Microwave vs. hot air: investigation into the drying characteristics and mechanisms of natural rubber

Abstract

Traditional drying relies on convection, radiation, and conduction, resulting in low drying efficiency, especially for natural rubber with elevated humidity, which is extremely likely to lead to severe product aging. In this study, clean and efficient microwaves were utilized to achieve rapid drying of natural rubber, and the microwave resonant cavity perturbation technique and low-field nuclear magnetic resonance (LF-NMR) technique were used to compare it with hot air drying. This approach aimed to clarify water migration pathways during microwave drying and highlight the advancements of this method. The results show that water in natural rubber contains two main forms—free water and bound water (T₂ spectra)—and the water content significantly affects the microwave absorption properties of natural rubber. T₂ relaxation analysis revealed different dehydration mechanisms: traditional hot air drying (480 min) preferentially removed free water (95.5 %), while bound water (5.9 %) was less, while microwave drying (25 min) could remove more evenly (83.4 % free water vs. 10.4 % bound water). Real-time MRI demonstrated microwave-induced reverse moisture migration (internal → external), explaining the fundamental reason for its high drying efficiency. The distinctive microwave drying technique led to a 14.9 % enhancement in the elongation at break of the dried rubber, when compared to traditional drying methods. In this study, the migration characteristics of water during conventional and microwave drying were elucidated through LF-NMR. This investigation not only unveiled the mechanisms underlying microwave drying but also provided a theoretical basis for the selection of the microwave drying process and process control.