Flow micro-calorimetry and FTIR studies on the adsorption of saturated and unsaturated carboxylic acids onto metal hydroxide flame-retardant fillers

Carboxylic acids, particularly fatty acids, are by far the most important surface modification systems for inorganic fillers used in the plastics and rubber industries. However, relatively little is understood regarding factors that affect their adsorption behaviour, including the interplay between acid molecular structure, adsorption conditions, and substrate chemistry. In this study, the adsorption (from n-heptane and toluene) of a range of saturated and unsaturated fatty acids, together with acrylic acid, onto aluminium and magnesium hydroxides has been investigated using flow micro-calorimetry and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). With C18 saturated fatty acids, the overall heat of adsorption per unit surface area of substrate was found to be affected by the structure of the alkyl tail. The orientation of the adsorbed unsaturated fatty acids was found to be affected by the number of double bonds in the molecule. Acrylic acid was found to be very strongly adsorbing and could displace isostearic acid from the filler surface. However, isostearic acid was found to adsorb on top of the layer of aluminium or magnesium acrylate. These findings have important implications particularly for mixed fatty acid treatment methods and the choice of fatty acid treatment for metal hydroxide flame-retardant fillers.