A must-read for technicians – about pinhole generation and solutions for powder coatings

Powder coatings, along with water-based coatings, light-curing coatings and high-solids coatings, are known as the four new environmentally friendly coatings
.
As an important branch of coating varieties, powder coatings are widely used in household appliances, general hardware, aluminum profiles and automobiles with
their high efficiency, environmental protection and economic characteristics, as well as excellent decorative and corrosion resistance.

Pinholes, or bristle pores, are small needle-like pores that are the online coatingol.
com
of fine coating defects that are prone to occur during the curing process.
Its appearance not only affects the aesthetics of the coating film, but also may seriously affect the corrosion resistance of the coating
.

Due to their unique application conditions, thermosetting powder coatings typically exhibit high curing temperatures (above 120°C), thick film thicknesses (more than 50 u), shorter curing times (less than 20 minutes), and high initial melt viscosity (solvent-free dilution).

Practice has shown that it is precisely because of the above characteristics that powder coatings are more likely to have pinholes
during the curing process than solvent-based coatings with lower initial viscosity.
It is worth mentioning that thermoplastic powder coatings have a relatively small
chance of pinholes due to the fact that the viscosity of the system does not increase.

Pinhole, as a type of coating defect, is especially obvious in high-gloss powder coatings, and low-gloss powder coatings, especially matte sand-grained powder coatings, are usually not obvious
.

How to prevent and eliminate pinholes in medium and high gloss powder coatings has become a topic that powder coating technicians must face (the following research is only for thermosetting high-gloss powder coating systems).

The causes of pinholes in powder coating films and their solutions

The formation of powder coating film pinholes is closely related to its unique melt curing process, therefore, to study the formation mechanism of powder coating film pinholes, it is necessary to understand the melt curing process
of powder coatings.

Powder coating, as the name suggests, is a powdery coating that is first adsorbed or accumulated on the surface
of the substrate in a loose structure by electrostatic spraying during the coating process.

After the spraying is completed, the workpiece enters the hot drying channel, the substrate and coating are heated and melted, and the original loose structure or accumulation mode is destroyed
with the melting flow of powder particles.

In particular, a local vortex effect produced by liquid flow during film formation is called the Bernard vortex
.

The essence of Benard's vortex is the change in surface tension caused by the change in viscosity accompanied by the melting and curing process of powder coating, causing the fluid with high viscosity and low surface tension to sink to the middle (recess) of the vortex, while the fluid with low viscosity and high surface tension rises to the periphery (convexity) of the vortex until the curing is completed
.

In this process, the gas (air) in the original loose accumulation void after coating will accumulate to form bubbles during the powder melting and collapse process, and small molecular gases from inside the coating or substrate will also accumulate to form bubbles and be discharged
.

As the viscosity of the system increases during curing, the bubbles that are trapped in the Benard vortex eventually form pinholes
during the discharge process.

Therefore, to prevent and eliminate pinholes in powder coatings, it is necessary to analyze the root cause of small molecules (bubbles) in the coating, and then prescribe the right medicine to prevent and solve the pinhole defects
of the coating film.

In the process of melting and curing of powder coatings, the volatile small molecules that are wrapped in powder coatings can be mainly divided into the following situations:

Powder coating native pinholes: air trapped in the coating

After spraying, the powder coating is accumulated on the workpiece in a loose structure, which makes a large number of gaps between the powder particles and the powder particles to be filled
by air.

With the increase of ambient temperature, the melting of powder coating particles leads to the collapse of this loose structure, because the thickness of the powder coating film is generally greater than 50μ (the loose structure after spraying is much larger than this thickness);

The powder particles in the middle position and heating up slowly melt slowly, so that the air between the particles is enveloped by the molten coating, and as the curing progresses, the viscosity of the system gradually increases, and the air wrapped in the coating leads to the formation of coating pinholes
.

This kind of film pinhole is a thermosetting powder coating due to its own characteristics, so strictly speaking, pinhole is the original defect
of powder coating.

In order to eliminate the above factors that cause native pinholes, the outgassing agent is a necessary raw material in the formulation of high-gloss powder coatings, and benzoin (benzene couple) is a highly effective degassing agent
to eliminate the above-mentioned pinholes.

