Polyvinyl alcohol (PVA) appearance of white powder, is a water-soluble polymer resin, has been widely used as adhesives in various industries, PVA has a strong cohesion, skin film flexibility, smoothness, oil resistance, solvent resistance and other characteristics of resistance, protective colloid, gas barrier, wear resistance and water resistance after special treatment, in addition to fiber raw materials, but also used for mass production of coatings, adhesives, paper processing agents. Emulsifier, dispersant, film and other products, is widely used in textile, food, pharmaceutical, engineering construction, wood processing, paper making, printing, agriculture, steel, polymer chemicals and other fields.
The polymerization degree of polymerase alcohol can be divided into four types according to molecular weight class: ultra-high polymerization degree, high polymerization degree, medium polymerization degree and low polymerization degree. -The high polymerization degree of polymerase alcohol is 250,000~300,000, the molecular weight of high polymerization degree is 170,000~220,000, the molecular weight of medium polymerization degree is 120,000~150,000, and the molecular weight of low polymerization degree is 25,000~35,000.
The alcoholysis of polyvinyl alcohol is usually 78%, 88% and 98%, the alcoholysis of phosphol is 98% to 100%, and the alcoholysis of partially alcoholysed polyethylene is usually 87% to 89%.78% is polyvinyl alcohol with low alcoholysis.
As the degree of polymerization increases, the viscosity of aqueous solution of the same concentration increases, but the solubility in water decreases; as the degree of alcoholysis increases, the solubility in cold water decreases and the solubility in hot water increases.
Alcohol solubility of 87% to 89% of the product water solubility is the best, either in cold water or in hot water can be quickly dissolved; alcohol solubility of 99% or more of PVA is only soluble in hot water above 95 ℃. As the degree of polymerization increases, the strength of the skin film on the surface of the aqueous solution increases.
In order to prepare aqueous solutions of oxidizing alcohols, it should be noted that the dissolution process of oxidizing alcohols is divided into two stages: solubilization and dissolution. The dissolution temperature and retention time vary with the degree of alcoholysis.
The dissolution temperature and holding time of fully alcoholized PVA are 95~100℃ and 2~2.5h, respectively, and the dissolution temperature of sorbitol with 87%~89% alcoholysis is 65~85℃, and the holding time is 0.5~85℃. 1.0h. Too high dissolution temperature may cause adverse effects.
The storage of PVA aqueous solution should pay attention to the following points.
1, in order to avoid the use of rust-prone containers to keep the aqueous solution, generally use stainless steel or plastic containers for storage, sodium nitrite or sodium benzoate is an effective rust inhibitor.
2, PVA aqueous solution will sometimes mold, in order to prevent mold, the use of salicylic acid, sodium dehydroacetate, potassium sorbate, sodium pentachlorophenol works best.
3, fully alcoholic PVA aqueous solution stored for a period of time, especially in winter under low temperature conditions, viscosity usually increases or even gelling, if the aqueous solution containing only PVA can be restored to flow. Raise the temperature and stir the solution.
4, a small amount (5-10% / PVA) of sorbic acid, thiocyanate or a high concentration of alcohol, can play the role of viscosity stabilizer, effectively prevent gelling.