Poly(vinyl chloride)-paste/clay nanocomposites: Investigation of thermal and morphological characteristics
Introduction
Many recent studies in the area of polymers have been focused on some frequently used plastic materials, e.g. polypropylene [1], [2], [3], polyethylene [4], [5], nylon 6 [6], [7], poly(methyl methacrylate) [8] and poly(ethylene oxide) [9], and on their final properties. These matrixes are modified with filler having at least one dimension of nanometre scale, 10−9. For example plate-like nano-clays, nanotubes and polyhedral nanoparticles can be used. Most often plate-like nano-clays are preferred [10], [11].
A well-known and widely used plate-like nano-clay is montmorillonite (MMT) which belongs to the group of 2:1 layered silicate [12], [13]. Montmorillonite possesses an enormous surface area (about 800 m2/g) in contrast with common micron-sized fillers. Moreover, thanks to its layers approximately 1 nm thick and enormous lateral dimension, it offers high aspect ratio. Previous attributes lead to large enhancements of mechanical, thermal and barrier final properties, comparable to those obtained for micron-sized fillers or neat polymers [10], [11], [14], [15], [16]. In addition, it has been demonstrated that modified natural clays, e.g. MMT, can act as fire-retardants by reduction of heat release and char formation [8]. For example, the presence of clay could delay the chain-stripping degradation of PVC, thanks to the char formation at the surface [17]. What must be stressed is that the potential property improvements usually depend on the degree of exfoliation and dispersion in the polymeric matrix, which was shown in the previous paper, and depends on the combination of proper chemical treatment and optimized processing conditions [18]. Furthermore, processing conditions play a cardinal role especially in case of PVC-paste/nanocomposite preparation.
Although PVC is one of the most widely used thermoplastics, its disadvantage is a low thermal stability. As a result, many studies have been done to understand the degradation behaviour [19], [20], [28], [31], [32]. In the presented papers, few possibilities have been mentioned as the reason for initial degradation [21]. Nevertheless, it has not been well established which of these parameters have the main influence on the evolution of HCl releasing. On the other hand, all researches agree with the idea of PVC degradation caused by HCl release followed by double bonds appearing in the backbone. A more complicated situation arises with organically modified clays. Here, the chemical modifier is thought to affect and facilitate dehydrochlorination of the PVC matrix [28], [31], [32]. To evaluate the thermal stability of PVC matrixes, well-established methods can be used. For instance, dehydrochlorination rates and the Congo Red (CR) determination [22], [23] are applied. Furthermore, FTIR can offer an efficient way to appraise the amount of HCl released [21], [24].
Even though many polymer/clay nanocomposites have been reported in recent studies, little attention, in contrast with polyolefins, nylon etc., has been paid to PVC, which is frequently used as a suspension of fine particles in a liquid plasticizer called plastisol.
This study follows a previous part and concentrates on thermal degradation investigation of prepared PVC-paste samples. Two modes were used to process nanocomposites: firstly, the effect of shearing was considered by using a high-shearing tool at room temperature; secondly, the effect of temperature was combined with shearing. Samples prepared this way were then investigated using thermogravimetric analysis; Congo Red and FTIR techniques completed with XRD and TEM analyses.
Section snippets
Materials
In this work, montmorillonite (MMT) obtained from Southern Clay Products was used as nano-filler. Its silicate layers are approximately 200 nm long and 1 nm thick; the interlayer spacing between stacked layers, denoted d001, is about 1 nm [25], [26]. Two different types of MMT were used: natural type labelled Cloisite® Na+, without chemical treatment, cation exchange capacity (CEC) 92.6 meq/100 g clay, which displays a polar character, and two organophilic MMTs modified with two types of quaternary
Thermogravimetric analysis
Thermogravimetric behaviour could be used as a proof of the interactions between the organic medium and inorganic nano-platelet surfaces. The samples presented in TGA traces were obtained using the method of shearing combined with temperature while processing.
TGA reveals that the thermal stability strongly depends on PVC-paste/nano-clay interactions. As can be seen in Fig. 3, TGA traces show different behaviour for nanocomposites as a function of the clay treatment. The temperature of the main
Conclusions
Partially intercalated and exfoliated PVC-paste/nanocomposites were obtained via a blending process. From general point of view, the process of PVC-paste preparation should bring no intercalation/exfoliation level in the case of presence of clay due to the processing conditions, although PVC from its nature can interact with the clays. Here, thanks to the novel method of the PVC-paste/nanocomposites preparation, the state of intercalation and/or exfoliation can be found. The chemical
Acknowledgements
The financial support of the Czech Ministry of Education, project MSM 7088352101 is gratefully acknowledged. This work was supported by grant of the Academy of Sciences of the Czech Republic (project AVOZ40500505). Special thanks belong to Prof. Jean Francois Gerard, Head of LMM, INSA de Lyon, for cooperation on the project and to Mgr. Ing. Dagmar Svobodova for her help.
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