Investigation of the matting effect of aromatic amines and zinc ion complexes in powder coatings

Investigation of the matting effect of aromatic amines and zinc ion complexes in powder coatings
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Abstract: Zinc 2-mercaptobenzothiazole (ZMBT) is widely used in the matting system of powder coatings. In this paper, the following hypothesis was put forward for the structural model of its matting: the structural model of “epoxy resin curing agent + zinc ion” can be used as a matting agent for epoxy resin powder coatings. Based on this assumption, two aromatic amines, 4,4-diaminobenzene methane (DDM) and 4,4-diaminodiphenylsulfone (DDS), were synthesized as complexes with zinc chloride, and the matting effects of the two aromatic amines and the corresponding zinc salts were compared with those of ZMBT. The results showed that both complexes had similar matting effects to ZMBT in powder coatings, but lower film gloss could not be obtained by compounding with other matting substances.

1 Introduction
Zinc 2-mercaptobenzothiazole (ZMBT) as the basis of the establishment of the powder coating matting system is widely used; the system matting effect is stable, the gloss can be adjustable, and at the same time, has excellent anti-yellowing performance, but the storage stability of powder coatings is not good. ZMBT is made of 2-mercaptobenzothiazole (MBT) in alkaline conditions and zinc ions complexed from the decomposition of the [1], in which MBT can be and epoxy group [1], the ring-opening addition reaction [1]. ring-opening addition reaction [2], which can be used as a curing agent or curing accelerator for epoxy resins. In order to investigate the matting mechanism of aromatic amine zinc salts on powder coatings, we put forward the following hypothesis: is it possible to achieve the matting effect when the curing agent of epoxy resin is combined with zinc ions?
Based on the above hypothesis, we selected two aromatic amine epoxy resin curing agents: 4,4-diaminobenzylmethane (DDM) and 4,4-diaminodiphenylsulfone (DDS), of which DDM is more reactive and can directly cure E-12 epoxy resin for powder coatings, while DDS is less reactive and can be reacted with liquid epoxy, and both of the aromatic amines can be effectively complexed with zinc ions [3]. Therefore, in this paper, we designed and synthesized the complexes of the above two epoxy resin curing agents with zinc ions (as shown in Fig. 1), and compared the matting effects of the two aromatic amine curing agents and the corresponding zinc salt complexes with the conventional zinc 2-mercaptobenzothiazole system.

2 Experimental Part
2.1 Raw materials
Zinc chloride (analytically pure), ethanol (95%), DDM, DDS, ZMBT, zinc dimethyldithiocarbamate (ZDEC, melting point 250°C), epoxy resin E-12, Xinzhongfa 50/50 polyester P5086, Anhui Excalibur 50/50 polyester SJ3#B, Yulin polyester YL5050, aluminum salts, barium matting, lightening agent, leveling agent, benzoin, carbon black etc. are all industrial grade.
2.2 Experimental equipment
Three-necked flask (500mL), oil bath with magnetic stirring, filter extraction device, twin-screw extruder, high-pressure electrostatic spraying equipment, constant temperature blast oven, coating film thickness meter, gloss meter, coating film impactor, metallurgical microscope and so on.
2.3 DDM/zinc ion complex preparation
The synthesis process of aromatic amine/zinc salt complex was as follows [4], 19.8 g DDM was added to the flask with 100 mL of ethanol and stirred in an oil bath at 60°C until complete dissolution, 14 g of zinc chloride was added to the solution, a large number of solids were precipitated immediately, and the reaction was continued to be stirred for 30 min, then cooled, filtered, washed, and dried in oven at 50°C for 2 h to obtain 30 g of yellowish powdered solids, i.e., DDM/ Zn2+ complex, yield 88.7%, melting point 320 ℃.
2.4 Preparation of DDS/Zn ion complexes
Add 24.8 g of DDS and 100 mL of ethanol in a flask, stir in an oil bath at 60 ℃ until completely dissolved, add 14 g of zinc chloride to the solution, stir the reaction for 2 h, then cool, filter, wash, and dry in an oven at 50 ℃ for 2 h to obtain 32 g of white powdery solid, i.e., DDS/Zn2+ complex, with a yield of 82.3% and a melting point of 315 ℃.
2.5 Powder coating and coating preparation
The experimental resin, curing agent and filler were mixed well according to the formula, and then incorporated into the extrusion → cooling and pressing → crushing → sieving → powder coating, and then electrostatically sprayed on the tinplate, with the thickness of 70~90 μm, and then cured at 200 ℃/12 min to obtain the coating film.