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Water Absorption

When an organic matrix composite is exposed to a humid environment or liquid, both the

moisture content and material temperature may change with time. These changes usually

degrade the mechanical properties of the laminate. The study of water absorption within

composites is based on the following parameters as a function of time: [4-26]

The temperature inside the material as a function of position;

The moisture concentration inside the material;

The total amount (mass) of moisture inside the material;

The moisture and temperature induced hygrothermal stress inside the

material;

The dimensional changes of the material; and

The mechanical, chemical, thermal or electric changes.

To determine the physical changes within a composite

laminate, the temperature distribution and moisture

content must be determined. When temperature varies

across the thickness only and equilibrium is quickly

achieved, the moisture and temperature distribution

process is called Fickian diffusion, which is

analogous to Fourier's equation for heat conduction.

Figure 4-13 illustrates some of the key parameters used

to describe the Fickian diffusion process in a

multilayered composite. The letter T refers to

temperature and the letter C refers to moistureconcentration.

Fick's second law of diffusion can be represented in

terms of three principal axes by the following

differential equation: [4-27]

=

+

+

c

tD

c

xD

c

xD

c

x

(4-7)

Figure 4-14 shows the change in moisture content, M, versus the square root of time. The

apparent plateau is characteristic of Fickian predictions, although experimental procedures haveshown behavior that varies from this. Additional water absorption has been attributed to the

relaxation of the polymer matrix under the influence of swelling stresses. [4-28] Figure 4-15

depicts some experimental results from investigations conducted at elevated temperatures.

Water Absorbtion Marine Composites

194

Figure 4-13 T i m e Va r y i n gEnvironmental Conditions in aM u lt i la y er e d C o mp o si t e[Springer, Environmental Ef-fects on Composite Materials]

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Structural designers are

primarily interested in

t h e l o n g t e r m

d e g r a d a t i o n o f

mechanical properties

w h en c o mp o si t es a r eimmersed in water. By

applying curve-fitting

p r o g r a m s t o

e x p e r i m e n t a l d a t a ,

extrapolations about long

term behavior can be

p o s t u l a t e d . [ 4 - 2 8 ]

Figure 4-16 depicts a 25

year prediction of shear

s t r e n g t h f o r g l a s s

p o l y es t e r s p e c im e n sdried after immersion.

S t r e n g t h v a l u e s

eventually level off at

a bo ut 6 0% o f t he ir

original value, with the

d e g r ad a t i on p r o c es s

accelerated at higher

temperatures. Figure

4-17 shows similar data

for wet tensile strength.

Experimental data at thehigher temperatures is in

relative agreement for

the first three years.

T ab le 4 -1 s ho ws t he

a p p a r e nt m a x i m um

moisture content and the

transverse diffusivities

for two polyester and

one vinyl ester E-glass

laminate. The numericaldesignation refers to

fiber content by weight.

The water content of laminates cannot be compared directly with cast resin water contents,

since the fibers generally do not absorb water. Water is concentrated in the resin

(approximately 75% by volume for bidirectional laminates and 67% by volume for

unidirectionals). [4-28]

195

Chapter Four PERFORMANCE

Figure 4-15 Time Varying Environmental Conditions in a Multi-layered Composite [Springer, Environmental Effects on Com-posite Materials]

Figure 4-14 Laminate Water Absorbtion Kinetics for Experi-mental Laminate Specimens [Pritchard, The Use of Water Ab-sorbtion Kinetic Data to Predict Laminate Property Changes]

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Table 4-1 Apparent Maximum Moisture Content and Transverse Diffusivities ofSome Polyester E-Glass and Vinyl Ester Laminates

[Springer, Environmental Effects on Composite Materials]

Substance Temp(C)

Maximum Moisture Content* Transverse Diffusivity

SMC-R25 VE SMC-R50 SMC-R50 SMC-R25 VE SMC-R50 SMC-R50

50% Humidity23 0.17 0.13 0.10 10.0 10.0 30.0

93 0.10 0.10 0.22 50.0 50.0 30.0

100%Humidity

23 1.00 0.63 1.35 10.0 5.0 9.0

93 0.30 0.40 0.56 50.0 50.0 50.0

Salt Water23 0.85 0.50 1.25 10.0 5.0 15.0

93 2.90 0.75 1.20 5.0 30.0 80.0

Diesel Fuel23 0.29 0.19 0.45 6.0 5.0 5.0

93 2.80 0.45 1.00 6.0 10.0 5.0

Lubricating Oil

23 0.25 0.20 0.30 10.0 10.0 10.0

93 0.60 0.10 0.25 10.0 10.0 10.0

Antifreeze23 0.45 0.30 0.65 50.0 30.0 20.0

93 4.25 3.50 2.25 5.0 0.8 10.0

*Values given in percent

Values given are D22x 10

7mm

2/sec

196

Water Absorbtion Marine Composites

Figure 4-16 Change of Moisture Con-tent with the Square Root of Time forFickian Diffusion [Springer, Environ-mental Effects on Composite Materials]

Figure 4-17 P r ed ic te d D ry S he arStrength versus Square Root of Immer-sion Time [Pritchard, The Use of WaterAbsorbtion Kinetic Data to Predict Lami-nate Property Changes]