Preparation,Properties and Water Resistance of Starch-based Foams Materials

Author:Meng Ling Zuo

Supervisor:yu long


Degree Year:2019





Starch is one such biodegradable and sustainable polymeric material obtained from natural feedstocks with good plasticity and biocompatibility,which has been widely studied.From economy and environmental protection point of view,starch materials have showed great potential to replace common plastic products,especially in packaging applications.However,starch materials have well recognized weaknesses acting as package materials,including poor mechanical properties and moisture sensitivity.Therefore,improving the weakness of starch material will efficiently expand their application.In the latest decade,with the rapid development of E-commerce and online shopping,the internal buffer packaging of goods during the transportation was widely used.Hence,the replacement of the plastic buffer,which requires lower mechanical properties and water resistance,with starch foam materials have showed great promising.In this thesis,the producing process of starch foams material was designed and studied,and the action mechanism of the moisture in the producing process and the influence mechanism on the properties of starch foams materials are explored.Furthermore,the gas evaporation modification method was proposed for decreasing the water sensitivity of starch foams materials.In order to further prepare waterproof starch material,a high-efficiency interface binder bonds waterproof coating to the surface of the starch material.The relevant research and results are as follows:The two-step extrusion method was firstly desigened and used to prepare starch foams material,which solved the difficulty of transportation and storage for low-density foams materials.At the same time,the mechanism of water as a plasticizer and a foaming agent in starch foaming process was systematically studied for the first time.It was found that the formation of starch foams materials exhibited different process models under different water content.In the critical moisture content range from 15-18 wt%,the cell structure of the starch foams materials transformed from open cell structure to closed cell structure,which significantly affected the expansion ratio and mechanical properties of starch foams materials.The reason of the this transformation was that lower moisture content led to higher glass transition temperature(Tg)and lower plasticizing strength of the starch material.In that case,the starch material was easier to form bubbles with open cell structure.Conversely,starch material with higher moisture content possessed lower plasticizing temperature and higher plasticizing strength,which was easier to form bubbles with closed cell structure.Meanwhile,the bubbles with closed cell structure prevented vapor volatilization in the processing of foaming by steam.Hence,a negative pressure could be formed inside the bubbles,which caused the bubbles shrinking during the starch foams material cooling down.For the bubbles with open cell structure,vapor volatilized instantly leaving bubbles during the foaming,and then stable open cell foams material was formed after cooling down.The prepared samples with 67KPa compressive strength are enough to package and protect the commodities.In order to further expand the application of the starch foams materials,the gas evaporation method was specifically designed to enhance hydrophobicity and improve water sensitivity according to the character of starch foams materials.A large number of free hydroxyl groups on the surface of Nano-SiO2 could react with polyhydroxy starch to form Si-O-C bonds obtaining better compatibility with starch,and could react with the hydrophobic modifier hexamethyldisilazane(HMDS)by generating stable Si-O-Si bonds.It was found that 8 wt%Nano-SiO2 particles was enough to blend with starch and excessive Nano-SiO2 particles added caused self-aggregation.Furthermore,the water absorption of the starch material was obviously reduced after forming strong Nano-SiO2-starch network.In addition,three methyl groups were exposed on the surface of starch materials after the HMDS steam treating,which significantly enhanced the hydrophobicity of the composite.The contact angle(CA)of the composite increased to about 120°and the water sensitivity of starch materials were decreased significantly.Based on this study,the sensitivity of starch material to water could be further improved and then developed waterproof material.It is first time that polyethylenimine(PEI)was used to act as the interface binder bonding hydrophobic acrylate epoxy soybean oil(AESO)with hydrophilic starch materials,where AESO coating prevented water from touching the starch films directly,and a biodegradable waterproof starch material was prepared.In this system,PEI could rapidly penetrate into the starch and retain on the surface of the starch films due to the hydrogen bonds between starch and PEI.At the same time,PEI with long chains linked the AESO coating to the starch film via a Michael addition reaction.In single lap shear testing,the interface adhesive strength between the starch film and the AESO coating was significantly improved after treated with PEI.The sample treated with 0.8 wt%PEI showed a mode of cohesive failure between AESO coating and starch film.This adhesive strength exceeded 5MPa could even destroy starch bulk during the tensile testing.The target product of AESO/starch waterproof film could maintain the original properties after soaking into water for 3h.In order to enhance the water resistance and bonding strength of the interface between the AESO coating and starch material,the oxidized starch was added to form ionic bonds with PEI.Ionic bonds,on the one hand,could bond more PEI on the starch material surface and then link with AESO coating;On the other hand,the ionic bond between macromolecules would form a water-resistant block copolymer,which prevented water from destructing the interface between AESO coating and starch film,and the complete interface layer and waterproof coating also prevented the starch material from water absorption and swelling further.Furthermore,hydrogen bonds were found to provide strong enough adhesive strength between the AESO coating and the starch materials during lap shear testing.The addition of ionic bonds enhanced their bonding strength further.Meanwhile,the tight bond between coating and starch could improve the mechanical properties of waterproof starch material.In summary,water acted as both plasticizer and foaming agent in starch foams material and was distributed and balanced to prepare starch-foams material with better properties.In this process system,15-18 wt%was the critical moisture value for starch material foaming,where the foams structure changed from open structure to closed structure significantly affecting the foaming ratio and mechanical properties of starch foam materials.These results were important to guide the production of starch foams materials in future.Furthermore,the method of water resistance modification for starch material was proposed according to its own characteristics and waterproof mechanism.A small amount of PEI was used as an interface binder to bond the AESO coating on the surface of the starch film to produce AESO/starch waterproof material showing strong waterproof performance that expanded the application field of environment-friendly starch-based materials further.