Polysilazanes for Coating Applications

Polysilazanes (PSzs) are an interesting class of polymers with a very reactive backbone of Si-N-Si, often containing reactive and non-reactive substituents on the Si and N positions. All this reactivity along with being liquid polymers makes them a very alluring subject for a broad number of applications including polymer-derived ceramics and coatings.

PSz coatings are well known for having great adhesion to a number of substrates and for being water repellent surfaces, making them attractive candidates for anti-adherent and anti-fouling coatings. However, along with the favourable properties comes inherent issues of non-ideal coating thicknesses due to a great dependence on perfect ambient curing conditions.

In this thesis, PSz has been explored in hybrid systems for a broad range of applications, with a particular focus on marine coatings. Thereby exploiting the great properties of PSz for improved industrial application by incorporation into silicon and carbon based hybrid systems as a minor component.

Firstly, an investigation of reactivity between alcohols and PSz by nuclear magnetic resonance (NMR) showed the formation of silyl ether bonds and the possibility of using PSz as a coupling reagent for reactive multi-functional hydroxyl silicones. A dynamic mechanical analysis (DMA) confirmed that the methodology could be used to produce non-metal catalysed silicone networks.

Secondly, in order to apply the previously developed methodology for marine coatings as a binder system, several important parameters were investigated. Initially, to reduce the curing time of PSz silicone systems a screening of relevant organic amines was conducted, which showed that amines with higher Brøndsted basicity and the ability to proton shuttle had a higher catalytic activity.

A formulated silicone coating based on PSz was prepared and compared to a commercial silicone coating, showing that the PSz system had improved adhesion to various substrates and that the final coating was chemically similar to the conventional silicone. These coatings were evaluated in outdoor exposure tests, confirming that no negative effects were introduced from using PSz as a silicone binder.

Thirdly, a PSz silicone system was investigated in preparation of silicone elastomers. The prepared elastomers could incorporate as much as 30 wt% hydrophobic silica filler as a reinforcing agent, which significantly improved the mechanical properties. By incorporating 10 % and 20 % of a linear silanol chain extender, the elongation at break could be maintained, while the Young’s modulus was significantly reduced, showing an easy way of tuning the mechanical properties.

Using off-stoichiometric amounts of PSz, pre-stretched elastomers could be prepared by reacting residual silazane groups in the network through a second high temperature curing at 180 °C in a pre-stretched state. Furthermore, the functionalisation of residual PSz in off-stoichiometric cured PSz silicone films was investigated.

The concept was proven by using both visible and fluorescent alcohol functional compounds for a simple surface functionalisation. As a means to create more amphiphilic surfaces, which could improve fouling release (FR) capabilities, the off-stoichiometric PSz silicones were functionalised with water and methoxypolyethylene glycol (mPEG).

However, a static outside exposure test showed no direct improvements in FR capabilities, though rub-off tests revealed that the adhesion of the foulants were weak for all tested compositions. Finally, an investigation into carbon-based hybrids networks revealed that certain hydroxyl functional compounds with a specific viscosity and reactivity could be used to prepare polyurethane foam mimics.

The density and compression modulus of these foams could be adjusted by changing the concentration of PSz, where higher amounts of PSz produced less dense and mechanically stronger foams.