How Does the Pigment Content (TiO₂ 0.5% vs. 2%) in a White PFA (Perfluoroalkoxy) Sheath Heater Change the UV Degradation Resistance When Used Outdoors with Occasional Sunlight Exposure?

White PFA sheath heaters with titanium dioxide (TiO 2 ) pigment are used for outdoor applications with minimal exposure to sunlight to reflect UV rays. UV protection is limited at 0.5% TiO2 and the polymer surface degrades (chalking, cracking) after 2,000-4,000 hours UV exposure. The UV resistance with 2% TiO 2 is significantly improved with the time to degradation being 8,000–12,000 hours. The TiO 2 particles absorb and scatter the UV light thus protecting the polymer chains.

The Mechanism of TiO2 UV Protection

TiO_2 is a UV-absorber having a bandgap of 3.2 eV (anatase) or 3.0 eV (rutile). It takes in UV light (300-400 nm) and changes the energy to heat. The particles additionally scatter UV light . This increases the travel length through the polymer . More absorption and scattering sites are provided by higher TiO₂ content. If the surface is not handled adequately TiO₂ can promote polymer breakdown. Recommended: Rutil TiO₂ coated with alumina/silica.

UV Degradation Quantified as a Function of the TiO2 Content

PFA has shown the following in QUV controlled accelerated UV testing (UVA-340 lamps, 60°C). The time to surface chalking at 0 % TiO₂ (natural PFA) is 500-1000 hours and the time to cracking is 1000-2000 hours. The time for chalking is 2,000–4,000 h and the time for cracking is 4,000–6,000 h at 0.5% TiO₂. With 1 % TiO2 the chalking time is 4,000-7,000 hrs and the cracking duration is 7,000-10,000 hrs. With 2% TiO₂, the chalking time is 8,000-12,000 hrs and the cracking time is 12,000-20,000 hrs. At 5% TiO₂, the chalking time is 15,000-25,000 hours and the cracking duration is 25,000-40,000 hours.

TiO2 Content Selection Guide For Outdoor Applications

TiO₂ Content (%) Time to Surface Chalking (hours) Time to Cracking (hours)Recommended for Outdoor Use
0 (natural) 500-1,000 1,000-2,000No 0.5 2,000-4,000 4,000-6,000 Limited 1.0 4,000-7,000 7,000-10,000 Yes (occasionally) 2.0 8,000-12,000 12,000-20,000 Yes
5.0 15,000-25,000 25,000-40,000 Yes (under way)
The Knowledgeable Specification

For a PFA heater, which is operated outside with intermittent sunshine, 2% rutile TiO2 content with surface treatment to inhibit photocatalytic activity is used. The engineer adds UV stabilizing pigment to PFA heaters to lengthen their outdoor life.