Sedayu, Bakti B (2020) Development of Semi-Refined k-Carrageenan-Based Films for Food Packaging Applications. PhD thesis, Victoria University.

Abstract

Carrageenan-based films exhibit some desirable properties for food packaging applications, but the high cost of source materials has been a considerable constraint for commercial development. This thesis presents a series of investigations designed to develop novel composite films for food packaging purposes utilizing a more economical and renewable raw material resource, namely semi-refined carrageenan (SRC). Additionally, the development and use of SRC as a biodegradable film-forming alternative to the current and commonly utilized refined carrageenan (RC) will also bring about benefits to the environment. Reinforcing the SRC with nanoclay and/or nanocellulose, lamination with poly(caprolactone), and surface photo-crosslinking are among the approaches used to enhance the SRC film properties. In the initial investigation, the preparation and characterization of SRC films plasticized with up to 50% (w/w) glycerol was performed using a solution casting method. The resultant film color and opacity increased with increasing levels of glycerol along with the moisture content, whereas the water vapor permeability decreased. The tensile properties of the SRC films improved significantly, particularly at glycerol additions greater than 30% (w/w). Moreover, the addition of glycerol improved the thermal stability and altered the surface morphology of the films. In general, the properties of the plasticized SRC films were comparable to those of refined carrageenan film counterparts. To overcome inherently poor water sensitivity and barrier properties of the SRC, film samples were reinforced with nanocellulose fibrils (NCF) and were benchmarked against similar films made from refined carrageenan (RC) with regard to the water sensitivity, physicomechanical and thermal properties. The level of NCF was varied from 1% to 7% (w/w) and, in general, the NCF reinforcement improved the overall properties of both the SRC and RC films including the water sensitivity and moisture barrier properties. Nonetheless, NCF inclusion in SRC film was less effective with regard to the mechanical and thermal properties enhancement compared with NCF inclusion in RC film. The enhancement in properties was attributed to the greater cohesiveness of the reinforced polymer structure and the crystalline regions formed in the structures of SRC and RC films by NCF incorporation. Semi-refined carrageenan film was also reinforced with nanoclay (NC) in combination with surface lamination using a thin layer of poly(caprolactone) (PCL) with a view to improving the barrier properties and hydrophobicity of the film and concurrently improving the mechanical properties. The water vapor permeability, moisture uptake, and water solubility decreased by 20%, 24% and 11%, respectively and the water contact angle increased from ca. 72° to 95° upon inclusion of the NC in the formulation. The tensile strength and elongation at break increased by 17.9% and 2.8% respectively, and the thermal stability also increased slightly. The PCL lamination was the main contributor to the enhanced barrier and mechanical properties of the films, whereas the NC inclusion contributed more to the enhanced thermal properties. In a further study, the surface regions of SRC film samples were photo-crosslinked with UV light of 0.46 W m-2 intensity using 6% (w/v) sodium benzoate solution as a photosensitizer. The surfaces were coated with the photosensitizer and exposed to the light source for 5, 10, 20 and 40 min. The effects of the surface crosslinking on the overall properties of the SRC films were investigated and related to the possible changes in the morphology of the substrate. The UV exposure and crosslinking were found to increase the crystallinity and the thermal stability of the films but with minimal colour changes. The mechanical properties were improved relative to the control sample with a ca. 42–55% increase in the tensile strength, ca. 142–144% increase in the modulus but with a concomitant ca. 50−52% decrease in the elongation at break. The crosslinking decreased the inherent moisture content in the films by ca. 50–52% and decreased the water vapour transmission rate by ca. 16–21% relative to the control. Changes in water sensitivity were observed with increases of ca. 35–44%, 18–22% and 21–22% in the water solubility, moisture uptake and water contact angle respectively. The latter increases were attributed to possible photodegradation products and the presence of residual photosensitizer that rendered the samples more hydrophilic. Under the conditions of the experiment, a UV exposure time of ca. 10– 20 min was found to be optimal in enhancing the mechanical and water barrier properties. These overall findings of the research have provided a most promising step towards the production of an economical food packaging material from SRC that has a minimal impact on the land environment.

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