**3. Off-shore structures**

Off-shore marine structures such as oil rigs and columns of bridges, underwater pipe line supports, etc., are conventionally made using reinforced cement concrete having steel rods as reinforcements inside the concrete. In some other cases, steel pipes, pillars, and column supports are used.

The underwater static steel structures are protected from corrosion and fouling by electrochemical protection system and paints. A new method of protecting the steel structure is to provide a wrap of composite as an outer lining, which is far more durable than painting and more maintenance-free. Steel pipes are used as a mandrel for a filament winding technique to provide a composite lining. Steel structures require underwater welding, etc., for repair and maintenance, which is very complicated and costly. The FRP lining provides a very convenient solution to reduce such maintenance cost and frequency of repair.

The concrete with diffused sea water generates more alkali, and the pH of this alkaline seawater increases from normal range of 8–8.3 to about 12–13 and

accelerates the corrosion of the steel reinforcement in the RCC structure. Therefore, FRP reinforcements are modern way of construction for higher durability and lesser maintenance. However, in a higher alkaline sea water environment, the FRP degradation is expected to be faster than in normal sea water.

For off-shore and maritime civil engineering structures, carbon fiber composites (CFRP) are preferred over glass fiber (GFRP) because of higher mechanical strength of CFRP. In addition, sea water uptake and degradation of GFRP in sea water are higher those in than CFRP. There are very few applications of GFRP in marine structures despite the fact that GFRP is cheaper compared with CFRP.

The durability is also dependent on the resin type and its interface bonding with the fiber. Generally, thermosetting polymers such as epoxy, polyurethane, phenolic resin, vinyl ester, and unsaturated polyester resins are used for composites. These resins and corresponding composites are to be evaluated for long period of sea water exposure in an RCC construction for durability. As accelerated studies might give some extrapolated figures of service life, but such studies cannot determine the effect of microorganisms on degradation of a composite. A very common example is sulfate-reducing bacteria (SRB) in the sea water. These organisms use sulfates dissolved in the sea (for example, MgSO4) for metabolism and produce hydrogen sulfide, which is highly corrosive to metals and may also increase the degradation of composites after settling onto the surface. The effect of such organisms is much more important for static structures rather than moving objects. Fouling by other micro and macro-organisms and subsequently the effect on the composite is another aspect of static structures. The protection from bio-fouling by application of anti-fouling coating is another subject of study. However, this type of coatings work either on toxin release mechanism or by providing a low surface energy coating. In case of toxin release coating, the toxin release depends on the hydrolysis and dissolution of the toxin in water, which is more effective in moving condition than stagnant water. Because of toxin depletion, the coating requires renewal after a certain time, mostly 3 years. In case of low surface energy coating, the effectiveness is far less for static structures, as this type of coating is quite good for moving objects, that too at certain minimum speed. However, the advantage is that the settlement of bio-fouling species on these low surface energy coating is very weak and can be removed by a soft cleaning mop. The second most important aspect of static structures is stress. Most supports and beams are under stress, small or large. The pre-stressed composite structure may have lesser service life compared with no-stress elements. The third consideration is fatigue. A bridge column, pipeline carrying liquids under the sea are subjected to vibrations. Hence, the composite elements are to be evaluated by fatigue for a predetermined frequency and number of cycles. This should be done in a repeated experiment and at a regular immersion period. The effect of pre-stress and vibration parameters may reveal some results, which could be different from normal static experiments.

## **4. Quality of a marine-grade resin and FRP**

A general understanding of large-scale application of FRP is that there can be three main alternatives for a techno-commercially viable thermoset selection, e.g., (1) unsaturated polyester resin (USP) cross-linked with styrene, (2) vinyl ester resin (VE) cross-linked with styrene, and (3) epoxy resin cross-linked with amine. Whereas, there can be two common fibers such as glass and carbon.

