CN113861642B

CN113861642B - PLA/PBF/POE-g-GMA/ZnO composite material for antibacterial food packaging and preparation thereof

Info

Publication number: CN113861642B
Application number: CN202111174584.0A
Authority: CN
Inventors: 刘文涛; 袁梦杰; 陈鸿燕; 鹿孟张; 陈金周; 何素芹; 刘浩; 黄淼铭
Original assignee: Zhengzhou University
Current assignee: Zhengzhou University
Priority date: 2021-09-16
Filing date: 2021-10-09
Publication date: 2023-06-16
Anticipated expiration: 2041-10-09
Also published as: CN113861642A

Abstract

The invention relates to the field of polymer composite materials and food packaging materials, in particular to an antibacterial food packaging composite material, a preparation method and application thereof. The method comprises the following steps: (1) modifying nano zinc oxide by using a silane coupling agent; (2) Mixing the modified nano zinc oxide with PLA (polylactic acid) by a master batch method to obtain a master batch; (3) And (3) uniformly mixing PLA, PBF, POE-g-GMA and the master batch by a melt blending method to prepare the PLA/PBF/POE-g-GMA/ZnO composite material. Meanwhile, the PLA/PBF/POE-g-GMA/ZnO composite material prepared by the invention can be pressed into a film by a vacuum film pressing machine and used for antibacterial food packaging. The composite material prepared by the invention has excellent mechanical property, and the composite film prepared by the composite material has excellent antibacterial effect, particularly has excellent fresh-keeping effect when packaging broccoli, and is expected to become a biodegradable food packaging material with low cost, simple preparation and good performance.

Description

PLA/PBF/POE-g-GMA/ZnO composite material for antibacterial food packaging and preparation thereof

Technical Field

The invention belongs to the fields of polymer composite materials and food packaging materials, and particularly relates to a PLA/PBF/POE-g-GMA/ZnO composite material for antibacterial food packaging, a preparation method and application thereof.

Background

The plastic produced by the prior art in China is prepared from petrochemical products such as polyethylene, polyvinyl chloride, polypropylene, polyvinyl alcohol and the like. The product is not degraded, so that white pollution is caused. And with the shortage of petroleum, the price of the petroleum rises again and again, and the price of the plastic produced by petrochemical industry rises again and again. Polylactic acid (PLA) is prepared by polymerizing lactic acid obtained by fermenting starch extracted from plants such as corn and the like, and is a bio-plastic which is of bio-based origin and can be completely biodegraded. The macromolecule framework structure of the waste polylactic acid material can be hydrolyzed or hydrolyzed into a small chain segment by microorganisms, and finally the waste polylactic acid material is degraded into water and carbon dioxide. The energy consumption of the polylactic acid material in the production process is only 20-50% of that of petroleum-based products, and CO generated after degradation is generated ₂ And more particularly, only half of the latter, and therefore, have received much attention from various industries, especially in the fields of packaging materials, clothing, automotive industry, electric appliances, etc. From the performance point of view, polylactic acid has three main defects which limit the industrialized development of the polylactic acid: poor toughness; the crystallization rate is slow, and the crystallinity is low; the melt strength is low. Therefore, modifying polylactic acid is a necessary means for improving its performance.

In the field of food packaging, most food packaging materials have limited antibacterial performance, are difficult to degrade, are buried in soil to seriously destroy sustainable utilization of land resources, and are poured into the ocean to destroy ecological balance of the ocean. Along with the gradual enhancement of the awareness of protecting the environment and saving resources of modern people, the expansion of the application range of biodegradable plastics such as polylactic acid in life is an urgent need to be solved.

Disclosure of Invention

Aiming at the problems and the defects existing in the prior art, the invention aims to provide a PLA/PBF/POE-g-GMA/ZnO composite material for antibacterial food packaging and a preparation method thereof.

In order to achieve the aim of the invention, the technical scheme adopted by the invention is as follows:

the first aspect of the invention provides a preparation method of a PLA/PBF/POE-g-GMA/ZnO composite material for antibacterial food packaging, which comprises the following steps:

(1) Modifying the nano zinc oxide by using a silane coupling agent to obtain modified nano zinc oxide;

(2) Preparing a PLA/ZnO film by adopting PLA and the modified nano zinc oxide obtained in the step (1), and crushing the obtained PLA/ZnO film for later use;

(3) And (3) carrying out melt blending on PLA, PBF, POE-g-GMA and the PLA/ZnO film prepared in the step (2) to obtain the PLA/PBF/POE-g-GMA/ZnO composite material.

More preferably, the PLA, PBF, POE-g-GMA is subjected to a drying treatment.

Preferably, the addition amount of the PBF is 5-30% of the mass of the PLA/PBF/POE-g-GMA/ZnO composite material.

Preferably, the silane coupling agent is hexadecyltrimethoxysilane.

Preferably, the addition amount of the hexadecyl trimethoxy silane is 2.5-10% of the mass of the modified nano zinc oxide.

Preferably, the addition amount of the modified nano zinc oxide in the step (2) is 0.5-2% of the mass of the PLA/PBF/POE-g-GMA/ZnO composite material.

Preferably, the preparation method of the PLA/ZnO film in the step (2) comprises the following steps:

1) Adding the dried PLA into a solvent for dissolution to obtain a PLA solution;

2) Adding the modified nano zinc oxide obtained in the step (1) into a PLA solution, and uniformly dispersing to obtain a PLA/ZnO mixed solution;

3) And transferring the PLA/ZnO mixed solution into a polytetrafluoroethylene mould, and drying after the solvent volatilizes to obtain the PLA/ZnO film.

