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POLYETHYLENE-CARBON BLACK NANOCOMPOSITES: MECHANICAL RESPONSE UNDER CREEP AND DYNAMIC LOADING CONDITIONS Matteo Traina, Alessandro Pegoretti and Amabile Penati University of Trento (DIMTI) and INSTM; Via Mesiano 77, 38050 Trento – Italy E-mail: matteo.traina@ing.unitn.it; On web: www.unitn.itmatteo.traina@ing.unitn.itwww.unitn.it INSTM Consorzio Interuniversitario Nazionale per la Scienza e Tecnologia dei Materiali Università degli Studi di Trento Dipartimento di Ingegneria dei Materiali e Tecnologie Industriali (DIMTI) VI CONVEGNO NAZIONALE SULLA SCIENZA E TECNOLOGIA DEI MATERIALI (June 12 th -15 th, 2007; Perugia)

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INTRODUCTION agglomerates primary particle aggregate Carbon black (CB) Carbon (graphene layers) Combustion or decomposition (C X H Y ) OAN (cm 3 /g) aggregate structure SSA (m 2 /g) particle diameter Microstructure: primary particles (diameter) specific surface area (SSA) measured by the BET (Brunauer– Emmett–Teller) method (ASTM D 6556-03) TEM analysis aggregates (structure) oil adsorption number (OAN) measured with the dibuthyl phtalate (ASTM D 2414-04) TEM analysis Other properties (…)

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INTRODUCTION Grade (Supplier) OAN [cm 3 /g] SSA [m 2 /g] CB105 Raven P-FE/B (Columbian Chemicals) 0.98105 CB226 Conductex 975u (Columbian Chemicals) 1.69226 CB231 Cabot XC72 (Cabot Corporation) 1.78231 CB802 Ketjenblack EC300J (Akzo Nobel) 3.22802 CB1353 Ketjenblack EC600JD (Akzo Nobel) 4.951,353 Carbon black (CB)

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SSA / OAN INTRODUCTION CB FILLED COMPOSITES Matteo Traina, Alessandro Pegoretti and Amabile Penati, Time-temperature dependence of the electrical resistivity of high density polyethylene - carbon black composites. Journal of Applied Polymer Science, in press. Matteo Traina, Alessandro Pegoretti and Amabile Penati, Processing and Electrical Conductivity of High Density Polyethylene – Carbon Black Composites. XVII Convegno Nazionale AIM (Napoli, September 11 th – 15 st, 2005)

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EXPERIMENTAL HDPE-CB composites VISCOELASTIC BEHAVIOR Creep tests DMTA tests COMPOSITE MORPHOLOGY constant filler content (1 vol%) Grade (Supplier)Properties HDPE Eltex A4009 (BP Solvay) MFI = 0.8 g/10min (190°C; 2.16kg) Density = 0.958 g/cm 3 (23°C) MATERIAL (polymeric matrix)

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EXPERIMENTAL HDPE-CB composites VISCOELASTIC BEHAVIOR Creep tests DMTA tests COMPOSITE MORPHOLOGY constant filler content (1 vol%) Effect of the SSA of CB matrix HDPE composite HDPE-CB226 composite HDPE-CB1353 Grade (Supplier) OAN [cm 3 /g] SSA [m 2 /g] CB105 Raven P-FE/B (Columbian Chemicals) 0.98105 CB226 Conductex 975u (Columbian Chemicals) 1.69226 CB231 Cabot XC72 (Cabot Corporation) 1.78231 CB802 Ketjenblack EC300J (Akzo Nobel) 3.22802 CB1353 Ketjenblack EC600JD (Akzo Nobel) 4.951,353 CB226 CB1353

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EXPERIMENTAL HDPE-CB composites VISCOELASTIC BEHAVIOR Creep tests DMTA tests COMPOSITE MORPHOLOGY constant filler content (1 vol%) Effect of the SSA of CB matrix HDPE composite HDPE-CB226 composite HDPE-CB1353 PROCESSING Melt compounding (Extrusion) Twin screw extruder (ThermoHaake PTW16) T = 130-200-210-220-220°C n = 12 rpm Effect of the degree of filler dispersion Multiple extrusions (up to 3 times)

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FILLER DISPERSION Extrusions3x HDPE-CB1353 500 µm HDPE-CB226 2x1x HDPE-CB composites >>> thin section (microtome) >>> optical microscope As the number of extrusions increases, as the degree of the filler dispersion is better. As the SSA decreases, as the degree of dispersion is better.