Service life of a structural element for marine vessels has to be minimum 25 years for reducing the investment for replacement and should be maintenancefree for at least 8 years to reduce the cost of refits in drydocks. For off-shore

### *FRP for Marine Application DOI: http://dx.doi.org/10.5772/intechopen.101332*

structures, where FRPs are used to make barrier for underwater cement concrete structures, the maintainability is even more difficult, requiring high service life without maintenance activity. Apart from the general physical and mechanical properties, an FRP for marine application must have additional characteristics of low moisture/sea water ingress, minimum hydrolysis, good bond strength between fiber and polymer, minimum physical damage of fiber and polymer due to water ingress and retention of mechanical properties even after prolonged sea water immersion. However, all the properties are primarily dependent on the matrix polymer and fiber and their interaction, secondary parameter being processing technique and fabrication methods to make flawless FRP components with fairly accurate dimensions, such as for a marine ball valve or pipe joint. Processing assumes larger importance since partially cured samples are prone to poorer physico-mechanical properties and higher degradation in water.

A significant improvement in properties of marine composites can be achieved by prepreg method and resin transfer molding (RTM) assisted by vacuum. Good compaction and high fiber volume fraction can be achieved by these processes. The process of prepreg molding is feasible where the resin-hardener reaction does not take place at ambient or storage temperature, and the curing is done at a fixed higher temperature. There are high-temperature reacting systems such as epoxy resin 4,4<sup>0</sup> methylene dianiline tetraglycidyl ether (TGDDM), to be cured with hardener such as 4,4<sup>0</sup> -diaminodiphenyl sulfone (DDS), modified polyamines, etc., which are used for making prepregs. The shelf life of such prepregs at storage temperature of �20°C is about 10–12 months, but few weeks at 20°C. The prepregs are cured in compression at above 100°C. However, prepreg system may not be possible for vinyl ester or polyester resins. RTM process requires low viscosity resin and hardener to facilitate good flow in the fabric stacked in the mold for proper wetting at all corners and contours. Trujillo et al. [6] reported the properties of RTM processed composites based on three common resins, i.e., epoxy, vinyl ester, and unsaturated polyester with glass and carbon fabrics. The flexural modulus of the glass composites was about 40GPa and about 110–120 GPa for carbon fabric composites, while the flexural strengths were seen to be in the range of 600–800 for glass composites and 1300–1400 MPa for carbon fabric composites.

Sea water absorption causes changes in the matrix by both plasticization and hydrolysis. Initial effect of water ingress is a plasticizing effect and swelling of the polymer matrix. The results are lowering of glass transition temperature due to plasticization and a possibility of debonding of the polymer-fiber interface due to swelling of the polymer. The initial effect of water ingress also causes hydrolysis of the fiber sizing and generates alkali (Na+ and K+) and the Fiber-polymer interface weakens. All these events result in reduction of ultimate strength and elastic modulus of an FRP.

On prolonged exposure, several chemical reactions may take place, such as hydrolysis of the polymer resulting in small molecules such as glycol, chain breaking, and release of low-molecular-weight polymer (especially polyester), release of the constituents of the resin (typically maleic/fumaric acid), release of styrene (cross-linker for polyester and vinyl ester), and extraction of these species from the FRP to the sea water. Prolonged water immersion of FRP may also cause mechanical damage to the fiber and polymer both, which may not be observed in short period, even in few months of exposure. SEM analysis of all FRPs irrespective of the fiber showed detachment of matrix from the fiber, which is the main reason for such drastic decrease in strength of the composite laminates as reported in literature.

The polymer plays the most important role in the hydrolytic durability of an FRP. As a special case of glass fiber reinforcement, the coated material used as coupling agent chemically degrades and causes weak interface of fiber-polymer. Therefore, a polymer-fiber combination is ultimately the consideration for optimization of hydrolytic properties.

