Polyvinyl Alcohol (PVA) Dissolution and Preparation

In industrial applications, polyvinyl alcohol (PVA) usually needs to be prepared as an aqueous solution to exert its properties. However, due to differences in PVA grades, degree of hydrolysis, and physical form, the dissolution process often encounters challenges such as clumping, foaming, or incomplete dissolution. This article will combine professional technical experience to detail the dissolution principles, operating methods, and defoaming techniques of PVA.

1. Dissolution Principles

The dissolution of PVA is a process of swelling followed by dissolution, and its efficiency is profoundly affected by molecular structure and physical form:

• Factors determining solubility: The solubility of PVA is mainly determined by its degree of hydrolysis, degree of polymerization, and shape.
• Effect of degree of hydrolysis: As the degree of hydrolysis decreases, the dissolution temperature of PVA decreases, and its water solubility increases.

          ◊ Fully hydrolyzed type: Highly dependent on temperature; below a certain temperature, it will not dissolve or will only partially swell.

          ◊ Partially hydrolyzed type: Although easier to dissolve, excessively high temperatures can easily lead to foaming and clumping.

• Effect of morphology: Powdered PVA (20-100 mesh) has a larger surface area, so its dissolution time is about half that of granular PVA.

2. Technical Issues in Preparing PVA Solutions

To prepare high-quality PVA solutions and avoid contamination, the following hardware and parameter settings must be considered:

2.1 Equipment Selection

A reaction vessel with a stirrer should be used. The material must be stainless steel, enamel, or iron lined with synthetic resin to prevent rust and chemical corrosion from contaminating the PVA solution.

2.2 Stirring Speed Control

The stirring speed needs to be precisely adjusted according to the PVA specifications and stirrer type:

• Double-winged spiral stirrer: 500-1000 rpm is recommended for fully hydrolyzed types; 100-300 rpm is recommended for partially hydrolyzed types.
• Frame stirrer: 80-150 rpm is recommended.
• Risk warning: Too low a speed can easily cause PVA to settle and clump; too high a speed can easily entrain air and produce a large amount of foam.

2.3 Heating Method

Direct steam injection heating (pressure 1-1.5 kg/cm²) is recommended, supplemented by jacketed steam heating to significantly shorten the time. Direct heating with an open flame is strictly prohibited to prevent scorching at the bottom of the container.

2.4 Suitable Temperature for Preparing PVA Solution

3. Dissolution Procedure

Following a scientific sequence of adding materials and increasing temperature can effectively prevent clumping:

• Preparation Stage: Add a measured amount of room temperature water (approximately 30°C is recommended) to the dissolution tank.
• Material Addition and Dispersion: Start stirring (a slightly higher speed is recommended), and slowly add the PVA. The slower the addition, the better, to prevent clumping.
• Swelling Treatment: Stir and disperse thoroughly for about 30 minutes to allow the PVA to swell completely.
• Heating and Dissolution: Gradually increase the temperature to the appropriate temperature according to the table above, and maintain the temperature while stirring for 1-2 hours. For partially hydrolyzed types, the heating should be slow to prevent foaming and overflow.
• Product Inspection: After obtaining a completely transparent solution, filter out impurities before use.

4. Foaming Principle and Defoaming Methods

Foaming is the most common interfering factor in PVA dissolution, especially common in medium and partially hydrolyzed products.

4.1 Foaming Mechanism

• Air Release: PVA is a porous substance, and its pores contain air and volatile substances such as methanol and esters remaining from production.
• Structural Differences: Partially hydrolyzed PVA has larger spatial voids than fully hydrolyzed PVA. After absorbing water, it releases the air in the pores, forming foam.
• Surface Activity: Partially hydrolyzed aqueous solutions have higher surface activity, which reduces the gas-liquid interfacial tension, and the solution has a certain viscosity, increasing the mechanical strength of the liquid film, making it difficult for the foam to disappear.

4.2 Defoaming Methods

• Physical Immersion Method: Before dissolution, soak and swell the PVA in cold water to pre-release the air in the pores, and then gradually increase the temperature. This can effectively suppress foam generation.
• Intermittent Operation Method: When foaming occurs, immediately turn off the steam and pause or reduce the stirring speed. After the foam disappears, gradually increase the temperature and stirring speed. Repeating this 2-3 times can significantly reduce foaming.
• Chemical Defoaming Method: If necessary, 0.01-0.05% (by weight of the solution) of a defoaming agent can be added, such as n-octanol, tributyl phosphate, or polyether defoaming agents.

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