Phosphonic acid Dyes

As in the case of chlorotriazines and vinylsulphones, dyes containing phosphonic acid groups had been prepared long before their utility as reactive dyes for cellulose was recognised [4]. The realisation that phosphonic acid derivatives could, under certain circumstances, react with alcohols to give phosphonate mono-esters was made in 1973 at the Stanford Research Institute. Subsequent development work by Burlington Industries and later ICI led to the commercial launch of the Procion T (ICI, now Zeneca) range of dyes in 1977. These dyes were unusual in being applied to cellulosic and blend fabrics under mildly acidic conditions (pH 5–6). This made them particularly suitable for polyester/cotton dyeing by the pad–dry–thermosol process, since the alkaline conditions normally required to fix conventional reactive dyes could be avoided.

The structures of many of these dyes were based on the versatile intermediate 3-aminophenylphosphonic acid attached to typical monoazo chromogens in various ways. One example was the 1:2 cobalt complex Procion Rubine T- 6BD Figure. These dyes were sold as aqueous solutions of the ammonium salts, which facilitated thermal dissociation to generate the free acid form. This in turn reacted with a carbodiimide, e.g. dicyandiamide, by one of two possible mechanisms, either or both of which may be operative. One mechanism involves formation of an O-acylisourea derivative of the dye phosphonate and the carbodiimide, the other proceeds via a dye phosphonic anhydride derivative. Dye fixation can occur by reaction of either of these derivatives with the hydroxy groups of cellulose.

More recently, the role of the carbodiimide in the dyeing process has been examined and it was concluded that the loss of this auxiliary by thermal decomposition was the most important factor limiting dye fixation. Later it was shown that fixation yields as high as 95% could be achieved by minimizing the decomposition of the carbodiimide.

A major attraction of the phosphonic acid dyes was the exceptional stability of the dye–fibre bonds, more stable than those from chlorotriazine dyes in acidic media and those from vinylsulphone dyes under alkaline conditions. Technical drawbacks of these first acid-fixing reactive dyes, however, included dye migration during drying, especially on heavy fabrics such as corduroy, and strength loss of the cellulosic fibres during thermofixation under the acidic conditions necessary for fixation. Consequently, the Procion T (ICI, now Zeneca) dyes were withdrawn in 1987, essentially for economic reasons.

Sulphatoethyl-sulphone and -sulphonamide Dyes

The Remazol (HOE) vinylsulphone dyes, containing the characteristic 2- sulphatoethylsulphonyl precursor grouping, are intermediate in reactivity between the high-reactivity heterocyclic systems, such as dichlorotriazine or difluoropyrimidine, and the low-reactivity ranges, such as aminochlorotriazine or trichloropyrimidine. Exhaust dyeing temperatures between 40 and 60°C may be chosen, depending on pH, since caustic soda is often selected to bring about alkaline hydrolysis of the precursor sulphate ester. These dyes are applicable by a wide variety of batchwise and continuous processes. The substantivity of many of these dyes is markedly lower than that of typical haloheterocyclic dyes. Not only has the vinylsulphone group, unlike the heterocyclic ring systems, little if any inherent affinity for cellulose, but the terminal sulphato group enhances the aqueous solubility of the precursor form before 1,2-elimination to the vinylsulphone. In contrast to the haloheterocyclic systems, the dye–fibre bonds formed by the vinylsulphone dyes are at their weakest under alkaline conditions.

For three decades the two most widely used reactive dyes have been those illustrated in Figure : Remazol Black B and Remazol Brilliant Blue R. The four solubilising groups in the precursor form of CI Reactive Black 5 confer high solubility but unusually low substantivity. It is a nearly symmetrical bis(sulphatoethylsulphone) structure and as these precursor groups lose their ionic charge by 1,2-elimination, the substantivity for cellulose is enhanced and the bis(vinylsulphone) structure formed shows excellent fixation efficiency under alkaline conditions. After fixation the inherently low substantivity of the unfixed bis(hydroxyethylsulphone) dye makes washing-off easy in a region of the colour gamut where this is often notoriously difficult.

CI Reactive Blue 19

The extremely attractive bright blue hue combined with excellent light fastness of CI Reactive Blue 19 could not be challenged by other reactive blue dyes for many years. The aqueous solubility of this dye is inherently low and depends on the zwitterionic 1-amino-2-sulpho grouping after 1,2-elimination of the sulphate ester has taken place. This has led to poor reproducibility and levelling problems, but nevertheless this dye has remained second only to Black 5 in terms of market share amongst reactive dyes.

In a recent investigation of the effect of low-frequency ultrasonic waves on the stability of the copper-complex phenylazo H acid dye Remazol Brilliant Violet 5R (Figure 4.8), reaction rates were recorded for the 1,2-elimination of the sulphato group to form the vinylsulphone and for hydrolysis of the latter to form the 2-hydroxyethylsulphone.

Certain members of the Hoechst range are designated Remazol D brands and these contain a 2-sulphatoethylsulphonamide precursor grouping formed by reacting the dye base with carbyl sulphate . As in the case of the conventional vinylsulphone types, 1,2-elimination occurs under alkaline conditions to give the reactive vinylsulphonamide group and this is capable of either reaction with cellulose or hydrolysis to the hydroxyethylsulphonamide according to the usual nucleophilic addition mechanism.

