Oxidative degradation of plastics

Many metals or ions of metals are catalysts of oxidative degradation of plastics. The conversion of peroxides (RiOO ) to hydroperoxides (RiOOH) via abstraction of hydrogen from an adjacent polymer molecule (R2H) is typically the rate-limiting step for the propagating chain reaction of oxidation... 

In this chapter, by the term weathering we mean fading because other aspects of weathering, first of all resulting from oxidative degradation of plastics and WPCs, were considered in the preceding chapter. 

UV absorbers generally decrease the oxidative degradation of plastics and plastic-based composites. However, they do not always significantly change the fading of WPC materials. For example, wood flour (50% w/w, 40 mesh)-fllled HDPE (MFI 0.72) after an accelerating weathering in a box for 3000 h increased its L value by 115% after... 

Antioxidants and their amounts effect oxidative degradation of plastics and plastic-based composite materials in the bulk, as it was shown in the preceding chapter. However, they do not seem to significantly effect fading of WPC materials, at least in many cases (see, for example. Table 16.9). GeoDeck deck boards, whether they do not contain antioxidants (the OIT of 0.2-0.4 min, see the preceding chapter) or are loaded with antioxidants (the OIT of 16-20) show the same fading, or rather a lack of it. 

The thermal and thermal-oxidative degradation of plastics were extensively studied. These investigations, however, aimed mainly at studies of stability rather than the flammability of plastics. 

Wet ashing with sulfuric acid This is a very common technique. It involves oxidative degradation of plastic with sulfuric acid. 

Beyond the photic zone, photodegradtion is much slower the cold, anoxic deepwater environment is not conducive to oxidative degradation of plastics. The same is true of the marine sediment where plastics are believed to ultimately accumulate. While, the occurrence of plastics debris in the sediment has been demonstrated, interaction of such debris with bottom-dwelling organisms is not well understood at the present time (Watters et al., 2010). 

Three mechanisms describe the oxidative degradation of plastics, and although they may act simultaneously, usually one of the mechanisms is dominant ... 

The effect of primary antioxidants is based on reactions with polymer radicals containing oxygen that are formed during auto-oxidative degradation of plastics. These very reactive radicals are transformed into inactive or less reactive compounds, thus causing the end of radical chain reaction. Fig. 1.13, Section 1.4.2. The antioxidants are consumed during this reaction. 

The three most eommon disinfectants in potable water are ehlorine, chloramines and chlorine dioxide. While these disinfectants are all oxidants, their uniqne eharacteristics can residt in a significantly different impact on the performance of plnmbing system components. In this paper, the chemistry and characteristics of the oxidants are discussed in the context of oxidative degradation of plastic piping system components. Testing strategies to ensure material performance in potable water applications are presented and reviewed. 

This type of degradation can also be referred to as photodegradation or ultraviolet (UV) degradation. It includes photo-oxidation. It produces some of the more familiar signs of degradation of plastics embrittlement, discoloration and loss of transparency. 

Antioxidants are fairly widely used at a 0.01-0.4% level to prevent or retard oxidative degradation of certain plastics. Polymers may be subject to various forms of oxidative attack during all stages of their life cycle, i.e. from polymerisation through processing to final end usage. The more used antioxidants include ... 

In addition to gasification, other oxidative treatments of plastic and rubber wastes, excluding total combustion, are described in this chapter. These methods, although relatively unknown, may be of great interest in the future for the chemical degradation of polymeric wastes. 

Typically, composite deck boards are porous. The pores are formed by steam and by volatile organic compounds (VOC) during extrusion. Composite boards are partly foamed, and the pores are typically opened and connected to each other, forming chains of cavities. That is why composite materials absorb water, unlike many plastics. Air oxygen flows in, through these pores, and effectively oxidizes composite materials from inside, particularly at elevated temperatures, which often takes place on decks. Water, which is always present in composite materials, serves as a catalyst for the oxidation. Metals, which are often present in composites (as constituents of colorants, lubricants, biocides, fillers), also serve as efficient catalysts of oxidative degradation of composites. As a result, rates of oxidative degradation of composites are 50-100 times faster than those for their constituent plastics. 

When one measures all three averages and the polydispersity in the course of oxidative degradation of a plastic, neat, or WPC, he/she has a better idea how the material degrades in terms of its molecular chains. 

Stress in plastic composites and profiles can decrease the free energy of activation of oxidation processes and, hence, speed up the oxidation degradation of the materials in the stressed areas. Besides, stress decreases local densities (specific gravities) of composites and thereby increases porosity and provides space for oxygen from air to diffuse in and oxidize the material from inside. ... 

In reality, the effect of solar radiation on plastics and composite materials can lead to many outcomes. Typically, but not always, UV light causes discoloration, or fading of plastic and plastic-based composite materials. That fading reflects oxidative degradation of the material at the very surface. It may lead to a damage of the bulk of the composite material, or it may not. This all depends on the amounts of antioxidants in the bulk of the material. Composite material with high amounts of... 

The most adeqnate laboratory tool to stndy the effect of UV light on plastic and composite materials is a weathering box, or an environmental chamber, weatherom-eter, among others. None of them matches precisely the spectrum of natural UV light however, they provide all four major components of oxidative degradation of materials at conditions of natnral exposnre UV light, heat, water, and oxygen. 

Photooxidation of plastics and wood-plastic composites (WPCs) was described in principal detail in the preceding Chapter 15. It was emphasized that photooxidation acts in a synergism with thermooxidation of the materials, speeding up an oxidative degradation of WPC products, particularly being exposed to direct sunlight. 

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