The food composite packaging bag is made by heat-sealing different films with special adhesives to overcome the inherent defects of a single film. In order to ensure that the film is free from the erosion of food and other contents and to ensure the preservation of the screen, PE, PET, PP, NY film, aluminized plastic film or aluminum foil are often used. This type of composite film is widely used in the packaging of beverages, pharmaceuticals, electronics, pesticides, and cosmetics, and has become an indispensable composite packaging bag for modern industry.
Composite packaging adhesive
In order to meet the requirements of the above food packaging bags, special adhesives have played a vital role. It must meet high adhesiveness to various materials, resistance to different media such as seasonings, fatty acids, oils and proteins in foods, high transparency, resistance to heat, pressure, and high-temperature retort sterilization during food processing (100 to 135°C) and The instant high temperature (220°C) during bag sealing, fast curing and harmless requirements. If used in cooking bag production, the above properties are indispensable; if used for general food bag production, performance requirements can be appropriately relaxed.
The excellent comprehensive performance of polyurethane adhesive makes it the first choice for adhesives for composite bags. It is used in more than 90% of food composite bags at home and abroad. According to information, in 2001 European polyurethane adhesive for composite films was approximately 37,650 tons, which accounted for almost 1/4 of the total amount of polyurethane adhesive used in the year. Japan's polyurethane adhesive for composite films in 2000 was 280,98 tons, accounting for about half of the total annual amount of polyurethane adhesives. The annual amount of polyurethane adhesive used for composite films in China is about 30,000 to 40,000 tons, of which about 10% is resistant to cooking. According to another report, from 2001 to 2002, due to the sluggish international market, manufacturers of adhesives and sealants were on the brink of trouble. However, the sales of composite film adhesives used in food and pharmaceutical packaging in the US market have always maintained an upward trend. The Asian market is even more Continued strong growth. It is estimated that in the next five years, the annual average growth rate of adhesives for food composite film bags will be 4%, and China will be 15% to 20%.
The Present Situation of Adhesive for Compound Packaging Bags in China
Since the 1980s, there have been 300 to 500 dry film production lines that have been introduced and self-designed and installed in China. The production of polyurethane adhesives has also grown rapidly. The sales situation is gratifying. They are basically two-component solution systems that can meet the production needs of domestic general-purpose composite films. High-performance varieties such as polyurethane adhesives used in aluminum/plastic composite films for cooking at 121°C or 135°C can only rely on imports for several years. Adhesive suppliers in Japan, Germany, and the United States have seized the Chinese market and sold them at high prices. This type of product. In recent years, some domestic enterprises and research institutes have actively invested human and material resources in the research and development of several successful varieties of cooking-resistant polyurethane adhesives and have achieved encouraging results. Liming Chemical Research Institute's RP-05 retort food composite adhesive is one of them. This kind of polyurethane adhesive can resist high temperature and resist all kinds of seasonings, fats and greases. It has been tested on many dry laminating machines from home and abroad. The resulting PET/Al/CPP tri-layer composite film is bright and smooth. Tunnels, patterns, bubbles and white spots. The compound bag was actually retort and sterilized by loading beef, chicken wings, etc., respectively, and was all broken. This shows that the properties of the polyurethane adhesive used in the retort-resistance composite bags are in line with the composite process requirements of the retort-resistance composite bags, and are equivalent to those of the corresponding imported products.
Development of environmentally friendly adhesives
Although the sample of the cooking bag made by the above composite film adhesive was detected without the precipitation of small molecules, the inspection according to the provisions of GB/T9683-88 complied with the food hygiene standards; the content of -NCO in the curing agent was less than 0.5%, which also met the requirements.
However, it is still a two-component solvent-based polyurethane adhesive, and there is still a need to transition to environmentally friendly adhesives. Foreign countries are actively developing solvent-free and waterborne polyurethane composite film adhesives, especially the former is more active. Companies such as DuPont, Henkel, Rohm & Haas, BASF, and Dainippon Ink Chemical Industry have products, but most of them are universal.
1. Solvent-free polyurethane adhesive
Increased environmental awareness has prompted people to actively develop solvent-free polyurethane adhesives, which are generally two-component adhesives composed of a relatively low molecular weight polyol and a polyisocyanate or polyisocyanate prepolymer and a polyol or polyamine. The application of this kind of composite film adhesive began in Germany in 1974 and was introduced in Japan in 1977. By 1997, dozens of solvent-free composite film production lines had been built. More than 50% of these adhesives have been used in Europe and the United States. The dry compound film industry has now developed to the 4th generation technology. Some high-performance varieties not only have good initial tack, but also are suitable for aluminum foil bonding. The company has successfully developed polyurethane adhesives with special structure resistant to high temperature cooking.
