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Polypropylene--clay nanocomposites: influence of low molecular weight polar additives on intercalation and exfoliation behavior.

INTRODUCTION AND OBJECTIVES

Polymer nanocomposites based on clay are two-phase materials in which clay particles are dispersed towards the nanometer (nm) range in the polymer matrix. The most commonly used clays are smectite group minerals such as the montmorillonite (MMT) type, which belongs to the general family of 2:1 layered silicates. Generally, inorganic MMT is modified with organic surfactant (e.g. alkyl ammonium salt) to make it compatible with polymers [1, 2]. Basically, polymer-clay composites are divided into three general types: phase-separated, conventional micro composites, intercalated nanocomposites (in which polymer molecules intercalate into clay galleries), and exfoliated nanocomposites, in which clay particles are separated into individual layers and dispersed evenly in the polymer matrix [3-5]. The latter two structures are the main interest in this article and they are considered as nanocomposites. Polymer-clay nanocomposites have received increased attention in recent years, since the Toyota research group first investigated polyamide 6/clay nanocomposites and it was found that mechanical and thermal properties are improved, in comparison with both conventional micro-scale composites and the unmodified polymer, at very low loading levels of clay [6, 7]. Since then a number of other polymer-clay nanocomposites such as those based upon polystyrene (PS), acrylic polymer, epoxy resins, and polycaprolactone have been reported in the scientific literature [8-11].

Polypropylene (PP) is one of the most widely used thermoplastic materials for a number of applications such as automotive and packaging because of its versatility, overall balanced properties, and its attractive property-cost ratios. However, it is difficult to get exfoliated and fully-dispersed clay particles in a PP matrix, because of the low polarity of conventional PP. This is mainly due to incompatibility, even between with organically modified clay and PP, which has no polar groups on its backbone. However, successful preparation of PP-clay nanocomposites (PPCN) has become a more realistic possibility by using functional oligomers as a compatibilizer [12, 13]. Such a compatibilizer should contain sufficient polarity to interact with silicate layers and it should also be easily mixed with the bulk PP [14]. The most commonly used compatibilizer for the preparation of PPCN is PP-MA [13, 15, 16]. By grafting polar maleic anhydride onto non polar PP, it becomes compatible with clay and it is also easily dispersed with the bulk PP, during compounding. It has been found that low molecular weight and high maleic anhydride containing PP-MA intercalates into clay galleries more effectively [17]. However, high maleic anhydride concentration leads to phase separation, therefore affecting the achievable mechanical properties of the PPCN [18, 19].

Low molecular weight additives containing polar groups are often used in PP formulations, for example, slip additives for extruded film or molded applications, to obtain specific surface properties in PP products. Generally, these additives are able to migrate onto the surface when PP artifacts cool and consolidate, due to the incompatibility with the bulk PP. Although in the recent past, extensive studies have been done on the preparation and properties of PPCN, less attention has been focused on how these low molecular weight polar additives affect the PPCN structure.

Therefore, an overall objective of this study was to determine the influence of low molecular weight polar additives (already present in PP) on intercalation and exfoliation behavior of clay, in PPCN. In addition, investigations on the effect of amide-type slip additive on PPCN structure, its accumulation on composite surface, together with the relationship between clay dispersability and rheology of PP-clay nanocomposites, were also important features of the research. With additional information of this type, technologies by which the physical properties of PPCN can be enhanced may be proposed more precisely.

MATERIALS AND EXPERIMENTAL METHODS

Materials

Two polypropylene homopolymer grades (HB671 and HB306) supplied by Borealis have been used in this study. The nominal melt flow index (MFI) of these two grades is 2 g/10 min, when tested at 230[degrees]C using 2.16 kg. HB306 contains antistatic and slip agents, whereas the other grade (HB671) does not contain low molecular weight polar additives. Maleic anhydride-grafted PP (Polybond 3200), containing 1% (by weight) of maleic anhydride, supplied by Crompton Corporation has been used as a functionalized, polymer-based compatibilizer. Montmorillonite clay modified with dimethyl dihydrogenatedtallow quaternary ammonium has been used to prepare PP-clay nanocomposites and was supplied by Southern Clay Products. An amide-type slip additive (AA) (supplied by Akzo Nobel Polymer Chemicals) was used as the low molecular weight polar additive.

