Главная Polymer Bulletin Properties of polyamide 6/clay nanocomposites processed by low cost bentonite and different organic...

Properties of polyamide 6/clay nanocomposites processed by low cost bentonite and different organic modifiers

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62
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DOI:
10.1007/s00289-009-0051-1
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June, 2009
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Polym. Bull. (2009) 62:791–800
DOI 10.1007/s00289-009-0051-1
ORIGINAL PAPER

Properties of polyamide 6/clay nanocomposites
processed by low cost bentonite and different organic
modifiers
D. Garcı́a-López Æ I. Gobernado-Mitre Æ
J. F. Fernández Æ J. C. Merino Æ J. M. Pastor

Received: 11 April 2008 / Revised: 18 July 2008 / Accepted: 10 February 2009 /
Published online: 19 February 2009
Ó Springer-Verlag 2009

Abstract Nanocomposites based on polyamide 6 have been directly prepared by
melt compounding, using modified low cost bentonites by three selected quaternary
ammonium cations, in particular quaternized octadecylamine (ODA), dimethyl
benzyl hydrogenated tallow quaternary ammonium (B2MTH) and dimethyl
hydrogenated ditallow quaternary ammonium (2M2TH). Thermal stability of
organic modifiers and organoclays has been studied by TGA and results allow
evaluating the degree of modifier incorporation into clay galleries. The influence of
the organic modifier on the morphology and properties of the obtained nanocomposites has been studied by X-ray diffraction and TEM analysis. Depending on the
degree of bentonite modification, different mechanisms were reported to explain the
improved mechanical properties of the resulting nanocomposites.
Keywords Nanocomposites  Polyamide 6  Organic modifier  Bentonite 
Melt compounding

D. Garcı́a-López (&)  I. Gobernado-Mitre  J. C. Merino  J. M. Pastor
Center for Automotive Research and Development (CIDAUT),
Technological Park of Boecillo, 47151 Valladolid, Spain
e-mail: garlop@cidaut.es
J. F. Fernández
Electroceramic Department, Instituto de Cerámica y Vidrio, CSIC. Kelsen 5,
28049 Madrid, Spain
J. C. Merino  J. M. Pastor
Dpto. Fı́sica de la Materia Condensada,
E.T.S.I.I. Universidad de Valladolid, 47011 Valladolid, Spain

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Polym. Bull. (2009) 62:791–800

Introduction
Polymer layered silicate nanocomposites based on thermoplastic polymers usually
display significant improvements in modulus and heat deflection temperature at
relatively low levels of;  reinforcement (3–8% wt). Other properties as permeability
and retardant enhancements with respect to the unmodified polymers are also
improved [1]. The vast majority of the work in nanocomposites has been focussed
on the use of montmorillonite type clays as nanoparticles. The mechanical
properties of the nanocomposites are strongly dependent on the degree of
exfoliation and dispersion of the bentonite aggregates in the polymer matrix. The
most pronounced changes in nanocomposite morphology as well as mechanical
properties result from altering the number and types of groups or tails attached to
the nitrogen atom of the modifier compound. Paul et al. [2, 3] found a significant
influence of the long alkyl groups on the exfoliation level. They show that a most
effective exfoliation is obtained with an organoclay having one alkyl tail in the
quaternary cation. On the other hand, Limpanart et al. [4] have found that the
surface coverage of the organoclay play a major role in controlling the type of the
final composite formation in Polystyrene-clay nanocomposites.
Once the organoclay is obtained, it may be added to the monomer before the
polymerization stage [5, 6] or directly to the polymeric matrix in a melt mixing
process [7] to produce the nanocomposite material. We have recently obtained [8]
that an excess of organic modifier negatively affects the properties of the melt
compounded nanocomposite. The thermal stability of the organic modifier is related
to the groups linked to the quaternary ammonium, and it must be previously
checked that the organic modifier is not decomposed during the compounding
process. Thus, modifier compounds should not only produce the exfoliation of the
clays but also remain thermally stable during the compounding process if in
principle the processing temperature is below the modifier decomposition
temperature.
The aim of this work is to compare the morphology and mechanical properties of
melt processed polyamide 6/layered-silicate nanocomposites based on different
organoclays by using a low cost bentonite raw material.

Experimental
Materials
The polymeric material used for the preparation of polyamide 6 nanocomposites
was a commercial PA 6 (Bergamid B 90, PolyOne); this matrix was characterised to
have a high viscosity. The clay raw material kindly supplied by Tolsa S.A. (Spain)
was sodium bentonite type clay with the following mineralogical composition
(dry% wt): 63 montmorillonite, 18 dolomite, 10 illite, 4 quartz, 3 calcite and 2
paglioclase. The purification process was previously described [8] and allows using
this clay having a cation exchange capacity (CEC) of 80 meq/100 g. The organic
modifiers were: octadecylamine (ODA), dimethyl benzyl hydrogenated tallow

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Polym. Bull. (2009) 62:791–800

793

quaternary ammonium (modified B2MTH) and dimethyl hydrogenated ditallow
quaternary ammonium (modified 2M2TH), where hydrogenated tallow (HT) is
*65% of C18; *30% C16; *5% C14. ODA with a molecular weight of 269 mol/
g was supplied from Aldrich with a purity[90%. Commercial B2MTH and 2M2TH
were supplied by KAO with a purity of [75 and [85%, respectively. Molecular
weights were 415 mol/g for B2MTH and 570 mol/g for 2M2TH.
The preparation method of the organophilic bentonite clays was also described in
a previous paper [8].
The nanocomposites were elaborated through an extrusion process using a
Leistritz 27 GL intermeshing twin screw extruder operating at 240–250 °C and
450 rpm in corrotating mode. The polymer matrix was added through the first feeder
and the clay through a side feeder. PA 6 granulates were dried at 80 °C for 8 h, and
the modified clay was dried at 60 °C for 24 h prior to blending in the extruder. After
being dried at 80 °C for 8 h, pellets of the nanocomposites were injection moulded
into test pieces for mechanical tests by using a Margarite JSW110 injection
moulder. The temperature of the cylinders was 240–250 °C and that of the mould
was 80 °C.
Characterisation
The morphology of the modified bentonites and nanocomposite fracture were
studied by using a field emission scanning electron microscope (FESEM) Hitachi
S4700 after gold coating. The gallery distance of the clays was evaluated by
measuring the d001 X-ray reflection in a Philips X’Pert MPD using Cu Karadiation.
The dispersion of the silicate layers in the polymers was evaluated in ultrathin
section by using Transmission Electron Microscopy (TEM) Jeol JEM 2000FX
Electron Microscope with 200 kV accelerating voltage. Mechanical properties were
evaluated using an Instron tester (Model 5500R60025) according to UNE-EN ISO
527-1 and 527-2. Heat deflection temperature (HDT) was measured in an HDTVICAT tester microprocessor, ATS-FAAR (Model A/3M) according UNE-EN ISO
75-1 and using a load of 1.8 MPa. For Notched Izod impact strength a pendulum
trademark Frank Model 53566 was used under UNE-EN ISO 180. Thermal stability
of the organoclays and clay contents in the nanocomposites were measured by
burning the samples in a Thermogravimetry Analysis Mettler Toledo Model
TGA851.

Results and discussion
The TG and DTG curves obtained for the modified bentonites are shown in Fig. 1
and Table 1 presents the percent weight loss. The thermal decomposition of these
nanoclays fits a four-step mechanism. Volatilization of the water molecules
adsorbed on the clays is first detected (*60 °C) and a further degradation of
inorganic crystallised water molecules is detected at 800 °C. Degradation steps at
200–450 °C are associated with the organic modifiers [8]. The decomposition
processes that take place between 150 and 350 °C may be related to free modifier

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Polym. Bull. (2009) 62:791–800

Weight loss (%)

794
105
100
95
90
85
80
75
70
65
60

BT-ODA
BT-B2MTH
BT-2M2TH

100

200

300

400

500

600

700

800

900

Temperature (°C)
0,10

DTG (dw/dT)

