Gurit Materials for Wind Turbine Blades
Introduction
Gurit has been supplying materials to the wind energy market for the manufacture
of composite blades since 1995 and has been actively involved in the development
of specialised blade materials as blade designs have increased rapidly in size and
complexity. Gurit has a product portfolio to cover all current manufacturing approaches
including wet lamination, resin infusion, and prepreg. With the advent of larger
blades and increasing Health and Safety awareness, the majority of blades are now
manufactured either by the resin infusion or prepreg process, and therefore whilst Gurit
also manufactures wet laminating resin systems, the focus of development has been
on prepreg and infusion technology and associated products.
Materials for Infusion Blades
¬ Infusion Resin: ¬ Structural Adhesives: ¬ Structural Core:
PRIME™ Infusion Family
Spabond 340™LV
G-Balsa, PVCell, Corecell™ T-Foam
Materials for Prepreg Blades
¬ Prepregs:
¬ Coatings:
¬ Structural Adhesives: ¬ Structural Core:
¬ Advanced Prepregs: WE91 Prepreg Family
CR3400 Process Coat
Spabond 340™LV
Corecell™ T-Foam
SPRINT™, SparPreg™
Gurit Materials Portfolio
Gurit’s development philosophy is based on the overall objective of reducing the cost
of blades, and therefore the Cost of Energy (COE) for wind power, by using advanced
materials and structural design to eliminate blade manufacturing processes or make
them as efficient as possible. The COE is also calculated over the entire lifecycle of a
Wind Turbine Generator (WTG) and therefore the cost of quality of a turbine blade is
also a key parameter. As a consequence Gurit materials are also developed to provide a
robust and consistent blade manufacturing process with exceptional quality.
As blade design becomes more sophisticated the number of products specified in
a bill of materials is increasing. As the industry drives to reduce the COE it has put
more emphasis to engineer out over-specified materials and replace with them with
more cost efficient alternatives that are fit for purpose. This has created more niche
applications for materials with more specific material characteristics and appropriate
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades cost base. In response to this market trend, Gurit is using its materials and engineering
know-how to identify, develop, and expand its product range to maintain its position as
the leading supplier of composites materials to the Wind Energy Market.
The maturing Wind Energy market has also driven the demand of product quality towards
6 sigma standards in order to further reduce process variability and subsequently the
COE. As a supplier to both the Aerospace and Automotive markets, Gurit understands
the importance in understanding customer needs through their Critical To Quality
requirements (CTQ’s) and therefore Gurit designs, manufactures and supplies products
accordingly by ensuring that stringent quality checks are made to provide a consistent
product.
A composite structure is by definition a mixture of very different materials and therefore
the properties and interaction between materials during processing need to be well
understood to produce a high quality component. Gurit supplies the full product portfolio
for the manufacture of infusion and prepreg blades and as a consequence is very well
placed to understand the complexities of blade material design and manufacture. This
section of the Wind Manual takes a look at some of the key products in the Gurit
portfolio for both infusion and prepreg blade manufacturing and highlights some relevant
technical features of the materials.
Materials for Infusion Blades
The four main components in the Bill of Materials (BOM) for an infusion blade are
the dry fabrics, the infusion resin, the core material, and the structural adhesive. The
manufacture of fibres, and/or the conversion of fibres into bespoke fabrics, are not part
of Gurit’s business strategy and therefore not discussed further. The remaining three
material categories are all areas of expertise for Gurit where considerable development
activity has been focused to provide a product range with market leading performance.
The product offering from Gurit in these three categories is as follows:
¬ Infusion Resin: ¬ Structural Adhesives: ¬ Core:
PRIME™ Infusion Family
Spabond 340™LV
G-Balsa, PVCell & Corecell™ T-Foam
Infusion Resin
Deciding whether a resin system is suitable for an infusion process requires careful
thought to be given to many different aspects of the production process including the
component size, ambient conditions, reinforcement, thermal & mechanical properties
and blade certification. The future optimization of the production process must also be
considered; mould utilization and cycle time is paramount to reducing cost. In order to
achieve this a robust infusion resin that offers a range of speeds that can be tailored to
continually improved processes is essential.
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades By recognizing this need to reduce cost through process optimization, Gurit is able to
offer such a versatile system through the PRIME™ infusion system, which has excellent
properties and a range of hardeners that can tailored to fit vitually any production process.
PRIME™ Infusion Resin Family
¬ Very low viscosity – minimising infusion time and blade production time
¬ Variable infusion times – a range of different speeds of hardener to fit any process
¬ Very low exotherm even in thick sections – extending mould life
¬ Suitable for infusing very large structures – even in hot climates
¬ Certified - Germanischer Lloyds approved
Infusing with PRIME™ Infusion Resin
PRIME™ Infusion Resin has a reduced viscosity resin and long working time, which
makes it ideal for infusing very large parts with complex reinforcements in one operation.
It maintains an exceptionally low exotherm characteristic, which allows thick sections
to be manufactured without risk of premature gelation due to the heat of exothermic
reaction. This low exotherm will also help to extend the life of the tools due to the
reduction in thermal cycling. The PRIME™ infusion system has been used successfully
for the single-operation moulding of components ranging from narrow carbon yacht
masts, up to 80’ yacht hulls and wind turbine blades. It achieves excellent mechanical
and physical properties from a moderate (50°C) postcure, offering the finished laminate
properties that lie between hand lamination and low-temperature cure prepreg.