Sustaining the growth of RMG sector

For a few months we have been receiving hardly any good news. Amidst the prevailing dread, our garment sector continues to enliven our hopes. Despite the political turmoil, the baggage of tragic Rana Plaza and Tazreen incidents, followed by prolonged workers' strike, garments exports performed significantly well with about 21 per cent increase in July-November this year over the performance in the same period last year. It was even 5.0 per cent higher than the target. This undeniably deserves a big applause! The cost of transportation to Chittagong port through roadways has mounted 600 to 700 times during the current political crisis, and therefore the reliance on air cargo has gone up sharply.

Bangladesh had excellence in textile industry for thousands of years. The country used to produce the finest fabric of the world called muslin. The present textile and garment making in the country, though not a continuation of the past legacy because of dynamic technological changes in manufacturing, product development and marketing, has placed the country at the forefront of the global industry. Currently, Bangladesh is the second largest exporter of readymade garments. Figures below present a view of the performance of the RMG sector in recent times as well as that of other important products:

Total exports have also gone up during this period, averaging 18 per cent change compared to that of the previous year. It also exceeded the overall export growth target by about three percentage points. The change in November 2013 over November 2012 was even higher, more than 25 per cent. Knitwear topped in terms of export volume ($4.9 billion) followed by woven garments ($4.75 billion). The other notable drivers of export growth were frozen food (mainly shrimp), vegetables, pharmaceuticals, leather, footwear and engineering equipment. On the other hand, some major products like petroleum bi-products, jute & jute goods, iron & steel, copper wire and electronic products witnessed big decline.

While this five-month statistics reveals cheerfulness in the external sector, a relevant and perhaps the most crucial question is whether this performance is sustainable for the rest of this fiscal. The World Bank has already cautioned with a lower predicted growth rate due to political chaos in the election period. However, this performance was achieved in the period of low implementation of Annual Development Programme compared to the previous year and low imports that helped boost forex reserve exceed $18 billion.

Recent news reports reveal that some buyers have shifted their purchase orders to India in December, one amounting to half a billion dollars given the prevalent circumstances. It gives a worrisome indication to the RMG exports that may decline in the coming months. The Bangladesh Garment Manufacturers and Exporters Association (BJMEA) has also forecast that the sector may experience a decline in the early months of the coming year.

Now we are indeed in a paradoxical situation: impressive achievement despite institutional, governance, infrastructural and democratic deficit. The country has come a long way overcoming many hurdles that could have turned the economy otherwise. What mystery underlies this spirit? A tentative answer could be the so-called 'intrinsic strength' of the economy. This would also lead to develop a theory of 'camel economy' that can store water to walk miles in the desert.

Nevertheless, no matter what would happen in the coming days, all we can do is offer best wishes for the manufacturing sector that contributed more than 95 per cent to total exports in the first five months of this fiscal. The factory owners have been showing their highest tolerance by continuing operation despite manifold uncertainties.

Now, the onus is on the public sector, especially the public exchequer, to relax the fiscal regulations so that the economic activities can get some respite. The NBR (National Board of Revenue) data show that nearly half of the anticipated income taxpayers have not yet submitted returns. Experts already predicted that the revenue would fall much short of the target. Given this awkward situation, the government must understand that this is not the time to provide cash incentive that would run the risk of opening a black hole. Rather, it can design a macro-prudent policy package to fuel the economy, especially the manufacturing sector keeping in mind its importance in exports and employment.

 At the moment it is the external sector, especially exports and workers' remittance, which seems to be the only ray of hope.

Inflation has not yet triggered despite large disruption in the supply chain. With uncertainty hovering threateningly over the economy, it is high time the scenario shifted to normalcy.

Published in: The Finacial Express
Writer: Mahfuj Kabir

               The writer is senior research                          

               Fellow, BIIS, Dhaka. mahfuzkabir@yahoo.com

Aminofluoro-s-triazine Dyes

A fluorine atom is used as the leaving group in the Cibacron F (CGY) range of 2- amino-4-fluoro-s-triazine dyes. The greater electronegativity of fluorine compared with chlorine results in a markedly higher level of reactivity for these dyes than for the 2-amino-4-chloro analogues. The substantivity and solubility of the dye structures can be modified considerably by careful introduction of appropriate substituents on the chromogen, the uncoloured arylamine and the bridging NH groups.

Re-activity: Moderate
Exhaust dyeing temperature: 40-60 C

Cl Reactive Red 3 or, Aminochloro-s-triazine Dyes

Reaction of a dichloro-s-triazine dye with an amine at about 25–40°C  produces a much less reactive 2-amino-4-chloro derivative as exemplified by CI Reactive Red 3. More energetic reaction conditions, typically 80°C and pH 11 for batchwise application, are necessary for efficient fixation on cellulosic fibres. Early studies of the relationships between structure and substantivity of aminochlorotriazine dyes revealed that the NH bridging groups linking the chromogen and the uncoloured arylamino substituent to the heterocyclic ring had marked effects on the solubility and dyeing properties of the dyes. Replacement of the simple NH imino group by an N-methylimino bridge tended to lower the substantivity for cellulose. The use of a sulphonated arylamine to form the uncoloured 2-arylamino substituent of a monochlorotriazine dye was helpful to enhance solubility and modify the dyeing behaviour.

DICHLORO-S-TRIAZINE DYES

In a typical dye of this type, such as CI Reactive Red 1, the partial polarisation of the ring C=N and C–Cl bonds, because of the greater electronegativity of the N and Cl atoms, makes the 2- and 4-chloro substituents susceptible to nucleophilic displacement, although these influences are muted by feedback of electrons from the NH bridging group linking the triazine ring to the henylazonaphthyl chromophore. Such dyes are stable in neutral solution at ambient temperature but subject to hydrolytic attack by hydroxide ions at alkaline pH and to autocatalytic hydrolysis under acidic conditions. To guard against this, a buffer is added to the solution to ensure stability during isolation and further buffer is added to the dyestuff paste before drying.

