What Are the Degree of Polymerization and Degree of Hydrolysis of PVOH?

The properties of polyvinyl alcohol (PVOH, PVA, poval) are largely determined by two key parameters established during the manufacturing process: the degree of polymerization (DP) and the degree of hydrolysis (DH). The DP strongly influences properties such as aqueous solution viscosity and film strength, while the DH has a significant impact on water solubility, water resistance, and gas barrier performance. Understanding these two parameters is essential for selecting the PVOH grade best suited to a given application.

This article first outlines how PVOH is manufactured, and then explains the definitions, calculation methods, and effects of the DP and the DH on material properties, in an easy-to-understand manner. It is intended to help users identify which parameters to focus on when considering the introduction or selection of PVOH.

Hydrolysis (saponification) is the reaction in which the acetyl groups (CH₃CO–) of PVAc are cleaved to form hydroxyl groups (–OH). PVOH is obtained through this reaction. Industrially, hydrolysis is typically carried out by alkaline hydrolysis, with methanol commonly used as the solvent.

By controlling how many acetyl groups are converted into hydroxyl groups during this reaction, PVOH grades with various degrees of hydrolysis (DH) can be produced. The DH has a profound effect on key properties such as water solubility, water resistance, and gas barrier performance (further details are explained in the next section).

The degree of polymerization (DP) indicates the length of the molecular chain, i.e., how many vinyl acetate units are linked within a PVOH molecule. It can be calculated using the numbers of repeating units “m” (hydroxyl units) and “n” (residual acetyl units) as shown on the right.

The DP is controlled during the polymerization step and has a major impact on final product performance.

Differences in the DP significantly influence the following properties. Please refer to this information when selecting the appropriate grade for your application.

• Aqueous Solution Viscosity: As the DP increases, molecular entanglement becomes more pronounced, resulting in higher solution viscosity. Low-DP grades exhibit low viscosity, are easier to handle, can be dissolved at higher concentrations, and provide greater formulation and coating-process flexibility.
• Film and Mechanical Strength: The tensile strength and tear strength of PVOH films generally increase with higher DP.
• Film Formation Stability: High-DP grades readily form strong films but may require careful control of dissolution and viscosity adjustment. Low-DP grades dissolve easily and enable uniform film formation, making them suitable for low-viscosity formulations and thin-film applications.

Differences in the DH significantly influence the following properties. Please refer to this information when selecting the appropriate grade for your application.

• Water Solubility: Lower DH result in easier dissolution in water and enable dissolution at lower temperatures. Fully hydrolyzed grades typically require heating to approximately 95 °C for dissolution. (However, if the DH becomes too low, the increased influence of residual hydrophobic acetate groups can reduce water solubility.)
• Gas Barrier Properties: Higher DH provide superior gas barrier performance. Fully hydrolyzed grades are therefore suitable for applications—such as food packaging—where gas barrier properties are required.
• Water Resistance: As the DH increases, the water resistance of dried films improves. Fully hydrolyzed grades are advantageous for applications exposed to moisture.
• Film Formation: The combination of DH and DP determines film strength, flexibility, and transparency.
• Surface Activity and Emulsification Stability: Partially hydrolyzed grades contain hydrophobic acetyl groups as well as hydrophilic hydroxyl groups, resulting in lower surface tension and making them suitable for use as emulsifiers and dispersants

While high water-soluble grades provide improved solubility, their water resistance after drying is generally lower than that of standard grades. When both high-water solubility and water resistance are required, formulations incorporating crosslinking agents can be effective. Kuraray also provides formulation support tailored to specific applications and areas of use.

For assistance with grade selection or formulation design, please feel free to contact Kuraray’s technical team via the inquiry form on this website.