The PRIME™ infusion system system is available with a range of hardeners, offering
a range of working times and cure speeds providing versatility to manufacture a wide
range of components. The cure speed can be characterised by a variety of techniques
such as gel time and pot life. These characteristics have been summarized below for
Gurit’s standard hardener range. Also provided are estimates for the time at which the
resin will stop flowing under vacuum, the earliest time that the vacuum can be removed
and the minimum time before de-moulding can occur at a range of temperatures.
However, it should be noted that these estimates are only valid for relatively thin
laminates without exothermic behaviour.
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades Fast Hardener
Properties
Slow Hardener
Extra Slow Hardener
20°C
25°C
30°C
20°C
25°C
30°C
20°C
25°C
30°C
Initial MixedViscosity (cP)
318338
219232
127134
308328
214228
172182
347369
220234
155165
Geltime – Tecam 150g in water (hr : min)
01:09
00:30
00:17
05:00
03:20
02:20
10:20
08:00
06:15
Pot life 500g in air (hr : min)
00:28
00:23
00:16
01:15
01:00
00:45
06:00
04:05
02:10
Latest flow under vacuum (theoretical, thin 03:10
film, hr : min)
02:40
01:50
05:20
04:50
04:10
10:50
08:50
06:40
Earliest vacuum offtime (theoretical thin
film) (hr : min)
04:15
03:20
02:15
07:30
06:30
05:50
15:10
11:30
08:50
Demould time (hr : min)
06:45
05:00
02:30
21:00* 15:30* 11:50* 88:00* 50:00* 28:00*
PRIME™ Infusion Properties
*Demoulding components made with Slow or Extra Slow Hardener should only be carried out after the part has received an
elevated temperature cure in the mould.
In recent years there has been a move to infusing larger blades in hotter climates. In
order to address this, Gurit has developed two additional hardeners specifically for
the Wind Energy market that are capable of achieving exceptional pot-lifes in excess
of 10 hours, whilst maintaining thermal and mechanical properties.
Matrix Performance
The cured PRIME™ resin matrix has been formulated to offer excellent mechanical
and thermal properties irrespective of the speed of hardener used. The table opposite
summarizes the key properties for each speed of hardener.
Hardener
Fast
Slow
Cure Schedule
Extra Slow
16hrs @ 50°C
Tg (DMTA - peak tan δ)
82.8
82.6
82.9
TgUlt (DMTA)
74-76
87-89
90-92
Tg1 (DMTA)
68-70
68-70
69-71
ΔH - DSC (J/g)
1.54
7.3
0
Estimated HDT
67
68
67
Tensile Strength (MPa)
75
73
69
Tensile Modulus (GPa)
3.2
3.5
3.5
Strain to failure %
4.1
3.5
3.1
Cured density (g/cm3)
1.153
1.144
1.132
Linear Shrinkage (%)
1.83
1.765
1.541
Barcol Hardness
21
27
25
PRIME™ Infusion Mechanical and Physical Properties
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades As part of the infusion process it is common to use 2 component mixing machines
capable of mixing to a high degree of accuracy – ideally less than ± 1 part for any epoxy
based infusion system. However, it is sometimes required that small quantities of resin
be mixed by hand or drill for repairs or other such applications that only require small
quantities of resin. Therefore, it is important to know that the infusion system is robust
to be mixed of ratio. The mechanical performance of the PRIME™ infusion system
using Fast hardener is maintained even when mixed off-ratio by up to ± 2 parts (subject
to customer requirements).
Effect of Mix Ratio on Mechanical Properties
The PRIME™ infusion system provides excellent laminate properties as shown in the
Table below, which considers a 5mm thick UD laminate manufactured using PRIME™
infusion resin and stitched UD fabric.
Property
Test Standard
PRIME™ Infusion Resin
Normalised Tensile Strength @ 53%
FVF (MPa)
DIN EN ISO 527
990
Normalised Tensile Modulus @ 53%
FVF (GPa)
DIN EN ISO 527
43.7
Normalised Compression Strength @
53% FVF (MPa)
DIN EN ISO 14126
645
Normalised Compression Modulus @
53% FVF (GPa)
DIN EN ISO 14126
42.7
ILSS (MPa)
DIN EN ISO 14129
68.0
PRIME™ Infusion Laminate Properties
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades Structural Core
As the Wind Energy Market has matured the optimization of blade design has become
critical as blade lengths and weights have increased. As a consequence the one core
fits all approach is no longer relevant for a modern wind turbine blade. Structural core
is used in both the structural shell and shear webs of a standard infusion blade design.
The material requirements for a structural core can vary considerably from shell to
shear web and also from one area of a shell to another. Therefore, to optimize the
design, weight and performance of a blade the blade designer may make use of a
number of core products.
In response to the demand for multiple core designs Gurit has recently extended its
product range and now covers PVC, Balsa and SAN core types all with a distinct range
of properties and attributes.