0,05
0,00
-0,05
-0,10
-0,15

BT-ODA
BT-B2MTH
BT-2M2TH

-0,20
-0,25
100

200

300

400

500

600

700

800

900

Temperature (°C)

Fig. 1 TG and DTG curves of the organoclays

Table 1 Thermogravimetric analysis values of organoclays
25–100 °C

150–350 °C

350–500 °C

750–850 °C

Mass
(%)

T
(°C)

Mass
(%)

T
(°C)

Mass
(%)

T
(°C)

Mass
(%)

T
(°C)

BT-ODA

0.77

59

16.85

238

16.84

394

2.41

790

33.68

BT-B2MTH

0.83

57

15.18

251

12.73

398

2.23

798

27.91

BT-2M2TH

0.70

59

20.41

308

13.29

408

1.95

799

33.70

Organoclay

Organic
modifier (%)

Temperature values were measured on the mid-point of the curve that corresponds to the decomposition
of the 50 wt% of the compound

and the modifier that is adsorbed on microfiller, and the degradation of the organic
modifier that is located in the clay galleries is detected at 350–500 °C. In this sense,
whereas for both ODA and B2MTH thermal stability is considerably increased
when they are interacting with the clay, a less significant increase was detected for
2M2TH. However, the decomposition temperature of the modifiers into the galleries
reached a quite similar temperature range, and therefore the increase in the
decomposition temperature should be related to the chemical bonding of the alkyl
tallow chains into the galleries of the inorganic layered silicates.
Considering that the weight loss detected between 350 and 500 °C corresponds
exclusively to the modifier that is located into the clay galleries, it is possible to
establish a ratio between the effectively incorporated organic modifier and the CEC
of the clay. The CEC must account for the available sites in the layered silicate.

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Polym. Bull. (2009) 62:791–800

795

Thus, we define this ratio as gallery effective modifier, Rgem = modifier into the
galleries/stoichiometric modifier required. For the nanoclays under study this Rgem
ratio was 0.95, 0.51 and 0.42 for ODA, B2MTH and 2M2TH, respectively.
Therefore, the higher the modifier molecular weight, the lower the incorporation to
the clay galleries. The increase of the amount of alkyl tails has a similar effect due
to their high stereochemical nature. The modifier that is not incorporated into the
clay galleries is probably adsorbed on the clay surface or on the impurity surfaces.
The decomposition temperature established the presence of modifier at the surface
and no other proofs until now were support such hypothesis. The decreasing of the
decomposition temperature in reference to the free modifier is thus attributed to a
catalyst effect of the clay surface. Possible interaction of the modifier with clay
surface must be occur through the surface hydroxyl groups resulting in a different
bounding than the one that may occurs in the galleries trough the basal oxygen of
the silica tetrahedral. It is important to remark that the total amount of modifier is
very similar in the nanoclays BT-ODA and BT-2M2TH, *33.7% wt, but the Rgem
was almost double for the first one because of the higher effectiveness of low
molecular weight modifier.
In Fig. 2a, the small angle X-ray diffraction (SAXD) patterns of the unmodified
and modified bentonites are shown. The treatment of clays resulted in intercalation
of the organic modifiers between the clay galleries, and thus a shift in the peaks to
lower 2h values is detected. The different increases in basal spacing indicate that the
interchange between the alkaline ions and the organic modifiers in the clays
galleries has a different extension and efficiency. The starting bentonite clay has an
interlayer of 1.21 nm that indicated the hydration process of the galleries because of
the purification process. For BT-ODA the presence of unmodified bentonite was
almost not detected, which is in accordance to their higher Rgem ratio that represent
a high amount of modifier into the clay galleries as measured by TGA, whereas a
significant amount of unmodified clay was detected for BT-2M2TH and BTB2MTH. On the other hand, BT-ODA XRD pattern shows two distinct peaks, which
reflects the presence of modifier intercalated into the clay galleries in structures that
gave a thicker interlayer space than the monolayer structure as can be account for
bimodal lamellar structure and paraffinic structure [9, 10]. B2MTH and 2M2TH
organoclays present a dissimilar behaviour probably due to the different molecular
structure of the organic modifiers. As higher the molecular weight higher the
interlayer space. Moreover, the presence of two alkyl tails shows a gradually
increase of the interlayer space with a less defined peak associated with a wider
distribution of interlayer spaces.
The X-ray diffraction patterns for the nanocomposites obtained by melt
compounding of PA6 and organoclays are shown in Fig. 2b. Similar gallery
distances are detected for the three nanocomposites, although a slight higher value
(3.46 nm) is detected for the nanocomposite with BT-2M2TH, indicating a higher
degree of intercalation of the polymeric chains into the clay galleries. The
exfoliation level for all the nanocomposites is difficult to measure by XRD but in
this case the presence of a peak at 8.9° that correspond to a fibrillar morphology
illite mineral indicates that the amount of non-swellable layered clay was almost
negligible. Note that this peak was also present in the raw material with very low

