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Impact Modification Of Thermoplastics


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  • Slide 47 seems to have a mistake. The 'ideal' case of a completely miscible blend would be a result of thermodynamic COMPATIBILITY.
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Impact Modification Of Thermoplastics

  1. 1. By Salman SHAHID Gul ZEB
  2. 2. <ul><ul><li>Toughness ? </li></ul></ul><ul><ul><li>Types of matrices. </li></ul></ul><ul><ul><li>Mechanism of toughening </li></ul></ul><ul><ul><li>Architecture of Rubber particles </li></ul></ul><ul><ul><li>Influence of Structure and properties of rubber </li></ul></ul><ul><ul><li>Thermoplastic elastomers </li></ul></ul><ul><ul><li>Styrenic block copolymers </li></ul></ul><ul><ul><li>Thermoplastic vulcanizates </li></ul></ul>Salman SHAHID Gul ZEB
  3. 3. <ul><li>Toughness is the deformation energy dissipated up to the beginning of failure.( by the frame work of fracture mechanics) </li></ul><ul><li>How to measure: </li></ul>Most accessible measurements are the notched Izod and Charpy protocols
  4. 4. <ul><li>1. Brittle amorphous polymers, such as PS and SAN, with low impact strengths </li></ul><ul><li>Pseudo-ductile engineering polymers, such as PC, PA, PI, PE, PP and PSF. </li></ul><ul><li>3. Polymers, such as PMMA, POM, and PVC, exhibiting fracture behavior intermediate between types 1 and 2. </li></ul>
  5. 5. <ul><li>The main aim of the rubber modification of thermoplastic homopolymers is to improve their toughness. </li></ul><ul><li>Methods to increase the toughness </li></ul><ul><li>Copolymerization </li></ul><ul><li>Incorporation of a second phase like other thermoplastics </li></ul><ul><li>Inorganic materials </li></ul><ul><li>Very small voids and spherical rubber particles. </li></ul><ul><li>The last mechanism is mostly used. </li></ul>
  6. 6. <ul><li>Rubber-like materials have long chains with higher flexibility and mobility which are joined in network. </li></ul><ul><li>Due to higher mobility the chain alter their configuration rather fast so able to bear higher loads. </li></ul><ul><li>On removal of the external forces, it goes back to original dimensions with non recoverable strain. </li></ul>
  7. 7. <ul><li>Molecular entanglements in a high molecular weight polymer. </li></ul><ul><li>Molecular entanglements locked by cross-linking. </li></ul>
  8. 8. <ul><li>A network is obtained by the linking of polymer chains together, and this linkage may be either chemical or physical. Physical linking can be obtained by </li></ul><ul><li>(1) Absorption of chains onto the surface of </li></ul><ul><li>finely divided particulate fillers; </li></ul><ul><li>(2) Formation of small crystallite </li></ul><ul><li>(3) Coalescence of ionic centers; and </li></ul><ul><li>(4) Coalescence of glassy blocks. </li></ul>
  9. 9. <ul><li>Well dispersed rubber particles are able to induce in the thermoplastic matrix different mechanisms of toughening: </li></ul><ul><li>1. crazing; </li></ul><ul><li>2. shear yielding and rubber particle cavitation; </li></ul><ul><li>3. combined crazing and shearing yielding. </li></ul>
  10. 10. <ul><li>Breaking of secondary bonds along the planes normal to the maximum tensile axis. </li></ul><ul><ul><li>Planner crack like defects </li></ul></ul><ul><ul><li>Stress whitening of material </li></ul></ul><ul><ul><li>High stress concentration </li></ul></ul><ul><li>Toughening particles > multiple craze </li></ul><ul><li>Formation elastomeric nature > prevents the growth of large crazes </li></ul>
  11. 11. <ul><li>Crazing: </li></ul><ul><li>Crazing is a brittle mechanisms leading to the small ductility of most base polymers </li></ul>TEM micrograph of crazing zone
  12. 12. <ul><li>Figure: Schematic representation of the crazing phenomenon. </li></ul><ul><li>( a) Crazed specimen subjected to a tensile force F. </li></ul><ul><li>(b) Section of a craze with fibrils, strained by the tensile force F. </li></ul><ul><li>(c) Multi craze mechanism induced by the presence of rubber particles in a rigid matrix. </li></ul>
  13. 13. <ul><li>In homogeneous polymers shear deformation consists of a distortion of the body shape without significant volume variation. </li></ul><ul><li>In toughened materials> diffused shear yielding, followed by rubber particle cavitation. </li></ul><ul><li>PVC, ABS, PC, PA, PE, PI and PSF undergo diffuse shear yielding. </li></ul>
