【作者】 李博；

【导师】 赵国群；

【作者基本信息】 山东大学 ， 材料加工工程， 2021， 博士

【摘要】 能源和环境为人类社会提供发展动力和生态基础。石油泄漏清理和含油废水处理是节能环保领域的重要课题。以吸油材料为载体的油水分离技术是上述课题的关键。现有吸油材料大多为传统石油基聚合物,存在过度消耗不可再生资源和造成塑料污染的弊端;同时,现有吸油材料的制备技术一般需要使用有毒的有机溶剂或化学试剂,易造成环境污染和危害生命健康。为此,本文提出利用以聚乳酸(PLA)为基体的生物聚合物和以超临界CO2为物理发泡剂的微孔发泡技术制备吸油材料的绿色方案。高性能吸油泡沫要求大发泡倍率和高开孔率。然而,利用微孔发泡技术制备大倍率聚乳酸开孔吸油泡沫仍面临诸多挑战和问题。第一,聚乳酸结晶与发泡间的关系非常复杂,发泡机理尚不明晰,泡孔结构调控困难。第二,现有研究大多通过材料改性手段提高聚乳酸发泡能力,不利于聚乳酸的生物降解和回收再用。第三,大倍率和高开孔率对熔体强度的要求在一定程度上相互冲突,导致现有聚乳酸开孔泡沫的发泡倍率较低。第四,现有研究基本未涉及聚乳酸微孔发泡材料的吸油性能,针对泡孔结构与泡沫疏水性等性能间关系的研究尚不系统。本文分别围绕升温发泡工艺和降温发泡工艺,研究了大倍率聚乳酸开孔泡沫的制备方法及其吸油性能。首先揭示了聚乳酸在升温发泡工艺中的晶体熔融行为和发泡机理,提出了基于预结晶和CO2诱导熔融制备层状泡沫和开孔吸油泡沫的新方法;在此基础上,提出了在升温发泡的加热阶段引入预等温冷结晶处理的新工艺,实现了发泡倍率和吸油能力的提高,建立了泡孔结构调控的绿色方法;最后,对降温发泡工艺进行了改进,制备了具有优异吸油性能的大倍率聚乳酸开孔泡沫,并利用生物可降解的聚丁二酸丁二醇酯(PBS)对聚乳酸进行了共混改性,拓宽了制备大倍率开孔泡沫的温度区间。主要工作内容和结果如下:(1)为揭示聚乳酸在升温发泡工艺中高压CO2下的晶体熔融和形貌演变规律及其对发泡行为的影响,采用原位可视化技术、非原位表征和发泡实验相结合的方法进行了研究,发现聚乳酸在高压CO2作用下能形成层状和非层状凝聚态结构,提出了一种基于预结晶和CO2诱导熔融调控层状和开孔结构的新方法,获得了具有良好吸油性能的开孔泡沫。研究发现,在较低温度区间,预结晶聚乳酸在高压CO2下能够形成具有熔融皮层和结晶芯部的层状凝聚态结构。随温度升高,熔融皮层的宽度和结晶芯部的熔融程度逐渐增加,直至整个试样完全熔融以形成均匀非层状的熔融态结构。利用上述晶体形貌演变规律,在低温和高温下分别制备了具有层状和非层状泡孔结构的聚乳酸泡沫。其中,高压高温下的低熔体强度和高气体含量更有利于泡壁破裂,进而可以得到非层状开孔泡沫,并且该泡沫表现出良好的疏水亲油性和吸油性能。(2)针对聚乳酸发泡能力不足和发泡倍率较低的问题,提出一种在升温发泡工艺的加热阶段引入预等温冷结晶处理的新方法,阐明了预等温处理对聚乳酸冷结晶、高压流变和发泡行为的影响机理,获得了大倍率泡沫的制备窗口,提高了泡沫的吸油能力。研究发现,预等温处理能够在加热阶段赋予聚乳酸更高的结晶度和熔融温度,从而有利于聚乳酸在CO2饱和阶段保留更多高熔点晶体,增强聚乳酸在CO2饱和阶段的粘弹性,从而促进泡孔长大和泡沫膨胀,进而提高各饱和压力下的发泡倍率,显著拓宽大倍率泡沫的制备窗口,将最大发泡倍率由6.4倍提高至17.7倍。引入预等温处理后,泡沫对各类油的吸附容量由0.8-1.0g/g增加至 2.6-6.6 g/g。(3)为调控聚乳酸泡沫的发泡倍率和泡孔形貌,建立了基于预等温冷结晶处理和升温发泡工艺的绿色方法,阐明了预等温处理条件对聚乳酸发泡前结晶行为、发泡行为、泡沫结晶行为的影响,获得了一系列发泡倍率和泡孔尺寸可调控的泡沫。研究发现,随预等温温度升高,发泡前的结晶度和熔点均逐渐升高,使得发泡倍率和泡孔尺寸均先增加后减小。随预等温温度升高,虽然CO2饱和阶段的高熔点晶体数量逐渐增加,但是与发泡倍率直接相关的应变诱导结晶效应先增强后减弱,使得泡沫最终的结晶度先增加后减小。随预等温时间增加,发泡前的结晶度先增加后稳定,使得泡沫的发泡倍率、泡孔尺寸和结晶度也表现出相似的变化规律。通过调节预等温条件,可制备发泡倍率约为2.4-10倍、泡孔尺寸约为2.6-22.5 μm的聚乳酸泡沫。(4)为提高聚乳酸开孔泡沫发泡倍率和揭示泡孔结构对泡沫疏水性等性能的影响,提出了一种改进的降温发泡工艺,阐明了该工艺中聚乳酸泡沫泡孔结构随工艺条件的演变机理,制备了大倍率开孔和闭孔泡沫以及一系列泡孔结构可调的泡沫,揭示了泡孔结构对泡沫疏水、隔热和压缩性能的影响,实现了优异的吸油和隔热性能。研究发现,在改进的降温发泡工艺中,聚乳酸在无定形态发泡时获得最大发泡倍率。与升温发泡工艺相比,降温发泡工艺的发泡温度窗口明显拓宽、发泡倍率显著提高。利用高温下低熔体强度和低温下高熔体强度能够分别获得开孔和闭孔泡沫。提高发泡倍率和减小泡孔尺寸,能够增强泡沫的疏水性。制备得到了发泡倍率高达43倍、开孔率高达97.9%的开孔泡沫,该泡沫对各类油的吸附容量高达10.9-31.2 g/g。制备得到了发泡倍率为60倍的闭孔泡沫,该泡沫的热导率低至31.7 mW/(m·K)。减小发泡倍率和泡孔尺寸均能够提高泡沫的压缩性能,通过减小泡孔尺寸,泡沫压缩强度和模量可分别提高大约900%和929%。(5)针对大倍率聚乳酸开孔泡沫制备温度高、区间窄的问题,提出了将改进的降温发泡工艺应用于PLA/PBS共混物泡沫制备的策略,揭示了 PBS含量对共混物相形貌、热行为、结晶行为和流变性能的影响规律,阐明了发泡温度和PBS含量对共混物发泡行为的影响机制,降低了大倍率开孔泡沫的制备温度,拓宽了制备温度区间,获得了优异的吸油性能。研究发现,PBS对PLA具有一定塑化作用,削弱共混物的熔融结晶。PBS能够降低共混物的熔体粘弹性,作为受力薄弱点能够诱导泡壁破裂,进而赋予泡沫蕾丝网状开孔结构,并拓宽大倍率开孔泡沫的制备温度区间、降低开孔温度。制备得到了发泡倍率高达43.6倍、开孔率高达98.2%的PLA/PBS开孔泡沫,其发泡倍率比现有研究提高了约315%。该PLA/PBS开孔泡沫的吸油容量达7.9-21.9 g/g,且对CCl4的吸附容量在20次循环使用过程中无明显衰减。更多还原

【Abstract】 Energy and environment provide the development power and ecological foundation for human society.Oil spill cleanup and oily wastewater treatment are important topics in the field of energy conservation and environmental protection.The oil-water separation technology with oil-adsorption material as the carrier is the key to the above-mentioned topics.Current oil-adsorption materials are mostly traditional petroleum-based polymers,which can result in excessive consumption of non-renewable resources and plastic pollution.In addition,toxic organic solvents or