Thermal Degradation of PMMA

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Thermal Degradation of PMMA

  1. 1. Thermal Degradation of Polymethyl methacrylate M.Sc. Ümit TAYFUN CHEM 512 DEGRADATION AND STABILIZATION OF POLYMERS METU 2007
  2. 2. Poly(methyl methacrylate) <ul><li>PMMA, is the most important commercial polymer of the acrylic class. </li></ul><ul><li>It is an amorphous, linear polymer valued for its hardness, rigidity, transparency and weathering resistance. </li></ul><ul><li>Glazing applications exploit good impact resistance and dull shards formed upon fracture. </li></ul>
  3. 3. Applications of PMMA <ul><li>Major applications of PMMA include: </li></ul><ul><li>Automotive industry - rear lamps, light fixtures </li></ul><ul><li>Acrylic sheet - bathtubs </li></ul><ul><li>Glazing – signs </li></ul><ul><li>Composites - sinks, basins and bathroom fixtures </li></ul><ul><li>C ontact lenses, bone cements </li></ul><ul><li>M embrane for dialysis </li></ul><ul><li>D ental restorations </li></ul><ul><li>Road Lines </li></ul>
  4. 4. Degradation <ul><li>At high temperatures the components of the long chain backbone of the polymer can begin to separate (molecular scission) and react with one another to change the properties of the polymer. It is part of a larger group of degradation mechanisms for polymers that can occur from a variety of causes such as: </li></ul><ul><li>Heat (thermal degradation and thermal oxidative degradation when in the presence of oxygen) </li></ul><ul><li>Light (photodegradation) </li></ul><ul><li>Oxygen (oxidative degradation) </li></ul><ul><li>Weathering (generally UV degradation) </li></ul>
  5. 5. Thermal Degradation <ul><li>The chemical reactions involved in thermal degradation lead to physical and optical property changes relative to the initially specified properties. Thermal degradation generally involves changes to the molecular weight of the polymer and typical property changes include : </li></ul><ul><li>􀂃 Reduced ductility and embrittlement </li></ul><ul><li>􀂃 Chalking </li></ul><ul><li>􀂃 Color changes </li></ul><ul><li>􀂃 Cracking </li></ul>
  6. 6. Thermal Degradation <ul><li>Many reactions occur in the polymer thermal degradation, as show below : </li></ul>mainchain scission side group scission disappear depolymerization
  7. 7. Thermal Degradation of PMMA <ul><li>At high temperature, polymers such as poly(methyl methacrylate) become thermally unstable, leading to degradation by depolymerization to yield a mixture of monomer and polymer. </li></ul><ul><ul><li>PMMA depolymerization is favoured at 300°C. External sources of radicals and defects in chemical structure make the material more susceptible to this mode of degradation </li></ul></ul><ul><li>Radical initiation is </li></ul><ul><li>thermolytic, leading to </li></ul><ul><li>fragmentation of tertiary </li></ul><ul><li>radicals to yields </li></ul><ul><li>monomer and equivalent </li></ul><ul><li>tertiary radical. </li></ul>PMMA Homolytic bond dissociation Tertiary alkyl radical Secondary alkyl radical fragmentation
  8. 8. <ul><li>Thermal degradation of PMMA at 300-4OO ° C is the chain radical reaction of depolymerization. The main volatile product of this process is MMA. Initiation of PMMA depolymerization proceeds via statistic rupture of macromolecule with formation of two free macroradicals and via disruption of the link, which is near the end double bond, with formation of an active macroradical and a relatively stable allyl radical </li></ul>
  9. 9. Thermal Degradation of PMMA <ul><li>There are two degradation reactions for PMMA </li></ul><ul><li>‘ fast' reaction, which is chain-end initiated, due to double-bonded chain-end polymer molecules </li></ul><ul><li>'slow' reaction, which is initiated at random </li></ul>The energy of activation for the fast reaction is much smaller than that of the slow reaction Degradation of isotactic and syndiotactic PMMA , which degrade by random initiation, have a first-order termination reaction
