Mechanical and barrier properties of PE and EVA/clay nanocomposites

D. MERINSKA *1,2, A. KALENDOVA 1,2, J. HROMADKOVA3, B.HAUSNEROVA2

1 Department of Polymer Engineering, Faculty of Technology, Tomas Bata University in Zlin, Nam.

T. G. Masaryka 275, 762 72 Zlin, Czech Republic 2 Centre of Polymer Systems, University Institute, Tomas Bata University in Zlin, Nad Ovcirnou

3685, 760 01 Zlin, Czech Republic 3 Institute of Macromolecular chemistry, Prague, Heyrovského nám. 2, CZECH REPUBLIC

Abstract: - Nanocomposite materials with layered clay used as nanofiller and polyethylene (PE) and copolymer ethylene and vinyl acetate matrix (EVA, the content of VA component 19 wt. %) were prepared by compounding the individual components in KO Buss kneader. The MMT Na+ and four types of commercial products such as Nanofil N 5 and N3000, Cloisite 93A and 30B were used as nanofillers. The quantity of all the above-mentioned nanofillers was 3 and 5 wt. % in relation to the content of montmorillonite. The aim was to evaluate the influence of clay type and concentration on nanocomposite mechanical and barrier properties. The morphology of nanocomposite samples was examined by means of XRD analysis illustrated by transmission electronic microscopy TEM. Furthermore, tensile strength and permeability for O2 and CO2 were observed.

Key-Words: nanocomposites, clay, barrier properties, polyolefins

1 Introduction Polyolefins is a group of polymers with a very low polarity character. Due to this fact, the mixing them with organic fillers is very difficult. It is very problematic to achieve a good compatibility of a polymer matrix with filler particle surface and by this to obtain required improvement of properties. This holds true for preparation of polymer nanocomposites1-5. Generally, polyolefins are not willing to form intercalated or even exfoliated nanocomposites in the presence of a layered silicate clay as smoothly as polar polymers such as poly(amide)s5,6. This problem can be solved or by using “compatibilizers” (maleated PP or PE) or by polar side-chains grafted to the polyolefin backbone have been utilized to aid in the intercalation and exfoliation of layered clays7,8. Nowadays also EVA polymer is used - the number of works based on the study of EVA copolymer and even EVA nanocomposites has become wider and wider10-12. Nanofiller combined with PE, PP or EVA copolymer matrix not only improve the gas barrier properties, but also increase a wide range of other useful properties, such as tensile strength and modulus, flammability resistance etc13-15. Another big advantage is the

possibility of easier recyclation in comparison with the common types of fillers. Ethylene vinyl acetate copolymer (EVA) is a type of polymer which can also be ranged to the group of polyolefins. EVA copolymers equal to long chains of ethylene hydrocarbons with acetate groups randomly distributed throughout the chains. Vinyl acetate is made from acetic acid. Ethylene is copolymerized with the vinyl acetate to form ethylene vinyl acetate copolymer. The above - described types of polymers are processed by several types of technologies – for example extrusion can be used to produce blown film, coated film, tubing, profiles and extrusion blow moulded products. Like PE blown films EVA ones are used in packaging industry. From this point of view, barrier properties are very important in case of this application. In practice, it is necessary in case of wrappings to prepare materials with balanced properties, either gas permeability or mechanical properties. Nanocomposites, as it is well known, are prepared by mixing polymer matrix with nanofiller. Generally, they represent layered silicate minerals - clays, where the thickness of the individual leaves is in nanometer size. The most frequently used type of nanoclay is montmorillonite (MMT)16-19. It is a layered mineral belonging to a group of clay minerals with octahedral

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and tetrahedral nets in the ratio of 2:1. In order to increase its exfoliation in the polymeric matrix, it is modified by the process known as organofilization or intercalation, i.e. the insertion of a suitable organic compound into MMT interlayer20-24. This way, a broader d-spacing as well as reduction of gravitational forces between the individual platelets of montmorillonite are reached. Consequently, this leads to the reduction of energy needed for the breakup of nanofiller into the individual nanoleaves in the polymeric matrix after the process of compounding25. The function of MMT nanoleaves in the polymer matrix can be observed from two points of view. Generally, the first one is the original effect of filler - the improvement of mechanical properties by increasing the filler surface. The effect of nanofillers rests on the ability to create this effect by much lower loading in comparison with common fillers. Another quality of nanoleaves is their positive impact on the orientation of long thick platelets in polymer matrix after compounding, where they are arranged in such a way that they create a gas barrier to the gas transmission. So, the gas permeability is lower. The principle of this phenomenon is shown in the Figure 1. Tab. 1 Properties of nanoclays

