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Other Organometallic Coupling Agents (Organo-Titanates and Zirconates)
Various other metals have groups similar to the silanols that could be used to react with filler surface hydroxyls, and the success of the organosilicon compounds has led to other organometallics being promoted for use as coupling agents. While they attempt to mimic the silane structures, they all have difficulty matching the hydrolytic and thermal stability of the silicon to carbon bond. The two principle types that may be encountered are based on titanium and zirconium.
The organo-titanates have aroused great interest and controversy in recent years. A recent review has been written by Monte (2010).
They can be regarded as derivatives of ortho-titanic acid, Ti(OH)4, and hence are commonly known as organo-titanates rather than by their systematic names. The controversy is over their real structures and whether they act as true coupling agents or merely as very effective dispersants.
The idealized structures written for the organo-titanates are similar to silanes with alkoxy functionalities again providing the filler-reactive precursor. However, unlike the silicon to carbon bond, the titanium to carbon bond is very unstable and cannot be used to permanently attach organic groups to provide polymer reactivity. Thus, all these substituents have to be attached through titanium oxygen bonds, and their chemistry is dominated by the hydrolytic sensitivity of these. One of the more successful approaches is to use chelate groups attached to titanium, particularly where a five- or six-membered ring is formed. Titanates are much more reactive species than silanes and can exhibit a variety of cross-linking and catalytic effects of their own in polymeric systems, over and above any due to filler treatment. This makes understanding their role in filled composites very complicated. They are also sensitive to photoreduction, which can lead to unwanted color effects. These problems are largely eliminated if titanium is replaced by zirconium, and hence, organo-zirconates analogous to the organo-titanates have been developed, but they are considerably more expensive to produce than the titanates and hence are unlikely to completely replace them.
Based on the proposed mode of action, one would expect best performance on the same filler types as for the silanes (i.e., heavily hydroxylated surfaces such as silica, silicates, and hydroxides). Surprisingly, they also seem to produce beneficial effects on other fillers, notably calcium carbonates and carbon blacks, but this may be due to improved dispersion, or to the titanate reactivity mentioned earlier, rather than any coupling action.
While the organo-silanes are generally based on three alkoxy groups, the main titanates only have one.
In general, the simplest organo-titanates used as filler modifiers are nominally triacyloxy isopropoxy derivatives. They thus contain three potential polymer- reactive groups (as opposed to one with the silanes). These can be simple hydrocarbon or one or more of them can contain reactive functionality. Two examples are shown below (Fig. 7).
Despite the uncertainties over mechanisms, the titanates do appear to give some useful effects.
The most notable effects are in giving significant viscosity reduction in highly filled systems while maintaining good final properties. This is largely due to improved dispersion, and they must be judged against other products, such as the fatty acids, which can do the same thing at much lower cost. Unfortunately, very little data exists on which one can properly judge the relative merits of the organo- titanates and other treatments.
Fig. 7 Typical reactive and nonreactive organo-titanate structures
The work of Sharma et al. (1982) is typical. They coated ground calcium carbonate with a nonfunctionalized titanate from toluene solution in a high-speed mixer. The filler was dry blended with polypropylene powder and compounded in a Buss Ko-Kneader. Test specimens were produced by both compression and injection molding. The coating was found to improve melt flow index, tensile elongation, and notched Izod impact strength relative to the uncoated filler. Scanning electron microscopy indicated better dispersion of the filler due to the presence of the titanate but gave no evidence of a chemical bond between the filler and matrix. Unfortunately, no comparison with a simple fatty acid treatment, which would be expected to give some, at least, of these benefits, was carried out.
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