Nanoscale Silica Particles

It is of interest to determine the flame retardant effectiveness of shapes or types of nanoparticles other than layered silicates, to find what shape or type of nanoparticle is most effective for improving the flammability properties of commodity polymers. In this chapter, flammability properties of nanocomposites containing nanoscale oxides such as nanoscale silica particles and metal oxides, polyhedral oligomeric silsesquioxanes (POSSs), and carbon-based nanoparticles such as graphite, single-walled carbon nanotubes (SWNTs), multiwalled carbon nanotubes (MWNTs), and carbon nanofibers (CNFs) are described and a flame retardant mechanism of these nanoparticles is discussed. 

FIGURE 10.1 TEM image of a PMMA-nanosilica nanocomposite (left), analyzed image (middle), and a histogram distribution of diameter (right). (From Ref. 15.) (See insert for color representation of figure.) 

FIGURE 10.2 Effects of the addition of nanosilica on the heat release rate of PMMA at 50 kW/m.  

---

C. Becker, H. Krug, H. Schmidt (1996) Tailoring of thermomechanical properties of thermoplastic nanocomposites by surface modification of nanoscale silica particles. Mat. Res. Soc. Symp. Proc. 435,237. 

In semicrystalline polymers, fillers may act as reinforcement, as well as nucle-ation agents. For example in PP, nanoscale silica fillers may nucleate the crystallization resulting in spherulites that show enrichment in particles in the center of the spherulite (Fig. 3.64). For a quantitative analysis of, e.g., filler sizes and filler size distributions, high resolution imaging is necessary and tip convolution effects [137-140] must be corrected for. The particles shown below are likely aggregates of filler particles considering the mean filler size of 7 nm [136]. 

A hierarchical integration of nanoscale textures by macromolecular building, is obtained in this way by grafting a newly designed highly fluorinated aldehyde and the last layer of amino-functionalized silica particles is then hydrophobized, creating a monomolecular layer via the formation of an imine function (Figure 28). 

Mieszawska, A. J., Nadkami, L. D., Perry, C. C., and Kaplan, D. L. (2010). Nanoscale control of silica particle formation via silk-silica fusion proteins for bone regeneration. Chem. Mater. 22, 5780-5785. 

Although the incorporation of microscale particles as fillers into polymers has been well explored scientifically, the decrease in size of particles to nanometers, and the simultaneous increase in interface area, results in extraordinary new material properties.In one such application, the flammability properties of polymers have been improved with the addition of nanoscale particles. These filled nanocomposites provide an attractive alternative to conventional flame retardants. At present, the most common approach to improving flammability is the use of layered silicates such as clays, as described in Chapter 3. However, there are many different shapes and types of nanoparticles. (Here, a nano scale particle is defined as having at least one dimension on the nanometer scale.) When all three dimensions are on the order of nanometers, we are dealing with true nanoparticles, such as spherical silica particles, having an aspect ratio of 1. Another type of nanoparticle has only one dimension on the nanometer scale. Such nanoscale... 

In the work of Rong et al. [108], various polymers were grafted on the surface of nanoscale silica filler particles through the simultaneous irradiation polymerization technique. In this way, the modified nanoparticles can be more effectively utilized in thermoplastics (such as PP) than conventional particulate fillers, when using the same direct compounding technology. 

---