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Mechanical properties of recycled polyethylene ecocomposites filled with natural organic fillers.

INTRODUCTION

Polymer composites have been subject to increasing interest, study, and utilization for some decades. One of the main categories of polymer composites is represented by polymers filled with glass fibers, which fit to a number of applications.

During the last years, however, the increase in environmental concern has pointed out how it is also necessary to reduce and rationalize the use of polymeric materials, not only due to their nonbiodegradability, but also because their production requires large amounts of oil, which is notoriously not renewable. All these issues have induced to look for alternatives.

Thus, the interest arose toward polymer composites filled with natural organic fillers, especially in conjunction with recycled and/or recyclable polymer matrices. This class of composites (sometimes indicated as "green composites") shows other interesting features. The first one certainly concerns the costs issues, which are quite reduced since natural organic fillers are extracted from usually abundant plants (often from wastes). But there are also operative advantages: these fillers are usually less abrasive than glass fibers to processing equipments, give rise to less concern regarding health of operators in case of inhalation, they can easily be incinerated, they allow to get interesting properties of the final composites (first of all, lightness, related to lower density if compared to mineral fillers). Finally, they usually have good thermal and acoustic insulating properties (because of the tubular structure of the natural fibers), they show particular aesthetical features, etc [1-12].

Scientific literature already cites several types of "green" composites, for instance: polyethylene with wood fibers and flour [13-16], corn starch [17-18], sago, tapioca and rice starch [19, 20], sisal fibers [5], kenaf fibers [17]; polypropylene with wood, sisal, hemp, kenaf fibers, and starches [12] etc.

With regard to industrial applications, the most used systems are filled with wood flour or fibers, as low cost fillers especially for PP and PE [1]. Some examples of application are door and window framing, furnishings, interior car panels, packaging, scaffolds, light panels, gardening items and, in general, all those applications that do not require particularly high mechanical resistance [1, 3, 21-23]. This also makes possible to conveniently use, in many cases, recycled polymers in place of virgin polymers.

Many efforts are made in the improvement of mechanical properties through the increase of adhesion between fiber and polymer, and of the filler dispersion within the polymer matrix. These two issues have great importance, and are currently faced through two distinct ways: the addition of a third component, which, because of its intrinsic characteristics, can improve the interfacial adhesion through the formation of bonds between filler and matrix, or on the other hand a chemical pretreatment of fibers (for example, through acetylation or MAPP molecules [24, 25]). The latter, at present time, gives rise to some perplexities with regard to industrial application, because of the cost of the chemical treatment that the fibers should undergo before use. Thus, it seems more practical to add small amounts of a third component to the polymer-filler mix during the feeding step of the processing equipments.

Literature mentions both techniques, regarding, for instance, utilization of polypropylene (or polyethylene) grafted with maleic anhydride, or fiber treatment with silane-based or anhydride-based chemicals [13, 26-28], or just a washing through immersion in an alkaline solution. The first two techniques seem to give rise to significant improvement to adhesion and dispersion, while alkaline solution treatment just improved filler dispersion in the matrix; this was easily observed through SEM investigation [26].

The aim of this work is to investigate on the behavior of a recycled polyethylene (RPE) in combination with wood flour (a widely used organic filler), in particular with concern to the enhancement of adhesion between the polymer matrix and the organic filler, through the addition of different copolymers (ethylene-vinyl acetate (EVA), ethylene-co-acrylic acid) or a maleated polyethylene wax, focusing the attention on mechanical properties and morphology of fracture surfaces.

EXPERIMENTAL

Materials

The polymer matrix (recycled polyethylene, RPE) used in this work comes from postconsume recycling from greenhouses. These were collected in the province of Ragusa (Sicily, where an important production is located). The recycled polymer is made of low density polyethylene (LDPE, 65-75%), linear low density polyethylene (LLDPE, 10-15%), and ethylene-vinyl acetate copolymer (EVA, 10-12%). There are also small amounts of inert fillers and UV stabilizers (about 2500 ppm) [29].

Filled samples were prepared using one of the most used natural organic fillers, that is, wood flour.

Flour was obtained from sawmill wastes and selected through two different sieves, thus obtaining two different size classes: SDc, with an average particle size from 250 to 1000 [micro]m and SDf, with particle size below 250 [micro]m.

