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 ?