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Thermo-physiological and comfort properties of Ugandan barkcloth from Ficus natalensis

Samson Rwawiire^ab & Blanka Tomkova^a

a Faculty of Textile Engineering, Department of Material Engineering, Technical University of Liberec, Liberec, Czech Republic.

b Faculty of Engineering, Department of Textile and Ginning Engineering, Busitema University, Busia, Uganda.

Published online: 23 Jan 2014.

To cite this article: Samson Rwawiire & Blanka Tomkova (2014) Thermo-physiological and comfort properties of Ugandan barkcloth from Ficus natalensis, The Journal of The Textile Institute, 105:6, 648-653, DOI: 10.1080/00405000.2013.843849 To link to this article: http://dx.doi.org/10.1080/00405000.2013.843849

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Thermo-physiological and comfort properties of Ugandan barkcloth from Ficus natalensis

Samson Rwawiire^a,b* and Blanka Tomkova^a

aFaculty of Textile Engineering, Department of Material Engineering, Technical University of Liberec, Liberec, Czech Republic;

bFaculty of Engineering, Department of Textile and Ginning Engineering, Busitema University, Busia, Uganda (Received 30 May 2013; accepted 9 September 2013)

The United Nations Educational, Scientiﬁc and Cultural Organization in 2005 proclaimed that Ugandan barkcloth largely produced from mutuba tree (Ficus natalensis) as a “Masterpiece of the Oral and Intangible Heritage of Humanity”. An exploratory investigation of thermo-physiological and comfort properties of barkcloth, a nonwoven material produced through a series of pummeling processes from mutuba tree in Uganda, is fronted. Barkcloth was extracted from the F. natalensis tree in Nsangwa village, Buyijja parish in Mpigi district, Central Uganda. Thermal conductivity, thermal diffusivity, thermal absorptivity, thermal resistance, fabric thickness, and peak heatﬂow density were measured using an Alambeta device, whereas a Permetest device was used for the measurement of the moisture vapour permeability and evaporation resistance. The study was carried out under relative humidity of 40% and at a laboratory room temperature of 24°C and the results show that the thermal conductivity is in the range of cotton fabrics rendering barkcloth from F. natalensis, a comfortable fabric. The lower value of thermal absorptivity of barkcloth compared to the value of cotton renders the fabric a warm feeling when in contact with the skin. Barkcloth had a higher moisture vapor permeability compared to cotton and other fabrics, meaning its clothing comfort properties are reasonable.

Keywords: barkcloth; Ficus natalensis; naturalﬁber; thermal conductivity; thermal diffusivity; thermal absorptivity;

thermal resistance; moisture vapor permeability

Introduction

The main purpose of clothing is to offer protection against various environmental stresses of which protection against climatic conditions is a major factor. Besides the task of protection, there is fashion and outward expression (Voelker et al., 2009).

Consumer’s behavior in regard to purchase of clothing is affected by demographic variables; however, clothing comfort is a universal requirement for most fabrics. Clothing comfort is a function of thermo-physiological factors such as thermal properties, water evaporation and water permeability. A worn fabric should be able to serve the purpose of heat insulation and also offer free movement of air/gases such that when the body temperature rises, cooling can be attained and when the temperature drops, the fabric should provide insulation against the rapid heat loss from the body to the surroundings.

In recent years, numerous studies have been conducted on clothing comfort from studying the normal properties affecting comfort to the development of mathematical models of heat and mass transfer through fabrics. Henry (1939) developed theories of heat and moisture transfer considering accumulation effects.

Ogniewicz and Tien (1981) developed a steady state model for convective and diffusive transport mechanisms in fabric. Fan and Cheng (2005) studied the heat and moisture transfer through clothing. Wu and Fan (2008) measured the moisture transport within the multi-layer clothing assemblies consisting of different types of batting. Numerous researches, Bhattacharjee and Kothari (2009), Ding, Tang, Song, and McDonald (2009), Li and Holcombe (1998), Stolwijk (1971), Urquhart and Wil- liams (1924), Woodcock (1962) and Xu and Werner (1997), on thermal comfort and modeling of clothing comfort have been done. Voelker and Kornadt (2009) used computational ﬂuid dynamics simulations to determine the temperature distribution of the human body, the heat ﬂux of the environment and the thermal comfort.

