Using Polymer Modified Asphalt Emulsions in Surface Treatments A ...

Sabbagh and Lesser (1998) note that the phase stability of CRM modified asphalt
emulsions is governed in large part by both particle size and morphology. In unstable
modified asphalt emulsions, polymer particles tend to coalesce, gradually increasing in
size over time until they become sufficiently large for creaming to occur (22). Sabbagh
and Lesser have experimentally determined the critical particle transition radius
(between coalescence and creaming) to be approximately 4 µm at 110º C for polyolefin.
With respect to morphology, polymer particles in unstable asphalt emulsions are
predominately teardrop-shaped, whereas those in stabilized asphalt emulsions are
characterized by either spherical and/or long-cylindrical shapes. The irregular, nonspherical
shaped polymer particles which characterize unstable modified asphalt
emulsions are commonly observed under high shear mixing conditions. Additionally,
the use of steric stabilizing copolymers has been shown to promote more
thermodynamically stable spherical polymer particle shapes (22). Sabbagh and Lesser
have noted that while polymer particle sizes in stabilized asphalt emulsions are
generally larger than those in unstable asphalt emulsions, the former are not more
susceptible to creaming. This is believed to be due to the increased density of the
particles in stabilized asphalt emulsions created by the use of steric stabilizers (22).
Thus, stabilized asphalt emulsions are those which are characteristically stable with
respect to both creaming and coalescence. Paradoxically, Sabbagh and Lesser have
shown comparable increases in fracture toughness and improved high-temperature
viscoelastic behavior with increasing polymer content for both stable and unstable
asphalt emulsions. This suggests that actual field performance is relatively insensitive to
initial polymer particle morphology.
CRM can be added as a dry ingredient to slurry mixes to avoid problems of phase
separation, but in such cases it serves primarily as a filler material. When used as filler,
CRM fails to form a pervasive matrix or network, and thus does not impart the cohesive
and viscoelastic benefits associated with most other forms of polymer modification.
One solution to the phase separation problems associated with CRM modifiers involves
the use of solvents to partially predigest the rubber particles prior to their introduction
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