118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure.This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point.These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order. The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility.The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13.118 ELECTRICAL THEORY OF ADBORPTTON The writer considers the double layer as consisting of a swface of rigidly fixed atoms under continuous bombardment of positively and negatively charged ions, any particular point on the rigid surface becoming in turn negative, neutral and positive, these conditions arisdg in any order.The observed contact difference is the average effect of these conditions. Where several kinds of atoms are present in the solution the average number of any one of them at the surface will depend on their concentbration, valency and mobility. The variation of contact Werence from negative to neutral and positive was observed with cotton and aluminium sulphate near the neutral point. These variations occurred during the same experiment, the readings being direct measurements of E.1I.F.s developed by filtration under pressure. This point would be covered by putting n2 = 1 and = 2 or 3 in Mukherjee’s equation No. 13. THE PHYSICAL PROPERTIES OF TEXTILE FIBRES I N RELA- TION TO TECHNICAL PROCESSES AND TO GENERAL COLLOID THEORY. B y S.A. SHORTER, D.Sc. (BRITISH RESEARCH ASSOCIATION FOR THE WOOLLEN AND WORSTED INDUSTRIES). Received May 16f4, 1924. CONTENTS. I. The Technically Important Properties of Fibres. 2. The Absorption of Water by Textile Fibres. 3. The Elastic Properties of Fibres. 4. The Finishing of Woollen and Worsted Fabrics. 5. A Theory of the Colloid Structure of Fibres. I. The Technically Important Properties of Fibres, In the course of their progress from the raw state to the finished fabric the different textile fibres go through a large variety of processes and their behaviour during these processes depends upon a correspondingly large number of factors. We can therefore consider the subject here in the crudest possible outline. Thus considered the processes of manufacture may be said to consist of (I) the formation of a coherent sliver from an ir- regular mass of fibres, (2) the thinning of this sliver (sometimes done in a large number of stages in the later of which twist is inserted) culminating in the process of spinning into yarn, (3) the weaving or knitting of this yarn into a fabric, (4) the ‘‘ finishing ” of this fabric, i.e.the conversion of the crude product of the loom into a saleable article. This outline is of course not true in its entirety of all cases of textile manufacture, but it fits the great majority of cases. The principal properties which govern the behaviour of fibres during the processes of manufacture may be classified under the following heads :- (I) The elastic properties (using the term in its widest sense, to relate to any kind of relation between deforming force and deformation). (2) The absorption of moisture and its effect on the elastic properties.(3) The surface structure of the fibre. The present address will be mainly concerned with (I) and (2). 2. The Absorption of Water by Textile Fibres. The absorption of water by textile fibres has been studied by a number of investigators 1 ~ 2 ~ f45. Though the work of Trouton and Pool indicated that the moisture content of wool is a function of the relative humidity only, the view put forward by Schloesing is generally accepted. Schloesing states that for a constant relative humidity the equilibrium regain decreases 228TEXTILE FIBRES AND COLLOID THEORY 229 with the temperature or, conversely, the atmospheric humidity necessary to produce a given moisture content increases with the temperature. As I pointed out some years ago t j this temperature effect is connected thermo- dynamically with the fact that the absorption of water is accompanied with the evolution of heat.I have recently used Schloesing’s data to calculate the heat of water absorption by means of the Kirchhoff equation 7 for the heat of dilution of a solution. The heat of absorption is very large when the fibre is dry, decreases as the moisture content of the fibre increases and approaches zero as the fibre approaches saturation.* There is a gradual change in the nature of the process of water absorption as the water content increases. The absorption of the first portions of water is the result of direct molecular attraction and takes place with a large evolution of heat.At this stage the vapour pressure of the water in the FIG. I. wool increases very slowly with the water content. The intensity of this attraction diminishes as the amount of water increases : the heat of absorp- tion diminishes and the vapour pressure begins to increase more rapidly. When the vapour pressure comes near to the saturation value the rate of increase diminishes more and more till the material becomes saturated. In the neighbourhood of saturation the process tends to resemble the osmotic absorption of water by a dilute solution-a process which, though it may require considerable force for its suppression, does not involve any appreciable evolution of