History

A company specialising in open mould glass-reinforced plastic (GRP) laminations for the automotive industries (e.g. lorry cabs) decided to diversify into small marine craft. With a short but demonstrably successful history they were subcontracted to supply hull mouldings for the military. These were moulded by hand lamination using chopped strand mat (CSM) and an unspecified grade of orthophthalic polyester resin. Pigmented marine grade isophthalic polyester was used for the external gel coat.

Within two years of service use, a small fraction of the hulls were observed to be severely blistered. Service records revealed a correlation between fresh water, hot climates and blistering.

Inspection and analysis

In the hull provided for inspection, blisters under the external gel coat ranged in size from a few mm to 25 mm in diameter. Blistering was restricted to the area of the hull below the water line. A cross section of a typical blister reveals a wall thickness of about 1 mm. This is the thickness of the gel coat.

The blister fluid was extracted with a hypodermic syringe and was analysed using various techniques. Gas liquid chromatography was used to identify organic solutes. In ascending order of concentration these were established as diethylene glycol, methyl ethyl ketone (MEK), acetic acid, styrene, benzaldehyde. The fluid had a pH of 4.5.

Atomic absorption spectroscopy was used to identify and quantify cations. Calcium ion concentration was 1500 ppm, aluminium was at 200 ppm, as was cobalt.

Failure diagnosis

Polyester resins are generally formulated with a glycol. Diethylene glycol or polypropylene glycol tend to be used in standard formulations, whilst others such as 2,2,4-trimethyl­1,3-pentanediol or 2,2-dimethyl-1,3-propanediol are preferred for marine grade resins. It is probable therefore that the main laminating resin is a standard orthophthalic polyester containing excess diethylene glycol.

Acetic acid, which is mainly responsible for the low pH of the blister fluid, is a by-product of the hydrolysis of polyvinyl acetate. The high concentration would suggest that emulsion bound CSM has been used as the reinforcement.

Benzaldehyde is a by-product of the oxidation of styrene. The standard reactive diluent for these resins is styrene monomer.

The higher than expected concentration of MEK and cobalt ions is significant. MEK is a soluble breakdown product of methyl ethyl ketone peroxide (MEKP), and MEKP/cobalt

(II) is the standard catalyst system for these polyester resins. It is probable that an excess of the catalyst has been used, either by mistake or to facilitate an increase in the cure rate.

The high concentration of calcium and aluminium ions is not unusual. E glass contains calcium and aluminium oxides, and these are leached out of the fibres by water and more rapidly by acids.

Slow permeation of water through the gel coat is inevitable. Whether this is sufficient to cause eventual blistering depends on many factors. The most important is the availability of water soluble species under the gel coat. Concentrated solutions will trigger an increase in the rate of water permeation by osmosis. In this case the immediate availability of excess cobalt salts and MEK probably initiates the process. The next phase is sustained by the hydrolysis of the polyvinyl acetate emulsion to acetic acid. The final stage is the acid leaching of water soluble metal salts from the glass fibres. These reactions, if sufficiently vigorous, will lead to a solute concentration, increased osmotic pressure, and eventually to blistering.

Lessons and consequences

1. Although the gel coat blistering phenomenon cannot be predicted with confidence, the service conditions, material variables and the fabrication procedures that promote it are reasonably well known within the marine construction industry. However in this case the fabricator had insufficient experience within the sector to avoid the risk of blistering. This should be born in mind when selecting a supplier. Suppliers with a short but successful history of fabricating products for a particular service environment should not be treated as equivalent to those whose products have (demonstrably) stood the test of time. Customers should insist that representative laminates be subjected to accelerated testing to assess blistering resistance. Typically this involves exposure of the gel coat to condensed water at 60 °C for several hundred hours. Resin suppliers will advise as to the level of resistance required for any given application.
2. Warm fresh water is the most detrimental service environment for GRP craft.
3. To defend against blistering in such an environment a powder bound reinforcement mat should have been employed rather than an emulsion bound reinforcement mat. Marine grade resins should have been used for the structural laminate as well as the gel coat. More consideration should have been given to catalyst levels.