  10. 10. <ul><li>PMMA is regarded as a polymer that depropagates to monomer as a result of thermal degradation up to 550 ° C </li></ul><ul><li>PMMA degrades predominantly by a depropagation process (as a reverse of the polymerisation process), the rate of which is first-order in weight loss </li></ul><ul><li>At lower degradation temperatures (340–361 ° C), the mechanism of thermal degradation was initiated by a mixture of chain end and chain scission processes, followed by depropagation </li></ul><ul><li>At higher temperatures, initiation was by a mixture of chain end and chain scission processes, followed by depropagation to the end of the polymer chain </li></ul>
  11. 11. Thermal Degradation of PMMA <ul><li>PMMA produces char. The process appears to be elimination of the methoxycarbonyl side-group, producing an unsaturated conjugated system. The amount of char yielded varies with molecular weight and temperature, and in some cases, it is estimated that 15% of the PMMA structure can degrade in this way </li></ul>
  12. 12. Thermal Degradation of PMMA <ul><li>The degradation of PMMA is a radical chain reaction that occurs in three irreversible steps: </li></ul><ul><li>Initiation :PMMA degrades randomly into two radicals by breakage of the bond in the position </li></ul>
  13. 13. Thermal Degradation of PMMA <ul><li>Depropagation: </li></ul><ul><li>The depropagation step consists of the production of the monomer from the newly created radicals </li></ul>This is the reverse of the propagation step in the polymerization process
  14. 14. Thermal Degradation of PMMA <ul><li>Termination: </li></ul><ul><li>The termination occurs by interaction of the pair of radicals to reform a polymer </li></ul><ul><li>This model, with the stationary-state assumption for all radical concentrations, leads to a rate equation that is first-order in polymer concentration </li></ul>
  15. 15. Thermal Stabilization of PMMA <ul><li>Non-crosslinking sucrose-based additives </li></ul><ul><li>Sucrose-based crosslinkers and additives </li></ul><ul><li>PS </li></ul><ul><li>Polysulfide polymers like poly styrene disulfide (PSD) and polystyrene tetrasulfide (PST) </li></ul><ul><li>Fullerene C[60] </li></ul>Thermal stability of PMMA increases with adding;
  16. 16. References <ul><li>K.Pielichowski, J.Njuguna, ‘’ThermalDegradation of Polymeric Materials’’ </li></ul><ul><li>Technical White Paper, ‘’Thermal Degradation of Plastics‘’ , Zeus Industrial Products, Inc. 2005 </li></ul><ul><li>Jellinek,H.H.G., ‘’Thermal Degradation of Polymethylmethacrylate: Energies of Activation’’, Jan. 1968 </li></ul><ul><li>B.J. Holland, J.N. Hay; “The kinetics and mechanisms of the thermal degradation of poly(methyl methacrylate) studied by thermal analysis-Fourier transform infrared spectroscopy”; Polymer 42 (2001) 4825–4835 </li></ul><ul><li>Grassie, N. and Scott, G., ‘’Polymer Degradation and Stabilization’’, Cambridge University Press, Cambridge, 1985. </li></ul><ul><li>Nobuyuki Higashi, Hideki Shiba, and Masazo Niwa;” Thermal Degradation of Poly(methyl methacrylate): Polymer with Head-to-HeadLinkages”; Macromolecules 1989, 22, 4652-4654 </li></ul><ul><li>Grassie, N.; Scott, G. Polymer Degradation and Stabilisation ; </li></ul><ul><li>Cambridge University Press: London, 1985. </li></ul><ul><li>8. Kannah Ganesh, Ramakrıshnan Lathan Kaushal Kıshore, Benny George, K. N. Ninan; “Stabilization of Thermal Degradation of Poly(methylmethacrylate) by Polysulfide Polymers”; Journal of Applied Polymer Science, Vol. 66, 2149–2156 (1997) </li></ul><ul><li>Giridhar Madras, J. M. Smith, and Benjamin J. McCoy;” Degradation of Poly(methyl methacrylate) in Solution”; Ind. Eng. Chem. Res. 1996, 35, 1795-1800 </li></ul><ul><li>Lewis E. Manring; “Thermal Degradation of Poly(methy1 methacrylate). 4. Random Side-Group Scission”; Macromolecules 1991,24, 3304-3309 </li></ul>

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