The Chemistry Of Wool
It is my privilege this evening to deliver the annual Presidential Address to the New Zealand Institute of Chemistry, a privilege which is at the same time a responsibility of which I am deeply conscious. In selecting as my subject the Chemistry of Wool, I have been influenced by the fact that we have had the pleasure in recent years of listening to expert surveys of chemical endeavour and achievement over a wide field, and it may not be inappropriate therefore this year to consider advances in knowledge of a particular subject, and one which concerns a nationally important commodity.
It is significant of the new relation which is emerging overseas between the University and Industry that much of the earlier fundamental scientific work on this industrially important commodity was carried out in the universities. This association of universities with industry in Great Britain began in the middle of last century, but it is only comparatively recently that the scientists and industrialists have discovered mutual interests and a common language. There is, in the first instance, the enforced interest that the University-trained scientist is needed increasingly in Industry, and it is the function of the University, whilst preserving all that is essential to a training on the fundamental aspects of science, so to adjust curricula to provide men able to fill the diverse needs of Industry. Moreover, particularly in Great Britain and in the United States, professors and lecturers in the science departments tend to be consulted by industry and so acquire a knowledge of industrial problems. There are those who tend to decry such association, but in my considered opinion, chemistry in the University has never suffered from its association with Industry, but, on the contrary, it has gained by having its field of interests extended. This in turn has resulted in a freshness of work in fundamental chemistry.
There has been no lack of freshness in fundamental work in chemistry in the period between the two wars. It has been a remarkably fruitful period, and not least in the field of natural products. New methods of separation of those complex mixtures, produced in the animal and vegetable kingdoms, have been devised and have initiated the now familiar cycle of isolation of the pure substance, determination of its constitution, its synthesis, and the syntheses of chemically-related substances. Here are fascinating chapters.
Whilst specialisation is increasingly necessary, yet it seems even more necessary for some resynthesis of the divisions of sciences
in the training of the chemical worker. It is, for example, much more necessary than formerly for the specialist in organic chemistry to know a good deal about physical chemistry and its techniques. This is also true as between the separate sciences. If the scientist is being recognised as useful in industry to day, it is because he has not over-specialised. He must know something of a wide field. Indeed, the time has come, even if it is not overdue, for the University course for the B.Sc. degree to extend over a period of four years, instead of the present three years. In this extended course, when it comes, there should be some provision not only for 31 broader basis of scientific study, but also for some training in lucidity of expression. Lucidity of thought and of expression is needed as never before, if the humanities and sciences are to become mutually appreciative of each other, and never before has this mutual appreciation been more desirable.
The advance in our knowledge of wool structure illustrates well the need for investigation of a wide front. Our increased knowledge of its atomic architecture is due to the close collaboration of workers in physics and chemistry, particularly in the first instance, in the University of Leeds. Chemically, wool belongs to that class of substances known as keratins, and is of similar composition to horn and feathers. By acid hydrolysis it is possible to obtain a mixture of amino acids from wool, from which it has been possible to separate and identify eighteen distinct amino acids.
The amino acids which have been thus isolated from wool are shown in Table I and the amounts obtained by various recent workers up to 1944 have been critically assessed by Astbury(1) in 1942.
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The composition in terms of amino acid content is not constant for the wool fibre is not a homogeneous structure. By treatment with concentrated ammonia at room temperature or by the action of buffered solutions of trypsin, it may be separated into its cellular elements. Three distinct parts may be recognised, the scale or cuticle, the cortex made up of long-shaped cells, and occasionally a medulla.
The cuticle cells are flat, oval-shaped bodies about 30 microns in diameter and about 1 micron thick, whilst the cortical cells are about 100 microns long. Work was carried out by Geiger(2) in 1944 on the composition of the scales, for by special treatment the scales may be left intact and subsequent analysis of this fraction reveals a higher cystine and lower arginine and tyrosine content. The higher cystine content of the scales was first recognised by Trotman and Sutton(3) in 1926. The composition of the fibre itself varies along its length, particularly as regards its cystine content, which appears to depend on nutritional factors when the fibre was being formed.
