Powder coating boiling problems and corresponding improvement measures

Abstract: This paper lists various boiling resistance problems of powder coatings in application, and gives corresponding reference schemes

1.

According to the composition of the coating, the types of thermosetting powder coatings are: epoxy powder coatings, epoxy/polyester powder coatings, polyester/TGIC (or HAA) powder coatings, polyurethane powder coatings, acrylic powder coatings

Among all categories of products, since polyester powders are the most sensitive to water and belong to the one with the worst water resistance, the requirements for boiling resistance are the most prone to problems in the field of polyester applications, and the inspection of coatings is more frequently coatingol.

For example, in the field of aluminum alloy doors and windows, because the products of the polyester system are used, the boiling resistance requirements in this field are more significant

Secondly, there will be some boiling problems in the epoxy/polyester field, but overall it is not a prominent contradiction

2.

1) Reasons for product formulation:

a.

a1: Different product systems:

The boiling resistance of different systems is different, and the root cause is that the structural units of the resin are different
.

For example, polyester resin due to the fact that the structure of the unit contains ester bonds, lack of sufficient benzene ring structure, contains residual hydroxyl groups and other reasons, its boiling performance is not as good
as epoxy resin water resistance.
(Figure 1: Molecular structure of polyester resin; Figure 2: Molecular structure of epoxy resin; ）

A2: The same polyester system, the monomer unit is different, the boiling resistance will be different:

For example, polyester resin, the synthesis of saturated polyester resin is the esterification and polycondensation reaction of polyacids and polyols, and there are many types of polyacids and polyols, acids are mainly PTA, IPA, adipic acid, fumaric acid, etc.
, and alcohols are mainly NPG, EG, DEG, propylene glycol and so on
.

Among them, the acid PTA and IPA have a benzene ring structure, the benzene ring molecular structure is stable, and the water resistance is strong, so the polyester synthesized by these two acids has high water resistance; Alcohols NPG and propylene glycol are also stable in their chemical structure, so the polyester synthesized also has strong water resistance
.

A3: Effect of polyester molecular weight:

In addition to its structure, the effect of molecular weight size on boiling resistance is also very obvious
.
The small molecular weight polyester resin has a small crosslinking density after curing, and cannot form a dense coating, so it will affect the boiling resistance
of the coating.

A4: Effect of polyester residual hydroxyl group amount:

End carboxyl polyester resin is the film-forming material of the current mainstream powder coating, the film-forming process is the carboxyl group of the polyester resin and the epoxy group or hydroxyl group provided by the curing agent to crosslink the reaction, thereby forming a thermosetting paint film
.

In this process, the residual hydroxyl group of the polyester resin and the curing agent epoxy group are difficult to add to the ether, and the carboxyl group is easy to react with the epoxy group to form an ester, so the residual hydroxyl group of the end carboxyl polyester is retained in the process of curing into a film in the powder coating
.

Table 1 shows that with the increase of polyester hydroxyl content, the boiling resistance of the powder coating paint film begins to decrease significantly, this is because after the end hydroxyl group adsorbs the water molecules, these water molecules can hydrolyze the ester groups close to it at high temperatures, resulting in the breakage of the macromolecular chains and the decrease of
the gloss of the paint film.

b.
Type of curing agent:

b1.
Different kinds of curing agents:

The curing agent, also known as the crosslinking agent, is also one of the main film-forming components of the polyester powder, which is a class of functional compounds that can be cured with a coating film with certain physical and chemical properties by crosslinking the polyester resin, so the curing agent must contain characteristic groups
involved in the reaction.

The curing agent of polyester resin is usually TGIC and HAA two types, TGIC is triglyceride isocyanurate, which is a heterocyclic polyepoxide (Figure 3), so it has good boiling resistance
.

The HAA curing agent, the curing functional group is a hydroxyl group (Fig.
4), and the carboxyl group of the polyester is reacted to generate an ester bond, and the water resistance of the ester bond is relatively poor
.

