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Solid freeform fabrication of epoxidized soybean
oil/epoxycomposite with bis or polyalkyleneamine curing agents
q
Z.S. Liu a,*, S.Z. Erhan a, P.D. Calvert b
a Food and Industrial Oil Research, NCAUR, ARS, USDA, 1815 N.
University Street, Peoria, IL 61604, United Statesb Arizona
Materials Laboratories, Department of Materials Science and
Engineering, University of Arizona, 4715 E. Fort Lowell Road,
Tucson, AZ 85712, United States
Received 13 June 2005; received in revised form 22 December
2005; accepted 13 January 2006
Abstract
Extrusion freeform fabrication has been used to make bars of
fiber-reinforced epoxidized soybean oil (ESO)/epoxy resin.
Freeformfabrication methods build materials by the repetitive
addition of thin layers. The mixture of epoxidized soybean oil
(ESO) and epoxyresin are modified with a gelling agent to solidify
the materials until curing occurs. The high strength and stiffness
composites are formedthrough fiber reinforcement. Glass, carbon and
mineral fibers are used in the formulations. It is shown that the
fiber orientation followsthe direction of motion of the write head
that deposits the resins and has a large influence on the
properties of the composite. In addition,the effects of curing
agents, curing temperature, epoxy/ESO ratio, and fiber loading on
mechanical properties of composites are studiedand reported. 2006
Elsevier Ltd. All rights reserved.
Keywords: A. Fibres; A. Polymer-matrix composites (PMCs); Solid
freeform fabrication; B. Mechanical properties
1. Introduction
Fiber-reinforced composites offer great potential for usein
aircraft and automotive primary structure. They are gen-erally
manufactured by using fibers as reinforcement andpolymeric resin as
a matrix. During the last few years, therehas been a growing
interest in the use of polymers obtainedfrom renewable resources
because advantages of these
polymers include their biodegradable properties and, inmany
cases, lower cost [1]. The importance of natural prod-ucts for
industrial applications also becomes very clear
with increasing social emphasis on issues of the environ-ment,
waste disposal, and the depletion of non-renewableresources. United
States agriculture produces over 20 bil-lion pounds of soybean oil
annually, only 590 millionpounds used in industrial application,
and frequentlycarry-over exceeds one billion pounds. The major use
ofsoybean oil is directed to food products such as saladand cooking
oil, shortening, and margarine. Less than
3% of soybean oil is utilized in non-food applications sincea
number of former vegetable oil markets were lost topetroleum
products. Development of economically feasiblenew industrial
products from soybean oil or commercialprocesses is highly
desirable. In our previous paper [2],we reported the preparation of
epoxidized soybean oil(ESO) based composites. These composites
reinforced withglass, short carbon, Franklin Fiber H-45, and Fillex
17-AF1 fibers demonstrated thermophysical and mechanicalproperties
comparable to petroleum-based soft rubberypolymeric materials. In
order to obtain a variety of viable
1359-835X/$ - see front matter 2006 Elsevier Ltd. All rights
reserved.
doi:10.1016/j.compositesa.2006.01.009
q Names are necessary to report factually on available data;
however,the USDA neither guarantees nor warrants the standard of
the product,and the use of the name by USDA implies no approval of
the products tothe exclusion of other that may also be suitable.*
Corresponding author. Tel.: +1 309 681 6104; fax: +1 309 681
6340.
E-mail address: [email protected] (Z.S. Liu).
www.elsevier.com/locate/compositesa
Composites: Part A 38 (2007) 8793
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polymeric materials ranging from elastomers to rigid plas-tics
meeting a wide variety of market demand, a combina-tion of ESO and
epoxy resin is considered.
Epoxy resins are widely used as polymeric matrix incomposites.
However, epoxy resins are similar to otherengineering resins in
that they are either brittle, notch-sen-
sitive, or both. For load-bearing purposes, this means thatthe
product may be subject to catastrophic failure. A majoreffort over
the years focused on improving the toughness ofepoxy structural
systems. As a result, epoxy resins havebeen used for structural
applications such as adhesives,encapsulation of electronic devices,
and composites suchas electrical laminates, aerospace parts and
automotiveparts.