The behavior of a commercial wood flour was tested as well, thanks to the kind availability of LA.SO.LE. (Udine, Italy). In detail, we used the "35" (diameter from 300 to 500 [micro]m), indicated with the abbreviation "SDc-LS", and the "200/150" (diameter from 150 to 200 [micro]m), indicated with the abbreviation "SDf-LS".

The average particle size and the average length-to-diameter ratio (L/D) of these filler are reported in Table 1.

RPE, was filled with both the sizes of wood flour (SDc, SDf), at 30 and 60% by weight.

It was also investigated whether small quantities of a polar polymer like EVA copolymer (supplied by Polimeri Europa (Italy), commercialized as Greenflex FC 45 and with a vinyl-acetate content of 14%) could improve interfacial adhesion between matrix and filler. In particular, two RPE + SDc systems were prepared with a filler content of 30 and 60% and EVA content of 5%.

Also, an alternative third component was tested: a poly-(ethylene-co-acrylic acid) copolymer commercialized by Exxon Chemical (USA) as "Escor 5001" (with an acrylic acid content of 6.2%), once more on the same kind of compounds, but in a lower weight percent (3%). Finally, this last way of sample preparation was used to check the behavior of Licocene PE MA 4351TP[R] produced by Clariant (Germany). This is a polyethylene wax grafted with maleic anhydride (viscosity at 140[degrees]C: 220 m Pa s, anhydride content about 7%), which does not alter the color of the final product. It should impart a significant adhesion improvement, and the low viscosity significantly simplifies processing. Furthermore, differently from other products, it should not undergo cross linking or decomposition during processing. It was tested in addition to compounds filled with 30 and 60% of both the types of wood flour. A schematic summary of the materials is shown in Table 2.

Finally, in order to try to get further understanding of the phenomena involving the RPE, this material was also compared with a virgin low-density polyethylene, LDPE (commercialized as Riblene by Polimeri Europa (Italy)), making some samples filled and not filled with coarse wood flour (at 30 and 60% by weight.).

Compounding and Testing

Before compounding, wood flour was dried in a 70[degrees]C heated oven for one night. Compounds were prepared by an industrial corotating twin-screw extruder (OMC, Italy, screw diameter D = 19 mm and length-to-diameter ratio LID = 35).

Processing conditions were thermal profile 120/130/140/150/160/170/180[degrees]C and screw speed 200 rpm. Then, the obtained materials were pelletized.

Specimens for tensile, impact, and heat deflection temperature (HDT) tests were made by compression molding by means of a Carver (U.S.A.) laboratory press, set at 180[degrees]C, molding time about 2-3 min.

Tensile tests were performed by a universal Instron (USA) mod. 4443 equipment, on specimens (thickness [approximately equal to] 0.5 mm, width = 10 mm) cut out from the above-mentioned compression-molded sheets, according to ASTM D882. Impact tests were performed on notched samples in Izod mode, by a CEAST (Italy) equipment (set at 2 J), following ASTM D256, and heat deflection temperature (HDT), according to ASTM D 2990-77 (flexural load 1.8 MPa, temperature increase rate 120[degrees]C/h) by an automatic CEAST (Italy) apparatus.

Finally, an additional analysis was performed using a laboratory twin-screw, counter-rotating compounder (Brabender DSK 42/77, Germany) with a thermal profile of 120/160/180[degrees]C and a rotating speed of 60 rpm, to directly compare the RPE, and the effects of EVA addition, to a virgin LDPE.

[FIGURE 1 OMITTED]

SEM analysis was performed by scanning the fracture surfaces of some samples by a Philips (Netherlands) ESEM XL30 equipment.

[FIGURE 2 OMITTED]

RESULTS AND DISCUSSION

Mechanical properties of RPE-wood flour systems (previously dried, according to the way described before) are reported in Table 3.

The properties of the samples filled with SDc and SDf are very close; the decrease of tensile strength is related to the increase of fragility of the material (as highlighted by the decrease of elongation at break); the fact that rigidity (quantified by elastic modulus and HDT) of the samples filled with coarse wood flour is slightly higher, could be explained considering that the average length-to-diameter ratio of its fibers is higher than the one of fine wood flour fibers (about 4.3 vs. about 3.2, see Fig. 1), and it is indeed well known that rigidity usually increases with increasing the L/D ratio of the inert filler [12, 30].