FIG. 1. The principle of gas transmission on polymer nanocomposite

matrix

The next quality which significantly influences nanocomposite properties is the number of MMT platelets, in other words the MMT concentration. It can be supposed, that the higher the MMT concentration, the better the improvement of properties. Unfortunately, in case of polyolefins with their problematic miscibility with nanofiller it is not so explicit. As it was mentioned above, the preparation of nanocomposites with PE matrix or a matrix of a similar nature2,3, such as the PE/vinyl acetate copolymer, is more complex than e.g. in case of PA or PES nanocomposites6,7,26. It is given by a different nature of both components, where MMT is highly polar while the polymeric matrix of the type mentioned above is rather nonpolar. In case of PE it is necessary to add the so-called compatibilizer (e.g. maleic PE or PP). As far as EVA copolymer matrix is concerned, the problem is partially solved by adding a certain amount of a more polar vinyl acetate groups. Even higher polarity EVA of created by vinyl acetate groups does not enable complete exfoliation of MMT particles. From this point of view, the higher MMT concentration does not improve the properties improvement, because with higher MMT concentration bigger montmorillonite particles can occur in polymer matrix. These can cause cracks and imperfect interfaces, thus no improvement can be expected. This article was focused on the preparation of PE and EVA copolymer clay nanocomposites with two different montmorillonite concentrations in order to evaluate the influence of various type and loading on prepared nanocomposite properties, such as morphology, mechanical properties and gas barrier properties.

Nanofiller

Medium particle

size Intercalant

D-spacing

Nanofil® 5

8 µm Distearyldimethyl amoniumchlorid

2.8 nm

Nanofil® SE

3000

10 µm It is not

available -

Cloisite® Na+

2 µm < 10%

6 µm < 50%

13 µm < 90%

- 1.25

nm

Cloisite 93

A

2 µm < 10%

6 µm < 50%

13 µm < 90%

methyl, dihydrogenated

tallow ammonium

2.36

nm

Cloisite

30B

2 µm < 10%

6 µm < 50%

13 µm < 90%

methyl, tallow, bis-2-

hydroxyethyl, quaternary ammonium

1.85

nm

A polymer matrix

Nanofiller platelets

Gas way

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2 Experimental

Nanofillers

Cloisite®Na+ (nontrerated MMT), Nanofiller N5 and N3000 Nanofils from Sudchemie Germany and Cloisites 93A and 30B from Southern Clay Chemistry, were used for filling EVA matrix. The quantity of all the above-mentioned nanofillers added to the polymeric matrix was 3 and 5 wt. % in relation to the content of MMT. The characteristics of the individual nanofillers are summarized in Table I.

Polyethylene DOWLEX 2035 E was supplied by company Dow Chemical Company. Maleated polyethylene was produced by Dow Chemical Company too. Copolymer matrix used: EVA copolymer – GREENFLEX FF 55 (VA content 19 wt. %, melt index 0.7, density 0.92 g/m3), from Polimery Europa. Mixture blending

PE and EVA mixtures with nanofillers were blended in one-screw extrusive KO BUSS machine at the temperature of 150;180 and 220°C, rpm 280 min-1. The process took place in Fatra Napajedla Company. The homogenized matter came out of an extrusive dye into a cooling bath in the form of a string and was subsequently granulated. The prepared granular was used for pressing plates. The pressing took place at the temperature of 200 and 230°C. For the purpose of tensile strength test, bodies with a blade on either side were cut out from these plates. There were 12 bodies prepared for each mixture. The same bodies were used for the preparation of samples for TEM and XRD tests. Film pressing

The next task of our experiment was to extrude films of the thickness of 0.05 mm for the measurement of barrier properties. The films were extruded on a pre-arranged assembly line consisting of the following parts: an extrusive screw Brabender machine, a special extrusive wide-slot dye, a cooling cylinder and a reel. The thickness of the film was regulated by the speed of the pullout on the one hand and cooling of the cylinder by an electric pump on the other.