Sue et al. [3] have reviewed rubber-toughening of epoxyresins.
This method has been most effective and can improvetoughness
substantially. Qureshi et al. [4] reported that theuse of 25%
epoxidized crambe oil (ECO) as reactive diluentsin bisphenol A and
cycloaliphatic epoxy compounds gave
improvement in resistance to fatigue crack propagationwithout
significant sacrifice in tensile or impact strengthand Youngs
modulus. Raghavachar et al. [5] recentlyreported rubber-toughening
epoxy thermosets with epoxi-dized crambe oil. They reported the
phase separation inthermoset matrices of diglycidyl ether
bisphenol(DGEBA)-4,4 0-diaminodiphenylmethane (DDM)-ECOand
improvement in toughness of the epoxy matrices. Frac-ture toughness
values of the epoxy thermosets wereincreased approximately 100% by
both 5% and 10% epoxi-dized crambe oil. It is clear that a
combination of epoxi-dized vegetable oil, epoxy resin and curing
agents can be
formulated to meet a wide variety of market demands.Soy-based
polymers will support global sustainability andprovide an
alternative to synthetic polymers for many man-ufacturing
applications [6].
Solid freeform fabrication (SFF) is a method of makingshapes
without molds. It is best known in its stereolitho-graphy forms as
a method of rapid prototyping. In stereo-lithography a laser
photopolymerizes successive thin layersof monomer to build up a
solid object. Extrusion solid free-form fabrication was developed
by the University of Ari-zona in collaboration with Advanced
Ceramic Research(Tucson, AZ) [7]. It functions essentially as a
three-dimen-sional (3D) pen plotter. A fine stream of liquid resin
orslurry is pumped from a syringe as it moves over the sur-face of
a hot-plate to trace out a layer of materials. The syr-inge then
moves up one step and writes the next lager as thefirst continues
to cure (Fig. 1). The shape to be producedmay be derived from a 3D
CAD program or from standarddrawing packages. This method has the
potential to pro-duce new materials and complex composites that
couldnot be made in any other way.
In this work, we report the preparation of ESO/epoxybased
composites by extrusion solid freeform fabricationmethod. High
strength and stiffness parts can be obtainedthrough fiber
reinforcement. These high performance com-
posites will be tested in agriculture equipment, the automo-
tive industry, civil engineering, marine infrastructure,
railinfrastructure, and the construction industry. Recentadvances
in genetic engineering, natural fiber development,
and composite science offer significant opportunities
forimproved materials from renewable resources withenhanced support
for global sustainability.
2. Experimental
2.1. Materials
The resin used as a co-matrix is EPON 828, providedby the Shell
Chemical Company (Houston, Texas).EPON 828 is a bisphenol
A/epichlorohydrin based epoxyresin, which is the most widely used
epoxy. ESO (7.0% oxi-
rane oxygen) was purchased from Alf Atochem Inc. (Phil-adelphia,
PA). Calcium sulfate microfiber, Franklin Fiber
H-45 used in the experiments was provided by the UnitedStates
Gypsum Company, (Chicago, IL). Wollastonitemineral fiber, Fillex
17-AF1 fiber is surface-modified wol-lastonite, an inorganic
mineral reinforcement. The Fillex
17-AF1 fiber was provided by Intercorp Inc. (Milwaukee,WI).
Glass fiber is milled E-glass (electric glass) fiber witha nominal
length of 1/32 in. Fiber diameter of 10 lm wasused in all
experiments. All above fibers were used asreceived. Short carbon
fiber was purchased from DupontCo. (Wilmington, DE) and chopped in
a coffee grinderfor 20 s to reduce the length. Optical microscopy
gave aver-age length from 0.25 to 0.12 mm. A previous study [8]
char-acterized length distributions, diameter distributions
andaspect ratios obtained for fibers chopped in this way. Cur-ing
agents, 2,2 0-(ethylenedioxy)-bisethylamine, JeffamineEDR-148,
polyalkyleneamine, Jeffamine D-230, and Jeff-amine T-403 were
provided by Huntsman Corporation,(Houston, Texas), and used as
received. Diethylenetria-mine (DETA) and triethylenetetramine
(TETA) 60% techwere purchased from Aldrich Chemical Inc.,
(Milwaukee,WI), and used without further purification. The
chemicalstructures of curing agents are shown in Scheme 1.
