Continuous Shrinkproofing Process (Superwashing)

            1.         INTRODUCTION

            Numerous processes are available for imparting shrink resistance to wool in all its different forms such as loose stock, tops, piece goods and garments. The overwhelming proportion of the world’s treated wool, however, is processed by various well-known, continuous techniques. The procedures available by which wool can be treated to Superwash levels of machine washability may be classified as:

                        (i). Resin process                      (ii) Oxidative process

            Wool treated by either approach may be used for the production of “Superwash” garments, but, in general, the performance of garments made from resin-treated wool is less dependent on the correct choice of constructional details than is the case of oxidatively-treated wool.

            Presently, more than 95% of Superwash wool is resin-treated and, of this, the majority is produced by one of the continuous techniques.

2.                  CONTINUOUS RESIN TREATMENTS

            Although the Bancora IFP process of Joseph Bancroft achieved some success, the CSIRO chlorine/Hercosett procedure is generally recognized as the first commercially accepted polymer process for the continuous treatment of tops. Early papers describe this system, which initially necessitated the use of a specially designed bowl for the prechlorination part of the process.  Subsequent communications detailed a pad chlorination procedure, but the most significant development which made this technology more widely acceptable to the wool processing industry was the applicability of the process to fairly standard, backwashing equipment.

2.1              The Continuous Chlorine/Hercosett Process

This process comprises the following operations applied sequentially to sliver or tops:

Acid                 Nuetralisation +                        Resin                Softener

ChlorinationàAntichlorination   à RinseàApplicationàApplication

These  are the basic requirements for satisfactory operation of the process, but various degrees of

sophistication are possible.  Indeed, the number and type of backwash bowls employed can vary considerably depending upon the investment available and the productive capacity required.  Some installations, for example, use 5, 6, 7 or 8 suction-drum bowls exclusively, while others utilize a mixture of suction-drum and standard backwash bowls.  Such a situation, far from being an undesirable complication, may be interpreted as a practical indication of the flexibility of the process.

The reactions and processing conditions involved at each stage of processing are conveniently discussed, in sequence, under the headings below.

2.l.l       Acid Chlorination

A pre-treatment based on a low level of chlorination is an essential part of the process.  The first stage, therefore, involves chlorination, usually with a solution of sodium hypochlorite, at a temperature of 15-20 degrees C and at a pH of 1.5-2.0.  The oxidizing properties of hypochlorite solutions are pH dependent with chlorine, hypochlorous acid and hypochlorite ions existing in equilibrium at a given pH value.  A dilute solution of sodium hypochlorite for example, contains essentially the hypochlorous ion OCL- above pH 8.5 and mainly undissociated hypochlorous acid, HOCl at pH 4.5, while at pH 2, free chlorine accounts for approximately 70% of the total active chlorine.

Under the conditions employed in the chlorine/Hercosett process, the bath contains mainly free chlorine and hypochlorous acid, both of which are capable of reacting with the wool.  There is claimed to be a considerable difference, however, between the results obtained by treatment with chlorine and with hypochlorous acid.

Both agents impart shrink resistance and produce the effects required of a pre-treatment for subsequent application of a resin but whereas chlorine is reputedly damaging, hypochlorous acid, on the other hand, is believed to cause only slight surface modification and to leave the scale structure of treated fibres almost intact.  Mains believes that hypochlorous acid is the major active ingredient in acidified hypochlorite solutions used for treating wool and suggests that hypochlorite ion or gaseous or dissolved chlorine molecules function only through the production of hypochlorous acid, i.e.

            CL2 + H20-------à   H+      +     CL-                +     HOCL

Clearly, as the wool reacts with the hypochlorous acid as formed above, the reactant solution becomes depleted and an equilibrium shift will take place to produce more hypochlorous acid molecules.

