Polypropylene–clay nanocomposites: effect of compatibilizingagents on clay dispersion

D. Garc�ııa-L�oopez a, O. Picazo a, J.C. Merino a,b, J.M. Pastor a,b,*

a Dept. F�ıısica de la Materia Condensada, ETSII Universidad de Valladolid, 47011 Valladolid, Spainb Center for Automotive Research and Development (CIDAUT), Technological Park of Boecillo, 47151 Valladolid, Spain

Received 26 June 2002; received in revised form 15 October 2002; accepted 21 October 2002

Abstract

In this work, polypropylene–clay nanocomposites are obtained and studied by using two different coupling agents,

diethyl maleate and maleic anhydride. Two different clays, a commercial montmorillonite (Nanomer I30.TC) and a

sodium bentonite purified and modified with octadecylammonium ions have also been used. The relative influence of

each factor, matrix and clay modification, can be observed from structural analysis (SAXS, TEM) and mechanical

properties. An explanation of the results is proposed according to the microstructure and chemical nature of the systems

and the thermodynamic interactions operating during nanocomposite preparation.

� 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Nanocomposites; PP; Clay; Modification; Compatibilization

1. Introduction

Polymer–clay nanocomposites are a new class of

materials which show improved properties at very low

loading levels compared with conventional filler com-

posites. Among these improved properties are mechan-

ical, dimensional, permeability, thermal stability and

flame retardant enhancements with respect to the bulk

polymer [1]. In order to obtain good interfacial adhesion

and mechanical properties the hydrophilic clay needs to

be modified prior to its introduction in most polymer

matrices which are organophilic. Clay modification is

generally achieved by ion exchange reactions of organ-

ophilic cations for sodium ions [2,3], and the polymer–

clay nanocomposites may be obtained mainly by three

methods: intercalation of a suitable monomer followed

by polymerization [4,5], polymer intercalation from so-

lution [6], and direct polymer melt intercalation [7].

Several polymer nanocomposites have been reported

up to date, such as polyamide 6 [8], polystyrene [9],

polyurethane [10] and epoxy resins [5]. Polypropylene

(PP) is one of the most interesting thermoplastic mate-

rials due to its low price and balanced properties.

However, due to the low polarity of PP, it is difficult to

get the exfoliated and homogeneous dispersion of the

silicate layer at the nanometer level in the polymer. This

is mainly due to the fact that the silicate clays layers have

polar hydroxyl groups and are compatible only with

polymers containing polar functional groups. Conse-

quently the matrix modification with polar oligomers is

necessary prior to modified clay introduction in order to

achieve nanometric dispersion of the clay [6,11]. When

nanometric dispersion of primary clay platelets is ob-

tained, the aspect ratio of the filler particle is increased

and the reinforcement effect is improved [12]. When

preparing nanocomposites by melt compounding, shear

alone is not enough to provide nanometric dispersion of

clay platelets, as an overall negative free energy in the

process needs to be obtained from entropic and enthalpic

European Polymer Journal 39 (2003) 945–950

www.elsevier.com/locate/europolj

* Corresponding author. Address: Dept. F�ıısica de la MateriaCondensada, ETSII Universidad de Valladolid, 47011 Valla-

dolid, Spain. Fax: +34-983-42-35-44.

E-mail address: jmpastor@fmc.uva.es (J.M. Pastor).

0014-3057/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0014-3057(02)00333-6

balance. Entropy decreases as the polymer chains be-

come constrained in between clay stacks. This has to be

balanced by a favourable enthalpic interaction between

clay and polymer.

In this work, two different polar coupling agents, di-

ethyl maleate grafted PP (PPgDEM) and commercial

maleic anhydride grafted PP (PPgMA) have been used.

The choice of diethyl maleate (DEM) as compatibilizing

agent has been made because of its high thermal sta-

bility, high boiling point and good compatibilization

with polyolefins, compared with other compatibilizing

agents. Furthermore, the low homopolymerization be-

havior of DEM, allows a better control of the func-

tionalization reaction. Maleic anhydride (MAH) has

been widely used as compatibilizing agent for this kind

of systems [2,4] and it is used as reference on this work.

The PP-clay nanocomposites have been prepared by

melt compounding with two different clays, commercial

modified montmorillonite, and sodium bentonite (BNa)

modified with octadecylammonium ions.

