Effect of ammonium orthophosphate on thermochemical transformations of hydroxyl-containing polymers

Abstract

Ammonium orthophosphate is an active pyrolytic additive for PVA fibre which can increase the yield of CF to 72% of the theoretical carbon content in the polymer. The conditions that ensure obtaining inactivated CFM with 60-85% sorption capacity, at the same level as BAU-grade industrially manufactured activated carbons were found.The wide development of studies on thermochemical transformations of polymers for obtaining a special class of carbon materials with unique properties, including the fibre form, is due to such promising areas of application as aerospace engineering, public health, treatment of gas-air and liquid media, etc. [1,2]. The specific features, as well as the complexity of the technology for manufacturing carbon fibre materials (CFM), consist of conducting rigorously controlled pyrolysis of an organic polymer with elevation of the temperature by steps in the range from 200 to 2500°C to eliminate heteroatoms from the polymer and retain carbon to the maximum, and in conditions of high-temperature treatment of the polymer, including carbon.Phosphorus-containing compounds are an effective pyrolytic additive in production of CFM for such carbonized polymers as hydrated cellulose (HC) and polyvinyl alcohol (PVA) [3][4][5]. On this basis, in studying the thermochemical transformations of these polymers, we selected ammonium orthophosphate, not previously used for these purposes, as the pyrolytic additive.Ammonium orthophosphate was applied on the fibre from aqueous solutions by impregnating it squeezing it out, and drying it at 100-110°C. The (NH 4 ) 3 PO 4 content in the fibre was varied from 2 to 8% by changing the concentration of the solutions and degree of squeezing out the fibre after the treatment.Thermal oxidation was conducted in a thermostat with a built-in fan in air medium while heating from 20 to 200°C at a temperature elevation rate of 3°C/min for 16 and 21 h. Carbonization was conducted in a nitrogen stream in a periodic reactor at a temperature elevation rate of 4°C/min and temperature of 900°C.The occurrence of thermochemical transformations was judged by the data from thermogravimetric (TGA), differential thermogravimetric (DTG), differential thermal analysis (DTA), and chemical methods of analysis. The DTA and TGA curves were obtained on a PaulikPaulikErdey derivatograph in air at a temperature elevation rate of 10°C/min from 20 to 1000°C.In studying the features of thermomechanical transformations of PVA fibres in the presence of (NH 4 ) 3 PO 4 , its content was varied from 2 to 8%, and the thermal oxidation temperature was varied within the limits of 170-200°C for a duration of thermal oxidation of 16 and 21 h.The TGA data for thermally oxidized PVA fibres in the presence of (NH 4 ) 3 PO 4 summarized in Table 1 and Figs. 1 and 2 suggest the important effect of ammonium orthophosphate on pyrolysis of PVA fibre. The presence of (NH 4 ) 3 PO 4 in the fibre decreased the initial decomposition temperature to 175-235°C and the maximum decomposition rate (υ max ) to 2 mg/min at 8% (NH 4 )...

General characteristics of chemical and structural transformations of polyvinyl alcohol fibre in conditions of high-temperature treatment

“…In addition to formation of double bonds, the number of acid groups in the polymer increases, and the ambient temperature has the dominant effect on the oxidation process. Their content increases from 2.5 to 6.8 mmole/g in the 180-200°C temperature region [5].…”