Conference PaperPDF Available

Sheep wool insulation for the absorption of volatile organic compounds

1
Young Researchers’ Forum III
Innovation in Construction Materials
12 April 2016 Paper Number 9
Sheep wool insulation for the absorption of volatile organic
compounds
E. Mansour, R. Marriott, G. Ormondroyd
BioComposites Centre, Bangor University
ABSTRACT
This paper summarises the quantitative analysis of the absorption of volatile organic compounds (VOCs),
namely gaseous formaldehyde, toluene, limonene and dodecane, by different wool types. VOCs are of
increasing concern due to their role as accumulating indoor air contaminants. The potential of sheep wool
insulation as a sustainable and natural solution to this problem is demonstrated. Different wool types were
tested via two different methods to assess their absorption ability with regard to these VOCs. For
formaldehyde analysis, samples were subjected to cycles of exposure to excess formaldehyde gas whilst the
weight gain was measured. For the other VOCs, a modified micro-chamber was used to flow the gases
through samples, followed by trapping and quantification of the non-absorbed VOCs using Tenax TA tubes
followed by their thermal desorption and gas chromatography. The data presented indicates that wool is able
to absorb a range of VOCs in differing amounts and that the amounts and types of VOCs absorbed were
dependent on the sheep breed and the processing of the wool. The data also shows that absorption occurs in
a distinctive manner, more or less as a function of percentage of the amount wool fibres are exposed to
rather in an indiscriminate absorptive manner; rather it buffers indoor VOC levels to smooth down spikes in
concentrations over time. The use of wool in building design may therefore have an important role not only in
imparting thermal efficiency but also in the improvement of indoor air quality.
1. INTRODUCTION
Indoor air quality (IAQ) has intrigued scientists
since the mid-1800s (von Pettenkofer, 1858).
Historical events such as the London smog of 1952
instigated substantial air pollution investigations,
and differences in the health of people working
indoors and outdoors were explored (Fairbairn and
Reid, 1958). There has been a mild interest in the
capacity of construction materials to contribute to a
better atmospheric environment (Braun and
Wilson, 1970); but the main studies investigating
volatile organic compounds (VOCs) in buildings
didn't start until relatively recently, with at least 50
studies conducted between 1978 and 1990 (Brown
et al., 1994). Unfortunately, indoor air pollution
remains a recognised socio-economic problem
(EEA, 2013; Franchi et al., 2006), potentially
causing the loss of a projected $10 billion to $20
billion annually of projected savings and
productivity gains in the US alone (Fisk and
Rosenfeld, 1997).
Based on further scientific findings, the World
Health Organisation (WHO) compiled a set of
statements emphasising the right to breathe
healthy indoor air and the obligations of responsible
authorities (WHO, 2000). According to the
European Respiratory Society, pollutants “may
have an important biological impact even at low
concentrations over long exposure periods”
(European Respiratory Society, 2013). VOCs are
frequently linked to what is termed “sick building
syndrome” (SBS), which refers to a range of
symptoms that include eye irritation, nasal
congestion, dry skin, headache, fatigue, and
difficulty in concentrating; for example, an
estimated cost of $1 million was incurred due to
SBS at the Environmental Protection Agency’s
(EPA) U.S. headquarters due to decreased
productivity (Wallace, 2001).
In response, the industry introduced a wide range
of ‘air cleaning/treating’ products to the market, and
the removal of both chemical and biological indoor
contaminants remains a subject of interest
(Carslaw et al., 2013). However, such devices can
be energy intensive, contribute to some other form
of contamination, and have a short operational life
span compared to the building’s life. It is possible
that a passive solution can overcome such
limitations.
Sheep wool fibre has emerged as a niche
construction product, specifically as an insulation
material. In addition to its desired thermal and
hygric properties that contribute to indoor comfort,
this keratinous construction material is known to
absorb volatile and very volatile organic
compounds (Seo et al., 2009). Curling et al.
showed via a quick and simple method that
gaseous formaldehyde is absorbed by sheep wool
(Curling et al., 2012).
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The aim of this study is to differentiate between the
quantities of a representative range of VOCs
absorbed by different wool types (Mansour et al.,
2016, 2015). This will assess the ability of wool in
the built environment to passively enhance indoor
air quality in addition to its usual uses.
2. WOOL SAMPLES AND VOC SELECTION
To represent the widest range possible, 4 v/VOCs
were taken into account in this study covering
boiling points from -19 to 216.2°C, a large polarity
range, and different chemical conformations:
formaldehyde (simple and polar molecule), toluene
(aromatic), limonene (cyclic and non-polar), and
dodecane (straight chain and non-polar). Wools
from a number of differing wool breeds were
selected for use in the formaldehyde absorption
study, with further comparison made to other
species, as detailed below. All samples were
obtained from commercially available sources.
