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

lock copolymer (15). Typical examples of block copolymer modifiers include SBS, SIS,
SB, ABS, and RET - with the most commonly used among these being SBS (a triblock)
owing to its desirable properties and comparatively low cost (16) (17). The elasticity
and strength benefits imparted by SBS modifiers are attributable to the molecule’s
rubbery polybutadiene (PB) “mid-blocks” capped at either end by polystyrene endblocks
which provide strength and rigidity (16). . Most block copolymer modifiers
behave as thermoplastic elastomers, returning to their original shape upon removal of
the loading stress.
Block copolymers are lower in molecular weight than typical formulations of SBR latex,
and generally consist of a comparatively narrow distribution of similar monomer chain
lengths. Whereas SBR latex chains are polymerically random in form, block
copolymers such as SBS and SB can exhibit a wide variety of regular and well-defined
molecular morphologies including linear, star-shaped, and radial structures (4).
When triblock copolymers such as SBS and SEBS are raised above the glass transition
temperature of their polystyrene end-blocks, these rigid domains soften, thereby
weakening the crosslinked structure of the polymer. At temperatures above 150º C,
block copolymers are pliable in molten form in contrast to NRL modifiers which begin to
undergo crosslinking at this temperature (3). Work by Wegan (2001) suggests optimal
mixing temperatures of approximately 180º C. for SBS modifiers (16). Because block
copolymers are workable at higher temperatures, the styrene domains comprising the
typical SBS modifier can readily be segregated under shear force during the milling
process, promoting the dispersion of individual chains throughout the asphalt binder.
Consequently, as the mixture is cooled and begins to break, these styrene domains
begin to reform, establishing a pervasive polymer network throughout the residual
asphalt matrix (3).
Stroup-Gardiner and Newcomb (1995) report that sufficient quantities of SBS polymer
modifiers are required to promote effective crosslinking during the cooling phase to
ensure that reactive portions of the styrene domains are close enough together to
permit bonding. Termed the “critical concentration” or “c*”, Stroup-Gardiner and
Newcomb recommend SBS contents of at least 2%, and in some cases > 4% by weight
17