Key aspects of powder coatingquality on architectural aluminium

Mr. Xuezhu PanPresident of Qualisino

Contents

1.Situation in China

2.Mill finish profiles to powder coating

3.Pretreatment processes

4.Powder coating materials

5.Coating processes

6.Summary

Production of aluminum profiles in 2017

Extrusion profiles

Finishedprofiles

Anodizingcoating

E.p. coating

powder coating

liquid coating

Output（million tons)

19.5 14.63 2.19 1.17 10.82 0.45

Ratio(%） 100 75.0 11.3 6.0 55.5 2.2

Statistics

• More than 650 applicators• Around 300 vertical powder coating lines• 18 vertical lines in one largest applicator’s plant• 10.82 million tons of powder coated aluminium profiles in 2017• Total profiles production 19.5 million tons• 550,000 tons of powder used for architectural aluminium

The quality of the powder coatings will be depended on the following factors:

1.Mill finish quality 2.Pretreatment processes3.Powder coating materials4.Coating processes

It is a systematic items from the all aspects.

There are some defects such as the convex-concave lines or die lines, pick-up,scraps, grease dirts on the profiles surface.

These defects would affect the appearance of the powder coatings to be rejected by the customers.

Mill finish quality：

The convex-concave lines may be the cross location in the section, see the picture as follow. After coating ,a line may be shown on the profiles surface.

convex-concave lines

Mill finish quality：

The convex-concave lines may happened in these sections as follow,the lines will be obvious after powder coating.

Mill finish quality：

The die lines may be the coarse in the surface. After coating ,a coarse srface may be seen on the profiles surface.

The pick-ups are the metal lumps on the surface caused by the friction between the aluminium and the die bearing .The pick-ups on the mill finish profiles can be changed into inclusions after powder coating . It would be the defects to be rejected by the user.

Mill finish quality：

The pick-ups electron images

Mill finish quality：

Scraps are the chippings after sawing the profiles ,if the scraps would be left on the surface , hard to be removed by the degreasing rinse and dipping into the coating , cause the poor coating quality.

Grease dirt may be a problem to the prtreatment to cause the detachment of the coatings.

Mill finish quality：

It is estimated that the Cr-free pretreatment lines may occupy 90% of the total powder coting lines in China. Many Cr-free pretreatment suppliers are certificated by Qualicoat.

Recently five years, it is no large negative report about the Cr-free pretreatment in the powder coating, shown that the Cr-free pretreatment processes are grown-up in China.

But the operation in the practice should be strict and the controlling data of production must be narrow and accurate.

More concern about the filiform and the weathering may be appeared for the requirements of the customers.

Pretreatment quality

The Cr-free pretreatment lines must be controlled strictly in all of the operation parameters.

Etching degree: ≥1g/m2 seaside ≥2g/m2

Rinsing : pH ,Conductivity, narrow range Cr-free conversion: pH ,Conductivity , conversion coat weight

narrow rangeAl3+ content, ≤ 0.1g/l

Concern on the bottom of the profiles in the vertical linges Vertical lines ,large mass production,

discharged the solution often. Check the powder coating adhesion onsite frequently.

Pretreatment quality

The powder coating materials quality is very important for the powder coating quality, it affects the weathering performances, integrity protection, resistance to moisture, coating processing

performances, etc.

The powder coating materials quality deponds on the composition , particle size, etc.

Powder coating materials quality

The weathering performances depond on the powder coating composition , such as the types of the resin, the ratio of valid resin, fills , additives ,etc,.

The more excellent weathering performances the powder coating have more valid resin added in the powder coating.

The valid resin could be measured indirectly by the checking the ignition residues after stoving . Example as follow table :

Table1. The relation between the weathering results(1 year) and the ignition residues

Powder coating materials quality

No. Class RAL powdertypes

original gloss

Gloss retention/%

∆E∆E

Requirement/≤

Gloss requirement/

≥

Igniton residue (maas)/%

42 Ⅱ 9010 bright 79.2 P 90.7 P 0.5 2.0 50 good 35.5

43 Ⅱ 9010 brigtt 65.7 L 46.9 P 0.8 2.0 50 bad 41.3

Picture 1. No.42 Powder RAL9010 ignition residues SEM diagraph(small particles)

Powder coating materials quality

Picture 2. No.43 Powder RAL9010 ignition residues SEM diagraph(coarse particles)

Powder coating materials quality

The powder coating integrity protection affects the powder coating quality. Otherwise the powder coating integrity protection depond on the powder coating materials quality.The corner coverage ratio may cause to the filiform on the aluminium windows and doors due to the un-integrity of the powder coating. See the picture as follow:

Powder coating materials quality

The applied example of water spot

The quality of the resistance to the moisture for the powder coating depond to the powder coating materials componets.

Powder coating materials quality

The results according to the water spot testing method from Qualicoat Specification 15th Edition Update sheet No.8

Powder coating materials quality

The quality of the powder coating have a huge influence to the powder coating spray processes, such as the deposition efficiency of powder spray , the coverage of the groove on the profiles. This property of the powder coating materials depends on the content of the resin , to be able to be determined indirectly by checking the ignition residues after stoving. The higher content of resin, the higher deposition efficiency of powder spray, the more better coverage for the profiles groove.

The particle size and size distribution of the powder coating can affect the deposition efficiency of powder spray , the coverage of the groove on the profiles too.

Powder coating materials quality

The particle size distribution curves of the powder coating

Coating processes

The coating processes have large effects to the powder coating, such as the curing processes, spray processes. Especially the curing temperature curves are important.

Summary• Mill finish profiles have great effects to powder coating quality .• Pretreatment processes have important influence to the powder

coating quality.• Powder coating materials have the lagest effects to the powder

coating quality.• Coating processes have very important influence to the powder

coating quality.• Therefore, the powder coating quality is the systematic items

depending on many aspects.

• Thanks a lot for your attention.