Preparation of PP-Clay Nanocomposites

PPCN compounds were prepared by melt blending of either HB671 (PP1) or HB306 (PP2) and organically modified clay (OMMT) in the presence of PP-MA, using a Haake Rheomix 600 torque rheometer, operating at 185[degrees]C (set temperature) and 100 rpm rotor speed. The mixing time was 6 min for all composites; this condition was predetermined as the most appropriate mixing time. The precise compositions of all the prepared composites are summarized in Table 1.

Characterization of PPCN Structure

X-ray diffraction (XRD) analysis was performed on a Bruker D8 diffractometer, using Cu K[alpha] radiation to determine the formation of nanocomposite structures and to determine interlayer spacing of the clay layers in the PP matrix, from the measured Bragg angles. XRD patterns of thin compression molded sheets of PPCN were obtained by scanning over a Bragg angle (2[theta]), ranging froml-10[degrees] at a rate of 0.01[degrees]/s. The conventional Bragg equation (n[lambda] = 2d sin [theta]) was used to calculate the interlayer spacing of clay in composite materials. The dispersability of clay particles in the PP matrix was evaluated by transmission electron microscopy (TEM), using a JEOL, JEM 200FX with an electron acceleration voltage of 200 kV.

Investigation of Surface Properties

Surface composition of the PP-clay composites (PPCC), prepared with the addition of AA, was examined qualitatively using contact angle measurements to characterize the surface of molded composite materials. Contact angle ([theta]), the angle that a liquid (of known surface tension) makes while resting at thermodynamic equilibrium on a solid, measures the wettability or spreadability of solid surface, and this characteristic varies greatly with the nature of solid surface. This was done using a Dataphysics OCA20 (an optical contact angle instrument) using pure distilled water and diiodomethane as liquid drops at room temperature (20[degrees]C). The molded samples were analyzed for contact angles 10 days following preparation, as the surface is saturated with AA during this period of time. The surface energies of each composite surface were also calculated as shown below, using contact angles ([theta]) of these two liquids, whose surface tensions are known. Values used were as follows: 72.8 mN/m (water) and 50.8 mN/m (diiodomethane) [20].

[[gamma].sub.LV](1 + cos[theta]) = 2([[gamma].sub.S.sup.d][[gamma].sub.LV.sup.d])[.sup.1/2] - 2([[gamma].sub.S.sup.p][[gamma].sub.LV.sup.p])[.sup.1/2]

where [[gamma].sub.LV] is the surface tension of a liquid, [[gamma].sub.S.sup.d] and [[gamma].sub.LV.sup.d] are dispersion contributions to surface energy of the solid and liquid, respectively, and [[gamma].sub.S.sup.p] and [[gamma].sub.LV.sup.p] are polar contributions to surface energy of the solid and liquid, respectively.

[FIGURE 1 OMITTED]

Melt Flow Index of PPCN

MFI is a pressure-imposed, capillary flow experiment and was used to study the relationship between low strain rate shear flow properties and clay structure in nanocomposites and the interaction between clay and PP matrix of PPCN prepared with the addition of PP-MA and AA. The blends of PP, PP-MA, and AA were used as control samples. To investigate the effect of PP-MA and AA on clay structure in PPCN and the interaction between clay and bulk PP, the 'control' polymer matrix effect should be excluded. Therefore, in this study, a normalized MFI (n-MFI) was calculated as shown below and was then compared to n-MFI of PPCC.

Normalized MFI = [MFI of composite]/[MFI of corresponding control].

The test temperature was set to 190[degrees]C and a dead weight load of 2.16 kg was applied. For each composite sample, the final results were the averages of two sets of measurements.

