Vacuum consolidation of thermoplastic composites for ... - DTU Risø

Proceedings of the 27 th Risø International Symposium
on Materials Science:
Polymer Composite Materials for Wind Power Turbines
Editors: H. Lilholt, B. Madsen, T.L. Andersen,
L.P. Mikkelsen, A. Thygesen
Risø National Laboratory, Roskilde, Denmark, 2006
VACUUM CONSOLIDATION OF THERMOPLASTIC
COMPOSITES FOR WIND TURBINE ROTOR BLADES
Aage Lystrup
Materials Research Department, Risø National Laboratory
DK-4000 Roskilde, Denmark
ABSTRACT
Technology and semi-raw materials for vacuum consolidation of thermoplastic
composites with continuous fibres are discussed, and the potential use of this
technology for wind turbine blades is demonstrated on a downscaled model.
1. INTRODUCTION
Basically, the vacuum consolidation of thermoplastic composite material is very simple.
All it takes is to vacuum bag the lay-up of the semi-raw material, heat it to the process
temperature and cool it back down to room temperature, in order to solidify the matrix
material (Lystrup 1997). Most thermoplastic polymers must be (completely) dry and
free of absorbed water before they are melted to avoid degradation caused by reaction
with water. If the material has not been dried prior to the consolidation process, a
drying step can be included in the process cycle as illustrated in Fig. 1. The process is
very attractive, since no additional pressure is applied, which means that no autoclave
is needed.
2. MATERIALS AND PROPERTIES
The success of this process has been demonstrated on commingled hybrid yarn of glass
fibres and PP fibres from Twintex (Twintex 2006), as well of glass fibres and L-PET
fibres from Comfil (Comfil 2006). L-PET is a modified low temperature (220 °C)
processing PET. Tension-tension fatigue properties for the two materials are shown in
Fig. 2 and compared with glass/PP manufactured from stacked layers of glass fibre
fabrics and PP films. The laminates are either made from balanced 0°#90° woven fabric
or from pure UD material (only [0°] fibre orientation), which are more relevant for
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wind turbine blades. In all cases, the fibre content is about 40 vol.-%. The fatigue
strength is higher for the Twintex material than for the Comfil material. It could be
explained by the fact, that the glass fibres in the Twintex material are straighter and
more aligned. The Twintex yarn is manufactured by pulling the glass fibres and
extruding the PP fibres simultaneously. The two types of fibres are simply merged,
commingled and spooled in one operation. The Comfil yarn is commingled in an air
texturing process where glass fibres and L-PET fibres are blown into each other to
achieve a thorough commingling, and this causes the glass fibres to be a little wrinkled.
The laminate manufactured by the film stacking technique has lower fatigue strength.
It is due to the presence of small voids and few dry glass fibres, especially in the
middle of the roving bundles, because it is difficult for the molten polymer to penetrate
the roving completely and wet all individual glass fibres. It emphasises the advantages
with commingled yarn and the importance of a thorough commingling of the two
types of fibres.
Shear fatigue properties were measured using short 3-point bending test specimens. As
shown in Fig. 3, the shear fatigue strength is higher for the Comfil material that for the
Twintex material, indicating higher bond strength between the L-PET matrix and the
glass fibres than between the PP matrix and the fibres, and/or a higher shear strength
of the pure L-PET compared to the pure PP.
Temperature [°C]
300
250
200
150
100
50
Oven temperature
Temperatures at
surface of laminate
Drying step
Evaporation of
absorbed water
0
0 1 2 3 4 5 6 7
Time [hour]
Temperature
inside laminate
Vacuum
Fig. 1. Typical process cycle for vacuum consolidation of thermoplastic composites. The
maximum process temperature depends on the type of thermoplastic matrix material.
0
-0.1
-0.2
-0.3
-0.4
-0.5
-0.6
-0.7
-0.8
-0.9
-1
Vacuum
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Max stress [MPa]
400
350
300
250
200
150
100
50
Vacuum consolidation of thermoplastic composites
Room Temp.
Tension-Tension Fatigue
Glass fibre / PP (Twintex)
Glass fibre / L-PET (Comfil)
Twintex - 0°#90° - Fabric
PP - Film Stacked
0°#90° - Fabric
1 000 10 000 100 000 1 000 000
Cycles
Twintex - 0°
Comfil - 0°
Comfil - 0°#90° - Fabric
Fig. 2. Tension-tension fatigue properties of glass/PP (Twintex) and glass/L-PET
(Comfil) for both 0°#90° woven fabric and pure UD fibre orientation. The arrows
indicate tests which were stopped before the specimen failed.
