Thermal resistance

Heat transfer through textiles and clothing involve conduction through air and fibres, radiation (both direct and from fibre to fibre), as well as convection (Ukponmwan, 1993). In a homogenous solid, the mechanism of heat transfer is through conduction, and the temperature profile is linear (Ukponmwan, 1993). The total heat transferred increases with the presence of radiation, but the temperature profile will depart from linearity due to the interaction between conduction and infra-red radiation (Ukponmwan, 1993).

According to Ukponmwan (1993), the transmission of heat through clothing and textiles occur in three forms:

1. Dry transmission (also referred to as conduction and includes a radiation element).

2. Diffusion of insensible perspiration.

3. Diffusion of liquid perspiration.

It is important to note that only the first two forms of transmission will operate under conditions of comfort. The presence of liquid, in the form of perspiration on the human skin, is associated with discomfort. Therefore, Ukponmwan (1993)

36 states that the total rate of heat transfer (Q) consists of a conductive ( ) and an evaporative component ( ):

Q = + (2-14) The resistance that a fabric provides against the movement of heat is crucial as it determines thermal comfort (Matusiak, 2013 and Ukponmwan, 1993). The air that is trapped within the structure of a fabric between the fibres and yarns, largely determines the thermal resistance of a textile (Matusiak, 2013). Thermal resistance is defined by the following equation (Matusiak, 2013):

=

Where: is the thermal resistance (m²K/W), h is the fabric thickness (m), and is the thermal conductivity (W/mK).

The thermal resistance of fabrics, made from the same fibres, depend on the thickness and compactness (such as weave and density of threads) of the particular textile (Matusiak, 2013). This is supported by Ho et al. (2011) and Ukponmwan (1993), who comment that thermal resistance is determined by fabric thickness, as opposed to the type of fibre used in the fabric. This is due to the fact that the volume of air enclosed in clothing materials are generally more than the volume of the fibres (Havenith, n.d.). In addition, when two fabrics are placed on top of each other, the thermal insulation will be higher compared to the individual fabrics due to the entrapment of more air between these two layers (Ho et al., 2011).

Starr (2002) confirms that the open spaces within a textile fabric largely determine thermal insulation, with the physical structure and size of fibres and yarns also influencing the overall insulating capacity. For example, a fibre, such as wool, has a crimp that provides more voids, pores and surface area for air, which increase the insulation of the fabric (Harnett, 1984 and Starr, 2002). In other words, the warmest fibre is usually the one that produces the thickest or bulkiest fabric (Ukponmwan, 1993). Furthermore, Harnett (1984) states that a fabric with an uneven or textured surface will have the ability to trap more air and provide more insulation than a fabric with a very smooth surface. For example, the brushing of a

37 fabric surface to increase its thickness and bluk will result in a higher thermal insulation due to the entrapment of air. This is particularly important when producing underwear, as it also increases the apparent warmth to touch (Holcombe, 1984).

As mentioned previously, external factors also play a significant role in the comfort of clothing. Qian (2005) and Ukponmwan (1993) state that external factors, such as wind penetration, can influence the level of thermal resistance of a clothing ensemble. For example, when a garment flaps due to wind, warm air is lost in the microclimate (the air gap between the skin and the garment assembly) and replaced with cold air, which reduces the thermal insulation of the garment ensemble as a whole (Qian, 2005). The design of a garment should also be considered, due to the fact that the transfer of air occurs through the openings of a garment, which influences thermal insulation (Qian, 2005).

Qian (2005) highlights the fact that the air gaps that exist between the skin, clothing, and clothing layers, determine the flow of air in the microclimate, due to the change in the size of the air gaps when the wearer moves. According to Matusiak (2010), this will have a major influence on heat exchange. Figure 6 illustrates the temperature distribution of a single shirt, to highlight the importance of garment fit. It is evident from Figure 6 that there are differences in temperature at different parts of the human body, mainly due to fabric folds at different distances from the body (Matusiak, 2010). The temperature along the perpendicular line L1, ranges between 26.16 and 31.77°C; whereas along the horizontal line, L12, it ranges between 25.47 and 31.19°C (Matusiak, 2010). The most important aspect highlighted by Figure 6, is that the difference in temperature between the extreme points of the shirt is higher than 5°C, even though the shirt is made from the same fabric (Matusiak, 2010).

38 Figure 6: Thermogram illustrating temperature distribution of a shirt

(Source: Matusiak, 2010)

Ukponmwan (1993) summarises the main factors that must be considered when studying the thermal properties of a textile as follows:

1. Thermal conductivity of fibres and the air contained within the fabric.

2. Specific heat of the fibre substance.

3. Textile thickness.

4. Textile density.

5. Textile surface in terms of fibre used, fabric construction, and finishing.

6. Area of contact between fabric and surfaces.

7. Heat loss by conduction from skin to fabric.

8. Heat loss by convection from skin through the fabric and from the fabric surface.

9. Heat loss by radiation.

10. Heat loss by evaporation of water from skin or fabric.

11. Heat gain due to water absorption by fabric.

12. External factors, such as temperature, relative humidity, or movement of surrounding air.

39 Ukponmwan (1993) states that the most important aspect to consider, from the factors listed above, is the ability of a textile to hold as large an amount of still air as possible, and then to retain this during use. Therefore, the linear relationship that exists between fabric thickness and thermal resistance is not significantly influenced by the type or blend of fibres (Ukponmwan, 1993).