Corecell™ T-Foam
¬ Environmental stability – High tolerance for heat and chemical exposure
¬ Built in toughness – High ductility and damage tolerance compared to cross-linked
PVC and Balsa
¬ Fine cell size – Resin absorption is very low, saving both weight and cost
¬ Superior uniformity – Low density variation
¬ Eliminating outgassing – Corecell™ eliminates the problems of foam outgassing
¬ Compatibility – Suitable for use with all polyester, vinylester and epoxy resins
¬ No inhibition - Corecell™ does not inhibit any epoxy resin curing mechanisms
¬ Handling – Tough and easy to machine
An Introduction to Corecell™
Corecell™ T-Foam has been developed as a technological step-change from traditional
PVC and Balsa structural core. Corecell™ T-Foam is an outstanding core material in
every application where balsa or PVC is commonly used. High mechanical toughness
and thermal stability give Corecell™ T-Foam excellent fatigue characteristics. This
reliability makes Corecell™ T-Foam a natural replacement for cross-linked PVC or balsa
in applications where a significant service life is required.
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades Fatigue Performance of PVC and SAN Foam
The formulation of Corecell™ T-Foam generates a remarkable thermal stability for a
polymer foam. At 100°C, conventional cross-linked PVC foams retain less than 20%
of their room temperature compressive properties, whereas Corecell™ T-Foam
retains almost 60%. This stability means that Corecell™ T-Foam structures exposed
to temperature variation will maintain their properties and cause very little core printthrough. The high temperature stability of Corecell™ T-Foam also means that it can be
used in manufacturing processes to at least 120°C with short durations during a cure
cycle to over 150°C. This makes it ideal for use with conventional prepregs and in some
liquid infusion processes where high resin exotherms can often be seen.
Corecell™ Cutting Patterns
Corecell™ T-Foam is available in every resin infusion format and is compatible with polyester,
vinylester and epoxy resin systems. One of the key variables of the infusion process is
the consistency of the resin flow from blade to blade, and the subsequent variation in blade
weight and quality. To overcome these phenomena, Gurit have developed customized cutting
patterns in the core material to optimise resin flow and distribution, whilst maintaining low
resin uptake within the structure and eliminating the need for sacrificial meshes.
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades A sum
Corecell™ with Infusion Knife Cut Pattern
This cut system uses a thin knife cut (typically 0.8mm) at a 30mm cut spacing on one side
of the sheet, coupled with the same pattern but in the perpendicular orientation on the
opposing face. Using the cuts in this way provides excellent flow characteristics, whilst
providing the conformability required to suit difficult mould shapes and curvatures. The
low cost characteristics of this type of pattern are enabled firstly by the material requiring
no glass scrim to be applied (often used to provide stability to heavily cut infusion core
patterns), and that no drilling or perforating is required – the depth of the cuts and the
regular 30mm by 30mm intersection provides a perfect infusion flow between the laminate
skins. In wind energy blade manufacturing, this cut configuration has been proven to:
¬
¬
¬
¬
¬
Reduce overall resin consumption
Provide consistently faster infusion times (as much as 20%)
Provide more repeatable infusion quality
Reduce dry areas caused by scrim cloth
Improve flow from perforations created by cross-cuts rather than drilled holes
As a practical example two test panels were manufactured simultaneously using the
two foam types: T400 and a PVC foam of an equivalent density. Both core materials
were prepared using the same kitting patterns and used in infusion panels manufactured
using PRIME™ 20 resin system and slow hardener. It was immediately apparent that
despite both infusions starting simultaneously, the Corecell™ panel infused before the
PVC. This is due to the surface characteristics of the Corecell™ foam and the low resin
drag of the foam.
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades Infusion Rate Testing Experiment
A summary of the finished panels is included in the table below.
Panel Status
Corecell T400 Panel (g)
PVC Panel (g)
Total dry weight before infusion
2152
2000
Total weight after infusion
4530
4625
Trimmed panel weight with mesh and
peel ply removed
3055
3170
Resin difference (of Type A)
-115
-
Resin difference per square metre
-250
-
Percentage of Type A
96.37%
100%
Resin Uptake Comparison
The Corecell™ panel had a higher initial weight due to the higher density of the T400.
But despite this the reduction in resin uptake, resulted with the final trimmed T400
panel being 115g lighter. This equates to 250g over a full square metre of the same
panel or up to 60kg in a 40 m blade.
G-Balsa*
¬ Excellent in-plane properties - high in-plane shear and compressive properties
¬ Renewable material – a natural product from a sustainable source
¬ Design-specific – ideal for existing blades designed using balsa as structural core
¬ Finished kits available – PVC or hybrid solutions can be machined to customer requirements
* G-Balsa only available in China.
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades PVCell*
¬ Wide Range of formats – available with standard densities and cut patterns
¬ Suitable for existing blade designs – any approved design using Diab or Airex PVC core
¬ Finished kits available – PVC or hybrid solutions can be machined to customer
requirements
¬ Manufactured on China mainland – ideal for domestic Chinese market
* PVCell only available in China.
Structural Adhesives
Of highest importance in advanced composite turbine blade applications is lifetime
mechanical performance of these large dynamic and active structures. Performance
of the structure through cyclic loading and often severe operational climates puts high
demands on all materials in their design, but even more so on the adhesive bonds
holding the major components together.