heat.g The variation of the rate of evaporation with the moisture content1* and the volume changes which occur when water is absorbed 11* I* are also significant in connection with the change of the nature of the process of water absorption.230 PROPERTIES OF TEXTILE FIBRES I N RELATION TO The moisture content of textile fibres is of importance technically and commercially, in the first place because water forms a variable proportion of a commodity which is sold by weight (hence the need for standards of (( condition ”) and, secondly, because the water content exerts an influence on the behaviour of the material during processing and on the properties of the final product.3. The Elastic Properties of Fibres. The elastic properties of fibres and yarns have been studied by a number of investigators 131 14, 151 16* who all attribute the peculiarities in the elastic behaviour of fibres to plasticity, and speak of (‘ permanent strains ” in fibres.In a recent work17 I dispute this conclusion, and put forward the view that textile fibres are much more perfectly elastic than the results of these investigators would seem to indicate, and that the apparent elastic im- perfection is largely due (in the case of wool almost entirely due) to the fact that the elastic extension or contraction is impeded by a resistance of a viscous nature. Such viscous or plastic material as exists in a fibre is, so to speak, in parallel with the elastic material and does not interrupt its con- tinuity. A dynamical model which forms a first approximation to the elastic behaviour of a fibre is shown in Fig.I . I t consists of a spiral spring S attached to a piston working in a cylinder of viscous liquid. The cylinder is perforated with a fine capillary channel and connected to the bottom end of the cylinder with a second spring So (which is much more extensible than S). This system, which given sufficient time is a perfectly elastic system, owes the peculiarities of its elastic behaviour to the slowness with which the spring So tends to come into equilibrium with S. Thus if the system be loaded, unloaded and reloaded we get an extension-tension diagram of the type shown in Fig. 2. The curve starts out from the origin in the direction OK corresponding to the extensibility of S alone. The slope of the curve increases owing to the increasing rate of extension So.On un- loading we obtain the curve AB which at first has a very small slope owing to the fact that So continues to extend till the two springs come to the same tension. At this point the unloading curve is parallel to OK, and beyond it becomes steeper and steeper owing to the fact that So is con- tracting more and more quickly as the tension of S diminishes. On re- loading the contraction of So continues for a time (till the tensions equalise), so that the reloading curve CDE is initially at a small slope, becomes steeper and finally approaches to coincidence with the prolongation of the loading curve. The curves obtained by New l3 and Matthew l5 for cotton and linen yarns are very similar to Fig. 2. The general arrangement of loading, unloading and reloading curves is precisely the same.The only difference is that with the yarns the extensibility decreases as the load increases owing to a straightening of the fibres and a general initial tightening up of the yarn structure. In the work referred to above17 I show that the stress-strain diagrams of fibres and yarns may be explained in terms of the dynamical model. 3.n particular, the spring So corresponds to elastic elements which are held in a state of strain by the resistance of a surrounding viscous medium. In the absence of external forces such strains will be released at a rate dependent on the viscosity of this medium. If this viscosity is high we may get internal strains of great persistence.TECHNICAL PROCESSES AND TO COLLOID THEORY 231 By means of stress-strain (tension-extension) diagrams it is possible to study the relation between these internal strains and the external stretching force.Thus if a given force be applied rapidly and maintained for a lengthy period we get a rapid extension followed by a slow one. In the case of the model, the process of approach to equilibrium would be very simple-the rate of approach would be proportional to the distance from the equilibrium. With animal hairs (wool, human hair, etc.) no such simple law is obeyed. The process of extension proceeds for a very lengthy period -days or even weeks. The explanation of this is, not as might be supposed, that the elastic elements are showing a plastic yield, but that the fibre con- tains elastic elements with very different degrees of damping, so that on the first application of an external force the more lightly damped elements ex- c .