The main reason why GFRP is not used in maritime civil construction applications is because sea water environment degrades the long-term mechanical properties of GFRP composites and interlaminar shear strength (ILSS). The glass fiberpolymer interface is strengthened by a coupling agent coated on the glass fibers and the process is called "Sizing." The sizing formulations are very complicated, may contain many different chemicals, and are proprietary to the manufacturers [7]. Most common are γ-amino propyl tri ethoxy silane (APTES), γ-glycidoxy propyl trimethoxy silane (GPTMS), γ-methacryloxy propyl trimethoxy silane (MPTMS), and vinyl tri ethoxy silane (VTES) having Si-OH groups on the fiber surface for improving the interface adhesion with the resin. The sea water diffused to the interface of fiber and polymer very quickly degrades the glass to produce alkaline oxides unless protected by sizing. Even then, prolonged immersion of the GRP with fiber with appropriate sizing may cause leaching of alkali oxides (sodium and potassium) from the surface of the fiber and degrade the composite mechanical property [8].

## **4.1 Epoxy thermoset-based FRP for marine application**

Epoxy resin is a versatile thermoset, widely used in many marine structures for many years. It has good mechanical properties, is highly polar and compatible to most fibers including metals, glass, carbon, Kevlar, and polybenzimidazole. Epoxy nanocomposites are gaining importance due to lightweight and high performance in some functional properties when used with carbon nanotubes, nanofibers, graphene, and also natural nanofibers. The conventional epoxy resin thermosets are somewhat brittle and, in many occasions, modifications are done either by physical mixing or chemical reaction onto the epoxy oligomer or use of high-molecularweight epoxy and/or the amine curing agent to make optimum tough thermoset. However, flexibilization means increase in free volume in the polymer and subsequent increase in moisture absorption. As such the degradation of conventional epoxy thermoset and composites is very widely studied by many researchers since last 45 years, for example, by Augl and Berger [9] in 1976 on carbon fiber-epoxy composites, McKague et al. [10], DeIasi and Whiteside [11], and Whitney and Browning [12] studied moisture diffusion in epoxy matrix and composite, during 1976–1978, to name a few. Similarly, Loos and Springer [13], Bohlmann and Derby [14], Shirrell [15] studied moisture diffusion and its effect on graphite epoxy composites way back during 19761979. Glass fiber-epoxy composites are most widely evaluated for effect of moisture or water or sea water absorption from those periods and are still being the subject of study. A few are listed here as references [16–30].

The glass transition temperature of cured epoxy matrix and composites is reduced from 120°C to as low as 66°C on a 2-month sea water exposure, but was observed to be almost constant around 85–88°C from 4 months onward till the end of the study period (12 months), as reported by Chakraverty et al. [31]. The authors explained this anomaly by probable osmotic effect of the bulky molecules of dissolved salts in sea water, which might have initially facilitated the creation of more free volume in the cross-linked epoxy matrix, but on prolonged exposure, deposition of these salts could have reduced the water ingress. Murthy et al. [32] have shown that the water uptake by epoxy-glass composite is more (about 0.9%) compared with 0.7% by epoxy-carbon composites after 12 months and remained unchanged. However, their study was limited to 16 months. The ILSS was reduced by 38% for epoxy-carbon and by 31% for 450 days at ambient temperature. SEM analysis revealed that the moisture penetration along the fiber/matrix interfaces

### *FRP for Marine Application DOI: http://dx.doi.org/10.5772/intechopen.101332*

caused interfacial debonding and consequently degradation of the interface. Espinel et al. [28] also showed that for an epoxy-glass composite, the saturation level of sea water was 0.4% at 25°C attained after 30 days. The tensile and flexural strength reduced by about 24% and 35% respectively after 90 days sea water immersion at 25°C, but observed that the strength did not decrease much after saturation of sew water. Contrary to these results, Murad et al. [25] showed that the sea water intake in epoxy-glass unidirectional composite was 2.5% after 12 months, but the strength and elastic modulus had no noticeable change compared with fresh sample. However, the fiber volume % was only 52. Wood and Bradley [20] also reported about 2.2% sea water uptake for 5 months at ambient temperature for an epoxy-glassgraphite hybrid composite, each layer fabricated by a similar process as filament winding, and hence the layers were unidirectional. The glass and carbon were in transverse direction to each other. However, the resin used had a 5% flexibilizer (rubber) and fiber volume % was 60. Komorek et al. [29] used fabrics of glass and carbon in epoxy resin. The bending strength was found to be 8% less for the samples immersed for 36 days at 15°C in sea water.

