Linen - the way ahead

William S Hickman

Although a ’traditional’ fibre, linen production has decreased substantially in recent years. In this article, William Hickman looks at the problems posed by linen, and research that is being carried out on linen fibres in order to secure its future.

I n November 19Y3, a series of presentations was made to a symposium entitled ‘The way ahead’, jointly organised by Solvay Interox and the Northern Ireland Textile Association (NITA). The presentations were used to show that, in spite of the environmental problems and the closure of the Linen Industries Research Association (LIRA), there is a way ahead for the linen industry. The title is used here to report Solvay Interox’s part in that presentation, but it should also be stressed that an important presentation was that given by the Institut fur angewandte Forschung (IAF) in Reutlingen, Germany. This is an academic group that could well become a replacement for LIRA.

Production The volume of linen produced over a 30 year period is shown in Table 1 [I]. These volumes are quite small compared, for example, to world cotton and total fibre production. Linen volume was around 4% of world fibre production in 1961, but has now dropped to around 2%.

An encouraging aspect for the future of linen production is that flax growth within the EU is being promoted. At this moment, highly branched plants are cultivated for oil production but, as world

Table 1

Production Year (kg x 1 08)

1961 6.5 1971 6.1 1980 6.3 1985 7.4 1986 7.2 1987 9.6 1988 9.2 1989 8.2 1990 7.2 1991 7.2 1992 7.1

Table 2

Cotton Average

-

Cellulose 88.0-96.0 Hemicellulose Pectins 0.7-1.2 Lignin Proteins 1 .l-1.9 Fats and Waxes 0.4-1.6 %Ash

Flax

Retted Unretted Jute ~

564 651 58 0-63 0 154 1 6 7 21 0-240 2 5 1 8 0 2-15 0 2 5 2 0 120-150

08-1 8 1 3 1 5 0 4-0 8

06-1 2

population increases, there must be increasing growth of textile fibre in temperate latitudes. A good choice for such a fibre is flax.

Constitution The bast fibres are generally not pure cellulose. Table 2 compares the constitution of cotton with flax and jute. The table clearly shows the relatively low cellulose content of bast fibres compared to that of cotton. In contrast, they have relatively high hemicellulose and lignin content. It is these components that form the ‘cement’ that glues the ultimate cells together to form a fibre. Pectins are also part of this cement; these are uronic acids salts. Scouring is usually carried out to remove these impurities but this can result in:

Weight loss Release of cations from salts into solution Loss in fibre strength.

If the released cation is iron, then this can lead to catalytic tendering during bleaching. The lignin content of flax is much lower that of jute. This lignin is difficult to remove in the wet processing of flax, and incomplete removal is responsible for the yellowing of the fibre after bleaching.

The morphology of flax has already been described as an assembly of ultimates cemented together within the fibres and an assembly of these fibres into bundles. These assemblies are prone to dislocation 121. It is the structure which makes linen feel like linen. Disruption oi the structure by, for example, scouring is referred to as cottonisation.

Table 3 compares the dimension of ultimates from several sources 131. The length, as shown, varies considerably, whereas the diameter is moderately constant from fibre to fibre. In flax tht. ultimates are much longer than those i n coir, jute, sisal and so on. The implication of this observation is that the ’cement’ c m be removed from flax with little effect oti

tensile strength as hydrogen bonding still occurs over the large contact areas of the ultimates. In the case o f the fibres with short ultimates, such bonding does not occur and removal of the ‘cement’ causcbs massive strength loss.

