Effect of processing temperature and mixing time on the properties of PP/GnP nanocomposites

https://doi.org/10.1016/j.polymdegradstab.2020.109321Get rights and content

Highlights

  • The effect of mixing conditions on properties of PP/GnP nanocomposites was evaluated.

  • Graphene nanoplatelets improve the thermo-mechanical resistance of the matrix.

  • The sample with the lower content of GnPexhibits a lower level of thermo-mechanical degradation.

Abstract

During processing of molten polymers the thermal and mechanical stress acting on the melt in presence of oxygen can induce degradation with a modification of the chemical structure of the polymers. This picture can become even more relevant if the melt is a multiphasic polymer system. In this last case, the effects of the degradation depend also on the presence of a second phase and on the interactions between the two phases.

In this work, the effect of the processing conditions, temperature and time, have been considered in order to investigate the thermo-mechanical and thermo-oxidative degradation of nanocomposites made by polypropylene and different contents of graphene nanoplatelets. Graphene nanoplatelets improve the stability of the polymer matrix and the sample with the lower content of nanofiller shows a lower level of degradation. The better resistance of the filled samples has been interpreted with a barrier effect to the oxygen of the nanofiller that hinders the transport of oxygen necessary for the oxidation of the polypropylene. The unexpected effect of the concentration of graphene nanoplatelets has been correlated with the agglomeration of the nanoplatelets that reduces the surface area of the filler and then the barrier effect to the oxygen.

Introduction

Among the many nanoparticles studied in recent decades, carbonaceous nanofillers have attracted a great interest from both an industrial and a scientific point of view because of their unusual and extraordinary physical properties. The peculiar chemical structure and huge aspect ratio of these carbonaceous particles impart very relevant thermal, mechanical, and electrical properties, exhibiting great potentials in applications, such as electrochemical devices, hydrogen storage and nanocomposites.

Graphene Nanoplatelets (GnP), recently developed, are short stacks of individual layers of graphite that often increase the tensile modulus of a composite material and are available at a low cost [1], [2], [3], [4], [5].

One of the most promising applications of GnP is their use in formulation of polymer-based nanocomposites [6], [7], [8], [9], [10], [11], [12], [13], [14]. The addition of GnP, even at low filler concentration (< 5 wt. %), can lead to the enhancing of the thermal, heat-resistance, mechanical and electrical properties of the polymer matrix due to both unique properties of these nanofillers and their exceptionally large specific surface area. Many researches have shown that the influence of these nanoparticles on the properties of this class of polymer based nanocomposites is of course related to a good dispersion achieved during processing and good adhesion of the filler within the matrix [15], [16], [17], [18]. However, in many cases the huge surface area, the incompatibility with the polymer matrix did not always allow obtaining the expected reinforcement of the matrix because good dispersion and good adhesion are not easy to obtain.

The effect of the processing conditions on the morphology and the final properties of nanocomposites has been studied by our research group [12], [19], [20], [21], [22], [23], [24], [25], [26], [27] and the obtained results suggest that the thermo-mechanical stress and the processing time are effective to alter the morphology of the nanocomposites made with both organomodified clay and carbon nanotubes, being able to align the nanofillers along the flow direction but also to modify the degradation kinetics of the nanocoposites in comparison with the pure matrix.

Although some papers [28], [29], [30], [31], [32] have investigated the improved thermal stability of GnP based nanocomposites, to our best knowledge, no scientific papers are reported in the literature concerning the possible thermo-mechanical and thermo-oxidative degradation during the preparation of the nanocomposites or during the processing necessary to obtain the manufactures. In this work polypropylene (PP) based nanocomposites with graphene nanoplatelets were prepared in different mixing conditions and their properties were investigated. In particular, the effect of mixing time and processing temperature on rheological and mechanical properties of PP/GnP nanocomposites was evaluated and correlated with the achieved morphology. Furthermore, the effect of the GnP content on the evaluated properties was studied.

Section snippets

Materials

The polymeric matrix used in this work is a sample of a polypropylene (PP) supplied by Carmel Olefins with trade name Capilene® E 50 E (M.F.I. = 1.8 g/10 min. at 230°C/2.16 Kg).

Graphene nanoplatelets (GnP) were supplied by XG Sciences Inc. (Lansing, MI, USA) with trade name xGnP®, Grade C. According to the manufacturer, the main characteristics of the GnP used in this work are the following: average diameter between 1 and 2 µm; average thickness lower than 2 nm and a specific surface area of

Results and discussion

In Fig. 1 the dimensionless curves of the torque as a function of the mixing time for all the investigated samples have been reported. The dimensionless values have been calculated by dividing the value of the torque at each time by that of the torque at 5 min to reach a thermal equilibrium and to eliminate the possible effects of the loading.

The torque curves decreases with time and this suggests that some degradation occurs during the mixing. Indeed, as already reported in literature [35] the

Conclusion

During processing the morphology and the molecular structure of molten polymer can be modified due to the application of mechanical and thermal stress. The level of degradation undergone depends, of course, on the level of the thermo-mechanical stress but also on the nature and molecular structure of the polymers and on the availability of oxygen during the processing. For composite polymeric systems the level of degradation depends also on the type of the filler and on its interactions with

Data availability

The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

This work has been financially supported from the Italian Ministry of University and Research (Grant PON03PE_00206_3 - NanoBioMat).

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