FUNDAMENTS OF DESIGN OF FABRICS AND GARMENTS WITH DEMANDED THERMOPHYSIOLOGICAL COMFORT by Prof. Lubos Hes, PhD., DSc, University of Liberec, Czech Republic, e-mail: firstname.lastname@example.org
1.HIGH ADDED VALUE CLOTHING AND GARMENTS DESIGNED BY THE APPLI CATION OF COMFORT SCIENCES Due to increasing sales of functional and protective clothing, clothing comfort and the related evaluation methods became very important in recent years. Comfort perceived by human senses: eyes, ears, touch, nose. Comfort defined as the absence of perceived pain- and discomfort. A new concept of garments with defined, but not maximal comfort level for young healthy people, sportsmen under training scheme etc.
1.1. Components of clothing comfort : psychological, sensorial (or tactile) and thermophysiological comfort. 2. PSYCHOLOGICAL COMFORT: geographical (climatic), economical, hi-storical, cultural, social and individual aspects. 2.1.Components of psychological comfort Climatic aspects: typical (daily) clothing should at first respect the climatic requirements Economical aspects: Resources, technology of food and objects manu-facture, skills, political system
Historical aspects: Inclination to products made of natural materials, to products simulating nature, to products of natural smell. Tradition in lifestyle and fashion Cultural aspects: religion, habits (in Arabic countries women are fully covered) Social aspects: age, qualification, social class, rank or position in this class Individual and group aspects: the effect of fashion, style, colours and lustre, trends, personal preferences
3.SENSORIAL COMFORT 3.1.Stephen's law, skin sensors: perceived sensation P (sound, lightning, climate) is proportional to the magnitude of physical stimulus I according to the relation: P = d I n (1) Skin temperature (Kraus & Ruffini) sensors: sensors for 38 to 43 O C, cold sensors for 15 to 35 O C, low sensitivity between 35 and 38 O C. Adaptability with acclimatisation.
Sensors for pressure and pain. No sensor for humidity (substituted by feeling of cold and pressure). Fig. 1 Schematic section view of human skin
1 ) Hair sheath 2 ) Hair 3 ) Smooth muscles 4 ) Fat gland 5 ) Skin vein 6) Sweat gland 7) Touch sensor 8 ) Higher temperature sensor by Ruffini 9 ) Vater-Paccini sensor of pressure and pull 10 ) Lower temperature sensor by Krause 11 ) Free ends of nerves
3.1.1.Sensory properties of fabrics and their division into mechanical, thermal and complex properties. Dynamic (Newton) force F [N/m 2 ]given by acceleration a [m/s 2 ] of a fabric mass m [kg/m 2 ]: F = m . a (2) and friction forces generated while wearing clothing. Ergonomic approach to garment design. 3.2. Survey of mechanical properties of fabrics . Basic equations: Hook's Law for extension and shear, typical load-extension curves for various fabrics, parameters, which determine the fibres and fabrics bending rigidity.
Mechanical properties of fabrics assessed manually when purchasing a cloth or garment in a shop: 1. Fabrics thickness and compressibility (between 2 fingers, immedia-tely) 2. Warm-cool feeling of fabric (within several seconds, by fingers or when fabric lies on the table) 3. Friction force and perfile (when moving a finger against fabric surface) 4. Bending rigidity (bending among 3 fingers) 5. Extensibility (pulling between both hands) 6. Shear rigidity (with both hands when fabric lies on the table)
Relation for the deformation y (in the axis perpendicular to the fabric plane) of an inclined fibre protruding from a fabric under the angle α between the fibre (of diameter 2r and length l) and a fabric, when the fibre is bent by a force F perpendicular to a fabric: y = F l 3 cos 2 α sinα / 3 E I (3) where the inertia momentum I [m 4 ] of a fibre is I = π d 4 / 64 (4)
The equations indicate, that the fibre deformation is proportional to the 3 rd power of the fibre length and indirectly proportional to the 4 th power of its diameter. That is why the micro-fibre fabrics, even if the protruding fibres are short, deform easily under pressure to certain extend and partially copy the acting body. Thus, the contact area is always large, the amount of heat taken away form the contacting body is high and the contact feeling is cool, in spite of the smooth and pleasantly soft surface. Similar phenomenon appears after the enzymatic or chemical treatment of fabrics: this relatively drastic action results in the disintegration of the fibre ending into several fine micro-fibrils, which behave as micro-fibres.
Then, the enzymatic treated fabrics reveal also cool feeling (high thermal absorptivity), accompanied by smooth, pleasant feeling. The bending rigidity B of such fabrics, however, is sometimes too low, as follows from the consideration placed at the end of this chapter. On the other hand, any mechanical treating of fabrics, like brushing or carding, brings the warmer feeling, because the original compact and smooth plane surface of dense woven fabrics with high mass and hence high thermal capacity is being replaced by the irregular surface featuring lower mass, irregular thickness of a structure composed of some soft and
easily deformed fibres, but some surface fibres are not split, and due to their relatively big diameter and short length they do not bend easily under pressure, thus conserving the surface less compressible, but full of thermal insulating air pores of low thermal absorptivity. 3.3. Yarn and fabrics hardness (compressibility) as a function of their packing coefficient μ A new expression for yarn hardness or compression modulus E c [Mpa] according to NECKAR (Prof. B. Neckar, Tech. Univ. of Liberec), k means the proportionality parameter in Mpa, depending on fibre material and processing,
μ O means the lowest possible level of packing, e.g. 0,8 for cotton yarns: E c =k3 μ 3 [1+2( μ / μ o ) 3 ]/[1-( μ / μ o ) 3 ] 4 (5) Bending rigidity of fabrics is an important fabric comfort parameter, since sometimes garments require low bending rigidity (silhouette skirts, pullovers, socks, all kinds of underwear), but good appearance e.g. of men's suits, trousers etc. is based on fabrics of higher and defined bending rigidity B. In the classical mechanics of elastic solids, the bending rigidity is given by a product of the purely material parameter E [Pa] called initial elastic modulus, and the inertia momentum I [m 4 ], given by the fabric structure and dimen-sions.
For the high density woven fabric of thickness h and of width b the fabric bending rigidity, under certain assumptions, the fabric bending rigidity may be estimated by the following expression: B = E . I , I = b.h 3 /12 (6)
3.4. Drape angle - a new method of drape determination One of the parameters, which characterises the wearing comfort of clothes is fabric drape. Due to easy way of its evaluation by means of the Cuisick’s instrument an increased attention is dedicated to this parameter, but the proper device is quite large and requires relatively complicated opto-electronic system. Also the proper measuring procedure is time consuming. That is why we cannot find this instrument or any similar drapemeter in the factory labs, they are used in the textile research units only.
3.4. Drape angle - a new method of drape determination One of the parameters, which characterises the wearing comfort of clothes is fabric drape. Due to easy way of its evaluation by means of the Cuisick’s instrument an increased attention is dedicated to this parameter, but the proper device is quite large and requires relatively complicated opto-electronic system. Also the proper measuring procedure is time consuming. That is why we cannot find this instrument or any similar drapemeter in the factory labs, they are used in the textile research units only.
Fig. 2 Measurement of Drape Angle by means of a special tool (table) and moving this sample towards the sharp (90 O ) corner of the table in such way, that the axis of the 90 o angle coincides with the warp or weft direction. The fabric motion stops, when the peak of the corner reaches the center of the sample. Then the fabric folds and forms a direct edge, whose inclination φ against the horizontal plane we measure. The sin φ value measured by means of simple ruler – see the Fig.2-then characterizes the level of drape. The fact, that this parameter, in some extend, does not depend on the length of the inclined fabric edge indicates,
that this inclination is a certain fabric property, which depends on composition, mass and structure of the fabric. A certain evidence, that this inclination may characterize the fabric drape results from the fact, that materials like paper with high shear stiffness do not fold in our test, they just bend, and hence do not create the drape edge. Theory of fabric drape According to the Niwa and Seto regression equation given below, DA = DA=C 0 + C 1 (B/W) 0,33 + C 2 (G/W) 0,33 (6)
the drape coefficient should depend not only on the fabric bending stiffness, shear stiffness and fabric mass, but also on the levels of their bending and shear hysteresis. Recently, the effect of fabric mechanical properties on drape coefficient was analyzed in 2003 by Militky and Hes. Experimental determination of correlation between Drape angle and Drape coefficient To confirm the principal correlation between the new Drape angle (DA) method and Drape coefficient (DC) according to the Cuisick´s method, 90 woven fabrics made of cotton, linen, viscose and their blends with nylon and polyester were tested by the mentioned procedures. Square mass of these samples varied from 50 g/m 2 to 390 g/m 2 .
Every sample was measured 8 times. whereas the axis of the table corner is perpendicular to the fabric edge Separate measurments along the weft and warp directions were made. The edge length in first series of measurement reaches 5 cm, in the second series it was 10 cm. From the presented figures follows, that best correlation of the new Drape Angle method with the Cuisick classical method exhibit the C/E sample orientation for the edge length 5 cm.
Fig. 3 - Correlation between the DC and DA methods
The correlation coefficient for the weft direction was R = 0,717, and for the warp direction the R reached 0,759. Unfortunately, we cannot directly correlate any of this direction with the Cuisick Drape coefficient, since the Drape coefficient involves the whole fabric. Therefore, we have also correlated the average value of DA for weft aned warp directions with the DC data, with the resulting level of correlation coefficient R = 0,767. Since this result was verified for very large set of fabrics, the practical verification of the new method can be considered as verified. Nevertheless, other unpublished results show, that this new simple and cheap method may emphasize more the effect of shear rigidity then the classical method,
which is a positive result, since the evaluation of shear rigidity of fabrics by other methods is quite complicated. 3.5. Methods of objective evaluation of sensorial comfort of garments and fabrics German method of objective evaluation of complex sensorial comfort of worn garments based on large experimental investigation. I is called index, independently of its dimension. The scales for TK (Wear Comfort, in German Tragekomfort) extends from 1 to 6, where 1 is the best level, 6 is the worst level).
where particular parameters mean: i mt index of water vapour transmission i o surface index n k number of contact points i k index of lepivosti i B index of wetting s bending rigidity Sensorial comfort ( 7)
Determination of thermophysiological comfort is similar: (8) i mt index of water vapour transmission F i dynamic vater vapour absorbtion [%] K d liquid moisture buffering coefficient T temperature buffering coefficient [K.min -1 ] K f moisture permeability [g.m -2 .hmbar -1 ] Numerical values of constants: 1 = -5,640 4 = -4,512 2 = -0,375 5 = -4,532 3 = -1,587 =11,553
K f moisture permeability [g.m -2 .hmbar -1 ] Numerical values of constants: 1 = -5,640 4 = -4,512 2 = -0,375 5 = -4,532 3 = -1,587 =11,553 Total comfort: TK tot = 0,35 . TK H + 0,65 . T K T
3.6.Objective measurements of tactile mechanical properties (FOM) of fabrics by means of Kawabata (KES) instruments and their correlation to Handle Values evaluation 4 measuring modulus, 16 parameters measured. Fabric friction and perfil measurement Fabric load/extension and shear force/shear angle dependences Fabric thickness and compressibility measurement Fabric beding rigididy dependence on curvature determination
Comparison of Hand Values with FOM parameters. Assesment of Total Hand Values. Kawabata-Niwa regression equations for fabrics. 3.6.1. Determination of friction coefficient of textile fabrics Friction coefficient belongs to the important parameters of textile fabrics, and its value affects both their behavior during confectioning, and their contact comfort parameter called handle. Feeling of friction influences customer´s opinion when buying new cloth for suits or skirts, and the possibility of its precise objective evaluation even in shops and markets would mean strong tool of textile marketing.
