Artificial Weathering using Oxygen Plasma Erosion

 Plasma Erosion Equipment

Scope and Field of Application

Previous work carried out by PRA has shown that when paint films are exposed to a low temperature oxygen plasma they degrade in a manner similar to that observed during natural exposure.

Oxygen plasma produces extremely rapid degradation and a few hours exposure is equivalent to several years natural weathering. It should be emphasised however that this method is still in the development stage and that the operating conditions of the plasma chamber need to be established for each type of paint tested.

Like all laboratory tests, the method described here has its advantages and limitations and these are summarised, in general terms, on our Testing Paints for Resistance to Weathering page.

The main disadvantages of the oxygen plasma erosion method are:-

The main advantages of the oxygen plasma erosion method are:-

In addition to this method you may also wish to consider the advantages and limitations of the enclosed carbon arc, fluorescent lamp and the xenon arc tests.

 

Summary of Method

The test is carried out using a Polaron Model PT 7300 Plasma Barrel Etcher. Polaron is now owned and managed by Quorum Technologies and they can supply equipment of this type.

The Plasma Barrel Etcher is designed for the oxygen plasma erosion of samples prior to examination using scanning electron microscopy. The test sample is placed in the chamber which is then evacuated and refilled with oxygen at low pressure (0.2 mbar). Radiofrequency power (50 to 150 watts at 13.56 Mhz) is then fed into the chamber via capacitor plates in order to generate an oxygen plasma.

A number of coated panels are produced for each system under test and these are exposed sequentially for increasing periods of time ranging from 5 to 90 minutes. The degree of degradation of the coatings is then measured either by gloss or degree of chalking. A plot of gloss or chalking versus time is then produced. Comparison of these plots enables the relative rates of degradation of the test systems to be compared.

There are a number of experimental variables including:-

All our experience to date has shown that it is not possible to create one set of conditions that is applicable to all classes of coatings. What we have been able to do is adjust the conditions so that we can replicate the degradation that occurs when the same class of coatings are exposed at the same location.

We have shown, for example, that the method is better than Q-UV weathering at predicting the exterior durability of polyester powder coatings. The oxygen plasma method produced in 90 minutes a degree of degradation equivalent to about 1 year in Scandinavia. This compared with the 2000 hours needed for the Q-UV method.

 

Sample Requirements

The technique involves exposing the samples to an oxygen plasma at low atmospheric pressure. Consequently it is best to avoid samples that have significant amounts of residual volatiles or entrapped air. We can test samples on wood provided they can be supplied as coated strips no more than 3 mm thick. Samples on paper are no problem but metal panels tend to heat up 80°C in the chamber. We have found glass microscope slides ( 76 x 51 mm ) to be the ideal substrate. Samples should not be smaller than this. The maximum size is 150 x 100 mm x 10 mm. We require 12 panels for each sample to be tested.

If you wish, we can prepare the test panels. The quantity of liquid paint we needs depends on the method of application. We will be pleased to advise you regarding our requirements.

 

Accreditation

Although PRA is accredited to ISO 17025 by the United Kingdom Accreditation Service (UKAS), we are not accredited to carry out this particular test.

 

Please contact Peter Collins for further details.

 

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