CI Reactive Red 1(Dichlorotriazine)

The dichlorotriazine dyes are highly reactive and can be readily fixed to cellulosic materials by pad–batch dyeing at ambient temperature or by batchwise methods at 30–40°C. This means that relatively small chromogens are preferred to ensure adequate mobility of dye on the fibre during the exhaustion stage. This requirement makes these dyes eminently suitable for bright dyeings but less satisfactory for deep tertiary hues, since the larger-size chromogens used for this purpose often fail to give acceptable performance by low-temperature application. A weakness with certain dichlorotriazine dyes, particularly red dyes based on H acid as coupling component, such as CI Reactive Red 1, is that under conditions of low pH the dye–fibre bond is broken by acid-catalysed hydrolysis, leading to deficiencies in fastness to washing or acid perspiration.

When partial hydrolysis occurs to form the 2-chloro-4- hydroxy-s-triazinylamino

species, the dye does not have a further chance to achieve fixation via the remaining chlorine atom. Under the alkaline conditions of the fixation stage, ionisation of the acidic 4-hydroxy substituent leads to a massive feedback of negativity into the triazine ring, causing total deactivation of the remaining 2-chloro substituent.
Controlled reaction of dichlorotriazine dyes with either amines or alcohols leads to two further classes of monofunctional dyes, the 2-amino-4-chloro- and 2-alkoxy- 4-chloro-triazines respectively. The latter are more reactive than the former but less reactive than the parent dichlorotriazine types. They are now only of historical interest, the 2-isopropoxy-4-chloro system forming the basis of the Cibacron Pront (CGY) range for printing. The bulky isopropoxy group was chosen in order to disrupt the planarity of the substituted triazine system and thus favour removal of unfixed dye from the printed fabric during the washing-off stage.
In a recent investigation, the relative reactivities of model 2-alkoxy-4- chlorotriazine dyes were compared. Surprisingly, the hydroxide ion preferentially displaced the alkoxy group rather than the chloro substituent. Increasing the size and electron-donating capacity of the alkoxy group resulted in a decreasing propensity for substitution, so that displacement of methoxide ion was 12 times faster than displacement of isopropoxide ion.
Reactivity: High
Exhaust dyeing temperature: which dyes is more  PH  will be less amount of used 8 or, 9 maximum 11 used.

MONOFUNCTIONAL REACTIVE DYE

Monofunctional systems:

Some of the most important monofunctional reactive systems contain only one possible reactive centre, such as the halogeno substituent in the aminohalotriazine dyes, or the activated terminal carbon atom in the vinylsulphone system. In others there are two equivalent replaceable halogeno substituents, as in the dichlorotriazine, difluoropyrimidine or dichloroquinoxaline heterocyclic ring systems. When one of these halogen atoms is displaced by reaction or hydrolysis, as in Scheme 4.2 for example, the reactivity of the remaining halogeno substituent is greatly decreased by the presence of the new hydroxy or cellulosyl substituent.

Dichloro-s-triazine dyes:

In a typical dye of this type, such as CI Reactive Red 1, the partial polarisation of the ring C=N and C–Cl bonds, because of the greater electronegativity of the N and Cl atoms, makes the 2- and 4-chloro substituents susceptible to nucleophilic displacement, although these influences are muted by feedback of electrons from the NH bridging group linking the triazine ring to the phenylazonaphthyl chromophore. Such dyes are stable in neutral solution at ambient temperature but subject to hydrolytic attack by hydroxide ions at alkaline pH and to autocatalytic hydrolysis under acidic conditions. To guard against this, a buffer is added to the solution to ensure stability during isolation and further buffer is added to the dyestuff paste before drying.

The dichlorotriazine dyes are highly reactive and can be readily fixed to cellulosic materials by pad–batch dyeing at ambient temperature or by batchwise methods at 30–40°C. This means that relatively small chromogens are preferred to ensure adequate mobility of dye on the fibre during the exhaustion stage. This requirement makes these dyes eminently suitable for bright dyeings but less satisfactory for deep tertiary hues, since the larger-size chromogens used for this purpose often fail to give acceptable performance by low-temperature application. A weakness with certain dichlorotriazine dyes, particularly red dyes based on H acid as coupling component, such as CI Reactive Red 1, is that under conditions of low pH the dye–fibre bond is broken by acid-catalysed hydrolysis, leading to deficiencies in fastness to washing or acid perspiration.

When partial hydrolysis occurs as in Scheme 4.2 to form the2-chloro-4- hydroxy-s-triazinylamino species, the dye does not have a further chance to achieve fixation via the remaining chlorine atom. Under the alkaline conditions of the fixation stage, ionisation of the acidic 4-hydroxy substituent leads to amassive feedback of negativity into the triazine ring, causing total deactivation of

the remaining 2-chloro substituent.

Controlled reaction of dichlorotriazine dyes with either amines or alcohols leads to two further classes of monofunctional dyes, the 2-amino-4-chloro- and 2-alkoxy- 4-chloro-triazines respectively. The latter are more reactive than the former but less reactive than the parent dichlorotriazine types. They are now only of historical

interest, the 2-isopropoxy-4-chloro system forming the basis of the Cibacron Pront

(CGY) range for printing. The bulky isopropoxy group was chosen in order to

disrupt the planarity of the substituted triazine system and thus favour removal of

unfixed dye from the printed fabric during the washing-off stage.

