Application of polydimethylsiloxane as a coating for the fouling mitigation of calcium carbonate

Fouling prevention is a key issue that is constantly a concern for many industries. From geothermal power generation to pharmaceutical production to marine transportation, fouling results in losses in process efficiencies, equipment breakdown and increased operational costs. There have been many different methods used to tackle the problem of fouling with varying levels of success.
This report centres on the use of a polydimethylsiloxane (PDMS) coating to mitigate calcium carbonate (CaCO3) formation. CaCO3 was the foulant chosen for this investigation as it is one of the most common in industrial processes that utilise freshwater or seawater. PDMS is an elastomer that is a combination of a silicone oil and a curing agent. PDMS was chosen as it is an elastomer that can have its mechanical properties altered through the curing ratio of the silicone oil to curing agent. Increase in curing ratio results in less crosslinks and therefore results in change in material stiffness. Therefore, samples of PDMS at different curing ratios were investigated to determine the effect of material stiffness on fouling. The range of ratio used initially was from 5:1 to 50:1, however, through material characterisation, this scope was narrowed to 10:1 to 30:1. The effect of curing ratios on material and surface properties were characterised.
To understand how the PDMS samples performed, bulk precipitation and surface scaling experiments were carried out. Bulk scale precipitation was observed using a traditional bulk jar test with high saturation ratio (SR) brine solution. Using a high SR, would significantly reduce the induction time in which crystals would form in the bulk and therefore drive towards crystal precipitation rather than surface crystallisation. SEM images were taken to analyse the surfaces. PDMS surfaces at curing ratio 10:1 in these tests had the lowest surface coverage despite having the larger crystal structures. Curing ratios of 20:1, 25:1 and 30:1 displayed similar results with low average crystal area and high surface coverage.
Surface scaling was investigated by using a visualisation cell and using a low SR brine solution. By using a low SR, the driving mechanism for crystal formation onto the surfaces was surface crystallisation. Images were taken at intervals and by using image analysis software, the surfaces were analysed. An increase in curing ratio resulted in an increase in average crystal count, decrease in average crystal area and an increase in average area coverage. PDMS of curing ratios 10:1, 15:1 and 20:1 all produced low results with average area coverage <5%. PDMS of curing ratio 30:1 performed poorly with an average area coverage of 20.6%. Understanding how crystals form onto the PDMS surfaces would help determine the feasibility of PDMS as an antifouling surface and how the surfaces can be optimised through curing ratio.