The importance of UV protection for polycarbonate sheets is well known to all PC sheet producers and their customers.
When production of polycarbonate sheets started over 30 years ago, producers were not aware of the necessity of a protection system against ultraviolet light from solar radiation. Later, customers began to complain about this failure after only a short service life.
In order to protect the sheets, producers first employed a protection system based on an acrylic lacquer layer enriched with UV-absorbers. The acrylic layer was sprayed on the sheet during the process or applied by a special roller system. The lacquer provided very good resistance to solar radiation and, in terms of UV protection, gave better performance when compared to the co-extrusion of a UV layer.
On the other hand, the reliability of the lacquer layer was very poor. In many cases, the layer was easily peeled off from the sheet, especially when the sheet was subject to high-stress circumstances. High stress could develop, for example, from extreme weather conditions or when the sheet was installed in a curved shape.
Bayer (today Covestro) was the first to develop the innovative idea of protecting the sheet with a thin polycarbonate layer enriched with UV absorbers and applied on the sheet by co-extrusion technology. This was a breakthrough in the production of polycarbonate sheets (solid and multi-wall) with extended service life.
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| Most sheet producers monitor and adjust the thickness of the UV layer with the aid of an optical microscope equipped with a UV lamp that emphasizes the co-extruded layer. Thickness measurements need to be conducted across the sheet's width to guarantee complete protection. |
Originally, co-extrusion of an additional polycarbonate layer on top of the UV protective layer was required due to the high level of plate-out of the UV absorber at the die exit prior to the calibration stage. Later, high molecular weight UV absorbers were developed (e.g. Tinuvin 360) that minimized evaporation/migration levels on the calibrator inserts.
Sheet producers discovered that optimal UV layer thickness should be in the range of 30 to 40 microns (see Figure). The UV absorber concentration in this layer should be at least 5% or 6%, though higher UV absorber levels resulted in better UV protection performance.
The rheological properties of the UV layer should match the base layer in order to achieve uniform layer distribution.
Nevertheless, rheology isn't the only factor which affects the layer distribution. The feed block design also has a very important effect. Use of a poor quality or a poorly designed feed block can result in uneven UV layer distribution. As a result, there is a reduction in the efficiency of the UV layer and the process becomes less economical.
Finally, the plate-out properties of the UV compound should be as low as possible to enable smooth production without interruptions for cleaning the calibrators. Few companies offer compounded UV polycarbonate for sheet protection. The main suppliers are the polycarbonate resin producers although several independent compounders offer these compounds as well. However, quality must be tested carefully before the material can be approved.
In short, the UV layer is the Insurance Certificate of the polycarbonate sheet. Therefore, sheet producers must pay careful attention to sourcing the UV cap layer from a reliable supplier. Not less important is implementing the compound in a reliable process that gives a uniform thickness across the entire sheet width.
Gabi Bar
Polycarbonate Product Manager | Tosaf Compounds
