The term “white specks” describes a condition in dyed cotton fabric where small nep-like forms can be seen as white or, more accurately, lighter shade specks on the surface of the finished fabric. This condition can exist in both woven and knitted goods.
White specks are undyed spots on dyed fabric, and are commonly caused by neps. According to the American Society for Testing and Materials (ASTM D123-96, 1996), a nep is “a tightly tangled knot-like mass of unorganized fibers.” This is to be differentiated from a mote, which is another impurity found in cotton consisting of a seed fragment encompassed by cotton fibers. Many researchers have studied neps over the decades, including types, formation, effects, and solutions.
Watson classified neps into two groups, mechanical and biological, and observed that mechanical neps are similar to the classical ASTM definition, where they are formed from mechanical actions on the fibers. They also reported that fibers with low micronaire values tend to form mechanical neps because the fibers are finer and less mature, and are therefore less rigid.
Biological neps are clumps of very immature fibers that can be found in seed cotton before mechanical processing has occurred. They also reported that fibers with low micronaire values (finer and immature fibers) tend to form mechanical neps because of the weak, poorly developed, less rigid fibers. Goynes et al. (1994) reported that because of low cellulose content of the undeveloped, flat, ribbon-like fibers, clumps of these fibers do not accept dye. Therefore, when a fabric is dyed, the mechanical and biological neps formed by fine or immature fibers create undyed spots in the finished fabric. These undyed spots are known as white specks.
Quality of a finished garment is determined by, among other things, the number of imperfections contained within the fabric. The more imperfections found in the cloth, the less value can be added to the product by the manufacturer. Since uniform surface color is a desirable aspect for fabrics, the inclusion of white specks is detrimental to fabric quality. White specks are a result of neps being included in the raw cotton product supplied to a processor or result from ensuing mechanical treatment.
The process of counting neps is very tedious and time consuming. Since the late 1930s, neps were counted manually using a back light (Helliwell, 1938) or a black background (Saco-Lowell, 1942). Even today, neps are being counted manually relying on visual inspection (Harrison and Bargeron, 1986; Hughs et al., 1988; Cheek et al., 1990; Smith, 1991). The manual counting is not only a time-consuming process, but also inconsistent and prone to error because it is very subjective.
Recently, many studies focused on automatic counting of neps and white specks. The Advanced Fiber Information System (AFIS) module has been used by many researchers to detect and count seed coat neps. Mor (1996) introduced a fiber contamination tester that detects sticky deposits by an electro-optical device and evaluates nonsticky parameters such as neps, trash, and seed coat fragment using an image processing system. Bel-Berger et al. (1994, 1995, 1996) used three different image processing hardware systems in analyzing the number and area of white specks found in dyed fabric. The area of white specks was calculated in terms of number of pixels.
The white speck counter developed consists of three major components: an illumination chamber, fabric transport mechanism, and image processing hardware and software. The illumination chamber is needed to provide uniform illumination on the fabric surface by blocking ambient light and furnishing a consistent light source. An aluminum roller is mounted on each side of the chamber so that a roll of dyed fabric can be mounted on a roller on one side of the chamber, and transported through the chamber to the other side onto the second roller. An image of the fabric is then captured by a black-and-white camera. Image analysis software counts the number of white specks in the image and measures the area of individual white specks.
The appearance of white specks on dyed and finished fabrics continues to be a sporadic and periodic problem for dyers, knitters, and spinners. Perhaps the most troubling aspect of this problem is the fact that its presence is not usually known until the fabric is dyed and finished. The severity of the occurrence can range from barely noticeable to rendering the material useless as first-quality goods. White specks are not normally visible in bleached or greige state goods.
White specks are actually small clusters of immature fibers (often “fused” together) which lie on the surface of the dyed fabric. Because these fibers are immature, or underdeveloped, their cell walls contain relatively little cellulosic material. This condition causes the fiber to take on a very flat and ribbon-like form. It is this flat form that, when seen on the surface of a dyed fabric, reflects light more efficiently than the surrounding fibers. This high reflectivity is perceived by the eye as being lighter in shade or, in some situations, as white specks.
All cottons (different varieties and bales) contain some amount of immature fibers. They are a natural product of the plant’s developmental physiology. It is only when these immature and underdeveloped fibers reach certain concentrations in a bale, or group of bales, that the problem of white specks becomes an issue. Depending on their form and/or concentration level, immature fibers may or may not actually produce a white speck occurrence. This is part of the dilemma facing yarn manufacturers…There is no way of absolutely predicting (or avoiding) a white speck outbreak.
With that said, there are some general rules of thumb (based on empirical data and actual experience) that a spinner can follow in order to lower the probability of producing yarns which contain potential white specks.
- For any given growth area and/or variety, higher micronaire values are less likely than lower micronaire values for producing white specks.
- Higher maturity ratios are less likely than lower maturity ratios for producing white specks.
- Stripper harvested cottons are more likely to produce white specks than spindle picked cottons. This is largely due to the tendency of the stripper to harvest bolls that are not fully mature.
