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Friday, June 11, 2010

Polyvinyl Acetal Adhesives

Polyvinyl acetal is the generic name of a group of polymers that are the product of reaction between polyvinyl alcohol and an aldehyde. In the preparation of these acetals polyvinyl acetate is used which in the course of the reaction is partially hydrolysed to the alcohol, thus the chain length of the acetate determine the chain length of the acetal. The monomer from of the vinyl acetals cannot be isolated.
An adhesives, the most important polyvinyl acetals are those from formaldehyde, namely the formal, and from butyraldehyde, the butyral, the properties of these polymers depending largely on their molecular weights and on the degree of hydrolyses of the acetate. Versatility as adhesives of the polyvinyl acetals is believed to be attributable to their terpolymer constitution, acetate and hydroxyl groups as well as acetal. In a study of the influence of the chemical composition on adhesion, it was observed that a decrease in degree of formalization (increase in hydroxyl groups) increase adhesive power almost linearly.

The properties of a number of grades of polyvinyl formal and butyral are shown on the table below:

                       Grade
Properties
7/70
12/85
7/95 S
15/95 S
15/95 E
Avg Molecular Weight
21,000
30,000
18,000
34,000
34,000
Hydroxyl Content, %
5-7
5-7
7-9
7-9
5-6
Acetate Content, %
40-50
20-27
9.5-13
9.5-13
9.5-13

Polyvinyl acetals are insoluble in water but are soluble in a wide range of organic solvents; the butyral is soluble in ethyl alcohol whereas the formal is insoluble but will dissolve in mixtures of ethyl alcohol and a non polar solvent such as toluene. The acetals have ill-defined softening points; that of the formal is often quoted as above 200oC, but it depend on the method of test. The high melt viscosity of the formal necessitates a moulding temperature of 175o-230oC, although softening starts below 100oC – the butyral a good deal lower.

Polyvinyl acetals, which appear to have been first used as adhesive in the bonding of mica, give high strengths as hot melt adhesives especially in gluing wood and metal.