An adhesive for food packaging and its use

Abstract

The present application discloses a kind of adhesive for food packaging and its application.The present application provides a kind of adhesive, it includes polyurethane, the polyurethane is prepared by the following method: polyol, polyester and isocyanate component are mixed to occur polymerization reaction.This adhesive has excellent bonding strength, oil resistance and high barrier performance etc., has wide application prospect in soft package, especially in the field of food packaging.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of adhesives for food packaging films, in particular to an adhesive for food packaging and its application. BACKGROUND

[0002] Food packaging, as an important part of the food industry, not only needs to ensure that the packaging material has sufficient strength, durability and sealing performance, but also requires it to effectively improve the gas barrier performance of the packaging film, especially the oxygen barrier performance, which is crucial to prolong the shelf life of food. Therefore, the adhesion performance and gas barrier performance of food packaging materials directly affect the packaging quality and the preservation effect of food.

[0003] Traditional food packaging adhesives are mostly solvent-based adhesives or hot melt adhesives, which have certain problems in environmental adaptability, durability and health and safety, such as harmful gas release during curing and poor adaptability to various materials. Therefore, it is of great significance to develop a polyurethane adhesive with excellent bonding strength, good debonding performance, high barrier property, environmental friendliness and wide adaptability to improve the performance and safety of food packaging materials.

[0004] Polyurethane adhesives are widely used in various industrial fields due to their adjustable hard segment and soft segment ratio, excellent adhesion and toughness, but their application in the field of food packaging still faces the challenge of how to optimize their performance to meet the requirements of food safety and packaging strength. In the prior art, although polyurethane adhesives are used in packaging, they still need to be further optimized and improved in terms of specific requirements for food packaging, such as oil resistance, debonding and barrier performance. SUMMARY

[0005] In order to overcome the shortcomings of existing food packaging adhesives in bonding strength, debonding performance and barrier performance, the present application provides a polyurethane adhesive for food packaging and its application. The adhesive not only has excellent bonding strength and debonding performance, but also meets the requirements of various environmental conditions such as oil resistance and aging resistance in food packaging, and has high oxygen barrier performance, thereby improving the overall performance of food packaging materials.

[0006] The technical scheme adopted by the present application is: 3-isocyanate propyl trimethoxysilane is grafted on the surface of hydroxyl-rich spherical alumina, and is doped with different types of isocyanate small molecules (such as hexamethylene diisocyanate, toluene diisocyanate, etc.) as a hard end, and a polyol containing active hydrogen (such as polyethylene glycol, polyoxypropylene glycol, polycaprolactone, etc.) is selected as a soft segment, the ratio of soft segment and hard segment is further adjusted, the binding force between the hard segment and the soft segment of the adhesive is enhanced, and a cross-linked structure is formed by reaction, so that the ideal adhesive performance is obtained. Through this optimized formula, the adhesive of the present application has high bonding strength, debonding characteristics and oxygen barrier property, and can be widely used in the bonding of food packaging materials.

[0007] The present application provides an adhesive for food packaging, comprising a polyurethane prepared by mixing polyols, polyesters and isocyanate components to undergo a polymerization reaction, wherein the isocyanate component comprises hydroxyl-rich spherical alumina grafted with isocyanate groups.

[0008] In the present application, the hydroxyl-rich spherical alumina grafted with isocyanate groups is hydroxyl-rich spherical alumina grafted with 3-isocyanate propyl trimethoxysilane.

[0009] In the present application, the hydroxyl-rich spherical alumina grafted with isocyanate groups can be prepared by the following method: (1) treating spherical alumina with an alkali to obtain hydroxyl-rich spherical alumina; preferably, the alkali is NaOH or KOH.

[0010] (2) reacting the hydroxyl-rich spherical alumina with a silane group-containing isocyanate to obtain hydroxyl-rich spherical alumina grafted with isocyanate groups; preferably, the silane group-containing isocyanate is 3-isocyanate propyl trimethoxysilane.

[0011] In the present application, the polyurethane can further comprise a catalyst, and the catalyst is dimethyl nitrosoamine.

[0012] In the present application, the polyol is polyethylene glycol.

[0013] In the present application, the polyester is polycaprolactone.

