COMPARATIVE STUDY OF THERMAL DECOMPOSITION OF BAST FIBERS AND SHIVES FROM A STALK OF STINGING NETTLE (URTICA DIOICA L.) PRETREATED WITH LIQUID HOT WATER (LHW)

D.V. Chashchilov, N.V. Bychin
DOI: 10.25699/SSSB.2022.41.1.011 Download PDF
Abstract: When using bast crops to obtain plant fibers, a large amount of a non-fibrous fraction is formed – bonfires. Bast fibers can be used as a reinforcing filler of polymer composite materials (PCM). The bonfire can be used to produce biochar, a solid residue of the pyrolysis process. In this regard, it is important to study the behavior of fibers and bonfires under thermal exposure. The stems of the dioecious nettle (Urtica dioica L.) of the spring harvest were the subject of an experimental study. The purpose is to study the process of thermal decomposition of bast fibers and bonfires. Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were used. Vegetable raw materials were cut into fragments from 120 to 150 mm long. Hydrothermal pretreatment was carried out – boiling in liquid hot water and rinsing with hot water. Cooking temperature 140 0C, duration - one hour, cooking module 1:10 (solid : liquid, mass.). The yield of the product after processing is 85.3%. The product was mechanically retted into bast fibers and bonfire particles, in the amount of 8.4% and 91.6%, by weight, respectively. Samples were tested by TGA, DTA in a synchronous thermal analyzer DTG-60 (Shimadzu Corporation, Kyoto, Japan), using aluminum crucibles, a weight of 3 mg, a nitrogen atmosphere, a flow rate of 40 ml/min, a heating rate of 100C/min, a maximum temperature of 6000C. It is established that the thermal decomposition of samples is three-stage: 1) at temperatures up to 1150C and 1000C, respectively, for bast fibers and bonfires, water evaporates; 2) up to a temperature of 3900C, the decomposition of thermolabile components of vegetable raw materials occurs, a sharp peak is observed at temperatures of 3630C and 3720C, respectively, for bast fibers and bonfires; 3) at temperatures above 4000C, a smooth decomposition of the most heat-resistant components is observed. The endothermic effects for bonfires in the first and second stages are two and one and a half times higher, respectively, than the effects for bast fibers. The water content is 2.7% and 5.2%, the weight loss in the second stage is 72.2% and 68.4%, in the third stage – 6.1% and 7.6%, the solid residue is 19.0% and 18.8%, respectively, for bast fibers and bonfires. The results can be used: 1) to determine the maximum processing temperatures of bast fibers of nettle dioecious, as a reinforcing filler of PCM, the temperature should not exceed 2050C; 2) to assess the suitability of nettle dioecious bonfires for biochar production, it is possible to carry out slow pyrolysis at temperatures up to 6000C, with an output of ≈19%; 3) to develop a promising integrated technology for processing plant raw materials.
Index terms: plant raw materials, plant fibers, pretreatment, reinforcing filler, natural fibers reinforced composites, thermogravimetric analysis, differential thermal analysis, experimental study, biochar, slow pyrolysis.

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