RESULTS AND DISCUSSION

Effect of Additives Present in PP on PPCN Structure

PPCN prepared with PP1 and PP2 in the presence of PP-MA were analyzed by the X-ray diffraction technique to investigate the formation of nanocomposite structure, in terms of interlayer spacing of clay in the PP matrix. Figure 1 shows the X-ray diffraction patterns of PPCN prepared with PP1 and PP2. Table 2 summarizes the Bragg angle position and inter layer spacing of clay in all PPCN calculated from (001) diffraction peaks. As shown in Fig. 1, PPCN prepared with PP1 (which does not contain polar additives) exhibits an X-ray diffraction peak shifted slightly towards a lower Bragg angle, resulting in higher interlayer spacing (see also Table 2), in comparison to pure organically modified clay. PP-MA intercalates into the clay galleries and as a result, the interlayer spacing is increased. However, on the other hand, PPCN (PP2-PB2-2 and PP2-PB6-2), which is prepared with PP2 (containing low molecular weight polar additives), shows a much more significant shifting of X-ray diffraction peaks (001) to lower angles, compared to that of PPCN prepared with PP1. Clay gallery spacing increases from a 30 [Angstrom] (as supplied) to around 39-40 [Angstrom] (Table 2). This could be due to the intercalation of low molecular weight polar additives (which are compatible with OMMT) and the compatibilizer. As a result of this cointercalation, interlayer spacing of clay is increased up to 39.4 and 40.1 [Angstrom] in PP2-PB2-2 and PP2-PB6-2, respectively. Hasegawa and coworkers [13] and also Kawasumi and coworkers [15] have reported that a higher content of PP-MA improves the intercalation and clay dispersability in the PP matrix. However, in the present study, addition of higher percentage of PP-MA (PP2-PB6-2) has not shown any significant difference of XRD peak position or intensity, compared to that of PP2-PB2-2, which contains a lower percentage of PP-MA. This implies that an optimum interlayer spacing was achieved with cointercalation of polar additives, which are present in PP2 and PP-MA.

Since the XRD results do not give any insight into clay dispersability, TEM micrographs of PPCN prepared with both PP grades were prepared, to evaluate the effect of low molecular weight polar additives present in PP2 on clay dispersability in PPCN structures. Figure 2 shows the TEM images, where the dark areas represent the clay particles and grey areas represent the continuous PP matrix, for PPCN prepared with PP1 and PP2. In Fig. 2a, for PPCN containing both low molecular weight polar additives and PP-MA, improved clay dispersability in PP2 matrix is shown, in comparison to the TEM image (Fig. 2b) of PP1-PB6-2, which contains only PP-MA. Both the compatibilizer and low molecular weight polar additives present in PP2 have intercalated into the clay galleries, therefore increasing the interlayer spacing of clay significantly, as shown in the XRD results. As a result, the forces between clay layers become weaker, enhancing the exfoliation of clay particles into smaller stacks (in comparison with PP1-PB6-2 containing only PP-MA), during the melt mixing process. These smaller clay stacks are dispersed evenly in the PP2 matrix due to the presence of compatibilizer. However, from both the XRD and TEM results, it is clear that complete exfoliation of clay was not achieved with either PP grades and therefore a mainly intercalated structure has been formed with very few completely exfoliated clay layers.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Comparison of Two PP Polymers

To confirm the effect of low molecular weight additives on intercalation and exfoliation behavior of clay, studies were continued by melt mixing each PP grade (PP1 and PP2) with clay in the absence of PP-MA. Figure 3 shows the X-ray diffraction patterns of PPCC prepared with each of these two PP grades (PP1 and PP2), relative to the uncompounded clay (OMMT). The comparison between the respective diffraction peaks of PP1-2 and PP2-2 composites clearly shows that the X-ray diffraction peak of PP2-2 composite is shifted significantly to a lower Bragg angle, resulting in a higher interlayer spacing. This X-ray diffraction result further confirms that low molecular weight polar additives (which are compatible with OMMT) intercalate into clay galleries and therefore increase the interlayer spacing. Interaction between the polar groups of these additives and silicate layers is likely to be the driving force for the intercalation of additives into clay galleries. The XRD peak of PP1-2 remains in the same Bragg angle position as pure OMMT, since only the phase-separated conventional micro composite is formed. However, the XRD peak of PP2-2 is narrower and has higher peak intensity compared to that of PP2-PB2-2 (in Fig. 1). This implies that although low molecular weight polar additives are able to diffuse into clay galleries, most of the organized clay structures remain unchanged. The stress applied during the melt mixing process is not sufficiently intense to separate the inorganic phase into smaller units, since there is no compatibilizer to transfer deformational energy from the matrix.

