Forming Technology And Properties Of Biomass Composites With Fully Open Cell Structure

Plastics is difficult to be biodegrade under natural conditions as its higher molecular weight and resistance to acids and alkalis.Plastic products are widely used in industry and household.The continuous accumulation of plastic wastes has brought serious environmental and ecological problems,hindering the sustainable development of the world.To deal with the environmental problems caused by plastics,a variety of response plans have been launched at home and abroad,including starch-based plastic products,polylactic acid products,and fully degraded biomass products.The raw materials of biomass products are renewable,and the products can be biodegraded in a short time.The development of biomass products that can replace plastic products has significant meaning for environment protection.The foaming material has light weight and good buffering,heat insulation,sound insulation and other properties.It is of great significance to use starch and plant fibers as raw materials to prepare biodegradable products with cell structure.In this paper,starch and plant fibers are used as mian materials,with the foam molding to prepare a green,light,and fully open cell structure biomass product with excellent performance.However,due to the insufficiency of the properties of biomass raw materials,the immature foaming process and equipment make it difficult to prepare biomass products with good performance.There are many technical problems to be solved in the preparation of biomass products.1)In term of raw materials,the phenomenon of starch retrogradation leads to poor mechanical properties of starch-based materials,which is unable to meet the requirement in its industrial applications.Poor compatibility between plant fibers and other polymer materials limits the application of plant fibers as high-strength biomass products prepared by strengthening systems.2)In foaming process,the mechanism of the growth and rupture of the bubbles in biomass products is not clear.And the dynamic change mechanism of the temperature field and pressure field of the open bubbles in the products at different growth stages is not clear.3)For foaming equipment,the mold is impossible to reserve foam space for bubble growth in the fixed direction,which limits the free growth of bubbles in the material and affects the properties of materials with open cell structure.4)In term of the product performance,the surface of biomass products is rough,with more gaps.And the biomass product has hydrophilicity,resulting in poor waterproof performance.In view of the above problems,the thesis focuses on the modification process of raw materials,the molding principle of products,the molding equipment of products,and the performance improvement of products.The forming process and performance of biomass composite materials with fully open cell structure are systematically studied.The thesis originates from the National Natural Science Foundation of China,"The mechanism and performance regulation of fully open cell structure of plant fiber/starch heterogeneous material products".The main research contents of this paper are as follows:The first part is starch plasticization-oxidation cross modification technology for starch regeneration(It is the research content in chapter 2).Aiming at the problem of starch regeneration,scholars have proposed various starch modification methods,including starch plasticization,oxidation,esterification,etherification,etc.The single modification of starch has not solved the problem of starch regeneration.Unlike the single starch modification process,the research content of this chapter introduces the oxidation process to the starch plasticization process.Plasticization-oxidation cross modification process for starch modification is proposed.The crystalline area of natural starch contains a lot of supercoiled structures.Under the action of high-temperature water,the supercoiled structure breaks the hydrogen bonds in the coiled structure,causing it to unwind,resulting in disorder.After the heat and water are lost,the starch molecules retrograde and reassemble It is an ordered crystalline structure.Different from the single plasticizing modification process of starch,this work introduced the oxidation process into the plasticizing period of starch.During the oxidation process,the electrons in the starch molecules were migrated,which resulted in the recombination of the hydrogen bond structure in starch.The crystalline region of natural starch contains a large number of supercoiled structures.The supercoiled structure disaggregates under the action of high-temperature water.The reorganization of starch hydrogen bonds makes the supercoiled structure disorganized after repolymerization.The area is reduced so that the starch recrystallization process is suppressed.The mechanical properties and waterproof properties of modified starch-based composites were studied.The effects of the modified starch microstructure and crystallinity changes on the macroscopic properties of the materials were also studied.The results showed that the crystallinity of modified starch was decreased significantly,from 42.5%to 0.8%.And the crystal area of starch was reduced,the amorphous area was increased,which benefitted combination of the plant fiber and starch.The mechanical properties of the composite got up,the tensile strength of material increased from 4.5 MPa to 7.9 MPa,the compressive strength increased from 5.7 MPa to 11.6 MPa.The second part is about compatibility between the cross-modified plant fiber and the starch(It is the research content in chapter 3).The outer surface of the plant fiber contains a large amount of wax,pectin and other substances,which makes it difficult to combine the plant fiber with other polymers.In addition,the polar functional groups contained