In a recent investigation, the relative reactivities of model 2-alkoxy-4-

chlorotriazine dyes were compared. Surprisingly, the hydroxide ion preferentially

displaced the alkoxy group rather than the chloro substituent. Increasing the size

and electron-donating capacity of the alkoxy group resulted in a decreasing

propensity for substitution, so that displacement of methoxide ion was 12 times

faster than displacement of isopropoxide ion [11].

FACTORS ASSOCIATED WITH FUNCTIONAL GROUP ATTACHMENT TO THE CHROMOGEN AND IMPORTANT REACTIVE SYSTEMS WITH TYPICAL BRAND NAME

Many reagents can be used to acylate cellulose when it is partially ionised under alkaline conditions but in the production of reactive dyes of commercial interest numerous factors other than the chemistry of such reactions have to be taken into account. Some of the most important are the following:

1. Economy – any reactive system selected as the basis of a range of dyes must enable them to be produced at acceptable cost.
2. Availability – the system selected must be free from patent restrictions, health hazards or other limitations to exploitation.
3. Facility – it must be possible to attach the reactive system to the dye chromophoric groupings readily in manufacture.
4. Storage stability – the dye containing the reactive groups must be stable to storage under ambient conditions.
5. Efficiency – the yield in manufacture of the reactive dye must be efficient and the dye fixation must be high under conventional conditions of application.
6. Bond stability – the dye–fibre bond must be reasonable stable under severe conditions of washing and durable finishing.

Only a few reactive systems (Table) have met these requirements sufficiently well to become commercially established in a significant segment of the market for reactive dyes on cellulosic fibres. In addition to these important types, several others have been marketed as alternative ranges that have failed to maintain a foothold in the market-place, or as individual members of established ranges where they show reactivity characteristics similar to one of the more important reactive systems. These systems of relatively minor significance include: 2-alkoxy- 4-chloro-s-triazine, 2,4-dichloropyrimidine-5-carbonylamino, 5-cyano-2,4-dichloropyrimidine, 5-chloro-2-fluoro-4-methylpyrimidine, 5-chloro-4-methyl-2- methylsulphonylpyrimidine, 1,4-dichloropyridazine-5-carbonylamino, 2-chlorobenzothiazole- 6-sulphonylamino, sulphatoethylsulphamoyl and sulphatopropionylamino.

During the early years of development of reactive dyes it was soon recognised that the important reactive systems could be classified into two distinct categories, depending on the mechanism of formation of the dye–fibre bond and the stability of this bond to subsequent treatments [8,10]. Those based on nitrogen-containing

System	Typical brand name
Monofunctional
Dichlorotriazine	Procion MX (Zeneca)
Aminochlorotriazine	Procion H (Zeneca)
Aminofluorotriazine	Cibacron F (CGY)
Trichloropyrimidine	Drimarene X (S)
Chlorodifluoropyrimidine	Drimarene K (S)
Dichloroquinoxaline	Levafix E (BAY)
Sulphatoethylsulphone	Remazol (HOE)
Sulphatoethylsulphonamide	Remazol D (HOE)
Bifunctional
Bis(aminochlorotriazine)	Procion H-E (Zeneca)
Bis(aminonicotinotriazine)	Kayacelon React (KYK)
Aminochlorotriazine-sulphatoethylsulphone	Sumifix Supra (NSK)
Aminofluorotriazine-sulphatoethylsulphone	Cibacron C (CGY)

REACTIVE DYES: HISTORICAL BACKGROUND AND SHORT DISCUSSION

It was not until the early 1950s, following the marketing in 1952 by Hoechst of two Remalan (HOE) vinylsulphone dyes capable of reacting with wool, that ICI was successful in devising a reactive dyeing process that enabled cellulose to be dyed with a trichromatic mixture of dyes under practical conditions. Cotton fabric was pretreated with alkali and dried before immersion in a solution of the highly reactive dichlorotriazine dyes. Various refinements of the process were necessary (adding salt to enhance substantivity, lowering the pH and buffering the dyebath to minimise dye hydrolysis) before these novel Procion (ICI, now Zeneca) dyes could be marketed in 1956.

Exploitation of the dichlorotriazine reactive system soon led to parallel development of the much less reactive monochlorotriazine dyes, readily made by a substitution reaction between an arylamine and the dichlorotriazine precursor. More stable padding liquors could be prepared using the aminochlorotriazine types and the range of reactivities offered by these two classes of dyes in combination with various alkalis greatly extended the scope of novel continuous dyeing methods for them.

At this stage, however, the limitations of continuous dyeing requirements became a temporary constraint on the adoption of reactive dyeing and attention turned to the development of batchwise methods. It was quickly demonstrated that optimal temperatures of dyeing should be sought (40°C or lower for dichlorotriazines, 70°C or higher for aminochlorotriazine dyes).

The major breakthrough came when it was realised that a neutral exhaustion in salt solution to achieve moderate uptake should precede the alkali addition to promote further exhaustion at a controlled rate determined by the dye–fibre reaction that proceeds at an optimal alkaline pH and temperature.

Over the decades since the commercial introduction of reactive dyes, their use has grown steadily rather than spectacularly. When they first appeared it was predicted that reactive dyes would largely replace azoic combinations, direct dyes and sulphur dyes, displace vat dyes from outlets where fastness to bleaching was not essential and eventually dominate the dyeing of cellulosic materials. This did not occur and even in the relatively sophisticated markets they do not account for more than 30% of all dyes consumed on cellulose. World-wide, the traditional uses of direct and sulphur dyes on woven cotton fabrics remain dominant and reactive dyes only account for about 10% of total consumption on this basis of comparison. In the USA, however, where vat dyes have been used preferentially for fast-dyed cottons, there has been a more gradual trend in favour of reactive dyes. Greater demand for brilliant hues, a shift towards cotton and cotton-rich blends for apparel and a greater prevalence of short-run lots in the US textile industry have all contributed to this trend.

