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Surface Treatment of Mica
During processing of siliceous products, molecular bonds are broken. The unsaturated terminal silicon and oxygen atoms react with water molecules to form hydroxyl groups. Whereas other water molecules can be absorbed, these water molecules cannot be fully removed by compounding, even under vacuum conditions at high temperature over a long treatment time. Typically (except talc) minerals have high polarity and surface tension (i.e., surface-free energy). On the other hand, typical polymers are lower in surface tension and less polar. Mixing the two is - simplified expressed - like mixing water (the mineral) with oil (the polymer) resulting in a high interfacial tension (due to the mismatch of polar and dispersive parts of surface tensions) and finally in a separation into two phases with minimized surface area. The resulting effects - taking into account that the mineral is a solid and the polymer (when being processed/compounded) is more or less a liquid - are agglomeration of the mineral particles leading to an uneven distribution of the latter and high interfacial tension resulting in easy cleavage of the interface between the two phases (after solidification) being the weakest part in the compound. Even worse, sometimes entrapment of air between mineral and polymer can be observed. Those voids are the nuclei for stress propagation.
By surface treatment of mineral fillers with silanes or silane-based compounds, those interfering effects at the interfaces between the polymer and the filler system can be minimized by - at least - adjusting the surface tensions and polarities of the surfaces.
Silanes are bifunctional chemicals that consist of stable organofunctional and hydrolyzable reactive terminal groups. The hydrolyzable group combines with the inorganic filler surface, while the organofunctional groups harmonize with the organic binder (polymer) system.
An important advantage of this method of incorporating already surface-treated fillers directly into a polymer system over “in situ” treatment is that the condensation by-products already escape during coating of the filler and do not get into and remain in the polymer system, as they do in the case of in situ post-silane treatment. Those condensation by-products (usually humidity and alcohols) typically weaken the polymer matrix. Additionally pre-coated fillers are easier dispersed into a polymer than uncoated ones tending to show much less agglomeration. To achieve an optimum chemical bond between the polymer and the functional filler, a silane specially adjusted to the polymer system must be applied to the surface of the filler.
Comparing the tough fracture of neat mica compounded in a polyamide with a surface-treated mica void can be identified between the mica platelets and the polymer matrix when using the neat mineral. Figures 4 and 5 show tough fracture SEMs of the two compounds.
Another comprehensive comparison of surface-treated and non-surface-treated mineral reinforcing fillers in polyamide is given by Hilgers and Mohr (Thorsten Hilgers 2010). Table 6 shows the reinforcing mineral fillers in comparison.
Tables 7 and 8 show the mechanical and thermal data at 20% (by weight) filler load in polyamide 6.
Fig. 4 Scanning electron micrograph; micronized muscovite mica without surface treatment in polyamide after a tough fracture
Fig. 5 Scanning electron micrograph; micronized muscovite mica with surface treatment in polyamide after a tough fracture
They conclude for mica:
The thermal and mechanical properties of polyamide compounds can be considerably improved by using platelet-shaped muscovite and phlogopite micas. In this way the following properties can be achieved:
Coated fillers are more easily incorporated into a polymer in comparison to uncoated ones.
An optimal effect between a polymer and the high-performance filler is achieved by a coating agent that is matched to the polymer system.
Table 6 Summary of the filler modifications of wollastonite, mica, and kaolin tested in polyamide 6
Table 8 HDT and shrinkage data of different reinforcing mineral fillers at around 20% (by weight) filler load in polyamide 6
Due to this the system, improving properties of the filler are optimally utilized in most cases. In particular tenacity can be improved by the use of silanized fillers.
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