[0014] In the present application, the isocyanate component is one or more of hexamethylene diisocyanate, toluene diisocyanate and hydroxyl-rich spherical alumina grafted with isocyanate groups.

[0015] In the present application, the isocyanate component is a mixture of hexamethylene diisocyanate, toluene diisocyanate and hydroxyl-rich spherical alumina grafted with isocyanate groups.

[0016] In the mixture, the mass ratio of the isocyanate component, hexamethylene diisocyanate, toluene diisocyanate and the hydroxyl-rich grafted isocyanate group spherical alumina is (3-7):(3-7):0.5; preferably 5:5:0.5.

[0017] In the present application, the mass ratio of the polyol, the polyester and the isocyanate component is 10:(4-6):(0.8-1.2); preferably 10:5:1.

[0018] The present application also provides a preparation method of the adhesive for food packaging, comprising the following steps: dissolving the above-mentioned polyurethane in a solvent to obtain the adhesive for food packaging; preferably, the solvent is ethyl acetate.

[0019] In the present application, the solid content of the adhesive for food packaging is 20-70% by weight; preferably 50% by weight.

[0020] The present application also provides the use of the above-mentioned adhesive for food packaging in food packaging bags.

[0021] On the basis of common general knowledge in the art, the above-mentioned preferred conditions can be combined arbitrarily, thereby obtaining various preferred examples of the present application.

[0022] The reagents and raw materials used in the present application are commercially available.

[0023] The positive progress effects of the present application are as follows: (1) By introducing a new hard end material and adjusting the ratio of hard segment and soft segment, the initial bonding strength of the adhesive is optimized.

[0024] (2) The stability of the adhesive in harsh environments such as anti-aging and oil resistance is enhanced, and the operability in practical application is ensured without affecting the bonding strength of the packaging material.

[0025] (3) An environmentally friendly and safe formula is used, which meets the requirements of food safety, and is not prone to side reactions during curing, is easy to operate and has better stability. The adhesive not only improves the overall performance of food packaging, but also has good market application prospects. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 It is the FT-IR spectrum of the hydroxyl-rich spherical alumina.

[0027] Figure 2 It is the FT-IR spectrum of the hydroxyl-rich spherical alumina grafted with 3-isocyanate propyl trimethoxysilane. DETAILED DESCRIPTION

[0028] The application will be further illustrated by the following examples without limiting the application to the examples described. The experimental methods in the following examples, unless otherwise specified, are selected according to the conventional methods and conditions, or according to the commercial instructions.

[0029] The application further relates to the adhesive composition according to the application for use as an adhesive for food packaging films. The application will be explained in more detail with reference to the following examples, which are not intended to limit the inventive concept.

[0030] General method for preparing the adhesive: Step one, preparation of hydroxyl-riched spherical alumina.

[0031] This step aims to introduce hydroxyl groups onto the surface of the spherical alumina through a hydroxyl-riching treatment, to improve its binding ability with 3-isocyanate propyl trimethoxysilane, thereby enhancing the cross-linking effect between the hard segment and the soft segment of the adhesive.

[0032] (1) Cleaning of the alumina surface: Spherical alumina (10 g, particle size: 0.2 μm) was added to 500 mL of deionized water and stirred thoroughly to remove impurities. The alumina was filtered out by vacuum filtration and rinsed with deionized water several times until no obvious impurities were present in the filtrate. The cleaned spherical alumina was dried in an oven at 120°C for 12 hours until it was completely dry, obtaining dried spherical alumina.

[0033] (2) Surface hydroxyl-riching treatment: 2 g of sodium hydroxide was added to 100 mL of deionized water and stirred to dissolve, obtaining a 2% (w / v) sodium hydroxide solution. The cleaned and dried spherical alumina (10 g) was added to a 500 mL beaker and 200 mL of the 2% sodium hydroxide solution was added. The beaker was placed on a heating plate and heated to 60°C, and the temperature was maintained for 4 hours of stirring to ensure that the sodium hydroxide solution was in full contact with the surface of the alumina. During the reaction, the alkaline conditions of the sodium hydroxide caused a large number of hydroxyl (OH) groups to be generated on the surface of the alumina, increasing the hydrophilicity of the surface.