Sheeps (Ovis aries) wool
o Swaledale: A hardy United Kingdom
(UK) hill sheep with a coarse durable
wool predominately used for home
furnishings and insulation.
o Welsh Mountain: A hardy UK hill breed
and the wool has been commonly used
for home furnishings.
o Light Herdwick: A hardy UK mountain
breed with the wool commonly used for
home furnishing and insulation.
o Drysdale: A New Zealand breed noted
for its coarse wool that is used in home
furnishings.
o Blackface: A UK mountain breed with
the wool used mostly for home
furnishing and tweed cloth
Comparison species
o Alpaca. (Vicugna pacos) A llama like
camelid, with the hair in this case
obtained from domestic animals raised
in the UK.
In addition, the Swaledale and Light Herdwick
samples were obtained and initially tested in their
scoured and unscoured state. Scouring of wool is
an alkali- and detergent- based washing process
used to remove contamination material, grease
and lanolin which could affect the properties of the
wool.
Based on the results of the formaldehyde study
further selections were made to reduce the number
of wool types tested to Swaledale, Blackface and
Light Herdwick for the other 3 VOCs.
3. FORMALDEHYDE ANALYSIS
Formaldehyde sorption analyses were performed
using DVS system (Surface Measurement
Systems, London, UK). Wool’s ability to absorb
formaldehyde was thus assessed by the use of
dynamic vapour sorption (Curling et al., 2012); a
method that shows good repeatability. A flow of
formaldehyde gas was produced by bubbling
nitrogen into a 9.25% solution of formaldehyde and
water. This flow can be adjusted to give differing
partial pressures of the formaldehyde in the test
chamber e.g. the amount of exposure to gaseous
formaldehyde increases with increasing partial
pressure. A micro-balance was used to detect any
uptake of moisture and formaldehyde by the fibre.
The sample was subjected to the following cycles
to calculate the weight of formaldehyde that the
wool was able to chemically bind with:
a. Sample was left to equilibrate at 0% RH; i.e. it
was not exposed to moisture or formaldehyde.
This sets its baseline weight. Equilibration at all
steps was based on a weight change of less
than 0.002% over 10 minutes.
b. Sample was left to equilibrate at 90% RH; i.e. it
was exposed to high levels of moisture and
formaldehyde where it sorbed both and gained
weight.
c. Sample was again equilibrated at 0% RH; at
this point it lost all the water it had sorbed. Any
weight gain relative to the sample’s state at
step 1 was therefore attributable to sorbed
formaldehyde.
d. Steps a to c were repeated several times to
determine the total sorption capacity.
4. LIMONENE, TOLUENE AND DODECANE
ANALYSIS
Emissions generated from products can be tested
using micro-chambers but to study the absorption
potential however, certain modifications were
applied to introduce gaseous toluene, limonene
and dodecane. Due to differences in partial
pressures, it was not possible to elute them from a
highly compressed state. Therefore, sources were
prepared containing the VOCs in their liquid form in
steel tubes, sealed on one side and stoppered with
a flexible plastic cover on the other side; this
allowed the slow release of the VOCs in their
gaseous form under controlled temperature and
pressure.
A flow of nitrogen gas, cleaned of any VOCs
already present, was passed through the ≈100ml
chamber containing the VOC sources. Thereafter
the flow was controlled at 2.5±0.1ml/min, with an
additional 2.5±0.1ml/min flow of clean nitrogen
added. The controlled flow was fed into a vertical
sample holder where the wool sample was held for
2.5 hours per run. This ensures that the VOCs
pass through the whole sample and do not just
interact with some of the surface of the sample.
Any VOCs that were not absorbed by the sample
were absorbed by 200mg Tenax TA contained in
89x6.4mm inert coated stainless steel Tenax TA
tubes (Markes Int.), which was in turn analysed
using a thermal desorber coupled to gas
chromatography coupled to a flame ionisation
detector (GC-FID, Perkin Elmer). The GC-FID was
previously calibrated using Tenax TA tubes injected
with known amounts of the three VOCs (7 differing
amounts for each VOC ran in triplicates covering a
3
range of 3.9ng to 312ng was used to obtain a linear
calibration fit forced through an intercept of 0 and
having adjusted R2 values of 0.9978 for toluene,
0.9989 for limonene and 0.9996 for dodecane).
The injection of known amounts into the Tenax TA
tubes was accomplished by injecting 0.5μl of
differing concentrations of the VOCs dissolved in
methanol whilst a clean nitrogen flow of 100ml/min
was introduced for 15 minutes.