  14. 15. <ul><li>TEM image showing the prevention of growth craze due to filled rubber particles. </li></ul>
  15. 16. Rubber particles Shear band crazing
  16. 17. <ul><li>The choice between both deformation mechanisms depends on: </li></ul><ul><li>Matrix’s chemistry (Secondary relation temp> see foot note) </li></ul><ul><li>Rubber phase </li></ul><ul><li>Generally, crazing prevails at </li></ul><ul><li>low temperatures </li></ul><ul><li>high deformation rates . </li></ul><ul><li>Shearing </li></ul><ul><li>Above the glass transition </li></ul><ul><li>Low deformation rates </li></ul><ul><li>Wu criteria </li></ul><ul><li>Polymers with a critical entanglement density above 0.15 mmol/cc should deform by shearing and below this critical level by crazing </li></ul>Crazing because the chains have not enough time to rearrange under the stress field. If have time then shear yielding
  17. 18. <ul><li>Elastic deformation </li></ul><ul><li>Plastic strain softening </li></ul><ul><li>3. Strain hardening of the yield zone </li></ul>
  18. 19. <ul><li>There are two extremes for architecture of Rubber particles. </li></ul><ul><li>Bulk or pelletized elastomeric compounds (homogeneous particles) </li></ul><ul><li>core/ shell particles </li></ul><ul><li>In Core/shell particles, cores is often formed from matrix material and is covered with a thin layer rubbery shell, which is grafted with an outer second shell. </li></ul>
  19. 21. <ul><li>Homogeneous Particles </li></ul><ul><li>Good toughening agent </li></ul><ul><li>Deleterious effects on matrix stiffness </li></ul><ul><li>Less stress whitening. </li></ul><ul><li>Good weatherability. </li></ul><ul><li>Better gloss </li></ul><ul><li>Core/shell particles </li></ul><ul><li>less efficient as toughening agents </li></ul><ul><li>Less deleterious effect on matrix stiffnes. </li></ul><ul><li>For transparent toughened polymers. </li></ul>Comparison
  20. 23. <ul><li>Glass transition temperature. </li></ul><ul><li>Independent of the type of deformation process, whether crazing or shearing, </li></ul><ul><li>T application > T glass transition of rubber </li></ul><ul><li>(mean should be in the BD transition). </li></ul>
  21. 25. <ul><li>In this context two thing are very important. </li></ul><ul><li>1. Adhesion </li></ul><ul><li>Good adhesion: Interdiffusion of phases thus good toughening </li></ul><ul><li>Agglomeration </li></ul><ul><li> large agglomerates are ineffective in toughening </li></ul>
  22. 27. <ul><li>In addition to adhesion grafting is also important for the dispersion of rubber particles. </li></ul>
  23. 28. <ul><li>In brittle homopolymers as in PS, crazing is the dominating deforming process. </li></ul><ul><li>It was observed experimentally that larger salami type particles play more better role than the small particles (at same composition) for same conditions. </li></ul>
  24. 30. <ul><li>In Pseudo-ductile materials shear is dominant mechanism of deformation. It is observed that smaller particles better handle the shear than large particles. </li></ul><ul><li>In semi ductile materials like PMMA better results were obtained with mixed size particles 300nm to 600nm. </li></ul>
  25. 31. <ul><li>Generally saying, polymers are inherently brittle and crazing is always dominant but if chemistry of the structure allows shearing (through secondary relaxation process) then shear yielding is dominant. </li></ul>
  26. 32. <ul><li>Rubber particles must be at least slightly cross-linked, otherwise the rubber phase loses its individual particular structure in processing and is transferred, e.g., to an interpenetrating network. But with increase of the degree of crosslinking the brittle strength of the material decreases. </li></ul>
  27. 34. <ul><li>A thermoplastic elastomer (TPE) is generally considered a bimicrophasic material that exhibits rubber elasticity over a specified service temperature range but at elevated temperature can be processed as a thermoplastic </li></ul>
  28. 35. Thermoplastic Elastomers (TPe) Tailor made properties by varying the ratio of two phases(hard and soft) . Upper service temperature  softening point of the hard phase. Low temperature properties  controlled largely by the soft segments.
