Direct Melt Extrusion Processing of Nylon 6/Clay

Nanocomposites Based on Pristine Montmorillonite

Ying Li, Zhaoxia Guo, Jian Yu*

Institute of PolymerScience andEngineering,Department ofChemicalEngineering, School ofMaterials Science andEngineering,Tsinghua University, Beijing 100084, ChinaE-mail: yujian03@mail.tsinghua.edu.cn

Received: January 8, 2005; Accepted: April 12, 2005; DOI: 10.1002/mame.200500012

Keywords: melt extrusion; nanocomposites; nylon 6; pristine montmorillonite

Introduction

In recent years, polymer/layered silicate nanocomposites

have attracted great interest, and numerous reports have

come forth in this field. Among them, nylon 6/clay nano-

composites may be the most prominent example since the

first demonstrationbyUsuki and co-workers.[1] In themean-

time, preparation methods have progressed from the initial

in situ polymerization to melt compounding, and partial

industrialization has been achieved for nylon 6/clay nano-

composites. Apparently, melt compounding is quite con-

venient for industrial production, and many corresponding

research reports have been published.[2–6]

As to melt compounding of polymer/clay nanocompo-

sites, it is generally considered that organo-treatment of clay

is the key issue for achieving the intercalation or exfoliation

nano-dispersion aswell as increasing the compatibilitywith

polymeric matrix. Nearly all of the reported results are bas-

ed on organophilic clays. However, if pristine clay is used to

prepare polymer/layered silicate nanocomposites instead of

organophilic clay, one prominent advantage is the drasti-

cally decreased production costs of the composite materi-

als, since the price of industrialized organo-clay is much

higher than that of pristine clay. In addition, its envir-

onmentally benign character based on the absence of large

quantities of industrial waste water is worth noting.

Alexandre et al. first reported the ‘‘one-pot’’ preparation

of EVA/clay nanocomposites starting from Naþ-montmor-

illonite.[7] Using a two-roll mill, the same intercalated

structure was obtained whether EVA-based nanocompo-

sites were prepared from an organo-modified montmor-

illonite or from aNaþ-montmorillonite by the addition of an

ammonium salt bearing long alkyl chains as a reactive

compatibilizer. Concerning the preparation of nylon 6/pris-

tine clay nanocomposites, Hasegawa et al.[8] reported the

preparation of nylon 6/Na-montmorillonite nanocompo-

sites by compounding nylon 6 with Na-montmorillonite

slurry; the acquired nanocomposites possessed almost

identical properties to those of conventional nylon 6/clay

nanocomposites prepared by dry-compounding of nylon 6

Summary: An original direct melt extrusion processing ofnylon 6/clay nanocomposites was reported based on pristine(Naþ-based) montmorillonite as well as a simple approachusing a typical two-screw extruder. By the application ofintercalation agents as the thermodynamic assistants, thismethod is as an appropriate procedure for industrializedmanufacture together with much lowered production cost.Interestingly enough, the synergistic effects of montmorillo-nite with other inorganic particulates was observed for thefirst time here.

X-ray diffraction patterns of pristineMMTand nylon6/MMTcomposites with grouped intercalation agents.

Macromol. Mater. Eng. 2005, 290, 649–652 DOI: 10.1002/mame.200500012 � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication 649

and organophilic clay. As indicated by the authors, the most

valuable advantage of this compounding process is that the

nanocomposite is prepared without any surfactants of the

clay minerals and additives. However, special processing

facilities were necessary for this method. Hu et al.[9,10]

reported the preparation of nylon 6/clay hybrids using a

direct melt intercalation technique, in which nylon 6 was

melt-mixed with clay using an ammonium salt bearing long

alkyl chains as a polymer/clay reactive compatibilizer.

Similar to Alexandre et al., whether for Naþ, Cu2þ or Fe2þ-

based montmorillonite, this intercalation technique involv-

ed comparatively long mixing times with a two-screw mill.

This communication investigates the direct melt extru-

sion processing of nylon 6/pristinemontmorillonite (MMT)

nanocomposites, just by the use of a simple blending and

extrusion procedure on a typical two-screw extruder. With

the addition of small molecular intercalation agents as well

as co-intercalation agents bearing functional groups, nylon

6/MMT nanocomposites with a better state of dispersion

have been prepared, and the synergistic effect of montmor-

illonite with the other inorganic particulates was primarily

discussed.