As for the commercial wood flour-filled samples, the higher L/D ratio could explain the significantly higher impact strength, and HDT, of the coarse flour when compared with the fine one. On the other hand, it is rather surprising that elastic modulus and also tensile strength are better in the fine wood flour-filled samples, even though the aspect ratio is higher in coarse wood flour filled ones. Therefore, granulometric distribution curves were plotted, by separating the wood flour samples through different sized mesh grids, in order to try to get a better comprehension of these rather unexpected phenomena (Fig. 2).

Maybe the different behavior highlighted earlier, between commercial wood flours and home-made ones, could be explained observing that on an average, commercial ones have higher percentages of bigger-sized particles and also that the curves of home-made flours are closer (each other) than those of commercial flours, especially in the field of smaller sizes.

All the adhesion promoters have been previously tested on the composites with the home made wood flour; the results of the tests are reported in Table 4.

It can be observed that, on the whole, the addition of EVA imparted some improvements, even though less than expected. Escor gave improvements similar to those obtained with EVA (slightly higher with regard to rigidity and tensile strength, very similar or slightly lower with regard to impact strength), Licocene granted a significant improvement in all the tensile properties (about 10-15%) but, on the other hand, there was not any significant improvement of impact strength. The overall improvement is probably due to a better dispersion of the organic filler within the polymer matrix and to the formation of chemical bonds between fibers and matrix.

With concern to the comparison between the RPE and the virgin LDPE, the tensile and impact tests gave the results as shown in Table 5.

As previously hinted, this kind of comparison was carried out in order to try to get, at least qualitatively, a deeper comprehension of the reasons why EVA did not impart great improvements to the mechanical properties. It showed that the change of polymer matrix did not impart any significant results variation; therefore, it seems possible that the natural decline of properties after recycling was, more or less, counterbalanced by the significant amounts (up to 10%) of EVA that are already contained in the RPE as delivered. This may also be an explanation that the addition of polar polymers, previously described, did not give great improvements constituting a "surplus" that, on the other hand, did not succeed much in creating more polar groups on the macromolecules. The only exception is represented, at least partially, by Licocene (but it should be considered that this additive shows some important differences from the others like, for instance, a low viscosity and a regular distribution of polar groups). After all, these conclusions seem to be confirmed by the fact that, adding some EVA to the virgin LDPE, in a quantity of 5% by weight, the material stiffens and improves its tensile properties, exactly what one could expect, while its addition to the RPE does not improve the global properties to the same extent, as seen before as well. The slight decrease of other properties (ductility) in the LDPE case seems in contrast with predictions (and the improvements of elastic modulus and tensile strength), but may be explained considering the acid nature of EVA, which may have caused some phenomena of embrittlement of the filler fibers [26].

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

The effectiveness of Licocene was then verified also using the commercial wood flour. The results of mechanical tests performed on these materials are reported in Figs. 3,4,5 and Table 6.

It can be observed that the overall behavior is similar to that of the same systems filled with noncommercial wood flour, but on the whole, there is a slight improvement of the global mechanical properties and of the reproducibility of results; this is probably due to the greater quality of the commercial filler, and may also be due to the fact that the granulometric curves show higher percentages of bigger fibers (as previously explained). Furthermore, it was observed that the influence of Licocene, in terms of improvement of a specific mechanical property, increases upon increasing the filler content, which is indeed the substrate on which the adhesion promoter acts. This allows good expectations in the vision of the use of high weight percents of natural organic filler.

An interesting tool for further investigation was provided by SEM analysis. SEM micrographs were obtained scanning the fracture surfaces of some RPE-wood flour specimens, coming out from both tensile and impact tests, and are shown in Figs. 6a-6n.

[FIGURE 6 OMITTED]

[FIGURE 6 OMITTED]

Figures 6a and 6b, show the fracture surfaces of RPE-30% coarse wood flour samples, coming from tensile tests, without(a) and with(b) addition of 3% Licocene. It is easy to observe that the wood fibers are better wet by the polymer in the second case (and the surface appearance is smoother), while in the first case some isolated fibers can be observed. In Figs. 6c and 6d, RPE-60% filled samples are shown, without and with Licocene: in the first case, it is possible to see some isolated and poorly wet wood fibers coming out of the smooth polymer matrix, while in the second case, the fibers appear better wet, and indeed the surface seems to be more isotropic, even though it could seem the contrary at a first glance. This could indicate a better, more uniform dispersion of the fibers within the matrix.

