Lowering Gas-phase Concentration of Formaldehyde in Indoor
Environment by a New Amino Silane Based Coating
Gourish Sirdeshpande*, Kenneth Caldwell and Michael Cook
Armstrong World Industries
2500 Columbia Ave.
Lancaster, PA 17603
*Corresponding
March 24, 2011
email: gsirdeshpande@armstrong.com
SUMMARY
Lowering indoor concentration of formaldehyde by active means is growingly an important
method to reduce the health risk due to formaldehyde exposure. In this work, a coating primarily
consisting of diamino silane, silica gel and calcium carbonate was shown to remove significant
amounts of formaldehyde from air. The performance of the coating was measured in small scale
environmental chamber by applying the coating to a ceiling tile using an accelerated test
procedure. Formaldehyde from a permeation device was continuously fed into the chamber.
Using procedures recommended in ASTM Methods, formaldehyde from output stream was
sampled and analyzed. It is shown that the reaction can be modelled by 1st order reaction with
respect to gas-phase formaldehyde concentration. The model predictions are validated by
carrying out experiments at different input formaldehyde concentrations and loading factors.
The model can then be used to predict percent reduction and longevity of reduction under actual
field conditions.
IMPLICATIONS
The new coating on interior surfaces can reduce peak level as well as long term exposure risks to
formaldehyde indoors. The development of a comprehensive method for evaluating the
performance of the coating has been reported in this study.
KEYWORDS
Formaldehyde reduction, Measurement and Modelling, Building Materials, Finishing and
coatings, Aldehydes
INTRODUCTION
Source control and increased ventilation are the most common techniques utilized to minimize
exposure to VOCs and formaldehyde (Herberger et al. 2009). Recently, many active methods
that remove formaldehyde and other VOCs from indoor air are being made available
(Salthammer et al. 2010). Photo-catalytic coatings, zeolite containing coatings, adsorbent such
as activated carbon are being used in building materials to provide this function. Photo-catalytic
coatings have been shown to produce harmful intermediates (especially if UV intensities are not
high) if oxidation reactions do not go to completion (Sherman and Hodgson, 2004). Moisture in
air reduces the effectiveness of adsorbents such as activated carbon. Building materials that
contain or are coated with chemically reactive compounds available commercially have
relatively short effective life (Ataka et al. 2004).
In this work, a coating primarily consisting of diamino silane, silica gel and calcium carbonate is
shown to remove significant amounts of formaldehyde from air. It is found that introducing
amine (-NH2) groups from different amine precursors as a linker between silica surface and
formaldehyde (or other aldehydes and carbonyl compounds) via a nucleophilic substitution
reaction yields higher aldehyde removal capacity. A multivalent metal carbonate such as calcium
carbonate was found to further aid the substitution reaction. Details are provided in a US patent
application (Barrows, Sirdeshpande et al. 2010). When formaldehyde reacts with amine, it is
converted into an inert organic-amino complex that is permanently retained in the coating.
2
~~𝑁𝐻2 + 𝐻2 𝐶 = 𝑂 → ~~𝑁 = 𝐶𝐻2 + 𝐻2 𝑂
Typically, the performance of such products is conducted following ISO Standard 16000-23
(ISO, 2009). With this method, half-life and saturation amounts are determined. However, the
outcome of such a measurement is not suitable for performance predictions under actual
conditions of use because the kinetics cannot be modeled from the results based on firstprinciples. In this work, an accelerated method is described. It is validated by comparing
predicted results with measurements carried out under varied conditions.
PERFORMANCE TESTING OF FORMALDEHYDE REDUCTION
Experimental Set Up
The coating is applied to a ceiling tile and then cured. The ceiling tile is placed in a small scale
emission test chamber. The ASTM D5116-10 standard guide was modified to test for VOC
reduction by introducing formaldehyde at constant rate into the chamber. The schematic of the
test set up is shown below.
Figure 1. Schematic of experimental set up.
