Development and characterization of starchpolyurethane hybrid materials for food packaging application
Creators: name: Tai, Nyok Ling
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 300604
subject: Food packaging, preservation and processing

description: Increasing awareness of negative impact of conventional plastic packaging and increasingly stringent environmental legislations are putting pressure on packaging industry to develop biobased biodegradable packaging materials. Due to easy availability, reasonably low cost and biodegradable nature of starch, packaging materials based on starch are attracting greater attention.  While starch is biodegradable and comes from renewable source; so far, there are no viable primary packaging is developed solely from starch. Packaging films produced solely from starch suffer from the inherent brittleness and poor water repelling properties. Hence, the physical and/or chemical structure and composition of starch must be modified to improve the physicochemical properties of starch materials before they can be used for as 'standalone' primary packaging. Starch-based biodegradable packaging materials can be developed using a range of synthetic, yet biodegradable polymers, such as polyurethane (PU).<br><br> Development of starch-polyurethane (starch-PU) hybrid materials is of greater research interest. This is because PUs can be specifically designed to make the biodegradable and also the flexible packaging produced from starch-PU hybrid materials possesses excellent mechanical properties. Also, chemical composition and physicochemical properties of PUs can be tailored, with relative ease, to increase their compatibility with hydrophilic starch. In legislative aspect, starch-PU hybrid and composite hybrid materials are approved by FDA for biomedical, pharmaceutical and food applications. In this context, the PhD study was aimed at developing starch-PU hybrid materials with suitable mechanical properties, hydrophobicity (water repellence) and biodegradability for packaging application by overcoming their incompatibility and microphase separation. <br><br>Despite above mentioned desirable properties, PUs are incompatible with starch as they are relatively more hydrophobic and lead to micro-phase separation and fail to yield desired properties.  This study hypothesises that by controlling the structure and function of PUs and applying different hybridisation methods (physical blending or chemical grafting), it is possible to overcome the incompatibility between starch and PU. The improved compatibility is expected yield starch-PU hybrid materials which can be readily converted into flexible packaging films. These packaging materials will meet property specifications required for packaging application at the same time they will be biodegradable. <br> <br>Initial focus of this study was to synthesise starch-PU hybrid materials using chemical grafting approach. Poly(ethylene glycol)-isocyanate (PEG-iso) crosslinker with end capped NCO end group was synthesised by reacting PEGs with hexamethylene diisocyanate (HMDI). PEG-iso crosslinker was covalently attached to starch. A good distribution or dispersion of starch and PEG-PU and strong covalent bonding between them was achieved. The introduction of PEG-iso crosslinker led to increased compatibility between starch and PU and resulted in improved hydrophobicity and flexibility of the starch-PU films. The synthesized starch-PU hybrid films were characterized to quantify the extent of miscibility, mechanical properties, hydrophobicity and establish structure-morphology relationship.  <br><br>This study further investigated the effect of molecular weight (Mw) of PEG-iso crosslinkers (Mw varying from 600 to 2050 Da) on the morphology and physio-mechanical properties of starch-PU films, with the aim to further improve the properties of starch-PU films. The results from this study showed that the Mw of polyols used in soft segments plays an important role in improving phase miscibility and physio-mechanical properties of the hybrid films. The phase miscibility was found to depend on the nature and extent of hydrogen bonding, degree of chain entanglement and crystalline structure of PEG-iso. The PEG-iso crosslinkers synthesised using PEG with 1000 and 1500 Da showed greatly improved miscibility with starch and provided a good mechanical and hydrophobicity. The degree of intermolecular hydrogen bonds between starch and PEG-iso crosslinker was quantified and their effect on the overall properties of starch-PU film was explained. The increase of hydrogen bonds indicated increase interactions and improve phase miscibility between starch and polyether urethane (PEG-PU) network.<br><br>Starch-PU hybrid materials were also produced using specifically designed physical mixing/blending process. A functionalised water dispersible anionic poly(ether-ester)urethane (AEEPU) was developed  by incorporating a mixed emulsifier containing Bis(hydroxymethyl) propionic acid (BMPA) and a small amount of  sodium dodecyl sulphate (SDS) in the formulation. AEEPU was then physically blended with starch. Due to water dispersible nature, AEEPU readily blended with starch. The compatibility or miscibility between starch and PU has very high due to physical chain entanglement and hydrogen bonding of ionic groups of AEEPU with starch. This increased miscibility aided by the ionic groups enabled better dispersion and intercalation of AEEPU into the starch matrix. The starch-PU hybrid materials produced by this physical blending/mixing produced films with transparency, hydrophobicity (contact angle) and mechanical properties comparable to that of low density polyethylene (LDPE).   <br> <br>The biodegradability of starch-PU films prepared by chemical grafting and physical blending/mixing methods were evaluated using ASTM D5988. The biodegradability, measured as carbon dioxide generation, of physically mixed and chemically grafted starch-PU hybrid films 72 % and 26 %, respectively. The biodegradability of starch (positive control) at the identical test condition was 86 %.   The biodegradation of starch-PU hybrid materials was found to depend on the degree of cross-linking caused by covalent linkages or physical entanglements, chemical groups present in the PU chains. The films produced by physically mixed starch-PU hybrids had weak physical bonding interaction between the starch and PU chains and also had higher starch content, all of which favoured increased water absorption and microbial action. The presence of polyester in the soft segment of AEEPU also contributed to the increased biodegradation. The filmed produced from these starch-AEEPU hybrid materials showed promised as biodegradable flexible packaging. <br><br>This PhD study probes the fundamental reasons that cause incompatibility between starch and PU proposes mechanisms through which it can be overcome. The advances made in this underpinning science enabled development of two (physical blending and chemical grafting) important technologies to produce starch-PU hybrid materials and flexible films from these materials.  Furthermore, this study contributes to the fundamental understanding of structure-morphology-property relationship and biodegradability of starch-PU hybrid materials films produced using both physical blending and chemical grafting approaches. The chemical composition and structural features of specifically designed PUs documented in this thesis will provide new avenues for developing novel hybrid materials of other biopolymers (e.g. cellulose and lignin) with PU.