- 4.a ii 4 C 0 E Tension FIQ.2. tend and, as time goes on, the extension of the more highly damped elements begins to show itself. We get a similar effect on removing the external force, and it is undoubtedly the extreme slowness of the recovery of the more highly damped elements that has given rise to the erroneous ideas of ‘‘ plasticity ” and “ permanent strains.” Similar considerations apply to the case where a fibre is held stretched to a definite length. We get an apparent elastic relaxation which, however, is very different from the effect contemplated in Maxwell’s theory of visc0sity.1~ I t is not the disappearance of a state of strain owing to molecular re-adjust- ment-it is merely the transference of a state of strain from lightly damped to highly damped e1ements.l 1 An important technical instance of this occurs when the “ shed ” of a loom is left open.232 PROPERTIES OF TEXTILE FIBRES I N RELATION TO Before going further into this theory we will consider the question of internal strains from the technical point of view.4. The Finishing of Woollen and Worsted Fabrics. The existence of latent strains in the wool fibre and the need for their elimination have been recognised in a vague way by the practical man for a long time. Many of the processes in cloth finishing are directly concerned with such strains. Greater definiteness was given to the theoretical basis of finishing by the work of Harrison,20 who showed (I) that wool fibres when dry (( exhibit a kind of plasticity in which the strains produced remain when the stress is removed but are accompanied by internal stresses,” (2) that such strains are released when the fibre is placed in cold water, (3) that wet fibres are truly elastic, (4) that in boiling water fibres are truly plastic.21 The fibres in a piece as it comes off the loom contain latent strains which are liable to be released when the cloth is wetted.These strains have been put in during carding, drawing, spinning, warping, etc. Any excessive tension, which according to earlier theories strains the fibre beyond its elastic limit, really results in the straining of highly damped elastic elements and therefore in latent strains of great persistence.To avoid the irregular shrinkage which might be caused by the release of such strains, the cloth is often crabbed or treated with boiling water while in the form of a tight roll. This eliminates these irregular strains by causing an internal readjustment, as in the annealing of steel or glass. The piece can now safely be washed in relatively cool water to get rid of oil and dirt. After washing the piece is fentered or dried in a stretched state. Since this drying is done at a temperature much below the boiling-point, the state of strain produced in tentering is not destroyed; it is merely rendered latent. This latent strain is largely eliminated by blowing (treat- ment with steam). To make certain that none of it is left in the cloth the process of London shnhking is sometimes carried out.This consists in wetting the cloth thoroughly and allowing it to dry-taking care to apply no more tension than is absolutely necessary. This process consists essen- tially of the release of latent strain (at this stage merely a uniform extension in length or width) which in the absence of external force disappears by the cloth shrinking. The finishing processes may therefore be said to involve (I) the action of cold water in releasing latent strain, ( 2 ) the action of boiling water or steam in destroying strain. 5. A Theory of the Colloid Structure of Fibres. In order to obtain a fibre structure which will reproduce in a general way the behaviour of the mechanical model shown in Fig. I we must suppose that the fibre consists of an elastic framework the interstices of which contain a viscous fluid.The divergences from a close quantitative correspondence with the model can be explained, as we have seen, by supposing that there are wide local variations in the viscosity of this fluid. The complete explanation of all the facts discussed in the preceding section is a very simple matter. All we have to do to explain the results (I), (2) and (3) of Harrison’ is to suppose that the viscous liquid which See the second paragraph of Section 4.TECHNICAL PROCESSES AND TO COLLOID THEORY 233 impedes the elastic framework is a gelatinous fluid whose impeding effect diminishes with increasing water content. To explain result (4) we must suppose that the elastic framework though unaffected by cold water has its elasticity impaired with boiling water, i.e.that it has an annealing tempera- ture of about rooo C. The rendering latent of strains by wetting, stretching, and drying stretched, such as occurs in tentering, consists in rendering mobile the gelatinous medium, stretching the framework, and rendering the medium viscous again by drying. The subsequent release of the strain by wetting again is, of course, due to the rendering mobile of the medium once more. To render the stretch permanent it would be necessary to attack the actual elastic framework with boiling water or steam. There is one experiment which illustrates the identity of the impeding action of the gelatinous medium with the viscosity of ordinary colloidal 0 Tension FIG.3. solutions. The resistance offered by the medium is diminished by exces- sive strain.22 This is shown by the difference between the tension-extension graph of a fibre the first time it is loaded and the second time. The first time the extensibility remains small up to quite high loads. The only extension which occurs is the small amount which can