Fillers effective barrier properties

Besides the coextruded laminate structure in Fig. 4b, cases c-f are also viable structures for some applications. Chapter 11(47) discusses the addition of inorganic fillers to EVOH copolymer to achieve large increases in barrier properties in some applications. The effects of different loadings of mica flake in several polymers other than EVOH was also recently reported to be effective (80). 

Fillers significantly increase the tensile properties of polysulfides. This is related to the type of filler, its particle size and the type of cure. A balance of filler particle size and type is required to achieve the optimum wetting and rheology to produce the most cost effective compounds. Consideration must be given to the pH of the filler, since this affects shelf stability or well as cure properties of the compound. Fillers must be inert and insoluble in the sealants s environment. Care must be taken that the filler is adequately dispersed to ensure optimum thixotropy and barrier properties. 

In the past decade, clay-based polymer nanocomposites have attracted considerable attention from the research field and in various applications. This is due to the capacity of clay to improve nanocomposite properties and the strong synergistic effects between the polymer and the silicate platelets on both a molecular and nanometric scale [2,3], Polymer-clay nanocomposites have several advantages (a) they are lighter in weight than the same polymers filled with other types of fillers (b) they have enhanced flame retardance and thermal stability and (c) they exhibit enhanced barrier properties. This chapter focuses on the polymer clay-based nanocomposites, their background, specific characteristics, synthesis, applications and advantages over the other composites. 

More recently nanoscale fillers such as clay platelets, silica, nano-calcium carbonate, titanium dioxide, and carbon nanotube nanoparticles have been used extensively to achieve reinforcement, improve barrier properties, flame retardancy and thermal stability, as well as synthesize electrically conductive composites. In contrast to micron-size fillers, the desired effects can be usually achieved through addihon of very small amounts (a few weight percent) of nanofillers [4]. For example, it has been reported that the addition of 5 wt% of nanoclays to a thermoplastic matrix provides the same degree of reinforcement as 20 wt% of talc [5]. The dispersion and/or exfoliahon of nanofillers have been identified as a critical factor in order to reach optimum performance. Techniques such as filler modification and matrix functionalization have been employed to facilitate the breakup of filler agglomerates and to improve their interactions with the polymeric matrix. 

The Nielsen model has been a popular theory, originally used to explain polymer lay nanocomposites. This model is used to describe the tortuosity effect of plate-like particulates of filled rubber polymer composite on the gas permeation. An increase in barrier properties of gas permeation of rubber polymer nanocomposites is a result of the impermeable nature of filler particles which creates a long path of penetrant molecule by directing them around the particle. 

A simple tortuous 2 D model developed by N ielsen to depict the effect of the size and aspect ratio a of platelet fillers with orientation perpendicular to the diffusion path on the barrier properties of the polymer composite related Eqs. (8.1) and (8.2) are found in Chapter 8. 

In recent years, lamellar nanofiUers have been established as the most important filler type for barrier and mechanical reinforcement. Dal Point et al. reported a novel nanocomposite series based on styrene-butadiene rubber (SBR latex) and alpha-zirconium phosphate (a-ZrP) lamellar nanofiUers. The use of surface modified nanofiUers improvement the mechanical properties. However, no modification of the gas barrier properties is observed. The addition of bis(triethoxysilylpropyl) tetrasulfide (TESPT) as coupUng agent in the system is discussed on the nanofiUer dispersion state and on the fiUer-matrix inteifacial bonding. Simultaneous use of modified nanofillers and TESPT coupling agent is found out with extraordinary reinforcing effects on both mechanical and gas barrier properties [123]. 

On the other hand, the imique barrier properties of nano-dispersed polymeric composites are of interest of polymer combustion due to specific laminar morphology. This type of structure is especially effective in comparison with the other forms of fillers because of the labyrinth effect . Researches in this area will allow defining an influence of diffusion of low-molecular products of pyrolysis on the process of micro-intumescence in a superficial layer of burning polymers. 

As mentioned previously, the addition of filler may also change the amount of crystallinity in the polymer. As polymer crystals are impermeable even to low molecular weight species, an increase in crystallinity also results in improved barrier properties, through increased tortuosity [54], This effect is expected to be especially prevalent for fillers that induce a high degree of transcrystallinity. 

Nontoxic Citrates Nontoxic citrate plasticizers derived from natural citric acid, such as triethyl citrate (TC), tributyl citrate (TBC), acetyl triethyl citrate (ATC), acetyl tributyl citrate (ATBC), and triacetine, have been shown to be effective plasticizers for PLA [27-29]. Some gas permeability tests have been performed to assess the potential use of PLA and nontoxic citrate plasticizer blends in food packaging and other applications. The effect of ATBC on PLA barrier properties was studied by Coltelli et al. [30] using PLA mixed with ATBC (10-35 wt%), followed by compression molding. Yu et al. [31] blended PLA/ATBC mixmres with carbon black (CB) to form electrically conductive polymer composites. Fourier transform infrared (FTIR) experiments revealed that the interaction between the PLA/ATBC matrix and the CB filler was increased by the addition of ATBC. Water vapor permeability values decreased with an increase in ATBC content (at constant CB levels). For example, at 30wt% CB, the WVP of the PLA decreased from 0.66 x 10 kgm/(msPa) (at 0% ATBC) to 0.10 X 10 kgm/(msPa) with the addition of 30% ATBC. 

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