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The Micromechanics of Deformation in Rubber-Toughened Thermoplastics

The Micromechanics of Deformation in Rubber-Toughened Thermoplastics. R. A. Bubeck Michigan Molecular Institute. January 14, 2010. Michigan Molecular Institute.

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The Micromechanics of Deformation in Rubber-Toughened Thermoplastics

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  1. The Micromechanics of Deformation in Rubber-Toughened Thermoplastics R. A. Bubeck Michigan Molecular Institute January 14, 2010

  2. Michigan Molecular Institute • A not-for-profit institute dedicated to advanced research, development and commercialization mainly in polymer technology in Midland, Michigan • Over sixty full-time and part-time employees • Major income sources: Grants and Contracts • Founded April 23, 1970 • Create new companies: • Dendritech, Inc. – 1991 • Oxazogen, Inc. – 1995 • Lion Compact Energy, Inc. – 1997-99 • Small subsidiary companies funded in part with SBIRs and STTRs. • Among current research projects is an effort to develop epoxy resins with BOTH improved impact resistance and superior VARTM processibility.

  3. Outline • Mass polymerization of rubber-toughened thermoplastics • Thermal stress contributions • Real-time small-angle X-ray scattering analyses of HIPS and ABS undergoing deformation • Particle size bimodality • Summary

  4. Continuous Bulk Polymerization Process for HIPS and ABS Graft formation, Mw advancement, and “2nd add” particles Phase inversion and particle sizing

  5. Stress and energy management in glassy polymers on the micro-scale via: • Crazes and crazing • Craze (bridging) fibril formation and viscous breakdown • Chain scission in fibrils and at fibril/matrix interface • Cracking • Shear yield • Ligament bending • Deflection of matrix material between toughening phases • Second phase cavitation • Often the “trigger” for the other modes

  6. “bulk” polymerized Composite gel Particles initiate and multiply crazes Donald and Kramer, 1982

  7. cavitation and crazing with core-shell particles TEM: R. Cieslinski

  8. Stresses around spherical particle • Triaxial configuration • Radial and tangential stresses (s) (Symbols: sqq = st; n= Poison’s ratio; E = moduli) Figure after Timoshenko and Goodier (1951), and others.

  9. Thermal Stresses in Rubber-Toughened Polymers • Thermal coefficient of thermal expansion of rubber >>> than matrix. • Rubber will try to contract more upon cooling than glassy matrix. • Net tensile (dilatational) stress in the rubber. • Net compressive stress on matrix. • Requires good adhesion between phases.

  10. Thermal Stress Models/Treatments for Rubber-Toughened Polymers • Manabe • Simplified configuration for particles used in combination with Timoshenko and Goodier equations for spherical inclusions in a matrix under stress. • S. Manabe, R. Murakami, M. Takayangi, Intern.J. Polymeric Mater.,1, 47-73 (1971). • R. A. Bubeck, C. B. Arends, E. L. Hall, J. B. Vander Sande, Polym. Eng. Sci., 2, 624 (1981) • Argon • A. S. Argon, R. E. Cohen, O. S. Gebizlioglu, C. E. Schwier, Advances in Polymer Science, 52/53, 275-334, Springer-Verlag (1983). • Bucknall • Uses Argon work referenced, viz.: M. E. Boyce, A. S. Argon, and D. M. Parks, Polymer, 28, 1680 (1987).

  11. Three basic simplified configurations Shells 0 and 2 are the same polymer for #1 Manabe Model Configurations

  12. Measuring Mechanical Gel Fraction with Composite Particles • Measure deflection of the sample with a cathetometer as a function of DT. • Composite “gel’ particles. Rubber-toughened sample Sample holder fashioned from same matrix material as sample.

