Surface energy and microstructural analyses of novel highly hydrophobic magnesium phosphate cement coatings

Magnesium phosphate cement (MPC) has attracted attention as a green coating material owing to its excellent adhesion and dense structure. However, MPC's natural hydrophilicity and poor water stability make it susceptible to infiltration by water containing harmful ions, limiting its application as a coating material. This study investigates the effect of polydimethylsiloxane (PDMS) on the wettability of MPC coatings with hierarchical structures. The chemical functional groups and surface morphologies of PDMS-modified MPC coatings were analyzed. The surface free energy of MPC surface was evaluated to understand the physicochemical properties of MPC surface. Static contact angles of MPC coatings with varying PDMS contents were measured using four polar and non-polar liquids. The most important and widely used models were applied to calculate the surface free energy of MPC coatings, assessing the suitability of each model. Wetting envelopes were constructed for the various surfaces to predict the wettability of the MPC with commonly employed liquids. The results indicate that the addition of PDMS successfully transformed MPC from hydrophilic to highly hydrophobic. The improvement in water repellency is due to the cross-linking reaction between PDMS and MPC, which lowers the surface energy through chemical modification. Additionally, the formation of hierarchical structures increases surface roughness, further improving hydrophobicity. Envelope curves were successfully designed to predict the wettability of the coating across a large range of liquids.