JEC COMPOSITES MAGAZINE - Issue #116 - October 2017 - 48

WIND ENERGY
resin

New products for longer blades help to cut cost

Konrad GEisslEr, Technical Service &
Development Manager Wind and Composites;
lEiF olE mEyEr, Lead Technical Manager
R&D Wind and Composites;
JEan-luc GuillaumE, Global Marketing
Director Wind and Composites
Olin

uring the last few years, the
political
environment
for
new wind-energy projects has
changed dramatically. Most countries
have moved away from fixed feed-in
tariffs to reverse bid auctions, leading
to fiercer, more direct and visible
competition vis-à-vis the levelized cost
of energy (LCOE). A closer look at the
current wind turbine market shows that
all major trends are driven by cost pressure. Among these, the most obvious
ones are:
- the move towards more cost-efficient
production, mainly through cuts to
the bill of materials and to cycle time;
- bigger turbines with longer blades for
both on- and offshore installations;
- improved reliability to reduce maintenance cost.
These trends have a direct impact on
the required material properties and on
the processability of any new infusion
resin and adhesive. Limitations of first
generation products become increasingly visible with the move towards
longer blades. Moreover, as knowledge
on composites grows, designers and
manufacturers can define required material properties for new blades much
more precisely.
All these aspects finally require changes in specification limits - or even

The wind turbine manufacturer's answer to increasing pressure to
reduce energy production costs is based mainly on longer blades,
faster production and improved reliability. Olin's new product portfolio
with toughened adhesives and an improved infusion system is specifically designed to meet these requirements by linking the proven
performance of epoxies with innovative features like enhanced crack
resistance or lower defect rate.
completely new specification items to
be considered. As an answer to these
changing conditions, Olin Epoxy has
developed a toughened, glass-fibre-free
adhesive to meet the current and
future requirements of wind turbine
blade manufacturers, and an optimized
infusion resin.
Bonding paste
Cracking is a problem often encountered when applying aggressive cure cycles to long blades, so the set of critical
requirements for the new AIRSTONE™
870 bonding paste included reduced
cracking, achieved by considerably increasing elongation at break (see Table
1 for details on mechanical properties).
Increased elongation at break also significantly extends the adhesive's fatigue
life, since the end points of real-life
fatigue cycles are determined by the
surrounding laminate. Hence, under
the same load conditions and with the
same laminate stiffness, only a lower
fraction of the break elongation will be
encountered by the adhesive. This will
increase the number of cycles before
fatigue failure occurs.
Apart from the higher elongation at
break, the material is also modified by
incorporating a toughener. This leads
to lower crack propagation, as can be
seen by the k1c value that is approximately tripled compared to a standard
adhesive.

jec composites magazine / N°116 October 2017

Furthermore, the material is highly
resistant to carbamation. As customers
shift to longer blades, the total bond
line length increases, and so application
times for the bonding paste can also
increase, leading to more stringent
requirements on open time. It goes
without saying that open time must
not have a significant influence on
bonding quality. However, between
application of the bonding paste to the
blade shell and closing of the mould,
the adhesive is left exposed to the atmosphere. Depending on humidity and
temperature conditions, carbamates
can form on the surface of the adhesive,
potentially and critically impairing the
adhesion between bonding paste and
shell laminate. As can be seen in Fig.1,
AIRSTONE™ 870 shows excellent
carbamation resistance, with remaining
lap shear strength clearly above 20MPa
when exposed to two hours open time
at increased temperature and humidity
during sample preparation.
For easier processing, and also to
reduce density, this bonding paste is designed to meet all requirements without
the need to add glass fibres. To achieve
their mechanical properties, traditional
wind-blade bonding pastes contain
glass fibres. However, glass fibres can
cause abrasion in mixing machines,
finally leading to higher maintenance
costs. Since this adhesive is glass-fibre
free, it is non-abrasive in the mixing

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #116 - October 2017