A calorimetric evaluation of the peel adhesion test

• Main Author: Goldfarb, Jay L
• Language: ENG
• Published: ScholarWorks@UMass Amherst 1992
• Subjects: Plastics
• Online Access: https://scholarworks.umass.edu/dissertations/AAI9305828

Peeling of pressure sensitive tapes and polymeric coatings bonded to aluminum substrates was analyzed from a thermodynamic perspective with the intent of determining how the energy expended in separating the bonded materials is consumed. The mechanical work expended and the heat dissipated during peeling were simultaneously measured using deformation calorimetry. The surfaces exposed by peeling were analyzed by electron microscopy and electron spectroscopy. The thermodynamic state of the peeled materials was analyzed using solution calorimetry. The thermodynamics of tensile drawing for polymeric materials identical to those deformed during peeling was studied using solution calorimetry, differential scanning calorimetry, deformation calorimetry and thermomechanical analysis. When polyimide coatings were peeled from aluminum substrates with a peel angle of 180$\sp\circ$, almost all of the mechanical energy was consumed by propagating the bend in the peeling coating. The fraction of the peel energy dissipated as heat was 48+/$-$1.3% and nearly all of the remainder was stored as latent internal energy in the peeled polyimide. When the bend is propagated through aluminum, which has a limited capacity to store latent internal energy, 100+/$-$2.7% of the mechanical energy is dissipated as heat. When pressure sensitive adhesive, PSA, backed with poly(ethylene terephthalate), PET, tape was peeled, the mechanical work was consumed by propagating the bend in the PET backing and by deforming the PSA layer. The fraction of the mechanical work of peeling which was dissipated as heat varied from 69-86% depending on the peel rate and the backing thickness. It was determined that the fraction of the peel energy, not dissipated as heat, was stored as latent internal energy in the PET backing. The energy stored in the backing is indicative of the total mechanical energy expended in deforming it. Studies of PET tensile deformation showed that 25-50% of the energy under the stress-strain curve is stored in deformed material. When a crack is introduced in a coating containing residual tensile stresses, a shear stress, which acts to delaminate the coating, is concentrated near the intersection of the crack and the coating-substrate interface. Stress driven delamination occurs with little bending deformation as compared to peeling and requires considerably less energy. For coatings with residual tensile stresses, a superior adhesion test was developed based on calculating the stored elastic energy released when the stressed coating delaminates surrounding a cut-through. Photographs of delamination in cut coatings were taken and the coatings were modeled using linear elastic finite element analysis to calculate the stored elastic energy released in the delaminated region surrounding the cut-through.