|Danny E. Akin, Ph.D., is a microbiologist with the Agricultural Research Service, U.S. Department of Agriculture, and works in the Quality Assessment Research Unit, Russell Research Center, Athens, Ga. He leads a research project that evaluates production, processing, and standards development for flax fibers. Akin chairs ASTM Subcommittee D13.17 on Flax and Linen.
D13.17 Subcommittee Meetings: Subcommittee D13.17 is open to anyone wishing to participate and contribute to developing standards for flax fibers. Currently, meetings are held twice each year as part of the meetings of Committee D13 on Textiles.
Standards for Flax Fiber
Flax, the source of linen and technical bast fibers, is a versatile crop that can be grown in a variety of climates throughout the world. Its scientific name, Linum usitatissimum L., translated as “linen most useful,” aptly describes this versatility. The commercially useful fibers of flax are bast fibers (fibers produced in the cortical region of the plant between the outer cell layers and lignified inner core tissues of the stem), which are used for textiles, composites, and specialty paper and pulp. In addition to the bast fibers, the flax plant is the source of linseed, from which is derived a highly prized industrial drying oil used in paints and varnishes and also a nutraceutical oil with high levels of 3-omega linolenic acid.
Retting and Its Influence on Fiber Quality
To obtain fibers from bast plants, the stems are first retted in a microbial process that partially degrades tissues and separates fibers from non-fiber material (see Figure 1 below). The degradation of pectin, which is a compound that binds cells and tissues together in plants, is key to the retting of flax. (1)
|Figure 1Scanning electron micrographs of flax stems.
Left: unretted cross section of stem showing bast fibers in bundles between the cuticle and woody core, that is, shives; right: dew-retted cross section of stem showing separation of fibers and fiber bundles from the non-fiber tissues.
Water retting, where bundles of stems are submerged for retting by anaerobic bacteria, is one of the best but most expensive methods for separating flax fibers. Rivers, lakes, ponds, and retting pits have been used for water retting. Warm water retting was practiced in the middle to late 20th century by the linen industry in Belgium. Currently, 80 percent of the flax fiber production in China is reportedly achieved by warm-water retting, but yields are low and quality moderate. (2) Cost as well as stench and pollution from the anaerobic bacterial fermentation caused the linen industry in western Europe to revert to the older method of dew retting.
Dew retting, which was practiced by Egyptians thousands of years ago, is enabled primarily by the actions of aerobic fungi on flax stems that have been pulled or cut and laid in swaths in fields. By the middle to latter part of the 20th century, dew retting had become the primary retting method for the major linen-producing regions of western Europe. It is generally agreed that the best fiber for linen comes from Normandy (northern France), Belgium, and The Netherlands, where the moisture and temperature is more conducive to dew retting than most other places and producers have experience and expertise for optimal retting and quality. Even under these conditions, however, fiber quality is variable and crop losses occur about one-third of the time.
Most flax fiber is produced under less ideal conditions and, as a consequence, commercial flax fiber can be extremely variable in quality. Dew retting is less expensive than water retting and eliminates pollution from fermentation. However, disadvantages exist and include 1) lower and less consistent quality fibers than with water retting, 2) restriction to particular regions where the climate is conducive to the growth of aerobic fungi, 3) the occupation of land for several weeks, and 4) soil and other contaminants present with the fiber.
Because of problems in both water and dew retting, chemical and enzymatic methods have been evaluated as improved methods and to reduce variability. Considerable research to find a replacement is ongoing, especially for dew retting, but no suitable alternative methods are yet used commercially. Fibers eventually produced by these newer methods will have to be compared with those produced by the established commercial methods, and objective standards will be valuable for such comparisons.
Processing and Flax Fiber Properties
Linen, which is made with the long, strong bast fibers from most of the length of the stem, has occupied a prominent place in textiles for centuries, and recently linen products have enjoyed a renaissance, especially in blended yarns. Although traditional linen in Europe is constructed with long-line fibers from scutching and hackling operations, many industry analysts indicate greater amounts of short staple fibers will be used in textiles for blending with cotton or other fibers. Traditionally short fibers, called tow, are produced as by-products of scutching or hackling and used in blends and for applications other than textiles. Processing lines, however, such as the Unified Line of Czech Flax Machinery or Lin Line of Temafa, produce a “total fiber” from flax stems, without traditional long line and tow products. Flax fibers may be “cottonized,” that is, refined and shortened for use in short stable spinning or for other, high-value applications.
