Ethylene vinyl alcohol copolymer, referred to as EVOH. It is the alcoholysis product of ethylene-vinyl alcohol copolymer undergoing saponification reaction or partial saponification reaction. EVOH is a highly crystalline substance, but because of its high barrier properties, better moisture absorption properties, good heat sealing properties, excellent mechanical properties, etc., it is widely used in food and pharmaceutical packaging.
High barrier properties of EVOH
EVOH resin has a special segment structure, that is, the hydroxyl group in the molecule and the hydrogen bond between the molecules have a strong bond with each other, and its cohesion is relatively strong, so the degree of molecular chain accumulation is high, and the gas diffusion coefficient in it is small. . In addition, EVOH is a highly crystalline thermoplastic resin, and small molecules cannot penetrate through crystals or crystal defects. In addition, EVOH also has a certain molecular chain stiffness that a good polymer barrier material has, and the space formed by free movement between molecules is small. The hydroxyl group in EVOH is polar, while the oxygen in the atmosphere is non-polar. Due to the similar principle of compatibility, oxygen cannot penetrate the resin. Therefore, EVOH film can not only be used to package easily oxidized food to prevent outside oxygen from entering, but also can be used as a barrier layer for inflatable packaging to prevent the escape of gas. EVOH resin also has strong oil resistance and organic solvent resistance. After being immersed in various solvents and oils at 68°F for 1 year, the percentage of weight increase is: 0% for solvents such as cyclohexane, xylene, petroleum ether, benzene and acetone, and 2.3% for ethylene glycol. It is 12.2% for methanol and 0.1% for salad oil. When EVOH film is used to package oily substances such as meat and sausages, it not only prolongs the shelf life of the food, but also protects the food from oil spills due to its good oil resistance.
Moisture absorption performance of EVOH
Due to the presence of hydroxyl groups in the molecular structure of EVOH resin, EVOH resin is hydrophilic and hygroscopic. When moisture is absorbed, the barrier properties of the packaging film will decrease. In order to improve its moisture absorption and reduce the influence of moisture absorption on the barrier properties of the film, the ratio of ethylene and vinyl alcohol was changed when preparing EVOH. Within a certain range, increasing the mole fraction of ethylene can not only improve the moisture-proof performance of EVOH, but also will not affect its barrier properties due to moisture absorption, and it is also conducive to the processing of EVOH resin. In addition, it is also possible to compound EVOH with PET or OPP on the outer layer through a compounding method. The barrier properties of the compounded product can be retained, and the problem of moisture absorption performance can also be solved. For example, this compound is used in the packaging of milk, beverages, fruit juices, etc. Craft.
Heat sealing performance of EVOH
The heat-sealability of EVOH is poor. On the one hand, the heat resistance of the vinyl alcohol unit in the EVOH resin is not good. On the other hand, because EVOH is a highly crystalline resin, the crystalline part of the film will cause the film to lose its heat-seal performance. At the same time, the heat-sealing of plastics is formed by the violent movement of macromolecules in the film at high temperature, and mutual penetration and mutual diffusion under pressure. However, the intra- and intermolecular cohesion of EVOH is relatively large, and the movement probability is not large, which is not conducive to The melting of molecules. Therefore, the heat-seal layer containing EVOH film generally needs to be heat-sealed with PE, and the current common processes include multi-layer co-extrusion one-time molding or coating process.
Mechanical properties of EVOH
Because the molecular chain is not flexible, EVOH has better mechanical strength, elastic modulus and tortuous performance, and has certain mechanical strength. As the world’s environmental protection requirements become more and more stringent, in the field of plastic recycling, EVOH resin has more advantages.