This article examines the influences of some polymer design parameters on block resistance of a low-VOC acrylic paint. The effects of surfactant choice, latex
particle size, gel content, and glass transition temperature (Tg) of the acrylic polymer are discussed.
Table 1—Low-VOC Acrylic Paint Formulation
Ingredient
Pounds
Gallons
EXPERIMENTAL
Acrylic Latexes
The acrylic latexes used in this study were prepared
by semi-continuous emulsion polymerization of an
emulsified monomer mixture containing butyl acrylate,
methyl methacrylate, methacrylic acid, and a wet adhesion monomer. Using the Fox equation, the Tg was varied
from – 10°C to 4°C by adjusting the ratio of butyl acrylate to methyl methacrylate. The latex particle size was
controlled by the amount of a seed latex or initial surfactant for particle generation. The particle size ranged from
0.096 to 0.22 micron, determined by Nanotrac UPA150.
Two anionic surfactants were employed in emulsion
polymerization at a level from 0.08 to 1. 8 parts per
hundred monomer (phm). The critical micelle concentrations (cmc) of surfactants A and B are 0.07 and 0.03
wt%, respectively. The surface tension at cmc is 29 and
33 dyne/cm, respectively.
Grind
Water ....................................... 231.1
Cellosize™ HEC ER 4400 ............... 5.0
Propylene glycol ........................ 10.0
Colloid 226-35 ........................... 7.0
KTPP ....................................... 1. 5
Triton™ CF- 10 Surfactant ............ 2. 5
Rhodaline 643 ........................... 1.0
Ammonium hydroxide, 28% ......... 1.0
TiPure R-706 .............................. 225.0
Polygloss 90 .............................. 25.0
27.70
0.40
1. 20
0.70
0.10
0.30
0.10
0.10
6.80
1. 20
Letdown
Acrylic polymer, 50% TS............... 425.0
Water ....................................... 97.0
RM 2020.................................... 10.0
Rhodoline 643 ........................... 1. 5
Ammonium hydroxide, 28% ......... 2.0
48.02
11.63
1. 14
0.21
0.26
Totals.......................................... 1044.6
99.86
Paint Formulations
Weight solids, ........................ 45. 8
Volume solids, ........................ 30. 3
PVC, .................................... 26. 2
VOC, g/L.................................... 37
A semigloss paint formulation with 26% pigment
volume concentration (PVC) and 30% volume solids
(VS) was used to evaluate block resistance of various
acrylic latexes. Table 1 shows the ingredients for the
low-VOC acrylic paint ( 37 grams per liter).
Table 2—ASTM Block Ratings
Block Resistance Test
The block performance indicating face-to-face adhesion of two paint films pressed together was rated
on the scale of 0 to 10 as defined by ASTM D 4946-89
( Table 2). The test paints were prepared on the Leneta
3B opacity charts using a 3-mil bird drawdown bar.
The films were dried in the constant temperature and
humidity (CTCH) environmental chamber for one day.
For room temperature (RT) block, two square strips of
2.54 cm x 2.54 cm paint films were pressed together
by a 454-gram weight. After 24 hr, the strips were separated and a numerical value was given according to
the scale in Table 2. For the elevated temperature (ET)
block test, the paint strips after one-day drying at CT/
CH were placed in a 120ºF oven under 1000-gram
weight for 30 min. The weight was transferred to the
paint films via a one-inch diameter rubber stopper,
generating approximately 2. 2 psi on the film strips. The
films were then allowed to cool for 30 minutes before
Block
Resistance
Numerical
Ratings
Type of Separation
Performance
10
no tack
perfect
9
trace tack
excellent
8
very slight tack
very good
7
very slight to slight tack good to very good
6
slight tack
good
5
m oderate tack
fair
4
very tacky; no seal
poor to fair
3
5–25% seal
poor
2
25 –50% seal
poor
1
50 –75% seal
very poor
0
75–100% seal