Experimental Part

Materials

A low viscosity grade nylon 6 (B3S, BASF) has been used asthe main matrix in this study. Purified sodiummontmorillonite(NB-900, Huate Group products, Zhejiang, China) with acationic exchange capacity of 95 meq/100 g and an interlayerspacing of 12.4 A has been used as the nanofiller precursor. Forthe sake of comparison, a high viscosity grade nylon 6([Z]¼ 3.0–3.5) from Yueyang Baling Chemical & SyntheticFiber Co., LTD (China) has also been employed. Epoxy resinE12, a diglycidyl ether of bisphenol Awith an epoxy value of0.12 eq/100 g, was supplied by Wuxi Resin Factory (China).Nanometer ZnO was supplied by Zhejiang Zhoushan Mingrinano material Co. (China), with an average diameter of 20 nm.

Preparation of Nylon 6/Montmorillonite Nanocomposites

After a preparatorymixing process, nylon 6, pristineMMTandthe intercalation agents were melt compounded using a two-screw extruder, with a screw speed of 120 rpm. The twin-screwextruder used had a screw diameter of 34 mm and a L/D valueof 28. The blending temperature profile was set to 220–235–235–235–230–200 8C.

Characterization

The morphology of the nanocomposites was examined usingwide-angle X-ray diffraction (WAXD) and transmission elec-tron microscopy (TEM). WAXD scans were performed oninjection-molded Izod bars, at a rate of 2 8/min using Cu Ka

radiation between1 to108 (2y). TEMobservationwas perform-ed on ultrathin sections of the composites.

Results and Discussion

As is well known, the intercalation or exfoliation of silicate

layers could not be achieved by merely melt compounding

of nylon 6 with pristine montmorillonite, especially when a

two-screw extruder was used. The residence time as well as

the shear force field are comparatively limited, making this

process more difficult. In the present study, the application

of small molecular intercalation agents was attempted with

the intention of decreasing the thermodynamic barriers

concerning the clay exfoliation. Generally speaking, the

selection of intercalation agents follows two basic rules:

first, the compounds should serve to swell the clay layers,

thus favoring the polymer chains to diffuse into the space

between the galleries; second, to improve the material pro-

perties, the intercalation agents used should possess certain

physical or chemical interactions with the polymer matrix.

Several kinds of small molecular compounds with varied

polarity have been selected as intercalation agents, includ-

ing H2O, alkylammonium, caprolactam, alcohols, esters. It

has been found that most of them could cause expanded

interlayer spacing in MMT, as verified by X-ray diffraction

patterns. In addition, the experimental data indicated

that the joint usage of two different species might lead to

better results. In the following discussion, H2O and an

ammonium salt bearing octadecyl chains (TA) were used in

groups, with weight ratios of 2 and 3 phr, respectively. In

these cases, nanocomposites with comparatively good

dispersion were achieved, as verified by TEM observation

(Figure 1).

Following the above results, the addition of co-intercala-

tion agent was further attempted for the composite system

investigated, in which our attention was mostly concen-

trated on hydrophilic polymers and an epoxy resin. The

experimental results showed that the epoxy resin was a

Figure 1. TEM micrographs of the nylon 6(B3S)/MMT 100/05composite with H2O and TA as the intercalation agents at (a) lowmagnification and (b) high magnification.

650 Y. Li, Z. X. Guo, J. Yu

Macromol. Mater. Eng. 2005, 290, 649–652 www.mme-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

comparatively better choice. As indicated in Figure 2, the

addition of H2O and TA clearly results in an intercalated

structure in the composite, in which the characteristic peak

forMMT (2y¼ 7.18) is broadened and shifts to 2.4 and 5.58,respectively. With the addition of small amount of E12

(0.6 phr), the characteristic peak further shifts to smaller

angles, indicating that a better nanometer dispersion has

been achieved. The TEM observation in Figure 3 further

confirmed this result. Note that with the same process pro-

cedure, nylon 6 matrix with higher viscosity is in favor of

the exfoliationofMMTlamellas,whichmightdue to strong-

er shear intensity involved in the processing.

