Flammability and Acetic Acid Emissions from Acetylated Wood under Well-Ventilated Burning Conditions

Abstract

Acetylation is a type of commercial wood modification used to enhance the durability of wood. Despite its adoption, especially in outdoor environments, there are mixed data on how acetylation affects the combustion of wood. This paper evaluates the differences in acetylated and untreated wood using a cone calorimeter in combination with Fourier Transform Infrared Spectroscopy (FTIR) to look for acetic acid vapors in the combustion gases. Two thicknesses of acetylated pine boards were tested and compared against an untreated board from the same genus. No differences were observed between the peak heat release between the acetylated and untreated boards. Likewise, there were no trends in the time to ignition between the acetylated wood and the control group. Differences were observed however in the chemical composition of the combustion products. An increase in acetic acid in the products of combustion was observed for the acetylated samples that corresponded with the peak heat release of the sample.

Full text

Citation: Hasburgh, L.E.; Zelinka,
S.L. Flammability and Acetic Acid
Emissions from Acetylated Wood
under Well-Ventilated Burning
Conditions. Forests 2023,14, 1186.
https://doi.org/10.3390/f14061186
Academic Editors: Edward Roszyk
and Magdalena Broda
Received: 4 May 2023
Revised: 5 June 2023
Accepted: 6 June 2023
Published: 8 June 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Technical Note
Flammability and Acetic Acid Emissions from Acetylated Wood
under Well-Ventilated Burning Conditions
Laura E. Hasburgh and Samuel L. Zelinka *
US Forest Service, Forest Products Laboratory, Madison, WI 53726, USA; laura.e.hasburgh@usda.gov
*Correspondence: samuel.l.zelinka@usda.gov; Tel.: +1-608-231-9277
Abstract:
Acetylation is a type of commercial wood modification used to enhance the durability
of wood. Despite its adoption, especially in outdoor environments, there are mixed data on how
acetylation affects the combustion of wood. This paper evaluates the differences in acetylated
and untreated wood using a cone calorimeter in combination with Fourier Transform Infrared
Spectroscopy (FTIR) to look for acetic acid vapors in the combustion gases. Two thicknesses of
acetylated pine boards were tested and compared against an untreated board from the same genus.
No differences were observed between the peak heat release between the acetylated and untreated
boards. Likewise, there were no trends in the time to ignition between the acetylated wood and the
control group. Differences were observed however in the chemical composition of the combustion
products. An increase in acetic acid in the products of combustion was observed for the acetylated
samples that corresponded with the peak heat release of the sample.
Keywords: modified wood; acetylation; fire performance
1. Introduction
Acetylation is a non-toxic method of wood modification used to enhance wood prop-
erties, especially resistance to fungal decay. Wood is reacted with acetic anhydride (or in
some cases ketene gas) and a 1:1 displacement reaction occurs where hydroxyl groups in
the wood cell walls are replaced with acetyl groups [
1
]. Fuchs [
2
] was the first to acetylate
wood in 1928; however, the commercialization of acetylated wood was not attempted until
the 1960s [
3
]. The first successful commercialization of acetylated wood was developed in
2003 in the Netherlands by Accys Chemicals [
4
] who now produce 60,466 m
3
of acetylated
wood per year [5].
It is generally accepted that acetylated wood treated to an uptake of 15%
−
20% weight
percent gain (WPG) is decay-resistant [
6
–
9
]. However, the mechanisms by which acetyla-
tion protects wood from fungi are still under debate [
10
–
12
] and it is possible that wood
treated to 20% WPG is not decay-resistant, but instead the kinetics are retarded or delayed
enough that decay is not a practical concern [
10
,
13
–
15
]. In addition to improving the
resistance of wood to fungal decay, acetylation also changes the physical properties of
wood. Since acetyl groups are larger than the hydroxyl groups they replace, the acetylation
reaction permanently bulks the wood. This irreversible swelling means that wood is more
dimensionally stable, i.e., it swells less in the presence of moisture and shrinks less when
the moisture is removed [16,17].
While acetylation has been extensively studied, very little is known about how acety-
lation affects the flammability or fire resistance of wood members. When exposed to high
temperatures, wood undergoes pyrolysis, a series of thermochemical reactions that change
the physical and chemical properties of wood [
18
]. These pyrolysis reactions lead to the for-
mation of char, an insulating carbonaceous layer that helps to protect the underlying wood
from further pyrolysis and char [
19
]. Reasonably, one would expect that the replacement of
hydroxyl groups with acetyl groups within the wood may affect pyrolysis. For example,
Forests 2023,14, 1186. https://doi.org/10.3390/f14061186 https://www.mdpi.com/journal/forests

