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method is acid addition, by which a strong acid solution is used to obtain an alginic acid gel. 17 In both methods, the separated solid form of alginate is commonly converted to a sodium alginate solution by reacting with sodium carbonate and subsequently precipitated with an organic solvent such as ethanol. 18 One of the most remarkable properties of alginates is their ability to form gels through coordination with divalent cations ( e.g. ,Ca 2+ , Sr 2+ , andBa 2+ ) or trivalent cations ( e.g. , Al 3+ andFe 3+ ) that enables their vast industrial utilization in turn. This mechanism, commonly known as the egg-box model, describes in general two antiparallel alginate chains that form poly- electrolyte complexes (PECs) by electrostatically interacting with positively charged ions through their negatively charged carboxyl groups. 19 A common, simple way to fabricate alginate-based materials is wet spinning to form a bre-like shape and adding dropwise an alginate solution into a calcium chloride bath to obtain cross-linked beads. Another bre-forming technique, including electrospinning, has demonstrated considerable potential in biomedical elds such as tissue engineering, wound healing, and drug delivery. The use of electrospun alginates and other naturally derived biopolymers to create sca ff olds has the potential to exploit an area of active and promising research. 19,20 Kobayashi et al. 11 produced alginate bres and made a paper sheet from them and mentioned that paper sheets were successfully formed from alginate bres without any binder. This discovery is interesting because there was also a paper- making trend shi ing to the use of non-wood bre materials. In this study, brown algae consisting of alginate bres from S. polycystum and L. japonica were used as substitutional bre sources to complement wood pulp bres to produce composite paper. Previous research has extensively covered the use of wood pulp in paper production and the properties of alginate bres in various applications. However, studies on the inte- gration of calcium alginate bres with wood pulp for composite paper production are among the few cases. The resulting composite paper can enhance properties, improve biodegrad- ability, and reduce environmental impact di ff erently compared to conventional plastic-based materials. This research aims to explore the preparation and characterization of composite paper from marine-sourced (blue carbon) alginate bres and wood pulp bres (green carbon) to nd out whether paper composites can serve as a viable alternative to plastics in various applications, including packaging. By using alginate bres as a component of the composites, the optimum conditions were pursued to improve its proper- ties, which were determined using scanning electron micros- copy (SEM), tensile strength, thermogravimetric analysis (TG), X-ray di ff raction analysis (XRD), and X-ray photoelectron spec- troscopy (XPS) as well as mechanical strength tests.
matrix of brown seaweeds and can be extracted from them. The sequence of the saccharide units consists of b -(1 / 4)- D -man- nuronic (M) and a - L -guluronic acid (G) blocks in varying proportions. Alginates are responsible for the springiness and mechanical strength required by seaweeds to survive in the ocean. 5 Alginates are widely used in various industrial sectors, including the food, textile, medical, and pharmaceutical industries. 6 In addition, the ame-retardant properties typical of alginates can open up a wide range of potential applications, including re ghters and military apparel, domestic furnish- ings like padding, interior ttings such as thermal and sound insulation for walls, and structural coatings for wooden beams. 7 Despite these potential bene ts, the literature on the ame retardancy of alginates remains relatively limited, as most research has focused on their biomedical applications. 7 Research on alginates and other polysaccharides has increased over the past decade due to growing awareness of the need for sustainable materials to replace fossil resource-based products. 8 Alginate can be cross-linked to form a hydrogel in the presence of divalent ions such as Ca 2+ . Based on the ability to form gels with metal ions, various alginate-based materials with di ff erent morphologies have been developed, such as alginate hydrogel beads, 9,10 alginate bers, 11,12 and so on. In Indonesia, S. polycystum is classi ed as a brown seaweed that is widely distributed. However, its utilization remains low and it has a low market value in Indonesia. 13 On the other hand, in Japan, one of the popular seaweed products is the sea kelp knownas Kombu . These kelps are large seaweeds that belong to the L. japonica species of brown seaweed. Kombu is o enused in avoured dishes, soup stocks, boiled vegetables, snacks, or seasoning for rice. 14 It is also used for other dishes such as Kombu tsukudani , Kombu dressing, or Kombu chips, and nally for feedstu ff or non-edible use as a material. Both S. polycystum and L. japonica are categorized as brown seaweeds with a high alginate content. The rst step in extracting alginate from seaweed involves drying the seaweed. The dried seaweed is then cut into small pieces. Size reduction of dried seaweed promotes extraction e ffi ciency because smaller particles have a higher surface area, leading to an increase in contact with the solvents during the pre-treatment or extraction stages. 5 For the pre-treatment process, Andriamanantoanina and Rinaudo 15 bleached Sargassum sp. with sodium hypochlorite (NaOCl) and obtained white alginates because the bleaching process removes the colour pigments present in Sargassum sp. Acid pretreatment is usually conducted before alkaline extraction. In brown seaweeds, alginic acid is mainly present as a calcium salt although potassium, magnesium, and sodium salts may also be present. 16 A er conversion to alginate acid during acid pre- treatment, insoluble alginic acid is formed and possibly con- verted into a soluble form, such as sodium alginate by reacting it with sodium carbonate. Subsequently, a crude sodium algi- nate solution was obtained. There are two methods for producing alginate a er brown seaweed is extracted. One method is calcium chloride addition, by which precipitation is carried out with a calcium chloride solution to obtain calcium alginate as a precipitate. The other
2. Materials and methods 2.1 Materials
Dried brown seaweed ( S. polycystum ) and dried Kombu ( L. japonica ) were purchased from markets in East Java, Indonesia,
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