Shear stress [MPa]
35
30
25
20
Room Temperature Short Beam 3-point Bending Shear Fatigue
Glass fibre / PP
Twintex - 0°
Glass fibre / L-PET
Comfil - 0°
15
10 000 100 000 1 000 000
Cycles
Fig. 3. Shear fatigue properties of glass/PP (Twintex) and glass/L-PET (Comfil) with
pure UD fibre orientation.
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Lystrup
3. PROCESS TECHNOLOGY
3.1 Cycle time. The necessary process temperature can be obtained either by built-in
heating elements in the mould or by heating the entire mould in an oven, and
therefore, there are practically no limits to the size of the components, which can be
produced. Therefore, it is also a potential technology for large wind turbine rotor
blades, if relatively long process cycle time is acceptable. The maximal laminate
thickness at the root-end section of a large rotor blade is expected to be around 100
mm, and the process cycle time for such a thick section in glass fibre/PP is about 24
hours, if heated in an oven with circulating hot air, as shown in Fig. 4. The required
process temperature of 190 °C in the middle of the thickness is reached after 15 hours, if
the temperature of the hot air in the oven is 200 °C. These measurements were
performed on a small but thick pyramid shaped test sample as shown in Fig. 5.
Temperature [°C]
200
150
100
50
Oven Temperature
Temperature
at surface of
laminate
Temperature
in the middle
of the laminate
0
0 5 10 15 20 25
Time [hour]
Vacuum
consolidation
of 100 mm thick
Glass/PP laminate
Fig. 4. Process cycle for vacuum consolidation of a 100 mm thick glass/PP laminate in
an oven with hot-air circulation.
Fig. 5. Pyramid shaped 100 mm thick glass/PP test sample for measuring the process
time for vacuum consolidation. At left: The 250 mm high lay-up with thermocouples
mounted at the top, bottom and inside in the middle of the laminate. At right: The 100
mm high consolidated laminate.
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Vacuum consolidation of thermoplastic composites
3.2 A complete blade in one process step. The assembly of a wind turbine blade from
parts is a major task in most current production methods. It is of interest to reduce the
number of steps in the process, preferably to one, so that a complete blade with webs
etc. is manufactured in one consolidation process. This one-step technology is used by
Siemens Wind Power for resin infusion of fibre composite blades with thermosetting
matrix, but the concept for using thermoplastic matrix was of interest for Siemens
Wind Power as well (Brøndsted, Lilholt and Lystrup 2005). Apart from the potential
time saving, it has also the advantage of reducing geometric inaccuracies introduced
during the assembly steps.
3.3 Compaction of the material. The consolidation pressure in vacuum consolidation is
limited to only 0.1 MPa. Therefore, in many cases, it becomes difficult to manufacture
components with complex geometry or sharp internal corners. The low pressure is not
sufficient to conform the material to the mould geometry. A series of simple tests in
small moulds have demonstrated this. The geometries of the test samples are shown in
Fig. 6. In order to compact a laminate into a mould with a curvature, the inner layers
will need to stretch or “flow”, by an amount proportional to the thickness change and
the curvature. The stretch-ability will depend on the lay-up, the fibre angle and the size
of the different fabrics in the laminate. An obvious way of improving the stretch
capability of a textile is to shrink it. That is effectively to compress the glass fibres
imparting local wrinkles to the fibres, which can be stretched again during the
manufacturing process. The thermoplastic fibres in the commingled material are
sensitive to heat, and can be shrunk by the use of a hot-air blower. By controlling the
time and temperature, a wide variety of shrink ratios can be achieved. The fibre fabrics
are shrunk locally, only where needed, and test has shown that this method is effective.
Fig. 6. Simplified geometries characteristic for wind turbine blades used for
development of techniques for lay-up and consolidation of material.
Manufacturing of sections simulating the trailing edge of a wind turbine blade
demonstrates that it is difficult to get a good consolidation in the acute angle. The
vacuum
bag is not able to reach into the very tip of the angle, and the flow of resin is
extremely limited. Consequently, a certain amount of porosity is observed. Therefore, it
may be necessary to accept a little higher matrix content at certain locations, in order to
entirely fill-up the fibre structure with matrix material, and produce a high quality
material with no or low porosity.
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Lystrup
The transition from the web to the shells of a rotor blade is another challenge.
