Technical Resources Area
Technical Resources Area |
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Gassing when electrostatic-spray coating over galvanised steel is a common problem. In addition the remedies that are often used with thermoset powder paints such as polyesters are not always appropriate for thermoplastic powders such as Plascoat PPA 571 ES.
Gassing usually occurs when air or water resides in the porous areas of the galvanised surface. It varies considerably with the quality of the galvanising and Plascoat would always recommend that recent, fresh consistent galvanised steel is used and that low quality or old metal should be avoided.
However, it is appreciated that this is not always possible and the following guide may help to reduce the effects of gassing when coating with Plascoat PPA 571 ES
A) Types of Gassing
There are principally two types of gassing that can occur in the coating over galvanised steel.
a) Air gassing
This is caused when air is resident in the galvanised layer generally due to its porosity. On heating in the cure oven after spraying, this gas expands and releases through the coating to form a small “volcano” or crater.
b) Water gassing
This is caused by small quantities of water either residing in the galvanised layer when it is porous, or residing in hydrated zinc or other metal oxides that can form on older corroded galvanised metal. This type of gassing manifests itself by a large number of small “dots” in the coating similar in nature to human skin “bubbling” after being in the sun too long – just prior to peeling!
If this form of gassing is excessive the smaller bubbles will join up and form a larger blister where all adhesion will be lost.
Both these forms of gassing can seriously affect the quality of the finish and considerably reduce the lifetime of the coated article. If it cannot be resolved alternative sources of galvanised steel should be found.
The following guide assumes the presence of pre-treatment which includes a degrease unit, zinc phosphate and a drying (pre-bake) oven prior to powder spraying.
B) White Rust
The substrate must be clear of white rust.
White rust will not only reduce adhesion due to its powdery nature but also it holds water much more than the metal itself due to the fact that it is a hydrated zinc oxide.
White rust is therefore also more difficult to dry in the drying oven as it holds onto the water.
On heating through the cure oven, therefore, this water will be released as vapour and will form gassing and water blisters.
White rust can be removed
reasonably well by using a phosphoric acid etch with degreaser as the first
stage of the pre-treatment. A suitable degrease/cleaner/acid etch is Pyroclean
205 at 68
°F
for 2 minutes. Pyroclean is supplied by Chemetall. The contact numbers
in England are:
Phone +44 (0) 1908 649333. Fax +44 (0) 1908 373939. Other country contacts
can be found at www.chemetall.com.
There are usually similar products produced by pre-treatment companies
world-wide.
C) Infra Red Ovens
The intense heat of an infra red oven can cause a “skin” to form on the surface of the polyolefin coating. This “skin” is more viscous can therefore hold in gas or water vapour.
In addition if the surface skin is viscous, it will not easily reform when a gas bubble/crater is created.
It is therefore recommended that any IR section to a cure oven be turned off. Alternatively, if it is possible to run only a few heaters, turn off every other heating element to reduce the heat intensity.
D) Pre-Baking
Pre-baking by using the drying oven is a common method to reduce gassing.
In general to reduce gassing from trapped air the metal temperature in the pre-bake should reach about 30°C (55°F) higher than the temperature reached by the metal in the cure oven.
If the metal temperature exceeds 280°C (536°F), then the zinc phosphate can start to deteriorate into a powder, which will in turn reduce adhesion of the coating to the substrate.
Plascoat would recommend therefore that the metal temperature on exit from the pre-bake oven is measured and the temperature of the oven itself adjusted accordingly.
E) Alignment of Articles in the Pre-treatment Section
Generally speaking, items such as pipes or large pieces of framework are best hung vertically on the jigs. Any water that remains after the phosphate/rinse will run off quicker if hung vertically. This will make the article more likely to be dry on exit from the drying oven.
F) Cure Oven Temperatures
In general the temperatures in the cure oven after spraying should be as low as possible to minimise further gassing. It is usually better to use lower temperatures and longer times than higher temperatures for shorter times.
The advantage of the former is that firstly gassing is less likely and secondly that should any occur there may be enough time left in the cure oven to “reseal” any craters that may have formed.
G) Include a light Grit-Blast in the Pre-treatment
A light grit-blast of galvanised steel sometimes reduces gassing. The light grit-blast will not only help to clean off any white rust or other oxides, but also open up surface cavities, which may be the cause of trapped air. Blast pressures should not exceed 40 psi (0.3 MPa) and fine copper slag, J-Blast or carborundum powder are preferred abrasives. Steel/iron abrasives can lead to bimetallic corrosion, particularly if grit rather than shot is used.
Pre-treatment for PPA 571, PPA 571 Application over galvanised steel Recommended pre-treatment over galvanised steel.
PB/09/08/02
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