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Friday, December 23, 2011

Peel Strength Properties of Adhesives

For various reasons, one of which concerns the surface treatment of the metal, peel strength is a property that is not as consistently reproduced in an adhesive as is tensile shear strength. This is the reason why the bend test of an overlap joint is often preferred to the peel test as a measure of flexibility. It is, however, certain that for two polymer adhesives the variation of peel strength with temperature depends largely on the behavior of the thermoplastic component. The usual effect of heat is first to create an increase and later a decrease in peel strength. A typical peel strength/temperature relationship of a polyvinyl acetal/phenolic adhesive is shown on the picture below. The curve shows the strength according to the Ciba (ARL) drum peel method, the climbing drum method gives a somewhat higher figure and an average value for this at 25oC is shown.

Conflicting figures have been given over the last 10 years, showing tensile shear strength/temperature relationship of acetal/phenolic adhesives. It is, moreover, difficult to make a comparison between polyvinyl formal and polyvinyl butyral unless regard is paid to the composition of the two polymers system, whether for instance the adhesive has been formulated for maximum peel strength or maximum hot shear strength. However, the curves shown in temperature for typical formulation. In each case the component ratio was 2 polyvinyl acetal to 1 phenolic resin solids. The systems were cured for 1 hour at 165oC, and this curing cycle may account for value which, while showing very good strengths at elevated temperature, are accompanied by rather below average peel strengths. In an example relating to use in aluminium honeycomb core it has been shown that the formal and butyral produce almost identical longitudinal shear/temperature curves.