in the plant fiber lead to incompatibility between plant fiber and starch.The alkali treatment of plant fiber can remove waxy pectin and other substances on its surface,which is beneficial to the combination of plant fiber and other polymers,but the alkalization process will cause environmental pollution.This work utilized the cross-modified plant fiber with hydrogen peroxide and urea at low temperature to achieve the compatibility between plant fiber and starch.In the natural environment,hydrogen peroxide can be decomposed into oxygen and water,and urea can be absorbed by plants for photosynthesis.The entire process is green,which avoids environmental pollution caused by high alkali treatment.The functional group changes of modified plant fibers were studied.The hydrogen bond model between modified starch and modified plant fiber was established.Their compatibility was verified by observing the microscopic characterization of modified plant fibers and modified starch composites.The modified plant fiber and modified starch were mixed to prepare the biomass composite with good mechanical properties.Compared with traditional packaging materials(EPE,EPS,corrugated board),the mechanical properties and greenness of the above composites were better than those of traditional packaging materials.The third part is the theoretical study on the bubble rupture of plant fiber/starch biomass composites(It is the research content in chapter 4).Bubble rupture is an inevitable stage in the formation of open cell structure,revealing the bubble wall rupture mechanism is beneficial to achieve parameterized control of open cell structure.Different from the traditional nuclear pore model and solute transport model,which reveal the mechanism of bubble wall rupture from the perspective of energy and wave,this part proposed the "local thinning-ultimate stress"model from the perspective of stress to reveal the bubble wall rupture mechanism of plant fiber/starch heterogeneous polymer.Based on the momentum balance equation of bubble growth,the mathematical model of ultimate stress was established.The elastic deformation leads to local thinning,and the local stress of starch bubble wall increased with the thinning of bubble wall.When the ultimate stress breaks the bubble surface tension,the bubble bursts.Different proportions of biomass slurry were prepared.The variable viscosity of slurry was controlled.And the bubble size of bubble rupture model was analyzed under different viscosities.The experimental values were compared with the calculated results of the above rupture model.The results showed that the calculated value was consistent with the experimental value.The "local shrinkage-stress limit" model analysis shows that the larger the viscosity,the larger the bubble radius when the bubble bursts.As the viscosity increases,the increasing trend of the bubble radius slows down,and the size of the heterogeneous structure has a great effect on the bubble burst.The smaller the foreign particles,the easier it is for the bubbles to burst.The fourth part of the research content is the unilateral cavity free foaming method and equipment(It is the research content in chapter 5).To meet the foaming process of plant fiber/starch biomass composites,innovative one-piece and distributed single cavity freestyle foaming method.And unilateral freestyle foaming mould equipment was developed.The foaming mould automatic springback after the composite slurry modeling.Large amounts of water vapor released as a source of pressure to control flexibly bubble growth inside the biomass materials.Foaming pulp cavity was formed upper the biomass composites,to realize the bubble growing freely and dip controllable pore size.At the same time,due to the diversity of the composition and distribution of biomass slurry,different foaming spaces are required for the growth of different compatible biomass materials,and the size of the reserved cavity in the upper part of the mold can be controlled.The matching of the compatible components of the foaming cavity and the biomass material is beneficial to the preparation of different biomass materials with excellent performance.At the same time,a certain pressure field is formed in the foaming cavity due to the gas overflow,which avoids the rapid growth of the bubbles and quickly merges and penetrates,thereby achieving flexible control of the foaming process.The fifth part is to improve the surface performance of biomass composites with fully open foam structure(It is the research content in chapter 6).Biomass materials have the characteristics of a sharp decline in mechanical properties after water absorption.There are a large number of holes,and moisture in the environment easily penetrates the surface of the biomass material with fully open cell structure,thereby affecting the performance of the biomass products.In order to further improve the surface performance of biomass products,high performance biomass composite films containing lignin were prepared.The biomass film was coated on the surface of the biomass composite material.At the same time,nanocellulose,as a dense enhancer,enhances the mechanical properties and surface performance of the biomass products.The above advanced biomass technology to prepare excellent performance biodegradable biomass products as replacement of plastic products is meaningful.It is of great significance to alleviate the environmental pressure caused by the accumulation of plastic waste and to realize sustainable development of the world.At the same time,the mentioned methods for modifying biomass materials such as starch and plant fibers can provide technical reference for the modification treatment of other biomass materials.The mentioned heterogeneous foaming theory and material foaming molding process can provide theoretical reference for the research work of other foaming materials.