Understanding and Control of Calcium Carbonate Crystal Growth on Model Substrates in Dishwasher

Abstract

This thesis presents the understanding and control of calcium carbonate crystal growth on model substrates under dishwasher conditions.
The study was conducted to understand why calcium carbonate crystals grow on the surface of kitchenware under dishwasher conditions, especially the severe deposition observed on plastic samples. Chapter 1 gives reviews on the current crystallization theories, with a particular focus on the current understanding of surface crystallization and mineral crystal growth control on the surface. Chapter 2 and Chapter 3 present the underlying reasons that promote the mineral growth under dishwasher conditions. Chapter 2 focuses on assessing how the different ingredients in the prototype dishwasher formula affect the ‘shine’ on the surface. Chapter 3 further explores the reason of shine loss by looking into the system parameters (pH, temperature and phosphate ions). Poly (methyl methacrylate) slide and soda-lime glass slides are used as two main model substrates. It is observed that the dishwasher conditions (high pH, high temperature, high Ca concentration), in any way, promotes the formation of calcium carbonate growth. The polymer, one of the ingredients in the prototype dishwasher formulation, encourages this process, especially for plastic samples.
Chemical inhibition methods are still the most efficient way of limiting mineral formation on the surface. Chapter 4 examines the X-ray crystal structures of a range of newly synthesized calcium compounds with oligocarboxylate candidate inhibitors so as to understand the exact structure and calcium binding mode of each potential inhibitor. Chapter 5 presents the screening results of inhibition performance for thirty-nine different phosphate-free inhibitors, in order to find an alternative phosphorus-free inhibitor to the currently used phosphate-containing inhibitor 1-hydroxyethane 1,1-diphosphoric acid (HEDP). Two phosphate-free, cyclic poly(carboxylic acids) inhibitors were found to be effective replacements.
Chapter 6 summarizes the main findings and conclusions towards the CaCO3 crystallization under dishwasher conditions and the structural characteristics of an efficient CaCO3 inhibitor. It was concluded that an effective inhibitor should achieve a critical charge density. While the exact structure requirements of an efficient inhibitor for CaCO3 is still unknown, factors such as balanced flexibility and rigidity in the backbone are desirable.