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Diffusion is a process by which matter is transported from one part of a system to another as a result of random molecular motions. Diffusion is a time-dependent process, where D is the diffusion coefficient. Upon assuming, a continuous mass Fig. 1 A carbon fiber reinforced vinyl ester (CF/VE) composites panel prepared at different orientations with final dimension sample of 200 mm by 25 mm consisting of four cross-stich plies of Devold LT 650 (12 k Toray’s Torayca T700 carbon fiber tow) and vinyl ester resin. The structure of each stitch bonded carbon fabric ply combines the fill face in 0˚ direction and warp face in 90 direction stitched with a polyester knitting thread used to form the panel of [fill face/

warp face]2S

Effect of Sea Water on Polymeric Marine Composites 131

distribution m(x,t), the steady-state flux (J) is controlled by Fick’s first law. More comprehensive details are given elsewhere [9].

J¼ Dom

ox ð1Þ

Under general conditions (non-steady state), which is more applicable to polymer composites, diffusion can often be described by Fick’s second law:

om

ot ¼ Do²m

ox² ð2Þ

The property degradation due to fluids may be recoverable or irreversible and a distinction between them can be made by observing characteristic features of weight gain data. Fluid sorption in polymers and polymeric composites can be broadly identified by means of five schematic curves relating weight gain to ffiffiffiffi

t^ p as presented in Fig.2. The solid curve marked in that figure applies to the linear Fickian (LF) behavior, which is often used to describe stable polymer systems exposed to fluid ingress. Curves ‘‘A’’ and ‘‘B’’ are typical variations corresponding to both neat polymers and fiber reinforced composites having recoverable fluid-induced degradations of certain material properties. Of these, type A variation corresponds to a case where weight-gain never attains equilibrium, such as for two phase diffusion, and type B represents the circumstance of two-stage diffusion.

Data represented by curves ‘‘C’’ and ‘‘D’’ are usually obtained for polymeric composites only. Of these, ‘‘C’’ accounts for the case of rapidly increasing fluid content, which is usually accompanied by damage growth that leads to material break down, large deformations, as well as occasional failure. Curve ‘‘D’’ accords with weight loss that is attributable to chemical or physical break down of the material. Materials that show a weight gain data as shown conceptually using curves ‘‘C’’ and ‘‘D’’ represent irreversible response, often associated with loss of material integrity and possible structural failure.

In this investigation, CF/VE facing samples 200 mm long and 25 mm wide and a given thickness (depending on the lay-up configurations) were initially dried in a

Fig. 2 Schematic curves representing a solid line, designated by LF,

corresponds to linear Fickian diffusion and four possible categories of non-fickian weight-gain sorption data

desiccator until their weight reached a constant value. Subsequently, weight gain data were recorded periodically after immersing the material in simulated sea water baths at 40C for a soaking duration of at least three months which cor-responds to saturation time for this polymeric composite material system. After a target duration of soaking, as suggested by ASTM D 5229 [11], these samples were removed from the water bath, padded dry to remove the surface moisture and then weighed on a Sartorius analytic balance within an accuracy of ± 0.1 mg to determine the percent weight gain (%M_gain) or moisture content using Eq.3 The wet condition (saturated sample) was achieved by immersing the carbon fiber reinforced vinyl ester samples in a water bath at 40C for at least 3 months prior to testing. The state of saturation was ascertained by monitoring the weight gain with time of soaking. The moisture content is plotted as function of the time ( ffiffi

pt ).

The diffusion coefficient, D can be obtained from the initial slope of a plot using Eq.4 if one were to assume that 1-D diffusion conditions are satisfied.

%M_gain¼M_wet Mdry

CF/VE coupon specimens went through special conditioning process to eval-uate potential degradations in mechanical properties. Considering time for reaching full saturation for these composites being 3 months, the specimens were immersed in temperature controlled water baths at 40C for 5 months before conducting mechanical property tests and water baths having sea water, distilled water, and deionized water were also used for selective studies. In this chapter, weight gain data as observed for considered fluid, specifically distilled, tap and sea water absorption is reported. Weight gain measurements were taken after 1, 4, 9, 16 and 24 h, followed by weekly intervals. Water absorption data suggests that on relative scale, these composite facings absorbed sea water the most, followed by distilled and tap water respectively as summarized in Fig.3. However, considering the significant scatter which is expected for this type of measurement approach, one can conclude that insignificant differences between three water types are noted for both saturated weight gain amount and temporal variation of fluid sorption behavior. Using a value of 0.5 % (Fig.4) for saturation weight gain condition corresponding to sea water sorption, using Eq.4for this data, diffusion coefficient of 7.5 9 10^-7mm²/s was obtained for this T700 carbon fiber based vinyl ester composite facing.