[0034] (3) Neutralization and washing: After the reaction was completed, the reaction solution was cooled to room temperature and slowly neutralized to neutral (pH = 7) using dilute sulfuric acid (concentration about 1 M). This step can remove excess sodium hydroxide and further enhance the stability of the hydroxyl groups on the surface of the alumina. After neutralization, the spherical alumina was separated by vacuum filtration and washed thoroughly with deionized water until the pH of the filtrate was close to 7. The cleaned alumina was further washed with ethanol for 3 times to remove residual water and impurities.

[0035] (4) Drying and Characterization: The washed hydroxyl-rich spherical alumina was placed in an oven and dried at 80°C for 12 hours to obtain the hydroxyl-rich spherical alumina powder. Fourier Transform Infrared Spectroscopy (FT-IR) analysis was used to characterize the surface of the hydroxyl-rich alumina to observe whether there were hydroxyl characteristic peaks (about 3200 cm -1 ) and other possible surface changes.

[0036] Through the above steps, the hydroxyl-rich treatment of spherical alumina was successfully achieved. A large number of hydroxyl groups were introduced onto the surface, enhancing its hydrophilicity and reactivity with isocyanate groups. The FT-IR spectrum showed a hydroxyl characteristic peak at 3200 cm -1 , proving that the surface was successfully enriched with hydroxyl groups. The treated spherical alumina could provide a good reaction basis for subsequent silane grafting reactions, thereby improving the performance of the polyurethane adhesive.

[0037] Step Two, Preparation of Grafted 3-Isocyanate Propyl Trimethoxysilane Hydroxyl-Rich Spherical Alumina.

[0038] The purpose of this step is to react the hydroxyl-rich spherical alumina with 3-isocyanate propyl trimethoxysilane (A-1120) to graft isocyanate groups onto the surface of the alumina, enhancing its bonding force with the polyurethane hard segment and further improving the performance of the adhesive.

[0039] (1) Alumina Surface Activation: Place the hydroxyl-rich spherical alumina (10 g, prepared in Step One) in a 50 mL dry round-bottom flask and add 30 mL of absolute ethanol. Add 5 mL of deionized water to the flask and stir until the hydroxyl groups on the surface of the alumina are activated, making them more active for reaction with 3-isocyanate propyl trimethoxysilane.

[0040] (2) Grafting Reaction: Add 3-isocyanate propyl trimethoxysilane (A-1120, 5 g) to the above alumina-ethanol-water mixed solution and stir until uniform. Continue stirring the reaction mixture at 60°C for 4 hours. During the reaction, the ethoxy groups of 3-isocyanate propyl trimethoxysilane undergo condensation reaction with the surface hydroxyl groups of the alumina, forming siloxane bonds (Si-O-Al).

[0041] (3) Post-Reaction Treatment: After the reaction is complete, cool the reaction mixture to room temperature. Wash the reaction product with deionized water and ethanol several times to remove unreacted silane and solvent. After filtration, dry the grafted alumina in an oven at 80°C for 12 hours to obtain dry grafted alumina powder.

[0042] (4) Characterization analysis: Fourier transform infrared spectroscopy (FT-IR) was used to characterize the grafted alumina. It was analyzed whether 3-isocyanate propyl trimethoxysilane was successfully grafted. The characteristic peak of the grafted isocyanate group appeared at 2270 cm -1 nearby.

[0043] Through the above steps, 3-isocyanate propyl trimethoxysilane was successfully grafted to the surface of the hydroxyl-rich spherical alumina. FT-IR spectrum showed that the characteristic peak of the isocyanate group at 2270 cm -1 appeared obviously, indicating that the grafting reaction was successful.

[0044] Step three, preparing the adhesive.

[0045] This example aims to prepare a polyurethane adhesive with excellent performance by dispersing and doping the grafted alumina with different types of isocyanate and carrying out a polymerization reaction with polyhydric alcohol. The adhesive has high bonding strength and good flexibility, and can be widely used in the bonding of food packaging materials.

