Development Status and Future of Solvent-Free Adhesives for Flexible Packaging
I. Solvent-Free Adhesives
HanGao has established dedicated Liofol laboratories in major global markets to ensure and enhance product performance, continually improving composite and coating technologies. Significant technological breakthroughs on reactive polyurethane in Liofol laboratories have led to the development of solvent-free adhesives.
Solvent-free adhesives are truly environmentally friendly adhesives with outstanding characteristics: (1) No solvents are used. Solvent-free adhesives, with 100% effective ingredient content, are applied using various coating systems without solvents. Therefore, there is no solvent evaporation during the entire composite process, no solvent residue in composite products, meeting food safety requirements, emission-free, and complying with emission reduction and environmental protection standards. (2) Not classified as flammable or explosive dangerous goods. They are safe, with no risk of fire or explosion, eliminating the need for explosion prevention measures for solvents. Solvent-free adhesives are not flammable or explosive hazardous goods, and they do not require explosion considerations in transportation and storage. Additionally, solvent-free adhesives have 100% solid content, requiring less storage space compared to solvent-based adhesives, which must consider flammability and explosiveness due to solvent content. (3) Lower coating amount Due to the better cold flow characteristics of solvent-free adhesives, the coating amount is generally between 0.8 and 2.5 g/m2. For special materials such as paper, the coating amount can reach 4 g/m2. In contrast, solvent-based adhesives typically have a coating amount of 2.0 to 4.5 g/m2. This highlights the cost advantage of solvent-free adhesives. (4) Relatively low molecular weight Solvent-free adhesives have a much lower molecular weight compared to solvent-based adhesives of the same level. This allows the adhesive to infiltrate, diffuse, and penetrate the substrate surface without the need for solvent involvement at certain suitable temperatures.
II. Types and Development History of Solvent-Free Polyurethane Adhesives
Solvent-free composite technology, equipment, and adhesives were developed in the 1970s, evolving from the first generation to the third generation.
The first generation solvent-free adhesive is a single-component moisture-curing adhesive. The chemical structure of single-component moisture-curing polyurethane solvent-free adhesive contains short-chain isocyanate end groups of polyester or polyether. The isocyanate group (-NCO) reacts with moisture in the environment, releasing CO2 for curing.
To overcome the drawbacks of single-component adhesives, two-component polyurethane solvent-free adhesives were developed. Depending on initial adhesion and steam resistance, two-component polyurethane solvent-free adhesives can be classified into the second and third generations.
The second-generation solvent-free adhesive was first applied to composite packaging materials in the early 1980s.
The third-generation solvent-free adhesive overcame the drawbacks of the second generation. It features high initial adhesion, low viscosity, low operating temperature, and no restrictions on composite substrates. It solves the adhesion problems of the second-generation adhesive to EVA and Nylon heat sealing. Issues such as wrinkles or easy peeling of highly barrier films have been resolved. It is suitable for the composite of various plastic films and foils, reducing the content of free diisocyanates. The performance of the composite film is excellent, and the adhesive strength and heat resistance of the adhesive have also been improved. Aluminum-plastic composite film products may have the performance of withstanding high-temperature cooking.
The main characteristics of third-generation products are as follows: (1) Adopting a counter-catalytic system. (2) Reducing the viscosity of the adhesive, improving the wetting properties of the adhesive on the film surface, and enhancing the coating dispersion performance. This not only improves the quality of solvent-free adhesive composite products but also increases production speed and efficiency.
Solvent-free polyurethane adhesive development has a history of nearly 30 years. Its products have gradually matured, and in the context of a green and low-carbon economy, solvent-free adhesives are favored in Germany, the United States, Japan, and worldwide. In developed countries such as Europe and the United States, the use of solvent-free polyurethane adhesives has increased from 10% 20 years ago to over half today. Since the introduction of the first solvent-free dry composite machine in Japan in 1977, in less than 20 years, it has developed to have nearly 100 solvent-free composite devices.
Two-component refers to using prepolymers terminated with isocyanate groups as component A and prepolymers containing hydroxyl groups terminated with hydroxyl groups as component B. They undergo urethane formation reactions, producing urethane methacrylate, which can be seen as the first step of the reaction. Excessive high-molecular-weight isocyanate groups on the chain react with moisture to produce urea, which can be considered as the second step of the reaction. The entire adhesive polymerization reaction is very complex, generally considered as the two reactions mentioned above. This can help and guide the composite production. The reaction mechanism is as follows:
Two-component solvent-free reaction mechanism As a pioneer in the market of solvent-free polyurethane adhesives, HanGao not only excels in today's third-generation universal solvent-free adhesive products, meeting and surpassing market requirements for high speed, cost savings, low energy consumption, and zero emissions; but also continuously innovates, developing intelligent fast-curing solvent-free adhesives that comply with international food safety regulations such as FDA, hybrid solvent-free adhesives with running speeds of up to 500 meters per minute, and simple solvent-free adhesives for low-viscosity room temperature applications. These innovations provide solutions for the ever-changing solvent-free adhesive market.
