Polyester powder coating curing agent
1 Melamine resin curing agent
Melamine resin is used as the curing agent for polyester powder coatings. Because its glass transition temperature is 40°C, the powder coatings obtained when the dosage is controlled below 10% of the polyester resin are still agglomerated. If the melamine resin is modified into a solid compound with cyclohexanone, toluene sulfonamide, phenol, etc., it can be used in powder coatings to improve its performance.
2 Blocked isocyanate curing agent
Zhang Baorui uses 2,4-toluene diisocyanate (TDI) to make blocked isocyanate curing agent. The appearance, mechanical strength and weather resistance of the coating film are all good. The powder coating has good flatness, gloss and physical properties, but it releases by-products such as sealant during curing. When the thickness of the coating film is too thick, it is easy to produce defects such as bubbles.
3 Triglycidyl isocyanurate curing agent
Triglycidyl isocyanurate (TGIC, Triglycidyl isocyanurate) is a white crystal with a melting point of about 95°C. It can be used with hydroxy polyester with an acid value of 30 to 60 to produce polyester powder coatings with good weather resistance. The powder coating has good mechanical properties, yellowing resistance, stain resistance, and chemical resistance. The disadvantage is that the curing agent has a stimulating effect on the skin and is relatively toxic.
4 Acid anhydride curing agent
The acid anhydride curing polyester powder coating is crosslinked to form a film through the addition reaction of the side chain hydroxyl group or the end group of the polyester resin and the acid anhydride, and the esterification reaction between the polyester resin and the hydroxyl group of the acid anhydride addition reaction. Due to the dehydration reaction in the reaction process, gas is generated when the coating film is baked, and the esterification reaction temperature exceeds 200°C, and the baking temperature is too high, so this curing agent is generally not used.
5 Hydroxyalkylamide curing agent
Hydroxyalkylamide is the general name of hydroxyalkylamide curing agents. In 2003, hydroxyalkylamide began to replace TGIC as the world's largest weather-resistant powder coating curing agent. The two basic varieties on the market today are: PrimidsQM1260 and PrimidsXL552.
The curing mechanism of Primid is the esterification reaction between its β-hydroxyalkyl group and the polyester carboxyl group, and water is a by-product of the reaction. The main disadvantage of powder coatings based on polyester and β-hydroxyalkylamide is that water will split to form bubbles during the curing reaction, and the thickness of the coating will cause pinholes and blisters. Slowing down the curing reaction speed can improve degassing, that is, maintaining a low melt viscosity for a relatively long period of time. So far, it is still believed that the reaction of Primid-type polyester powder coatings is not adjustable. Zhang Jun and others studied a new type of curing agent, its representative products PL1174, XL-552. The hydroxyl equivalent of XL-552 is 82~86g/eq, the melting point is above 120℃, the hydroxyl equivalent of PL 1174 is 90~125g/eq, the melting point is 90~110℃, and the average molecular weight is 350. The coating system is a polycondensation reaction, which limits the thickness of the coating film and reduces the corrosion resistance and salt water resistance.
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