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Previously, dry lamination was the main method used by domestic flexible packaging enterprises. However, with the rapid development of the economy and the gradual maturation of various conditions, domestic solventless lamination and solventless polyurethane adhesives are facing unprecedented opportunities. Especially after the introduction of the new national standard GB/T 10004-2008 "Plastic Laminated Films and Bags for Packaging Use, Dry Lamination, and Extrusion Lamination", which started to be implemented from August 1, 2009 (the total amount of solvent residue ≤5mg/m2, benzene solvents must not be detected), more and more flexible packaging enterprises began to pay attention to solventless lamination, and some have started to introduce solventless lamination production lines. As these imported solventless lamination production lines are successfully used in flexible packaging enterprises, it is not only hygienic and environmentally friendly but also brings tangible economic benefits to the companies, driving even more flexible packaging enterprises to opt for solventless lamination production lines. With the rapid increase of solventless lamination production lines, the solventless lamination process is rapidly developing domestically.
A few years ago, the solventless adhesives on the domestic market were mostly general-purpose products, with hardly any application of other high-end products seen. As the popularity and application of the solventless lamination process increase, solventless lamination adhesives also need to develop towards more targeted high-end products.
The development trend of solventless adhesives for plastic flexible packaging mainly includes several categories: general-purpose solventless lamination adhesives, high-temperature steam-resistant solventless lamination adhesives, medium-resistant solventless lamination adhesives, single-component solventless lamination adhesives, and UV-curable solventless lamination adhesives.
Undoubtedly, general-purpose solventless lamination adhesives are the main adhesives used in solventless lamination. Especially when the solventless lamination process is just beginning, flexible packaging manufacturers have just introduced solventless lamination production lines, and the entire lamination process is still in a phase of familiarization and understanding. They start from the simplest, most general materials and gradually become familiar with the solventless lamination process. Only when they can fully grasp the essentials of the solventless lamination process will they confidently use high-end products in solventless lamination.
As the first type of solventless adhesive that flexible packaging manufacturers come into contact with at the beginning of the solventless lamination process, general-purpose solventless lamination adhesives must meet the following requirements:
General-purpose solventless lamination adhesives must first be convenient to operate, including having appropriate room temperature component viscosity and mixed viscosity, and providing enough working time. Many flexible packaging manufacturers initially adopt manual mixing when they introduce solventless lamination production lines. If the viscosity of the solventless adhesive itself is high, it will not be conducive to uniform mixing of the adhesive and may lead to large errors in the proportion of the two components, causing poor peel strength in parts of the compound product or stickiness. Using a mixing machine for mixing does not require high initial viscosity. Moreover, sufficient working time is very necessary for both manual and machine mixing, as the composite process includes first and second lay-downs and roll changing, as well as emergencies that may occur during the process, all of which require a certain amount of time. If the working time is too short, a rapid rise in adhesive viscosity could affect product quality and, in severe cases, damage solventless lamination equipment; generally, the working time of solventless adhesives should not be less than 30 minutes.
One of the most prominent advantages of solventless lamination is its efficiency and fast lamination speed, which requires solventless adhesives to have broad applicability. The variety of products is a prominent feature of flexible packaging products, and flexible packaging manufacturers often receive different orders for laminating different products. If they have to switch to different types of adhesives for each product, then just cleaning the solventless lamination equipment would be a significant burden for flexible packaging manufacturers. Cleaning solventless lamination machines generally takes 1~2 hours, and for workers new to the solventless lamination process, it may take even longer. If a mixing machine is used for mixing, cleaning the machine is even more troublesome and may require an entire shift for the cleaning work, which makes it difficult to reflect the efficiency advantage of solventless lamination.
Therefore, for solventless adhesives, they must be usable with most common flexible packaging materials, including transparent materials, printed materials, and aluminum-coated materials. Solventless adhesives can be divided into general-purpose products and some high-end products (such as whether they are resistant to cooking, resistant to pesticides, and other corrosive media). In general-purpose products, it's not advisable to further divide them into extremely specific categories (such as dedicated for instant noodles, transparent materials, etc.), as this would cause unnecessary trouble for flexible packaging users during use.
The post-application curing temperature of solventless adhesives is generally controlled at 40°C to 50°C, not much different from the curing temperature of dry lamination; the curing time should be controlled at about one day, as too long a curing time not only increases energy consumption but also affects the normal processing of subsequent steps in laminated flexible packaging materials.
