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

• We report the ability of structurally differentiated MgAl-LDHs to cure XNBR.
• The properties of the filled XNBR are strongly influenced by the type of LDH.
• XNBR/MgAl-LDH composites show an additional high temperature relaxation peak.
• XNBR filled with LDH with a Mg/Al ratio ≈ 2:1 shows thermoreversible transparency.
• Highly anisotropic LDH contributes to great improvements in tensile strength.

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 (Tg) 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.

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