The melt compounding of polymer and clay is a process

prominently dominated by enthalpy changes. Accordingly,

the intercalation of macromolecular chains and interspatial

expansion of the layered silicate are the two crucial factors

for this process. For the present research, the enthalpy

changes, DH, mainly correspond to the interactions be-

tween polyamide macromolecular chains and the clay;

whereas the entropy changes, DS, mostlywere ascribed to

the changes in the restriction state among the small molec-

ules and the macromolecular chains. Obviously, the inter-

action of pristineMMTwith polymermatrix could beworse

than that of nylon 6/organoclay composites. However, it is

worth noting that, when considering the strong polarity of

MMT surface as well as the characteristics of both polarity

and strong hydrogen-bonding interactions for nylonmatrix,

there must be an attractive energetic interaction for the

polyamide chains with the pristine surface of the silicate.

On the other hand, the application of small molecular inter-

calation agents could enhance the thermodynamic driving

power for the intercalation or exfoliation of layered silicate

or both. In addition, as indicated in some corresponding

reports,[11,12] it is easy for the epoxy resin to enter the sili-

cate layer and consequently enlarge the lamella space. At

the same time, the reactions between the epoxy end

functional groups and the polyamide chains may arise in

the melt-compounding process, which can speed up the

intercalation of polyamide chains into the silicate layers and

favors the improvement on the composite properties.

Apart from the employment of different intercalation

agentsmentioned above, onemeaningful finding in the pres-

ent study is that the co-addition of other inorganic parti-

culates with MMT could facilitate the delamination or

exfoliation of silicate layers, includingZnO,Al2O3,CaCO3.

The X-ray diffraction patterns and the corresponding TEM

image of nylon 6/MMT/ZnO nanocomposites are depicted

in Figure 4 and Figure 5, respectively, for the nano-ZnO

example.

Figure 2. X-ray diffraction patterns of pristine MMT and thenylon 6(B3S)/MMT 100/05 composites with grouped intercala-tion agents; here, H represents H2O and E represents epoxy resin.

Figure 3. TEM micrographs of the nylon 6/MMT 100/05composites with H2O and TA as the intercalation agents and theepoxy resin as the co-intercalation agent: (a) low viscosity nylon 6(B3S) and (b) high viscosity nylon 6 at low and high magnifica-tions.

Direct Melt Extrusion Processing of Nylon 6/Clay Nanocomposites Based on Pristine Montmorillonite 651

Macromol. Mater. Eng. 2005, 290, 649–652 www.mme-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Compared to the base composite system containing

TA (3 phr) as the intercalation agent, further addition of

nano-ZnO induced the shift in characteristic peak forMMT

to smaller angles in the X-ray patterns; this effect became

more distinct as the quantity of nano-ZnO increased. The

TEM micrographs of the nylon 6/MMT/ZnO compo-

site with TA have indicated quite a good state of dispersion

in both the silicate layer and the nano-ZnO particulates.

This phenomenon has not been documented until now

and needs to be investigated further to give a reasonable

explanation.

Conclusion

From the discussion mentioned above, a conclusion might

be drawn that for the preparation of nylon 6/pristine MMT

nanocomposites via melt extrusion, the thermodynamic ac-

cessorial functions of intercalation agents play an important

role; the feasibility of this research design has been fully

attested. The comparison of various properties between

nylon 6matrix and nylon 6/MMTnanocomposites prepared

by the addition of intercalation agents aswell as the detailed

presentation concerning the synergistic effects of mont-

morillonite with other inorganic particulates will be discus-

sed in a forthcoming paper.

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Figure 5. TEM micrographs of nylon 6(B3S)/MMT 100/05composite with intercalation agent TA and (a) 1-phr nano-ZnO;(b) 2-phr nano-ZnO at low and high magnification.

Figure 4. X-ray diffraction patterns of pristine MMT, nylon6(B3S)/MMT composite with TA, and nylon 6(B3S)/MMT/ZnOcomposites with TA.

652 Y. Li, Z. X. Guo, J. Yu

Macromol. Mater. Eng. 2005, 290, 649–652 www.mme-journal.de � 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim