Talk about how to improve the storage stability of powder coatings for aluminum profiles

Abstract: This paper mainly analyzes the reasons for the abnormal storage stability of powder coatings for aluminum profiles, and provides some ideas on how to improve the storage stability of powder coatings, and provides some precautions
for powder coating users in the application process.

preface

In the application of powder coatings, especially in the humid and stuffy south, powder coating users often find powder agglomeration when they are about to spray out of the box
.

At this time, the spraying staff needs to screen the powder, which affects the efficiency of spraying and the paint coatingol.
com
online.
There are also some special powders, after being stored for a certain period of time under not very ideal environmental conditions, spraying and curing into a film, and there are abnormal gloss of the coating film, rough surface, decreased mechanical properties, poor boiling water resistance and other phenomena
.

At this time, it is often judged as unqualified powder, which is what we often call powder deterioration
.
The reasons for the above phenomena are mainly the particularity of the powder itself and the poor storage stability caused by improper storage, and from the mechanism analysis, there are mainly physical changes and chemical changes
.

This paper mainly analyzes the reasons for the abnormal storage stability of powder coatings for aluminum profiles from the mechanism of physical changes and chemical changes, and provides some ideas
on how to improve the storage stability of powder coatings.

At the same time, some precautions for the application process are provided to powder coating users, and finally the storage period of powder coatings is relatively extended, reducing unnecessary troubles
for users in the spraying process.

Second, the cause analysis

Powder coatings change slowly during storage, mainly because resins, hardeners and various additives are mixed together
by extrusion and melting during powder coating production.

Therefore, after storage in a certain environment for a period of time, the mixed raw materials themselves or each other will produce some slight changes, so that the powder coating can not be used
normally.

Usually after the storage time is exceeded, the powder will appear agglomeration, poor fluidity and other phenomena, and the coating film will produce abnormal gloss, poor leveling, rough surface and other changes
.

We usually think that the poor storage stability of powder has two reasons
.

First, the physical state of the powder has changed, which is due to the powder coating being at a high temperature for a long time (close to or greater than the glass transition temperature of the powder), the powder is converted from the glass state to the high elastic state, resulting in the powder softening and stickiness, which is manifested as powder agglomeration or aggregation, which is difficult to fluidize and disperse in the fluidization barrel, affecting the normal spraying construction
.

Another involves a change in the chemical activity of the powder, due to the addition of reactive matting agents, matting curing agents and other components in some special powder coatings, which will lead to an increase
in the activity of the powder.

Under certain time and temperature conditions, the powder produces a partial chemical reaction, causing the powder to deteriorate, usually manifested as changes in the leveling and gloss of the coating film, and sometimes the physical and chemical properties of the coating film are partially reduced
.

The specific influencing factors are analyzed as follows:

1.
Glass transition temperature of powder coating

After a period of storage, the physical state changes such as agglomeration or aggregation of powder coatings are mainly caused by powder softening and stickiness, and powder viscosity is mainly because the ambient temperature of powder storage is close to or higher than the glass transition temperature
of powder.

The glass transition temperature refers to the temperature at which the polymer changes from a highly elastic state to a glassy state, which refers to the transition temperature of an amorphous polymer (including the amorphous part of a crystalline polymer) from a glass state to a highly elastic state or from the latter, which is the lowest temperature at which the amorphous polymer macromolecular chain moves freely, usually expressed by Tg, and varies with the method and conditions of determination
.

Figure 1 shows the deformation curves
of linear amorphous polymers at different temperatures.
Below Tg, the polymer is in a glassy state, and neither the molecular chain nor the chain segment can move, but the atoms (or groups) that make up the molecule vibrate
at its equilibrium position.

In Tg, although the molecular chain cannot move, the chain segment begins to move, showing high elasticity
.
(1) Therefore, although the storage temperature of powder coatings is often below its Tg, agglomeration can also occur due to the movement of low molecules and the vibration of atoms (or groups) in resin molecules
.

Figure I

From the above analysis, it is known that the higher the glass transition temperature of the powder coating, the better
its storage stability.
The glass transition temperature of powder coating is determined by the various components of powder coating, mainly determined by the glass transition temperature of resin and curing agent, because the content of these two in powder coatings generally accounts for about
60% of the total.

Most of the commonly used additives are polymer compounds with small molecular weight, and the glass transition temperature is low, which will reduce the Tg of the powder coating, but the amount of most additives is very small, and the final Tg of the powder has little effect
.

The inorganic substances (such as fillers and inorganic pigments) in powder coatings have a certain contribution
to the final Tg improvement of powder.
As can be seen from Table 1, different types of powder coating Tg are different, and there are some differences
in the same type of powder coating Tg.
In general, the sand-grained powder Tg is higher
than the flat powder.

The powder Tg in Table 1 is measured by METTLER TOLEDO DSC1 in Switzerland, and the resin Tg refers to the Tg of the resin used in the corresponding sample powder, which is calculated
by the following formula.

1/Tg=W1/Tg1+W2/Tg2+.
.
.
Wn/Tgn

Where: Tg-glass transition temperature of resin in powder formulation, K; The glass transition temperature of each resin in Tgn powder coating, K, is provided by the resin manufacturer's test report; Wn- is the mass fraction
of each resin in the powder coating.

As can be seen from Figure 2, there is a certain positive correlation between the Tg of the powder coating and the resin Tg used in the formulation, that is, the higher the Tg of the resin, the higher
the Tg of the corresponding powder.

Most of the difference between powder Tg and resin Tg is about 10 °C (except for sand pattern 1).

Although increasing the Tg of the resin improves the storage stability of the coating, the increase in the Tg of the resin makes the coating film brittle and hard, reduces impact resistance and flexibility [1], and deteriorates
the coating film leveling.