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HDPE-CB226, 1x HDPE-CB composites >>> ultra-thin section (cryo-ultramicrotome) >>> transmission electron microscope >>> PRELIMINARY RESULTS HDPE-CB226, 2x CB226 FILLER DISPERSION As the number of extrusions increases, as the degree of the filler dispersion is better.

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MOLECULAR WEIGTH DISTRIBUTION HDPE Size Exclusion Chromatography (SEC) 1,2,4 trichlorobenzene (TCB) at 140°C IP MWMW HDPE HDPE-CB The HDPE undergoes a progressive thermo-mechanical degradation during the extrusion processes.

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EFFECT OF MULTIPLE EXTRUSIONS: 3x > 2x > 1x HDPE > HDPE-CB226 > HDPE-CB1353 EFFECT OF THE FILLER: HDPE > HDPE-CB226 > HDPE-CB1353 3x > 2x > 1x CREEP: GENERAL COMPARISON Creep tests: 30°C, 10 MPa extruded 1x extruded 2x extruded 3x

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HDPE-CB composites VISCOELASTIC BEHAVIOR Creep tests DMTA tests COMPOSITE MORPHOLOGY constant filler content (1 vol%) Effect of the SSA of CB matrix HDPE composite HDPE-CB226 composite HDPE-CB1353 Effect of the degree of filler dispersion Multiple extrusions (up to 3 times) HDPE 1x HDPE 2x HDPE 3x HDPE-CB226, 1x HDPE-CB226 2x HDPE-CB226 3x HDPE-CB1353 1x HDPE-CB1353 2x HDPE-CB1353 3x DEGRADATION PHENOMENA HDPE, 1x HDPE, 3x HDPE 1x HDPE 3x HDPE-CB226 3x HDPE-CB1353 3x FILLER EFFECT HDPE, 3x HDPE-CB226, 3x HDPE-CB1353, 3x

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CREEP RESISTANCE DEGRADATION: HDPE 3x < HDPE 1x FILLER EFFECT: HDPE < HDPE-CB226 < HDPE-CB1353 These effects are evident at long time, while at short time the curves are almost superimposed. CREEP: MASTER CURVES HDPE-CB @ 30°C HDPE @ 30°C Creep test: temperature = 30 90°C stress = 3 MPa (linear viscoelasticity) ANALYSIS OF THE DATA: Time-Temperature Superposition Principle (temperature spectrum master curve)

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LOG-linear IN GENERAL: linear decreasing in bi-logarithmic scale the most differences is present at short time ( 10 5 s) the curves are superimposed CREEP: CREEP RATE Master curves linear viscoelasticity Creep tests constant load/stress LOG-LOG strain rate: Creep rate AT SHORT TIME: DEGRADATION: HDPE 3x > HDPE 1x FILLER EFFECT: HDPE > HDPE-CB226 > HDPE-CB1353)

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CREEP: RETARDATION SPECTRA HDPE-CB @ 30°C HDPE @ 30°C The retardation spectrum translates: DEGRADATION: HDPE 3x < HDPE 1x FILLER EFFECT: HDPE < HDPE-CB226 < HDPE-CB1353 Linear viscoelasticity: es. Maxwell generalized model retardation time distribution Retardation spectrum (first-order approximation)

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The elastic components don’t change in a meaningful way. CREEP: ISCOCHRONOUS COMPLIANCE Comparison of the isochrone compliance (@ 2000s) as a function of the temperature The compliance is divided in: elastic component (instantaneous), D E viscoelastic component (time dependent), D V HDPE @ 2000s, D V HDPE-CB @ 2000s, D V D(t=2000) = D E + D V D E = D(t=0s) D V = D(t=2000s) – D(t=0s) The viscoelatic components: DEGRADATION: HDPE 3x > HDPE 1x (<70°C) FILLER EFFECT: HDPE > HDPE-CB226 > HDPE-CB1353