            CL2   +   H2O ------à     H+     +     CL-     +   HOCL

In  view of the paucity of precise data, therefore, the term chlorination will be used in what follows to cover the treatment of wool with chlorine or with a reactive, chlorine containing chemical.

Under the conditions employed in the first bowl, chlorination occurs preferentially at the surface of the fibre and in such a way that damage and yellowing are minimized almost completely.  The pretreatment must be level across the width and along the length of the web of wool as well as being uniform through the cross-section of individual slivers.  A parallel presentation of 30-40 slivers (each weighing 20-30 g/m) to the chlorinating liquor is important to ensure uniformity of treatment.

The most common procedure for chlorinating in this way is to use a Fleissner suction drum backwasher bowl, in which the liquor is drawn from the bowl, through the web of wool and then out at the ends of the perforated drum, back into the bowl.  The liquor consists of dilute sulphuric acid, sodium hypochlorite and an acid-stable wetting agent.  Quantities of each are pumped continuously into the bowl to provide the correct level of chlorination appropriate to the processing speed and the quality of the wool being treated.

                                                            Linear Processing        

                    Quality         Fiber Fineness          Speeds Possible           Level of Chlorination

 

64’s                 21u                              5 – 7 m/min                  1.9-2.1 %

 

58’s                 26.5u                           7 _ 10 m/min                1.7–1.9 %

 

     Calculated volumes of chemicals are normally supplied to the bowl via accurate, piston-type dosing or metering pumps, and a variation in pH outside the limits 1.4-1.5 seldom occurs.  In some countries, where the ambient temperature exceeds 20-25 degrees C, refrigeration is used to maintain a low bath temperature, in order to prevent colour change in the treated wool, but in most situations, however, no temperature control is required.

 

     During the treatment, the interaction between sulphuric acid and sodium hypochlorite results in an accumulation of various degradation products and soluble salts in the bowl.  These include sodium chloride, sodium sulphate, hydrolysed protein and oils which, in time, can reduce the efficiency of the chlorinating reactants.  To counteract this, the contents of the bowl are either changed routinely (say every 2 hours) or a controlled, additional supply of water is metered into the bowl to effect constant replenishment of the liquor during processing.

 

     The small amount of chlorine gas which accumulates at the surface of the liquor is corrosive, and high-chrome stainless steel is most suitably used in the construction of the bowl. Excess gas is removed routinely by means of a suction hood situated over the first few bowls or by using a system which aspirates the gas directly above the chlorinating liquor.

 

     The two requirements mentioned, corrosion resistance and a suction drum bowl, cannot always be met, often because of the large capital outlay needed.

 

     Specific advantages of a pad chlorination procedure are:

 

(i)                  Faster chlorination is possible, up to linear processing speeds of 14 m/ minute.

(ii)                The procedure is simpler and more flexible.

(iii)               Stoppages during processing are minimal since a breakdown of short duration during chlorination does not necessitate stopping the backwashing bowls used in subsequent stages, if a storage scray is sited between the pad mangle and the bowls.

(iv)              Often slightly whiter wool results.

(v)                To make the basic chlorine/Hercosett technique applicable to a wider range of shrink-proofing/backwasher installations.

 

                 The two most popular chlorine donating compounds used for shrink-resist treating wool are sodium hypochlorite and certain alkali metal salts of dichloroisocyanuric acid (DCCA).  In use these are mixed with acids and wetting agents in, or prior to entering, the pad nip.  Until  recently it has been difficult however to prepare stable, concentrated solutions of DCCA at low pH values owing to the precipitation, below pH 3.5, of cyanuric acid and cyanurates.  A technique developed by SAWRTI utilizes a mixture of a mineral acid and an organic acid such as acetic acid, to stabilize concentrated, low pH solutions of DCCA.  These solutions are stable at pH 1.5-2.5 for a period long enough to allow the continuous chlorination of wool to be carried out in a pad mangle.

                 In trials, this system has afforded excellent results but suffers, unfortunately, from two disadvantages.  First, DCCA is expensive compared to alternative chlorine sources and the relatively high concentration of acetic acid that is required increases further the cost of the pre-treatment.  Secondly, the presence of sodium chloride in mill water reduces the stability of the solution, even if steps are taken to mitigate this.  Some improvement has been achieved by using potassium DCCA rather than sodium DCCA, by using different wetting agents depending on local circumstances and by using low speed stirring to mix the reactants prior to their injection into the pad mangle.  The success of these modifications, however, depends to a large extent on the salt concentrations prevailing in the mill water at the time.

2.1.2          Neutralization and Antichlorination

     After chlorination, by which ever system is used, the residual acid and chlorine must be deactivated by rinsing (optional depending on the availability of a bowl), followed by neutralization and antichlorination.  This treatment is effected in a bowl usually containing sodium carbonate and sodium sulphite or metabisulphite.  When the chlorination reaction is followed by either a bisulphite or a thioglycollate antichlorination treatment, the absorption of resin is increased substantially.  It is also suggested that the affinity of the resin for the pretreated wool is greatest when sodium bisulphate is employed.

     In practice sodium sulphite, sodium bisulphate or a mixture of both are the most popular antichlorinating agents used commercially.  Since sodium bisulphate is converted to sodium sulphite under the alkaline conditions employed for neutralization of the acid, there is probably little to be gained from using bisulphate which is, generally, more expensive.  It is desirable that the temperature of the chlorination liquour should be maintained at approximately 25-30degreesC, with automatic temperature control equipment, to create conditions which will produce a slight bleaching effect.  This will counteract any slight yellowing which, with certain wools, may have occurred in the chlorination bowl.  The pH in this bowl is kept at a value of 8.5-9.5, most conveniently with automatic pH control equipment.

2.1.3          Intermediate Rinsing

     Rinsing after neutralization is very important to prevent the accumulation of sulphite in the resin bath where it retards the rate, and therefore the extent, of the exhaustion of the resin on to pretreated wool.  The effect of sulphite contamination of the resin bowl on treated wool properties such as shrink-resistance and dyeing behaviour is described elsewhere.  The flow of water into the rinse bowl depends on the production rate and is likely to be 500-1,000 1/hr.  Sulphite analysis by titration will confirm whether this is adequate.  A continuous rinse is ensured by supplying the water via a flowmeter and a solenoid valve connected electrically to the machine drive.  In this way, rinsing will occur all the time the backwasher is running.  The processes of chlorination, neutralization plus antichlorination and rinsing may together be called the “pre-treatment”, preceding the application of the resin.  Pretreatment serves various purposes:

(i)                  To create charged sites on the surface of each fibre to which oppositely charged resin molecules are attracted, and with which they may become covalently bonded.

(ii)                To raise the surface tension of the surface of each fibre to a value higher than that of the resin and thus facilitate spreading of the resin during treatment and drying.

(iii)               To provide a low level of shrink-resistance which is enhanced considerably by the    subsequent application of the resin.

2.1.4    Application of Resin

The resin is applied preferably in a suction drum bowl to ensure uniform application through the cross-section of the slivers.  A cationic polyamide epichlorhydrin resin, most usually Hercosett 57, (Hercules Chemical Company) is used.  The bowl contains this resin and sodium bicarbonate and is maintained at a constant temperature in the range 35-40degreesC.  It is undesirable to allow the temperature to fall below 30degreesC or the exhaustion rate of the resin becomes impaired.  During processing, resin is supplied to the bowl at the rate of 2% resin solids on the weight of wool by an accurate metering pump.  The pH of the bath is maintained at 7.5-8.0 by the addition of sodium carbonate solution, via pH control equipment.  Incomplete treatment with the resin, attributable to poor control of the parameters outlined above, is manifested by inferior staining tests (q.v.), inadequate shrink-resistance, hard tops and dyeing difficulties.

