Zeitschrift Kunststofftechnik Journal of Plastics

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Zeitschrift Kunststofftechnik
Journal of Plastics Technology
© 2012 Carl Hanser Verlag, München
www.kunststofftech.com
archival, peer-reviewed online Journal of the Scientific Alliance of Polymer Technology
archivierte, peer-rezensierte Internetzeitschrift des Wissenschaftlichen Arbeitskreises Kunststofftechnik (WAK)
www.plasticseng.com, www.kunststofftech.com
handed in/eingereicht:
accepted/angenommen:
11.01.2012
30.04.2012
M. Sc. Michaela Kersch1, Dr.-Ing. Felipe Wolff Fabris1, Dipl.-Chem. Marieluise Stumpf1,
Dipl.-Chem. Florian Richter2, Prof. Dr. Hans-Werner Schmidt2,3, Prof. Dr.-Ing. Volker
Altstädt1,3
a
Polymer Engineering, University of Bayreuth, Universitätsstraße 30, 95447 Bayreuth,
Germany
b
Macromolecular Science I, University of Bayreuth, Universitätsstraße 30, 95447
Bayreuth, Germany
c
Bayreuther Institut für Makromolekülforschung (BIMF)
Barrier Properties of Polyamide 12 Films
Nucleated with Talc
In this study the influence of talc on the permeability of polyamide 12 films was investigated. Samples
were prepared by injection molding of plates and subsequent compression molding. DSC
measurements and optical microscopy were used to determine influences of the additive on the
degree of crystallinity and spherulite size. The permeation of oxygen, carbon dioxide and water vapor
was measured and relationships between morphology and barrier properties of neat and talcnucleated PA12 films were evaluated. A finer and more homogeneous spherulite structure was
achieved when talc was added. Even though talc did not increase the degree of crystallinity, a
remarkable improvement of barrier properties of up to 50 % was achieved.
Barriere Eigenschaften von Polyamid 12
Filmen mit Talk als Nukleierungsmittel
Im Rahmen des Beitrags wurde der Einfluss von Talk auf die Permeabilität von Polyamid 12 Filmen
untersucht. Die Probenherstellung erfolgte durch Spritzgießen und anschließendes Pressen. Der
Einfluss des Additivs auf die Kristallinität und Spherulitgröße wurde mittels DSC und optischer
Mikroskopie untersucht. Permeationsmessungen für Sauerstoff, Kohlenstoffdioxid und Wasserdampf
wurden durchgeführt und Zusammenhänge zwischen Morphologie und Barriereeigenschaften der
reinen und mit Talk nukleierten Filme aufgestellt. Durch die Zugabe von Talk wurde eine feinere und
gleichmäßigere Spherulitstruktur erhalten und obwohl keine Erhöhung des Kristallinitätsgrads auftrat,
wurde eine bemerkenswerte Erhöhung der Barrierewirkung von bis zu 50 % erzielt.
© Carl Hanser Verlag
Zeitschrift Kunststofftechnik / Journal of Plastics Technology 8 (2012) 4
Kersch, Altstädt et al.
Barrier Properties of PA12
Barrier Properties of Polyamide 12 Films
Nucleated with Talc
M. Kersch, F. Wolff Fabris, M. Stumpf, F. Richter, H.-W. Schmidt, V. Altstädt
1
INTRODUCTION
Polyamides are one of the most important classes of engineering plastics due to
their unique properties [1]. These polymers exhibit high temperature, electrical
and chemical resistance and good mechanical behavior such as high tensile
strength in combination with high toughness and high abrasion resistance.
Therefore they are widely used for many engineering applications. Polyamides
also play a major role in the packaging industry because of their low gas
permeability [2,3]. The ease with which gases and vapors permeate through a
polymer sample, is of critical importance in packaging, but also relevant for
other fields like construction, the pharmaceutical industry, agriculture, tire
manufacturing, etc [4].
To indentify appropriate materials for these applications, barrier properties can
be characterized by measuring the transmission rate of gases and vapors
through the material [5-7]. The mass or volume transfer of a permeate through a
sample can be quantified by the transmission rate TR (eq. 1) which expresses
the amount of gas or vapor passing through a unit of area under set conditions:
TR 
(E e  E 0 ) Q
A RL
(1)
where Eo stands for the steady state test TR level, Ee for “zero” TR, RL for the
specimen area, Q for a calibration constant and A for the value of load
resistance [8]. In terms of units, transmission rate is expressed by eq. 2:
TR 
amout of substance
area  time
(2)
This value can be automatically recorded by standard equipment. For better
comparability this value can be normalized to the thickness of the film and the
difference between the partial pressure of the permeate on the two sides of the
film [8]. This value is called permeability coefficient P (eq. 3):
P
TR
amout of substance
d
 thickness
p
area time pressure
(3)
In the case of vapors as permeates, the characteristic values are calculated
Zeitschrift Kunststofftechnik 8 (2012) 4
332
based on mass transfer instead of volume transfer [9].
It has been reported that the permeability of polymers is generally influenced by
their chemical nature [10,11], free volume [12-20], crystallinity [10,21-26], chain
orientation [10,23,27-29], humidity [11,21,30], size of the migrant and migrant
clustering [10,31], additives like clay [32-36] or antiplasticizers [14,37,38], blend
morphology [2,39] and different treatments like conditioning [40] or surface
treatment [41,42].
PA6 and PA6.6 are the most popular choices for packaging materials among
polyamides and research has focused mainly on these materials. They offer
good mechanical properties and high temperature stability. PA12 on the other
hand has a higher impact strengh and exhibits less moisture absorption than
PA6 and 6.6 due to fewer amide groups and therefore offers better dimensional
stability at changing humidity conditions. However, relatively little research has
been done on the permeability of PA12. It has been found that the barrier
properties of PA12 can be improved by incorporating clay in the samples
[32,33,35,43]. Alexandre et al. investigated the transport of small molecules
through polyamide 12/ montmorillonite nanocomposites with a clay volume
fraction of 0 -5 %. Nitrogen, water and toluene were used as permeates for
these studies. A clear decrease of nitrogen permeability with increasing clay
content was found and explained mainly by the increase of tortuosity of the
diffusion path [33]. For water and toluene the behavior was more complex.
Despite the tortuosity effect, water permeability increased at clay contents
above 2.5 % and toluene permeability increased with increasing clay fraction.
This was explained by the different interaction of the water and toluene
molecules with fillers according to their hydrophilic/hydrophobic character.
Moreover, the plasticization effect of water and toluene in the matrix played an
effect [32,33,43].
Meng et al. investigated the hydrogen permeability of polyamide 12/
montmorillonite nanocomposites as a function of mixing time of the composites.
A decreasing permeability with increasing mixing time was found and explained
by a better dispersion of the clay in the polymer matrix at higher mixing times
[35].
Dreux et al. modified the surface of polyamide 12 samples by CF4 plasma
treatment to improve water and toluene barrier properties. The achieved lower
permeability for both permeates was explained by a more hydrophobic surface
and therefore less attraction to the water molecules and a low affinity of
fluorinated surface groups to toluene [41,42].
Quite surprisingly, even though talc is a commonly used additive and is known
to nucleate polyamides [44], to our knowledge no work can be found in the
literature regarding the influence of talc on the permeation behavior of
polyamides. Since nucleating agents have a major influence on the crystalline
structure by altering the spherulite size dramatically, changes in the permeation
behavior are expected. Therefore the objective of this work is to evaluate the
relationship between morphology and barrier properties of neat and talcZeitschrift Kunststofftechnik 8 (2012) 4
333
nucleated PA12 films with respect to carbon dioxide, oxygen and water vapor.
Structure and morphology, modified by the additive are correlated with the
barrier properties in order to establish structure-property relations.
2
EXPERIMENTAL
2.1
Materials
Polyamide 12 (PA12) (Vestamid L1600) without stabilizers or other additives
was supplied as pellets by Evonik Degussa GmbH. The talc used in this study
(Micro-Talc IT Extra, Mondo Minerals OY) had a platelike shape, a median
particle size of 1.7 µm (d50%) and a specific surface area of 9.2 m2/g. The
polymer was dried at low pressure (1 mbar) at 80 °C for 24 h prior to
processing.
2.2
Sample Preparation
For preparing the polymer-talc mixtures PA12 was powdered, mixed with 2 wt%
of talc in a glass bottle and dry blended for 24 h with a rotating mixing
equipment. The powder was afterwards compounded under nitrogen
atmosphere in a co-rotating twin-screw microcompounder (DSM Xplore) at a
rotational speed of 50 rpm and a temperature of 230 °C for 4 minutes. The melt
was discharged and filled directly into the injection molding piston. The different
concentrations were prepared by subsequently diluting the initial nucleating
agent concentration with neat PA12. Circular plates with 25 mm diameter and a
thickness of 1 mm were injection molded in a DSM-Injection Molding Machine
using a conic mold. The process was carried out at 220 °C with a mold
temperature of 40 °C and an injection pressure of 6 bar. Films were then
prepared by compression molding. The PA12 plates were, together with a
100 µm-thick metal frame, placed between two metal platens both covered with
a polyimide and a polytetrafluoroethylene (PTFE) film, figure 1. This assembly
was placed in a hydraulic press and heated at 210 °C for 4 minutes at ambient
pressure. The pressure was then increased to 350 kPa and 520 kPa for
1 minute each. The films were cooled down in a cold press with running water
at 16 °C. The obtained samples had a thickness of 100 µm (± 7 µm).
334
Figure 1: Assembly for compression molding of films
2.3
Thermal Analysis
The transition temperatures and the degree of crystallinity of the samples were
determined using a Mettler Toledo DSC/SDTA 821. The samples were heated
from 25 to 210 °C at a heating rate of 10 K/min under nitrogen atmosphere, held
at 210 °C for five minutes and cooled down to 25 °C at a cooling rate of
10 K/min. The degree of crystallinity was determined from the melting enthalpy
using equation 5 [22]:
Xc 
H m  H c
H m,0
(5)
with Hm as the measured melting enthalpy, Hc as the recrystallization
enthalpy obtained during the heating scan and Hm,0 as the melting enthalpy of
a 100% crystalline PA12 (233,5 J/g) [45]. The amount of talc was taken into
consideration when calculating the degree of crystallinity.
2.4
Optical Microscopy
The morphology of the samples was characterized using an optical microscope
with crossed polarizers (Nikon, DIAPHOT 300) equipped with a digital camera
(Nikon, DMX1200).
2.5
Permeability Measurements
The permeation properties of the films for oxygen, carbon dioxide and water
vapor were carried out at 23 °C and ambient pressure (50 % r.h. for oxygen and
335
water vapor, 0 % r.h. for carbon dioxide) using equipment from MOCON:
MOCON OX-TRAN® 2/21 for oxygen (according to ASTM D-3985), MOCON
PERMATRAN-C® 4/41 for carbon dioxide (according to ASTM F-2476-05), and
MOCON PERMATRAN-W® Model 3/33 for water vapor (according to ASTM F1249). The sample thickness was 100 µm (± 7 µm) and the test area was 5 cm2.
The measurements were carried out for 24 hours on at least four samples for
every set of conditions. In order to calculate the permeation value from the
transmission rate, the thickness of the films was measured at five points
distributed over the entire test area and an average was calculated. Diffusion
and solubility of carbon dioxide were determined with the permeation equipment
according to the time lag method [6] whereas for water the mass uptake of the
samples determined in a sorption experiment was used to calculate diffusion
coefficients D and solubility according to equation 6 [36]:
4
Dt
Mt


Mm
 h2
(6)
with Mt as the relative weight gain, Mm as the equilibrium relative weight gain of
the sample, t as the sorption time and h as the sample thickness.
3
RESULTS AND DISCUSSION
It is well known that migrant substances move through a material depending on
their nature. This process is governed by the solubility of the permeate in the
polymer, the size of the migrant, the free volume and the chain mobility in the
polymer matrix [10]. To evaluate these relations in the case of PA12, the
influence of the size of the permeate and possible interactions with the polymer
matrix were analyzed. Figure 2 presents the results of the influence of the
permeate type on the permeation through the films.
336
Figure 2: Permeability coefficients of the different permeates through neat
PA12 films
Oxygen showed the lowest permeability (0.83 x 10-3 mol mm/(m2 day atm))
followed by carbon dioxide (3.5 x 10-3 mol mm/(m2 day atm)). Water vapor
showed much higher permeability coefficients (18 x 10-3 mol mm/(m2 day atm)).
The polarity of the permeate defines the affinity of the polymer matrix towards
the permeate [33]. Since polyamide contains highly polar amide groups there
are strong H-bonding interactions with the polar water vapor. Carbon dioxide
and oxygen are less polar and therefore only much weaker interactions
between polymer and permeate take place. Carbon dioxide however has a
quadrupolar character and therefore interacts with the polar groups in the
polymer [46].
It is known that talc can act as a nucleating agent for polyamides [44] and
therefore influence their crystal structure which, in turn, can have an impact on
the permeability [11]. Since crystalline regions are generally inaccessible to
permeates, the degree of crystallinity and the size and orientation of the
crystallites are among the most important parameters that influence barrier
properties of a polymer film [47]. Therefore it is important to investigate
structure-property relations between additive, morphology and permeability.
337
Figure 3: Degree of crystallinity of the neat film and the samples with different
concentrations of talc
Figure 4: DSC thermograms of neat PA12 and PA12 with different concentrations of talc
The DSC measurements revealed that the incorporation of talc only had a minor
influence on the degree of crystallinity of the samples, figure 3. This is in
338
agreement with the results obtained by other groups where nucleation agents,
fibers or clays did not change the degree of crystallinity of polyamides
[33,43,48]. But even though the degree of crystallinity was not affected, the
nucleating effect of the additive can be clearly observed as the crystallization
temperature of the samples containing talc (159 °C for 0.5 and 1 % talc, 160 °C
for 2 % talc) was significantly higher than of the neat PA12 (152 °C), figure 4.
Studies with a polarization microscope showed that talc has a clear influence on
the size of the spherulites as a result of the nucleation of the additive. A finer
and more homogenous crystal structure was obtained for the samples
containing additive in comparison to the neat PA12 samples, figure 5.
Figure 5: Optical micrographs of films of neat PA12 (left) and PA12 with
0.5 wt% talc (right)
With respect to the permeability of carbon dioxide, oxygen and water vapor, a
significant decrease for all three permeates was found when talc was added,
figures 6 – 8. Additionally a much lower standard deviation of the permeability
values for the nucleated samples was observed, which can be explained by the
better homogeneity of the samples containing talc. The permeabiltiy of carbon
dioxide was reduced from 3.5 x 10-3 mol mm/(m2 day atm) for the neat material
to 2.2 x 10-3 mol mm/(m2 day atm) for the samples containing 0.5, 1.0 and 2.0
wt% of talc. The permeability of oxygen could be reduced from 0.83 x 10-3
mol mm/(m2 day atm) to 0.58 x 10-3 mol mm/(m2 day atm) by adding talc. The
highest reduction of up to 50 % was achieved for the permeation of water
vapor from 18 x 10-3 mol mm/(m2 day atm) of the neat material to
9 x 10-3 mol mm/(m2 day atm) for the samples containing 1 wt% (2 wt%) talc.