Attempts to mould the results of amino-acid analysis of this heterogenous structure into a Bergmann-Niemann frequency distribution must therefore be accepted with caution. Since 1943 electron micrographs have been obtained which have further emphasised the heterogeneity of the structure. Last year Mercer and Rees(4) found that the cuticle cell fragments obtained by grinding or scraping show a relatively smooth surface, whereas cells isolated by the action of trypsin possess a clearly-defined surface structure which is pitted or honeycombed. These preserve their appearance even after prolonged retting in enzyme solutions. Mercer terms the digestible material as k1 and the resistant component as k2. When the whole fibre is treated with trypsin, the intercellular cement, holding the cuticle to the cortex and the cortical cells together, is also removed so that there is a close similarity of the k1 of the cuticle with the intercellular cement. This intercellular cement was given the name “lanain” by Haller and Hall(6) in 1936, who found cystine and cystine groups absent.
Electron micrographs of fragments of the cortical cells(3) obtained by grinding show dense fibrils and a less dense interfibrillar component which is apparently digested by trypsin, whilst the fibrils are not. The interfibrillar component is reported as amorphous and solid, but can be drawn into threads in a manner closely resembling the behaviour of a viscous rubber solution.
In the light of these facts, there appears little hope that chemical work based on isolation of molecular fragments may lead to complete knowledge of the structure of keratin. Nevertheless, very interesting work is now proceeding on the partial hydrolysis of wool and the identification of the dipeptides and tripeptides which result. This investigation has been developed by Synge and Martin(7) and may serve to check structures which have been advanced on other grounds. These workers at the Wool Industries Research Association, Torridon, have developed a technique described as partition chromatography. The basis of this method is
the fact that if an aqueous solution of amino acids is extracted on a counter-current principle with a solvent immiscible with water, the various amino acids tend to be carried forward by the solvent at different rates depending on their relative partition coefficients. This was developed by Gordon, Martin and Synge(7) in 1943, into the use of a silica column in which the water phase is held on the silica and the amino acids applied to the top of the column and some suitable non-aqueous solvent allowed to percolate down. In this way the acids gradually moved down the column. Later, cellulose in the form of strips of filter paper was used as the substrate on which absorbed water was held immobile whilst various solvents partially miscible with water were used to carry the amino acids down the paper.
Since no single solvent gives complete separation of the amino acids, the paper is sometimes treated first with a solvent such as collidine and then turned through a right angle and treated with another solvent such as water-saturated phenol producing a two-dimensional chromatogram. The paper after drying is sprayed with ninhydrin, which forms coloured complexes with the amino acids. The identification of any particular amino acid is possible by running a duplicate chromatogram using a known acid, its position at a given time being characteristic of that acid. The position is determined by the rate of movement of non-aqueous solvent and the distribution coefficient of the substance between the two solvents.
In examining partial hydrolysates, the peptide is revealed by a colour reaction and the colored paper strip is used as a guide to cutting a second paper strip run at the same time and the peptide is then washed off, hydrolysed, and the free amino acids again examined and identified in this way(8) The results indicate that wool is very complex; for example, glutamic acid may have attached to it in the dipeptide fragment, residues of any of the following: glutamic acid, aspartic acid, serine, tyrosine, glycine, alanine, leucine and possibly valine. There is as yet no indication of any regular sequence of amino acids in the polypeptide chain.
What, however, has been recognised for some time from the chemical work is that since all eighteen amino acids isolated are α-amino acids, the structure of the parent protein must be based on Fischer's theory of polypeptide links.