A unique study on fatigue and sea water aging of epoxy-glass and epoxy-Kevlar composite was done by Menali et al. [33]. The authors studied the effect of sea water (artificial) immersion (40 days) after fatigue for 100–50,000 cycles for these composites. There was about 19% reduction in tensile strength for the Glass-epoxy composite samples and about 15% for Kevlar-epoxy samples which had undergone 50,000 cycles of straining and aged in sea water for 40 days. The stiffness of the composite laminates was also degraded by almost similar extent. This result, when compared with that of Komorek [29], clearly shows the additional degradation under cyclic loading.

There is another interesting review report by Li et al. [34] on effect of alkaline sea water (pH at 12–13) for pre-stressed FRP laminate and FRP tendons. The alkaline sea water simulates the property of the sea water sea sand concrete (SWSSC), which is now very much used in civil construction of marine static structures such as off-shore platforms. The authors compiled several results by some researchers. It is seen from the review article that the alkaline SWSSC at pH of 12–13 has a higher degrading effect under such condition.

A comprehensive study was done on the effect of sea water immersion at various temperatures for an epoxy thermoset plaque and its E-glass fabric composite having 55% fiber by volume. The report is not for publication. The composite samples were made by vacuum bagging process followed by compression molding at 120°C. The curing of plaque and composite was done after thorough degassing of the resinhardener mix. It was observed that after 360 days of immersion, the flexural strength reduced from about 90 MPa to about 65 MPa, and the dynamic flexural modulus was reduced from about 3.20 GPa to about 2.5 GPa at 30°C in natural sea water. The E-glass composites of the same resin were seen to deteriorate in flexural strength and modulus. The strength reduced from 250 MPa to about 180 MPa, and the dynamic modulus reduced from 8 GPa to about 5.5 GPa. The results clearly show the effect of debonding of the fiber from the epoxy matrix interface thereby drastically reducing the loadbearing capability. The water had a plasticizer effect too, as the glass transition temperature changed from 60 to 62°C to about 52–54°C in 12 months, and the SEM micrograph showed separation of the fiber from the matrix at the interface very clearly. However, the effect of the microbes on degradation could not be quantified separately.

The studies done so far indicate a common observation and conclusion that the degradation of epoxy-based composites is significantly high in terms of delamination, loss of mechanical properties and glass transition on exposure in sea water even for a year. The initial moisture ingress has a plasticizing and swelling effect,

due to which the glass transition temperature reduces with a drop in mechanical properties. In prolonged exposure, the water molecules chemically react with the resin (hydrolysis) producing small chemical substances, which tend to diffuse out of the resin, causing blisters. Also, various salt components of the sea water may affect the moisture absorption rate compromising some properties of FRP in sea water. It is also known that the effect of sea water on glass fiber reinforced composites differs according to the type of matrix and fiber. The mode of failure of glass/epoxy composite is altered from a brittle matrix and ductile fiber to ductile matrix and brittle fiber. However, in some opinion, the strength stabilizes after the absorbed moisture attains saturation.

In construction of FRP elements of ships, the items that are not in continuous immersed condition such as superstructures, ladders, stanchions, guard rails, etc., are better designed with toughened epoxy resin and carbon/glass fabric composites since the degradation is limited in the atmosphere and the composites can have sufficient strength, reasonable glass transition temperature even after the toughening process of the resin. For naval ships of stealth features, carbon fiber and nanocarbons cannot be used as the radar reflection will be increased. For elements to be used underwater, epoxy resin is not that superior to the vinyl ester class of resins.

