Preparation, Bleaching, Dyeing and Finishing of Linen

FREDERICK R. W. SLOAN

Kirkpatrick of Ballyclare Ltd Co. Antrim N. Ireland

Linen has gradually been losing its posi- tion as an apparel fabric since the 1950s and as a consequence has become an insignificant commercial target for dye manufacturers. There have thus been no worthwhile studies of dyeing kinetics and dyeing behaviour in this field. Too often the comment, ‘linen dyes like cotton’, is heard and, although this is true to some extent, the large difference in morphol- ogy between these two fibres‘affects the dyeing kinetics, not to mention the effects of linen preparation.

The emergence of linen as a compo- nent of blends has stimulated consider- able interest in preparation, bleaching and finishing as evidenced by the number of papers published in the last few years. I t is of interest to note that the first reference to blending is in the Bible - Deut. 22-11 [ l ] refers to the blending of wool and linen. The Jewish Laws were often functional as well as religious and it is possible that the advantages of using both fibres in a blend without the dis- advantages of either alone were rec- ognised. However, the method then of preparing, etc., linen cloth was to beat it with stones in the river, followed by treatment with sour milk; this would have felted the wool - hence the law.

Linen is a unique fibre, although an apparent paradox in that it is both inextensible and flexible. Its structure is essentially a highly crystalline arrange- ment of anhydroglucose units, which accounts for its inextensibility. The flex- ing or bending takes place at the nodes, which are natural bending points distrib- uted 0.03 mm apart along the fibre to enable the flax plant to bend under the forces exerted by wind and rain. These nodes are also areas of weakness, account- ing for the relatively poor abrasion resist- ance of the fibre and its propensity to crease. They also account for the very rapid absorption and desorption of water, providing the unique cool handle of linen. It is of interest to note that linen has the highest measured response to chemical

crease-resist treatments; embrittlement at the nodes, however, is a limiting factor.

Retting Linen fibre is obtained from the flax plant Linium Usitatissimurn by a compli- cated process of chemical or biochemical and mechanical treatments to separate the fibre from the woody core. The older method of water retting is now almost extinct owing to its high cost. The method currently in widespread use is ‘dew retting’, i.e. allowing the straw to lie on the ground until the combined action of weather and fungus degrades the woody core. A further objective of rett- ing is to partially break down the lignin or binding material which is approx- imately 2% of the total fibre assembly. This enables the primary fibres to separate so that they are free to draft when drawn during preparation and wet spinning.

Because of the non-uniform nature of ‘dew retting’ along the stem of the plant from root to tip and the difficulty of gathering the straw at precisely the right moment, there has been a tendency to under-rett. Consequently, the requisite amount of fibre separation is then obtained by chemical treatment at the spinning stage. Suitable methods have been developed by linen research estab- lishments in France and Northern Ireland [2]. The difficulty, however, is to obtain a standard formula applicable to all types of flax. The technique is relatively simple; roving is prepared in the usual way, but wound on to stainless steel or plastic bobbins. The bobbins are then treated with mild alkali on equipment identical to that used for bleaching yarn; bleaching can also be carried out at this stage. Dew-retted flax is more difficult to bleach and in some instances, when the fibre is very dark in colour, bleaching is extremely difficult and costly. The dark colour is associated with a fungus called Altanaria, which must be avoided. It appears under damp conditions and when the retting process is well advanced. Over the past ten years continuous efforts have been made to obviate the retting process by scutching the flax green. Problems associated with predictability and repro- ducibility have so far not been solved

and, moreover, the economics at present prices for fibre are not encouraging.

Recently Neenan [3] has investigated the feasibility of producing linen by the dew-retted route in Eire. His work indi- cated that in certain areas the correct combination of moisture and heat pro- duced a similar fungus suitable for dew retting to that obtained in Northern France, the most important of which is Chdosporiurn .

