Clay-based heat insulator composites: Thermal and
water retention properties
Kamal Al-Malaha,⁎, Basim Abu-Jdayilb
aDepartment of Chemical Engineering, Jordan University of Science and Technology, Irbid, Jordan
bVisiting Associate Professor at Department of Chemical Engineering, University of Arab Emirates, Al-Ain, United Arab Emirates
Received 25 August 2006; received in revised form 2 January 2007; accepted 8 January 2007
Available online 17 January 2007
The formulation of unsaturated polyester composite as an insulating material that gives the best in terms of thermal and water
retention properties was investigated as a function of filler type and content. Different types of local fillers were used in the
formulations. Bentonite-based unsaturated polyester composite which is denoted as BBUPEC was found to have stable and
compatible thermal, physical, and chemical properties. BBUPEC thermal conductivity, k, values lie between 0.1 and 0.2 W/(m K). It
was found that at 50 wt.% filler content and 40 wt.% polyester content, k of BBUPEC is minimum. Calcium carbonate-based
and dissolution tookplace.In termsof waterretention value,citric acidand NaOH impregnationvalues,one could say thatbentonite-
thermal conductivity and physical and chemical stability and with such inexpensive and abundant fillers from natural resources, they
pose a potential thermal insulating material. Sandwiching of BBUPEC in wall structures by one-third of the total thickness will
significantly reduce the overall heat transfer coefficient in home and industrial applications by at least 50%.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Thermal insulator; Jordanian bentonite; Composite material; Unsaturated polyester; Clay; Water retention; Thermal conductivity
There is a necessity for finding alternative thermal
insulating materials to preserve energy by minimizing
energy losses. Jordan suffers from a cold and humid
weather during winter season, which requires minimi-
zation of energy loss from the shelters to the environ-
ment. Home shelters are usually heated by a kerosene-
based stove and to a lesser degree by central, oil-burned
heaters. On the other hand, there is also a necessity for
air-cooling as some temperature spikes occasionally
occur during summer. Therefore, thermal insulation is
becoming an essential element in home shelters and in
commercial and governmental buildings, and is com-
monly used between wall cavities.
Insulation materials can be made in different forms
including loose-fill form, blanket batt or roll form, rigid
form, foamed in place, or reflective form. The choice of
the proper insulation materials type and form depends
on the type of application as well as the desired materials
physical, thermal and other properties (Al-Homoud,
2005). Some typical properties of insulating materials
that are considered as a must in terms of mechanical,
Applied Clay Science 37 (2007) 90–96
⁎Corresponding author. Tel.: +962 27201000.
E-mail address: firstname.lastname@example.org (K. Al-Malah).
0169-1317/$ - see front matter © 2007 Elsevier B.V. All rights reserved.
physical, and thermal properties are: low thermal
conductivity, prevention of water leak, ease of handling
and machining, durability and light weight, fire
resistance, and safe/healthy use and installation. In
addition, the thermal insulation of buildings must be
cost effective; i.e., reduction in running cost due to
energy saving should outweigh the increase in fixed cost
within the lifetime of the building.
Al-Sanea (2002) found that the inclusion of a 5-cm
thick molded polystyrene layer reduced the roof heat-
transfer load to one-third of its value in an identical roof
section without insulation, while using a polyurethane
layer instead, reduced the load to less than one-quarter.
cork and gypsum. They found that cork-gypsum com-
posites are characterized both by low thermal conduc-
tivity andlowdensity.Ontheother hand,themechanical
properties of cork–gypsum composite were poor. Such a
composite material was suggested for use in building
applications as partitions.
Kumar (2003) examined hollow blocks made of fly
ash–lime–phosphogypsum (FaL-G) composites, as
potential thermal insulating materials. Fly ash is rich
in silica and to a lesser degree in alumina; lime being
rich in calcium oxide; and calcined phosphogypsum in
calcium sulfate di-hydrate. It was found that water
absorption of FaL-G hollow blocks was between 19.2%
and 37.2 wt.%, with a minimum value at 30 wt.% of fly
ash and a maximum value at 80 wt.% fly ash content.
Marcovich et al. (2001) examined composites made
from unsaturated polyester/styrene thermoset matrix and
woodflour. Using scanning electron microscopy (SEM),
they showed that at low filler loading of woodflour, the
resin managed to fill in the gaps between fibers of filler,
completely the filler, hence empty capillaries were found.
Bureau et al. (2001) examined the effect of styrene
content on fragility of unsaturated polyester resins.
