Comparative Functionalization of Seaweed-Derived Polysaccharides: Esterified Agar–Phenylalanine as a Promising Candidate for Bio-Based Packaging

Plastic pollution has become a critical global issue, driving the demand for biodegradable alternatives. Seaweed-derived carbohydrates, such as agar and κ-carrageenan, are renewable, biodegradable materials with potential for biobased packaging applications. In this work, these polysaccharides were chemically modified with phenylalanine amino acids through amidation and esterification reactions to improve their hydrophobicity. Agar was successfully modified using both amidation or esterification methods. However, amidation required multiple steps and was a more complex functionalization process, resulting in a lower degree of substitution compared to esterification. An agar–phenylalanine derivative was synthesized via esterification and subsequently processed into films. These films were evaluated for their physical, mechanical, and barrier properties. Results showed that increasing the polymer concentration in agar–phenylalanine films improved tensile strength, stiffness, and flexibility. In contrast, higher glycerol content, introduced as a plasticizer, reduced strength and stiffness but increased polymer film flexibility, as indicated by greater elongation at break. The agar–phenylalanine films exhibited mechanical performance between flexible and rigid plastics, outperforming common flexible plastics such as HDPE and LDPE but below more rigid ones like PET, demonstrating their practical potential. The films were clear and suitable for packaging, and when applied as a coating on paper, they provided excellent water resistance. Coated paper showed strong liquid resistance against substances such as ketchup, vinegar, milk, hot coffee, water, and cola. These findings highlight the potential of esterified agar derivatives as promising candidates for eco-friendly packaging applications.