Selected fabrics have high thermal performance and thermal response as insulators, which can be applied to a range of exposure temperatures of 40-200oC, say Zeinab S Abdel-Rehim, M M Saad, M El-Shakankery and I Hanafy.

Many development applications for the new materials such as textile fabrics used as thermal insulators require a full study of its thermal insulating properties at different operating conditions. One of the most important of these studies is the effect of temperature with thermal conductivity and material density on the response of the textile fabrics as insulators.

Thermo-insulating properties of perpendicular-laid versus cross-laid lofty nonwoven fabrics are presented by Oldrich et al^[1]. In their study, the relationship between the thermal conductivity and material density of samples was studied. They concluded that the thermal conductivity decreases with increasing material density. Morris^[2] presented a study of thermal properties of textiles and concluded that their thermal conductivity increases with density, based on his observation that when two fabrics are of equal thickness, the one with a lower density has the greater thermal insulation.

However, he reported that there is a critical density of about 60.0 kg/m³, below which the convection effects become dominant and the thermal insulation falls. Recently, the heat flux sensor was used to measure the thermo-insulating properties of textiles in an apparatus called the Alambeta^[3]. The thermal properties of fabric insulators are investigated by Ukponmwan^[4]. Heat and mass transfer analysis of textile fabrics are presented in many researches^[5-8].

In these researches, the effect of operating parameters such as temperature, humidity and heat & mass transfer coefficients are examined by mathematical and experimental studies. A model of heat and water transfer through layered fabrics was developed by Fohr et al^[9]. They aimed at studying the effect of weather conditions and human activities on the selection of clothing. Their model takes into account the occurrence of condensation or evaporation in accordance with the environmental conditions and their variations.

Thermal expansion behaviour of hot-compacted woven polypropylene and polyethylene composites was studied by Bozec et al^[10]. Compression and thermal properties of recycled fibre assemblies made from industrial waste of seawater products are presented by Sukigara et al^[11]. In their study, the effective thermal conductivity of fibre assemblies with the steady-state, and parallel-plates was measured. Their results showed the lower effective thermal conductivity of recycled fibre assemblies than pure wool fibre assemblies, which indicate that the effect of heat radiation on thermal conductivity cannot be ignored.

In this work, the heat transfer through two different fabrics -- polyester and polypropylene is studied. The experiments are carried out using a special test-rig to study the thermal behaviour of the selected textile fabrics used as thermal insulators in many applications. Temperature, density, thickness and weight are measured for the selected textile fabrics used as case study. The thermal insulation properties of the selected textile fabrics are calculated and studied with respect to the importance of operating conditions such as; inlet temperature, thickness, weight and density. The comparison between the selected textile fabrics as thermal insulators according to certain operating conditions is given. On the basis of this study, some applications of these materials are considered.

Governing equations

The heat energy can be transferred through the textile fabrics by conduction, convection and radiation that easily explainable phenomena such as heat exchange in porous media. Basic concepts of the heat transfer through fabrics are explained as follows:

Thermal conductivity The heat transfer by conduction depends on their heat conductivity, ie, their capacity of transferring heat from a warmer medium to a cooler one. The main characteristics of heat conductivity are:

Conductivity factor ? [W/(mo C)] expresses the heat flow (Q), W, passing in 1 h through area (A) of 1 m2 of the fabric thickness (L) at a temperature difference (T1 - T2) of 1o C, as given in the following equation:

 ? = Q L / A t (T1 - T2) (1)

Heat transfer co-efficient K [W/m^{2 o}C] expresses the heat flow passing during 1 h through 1 m2 of fabric with actual thickness, (L) and difference temperatures of two media (air and fabric) 1^oC, as the following equation:

K = Q / A t (T1 - T2) (2)

Specific heat resistance (r)

The specific heat resistance, r ((m ^oC) / W) is a characteristic inverse to the heat transfer factor, ? ?as the following equation:

r = 1/ ? ? = A t (T1 - T2) / (Q L) (3)

Heat resistance, (R)

The heat resistance, R (m^{2 o}C / W) is a characteristic inverse to heat transfer co-efficient, K as the following equation:

R = 1/K = A t (T1 - T2) / (Q) (4)

The specific heat resistance, ?r and the heat resistance, R characterise the heat capacity of the fabrics to impede the transfer of heat through them.

Thermal resistance, (Rth)

The thermal resistance, Rth of textile fabrics is a function of the actual thickness of the material and the thermal conductivity, K. This function is given by the following relationship:

R_th = L / k, ((m^{2 o}C) / W) (5)

Where L is the actual thickness of the sample, m.

Heat flow, (Q)

 The heat flow, Q, through the textile fabric is given as the following:

Q = - k A (T1 - T2) / L (6)

Where A is surface area exposed to the hot air, T1 is the initial air temperature and T2 is the transient air temperature.

