Polyurethanes for flexible packaging composite films have evolved with the development of flexible packaging. The flexible packaging composite film utilizes different water and air barrier properties, lightfastness and printability of different composite layers, and then combines the different characteristics of these composite layers to exhibit a good overall performance in the food, medical, electronic and electrical industries. Widely used. The composite film flexible packaging has the characteristics of high strength, airtightness, good shading, waterproof and moisture resistance, etc., so as to meet the requirements for preservation, color retention, and taste protection of the packaged product. With the vigorous development of China's food and beverage, pharmaceutical and other industries, the demand for flexible packaging composite films has increased rapidly. Different composite layers of flexible packaging composite films are often compounded using adhesives. The commonly used composite process in China is dry compounding, which is characterized by low equipment investment, high product quality, simple process, and high production efficiency. In the dry composite production process, the adhesive is one of the key factors affecting the quality of the composite film.
In the early stage of the development of domestic flexible packaging, the adhesive used for compounding often uses resin-type one-component adhesives. The adhesive uses benzene-based solvents and has high toxicity. At the same time, adhesives have poor temperature resistance and are quickly replaced by polyurethane adhesives. Polyurethane adhesives have excellent overall performance, low temperature resistance, boiling resistance, flexibility, transparency, and excellent adhesion to various materials. They are the most commonly used adhesives for the production of composite films. Here we introduce a dry composite Polyurethane adhesive with high solids content (75%), good low temperature fluidity, strong initial tack, and storage stability.
1 Technical route
The experimentally produced main agent (abbreviated as component A) is obtained by the use of a liquid amorphous mixture of dihydric alcohols and dibasic acid polyesters with a certain amount of TDI chain extension to obtain the appropriate viscosity and viscosity by changing the ratio of -NCO/-OH. Viscosity; then adjust the amount of curing agent (B component) to improve the cohesive strength of the adhesive.
2 experimental part
2.1 Raw materials
Phthalic anhydride, industrial grade, import; adipic acid, industrial grade, DuPont; ethylene glycol, industrial grade, Maoming Petrochemicals; diethylene glycol, industrial grade, Maoming Petrochemicals; toluene diisocyanate (TDI80/20), industrial Products, Bayer, Germany; trimethylolpropane (TMP), industrial grade, imported; ethyl acetate, industrial grade, Shunde.
2.2 Synthesis Experiment
(1) Synthesis of Polyester polyols. The phthalic anhydride, ethylene glycol, diethylene glycol, adipic acid, and the catalyst are metered into a reaction vessel and the material is melted when the nitrogen gas is warmed to 140° C. and the material is melted. The stirring is started and the reaction starts to react. At this temperature, the temperature is kept for 1h, and then the temperature is increased. The temperature of the material is gradually increased to 210°C. After reaching the theoretical amount of water, the nitrogen gas is stopped. After the polycondensation reaction is performed by vacuum, the acid value is sampled and analyzed, and the material is discharged after the pass. The hydroxyl value is controlled at 60°C. about.
(2) Synthesis of polyurethane (component A). The polyester polyol, toluene diisocyanate and part of the solvent are added to the reactor and heated to heat. After a constant temperature for several hours, the terminator and other additives are added, and the residual solvent is added to dilute and reduce the temperature. The adhesive main agent can be obtained after uniform mixing ( The A component) has a solid content of 75±2% and a viscosity of 3.0 to 5.0 Pa.s.
(3) Synthesis of curing agent (component B). TDI and part of the solvent were added to the reactor and then trimethylolpropane was added dropwise. Then gradually warmed to 110 ° C for 2h, add the solvent and cool down to control the solids content of 75 ± 2%.
2.3 Peel strength test
The A component and the curing agent B component are mixed in a certain proportion, and a certain amount of ethyl acetate is added for dilution to obtain a Polyurethane Adhesive solution (control working concentration is 40%). The coating was performed on a dry laminator (the sample film was OPP/CPP). Control the amount of glue at 3g/m2 (dry basis). The composite was cured in a constant temperature oven at 50°C. The adhesive strength after 5 minutes and 24 hours of curing was tested. Specimen size: 15mm x 200mm; Test method: 180° peel test at a speed of 100mm/min on a tensile tester.
3 Results and Discussion
(1) Analysis of peel strength test results. The results of the composite film peel strength test are shown in Table 1. From the peel strength test results, the initial peel strength of the adhesive is not very good. The initial tack of the adhesive can be improved by adding additives.
(2) Product storage stability. From Table 2, it can be seen that the viscosity of the product remained basically unchanged for 6 months.
4 Conclusions (1) The polyurethane adhesive obtained by the experiment has a solid content of 75% and a viscosity of 3 to 5 Pas. It can be used at a concentration of 40% in solids, and ensures good sizing properties. And have good initial tack and peel strength. At present, some commonly used two-component polyurethane adhesives, due to their high viscosity and working concentration of about 25%, require more diluent, which increases the cost of use and also increases environmental pollution.
(2) The adhesive obtained in the experiment has better stability. There was no significant change in viscosity within 6 months.
(3) The conditions for use of the polyurethane adhesive: the diluent is ethyl acetate, the working concentration is 40%, the mass ratio of the main agent to the curing agent is 10:2.0, the curing temperature is 50-60° C., and the curing time is not less than 24 h.