With increasing interest in the use of natural fibers for composites in the automotive sector and other large industrial users, there is potential to obtain total flax fibers from diverse sources, some of which may be cottonized. For example, linseed straw, which is now a waste product of the linseed industry, is available in large amounts and currently has little value (a small percentage is used for specialty paper). Fibers from linseed straw are considered inferior in quality to that from fiber flax. The use of flax from such diverse, non-traditional sources further indicates the need for uniform, objective standards for judging fiber quality for optimal processing and applications.
Historical Perspective and Interest in Standards for Flax Fiber
Flax is likely the oldest textile fiber known, with evidence of production dating back 7,000 or more years. Surprisingly, however, objective standards do not exist for the most part. To the author’s knowledge, only one international standard for flax exists (ISO 2370, Textiles determination of fineness of flax fibres-permeametric methods, 1980). Flax fiber is traditionally bought and sold by the subjective judgment of experienced graders who appraise by look and feel; this is called organoleptic testing. Various classification schemes that include the source (for example, Belgium, France, Russia, or China), processing history (for example, water or dew retted), or application (for example, warp or weft yarn) have been used.
Grading systems for traditional linen assess fineness, length and shape of fibers, strength, density, luster, color, handle, parallelism, cleanliness, and freedom from neps and knots. The linen industry, which is small, sufficient for limited needs, and self-contained, has not actively promoted the development of objective standards and continues to rely upon organoleptic methods. Within particular countries, the measurement of flax fibers is done by more or less consistent means and, therefore, a limited classification system may exist. Various grades of flax fibers for various uses (for example, cottonized fibers) are identified for marketing within a company.
Without standards, manufacturers lack information on how to set equipment for optimum production, which affects efficiency (for example, through downtime) and product quality, or how best to use available resources. Natural fibers such as flax and linen are, by their nature, variable. Production in extremely different climates and under myriad production systems (as described above) further contributes to variations in fiber properties and quality. This variation in characteristics and potential for large and expanded uses in a variety of industries (for example, parts for automobiles) has resulted in considerable interest in the development of standards for flax fibers. (3) The Cost (European Cooperation in the Field of Scientific and Technical Research) Action 847 (Textile Quality and Biotechnology) of the European Union stated an objective of acquiring knowledge “to set up quality standards for assessing” flax fiber.
The Center for American Flax Fiber, a not-for-profit organization dedicated to promoting flax fiber in North America, led the way in requesting the establishment of a subcommittee to develop flax standards within ASTM International Committee D13 on Textiles. In 1999, Subcommittee D13.17 on Flax and Linen was officially formed and began meeting biannually as part of Committee D13.
Flax properties identified for standardization included strength, length, fineness, color, and trash (non-fiber components). Task groups within D13.17 were established to address each of these properties, with task groups for terminology and industry liaison added later.
Development of Specific Standards Under D13.17
D 6798, Terminology Relating to Flax and Linen Lengthy discussions over several meetings were required to reach agreement on precise terminology. Research was undertaken to assure that terms were not counter to accepted language in Europe or other regions with a long history of flax production. The standard was approved in 2002.
D 6961, Test Method for Color Measurement of Flax Fiber The natural color of flax fiber is light amber. Retting methods, however, influence the color of processed fibers. Water retting results in a light-colored fiber. Dew retting, in contrast, imparts hues from gray to black to the fibers, depending upon the extent of retting among other factors (see Table 1 below). Experimentally produced enzyme-retted or chemical-retted fibers are very light due to some bleaching action of the chemicals.
|Table 1 CIELab color values of dew-retted, water-retted and enzyme-retted flax fibersa
|Dew retted (N=3)
||59.428 ± 1.369 a
|11.080 ± 1.663 a
|Water retted (N=2)
||67.541 ± 1.048 b
|14.540 ± 0.488 b
|Enzyme retted (N=6)
||72.019 ± 3.259 b
|16.300 ± 1.565 b
|L is the amount of lightness from black (0) to white (100)
a* is redness/greenness value (the higher the number the redder the sample)
b* is yellowness-blueness value (the higher the number the yellower the sample)
abc values within columns with different letters differ at P<0.05.
