Surface and Interfacial Structure of Release Coatings for Pressure Sensitive Adhesives I. Polyvinyl N-Alkyl Carbamates

Abstract

Polyvinyl N-alkyl carbamates belong to the general class of long alkyl side chain polymers. Such polymers are commonly used as release coatings for pressure sensitive adhesive tapes. In this paper the bulk, surface, and interfacial structures of polyvinyl N-alkyl carbamates having either decyl or octadecyl side chains are examined. The bulk structures and thermal transitions were characterized using X-ray scattering and differential scanning calorimetry. Dynamic mechanical thermal analysis was used to investigate thermal transitions and rheology (i.e., segmental mobility) of the polyvinyl N-alkyl carbamates. The surface energies of polyvinyl N-alkyl carbamate coatings were determined using contact angle methods, while X-ray photoelectron spectroscopy and static secondary ion mass spectrometry were employed to characterize the near-surface compositional profiles of the coatings. The peel force provided by the polyvinyl N-alkyl carbamate coatings, as a function of aging time and temperature, was measured for a tape having an acrylic acid containing alkyl acrylate based pressure sensitive adhesive. The changes in peel force with aging time and temperature were related to the ability to maintain a stable interfacial structure between the PSA and polyvinyl N-alkyl carbamate coatings. Changes in the interfacial composition upon aging were characterized by comparing the surface compositions of the PSA and polyvinyl N-alkyl carbamate coatings initially, prior to contact, as well as after aging and peeling them apart. The increase in peel force upon aging can be attributed, in large part, to a restructuring at the PSA/polyvinyl alkyl carbamate interface. Energetically favorable acid-base interactions between the basic urethane and acetate groups in the polyvinyl alkyl carbamates and the acrylic acid groups in the PSA provide a driving force for the restructuring. If the segmental mobility within the polyvinyl alkyl carbamate is sufficient, restructuring can occur, leading to increased concentrations of these groups at the PSA/polyvinyl alkyl carbamate interface, resulting in higher attractive forces and greater adhesion. The propensity for the polyvinyl N-alkyl carbamate coatings to restructure upon contact with a polar medium was also characterized by monitoring the receding contact angle of water, as a function of water contact time and temperature. A good correlation is seen between the ability of the polyvinyl alkyl carbamate coatings to provide a low peel force for the acrylate PSA tape and the ability of the coatings to maintain a high water receding contact angle.