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Phase structure and mechanical properties of PP/EPR/CaC[O.sub.3] nanocomposites: effect of particle's size and treatment.

INTRODUCTION

In the last few years, the production and use of polypropylene have shown the highest growth rate after that of polyethylene. Polypropylene-based materials are increasingly attractive because of their low cost, processability, and good balance of properties.

PP composites with high modulus and impact strength are highly desirable for automobile applications. For example, car bumpers are generally made of impact modified poly (propylene) (PP), ethylene propylene rubber (EPR) or ethylene propylene diene monomer (EPDM).

Calcium carbonate which is abundantly found in nature is low cost filler which has large size distribution, higher stiffness, rigidity and viscosity and improves the productivity of PP. Although it is obvious that a stiff particle guarantees a high rigidity of thermoplastic composites, there are contradictory results about the ability of Nano-sized calcium carbonate to toughen thermoplastics such as Polyethylene [1], PP homopolymer [2], isotactic polypropylene [3], recycled PP [4]. Few articles studied the synergetic effect of calcium carbonate to toughen PP copolymer [5, 6]. A complex mixture containing calcium carbonate was studied to solve recycling problem, and to facilitate the recovery of PP composites with simplified sorting and lower levels of purity [7]. Nano-CaC[O.sub.3] fillers were mixed to (PP/EPR) based formulations in the perspective of processing more complex blends when recycling [8]. Calcium carbonate was also considered as impurity in postconsumer PP [9].

The ultimate properties of PP/CaC[O.sub.3] composites depend certainly on many factors related to the particles [10]: specific energy, state distribution, and stiffness, to the matrix type [11]: homopolymer, copolymer, and to the interface and/or the interphase properties [12]. Processing conditions [13] are also essential to ensure a better dispersion and homogenous distributions. The particles sizes and morphology have a great influence on the toughness of Nano composites. Aggregates have a bad influence on mechanical properties [14]. Thio et al. [15] and Zuiderduin et al. [16] showed that with calcium carbonate fillers of diameter 70 nm, Izod impact strength is not improved, whereas Chan et al. [17] succeeded to toughen PP with particles of only 44 nm in size. Large shear forces and small surface energy favor homogenous distribution while small particles have strong tendency to agglomerate. Stearic acid was frequently used [18] to achieve good dispersion [19] by reducing the specific energy and the magnitude of adhesives forces. However, coating does not eliminate aggregations completely [20] and could modify the interfacial interaction with the matrix [21]. Few studies have dealt with the efficiency of nanoparticles in toughening copolymer matrix and the complexity of the resulting morphologies.

The aim of this paper is to study complex blends (PP/EPR) based formulations containing nano-CaC[O.sub.3] fillers in a perspective of recycling. The research focused on the effectiveness of rigid particles on PP/EPR/CaC[O.sub.3] composites and investigated the effect of surface treatments and particle size by dynamic analysis and SEM observations. Mechanical performance and morphology relationships were also studied.

EXPERIMENTAL

Materials

The kind of PP used in this study was a polypropylene copolymer mainly used for bumper injection, supplied by BP Petrochemicals (PPC 7712). Its main characteristics are listed in Table 1. The Nano fillers were two commercial grades of calcium carbonate (Socal[TM] 322 and Socal[TM] 31) from Solvay, having an average particle size of 50 nm and a specific surface of 26 [m.sup.2]/g; Socal[TM] 322 was treated with stearic acid to ensure a better dispersion of fillers. Table 2 contains their most important characteristics. Microsized calcium carbonate MILLICARBOG[TM] produced by Orgon has a 2.7 [micro]m average diameter and a 2.5 [m.sup.2]/g specific surface. The CaC[O.sub.3] particles have an aspect ratio close to unity and do not have sharp edges (Fig. 1).

Fillers were first dried in an oven for 9 hours at 60[degrees]C; PP was melt compounded with respectively 3%, 10%, and 20% weight percent of fillers in a co-rotating Brabendci[R] twin-screw extruder with a temperature profile (200[degrees]C, 210[degrees]C, 220[degrees]C) and a screw rotation speed of 60 rpm to obtain a better dispersion of fillers. Compounds were then granulated and injection molded with an Engel ES 8035 machine into standard dumb-bell tensile bars.

