Waterborne polyurethane adhesive and its application in packaging

Polyurethane adhesives have the advantages of high adhesive strength, high peel strength, impact resistance, ultra-low temperature resistance, oil resistance, and good abrasion resistance. Their application is more and more extensive. However, the solvent-based polyurethane adhesive is still used more often. As people pay more and more attention to the environment, the demand for reducing volatile organic compounds (VOCs) is increasing. Waterborne polyurethane adhesives are an inevitable substitute for solvent-based polyurethane adhesives. trend. Waterborne polyurethanes were first produced by West German P. Schlack in 1943; polyurethane emulsions were industrialized for the first time in 1967 and were introduced in the U.S. market; Bayer Company took the lead in 1972 to use waterborne polyurethane as a leather finishing agent, making it an important starting point. commodity. In recent years, patents and papers on waterborne polyurethanes emerge in endlessly. New types of waterborne polyurethanes are emerging and their performance continues to increase. They have been widely used in coatings, leather, and printing industries, and have also begun to be used in the packaging industry. The importance of adhesive in the 21st century is obvious. The author reviewed the effects of raw materials and their amounts on the performance of emulsions and the progress of waterborne polyurethane adhesives in order to provide some ideas and inspirations for the formulation of waterborne polyurethanes.

1, raw materials

2. Development of Waterborne Polyurethane Adhesives

Some researchers added a mixture of polyester polyols and polyether polyols in a certain proportion to obtain a polyurethane prepolymer with better performance. It is reported that when the polyester/polyether is 35/65, the tensile strength, breaking strength, elongation and hardness of the sample are all high. The equivalence ratio of isocyanate to alcohol hydroxyl has a great influence on the dispersibility of the self-emulsifying aqueous polyurethane, the hardness, elongation, resilience and permanent set of the film. When nNCO(total)/nOH(total) tends to 1, the molecular weight of the prepolymer formed is large, which tends to lead to an initial large viscosity and difficult dispersion; when nNCO(total)/nOH(total) is large, the prepolymer The small viscosity is favorable for dispersion, but the remaining NCO during emulsification reacts with weak basic amines to form an excessive urea structure and gum or even gel occurs. Zheng Yu et al. found that nNCO (total)/nOH (total) was controlled between 1.2 and 1.8 to obtain a stable emulsion. In this range, the greater the nNCO (total)/nOH (total) value, the harder the film; the smaller the nNCO (total)/nOH (total) value, the softer the film. Frisch reacted with hydroxy-terminated polybutadiene, isophorone diisocyanate (NCO/OH=1.8:1) and dimethylolpropionic acid (DMPA/OH=1.8:1), neutralized with triethylamine and Dispersed in water and extended with ethylenediamine, the resulting dispersion had a viscosity of <50 cps, a solids content of 37.6%, an elongation strength of 1920 PSI, and an elongation of 130%. In addition, several articles reported that the best nNCO (total)/nOH (total) was 1.2.

Li Yanke et al. studied the effect of R value (ie, nNCO(init))/nOH (initial) on the properties of the emulsion. The results showed that the tensile strength of the film increased with the increase of R value, and the elongation at break and the water absorption rate varied with R. When the value increases and decreases, the thermal stability of the membrane first increases and then decreases with the increase of the R value. Based on the comprehensive research results, when R=3.5, the membrane performance is best. Sun Guoqing et al. found that the introduction of small molecular weight 1,4-butanediol is beneficial to the reduction of free TDI content; nNCO (total)/nOH (total) is 1.8, the reaction temperature is 60 to 80°C, and the amount of organotin catalyst is 0.1. At the time of %-0.2%, the free TDI mass fraction of the product can be less than 1% after only 1.5 hours of reaction. Zhang Hongtao and others studied the effect of alcohol chain extenders on the properties of waterborne polyurethanes. Compared with ethylene glycol, 1,4 butanediol and trimethylol propane, it was found that the use of ethylene glycol and 1,4 butanediol chain extender increased, the emulsion particle size decreased, and the homogeneity increased; and trimethylolpropane The particle size and dispersibility increase and the stability decreases. The trifunctional alcohol chain extender TMP can improve the water resistance of the film, and the film has good tensile strength and elongation at break.

