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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, Trento – Italy On web: 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 ) TEM analysis aggregates (structure) oil adsorption number (OAN) measured with the dibuthyl phtalate (ASTM D ) 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) CB226 Conductex 975u (Columbian Chemicals) CB231 Cabot XC72 (Cabot Corporation) CB802 Ketjenblack EC300J (Akzo Nobel) 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 = 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) CB226 Conductex 975u (Columbian Chemicals) CB231 Cabot XC72 (Cabot Corporation) CB802 Ketjenblack EC300J (Akzo Nobel) 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 = °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-CB 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 30°C 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 30°C 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 2000s, D V 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 HDPE-CB226, 3x HDPE-CB1353, 3x DMTA tests: temperature = -130 130°C frequency = 1 Hz Relaxation phenomena ( , )

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DMTA: MASTER CURVES 30°C 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 30°C 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 1720 cm -1 ) intensity normalized by the peak 1300 cm -1 (skeletal C-C 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) Caceres CA, SV Canevarolo. Calculating the chain scission distribution function (CSDF) using the concentration method. Polymer Degradation and Stability 86 (2004) 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, 30°C HDPE, 30°C HDPE-CB226, 30°C HDPE-CB1353, 30°C

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CREEP: ISCOCHRONOUS COMPLIANCE 0s, D E 0s, D E 2000s, D V 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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