Developments in Waterborne Acrylic Coatings for Concrete Highway Barriers

16 Pages·2007·34.75 MB·English

by  Jerry Fabich

D e v e l o p m e n t s i n
Waterborne Acrylic Coatings
for Concrete Highway Barriers
Anne M. Bacho and Leo J. Procopio
Rohm and Haas Company
Spring House, PA
environmental regulations on paint and
coatings manufacture and application is
perhaps most obvious in the lowering of
volatile organic compound (VOC) limits.
Waterborne coatings are one way in
which coating manufacturers are trying
to meet the strict limits. Wat e r b o r n e
acrylic coatings also offer the advantages
of lower toxicity, lower odor, lower or no
risk of fla m m a b i l i t y , easy and safe clean-
up, and less hazardous disposal relative
to solvent-borne coatings. And water-
istockphoto
borne acrylics have also shown excellent
pe r f ormance in real world scenarios and
T he durability of concrete is in the aggregates to form an alkali-silica are being used increasingly in light- and
an important concern for gel. As this gel swells, causing an increase medium-duty industrial maintenance
owners of concrete highway in pressure and expansion and cracking ap p l i c a t i o n s . 2
barriers and dividers (some- of the aggregate, the concrete cracks and In this article, we describe the develop-
times known as Jersey barri- shows signs of spalling.1Such deteriora- ment of a new waterborne elastomeric
ers). Concrete can provide tion ultimately adds costs for mainte- acrylic resin designed specifically to sur-
good performance through- nance and replacement of the concrete vive the thermal rigors a coating for high-
out the service life of the barrier. Good barriers. To prevent this deterioration, way barriers must endure. The article
concrete can have an extremely long life many DOTs in the United States specify a begins by explaining how an elastomeric
span under the right conditions. pigmented coating for concrete barriers waterborne acrylic resin became the foc u s
Wat e r , although important for con- on their Qualified Products List (QPL). of the study. The article then describes
crete hydration and hardening, can also The specified systems include both testing of a coating based on the new resin
play a role in decreasing the durability smooth and textured finishes, and range against a variety of commercially avail-
of concrete. Water not consumed in the in technology from solvent-borne (includ- able coatings for highway barriers.
hydration reaction will remain in the ing coatings based on epoxies, acrylics
microstructure pore spaces and create and vinyltoluene resins) to waterborne Direction of the Study
tiny capillaries. In areas of the country (including acrylics). The study described in this article was
that are subjected to freezing and thaw- With increasingly stringent health, initiated with the goal of developing a
ing, water containing de-icing chemicals sa f e t y , and environmental regulations, waterborne acrylic coating that has good
is drawn into the concrete through waterborne coatings systems offer coat- pe r f ormance over concrete and that
these capillaries. This external source of ings users a safe and effective method for might be suitable for the particular appli-
alkali reacts with the amorphous silica protecting their structure. The impact of cation of concrete highway barriers. To
42 JPCL January 2007 www.paintsquare.com
Industrial Architectural Architectural Elastomeric Traffic Paint
Resin #1 Resin #1 Resin #2 Resin #1 Resin #1
Tg = 35 C Tg = 20 C Tg = 20 C Tg = - 35 C Tg = 25 C
determine the direction of the investiga- Figure 1: Examples from the initial screen - occur when water, trapped in the cement
ing of various waterborne acrylic technolo -
tion, several commercially available paste or absorbed by porous aggregate,
gies. Coatings on concrete were evaluated
waterborne resins were evaluated in after cyclic freeze/thaw testing. Testing was freezes. The resulting expansion and con-
coatings over concrete for resistance to done on films previously tested for adhesion. traction creates stress throughout the
spalling in the thermal cycling protocol. Photos of each block represent 20 cycles. All concrete. When the pressure from the ice
resins were formulated into the same mason-
The resins were chosen to span the vari- exceeds the strength of the concrete, the
ry coating formulation.
ous acrylic technologies currently used concrete weakens and fails. Freezing also
in architectural and industrial coatings, causes aggregate failure, especially close
and were formulated into the same fying the existing elastomeric water- to the exposed surfac e .
masonry coating formulation. borne acrylic to make it suitable for high- The ASTM C 672 method used con-
Initial resins tested included those way barrier service. Because of their low sists of applying the coating by brush or
designed for industrial maintenance, Tg and corresponding low minimum film airless spray to a 4” x 6” concrete block,
architectural, building products, and traf- formation temperature (MFFT), elas- and drying at ambient conditions for 1
fic/road marking coatings. The coatings tomeric acrylic resins can be for m u l a t e d week. Caulk is applied around the edge of
based on industrial maintenance, archi- at very low VOC levels, often less than the block to form a dam to hold a 1⁄”
4
tectural, and traffic paint binders proved 50 g/L. They can also be applied in fai r l y layer of water on the coated surfac e .
