Halloysite Nanotubes Rubber Composites

Abstract

Rubber composites based on halloysite nanotubes (HNT) have attracted tremendous attention during the last decades owing to the improved mechanical, dynamic mechanical, and thermal properties [1–3]. Before exploring the various aspects of HNT/rubber composite systems, we first briefly describe the HNT. It was first introduced by Berthier long back in the year of 1826 [4]. HNTs are a type of naturally occurring silicates with rolled nanotubular or spiral morphology, which have an analogous chemical structure of kaolinite [5]. They are naturally occurring and economically viable, and many countries such as China, France, Belgium, and New Zealand have deposits of these clay minerals. The major origin of HNT is a clay mineral obtained from a weathering product of granitic and rhyolitic volcanic rocks. The chemical composition of HNT is reported as Al2(OH)4Si2O5∙nH2O with n 1⁄4 0 or 2 for the anhydrous and the fully hydrated halloysite, respectively. The nanotubular structure was evolved from kaolinite by rolling up the layers under natural conditions. Most of the HNTs are multiwall or double-wall nanotubes [6]. Usually, the inner diameter of HNT is in the range of 50–80 nm and length of about 1,000 nm. Based on the state of hydration, HNTs are classified in two categories: hydrated HNTs with a crystalline structure of 10 A (d001) spacing and dehydrated ones with 7 A (d001) spacing [7]. HNTcontains two different types of hydroxyl groups, inner and outer hydroxyl groups, which are positioned in between the layers and on the surface, respectively. Attributed to the multilayer structure, most of the hydroxyl groups are inner groups, and only a few hydroxyl groups are located on the surface of HNT. The surface of HNT is mainly composed of outer O–Si–O (silanol) groups and O–Al–O (aluminol) groups situated inside the lumen. It is interesting to note that the density of surface hydroxyl groups in HNT is rather lower compared to other silicates clay minerals, for instance, kaolinite and montmorillonites (Fig. 1).