2.2 Development Trend of Starch Adhesives
(1) The main development trend of starch adhesive is to modify it by adding polymer materials, so that the parameters such as water resistance, drying speed, initial viscosity and storage time are significantly improved.
(2) In recent years, the study of domestic starch adhesives has mostly adopted the method of adding some thickeners to form composite starch glues to improve the water resistance, adhesive strength and drying speed.
(3) In order to overcome the shortcomings of viscosity stability of liquid starch adhesive, not easy to exist for a long time, and inconvenient transportation, the research of solid starch adhesives is also widely carried out.
3 PVA Adhesives
Polyvinyl alcohol (PVA) is a common water-soluble polymer that is a white powdered resin derived from the hydrolysis of polyvinyl acetate. PVA has good adhesion to porous, hydrophilic surfaces such as paper, textiles, wood and leather surfaces. The wet tackiness of polyvinyl alcohol is commonly used and it is widely used as an adhesive after modification.
3.1 Comparison of PVA Adhesive and Starch Adhesive
Although the country stipulated in 1984 that the export of food packaging boxes should be switched to corn starch glue. However, starch adhesives have problems that cannot be ignored: (1) Product quality is not stable. Corn starch is generally oxidized by various units. The control of the process is difficult. Changes in the weather can easily lead to the formation of jelly and mildew, making some cartons of enterprises unstable. (2) Slow air drying and poor water resistance.
For many disadvantages of starch adhesives, PVA adhesives have the advantage of being popularized. First, the adhesive strength of PVA adhesives is better than that of starch adhesives, depending on their structure. Polyvinyl alcohol formula 
It is a simple straight chain, so that the molecular chain is soft and conducive to rotation, so the fluidity of the adhesive is better. Moreover, the hydroxyl group is on the outside and is easily combined with the cellulose in the paper to form a hydrogen bond, so that the adhesiveness is strong, so its adhesive strength is high. The molecular formula of starch is (C6H10O5)n, and there is a ring structure in the molecule, which makes the molecular chain inconvenient to rotate, so its rigidity increases and the adhesive has poor fluidity. And its hydroxyl is in the ring. Therefore, it is not conducive to the formation of hydrogen bonds between the hydroxyl groups in the adhesive and the paper fibers, so that the adhesion is not tight and the adhesive strength is relatively low.
Second, the production process is simple. Because: PVA adhesives affect the quality of factors, climate change is not. Again, PVA adhesive has high bonding strength, especially the initial adhesion is obviously better than other adhesives. Compared with ordinary starch adhesives. PVA adhesive has fast drying speed, strong adhesive force, good fluidity, strong antifreeze, and can hold liquid for more than two months. It is a composite carton adhesive with excellent comprehensive performance.
3.2 Development Trends of PVA Adhesives
Polyvinyl alcohol (PVA) is a kind of polyhydroxy polymer compound, its outstanding advantages are water-soluble, non-toxic and harmless, and raw materials are easily available, so it is widely used. However, PVA has poor water resistance. When the requirement is high, the bond strength is not enough. Therefore, how to modify PVA to ensure its water resistance is the development trend of PVA adhesives. The usual improvement is to use a chemical reaction with the -OH group on the PVA chain, for example: condensation with aldehydes; dehydration cross-linking with thermosetting resins; and the bifunctional compounds dihydroxymethylurea, glyoxal, oxalic acid, etc. Cross-linking reaction: Add crosslinker borax, boric acid, sulfate and so on. In recent years, PVA has been cross-linked with diisocyanate to greatly improve the bonding performance and water resistance of PVA glue, and has been successfully used in the wood processing industry.
Currently. The application of PVA adhesive in the carton industry mainly includes: (1) bonding between corrugated cardboard and (2) coating of corrugated cardboard.
4 Conclusion
Packaging adhesives can be roughly divided into two types, one for the production of packaging materials such as cardboard, corrugated board, and composite paper. The other applies to labels and seals. With the development of the packaging industry, the demand for adhesives will inevitably increase, so the types of adhesives will continue to change, the performance will continue to increase, and they will develop toward economical environmental adhesives.
Source: Packaging and Food Machinery