From a number of investigations it has been proven that all the amino acids which have been isolated have the same relative stereochemical configuration. The hydrogen atom, amino group, carboxyl group and the R. group are arranged around the carbon atom in the same way for each acid. The result of this is that in the polypeptide the R groups along the extended polypeptide chain stick out alternately up and down. If, as appears feasible, a polypeptide may be synthesised in vivo at a fat-water interface, one may expect polar groups to be attached on one side, i.e., from the water phase and non-polar groups from the non-aqueous phase. This would result in an alternation of polar and non-polar groups as side chains along the polypeptide chain and should the polypeptide chain be folded the polar groups would oppose each other in one
fold and non-polar groups would oppose each other in the next fold. That may well be so for the soluble proteins, but, as the recent results obtained by Martin demonstrate, it does not apply to the polypeptide chains of wool. For if polar and non-polar side chains alternate, no dipeptide consisting of two glutamic acid residues or of glutamic and aspartic acid residues should be possible.
Cystine, which is a di-α-amino acid, may be a part of two such chains, as was first suggested by Astbury and Street in 1931.(9) Other formulations in which the cystine is only partially built into polypeptide chains are unlikely. For if the cystine were a member of only one polypeptide chain, processes that break the di-sulphide bond would result in elimination of half the sulphur. This does not occur. If either or both of the amino groups were unconfined, treatment of the wool with nitrous acid would destroy half or the whole of the cystine, whereas only a small proportion of cystine is destroyed by this treatment. Moreover, there cannot be many free carboxyl groups since the free carboxyl groups in wool can be accounted for to a first approximation by its content of glutamic and aspartic acids. Such cross linking in which the cystine is a part of two polypeptide chains may account for the unusual insolubility of keratin protein and an analogy may be drawn with the solubility properties of cross-linked thermo-hardening plastics, where increase of cross linking decreases the solubility.
Other possible cross linkages between polypeptide chains arise from the fact that some of the amino acids are basic and others acidic. Aspartic acid is present in 100 gm. of wool according to Speakman and Townend(10) to the extent of 0.108 gm. residues and glutamic acid to the extent of 0.0507 residues. (Astbury gives 0.0959 and 0.0494 gm. residues.) From this total of 0.1587 gm. residues must be subtracted the acid residues present as amides. Thus aspartic acid may be present as asparagine H2N.OC.CH2CH (NH2)COOH and glutamic acid as glutamine H3N.OC.CH2CH2 (NH2)COOH. The amide nitrogen present in 100 gm. of wool is approximately 0.081 equivalents, leaving 0.077 residues of aspartic and glutamic acids in which the terminal carboxyl group is free.
The basic amino acids, lysine, arginine and histidine, are slightly in excess of this and amount to 0.0819 residues. Should the acidic and basic residues be suitably placed in relation to one another, salt formation may occur. These cross links are illustrated in Fig. 1.
The presence of the basic and acidic residues in the ionised state as shown in the suggested model has been confirmed by measurement of the heat of reaction with hydrochloric acid. If the acid reacts with unionised amine, RNH2, the heat evolution at room temperature is approximately 11,000 cals. per mol. If, on the other hand, it reacts with ionised carboxyl groups, RCOO-, the heat evolution is equal and opposite in sign to the heat absorbed when a carboxyl group ionises, which is 1,000—2,000 cals./mol. Values of the heat change observed vary from 2,000 to 3,500 cals./mol. evolved(12), which confirm the presence of the groups in their ionised forms RCOO− and RNH3+ and the existence of salt linkages.
On this view the groups present in the wool which combine with acid are the ionised carboxyl groups which, from the aspartic acid and glutamic acid content less the amide content, amount, as we have seen, to 0.077 equivalents/100 g. By titration of wool it is found(13) that 100 g. of wool combines with 80 c.cs. of N-hydrochloric acid, i.e., with 0.08 equivalents of acid: thus the analytical figures for ionised carboxyl groups deduced from amino acid analysis very nearly total the acid combining value of wool found by titration. The salt linkages appear to be reasonably well confirmed. Other cross links, in addition to those formed by cystine and by salt formation, probably exist due to hydrogen bonding.