Unfortunately, common instruments for the friction assessment are too large, and their operation is cumbersome. The aim of research based on the above mentioned torque measuring instrument was to design a small portable instrument which is easy to operate. New measuring method of the fabric fric tion measurement The principle of the instrument depends in manual application of mechanical torque momentum by means of hand rotation and the measurement of the peak value of this momentum applied in the specific friction torque measuring tool.
The measuring unit was designed in mechanical version, electronic analogue version and also in electronic digital version. All instrument versions should exhibit the possibility of recording the peak value of the applied torque momentum. The mechanical version, equipped by the testing needle (1) used also for the hardness measurements in yarn packages, is displayed in the Fig. 4. The principle of recording the peak torque momentum depends in the use of torque spring inside the instrument body (2).
Fig. 4a. The measuring unit equipped by a needle for the measurement of the package hardness 1 2 3 4
The more is turned the instrument handle (3), the higher is the torque momentum in the main shaft of the instrument. The maximum angular displacement, and hence the maximum momentum, is then displayed by the extreme position of the slippage dial (pointer). The electronic versions of the instrument are based on torque the ultra-thin wall tube, whose small angular deformation is measured by means of special strain gages, fixed on the tube wall under the 45 o slope. The strain gages are fully temperature compensated. One end of the tube is manually turned, the other end carries the proper measuring tool.
The strain gages signal is conducted to the Wheatstone bridge and the processed by means of a digital micro-controller or PC together with an A/D converter. As shown in Fig 4b, surface of the ring shaped body of diameters D and d, rubs against the flat surface due to the force P. The torque momentum M of this dry clutch and consequently the friction coefficient are given by the following equations :
Fig. 4b Geometry of the friction ring used in the tester P d D
The force P causing the pressure p results from the mass of the measuring ring (here, the mass inertia affects partially the level of the momentum M at the beginning of the measurement) or may be assured by other, recently developed method, which is not negatively influenced by the ring mass. 3.7. FOM by means of the FAST instruments and related snake diagrams
3.8. Non traditional methods of FOM Pulling a fabric through a short hollow cylinder of approx. 15 mm diameter and recording the pulling force as a function of the fabric displacement reflects the effect of several mechanical properties, but the specific calibration is almost impossible. Also pulling a fabric strip by a moving straight edge into a gap of certain width (Handle-O-Meter) reveals a mixed effect of fabric bending rigidity, compressibility and surface friction.
L. Hes and Yi Li (Hong Kong Polytechnic University) designed and manufactured an intelligent Hand Simulator , which should measure in one step and on one sample all thermal and mechanical fabric characteristic commonly detected by a customer when buying the cloth and compare these characteristic with the subjective ones.
3.9. Instruments For the Evaluation of Thermal Contact Feeling of Textile Fabrics Warm-cool feeling means the feeling we get when the human skin touches shortly any object, in our case textile or other fabric used in clothing, furniture or carpets. It was found, that this parameter characterises well the transient thermal feeling which we get in the moment, when we put on the undergarment, shirts, gloves or other textile products. Since this feeling strongly affects the choice of people when buying the clothes or gar-ments, the objective assessment of this feeling became very important in the last decade.
The first instrument, which was able to evaluate the warm-cool feeling of fabrics objectively, was developed by YONEDA and KAWABATA in 1983. They have introduced the maximum level of the contact heat flow qmax [W/m2K] as a measure of this transient thermal characteristics, and KAWABATA has published the first objectively determined values describing the thermal-contact properties of textile fabrics. Their instrument, called THERMO-LABO, was commercialised. In 1986 an other instrument for the objective evaluation of warm-cool feeling of fabrics, but of different concept, was completed at the Technical University in Liberec.
This computer controlled semi-automatic instrument called ALAMBETA calculates all the statistic parameters of the measurement and exhibits the instrument auto-diagnostics, which avoids faulty instrument operation. The whole measurement procedure, including the measurement of thermal con-ductivity , thermal resistance R, qmax, sample thickness and the results evaluation, lasts less than 3 -5 min. As the objective measure of warm-cool feeling of fabrics, so called thermal absorptivity b [Ws1/2/m2K] was introduced. The meaning of this parameter (formerly used in the civil engineering and ergonomics) is explained in next paragraph.
Provided that the time of thermal contact between human skin and a fabric is short, textile fabric was idealised to a semi-infinite body of thermal capacity c[J/m3] and initial temperature t2. Transient temperature field between human skin (characterised by a constant temperature t1) and a fabric is then given by the following partial differential equation (t / ) = a ( 2t / x2) (9)
and can be used for the calculation of the initial level of heat flow q passing between the skin (characterised by a constant temperature t1) and textile fabric according to the next equation, whose derivation for the boundary condition of 1st order is similar to derivation of the Eq. (10): q dyn = b ( t 1 - t2 ) / ( )1/2 (10)
Thus derived thermal absorptivity b [Ws1/2/m2K] is given by the relation: b = (c)1/2 (11) As it can be see, the level of thermal absorptivity depends neither on the temperature gradient between the fabric and skin, nor on the measurement time. This value just depends on the contact pressure, which also correspond to the real situation. The pressure is adjustable.
The simplified scheme of the instrument is shown on Fig. 6. The principle of first version of this instrument depends in the application of ultra thin heat flow sensor 4 , which is attached to a metal block 2 with constant temperature, which differs from the sample temperature. When the measurement starts, the measuring head 1 containing the mentioned heat flow sensor drops down and touches the planar measured sample 5 , which is located on the instrument base 6 under the measuring head. In this moment, the surface temperature of the sample suddenly changes and the instrument computer registers the heat flow course. Simultaneously, a photoelectric sensor measures the sample thickness.
All the data are then processed in the computer according to an original programme, which involves the mathematical model characterising the transient temperature field in thin slab subjected to different boundary conditions. To simulate the real conditions of warm-cool feeling evaluation, the instrument measuring head is heated to 32ºC (see the heater 3 and the thermometer 8) , which correspond to the average human skin temperature, while the fabric is kept at the room temperature 22ºC. Similarly, the time constant of the heat flow sensor, which measures directly the heat flow between the automatically moved measuring head and the fabrics, exhibit similar value (0,07 sec), as the human skin. Thus, the full signal response is achieved within 0,2 sec.
Fig. 6 Principle of the ALAMBETA instrument H 1 3 2 8 6 4 5 9 7 10 11
The validity of thermal absorptivity as a new warm-cool feeling parameter of fabrics was confirmed by several tests where the results of relative subjective feeling of 100 persons were compared with the values of thermal absorptivity found by means of the ALAMBETA instrument. It was found, that practical values of thermal absorptivity of dry fabrics range from 20 to 300, – see Tab. 1. The higher is this value, the cooler feeling represents.
Tab. 1 ALAM-BETA EFFECT OF FABRIC STRUCTURE, COMPOSITION AND TREATMENT ON THE LEVEL OF THERMAL ABSORPTIVITY b [Ws1/2/m2K], contact pressure 200 kPa 20- 40 Micro-fibre or fine PES fibre non-woven insulation webs 30- 50 Low density raised PES knits, needled and thermally bonded PES light webs 40- 90 Light knits from synthetic fibres (PAN) or textured filaments, raised tufted carpets 70–120 Light or rib cotton RS knits, raised wool/PES fabrics, brushed micro-fibre weaves
100-150 Light cotton or VS knits, rib cotton woven fabrics 130-180 Light finished cotton knits, raised light wool woven fabrics 150-200 Plain wool or PES/wool fabrics with rough surface 180-250 Permanent press treated cotton/VS rough weaves , dense micro-fibre knits 250-350 Dry cotton shirt fabrics with resin treatment, heavy smooth wool woven fabrics 300-400 Dry VS, Lyocell, silk weaves, smooth dry resin-free heavy cotton weaves (denims) 330-500 Close to skin surface of wetted (0,5 ml of water) cotton/PP (or spec. PES) knits
450-650 Heavy cotton weaves (denims) or wetted knits from spec. PES fibres (COOLMAX) 600-750 Rib knits from cotton or PES/cotton, knits from micro-fibres, if superficially wetted 750 Other woven and knitted fabrics in wet state 1600 Liquid water (evaporation effect not considered)
As results from the table, the thermal - contact feeling of the fabrics is strongly affected by their structure and composition. It was found, that fibres and fibre polymers of higher moisture regain, provide also cooler feeling. Therefore, the warmest feelings can be achieved at fabrics made from PVC, PP, PAN, whereas viscose, flax, cotton and PAD fibers show the coolest feeling. Which feeling is better, depends on customer: for hot summer garments cooler (cotton) feeling is demanded, whereas in the north of Europe warmer clothing, based on the PES/wool is preferred.
An important aspect of the “warm-cool” feeling evaluation is the change of this feeling when the textile product gets wet. Because the time of the warm-cool feeling evaluation of samples in the ALAMBETA instrument is very short, less than 3 minutes, the evaluation of humid samples is reliable (the sample does not turn dry during the measurement). Because the thermal conductivity and thermal capacity of water is much higher than these of dry textile structure, the negative feeling of coolness of garments moistened by sweat can exceed 1000.
Since the thermal absorptivity is mainly the superficial property, its level can be changed by any superficial or finishing treatment, like raising, brushing and coating. The instrument was commercialised by the Czech SENSORA company.
3.10. Moisture absorbtivity of fabric Many people believe, that 100% cotton underwear (t-shirts) provide the best thermal contact comfort, even in hot days, due to its easy and fast sweat sorption (wetting), and their experience based on common life of a clerk or a bussinesman confirms this statement. Nevertheless, when the wearer has to exhibit some physical effort, the excess of the sweat keeps accumulated in the cotton fabric in the proximity of sweating glands for long time and causes thermal discomfort. On the other hand, when we wear the PES/cotton fabrics in hot day, the thermal discomfort, appears as well (even without physical effort), but such fabric gets dry soon.
Some customers believe, that both fabrics differ in their water vapour permeability mainly. In order to explain the effect of this parameter, various underwear fabrics were measured in this study. From the measurements made on the PERMETEST (Sensora) instrument (see in Tab. 2) resulted, that water-vapour permeability of the measured underwear fabrics depends more on their mass per area then on their composition, and that in all cases the relative vapour permeability was very good, exceeding 15%.
The next parameter in question is the moisture sorption capacity (absorbency) of shirt fabrics. There are some other methods to determine this parameter. Nevertheless, the moisture absorbency characterises just the specific moisture retention corresponding to the state of full saturation of the fabric volume by water or sweat, and is directly proportional to the fabric mass. No transient aspects are considered here, and no different boundary conditions of moisture transmission between the skin and a fabric are respected. A survey of other techniques to measure transplanar liquid transport into fabrics published Kissa in 1995.
