SparPreg™
The main load bearing structure of a wind turbine blade is the spar component which is either integrated into a structural shell as a spar cap, or constructed in parallel production to the shell as a separate spar structure complete with shear webs.
What is common to both approaches is that the utilization of unidirectional fibre (UD), glass or carbon, to provide bending strength and stiffness. The quantity of UD material required is significant, resulting in laminate sections up to 50mm thick towards the root section. This provides some technical challenges when considering fibre alignment, resin content, void content, deposition rate, exotherm control, and connection to the shear web using multiaxial materials.
Furthermore, as blade size has increased and carbon fibre is used more widely, the strength requirements have become the main design driver placing even more focus on fibre alignment and high quality laminates with low void content.
With the current focus in the market to look at the cost of quality for blade manufacturing there has been an increasing demand for products that produce a consistent quality for a competitive overall blade cost.
One of the key areas for improvement was that of the spar cap where the associated difficulties in processing had lead to variability in part quality and/or expensive engineering solutions. Using Gurit’s experience in air breathe materials and solutions, namely SPRINT®, a new unidirectional material has been developed known as SparPreg™.
This product has been specially formulated to facilitate the removal of inter-ply (between the plies) air within a UD stack using standard vacuum bag processes, and without intermediate debulking stages. SparPreg™ is also designed to be fully compatible with SPRINT® multiaxial materials, providing a very versatile material solution for all structural spar concepts.
Whilst being an ideal replacement for any UD prepreg used in spar manufacture where air removal is a problem, there is also a place for SparPreg™ in an infusion process.
There are many techniques for the manufacture of spar caps including wet laminating and infusion of woven UD fabrics, wet tow placement, and prepreg. These are often manufactured off-line to enable full quality validation of this critical component before integration with the shell structure.
One of the key issues with spar cap manufacture is the attainment of low void content with large UD cap thickness. The problem is compounded by the low breathability of UD materials, making them notoriously difficult to infuse, or in the case of prepreg remove the interplay air. These issues are normally overcome by integrating multi-axial materials within the UD laminate stack to provide faster infusion rates, and to facilitate air removal from a prepreg stack.
SparPreg™ provides another, hybrid solution that can help to reduce void content and eliminate the need to conduct timely UD infusions. SparPreg™ can be used to manufacture fully cured spar cap components that can subsequently be introduced into the shell laminate for the final infusion of the shell laminate.