This building up of the primary chemical structure was assisted considerably by the X-ray investigations of Astbury. In these well-known investigations a homogeneous beam of X-rays was directed at wool or hair fibre clamped with its axis vertical. Owing to the multitude of cortical cells lying with their long axis vertical within the fibre, the X-ray diffraction picture obtained resembles those in which a crystal is rotated about a vertical axis, for in the fibre there will always be some of the crystallites correctly oriented to the beam and so X-ray diffraction will occur. The diffracted X-ray will reveal itself as spot or spots on a photographic plate suitably located at a distance behind the fibre. Using X-rays of constant wave-length it is a simple consequence of the theory that the greater the displacement of the X-ray beam from its undeflected path, the smaller must be the distance between the reflecting planes. With the fibre vertical, spots on the equator of the X-ray photograph measure the spacing between vertical chains of atoms while spots on the meridian have reference to the vertical repeat pattern. The sharpness or, on the other hand, the diffuseness of the spot is well known to be an indication of the crystal size, for the smaller the crystal, with its fewer number of reflecting surfaces, the less sharply does it reflect and consequently the more diffuse is the reflected spot. Examination of the X-ray photograph of silk shows that there is in such fibres a definite horizontal as well as definite vertical pattern. Moreover, since the spots along the equator are much broader and more diffuse than they are along the meridian, the photograph indicates that the reflecting crystallites are much smaller in a horizontal direction than they are in a vertical direction. Thus the physical picture of a silk fibre is of a number of long, thin particles lying closely side by side and pointing along the length of the fibre. If the crystal particles pointed in all directions, the resulting photograph would be a ring as occurs when a photograph is taken of a strip of photographic film which contains silver bromide crystals pointing at random in all directions.
The spacing of the vertical repeat pattern in silk fibre, obtained from the sharp line on the meridian, corresponds to 3.5A, which is just the calculated distance for a fully extended amino acid residue. It would thus appear that the thin crystallites which lic along the silk fibre are composed of bundles of extended polypeptide chains. When wool is examined, the photograph shows, as does the silk fibre, a much sharper spot on the meridian than on the equator
indicating long, thin crystallites lying lengthwise along the fibre. The vertical spacing is, however, quite different from that of silk and corresponds to a repeat at 5.1 Å. When wool is stretched, however, the vertical repeat alters and occurs at 3.33 Å, i.e., it corresponds to the repeat in an extended polypeptide chain. Unstretched wool, or a-keratin, thus undergoes a change in molecular folding on being stretched, giving β-keratin, and this change is a reversible one. The spot on the equator of the photograph of α-keratin corresponds to a separation of 9.7 Å, which has been, identified with the lateral separation of the polypeptide chains. Thus if one likens the molecular model for wool to a ladder with cross links as rungs, the 9.7A corresponds to the distance between the sides. β-keratin also shows the 9.7 Å spacing, i.e., the distance between the sides of the ladder is not altered in the change from α to β-keratin, but in addition a new spot has appeared indicating a spacing of 4.65 Å as shown in Fig 2. This is identified as the thickness of the main chains which form the sides of the ladder when in their extended state. Thus the folding and unfolding of the polypeptide chains of wool in the α–β transformation occurs in planes at right angles to the plane of the ladder. In order to accommodate the side chains and cross linkages of varying length the fold has been revised by Astbury and Bell in 1941(26) following-criticism by Newrath of Astbury's earlier model. The side view of the ladder in its folded or α-form is shown in Fig. 3. The R groups, some of which will form cross links between neighbouring chains, are at right angles to the plane of the diagram and project alternately up and down.
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The X-ray dimensions of β-keratin, if the interpretation is correct, are those of an average amino acid residue. Thus the mass of one residue = 9.7 × 4·65 × 3·33 × 10−24 × density. The density of wool = 1·3 gms./c.c. The average gram residue will be the weight of N average residues = 6·03 × 10−23 × 9·7 × 4·65 × 3·33 × 1·3 × 10−24 = 118. Therefore the number of gram residues in 100 gm. wool = 110/118 = 0·85, cf. 0·8597 found. Thus the amino acid analysis of wool is satisfactorily confirmed as complete.
