effects of moisture absorption on polymer composites and...

Effects of Moisture Absorption on Polymer Composites and Adhesives

W R BroughtonNational Physical Laboratory

TMAN EventMeasurement & Control of Moisture in Materials

4th February 2009

Tuesday, 17 March 2009

2

Background

Composites

Adhesive Joints

Conclusions

Content


3

Boyne BridgeGRP Cladding

Wind TurbineOff-shore

Chemical Plant + Storage

Courtesy of White Young Green

Chemical + Water TreatmentOver 30 year service

Applications


4

Applications


5

Growing demand for manufacturers to guarantee product life expectancy

– Particularly where inspection and/or maintenance can be difficult or failure catastrophic

Effective use of composites and adhesives for use in hostile (hot/wet) environments hinges on the availability of validated design data that is applicable for the entire structure service life (often 20-50 years, or longer)

Reliable in-situ, real-time techniques for measuring moisture content and distribution in composite and bonded structures

Requirements


6

Osmotic BlisteringGlass Fibre-Reinforced Plastic (GRP)

GRP Boat Hull


7



8


Water vapour permeates the structure

As water diffuses into the composite it reacts with any hydrolysable components inside the laminate to form tiny cells of concentrated solution

Under this osmotic process, more water is drawn through the semi-permeable membrane of the laminate in an attempt to dilute the solution

The water can increase the fluid pressure of the cell substantially (up to 50 atmospheres), which eventually distorts or bursts the laminate and leads to a blistering of the surface

Damage can be very extensive requiring major repair or the replacement of the structure


9

BRE Study

Crazing of surface

Deterioration of gel-coat

Un-even discolouration


10

Environmental DegradationPossible Outcomes

Time

Prop

erty

Required Value

acceptable

unacceptable

catastrophic


11

Moisture Effects on Thermoset Resins

Swelling, weight gain and cavity formation

PlasticisationReduction in glass transition temperatureLower operating temperaturesReduction in stiffness and strength properties

Chemical LeachingLoss of fillers, catalysts, hardeners, pigments or fire retardants


12

Glass Transition TemperatureMoisture Conditioned Polyester Resin

0.0 0.5 1.0 1.5 2.0 2.50

10

20

30

40

50

60

70

25oC immersion

40oC immersion

60oC immersion line of best fitG

lass

Tra

nsiti

on T

empe

ratu

re (o

C)

Moisture Content (%)


13

E-Glass (electrical)Relatively low cost (£1-2/kg)Most common form of fibre-reinforcement used for PMCsSensitive to moisture and other chemicals

C and E-CR Glass (chemical)Good chemical resistance to chemical attack

Carbon FibresHigh resistance to corrosion, moisture, creep and fatigueLow production volumes/relatively high price

Aramid FibresSensitive to moisture

Fibre Types


14

Leaching of alkali oxides (sodium and potassium oxide) from the fibre surfaceFormation of surface micro-cracks ⇒ stress concentratorsGlass fibres slowly decompose/dissolvePermanent loss of strength (even after drying)

Strength reduces by 20% in de-ionised water at room temperature after 3 weeks exposure

Degradation accelerates as temperature and stress increaseDe-ionised water more aggressive than tap water and saltwater

Moisture Effects on E-glass Fibres


15

F922 Epoxy Unidirectional E-glass/F822

Stress Rupture of E-glass Fibres

fUTS

APP tlogk1−=σσ


16

Physical EffectsCapillarity effects on fluid uptakeFibre/matrix interfacial breakdownSwelling and weight gainMicro-crackingFibre/matrix interfacial breakdownLoss of fillers, catalysts, hardeners, pigments or fire retardantsOsmotic blistering/cavity formation

Mechanical/Physical Property EffectsReduction in glass transition + operating temperaturesReduction in stiffness and strength propertiesReduction in fatigue performanceShortened life expectancy

Moisture Effects on PMCs


17


Moisture

h

Moisture

y

xz

( ) %100xW

WWMDRY

DRYWET −=

Moisture Diffusion


18


Fickian Diffusion

( )( )

2

12

12

ttMMMh

16D ⎟

⎟⎠

⎞⎜⎜⎝

⎛

−−π

=∞

0 600

4

t11/2 t2

1/2

M2

M1

moisture equilibrium

Moi

stur

e C

onte

nt (w

t %)

Square Root of Time (t)1/2D is Diffusion coefficient

M is moisture content (wt%)

M∞ is equilibrium moisture concentration

t is exposure time


19

0 20 40 600

1

2

3

4


Experimental Fickian Curve

Moi

stur

e C

onte

nt (w

t %)

Square Root of Time (hrs)1/2

0 20 40 600.0

0.2

0.4

0.6

0.8

1.0


Experimental Fickian Curve

Moi

stur

e C

onte

nt (w

t %)




