Barrier properties polypropylene nanocomposites

The mechanism of the improvement of thermal stability in polymer nanocomposites is not fully understood. It is often stated [126-129] that enhanced thermal stabihty is due to improved barrier properties and the torturous path for volatile decomposition products, which hinders their diffusion to the surface material where they are combusted. Other mechanisms have been proposed, for example, Zhu et al. [130] recently proposed that for polypropylene-clay nanocomposites, it was the structural iron in the dispersed clay that improved thermal stability by acting as a trap for radicals at high temperatures. 

Polypropylene nanocomposites have attracted more and more interest in flame retardant area in recent years due to their improved fire properties [18-20], It is suggested that the presence of clay can enhance the char formation providing a transient protective barrier and hence slowing down the degradation of the matrix [19,20],... 

Ozcalik O, Tihminlioglu F. Barrier properties of com zein nanocomposite coated polypropylene films for food packaging applications. J Food Eng 2013 114 505-513. 

Incorporation of silicate nanolayers in semi-crystalline polymers like polypropylene ean have a two-fold effect on the barrier properties, (1) well oriented large aspeet ratio platelets will increase the tortuosity of the diffusion path and (2) the nanolayers will affect the crystalline order (size and interlamellar spacing) and possibly affect the barrier properties. The extent of orientation is greater in blown film than in extmsion cast film and this leads to similar trends in barrier properties of polypropylene nanocomposites with 7wt.% 1.3 IPS (silated) clay as reported by Qian et al. With cast films, the nanoeomposite had a lower permeability to oxygen by a faetor of 1.5 compared to neat polypropylene. With blown films, the nanocomposite permeability to oxygen was lower by a factor of 2.5 compared to neat polypropylene. However, Ellis and D Angelo were able to prepare only intercalated polypropylene nanocomposites with the same 1.31 PS and obtained no improvement in permeability to a solvent over that for neat polypropylene. This underlines the greater sensitivity of barrier performanee to the level of dispersion and orientation. 

A multilayer coextrusion die was used to create polypropylene (PP) / organoclay nanocomposites and to study the stress effects on clay dispersion. Nanocomposites contained 5 wt% maleic anhydride grafted PP compafibilizer and 1-5 wt% organoclay. Nanocomposite layers were extruded opposite talc fill PS before being separated. Individual PP nanoconposite layers were tested for improvements in mechanical, thermal and barrier properties and were compared to theoretical predictions in order to determine an aspect ratio for the organoclay. Aspect ratios from theory were compared to those measured directly through TEM. A PP nanocomposite / elastomer system was also considered. 

In recent years polypropylene (PP) nanocomposites have attracted great interest in academia. The attractiveness of this new class of material hes in the large improvements in both mechanical and thermal properties, as well as in gas barrier and fire resistance (Grunes et al., 2003). Nanofillers, being additives of nanometre scale, are dispersed in PP matrix, offering multifunctional and high-performance pwlymer characteristics beyond those possessed by traditional filled materials. 

Intercalated nanocomposites are usually formed by mixing in the melt or in situ polymerisation whereas exfoliation may require more complex processing depending on the properties of the clay (Usuki et al, 1993). However, such layered silicate-based polymer nanocomposites have attracted considerable recent interest after the commercialisation of polypropylene-and nylon-6-based materials (Krishnamoorti and Yurekli, 2001, Kiersnowski and Piglowski, 2004). The major barrier to commercialisation has been developing techniques to ensure a reliable and reproducible product which has now been addressed for clay-based composites some thirty or so years after they were first developed. 

This book covers both fundamental and applied research associated with polymer-based nanocomposites, and presents possible directions for further development of high performanee nanocomposites. It has two main parts. Part I has 12 chapters which are entirely dedicated to those polymer nanocomposites containing layered silicates (clay) as an additive. Many thermoplastics, thermosets, and elastomers are included, such as polyamide (Chapter 1), polypropylene (Chapter 4), polystyrene (Chapter 5), poly(butylene terephthalate) (Chapter 9), poly(ethyl acrylate) (Chapter 6), epoxy resin (Chapter 2), biodegradable polymers (Chapter 3), water soluble polymers (Chapter 8), acrylate photopolymers (Chapter 7) and rubbers (Chapter 12). In addition to synthesis and structural characterisation of polymer/clay nanocomposites, their unique physical properties like flame retardancy (Chapter 10) and gas/liquid barrier (Chapter 11) properties are also discussed. Furthermore, the crystallisation behaviour of polymer/clay nanocomposites and the significance of chemical compatibility between a polymer and clay in affecting clay dispersion are also considered. 

Retort is a high temperature sterilization process that is used to prolong the shelf life of military rations. Ethylene co-vinyl alcohol (EVOH) /montmorillonite layered silicate (MLS) nanocomposites were co-extruded with retort grade polypropylene (PP) into a multilayer cast fdm to determine if the addition of MLS to EVOH improved barrier, mechanical, thermal and retort properties. The PP/EVOH-MLS/PP structure showed an improvement in some properties such as water vapor transmission rate. Young s modulus, and seal strength before retort in comparison to the PP/EVOH/PP structure however, the improvement in properties was lost after the retort process. 

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