Preferably, the melt blending temperature in step (3) is 180 ℃.

More preferably, in the step (3), the materials are melt blended by using a torque rheometer, wherein the three sections of the torque rheometer are set to be 180 ℃ and the rotating speed is 60rpm; the machine pre-heat time was 10min.

More preferably, the drying temperatures are all 80 ℃.

More preferably, the preparation method of the modified nano zinc oxide in the step (1) comprises the following steps:

(a) Uniformly dispersing nano zinc oxide in a solvent, refluxing for 1h at 40 ℃ and then performing ultrasonic dispersion for 20min;

(b) Adding water, regulating the pH value of the solution to 2, refluxing for 1h, adding a silane coupling agent, stirring and refluxing for 1h at 40 ℃, regulating the pH value to 10, and refluxing for 2h to obtain a reflux product;

(c) Taking out the reflux product, centrifuging, removing supernatant, taking the centrifugated product, placing into a polytetrafluoroethylene die, and drying at 80 ℃ for overnight to obtain the modified nano zinc oxide.

More preferably, the solvent in the step (a) is absolute ethanol.

Preferably, the solvent in the step 1) is chloroform, and the dissolution mode is heating dissolution, and the heating temperature is 40-60 ℃; the dispersion mode in the step 2) is stirring and ultrasonic treatment, the stirring time is 15-30 min, and the ultrasonic treatment time is 10-15 min; the drying time in the step 3) is 12-24 h.

The second aspect of the invention provides a PLA/PBF/POE-g-GMA/ZnO composite material prepared by the preparation method.

The third aspect of the invention provides an application of the PLA/PBF/POE-g-GMA/ZnO composite material in antibacterial food packaging.

Preferably, the specific operation of applying the PLA/PBF/POE-g-GMA/ZnO composite material to antibacterial food packaging is as follows: and carrying out vacuum film pressing treatment on the prepared PLA/PBF/POE-g-GMA/ZnO composite material to obtain the PLA/PBF/POE-g-GMA/ZnO composite film.

More preferably, the vacuum film pressing treatment specifically comprises the following steps: spreading the prepared PLA/PBF/POE-g-GMA/ZnO composite material between two polytetrafluoroethylene films, transferring the two polytetrafluoroethylene films and the composite material spread therein into a vacuum film pressing machine for film pressing, and removing the upper polytetrafluoroethylene film and the lower polytetrafluoroethylene film after film pressing is finished to obtain the PLA/PBF/POE-g-GMA/ZnO composite film.

Preferably, the PLA/PBF/POE-g-GMA/ZnO composite film has a thickness of 70 mu m.

Compared with the prior art, the invention has the following beneficial effects:

(1) The PLA/PBF/POE-g-GMA/ZnO composite material prepared by the invention has the main components of PLA and furyl polyester (poly-2, 5-butylene furandicarboxylate, PBF) which are biodegradable materials, and the PBF is a strong and tough material type. The PBF is added into PLA, so that the composite material has higher elongation at break, and simultaneously, the tensile strength and the elastic modulus are less reduced, and further, the composite material has excellent mechanical properties. When the addition amount of the modified zinc oxide (m-ZnO) in the composite material is 0.5wt%, the elongation at break of the composite material reaches 132.1%, which is about 5 times that of PLA, and meanwhile, the tensile strength is 77.4MPa, the Young modulus is 1167.1MPa, the mechanical property is good, and the balance of rigidity and toughness is achieved.

(2) According to the invention, by comparing two silane coupling agents, the silane coupling agent hexadecyl trimethoxy silane (HDTMS) is finally selected to modify the nano zinc oxide, the prepared modified nano zinc oxide has good hydrophobicity and lipophilicity, can achieve a nano-level dispersion effect in an organic solvent, and then the modified nano zinc oxide is well dispersed in a composite material by combining a master batch method. Meanwhile, the crystallinity of the PLA/PBF/POE-g-GMA/ZnO composite material prepared by the method is obviously increased, compared with the PLA material, the number of spherulites is increased, and the gas barrier property is obviously enhanced.

(3) The PLA/PBF/POE-g-GMA/ZnO composite film prepared by the invention has obvious antibacterial effect. When the addition amount of the modified nano zinc oxide is 1wt%, the antibacterial rate of the PLA/PBF/POE-g-GMA/ZnO composite film on staphylococcus aureus and bacillus subtilis respectively reaches 99.9% and 99.5%, and the escherichia coli has lower survival rate. When the addition amount of the modified nano zinc oxide is 1.5wt%, the antibacterial rates of the PLA/PBF/POE-g-GMA/ZnO composite film on the three bacteria respectively reach 99.2%, 100% and 99.9%.

(4) The PLA/PBF/POE-g-GMA/ZnO composite film prepared by the invention can be used for food packaging. After nano zinc oxide is added, the sensory quality evaluation of the broccoli packaged by the PLA/PBF/POE-g-GMA/ZnO composite film is better, the quality loss rate is lower, the change of relative conductivity and chlorophyll content is smaller, and the fresh-keeping effect of the broccoli can be achieved.

