How much do you know about modified PVA high barrier film?
1. The characteristics of polyvinyl alcohol (PVA)
Polyvinyl alcohol (PVA) is a hydrolysis product of polyvinyl acetate. It has excellent film-forming properties, colorless and transparent film, oil resistance, organic solvent resistance, and stability to bacteria and sunlight. As a film production process, the following characteristics must be Attract enough attention.
01
The solvent of PVA is water, but its solubility in water is largely affected by the degree of polymerization, especially the degree of alcoholysis. The completely alcoholysis PVA dissolves very little in water. When the alcoholysis degree is less than 88%, it is almost completely dissolved at 20°C and normal temperature, but as the alcoholysis degree increases, the solubility greatly decreases.
02
The viscosity of the polyvinyl alcohol aqueous solution with high alcoholysis degree changes due to the different storage conditions, and the storage temperature directly affects the viscosity. Experiments have proved that the viscosity drops significantly at 20-50°C, the viscosity drops slightly above 50°C, and it remains stable above 80°C. The viscosity of the PVA aqueous solution increases with the increase of the PVA concentration. The surface of the PVA aqueous solution is very easy to crust in the static state, which is extremely unfavorable to the coating process.
03
PVA film has excellent oxygen barrier properties under dry conditions, and its oxygen permeability coefficient is the lowest among various resin films. There are a large number of hydroxyl groups (OH) on the molecular chain of PVA. These hydroxyl groups can easily form hydrogen bonds with water molecules in a wet environment, resulting in changes in the structure of PVA aggregates and greatly reducing the barrier properties of PVA. As the relative humidity rises, its oxygen transmission rate increases significantly.
04
The PVA film will swell and fall off when encountering water. If the coated composite film is exposed in a humid environment, its high oxygen barrier performance will be lost.
In the early 1990s, some domestic universities and research institutes made PVA resin into a hydrosol, and made a PVA coated film using a coating compound process. In order to stabilize the barrier properties of the PVA film under high humidity conditions, a masking film was used. Technology produced BOPP/PVA.
PE coating film, its essence is to sandwich the PVA that is afraid of water between two layers of films with better moisture barrier properties, which shields the influence of moisture on PVA. The PVA coated composite film produced by this process can maintain its barrier properties, but the process is complicated, low in efficiency, high in cost, and unstable in performance.
2. Modification of polyvinyl alcohol
In order to improve and stabilize the wet barrier properties of PVA, simplify the production process, reduce production costs, and bring the PVA composite film with superior performance to the market, researchers have modified PVA.
The reason why polyvinyl alcohol is not water resistant is that it has a hydrophilic hydroxyl group (OH). If the hydroxyl group can be properly blocked and water resistant groups can be attached, the water resistance of the PVA film can be improved. PVA contains hydroxyl groups, which can cause all typical reactions of polyols. Researchers have selected a variety of compounds that can be molecularly cross-linked with OH in PVA. The modification effect and processability are not ideal. Finally, a pair of Melamine resin modified liquid "868", which has no toxic and side effects to human body.
Because "868" is a multifunctional polycondensate, it can be moderately cross-linked with the hydroxyl group in PVA when the amount of addition is not large, so that PVA can form a tough three-dimensional structure coating, which stabilizes PVA. The air tightness under the wet conditions improves the water resistance.
The modified PVA glue does not form skin at room temperature, and the viscosity does not rise within the time allowed for production. It realizes the glue preparation at room temperature and the coating at room temperature, which provides a relaxation for the production of modified PVA coated composite film. The process conditions.
Coating process: Due to the high polarity of the modified liquid, the adhesion between the modified PVA and the base film is improved, and the adhesion between the layers can reach an inseparable degree. It can be directly coated on the surface of PE, BOPP, BOPA, BOPET and other films. Cloth, constitute modified PVA/PE, modified PVA/BOPP coated composite film, the production process is as follows:
PE unwinding-→ modified PVA coating-→ drying-→ winding-→ curing-→ slitting-→ inspection-→ storage
The modified PVA/PE film can be color printed on the PVA surface without any treatment, and the ink adhesion is very strong.