2. Experimental

2.1. Materials

The materials used for the preparation of PP nano-

composites are commercial PP (Stamylan 17M10,

DSM), two different coupling agents, a commercial

PPgMA oligomer (Polybond 3200) with 1.2 wt.% of MA

from Uniroyal Chemical and PPgDEM whit 0.9 wt.% of

DEM prepared by us. Two different clays were used, a

commercial organophillic montmorillonite (I30.TC)

from Nanocor and a BNa from BENESA (Spain)

modified with octadecylammonium ions (BC18) fol-

lowing the method described on the literature [2,3].

2.2. Diethyl maleate modification of polypropylene

PP was functionalized with DEM using dibenzoyl

peroxide as catalyst by reactive extrusion in a Leistritz

27GL double corrotating screw. Functionalization was

achieved in two stages. In a first step, PP and DEM were

mixed, feeding the compatibilizer dissolved in acetone

through a side feeder, bymeans of a peristaltic pump. In a

second stage, the catalyzer as an acetone solution

was added in order to initiate the grafting reaction. The

obtained material is washed with acetone to extract the

unreacted functionalizing agent. The functionalized

polymer is characterized by ATR-FTIR with a calibrate

curve obtained with DEM and iso-heptane, in order to

calculate the functionalization grade. A more description

of the process can be found in the literature [13–16].

2.3. Preparation of polypropylene–clay nanocomposites

Nanocomposites were obtained by previous prepara-

tion of a masterbatch by mixing the compatibilizing

agent MAH or PPgDEM and the clays in a Leistritz 27

GL intermeshing twin screw extruder operating at 190–

210 �C and 50 rpm in corrotating mode. The clay was

added through a side feeder. Subsequently, the desired

amount of pure PP, masterbatch and grafted PP were

mixed in the twin screw extruder at 190–210 �C and 250rpm. The composition of nanocomposites is detailed in

Tables 1 and 2.

After being dried, pellets of the nanocomposites were

injection molded into test pieces for mechanical tests by

using an injection molder Margarite JSW110. The tem-

perature of the cylinders was 190–200 �C and that of themold was 40 �C.The contents of the inorganic clay of the nanocom-

posites were measured by burning the samples in a

Thermogravimetry Analysis Mettler Toledo Model

TGA851.

2.4. Evaluation of microstructure

X-ray diffraction of the clays and nanocomposites, in

order to evaluate the evolution of the clay d0 0 1 reflec-tion, was performed in a Philips X�Pert MPD using Cu

Ka radiation. Transmission electron micrographs were

Table 1

Composition and mechanical properties of PPgDEM nanocomposites

PP

(wt.%)

PPg

(wt.%)

Type of clay Clay

(wt.%)

TGA clay

contents

(wt.%)

Modulus

(MPa)

Tensile

strength

(MPa)

Notched Izod

impact strength

(kJ/m2)

100 – – – – 1828� 33 34:3� 0:9 3:3� 0:391 9 – – – 1799� 24 35:2� 0:2 2:5� 0:479 21 – – – 1658� 39 34:8� 0:1 2:8� 0:388 9 Bentonite 3 2.0 1780� 54 34:7� 0:4 3:2� 0:472 21 Bentonite 7 5.0 1869� 49 33:2� 0:3 3:4� 0:388 9 Nanomer

I30.TC

3 2.1 1902� 43 35:0� 0:6 2:8� 0:3

72 21 Nanomer

I30.TC

7 5.7 2065� 22 34:1� 0:5 2:6� 0:2

946 D. Garc�ııa-L�oopez et al. / European Polymer Journal 39 (2003) 945–950

taken from 100 nm microtomed sections of the com-

posites cut with a Reichert–Jung Ultracut E microtome,

using a Jeol JEM 2000FX Electron Microscope with 200

kV accelerating voltage.

2.5. Evaluation of mechanical properties

The properties of the resulting nanocomposites were

measured as follows: Young�s modulus and tensile

strengthweremeasured according toUNE-EN ISO 527-1

and 527-2 with a Instron Model 5500R60025. For not-

ched Izod impact strength a pendulum trademark Frank

Model 53566 was used under UNE-EN ISO 180.

3. Results and discussion

Tables 1 and 2 show TGA clay content and me-

chanical properties of the composites obtained, specifi-

cally Young�s modulus, tensile strength and notched

Izod impact strength. The analysis of the trends on

mechanical properties gives information about the effect

of both compatibilizing agent and clay.

The actual clay content, as inorganic fraction, was

found by TGA analysis in different parts of the samples.

The clay percentage by weight is close to 5% in the

samples where 7% clay was added during nanocom-

posites preparation. In 3% samples, the final clay con-

tent was around 2%.