Thixo-tropic agent, Aerosil R805 was obtained from Degussa
Corp. (Ridgefiled Park, NJ).
Fig. 1. Sketch of the extrusion freeform fabrication
apparatus.
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2.2. Forming of composites
ESO and EPON 828 were mixed well in the ratio of1:0.3. Vacuum
was applied to remove air bubbles at55 C for 30 min. Then mixture
was mixed with AerosilR805, 10 g /100 g (ESO + EPON), and fibrous
fillers. Themixture was degassed in vacuum system at 55 C for30
min. The fiber filled slurries showed a yield point, suchthat
formed parts hold their shape until cured. The mixturewas removed
from the oven, and cooled to room tempera-ture. Curing agent, 23.3
g/100 g (ESO + EPON) was thenadded, and after proper mixing, the
paste was put into20 cc plastic syringe. Bars 75 mm 8 mm 4 mm
wereformed by deposition of five layers, and subsequently curedat
100 C for 24 h, then at 180 C (Jeffamine curing agents)for 48 h, or
at 150 C (TETA or DETA curing agent) for48 h.
2.3. Solid freeform fabrication
Solid freeform fabrication was conducted using anAsymtek model
402 fluid dispensing system, equipped withsmall stepper motors
(Oriel stepper mike) to drive the deliv-
ery syringe. The Asymtek and syringes were controlled by a
program written in Microsoft Quick Basic. Solid bar sam-ples
were written as a series of lines.
2.4. Scanning electron microscopy
Scanning electron microscopy (SEM) was performed to
investigate the fracture surface of the composites and
inter-face between the filler and the polymeric matrix. The
SEMphotographs were taken with a JEOLJSM 6400 V instru-ment. The
specimens were previously coated with gold.The microscope was
operated at 5 kV.
2.5. Mechanical testing
The mechanical properties testing were done using athree point
bending test method with an Instron model1100 testing machine. The
tests were carried out at roomtemperature. Because samples for
three point bending testare rectangular shapes, ASTM D-90 test
standard was fol-
lowed. The support span length was modified to 40 mm(according
to ASTM-90, the support span length of50 mm was recommended). The
standard formula for themodulus, E and strength, U in three point
bending of abeam was used
E PL3=4bd3d; U 3PL=2bd2
Here Pis equal to the break load, L is the support span, d isthe
deformation at the center under load P, dis the sampleheight, and b
is the sample width.
3. Results and discussion
3.1. Composite morphology
Scanning electron microscopy (SEM) was performed tocharacterize
the morphology of soybean oil-based filledcomposite materials. In
our previous paper [2], we reportedSEM micrographs of four
different kinds of fibers: (1) thecommercial milled glass fibers
with a nominal length of1/32 in. and fiber diameter of 10 lm, the
aspect ratio distri-bution reported according to Peng [9] shows two
mainpeaks at values of 3 and 7; (2) calcium sulfate
microfiber,Franklin Fiber H-45 with average length of 6075
lm,average diameter of 1.52 lm, average aspect ratio of 40;(3)
Wollastonite mineral fiber, Fillex 17-AF1 with averageaspect ratio
of 20, diameter of 2.5 lm; (4) the carbon fiberwas grounded for 20
s in coffee ground, giving an averagelength from 0.25 to 0.12 mm.
Those fibers were used inall the experiments. Fig. 2 shows SEMs of
freshly fracturedsurface for ESO/EPON composite filled with four
types offibers. Fig. 2(a) corresponds to milled glass fiber. Fig.