Although the dyeing cycles of direct and reactive dyes are broadly similar, a major difference becomes apparent when unfixed or hydrolysed reactive dye has to be washed off thoroughly in order to achieve the desired superior wet fastness of the reactive dyeing. As much as 50% of the total cost of a reactive dyeing process must be attributed to the washing-off stages and treatment of the resulting effluent. This aspect of the process should be recognised as a major limitation that prevents reactive dyes from achieving the degree of success that was predicted for them at the time of their discovery. Certain other deficiencies are associated with the limited stability of specific types of dye–fibre bond to various conditions of treatment of the dyed fibres.

Research into novel reactive dyeing systems and application methods has remained highly active and some notable developments have taken place during recent years. Aminofluorotriazine dyes have joined the vigorous competition between the major reactive systems, and bifunctional systems reacting by two distinctly different mechanisms have also appeared. Bis(aminonicotinotriazine) dyes that react with cellulose under neutral conditions and reactive dyes containing phosphonic acid groups capable of fixation under hot, dry, acidic conditions represent two quite different novel approaches to reactive dye fixation that have been commercialised.

Pretreatment of cellulosic materials with cationic agents of various kinds that enhance uptake of anionic dyes and facilitate the fixation of reactive dyes in the absence of either salt or alkali has also attracted considerable interest. Novel chromophores, cold-dissolving granules and liquid brands, improved automation of application methods and greater awareness of the environmental impact of reactive dye wastes have all received attention over this recent period.

INTRODUCTION TO REACTIVE DYES

Discovery of the first direct dye, depended on various techniques for depositing insoluble colorants inside the amorphous regions of the cellulosic polymer. For a further four decades, this remained the only feasible method of achieving dyeings of high fastness to washing on cellulosic textiles.

During this period much fundamental work on the structure of cellulose and the morphology of cellulosic fibres was undertaken and numerous ethers and esters of cellulose were prepared. It would be surprising if none of these had involved the formation or attachment of coloured sidechain substituents on the cellulose chain. Although such necessarily complex and esoteric reactions did indeed confirm the formation of covalent bonds between typical chromophoric groups and the hydroxy groups in the cellulose molecule, they remained essentially of academic interest only.

Surprisingly, few attempts were made to adapt such reactions or to develop appropriate reagents that would allow such derivatives to be formed under typical dyehouse conditions. Some of the drawbacks of the treatments applied in this period, apart from their multi-stage complexity or the use of costly and hazardous solvent media, were degradative attack of the cellulose chains by some of the vigorous reagents or reaction conditions necessary, or sensitivity of the colorant–fibre bond to hydrolytic attack during subsequent handling or storage of the coloured product.

FEATURES OF A TYPICAL REACTIVE DYE MOLECULE

The four characteristic features of a typical reactive dye molecule are:

1. The chromophoric grouping, contributing the colour and much of the substantivity for cellulose;
2. the reactive system, enabling the dye to react with the hydroxy groups in cellulose;
3. a bridging group that links the reactive system to the chromophore;
4. one or more solubilising groups, usually sulphonic acid substituents attached to the chromophoric grouping.

In a few cases the reactive grouping is attached directly to the chromophore and most reactive systems contain a heterocyclic ring that contributes some substantivity for cellulose. The nature of the bridging group and other substituents on the heterocyclic ring greatly influences the reactivity and other dyeing characteristics of such dyes. The sulphatoethylsulphone precursor of the vinylsulphone reactive group contributes significantly to the aqueous solubility of reactive dyes of this type.

Many reagents can be used to acylate cellulose when it is partially ionized under alkaline conditions but in the production of reactive dyes of commercial interest numerous factors other than the chemistry of such reactions have to be taken into account.

BACKED FABRICS

BACKED FABRICS:

The backed fabrics are those types which employ a face and back weave alternatively on the two sides of the cloth. These weaves may be of a reversible or non reversible nature. These types of fabrics are mainly constructed for two purposes :

• Increasing the warmth retaining qualities of the cloth
• Secure a greater weight and substance that can be acquired in a single structure which is equally fine on the surface.

A heavy single cloth can only be made by using thick yarns in conjunction with which it is necessary to employ only a comparatively few threads per unit space. A heavy single texture appears to be coarse in appearance. By interweaving threads on the underside of a cloth it is possible to obtain any desired weight combined with the fine surface appearance of a light single fabric.

The purpose of inserting threads in forming a back to a face fabric is only to give additional weight. One of the advantages of the backed construction is that the extra weight can be obtained in an economical manner, since material which is inferior to the face yarns may be used on the underside. Backed cloths are constructed on both the backed weft and backed warp principle. In the case of warp backed cloth there are two series of warp threads and one series of weft threads, and in the case of weft backed cloth there are two series of weft threads and one series of warp threads.

PRINCIPLE OF CONSTRUCTION:

The construction of backed fabrics involves the following stages,

1. The face and back threads are marked out on design paper. They are marked out according to the order of insertion.
2. The face weave is inserted on the face threads only using normal convention for warp backing and reverse convention for weft backing.
3. The back weave is inserted on back threads only using the normal and reversed convention. A mark is placed on the back weave between two long floats of the face weave. This hides the binding marks of the back weave by covering float on the face.

