Aqueous polyacrylic acid based gels: physicochemical properties and applications in cultural heritage conservation

Abstract

Solutions of polyacrylic acid form strong gels upon the addition of bases. Two different gels have been studied by differential scanning calorimetry (DSC) and rheometry in order to examine both their viscoelastic behaviour and the fraction of the free solvent in the system. These properties are important from a technological point of view, because of their application in cultural heritage conservation field as cleaning agents. When the continuous phase is constituted of water the study was focused on the influence of pH on the gel properties. Rheological properties have been studied as a function of the NH₃ concentration, the base necessary to stabilise the aqueous gel. The rheological behaviour of the system polyacrylic acid-water-ammonia, has been characterised using rotating and oscillating measurements. Dynamic (oscillatory) shear tests were carried at 25 °C in the frequency range 0.01–100 Hz; rheological parameters (such as loss modulus G′′ and storage modulus G′) were also correlated to the pH of the blends. To vary the gel strength, different ammonia concentrations were used, while keeping the temperature and cross-linker concentration constant. Both the loss and storage moduli increased with the NH₃ concentration for this gel system. The viscosity and the elastic modulus G′ increased when the pH is between 3 and 6. When all the acidic groups have been neutralised by a solution of 0.1 M NH₃ G′ reaches a maximum (pH=6.9); further increases of pH produced a decrease of G′. The excess NH₃ decreases the fraction of bound water within the system lowering the strength of the network.

Acknowledgments Thanks are due to the Consorzio Interuniversitario per lo Sviluppo dei Sistemi a Grande Interfase, CSGI, for financial support.