- Removing more waste through cleaning and carding (especially under the lickerin and fine openers) can minimize, or make an unacceptable situation acceptable. This suggestion should also imply that the introduction of reclaimed waste is a high risk activity for introducing white specks.
- Maintaining a higher nep reduction factor on all cards can be very effective in minimizing white speck problems
These suggestions, even if followed to the letter, are still no guarantee that problems with white specks can be completely avoided or eliminated. This is especially true if the concentration of white speck producing material is high enough, but even a severe problem can be improved by their implementation.
The real crux of this very costly and frustrating situation is that there is no definitive or quantitative means of identifying, absolutely, the potential for this problem before it actually appears in dyed fabric. With all the indicators available to fiber users, none offer the ability to positively warn of a white speck outbreak. For this reason, many spinners, knitters, and dyers will perform sample dyeings on a given lot of yarn, which has been knitted into a small amount of fabric expressly for that purpose.
Since it should be clear at this point that complete avoidance is not possible, then it should also follow that the responsibility for white speck occurrences is very difficult to assign to one party in the production chain. If the conditions are shown to be favorable for white specks to appear, all parties must communicate quickly so that alternative processing and production decisions can be made in a timely manner.
Using known white speck containing fabrics for only bleached whites is one possible recourse. There are also many dyes that do a better, or worse, job of actually covering the problem. Dye selection (and shade choices) alone can prove to be a very effective means of dealing with this serious issue. Caustification of white speck infected fabrics has also shown to be quite successful.
There is no one, single best answer to this very frustrating issue. But, with the understanding and cooperation of all those involved, there may be found some fair compromises that could very well turn an unacceptable situation into one of shared acceptability.
A SIMPLE METHOD FOR DETECTING WHITE SPECK POTENTIAL IN UNDYED COTTON
W. R. Goynes, B. F. Ingber, and P. D. Bel-Berger
USDA, ARS, Southern Regional Research Center
New Orleans, LA
Dyeing imperfections that appear as white specks on cotton fabrics that have been dyed deep shades are a major problem in the textile industry. The presence of these imperfections in raw cotton is not evident since they only show up after dyeing. Processing through fabric dyeing results in both time and product losses when white specks are present. Approaches for eliminating or minimizing the problem include plant breeding, changes in growing and harvesting procedures, and additional finishing during dyeing. None of these provide immediate cost free solutions. A method of screening samples for dye defect potential before processing would allow mills to divert affected cotton batches to non-problem products. In this paper, a simple light microscopy process is described for screening undyed fabrics, yarns, and sliver. This darkfield procedure discriminates between common fiber tangle neps that are not dye resistant, and those that consist of bundles of extremely thin-walled fibers that will not dye.
The nature and effect of undyed white defects in cotton fabrics has been extensively investigated . These defects have been confirmed to be bundles of extremely immature (undeveloped) fibers that come in with ginned cotton, and though some are removed in cleaning processes many are carried through processing to the final fabric, and become apparent on dyeing. Processing can affect apparent size and number of defects. Growing location and conditions can influence the number of defective fiber bundles in a harvested lot. Variety also is responsible for amount of defects in a lot. Breeding programs can possibly decrease the pre-disposition for production of motes, which in turn produces undeveloped fibers. Effects of environmental conditions in open fields are difficult to control.
It is possible to adjust dye formulations for variations in overall bulk maturity, but it is difficult to achieve even dyeing when concentrated areas of undeveloped fibers are present in lots of otherwise average maturity. Therefore, the most immediate solution to the problem would seem to be a system for predetermining presence of large quantities of undyeable materials. If such tests could be developed for incoming lots, then those with high speck potential could be rejected, or at least diverted to non-problem uses. Detection even at the yarn or pre-dyed fabric level would at least prevent use of white speck goods for dark textiles. Therefore, we attempted to develop a method to “see” white specks in undyed cotton.
Materials and Methods
Samples used in the study were from a series of cottons especially grown for white speck studies . They were grown under irrigated conditions in a field in the San Joaquin Valley in California, and included a commercial Delta Upland (DP-90), a Mississippi hybrid (ST-825), and two Acalas (EA-C 30, early maturing; and EA-C 32, a Prema). Each sample was available as bale cotton, sliver, yarn, undyed fabric, and dyed fabric. For microscopical examination, a wide field stereo zoom light microscope equipped with substage darkfield illumination was used. Observations were made at 10-20X magnification. Identified defects were marked, cut from the sample, and prepared for examination at higher magnification using a scanning electron microscope (SEM).
When textile fabrics are examined using surface lighting with a stereo light microscope, it is possible to see the weave of the fabric, outlines of fibers within yarns, and both contamination and fiber defects on the fabric surface. Contamination defects such as seed coat fragments, and leaf and bract materials appear very dark, and can easily be segregated from fiber defects. One type of fiber defect consists of individual fibers that have been tangled with other fibers during processing . These defects do not normally cause dye defects. Another type of fiber defect is caused by bundles of undeveloped fibers. Both of these fiber defects can be seen using surface lighting. However, it is not possible to distinguish them as two different types using surface illumination, so determinations of white speck potential cannot be made from such observations.
