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Thermoplastic Composite Wind Turbine Blades
Kjelt van RijswijkHarald Bersee
Delft University of TechnologyFaculty of Aerospace EngineeringDesign and Production of Composites Structures
There is a wind of change blowing in the Netherlands
Contents
• Introduction:• Large Blades• Materials, Design & Manufacturing
• Vacuum Infusion of Thermoplastic Composites:• Anionic Polyamide-6 Resin• Infusion Process & Equipment
• Composite Properties:• Static Properties• Dynamic Properties
• Conclusions
There is a wind of change blowing in the Netherlands
Large Wind Turbine Blades
• Dedicated Offshore Wind Power Systems:• Stronger and more constant wind• Increasingly large blades to increase power output per
turbine and reduce cost per kWh• No noise-pollution and aesthetical issues
• Larger blades require:• Materials with higher specific properties (E/ρ, σ/ρ):
– Carbon fiber based composites
• More efficient structural designs
mblade ∼ (Rblade) 3mblade ∼ (Rblade) 32.35
There is a wind of change blowing in the Netherlands
Alternative Structural Design
Re-introduction of ribs:– Higher structural efficiency (E/ρ)– Reduces buckling of the spar– Provides attachment points and load paths for smart
actuators/control surfaces and sensors– Not entirely new
There is a wind of change blowing in the Netherlands
Thermoplastic Composites
Current blade manufacturing technology:– Thermoset composites– 2 skins and 1 spar assembled through structural bonding– Vacuum infusion or pre-pregging of individual parts
There is a wind of change blowing in the Netherlands
Thermoplastic Composites
Alternative blade manufacturing technology:– Thermoplastic composites– 2 skins, 1 spar and many ribs assembled through welding– Rubber forming and diaphragm forming of individual parts
Pre-cut laminate sheet material
Infra red heating panels
Rubber die
Metal die
Rubber press
Final thermoplastic composite part
Heating element
Clamp connection
Welded parts
Voltmeter
Ampmeter
There is a wind of change blowing in the Netherlands
Thermoplastic Composites
• Additional advantages:– Better impact properties– Do not turn brittle at low temperatures– Unlimited shelf-life of the raw materials– Fully recyclable (environmental and economocal benefits)
• Drawbacks:– Poor fatigue performance due to poor fiber-to-matrix bond– Requires introduction of new and expensive technology– High material costs due to the need for intermediate
processing steps– High processing temperatures (>200°C)– Melt processing limits achievable part size and thickness
There is a wind of change blowing in the Netherlands
Vacuum Infusion of Thermoplastic Composites
Reactive processing:– Manufacturing of larger, thicker and more integrated
thermoplastic composite parts– Improved chemical bonding due to in-situ polymerization of
the matrix around the fibers– Manufacturing of parts directly
from the monomer– Commonly used technology
for blade manufacturing
There is a wind of change blowing in the Netherlands
Vacuum Infusion of Thermoplastic Composites
Anionic Polyamide-6:– AP Nylon®, Brüggemann Chemical– World wide availability– Price/performance (2-3 €/kg)– Viscosity (10 mPa.s)– Processing temperature (150-180°C)
0,001
0,01
0,1
1
10
100
1000
10000
100000
0 50 100 150 200 250 300 350 400 450
Processing temperature [ºC]
Mel
t vis
cosi
ty [P
a·s]
epoxyvinylester
polyester
PMMA
PA-6
PBT
PA-12
PEK
ETPUPC
PMMAPA-12
PA-6
PBTPPS
PES
PEI
PEEK
PEKK
Reactive processing of thermoset resins
Reactive processing of thermoplastic resins
Melt processing of thermoplastic polymers
0,001
0,01
0,1
1
10
100
1000
10000
100000
0 50 100 150 200 250 300 350 400 450
Processing temperature [ºC]
Mel
t vis
cosi
ty [P
a·s]
epoxyvinylester
polyester
PMMA
PA-6
PBT
PA-12
PEK
ETPUPC
PMMAPA-12
PA-6
PBTPPS
PES
PEI
PEEK
PEKK
Reactive processing of thermoset resins
Reactive processing of thermoplastic resins
Melt processing of thermoplastic polymers
There is a wind of change blowing in the Netherlands
Anionic Polyamide-6
Control of Reaction Rate
0
20
40
60
80
100
0 5 10 15 20 25 30 35 40
time [min]
Deg
ree
of c
onve
rsio
n [%
] .
