Adhesion Phenomena between Solid Particles and Polymers
Promotion date: 26 May 2002
What was your PhD-research about?
Adhesion of small particles, of 10 to 20 micrometers. The research was financed by the Dutch Polymer Institute and conducted in cooperation with Oce-Technologies in Venlo. This company has an interest in obtaining knowledge about the adhesion between toner particles and the transportation belts in copy machines at a microscopic scale. Until now they have tried to enhance the technique using macroscopic observations, because a satisfactory theoretical foundation and experiments on the submicron level lacked. The toner particles have an irregular shape. In addition, the particles consist of a mix of different kinds of materials, among others polymers and iron, and therefore it is difficult to describe their behavior theoretically. Statistical methods are recommended to be able to predict the particle's behavior.
We focussed on the investigation of adhesion phenomena at a (sub-)microscopic length scale, and particularly on the influence of the individual variables. So we varied the individual parameters in experiments, while controlling the others, in order to make a good theoretical prediction of quantitative contributions to adhesion. The variables that play a role in the adhesion between a particle and a substrate are the humidity, the temperature, the surface energies of both particle and substrate and the properties of the substrate, of which the roughness of the surface probably is the most important one in the case of hard materials.
The humidity we tried to adapt in a controlled manner. Until at a certain moment the professor got red-faced because he could hardly see his Atomic Force Microscope (AFM) through a haze of humidity.
Furthermore, we experimented with the roughness of the particle and the substrate by etching silicon in a cleanroom. This provided us a series of model substrates. The same apparatus we used for adhesion experiments, an AFM, we used for the characterization of the particle's roughness. The adhesion between the particle and a substrate can be measured by landing on it with the particle glued onto the end of an AFM cantilever. This is an AFM technique used in an inverted way, a very easy approach.
Another question we had to try to answer was of what material the outer shell of a toner particle consists. It is known that it mainly contains a polymer and iron, but which one of those is really present at the outside. It appeared that the polymer forms the outer shell, while the iron provides some rigidity. If there happens to exist some roughness on the outside of the particle, the iron makes it sturdy and that it continues to exist. And because the particles are mainly ruggedly shaped on a submicroscopic scale; this makes the contact area to decrease and therefore it will be harder for the particles to stick onto the substrate. This effect also adds to the unpredictability of the adhesion forces.
The surface tension we measured using an inverse gas chromatograph (IGC). A gas containing known molecular species, probes, was blown into the chromatograph containing a column packed with the toner material of interest. From the difference in retention volume of probe molecules and an inert probe entering and leaving, combined with the known properties of the probe molecules, the surface tension can be derived.
We also varied the temperature of the substrate to investigate the temperature dependence of particle adhesion. Some people raised the temperature extremely, until they realized that the piezocrystal, that positions the substrate with respect to the tip or particle at the end of the AFM cantilever, might not like these kinds of temperatures.
Has it been a theoretical oriented, or a more applied investigation?
I always kept the application in mind, but with the idea of developing further the theory behind it. I looked for example at the adhesion between toner particles and the PDMS-rubbers that transport the particles from the photoconducting plate to the substrate. These rubbers happen to be very sticky for the particles, so that transportation is made possible in the first place. But why is this material that sticky for the toner particles? It has a very low surface tension, which is a property that normally is not related to sticky materials. It turned out to depend on the amount of deformation. Because the rubbers are deformed at a certain velocity and to a certain extent, a lot of energy gets dissipated. Because a particle gets heated during transportation it also suffers a deformation, with corresponding energy dissipation. .In the end we found that an existing theory for an elastic, compliant particle on a rigid material can also be used in this case of a rigid particle on an elastic, compliant surface.
We also varied the speed of the process in a controlled manner by removing the tip or particles of interest at the end of the AFM cantilever off the surface. For being able to do this, we had to adapt the AFM, but fortunately my predecessor already had done this. The conclusion of this was that the particle sticks better onto the surface, if you add an extra time step, increase the loading, or increase the velocity.
How did you personally experience the period of your PhD-investigation?
Of course you start very enthusiastically with the project, but after a while things don't go your way. At first sight the investigation does not yield what you expect. What you thought to be important appears not to be. And after that you encounter parameters of which you find out that they do matter. Most of the times it is nonsense if you expect already to precisely know in what direction your investigation will go. That is exactly conducting research.
After a while I discovered that I lacked equipment. Luckily someone else was doing similar investigations, but on a larger scale. He needed the same equipment and developed it. Fortunately I could use that. If I had had to develop it all by myself, it would have costed me some 2 years.
The IGC we did develop ourselves. It is a known technique, but I automated it. I did an CTI-subject, so it was a good opportunity to use that knowledge. You will always encounter that you'll have to make a certain step to computer science, in particular towards automation.
How did you like conducting the investigation?
The atmosphere in our group was rather good. Also de MESA+-days and lectures I liked a lot. For the future it might be a good idea to bring the real estate, the buildings closer together, because you can really see that faculty of applied chemistry is a bit independent of the other groups.
What plans for the future do you have?
Conducting research is good, provided that you have a guarantee of progress. No progress is frustrating. In this moment I can imagine myself moving in the direction of coatings or adhesives. In this case you are dealing with physical behavior on surfaces. There is an aspect of materials science, but also an application. That is what has my interest.