Drawings

FIG. 1 is a graph showing a comparison of sedimentation of suspensions of nano zinc oxide before and after modification, wherein a is an m-ZnO suspension of 5wt% HDTMS and b is an un-ZnO suspension;

FIG. 2 is an agar plate diagram of the inhibitory effect of PLA/PBF/POE-g-GMA/ZnO composite film prepared from nano zinc oxide with different contents on Escherichia coli;

fig. 3 is a graph showing preservation effect of broccoli stored for 0 to 5 days, wherein a is a graph showing sensory quality change, b is a graph showing mass loss rate change, c is a graph showing relative conductivity change, and d is a graph showing chlorophyll content change.

Detailed Description

The present invention will be further described in detail below with reference to the accompanying drawings by way of examples in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of the invention.

It should be noted that, in the case of no conflict, the embodiments and features in the embodiments may be combined with each other. The present application will be described in detail below with reference to the accompanying drawings in conjunction with embodiments.

Example 1

A preparation method of a PLA/PBF/POE-g-GMA/ZnO composite film for antibacterial food packaging comprises the following steps:

1. preparation of modified nano zinc oxide

(a) Uniformly dispersing 10g of nano zinc oxide in 100mL of absolute ethyl alcohol solvent, carrying out ultrasonic oscillation for about 15min, and carrying out ultrasonic dispersion for 20min after refluxing for 1h at 40 ℃.

(b) Proper amount of water was added, ph=2 was adjusted with 0.1mol/LHCl solution, then, after refluxing at 40 ℃ for 1 hour, hexadecyltrimethoxysilane (HDTMS) was added dropwise into the flask, after refluxing at 40 ℃ with stirring for about 1 hour, ph=10 was adjusted with NaOH, and refluxing at 40 ℃ for 2 hours, to obtain a refluxed product.

(c) Taking out the reflux product, centrifuging for 5min, removing supernatant, taking the centrifugal product, placing into a polytetrafluoroethylene mold, and drying in an oven at 80 ℃ for overnight to obtain the modified nano zinc oxide (m-ZnO). In the modified nano zinc oxide, the content of HDTMS is 5wt%.

2. Preparation of PLA/ZnO film

1) And (3) drying PLA at 80 ℃, dissolving 10g of dried PLA in 100mL of chloroform, heating and stirring at 40 ℃ until the PLA is completely dissolved, and obtaining a PLA solution.

2) And (2) adding 0.4g of the modified nano zinc oxide prepared in the step (1) into the PLA solution, stirring for 15min, cooling to room temperature after the solution is uniformly white, and performing ultrasonic dispersion for 15min to obtain the PLA/ZnO mixed solution.

3) And transferring the PLA/ZnO mixed solution into a polytetrafluoroethylene mould, volatilizing in a fume hood overnight, and drying in an oven at 80 ℃ for 12 hours to obtain the PLA/ZnO film.

3. Preparation of PLA/PBF/POE-g-GMA/ZnO composite material

Turning on a torque rheometer (model: CTR-300), wherein the three sections of the torque rheometer are set to be 180 ℃ and the rotating speed is 60rpm; the machine pre-heat time was 10min. After the preheating of the torque rheometer is finished and the rotation speed is stable, 22g PLA, 8g PBF (manufacturer: ji plastic new material technology Co., ningbo Co., ltd.) and 2g POE-g-GMA (glycidyl methacrylate grafted polyolefin elastomer, manufacturer: shanghai Co., ltd.) dried at 80 ℃ are added into a material port of the torque rheometer.

And (II) after the rotation speed of the torque rheometer is stable, cutting the PLA/ZnO film prepared in the step (2) into pieces and adding the pieces into a material opening of the torque rheometer.

And (III) taking out the materials obtained by blending in the torque rheometer after the torque is balanced, and shearing to obtain the PLA/PBF/POE-g-GMA/ZnO composite material, wherein the composite material is composite particles.

4. Preparation of PLA/PBF/POE-g-GMA/ZnO composite film

And (3) paving the composite material prepared in the step (3) between two layers of polytetrafluoroethylene films, directly putting the two layers of polytetrafluoroethylene films and the composite material paved therein between two layers of iron plates under the condition of not using a die, and transferring the composite material into a vacuum film pressing machine for film pressing. And after the film pressing is finished, taking down the two polytetrafluoroethylene films to obtain the 70 mu m PLA/PBF/POE-g-GMA/ZnO composite film.

Example 2

The preparation method of PLA/PBF/POE-g-GMA/ZnO composite film for antibacterial food packaging is basically the same as that of example 1, except that: in the modified nano zinc oxide obtained in the step 1, the content of the HDTMS is 2.5wt%.

Example 3

The preparation method of PLA/PBF/POE-g-GMA/ZnO composite film for antibacterial food packaging is basically the same as that of example 1, except that: in the modified nano zinc oxide obtained in the step 1, the content of HDTMS is 7.5wt%.

Example 4

The preparation method of PLA/PBF/POE-g-GMA/ZnO composite film for antibacterial food packaging is basically the same as that of example 1, except that: in the modified nano zinc oxide obtained in the step 1, the content of the HDTMS is 10wt%.

Example 5

The preparation method of PLA/PBF/POE-g-GMA/ZnO composite film for antibacterial food packaging is basically the same as that of example 1, except that: the addition amount of the modified nano zinc oxide in the step 2) is 0.2g.