Fabric Preparation and Bleaching The conventional method of preparing and bleaching linen involves a plurality of treatments: a lime boil followed by hydrochloric acid sour, alkaline boils (soda ash), acid chemic (hypochlorite at pH 4-5), a further alkaline boil (soda ash) which removes the chlorinated sprit (remnants of the woody core of the flax stem), and, finally, a hypochlorite bleach (pH 9- 10). T h s complicated sequence has been streamlined over the years. The lime boil has been replaced by a caustic soda-soda ash boil. Sodium chlorite, hydrogen peroxide and sodium hypochlo- rite are used as bleaching agents; niorc often the first two are used in sequence. Linen can be bleached white from the grey state without any prior boiling treat- ment by using higher concentrations of sodium chlorite (20-30 gl-’ at 85°C and pH 3.5-4); the impurities in grey flax activate sodium chlorite. A novel method of activating this bleaching agent using water-soluble polyesters has been des- cribed [4]. When sodium chlorite alone is used it is essential to follow with a good emulsifying boil to obtain a stable white.

The influence of retting on chlorite or peroxide bleaching of linen has been studied by Boute [S]. He has shown that changes in colour and chemical constitu- tion during dew retting account for dif- ferences in behaviour on subsequent bleaching. Ruiker [6] has considered the morphology of linen and the nature and amount of impurities in relation to modern trends in scouring and bleaching. Van Lancker [ 7 ] has studied the effect of different fabric treatments on the moisture absorption of linen. Lambrinou [8] has examined the effect of 14 dif- ferent bleaching sequences on the struc- ture of the fibre by photoniicrography.

12 REV. PROG. COLORATION VOL. 5 1974

She observed that optimum results were obtained with a mixture (3: 1) of soda ash and caustic soda, followed by a chlorite and peroxide bleaching sequence.

The mechanism of bleaching with sodium hypochlorite and sodium chlorite is not yet fully understood, notwithstand- ing long association with usage under varying conditions. The formation of labile ions or free radicals is a likely mechanism. Peters [9] has examined the various theories from consideration of the kinetics involved.

The modern, more streamlined, methods result in a lower weight loss (around 15%) than with the older methods (about 25%). This is due to only partial removal of the hemicelluloses, which are low-molecular-weight poly- saccharides and polyuronides. These are sensitive to alkaline treatments and are present in the linen fibre to the extent of about 18%. The absence, or otherwise, of hemicellulose would appear to have little effect on dyeing behaviour. From studies carried out by Sloan [ 101 there is evidence that removal of hemicellulose leaves the fibre more vulnerable to damage by subsequent exposure to acid o r oxidising conditions, particularly during resin treatment and chemical crosslinking under anhydrous acid condi- tions. Nothing is known of the distribu- tion of the hemicellulose; it is probable that it is concentrated in the nodal regions.

A problem that may arise when chlo- rite or peroxide (or both) is used as the bleaching agent is that of the presence of ‘sprit’. This term is commonly used to describe remnants of the woody core of the flax stem present in small amounts. It contains some short fibres and has a higher lignin content than the rest of the fibre. The dyeing properties of sprit [ 1 I ] may differ from those of the linen fibre; it dyes more heavily (Alcian Blue 8GX), lighter (some vat and reactive dyes) or equally (selected vat dyes) compared with linen. When fibre is obtained by the turbine-scutched route and is well machined there is less possibility of sprit being present (it is a common hazard, however, with tow-type scutching). The only effective way to remove it is by treatment with acid hypochlorite.

Mercerising Mercerising is an important part of linen preparation and it is a matter of concern that it is not more widely used. Swelling takes place at the nodes and as a result

there is a significant increase in abrasion resistance. It is also used to disembrittle linen finished with crease-resist resins or chemical crosslinking agents. Mercerising of linen fabrics is beneficial in the follow- ing ways:

1. It significantly improves abrasion resistance.

2. It covers the reediness in cloth associ- ated with yarn unlevelness - a feature of linen yarn.

3. It improves colour yield.

The use of liquid ammonia as an alterna- tive to caustic soda solution of mercer- king strength has been investigated at L.I.R.A. [ 121. Behaviour was similar to that observed with caustic soda, but the maximum shrinkage was somewhat less. T h e appearance, however, of the ammonia-treated dyed fabric was cleaner, and dye penetration superior.