And, since HAA itself is a hydrophilic compound, if it is present in excess of the system, it can also cause the coating film to be slightly less resistant to hydrolysis than TGIC
.

Therefore, the powder coating formed by HAA curing agent is better
boiled than TGIC curing.

b2.
The same type of curing agent, curing agent of different manufacturers:

The quality of curing agent manufacturers on the market is uneven, in order to improve the boiling water resistance of outdoor polyester powder, it is crucial
in the selection of curing agent manufacturers.

The possible reason is that there is a certain chloride ion residue in TGIC, and the level of chloride ion content will directly affect the curing degree of the powder coating, and eventually lead to the difference in the boiling water resistance of the coating film (see Table 3).

c.
Crosslinking density

c1.
The root cause of the acid number size affecting the water resistance of the coating:

The size of the acid number fundamentally determines the crosslinking density of the coating, and the higher the acid number, the higher the coating can form a coating with higher crosslinking density, and the water resistance of the corresponding coating will be better
.
Table 4 follows:

c2.
The purity of the curing agent also affects the crosslinking density:

This principle refers to b2, which will not be repeated
here.

d.
Pigment filler:

d1.
Pigments: Generally, the water resistance of 100% pure pigment stacking coatings will not have obvious effects
.
Some pigments are treated with surface treatment agents, and if the surface treatment agent is a hydrophilic surfactant, it will greatly reduce the water resistance
of the paint.

For example, some azo pigments themselves have poor cooking resistance, and if they contain hydrophilic surface treatment agents, the impact on the coating when boiled will be greater
.

In addition to the influence of hydrophilic treatment agents on the surface of the pigment, there are some pigments, if the molecules are low, especially the low molecular compound pigments with asymmetrical molecular structure, the impact on the cooking resistance of the coating will also be relatively large
.

d2.
Fillers: The varieties of fillers commonly used in powder coatings are barium sulfate (barite powder, precipitated barium sulfate), calcium carbonate, sericite and kaolin
.

In addition to calcium carbonate, these fillers are very resistant
to water.
Barium sulfate has good water resistance and weather resistance, which can enhance the leveling and light retention of powder coatings, and has good compatibility
with all pigments.

It can make the powder coating achieve the ideal coating thickness in the spraying process, and the powder rate is high and the price is cheap, so it is the most used filler
.

Sericite is dioctahedral, that is, it is composed of two layers of silicon-oxygen tetrahedral and a layer of aluminum-oxygen octahedron, and the reactive oxygen species and additional anions of the tetrahedron are located in the center of the two-layer hexagonal network, which constitutes an active crystalline water group
.

Therefore, sericite is resistant to high temperatures, strong chemical resistance, and has an excellent shielding effect on ultraviolet rays
.
Its fine phosphorus flake crystal structure improves the density of the coating and prevents moisture from penetrating
.
It should be used in coatings, which can greatly improve the weather resistance and water permeability of the coating film, while the dye particles are easy to enter the sericite lattice layer, so that the color continues for a long time
.

Therefore, the water resistance of sericite is better than barium sulfate, so in products with higher water resistance and weather resistance, sericite can be used to replace barium
sulfate.
Kaolin is similar in structure to sericite, and its water resistance is similar, which will not be repeated
.

2) Degree of curing:

The degree of curing of the coating fundamentally determines the basic properties of the coating including boiling resistance, and only sufficient curing can ensure the high crosslinking density of the coating, thereby reducing the entry of water molecules into the inside of the
coating.

Powder coatings are manufactured with little chemical reaction, but crosslinking reactions occur when used by downstream customers to generate a coating film
with certain physical and chemical properties.

Therefore, in order to produce a qualified product, it is necessary to choose the appropriate curing conditions
.
At a lower curing temperature, the reaction between the resin and the curing agent is not complete, and the crosslinking density cannot be reached, so the boiling water resistance of the coating film is very poor, and other properties cannot meet the requirements
.