2(b)corresponds to Franklin Fiber H-45. Fig. 2(c) corre-sponds to
Fillex 17-AF1 fiber. Fig. 2(d) corresponds tocarbon fiber. SEM
photographs show the well-developedinterfacial interaction between
fiber (Franklin Fiber
H-45 or Fillex 17-AF1 fiber) and matrix. Clearly, for glass
fiber and carbon fiber, there are fiber breakages due to the
H2N CH CH2 O[ CH2 CH
CH3 CH3
]x NH2
Jeffamine D-230
CH2
CH2
O CH2
O CH2
O CH2
CH
CH
CH
[
[
[
(CH3) ]x
NH2
NH2
(CH3) ]y NH2
(CH3) ]z
CH3CH2 C CH2
Jeffamine T-403
H2N CH2 CH2 O CH2 CH2 OCH2 CH2 NH2
Jeffamine ERD-148
H2N CH2 CH2 NH CH2 CH2 NH2
Diethylenetriamine (DETA)
H2N CH2 CH2 NH CH2 CH2 NH CH2 CH2 NH2
Triethylenetetramine (TETA)
H ( NH CH2 CH2 )n NH2
Polyethylenimine, linear
Scheme 1. Chemical structures of curing agents.
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polyether chains. The reactivity of amines located on sec-ondary
carbon atoms is lower than that of amines locatedon primary carbon
atoms. That is the reason the reactivityof Jeffamine curing agents
in the order of EDR-148 > T-403 > D230. By comparison of
triethylenetetramine(TETA) and diethylenetriamine (DETA) to
Jeffamine cur-
ing agents, TETA and DETA curing agents provide com-posites with
better mechanical properties. This is becausethat three are two
primary amine groups located on pri-mary carbon atoms at the ends
of an aliphatic polyiminechain in TETA and DETA molecules. At the
same time,there is one secondary amine group in DETA moleculeand
two secondary amine groups in TETA molecule. Thosesecondary amine
groups also take part in reaction and for-mulate a network
structure of composites. The networkstructure of composites would
show the stronger mechani-cal properties.
3.4. Effect of fiber type
The incorporation of disperse fibers into polymers canimprove
the mechanical and thermal properties of materi-als required for
engineering applications. This change inthe mechanical and thermal
behavior is due to several fac-tors such as variation in the
mobility of the macromole-cules in the boundary layers, the
orientation influence ofthe fiber surface, effect of fibers on the
structure of thepolymers, as well as the different types of
fiber-polymerinteractions. In order to study the effect of
different typesof fibers on the mechanical properties, the
experimentswere carried out to make test bars. The composite
formu-
lation is ESO, 52.0 vol%, EPON, 13.5 vol%, and fiber,18.7 vol%
with Jeffamine EDR-148 curing agent, 23.3 g/100 g (ESO + EPON). The
mechanical properties measure-ment results are presented in Table
3. Among the differentfibers, glass and carbon fibers show better
reinforcingeffects than mineral fibers. These results are probably
dueto glass and carbon fibers themselves having high strength.For
example, E glass fiber has a tensile strength of about3 GPa and a
modulus that approaches 100 GPa. They exhi-bit the large
contribution to the strength and rigidity of thereinforced
composites. The principal advantages of glassfibers commonly used
as reinforcing fibers for polymermatrix are their high strength and
their low cost. FranklinFiber H-45 used in this study has average
6075 lm
length. Specific gravity is 2.452.55 g/cc. As a microfiber,it
shows moderate reinforcement. Advantages of this min-eral fiber are
their cheap price and good temperature stabil-ity. Wollastonite is
a semi-reinforcing mineral fiber. Thesurface of the fiber is
modified. Their properties of bondingat the interface between resin
matrix and inorganic miner-
als are improved. Their mechanical properties, such as ten-sile
strength and modulus have not been presented, but it isprobably
similar to most microfibers with a modulus ofapproximately 100 GPa,
but relatively low tensile strength.Wollatonite provides many
processing performance bene-fits, including increased stiffness and
strength, improvedheat distortion temperature, low coefficient
expansion,and ease processibility at considerably lower cost.