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Glass transition temperature: DEGRADATION: HDPE 3x < HDPE 1x (-10°C) FILLER EFFECT: HDPE < HDPE-CB (+4°C) DMTA: GENERAL COMPARISON MaterialT g =T [°C] HDPE, 1x-98.8 HDPE, 3x-108.9 HDPE-CB226, 3x-104.3 HDPE-CB1353, 3x-103.8 DMTA tests: temperature = -130 130°C frequency = 1 Hz Relaxation phenomena ( , )

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DMTA: MASTER CURVES HDPE-CB @ 30°C HDPE @ 30°C DMTA test: temperature = -20 130°C ( relaxation) frequencies = 0.3 30 Hz ANALYSIS OF THE DATA: Time-Temperature Superposition Principle (temperature spectrum master curve) The DMTA results are analogous to the CREEP results. Storage modulus: DEGRADATION: HDPE 3x < HDPE 1x FILLER EFFECT: HDPE < HDPE-CB226 < HDPE-CB1353

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The relaxation spectra (DMTA) are consistent with the retardation spectra (CREEP) and very similar to the MWD data for the HDPE. DMTA: RELAXATION SPECTRA HDPE-CB @ 30°C HDPE @ 30°C Linear viscoelasticity: Relaxation spectrum (first-order approximation) DEGRADATION: HDPE 3x >narrow> HDPE 1x FILLER EFFECT: longer relaxation times for HDPE-CB

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ACTIVATION ENERGY ACTIVATION ENERGY of “ ” relaxation [kJ/mol] various method of calculation CREEP shift factor (Arrhenius equation) DMTA shift factor at high temperature (50 100°C) (Arrhenius equation) DEGRADATION: HDPE 3x < HDPE 1x FILLER EFFECT: HDPE < HDPE-CB

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CONCLUSIONS The creep resistance (in general the viscoelastic behaviour) of the HDPE-CB composites is strictly dependent: ● on the CB type as the SSA increases as the creep resistance increases >>> The filler-matrix interaction hamper the chain motions elastic/viscoelastic components of compliance activation energy, retardation/relaxation spectra creep rate. ● on the level of dispersion of the filler in the polymer matrix as the filler dispersion is improved as the creep resistance increases >>> The improved dispersion enhances the filler-matrix interaction, i.e. the effective surface area. ● on the degradation of the polymer matrix as the matrix degrades as the creep resistance decreases

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ADDITIONAL MATERIAL

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FT-IR spectra: degradation phenomena (oxidation) carbonyl peak (C=O @ 1720 cm -1 ) intensity normalized by the peak intensity @ 1300 cm -1 (skeletal C-C vibrations) @ 720 cm -1 (methylene –(CH 2 ) n - rocking OXIDATIVE DEGRADATION: the most part of the phenomenon takes place during the first extrusion the oxidative phenomena are more intense for the HDPE-CB composites (HDPE

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DEGRADATION: THERMAL ANALYSES Thermal analyses: Differential Scanning Calorymetry (DSC) 0-200°C, +10°C/min, N 2 flux Thermogravimetric Analysis (TGA) 0-600°C, +10°C/min, N 2 flux The extrusion induces a meaningful change of crystallinity only after the first extrusion on HDPE. The thermal stability of the composites HDPE-CB increases in comparison with the HDPE of about 5°C. In particular: HDPE

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MWD DEGRADATION: MOLECULAR WEIGTH From the MWD to the CSDF: Canevarolo SV. Chain scission distribution function for polypropylene degradation during multiple extrusions. Polymer Degradation and Stability 709 (2000) 71-76 Caceres CA, SV Canevarolo. Calculating the chain scission distribution function (CSDF) using the concentration method. Polymer Degradation and Stability 86 (2004) 437-444 Number of chain scissions Chain scission distribution function (CSDF) averagefor each MW The extrusion induces the scission of the high MW chains and the branching/cross- linking of the low MW chains. The intensity of these phenomena (after each extrusion) decreases (1x>2x>3x). The shape of the CSDF curve gives information on the type and intensity of the degradation. CSDF

