Overview of main varieties of powder coating curing agents

Foreword: Powder coating is a new type of solvent-free 100% solid powder coating

At present, the main domestic powder coatings market is dominated by low to mid-range, polyurethane powder coatings and acrylic powder coatings with better weather resistance and decorative properties account for a relatively low share, and there is relatively large room for development .

Most powder coatings are thermosetting and require the use of curing agents

1.

Glycidyl ester curing agents have epoxy groups and are mainly used in conjunction with carboxyl polyester resins

The three epoxy groups of TGIC make it have high activity and crosslinking efficiency, and the stable triazine ring gives the coating good heat and weather resistance

TGIC cured polyester powder coatings are widely used because of their adjustable curing characteristics, good mechanics and weather resistance

However, because it is irritating and suspected of being carcinogenic, it is gradually banned by the European Union, while the US market is still in use, so it will have a certain market for a long time

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

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

Compared with TGIC, PT190 eliminates ecological toxicity, but still has certain contact toxicity and severe irritation
.
In addition, due to its low content of functional groups, which reduces the reactivity, it is necessary to add an accelerator during use
.

Tri-β-methylglycidyl isocyanurate (tri-β-methylglycidyl isocyanurate) is a substitute for TGIC produced by Nissan, brand MT239
.
Nissan believes that its toxicity is much lower than TGIC
.
Due to the methyl group at the β position, its reactivity is lower than that of TGIC
.

In addition, glycidyl methacrylate (glycidyl methacrylate) polymer is also used as a curing agent for powder coatings, and can be used as an alternative to TGIC in some areas
.

2.
Hydroxyalkylamide curing agent

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

Figure 1 Hydroxyalkylamide curing agent

The curing principle of HAA is to use the hydroxyalkyl group at the β-position and the carboxyl group on the resin for esterification and dehydration
.
Its esterification reaction activity is very high, no need to add catalyst
.
From the perspective of toxicology, both HAA curing agents are completely safe
.

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

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

For thick coatings, pinholes and blisters may be caused
.
To overcome these problems, it is necessary to slow down the curing speed of the coating, so that the coating can be maintained for a longer time under the condition of low melt viscosity, and allow time for the escape of water molecules
.

3.
Isocyanate curing agent

As a kind of powder coating, polyurethane powder coating has many excellent properties.
Its coating film has high gloss, excellent decorative properties, strong wear resistance, good adhesion, and at the same time has good weather resistance, corrosion resistance and 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
.

In many application fields, including industry, agriculture, transportation, and aerospace industry, the powder coating can give full play to its advantages
.

The curing of hydroxy polyester resin with isocyanate curing agent is an important variety of powder coatings
.

However, the activity of the isocyanate group is very high, and the blocking agent BH (as shown in Figure 2) needs to be used to block the isocyanate group during the preparation of the powder coating, and then unblock it when the crosslinking is cured
.
The common ones are trimers of IPDI (isophorone diisocyanate) (Figure 3)
.

The biggest disadvantage of polyurethane powder coatings is that when they are cured into a film in the oven, they release the sealant, causing pollution to the environment
.

In addition, when the coating film is too thick, pinholes or bubbles are easily generated due to the release of the sealant
.
For this reason, it is necessary to minimize the amount of sealing agent and reduce the emission to the atmosphere, and consider the use of non-toxic sealing agent
.

In order to reduce the amount of blocking agent, the self-condensation of IPDI can also be used to form a uretdione structure, which can decompose and release isocyanate groups at a certain temperature (Figure 4)
.

The β dicarbonyl compound diethyl malonate can block isocyanate (Figure 5) and cure at a low temperature of 120-140 oC.
It can be used in epoxy and its mixed formulations.
It is a promising low temperature Curing agent
.

The isocyanate blocked by oxime (usually methyl ethyl ketoxime) (Figure 6), oxime is precipitated during curing, there is no unpleasant smell, and the curing temperature is lower.
It is a promising low-temperature curing agent and a hydroxyl-terminated poly Ester is used in polyurethane powder coatings, but there is a problem of yellowing
.

4.
Carboxylic acid curing

Carboxylic acid curing agents are used for curing epoxy resins, epoxy-containing polyester or acrylic resins
.
Carboxyl-terminated polyesters used for curing of polyester-epoxy powder coatings are also classified in this category, although polyester resins may account for the majority of coatings
.

The carboxyl-terminated polyester resin and epoxy resin are cross-linked and cured.
The epoxy resin can reduce the cost and endow the paint film with corrosion resistance and water resistance.
The polyester resin can improve the flexibility and weather resistance of the paint film
.

The synthesis of carboxyl-terminated polyester is generally divided into two steps.
First, hydroxyl-terminated polyester is synthesized, and then further esterified with trimellitic anhydride to obtain carboxyl-terminated polyester
.

Small molecule acid 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 in powder coatings due to their strong hygroscopicity and strong irritation to the human body.
The curing agent
.

The typical representative of small molecule carboxylic acid as curing agent of powder coating is dodecanedioic acid (DDDA)
.

Acrylic powder coatings using it as a curing agent are the best weather-resistant powder coatings and can be used in automotive topcoats
.
According to reports, European SMART models use acrylic powder coatings
.

5.
Amine curing agent

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。

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。

7

（dicyandiamide，DICY），207~209oC，200 oC/30 min
。

DICY，
。

，
。，－（Ortho-tolyl-biguanide），136 oC，A
。

，，
。160 oC/20 min-180 oC/10 min
。

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。

、
。（Sebacic dihydrazide），185 ~190 oC
。180 oC/15 min170 oC/20 min
。，
。

The 2-position substituted imidazole is also used in a large amount in epoxy powder coatings and is mainly used as an accelerator
.

Commonly used varieties are 2-methyl imidazole, 2-propyl imidazole, 2-propyl imidazole, and 2-phenyl imidazole.
, Undecyl imidazole (undecyl imidazole) and heptadecylimidazole (Heptadecylimidazole)
.

The substitution of the alkyl group and benzene ring at the 2-position has steric hindrance to the active sites (secondary and tertiary amino groups) on the imidazole molecule, thereby reducing its reactivity, while the long chain helps to improve the compatibility with the resin , Increase the flexibility of the cured product
.

6.
Polyphenol curing agent

Polyphenol curing agents mainly refer to phenolic resins, whose phenol groups can react with epoxy groups at high temperatures
.

Phenolic resin and epoxy resin have good compatibility, the resulting paint film has good adhesion, flexibility, impact resistance, high brightness, leveling and anti-corrosion properties, good storage stability, and reactivity can be passed through accelerators coating system to be adjusted, non-toxic, it can be used in food, drinking water and other direct contact
.

The main disadvantage of phenolic resin used in epoxy powder coating curing agent is that the system has poor yellowing resistance and is not suitable for making white coatings
.
At the same time, the presence of aromatic ether groups will further reduce the UV resistance of epoxy resins
.

7.
Conclusion

The development of powder coatings is bound to be accompanied by the development of curing agents
.
Taking into account the storage stability of coatings, low-temperature curing energy-saving, high efficiency and low dosage, thinning of the coating film and good decoration, mechanics, anti-corrosion and weather resistance are issues that must be considered when improving and developing new curing agents
.