3.5. Influence of fiber orientation on flexural modulus
The strength and moduli of fiber-reinforced compositesdepends on
fiber volume fraction, aspect ratio, and orienta-
tion. There are good theoretical treatment for the effect
ofaspect ratio and volume fraction on modulus [11], but
ori-entation is more difficult to control. It is strongly
influencedby processing methods and by local flow conditions in,
forinstance, injection-molded parts. As a result, changes in
vol-ume fraction and aspect ratio will change the degree of
ori-entation. Also, theoretical treatments are not
verysatisfactory. Peng and Calvert [12,13] studied
orientationeffects in freeformed short-fiber composites. They
sectionedand polished samples of epoxy/glass composites cross
thewrite direction. Optical microscopy was used to measurethe major
and minor axes of the elliptical fiber sections.
They discovered that fiber orientation corresponds closelyto the
machine direction during sample preparation. Inglass fiber/epoxy
composites, they found that 90% of fibersare within 10 of the
machines write direction. By writing aseries of test bars with
write axes at different angles to thelong axis, they can vary the
modulus by approximately afactor of three. In ESO/epoxy system, we
prepared test barsby writing at varying angles relative to the axis
of the testbars to investigate effects of fiber orientation on the
proper-ties of composites. The composites formulation is
ESO/EPON/Jeffamine EDR-148 reinforced with 23.4 wt.% ofFranklin
Fiber H-45. The effect of this orientation on flex-ural modulus is
shown in Table 4. It can be seen that mod-ulus is much greater
parallel to the writing direction thancross this direction. The
modulus can be varied by approx-imately a factor of three and a
half. Peng [13] discussed dis-tribution of fiber orientation. There
is a distribution of fiberorientation from small to major depending
on the writespeed. At lower write speed, such as 250 step s1, and
pumpspeed at 16 lm s1, there is a major distribution. But athigher
writ speed of 750 step s1 there is a small fiber distri-bution. In
this fiber orientation study, higher writing speedof 750 step s1
used, we expected that there is a small distri-bution of fiber
orientation. An increase in modulus asshown in Table 4, is resulted
in increasing fiber orientation.
This is significant because the composite modulus is at
least
Table 3Effect of different types of fibers
Fiber Flexuralmodulus (GPa)
Flexuralstrength (MPa)
Strain atbreak (%)
E-glass 1.28 22.3 3.2Carbon 1.06 21.0 6.6Franklin 0.97 21.0
2.2Wollastonite 0.77 19.5 4.2
Conditions: ESO, 52.0 vol%, EPON 828, 13.5 vol%, and fiber, 18.7
vol%,in the composite formulations with Jeffamine EDR-148, 23.3
g/100 g
(ESO + EPON).
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as sensitive to orientation as to fiber aspect ratio and vol-ume
fraction.
3.6. Effect of EPON 828/ESO ratio
The experiments were carried out for investigating influ-
ence of EPON/ESO ratio on mechanical properties of
thecomposites. The results are presented in Table 5. It wasfound
that flexural modulus increases with the EPON con-centration
increases. This indicates that EPON resin has adetermining
influence on mechanical properties of the com-posites. In the two
component matrix system, ESO is a softsegment, and EPON epoxy is a
hard segment. An increasein the EPON resin concentration provides
the compositewith a higher flexural modulus. However, when
theEPON/ESO weight ratio is more than 0.3 to 1, phase sep-aration
was observed. This is because EPON has a muchhigher reactivity with
curing agent Jeffamine EDR-148
than ESO, because of the location of epoxy groups; itreacts
first with curing agent and forms a rigid EPON-Jeff-amine phase.
Peng [12] has studied the physical propertiesof composites prepared
by pure EPON 828 resin and Jeff-amine EDR-148 with glass fiber. The
flexural modulusand flexural strength of the composites can reach
to6.3 GPa and 109 MPa, respectively. The composites pre-pared by
pure EPON 828 resin with Franklin fiber wouldhave better mechanical
properties. The brittle and notch-sensitive properties of epoxy
composites need to beimproved. It is well know that soybean oil is
a mixture oftriglycerides formed by different saturated and
unsaturatedfatty acids. All C@C bonds in the unsaturated fatty
acids
are located in the middle of fatty acid chain. Hence,
reac-tivity of internal epoxy group of ESO obtained by the
epoxidation of double bonds is slower, compared to theepoxy
group on the end of epoxy resin chains. This isdue mainly to the
diffusional restrictions.