Nevertheless, all the found measuring methods are not suitable for simple standard measurements of transient fabric wetting, due to quite complicate preparation of the measurements and poor dynamic properties of some of these methods. Moreover, the reduced comfort caused by wearing e.g. the PES/cotton shirts in hot day is felt mainly in the moment, when the suddenly wetted fabric touches the skin. Consequently, local cool feeling occurs, which is considered unpleasant. Within the con-tact time, heat is transferred by conduction through a thin intermediate layer, created by wet outstanding fibres. Thus, the boundary condition approximates to the heat transfer of 1st order, which should be respected within a measuring method in question.
Therefore, the first objective of the research work was to develop a method of an indirect experimental determination of the so called surface moisture absorptivity B , whose higher level apparently increases the contact comfort of wet fabrics and on the contrary. Such parameter should present an integral factor, embracing the effect of moisture surface adhesion (given by the contact angle ) and the moisture conduction (depending on capillary forces). The highest surface moisture absorptivity appears in the moment, when the adhesion and conduction mechanisms, which in some cases act again each other, create a specific synergic effect.
Introduction of Moisture Absorptivity The amount of liquid inside any porous structure or textile fabric can be expressed in terms of the fabric free volume saturation s. Thus, for s = 0 the fabric is dry, and for s = 1 all the pores are full of a liquid. In this case, the saturation propagation within a fabric, either along its surface, but also perpendicularly to its surface, can be characterised by the classical partial differential equation of diffusion processes: ( s / ) = A ( 2s / x2) (12)
where A [m2/sec] is so called moisture diffusivity. This parameter is for textile fabrics sometimes moisture dependent due to swelling. The solution of equation of this kind for A = const is generally known. If we consider just short time moisture conduction, then we can convert a textile fabric to a semiinfinite body, where the 1st order boundary condition is applied. In this case, the moisture saturation propagation in the x direction is given by the equation s = erfc (x /2 A1/2 1/2) (13)
The experimental determination of the moisture diffusivity from the moisture propagation along the measured fabric is possible. Unfortunately, the moisture diffusivity in this form does not characterise the volumetric capacity V of the fabric expressed in this case in m3/m 2 s to conduct the moisture (sweat) from the contacted skin away towards a fabric interior. To cope with this task, a Darcy law modified for the saturation gradient should be introduced as follows: V = - s ( s/ x) (14)
where s [m2/s] is the volumetric moisture flow conductivity, which is proportional to the fabric permeability. In the next step, we should remind, that in the first Fick´s diffusion law, which is used to express the mass flow in the form formally identical with Eq. (14), the same diffusion coefficient D occurs, as in the second Fick´s law for transient mass transfer by diffusion. By simplifying the problem solved to a simple diffusion, we can express the moisture flow conductivity in Eq. (14) s by means of the moisture diffusivity A. From applying this relation in equation (13) follows: V = A1/2( s/ 1/2 1/2) (15)
The first term in this equation fully characterises the fabric ability to absorb the moisture from any moist surface which contacts the fabric. Then this so called moisture absorptivity B [m3 s1/2] is defined by the next relation: B = A1/2 (16)
Many researchers have already measured the time-dependent longitudinal wicking of fabrics. From these results, the moisture diffusivity A could be determined and its square root used for the calculation of the spontaneous moisture uptake according to Eq. 15. Nevertheless, this approach may produce inaccurate results, since longitudinal wicking rates not always correlate with the corresponding transplanar ones, due to the complexity of the wicking processes, which besides the diffusion processes include capillary penetration of moisture inside fabrics, and also moisture absorption of the fibre surface.
Indirect Method of the Moisture Absorptivity Measurement The suggested method is based on the objective evaluation of warm-cool feeling perceived by a wearer of a cloth, which suddenly comes into contact with a wetted skin. In this moment, the cotton fabric absorbs the liquid sweat rapidly, and conducts it away from the fabric surface towards to the fabric inerroir. Due to high adhesion forces, the sweat keeps accumulated in the fabric close the places where the sweat was generated. If the amount of sweat is not too high, within a short time the moisture concentration close to the fabric contact surface reduces, and the wearer feels the pleasant contact of nearly dry fabric.
The other mechanism of achieving the pleasant dry feeling of underwear and shirts is based on the use of PES microfibres, which, due to higher surface, absorb in some extend the humidity also, but the liquid sweat is rapidly distributed by capillary forces in larger area surrounding the perspiration zone, thus reducing the average relative humidity of fabric under the limit, which would result in unpleasant wet feeling. Unfortunately, this mechanism requires also some additional dymamic contact forces typical for sport activities. In the case of blended fabrics containing too much poorly absorbing PES fibres of common section and fineness, the sweat keeps adhered on the skin, and provokes an unpleasant cool feeling due to sweat evaporation.
The suggested method is based on the objective evaluation of cool feeling effect within an experimental procedure, which simulates the real fabric wearing conditions described above. Methodology of the Indirect Measurement of the Fabric Moisture Absorptivity The intention of this research was to characterise the contact comfort felt by a wearer of a shirt during a hot day. For this purpose, a special thin interface fabric was found, which should simulate the effect of a sudden sweat discharge on the skin.
This sweat simulator should be thin, in order not to influence (in dry state) the thermal capacity of the measured fabric, but this interface fabric should absorb a certain amount of liquid injected in the centre of this interface fabric and it should distribute the liquid fast and uniformly within a circle of approx. 50 mm diameter. After some trials, a thin PES Coolmax knit was found to fulfil all demands.
At the beginning of the measurement, the ALAMBETA instrument is switched on and the measured underwear is placed on the measuring base of the instrument. Then, the volume of 0,2 ml of water (containing detergent) was injected on the centre of the interface fabric surface, covered by the viscose fibres. Within one minute, the liquid distributed uniformly within a circle of 45-50 mm, and stopped. When this occurred, this interface fabric was inserted into the space between the measured sample and the centre of the measuring head of the instrument - see Fig. 7. At the same time, the interface fabric and the measuring head of the instrument dropped down towards to the measured underwear or shirt fabric.
Within a few seconds, the liquid from the interface fabric was more (in case of pure cotton shirt) or less (in other cases) taken away by absorption into the lower fabric. If this fabric exhibits low absorption, the thermal capacity of the interface fabric is maintained quite large and the initial level of thermal absorptivity b is significantly higher. In the case of measurement of “warm-cool” feeling on pure cotton fabrics, characterised by higher moisture absorptivity, the moisture is rapidly distributed within the whole thickness of the fabric, so that the interface fabric gets nearly dry, and the instrument shows a lower level of the resulting thermal absorptivity.
Fig. 7 - Simulation of the underwear wetting and wicking by means of the ALAMBETA instrument Guiding shaft Measuring head Interface fabric Upper heat flow sensor Fabric fated Fabric Bottom sens. sensor
Theoretical Analysis of the Underwear Wicking and Wetting Effect on Cool Feeling In the new version of the ALAMBETA, not only one, but two heat flow sensors are applied, as shown on Fig. 7- see the second sensor. This enables to simplify the heat flow signals evaluation and moreover, the instrument can check the level of heat, which is during the measurement conducted away (in the surrounding air) from the sample. This increases the measurement precision.
During the measurement, the computer integrates the heat Q[W] passing through both heat flow sensors, which is accumulated inside the measured sample, according to the Eq. 17: Q = q( ) d = q 1 –q o ) d (17)
The measurement finishes for the time level T, when q 1 (T) equals q o (T). Then, the heat Q causes the increase of the average temperature inside the measured sample, as follows: Q = m o c o (t 1 – t o ) (18)
This equation then yields the surface related heat capacity m o c o [J/ m 2 ]. Simultaneously, the sample thickness h[mm] is measured and used for the determination of the fabric heat capacity o c o [J/m 3 ] from the equation o c o = m o c o /h (19)
The consequent steady state measurement of heat flow passing through the sample then enables the easy determination of the coefficient of thermal conductivity [W/m.K] and the fabric thermal resistance R[m 2 K/W]. The warm-cool feeling parameter - thermal absorptivity, then follows from Eq. 11.
In the next step, the mentioned measuring principle will be applied in the simple analysis of the wetting and wicking simulation. In this case, heat balance of the space between both heat flow sensors should include the effect of heat capacity per area m 1 c 1 of the interface fabric and ist initial moisture M w c w , provided that the measuring head has just dropped down and completed the thermal contact between the interface and underwear fabrics: Q M = (m o c o + m 1 c 1 + M w c w )(t 1 – t o ) (20)
Within a few seconds, the moisture will be absorbed by the underwear and distributed in its volume. In fabrics exhibiting good moisture conduction the sweat will be transported by the capillary action outside the area of heat flow sensors. Hence, the effective moisture content in central part of the underwear will be reduced to lower level m w , thus reducing the volumetric thermal capacity of the system consisting of interface fabric and underwear. The integral heat detected by both heat flow sensor will be as follows: Q m = (m o c o + m 1 c 1 + m w c w )(t 1 – t o ) (21)
Because m w M w , and due to the fact, that the specific heat of water c w is very high – approx. 3 times higher than that of fabrics, even small differences in the moisture amount absorbed form the sweat simulating fabric and conducted away form the sensing area of heat flow sensors will result in relative big changes in heat capacity of various tested fabric system and hence in big changes in their thermal absorptivity levels. In fact, the resulting sensitivity will be even bigger, due to varying evaporation effects, which were not considered in this simple analysis (the more moisture keeps in the interface fabric after contact, the cooler is ist surface, and the higher is the resulting thermal absorptivity).
Experimental Results The composition of the investigated plane fabrics varied from 100% cotton to 100% PES or PP fibres. Medium values of the results are shown in the following Table 2. Tab. 2. Cool feeling (thermal absorptivity) of various fabrics measured by the ALAMBETA instrument when simulating their sudden thermal contact with wetted skin, pressure 200 Pa
SAMPLE COMPOSITION AND STRUCTURE Thick- ness H[mm] CV up 3% Thermal conducti- vity [mW/mK] CV up 3% Relative water va- pour per-meabili- ty % CV up 6% Thermal absorp- tivity b [Ws1/2/ m2K] CV up 5% 45% cotton, 45% PP +PAD Italian 2 layers smart knit 1.15 105 41,4 415 50% cotton 50% PP spec. struct. Czech smart knit 0,66 100 43,0 421 100% spec. Section PP + common PP, Czech 1 layer smart knit 1,21 112 40,2 430
Results Evaluation With an increasing portion of PES fibres in common woven shirt fabrics increases the unpleasant cool feeling (i. e. increases thermal absorptivity) when worn in conditions of surface wetting, which matches the practical experience of wearing the tested shirts. Special smart fabrics with improved thermal comfort properties like double layered knits or T shirts knitted from Coolmax or Coolplus (Taiwan) modified PES fibres reveal more pleasant contact feeling in conditions of superficial wetting.
Exceptionally some cotton/PES blend fabrics made from common fibres may exhibit relatively good thermal contact comfort in the wet state, even with quite high portion of PES fibres, due to some unknown effect or due to a special fabric structure (confirmed by wearers). Cotton shirt weaves containing too much chemical agents deposited inside the fabric may show worse contact comfort feeling in the wet state, in spite of the fact, that their steady-state water vapour permeability keeps very high.
