Crosslink Network And Properties Of Rubbers Reinforced By In Situ Zinc (Magnesium) Dimethacrylate

Rubbers can be reinforced by metal salts of unsaturated carboxylic acids in the presenceof peroxides，showing excellent mechanical properties. The ionic cross-links are formed bythe metal salts graft-polymerized onto the rubber chains, while the poly-(metal salt) chainsseparate from the rubber matrix and aggregate into nano-particles, forming salt cross-links.Thus metal salts of unsaturated carboxylic acids reinforced rubbers contain conventionalcovalent cross-links and ionic cross-links and exhibit special crosslink structures. A largenumber of studies on the preparation, structure and properties of metal salts of unsaturatedcarboxylic acids reinforced rubbers have been carried out. In order to explain the excellentmechanical properties, researchers also have put forward various structure models of metalsalts of unsaturated carboxylic acids reinforced rubbers. However, there are few of studiesfocus on the crosslink network evolution of this kind of rubber composite.In this dissertation, zinc dimethacrylate (ZDMA) and magnesium dimethacrylate(MDMA) were prepared in situ with zinc oxide (ZnO)/methacrylic acid (MAA) and magnesia(MgO)/MAA, respectively. Natural rubber (NR), nitrile rubber (NBR) and styrene-butadienerubber (SBR) were selected as rubber matrixes. The crosslink network evolution andproperties of NR/ZDMA system, NR/MDMA system, NBR/ZDMA system and SBR/ZDMAsystem were studied using cure-curves, X-ray diffraction (XRD), fourier transform infrared(FT-IR) spectroscopy, dynamic mechanical analysis (DMA), differential scanning calorimetry(DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM),rubber processing analyzer (RPA), extraction experiments and element analysis, crosslinkdensity determination, and mechanical properties test, etc. In addition, polypropylene(PP)/NBR/ZDMA ternary composites were successfully prepared through dynamicvulcanization, using dicumyl peroxide (DCP) as a curing agent. The effect of maleicanhydride grafted polypropylene (MA-g-PP) on mechanical properties of the PP/NBR/ZDMAcomposites was investigated.The results showed that the NR/ZDMA and NR/MDMA system had the similar crosslinknetwork structures and the evolution process. The apparent activation energy of ZDMA (orMDMA) was far lower than that of NR. Therefore, ZDMA (or MDMA) had the priority to capture free radicals and polymerized rapidly. After ZDMA reaching a considerableconversion, the NR matrix initiated the substantial covalent crosslinking reaction. Graftpolymerized ZDMA (g-PZDMA)(or graft polymerized MDMA (g-PMDMA)) was the mainlyform of ZDMA (or MDMA) in the vulcanizates, which resulted in an advanced ioniccrosslink network. A primary “ionic crosslink network” was formed at the initial stage ofvulcanization: the continuous covalent crosslink network was not formed, but the ioniccrosslinks, cooperating with some primary covalent crosslink points and the covalent chainsof PZDMA (or PMDMA) between two NR molecules, played an important role in supportingthe crosslink backbone of the composite in this period. A higher cure temperature resulted in alower ionic crosslink density, while a lower cure temperature resulted in a lower covalentcrosslink density. A suitable cure temperature, namely155℃, resulted in the optimumcovalent/ionic crosslink ratio and the best comprehensive properties of the NR/ZDMAvulcanizates. Above100°C, the decrease of mechanical properties was mainly attributed tothe weakening of the effective ionic crosslinks. The ionic bonds were easy to slip duringstretching. However, new ionic bonds were formed rapidly at current position, whichmaintained the instantaneous structure to fit the stress.In NBR/ZDMA system, the apparent activation energy of ZDMA was slightly lower thanthat of NBR. As a result, the polymerization of ZDMA and the crosslinking of NBR tookplace simultaneously at170℃. At this time, the covalent crosslink density grew rapidly in thefirst1minute, whereas the ionic crosslink density increased gradually during the whole curingprocess. Homo-polymerized ZDMA (h-PZDMA) proportion was much higher than g-PZDMAin NBR/ZDMA vulcanizates, which resulted in a low ionic crosslink density. The result ofcrosslink density measurement at150℃indicated that ZDMA promoted the evolutionprocess of the covalent crosslink network of NBR matrix.As for SBR/ZDMA system, the polymerization of ZDMA and crosslinking of SBR alsotook place simultaneously at170℃. At this time, both the covalent crosslink density and theionic crosslink density grew rapidly in the first few minutes. The crosslinking process of theSBR matrix was not affected by the ZDMA content. When the curing temperature wasreduced to120°C, the decomposed DCP was very limited in the first several minutes. Then,ZDMA was favored to capture those limited radicals to generate PZDMA radicals which could initiate the crosslinking of SBR matrix.The addition of ZDMA into PP/NBR blends prepared via peroxide dynamicvulcanization improved the compatibility between NBR dispersed phase and PP continuousphase. The resultant PP/NBR/ZDMA composites showed better mechanical properties.Furthermore, the addition of ZDMA reduced the size of crosslinked NBR particles.Incorporation of MA-g-PP further improved the compatibility between PP and NBR phase,which contributed to the further increased mechanical properties of the resultant composites.