The defoaming mechanism of benzoin is very complex, in addition to eliminating the above pinholes, benzene marriage also has a certain effect
on the elimination of pinholes caused by other factors.

It should be noted that although benzoin is a very effective powder coating defoamer degassing agent, it cannot solve all powder coating pinhole problems
.
Even if the problem of the primitive outgassing of powder coatings still needs attention:

A.
Benzoin is easy to decompose and cause yellowing of the coating film under heating, and the addition of too much benzoin will bring discoloration to
light-colored powder coatings.

B.
With the increase of the thickness of the coating film, especially when it exceeds 120μ, even if a large amount of benzoin is added, obvious pinholes (thick film pinholes)
will usually appear on the surface of the coating film.
Such thick film pinholes need to be eliminated by adding other types of defoamers in combination with benzoin
.

C.
Benzoin can not completely eliminate the pinholes in some low-temperature curing powder coatings, in order to reduce the curing temperature, a curing promoter is usually added to the polyester/TGIC system or polyester/epoxy mixed powder coating system, resulting in a rapid increase in the melt viscosity of the system during heating curing;

As a result, a large amount of gas is trapped in the coating and cannot be completely released, resulting in pinholes
.
Practice shows that benzoin cannot completely solve this problem and also needs to be solved
with other defoamers.

Volatile small molecules produced by the curing reaction of powder coatings

Powder coating curing reactions can be divided into two categories, one is direct reaction without small molecule release, such as carboxyl and epoxy reactions
, and hydroxyl groups and unblocked isocyanate groups.

At present, polyester/epoxy hybrid indoor powder coatings, polyester/TGIC outdoor powder coatings, GMA acrylic resin/DDDA transparent powder coatings, pure epoxy powder coatings, etc.
are widely used in the market, and the curing reaction does not produce additional small molecules
during the curing process.

Another type of curing reaction releases small molecules, such as polyester/β-hydroxyalklamide outdoor powder coatings, hydroxypolyester/tetramethoxymethylglycena powder coatings, and hydroxyl/blocking isocyanate (e.
g.
Evonik B1530) powder coating systems;

Water, methanol and isocyanate blockers
are released during the curing process.
The small molecules released by the curing reaction converge into bubbles and are discharged from the coating film
.

However, due to the increase in viscosity of the powder system and the problem of film thickness, some small molecules cannot be released in time and are wrapped in the coating, resulting in pinholes
.
Therefore, this kind of powder coating not only has native pinholes to solve, but also additional small molecular bubbles that are generated and accumulate need to be eliminated
.

Of course, it is worth mentioning that due to the high unsealing temperature of B1530 sealant, about 160 °C, the system has enough time and low viscosity to level and release the enveloping gas before unsealing, and the pinhole problem of the B1530 curing system is not particularly obvious
.

Practice shows that benzoin alone cannot significantly solve this pinhole problem
.
The elimination of pinholes in such powder coatings, in addition to the use of benzoin, also needs to be used in
conjunction with other defoamers and degassing agents.

Substrate factors

Substrates are another important cause of pinholes in powder coating films, and porous substrates, such as cast aluminum and cast iron, are high incidence areas
for pinhole problems in powder coatings.

The reason is that there is a large amount of air in the porous substrate itself, or a large number of volatile substances (such as undried moisture) in the voids;

After the powder coating is completed, the air or volatile substances in the void are closed by the molten powder coating during the heating process, and the viscosity of the system increases rapidly during the coating curing process, so that the gas in the void is not released from the coating, resulting in pinholes
.

To eliminate such pinholes, first of all, the porous substrate can be preheated at a higher temperature and longer before spraying, drying the substrate as much as possible, and making the powder coating have a low initial melt viscosity due to the high temperature of the substrate, which helps to exhaust
the gas in the substrate.

In powder manufacturing, on the one hand, certain substances that can improve the wetting performance of the powder coating substrate can be added to the powder coating formulation, so that the molten coating can quickly penetrate into the porous substrate and force out the gas
in the void as soon as possible when the initial melting viscosity of the system is low.

On the other hand, resins (film-forming substances) with lower melt viscosity should be selected, or certain substances
should be added to the powder formulation that can significantly reduce the melting viscosity of the coating.

