Filler II for the hottest food packaging film

Fillers for food packaging film (II)

II. Functional fillers

1 It is an important mission of food packaging film to prevent food deterioration and extend the shelf life of food with fillers for barrier and fresh-keeping film. The freshness of food is closely related to the barrier of packaging film to moisture (humidity), oxygen, short band visible light and ultraviolet light. Generally, food packaging film has small water permeability and small oxygen permeability. PVDC is a plastic with low water permeability and oxygen permeability. PVDC film and the so-called k-coated film coated by PVDC are the most common. But now, due to the dioxin and environmental hormone problems of PVDC film, the food market keeps away from it and continues to turn to materials with better environmental properties. The high barrier (high oxygen barrier) materials that replace PVDC are PVA and EVOH, etc., but the gas barrier of this kind of hydrophilic plastic film is greatly affected by humidity. Under high humidity conditions, It is difficult to obtain high barrier. In order to obtain higher barrier materials, there are organic/inorganic composites, liquid crystal polymer alloys, new substrates (such as phenoxy resin), etc. among them, the research work of using special sheet filler nanocomposites to improve barrier is quite active

(1) nano clay composite

it is an inorganic/organic composite material that uses a special polymerization method to make the flat silicate layered compound (clay) disperse in polymers such as nylon at the nano level. The specific method is to use montmorillonite sodium salt as clay, exchange the Na in it with quaternary amine salt, and modify the interlayer organically. Then, through the interlayer polymerization of caprolactam, nylon 6/clay composite with montmorillonite like micro dispersion is obtained (see Figure 3). The gas permeability of this composite depends on the vertical and horizontal comparison of the volume of clay and clay particles (the ratio of average length to diameter). If the two values are large, the barrier property is also improved. Montmorillonite has large surface area (750m2/g) and high aspect ratio (200 ~ 1000), so the effect of improving barrier property is better. The mechanism of improving the barrier property is believed to be that the differential dispersed sheet filler in the substrate acts as a barrier to gas diffusion, making the gas diffusion route longer; When 2% montmorillonite is dispersed in nylon, the barrier to gas is about twice as high

recently, it has been studied that nylon/clay composite films are prepared by using an extruder with melt mixing instead of special polymerization to disperse sheet fillers. In nylon 6, adding 2.5% montmorillonite, the permeability is reduced to 60% of the original; Adding 7.5% montmorillonite, the oxygen permeability is reduced to 40% of the original. In addition, with the increase of the amount of montmorillonite, the water vapor transmission and ultraviolet transmission also decreased, while the tensile strength, tensile modulus, bending modulus, load radial deformation temperature and so on increased. Although montmorillonite is easy to disperse in polar polymers such as nylon, it is difficult for low polar polymers such as polyolefins to enter the montmorillonite layer, and good results have not been achieved. It seems that in the future, while studying the dispersants that promote the dispersion of montmorillonite in the polymer and the melt mixing process conditions, it is also a subject to develop organically modified montmorillonite that can withstand the melt mixing temperature. In addition, although the nanocomposite gas barrier film has the advantage of not requiring secondary processing, its gas barrier property is worse than that of evaporated aluminum oxide or evaporated silicon oxide film, which is still imperfect and needs to be further improved in the future

(2) oxygen absorbing material

plastic has limited barrier property. In order to make plastic products with high barrier property, there is a method of filling oxygen absorbing agent in plastic to make it difficult for oxygen to enter the packaging container through the container wall. This method of filling oxygen absorbing agent has been used to produce plastic diaphragms for containers. Oxygen getter is a fine reduced iron powder with special treatment. Because it is black, it is used for the inner layer, while the outermost layer is covered with white. The effect of oxygen absorbing barrier agent is shown in Table 4. From the data in the table, it can be seen that under high temperature and high humidity conditions, compared with Veoh and PVDC, the oxygen absorbing barrier material is also quite excellent. This kind of oxygen cutting diaphragm, 10 × 10cm size with 30ml oxygen absorption capacity

recently completed the attempt to manufacture oxygen absorbing barrier materials with easily oxidized organic polymers. The initial attempt was the "oxbar" oxygen absorbing packaging container successfully developed by CMB company in 1989 with mxd16 as the oxygen absorbing layer of the combination of easily oxidized organic polymers and cobalt compound catalysts. According to the literature, organic polymers containing double bonds and tertiary hydrogen, such as polybutadiene, polypropylene, etc., are used for double bonds that are easy to oxidize. To confirm the specific cause of the failure of the change-over valve, it can be removed for experimental inspection and the existence of tertiary hydrogen atoms. It can also be used as the base material of oxygen absorption composition. In order to improve the oxygen absorption performance, it can be used together with cobalt catalyst; This kind of oxygen absorbing polymer can collect low molecular oxidation products that will produce odor during oxidative degradation. Therefore, porous inorganic particles (such as zeolite powder or amorphous silica powder) mixed with low molecular products that absorb odor form an odor absorbing layer to eliminate odor, as shown in Figure 4. Amorphous silicon oxide for odor absorption; The average particle size is 20 μ M below, the average pore diameter is between 20a ° and 30A °. Of course, the combination of PET layer (or polylactic acid layer) with good barrier to odor 4 sample substances can also be used next to the oxygen absorption layer to prevent odor from entering the container (or escaping), but the cost is high

(3) filler for air control film (MA film)

