Characterization of waterborne polyurethane adhesives containing different amounts of ionic groups

Several research studies had been devoted to the structure–properties relationship in solvent-based polyurethane adhesives [9–17]. However, the literature dealing with the synthesis and characterization of aqueous PUD is relatively scarce, and the majority of studies have been made in industrial laboratories, so they have not been published in the scientific literature. Moreover, the few studies carried out about this subject mainly relate the polyurethane ionomer structure to physical characteristics of the dispersion and to the mechanical, thermal and dynamical–mechanical–thermal properties, but to our knowledge none of them have been devoted to the adhesive properties, and less even, in the footwear industry. Therefore, the objective of this study was to prepare different polyurethane ionomers and establish a relationship between their structure and adhesion properties. More precisely, the influence of the ionic groups content on the properties of the polyurethane ionomers was considered.
The ionic groups in the ionomer impart the ability of dispersion of polyurethanes into water, normally producing stable dispersions. In water, the stabilizing effect of the ionic sites is due to the formation of tiny spheres which contain a core of aggregate hydrophobic segment and a boundary layer carrying the ionic groups. The result is a surprisingly stable hydrosol or aqueous dispersion [2]. The properties of the polyurethane ionomers are mainly determined by the interactions between the hard and soft segments and by the interactions between the ionic groups
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(Directive 99/13/EC) with respect to the emission of organic solvents to the atmosphere. Waterborne PUD offer an efficient alternative to solvent-based adhesives, the most commonly used in the upper to sole bond in footwear industry.
$Presented in ‘‘EURADH 2004’’, Freiburg im Breisgau (Germany), 5–9 September 2004.
ÃCorresponding author. Tel.: +34 96 5903977; fax: +34 96 5903454. E-mail address: jm.martin@ua.es (J. Miguel Martı´ n-Martı´ nez).
Recent studies [3–7] demonstrated that the polyurethane ionomer properties are influenced by the ionic groups content, the hard–soft segment molar ratio, nature and molecular weight of the macroglycol, the type of chain extender, the degree of neutralization of carboxylic acid groups or the nature of the counter-ions, among other factors, since they determine the hard–soft segment interactions, as well as the interactions between the ionic groups. Previous studies [3,4,7,8] have shown that the increase of the ionic sites concentration produces a decrease in particle size of the polyurethane dispersion and an increase in hardness, moduli, glass transition temperature (Tg) and tensile strength, whereas a decrease in molecular weight and elongation at break is obtained.
Currently, a great interest in the development of aqueous PUD has come out in the footwear industry because of their lack of flammability and toxicity that comply with the restricted environmental legislation
M. Angeles Pe´ rez-Limin˜ anaa, Francisca Ara´ n-Aı´ sa, Ana M. Torro´ -Palaua, A. Ce´ sar Orgile´ s-Barcelo´ a, Jose´ Miguel Martı´ n-Martı´ nezb,Ã
aINESCOP, Footwear Research Institute, 03600 Elda, Alicante, Spain. bAdhesion and Adhesives Laboratory, University of Alicante, 03080 Alicante, Spain.
0143-7496/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijadhadh.2005.02.002
lateral or terminal groups (non-ionomer) during the synthesis [1].
Keywords: A. Polyurethane; water based. C. Rheology; thermal analysis
1. Introduction
A waterborne polyurethane adhesive is constituted by a linear thermoplastic polyurethane which is dispersable into water due to the presence of ionic groups in its structure (i.e. a polyurethane ionomer), which act as an internal emulsifier. These ionic groups could be cationic (quaternary ammonium groups) or anionic (carboxylate or sulfonated groups), although aqueous polyurethane dispersions (PUD) can also be prepared by inserting hydrophilic segments (such as polyether chains) as
Accepted 14 February 2005 Available online 31 March 2005
Abstract
Waterborne polyurethane adhesives are an interesting alternative to the current solvent-based polyurethane adhesives used in footwear industry. In this study, different aqueous polyurethane dispersions were prepared by using the prepolymer mixing process. The ionic groups content in the polyurethane ionomer structure was varied by changing the amount (5 and 8 wt% (with respect to the prepolymer weight) of the internal emulsifier—dimethylolpropionic acid (DMPA). The decrease in the DMPA content slightly increased the particle size in number due to the decrease in both the hydrophilicity of the polyurethane ionomer, and the electrolytic stability of the aqueous polyurethane dispersion. Furthermore, the lower the DMPA amount, the lower the hard segment content in the ionomer and the higher the crystallinity of the polyurethane; however, the resistance to flow under temperature (i.e. thermoplasticity) was reduced. The resistance to thermal degradation of the polyurethane ionomer increased by decreasing the DMPA content due to the lower hard segment content. Finally, high initial adhesive strength was obtained and the adhesion to PVC increased as the DMPA content in the polyurethane ionomer decreased. r 2005 Elsevier Ltd. All rights reserved.