the ratings of film separation were given. The test was
run in triplicate and the average value was reported.
RESULTS AND DISCUSSIONS
Surfactant Concentration
The concentration of surfactant A was varied from
0.8 to 1. 8 phm. Figure 1 shows that the one-day room
temperature block resistance of the paints increases
with increasing surfactant concentration. At 0.8 phm
surfactant concentration, the acrylic polymer with a
low Tg (= – 10°C) received a poor block rating of 2.
When surfactant concentration was increased to 1. 8
phm, the polymer of identical composition yielded significantly better block resistance. The numeric value of
7 represents reasonably good block performance for a
polymer with MFFT < 0°C.
The data in Figure 1 suggests that surfactant enrichment at the air-film interface potentially contributed
to the improved block resistance at higher surfactant
concentrations. Exudation of surfactant to the film
surface during latex film formation has been a topic of
numerous publications. 16-19 Surfactants move with the
evaporating water toward the film surface as particle coalescence proceeds. Surfactant migration is also evident
in the latex systems studied here. The atomic force micrography (AFM) image shows long, thin lamellar structure covering the latex film surface. X-ray photoelectron
spectroscopy (XPS) of film surface and cross-section confirmed that surfactant concentration is higher on surface
than in bulk, consistent with a previous publication. 16
The surfactants that bloomed to the top of film
surface function as a barrier layer that interferes with
polymer interdiffusion across the interface. This effect
is similar to the benefit of fluoroadditives reported in
the literature. 13 Adhesion between the two contacting
Figure 1—One-day room temperature block vs. surfactant
concentration.
10
Tg = - 10°C
8
1d-RT block
4
6
2
0
0.0
0.5
1.0
Surfactant A, (phm)
1. 5
films is reduced and block resistance is improved.
The elevated temperature block or hot block was poor.
This is not surprising given the low Tg of the polymers.
Soft binders deform readily at elevated temperature, resulting in increased contact area. High temperature and
high pressure promotes polymer diffusion across the
interface due to greater contact area and polymer chain
mobility. Consequently, blocking is more severe at the
elevated temperature.
Latex Particle Size
Particle size plays an important role in the latex film
formation. Small particles form films with less void
volume because of higher packing efficiency. 20 In a film
formation study of poly(styrene-co-n-butyl acrylate)
latex, Niu and Urban found that surfactant exudation
to film surface is only detected in larger particle size
latex. 21 They concluded that a tighter film formed by
smaller particles has less space for surfactant molecules
to migrate, minimizing preferential location of surfactant on latex film surface. The beneficial effect of block
improvement, therefore, may be diminished.
In this experiment, the total amount of surfactant
used was kept constant at 1. 8 phm. The latex particle
size control was accomplished by adjusting the seed
latex usage or initial surfactant concentration during
latex preparation. Similar block resistance was obtained
for the paints based on latexes of different particle size
ranging from 0.096 to 0.22 micron (Figure 2). Particle
size did not seem to affect the block resistance of the
low-VOC acrylic paints. This is probably because surfactant migration during the film formation of a formulated paint differs somewhat from the process of neat
latex particles. Previous studies have focused extensively
on coalescence of pure latex systems and the factors affecting surfactant distribution during and after the film
formation process. One can imagine that there may be
Figure 2—One-day room temperature block vs. latex particle size.
10
8
1d-RT Block
4
6
Tg = - 10°C
Surfactant A = 1. 8 phm
2
0
0.00
0.05
0.10 0.15
Particle Size, (micron)
0.20
