Case Study - Leaking Skylights

Smithers Rapra were asked to comment on the failure of skylights that resulted in water ingress.

History

During a period of high winds & rainfall an office complex suffered serious disruption & consequential losses as the result of water damage. Entry was via six skylights that had been installed for five years.

Inspection

The skylights had been manufactured by vacuum forming PVC sheet. Severe discoloration & reduced transparency indicted advanced photo-oxidative degradation. A lack of discoloration of material protected by the peripheral gasket confirmed the diagnosis.
As shown in Figure 1, the product had also suffered from distortion. An edge had retracted & this was the entry point for rain water.

Failure Diagnosis

The distortion of the skylight was caused by heat reversion, which is classed as a 'memory effect'. The sheet had been moulded at modest temperatures & the significant residual stresses were distributed in such a manner as to promote reversion back to a flat sheet. This change will only occur at elevated temperatures or more specifically as temperatures approach the glass transition temperature (Tg) of the material. The claimed Tg of rigid PVC is ~ 80 °C.
The surface temperature of a failed skylight was measured on a cloudless day. With an air temperature of 29 °C the surface temperature of the PVC reached 63 °C. This high temperature is due to solar energy absorption, greatly enhanced by the UV induced opacity of the material.

Although 63 °C is comfortably below the claimed Tg it should be emphasised that Tg reduces with increasing test or exposure period. Most published data result from short-term tests (a few minutes) whilst problems in service can accumulate over months or years. It is likely that the effective Tg of the PVC is less than 70 °C over a time-scale of months.

Lessons & Consequences

1. The critical temperature for heat reversion decreases with exposure time. Therefore at constant temperature the distortion of products may not occur in the short-term but may develop in the longer term.
2. In this case & in addition, the service temperature increased with exposure time due to discolouration & a reduction in transparency. The accumulation of dust & dirt would have induced a similar effect.
3. Separate tests to determine thermal & UV stability would not have revealed the antagonistic synergy between the two.
4. Polycarbonate with its higher heat reversion resistance was recommended as the replacement.