When the contents of styrene increases, they found that
the molar ratio of maleic anhydride/styrene controls the
length of the styrene chain that will be connected to the
unsaturated polyester. Using differential scanning calo-
rimetry (DSC), they found that the increase of the glass
transition temperature observed for their samples in the
range of styrene content included between 25% and
50% (w/w) is due to an increase of the average length of
the styrenic chain, hence, the fragility of these networks
increases as the styrene content increase.
In this research, focus is made on the formulation of
polyester–clay composite as an insulating material that
gives the best in terms of thermal and mechanical
properties. Different types of local fillers were used in
the formulations. In this paper, thermal conductivity and
water absorption capacity as a function of filler type and
content will be addressed. There are valuable fillers
available in Jordan, which can be used in this aspect; for
example, limestone and clays (smectite, illite, and
kaolinite). Clay materials are available in abundant
amounts in Al-Azraq area with a projected amount of
17 million metric tons.
2. Experimental methods
Polyester used in this study was obtained from the Inter-
mediate Petrochemicals Co., Jordan. Unsaturated polyester
was chosen because of their ease of handling, low water ab-
sorption values, low cost and its rapid curing with no gases
evolved (Wordingham and Reboul, 1968; Billmeyer, 1984).
The type of polyester used is HM190 with a styrene content of
33–35%, an acid content of 21–23% and a viscosity of 300–
400 mPa s.
Three different bentonite samples obtained from different
areas in Jordan and one feldspar sample was used in this study
as fillers. The International Company for Ceramics (Mufraq–
Jordan) supplied us with the feldspar sample. The chemical
analysis of the fillers used is shown in Table 1. In addition,
commercial calcium carbonate obtained from the Jordan
Carbonate Company (Amman–Jordan) was used for the sake
of comparison. Calcite is the natural form of calcium carbonate
which is widely used in plastics as filler due to its high
dispersability, low oil absorption characteristics, higher impact
resistance, smooth surface finish, easy processing and
excellent dimensional stability. Pure calcite is a relatively
soft material (Moh hardness 3.0) with a specific gravity of 2.7.
3. Sample preparation
The fillers were crushed and then screened to a grain
size of 0.5 mm. To overcome the low degree of cross-
linking in polyester, styrene (C6H5CHCH2) was added.
Styrene was chosen because of its compatibility with the
polyester. Its low viscosity makes the mixing process
(unsaturated polyester/styrene with filler) easier. For the
was used because of its solubility in styrene and when
mixed with the accelerator (cobalt–octoate) decomposi-
tion occurs at room temperature to give free radicals.
Different filler contents (25–60 wt.%) at a constant
styrene/polyester ratio (18 wt ratio) and different styrene
contents (5–12 wt.%) with constant filler content of
50 wt.% composites were prepared. The composition of
the prepared composite was chosen upon the results of
our preliminary study (Abu-Jdayil et al., 2002). The
91 K. Al-Malah, B. Abu-Jdayil / Applied Clay Science 37 (2007) 90–96
composites were prepared at room temperature using a
high viscosity mixer. Then the mixture was poured in
the suitable mold prepared from stainless steel under
mechanical vibration to get rid of the air bubbles that
may occur while pouring. Different types of molds, with
different shapes and sizes, were fabricated to meet the
requirements of the tests that were performed on the
prepared composites. The interior surface of the molds
was coated with paraffin wax and poly vinyl acetate to
prevent sticking of the sample with the mold.
3.1. Thermal conductivity test
The Hilton B480 Thermal Conductivity of Buildings
& Insulating Materials Unit was used to measure the
thermal conductivity of the prepared composites. The
apparatus is available at the University of Applied Sci-
ence/Jordan. The measurement conditions followed the
standard methods reported by ISO 8301. The steady state
method was used in these measurements, where the
thermal conductivity was determined frommeasurements
of the temperature gradient in the composite material and
of the samples were 300 mm×300 mm×10 mm. The
measurementswereperformed induplicatesand the aver-
age value was reported. Itshouldbepointed out thatinall
is, on the average, less than 5% of the measured value.
3.2. Water retention
This test was performed according to the standard
test ASTM D-570-81. Distilled and tap waters were
used in this test. The test specimens were of the form of
a bar 42 mm long by 35 mm wide by 20 mm thickness.
The specimen was placed in a container of water at room
temperature, and rested at its edge and entirely
immersed. At the end of 24 h the sample was removed
from water, wiped free of surface moisture with a dry
cloth, weighed to the nearest 0.001 g immediately, and
then replaced in water. The weighing was repeated at the
end of the first week and every week thereafter and for
4 weeks. This time was enough to reach the saturation
(equilibrium) condition, where no change in the sample
weight was noticed. During this period, the specimens'
weight difference was recorded at different times.