It is of interest to note that, if yarn is treated with liquid ammonia by the Pro- grade process (J . & P. Coats), subse- quently woven into a cloth and finished with a reactant resin, no worthwhile improvement in abrasion resistance is obtained. The same considerations apply to yarn treated with caustic soda. Investi- gations of the effects obtained with liquid ammonia at very low temperatures may prove rewarding.

Recently Kane [13] examined the application of various dyes to different fibres from anhydrous ammonia at -40°F. The results obtained were com- parable to those obtained by the conven- tional route. This could prove of interest where conserva t ion of water is mandatory.

Finishing In finishing the high measured response of linen fabric to crease-resist and durable-press finishes continues to stimu- late research. Clearly, if the problem of severe embrittlement could be minimised sufficiently, linen would have outstanding performance as a crease-resist fabric. It is believed, however, that most of the resin or chemical reactant is concentrated in the nodal regions. Moreover, because of the ease with which moisture can enter and leave this area, migration of reactant during drying is a much more acute problem as compared with cotton. Clearly, the reactant is limited in its migration by either reaction with the cellulose or increase in molecular size by polymerisation. The same, however, can-

not be said of the catalyst, which migrates freely, resulting in a concentra- tion of acid at the crowns (high spots in the fabric) of the yarns. A water-soluble polyester catalyst produced from tri- ethylene glycol and citric acid was uti- lised by Sloan [I41 to overcome this problem with some degree of success. The best results were obtained using a carba- mate type resin. An advantage of this method is that it does not require a subsequent mercerising treatment. How- ever, on using . the process in bulk improvements were marginal. It remains difficult, therefore, to improve signifi- cantly upon the original Tootal, Broad- hurst Lee process for producing a crease- resist linen. The author has found that optimum results are obtained by omitting the pre-mercerising stage and allowing all of the shrinkage to take place at the subsequent mercerising stage; use of carbamate resins with the citric acid- polyester catalyst gave the best overall result. It has long been recognised from studies at L.I.R.A. that the type of flax, degree of fineness and retting procedure all have an important influence on the final result.

Demand for durable-press properties in linen tablecloths resulted in an investi- gation by L.I.R.A. [15]. The results indicated that linen was unacceptably embrittled at the level of durable-press rating required, e.g. 3-4. Notwithstand- ing the limitations indicated, considerable yardages of 100% linen have been pro- duced using a simple bifunctional cross- linking agent together with a stiffening agent. The results, however, at best could only be described as marginally accept- able. The use of blends of cellulose and man-made fibres, however, enabled materials to be made which gave reasona- ble performance.

The finishing performance of linen in relation to fabric construction, bleaching treatment and absorbency has been investigated by Lambrinou [ 161. She also studied the influence of fibre type, bleaching methods and finishing processes on abrasion resistance and, finally, showed the relation between crease-resist level and loss in tensile strength and changes in water regain [ 171 . Aluminium chlorohydroxide [ 181 is a useful chenu- cal for broad applications of various types of crease-resist treatment under moist- cure, mild-cure and hard-cure conditions, and is particularly useful for producing a low-temperature durable-press finish on linen tablecloths. ln recent years there

REV. PROG. COLORATION VOL. 5 1974 13

has been considerable interest in vapour- phase crosslinking using formaldehyde and gaseous hydrochloric acid; this route offers possibilities for linen since it avoids anhydrous conditions*. Very few methyl- ene crosslinks are required to produce a substantial increase in crease resistance. Recent research in the use of diepoxides from organic solvents for imparting crease resistance to cotton is worthy of exami- nation in relation to linen and undoubt- edly work along these lines will take place. Grafting a thermoplastic polymer by irradiation produced negative results on linen [ 191 .

Blends The emergence of linen as a component of blends represents perhaps a turning point in the decline of the industry. Terylene (IC1)-linen blends (50:50) were the first to appear. The blend yarns were spun on both the wet and the dry systems. With wet spinning the polyester was stretch-broken at the reach on the frame, with the result that it migrated to the core of the yarn. With dry-spun yarns there was no stretch-breaking; resulting from alkaline treatments of the cloth formed from the yarns, the linen migra- ted to the core on shrinking. The dry- spun yarns had different dyeing and finishing properties and difficulties arose if they were inadvertently mixed.