Solventless adhesives for food flexible packaging are just starting, and must have good stability to withstand market tests. It can be said that stable and reliable products are the lifeline for enterprises to survive and develop.
High-temperature steam-resistant bags refer to the laminated bags made by bonding materials such as PET, PA, Al, CPP, etc., with steam-resistant adhesives for packaging food, medical instruments, etc., that can withstand high-temperature steam sterilization, also known as cookable flexible packaging or soft cans. Flexible packaging food, with its lightweight, convenience, long shelf life, hygiene, easy storage, and easy opening features, meets the needs of people's daily lives.
Cooking bags are divided into three grades according to the sterilization level:
(1) 121°C medium-temperature cooking bags: Most foods require 121°C for 40min of cooking, special foods, such as beef need 121°C for 60min sterilization, while seafood and soy products generally need only 121°C for 20 minutes;
(2) 135°C, up to 20min of high-temperature cooking can achieve sterile packaging;
(3) 145°C ultra-high-temperature cooking, generally 2~3 minutes, at most 3~5 minutes, is enough to completely eliminate the most heat-resistant harmful bacteria (spore-forming botulinum bacteria).
So far, general-purpose solventless lamination adhesives (i.e., below 100°C, boiling sterilization) have already obtained a certain market application in China, while there are very few solventless two-component polyurethane steam-resistant adhesive products, especially those resistant to 135°C high-temperature steam cooking, domestically and internationally. If solventless lamination is to completely replace the traditional dry lamination process, developing steam-resistant solventless adhesives is essential.
As the application range of laminated flexible packaging materials continues to expand, they are not only used in packaging for food and pharmaceuticals but also in packaging for daily necessities, cosmetics, laundry products, sanitary products, pesticides, and certain chemical drugs, making the demand for functional requirements more urgent. Research and development of solventless adhesives resistant to acids, alkalis, spicy, salty, oily substances, as well as resistant to benzene, toluene, xylene, esters, hydrocarbons, alcohol, surfactants, organic solvent mixtures with less than 20% DMF, pesticides, etc., are future trends in the development of solventless adhesives. After full maturation, the laminated film should be able to package liquid pesticides containing toluene, xylene, a small amount of DMF, alcohol, esters, hydrocarbons, etc.
The main advantage of single-component solventless adhesives is their convenience of use, with no need for onsite mixing, possible application at higher temperatures, longer shelf life, and higher initial adhesion compared to two-component solventless adhesives, requiring less strict roll-up conditions. The disadvantage is that it relies on the moisture in the air for curing, which is greatly affected by the environment (air humidification may be necessary to ensure sufficient maturation of the adhesive); carbon dioxide is generated during the curing process, which can easily produce bubbles in the product if not promptly removed.
The future development direction of single-component solventless lamination adhesives is mainly for use in paper-plastic composite products because paper easily absorbs glue in the paper-plastic composite process. If a general-purpose solventless adhesive is used due to its low viscosity, it may cause serious glue absorption, affecting the quality and appearance of the composite product. A single-component solventless lamination adhesive is applied at high temperatures, and after composite, when the temperature drops, the adhesive viscosity rapidly increases, which can effectively solve this problem.
UV-curable solventless lamination adhesives not only have the characteristics of fast curing, transparent products, high bonding strength, and good flexibility of the finished glue but also have the advantages of being solvent-free, non-toxic, non-corrosive, energy-saving, and environmentally friendly. They will be the new generation of environmentally friendly products partially replacing traditional solventless lamination adhesives. UV-curable adhesives can be used in the rapid production of laminated plastic/plastic, plastic/paper, plastic/metal film products, etc., featuring convenient production and high efficiency, which are very suitable for the rapid laminating production process of high-speed production lines and will be the ultimate trend in the development of solventless lamination.
With its unique advantages, along with the guidance of domestic laws and regulations, solventless lamination technology will surely become the development trend of domestic flexible packaging enterprises and will gradually replace traditional dry lamination technology. Meanwhile, solventless adhesives will also gradually develop from the original single general-purpose solventless lamination adhesives towards high-end products. Besides general-purpose solventless lamination adhesives, high-temperature steam cooking, aluminum foil specific, and single-component solventless lamination adhesives have already been used in the flexible packaging composite market. Medium-resistant solventless lamination adhesives and UV-curable solventless lamination adhesives have been developed and researched by domestic enterprises, and it is believed that this will accelerate the promotion and development of solventless lamination technology in China.