Hao Guangjie reacted with maleic anhydride and polypropylene glycol (molecular weight 1000) to produce carboxylate polyol, and then reacted with TDI 80, crosslinker, chain extender, etc. to prepare a carboxyl-containing polyurethane prepolymer and then dispersed it. In the aqueous solution of triethanolamine, aqueous polyurethane was obtained. The effect of the type and amount of chain extender on the properties of the resin was studied. It was found that the amino chain extender had better mechanical properties than the hydroxyl chain extender, but the particle size of the emulsion was larger; the use of bisphenol A as a chain extender not only allowed Improve the mechanical properties of the resin, but also can increase the glass transition temperature of the resin, improve the temperature range of the resin leather state, but has little effect on the particle size of the emulsion. Ding Li and others used polyether PPG 220 (molecular weight = 2000) and isophorone (IPDI) as the main raw materials, and dimethylamino propionic acid (DMPA) as the hydrophilic monomer to prepare the polyurethane emulsion adhesive. Effect of DMPA Content and Feeding Methods on the Properties of Emulsion and Its Adhesive Films . It is pointed out that the emulsion has high stability only when the DMPA content is in the range of 3% to 8%. Under the premise of ensuring a stable emulsion, the higher the DMPA content, the higher the viscosity of the emulsion, the smaller the particle size, and the narrower the particle size distribution; however, the mechanical properties of the membrane tend to increase, such as increased tensile strength and tensile shear strength. , And the elongation at break decreases. However, too much hydrophilic component content is not conducive to the film's water resistance, so in general under the premise of taking into account the emulsion stability, the content of hydrophilic groups should be controlled. The addition of DMPA also affects the properties of the emulsion and its membranes. The experimental results show that the two-step method (the first reaction of polyether and diisocyanate and then added hydrophilic chain extender DMPA) is a one-step method (polyether, diisocyanate and hydrophilic chain extender are added into the reactor at one time). The obtained emulsion has better dispersibility, but the mechanical properties of the film are not as good as the one-step method.

Li Wen'an reacted with malonic acid and diethylene glycol to obtain a polyester glycol, which was then reacted with TDI and DMPA to synthesize an aqueous polyurethane dispersion. The coating was further prepared and the performance of the neutralizer on the film was discussed. The effect of triethylamine as a neutralizing agent was found to be that the film formed by cross-linking was whitened and the transparency was poor, and satisfactory results were achieved with ethanolamine. Guo Qi et al used polyether polyol, TDI, DMPA, neutralizer TEA, and self-made chain extender DE901 to obtain water-borne polyurethane. The effect of the amount of salt-forming agent triethylamine on the properties of polyurethane emulsion was studied and found that TEA When the molar ratio of DMPA is less than 0.7:1, the obtained emulsion has poor stability, but as the amount of TEA increases, the viscosity of the emulsion after several days of standing will gradually increase. Therefore, it is preferred that the molar ratio of TEA to DMPA be (0.73 to 0.78):1.

3 Application

Composite film flexible packaging can play a role in shielding, printing and heat sealing, and partially replaces packaging materials such as glass, tinplate, and paper, and has been widely used in the packaging of food, medicine, cosmetics, and office consumer goods. The domestic composite film is a packaging material developed in the late 1970s and early 1980s, and is mainly used in the field of food packaging. Of the composite film adhesives, 90% are polyurethane adhesives. Compared to solvent-based polyurethane adhesives, waterborne polyurethanes have the advantages of no pollution and low cost. In the field of food packaging, water-based adhesives are an inevitable trend instead of solvent-based adhesives. According to reports, water-based polyurethane adhesives used in food packaging composite films in Europe are about 2.4 x 106 kg per year, and will continue to grow in the coming years. However, in order to facilitate the application, low viscosity is required, and the relative molecular weight of the corresponding polyurethane is small, resulting in insufficient initial viscosity. Therefore, research and development of low viscosity polyurethane adhesives with large cohesive forces have become a research hotspot. The compound modification of waterborne polyurethane is a common and effective solution. Toho Chemical Industry Co., Ltd. has developed an epoxy compound modified ionic water-based polyurethane adhesive, which greatly enhances its adhesive properties and is widely used for the bonding of composite films for food packaging. Waterborne polyurethane compound modified materials are commonly found in epoxy, silicone, and acrylic emulsions. Epoxy resin has the advantages of strong adhesion, high strength and good thermal stability. It can be used in leather finishing agents after being compounded with water-based polyurethane to improve the adhesion of coating agent to the substrate, coating brightness, mechanical properties of the coating, Heat resistance and water resistance, etc., but also glued polypropylene film, used in food packaging. Silicone resins have excellent high temperature resistance, weather resistance and air permeability, and have been widely used in the modification of polyurethane materials. Acrylic Modified Polyurethane Emulsion makes polyurethane excellent in tensile strength, impact strength, flexibility and wear resistance, combines the advantages of good adhesion and low cost of acrylic resin, and is known as “3rd generation waterborne polyurethane "There is a great potential for development in the bonding of packaging materials such as foods and pharmaceuticals.

4 Conclusion

The study of water-based polyurethane adhesive in China started late. Domestic research is still in the initial stage of development. The level of applied research is not high, the number of manufacturers is small, and the variety is single, and the scale is not large, especially in high-performance, low VOC content, multi-functional There is still a large gap between the product quality of waterborne polyurethane adhesive research and foreign countries, so it is necessary to strengthen research in this area in order to shorten the gap with foreign countries.

Author: Xie Xiaowei Fu and green, Wong Hung, Huanqin

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