This model of the wool crystallite lying with its long axis in the direction of the length of the fibre may perhaps be over simplified. Nevertheless, it has provided a valuable working hypothesis and has offered an explanation of that very valuable property of the wool fibre, namely, its extensibility and permanent set.
When wool or hair is steamed and stretched at 100°, it is possible to obtain 100 per cent extension. If the fibre is dried in this stretched state, it retains its extended form. This is termed temporary set, because when the fibre is wetted, the fibre contracts. If, however, the fibre is maintained in the stretched position for two or three minutes at 100° and then allowed to relax, it shows super-contraction, contracting to a length less than its original length. Alternatively, if it is steamed at 100° in the stretched state for more than three minutes, it gradually acquires a permanent set.
The fibre will not then contract even on boiling in dilute acid or on immersion in alkali.
According to Speakman, the reaction stage which leads to supercontraction is the hydrolysis of the di-sulphide link between the polypeptide chains: W–S–S–W + H2O = WSH + WSOH. The polypeptide chains are then free to fold up when the strain is removed, particularly if the lateral cohesion of the polypeptide chains is reduced by the intrusion of water molecules. If the extended fibre is steamed for over three minutes it is suggested that new bonds are formed between the terminal—SOH groups and free amino groups: WSOH + H2NW = WS.NH.W + H2O, which
tend to fix the polypeptide chains in the extended state, so conferring permanent set. Supporting this view is the appearance and detection of free SH groups in the fibre after permanent set is conferred. Further, the amino groups would appear to be involved in the conferring of permanent set, for if the wool is deaminated, using sodium nitrite and acetic acid, permanent set is not possible. The new linkages which cause permanent set cannot be simply salt linkages or new di-sulphide linkages, for both these are easily split by alkali or by boiling acid to which the permanent set is stable.
The nature of these cross linkages which confer permanent set is, however, still under discussion. The sulphenic acid, WCH2SOH, formed by di-sulphide hydrolysis, may yield an aldehyde under alkaline conditions, liberating sulphide which can easily be recognised: WCH2SOH = WCHO + H2S. The aldehyde may condense with amino groups to give linkages of the type WCH : NW, but so far no products have been isolated from wool hydrolysates which contain this grouping. Consequently the existence of such linkages may be open to doubt. Horn, Jones and Ringel in 1941 isolated from alkali-treated wools a new amino acid, lanthionine, having the formula, HOOC.CH(NH2)CH2S.CH2.CH(NH2) COOH, for which the following mechanism of formation appears likely(10), hydrolysis of the di-sulphide occurring, followed by W.CHCH2SOH = WC: CH2 + H2O + S and WC: CH2 + HS.CH2CHW = WCH.CH2SCH2.CHW, giving lanthionine on hydrolysis. This is the only product isolated which is evidence for a new cross linkage brought about by simple treatment of the wool fibre. Its formation may play a part in the permanency of the set of an extended fibre.
Useful in the assessment of the degree of cross linkage have been measurements of the work of extension of single fibres. This work of extension is obtained by plotting the standard load-extension curves and evaluating the area under the curve. Cross linkage breakdown is attended by reduction in the work of extension. Thiol-acetic acid, SHCH2COOH, is a reducing agent which reduces the di-sulphide group to two thiol groups, i.e., it breaks the di-sulphide cross-link and the result is a marked decrease in the work of extension.
W–S–S–W + 2HS.CH2COOH = WSH + HSW + (SCH2 COOH)2. Similarly, in acid solution, the ionised carboxyl group is neutralised and the electrovalent salt linkage disappears. The extension of the fibre is therefore easier also in acid solution.
