Effects of Different Cross-linking Agents on Structures and Properties of Hydroxypropyl High Amylomaize Starch Films by Melting Extrusion

Author:Qin Yang

Supervisor:dong hai zhou


Degree Year:2019





The non-biodegradable characteristic of conventional petrol-based plastic has caused serious environmental pollution and ecological disasters,threatened the survival of mankind and sustainable development.Biodegradable plastics have been recognized as the most promising alternative to conventional petroleum-based plastics.Up to now,among the various of degradable plastic products,starch based degradable plastic is the most potential natural polymer material.Nevertheless,by comparing with petroleum-based plastics,the poor mechanical property and sensitivity to water limited the promotion and application of starch-based plastics.In this study high amylomaize starch,which could form a plastic film with better machinal property,was been chosen as a matrix and several advanced analysis methods,such as SEM、13C-SSNMR、ATR-FTIR、SLS/DLS、XRD,were been employed to study the effect of different cross-linking agents on structure and property of hydroxypropyl high amylomaize starch film by melt extrusion.The experiment clarified the differences of molecular structure,intermolecular interaction and crystalline structure between each hydroxypropyl high amylomaize starch films,revealed the effect of different cross-linking agents on the mechanical property of hydroxypropyl high amylomaize starch film and the improvement mechanism of cross-linking agents on the properties of hydroxypropyl high amylomaize starch films in melt extrusion,in order to provide theoretical and technical support for the development and application of high performance hydroxypropyl high amylomaize starch film.The main results were as follow:(1)Alkali-treatment had a obviously effect on the property of high amylomaize starch film,as there was a increase of the elongation at break value(EAB)of the high amylose starch film as the addition of NaOH increased which also lead the tensile strength(TS)decreased.The film with 2%NaOH had TS value at 10.03 MPa and EAB value at 40%;by adding 10%NaOH,HAS film had TS value at 7.69 MPa and EAB at 60%.(2)Adding 2%NaOH would be helpful for rearranging starch molecular chains to form an Eh-type structure that was a single helix structure with large inner space and making the molecular chains easily slide.Moreover,the recrystallization of high amylomaize starch was retarded by NaOH;Adding 10%NaOH could oxidize the hydroxyl group of starch into a multi-carbonyl molecule with severl small fragments which were formed by alkaline hydrolysis starch chain during melting extrusion.Moreover,the effect of 10%NaOH would cause cross-link between starch chains during melting extrusion.(3)The cross-linking reaction between citric acid(CA)and hydroxypropyl high amylomaize starch(HPAS)was achieved by reaction extrusion while preparing thermoplastic starch.This could short the steps from modifying HAS to preparing starch film and save production time,reduce energy consumption and improve production efficiency.(4)The HPAS-CA film which prepared at pH=5.66 had a TS value at 7.20 MPa,and EAB value at 94.53%.The TS value of the film had an obviously decrease as the pH decreased.However,lower pH condition was beneficial for preparing high water resistance HPAS-CA films as water vapor permeability(WVP)of the film prepared at pH=4.83 was 2.51×10-12g·m·m-2·s-1·Pa-1.(5)The CA would prefer neutralize NaOH during reactive extrusion at alkaline condition and promoted the gelatinization of starch.The weak acid condition would be more suit for esterification and forming and di-eater cross-linked structure between CA and HPAS which would reinforce the film;The acidolysis was enhanced as pH decreased which degraded starch molecule and lower the properties of starch film.(6)The type of crosslinking agent had a significant effect on the mechanical properties of hydroxypropyl high amylomaize starch-citric acid film.the HPAS-CA film modified by glutaraldehyde had TS value at 8.54 MPa and EAB at 69%;while HPAS-CA film modified by sodium trimetaphosphate had TS value at 6.42 MPa and EAB at 85%.(7)The glutaraldehyde had a aldehyde group on eachα-C which could form an“H”shapede cross-linked structure between starch chains.That would be conducive to the orderly arrangement of starch molecular chains and stable and stabilization the conformation of starch chain during the film-forming process and incresed its TS value.While the epichlorohydrin could form an“I”shaped starcuture or inter-molecular cross-linked structure that promoted irregular agglomeration of starch chains.The epichlorohydrin had a poor effect on starch enhancement compared with glutaraldehyde.However,sodium trimetaphosphate had an heterogeneous reaction with HPAS that decreased the properties of the film.(8)Hydroxypropyl methyl cellulose(HPMC)with different molecular wight reinforced HPAS-CA film while they decreased the EAB value dramaticly;and the TS of the films decreased while the molecular weight(Mw)of HPMC increased.The HPMC with Mw at 10,000lead the HPAS-CA film had the largest TS value at 10.34 MPa,and a lower EAB at 16%.(9)HPAS had a poor compatibility with HPMC during the extrusion.However,the HPMC with lower molecular weight maintained a linear crystal structure in HPAS-CA film which strengthen the TS valus as skeleton;And the HPMC with higher molecular weight preferd to random reunion and form amorphous with starch chain which would larger the space between molecular and decreased the TS value of films.(10)By adding appropriate additives,the molecular structure and crystal structure of high-amylose corn starch were destroyed during melting extrusion,and the molecular chain arrangement and crosslinking structure were formed,which was beneficial to the improvement of tensile strength of starch film.An optimal combination for preparing high performance biodegradable starch film was hydroxypropyl high amylomaize starch as base material,citric acid/glutaraldehyde as crosslinking agents,the HPMC with lower molecular weight as enhancer.