ARID ZONE JOURNAL OF ENGINEERING, TECHNOLOGY &
ENVIRONMENT

AZOJETE June 2020. Vol. 16(2):415-422
Published by the Faculty of Engineering, University of Maiduguri, Maiduguri, Nigeria.

Print ISSN: 1596-2490, Electronic ISSN: 2545-5818
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Corresponding author’s e-mail address: abdulrahimat@unimaid.edu.ng 415

ORIGINAL RESEARCH ARTICLE
THERMAL AND MECHANICAL PROPERTIES OF CLAY AND DROP-OFF OF

CEIBA PENTANDRA (KAPOK) PLANTWOOL MIXTURE

A. T. Abdulrahim1*, S. Abdulkareem2, S. A. Abdulraheem1 and W. J. Mohammed1
1Department of Mechanical Engineering, University of Maiduguri, P.M.B. 1069 Maiduguri, Nigeria

2Department of Mechanical Engineering, University of Ilorin, P.M.B. 1515, Ilorin, Nigeria

*Corresponding author’s email address: engrabdulrahimat@gmail.com, abdulrahimat@unimaid.edu.ng

1.0 Introduction
Thermal insulations play a major role in the design and production of energy efficient process,
devices and systems because of their ability to retard heat transfer by acting as barriers in the
path of heat flow. The variety of insulation materials available are mostly those that are
primarily made of fiberglass, mineral wool, polyethylene, form, or calcium silicate (Cengel,
2007; Holman, 2010). It is important to note that synthetic fiber composites are being replaced
by environmentally friendly materials such as natural fiber because of their low weight, density,
cost, renewable nature among others (Ayugi, 2011). Natural fiber low to zero toxin ratings, easy
to reuse and dispose with significant health benefits have also been recognized as additional
advantages (Andy et al., 2011).

Drop-off of Ceiba pentandra plant (Kapok) wool have been reported in literature as a material
for thermal insulation. Studies carried out by Abdulkareem et al. (2016) and Abdulkareem et al.

ARTICLE
INFORMATION

ABSTRACT
This study is aimed at the investigation of the effects of the addition of clay to
drop-off of Ceiba pentandra (Kapok) plant wool on the thermal and mechanical
properties of the materials produced. Because of the considerable increase in the
use of insulations in many heat devices, process and systems, and dependence
on imported insulations, the need to process natural fibre insulations to form
rigid or semi-rigid material using appropriate and sustainable binding material
cannot be over emphasized. For this study, Kapok wool and clay mixture were
investigated. Five samples were prepared: Sample A was 100% Kapok wool
while samples B, C, D, and E were mixture of Kapok wool and Clay in water
with varying percentage of clay quantities (Kapok to Clay ratio; B 2:1, C 2:1.5, D
2:2.5, E 2:3.5). The samples were prepared into cylindrical shapes, compressed
and dried. The Thermal conductivity meter on Armfield Heat transfer equipment
was used to find the thermal conductivity of each sample, while the thermal
diffusivity and thermal resistivity of samples were determined using appropriate
formula. Universal Testing Machine (Model: FS 50AT) was used to determine
the stress at Yield and Young Modulus of elasticity for samples B, C, D, and E.
Results show that the density of sample increases with increase in clay content.
Thermal conductivities of the samples after the addition of clay ranges between
0.027 (W/mK) and 0.013 (W/mK). Reversed was the case for Thermal
Resistivity, the value of thermal resistivity first decreases with addition of clay
content (sample B) and then started increasing with increased clay content in the
samples, and the values range from 37.037 to 76.923mKW-1. Thermal
diffusivity of sample decreases from 1.2762 x10-3 m2/s to 0.2810 x10-3 m2/s
with increasing clay content. Results also show that the Stress at Yield of samples
range from 0.012 N/mm2 to 0.022 N/mm2 while the Young Modulus of
elasticity range from 0.113 N/mm2 to 0.222 N/mm2. Kapok wool – Clay
combination of ratio 2:3.5 (Sample E) have the highest value of thermal
resistivity and lowest thermal diffusivity, though with high density, which suggests
it to be the best among samples in the study for insulation purposes. The
products are recommended for use as insulations of thermal storage for medium
temperature applications.

© 2020 Faculty of Engineering, University of Maiduguri, Nigeria. All rights reserved.