Rheological Testing

Rheological measurements were carried out with a dynamic stress controlled rheometer Gemini 2000 from Bohlin Instruments (Cranbury, NJ, USA). It was equipped with a plate/plate geometry (d = 25 mm), a gap of 1.8 mm at a 220[degrees]C temperature under nitrogen atmosphere.

Oscillatory shear measurements were performed over a frequency range of 0.01 to 100 Hz. The strain amplitude was fixed to 1.8%. This strain value was decided by an amplitude sweep test to guarantee a linear viscoelastic response.

SEM Observations

Scanning Electron Microscopy observations were conducted with a Jeol JSM 6460LV electron scanning microscope at an acceleration voltage microscope of 15 kV. Prior to examination, samples were dipped in liquid Nitrogen and the fractured surface was gold-coated using Sputter Coater. To improve the contrast between PP matrix and EPR phases, the fractured surface were etched by heptane at 50[degrees]C for 2 hours.

DSC Analysis

Nonisothermal melting and crystallization behavior of the neat PP and Nanocomposites was measured using a Mettler-Toledo DSC 822 (Mettler-Toledo, Viroflay, France) thermal analyzer under nitrogen atmosphere. Microtomed sections of tensile specimens were used as samples. The samples typically weighed 10 mg. Test conditions were as follows: scan rate 10[degrees]C per minute. The temperature is between 20[degrees]C and 200[degrees]C. The crystallinity percent was calculated from the following formula:

% crystallinity = D[H.sub.melting]/D[H.sub.[infinity]] X 100 (1)

[DELTA][H.sub.melting] is the melting enthalpy of the sample as obtained from the DSC thermogram; for composite samples, this value is corrected by the weight fraction of PP in the sample.

[DELTA][H.sub.[infinity]] the value of the enthalpy of fusion for a 100% crystalline polypropylene. The value was taken as 209 J/g.

The melting enthalpy is taken after the second scan on heating. At least two experiments were conducted for each sample.

Tensile Test

Tensile properties were determined using a MTS Synergie RT1000 (MTS, Eden Prairie, MN, USA) testing apparatus equipped with a HTE extensometer with a nominal length of 49.7 mm. Two crosshead speeds were used: 50 mm/min for the elongation at break and 2 mm/min to calculate the Young modulus. Tensile specimens were injection molded according to ISO 529. At least five samples were conducted for each tensile test.

Notched Izod Impact Test

Notched Izod impact tests were performed using a Tinuis Olsen machine at ambient temperature. A single-edge V-shaped notch (width = 4 mm, depth = 2 mm, 45[degrees], tip radius = 0.25 mm) was milled in the sample for the Izod impact test.

RESULTS AND DISCUSSION

Rheological Properties and Morphology

SEM Observations. SEM micrographs obtained with three kinds of calcium carbonate--nanometric untreated (ncc31), nanometric modified by stearic acid (ncc322), and micronic ([mu]cc)--are presented in Fig. 2. For samples with nanofillers, we can observe the presence of many agglomerates (soft flocks) or aggregates (hard agglomerates), the number of which increases with increasing weight fraction of fillers. Nanoparticles treated with stearic acid seem to form smaller clusters than untreated ones. It appears that surface treatment reduces the agglomeration but does not hinder it totally. Stearic acid coating substantially reduces the surface energy of calcium carbonate particles from 210 to 40-60 mJ/[m.sup.2] [22]. Theoretically, stearic acid molecules lie perpendicular to the filler surface to form a closely packed layer with a thickness of about 2.5 nm [23], Samples with microparticles do not show such agglomerates, probably as they have lower specific surface (2.5 [m.sup.2]/g). Shear forces could therefore easily disperse them in the matrix. Aggregations were shown to start around 6 [m.sup.2]/g specific surface areas in CaC[O.sub.3]/ PP composites when fillers are not coated [24].

