Use of hydrazine salts as modifying agents for polyacrylonitrile in the preparation of carbon fibres

Abstract

Polyacrylonitrile (PAN) fibres are used as a raw material for the preparation of highmodulus and high-strength carbon fibres [i]. Sometimes, with the objective of cutting down the time for thermal stabilization of the fibres, they are treated in the presence of cyclization catalysts. These catalysts are substances which aid in the dehydrogenation of hydrocarbons or the dehydrogenation of hydroxyl-containing organic compounds, for example ZnCI2, AICI3, or FeCI3 [2]. Compounds of the Lewis acid type [3], acids, or alkalis can serve as catalysts [i]. In our research we used salts of hydrazine as modifiers-- the hydrochloride and the sulfate, and also combinations of hydrazine salts with caustic soda. Modification of the PAN fibres was carried out at 343°K in solutions whose concentration was from 0.5 to 20% by wt. The length of the treatment was i ho When the salt-NaOH system was used, the ratio of the components in the solution was varied from i:i to 2:1. After treatment, the fibre was subjected to oxidation in a heating chamber. The color of the fibres hardly changes if they are treated with the pure salts. Addition of alkali leads to a change in color from white to dark yellow. For comparison we carried out experiments on modification of fibres with caustic soda solutions alone. At a solution concentration greater than 0.5%, the morphological structure of the fibre is destroyed. Treatment of PAN fibres with hydrazine salts leads to an increase in the strength of the carbon fibres (Fig. i)o Treatment with salts at low concentration, 0.5-1% by wt., exerts the most important effect on the strength of the carbon fibres; the increase in their strength on treatment with 15 or 20% solutions of the salts is slight° The strength of the carbon fibres obtained by modification of PAN fibres with pure hydrazine, salts remains below the strength of the fibre obtained asa result of adding the salt--NaOH system. The difference in strength of the carbon fibres at various ratios of the components in this system is small; it is more prominent when the PAN fibres are modified with hydrazine sulfate° Within definite limits, the additions of alkali aids in raising the pH of the medium and favorably affects the quality of the carbo~ fibres, Treatment in an alkaline medium in combination with the cross-linking action of the hydrazine [4],which is liberatedondissociation of the salt, leads in the end to an increase in the strength of the carbon fibres. This fact may be explained by structural changes in the PAN fibres during the modification process. These changes are recorded on a differential scanning calorimeter, where the thermal effects of the cross-linking process are determined, which are characterized by the specific enthalpy values: Fibre AH, J/g