The short curing time of the resin and the curing agent crosslinking reaction is not complete, and the crosslinking density of the coating film has not reached the due crosslinking density, so the boiling water resistance of the coating film is also very poor
.

The aluminum profile factory complained that a powder coating supplier defilms a batch of products after boiling, which is likely to be related
to the low actual effective curing temperature of the oven or the short effective curing time.

As shown in Figure 5, we take a product with a curing temperature of the longitudinal axis and a curing time for the curing window diagram
made of the horizontal axis.
It can be seen from the window diagram that the curing temperature is too low or the curing time is too short to show the state of insufficient curing, and in this curing state, the coatings show different degrees of poor water resistance
.

3) Boiling aids:

When there is a problem with the coating boiling, it is sometimes effective
by adding an appropriate amount of boiling aid.

The mechanism of action of boiling resistance may be that it can increase the crosslinking density of the coating to a certain extent, or it can enhance the adhesion of the coating, or it can provide a better hydrophobic surface to improve the boiling resistance
of the coating.

The principle of action of boiling aids of different manufacturers is slightly different, but water resistance additives can only play their due role
when the boiling performance needs to be slightly improved.

When a coating is cured by polyester or curing agent or cross-linked density, it does not fundamentally improve the boiling performance
.
As shown in Table 5:

4) Pretreatment and plates:

Poor pretreatment or sheet material can lead to very serious boiling problems: for example, the coating is blistering or many small bubbles or the coating falls off in strips
.
Since this article focuses on the coating itself, the boiling resistance caused by pretreatment and plate, this article will not be expanded
in detail.

3.
Boiling phenomenon and countermeasures of polyester type products

First of all, compare the water resistance requirements and inspection standards
of polyester products in the field of architectural aluminum profiles.
See Table 6 below:

1.
Foaming and wrinkling:

1) Raise small blisters the size of small yellow rice grains (see part 6 with pictures):

Such problems are generally improved by adjusting the recipe:

a.
Spelled into the high weather-resistant level of polyester (high weather-resistant polyester with an acid value of >30, and the water resistance is generally good); (Due to the high weather resistance requirements, the dibasic acid in the formulation will use a large amount of IPA, that is, isophthalic acid)

b.
Add an additive to improve the crosslinking density, more precisely to add a product with a very low equivalent energy reaction with polyester,

c.
Replace the polyester with stronger boiling resistance (other polyesters with the same level of performance, but with better water resistance);

d.
Add part of epoxy resin, content 2-5%, according to the specific product to determine the formula;

e.
Partial use of fillers such as sheet mica to reduce the penetration of water in the coating;

2) Big blisters (see part 6 with picture):

Similar to mung bean grains, soybean kernels or corn kernel-sized bubbles, this kind of problem is generally related to the customer's incompleteness before treatment, there are residues on the surface of the substrate, such as there may be oil stains or pre-treatment nights, at this time changing the formula is basically useless, it is recommended that customers contact the pre-treatment manufacturer
.

However, in some cases, the pretreatment meets the requirements, and the coating formulation uses a system with strong water resistance, and very large bubbles can still be seen, at which time the formula adjustment may also help customers solve the problem
.

However, the direction of adjustment is no longer to increase the crosslinking density, but to the opposite direction, that is, to reduce the crosslinking density, and select polyester
with a lower acid value.
The reason for this adjustment is that we believe that sometimes the density of the plate is not enough, especially in the extrusion process of some aluminum profiles, it is easy to have small gaps
in the micro level.

In the process of pre-treatment, the pretreatment liquid and moisture will enter these gaps, although there is a baking link in the pre-treatment process, but the remaining liquid in the substrate gap can not be 100% fully released, and the result is that after spraying the powder, if the coating is formed by the coating crosslinking density is very good
.

When boiling, the moisture remaining in the crevices and the pretreatment liquid do not have an effective channel to overflow, then under the action of heat and pressure, the coating will be peeled off from the substrate to form a large bubble
.

