Preparation Of SPEEK/PE Modified Atmosphere Composite Membrane For Lettuce Preservation

The loss rate of fresh fruits and vegetales in china is over 30%,preservation technology of fresh fruits and vegetables become the key technical problem to be solved urgently in the processing,storage and transportation of agricultural products.The modified atmosphere preservation technology can adjust the gas concentration in the storage environment to keep it at a suitable level for a long time,thus can prolong the storage period of fruits and vegetables.In this paper,sulfonated poly ether ether ketone(SPEEK)was prepared by post-sulfonation modification method using poly ether ether ketone(PEEK)as raw material.SPEEK modified atmosphere membranes with different sulfonation degree were prepared by solvent evaporation method and the influence rules of sulfonation degree on the microstructure,moisture permeability,gas permeability,hydrophilicity and light transmittance of SPEEK membrane were investigated.SPEEK/PE modified atmosphere composite membranes with different sulfonation degree were prepared by coating method,and the moisture permeability and gas permeability of composite membranes were analyzed.The SPEEK/PE modified atmosphere membrane with high sulfonation degree was applied to the preservation of lettuce.The main work and research results of this paper were as follows:(1)Study on modification of PEEK materials.SPEEK with different sulfonation degree was prepared by sulfuric acid post-sulfonation modification method and the material properties of PEEK before and after modification was studied.Results indicated that the degree of sulfonation increased first and then slightly decreased with the reaction temperature and time increasing.HNMR,FTIR,XRD Characterization results indicated that the presence of sulfonic acid groups on the backbone of the PEEK molecule.The hydrophilicity of SPEEK has been greatly improved.At 4℃,the degree of sulfonation had no effect on the hydrophilicity of SPEEK,and with the increase of temperature,the higher the degree of sulfonation was,the better absorption of water it performed.Compared with PEEK,the thermal stability of SPEEK material decreased,and weight loss started at about 200℃.But the solubility was greatly improved.It can be dissolved in some organic solvents and the SPEEK membrane can be prepared at room temperature,which reduced the difficulty of thermal processing of the material and the cost of membrane formation.(2)Preparation and gas permeability of SPEEK membrane.The SPEEK modified atmosphere membraen was prepared by solvent evaporation method and the microstructure and properties were characterized.Result showed the degree of sulfonation had no effect on the dense microstructure of the membrane,but had significant effect on the light transmittance The SPEEK membrane with different sulfonation degree had obvious difference in the permeability of CO2 and O2.When the degree of sulfonation was 1.15,the permeability coefficient of SPEEK membrane were 1.16×10-13 cm3·cm/(cm2·s·Pa)for CO2 and 1.54×10-14 cm3 cm/(cm2·s·Pa)for O2.By controlling the degree of sulfonation,SPEEK membranes which had different separation properties for CO2 and O2 can be prepared.The degree of sulfonation increased from 0.25 to 1.15,and the separation factor of the SPEEK membrane to CO2/O2 increased from 0.49 to 7.53.The degree of sulfonation of SPEEK had no significant effect on the permeation of water vapor.The permeability coefficient of water vapor of SPEEK membrane with different sulfonation degree reached 1.30×10-12 g cm/(cm·s·Pa)and the Moisture permeability was much better than the same type of membrane(3)Preparation and gas permeability of SPEEK/PE modified atmosphere composite membrane.SPEEK/PE composite membrane was prepared by uniformly coating different sulfonation degree SPEEK solution on the surface of PE membrane and the gas permeability was studied in detail.The permeability coefficient of PE membrane for CO2 and O2 were 4.51×10-12 cm3·cm/(cm2·s·Pa)and 1.53×10-12 cm3·cm/(cm2·s·Pa)respectively Compared with SPEEK membrane,the permeability coefficient of SPEEK/PE composite membranes with different sulfonation degree for CO2 and O2 decreased slightly.The separation coefficient of SPEEK/PE membrane for CO2/O2 was increased from 0.73 to 5.58 The permeability coefficient of PE membrane to water vapor was 2.43×10-14 g·cm/(cm·s·Pa),and the permeability coefficient of SPEEK/PE composite membrane with different sulfonation degree was less than 2.65×10-14 g·cm/(cm·s·Pa).Compared with the SPEEK membrane,the water vapor transmission coefficient was reduced by two order of magnitude SPEEK/PE composite membranes retained both the high separation factor of SPEEK membrane and the low moisture permeability of the PE membrane(4)SPEEK/PE Modified Atmosphere Composite Membrane for Lettuce Preservation.The high sulfonation degree SPEEK/PE modified atmosphere membrane was applied to the modified atmosphere preservation of lettuce,and their sensory quality and physical and chemical changes in the storage were analyzed.The composite membrane group showed the best sensory quality and still had good senses after 24 days of preservation.And it was followed by the ordinary PE group,in which yellow occurred on the 8th day and local decay occurred on the 12th day.While on the fourth day,the control group showed a large area of yellowing and severe water loss.The composite membrane can regulate the formation of low oxygen(4～5%,v/v)and high carbon dioxide(10～12%,v/v)gas environment and significantly reduce the respiratory intensity of lettuce.Compared with the normal PE group,the respiratory intensity decreased by 16.7%.On the 16th day,the weight loss rate of lettuce with the composite membrane package was 16.7%lower than the ordinary PE group.On the 9th day,the chlorophyll content of the lettuce in the control group was less than 0.024 mg/g,which has lost its edible value.The chlorophyll content of the composite membrane packaging was higher than the control group,and was 33%higher than the PE group.The high sulfonation degree SPEEK/PE modified atmosphere membrane had a good atmosphere preservation effect on lettuce,and the preservation period of the blank and ordinary PE group was extended by 12 days,20 days respectively.