Mid Sweden University

The objective with this work is to compare the adhesive behavior for latex polymers of different Glass transition Temperatures (T_g) at different length scales and with different contact times. This is accomplished by two techniques: AFM colloidal probe force measurements and JKR-measurements. The aim is to compare the results from these two techniques and relate them to the interaction of the latex polymers towards oxidized silicon wafers and silica/cellulose probes. Theory suggests that altering the short timeframes used in the colloidal probe technique does not affect the ranking of the adhesion for the different polymers, but for the macroscopic JKR-technique it influences the measured work of adhesion. It is therefore important to let the system reach a steady state before assuming complete spreading and adhesion. AFM and JKR measurements showed the same trends where the polymer with lowest content of styrene has the lowest T_g and the highest adhesion, due to the larger polymer chain mobility.

The spreading of single styrene-acrylic latex particles on silicon oxide surfaces was studied using atomic force microscopy (AFM). Three latexes with different glass transition temperature (Tg) were used and the effects of temperature, time and preparation method were investigated. Particle sizes and shape were measured with AFM and the contact angles were calculated. The observed rate for the spreading of latex particles was low and it took several days before the particles reached steady state, even at temperatures well above their Tg. The experimental particle spreading results deviated with two orders of magnitude from predictions using the WLF equation for polymer diffusion. The deviation could be attributed to polymer-surface interactions that slowed down the particle spreading. The work of adhesion was calculated using two models. The results from using the regular Young-Dupré equation and a modified version of this equation that also included the mechanical properties (E-modulus and Poisson�s ratio) of the latexes, were compared. For soft latex particles the results from the two models agreed well and were of the order of 75 J/m2, but for glassy latexes the Young-Dupré equation underestimated the work of adhesion.

Time-dependent changes in the surface properties of coated papers were studied as the evolution of surface hydrophobicity of laboratory and commercially coated papers. We measured the apparent contact angles on the papers during several weeks post-production. Hydrophobicity upon aging increased for all samples made from traditional coating colors on mechanical base stock and on base stock made from cotton linters. Accelerated aging by heat treatment intensified the increase of the apparent contact angles and accelerated the time-dependent behavior. A key mechanism in these changes may be the spreading of latex particles in a coating.