| Due to the environmental problems caused by the use of non-biodegradable plastics and shortages in petroleum resources, attention has gradually shifted to natural and biodegradable plastic alternatives which utilize environmentally friendly, renewable and natural polymer materials. Starch is a product of photosynthesis from plants. As the second most abundant renewable resource in the modern world, it is a powerful alternative to traditional non-biodegradable plastics due to its cost-effectiveness, widespread availability and biodegradability. However, the brittle quality and water sensitivity of starch limit its applications. Polyurethane, as a tough and hydrophobic material, can used to modify and effectively improve the properties of starch. In order to improve the compatibility of hydrophobic polyurethane and hydrophilic starch, we built on our previous work by using water as a plasticizer in combination with mixer or extrusion technology. Polyurethane micro-particles were cross-linked to starch matrix by urethane linkages. In addition, the effect of different polyurethane structures, starch structures, starch/polyurethane material interface processing methods and compatibility/performance relationships were studied through various characterization tests.This work was highly innovative in several respects. Firstly, based on our previous work, a simple, efficient and environmentally friendly method was used to prepare a series of starch/polyurethane materials joined by covalent bonds. A new theory of multifunctional polyurethane particles was thereby proposed. Secondly, the structure of soft segment of polyurethane was changed to synthesize polyurethane prepolymers. The influence of the structure of polyurethane prepolymers on the interface structure of starch/polyurethane and the subsequent performance of modified starch was then studied. Thirdly, the influence of different processing methods (mixer and extrusion) on the structure and properties of modified starch by the castor oil based polyurethane was studied. Fourthly, the amphiphilic modified maltdextrin with the highly-efficient performance (low molecular weight starch) was then prepared. Lastly, styrene-butadiene rubber (SBR) was used to synthetize polyurethane prepolymers and to prepare modified starch with high hydrophobicity and toughness.The main contents and brief conclusions of this paper can be summarized as follows:(1) effects of NCO content on the structure and properties of modified starch. Polyurethane prepolymers (PUPs) with various NCO content proportions were synthesized by castor oil and4,4’-methylenedi-p-phenyl diisocyanate (MDI) and then mixed reactively with a starch-water mixture to prepare modified thermoplastic starch (TPS) in an environmentally friendly manner. The subsequent results indicated that the grafting ratio of PUPs used for preparing the modified TPS was99.8%. It was therefore deduced that the addition of polyurethane prepolymers can obviously improve the hydrophobic properties of modified starch. As the NCO content of the PUPs increased, the amount of urethane linkage between PU microparticles and the starch matrix showed corresponding increases. In addition, improved compatibility was also seen between the two polymers, resulting in improvements in tensile properties. The amount of urethane linkage between PU and starch was thus pinpointed as an important factor for improving the compatibility and properties of modified TPS.(2) Effects of molecular weight of softsegment on the structure and properties of modified starch. Polyurethane prepolymers (PUP) were synthesized using different weight-average molecular weights (Mw) of poly-1,4-butylene glycol adipate diol (PBA). They were then used to prepare modified thermoplastic starch (TPS) in an intensive mixer. The decreasing molecular weights of soft segments in PUP led to increasing NCO content in PUP and better compatibility between the two polymers, resulting in improved tensile properties and decreased crystallization. Polyurethane prepolymers (PUP) synthesized by low molecular weight polyols for modifying thermoplastic starch (TPS) can thus be considered beneficial for improving compatibility between TPS and PUP and enhancing the properties of modified starch material.(3) Effects of hydroxyl numbers of polyols on the structure and properties of modified starch. Three different types of polycaprolactone (PCL) were used to synthesize polyurethane prepolymers (PUPs):polyols-PCL diol, PCL triol and PCL tertol. The obtained PUPs were then used to modify thermoplastic starch (TPS) in an intensive mixer. As the molar number ratio of isocyanate to hydroxyl group (NCO/OH) was2.0, the amount of NCO increased with an increase in OH group in polyols and the compatibility between polyurethane and starch was thus improved. The branched polymers were easily synthesized by polyols, and the cross-linking degree of polyurethane and starch therefore increased with the increase of OH group in polyol. As a result, the mechanical properties of modified starch were improved and thermal stability increased. It is worth noting that polyurethane prepolymers with high content will generally reduce the breaking strength of modified starch; however, PCL tetraol-based PUP were shown to simultaneously improve the tensile strength and elongation of modified starch due to the high degree of cross-linking in starch PCL tetraol-based PUP. Therefore, good compatibility and superior performance was achieved through the use of PCL tetraol-based PUP in the preparation of modified starch.(4) Effects of structure of polyurethane on the structure and properties of modified starch.. Rubber-based polyurethane prepolymer was prepared by styrene butadiene rubber diol to modify starch in an intensive mixer. Compared with polyester polyols, the rubber showed better hydrophobicity and toughness and could therefore create more significant improvements in the properties of starch. Polyurethane particles and starch had an obvious phase separation when the content of rubber-based polyurethane was10%and20%, which was inconsistent with the modified starch by polyester based polyurethane results. However, polyurethane showed good compatibility with starch when the content of rubber-based polyurethane was30%. Compared with pure starch, elongation at break for modified starch increased from2.0%to38.4%. Rubber-based polyurethane was therefore used successfully to prepare modified starch with excellent hydrophobicity and toughness.(5) Effects of molecular weight of starch on the structure and properties of modified starch. Maltodextrin is one of the hydrolysis products of starch. In this method, modified maltodextrins were successfully prepared from maltodextrin and castor oil based polyurethane prepolymer (PUP) in an intensive mixer. The results established the grafting ratio of PUP at99.6%. Good compatibility was achieved between the PU particles and maltodextrin matrix, which can be attributed to the urethane bonding interaction. Compared with raw maltodextrin, the modified maltodextrin exhibited improved strength, hydrophobicity and thermal stability. These properties were further improved with increased PU content. The modified maltodextrin emulsion was milk white, indicating that it was amphiphilic. Because the molecular weight of maltodextrin is low than starch, the modified maltodextrin emulsion has better stability than that of modified starch.Through this work, an efficient and environmentally friendly method has been provided for the preparation of amphiphilic maltodextrin.(6) Effects of processing technique on the structure and properties of modified starch. On the basis of previous mixer studies (solid-state reaction), castor oil-based polyurethane prepolymers were prepared through extrusion (continuous reaction) to modify starch. The effect of different processing methods (different residence time) on the structure and performance of modified starch was studied. The grafting efficiency of the polyurethane (PU) was determined at98.6%, indicating that all polyurethane prepolymers reacted with starch. In the process of the reaction in extruder, polyurethane (PU) formed a large amount of particles with NCO group under the action of strong shear force. The particles accelerated the reaction between starch and polyurethane, rendering the effect of residence time on graft efficiency inconsequential. Electron microscopy (SEM) results also demonstrated good compatibility between polyurethane particles and starch. The hydrophobic properties, mechanical properties and thermal stability of starch strengthened with increasing PU content. Due to the short retention time (1minute), the mechanical properties of modified starch prepared by extruder decreased in comparartion with the modified starch prepared by mixer.The method therefore proved highly efficient in the successful continuous production of modified starch, showing great potential for future application.In conclusion, a simple and efficient method has been proposed to prepare a series of modified starches with covalent bonds. The relationship between interface structure and the properties of modified starch was illuminated, providing new insight into methods for the preparation of modified starch with hydrophobicity and toughness. It is expected that there will be wide implications for further research and for the application of starch materials. |
