Natural adhesives are commonly used in our daily life. According to different sources, they can be classified into animal adhesives, plant adhesives, and mineral adhesives. Among animal adhesives, there are hide glue, bone glue, shellac, casein glue, albumin glue, fish bladder glue, etc.; among plant adhesives, there are starch, dextrin, rosin, gum arabic, natural rubber, etc.; among mineral adhesives, there are mineral wax, asphalt, etc. Due to their abundant sources, low cost, and low toxicity, they are widely used in furniture, binding, packaging, and handicraft processing.

Starch Adhesives
After entering the 21st century, good environmental performance of materials has become a major feature of new materials. As a non - toxic, harmless, low - cost, biodegradable, and environmentally friendly natural renewable resource, starch is increasingly widely used in various industries.
Especially in recent years, the production technology of the world's adhesive industry is developing towards energy conservation, low cost, harmlessness, high viscosity, and solvent - free. As a green environmental - friendly product, starch adhesives have attracted extensive attention and high - level emphasis in the adhesive industry. Regarding the application and development of starch adhesives, the starch adhesive made from oxidized corn starch has a promising future and is the most studied and applied. Currently, the main application of starch as an adhesive is in paper and paper products, such as sealing and pasting cartons and boxes, labeling, surface sizing, envelope gluing, and multi - layer paper bag bonding.
The following is an introduction to several common starch adhesives:

Oxidized Starch Adhesives
An adhesive made from a mixture of modified starches with low degree of polymerization containing aldehyde and carboxyl groups and water, which is gelatinized by heating or at room temperature under the action of an oxidant, is an oxidized starch adhesive. After oxidation, starch forms oxidized starch with water - solubility, wettability, and adhesive properties. If the amount of the oxidant is small and the degree of oxidation is insufficient, the total amount of new functional groups generated by starch decreases, resulting in an increase in the viscosity of the adhesive, a decrease in the initial adhesion, and poor fluidity. If the amount is too large and the oxidation is excessive, the viscosity and initial adhesion of the adhesive will decrease. The oxidation reaction time has a significant impact on the acidity, transparency, and hydroxyl content of the adhesive. As the reaction time prolongs, the degree of oxidation increases, the carboxyl content increases, the product viscosity gradually decreases, but the transparency becomes better.

Esterified Starch Adhesives
Esterified starch adhesives belong to non - degradable starch adhesives. They are obtained by the esterification reaction between the hydroxyl groups of starch molecules and other substances, endowing starch with new functional groups, thereby improving the performance of starch adhesives. Due to partial cross - linking of esterified starch, the viscosity increases, the storage stability is better, the moisture - proof and anti - mildew properties are improved, and its adhesive layer can withstand high and low temperatures and their alternating effects.

Grafted Starch Adhesives
The grafting of starch is to generate free radicals in the starch molecular chain by physical and chemical methods. When encountering polymer monomers, a chain reaction occurs, and a side chain composed of polymer monomers is formed on the main chain of starch. Taking advantage of the fact that both polyethylene and starch molecules have hydroxyl groups, hydrogen bonds can be formed between polyvinyl alcohol and starch molecules, playing the role of "grafting" polyvinyl alcohol and starch molecules. Thus, the prepared starch adhesive has better adhesiveness, fluidity, and anti - freezing properties. Since starch adhesives belong to natural polymer adhesives, they are widely studied and applied due to their low cost, non - toxicity, odorlessness, and no pollution to the environment. Currently, starch adhesives are mainly applied to paper, cotton fabrics, envelopes, labels, and corrugated cardboard.

Cellulose - based Adhesives
Cellulose Ether Derivatives as Adhesives
The main cellulose ether derivatives used as adhesives include methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, etc.
Ethyl cellulose (EC): It is a thermoplastic, water - insoluble, non - ionic cellulose alkyl ether. It has good chemical stability, is resistant to strong alkalis, has excellent electrical insulation and mechanical rheology, and has the characteristics of maintaining strength and flexibility at high and low temperatures. It is easily compatible with wax, resins, plasticizers, etc., and is used as an adhesive for paper, rubber, leather, and fabrics.
Carboxymethyl cellulose (CMC): It is an ionic cellulose ether. In the textile industry, CMC is often used to replace high - quality starch as a sizing agent for fabrics. Textiles coated with CMC can increase the softness, and the printing and dyeing performance is also greatly improved. In the food industry, various cream ice - creams added with CMC have good shape stability, are easy to color, and are not easy to soften. As an adhesive, it is used in the manufacture of pencils, cartons, paper bags, wallpaper, and artificial wood.
Cellulose Ester Derivatives
The main cellulose ester derivatives are nitrocellulose and cellulose acetate.
Nitrocellulose: Also known as cellulose nitrate, due to different degrees of esterification, its nitrogen content is generally between 10% and 14%. The one with a high content is commonly known as guncotton, which was once used in the manufacture of smokeless and colloidal gunpowders. The one with a low content is commonly known as collodion. It is insoluble in water but soluble in an ethanol - ether mixed solvent, and the solution is collodion. Since a tough film is formed after the solvent of collodion volatilizes, it is often used for bottle - mouth sealing, wound protection, and the manufacture of celluloid, the first plastic in history. If an appropriate amount of alkyd resin is added as a modifier and an appropriate amount of camphor is added as a toughening agent, it becomes a nitrocellulose adhesive, which is often used for bonding paper, cloth, leather, glass, metal, and ceramics.