  29. 36. Rigid domain (physical crosslinking) Soft amorphous domain
  30. 37. <ul><li>(1) Simple processing. </li></ul><ul><li>(2) Shorter fabrication times. </li></ul><ul><li>(3) There is little or no compounding. </li></ul><ul><li>(4) reusing scrap as with thermoplastics. </li></ul>Advantages of TPes <ul><li>They melt at elevated temperatures </li></ul><ul><li>They may require drying before processing </li></ul><ul><li>There is a limited number of low modulus compounds </li></ul>Disadvantages of TPe
  31. 38. <ul><li>1- Styrenic block copolymers(SBCs) </li></ul><ul><li>2- Dynamic vulcanizates (TPVs) </li></ul><ul><li>3- Polyamide TPEs </li></ul><ul><li>4- Copolyester elastomers (COPEs) </li></ul><ul><li>5- Thermoplastic polyolefins (RTPOs); and </li></ul><ul><li>6- Ionomeric TPEs </li></ul>
  32. 39. <ul><li>A substantial portion of industrially produced TPEs is represented by block copolymers, consisting of two or more polymer chains attached at their ends. Most block copolymers are prepared by Anionic polymerization and controlled polymerization . </li></ul>
  33. 40. <ul><li>Styrenic block copolymers (SBCs) are based on </li></ul><ul><li>simple molecules of the type A–B–A, where A is polystyrene and B is an elastomeric segment </li></ul><ul><li>The most common structure of SBCs is that where the elastomeric segment is a polydiene </li></ul><ul><li>Polybutadiene </li></ul><ul><li>Polyisoprene </li></ul><ul><li>Example SBS, SEBS, SIS </li></ul><ul><li>S : Styrene </li></ul><ul><li>B : Butadiene </li></ul><ul><li>EB : Hydrogenated butadiene </li></ul>
  34. 41. Schematic of a styrene–butadiene–styrene block copolymer
  35. 42. Changes in morphology of an A–B–A block copolymer as a function of composition
  36. 45. <ul><li>Two types of compatibility </li></ul><ul><ul><li>Thermodynamic </li></ul></ul><ul><ul><li>Technological </li></ul></ul>
  37. 46. <ul><li>If polymers are thermodynamically compatible, i.e. miscible , their intimate mixture exists as a single phase. For this case </li></ul><ul><li>Unlike the case of monomeric materials, the entropy of mixing of polymers is very low. </li></ul><ul><li>it would be best that the enthalpy of mixing, be negative (i.e., that mixing be exothermic). </li></ul><ul><li>It would be required that unlike polymer molecules associate with one another more strongly than do like polymer molecules </li></ul>Polymers are rarely Thermodynamically compatible
  38. 47. <ul><li>Ideal elastomeric rubber-plastic blend would comprise finely divided rubber particles dispersed in a relatively small amount of plastic </li></ul><ul><li>Practically the “ideal” case proposed above could arise as a result of the polymers being thermodynamically incompatible </li></ul><ul><li>low T g of the rubber phase would be maintained because of the relative purity of the rubber phase; yet the high T g of the hard phase could be retained for structural integrity over a useful temperature range </li></ul>
  39. 48. <ul><li>If two polymers are said to be technologically compatible , it merely means that their blends are technologically useful. </li></ul><ul><li>Technological compatibilization, then, is any process that improves the properties of a blend to make it more useful. </li></ul><ul><li>Compatibilization techniques for improving such mixtures may be mechanical or chemical in nature </li></ul><ul><li>Such techniques generally do not make the mixtures become miscible, i.e compatible in thermodynamic sense </li></ul>
  40. 49. <ul><li>We can improve the properties of blends prepared by simple melt blending by </li></ul><ul><ul><li>Dynamic vulcanization </li></ul></ul><ul><ul><li>(the process of crosslinking the rubber phase during its melt-mixing with the plastic material) </li></ul></ul><ul><ul><li>Technological compatibilization </li></ul></ul><ul><ul><li>(by addition (or in situ formation) of small amounts of block copolymers, which contain blocks of each of the polymers to be compatibilized) </li></ul></ul>
  41. 50. <ul><li>Mixing as well as selective crosslinking of the rubber are superimposed processes that happen in the melt-mixing process called dynamic vulcanization. </li></ul><ul><li>Rubber particles cross-linking </li></ul><ul><li>Embedded in less viscous thermoplastic component </li></ul>
  42. 51. <ul><li>Lower permanent set </li></ul><ul><li>Improved mechanical properties (tensile </li></ul><ul><li>strength, elongation at break) </li></ul><ul><li>Better fatigue resistance </li></ul><ul><li>Lower swelling in fluids, such as hot oils </li></ul><ul><li>Higher melt strength </li></ul><ul><li>Improved utility at elevated temperatures </li></ul><ul><li>Greater stability of phase morphology in the </li></ul><ul><li>melt </li></ul><ul><li>Greater melt strength </li></ul><ul><li>More reliable processing characteristics in </li></ul><ul><li>melt processing. </li></ul>
  43. 53. Torque-time characteristics of a dynamic vulcanization process in an internal mixer. PP/EPR 40:60, peroxidic cross-linked. Points a to l: times of sampling.