The percentage water retention (WR%) for unsatu-
rated polyester composites was calculated using
WR% ¼weight of equilibrated sample−weight of dry sample
weight of dry sample
It is a measure for percent relative increase in weight
due to water imbibement or retention within the solid
3.3. Resistance to chemical reagents
This test was performed according to the test ASTM
D-543-84. Two chemical reagents were used, namely;
citric acid and sodium hydroxide solutions. 104 g of
citric acid crystals were dissolved in 935 mL of water to
produce a solution with 1% concentration. On the other
hand, 107 g of NaOH in 964 mL of distilled water to
produce a 10% NaOH solution. Then the same
procedure of water retention was followed.
The percentage acid or base impregnation for un-
saturated polyester composites was calculated using
acid ðbaseÞ impregnation %
¼weight of equilibrated sample−weight of dry sample
weight of dry sample
It is a measure for percent relative increase in weight
of the solid matrix due to acid or base impregnation.
It should be pointed out that in all experiments
carried out the standard error of measurement is, on the
average, less than 5% of the measured value.
4. Results and discussion
The thermal conductivity coefficient of un-saturated
polyester was experimentally found to be 0.1 W/(m K).
Chemical analysis of used fillers
Filler type SiO2(wt.%) Al2O3(wt.%)Fe2O3(wt.%) MgO (wt.%)CaO (wt.%) Na2O (wt.%)K2O (wt.%)H2O (wt.%)
92 K. Al-Malah, B. Abu-Jdayil / Applied Clay Science 37 (2007) 90–96
Increasing the filler content, in general, causes an
increase in thermal conductivity coefficient, see Fig. 1.
This may be due to the higher conductivity of the filler.
The thermal conductivity of most mineral fillers is about
and their incorporation considerably increases the
conductivity of a composite (Rothon, 1999). For exam-
ple, pure, dense MgO and pure, dense Al2O3do have
thermal conductivity values about 38 W/(m K) at room
(Rothon, 1999). Bentonite and feldspar used in our study
are rich in silica and alumina, see Table 1. If the heat
thermal conductivity of a substance, then the overall
sum of the individual resistance for each constituent
while weighted by their volume (or weight) fraction.
þ N þwN
wherewirepresents theweight fractionof species i andki
is the thermal conductivity, summed over all entering
constituents of a composite.
Eq. (3) will be applied, first, to the filler itself, and
second, to the Bentonite-Based Unsaturated PolyEster
Composites (BBUPEC). For the filler itself, it will be
metal oxides, silica, and water. Thus, the average thermal
conductivity for typical filler, like bentonite 1, will be:
kbentonite1¼ 3:5 W=ðm KÞð4bÞ
Applying Eq. (3) to, for example, bentonite 1-based
unsaturated polyester will result in:
kBBUPEC¼ 0:2 W=ðm KÞð5bÞ
Thus, the thermal conductivity value of BBUPEC is
around 0.2. Such a value is quite comparable with the
range of empirical values reported in Fig. 2, for instance.
The experimental value at the same composition is
about 0.13 W/(m K). The presence of voids (or, poro-
sity) is expected to be behind an experimental value that
is less than that predicted by Eq. (3).
Compared with building bricks (or cement plaster)
which have thermal conductivity of 0.72 W/(m K), with
concrete (stone) which has a thermal conductivity of
0.93 W/(m K), and with reinforced concrete with a
thermal conductivity of 1.73 W/(m K), BBUPEC's have
thermal conductivity values between 0.1 and 0.2, at
50 wt.% filler content and about 40 wt.% polyester. If an
average resistance, using Eq. (3), is taken to the afore-
mentioned materials without incorporating BBUPEC,
A kaverageof 1.0 W/(m K) is obtained. On the other
hand, if a wall is built with such building materials
(building blocks, concrete and reinforced concrete)
while this time BBUPEC comprises one-third of the
Fig. 1. Effect of filler content on the thermal conductivity of unsa-
turated-polyester composite materials.
Fig. 2. Effect of polyester content on the thermal conductivity of
unsaturated-polyester composite materials.
93K. Al-Malah, B. Abu-Jdayil / Applied Clay Science 37 (2007) 90–96
wall thickness while the rest of the wall is made of the
previous materials, then
which means that the new value of kaverage will be
0.427 W/(m K). If 0.1 W/(m K) is used instead of 0.2 for
BBUPEC, then kaveragewill be 0.25 W/(m K).
Consequently, constructing a wall made of BBUPEC
that comprises one-third a wall thickness, the minimum
will be about 57% and the maximum will be about 75%.
