Discussion on Adhesive for Food Flexible Packaging Compound Film (I)

Composite film packaging materials have advantages such as high strength, good airtightness, fireproof shading, heat-resistant sealing, etc. Since the advent of the 1950s, they have been widely used in the food industry. However, the quality of food flexible packaging is not only related to the nature of the composite film, but also depends on the nature of the adhesive used. This article explains the choice of adhesives and the advantages and disadvantages of many adhesives from the perspective of their adhesives. 
First, the choice of adhesives for food flexible packaging films
There are many factors to consider when choosing adhesives for food flexible packaging. In general, the following conditions should be met as a composite adhesive:
1 No odor, non-toxic; 2 No harmful extracts appear after packaging food; 3 Applicable temperature requirements for food storage; 4 Good weather resistance, no yellow foaming, no chalking delamination; 5 Oil, fragrance, vinegar and alcohol ; 6 does not erode the printing pattern ink, and has a higher affinity for the ink.
In addition, flexibility, operability, durability of adhesion, resistance to media corrosion, and hygienic safety cannot be ignored. For example, in terms of resistance to media corrosion, foods contain salts, acids, spicy, spices, alcohols, water, sugars, fatty acids, etc., and their characteristics are different, but one commonality may penetrate into the adhesive layer through the inner layer of the composite film. , Corrosion damage, resulting in delamination of the packaging, rupture and failure. For this reason, the adhesive must have the ability to withstand the erosion of the above substances and always maintain sufficient adhesive peel strength.
Second, adhesives for food flexible packaging films  1  polyurethane adhesive 
Polyurethane (PU) is a kind of block compound with oligo-polyol as soft segment, isocyanate and chain extender as hard segment. Its chemical structure contains urethane bond, ester bond, urea bond, etc. The key gives it excellent adhesion and film forming properties. However, different types of polyurethane adhesives also have shortcomings.
1 Ester-soluble polyurethane adhesives At present, there are about 1,000 composite film production lines at home and abroad. The vast majority use solvent-based polyurethane adhesives. The solvent-based adhesives here refer to ester-soluble adhesives. This adhesive has a long history. Mature technology, low prices. Almost 100% of the world uses polyurethane adhesive as a dry composite adhesive to make foods. It can be seen that polyurethane adhesives play an important role in food packaging.
Composite film polyurethane adhesives for ordinary packaging generally include dry food packaging adhesives and plastic food packaging adhesives that resist 70 to 100° C. boiling sterilization. This type of adhesive is a two-component ester-soluble polyurethane adhesive. The main agent is a hydroxyl group-containing polyurethane polyol modified with an aromatic isocyanate based on a polyether-polyester polyol, and the curing agent is a combination of aromatic isocyanate and trimethylolpropane. Ordinary polyurethanes cannot completely adapt to foods with corrosive media, such as salty, sour and spicy mustard, hot and sour dishes, and Sichuan cuisine. The new PU-500 food grade polyurethane adhesive has the advantages of high solid content, low viscosity, good initial adhesive strength, acid resistance, spicy resistance, salt resistance, and high peel strength, which can make up for a major defect of ordinary polyurethanes.
Composite film polyurethane adhesives for transparent cooking bags and aluminum foil cooking bags generally include composite film adhesives resistant to retort sterilization at 121°C and composite film adhesives resistant to high temperature retort sterilization at 135°C. In order to improve the retort resistance of polyurethane adhesives, it is usually to change the molecular structure of the soft segment polyester and hard segment, introduce hot and humid high temperature segments, change the type of crosslinker, adjust the density of the crosslinker, and even use high polymers. The co-mixing forms an interpenetrating polymer network approach to solve. For example, two companies in Japan have developed a series of cooking-resistant polyurethane adhesives. Polyester polyols with a high molecular weight synthesized by mixing dibasic acids and mixed glycols react with a curing agent and simultaneously add epoxy resins, phosphoric acids, and anhydrides. Compounds such as amines have greatly improved the high temperature resistance and water resistance of the rubber, and also improved the acid resistance and oil resistance of the rubber.
Although the ester-soluble polyurethane adhesive has good performance, mature technology, and many varieties, it still has some shortcomings, such as high cost and toxicity, and the large amount of solvent it emits pollutes the environment, and the composite material may hydrolyze over time as it releases. The carcinogen TDA (aminoethane sulfonic acid diacetic acid). It greatly jeopardizes the health of operators. Therefore, as the awareness of environmental protection in the country increases, the use of this type of adhesive is declining. Nowadays, the packaging of such adhesives in developed countries in Europe and America has dropped from 80% 10 years ago. Now 30%.
2 Alcohol-soluble polyurethane adhesive 醇 Alcohol-soluble polyurethane adhesive uses low-cost industrial alcohol as solvent, low production cost, good hygienic performance, harmless to workers, and no pollution to the environment, and is very suitable for domestic soft packaging production for export goods. New Oriental Group Huaguang Resins Co., Ltd. has developed and launched U-7750/H-100 alcohol-soluble polyurethane adhesive through nearly one year of research and development. The performance index has reached the requirements of practicability, safety, and hygiene. Alcohol-soluble adhesives can be cured at room temperature, have a good initial adhesion, and the later adhesive strength is higher, the composite film has good transparency, is insensitive to water vapor and low residual solvents.
Its deficiency lies in the fact that it cannot be used for cooking, it is not resistant to high temperatures of 100°C, and it cannot be packed with corrosive products such as pickled mustard. The biggest drawback is that the deployment of a good glue can only be used on the same day, if the remaining can only be diluted several times after the preservation, the next day production, as a diluent. If the glue has whitened or thickened, it cannot be used.
3Waterborne Polyurethane Adhesives At present, polyurethane adhesives are mainly solvent-based. Organic solvents are flammable, explosive, volatile, and have large odors. They cause air pollution and other public hazard problems when used, prompting researchers around the world to develop waterborne polyurethane adhesives as The direction of future development. Waterborne polyurethane (APU) is a binary colloid formed by dispersing polyurethane (PU) fine particles in the aqueous phase. It was first developed by PSchlack in 1942. It can be made into the form of water-soluble, latex or emulsion. In the production process of composite packaging, it discharges 100% of water vapor instead of harmful solvents and does not produce any odor. Therefore, it is generally welcomed by people.
American Rohm and Haas company's water-based adhesives are mainly single-component and two-component two major product lines, one-component water-based adhesives are mainly used in the expansion of food, biscuits, candy, condiments and other light packaging . The two-component waterborne adhesives break through the technical problems of peel strength, heat resistance and chemical resistance, and can be used for high-end products such as steamed foods and liquid packaging. Waterborne polyurethane adhesives are soft and tough, and are particularly suitable for the manufacture of laminated composite films for food packaging.
Advantages of waterborne adhesives are: Viscosity does not change significantly with changes in the molecular weight of the polymer, which can quantify polymer macromolecules to increase their cohesive strength; viscosity is generally greater than that of a solvent type; easy to mix with other resins or pigments to improve performance , Reduce the cost; Non-combustible, non-toxic, suitable for the substrate easily eroded by organic solvents; Easy to formulate and easy to clean. The disadvantages are: the drying process is slow, the energy consumption is large; the initial viscosity is worse than the solvent type; the cost is high and some properties are inferior to the solvent type.

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