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FRACTURE BEHAVIOUR: EWF Essential Work of Fracture (EWF) Specific total work of fracture w f w f = W f / LB = w e + w p L (under plane stress) wfwf L wewe PURE PLANE- STRESS REGION displacement load F max u ini u max B H L W DENT samples tensile test to fracture w e = the specific essential work of fracture, i.e. the work dissipated in the process zone close to the crack tip ( ██ ); w p = the specific non-essential work of fracture, i.e. the work responsible for the plastic deforma- tion outside the fracture-process zone (██). w e calculation WfWf

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FRACTURE BEHAVIOUR: EWF HDPE-CB HDPE as the number of extrusions increases, as the fracture toughness decreases (w e ) HDPE-CB as the number of extrusions increases, as the fracture toughness of the HDPE-CB composites in comparison to the HDPE toughness (with the same number of extrusion) increases (w e /w e,HDPE ). This phenomenon has a different dynamic with different CB (i.e. different SSA). Essential Work of Fracture (EWF): DENT specimens (L = 5 15 mm; H = 6 mm) crosshead speed = 12 mm/min at room temperature (23°C) Fracture toughness (plane stress) Matteo Traina, Alessandro Pegoretti and Amabile Penati, Fracture behaviour of high density polyethylene – carbon black composites evaluated by Essential Work of Fracture approach. 8° Convegno Nazionale AIMAT (Palermo, June 27 th – July 1 st, 2006)

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Creep test: temperature = 30 90°C stress = 3 MPa (linear viscoelasticity) ANALYSIS OF THE DATA: Time-Temperature Superposition Principle (temperature spectrum master curve) CREEP: MASTER CURVES PRELIMINARY TEST: Creep tests: 30 and 75°C, 3 10MPa Isochronous curves: 2000 s deviation from linearity over 6 MPa increasing creep resistance for HDPE 1x increasing creep resistance for the compo- sites at all the tested stresses (HDPE

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CREEP: TEMPERATURE SPECTRA HDPE, 1x HDPE, 3x HDPE-CB226, 3x HDPE-CB1353, 3x temperature

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CREEP: MASTER CURVES HDPE, 1x @ 30°C HDPE, 3x @ 30°C HDPE-CB226, 3x @ 30°C HDPE-CB1353, 3x @ 30°C

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CREEP: ISCOCHRONOUS COMPLIANCE HDPE @ 0s, D E HDPE-CB @ 0s, D E HDPE @ 2000s, D V HDPE-CB @ 2000s, D V

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DMTA: POLYETHYLENE RELAXATIONS storage and dissipative modulus loss factor “ ” transition 30 60°C Relaxation phenomena in the crystalline regions of the polymer “ ” transition -30 20°C Relaxation phenomena in the side- branching of the polymer (if present) YES: LDPE NO: HDPE; LLDPE “ ” transition (glass transition) -120 -110°C Micro-Brownian motion of long chain segments in the amorphous regions of the polymer

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The activation energy decreases after 3 extrusions for the HDPE. The activation energy generally increases for the HDPE-CB composites in comparison to the HDPE (shift factor). Only in the case of the E’’ method the activation energy clearly decrease. ACTIVATION ENERGY ACTIVATION ENERGY [kJ/mol] various method of calculation CREEP shift factor (Arrhenius equation) DMTA peak of E’’ (Arrhenius equation) DMTA shift factor high temperature (50 100°C) (Arrhenius equation) DMTA shift factor low temperature (0 25°C) (Arrhenius equation) An increase of the activation energy from the shift factor (under/over the relaxation temperature) is related to a reduced mobility of the polymer chains. A change of the activation energy from the Arrhenius plot is directly related to the relaxation.

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1 ASTM : American Society of Testing and Materials.

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