Preparation And Application Of Maleic Anhydride Grafted Natural Rubber

The grafting of maleic anhydride (MAH) to natural rubber (NR) was one of the earliesttry for the chemical modification of NR. Studies on maleic anhydride grafted natural rubber(NR-g-MAH) were reported in the literature early in1940s, mechanisms of the graftingreaction between NR and MAH and structures of the grafting product were proposed. Afterthat, few works about the synthesis and application NR-g-MAH were reported for unkownresons. Until2000, NR-g-MAH was used as compatibilizer by some researchers to the blendsof NR with polar rubber and plant fibers (cotton fiber, rice husk fiber, et al) reinforced NRcomposites. However, there was little report on systemic research of NR-g-MAH preparationprocess and the application of NR-g-MAH was urgent to broaden.New problems stem up with the development of rubber industry, for example therequirements for high performance tire, which make short fiber an important ingredient inrubber industry. Short fiber reinforced rubber composites can be used to produce various partsof tire. The puncture resistance, tear resistance, anti skip chunking ability of the tire can beimproved, while the weight of the tire, rolling resistance, fuel consumption, tire heataccumulation and noise decreased. But the intermolecular forces of nylon short fiber,polyester fiber, cellulous and all kinds of plant fibers are quite high which makes the shortfibers difficult to disperse in the rubber matrix and the interface between those polar fibersand NR is weak. It is assumed that the maleic anhydride groups of NR-g-MAH canchemically attached to the fiber on one hand, which reduced interfacial tension and retardedfiber coalescence during mixing. On the other hand, the backbone of NR-g-MAH will formstrong entanglement with the NR matrix and realize covulcanization, offer suitable softinterfacebetween fiber and rubber matrix. So, NR-g-MAH may have potential application inthe field of high perforemences tire.NR-g-MAH can be prepared in solution or in solid state. Compared with solutiongrafting, mechanical method is a more economical, environmental protection and highefficiency way to prepare NR-g-MAH, which has the advantages of mild reaction condition,without solvent, convenient operation and simple post treatment. In this dissertation,NR-g-MAH was synthesized by mixing MAH and NR in solid state in a torque rheometerusing dicumyl peroxide (DCP) as initiator. The functionalization of NR with MAH wasconfirmed by Fourier transform infrared spectroscopy (FTIR) and solid state nuclear magneticresonace (13C-NMR.). Quantification of grafting yield and grafting efficiency was performedusing titration method. Influences of grafting reaction conditions such as dosages of MAH and DCP, rotor speed, reaction temperature and reaction time on both grafting yield andgrafting efficiency were studied. The optimum grafting conditions obtained in this researchwere5wt%of MAH and0.75wt%DCP were mixed with NR in an internal chamber at130℃for3minutes, with the rotor speed of50rpm. The grafting yield, grafting efficiency and gelcontent are1.4%,30.6%and63.1%respectively. Reaction kinetics of MAH grafting onto NRwas studied by temperature-programming differential scanning clorimity (DSC) and theactivity energy of the grafting reaction calculated by Flynn-Wall-Ozawa(FWO) method was128KJ mol.Firstly, the self-prepared NR-g-MAH was used as a compatibilizer in short nylon fiberreinforced natural rubber composites (NR/SF). The maleic anhydride groups of NR-g-MAHcould react with the amine groups of PA6fibers to form graft copolymers in the interfacialregions, which was confirmed by FTIR. Composites with different short nylon fiber loadings(0,5,10,15and20phr) were compounded on a two-roll mill and the effects of theNR-g-MAH on the tensile and thermal properties, fiber–rubber interaction, as well as themorphology of the NR/SF composites were investigated. At equal fiber loading, theNR-g-MAH compatibilized NR/SF composites showed improved tensile properties,especially tensile modulus at100%strain which was about1.5times of the correspondingun-compatibilized ones. The equilibrium swelling tests proved that the incorporation ofNR-g-MAH increased the interaction between the nylon fibers and the NR matrix. Thecrosslink density measured with NMR techniques showed that the NR-g-MAH compatiblizedcomposites had lower total crosslink density than the corresponding un-compabilized ones.Morphology analysis also confirmed better dispersion and interfacial bonding between thePA6fibers and NR matrix in NR-g-MAH compatibilized NR/SF composites. All these resultssignified that the NR-g-MAH could act as a good compatibilizer of NR/SF composites, just astheoretically proposed.Similar results were obtained when the self-prepared NR-g-MAH was applied to shortPA6fiber and Carbon black filled natural rubber composites (NR/CB/SF). With the additionof NR-g-MAH, the optimum cure time t90of the NR-g-MAH modified NR/CB/SF wasdelayed and the scorch stability was improved. The resistance to toluene swelling increasedwhile water absorption decreased. The mechanical properties such as tensile strength, tearstrength, stress at set extension and hardness were improved. For example, when the fiberloading is5phr, the tensile strength and tear strength of the NR-g-MAH modified NR/CB/SFcomposites were increased35.8%and53.5%respectively. Dynamic mechanical analysisshowed that the NR-g-MAH modified NR/CB/SF composites had higher storage modulus and lower tanδ.At last, the self-prepared NR-g-MAH was applied in NR/CaCO3composites. Theanti-swelling property in toluene and mechanical properties of NR/CaCO3composites werealso improved by NR-g-MAH. Stronger interconnection between CaCO3and NR matrixformed by NR-g-MAH constrained the mobility of the NR chains greatly which made thestress relaxation speed slower and stress softening effect more obvious.