Toner-silica adhesion

Keywords pyrogenic silica, toner, toner-silica adhesion, scanning force microscopy (SFM)... 

Silica Adhesion on Toner Surfaces Studied by Scanning Force Microscopy (SFM) 911... 

M. Heinemann, U. Voelkel, H. Barthel, S. Hild, S. Koenig, S.-C. Imhof, Silica adhesion on toner sur ces studied by scanning force microscopy, IS T s NIP18 International Conference on Digital Printing Technologies, 2002, 651. 

PFM experiments performed with surface-modified tips allow steady imaging of polymer blends with respect to the pull-off forces. Comparing non-modified, hydrophilic tips with modified hydrophobic tips reveals the inversion of the difference between the adhesive forces, indicating even an inversion of the strength of interactions between hydrophilic and hydrophobic polymer surfaces and the SFM tip or silica particles. It could also be shown that the hardness of the silane layer influences the measured pull-off forces. The harder HMDS modification leads to lower adhesive forces, like the softer PDMS modification, confirming the results obtained for toner-silica particles. 

Keywords toner, silica, SFM, material contrast, adhesive properties... 

The focus of this study is on characterization of the toner surface morphology and description of the morphology of toner-silica interfaces. To determine the adhesion forces at toner-silica interfaces the distribution of silica particles on toner surfaces has been analyzed with respect to the toner composition and the chemical properties of the silica particles. 

However, PDMS-modified silica aggregates seem to coat nearly the entire toner particle. Their topographical diameter is doubled compared to the HMDS-coated particles, but the diameter estimated from the phase image is very similar to the one foimd for the HMDS-modified ones. The maximum phase shift is reduced compared to the samples above. This indicates that the silica aggregates are covered by a soft material - the silylation layer. This soft material covers not only the particles but also the resin surface in between them. This can be seen by the decreased phase shift in between the silica particles compared to the pure resin. As in sample (Fig. 5b), probably the PDMS silylation layer interacts with the toner resin surface, increasing the overall adhesion. 

Comparing PDMS/HMDS- and HMDS-modified silica particles, a lower phase shift of up to 70° has been observed for the PDMS/HMDS silicas, and for the HMDS silicas a higher phase shift of up to 90° has been found. Differences in phase shift values indicate the impact of surface modification on the local hardness of the silica particles. PDMS modification leads to a softer, polymer-like grafting, whereas pure HMDS modification oidy increases the hydrophobicity by a hard monolayer formation of trimethylsiloxy groups. HMDS-treated silicas seemed to show a weaker interaction with the toner resin surfaces. In contrast, PDMS/HMDS-treated silicas show stronger adhesion to the toner resin surfeces, so they can easily be imaged at high resolution. 

Surface modification by PDMS cause a polymeric surface layer on the silica particles leading to higher adhesion due to softening of the toner surface and increase in contact area. Chemically prepared toner shows higher interaction than the other resins. 

Dispersing particles directly on a support like sticky tape will provide the surface of silica-loaded toner without additional external changes. To avoid displacement of the particles during SFM imaging, they have to adhere properly to the supporting sur ce. One possibility is to disperse them by air flow on sticky tapes (Tesa, double-sided, Beiersdorf). Because the adhesive polymer matrix of the tape covers the particles after one day of storage, images have to be taken only on fleshly prepared samples. 

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