| Sea-sland fiber is a high value-added,high-performance composite fiber with a wide range of applications in artificial suede,peach skin suede,synthetic leather,and filter absorbing materials.The current benzene deweighting and alkali deweighting used for sea-island fiber splitting have problems such as high energy consumption,environmental pollution and waste of resources,so researchers have started to investigate the green and non-toxic water-soluble polyvinyl alcohol(PVA)as a sea phase component to achieve composite melt spinning of seaisland fiber,followed by water-soluble splitting of the fiber.However,PVA is difficult to melt spinning due to strong inter/intra-molecular hydrogen bonding resulting in a very close melting point and thermal decomposition temperature.In addition,the recycling of PVA from the hydrolysis fiber waste water is a major challenge.Based on this,in this paper,PVA was modified with dipentathritol and pseudo ionic liquid as plasticizers,supplemented by lubricants and antioxidants,to prepare plasticized modified PVA,and then modified PVA fibers were obtained by melt spinning.The effect of the plasticising system on the melting point,thermal decomposition temperature and melt fluidity of PVA was studied,and the optimum ratio of each component of the plasticising system was clarified,and the effect of the plasticising system on the hydrogen bonding structure,crystallinity,compatibility and spinnability of PVA was clarified.The water solubility of PVA was investigated,and the "anhydrous sodium sulphate/sodium polyphosphate/boric acid" coagulant system was developed.The optimal recycling process of PVA was defined,and the film-forming,thermal,mechanical and flame retardant properties of recovered PVA were characterized.The contents are as follows:(1)In order to solve the problem of melt spinning processing of PVA,dipentaerythritol was used as plasticizer with calcium stearate and antioxidant B225 for plasticizing modification of PVA,to break the intramolecular/intermolecular hydrogen bonding,reduce the regularization degree of PVA molecular chain,reduce its melting point,increase its thermal decomposition temperature,and realize the high temperature melt spinning processing of PVA.The results showed that when the contents of dipentathritol,calcium stearate and antioxidant B225 were15%,3% and 1%,respectively,and the pelleting temperature was 200 ℃,the melting point of PVA was reduced to 178.2 ℃,and the thermal decomposition temperature was increased to301.3 ℃,providing a processing window of 123.1 ℃.The compatibility between plasticizer and PVA was good,with no migration and precipitation.The modified PVA could be processed smoothly at the spinning temperature 200～260 ℃,and the resulting fiber was smooth and uniform.The modified PVA fiber had good water solubility and could be completely dissolved in about 1 min under the condition of bath ratio of 1:30 and water temperature of 70 ℃.The research laid a solid foundation for promoting the development of PVA sea component and the preparation of water-soluble sea-island fiber.(2)As polyols are prone to decomposition and dehydration at high temperatures,in order to slow down the decomposition of plasticisers at high temperatures and to improve the thermal stability of modified PVA systems,choline chloride(Ch Cl)and xylitol(Xyl)were designed to synthesise thermally stable pseudo ionic liquids(PILs)as plasticisers supplemented by calcium stearate and the antioxidant B225 to plasticise PVA and to achieve melt spinning of PVA.The results showed that the thermal stability of PILs was much greater than that of Xyl.PILs could produce hydrogen bonding with the hydroxyl group of PVA,which broke the original hydrogen bonding of PVA and lowered its melting point and crystallinity.When the molar ratio of PILs was 1:2 and the content was 12.5%,the maximum thermal decomposition temperature of modified PVA was 291.2 ℃,the melting point was lowered to 169.4 ℃,providing a processing window of 121.8 ℃.The PILs had good compatibility with PVA.The modified PVA could melt spinning at 200～250 ℃,and the resulting fiber was longitudinally straight,uniform in thickness and smooth;the modified PVA fiber could be completely dissolved in about 52 s at a bath ratio of 1:30 and a temperature of 70 ℃,had excellent water solubility.Compared with dipentaerythritol,the amount of PILs used as plasticisers was less.(3)For the recycling of PVA in wastewater,the anhydrous sodium sulphate/sodium polyphosphate/boric acid coagulant system was developed for the recovery of modified PVA wastewater with a mass fraction of 3.33%.The effect of anhydrous sodium sulphate,sodium polyphosphate and boric acid on the recovery of PVA was investigated by combining singlefactor experiments with response surface optimisation experiments,and the optimum recovery process for PVA was obtained as follows: the amounts of anhydrous sodium sulphate,sodium polyphosphate and boric acid were 0.471 g,0.414 g and 0.435 g respectively,at which the recovery of PVA was up to 79.26%,the precipitated water was clear and the recovered PVA was presented as gel agglomerate.The recovered PVA had good film formation and mechanical properties with a high transparency of 84.2%,a breaking strength of 17.9 MPa and an elongation at break of 61.8%.The heat release rate peak temperature of the recycled PVA increased to 434.5 °C,the LOI value increased from 19.5% to 25.1% and the flame retardant property was improved.The research provided a theoretical basis and technical support for the recycling of PVA after hydrolysis of sea-island fiber. |
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