Properties of Polyvinyl Alcohol
Polyvinyl alcohol (PVA) is an odorless, white, flaky, flocculent, or powdery solid. It is readily soluble in water, slightly soluble in dimethyl sulfoxide, but insoluble in gasoline, kerosene, vegetable oil, benzene, toluene, dichloroethane, carbon tetrachloride, acetone, ethyl acetate, methanol, ethylene glycol, etc.
The degree of polymerization of polyvinyl alcohol (PVA) is classified into ultra-high degree of polymerization (molecular weight 250,000–300,000), high degree of polymerization (molecular weight 170,000–220,000), medium degree of polymerization (molecular weight 120,000–150,000), and low degree of polymerization (molecular weight 25,000–35,000). The degree of alcoholysis is typically 78%, 88%, and 98%. The degree of alcoholysis for partial alcoholysis is typically 87%–89%, while the degree of alcoholysis for complete alcoholysis is 98%–100%. PVA product model names are typically preceded by the thousands and hundreds digits of the average degree of polymerization, followed by the percentage of degree of hydrolysis. For example, PVA 17-88 indicates PVA with a degree of polymerization of 1700 and a degree of hydrolysis of 88%. Generally, as the degree of polymerization increases, the viscosity of the aqueous solution increases, and the strength and solvent resistance of the film also increase, but water solubility and elongation after film formation decrease.
The relative density (25℃/4℃) of polyvinyl alcohol (PVA) is 1.27–1.31 (solid) and 1.02 (10% solution). Its melting point is 230℃, and its glass transition temperature is 75–85℃. It undergoes a color change when heated above 100℃ in air. Heating to 160–170℃ causes dehydration and etherification reactions, reducing solubility. PVA begins to decompose at 200℃.
Manufacturing Methods of Polyvinyl Alcohol (PVA)
In the production of polyvinyl alcohol (PVA), alkaline alcoholysis processes are mainly divided into wet and dry methods, commonly referred to as the high-alkali method and the low-alkali method, respectively.
In the wet alcoholysis process, the methanol solution of PVAc contains 1%–2% water, and the sodium hydroxide catalyst is also prepared in the form of an aqueous solution. This method uses a large amount of alkali, resulting in a high alkali concentration in the reaction solution; therefore, it is also called the high-alkali method. The advantages of the high-alkali method are fast alcoholysis speed and high equipment capacity. However, the disadvantage is that there are more side reactions producing sodium acetate, leading to a higher sodium acetate content in the PVA product, resulting in low PVA purity and high ash content, thus affecting product quality.
Dry alcoholysis refers to the PVAc methanol solution containing less than 1% water, and the alcoholysis reaction is carried out under almost anhydrous conditions. This method uses less alkali, with the amount of sodium hydroxide used being only one-tenth that of the wet alcoholysis method. In addition, the side reaction producing sodium acetate is greatly reduced. Therefore, dry alcoholysis has become the main process for PVA production. However, the dry alcoholysis reaction rate is slower. During the dry alcoholysis process, the concentration of PVAc in the methanol solution is approximately 35%, and the reaction is carried out at 40°C for 15-20 minutes.
Quality Control and Production of Anhui Liwei Polyvinyl Alcohol
Anhui Liwei Polyvinyl Alcohol is produced under carefully controlled hydrolysis and polymerization conditions to ensure consistent molecular weight, hydrolysis degree, and performance stability. The degree of hydrolysis directly affects the hydroxyl content of the polymer chain, influencing key properties such as water solubility, film strength, and adhesion. At Anhui Liwei, advanced production processes and strict quality control systems are applied to monitor critical parameters throughout manufacturing. By optimizing catalysts, reaction conditions, and purification steps, Anhui Liwei Polyvinyl Alcohol products are designed to meet the requirements of various industrial applications, including adhesives, coatings, films, paper processing, and textile sizing.