The reinforcing effect of crosslinkable waterborne polyurethane/polysiloxane composite emulsion by aqueous sol-gel method

A highly branched polysiloxane (S2) was prepared by an aqueous sol-gel process and used as an underlying crosslinker to modify silanized waterborne polyurethane (WPUS), and a novel crosslinked waterborne polyurethane (WPU)/polysiloxane composite with low VOC features was obtained. The good dispersibility of polysiloxane (S2) was proved by both TEM and DLS analyses, implying that the polysiloxane had an excellent intermiscibility with the WPU. The larger silsesquioxane network in the composite was proved by XPS, which was generated from the condensation of the Si-OH groups of polysiloxane and WPUS. On the basis of tensile results, the content of polysiloxane had a significant impact on the mechanical properties. The reinforcing and toughening synergy effect was found in the composite with a low concentration of S2. According to micrographs of the fracture surface, the dominating toughening mechanism was dependent on microcracks, while the high concentration of polysiloxane turned into silica particles in the WPU matrix, resulting in force concentration and degradation of the mechanical properties. However, the Young's modulus increased with the increasing content of S2. Additionally, the incorporation of the crosslinked silsesquioxane structure also improved the hydrophobic properties and thermal stability of the composite.