Fast systemSlow system
infusion
cure CAPROLACTAM
ACTIVATOR C20 INITIATOR C1
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Simultaneous Polymerization & Crystallization
CN-
O
CN
OC
NH
O
CN
OC
NH
ON-
CNH
O+
CN
OC
NH
ON-
CNH
O
CNH
O
+C
N
OC
NH
O
NHC
NH
O
CN-
O
+
CN-
O
CN
OC
NH
O
CN
OC
NH
ON-
CNH
O+
Initiator Activator
Monomer
PA-6
n
CN
OC
ONH
CNH
O
*
O
NH O
HN O HN
HN
NHO
O
H
O HN
NH O
O
Hydrogen bonding between amide groups
Grouping of polymer chains into Switchboards
Stacks of switchboards forming spherical conglomerates calledspherulites
O
NH O
HN O HN
HN
NHO
O
H
O HN
NH O
O
Hydrogen bonding between amide groups
Grouping of polymer chains into Switchboards
Stacks of switchboards forming spherical conglomerates calledspherulites
O
NH O
HN O HN
HN
NHO
O
H
O HN
NH O
O
O
NH O
HN O HN
HN
NHO
O
H
O HN
NH O
O
Hydrogen bonding between amide groups
Grouping of polymer chains into Switchboards
Stacks of switchboards forming spherical conglomerates calledspherulites
Tpol < Tc ⇒Polymerization
and crystallization occur
simultaneously.
Tpol < Tc ⇒Polymerization
and crystallization occur
simultaneously.
There is a wind of change blowing in the Netherlands
Simultaneous Polymerization & Crystallization
Selecting the optimum process temperature:– Too low:
polymerization stops
– Too high:Crystallization difficult
– Much too high:Crystallization stops
Crystal
MonomerCrystal
Long polymer chain
Branched polymer chain
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Simultaneous Polymerization & Crystallization
Selecting the optimum process temperature:
– Optimum temperature:Continuous polymerization &Crystallization Short polymer chain
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Anionic Polyamide-6
Tensile properties of the neat resin
σ2
σ1
0.1
σ
ε0.2 εf
σm
Compared to injection molded PA-6
Condition Young’s modulus [GPa]
Maximum strength [MPa]
Strain at failure [%]
23ºC, dry 4.2 (+ 41%) 96 (+ 14%) 9 (-) 23ºC, 50% RH 2.1 (+ 59%) 61 (+ 4%) 28 (-)
80ºC, dry 1.6 (+ 65%) 51 (+ 32%) 29 (-)
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Vacuum Infused APA-6 Composites
110°C
110°C
160-180°C
60 minutes
250 mbar
There is a wind of change blowing in the Netherlands
Vacuum Infused APA-6 Composites
100
120
140
160
180
200
0 5 10 15 20 25 30
Time [min]
Tem
pera
ture
[ºC
]
APA-6 140ºCAPA-6 150ºCAPA-6 160ºCAPA-6 170ºCAPA-6/GF 180ºCAPA-6/GF 170ºCAPA-6/GF 160ºCExothermic reactionExothermic reaction
Processing temperatures:
Neat APA-6: 150-170°C
APA-6 Composites: 160-180°C
Processing temperatures:
Neat APA-6: 150-170°C
APA-6 Composites: 160-180°C
There is a wind of change blowing in the Netherlands
Static mechanical properties
Physical properties
Vacuum Infused APA-6 Composites
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Vacuum Infused APA-6 Composites
050
100150200250300350400
1.0E+02
1.0E+03
1.0E+04
1.0E+05
1.0E+06
1.0E+07
n
S [M
Pa]
APA-6 (180ºC)PA-6Epoxy
Reactive processingReactive processing
Melt processingMelt processing
Fatigue performanceFatigue performance
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Conclusions
For increasing blade lengths, switching to more efficient structural designs is inevitable: the re-introduction of ribs is suggested.
For rib/spar/skin-structures, thermoplastic composites are favoured over thermoset composites. Parts can be rapidly melt processed and assembled through welding. Blades will be fully recyclable.
Vacuum infusion of thermoplastic composites is introduced to overcome the classical drawbacks of these materials: limited size and thickness of parts, poor fatigue resistance, expensive materials.
The cure of a semi-crystalline thermoplastic resin is more complicated than of a thermoset resin.
There is a wind of change blowing in the Netherlands
Conclusions
AP Nylon® has a low viscosity (10 mPa⋅s), good availability, a low price (2-3 €/kg), and a relatively low processing temperature (150-180°C).
Reactively processed PA-6 outperforms melt processed PA-6 in all temperatures and humidities tested.
Static properties of APA-6 composites are better than of their HPA-6 and epoxy counterparts in dry conditions. When moisture conditioned, the performance of APA-6 composites drop rapidly, which is caused by the low conversions and high void contents.
Reactive processing of thermoplastic composites results in a strong interfacial bond strength and leads consequently to better fatigue performance compared to melt processing.