[0046] (1) Preparation of adhesive raw material mixture: 0.5 g of 3-isocyanate propyl trimethoxysilane grafted hydroxyl-rich spherical alumina (prepared in step two) was added to a 250 mL flask, 30 mL of toluene was added, and stirred until the grafted alumina was uniformly dispersed in the solvent. At the same time, 10 g of hexamethylene diisocyanate (HDI) and toluene diisocyanate (TDI) with different mass ratios were weighed into two different flasks, and 10 mL of toluene was added to dissolve completely. Then, the HDI and TDI solutions were mixed in proportion to obtain a mixed isocyanate solution.

[0047] (2) Isocyanate dispersion doping: The mixed isocyanate solution was slowly added to the grafted alumina dispersion, and stirred until the isocyanate molecules were uniformly dispersed on the surface of the grafted alumina. The temperature was controlled at about 30°C, and stirring was maintained for about 30 minutes to ensure uniform dispersion of the isocyanate molecules on the surface of the grafted alumina. The mixture was transferred to an oven and dried at 80°C for 12 hours to remove excess solvent toluene, obtaining grafted alumina powder doped with isocyanate.

[0048] (3) Preparation of polyurethane adhesive by polymerization reaction: In a 250 mL flask, take 10 g of polyethylene glycol (PEG-2000) and 5 g of polycaprolactone (PCL-1000), add 60 mL of toluene, and stir until dissolved. This step can effectively ensure that the polyol is fully dissolved in the solvent and provide a uniform reaction environment for the polymerization reaction. Heat the above solution to 70°C and add 0.2 g of dimethyl nitrosamine (DMAPA) as a catalyst, continue to stir for 30 minutes to ensure the activation of the polyol mixture. Slowly add the isocyanate-doped grafted alumina powder (1 g) and continue to stir the reaction. At this time, the polymerization reaction between the isocyanate groups and the polyols will occur, forming a polyurethane skeleton. During the reaction, the isocyanate groups adduct with the hydroxyl groups in polyethylene glycol and polycaprolactone to form polyurethane chains. The reaction is continued at 70°C for 2 hours to ensure complete polymerization.

[0049] (4) Removal of solvent and preparation of final product: After the reaction is complete, cool the reaction system to room temperature, wash the reaction product with deionized water to remove residual catalyst, solvent and unreacted raw materials. Then, transfer the adhesive solution to an oven and dry at 80°C for 12 hours, then add ethyl acetate to prepare an adhesive solution with a solid content of 50% by weight, thereby obtaining a polyurethane adhesive product.

[0050] Example 1: Adhesive sample A: According to the general method for preparing the adhesive, HDI and TDI are mixed in a ratio of 1:1, the temperature is 70°C, and the reaction time is 2 hours, and the prepared adhesive is obtained.

[0051] Example 2: Adhesive sample B: According to the general method for preparing the adhesive, the ratio of HDI to TDI is adjusted to 1:2, and the reaction temperature is increased to 80°C; the reaction time is 2 hours, and the prepared adhesive is obtained.

[0052] Example 3: Adhesive sample C: According to the general method for preparing the adhesive, the reaction time is extended to 3 hours, the temperature is kept at 70°C, and the ratio of HDI to TDI is kept at 1:1; the prepared adhesive is obtained.

[0053] Adhesive 4: Adhesive sample D: According to the general method for preparing the adhesive, the reaction time is shortened to 1 hour, the temperature is kept at 70°C, and the ratio of HDI to TDI is kept at 1:1; the prepared adhesive is obtained.

[0054] Example 5: Adhesive sample E: According to the general method for preparing the adhesive, the reaction time is shortened to 1 hour, the temperature is kept at 70°C, and the ratio of HDI to TDI is kept at 2:1; the prepared adhesive is obtained.

[0055] Example 6: Adhesive sample F: According to the general method for preparing adhesive, the reaction time was extended to 3 hours, the temperature was maintained at 70 °C, and the ratio of HDI to TDI was maintained at 2:1; the prepared adhesive.

[0056] Example 7: Adhesive sample G: According to the general method for preparing adhesive, but without using the grafted 3-isocyanate propyl trimethoxysilane hydroxyl-rich spherical alumina raw material, the ratio of HDI to TDI was 1:1, the temperature was 70 °C, and the reaction time was 2 hours; the prepared adhesive.

