The difference between pinhole and shrinkage hole and its treatment

Pinholes, shrinkage holes are more common drawbacks in the use of powder coatings, their existence not only affects the appearance quality of the product, but also seriously affects the mechanical properties and protective properties of the product, and most of the corrosion of the protected materials begins to extend from this, thus losing the use value of
powder coatings.
Therefore, in the process of powder coating, it is of practical significance
to reduce the occurrence of pinholes and shrinkage holes as much as possible.

Pinhole refers to the time from melting to curing of powder coating, the gas in the powder coating passes through the high-viscosity, nearly closed elastic resin layer from the bottom layer to the surface of the coating, and fails to escape and the defective coating is coatingol.
com
.
It is formed on the surface of the paint film, which is caused by the rapid evaporation of the solvent during construction or the bubbles in the coating film cannot be leveled after overflowing, resulting in needle-like holes
after drying.

The surface defects of shrinkage hole coating film are mainly depression, pinhole, and shrinkage or thick edge of corners
.
There are two cases of surface depression of the coating film, one is a circular depression and the other is a hexagonal polygonal depression
.
The depression on the surface of the coating film is caused by the surface tension gradient, due to the change of coating composition and temperature change resulting in uneven surface tension, the fluid flows from low surface tension to high surface tension, resulting in the formation of depression on the surface of the fluid, also known as the Maragoni effect, and finally the edge bulge, the center sinks into a circular crater, or the edge bulges, the center sinks into a hexagonal groove Bernaldo vortex
。 There is a low surface tension substance in the center of the crater, and there is a surface tension difference between it and the surrounding coating, and this difference is the driving force of the formation of the crater, which causes the surrounding liquid fluid to diverge from it (crater hole source) and flow into a depression
.

01Several characteristics of shrinkage holes

NO.
1 Shape features

1) There is a core - there is a core particle in the middle of the shrinkage hole, which is hairy or granular, and the shrinkage hole is large and deep, exposing the substrate;

2) No core - there is no particle in the middle of the shrinkage hole, mostly located on the surface of the
coating.

NO.
2 Distribution characteristics

1) Uniform distribution - shrinkage holes are evenly distributed in all parts of the painted workpiece;

2) There is a characteristic distribution - the part is very concentrated, and even manifested as a comet;

3) Anisotropy - when the sprayed workpiece has an upward plane, there are more
shrinkage holes on the upper surface.

NO.
3 Time characteristics

1) When the temperature changes greatly in spring and autumn, it is easy to have the problem of shrinkage;

2) It is easy to occur when the compressed air drying equipment is abnormal;

3) It is easy to occur when the spraying equipment has a long downtime and restarts;

4) After the shrinkage problem occurs, the raw materials in the equipment are cleaned out, and the problem will be alleviated or solved
.

NO.
4 Formation Cause

The paint forms shrinkage holes during the application drying process, and there are problems with the paint itself and the cleaning of
the substrate.
Due to the existence of low surface tension droplets in the coating itself, or the surface to be coated has a low surface tension area due to pollution, resulting in uneven surface tension, the coating flows from low surface tension to high surface tension under the action of poor surface tension, resulting in the formation of holes in the center depression - shrinkage holes
.
We attribute the unevenness of surface tension to the internal cause
of crater formation.
In fact, some properties of the coating itself, such as coating viscosity, thixotropy, coating drying speed and coating film thickness, can aggravate or weaken the flow capacity of the coating fluid, thereby aggravating or weakening the degree of shrinkage, and we classify these factors as external causes
of shrinkage pores.
Internal causes are inevitable conditions for shrinkage, and external causes can appropriately control or aggravate the degree of
shrinkage.

After coating construction, in the process of drying and film-forming, the surface solvent volatilizes, the concentration of the surface polymer increases, and the viscosity of the coating increases, which will lead to surface tension and surface density exceeding the body, forming depressions, in general, thicker coating film (>4mm) liquid coating, mainly density gradient driven flow, thinner liquid coating is surface tension gradient control flow (Figure 1), forming a shrinkage hole
.

02 Characteristics of pinholes

Pinholes are produced by the discharge of gases (air or gas produced by reactions, etc.
), and shrinkage holes are caused by poor wetting, such as the gap between the surface tension of the substrate and the top coating is too small, or the substrate is contaminated, and some are polluted by matting agents or reeling agents in the upper coating - this kind is generally evenly distributed in the coating film, and there is no regular occurrence of when and where, and the shape is very small point-like shrinkage holes
.

Causes of formation:

1 Reasons for the formulation and manufacturer of the coating
.
Poor refining of varnishes, improper selection and mixing ratio of solvents, poor dispersion of pigment fillers, air bubbles and water vapor
are brought in in paint production.

2 The storage temperature is too low, so that the miscibility of the resin becomes worse, the viscosity rises or local precipitation occurs, which is easy to cause particles or pinhole defects (special asphalt coatings)

3 Stirring vigorously for a long time, mixing air into the paint, forming countless bubbles
.

4 The humidity of the construction environment is too high, the oil-water separator of the spraying equipment fails, the air is not filtered, and the moisture is brought into the spray with the air pipe during spraying, causing pinholes and even blisters
on the surface of the coating film.
The pressure is too high and the distance is too high when spraying, which destroys the solvent balance
of the wet coating film.
The force used during brushing is too large, and the speed is too fast when roller coating, so that the resulting bubbles cannot escape
.

5 After coating, in the solvent volatilization to the initial film-forming stage, due to the solvent volatilization too fast, or construction at higher temperatures, especially by high temperature baking, the coating film itself does not have time to fill the gap, forming a series of small holes, that is, pinholes
.

6 Improper surface treatment of the coated object, coating on the oily surface
.
The steel has a high moisture content, putty and seventh undry
.
The coating film is too thick at one time, the solvent cannot be volatilized in time, is wrapped in the coating, and after a period of time, the volatilization escapes to form pinholes
.