The main function of the barrier layer is to reduce or delay the ingress of the environment into the structural laminate. It has been shown that the flexural strength degradation of laminates is proportional to the amount of water going through the laminates, regardless of time. Since water permeation is controlled by solubility and diffusion processes and diffusion rate increases exponentially with temperature, it is likely that the solubility of water in a resin matrix is the primary factor controlling laminate degradation up to about 60°C, above which temperature activated diffusion becomes rate controlling.
Regester showed that neither hydrochloric acid nor sulphuric acid fully penetrated a 2.5 mm thick GRP barrier layer after 6 months at 100°C and that sodium chloride, with the same chloride ion concentration as the hydrochloric acid, penetrated to a much lesser extent. It was concluded from this that sulphate anions penetrate primarily by wicking along the fibre-resin interface, whereas chloride anions diffuse through the resin matrix and, in the case of sodium chloride, sodium cations are easily polarised, increasing their effective diameter and decreasing their diffusion rate.
Since electrical neutrality must be maintained within the laminate, then the slowest moving ion will determine the penetration rate. Hence, seawater has less effect on laminate properties than distilled or tap water. It, therefore, follows that the thickness of the barrier layer and the resistance of its materials of construction delay environmental penetration, thereby protecting the structural laminate from possible degradation and failure.
In the chemical plant industry, barrier layers used for protecting GRP are either thermoplastic liners or GRP liners manufactured in a resin-rich fibre reinforced form using resin with resistance to the environment to be contained.
For mild operating conditions a clear gelcoat, backed up with a surface tissue reinforced layer will provide acceptable chemical resistance. However, for more corrosive environments a single or double glass or synthetic veil reinforced liner should be allowed to cure before the CSM reinforced part of the barrier layer is fabricated to provide a non-structural liner with a minimum thickness of 2.5 mm.
Solvents are especially aggressive towards GRP but, again, it has been shown that the correct choice of barrier layer can provide a long term solution for the storage of fuels in underground tanks providing the necessary corrosion resistance to both the internal environment and the external, often aggressive, soil conditions has been taken into account. In order to achieve the highest level of solvent resistance it is essential to use fully cured (involving a high temperature post cure), highly cross-linked resin systems. The level of degradation of GRP in contact with solvents results in varying levels of swelling, absorption, whitening, surface crazing and cracking depending upon the type of solvent or blend of solvents in contact with it. Often blends of solvents are more aggressive, because of synergistic solvent effects, than the individual solvent components alone. A particular example of this problem is a test fuel blend of octane, toluene and methanol which is far more aggressive to GRP than the component solvents alone.
However, highly cross-linked isophthalic acid based polyester resin has been shown to be very resistant to this solvent blend and hence, suitable for the manufacture of underground petroleum storage tanks. Again, the correct choice of surface tissue and liner construction enhances the long term performance of GRP petroleum storage tanks.
Published courtesy of Dr L S Norwood, Scott Bader Company Ltd
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