Fickian Diffusion


20


Diffusion coefficients parallel and perpendicular to the fibres may be estimated using:

Dm = diffusivity of the matrix, Vf = fibre volume fraction

Unidirectional E-glass/F922

Diffusion Coefficients- Anisotropic Solids

Dm = 8.3 x 10-1 m2/s

D22 = 6.3 x 10-2 m2/s (experimental)

D22 = 7.3 x 10-2 m2/s (predicted)

( )

mf

22

mf11

DV21D

DV1D

⎟⎟⎠

⎞⎜⎜⎝

⎛

π−=

−=


21

Moisture Distribution

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

M∞1 year

1 month

1 week

1 day

Moi

stur

e C

onte

nt (w

t %)

Position through plate thickness, h (mm)

⎥⎥⎦

⎤

⎢⎢⎣

⎡⎟⎠⎞

⎜⎝⎛−−=

−−

=∞

75.0

2i

i

hDt3.7exp1

MMMMG


22

Moisture Distribution - GRP Laminate (10 mm) Water Immersion @ 23°C

(1) 10 days

(2) 100 days

(3) 500 days

(4) 1000 days

(5) 2500 days


23


0 20 40 60 800.0

0.5

1.0

1.5

2.0

Temperature Effects on Moisture Diffusivity Unidirectional T300/924 Carbon/Epoxy

25 oC 40 oC 60 oC

Moi

stur

e C

onte

nt (w

t %)



24


T/ko expDD −=

Moisture diffusivity increases with temperature

Do = 0.327 mm2s-1 and k = 4,669 K

3.0 3.1 3.2 3.3 3.41E-8

1E-7

1E-6

Temperature Effects on Moisture Diffusivity Unidirectional T300/924 Carbon/Epoxy

Diff

usiv

ity, D

(mm

2 s-1)

1/T x 103 (1/K)


25


0.0 0.5 1.0 1.50

100

200

300

400

500

Moisture Effects on Tg

T300/924 E-Glass/913G

lass

Tra

nsiti

on T

empe

ratu

re (K

)

Moisture Content (wt%)


26


Tgd is the Tg of the dry material

Tgw is the Tg of conditioned (or wet) material

M is the amount of moisture absorbed (wt %)

g is the temperature shift (in K) per unit moisture absorbed

g is 28.9 K for E-glass/913 and and 36.8 K for T300/924

Corresponding values of Tgd are 430 K and 482 K

gMT = T gdgw −

Moisture Effects on Tg


27


0 10 20 30 40 500

200

400

600

800

1000

1200

1400

Moisture Effects on Tensile StrengthUnidirectional E-glass/913 and E-glass/F922

E-glass/913 E-glass/F922R

esid

ual T

ensi

le S

tren

gth

(MPa

)

Exposure Time (days)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.40

200

400

600

800

1000

1200

1400

Res

idua

l Ten

sile

Str

engt

h (M

Pa)

Moisture Content (wt%)


28

F922 Epoxy Unidirectional E-glass/F822P is material property at the test temperature T

Po is the initial property (un-aged) value of the dry material at room or reference temperature To

n is an empirical constant experimentally derived

n is 1 for E-glass/913 and 0.3 for T300/924

Tgd > To and Tgw > T

Temperature Effects on Transverse Flexure Properties

n

ogd

gw

o TTTT

PP

⎟⎟⎠

⎞⎜⎜⎝

⎛

−−

=


29


t1/2 is time required for the tensile strength to degrade to half its original value (half-life) at each temperature

A and k are material constants determined experimentally

T is the ageing (or conditioning) temperature

Unidirectional E-glass/Polyester

Temperature Effects on Tensile Strength

A = -430 and k = 1.48 x 105

T/A + k = t ln 2/1


30


0.0028 0.0030 0.0032 0.00340

10

20

30

40

50

Tensile Strength Half-Life vs. TemperatureE-glass/Polyester

t 1/2 (d

ays)

1/T (K-1)


31


2.0x10-5 3.0x10-5 4.0x10-5 5.0x10-50

1

2

3

4

Tensile Strength Half-Life vs. DiffusivityE-glass/Polyester

ln t 1/

2 (d

ays)

Transverse Diffusivity (mm2s-1)


32

Empirical ApproachNon-Dimensional Temperature

P Material property (e.g. longitudinal tensile strength)

PO Initial property value of the dry material at room or reference temperature To (296 K)

T Test temperature (K)

Tg Glass transition temperature of the material

g Temperature shift (in K) per unit moisture absorbed

M amount of moisture absorbed (wt %)

gMTTwhereTTTT

PP

gdrygwet

n

ogdry

gwet

o

−=⎟⎟⎠

⎞⎜⎜⎝

⎛

−−

=


33


0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

Transverse PropertiesUD E-glass/913 Epoxy

modulus strength

P/P o

(Tgw - T)/(Tgd - To)


