Dyes dyeability
Dyeability As a prerequisite for the use of textile fibres. One of their most important properties is dyeabilily. Consequently, the range of application of some synthetic fibres has been, and still is limited either because they have poor dyeability or the problem of dyeing these fibres has not yet been satisfactorily solved. For example, when 21/2 acetate fibres were first introduced. they were not successful for a long time because they could not be dyed with the known dye classes at that time, Indeed, it was only after the discovery of disperse dyes that acetate fibres gained wide acceptance. The same problem applies to polypropylene fibres although in this case, appropriate modifications have provided a partial solution. Comparable examples are the attempts to develop polyester fibres with a higher dye uptake as well as the efforts to produce a polyamide fibre with diff erential dyeing characteristics (— Differential dyeing) which has had a favourable influence on their range of application. Some textile fibres may be dyed with several technologically different dye classes capable of achieving relatively good color fastness: others. however, can be dyed with one particular dye class only (and. even then. not without difficulty in some cases).
In practice the dyeability of a textile fibre is determined by the rate of dyeing and the degree of saturation which can be achieved. For example the different dyeing properties of mercerized cotton compared to non- mercerized cotton can be characterized by results from the time of half dyeing (i.e. the reciprocal of the rate of dyeing) and the saturation concentration of the substantive dye.
Dyeability is dependent on the following factors:
a) Chemical composition of the fibre: fibres of native and regenerated cellulose differ from each othei for example in their physic 0-mechanical structure but. despite this. they can be dyed with the same classes of dye. The same situation applies to protein fibres (a gram equivalent of wool is 1200 g and that of silk is 4200 g for reactions as bases) as well as for polyam ide and polyester fibres.
b) The coloristic dye class resp: the entire dyeing syst em including additions to the dyebath: certain dye classes are only suitable for one particular type of fibre. i.e. either the dye is not capable of dyeing certain fibres or can only dye them very slightly resp stain the fibre this can however, be caused by tin- suitable dyeing conditions. e.g. by using an inappropriate dyebath pH.
c) The geometrical form of the fibre: the fibre dimens ions as well as the morphological and histological structure of the fibre (the cuticle in cotton. the scale layer of wool and the skiii effect in viscose fibres) and the fine, or supra molecular structure of the fibre, whereby each change in fine structure results in a change in dye uptake: e.g. mercerized or non- mercerized cotton different regenerated cellulose fibres, drawn or heat-set synthetic fibres.
Those changes which, for example. involve changes in the ratios of crystalline. oriented and amorphous yeg ions due to the ageing of cotton or changes in the pore size, can be attributed to changes in the accessibility of the fibre to certain reagents. The relationship between fibre accessibility A% and the degree of crystallinity K% can be derived as follows:
A=(lOO-K)+aK
a is a constant which is related to the crystalline part of the fibre. It can have values from 0—1. An ideal subs tance with hundred percent accessibility would be represented by a = 1. The first part of the formula repres ents the amorphous part of the fibre p. and the second part represents the accessible surface of the crystallites (if a = 0. this surface would be inaccessible):
A=p+ctK
In the case of hiah tenacity viscose fibres K can vary, for example between the limits of 33—49%. a = 0.09—0.67. The accessibility A represents an appropria te numerical criterion of dyeability. It is an absolute parameter which represents a previously given property of the fibre and in general gives expression to the req uirement for its dyeability in achieving dark shades or black within an acceptable dyeing time provided the dye has adequately high build up properties. The fact that in isothermal dyeing dyeability decreases with the time of dyeing and that a fibre has a greater affinity for the dye at the beginning of the dyeing process than at the end is well known to every dyer. It is also possible to assess dyeability from the standpoint of the momentary quantity and rate of dye uptake by the fibre at a given time. The relative dyeability is always a kinetic parameter which is related to a specific dyeing system. If it is assumed that at time t. a dye concentration c1 exists in the fibre then tile reciprocal of this parameter Ct’ is the dilution of tile dye in tile fibre at tune t. At the start of dyeing the dilution reaches a value which inc reases beyond all limits (apart from the dye adsorption on the fibre surface at time t = 0: the dilution has a value of c1).