Example 6

The preparation method of PLA/PBF/POE-g-GMA/ZnO composite film for antibacterial food packaging is basically the same as that of example 1, except that: the addition amount of the modified nano zinc oxide in the step 2) is 0.6g.

Example 7

The preparation method of PLA/PBF/POE-g-GMA/ZnO composite film for antibacterial food packaging is basically the same as that of example 1, except that: the addition amount of the modified nano zinc oxide in the step 2) is 0.8g.

Example 8

The preparation method of PLA/PBF/POE-g-GMA/ZnO composite film for antibacterial food packaging is basically the same as that of example 1, except that: the heating temperature in the step 1) is 50 ℃; the stirring time in the step 2) is 20min, and the ultrasonic time is 12min; drying for 18h in step 3).

Example 9

The preparation method of PLA/PBF/POE-g-GMA/ZnO composite film for antibacterial food packaging is basically the same as that of example 1, except that: the heating temperature in the step 1) is 60 ℃; the stirring time in the step 2) is 30min, and the ultrasonic time is 10min; and 3) drying for 24 hours in the step 3).

Comparative example 1

The preparation method of PLA/PBF/POE-g-GMA/ZnO composite film for antibacterial food packaging is basically the same as that of example 1, except that: the preparation of step 1 and step 2 is not performed; in the step (I), the PLA was added in an amount of 38g and the PBF was added in an amount of 2g, and the preparation in the step (II) was not performed.

Comparative example 2

The preparation method of PLA/PBF/POE-g-GMA/ZnO composite film for antibacterial food packaging is basically the same as that of example 1, except that: the preparation of step 1 and step 2 is not performed; in the step (I), the PLA addition amount was 36g and the PBF addition amount was 4g, and the preparation in the step (II) was not performed.

Comparative example 3

The preparation method of PLA/PBF/POE-g-GMA/ZnO composite film for antibacterial food packaging is basically the same as that of example 1, except that: the preparation of step 1 and step 2 is not performed; in the step (I), the PLA was added in an amount of 32g and the PBF was added in an amount of 8g, without carrying out the preparation in the step (II).

Comparative example 4

The preparation method of PLA/PBF/POE-g-GMA/ZnO composite film for antibacterial food packaging is basically the same as that of example 1, except that: the preparation of step 1 and step 2 is not performed; in the step (I), the PLA was added in an amount of 28g and the PBF was added in an amount of 12g, without carrying out the preparation in the step (II).

Comparative example 5

The preparation method of PLA/PBF/POE-g-GMA/ZnO composite film for antibacterial food packaging is basically the same as that of example 1, except that: in the step (b), the modification is carried out without using a silane coupling agent, namely without adding the silane coupling agent; obtaining unmodified nano zinc oxide (un-ZnO) in the step (c); the unmodified nano zinc oxide is used in the step 2) to prepare the PLA/ZnO film.

Comparative example 6

The preparation method of PLA/PBF/POE-g-GMA/ZnO composite film for antibacterial food packaging is basically the same as that of example 1, except that: the silane coupling agent used in the step (b) is gamma-aminopropyl triethoxysilane (KH-550), and the content of the silane coupling agent in the modified nano-zinc oxide is 5wt%.

Comparative example 7

The preparation method of PLA/PBF/POE-g-GMA/ZnO composite film for antibacterial food packaging is basically the same as that of example 1, except that: the silane coupling agent used in the step (b) is KH-550; in the modified nano zinc oxide, the content of the silane coupling agent is 7.5 weight percent.

Comparative example 8

The preparation method of PLA/PBF/POE-g-GMA/ZnO composite film for antibacterial food packaging is basically the same as that of example 1, except that: the preparation of step 1 and step 2 is not performed; the amount of PLA added in step 3 was 32g, and the preparation in step (II) was not performed.

Comparative example 9

The preparation method of PLA/PBF/POE-g-GMA/ZnO composite film for antibacterial food packaging is basically the same as that of example 1, except that: the preparation of step 1 and step 2 is not performed; in the step (I), PBF and POE-g-GMA are not added, the PLA is used in an amount of 40g, and the preparation in the step (II) is not carried out.

Discussion of the amount of PBF

In order to investigate the influence of the PBF content on the mechanical properties of the composite materials, the inventors respectively compared PLA/PBF/POE-g-GMA/ZnO composite materials prepared in examples 1 to 4 as test samples, and the corresponding PBF contents are respectively 5wt%, 10wt%, 20wt% and 30wt%, and the following tensile and impact tests are performed: the test specimens were pressed into dumbbell-shaped bars (tensile bars, specification: 2X 35X 0.5 mm) and rectangular bars (impact bars, specification: 75X 20X 1 mm) with a vacuum film press, and tensile test was performed on a universal tensile tester with a plastic-film tensile protocol at a tensile speed of 5mm/min. The thickness was averaged at 3 points per spline, and 5 replicates were run per group. A45V-shaped notch with the depth of 4mm is firstly ground on one side of an impact spline by a notch sampling machine, and the impact strength of the notch is measured on a cantilever Liang Baichui impact measuring instrument. The results are shown in Table 1. Experimental results show that the mechanical properties of comparative example 3 are best, and the subsequent preparation of the composite material is carried out when the PBF addition amount is preferably 20 wt%.

