Activators
“Zinc oxide (ZnO), as well as other divalent metal oxides such as CaO and MgO is commonly referred to as an activator for the cure of rubber compounds containing accelerators, especially those containing sulfur. More precisely, one molecule of cationic component (e.g. ZnO) combines with two molecules of a fatty acid such as stearic acid (C.sub.17H.sub.35COOH) to form a salt or soap such as zinc stearate (Zn(C.sub.17H.sub.35COO).sub.2) and water. The soap (e.g., the zinc stearate) is believed to be the activator for cure.”
“During mixing and/or cure, the ZnO (or other cationic component) and fatty acid react to form the soap, which activates the sulfur cure. In actual practice, a stoichiometric excess of the cationic component to fatty acid is employed in the rubber compounds to obtain sufficient activation.”
“The excess of cationic component such as ZnO in the cured rubber tends to weaken the physical properties of the cured rubber. Additionally, zinc tends to leach out of cured rubber compositions over time, which could lead to concerns over contamination since zinc oxide is considered to be an eco-toxin.”
Recent US Patents
10/19/2010
7,816,443
Rubber compositions with activated sulfur cure
Wilson of Nike, Oregon have developed activators that avoid reaction during mixing by avoiding an activator soap (or of components that can form the activator soap in situ) and silica filler together in process steps where the temperature would tend to be above 100 C. The use of ZnO for example can be reduced by as much as 90.about.95% in a typical rubber compound. (RDC 1/24/2011)