In reversible structures the binding marks of the face weave should be equally well concealed on the back. This is achieved by a suitable choice of face and the back weaves. 

Warp faced weaves are more suitable for warp backing and weft faced weaves for weft backing , while certain square faced weaves can be successfully applied to both structures. In order to get a well covered face in the back cloth, correct settings are very important as without sufficient density of the face threads, the binding marks of the back weave cannot be covered, no matter how clearly they are placed.

WARP BACKED FABRIC:

These fabrics are produced by alternately weaving two similar or different warp faced weaves. The objective of such a technique is to get greater thickness or mass of the fabric without using coarser yarns. For constructing warp backed fabrics two systems of warp and one system of weft is required. One series of warp threads constitute the face warp and the other constitutes the back warp. Obviously two warp beams are required. The ratio of the face to back warp threads is generally 1:1. Sometimes a ratio of 2:1 is also adopted.

The first step in the construction of warp backed fabric is the selection of the face weave. The next step is to choose the back weave. The back weave is selected so as to leave long weft floats on the back side in order to lower the back warp threads. Hence a warp faced weave is chosen for both the face and back threads.

A design of warp faced back weave is shown in Fig. 11.1 below.

Design of warp backed weave

A 3/1 twill is chosen as the base weave for both the face and back weaves. At A is shown the face weave and at C is shown the back weave. The design at C is a 3/1 twill as seen from back side and is 1/3 as viewed from the face side. For clarity the face and back warps are denoted by arabic and roman numerals respectively. The figure B shows the warp way cross section with the first pick as reference to show the manner of interlacement. As can be seen from this cross section, the first pick of weft goes below the face warp threads 1, 2 and 3 and above 4 respectively. The weft also goes above the back warp threads I, III and IV and below II respectively. It can be seen that the warp thread II is the binding point for the weft. This has been chosen since the binding point comes in the middle. The point of intersection of the weft thread 1, 2, 3 and 4 with the back warp threads I, II, III and IV respectively is denoted by the circled cross mark in diagram C. 

The face and back warp threads are arranged alternately in the ratio of 1:1 as shown at D. At E is shown the warp way cross section of the warp backed fabric. It is to be noted that this is the same as the one shown at B. The weft way cross section is shown at F. At G is shown the complete weaving plan of the warp backed design. The draft used here is a divided draft, since two sets of warp threads are used in the design.

WEFT BACKED FABRICS:

In these types of fabrics two series of weft threads and one series of warp threads are used. A drop box is necessary for the purpose. The purpose of introducing back weft thread is to obtain additional weight or thickness of fabric. The face weft threads are placed in the upper layer of the fabric and the back weft threads are placed in the lower layer of the fabric.

As in the case of warp backed weave, the first step is selection of the base weave. This may be either a warp or weft faced weave. A weft faced weave is suitable since it has longer warp floats on the back side.

A design of weft faced back weave is shown in Fig. 11.2

Design of weft faced back weave

At A is shown a 1/3 twill which is weft faced. B shows the weft way cross section of the weft backed design. As in the case of warp backed fabric the most suitable stitching point is in the middle of the float. The face and back weft are denoted by arabic and roman numerals respectively. The binding point of the first warp thread is by lowering below the weft thread III. C shows the back weft design with the suitable stitching points based on Fig. B. D shows the final design of the weft backed fabric by alternating the face and back weaves in the ratio of 1:1 weft way. F shows the weft way cross section which is the same as B. E shows the warp way cross section. G shows the weaving plan of the design. Since only one series of warp threads is used a straight draft is employed.

COMPARISON BETWEEN WARP BACKED AND WEFT BACKED FABRICS:

Weft backed fabrics	Warp backed fabrics
1. Softer and more lofty handling cloth can be obtained. This is due to weft containing less twist and being under less tension than the warp.	1. Less softer and loftier handle when compared to weft backed.
2. Requires one warp beam and drop box (2 ¥ 1)	2. Requires two warp beams and no drop box.
3. Costlier to produce due to more picks/cm	3. Cheaper to produce owing to less picks/cm
4. Impossible to produce a solid appearance.	4. A more solid appearance can be given to the cloth by the formation of stripe patterns on the underside.
5. Lower strength warp way	5. Greater strength warp way.
6. Inferior from structural point of view	6. Superior from structural point of view.
7. Low quality of backing yarn can be used in weft due to less strain on yarn	7. Low quality of yarn cannot be used in warp due to greater strain in weaving
8. Drawing in is cheaper due to less number of ends	8. Drawing in is a costlier operation since there are more number of ends
9. Drafts are simpler	9. Drafts are usually more complicated, and a greater number of healds are required in producing similar effects
10. The standard orders of arranging the picks are – 1 face to 1 back, 2 face to 1 back, 3 face to 1 back, 2 face to 2 back, 4 face to 2 back	10. The standard order of arranging the ends in warp backed cloths are – 1 face to one back, 2 face to 1 back and 3 face to 1 back.

END USES:

Backed fabrics find uses in shawls, heavier dress materials, overcoats etc.

MOCK LENO WEAVES

MOCK LENO WEAVES:

Mock lenos, also known as imitation lenos are a variety of weaves of ordinary construction which produce effects that are similar in appearance to the gauze or leno styles obtained with the aid of doup mounting. These weaves are generally produced in combination with a plain, twill, satin or other simple weaves or even with brocade figuring, to produce striped fabrics, which bear a very close resemblance to true leno fabrics. Two kinds of structures are produced by the weaves,

• Perforated fabrics which imitate open gauze effects
• Distorted thread effects which imitate spider or net leno styles.

