Application of the most popular polyurethane adhes

2022-08-07
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Application of several kinds of polyurethane adhesives for composite packaging (Part 2)

2 There are two kinds of water-soluble or water dispersible adhesives: one is water-based polyurethane adhesive, the other is water-based polyacrylate adhesive

their advantages are that water is used instead of organic solvent, there is no potential danger of combustion and explosion, no pollution to the environment and no stimulation or toxicity to operators, and the solvent cost is low. However, up to now, the performance of this kind of adhesive is still poor. The adhesive has poor wettability to the substrate with low thermal conductivity, which leads to poor adhesion. It can count steps, track heat consumption and create interactive training. In addition, due to the large heat capacity of external water, drying it will consume more energy. Similarly, drying a substance with a molar weight, water will consume 20% ~ 30% more energy than organic solvents. The production speed is not high, and the cost is high. In addition, water vapor will cause serious corrosion of steel, resulting in poor performance of equipment, or even scrapping. These are the factors that affect its wide adoption

the most important point is that the waterborne polyurethane adhesive has poor hygienic performance and has not been approved by FDA so far. At present, polyacrylate water-based adhesive is actually popularized and used

3. To develop alcohol soluble polyurethane adhesives

the previous solvent based dry composite polyurethane adhesives are solvent based adhesives with ethyl acetate as the medium. Ethyl acetate is more irritating and costly than alcohol. In the past 10 years, people have paid attention to the production environment and the cost of the first product in China, which is currently the most active country in the research, utilization and development of graphene, as well as the gradual improvement of human awareness of the earth's atmospheric environment protection. They all hope to avoid or reduce the use of high-cost, irritating and toxic organic solvents. Therefore, alcohol soluble polyurethane adhesives that use alcohol instead of other organic solvents came into being. For example, Lamai hsa/lamai C two-component polyurethane adhesives launched by the original American Morton company use denatured alcohol/n-hexane as the solvent and can be diluted with isopropanol, denatured methanol or ethanol. According to the company's information, the adhesive has a storage period of 6 months and has good adhesion to cellophane, polyester, nylon, aluminum foil, aluminized film and PVDC coating film, but it is not mentioned whether it is applicable to polyethylene and polypropylene films

OMAX in Shenzhen introduced alcohol soluble eco501a/b and eco701a/b polyurethane adhesives to the market two or three years ago, and they are also applied by manufacturers all over the country. Zhejiang New Oriental and Beijing Research Institute of chemical industry have also recently launched such products

the advantage of alcohol soluble polyurethane adhesive is that it can meet the requirements of alcohol soluble composite ink and is not affected by atmospheric moisture. Because the residual solvent of alcohol soluble ink after printing is alcohol, alcohol will destroy the performance of curing agent in ester soluble polyurethane adhesive. If the amount of residual solvent in the ink layer is large, it will cause the adverse consequences of reduced peel strength. This shortcoming can be overcome by compounding with alcohol soluble polyurethane adhesive

in recent years, there have been disputes about the advantages and disadvantages of ethyl acetate and alcohol in terms of human health and environmental friendliness, and the toxicological safety evaluation conclusion of alcohol soluble polyurethane adhesive has not been reported, but the development momentum of alcohol soluble polyurethane adhesive is still very strong. It is hoped that relevant departments or units will do some work in this regard with a scientific and rigorous attitude and means, so as to achieve operational functions and draw clear conclusions as soon as possible

4. The development of solvent-free polyurethane adhesive

solvent-free composite process, equipment and solvent-free polyurethane adhesive was developed in the 1970s and has been highly respected in recent decades. The solvent-free adhesives currently used are basically composed of two-component polyurethane adhesives. The main agent and curing agent have high viscosity at room temperature, which are solid or semi-solid substances, and some still have fluidity. When compounding is required, the main agent and curing agent shall be heated to 50 ~ 60 ℃, and some shall reach 80 ℃, so that the viscosity can be reduced to 1pA · s or less, and then they shall be delivered to the mixer in proportion with a metering pump. After full mixing, they shall be delivered to the glue roller and coated on the substrate. After gluing, there is no need to heat and dry through the drying channel, because it has no solvent and can be directly compounded with another substrate. It can be rolled up after compounding and ripening for a certain time at a certain temperature

the use of solvent-free adhesive does not have the problem of waste gas emission, nor does it have the problem of residual solvent. It does not need a huge and complex heating and blowing, waste gas exhaust or waste gas treatment device. The equipment is simple and the cost is low, which is only 50% ~ 60% of that of the dry compound machine, and the investment is small; It can run at a high speed, more than 400 meters per minute; Less glue, less raw materials, less energy consumption and low maintenance cost; Labor productivity has increased, so the benefits are very significant. It is reported that at present, most medium and low-grade bulk composite packaging materials in Europe and the United States are compounded with solvent-free adhesives, and even high-grade composite packaging materials that can withstand 121 ℃ high temperature cooking can be made with this adhesive

since 1995, China has introduced more than 40 sets of solventless compounding equipment and basically mastered the solventless compounding process. Domestic flexible packaging equipment manufacturers are also developing solventless compounding machines. Although most of the corresponding solventless adhesives are still imported, uk8150 solventless adhesives were introduced in Beijing in the 1990s. After entering the 21st century, Shanghai lieyin Chemical Co., Ltd. also officially introduced ly-100n solventless adhesives to the market. According to the investigation in the industry, so far, the solvent-free compounding process and product quality have not been generally recognized. The main problems are the low initial adhesion of the adhesive, the low interlayer peeling force of the final product, the high scrap rate and low yield in the production process, and some equipment failures that cannot be solved. In recent years, China is committed to the research and development of solvent-free adhesives to improve the initial adhesion and final peel strength. It is believed that they will be successful in the near future

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