Natural Antioxidant Hydroxypropyl Methyl Cellulose Water Soluble Packaging Film

Abstract : AOB/HPMC composite water-soluble packaging film with antioxidant properties was prepared by adding bamboo leaf antioxidant (AOB) to hydroxypropyl methylcellulose (HPMC) film-making liquid by casting method. The changes in functional groups and crystallization of the composite film were characterized by infrared absorption spectroscopy, X-ray diffraction, and thermogravimetric analysis. The light transmittance and haze of the sample film were measured. The DPPH free radical scavenging experiment was used to analyze the film Antioxidant properties. The effects of AOB addition on the antioxidant properties, water solubility, mechanical properties and optical properties of composite packaging films were studied. The experimental results show that the addition of AOB does not destroy the groups of HPMC itself, but only disturbs the arrangement of molecular chains, resulting in a decrease in the crystallinity of the AOB/HPMC composite film, an increase in thermal decomposition temperature, enhanced thermal stability, and oxidation resistance. properties; and with the increasing amount of AOB, the water solubility of the composite water-soluble packaging film continues to increase, the mechanical properties and optical properties decrease, but the decline is not large, and the oxidation resistance increases first and then decreases. When the AOB When the added mass fraction was 0.03%, the scavenging rate of DPPH free radicals of the composite film reached the maximum value, which was 89.34%.

Key words: hydroxypropyl methylcellulose; antioxidant properties; water solubility; packaging film

0.Preface

In recent years, with the continuous improvement of people's requirements for food packaging, various new packaging technologies have been developed and applied, among which anti-oxidation packaging is the most promising packaging technology. The so-called anti-oxidation packaging is a packaging technology that adds antioxidants to food packaging materials to improve the packaging environment of the packaged food, slow down the oxidation rate of food, and extend the shelf life of food. Packaging material researchers have prepared various anti-oxidative packaging films by adding antioxidants to commonly used plastic film materials, such as polyethylene, polypropylene, ethylene-vinyl alcohol copolymer, etc. corresponding research. At present, the application of synthetic antioxidants is relatively common, but there are still many controversies about the safety of synthetic antioxidants. Therefore, it is of great practical significance to use safer antioxidants to make anti-oxidation packaging materials.

Natural antioxidants have received extensive attention from packaging material researchers because of their high safety and benefits to the human body. For example, C. Wessling added vitamin E to low-density polyethylene materials, and found that the improved material can effectively inhibit the oxidation of linoleic acid emulsion, but low-density polyethylene materials are not degradable, and there are serious environmental pollution problems; and E . Portes et al. prepared chitosan films with antioxidant properties by adding natural antioxidant tetrahydrocurcumin to chitosan films.

Antioxidant of bamboo (AOB) is a natural antioxidant with good water solubility, which has been listed in GB 2760-2014 "National Food Safety Standard Food Additive Use Standard" and approved to be used as a natural food antioxidant , can be used as an additive for baking, frying and other foods.

As a natural cellulose derivative, hydroxypropyl methylcellulose (HPMC) has good water solubility, good film-forming properties and excellent stability, and is widely used in cosmetics, medicine and food and other fields. At present, it has been reported that many scientific researchers at home and abroad use HPMC as a film coating agent to apply it in the preservation of fruits such as plums, citrus, grapes, and red bayberry; Research on the preparation of composite packaging film by blending glucomannan and others. However, there are few reports on the anti-oxidation water-soluble packaging film based on HPMC and the effect of antioxidant on its performance. Therefore, this paper intends to use bamboo leaf antioxidants as natural antioxidant additives to prepare HPMC water-soluble composite packaging films containing different mass fractions of bamboo leaf antioxidants, and use Fourier transform infrared spectroscopy (FTIR) and wide-angle X-ray Diffraction (X-ray diffraction, XRD) instrument, thermogravimetric analysis (thermo gravimetric analysis, TGA) instrument, etc. research the antioxidant properties, water solubility, mechanical properties, optical properties, etc. The application provides a certain reference basis.