From Akin, D. E., H. H Epps, D. D. Archibald, and H. S. S. Sharma. 2000. Color measurement of flax retted by various means. Textile Res. J. 70(10):852-858.
In addition to lightness, color measurement systems can show other color scales, such as the red-green and yellow-blue, and thereby provide other information. In one study, the water- and dew-retted fibers differed in yellow values, while enzyme-retted flax showed more redness when the chelator levels increased in an enzyme-retting formulation (Table 1). In practical usage, much of the fiber sold for linen is blended among harvests to have color consistency in the final product. The use of a standard method for color values could help in blending for particular properties of a fiber sample arising from a variety of sources and processing methods.
The use of CIELab measurements (developed by Commission Internationale de l’Éclairage) provided an established means for objective color determination using three factors: black to white (L* value), green to red (a* value), and blue to yellow (b* value). (4) With this method, problems related to color matching can be more objectively addressed to provide better use of flax from a broad production system. This standard was approved in 2003.
D 7025, Test Method for Assessing Clean Flax Fiber Fineness Fineness is one of the most important properties for textile fibers. A European effort in the 1970s resulted in ISO 2370, which estimated fineness based on air flow. To our knowledge, this standard has not been updated since 1980. An air flow test was developed (5) based on a modified cotton micronaire system and using a series of flax fiber grades purchased from the Institut Textile de France Lille (now the Institut Francais Textile Habillement). This test provides a number as a comparative score for ranking fibers. This ranking showed good agreement with fiber widths determined by image analysis (see Table 2 below). This standard was approved in 2004.
|Table 2 Image analysis and airflow fineness measurements of flax fibers
|Frequency of occurrence of fiber widths by image analysis:
|a IFS Standards (and airflow values for fineness) from Institut Francais Textile - Habillement.
b Modified cotton micronaire method using 5.0 g flax fibers cut to 2.5 cm, which resulted in a reading within the accepted range for the micronaire.
Adapted from Akin, D. E., I. R. Hardin, L. L. Rigsby, and H. H. Epps. 1999. Properties of enzymatically retted flax for linen fiber, pp. 486-492. Book of Papers, American Association of Textile Chemists and Colorists, Research Triangle Park, N.C.
D 7076, Test Method for the Measurement of Shives in Retted Flax The presence of non-fiber, trash particles is particularly troublesome in high-value products like textiles. Production efficiency, for example, the spinning of yarn without interruption, and final product quality are both diminished with trash. One of the particular problems with flax is the close association of non-fiber tissues with fibers. After retting and subsequent cleaning, shives and cuticularized epidermis often still remain with the fiber. The amount of trash (that is, non-fiber materials) depends upon the quality of retting to a large extent. Flax fibers are mostly cellulose, that is, around 65-89 percent, with other non-cellulosic sugars present. The shives contain substantially more aromatics and lignin than fibers, and the different chemistries of these components provide a relatively easy way to differentiate between them. Table 3 below shows variations in chemical components of bast fibers and shive.
|Table 3 Carbohydrate and aromatic constituents in flax fractions
|| Non-cellulose carbohydrates
|Data adapted from Akin, D. E., G. R. Gamble, W. H. Morrison III, L. L. Rigsby, and R. B.Dodd. 1996. Chemical and structural analysis of fiber and core tissues from flax. J. Sci. Food Agric. 72:155-165.
A model was developed using near infrared spectroscopy and a series of mixtures with exact proportions (by weight) of ground fiber and shive. While this model is based on shive material, the presence of cuticularized epidermis, having a high level of wax and cuticle along with aromatics, may dictate another model, or modification, in further refinements of the method. This standard was approved in 2005.
The standards for color, fineness, and shive content were based on intralaboratory data; obviously, there is no bias for different laboratories with these standards. Their acceptance is valid for five years. After that time, interlaboratory data are required for validation. Round robin collaborators are being contacted to establish future round robin tests.
