Selection of Catalysts for Polyurethane Powder Coatings

In the reaction process of polyurethane resin there are mainly -NCO/-OH, -NCO/H2O, -NCO/-NCO three types of reactions, in order to control the reaction in favor of the designed direction, shorten the reaction time, in its synthesis process, need to add the appropriate catalyst.

In polyurethane powder coating, the main choice is -NCO/-OH reaction, generally choose organic tin as catalyst, more widely used such as: stannous octanoate, dibutyltin dilaurate, etc. However, with the increasing demand for environmental protection, many application industries downstream control of tin catalysts are becoming more and more strict, the use of restrictions, so the development of new environmentally friendly catalysts need to be resolved, at present The common types of environmental catalysts in the market are organic bismuth and organic zinc, but these two types of catalysts are also limited by their low activity in use.

① According to the chemical composition of corrosion inhibitors, it can be divided into inorganic corrosion inhibitors and organic corrosion inhibitors.
Inorganic corrosion inhibitors: phosphates, borates, molybdates, zinc pigments, other pigments,
Organic corrosion inhibitors: basic sulfonates, two benzal naphthalene sulfonates, carboxylates, nitro acid salts, organic nitride zinc salts, other zinc salts and coordination compounds, chelating Chemical additives, amines and amine salts, phosphonates, silanes, titanates and other additives.
② According to the mechanism of action, it can be divided into: anode type corrosion inhibitor, cathode type corrosion inhibitor, and mixed type corrosion inhibitor.

Mechanism of corrosion inhibitor
The corrosion process of metal in the medium is composed of two ptfe powder chain conjugated electrochemical reactions. The two electrochemical reactions are the anode reaction and the cathode reaction. The corrosion inhibitor can inhibit one or both of the anode and cathode reactions, and can reduce the corrosion rate.

Anode corrosion inhibitor
Anode type corrosion inhibitor refers to the corrosion inhibitor that can inhibit the anode reaction of the corrosion battery, as shown in Figure 22-1. When no corrosion inhibitor is added, the anodic polarization curve of steel is curve 1, and the cathodic polarization curve is curve 3. The two polarization curves intersect at S, and the corresponding corrosion potential is pe. The corrosion current density is ic. At this time, the metal is in an activated state in the corrosive medium, and the intersection point S of the anodic polarization curve and the cathodic polarization curve is in the activation zone, corresponding to a large corrosion current ic, a very negative corrosion potential vs, and the metal is in a corrosive state. After adding the oxidizing metal salt such as chromate, the anodic polarization curve becomes curve 2. The schematic diagram of the action principle of the cathodic and anodic inhibition type corrosion inhibitor a] The polar reaction does not matter.