chemical reagents are generally used in their preparation technologies,causing environmental pollution and endangering health.Herein,a green solution was proposed to prepare oil-adsorption materials by using poly(lactic acid)(PLA)-based bio-polymers and microcellular foaming technology with supercritical CO2 as physical foaming agent.High-performance oil-adsorption foams require large expansion ratio and high open-cell content.However,there are still many challenges and problems in the preparation of high-expansion PLA open-cell foams applied in oil-adsorption by using microcellular foaming technology.First,the relationship between the crystallization and foaming of PLA is very complex,the foaming mechanism is still unclear and it is difficult to regulate the cell structure.Second,many material modification methods have been used to improve the foaming ability of PLA,but these methods make against the biodegradation and reuse of PLA.Third,the requirements of high expansion ratio and high open-cell content on melt strength are in conflict to a certain extent,resulting in low expansion ratios of the existing PLA open-cell foams.Fourth,current researches basically do not involve the oil-adsorption performance of the PLA microcellular foam,and the relationship between the cell structure and the hydrophobicity or other performance of PLA foam has not been systematically studied.In this work,the preparation methods and oil-adsorption performance of high-expansion PLA open-cell foams were investigated based on the heating foaming process and cooling foaming process,respectively.Firstly,the crystal melting behavior and foaming mechanism of PLA in the heating foaming process were revealed,and a new method based on pre-crystallization and CO2-induced melting was proposed to prepare the layered foam and open-cell oil-adsorption foam.On this basis,a new process of introducing pre-isothermal cold crystallization treatment in the heating stage of heating foaming was proposed,the expansion ratio and oil-adsorption capacity were improved,and a green method to regulate the cell structure was established.Finally,a modified cooling foaming process was proposed to prepare high-expansion PLA open-cell foam with excellent oil-adsorption performance,and the PLA was further modified by blending with biodegradable poly(butylenes succinate)(PBS)to broaden the temperature range of preparing high-expansion open-cell foam.The main research contents and results are as follows:(1)Towards revealing the crystal melting and morphology evolution laws of PLA under high pressure CO2 in the heating foaming process and their influence on foaming behavior,the research was carried out by combining in-situ visualization technology,ex-situ characterization and foaming experiments.It was found that PLA can form layered and non-layered condensed structure under the action of high pressure CO2.A new method based on pre-crystallization and CO2-induced melting was proposed to regulate the layered and open-cell structure,and open-cell foams with good oil-adsorption performance were obtained.It was found that in the low temperature range,pre-crystallized PLA can form a layered condensed structure