To achieve the best properties in pure adhesive bonds, there are trade offs in matching
the ultimate cohesive and adhesive strength of the adhesive, with its ability to absorb
shock or deformation before failure. Some of these trade offs are due to fundamentals
of polymer chemistry that can only be overcome at extremely high formulation cost or
with complex application processes. The challenge is to find a solution that is compatible
with wind turbine blade economics.
The choice of adhesive is often based on these mechanical properties. However, for
structures of the size being considered in wind turbine blade, manufacturing issues are
always key considerations. This should include:
¬ Preparation of surfaces to be bonded
¬ The time for dispensing, mixing and application of adhesive in relation to adhesive
gel speeds:
¬ Open assembly time – the time between the adhesive mixing and application to
the first bonding surface before the components have to be clamped together and
how this varies with ambient temp and humidity changes - avoiding “dry joints”
¬ Closed assembly time - how long the adhesive continues to flow and compress
to ensure intimate contact and allow component repositioning while ensuring the
glueline meets the expectation of the structural calculations
¬ The cure speed at given temperatures, the resulting thermal shrinkage as well as
thermal softening point of the adhesive will all influence the design of the curing
and demoulding cycle
Spabond 340 LV
¬ Excellent gap filling properties – ideal for variable bondline thicknesses
¬ High strength and toughness industrial adhesive, even when mixed up to 5 parts off-ratio
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades 10
¬ Variable Clamp Times – three hardener speeds to give a range of working times
¬ Low exotherm and shrinkage – minimizing risk of cracking
¬ High Temperature Resitance – high performance up to 75°C
Mixing and Application
Spabond 340LV is a toughened, thixotropic adhesive system with a simple 2:1 by
weight and by volume mix ratio. It is designed for use with a mixing machine and
has a sag resistance of 30mm on a vertical surface. The range of hardeners available
make this product ideal for bonding small or large structures. The resin & hardeners are
pigmented to give a visual indication of mix quality. The robust formulation ensures that
the high strength and toughness performance of Spabond 340LV can be achieved even
off-ratio by up to 5 or 10 parts (subject to customer requirements).
Effect of Mix Ratio on Mechanical Properties
Structural adhesives used in wind turbine blade manufacture are often applied in thick
bond-lines up to 40 or 50mm in diameter and often in vertical areas of the blade. This
demands that the adhesive has good wet adhesive and thixotropic properties to ensure
that it remains in place up until the point at which the to parts are brought together.
Spabond 340LV offers not only excellent sag resistance, even at high temperatures but
also has a remarkable shear recovery after being dispensed through a mixing machine.
This is a property that can be overlooked, but forcing an adhesive through a static or
dynamic mix head places high shear forces onto the adhesive in order to achieve a good
mix quality. However, this process also has the tendency to ‘shear thin’ the adhesive
which reduces the thixotropy (sag resistance) of the material. Once dispensed the
adhesive is no longer under a shear load, but it takes time for the adhesive to achieve
it’s original thixotropic properties – which could lead to the adhesive sagging or falling
from the point of application.
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades 11
Blade weight
The high volumes of adhesive used in the turbine blade assembly raise another
factor in choice of adhesive, namely consumption. Not only is it vitally important 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 significantly influence total weight of adhesive in a typical blade by as much
as 50-100Kg. As blade makers push larger stretched blades onto existing unchanged
turbine platforms to pick up lighter winds, keeping blade weight reduced in this way
can be critical.
This highlights another unusual characteristic of Spabond 340LV adhesive, that it has a
considerably lower density when compared to an equivalent glass toughened adhesives
as the table below shows.
Sample
ρresin
ρhardener
Mix Ratio (R:H) ρmixed
Spabond 340LV with Extra Slow Hardener
1.120 g/cm3
1.060 g/cm3
100 : 50
1.099 g/cm3
Equivalent Glass Filled Adhesive
1.270 g/cm3
1.110 g/cm3
100 : 45
1.216 g/cm3
Mixed Density Analysis of Spabond 340
A lower density can not only reduces the overall weight of the blade, but as the industry
tends to buy structural adhesives by the kg, Spabond 340LV is also more cost effective
per unit volume.
Reducing blade costs
In recent years there have been significant improvements 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 has been the main focus of development
work to assist in reducing these cycle times to achieve higher output. However, the
heating and cooling processes used to speed the curing cycle can lead to thermal
variation in the adhesive joints between the components, resulting in built-in stresses
which may be large, especially in thick bondlines used in rotor blade assembly. These
stresses can affect the lifetime fatigue performance.
Spabond 340 LV was formulated to provide excellent toughness by preventing crack
propagation both in static and dynamic fatigue stress states. The single lap shear
fatigue performance of Spabond 340 LV resin with Extra Slow hardener compared
to a typical glass filled/epoxy based adhesive is shown in the schematic below. The
substrates were made of epoxy/glass UD laminates and the testing was performed at
5Hz and at R=0.1.
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades 12
Fatigue Performance of Spabond 340 LV
Rotor blade manufacturers have detailed specification for their adhesive to try and
address as many manufacturing and in-service risks as possible. This combination
of properties required 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 temperature (Tg). Such fast
heating and cooling also requires a tough adhesive to prevent thermal cracking, a property
that usually reduces Tg. This becomes particularly important in large blade production
since dimensional variation in moulding 30 - 50m long composite components can be
significant. Careful consideration is given to these thicker adhesive regions that will
show even higher cooling shrinkage stresses.
