| 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 ?m 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. |
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