Study On Properties And Vulcanization Kinetics Of Natural Rubber Coagulated By Microorganisms

Natural rubber latex is a white milky liquid which outflow from Hevea brasiliensis tree. In order to meet with the requirement of industry application, natural rubber latex is always prepared into raw rubber. There are many methods for natural rubber latex coagulation, such as acid coagulation, natural coagulation, inorganic salt coagulation, microwave coagulation, and microorganisms coagulation, etc. For a long time, Chinese usually use acid coagulation to prepare natural rubber, which not only causes a severe environmental pollution, but also the production can’t be applied to produce performance tires. However, microorganisms coagulation doesn’t need add ammonia to save, and add acid to coagulate, thus the cost of production and the emission of pollutants are reduced. The most important is that the rubber prepared by microorganisms coagulation is especially suitable to produce performance tires. Therefore, microorganisms coagulation processing has a potential application in natural rubber industry. In this paper, one kind of microorganisms is applied to prepare microorganisms coagulate natural rubber, named NR-m. The physical and chemical properties, vulcanization kinetics, crosslinking density, curing characteristics and mechanical properties of NR-m are investigated comprehensively. Moreover, the processing property of NR-m is tested by rubber process analyzer. In addition, thermal degradation and thermooxidative degradation properties of NR-m are studied by thermogravimetric analyzer. All investigations of NR-m here are compared with acid coagulate natural rubber, named NR-a.The results show that NR-m has a higher plasticity before aging, ash content and number average molecular weight, while the content of volatile matter, impurity and nitrogen, and plasticity retention index is low and the molecular weight distribution is narrow. The maximum torque of mixed NR-m and curing rate index is higher, t10and t90is shorter than that of NR-a.As to the cured rubber, the mechanical properties of cured NR-m is better than cured NR-a, such as tensile strength and tear strength of cured NR-m is stronger25.5%and34.8%than that of cured NR-a, respectively. But the tensile strength and tear strength of cured NR-m decreased7.6%and12.6%after aging. The rate of magnetizing attenuation with time of NR-m is faster than NR-a according to nuclear magnetic resonance and the crosslinking density of NR-m is higher than that of NR-a.Thermogravimetric analysis show that thermal degradation of NR is a first order reaction, which happens from340℃to440℃. The rate of terminate degradation(Cf) of NR-m and NR-a is97.11%and95.89%, respectively. T0、Tp and Tf of NR-m are higher than NR-a indicate the thermal stability of NR-m is superior to NR-a. While the thermooxidative degradation of NR can be divided into two stages. The first stage is the main degradation stage happens from250℃to450℃, the Cf of NR-m and NR-a in this stage is79.98%and80.28%separately. The second stage happens from450℃to550℃, the Cf of NR-m and NR-a in this stage is99.63%and99.68%, respectively. The characteristic temperatures T0、Tp and Tf of NR-m are lower than NR-a at each stage tell us the anti-heat and oxygen aging property of NR-m is worse than NR-a.Strain sweep tested by rubber process analyzer indicate that the elastic modulus(G’) decrease with strain’s increase, and G’of NR-m is higher than that of NR-a. With strain increased, elastic torque(S’) of NR increased to the maximum at first, then decreased and S’of NR-m is higher than NR-a. Tanδ of NR is the same trend as G’, but the tanδ of NR-m is lower than NR-a. When samples used to test temperature sweep, G’and tanδ have the same tendency as strain sweep. So it can be concluded that the processing property of NR-m is inferior than NR-a, and the internal friction of NR-m is lower than that of NR-a.The curing time from tML to tMH can be divided into three stages, tML to ts is intuction stage, ts to t90is thermal curing stage and t90to tMH is flat curing stage. With the increase of curing temperature, both scorch time and optimum curing time of samples reduced obviously. The temperature dependence of NR-a is stronger than NR-m at intuction stage and thermal curing stage, while NR-m is more sensitive to temperature than NR-a. The curing reaction rate constant increases with increasing temperature and the curing reaction rate constant of NR-m is bigger than NR-a at the same stage under the same temperature. At flat curing stage, the reaction order of NR-m and NR-a is0.70-0.72and0.73-0.89, respectively, which related to the crosslinking bond fracture and rearrangement at the end curing stage. NR-m needs more vulcanizing activation energy than NR-a at each stage, especially in the flat curing stage, due to the vulcanizing activation occurs difficultly and it mainly dependence on temperature in the end of this stage.