occur without shear of the viscous medium. Quite suddenly the resistance of the medium gives way and the extensibility increases very rapidly. If now the load be removed and, after a few minutes, reloading is commenced, the extensibility begins to increase at a smaller load than before, and in a much more gradual manner. The difference between the two graphs is illustrated in Fig. 3. This increased extensibility at low loads slowly diminishes, but the fibre takes a long time (several days) to get back to its original state.The theory is illustrated in a very perfect manner by wool and the animal hairs. In these the two portions are very clearly differentiated in properties. The elastic portion is very resistent, and the viscous portion234 PROPERTIES OF TEXTILE FIBRES IN RELATION TO very susceptible, to the action of water. Other fibres give similar stress- strain diagrams. Thus the types of diagram showing the instantaneous elastic yield and the slow yield and the corresponding recoveries may be obtained with the artificial silks, though in certain samples examined the elastic framework is attacked even by cold water, so that it is difficult to show the release of internal strains by wetting.This was tested in a new form of autographic machine 18 in which the effects of friction and inertia are reduced to a minimum so that the stress-strain relationships holding during rapid changes can be investigated. In this machine the recording pen is attached to the spring which measures the tension of the yarn, and records its vertical motion. The paper moves horizontally at a rate equal to that of the lower end of the yarn (the upper end of which is attached to the spring). I t will be readily seen that the machine gives a tension-extension diagram in which the extension axis is horizontal and the tension axis makes an angle of 45’ with the horizonal. Fig. 4a refers to a yarn which was stretched in the “dry” state (at about 7 per cent.regain). The carriage carrying the paper (and regulating the motion of the lower end of the yarn) was moved rapidly forward (A), a rest of a minute was made during which the yarn (which had extended In Fig. 4 are shown two graphs relating to a cotton yarn. FIG. 4.-1612’s Cotton Yarn. rather over 3 cm.) extended further. Other rests of a minute were made (B, C, D) and then the carriage was moved back till the tension became zero (E) with a residual (not permanent) strain of nearly 3 cm. A pause of one minute was made during which the yarn contracted and pulled the pen a short distance downwards. The tension was again adjusted to zero and another pause of one minute was made (F) during which the further contraction was barely perceptible. The carriage was then moved back to its starting place, the pen returning to 0, and the fibre hanging slack between the clamps.Next morning (about eighteen hours afterwards) the carriage was brought forward and then returned to its starting place, so that the pen described a small cycle (G) which indicated a residual extension of about 1-5 cm., and a recovery during the eighteen hours of nearly I cm. The yarn was then sprayed with water till it was thoroughly wet. I t was seen to tighten up between the clamps, and on describing a short cycle (H),. perfect recovery was indicated.“ The pen did not, of course, retrace its original path, as the extensibility of the wet yarn is much greater than that of the dry yarn. Other (unstretched) samples of the same yarn did not contract on wetting.TECHNICAL PROCESSES AND TO COLLOID THEORY 235 Fig. 4b relates to another sample of the same yarn which was soaked in water before testing. A pause of one minute was made at the end of the outward journey of the carriage (K). I t will be seen that the yarn is much more extensible. The residual extension was about I '4 cm. immedi- ately after unloading. This diminished to 0.3 cm. in about three minutes (L) and did not diminish any more even after several hours although the yarn was kept met. In this case therefore there seems to be a small permanent extension. REFERENCES. 1 Schloesing, Bul. SOC. Encour. Indust. Nat., 1893. 2 Hartshorne, Traits. of the Ntw England Cotton Manufac. Assn., September, 1905 ; 2 Trouton and Pool, Proc. Roy. SOC., An, p. 292, 1906. 4 Masson and Richards, Proc. Roy. SOC., A?8, p. 412, 1906. 5 Shorter and Hall, ?ow. Tex. Illst., June, 1924. 6 Shorter, Jour. SOC. Dyers and Col., December, 1920. TKirchhoff, Pogg. Ann., 103, p. 177, 1858. SShorter, YOUY. Tex. Inst., June, 1924. 'J Shorter, Jour. SOC. Dyers and Col., September, 1923. 10 Fisher, Proc. Roy. Soc., IO@, pp. 139 and 664, 1923. 11 H. R. Hirst, Publication No. 17, B.R.A.W.W.I. 12A. T. King, Publication No. 19, B.R.A.W.W.I. l3New, your. Tex. Inst., 1922, 13, p. 25. l4 Barratt, ibid., p. 45. l5 Matthew, ibid., p. 17. 16 Pierce, Jour. Tex. Inst., November, 1923. I7Shorter, Your Tex. Inst., 1924, 15, p. T207. 18 Shorter and Hall, Jour. Tex. Inst., 14, p. T493. l9 Maxwell, Phil. Trans., 156, p. 49, 1867. mHarrison, Proc. Roy. Soc., 94A, p. 460,1918. "See also Publication No. 12, B.R.A.W.W.I. !B Garrett, Dissertation, Heidelberg, 1903 ; see also Ostwald, Colloid Chemistry , April, 1911. p. 160.