4.Thermophysiological clothing comfort - principles Fig. 8 Thermoregulation system of human body
4.1.Environmental parameters of human life: air relative humidity φ , air velocity v A , dry thermometer (or air) temperature t A, wet bulb (thermo-meter) temperature t WB (strongly dependent on φ and v A , and globe temperature t G , which is measured in centre of big black globe, thus expressing the effect of solar radiation. The integral environmental effect expressed in terms of the wet bulb globe temperature t WBG : t WBG = 0,7 t WB + 0,2 t G + 0,1 t A (21)
Examples of groups of environmental parameters, which offer the thermo-physiological comfort under various physical activities, provided that the level of the radiation temperature (emitted e.g. by warmer walls) does not exceed the dry air (environmental) temperature t A for more then 2 O C:
Administrative work t A = 21 O C ± 3 O C, φ = 55% ± 15%,v A = 0,1 m/s Light manual work, standing t A = 19 O C ± 3 O C, φ = 55% ± 15%, v A = 0,2 m/s Heavy manual work t A = 18 O C ± 3 O C, φ = 50% ± 15%, v A = 0,4 m/s Very heavy work t A = 7 O C ± 3 O C, φ = 50% ± 15%, v A = 0,5 m/s
4 . 2. Fundaments of human thermal physiology Human body as a thermal engine with the efficiency = (5-25%). Fig. 9 Human body as a thermal machine
Muscles transforming chemical energy into labour L [J] (up to 50x increase from the resting level). Energy carriers: carbohydrates (18 kJ/g), fats (40 kJ/g) and proteins (19 kJ/g). Food processing: stomach absorption in small intestines transport by blood transformation into energy in cells or storing as glycogen (C 6 H 10 O 5 ) or fat.
Muscles work: energy input converts adenosine diphosphate -ADP into adenosine triphosphate (ATP). Most easily used energy carrier: glucose . Aerobic metabolism (most effective): C 6 H 12 O 6 + 6O 2 = 6 CO 2 + 6 H 2 O + energy (690 kcal/mole). Anaerobic metabolism: lactic acids released. Energy storage: fat, 16-22% of the body weight in a man, 22-34% in a woman (15-20% in sportsmen). Much less stored energy in glycogen and glucose. Protein amino acids used for energy just in vital conditions (death of cells involved).
Basal metabolism: approx. 1,1 W /kg of body weight, minimum metabolic power M min reaching 50-100 W., heart rate 60-80 /min. Resting metabolism: 1, 25 W /kg of body weight, corresponding to the oxygen consumption 0,0035 L /min/kg of body weight (which is called 1 MET). Heavy work requirements: even more then 10 W /kg of body weight, heart rate exceeding 120 /min, muscles consuming up to 70% oxygen available, brain always 5%, internal organs suffering. Heart pumping rate: from 25 litre/min to 40 litre/min for sportsmen. Temperature set points in hypothalamus: 37 O C for core, 33 O C for skin.
Temperature limits: over 45 O C:coagulation of proteins, 0 O C:breaking cell aparts due to ice crystals. Deviation of core temperature 37 O C ± 2 O C affects body functions, deviations ± 6 O C are lethal. Sudomotor nervous pathways control the sweat glands activities only, the vasomotor system brings about the vascular dilation, constriction or shunting, thus affecting the heat distribution throughout the body.
Sweating level m p [kg/min, up to 10 kg/day] as the function of real skin (t S > 32 O C) temperature and core (nuclei) temperature (t N > 37 O C) due to increased environmental temperature t E m p = F 1 (t S - 32 ) + F 2 (t N - 37) (22)
4.2.1.Definition of thermal comfort for lying or resting human body: thermal equilibrium, no muscular shivering nor vasodilatation, no principal sweating (relatively dry skin), skin temperature between 32 and 34 O C, no heat storage or loses. Changes of stored (accumulated) heat: Δ Q AC =c spec .(0.35 Δ t S +0.65t N ),c spec =3300J/kg.K (23)
4.3. Equation of steady - state thermal equilibrium of human body expressed in heat/time [J/sec] units, it means in power Q [W] units. (M min + L/ η - L)/A Du = (M - L)/ A Du (24) (M-L)/A Du ± q cond ± q conv ± q rad - q res,c - q res,e - q ins - persp = 0 (25)
Meaning of new symbols: A Du is the surface of the average human body, 1,8m 2 . q conv heat flow [W/m 2 ] by convection from the skin surface q conv = α F cl (t* sk – t a ) A Du (26) α = 2,38 (t* sk – t a ) 0, 25 for natural convection α = 3,5 + 5,2 v ar for forced convection at v ar 0-1 m/s α = 8,7v ar 0,6 for forced convection at v ar over 1 m/s
q cond given by Eq. 28, at walking just 5 -10 w. q rad given by Eq. 35 q res,c cooling by respiratory convection q res,c = c p V a (t ex – t a ) A Du , which can be expressed through metabolic power M as q res,c = 0,0014 M (t ex – t a )
q res,e cooling by evaporation at respiration q res,e = L V a (W ex – W a ) A Du , which can be expressed through metabolic power M as q res,e = 0,0173 M ( p ex – p a ) q ins cooling by insensible and permanent evaporation from skin pores, approx.0,15 W/1 kg of body mass q persp intensive cooling by means of principal sweating glands controlled by brain hypothalamus (sudomotor pathways) through adrenalin level, and by means of smaller glands in
palms and soles q persp = w (p wv,sat – p wv,out )/R evap,tot w means here skin wettedness, given by fraction of the wetted skin skin surface related to total skin surface The heat and moisture transfer mechanisms applied in the human body thermal balance are explained in the next text in more detail.
4.4. Fundaments of heat transfer between human body and environ- ment by conduction, convection and radiation 4.4.1. Principal relations describing the heat conduction : Fourier's law, expressing the proportionality among heat flow q [W/m 2 K], thermal conductivity λ [W/m.K] and temperature gradient Δ t/ Δ x: q = - λ . Δ t/ Δ x (27)
Relation for thermal resistance R [m 2 K/W] of fabrics, thin air layers and other plane materials of thickness h [m]: R = h / λ (28) Thermal resistance of air layer in clothing: maximum for h = 5mm. Total thermal resistance of clothing R CL consisting of full area individual layers: R CL = R 1 + R 2 + R 3 + . . . (29)
Fig. 10 Heat flow through clothing layers body core transfer of heat, moisture and air moisture air environment air layers laylaylayers vzduchu I II III skin surf. underw. med. fabric outer fabric
Total heat flow - heat power Q * [W] through a clothing of area A CL by conduction within the temperature gradient Δ t = t S - t E is then given by the equation Q = A CL . q = Δ t . A CL / R TOT , where R TOT = R CL + R E (30) 4.4.2. Principal relations describing the heat convection : Heat is transferred by particles of fluids moving with the velocity v [m/s]. The thermal boundary layer thickness δ is thick for the laminar fluid flow and becomes thinner for the turbulent flow,
when the Reynolds dimensionless number Re exceeds 2300 for any object of characteristic dimension d [m]. Re = vd/ ν , where ν [m2 / s] means the dynamic viscosity of the fluid.
The heat transfer coefficient α C [W/m 2 K] is relatively low for natural convection, and increases for forced convection. For the conditions typical for the use of clothing, the heat transfer coefficient can be also given by a simplified equation for all air velocities α C = 8,3 √ v (30) The Newton's law for the heat flow transferred by any kind of convection or conduction is as follows: q = α C (t 1 - t 2 ) or q = (t 1 - t 2 ) /R cl (31)
where R cl means thermal resistance of garment or clothing. Convection thermal boundary layer presents important external thermal resistance R bl = 1/α C (32) which should be included into the total thermal resistance R TOT . Sometimes we should also consider the heat loses by radiation, given by the linear radiation heat transfer coefficient α rad .
4.4.3. Principals of heat transfer by radiation Generally, the heat flow passing through clothing layers by IR radiation may reach up to 10 - 15% of the total heat flow. In hot days or countries, solar radiation, both visible and invisible, causes principal thermophysiological discomfort. Radiation UV, μm Infrared waves Ultrashort Short Radiofequences γ , X 0,19-0,38 0,75 - 1000 μ m 1mm - 1 dm 0,1-2m 2 - 1500 m Visible light 0,35 - 075 μm Log wavelength λ -> Fig. 11 Spectrum of electromagnetic radiation
Radiation heat is transferred by visible (light) and invisible electromagnetic waves.The visible part of the electromagnetic spectrum involves the wavelength Λ = 0.4-0,75 micrometer ( μ ),where sun emits approx. 50% of its thermal energy. White garments reflects a good part of this thermal energy. The resting 50% is radiated in the invisible infrared (IR) part of the spectrum (0,75 - 100 μ ), mainly in the near infrared part of the spectrum (till 2 μ ). Here, the degree of reflection ρ <1 already cannot be characterised by a white colour - we cannot distinguish here any colours, but any smooth surface reflexes IR radiation better can a harsh, coarse surface. The-refore, the protecti-ve clothing against heat should be white (or of polished metal), and smooth.
According to the Wien's law, the lower is the absolute temperature T[K] of the heater, the shorter is the wave-length Λ MAX [ μ ] corresponding to the maximum level of the emitted energy, as follows: Λ MAX . T = 2890 (33) Thus, heat flow transferred by radiation between the sun and humans reaches its maximal level for the green light (0,55 μ m),whereas clothed humans lose energy towards the common environment at the wavelength approx. 10 μ m. Some special fibres contain ceramic particles,
which absorb the visible radiation with the degree of emissivity (or absorbance) ε≈ 1, whereas for the IR radiation this dimension less parameter is substantially lower then 1.
When calculating the heat flow q [W/m 2 ] transferred by IR radiation between two clothing layers (garments), we can use the relation for parallel planes with the emissivity levels ε 1 , ε 2 and kept at temperatures T 1 and T 2 in IR permeable environment as follows ( σ = 5,67 x 10 -8 is the radiation constant): q = σ (T 1 4 - T 2 4 ) / [ (1/ ε 1 )+ (1/ ε 2 ) - 1] (34)
In order to reduce the heat transferred through clothing by radiation (e.g. in sleeping bags), the textile layers can be coated by the vacuum deposited aluminium. Radiation heat flow transferred between a (clothed) human of surface absolute temperature T S and a homogeneous, cooler environment of the average absolute temperature T E is given by en expression q = σε S (T 1 4 - T 2 4 ) 4 σε S [(T 1 + T 2 )/2] 3 (t 1 - t 2 ) (35)
From the analogy with the convection heat transfer, the linear radiation heat transfer coefficient receives the form α rad = 4 σε S [(T 1 + T 2 )/2] 3 (36) High thermal resistance of nonwoven fabrics is a frequent reason of their applications in protective clothing, sleeping bags and related technical textiles. New standards and higher requirements result in the necessity of determination of thermal resistance of these insulation layers with higher precision. Therefore, several new measuring instruments and methods appeared recently to serve for these purposes. Majority of them is based on the evaluation of steady
or unsteady heat flow passing between two plates, which embrace the measured fabric. Should the thin or low density fabrics be measured, the portion of the heat transferred between the plates by infrared radiation may reach 10-30% of the total heat flow. Convection is generally negligible. In this case, the radiation properties of the plates should affect the effective thermal resistance of the nonwoven insulation layer. The first objective of the next study is to determine the portion oh heat, which, in common textile fabrics, is transferred by infrared radiation. The main objective of the paper is, however, the experimental determination of the effect of the
surface emissivity of the measuring plates of the thermal insulation measuring instrument, on the measured thermal conductivity and thermal resistance of selected relatively thin nonwoven fabrics made of different materials. The experimental procedure is based on a new computer-controlled measuring instrument, which measures the steady and transient thermal characteristics of non-metallic materials within one step. A brief description of this instrument is given in the next chapter, along with the brief theoretical analysis of the problem.
