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Monitoring Cure in High Performance Composites Ivana Partridge
www.bris.ac.uk/composites
2/19 Thermosetting resins and their composites
resin
carbonfibres
• The curing reaction of the resin is a critical process in composites manufacturing.
• Physico-chemical effects can
be followed by process monitoring techniques, such as dielectric sensing
3/19 Thermosetting resins and cure reaction
Curing reaction Liquid
Unreacted resin
Liquid
Gelation
Rubber
Vitrification
Glass
+ heat
Resin chemistry complex and usually unknown Monitoring achieved through following changes in mobility of the growing macromolecular networks
4/19 Thermosetting resins and cure reaction
Temperature
Time
ViscosityGlass Transition Temperature
MaximumFlow
Gelation Vitrification
TTT diagram of resin system
5/19 Cure process – critical phenomena/process milestones – Point of maximum flow
Viscosity of the resin is minimum à minimum flow resistance. Manufacturing: Onset of reaction, practical end of mould filling in Liquid
Composite Moulding, onset of pressure application in autoclave – Gelation Transition from the liquid state à rubber state
Matrix does not flow after gelation, no-return point in cure ! Manufacturing: Fibre wetting , phase separation processes stopped by
gelation – Vitrification Transition from the rubber state à glass state - Polymerisation reaction
essentially stops. Manufacturing: Final glass transition temperature reached - defines
upper temperature limit for material usage
6/19 Dielectric measurements
Charged species contributing to signal
The charged species respond differently at various frequencies in the spectrum
Main contributions in resin systems:
from ions and interfaces
No field, E=0
Applied field, E>0
7/19 Dielectric measurements
Signal analysis and property derivations
Moving from circuit properties to material
properties
Circuit analysis Time Volta
ge
Time Cur
rentτ
v(t) = Vm sin (ωt)
i (t) = Im sin (ωt + θ)
θ,)(/)( Ztitv =
( ) ZZZ ʹ′ʹ′ʹ′→ ,,θ
( ) pp RCZ ,, →θ
σεεε ,,, ʹ′→ʹ′ʹ′ʹ′→pp RC
εε
ε
ʹ′→ʹ′ʹ′
ʹ′ʹ′→
geometryfC
geometryfR
p
p
,,,
,,
C-R parallel circuit analysis
Interdigital sensor calibration
Derivation of conductivity
8/19 Dielectric cure monitoring
Impedance representation
20
logf
logZ
', log
Z''
Electrode polarisation
Charge migration
Dipolar relaxation
n Minimum, maximum and shoulder in the imaginary impedance spectrum
n Two plateaus in the real impedance spectrum
9/19
-0.1
0.2
0.5
100 150 200Temperature(oC)
Heat
flow
(W/g
)
-0.09
-0.05
-0.01
0.03
dlog
[Z"(
ohm
)]/dt
(s)
1 °C/min
0.5 °C/min
0
0.2
0.4
0.6
0.8
1
100 150 200 250Temperature (oC)
Norm
alis
ed c
onve
rsio
n
DSC-0.5 °C/min
IIM-0.5 °C/min
DSC-1 °C/min
IIM-1 °C/min
Commercial epoxy
Dielectric cure monitoring
Degree of cure estimation under dynamic conditions
10/19
Product Certification During Manufacturing Dielectric sensor: flat interdigital layout of capacitor, creating fringing (curved) electric field as the terminals are subjected to alternating voltage
Sensor development
W L
t D 1.4D
Electric field Electrode
Thin polymer film substrate – suitable for embedded sensors
Ceramic substrate–suitable for tool mounted, reusable sensors (photo courtesy of INASCO)
Always require protection against shorting out on conductive fibres
11/19
RTM moulds
(IAI, Israel) autoclave (Bombardier UK)
pultrusion
(Excel, UK)
Application to composites processing
Installation of monitoring system in composite processing tools
Underside of tool Inside autoclave
outside autoclave cure monitoring system
Sensor at die exit
inside mould
underside of mould
(Images supplied by INASCO)
12/19
Dry FabricImpregnatedArea
Inlet Outlet
Resin flow monitoring in Resin transfer moulding
Dielectric lineal sensor in non conductive reinforcement
0
20
40
60
80
0 1000 2000 3000Time (sec)
Rea
l len
gth
(cm
)
-1
0
1
2
3
4
5
Imag
inar
y le
ngth
(cm
). Visual
100 kHz Re
100 Hz Re
100 kHz Im
100 Hz Im
13/19 Modelling and monitoring
Cure Monitoring
Gelation Vitrification
Degree of cure
Strain
Heat Transfer Model
Cure Kinetics Model
Chemorheology and
Structure Models
Strain Development Model
Strain Monitoring
Flow Model
Filling progress Flow Monitoring
14/19 Towards feedback loop control
INASCO dielectric cure monitoring equipment and controlled oven, Airbus UK/Na>onal Composites Centre
Cure monitoring system also controls the oven hea>ng/cooling. Material proper>es in current database: • Degree of cure • Viscosity advancement • Tg advancement User defined cure cycle Material property evolu>on in real >me.
15/19 Matrices for high performance composites
Or ‘phase inverted’ structure such as in TGDDM epoxy/ PEI thermoplastic
Thermoplastic particles occluded within epoxy matrix (low concentration of thermoplastic)
Initial work at Cranfield prompted by interest in phase separation. Using laboratory equipment, a well pre-characterised epoxy-rubber blend and Dek-Dyne sensors ($50 a piece, non-reusable !!)
16/19 Phase separation in thermoset - thermoplastic resin blend
heat
time
Liquid epoxy & hardener and Dissolved thermoplastic Solid toughened epoxy
17/19
18/19 Dispersion monitoring in epoxy/CNT
0
1
2
3
4
5
6
0 1 2 3 4 5 6
log ε'
log frequency(Hz)
0 J/g 80 J/g 200 J/g 350 J/g 1300 J/g
Liquid epoxy + 0.25% MWNT 200 µm
200 µm
200 µm
Addi7onal polarisa7on due to interfaces created by ultrasonica7on
Ø New interface results in an addi5onal relaxa5on mechanism Ø Strength of relaxa5on can provide a metric of dispersion Ø Response controlled by the shape factor of dispersed phase and volume frac5ons Ø Details see Dr A Skordos at Cranfield
University
19/19
Dielectric cure monitoring usable to reduce cure cycles and provide certification on-line; possibility of feedback-loop process control, management of residual stresses Next generation aircraft – composite structure performance really critical ?? If so…………may come back to phase separation monitoring in aerospace grade resin blends…….. Driving dispersion quality in future commercial nanocomposites…….
Future applications ?