Forests 2023,14, 1186 2 of 7
acetyl groups may be cleaved from the polymeric chains during pyrolysis forming acetic
acid; in this case, this endothermic process may retard the combustion kinetics compared to
untreated wood. On the other hand, the liberated acetic acid in the volatiles is flammable
and the combustion of this volatile may cause the wood to experience more rapid burning.
Even though acetylated wood has clear chemical differences from untreated wood,
relatively little is known about its flammability. Only four previous studies have examined
the interaction of acetylated wood with fire [
20
–
23
]. Importantly, the literature on how
acetylation affects the flammability of wood is inconsistent; two of these studies found that
acetylated wood was less flammable than untreated wood and the other two studies came
to the opposite conclusion. The inconsistent findings could be due to the different reaction-
to-fire tests used, the initial wood species that was acetylated, and/or the comparative
control materials.
Two researchers utilized ISO 11925-3 to study the flammability of acetylated wood [
24
].
In these tests, a burner was placed on the wood surface for one minute and then re-
moved. The fire performance is evaluated by measuring the time of flaming combustion,
glowing combustion, and the charred area. Mohebby et al. [
21
] examined acetylated
wood and noted that at higher levels of acetylation, the time of flaming combustion de-
creased and the time of glowing combustion increased. Similar results were observed by
Papadopoulos et al. [
22
] for orientated strand board (OSB) made with acetylated chips.
Both studies claimed that since the time of flaming combustion decreased as the level of
acetylation increased, acetylation decreases flammability. However, it should be noted
that the amount of glowing combustion increased with acetylation and, therefore, the total
combustion time (flaming + glowing) remained about the same for all samples tested.
In contrast, Morozovs and Buksans used a radiant panel flooring test (ISO
9239-1) [
25
] and the cone calorimeter (ISO 5660-1) [
26
] to examine the flammability of
acetylated wood [
20
]. They observed that acetylated wood had increased flame spread
when compared to an untreated control in the radiant panel test. While the heat release rate
was not significantly different between the treatments, the acetylated wood had a longer
time to flameout. Rabe et al. [
23
] also used the cone calorimeter to evaluate the flammability
of acetylated wood. They found that acetylation did not improve the thermal stability or
flammability of wood, but rather resulted in a shorter time to ignition and an accelerated
heat release rate. They noted that the acetyl groups inherent in the acetylation process
act as combustible volatiles, increasing the effective heat of combustion of the pyrolysis
products that resulted in the observed increases in burning speed.
One commonality across the literature was that large amounts of “blue smoke” were
produced in the combustion of acetylated wood, although Morozovs and Buksans found
that acetylated wood had a lower total smoke release in the cone calorimeter test. The blue
smoke was attributed to acetic acid released in the pyrolysis of acetylated wood.
The release of acetic acid during the pyrolysis process likely affects the flammability of
acetylated wood. While blue smoke was observed in several tests, Rabe et al. concluded that
the acetic acid produced during pyrolysis was rapidly consumed by the fire and contributed
to the more rapid consumption of the acetylated wood compared to the controls. Despite
its presumed importance in the combustion of the modified wood, evolved acetic acid has
not been measured during any of the previous fire tests on acetylated wood.
In this paper, we examine the flammability of acetylated wood using a cone calorime-
ter. Furthermore, we use Fourier Transform Infrared Spectroscopy (FTIR) to observe the
products of combustion during flammability testing; specifically looking at acetic acid
to see if there is any correlation between acetic acid in the products of combustion and
differences in the heat release rate signatures of the different materials.
2. Materials and Methods
Three different sample types were tested: two types of acetylated wood and one
control. All samples were developed with pine (Pinus spp.) and had a surface area of
100 mm by 100 mm. The term species in this article in some contexts is used to refer

