Characterisation of peracids
Peroxides are of interest to the detergents industry for use as bleaching agents. However, peroxides have a history of being potentially or actually explosive. In this work two peracids were studied which were thought to be of relatively high stability. The first peracid was a single aliphatic di-peroxy acid, and the second peracid was a homologous series of aliphatic phthalimido monoperoxy acids. The aim of this project was to examine the stability of these peracids under certain stimuli, such as mechanical impact, electric fields and thermal sources. Also, it was hence hoped to be able to suggest how to design stable peracids suitable for the detergents industry. It was found that none of the peracids would suffer dielectric breakdown when subjected to fields as high as 50 MVm-1. Impact studies indicated that the phthalimido peracids were insensitive to impact, showing no signs of ignition. The aliphatic di-peracid, however, did show signs of ignition when sensitised with grit, though propagation of the reaction from these sites failed. With appropriate confinement, however, a reaction could be made to propagate readily, as seen in experiments with the aliphatic di-peracid confined in plastic tubes. In similar experiments with the phthalimido peracid 'PAP5', propagation of the reaction failed. It was concluded that the peracids studied were impact insensitive, though under the right conditions a reaction could be sustained in the aliphatic di-peracid. The slow and controlled decomposition of the phthalimido peracids was characterised by thermal analysis. During these studies, it was found that existing analysis methods were not flaw-free, hence a new improved analysis method was developed. The validity of this method was verified by analysing experimental data from a well studied and well documented system: the decomposition of barium azide. On applying this method to differential scanning calorimetry and thermogravometry data from experiment s on the phthalimido peracids, it yielded physical mechanisms of reaction which were in good agreement with observations made under a hot-stage microscope. The activation energies for the members of the homologous series of phthalimido peracids indicated a minimum at the point where there is a change in the mode of hydrogen bonding, and it is suggested that such bonding is important to the relatively high stability of the group. Based on the results of thermal analysis, spect roscopy and macroscopic observations, it is proposed that the rate-determining step during thermal decomposition is the break-away of molecules from the semi-infinite hydrogen bonded chains, or breakage of bonds in the proximity of the hydrogen bonds at the peracid head. It is suggested that the strength of the hydrogen bonded chains is reflected in the activation energies determined, and is responsible for the high stability of the phthalimido peracids. It is concluded that a considerable understanding of the behaviour of these peracids has been achieved in this dissertation, which may help in the manufacture ofperacids suitable for use in the detergents industry.