2.1.4          Application of Softener

The wool is then passed into the last bowl in which softener is applied.  Suitable products are cationic or nonionic softeners and an add-on of 0.2-0.3% softener solids on weight of wool appears to be adequate for most treated wools.  Besides imparting a marked softening effect, passage of the wool through this bath also removes any slight excess, unbonded resin, thus reducing the development of fibre-fibre bonding in the subsequent drying operation.  Resin migration and the associated phenomenon of fibre-fibre bonding is influenced by the type of softener used and, particularly, by the temperature of the softener bath.  A pH of 7.5 and a bath temperature of 40-45degreesC is recommended to ensure:

(i)                  Maximum statement of liquor from the wool, attributable, possibly, to the reduced viscosity of the hot solution of softener.

(ii)                The bound and absorbed water entering the drier is warm and therefore more easily evaporated.

(iii)               Adequate drying can occur at lower temperature (60-80 degrees C) to minimize overdrying, subsequent processing problems and any possibility of heat yellowing.

(iv)              An interesting feature of some cationic softeners applied at this temperature is that a marked increase in bulk of the treated wool is often apparent after processing.  This may be related to an observation that, in general, cationic softeners facilitate faster uptake of atmospheric moisture by Hercosett-treated wool than similar wool softened with nonionic or anionic products.  Where cationic softeners are employed, however, it is particularly important that wool destined for top dyeing should be pre-scoured prior to the application of dyestuffs, as has been normal practice previously.

 

2.1.5          Drying

 

A high temperature cure during drying is not necessary, but it is important that the wool be not physically damp to the touch after drying.  A regain of 8-12% after treatment is satisfactory, though there is some belief that initial drying, to a level lower than this, may improve the rubbing fastness of treated wool which is subsequently dyed.  In some installations, the treated wool is reconditioned by a second application of softener and a second stage of drying to a higher regain, or by the application of steam or water prior to, or during subsequent processing.

2.2                The KROY/Hercosett Process

2.2.1        Kroy Chlorination

A revolutionary method of chlorinating wool, based on a novel principle of vertical, deep immersion, has been developed by Kroy Unshrinkable Wools Ltd. of Toronto.  The degree and uniformity of chlorination imparted by treatment in the “Deepim” machine is sufficient, in itself, to produce resistance to shrinkage up to “machine washable” standards, or to serve as a pretreatment for the subsequent application of shrinkproofing resins e.g. Hercosett 57, for the attainment of Superwash levels of machine washability.

The machine is a compact, totally enclosed unit occupying little floor space and requires minimum installation modifications and facilities.  Usual additional hardware such as stock tanks, metering pumps, loading valves etc., are not required.

Sliver transport is achieved by two endless, polypropylene-mesh belts which ensure that everything entering the machine is carried through without fibre loss, roller lapping or problems associated with broken ends.  Wool silver or tops ranging in weight from 20-30g/m (or greater) are fed into the machine at a speed governed primarily by subsequent drying capacity.  It is claimed that the machine will chlorinate wool at speeds from 5-20m/min with equal effectiveness over the whole speed range.

The number of slivers entering the machine is immaterial to the effectiveness of the process; from one to forty (or more) can be treated, depending on the sliver weight.

There are only three controls associated with this machine viz., the speed of processing, the flow of water into the machine and the rate of flow of chlorine gas.  The gas and water are mixed in a special injector and hypochlorous acid (the chlorinating agent largely involved in this treatment of wool) is generated accordingly:

CL2    +          H20 -à           HOCL             +          HCL

Hypochlorous acid reacts rapidly with wool, particularly in the acid environment created by the production of hydrochloric acid as outlined above.  A pH of 2.0-2.5 is usual during the chlorination process.  Traditional reagents such as mineral acids, chlorine retarding compounds, stabilizers, inhibitors and wetting agents are not used in this process.