34


Kitagawa power-law relationship between shear yield stress, τ, and shear modulus, G, for glassy polymers is:

To is the reference temperature (K) – (e.g. 23°C or 296K)

τo is the shear yield stress at To

G is shear modulus

n is an empirical constant experimentally derived

Kitagawa Power-Law RelationshipShear Properties

n

o

o

o

o

TGGT

TT

⎟⎟⎠

⎞⎜⎜⎝

⎛=

ττ


35

Shear Modulus Shear Strength

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

AS4/3501-6 XAS/914 T300B/R23 APC-2

P/P o

(Tgd-Topr)/(Tgd-Tr)

0.0 0.2 0.4 0.6 0.8 1.00.0

0.2

0.4

0.6

0.8

1.0

AS4/3501-6 XAS/914 T300B/R23 APC-2

P/P o

(Tgd-Topr)/(Tgd-Tr)

In-Plane Shear PropertiesUnidirectional Laminates


36

Kitagawa Power-LawIn-Plane Shear PropertiesUnidirectional CFRP Laminates

0.0 0.1 0.2 0.3 0.40.0

0.1

0.2

0.3

0.4

n = 1

-log

(T0τ

/Tτ 0)

-log (T0G/TG0)

AS4/3501-6 XAS/914 T300B/R23 APC-2


37

GRP Pultruded Rods

Fibre Volume Fraction (Vf)Well bonded: 56.2 ± 0.7Poorly bonded: 55.8 ± 0.8

Glass Transition Temperature (Tgdry)Well bonded: 118.2 °CPoorly bonded: 122.2 °C


38

GRP Pultruded Rods – Flexure PropertiesMaterial Moisture Content

(%) Flexural Modulus

(GPa) Flexural Strength

(MPa) Dried at 50 °C Well Bonded Poorly Bonded

0.00 0.00

33.8 ± 0.8 30.1 ± 1.1

853 ± 39 371 ± 56

1 Month Well Bonded Poorly Bonded

0.16 ± 0.07 0.27 ± 0.15

36.0 ± 1.1 29.2 ± 0.8

871 ± 61 281 ± 6

3 Months Well Bonded Poorly Bonded

0.27 ± 0.04 0.83 ± 0.22

36.1 ± 1.4 28.3 ± 1.9

866 ± 52 298 ± 31

Flexural stiffness and strength reduced due to poor fibre/matrix interfacial strengthPoorly bonded systems tend to absorb higher levels of moisture – deionised water at 23°C


39

Fibre Volume Fraction (%)Well bonded: 56.2 ± 0.7Poorly bonded: 55.8 ± 0.8

Flexural Modulus (GPa)Well bonded: 33.8 ± 0.8Poorly bonded: 30.1 ± 1.1

Flexural Strength (MPa)Well bonded: 853 ± 39Poorly bonded: 371 ± 56

Interlaminar Shear Strength (MPa) – core materialWell bonded: 54.6 ± 3.1Poorly bonded: 20.0 ± 1.4

Moisture Content (%) – 3 months in deionised water at 23°CWell bonded: 0.27 ± 0.04Poorly bonded: 0.83 ± 0.22Poor Good

GRP Pultruded Rods – Test Data


40

GRP Pultruded Rods

Poor Good

Blisters or residue sizing droplets on surface of the fibre

Interphase region< 50 nm (well bonded)100-350 nm (poorly bonded)


41

5 µm

0 µm

2.5 µm

5 µm0 µm 2.5 µm

0.00 nm

57.33 nm

5 µm

0 µm

2.5 µm

5 µm0 µm 2.5 µm

5.410 V

7.331 V2 µm

0 µm

1 µm

2 µm0 µm 1 µm

5.623 V

7.302 V1 µm

0 µm

0.5 µm

1 µm0 µm 0.5 µm

5.577 V

7.268 V

1 µm

0 µm

0.5 µm

1 µm0 µm 0.5 µm

0.00 nm

24.84 nm

2 µm

0 µm

1 µm

2 µm0 µm 1 µm

0.00 nm

29.99 nm

Topography

Phase Images

5,2 and 1µm Phase Images of a portion of a unidirectional GFRP specimen with poor interfacial bonding

Phase Image of interface region for poorly bonded sample

5µm scan size 2µm scan size 1µm scan size

Glass

Matrix

Glass

Matrix


42

Property Predictions - E-glass/Vinylester

Undamaged – core materialE11c = 48.1 ± 1.3 GPaM/41.90 GPaP

S11c(T) = 1216 ± 83 GPaM

G12 = 3.88 GPaP

S13 = 54.6 ± 3.1 MPaM

S11c(F) = 1216 ± 83 GPaM

Degraded by 100% – core material E11c = 43.5 ± 1.3 GPaM/41.87 GPaP

S11c(T) = 540 ± 52 GPaM (0.44S11c(T))G12 = 1.44 GPaP (0.37G12

P)S13 = 20.0 ± 1.4 MPaM (0.37S13)

