Thermoplastic Composite Wind Turbine Blades

Kjelt van RijswijkHarald Bersee

Delft University of TechnologyFaculty of Aerospace EngineeringDesign and Production of Composites Structures

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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

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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

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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

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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

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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

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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

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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

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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

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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.

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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

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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

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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.

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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.

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Questions?