In this phenomenon, if the spray plant can improve its substrate formation process, it will be better
to further compact the substrate.
Many times the spray plant is reluctant to improve its profiles, then it is possible to consider reducing the crosslinking density on the powder to meet customer expectations, but the principle and possible problems need to inform the customer
.

2.
Adhesion

Problems with adhesion after boiling (see Part 6 with drawings) are generally due to pre-treatment or formulation, and some are also due to
plate.
Formula adjustment is generally as follows:

a.
The use of polyester with a slightly larger acid value, generally low acid value of polyester resin, boiling ability is very poor, do not choose the acid value of about 20 alone, the best acid value is less than 30 do not become the main component
of the curing system.

b.
Replace the polyester manufacturer, even if the acid value is above 30, the polyester produced by different manufacturers, the boiling capacity will be very different, you can choose more powerful suppliers to provide polyester;

c.
Upgrade the polyester type, the higher the general weathering level, the better the boiling capacity, so consider switching to a higher weather-resistant grade and a boil-resistant level of polyester under the conditions allowed by the cost; or add a higher boil-resistant level of polyester according to a certain proportion;

d.
Add adhesion additives or salt spray additives, which are somewhat helpful to the formulation but not as thorough
as improving the main body resin.

3.
Color change problem: generally divided into three aspects:

1) Severe discoloration

Visually the overall apparent whitish (see part 6 with the drawings), which is generally a problem
of polyester resin selection.
If this problem occurs, replace the polyester resin
.
Improvements in several ways:

a.
Use polyester with a slightly larger acid value, generally low acid value of polyester resin, boiling resistance is very poor, the same, do not choose the acid value of about 20 alone, the best acid value below 30 do not become the main component
of the curing system.

b.
Replace the polyester manufacturer, the same, even if the acid value is above 30, the polyester produced by different manufacturers, the boiling capacity will be very different, you can choose more powerful suppliers to provide polyester;

c.
Upgrade the polyester type, the higher the general weathering level, the better the boiling capacity, so under the conditions of cost allows, consider changing the weathering level and the boiling level of polyester; or add the better boiling grade polyester according to a certain proportion;

2) Slight discoloration, but irregular flowering on the surface (see part 6 with the accompanying figure):

a.
Add some boiling resistance additives to the formula, and many suppliers of boiling resistance additives can solve the problem of coating boiling flowers;

b.
Add an additive to increase the crosslinking density, more precisely to add products with a very low equivalent that can react with polyester;

c.
Replace the polyester with stronger boiling resistance;

d.
Add part of the epoxy tree monthly purpose, the content is 2-5%, according to the specific product to determine the formula;

e.
Pigment problem, replace the pigment with stronger boiling / weather resistance

4.
Boiling phenomenon and countermeasures of epoxy-polyester type products

Because epoxy resin has very good water resistance, epoxy polyester products have few
problems in boiling compared to outdoor polyester products.

However, we also encounter a little special situation in a large number of cases: for example, when boiling, there are some small bubbles on the surface or epoxy polyester products as primer products, and when the topcoat is sprayed and then boiled, the bubbles may spread
from the bottom epoxy polyester layer.

When this phenomenon occurs, the epoxy polyester layer can be changed to a pure epoxy layer, or the inorganic chlorine of the epoxy resin can be limited, and the volatilization of the epoxy resin can be controlled or replaced by a two-step epoxy
.

5.
Conclusion

In the case of environmental protection more and more need to pay attention to, powder coating is an important trend in the development of aluminum profiles at this stage, so everyone will pay more attention to
the performance of powder coatings.

Improving the water resistance of powder coatings, like many other performance requirements, will get better and better
as the industry develops.

Of course, many processes in powder coating production also have many places that need to be improved, requiring manufacturers to continuously transform the existing processes to improve the level of surface treatment of domestic aluminum profiles, promote the development of the industry to a newer, deeper, higher direction, and meet the requirements of
consumers' higher living standards.

6.
Drawings