Farnworth (1986) developed a model for the combined diffusion of heat and moisture through clothing. Wissler and Havenith (2009) developed a model for heat and moisture transport in multi-layer garments. Fauland, Lenz, Rohrer, and Bechtold (2012) developed models for moisture assessment of bandages. Borreguero, Rodri- guez, and Gonzalez (2013), Yoo, Lim, and Kim (2012), and Zhao et al. (2013) studied the effect of application of phase change materials on the thermal properties of

*Corresponding author. Email: rsammy@eng.busitema.ac.ug

–653, http://dx.doi.org/10.1080/00405000.2013.843849

Ó 2014 The Textile Institute

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phase change material garments. ISO 9920:2009 is the current standard used in the evaluation of clothing thermal characteristics in steady-state conditions; however, the drawback is that it does not consider effects of clothing such as water adsorption, buffering or tactile comfort, inﬂuence of rain, special protective clothing, and insulation.

Uganda is the leading cotton grower and exporter in East Africa. Apart from cotton, the country also boasts of other ﬁber-producing plants such as many varieties of banana, pineapple, nettle, sisal, okra, sansevieria, barkcloth, etc. with high potential of becoming textile ﬁber producing plants.

Barkcloth clothing is believed to have originated in South China and the technology of extraction of barkcloth has been conﬁrmed with archeologists discov- ering grooved stones in Xiantouling site of Shenzhen similar to grooved hammers used today. It is believed that the extraction of barkcloth in ancient China spread to Taiwan, Philippines, Africa, Central America, and Oceania. Barkcloth produced from Polynesia is derived from the bast of mulberry, whereas in Uganda the felt is derived from Ficus trees all being from Moraceae family.

According to United Nations Educational, Scientiﬁc and Cultural Organization (UNESCO), Barkcloth has been in production in Uganda for over six centuries;

however, the nonwoven fleece, which is produced through a series of pummeling processes, has been confined to cultural regalia worn at coronation of kings by Baganda a tribe in Central Uganda and also utilized during funerals and other artifacts. The technology transfer of barkcloth production from the elder to the youth has been impeded by rural to urban migration of the youth and influence to modernization. That notwith- standing, in 2005, UNESCO proclaimed barkcloth as a

“Masterpiece of the Oral and Intangible Heritage of Humanity”. In the 70s and 80s, production of barkcloth was banned in Uganda and was revived in the 90s.

Barkcloth terracotta in color from Ficus natalensis and Antiaris toxicaria is now largely produced in Uganda and its use in apparel and fashion is a new sensation as shown in Figure 1.

In this study, an exploratory investigation of thermo- physiological and comfort properties of barkcloth, a widely used textile fabric in Uganda, are studied.

F. natalensis trees grow naturally in Central Uganda and do not need fertilizers. Trees preserved for the purpose of barkcloth production are well tendered such that the stem has no roots to propagate on it. Despite the fact that barkcloth has been around dating back as far as thirteenth century, there has been limited data or no scientiﬁc study on barkcloth. Therefore, in this study, for the ﬁrst time we present the thermo-physiological and comfort properties of barkcloth.

Materials and methods Extraction

The extraction of the naturally occurring nonwoven starts with scraping off the surface layer of the trunk to expose the fresh raw bark using a sharp blade. The blade is held Figure 1. A buganda tribe lady from Uganda dressed in barkcloth.

The Journal of The Textile Institute 649

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at an angle such that only the surface layer is removed and also avoids damaging the tree and fresh bark as shown in Figure 2(a). A ring is then cut with a knife on both ends of the scraped stem that reﬂected the length of the barkcloth that was to be produced. At the same time, a vertical slit is made from the top of the stem to the bottom. With the help of a wedged tool, locally known as ekiteteme, the innermost part of a banana stem is carved out. The bark is easily peeled off starting from the base slowly moving upwards.