In order to improve the wetting performance of powder coatings on substrates, certain compounds
containing polar groups (such as amide groups, hydroxyl groups, etc.
) can be added to the system.

These compounds can not only help to wet the substrate, but also help reduce the melting viscosity of the system, similar to solvents or thinner in liquid coatings, which can be called "solid solvents" because they exhibit the characteristics of solids
in powder coatings.

"Solid solvent" is a very beneficial attempt to solve such problems, of course, in order not to affect the anti-corrosion performance of the system, such "solid solvent" can not be a non-reactive and water-soluble compound, and the amount added cannot be too large
.

It is worth mentioning that the discovery of certain reactive "solid solvents" may be very outstanding
.
This type of "solid solvent" mainly exhibits two characteristics:

First, solid substances with low molecular weight and low initial melt viscosity; Second, it can participate in chemical reactions, but it does not necessarily react with the original powder coating system, but a self-consistent reaction system
.

Typical representatives, such as closed or unblocked isocyanate curing agents (such as B1530, B1540), their introduction can participate in the reaction of residual hydroxyl groups in the system, which can prolong the gelling time of the system, reduce the melt viscosity, and improve the crosslinking density
.
Such substances are also helpful in eliminating pinholes
.

The powder is damp

Unless the conditions are extremely harsh, under normal circumstances, powder coatings are not susceptible to moisture
.
Powder moisture is prone to occur, often due to the powder being stored at low temperatures for a long time and suddenly entering a high temperature and humid environment
.
Due to the low temperature of the powder coating after unpacking, it is easy to condense the moisture in the air and get damp
.

After severe moisture in powder coatings, a large amount of moisture is introduced into the system, which is discharged from the coating during the baking process, and some of the gases that do not have time to be discharged are expressed in the form of very fine pinholes, usually in the form of fog
shadows.

Such problems can only be prevented
.
The first is to ensure the normal storage conditions of the powder, and the second is to avoid ultra-low temperature storage as much as possible, if it is really necessary to store at low temperature, the powder should be allowed enough time to return to normal temperature before opening the box to avoid moisture
.

Factors of powder raw materials

The content of small molecules in raw materials is too high, which is also an important factor
in the pinholes of the coating film.
On the one hand, the volatile content of the powder resin itself is too high, especially the content of small molecules with high boiling point is too high; On the other hand, the powder raw material is damp and absorbs a lot of moisture
.

The pinholes caused by the above factors are generally very fine, which is mainly manifested as fog shadows
on the surface of the coating film.

Choice of defoamer

Benzoin is the most effective defoamer for high-gloss powder coatings, effectively eliminating native pinholes
at normal film thicknesses (60-90μ).

Benzoin also has a certain synergistic effect on pinholes caused by other causes, but it is often limited
.
A large amount of benzoin use will also bring about the problem of coating yellow, and benzoin itself is also a substance that decomposes and sublimates by heating, and too much use will bring negative effects
.

Defoamers containing amide groups and hydroxyl groups can effectively improve the wetting performance of coatings on substrates, and can also improve the wettability of resins to pigments, especially inorganic pigment fillers, reduce the melting viscosity of coatings, and effectively eliminate the cells
caused by various reasons.

Of course, it is also necessary
to choose a resin with low viscosity and gelling time for serious cell situations.

Transparent powder defoamer: As a special variety of powder coating, the principle and defoaming method of pinhole generation of transparent powder are the same
as ordinary powder coatings.

In particular, the transparent powder is a highly transparent system, and the whole system is optically isotropic without serious phase separation
.

Therefore, in the defoaming process, the added defoamer must be fully compatible with the
system.
In this sense, such defoamers need to be carefully selected
.

It should be noted that the addition of a large number of non-reactive defoamers is harmful to the performance of the coating and requires caution in the amount of addition
.

In short, the formation of pinholes (cells) in powder coatings is multifaceted, and the fundamental reason is that as the viscosity of the system increases during curing, the small molecule gas that is wrapped (Entrapped) in the coating gathers and does not have time to release
.

The sources of small molecules trapped in the coating include: loosely accumulated air during powder coating coating (powder coating originality), small molecules such as water and methanol generated by certain curing reactions, small molecule gases from inside porous substrates, and small molecule gases introduced due to improper storage of powder raw materials or powder products
.

Workarounds can be taken for different causes: