C.-E. PELIN et al.: MECHANICAL PROPERTIES OF POLYAMIDE/CARBON-FIBER-FABRIC COMPOSITES
723–728
MECHANICAL PROPERTIES OF
POLYAMIDE/CARBON-FIBER-FABRIC COMPOSITES
MEHANSKE LASTNOSTI KOMPOZITNE TKANINE IZ
POLIAMID/OGLJIKOVIH VLAKEN
Cristina-Elisabeta Pelin1,2, George Pelin1,2, Adriana ªtefan1, Ecaterina Andronescu2,
Ion Dincã1, Anton Ficai2, Roxana Truºcã3
1National Institute for Aerospace Research "Elie Carafoli" Bucharest- Materials Unit, 220 Iuliu Maniu Blvd, 061126 Bucharest, Romania
2University Politehnica of Bucharest, Faculty of Applied Chemistry and Materials Science, 1-7 Gh. Polizu St., 011061 Bucharest, Romania
3S.C. METAV Research & Development S.A., 31 C.A. Rosetti St., 020011 Bucharest, Romania
bancristina@gmail.com, ban.cristina@incas.ro
Prejem rokopisa – received: 2015-07-01; sprejem za objavo – accepted for publication: 2015-09-10
doi:10.17222/mit.2015.171
This paper presents the production of carbon-fiber-fabric-reinforced laminated composites based on a polyamide 6 matrix using
a multiple-stages technique that involves polymer dissolution in formic acid followed by fabric impregnation and high-tempera-
ture pressing. The polyamide/solvent ratio’s influence on the interface and mechanical properties is discussed, analyzing three
PA6 weight contents of (10, 20, and 30) % in a formic acid solvent. The mechanical behavior of the obtained laminated compo-
sites is evaluated using tensile and 3-point bending tests and the fracture cross-section is analyzed using microscopy
investigation techniques in order to evaluate the fiber-matrix interface and the composite fracture mechanism. The results show
that the best mechanical performance is obtained when using a solution of 20 % mass fraction of polyamide in formic acid, as
this leads to the formation of a uniform polymer layer that is able to completely embed the fibers that constitute the fabric and
create a strong mechanical interface within the composite.
Keywords: polyamide 6, carbon fiber, mechanical properties, polymer/solvent ratio, mechanical interface
^lanek predstavlja izdelavo laminatnega kompozita na osnovi poliamida 6, oja~anega s tkanino iz ogljikovih vlaken, z uporabo
ve~stopenjske tehnike, ki vklju~uje raztapljanje poliamida v mravljin~ni kislini ter impregnacijo tkanine in stiskanje pri visoki
temperaturi. Razlo`en je vpliv razmerja poliamid/topilo na stik in mehanske lastnosti, z analizo treh masnih vsebnosti PA6 (10,
20, 30) % v mravljin~ni kislini. Mehansko obna{anje dobljenega laminiranega kompozita je ocenjeno z nateznim preizkusom in
s 3-to~kovnim upogibnim preizkusom, presek preloma pa je analiziran z mikroskopsko tehniko, da bi ocenili stik z vlaknato
osnovo in mehanizem preloma kompozita. Rezultati ka`ejo, da je najbolj{a mehanska zmogljivost dose`ena pri uporabi
raztopine z 20 % masnim dele`em poliamida v mravljin~ni kislini, ker to povzro~i nastanek enakomernega polimernega sloja, ki
lahko popolnoma obda vlakna tkanine in ustvari mo~an mehanski stik v kompozitu.
Klju~ne besede: poliamid 6, ogljikovo vlakno, mehanske lastnosti, razmerje polimer/topilo, mehanski stik
1 INTRODUCTION
In recent decades the use of composite structures in
both aeronautic and automotive applications has in-
creased tremendously. Nowadays, composites represent
approximately 50 % of the structure of the Airbus A350
XWB and the Boeing 787 Dreamliner, resulting in 20–25
% reduction in fuel consumption.1 Most composite struc-
tures are based on thermoset matrix fiber composites, but
nowadays there is an increasing interest in replacing the
thermoset with a thermoplastic matrix. This trend is due
to problems arising from thermoset composites, such as
the high costs of raw materials, high energy consump-
tion, extended processing times, non-visible damage,
complex repair procedures, recycling difficulties and sig-
nificant amounts of scrap.2–3 A thermoplastic matrix of-
fers facile recycling possibilities, lower costs and more
flexible processing routes3–6, making them a promising
solution for the shortcomings of thermoset composites.
Commercial carbon-fiber-fabric-based composites for
the transport industry obtained using melt and solvent
impregnation with thermoplastic matrixes use PEEK,
PPS or PEI7–8, which are high-performance polymers that
require over 300 °C for the processing temperature, lead-
ing to high costs. This study focuses on an engineering
polymer, with a high potential, i.e., polyamide 6. Most
literature studies present polyamide 6 composites rein-
forced with short fibers (carbon, glass or aramid) pro-
cessed by melt extrusion9–12, a few studies present fab-
ric-reinforced polyamide 6 composites13–14, because of
the technological processing difficulties arising from fab-
ric-impregnation issues generated by the molten polymer
and fiber wet out.15 Moreover, the data concerning the
optimum solution viscosity range and the optimum poly-
mer/solvent ratio when using the solvent-impregnation
method is very briefly discussed, although its importance
is underlined16–18, as the rheological properties of the ma-
trix are important for establishing the composite’s pro-
cessing parameters. This paper presents the production
of carbon-fiber-fabric-reinforced polyamide 6 matrix
laminated composites using polymer dissolution fol-
lowed by fabric solvent impregnation, solvent removal
Materiali in tehnologije / Materials and technology 50 (2016) 5, 723–728 723
UDK 67.017:621.315.614:677.494.675 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(5)723(2016)
and thermal pressing. The processing and final properties
of fabric-reinforced composites depend on several fac-
tors, one of them being the viscosity of the matrix used
to impregnate the fabric. In this context, the novelty of
the study is represented by an optimization study con-
cerning the PA6/formic acid (polymer/solvent) ratio’s di-
rect influence on the mechanical properties of the final
composites, analyzing the solution’s viscosity and its di-
rect effect on the fiber/matrix interface and, conse-
quently, on the failure mode of the composites. The me-
chanical behavior of the obtained laminated composites
was evaluated using tensile and 3-point bending tests, the
fracture cross-section being analyzed using microscopy
techniques to evaluate the fiber-matrix interface and the
composite fracture mechanism and to establish the opti-
mum polymer content for the impregnation solution. The
results indicate that at an optimum polymer/solvent ratio
of 20 % of mass fractions, the obtained materials possess
the highest tensile and flexural properties.
2 EXPERIMENTAL SECTION
2.1 Materials
The matrix was polyamide 6 (PA6) pellets supplied
by SC ICEFS Sãvineºti, while the solvent was formic
acid 85 % analytical grade, purchased from Chemical
Company, Romania. The reinforcing agent was a car-
bon-fiber-fabric twill weave (FC) produced by Chemie
Craft, France, 3 K warp, with 193 g/m2 fabric areal
weight and 1.7 g/cm3 fiber density.
2.2 Obtaining process
The procedure resembles a process involving fabric
impregnation with thermoset resins, but it is adjusted to
allow the use of polyamide pellets as the raw material.
The dried polymer pellets were dissolved in 85 % formic
acid in three different concentrations, i.e., 1(0, 20 and
30) % (polymer weight/solvent volume), under mecha-
nical stirring for 4 h. Each ply of the carbon-fiber fabric
was impregnated with the solution and stacked up in
groups of five layers. The solvent was removed at room
temperature for 48 h and additional traces were removed
at 80–100 °C for 8 h. Each laminated composite was
pressed using a CARVER hot platens press, following an
established temperature program, with a linear tempe-
rature increase from 25 °C to 230 °C and 5 min dwell
periods at (230, 235, 240 and 245) °C. Because the
fabric layers were semi-impregnated with polymer after
the solvent’s removal, the high-temperature pressing did
not generate impregnation difficulties that appear during
standard polymer-melt impregnation because of the fiber
wet-out difficulties.19 The cooling took place under
pressure down to room temperature. There were obtained
laminated plates differing in the PA6 content solution
used, referred to as PA6(10%)/5FC, PA6(20%)/5FC and
PA6(30%)/5FC, with an average fiber volumetric ratio of
66 %, that were processed into tensile and flexural shape
specimens.
2.3 Testing and characterization
The viscosity of the different concentration solutions
used for the impregnation was measured using an
Ubbelohde capillary-tube viscometer (Cannon CT-1000).
The PA6 matrix was subjected to FTIR spectroscopy
analysis (ThermoiN10 MX Mid Infrared FT-IR Micro-
scope) operated in ATR mode and scanning electron mi-
croscopy (QUANTA INSPECT F). The carbon-fiber
laminates were mechanically tested (INSTRON 5982
machine) in tensile conditions according to SR EN ISO
527-220 at a 5 mm/min tensile rate on dumbbell speci-
mens and flexural tests, according to SR EN ISO 1412521
at 2 mm/min, conventional deflection and span length on
rectangular specimens. The fracture cross-section was
analyzed using SEM and the fracture mode was evalu-
ated using optical microscopy (Meiji 8500).
3 RESULTS AND DISCUSSION
3.1 Viscosity measurement
The solutions containing different contents of dis-
solved polymer used to impregnate the fabric has to have
optimum viscosity, as it distributes the polymer through
the fibers.22 The effect of the PA6 content dissolved in
formic acid on the kinematic viscosity of the solution
was studied at room temperature. Figure 1 presents the
kinematic viscosity values of the solutions with three dif-
ferent PA6 weight contents, after 4 h of mechanical stir-
ring. The viscosity increases dramatically with polymer
content, from 34.1 mm2/s for 10 % PA6 in formic acid to
173 mm2/s for 20 % and 898 mm2/s for 30 %. As the
polymer content is increased, the viscosity increases ex-
ponentially by approximately five times compared to the
C.-E. PELIN et al.: MECHANICAL PROPERTIES OF POLYAMIDE/CARBON-FIBER-FABRIC COMPOSITES
724 Materiali in tehnologije / Materials and technology 50 (2016) 5, 723–728
Figure 1: Kinematic viscosity as a function of polyamide 6 weight
content in an 85 % formic acid solution
Slika 1: Odvisnost kinemati~ne viskoznosti od vsebnosti poliamida 6
v raztopini 85 % mravljin~ne kisline
previous value. The dramatic increase for the 30 % con-
tent is due to the fact that this value is very close to the
homogenous phase-formation limit of a polyamide/for-
mic acid system.23–27 The rheological properties affect
the impregnation degree and the fiber/matrix interface. If
the impregnating solution viscosity is too low, it will
pass through the fabric, resulting in polymer coverings
that are too thin, while a too high viscosity will not en-
sure uniform and large fiber-matrix contact surfaces, as
any penetration through the fibers will be difficult.28
3.2 FTIR spectroscopy
FTIR spectroscopy was performed on polyamide
films obtained after solvent removal from the three dif-
ferent solution concentrations to evaluate the chemical
structure and the eventual solvent traces. Figure 2 pres-
ents the spectra of the three PA6 samples compared to a
pure PA6 pellet, all the spectra showing the characteristic
peaks of polyamide 6.29,30 There are no significant
changes in the spectra of the samples compared with the
pure pellet, as no supplementary peak appears, and there
are no traces of unremoved solvent, confirming that the
polyamide’s chemical structure was not altered by the
solvent’s presence and it was fully dissolved. The small
modification of the bands from (689.5, 1201.4 and
1464.7) cm–1 can be assigned to the restructuring of the
polymer as a result of solubilization followed by crystal-
lization.31 The 1200 cm–1 and 1465 cm–1 peaks corre-
spond to the amide V and CH2 bending vibrations, re-
spectively, in polyamide  or  form, which is
commonly obtained when the processing of PA6 in-
volves slow cooling32, as is the case here.
