Thermal Morphological and Mechanical Characterization of Carbon Nanofiber

Thermal, Morphological, and Mechanical Characterization of Carbon Nanofiber Reinforced Low Density Polyethylene Author: Camille Hebert Chemical Engineering Undergraduate Research Assistant Laboratory for Composite Materials Advisor: Ahmed Khattab, Ph. D. Director Laboratory for Composite Materials Co-Advisor: William Chirdon, Ph. D. Department of Chemical Engineering College of Engineering University of Louisiana at Lafayette

Presentation Outline: p Polymer Nanocomposites p Carbon Nanofibers (CNF) p Low Density Polyethylene (LDPE) p Materials Processing (CNF/LDPE) p Thermal & Morphological Characterization p Tensile Testing p Mechanical Characterization p Conclusions/Discussion

POLYMER NANOCOMPOSITES Composite materials are typically composed of a physically bonded matrix and filler, whose properties are independently preserved while significantly enhancing those of the overall composite. p The basis for obtaining these specific properties with carbon nanofiber reinforced polymer composites involves various aspects such as filler/matrix compatibility, proper dispersion of CNFs within the matrix, and the crystal structure of the polymer. p

POLYMER NANOCOMPOSITES p p Nanoreinforced polymer composite materials are widely used in the fields of Aerospace, medical, sports, avionics, engineering, etc. Increasing demand for such material production is due to advanced properties such as high thermal resistance, high strength and stiffness to weight ratio, fracture resistance, and lowered density. Courtesy of google images

Carbon Nanofibers p p Advantageous nanofiller due to their enhanced electrical conductivity, mechanical reinforcement, and relatively easy fabrication. Economical alternative that offers substantial composite improvement when compared to carbon nanotubes. CNFs display superior interfacial bonding with the polymer matrix, which is vital in optimizing the overall composite properties. Enhances strength, thermal stability, electrical conductivity, hardness, UTS, etc.

Structure of Carbon Nanofiber Typical diameter range: 50 -200 nm. p These fibers are usually long hollow filaments comprised of conical layers of graphitic carbon planes. p This unique structure helps to enhance the conductive properties of this nanomaterial. p www. google. com http: //quantum. soe. ucsc. edu/research/old/ent. html

CNF Production & Nomenclature p Typically produced by catalytic chemical vapor deposition of a hydrocarbon or CO over a surface of metal or metal alloy catalyst: (gas decomposition→carbon deposition→fiber growth) PR-24 -XT-LHT & PR-24 -XT-PS (Pyrograf Products, Inc. ): • PR-24: thin layer of disordered carbon and fiber diameter ~100 nm • XT: improved density debulking method (enhances process handling and dispersion) • LHT: 1500°C heat treatment • PS: pyrolitically stripped CNF (heat treatment that removes unwanted tars and hydrocarbons from fiber surface) SEM image of neat vapor grown carbon nanofibers.

Processing Challenges • Most critical processing technique is dispersion of CNFs within the polymer matrix • Improper dispersion hinders overall composite properties, and can lead to break down • CNF agglomeration tendency (Van der Waals/low sol. )

Low Density Polyethylene Thermoplastic - physical bonds between polymer chains, can be repeatedly processed p Semi-crystalline polymer- amorphous regions of structure and highly branched chains allow for better CNF dispersion within matrix p Retains toughness and pliability over wide temp range p http: //www. google. com/imgres

Material System p EM 460 is the LDPE used in this study provided by Westlake Polymers Corporation: • • Ultimate Tensile Strength=13. 1 MPa (1900 psi) Elongation Yield=120% Young’s Modulus=234. 4 MPa (34000 psi) Melt Index=27 g/10 min. p Vapor-grown carbon nanofibers, manufactured by Pyrograf Products, Inc. , are used as the reinforcing agent in this study: • • Average Diameter=120 nm Length=30, 000 -100, 000 nm Tensile Modulus=600 GPa (on average) Tensile Strength=~7 GPa.

Materials Processing CNF/LDPE composites with different CNF weight percentages were prepared by single-screw extrusion followed by injection molding. p LDPE pellets mixed with 0. 25, 0. 75, 1. 00, 2. 00, 3. 00 wt% of CNF. p CNF were dispersed into the LDPE by extrusion using a Killion single screw extruder. p The extruded CNF/LDPE pellets were fed to a Morgan -Press plastic injection molding machine to mold tensile specimens according to ASTM D 638. p

Thermal Characterization Thermal properties of the molded CNF/LDPE composites were determined by using a differential scanning calorimeter (DSC). p Measurements were carried out in a DSC 131 apparatus produced by SETARAM, Inc. p The samples were cut in the form of thin discs, with weight ranges from 10 mg to 13 mg. p Specimens were analyzed over a temperature range from 30°C to 200°C. A heating rate of 10 °C/min was used without sweeping gas. The temperature was then held at 200°C for 1200 seconds. The specimens were then cooled to 30 °C at a scanning rate of 10 °C/min. p

Thermal Analysis Definitions… p p p p Onset temperature- initial melting/cooling point of specimen; Peak temperature- temperature at which rate of melt or crystallization is fastest Heat of fusion- energy required to melt crystalline component Crystallization temperature- initial cooling/crystallization point of sample Glass transition- (not genuine phase transition) characterizes a “softening” of polymer; defines a point of “relaxation” transition Enthalpy- quantity of a system’s heat exchange; amount of stored/internal energy (total nrg internal+ nrg required by system to displace it) % crystallinity- overall amount of crystalline component in relation to amorphous content

