Studies On Structure And Properties Of Natural Rubber Coagulated By Enzymes

Natural rubber,a kind of organic polymer material with excellent comprehensive properties is obtained from Hevea brasiliensis.The fresh natural rubber latex consists of rubber hydrocarbon,water and also certain amount of non-rubber substances.In practices,an available coagulation technique had to be applied to process the fresh natural rubber latex into raw rubber by destroying the colloid stability of latex.The coagulation method of fresh natural rubber latex is one of the key techniques for preliminary process of natural rubber.The commonly available coagulation can induce the loss of non-rubber contents and result in the variations of natural rubber performances with dark color and dense odor which have affected the quality of natural rubber greatly.In this work,the coagulation effects of enzymes on fresh natural rubber latex and their effects on the structure and performances of natural rubber were investigated to provide a new coagulation method for preliminary process of natural rubber,which will be of great significance for improving natural rubber quality and upgrading and updating the natural rubber process technology of China.The fresh natural rubber latex was coagulated by adding the same amount of lysozyme,alkaline protease and papain,and compared with that obtained by coagulating with acetic acid.The effects of different coagulation methods on the properties were studied.The results show that different kinds of enzymes are potential to be used to coagulate the natural rubber latex,but the protein contents of natural rubber obtained,however,have been varied greatly.The protein content of lysozyme coagulation natural rubber is higher than that of acid coagulation one and the characterization absorption of proteins can be observed obviously from the FTIR spectra.The vulcanization rate of lysozyme coagulation natural rubber is accelerated slightly as compared to that of acid coagulation natural rubber.The vulcanizate is provided with high cross-linking density and high mechanical properties with a tensile strength of 30.22 MPa.The anti-oxidative behaviour has been improved with a plasticity retention index of 88 and a tensile strength retention ratio of 81.37% after aged in air atmosphere.The compression heat build-up of lysozyme coagulation natural rubber is also higher slightly than that of acid coagulation natural rubber.The protein contents of natural rubber obtained by coagulating natural rubber latex with alkaline protease and papain are much lower than that of acid coagulation and the characterization absorptions of proteins have been nearly disappeared on the FTIR spectra.The anti-oxidative behaviors have declined with obvious decrements in plasticity retention index and the vulcanization rate become fast obviously.As compared with the acid coagulation natural rubber,the cross-linking density and the glass-transition temperature of vulcanizates are reduced slightly,but the compression heat build-up become low greatly.The effect of lysozyme adding amount on the properties of natural rubber was further investigated.The protein content,plasticity retention index and Mooney viscosity are increased gradually with the lysozyme adding amount increased.Compared with those of the natural rubber coagulated by acetic acid,the protein content of the natural rubber coagulated by adding lysozyme is increased and the vulcanization rate,cross-linking density,mechanical properties and the anti-oxidative behavour are improved.The vulcanization rates first accelerate,but slow down gradually,and the mechanical properties are also increased at first and then decreased gradually by increasing the adding mount of lysozyme.When the adding amount of lysozyme is 0.3 per hundred rubbers,the natural rubber exhibits short vulcanization time and high tensile strength of 28.10 MPa and tear strength of 37.17 kN/m.The thermal stability and the compression heat build-up of vulcanizates are increased,while the glass-transition temperatures are decreased gradually.The natural rubber with maximum elastic modulus and minimum loss factor can also be obtained when the adding amount of lysozyme is 0.3 per hundred rubbers.