Modifications Of Microcrystalline Cellulose And Its Application In Thermoplastic Starch Composites

Microcrystalline cellulose（MCC）has advantages of low cost，low density，high elasticrenewable and biodegradable，it can be used as a reinforcement to prepare composites.Thermoplastic starch（TPS），as a renewable，inexpensive and biodegradable natural polymer，is one of the most promising biodegradable materials. But the mechanical properties of TPS issensitive to the humidity of environment，which limits its applications. When MCC was usedas a reinforcement，the properties of MCC/TPS composite can be improved. However，MCCtends to self-aggregate due to the hydrogen bonding ineraction from the surface carryinghydroxyl groups，which causes uniformly dispersion of MCC in TPS matrix and hinders theimprovements of mechanical properties，water resistance and water barrier properties ofMCC/TPS composites.In this study，MCC was treated by mechanical ultrafine processing and sulfuric acidhydrolysis， and modified with soybean oil through esterification to improve thehydrophobicity. The effects of reaction time，soybean oil concentration were investigated inorder to control the degree of the modification. The modified MCC was used to prepare MCCreinforced TPS composites. The effects of particle loading， particle size and surfacemodification on the mechanical properties and water barrier properties were investigated. Thechanges of mechanical properties and water barrier properties to the content and size of MCCwere obtained to provide some essential scientific date for applications.Laser particle sizer was used to analysis the size and distribution of MCC. The resultsindicated that the mechanical ultrafine processing and sulfuric acid hydrolysis reduced thesize of MCC. The size of MCC decreased with increasing of rotation speed during mechanicalultrafine processing and prolongation of treating time. But after60min treatment，the size ofMCC decreased very slowly.The esterification mofification of MCC was characterized by FTIR spectroscopy. The results indicated that MCC was modified with soybean oil through esterification. The heatingtime and soybean oil concentration did not have a significant effect on the esterification extentof MCC at110oC. The MCC obtained from mechanical ultrafine processing was easier to beesterified with soybean oil. The washing experiment in boiling water showed that thepreparation of the MCC/TPS composites would not damage the esterification of MCC.The crystalline structure of MCC was characterized by X-ray diffraction (XRD). Theresults indicated that the mechanical ultrafine processing and sulfuric acid hydrolysis did notdestroy the crystalline structure of MCC. XRD measurements aslo confimed that theesterification modification just occurred on the surface of the MCC，because there was noalteration in crystalline structure of MCC after the modification.The surface polarity of MCC was characterized by wettability experiments. The resultsindicated that the esterification modification affected the wettability of MCC. Afteresterification，the MCC obtained from mechanical ultrafine processing displayed a higheraffinity for low polar solvent. With increasing of esterification reaction time，the MCCobtained from mechanical ultrafine processing displayed a higher affinity for lower polarsolvents.The mechanical properties，water absorption behaviour，water barrier properties andwater contact angle of MCC/TPS composites were measured. The results suggested that as thecontent of MCC increasing，the tensile strength and Young’s modulus increaseed and theelongation at break decreased. The modified MCC aslo improved the tensile strength andYoung’s modulus of the composites. For the composites with esterification modified MCC，the tensile strength increased at first and then decreased，the Young’s modulus increased andthe elongation at break decreased as the content of the modified MCC increasing. The resultsshowed that the Young’s modulus was largely dependent on the content of MCC and was notremarkablely influenced by the modification of MCC. Addition of MCC and modified MCCreduced the water absorption of the composites，and the water absorption of the compositesdecreased as the content of MCC increasing. Addition of MCC did not influence the watercontact angle of the composites remarkably. The size of MCC had a significant effect on theYoung’s modulus and water barrier property of the MCC/TPS composites.