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TiO2: A Key Ingredient to a Long-Lasting Reflective Roof

May 15, 2014

Cool roofs: Although some believe them to be a relatively
new phenomenon, the truth is, they have been
around for a long time—decades for sure, and perhaps
for centuries. In many warm-weather countries such
as Greece, Spain, and Bermuda, folks have long been
painting their roofs white and have understood the
benefits of reflecting the sun’s energy back into the atmosphere rather
than heating the house or building.
In the United States, cool roofs started to surface in the mid-1970s
with polyvinyl chloride (PVC) single-ply roof membranes and, later
in the late 1990s, with thermoplastic olefin (TPO) roof membranes.
According to industry statistics, PVC and TPO roofing reflective membranes
have become the fastest-growing segment of the commercial
roofing market. It is estimated that thermoplastic membranes have
increased in square footage sold by 136% from 2001 to 2012. Darkcolored
membranes such as ethylene propylene diene monomer
(EPDM), modified bitumen, and built-up roofing (BUR) have lost an
estimated 30% in square footage sold in the same time frame. White
EPDM and more-reflective modified-bitumen roofing membranes are
also being promoted as energy-saving cool
roofs.
Another area of growth in reflective roofing
is roof coatings. Although roof coatings
have been around for decades, their formulations
are evolving and their usage increasing.
The early 1970s saw the introduction of the
first acrylic-based white elastomeric roof
coatings in the United Kingdom. The acrylic
technology provided a durable, flexible film
that typically was pigmented with titanium
dioxide (TiO2) to provide a reflective, “cool”
surface. It is estimated that the use of reflective
roof coatings that use TiO2 has grown
8-10% annually in the last ten years (Figures
1 and 2).
TiO2 is a white pigment used in a
variety of applications to impart opacity,
whiteness, brightness, and UV durability.
TiO2—its unique crystal structure,
atom configuration, high refractive
index, density, and very efficient scat-
Ma r c h 2 0 1 4 I n t e r f a c e • 1 9
Figure 2 – Global demand for roof coatings with TiO2.
Figure 1 – 5.2 million metric tons of TiO2 were consumed by
diverse markets in 2013.
tering power—allows the material to have superior
optical properties. Due to the high reflective index of
TiO2, there are few if any comparable substitutes for the
majority of end-use markets. TiO2 uniquely reflects the
infrared portion of the solar spectrum, maximizing solar
reflectivity.
There are two commercial processes in the manufacture
of TiO2: the sulfate process and the more modern
chloride process (see Figure 3). In the sulfate process,
extracted titanium ore or slag is fed into the feedstock
digestion tank, then goes through hydrolysis, followed by
calcination, along with postmanufacture treatment. In the
chloride process, the slag feedstock will undergo a chlorination
reaction. The raw, intermediate titanium tetrachloride
(TiCl4) is to be purified prior to an oxidation to
the final product. The chloride process is a cleaner and
environmentally preferable route, as fewer wastes are
generated in the process.
Virtually all reflective (TPO and PVC) roof membranes
and roof coatings made in North America use TiO2 produced
using the chloride process. TiO2 is also used to
2 0 • I n t e r f a c e Ma r c h 2 0 1 4
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August 2014 Insulation May 15, 2014
September 2014 Building envelope issues June 13, 2014
October 2014 Roofing failures July 15, 2014
November 2014 Traditional materials August 15, 2014
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Figure 3 – Flowchart of chloride process in titanium
dioxide production.
Figure 4 – Weight loss vs. TiO2 levels.
make the top surface of EPDM white and,
in many cases, to make modified bitumen
more reflective.
Besides making things white, TiO2 provides
a significant weathering benefit for
long-term applications such as single-ply
roof membranes. In basic terms, the more
TiO2 a product has in it, the better it will
weather. Laboratory testing indicates that
the more TiO2 there is in the exposed top
surface of the membrane, the less weight
loss occurs in the membrane (see Figure 4).
As TiO2 content increases, the effectiveness
of protection from radiation increases, and
the loss/degradation of binder/resin caused
by radiation decreases.
One of the best ways to improve the
weathering of a PVC or TPO membrane is
to increase the loading of TiO2 in the top
surface.
TiO2’s inherent characteristics, such as
high brightness, opacity, neutral and blue
undertone, solar reflectivity, chalk resistance,
weather durability, lightfastness,
gloss, and mechanical property retentions
can be built into the manufacturing process.
Other surface characteristics, such as
compatibility to roofing substrate, optimized
dispersion, and ease of processing must be
further engineered to meet the final requirements
of roofing membranes.
