Synthesis and characterization of admicelled natural rubber with poly(methyl methacrylate)-co-poly((3-trimethoxysilyl) propyl methacrylate) and evaluation of the properties of its vulcanizates
Admicelled natural rubber (adNR) vulcanizates were prepared from NR modified
by coating with poly(methyl methacrylate)-co-poly((3-trimethoxysilyl) propyl methacrylate),
PMMA-co-PMPS shell. The core-shell structure of adNR was achieved after admicellar
polymerization, using PMMA at 10 parts per hundred rubber (phr) and PMPS at 0-50 phr
miscible in the mixture. During admicellar polymerization, hydrolysis and condensation
of PMPS simultaneously with the free radical copolymerization of MPS and MMA give
rise to the crosslink structure in the PMMA-co-PMPS shell, and these formations were
assessed by FT-IR and H-NMR. The thermal stability of adNR was enhanced by the Si in
PMMA-co-PMPS shells acting as an efficient diffusion barrier that hindered random scission
of the polymer chains. The adNR vulcanizates were prepared by using conventional
sulfur vulcanizing system. The mechanical properties, dynamic mechanical properties and
swelling resistance improved with vulcanization and these were further enhanced by the
PMMA-co-PMPS in the coating shell. The PMMA coating (adNR with PMMA 10 phr, P10),
also with a low amount of PMPS (adNR with PMMA 10 phr and PMPS 12 phr, P10-S12),
did not significantly affect the ozone resistance. However, ozone resistance then increased
with further PMPS content (adNR with PMMA 10 phr and PMPS 12, 37 and 50 phr,
respectively; P10-S25, P10-S37 and P10-S50). This high ozone resistance of adNR arises
from the poor access of ozone to the double bonds in NR, protected by the encapsulating
coating.