TABLE 1 influence of PBF usage on mechanical Properties of composite materials

(II) selection of silane coupling agent

In order to investigate the influence of the types and the amounts of the silane coupling agents on the hydrophilicity and the lipophilicity of the nano zinc oxide, the inventors used the modified nano zinc oxide prepared in example 1, example 2, example 3, comparative example 5, comparative example 6 and comparative example 7 as test samples, respectively, the corresponding types and the amounts of the silane coupling agents were HDTMS 5wt%, HDTMS 2.5wt%, HDTMS7.5wt%, 0wt%, KH-5505wt%, KH-5507.5wt%, respectively, and carried out the following experiments: measuring a water-air contact angle by a static contact angle tester, and reflecting the hydrophilicity and hydrophobicity of the surface of the sample; samples were tested for lipophilicity by a lipophilicity assay. The results are shown in Table 2.

TABLE 2 influence of silane coupling agent types and amounts on hydrophilicity and lipophilicity of nano zinc oxide

Numbering device	Contact angle (°)	Degree of lipophilicity (%)
			Example 1 (5 wt% HDTMS)	145	58.5
Example 2 (2.5 wt% HDTMS)	85	39.0
			Example 3 (7.5 wt% HDTMS)	147	60.9
Comparative example 5 (without silane coupling agent)	25.5	0
			Comparative example 6 (5 wt% KH-550)	39	19.3
Comparative example 7 (7.5 wt% KH-550)	41	23.0

The general formula of the silane coupling agent is RSiX ₃ Wherein R represents a reactive functional group having reactivity or affinity with the polymer molecule. During the research, we found that the silane coupling agent can play a role of a 'molecular bridge' in the material: silaneThe hydroxyl generated after the hydrolysis of the group can react with the functional group on the surface of the powder; meanwhile, another part of groups can be physically entangled or chemically reacted with the organic polymer to connect the inorganic powder filler with the polymer so as to realize coupling. In the coupling process, X is hydrolyzed to form silanol, then reacts with hydroxyl groups on the surfaces of the inorganic powder particles to form hydrogen bonds and is condensed into Si-M covalent bonds (M represents the surfaces of the inorganic powder particles). Meanwhile, silanol in each molecule of the silane is mutually associated and oligomerized to form a film with a net structure to cover the surface of the powder particles, so that the surface of the inorganic powder is organized.

As is clear from Table 2, un-ZnO is relatively hydrophilic because of the rich hydroxyl groups on the surface, has a contact angle of 25.5℃and is unfavorable for uniform dispersion in the polymer and has poor compatibility. When the silane coupling agent is HDTMS, the contact angle and the oleophilic degree of the nano zinc oxide tend to be increased firstly and then unchanged along with the increase of the content of the HDTMS, and the balance is achieved when the addition amount of the HDTMS is 5%, which means that the number of the silane coupling agents on the surface of the powder is saturated. This is because hydroxyl groups on the surface of nano zinc oxide cannot completely undergo association reaction with a silane coupling agent due to steric hindrance, and physical adsorption of a monolayer is formed on the surface of zinc oxide, and then surface micelles are formed. The modification effect of KH-550 is obviously lower than that of HDTMS, and the contact angle of m-ZnO modified by KH-550 and un-ZnO is not changed greatly because HDTMS is hexadecane carbon chain, and compared with KH-550 containing two alkanes, the hydrophobicity is stronger. Therefore, the nano zinc oxide is modified by using HDTMS, and better oleophilic and hydrophobic effects can be achieved when the dosage is 5wt%.

(III) discussion of the amount of silane coupling agent

In order to investigate the influence of the amount of the silane coupling agent on the dispersing effect of the nano zinc oxide, the inventors made the following experiments, respectively:

the modified nano zinc oxide prepared in the examples 1, 2, 3, 4 and 5 is used as a test sample, and the content of the corresponding silane coupling agent is 5wt%, 2.5wt%, 7.5wt%, 10wt% and 0wt% respectively; and respectively taking 0.1g of nano zinc oxide sample, adding a proper amount of ethanol dispersion liquid, placing in a four-way cuvette, performing ultrasonic oscillation for about 15min until particles are uniformly dispersed and the dispersion liquid is semitransparent, and then testing the particle size of the nano zinc oxide by adopting a laser particle sizer. The results are shown in Table 3.

The modified nano zinc oxide prepared in example 1 and comparative example 5 were used as test samples, and the apparent test was performed using suspension sedimentation and transmission electron microscopy, and the results are shown in fig. 1. When the apparent test is carried out by using a transmission electron microscope, a small amount of nano zinc oxide sample is taken in a transparent sample bottle, ethanol is added, ultrasonic treatment is carried out for 10min to disperse the nano zinc oxide sample, a 2 mu L suspension is removed by using a microinjector on a copper mesh, and a microscopic morphology graph is observed and recorded by using the transmission electron microscope.

TABLE 3 influence of silane coupling agent usage on average particle size of nano zinc oxide

Numbering device	Average particle diameter (nm)
		Example 1 (5 wt%)	90.47
Example 2 (2.5 wt%)	122.97
		Example 3 (7.5 wt%)	111.01
Example 4 (10 wt%)	136.21
		Comparative example 5 (without silane coupling agent)	205.02

The nano zinc oxide has larger specific surface area and surface potential energy due to small size effect and surface effect, and particles are easy to agglomerate so that the size is increased. The particle size of the nano zinc oxide before modification is mainly distributed between 90 and 460nm, and the number of particles with the particle size of about 200nm is the largest (as shown by the average particle size of un-ZnO in Table 2 being 205.02 nm). As is clear from Table 3, as the HDTMS content increases, the m-ZnO particle size decreases and then increases, and the number of particles having a particle size of about 100nm is the largest. Wherein, the 5% HDTMS particle size is distributed in a smaller area, and the average particle size is 90.47nm which is calculated by software, thereby meeting the requirement of the nano particle size of 1-100 nm. This is due to the silane coupling agent coating the zinc oxide surface to reduce agglomeration with each other. As the addition amount of the coupling agent increases, the particle size is increased by coating the coupling agent layer by layer.