5. RESULTS AND DISCUSSION
In comparing the uptake of the VOCs it should be
remembered that two different methods were used.
The formaldehyde sorption by DVS utilised an
excess of formaldehyde and measured the
cumulative uptake throughout multiple cycles of
absorption/desorption, i.e. the wools’ total capacity
to absorb gaseous formaldehyde; however, the
micro-chamber approach utilised a constant flow of
relatively low concentrations of limonene, toluene
and dodecane (in the ng of VOC per g of wool
range) and does not represent the wools’ total
capacity to absorb these three VOCs but simulates
real life situations. Figure 1 shows the amount of
formaldehyde absorbed per kg of different wool
types. It is evident that both wool type and condition
(scoured or unscoured) have an effect on wool’s
ability to absorb formaldehyde. It was also noticed
that there is a general trend that the more darkly
pigmented the wool fibre is, the higher its sorption
capacity. Further research will determine if there is
a correlation between fibre pigmentation of the
same wool source.
Figure 1. Mass of formaldehyde chemically bound by different
wool types per kg of wool.
Figure 2 shows the amounts of toluene, limonene
and dodecane absorbed per g of some wool types.
As in the case of formaldehyde, it was evident that
both wool type and condition (scoured or
unscoured) had an effect on absorption. It is
noteworthy that in the case of unscoured wools
types, the total amount of limonene and dodecane
that it was subjected to was completely absorbed,
unlike the case of scoured wools which show
residual peaks of unabsorbed material in their
chromatograms.
Figure 2. Mass of toluene, limonene and dodecane absorbed
by different wool types.
When the absorption of the studied VOCs is
compared across the range of different wool types
a trend can be observed based on the polarity of
the VOC. For example, in the case of
formaldehyde absorption, Swaledale wool absorb
significantly less than Blackface. However, looking
at the increasingly non-polar VOCs, Swaledale is
seen to absorb more limonene and significantly
more dodecane than Blackface does. This
indicates that the polarity of the surface may be
different between the wool types, leading to
different levels of interactions between different
VOCs depending on their polarity. Wool product
producers may be able to take advantage of this
phenomenon and create tailored products that
absorb a specific range of those VOCs which the
air in a certain type of building is known to contain
high levels of.
Unscoured wool was observed to absorb more of
the tested VOCs than its scoured counterpart. This
could be due to the presence of lanolin or
contaminants that are removed during the scouring
process, but it could also be due a modification of
the wool surface due to the scouring process.
Further tests were carried out to see the effect of
the amount of VOCs wool fibre is exposed to in
relation to its absorption. Using Light Herdwick
wool, it was noted that increases in the amount of
VOCs, most notably limonene and dodecane, the
sample was exposed to at the same conditions
were accompanied with increase in absorption,
even at a much higher level, as seen in Figure 3.
When we examine the percentage of the VOCs
absorbed as a function of the total amount the wool
was exposed to, we see that, within significance,
the percentage is the same for each VOC as
illustrated in Figure 4.
4
Figure 3. Mass of toluene, limonene and dodecane absorbed
by Light Herdwick wool at different levels of exposure.
Figure 4. Percentage of toluene, limonene and dodecane
absorbed by Light Herdwick wool as a function of total amount
of exposure.
5. CONCLUSIONS
The data presented in this paper shows that wool,
a natural and sustainable material, is able to
absorb a range of potentially harmful chemicals
from the indoor environment. This has important
considerations for the prevention or reduction of
sick building syndrome at a time when this issue is
becoming more prevalent. The data indicates that
the breed of sheep that provided the wool may
have an important effect on the absorption of
VOCs. Further observations is that the absorption
by scoured wool is discriminate, equating to a
percentage of the amount of VOC the fibres are
exposed to; this indicated that sheep wool
insulation does not simply absorb till a maximum
limit and stops, but rather it buffers indoor VOC
levels to smooth down spikes in concentrations
over time. The use of wool in building design may
therefore have an important role not only in
imparting thermal efficiency but also in the
improvement of indoor air quality.
5. ACKNOWLEDGMENTS
The research leading to these results has received
funding from the European Union's Seventh
Framework Programme (FP7/2007-2013) for
research, technological development and
demonstration under grant agreement no 609234.
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... They concluded that the wool both chemi-and physi-sorbed the formaldehyde, thus acting both as a scavenger and a buffer that would release the formaldehyde back into the atmosphere when levels were low. The research into this area was expanded by Mansour et al. (2016aMansour et al. ( & 2016b in which different varieties of sheep's wool were assessed for their use as a VOC absorbent. The experiments used limonene, dodecane and toluene as model compounds, representing VOCs with different polarities. ...
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