If the fabrics are examined using dark field lighting, there is an obvious difference in the two types of fiber defects. Dark field illumination is accomplished using a substage lighting system, and a field stop that blocks the path of the light beam that normally is projected through the sample. This system forms a hollow cone of light that travels around the field stop. A ground glass or filter system can be used to decrease the intensity of the light. With small objects scattered on a clear field, the field appears black, and the sample appears self illuminated because the light observed is that transmitted to the objective lens by the sample itself. Thus the nature of the sample determines the brightness of the object in the observed field. If samples containing fiber defects are examined first using surface illumination to show presence of the defect, then the lighting is switched to darkfield, an immediate differencev can be seen in the fabric image.
Yarns appear with bright edges because they are thinner at the edge, and more light is transmitted. Differences can also be seen in thick and thin yarns because of the amount of light that passes through them. Thick, non-fiber neps (usually plant parts) appear completely dark, and those containing only thin areas of seedcoat may appear gold or orange. Of greater significance, differences can be seen between fiber neps. Tangled fiber neps blend into the yarn and are hardly seen, but defects formed from clumped, undeveloped fibers appear as a shadow on, or in the yarn. The tangled network of the thin-walled fibers can be seen. This difference is subtle, and careful observation is required to become familiar with the differences in appearance. However, switching back and forth between surface and darkfield, subsurface lighting shows the obvious differences in tangled fiber neps and undeveloped fiber neps. To verify that defects identified as undeveloped fiber clumps were actually the same undyeable white defects that were found in dyed fabrics, the defects identified by dark field microscopy were cut from the fabric and examined using scanning electron microscopy.
Results of these examinations showed that all examined defects were composed of undeveloped fiber clumps. Although detection of white speck potential of fabrics is of great significance because it would prevent dyeing of fabrics that would be unusable, detection of these defects at earlier stages of processing would be of even greater value. Therefore, a procedure was devised for examination of yarns using darkfield illumination. Yarns are more difficult to examine at low magnifications than are fabrics. Even in samples of high white speck content, an individual defect may only be found once in a 36 inch length of yarn. Therefore, yarns must be moved rapidly through the viewing field because a large portion of the yarn has no defect. This was accomplished by locating a spindle containing yarn on the left side of the microscope stage, pulling the yarn across the stage so that it was visible through the binoculars, and rolling the examined yarn onto a dowel attached to the right side of the stage. Turning the dowel pulls yarn from the spindle, across the stage and rerolls it onto the dowel while the yarn is being observed. When a defect is detected, that section of the yarn can be clipped and prepared for examination by SEM. As with defects found in fabrics, those cut from yarns and examined by SEM were also identical to white speck defects on dyed fabrics. Similar examinations were made on cotton in the sliver form.
Sliver was flattened and thinned so that light could pass through. A cast aluminum plate with a 2 in2 opening was placed over the sample to maintain the proper location of the examined area. Dark defects detected by darkfield microscopy were removed and examined by SEM and were shown to be the expected bundles of undeveloped fibers.
Of the four specifically grown cotton varieties, the EA-C 32 sample was found to have the highest number of white defects as shown by image analysis, and EA-C 30 the lowest . In developing the darkfield procedure for dye defect surveys, these two samples were compared. Results indicated significantly more defects in the EA-C 32 sample than in the EA-C 30, which is consistent with data from image analysis on dyed fabrics. This method provides a means of determining presence of dye defects in undyed cotton. Fabrics with high and low defect counts can be distinguished. However, standardization of the method would require a sufficient number of samples from different sources to be examined to determine a thresh hold level of defects that would make fiber lots unusable for dark shade dyeing.
1. Defects that appear as white specks on dark cotton fabrics are bundles of undeveloped, extremely thin-walled fibers.
2. Although many fiber-bundle defects can be seen on fabric surfaces using low magnification widefield light microscopes, all fiber defects do not become dye defects. Fiber defects that become white specks and those that do not cannot be distinguished using normal surface lighting.
3. Use of substage, darkfield lighting distinguished potential white speck defects from other fiber bundle defects in undyed samples. White specks defects appeared as dark shadows in fabrics, yarns, and sliver. The nature of the detected defects was verified by removing the defects from the sample and examining in the SEM. All defects detected by darkfield microscopy were confirmed to be thinwalled fiber bundles that cause undyeable defects.
4. Therefore, low magnification darkfield microscopy provides a simple procedure for predetermining white speck potential in undyed cotton samples.
A SIMPLE METHOD FOR DETECTING WHITE SPECK POTENTIAL IN UNDYED COTTON ,W. R. Goynes, B. F. Ingber, and P. D. Bel-Berger, USDA, ARS, Southern Regional Research Center ,New Orleans, LA
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