Submitted 10 March, 2020
Revised 22 May, 2020
Accepted 27 May, 2020

Keywords:
Kapok wool
Clay
Thermal conductivity
Thermal Diffusivity
Modulus of Elasticity

mailto:engrabdulrahimat@gmail.com
mailto:abdulrahimat@unimaid.edu.ng
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Abdulrahim et al: Thermal and Mechanical Properties of Clay and Drop-Off of Ceiba Pentandra (Kapok) Plant Wool Mixture.
AZOJETE,16(2):415-422. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng

Corresponding author’s e-mail address: abdulrahimat@unimaid.edu.ng 416

(2015a,b) on the investigation of thermal insulation properties of Biomass composites involving
Kapok wool, Sugarcane bagasse, Coconut fibre, Luffa Cylinderica and Cellulose using Molasses
as binder revealed the potentials of composites from these biomass materials as insulation
materials because of their high thermal resistivity and low thermal conductivity. Ali and Zeitoun
(2012) have also investigated Apple of Sodom Fibre (Calotropis procera) as insulation material,
and its potential as a good insulator has been established.

Because of the considerable increase in the use of insulations in many heat devices, process and
systems, there is growing needs for the development of new and more effective as well as
affordable insulation materials. The need for the use of locally available materials that will
eliminate the problem of importation of insulators from other countries cannot be over
emphasized. Natural fibre insulations can be process to form rigid or semi-rigid rolls or loose
insulation material for a variety of uses. Clay could be a binding agent for processing natural
fibre into a rigid or semi-rigid insulation materials because of its fine particle size and plastic
nature as well as its refractory tendencies (Lewis, 2013; Ashby and Jones, 1998). Studies have
shown the availability of clay in Nigeria as well as its uses for pottery, mud building of local
houses, refractories and glaze making (Adams et al., 2018; Adams et al., 2017; Counts et al.,
2000). To this effect, this study is aimed at the investigation of the effects of the addition of clay
to drop-off of Ceiba pentandra (Kapok) plant wool on the thermal and mechanical properties of
the insulation materials produced.

2 Materials and Methods
2.1 Samples preparation
The locally available insulating materials that have been considered in this study are Kapok
wool and clay. Kapok pods were obtained at the University of Maiduguri, Maiduguri, Nigeria
from Kapok Trees. The Kapok wool was removed from its pod (Figure 1a) and hand picking of
the kapok seed in the Kapok wool was done and it was then brushed to remove other unwanted
particles (Figure 1b). The Kapok wool comprise 64% cellulose, 13% lignin and 23% pentosane
(Kobayashi et al., 1977).

Figure 1: Kapok Wool Source and Processing: (a) Kapok Pod; (b) Kapok Wool, Seed, Exocarp
and other unwanted particles

Clay was collected from a deposit in Maiduguri, Nigeria which was further dried and finally
crushed down to smaller sizes. The clay was sieved by hand through a 500 µm sieve to remove
coarse particles and foreign materials that might have been present in the clay. The chemical
compositions of clay sample were determine using Single beam spectrophotometer (with test
kits, hot plate and furnace) at the Department of Geology, University of Maiduguri, Maiduguri,
Nigeria.

Samples were weighed using an Electronic Kitchen Scale Model: SF 400. Five samples of the
materials were prepared. Sample A is 100% Kapok wool while samples B, C, D, and E are
mixture of Kapok wool with Clay mixed in water with varying percentage of clay quantities as
shown in Table 1. The samples were prepared into cylindrical shapes of thickness (t) in m and
diameter (d) in m. The volume (V) is given by equation 1:

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tRV 2 (1)

where: 2
dR 

Table 2: Composition of samples by mass

S/No. Sample Kapok (g) Clay (g) Water (ml)
1. A 20 - 200
2. B 20 10 200
3. C 20 15 200
4. D 20 25 200
5. E 20 35 200

A 110mm diameter mould was used to mould the samples. Sample A was achieved by adding
200ml of water to the 20g of Kapok wool and vigorously mixing the contents until the Kapok
wool was soaked in the water. For samples B, C, D and E, 200ml of water was mixed with the
corresponding mass of clay for each of the samples and them mixed with the Kapok wool. The
mixture was poured into the mould and compressed by a total load of 196 N. The samples were
left for one day in the mould, then removed and sun dried for 2 days. The samples were oven
dried for 9 hours at 105 oC before tests were carried out. This was done to eliminate moisture
content if present in the samples. A total number of 5 samples were produced. The samples are
as shown in Figure 2.

Figure 2: Dried Samples of A, B, C, D, and E

Determination Density of sample
The density of the sample was obtained from Equation 2 according to Abbott (1980):

Volume
MassDensity  (2)

Thermal conductivity
The Thermal conductivity meter on Armfield equipment (Model: HT12) with Heat transfer
service unit (Model: HT10XC) equipment were used to check the thermal conductivity of the
materials. The probe consists of single heater wire and thermocouple. When constant electric
power (energy) is given to the heater, the temperature of the wire will rise in exponential
progression. Temperature rising with time axis scaled in logarithm. The angle of this line
increases if the sample has less thermal conductivity (TC) and decreases if it has higher TC.