  13. Radial and Tangential Thermal Stresses3-Layer Concentric Sphere Model(Manabe-matonis) fg = mechanical gel fraction; fr = volume fraction of second phase; B = bulk modulus; G = shear modulus; a = coefficient of thermal expansion

  14. High Impact Polystyrene w/Particles of Different Gel Fractions vs. Environmental Stress Crack Resistance HIPS A fg = 0.29 ~3 MPa 1 mm HIPS B fg = 0.23 Vol. Avg. Particle Size ~ 3 mm 10 wt. % Rubber

  15. Minimum Stress to Cause Dilation • Estimated using axially symmetric stress distribution for the spherical case (See, Timoshenko and Goodier) • A function of particle morphology and elastomer Vol. %. ~ 3 MPa

  16. Real-time Small Angle X-ray Scattering Scheme Cornell High Energy Synchrotron Source ‘hard’ X-rays

  17. Experimental Scattering Geometry Experimental scattering geometry “Ideal” scattering pattern projected onto the Reticon detector array

  18. SAXS and Partial Beam Transmission Scans for a 7 wt% Rubber HIPSDeformation rate = 4.2 cm/s; Time resolution = 18.2 ms/scan Fracture Counts Pixels

  19. A typical 1-dimensional array scan showing the attenuated primary beam, the scattering from crazes, and the Porod region

  20. Deformation Modes vs. Time in a 7 wt.% Rubber HIPS at Two Deformation Rates < < D > D > εT εT εNCR εNCR εCR εCR RTSAXS analysis of HIPS-1 at 7.1 s-1. (a) Plots of engineering stress and average craze fibril diameter (D) (nm). (b) Plots of total plastic strain (εT), non-crazing strain (εNCR), and strain due to crazing (εCR). RTSAXS analysis of HIPS-1 at 31.3 s-1. The figure layout and labeling are identical to those to the left.

  21. RTSAXS of a 22 wt% Rubber ABS at a Deformation Rate = 8.6/s < D > εT εNCR Time After Initiation of Impact, (ms) εCR

  22. Real-time Small-Angle X-ray Scattering (RTSAXS) of HIPS and ABS: • Non-craze contribution initiates before • crazing. • Particle cavitation occurs before crazing. • Non-craze (ligament bending, microshear) • contributions are greater than the crazing • contribution. • Average craze fibril diameter constant with • time. R. A. Bubeck, D. J. Buckley, Jr., E. J. Kramer, H. R. Brown, J. Mater. Sci., 26, 6249 (1991).

  23. Particle size RTSAXS analyses for An 11 wt% rubber ABS • Tensile creep rate = 0.01/s • Tensile impact rate = 50/s

  24. Particle size RTSAXS analyses for An 11 wt% rubber ABS, Cont’d: • Tensile creep rate = 0.01/s • Tensile impact rate = 50/s

  25. Bimodal Rubber particle Effects Tensile direction • ABS example – 17 wt% elastomer • Large gel particles ‘trigger’ the smaller core-shell particles • Crazing • Cavitation • Micro-shear between smaller 1 mm

  26. ABS Impact Map – Effect of Particle Size biModality Ratio vs. Toughness via instrumented Driven mandrel impact (11 wt% rubber; 0.2 mm primary particle size)

  27. ABS Impact Map – Effect of Particle Size biModality Ratio vs. Toughness via instrumented Driven mandrel impact (17 wt% rubber; 0.2 mm primary particle size) 1/1 Particle Ratio

  28. Summary • Thermal stress is an important influence on the mechanical properties of rubber-toughened polymers. • Decreasing glassy occlusions in the gel particles increases compressive thermal stresses imposed upon the matrix • Non-crazing modes of deformation exceed the importance of crazing, even in traditional rubber-toughened ETPs. • Craze morphology remains globally stable up to fracture

  29. Further reading: • D. Henton and R. Bubeck in “Polymer Toughening”, C. B. Arends, ed.; Marcel Dekker, New York, 1996. • V. A. Matonis, Polym. Eng. Sci., 9, 90 (1969) • R. A. Bubeck, D. J. Buckley, Jr., E. J. Kramer, H. R. Brown, "Modes of Deformation in Rubber-Modified Thermoplastics During Tensile Impact," J. Mater. Sci., 26, 6249 (1991).

  30. Thanks: • Chuck Arends (Deceased) • Don Buckley, Jr. (GE) • Hugh Brown (Univ. of Wollongong) • Ed Kramer (U.C.S.B.) • Staff at C.H.E.S.S.

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