Jacques Lechat

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.

RHEOLOGICAL PROPERTIES OF HOT-MELT PRESSURE-SENSITIVE ADHESIVE (HMPSAs) BASED ON STYRENE–ISOPRENE COPOLYMERS. PART 2: INNOVATIVE MOLECULAR DESIGN FROM PREDICTIVE FORMUlATION

This article is the second in a series that deals with the viscoelastic properties of Hot-melt pressure-sensitive adhesives (HMPSAs) based on formulations of block copolymers and tackifying resins. The viscoelastic properties of HMPSAs govern, to a large extent, their adhesion, processing, and end-use properties. In the first part of this article, we present a brief description of the rheological behavior of styrene isoprene styrene–styrene isoprene [SIS–SI] copolymer blends at room temperature. We then present an original approach that may lead to the design of new block copolymers (tetrablock and radial copolymers) that mimic the rheological behavior at room temperature of optimized SIS–SI blends used in adhesive formulations. We describe the concept and calculations that lead to the design of the characteristics of these new molecules. In the third part of this article, we discuss in detail the rheological behavior of these new block copolymers compared with the observed behavior of equivalent SIS–SI. In the last part we also demonstrate how the molecular model of the rheological behavior developed in the first article of this series can be applied to these new molecules. We propose, in particular, to apply the blending law (presented in the first article) on the complex shear modulus instead of the relaxation modulus, which simplifies calculations and even leads to a better agreement with experimental data. As a conclusion, we show how this original approach can bring really innovative solutions for the formulation of adhesives with specific properties by using molecular concepts of viscoelasticity.