PERFORATED FABRICS:

These are constructed by reversing a small unit of the weave. The weaves are in sections and tend to oppose each other. The outer threads of adjacent sections tend to be forced apart. The manner of interweaving in each section permits the threads to readily approach each other. Fig. 10.1 shows the various types of perforated fabric designs. 

Design of perforated fabric

Figures A, B and C show the 3 ¥ 3, 4 ¥ 4 and 3 ¥ 5 imitation gauzes. The warp threads run in groups with a space between, and are crossed by weft threads which are grouped together in similar manner. The designs A, B and C are dented 3, 4 and 5 ends respectively per split as shown above the plans. arrows above the denting plans indicate the positions of the empty splits.

The DISTORTED THREAD EFFECTS:

The weaves of this category are so arranged to distort certain threads in either the weft or the warp, or in both weft and warp. The distorted thread effects are shown in Figs. 10.2 and 10.3.

Design of warp way distorted mock leno weave

Design of weft way distorted mock leno weave

In Fig. 10.2, the ground structure is plain weave, and the fourth and eleventh ends, which are distorted, float over all the plain picks, but pass under the fourth and eleventh picks. The latter float over one group of plain ends, and under the next group in alternate order. The distorted ends are placed on a separate beam and are given in more rapidly than the ground ends and hence they are drawn towards each other where the picks four and eleven, float over the ground ends. As the latter floats occur in alternate order, the ends are drawn together in pairs, and then separated, as indicated by the zig zag lines.

The distorted warp effects are chiefly used in combination with other weaves in stripe form. When used in stripe form the ends which form the zig zag effect should be somewhat crowded in the reed.

Figure 10.3, shows a distorted weft design. The design is arranged with plain ground similar to that in fig. 10.2. The floating ends pass over all the distorted picks, and alternately over the ground picks between. Therefore the distorted picks, which float over all the ground ends, are alternately drawn together and separated, as shown by the zig zag lines of Fig. 10.3.

END USES OF MOCK LENOS:

Mock lenos find uses in canvas cloths, cheap fabrics for window curtains, light dress fabrics, blouses, aprons etc. In many cases, they are generally employed in combination with other weaves.

WELTS AND PIQUES

WELTS AND PIQUES:

Welts and piques are characterized by more or less pronounced ridges and furrows producing a series of ribs, welts or cords with a surface tissue of the plain calico weave, and extending in parallel lines transversely across the width of the fabric, i.e., in the direction of the weft threads. A pique structure consists of a plain face fabric composed of one series of warp and one series of weft threads, and a series of back or stitching warp threads. The stitching ends are placed on a separate beam which is heavily weighted to provide greater tension to the stitching warp.

The loom equipment necessary for manufacturing pique structures are a dobby loom with two warp beams (one for face warp under normal tension and another for stitching warp under heavier tension), a fast reed beat up mechanism and drop boxes ( 2 ¥ 1 or 2 ¥ 2), for wadded designs. 

The tight stitching ends are interwoven into the plain face texture, with the result that the latter is pulled down and an indentation is formed on the surface. In order to increase the prominence of the unstitched portions of the cloth, wadding picks are normally inserted between the tight back stitching ends and the slack face fabric.

Pique fabrics are mainly manufactured entirely of cotton woven in the grey or natural state and then bleached. They are produced in a variety of different textures, according to the purpose for which they are intended.

STANDARD QUALITY PARTICULARS:

                  Face warp : 30s - 60s

                  Stitching warp : 20s - 2/30s

                  Weft : 40s - 70s

                  Ends/inch : 92 - 132

                  Picks/inch : 96 – 152

CLASSIFICATION OF WELT STRUCTURES:

Welt structures are classified as shown below:

ORDINARY WELT STRUCTURES:

In these types of welt structures the indentations form continuous sunken lines which run horizontally in the cloth. The number of face picks in the width of a cord is varied according to requirements, but usually the number of consecutive picks that are unstitched should not exceed twelve. Figure 9.1 shows the design of ordinary welt structures.

Design of ordinary welt structure

In the above figure, is shown some ordinary welt structures. Figs. A, C and E show the first stage in the construction of ordinary welt structures and Figs. 9.1 B, D and F show the corresponding final designs. The three different welt designs shown above are constructed on repeats of 6, 8 and 18 picks respectively. The ratio of the face to stitching warp is 2 : 1. The stitching ends are indicated by shaded squares. The ends are arranged in the order of one face, one stitching and one face, in each split of the reed. In the final designs B, D and F, the solid marks indicate the lifts of the tight stitching ends into the plain face texture on two consecutive picks.

WEFT WADDED WELTS:

In the case of welt structures wadding threads can be introduced weft way. The object using the wadding threads is to enhance the prominence of the horizontal cords, and to make the cloth heavier. The wadding weft is coarser than the ground weft and is inserted as a pair of picks at a place. This is achieved with looms provided with multiple shuttle boxes at one side only. The face ends are lifted over the wadding picks, while the stitching ends are left down. Sometimes, the same kind of weft is used for both the face and the wadding. In such cases looms with a single box at each side are employed, and in such cases, one wadding pick at a place may be inserted.

Wadding picks are inserted only as extra picks and the take up motion is either rendered inoperative on wadding picks, or it is worked out in terms of the face picks only. Fig. 9.2 shows the various designs of weft wadded welt structure.