Thermal conductivity of textile fabrics Thermal-insulation properties belong to the basic properties of textiles fabrics and so they have been studied and measured very thoroughly. Similarly, the theoretical analysis of heat transfer through fabrics was carried out by several investigators and the most recent papers were published by FARNWORTH, CAPS and UMBACH, HES and STANEK. It was found that the mechanisms of transfer of heat through textile fabrics depend mainly on thermal conduction and radiation. This was confirmed during an extensive experimental investigation of heat transfer through woven and nonwoven fabrics, which was conducted at the Technical University of Liberec,
where the Grasshoff number Gr, describing the effect of free convection, was always lower than 1000. Based on new theory and a new high precision measuring instrument, HES and STANEK derived the following formula for thermal conductivity of textile fabrics with low density: λ = A 1- + (37)
Here, the first term on the right hand side expresses the transmission of heat by conduction in air gaps (proportional to thermal conductivity λ A of air) and through the polyester fibres (with a thermal conductivity λ PES 15 times higher than that of air) oriented parallel with the surface. The second term shows the heat conduction through the fibres oriented perpendicular to the fabric surface. The term μ represents the filling coefficient of the fabric, and ν is the (idealized) portion of fibres oriented vertically to the isotherms in the fabric.
For the purpose of this paper, the more important factor is the last right hand side term, expressing the heat conducted by radiation, where the classical dependence of heat flow on the 4th power of temperature can be approximated by a linear one: rad = (38)
Here, h is the thickness of the fabric, σ is the radiation constant, T the ave- rage temperature in in the fabrics and ε and r the emissivity and radius of fibres. As shown later, the portion of heat flow transferred by radiation does not exceed 20% of the total heat flow. Thus the linearity of the mathematic model is conserved and the following relation can be used: = cond + rad (39)
Nevertheless, in spite of the fairly low effect of rad on total , the investigation of factors influencing rad is important, because the technological ways to reduce cond only (in order to increase the insulation power of fabric) have already been exhausted. As a result, many researchers are now trying to reduce the rad of textiles. Because of the low contribution of rad to the total level of λ, the investigation of this factor creates strict demands on the sensitivity and precision of the experimental technique.
The instruments generally used for the measurement of thermal conductivity and thermal resistance R of thin layers are not sufficiently precise, because the changes in heat flow caused by the low thermal resistance of fabrics are nearly undetectable for classical instruments of the BOCK (large skin model) type. A recently developed instrument ALAMBETA, does not exhibit this problem. Besides that, the new instrument enables also the measurement of transient thermal characteristics of textile fabrics, where one of these characteristics can be used for objective evaluation of warm or cool feeling. This is important during short contact with the fabric, or when wearing some fabrics (like trousers) which come into intermittent thermal contact with our skin.
Theoretical background of the experimental procedure To investigate the portion p of radiation heat flow transferred through textile fabric, the next procedure can be applied: The method is based on the fact, that rad increases with the mean temperature T in the layer. Therefore, for two different mean temperatures T (supposing T1=300 K and T2=315 K, which can be adjusted at the instrument) and after applying Eq. (3) we get: for T =T1 cond + rad(T 1 ) = 1 (40)
for T=T2 cond + air + rad (T1) . (T 2 /T 1 ) 3 = 2 (41) Subtracting both equations, we obtain rad = ( 2 - 1 + air / [( T2 / T1 ) 3 - 1] (42) The effect of increased mean temperature on thermal conductivity of the air trapped in fabrics was considered by increasing cond in Eq.9 by air = 0.00055 W/m.K, (43) which also reflects the relative portion of air in the fabric (about 70%).
4.5. Fundaments of water vapour transfer between human body and environment Moisture (mass) can be transferred either by conduction or by convection. The transfer force of water vapour in clothing systems is the gradient between the saturated vapour conentration or saturated (maximal) partial pressure p WSAT [Pa] at the human skin the actual environmental water vapour concentration or its partial pressure p WE [Pa]. The inverse ratio of these parameters multiplied by 100 we call relative humidity φ [%]. In countries, where the φ is lower than 60 - 70 %, humans can reach the conditions of reasonable thermo-physiological comfort (due to the efficient sweating) even under high air temperatures (in deserts).
When the air relative humidity φ exceeds 80-85%, than no state of comfort is attainable at air temperatures over 35 O C. Moisture (mass) transfer by conduction The amount of transferred vapour (mass) m * [kg/m 2 . s] is proportional to the diffusion coefficient D p [kg/m.s.Pa] and to the partial pressure gradient Δ p parc / Δ x (44) According to the Fick's law: m * = - D p . Δ p parc / Δ x = - D p . (p WSAT - p WE ) (45)
Instead of the water vapour pressure gradient, also the mass concentration gradient C [kg H 2 O/ 1kg humid air] can be used in the above mentioned expression: m * = - D C . Δ C / Δ x = - D C . (C WSAT - C WE ) (46) The correlation between both forms of the diffusion coefficient is then given by the state equation of gases, comprising the molar density of water vapour M W , universal gas constant R and absolute vapour temperature T: D p = D C . M W /RT (47)
Vapour is transferred by conduction (diffusion) through fabrics and gaps in garments or in clothing systems. If there are no pumping effects of free convection in clothing systems, the water vapour resistance R W of gaps, given by the equation R WP = h / D P or R WC = h / D C (48)
Can reach relatively high values. In textile fabrics consisting of pores (channels), which present big barriers for the moisture transfer, due to the fabric surface porosity ε < 1 and increased channels length (given by the tortuosity factor ξ >1), the water vapour resistance R WF of fabrics can be very high, according to the next expression: R WP = ξ .h / ε .D p (4 9)
Therefore, due to the larger porosity, open fabrics like knitted ones naturally offer much higher water vapour permeability or lower water vapour resistance then the woven fabrics Moisture (mass) transfer by convection The relation for the mass transferred by convection is similar to the Newton Law: m * = β p (p WSAT - p WE ) = β C (C WSAT - C WE ), β p = β C M W /RT (50)
Similarly, as the convection heat transfer coefficient α increases with the air velocity, the convection mass transfer coefficient β p [kg/m 2 s Pa] is also proportional to the air velocity. Due to analogy between the heat and mass transfer, the convection mass transfer coefficient β C for low air velocities can be calculated by means of the Lewis Law: α = β C . c pA (51) Here, c pA [J/kg.K] is the specific heat of the humid air.
4.6. Fundaments of wetting and wicking of textile fabrics. Contact angle, adhesion. 4.7.Simple thermal model of a clothed human body Relation for the clothing/garment total thermal insulation R cl [m 2 K/W] in cold conditions, when no principal transpiration is involved, and homogeneous, full body covering textile layer is considered: q =[0,75(M-L)]/A cl =(33-t ext )/[(R cl .v)+1/( α conv + α rad )] (52)
Here, the effect of ventilation and body movement on thermal loses of a body is respected by means of the ventilation coefficient v = F(air velocity, fabric air permeability), v < 1. Factor 0,75 in the first term reflects the heat loses by insensible evaporation, respiration and cocuction.
5.Thermophysiological clothing comfort - evaluation 5.1. Measurement of thermal resistance and of warm-cool feeling of fabrics, both in dry and wet state, by means of the ALAMBETA computer-controlled device - see in Chapter 3.9. Fig.12 Measuring facility
5.2. Evaluation of warm-cool feeling in simulated conditions of medium and intensive sweating - see in Chapter 3.10. 5.3. Measurement of water vapour resistance (in dry and wet state) and heat of absorption of fabrics, by means of the fast PERMETEST instrument Water vapour permeability of fabrics presents, along with fabric thermal resistance, the most important characteristic of clothing comfort .
That is why increased attention is paid to this parameter in recent decades. Testing of this parameter in official laboratories and the used instruments are generally costly, time consuming and requires of ten special samples cut from pieces of fabrics. If these are large, their price can increase substan-tially the total price of each measurement. Moreover, the necessity of specially sized samples avoids the non-destructive measurements on tailored garments due to high price of the completed products. That is why a new friendly testing method and corresponding measuring device PERMETEST that presents a small „skin model“ were developed in the nineties.
The istrument was comerci-ased by the Czech SENSO-RA Company, and under standard laboratory condi-tions (at 22 °C and relative humidity 55%–60%) it offers reasonable precisi-on of measurement. Re-sults of measurement are expressed in units defined in the ISO Standard 11092. The main advantage of this instru-ment is the fast and non-destructive testing of wa-ter-vapour and thermal resistance / permeability of textile fabrics.
Principle of the PERMETEST instrument Slightly curved porous surface is moistened (either continuously or on demand) and exposed in a wind channel to parallel air flow of adjustable velocity (Fig.12). A tested sample is located in a small distance from the wetted area of diameter about 80 mm and characterized by high thermal conductivity. The amount of evaporation heat of liquid water taken away from the active porous surface is measured by a special integrated system. Thus, very low time constant of the whole system was achieved, resulting in short measurement time – full signal is registered within several minutes.
Besides basic elements described below (see also Fig.12) the device consists of water dosing syringe, an industrial digital temperature controller, a consumer ambient digital thermometer joined with relative humidity meter, a chart recorder and a supply unit. The core system can be heated to temperature exceeding the room temperature or can be kept at the room temperature to maintain the isothermal working conditions.
At the beginning of the measurement, heat flow value q’ ho without a sample is saved. If water was regularly distributed and the head temperature was properly controlled the signal becomes quite stable but will include some small turbulent variations which cannot be avoided. In the next step, the measuring head pulls down and a sample is inserted between the head and the cutout in the wind channel. Then the measuring head moves back to the channel and squeezes the sample. After short period when the signal reflects the effect of different temperature of the sample, the signal becomes steady and new value q’ hs which quantifies heat loses of moist measuring head covered by a sample is read.
Relative water vapour permeability of the textile sample r wv is calculated from the formula - Eq. 53: (53)
When the instrument should measure the water vapour resistance according to the ISO 11092 Standard then a cellophane foil permeable for the water-vapour but not permeable for the liquid water is put to cover the head surface. Application of the same procedure as above gives two values q“ h0 and q“ hs . The demanded water vapour resistance of the sample R wf then follows from the equation (54) (54)
The values p“ wv and p wv in this equation represent the water vapour saturate partial pressure valid for ambient temperature a and actual partial water vapour pressure in a laboratory. Relative humidity expresses a relation between vapour densities and also pres-sures (55)
so the equation (55) is rewritten (56) using function p“ wv = g( a ) which is given in the table of water vapour properties.
