Photochemical degradation discoloration

Wood is an anisotropic material that undergoes uneven dimensional changes and, under extreme variations of environmental conditions, becomes distorted and warped (see Chapter 10). Exposed to the atmosphere, wood is also susceptible to the mechanical forces of wind and rain, and the effects of solar radiation the latter, in particular, causes discoloration initially, and then photochemical degradation, which often results in the wood s total decomposition. Wood is also prone to consumption by bacteria, fungi, insects, and rodent animals (Unger et al. 2001). 

Alternatively, the container may contain an opacifying agent, e.g., titanium dioxide. The use of multilayered bags was demonstrated to inhibit photochemical decomposition of vitamin E in TPN fat emulsions (Allwood and Martin, 2000). PVC films discolor on irradiative exposure due to photochemical degradation of the polymer (Hollande and Laurent, 1997). Plastic containers for parenteral use may contain several additives, e.g., antioxidants, stabilizers, plasticizers, lubricants, impact modifiers, and coloring matter when justified and authorized. In an appendix, the European Pharmacopoeia presents a list of plastic additives that may be used (European Pharmacopoeia, 2002). The additives should not be extracted by the contents in such quantities as to alter efficacy or stability of the product or to present any risk of toxicity (European Pharmacopoeia, 2002). However, organic additives extracted in concentrations below the detection limits of the analytical methods authorized by the European Pharmacopoeia may be sufficient to initiate photosensitized reactions in the formulation. 

The role of sequence distribution on photochemical discoloration SAN has recently been reported. It was shown, using FT-IR, that chemkal changes occurring during photochemical degradation are a result of attack of the SAS triad sequences [77], Fiuther study revealed that the chemical attack was oamrring specifically at the styrene repeat units within the SAS tri [78]. 

Major components of wood are cellulose, lignin and hemicellulose, and all of them are susceptible of photochemical degradation, behavior manifested by discoloration and loss of gloss and lightness, roughening and loss in mechanical properties [22, 36]. 

A special mention has to be made concerning the PVC-wood composites subjected to accelerated weathering they retained their initial mechanical properties in a higher proportion than the originating polymer [35, 58, 59]. The possible explanation of this behavior may be that the photo-oxidation takes place only at the surface of the composite where wood particles, considered as a chromophore material because of the presence of carbonyl groups, undergo photochemical degradation. Hence, all composite samples exhibited a more intense discoloration than the neat PVC. 

The SMP based syntactic foam may be used in various lightweight composite stmctures. Most of the time, the structures may be used outdoors with uncontrolled environmental attacks. The most obvious environmental attacks include ultraviolet (UV) radiation, moisture, temperature, and combinations of these single factors, such as hydrothermal attacks. Polymers are extremely sensitive to these environmental attacks due to the photophysical and photochemical effects. Often, the combination of UV radiation with oxidative and hydrolytic factors leads to more severe degradation than that from a single factor. The damage may range from mere surface discoloration to extensive loss of mechanical properties. 

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