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

In test mixes where 7% EVA was pre-blended with the asphalt binder, Wegan reports
observing the formation of a partial, yet distinct polymer network structure. Test
samples containing 18% pre-blended EVA exhibited an even greater degree of polymer
network formation. These results appear to suggest that in contrast to the findings of
the BASF and Forbes studies, modified asphalt binders produced by pre-blending may
produce a cross-linked network structure, providing that the polymer content is
sufficiently high. However, no information is provided by Wegan with respect to
comparing the performance of high polymer content pre-blended binders to
conventionally modified lower content mixtures, or whether the increased materials cost
of this form of pre-blend justifies its use.
Hussein (2005) has examined the impact of polymer-asphalt blending time on PMA
performance for varying molecular weight LDPE and EVA additives. Figure 14
summarizes the change in complex shear modulus for various modified and neat
asphalts relative to mixing time. Polymer modified mixes exhibit significant and welldefined
increases in complex shear modulus (G*) as mixing time is lengthened, until a
critical point is reached where upon these improvements begin to stabilize. For
example, the steady-state points for 8% LDPE1, 8% EVA1 and 8% EVA2 are
approximately 30, 15, and 20 minutes, respectively. In contrast, neat asphalt exhibits a
virtually flat-line G* response over the same period. Hussein proposes that the point
which represents stabilization in the magnitude of G*, is indicative of the optimal
blending time for that polymer-modified mixture. Results indicate that the optimal
blending time for EVA-modified binders was generally less than for LDPE-modified
mixtures, owing in part, to the lower weight-average molecular weights of the former
(35). Hussein also found that binders containing low vinyl acetate content EVA
additives exhibited the best high temperature susceptibility and long-term storage
stability of the mixtures tested. However, little if any benefit was identified for these
polymer additives at low temperatures.
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