In the case of thermal resistance determination of dry textile samples the whole procedure is identical again but the measuring head is dry and its temperature should be maintained at 32 C or 35 C that makes temperature difference to ambient air . The thermal resistance of the sample R t then yields the equation (57)
5.4 . Non- destructive testing of thermal comfort properties of garments Testing of thermohysiological properties of protective clothing on thermal manniquins is the ideal way of testing, but this procedure is very costly. In some cases, we need to know or verify just some specific fabric parameters like thermal resistance or water vapour permeability. Classical methods of testing, unfortunately, require cutting of samples of certain dimensions, which results in damaging the clothing. Fortunately, two relatively new commercial instruments called ALAMBETA and PERMETEST allow the non-destructive determining of the above mentioned principal comfort para-meters.
The commercial instruments called ALAMBETA and PERMETEST for the evaluation of thermophysiological comfort of garments feature special design of small sensitive elements, which are surrounded by relative large isothermic areas or by areas of constant humidity. This patented solution avoids to some extent the so called edge – effects and brings certain inde-pendence of the measured results on the sample dimensions.
In practical tests, any part of the protective garment can be inserted into the measuring zone of the instruments, and the garment is not damaged during the measurement. Naturally, the results recorded on large garments may partially differ from the results measured on standard samples. That is why the second objective of the paper is the analysis of the effect of sample dimensions on the data determined by means of the mentioned instruments.
The instrument is not delivered with any climatic device, because its use under standard laboratory conditions (at 22 o C and relative humidity 55%) offers reasonable precision of measurement. Results of measurement are expressed in units defined in the ISO Standard 11092.
5.4.1.Experimental results and their evaluation Thermal resistance All the samples were woven in plain weave and composed of 100 % natural and synthetic polymers in grey state. Their square mass varied from 125 to 160 g/m 2 . For thermal measurements, 9 different samples was used, and the number of fabric layers used in the measurement varied from single layers up to 5 layers of fabrics.
Figs 14 and 15 The effect of sample dimensions on thermal resistance and conductivity of PES fabrics measured by the ALAMBETA instrument
The first group of measurement embraced the so called “cut” circular samples with the diameter equal to the diameter of the instrument measuring head (113 mm), whereas the second series of measurement were performed on “endless” it means very large fabrics.
All the samples were measured 5 times in different places, and the CV values (%) were determined. The detailed analysis reveals, that the differences between thermal conductivity and thermal resistance values for “cut” and “large” samples in 5 studied cases did not exceed 3%, in 2 cases 5% and in the only case (for PAD 6) 9%. As regards thermal resistance, even for 8 polymers, the differences between cut and large samples were lower than 3%, just for cotton sample the discrepancy was 10 %. Variation coefficients in most cases did not exceed 3%, hence, the measurement seemed to be quite reliable. It can be stated, that in all these cases, the suggested non-destructive measurements were reasonably justified.
Water vapour permeability The PERMETEST instrument requires samples, which are larger then the diameter of wetted area (60 mm in our case, but at new instruments it was increased to 80 mm), due to fixing the sample in circumferential clamps. To assure the defined dimensions of the permeated area (to simulate the cut samples), the fabric non-permeability was achieved by means of printing of impermeable polymer in the form of circle of diameter 60 mm. Diameters of other circles were 80 mm, 100 mm and the full large sample.
Fig16. The effect of sample dimensions on the measured relative water vapour permeability of thin woven fabrics similar in structure but differing in composition
Experimental results in terms of relative water vapour permeability (where 100% presents the „permeability“ of free measuring surface) are displayed in the next Fig.16. All the results present the average values from 10 measurements on each sample. Variation coefficients in most cases did not exceed 5%, which confirms good measurement precision for this kind of measurement.
From Fig. 16 follows, that the lowest levels of permeability were found for samples of diameter 60 mm, which coincides with the wetted measuring area of the instrument. If the sample dimensions exceed the measuring area of the instrument, then the determined water vapour permeability increases by 4-9%. This effect can be explained by planar conduction of condensed vapour from the boundary of the measuring area towards fabric edges. Lowest permeability increase (up to 6%), as expected, was found for fabric made of common synthetic polymers and blends.
The determined increase of water vapour permeability is in most cases small enough to permit the non-destructive measurements of commercial textile products in cases when customers require to confirm the properties of their goods or for comparative tests of various heavy fabrics also.
5.5. A new principle of evaluation of thermal comfort of clothing based on thermal mannequins, small thermal-comfort instruments and data storage in PC Thermal mannequin simulate a human body as a thermal machine divided into up 17 independently heated segments, which keeps (by means the PC control) their surface (skin) temperature t S at the average level of 33 O C, and which enables exact measurement of an electric power P [W] required for this relatively truly simulation of heat distribution in the human body. From these values, the PC calculates the levels of individual superficial heat fluxes q i of the mentioned segments.
First, heat fluxes q in for the naked mannequin should be measured and used for the calculation of the exterior resistances R EN of the naked body: R EN, i = (t Si - t E ) / q N, I (58) In the next step, the mannequin is dressed and total thermal resistances R TOT,,i will be determined by similar procedure: R TOT, i = (t Si - t E ) / q TOT, I (59)
The differences between the both above given measurement present the demanded individual clothing resistance levels R CL, i = R TOT, i - R EN, I (60)
Up to this point, the common procedure was described. Hes (1999) proposed to use in the next step also small table instruments like ALAMBETA, in order to determine the sum of thermal resistance values of densely layered garments R G,i covering without air gaps an individual segment of thermal mannequin. The difference between R CL, i and R G, i then will present the thermal resistance of air gaps in clothing corresponding to individual segment: R A, i = R CL, i - R G, I (61)
In the study made by Hes, Graveiro and Gameiro (Coimbra 1998), the air layers under absence of wind presented 22 - 40% of the full women's clothing resistance (blouse, trousers) for the standing mannequin. When air parallel flow was included, the resistance of air gaps reached 35 - 65 % (thermal resistance of garments decreased). For the sitting mannequin, the decrease of the garment resistance due to the ventilation effects was lower.
These values then correspond with certain (not too high) precision to the chosen style, fit and size of clothing system (suit). Presently, with the advanced PC recording of all the data about clothing manufacture, should be possible to find the correlation procedure stored in PC, which should enable the fast evaluation of thermal resistance of selected clothing,
The input data will include, besides the proper characteristics of the clothing style, fit and size, also the thermal resistance values of individual garments. The effect of water vapour permeability of garments on clothing comfort cannot be determined directly by means of the described thermal mannequin and the related procedure. Fortunately, the processes of heat and vapour transfer are similar. Therefore, once we know the average thickness of the air gaps, and when we can determine the water vapour permeability of individual garments (e. g. by means of the PERMETEST instrument),
it is relatively easy (for the specialist) to calculate the total water vapour resistance of clothing, following the rules given in Chapter 4.5. Nevertheless, the effects concerning the moisture condensation in garments and the generation of moistening and absorption heat, cannot be respected in such simplified procedure. Therefore, in some countries, the sweating mannequins are also used. However, the price of these thermo-physiological: Recording, checking and investigation of the personal comfort simulators is very high and the mannequin should be operated by a skilled engineer.
5.6. Personal thermo-physiological comfort sensoring chips worn under clothing or even implanted which respond on external radiofrequency inquiry. Development project started at the Minho University, Portugal. Objectives health and comfort level during and after work and exercise. Analysis of the effect of the clothing worn in real life conditions on the personal comfort level. Long time study of working conditions and lifestyle and their influence on personal efficiency. Recorded parameters: skin temperature, skin humidity, skin surface heat flow, pulse.
6.Thermophysiological clothing comfort - design 6.1.Selected parameters of yarns and filaments, which affect the thermo-physiological comfort of fabrics As already explained, fine surface fibres, even if short, offer smooth and pleasant, but sometimes too cool feeling, as well as and long smooth surface fibres without any axial curling. Short medium fineness surface fibres may provide warmer feeling, but due totheir relative low flexibility, they may scratch the skin (like PES/cotton underwear). Why animal hairs, even if long, give frequently warm and smooth feeling? The animal hairs are frequently curly, and their endings are tapered.
Therefore, the total contacting area is not large, and the endings bend easily. Moreover, the surface structure is irregular and contains a lot of pores. Hairs also are not circular in their section. Any trial to simulate the natural comfort feeling by means of fabrics made of synthetic fibres should follow this rules, e. g. by irregular yarn texturing, by irregular yarn section and also by irregular surface raising. If a special synthetic yarn consisting of continuous filaments may contain filaments with very weak places, which could be cut during an intensive raising, then possibly a fabric surface may better simulate the animal contact feeling.
6.2. The effect of chemical composition and mechanical structure of shirt fabrics on appearance and complex comfort properties of shirts 6.3.Theory and design of ever-dry double-layered fabrics composed of cotton and PP textured filaments Capillary pressure Δ P, causing the liquid moisture flow generally form the big pores of the equivalent radius R to small pores of the equivalent radius r is proportional to the water surface tension γ and cosine function of the contact angle Θ , according to the relation
Δ P = 2 γ [(p r . cos Θ r/ r) - p R . cos Θ R / R)] (62) Here, the term p presents the increase of the inner surface of the capillary channel. If some fibre surface treatment was achieved, which increases the fibre roughness (like laser treatment), the capillary pressure should increase and hence the treated fabrics should exhibit higher wicking properties.
As the general rule should be mentioned, that to achieve the good wicking properties (high moisture absorptivity), the yarn structure should be compact and the space among the specially profiled fibre section should be as small as possible. The outer (cotton, viscose or at best Lyocell fibre) layer should exhibit higher suction force than the skin contacting (PP or special PES) layer, but adhesion forces (cos Θ ) in the skin contacting layer should be lower.
Fig. 19 Examples of filaments (Moira TG 900, Coolmax) creating smal l channels in twisted bundles, which conduct moisture well
6.4. The use of Phase Changing Materials in thermal protective clothing Recently, sport clothes with higher thermal capacity, which provide temporary protection against overheating whether caused by stay in hot surroundings or by higher production of metabolic heat during high sport or work strain, appeared on the world market. Protective function of those products is based on heat absorption during phase change in so called “phase change materials” (PCM) which are put inside protection layer of the special clothes.
Dynamics of fabric heating While dressing especially of underwear with temperature different from body temperature we feel effect of heat accumulation in the clothes, which is given by area related thermal capacity of clothing C [J/m 2 ] calculated as a product of specific heat c [J/kg] and surface density of the fabric M [kg/m 2 ]. Thermal conductivity of textile material takes share on the overall thermal-contact perception of the fabric. Prof. Kawabata first pointed out the importance of dynamics of thermal-contact perception as a part of feel or hand. Resulting parameter called thermal absorbtivity b [Ws 1/2 /m 2 K] introduced by Hes can be determined with commercial apparatus ALAMBETA.
Thermal capacity of clothes has to defend the body from sudden temperature changes in the environment, for example when leaving air-conditioned space and entering to tropical atmosphere. In fact, this balancing effect is weak and short. Wool clothes provide much higher “buffer” effect due to vaporization heat of absorbed water, but only in case if high temperature of surrounding air comes with low humidity. Yet this is not a usual case. That is why fabrics based on heat accumulation by means of phase change appeared on the market.
