Transcript Slide 1

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:
 Polymer
Nanocomposites
 Carbon Nanofibers (CNF)
 Low Density Polyethylene (LDPE)
 Materials Processing (CNF/LDPE)
 Thermal & Morphological Characterization
 Tensile Testing
 Mechanical Characterization
 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.
 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.
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POLYMER NANOCOMPOSITES
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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
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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.
 These fibers are usually long hollow filaments comprised
of conical layers of graphitic carbon planes.
 This unique structure helps to enhance the conductive
properties of this nanomaterial.
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www.google.com
http://quantum.soe.ucsc.edu/research/old/ent.html
CNF Production & Nomenclature
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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 ~100nm
•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
 Semi-crystalline polymer- amorphous regions of
structure and highly branched chains allow for better
CNF dispersion within matrix
 Retains toughness and pliability over wide temp range
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http://www.google.com/imgres
Material System
 EM
460 is the LDPE used in this study provided by
Westlake Polymers Corporation:
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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.
 Vapor-grown
carbon nanofibers, manufactured by
Pyrograf Products, Inc., are used as the reinforcing
agent in this study:
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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.
 LDPE pellets mixed with 0.25, 0.5, 0.75, 1.00, 2.00,
3.00 wt% of CNF.
 CNF were dispersed into the LDPE by extrusion using
a Killion single screw extruder.
 The extruded CNF/LDPE pellets were fed to a
Morgan-Press plastic injection molding machine to
mold tensile specimens according to ASTM D638.
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Thermal Characterization
Thermal properties of the molded CNF/LDPE
composites were determined by using a differential
scanning calorimeter (DSC).
 Measurements were carried out in a DSC 131
apparatus produced by SETARAM, Inc.
 The samples were cut in the form of thin discs, with
weight ranges from 10 mg to 13 mg.
 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.
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Thermal Analysis Definitions…
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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
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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…
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Zones 1 & 3: endothermic, denote melting
Zones 2 & 4: exothermic, denote cooling
3 composite samples of each weight percentage CNF (neat, 0.25, 0.5, 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
 Fracture
morphological analysis was performed using
a scanning electron microscope (SEM). The
VGCNF/LDPE composites were observed under JSM
6300 SEM.
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-1.00wt% CNF Content...
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visual comparison of a neat LDPE matrix to increasing wt% CNF filler in
LDPE matrix(20x mag):
Notice the automatic increase in the degree of LDPE nucleation due to the
introduction of CNF into the matrix...
Neat LDPE
LDPE+0.25%CNF
LDPE+0.75%CNF
LDPE+0.5%CNF
LDPE+1.0%CNF
Thermal/Morphological Conclusions
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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
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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.
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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/EducationResources/CommunityCollege/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
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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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