A 65 L. Emission Chamber maintained at 23C  2 C and relative humidity of 50%  5% is
used for the test. A small fan is used inside the chamber to keep the air well mixed. The air
velocity across the face of the ceiling tile is about 0.1 m/s which transltes to a mass transfer
coefficient of about 15 m/hr. Pure air (less than 2 μg/m3 of background formaldehyde) is
humidified and then mixed with known amount of formaldehyde which is generated by a
permeation oven. The permeation oven has a NIST traceable permeation tube, maintained at
constant temperature which generates formaldehyde at constant rate. The humidified mixture of
air and formaldehyde is supplied to the emission chamber at a constant rate of 1.3 air changes per
hour. The concentration of formaldehyde in the inlet air to the chamber can be varied by using
different permeation tubes in the permeation oven. Formaldehyde concentration in the inlet air
was varied from of 1.6 to 0.1 ppm. Tests were conducted with sample sizes corresponding to a
3
load factor of 0.1 – 0.4 m-1. Formaldehyde concentration from inlet and outlet stream are
sampled and analyzed following ASTM Standard Method D 5197-03.
Experimental Procedure
An empty chamber is allowed to equilibrate for 24 h. during which humidified air mixed with
formaldehyde is passed through the chamber. The concentration of formaldehyde in the outlet
stream is measured after 24 h. This concentration is considered to be the constant inlet
concentration, Ci, for the entire duration of the experiment. A 76 mm. x 86 mm. tile sample is
suspended in the chamber on a stand. The sample size corresponds to load factor of 0.1 m-1.
Formaldehyde concentration in the out let stream, Co, is measured starting 24 h. after the sample
is placed in the chamber at regular intervals until the outlet concentration reaches 90% of the
inlet concentration. The procedure is repeated for multiple samples. The empty chamber
concentration measured in the beginning of each run is used to check the steadiness of the
permeation oven output.
RESULTS AND DISCUSSION:
Figure 2. Results from chamber measurements. Loading=0.1m-1, ACH=1.3hr-1, Conc=1.6ppm
A combination of high inlet concentration of 1.6 ppm and small load factor of 0.1 m-1 is used to
accelerate the test to determine the total reduction capacity of the coating on the ceiling tile. The
difference between the inlet and the outlet concentrations (shaded region in the Fig.2) is a
measure of the amount of formaldehyde removed by the ceiling tile over 312 h. Figure 2 also
shows formaldehyde removed by a ceiling of same size without the active coating. Without the
coating, the formaldehyde adsorbed by the ceiling is almost negligible. It was verified that
formaldehyde consumed by the coating is not re-emitted indicating that formaldehyde has
reacted chemically with the amine groups.
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It is seen from the Figure 2 that the magnitude of reduction decreases over time as formaldehyde
reacts gradually with diamino silane. It is well known that in these gas-solid heterogeneous
systems, the reaction rate is primarily controlled by mass transfer rate at the coating’s surface.
Formaldehyde is first adsorbed on the surface, reacts via a nucleophilic substitution reaction with
the primary and secondary amine (-NH2) groups that are readily available on the surface of the
coating. The reduction rate, therefore, is high during the initial period of the experiment. As the
surface amine groups get consumed by reaction with formaldehyde, the reaction rate slowly
decreases as formaldehyde molecules diffuse through the coating, reacting with the amine groups
that are beneath the surface of the coating. Because the reaction rate of formaldehyde with the
amines is fast (Weintstock et al. 2005), relative to the duration of the experiment, a quasi-steady
state model can be applied and rate of formaldehyde reduction is given by,
𝑅𝑎𝑡𝑒(𝑡) = 𝑄𝐶𝑖 (1 −
𝐶𝑜 (𝑡)
𝐶𝑖
)
(1)
The total amount formaldehyde removed is obtained by numerically integrating the above trace
according to the equation below:
312
𝑅𝑒𝑚𝑜𝑣𝑒𝑑𝑡𝑜𝑡𝑎𝑙 = ∫0
𝑄𝐶𝑖 (1 −
𝐶0 (𝑡)
𝐶𝑖
) 𝑑𝑡;
𝑅(𝑡) = (1 −
𝐶𝑜 (𝑡)
𝐶𝑖
) × 100
(2)
R(t) is percentage of formaldehyde removed by the tile at any given instant. The total
formaldehyde removed in the above example at a loading factor of 0.1m-1 is calculated to 17 mg.