with a molten skin and a crystalline core under high pressure CO2.As the temperature increases,the width of the molten skin and the melting degree of the crystalline core gradually increase until the entire sample is completely melted to form a uniform non-layered molten structure.Based on the above crystal morphology evolution,layered and non-layered PLA foams were prepared at low temperatures and high temperatures,respectively.Specially,the low melt strength and high gas content under high pressures and high temperatures are more conducive to the rupture of cell wall,and then the non-layered open-cell foam can be obtained.The open-cell foam exhibits good hydrophobicity,lipophilicity and oil-adsorption performance.(2)Aiming at the problems of weak foaming ability and low expansion ratio of PLA,a new method of introducing pre-isothermal cold crystallization treatment in the heating stage of heating foaming process was proposed.The influence mechanism of pre-isothermal treatment on the cold crystallization,high-pressure rheology and foaming behavior of PLA was clarified.The preparation window of high-expansion foam was obtained,and the oil-adsorption ability of foam was improved.It was found that pre-isothermal treatment can endow PLA with higher crystallinity and melting temperature during the heating stage,which is beneficial to retain more crystals with high melting point in the CO2 saturation stage and enhance the viscoelasticity of PLA.Thus,pre-isothermal treatment can promote cell growth and foam expansion,improve the expansion ratio under various saturation pressures,and significantly widen the preparation window of high-expansion foam.After introducing pre-isothermal treatment,the maximum expansion ratio can be increased from 6.4 to 17.7.After introducing pre-isothermal treatment,the adsorption capacities of foam for various oils can be increased from 0.8-1.0 g/g to 2.6-6.6 g/g.(3)To regulate the expansion ratio and cell morphology of PLA foam,a green method based on pre-isothermal cold crystallization treatment and heating foaming process was established.The effects of pre-isothermal treatment conditions on the crystallization behavior before foaming,foaming behavior,and crystallization behavior of foam were clarified.A series of foams with tunable expansion ratio and cell size were obtained.It was found that,as the pre-isothermal temperature increases,the crystallinity and melting point before foaming gradually increase,and thereby the expansion ratio and cell size first increase and then decrease.With the pre-isothermal temperature increasing,although the crystals with high melting point in the CO2 saturation stage increases,the strain-induced crystallization effect directly related to the expansion ratio first strengthens and then weakens,causing the final crystallinity of foam to increase first and then decrease.As the pre-isothermal time increases,the crystallinity before foaming increases first and then becomes stable,making the expansion ratio,cell size and crystallinity of foam also show similar variation laws.By adjusting the pre-isothermal conditions,PLA foams with expansion ratios of about 2.4-10 and cell sizes of about 2.6-22.5 μm can be prepared.