Evaluation of the samples prepared

X-ray diffraction analysis

X-ray diffraction analysis was performed on URD 6 diffractometer in FT UTB, Zlín. The measurements were made in the reflective mood in the range of 2Θ counting for 2-30° with a step of 0.02 for the evaluation of both powdered nanofillers and polymeric nanocomposites of our own production.

Transmission electronic microscopy

For the purpose of TEM, ultra-thin cross-sections prepared on a special cutting device LEICA ULTRACUT UCT ultracryomicrotome at the temperature of -110°C were used. TEM was performed on a JEM 200CX (JEOL) device at 100 kV. The scans taken were digitised by means of a computer-controlled digital camera DXM1200 (Nikon). Gas barrier properties

The observed nanocomposite molded films with the thickness of about 50 µm were used for the measurement of O2 and CO2 permeability, the equipment based on ČSN 64 0115, method of a constant volume.

3 Results and disscusion

XRD. In order to discover the result of montmorillonite exfoliation in used matrices, XRD patterns, summarized in Fig. 2-3, have been taken. According to the recorded results it is possible to observe that no complete montmorillonite exfoliation has taken place. All plotted patterns show at least one peak which belongs to the MMT. Also, it is surprising that peaks are similar for both nanofiller concentrations – the example of EVA samples (fig. 3). To sum up, the complete exfoliation of nanofiller has not taken place in any mixture prepared. Moreover, the result is not influenced by MMT concentration and this holds true for all nanofillers observed.

2 4 6 8 10 12

Intensity (a.u.)

unfilled PE

PE + Cloisite 25 A

PE + Cloisite 93A

PE + Cloisite 30B

PE+Nanofil 5

FIG. 2. XRD patterns of PE

PE+Nanofil3000

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As discussed below, despite the incomplete exfoliation shown by XRD measurement, a certain improvement of some qualities observed has taken place.

0 4 8 12 16

Intensity (a.u.)

unfilled EVA

EVA + Nanofil 5 - 3%

EVA + Nanofil5 -5%

EVA + Cloisite 93A - 3%

EVA + Cloisite 93A - 5%

FIG. 3. XRD patterns EVA copolymere

TEM. TEM was used in order to complete the information about morphology. Fig. 4 show TEM scans of PE and EVA samples. All showed pictures are for 5% loading.

PE + 30B

PE + 93A

EVA + 93A

The best result can be observed for PE with Cloisite 93 A what is confirmed also later by the observation of mechanical or barrier properties.

Mechanical properties. The recorded values are arithmetic means of ten measurements.

The only quantity recorded here is the tensile strength (see Fig. 5). From the graph it is possible to say that for PE matrix the most suitable nanofiller is type of Nanofil on the other hand for the EVA matrix Cloisite 93A seems to be the best one. It can be connected with a little bit different polymer matrix properties.

5

10

15

20

25

30

FIG. 5. Graph of tensile strength

FIG. 4. TEM pictures

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The value of tensile strength was higher in about 20%.

Permeability. As mentioned above, PE and EVA copolymer are polymers used in packing industry. Therefore, our study also covers the values of permeability for O2 and CO2, which are mediums that should be prevented from penetrating through film, e.g. in case of food wrapping.

In this instance, films extruded on a flat extrusive dye were used as samples for the purpose of measurement.

The evaluation of the O2 permeability (see Fig 6) shows for PE matrix the nanofiller Cloisite 30 B is totally unsuitable, similar result can be seen for EVA matrix. For all the other used nanofillers the result of permeability observation was better in the comparison of unfilled matrix. In Fig. 7 the measurement of CO2 permeability is presented. For PE matrix the result is the same like for O2 observation. Unfortunately, in case of EVA no nanofiller showed the significant lowering of CO2 permeability.

FIG. 6. Graph of O2 permeability

FIG. 7. Graph of CO2 permeability

4 Conclusion PE and EVA matrices with four types of commercial clay nanofillers were prepared and observed. Because of using of mentioned materials for food packaging, the morphology of prepared samples, mechanical properties and barrier properties were checked. Although the TEM and XRD measurement did not show the full exfoliation of nanofiller in the polymer matrix, the very good result in case of mechanical and barrier properties were found, especially for PE matrix, where except of Cloisite 30B all the other used nanofiller brought significant improvement of properties.

Acknowlegement:

This research was supported by the project FR TI4/623 and CZ.1.05/2.1.00/03.0111. References:

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