Reduced wool containing free –SH groups may react with alkyl halides(17) to give chemically modified wool: W–SH + RX = W–S–R + HX. Dihalides are also capable of reacting with reduced wool forming new cross links in which the sulphur atoms of the cystine are connected by short hydrocarbon chains:
2W–SH+ (CH2)nX2 = W–S–(CH2) n–S–W+2HX.
Reduced wool treated by dihalides such as ethylene dibromide has a work of extension and breaking strength not far different from that of untreated wool, whilst reduced wool or reduced and methylated wool has a work of extension about two-thirds that of untreated wool. The formation of these new cross links by the
action of dihalides confers increased alkali resistance and renders the wool, as was shown by Harris and co-workers, resistant to moth attack. This is but one of the many methods of rendering wool resistant to moth attack, but is one specially interesting in that the digestive system of the moth larvae, whilst capable of splitting the di-sulphide bond, cannot apparently split the more stable thio-ether linkages in this chemically modified wool.
Much work has been done on the mechanism of the permanent set of keratin fibres, and its importance in the technology of finishing of wool fabrics is considerable. In view of the heterogeneous. nature of wool it is of interest to note that the property of super-contraction and of set has been connected by Woods(18) in 1938 with the cortical cells. Woods separated cortical cells and by appropriate treatment was able to cause them to super-contract and also to assume both temporary and permanent set. A similar conclusion has recently been reached by other workers.(19)
Some reference must now be made to work which has been carried out on wool with a view to prevention of shrinkage. From the textile point of view, shrinkage or felting is, of course, one of the very valuable properties of the wool fibre, for it allows of the production of cloth in which the separate fibres have become interlocked, giving a uniformity of surface, in which.the spaces between the yarns have been eliminated. The technological problem is to utilise this beneficial property where needed and to control it where it is disadvantageous. An illuminating experiment is to tie a staple of greasy wool in the middle and then wash or “scour” it. The portion towards the base felts rapidly, whilst the portion towards the tip which is free from root ends remains unfelted. Felting is caused by the migration of the fibre in the direction of its root end. The surface friction of the fibre can be measured in various ways(20) and it is less from root to tip than it is in the opposite direction. This has for a long time been attributed to the scale structure of the fibre, and the scales have been assumed to exert a ratchet-and-pawl action. The electron microscope indicates this physical structure very clearly and the way the scale edge is rounded off in certain anti-shrink treatments. This difference in friction with and against the scales correlates with the ability to felt as recognised in practice, and until recently all anti-shrink treatments have resulted in reducing or eliminating this directional frictional effect.
The common anti-shrink treatment is treatment with a solution of hypochlorite or with aqueous or gaseous chlorine. Recently, with Miss R. Mauger, a New Zealand Wool Board Research Fellow at the University of Otago, an attempt is being made to elucidate more fully the chemistry of wool chlorination. The method of approach has been to examine the separate amino acids after protection of the amino groups by acetylation. It is, of course, recognised that bi-molecular reactions are possible in solution which could not occur in a rigid structure owing to spatial considerations. Hypochlorite solutions react with amide groups and with the guanidine portion of arginine to form chloroamines. The reaction with a substance such as methyl guanidine is very fast and, as in
certain other cases investigated, the reaction involves the hypochlorite ion. In acid solution this reaction rate decreases as the aonisation of the hypochlorous acid decreases. The oxidation of diacetyl citizen is also a very fast reaction, apparently involving the unionised hypochlorous acid, but in buffered solutions the rate depends to a marked extent at constant pH on the nature and concentration of the buffer solution. The end product of the reaction appears to be acetyl cysteic acid. Taking 1 mol. of diacetyl citizen and 8 mols. of hypochlorous acid, 5 mols. of hypochlorous acid are rapidly used up, a further 2 mols. form chloramides by reaction with the NH.CO groups present, leaving 1 mol. of residual hypochlorous acid. Simultaneously the acid titre increases by 7 mols., i.e., by 5 mols. of hydrochloric acid and 2 mols. of acetyl cysteic acid.
