Nanofillers and Nanocomposites

Rather recently, so-called nanoparticles were introduced into the field of additives for plastics, composites in general and WPCs in particular. Nanoparticles are employed in amounts typically below 10%, or more often below 5%, so they are not real fillers but rather additives. However, because even in such small concentrations they sometimes improve properties of materials, some call them nanofillers.  

Nanoparticles or nanofillers are collective terms for modified layered silicates (organoclay), graphite nanoflakes, carbon nanotubes, and a number of materials dispersed in the polymer matrix, when the particles size is in order of nanometers (one thousands of micron), or tens of nanometers. A plastic filled with nanoparticles, typically in the range of 2-10% (w/w) is called a nanocomposite. 

There are two basic types of nanocomposites, in which particles are intercalated or exfoliated. In an intercalated composite the nanodispersed filler still consists of ordered structures of smaller individual particles, packed into intercalated structures. Exfoliated particles are those dispersed into practically individual units, randomly distributed in the composite. Layered silicates, such as montmorillonite clays or organoclays, can be used in nanocomposites. Because clays are hydrophilic and polyolefines are hydrophobic, it is not easy to make a nanocomposite based on polyethylene or polypropylene because of their natural incompatibility. 

When added to WPCs, layered nanoparticles typically do not improve flexural or tensile strength (though there were some reports on the beneficial effect of nanoparticles on flex strength), but significantly increase flexural modulus (stiffness). Sometimes they even increase water absorption by the WPC, making the final material worse in this regard [24]. 

TABLE 4.28 Effect of a nanoclay on flexnral strength and flexnral modnlns of polypropylene-based WPC containing 50% (w/w) maple wood flonr (80-mesh) [24]. No coupling agent added 

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Nanofillers and nanocomposites, 154, 476 Nano-particles as flame retardants, 476 Nanoparticles, 79, 126, 154, 155... 

New families of nanofillers and nanocomposites are opening up performance reserves of plastics, rubbers and dispersions. Even very low nanofiller % by volume can suffice to alter the property profiles of polymeric materials and functional additives in significant ways. The application spectrum extends from new construction materials to a diversity of functional polymers [77, 106],... 

For the comprehension of mechanisms involved in the appearance of novel properties in polymer-embedded metal nanostructures, their characterization represents the fundamental starting point. The microstructural characterization of nanofillers and nanocomposite materials is performed mainly by transmission electron microscopy (TEM), large-angle X-ray diffraction (XRD), and optical spectroscopy (UV-vis). These three techniques are very effective to determine particle morphology, crystal structure, composition, and grain size (48). 

ENGAGE is an ethylene-octene copolymer. Ray and Bhowmick [70] have prepared nanocomposites based on this copolymer. In this study, the nanoclay was modified in situ by polymerization of acrylate monomer inside the gallery gap of nanoclay. ENGAGE was then intercalated inside the increased gallery gap of the modified nanoclay. The nanocomposites prepared by this method have improved mechanical properties compared to that of the conventional counterparts. Preparation and properties of organically modified nanoclay and its nanocomposites with ethylene-octene copolymer were reported by Maiti et al. [71]. Excellent improvement in mechanical properties and storage modulus was noticed by the workers. The results were explained with the help of morphology, dispersion of the nanofiller, and its interaction with the mbber. 

Compared to other models (e.g., Voigt-Reuss, Halpin-Tsai, modified mixture law, and Cox), the dilute suspension of clusters model promulgated by Villoria and Miravete [255] could estimate the influence of the dispersion of nanofillers in nanocomposite Young s modulus with much improved theoretical-experimental correlation. 

The fire toxicity of each material has been measured under different fire conditions. The influence of polymer nanocomposite formation and fire retardants on the yields of toxic products from fire is studied using the ISO 19700 steady-state tube furnace, and it is found that under early stages of burning more carbon monoxide may be formed in the presence of nanofillers and fire retardants, but under the more toxic under-ventilated conditions, less toxic products are formed. Carbon monoxide yields were measured, together with HCN, nitric acid (NO), and nitrogen dioxide (NO2) yields for PA6 materials, for a series of characteristic fire types from well-ventilated to large vitiated. The yields are all expressed on a mass loss basis. 

