M. Hulden, HYDROPHOBICALLY-MODIFIED URETHANE ETHOXYLATE (HEUR) ASSOCIATIVE THICKENERS .2. INTERACTION WITH LATEX, Colloids and surfaces. A, Physicochemical and engineering aspects, 88(2-3), 1994, pp. 207-221
The rheological behaviour of model latexes thickened with hydrophobica
lly modified ethylene oxide-urethane block copolymers, i.e. HEUR type
associative thickeners, is described. The latex particle size, as well
as the type and level of surfactant used, greatly affect the thickeni
ng efficiency of the HEUR. Measurements of the electrophoretic mobilit
y of the latex particles at different HEUR concentrations indicate ads
orption of the thickener in the absence of surfactant and in the prese
nce of nonylphenol ethoxylate, NPE10. However, the HEUR thickeners do
not adsorb to the latex particles in the presence of sodium dodecyl su
lphate (SDS) at levels above the CMC of the surfactant. This can be ex
plained by SDS-HEUR interactions in the solution. Both adsorption and
rheological measurements support a latex-HEUR association mechanism, a
ccording to which the particles are incorporated through adsorption of
their hydrophobic end-groups into a three-dimensional transient thick
ener network. At a given volume fraction, latexes with a small particl
e size present a larger number of potential crosslink points at shorte
r distances from each other. This leads to a synergistic increase in v
iscosity on addition of HEUR to the latex. A HEUR-thickened latex that
exhibits such a synergistic viscosity increase is more shear thinning
and elastic than an aqueous HEUR solution of comparable low-shear-rat
e viscosity. This may be explained by the assumption that the latex pa
rticles introduce stronger crosslink points in the network in addition
to the micelle-like hydrophobic aggregates which act as crosslink poi
nts in aqueous solutions of HEUR. Variations in the chemistry of the H
EUR affect both the level of the low-shear-rate viscosity and the shea
r thinning behaviour of the thickened latex. Octadecyl-terminated HEUR
s produce latex systems that are more shear thinning and elastic than
HEURs with shorter hydrophobic modifications (i.e. with pentadecyl or
9-heptadecenyl terminations). However, the latter are less efficient i
n increasing the viscosity at low and medium shear rates. The effect o
f the molecular weight of the HEUR is mainly to increase the length of
the network junctions between the crosslink points. As a result the g
reatest thickener efficiency is obtained with medium molecular weights
. The effects on the rheology of thickened latex obtained by varying t
he HEUR composition parallel the effects seen in aqueous solution. Thi
s does not mean that the solution properties dominate in the rheologic
al behaviour, but rather that both types of crosslinks present in the
HEUR-latex network are affected in similar ways by variations in the H
EUR chemistry. This is reasonable, as both crosslinks are a product of
hydrophobic interactions.