Generally, dry-spun blends could only be spun to relatively coarse counts (18s metric) limited by the denier of the linen compound fibres.

Empirical methods of dyeing and finishing new blends were developed by ICI and L.I.R.A. Recently, Opitz [20] studied the effect of weft tension in blends during dyeing and finishing, partic- ularly with crosslinking agents, and indi- cated that improvements in weft tear strength could be obtained by careful control of tension throughout. The devel- opment of a bleached, peptonised, chemi- cally pure linen fibre (Linron) by Kirkpatrick of Ballyclare [21] opened up completely new vistas for linen as a blend component. The precise chemical route of producing Linron has not been pub- lished, but essentially it is a treatment that does not use alkali and allows fibril- lation of the compound fibre strands to

*It has recently been drawn to my attention (Chem. in Britain, 9 (Sept 1973) 424) that bis-chloromethyl ether, a highly toxic carcinog- enic compond, may be formed whenever formaldehyde and hydrochloric acid come into contact.

take place under the forces of dynamic and static friction such as occur in varying degrees on all spinning systems. Thus it was possible to break down the compound fibres by a dry-spun route in much the same way as occurs on wet spinning. Hence, for the first time a bleached and purified linen fibre was presented to the spinner which was not unlike a synthetic-polymer fibre and had a reproducible and predictable behavioui-.

The fibre assembly in a linen yarn is essentially a high-density packing. This presents difficulties with liquor penetra- t i on and uniformity of treatment throughout the cross-section of the yarns, which in turn produces problems in dye- ing and finishing. These are overcome by the use of Linron.

The greatest success for Linron has been in blends with wool and it has enabled wool to be acceptable in spring- and summer-wear. It is interesting to note that in such blends (up to a 40:60 ratio) there is no apparent decrease in crease- resist angle as compared with 100% wool. Apart from the coolness and crispness of this blend, the emotional and prestige appeal associated with these two fibres is undoubtedly a factor in its commerciai acceptance. Clearly, as spinning costs increase it may not be economic to spin grey flax, when up to 25% of the weight is subsequently to be removed; nor can it be economical to bleach, say, a poly- ester-Linron (67:33) blend yarn in cloth form when only approximately one-third of it requires the treatment.

The technical advantages, however, of using Linron cannot be achieved by any other route. Thus, for example, the quality control and larger surface area produce higher abrasion resistance. In addition, because the fibre is stabilised there is little or no migration during subsequent wet treatment, although treat- ment with alkali of mercerising strength will cause the Linron to move towards the core of the yarn. Naturally such treatment cannot be used in blends with wool. Fine yarns up to 45s metric count have been produced on the cotton system of spinning in a blend of polyester- Linron (67:33). So far it has not been possible to obtain similar results utilising green flax (unretted), so it must be assumed that the retting process, particu- larly by fungus, plays an important part.

With wool- and acrylic-linen blends this is the only route possible, since the normal method of preparing and bleach- ing the linen component, expecially to

remove sprit, would degrade the acrylic component and seriously destroy the wool component. Recommendations for finishing polyester-Linron blends are available from Kirkpatrick [22].

Dyeing The lack of published work or detailed studies of the dyeing of linen and blends has already been mentioned. Much practi- cal information has been gathered over the years and is available in the form of pattern cards and technical information manuals from various dye manufacturers.

It is essential that linen or linen-blend material is well prepared and free from creases before dyeing. Continuous (pad- steam) and discontinuous (pad-batch) vat dyeing of linen fabrics is still standard practice. The discontinuous method, however, is the one in most general use because of the lack of long runs per colour owing to the decline of linen as an apparel fabric. Phillips [23] has given an account of the method from the practical dyer’s point of view. Generally, it is necessary to select dyes of low strike and high rates of diffusion if well-penetrated dyeings are required, particularly when solublised vat dyes are employed. Dark colours such as navy and black can be dyed economically with sulphur dyes.

Reactive dyes, e.g. Procion (ICI), are particularly suitable for dyeing liner: because of their high rates of diffusion. A number of techniques are used in practice:

1. Pad-develop (beam or jig) 2. Pad-batch 3. Continuous dyeing 4. Beamdyeing

Because of the lack of long runs per colour techniques 1 and 2 are most commonly employed. Reactive dyes give well-penetrated dyeings of excellent quality. The skittery effect sometimes associated with vat dyeings is notably absent.

