Study On Microwave Drying Process Optimization And Curing Properties Of NR

Natural rubber (NR), which is from a renewable natural resource with good comprehensive performance, has many features such as high flexibility, high strength, high elongation, wear-resistant and so on, and has found wide application in aerospace, national defense industry, heavy vehicles, aircraft tires, medical devices playing a very important role in the national economical construction. With the ever-growing consumption and shortage, NR has become one of the scarce strategic resources following petroleum, iron ore and non-ferrous metals in China.Raw NR is directly made from field NR latex or preserved NR latex. The preparation of raw NR is usually by the following process:the NR latex is first diluted to a certain concentration, consequently coagulated with acid or microorganism, then sheeted, dried, packaged and other mechanical treatment. Generally, raw NR includes several types like smoked sheet, crepe, granule rubber and so on and each type is of some difference in the production process.. Drying is a.crucial process in the production of standard rubber. The researchers in the Agricultural Product Processing Research Institute, Chinese Academy of Tropical Agricultural Sciences have applied microwave to dry NR, and systematically studied the microwave drying characteristics and properties of dried NR using a self-designed NR microwave drying equipment. It is found that, the drying time is greatly reduced and the properties of products are improved by microwave drying. Basing on the previous studies, we systematically studied the effects of multi-factors on the microwave drying process through orthogonal experiment in this work, by investigating the optimal process for microwave drying, simulating the relationship between the moisture content in NR and microwave drying time under multi-effect conditions, studying the vulcanization kinetics of NR dried by microwave, and comparing the properties of raw NRs dried by hot air and microwave.The research began with the single factor experiment at first, including the effects of microwave power density, rubber thickness, initial moisture content and dehydration wind speed on the microwave drying characteristics of wet NR and the out-appearance and morphology of dried NR. The results showed that, the best range of factor for microwave drying of wet NR is as follows:the microwave power density is 30.4-34.2w/dm3; the rubber thickness is 5-15mm; the initial moisture content is10-20% and the dehydration wind speed is 1.51-2.52m/sOrthogonal experiment of L9 (34) was designed according to the results of the single factor experiment,, targeting on the combination of the drying time, the mooney viscosity of raw NR, the mechanical and aging properties of NR vulcanizates as the preference of drying factors. The results of the experiments were treated using the software SAS 9.0 for the range and variance analyses to get the following optimal combination of microwave drying of wet rubber:the microwave power density is 30.4w/dm3; the rubber thickness is 5mm; the initial moisture content is10% and the dehydration wind speed is 2.52m/s. The microwave drying process resulting from the combination of factors is less time-consuming, high efficiency, low energy consumption with high mooney viscosity of raw NR, and excellent, mechanical and aging properties of NR vulcanizates.After investigation of the drying model for microwave drying of wet NR, it is found that the changes of the water content in NR can be expressed by use of the following single diffusion model of MR=Aexp (-kt),. A=exp (37.81050-2.35485X1-0.20883X2+0.11269X3+0.03639X12+0.00857X22-0.00501X32+0.07224X42); k=-(17.46609+1.00197X1+1.00849X4-0.01568X12+0.00065801X22+ 0.00052824X32-0.28665X42);Where X1 is the microwave power density (P), X2 is the rubber thickness (D), X3 is the initial moisture content (Aw) and X4 is the dehydration wind speed (v)The studies on the vulcanization characteristics of NR dried by microwave shows that, at the same vulcanization temperature, the induction time tc10 and optimum cure tc90 of microwave dried sample were obviously less than those of hot air dried samples, but their rate constants is of a opposite relationship.The maximum and minimum torques showed a tendency of linearly reduction with the rise of the temperature,,and at the same vulcanization-temperature, the MH of the sample dried by microwave is higher than that of sample dried by hot air, but their ML are of an opposite relationship. The activation energy of vulcanization reaction of the sample dried by microwave is lower than that of sample dried by hot air.SEM, RPA and TG/DTG were used to study the structures and related properties of NRs dried by microwave and hot air, respectively. The results showed that the microwave-dried NR has good comprehensive properties.The infrared thermal and degradation dynamics studies on the microwave-dried mildew-eroded and un-eroded nature rubbers showed that the reduction in the thermal stability of the nature rubber eroded by mildew is due to the decomposition of proteins in the NR.