Study on Ageing Properties of Microwave Drying Natural Rubber
|Keywords||Natural rubber Microwave drying Hot-air drying Nature drying Ageing property|
Natural rubber (NR) is one polymer obtained from Hevea brasiliensis, the component of which was poly cis-1,4-isoprene. The fresh latex was coagulated using acid to obtain coaglum, then crepped and guanulated, the moisture content of granulates was about30%, Hot-air generated from fuel burning was used to dry wet NR at home and abroad. High temperature, long-time leaded an incomplete drying and melt tacky of NR. There were lots of unsaturated double bonds in NR molecular structure, which rendered it particularly sensitive to oxygen in the presence of heat during the whole hot-air drying process, causing a bad quality of NR. So drying is an important procedure in NR processing.Microwave was used to dry wet NR, the ageing propeties of microwave drying NR were studied at different drying facters (microwave power density, dry thickness and initial moisture content) in this subject.Drawn from the researches, the cut traces during granulating of wet NR sheet could be remained of microwave drying, loose-structure, dales which internal water migrated to surface of NR granules were also visible. However, the surface of hot-air drying NR granules seemed to be wrapped by a layer of thin-film, the entercloses of water migration were blocked. The property index of NR dried by hot-air, nature and microwave confirmed to GB/T8081-2008, and Po, PRI of NR dried by microwave were16.7and7.84higher than hot-air drying (superior to national standard), VR of NR dried by microwave was higher (the value of VR was5.79unit higher than hot-air), molecular weight, crosslink density and thermal-oxidative temperatures were more higher, indicating an excellent thermal-oxidative resistance properties. As for NR dried by different microwave factors, microwave power density influenced the molecular weight and Vr slightly. The influence of dry thickness on molecular weight was most obvious, the molecular weight of NR reduced with dry thickness increment (the Mn reduced26.28%from2cm to6cm), VR reduced8.86unit. The influence of initial moisture content of wet NR on molecular weight and VR was indistinctive, the lower initial moisture content of wet NR, the bigger the molecular weight of NR after dried. The influence of microwave power density and dry thickness on viscoelastic properties of NR after dried was obvious, at the same sweep frequency, S’, S", G’ reduced, tanO increased with the dry thickness increment, while the influence of different initial moisture content on viscoelastic was unconspicuous. FTIR, RPA and TG tests indicated that ageing resistant of NR dried by microwave was superior to that dried by hot-air and nature. Drawn from the mechanical properties tests, the ageing resistant of NR dried by microwave improved greatly:rate of tensile strength change (-23.11%) and rate of elongation at break (-18.61%) before and after aged were remarkable higher than that of hot-air drying (-34.61%,-28.42%), closed to that of nature drying. The storage lifetime of microwave drying NR at ambient temperature was2.9years longer than hot-air drying, closed to nature drying.