## **4.2 Vinyl-ester-based FRP composites**

Vinyl ester resins are most commonly used for marine composites for two main reasons, the mechanical strength retention on prolonged exposure in sea water and the strength is comparable to epoxy composites and higher than polyester-based composites. The resin has inherent resistance to water diffusion and consequently lesser effect on its glass transition and strength. For large ship structures, vinyl ester resin is a better thermoset due to suitability for processing large items such as hulls, using vacuum infusion due to its low viscosity, apart from its durability in marine environment.

Conventional vinyl esters are having aromatic backbone of epoxy base and the double bond of the unsaturated ester is cured by styrene, exactly the same process as a polyester resin. The presence of the higher content of stiff aromatic epoxy backbone provides the higher mechanical strength compared with phthalic-acidbased polyesters. The higher aromatic content also restricts the diffusion of fluids. Unlike epoxy matrix cured by amines, the vinyl ester matrix is cured by hydrophobic monomer styrene, and hence, the water ingress is lesser than epoxy resin.

VE-CFRP and VE-GRPF have different strength ratios depending on the mode of force application. Wonderly et al. [35] compared these two types of composites in terms of tensile strength and found that CFRP was about 850–950 MPa and was 1.6–1.75 times higher than GFRP, but the open hole tensile strength was comparable at about 250–265 MPa, and compression strength of GFRP was about 330–360 MPa for CFRP and was about 17% lower than GFRP. Transverse tensile strength of CFRP was also about 75% of GFRP. One interesting study was done by the authors on ballistic impact test, which is important for military application. At a comparable areal density, the specific energy (J/kg/m<sup>2</sup> ) required to penetrate the panels for CFRP was higher by about 25% compared with GFRP for identical muzzle velocity.

In general, the glass transition temperature of a vinyl ester matrix is about 115–120°C. The flexural modulus and strength of a vinyl ester plaque are about 3.0–3.5 GPa and 80–120 MPa respectively, almost same as epoxy plaque. GFRP of vinyl ester has flexural modulus of about 10–12 GPa and flexural strength of about 270–300 MPa, ILSS of about 30–35 MPa, depending on the fiber type and fiber volume fraction in the composite. The CFRP of vinyl ester has much higher strength and modulus, about 3–3.5 times higher than GFRP at the identical volume fraction of fiber.

Water diffusion studies show on an average 0.6% water intake at equilibrium for GFRP and about 0.4% for CFRP, which are almost half of corresponding figures for epoxy CFRP. Murthy et al. [32] showed that the sea water saturation levels in both GFRP and CFRP of vinyl ester are about 0.7% and approximately 0.4% respectively, and there was no reduction in the total weight of the samples even after 450 days of immersion. The interlaminar shear strength was reduced by about 35% after 365 days for both CFRP and GFRP of vinyl ester. Similar extent of degradation was observed for flexural strength. The reduction of tensile strength was about 30% for the same period of immersion. However, it is observed that the mechanical properties and the water uptake almost became steady after 365 days. The authors showed that after immersion in artificial sea water for 450 days, the strength reduced by about 35% for both the composites. Similarly, the ILSS also reduced by about same extent. CFRP is marginally better in ILSS on aging in sea water. The maximum water intake for CFRP was about 0.4% compared with about 0.48% for GFRP.

A study by Mungamurugu et al. [36] showed about 1% water absorption at 20°C for vinyl ester GFRP composite for glass fiber volume of 58% compared with about 0.75% for the plaque after 450 day and the reduction in flexural strength (original 250 MPa) by about 25% for the composite after 300 days.

However, most experiments reported in literature are done with artificial sea water, and the effect of the microorganisms and of the evolved materials due to the metabolism of the microbes present in sea water was not possible to observe. Therefore, the drastic decrease in mechanical strength for thermosets resin plaques is due to reaction with water and hence loss of molecular integrity of the cross-linked matrix.