[0057] Effect example: Performance test of adhesives prepared under different conditions.

[0058] This example evaluates the bonding performance of adhesives prepared under different conditions through comprehensive performance testing. The following aspects are mainly investigated: viscosity of adhesive solution, adhesion strength, initial adhesion after aging, peel strength after sterilization, and content resistance. The samples selected in this example are representative samples and are not limited to the sample preparation conditions of the present application.

[0059] Materials and reagents: Test substrates: aluminum foil (domestic, thickness 50 μm), polyethylene terephthalate (PET) film (domestic, thickness 100 μm), cast polypropylene (CPP) film (domestic, thickness 80 μm).

[0060] Reagents and solvents: deionized water, ethanol, toluene.

[0061] Test equipment and tools: rotational viscometer (Brookfield, DV-II), tensile testing machine (Instron, 5965), oxygen transmission rate tester (MOCON, OX-TRAN 2 / 40), water vapor transmission rate tester (MOCON, AQUATRAN 3 / 40), oven, constant temperature and humidity chamber, titration flask, ultraviolet sterilization box (UltraClean 1000).

[0062] (1) Adhesive sample preparation: The preparation methods of samples A, B, C, D, E, F and G refer to the steps in Example 3, but the ratio of HDI to TDI, the reaction temperature and the reaction time are adjusted under different reaction conditions. Ensure that each sample has sufficient curing degree and that the solvent and unreacted components are completely removed.

[0063] (2) Preparation of food packaging film samples: First, the adhesive was uniformly coated on the surface of PET and CPP films, and the coating amount of solid components was adjusted to 4 g / m2. Then, the PET and CPP films coated with the adhesive were dried at 80°C for 10 minutes to obtain the PET and CPP films coated with the adhesive. Next, the dark side of the aluminum foil was covered on the surface of the PET film coated with the adhesive, and the bright side of the aluminum foil was covered on the surface of the CPP film coated with the adhesive; and the two films were pressed together using a flat pressing device at a closed pressure of 50°C and 1.0 MPa for 30 minutes. Finally, the pressed film was aged at 50°C for 3 days to obtain the final food packaging film sample.

[0064] (3) Adhesive performance test: ① Adhesive solution viscosity test: The viscosity of each adhesive solution was measured using a rotational viscometer (Brookfield, DV-II), and the test conditions were as follows: measurement temperature: 25°C, rotation speed: 10 rpm, sample amount: 2 mL.

[0065] ② Adhesion strength test: The adhesion strength of different adhesive samples was tested using a tensile testing machine. The food packaging films prepared with different adhesives were cut into 15 mm wide samples and subjected to 90° tensile peeling test. The test conditions were as follows: test speed: 300 mm / min, test temperature: 25°C (room temperature). The maximum tensile strength was recorded.

[0066] ③ Initial adhesion test after aging: The food packaging films prepared with different adhesive samples were cut into 15 mm wide samples and placed in an oven at 60°C for aging treatment for 48 hours. Then, the initial adhesion after aging was evaluated, and the test method was as follows: the adhesion strength of the adhesive to PET film and aluminum foil after aging was measured using a tensile testing machine. The initial peeling strength was recorded using a peeling test (tensile speed: 300 mm / min).

[0067] ④ Peeling strength test after sterilization: The food packaging films prepared with different adhesive samples were placed in a UV sterilization box for sterilization treatment, and the sterilization conditions were as follows: UV wavelength 254 nm, sterilization time 30 minutes. After sterilization, the peeling strength of the adhesive was evaluated by a peeling test (tensile speed: 300 mm / min).

[0068] ⑤ Oxygen barrier performance (oxygen transmission rate) test: The food packaging films prepared with different adhesive samples were cut into 15 mm wide samples, and the oxygen transmission rate was tested using a constant pressure method. The test conditions were as follows: oxygen flow rate 0.2 mL / min, test temperature 25°C (room temperature), test time 4 hours. The barrier performance of the food packaging film was evaluated by the oxygen transmission rate result (unit: cm 3 / m²·24h·0.1MPa), and the lower the value, the better the barrier performance.