The special feature of the Kroy chlorination procedure is that every fibre in the sliver is chlorinated uniformly, irrespective of fibre diameter, wool type, sliver weight etc.  Moreover, the efficiency of chlorination does not appear to be affected by the presence of oil or twist in the slivers—hitherto major obstacles in the achievement of uniform chlorination when alternative treatments are employed.

The uniformity of treatment is achieved by passing the sliver, in a near vertical manner, into the chlorinating liquor.  The combined effects of capillarity and hydrostatic pressure which develop as slivers are progressively drawn deeply down the bath, ensure that every fibre is surrounded by a layer of liquor containing the chlorinating agent, hypochlorous acid.

Traditionally, chlorination of wool has required a neutralization and antichlorination process involving, usually, sodium carbonate or bicarbonate and sodium sulphite or bisulphate.  In some processes, the presence of sulphite has various advantages.  In the chlorine/Hercosett process it is necessary evil.  Its presence is a potential source of problems because of the possibility of sulphite contamination of the resin bath.

With the Kroy system, a neutralization/antichlorination procedure may not be required, since residual chlorine is undetectable on the treated wool and the pH of the emerging wool is in the region of 4.0-4.5

Special features of the machine are its inbuilt capacity to deal effectively with gaseous and liquid effluents.  Any small amount of unreacted chlorine gas escaping from the reaction zone is collected within the machine for purification and disposal.  This is achieved by drawing the gas through a solution of sodium hydroxide whence the gaseous effluent is “scrubbed” free of chlorine.  When the machine is not in use, for example, during the evening or at weekends, a built-in system automatically switches the fan on periodically to ensure that any accidental leakage of gas from the cylinders is not allowed to build up in the atmosphere to uncomfortable levels.

As outlined above, a by-product of the reaction of chlorine with water is hydrochloric acid.  The overflow, amounting to only a few litres/minute is passed through a filter bed constructed of marble chips where the acidity is neutralized prior to disposal.

2.2.2        Resin Application

The Deepim machine can be utilized in two ways in a chlorine/Hercosett process.  One approach is to use the Kroy machine for pretreatment and follow this with a two or three bowl backwash set for applying resin and softener as outlined previously.

Another approach is to use a simplified machine for applying resin and softener by the principle of deep immersion.  Both techniques are presently undergoing evaluation on a pilot plant and commercial scale and the results look most promising.

Initial trials have shown that linear processing speeds of the order of 15m/min are possible without detriment to the performance or aesthetics of the treated wool.

2.3            Dylan GRC Process

The Dylan GR continuous (GRC) process of Precision Processes (Textiles) Ltd. (PPT) involves the simultaneous treatment of wool with chlorine and a polymer.  The treatment is carried out on a pad shrink-resist/backwash installation, preferably comprising four bowls.  Little published information is available but it is believed that British Patent Specification 1430595 (1976) relates to this process.  In this technique, prechlorination and resin application is carried out simultaneously in a horizontal pad mangle and a backwashing set is used for rinsing, neutralization and softening.

Dylan ‘Auxiliary’ or alternatively sodium or potassium dichloroisocyanurate applied under acid conditions is preferred for chlorination and is used in conjunction with appropriate stabilizers and inhibitors to minimize the precipitation of cyanurates and to prevent reaction between the chlorinating agents and the polymer.

The latter, having a plurality of cationic ionizable sites per molecule is defined as one having the following properties:

(i)                  It is soluble or dispersible in the chosen oxidizing agent solution.

(ii)                It is inert to the oxidizing solution, or if chemical or physical instability develops when the components are mixed together, this instability can be prevented by the addition of suitable stabilizing agents.

(iii)               It is capable of exhausting on to the wool fibres from the aqueous medium.

(iv)              The stability of the polymer solution or dispersion and the ability of the polymer to exhaust on to the wool fibres are not destroyed by the addition of any wetting agent that may be necessary to obtain adequate penetration of the fluid into the sliver.