C.-E. PELIN et al.: MECHANICAL PROPERTIES OF POLYAMIDE/CARBON-FIBER-FABRIC COMPOSITES
Materiali in tehnologije / Materials and technology 50 (2016) 5, 723–728 725
Figure 4: SEM images of the fracture cross-section of laminated composites: a), b) PA6(10%)/5FC, c) PA6(20%)/5FC
Slika 4: SEM-posnetki preseka preloma laminiranega kompozita: a), b) PA6 (10 %)/5FC, c) PA6 (20 %)/5FC
Figure 2: FTIR spectra of the pure PA6 pellet and dried samples
based on different PA6 contents dissolved in 85 % formic acid: PA6
(10 %), PA6 (20 %), PA6 (30 %)
Slika 2: FTIR-spektri ~istega peleta PA6 in posu{enih vzorcev z
razli~no vsebnostjo PA6, raztopljene v 85 % mravljin~ni kislini: PA6
(10 %), PA6 (20 %), PA6 (30 %)
Figure 3: SEM images of matrix samples used to form the laminated composites: a) PA6(10 %), b) PA6(20 %), c) PA6(30 %)
Slika 3: SEM-posnetki osnove vzorcev, uporabljenih za laminiran kompozit: a) PA6 (10 %), b) PA6 (20 %), c) PA6 (30 %)
3.3 SEM analysis
SEM analyses were performed on the matrix samples
in the form of films to evaluate the morphology and ho-
mogeneity. Figure 3 presents the images of dried sam-
ples obtained after the dissolution of (10, 20 and 30) %
PA6 into formic acid. As the PA6 content increased up to
30 %, there are visible areas of undissolved polymer; this
higher concentration value being close to the weight con-
tent limit of PA6 in formic acid23–26, it probably needs
different process parameters for a complete dissolution
(e.g., a longer homogenization time).
SEM investigations were performed on the fracture
cross-section of the tensile tested laminates. Figure 4 il-
lustrates the samples with a matrix obtained by dissolv-
ing 10 % PA6 (Figure 4a and 4b) and 20 % PA6 (Figure
4c). In PA6(10%)/5FC (Figure 4a) there are several ar-
eas where a thin polymer layer uniformly covers the fi-
bers of the fabric, but there are also a few areas where
the polymer layer is partially detached from the fiber sur-
face and there are uncovered fibers (Figure 4b). The dif-
ference is significant for PA6(20%)/5FC, in which case
the polymer is distributed in a solid layer that covers the
entire fiber surface, but its thickness is not as uniform as
in PA6(10%)/5FC. Each carbon fabric ply is covered
with its own polymer layer. The polymer ductile fracture
is distinguished from the fiber’s fragile fracture.
3.4 Mechanical testing
Table 1 illustrates the average values of the strength
and elasticity modulus, obtained after tensile and flexural
testing of the obtained composites based on five carbon
fabric plies. The highest mechanical performance in both
the tensile and flexural testing is presented by the sample
based on PA6(20%). PA6(20%)/5FC has a 60 % higher
tensile strength compared to PA6(10%)/5FC, while the
flexural strength is approximately 70% higher. These
samples also have a superior rigidity, showing a modulus
of elasticity that is higher by 35–40 %. The
PA6(30%)/5FC samples showed lower tensile and flex-
ural strengths and tensile moduli compared with
PA6(20%)/5FC, but higher than the PA6(10%)/5FC,
while the flexural modulus had the lowest average value
for the entire series. These inconsistent results in
PA6(30%)/5FC are most likely due to the high viscosity
of the impregnating solution that was probably not able
to uniformly distribute on the surface of the fiber fabric,
supplemented by possible undissolved polymer sites,
which although they were melted during thermal press-
ing, could also lead to non-uniform polymer layer sites at
the microscopic level. These issues generate
inhomogeneity and stress-concentration sites that de-
crease the rigidity.33–34
Table 1: Mechanical properties of composites based on PA6 (dis-
solved in different contents relative to the solvent) and five plies of
carbon fiber fabric
Tabela 1: Mehanske lastnosti kompozita na osnovi PA6 (raztopljene
razli~ne koli~ine v topilu) in petih plasti tkanine iz ogljikovih vlaken
Sample
Tensile
strength
(MPa)
Young’s
modulus
(GPa)
Flexural
strength
(MPa)
Young’s
flexural
modulus
(GPa)
PA6(10%)/5FC 339.2 45.5 436.7 38.3
PA6(20%)/5FC 540.5 63.6 732.6 51.4
PA6(30%)/5FC 505 56.3 571.6 38.1
Overall, the mechanical results show that all the ob-
tained carbon fabric/PA6 laminated samples are superior
to the ones exhibited by long-carbon-fiber-reinforced
PA635–37 that show a tensile strength between 240 and
300 MPa, a flexural strength between 330 and 500 MPa,
and Young’s modulus in the range 25–40 GPa for tensile
and 20–30 GPa for flexural. Also, PA6(20%)/5FC me-
chanical properties are comparable with those presented
by carbon-fiber-fabric-epoxy composites38 with extended
applications in aeronautics.
