Studies On The Rapid And Efficient Method For Synthesizing Fiber-grade Polylactide

Polylactide,which can be synthesized from renewable resources,is an environmental friendly polymer.It has been recently proposed as a substitute for petrochemical-based polymers due to its renewable raw materials,biodegradability,good processability and mechanical properties.For many application forms,polylactides used for preparing fibers have the highest added value.However,spinning is also the most strict with properties of polylactide.Recently the development bottleneck of polylactide is its synthesis method.Therefore,polylactide and its fiber had been classified as the key objects in the"Made in China 2025"strategy and the 13th Five-Year Plan of textile industry.At present,the industrial synthesis of PLA is generally carried out using kettle or tower reactors via ring-opening polymerization of lactide.However,these methods usually require longer polymerization time,which will lead to high energy consumption and carbon emission.Hence,the aim of this thesis was to develop some rapid and efficient methods for synthesizing fiber-grade polylactide.For this purpose,the twin-screw reactive extrusion technique for synthesizing polylactide was investigated firstly in the thesis.It was found that the effect of the complex catalyst system on the polymerization was superior to that of the single catalyst.Under the condition of this thesis,the complex catalyst system consisting of stannous octoate as the main catalyst and triphenylphosphine as the cocatalyst showed a better catalytic effect on the ring-opening polymerization of L-lactide.Combining the analysis of the molecular weight and its distribution,FT-IR,optical purity,¹H-NMR and ¹³C-NMR results,it was further found that the molecular weights of the polylactide products increased firstly and then decreased with the increasing of the polymerization temperature,reaction time and screw speed.Moreover,a higher polymerzation temperature,a longer polymerization time or a faster screw speed could intensify the side reactions,which resulted in deterioration of polylactide properites including the molecular weight and its distributuion,monomer conversion,etc.Under the optimal conditions,that is,when the screw speed was 110 rpm,the polymerization temperature was 170°C and the reaction time was90 min,the poly（L-lactide）product with a weight-average relative molecular weight of 84000,a polydispersity index of 2.10 and an optical purity of 93%could be obtained by using the reactive extrusion technique.The results of optical activity of the poly（L-lactide）product obatined by¹H-NMR and ¹³C-NMR were in good agreement with those obtained by polarimeter.Furthermore,the kinetics of ring-opening polymerization of lactide catalyzed by the complex catalyst system in the co-rotating twin-screw extruder was studied.The results showed that the ring-openging polymerization of lactide is first-order in monomer and catalyst concentration.And its apparent activation energy by using the twin-screw extruder as reactor was 60.9 kJ/mol and was declined by 33%compared with that obtained by using traditional reactor,indicating that the twin-screw reactive extrusion technique was a rapid and energy-saving method for synthesis of polylactide.Although the above technique could be used for rapid synthesis of polylactide,the performance of the polylmer products,such as the molecular weight and its distributuion,optical purity,etc.,still need to be improved.For this purpose,a tubular static mixing reaction technique was developed and the polylactide products with better properties were expected.In this technique,the ring-opening polymerization of L-lactide were carried out in the tubular static mixing reactor,which was a loop reactor consisting of 4 well-designed and corrugated plate-type static mixers.Through the analysis of the the molecular weight and its distributuion,optical purity and monomer conversion of resultant polymers,it was found that less impurity content and higher optical purity of L-lactide were advantageous for the synthesis of polylactide with better properites.The molecualr weights and monomer conversions of the polylactide products increased firstly and then decreased with the increasing of the catalyst dosage,polymerization temperature and reaction time.Moreover,the flow rate of reactants in the static mixing reactor also had an important effect on the properties of polylactide products,such as the molecular weight and its distributuion,monomer conversion,etc.A slow flow rate of materials leads to an insufficient mixing of reactants.Consequently,it is difficult to get high qulitiy polylactides.However,the shear rate of reactants sheared in the reactor increased with the increasing flow rates of materials.The excessive shear rates could result in a decrease of polylactide performance.Therefore,a proper flow rate of materials needed to be selected for ensuring the efficient mixing of reactants and avoiding the above problems.Finally,by using the tubular static mixing reaction technique and when the flow rate of reactants was controlled to be 3.7 mm/s,poly（L-lactide）products with a weight-average relative molecular weight higher than 140000,a polydispersity index of 1.54 and an optical purity of 97%could be obtained at 170°C in 120 min or by 5 to 6 circulation flow of the reactants in the loop reactor.The results indicated the tubular static mixing reaction technique was feasible,fast