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Polym. Bull. (2009) 62:791–800

(a)

3.11 nm

Intensity (a.u)

3.75 nm
1.59 nm
1.86 nm

BT-ODA
BT-2M2TH
BT-B2MTH

1.21 nm

2

3

4

5

6

7

BT-Na

8

9

10

2θ (Degrees)

(b)

3.0 nm

Inyensity (a.u)

3.0 nm
PA 6-BT-ODA

3.46 nm

PA 6-BT-B2MTH

PA 6-BT-2M2TH

2

3

4

5

6

7

8

9

10

2θ (Degrees)
Fig. 2 XRD patterns of a the organoclays and b the PA 6 nanocomposites

intensity (Fig. 2a). However, the presence of a broad peak at 2 nm for PA-BT2M2TH indicated that a higher interlayer gallery collapses without intercalation of
polymeric chains. By the contrary for the PA-BT-B2MTH there is an increase of the
interlayer distance from 1.86 to *2 nm. PA-BT-ODA showed the better exfoliation
if take into account that low presence of monolayer structures remains after the
compounding.
Transmission electron microscopy micrographs of the PA 6 nanocomposites are
shown in Fig. 3. Most platelet thickness deduced from these photomicrographs is
slightly higher than that of a single clay layer, indicating that organoclays are not
fully exfoliated. In the TEM micrographs was not possible to distinguish the illite
clay and the layered clay. However, an intercalated–exfoliated structure is observed,

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797

Fig. 3 TEM micrographs nanocomposites (980,000) of: a PA 6-BT-ODA; b PA 6-BT-B2MTH; and
c PA 6-BT-2M2TH

confirming XRD results. Organoclays became smaller and were dispersed more
uniformly in PA 6-BT-ODA nanocomposite, in agreement with the XRD, showing
an intercalated structure with a high level of exfoliated silicate layers. PA
6-BT-2M2TH nanocomposite presents a similar morphology but with a lower
exfoliation grade, and an important amount of piled up platelets is observed. This

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Polym. Bull. (2009) 62:791–800

can be observed on the XRD results, where a peak at 2h = 5° is observed,
corresponding to an intercalated structure of the clays. PA 6-BT-B2MTH
nanocomposites show an intermediate behaviour that corresponds to the presence
of two XRD peaks related with two different intergallery sizes; one of them
corresponds to the structure of the modified clays and the other one to the
intercalated structure produced during the extrusion process.
For PA 6-BT-2M2TH nanocomposite, it can be observed that the layers slide
between them and resulting in an increase of the effective length [11]. The TEM
evidences of this process are associated with the increasing of platelet length as well
to the bifurcation and partial splitting of platelet stacks. This intercalated–exfoliated
structure may be due to the fact that bentonite did not complete the cation exchange
between the sodium cations and the organic modifier during the organoclay
modification process as previously evidenced. Thus, the modifier located on the
surface of the platelet stacks seems to restrain the intercalation of polymer and thus
the exfoliation of the silicate layer because of their affinity with the polymer. In this
sense, the organic modifier on the surface of the platelets stabilizes the platelet
stacks and the organic modifier that is effectively incorporated into the clay galleries
promotes the sliding between silicates layers resulting in deformable intercalates. In
addition, there is some white type phase forming irregular form with smooth
contour and nanosize in Fig. 3b and c that could also be attributed to free modifier.
Such phase is organic in nature as confirmed by EDX and their absence in the PABT-ODA nanocomposite probes that neither is related to TEM sample procedure
neither appeared in nanocomposites with most of the modifier located in the clays
galleries.
Tensile modulus, HDT and Notched Izod impact strength values of PA6 and PA
6-organoclay nanocomposites are shown in Table 2. It can be observed that the
presence of organoclays leads to substantial improvement in stiffness and heat
deflection temperature (HDT), which correlated with a reduction in Izod impact
strength in all PA 6 nanocomposites. Higher increases are obtained for both PA 6BT-2M2TH and PA 6-BT-ODA nanocomposites (54% in tensile modulus and
*35% in HDT) in comparison with PA-BT-B2MTH. It is interesting to remark that
similar tensile modulus was observed for pure montmorillonite type organoclays [8]
and the only difference was found in the HDT values. Previously, it was concluded
that the presence of free modifier contributed to decrease only HDT parameter [8],
but this contribution is slight. In such a case difference in clay platelet aspect ratio