The most common method used in practice for the determination of the dif-fusion coefficient is outlined by refs [11,12] and requires periodic monitoring of weight gain data to calculate the moisture diffusivity constant. Weight gain data in linear region corresponding to shorter time scales has considerable data and requires linear regression on noisy data to interpret slope leading to large error in the estimation of diffusion coefficient for polymeric composites [13]. Moreover,

Effect of Sea Water on Polymeric Marine Composites 133

the anisotropic nature of composites introduces additional errors to diffusivities calculated by conventional methods using simple 1-D model.

One should expect considerable scatter in actual weight gain data using the method described above and an example is shown for two types of composites in Fig.5. Availability of this type of data associated with long-term weight gain for polymer composites is rare in the literature and it is important to recognize onset of permanent damage from such simple, but time consuming measurements. Both types of composites prepared using unidirectional [06] AS4/3501-6 and [06] IM7/8551-7 graphite/epoxy coupons with dimensions of 200 mm and 25 mm (manufactured in a vacuum press following Hercules corp specification) exhibits a type ‘‘D’’ curve discussed earlier, indicating the onset of permanent damage in composites. If the data was not acquired for long-enough duration, once could have easily considered these materials as stable when exposed to sea water.

Authors have recently developed an experimental procedure to allow mea-surement of moisture uptake during adsorption/desorption kinetics with a Fig. 3 Summary of weight

gain data for specimens immersed in sea, distilled, and tap water

Fig. 4 Weight gain data for CF/VE immersed in sea water over 3 months

resolution of 0.1 lg using a highly precise gravimetric microbalance system as shown in Fig.6. Water vapor adsorption experiments were performed by gravi-metric techniques. The experimental setup consists of a high-sensitivity micro-balance (Cahn Digital Recording Balance, DRB-200), customized sampling and gas handling system, and a data acquisition system. A custom-made 750 mm long and 75 mm inner diameter quartz chamber is fitted to the balance on the sampling side. The sample pan is attached using a nickel–chromium wire mesh to reduce mass transfer resistance between solid–fluid phases. The sample pan is suspended inside the quartz chamber by a Ni–Cr wire, which does not absorb moisture. The quartz chamber has an inlet gas port at the bottom and several exit ports along its length. The gas generation system consists of ultrahigh purity N₂(99.999 % UHP N₂) as carrier gas. The carrier gas is initially passed through a gas drier/desiccator, containing anhydrous CaSO₄, to remove any trace gas moisture. Mass flow con-trollers are used to control the gas flow rates. A portion of carrier gas is allowed to purge through double-distilled water in a fritted glass bubbler creating a saturated vapor or wet stream of carrier gas. Subsequently the wet carrier gas is mixed with remaining dry carrier gas to generate controlled water vapor concentrations in the carrier gas streams. The concentration of water vapor is monitored by a relative humidity probe (Cole-Parmer Instrument, Digi-Sense) upstream of the sample pan which provides the feedback control for the flow rates of wet and dry streams to maintain a target relative humidity at the sample location. The gravimetric balance is coupled with a data acquisition system that gathers mass, time and temperature data. Figure7shows the typical absorption results of a CF/VE sample exposed to approximately 95 % relative humidity for 300 h. When compared with Fig.4, percent weight gain data in Fig.7is accurately captured and the initial weight gain data shows proportional increase without scatter. Note that it cannot be considered as one-dimensional diffusion due to the relatively small width with respected to the thickness of CFVE sample employed for this study. Therefore, interpreting this Fig. 5 Five year sorption data for [0₆] AS4/3501-6 and [0₆] IM7/8551-7 coupons immersed in simulated seawater at 34C

Effect of Sea Water on Polymeric Marine Composites 135

data using simple 1D diffusion model is simplistic at best. Since the moisture can enter the specimen through the edges, D for homogeneous material including edge effects yields [12]:

D¼ p h 4M_m

 2

M2 M1

ffiffiffiffi t₂ p pffiffiffiffit₁

 2

1þh l þh

n

 2

ð5Þ where h = sample thickness (mm), n = sample width (mm), and l = length (mm).

Considering the weight gain data corresponding to 95 % relative humidity exposure shown in Fig.7b, using sample geometry, and Eq.5, diffusion coefficient

Fig. 6 A setup of gravimetric microbalance system coupled with a DAQ system

of 5.8 9 10^-5mm²/s is obtained considering 0.17 % for saturation condition after 2 weeks.

The diffusion coefficient calculated by one-dimensional approach is accurate only if the correct slope of the weight gain versus ffiffi

pt

at t = 0 is obtained. If one were to calculate Fickian mass diffusion due to combined edge and anisotropic effects, the analytical solutions proposed in refs [13,14] can be considered.

3 Mechanical Property Degradation Due to Sea Water