Polyvinyl alcohol Properties

Polyvinyl alcohol Properties

1. chemical properties

There are a large number of secondary hydroxyl groups on the main chain macromolecule of polyvinyl alcohol, which has many similarities with cellulose in terms of chemical properties. Polyvinyl alcohol can react with various acids, acid anhydrides, acid chlorides, etc. to generate corresponding esters of polyvinyl alcohol. However, its reaction ability is lower than that of general low molecular alcohols. The etherification reaction of polyvinyl alcohol is easier to carry out than the esterification reaction. After the etherification reaction, the intermolecular force of polyvinyl alcohol is weakened, and the strength, softening point and hydrophilicity of the product are all reduced. Adding a small amount of boric acid to polyvinyl alcohol aqueous solution will significantly increase its viscosity, and this change is closely related to the pH value of the medium. When the pH of the medium is more alkaline, the intermolecular reaction between boric acid and polyvinyl alcohol will cause the viscosity of the solution to increase sharply, resulting in the formation of a gel. Polyvinyl alcohol aqueous solution reacts with sodium hydroxide, and its viscosity increases faster than adding boric acid. Therefore, aqueous sodium hydroxide solution can be used as a coagulant for polyvinyl alcohol spinning. Under the action of acid catalyst, polyvinyl alcohol can undergo acetalization reaction with aldehyde. The acetalization reaction can be carried out in both homogeneous and heterogeneous phases. However, the acetalization group of the product obtained by the homogeneous reaction is evenly distributed, and the strength, elastic modulus and heat resistance of the acetalization product are all reduced. When a heterogeneous reaction is carried out, under proper control conditions, since the acetalization group is unevenly distributed and mainly occurs in the amorphous region, it has little effect on the mechanical properties of the product, and the heat resistance is still low. improved.

2. thermal performance

Polyvinyl alcohol softens after being heated (210-215°C), but in general, it decomposes before melting. Polyvinyl alcohol does not change significantly when heated to below 140°C. When heated to above 180°C, polyvinyl alcohol obtained by alkaline alcoholysis begins to change, macromolecules are dehydrated, and conjugated double bonds are formed on long chains. , and gradually darken its color. 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.

3. physical properties

The filling 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 polyvinyl alcohol is difficult to determine directly because its decomposition temperature in air is lower than the melting temperature. Its melting point is measured at about 230°C by indirect method. Polyvinyl alcohols with different stereotactic degrees have different melting points, among which S-PVA (syndiotactic) has the highest melting point, A-PVA (atactic) is the second, and I-PVA (isotactic) is the lowest. The glass transition temperature of polyvinyl alcohol is about 80°C. The glass transition temperature is not only related to the measurement conditions, but also related to its structure. For example, as the syndiotacticity of polyvinyl alcohol increases, the glass transition temperature increases slightly. When the residual acetate and water content in polyvinyl alcohol increase, the glass transition temperature will decrease accordingly.