Study on Adhesive for Food Flexible Packaging Composite Film

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 focuses on the selection of adhesives and the use of various adhesives.
First, the selection of adhesives for food flexible packaging films The choice of adhesives for food flexible packaging should consider a variety of factors, in general, as a composite adhesive should meet the following conditions:
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, foods containing salt, acid, spicy, spices, alcohols, water, sugar, fatty acids, etc., have different properties, but commonality may penetrate the adhesive layer through the inner layer of the composite film. Corrosion damage, resulting in delamination of the packaging bag, cracking 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, food flexible packaging film adhesive 1. Polyurethane adhesives 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 polar bond 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.
For general packaging composite film polyurethane adhesives, generally include dry food packaging adhesives and plastic food packaging adhesives that resist 70-100°C boil sterilization. This type of adhesive is a two-component ester-soluble polyurethane adhesive. Its main agent is a hydroxyl group-containing polyurethane polyol modified with aromatic or isocyanate by a polyether or polyester polyol. The curing agent is a combination of aromatic isocyanate and trimethylol propane. 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 JPU-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 common 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, high toxicity, and the large amount of solvent it emits pollutes the environment, and the composite material may hydrolyze over time, releasing a kind of TDA (aminoethane ethane diacetic acid), a carcinogenic substance, greatly jeopardizes the health of operators. Therefore, with the increasing awareness of environmental protection in China, the use of this type of adhesive has been declining. Nowadays, such adhesives are used in developed countries in Europe and America. The packaging has dropped sharply from 80% 10 years ago to 30% today.
2 Alcohol-soluble polyurethane adhesive Alcohol-soluble polyurethane adhesive uses low-cost industrial alcohol as a solvent, which has low production cost, good hygienic performance, harmless to workers, and no pollution to the environment, and is 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 a good transparency, insensitivity 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.
3 Waterborne 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 hazards when used, prompting researchers from all over the world to develop waterborne polyurethane adhesives for future use. The direction of development. Waterborne polyurethane (APU) is a binary colloid formed by dispersing polyurethane (PU) fine particles in an aqueous phase. It is composed of P. Schlack was first developed 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 lower than that of 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.
4 Solvent-Free Polyurethane Adhesives The solvent-free composite process for the manufacture of composite film packaging materials has become a well-known and mature application technology. Drying with a solvent-free adhesive, in addition to the advantages of using solvent-based adhesives for dry compounding, it also has the following features: no organic solvents are used, the cost is reduced; there is no environmental pollution caused by the volatilization of organic solvents; Energy consumption; Composite products have no residual solvent damage, and eliminate the solvent erosion of printing inks; No organic solvents, eliminating the composite substrate susceptible to solvent and high temperature drying damage, so that the composite membrane structure, good dimensional stability The equipment is simple and the floor space is small, saving investment.
For example, the solvent-free two-component polyester-castor oil polyurethane adhesive developed by China Aerospace Corporation 42 has selected polyester as the component of the solvent PU adhesive, and used low-cost and abundant sources of castor oil and toluene diisocyanate (TDI). ), 4,4 '-dibenzoyl diisocyanate polyaddition, the resulting isocyanate-based PU prepolymer as component B; the two components were mixed and made. This adhesive is characterized by: suitable viscosity, and can be adjusted by adjusting the polymer composition and molecular weight, its storage period of 1a or more; a variety of composite film materials, high bonding strength, high temperature (130 °C × 8h); cost Lower, the application prospects in food packaging are broad.
2. Thermoplastic Elastomer Food Bag Adhesive Guangdong Chaoan Jiangyi Chemical Co., Ltd. uses SBS (styrene-butadiene-styrene block copolymer) thermoplastic elastomer, rosin, orange aromatic sugar and various chemical raw materials to synthesize A new type of composite film adhesive, its main raw materials are SBS thermoplastic elastomer, rosin, phenol, formaldehyde, zinc oxide, glycerin, toluene, ethyl acetate, antifreeze, antifoaming agent, T90 resin (a terpene resin ). Its products are single-component packaging, easy to use, characterized by: 1 good performance, long service life. After bonding, boiled in boiling water at 100°C for 1 hour without peeling; in the 0°C environment, no hardening, no deinking, no effect on printing ink; 2 good drying performance, when the composite film is not heated, it can be completely after 24h Self-drying, high transparency, and suitable for various films; 3 non-toxic, no odor, no pollution, in line with food hygiene requirements, the preparation process does not produce harmful substances that pollute the environment.
3. Controllable and breathable packaging adhesive: A good gas barrier acrylic/chlorinated polyethylene latex or acrylic/chlorinated polyethylene/ethylene latex with a poor gas barrier acrylic/ethylene adhesive Mixing in different proportions and then coating it on a very breathable film (eg polyolefin) gives a composite film with controlled gas permeability. Adjusting the ratio of the two adhesives can be used both as a food package that requires low air permeability and as a fruit and vegetable package that requires high air permeability.
It can be seen that adhesives for food flexible packaging films have attracted much attention, and new types have also been increasing. At present, almost all manufacturers still use solvent-based polyurethane adhesives, although their excellent performance is outstanding, but because of its toxicity, have adverse effects on human health and the environment, it can not meet the social development requirements. While other new types of adhesives can eliminate toxicity to a certain extent, their performance and cost are far inferior to solvent-based adhesives. Therefore, the adhesive for this application needs the joint efforts of researchers to realize its multifunctionality and improve the stability of product performance.