For environmental sustainability, the debarked stem is wrapped with banana leaves, Figure 2(b), which act as bandages to prevent dehydration; these are usually removed after a week giving way for growth of fresh bark. Figure 4 shows the detailed process of production of barkcloth. The extracted bark is then burnt using dried banana leaves to soften it prior to the pummeling process which includes different well-designed wooden grooved hammers. Pummeling is usually done under shade to prevent direct sunrays from creating creases in the barkcloth.

After pummeling, the barkcloth is sun-dried for 3 h every day for 6 days giving it a rich deep red–brown color and then repounded to smoothen the cloth surfaces. Drying involves stretching the wet fresh barkcloth using heavy loads at its perimeter to retain its dimensions on drying.

Thermo-physiological properties

The thermo-physiological properties were evaluated using ISO 11092 (EN 31092) standard with laboratory room temperature at 24°C and at relative humidity (RH) of 40%. The Alambeta instrument (Hes & Dolezal, 1989) was used to measure the thermal conductivity, thermal diffusivity, thermal absorptivity, thermal resistance, sample thickness, and peak heat flow density. The principle of operation of Alambeta applies a heat flow sensor attached to a metal block with constant temperature which differs from the sample temperature. When the measurement starts, the measuring head with the heat flow sensor drops down and touches the sample placed

on the instrument base directly under the measuring head. A photoelectric sensor is used to measure the sample thickness. In order to simulate the real conditions of warm–cool feeling evaluation, the instrument measuring head is heated to 32°C which corresponds to the average human skin temperature, while the fabric is kept at the room temperature.

The Permetest instrument (Hes & Dolezal, 1989) was used to measure the relative water vapor permeability and evaporation resistance. The measuring head of the device is covered by a resistant semipermeable foil, which avoids wetting the sample.

Results and discussion

The used barkcloth sample weight is 123 g/m². The thermo-physiological properties are shown in Table 1.

Barkcloth morphology is shown in Figure 3.

Thermal conductivity coefﬁcient, λ [W/m K] is the amount of heat transmitted through a unit area of material having unit thickness within a second due to a unit temperature gradient. Metals have the highest thermal conductivity, whereas polymers have low thermal conductivity ranging from 0.2 to 0.4 W/m K; textile structures ranging from 0.033–0.01 W/m K; and steady air at 20°C is 0.026 W/m K. Thermal conductivity of water is 0.6 W/m K, which is 25 times more than that of air; therefore, the presence of water in textile materials is uncomfortable. Thermal conductivity of barkcloth is

Figure 2. (a) A man dressed in barkcloth harvesting bark from mutuba tree. (b) Protecting the tree (image courtesy of Fumiko Ohinata/UNESCO).

Table 1. Thermo-physiological properties of barkcloth.

Property Value CV [%]

Thermal conductivity [W/m K] 0.0357 2.8 Thermal diffusivity [m²/s] 10⁶ 0.197 19.7 Thermal absorptivity [Ws^1/2/m²K] 81.4 10.8

Thermal resistance [m²K/W] 0.034 9.7

Sample thickness [mm] 1.21 10.4

Peak heatﬂow density [Wm²] 10³ 0.234 16.7 Relative moisture vapor permeability [%] 66 8.9 Evaporation resistance [Pa m²] 4.4 25.3

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0.0357 W/m K. This low level of thermal conductivity is attributed to the fabric structure being a natural nonwoven material with some pores arising from the fabric

handling and rigorous manual production processes; the high porosity is a haven for entrapped air which lowers the thermal conductivity of the fabric.

Figure 3. Morphology of barkcloth. (A) Transverse section. (B) Top surface. (C) Bottom surface. (D) Microﬁber.

Figure 4. Extraction of barkcloth from mutuba tree: (A) and (B) scraping off surface layer. (C) Debarking the tree using a banana stalk. (D) Peeling off the bark. (E) Pummeling process using grooved wooden hammers. (F) Dried and ﬁnished barkcloth. Images courtesy of Fumiko Ohinata/UNESCO.

