**4.3 Unsaturated-polyester-based FRP composites**

Unsaturated polyesters (USPs) are also widely used in marine construction since it is very cost-effective, easy to process by vacuum-assisted resin transfer process due to low viscosity. Easy to cure, and intricate shapes can be made with a large variety of USP. The oligomer resin is cured conventionally by styrene in presence of catalysts such as methyl ethyl ketone peroxide (MEKP) and in some cases added accelerators such as a cobalt salt or those based on a tertiary amine. The cross-link density and corresponding mechanical properties are controlled by styrene content and the unsaturation in the oligomer. There are new USPs developed where styrene is replaced by acrylic monomers such as tri- or tetraethylene glycol dimethacrylate (TEGDM) [37], which are comparatively less toxic than styrene. The generalpurpose and marine-grade USPs are synthesized with different glycols such as isomers of pentyl glycol and isomers of phthalic acid with small amount of an unsaturated acid such as fumaric acid or malic acid. The average flexural strength of USPbased GFRP is about 250 MPa with 58–60% glass fiber by volume.

A study on long-term natural sea water immersion of USP was done by Norwood [38]. The USPs of orthophthalic-acid-based marine resin and isophthalic acidneopentyl glycol (IST-NGP)-based marine-grade resin were used with about 2.25:1 ratio of CSM: resin by weight in the form of chopped strand mat (CSM) and 1:1 ratio by weight for woven roving (WR): resin. The surface tissue coating of the same resin with 5% filler content and a gel coat was used to reduce water permeation. The study revealed that the IST-NGP-glass composite showed best water resistance in terms of appearance of blisters. The best performance was of high HDT (heat deflection temperature) IST-NGP where the blister formation was seen only after 200 weeks, while orthophthalic-acid-based conventional marine resin (medium HDT) showed blisters in about 52 weeks as the best performance. The conclusion in that study was significant for subsequent research on marine-grade FRPs. It was suggested to use a tissue layer of about 5% (by weight) CSM in the IST-NGP resin (high HDT grade marine resin) over the outer layer of the composite and a top layer of the gel coat (white) to ensure longer life in continuous sea water immersion for at least 4 years. However, the study was restricted to only blister formation, but did not indicate change in mechanical properties on sea water aging.

Mechanical properties for short period were investigated by Espinel et al. [28], which revealed that the tensile strength for USP-glass FRP reduced by 20% after 125 days immersion, and interestingly, while the tensile strength attained a constant value after 30 days of saturation, the transverse strength in flexure continued to decrease till 125 days, indicating that the fiber-polymer delamination is more observed if flexural properties are considered. The reason for difference in behavior in these two modes is that the interface delamination affects the bending loadbearing capacity, while in tension, the maximum load is taken by the fiber as such. Unless the fibers are damaged to very high extent as to break down below a critical length, the longitudinal strength will not decrease significantly. This is perhaps the reason for most researchers to measure flexural properties of composites rather than tensile for sea water aging study.

Kootsookos et al. [39] studied the sea water durability of GFRP and CFRP based on USP containing about 32–35% fiber by volume. The flexural modulus of GFRP was about 50% of that for CFRP. The water uptake trend was similar to many other observations, a peak water uptake of about 0.75% for GFRP and 0.5% for CFRP after 16–20 days. However, the water ingress curve had a negative slope after 20 days for both composites. The corresponding flexural modulus of GFRP showed an initial increase, and then finally after 145 days, there was no significant change. Whereas the modulus for CFRP initially decreased and ultimately the reduction is also negligible. It was opined that the weight reduction after peak water uptake is due to hydrolysis and loss of small molecules, but the modulus did not change much for the period of study (145 days). However, the flexural strength of GFRP was seen to reduce considerably, by about 33% but that for CFRP did not change significantly. The performance of the CFRP in sea water aging was seen to be much superior to the GFRP based on polyester resin.

Loos et al. [40] studied hydrothermal effect on USP-based GFRP using distilled water and saturated NaCl solution at 32°C and 50°C. The authors observed that at 32° C, the weight increase is continued with time till saturation value of 3.5–3.6% on 100 days and remained constant thereafter (till 150 days) when immersed in distilled water. For immersion at 50°C, the weight change was having a negative slope after about 50 days. Similar observations were also made by Fraga et al. [41], who studied hydrothermal aging and its effect on interlaminar shear strength and dynamic mechanical properties of GFRP made from isophthalic-acid-based USP with styrene as cross-linker. After 12 days exposure in water at 80°C, the weight change showed a negative slope indicating release of silane coupling agent (sizing of fiber) and also small organic molecules due to hydrolysis of the resin at the elevated temperature. The shear modulus was reduced by about 50% for the composite at 80°C at the end of the study period (1000 h). The glass transition did not change significantly, and the dynamic modulus increased by about 30% but flexural modulus reduced by about 25% at 80°C after about 400 h but stabilized thereafter till 1000 h of study.

Although an USP made of isophthalic acid and neopentyl glycol meets the requirement as a marine-grade resin, as its water resistance is much better than the other USPs, but on a comparison with epoxy resin and vinyl ester resin, the strength of the USP is quite lower, which necessitates a thicker section of a component, say hull of a boat, and consequently there is a possibility of more defects, enhanced water ingress, and faster damage.
