Synthesis and control of crosslinked poly(acrylic acid) based viscosity modifiers using dense phase carbon dioxide as a solvent

Abstract

We investigate the clean synthesis of a cross-linked poly(acrylic acid) viscosity modifier using supercritical CO2 to replace more hazardous volatile organic solvents that are typically used for this process. The polymers were analysed by aqueous swell ratio studies to demonstrate the effect of process conditions such as pressure and temperature on the cross-link density of the materials. The reactions were optimised to yield high swelling polymers of up to 134 g water /g polymer. Dynamic mechanical analysis (DMA) was also applied to investigate the mechanical properties of the synthesised cross-linked poly(acrylic acid) materials and these data were directly correlated with swell ratio. In addition, rheological studies demonstrate that the hydrated gels are comparable with commercially available equivalents. Introduction:Cross-linked poly(acrylic acid) (PAA), otherwise known as a carbomer, is a material commonly used as a superabsorbent polymer, or a thickener for cosmetic products. Conventionally, carbomers are produced via precipitation polymerisation using solvents such as benzene,[1, 2] DCM,[3] or more recently co-solvent combinations of ethyl acetate and hexanes.[4] Significant efforts have been focussed upon the use of less toxic solvents largely because of the regulatory requirements in personal care and cosmetics and also because complete removal of residues is highly energy intensive. Supercritical CO2 (scCO2) has shown promise as an alternative to volatile organic solvents for polymerisations and polymer processing. [5-12] Indeed, several studies have investigated the use of scCO2 for the polymerisation of acrylic acid (AA) to produce cross-linked PAA without the use of VOCs. One of the first examples was a BASF patent filed in 1987, [13] which demonstrated that Poly(acrylic acid-co-methacrylic acid) viscosity modifiers (viscosity max ~ 26 Pa.s, spindle rpm 5.5, pH 8) can be produced from scCO2 at temperatures of 60 -110 °C and pressures of 120 -200 bar. In 1995 Romack et al. investigated polymerising AA through this approach, setting a fixed temperature of 65 °C, whilst varying the pressure from 165 -345 bar, but noting no effect on molecular weight, dispersity, particle size or morphology.[14] Further studies conducted by Tao et al. demonstrated the synthesis of crosslinked PAA in scCO2 at 50 °C, before then scaling up to a promising semi-continuous process at 70 °C. [15] Rheological experiments on hydrated PAA gels demonstrated a thickening effect (viscosity from 100 Pa.s at 0.01 s -1 to 0.1 Pa.s at 100 s -1 ), but without evidence of commercial applicability. Under certain conditions there was significant formation of PAA coagulum that could not be dispersed in water and hence did not contribute to increasing the viscosity of the aqueous PAA dispersions. A model of particle formation of PAA in the polymerisation was proposed suggesting that control of molecular weight and reaction temperature played an important role in determining PAA particle size and suggesting that agglomeration of such particl...

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Synthesis, Characterization, and Application of Novel Polyacrylate Thickeners for Textile Printing

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