Wet processing The impurity content and morphology have an effect on wet processing, and thts impurities are generally removed during the preparation stage. This is a process which must be carried out carefully in order to maintain a balance between the required properties of whiteness,

170 JSDC VOI,UME 110 ~ ~ A Y J L J N E 1994

Table 3

1810 - 2200

I 2140 - 21 00 1790 2170 - I

1640 - 2230

Aspect ratio

-

Manila Coir Flax True Hemp Sunn Hemp Jute

Jute

Kenaf Ramie Mesta Sisal Urena

(Cor. Capsularis)

(Cor. Olitorius)

4.6-5.2 17.0-21.4 0.9-1.2 16.2-19.5 27.4-36.1 17.8-21.6 8.3-14.1 17.0-22.8 3.7-6.0 23.0-35.0

1.9-2.4 16.6-20.7

2.3-3.2 15.9-1 8.8 2.0-2.7 17.7-21.9 125.0-1 26.0 28.1-35.0 2.6-3.3 18.5-20.0 1 .8-3.1 18.3-23.7 2.1-3.6 15.6-1 6.0

255 60

1600 560 170

115

160 120

4000 150 120 180

Table 4

Kier boil Lime 10-12% 0.w.g. 8 hat 120°C Cistern sour Kier ash boil Soda ash 12.5 g/l18 h at 60°C Cistern chemick Av. CI, 1.7 g/l2 h at pH 4.3 Kier ash boil 6.5 g/l 18 h at 60°C Chemick bleach Av. CI, 0.6 g/l cold pad-batch overnight

Soda ash

Soda ash 2.0 g/l

absorbency and the like, and undesirable effects such as chemical damage, handle modification, and high weight and strength loss. Traditionally, long, slow processrs were carried out to obtain the correct balance. In the case of flax, this usually consisted of chlorination, extraction of the chlorinated lignin by scouring in soda ash and then bleaching, preferably with alternate chlorite and peroxide stages or even a hypochlorite bleach. Process details, from Solvay Interox customer files, are shown in Table 4.

In 1970, HASF carried out some pioneering work on the replacement of this traditiond process [4]. The work investigated the whiteness and damage that resulted from scouring in one of three ways (soda ash, caustic soda and caustic soda plus Lufibrol KB) and then bleaching with either hypochlorite, sodium chlorite (C) or hydrogen peroxide (P). The results are summarised in Figure 1. The numbers at the top of each bar are DP (degrce of polymerisation) values and are a measure of chemical damage. A value above 2000 is considered satisfactory. In general, the scour can be improved by adding a suitable auxiliary, in this caw one that contains a chelating agent and thus sequesters any cations released from the pectins. Chlorite gives

the highest whiteness but also the highest damage. All DP values are, however, acceptable.

If these samples are rebleached to improve the whiteness, then Figure 2 shows how combinations of bleaching stages (e P+C, C and C+P) affect the final bleached result. Hypochlorite was also evaluated, but is not reported in view of the AOX problem.

Weight loss was also investigated. As caustic soda concentration in the saturator increases from 0 to 40 g4, then weight loss increases almost linearly from 0 to 18%. Concentrations above 60 g'l give a virtually constant weight loss of around 22%,

Problems The premise on which this paper was written is that there are problems with the existing processes. These are:

Organohalogen compounds - AOX [5] Post-yellowing of the bleached fabrics or fibres Cottonisation.

The first is caused by reaction of chlorine- containing compounds with organic materials, primarily lignin [6]. The measurement of this residual lignin is difficult. It can be 'assessed by spotting

80

g! 60 d

40 0,

U

- c

20

0 Soda Caustic Caustic + Lufibrol

Hypochlorite + Peroxide Chlorite

Figure 1 Reflectance from various pad-roll bleaches

Soda Caustic Caustic + Lufibrol

m P U P + C o c U C + P

Figure 2 Reflectance from various pad-roll bleach combinations

JSDC VOLUME 110 MAY/JUNE 1994 171

with phloroglucinol dissolved in hydrochloric acid [7]. Quantitative determination can be achieved by sulphuric acid extraction.

Before proceeding to solutions for these problems, it is worth considering flax in a wider context. Flax is a natural product which is released from the plant by mechanical (decorticisation) and biochemical (retting) processes. I t is, therefore, a variable material. IN has investigated which parameters affect plant growth and maturity in an attempt to minimise this variability, using microscopy, chemical analysis and modern data analysis (factor analysis) in statistically planned trials to improve spinning and weaving efficiency. It is also well equipped to provide integrated solutions to wet processing problems.

