A new type of polyurethane adhesive was prepared by using small molecule polyacids and small molecule polyols as the basic raw materials to prepare prepolymers. During the chain extension process, hyperbranched polymers and HDI trimers were introduced into the polyurethane structure. The test results show that the adhesive prepared in this study has suitable viscosity, a long adhesive disc life, can be quickly cured at room temperature, and has good bonding properties, heat sealing strength and thermal stability.

Composite flexible packaging has the advantages of exquisite appearance, wide application range, convenient transportation, and low packaging cost. Since its introduction, it has been widely used in food, medicine, daily chemicals, electronics and other industries, and is deeply loved by consumers. The performance of composite flexible packaging is not only related to the film material, but also depends on the performance of the composite adhesive. Polyurethane adhesive has many advantages such as high bonding strength, strong adjustability, and hygiene and safety. It is currently the mainstream supporting adhesive for composite flexible packaging and the focus of research by major adhesive manufacturers.

High-temperature aging is an indispensable process in the preparation of flexible packaging. With the national policy goals of "carbon peak" and "carbon neutrality", green environmental protection, low-carbon emission reduction, and high efficiency and energy saving have become the development goals of all walks of life. The aging temperature and aging time have a positive effect on the peel strength of the composite film. Theoretically, the higher the aging temperature and the longer the aging time, the higher the reaction completion rate and the better the curing effect. In the actual production application process, if the aging temperature can be lowered and the aging time can be shortened, it is best not to require aging, and slitting and bagging can be carried out after the machine is off. This can not only achieve the goals of green environmental protection and low-carbon emission reduction, but also save production costs and improve production efficiency.

This study is intended to synthesize a new type of polyurethane adhesive that has suitable viscosity and adhesive disc life during production and use, can cure quickly under low temperature conditions, preferably without high temperature, and does not affect the performance of various indicators of composite flexible packaging.

1.1 Experimental materials Adipic acid, sebacic acid, ethylene glycol, neopentyl glycol, diethylene glycol, TDI, HDI trimer, laboratory-made hyperbranched polymer, ethyl acetate, polyethylene film (PE), polyester film (PET), aluminum foil (AL).
1.2 Experimental instruments Desktop electric constant temperature air drying oven: DHG-9203A, Shanghai Yiheng Scientific Instrument Co., Ltd.; Rotational viscometer: NDJ-79, Shanghai Renhe Keyi Co., Ltd.; Universal tensile testing machine: XLW, Labthink; Thermogravimetric analyzer: TG209, NETZSCH, Germany; Heat seal tester: SKZ1017A, Jinan Qingqiang Electromechanical Co., Ltd.
1.3 Synthesis method
1) Preparation of prepolymer: Dry the four-necked flask thoroughly and pass N2 into it, then add the measured small molecule polyol and polyacid into the four-necked flask and start stirring. When the temperature reaches the set temperature and the water output is close to the theoretical water output, take a certain amount of sample for acid value test. When the acid value is ≤20 mg/g, start the next step of reaction; add 100×10-6 metered catalyst, connect the vacuum tail pipe and start the vacuum pump, control the alcohol output rate by the vacuum degree, when the actual alcohol output is close to the theoretical alcohol output, take a certain sample for hydroxyl value test, and terminate the reaction when the hydroxyl value meets the design requirements. The obtained polyurethane prepolymer is packaged for standby use.
2) Preparation of solvent-based polyurethane adhesive: Add measured polyurethane prepolymer and ethyl ester into a four-necked flask, heat and stir to disperse evenly, then add measured TDI into the four-necked flask, keep warm for 1.0 h, then add the homemade hyperbranched polymer in the laboratory and continue to react for 2.0 h, slowly add HDI trimer dropwise into the four-necked flask, keep warm for 2.0 h, take samples to test the NCO content, cool down and release the materials for packaging after the NCO content is qualified.
3) Dry lamination: Mix ethyl acetate, main agent and curing agent in a certain proportion and stir evenly, then apply and prepare samples on a dry laminating machine.

1.4 Test Characterization
1) Viscosity: Use a rotational viscometer and refer to GB/T 2794-1995 Test method for viscosity of adhesives;
2) T-peel strength: tested using a universal tensile testing machine, referring to GB/T 8808-1998 peel strength test method;
3) Heat seal strength: first use a heat seal tester to perform heat seal, then use a universal tensile testing machine to test, refer to GB/T 22638.7-2016 heat seal strength test method;
4) Thermogravimetric analysis (TGA): The test was carried out using a thermogravimetric analyzer with a heating rate of 10 ℃ /min and a test temperature range of 50 to 600 ℃.

2.1 Changes in viscosity with mixing reaction time The viscosity of the adhesive and the life of the rubber disc are important indicators in the product production process. If the viscosity of the adhesive is too high, the amount of glue applied will be too large, affecting the appearance and coating cost of the composite film; if the viscosity is too low, the amount of glue applied will be too low, and the ink cannot be effectively infiltrated, which will also affect the appearance and bonding performance of the composite film. If the life of the rubber disc is too short, the viscosity of the glue stored in the glue tank will increase too quickly, and the glue cannot be applied smoothly, and the rubber roller is not easy to clean; if the life of the rubber disc is too long, it will affect the initial adhesion appearance and bonding performance of the composite material, and even affect the curing rate, thereby affecting the production efficiency of the product.

