How much do you know about the properties and applications of polyvinyl alcohol

Polyvinyl alcohol (PVA) is an organic compound with the chemical formula [C2H4O]n. It has the appearance of a white flake, flocculent or powdered solid and is odorless. Soluble in water (above 95℃), slightly soluble in dimethyl sulfoxide, insoluble in gasoline, kerosene, vegetable oil, benzene, toluene, dichloroethane, carbon tetrachloride, acetone, ethyl acetate, methanol, ethylene glycol, etc. Polyvinyl ALCOHOL IS AN IMPORTANT CHEMICAL RAW material, USED IN the manufacture OF POLYVINYL acetal, gasoline resistant pipeline and VINylon, fabric treatment agent, emulsifier, paper coating, adhesives, glue, etc.

The nature of the

Soluble PVA is soluble in water, the higher the water temperature, the greater the solubility, but almost insoluble in organic solvents. The solubility of PVA varies with the degree of alcoholysis and polymerization. Partially alcoholized PVA and PVA with low degree of polymerization dissolve very quickly, while fully alcoholized PVA and PVA with high degree of polymerization dissolve more slowly. As a general rule, the effect of alcoholysis on the solubility of PVA is greater than that of polymerization. The dissolution process of PVA is carried out in stages, namely: affinity wetting, swelling, infinite swelling and dissolution.

Chemical properties

There are a large number of secondary hydroxyl groups on the main chain macromolecules of polyvinyl alcohol, which have many similarities with cellulose in chemical properties. Polyvinyl alcohol can interact with a variety of acids, anhydride, acyl chloride, etc., to produce the corresponding polyvinyl alcohol ester. However, its reactivity is lower than that of general low molecular alcohols. The etherification reaction of polyvinyl alcohol is easier than esterification reaction. After the etherification reaction, the intermolecular force of PVA was weakened, and the strength, softening point and hydrophilicity of the product were reduced. When a small amount of boric acid is added to the water solution of PVA, the viscosity will increase obviously, and this change is closely related to the pH value of the medium. When the pH value of the medium is more alkaline, boric acid and polyvinyl alcohol have intermolecular reaction, so that the solution viscosity increases, so that the formation of gel. The reaction of polyvinyl alcohol solution with sodium hydroxide increases the viscosity faster than adding boric acid. Therefore, sodium hydroxide aqueous solution can be used as a coagulant for PVA spinning. Under the action of acid catalyst, polyvinyl alcohol and aldehydes can undergo acetylation reaction. Acetylation can be carried out in either homogeneous or heterogeneous phases. However, the acetalylation groups of the products obtained by homogeneous reactions are uniformly distributed, and the strength, elastic modulus and heat resistance of the acetals are reduced. When the heterogeneous reaction is carried out, THE mechanical properties of the products are little affected, and the heat resistance is improved, because the distribution of the acetylation groups is not uniform and mainly occurs in the amorphous region under the appropriate control conditions.

Thermal performance

When PVA is heated, it softens (210 ~ 215 ° C), but in general, it dissolves before melting. When PVA is heated below 140℃, it does not change obviously. When it is heated above 180C, PVA obtained by alkaline alcoholysis begins to change, the macromolecule dehydrates, the conjugated double bond is formed on the long chain, and the color of PVA becomes darker gradually. At this time, its physical properties also change, such as the original water solubility disappears, the elastic modulus increases significantly, and gradually becomes hard and brittle.

Physical properties

The packing density of polyvinyl alcohol (PVA) is about 0.20 ~ 0.48g/cm3, and the refractive index is 1.51 ~ 1.53. The melting point of PVA is difficult to determine directly because its decomposition temperature in air is lower than the melting temperature. The melting point was found to be about 230℃ by indirect method. Polyvinyl alcohol with different degree of vertical gauge has different melting points, among which S - PVA(intergauge) has the highest melting point, A - PVA(random gauge) has the second, I - PVA(equal gauge) has the lowest. The glass transition temperature of polyvinyl alcohol is about 80℃. The glass transition temperature is not only related to the measurement condition, but also to its structure. For example, the glass transition temperature increases slightly with the increase of the inter-gauge of PVA. The glass transition temperature will decrease with the increase of residual acetic acid and water content in PVA.

Application performance

Polyvinyl alcohol (PVA) is a kind of high polymer, which is made from vinyl acetate (VAC) by polymerization and alcoholysis. It has good adhesion, film formation, oil resistance, colloid protection, and has the characteristics of pollution-free and degradability, which are rare in high polymer materials. Compared with other chemical products, the development space of PVA and its derivative products is very broad, and new applications and new varieties are still under development.

PVA product traditional application field, except for synthesis of vinylon fiber raw materials, mainly used in adhesives, textile pulp, paper, etc., as the new technology, new technology, the continuous application of new USES and discover, in the film, soil conditioner, food packaging, building materials, medicine, leather, paper, electronics, environmental protection and other industries also has a broad application prospect.

Production methods

Prepared by partial or total hydrolysis of polyvinyl acetate. The specific process is the reaction of polyvinyl acetate with methanol under the action of chamber

Polyvinyl alcohol in China mainly for the production of polymer additives, fabric paste, whalen fiber and adhesive, etc., with the development of our economy, and high density of high-grade textile exports increased, high-grade paper industry, oil industry development speed, the automotive industry to flourish, automobiles, building safety glass demand rapid increase, These industries provide a broad space for the development of new PVA products. At the same time, a large number of domestic infrastructure construction for the adhesive market to provide development opportunities, electronics, paper, medicine, fine chemical industry steady development, but also further promote the rise of PVA demand.