3.5 Fractography
Optical microscopy images were recorded on the
fracture region of the laminated samples to establish the
fracture mechanisms that led to the failure and to evalu-
ate their behavior when tested under tensile and flexural
loads. Figure 5 presents the fracture region of represen-
tative specimens from each sample tested in tensile. The
identified fracture mechanisms are marked, all of them
being classified as classical mechanisms presented by 39–40:
(1) crack propagation, (2) layer de-bonding, (3) fiber
breakage, (4) fiber pull-out, (5) ply breakage. The
PA6(10%)/5FC and PA6(20%)/5FC fractures resemble
as three layers are broken by fiber breakage in the same
area, the fracture causing layer de-bonding to the next
C.-E. PELIN et al.: MECHANICAL PROPERTIES OF POLYAMIDE/CARBON-FIBER-FABRIC COMPOSITES
726 Materiali in tehnologije / Materials and technology 50 (2016) 5, 723–728
Figure 5: Fracture regions of tensile-tested specimens: a) PA6
(10 %)/5FC, b) PA6 (20 %)/5FC, c) PA6(30%)/5FC
Slika 5: Podro~ja preloma nateznih preizku{ancev: a) PA6
(10 %)/5FC, b) PA6 (20 %)/5FC, c) PA6 (30 %)/5FC
layers, with more pronounced de-bonding in
PA6(10%)/5FC. PA6(30%)/5FC presented the most
destructive failure, involving several mechanisms
including fiber pull out and interlayer crack propagation,
leading to delaminated areas. This can be explained by
the high viscosity of the solution that led to non-uniform
matrix layers, creating stress-concentration sites. In
flexural testing (Figure 6), PA6(10%)/5FC and
PA6(20%)/5FC did not present any layer rupture until
conventional deflection, but the PA6(30%)/5FC
presented the rupture of one external layer that
de-bonded on a longer length. It is important to mention
that although PA6(10%)/5FC presented in general the
lowest mechanical performance, its fracture mode was
not as destructive as for PA6(30%)/5FC. The optical
images complement both the mechanical test results and
the SEM studies.
4 CONCLUSIONS
The study presents the production of carbon-fab-
ric-reinforced laminated composites based on the engi-
neering polymer polyamide 6 as the matrix using a mul-
tiple-stages technique that involves fabric solvent
impregnation with a formic acid solution that contains
different contents of dissolved polymer, solvent removal
and high-temperature pressing. The aim of the study was
to evaluate the polymer/solvent (PA6/formic acid) ratio’s
influence on the mechanical interface within the compos-
ite and on the mechanical properties. Three polymer con-
tents in formic acid were used (10, 20 and 30) %
weight/volume, with the rheology tests showing that the
solution viscosity increases exponentially. A lower con-
tent solution led to a slightly uniform polymer layer that
covered the fibers of the fabric, ensuring large contact ar-
eas, but because the thin layer was too weak, the lami-
nates’ mechanical performance was lower. The highest
content solution (30 %) had an extremely high viscosity
and most likely did not distribute uniformly, probably
generating stress-concentration sites that resulted in a
very destructive failure during the mechanical testing.
The study concludes that the optimum content is
20 % polymer dissolved into the solvent, as it leads to
medium-viscosity solution that supports polymer pene-
tration through the fibers and forms polymer layers with
suitable thickness and uniformity on the fiber surface.
This ensures high contact areas, a strong fiber-matrix in-
terface and, consequently, an optimum fiber-matrix load
transfer, leading to high mechanical performances in ten-
sile and bending and failure modes that do not exhibit a
high degree of delamination. The results show that at an
optimum polymer/solvent ratio, these composites can
represent potential solutions as materials for high me-
chanical performance in aeronautics and automotive ap-
plications.
Acknowledgments
This work was funded by the Sectoral Operational
Programme Human Resources Development 2007-2013
of the Ministry of European Funds through the Financial
Agreement POSDRU/159/1.5/S/132397 and by Roma-
nian Ministry of Education through the PN-II-PT-PCCA-
168/2012 project: “Hybrid composite materials with
thermoplastic matrices doped with fibres and disperse
nano fillings for materials with special purposes”.
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