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Mechanism of Phase Transition from Layer to Hexagonal Form

Figure 1.5 is a schematic illustration of the layer filling mechanism. This mechanism is based on the intercalation process of the surfactant and silicate layers. The cations of the surfactant are inserted between the two silicate layers by ion exchange, the silicate layers then fold around the surfactant and condense into a hexagonal MQTB structure.

• Mechanism of phase transition from layered to hexagonal form

Silicate layer

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Silicate Surfactant

Figure 1.6. Mechanism of phase transition from layered to hexagonal form

Figure 1.6 is a schematic illustration of the phase transition mechanism from layered to hexagonal. This mechanism assumes that the silicates are first arranged in thin layers and that, due to electrostatic interactions with silicate anions, the surfactant cations are interspersed between the silicate layers.

MQTB silica material with crystal wall

Compared with zeolite microporous materials, MQTB materials have larger capillary sizes and high order, which allows large molecules to easily diffuse into the capillaries to participate in reactions (heavy fraction cracking and chemical transformation in high viscosity environments). However, the amorphous nature of the capillary wall and very weak acidity, low hydrothermal stability, so MQTB materials cannot respond to reactions under harsh conditions.

c- Bentonite [77]

* Chemical composition

Bentonite is a natural clay mineral, the main component of which is montmorillonite with the general chemical formula Al 2 O 3 .4SiO 2 .nH 2 O and some other clay minerals such as saponite-Al 2 O 3 .[MgO].4SiO 2 .nH 2 O; nontronite- Al 2 O 3 .[Fe 2 O 3 ].4SiO 4 .nH 2 O; beidellite - Al 2 O 3 .SiO 2 .nH 2 O. In addition, people have also found that bentonite contains some other clay minerals, alkaline salts and organic substances.

When analyzing the chemical composition of bentonite, in addition to the elements silicon and aluminum, people also discovered the presence of the elements Fe, Ca, Mg, Ti, K, Na... In which, the water content n = 4÷8; The ratio Al 2 O 3 : SiO 2 is from 1:2 to 1:4.

The chemical composition of bentonite greatly affects its structure, properties and bioavailability.

their usability.

* Bentonite crystal structure

Montmorillonite (bentonite) is a naturally occurring aluminosilicate with a 2:1 dioctahedral layer structure. The crystal structure of bentonite is composed of two tetrahedral networks linked together with an octahedral network in between to form a structural layer. Between the structural layers are exchangeable cations and adsorbed water.

Each structural layer is developed continuously in space along the a and b axes. The structural layers are stacked parallel to each other and self-interrupted along the c axis, the cation layers and adsorbed water form a three-dimensional spatial network of bentonite crystals.

The thickness of a bentonite structural layer is 9.6A o . If the cation exchange layer is included,

exchange and water adsorption, the thickness of the layer is about 15A o . Figure 1.7 shows the layered structure model of montmorillonite. When the crystal network is neutralized.

b

c

a

Figure 1.7. Spatial diagram of the structural network of montmorillonite

1.3.2. MAP packaging manufacturing technology

Modified atmosphere packaging films are typically produced by the blow extrusion method using thermoplastic resins. The extrusion process is as follows: Screw

Rotating in a heated, fixed circular cylinder and in the groove between the screw and the cylinder, the oriented plastic mass will be melted, softened, transported forward by the screw and through the shaped gap of the extrusion head, it is pushed out into the product [78].

In addition to single screw extruders, multi-screw extruders are also used. Among the multi-screw extruders, the twin-screw extruder is particularly useful for processing powdered plastics, especially for PVC. In principle, all thermoplastics can be extruded, but the molten plastic mass must have a certain hardness. Plastics with low molten solids due to their chemical structure can only be extruded when there is a very large polymerization or the addition of suitable fillers. Extrusion is used for large-scale processing of mainly hard PVC, soft PVC, PE and PP.

- Main components of the extruder:

+ Engine

+ Gear box

+ Screw and cylinder

+ Feeding unit

On the cylinder are arranged many heating zones, each zone can determine the temperature separately, and can be adjusted. Depending on each case, in addition to the heating zones, people also install additional cooling components, serving the production of more flexible temperature adjustment. The feeding chamber is always cooled to prevent the plastic from melting near it, avoiding affecting the feeding of the machine.