MA film is a film that can change the composition of the air atmosphere in the film, also known as air control film or air conditioning film. By changing the composition of the air in the film to stabilize it within the desired range, the purpose of inhibiting the respiration of fruits and vegetables, inhibiting the decomposition of chlorophyll, inhibiting ethylene gas and so on can be achieved, so as to improve the fresh-keeping effect. Ma film is easy to use and low cost, so it is widely used in vegetable preservation. Control the composition of oxygen and carbon dioxide in the film bag (appropriately increase the proportion of carbon dioxide and moderately reduce the proportion of oxygen to inhibit the respiration of fruits and vegetables), rely on the pore size and quantity of pores (pinholes or micropores) on the film, and the removal of harmful gases such as ethylene depends on the specific fillers added to the thin film, such as porous activated carbon, coral, zeolite, Taguchi, clay, etc., among which the FH film with Taguchi as the filler, It is said to have a good fresh-keeping effect, which has a certain impact in Japan

in the future, the development of inhibiting ripening films hopes to really absorb ethylene fillers, supplemented by oxidation catalysts and photocatalysts (titanium dioxide particles). However, in this case, incomplete decomposition or decomposition of film substrate will be caused, resulting in odor and safety problems; Recently, Photocatalyst (titanium dioxide) and adsorbents such as zeolite were microencapsulated through porous silica. The average particle size is 7 ~ 10 μ M deodorizing filler is added to polyolefin film, which has a long-term deodorizing effect on ammonia and other substances, and its effect is above that of activated carbon. However, due to the large particle size of the filler, the transparency of the film is poor, and the price of the filler is expensive, it has not been applied in practice

2. Antibacterial fillers

in Japan, food poisoning caused by pathogenic Escherichia coli "0-157" and microbial accidents in food production lines were used as wedges. In order to control harmful and dangerous microorganisms, antibacterial wrapping films filled with organic or inorganic antibacterial fillers were developed in the early 1990s. In order to solve this problem, antibacterial agents can be roughly divided into inorganic antibacterial agents (including silver and zinc plasma), organic antibacterial agents (quaternary ammonium salt, etc.) and natural antibacterial agents (chitin, deacetylated polysaccharide, juniper alcohol, Shanyu vegetable, etc.). Their main advantages and disadvantages and the main manufacturers of antibacterial films are summarized in Table 5

as shown in the table, among the antibacterial agents used in food packaging film, inorganic antibacterial agents have higher safety and better heat resistance than organic antibacterial agents; Compared with natural antibacterial agents, the price is low, so inorganic antibacterial agents, especially silver antibacterial agents, are often used. Almost all silver antibacterial agents are porous inorganic fillers impregnated (or adsorbed) with silver ions, and their substrates can be zeolite, aluminum phosphate or apatite, etc

it must be noted that silver ions will be produced due to the action of light, so as to prevent dust from entering the internal active oxygen, causing organic matter oxidation, and silver ions will turn into silver of 1 valence and change color; Silver ions can also react with halogen ions, so the antibacterial film filled with silver antibacterial agents has the disadvantage of easy discoloration. In order to eliminate this shortcoming, silver antibacterial agents have been improved, but because the discoloration problem is the nature of silver ions, it is limited to inhibit discoloration and maintain their antibacterial properties

3. The proper disposal of packing waste for degradable film is an important topic in the current environmental protection work. In order to meet the needs of environmental protection, the development and application of degradable plastics have attracted attention in recent years, among which mixed biodegradable plastics have been put into practical application with good results. Mixed biodegradable plastics, with starch as the filler (biodegradable agent, biodegradable "disintegrating" agent) and added to polyolefins (polyethylene or polypropylene), can maintain the equivalent physical and mechanical properties of ordinary polyolefin plastic products and meet the needs of use; Once used, it is scattered in the natural environment as waste, or buried in the soil (or in the composting environment). The starch decomposes rapidly under the action of microbial secretion amylase, and the disappearance of starch makes the plastic products form a porous structure, and the mechanical strength decreases, resulting in fragmentation. The porous structure and fragmentation greatly increase the contact surface between plastic products and the environment, thus promoting the subsequent degradation processes such as oxidation and hydrolysis of polyolefins, and finally degrading polymer compounds into low molecular products that can be absorbed by the environment

in order to improve the compatibility between starch and polyolefin and obtain degradable plastics with excellent mechanical properties, it is usually necessary to modify starch (such as hydrophobic treatment); In addition, in order to promote the photooxidation and thermal oxidation reactions of polyolefin polymer compounds and accelerate the fracture of C-C long chains of polymers, specific automatic oxidation catalysts, such as transitional element (iron, copper, vanadium, etc.) compounds or unsaturated fatty acids, fatty acid esters, etc., are often added when starch fillers are added

table 6 shows the comparison of the physical and mechanical properties of starch filled biodegradable plastics (80% llppe + 20% of "デグラノンンン}" *) and LLDPE films. From the table, it can be seen that the physical and mechanical properties of starch filled biodegradable plastics can reach the level close to that of ordinary polyolefin plastics; Figure 5 shows the change of polyolefin molecular weight in the film with time during composting of starch filled biodegradable plastics. Under composting conditions, the degradation rate of starch filled biodegradable plastics is extremely significantly improved compared with ordinary polyethylene: the peak molecular weight distribution of biodegradable polyethylene containing 10% of "デグラノボン" is about 70000 before composting, After 5 days of composting treatment, it fell to about 30000 (after 18 days of composting treatment, it fell to 7000), and the molecular weight peak of ordinary polyethylene with a molecular weight of 70000 was still 70000 after 5 days of composting treatment, and the effect of molecular weight decline was basically not seen. Moreover, according to the literature, it has been confirmed by C 14 tracer atom tracking test that the polyethylene macromolecular chain in the above starch filled biodegradable plastics can be finally inorganic (that is, the carbon atom in the polyethylene macromolecular chain is finally converted to carbon dioxide) [2]

4 ﹐ absorb harmful substances