(4)Towards improving the expansion ratio of PLA open-cell foam and revealing the influence of cell structure on the hydrophobicity and other properties of foam,a modified cooling foaming process was proposed.The evolution mechanism of the cell structure of PLA foam with the process conditions in this process is clarified.High-expansion open-cell and closed-cell foams and a series of foams with tunable cell structure were prepared.The influence of cell structure on the hydrophobicity,thermal-insulation and compression properties of foam was revealed.And excellent oil-adsorption and thermal-insulation performance were obtained.It was found that the maximum expansion ratio in the modified cooling foaming process is obtained when the PLA is foamed in an amorphous state.Compared with the heating foaming process,the cooling foaming process shows significantly wider foaming temperature windows and higher expansion ratios.The open-cell and closed-cell foams can be obtained at high temperatures and low temperatures,respectively.Increasing the expansion ratio and reducing the cell size can improve the hydrophobicity of foam.The open-cell foam with an expansion ratio of 43 and an open-cell content of 97.9%was prepared,and the open-cell foam shows adsorption capacities for various oils of 10.9-31.2 g/g.The closed-cell foam with an expansion ratio of 60 was also prepared,and the closed-cell foam exhibits a thermal conductivity of 31.7 mW/(m·K).In addition,reducing the expansion ratio and cell size can improve the compressive performance of foam.By reducing the cell size,the compressive strength and modulus of foam can be increased by about 900%and 929%,respectively.(5)Aiming at the problems of high preparation temperature and narrow preparation temperature range of high-expansion PLA open-cell foam,a strategy of applying the modified cooling foaming process to prepare PLA/PBS blend foam was proposed.The effects of the PBS content on the phase morphology,thermal behavior,crystallization behavior and rheological properties of the blend was revealed.The influence mechanism of the foaming temperature and PBS content on the foaming behavior of the blend was also clarified.The purpose of reducing the preparation temperature of high-expansion open-cell foam and widening the preparation temperature range was realized,and excellent oil-adsorption properties were obtained.It was found that the PBS has a certain plasticizing effect on the PL A and can weaken the melt crystallization of the blend.The PBS can also reduce the melt viscoelasticity of the blend and work as weak points of stress to induce the rupture of cell wall,thereby endowing the foam with lace-like open-cell structure,widening the preparation temperature range of high-expansion open-cell foam,and reducing the cell-opening temperature.The prepared PLA/PBS open-cell foam shows an open-cell content of 98.2%,and exhibits an expansion ratio of 43.6,which is about 315%higher than that reported in literatures.The PLA/PBS open-cell foam possesses oil-adsorption capacities of 7.9-21.9 g/g,and its adsorption capacity for CCl4 shows no obvious attenuation during 20 adsorption-desorption cycles.更多还原