Quality Assessment of Textiles

Inappropriate treatment of textiles during production and use can cause chemical, mechanical and thermal damage or damage due to microorganisms; each has different effects and greatly reduces the serviceability of the textiles. Usually the cause of damage cannot be determined by purely visual evaluation. Textile microscopy, in contrast, often provides exact results within a very short time, which can be decisive in the correction of future production.

Wool fibers have an extremely complicated structure and characteristic scales, Fig. 26, by which they can be distinguished from other fibers. Apart from this, structural abnormalities, i. e. damage, can be detected quite easily under the microscope. Staining reactions with Cotton Blue-lactophenol [11], Neocarmin W (Fesago, Heidelberg) or Pauly reagent [12] simplify the recognition of wool damage. With Cotton Blue-lactophenol, damaged wool fibers are dyed blue, Fig. 27. With Neocarmin W — a dye solution used to distinguish various fibrous materials — undamaged wool is dyed yellow and damaged wool orange. It is very easy to perform this staining reaction. The sample is placed into the dye solution for 5 minutes at room temperature and then rinsed. However, slight damage is either not indicated or only indistinctly. The reagent responds best to badly alkaline-damaged wool, Fig. 28.

During textile finishing, fiber damage is often caused by abrasion [31], resulting e. g. in grayed spots and light streaks. This damage is often only recognized in a later finishing stage when large quantities of material have already been damaged. Natural fibers and cellulose regenerated fibers are more susceptible to mechanical damage than synthetics; however, synthetics also suffer damage.

A typical characteristic of synthetic fibers is their thermoplasticity. In order to provide the material with optimal use characteristics, heat treatment in the thermoplastic range cannot be avoided during manufacturing, finishing and garment production. Irreversible damage can only be prevented if the softening range of the fiber polymers is considered thoroughly. In addition, synthetics can suffer irreversible deformation during all processes which generate friction heat. Pressure or impact, causing a temperature increase, can also lead to the deformation of synthetics [36, 37].

Differences in the yarn count, yarn twist and plytwist create different yarn volumes. They are generally referred to as yarn differences and are noticeable as streaks or bars parallel to the threads in woven fabrics and knitwear. They must be attributed to purely optical effects. During examination in reflected light they are caused by the fact that adjacent, more voluminous yarns reflect light to a greater extent. The observer thus gains the impression that there are lighter streaks in these areas than in those with larger gaps between the threads. Here, light is reflected to a lesser extent, thus giving the impression of a darker dyeing or a darker color streak. In transmitted light, conditions are exactly the opposite. This is schematically illustrated in Fig. 173 and 174. However, luster differences between more or less intensively twisted yarns can lead to differences in lightness [44]. There are several possibilities for macroscopic and/or microscopic detection of yarn differences:

Tight threads cause a wavy and/or blister-shaped contraction of the fabric [48]. In addition, they can lead to a more or less distinct streak formation. Greater tension of individual weft yarns or warp threads forces the neighboring, slightly stretched threads to be incorporated into the fabric in waves.

Oils, greases and waxes (e.g. from spin finishes, lubricants, coning oils, sizing waxes and loom oils) which are not removed before dyeing can cause reserving and, as with sizing residues, lead to dyeing unlevelness. Precipitated dye or undissolved dye particles cause dye stains. Inappropriate finishes lead to the formation of chalky marks when the fabric is scratched. Oligomer and lime deposits result in graying and light stains on dyed and printed fabric.

Chemical, mechanical and thermal damage, described in the preceeding chapters, and biological damage that will be dealt with in chapter 9, has recurring causes which (as experience shows) can almost certainly be detected by microscopic examination. In practice, other defects occur which can be caused by various chance events. To determine the causes of these defects in finished textiles, considerable experience, a “criminalistic” intuition and thorough examination of the company-internal conditions are necessary. In these cases, the sense of achievement of the textile microscopist is especially significant. Examples will demonstrate some practical problems encountered.

All natural fibers can be damaged by microbial attack; the fibers themselves serve as a nutrient substrate for microorganisms. In the case of synthetic fibers, the lubricants, sizings, softening agents and finishes used during textile manufacturing assume this function [51]. Favorable preconditions for growth of microorganisms are particularly created by the combination of heat and high atmospheric humidity during storage in still air. During summertime, the hazard of damage by microorganisms is very high in sizing rooms, dyehouses and finishing plants because high room temperatures and high relative humidity prevail in the wet areas. Textiles with an excessive moisture content are frequently packaged in sheets or plastic bags. On the other hand, bottlenecks occuring at drying aggregates in the finishing plants often account for the fact that pieces which are spin-damp or improperly dried must be left over the weekend or the works holidays [52].

Poultry feathers give very good protection against the cold. At the present time, they are not only used as filling material for bedding but are also highly sought after as padding material for sleeping bags and garments such as sportswear for mountain climbers, anoraks, padded jackets and coats. These uses also raise the question of care treatments for this material. For example, pillows filled with poultry feathers are subjected to a so-called disinfectant laundering at 95°C when used in hospitals. In this case special finishes and, above all, suitable detergents and laundering processes are necessary in order to avoid drastic damage to the natural product.