Table 2 Properties of PA 6 nanocomposites
Sample

Clay
treatment

Clay TGA clay
Tensile HDT (°C) Notched izod
(wt%) content (wt%) modulus
impact strength
(MPa)
(kJ/m2)

PA

–

–

PA 6-BT-ODA
PA 6-BT-B2MTH
PA 6-BT-2M2TH

123

–

2,637

48

6.67

5% solid spray
5
dried ? milling 5

3.4

4,060

66

3.84

4.0

3,893

62

4.16

5

3.5

4,000

66

4.82

Polym. Bull. (2009) 62:791–800

799

Fig. 4 FESEM micrograph of fresh nanocomposite fracture of PA 6-BT-2M2TH

could be contribute as well. So the question that arises at that point is as follow: are
there different mechanisms that contributed to mechanical reinforcement of
polymer/clay nanocomposites? Maybe the answer is quite complex and out of this
paper but the experimental results obtained stated that probably more mechanisms
than the inorganic-polymer interphase reinforcement could be involved. This
mechanism was supported by the fact that this interphase produces a higher thermal
stability of the nanocomposites, that is the polymer-inorganic interphase stiffness
the polymer. Two possible contributions could appointed: (a) crack deflection due to
presence of inorganic nanofiller as shown in micrograph of Fig. 4. The inorganic
particles with higher module than the matrix acted as crack energy adsorption and
produce an enhancement of the crack tortuosity. This mechanism is more efficient
when the inorganic particles are under compressive stress as evidence with the
presence of bridges at the tip of the crack. This type of fracture behaviour occurs in
the different nanocomposites studies and is quite different than the more cohesive
fracture behaviour usually observed in pure PA6 polymers. (b) The higher aspect
ratio of deformed intercalates in samples with low Rgem ratios due in part to higher
flexibility of this nanostructures. The higher dispersion of the layered silicates were
translated to lower values of the Izod impact strength for the PA 6-BT-ODA
nanocomposites indicating higher rigidity of the structure. The differentiation of the
proposed possible mechanism is thus quite difficult because all samples shown
similar tensile modulus and all of them could contribute simultaneously. The
presence of intercalated structures seems to be on the base of proposed mechanisms.

Conclusions
An intercalated–exfoliated structure has been evidenced by XRD and TEM analysis
for PA 6 nanocomposites formed with bentonites modified with ODA, 2M2TH and

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Polym. Bull. (2009) 62:791–800

B2MTH modifiers. The chemical structure of the organic modifier used for
bentonite treatment, related to its thermal stability and its ability to penetrate into
the clay galleries has a meaningful influence on organoclay exfoliation. However,
the similarities on mechanical properties of PA 6 nanocomposites stated that a
different mechanism could be taken into account. A possible crack deflection
mechanism based on nanofiller presence was observed in PA6/organobentonite
nanocomposite.
Acknowledgments This work is supported by the Ministerio de Educación y Ciencia [program
MAT2007-66845-C02-01, Program MAT2005-06627-C03 and the European Social Foundation (ESF)
Torres Quevedo Program].

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