Study On The Improvement Of Mechanical Properties Of Lyocell Fiber Used As Precursor Of Carbon Fiber

In this paper, the development and basic feature of carbon fiber, especially cellulose-based carbon fiber and its precursor were briefly introduced. The present situation of improvement of mechanical properties of Lyocell fiber was reviewed. The methods for determining molecular weight (MW) and molecular weight distribution (MWD) were introduced with emphasis. The MW and MWD of cellulose pulps, which are suitable for producing Lyocell precursor for carbon fiber, were studied by gel permeation chromatography (GPC). The MWD and polydispersity indices (PDI) of cellulose were also investigated from the dynamic rheological data of cellulose/ NMMOH2O solutions. The comparison of the results from rheological data and GPC shows that it is feasible to predict the MWD of cellulose by using rheological method. Moreover, heat treatment of Lyocell fiber and Lyocell fiber modified by carbon black were applied to improve the mechanical properties of Lyocell precursor, and the effects of heat treatment and the addition of carbon black on the structures and properties of Lyocell fiber and resulted Lyocell-based carbon fiber were also studied.There are many reports about determining the MWD of cellulose by GPC which using 0.5% LiCl in N, N- dimethylacetamide (DMAc) as the eluent. In this paper, MW and MWD of different kinds of cellulose are determined by GPC accessory of High Performance Liquid Chromatography (HPLC). The results show that the mechanical properties of Lyocell fiber are improved with the increasing of the degree of polymerization (DP) of cellulose pulp. The MWD of cellulose for producing textile Lyocell fibers is different from that for producing Lyocell precursor for carbon fiber. The DP of cellulose for producing textile Lyocell fiber can't be too high, and MWD should be broad. However, cellulose for producing Lyocell precursor for carbon fiber should have a high DP and moderate MWD. It is found that the spinnability was poor only using the higher DP (DP 1400) pulp as material, on the other hand the spinnability of blended pulp is improved and the mechanical properties of obtained Lyocell fiber are very well by adding a smallamount of moderate DP (DP500-800) pulp. The MWD of this blended pulp becomes broader and the peak of high MW has no obvious change, whereas the contents of moderate and low MW increase and the low MW peak appears. This high MW section endues the product with good mechanical properties, and the moderate and low MW sections act as the plasticiser, which makes the process easier.Generally speaking, the MWD predicted from the dynamic rheological data of cellulose/NMMO-H2O solutions are consistent with those from GPC. Although the differential MWD obtained from rheological method is bell-shaped and can't describe the detail of MWD of cellulose as GPC, it is feasible to compare the MWD of cellulose from different pulps by using rheological method. Moreover, the method proposed by Llorens, which could predict the PDI of polymer, is also used to calculate the PDI of cellulose from the rheological data of cellulose/NMMOH20 solution. Although the results predicted by rheological method are slight higher than those of GPC, the order of change of PDI determined by rheological method and GPC is identical. Therefore, it is feasible and convenient to predict and compare the relative MWD and PDI of cellulose by using rheological method.The Lyocell fibers are heat treated with keeping a constant length in an experimental apparatus in our laboratory. The results show the tensile strength and initial modulus of treated Lyocell fiber increase sharply, whereas the elongation at break decreases. The effects of temperature and time of heat treatment on the mechanical properties of Lyocell fiber are analyzed. It is found that the mechanical properties reach maximum when the treat temperature is 160℃ and the processing time is 12~25s. Moreover, applying tension on the fiber during the heat treatment could improve further the tensile strength and initial modulus of Lyocell fiber, while the elongation at break of fiber dec...