4
6
2
0
0
0.2
CTA, (phm)
more passages for surfactant migration in a pigmented
paint system than in the neat latex films. The polymers
included in Figure 2 have the same Tg; equal contribution of polymer bulk property to block resistance can
be expected. Since block resistance has both bulk and
surface components, the data thus suggests that good
film formation facilitated by low MFFT (< 0°C) resulted
in similar surface morphology after one day drying at
room temperature, in spite of different particle sizes.
Gel Content and Molecular Weight
Polymer composition, gel content, and molecular
weight have profound impact on end-use performance.
Gel content is the insoluble fraction of the dried polymer in a solvent, typically tetrahydrofuran (THF). In
addition to chemical crosslinking, chain transfer to
polymer can also introduce high gel content. In emulsion polymerization of butyl acrylate, gel is produced
by both intermolecular and intromolecular transfer to
polymer. 22-24
In the presence of a chain transfer agent (CTA),
transfer of propagating radicals to the CTA dominates
all other chain transfer processes. The degree of branching and, consequently, the amount of gel material is
suppressed.25 This is also true for the acrylic polymers
in this study. The gel content of the acrylic polymer
without CTA was approximately 65%. When CTA was
present at 0.2 phm, the gel content was reduced to 1%,
indicating the resulting polymer was substantially free
of insoluble gel.
The amount of gel indicative of degree of branching
has important implications for particle coalescence during film formation and polymer adhesion or interdiffusion in the block test. Using fluorescence resonance
energy transfer (FRET) technique, Winnik et al. was able
to prove that long-chain branching alters polymer
1d-RT 1d-ET
Surfactant A = 1. 8 phm
8
Block Rating
4
6
2
0
- 10.0
- 4. 5
Fox Tg, (ºC)
4.0
diffusion rates in the film formation of poly(butyl
acrylate-co-methyl methacrylate) latex. 26 Figure 3
compares the room temperature block performance
of the polymers prepared with and without CTA. The
polymer with high gel content exhibits less blocking.
Even though the polymer matrix with no branching
may allow greater surfactant mobility, it also enhances
film adhesion or polymer interdiffusion across the film
interface. The net result is that the polymer with low
gel content yielded lower block resistance, as shown in
Figure 3.
Glass Transition Temperature of Polymer
The glass transition temperature of polymer is a
characteristic property of the polymer. The Tg of the
acrylic polymers was varied from – 10 to 4°C by changing the ratio of butyl acrylate to methyl methacrylate.
This narrow range is chosen to ensure adequate film
formation of the low-VOC paint formulations. Figure
4 presents the one-day room and elevated temperature
block results. Increasing Tg did not effect significant
change in the polymer’s blocking behavior at room
temperature. However, the hot block performance is
improved considerably when the polymer Tg is increased to 4°C. This result suggests that polymer Tg
plays a more important role in block resistance at elevated temperature.
Increasing Tg by increasing MMA content in the latex
has several effects. First, it improves polymer cohesive
strength and decreases chain mobility, both of which
should improve block resistance. At the same time,
there is less free volume in the higher Tg polymer system. Surfactant migration is restricted, which lessens the
contribution of surface-concentrated surfactant to block
resistance. Secondly, increasing MMA reduces BA branching, which changes the diffusion rates of both polymer