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Abdulrahim et al: Thermal and Mechanical Properties of Clay and Drop-Off of Ceiba Pentandra (Kapok) Plant Wool Mixture.
AZOJETE,16(2):415-422. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng

Corresponding author’s e-mail address: abdulrahimat@unimaid.edu.ng 418

Therefore, TC of a sample was determined from the angle of the rising temperature graphic line
(Armfield equipment model HT12 manual).
Thermal conductivity of sample, k [W/mK] is expressed as given in Equation 3 as reported by
Tripathi et al. (2016), Eeday et al. (2014) and Rodriguez et al. (2011):

 12

1

2

4 TT
t
tqIn

k














(3)

where: q is generated heat per unit length of sample/time [W/m]
t1 and t2 are measured time length [sec]
T1 and T2 are Temperatures at t1and t2 respectively [K]

Thermal Resistivity
The Thermal Resistivity of samples were calculated from Equation 4 according to Satta and
Steve (2008):

Thermal Resistivity,
k

r 1
 (4)

Thermal diffusivity
The Thermal diffusivity of samples were calculated using the expression given by Incropera et al.
(2007) as in Equation 5:

pC
k


 (5)

where: Thermal Diffusivity, α (m2/s),
ℓ is the density (kg/m3) of the material,
Cp is the specific heat capacity (J/gK).

The plot of natural logarithm of change in temperature versus reciprocal of time ( In δT vs 1
t
)

were drawn for the samples, and heat capacity of each sample was obtained from the intercept of
the plots.

Compressive Test
The compressive stress test was determined by the use of Universal Testing Machine
(Testometric Type: DBBMTCL-5000kg; Serial No. 38140; Model: FS 50AT) at the University
of Ilorin, Ilorin, Nigeria for samples B, C, D and E. On sample A, the study of effect of
compression stress on the sample was not done since it is 100% Kapok wool. For other samples
(B, C, D, E), the samples were mounted one at a time and test speed of 2mm/minute was applied
until samples failed after it has reached yield point. The stress at Yield and Young Modulus of
elasticity were then recorded.

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3 Results and Discussion
Table 2 show the chemical composition of kapok wool
Table 2: Chemical compositions of the kapok wool and clay

Component Chemical Composition (%)
Clay Kapok wool

Silica (SiO2) 55.71 -
Alumina (Al2O3) 19.86 -
Ferric Oxide (Fe2O3) 0.74 -
Titanium oxide (TiO2) - -
Lime (CaO) 7.7 -
Magnesium oxide (MgO) 1.68 -
Potash (K2O) 7.23 -
Soda (Na2O) 0.52 -
Manganese oxide (MnO) 1.61 -
Phosphorous pentaoxide (P2O5) 1.19 -
LOI 2.8 98.57
Carbon (C) - 0.99
Hydrogen (H) - 0.41
Nitrogen (N) - 0.01

Note: Silica and Alumina were determined by XRF while others by ICP-OES.
Table 3 shows the results of the Density, Thermal conductivity, Thermal Diffusivity and
Thermal Resistivity of the samples.
Table 3: Density, Thermal conductivity, Thermal Diffusivity and Thermal Resistivity of
Samples of kapok wool-clay mixture

Sample Density,ρ
(g/cm3)

Thermal Conductivity
(W/mK)

Thermal Diffusivity
(m2/s)

Thermal Resistivity
(mKW-1)

A 0.067 0.0177 1.2762x10-3 56.497
B 0.125 0.027 1.1232 x10-3 37.037
C 0.240 0.026 0.7085 x10-3 38.462
D 0.275 0.021 0.5755 x10-3 47.619
E 0.700 0.013 0.2810 x10-3 76.923

From Table 3, the density of sample increases with increase in clay proportion as reflected in
samples B, C, D and E. Sample A has the lowest density while sample E has the highest density.
The addition of clay to Kapok wool first caused a rise in thermal conductivity of the sample as
shown in Table 4 in the case of sample B, and with increase proportion of clay content, the
thermal conductivity decreases as reflected in samples C, D and E. Reversed were the case for
Thermal Resistivity: the value of thermal resistivity first decreases with addition of clay content
(sample B) and the started increasing with increase in clay content in the sample. This is an
indication that Samples C, D, and E have improved insulation properties proportion to increase
clay contents.
Table 4 shows the results of the stress at yield and the modulus of Elasticity at failure for the
samples.
Table 4: Stress at Yield and Modulus of Elasticity of Samples at Failure

S/No. Sample Stress at Yield (N/mm2) Young Modulus of elasticity (N/mm2)
1. A - -
2. B 0.012 0.113
3. C 0.015 0.148
4. D 0.020 0.151
5. E 0.022 0.222

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AZOJETE,16(2):415-422. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng

Corresponding author’s e-mail address: abdulrahimat@unimaid.edu.ng 420

From Table 4, it can be observed that both the stress at yield and the modulus of elasticity
increase as the clay content in the sample increased.

