Influence Of Acetone Solutes From Natural Rubber On The Structure And Performance Of NR Composites

Natural rubber is an important strategic material for its unique high elasticity,which consists of 95%polyisoprene and 5%non-isoprene compounds.Normally,the modification of NR and its composites are concentrated upon the grafting of rubber molecular chains or surface treatment of inorganic filler,never concerned with the effect of non-isoprene compounds.For the recent two decades,the climax of the study on high performance tire could be seen all over the world that promotes the research progress of natural rubber.According to the needs of the development of high performance tire,acetone solutes(AS)from NR was taken and its structure and constitution were also investigated.Simultaneously,the influences of AS on the interface structure and properties of unfilled NR,NR/CB and NR/silica composites were studied as listed below:(1)The composition and structure of AS from four countries(Thailand,China,Malaysia,Myanmar)were illustrated systematically through the high resolution pyrolysis gas chromatography-mass spectrometry(HRPyGC-MS),thermogravimetric analysis(TGA)and Fourier transform infrared spectrum(FTIR).The results showed that all AS presented two degradation stages,which means two decomposition courses existed in different temperature regions.The compounds in the first degradation stage are higher fatty acid and aliphatic ester,the isoprene and isoprene dimer are contained in the second stage reflecting the existence of low molecular weight polyisoprene.Besides,the compositions of different AS are similar,but the relative amounts of specific components are different obviously.The relative amounts of linoleic acid and stearic acid in different AS were analyzed as AS-STR>AS-WF>AS-SMR>AS-MRSS through establishing the internal calibration curves.Therefore,a new effective method of representing the thermal decomposition behaviors of AS from NR was found.(2)The effects of AS on the curing,mechanical and dynamical properties of unfilled NR were investigated.The results showed that,with the increase of AS,the curing time of unvulcanized compound was decreased firstly until it achieved a threshhold value,the mechanical properties were weakened and the hysteresis loss and rolling resistance of NR vulcanizates increased.These are caused by the higher fatty acid and low molecular weight NR in AS confirmed by HRPyGC-MS method.The strain-induced crystallization behaviors of unfilled NR vulcanizates were promoted significantly for the raises of crystallinity index and lateral direction crystallite size when moderate AS was added.(3)The influences of AS on the structure and properties of NR/CB composites were studied.The results demonstrated that,comparing with the composites without AS,the material performed with the lowest rolling resistance,improved compression heat generating and wear resistance when AS was 2 phr,as well as the optimal mechanical properties.Meanwhile,the tan?value at 60 ~0C was increased via increasing AS.It was found that the dispersion of CB in NR matrix was improved by AS leading to an increase of contact areas among NR-silica which is benefit for the improvement of rubber-filler interface interaction.Oppositely,the crystallite size on CB surface was increased via adding AS resulting in a decrease of the number of high energy active points that leads to the reduction of adsorption capacity of CB to rubber molecules overall.The threshhold value of AS was 2 phr before which the former one is in charge of the properties of NR/CB composites,and the latter one could be more critical to the performance of composites when AS was added more than 2 phr.Besides,AS could make a great impact on the strain-induced crystallization behaviors of NR/CB vulcanizates.(4)Through the investigation of the structure and performance of NR/silica composites modified by AS,it obtained an optimal performance when AS was 4 phr,where the tensile strength was increased by 75%and rolling resistance was decreased by 38.1%comparing with unmodified system.Simultaneously,the lowest hysteresis loss and improved wear resistance were achieved,but a delay effect on curing behaviors.It was explored that the reactions between oxygen-containing groups as higher fatty acid or aliphatic ester and silicon hydroxyl on silica surface were hydrogen bond and covalent bond which made a promotion of silica dispersion in NR matrix assisted by low molecular weight polyisoprene.This effect made an increase of constrained regions of rubber phase and enhanced the rubber-filler interface interaction.In addition,the promotion of strain-induced crystallization behaviors of NR/silica vulcanizates under high extension ratio by AS should be also responsible for the enhancement of mechanical properties of composites.(5)The impacts of low molecular weight polyisoprene and higher fatty acid as linoleic acid(LA)and stearic acid(SA)on the structure and properties of NR/silica composites were illustrated.It was found that higher fatty acid was in charge of the improvement of interface interaction and dynamic properties of NR/silica composites.The rolling resistance and hysteresis loss were decreased with the increase of fatty acid and an optimal mechanical performance was obtained as well.LA exhibited a better performance than SA on promoting the rolling resistance and hysteresis loss of composites under the same condition.The hydrogen bond and covalent bond were shaped between carboxyl of fatty acid and silicon hydroxyl on silica surface,which improved the silica dispersion and strengthened the rubber-filler interface interaction.The alkene?-H of LA could participate in sulfur vulcanization enhancing the interface interaction more significantly than that of SA.(6)The basic formulas of NR/silica composites were designed optimally via combining the advantages of ENR and higher fatty acid together.The results demonstrated that the modulus,tensile strength and hardness of modified composites were better than that of unmodified composites.The blending of ENR or ESH(ENR/silica hybrid)and SA or LA could significantly decrease the tan?value at 60 ~0C and increase the tan?value at 0 ~0C simultaneously,which is meaningful to the high performance tire with high wet skid and low rolling resistance both.ENR+SA is good for the reduction of tan?value at 60 ~0C to obtain a low rolling resistance,while ESH+LA is more suitable than others for increasing the tan?value at 0 ~0C.Fortunately,the addition of ESH accelerated sulfur vulcanization behavior of NR/silica composites.