Microparticle Adhesion In In Xerography
In this Thesis, IGC and AFM-based adhesion measurements were combined to obtain insight into the technologically relevant xerographic toner particle adhesion to surfaces. New concepts for the quantitative characterization of the adhesion of composite toner microparticles were applied. The toner particles were chosen based on their relevance for the xerographic process developed by Océ-Technologies B.V. (Venlo, The Netherlands). The concepts were applied to investigate the parameters, which were considered most relevant for the particular xerographic process and for the corresponding machine's lifetime performance. Atomic force microscopy (AFM) was used to measure the in situ force an individual particle 'feels' when it is gently brought into contact with model substrates, like hard silicon (as a model surface for the hard photoconductive plate), and poly(dimethylsiloxane) (PDMS) (a commercially applied transfer medium in the copier). Three different kinds of model toner particles were used: (i) particles made of pure polyester; (ii) composite particles consisting of the polyester and filled with magnetic iron domains; (iii) particles as described under (ii), but covered by a shell of carbon particles. The model materials included custom-made, and spherically shaped toner particles. The spherical particles were chosen in order to exclude the influence of a non-spherical geometry, and to enable quantitative comparison with adhesion theories. The difference between operating temperature and the particle's glass transition temperature, was concluded to be the main control variable for toner transfer in the second part of the toner pathway, i.e. the transfer of the toner to the paper copy. The surface roughness of both the particle and the substrate was concluded to be the main source of discrepancy between experimental removal force values and theoretical ones. The reinforcing effect of the iron domains, preventing plastic deformation of the particle surface asperities, was proposed to cause the difference found between the adhesion of pure and iron-filled polyester spheres on hard substrates. Surface energy values used in the adhesion force calculations of toner particles were obtained with a custom-developed chromatograph, using the inverse gas chromatography (IGC) technique. The toner particles of interest were packed in gas chromatographic columns. The surface energy values of the particles were investigated by the injection of pure and known molecular probes such as n-alkanes, starting from sub-ambient temperatures up to 40 °C. The surface energy of the main component of the composite toner particle, a polyester, turned out to be the dominant, adhesion determining, factor of the chemically heterogeneous toner surface. Chapter One introduces the applied and fundamental questions regarding the materials and processes under investigation. The fundamental physicochemical parameters relevant to the materials and xerographic process were identified. The materials conditions for the toner particles were given and the custom-made particles, which were used as 'model' particles were introduced. AFM, introduced in this Chapter, proved to be an important technique for determining particle adhesion. In the so-called AFM pull-off force experiment the force necessary to remove a particle that is attached to an AFM cantilever spring can be determined. The IGC technique, which was chosen for surface energy studies of powder particles, was also introduced in this Chapter. Chapter Two describes the theory of adhesion measurements by AFM. This technique can be employed to investigate both surface topology and properties such as in situ interaction forces between the probe and the local substrate surface. The AFM pull-off experiments provided adhesion values for individual particles and not an average value as obtained with other techniques. Special attention was paid to the characterization of roughness and its effect on adhesion. In addition, a short literature overview was presented. In Chapter Three the device and method employed to attach microparticles to AFM cantilevers are described. The optical interference signals distorting the AFM force-displacement curves were significantly reduced by replacing the commercial laser. The nonlinear behavior, which is observed for large cantilever bending using the optical lever technique, was accounted for by a calibration procedure. A conversion algorithm for the photodetector signal to force values was proposed using 'dynamic sensitivity' values. Chapter Four considers the effect of surface roughness on adhesion force values. The roughnesses of both the substrate and the particle probe were characterized by AFM. Surface roughness was found to be the dominant factor for the mismatch between experimental values and theoretical predictions. Roughened silicon model substrates were used in pull-off force experiments with polyester spheres and iron-containing toner particles. The values measured were compared with the predicted values obtained from a new particle adhesion model, proposed by Rabinovich et al. This model includes the topology of the substrate, which was obtained from AFM roughness analyses. The surface energy of the particles, which is also included in the model, was determined by IGC experiments in Chapter Eight. Good agreement was found between theory and experiment for silica and polyester spheres on silicon. In contrast, no agreement was found for the iron-containing toner spheres. This disagreement was ascribed to the non-deformable iron domains that form hard asperities at the particle's surface. The contact mechanics of toner particles and AFM cantilever tips brought into repeated contact with PDMS networks are discussed in Chapter Five. From different time and velocity-dependent pull-off force experiments it was concluded that the surface of elastic PDMS networks (G"?0) behaves viscoelastically. At low speeds the adhesion is independent of the speed and corresponds to the JKR static approximation. In the high-velocity regime adhesion and velocity could be related by a power law. The increase in contact time independently contributed to an increase of the pull-off forces. In addition, an increase in loading force on the probe on the PDMS substrate contributed to a higher practical adhesion. The results are in agreement with recent theoretical predictions by Attard for compliant spheres on hard, flat substrates. In addition, energy dissipation, due to viscous behavior at the surface, could be increased tremendously by adding unbound, non-reactive, PDMS chains (> 5 wt%) to the bulk prior to cross-linking. In Chapter Six the influence of temperature and relative humidity (RH) on adhesion in toner-silicon and toner-PDMS systems are discussed. Above 50 °C the pull-off forces were undetectable with the applied configuration, whereas below 50 °C an Arrhenius behavior was demonstrated for the toner adhesion on silicon. Pull-off forces in the toner-silicon system at room temperature increased, from a constant level of 100 nN at an RH value of up to 70%, to 450 nN at an RH of 85%. In frequency and loading force dependent pull-off force measurements between toner and PDMS, a maximum variation in the pull-off force of only 5% was observed upon varying RH between 33% and 95%. The theory behind IGC is summarized in Chapter Seven. The equations recommended in the literature for the derivation of the (global) surface energy value, the specific and the non-specific value, were discussed, and the corresponding theoretical assumptions and limitations were pointed out. The determination of adsorption isotherms of the probe molecules on the stationary phase and the construction of the energy site distribution function were explained. The instrumental design and the automation of the setup have also been included. Using the instrumentation presented in Chapter Seven the surface energy properties of different toner particles are investigated and discussed in Chapter Eight. The values obtained were comparable to the values obtained from sessile drop experiments. The dispersive component of the surface energy did not vary significantly between the three different kinds of toner particles. Several indications for chemical surface heterogeneity were found: (i) the difference between the dispersive surface energy determined by IGC and the relatively low total surface energy as determined by droplet analysis; (ii) the strong tailing behavior of the peaks of the injected polar probes; (iii) the decrease of the net retention volume with increasing n-alkane concentration; and (iv) the broad energy site distribution function as determined by finite concentration IGC for n-pentane and n-heptane as molecular probes for iron-filled toner and pure polyester toner, respectively. In Chapter Nine the results and conclusions reached in this Thesis are combined and extended with respect to their practical relevance for xerography. The reported observations in the previous Chapters were compared to practical values and phenomena encountered in the xerographic process. The limitations of the AFM data obtained were stressed with respect to the complexity of the practical process conditions and particle properties. The Chapter ends with an outlook of the future research in this active field of materials surface science and technology.