Investigation of thermal conductivity and mechanical properties of polyurethane urea nanocomposites for bend stiffener application

Okolo, Chinyere (2024) Investigation of thermal conductivity and mechanical properties of polyurethane urea nanocomposites for bend stiffener application. Doctoral thesis, Northumbria University.

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Abstract

Subsea bend stiffeners provide support for flexible risers and cable umbilicals when connected to topside rigid structures. These bend stiffeners, which can be in air, in water, or partly in both, are typically made of flexible polymeric material, which provides suitable stiffness but also a high degree of thermal insulation. While subsea thermal insulation is generally not a concern, there is an issue when it comes to majority of bend stiffeners being connected to the topside of a vessel, as a portion of the bend stiffener is exposed to high-temperature atmospheric conditions. This temperature increase, exacerbated by prevailing weather conditions, is further compounded by the presence of electric cables, which contribute to joule heating within the bend stiffener.

Thermal studies report that thermal insulation can be a problem in high temperature umbilicals as it creates a length of umbilical within the bend stiffener at significantly higher operating temperature than the other areas in seawater. This may result in overheating of the electrical cores and other polymeric components, which over time can reduce the design life of such components resulting in increased maintenance costs and in some cases downtime.

Therefore, the objective of this research was to develop the thermal performance whilst maintaining the mechanical performance for graphene reinforced polyurethane-based nanocomposites. Specifically, the objective was to increase the thermal performance of a commercial based polyurethane with the potential of increasing its performance envelope for use in subsea bend stiffeners.

In terms of the application, polyurethane, and its derivatives on their own possess excellent tensile properties. However, the weak link is their poor thermal conductive performance and in bend stiffener operation, they tend to overheat. In order to impart multi-functional behaviour to the polyurethane-based matrix, the resin was reinforced using two different grades of conductive graphene derivatives.

In the first case, after the fabrication of the pristine polyurethane derivative, the graphene derivative was dispersed in amine component of the polyurethane resin at loading levels, 0.5-1 wt.%, using bath sonication. Incorporating the graphene derivative resulted in lower tensile strength at the smallest concentration (0.5 wt.%) but the strength increased as the concentration increased past 0.5wt.% graphene nanoplatelets. At 0.5 wt.%, GNPs in the resin decreased the tensile strength by up to 20%. The weak interface was triggered by the poor bonding strength between the graphene derivative and the polyurethane-based matrix. However, the thermal conductivity in these nanocomposites exceeded that of the matrix by 18% at the highest concentration.

In the second case the addition of the second type of graphene derivative to the resin yielded a better balance of thermal and mechanical properties. Adding 0.3 wt.% of the nanofiller to the polyurethane-based resin yielded approximately a 10 % increase in the tensile strength and a 5% increase in the thermal conductivity. In these samples, the thermal conductivity increased with the graphene loading and the increase was influenced by the dispersion technique, the loading, and the structure of the graphene derivative. In addition, the results show better interfacial bonding in the second graphene polyurethane-based system.

In general, the findings of this research have revealed that even without chemical modification of GNPs, enhanced multi-functional performance can be achieved with the right dispersion and fabrication technique. Bath sonication as a technique appears to be too simplistic to manage the dispersion of GNPs on the nanoscale. However, the non-contact planetary technique was sufficient to contribute to the process of improving the thermal conductivity and mechanical performance of the polyurethane-based resin.

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