Overview of the main varieties of powder coating curing agents

preface

Powder coating is a new type of solvent-free 100% solid powder coating. It has the characteristics of solvent-free, pollution-free, recyclable, environmental protection, energy and resource saving, labor intensity reduction and high mechanical strength of coating film, and is one of the fastest growing environmentally friendly coating varieties.

At present, the main domestic powder coating market is mainly low-to-mid-range, and the share of polyurethane powder coatings and acrylic powder coatings with good weather resistance and decorative properties is relatively low, and the development space is relatively large.

Most powder coatings, on the other hand, are thermoset and require the use of a hardener. The matching of hardener and resin is crucial for various properties such as storage stability, curing speed and application leveling performance, film appearance, and film mechanics and weather resistance. This time, the editor here is a review of the main curing agent varieties used in powder coatings.

1. Shrinkage dry grease curing agent

Glycidyl ester curing agents have epoxy groups and are mainly used with carboxyl polyester resins. The most important of these hardeners is triglycidyl isocyanurate (TGIC).

TGIC has three epoxy groups that make it highly active and crosslinked, and the stable triazine ring gives the coating good heat and weather resistance

TGIC cured polyester powder coatings are widely used due to their adjustable curing properties, good mechanical and weather resistance.

However, due to a certain irritation and suspected carcinogenicity, it is gradually banned by the European Union, while the US market is still in use, so for a long time, there will also be a certain market.

In order to avoid some of the shortcomings of TGIC, various companies have also introduced some structurally modified glycidyl ester curing agents.

Among them, the mixture of triglycidyl trimellitate and diglycidyl terephthalate is a curing agent developed by Huntsman, and the trade name is PT910.

Compared with TGIC, PT190 eliminates ecotoxicity, but still has some contact toxicity and severe irritation. In addition, due to its low functional group content, the reactivity is reduced, and an accelerator needs to be added when it is used.

Tri-β β-methylglycidyl isocyanurate is an alternative to TGIC manufactured by Nissan under the grade MT239. Nissan considers it to be much less toxic than TGIC. Due to the presence of methyl groups at the β position, its reactivity is lower than that of TGIC.

In addition, glycidyl methacrylate polymers are also used as powder coating curing agents, which can be used as a substitute for TGIC in some fields.

2. Hydroxyalkyl amide curing agent

At present, hydroxyl alkyl amide (HAA) is gradually replacing TGIC as the first major type of curing agent for weather-resistant powder coatings, and its main varieties are Primid XL552 and Primid QM1260 (see Figure 1).

The curing basis of HAA is to use the hydroxyalkyl group at the β position and the carboxyl group on the resin to carry out esterification and dehydration reaction. Its esterification reaction activity is highly high and does not require the addition of catalysts. From a toxicological point of view, both HAA-based hardeners are completely safe.

The only difference in the molecular structure of QM1260 compared with XL552 is that there is an extra methyl group on the carbon atom where the hydroxyl group is located, and the presence of steric hindrance effect may slightly reduce the reactivity of the hydroxyl group, but it can play a certain shielding role in the ester group formed after curing, thereby improving its hydrolysis stability and inhibiting the yellowing of the paint film.

The biggest disadvantage of powder coatings prepared with HAA as curing agent is that water molecules are released during the curing process, which will affect the compactness of the paint film.

In the case of thick coatings, pinholes and blistering may occur. To overcome these problems, it is necessary to slow down the curing rate of the coating so that the coating remains longer at low melt viscosity, allowing time for water molecules to escape.

3. Isocyanate curing agent

As a kind of powder coating, polyurethane powder coating has many excellent properties, such as high gloss, excellent decorative performance, strong wear resistance, good adhesion, and good weather resistance, anti-corrosion, electrical properties and mechanical properties;

It can be said that it takes into account the advantages of epoxy and acrylic powder coatings, and has the advantages of non-toxicity compared to pure polyester powder coatings.

The powder coating can be used in many applications, including industry, agriculture, transportation, and aerospace.

The use of isocyanate curing agent to cure hydroxyl polyester resin is an important variety of powder coatings.

However, the activity of the isocyanate group is very high, and the isocyanate group needs to be sealed with a sealant BH (as shown in Figure 2) in the preparation process of powder coating, and then unsealed during cross-linking curing. A trimer of IPDI (isophorone diisocyanate) is common (Figure 3).

The biggest disadvantage of polyurethane powder coating is that when it cures into a film in the oven, it releases a sealant, causing pollution to the ambient atmosphere.

In addition, when the coating film is too thick, pinholes or bubbles are prone to occur due to the release of the sealant. To this end, it is necessary to minimize the amount of sealant and reduce the emission to the atmosphere, while considering the use of non-toxic sealants.

In order to reduce the amount of sealant, the self-shrinkage of IPDI can also be used to form a ureadione structure, which can decompose and re-release the isocyanate group at a certain temperature (Figure 4).

Diethyl malonate, a β dicarbonyl compound, can block isocyanates (Figure 5) and cure at low temperatures of 120-140 oC, making it a promising low-temperature curing agent for epoxy and its hybrid formulations.

The isocyanate (Fig. 6) enclosed by oxime (generally with butanone oxime) is precipitated during curing, and there is no unpleasant smell and low curing temperature, which is a promising low-temperature curing agent, and is used in polyester with hydroxyl end for polyurethane powder coating, but there is a yellowing problem.

4. Carboxylic acid curing

Carboxylic acid curing agents are used for the curing of epoxy resins, epoxy-containing polyester or acrylic resins. Carboxylate polyesters used for curing polyester-epoxy powder coatings also fall into this category, although polyester resins may be the majority of the coatings.

The carboxy-based polyester resin and the epoxy resin are cross-linked and cured with each other, the epoxy resin can reduce the cost, give the paint film corrosion resistance and water resistance, and the polyester resin can improve the flexibility and weather resistance of the paint film.

The synthesis of terminal carboxyl polyester is generally divided into two steps, first synthesizing hydroxyl terminal polyester, and then using trimellitic anhydride to further esterify to obtain terminal carboxyl polyester.

Small molecule anhydrides such as phthalic anhydride and trimellitic anhydride can also be used as curing agents for epoxy resins in the presence of accelerators, but they are rarely used as curing agents for powder coatings due to their strong hygroscopicity and strong irritation to the human body.

A typical representative of small molecule carboxylic acids as powder coating curing agents is dodecanedioic acid (DDDA).

Acrylic powder coatings with it as a curing agent are the most weather-resistant powder coatings and can be used for automotive topcoats. According to reports, the SMART model in Europe uses acrylic powder coating.