PNCs are materials composed of a polymer matrix with nanometer-size particles dispersed in it. It is the size and shape of the dispersed ultra-fine particles that give PNCs their performance advantages with respect to conventional polymer composites. Most PNCs are manufactured with 2 to 6 wt% of clay nanoplatelets--these materials dominated the Symposium. Other types of PNCs include those containing high aspect ratio carbon nanotubes (CNTs)--these materials are more recent, being developed for specific applications. The incorporation of well-dispersed nano-particles results in increased stiffness, strength, barrier properties, flame, and heat resistance, without loss of impact strength. In a crystallizable polymer matrix, the clay platelets may promote faster crystallization that results in finer crystals and higher crystallinity. Because the particle sizes are of the order of the wavelengths of visible light, the nanoparticles have little effect on transparency. The materials are slowly being introduced to the transport, packaging, and other industries.
Both experiments and computer simulations indicate that dispersion of nanoparticles is relatively simple for hydrophilic polymers, somewhat more difficult for polar ones like polyamides and epoxies, and much more difficult for hydrophobic, nonpolar resins like polypropylene, polyethylene, and styrenics. Thus, in spite of the fact that the first patent on PNCs was issued in 1950 and the polyamide-type had its market debut in 1989, industrial acceptance has been slower than originally expected (GM introduced TPO-based PNC in 2002 for a step-assist and this year for side moldings). For example, the USA production of PNCs in 2004 was projected by BCC Corp. to be 25 kton valued at $195 M, i.e., at a cost of US$7.8 per kg! The high cost, caused largely by the high price of organoclay, explains the slow acceptance by the plastics industry. To cure this problem there is a tendency to go directly from purified, inorganic clay to PNC in a single processing step. The technology was initially used for PNC with hydrophilic polymers, then with PA-6, and (as discussed during the Second Symposium by Kato et al.) with PP. Noteworthy, elimination of ammonium cation in the direct wet process also solves two intercalant-related problems: toxicity and thermal stability. The most common method for PNC preparation is melt compounding.
Considering that nanocomposites offer significant improvement of polymer performance, it is a historical necessity that PNCs will progressively replace neat resins in their direct use as well as in composites, blends or foams. The key is the economics--in some cases (e.g., weight or flammability reduction) PNCs with improved performance may justify their increased cost. However, only a serious reduction of nanocomposite production cost will lead to large-scale use of PNCs in various applications. Polym. Eng. Sci. 44:1197, 2004.
[c]2004 Society of Plastics Engineers.
Published online in Wiley InterScience (www.interscience.wiley.com).
L. A. Utracki and K. C. Cole
Boucherville, April 2004