ENVIRONMENTAL CONDITIONS AFFECTING PLASTICS

Table 4-5. ENVIRONMENTAL CONDITIONS AFFECTING PLASTICS 277 General Guide for Plastics Behavior in Sterilization Peformance ... 

Of the examples considered above, two are of phenotypic diversity in a life-history trait where the life-history trait under consideration is clearly a facultative phenomenon. That is, for developmental route in S. ratti and for arrested development, there are distinct, mutually exclusive developmental routes. Thus, diversity in these traits between different parasite lines is relatively easy to observe, as is the response to selection. Both these traits are, in part, affected by environmental conditions and so are phenotypically plastic. For S. ratti, variation in the sensitivity of this plasticity can also be seen. Although environmental sensitivity of arrested development is as yet uninvestigated, by analogy with S. ratti it is likely to vary. 

Many materials used for food and beverage packaging have characteristic odors or sensory active compounds (Torri et ah, 2008). The intensity and description of the odor may be affected by the number and type of volatile compounds that are released under environmental conditions at the time of evaluation. Chemical composition of the material and polymer morphology may play a role in the sensory characterization. Sensory descriptors do not define a specific chemical compound but may be related to different compounds, a blend of compounds, and even a limited concentration range of a compound or class of compounds. For example, frans-2-nonenal in water changes in sensory (taste) description from "plastic (0.2 gg/1) to "woody" (0.4-2.0 p.g/1), "fatty" (8-40 pg/1), and "cucumber" (1000 gg/1) (Piringer and Ruter, 2000). Such terms are descriptive of the sensation and perception by human response to the chemical stimuli (Table 2.1). 

It is necessary to thermally cure the coated plastic in order to achieve full crosslinking of the air dried film. The degree of crosslinking affects not only the abrasion resistance of coating, but also the adhesion and the long term performance under environmental conditions. The temperature and time of cure is also dictated by the thermal stability and properties of the plastic substrate. The following table lists recommended cures for various plastics. 

Polyethylene (PE) is a member of the polyolefin family, which also includes PP and various plastics with different molecular linearily, densities, polymerization processes, and substitution types. PE densities are relatively low with values ranging from 0.940 to 0.970 g/cm for HOPE and from 0.916 to 0.940 g/cm for LLDPE. Typically, these PEs not only have good processability (e.g., can be converted into bags, films, and bottles) but also exhibit an excellent water vapor barrier property, which is required for many water-sensitive food products such as dried and liquid foods. However, this type of plastic is not appropriate for easily oxidized food products due to its low oxygen barrier property. The properties of polyolefins can be significantly affected by environmental conditions and physical factors, such as the density, crystallinity, presence of free volume, polarity, humidity, and temperature [44]. 

Plastics are affected by stress, impact, temperature, and environmental conditions and these are all time dependent. Long-term behavior is the subject of another book in this series and is not discussed further here. 

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