For many blade manufacturers who have had to avoid brittle adhesive cracking, the
preferred option has been to address the risk of cracking by lengthening cure cycles to
provide lower thermal stresses*.
*Lengthening cure cycles has a detrimental effect on productivity and is therefore a considerable
compromise. Typical cure development data is shown overleaf for Spabond 340 LV.
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades 13
Thermal Properties of Spabond 340 LV
Materials for Prepreg Blades
A prepreg blade typically comprises of a root and spar manufactured using UD glass
or carbon prepreg, biax prepreg & structural core; and a shell using multiaxial prepreg,
structural core and an in-mould process coat. These components are then assembled
using a structural adhesive (for female box spar construction) such as Spabond 340LV or
a non-structural PU adhesive (for male mandrill box spar construction).
Gurit has a long history supplying prepreg, core, processs coats and structural adhesive
for the manufacture of prepreg blades, as well as leading the way in developing advanced
prepreg technology. The product offering from Gurit in these five categories is as follows:
¬ Prepregs: WE91 Prepreg Family
¬ Coatings: CR3400 Process Coat, UV stable Top Coat
¬ Structural Adhesives: Spabond 340 LV
¬ Structural Core: Corecell™ T-Foam
¬ Advanced Prepregs: WE91LE, SPRINT®, SparPreg™
Prepreg Technology
Prepreg technology is used in applications where weight, performance and consistency
of quality are prerequisites. Wind Energy prepregs are characterized by their high areal
weights, cost-effective resins, high deposition rates, and simplistic fibre orientation:
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades 14
¬ Glass/epoxy multiaxials (300-2000gsm)
¬ Glass/epoxy unidirectional (600-1600gsm)
¬ Carbon/epoxy unidirectional (300-800gsm)
Choosing prepreg for wind turbine blade manufacture, effectively reduces the complexity
of the component manufacture by eliminating mix ratio, mix quality, resin content, fibre
alignment and impregnation quality issues.
WE91 Prepreg Family
¬ Reduced Cycle Times - lower peak exotherm in thick sections
¬ Diuron-free - improved Health and Safety
¬ Versatile - long out-life at room temperature & available with a range of
reinforcements
¬ Recommended cure between 85ºC and 120ºC - excellent laminate quality, low
bleed
An Introduction to the WE91 Prepreg Family
The WE91 series is part of the WE and WT range of prepreg, SPRINT®, and AIRSTREAM™
products. This unique product range provides technically and commercially competitive
engineering materials, ideal for use either solely, or in conjunction with other SP
products from within the range.
The WE91 series is a high flow, Diuron free epoxy prepreg ideally suited to the
manufacture of thick sections. It can be cured at temperatures as low as 85°C, but can
also be used for the rapid manufacture of components through its 45-minute cure at
120°C. All of this can be achieved together with an out-life of 60 days at 21°C.
The WE91 series is designed for vacuum bag processing and offers excellent mechanical
performance on glass and carbon fibre reinforcements. WE91 is pre-impregnated into
three types E-glass fibre, unidirectional, biaxial and triaxial, all of which are manufactured
in large volumes in order to make it a cost-effective composite building block for a
range of applications. Other WE91 products include pre-impregnated peel ply, and
needlemat.
The unidirectional glass prepreg uses E-glass in 600 - 1600g fibre weights. This provides
a very economical way of laying down a large thickness of high performance material.
It is particularly suitable as the primary composite material in structures which are
subjected to longitudinal compression and bending, such as masts, poles and other
beam like structures. It can be supplied in widths of up to 1400mm.
The unidirectional carbon prepreg is suited for use when high mechanical properties are
required, it is available from 300 to 800g fibre areal weights.
The biaxial prepreg is a ± 45º stitched E-glass fabric using a fibre weight of between
300 and 1800g. This material can either be used alone as a thick drapeable fabric or as
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades 15
a secondary product in conjunction with the unidirectional product, where it imparts
tensile and torsional strength and shear stiffness. It can be supplied in widths of up to
1400mm wide.
The triaxial prepreg is a ± 45º biaxial E-glass stitched to unidirectional fabric giving a total
fibre weight of 900 - 1800g. This material can be used as a thick drapable fabric. The
triaxial prepreg is available with a glass tissue on the biax side, which helps to prevent
print-through onto in-mould process or top coats.
Tack
Tack is a very important characteristic of a prepreg. It is a surface sensitive and
viscoelastic property, which depends on both the resin and fibre properties, the
degree of impregnation, and the process used to define tack. Tack is essential when
manufacturing prepreg components as it helps to ensure that the prepreg and core
materials stay in place whilst the lay-up and vacuum application is completed. However,
differences in tack and impregnation can also cause final parts to perform differently
than expected. This can include costly problems such as de-laminations of prepreg
plies, wrinkling and voids. To prevent such problems occurring, it is important to choose
the right level of tack for the each material in the laminate stack.
There are two tack variants of the WE91 resin matrix; WE91-1 (high tack) typically used
for UD glass or biaxial prepeg and WE91-2 (medium tack) used for biaxial, triaxial or UD
carbon prepreg.
