Development and Application of HAA Weather-resistant Powder Coatings

This article mainly introduces two powder coating Primid system improved curing agent products QM 1260 and SF4510 developed by Swiss EMS company to replace XL552 curing agent.

Through the comparison of experiments, the application of QM 1260 and SF4510 curing agents has solved the problems of yellowing, gas bake resistance and hydrolysis resistance of the powder coating film, and the problem of thick coating with pinholes.

01

Preface

Weather-resistant powder coating polyester/TGIC type has no volatile matter produced during the curing reaction, so it can be made into a thick film without pores, has good weather resistance, and has an excellent film appearance.

However, the main disadvantage of polyester/TGIC powder coatings is that the curing agent TGIC is relatively toxic and irritating to the skin.

The polyester/HAA (hydroxyalkyl amide) system powder coating that replaces TGIC curing agent has experienced more than ten years of tortuous development.

More than half of them are used in Europe.

The HAA system has outdoor durability equivalent to that of the TGIC series powder.

Whether it is the increase of HAA users or the decrease of TGIC and PU users, they all start from caring for human health, reducing pollution to the global environment and saving energy.

HAA system powder coating can achieve low temperature curing, non-toxic, good storage stability of the coating, low cost of coating formulation, HAA reaction activity is stronger than TGIC, and can be adapted to friction gun spraying.

Although the HAA system has advantages in health and environmental protection, some physical and chemical properties restrict its development.
Hydroxyalkylamide releases water and other small molecular compounds in the process of curing polyester.
If the coating is too thick, pinholes are prone to appear, and the heat resistance is worse than that of TGIC.

The surface of the high-gloss flat powder coating prepared by hydroxyalkylamide is not dense enough, which affects the decoration.
It is mostly used for the production of sand grain, hammer grain and other powders that do not require a smooth appearance.
Therefore, it is necessary to focus on research and development to solve technical problems such as thick coating pinholes, yellowing, and poor acid rain resistance.

The representative product of HAA curing agent is XL552.
Its weak points are leveling, yellowing, and pinholes, which are also key issues that powder coating manufacturers, coating application companies and powder coating technicians care about.

Through research and practice in ROYAL powder coating company in Turkey, using the improved curing agent product of Primid system developed by Swiss EMS company, QM 1260 and SF4510 completely solve the yellowing, gas bake resistance and hydrolysis resistance of XL552 and thick coating.
There is a pinhole problem.

02

The characteristics of improved HAA curing agent

QM 1260 mainly solves the yellowing, gas baking and hydrolysis resistance problems of XL552

The curing agent QM1260 is a compound containing polyhydroxyl and alkyl functional groups, which is a kind of HAA, and can also replace triglycidyl isocyanurate (TGIC) as a curing agent for weather-resistant powder coatings;

Used for cross-linking of hydroxyl-containing polyester resins and polyacrylic resins, it can prepare polyester powder coatings without biological mutation, better weather resistance, and non-toxic.
This is mainly due to the unique chemical structure of QM1260.

The hydroxyl group of curing agent QM1260 is at the β position, which is easily affected by N atoms and has very high activity.
It can be used as a chemical intermediate and is easy to react with carboxyl groups.
It can still interact well with carboxyl groups without adding a catalyst.
The polyester resin and polyacrylic resin are cured.

In addition, the powder coatings cured by QM1260 curing agent have better impact resistance and better leveling properties than powder coatings cured with TGIC.

At the same time, because HAA has a higher melting point, the curing agent QM1260 can be very effective and energy-saving, and it can be completely cured at a minimum of 150°C/20-25min.

The curing agent QM1260 system powder coating has good frictional electrification, because it has a hindered amine structure and can reduce the spraying voltage.
It can also be applied with a rubbing spray gun.

Product indicators: ①Gardner color is less than 0.
5; ②Moisture content is less than 1.
0%; ③Melting point range is 100-115℃; ④Hydroxyl value is 550-650mg/g.

SF4510 mainly solves the problem of pinholes in thick coating

SF4510 is a reactive cross-linking agent.
It can be selected from a wide range of carboxyl polyester resins with acid values ​​ranging from 20 mg/g to 80 mg/g.
It is especially suitable for high-gloss powder coatings;

Among the hydroxyalkylamide powder coatings, not only are environmentally friendly and pollution-free and can withstand the Florida exposure test, but also have better fluidity, excellent degassing performance, can meet higher film thickness, and can improve Charge performance and excellent baking resistance.