Actually, the nature of the matrix, a copolymer, provides composites with very complex morphologies since the PP/EPR/ CaC[O.sub.3] is a ternary composite. A two-phase morphology is clearly visible in PP copolymer where the EPR phase was extracted by heptane. Dark holes correspond to the EPR phase (Fig. 3). The average size of EPR nodules in PP copolymer is about 0.62 [micro]m. Nanoparticles (treated ones) are embedded on the elastomeric phase forming core shell structures (Fig. 4). At higher filler (10%) content, a unique phase structure was found as large amounts of CaC[O.sub.3] particles agglomerate around EPR particles (dark holes) and pervade over the matrix.

A separated structure is formed when untreated fillers are used, which may be because of poor adhesion between CaC[O.sub.3] particles and PP matrix. Comparisons of the diameter of EPR nodules show that their sizes increase with untreated nanofillers. This may be because of the coalescence of EPR phase as seen in Fig. 5.

Rheological Properties. To further understand the state distribution and percolation of particles, the flow behavior of filled PP was investigated. The low stresses involved in dynamic rheometric test induced small strains on the sample and therefore could give enough sensitive results to the study of the filler percolation and aggregation [25, 26].

Viscosity of neat PP versus frequency exhibits a plateau at lower frequencies. This behavior is similar to that of a Newtonian fluid. Viscosity increases further with increasing filler content (Fig. 6). Non-Newtonian behavior is a direct consequence of interfacial interaction between fillers and matrix, fillers and fillers, and cluster networks [19], This behavior is evidenced by the increase of number of clusters and the decrease of the interparticle distances, as revealed by the SEM micrographs.

The highest viscosity is observed with uncoated Nanofillers; coated Nanofillers give slightly lower values; the viscosity of Nanocomposites with microsized fillers is much lower, as it can be observed in Fig. 6. The higher surface energy of nanoparticles induces more interactions between particles and so more agglomerates and aggregates and hence an increase in the Nanocomposite viscosity.

The decrease in viscosity noticed in case of coated particle may be because of a reduction of the immobilized polymer fraction because of the lower interfacial tension between the solid particles and surrounding liquid phase [20]. Another effect of the stearic acid is to decrease the particle surface energy reducing the particles' tendency to agglomerate as well as the attraction forces between them and the polymer, leading to lower viscosity [27].

The shear thinning behavior at low frequencies can be expressed as a power law relation:

[[eta].sup.*] = k[omega]n

[[eta].sup.*] is the dynamic viscosity, k is a sample specific pre-exponential factor, [omega] is the oscillation frequency in the frequency sweep test, and "n" is the shear thinning exponent.

n is a semiquantitative measure of the filler dispersion in polymer phase [25].

Power law exponents of the composites with nanotreated, untreated fillers and microfillers are listed in Table 3. We can compare the values for the different Nanocomposites prepared at the same composition and processing conditions in order to understand how filler dispersion differs depending on the filler size and treatment. It is seen that "n" value of the untreated CaC[O.sub.3] is higher than that of the treated ones. It could be expected that the filler treatment yields better dispersion into polymer phase.

This can be explained by the interaction and agglomeration because of attractive Vander Waals forces. Owning to their lower surface area, micro fillers show better dispersion even at higher contents.

Since small-amplitude oscillatory shear does not significantly deform the microstructure of the complex fluid, it allows studying the morphology of composites. A significant enhancement of dynamic moduli over the whole frequency was observed in Fig. 7.

At lower frequency, the slope of storage modulus vs. frequency decreases as the treated nano CaC[O.sub.3] content increases. Thus for 20% fillers, the modulus no longer depends on frequency. This is often observed with high surface area and high loading [28]. The volume fraction at which this observed plateau appears to depend on the stiffness, the particle size, and the interfacial interaction. The low-frequency plateau corresponding to a solid-like behavior has been often attributed to the presence of a particle network [29, 30].