Much has been written about the benefits
of cool roofing. A study conducted
by Akbari and Levinson of the Heat Island
Group of Lawrence Berkeley National
Laboratory (LBNL) confirmed that a cool roof
lessens the flow of heat from the roof into
the building, reducing electricity demand
for space cooling in conditioned buildings.
According to the authors of the study, substituting
a weathered cool roof (solar reflectance
of 0.55) for a conventional darkercolored
roof (solar reflectance of 0.20) could
have a far-reaching energy saving impact
per square meter of roof area, annually,
averaged across the U.S. (see Table 1).
LBNL estimated that retrofitting 80% of
the 2.58 billion m2 of conditioned commercial
buildings in the U.S. would yield annual
energy cost savings of $735 million. It would
also offer an annual carbon dioxide (CO2)
reduction of 6.23 million tons (offsetting
the annual CO2 emissions of 1.2 million
typical cars), an annual nitrogen oxides
(NOx) reduction of 9.93 kt (offsetting the
annual NOx emissions of 0.57 million cars),
an annual sulphur oxides (SOx) reduction of
25.6 kt (offsetting the annual SOx emissions
Ma r c h 2 0 1 4 I n t e r f a c e • 2 1
Table 1
Cooling Heating Energy CO2 NOx Sox Hg
energy energy cost reduction reduction reduction (mercury)
savings penalty savings (kg/m2) (g/m2) (g/m2) reduction
(kWh/m2) (therm/m2) ($/m2) (μg/m2)
5.02 0.065 0.356 3.02 4.81 12.4 61.2
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mercury (Hg) reduction of 126 kg.
Beyond energy savings, cool roofs contribute
to the reduction of the urban heat
island effect, a source of smog and other
environmental burdens.
Despite the exceptional track record of
some cool-roof technology over decades, certain
individuals and some companies within
the low-slope roofing industry have recently
attempted to cast doubts on the fundamental
science behind cool roofs and the energy
impact of cool roofs in northern climates.
The energy benefits of reflective roofing in
southern climates have come to be broadly
accepted and recognized. If one accepts that
cool roofs provide energy saving benefits in
cooling-dominated climates, the intuitive
corollary is that reflective roof surfaces
are disadvantageous in heating-dominated
climates. One might assume that black,
minimally reflective surfaces will absorb
the sun’s energy in the winter, presumably
reducing heating energy loads.
The answer is not quite as simple as it
might first appear. Winter days are shorter
than summer days, with more overcast
skies. Most importantly, the sun is much
closer to the horizon in the winter and
generates much less heat in the northern
states. Winter solar irradiance is typically
20% to 35% of the summertime irradiance
for a given location; therefore, a roof surface
receives three to five times more sun in
the summer than in the winter in northern
states. Additionally, in many northern
states, roofs will be covered by a highly
reflective blanket of snow for extended periods
of time, further reducing the impact of
a darker-colored roof surface on heating
energy.
We believe that a roof should not be
selected or rejected based on color alone.
The equation for a long-lasting roof = proper
roof design + time-proven materials + quality
installation + ongoing maintenance.
The reflective nature of TiO2 reduces the
heat load from ultraviolet sunlight, resulting
in a longer-lasting roof membrane while producing
cooling energy-saving benefits.
REFERENCES
Ronnen Levinson and Hashem Akbari,
“Potential Benefits of Cool Roofs on
Commercial Buildings – Conserving
Energy, Saving Money, and Reducing
Emissions of Green House Gases
and Pollutants,” Lawrence Berkeley
Laboratory, December 2007.
2 2 • I n t e r f a c e Ma r c h 2 0 1 4
Ramen Chiu has
worked in the polymer
industry for 27
years. He is currently
employed by
Kronos Worldwide,
Inc. as a technical
service engineer.
He has worked on
TPO, EPDM, and
vinyl roofing materials.
Chiu holds a
BS in chemical engineering and an MS in
polymer technology. He is a member of The
American Institute of Chemical Engineers,
Society of Plastics Engineers, and The
American Chemical Society.
Ramen Chiu
Brian Whelan is
the senior VP of
the roofing business
unit of Sika
Sarnafil and president
of Sarnafil
Services. A graduate
of Harvard
University’s Business
School PMD
Program, he has
a degree in architectural
technology. Whelan is a director
for the Center of Environmental Innovation
in Roofing and has participated on various
NRCA committees. He has served on
the board of SPRI and as chairman of its
Thermoplastic Subcommittee. He was on the
original board for RICOWI and has been a
member of RCI since 1989. Whelan jointly
owns three patents on hot air welding of
thermoplastic membranes and profiles.
Brian J. Whelan
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