Forests 2023,14, 1186 3 of 7
to commercial groups that include multiple species, such as the Southern pine group.
The control samples were cut from Southern pine dimension lumber produced in the
United States with a grade stamp of “select structural” The Southern pine group is a
commercial species group which contains four species that cannot be separated through
wood anatomy [
27
]. The control samples were 37 mm thick. The first acetylated sample,
hereafter referred to as Ac1, was obtained from a commercial supplier and was radiata
pine. The thickness of the Ac1 sample was 53 mm. The second acetylated wood sample,
Ac2, was also radiata pine but obtained from a second distributor with a sample thickness
of 19 mm. Both Ac1 and Ac2 were acetylated by the same commercial acetylation process
and were sold as the same tradename. Each sample group was tested in triplicate (n= 3).
The flammability of the acetylated and control groups was tested using a cone calorime-
ter (FTT iCone Mini, East Grinstead, West Sussex, UK) according to ASTM E1354 [
28
]. All
cone calorimeter tests were conducted in the horizontal orientation and at a constant heat
flux exposure of 35 kW m
−2
. This heat flux is commonly used for exploratory testing.
A standard holder frame was used for each test which allowed an exposed surface of
88.4 cm
2
for each specimen. The sides and bottom of the sample were wrapped in alu-
minum foil and the foil-wrapped samples were placed on a ceramic fiber blanket in the
holder. A spark ignitor provided the piloted ignition source. Prior to testing, the samples
were conditioned for six months in a 21
◦
C/50% relative humidity chamber. The tests were
terminated two minutes past either the end flaming combustion or when the specimen
had been consumed, whichever was longer. To terminate the tests, the heat flux source
was blocked, the specimens removed, and water was applied. The heat release rate (HRR)
was calculated using the oxygen consumption method [
29
] by measuring the amounts of
O
2
, CO, and CO
2
in the products of combustion and the mass loss of the specimen was
also measured. Key flammability parameters were tabulated for all samples including the
heat release rate, total heat release, time to ignition, mass loss rate, total smoke release,
and effective heat of combustion. However, differences in sample thicknesses between the
groups will affect the duration of the tests, features of the heat release rate curves, and
cumulative properties such as the total heat release between samples. Therefore, because of
the differences in thickness, the analysis was restricted to the initial peak heat release rate
and time to ignition between samples.
In addition to the O
2
, CO, and CO
2
used to calculate the heat release rate, the composi-
tion of the combustion gases was monitored using Fourier Transform Infrared Spectroscopy
(FTIR) (Protea atmosFIR AFS-A-15, Middlewich, UK) which was coupled with the cone
calorimeter. This FTIR utilizes a multi-pass gas cell interferometer operating at 180
◦
C,
with a path length of 4.2 m and spectral resolution of 4 cm
−1
. This set up allows for
the simultaneous identification and quantification of 35 gaseous compounds including
CH
3
COOH (acetic acid). The acetic acid data were calculated at a frequency of 0.25 Hz
from the spectra. The acetic acid signal contained an appreciable amount of noise and, to
better see the signal, the acetic acid data were smoothed by using a moving average filter
over 22 points (or approximately 1.5 min) via the “smooth” function in MATLAB (Natick,
MA, USA).
3. Results and Discussion
The initial peak heat release rate (PHRR) and time to ignition (Tig) for the various
groups are presented in Figure 1. Since the sample thicknesses varied across groups, it was
not possible to compare other metrics such as the total heat release and information related
to the second heat release rate peak. The graphs present the data from each replicate with
the solid lines representing the range of measured values.