In order to further improve the effect of packaging and decoration, it can also be printed on BOPP and other substrates, then coated and compounded with modified PVA, and further processed into BOPP/modified PVA/PE film.
Three, coating compound equipment
The multifunctional coating unit suitable for the production of modified PVA coated composite film adopts advanced structures such as reverse roller coating head, hot air convection drying, and micro-tension conveying. It can be used to produce BOPP/modified PVA/PE, BOPP/PE, etc. in one step. PET/PE composite film; it can also be used in the production of self-adhesive and protective film, achieving the design goals of stable operation, uniform coating, energy saving and one machine with multiple functions.
When the length of the drying tunnel and the air supply of the coating laminating unit are fixed, the drying speed of the coating film is controlled by the coating thickness, the temperature of the drying tunnel and the film speed. The thicker the coating, the lower the temperature of the drying tunnel. , The slower the drying speed. In order to ensure that the coating is dry, the film running speed is bound to be very low, losing the meaning of industrial production.
The coating thickness is 5-8 microns, and the process conditions for the PVA coating to pass quickly in the 100 ℃ drying tunnel are not only conducive to improving the crystallinity of PVA, ensuring the air tightness and water resistance of PVA, but also conducive to improving production efficiency. The production capacity of this unit is 1,000 tons per year.
Fourth, PVA modification effect
Compared with the unmodified PVA coating film, the modified PVA coating film has significantly improved adhesion, wet barrier properties and water resistance.
Five, the characteristics of modified PVA high barrier film
1. Use the melamine resin modification liquid "868", which is non-toxic to human body and has no side effects, to appropriately crosslink PVA. While retaining the advantages of dry barrier performance, transparency and flexibility, PVA also improves the substrate Adhesiveness, eliminating the need for pre-coating and adhesives, especially outstanding is that compared with unmodified PVA, the wet barrier performance is significantly improved, and the whole set of technologies is advanced.
2. Compared with the unmodified PVA process, the "868" modified PVA eliminates the need for pre-coating and the use of binders, which can reduce equipment investment and product costs. The modified PVA hydrosol solves the problem of surface skinning under static conditions and viscosity increase under production conditions, cancels the masking process, and makes the production process reasonable.
3. Compared with unmodified PVA, the modified PVA coating layer has no swelling or shedding after being immersed in water for 24 hours, and the key indicator oxygen permeability has reached the international advanced level.
Six, application
The application of modified PVA high-barrier film is inseparable from its characteristics, here is a brief introduction.
1. The modified PVA film has good transparency and gloss. It is especially suitable for the sealing of clothing, shoes and hats, leather and leather products, arts and crafts, manuscripts, and files. It is used with deoxidizers to prevent mildew, insects, and oxidation. The effect of discoloration.
2. The modified PVA film has strong polarity and good ink adhesion during printing. With cellophane type printing ink, you can get beautiful printing effect.
3. Good oil resistance and organic solvent resistance. It is insoluble to animal oils, vegetable oils, mineral oils, alcohols, ethers, ketones, and esters. It is suitable for packaging of chemical raw materials and intermediates and packaging of oily food.
4. It has good aroma retention, and can be widely used in the packaging and sealing of tea, coffee, curry powder, chili powder, pepper powder and other spices. It is used for filling mustard, so that the smell can not be seen.
5. It has excellent gas barrier properties, especially extremely low oxygen permeability, and is suitable for packaging and sealing the following items.
A. It can be used for the packaging of small grains, oilseeds, dried seafood, precious Chinese medicinal materials, tobacco, etc., and used with deoxidizers or vacuuming, and its anti-mildew, insect-proof, and anti-fading quality and fresh-keeping effect is particularly obvious, and it is used for sealing High-fat foods have the effect of preventing the oxidation of fat from "ha" and maintaining the original flavor of the food.
B. It is used for the comprehensive storage of radio components and complete machines, precision machinery, ordnance, radar, ammunition, and large missiles. It has a unique anti-oxidation and corrosion function.
C. The optical instruments, lenses and other accessories are comprehensively sealed to prevent mold and fogging.
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