Effect of alkyl-disubstituted ureido silanes with different alkyl chain structures on tracking resistance property of addition-cure liquid silicone rubber

Ureido silane is considered to be a novel tracking resistance additive for effectively improving the tracking resistance property of addition-cure liquid silicone rubber (ALSR). In this work, a series of alkyl-disubstituted ureido silanes (ADUSs) with different alkyl chain structures were synthesized by the transetherification of allyl alcohol (AA) and alkyl-disubstituted ureido siloxanes from the nucleophilic addition between alkyl-disubstituted amides and 3-isocyanatopropyltriethoxysilane (ICPES). The effect of ADUSs with different alkyl chain structures on the tracking resistance property of ALSR was investigated by inclined plane test (IPT), thermogravimetry analysis (TGA), thermogravimetry-Fourier transform infrared spectrometry (TG-FTIR) and energy dispersive spectroscopy (EDS). The results showed that ADUSs could effectively improve the tracking resistance property of ALSR, especially for (γ-diisopropylureidopropyl) allyloxyethoxysilane (DIPUPAS), (γ-dibutylureidopropyl) allyloxyethoxysilane (DBUPAS) and (γ-diisobutylureidopropyl) allyloxyethoxysilane (DIBUPAS). ALSR incorporated with DIPUPAS, DBUPAS and DIBUPAS passed the inclined plane test (IPT) at the alternating voltage of 4.5 kV and the average erosion mass was only 3.8%, 3.4% and 4.4% of the virgin ALSR, respectively. TGA and TG-FTIR results revealed that DIPUPAS, DBUPAS and DIBUPAS could also effectively improve the thermal stability of ALSR. Accompanying with the weakened oxidation reactions of methyl groups in molecular chains, the formed cyclic oligomers decreased. Meanwhile, the release of methane and cross-linking reaction of the silicone rubber chains were enhanced, leading to the increase of residue under high temperature and the formation of a ceramic barrier layer to protect the silicone rubber matrix. EDS results showed that the carbon content in ceramic barrier layer decreased, which was beneficial to prohibit the development of tracking.