Due to the continuous increase in the speed of laminating machines, making it difficult for general-purpose adhesives to adapt to curing speed, thanks to HanGao's research results and rich experience, the fourth generation of ultraviolet-curing solvent-free adhesives has emerged. Currently used cationic curing products are mainly acid-catalyzed epoxy systems. Under ultraviolet irradiation, the photoinitiator of the adhesive decomposes, initiating the polymerization of the epoxy segment to form a three-dimensional cross-linked structure of epoxy resin. With the appropriate ultraviolet generator, this adhesive can quickly laminate polyester films and achieve high bonding strength in minutes to an hour, facilitating subsequent processing.
III. Characteristics of Solvent-Free Composite Equipment
Solvent-based and solvent-free composite equipment differ in several aspects. On the one hand, the coating systems and glue supply systems of solvent-free composite equipment are different. On the other hand, solvent-free composite equipment does not have a drying path, has a compact structure, low energy consumption, low noise, and a small footprint.
Structure and schematic diagram of solvent-free composite equipment: Compared with solvent-based composite equipment, solvent-free composite equipment has the following advantages: (1) No drying path, low energy consumption. Solvent-based composite machines generally have an installed drying path capacity of 150 to 250 kW, with an average annual power consumption of 500,000 to 700,000 RMB. Even with steam drying, energy consumption is not negligible. The overall power consumption of solvent-free machines is lower, with no drying path. According to information provided by Nordmeccanica in Italy, the energy consumption of solvent-free machines is almost one-twentieth of that of solvent-based machines. (2) Fast lamination speed. Solvent-free lamination does not require drying, eliminating residual solvent constraints, resulting in high production efficiency, with an average lamination speed of 250 m/min. It can reach a maximum of 600 m/min, and one lamination machine can meet the production of two printing machines. (3) High degree of automation. Solvent-free laminating machines use dedicated equipment for automatic glue feeding, automatic control of the ratio of components A and B, and automatic temperature control. Coating amounts can be adjusted online, and the equipment is compact, saving labor. Liofol laboratories have industrial-scale solvent-free laminating machines for real-world testing of materials and solvent-free adhesive systems, developing optimal formulations and customized solutions for various application areas.
IV. Key Points of Solvent-Free Composite Process
Currently, commonly used substrates for solvent-free lamination include BOPP, PET, OPA, VMCPP, VMPET, CPP, PE, aluminum foil, etc. In general, rigid and well-coating-performing materials (PET, BOPP, OPA, VMPET, etc.) are placed in the main unwinding position, while stretchable materials (PE, CPP, VMCPP, etc.) are placed in the secondary unwinding position. However, this is not fixed and can be flexibly chosen based on actual production conditions. For example, when printing film is laminated with aluminum-plated materials, to ensure the quality and production efficiency of lamination, aluminum-plated materials can be placed in the main unwinding position.
Solvent-free adhesives mainly include single-component moisture-curing adhesives, two-component cold-coating solvent-free adhesives, two-component reverse heat-coating solvent-free adhesives, and UV-curing adhesives. Several factors need to be considered when choosing an adhesive, such as the type of packaging contents and the type of film material used. If printing ink comes into contact with the adhesive, the compatibility between the two should also be considered. Additionally, factors such as peel strength requirements and heat sealing conditions have a significant impact on adhesive selection.
It is also essential to consider two issues: firstly, MDI-type isocyanate compounds will gradually migrate through the inner film and react with moisture to form a polyurea heat-sealing layer, affecting the heat-sealing quality of packaging bags. This issue is more likely to occur when using two-component adhesives, especially if the quality of the polyethylene film is poor or if the adhesive selection is improper. Secondly, when films with high levels of slip agents are used, the migration of slip agents into the adhesive layer may lead to problems such as poor peel strength, poor heat sealing, and increased friction coefficient, affecting operability on the packaging production line. Therefore, when laminating such films, it is essential to choose a suitable adhesive.
V. Prospects and Outlook for Solvent-Free Adhesives
After decades of development and application, solvent-free and solvent-free composite technologies have become mature. Using solvent-free adhesives to laminate films can reduce energy consumption, increase production efficiency, improve product quality, and lower overall production costs. Due to the absence of solvents, solvent-free laminates contribute to environmental protection and clean production, ensuring that the lamination of food and pharmaceutical packaging is contamination-free.
With the increasing emphasis on the quality of composite products, awareness of food safety and hygiene, and the strengthening of energy-saving and environmental protection, solvent-free lamination is expected to receive more attention from professionals in the composite packaging industry. The successful operation of existing solvent-free composite equipment in the country, acting as a demonstration, will accelerate the transformation of the flexible packaging industry from solvent-based to solvent-free lamination. With the willingness of companies in the film industry to understand, research, and use solvent-free lamination, coupled with the promotion of energy-saving and emission reduction policies in the food and pharmaceutical industries, solvent-free lamination is expected to experience vigorous development, becoming the main lamination method in the film industry in China.