| Recently,technical bottleneck has been formed for high performance carbon fiber production in China;the main contributing factor is the restriction of low-level precursor.Stretching is an important way to decrease fiber linear density,improve fibril orientation degree and tensile strength;in specialty,stretching in saturated steam condition(denoted as SSS)is suitable for producing high-level precursors with high tensile strength and orientation degree as it can achieve a very high stretching ratio.Thus,in this dissertation,SSS is conducted on a modified stretching apparatus for the widely used and projective PAN-based carbon fibers;in depth,the influence of SSS process on carbon fiber precursor structures and properties is also investigated,which may devote some technical guidance and theoretical reference for high performance o carbon fiber precursor production.Various hot stretching processes have been conducted for self-made PAN fiber precursors;then,the structural and mechanical properties of stretched PAN fiber precursors are comparatively studied after tests by various apparatus such as fiber fineness tester,fiber tensile tester,XRD,SEM and optical microscopes.Results show that the stretched precursor linear density and breakage elongation decrease,while the stretched precursor tensile strength,modulus and orientation degree increase as the stretching ratio increases for the fiber stretching in 80 ℃ hot water;excessive stretching ratio(larger than 2.10)can lead to structure damage of precursor.Comparatively,stretching in boiling water,with an ultimate stretching ratio 2.15,yields better results of stretched precursor than that in hot water.In dry-hot stretching condition,precursor tenacity increases as the temperature and stretching ratio increase;however,the effect of the stretching ratio is more significant than that of temperature.Therefore,the temperature and stretching ratio simultaneously increase to higher the precursor tenacity;the suitable temperature is proved to be within 175 ℃for dry-hot stretching process.In order to conduct an effective and reliable stretching in saturated steam condition(denoted as SSS),we have modified the existed SSS apparatus by improving SSS device insulation and sealing(of pressure),optimizing the design of clamshell-style stretching parts.Consequently,the difficulty in carbon fiber sample installation is solved;precise temperature control is successively achieved.The modified SSS apparatus is applied to conduct the stretching of carbon precursors;then responding stretched carbon precursor structure and properties are studied and discussed.Results indicates that increasing temperature-pressure of pre-heating section and full-heating section within the range that precursor can withstand results in an increase of precursor real stretching ratio as well as precursor tensile proprieties.Under the same SSS process,stretching temperature,pressure and stretching ratio should be matched(i.e.,a higher stretching ratio should corresponds to a higher temperature and a larger pressure)within the maximum limit that starts to damage fiber structure.The relative humidity of saturated steam has an influence on the precursor fuzziness number and stretching ratio;according to experimental results,the appropriate saturated steam relative humidity for pre-heating section is ranged as 5-10%,for full-heating section ranged as 15-25%.A computation method of grey relation analysis was used to study the influence of stretching parameters on the tensile strength of polyacrylonitrile precursors.It is shown that the stretching ratio of PAN precursors is the firstly important parameter.The results also indicate that the effect of pressure of full-heating section,pressure of pre-heating section,moisture of full-heating section,moisture of pre-heating section is according to the sequence from great to less.Eventually,the best precursor(with tenacity as high as 7.71cN/dtex)has been gained by setting the optimum SSS parameters on our modified stretching device. |
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