Cysteic acid has also been found in wool hydrolysates after chlorination, and there appears general agreement that the splitting and oxidation of the disulphide bond is an essential part of many non-shrink processes. The production of the strongly ionised sulphonic acid group on the surface of the wool cause gelatinisation of the surface by swelling. When wool is immersed in chlorine water, there are sacs formed along the surface of the fibre. This is the Allworden reaction(21) and may be explained by the diffusion of chlorine through the k1 layer which contains little or no citizen followed by reaction of chlorine with some of the citizen of the k2 layer to form cysteic acid. Cysteic acid side chains ionise and give a Donnan equilibrium effect of excess diffusible ions inside the k1 layer with consequent swelling. Surface swelling and gelatinisation will remove the directional frictional effect and thus provide an explanation of the shrinkage resistance. In acid solution, where the excess of diffusible ions inside the surface layer is reduced, swelling will be minimised and shrinkage may still occur even when it does not occur in mildly alkaline soap solutions.
According to Phillips(22) who has recently divided the combined cystine of wool into four fractions, A, B, C, and D, which differ in reactivity and mode of reaction with alkalies and other substances, it is the (A+B) cystine fraction which appears to be concerned in shrinkage properties. The (A+B) fractions, amounting to 50 per cent of the total cystine, are converted by alkalies to lanthionine in a way already indicated. Wools which have been first treated with alkali and subsequently chlorinated do not acquire shrinkage resistance. This suggests that the cleavage and oxidation of the (A+B) cystine is an essential step in destroying the felting power of wool.
The explanation advanced for the differences in behaviour of the four fractions of combined cystine is that they arise through variation in side-chain environment and there is some evidence that the (A+B) fractions are associated with an environment of polar side chains whilst C and D are associated with non-polar chains.
Chlorination by gaseous chlorine(27) provides a more uniform treatment than wet chlorination. The latter tends to cause overtreatment of the more accessible fibres. This method of dry chlorination
to-day may be followed by the action of papain leaving a wool which is scaleless, though the fibres are otherwise intact. The wool is soft and possesses a silk-like gloss and is completely non-feltable. It is so completely non-felting that it may be used as a filling for washable quilts and cushions.
So far it is the directional frictional effect which has been emphasised. However, in order that felting may occur, the wool fibre must be able to extend and to contract after extension. Consequently, unshrinkability may be realist by increasing the resistance of the fibre to extension or by decreasing its power of recovery. This has been achieved by Barr and Speakman by increasing the number of cross-linkages between the peptide chains.(23) Mercuric acetate and benzoquinone have been used for this purpose. Although the frictional properties of the fibres were unaffected, a high degree of unshrinkability was obtained by such treatment.
Another method of approach has also been made by Speakman.(24) This consists in building up a film of polymer on the reactive side chains of the wool using suitable monomers and so masking the scale structure. Anhydrocarboxy glycine appears to be particularly suitable in this connexion, for the polymerisation is initiated by water and the film produced is protein-like in character.
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W—NH2+nCO—CH2—CO2—NH = W.NH(—CO—CH2—NH—)n H+nCO2
Since the film is protein-like, the fibre should retain the characteristic properties of wool apart from its ability to felt. A noteworthy result of this treatment is increased abrasion resistance, addition of 4 per cent. of anhydro-carboxy glycine conferring an increased wear resistance of 300 per cent.