There are also more than a dozen of solventless composite film production lines in China. In 1984, the Beijing Chemical Research Institute introduced a non-solvent polyurethane adhesive production technology from the Henkel company in Germany, which can be used for the production of dry compound films. As there was only one solventless laminator in China at the time, it was not officially produced for a long time. Driven by environmental laws and regulations, in 1990, various complex production lines were introduced throughout the country. However, due to certain technical problems, the operating rate is insufficient.
The author believes that the development and application of solvent-free composite adhesives is undoubtedly a direction for the development of adhesives, but there are also many problems. To sum up, there are the following points:
(1) Although the amount of rubber applied is small (only 1.5g/m2), the unit price is too high, and the ordinary plastic is about 65 yuan/kg, which has a high production cost.
(2) In order to improve the wettability of the adhesive to the film, the viscosity of the adhesive is required to be low, and the consequence is that the initial cohesion is low, the adhesiveness is not high, and the curing speed is not fast enough, which affects the composite production efficiency. This is attributed to the low relative molecular weight of the polyurethane or polyester base, which is generally controlled at 3000-4000. If the relative molecular weight is increased, the viscosity is too high and uniform coating on the composite film is not easy, which greatly affects the quality of the composite film. The viscosity should generally be controlled at 2000 to 3000 mPa.s. The viscosity can be reduced by increasing the coating and compounding temperature (>40°C), but the safety of the curing agent polyisocyanate must be considered.
2. Water-based adhesive
Waterborne adhesives are another type of environmentally friendly adhesives. Waterborne acrylate adhesives have been developed in foreign countries, and isocyanates are mostly used as curing agents (isocyanates are used as curing agents in the category of polyurethane adhesives). Due to 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. However, there are still some problems in film-film composite acrylate adhesives, such as:
(1) General technical performance, especially on printed film can only be used for low-end products. Because the general-purpose water-based acrylate adhesive has a low peel strength on the printed film, the peel strength of the composite film depends on the type of printing ink. The food packaging film is made of nitrocellulose ink. Unfortunately, it is incompatible with general-purpose water-based acrylate adhesives. The composite film is easy to peel and its strength is only (0.1-0.5)N/15mm. This phenomenon does not occur with solvent-based and solvent-free polyurethane adhesives because when these two adhesives migrate after the film is transferred to the ink layer, they can still act as an ink binder, producing a cohesive force with the printing ink layer.
(2) Poor strainability, high relative molecular weight of the film, low gloss, large energy consumption during drying, and bubble generation when the composite film is formed.
Hydrogen bonding and good crosslink density are two key elements for obtaining good composite adhesion.
Because of the nature of the chemistry, waterborne acrylates do not have strong hydrogen bonds, so most acrylate adhesives have low crosslinkability. However, one-component acrylate adhesives generally have high solids content, high shear strength, high mechanical flow, and a long pot life, which is beyond the reach of other aqueous adhesives. To improve the adhesion of waterborne acrylate adhesives while maintaining their excellent properties, Rohm & Haas has developed a new two-component acrylic/urethane hybrid. Due to the formation of an interpenetrating network structure and a urethane reaction cross-linking point in the molecule of the system, it has superior cohesive strength, peel strength, moisture resistance, heat resistance, and resistance to contents as compared with the one-component acrylate adhesive. Adhesive properties are comparable or better than polyurethane water-based adhesives. This superior performance, combined with its low price, makes it highly competitive in food composite film packaging applications.
China also has acrylate adhesives for the production of composite films, but the performance is general. The chemical structure of the main agent must be improved.
Waterborne polyurethanes can also be used for the compounding of composite films. To improve its performance, water-dispersible polyisocyanate technology has been developed at home and abroad.
3. Other environmentally friendly adhesives
Other environmentally friendly adhesives for composite films include: UV-curable and electron beam-curable, moisture-curable polyurethane hot-melt adhesives and alcohol-soluble adhesives. Among them, alcohol-soluble polyurethane adhesives have been produced by Occidental Industries (Shenzhen) Co., Ltd., which is dedicated to plastic film/aluminum-coated film.
China's universal solvent-based polyurethane composite film adhesives have been produced in large quantities, and high-performance solvent-based polyurethane adhesives that withstand 120 to 135°C for cooking compound bags have also been successfully developed. Environmentally-friendly adhesives such as alcohol-soluble polyurethane aluminum-plated composite adhesives, solvent-free polyurethane adhesives, and water-based adhesives are all produced. However, due to price issues or due to quality problems, they have not been widely adopted as solvent-based adhesives. In the future, it is necessary to actively organize strengths to start from the chemical structure of the resin itself and to develop environmentally friendly composite film adhesives that meet various requirements.