The properties for which standards have already been developed are some of those commonly required for typical fiber applications. Standards for fiber strength and length are two others that require future action.
For research, strength has been tested with the stelometer. The use of high volume instruments, or HVI, for the cotton industry could provide a more rapid method if modifications could be made for use of flax fiber. Assessing the force necessary to break a certain fiber mass could be considered for a variety of methods.
Fiber length may be more problematic. While long-line fiber for high-value textiles has a minimum length of about 50 cm, tow or “total fiber,” in which fiber is non-uniform and non-aligned from the whole plant, could be extremely variable. Methods used for cotton or with image analysis systems will likely provide the starting point for fiber length standards in flax.
Recent attempts have been made to use rapid, spectroscopic methods to assess flax fiber quality in place of more time-consuming methods. Models using near-infrared reflectance spectroscopy have been used to determine fiber content in intact stems as well as the degree of retting. Near infrared spectroscopy using particular wavelength ranges has been used to assess flax fiber fineness (using calibration data from derivative thermogravimetric analysis and airflow methods). (6) The spectroscopic methods require calibration sets from some other assessment method, for example, wet chemical, strength, and fineness. These methods offer a potential strategy, by rapid and nondestructive means, to develop standards for an objective classification system for flax fibers.
Unlike many fibers, flax does not have a uniform set of objective standards to judge quality. The broadening use of flax and production from diverse sources call for objective standards for efficient processing, application, and marketing potential. The traditional linen industry depends primarily upon long-used organoleptic tests for quality. Interest is strong, however, for uniform and objective tests to measure properties of flax fiber that could be used in myriad applications. Subcommittee D13.17 has been active since 1999 in developing standards for flax fibers, and to date four standards have been produced. Existing fiber methods have been adapted to flax where appropriate, but new technologies are being used for rapid, nondestructive testing.
The author gratefully acknowledges the following: P. A. Annis and H. H. Epps, Department of Textiles, Merchandising and Interiors, University of Georgia, in leading efforts for standards on terminology and a color test method, respectively; J. A. Foulk, Cotton Quality Research Station, ARS-USDA, Clemson, S.C., for leading the effort for a fineness test method; W. H. Morrison III for leading, with assistance from F. E. Barton II and Miryeong Sohn, work toward a trash (non-fiber) standard; D.D. McAlister, Cotton Quality Res. Station, ARS-USDA, Clemson, S.C., for support and general review; and R. B. Dodd, Clemson University; Brad Reed, Cotton Quality Research Station, ARS-USDA, Clemson, S.C.; Luanne L. Rigsby, Quality Assessment Research Unit, ARS-USDA, Athens, Ga.; and Alvin Ulrich, Biolin Research Corp, Canada, for numerous contributions on research and evaluations related to standards.//
(1) Sharma, H.S.S., and C.F. Van Sumere (editors).The Biology and Processing of Flax, M Publications, Belfast, Northern Ireland. 1992.
(2) Euroflax Newsletter, vol 21 (1), Information Bulletin of the FAO European Cooperative Research Network on Flax and Other Bast Plants, Institute of Natural Fibers, Poznan, Poland. 2004.
(3) Van Dam, J.E.G., G.E.T van Vilsteren, F.H.A Zomers, W.B. Shannon, and I.T. Hamilton. Industrial Fibre Crops, Increased Application of Domestically Produced Plant Fibres in Textiles, Pulp and Paper Production, and Composite Materials, ATO_DLO, Wageningen, The Netherlands, 1994.
(4) Epps, H.H., D.E. Akin, J.A. Foulk, and R.B. Dodd. 2001. Color of enzyme-retted flax fibers affected by processing, cleaning, and cottonizing. Textile Res. J. 71:916-921.
(5) Akin, D.E., L.L. Rigsby, and W. Perkins. 1999. Quality properties of flax fibers retted with enzymes. Textile Res. J. 69:747-753.
(6) Faughey, G.J., and H.S.S. Sharma. 2000. A preliminary evaluation of near infrared spectroscopy for assessing physical and chemical characteristics of flax fibre. J. Near Infrared Spectrosc. 8:61-69.