1. Experimental part

1.1 Experimental materials

Hydroxypropyl methylcellulose: the mass fraction of methoxyl group is 26.04%, the mass fraction of hydroxypropoxyl group is 6.08%, the viscosity is 6 980 mPa·s, produced by KIMA CHEMICAL CO., LTD;

Bamboo leaf antioxidant: food grade, produced by Henan Yuzhong Biotechnology Co., Ltd.;

1,1-diphenyl-2-trinitrophenylhydrazine (1,1-diphenyl2-picrylhydrazyl radical 2,2-diphenyl-1-(2,4,6-trinitrophenyl)hydrazyl, DPPH): Analytical grade, Shanghai Produced by Polyene Biotechnology Co., Ltd.;

Anhydrous ethanol: analytically pure, produced by Hangzhou Gaojing Fine Chemical Co., Ltd.;

Distilled water: homemade.

1.2 Instruments and equipment

Nicolet 5700 Fourier transform infrared spectrometer, produced by American Thermoelectric Company;

ARL XTRA type X-ray diffractometer, produced by Japan Electron Optics Laboratory (JEOL);

PYRIS-1 thermogravimetric analyzer, produced by Perkin-Elmer, USA;

Lambda 900 ultraviolet spectrophotometer, produced by Perkin Elmer Company of the United States;

INSTRON (5943) Miniature Electronic Universal Tensile Testing Machine is produced by Instron (Shanghai) Testing Equipment Trading Co., Ltd.;

WGE-S light transmission haze tester, produced by Shanghai Precision Scientific Instrument Co., Ltd.;

EP114C electronic balance, produced by Shanghai Precision Instrument Co., Ltd.;

HK7-02 constant speed magnetic stirrer, produced by Shanghai Leici Chuangyi Instrument Co., Ltd.;

DHG-9140A type electric heating constant temperature blast drying oven, produced by Shanghai Jinghong Experimental Equipment Co., Ltd.

1.3 Preparation of samples

Hydroxypropyl methylcellulose water-soluble packaging films with different additions of bamboo leaf antioxidants were prepared by solution casting method. uniform; then add a certain proportion (mass fraction added is 0, 0.01%, 0.03%, 0.05%, 0.07%, 0.09%) of bamboo leaf antioxidants to the hydroxypropyl methylcellulose film-forming solution, and continue to stir until Mix well; stand at room temperature for 3-5 minutes (degassing), and then obtain HPMC film-forming solution containing different mass fractions of bamboo leaf antioxidants; finally, inject the film-forming solution into a glass petri dish to form a film, and Put it in a blast drying oven at 40-50 ℃ and dry it. After peeling off the film, put it in the drying oven and store it for later use.

Hereinafter, the prepared bamboo leaf antioxidant/hydroxypropyl methylcellulose composite water-soluble packaging film is referred to as AOB/HPMC film, and the hydroxypropyl methylcellulose water-soluble packaging film as a reference sample is referred to as HPMC film.

1.4 Testing and Analysis

1.4.1 FTIR analysis

FTIR is a powerful tool for characterizing functional groups contained in molecular structures and for identifying functional groups. The infrared absorption spectrum of the packaging film used in the experiment was measured by Fourier transform infrared spectrometer. In this experiment, the thin film method was adopted, the scanning range was 500-4 000 cm-1, and the number of scanning was 32. And the sample film was dried in a drying oven at 50 °C for 24 h before the infrared spectrum measurement.

1.4.2 XRD analysis

XRD is the analysis of the crystalline state of substances at the molecular level, and is determined by X-ray diffractometer. The measurement conditions are as follows: the X-ray source is nickel-filtered Cu-kα line (40 kV, 40 mA); the scanning angle is 0°~80° (2θ); the scanning speed is 6 (°)/min.

1.4.3 TGA analysis

The purpose of thermogravimetric analysis is to study the thermal stability of materials. In this experiment, a thermogravimetric analyzer was used for thermogravimetric analysis. The tested samples were first dried in a vacuum oven, and then loaded into an alumina crucible for thermogravimetric testing. During the test, the temperature rose at a rate of 20 °C/min. The test range was 25~500 ℃; air is used as the protective gas, and the test is carried out at a rate of 40 mL/min to study the weight loss of the film as the temperature increases, and analyze the thermal stability of the film through the TGA curve.

