the credit crunch pushes blade demand below supply levels, getting a durable
product made reliably and efficiently has never been more important. Adhesive
can play a key role in this goal.
In turbine blade binding, lifetime mechanical performance is
crucial. Cyclic loading and severe operational climates put high demands on all
materials in a turbine blade's design, especially the adhesive bonds holding
the major components together.
To achieve the best pure adhesive bond properties, sacrifices must be made in
matching the ultimate cohesive and adhesive strength of the adhesive and its
ability to hold two substrates together with its ability to absorb shock or
deformation before failure. Some of these tradeoffs are due to fundamentals of
polymer chemistry that can only be overcome at extremely high formulation costs
or with complex application processes. The challenge is to find a solution that
is compatible with turbine blade economics.
Structurally, it is important to not only have a strong bond but to match
substrate and adhesive elasticity performance to ensure stresses are managed
across a component assembly joint and not concentrated into the joint area. Anyone
who has tried using a rigid adhesive to repair leather shoes will understand
the need for this compatibility.
An adhesive is often first chosen on these mechanical properties. However,
because of the size of structures for wind turbine blades, a couple of
manufacturing issues must be considered:
- Preparation of surfaces to be bonded through each repetition of the
process. The time for dispensing, mixing and application of adhesive in
relation to adhesive gel speeds should be considered, including open and closed
- The cure speed at given temperatures, the resulting thermal shrinkage
and the adhesive’s thermal softening point will all influence the design of the
curing and demoulding cycle.
Reducing Turbine Blade Costs
Depending on delivery transport distances, turbine blades
can represent 15-25% of the total cost of a turbine, and, therefore, the total
cost of the electricity eventually produced. Much of this cost can be improved
if significantly higher production can be obtained from the same factory
investment. For instance, a 10% increase in cycle time out of a blade mould
will not only mean 10% more blades at the end of the year, but 10% less
overhead cost allocation. In times of high growth, such productivity gains can
delay the need for new plant investment. In times of lower demand, such
productivity gains can beat the competition and score the rare contract.
Significant improvements have taken place in the overall mould cycle time for blade
component manufacture. Assembly of the basic constituent components, such as
the sparcaps, shear webs, shells and roots, have been the main focus of most
development work to assist in reducing these cycle times to achieve higher
output. The heating and cooling processes used to speed the curing cycle can
lead to severe thermal variation in the adhesive joints between the components,
resulting in built-in stresses that may be large, especially in the often thick
bondlines used in rotor-blade assembly. These stresses can affect lifetime
Rotor blade manufacturers want their adhesive to address all risks. This
combination of properties is a constant trade-off between cure progression for
strength, shrinkage stresses caused by heat and exotherm, and the ability to
achieve a correct thermal resistance, often expressed as a glass-transition
). Such fast heating and cooling also
requires a tough adhesive to prevent thermal cracking, a property that usually reduces
. This becomes particularly important in large blade
production, since dimensional variation in moulding 30- to 50-meter long
composite components designed to fit together can require gluelines to fill in
gaps that may be considerably thicker than needed. Careful consideration is
given to these thicker regions that will show even higher cooling shrinkage
For blade manufacturers who have to avoid brittle adhesive, the preferred
option has been to address the risk of cracking by lengthening cure cycles to
provide lower thermal stresses. This unnecessarily reduces the productive
turnaround speed of the mould. Choosing toughened adhesives like Gurit’s
Spabond 340 minimizes this need for longer cycles, thus restoring production
capacity and opening up the possibility to review further cycle improvements.
Cross-section of a turbine blade.
Monitoring Adhesive and Blade Weight
high volumes of adhesive used in turbine blade assembly bring to mind another
consideration when choosing an adhesive - consumption. Not only is it crucial
to consider the cost per volume (not per mass) of an adhesive, but also the
rheology or flow characteristics of the product, which may significantly affect
the shape and size of the adhesive bondline and the amount of “squeeze out,” or
material wasted by inadvertent flow away from the desired position. Choosing
the right adhesive for both density and flow can affect the total adhesive
weight in a typical blade by as much as 50-100 kg. As blade makers push larger,
stretched blades onto existing turbine platforms to pick up lighter winds,
keeping blade weight low becomes a critical concern.
Consider Blade Size
blades get larger, price becomes increasingly dependent on transportation
costs. Before long, sophisticated modular designs with mid-length in-field
joints will be used, allowing blades to be delivered in transportable pieces.
While mechanical joints have many practical features for site assembly, the
smooth, larger areas of adhesive-bonded joints help to manage stress concentration
at these joints. Even mechanical fittings will have to be bonded to the
composite at some point, as many blade root studs are today. The challenge of
making this field-practical is one that again involves trade-offs in adhesive
design but offers financial rewards if solved by blade producers and adhesive
engineers working together at the design stage.
Assembly Joint Size Critical to Success
The assembly joint design is a critical (and often
overlooked) element of successful bonding; blade design is no exception.
Adhesives work best if peel stresses are kept away from the joint, but this is
not easy. Localized joint design for leading edge, spar cap and other joints
can have a dramatic effect. This is often an area where most adhesive suppliers
leave the clients’ designers and manufacturing engineers to provide solutions
on their own. With its experience in raceboat and wind blade engineering, Gurit
offers a large structural engineering service that can recommend designs for
the adhesive joint profile to optimize component structure while integrating
By carefully designing the structure and its joint elements, as well as
considering the production process, over-engineering can be avoided. Thus,
material consumption, time and energy can be efficiently used in the blade production
to avoid waste.
For more information, contact Peter George, Head of Business
Development, Gurit UK, St. Cross Business Park, Newport, Isle of Wight, United
Kingdom PO30 5WU; phone +44 (0) 1983-828-000 ext. 448; fax +44 (0)
1983-828-100; e-mail email@example.com; or visit www.gurit.com.