The textile fabrics have two thermal functions; they prevent air movement and provide a shield against radiant-heat losses. Within the limit before heat conducted by fibres becomes dominant, the more densely fibres are arranged within the fabrics, the better that they will fulfil these two functions.

Energy equation

The energy equation for textile fabric is simply the transient heat conduction equation with a heat radiation source term, this equation is given as:

-------(7) Where k, ?, CP, T and t are the thermal conductivity, density which it was calculated as ? ?= ?/L (? is the basic weight of the sample), specific heat, temperature and time for the selected fabrics, respectively. X is X-axis and is given as: 0 = X = L, 0 = t = 8 (8)

Where, L is fabric thickness. qr is the heat flux by radiation at any point within the fabric and can be written as,[12]:

qr (x) = 4s To3 (T1 - T2) at 0= X = L (9)

Where Sigma is the Stephan-Boltznan constant and equals 5.67 x 10^-8 W/m²K⁴ and T_o is the mean temperature in our experimental (T_o = 298 K). Figure (1) shows the schematic drawing of the fabric insulator model.

Thermal Insulating Value (TIV)

(TIV) represents the efficiency of the textile fabric as an insulator. It is defined as the percentage reduction in heat loss from a hot surface maintained at a given temperature. The (TIV) increases to 100% when a "perfect" insulator is obtained. (TIV) of textile fabric depending upon the thermal conductivity of the fabric, the thickness of the assembly and the thermal emission characteristics of the surface fabric. It is expressed as a percentage, which represents the reduction in the rate of heat loss due to the insulation, relative to the heat loss from the surface.

Thus, the following relation represents this value:

(TIV) % = 100 [ 1 - (Kt /? eo) / (L + (Kt /? e1) ] (10)

Where eo and e1 are emissivity of one surface of the insulator (textile fabric) and the other surface, respectively.

A typical value of emissivity of textile fabric is 2.06 cal / m² s^o C. The conversion of (TIV) to the tog unit can be written as the following:

(TIV) % = 100 [1- (Io / I1)] (11)

where Io and I1 are tog values of unclothed and clothed bodies, respectively, where 1 tog=0.418 m²s ^oC/cal.

Table 1 gives the calculated values of the (TIV), % for the samples of the selected fabrics.

Table 1: Thermal insulating values (TIV) of the selected fabrics

Experimental work

In order to investigate the heat transfer and thermal behaviour of textile fabrics as thermal insulators, especially experimental test-rig was designed and constructed to measure the temperature variation with test time through the selected textile fabrics during the heat exchange process between the inlet hot air and the fabric sample.

Experiments are carried out on two of nonwoven fabrics. The fabric samples are prepared by drying rout web formation technique and produced on the needle-punching machine. A group of samples made from polyester fibres with different weight per unit area, another group made from polypropylene with the same weight. The fabric samples are subjected and exposed to different levels of heat in the emission side (the heat source side) and then the temperature are measured in the other side of the fabric sample in order to evaluate its thermal resistance and behaviour as thermal insulator.

Tables 2 and 3 give the numerical values of parameters for the samples of the selected textile fabrics (polyester and polypropylene, respectively) and inlet heat exposure levels as temperatures that are used in the presented study.

Results and discussion

From the results of laboratory experiments and calculation, it is found that thermal insulating properties of textile fabrics (?, K, CP) affect the insulation response. Figures 2 to 24 illustrate the thermal response and behaviour of the selected fabrics (polyester and polypropylene) that used in this work as thermal insulators.

Temperature variations with time for polyester samples (1-5) at different exposure temperatures (40, 80, 120, 160 and 200^oC) through 25 experiments are plotted in Figures (2-6). From the figures, it is found that the fabric temperature (TF) variations increase rapidly in the initial stage of the exposure temperature. This may be, because the temperature difference between the fabric sample and the exposure hot air is high in the early stage of the exposure process. Through 20 experiments, temperature variations with time of polypropylene samples (1-4) at different exposure temperatures are shown in Figures 7-10.

Figures 11 and 12 show the effect of polyester and polypropylene thickness on fabric temperature (TF), respectively. It is found that higher fabric thickness means good insulation. Specific heat resistance for the selected fabrics is shown in Figure 13. It is found that polyester samples have higher specific heat resistance than polypropylene samples. Also, thermal resistance of the selected fabrics is shown in Figure 14.

From these figures, the polyester has higher specific heat resistance and thermal resistance than polypropylene. This may be, because the thermal conductivity of the polyester is lower than polypropylene. Figures 15 to 22 give the effect of exposure temperatures on the heat flow through the polyester and polypropylene for the samples (1-4) each of them. It is found that the heat flow rises rapidly during the early stage of the hot air exposure to the fabric. This is due to the high temperature difference between the fabric surface (cold) and the hot air. Also, high exposure temperature means high heat flow through fabric.