This estimates to a formaldehyde removal capacity of 2.6 g/m2 of ceiling tile. Many such
experiments were conducted over several samples and the average capacity of the tile is
determined to be 2.5 ± 0.3 g/m2.
Scale-up for larger sample size and lower formaldehyde concentration
Experiments we are also conducted at inlet concentration of 0.1 ppm and at load factor of 0.4m-1.
This load factor is similar to the load factor of a ceiling in a typical building. Results are used to
model and validate the reaction rate obtained by the accelerated test. The validated model can
then be used to estimate formaldehyde reduction under real life situations.
It is proposed that for a given application rate (g/m2) of the coating, the fractional reduction, R(t),
at any instant of time primarily depends on the amount of active groups of diamino silane
available at that given time. This in turn depends on the initial amount of diamino silane, age of
the ceiling tile and its history to exposure of formaldehyde. The fractional reduction is not
dependent on the inlet concentration of formaldehyde.
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Figure 3. Results at Loading=0.4m-1,1.6 ppm.
Figure 4. Inlet conc. 0.1 ppm and 1.6ppm
Figure 3 shows comparison of reduction experiment at 0.1 m-1and 0.4 m-1 load factor at same
inlet concentration of 1.6 ppm. In both the instances, the application rate of the coating (g/m2) is
the same. Because the specimen area is increased by 4 times, the amount of active ingredient
available is also quadrupled. So one would expect that the total amount of formaldehyde
removed at a loading factor of 0.4m-1 should be 4 times that at 0.1m-1. Applying the equation
[2], the amount formaldehyde removed at 0.4m-1 loading factor is 70 mg. which is
approximately 4 times the reduction at 0.1m-1 (17 mg.). In fact, this relationship is true at any
fractional conversion R(t). An example is shown at R(t)=50% in Figure 3. The amount of
formaldehyde reacted at loading ratio of 0.1 m-1 at R(t)= 50% is 12.5 mg. (as indicated by the
area of shaded line) and the amount of formaldehyde reacted at loading ratio of 0.4 m-1 at R(t)=
50% is 58 mg which within the sample error of  15%. This shows that any R(t) the total
amount of formaldehyde reacted is directly related to the amount of active ingredient available.
The other way to interpret this observation is that at the same input conditions of Q and Ci, a tile
at loading ratio of 0.4m-1, will approximately last 4 times as much as a tile at a loading factor
0.1m-1.
Figure 4 shows the comparison of reduction at inlet concentration of 0.1 ppm and 1.6 ppm at the
same 0.4 m-1 load factor. This experiment was run over 7800 h. Comparing the Figures 3 and 4,
reduction curves have a similar shape. The dashed curve is the predicted reduction from the
result of measurement at 1.6 ppm. The predicted curve is simply obtained by a linear translation
in time of the fraction reduced by the ratios of the two inlet concentration as shown by the
equation below
[𝑡𝑅 |0.1𝑝𝑝𝑚 ]𝑝𝑟𝑒𝑑𝑖𝑐𝑡𝑒𝑑 = 𝑡𝑅 |1.6𝑝𝑝𝑚 ×
An example is shown in the graph at
𝐶0
𝐶𝑖
𝐶0
𝐶𝑖
1.6𝑝𝑝𝑚
= 𝑡𝑅 |1.6𝑝𝑝𝑚 × 16
0.1𝑝𝑝𝑚
(3)
=40%,(R=60%) from actual measurements,
= 40%; 𝑅 = 60%; 𝑡0.1𝑝𝑝𝑚 = 410 ℎ. ; 𝑡1.6𝑝𝑝𝑚 = 6300 ℎ. ;
𝑡0.1𝑝𝑝𝑚
𝑡1.6𝑝𝑝𝑚
=
6400
410
= 15.6
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The amount of formaldehyde reacted during these times at 1.6 ppm and 0.1 ppm, both at loading
ratio of 0.4 m-1, is calculated by numerically integrating the equations below,
6300
𝐴𝑚𝑡. 𝑅𝑒𝑎𝑐𝑡𝑒𝑑0.1𝑝𝑝𝑚 = 𝑄𝐶𝑖=0.1𝑝𝑝𝑚 ∫0
410
𝐴𝑚𝑡. 𝑅𝑒𝑎𝑐𝑡𝑒𝑑1.6𝑝𝑝𝑚 = 𝑄𝐶𝑖=1.6𝑝𝑝𝑚 ∫0
𝑅(𝑡)𝑑𝑡 = 55 mg.