Young�s modulus of DEM composites versus MAH

composites, comparing the corresponding samples with

the same composition, gives higher performance for the

latter in all cases. It can also be found the influence of

the kind of clay on mechanical performance, as all

samples containing commercial clay have higher mod-

ulus than the samples containing a raw bentonite mod-

ified by us. This difference in mechanical performance

shows how important is the nature of the polyolefin

grafting and the clay treatment process. MAH is more

polar than DEM. DEM has an open structure in which

the dipole moment can be close to zero due to transoid

conformations. MAH is a rigid five membered ring with

permanent dipole moment. Due to this effect, MAH is a

better compatibilizing agent, because the polar interac-

tions with the polar clay are more favourable compared

with DEM. Another feature which may explain the

improved properties of MAH nanocomposites versus

DEM is the imide bond formation [17]. The modified

clay surfactant, octadecylamine cations, exists in an acid-

base equilibrium, being able to react as a nucleophile

with the carbonyl groups on the grafting agent. The

reactivity of MAH carbonyl groups towards this kind of

reactions is higher than in the case of DEM, due to ring

strain. On the other hand, the commercial clay of

montmorillonite has very high purity, being very ho-

mogeneous. The clay modified by us, is a low purity

bentonite, which has many inhomogeneous aggregates,

due to calcium content, and siliceous impurities. Al-

though many of the impurities are eliminated during

modification process, due to flocculation of the clay and

the differences in specific gravity between clay and sili-

ceous minerals, and also during screening through a 63

lm sieve, some micrometric impurities can be found,

which act as stress concentrators, allowing crack initia-

tion and propagation, decreasing consequently the me-

chanical performance of the nanocomposite. Another

important factor is the aspect ratio of the clay, which is

variable depending on the ore from which the clay is

obtained. Higher aspect ratios offer improved mechani-

cal performance and specially improved heat deflection

temperature.

This shows how both matrix modification, and clay

modification and quality have a direct influence on

mechanical properties of the obtained nanocomposites.

Tensile strength values are not profoundly changed

either by the kind of grafted PP or by clay content.

However, it can be found a small decrease on the values

as the content of grafted polyolefin increases. MAH

containing samples show higher tensile values than

DEM samples. Again the best mechanical performance

Table 2

Composition and mechanical properties of PPgMAH nanocomposites

PP

(wt.%)

PPg

(wt.%)

Type of clay Clay

(wt.%)

TGA clay

contents

(wt.%)

Modulus

(MPa)

Tensile

strength

(MPa)

Notched Izod

impact strength

(kJ/m2)

100 – – – – 1828� 33 34:3� 0:9 3:3� 0:391 9 – – – 1797� 81 36:0� 0:4 2:4� 0:179 21 – – – 1672� 36 35:4� 0:2 2:4� 0:288 9 Bentonite 3 2.6 2024� 43 36:8� 0:2 2:2� 0:272 21 Bentonite 7 4.8 2130� 56 35:5� 0:3 1:9� 0:488 9 Nanomer

I30.TC

3 2.4 2282� 27 36:8� 0:4 2:5� 0:3

72 21 Nanomer

I30.TC

7 4.5 2597� 34 36:2� 0:1 1:2� 0:2

D. Garc�ııa-L�oopez et al. / European Polymer Journal 39 (2003) 945–950 947

is found in MAH nanocomposites as expected from

modulus results.

Notched Izod impact strength values show in general

a decrease on the impact strength of the composites as

the content of grafted polar agent increases. When DEM

is used as compatibilizer, the nanocomposites show little

variation in their properties. However the increase of the

MAH in the PPgMAH nanocomposites produces an

increase of the modulus and stiffness and a decrease of

the impact strength.

The analysis of the mechanical properties of the

nanocomposites show clearly the influence of the kind of

compatibilizing agent in the final properties of the ma-

terials. Young�s modulus and impact strength are greatlyinfluenced by the content of coupling agent and also by

the quality of the clay. The effect of the kind of clay can

only be seen when MAH is used, as DEM has a low

compatibilizing effect due to its low polarity. Because of

this, the clay can not be well dispersed in the polymer

matrix.