1.4.4 Determination of antioxidant properties

In order to test the oxidation resistance of the prepared HPMC film and AOB/HPMC film, the DPPH free radical scavenging method was used to measure the scavenging rate of the film to DPPH free radical, so as to indirectly measure the oxidation resistance of the film.

1) Preparation of DPPH solution. Under the condition of avoiding light, take 2 mg of DPPH, dissolve it in 40 mL of ethanol solvent, treat it with ultrasound for 5 min, and shake it sufficiently to make the upper and lower parts uniform. Put the mixed DPPH solution in the refrigerator, and refrigerate at 4 °C for later use.

2) Determination of free radical scavenging rate. Refer to the experimental method of Fan Chen et al., and slightly modify it. First, add 2 mL of DPPH solution into a test tube, then add 1 mL of distilled water, shake and mix thoroughly, measure the absorbance value of the solution at 523 nm with a UV spectrophotometer, and record this value as A0. Then take 2 mL of DPPH solution and add it to the test tube, then add 1 mL of HPMC thin film solution, shake and mix thoroughly, measure the absorbance value of the solution with a UV spectrophotometer, and record this value as A. Water was used as the blank control during the test, and 3 parallel data were taken for each group.

1.4.5 Determination of water solubility

Cut a sample film of 30 mm×30 mm and a thickness of about 45 μm, add 100 mL of water to a 200 mL beaker, place the film in the center of the water surface, and measure the time for the film to completely disappear. Each sample was measured 3 times, and the average value was taken, and the unit was min.

1.4.6 Determination of mechanical properties

The tensile strength and elongation at break of a film are often used as criteria for judging its mechanical properties. Tensile strength refers to the maximum tensile stress suffered by the film sample when it is pulled off, in MPa; elongation at break is the ratio of the length of the film when it is pulled to the original length, in %. Using a miniature electronic universal tensile testing machine, in accordance with the requirements in GB 13022-1991 "Test Method for Tensile Properties of Plastic Films", the temperature of the film is 25 ℃ and the relative humidity (relative humidity, RH) is 50%. Mechanical properties, and samples with uniform thickness, clean surface and no impurities are selected for testing. Each sample was tested 3 times and the average value was taken.

1.4.7 Determination of optical properties

Optical properties are an important parameter for evaluating packaging films, mainly including light transmittance and haze of the film. The light transmittance and haze of the sample films were measured by a light transmission haze tester. Select a test sample with a clean surface and no creases, gently place the sample on the test frame, fix it with a suction cup, and measure the light transmittance and Haze, each sample was tested 3 times, and the average value was taken.

1.5 Data processing

Excel was used to process experimental data, and Origin software was used to draw graphs.

2. Results and Discussion

2.1 FTIR analysis

The characteristic skeleton vibration peaks of bamboo leaf antioxidants are mainly concentrated in the region of 2 010-2 500 cm-1, which is the characteristic absorption peak region of the stretching vibration of its triple bond and cumulative double bond. Among them, the region of 2 280-2 265 cm-1 is the characteristic absorption peak area of stretching vibration of -N==C==O.

The characteristic skeleton vibration peaks of HPMC films are mainly concentrated in the two regions of 2 600-3 700 cm-1 and 750-1 700 cm-1. Among them, the absorption band around 3 418 cm-1 is caused by the stretching vibration of the O—H bond, the characteristic absorption peak at 2 935 cm-1 is the stretching vibration of the -CH2- group, and the primary and secondary band at 1 050 cm-1 The stretching vibration characteristic absorption peaks of —C—O—C— and —C—O— on the hydroxyl group, 1 657 cm-1 is the characteristic absorption peak produced by the hydroxyl group on the benzene ring during the stretching vibration of the skeleton, and at 945 cm-1 1 is the rocking absorption peak of -CH3. The absorption peaks at 1 454, 1 373, 1 315, 945 cm-1 are assigned to the characteristic absorption peaks of asymmetric, symmetric deformation vibration, in-plane and out-of-plane bending vibration of CH3, respectively.