Figure 1.8. Schematic diagram of extruder

The equipment used for blowing film includes an extruder with a film blowing head, a cooling ring, a film flattening device, a film pulling shaft system, and a winding device. For film blowing technology, people often use a perpendicular extrusion head and the product is pulled up vertically. With such a solution, heavy machines and equipment are placed on the workshop floor, while the pair of pulling shafts and the film flattening device are mounted on a suitable frame. The advantage of this method is that the weight of the film will not affect the molten plastic mass coming out of the extrusion head. From the extrusion head, the plastic film is extruded in the form of a thin tube, then blown to the desired size. The air used for blowing is led in through a tube through the extrusion head hole.

In order to increase the cooling rate, the hot air inside the film bag needs to be replaced by cold air continuously. On the outside of the film bag, a cooling ring is used to supply cold air for cooling. The cooled film bag needs to be flat, punctured if necessary, and then rolled up. In films that need to be printed and re-sealed before being used for packaging, a very small variation in thickness is allowed. In order to regulate the change in film thickness, people rotate back and forth or use a mechanism

Pull the film or extruder at an angle of about 27 0 slowly. The back and forth

This is necessary because if the film is rolled up in the same position and there is an increase in film thickness, it will cause wrinkling on the roll of material.

By blown film technology, it is possible to produce multilayer films, in which case of course multiple extruders are required, followed by a film blowing head from which different molten plastic streams are stacked into layers [78].

1.3.3. Method of adjusting air permeability through MAP membrane

1.3.3.1. Adjusting film thickness

The decisive factor for the preservation efficiency of packaging film is the gas and water vapor permeability of the film. To achieve good gas permeability, there are many methods such as adjusting the film thickness, forming the film by the combined extrusion method, and the perforation method. Film production by the extrusion blowing method can adjust the film thickness by changing the distance and rotation speed of the roller and adjusting the forming head. In addition to this method, the gas permeability of the film can be adjusted by the combined extrusion method. This is essentially a method of extruding multiple films at the same time with different extrusion heads and joining these films together when they are still in a highly soft state. By this method, a multilayer film can be obtained that can combine films with different properties in a uniform film structure.

To achieve the best results in extrusion, the films can be oriented in advance before combining them together to obtain the most suitable film structure. In this way, the air-barrier properties of the film can also be adjusted. However, this method also has its own limitations. When the film is too thick, the permeability of O 2 and CO 2 through the film is low, then the film becomes airtight, the fruit's respiration produces water vapor, the water vapor does not escape and will stick to the surface of the fruit, causing the fruit to spoil faster. Films that are too thin are also not convenient for storage because they are not durable and are not convenient for long-distance transportation.

1.3.3.2. Punching method

The gas exchange permeability of the film can also be adjusted by perforation. This method involves punching holes of a certain size on the packaging film to achieve the appropriate gas exchange permeability for the fruit being stored. The holes can be of various shapes such as round, square, pentagonal, etc.

, hexagonal, elliptical… with different sizes. Normally, there are about 5 to 250 holes per 1cm 2 , even more. The hole diameter is from 0.01 to 0.25cm. By adjusting the size and number of holes, the gas exchange permeability of the membrane can be adjusted. To obtain the best membrane properties, the perforated membrane

often used in combination with other types of packaging such as sticking on non-perforated film or sticking perforated films together in an oriented manner. Perforating the film often satisfies the need for humidity control but is not satisfactory in maintaining a variable atmosphere around the product due to its high permeability. The perforation method also faces many difficulties in practical application due to its rather complicated technique [79].

1.3.3.3. Addition of gas permeability adjusting additives

Another method to adjust the gas permeability of MAP films is to introduce additives during the film manufacturing process. These additives are usually inorganic compounds based on silica or alumino-silicates (zeolites) such as clay and natural clay minerals (bentonite) [80]. The additives change the gas permeability of the film so that it interacts with the metabolic activity of fresh fruit to change the atmosphere around it. The additives are characterized by the silicon/aluminum ratio, capillary diameter, specific surface area, specific gravity and must meet three criteria: inert, porous and able to physically bind with gases such as O 2 , CO 2 , C 2 H 4 … These additives are hydrophilic, absorbing water, ethylene, carbon dioxide and other gases. These additives must also have a high porosity, capable of chemically or physically promoting the exchange of various gas molecules produced or used by the fruit in a way that ensures that O 2 is not completely depleted from the product atmosphere and that CO 2 does not increase to levels that cause spoilage. The presence of the additive affects the relative permeability of O 2 , N 2 , CO 2 , H 2 O, C 2 H 4 compared to conventional films, allowing for a more continuous and better adjustment of the variable atmosphere surrounding the fruit. The mechanism by which the permeability is affected is due to the physical properties of the additive and its interaction with the plasticizer. The plasticizer layer surrounding the additive particles is capable of controlling the permeability of various gases. Molecular sieves in the additive selectively control the movement of gases from within the film adjacent to the fruit to the outside atmosphere. Capillaries in the additive allow for bidirectional flow and by controlling