and highly efficient for synthesis of high quality polylactide.On the basis of above studies,the kinetics of ring-opening polymerization of L-lactide catalyzed by the stannous octoate and triphenylphosphine complex catalyst system in a tubular static mixing reactor was further investigated.The experimental results showed that the polymerization of lactide was also first-order in monomer and catalyst concentration.Furthermore,its apparent activation energy was much lower（58.0kJ/mol）,indicating that the tubular static mixing reaction technique was a more rapid and energy-saving method for synthesizing polylactide.The above studies showed that the tubular static mixing reaction technique and reactive extrusion technique were both rapid,efficient and energy-saving methods for synthesis of polylactide.However,by contrast,the polylactide polymers with higher molecular weight,optical purity,melting point and monomer conversion,narrower molecular weight distribution could be obtained by using the tubular static mixing reaction technique.Moreover,the dominant time of chain propagation reaction in its ring-opening polymerization of L-lactide was longer.This was caused by the difference between the structures and mixing mechanisms of the twin-screw extruder and tubular static mixing reactor.It was found that the mixing of materials in the twin-screw extruder was mainly relied on the high shearing action of materials sheared in the intermeshing region of the kneading blocks.But in this intermeshing region,the share rate was as high as 700s^-1.The overmuch shear rates probably promoted a remarkable local over-heating in the reactants,which may result in decrease of the properties of polylactide products.By contrast,the shear rates of the materials sheared in the tubular static mixing reactor were relatively low(when the flow rate of the materials was 3.7mm/s,the shear rate was 48.3^-1).Moreover,the unique structure of corrugated plate-type static mixers in the tubular static mixing reactor can provide a uniform and three-dimensional mixing with no dead space for mixing reactants,which may avoid local over-heating and promote the mixing performance of the reactants.Furthermore,in order to investigate the mixing charateristic of the corrugated plate-type static mixers,it was further found that the number of striations of reactants separated by one static mixer used in the thesis was up to1.6×10²⁰.Additionally,it was also found from the simulation results of ANSYS FLUENT software that the intersecting flows of reactants could be formed in the flow channel intersection of the static mixer and they could significantly enhance the mixing degree of the reactants.Therefore,a lower shear rate,a better mixing performance of materials,and a fuller polymerization reaction were achieved by using the tubular static mixing reactor,which resulted in better qulities of polylactide products.It was also found in the thesis that the polymerization time for synthesizing polylactide was shortened to 120min by using the tubular static mixing reaction technique.Furthermore,the properties of polylactides obtained from the tubular static mixing reaction technique,such as the optical purity,molecular weight and its distribution,were comparable with imported fiber-grade raw materials and could meet the requirements of melt-spinning.In addition,in order to determine the melt-spinning conditions of polylactide,the rheological properties of polylactides were studied.Moreover,the prediction method of the molecular weight and its distribution of the polylactides by using their dynamic rheological properties were also explored in this thesis.The results showed that the molecular weights and their distributions of polylactide determined by the dynamic rheological method had a good consistency with the results obtained by the GPC method.On the basis of above studies,using the polylactides prepared in this work as the spinning raw materials,the polylactide fibers were obtained by melt-spinning and hot-drawing process.It was found that a higher molecular weight,a narrower molecular weight distribution and a higher optical purity of the spinning raw materials were beneficial for improving the mechanical properties of the polylactide fibers.Moreover,spinning temperature,winding speed,stretching temperature and draw ratio also had important effects on the properties of polylactide fibers.The results showed that the tensile strengths of polyalctide fibers increased firstly and then decreased with the increasing of the stretching temperatures and draw ratios.Furthermore,the as-spun polylactide fibers prepared by melt-spinning were drawn for 4 times at 80°C.Finally,polylactide fibers with a tensile strength of 3.7 cN/dtex and an elongation at break of 31%were obtained and these properties of the fibers had achieved the standards of the related properties of commercial polylactide fibers and the superior qualities of the polylactide fiber in textile industry.The results from the thesis showed that a tubular static mixing reactor was established.Moreover,the tubular static mixing reaction technique was a rapid and efficient method for synthesizing fiber-grade polylactide.By using this technique,fiber-grade polylactide with a good melt-spinnability could be obtained in a relatively short reaction time.Therefore,the studies in this thesis showed considerable realistic significance for promoting the industrialization of polylactide fiber and its raw materials.