Strong bonding in biocomposites is achieved using bio-based adhesives and surface treatments that enhance interfacial adhesion between the matrix and reinforcement.

Polysaccharide-Based Binders

Polysaccharides are long-chain carbohydrates derived from natural sources like plants and seaweed. Common examples include starch, cellulose, guar gum, and alginates. Starch, extracted from sources like corn, potato, or tapioca, is widely used due to its abundance and adhesive properties. It can form strong hydrogen bonds upon gelatinization. Cellulose and its derivatives such as carboxymethyl cellulose (CMC) or methylcellulose are used for structural support and binding in composite materials. Seaweed-derived polysaccharides like agar and sodium alginate offer gelling capabilities and are used in lightweight, biodegradable composites. These binders are valued for their biodegradability and renewability, although their water sensitivity can be a limitation unless chemically modified.

Protein-Based Binders

Proteins from plant and animal sources serve as excellent natural adhesives. Soy protein is one of the most commonly used binders in biocomposites and wood adhesives. It is biodegradable and can be chemically modified to improve bonding and moisture resistance. Casein, a milk-derived protein, has historically been used in glues and paints and offers strong adhesion and water resistance. Wheat gluten, rich in protein content, forms cohesive and elastic films, making it a suitable binder when thermally or chemically treated. These protein-based binders are especially useful in packaging, construction, and agricultural composites, though they may be sensitive to pH and microbial degradation without proper treatment.

Lignin and Tannin

Both lignin and tannin are phenolic compounds derived from wood and plant extracts. Lignin, a byproduct of the paper and pulp industry, is thermoplastic and aromatic in nature, which allows it to act as a natural binder in biocomposites and fiberboards. It can improve water resistance and durability when blended with other components. Tannin, extracted from tree barks like mimosa or quebracho, is widely used in natural wood adhesives. It forms strong covalent bonds with proteins and fibers and shows good performance in humid environments. These binders are particularly suitable for structural and outdoor applications due to their natural durability and resistance to microbial attack.

Other Natural Binders

This category includes a diverse set of materials like natural latex, beeswax, plant resins, and chitosan. Natural latex, derived from rubber trees, is flexible, water-resistant, and used in coatings and flooring. Beeswax and other plant-based waxes provide hydrophobic properties and are typically used in surface treatments or packaging composites. Tree resins, such as pine resin, have long been used in varnishes and adhesives due to their sticky and durable nature. Chitosan, obtained from the deacetylation of chitin found in crustacean shells, is antimicrobial, biodegradable, and capable of forming cohesive films. These binders are often chosen for their specialty properties like flexibility, antimicrobial action, or environmental resistance.