Controlling different gases at a certain rate relative to the molecular structure and desired properties, it is possible to establish CO 2 around the fruit at a level that affects the respiratory rate, reducing the metabolic rate leading to aging. At the same time, it allows O 2 to pass back from outside the product packaging at a rate that it is used inside the packaging corresponding to the reduction in metabolic rate. This mechanism ensures the maintenance of the atmosphere around the product, ensuring that the fruit remains alive and does not continue the state that leads to anaerobic spoilage due to lack of oxygen.

For MAP films made from LDPE, LLDPE, HDPE, the film thickness can be in the range of 10 to 150µm but is usually in the range of 25-50µm. The particle size of the activated inorganic additives must be uniform and usually over 50% must be in the range of 15-50µm and the maximum size must not be larger than the film thickness.

When using ceramic additives, the packaging film typically contains about 10% very fine ceramic powder and manufacturers claim that this material emits far infrared radiation or absorbs ethylene to extend the shelf life of foods. Ceramic-additive films have higher CO 2 to O 2 permeability ratios (3.6-5.0) and ethylene to O 2 permeability ratios (1.5-1.8) than conventional LDPE films, especially at low temperatures. These permeability ratios are particularly important for modeling MAP films for fresh produce.

Dirim and his colleagues used zeolite additives with three different film-forming methods to fabricate LDPE-based MAP films [80]. In the hot-press method, zeolite particles were introduced into the surface of an LDPE film placed between two plates that were controlled by temperature and pressure. This method did not yield satisfactory results due to the uneven distribution of zeolite and low mechanical strength. The resulting film resembled a sheet of kraft paper rather than a plastic film. Another method was to mix zeolite additives with molten PE or PE solution. In this method, zeolite particles were mixed with molten PE resin and then spread or coated on a specialized equipment. This method yielded uneven films due to the material sticking to the coating knife (the material solidified when cooled rapidly). The reason for the failure was that this process required high temperatures, close to the melting temperature, while coating or spreading needed to be done at room temperature with available equipment. PE can also be dissolved in a solvent (usually xylene) after

then mix zeolite in the solution. However, the resulting mixture is viscous like glue, so it is difficult to coat or spread the film at the operating temperature of the equipment. The extrusion blow method is most commonly used in industry, creating films that meet the requirements of use. However, it is necessary to study and adjust the size and content of additives to ensure that the film is evenly distributed with additives because there is no equipment specifically designed for composite films, so with additives of large particle size or content, sedimentation will occur and form larger blocks, causing defects on the film.

The ability to extend the shelf life and preserve fruits depends on the film thickness, additive content and particle size. Currently, the method of producing MAP films with the addition of additives to adjust the air permeability through the film is widely studied and applied.

1.3.4. Application of MAP packaging to preserve fresh fruits and vegetables after harvest

1.3.4.1. Overview of fruit preservation using MAP

Effect of MAP on physiological conditions and fruit storage: Modified atmosphere packaging affects the physiological properties of fruits and vegetables. Quality parameters such as retention of pigments, glutathione, ascorbic acid, sugars, sugar alcohols, amino acids are also affected during MAP storage. During modified atmosphere storage, the concentrations of O 2 , CO 2 and C 2 H 4 in plant cells determine the physiological and biochemical responses of that cell. The benefits of MAP for a given fruit or vegetable can be predicted from information on the underlying causes of spoilage and the known effects of these causes such as respiration, compositional changes, transpiration, physiological disorders, pathological spoilage. The reduction in respiration rate associated with the reduction in ethylene results in the retention of pigments (chlorophyll, lycopene, etc.), structure (less softening and ligninization), and sensory properties of fruits.

Due to the advantages of MAP, there have been many studies in the world applying MAP in preserving vegetables and fresh foods. David O'Beirne has studied the combination of MAP with cold storage applied to preserve beef, poultry and some fresh vegetables such as apples, potatoes, lettuce [78]. Based on the study of the gas barrier properties of the film and changes inside the package such as gas concentration

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