Incorporation of a polar filler such as silica into a
nonpolar
rubber matrix is challenging and energy consuming due to their large
difference in polarity. Epoxidation of carbon–carbon double
bonds in unsaturated rubber, especially for rubber with low unsaturation
such as butyl rubber, is an effective method to introduce polar functional
groups to the rubber macromolecules for better filler dispersion.
Although different epoxidation reagents including hydrogen peroxide
(H2O2), peracid, and meta-chloroperoxybenzoic
acid (mCPBA) have been previously reported, these reagents have different
drawbacks. In this article, a metal-free epoxidation reagent, dimethyl
dioxirane (DMDO), generated from acetone and Oxone is explored for
efficient epoxidation of rubber with low unsaturation. The effects
of the addition manner of the reactant Oxone and buffer sodium bicarbonate
(NaHCO3) and reaction temperature on the epoxide formation
are studied. Compared to peracid, a faster and more efficient epoxidation
without the generation of a ring-opened product is achieved when DMDO
is used as the epoxidation reagent. Furthermore, it is found that
the epoxidation using DMDO is not sensitive to the water concentration
in the rubber solution up to 20 wt %. The addition of quaternary ammonium
salt as a phase transfer catalyst not only improves the conversion
but also further increases the water tolerance to 25 wt %. The reaction
conditions for preparation of epoxidized butyl rubber with different
percentages of epoxide group are optimized by Design of Experiments
(DoE). At the end, improved dispersion of silica in the matrix of
epoxidized butyl rubber is achieved, as revealed by the rubber process
analyzer (RPA) and atomic force microscopy (AFM).