Cause of pinhole in powder coating polyester and how to improve it

Cause of pinhole in powder coating polyester and how to improve it

Li Zhongliang, Wang Miao (Wuhan Yincai Technology Co., LTD.)

 abstract 

The anti-interference and anti-pollution ability of polyester resin in powder coating is different, which mainly depends on the amount of isophthalic acid and ethylene glycol used, which represents the ability of anti-pollution of this polyester. This paper propose a solution of how to balance the cost and the ability of anti-pollution.

I. Causes of shrinkage

To resolve shrinkage cavities, we must first understand their formation mechanism. Simply speaking, shrinkage holes are caused by poor anti-pollution, the theoretical presence of highly incompatible materials. This occurs when a small unmoistened particle at the bottom interacts with incompatible resin layers, creating a pronounced subsidence vortex. Sublime substances prone to cavity formation typically belong to low surface tension categories.

The formation of shrinkage holes has internal and external causes

1.1 Regarding internal factors, for instance, when using the same formula powder material in a well-mixed extruder, the resulting powder exhibits better anti-contamination properties and improved wettability. This phenomenon becomes particularly evident in extruders with lower precision or aged components. The reason why single-extrusion compresses more holes while double or triple extrusions compress far fewer holes lies precisely in this mechanism.

2.2 External causes of pinhole formation, condensation of aerosol (water vapor and oil and gas), airborne dust (powder of different brands of polyester, silicone powder, acrylic powder) can be caused.

Internal factors cannot be completely eliminated, but external causes can be minimized through measures like regular de-watering and degreasing purification at spray booths, along with maintaining hygiene in production facilities and spraying environments. For powder formulation, selecting polyester brands and adding additives can enhance film anti-stain properties. The primary additive is flow promoter (commonly called leveling agent), which effectively eliminates orange peel but shows weaker shrinkage hole reduction. A wetting enhancer (also known as brightener) is then added to address shrinkage holes, though it doesn't eliminate orange peel. Typically, leveling agents alone suffice, but their combination becomes necessary when shrinkage holes are likely to occur.

II. Factors affecting the anti-soiling properties of polyester resin

Polyester resin manufacturers have many models, but the production of polyester process is not very different, the quality of anti-pollution depends on the selection of raw materials. Some polyester has good anti-soiling properties and only needs to add a leveling agent. Some polyester has poor anti-soiling properties and needs to add a brightening agent.

How can we effectively utilize polyester raw materials and ensure their synergistic effects to minimize shrinkage cavities? To address this, we must first understand the essential parameters of polyester resins used in powder coatings. These parameters—including average functionality (fn), acid value (AV), number-average molecular weight (Mn) (2000-6000), glass transition temperature (Tg) (55-70°℃), softening point (95-115°℃), and viscosity index (VIS)—must be maintained within specific ranges to guarantee storage stability and processing integrity. The viscosity index is calculated as fn=AV * Mn/56100, with both viscosity and Tg being directly proportional to molecular weight and Mn values. Critical factors affecting coating performance—such as water resistance, weather resistance, heat resistance, light retention, impact resistance, stain resistance, and leveling properties—are determined by the selection of appropriate raw materials.

The following table shows the effects of various acid raw materials on synthetic resin:

Among dicarboxylic acids, isophthalic acid (IPA) demonstrates the highest stain resistance when used exclusively, commonly referred to as ultra-resistant polyester. Therefore, the usage amount of IPA serves as a key indicator of this polyester's stain resistance. While other acid-based raw materials also exhibit good stain resistance, their application levels are subject to specific limitations.

The following table shows the effects of various alcohol raw materials on synthetic resin:

Among alcohol-based raw materials, ethylene glycol and diethylene glycol are relatively inexpensive. However, excessive use of diethylene glycol significantly reduces the resin's glass transition temperature. To control resin costs, ethylene glycol is primarily used in larger quantities. As the usage increases, the film's stain resistance noticeably declines, leading to visible pinholes. This makes eliminating shrinkage holes the primary challenge in this process.

As discussed above, selecting weather-resistant polyester without additives can improve stain resistance. The primary reason for polyester's poor stain resistance lies in its heavy reliance on ethylene glycol as a raw material. While this approach reduces costs, it inevitably compromises key performance characteristics such as weather resistance, impact resistance, and stain resistance.

III. How to improve the anti-soiling of polyester resin

The above describes the main reasons why polyester is prone to crater formation. The use of ethylene glycol is used more to reduce the cost, and other raw materials with better anti-fouling properties are less expensive. Next, we will try to find ways to eliminate contraction cavities 。

First of all, the design of polyester formula can be appropriately added branched monomer raw material acid or alcohol to improve the molecular weight. Large molecular weight has better anti-soiling, so as to appropriately improve the anti-soiling of polyester in powder coating and reduce the generation of shrinkage.

Secondly, the lightener is used in the powder coating formula to eliminate shrinkage. The following is the test of polyester with the same raw material with acid value of 30-35:

Note: 8% ethylene glycol content refers to the total amount of raw materials

As can be seen from the above table:

1. The polyester A without ethylene glycol raw material has good anti-fouling properties, so it can not be used to add a brightener or shrinkage. However, adding a brightener will affect the leveling of the coating film.

2. The linear ethylene glycol content of 15% polyester B has obvious shrinkage holes in the coating film without adding brightener, and the shrinkage holes can be eliminated only by adding brightener.

3. The content of branched ethylene glycol is 8% polyester C. Without adding optical brightener, the shrinkage hole is less than B, indicating that the anti-pollution is improved. Adding optical brightener also has no shrinkage hole.

                                                                        A                                              B                                       C

Powder compatibility test (50:50):

In previous compatibility testing, we predominantly used polyester materials with excellent anti-fouling properties, particularly those from different manufacturers. The results revealed that when two powders with poor compatibility were mixed, the resulting coating maintained uniform flatness but exhibited reduced gloss and developed a fog-like surface. As shown in the table, ethylene glycol-containing polyester demonstrated distinct compatibility issues: while the fogging phenomenon remained subtle, orange peel texture became pronounced, with the latter becoming more evident as the ethylene glycol concentration increased.

The above is  test with 8% ethylene glycol consumption. What if we continue to increase the proportion of ethylene glycol . What will happen? The following is the extrusion test of branched ethylene glycol content 15% polyester D:

As can be seen from the above table, with the increase of ethylene glycol consumption, adding a light enhancer in the extrusion formula can only partially eliminate shrinkage. At this time, we need to choose a stronger anti-interference C-2122 (Wuhan Yincai) and add it into the polyester synthesis to ensure more uniform mixing. The following is the extrusion test experiment:

It can be concluded from the above experiments that polyester D with poor anti-interference has exceeded the ability of the light enhancer to eliminate shrinkage holes, and only when polyester D is heated and C2122 is added in the molten state can shrinkage holes be eliminated in the coating film.

If the ethylene glycol content continues to increase, the effectiveness of C2122 in eliminating shrinkage cavities will reach a limit. When the ethylene glycol content reaches 20% or higher, the resin should be prioritized for use in indoor 70/30 mixed formulations. Alternatively, the resin's acid value can be increased (e.g., designed for 45-55 acid value) for application in indoor 60/40 powder coatings, thereby reducing the resin's usage ratio.

 Conclusion

The experimental results indicate that polyester formulations containing higher ethylene glycol content require the incorporation of branching-type additives (such as trimellitic anhydride TMA or trimethylolpropane TMP) to enhance molecular weight. When combined with Wuhan Yincai's anti-interference agent C2122, this formulation achieves polyester with superior stain resistance.