Blends of hard (Tg similar to 60 degrees C) and soft (T-g similar to 0 degrees C) latexes were studied as a function of particle size ratio (R(soft)/R(hard)) and blend ratio (mass soft phase/mass hard phase). Addition of hard phase latex to the soft film forming latex significantly improved block resistance, even at blend ratios as low as 70/30. Film properties were not sacrificed, except at high concentrations of the hard phase (50/50). For a given blend ratio, the particle size ratio had a dramatic effect on the block resistance. For a 70/30 blend ratio, the block resistance of a blend with R(soft)/R(hard) = 4.0 was equivalent to that of a control latex having the same overall composition, but with a minimum film temperature 20 degrees C higher than the blend. The phenomenon can be explained in terms of the bulk and surface contributions to adhesion. The hard phase increases the elastic modulus (G') of the film. The magnitude of G' was found to increase with increasing R(soft)/R(hard), an effect that is consistent with percolation theory. The effect of particle size ratio on the surface contribution to adhesion can be explained by particle packing. Visual models indicate that a high apparent surface concentration of hard particles would be expected for a large value of R(soft)/R(hard), given ideal packing conditions. This effect was confirmed by scanning electron microscopy.
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