[0069] (6) Water barrier property (water vapor transmission rate) test: cut the food packaging film prepared by different adhesive samples into 15 mm wide samples, simulate the regular storage environment, and use the equilibrium method to test the water vapor transmission rate. The test conditions are as follows: test temperature 25°C (room temperature), humidity 50% RH, test time 4 hours. The water barrier property of the food packaging film is evaluated by the water vapor transmission rate result (unit: g / m²·24h), the lower the value, the better the barrier property.

[0070] (7) Content resistance test: each adhesive is coated on the surface of PET film or CPP film and aluminum foil, respectively, to form a PET film + adhesive + aluminum foil + adhesive + CPP film structure. After the adhesive is cured for 24 hours, the content resistance test is performed. The sterilized food packaging bag is stored with the fake food in a 40°C environment for 14 days, and the packaging bag remains unopened. After the storage period, the food packaging bag is cut open and the bonding condition between the PET film and / or CPP film and aluminum foil is visually inspected to determine whether delamination (such as lifting or peeling) occurs. The delamination condition is evaluated as follows: excellent (no delamination), medium (slight delamination), and poor (obvious delamination).

[0071] The test results are as follows:

[0072] Based on the test results of the above seven samples, samples A and B perform best overall, especially in initial adhesion, adhesion strength, peel strength after sterilization, and oxygen and water vapor barrier properties, which show excellent performance. They are suitable for application scenarios that require high adhesion of packaging film and may be subjected to sterilization, and can effectively block oxygen and water vapor to prevent food from being oxidized and deteriorated.

[0073] Samples D and E perform poorly in multiple tests, significantly lower than other samples, which may be due to insufficient cross-linking reaction caused by too short reaction time, and weak internal forces caused by insufficient cross-linking reaction.

[0074] The hard segment component in the control sample G does not contain grafted alumina, and the other preparation conditions are consistent with those of sample A. The results show that the addition of grafted alumina adjusts the hard segment component, which greatly improves the adhesion indicators of the adhesive and the oxygen and water vapor barrier properties.

[0075] Comprehensively, the adhesive has obvious advantages in multiple performance indexes, can keep strong initial adhesion and adhesion strength after aging, has excellent peeling strength after sterilization treatment, and has high barrier effect on oxygen and water vapor. By reasonably adjusting the viscosity of the adhesive solution and the coating process, the food packaging demand can be accurately controlled, and the adhesive has high market application potential.

Claims

1. An adhesive for food packaging, characterized in that, It comprises polyurethane, which is prepared by mixing polyol, polyester and isocyanate component to occur polymerization reaction, wherein the isocyanate component is a mixture of hexamethylene diisocyanate, toluene diisocyanate and hydroxyl-rich spherical alumina grafted with isocyanate groups, wherein the ratio of hexamethylene diisocyanate and toluene diisocyanate is 1:2; The hydroxyl-rich spherical alumina grafted with isocyanate groups is hydroxyl-rich spherical alumina grafted with 3-isocyanate propyl trimethoxysilane; The mass ratio of the polyol, the polyester and the isocyanate component is 10: (4-6): (0.8-1.2).

2. The adhesive for food packaging according to claim 1, wherein The hydroxyl-rich spherical alumina grafted with isocyanate groups is prepared by (1) treating spherical alumina with alkali to obtain hydroxyl-rich spherical alumina, wherein the alkali is NaOH or KOH; (2) reacting the hydroxyl-rich spherical alumina with silane group-containing isocyanate to obtain the hydroxyl-rich spherical alumina grafted with isocyanate groups; and the silane group-containing isocyanate is 3-isocyanate propyl trimethoxysilane.

3. The adhesive for food packaging according to claim 1, wherein The polyurethane further comprises a catalyst, and the catalyst is dimethyl nitrosoamine.

4. The adhesive for food packaging according to claim 1, wherein The polyol is polyethylene glycol; and / or the polyester is polycaprolactone; and / or the isocyanate component is hexamethylene diisocyanate, toluene diisocyanate and hydroxyl-rich spherical alumina grafted with isocyanate groups.

5. The adhesive for food packaging according to claim 1, wherein The mass ratio of the polyol, the polyester and the isocyanate component is 10:5:

1.

6. Use of the adhesive for food packaging according to any one of claims 1-5 in a food packaging bag.

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