Recently, new approaches on flame retardancy deal often with nanofillers and in this section some examples of improvements of fire behavior of polymeric foams obtained by use of nanoclays or nanofibers will be shown. Much more details on flame retardancy of polymeric nanocomposite may be found elsewhere as for example in the book edited by A. B. Morgan and C. A. Wilkie105 or in scientific review.106 Polymer nanocomposites have enhanced char formation and showed significant decrease of PHRR and peak of mass loss rate (PMLR). In most cases the carbonaceous char yield was limited to few weight %, due to the low level of clays addition, and consequently the total HRR was not affected significantly. Hence, for polymer nanocomposites alone, where no additional flame-retardant is used, once the nanocomposite ignites, it burns slowly but does not self-extinguish... 

The seeond from the mentioned eoneeptions has shown that nano- and microsystems differ by density fluetuation which does not exists in the first but in the second. The last eireumstance assumes that for the considered nanocomposites, density fluctuations take into account nanofiller and polymer matrix density difference. The transition from nano- to microsystems is realized in the ease when the deformed material volume exceeds nanofiller partiele aggregate and surrounds it layers of pol5mier matrix combined volume [49]. 

Figure 5.93 depicts the procedure for making the nanofiller and the polymer nanocomposite by the aforesaid method. Firstly, the sodium montmorfllonite (Na" MMT) is modified by impregnation with a... 

As the adduced above data have shown, the polymer nanocomposites with three main types of inorganic nanofiller and also polymer-polymeric nanocomposites melt viscosity caimot be described adequately within the fiamework of models, developed for the description of microcomposites melt viscosity. This task can be solved successfully within the framework of the fractal model of viscous liquid flow, if in it the used nanofiller special feature is taken into account correctly. Let us note that unlike microcomposites nanofiller cotents enhancement does not result in melt viscosity increase, but, on the contrary, reduces it. It is obvious, that this aspect is very important from the practical point of view. 

The preparation methods have crucial impact on the dispersion of the nanofillers and the final properties of the nanocomposites. In this section, the morphology of the PP nanocomposites and various nanocomposite properties will be discussed. 

Hence, the stated above results have shown, that elasticity modulus change at nanoindentation for particulate-filled elastomeric nanocomposites is due to a number of causes, which can be elucidated within the frameworks of an harmonicity conception and density fluctuation theory. Application of the first from the indicated conceptions assumes, that in nanocomposites during nano indentation process local strain is realized, affecting polymer matrix only, and the transition to macrosystems means nanocomposite deformation as homogeneous system. The second from the mentioned conceptions has shown, that nano- and micro systems differ by density fluctuation absence in the first and availability of ones in the second. The last circumstance assumes that for the considered nanocomposites density fluctuations take into account nanofiller and polymer matrix density difference. The transition from nano to Microsystems is realized in the case, when the deformed material volume exceeds nanofiller particles aggregate and surrounding it layers of polymer matrix combined volume [49]. 

Considering the low physical characteristics of biopolymers, fillers are recommended for the reinforcement of their electrical, mechanical and thermal properties. Following the discovery of CNT, much work has been done regarding their application as fillers in other polymers, for improving the properties of the matrix polymer. At first CNT were used as a filler in epoxy resin, by the alignment method. Later on, numerous studies have focused on CNT as excellent substitutes for conventional nanofillers in nanocomposites and recently, many polymers and biopolymers have been reinforced by CNT. As already mentioned, these nanocomposites have remarkable characteristics, compared to the bulk materials, due to their imique properties. 

Tailoring the properties of polymers with the inclusion of nanometric carbon depends on many factors. Among them, the parameters most taken into account are (a) the size, structure and distribution of the nanofiller in the matrix and (b) the interface between the nanofillers and the matrix. This chapter focuses mainly on the effects that functionalization and concentration of nanofillers have in the storage modulus and tribological properties of polymer nanocomposites reinforced with NDs, CNTs and graphene, describing briefly the hardness and scratching performance achieved in these nanocomposites. 

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