3.7. Effect of fiber loading
At conditions of EPON/ESO in the ratio of 0.3:1, fumedsilica, 10
g/100 g (ESO + EPON), and Jeffamine EDR-148,23.3 g/100 g (ESO +
EPON), the flexural modulus of com-posite as a function of Franklin
Fiber H-45 loading is pre-sented in Fig. 3. It is observed that the
increase in fibercontent leads to an increase in the flexural
modulus. Inthe case of fiber-reinforced composites, it is well
knownthat there exits a critical aspect ratio at which the
mechan-ical properties of the composites are maximized. This
crit-ical aspect ratio depends on the volume fraction of the
fiberand also on the ratio of the modulus of fiber to matrixmodulus
[14]. When the volume fraction increases beyond
this limit, the slurry is too viscous and will not flow
throughthe needle. Therefore data for higher Franklin Fiber
H-45contents are not available.
4. Conclusions
The application of solid freeform fabrication to rein-forced
composites has been explored. Fiber reinforcedepoxidized soybean
oil/epoxy composites can be formedwith high strength and stiffness.
Different fiber types, E-glass fiber, carbon fiber, and mineral
fibers are used in rein-forced composites. It was found that glass
fiber and carbonfiber show better reinforcing effects than mineral
fibers. Bywriting a series of test bars with write axes at
different anglesto the long axis, modulus can be varied by
approximately afactor of three and a half. This is significant
because com-posite modulus is at least as sensitive to orientation
as tofiber aspect ratio and volume fraction. By studying the
reac-tivity of different curing agents, it shows the Jeffamine
cur-ing agents in the order of EDR-148 > T-403 > D230.
DETAand TETA curing agents provide the composites withhigher
physical properties. The reinforcement is moremarked at elevated
temperature. Further, the prospect ofusing solid freeform
fabrication method for composite fab-rication from naturally
derived matrix materials has many
potential applications in making shapes that cannot be
Table 4Effect of orientation on EPON 828/ESO/Jeffamine EDR-148
Reinforcedwith 23.4 wt.% Franklin Fiber H-45
Fiberorientation
Flexuralmodulus (GPa)
Flexuralstrength (MPa)
Strain atbreak (%)
0 0.97 21.0 2.230 0.42 14.0 5.0
45 0.34 16.0 5.060 0.32 22.0 4.390 0.29 8.1 2.2
Conditions: ESO, 47.6 wt.%, EPON, 14.5 wt.%, and Jeffamine
EDR-148,23.3 g/100 g (ESO + EPON).
Table 5Effect of EPON 828/ESO ratio
EPON/ESO ratio(weight)
Flexuralmodulus (GPa)
Flexuralstrength (MPa)
Strain atbreak (%)
0.30:1 0.97 21.0 2.20.22:1 0.71 16.3 3.30.15:1 0.26 12.1 3.6
Conditions: Franklin Fiber H-45, 23.4 wt.%, Jeffamine EDR-148,
23.3
g/100 g (ESO + EPON) and fumed silica, 10.0 g/100 g (ESO +
EPON).
0
0.4
0.8
1.2
1.6
15 20 25 30 35 40 45
Fiber content (wt%)
Flex
uralModulus(GPa)
Fig. 3. Flexural modulus of composites as a function of Franklin
Fiber
H-45 fiber content.
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machined. The resulting composites have sufficient mechan-ical
properties to be used in a wide variety of areas, such
asagriculture equipment, civil engineering, the automotiveindustry
and the construction industry.
Acknowledgement
The authors gratefully acknowledge Dr. Arthur Thomp-son for help
in SEM, and Ms. Jiong Peng for help in threepoint bending test.
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