Thermal Properties

The values of fusion, crystallization heats, and temperature of PP and related composites are shown in Table 4. Results show that PP and PP/CaC[O.sub.3] composites have similar melting and crystallization temperatures that are confirmed with others studies [16]. However, a slight increase in melting temperature is observed for 20 wt% fillers. Indeed, melting temperatures are affected by the flexibility of matrix chains. It is clear that higher content of fillers causes less flexibility of chains and leads to increased melting temperatures. Enthalpy curves do not show any trace of a beta phase as it was observed in other polypropylene [31].

The crystallinity percent increases quite linearly with increasing filler content. The homogeneity of results in the case of coated CaC[O.sub.3] composites may be related to the effect of stearic acid in proving the distribution state of particles. Microcomposites show a higher increase in crystallinity than Nanocomposites. It may be that Nanoparticles of CaC[O.sub.3] with the same weight content as microparticles are so huge that only a small fraction served as nucleating sites; most particles restrict molecular movements and hinder orderly packing of molecular segment [32].

The PP copolymer matrix reduces the nucleating efficiency of CaC[O.sub.3] contrary to other work [33] as a result of interaction and encapsulation by the EPR phase.

From these results we can say that CaC[O.sub.3] has a very weak nucleating effect on the crystallization of PP, and that the increase in crystallinity percents depends on physical and topological factors.

Tensile Properties

Experimental values of Young's modulus for three composites against calcium carbonate content are shown in Fig. 8. The tensile properties of the composites are summarized in Table 5. A significant and almost linear improvement in the modulus of composites can be observed, especially with untreated calcium carbonate fillers ncc31. This increase in modulus is quite reduced in the case of micro composites and ncc322. This could be explained by to the fact that the modulus is measured before any significant plastic deformation takes place and so, it does not take into consideration the interactions between the fillers and the polymer matrix. Therefore, an increase in the modulus of PP can only be caused by (1) the substitution of PP by the largely more rigid filler and (2) the restriction in the mobility and deformability of the matrix by the fillers. Differences in Young's modulus for all particles are more visible at higher contents. They could be attributed to the morphology formed. Surface treatment and particle size have indirect effects on the elastic properties by influencing the state distribution on the matrix. Moreover, in the case of treated particles, the formation of core-shell morphology reduces the rigidity of particles and increases the effective volume of soft particles.

SEM micrographs of fracture surfaces (see Fig. 9) across the tensile specimen on the necking zone show many voids surrounding calcium carbonate particles due the debonding of fillers that is considered beneficial for increasing toughness of many semi-crystalline polymers [15].

Visual observation of deforming specimens of modified particle shows that yield and plastic flow are accompanied by strong whitening of the blend in the necking zone. This suggests extensive debonding of the particles from the matrix. Deformed specimen prior to the yielding point shows the detachment of particles from matrix (Fig. 9A). The debonded particles facilitate both plastic deformation and shear yielding behavior.

A comparison of ultimate elongation of composites demonstrates that fillers induce a decrease in elongation at break, which becomes very substantial for some samples with large CaC[O.sub.3] weight fractions. Moreover stearic acid has a pronounced effect in increasing elongation at break. The decrease in elongation is related to the agglomeration and aggregation of nanoparticles, especially in the case of untreated fillers. These imperfections initiate fracture encountered by the propagating neck. However, for microcomposites, elongations at break vary widely and reach higher values than for Nanocomposites.

Untreated particles exhibit a good adhesion to the matrix, even at higher plastic deformation. In fact, stretched fibrils could be a sign of plastic deformation. Fibrils surrounded by untreated particles indicated good adhesion of calcium carbonate with the polymer matrix.

At higher content, untreated particles have a strong tendency to build agglomerates which minimize interfacial interaction between particles and PP. However some particles have a bridging effect between matrix and other agglomerates (Fig. 9B). In fact, the debonding favored by coating agent, creates more voids on the matrix, and reduces the sensitivity towards crazing. The shear yielding becomes operative and composite become able to absorb large quantities of energy up to fracture.