Resin Content
Resin content is another very important controlling parameter for the processing of
prepregs. In high performance prepregs, the resin content is determined by the
trade-off, for optimal mechanical properties, between higher fibre packing density
and void content. This tradeoff varies for the consolidation pressure. In high pressure
autoclave processing, lower resin contents can be reached before the void content
starts to increase significantly. In vacuum consolidated glass prepregs typical of
those manufactured at Gurit, resin content is usually limited to a minimum of around
27% by weight, corresponding to a FVF of 59%. Below this value the void content
rises sharply, as shown by the figure below for a biax laminate, which has a dramatic
effect on resin-dominated mechanical properties such as inter-laminar shear and
compression strength.
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades 16
Effect of Resin Content of Laminate Void Content
Conversely, a high resin content can also cause high void content as it becomes more
difficult to evacuate air inside the prepreg stack when there is excessive resin between
the plies. However, it is less common for a high resin content to be required and tends
to be used in single ply laminates or next to core materials whereby excess resin is
required for bonding.
The table below shows a rough guide to the resin contents available for WE91 prepregs.
Resin System
Reinforcement
WE91-1
UD Glass
32%
WE91-1 or -2
Biaxial Glass
35 – 50%
WE91-2
Triaxial Glass
35 – 50%
WE91-2
UD Carbon
32%
Drape
Drape is the ability of a prepreg to conform over a tool surface to the required shape. This
is an important factor in prepregs since they offer very limited drapability relative to wet
laminated or infused reinforcements due to the interaction between the fibre and the
higher viscosity resin. It is important to ensure sufficient drape to minimise bridging of the
prepreg between the tool or neighbouring plies, whilst ensuring that the resin viscosity is
high enough not to be squeezed out of the prepreg when in a roll or during lay-up.
Drape decreases with decreasing application temperature since resin viscosity
increases; therefore it is important to ensure that good drape is attainable down to
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades 17
the lowest application temperatures experienced for that prepreg. Similarly, higher
application temperature will tend to provide much better drape, although it will also
increase tack. Gurit’s prepreg resins have been tailored to work in real-life factory
environments. Drape is also closely linked to the fabric architecture and in some
instances such as triaxial fabric the stitching can restrict the drapability of a fabric.
Gurit’s WE91-2 resin system isformulated to provide exceptional drape performance
without compromising tack or fibre wet-out.
Out-life
Prepregs usually have a catalytic resin matrix, which reacts above the initiation
temperature of approximately 75 °C. However, some activity is initiated during the
prepregging process and therefore the prepreg matrix will cure very slowly even at low
temperatures (0oC), staging the product.
Staging increases the viscosity of the prepreg matrix and it is therefore very
important to use the prepreg before the resin stages to a point where resin flow
is reduced below a critical point. This critical flow, which corresponds to a critical
viscosity, corresponds to the point where resin flow during the cure is limited to
the degree that the mechanical properties of the cured laminate will be affected.
In all composite structures a level of matrix flow will be required to ensure wetting
of adjacent prepreg plies and other laminate features such as foam surfaces and
cavities. It must be ensured, therefore, that for optimal mechanical performance the
‘out-life’ of prepreg must not be exceeded under the specific storage temperature.
The out-life is determined by measuring the time for the viscosity of fresh matrix to
increase to this critical point at a given storage temperature. Recommended storage
temperatures for WE91 prepreg are shown below.
Outlife
At -18°C (months)
18
At +5°C (months)
6
At 21°C (days)
60
Reactivity
The reactivity of a prepreg is fundamentally controlled by the type, proportion and
quantity of catalyst present in the matrix, which defines the total enthalpy of cure at
a certain mix ratio. The chosen catalyst and mix ratio is specific to the application of
the prepreg, and it must be ensured that for the curing of thick sections, the reactivity
is minimised to prevent excessive heat being generated by the reacting resin during
curing – this is known as an exotherm.
When characterising a resin system, it is important to measure the total enthalpy of
reaction, the onset and endset reaction temperatures, and the peak height, using a
DSC with a standard cure. These parameters enable the behaviour of the system to be
predicted in large laminate sections, by comparing against benchmark values.
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades 18
The glass transition temperature or Tg of a prepreg resin is another important parameter
to characterise a resin matrix system, since it is indicative of the crosslink density, and
therefore potential heat output during reaction.
WE91 Thermal properties (20ºC-250ºC @ 10°C/minute)
Enthalpy (J/g)
250 ± 50
DSC Tg2 (°C)
110 - 125
Mechanical Properties
Various critical mechanical properties are measured to characterise the mechanical
performance of a prepreg. These include normalised tensile strength and tensile
modulus, inter-laminar shear strength and compressive strength. Other secondary
properties can also be considered such as foam and gelcoat adhesion which tend to be
evaluated as part of a complete laminate design. In order to provide an overview of the
mechanical performance of WE91 products, a Summary Table is provided below with
some typical prepregs.