Comparing the infrared spectra of HPMC film and AOB/HPMC film, it can be seen that AOB is used to modify HPMC. With the addition of AOB, the positions of the characteristic peaks of AOB/HPMC film do not shift, indicating that the addition of AOB does not destroy HPMC itself. group. The characteristic peaks of AOB/HPMC films at 2 935 cm-1 and 1 050 cm-1 did not change significantly, but the stretching vibration peak of O—H bond weakened near 3 418 cm-1. This shows that the number of hydroxyl groups in the film decreases, and the change of the peak shape is mainly caused by the change of the adjacent methyl or methylene bands induced by hydrogen bonds. It can be seen that the addition of AOB has an effect on the intermolecular hydrogen bonds. From the infrared spectrum of the AOB/HPMC thin film, it can be known that after adding AOB, a new absorption peak appears in the curve near 2 270 cm-1. Comparing the infrared spectrum of AOB, it can be seen that this absorption peak is the The characteristic absorption peaks of triple bond and cumulative double bond stretching vibration near cm-1 provide a basis for the anti-oxidation properties of AOB/HPMC films.

2.2 XRD analysis

The XRD spectrum of the HPMC film has three diffraction peaks located at 24.6°, 32.6° and 47.5°, respectively. Among them, there is a diffuse broad peak near 2θ=24.6°, the diffraction peak near 32.6° is the crystallization peak of HPMC film, and a small diffraction peak near 47.5°. With the addition of AOB, in the XRD spectrum of the AOB/HPMC composite film: the diffraction peak near 24.6° weakens, the crystallization peak near 32.6° weakens obviously, and the diffraction peak near 47.5° basically disappears. This result indicated that with the addition of AOB, the crystallinity of AOB/HPMC films decreased, which indicated that the addition of AOB disrupted the order of HPMC molecular chains.

2.3 TGA analysis

The thermal weight loss process of HPMC film and AOB/HPMC film can be divided into two stages: the first stage is the weight loss caused by the loss of bound water in the material, and the weight loss in this stage is small; the second stage is caused by the HPMC film under high temperature. The weight loss caused by the decomposition, the weight loss at this stage is relatively large. It can be seen from the thermogravimetric analysis curve that the thermogravimetric loss of the film mainly occurs in the range of 250-350 ℃, and there is only one decomposition point, indicating that both the HPMC film and the AOB/HPMC film are decomposed in one step.

Mutually

Compared with the pure HPMC film, the initial decomposition temperature of the AOB/HPMC film was increased by only 3 °C; however, the decomposition temperature T20% of its thermal decomposition was increased by 12 °C. This result indicated that the addition of the antioxidant AOB increased the thermal decomposition temperature of the AOB/HPMC film. This indicates that the AOB/HPMC film has better thermal stability than the pure HPMC film.

2.4 Antioxidant properties

In order to investigate the effect of different AOB additions on the oxidation resistance of AOB/HPMC films, the composite films prepared when the AOB content was 0, 0.01%, 0.03%, 0.05%, 0.07%, and 0.09% were studied respectively. DPPH free radical scavenging rate.

The addition of AOB antioxidants significantly improved the scavenging rate of HPMC films for DPPH free radicals, that is, improved the oxidation resistance of the films. And with the increase of the mass fraction of AOB added, the scavenging of DPPH free radicals by the AOB/HPMC composite film increased firstly and then decreased gradually. When the added mass fraction of the antioxidant AOB is 0.03%, the AOB/HPMC film has the best effect on the scavenging rate of DPPH free radicals. At this time, the scavenging rate of DPPH free radicals reaches 89.34%. It can be seen that the AOB/HPMC film at this time The anti-oxidation performance of the film is the best; when the mass fraction of AOB added is 0.05% and 0.07%, the scavenging rate of AOB/HPMC film for DPPH radicals is slightly higher than that of the 0.01% group, but significantly lower than that of the added mass fraction A group with a score of 0.03%. This may be due to the addition of excessive natural antioxidant AOB, which leads to the aggregation of AOB molecules, so that its distribution in the composite film is uneven, which affects the antioxidant effect of AOB/HPMC film. It can be seen that the AOB/HPMC film prepared in the experiment has good oxidation resistance, and when the mass fraction of AOB added is 0.03%, the oxidation resistance of the obtained AOB/HPMC film is the strongest.