Figures 23 and 24 show the surface plot and contours of measured fabric temperatures for 100% of polyester and 100% of polypropylene nonwoven fabrics, respectively. The figures give the influence of nonwoven fabric weight and time on the temperature variations when it exposes to different temperatures (40, 80, 120, 160 and 200^oC). It is found that the temperature variations of the fabric increased with increasing of time and decreased with fabric weight up to a certain limit, beyond its optimum level. This may be due to the fibre quantity that increased with increasing of fabric weight; in other words, the compactness of nonwoven fabrics increased with increasing basically weight and consequently the fabric thickness. This is the reason of thermal behaviour of the fabrics. As the results of ANOVA-two way, the relations between temperature variation of the polyester and polypropylene fabrics, respectively, weight and time are given as the following, (Figures 23 and 24):

Z = 14.68 + 0.035 * X + 0.66 * Y - 0.0 X*X - 0.0 X*Y - 0.003*Y*Y

Z = 21.55 + 0.036 * X + 0.511 * Y - 0.0 X*X - 0.0 X*Y - 0.003*Y*Y

Conclusion

Based on the calculations and experiment results of the selected fabrics that were used as thermal insulators, the following conclusions can be drawn:

The laboratory experiments and calculation have shown that the selected textile fabrics can be used as good thermal insulators in the range of exposure temperatures of 40-200^oC.

The study concludes that the selected fabrics have high thermal performance and thermal response as insulators. The effect of fabric thickness on the fabric temperature variations has obviously significance that the higher thickness means good thermal insulation.

Both the thermal conductivity and thermal resistance of all selected fabric samples increases with increasing of fabric density. Fabric thickness affects the transient fabric temperatures, and that fabric temperature variation decreases with increasing fabric thickness.

The exposure temperature affects the heat flow through the selected fabrics, and the heat flow increases with increasing exposure temperatures. The temperature variations of the fabric increase with increasing of time and also decrease with fabric weight up to a certain limit, beyond its optimum level.

References

1. Oldrich Jirsak: Thermo-Insulating Properties of Perpendicular-Laid Versus Cross-Laid Lofty Nonwoven Fabrics, Textile Res J, Vol 20, No 2, pp 121-128, 2000.

2. Morris G J: Thermal Properties of Textile Materials, Tex-tile Inst J, Vol 44, T 449, 1953.

3. Hes L, Araujo M and Djulay V: Effect of Mutual Bonding of Textile Layers on Thermal Insulation and Thermal Contact Properties of Fabric Assemblies, Textile Res J, Vol 66, pp 245, 1996.

4. Ukponmwan J O: The Thermal-Insulation Properties of Fabrics, Textile Prog, Vol 24, 1993.

5. Sang IL Park: Heat and Mass Transfer Analysis of Fabric in The Tenter Frame, Textile Res J, Vol 67, No 5, pp 311-316, 1997.

6. Ammar A S and M El-Okeily: Heat Transfer Through Textile Fabrics: Mathematical Model, Math and Computer Modelling, Vol 12, Issue 9, pp 1187, 1998.

7. C V LE and N G LY: Heat and Mass Transfer in the Condensing Flow of Steam Through An Absorbing Fibrous Medium, Int J of Heat and Mass Transfer, Vol 38, No 1, pp 81-89, 1995.

8. Li Y and B V Holcombe: Mathematical Simulation of Heat and Moisture Transfer in a Human-Clothing-Environment System, Textile Res J, Vol 68, No 6, pp 389-397, 1998.

9. Fohr J P, D Couton and G Treguier: Dynamic Heat and Water Transfer Through Layered Fabrics, Textile Res J, Vol 72, No 1, pp 1-12, 2002.

10. Bozec Y Le, S Kaang, P J Hine and I M Ward: The Thermal-Expansion Behaviour of Hot-Compacted Polypropylene and Polyethylene Composites, Composites Science and Technology, Vol 60, Issue 3, pp 333-344, Feb 2000.

11. Sukigara S, H Yokura, and T Fujimato: Compression and Thermal Properties of Recycled Fibre Assemblies Made from Industrial Waste of Seawater Products, Textile Res J, Vol 73, No 4, pp 310-315, 2003.

12. Holman J P: Heat Transfer, 6th ed, Mc Graw-Hill Book Company, NY, pp 373-472, 1986.

Note: For the detailed version of this article please refer the print version of The Indian Textile Journal - Sept 2006.

Zeinab S Abdel-Rehim is Associate Prof, Mechanical Engineering Dept; and M M Saad and M El-Shakankery are Professors with the Textile Department, National Research Centre, Dokki, Giza, Egypt. I Hanafy is Associate Prof, Helwan University, Cairo, Egypt.