QUESTIONS 1. Describe how ultraviolet light degrades plastics and why it does no
ID: 1766673 • Letter: Q
Question
QUESTIONS 1. Describe how ultraviolet light degrades plastics and why it does not generally degrade 2. Describe the processes of solvent/solute interaction that lead to swelling and, in some 3. Explain the relationship between plastic crystallinity and sensitivity to solvent attack. metals. polymers, dissolving of a plastic by a solvent. tionship between plasticizers and the glass transition temperature? more readily solvated than large molecules. 4. Explain why plasticizers increase flexibility and elongation in plastics. What is the rela- 5. Explain in thermodynamic and molecular structure terms why small molecules are 6. Discuss the physical factors in a plastic that afiect permeability. 7. Describe the key features of a polymer that would make it electrically conductive. 8. Identity three tests for thermal properties of plastic materials. 9. What is the limiting axygen index (LOl)? Which buns more easily, a material with an LOI of21 or of 17? 10. 11. Explain the flammability of aromatic versus aliphatic molecules fro Explain why laboratory identification of a piastic so often involves burning the plastic. m an energy perspective, assuming the bond strength of a C-C bond is approximately twice as much as the bond strength of a C-C and also that the bond strength of a hydrogen at- tached to benzene is about 20% higher than the bond strength of hydrogen attached to propylene 12. Low-density poyethylene is normally translucent. If a piece of this material is bent into a tight radius, it translates less light (becomes chalsy white). What is this phe- nomenon called? Explain why this happens. Expiain why the material often breaks after repeated bends.Explanation / Answer
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Question 1
Ultraviolet light is a part of electromagnetic spectrum having its wavelength in the range from 100 to 400 nm. This typical spectrum is composed of three sections namely, UVA, UVB and UVC lights. The effect of UV light on any material depends on many factors such as duration of exposure, type of material, molecular structure of that material, intensity of UV lights in W/m2 etc.
When plastics are exposed to the UV lights, plastic molecules absorb the energy in the form of light and thus initiate a chemical reaction. As a result of molecules having higher energy levels, transient excited states are created whose chemical and physical properties greatly vary from those of the original molecules. This process is called as ‘Photochemical reaction’. Energy of UV lights absorbed by plastic molecules excites the photons creating free radicals. In the presence of oxygen in the air, these free radicals react with it and form oxygen hydro-peroxides. These hydro-peroxides in turn break the bonds between the monomers of plastic and thus form a brittle structure of plastic (an isotope). This process is called as photo-oxidation. The change in plastic can be observed by yellowing of plastics.
Same as in plastic, the molecules of metals also absorb the energy from the light and get converted into highly excited energy states. However, the energy absorbed by molecules is sometimes, not sufficient to free the metal molecules from bonds. This is due to the fact that metal molecules are joined together by covalent bonds which are very strong. Thus even in the presence of oxygen, the photo-oxidation reaction cannot take place. Therefore, many metals are not affected by the UV lights. The UV effect on metals depends on how strong the covalent bond between metal molecules is.
This is the reason ultraviolet light degrades the plastic not generally the metals.