2.5 Water solubility

The water solubility of HPMC films was significantly affected by different AOB additions. After adding the antioxidant AOB, as the mass fraction of AOB increases, the water-soluble time of the film is shorter, indicating that the water solubility of the AOB/HPMC film is better. That is, the addition of AOB improved the water solubility of AOB/HPMC films. From the previous XRD analysis, it can be known that after adding AOB, the crystallinity of AOB/HPMC film decreases, making it easier for water molecules to enter the AOB/HPMC film, so the water solubility of AOB/HPMC film is improved to a certain extent.

2.6 Mechanical properties

Different AOB additions have a significant effect on the mechanical properties of the film. After adding AOB, the tensile strength of the composite film decreased with the increase of AOB content, and the decreasing range became smaller and smaller, while the elongation at break increased rapidly and then gradually decreased with the increase of AOB content. When the mass fraction of AOB added was 0.01%, the elongation at break of the composite film reached the maximum value, about 45%. It can be seen that the addition of AOB has a more obvious effect on the mechanical properties of HPMC films.

The XRD analysis results show that the addition of AOB will reduce the crystallinity of AOB and HPMC films, thus reducing the tensile strength of AOB/HPMC films. Due to the existence of intermolecular bonds, the fluidity of the macromolecular chain is poor, and the HPMC film contains a rigid structure, and the film is brittle. AOB has good mutual solubility and water solubility. In the process of miscibility with the macromolecular solution, Weaken the force between the molecular chains, soften the rigid structure of the HPMC film, and make the AOB/HPMC film soft, thus, the elongation at break of the AOB/HPMC film increases. As the amount of AOB added continues to increase, the elongation at break of the AOB/HPMC film decreases, because the AOB in the AOB/HPMC film makes the gap between the macromolecular chains large, and there is no entanglement point between the macromolecules. It is easy to break when the force is applied, so that the elongation at break of the AOB/HPMC film is reduced.

2.7 Optical properties

With the increase of the mass fraction of AOB added in the AOB/HPMC film, the light transmittance decreased slowly at first, and then decreased rapidly when the mass fraction of AOB added exceeded 0.05%; the haze first increased slowly and then increased rapidly. When the added mass fraction of AOB does not exceed 0.05%, the light transmittance and haze of the AOB/HPMC film change slowly; when the added mass fraction of AOB exceeds 0.05%, the light transmittance and haze change quickly . Therefore, the added mass fraction of AOB in the composite film should not exceed 0.05%.

3. Conclusion

A new type of natural antioxidant packaging film was prepared by solution blending and casting film-forming method with bamboo leaf antioxidant (AOB) as natural antioxidant and hydroxypropyl methylcellulose (HPMC) as film-forming matrix. The change of functional groups and crystallization of the film were characterized by infrared absorption spectrometer, X-ray diffractometer and thermogravimetric analyzer, and the light transmittance and haze of the sample film were measured by a light transmission haze tester. The anti-oxidation performance of the film was analyzed by the removal experiment, and the following conclusions can be drawn:

1) The AOB/HPMC composite film prepared in this experiment, with the addition of AOB, did not destroy the groups of HPMC itself, but disturbed the arrangement of HPMC molecular chains, so the crystallinity of the composite film decreased, and the thermal decomposition temperature increased. Enhanced stability and functional properties of anti-oxidation.

2) When the mass fraction of AOB added was 0.03%, the prepared AOB/HPMC composite film had the best scavenging rate for DPPH radicals, which was about 89%, which was 61% higher than that of the HPMC film without adding AOB; Performance was also significantly improved; mechanical and optical properties decreased, but not by much.

3) The improvement of the anti-oxidation performance of AOB/HPMC composite water-soluble packaging film materials can expand its application in food packaging, and because of its good anti-oxidation performance, it has a good application prospect.