This principle was first used in civil engineering - under roofs of “intelligent” houses there were put closed containers with PCM materials. The heat accumulated during hot days warmed the whole house during cold nights. Dr Barbara Pause published this principle first after the suitable way of application of these materials in fabrics was solved. The mostly used materals - alkens - are products of organic chemistry and their melting temperature lies usually between 15 and 40 o C. As an example eicosan can be used, with melting temperature 36,1 o C.
Due to intensive marketing, these clothes are known to public and they found their customers. However, producers of the fabrics are not able to characterize the effect of the proclaimed heat protection in simple manner. There is growing suspicion among textile specialists that the protection is not necessarily proportional to high price of these “performance” fabrics and garments. These wax-like materials are encapsulated in small beads of micrometer range diameter and can be deposited and fixed inside any textile structure (e.g. in non-wovens) or on the fabric surface e.g. by means of resins.
They exhibit relatively high phase change heat L [J/m 2 - when considering the concrete mass applied in the textile layer] when they, due to higher environmental temperature t E get melted, or when they are subjected to cooling in the melted state. In both cases, during the phase change time τ PC [sec], the PCM particles keep the textile layer containing these particles on the phase change temperature t PC for the period of tens of minutes. It they are used e.g. for protection of the human skin of temperature t S against hot environment (hot pot when cooking) inside a protection glove, consisting of 2 textile layers of interior and exterior thermal resistances R P and R E with the PCM layer is located between these layers,
Heat flow q o [W/m 2 ] to the skin (t S 33 C) without protection: q o = (t E – t S ) / (R E + R PC + R PROT ) (63) then the protective function of such glove can be, under certain simplifications and for R P >> R E , analysed in the next rext.
Heat flow q p [W/m 2 ] to the skin with protection (t PC 35-38 C): q P = (t PC – t S ) / (0,5 R PC +R PROT ) (64) If PCM protection appears, then q P q O , Heat flow to PCM layer during the time of protection OCH : q PCM = (t E – t PC ) /(0,5 R PC +R E ) (65)
Total heat L [J/m 2 ] necessary for complete melting of PCM layer: L = q PCM PC (66) Time of thermal protection provided e. g. by protection glove determined under precondition (in the practice only hardly feasible) that temperature of PCM layer is for certain time in the whole PCM layer constant: PC = L / q PCM = L. R E / (t E – t PC ) (67)
From this simplified analysis follows that level of protective function of PCM is strongly affected by the level of phase change temperature and the level of thermal resistance, and that the most important factor is mass of PCM elements inserted into active layer of clothes. If their weight portion is low (lower than 30%), the outside penetrates through the PCM layer to inside layers which are in contact with skin and the second part of the equation is invalid. To get really effective PCM heat protection, we have to use thick textile layers, which are less flexible and consequently less comfortable.
New method for evaluation of thermal efficiency of PCM protection New instrument called PC Tester is in some extend based on commer-cial equipment ALAMBETA for the measurement of thermal-contact and thermal thermal-isolation characteristic of fabrics - see Fig. 20: Fig. 20 Scheme of the PC Tester instrument
The instrument consists of two blocks – boxes with different temperatures. The temperature of the first one, so called SKINBOX, is kept on skin temperature t sk by means of classical circulation thermostat 2. The second one is HOTBOX 3; it is heated or cooled electrically. The temperature in this case is kept by digital regulator or computer 4 on different level t E . Tested fabric 5 containing PCM elements is located between sensing areas of both boxes, and in the course of the testing proccess it is surrounded by two flat textiles 6 and 7 which simulate both effect of thermal resistance of underwear with respective air gaps R PROT and effect of total thermal resistance R E between PCM layer and the environment with temperature t E .
When evaluating effect of PCM layers it is necessary to prepare the PCM layer with thermal resistance R PC as well as the simulation layer with the same thermal resistance R SIM, but without PCM elements.
Evaluation of the efficiency is started by allocation of layers 4, 5 (SIM) and 6 between sensing surfaces of both boxes and their bringing into mutual thermal contact. The computer begins to register level of heat flow q[W/m 2 ) passing through sensing surface of the SKINBOX. As it is evident from Fig. 21, the heat flow reaches its maximum q max0 in short time 0 not exceeding several seconds, because effective surface thermal capacity of fabrics, given by product of specific heat c [J/kg] and surface density of fabric [kg/m 2 ] is very low. The final value of thermal resistance is then given by the relation (68).
In next step the textile fabric 5(SIM) replaces protective layer 5(PCM) in the measured assembly and measurement is repeated. In this case the increase of the level of heat flow is slower, because of the effect of heat accumulation needed to accomplishing of the phase change. In theory, the above mentioned accumulation of heat should be constant for some time and consequently, the heat flow should not change in the meaning of the relation (68). Time of protection should be characterized by the relation (66 ). In reality, there is no “plateau” with constant value q at the registration of the heat flow going though the system with PCM layers, because PCM layer is not continuous in usual application in textiles.
. A non reduced heat flow penetrates between fibres from outside layer, and as a result of it the curve of heat flow is smooth and continuous and reminds again an exponential one. How can we then simply evaluate time of protection? In physics, the exponential curves characterize a lot of natural processes, as e.g. radioactive isotope decay. For simple expression of the radiation intensity drop, the radioactive half time was introduced, which is given by time necessary for drop of radiate level intensity to half. This parameter is useful, because it is clear for understanding. Similarly, analogy of this parameter will be used for evaluation of the thermal protective efficiency of fabrics containing PCM elements.
Fig. 21 Time course of heat flow passing through the simulated skin during the evaluation of thermal efficiency of PCM fabrics in the PC Tester
As the time of protection PC will be appointed the time in which heat flow will be lower or equal to one half of the maximum rate of heat flow q max,o achieved when measuring the fabric simulating the protective layer, but without PCM elements. Because thermal resistance R of fabrics is given by the known relation R = (t E – t ask ) / q, (68) then the time of protection means in reality also the time for what effective thermal resistance of protective fabric is at least twofold in comparison with the same layer without PCM elements.
Experimental results First prototype of the equipment was made in cooperation with Prof. R. Gomes at the University of Minho. During the preliminary tests the PCM layer (melting point 28°C) was surrounded by textiles with the same thermal resistance R (about 0,1m 2 K/W) and the quantity of PCM elements was raised from 0 to 50% of the sample weight. Time necessary to reach the 50% level of q max,o with raising proportion of PCM was increasing almost linearly to 620 sec, which in some extend proves the validity of relation .
Another prototype of the device PCM tester was build recently in the Textile Research Institute in Taipei, Taiwan, and the method was once more verified, by providing similar results. A priority of the described method of evaluation of thermal efficiency of PCM fabrics is covered by an US patent since 2003.
6.5. Textiles changing their absorption of infrared radiation heat according to the solar radiation level Maximum level of heat flow coming from sun (equator, midday, no clouds): 1400 W/m 2 . Solar heat flow q S in warm countries (Portugal, Spain, Italy) in summer midday: 900 W/m 2 The heat flow reaching clothed person q SC depends on the angle between the rays and the line perpendicular to the surface and then some part of the flow is absorbed, some part reflected and the rest passes though the clothing, according to the next relation: q SC = q S . cos = . q SC + . q SC + . q SC (69)
Some liquid crystals dyestuffs or coatings are black in the range from common comfort temperatures till some temperature limit, e.g. 35 O C. Over the temperature limit, they turn grey and later even white. Thus, their surface emissivity decreases, and the visible part of solar thermal radiation gets reflected due to increased reflectivity . The reduction of the absorbed radiation flow may be up to 50%. Unfortunately, these intelligent coatings are still quite expensive.
6. 6.Application of semi-permeable fabrics, membranes and fabric coatings to achieve the reasonable permeability for water vapour and simultaneously no permeability for water drops. The protection against wind also reduces substantinally the heat loses by convection. Intelligent breathable but waterproof fabrics should allow to pass up to 2000-2500 g/m 2 of vaporized sweat per day at low physical activity and up tp 4000-5000 g/m 2 /day at high energy production. This water vapour permeability naturally depends on the outer air relative humidity and temperature.
Fig. 22 An example of the "windstopper" fabric, NO WIND PRO 600
Principal division of windproof and waterproof wv permeable fabrics 1.dense woven fabric (up to 7000 yarns/cm), pore size 10-3 m made of mikrofibre yarns PES, PAD 2. coated basic material, microporous layer, pore size 2-3 m by mechanical microperforation: direct or indirect (crashed foam), coagulation procedure: dry or wet with hydrophilic coating, pore size 0,001 m
3. lamination by membranes, which are manufactured as folies and fixed on basic woven material: microporous, hydrophobic foils, pore size 3-0,1 m hydrophylic film, pore size 0,001 m
Combination of both procedures. The membrane can be also free. Laminates may constost of up to 3 layers. Laminated fabrics should exhibit next thermal comfort and wearing properties: high water vapour permeability low wind penetration resistance against hydrostatic pressure (up to 15 m water column) low garment mass low bending and shearing rigidity, soft handle.
Fig. 24 Measurement of windproof fabric surface temperature by the IR camera. Without the windstopper membrane the outer air cools the jacket surface and its temperature is lower.
Also abrasion resistance is required, stability of properties after repeated washing and washing cleaning, 6.8. Military garments. Overcoat "invisible" in visible light consists of optical camera, which detects the image of the background behind (in front of) of the wearer and this image is projected on the special reflecting garment in proper magnitude. Thus, when we imagine an observer located in front of the protected soldier, and watching him, the observer should see the missing part of the background projected on the soldier garment.
Current camouflage military garments for visible and near infrared range This garments offer certain invisibility in homogeneous environment like forest, meadow or desert, since their average emissivity corresponds to the background. The applied NIR of personal night vision systems was up to 1 micrometer, at present the range increases till 1,2 micrometer. Military garments invisible for low temperature (medium infrared) detectors (detectors of own heat radiation), due to very low surface emissivity. Here, the total radiation heat flow (in spite of higher body temperature compared with the environment) is too weak to make difference against the signal from the surroundings.
6.9 .Examples of civic protective clothing against high solar radiation Clothing protecting against radiation heat should prevent the passage of IR rays, but simultaneously should allow the creation of the free convection vertical streams in the clothing, which take away the moisture and heated air. Thus, the clothing should be bulky and porous in the space next to skin, but external surface should be dense enough to stop the solar radiation. Moreover, the body motion should perform the pumping effect inside the garment system, in order to take away the stagnant humid air.
Indian sari is an example of such clothing. The used special woven fabric is be very porous but rigid, so that the fabric folds may create elastic air channels which get deformed when walking. Also Arabic burnus or Greek tunica satisfy most of the above mentioned requirements. A new protective clothing patented in Czech Republic exhibits, except other features, many permanent vertical channels, which support the intensive humid air evacuation from the clothing.
7. SMART TEXTILES ( by Prof. L. Van Langenhove, Univ. of Ghent) Smart textiles are able to sense stimuli from the environment, to react to them and adapt to them by integration of functionalities in the textile structure. The stimulus as well as the response can have an electrical, thermal, chemical, magnetic, or other origin. Advanced materials , such as breathing, fire-resistant or ultrastrong fabrics, are according to this definition not considered as intelligent , no matter how high-technological they might be.