With regard to impact test specimens, 60% filled samples are shown in Figs. 6e and 6f: there is a slight improvement of fiber wetting and uniformity of fracture surface with the addition of Licocene, and this could explain the slight increase in impact strength; however, in other cases it was not followed by any significant variation, as seen in the above table. Maybe in these cases there was only an improvement of dispersion, but not of adhesion.

The further four figures (Figs. 6g to 6j) show samples filled with 30% fine wood flour, with similar conclusions as above; in particular, the first two show well how the addition of Licocene (Fig. 6h) improves the uniformity of fracture surface by significantly reducing the amount of voids.

Finally, Figs. 6k-6n show samples filled with the homemade wood flour "SDc" previously illustrated; the first two come from 30% filled samples without and with Licocene, respectively. It can be seen in Fig. 61 that there is no presence of isolated fibers and the surface is quite smoother; the last two come from 60% filled samples, once more without and with Licocene: also in this case, Licocene seems to have imparted advantages to the surface in terms of smoothness and uniformity, which can be related to improved dispersion. Other micrographs have been taken (both from commercial and home-made wood flour filled specimens) and they confirm the trends and conclusions reported here.

Therefore, it can be concluded that the significant improvements of tensile properties are probably related to a better dispersion of the organic fibers within the polymer matrix, and to the formation of chemical bonds between polar filler particles and nonpolar matrix (and thus, an improved adhesion); the somewhat unexpected results concerning some of the impact strength samples may be related to an increase in dispersion but not in adhesion, or on the other hand, adhesion improvement may have occurred but not enough to guarantee a higher resistance to the propagation of impact fracture. However, it should be pointed out that also other researchers found just little (or even negligible) variations of impact strength with the addition of maleated adhesion promoters [26, 31]; in particular, this seems especially related to notched impact strength, rather than unnotched. A possible explanation may be found considering that it has been observed that maleated compatibilizers mainly influence the crack initiation stage rather than the crack propagation phase [32], but at the same time it is known that notched impact energy is a measure of crack propagation [33], and therefore it may be not influenced significantly by the addition of these compatibilizers. To mitigate crack propagation (rather than initiation), it seems better to rely on impact modifiers, such as elastomer-based impact modifiers [31. 32].

CONCLUSIONS

The addition of organic fillers to post-consumer recycled polymer matrices causes an increase of elastic modulus (and thus of rigidity) and of thermo mechanical resistance, while a reduction of ductility is observed.

These effects were more remarkable upon increasing the filler content. Wood flour, a very cheap filler, showed to have interesting features. In particular, the commercial product showed good characteristics of uniformity and quality, and the coarse one seems more suitable if the impact strength is preferred to tensile strength.

Attention was also focused on investigation of possible improvements of mechanical properties, through the addition of polar polymers like the EVA copolymer, or the ethylene-co-acrylic acid copolymer. These imparted little improvements to the overall properties of the final composites, but slightly lower than expected; therefore, it was decided to experiment other more specific adhesion promoter, like Licocene PE MA TP 4351. The tests, performed on systems composed by RPE and wood flour, processed through an industrial twin screw extruder, have highlighted a significant improvement of tensile properties, and in particular, this additive showed to behave even better when high filler contents are used. This is probably due to better filler dispersion and wetting within the polymer matrix, and seems to be confirmed also by SEM micrographs; however, notched impact properties in some cases did not change significantly. This may be related to a poor influence of the adhesion promoters on crack propagation mechanism.

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F.P. La Mantia, M. Morreale

Universita di Palermo, Dipartimento di Ingegneria Chimica dei Processi e dei Materiali, Viale delle Scienze, 90128 Palermo, Italy

Correspondence to: F.P. La Mantia; e-mail: lamantia@dicpm.unipa.it
TABLE 1. Average particle size and length-to-diameter ratio for all the
filler samples.

