Elsevier

Thermochimica Acta

Volume 429, Issue 1, 1 May 2005, Pages 13-18
Thermochimica Acta

Thermal stability analysis of organo-silicates, using solid phase microextraction techniques

https://doi.org/10.1016/j.tca.2004.11.020Get rights and content

Abstract

An analysis of thermal degradation products evolved during the melt processing of organo-layered silicates (OLS) was carried out via the use of a solid phase microextraction (SPME) technique. Two commerical OLSs and one produced in-house were prepared for comparision. The solid phase microextraction technique proved to be a very effective technique for investigating the degradation of the OLS at a specific processing temperature. The results showed that most available OLSs will degrade under typical conditions required for the melt processing of many polymers, including thermoplastic polyurethanes. It is suggested that these degradation products may lead to changes in the structure and properties of the final polymer, particularly in thermoplastic polyurethanes, which seem significantly succeptable to the presence of these products. It is also suggested that many commercially available OLSs are produced in such a way that results in an excess of unbound organic modifier, giving rise to a greater quantity of degradation products. All OLSs where compared and characterised by TGA and GC–MS.

Introduction

Recently, much attention has been paid to polymer-layered silicate nanocomposite materials, and more specifically to their production via commercially available processing methods, such as melt compounding [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Nanocomposites are attractive as they have been shown to enhance mechanical, thermal, barrier and flame retardant properties over that of the host polymer [11], [12], and these effects can be observed at very low loadings of organo-layered silicate (OLS), normally between 1% and 10% by weight.

Layered silicates offer an effective form of nanofiller for nanocomposites, due to their high surface area, high aspect ratio of individual platelets, and the capacity be readily modified with a variety of organic surfactants to allow for improved compatibility between the silicate and the polymer. Most typically, the surfactants employed are alkyl ammoniums or phosphoniums, however recently imidazoliums have also been investigated [2].

Processing of these nanocomposites can be achieved by melt intercalation, in situ polymerization or solvent casting. The method used strongly depends on the host polymer system and the end application of the product. Melt intercalation has the distinct advantages of being a scalable and continuous process, and it is suitable for many thermoplastics. However, this method exposes the silicate to significant temperatures for extended periods (residence time in the extruder), and may result in the degradation of the surfactant, which may in turn affect the performance of the final product. The degradation of OLS and the production of these degradation products are often overlooked, but must be considered in order to understand how the degradation products might affect the final nanocomposite properties and long term performance.

The analysis of thermal degradation products would traditionally require the use of solvent extraction techniques [13] or necessitate expensive combined TGA–GC–MS instrumentation. Recently, solid phase microextraction (SPME) has emerged as an effective alternative for the study of the low molecular weight compounds released by polymers whilst they undergo degradation [13], [14]. This has been shown to provide better sensitivity for headspace extraction than traditional static headspace techniques. The disadvantages of the SPME approach are that it is an equilibrium-based technique, so the concentration of products adsorbed on the fibre's surface will not be a direct indicator of concentration in the headspace. If used in a static equilibrium environment, then the amount of analyte coating the surface of the fibre should be directly proportional to the concentration in the headspace [14].

In this study, the thermal degradation products of two commercially available OLS and one analogue OLS material prepared by us were investigated. This was done by exposing the OLS to a typical processing temperature (210 °C) employed for thermoplastic segmented polyurethanes (TPU), which our group have investigated. A simple novel analysis technique employing the use of solid phase microextraction fibres was used to obtain the vapour products, providing an alternative to the expensive equipment normally required to perform this type of study, such as a TGA–GC–MS [7].

Section snippets

Materials

Montmorillonite based OLS from the Southern Clay Products Cloisite™ series were examined. These were Cloisite™ 15A, Cloisite™ 30B. Cloisite™ 15A has a modifier loading of 125 meq/100 g, and Cloisite™ 30B has a loading of 90 meq/100 g. The tallow groups on the modification of both these materials have a predicted composition of ∼65% C18; ∼30% C16; ∼5% C14.

Another organoclay was prepared for comparison by modifying Cloisite™ Na+ (with a CEC of 92.6 meq/100 g) with Ethoquad O/12 available from Akzo

Results and discussion

The thermal stability of OLS variants and degradation products will be discussed with reference to the processing conditions for thermoplastic polyurethane nanocomposite materials, and in comparison with pristine silicate.

Conclusions

The use of SPME fibres in conjunction with a GC–MS, has been shown to provide a quick and cost effective method for the analysis of the degradation of an OLS, for a given operating temperature. This technique is, therefore, useful for investigating the thermal stability of OLS candidates being considered for melt processing of nanocomposites.

The release of organic compounds during processing, as was simulated by conditions in this research, may have significant impact on the performance of the

Acknowledgements

The authors wish to acknowledge Bradley Finnigan for helpful discussions, the Division of Chemical Engineering for a departmental Ph.D. scholarship, and the UQ Research Development Grants scheme for support.

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