The modification mechanism of
Polyvinyl Alcohol (PVA) and the production process of modified PVA coating composite are discussed. The results show that the modified PVA achieves the advanced technology of direct coating and compounding on PE without pre-coated anchoring agent, simplifies the production process and reduces the production. Compared with the unmodified PVA, the wet performance is obviously improved and the anoxic performance is improved. To reach the international advanced level.
Keywords: PVA modification; coating; oxygen barrier; moisture resistance
Polyvinyl alcohol, polyvinylidene chloride and acrylonitrile are by far the best resins with three barrier properties. Polyvinyl alcohol (PVA) is a hydrolysate of polyvinyl acetate. Its molecular formula is:{- CH2 - CH-CH2 -}n|
In the early 1990s, some domestic universities and research institutes made PVA resin into a hydrosol, and a PVA coated laminated film was made by a composite coating process, such as BOPP/PVA/PE, to coat PVA on the PE surface. Products, such as PVA/PE, are rare in the market because unmodified PVA has four characteristics.
(1) Hermeticity. The unmodified PVA film has excellent oxygen barrier performance under dry conditions, and as seen from Table 1, its oxygen permeability coefficient is the lowest among various resin films.
Table 1 Air permeability coefficient of various resin films
Thin film name PVAPVDC Nylon 6 PETPE oxygen permeability coefficient (ml·cm/cm2·s·cmHg) 6.24×10-175.3×10-133.8×10-12~7.1×10-132×10 -122.5×10-10
However, there are a large number of hydroxyl groups (OH) in the molecular chain of PVA. In the wet environment, these hydroxyl groups easily form hydrogen bonds with water molecules, resulting in changes in the aggregation structure of PVA, which greatly reduces the barrier properties of PVA.
(2) Water resistance. The PVA film swells and detaches when it encounters water. If this coated composite film is exposed to a humid environment, its high oxygen barrier performance will be lost.
(3) processability. The viscosity of PVA hydrosol is directly related to temperature and storage time. At 20 to 50°C, the viscosity decreases more, the viscosity decreases at 50°C or more, and it tends to stabilize at 80°C or more. The viscosity of PVA hydrosol increases with increasing PVA concentration.
(4) Surface crusts. In addition to the characteristic of increased timing, the PVA hydrosol is very susceptible to crusting in the static state. This crust is extremely unfavorable for coating.
Without solving the above problems, PVA will be restricted as the promotion and application of barrier packaging materials.
1 Interruption technology To stabilize the barrier properties of PVA film under high humidity conditions, BOPP/PVA/PE coated composite films have been produced using blocking technology. The essence of this technology is to sandwich PVA, which is afraid of water, in two layers with better moisture resistance. In the middle of the film, the influence of moist moisture on PVA is interrupted. The production process is as follows:
PVA Sol
With adhesive constant temperature cycle adhesive PE film roll ↓ ↑ ↓ ↓
BOPP film roll → pre-coated adhesive → dry → coated PVA glue → dry → adhesive coating → drying → lamination → winding → ripening → slitting → inspection → storage
The PVA coating composite film produced by this process can maintain its barrier properties, but the process is complicated, the efficiency is low, and the cost is high.
2 PVA modification is to improve and stabilize the wet barrier properties of PVA, simplify the production process, reduce production costs, make the PVA composite film with superior performance go to market, and modify the 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 closed and the water-resistant group is connected, the water resistance of the PVA film can be improved. PVA contains hydroxyl groups, can play all typical reactions of polyols, select a variety of compounds that can be cross-linked with OH in PVA, its modification effect and processability are not ideal, and finally developed a success for the human body The melamine resin modification liquid [-868" with toxic side effects, because [868" is a multifunctional polycondensate, can be moderately cross-linked with the hydroxyl group of PVA when the amount is small. PVA forms a strong three-dimensional structural coating, which stabilizes the airtightness of PVA in wet conditions and improves water resistance.
The modified PVA glue liquid does not crust at normal temperature, and there is no increase in viscosity during the allowable time of production, and normal temperature dispensing and room temperature coating are realized, providing a production of modified PVA coated composite film. Loose process conditions.
3 Coating process Due to the greater polarity of the modified liquid, the adhesion between the modified PVA and the base film is improved, and the interlaminar bonding force can reach an inseparable degree. The coating can be directly coated on the surface of PE, BOPP, etc. PVA/PE, modified PVA/BOPP coating composite film, production process is as follows:
Plastic Glue
PE film roll → modified PVA coating → drying → winding → ripening → slitting → inspection → storage
The modified PVA/PE film can be printed on the PVA surface without any treatment, and the ink adhesion is very strong.
In order not to increase the effect of packaging and decorating, it is also possible to first print on BOPP and other substrates, and then apply the composite with modified PVA to further process BOPP/modified PVA/PE composite film.
4 The coating equipment was designed and manufactured by itself to produce a multi-functional coating compounding unit suitable for production of modified PVA coated composite film. The unit adopts advanced structure such as reverse roller coating head, hot air convection drying and micro-tension conveying, etc. Production of BOPP/modified PVA/PE coated composite film; modified PVA/PE coated composite film and nylon/PE, BOPP/PE, PET/PE composite film can be produced at the same time; it can also be used for self-adhesive and protection The production of membranes achieves the design goals of stable operation, uniform coating, energy saving and multi-purpose use, as well as control of coating thickness, drying tunnel temperature, and operating speed.
When the length of the drying tunnel and the air supply volume of the composite coating unit are fixed, the drying speed of the coating film is controlled by the coating thickness, the drying tunnel temperature and the film-moving speed. The thicker the coating, the lower the drying tunnel temperature, The slower the drying speed is, in order to ensure that the coating is dry, the running speed of the film is bound to be very low, losing the significance of industrial production. The process conditions for coating thickness of 5-10 μm and PVA coating to pass through in a drying tunnel at 100°C are obtained. This not only helps to increase the crystallinity of PVA, ensures the airtightness and water resistance of PVA, but also improves the production efficiency. The unit has a production capacity of 1,000 tons/year.
5 Modification effect The modified PVA coating composite film has passed a small-scale test and a mid-test, and has been built with an annual production capacity of 1,000 tons. The products put on the market are well received by users. Compared with the unmodified PVA coating film, the modified PVA coating film has significantly improved adhesion, wet barrier properties and water resistance, and the test data are shown in Table 2.
Table 2 Test data of adhesion, water resistance and barrier property of unmodified PVA and modified PVA coating
Test Index Unmodified PVA Coating Modified PVA Coating Test Unit Adhesion PVA Coating Peeling PVA Coating No Peeling Shanghai Pharmaceutical Administration Drug Testing Water-Resistant PVA Coating Swelling, Shedding Soaking in Water for 24h, Swelling Shedding phenomenon. PVA coating does not swell or detach, soaks in water without soaking for 24 hours. Phenomenon: Shanghai Institute of Chemical Medicine, Institute for Drug Control, Nanjing University, School of Chemical Engineering, oxygen permeation ml/m2·24h·atm, water vapor permeation g/m2·24h5.89
Saint Petrochemicals Limited