Appropriate viscosity control and the life of the adhesive disc are important parameters for the good use of adhesives. According to production experience, the main agent, ethyl acetate and curing agent are adjusted to the appropriate R value and viscosity, and the adhesive is rolled in the adhesive tank with a rubber roller without applying glue to the film. The adhesive samples are taken at different time periods for viscosity testing. Appropriate viscosity, appropriate life of the adhesive disc, and rapid curing under low temperature conditions are important goals pursued by solvent-based polyurethane adhesives during production and use.

2.2 Effect of aging temperature on peel strength The aging process is the most important, time-consuming, energy-intensive and space-intensive process for flexible packaging. It not only affects the production rate of the product, but more importantly, it affects the appearance and bonding performance of composite flexible packaging. Faced with the government's goals of "carbon peak" and "carbon neutrality" and fierce market competition, low-temperature aging and rapid curing are effective ways to achieve low energy consumption, green production and efficient production .

The PET/AL/PE composite film was aged at room temperature and at 40, 50, and 60 ℃. At room temperature, the peel strength of the inner layer AL/PE composite structure remained stable after aging for 12 h, and the curing was basically completed; at room temperature, the peel strength of the outer layer PET/AL high-barrier composite structure remained basically stable after aging for 12 h, indicating that the high-barrier film material will affect the curing of the polyurethane adhesive; comparing the curing temperature conditions of 40, 50, and 60 ℃, there was no obvious difference in the curing rate.

Compared with the mainstream solvent-based polyurethane adhesives in the current market, the high-temperature aging time is generally 48 hours or even longer. The polyurethane adhesive in this study can basically complete the curing of the high-barrier structure in 12 hours at room temperature. The developed adhesive has the function of rapid curing. The introduction of homemade hyperbranched polymers and multifunctional isocyanates in the adhesive, regardless of the outer layer composite structure or the inner layer composite structure, the peel strength under room temperature conditions is not much different from the peel strength under high-temperature aging conditions, indicating that the developed adhesive not only has the function of rapid curing, but also has the function of rapid curing without high temperature.

2.3 Effect of aging temperature on heat seal strength The heat seal characteristics of materials and the actual heat seal effect are affected by many factors, such as heat seal equipment, physical and chemical performance parameters of the material itself, heat seal time, heat seal pressure and heat seal temperature, etc. According to actual needs and experience, a reasonable heat seal process and parameters are fixed, and the heat seal strength test of the composite film after compounding is carried out.

When the composite film is just off the machine, the heat seal strength is relatively low, only 17 N/(15 mm). At this time, the adhesive has just begun to solidify and cannot provide sufficient bonding force. The strength tested at this time is the heat seal strength of the PE film; as the aging time increases, the heat seal strength increases sharply. The heat seal strength after aging for 12 hours is basically the same as that after 24 and 48 hours, indicating that the curing is basically completed in 12 hours, providing sufficient bonding for different films, resulting in increased heat seal strength. From the change curve of heat seal strength at different temperatures, it can be seen that under the same aging time conditions, there is not much difference in heat seal strength between room temperature aging and 40, 50, and 60 ℃ conditions. Aging at room temperature can completely achieve the effect of high temperature aging. The flexible packaging structure composited with this developed adhesive has good heat seal strength under high temperature aging conditions.

2.4 Thermal stability of cured film During the use of flexible packaging, heat sealing and bag making are required. In addition to the thermal stability of the film material itself, the thermal stability of the cured polyurethane film determines the performance and appearance of the finished flexible packaging product. This study uses the thermal gravimetric analysis (TGA) method to analyze the thermal stability of the cured polyurethane film.

The cured polyurethane film has two obvious weight loss peaks at the test temperature, corresponding to the thermal decomposition of the hard segment and the soft segment. The thermal decomposition temperature of the soft segment is relatively high, and thermal weight loss begins to occur at 264°C. At this temperature, it can meet the temperature requirements of the current soft packaging heat sealing process, and can meet the temperature requirements of the production of automatic packaging or filling, long-distance container transportation, and the use process; the thermal decomposition temperature of the hard segment is higher, reaching 347°C. The developed high-temperature curing-free adhesive has good thermal stability. The AC-13 asphalt mixture with steel slag increased by 2.1%.

3)When the steel slag content reaches 100%, that is, when the single particle size of 4.75 to 9.5 mm completely replaces the limestone, the residual stability value of the asphalt mixture is 85.6%, which is 0.5% higher than that of AC-13 asphalt mixture without steel slag; the splitting strength ratio is 80.8%, which is 0.5% higher than that of AC-13 asphalt mixture without steel slag. The addition of appropriate amount of steel slag can effectively improve the residual stability and splitting strength ratio of AC-13 steel slag asphalt mixture, and can effectively improve the water stability of asphalt mixture.

1)Under normal use conditions, the initial viscosity of the solvent-based polyurethane adhesive prepared by introducing homemade hyperbranched polymers and multifunctional polyisocyanates is around 1500mPa·s, which has good viscosity; the life of the adhesive disc reaches 60 min, which can fully meet the operating time requirements of flexible packaging companies in the production process.

2)It can be seen from the peel strength and heat seal strength that the prepared adhesive can cure quickly at room temperature. There is no big difference in the curing speed at room temperature and at 40, 50, and 60 ℃, and there is no big difference in the bonding strength. This adhesive can be completely cured without high temperature and can cure quickly.

3) TGA analysis shows that the adhesive has good thermal stability and can meet the temperature requirements during production, transportation and use.