Looking over Liquid Silicone Rubbers: (1) Network Topology vs Chemical Formulations

This study proposes a comprehensive study on liquid silicone rubber (LSR) formulations to unravel which components (among functional polydimethylsiloxane polymers and modified silica fillers) improve the mechanical properties of the final materials. In this first part, various industrial products have been deformulated using conventional chemical analyses. The silica content and their surface chemistry were assessed by TGA. Architecture and molar mass of polymers were deduced from ²⁹Si NMR and SEC in toluene, respectively. Relative concentrations of hydride and vinyl reactive groups and stoichiometric imbalance (<i>r</i> = <i>n</i>_SiH/<i>n</i>_SiVi) were quantified by proton NMR. Stoichiometric imbalance is slightly higher than 1.5 for cross-linker with hydride functions well redistributed along the chain, whereas for some formulations, <i>r</i>’s as high as 3.7 were implemented. These variations has strong implications on the cross-linking density of the final material, since the remaining hydride groups react together and decrease the molar mass between cross-links. From the comparison between formulations, it was shown that hardness adjustment is mainly performed by playing on two parameters: filler content and molar mass between cross-linking points for hardness ranging from 20 to 30 Shore A. Above this limit, it is necessary to modify the silica surface with reactive groups, such as vinyl functions. Surprisingly, two formulations were shown to use a dual cross-linking catalysis systems, peroxide and platinum, leading to efficient and full cure even at lower temperature (typically 140 °C). Network topologies were estimated from the predicted chemistry of the materials in a final discussion part

Looking over Liquid Silicone Rubbers: (2) Mechanical Properties vs Network Topology

In the previous paper of this series, eight formulations were analyzed under their uncross-linked forms to relate liquid silicone rubber (LSR) chemical compositions to material network topologies. Such topologies were confirmed by swelling measurements and hardness evaluation on vulcanized samples. In this article, characterization of cross-linked materials is further done using different mechanical measurements on final materials, including dynamic mechanical analysis, compression set, stress–strain behavior and tear resistance. It was shown that the compression set value is mainly related to the chains motion: increasing the filler–polymer interactions and/or decreasing the dangling/untethered chains content positively impact the compression resistance. Elongation at break depends on the molar mass between cross-linking points, showing an optimum value set at around 20 000 g mol^–1, i.e., the critical mass between entanglements. The distribution of elastic strands into the network has strong implications on the stress–strain curves profiles. By generating bimodal networks, the ultimate properties are enhanced. The materials cured by hydride addition on vinyl groups catalyzed by peroxide exhibit poorer compression set and tensile strength values, respectively, because of post-cross-linking reaction and broad polydispersity index of elastic network chains