL approved Araldite epoxy resin systems
Araldite LY 1568/Aradur 3489 is an example of a new epoxy-based resin infusion system from Huntsman which claims to offer more control in the manufacturing process. The combination of its low mix viscosity (200-300mPas at 25°C) and its low exothermic reaction and long pot life (850-950min) is said to provide a major benefit for its users in the infusion of long or thick composite parts. Araldite LY 8615/Aradur 8615 and Araldite LY 8615/XB 5173 are examples of the latest high temperature infusion systems which are said to provide advantages for high temperature composite blade mould production. It claims to withstand significantly higher temperatures than the curing temperature of the component production process.
Both systems can be processed from room temperatures up to 40°C and offer heat resistance up to 180-200°C. The Tg can reach over 210°C after post curing, making these systems particularly suitable for the production of blades made of high performance prepregs, requiring high post cure cycles.
The latest approved Araldite epoxy resin system is Araldite AW 4856 / Hardener HW 4856 designed to improve productivity and increase fatigue resistance for wind blade shell bonding and repairs. As a wind blade spins, it is subject to static and dynamic forces that vary along the blade from the supporting root to its tip. To account for these diverse loads, this adhesive system has been formulated to deliver increased toughness, offering good load bearing strength and a lap shear strength of 22-25MPa. In comparison to the previous adhesive from the range (which had a linear shrinkage of 1.5 to 2.0%), this adhesive has reduced linear shrinkage in the range of 0.2 to 0.6%, claims Huntsman. This would serve to reduce internal stress during curing.
This adhesive’s toughness and fracture properties have also been enhanced. The fracture toughness, in other words the ability of a material containing a crack to resist fracture, has increased from 1.2-1.3MPa.m1/2 to 2.9-3.2MPa.m1/2 – a major benefit for the blade service life once installed.
In combination, this system’s good thixotropic and low exothermic properties lead to a long pot life of 240min at 23°C, fast strength build-up and reduced curing cycle capabilities. The standard recommended cure time of 5h at 70°C could be reduced to 2-3h at 70°C with the necessary minimal mechanical properties – including lap shear strength, Tg and E-Modulus, states Huntsman.
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