Wrinkling of epoxy powder coatings

Abstract

Differential scanning calorimetry (DSC) and mechanical profilometry were used to study wrinkle formation in curing epoxy powder coatings. Powder coating formulations were studied that contained solid epoxy resins, methylene disalicylic acid (MDSA) crosslinker, and an amine-blocked Lewis acid catalyst. Both the crosslinker (MDSA) and the amine-blocked catalyst are required for wrinkle formation. Evaporation of the blocking amine from the free surface of the coating generated a depthwise gradient in the extent of polymerization and crosslinking, and hence in the degree of solidification, as evidenced by the formation of a mechanical skin prior to wrinkling. It is hypothesized that compressive elastic stress develops in the still swellable skin when unreacted low-molecular-weight material from beneath diffuses up into the monomer-or oligomer-depleted crosslinking skin and swells it. This compressive stress, if above a critical value, buckles the skin to produce wrinkles. Experimentally observed compositional requirements for wrinkle formation were consistent with the proposed mechanism. The size of the wrinkles can be controlled by varying formulation parameters such as the amount of catalyst or crosslinker. Increasing the amount of catalyst decreased both the wavelength and the amplitude of the wrinkle pattern. Increasing the amount of crosslinker initially increased the amplitude of the wrinkles; after reaching a maximum level, the wrinkle amplitude decreased. DSC was a useful tool to understand the critical reactions responsible for wrinkling in this system. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 116 -129, 2005

Wrinkling of a nanometric glassy skin/crust induced by drying in poly(vinyl alcohol) gels

“…By stretching the film or controlling the relative motion of the substrate and the ion beam or the duration of exposition, the induced strain and, thus, the width, the amplitude and the length scale of the wrinkles can be controlled. Wrinkling is also observed for different bilayer systems such as metal capped polymers [12][13][14] where the strain is induced by heating, twolayer polymeric coatings made of a glassy top layer and a rubbery bottom layer, 15 cured coatings where the top stiff layer is due to a gradient of solidification in depth 16,17 or ultrathin polymer films bonded to a compliant substrate. [18][19][20] The hydrogels can absorb reversibly a large quantity of water and thus they can undergo a large volume change during swelling-deswelling.…”

Fabrication and Applications of Wrinkled Soft Substrates: An Overview

“…In the case of homogenous and gradient polymerized films the experimental parameters like reaction temperature, the concentration of catalyst, amount of crosslinking agent, etc. play an important role in determining the wrinkling dimensions [110] . In addition to that chemical composition and physical parameters like film thickness clearly influence wavelength of surface wrinkles formed [88] .…”

“…play an important role in determining the wrinkling dimensions. [110] In addition to that chemical composition and physical parameters like film thickness clearly influence wavelength of surface wrinkles formed. [88] Also, in certain cases in order to create uniform patterns, the wrinkle formation in selective positions has to be suppressed.…”

Advances in polymer‐based matte coatings: A review

“…To date, four methods prevail in the preparation of matting powder coatings, which we will describe below. [75][76][77] The first method is the introduction of matting additives. Powder coatings can achieve the desirable gloss reduction by the addition of extenders, such as silica, talc, barium sulphate, and aerosil.…”