PP/CaC[O.sub.3] Nanocomposites at low filler content (wt. 3% ncc31) are characterized by ductile fracture. The SEM micrographs of fractured surface are presented in Fig. 10. We can observe that fibrillation is concentrated in the centre of the fractured surface. Additionally the microstructure of the sample appears porous and consists of severely deformed fibrils with large and shallow voids. Severely deformed fibrils are expected to more scatter light and to exhibit higher stress whitening.

Fracture surface of 10% ncc31 sample exhibits a lot of heterogeneities on various areas at the necking zone (Fig. 10); fibrils represent almost the whole fracture surface. This can explain the dispersion of elongation at break values.

Nanocomposites with higher ncc31 content (wt20%) show brittle fracture, without fibrillation.

Impact Resistance

Izod impact results are shown in Fig. 11. Different factors justify the decrease in the absorbed energy: the nature of the PP matrix (ethylene-propylene copolymer), the size, distribution and filler dispersion, the crystal content, the interfacial adhesion between the filler and the polymer.

A decrease in impact strength with filler content is observed for all Nanocomposites. But we can observe that, up to 10 wt% fillers, the impact strength of PP/ncc322 is three times higher than that for (PP/ncc31) Nanocomposites. This can be explained by the difference in morphologies formed.

Core shell structures observed with ncc322 are expected to have an effect on the composite toughness. First, we can suppose that treated particles interact directly with the soft or rubbery component of the PP matrix and then affect its ability to absorb energy during impact [23]. Second, core shell structures increase the effective volume fraction of rubber nodules and can make a bridge between rigid particles and then facilitate the shear yielding and so enhance the impact strength. In the case of untreated fillers, the higher number of agglomerates and larger nodule size contribute to lower the impact strength, particularly for high filler contents (10-20 wt%). These agglomerates tend to weaken the composites [34] as observed in Fig. 12.

Moreover the fact that CaC[O.sub.3] nanoparticles do not toughen the PP matrix may be explained by its poor ability to nucleate the [beta]-phase crystals around the CaC[O.sub.3] nanoparticles [31].

Composites with microparticles show better impact resistance than that with untreated particles but some what lower than that of treated ones. Voids created by nanoparticles debonding are originally smaller and more stable than those created by microparticles, avoiding embrittlement [34]. Decrease in impact strength may also be because of the broad size distribution of particles, between 1 and 10 [micro]m (cf Table 2) [35].

CONCLUSION

Effect of nanoparticle treatment and particle size of calcium carbonate on the mechanical, rheological, and morphological properties of PP/EPR/CaC[O.sub.3] were investigated.

Particle size and treatment affect indirectly the flow behavior by influencing the morphology and dispersion of particles. Rheological results show that power law exponent is a semi-quantitative measurement of the filler dispersion.

Filler treatment does not hinder agglomeration but show a tendency to build more complex structures with EPR nodules which reduce the rigidity and improve impact strength, compared to untreated particles.

CaC[O.sub.3] particles show a very weak nucleating effect on the crystallization of PP that its related to the more complex morphology formed.

A detailed investigation of fracture mechanisms confirms a brittle behavior for all particles. Low impact toughness seems related to the synergy between EPR and rigid particles.

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Ahmed Elloumi, (1,2) Sylvie Pimbert, (3) Chedly Bradai (1,2)

(1) LAboratoire Des Systemes Electro Mecaniques(LASEM), Ecole Nationale D'ingenieurs De Sfax, Tunisie

(2) Universite De Sfax, Tunisie

(3) Laboratoire D'ingenierie Des MATeriaux De Bretagne (LIMATB), Universite De Bretagne Sud, 56 321 Lonent, France

Correspondence to: S. Pimbert: e-mail: sylvie.pimbert@univ-ubs.fr

DOI10.1002/pen.24177

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. PP impact modified properties.

Properties                                    Values

Density (g/[cm.sup.3])(ISO 1183)              0.905
Tensile strength at yield (MPa) (ISO 527-2)     23
Melt flow index (gr/10 min)                     13
  (ISO 1133,230[degrees]C/2.16 kg)
Tensile modulus (MPa) (ISO 527-2)              1200
Charpy impact strength (notched)               >40
  (kJ/[m.sup.2]) (ISO 179) at 23[degrees]C
Melting point ([degrees]C)                     165

TABLE 2. Calcium carbonate properties.

Trade name          MILLICARB-OG    Socal[TM] 31    Socal[TM] 322

Mean particles      2.7 [micro]m    0.07 [micro]m   0.05 [micro]m
  diameter (dp)        (1-10)        (0.05-0.1)      (0.04-0.07)
Specific surface         2.5             20              26
                    ([m.sup.2]/g)   ([m.sup.2]/g)   ([m.sup.2]/g)
Density                  2.7            0.28            0.21
Structure           Rhombohedral    Rhombohedral    Rhombohedral
                       calcite         calcite         calcite
Special                  Low         High purity     Hydrophobic
  characteristics     roughness                       treatment

TABLE 3. n value at low frequencies
(between [10.sup.-2] et 10-1 Hz).

Filler wt%   PP ncc31   PP ncc322   PP [micro]cc

3              0.11       0.10         0.015
10             0.70       0.18          0.06
20             1.2        0.61          0.10

(n value for pure PP is 0.009).

TABLE 4. Thermal properties for different PP/CaC[O.sub.3] composites.

                                                   [DELT]
                     [T.sub.f]      [T.sub.f]      [H.sub.c]
                     ([degrees]C)   ([degrees]C)   (J/g)

Virgin PP            166.44         121.76         69
PP 1 wt% ncc322      167.11         120 85         70.5
PP 3 wt% ncc322      167.42         121.96         73.8
PP10 wt% ncc322      167.45         122.24         79.5
PP20 wt% ncc322      167.78         122.42         83.5
PP3 wt% [micro]cc    167.92         121.82         76
PP10 wt% [micro]cc   167.98         121.60         77
PP20 wt% [micro]cc   166.64         122.4          86.7
PP 1 wt% ncc31       165.83         122.45         75.8
PP 3 wt% ncc31       166.17         122.66         81.3
PP10 wt% ncc31       166.78         122.75         72.6
PP20 wt% ncc31       168.28         122.24         79.6

                     [DELT]
                     [H.sub.f]
                     (J/g)       X (%)

Virgin PP            69.3        33
PP 1 wt% ncc322      71.92       34
PP 3 wt% ncc322      75.12       36
PP10 wt% ncc322      79.3        38
PP20 wt% ncc322      84.28       40
PP3 wt% [micro]cc    77.3        37
PP10 wt% [micro]cc   78.2        37.5
PP20 wt% [micro]cc   86.55       41.5
PP 1 wt% ncc31       77          36.8
PP 3 wt% ncc31       79.8        38
PP10 wt% ncc31       73.27       35
PP20 wt% ncc31       81.82       39

TABLE 5. Mechanical properties for
different PP/CaC[O.sub.3] composites.

                     Young's   Elastic    Strain at    Strain to
                     modulus    limit       yield        break
                      (MPa)     (MPa)        (%)          (%)

PP V                 969       21.7       7.6             158
PP1 wt% ncc332       1035      19.5       7.2
PP3 wt% ncc332       1081        21       6.1             160
PP10 wt% ncc322      1136      20.7       5.8             82
PP20 wt% ncc322      1253      19.7       4.1            38.6
PP1 wt% ncc3l        1089      21.2       6.5            56.8
PP3 wt% ncc3l        1119        21       6.8            47.2
PP10 wt% ncc31       1165      20.6       5.7             47
PP20 wt% ncc31       1352      19.8       4.1            16.5
PP3 wt% [micro]cc    1034      20.5       7.20        419 and 229
PP10 wt% [micro]cc   1100      19.8       6.80        379 and 100
PP20 wt% [micro]cc   1139      18.642     6.60        174 and 147
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Author:Elloumi, Ahmed; Pimbert, Sylvie; Bradai, Chedly
Publication:Polymer Engineering and Science
Date:Dec 1, 2015
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