Reinforcements
Prepreg
1600g UD
Glass
600g
UD Low
Modulus
Carbon
600g
Fleeced
Biax
1200g
Fleeced
Triax
Test Standard
Resin System
WE91-1
WE91-2
WE91-2
WE91-2
-
Resin Content (%)
32
35
43
43
-
Tack
High
Medium
Medium
Medium
-
Fibre Weight (g/m2)
1600
600
600 + 50
Fleece
1200 + 50
Fleece
-
Prepreg Aerial Weight (g/m2)
2353
923
1140
2193
-
0° Tensile Strength (MPa)
1185
1760
135
600
BS EN ISO 527
0° Tensile Modulus (GPa)
50
120
15
30
BS EN ISO 527
0° Tensile Strain to Failure (%)
2.4
1.45
0.76
2
BS EN ISO 527
0° Compressive Strength (MPa)
-
1040
435
-
ISO 14126
0° Compressive Modulus (GPa)
-
115
25
-
ISO 14126
0° Compressive Strain to Failure (%)
-
0.95
1.45
-
ISO 14126
0° ILSS (MPa)
60
75
-
40
BS EN ISO 14130
45° Tensile Strength (MPa)
-
-
490
380
BS EN ISO 527
45° Tensile Modulus (GPa)
-
-
25
25
BS EN ISO 527
45° Tensile Strain to Failure (%)
-
-
1.45
1.5
BS EN ISO 527
45° ILSS (MPa)
-
-
35
40
BS EN ISO 14130
WE91 Prepreg Mechanical Properties
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades 19
One of the key characteristics of prepregs is that the fibre alignment is effectively
‘locked’ during the impregnation stage of manufacture. This means that the fibre is
less likely to be distorted during the lay-up stage and negates the need for additional
reinforcements, and in particular stitching in UD laminates. This can be understood
more clearly by considering the comparative mechanical performance of a UD laminate
using prepreg and infusion production process, summarized in the Table below.
Property
Prepreg
Infusion
% Change
Normalised Tensile Strength @
53% FVF (MPa)
Mean
1010
989
-2.11%
Normalised Tensile Modulus @
53% FVF (GPa)
Mean
44.1
43.7
-1.01%
Normalised Compression
Strength @ 53% FVF (MPa)
Mean
1036
644
-37.81%
Normalised Compression
Modulus @ 53% FVF (GPa)
Mean
42.7
42.8
+0.2%
Comparison of Prepreg and Infusion UD Properties
Whilst tensile and inter-laminar properties are comparable, the compressive strength can
be greatly reduced due to fibre buckling caused by distortion due to fabric stitches.
Coatings
The manufacture of prepreg blades typically requires an in-mould UV stable gel-coat (as
the finished surface of the blade) or process coat (as a substrate suitable for painting).
However, most prepreg blade production processes will require leading and trailing edges
to be filled and faired post-bonding as a minimum. Furthermore, whilst a UV stable gelcoat
helps to protect the laminate, if a painting scheme is to be used as the final coating of the
blade then the complex formulation will add unnecessary cost to the process.
As well as the process savings with regards to finishing time, a process coat also acts to
protect the mould surface and will help to extend the life of the tool and increase yield.
Gurit is a leading manufacturer and supplier of epoxy coating products for the wind
energy market with well established UV-stable top coats and process coats. For larger
blades there has been a trend to use process coats with PU paints and therefore only
proces coats is discussed further.
CR3400 Process Coat
CR 3400 is an in-mould epoxy process coat for epoxy laminates, and is designed to
be used as the base for the subsequent application of a paint scheme. CR 3400 is
therefore formulated to be easily sandable so that once released, the CR 3400 surface
can be readily keyed prior to the application of the paint system. This feature gives
additional benefits in that any minor surface defects caused by laminate print-through
or mould imperfections can easily be sanded away.
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades 20
CR 3400 is applied into a mould in the same way as other epoxy gelcoats, and has
similar handling characteristics. The product has a considerably longer overcoating
window than other gelcoats - which means that the system can also be used in a
very flexible manufacturing environment. However, care must be taken to validate
the process window as factory humidity and potential contamination are also critical
factors.
Property
15°C
20°C
25°C
30°C
Initial Mixed Viscosity (cP)
7655
4780
2930
1830
Pot Life - 500g Mix in Air (hrs:mins)
-
01:00
-
00:45
Sag Resistance (μm)
550
400
280
200
Tack-off Time (hrs:mins)
05:50
05:00
04:20
03:40
Latest Overcoating Time (hrs:mins)
61/2
24
16
12
Min. Rec. Thickness (μm)
300
300
300
300
CR3400 Processing Properties
CR 3400 is designed to be backed up with an epoxy laminate and is compatible with
Gurit’s WE91 prepreg family and SPRINT® materials.
Adhesives for Prepreg Blades
For some box beam designs (male mandrel), a structural adhesive is not required to
bond the beam and shells together. These types of design typically use low cost, low
performance adhesives like PU. For female moulded spar designs, a structural adhesive
is still required to bond the two spar sections together. For this type of prepreg blade,
Gurit supplies Spabond 340LV adhesive, which is discussed in more detail in “Materials
for Infusion Blades” earlier in this section.
Structural Core
As the Wind Energy Market has matured the optimization of blade design has become
critical as blade lengths and weights have increased. As a consequence the one core fits all
approach is no longer relevant for a modern wind turbine blade. The material requirements
for a structural core can vary considerably from shell to shear web and also from one area
of a shell to another. Therefore, to optimize the design, weight and performance of a blade
the blade designers may make use of a number of core products.