                       Preferred polymers are Primafloc C7 and Primafloc C5 (Rohm and Haas) but it is suggested that any cationic polyelectrolyte conforming to the criteria listed above will be suitable.

2.4            The SAWTRI/DCCA Aminoplast Resin Treatment for Wool Tops

A technique for chlorinating wool with DCCA followed by the application of an alkylated methylolmelamine resin in the form of an acid colloid, has been developed at SAWTRI.  The results of laboratory and mill trials are reported to be good with no adverse effect on the handle, dyeing or processing performance of the treated wool.

2.5            The USDA Ozone/Hercosett Process

An apparatus has been developed by the Western Regional Research Laboratory of the USDA for the continuous treatment of top.  In this process, dampened wool is pretreated with ozone, generated electrically on site, prior to the application of Hercosett resin by a conventional procedure.  The limited information available suggests that high levels of shrink-resistance can be achieved with this process. The cost of developing large scale equipment of this type and the problems associated with the use of ozone are, however, likely to preclude commercialization of this process.

3.                  FUTURE REQUIREMENTS FOR CONTINUOUS POLYMER PROCESSES

An obvious feature of those continuous shrinkproofing treatments for tops, which have achieved any degree of commercial acceptance, is that an oxidative pretreatment is an essential part of the process.  Whatever oxidizing agent is used and by whatever means it is applied, it is generally recognized that the pretreatment is necessarily degradative.  With adequate control, however, the damage or “modification” can be kept to very low levels and in present techniques, is very much less that that incurred in earlier oxidative processes.  This is attributable to various effects.  One of the major reasons is that a loser level of oxidative pretreatment is required since most of the antifelting effect in a chlorine/resin treatment obtains from the effectiveness of the resin.  Secondly, the application of resin to pretreated wool generally ameliorates any reduction in physical properties associated with the pretreatment, as well as imparting some protection against the hydrolytic effects of boiling dyebaths and enzyme detergents.  It also improves performance characteristics such as abrasion- and pill-resistance.

Nonetheless, researches are actively attempting to obtain shrinkproof effects on tops by resin treatments which do not require an oxidative pretreatment.  With the introduction of more effective, less damaging methods of pretreatment e.g. Kroy chlorination, which can operate at high speeds, the chance of developing replacement resin processes not requiring pretreatment, is becoming more remote.  Some of the requirements of candidate polymers for such an application would include the following:

 

(i)                  The resin must have a sufficiently low surface tension to spread easily over fibre surfaces and be applicable from long or short liquors, and at speeds up to 10-15 m/min.  The applied resin must not be removed from the fibre surface during subsequent wet processing (e.g. rinsing, softening) but must be removed easily from the sliver interstices, so that fibre/fibre bonding in drying is minimized.  Excessive interfibre bonding creates unacceptable difficulties in subsequent gilling and drawing operations and has a drastic effect on fibre length and the handle of the treated top.

(ii)                The shrinkproofing effect must operate by a mechanism other than that of                  fibre/fibre bonding because bonds of this type are destroyed in subsequent processing.

(iii)               Encapsulation of fibres in a sheath of polymer is not sufficient, in itself, to make the wool shrinkproof.  Certain, highly effective, polymers in solvent-batch shrinkproofing treatments have been applied to tops from low surface tension, organic solvents but have failed to effect shrinkproofing when the wool was subsequently gilled.

(iv)              Candidate polymers must not interfere with subsequent processing including gilling, re-combing, drawing, spinning, dyeing, knitting, etc.

(v)                Candidate polymers must possess adequate properties of adhesion to resist removal during processing, abrasion and wear.

                               These observation suggest that an oxidative pretreatment is a necessary operation for a successful top treatment.