to be much too hard to survive the thick films (e.g., one coat at 20 mils of Water is ponded on the surface and
expansion and contractions of the con- dry film thickness) without mudcracking, frozen at -18 C (0 F) for 16 hrs. The block
crete during thermal cycling. The coat- which is very difficult to achieve with is removed from the freezer, and approx-
ings cracked, wrinkled, lost adhesion, higher Tg waterborne acrylic resins. imately 6 g of sodium chloride is sprin-
and in some cases almost completely Modifications to the elastomeric acrylic kled on top of the ice. The ice is allowed
delaminated. The best performing coat- binder have led to the development of a to melt for 4 hrs, and then the resulting
ing was based on a waterborne elas- new resin. A coating based on this exper- solution is rinsed off the surface with
tomeric acrylic resin. Although it still imental waterborne acrylic polymer was fresh water. The block is dried for 4 hrs
showed some minor blistering of the then made and compared to commercial and rated for blistering, wrinkling, and
film, there was no evidence of the film controls in testing on concrete sub- damage to the coating or concrete. The
cracking or splitting. Results of this early strates. The test method is described in cycle is then repeated. On weekends, the
screening can be seen in Fig. 1. the next section. samples are left in the freezer, so one
It appeared from this early screening week yields five cycles.
study that a flexible resin with a low Test Method Separate experiments were run with a
glass transition temperature (Tg) would Coatings were tested on concrete using modification to the ASTM method to
be especially suited to withstand the ASTM C 672, a test method for spalling allow for another factor of exterior
forces of expansion and contraction resistance of concrete surfaces exposed weathering – exposure to UV light. The
which the concrete substrate undergoes to deicing chemicals. Spalling, also called coatings were first subjected to a week of
during thermal cycling; hence, we scaling, is the loss of surface material in exposure in an accelerated weathering
focused our development work on modi- patches of varying size. Damage can cabinet that includes UV exposure, and
www.paintsquare.com JPCL January 2007 43
Number of Thermal Cycles
50 42 10 20 50
Experimental Commercial Commercial Commercial Commercial
WB Elastomeric Acrylic SB 2K Epoxy SB Vinyltoluene Acrylic (textur ed ) SB Acrylic WB Acrylic (higher Tg)
Figure 2: Comparison of experimental waterborne elastomeric coating and
commercial systems. Coatings on concrete were evaluated after cyclic
freeze/thaw testing (ASTM C 672). Testing was done on undamaged films.
Photos of each block were taken at the number of cycles listed.
with an undamaged in the testing on concrete included a sol-
coating was always vent-borne two-component epoxy, a sol-
run for comparison. vent-borne textured coating based on a
The concrete test vinyltoluene acrylic resin, a solvent-
substrates included borne acrylic, and a waterborne acrylic
standard concrete coating based on a higher Tg resin. All of
made according to the commercial systems included in this
ASTM C 192 stan- study are products which appear on
dards for a 4000 psi Qualified Product Lists (QPL) of various
mix and aged over United States transportation agencies for
28 days. Natural use over concrete highway barriers.
Fig. 3: Mode of failure of the textured, solvent-borne vinyltoluene acrylic
coating. Close-up of the flaking mode of failure observed for the textured, sand was used as Experimental waterborne acrylic binders
solvent-borne vinyltoluene acrylic coating after 10 cycles of the cyclic the fine aggregate were formulated into a white topcoat for -
freeze/thaw test (ASTM C 672). Close-up corresponds to the concrete and crushed stone mulation (36% PVC, 58% volume solids)
block pictured in Fig. 2.
(3⁄ in.) was used for using titanium dioxide and calcium car-
8
then to a week of the thermal cycling (5 the coarse aggregate. Concrete prepared bonate pigments.
cycles) as described above. This process according to ASTM C 192 and aged out- The commercial coatings were applied
continued until the coatings had experi- doors for one year was also used to sim- at their recommended film thicknesses.
enced 10 weeks of weathering exposure ulate the painting of weathered concrete. The solvent-borne acrylic and water-
and 50 freeze/thaw cycles. The weather- In addition, high water content concrete borne acrylic (based on higher Tg resin)
ing exposure consists of a repeating cycle was prepared using a 10% excess of were applied in two coats in order to
of 8 hrs exposure to UV-A (340 nm) light, wa t e r , and concrete with a high salt con- obtain the desired film thickness.
followed by 4 hrs condensation. tent concrete was prepared by adding
The thermal cycling tests were done as 0.2% sodium chloride to the concrete RE S U L TS AND DISCUSSION
described above with blocks having mix. For the high water and salt content Results of the cyclic freeze/thaw test
undamaged coatings, as well as on blocks concrete, a 6/2/1 ratio of a (ASTM C 672) are shown pictorially in
that had a damaged coating. Coatings sa n d / P ortland cement/water mixture Figs. 2 through 4.
were damaged by scoring with a knife was used, and the coarse aggregate was The cyclic method involves ponding
and the removal of any poorly adhered omitted. water on the surface of the coating, freez-
coating during the test. A control block Commercially available coatings used ing overnight, and then thawing with the
44 JPCL January 2007 www.paintsquare.com

Number of Thermal Cycles
50 25 15 10 50
Experimental Commercial Commercial Commercial Commercial
WB Elastomeric Acrylic SB 2K Epoxy Acrylic (textured) SB Vinyl toluene SB Acrylic WB Acrylic (higher Tg)
aid of deicing chemicals, in this case with exposed aggregate after 30 cycles. based on a higher Tg binder also showed
sodium chloride. An uncoated concrete The experimental waterborne elastomer- no damage after 50 cycles. The good per-
block, run as a control, began to show ic acrylic coating performed very well in formance of the waterborne systems is
scaling of the concrete surface after only this test, and after 50 cycles showed no probably related to their good fle x i b i l i t y ,
6 cycles, and had heavy deterioration signs of damage. The waterborne acrylic as the failure mechanism for the solvent-
46 JPCL January 2007 www.paintsquare.com
Fig. 4 (facing page): Comparison of experimental
waterborne elastomeric coating and commercial
systems using damaged coatings. Coatings on
concrete were evaluated after cyclic freeze/thaw
testing (ASTM C 672). Testing was done on films
that were first damaged due to adhesion testing,
i.e., scoring. Photos of each block were taken at
the number of cycles listed.
borne systems appears to be initial
cracking and flaking of the film, which
then exposes the bare concrete.
The better adhesion of the waterborne
acrylics is another likely factor in their
good performance. As shown in Fig. 2,
the solvent-borne systems began to show
damage to the coating and concrete at
much earlier times. Although difficult to
see from the photograph in Fig. 2, the
textured vinyltoluene acrylic started to
fail by flaking of the paint after only 10
cycles. A close-up of the failure mode at
10 cycles is shown in Fig. 3, and after 25
cycles, 100 percent of the coating sur-
face showed this type of failure. The two-
component solvent-borne epoxy dis-
played film cracking and adhesion loss
af ter 42 cycles. The solvent-borne
acrylic coating failed completely at even
earlier times, and after only 20 cycles
much of the coating had flaked off the
concrete.
Similar results were obtained on coat-
ings that had been damaged prior to the
cyclic testing (Fig. 4). Even after having
been scored by a knife, the experimental
elastomeric waterborne coating per-
formed very well with no blistering or
adhesion loss after 50 cycles. The sol-
vent-borne two-component epoxy and
solvent-borne acrylic showed similar fai l -
ure mechanisms as for the undamaged
films, but only at much shorter times.
The cuts in the film no doubt allowed
faster water penetration, resulting in a
greater expansion and contraction of the
concrete. The textured vinyltoluene
acrylic began to fail through the same
flaking mechanism pictured in Fig. 3, but
at approximately 15 cycles.
Exposure to UV light is another of the
www.paintsquare.com JPCL January 2007 47
Number of Thermal Cycles / Weeks of UV-A Exposur e
50 cycles / 10 wk 30 cycles / 6 wk 30 cycles / 6 wk 30 cycles / 6 wk
Experimental Commercial Commercial Commercial
WB Elastomeric Acrylic SB 2K Epoxy SB Acrylic WB Acrylic (higher Tg)
forces to which exterior coatings are tomeric acrylic resin, in a simple, 36 much faster rate. The commercial water-
subjected. It was decided to modify the PVC, 58% volume solids, white for m u l a - borne system based on a higher Tg
ASTM C 672 test to also include UV tion, was compared to the same commer- acrylic resin, which performed very well
exposure, to better simulate the stresses cially available coatings used in the pre- in the previous test after 50 thermal
which a film may experience in actual vious test. The solvent-borne commercial cycles, did start to show some cracking
weathering. A week of UV-A exposure controls, including a two-component in this test after 8 weeks of UV-A expo-
was added to the test after every 5 ep o x y , a textured vinyltoluene acrylic, sure and 40 thermal cycles. The experi-
freeze/thaw cycles. and an acrylic displayed the same modes mental elastomeric acrylic showed no
The experimental waterborne elas- of failure (cracking and peeling) but at a blistering, cracking, or splitting of the
48 JPCL January 2007 www.paintsquare.com
Fig. 5 (facing page): Experimental elastomeric
acrylic and commercial coatings after ASTM C
672 thermal cycling and UV-A exposure on high
salt content concrete. Coatings on concrete made
with 0.2% salt added to concrete mixture.
Fig. 6: Example of small cracks seen in the com-
mercial waterborne acrylic based on a higher Tg
resin.Close-up of the surface cracking mode of
failure observed for the harder, commercial water-
borne acrylic coating after 30 cycles of the cyclic
freeze/thaw test (ASTM C 672) and 6 weeks of
UV-A exposure. Close-up corresponds to the con-
crete block pictured in Fig. 5. Scale shows one
millimeter per division.
films after 10 weeks of UV-A exposure
and 50 thermal cycles.
To stress the experimental system
even more and also mimic the real world
variability of concrete, a test series was
pe r f ormed over various types of con-
crete. A control concrete was made
using a 6/2/1 Sand/Cement/Wat e r
composition. A very porous concrete
was made by increasing the water level
10%. To increase the alkali/aggregate
reaction, 0.2% sodium chloride was
added to another batch of concrete,
which did produce surface crazing of the
dried concrete. Finally, since it is known
that concrete hardens as time passes,
and the hydration reaction gets slower
and slower as the calcium silicate
hydrate forms, a control batch of con-
crete was aged outdoors for 12 months.
The experimental waterborne coating
was again compared to commercially
available systems recommended for con-
www.paintsquare.com JPCL January 2007 49
Number of Thermal Cycles / Weeks of UV-A Exposur e
50 cycles / 10 wk 20 cycles / 4 wk 20 cycles / 4 wk 25 cycles / 5 wk
Experimental Commercial Commercial Commercial
WB Elastomeric Acrylic SB 2K Epoxy SB Acrylic WB Acrylic (higher Tg)
Fig. 7 (above): Experimental elastomeric acryl i c
and commercial coatings after ASTM C 672 ther-
mal cycling and UV-A exposure on very porou s
co n c r ete. Coatings on porous concrete, made with
10% additional water in the concrete mixture.
Fig. 8 (right): Close-up of the surface cracking
mode of failure. Close-ups correspond to the
concrete blocks pictured in Fig. 7. Commercial Commercial Commercial
SB 2K Epoxy SB Acrylic WB Acrylic (higher Tg)
50 JPCL January 2007 www.paintsquare.com
crete highway barriers.
The uncoated concrete blocks, ran as
controls, began to show scaling of the con-
crete surface after only 5 freeze/thaw
cycles and 1 week of UV-A exposure. All
uncoated blocks were heavily damaged
af ter 10 thermal cycles and 2 weeks of
UV -A exposure. Figures 5 and 6 show
the results for the coatings on the “high
salt”concrete. The experimental elas-
tomeric waterborne acrylic coating per-
formed very well, and after 50 cycles
showed no signs of damage. The two-com-
ponent epoxy and the solvent-borne
acrylic coatings displayed severe film
cracking and adhesion loss after 30
cycles. The commercial waterborne
acrylic did have many hairline cracks by
30 cycles, an example of which can be
seen in Fig. 6. The “high water” concrete
produced similar results, although not as
dramatic, and can be seen in Fig. 7. The
2K epoxy system and the solvent-borne
acrylic have many hairline cracks which
can be seen in Fig. 8. The concrete that
had been aged for 12 months showed
excellent strength and durability. All
coatings over the aged concrete per-
formed quite well, suggesting that in
repaint situations or when painting previ-
ously unpainted concrete which has
weathered, the choice of coating is less
critical than when painting new concrete.
Of ten, the base of a highway barrier is
striped with a traffic paint containing
re f lective beads as a safety feature.
Although commercial traffic paints per-
form very well on concrete and asphalt
roads and pass numerous freeze/thaw
cycles in the field, there have been some
reports that they can lose adhesion to the
smooth, vertical concrete highway barri-
ers. However, if the entire barrier were
painted with the experimental elastomer-
ic acrylic, then adhesion of the traffic
paint to the bare concrete is not critical.
To test this concept, a concrete block was
coated by airless spray with the new elas-
tomeric acrylic and dried vertically
be f ore it was striped with a commercially
www.paintsquare.com JPCL January 2007 51