THINK BIG: THE CASE AGAINST RELYING JUST ON NANOCELLULOSE TO REINFORCE PLASTIC COMPOSITES

摘要

By incorporation of cellulosic reinforcements into plastic matrices, several desirable goals can be achieved. The resulting wood-plastic composites (WPCs) tend to have higher modulus, lower density, and less tendency to sag in comparison with the matrix phase by itself, while still allowing the resulting material to be cut or milled. Though many of the same goals can be achieved using glass fibers, such composite products are heavier and more abrasive. Though it is well known that cellulosic materials are generally too hydrophilic to adhere well to common thermoplastic materials such as polyethylene, such deficiencies can be overcome by use of compatibilizers, e.g. ethylene-maleic anhydride. Though WPCs, prepared with macroscopic cellulosic reinforcement, have some problems - such as a tendency for the cellulosic material to rot in places where it is exposed to moisture - the products have become widely used, for instance in the manufacture of park benches and playground equipment from recycled plastics and wood fibers. Recently many researchers have proposed that the same goals can be achieved by use of nanocellulose. Based on recent publications, there appears to be feverish interest in anything having the prefix "nano". This includes the use of nanocellulose of various types in plastic composites. Three main theories, if true, might justify using nanocellulose for such a purpose: (a) the idea that nano-sized reinforcements can have a more pervasive effect due to their higher surface area and presumably much more intimate interactions with the matrix; (b) the idea that extremely small reinforcing elements can be processed more easily, resulting in a more uniform composite with better properties; and (c) the idea that a hybrid composite having at least two different kinds of reinforcements, one of which may be of nano scale, can achieve results superior to ordinary composites. Evidence to support any of these theories is scant. One of the most serious problems facing nanocellulose-plastic composites is a very high cost to achieve good interfacial adhesion due to the high surface area. Other key problems include the viscous nature of the nano-scale mixtures, thermal decomposition, and the undesirability of the mixtures as source material for high-volume recycling operations. Such considerations suggest an overall greater likelihood of practical success in the future when using larger-size cellulose-based reinforcements. Nevertheless, the inherent uniformity of cellulose nanocrystals (CNCs) may make it a promising additive for preparation of "hybrid composites," in which two kinds of reinforcements are used in combination.