Product indicators: ①Gardner's color is less than 1.
0; ②The moisture content is less than 1.
0%; ③The hydroxyl value is 550-650mg/g.

03

Formula design of improved HAA weather-resistant powder coating

One self-evident important performance index of weather resistant powder coatings is weather resistance.
The main factor that causes coating aging or damage is sunlight, especially 250-380nm ultraviolet radiation causes the degradation of the coating film polymer, which has the most serious damage to the coating, so this must be considered in the formulation design.

It is not the use of outdoor polyester and outdoor curing agent.
If any kind of material (including additives, pigments and fillers) is used indiscriminately, not only will it not increase the weather resistance, but some will accelerate the aging process.
So, how to design products for weather-resistant powder coatings?

polyester resin

The original intention of the weather resistant powder coating design is to maximize the weather resistance of the film-forming material to make it perfect.
Therefore, the choice of resin and crosslinking agent for this type of powder coating is very critical.

Weather-resistant powder coatings require that the selected polyester resin must have high weather resistance and the most stable product quality, without adding any catalyst that affects weather resistance.
Under this circumstance, only a small number of polyester resin manufacturers can do it.

Choose CRYLCOAT® 4659-0 super weather-resistant carboxyl functional polyester resin from Allnex (formerly CYTEC) Industries of the United States, combined with Primid to prepare non-chalking powder coatings, suitable for Primid systems that require flexibility.

CRYLCOAT® 4659-0 technical indicators: acid value 34mg/g, softening point 118-120°C, glass transition temperature (Tg) 59°C, curing temperature 190°C.
It is also suitable for TGIC and HAA curing agents.

Crosslinking curing agent

Figure 1 Hydroxyalkylamide curing agent

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

The curing mechanism of HAA is to use the hydroxyalkyl group at the β-position and the carboxyl group on the resin for esterification dehydration reaction, the esterification reaction activity is very high, and no catalyst is required.
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 a 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 have a certain effect on 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 curing agent is that water molecules are released during the curing process, which will affect the compactness of the paint film.
For thick coatings, it may cause pinholes and blisters.

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.

Weather-resistant powder coatings are also very strict in calculating and using the amount of resin and curing agent, and the degree of crosslinking of the cured film-forming substance must reach the best value.

Therefore, in this case, the polyester resin and the cross-linking agent have very high quality and stability requirements, and their technical indicators are also very high.

The amount of resin and curing agent must be safely calculated by equivalent to obtain the best ratio, rather than estimation.
Only weather-resistant powder coatings with a high degree of cross-linking can effectively ensure their high weather resistance.

Use and selection of additives

Weather-resistant powder coatings also have higher requirements for the selection of additives.

(1) Benzoin

As a degassing agent, it has a yellowing tendency and is basically consumed in the curing process of powder coatings.
However, it is necessary to choose quality-guaranteed benzoin.
For domestic benzoin that contains many by-products, yellowish benzoin should be kept away.

(2) Leveling agent

Still choose acrylic leveling agents, such as ESTRON's PV88, P67, etc.
, Solutia's Modaflow Ⅲ, BYK's BYK 360, etc.
The domestic leveling agent with more impurities or low purity should be carefully selected, especially the light-attracting agent contained in the leveling agent will cause the weather resistance of the powder coating to be greatly reduced.

(3) Wax powder  

It is a commonly used additive in powder coatings, so only micronized PTFE wax can be selected in weather-resistant powder coatings.
For other wax powders, they will be excluded from the optional range, especially modified matting waxes, which have a greater impact on weather resistance, and should not be used.

(4) EP520  

It is made by the original factory of Clariant Chemical Company in Switzerland.
It has high hardness polyethylene and polytetrafluoroethylene copolymer fine powder wax.
It has anti-oxidation and white appearance, without fish eyes, and can be used as a high-efficiency external lubricant.
It is suitable for various coating systems that require improvement of surface gloss and feel.

(5) Antioxidant, ultraviolet absorber and light stabilizer  

This is a type of additive that must be selected in weather-resistant powder coatings.
In colored non-transparent powder coatings, UV absorbers can be used without choice, but antioxidants and light stabilizers must be selected and used, and the amount used should be appropriate.
It is not easy to be too much or too little.

