Modiifcations Of Starch Nanocrystals And Application In Thermoplastic Starch Composites

In the last two decades, there have been greater efforts to develop biodegradablepolymers and products from renewable resources for replacing non-degradablepetrochemical-based materials. Thermoplastic starch (TPS) has received considerableattention because of its biodegradability, availability from renewable resources and low cost.Products of TPS can be manufactured using technology already developed for the syntheticplastics and have found applications to replace the synthetic plastics in some marketsespecially in packaging industry. However, the hydrophilic nature of starch leads tomechanical properties of TPS sensitive to humidity and poor moisture barrier. Thesedisadvantages hinder the applications of TPS materials which may be improved by addingreinforcing fillers with at least one nanoscale dimension (nanoparticles), formingcomposites.By submitting native starch to acid hydrolysis at temperature below the gelatinizationtemperature of starch, the amorphous regions in starch granules are hydrolyzed allowing theseparation of nanoscale crystalline residues. Because of its unique properties such as thenanoscale platelet morphology, intrinsic rigidity, high crystallinity and low permeability,starch nanocystals (SNC) have been used as ideal reinforcements to prepare TPSnanocomposites to improve mechanical properties and moisture barrier. In this paper, SNCobtained from acid hydrolysis of waxy maize starch were modified through crosslinking oresterification or dual modification of crosslinking and esterification to reduce hydrophilicity of SNC and adjust surface polarity of SNC. TPS nanocomposites were prepared by castingprocess using modified SNC as the fillers in glycerol-plasticized corn starch matrix. Themechanical properties and moisture barrier properties of the SNC reinforced TPSnanocomposites were optimized by controlling the degree of modification of SNC.SNC were successfully modified through crosslinking with sodium hexametaphosphate(SHMP), borax and glutaraldehyde (GA) in water at temperatures below the gelatinizationtemperature of starch. The crystalline structure of SNC was completely or partially preservedafter the crosslinking modification and the crosslinking reaction may only occur on thesurface of SNC. The ester groups were introduced onto the starch molecule throughcrosslinking with citric acid (CA). The crystalline structure of SNC was completelydestroyed after modification with CA aqueous solution, but the crystalline structure could bepartially preserved with the CA aqueous solution adjusting the pH to3.5or CA ethanolsolution. Transmission electron microscopy (TEM) showed that, after crosslinkingmodification, the morphology of SNC changed and the aggregation between SNC due to thehydrogen bonding significantly reduced. The SNC crosslinked with SHMP or borax could bewell dispersed in water. The SNC crosslinked with GA or CA could be well dispersed notnoly in water, but also in lower polarity organic solvents such as chloroform,dichloromethane. These results suggested that it is possible to reduce the hydrophilicity ofSNC and provide it hydrophobicity by selecting crosslinking agents with different functionalgroups to react with SNC.SNC were modified through esterification by using dodecenyl succinic anhydride(DDSA), octenyl succinic anhydride (OSA) and actic anhydride (AA) at temperatures belowthe gelatinization temperature of starch. The ester groups were introduced onto the starchmolecule and the crystalline structure of SNC was partially preserved after esterificationmodification. Esterification modification of SNC with OSA having shorter chain ofn-alkenyl group was more easily to yield a higher degree of substitution of hydroxyl groupsthan with DDSA at the same reaction conditions. After esterification modification, thepolarity of SNC reduced and SNC can be well dispersed in water and organic solvents suchas chloroform, dichloromethane and toluene, suggesting that the esterified SNC has amphiphilicity. Esterified SNC with higher degree of substitution of hydroxyl groups wasmore easily dispersed in organic solvents. Esterification modification of SNC with ASA wasbetter than with AA for reducing the hydrophilicity of SNC and the aggregation betweenSNC at similar degree of substitution of hydroxyl groups.The crosslinking SNC were further modified through esterification with AA, DDSA orOSA. The crystalline structure of SNC was partially preserved after the dual modification ofcrosslinking and esterification. Dual modification of SNC through crosslinking andesterification was better than single crosslinkng modification for reducing the polarity of theSNC, and was more easily to yield a higher degree of substitution of hydroxyl groups andmore effectively to reduce the the polarity of the SNC than single esterification modification.The addition of SNC significantly improved the tensile strength and Young's modulusof TPS composites. The addition of crosslinked SNC significantly increased the tensilestrength and Young's modulus of TPS composites and the elongation at break at75%RHand95%RH. Compared to SNC self-reinforced TPS composites, the tensile strength andelongation at break of crosslinked SNC reinforced TPS composites increased, but theYoung's modulus was almost unchanged. The addition of esterified SNC or dual modifiedSNC improved the tensile strength, Young's modulus and elongation at break of TPScomposites, but compared to SNC self-reinforced TPS composites, it led to a drasticdecrease of tensile strength and Young's modulus. The tensile strength and Young's modulusof dual modified SNC reinforced TPS composites were significantly less than those ofcrosslinked SNC reinforced TPS composites, but elongation at break was greater. Thepresence of SNC or modified SNC decreased the rate of water vapor transmission (WVT)and the water vapour permeability (WVP) of TPS composites. Moisture barrier properties ofdual modified SNC reinforced TPS composites were significantly better than crosslinkedSNC reinforced TPS composites and esterified SNC reinforced TPS composites. In the lowRH area, the equilibrium moisture contents of SNC self-reinforced TPS composites andmodified SNC reinforced TPS composites were very similar to those of TPS, but in the highenvironment, the equilibrium moisture contents of them were lower than those of TPS. Dualmodification was better than crosslinking modification and esterification modification for reducing the sensitivity of SNC reinforced TPS composites to ambient humidity.