TEM analysis was also performed on both PP1-2 and PP2-2 composites to study the clay dispersability within the PP matrix. Figure 4a shows significantly larger clay particles, which are not intercalated, and a conventional 'micro-composite' structure is formed, presumably due to the absence of PP-MA. This TEM image is agreeable with the result obtained from the X-ray diffraction (Fig. 3). On the other hand, Fig. 4b shows comparatively smaller clay particles than Fig. 4a, consistent with a higher interlayer spacing of clay in PP2, and so the clay particles are separated into smaller units during the mixing process. However, in comparison with PPCN prepared by incorporation of PP-MA (Fig. 2), clay particles in PP2-2 composite are comparatively larger, indicating that they are not dispersed homogenously throughout the PP2 matrix, in the absence of PP-MA compatibilizer. Although these low molecular weight polar additives present in PP2 are able to intercalate into clay galleries, they are not compatible with PP (in the same way as PP-MA), and therefore clay particles do not distribute homogenously throughout the matrix.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Effect of Amide Type Slip Additive on PPCN Structure

It has been shown from XRD and TEM results that both additives (slip and antistatic agents), which are present in PP2 grade, intercalate into clay galleries, and so AA was then used to study the effect of its concentration on intercalation behavior of clay. PP1 (which initially does not contain any polar additives) was melt mixed with OMMT and AA in the absence of compatibilizer to investigate the intercalation behavior of this additive into clay galleries.

Figure 5 shows X-ray diffraction peaks of PP1-clay composites prepared with different concentrations of AA. XRD results clearly show the shifting of all peaks to a lower Bragg angle when AA of increasing concentration is incorporated into the PP-clay composites (in comparison with the original OMMT). Table 3 summarizes the diffraction peak position and the calculated interlayer spacing ([d.sub.001]) of clay in each composite material. The highest interlayer spacing was achieved by the addition of 0.5% (by weight) of AA. However, a further increase of additive concentration is not effective in terms of modifying the interlayer spacing, probably because of the migration of additional slip additive onto the surface, as occurring in pure PP. From this result, it is clear that functional (amide-type) slip additives intercalate into the clay galleries, resulting in higher gallery spacing, and the presence of the polar amide group of the additive is likely to be the driving force for the migration process that creates enhanced intercalation of the clay galleries.

However, in comparison with the XRD results for PP2-2 composite (Fig. 3), interlayer spacing of PP1-clay composite (37.44 [Angstrom]) containing 0.5% of AA is smaller than that of PP2-2 (38.7 [Angstrom]). This would suggest that not only slip agents but also antistatic agents intercalate into the clay galleries, resulting in a higher clay gallery space in PP2-2 composite. PP1 was melt mixed with clay in the presence of both compatibilizer and AA, and TEM image of this PPCN was observed to find out the effect of this additive on clay dispersability in PP1 matrix.

As shown in Fig. 6a, clay particles are separated into much smaller stacks and they are dispersed homogenously throughout the PP1 matrix when AA is added, in addition to PP-MA. The improved clay dispersability of PP1-PB2-AA0.5-2 (Fig. 6a) in comparison with that of PP1-PB2-2 (Fig. 6b) can be attributed to the cointercalation of AA with PP-MA, and an increased clay dispersability during the melt mixing process is also achieved. From this result, it can be further confirmed that although the addition of AA has a positive impact on clay dispersability in PP matrix, the compatibilizer (PP-MA) is also necessary to achieve a more homogenous dispersion of clay in the PP matrix.

