Critical importance of coating UV curing GRP for resistance against UV degradation

Posted on: 28th Feb 2019

A review of the various websites of manufacturers of UV curing GRP describe UV resistance as "good resistance to UV" and "UV resistance - excellent" whilst others simply remain silent on the matter. None put forward any test results or quantified performance.

UV curing GRP has even been supplied to projects where the specification demands a forty year life for the cladding system.

ULVA does not make such claims but rather relies upon proper research and independent testing.

In 2014, as part of its product development programme, ULVA commissioned an independent study of the most commonly used UV curing GRP (test report D10508 dated 8 January 2015) to ISO20340 for a duration of 4,200 hours. One of the pass/fail criteria mandated by NORSOK under this test regime is that samples should retain 90% of original physical properties after 4,200 hours weathering. Within the study, one sample was uncoated and four samples each had different coatings. The uncoated sample lost 40% of its physical properties after only 1,100 hours whilst all of the coated samples retained in excess of 90% or original physical properties.

The report did, however, observe that each of the coatings was breaking down and a re-coating would be required for UV resistance to be maintained.

A more recent independent study was undertaken to ASTM G154 cycle 1 using UVA340 bulbs and in irradiance of 0.77W/m2, incorporating an 8 hour UV cycle at 60C with 4 hour condensation cycle at 50C. The duration of exposure was 4,200 hours (test report J1806 dated 18 February 2019). The uncoated weathered sample showed 69% retention of stress at break and 63% retention of strain after UV exposure whilst the coated example retained a higher stress at break and strain at break with 94% retention of stress and 74% retention of strain after UV exposure.  

The types of resins used in UV curing GRP have inherently poor UV resistance. However, because it is necessary for light to pass all the way through the GRP layer in order to activate the photo-initiator used in the curing process it is not possible to add sufficient UV absorbers or inhibitors that would normally be used to enhance UV resistance. The result is that uncoated UV curing GRP is subject to breakdown and failure which can be rapid in high UV exposed locations.

The attached photographs show a typical example of this failure mode from a moderate UV location. The resin at the upper (most exposed) area of the cladding has been degraded by UV exposure leaving only the residue of glass rovings which become porous allowing water to enter. The lower (less unexposed) area remains sealed thereby trapping the water inside leading to accelerated corrosion.  

The wet insulation in the lower part of the system allows the cold service temperature to be transmitted to the surface of the cladding which, in the warm and humid environment, generates constant surface moisture, hence the algae growth.

It is ESSENTIAL that where UV curing GRP is applied, regardless of manufacturer/brand that it is coated for the prevention of UV degradation.  Asset management plans must include provision to periodically re-coat all areas to maintain UV resistance, the frequency of which will be determined by local UV intensity.

Copies of reports referenced are available upon request.

Ulva Insulation Systems Ltd Countering corrosion under insulation (c.u.i)