Reaction rate estimation of controlled-release antifouling paint binders: Rosin-based systems

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 μg Zn2+ cm−2 day−1 at 25 °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 °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:(−rZnR)=k1COH−a−k−1CR−bCZn2+cμgZn2+(cm2film)dayk1=Ae(−Ea/RT)μgZn2+(cm2film)day1molak−1=k1COH−aLZnR2b−2(CZn2+)eqb+c−3μgZn2+(cm2film)day1molb+cwhere the natural logarithm of the pre-exponential factor, ln (A), is 18.0 ± 2.5 (the unit of A being the same as k1), the activation energy, Ea, is 18.5 ± 6.0 kJ/mol and the reaction order with respect to the hydroxide ion concentration, a, is 0.86 ± 0.42. LZnR is the estimated solubility product of the ZnR resin which has a value of 3.1 × 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 μg CuR cm−2 day−1 at 25 °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.