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Thermal absorptivity, b [Ws^1/2/m²K] of fabrics was introduced characterizing thermal feeling (heat flow level) during short contact of human skin with the fabric surface. Provided that the time of heat contact t between the human skin and the textile is shorter than several seconds, the measured fabric can be simplified into semiinfinite homogenous mass with certain thermal capacity ρc [J/m³] and initial temperature t2. Unsteady temperature field between the human skin (with constant temperature t1) and fabric with respect to boundary conditions offers a relationship, which enables to determine the heatflow q [W/m²] through the fabric:

q¼ aðt1 t2Þ=ðptÞ¹⁼²; b¼ ðkqcÞ¹⁼²;

where ρc [J/m³] is the thermal capacity of the fabric and the term α represents the thermal absorptivity of fabrics.

The higher the thermal absorptivity of the fabric, the cooler its feeling upon contact with the skin. In textile applications, the thermal absorptivity ranges from 20 Ws^1/2/m²K for ﬁne nonwoven webs to 600 Ws^1/2/ m²K for heavy wet fabrics. Barkcloth had thermal absorptivity of 81.4 Ws^1/2/m²K, which was lower than the values of reported for cotton by Chidambaram, Govindan, and Venkatraman (2012), Demiryürek and Uysaltürk (2013) and Oglakcioglu and Marmarali (2007). This gives a warm feel to barkcloth fabric when in contact with the skin.

Thermal resistance, R [m²K/W] is the measure of a material’s resistance to the ﬂow of heat. It depends on fabric thickness h and thermal conductivity k: R = h/k;

therefore, the fabric thickness inﬂuences the thermal resistance. In sharp contrast, barkcloth thermal resistance of 0.034 m²K/W is attributed to the thickness of the samples compared to lower values registered by cotton fabrics by Chidambaram et al. (2012), Demiryürek and Uysaltürk (2013) and Oglakcioglu and Marmarali (2007). From the experiments conducted, the inverse proportional relationship of thermal resistance and thermal conductivity was fulﬁlled.

Thermal diffusivity, a [m²s¹] is the measure of the rate at which thermal heat is transferred. Materials with higher thermal diffusivity quickly adjust their base temperature to that of the surroundings. Barkcloth thermal diffusivity was recorded as 0.197 10⁶m²s¹, slightly higher than the values of cotton single jersey knitted fabrics reported by Chidambaram et al. (2012).

Relative water vapor permeability is the ability of the material to transfer moisture from the body to the surroundings. Water vapor permeability is an important clothing comfort term because a fabric should be able to serve the double function of transferring internal

moisture, which may be sweat, to the surface and also aide its evaporation to the surroundings. This process can occur through capillary motions of moisture through the fabric ﬁbers or through the pores of the fabric. At higher levels of sweating, the sweat wets the fabric and mass transfer can occur from the previous processes described. The average water vapor permeability of barkcloth was 66%; however, the measured values were in the range of 60–75%. The higher values of water vapor permeability are due to porosity and the nature of barkcloth.

Conclusion

This study has shown the thermo-physiological and comfort properties of barkcloth from Uganda. The reader is informed of the fact that barkcloth is a natural nonwoven fabric; the climatic conditions, type of soils, and the area position of the fabric as it is extracted from the tree inﬂuence the overall mechanical, thermal, and comfort properties. The results of this study are valid for the conditions of RH of 40% and at laboratory room temperature of 24°C.

Chidambaram et al. (2012), Demiryürek and Uysaltürk (2013) and Oglakcioglu and Marmarali (2007) reported that 100% cotton single jersey knitted fabrics had thermal conductivity ranging between 0.0357 and 0.0501 W/m K.

Marmarali et al. (2009) showed the thermal conductivity of tetra-channel polyester, high functional polyester, and blend of natural and synthetic fibers to lie in the range 0.0382–0.0446 W/m K, which is comparable to the measured value of barkcloth rendering the barkcloth from F. natalensis, a comfortable fabric. The lower value of thermal absorptivity of barkcloth, compared to the value of cotton, shows that the fabric has a warm feeling when in contact with the skin. Barkcloth had a higher water vapor permeability compared to cotton and other fabrics meaning its clothing comfort properties are reasonable. It has, therefore, been shown that the fabric which has been in production for the past six centuries fulfills all the requirements for thermal clothing comfort and its applications in engineering is a virginfield of research.

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