Forests 2023,14, 1186 4 of 7
Forests 2023, 14, x FOR PEER REVIEW 4 of 7
No obvious trends can be observed in either the PHRR or Tig data between the acet-
ylated wood and the control samples. The PHRR of the replicates for the Ac2 group were
lower than those for the control group; however, the Ac1 group had measured values both
above and below those measured for the control group. The heat release rate depends
upon several material properties including density, heat of combustion, and thermal con-
ductivity. Variations in the sample density between the groups may have contributed to
the lower PHRR observed for the Ac2 group. All replicates of the Ac2 group had a longer
Tig than either the control or Ac1 group; however, the measured Tig for the Ac1 group fell
between the highest and lowest Tig measured for control specimens.
While further testing is needed to determine the statistical significance between these
treatments, acetylation does not appear to cause a large shift in these measurements of
flammability in either direction.
Figure 1. Initial peak heat release rate and time to ignition for the three sample groups tested. Ab-
breviations: Con = control; Ac1 = acetylated group; 1 Ac2 = acetylated group 2.
A more complete picture of the flammability and acetylation can be obtained from
Figure 2 which plots the average heat release rate of all replicates as a function of time.
The boom half of Figure 2 contains the amount of acetic acid measured in the combustion
gases from the cone calorimeter during the tests. The HRR curves for each sample group
exhibited features standard for wood-based materials with a sharp PHRR. This sharp ini-
tial peak is accredited to the ignition and rapid exothermic reaction for uncharred surface
material [30–32]. After this peak, the HRR reduces once char forms and eventually leads
to a second peak. The second peak reflects the thermal wave reaching the unexposed side
of the specimen and can be affected by many variables including the density, heat flux,
material thickness, and backing product [33].
The HRR curves are very similar across the control group and both groups of acety-
lated wood. The largest differences occur in the placement and magnitude of the second
peak in the heat release rate curve. The second heat release rate peak is sensitive to factors
such as the thickness and type of insulating substrate but is largely aributed to specimen
burn-through [33]. Therefore, it is not surprising that the Ac2 group, which was the thin-
nest sample, had the earliest and most pronounced second peak.
Figure 1.
Initial peak heat release rate and time to ignition for the three sample groups tested.
Abbreviations: Con = control; Ac1 = acetylated group; 1 Ac2 = acetylated group 2.
No obvious trends can be observed in either the PHRR or Tig data between the
acetylated wood and the control samples. The PHRR of the replicates for the Ac2 group
were lower than those for the control group; however, the Ac1 group had measured
values both above and below those measured for the control group. The heat release
rate depends upon several material properties including density, heat of combustion, and
thermal conductivity. Variations in the sample density between the groups may have
contributed to the lower PHRR observed for the Ac2 group. All replicates of the Ac2 group
had a longer Tig than either the control or Ac1 group; however, the measured Tig for the
Ac1 group fell between the highest and lowest Tig measured for control specimens.
While further testing is needed to determine the statistical significance between these
treatments, acetylation does not appear to cause a large shift in these measurements of
flammability in either direction.
A more complete picture of the flammability and acetylation can be obtained from
Figure 2which plots the average heat release rate of all replicates as a function of time. The
bottom half of Figure 2contains the amount of acetic acid measured in the combustion
gases from the cone calorimeter during the tests. The HRR curves for each sample group
exhibited features standard for wood-based materials with a sharp PHRR. This sharp initial
peak is accredited to the ignition and rapid exothermic reaction for uncharred surface
material [
30
–
32
]. After this peak, the HRR reduces once char forms and eventually leads to
a second peak. The second peak reflects the thermal wave reaching the unexposed side
of the specimen and can be affected by many variables including the density, heat flux,
material thickness, and backing product [33].
The HRR curves are very similar across the control group and both groups of acetylated
wood. The largest differences occur in the placement and magnitude of the second peak
in the heat release rate curve. The second heat release rate peak is sensitive to factors
such as the thickness and type of insulating substrate but is largely attributed to specimen
burn-through [
33
]. Therefore, it is not surprising that the Ac2 group, which was the thinnest
sample, had the earliest and most pronounced second peak.
Although the HRR curves across all three groups are similar and exhibit the typical
two-peak behavior, differences can be seen in the acetic acid detected by the FTIR. Both the
Ac1 and Ac2 groups exhibit a peak of acetic acid emissions that is slightly offset from the
initial peak in the heat release rate curve. In examining the acetic acid concentration as a
function of time, it is important to mention that the data in Figure 2contain time averaging