The extent of intelligence can be divided in three subgroups /5/: - passive smart textiles can only sense the environment, they are sensors; - active smart textiles can sense the stimuli from the environment and also react to them, besides the sensor function, they also have an actuator function; - finally, very smart textiles take a step further, having the gift to adapt their behaviour to the circumstances.
To fulfil the above presented tasks, two components need to be present in the textile structure in order to bear the full mark of smart textiles: a sensor and an actuator , possibly completed with a processing unit which drives the actuator on the basis of the signals from the sensor.
Although smart textiles find and will find applications in numerous fields, this presentation is limited to clothing. It involves for example wearable smart textiles meant for medical applications, designed to fulfil certain functions, but apart from that without any fringes. Also casual clothing is possible, which is expected to be functional as well as fashionable. It also embraces sports clothing, where the comfort factor is even more critical. Finally, smart textiles could be sold as a gadget, where the intelligent character will be more accessory than useful but in any case extremely visible.
Initially, smart clothing will find applications in those fields where the need for monitoring and actuation can be of vital importance, such as medical environment, and with vulnerable population groups, in space travel and the military. However, as experience and and familiarity will increase and hence breaking down barriers, the field of application will in the long term definitely widen to more daily applications such as sports and leisure, the work environment and so on.
State of the art The first generation of intelligent garments was based on conventional materials and components and garments were designed to fit in the external elements. They can be considered as e-apparel, where electronics are added to the textile. A first successful step towards wearability was the ICD+ line at the end of the 90ies, which was the result of co-operation between Levi´s and Philips. The line´s coat architecture was adapted in such a way that wxisting aparatuses could be put away in the coat: a microphone, an earphone, a remote control, a mobile phone and an MP3 player. The coat construction at that time did require that all these components, including the wiring,
The most obvious thing to do was integrating the connection wires of the different components into the textile. To this end, conductive textile materials are appealed to. Infineon has developed a minituariased MP3 player, which can easily be incorporated in a garment. The complete concept consists of a central microchip, an earphone, a battery, a download card for the music and an interconnection of all these components through woven conductive textiles. Robust and wash-proof packing protects the different components. were carefully removed from the coat before it went into the washing machine. The limitation as to maintenance caused a high need for futher integration.
No matter how strongly integrated, the functional components remain non-textile elements, meaning that maintenance and durability are still important problems. 5 functions can be distinguished in an intelligent suit , namely: - Sensors - Data processing - Actuators - Storage - Communication
They all have a clear role, although not all intelligent suits will contain all functions. The functions may be quite apparent, or may bean intrinsic property of the material or structure. They all require appropriate materials and structures, and they must be compatible with the function of the clothing: comfortable, durable, resistant to regular textile maintenance processes and so on. Sensors Sensors detect certain signals and transform them into another signals that can be read and understood by a predefined reader, which can be a real device or a person.
When designing new textile sensors , the art will be to specify the concepts of transformation that make it possible to turn the signal one wants to measure into (the variation of) a signal one can measure (in most cases the latter will be an electric signal). Possibly, intermediate transformations may be necessary, although these must be minimised. As for real devices, ultimately most signals are being transformed into electric ones. Electroconductive materials are consequently of utmost importance with respect to intelligent textiles.
Nevertheless, when looking at possibilities of transformation, e.g. from optical to electrical, from thermal to mechanical, from mechanical to electrical, from thermal to optical, and combinations of subsequent tranformations it will be clear that there is an enormous potential of sensors that are just waiting to be developed for a huge range of textile applications. Of course, apart from technical considerations, concepts, materials, structures and treatments must be focusing on the appropriateness for use in or as a textile material. This includes criteria like flexibility, water (laundry) resistance, durability against deformation, radiation etc.
Materials that have the capacity of such a transformation are for instance: ● Thermocouple: from thermal to electrical ● The Softswitch technology: from mechanical (pressure) to electrical. ● It uses a so-called „Quantum Tunnelling Composite (QTC)“. This composite has the remarkable characteristic to be an isolator in its normal condition and to change in a metal-like conductor when pressure is being exercised on it.
Depending on the application the pressure sensitivity can be adapted. Through the existing production methods, the active polymer layer can be applied on every textile structure, a knitted fabric, a woven fabricm or a nonwoven. The pressure sensitive textile material can be connected to existing electronics. ● Fibre Bragg Grating (GBG) sensors: from mechanical through optical to electrical. This is a type of optical sensors receiving a lot of attention the latest years. They are uswed for the monitoring of the structureal condition of fibre-reinforced composites, concrete constructions or other construction materials. At the Hong Kong Polytechnic Uniresity,
several important applications of optical fibres have been developed for the measurement of tension and temperature in composite materials and other textile structures /4/. FBG sensors look like normal optical fibres, but inside they contain at a certain place a diffraction grid that reflects the incident light at a certain wavelength (principle of Bragg diffraction) in the direction where the light is coming from. The value of this wavelength linearly relates to a possible elongation or contraction of the fibre. In this way, The Bragg sensor can function as a sensor for deformation.
Data processing Data processing is one of the components that are required only when active processing is necessary. So far, nop textile materials are available that can perform this task. Pieces of electronics are still necessary. However, they are available in miniaturised and even in a flexible form.
Research is going on to fix the active components on fibres (Ficom project). Many practical problems need to be overcome before real computing fibres will be on the market: fastness to washing, deformation, interconnections, etc. Actuators Actuators respond to an impulse resulting from the sensor function, possibly after data processing. In a sense, actuators are similar to sensors in that they also transform the impulse signal into a response signal. Actuators make thing move, they release sunstances, make noise, and many others.
Shape memory materials are the best-known examples in this area. They transform thermal energy into motion. In a cold state, or beneath the transition temperature, a shape memory alloy can easily be deformed and the material will keep this shape. If the material is heated above the transition temperature, it will return to its original shape. The material so to speak has „memorised“ this shape. Because of its ability to react to a temperature change, a shape memor alloy can be used as an actuator and links up perfectly with the requirements imposed to smart textiles. A common shape memory alloy is Nitinol . It consists of a mixture of nickel and titanium.
Shape memory alloys exist in the form of threads, which makes them compatible with textile materials. Although shape memory polymers are cheaper, they are less frequently applied. This is due to the fact that they cannot be loaded very heavily during the recovery cycle. Until now, few textile applications of shape memory alloys are known. The Italian firm, Corpo Nove, in cooperation with d´Appolonia, developed the Oricalco Smart Shirt. The shape memory alloy is woven with traditional textile material resulting into a fabric with a pure textile aspect. The trained memory shape is a straight thread. When heating, all the creases in the fabris disappear. This means that the shirt can be ironed with a hair dryer.
Real challenges in this area are the development of very strong mechanical actuators that can act as artificial muscles. Performant muscle-like materials, however, are not yet within reach. ● Materials that release subsrances already have several comercial applications. However, actively controlled release is not obvious. Chemical products (in the widest sense) can be released in a controlled wayusing two main principles:
● The textile material contains one or many tanks (massive fibre, m icro or nanocapsules, micro or nanopores) whereby release is controlled by an intelligent barrier. Micro-encapsulation is a wellknown example of this category. ● Products are chemically bound to the substrate, these bonds are broken down by a predetermined factor, or
Obviously, controlled release opens up a huge number of applications as drug supply systems in intelligent suits that can also make an adequatediagnosis. Mechanical and chemical actuators are clear examples, but of course one can think of many other types, again for a huge number of applications.
Storage Smart suits often need some storage capacity. Storage of data or energy is most common. Sensing, data processing, actuation, communication, they usually need energy, mostly electrical power. Efficient energy management will consist of an appropriate combination of energy supply and energy storage capacity.
Energy supply is also based on transformation, in this case of one type of energy into another one. Sources of energy that are available to a garment are for instance body heat, mechanical (elastic from deformation of the fabrics, kinetic from body motion), radiation, etc. - Infineon had the idea to transform the temperature difference between the human body and the environment into electrical energy by means of thermogenerators. The prototype is a rigid, thin micromodule that is discretely incorporaed into the clothing. The module itself is not manufactured out of textile material. However, the line of thought is introduced.
The Infineon thermogenerator delivers 1,6 microwatt/cm 2 and is washable thanks to plastic packing. - The use of solar energy for energy supply is also thought of. At the University of California, Berkeley, a flexible solar cell is developed which can be applied to any surface .
As mentioned before, energy supply must be combined with energy storage. When hearing this, one thinks of batteries. Batteries are becoming increasingly smaller and lighter. Even flexible versions are available, although less performant. Currently, the lithium-ion batteries are found in many applications.
Communication For intelligent textiles, communication has many faces: commu-nication may be required - Within one element of a suit, - Between the individual elements within a suit, - From the wearer to the suit to pass instructions, - From the suit to the wearer or his environment to pass information.
Within the suit , communication is currently realised by either optical fibres, eitherconductice yarns. They both clearly have a textile nature and can be built in the textile seamlessly, the advantages and disadvantages of both carriers are well known: optical fibres are light and insensitive to EM radiation. The transport does not cause production of heat. On the other hand, the signals have to be transformed into electrical onesat least in one point.
Communication with the wearer is possible for instance by the following technologies: - For the development of a flexible textile screen, the use of optical fibres is obvious as well. France Telecom has managed to realise some prototypes (a sweater and a backpack). At certain points, the light from the fibre can come out and form pixels on the textile surface. The textile screen can emit static and dynamic colour images. The resolution is extremely low however. Nevertheless, in this way, these clothes are uplifted to a first generation of graphical communication means. - Pressure sensitive textuile materials allow putting in information, provided a processing unit can interpret the commands.
Communication with the wider environment does not allow direct contact, so wireless connections are required. This can be achieved by integrating an atenna. The step was also taken to manufacture his antenna in textile material. The advantage of integrating antennas in clothing is that a large surface can be used without the user being aware of it. In the summer of 2002, a prototype was presented by Philips Research Laboratories, UK and Foster Miller, USA on the International Interactive Textiles for the Warrior Conference (Boston, USA). The researchers made use of conductive yarn on the basis of copper and nylon. Moreover, existing production methods were appealed to construct the textile antenna.
Requrement for further research The potential of intelligent textiles is huge. One can think of many applications for each of the examples given above. The other way around, starting from an application, the basic concepts have to be defined and evaluated for their use in or as a textile product. Selection of materials, structure and production technology are the first step in the design. The actual research phase will be long and hard for many cases.
Basic items that need to be addressed to come to a real breakthrough and to innovation are: - Transformation and conversion mechanisms; - New materials; - New structures that can offer the requested function. Conductive materials, and more specifically inherently conducting polymers or ICPs, are already being used in many applications: antistatic working, EMI shielding, heating, transport of electrical signals, etc.
These are fascinating, dynamic, molecular systems suitable for applications in many domains of intelligent clothing: polymer batteries, solar energy conversion, biomechanical sensors, Some materials are already available, be it at laboratory level. Some substantial disadvantages, which have to be overcome, are the instability of the polymer in the air, the weak mechanical properties and the difficult processing. However, in the United States one has managed to spin the first polyaniline fibre.
Another class of materials that will play a major role in many intelligent clothes are optical fibres . They are well-known from application in electronics, but the range of deformations to deal with in textile applications is of a different order and causes problems thar restrict the number of applications at present.