Biofouling on ship hulls is prevented by the use of antifouling (A/F) paints. Typically, sea water soluble rosin or rosin-derivatives are used as the primary means of adjusting the polishing rate of the current chemically active self-polishing paint systems to a suitable value. Previous studies have shown that mathematical coating models based on a fundamental knowledge of the underlying mechanisms of A/F paints is a promising tool for accelerated product testing at different operational conditions of a sailing ship or a paint rotor. Such models can also be used for generation of ideas aiming at product optimisation and innovation (e.g. incorporation of natural active agents). This study seeks to attain scientifically founded knowledge of the reaction mechanisms and the rate of reaction with sea water of a Zn-carboxylate of a synthetic rosin compound. The kinetic expression attained can be used as input to mathematical models describing the behaviour of rosin-containing tin-free A/F paints. The experimental procedures developed can be easily implemented by marine paint companies for the screening of novel controlled-release binder materials for A/F paints. As a first step, it is demonstrated that the degradation of this Zn-containing rosin-derivative by sea water plays a key role in the polishing mechanism of paints formulated with such a resin. Then, the relevant literature available on the sea water behaviour of rosin and rosin-based binders is reviewed. Subsequently, two experimental procedures for the reaction rate estimation of the selected rosin-derived resin are presented; one is based on a gravimetric approach while the other uses flame atomic absorption spectroscopy (FAAS) to determine the total Zn2+ released by the resin. Both methods yield well-defined reaction conditions and sufficiently high accuracies. The latter is important because very low steady state reaction rates (about 0.70 +/- 0.26 mu g Zn(2+)cm(-2)day(-1) at 25 degrees C and pH 8.2) are measured. Steady state reaction rates of Cu2+- and Mg2+ -derivatives are also determined and discussed. The experimental procedures developed are used to investigate the influence of NaCl concentration (12-52 g/l), pH (7.8-8.5) and sea water temperature (10-35 degrees C) on the rate of reaction of the Zn-carboxylate. Within that range of sea water conditions, the following kinetic expression is found to describe the steady state Zn2+ release rate resulting from the reaction of the Zn-carboxylate with sea water: [GRAPHICS] [GRAPHICS] [GRAPHICS] where the natural logarithm of the pre-exponential factor, In (A), is 18.0 +/- 2.5 (the unit of A being the same as k(1)), the activation energy, E-a, is 18.5 +/- 6.0 kJ/mol and the reaction order with respect to the hydroxide ion concentration, a, is 0.86 +/- 0.42. L-znR is the estimated solubility product of the ZnR resin which has a value of 3.1 x 10(-12) (mol/l)(-3) (about 6 mg Zn2+/l in equilibrium). The low value of the activation energy is believed to result from the complex reaction mechanisms hypothesised rather than pointing at a certain diffusion control in the reaction rate experiments. The reverse reaction is found not to affect the hydrolysis rate within the pores, of antifouling paints significantly. It is concluded, from the reaction mechanism proposed, that the observed partial exchange of Zn2+ for Cu2+ in the resin structure during paint dispersion and immersion results in a lower reaction rate compared to the pure ZnR. Cu-carboxylate has a reaction rate of about 5.8 +/- 1.0 mu g CuR cm(-2) day(-1) at 25 degrees C and pH 8.2. The presence of Mg and Na compounds (probably Mg- and Na-resinate) in the solid paint film has also been detected, and will influence the reaction rate by modifying the ZnR exposed surface area. (c) 2005 Elsevier B.V. All rights reserved.
Progress in Organic Coatings, 2005, Vol 53, Issue 4, p. 256-275