Determination of Surface Energy

Generally, when functional slip additives are incorporated into pure PP, migration onto the surface occurs and a thin additive-rich layer is formed on cooling, due to incompatibility with the PP matrix; as a result, the surface composition is changed, resulting in different surface energy and friction properties. Contact angle is sensitive to the chemical nature of polymer surfaces. Water and diiodomethane contact angles on PP1-clay composite substrates prepared with the addition of different concentrations of AA were measured, and surface energies of each composite were calculated according to contact angles to characterize the accumulation of this additive on the composite surface.

Figure 7 shows the water contact angles of PP-clay composites prepared with different concentrations of AA and their respective PP-AA blends. Water contact angles of PP-AA mixtures were decreased with an increase of AA and reach steady values with further increases of the additive concentration above 1%. This implies increased wettability of PP-AA blend surfaces as contact angle decreases because of the migration of AA onto the surface, as expected. However, by adding clay into PP-AA blends, water contact angles are increased dramatically, especially at low concentrations where contact angle becomes almost the same as pure PP1, compared to that of their respective PP-AA blends.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

The migration of AA and its accumulation on the surface of PP1-AA-clay composites can be further verified by the calculated surface energy values (Fig. 8) obtained for PP-AA-clay composites and the equivalent matrix formulation (PP-AA). Surface energy of PP1-AA is increased with the increase of AA towards a steady value of just below 32 m [Nm.sup.-1]. However, the surface energies of PP1-AA-clay composites show lower values compared to those of the respective matrix, across the additive concentration range. AA migrates onto the surface in pure PP, as expected, and increases the surface energy due to the presence of the polar group (N[H.sub.2]) in this additive and its increased surface concentration. Surface energy reaches steady values when the additive concentration exceeds around 0.7% by weight, as the surface becomes saturated with AA-rich species. However, by adding clay into PP1-AA blends, the surface energy becomes lower (in comparison to the respective PP1-AA blends at similar additive concentration), since most of AA interacts with clay particles, rather than migrating onto the surface. Especially at low concentrations of AA, surface energy assumes similar values as pure PP1, because all the available AA intercalates into the clay galleries. We have no clear explanation at present as to why the PPCN compound containing 0.5% AA shows a slightly lower surface energy than unmodified PP. In Fig. 8, surface energy is then increased with an increased AA concentration (at concentrations greater than 0.7 wt%) if a proportion of the AA is able to migrate to the surface. These contact angle measurements confirm the results obtained with XRD (Fig. 5), that slip additives containing polar groups diffuse into clay galleries, interact with them, and promote particle intercalation.

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

Analysis of PPCN by Melt Flow Index

PPCN prepared with the addition of AA and increasing concentration of clay were analyzed by MFI to study the relationship between melt rheology and clay dispersability in the PPCN. It is known that the viscous behavior of a PPCN composite (at any given particle concentration) is sensitive to the aspect ratio of filler and filler-matrix interactions, especially at low shear rates. However, it is not only the clay structure but also the viscous properties of the polymer matrix that also influences the measured MFI. Therefore, to investigate clay structure in PP and its interaction with the continuous PP matrix, a "normalized" MFI (defined as shown in the experimental section) was calculated to present comparative data. Zhu and Xanthos have also used MFI measurements successfully to study the relationship between theology and morphology of clay in PPCN [21]; their results showed that MFI data are able to provide an indication of exfoliation and dispersion of clay in the PP matrix.

Figure 9 shows how the normalized MFI changes with increasing clay concentration, for both PP1-PB-AA-clay nanocomposites and PP1-clay composites. The composition of each PPCN prepared with the addition of AA is shown in Table 1. When PP is melt blended with clay in the absence of both PP-MA and AA, the n-MFI does not change when clay loading is increased, because clay particles remain in their original state and there is also no significant interaction between clay particles and the polymer matrix. As a result, the clay simply acts as a conventional diluent filler in the PP matrix, having little influence on shear flow properties at such a relatively low concentration. However, on the other hand, when PP-MA and AA were also added into PP-clay composites, the n-MFI shows a steady reduction with increased clay loading (from 2 to 6% by weight) in Fig. 9. This demonstrates a viscosity increase due to the exfoliation of clay into smaller stacks with the addition of PP-MA and AA, hence increasing the aspect ratio and specific surface area of clay particles in the nanocomposites. A steady reduction of n-MFI with an increased clay concentration further implies that the interaction between clay and the modified PP matrix is achievable.

