PP and PBT composites filled with sepiolite: Morphology and thermal behaviour
Introduction
In recent years, polymer nanocomposites have attracted great interest because they frequently exhibit remarkably improved mechanical and physical properties when compared either to the polymer matrix alone or to conventional micro- and macro-composite materials. In fact, the dispersion of nanofillers having very high surface area frequently improves, even at low loadings (⩽5%), the physical and mechanical properties of polymers [1], [2], [3], [4], [5]. Higher modulus, optical transparency, increased strength, enhanced barrier properties, lower combustion rate and higher thermal stability are typical features of polymer nanocomposites [6], [7], [8], [9], [10], [11].
Most of the literature regarding nanocomposites is devoted to lamellar layered silicates [3], [8], [12], [13].
This paper deals with sepiolite, which is a layered hydrated magnesium silicate characterised by a needle like morphology and very high surface area (BET 374 ± 7 m2/g) as compared to layered phyllosilicates (BET 82 ± 1 m2/g) [14].
The addition of sepiolite generally leads to an improvement of the mechanical properties in polymers. A good adhesion with polymer matrices like PP and interesting compatibilizing effects on polymeric alloys have been found [15], [16], [17], [18], [19], [20], [21]. Furthermore, surface treatments of sepiolite with specifically designed organic surfactants were found to enhance sepiolite compatibility with polymers [22], [23], [24], [25].
The above achievements are mainly attributed to the large concentration of surface silanols, spaced every 5 Å along the length of sepiolite needle [26], [27], [28], that are easily available for coupling reactions with both polymers and organic surfactants. Moreover, this very high density of silanol groups (2.2 groups/100 Å2 [26], [29]) suggests that also other forces such as hydrogen bonding and van der Waals interactions are largely involved in the adsorption phenomena at the interface.
This excellent adhesion/compatibility with polymeric matrices and the strong anisotropy of this mineral account for the excellent reinforcing effect on polymers, increasing the elastic modulus of the final compounds.
Unfortunately, recent studies on thermal stability of polymer/sepiolite systems have shown a strong catalytic effect on the degradation of polyolefins, underlining the drawbacks of the large presence of hydroxyl groups on sepiolite surface [30].
Recent studies have demonstrated that a variety of sepiolite surface modification processes are possible. This paper will show what is the effect of sepiolite modification on the morphology and the thermal properties of the final composites.
Sepiolites, either pristine or organically modified by adsorption of ammonium salts and amine or by silane grafting, have been added to polypropylene and poly (butylene terephthalate); the properties of the resulting composites are reported in this paper, with a particular attention to the thermal characteristics. As both the polymers used are semi-crystalline, the influence on the morphology of the crystalline phase was studied.
Section snippets
Materials and methods
Two commercial polymers were used: polypropylene (PP) Moplen HP500H from Basell and poly (butylene terephthalate) (PBT) PBITER TQ 8,5 from P-group.
Micronized pristine and organophilic sepiolites with different surface treatments were kindly provided by Tolsa SA (Spain).
Sepiolite is a nonswelling clay constituted by a magnesium octahedral sheet in between two layers of silica tetrahedra, which extend as a continuous layer with an inversion every six units. This inversion creates a discontinuous
Morphology
From Fig. 2, we can observe that the composites show the typical XRD diffractogram (Fig. 2c–e) of the monoclinic α phase of iPP [38], [39], [40]; while only the most intense diffraction peak of sepiolite (1 1 0) is visible at 2θ = 7.3° (Fig. 2b), because of the dilution effect due to the low amount of filler (3 wt.%). The 2θ position of this peak is coincident with that of the pristine filler as it is characteristic of the mineral lattice (Fig. 1) [31], [32], [33], [34], [35], [36]. Indeed it is due
Conclusions
Primary particle aggregates are visible in composites prepared in internal mixer, which cannot be further dispersed by increasing mixing time above 3 min. A good adhesion with both polymer matrices PP and PBT is found. Surface treatments of sepiolite with organic surfactants both adsorbed and grafted do not significantly improve sepiolite compatibility with these polymers.
Sepiolites have a certain nucleating effect on both polymer matrices, more relevant in the case of PP. In addition, in
Acknowledgement
This study was carried out in the frame the STRP European Research Program “NANOFIRE”, No. 485631, in the 6th Framework Program.
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