Reinforcement Of Rubber With Multi-Walled Carbon Nanotubes

Because of the unique structure of carbon nanotubes (CNTs), they have excellentelectrical, thermal and mechanical properties. Various studies have focused on thereinforcing effect of CNTs on rubbers, however, the mechanical properties of therubber/CNTs composites were not as good as expected owing to the poor dispersionof CNTs in the rubber and the weak interactions between the CNTs and the rubbermatrixes. Therefore, how to adequately disperse CNTs in rubbers and enhance theinterfacial bonding between CNTs and matrixes are key points for fully revealing thefascinating properties of CNTs in rubber composites.Initially, the effect of multi-walled carbon nanotubes (MWNTs) on the properties ofhydrogenated acrylonitrile-butadiene rubber (HNBR) was investigated. The tensilestrength, modulus and the elongation at break of the composites were significantlyimproved with respect to the minimal loading of MWNTs. Specifically, the tensilestrength of the HNBR/MWNTs (100/4) composite was double that of the HNBR. Theactivation energy of the curing reaction for the HNBR composites reduced after theaddition of MWNTs. The storage modulus and loss modulus of the compositesincreased with increasing MWNT content. The glass transition temperature (Tg) of thecomposites shifted towards higher temperatures with the presence of MWNTs. Theelectrical conductivity and dielectric constant of the HNBR increased after theaddition of MWNTs. However, some MWNTs aggregates appeared in the fracturesurface of HNBR/MWNTs (100/4) composites, which restricted the reinforcing effectof MWNTs on HNBR.The effect of various metal compounds on the mechanical and curing properties ofHXNBR were studied. HXNBR/metal compound composites were observed to haveshorter optimum curing time and higher curing rate indexes compared with unfilledHXNBR. Metal compounds were shown to improve the mechanical properties of theHXNBR owing to the ionic bonding. Magnesium oxide (MgO) displayed the optimalreinforcing effect among the metal compounds evaluated, followed by zinc oxide (ZnO), sodium hydroxide (NaOH) and sodium carbonate (Na2CO3). After the additionof MWNTsCOOHs to the HXNBR/metal compound system, the mechanicalproperties of the composites were further improved.Therefore, the properties of HXNBR/MgO/MWNTsCOOH compounds wereinvestigated. The tensile strength and modulus of the HXNBR were significantlyincreased after the addition of both MgO and MWNTsCOOH. Similarly the tensilestrength and modulus were found to demonstrate improved benefits in comparison tothe combination of the two composites separately filled with MgO andMWNTsCOOH. The addition of MgO or MWNTsCOOH had little effect on thestorage modulus of HXNBR, however, the storage modulus significantly increasedafter the addition of both fillers. The peak of the loss modulus of theHXNBR/MgO/MWNTsCOOH composite shifted towards much higher temperaturescompared with HXNBR filled with MgO or MWNTsCOOH independently of theformer. Therefore, MgO could act as coupling agent to link the HXNBR andMWNTsCOOH which enhanced the interfacial bonding so that it influenced theproperties of the composites.MWNTs and nano-MgO mixtures were also used to reinforce the HXNBR tofurther improve the mechanical properties of the composites. Magnesium hydroxide(Mg(OH)2) was prepared by a homogeneous precipitation method in the presence ofMWNTs. A MWNTs-MgO mixture was yielded after the calcination ofMWNTs/Mg(OH)2in which the MgO obtained was cubic in nature. The crystallinityof the MgO was improved with increasing calcining temperature. The MgO obtainedin the presence of MWNTs had smaller crystal size, less refined crystallinity and asmaller crystal lattice parameter compared with MgO obtained without MWNTs.Microscopic images revealed that some MWNTs were disentangled during theprocessing and that the interpenetrating structure of MgO and MWNTs was formed inthe mixture. MWNTs-MgO could significantly improve the mechanical properties ofHXNBR. The tensile strength of the HXNBR/MWNTs-MgO composite was45MPawhich was more than three times higher than that of unfilled HXNBR. The Tgof theHXNBR/MWNTs-MgO composite was approximately10oC higher than that ofHXNBR, indicating there was a strong restricted effect on the movement of HXNBRchains by the MWNTs-MgO mixture.MWNTsCOOH functionalized by ZnO (MWNTs-ZnO) were prepared. Theresulting ZnO isolated was determined to be hexahedron in nature. ZnO obtained in the presence of MWNTsCOOH had a smaller crystal size, less refined crystallinityand similar crystal lattice parameters compared with ZnO obtained without MWNTs.Microscopic images revealed that the ZnO was attached on the surface ofMWNTsCOOH. HXNBR filled with MWNTs-ZnO demonstrated improved modulus,stress relaxation behaviour and a stress softening effect compared with HXNBR filledwith only MWNTsCOOH or ZnO.Similarly, MWNTs were functionalized with silane to improve the interfacialbonding between MWNTs and HXNBR. MWNTs were first coated with silicon oxideby a sol-gel process, and then reacted with vinyl trimethoxy silane (A171). Thefunctionalization of the MWNTs by silicon oxide and silane was characterized byFourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA),transmission electron microscopy (TEM) and energy dispersive X-ray analysis.Silanized MWNTs had an enhanced reinforcing effect on the HXNBR in comparisonto that of pristine MWNTs. The tensile strength of the composite increased roughly50%after the addition of2phr silanized MWNTs. The effect of the MWNTsfunctionalized by three different silanes (A171,2,3-epoxypropoxypropyltrimethoxysilcane,γ-Methacryloxypropyltrimethoxy silane) on the mechanical properties of thecomposites was also investigated. MWNTs functionalized by A171displayed themost significant reinforcing effect on the mechanical properties of the HXNBRcomposite compared with MWNTs functionalized by the two alternative silanes. Theproperty differences were attributed to the differing interactions between the variableorganic group chains of the silanes and the HXNBR. HXNBR/silanized MWNTscomposites had increased stress softening effects, higher storage modulus and lossmodulus compared with HXNBR/MWNTs composite. The filler-rubber interactionwas also studied by Kraus plots. These plots indicated that the interaction betweensilanized MWNTs and HXNBR was stronger than that between pristine MWNTs andHXNBR.The interaction between styrene-butadiene-styrene tri-block copolymers (SBS) andMWNTs during melt mixing and the properties of SBS/MWNTs composites wereinvestigated. SBS/MWNTs composites were prepared by solvent and melt mixing,respectively. The interactions between SBS and MWNTs occurring during meltmixing were characterized by the solubility of the MWNTs in tetrahydrofuran,dynamic mechanical thermal analysis, X-ray photoelectron microscope, ultravioletspectra and TEM. Both the dispersion of MWNTs in SBS and the mechanical properties of the composites prepared by melt mixng were significantly improvedcompared with that by solution mixing. The presence of MWNTs leaded to anincrease of the Tgof polystyren and polybutadiene domains. The thermal degradationbehavior of SBS and SBS/MWNTs composites prepared by solution and melt mixingwere investigated using non-isothermal TGA. The kinetic parameters for degradation,such as activation energy, pre-exponential factor A, and reaction order were evaluated.The activation energy increased after the addition of MWNTs and the SBS/MWNTscomposite prepared by melt mixing displayed the highest value independent of theapplied methods compared with SBS and the composites prepared by solution mixing.The gases evolved during thermal degradation under a nitrogen atmosphere werestudied by FTIR coupled with TGA. The thermal degradation rate of theSBS/MWNTs composite prepared by melt mixing was lower than that of the SBS andSBS/MWNTs by solution mixing.