Development and Application of Low Viscosity Composite Adhesives

Development and Application of High Solids, Low Viscosity Composite Adhesives

Dry compounding is the process of using a dry compounding device to bond two or more films together using an adhesive. The amount of adhesive used in dry compounding is small, accounting for about 10% of the material cost, but it has a very important influence on the quality performance of the composite film. It determines the peel strength of the composite film, and affects the transparency of the composite film, heat seal strength, heat resistance, resistance to media and so on. Domestic adhesive production companies have matured and their technological development capabilities have been greatly enhanced. Representative companies such as Zhejiang New Oriental, Beijing Chemical Research Institute and Beijing Gaomeng have emerged. These enterprises have strong research and development capabilities and their products are increasing. Solid, low viscosity and functional development.

The use of high solids and low viscosity adhesives has become an important development direction for dry composite adhesives. Previous two-component polyurethane adhesives, whose solid content of the main agent was 35% or 50%, have now been increased to 75% or 80%. 75% solids two-component polyurethane adhesives account for 60% of the market, and the market for 50% solids two-component polyurethane adhesives is shrinking to less than 30% of the market. Due to the characteristics of high solids and low viscosity, it is possible to use high solids content for high-speed composite machining.

Using the high solid binder at the same coating weight can effectively reduce the amount of solvent used, and has the following advantages: (1) reduce solvent consumption; (2) reduce drying load, increase production efficiency, and reduce energy consumption; (3) Reduce the residual solvent.

However, increasing the solids content (working concentration) also brings some problems:

(1) The uniform coating of the adhesive is difficult due to the drastic change of the viscosity when the solvent evaporates;

(2) As the activation period is short, it is easy to cause blockage, and it is difficult to ensure a uniform coating amount. But also easily lead to poor appearance;

(3) Since the binder has a small molecular weight (low viscosity), the cohesive force is small, and processing problems such as tunneling are easily caused. At present, some adhesive manufacturers in China have produced high-solids, low-viscosity products with high initial tack by adjusting the molecular weight and molecular structure.

Development and application of water-soluble adhesives

The advantage of water-soluble binders is that water replaces organic solvents, there is no potential risk of combustion explosion, there is no pollution to the environment and the operator is poisonous, and the cost is low. However, this adhesive has poor coating properties and poor wettability to the composite substrate (because the surface tension of water is 72 mN/m, and the surface tension of the substrate is mostly only about 38 mN/m). Adhesion is not high. In addition, the heat capacity of water is large, and it takes more energy to dry the ink. This not only limits the production speed, but also increases the production cost. At the same time, water vapor also causes certain corrosion to the equipment. These factors, to a certain extent, limit the widespread use of aqueous adhesives.

Development of solvent-free composite process

Solvent-free adhesives are basically also composed of two-component polyurethane adhesives, whose main agent and curing agent have higher viscosity at room temperature but still have fluidity and are semi-solid substances. When compounding is required, the main agent and the curing agent are mixed in proportion, placed in an adhesive tray having a heat-retaining function, heated to a viscosity to a suitable degree, and then passed through a heat-insulating net pattern. Roller is applied to the substrate. After coating, it is not necessary to heat and dry directly to compound with another substrate. The solvent-free composite process does not have the problem of exhaust emissions. It does not require large and complex heated blasts and exhaust air exhaust devices. The equipment is simple and energy consumption is reduced. The solvent-free compounding process is simple, the equipment covers an area that is small, it is appropriate to reduce costs, save resources, and protect the environment. Therefore, it will be the main direction for the development of composite processes in the future. In recent years, with the strengthening of product quality and environmental protection awareness in China, solvent-free compounding has become more and more popular among people in the composite film packaging industry in China. Many domestic solventless laminators have begun to operate. At present, most middle and low-grade plastic flexible packaging materials can be produced with solvent-free adhesives, but due to factors such as the quality of composites, they cannot completely replace the dry composite process.

Although the development of adhesives is ultimately going to be solventless, because solventless adhesives require dedicated solventless laminators, the investment in equipment is large, and it takes a long time for large-scale deployment. Therefore, solvent-based polyurethane adhesives are still the mainstream products on the market.

Development and Application of Alcohol-soluble Polyurethane Adhesives

In the field of plastic flexible packaging, currently developed non-organic solvent type adhesives (including solvent-free and water-based polyurethane adhesives, etc.) have not fully reached the level of solvent-based adhesives, so solvent-based polyurethane adhesives will still be Exists for a longer period of time. Alcohol-soluble polyurethane adhesives will gradually occupy a certain market share because they meet the green environmental protection requirements and have the basic advantages of two-component, solvent-based polyurethane adhesives.

Most of polyurethane adhesives use ethyl acetate as a diluent, whereas alcohol-soluble polyurethane adhesives can use ethanol as a diluent, ethanol has lower toxicity compared to ethyl acetate, and in the usual case the solvent Less residue, and cheaper ethanol, can effectively reduce the composite cost. In addition, the alcohol-soluble polyurethane adhesive and the alcohol solvent ink have good compatibility and adhesive performance to the alcohol-soluble ink, and at the same time, it can save curing costs because it can be cured at room temperature.