The sedimentation comparison chart of the suspensions of nano zinc oxide before and after modification is shown in figure 1. In the figure 1, after the m-ZnO suspension with 5% of HDTMS is left for 3 hours, the m-ZnO is still uniformly dispersed in ethanol, and the m-ZnO suspension is in cup sticking phenomenon due to good lipophilicity, the overall phenomenon is basically not settled, and the dispersion degree of the m-ZnO calculated by a formula reaches more than 99%. In FIG. 1, b shows the dispersion of un-ZnO in ethanol, showing that the settling velocity of un-ZnO is relatively high, the particle deposition phenomenon occurs at the bottom, the top suspension is in a semitransparent state, and the dispersion degree of m-ZnO is 60% as calculated by a formula. This is because the particle diameter of the nano zinc oxide is increased due to collision and agglomeration among particles, so that gravity is increased to cause the particles to sink.

In order to prove the modification effect of the silane coupling agent HDTMS on the nano zinc oxide from a microscopic angle, a transmission electron microscope is used for observing the microscopic morphology of a sample, and the modified nano zinc oxide agglomerated large particles are opened, so that the aggregation phenomenon is obviously improved.

In conclusion, better dispersing effect can be achieved when the using amount of the HDTMS is 5 weight percent.

(IV) Performance test

1. Analysis of mechanical properties of PLA/PBF/POE-g-GMA/ZnO composite material

To investigate the effect of nano zinc oxide usage on the mechanical properties of PLA/PBF/POE-g-GMA/ZnO composites, the inventors used the composites prepared in examples 1, 5, 6, 7, 5, 8, 9 as test samples, respectively, with the corresponding nano zinc oxide usage of m-ZnO 0.4g, m-ZnO 0.2g, m-ZnO 0.6g, m-ZnO0.8g, un-ZnO 0.4g, unused nano zinc oxide (pure PLA material), respectively, and performed the following experiments: the test specimens were pressed into dumbbell-shaped bars (tensile bars, specification: 2X 35X 0.5 mm) and rectangular bars (impact bars, specification: 75X 20X 1 mm) with a vacuum film press, and tensile test was performed on a universal tensile tester with a plastic-film tensile protocol at a tensile speed of 5mm/min. The thickness was averaged at 3 points per spline, and 5 replicates were run per group. A45V-shaped notch with the depth of 4mm is firstly ground on one side of an impact spline by a notch sampling machine, and the impact strength of the notch is measured on a cantilever Liang Baichui impact measuring instrument. The results are shown in Table 4.

TABLE 4 influence of the amount of Nano Zinc oxide on the mechanical Properties of PLA/PBF/POE-g-GMA/ZnO composite

As can be seen from table 4, as the zinc oxide content increases, the crystallinity of the composite material gradually increases. The increase in crystallinity of the composite results in a decrease in elongation at break and impact strength, and the material tends to be ductile to brittle. But compared with pure PLA material, the PLA/PBF/POE-g-GMA/ZnO composite material has obviously improved elongation at break and impact strength, and the material has the beneficial effects of being strong and tough. When the addition amount of the modified nano zinc oxide is 1wt%, the PLA/PBF/POE-g-GMA/ZnO composite material can achieve better 'strong and tough' performance.

2. Analysis of antibacterial Properties of PLA/PBF/POE-g-GMA/ZnO composite film

In order to investigate the effect of nano zinc oxide usage on the antibacterial effect of PLA/PBF/POE-g-GMA/ZnO composite films, the inventors used composite films prepared in examples 1, 5, 6, 7, 5 and 9 as test samples, respectively, with the corresponding nano zinc oxide usage of m-ZnO 0.4g, m-ZnO 0.2g, m-ZnO 0.6g, m-ZnO0.8g, un-ZnO 0.4g, and unused nano zinc oxide (pure PLA material), respectively, the following experiments were performed: the antibacterial rate of the composite film to the escherichia coli is calculated by using a plate counting method through an agar plate graph of bacteria, and all operation steps of the experiment are carried out in a sterile environment. In the experiment, bacterial liquid is firstly cultured on the surface of a sample, then elution treatment is carried out, and then the eluent is coated, cultured and counted on a solid culture medium, wherein the composition of the solid/liquid culture medium is shown in table 5. The whole testing process is carried out in four days, and the specific operation steps are as follows:

the first day: solid cultures were prepared according to the formulation in Table 4, based on conical flasks, placed in a sterilization chamber and sterilized, and then poured into petri dishes while hot, until the agar slowly solidified. Taking out the conical flask containing bacteria from the refrigerator, dripping a few drops of culture solution into a solid base, uniformly smearing the sterilized coating rod, and then placing the coated coating rod in a 37 ℃ incubator for culturing for 24 hours.