I would like in conclusion to refer to one other physical property of wool, namely, its equilibrium with water vapour. Fabrics made up of fine fibres expose a large surface to the air and consequently come very rapidly to equilibrium with the air in their immediate vicinity. A loose yarn, and a loose yarn is characteristic of wool, obviously presents more surface than a yarn which is tightly twisted. The surface of 1 lb. of wool is easily calculated, knowing the density of wool and the diameter of the fibre. Taking 1 lb. of a fine wool, such as 64's quality of diameter of 20 microns, the area of the fibres is 800 sq. ft. A suit usually weighs about 3 lb. and represents a fibre surface area of 2,400 sq. ft. This large area means a rapid reaction to any change in atmospheric humidity. This, however, does not mean that a mass of wool rapidly comes. to equilibrium with surrounding air, for the surrounding air takes time to displace the air entrapped by the wool and what is termed conditioning is a slow process. Wool may absorb up to 33 per cent. of its weight of water and to a first approximation the water absorbed is dependent on the relative humidity of the atmosphere. The curve obtained by plotting water absorbed against relative
humidity is only slightly dependent on temperature. Thus the effect of temperature on the water absorbed may be deduced from the changes in the atmospheric relative humidity.
The actual amount of water vapour in the air in a well-ventilated room is the same as it is out of doors, i.e., the absolute humidity is approximately the same, but since a room in winter is heated, the relative humidity of the air inside is much less than outside. A suit containing 3 lb. of wool, if in equilibrium with typical indoor conditions, in winter would absorb roughly ½ lb. of water if allowed to recondition out of doors. The heat evolved when ½ lb. of water is absorbed by wool is 600 B.T.U.'s, which is as much heat as a man's body normally generates in 1 ½ hours. This remarkable evolution of heat is a great protection against the temperature drop on proceeding out of doors, particularly if the heat is liberated gradually. It is important that the heat be evolved at a suitable rate, for it would be useless if liberated immediately or too slowly. If at constant absolute humidity the temperature of the air is reduced, its relative humidity is increased, water is absorbed, and both fabric and the entrapped air are warmed. This heat may be radiated, or entrapped air may be replaced by other air. Thus the rate at which heat is given up depends partly on the rate of passage of air through the fabric. The more rapid the passage of air through the fabric, and the more likely the sensation of cold, the more rapidly is heat generated. So long as the fabric is absorbing water from the perfusing air, the air and fabric will be warmed. Looked at from another viewpoint: the percentage of water in the fabric will change only slowly, because the fabric contains so much more water than a corresponding volume of air. The water content of the air in the fabric is controlled by the water content of the wool. Hence a conditioned fabric will tend to keep any air passing through it at the relative humidity at which it was conditioned. Thus if the fabric is conditioned, i.e., is in equilibrium with air, at an indoor temperature of 65° and a relative humidity of 45 per cent., and cold air at 40° and relative humidity at 95 per cent, is then blown through the fabric, the air must emerge at the original relative humidity of 45 per cent. Since some of the water in the air is removed by the wool, the air reaches the relative humidity of 45 per cent, at a temperature intermediate between 65° and 40° F. The method of calculating this temperature was due to Cassie(25) in 1940 and has been shown to agree well with experiment. In the case considered, the cold air by passing through the wool is warmed to about 52°, approximately halfway between the indoor and outdoor temperatures. Obviously, the greater the amount of water which can be taken up as the relative humidity of the atmosphere increases, the longer can the fabric give out heat and afford protection. From this angle wool is in an advantageous position as a textile fibre.
It is interesting to note that if wool be conditioned indoors at 65° in a badly ventilated room where the relative humidity is high, removal of the wool to a cold outdoor temperature at the same relative humidity will not be attended by any further absorption
of water vapour and, consequently heat will not be generated; under such conditions the heat-generating properties of wool are in abeyance.
In this address I have attempted to bring together some of the advances which have been made in our knowledge of the wool fibre and to indicate the lines along which advance has occurred. The study of a particular problem frequently leads to results which illuminate a wider field, and the investigation of wool fibre is an example of this. From a study initiated by technological considerations, new chapters have been added to the chemistry of proteins and the essential similarity of the protein of wool fibre, the myosin of muscle, and also of soluble protein after denaturation has been discovered. The study of the wool fibre is proving a valuable bridge-head to that still largely unknown country of protein chemistry.