Effects of unmodified layered double hydroxides MgAl-LDHs with various structures on the properties of filled carboxylated acrylonitrile-butadiene rubber XNBR

The application of magnesium aluminum layered double hydroxide (MgAl-LDH) as a filler and crosslinking agent in carboxylated acrylonitrile-butadiene rubber (XNBR) enables production of mechanically strong and transparent elastomer composites without requiring the addition of standard curatives, such as zinc oxide, or a sulfur cure system, which is an important ecological and economical advantage. In the present work, the effects of LDHs with various structures, specific surface areas and Mg/Al ratios on the curing behavior, crosslink density, mechano-dynamical properties, transparency, thermo-optical properties, barrier properties and morphology of XNBR composites were investigated. The compounds were prepared by the melt-mixing method, without using any conventional curatives and were crosslinked with MgAl-LDH. Layered double hydroxides were added to the rubber at 2.5, 5, 10,20 and 30 parts per hundred rubber (phr). The incorporation of LDH with a high aspect ratio of nanoplatelets had the most significant impact on the glass transition temperature (T-g) of the XNBR matrix and led to the greatest improvements in mechanical properties, cure degree and barrier properties of the composites, even at low concentrations of 5-10 phr of LDH. In addition, it was found that the optical properties of XNBR/LDH composites are reversible, depending on temperature and the type of MgAl-LDH. The addition of high levels of LDH with a Mg/Al ratio 2:1 to the XNBR matrix produced composites that exhibited thermoreversible transparency. In contrast, the application of aluminum rich LDH resulted in opaque materials, whereas XNBR composites filled with Mg-rich LDH exhibited transparency but did not show thermochromic behavior. (C) 2014 Elsevier Ltd. All rights reserved.