Chinox 1010 antioxidant: It has the advantages of low volatility, high thermal stability, long durability, non-coloring, non-polluting, non-toxic, and can prevent thermal aging degradation.

Chinox 326 ultraviolet absorber: can protect the coating film from ultraviolet radiation damage, has excellent thermal stability and light stability, so it can reduce the possibility of coating film fading.
Its protective effect on pigments is also very good, which meets the optical requirements of sunglasses.

Other additives: The range of options is not wide, and it can only be used after repeated experiments when choosing.

Colorant

Among weather-resistant powder coatings, coloring pigments are a very important part of the formulation.
Because the weather resistance of such powder coatings is higher than that provided by organic pigments, almost all organic pigments are not within the usable range of weather-resistant powder coatings.

The weather resistance of inorganic pigments is relatively good, but the weather resistance of different inorganic pigments is also very different, and the inorganic pigments with poor weather resistance are not in the selection range.
Those inorganic pigments that are liable to lose moisture or change color under heating conditions (such as 200-300°C) should be prohibited from being used.

Studies have shown that the pulverization of powder coatings mainly starts from the pigment particles, especially where the pigment forms agglomerates.
This is a photochemical process.
Therefore, in addition to selecting the type of pigment for the weather-resistant powder coating system, the dispersibility of the pigment must also be excellent.

(1) CR-826 titanium dioxide is produced by Komeiji's chlorination process.
It is coated with silicon oxide and aluminum oxide and has extremely high weather resistance (TiO2 content 93%).
It has excellent optical properties and excellent Coloring power and hiding power, excellent weather resistance and high gloss effect.

(2) Ultramarine blue G58 is an inorganic pigment produced by nubiola.
It is an artificial lapis lazuli pigment with a pure, bright and reddish blue.

Its characteristic is non-toxic, it has very reliable safety performance both in the production process and in the final application process, and still has high safety after a long period of market circulation;

Comply with most stringent health and safety standards (including standards of the United States, European Union and Japan), suitable for food packaging, toys, cosmetics, etc.
;

It has excellent low heat resistance (350 degrees Celsius), light stability and weather resistance.
Products with excellent low alkalinity and acid resistance can also be used in alkaline environments.

Table 1 Reference formula of Primid powder coating (parts by mass)

Physique filler

The recommended bulk fillers are mainly precipitated barium sulfate, other fillers with layered structure such as kaolin, talc, mica powder, organic bentonite, and high-hardness bulk fillers such as alumina.

In short, the range of options is generally relatively wide.
Calcium carbonate is easily corroded by acidic substances due to its alkalescence, so it is not easy to be used in weathering powder coatings.

Regarding the choice of physical fillers for weather-resistant powder coatings, no matter which physical filler is selected, it should be tested and tested as much as possible, and must not be used blindly.
 

04

Coating preparation

The above-mentioned raw materials are matched in a reasonable proportion to form a HAA weather-resistant powder coating formula (Table 1), which is placed in a premix tank for high-speed dispersion for 5-10 minutes, and then melted and mixed by an extruder at high temperature, cooled, coarsely pulverized, and finely pulverized by ACM.
After passing through a sieve of 89μm (180 mesh), a high-performance weather-resistant powder coating is obtained.

05

Spraying process

Use the high-voltage electrostatic spray gun (50～70kV) or rubbing spray gun to spray the prepared improved HAA weather-resistant powder coating on the phosphated 1.
2mm thick cold-rolled steel plate, and then pass 200℃/8min or 180℃/10min or Bake and cure at 165℃/20min.
The performance index of the coating film is shown in Table 2.

Table 2 Test results of coating film (for reference only)

It can be seen from the table that the improved QM1260 and SF5410 curing agents used in Prmidi system powder coatings have basically solved the problems of baking yellowing and thick coating pinholes.

The test data shows that QM1260 and SF5410 curing agents can be used together to achieve the desired effect.
 

06

Concluding remarks 

The application of QM 1260 and SF4510 curing agents completely improved the coating drawbacks of the XL552 system.
However, we also look forward to the localization of new curing agents for weather-resistant powder coatings, and the early coming out of varieties with better quality, more suitable cost performance, and healthier and more environmentally friendly products.

It is hoped that the upstream and downstream of the industry will develop in coordination, increase the development of outdoor weather-resistant curing agents, and replace TGIC toxicity and HAA yellowing and pinhole problems with non-toxic new varieties.