New aspects of migration and flame retardancy in polymer nanocomposites

doi:10.1016/j.polymdegradstab.2008.02.021

Polymer Degradation and Stability

Volume 93, Issue 11, November 2008, Pages 1986-1995

https://doi.org/10.1016/j.polymdegradstab.2008.02.021 Get rights and content

Abstract

Annealing of pristine polypropylene blended with the organomontmorillonite (OMMT) at temperatures of 180–340 °C under a stream of nitrogen and of nitrogen–air mixtures is investigated. The oxidative annealing brings about the dispersion of the OMMT in the polypropylene and the formation of a nanocomposite structure. This is evidenced by the increase in the interlayer distance ‘d’ as measured by small angle XRD, with time of annealing and with the weight percent of air. This indicates progressive intercalation of the polymeric matrix into the clay gallery and subsequently exfoliation. The degree of exfoliation is estimated by the extent of migration determined spectroscopically on the surface of the annealed sample. The accumulated clay on the surface due to migration hinders the penetration of the oxygen into the annealing melt as expressed by the decrease in the rate of migration with the increase in the air concentration. This indicates the increase in ageing and storage stability of nanocomposites with increase in the extent of migration. The extent of migration is proportional to the polar carbonyl groups formed on the matrix. The energy of activation of the migration was found to be 37.82 kJ/mol indicating that the rate-determining step of migration is diffusion controlled reaction. The penetration of oxygen into the melt is the first of five steps, followed by oxidation, intercalation, exfoliation and migration. Monitoring the migration with increase in the temperature enables the observation at 275 °C of the transition of the nanocomposite structure to noncolloidal microcomposite. Increasing the annealing temperature above 300 °C brings about a slow, low-temperature combustion and formation of a new kind of char on the surface of the sample.

Introduction

Polypropylene (PP) appears to be the most widely investigated polymer for use in the preparation and application of nanocomposites. PP is a highly hydrophobic and nonpolar polymer and cannot be used as a matrix for the dispersion of inorganic hydrophilic clays in order to prepare nanocomposites without a compatibilizing modification [1]. The modifications described in the literature are time consuming and of high cost. The most prevalent modification used at present consists of grafting maleic anhydride (MA) onto PP. However, this treatment is connected with a number of complications including side reactions such as β-scission, chain transfer, coupling and above all [2], [3], [4], [5], severe decrease of the molecular weight. One of the aims of this study is to explore the possibility of dispersing organophilic layered silicate (OLS) in pristine PP in the presence of air, but without any previous pre-treatment.

The interaction of pristine PP and of PP/OLS with oxygen has been studied by several investigators [6], [7], [8], [9], [10], [11], [12]. These studies were concerned mainly with the degradation of the polymer by thermal oxidation. The extent of oxidation was monitored spectroscopically by determining carbonyl groups on the surface of the samples. The above studies were not concerned with investigating the effect of oxidation on the structure of PP based nanocomposites, and treated nanocomposite samples as stable, unchanged moieties of polymer/OLS (POLS) [13], [14]. A preliminary study on the effect of air oxidation of poly(ethylene-co-vinyl acetate) (EVA) was reported in the literature, and whereas changes in the nanocomposite structure were noticed, they do not pertain to PP nanocomposites due to the differences in chemical structure between the two matrix polymers [15].

A study of the effect of oxygen on the structure of PP/OLS was carried out in this lab and recently published [16]. It was shown that whereas the structure of PolyOne PP (Nanoblend 1001 concentrate, a 60% compatibilized PP and 40% organoclay, obtained from Polyone Corporation) does not change upon annealing with N₂, profound changes occur in the PP/OLS structure upon annealing under a stream of air. These changes were monitored by XRD and by migration of clay to the surface as determined by ATR-FTIR [16], [17]. Additional results on the effect of oxygen during annealing of maleic anhydride grafted PP were also recently obtained [18]. In this study a limited concentration of air of up to 25% of the purging gas was used, and effects of two structure modifiers were studied simultaneously, one of the grafted MA, and the other, of the oxygen. In the present study the effects of increasing concentrations of air up to 100% of the purging gas were studied on pristine PP without any other compatibilizers.

The experiments in the present paper were carried out at a range of temperatures from 180 to 340 °C. In this temperature range profound changes in the structure of the nanocomposite occur beginning with mild oxidation in the low-temperature range and culminating with a slow combustion and char formation at the highest temperature.

Section snippets

Materials and preparation of samples

A polypropylene homopolymer, Petrothene PP 31KK01, having melt flow rate of 5 g/10 min, was obtained from Equistar Chemicals (Houston, Texas), and was used as-received. The clay used in the study is NANOMER^®-I.44P, which was purchased from Nanocor Company. It is an onium ion modified montmorillonite clay containing 60% MMT (CAS No. 1318-93-0) and 40% dimethyl dialkyl (C14-18) ammonium organic modifier (CAS No. 61789-80-8). The preparation of PP/clay nanocomposites containing 5 wt.% of NANOMER^®

Migration vs. concentration of oxygen

A series of experiments which involved annealing under pure N₂ and at several concentrations of air in the N₂ purging gas were carried out at 190 °C for 60 min. Five nitrogen–air mixtures with increasing concentrations of O₂ were used for purging the samples: 6.25, 12.5, 25, 50 and finally 100% which corresponded to 13.2 g in only air atmosphere. The results of these five experiments are presented in Fig. 2 where each of the data points pertains to one experiment in which a specified amount of air

Conclusions

(1)
The extent of migration upon annealing at 190 °C increases with the increase in the concentration of air in the nitrogen stream used for purging the sample whereas the rate decreases. This decrease is explained by decrease in rate of diffusion of oxygen due to the increased concentration of clay on the surface following the migration. The oxygen molecules cannot penetrate the clay particles and their diffusion path is longer.
(2)
The extent of migration is proportional to the number of carbonyl

Acknowledgements

Partial support for this work was provided by NSF (DMR 0352558) and from National Institute of Standards and Technology (NIST 4H1129). The authors are grateful to Jeffrey Gilman, Marc Nyden, Eli M. Pearce, Kalle Levon, Mauro Zammarano and Avraham Mey-Marom for useful discussions.

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New aspects of migration and flame retardancy in polymer nanocomposites

Abstract

Introduction

Section snippets

Materials and preparation of samples

Migration vs. concentration of oxygen

Conclusions

Acknowledgements

Polym Degrad Stab

Appl Clay Sci

Polym Sci USSR

Polym Degrad Stab

Polym Degrad Stab

Polym Degrad Stab

Polym Degrad Stab

Polymer

Chem Mater

Macromolecules

Macromolecules

Macromolecules

Macromolecules

J Mater Sci