Coating with inherent sensing functionality based on dielectric elastomer

Marine biofouling causes serious economic and ecological problems for the marine industry. Due to great antifouling properties, silicone elastomers are materials which have dominant role in antifouling coatings for marine applications. As dielectric materials, silicone elastomers have a role in dielectric elastomer transducers (actuators, generators and sensors).

New antifouling technologies combining these two roles of silicone elastomers have already been demonstrated. The main objectives of these technologies are the prevention and detachment of biofouling. However, the biofouling detection is still only visual, and therefore, it is of great importance to develop new fouling detecting technologies.

The goal of this project is development of a detection technology based on a silicone elastomer, which will be deployed directly on the marine surface susceptible to biofouling. In order to fulfill this ambitious goal, different sensor models for detecting biofouling are proposed and investigated. To ensure constant performance of the silicone elastomer transducers under continuous deployment, the softening effects of silicone elastomers, which can alter their properties and thus the performance of the soft transducer, are investigated.

Initially, soft and hard commercial silicone elastomers and two blends of commercial silicone elastomers are extensively investigated in order to obtain details regarding their chemical structure. Afterwards, the mechanical properties of these silicone elastomers are examined, namely their softening behaviour due to the Mullins effect.

The Mullins effect in different silicone elastomers is quantitatively compared using a strain-energy function. Ultimate stresses, ultimate strains, and Young’s moduli are obtained from uniaxial tensile tests. The point of softening is shown to greatly depend on both elastomer type and its strain history.

Furthermore, a significant permanent set is observed in the softest commercial formulations. Subsequently, three models for detecting biofouling are proposed and investigated. Detecting biofouling in the first and second sensor model relies on the mechanical deformation of the dielectric elastomer (DE) under applied hydrostatic and dynamic water pressure, respectively.

Biofouling induces stiffening effect on the surface of the DE, and therefore different DE deformations under applied pressure. Simulations of the first two proposed models are performed using the finite-element-method (FEM) software ANSYS. Soft and hard commercial silicone elastomers are investigated to be used as a dielectric material for the first sensor model.

A study of geometrical parameters is performed. The FEM analysis predicts the deformation of the sensors under applied pressure. Data show low sensitivity of the sensors to applied pressure, indicating that the proposed sensor model cannot be used for detecting biofouling. Afterwards, a hard commercial silicone elastomer is investigated as a dielectric material for the proposed geometry of the second sensor model.

Two positions on the proposed DE geometry are examined as potential positions for the stretchable sensor. Deformation of the areas of those two positions is analysed. Data show small deformation of the areas under applied dynamic pressure, indicating that low change in the sensor’s capacitance may be expected under applied pressure, and thus their low sensitivity.

Finally, a third sensor model for detecting biofouling is proposed. In the third sensor model, a sensing capability is added to the actively deformed DE surface. A device consists of a copper plate, a Kapton sheet, and a thin silicone film immersed in conductive 3.5 wt% NaCl solution, which acts as one electrode, with the copper plate being the second electrode.

The electrical measurements are performed to detect surface deformations. Biofouling attachment to the surface of the silicone elastomer causes a change in the surface stiffness of the silicone elastomer, and as a result the threshold voltage necessary to develop the surface deformations increases. The devices are exposed to UV light in order to simulate the biofouling induced stiffening effect.

Influence of UV light on gel fraction and the mechanical and dielectric properties of silicone elastomer is also investigated. Furthermore, an investigation of chemical structure and the mechanical and dielectric properties of the DE before and after exposure to the artificial sea water for 250 hours and at different temperatures is performed.

Since the DE is an incompressible material, and since it is bonded to a rigid substrate, voltages below the creasing threshold created no deformation in the film, and therefore no change in the capacitance. Above the voltage threshold, creasing instabilities appear on the surface of the silicone, thus increasing the capacitance of the device.

The exposure of the DE to artificial sea water is shown not to have influence on the chemical structure and the mechanical and dielectric properties of the DE. On the other hand, exposure of the DE to UV light significantly changes the gel fraction and the mechanical and dielectric properties. Furthermore, surface deformations are not observed after 48 hours of UV exposure.

However, the new onset voltage is not detected herein, since electrical breakdown occurs before the formation of creasing instabilities.