Increasing equilibrium solubility of the fluid in the polymer (see Section 5.1),
• Decreasing molar volume of the fluid,
• Mechanical stress in the polymer.
The equilibrium solubility of fluids in polymers increases with the application of tensile
stress and decreases under the influence of compressive stress in the polymer. The sensitivity
to tensile or dilational stress is responsible for the local plasticisation ahead of intrinsic
defects, crazes, and cracks, and therefore an essential stage in the process of environmental
stress cracking. The loss of plasticiser due to compression is partly responsible for the
decline in performance of gaskets and seals. The rate of loss can be surprisingly high even
under modest compression. In one recorded example involving DOP plasticised PVC
catheters [23], the pressure of shrink wrap packaging film caused such a high rate of
exudation that drops of DOP were visible to the naked eye and the product was unsaleable.
In another example, shrink wrapped acrylonitrile-butadiene-styrene (ABS) telephones were
rendered unsaleable by surface marring and loss of gloss, where the plasticised PVC cordage
made pressure contact with the moulded casings [24]. On this evidence it would be prudent
to assume, until proved otherwise by testing, that fluid absorption and desorption is
‘spongelike’, i.e., a significant proportion of the absorbed fluid is not intimately mixed
with the polymer but contained within interconnecting pores.
The diffusion of water through gel coated glass fibre reinforced laminates is the first step
that leads to surface blistering. This is the most common cause of environmental
degradation in glas reinforced plastic (GRP) boat hulls and other marine products. Water
diffuses through the gel coat and dissolves oxides in the glass, emulsions used to size and
bind the glass, residual catalysts, etc. The concentrated solution behind the semipermeable
gel coat promotes increased diffusion of water via osmosis and the increase in the hydraulic
pressure of the solution eventually results in fluid filled surface blisters. These demand
removal and repair not only for aesthetic reasons but also because they represent a breach
in the resin rich corrosion barrier. Water will only slowly attack glass fibres and the bond
between fibres and resin. However the water-based solution is generally acidic and the
rate of internal attack following blistering can be very rapid (see Section 5.4).
The time in service to develop blisters can be as short as a year and as long as 30 years.
It is known to depend significantly upon the quality of workmanship, and in particular
on maintaining a well cured gel coat of sufficient thickness. Laminating with damp glass
fibre is also known to reduce the time to blistering. Material qualities are important.
Water soluble emulsions (e.g. polyvinyl acetate) used to bind glass mat will accelerate
the problem, as will standard resins (whether these are ortho- or isophthalic polyesters)
that have not been formulated for marine applications.