The chemical composition of the clay as presented in Table 2 showed significant amounts of
SiO2 and Al2O3 (55.71 and 19.86% respectively). The clay can be used as a binder with this level
of Silica and Alumina contents, and it is plastic when moist but hard when fired (Lewis, 2013).
The results obtained revealed that the value of the thermal conductivities of the samples were
found to first increase from 0.0177 W/mK to 0.027 W/mK with Kapok wool to Clay ratio 2:1
while the density increases from 0.067 to 0.125 g/cm3. The loose fibrous nature of Kapok wool
enhances its conductivity and with limited amount of clay added might brought about the
increase since clays have plate-like molecules with charges on their surfaces (Ashby and Jones,
1998). More so at the low level the clay may just be serving as a binder and not as a
reinforcement. Similar report was made by Ali and Zeitoun (2012) when they asserted that low-
density fibers (Calotropis procera) with no resin (loose fibres) have a lower thermal conductivity
than the ones made in a board form using Phenol-Formaldehyde resin. The addition of more
clay to obtain a Kapok Wool to Clay ratio 2:1.5 brought about a reduction in thermal
conductivity to 0.026 W/mK while the density increases to 0.240 g/cm3. The solid clay particles
might have been finely dispersed through air spaces within the Kapok wool thereby preventing
heat transfer by convection through the air in the void spaces as well serving as reinforcement.
And subsequent increases in clay content (Kapok Wool to Clay ratio 2:2.5) reduces the thermal
conductivity while it increases sample density. Reduction in thermal conductivity was also noted
at Kapok Wool to Clay ratio 2:3.5. This may be due to the evacuation of air in the space within
the Kapok wool due to increasing clay content which could have reduce the effective thermal
conductivity of the system as supported by Incropera et al. (2007) report. The stress at yield and
the modulus of elasticity of the samples were found to increase with increasing clay content in
sample. Since clay also served as binder thereby binding the loose Kapok wool together and
improve its strength. Results show that the Stress at Yield of samples range from 0.012 N/mm2

to 0.022 N/mm2 before failure while the Young Modulus of elasticity range from 0.113 N/mm2

to 0.222 N/mm2 before failure.

The addition of clay in order to enhance the thermal properties showed on the average the
decrease in thermal conductivity which is good because it indicated that the proposed materials
produced are good insulators. Samples with high thermal resistivity are desirable for insulation
purposes. Thermal diffusivity was found to decrease which is good because materials with high
thermal diffusivity adjust rapidly to ambient temperature because heat is quickly connected.
Materials of high thermal diffusivity will respond quickly to changes in their thermal
environment, while materials of small thermal diffusivity will response more sluggishly, taking
longer to reach a new equilibrium condition (Incropera et al., 2007).

4 Conclusion
This study investigated the effects of varying proportion of clay contents mixed with Kapok
wool on thermal and mechanical properties of materials produced. In this way, Kapok wool
which is loose in nature have been processed to form rigid or semi-rigid insulation materials.
Kapok wool – Clay combination of ratio 2:3.5 (Sample E) appear to have the highest value of
thermal resistivity of 76.923 mKW-1 and lowest thermal diffusivity of 0.2810 x 10-3 m2/s, though
with high density, which suggests it to be the best material among samples in the study for
insulation purposes. While Kapok wool – Clay combination of ratio 2:1.5 (sample B) having
thermal resistivity of 37.037 mKW-1 is the least in the group. The results obtained shows the
possibility of having a combination of two different insulation materials (Kapok wool and Clay)
to have desired thermal properties and strength. This study has established that the Kapok wool-
Clay mixture is a possible solution to meet insulation needs, and the insulators produced have a
lower-cost of production and sustainable. The insulation materials produced can be use in
thermal storage for medium temperature applications.

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Acknowledgements
The authors would like to acknowledge the TETFund through the IBR Research Grant No.
TETFUND/DESS/UNIMAID/MAIDUGURI/RP/VOL.II at University of Maiduguri, Maiduguri,
Nigeria for providing financial support for the study. The authors also appreciate Mr. M. Ndagi
of the Department of Mechanical Engineering, University of Ilorin, Nigeria for his assistance
during the study.

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AZOJETE,16(2):415-422. ISSN 1596-2490; e-ISSN 2545-5818, www.azojete.com.ng

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