The positive effect of low molecular weight slip additive and PP-based compatibilizer on clay dispersability is further verified from these results and which are agreeable with TEM images of PP2-clay composites (Fig. 4b) and PP1-PB-AA-clay nanocomposites (Fig. 6a). The steady reduction of n-MFI would suggest that PP molecules diffuse into clay galleries with PP-MA and AA and, as a result, an intercalated nanocomposite structure is more likely to be formed.

CONCLUSIONS

In this study, the effect of low molecular weight polar additives on PPCN structure was investigated using a small-scale batch mixer and subsequent analysis, using a range of characterization techniques. The experimental results showed that polar additives, some of which are present in commercial PP grades, are able to intercalate into the clay galleries, resulting in a higher interlayer spacing and promoting enhancement of clay dispersion in the PP matrix. By adding a specific polar additive (amide-type slip agent) into PPCC demonstrates intercalation of this additive into the clay galleries; yet it does not disperse

homogeneously into the bulk PP matrix in the absence of a functional PP-MA compatibilizer, since these additives are not directly compatible with PP. The PPCN prepared by incorporating both PP-MA and AA show improved dispersion of clay compared to PPCN prepared with only PP-MA. Contact angle measurements verified that amide-type slip additive intercalates into the clay galleries and interacts with silicate layers, rather than migrating to the surface of the molded PP artifact. A normalized MFI, which characterizes capillary shear flow behavior at low shear rates relative to the matrix polymer, is an appropriate parameter with which to study the exfoliation of clay and its interaction between PP matrix in PPCN. A mainly intercalated nanocomposite structure, with improved clay dispersion, is formed by the addition of AA in addition to PP-MA; a reduction of the PP-MA ratio in PPCN is also helpful, in this respect.

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U.N. Ratnayake, B. Haworth

Institute of Polymer Technology and Materials Engineering (IPTME), Loughborough University, Loughborough LE11 3TU, United Kingdom

Correspondence to: B. Haworth; e-mail: b.haworth@lboro.ac.uk
TABLE 1. Composition of PP-clay nanocomposites (PPCN) and PP-clay
composites (PPCC (a)) prepared with two commercial grades of PP.

 PP1 PP2 PP-MA AA OMMT
Composite sample (wt%) (wt%) (wt%) (wt%) (wt%)

PPCN
 PP1-PB2-2 96 -- 2 -- 2
 PP1-PB6-2 92 -- 6 -- 2
 PP2-PB2-2 96 2 -- 2
 PP2-PB6-2 -- 92 6 -- 2
 PP1-PB2-AA0.5-2 95.5 -- 2 0.5 2
 PP1-PB4-AA1-4 91 -- 4 1 4
 PP1-PB6-AA1.5-6 86.5 -- 6 1.5 6

PPCC
 PP2-2 -- 98 -- -- 2
 PP1-2 98 -- -- -- 2
 PP1-4 96 -- -- -- 4
 PP1-6 94 -- -- -- 6

(a) The term PPCC is used to denote composites that do not contain
compatibilizer or amide additives.

TABLE 2. Interlayer distance of clay in PPCN prepared with the PP-clay
formulations shown in Figure 1.

 Interlayer spacing
Composite material Bragg angle (2[theta]) ([d.sub.001])([Angstrom])

OMMT 2.92 30.2
PP1-PB6-2 2.76 32.0
PP2-PB2-2 2.24 39.4
PP2-PB6-2 2.20 40.1

TABLE 3. Interlayer distance of clay in PP-clay composites prepared with
different concentrations of AA.

Composite Bragg angle Interlayer spacing
material (2[theta]) ([d.sub.001])([Angstrom])

OMMT 2.92 30.2
PP1-AA0.3-2 2.64 33.46
PP1-AA0.5-2 2.36 37.44
PP1-AA0.7-2 2.42 36.48
PP1-AA1-2 2.36 37.44
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Date:Aug 1, 2006
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