𝑅(𝑡)𝑑𝑡 = 52 mg.
(4)
(5)
This amount is about one-fourth of amount reacted at 1.6 ppm and loading ratio of 0.1m-1 at
R(100)=60% (from Fig. 2) as shown by the calculation below.
100
𝐴𝑚𝑡. 𝑅𝑒𝑎𝑐𝑡𝑒𝑑1.6𝑝𝑝𝑚,0.1𝑚−1 = 𝑄𝐶𝑖=1.6𝑝𝑝𝑚 ∫0
𝑅(𝑡)𝑑𝑡 = 12.9 mg.
(6)
The agreement is within experimental error. It is seen from Fig. 4 that the predicted and the
measured reduction ratio agree well in range of R measured from 93% - 35%. Thus, the
mechanistic model to describe the reduction of formaldehyde by the coating can be generalized
for all loading ratios and inlet formaldehyde concentrations.
Longevity of performance under typical room formaldehyde concentration
Although these experiments were carried in an emission chamber, it is expected that the results
would be similar in a room/building under similar conditions. In an actual building, there are
many sources of formaldehyde and conditions such as ventilation rate, load factors, temperature
vary. While it is difficult to estimate the performance of such a product without accurately
knowing these factors over time, it is possible make a simple estimate of the benefit of such a
product based on average conditions. From the above results, it can be generalized that a ceiling
tile with this coating, can remove on an average 50% of formaldehyde generated in a room air
over its life time. The life of such a ceiling tile will however depend on the amount of
formaldehyde in the building and loading ratio of the ceiling product. The BASE (US EPA,
2010) study of 100 randomly selected buildings conducted by EPA found that the average
concentration of formaldehyde is about 0.013 ppm. Based on this average formaldehyde
concentration in a building and an installed ceiling load factor of 0.36 m-1, it is estimated that
there is enough reactive material in the ceiling tile to last for over 10 years. The primary
assumption made in this calculation is that the master transfer coefficients in a building are
similar in magnitude to that in chamber.
CONCLUSIONS
Extensive testing of the new coating on a ceiling tile in the laboratory has shown to remove
significant amount of formaldehyde from the air. It is shown with high formaldehyde loading
and a small sample size the test can be accelerated in an emission chamber. The results from the
accelerated test can be scaled for other inlet formaldehyde concentrations and loading ratio by
linear ratio of loading factors and inlet concentration. The percent reduction in formaldehyde is
independent of the inlet concentration. The reduction at any time depends on the amount of
active diamino silane available for reaction. When a ceiling tile with such a coating is used in the
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entire building, there is enough capacity to achieve an average of 50% reduction in room
formaldehyde concentration over 10 years at average levels (0.013 ppm) of formaldehyde.
Although the performance of this coating is demonstrated on a ceiling tile, in principle, the
coating can be applied to any interior finish surface to accrue similar benefits. With the use of
such a coating, other expensive and energy intensive mitigation methods such as increased
ventilation rates can be avoided.
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CS-4313-411
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