Similar effects have been observed by Gill et al. for HDPE and talc [49]. The
change in permeation was explained by the relatively high aspect ratio of the
additional impermeable sites and the resulting increase in tortuosity of the
diffusion path, figure 9. This effect has also been explained in theory by Nielsen
for polymers filled with platelike particles [50].
339
Figure 6: Permeability coefficients of carbon dioxide through neat PA12 films
and samples with different amounts of talc
Figure 7: Permeability coefficients of oxygen through neat PA12 films and
samples with different amounts of talc
340
Figure 8: Permeability coefficients of water vapor through neat PA12 films and
samples with different amounts of talc
Figure 9: Schematic illustration of increased path of diffusion through a sample
containing impermeable particles with high aspect ratio
Whereas the permeability of oxygen and carbon dioxide was already
significantly reduced by the addition of 0.5 wt% talc and higher concentrations
did not improve the barrier properties any further, the permeability of water
vapor only slightly decreased in the samples containing 0.5 wt% of talc and a
clear reduction of permeability was only seen at additive amounts of 1 wt% and
higher. Considering the tortuosity of the diffusion path as the main factor for the
reduced permeability in the samples containing talc, one would expect that a
higher amount of additive would lead to a further reduction in permeability. This,
however, was not observed for oxygen and carbon dioxide where the
permeability reached a plateau. Water permeation however was further reduced
at higher additive concentrations.
341
Another factor that could be considered is reduced chain mobility in the
amorphous parts of the sample due to shorter chain lengths between crystalline
parts in the presence of talc and around the talc particles. The reduction of
chain-segment mobility is known to play a role in polymer clay nanocomposites
[33,51]. Even though talc and nano-clay can only be compared to a certain
extend, the plate-like structure of both clay and talc provides a high surface
area and therefore a high degree of additive-polymer interaction [20]. A higher
number and smaller size of crystallites in the samples containing talc shortens
the distance between crystalline regions and therefore also shortens the chain
length in the amorphous regions, which can cause a reduction in chain mobility.
The presence of the talc particles themselves can also restrain the mobility of
the polymer chains surrounding the additive particles, in particular if nucleation
occurs.
To discriminate between effects caused by changes in the crystalline structure
of the polymer and tortuosity effects resulting from the additive, diffusion and
solubility of carbon dioxide (as a representative of the two gases) and water
were determined, table 1 and 2. Changes in the crystalline structure would
mainly affect the solubility of the permeates, whereas changes in the diffusion
coefficient can be mainly related to the presence of additional impermeable
sites that increase the tortuosity. The solubility of carbon dioxide in the samples
did not change from the neat samples to those containing different amounts of
talc, but a clear decrease in diffusion could be observed when talc was added.
No further decrease could be achieved at higher additive concentrations
compared to 0.5 wt%. No noticeably change in solubility could be observed for
water vapor either. But unlike for carbon dioxide, the diffusion coefficient did not
change (within the standard deviation) and was only decreased at higher
concentrations of talc. Taking these results into account, it can be concluded,
that the increase in diffusion path resulting from the addition of the high-aspect
ratio talc particles is the main reason for the reduced permeability.