The following day: the liquid culture medium prepared according to the formula shown in Table 4 is placed into a sterilizing box for sterilization, taken out, cooled under an ultraviolet lamp to avoid contamination by mixed bacteria, and packaged in a plurality of culture dishes. Scraping out a proper amount of bacteria from the solid base prepared on the first day by using the sterilized inoculating loop, transferring the bacteria into the freshly prepared liquid base, shaking the liquid base uniformly, and sucking a proper amount of bacteria liquid for dilution by 1000 times. Respectively dripping an equal amount of diluted bacterial liquid on the surface of a sample, then attaching a sterilized PE film, putting the sample into an empty culture dish with marks, and culturing for 24 hours in a culture box at 37 ℃.

Third day: preparing a solid culture medium, eluting bacteria growing on the surface of a sample in the next day by using normal saline, transferring a proper amount of eluent, dripping the eluent into the solid culture medium, uniformly smearing the eluent on a sterilized coating rod, and placing the solid culture medium in a 37 ℃ incubator for culturing for 24 hours.

Fourth day: the growth of bacteria on the surface of the solid medium was observed and recorded by photographing. The antibacterial rate of the PLA/PBF/POE-g-GMA/ZnO composite film is shown in Table 6.

TABLE 5 solid/liquid Medium composition Table

Reagent(s)	Liquid medium composition	Solid medium composition
			Physiological saline	100mL	100mL
Sodium chloride	0.5g	0.5g
			Yeast extract powder	0.5g	0.5g
Peptone	1g	1g
			Tween 80	Proper amount of	0
Agar-agar	0	1.5g

TABLE 6 Effect of nano Zinc oxide dosage on antibacterial Effect of PLA/PBF/POE-g-GMA/ZnO composite film

As can be seen from table 6, the modified nano zinc oxide has significantly better antibacterial effect than the unmodified nano zinc oxide, probably because the modified nano zinc oxide has better dispersibility and the antibacterial rate is effectively improved. The PLA/PBF/POE-g-GMA/ZnO composite film has the antibacterial effect effectively improved along with the increase of the addition amount of the modified nano zinc oxide, and the antibacterial effect is near to the saturation state when the addition amount of the modified nano zinc oxide is 0.4g (namely 1wt percent m-ZnO). Wherein FIG. 2 is an agar plate of PLA/PBF/GPOE/ZnO composite film against E.coli. Therefore, when the addition amount of the modified nano zinc oxide is 1wt%, the PLA/PBF/POE-g-GMA/ZnO composite film can achieve better antibacterial effect.

3. Fresh-keeping effect analysis of PLA/PBF/POE-g-GMA/ZnO composite film on broccoli

In order to investigate the effect of nano zinc oxide usage on the preservative effect of PLA/PBF/POE-g-GMA/ZnO composite film on broccoli, the inventors used composite films prepared in examples 1, 5, 6, 7, 5, 8 and 9 as test samples, respectively, with the corresponding nano zinc oxide usage of m-ZnO 0.4g, m-ZnO 0.2g, m-ZnO 0.6g, m-ZnO0.8g, un-ZnO 0.4g, unused nano zinc oxide (pure PLA material), and the results are shown in FIG. 3.

(1) Sensory quality evaluation: paper tape is torn off every day during the period of broccoli storage (0-5 days) according to national agricultural standard NY/T941-2006, a broccoli sample is taken out for observation, scoring is performed according to the color, smell and the like thereof, and the average value of 5 persons of scoring is taken as a sensory quality evaluation score.

(2) Mass loss rate change: the broccoli mass was weighed daily with an analytical balance, and the mass loss rate (weight loss rate) was calculated according to formula (1).

(3) Relative conductivity: firstly, cutting 1g of flake from a sample, putting the flake into a weighing bottle, adding 20mL of deionized water, vibrating for 5min, standing for 3h, and measuring the conductivity at the moment and marking as gamma ₁ The method comprises the steps of carrying out a first treatment on the surface of the Transferring the liquid into a small beaker, boiling in 140 deg.C oil bath to kill fruit and vegetable tissue, taking out, cooling to room temperature, and measuring conductivity gamma ₂ . Relative conductivity (gamma) _e ) The formula of the formula (2) is shown in the specification.

(4) Chlorophyll content: the absorbance of the acetone chlorophyll solution extracted from broccoli was measured with an ultraviolet spectrophotometer, wherein the maximum absorption peaks of the acetone extracts of chlorophyll a and b were at wavelengths 645nm and 663 nm. And pouring chlorophyll into a cuvette by adopting a photometric measurement mode, measuring absorbance at different wavelengths by taking acetone as a reference, and calculating the chlorophyll content according to the formula (3) and the formula (4).

ρ _T ＝ρ _a +ρ _b ＝20.29A ₆₄₅ +8.05A ₆₆₃ (3)

In the formula (3): ρ _a (mg/L) and ρ _b (mg/L) represents the mass concentrations of chlorophyll a and chlorophyll b, respectively. A is that ₆₆₃ And A ₆₄₅ Refers to the absorbance values of the sample at wavelengths 663nm and 645 nm. The specific operation steps are as follows: the paper tape was torn off, 1g of the paper was cut from broccoli, added to an agate mortar, and 2mL of acetone was added to finely grind the paper tape until the sample became a slurry. Pouring into a beaker, adding acetone to 20mL, and standing in the dark for about 24h due to chlorophyll absorption. Washing the sand core funnel with acetone, and filteringFiltered into glass sample vials, labeled with the name. Finally, the absorbance of the sample at the wavelengths 663nm and 645nm is measured by an ultraviolet spectrophotometer, and the acetone blank is deducted.