Sample Average particle size ([micro]m) Average L/D

SDc 250-1000 4.3
SDf <250 3.2
SDc-LS 300-500 5.7
SDf-LS 150-200 2.5

TABLE 2. Summary of the investigated materials and related identity
codes.

Material Code

Recycled polyethylene RPE
RPE + 30% coarse wood flour RPE30SDc
RPE + 30% fine wood flour RPE30SDf
RPE + 60% coarse wood flour RPE60SDc
RPE + 60% fine wood flour RPE60SDf
RPE + 30% coarse wood flour + 5% EVA RPE30SDc5EVA
RPE + 60% coarse wood flour + 5% EVA RPE60SDc5EVA
RPE + 30% coarse wood flour + 3% Escor RPE30SDc3Escor
RPE + 60% coarse wood flour + 3% Escor RPE60SDc3Escor
RPE + 30% coarse wood flour + 3% Licocene RPE30SDc3Licocene
RPE + 60% coarse wood flour + 3% Licocene RPE60SDc3Licocene
RPE + 30% commercial coarse wood flour + 3% RPE30SDc-LS 3Licocene
 Licocene
RPE + 60% commercial coarse wood flour + 3% RPE60SDc-LS 3Licocene
 Licocene
RPE + 30% commercial fine wood flour + 3% RPE30SDf-LS 3Licocene
 Licocene
RPE + 60% commercial fine wood flour + 3% RPE60SDf-LS 3Licocene
 Licocene

TABLE 3. Mechanical properties.

Fiber (wt%) SDc-LS SDf-LS SDc SDf

Elastic modulus (MPa)
 0 149 149 149 149
30 320 491 351 338
60 551 788 589 520

Tensile strength (MPa)
% SDc-LS SDf-LS SDc SDf

 0 12 12 12 12
30 7.5 8.9 7.1 7
60 6.4 7.2 6 6.3

Elongation at break (%)
% SDc-LS SDf-LS SDc SDf

 0 350 350 350 350
30 7.6 11.3 6.5 6.2
60 1.6 3.2 1.6 2.4

Impact strength (J/m)
% SDc-LS SDf-LS SDc SDf

 0 NB NB NB NB
30 100 64 81 80
60 50 41 57 58

HDT ([degrees]C)
% SDc-LS SDf-LS SDc SDf

 0 30 30 30 30
30 50 43 42 41
60 57 51 51 48

NB = no break.

TABLE 4. Mechanical properties after the addition of various additives.

 Filler
Property (wt%) SDc SDc5EVA SDc3Escor SDc3 Licocene

Elastic modulus, 0 149 149 149 149
 E (MPa) 30 351 359 398 400
 60 589 597 673 788
Tensile strength, 0 12 12 12 12
 TS (MPa) 30 7.1 7.3 7.9 10.3
 60 6 6.9 7.4 11.6
Elongation at break, 0 350 350 350 350
 EB (MPa) 30 6.5 6.1 7 8.5
 60 1.6 1.8 1.9 2.8
Impact strength, 0 NB NB NB NB
 IS (J/m) 30 81 85 75 78
 60 57 58 55 58
HDT ([degrees]C) 0 30 30 30 30
 30 42 41 44 47
 60 51 46 52 53

NB = no break.

TABLE 5. RPE-LDPE comparison.

 E (MPa) [sigma] (MPa) [epsilon] (%) IS (J/m)

RPE + 30SDc 300 6.5 8 85
RPE + 60SDc 515 5.5 2 47
LDPE + 30SDc 316 7 11 82
LDPE + 60SDc 530 4.9 2.2 46
LDPE + 30SDc + 5EVA 478 9.3 10 74
LDPE + 60SDc + 5EVA 880 6.6 1.6 43
RPE + 30SDc + 5EVA 304 7 8.5 75
RPE + 60SDc + 5EVA 590 6.5 2.1 41

TABLE 6. Impact strength vs. filler content for samples with and without
Licocene.

Filler (wt %) SDc-LS SDc-LS 3 Licoc. SDf-LS SDf-LS 3 Licoc.

 0 NB NB NB NB
30 100 97 64 65
60 50 60 42 53

NB = no break.
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Author:La Mantia, F.P.; Morreale, M.
Publication:Polymer Engineering and Science
Date:Sep 1, 2006
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