Diffusion Coefficient
10-12 [m2/sec]
Solubilty
[cm3/cm3]
PA12 neat
2.18 (± 0.68)
0,19 (± 0.05)
0.5 wt % talc
1.36 (± 0.04)
0,19 (± 0.01)
2 wt% talc
1.47 (± 0.29)
0.18 (± 0.03)
Table 1:
Diffusion coefficients and solubility of carbon dioxide in PA12
342
Diffusion Coefficient
10-13 [m2/sec]
Solubilty
10-2 [g/g]
PA12 neat
3.68 (± 0.36)
0.95 (± 0.03)
0.5 wt % talc
4.55 (± 1.26)
0.87 (± 0.05)
2 wt% talc
1.32 (± 0.27)
0.97 (± 0.06)
Table 2:
Diffusion coefficients and solubility of water vapor in PA12
As a result of the hydrophility of the polymer matrix, the formation of hydrogen
bonds, the hydrophobicity of the additive and the plasticizing effect of water, the
permeation of water vapor through the polyamide samples is much more
complex than that of the less polar gases carbon dioxide and oxygen. Therefore
more interactions and effects have to be taken into account. Despite that, the
main effects of talc on the barrier properties of the polyamide samples that were
explained above could also be observed for water and resulted in a clear
reduction in the permeability of water vapor.
4
SUMMARY
The influence of talc as an additive on the permeability of carbon dioxide,
oxygen and water vapor through polyamide 12 films was discussed in this
study. Even though talc had no influence on the degree of crystallinity of the
samples, a change in morphology could be observed. The samples with
additive showed a much finer and more homogenous spherulite structure than
the neat ones. It was shown that the permeability of the polyamide films strongly
depended on the nature of the permeate and the amount of additive.
Much higher permeation rates were observed for water than for the two nonpolar gases. This was explained by the strong interaction between polymer and
water molecules through hydrogen bonding.
It could be shown that an increase in diffusion path was the main reason for the
reduction in permeability.
5
ACKNOWLEDGEMENTS
The authors are grateful to the German Research Foundation (DFG) for
financial support within the Collaborative Research Center 840 (SFB 840),
project B4. We thank Anais Graterol and Ute Kuhn for their help with the
measurements.
343
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349
Keywords:
barrier, additives, gas permeation, polyamides, structure-property relations
Stichworte:
Barriere, Additive,
Beziehungen
Gas
Permeation,
Polyamide,
Author/Autor:
M. Sc. Michaela Kersch
Dr.-Ing. Felipe Wolff Fabris
Dipl.-Chem. Marieluise Stumpf
Prof. Dr.-Ing. Volker Altstädt
Polymer Engineering
University of Bayreuth
Universitätsstraße 30
95447 Bayreuth
Dipl.-Chem. Florian Richter
Prof. Dr. Hans-Werner Schmidt
Macromolecular Chemistry I
University of Bayreuth
Universitätsstraße 30
95447 Bayreuth
Editor/Herausgeber:
Europe/Europa
Prof. Dr.-Ing. Dr. h.c. Gottfried W. Ehrenstein, verantwortlich
Lehrstuhl für Kunststofftechnik
Universität Erlangen-Nürnberg
Am Weichselgarten 9
91058 Erlangen
Deutschland
Phone: +49/(0)9131/85 - 29703
Fax.:
+49/(0)9131/85 - 29709
E-Mail-Adresse: [email protected]
Publisher/Verlag:
Carl-Hanser-Verlag
Jürgen Harth
Ltg. Online-Services & E-Commerce,
Fachbuchanzeigen und Elektronische Lizenzen
Kolbergerstrasse 22
81679 Muenchen
Phone.: 089/99 830 - 300
Fax: 089/99 830 - 156
E-mail: [email protected]
Struktur-Eigenschafts-
E-Mail:
[email protected]
Webseite:
www.polymer-engineering.de
Tel.: +49(0)921/55-7471
Fax: +49(0)921/55-7473
The Americas/Amerikas
Prof. Dr. Tim A. Osswald,
responsible
Polymer Engineering Center,
Director
University of Wisconsin-Madison
1513 University Avenue
Madison, WI 53706
USA
Phone: +1/608 263 9538
Fax.:
+1/608 265 2316
E-Mail: [email protected]
Editorial Board/Beirat:
Professoren des Wissenschaftlichen
Arbeitskreises Kunststofftechnik/
Professors of the Scientific Alliance
of Polymer Technology
350

Zeitschrift Kunststofftechnik Journal of Plastics

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