What undesirable phenomena about "surface tension" when making powder?

Coated orange peel

01

Coated orange peel is a local vortex effect of liquid flow during the film formation process of the coating, also known as Bernard's vortex, which is due to the change in the viscosity of the powder coating when melting leads to a change in surface tension, so that it is formed: the high viscosity and low surface tension of the coating sink to the middle of the vortex, (that is, the later depression); The low viscosity and high surface tension coating rise to the periphery

02

In the choice of leveling agent for powder coatings, there should be two effects, namely wetting effect and leveling effect, the wetting effect is to increase the system when the powder is at about 100 ° C, leveling combination, at this time the surface tension is as small as possible, that is, greater than 150 ° C when the leveling effect should be the main, the surface tension of the powder should be larger, to increase the leveling

03

During the selection of powder coating leveling agents, the SITA surface tension meter can be used to dynamically measure the surface tension of the change, preventing the phenomenon that the paint tension is too large or too low

Coating shrink holes

01

Coating shrinkage refers to special defects

02

When the surface tension or surface energy of the powder coating and the substrate is not adjusted well, if the surface of the substrate can not match the surface tension of the coating film is not too serious, or the coating is very thick, there may be poor adhesion phenomenon, and in severe cases, delamination
may occur.
In addition, we sometimes encounter some flower spots when making metal powder, which may be that we are spraying substances
that bring in some low surface tension.

03

Surface tension as a key factor in coating film formation, the reasonable use of surface tension meter for raw material screening and formula optimization, can solve many film effect problems, to create a more perfect paint coating performance
.

04

In order to reduce the effect of shrinkage of the coating, the use of a wetting agent with low surface tension can effectively wet and well disperse the particles
that cause the shrinkage of the coating.

05

In addition, our powder coatings sometimes need to screen text or patterns on the top, glue, or spray a layer of paint on the surface, often the fastness is not enough, which is mainly because the surface tension of the powder coating is too low, and the wax powder in our formula is a kind of additive, which also reduces the surface tension of the coating while providing smoothness, resulting in instability
of the silk screen.

Pinhole fine lines

01

This is due to the optical phenomenon
formed by the diffusion of light due to the many pinholes or fine textures on the surface of the coating film.
Eliminate pinholes or these subtle textures, and the fogging problem is solved
.

02

Statistics and analysis found that most of the pinholes and fine lines are caused by
interspersing and interfering with different polyesters after the material is extruded.

03

In addition, it was found that these problems were more likely to occur when making small samples
, especially in the case of small unloading volumes.
The greater the difference in gelling time, the more serious
the interference between powders.
When this mixed powder is extruded again, the interference phenomenon is very small, and some are even eliminated
.

04

In the production, it was also found that the use of epoxy resins from the same manufacturer and the same type of polyester, in the process of mass production of a powder coating, there will also be pinholes and fine lines on the surface of the coating film from time to time, more and less
.
In addition, the high volume concentration of pigment filler is prone to this phenomenon
.

05

In addition, when sampling spray plates, sometimes the highlighter plate is very smooth when it is just taken out of the oven, and as the plate surface gradually cools, some plates will show pinholes or fine lines
.

06

The powder is heated up to a certain temperature when it begins to melt, due to the exchange of heat and the action of the surface tension difference to produce countless small turbulence, with the rise of temperature, the powder continues to melt, turbulent, leveling, and at a certain temperature, the resin begins to crosslink, the viscosity of the paint begins to increase, with the increase of temperature and the continuation of time, the viscosity of the powder coating is getting larger and larger, the rate of turbulence and surface tension balance is getting slower and slower until it stops, and the powder coating begins to appear gelling state.
Then with the increase of temperature or the continuation of the holding time, the rate of crosslinking of the resin tends to be infinite - film
formation.

07

When different powders are mixed, if the curing rate of the two (or above) is different, then part of the glue solidifies first, while the other part is still turbulent, so the two form an interface, the coating film is all cured after the interface is fixed, bumps or folds appear, shadows are formed when illuminated, if the interface is very small bumps or folds can not be seen, and then the large naked eye is pinhole, large enough when the naked eye is fine lines
.
Squeeze it out again, and the interface becomes less dispersed
.

08

If the kneading and mixing effect of the extruder is not good, especially when the volume concentration of the powder pigment filler is high, the dispersion of the pigment filler is very uneven, and after the powder is powdered, some of the powder pigment filler content is low, and some are very high
.
The low one is easy to form turbulence when baking, and the high one is not easy or cannot form turbulence, thus forming an interface, causing bumps or folds, and the phenomenon of
pinholes or fine lines appears.

09

The purchased resin, between different batches, the indicators of resin reaction group content vary greatly, often resulting in inaccurate formula, resulting in curing into a film, the coating film contains different degrees of unreacted thermoplastic resin
.
Or the resin of the scrubbing machine is also involved in the grinding when making a small sample, so that a certain resin in the powder coating is
excessive.

10

In the change of hot and cold, because the shrinkage rate of the thermoplastic resin is greater than the shrinkage rate of the thermosetting resin, therefore, after cooling, the coating film is cooled, due to the difference in shrinkage rate, a convex or fold is formed, so that the coating film produces pinholes or fine lines
.
This is also why the high-gloss board is very smooth when it is just taken out of the oven, and as the plate surface gradually cools, some plates will show pinholes or fine lines
.

11

This problem is more common
in the production of powder coatings.
In order to reduce or eliminate pinholes or fine lines, on the one hand, it is necessary to carefully clear the machine, focusing on cleaning the part after the tablet roll, focusing on the grinding chamber, especially when
changing large varieties.
On the other hand, the selection of raw materials should be noted, especially the pigment filler, it is best to choose a regular manufacturer, choose a stable, temperature-resistant, easy to disperse products
.
The formula should be reasonable, and the amount of filler should be determined according to the effect and process of
the extruder.