Effect Of Molecular Weight And Distribution Of Natural Rubber On Properties

Natural rubber(NR)is prepared from a fresh latex of the Hevea brasiliensis through solidification,compression,drying and other processes.Natural rubber is one of the four important basic industrial raw materials with excellent comprehensive properties,which is indispensable and widely used in life,production and national strategic materials.In 2018,the world's total natural rubber production was 13.96 million tons,but the global demand for natural rubber reached 14.01 million tons,and the demand gap was 57,000 tons.In China,natural rubber production has been hovering around 800.000 tons,but annual consumption is close to 5 million tons,more than 80%dependent on imports.In addition,the quality of domestic rubber is also very different from that of foreign rubber.The quality and performance of raw rubber have a significant impact on the compounding and mixng of the rubber compound,and directly affect processability and finished product properties of the rubber compound.The most fundamental influencing factor is the molecular weight and distribution of natural rubber.In this paper,all the latex solidified natural rubber(SCRWF)were prepared by mechanical shearing force to produce five different rubber compounds.The gel permeation chromatography method was used to test and prove the change of relative molecular weight and distribution of SCRWF after treatment.The plasticity properties such as initial plasticity(P0),plastic retention(PRI),Mooney viscosity and stress relaxation time of the rubber samples were compared with the crosslink density of the rubber and vulcanizate.The rheological properties of the vulcanizate,mechanical properties and thermodynamic properties were conducted to characterize,and the processability was analyzed to explore the relationship between the molecular weight,distribution and properties of different SCRWF.The results show that the Mn of SCRWF-a is 60.3×104,Mw is 152.7×104,and Mz is 274.2×104.After treatmen,the Mn of SCRWF-b is 50.6×104,and Mw is 132.6×104.Mz is 251.8×104 SCRWF-c has Mn of 39.7×104,Mw is 90.4×104.M,is 156.0×104 SCRWF-d has Mn of 30.5×104,and Mw is 69.1×104.M-z is 117.2×104,SCRWF-e has M,of20.7×104,Mw is 57.7×104,and Mz is 104.0×104,and the molecular weight distribution curve generally shifts from the high molecular weight region to the low molecular mass region,and the small shoulder peak intensity of the low relative molecular mass region increases.As the molecular weight decreases and the molecular distribution becomes wider,the quality of the raw rubber is clearly affected—the P0 value is significantly reduced.On the contrary,the PRI value is continuously increased with the greater impact of adverse factors in process of processability,storage and transportation.The stress relaxation time is reduced from 22.5 s of SCRWF-a sample to 1.1 s of SCRWF-e sample,and the physical properties of the processed product are stable and durable,and the service life is prolonged.The crosslink density data of raw ubber and vulcanizate correspond to the change of molecular weight and distribution.The higher crosslink density corresponds to the shorter rubber molecular chain between the cross-linking points,that is,the smaller relative average molecular mass Me between the cross-linking points,resulting in the faster relaxation rate and the shorter relaxation time t2.The mechanical properties analysis demonstrate that the SCRWF-a sample has high physical and mechanical properties,tensile strength of 19.33 MPa,tear strength of 32.84(kN·m-1)and volume wear of 1.212[cm3·(1.61km)-1 low wear.TG-DTG analysis shows that there are some differences in the thermal stability of the rubber samples with different molecular weights,which were SCRWF-a>SCRWF-b>SCRWF-c>SCRWF-d>SCRWF-e.DSC analysis shows that the glass transition temperature(Tg)of the rubber is independent of the molecular weight within a certain molecular weight range.RPA analysis indicate that in the frequency scan,as the increased frequency of 5 colloid loss factor(tan ?),the linear decreases and the elastic modulus(G)increases significantly.In the strain scan,as the strain increases 5 colloid loss factor(tan?)linear increases with the elastic modulus(G')decrease.In the temperature scan,with the increase of temperature of 5 colloid loss factor(tan?)linear increases,and the elastic modulus(G')decreases significantly.after that.the balance is sligchtly reduced.The common modes of frequency scanning,strain scanning and temperature scanning are that the rubber with higher molecular weight has smaller loss factor(tan?)and larger elastic modulus(G'),indicating that the larger the molecular weight,the smaller the tan ?,the larger the G'.The SCRWF-a sample with higher molecular weight has the worst processability.In the acrual processing and production process,the raw rubber sample is more difficult to be sheared and broken by the processing equipment,and the initial energy consumption is high.