Peeling of PSAs on Viscoelastic Substrates: A Failure Criterion

This work deals with the study of the viscoelastic and adherence properties of pressure-sensitive adhesive (PSA) formulations dedicated to medical applications. We have developed a specific viscoelastic substrate to measure the adherence properties of PSAs that mimics adhesion on human skin. In the present article, we describe several experiments dedicated to a better understanding of adhesion on viscoelastic substrates without discussing specifically the case of human skin. In this way, we have studied different model adhesive formulations based on real medical formulations, and we have related the rheological behavior to the adherence properties obtained on different substrates to study the various specific effects due to the viscoelasticity of soft substrates. We propose from this study a failure criterion that allows one to derive a reasonable estimate of the peeling transition rate from cohesive to interfacial or stick–slip failure.     

Rheology and Adherence of Pressure-Sensitive Adhesives

We have studied the relationship between rheological and peeling properties for hot-melt pressure-sensitive adhesives based on homopolymers or copolymers blended with tackifying resins. In this article, we particularly try to demonstrate that it is possible to define a quantitative link between rheology and adherence when the model formulations are deposited on substrates with strong (thermodynamic) adhesion. We describe the experimental results obtained on these model formulations and discuss the quantitative relationships obtained. In the case of “adhesion modulation” (derived from different treatments of the substrates), we show that the relationships become much more complicated, even with the same model adhesives. At the end, we discuss on the competition between adhesion and dissipation in the case of poor adhesion.     

Ultimate Tensile Stress over a Zone: A New Failure Criterion for Adhesive Joints

The objective of this work was to develop a criterion for predicting the static strength of adhesively-bonded joints. Although there is a large body of literature on this subject, no satisfactory criterion has been proposed in any publication to date. To obtain a criterion, finite element models of widely differing joint geometries were developed, in which the stresses in the adhesive bonds were calculated in great detail. These were then compared with test measurements. After examining two toughened epoxy adhe-sives, the authors have developed a failure criterion that predicts failure loads to within approximately 15% of measured values. This is: that the maximum principal stress must exceed the ultimate tensile stress of the adhesive material over a finite zone normal to the direction of maximum principal stress. The size of this zone is a property of the adhesive that can be determined from a combination of analysis and test.     

Rheological properties of hot melt pressure-sensitive adhesives based on styrene--isoprene copolymers. Part 1: A rheological model for [sis-si] formulations

The viscoelastic properties of hot melt pressure-sensitive adhesives (HMPSA) based on formulations of block copolymers and tackifying resins have been studied in detail, through the variation of the complex shear modulus, G*, as a function of frequency, y . In this first article, we analyze the individual behavior of the components of HMPSA blends: (1) the two copolymers, styrene-isoprene (SI) diblock copolymer and styrene-isoprene-styrene (SIS) triblock copolymer and (2) two tackifying resins. The viscoelastic behavior of the overall formulation is also presented. We have mainly studied the effects of (1) the molecular characteristics of the SI and SIS copolymers and (2) the composition of the blends (mainly the effect of SI content, S content in SIS and SI, resin content) on the viscoelastic properties. A theoretical approach based on concepts of molecular dynamics leads to a model which describes reasonably well the linear viscoelastic properties of individual components and their formulations. Our systematic study can be used to design new copolymer molecules which can mimic the rheological behavior and end-user properties of regular formulations at room temperature.     

The Definition of the Locus of Failure on Ceramic Substrates: The Benefit of Monochromated XPS

The use of X-ray photoelectron spectroscopy (XPS) to assess the locus of failure of a photo-cured resin on an alumina substrate is reported. It is shown that the carbon 1s spectra obtained with conventional (achromatic) and monochromatic photon sources are markedly different. The spectrum obtained with the monochromatic source reveals the fine structure associated with the polymer whilst the spectrum recorded with the conventional source is distorted by differential charging. This observation has important ramifications when XPS is used for the definition of the locus of failure of organic coatings, or adhesives, applied to insulating substrates such as ceramics.     

Rheological Properties of Hot Melt Pressure Sensitive Adhesives (HMPSAs) Based on Styrene-Isoprene Copolymers, Part 3: Rheological Behavior of Different Block Copolymers With High Diblock Content

The processing and application properties of hot-melt pressure-sensitive adhesives (HMPSA) are governed, to a large extent, by their rheological properties. Coating of the HMPSA is performed at high temperatures in the molten state. At room temperature, the adhesive satisfies the Dahlquist criterion and, consequently, has permanent tack. We have particularly studied the full formulations based on triblock and diblock copolymers, and also those based on newly designed molecules, such as tetrablock or radial copolymers. We have demonstrated in the previous articles of this series that, for these systems, the volume fraction of the free polyisoprene is the most important parameter that drives the tack performances by controlling the level of the secondary elastic plateau modulus observed in the low-frequency range. To improve the end-user properties, we have increased the diblock content in the blends. We describe here the dynamic mechanical properties, at room temperature, of the pure copolymer blends (i.e., without addition of a tackifying resin) and the full HMPSA formulations. We focus particularly in this article on blends that contain a high diblock content. The effect of the morphology of the diblock copolymer on the rheological behavior of the adhesive is discussed in detail. Finally, we propose a model, based on molecular dynamics concepts, which describes the rheological behavior in a very wide range of frequencies for all copolymers and full formulations of the study.