PVB

Описание

Polyvinyl butyral (PVB) is a synthetic resin obtained by the condensation reaction of PVA and butyraldehyde in the presence of acid catalyst. Since the acetalization reaction cannot be carried out completely, the molecular chain of PVB contains three functional groups, butyral, alcohol hydroxyl and acetate, the latter two of which come from PVA. The chemical structure is:
The proportion of these three groups can be controlled by changing the reaction conditions, but they are randomly distributed in the long molecular chain. The proportion and distribution of the three are different, and the properties of PVB are also different.

Химические свойства

PVB resin is white or slightly yellow amorphous powder, with a particle size of 40–200 μm, fluidity and a density of 1.11 kg.m–1. It is a thermoplastic material, the softening temperature is 60–65°C, the glass transition temperature is 70–78°C, the molding temperature is 120–170°C, and the temperature adaptation range is 10–80°C.
PVB resin is insoluble in water, but soluble in a variety of organic solvents such as alcohols, esters, benzene and ketones. It is miscible with various plasticizer including phthalates, phosphates, and fatty acid ester. It has good compatibility with various resins such as phenolic resin, urea–formaldehyde resin, melamine resin, epoxy resin, nitrocellulose and natural resin. By mixing these resins in an appropriate ratio, the physical and chemical properties are improved, and the cost is reduced.
PVB resin has certain chemical stability, but it can be decomposed in acidic or alkaline (especially acidic) medium. It starts to decompose thermally after being heated to 100°C, and will be almost completely decomposed at 200–240°C. Hydrolysis can occur in dilute acid, at this time intermolecular cross-linking can occur, a small amount of double bonds is formed in the chain, and the acetate group will be further hydrolyzed. Heated in concentrated acid to form a transparent solution, while decomposing into aldehydes. Upon heating in dilute alkali solution with alkali concentration less than 1%, the free acid in PVB resin is neutralized and the thermal stability of the resin is improved. However, when PVB resin is heated with 3% alkaline solution to boiling, the acetate groups can be further hydrolyzed and saponified.

Использование

The main application of poly(vinyl butyral) is as interlayers in safety glass laminates. In this application, a pure (colourless, light-stable) product with high hydroxyl content (for good adhesion) is required. Such material is preferably prepared from poly(vinyl alcohol) rather than directly from poly(vinyl acetate). In a typical process, completely hydrolysed poly(vinyl alcohol) is suspended in ethanol and then butyraldehyde and a small quantity of sulphuric acid (catalyst) are added. The mixture is heated at 800℃ for 6 hours. Water is then added to the product with rapid agitation and the poly(vinyl butyral) is precipitated as granules, which are washed with alkali and dried.
Poly(vinyl butyral) is generally softer than poly(vinyl formal) with lower softening point, hardness and tensile strength. In commercial polymers the hydroxyl content exerts a major influence on the solubility characteristics. Most grades are soluble in alcohols, glycol ethers and dioxan and as the hydroxyl content decreases the polymers become more soluble in ketones and esters.
Poly(vinyl butyral) used in safety glass generally has 78-80% butyral content, 18-19% hydroxyl content and less than 2% acetate content. For this application the polymer is plasticized with an ester such as dibutyl sebacate to the extent of about 30 parts of plasticizer per 100 parts of polymer; the compound is then sheeted by calendering. To produce safety glass, the sheet is placed between two pieces of glass and the laminate is then bonded under heat and pressure.