1. Q: How is polyvinyl alcohol PVA processed?
A: The production line of PVA takes vinyl acetate as monomer, azodiisobutyronitrile as initiator, solution polymerization to produce polyvinyl alcohol (PVAc) intermediate by way of solution polymerization. Then through alcoholysis on the condition of base catalyst, a series of PVA can be produced.
2. Q: What is the property Index of PVA?
A: 0.42~0.52 g/ml
3. Q: What is PVA polyvinyl alcohol?
A: PVA is a solid substance, colorless and scentless in the shape of floccus, flake or granule. As it contains plenty of hydroxyl groups in its molecule, PVA is easily soluble in water as well as in organic compounds also containing hydroxyl such as glycerol, glycol, acetic acid, acetaldehyde and benzoic acid, except for general organic solvents in which it is insoluble. When heated up to 130-140, PVA gets yellow without essential changes in nature; when at 160for a longer time, it gets darker; at 200, inter-molecular dehydration takes place with lower water solubility; at above 200, intra-molecular dehydration takes place; nearly at 300, PVA is decomposed into water, acetic acid, acetaldehyde and crotonaldehyde. The bulk densities of PVA vary depending on methods of alcoholysis: 0.2~0.27g/ml for high alkali alcoholysis and 0.42~0.52 g/ml for low alkali alcoholysis.
4. Q: Is polyvinyl alcohol PVA soluble?
A: PVA is water soluble with transparent solution. Most PVA series are used while dissolved in water. Its solubility depends mainly on degrees of polymerization and alcoholysis. At the same temperature, the solubility increases along with reduction of the degree of alcoholysis, and decreases with increase of the viscosity; for the PVA series with the same degree of polymerization, if their degrees of alcoholysis are low, their temperatures get low upon completion of dissolution, and vice versa; for the PVA of the same alcoholysis, if the degrees of polymerization are high, their temperatures get high upon completion of dissolution, and vice versa. The PVA at the degrees of alcoholysis above 99 (mol/mol)% depends greatly on temperature changes, that is, dissolution or partial swelling may not take place at a particular temperature.
5. Q: What are the ways of PVA’s dissolution?
A: Add PVA slowly while stirring for sufficient decentralization and swelling before heated. 10- to 30-minute stirring to swell PVA at the ambient temperature can efficiently shorten the dissolving duration.
When heated to temperatures above 90-95, the complete alcoholysis type PVA can be fully dissolved within 30-60 minutes approximately; for the medium alcoholysis type PVA, when heated up to above 80-90, within 30-60 minutes approximately; the partial alcoholysis type, up to 60and then below 30, for about one hour to clear solution. In order to have a better effect of use, the completely dissolved PVA should be screened with a mesh of 100-120 m.
When dissolved, the PVA of medium and partial alcoholysis types blisters easily. Stirring during dissolution at the speed of 60-90 rotations per minute ensures that the temperature will not increase too fast, otherwise foam and spillage may easily arise. If there are too much foam during dissolution, the steam can be closed, and the stirring be either slowed down or stopped. Gradual increase of the temperature and stirring up may be followed after the foam gradually disappears. Generally, after two or three times of repetition of such procedures, the quantity of foam can be dramatically decreased and the certain dosage of all foam-removing additives is just 0.008-0.04% by proportion to the consumption of PVA. The commonly used foam-removing agents include tributyl phosphate. Storage after dissolution may result in formation of a layer of membrane on the surface of the paste and affect inconvenience in use. To avoid this, first use 2/3 cold water to dissolve PVA, then after dissolution, add the rest 1/3 cold water, and stir solution up until it cools down.
Compared with other water soluble macromolecule substances (CMC), the PVA solution is a low viscosity polymer very stable at the ambient temperature. It does not denature during storage. With its alcoholysis equal or greater than 99 (mol/mol)%, it can turn into gel at a low temperature, and easily turn into solution once it is heated. In order to prevent the size paste from contamination in the process of dissolution, stainless steel dissolving vessels are usually used; jacketing or water bathing can be the heat source, and heating can be achieved from direct steam, but heating from direct fire may lead to paste burning at the bottom of the kettle and should be avoided where possible.
At the ambient temperature, add a certain amount of cold water into dissolving vessels. If steam is blown directly into the vessel to heat the water, 10-15% of water can be saved due to production of condensate. To avoid lumping, direct use of hot water for dissolving should be avoided where possible.
5. Q: How is our PVA packed?
A: LIWEI CHEMICAL CO. LTD brand PVA is packed in laminated plastic woven bags each containing 12.5kg of floccules products and 20Kg of flake products (net weight), with manufacture dates, work shifts, batch numbers and anti-falsification labels sewn on the bags.
6. Q: What is the storage method of PVA?
A: PVA can be storaged for a long period without denaturalization unless it contacts water directly. During the storage, volatilization matter may slightly cause both moisture and weight loss but that will not affect the quality of PVA. Then, when the volatilization matter gets low, PVA absorbs easily moisture and adds weight to it.
If it is not contaminated, PVA solution does not rot or deteriorate easily which makes it easier for storage than any other macromolecular solution.
PVA 24-99, PVA18-99, PVA17-99, PVA 13-99 series are high viscosity and complete alcoholysis types. If they are made into a paste with a concentration higher than 8%, they can be stored at the ambient temperature lower than 20with increased viscosity, forming gel due to molecule crystallization without any change in nature. Their fluidity can be restored to the original after heated and stirred. To avoid such happenings, in winter the frost resistance type PVA can be selected, or similar products with alcoholysis degrees of 88-97mol/mol%, which perform slightly poor in PVA water resistance, or by combination of complete and partial alcoholysis types for freeze resistance.
For the PVA with a degree of alcoholysis smaller than 99 (mol/mol)%, its viscosity is consistent with few possibilities to gel at low temperatures.
Stainless steel tanks are suitable for storage of PVA paste. If only iron or carbon steel containers are available, 0.05% sodium nitrate or sodium benzoate can be added into the PVA paste to avoid rusting. Given a long period of storage, the untreated PVA solution in water mildew and rot easily, when some 0.01%-0.05% antiseptic can be added, e.g. salicylic acid.
7. Q: What is the standard of our polyvinyl alcohol PVA?
A: LIWEI CHEMICAL CO. LTD brand polyvinyl alcohol applies to the enterprise standard Q/YWJ 5042-2007 for PVA Resin.