Photooxidation and Fading

Fading is generally an accepted featnre of WPCs, probably becanse people get nsed to it with common wooden decks hence, this phenomenon has a kind of grandfather status. However, some composite materials fade less than others, and some mnch more (Fig. 1.46). 

Clearly, cnstomers generally prefer to have their deck not fading at all. However, they are either not informed on the prospective fading or do not know that some WPC practically do not fade, or accept the fading as it is. When the sun irradiation on their deck is nniform thronghont the day, it does not create a problem. However, in many cases just after several months the difference in color of their deck is too noticeable (see Fig. 1.1). 

Fignre 1.46 shows a difference in fading (in terms of lightness) of 32 commercially available WPC deck boards after 1000 h of the accelerated weathering. A difference between AL (on the Hnnter Lab color scale) is between 0.4 and 35 L units. In a simplified manner, one nnit is the first shade difference that the naked eye can 

It is rather difficult, if possible at all, to quantitatively translate the fading in the weathering box to the real world. However, some very approximate comparisons can be made. Depending on the material color, one day in the weathering box under standard conditions (340 nm, 0.35 W/m, 102 18 cycle, 63°C black panel temperature) often corresponds to 9 + 4 days of natural weathering in the U.S. Midwest and New England. This figure is often called the acceleration factor. In Arizona and Florida, the acceleration factor is about 40% of the above, that is, around 3.5. 

Fading of composite materials depend on many factors, some of them are related to the WPC composition (wood fiber content, type of cellulosic fiber, amount of UV stabilizers and antioxidants and amount and type of colorants) and some to the outdoor conditions (covered or open deck and amount of moisture on the deck and other climatic conditions). It does not appear that processing of WPC and the profile manufacturing noticeably affect the material fading. 

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Hindered Phenols. Because of the relationship between photooxidation and discoloration, we became interested in the effect of hindered phenols which are well-known oxidation inhibitors. A preliminary study was carried out to determine the relative effectiveness of a variety of such compounds, many of which are commercially available. The additives were incorporated into polystyrene at 0.1% concentration, and 50-mil molded plaques were exposed to a carbon-arc Fade-ometer. 

Minerals often contain admixtures (such as free metals) which catalyze thermo-and/or photooxidation of the filled plastic. This subject will be considered in more detail in Chapter 15. Here we give just two examples of fading of CaCOs-fllled HDPE and polypropylene, with 76 and 80% w/w of the filler, respectively. The matrix had the melt flow index equal to 1 g/10 min (HDPE) and 8 g/10 min (polypropylene). Ashing of the both filled plastics at 525°C gave ash content of... 

However, photooxidation has its own specific manifestations, and, first of all, changing the material s color or fading (Fig. 16.1). Hence, we will focus in this chapter primarily on fading, what causes it, how to characterize it, and how to minimize it. We will also show how much commercial composite deck boards differ in terms of fading. 

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