Application of Polyurethane Adhesive for Several Kinds of Packaging (in)

2. Towards water soluble or dispersible adhesives

There are two types of these adhesives: one is an aqueous polyurethane adhesive and the other is an aqueous polyacrylate adhesive.

Their advantage is the use of water instead of organic solvents, there is no potential risk of combustion explosion, nor will it cause environmental pollution and stimulation or poisoning of operators, and the cost of solvents is low. However, the performance of such adhesives has been poor so far, and the adhesive has poor wettability to the substrates to be bonded, resulting in low adhesion. In addition, the heat capacity of water is large. To dry it, it consumes more energy. Similarly, when it is dried to a molecular weight material, water consumes 20% to 30% more energy than organic solvents. The production speed is not high and the cost is also high. Taller. In addition, water vapor can cause serious corrosion to steel, leading to deterioration of equipment performance and even scrapping. These are the factors that affect its widespread adoption.

The most important point is that the waterborne polyurethane adhesive has poor hygienic properties and has not yet been approved by the FDA so far. The current practical promotion is the use of polyacrylate-based aqueous adhesives.

3. Towards the development of alcohol-soluble polyurethane adhesives

Conventional solvent-based dry compound polyurethane adhesives are solvent-based adhesives using ethyl acetate as a medium. Ethyl acetate is more irritating to humans than alcohol and costs more. In the past 10 years, people have paid attention to the production environment and the cost of products, and the gradual improvement of the entire human being’s awareness of environmental protection in the earth’s atmosphere, and hoped that no expensive or stimulating toxic organic solvents would be used or reduced. Alcohol-soluble polyurethane adhesives that used alcohol instead of other organic solvents came into being. For example, the LamaI HSA/LamaI C type two-component polyurethane adhesive originally introduced by Morton of the United States was made of a denatured alcohol/n-hexane solvent. Can be diluted with isopropanol, denatured methanol, or ethanol. According to the company's data, the adhesive has a storage period of 6 months and has good adhesion to cellophane, polyester, nylon, aluminum foil, aluminized film, and PVDC coated film, and to polyethylene and polypropylene film. Whether it is applicable but not mentioned.

The European and American companies in Shenzhen introduced the alcohol-soluble ECO501A/B and ECO701A/B polyurethane adhesives to the market two or three years ago. Zhejiang New Oriental and Beijing Chemical Research Institute have also recently launched such products.

Alcohol-soluble polyurethane adhesives have the advantages of being compatible with the requirements of alcohol-soluble composite inks and also not affected by atmospheric moisture. Since the residual solvent after the printing of the alcohol-soluble ink is alcohol, the alcohol has to destroy the performance of the curing agent in the ester-soluble polyurethane adhesive. If the amount of residual solvent in the ink layer is large, the peeling strength will be adversely affected. Alcohol-soluble polyurethane adhesives can overcome this disadvantage by recombination.

In recent years, there have been controversies regarding the superiority of ethyl acetate and alcohol in terms of human health and environmental friendliness. The toxicological safety evaluation of alcohol-soluble polyurethane adhesives has not been reported yet, but alcohol solubility The development momentum of polyurethane adhesive is still very fierce. It is hoped that relevant departments or relevant units will use scientific and rigorous attitudes and means to do some work in this area and draw a definite conclusion at an early date.

4. Towards solvent-free polyurethane adhesives

The solvent-free compounding process, equipment, and solvent-free polyurethane adhesives developed in the 1970s were highly respected in the last decade or so. The solvent-free adhesives used today are also basically composed of two-component polyurethane adhesives. Their main agents and curing agents have high viscosity at room temperature and are solid or semi-solid substances, and some still have fluidity. When compounding is required, the main agent and curing agent are first heated to 50-60°C, and some are to reach 80°C, so that the viscosity is reduced to 1 Pa·s or lower, and then transferred to the mixer in proportion with a metering pump. After being fully mixed, it is then transferred to a sizing roll and coated on a substrate. After being glued, it is not necessary to heat and dry it through a drying tunnel, because it does not have any solvent itself and can be directly compounded with another substrate. The compound is rolled up and aged at a certain temperature for a certain period of time.

The use of solvent-free adhesives does not have the problem of exhaust emissions, there is no problem of residual solvents, there is no need for large and complex heating blasts, exhaust air or exhaust gas treatment devices, equipment is simple, and the cost is low, only the dry composite machine 50% to 60%, low investment; high speed operation, up to 400 meters per minute; less glue, raw materials, energy consumption, low maintenance costs; labor productivity increased, so the benefits are very significant. According to reports, most of the middle and low-end bulk composite packaging materials in Europe and the United States are produced with solvent-free adhesives, and even high-grade composite packaging materials that are resistant to high-temperature cooking at 121°C can be manufactured using this adhesive.

Since 1995, China has introduced nearly 40 sets of solvent-free composite equipment, basically mastering the solvent-free composite process, and domestic flexible packaging equipment manufacturers are also developing solvent-free laminators. Although most of the solvent-free adhesives that are compatible with them are still imported, Beijing introduced UK8150 solvent-free adhesives in the 1990s. After entering the 21st century, Shanghai Lianyin Chemical Co., Ltd. also has LY- 100N solvent-free adhesive was officially introduced to the market. According to industry surveys, the quality of solvent-free compounding processes and products has not been universally recognized so far. The main problems are the low initial adhesive strength of the adhesive, the low interlaminar peel force of the final product, and the waste during the production process. High rates, low yields, and some equipment failures cannot be solved. In recent years, research and development of solvent-free adhesives that are dedicated to improving initial tack and final peel strength are believed to be successful in the near future.