Abstract:
To explore the substitutability between wormwood stem and wood, the transverse compression and bending properties of wormwood stem under natural state and microwave drying are studied, and the microstructural changes before and after microwave drying are analyzed. Microwave are common way of drying and straightening process of plant material, which is widely used in wood processing industry in order to improve the mechanical properties, permeability and microscopic structure of wormwood stem for the processing of disposable chopsticks. In this paper, we selected wormwood dry stems with 6 mm diameter and 210 mm length, which were soaked until the moisture content was about 55% for moisture content test under the microwave drying. At the same time, to reveal the changes of mechanical properties of wormwood stem before and after microwave drying, the following aspects of tests were carried out: 1) Using MTS universal testing machine to test the radial compression characteristics of wormwood stem before and after microwave drying, to determine the feasibility of mechanical processing; 2) In order to obtain the force condition of chopsticks in a typical application environment, testing the bending properties of wormwood stem before and after microwave drying to determine the application feasibility of herbal chopsticks. Experimental data showed that under the process of microwave drying for wormwood stem, the moisture content change could be roughly divided into 3 stages: Speed-rising drying, speed-constant drying and speed-falling drying stage. In this test, the speed-rising drying stage of wormwood stem was about 120 s, lower than the wooden material preheating time; the moisture content of wormwood stem fell by 16% in this stage, which showed that a large number of cell wall crack had not formed due to the moisture migration. The stage of microwave drying with constant speed for wormwood stem was not obvious, and the drying was mainly in the speed-falling drying stage. As microwave time became longer, the moisture content was reduced gradually and the moisture content decreased rapidly in the early 120 s. In the 700 W microwave oven, the moisture content was reduced from 55% to 10% within 420 s (about 40 g). Decreasing rate of moisture content gradually slowed down after 420 s, approaching 5%. As the microwave drying extended, the pit structure of cell wall was destroyed, which released the wormwood stem growth stress and avoided the structure deformation caused by the stress release. However, the fibrous tissue structure was compact for the wormwood stem without microwave drying, whose microstructure determined it had high moisture content and large growth stress. In the process of external grinding, compared with the disposable chopsticks with the diameter of about 5-6 mm for daily life, the middle part of wormwood stem was the best for processing disposable chopsticks. The minimum radial pressure after microwave drying was 630 N, which was much greater than the external grinding force. The result showed that using wormwood stem through the processing of microwave drying and straightening as raw materials to get ideal herbal chopsticks was feasible. To verify the using feasibility of herbal chopsticks, the loading way of three-point bending was used to test bending properties of wormwood stem. The toughness of wormwood stem increased after microwave drying, and the herbal chopsticks could withstand the bending load of up to 41 N, which was enough to meet the daily needs (the maximum was about 6-10 N). The results provide a theoretical basis for processing disposable chopsticks using wormwood stalks as raw material.