Polyvinyl alcohol (PVA) is an organic compound that is an odorless, white, flaky, or powdery solid. It is soluble in water but insoluble in gasoline, kerosene, vegetable oil, benzene, toluene, ethylene dichloride, acetone, ethyl acetate, methanol, and ethylene glycol. It is slightly soluble in dimethyl sulfoxide (DMSO). Polyvinyl alcohol is an important chemical raw material used in the manufacture of polyvinyl acetal, gasoline-resistant pipes, vinylon synthetic fibers, fabric treatment agents, emulsifiers, paper coatings, and adhesives.
The physical properties of PVA are influenced by its chemical structure, degree of alcoholysis, and degree of polymerization. There are two chemical structures in the PVA molecule: 1,3-ethylene glycol and 1,2-ethylene glycol, but the predominant structure is the 1,3-ethylene glycol structure, also known as the "head-tail" structure. Polyvinyl alcohol (PVA) has a degree of polymerization (DP) of 250,000-300,000, a high DP (170,000-220,000), a medium DP (120,000-150,000), and a low DP (25,000-35,000). The degree of hydrolysis generally ranges from 78%, 88%, and 98%. Partial hydrolysis typically ranges from 87% to 89%, while complete hydrolysis ranges from 98% to 100%. The average DP is usually expressed with the thousands and hundreds digits preceding it, followed by the percentage. For example, 17-88 indicates a DP of 1700 and a solubility of 88%. Generally speaking, increasing the DP increases the viscosity of the aqueous solution, improving the strength and solvent resistance of the film, but decreasing its solubility in water and elongation after film formation.
It is used in the manufacture of water-soluble adhesives. It is also used as a modifier for starch adhesives. It can also be used in the preparation of photosensitive adhesives and benzene-resistant sealants. It is also used as a release agent and dispersant. Polyvinyl alcohol 17-92, abbreviated as PVA17-92, comes in white granules or powder. It is readily soluble in water, with a dissolution temperature of 75-80°C. Other properties are similar to those of PVA17-88. It is used as an emulsion stabilizer in emulsion polymerization and in the manufacture of water-soluble adhesives.
Polyvinyl alcohol 17-99 is primarily used in the manufacture of high-viscosity polyvinyl formal butyral and is widely used as a dispersant for sizing materials. 17-99 is used in the manufacture of aqueous solutions of polyvinyl formal.
Formaldehyde is a gas at room temperature and highly toxic. However, its solution exhibits some corrosion protection. Formaldehyde can undergo condensation reactions with other carboxyl and hydroxyl compounds. It has a wide range of applications, including synthetic resins, surfactants, plastics, rubber, leather, papermaking, dyes, pharmaceuticals, pesticides, photographic film, explosives, building materials, and disinfection, fumigation, and corrosion prevention processes. However, its use must be limited and standardized.
Exceeding these standards and limits can have adverse effects. In fine chemicals, polyvinyl alcohol and formaldehyde react under certain conditions to form a polycondensation compound with low fluidity and a certain viscosity, forming an adhesive. However, due to the influence of the degree of polymerization, this adhesive can sometimes exhibit high fluidity but poor adhesive properties. This can sometimes be unsatisfactory for practical applications, and starch is often added to improve its performance. Starch is a nutrient stored in plants, found in seeds and tubers. Various plants contain high levels of starch, ranging from 62% to 86% in rice, 57% to 75% in wheat, 65% to 72% in corn, and 12% to 14% in potatoes. Cassava starch is particularly suitable for foods requiring refined flavors, such as cake puddings and pastry fillings. Its high viscosity allows it to be used as a thickener, binder, bulking agent, and stabilizer. It is also a raw material for chewing gum and jelly.
Aqueous solutions of starch can gelatinize at a certain temperature, forming a colloidal solution with a certain degree of adhesiveness. Starch has numerous industrial applications, particularly because it contains numerous hydroxyl groups, which can react with aldehydes, carboxyl groups, and other groups through dehydration condensation. Therefore, starch is widely used as a modifier in adhesives. This can alter the properties of adhesives, increasing their fluidity or enhancing their adhesive strength. Starch is a polyhydroxyl natural organic polymer, polyvinyl alcohol is also a polyhydroxyl polymer, and formaldehyde contains aldehyde groups. The three can condense with each other to form a higher molecular weight polymer. Adding a small amount of starch to the condensation reaction between polyvinyl alcohol and formaldehyde can result in an adhesive with moderate and stable viscosity, strong gluing ability, quick drying, good water resistance, and easy storage. In particular, the enhanced condensation with formaldehyde during the reaction significantly reduces the amount of unreacted formaldehyde in the reaction product, reducing free formaldehyde and the harmful effects of formaldehyde on the human body during use, which is environmentally friendly and healthy. However, if too much starch is added to the reaction, the starch itself can gelatinize at a certain temperature, forming a very viscous gelatin. This also significantly increases the viscosity of the resulting adhesive, making it difficult to flow, reducing adhesion, making it difficult to apply, drying quickly, and difficult to store. Therefore, starch can be used as a modifier in the condensation reaction of polyvinyl alcohol and formaldehyde, but the dosage must be based on the requirements of the desired product. It cannot be too much or too little, as this will have a great impact on product quality. Therefore, the right amount is best.
