Thermally Induced Surface Deformations of Polypropylene Monofilaments

Abstract

A coupling of two experimental stress analysis techniques, grid analysis and brittle coating analysis, has been used to study surface deformations of rectangular cross-section polypropylene monofilaments held at fixed length during heating and subsequent cooling. The brittle coating used was a very thin film of aluminum deposited by vacuum evaporation. A masking technique was used to deposit the coating on the fiber surface as a series of aluminum stripes. Small surface strains were revealed by cracking and buckling of the aluminum coating while large strains were identified by macroscopic distortion of the aluminum stripes. The filaments investigated included drawn specimens as well as "as spun" fiber. After heating to 166°C, frozen surface strains in the undrawn filament were relieved, resulting in longitudinal buckling deformations of the aluminum coating. In the drawn filaments, also heated to 166°C, relief of frozen strains resulted in macroscopic bowing of the stripes as well as longitudinal buckling of the coating. The thermally initiated deformation was greatest in the more highly drawn filament. On cooling, volume contraction, probably due to recrystallization, caused further filament deformation. The latter may play an important role in causing residual stress buildup at bond interfaces in thermally induced fiber bonding.