Acrylics have been the rising stars of the caulk market
during the last decade. Just as acrylic paints are now matching
or beating the performance of the best oil-based paints (and
winning over skeptics along the way), acrylic caulks are
starting to move in where previously only silicones or
polyurethanes dared to go. Choosing a bedding caulk for
installing door and window units is a no-brainer — you
grab a tube of a reasonable quality latex caulk and have at it.
Likewise for any interior trim and painting caulking —
nothing beats the workability of a latex formulation. Latexes
tool with a finger and clean up with water; plus they’re
nontoxic and produce little odor, making them ideal for indoor
work. They stick well to a variety of materials and can be
High-quality latex acrylics win the prize for user
friendliness, and claim to meet the same performance standards
as some silicones and polyurethanes.
But are they up to heavier-duty work — a tile shower
or exposed outdoor caulking? The answer is yes, as long as you
choose the product carefully and give it some protection as it
Older-generation latex caulks have less flexibility —
+/-7.5% movement capability. The new high-performance acrylics
are formulated with a high percentage of plasticizers, so many
are capable of +/-25% movement. This doesn’t come close
to the ability of silicones and polyurethanes, some of which
can withstand 100% elongation and 50% compression (+100%/-50%
movement), but it’s adequate for most residential
demands. The plasticizers in these new-generation acrylics are
nonvolatile — that is, they don’t evaporate as the
caulk ages, so the caulk remains flexible indefinitely.
Top-performing acrylics also have good UV resistance and
aren’t susceptible to adhesion loss from water.
So why not toss all those messy silicones and urethanes and
move over to acrylics? There are a couple of reasons. For one
thing, acrylics are much slower to firm up as they cure than
urethanes or silicones, leaving them more vulnerable to the
weather. Much of the ASTM testing is done after the caulks have
been able to cure for days or even weeks. But on a remodel, an
exterior caulk bead goes into service from the moment
To test how these softer acrylics would handle a surprise
rainstorm, I ran beads for six different acrylic caulks on a
piece of painted sheet metal, along with sample beads of
silicone, polyurethane, and Kraton-based products. I let all
the beads cure for an hour outside at 70°F, then hit them
with spray from a garden hose for five minutes. Of the six
acrylics, all were deformed by the spray; in two cases the bond
lines were smeared and weakened, and in one case the caulk
partially dissolved and ran down. The other three types of
caulk were virtually unaffected.
This was no white-smocked, scientific test by any means, but
it supported my suspicions that compared with other types of
caulk, these soft acrylics might require special care to be
able to cure properly in exterior joints where movement is
anticipated. (For light caulking of nonmoving joints before
painting, however, acrylics are fine as long as the weather is
cooperating.) Acrylic caulks also have to be protected from
freezing in the package and can’t be applied below
40°F. However, once they’re in place and cured, many
remain flexible at very low temperatures. DAP’s Formula
230, for instance, has a service temperature range from
-30°F to 180°F.
With care, I would also use a top-of-the-line acrylic to
caulk the expansion joints in a tiled shower — another
fairly severe environment. Allowed to cure properly, these
caulks are up to the task. Tile guru Michael Byrne swears by
Color Caulk’s Pro Line Class A — an acrylic caulk
that meets C 920 and comes in specially mixed colors, and
sanded if desired, to match most commercially available grouts.
Always use a caulk with mildewcide for kitchen and bath
Because of their water content, latex acrylics will shrink
as they cure — up to as much as 30% by volume. For a
tight waterseal, you may have to go back and add more
What does siliconized mean? In choosing a high-performance
acrylic, don’t be misled by the term "siliconized." All
caulks are "siliconized," which refers to the addition of small
amounts of adhesion promoters called silanes (a form of
silicone). Early on, the term was a marketing ploy that let
latexes piggyback on the reputation of silicone. Nowadays, with
100% acrylics coming into their own, the term is just as likely
to be used on a medium- or even low-quality latex caulk.
Because there are so many grades of latexes (including
polyvinyl acetates, vinyl acrylics, and 100% acrylics), you
will have to choose carefully to find a top performer. Look for
an acrylic with the ASTM C 920 Class 25 designation, which
+/-25% movement capability. Don’t confuse this with the
marketing claim of "25% total movement," which indicates
Kratons, or Block
Kratons are the new kid on the caulk block. These are typically
the "super-clear" caulks — products such as
Sashco’s Lexel, Geocel’s Pro Flex, and OSI’s
Quad. The strength of these caulks is their versatility. They
stick to just about everything (they’ll melt rigid foam,
however), and can be applied to damp substrates and in
temperatures down to 10°F or lower. And there’s no
doubt about their clarity, if it’s a clear caulk you
need. (I hate to take sides in the caulk wars, but I found Quad
to be the clearest of all.) These caulks skin very quickly, but
can typically be wet-tooled with soapy water.
New solvent-based "block copolymer" synthetic
rubbers are the backbone of these caulks. The products
are known for good performance and versatility, but the
fumes can be strong and persistent.
For me, the biggest drawback to the Kraton products is their
heavy solvent odor. Despite some marketing claims to the
contrary, these are not caulks to be used indoors —
unless you’ve got at least a week for ventilation before
occupancy. One tech support person told me that sensitive
individuals can smell the solvents for months. A second
drawback is shrinkage: After the solvents flash off, the bead
is as much as 35% smaller. The caulk will meld to itself,
however, so if shrinkage is a problem you can add more
Kratons can be painted, and they’re easy to color
— a fact that OSI has used to advantage in its Quad line,
which is available in more than 100 colors to match standard
shades of vinyl and aluminum siding.
I could probably live without Kraton products if I had never
heard of them, but I might keep a tube of Geocel’s Pro
Flex in the truck, for one reason: It claims to stick to oily
surfaces — a unique claim in the caulk market, as far as
I know. This could come in handy if you have to get caulk to
stick to a concrete foundation that has traces of form release
oil on it. I’m not ready to bet the ranch that it will
work, but it’s worth a try.
Get Your Hands Sticky
So who can you believe when it comes to caulk claims?
Ultimately, you have to get your hands on the product and try
it, then go back and see how it performs over time. Your own
house may be your best test site. If you’ve got a
question about how a caulk will work in a given application,
call tech support by all means, but don’t ignore your own
field test with the actual substrate materials; it’s the
only way you can be sure.
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It's getting harder to find a good,
old-fashioned "caulk" on the store shelves anymore —
seems they've all gone off and become "sealants." The term
sealant implies higher performance — a material that not
only sticks well to a variety of substrates but that withstands
the weather and is flexible enough to stretch and compress with
building movement without losing adhesion or tearing. Silicones
and urethanes are invariably called sealants. Among the latex
brands, the terminology is mixed: Lower-performing compounds
are ore likely to be called caulks (for example, "painter's
caulk"), while 100% acrylic products moght be dubbed either a
caulk or a sealant. Dap's 230m acrylic is labelad an "advanced
latex sealant" while M-D's Maxx 5000 is touted as "an advanced
professional sealant" and our best acrylic caulk.
This inflationary language is more than just marketing hype
— it has to do with terminology adopted by the ASTM
(American Society for Testing and Materials) C-24 committee,
the standard writer for the sealant industry. ASTM C-834 is the
"Standard Specification for Latex Sealants," a minimun standard
for older generation caulks; ASTM C-920 is the much more
rigorous "Standard Specification for Elastomeric Joint
Caulk code: ASTM C-834 is the standard for latex caulks;
ASTM C-920 is the spec for sealants for moving joints.
TT-S-00230C is a federal standard similar to, and being
replaced by, C-920.
If a caulk meets the requirements of either (or both) of
these standards, the manufacturer is likely to use the name
"sealant" on the label.
The C-920 standard was originally created for polysulfides,
polyurethanes, and silicones — rugged sealants that met
the demands of high-rise curtain wall construction. To pass
C-920, caulk samples on glass, aluminum, and concrete are
exposed to a battery of tests, including being heated in ovens,
frozen, immersed in water for a week, and out through
numerous cycles of compression and elongation, without showing
adhesive or cohesive failure. As acrylics have improved, they
are also proving able to meet this standard —
Don Jackson is JLC’s managing editor.