Appendix A Problems With Fiber Reactive Dyeings Not Repeating - .A Look at Five Typical Cases

The first part of this article (Mar. 8; ADR) presented some fundamenta aspects of dyeing quality control ir general. This part (11) present: specific details of controlling batct jye processes. Critical quality con lrol parapeters are discussed foi tarious dye classes, equipment xocesses, and substrates. Also Yiagnostic tests and repair pro :edures are presented. The final pari )f this article will concentrate on :ontinuous dyeing. ntroduction Many different classes of dyes are applied to various substrates by batch methods in mill-specific situa. tions using a wide variety of equip ment. It would be impossible to review each and every commercially important combination of circumstances in depth; therefore, informa. tion presented herein is intended primarily as a guide for troubleshooting in a variety of bath dyeing production situations. Dyeing defects Various types of defects may occur during batch dyeing and particular combinations of processes, equipment, dyes or substrates may be more or less susceptible to specific types of defects. Most production dyers cite various defects, such as those listed in Table I, as their major problem areas. However, :here is no question that the major iroblem which occurs in. batch dyetditor's Notes-Third in a series of len articles to be prepared by Dr. Brent Smith on the general theme, "Troubleshooting In Wet Processing". ing i s poor shade repeats (off shade dye i n g s). In addition to production which gets out of the dyehouse with an unacceptable shade, poor shade repeats actually are the underlying cause of a substantial portion of physical damages, uneven dyeings, and foreign deposits. These defects frequently occur when a shade does not repeat properly and requires corrective action such as dye or chemical adds, extra run time, boiling down, stripping, redyeing, andlor overd yeing . Correct ivelre pai r procedures require extra time and processing; hence, the risk of physical damage is greater. Practices such as stripping or adds increase the risk of uneven dyeing and bath instability. Consideration of dyeing economics and losses associated with dyeing defects must take into account the relative value of the substrate (frequently several dollars per pound), versus the actual labor, overhead, dye, and chemical cost for dyeing (usually less than $0.50 per pound for most shades). Thus, a relatively large savings in dye cost can be quickly offset by even a minor increase in defect level and the associated losses such as the loss of expensive substrate. For example, a 10% cost reduction on a dye cost of $0.25 per pound would be offset by an increase of only 1% off-quality on a $2.50 substrate. In addition, that 1% off quality might be reworked or redyed by a costly procedure (causing additional economic loss) which has a higher risk and lower chance to produce acceptable product than first-run dyeings. Thus the dyer's priority must be to avoid producing defects. To avoid defects, the first step must be to pro. duce consistent shade repeats. There is a very widespread impres sion that avoiding defects, and espe. cially improving shade repeats, is a simple matter of discipline. Although supervision, good maintenance, and proper operation of equipment are important, it is also very important to put quality (not cost reduction and short cuts) as the number one priority, Management pressure to reduce cost or increase production beyond reasonable limits (for example, by overloading eq u i pmen 1) is frequently counterproductive. Dye selection The exhaust dyer has a wide variety of colorants from which to select his dye recipe. Many considerations enter into selection of dye class, subclass, and specific dyestuffs. A typical selection protocol is shown in Table II. Sometimes special situations such as unusual blend, substrates, constructions, or production volume arise which require complete review of the entire formulation protocol. In these cases, it is important to make a careful evaluation and 7ot to treat the situation as "routine." To avoid trouble, these situations nust be identified in advance and iandled properly in terms of equipnent, process and dye selection, quality control requirements, cost analysis, and expectations for offquality and re-working. Using a cer,ain set of dyes, procedures, cost analysis, etc., from habit is sure to ead, sooner or later, to trouble. Examples of such situations are rot difficult to find. One example vhich frequently occurs is the design I f difficult-to-dye blends, especially hree fiber blends, by stylists who do lot realize the difficulties involved. 'he dyeing behavior of many binary