  44. 55. Effect of mixing time on the phase morphology of Brabender-mixed EPDM/BR blends (a) 5 min (3300X), (b) 15 min (3300X), (c) 30 min (10,000X)
  45. 56. <ul><li>Rubber-plastic blends have generally been prepared by melt-mixing techniques. </li></ul><ul><li>Melt-mixing has been accomplished by various mixing devices </li></ul><ul><ul><li>Two-roll mills </li></ul></ul><ul><ul><li>Twin-screw extruders </li></ul></ul>
  46. 57. <ul><li>In some blends, the rubber can be slightly cross-linked by the action of an organic peroxide. </li></ul><ul><li>A disadvantage of the process of vulcanizing rubber before mixing it with polyolefin is that the compositions generally contain rather large rubber particles. </li></ul>
  47. 58. <ul><li>After sufficient melt-mixing to form a well-mixed blend, vulcanizing agents (curatives, crosslinkers) are added. </li></ul><ul><li>Vulcanization then occurs while mixing continues. </li></ul><ul><li>The more rapid the rate of vulcanization, the more rapid the mixing must be to ensure good fabricability of the final blend composition. </li></ul>
  48. 59. PP, PE, PA, SAN, ABS, PC, and PS. Diene rubber, such as NR, SBR, PBD, BR, EPDM.
  49. 60. Continuous PP phase Dispersed EPDM First commercial TPV: PP-EPDM ( Santoprene® ) Crystalline Tm ~ 160°C
  50. 61. Stress-strain behavior of a non- reactive and a dynamic vulcanized PP-EPR blend: PP/EPR 40:60. Effect of polypropylene concentration of EPDM/PP thermoplastic vulcanizate
  51. 65. <ul><li>Modes of reuse : </li></ul><ul><li>Use as generic plastic > recycled TPE + Virgin </li></ul><ul><li>Use of mixed plastic > e.g improve properties of TPOs </li></ul><ul><li>Use in energy recovery > little sulfur, better incineration </li></ul>
  52. 66. <ul><li>Properties equal to thermoset elastomers </li></ul><ul><li>Improved processing & increasing fabrication methods </li></ul><ul><li>Tailor made properties > transparency, adhesion and compatibility </li></ul><ul><li>Struggling for potential application > Artificial implants </li></ul><ul><li> > Biological adhesives </li></ul><ul><li> > Soft tissue replacements. </li></ul>
  53. 67. <ul><li>Handbook of Elstomers 2 nd edition, Anil K. Bhowmick H. L. Stephens 2001 Mercel Dekker . </li></ul><ul><li>Mechanical properties of polymers based on Nanostructure and Morphology edited by G. H. Michler F. J. Baltá-Calleja </li></ul><ul><li>Handbook of Thermoplastic Elastomers by Jiri George Drobny Drobny Polymer Associates. </li></ul><ul><li>Micro and Nanostructured Multiphase Polymer Blend Systems Phase Morphology and Interfaces Edited by Charef Harrats, Sabu Thomas and Gabriel Groeninckx . </li></ul><ul><li>Polymer Blends Handbook 3 rd volume edited by L. A. Utracki </li></ul><ul><li>Current Topics in Elastomers research edited by Anil K. Bhowmick. </li></ul><ul><li>Modern Styrenic Polymers: polystyrenes and styrenic copolymers edited by Jhon Scheirs ExcelPlas. </li></ul>