Fig. 2 shows the effect of polyester content on the
thermal conductivity coefficient for the specimen
containing 50 wt.% filler content. It should be noted
that increasing the polyester content will be at the
expense of styrene content. In the case of calcium
carbonate, feldspar and bentonite 3 increasing the
polyester content from 37.5 to 40.0 wt.% causes a
decrease in the thermal conductivity coefficient of the
composite to reach its minimum within the examined
range. At low polyester content (i.e., 37 wt.% or less) it
fill in the gaps within the filler, in the presence of a
relatively high filler load, which will result in void
formation in specimens and therefore the thermal con-
ductivity coefficient is reduced. Increasing the polyes-
in the thermal conductivity. This increase is more likely
due to the co-polymerized polyester/styrene products,
which act as strong binders and hold the grains of
bentonite together into one solid mass.
Fig. 3 gives the distilled water retention percent for
different composites as a function of filler content., In
general, WR% value of composite is low and it in-
creases with increasing bentonite content for bentonite 2
and 3. On the other hand, bentonite 1 shows a reverse
behavior where the water retention decreases with
increasing filler content. This behavior of bentonite 1
was also noticed when the distilled water was replaced
Fig. 5. Effect of polyester content on the distilled water retention of the
Fig. 4. Effect of filler content on the tap water retention of the
Fig. 3. Effect of filler content on the distilled water retention of the
Fig. 6. Effect of filler content on the citric acid impregnation for the
94K. Al-Malah, B. Abu-Jdayil / Applied Clay Science 37 (2007) 90–96
by tap water, see Fig. 4. The comparisonbetween Figs. 3
and 4 reveals that the type of water does not play a
significant role in the amount of water retained. The
greatest difference between the distilled and tap water
retention was recorded for bentonite 2.
From Fig. 5, it is noticed that increasing the polyester
content does not cause a pronounced change in the water
retention percentage. It is expected that in the examined
polyester content range, the cross-linking process is very
efficient, which reduces voids needed for water reten-
tion (Abu-Jdayil et al., 2002). The results of water
retention are comparable with those reported by Ismail
et al. (1999).
Fig. 6 illustrates variation of citric acid impregnation
capacity of the composite specimens as a function of
filler content. It is clear that increasing the filler content
decreases the citric acid absorption to reach a minimum
at the filler content of 50 wt.%. It is believed that at this
filler content the composites possess the best network
structure. However, the calcium carbonate-based com-
posites show a negative absorption percentage, which
means that the citric dissolves parts of the composite. It
should be mentioned that the percentage of citric acid
absorption is comparable with that of water. Increasing
the polyester content decreases generally the citric acid
impregnation, as shown in Fig. 7.
As shown in Fig. 8, the NaOH impregnation
decreases with increasing filler content to reach a
minimum value at a filler content that lies between 40
and 50 wt.%. On the other hand, the prepared
composites show a greater resistance against NaOH
than citric acid. For example, in the case of a bentonite 2
based composite with a filler content of 50 wt.%, the
citric acid impregnation is 2.9% while the NaOH
impregnation is 1.0%. Bentonite 3 shows the best
resistance against the NaOH. Except for bentonite 1
composite, the NaOH impregnation percent is indepen-
dent of polyester content in the range examined, see
Fig. 9. It should be mentioned here that it is really hard
for us to compare our results for water, acid, and base
retention values with those of other investigators,
simply, because, the operating conditions in terms of
immersion or equilibration time, type of composite, and
temperature are different.
Natural clay (bentonite) can be utilized to manufac-
ture a stable and compatible composite material, which
was denoted as BBUPEC. In general, with 50 wt.% filler
content and 40 wt.% polyester content, the thermal con-
ductivity of un-saturated polyester composite showed a
minimum thermal conductivity, k. At the afore-men-
tioned condition, calcium carbonate-based composite
showed the minimum value of k which is 0.1 W/(m K),
followed by bentonite-1-based and bentonite-2-based
composites, followed by bentonite-3-based, and finally
followed by feldspar-based composites. However, in
Fig. 9. Effect of polyester content on the NaOH impregnation for the
Fig. 8. Effect of filler content on the NaOH impregnation for the
Fig. 7. Effect of polyester content on the citric acid impregnation for
the composite materials.
95K. Al-Malah, B. Abu-Jdayil / Applied Clay Science 37 (2007) 90–96
terms of citric acid impregnation, calcium carbonate-
based composite was not stable and dissolution took
place. In terms of water retention value, citric acid and
NaOH impregnation values, one could say that benton-
ite-3-based composite was the best among BBUPEC.
Consequently, one would say that bentonite-based
composites show good characteristics in terms of
thermal conductivity and physical and chemical stability
and with such cheap and abundant fillers from natural
resources, they show a promising thermal insulating
material both for domestic and industrial applications.
BBUPEC Bentonite-Based Unsaturated Poly-Ester
This research was funded by the Higher Council for
Science and Technology, Amman–Jordan.
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