Simultaneously with the compaction of the material in the shells, the material in the
webs must be stretched, and some
means of providing a compacting pressure for the
shell material in line with the web must be applied. This can be achieved by using PMI
foam, as illustrated in Fig. 7, which is able to re-foam. The foam expands when a
certain temperature is reached, depending of the type of PMI foam, and provides the
compacting pressure for the reinforcement. For one quality of PMI foam foaming
commences at around 200 °C. The amount of additional expansion and pressure
depends on time, temperature and constrained conditions. This technique can also help
solving the problem at the trailing edge.
Fig. 7. Test of technique using expanding foam to consolidate material at difficult
geometries. At left: Transition from the web to the shell. At right: Trailing edge.
3.4 Handling and lay-up of materials in the mould. The geometry of the finished blade,
with a length over 40 m and a weight of several tonnes, puts severe limits to how the
manufacturing
can be done. Test on a short section (0.5 m) of a turbine blade has
shown that a combination of outside female and temporary inside male moulds is very
promising. The material for one side of the blade is placed in a bottom female mould.
Webs, vacuum bag material, and the temporary male mould parts are then placed in
the female mould on top of the laid-up material, and the material for the "top" shell is
placed on the male mould sections, as shown in Fig. 8. Then the mould is closed by a
second female mould, the vacuum bags are sealed to the mould, and a vacuum
sufficient to hold the material in place is drawn between the vacuum bags and the
mould surface. The temporary male mould sections used to hold the fibre materials in
place during lay-up are removed, and the wing section is ready for consolidation, as
shown in Fig. 9, together with various types of finished test sections. Temporary male
mould parts made from soft foam are very convenient, especially when the different
parts are enclosed in individual vacuum bags. The stiffness of the foam parts is
sufficient to support the material and hold it in place during lay-up and sealing
procedure, and by pulling a slight vacuum on a bag containing a foam section, its
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Vacuum consolidation of thermoplastic composites
volume is reduced drastically, which facilitates the removal of the male mould part
from the female mould. The female mould is made of Aluminium. It is light, easily
manufactured, and conducts heat well.
Fig. 8. Lay-up of material in one-half of a female mould using temporary male moulds
to support the materials, which later will be pressed towards the other half of the
female mould. The glass/PP fabrics at the far left of the picture are folded to the top of
the lay-up before the top-female mould is placed and assembled with the bottom
female mould.
Fig. 9. At left: The completed vacuum bagged
lay-up test section with two webs in
place ready to be heated and consolidated. At right: Various types of finished test
sections.
4 CONCLUSIONS
The potential use of vacuum consolidation of thermoplastic composites for wind
turbine blades has been demonstrated.
The process technology for making a complete wind turbine blade in one process step
has been developed.
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Lystrup
The fatigue properties of two types of semi-raw materials have been investigated. That
is commingled glass fibres and PP fibres from Twintex, and glass fibres and L-PET
fibres from Comfil.
The Twintex material has the highest tension-tension fatigue strength, whereas the
Comfil material has the highest shear fatigue strength.
The process cycle time for vacuum consolidation of a 100 mm thick laminate is about 24
hours. That is what it takes to heat up the centre of the laminate to the process
temperature of about 190 °C, and to cool it back down to room temperature in an oven
with circulating air.
The laid-up fabrics must be able to stretch in order to obtain full compaction of the
material into a concave
curvature. Stretchable fabric was created by pre-shrinking it
prior to lay-up. The shrinking can be done by heating the fabric, because the polymer
fibres shrink when heated. The location and amount of shrinking were easily controlled
by the use of a hand-hold hot-air blower.
Compaction of the material into the sharp tip of the trailing edge and of the shell
laminate, where the webs are connected to the shell, can be achieved by the use of
foam, which expands when heated to the process temperature.
A technique for placing and vacuum bagging all the fabrics for a complete blade,
including webs, inside a female mould has been developed and demonstrated. The
laid-up fabrics are temporary supported by soft-foam male mould parts, which are
removed after the female mould is closed and the material is hold in place by the
vacuum.
ACKNOWLEDGEMENTS
Thanks to Siemens Wind Power A/S, Denmark, for the collaboration within this work,
EU-JOULE program for financial support, and B. S. Johansen, Risø National
Laboratory, for planning and conducting a part of the work.
REFERENCES
Brøndsted, P., Lilholt, H., and Lystrup A. (2005). Composite materials for wind power
turbine blades. Annual Review of Materials Research, 35,
505-538.
Comfil (2006). http://www.comfil.biz
Lystrup, A. (1997). Processing technology for advanced fibre composites with
thermoplastic matrices. 18 th Risø International Symposium on Materials Science 1997
– Polymeric Composites – Expanding
the Limits, Risø, Denmark, 69-80.
Twintex (2006). http://www.twintex.com/
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