Assessment of Lightfastness

Q-UV weatherometer

Scope and Field of Application

Lightfastness is a term used to describe the resistance of a material to colour change when exposed to sunlight or to artificial light sources. When paints are exposed outdoors they generally become less glossy and more prone to cracking and flaking; they may also show colour changes or fading. Paints exposed indoors to sunlight that has passed through window glass generally degrade less rapidly the reasons being that they are not exposed to rainfall nor to the ultraviolet region of the solar spectrum ( glass does not transmit very much UV radiation).

In addition to paints, we can test other coloured items including printed material, textiles and plastics and these can be exposed directly to the light source or tested behind glass.

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

 

Summary of Method

We offer an in-house method based on the Q-UV fluorescent lamp weatherometer. In our experience, when this equipment is fitted with Q-UVA lamps, it takes between 400 and 500 hours to producing fading equivalent to 6 months natural exposure behind window glass in the UK. Between 700 and 1000 hours exposure are needed to produce fading equivalent to 1 year of natural exposure under the same conditions.

We use the Blue Wool Scale method to quantify the radiation dose and as a general guide, a sample of Blue Wool Number 7 fades to Grey Scale 4 after one year of natural exposure behind window glass in the UK.

The Blue Wool method is only suitable for quantifying internal exposure. If you require lightfastness tests to be carried out on samples intended for exposure outdoors, please consider the advantages and limitations of our carbon arc, fluorescent lamp and xenon arc methods for artificial weathering.

In addition to our in-house method, we offer a British Standard method based using xenon arc equipment.

 

Sample Requirements

Ideally 150 x 75 mm panels not more that 5 mm thick, although we can accommodate a limited number of 150 x 100 mm panels provided they are less than 5 mm thick. If you have smaller samples you can fix them to 150 x 75 mm panels provided the overall thickness does not exceed 5 mm. If your samples are larger, you may wish to consider the enclosed carbon arc method.

We recommend that samples are run in triplicate.

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.

 

Back to Paint Testing Laboratory Page

Back to Testing Paints for Resistance to Weathering Page

Back to top of page