Forests 2023,14, 1186 5 of 7
of the data to smooth out the high amount of noise in the data. Therefore, some of the
observed shift in the acetic acid peak from the heat release peak may be a result of the
smoothing routines applied to the acetic acid data. Both Ac1 and Ac2 exhibit peaks above
1 part per million (ppm) before tapering off throughout the test. In contrast, the acetic
acid in the combustion gases of the control group exhibited minor fluctuations around and
under 0.5 ppm.
Forests 2023, 14, x FOR PEER REVIEW 5 of 7
Figure 2. Heat release rate (HRR) curves for acetylated and control wood (top). Acetic acid concen-
trations measured in the combustion gases with FTIR (boom). Note differences in scale on x axis.
Although the HRR curves across all three groups are similar and exhibit the typical
two-peak behavior, differences can be seen in the acetic acid detected by the FTIR. Both
the Ac1 and Ac2 groups exhibit a peak of acetic acid emissions that is slightly offset from
the initial peak in the heat release rate curve. In examining the acetic acid concentration
as a function of time, it is important to mention that the data in Figure 2 contain time
averaging of the data to smooth out the high amount of noise in the data. Therefore, some
of the observed shift in the acetic acid peak from the heat release peak may be a result of
the smoothing routines applied to the acetic acid data. Both Ac1 and Ac2 exhibit peaks
above 1 part per million (ppm) before tapering off throughout the test. In contrast, the
acetic acid in the combustion gases of the control group exhibited minor fluctuations
around and under 0.5 ppm.
The FTIR data In Figure 2 clearly indicate differences in the amount of acetic acid
liberated from the acetylated and control wood during combustion. Acetic acid is clearly
present in the combustion gases of acetylated wood. In general, the amount of acetic acid
measured in the experiment was related to the heat release rate. This measurement of
evolved acetic acid was consistent with previous observations of the flammability of acet-
ylated wood. However, previous work had suggested that this release of acetic acid may
reduce the flammability of the wood by absorbing energy, whereas other work had sug-
gested that the acetic acid may increase the flammability by adding additional volatiles to
be consumed in the fire. Although acetic acid could be measured in the cone calorimeter
combustion gases of acetylated wood in these experiments, the release of acetic acid ap-
pears to not have a large effect on the flammability of the acetylated wood.
Compared to the broader literature on the flammability of wood, fewer studies have
been published on the levels of various chemical species in the products of combustion.
FTIR is commonly used in conjunction with thermogravimetric analysis to look at gases
released during thermal decomposition, although it should be noted that the exposure
conditions vary greatly between TGA and the cone calorimeter [34]. Limited work has
been conducted analyzing the products of combustion using a cone calorimeter for wood
and wood-based materials [35–37]. These experiments used solvent absorption to look for
Figure 2.
Heat release rate (HRR) curves for acetylated and control wood (
top
). Acetic acid con-
centrations measured in the combustion gases with FTIR (
bottom
). Note differences in scale on
x axis.
The FTIR data In Figure 2clearly indicate differences in the amount of acetic acid
liberated from the acetylated and control wood during combustion. Acetic acid is clearly
present in the combustion gases of acetylated wood. In general, the amount of acetic
acid measured in the experiment was related to the heat release rate. This measurement
of evolved acetic acid was consistent with previous observations of the flammability of
acetylated wood. However, previous work had suggested that this release of acetic acid
may reduce the flammability of the wood by absorbing energy, whereas other work had
suggested that the acetic acid may increase the flammability by adding additional volatiles
to be consumed in the fire. Although acetic acid could be measured in the cone calorimeter
combustion gases of acetylated wood in these experiments, the release of acetic acid appears
to not have a large effect on the flammability of the acetylated wood.
Compared to the broader literature on the flammability of wood, fewer studies have
been published on the levels of various chemical species in the products of combustion.
FTIR is commonly used in conjunction with thermogravimetric analysis to look at gases
released during thermal decomposition, although it should be noted that the exposure
conditions vary greatly between TGA and the cone calorimeter [
34
]. Limited work has
been conducted analyzing the products of combustion using a cone calorimeter for wood
and wood-based materials [
35
–
37
]. These experiments used solvent absorption to look for
specific toxic gases in the products of combustion, namely hydrogen cyanide. Previously,
acetic acid was identified as one of the condensates in wood smoke under smoldering con-
ditions [
38
]. However, the current work shows that even under well-ventilated conditions
within the cone calorimeter, acetic acid is evolved from acetylated wood.

Forests 2023,14, 1186 6 of 7
The results from these experiments suggest that acetylation does not have a large
impact on the flammability of wood. However, cone calorimeter experiments are conducted
in a highly ventilated environment where the sample has ample access to oxygen. While
acetylation does not appear to greatly impact the flammability of wood in cone calorimeter
tests, it may be that acetylation has a greater impact on smoldering combustion and
differences may be observed in a less oxygen-rich environment.
4. Conclusions
The goal of this work was to examine the flammability of acetylated wood and further
examine the relationships between the release of acetic acid from acetylated wood during
combustion and changes in the heat release rate.
The results showed that the heat release rate curves were similar between the acety-
lated and untreated wood. From a practical standpoint, these results suggest that the
flammability of acetylated and untreated wood are similar. FTIR revealed acetic acid emis-
sions during combustion for acetylated wood; however, these emissions did not correspond
with a distinctive feature of the heat release rate curve. In all cases, the total amount of
acetic acid in the smoke was small, a maximum of 2 parts per million.
The cone calorimeter utilized in the present study produces well-ventilated burning
conditions. Future work will include evaluating the acetylated wood in a controlled
atmosphere cone calorimeter to determine the toxicity of fire effluents in an under-ventilated
condition more properly.
Author Contributions:
Conceptualization, L.E.H. and S.L.Z.; formal analysis, L.E.H. and S.L.Z.;
writing—original draft preparation, L.E.H. and S.L.Z.; writing—review and editing, L.E.H. and S.L.Z.;
visualization, S.L.Z. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.
Data Availability Statement: Data can be obtained by contacting the authors.
Conflicts of Interest: The authors declare no conflict of interest.
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