Fox [24] has compared the dyeing behaviour of unmercerised linen using Procion M dyes with other fibres, e.g. cotton, rayon, Vincel 28 (Courtaulds) and jute. The highest rate of dyeing is shown by cotton and the lowest by rayon. Linen was intermediate between cotton and rayon on the one hand and Vincel 28 and jute on the other. Measure- ment of substantivity showed also that linen was intermediate between cotton and rayon. The rate of levelling, using C.I.

14 REV. PROG. COLORATION VOL. 5 1974

Reactive Red 8, was much lower on linen than cotton and considerably lower than on 1.5-den rayon. Measurement of strike again showed that it was much lower on linen than on cotton and low-denier rayon.

Theoretical and practical aspects of batchwise dyeing with reactives have been discussed by Marshall [25]. The influ- ences of finishing using bifunctional amine reactants or resins have to be taken into consideration. The latter can effect changes in light fastness and promote unexpected colour changes associated with photoreduction of certain dyes.

Pfoff [26] has studied the effects of different resin catalysts on a range of vat, direct and reactive dyes. He has shown that changes in light fastness and photo- tropy are predictable from considerations of resin structure, but changes in colour are not so related. The ‘after mercerising’ process can have a profound effect on colour, particularly with vat dyes; most of these, however, are noted in dyestuff manufacturers’ pattern cards.

With polyester-linen blends the pad- thermofix method employing vat or solubilised vat dyes and disperse dyes is commonly in use, although the mecha- nism of dyeing is complicated. The dis- perse dye is first adsorbed by the cellu- losic component and then transferred by vapour-phase migration [27] to the poly- ester. Practical aspects have been consi- dered by Fox. et el. [28]. The partition coefficients in the dyeing of linen- polyester blends have been investigated by Kruglov et el. [29]. They found a relationship between pH over the range 2-9 and the partition coefficients of vat dyes. A useful practical summary of the preparation, dyeing and finishing of poly- ester-linen (50:50) blends has been given by Bruggeman [30].

With Linron-polyester blends more uniform colouring of the Linron com- ponent is achieved with better penetra- tion. This is undoubtedly due to the more uniform preparation of the fibre and the greater surface area resulting from break- down of the fibre bundles. Soledon (ICI)

solubilised vat dyes wdl cover both com- ponents in light and pastel colours. Vat or reactive dyes in conjunction with disperse dyes can be used over a wide range of colours. Informa tion on dyeing these blends is available [31].

Wool-Linron blends can be dyed using Procion dyes [32] (under condi- tions that do not damage the wool) for the cellulosic component and, subse- quently, acid or reactive wool dyes for the wool. Solid contrasting or tone-in- tone effects can be obtained.

Acrylic-Linron blends are used mostly in knitted fabrics. The combina- tion of a shrink-stable fibre such as Linron with a high-shrink fibre such as a bulked acrylic fibre enables the Linron component to move to the outside of the yarn, thereby producing a maximum linen affect without the disadvantage of slubs, etc., which would present knitting problems, particularly on the finer-gauge machines. The usual blend level is 70:30 acrylic-Linron, but because of fibre migration the appearance of the yarn is more linen-like. Reactive dyes in conjunc- tion with disperse dyes are recommended

Considerable quantities of natural grey flax are used in blends with poly- ester and acrylic fibres to obtain coloured effects. This is a dangerous practice because of the poor fastness to light and bleachmg of the natural linen component. A joint investigation by ICI and Kirkpatrick has shown that Linron can be dyed a natural colour, e.g. ecru, at the fibre stage using Soledon dyes [34]. The presence of alkali promoted intense fibril- lation, so normal vat dyes could not be used for this work.

WI.

The Way Ahead The Linron concept has opened up possi- bilities and outlets for linen in a plurality of blends and blend levels never before thought possible. Today, blends with cotton, wool, polyester and acrylic fibres are well established over the complete range of spinning systems.