Due to the higher thermal performance required for prepreg blade processes, foams
with good thermal stability are required. Gurit supplies two foam types for prepreg
applications in the Wind Energy market; Corecell™ and G-PET.
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades 21
Corecell™ T-Foam
¬ Environmental stability – High tolerance for heat and chemical exposure
¬ Built in toughness – High ductility and damage tolerance compared to cross-linked
PVC and Balsa
¬ Fine cell size – Resin absorption is very low, saving both weight and cost
¬ Superior uniformity – Low density variation
¬ Eliminating outgassing – Corecell eliminates the problems of foam outgassing
¬ Compatibility – Suitable for use with all polyester, vinylester and epoxy resins
¬ No inhibition - Corecell does not inhibit any epoxy resin curing mechanisms
¬ Handling – Tough and easy to machine
G-PET
Gurit has recently added G-PET to its portfolio of structural cores. PET foam is a low
cost core that is manufactured using low cost materials and the efficient extrusion
manufacturing process. PET extruded foam has reasonable mechanical properties but
at higher densities is suitable for Wind Applications. Therefore, where design is not
weight critical, PET is a cost effective solution and as a result has found increasing
adoption in prepreg blade manufacture.
¬
High process temperatures
¬
Excellent chemical resistance
¬
Good adhesion
¬
Cost-effective, especially in larger thickness
¬
Excellent mechanical properties
¬
Recyclable
¬
Compatible with all types of composite processes
Advanced Prepregs
Gurit’s long history supplying prepregs to the Wind Energy industry has also allowed
Gurit to lead the way be introducing the next generation of prepregs specifically
designed to make turbine blade manufacture faster, easier and cheaper. This section
presents some of the advanced prepregs developed by Gurit and in use in turbine blade
manufacture today.
WE91LE
Wind Energy prepreg laminates are typically cured with a dwell at 80-90°C to control the
exotherm that can result in thick sections, before being cured at higher temperatures
to achieve the optimum thermal and mechanical properties. When cured in a laminate
without such a dwell, Gurit’s WE91LE exhibits a significantly lower peak exotherm
temperature compared to standard products. This characteristic can enable further
cure optimisation for new and current processes to further reduce cycle times and
increase productivity.
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades 22
Exothermic Behaviour of WE91 Prepregs
SPRINT®
With the advent of large blades, new prepreg technology was required to meet
the increasing technical challenges in producing high quality components. Gurit’s
award winning, patented SPRINT® technology, was ideally suited to fill this need
and continues to lead the market as a composites technology platform for very large
structures. Available in multiaxial or woven formats, SPRINT® enables the production
of thick section laminates of very high quality (very low void content) using standard
curing and vacuum bag operations. The product can also be used interchangeably
and is compatible with other SPRINT® and prepreg products to produce very high
quality components.
Some of the key benefits of SPRINT® are:
¬ Autoclave quality but from vacuum bag
processing
¬ Excellent materials handling
¬ Good health and safety
¬ Fast lay-up
¬ No De-bulking required
¬ Reliability and repeatability
¬ Excellent laminate quality
¬ Reduced Costs
SPRINT glass laminate – 0 - 0.5% void content
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades 23
SPRINT® is available in two main formats:
1. Standard SPRINT® - whereby the
SPRINT® resin is sandwiched by a woven
or multiaxial fabric on either side as shown
in the diagram (left). This product can be
supplied on large rolls or smaller tapes
for spar winding applications. Another
advantage is that this product can be
supplied without a backer and reduce
waste handling & disposal costs.
SPRINT Schematic
2. Single Sided SPRINT® - typically for heavier multiaxials, the SPRINT® resin is
applied to one side of the reinforcement and to help with the lay-up process, a tack
film can be applied to the other side. This product is particularly effective when used
as the first layer applied to a gel or process coat in order to eliminate any surface
defects that can occur whilst processing and reduce the overall finishing time.
Another common application for SPRINT® is as an additional air evacuation system
within an existing prepreg stack. It is common to produce thick UD laminates for sparcap applications using heavy, high tack prepregs that cause excessive air entrapment
between each ply. By interleaving every 2nd, 3rd or even 4th ply with a biaxial SPRINT®,
the resulting void content can be significantly reduced.
The novel SPRINT® concept opens up a new range of possibilities for optimizing not
only the laminate but also the production process. More often than not any area of the
prepreg lay-up that suffers from poor consolidation or a high level or air entrapment can
be reduced or eliminated using SPRINT®. It is even possible to use a SPRINT® as the
final layer of the lay-up to act as the breather to consolidate the laminate. This not only
allows for a breather to be re-used, reducing the cost of consumables per blade, but with
minimal resin bleed the resin content could also be reduced to save weight and money.
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. 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
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades 24
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® and Airstream®,
a new uni-directional 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
Void Content Analysis
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 interply 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 for infusion blades. As woven UD and
multiaxial layers are not required with UD prepreg, the mechanical performance of
Sparpreg™ is far superior to the wet system equivalent. Processing is also simplified
and robust, enabling rapid deposition and curing of the cap with a very low defect rate.
Although the raw material costs are higher for prepregs, the processing savings and
reduced weight of the blade make Sparpreg™ spar caps an attractive option.
WE Handbook- 5- Gurit Composite Materials for Wind Turbine Blades 25