Tile dilution of solutions behave in an analogous manlier; before a substance dissolves in a solvent, tile dilution is infinite (since tile reciprocal value. i.e. the concentration, is equal to 0): with increasing concentrat ion tile dilution decreases (not. however, to 0 but to a certain value which corresponds to the dilution of the substance ill a saturated solution). Tile rate of dyeing at any time of dyeing t is proportional to this dye dilution:
Dc=K
dt= Ct
In practice the dyeability of a textile fibre is determined by the rate of dyeing and the degree of saturation which can be achieved. For example the different dyeing properties of mercerized cotton compared to non- mercerized cotton can be characterized by results from the time of half dyeing (i.e. the reciprocal of the rate of dyeing) and the saturation concentration of the substantive dye.
Dyeability is dependent on the following factors:
a) Chemical composition of the fibre: fibres of native and regenerated cellulose differ from each othei for example in their physic 0-mechanical structure but. despite this. they can be dyed with the same classes of dye. The same situation applies to protein fibres (a gram equivalent of wool is 1200 g and that of silk is 4200 g for reactions as bases) as well as for polyam ide and polyester fibres.
b) The coloristic dye class resp: the entire dyeing syst em including additions to the dyebath: certain dye classes are only suitable for one particular type of fibre. i.e. either the dye is not capable of dyeing certain fibres or can only dye them very slightly resp stain the fibre this can however, be caused by tin- suitable dyeing conditions. e.g. by using an inappropriate dyebath pH.
c) The geometrical form of the fibre: the fibre dimens ions as well as the morphological and histological structure of the fibre (the cuticle in cotton. the scale layer of wool and the skiii effect in viscose fibres) and the fine, or supra molecular structure of the fibre, whereby each change in fine structure results in a change in dye uptake: e.g. mercerized or non- mercerized cotton different regenerated cellulose fibres, drawn or heat-set synthetic fibres.
Those changes which, for example. involve changes in the ratios of crystalline. oriented and amorphous yeg ions due to the ageing of cotton or changes in the pore size, can be attributed to changes in the accessibility of the fibre to certain reagents. The relationship between fibre accessibility A% and the degree of crystallinity K% can be derived as follows:
A=(lOO-K)+aK
a is a constant which is related to the crystalline part of the fibre. It can have values from 0—1. An ideal subs tance with hundred percent accessibility would be represented by a = 1. The first part of the formula repres ents the amorphous part of the fibre p. and the second part represents the accessible surface of the crystallites (if a = 0. this surface would be inaccessible):
A=p+ctK
In the case of hiah tenacity viscose fibres K can vary, for example between the limits of 33—49%. a = 0.09—0.67. The accessibility A represents an appropria te numerical criterion of dyeability. It is an absolute parameter which represents a previously given property of the fibre and in general gives expression to the req uirement for its dyeability in achieving dark shades or black within an acceptable dyeing time provided the dye has adequately high build up properties. The fact that in isothermal dyeing dyeability decreases with the time of dyeing and that a fibre has a greater affinity for the dye at the beginning of the dyeing process than at the end is well known to every dyer. It is also possible to assess dyeability from the standpoint of the momentary quantity and rate of dye uptake by the fibre at a given time. The relative dyeability is always a kinetic parameter which is related to a specific dyeing system. If it is assumed that at time t. a dye concentration c1 exists in the fibre then tile reciprocal of this parameter Ct’ is the dilution of tile dye in tile fibre at tune t. At the start of dyeing the dilution reaches a value which inc reases beyond all limits (apart from the dye adsorption on the fibre surface at time t = 0: the dilution has a value of c1).
Tile dilution of solutions behave in an analogous manlier; before a substance dissolves in a solvent, tile dilution is infinite (since tile reciprocal value. i.e. the concentration, is equal to 0): with increasing concentrat ion tile dilution decreases (not. however, to 0 but to a certain value which corresponds to the dilution of the substance ill a saturated solution). Tile rate of dyeing at any time of dyeing t is proportional to this dye dilution:
Dc=K
dt= Ct
Tile dyeability of the fibre at time t is the dye dilution related to a unit of time. It is infinite at tile beginning (if the dye adsorption at time t = 0 is disregarded) and decreases with the time of dyeing (usually very quickly) and is equal to 0 at equilibrium. According to experience, the fibre is now no longer cap able of absorbing any dye from the dyebath (only all interchange of dye particles between the fibre and the dyebath takes place as a result of the dynamic equilibrium which exists in reversible processes).
Dye absorption index (fibre affinity index SF), fibre characteristic which gives information on the max dyestuff affinity of an acrylic fibre for cationic dye stuffs.
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