Differential Scanning Calorimetry p p p Thermodynamic analysis that investigates the change in enthalpy (internal energy) between two states of a substance. Measures heat flow as a function of temperature and time. Useful in determining important thermal properties (previous definitions) • Endothermic processes(melting) increase enthalpy, since energy is being absorbed by the system. • Exothermic processes (cooling/crystallization) decrease enthalpy since energy is released by the system. • Observed heat of fusion can be determined by integrating melt or crystallization peaks. http: //www. google. com

DSC Zones… p p p Zones 1 & 3: endothermic, denote melting Zones 2 & 4: exothermic, denote cooling 3 composite samples of each weight percentage CNF (neat, 0. 25, 0. 75, 1. 00, 2. 00, 3. 00) were analyzed in each zone. Average DSC results with standard deviations for VGCNF/LDPE composites and neat LDPE from cooling zone Sample Code ∆Hfobs (J/g) Crystallinity (%) Onset Temp. (°C) Peak Temp. (°C) Pure LDPE 101. 53(± 2. 35) 36. 26(± 0. 84) 98. 30(± 0. 11) 95. 58(± 0. 09) LDPE+0. 25 wt%VGCNF 92. 43(± 1. 50) 33. 09(± 5. 55) 102. 07(± 0. 05) 98. 61(± 0. 12) LDPE+0. 5 wt%VGCNF 100. 61(± 2. 79) 36. 11(± 1. 00) 102. 41(± 0. 08) 98. 67(± 0. 10) LDPE+0. 75 wt%VGCNF 99. 46(± 4. 09) 35. 79(± 1. 47) 102. 48(± 0. 08) 98. 61(± 0. 15) LDPE+1. 0 wt%VGCNF 96. 68(± 3. 01) 34. 88(± 1. 09) 102. 66(± 0. 08) 98. 93(± 0. 13) LDPE+2. 0 wt%VGCNF 91. 24(± 4. 68) 33. 25(± 1. 70) 102. 79(± 0. 09) 98. 60(± 0. 11) LDPE+3. 0 wt%VGCNF 96. 92(± 2. 40) 35. 69(± 0. 88) 103. 28(± 0. 07) 99. 12(± 0. 20)

Data Interpretation: (onset temperature) Zone 2: (98 -103°C) Zone 4: (98 -103°C) • It is proposed that CNFs serve as a nucleating agent for the crystallization of LDPE, which causes higher onset temperatures of crystallization. • Nucleation of the LDPE by the CNF is evident in the graphical jumps. Only a very small amount of CNF is needed to induce nucleation. • Higher onset temp=sooner crystallization • Higher onset temperatures indicate that composites should solidify sooner in a cooling process. • Sooner crystallization=faster processing=potential $$$

Morphological Characterization p Fracture morphological analysis was performed using a scanning electron microscope (SEM). The VGCNF/LDPE composites were observed under JSM 6300 SEM image of (a) VGCNF/LDPE composites with 0. 1 wt% VGCNF, (b) cross-section of VGCNF/LDPE composites with 3. 0 wt% VGCNF.

Neat LDPE-1. 00 wt% CNF Content. . . p p visual comparison of a neat LDPE matrix to increasing wt% CNF filler in LDPE matrix(20 x mag): Notice the automatic increase in the degree of LDPE nucleation due to the introduction of CNF into the matrix. . . Neat LDPE+0. 25%CNF LDPE+0. 75%CNF LDPE+0. 5%CNF LDPE+1. 0%CNF

Thermal/Morphological Conclusions p p p The crystallization onset temperatures of CNF/LDPE composites were higher than those of pure LDPE, which means adding CNF to LDPE causes the polymer to solidify sooner on cooling. This has the processing advantage of potentially reducing the hold time for a mold during injection operations, but has the disadvantage of a reduced amount of time to fill the mold under a given set of processing conditions before solidification occurs (easily alleviated). Morphological analysis of the fracture surface of CNF/LDPE composites showed that CNF were oriented in the direction of the injection inside the mold with a well-dispersed CNF distribution. However, some agglomerates were also observed.

Mechanical Characterization p Tensile testing (ASTM) used to obtain mechanical properties of specimens with varied CNF loadings: • UTS • Stress • Strain • Young’s Modulus • Toughness Standard dog-bone shape of specimen promotes fracture in the gauge length. p

Components of a Stress-Strain Curve: • UTS-maximum allowable stress (before necking) • Yeild Strength-onset of plastic deformation • Young’s Modulusuniaxial stress: uniaxial strain (stiffness) http: //www. ndt-ed. org/Education. Resources/Community. College/Materials/Mechanical/Tensile. htm

Interpretation of Tensile Test Data According to these graphs: • UTS displayed an overall increase of 15% (avg) • Strain decreased linearly with increasing CNF loading • 44% overall increase in Young’s Modulus (avg)

Mechanical Conclusions p p Tensile testing data reveals a significant enhancement of overall composite properties when CNF is introduced to the polymer matrix. UTS and modulus of elasticity increased with increased CNF loading. (improved polymer strength) CNF reinforcement reduced LDPE ductility, while improving strength. A resulting advantage is that (at smaller loadings) CNFs can potentially bridge across any developed microcracks due to loading, which would result in resistance to crack propagation.

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