Design of weft wadded welt structure

Fig. A, B and C show the design of weft wadded welts repeating on 8, 10 and 24 picks respectively. The stitching warp is indicated by the solid shade, the wadded thread by circled cross mark and the plain threads by cross mark. As can be seen from the designs, the stitching takes place on three picks. Figure D shows the weft way cross sectional view of design A.

FAST BACK WELTS:

In these types of structures the stitchings are interwoven in plain order with all, or some wadding picks. Whereas in ‘loose back’ type of structures (previous two types) the stitching ends are only lifted to form the indentations. In case of fast back welts, the reduction of the float length of the stitching ends on the back of the fabric helps to produce a more serviceable cloth less liable to accidental damage. Fig. 9.3 shows the design of a fast back welt structure.

Design of fast back wadded welt structure

Figure A shows the design of a fast wadded welt structure and figure B, shows the weft welt cross section. The numbered threads represent the face and stitching warp.

END USES OF WELTS:

Welts find uses in shirtings, ties and vestings.

BEDFORD CORDS

BEDFORD CORDS:

The bed ford cords are a class of weaves that produce longitudinal warp lines in the cloth with fine sunken lines in between. They are constructed on a pair of picks or alternate picks. The cord weave is alternated by plain weave weft way. The cord effects so produced enable to bring out stripe effects in solid colours. Generally cotton and worsted yarns are used in the production of bed ford cords. Cotton is used in weaving of lighter textures while worsted is used in weaving of heavier textures. In the design of bed ford cords, two series of warp threads are considered. The first group constitutes the face threads which weave as cord and plain weave on alternate or pair of picks. The other group of threads known as cutting ends weave as plain. The cutting ends separate the neighbouring cords. The cords may be

alternated by plain or twill weave weft way.

Sometimes special threads known as wadding threads are introduced in between the normal warp threads. The purpose of this is to increase the prominence of the cords and also to increase the weight, bulk and strength of the fabric. The wadding threads never interlace with weft, but lie perfectly straight between the ridges of their respective cords and the floating weft at the back. Generally wadding threads are of considerably coarser counts of yarn than the principal or face warp threads, and since they never interlace with weft but remain straight, their construction during weaving is nil. This condition necessitates the wadding threads being wound upon a separate warp beam, and held at greater tension than face warp threads during weaving.

CLASSIFICATION OF BED FORD CORDS:

STANDARD QUALITY PARTICULARS:

The following constructional particulars are suitable for Bedford cords used as worsted dress fabrics.

          Warp : 2/20s

           Weft : 18s

           Ends/inch : 92

           Picks/inch : 82

The following constructional particulars are suitable for a cotton twill faced Bedford cord (London cord).

      Warp : 14s

      Weft : 20s

      Ends/inch : 86

      Picks/inch : 78

LOOM EQUIPMENT:

A dobby loom with fast reed and heavy beat up is suitable for manufacturing bed ford cord fabrics.

PLAIN FACED BED FORD CORDS:

In this type, the cord or rib effect is produced by alternating plain weave with the cord either on alternate picks or a pair of picks. Fig. 8.1 shows the construction of a plain face Bedford cord on a pair of picks, and Fig. 8.2 shows a Bed Ford cord constructed on alternate picks.

Construction of plain faced Bedford cord on a pair of picks

In Fig. 8.1 A, is shown the repeat size of the Bedford cord. The repeat is split into cutting ends and face ends. The cutting ends weave plain and the face ends weave the cord. In Fig. A the insertion of the cutting ends are shown in the figure B, the insertion of the face ends are shown. Figs. C, D, E and F show the design, draft, peg plan and the cross section of the Bedford cord. At figure F, the interlacement of the various picks in the repeat with the face and the cutting ends are shown. In the example above the ratio of face ends to cutting ends
 is 2 : 4.

Construction of a plain faced bedford cord on alternate picks

Fig. 8.2, shows the construction of a plain faced bed ford cord on alternate picks. Fig. A, shows the face and cutting threads. Fig. B, shows the insertion of plain weave on alternate picks to obtain the Bedford cord design.

Some wadded threads are introduced in between the face threads in order to increase the weight of the fabric or enhance the cord effect. The wadded threads so introduced will usually be coarser than the face threads and made of a cheaper material. A typical example is shown in Fig. 8.3.

Construction of a plain 3 faced wadded Bedford cord design

At A, is shown the design of plain faced wadded bed ford cord. The wadded threads are introduced at the middle in between the face threads. For the purpose of differentiation, the face, cutting and wadded threads are indicated by separate notations respectively. At C is shown the warp way cross section of the design.

TWILL FACED BED FORD CORD:

In this type of cord, a twill weave is used instead of a plain weave, along with the cord or rib weave to get a better effect. In this type, the warp is brought more prominently to the surface. Figure 8.4, shows the design of a twill faced bed ford cord.

Construction of a twill faced Bedford cord design

At figure A is shown the twill faced Bedford cord. The twill weave is inserted on alternate picks. At B, is shown the basic twill weave, which is a 1/3 twill. The repeat size of the cord is 16 ¥ 16 including the face and cutting ends.

Wadding threads can also be introduced as in the case of plain faced Bedford cords. This is shown in Fig. 8.5.

Design of a wadded twill faced bedford cord

At A, is shown a wadded twill faced bed ford cord design. A 2/1 twill has been chosen(figure B) and inserted with the cord. The wadding threads are inserted in between the face threads and work with the cord threads. The wadded threads do not inter weave with the picks.

END USES OF BED FORD CORDS:

Bed ford cords find a wide range of applications such as dress materials, military dresses, suitings, woolen and worsted fabrics (heavy type).