FIG. 3 is a graph showing the results of the fresh keeping evaluation experiment in the above-mentioned 4. As shown in fig. 3 a and b, as the storage time of the broccoli becomes longer, the sensory quality such as the odor of the color of the broccoli Lan Huase tends to decrease, and the decrease tends to be slow and fast; the mass loss tends to increase and then not change. And the sensory quality of the broccoli is reduced more slowly with the increase of the content of the modified nano zinc oxide in the composite film, and the reduction proportion is smaller; the smaller the mass loss and the higher the content the smaller the loss. But the sensory quality of the broccoli packaged with 0.4g un-ZnO, composite film without nano zinc oxide and nano zinc oxide (PLA material) is greatly reduced and the quality loss rate is great.

The relative conductivity and chlorophyll content can show a basic index of plant cell membrane permeability, namely when the cell membrane is destroyed, the cell membrane permeability is increased, so that the cell electrolyte and chlorophyll are extravasated, and the conductivity of the plant cell extract is increased, and the chlorophyll content in the cells is reduced. When the cell dies, chlorophyll is released from chloroplasts, the released chloroplasts are unstable, and light, acid, alkali, oxygen, oxidant and the like can decompose the chlorophyll. Thus, the greater the experimentally measured relative conductivity, the lower the chlorophyll content, indicating greater cell membrane permeability and higher cell death rate. As shown in fig. 3 c, the relative conductivity of broccoli tends to increase as the storage time of broccoli becomes longer, but the composite film containing nano zinc oxide significantly slows down the trend, which indicates that the composite film added with nano zinc oxide has a certain slowing effect on cell death. As shown in fig. 3 d, the chlorophyll content in the broccoli gradually decreases as the storage time of the broccoli becomes longer, and the freshness-retaining effect of the broccoli is increased by the composite film as the content of the modified nano zinc oxide in the composite film increases. In conclusion, the PLA/PBF/POE-g-GMA/ZnO composite film has good effect on the preservation of the broccoli.

In conclusion, the invention effectively overcomes the defects in the prior art and has high industrial utilization value. The above-described embodiments are provided to illustrate the gist of the present invention, but are not intended to limit the scope of the present invention. It will be understood by those skilled in the art that various modifications and equivalent substitutions may be made to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention.

Claims

1. The preparation method of the PLA/PBF/POE-g-GMA/ZnO composite material for antibacterial food packaging is characterized by comprising the following steps:

(3) Melt blending PLA, PBF, POE-g-GMA and the PLA/ZnO film prepared in the step (2) to obtain a PLA/PBF/POE-g-GMA/ZnO composite material;

wherein, the silane coupling agent in the step (1) is hexadecyl trimethoxy silane; the addition amount of the hexadecyl trimethoxy silane is 2.5-10% of the mass of the modified nano zinc oxide; the addition amount of the modified nano zinc oxide in the step (2) is 0.5-2% of the mass of the PLA/PBF/POE-g-GMA/ZnO composite material; the addition amount of the PBF in the step (3) is 5-30% of the mass of the PLA/PBF/POE-g-GMA/ZnO composite material;

the preparation method of the modified nano zinc oxide in the step (1) comprises the following steps: (a) Uniformly dispersing nano zinc oxide in a solvent, refluxing for 1h at 40 ℃ and then performing ultrasonic dispersion for 20min; (b) Adding water, regulating the pH value of the solution to 2, refluxing for 1h, adding a silane coupling agent, stirring and refluxing for 1h at 40 ℃, regulating the pH value to 10, and refluxing for 2h to obtain a reflux product; (c) Taking out the reflux product, centrifuging, removing supernatant, taking the centrifugated product, placing into a polytetrafluoroethylene die, and drying at 80 ℃ for overnight to obtain the modified nano zinc oxide.

2. The method for preparing PLA/PBF/POE-g-GMA/ZnO composite material for antibacterial food packaging according to claim 1, wherein the method for preparing PLA/ZnO film in the step (2) is:

3. The method for producing PLA/PBF/POE-g-GMA/ZnO composite for antibacterial food packaging according to claim 2, wherein the melt blending temperature in the step (3) is 180 ℃.

4. The method for preparing PLA/PBF/POE-g-GMA/ZnO composite material for antibacterial food packaging according to claim 3, wherein the solvent in the step 1) is chloroform; the dispersion mode in the step 2) is stirring and ultrasonic treatment, the stirring time is 15-30 min, and the ultrasonic treatment time is 10-15 min; the drying time in the step 3) is 12-24 h.

5. A PLA/PBF/POE-g-GMA/ZnO composite material prepared by the preparation method of any one of claims 1 to 4.

6. Use of a PLA/PBF/POE-g-GMA/ZnO composite material according to claim 5 in antibacterial food packaging.

7. The use of the PLA/PBF/POE-g-GMA/ZnO composite material in antibacterial food packaging according to claim 6, characterized in that the specific operations of using the PLA/PBF/POE-g-GMA/ZnO composite material in antibacterial food packaging are: performing vacuum film pressing treatment on the prepared PLA/PBF/POE-g-GMA/ZnO composite material to obtain a PLA/PBF/POE-g-GMA/ZnO composite film; the thickness of the composite film is 70 mu m.