Potential solutions The cottonisation problem, as already indicated, can be solved by scouring correctly. Stazione Sperimentale per la Celluloza stated at the outcome of its part of the 'Eurolin' project that there is an interaction between scouring and bleaching that can cause a wide spectrum of cottonisation effects [2]. The other problems can simply be eliminated by replacing the chlorine-containing bleaching agent, be it hypochlorite or chlorite, with another agent that is also able to delignify fully. No mention was made of delignification in a recent paper presented to the American Association of Textile Chemists and Colorists [8], but it did show that peracetic acid/peroxide combinations can replace hypochloritel peroxide and achieve the same whiteness as the chlorine route. These results quoted are summarised in Table 5.

Earlier work from Russia suggested

Table 5

State Reflectance (%)

Grey linen 20 Ash scoured 30 Peroxide bleached 65 Peracetic acid process 77 Hypo/Peroxide bleached 67

Table 6

Subitol LSN 1 1 1 1 Stabicol A 1 1 1 1 Soda ash 2 2 to pH 8 to pH 8 Proxitane 1507 15 15 Hydrogen peroxide 10 10

Subitol LSN Stabicol A Soda ash Proxitane 1507 Hydrogen peroxide

1 1 1 1 to pH 8 1 15

10

Reflectance (%, Elrepho) 37.9 64.7 42.8 59.5 Phloroglucinol stain Darkred Clear Dark red Pink

that peroxide itself was a good delignification agent and was, in fact, better, than peracetic acid [9]. The work was carried out on lignin extracted with 72% sulphuric acid which was bleached by either chlorite (pH 3.6,l-3 h), peroxide (pH 11.8,l-3 h), peracetic acid (pH 6,l-3 h) or hypochlorite (pH 8,l-2 h). The temperature was 80°C except for the hypochlorite stage which was at carried out at 20°C. The effectiveness of the treatment was assumed to be reflected by an increase in water-soluble oxidation products. Chlorite gave 37-66% solubility, peroxide 54-5796, peracetic acid (PAA) 34- 48% and hypochlorite 2&26%.

Our work on pulp delignification with peracetic and other peracids does not agree with the Russian findings and this is demonstrated in the winch bleaches shown in Table 6 (liquor ratio lO:l , 60 min, 80°C) on 100% linen woven fabric using 15% PAA (Proxitane 1507).

Conclusion The morphology and constitution of flax make it a unique fibre which is likely to increase in volume as world population increases, but it is difficult to bleach by virtue of its lignin content. This difficulty in bleachability is likely to worsen as chlorine containing bleaching agents are withdrawn from use on the grounds of AOX formation.

It is l i l y that, in the future, peracids will be able to replace these agents both as bleaching and delignification agents. There is, however, much work still to be

done to develop production processes and this would be made much easier if the fibre were a more homogeneous, amorphous substrate. These are qualitiea which the IAF work on flax maturity nnd 'steam disintegration' should be able to confer.

I do believe, with the help associations like NITA, academic groups l&e IAF and the chemical producers like Solvay, that there is 'a way ahead' for linen.

References 1. 2.

Wool facts, IWS Publication March 1992. C Blum and J G Wurm (ed), E 1 4 r o p w t i Textile Research: Compf:titiveness t/Irou;,df innovation, Elsevier, London (1985). D H Bowen, Structure and MorphokJs$y of Vegetable Fibres, IJIRA Golden Jubilee Seminar, 6-7th Nov 1987.

4. K-H Riicker, Melliand Extilber., 51 (1970), 1085.

5. W Sebb, Textil Praxis, 44 (1989) 841. 6. G Schulz, Textil Praxis, 45 (1990), 40. 7. W Garner, Textile Laboratory Munrcul

(1967). 8. N Steiner, AATCC Technical Corr~~rc~tic~i~

1993, Book ofPapus, 214. 9. V I Lebedeva, Tech of TL'xtile Industry

USSR (English version), 1 (1969), 117.

3.

William S Hickman is textile market specialist at Solvay Interox R & D, i Widnes, Cheshire, UK.

172 JSDC VOLUME 110 MAY~JLJNE 1994