Characterizing the microstructure of rubber blends
RC has a unique expertise for characterizing the microstructure of rubber blends and composites, such as rubber/organoclay nanocomposites. David Lowe, from our Advanced Materials & Product Development Unit, recently presented some results from a nanotechnology research project investigating the use of organoclay nanomaterials as reinforcing fillers for natural rubber.
To view David Lowe’s presentation click here.
Organoclays are nanoclays modified with organic interlayer cations, normally quaternary ammonium cations with large hydrocarbon substituents. A commonly used clay is montmorillonite, which consists of silicate sheets about 1 nm thick and 30-500 nm in diameter. The organic cations have the dual effects of making the nanoclay more hydrophobic and physically increasing the separation of the clay sheets. This helps the sheets to delaminate within a polymer.
At low levels, the organoclays can have profound effects on polymer properties, particularly mechanical, barrier and flame-retardant properties. These effects are dependent on the microstructure of the nanocomposite, especially the extent of delamination, or exfoliation, of the silicate sheets. In order to understand, control and optimize these effects, characterization of the microstructure of the nanocomposite is essential. This was achieved by Transmission Electron Microscopy (TEM) and Network Visualization , a technique developed in the Rubber Consultants laboratories.
Figure 1 shows a TEM micrograph of an organoclay (Nanofil ® 8) in natural rubber. The organoclay is partially exfoliated. Although there are a few individual silicate layers, most of the organoclay is in the form of tactoids, consisting of several layers, spaced about 3.5 nm apart. The exfoliation is insufficient for more than limited effects on barrier properties, leading to a modest decrease in air permeability.
Figure 1
‘Network visualization’ microscopy was also performed on rubber/organoclay composites, with and without added silane coupling agent, to establish the nature of any interaction existing between the natural rubber network and the clay layers. The basic process of network visualization is:
• Swell a vulcanizate in styrene
• Polymerize the styrene
• Stain the rubber network
• Visualize using TEM.
This technique has previously been used to identify regions of ‘weakness’ in rubber blends or ‘composite’ materials, as the styrene can preferentially swell into areas of lower crosslink density or where interaction between interfaces is poor. As examples, Rubber Consultants has applied this technique, on behalf of clients, to study the interactions between rubber and silica 1, and rubber and brass 2.
Figure 2 Figure 3
Figures 2 and 3 show network visualisation micrographs of nanocomposites with (Fig 2) and without (Fig 3) a silane coupling agent (Si 69). The styrene preferentially polymerises between the organoclay and the rubber network, leading to polystyrene voids surrounding the organoclay nanoparticles. T he voiding is reduced in the compound containing the coupling agent, demonstrating an increased amount of rubber-filler interaction. The alignment of the clay visible in the micrograph without coupling agent occurs during milling and curing under pressure. This alignment is not apparent when the coupling agent is added. It appears that at least some coupling of the organoclay particles to the rubber has occurred during mixing, reducing the clay’s freedom to move. The network visualisation studies also provide evidence that the organoclays are connected to the rubber network at the ends of the layered sheets, where access to the intercalated organic groups is possible.
1 L. Ladouce, Y. Bomal, L. Flandin and D. Labarre, Paper No. 33, ACS Rubber Division Spring Meeting, Dallas, April 4-6, 2000; Rubber Chem. Technol., 76, 145 (2003).
2 W.S. Fulton, Paper No 122, ACS Rubber Division Fall Meeting, Cleveland, Ohio, October 16-19, 2001; Rubber Chem. Technol., 78, 426 (2005).
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