Measured – m, predicted - p


43

Steam Autoclave - 2.2 bar/136 °CMaterial Moisture Content

(wt %) Tensile Strength

(MPa) E-glass/F922 Dry 0.00 1087 ± 29 70 °C/85% RH (6 weeks) 1.00 763 ± 42 Steam autoclave (24 hrs) 2.09 654 ± 22 Steam autoclave (48 hrs) 2.94 579 ± 47 Steam autoclave (72 hrs) 2.83 585 ± 50 HTA/F922 Dry 0.00 1684 ± 132 70 °C/85% RH (6 weeks) 1.06 1728 ± 132 Steam autoclave (24 hrs) 2.12 1691 ± 91 Steam autoclave (48 hrs) 1.90 1725 ± 42 Steam autoclave (72 hrs) 2.20 1784 ± 94


44

Single-Lap JointAluminium/Epoxy

Load

Gauge length: 112.5 mm

50 mm 50 mm12.5

0.25 mm1.6 mm

(Width: 25 mm)


45

Moisture Effects on Joint StrengthAluminium/Epoxy Single-Lap Joint

0 10 20 30 40 500

50

100

150

200

250

300

350

25 oC

40 oC

50 oC

60 oC

70 oC

Failu

re L

oad

(N/m

m)

Exposure Time (days)


46

Effects of Moisture on Elastic PropertiesEpoxy Adhesive (Water Immersion)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

0 50 100 150 200 250 300

Conditioning Time (hours)

Moi

stur

e C

onte

nt, M

(%)

AV119 - Water Immersion at 60°C

1500

1700

1900

2100

2300

2500

2700

2900

3100

3300

0.0 1.0 2.0 3.0 4.0 5.0 6.0

Moisture Content (%)

E (G

Pa)

0.30

0.32

0.34

0.36

0.38

0.40

v

Young's Modulus

Poisson's Ratio


47

Effects of Moisture on Tensile StrengthEpoxy Adhesive (water Immersion)


48

Predicted Moisture Concentration DistributionsAluminium/Epoxy Single-Lap Joint

0

2

4

6

8

10

12

0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5

Distance from the edge to the centre of the overlap (mm)

Moi

stur

e C

onte

nt (%

)1 hour water immersion

6 hours water immersion

1 day water immersion

2 days water immersion




49

Predicted Shear Stress DistributionsAluminium/Epoxy Single-Lap Joint

0

2

4

6

8

10

12

14

16

18

20

22

24

0 1.25 2.5 3.75 5 6.25 7.5 8.75 10 11.25 12.5Distance from edge (mm)

S12

Stre

ss (M

Pa)

No conditioning

1 hour water immersion

6 hours water immersion

1 day water immersion





50

Predicted Plastic Strain DistributionsAluminium/Epoxy Single-Lap JointImmersed in Deionised Water @ 60°C

Unconditioned 12 days immersion


51

Effects of Stress on Moisture AbsorptionEpoxy Adhesive


52

Effects of Pressure on Moisture AbsorptionEpoxy Adhesive


53

Dielectric measurements to monitor water content in adhesive joints

Optic fibre systems – Embedded Fibre Bragg Gratings for in-situ sensing of chemical species (including water) – composites and adhesive joints

Improved predictive analysis – fracture mechanics based FE modelling incorporating interfacial degradation

Future Trends


54

Fibre/matrix interfacial bond strength is critical to structural and long-term performance

Tests exist that can be used differentiate between well and poorly bonded composite systems

Continuous Immersion in Deionised Water

Analytical/semi-empirical relationships - relate degree of material property degradation with level of degrading agent

Enabling extrapolation for longer exposure times under more benign (realistic) conditions

Embedded sensors, such as Fibre Bragg gratings are now being used to determine expansion or swelling due to moisture and chemical species concentrations

Conclusions


55

Concern regarding the relationship of the results of accelerated ageing tests for polymeric materials and the actual service performance

Few test methods or standards exist that predict life expectancy with confidence

A major challenge is to ensure that performance testing for determination of chemical resistance (level of degradation) is based upon a set of “quantitatively”measurable criteria

Conclusions


56

AcknowledgementsSpecial thanks to:UK Department of Innovation, Universities and Skills National Measurement System Policy UnitJohn Hartley (Exel Composites Ltd)Peter Thornburrow (Owens Corning)NPL Colleagues: Bruce Duncan Sam Gnaniah Maria LodeiroNeil McCartney Gordon Pilkington Graham SimsTim Young


57

Thank You

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