This paper investigates the water absorption of polyamide 6. The high amount of absorbed water in the polymer and the large resulting decrease in the glass transition temperature (Tg) leads to a non Fickian water diffusion when samples are immersed, which is a significant difficulty when trying to model the water profile in thick specimens. The aim of this study is to be able to model this particular behaviour based on physical considerations. First, it is shown that the non Fickian water diffusion is caused by an increase in the diffusivity during water absorption. Two cases are then identified; one below Tg where the diffusivity is described using an Arrhenius law and one above Tg based on the free volume theory. Then, these two laws are implemented in a specific model that is able to describe the non Fickian water diffusion over a wide range of temperatures.

Ocean pollution by microplastics, i.e. small pieces of plastic of less than 5 mm, is one of the major concerns for the future of our planet. Secondary microplastics formation is due to fragmentation of macroplastic waste. This fragmentation can be attributed to environmental loadings such as waves, winds and tides, coupled with a change in mechanical properties of polymers induced by UV and seawater ageing. This study aims to characterize and understand changes in the mechanical behaviour of Polyethylene Terephthalate (PET) induced by hydrolysis, especially for high degradation levels. Thin films (200 microns) of PET were aged in water at temperatures from 110 °C to 80 °C for up to 150 days. Embrittlement occurs with chain scission during hydrolysis when molar mass of the polymer falls below 17 kg/mol. When the polymer is brittle, i.e. for high levels of degradation, the stress at break decreases linearly with the molar mass, and can be described by a simple mathematical expression.

Thermoplastic matrix carbon fibre composites offer considerable potential for underwater applications. Various material options exist but there are questions concerning the tension/compressive behaviour and water sensitivity of the less expensive polymers (e.g. polyamides) for these applications. The aim of the current work is to model water diffusion and its effect on the mechanical properties of thick carbon fibre reinforced polyamide-6 composite cylinders immersed in sea water for deep sea applications. To provide the data for such a model, thin specimens (2 mm thick) have been aged under different humidity conditions and tested in tension and compression. As water enters the composite, a significant reduction in the laminate properties is observed. An empirical relationship that links matrix-dominated properties to water content is presented and can be used for modelling purposes.More than 70% of the oceans remain unexplored. With an average depth of over 3800 meters, it is necessary to design exploration devices that are able to withstand high hydrostatic pressures. The lighter these pressure vessels are the more equipment and measuring instruments they can carry, so composites have been used underwater for many years [1]. The use of composites for pressure hulls of underwater vehicles and submarines has been an ongoing research topic for many years, since the early work in the UK by Smith and colleagues in the 1970"s [2]. The use of glass and carbon reinforced thermoset composite materials for deep-sea applications needs a thorough understanding of the behaviour of these materials under hydrostatic pressure.However, the ability to predict the implosion pressures of such materials and under such loadings (bi-axial compression) is not an easy task, as was demonstrated by the results of the World Wide Failure Exercise [3].Applications for thick thermoplastic cylinders for underwater applications are rarer, even though there is an increasing number of thermoplastic matrix polymers available on the market such as polypropylene (PP), polyamide (PA), polyphenylene sulphide (PPS), polyetheretherketone (PEEK), etc. They offer possibilities for forming by local heating, attractive mechanical properties, good environmental resistance and the potential for end of life recycling. More ductile and reparable by melting, they offer a real potential for greatly improved devices. Studies have been conducted in the USA on carbon/PEEK cylinders [4], [5] and by the author [6] and were found to be very promising as they imploded at comparable pressures to those of their carbon/epoxy counterparts. However, it is not clear to what extent the compressive behaviour of thermoplastic composites in the presence of water will limit the operating depth of an underwater pressure vessel, especially for carbon/polyamide 6 (C/PA6) composites. The latter are attractive as they are much less expensive than composites based on high performance matrix polymers such as PEEK.The mechanical behaviour of composite materials has been extensively expl...