4.                  OXIDATIVE PROCESSES

Two possibly similar systems are currently in use on a small scale for the shrinkproofing of wool to Superwash levels by oxidative treatment.  These are the Dylan FTC process of PPT Ltd., and the Sitt treatment of Tintoria Sitt (Prato) Italy. BP 1,475,367 describes what is believed to be the basis of the FTC process.  According to the patentees, tops are chlorinated in phosphate-buffered, near neutral solutions of sodium hypochlorite, in a horizontal pad mangle.  Particular attention is paid to the selection of the wetting agent used in this process.  Following padding, the wool is subjected to an “acid shock” followed by rinsing, antichlorination, rinsing and softening.

Presently, no details of the Sitt treatment are available, but it is thought to be substantially similar to the Dylan FTC process.

5.                  PROCESS CONTROL OF SHRINKPROOFING TREATMENTS

Quality control during processing is simple and reliable and involves taking samples during various stages of the treatment and conducting the following subjective tests:

STAINING TEST:     Samples of wool are obtained from the bulk after pretreatment and resin application and immersed in cold solutions of dyestuffs.  A visual assessment of the depth of staining correlates well with the level and uniformity of treatment.

HAND WASH TESTS TO ASSESS SHRINK-RESIST PERFORMANCE:  A simple hand wash test of treated silver, followed by an assessment of the ease with which fibre can be separated and removed from the mass of fibre, provides a good indication of the treatment’s effectiveness.

QUANTITATIVE TESTS:  A quantitative measure of the shrink resistance of the treated wool is obviously required and various test methods are available.  Most suffer from the disadvantage of being time-consuming.  In some cases 4 or 5 hours are required, in which time or course, a considerable quantity of wool can be processed.  The ideal test, therefore, should be quick, easy and meaningful, not involve expensive or sophisticated equipment, use equipment widely available and be cheap to carry out in terms of labour, time and material.

Most wash tests are now performed in an International Cubex machine in which samples are washed for times varying from one to three hours. The  samples may be sliver, plaited sliver, coarse knitted fabric or single jersey fabric. More detailed information will be published in part IV of this review.

6.                  SUBSEQUENT PROCESSING

Following shrinkproofing with any of the treatments detailed above, it is usual to apply between 0.5-1.0% of a suitable, compatible, water soluble lubricant (or a mixture of lubricants) to the treated wool.  It is important to ensure that selected products have been evaluated for their compatibility with shrinkproofed wool and in general, products which are readily removed during wet processing and which do not contain mineral oils, are satisfactory.  Certain oils drastically reduce the effectiveness of the shrink proofing treatment. And most manufacturers of spinning aids are now aware that their untested products require thorough evaluation before being recommended for application to shrinkproofed wool.

Contamination of the shrinkproofed wool by untreated fibre must also be avoided at all stages processing following treatment, since it has been shown that an admixture of less that 1% untreated wool can seriously impair the surface appearance of treated samples following washing.  Fabrics manufactured from contaminated yarn exhibit a phenomenon called “spot felting”.  As its worst, this is manifested by a boucle-like appearance and when less severe, by small spots or felted areas which are detectable on the surface of a washed sample which is otherwise, often acceptable as far as overall shrink resistance is concerned.

A similar phenomenon is observed also in washed samples made from treated wool containing a minor proportion of inadequately, or incompletely-treated fibre.  In this case, the level of contamination which results in the development of spot felting cannot be ascertained precisely because of the complex relationship which is likely to exist between the proportion of incorrectly processed wool in the blend, the nature of the inadequacy and the surface appearance of samples after washing.

In ecru wool, it is sometimes possible to distinguish between spot felting associated with contamination by untreated wool from that caused by the presence of incorrectly-treated fibre.  This can be achieved by using various staining tests in conjunction with a light microscope examination.  Unfortunately, with dyed material, there is, at present, no acceptable diagnostic procedure.

The increasing use of automatic control, however, is reducing the possibility of defective processing to very low levels indeed and some of the equipment now available is particularly well suited to continuously shrink proofing operations.