Linen as a component of blends has much to offer. Traditionally it is consi- dered to be the most comfortable fabric to wear under hot, humid conditions and the properties that make it so can be transferred to blends. Spencer-Smith [35] has investigated these properties - the high density, allowing relatively heavy material to be constructed which has low heat and water-vapour resistance, the high rate of moisture absorption and the total amount of moisture absorbed without imparting a feeling of dampness. These three factors, together with high thermal reflectance, were found to be important. He also observed a buffering action which allows a large temporary increase in heat loss, providing increased comfort under tropical conditions [36].

Solvent scouring and dyeing are unlikely to prove of much value except where water is scarce; organic solvents will not remove lignin. Application of crosslinking agents from solvents is likely to prove of interest.

Vacuum-impregnation techniques [37] are likely to be of value, because linen and linen-blend fabrics lend them- selves to padding techniques; penetration and wetting problems are not infrequent in conventional dip-nip systems of impregnation.

The possibility of applying crease- and flame-resist finishes to the fibre before spinning is worthy of investiga- tion.

Although linen is used to obtain surface appeal on cloths, it is of interest to note that polyester-Linron blends for double-knit fabrics are now well estab- lished in the U.S.A., in spite of the lack of interest of American producers in surface effects. The cool, comfortable handle given to the cloth is the prime factor.

I t remains to be seen whether the interest now shown in linen will promote more detailed research. Linen, which is as old as the hills, is now as modern as the hour - a factor that must promote its acceptability and use.

References

1. Dr Moffat’s translation. 2. Lambeg Industrial Research Associa-

tion. 3. ATPUL and Neenan, Sci. Proc. Royal

Dublin SOC., 3 (14 Dec 1973) 201. 4. Sloan, BP 821,168 (1958).

5 . Boute, Bull. Inst. Text. France, No.

6. Riiiker, Melliand Textilber., 51

7. Van Lancker, lndustrie Textil Balge,

8. Lambrinou, Melliand Textilber., 52

24 (July-Aug 1970) 637.

(1970) 1085.

13 No. 5 (1972) 41.

(1971) 1184.

9. Peters, ‘Textile Chemistry’ Vol. I1 (Amsterdam: Elsevier, 1967).

10. Sloan, unpublished work. 11. Fox, private communication. 12. L.I.R.A. Memo No. 2069 private

13. Kane, Amer. Dyestuff Rep., 64 (May reports.

1973) 27.

REV. PROG. COLORATION VOL. 5 1974 15

14. Sloan, BP 877,582 (1958). 15. L.I.R.A. Memo No. 1848 private

16. Lambrinou, Melliand Textilber., 51

17.Idem, ibid., 51 (1970) 815; 1241;

18. Thonig and Schmidt Textil Praxis, 27

19. Spencer-Smith, L.I.R.A. private

20. Opitz, Textil Praxis, 27 (1972) 166. 21. Chem and Ind., (1 1 Mar 1967) 378. 22. Kirkpatrick of Ballyclare, Tech. Bull.

reports.

(1970) 930.

930.

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1969) 19; Amer. Dyestuff Rep., 58 (Aug 1969) 19.

26. Pfoff, Z. Fur die ges. Textilind., 71 (1969) 616.

27. Bent, Flynn and Sumner, J.S.D.C., 85 (1969) 606.

28. Fox, Glover and Hughes, ibid., 85 (1969) 614.

29.Kruglov, E r u k n i m o v i t c h and Kopelman, Tekstil. Prom., No. 6 (June 1971) 50.

30. Bruggeman, Bull. Inst. T k t . France,

3 1. Kirkpatrick of Ballyclare, Tech. Bull.

32. Idem, Tech. Bull. No. 1. 33. Idem, Tech. Bull. No. 2. 34. Idem, Tech. Bull. No. 6. 35. Spencer-Smith, Shirley International

Seminar, Textiles for Comfort (June 1 97 1).

36. Idem, private communication, Text. Research J., 36 (1966) 855.

37. Fox, Amer. Dyestuff Rep., 61 (Mar 1972) 48; J.S.D.C., 89 ( 1 973) 474.

No. 25 (1971) 161.

No. 4.

16 REV. PROG. COLORATION VOL. 5 1974