Fig. 1 displays the small angle X-ray diffraction pat-

terns of both modified and unmodified montmorillonite

and bentonite clays. It can be observed the d0 0 1 peakshift to lower angles, corresponding to an increase on

the basal spacing of the clays by exchange of interlayer

sodium with onium cations. The basal spacing on

commercial clay (I30.TC) moves from 1.24 nm for so-

dium montmorillonite (MMNa) to 2.52 nm for the

modified clay. BNa basal spacing moves from 1.36 to

2.99 nm by surfactant exchange. However, this modified

bentonite (BC18) shows a much broader peak compared

with the commercial sample. This is due to the inho-

mogeneous distribution of the surfactant between the

layers of the clay, presenting a broader range of inter-

layer distances depending on the extent of ion exchange.

In the nanocomposite X-ray diffraction patterns (Fig.

2) it can be observed an increase in intergallery spacing,

as the d0 0 1 peak shifts to lower angles. This increase isdue to the PP and functionalized PP intercalation be-

tween clay platelets. The nanocomposites obtained with

DEM functionalized PP show, for BC18 clay an inter-

gallery spacing of 3.26 nm, and for I30.TC clay the in-

tergallery spacing moves to 2.90 nm. When using MAH

functionalized PP, values of 3.21 nm for BC18 and 2.82

nm for I30.TC are obtained. In this diagram, we can also

observe how the commercial clay X-ray pattern shows a

much more defined peak than the clay modified by us,

for both fuctionalizating agents used. This much more

defined peak for the commercial clay is also observed in

the modified clays X-ray patterns.

TEM images (Fig. 3) are in good agreement with the

observed mechanical properties. MAH nanocomposites

(a) and (b) show a higher degree of disordered structures

and exfoliated layers than DEM nanocomposites (c) and

(d). It can also be observed how the structure of MAH

nanocomposites is intermediate between an intercalated

and an exfoliated state, with stacks of disordered layers

of around 200 nm and smaller aggregates containing

between 2 and 10 clay platelets.

DEM nanocomposites are more ordered and closer to

a tactoid structure, with certain degree of intercalation

in the outer layers of clay aggregates. From TEM images

it can be appreciated a greater intergallery spacing in

MAH nanocomposites than in DEM modified samples.

The two different clays (a,c) and (b,d) also show differ-

ences in mechanical properties and TEM images.

Modified bentonite, improves mechanical performance

in a lower extent than the commercial modified mont-

morillonite, due to the lower extent of modification and

low homogeneicity. In Fig. 3d, a spherical impurity of

about 50 nm can be observed, which has been found to

be a rutile (TiO2) by X-ray elemental analysis coupled to

TEM. Also, the absence of sodium or calcium ions has

been confirmed by this technique, confirming the com-

Fig. 1. X-ray diffraction patterns of the clays of unmodified

montmorillonite (MMNa) and bentonite (BNa) and modified

montmorillonite (I30.TC) and bentonite (BC18).

Fig. 2. X-ray diffraction patterns of nanocomposites PP/DEM/

BC18, PP/DEM/I30.TC, PP/MAH/BC18 and PP/MAH/

I30.TC.

948 D. Garc�ııa-L�oopez et al. / European Polymer Journal 39 (2003) 945–950

plete exchange of those cations by octadecylammonium

ions in the BNa modified by us.

4. Conclusion

In this study DEM and MAH modified PP nano-

composites have been obtained and two different clays

have also been used. Although the commercial clay

outperforms octadecylammonium treated bentonite,

differences in mechanical properties when using different

clays are smaller if DEM is used instead of MAH. This

is a consequence of the very low degree of compatibili-

zation between the polymer matrix and the clay. Clay

dispersion and interfacial adhesion are greatly affected

by the kind of matrix modification.

Fig. 3. TEM images of (a) MAH/I30.TC; (b) MAH/BC18; (c) DEM/I30.TC; (d) DEM/BC18. Sample composition: PP/PPg/Clay (72/

21/7).

D. Garc�ııa-L�oopez et al. / European Polymer Journal 39 (2003) 945–950 949

Clay modification and processing conditions are not

enough to provide an appropriate nanometric dispersion

of clay layers and an homogeneous distribution of the

clay in the samples. This might be due to several issues

related with thermodynamic interactions in the modified

clay–matrix–oligomer system. DEM has a lower polarity

compared with MAH, providing a less effective inter-

action with the polar components of the clay. The re-

activity of MAH towards the modifying agent is greater

than in the case of DEM. Both factors give as result

better interfacial adhesion and subsequent mechanical

performance for MAH nanocomposites.

Clay and matrix modification are synergistic factors

which need to be properly modulated in order to obtain

the desired final properties on this kind of non-polar

polymer based nanocomposites.

Acknowledgement

This work is supported by CICYT (program

1FD1997-2025-CO2/MAT).

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