Functional Polymer Materials

Polymer-based Adhesives

Pressure sensitive adhesives (PSA) stick to a huge variety of materials by applying a slight pressure. Based on polymer melts, the rheological properties of PSA play a key role in the phenomenological appearance of tackiness. From the mechanical point of view, a PSA polymer melt possesses a visco-elastic nature. At long time scales the viscous aspect dominates and enables the adhesive to achieve an intimate contact to the surface of a solid specimen, adapting to the latter surface profile. On the other hand the elastic properties, mainly gaining in importance at short time scales, allow the adhesive to sustain short time shear forces. The fact that PSA combine both aspects at the same time distinguishes them from other types of adhesives stepping from a fluid state to a solid state by for example a change in temperature or through a chemical reaction. The lack of hardening enables in PSA application a desired, controlled release of the adhesive bond. One of the most prominent examples in daily life are stick-on notes, in addition making use of the ability to undergo several cycles of bonding and detaching. Scientifically, the quality of adhesion can be quantified in the so called tack test. A probe punch, like a flat-ended, rigid cylinder, is pressed with a defined force onto a PSA film. After maintaining the force for a well defined time the punch is withdrawn from the surface with a fixed velocity. During the whole process the force needed to sustain the constant retraction velocity is probed as function of the distance between film and punch. Usually, the resulting curves show characteristic features such as a sharp maximum followed by an extended force plateau. Even up to film-punch distances of multiple film thickness a non zero force value is detected. The geometry of the tack test with a flat ended cylinder ensures, contrary to a comparative test with a spherical indenter, an uniform spacing of substrate and punch. Despite the homogeneous elongation throughout the contact area a highly heterogeneous structure of cavities and fibrils develops in the polymer. In other words, the material has to face the challenge to occupy a rapidly increasing volume and to respect its low compressibility as well. The debonding process during the tack test was analyzed in detail by optical microscopy from both underneath, through a transparent substrate, and from the side.

The tack of polymer surfaces is determined by a two-stage process of bond formation and bond separation. Bond formation between polymer surfaces are governed by the bonding force, the roughness of the surfaces, the bonding time and the temperature. The quantities which influence bond separation are debonding rate and temperature. In order to improve the tack of polymers which are used as adhesives, the mechanisms of bond formation and separation have to be understood. We want to combine the mechanical test with scattering experiment in transmission as well as in reflection geometry. With USAXS experiments the fibrils created during the debonding are observable. The GISAS technique will give access to a statistical description of the related surface morphologies. The utilization of scattering will introduce a new experimental method into this scientific area.

So far, four different characteristic stages of adhesive failure have been discovered by the means of optical techniques (see figure 3). Firstly the polymer film is elongated homogeneously in the direction of tension accompanied by a sharp increase of the force. As traction proceeds cavities are introduced into the film locally. Accompanied with a stress release the appearance of cavities corresponds to the force maximum. As the cavities expand laterally, the force promoting debonding suddenly decays to a comparatively low, but non zero value. A plateau of constant height in the force versus distance curve follows and the cavities already occupying most of the nominal contact area now expand mainly vertically. As the cavities remain well separated the polymeric material in between still provides a local connection between the punch and the substrate. With proceeding debonding the film transforms out of a foam like state to a fibrillar structure. Finally when air rushed in from the outer border of the vertically expanded film the measured force vanishes to zero. Thanks to investigations of PSA failure by optical microscopy a detailed understanding of the highly non linear force-distance curve in the test has been obtained. Nevertheless all experiments are restricted towards small length scales by the optical resolution limit. In order to overcome this limit we applied x-ray scattering techniques.

Selected Publications:

  • 1. P.Müller-Buschbaum, T.Ittner, W.Petry
    Tackiness of pressure sensitive adhesive: an ultra small angle x-ray scattering investigations;
    Europhys. Lett. 66, 513 (2004)
  • 2. E.Maurer, S.Loi, D.Wulff, N.Willenbacher, P.Müller-Buschbaum
    Microscopic structure in pressure sensitve adhesives: An ultra small angle x-ray study;
    Physica B 357, 144 (2005)
  • 3. E.Maurer, S.Loi, P.Müller-Buschbaum
    Debonding of pressure sensitive adhesives: A combined tack and ultra-small angle X-ray scattering study;
    in "Adhesion - Current Research and Applications", edt. W. Possart, Wiley-VCH, 421-434 (2005)
  • 4. P.Müller-Buschbaum, T.Ittner, E.Maurer, V.Körstgens, W.Petry
    Pressure sensitive adhesive blend films for low tack applications;
    Macromol. Mat. Eng. 292, 793 (2007)