Picture table 1 Resin Tg and powder Tg of different powders

Fig.
2 Resin Tg and powder Tg of different powders

Therefore, when designing powder coating formulations for aluminum profiles, try to choose resins with high Tg in the case of ensuring the appearance of the coating film; In addition, without affecting the performance of powder coatings and coating films, appropriately increasing the content of pigment fillers in the formula can increase the glass transition temperature
of powder coatings.

2.
Reactivity of powder coating system

In powder coatings, mainly due to the direct contact between the resin and the curing agent, a chemical reaction will occur under certain conditions for a long time, resulting in deterioration of the powder coating, usually manifested as changes in the leveling and gloss of the coating film, and sometimes the physical and chemical properties of the coating film (such as boiling water resistance, mechanical properties) are partially reduced
.

The greater the reactivity of the powder coating system itself or the higher the ambient temperature of storage, the chemical change
of the powder system will be accelerated.

The
reactivity of powder coatings is mainly determined by the acid/hydroxyl value of the polyester resin, the amount of accelerator and the amount of reactive matting agent or matting curing agent.

In general, the higher the acid value/hydroxyl value of polyester resin, the more functional groups it reacts, the greater the reactivity, and the macroscopic performance is that the gelling time of the powder system is short
.

For example, the powder gelling time of the high hydroxyl value resin in polyurethane powder is significantly shorter than that of the low hydroxyl value resin, the storage environment requirements are more stringent, it is very easy to deteriorate, and the storage cycle is extremely short
in practical application.

The acid value of polyester resin is not much different, but the reactivity of different types or manufacturers of resin is also different, such as the resin generally used for wood grain powder is more reactive than the resin for ordinary powder, so the general wood grain powder is placed for a period of time, the physical state of the powder will not agglomerate, but it will be difficult to tear the paper phenomenon
when transferring.

This is that due to the high reactivity of wood grain powder, a small part of the powder has slowly reacted and deteriorated
.
Therefore, when designing the formulation, it is important
to select the appropriate acid number and active resin according to customer requirements.

For semi- and matte formulations, matting agents or matting curing agents will be added to the formula, and most of the physical matting agents on the market today are internal reaction balance matting agents, and the external performance is that there is no need to consume curing agents
.

For matting curing agent, it is to directly consume the curing agent to participate in the reaction, so these two types of additives will lead to the chemical activity of the powder coating becoming stronger, the gelling time is shorter, and the powder coating will be more likely to undergo chemical changes
during storage.

Therefore, the storage stability
of powder coatings can be improved by selecting matting agents that do not affect the reactivity of powder coatings or by adjusting the formula to minimize the amount of reactive matting agents or matting curing agents.

For some special formulations will use accelerators and other substances, the external performance is that the gelling time of the powder coating is significantly shorter, indicating that the accelerator will make the powder coating system activity significantly stronger, resulting in the powder coating is easy to deteriorate, so try to avoid using
it.

3.
Powder coating particle size distribution

The particle size of powder coatings is usually controlled between 30-40μm D50, if there are more fine powders below 10um, their specific surface area will increase rapidly, and the contact surface between powder and powder will also increase, which is easy to gather into agglomerates
.

Especially in the case of hot and humid summer, the powder is more likely to agglomerate, in the fluidization barrel, it is difficult to fluidize, and a uniform mist cannot be formed, resulting in uneven powder from the spray gun and a decrease
in spraying efficiency.

Therefore, controlling the particle size distribution of powder, especially reducing the content of fine powder, can also improve the storage stability
of powder coatings.

Third, the method to improve the storage stability of powder coatings

From the perspective of powder coating itself, through the above analysis, we can improve the storage stability
of powder coatings for aluminum profiles from the following aspects.

1.
Without affecting the performance of powder coating and coating film, try to choose a resin with high Tg, and appropriately increase the content
of color filler in the formula.

2.
According to the performance of different products and customer requirements, select the appropriate acid value and active resin, strictly control the type and amount of matting agent, and avoid the use of accelerators
.

3.
Use less and use other auxiliaries with caution, communicate with raw material manufacturers, and improve the storage stability
of raw materials.

4.
During powder production, control the particle size distribution, especially to reduce the content of
fine powder.
After the powder is milled, wait for the powder in the container to cool before sealing or ventilation in the workshop to prevent the powder from being too hot
.

5.
In the manufacturing process of powder coating, fumed silica, alumina C and other loosening agents
with large specific surface area and strong moisture absorption capacity are added externally.

The isolation layer between the powder coating particles is formed, which reduces the chance of collision and agglomeration of powder coating particles, and makes the powder coating not easy to agglomerate, thereby improving the storage stability
of powder coating.

Because it is necessary to balance the performance of powder coatings and coating films, the Tg of powder coatings cannot be designed very high
at the current level of technology.

Therefore, after the powder coating is produced, its storage stability has been fixed, in order to extend its storage period, the user's storage environment is also very important
.
For powder coatings for aluminum profiles, it is recommended that users should meet the following conditions during storage, and use the
powder in as short a time as possible.

Storage temperature: not more than 30 °C, preferably below
25 °C.

Storage environment: should be kept ventilated and dry to prevent direct sunlight from irradiating the powder; The location where the powder is stored is not allowed to be close to the fire and away from heat sources
.

Other precautions: to prevent water accumulation or dripping in the warehouse, powder coating packaging boxes need to be placed on the shelf or on the wooden board isolated on the ground, and the stacking height should not exceed 4 layers
.

IV.
Conclusion

To improve the storage stability of powder coatings, not only powder coating raw material manufacturers need to design raw materials with high Tg, but also powder manufacturers need to design formulas and control the production process in a targeted manner, and also need powder coating users to improve
the storage environment.