Adhesives: Towards environmental protection

With the development of China's industrial and agricultural production and export trade, people's pace of life is accelerating, consumption levels are increasing, travel and tourism are increasing, and food consumption concepts and methods are changing accordingly. Compound bag food (also known as soft canned food) is adapting to people's daily needs due to its characteristics of light weight, convenience, long shelf life, hygiene, easy storage and transportation, easy disassembly, less waste and good shelf effect Favored by people.

Food composite packaging bags are made by compounding films with different characteristics with special adhesives and heat sealing them together to overcome the inherent defects of a single film. In order to protect the film from the erosion of food and other contents and to ensure the shielding and preservation effect, PE, PET, PP, NY film and aluminized plastic film or aluminum foil are often used. This type of composite film is widely used in the packaging of beverages, medicine, electronics, pesticides and cosmetics, and has become an indispensable composite packaging bag in modern industry.

Adhesive for composite packaging bags

In order to meet the requirements of the above food packaging bags, special adhesives play a vital role. It must meet high adhesion to various materials, resist erosion of different seasonings in food, fatty acids, fats and proteins and other media, high transparency, resistant to heating, pressurization, high temperature cooking and sterilization (100 ~ 135 ℃) and The instantaneous high temperature (220 ℃) ​​during bag making and sealing, fast curing speed and harmless requirements. If it is used in the production of cooking bags, the above properties are indispensable; if it is used in the production of general food bags, the performance requirements can be appropriately relaxed.

The excellent comprehensive properties of polyurethane adhesives make it the preferred variety of adhesives for composite bags, and it is used by more than 90% of food composite bags at home and abroad. According to information, in 2001, the polyurethane adhesive used for composite films in Europe was about 37,650 tons, accounting for almost 1/4 of the total amount of polyurethane adhesives in that year. In 2000, the polyurethane adhesive used for composite films in Japan was 28098 tons, accounting for about half of the total annual amount of polyurethane adhesives. The annual dosage of polyurethane adhesives used for composite films in China is about 30,000 to 40,000 tons, of which about 10% are retort-resistant adhesives. It is also reported that from 2001 to 2002, due to the sluggish international market, manufacturers of adhesives and sealants are on the verge of difficulties, but the sales of composite film adhesives for food and pharmaceutical packaging in the US market have always maintained an upward trend, and the Asian market Continued strong growth. It is estimated that in the next 5 years, the global average annual growth rate of adhesives for food composite film bags will be 4%, and China will be 15% to 20%.

Present status of adhesives for composite packaging bags in China

Since the 1980s, China has successively introduced and designed and installed 300-500 dry-process composite film production lines. The production of polyurethane adhesives has also developed rapidly, and the sales situation is gratifying. They are basically two-component solution systems, which can meet the production needs of domestic general-purpose composite films. Polyurethane adhesives for the production of aluminum / plastic composite films resistant to cooking at 121 ° C or 135 ° C can only rely on imports a few years ago. Adhesive suppliers in Japan, Germany, and the United States have seized the Chinese market and sold them at high prices. Such products. In recent years, certain domestic enterprises and research institutes have actively invested in manpower and material resources to research and develop several varieties of retort-resistant polyurethane adhesives, and achieved gratifying results. Liming Chemical Research Institute's RP-05 adhesive for retort-resistant food composite film is one of them. This type of polyurethane adhesive can withstand high temperature and resist the erosion of various food seasonings, fats and greases. After being tested by multiple dry laminating machines at home and abroad, the obtained three-layer PET / Al / CPP composite film has a bright and smooth appearance, without Tunnels, patterns, bubbles and white spots. The compound bags were filled with beef, chicken wings, etc., and were cooked and sterilized. This shows that the performance of the developed polyurethane adhesive for retort-resistant composite bags conforms to the compound process requirements of retort-resistant composite bags, and is comparable to the performance of corresponding imported products.

Development of environmentally friendly adhesives

Although the sample of the cooking bag made of the above-mentioned composite film adhesive has no small molecule precipitation; it is tested according to GB / T9683-88 and meets the food hygiene standards; the content of -NCO in the curing agent is less than 0.5%, which also meets the requirements.

However, it is still a two-component solvent-based polyurethane adhesive, which still needs to transition to an environmentally friendly adhesive. Foreign countries are actively developing solvent-free and water-based polyurethane composite film adhesives, especially the former is more active. DuPont, Henkel, Rohm & Haas, BASF, Dainippon Ink Chemical Industry and other companies already have products, but most of them are universal.

1. Solvent-free polyurethane adhesive

The enhancement of environmental awareness has prompted people to actively develop solvent-free polyurethane adhesives, which are generally two-component adhesives composed of polyols and polyisocyanates or polyisocyanate prepolymers with relatively low molecular weight and polyols or polyamines. The application of this type of composite film adhesive began in Germany in 1974. Japan introduced this type of technology in 1977. By 1997, dozens of solvent-free composite film production lines have been built. More than 50% of this type of adhesive has been used in Europe and America. The dry composite film industry has now developed to the fourth generation technology. Some high-performance varieties not only have good initial adhesion, but also are suitable for aluminum foil bonding. Existing companies have successfully developed high-temperature cooking-resistant polyurethane adhesives with special structures.

There are more than a dozen solvent-free composite film production lines in China. The Beijing Research Institute of Chemical Industry introduced the production technology of solvent-free polyurethane adhesive from Henkel Company of Germany in 1984, which can be used for the production of dry composite film. Since there was only one solvent-free compound machine in China at that time, it was not officially produced for a long time. Driven by environmental protection laws and regulations, in 1990, all parts of the country also introduced corresponding composite production lines. However, due to certain technical problems, the operating rate was insufficient.

The author believes that the development and application of solvent-free composite adhesives is undoubtedly a direction of adhesive development, but there are many problems. In summary, there are the following points:

(1) Although the amount of glue applied is small (only 1.5g / m2), its unit price is too high. The common glue is about 65 yuan / kg, and the production cost is high.

(2) In order to improve the wettability of the adhesive to the film, the viscosity of the adhesive is required to be low. The consequence is low initial cohesion, low adhesion, and insufficient curing speed, which affects the compound production efficiency. This is due to the low molecular weight of the polyurethane or polyester main agent, which is generally controlled at 3000 to 4000. If the relative molecular weight is increased, the viscosity is too high, it is difficult to uniformly coat the composite film, which greatly affects the quality of the composite film. The viscosity should generally be controlled at 2000 ~ 3000mPa.s. The viscosity can be reduced by increasing the coating and compounding temperature (> 40 ° C), but the safety of the polyisocyanate curing agent needs to be considered.

2. Water-based adhesive

Water-based adhesives are another type of environmentally friendly adhesives. Foreign countries have developed water-based acrylate adhesives, mostly using isocyanate as curing agent (using isocyanate as curing agent is considered to be a category of polyurethane adhesive). Because of its abundant raw materials, low price, high solid content and low viscosity in North America, it has successfully occupied a large share in the composite film market. But so far, there are still some problems with film-film composite water-based acrylate adhesives, such as:

(1) General technical performance, especially for low-end products on printed films. Because the general-purpose water-based acrylate adhesive compounded on the printed film has a low peel strength, of course, the peel strength also depends on the type of printing ink. Nitrocellulose-based inks are widely used in food packaging film. Unfortunately, it is very incompatible with general-purpose water-based acrylic adhesives. The composite film is easy to peel and its strength is only (0.1 ~ 0.5) N / 15mm. This phenomenon does not occur on solvent-based and solvent-free polyurethane adhesives, because when these two adhesives compound the film and migrate to the ink layer, they can still act as a binder for the ink, producing cohesion with the printed ink layer.

(2) Poor strainability, high relative molecular weight and low gloss of the adhesive film, high energy consumption during drying, and air bubbles in the composite film.

Hydrogen bonding and good cross-linking density are the two key elements to obtain excellent composite adhesion.

Due to the chemical nature, aqueous acrylates do not have strong hydrogen bonds, so most acrylate adhesives have low crosslinkability. However, single-component acrylate adhesives usually have high solids content, high shear strength, high mechanical fluidity and long pot life, which are unmatched by other water-based adhesives. In order to improve the adhesion of waterborne acrylate adhesives while maintaining its excellent properties, Rohm & Haas has developed a new two-component acrylic / urethane hybrid. Due to the formation of interpenetrating network structure and urethane reaction crosslinking point in the molecule of this system, it has superior cohesive strength, peel strength, moisture resistance, heat resistance and content resistance compared with single component acrylate adhesive ; Compared with polyurethane water-based adhesives, its bonding performance is equivalent or better. This superior performance, combined with its low price, makes it highly competitive in food composite film packaging applications.

There are also acrylic adhesives used in the production of composite films in China, but their performance is average. Improvement must be started from the chemical structure of the main agent.

Water-based polyurethane can also be used in the lamination of composite films. In order to improve its performance, domestic and international are developing and applying water-dispersible polyisocyanate process technology.

3. Other environmentally friendly adhesives

Other environmentally friendly adhesives for composite films include: UV curing and electron beam curing, moisture curing polyurethane hot melt adhesives and alcohol-soluble adhesives. Among them, alcohol-soluble polyurethane adhesives have been produced in Europe and the United States Industrial (Shenzhen) Co., Ltd. and other organizations, it is dedicated to plastic film / aluminum film.

Adhesives for general solvent-based polyurethane composite films in China have been mass-produced, and high-performance solvent-based polyurethane adhesives for composite bags that are resistant to 120-135 ° C cooking have also been successfully developed. Environmentally friendly adhesives such as alcohol-soluble polyurethane aluminized film composite adhesives, solvent-free polyurethane adhesives and water-based adhesives are all produced. However, due to price problems or quality problems, it has not been widely adopted like solvent-based adhesives. In the future, it is necessary to actively organize forces to start from the chemical structure of the resin itself and develop environmentally friendly composite film adhesives that meet various requirements.

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