|Dyeing of Sisal and other Plant Fibres: A Handbook for Craft Instructors (NRI)|
|Part 1: Basic information and essential requirements|
The main purpose of dyeing is to make the finished goods more attractive. However, uneven dyeing and the presence of loose dye will detract from the appearance. Moreover, the customer will not be pleased if a shower of rain causes the colour to run, spoiling the design and possibly his clothing. Also the goods must be competitive with similar products, and if dyeing adds considerably to the cost of the article or the colours are not sufficiently durable, the prospective purchasers may well turn to alternative products. Cost and durability are both affected by dyeing technique.
It is not possible to describe one ideal dyeing technique, since the choice depends on the customer's requirements and the circumstances of the individual dyer. However, there are certain basic rules that must be followed, and all techniques involve a compromise between quality and economics. The essential steps are given below and form the basis of the dyeing instructions:
1. Select material of uniform colour for dyeing.
2. Choose the colour and decide what fastness properties are required for the end product; then select the cheapest dyestuffs which give these. (For example, it is not necessary to use dyes of high light fastness for party hats that will be used only once-though such an article needs to be moderately water fast to prevent a shower of rain from staining other articles).
3. Choose a dyeing method suitable for use with the chosen dyes. Three methods are mentioned in this handbook:
(i) Exhaust dyeing in which a batch of fibre is dyed from a liquor that is used only once and then discarded. This technique enables dyers to produce matching colours on different batches of fibre with relative ease (provided that the composition of the dye liquor and the dyeing conditions are carefully controlled) but obviously leads to the wastage of chemicals that are not consumed in the dyebath. However, provided that dyes which exhaust well are used, the loss of dyestuff is negligible and other chemicals used in the dyebath are relatively cheap.
(ii) Standing bath dyeing is basically similar to exhaust dyeing. However, after the first dyeing the dyebath is replenished with dyestuff and chemicals and a further dyeing is carried out. This replenishment may be repeated several times. Experience is required to calculate the exact amount of dyestuff and chemicals needed for replenishment.
(iii) Pad-batch dyeing is a recently-developed method based on the use of reactive dyes (see p. 43 and Glossary). It is often (as described by Canning et al., 1977) carried out in the cold and uses low liquor to fibre ratios (1: 1 compared with 20: 1 for exhaust dyeing).
When using either the exhaust or standing bath method:
4. Select dyestuffs that exhaust well onto the material. (If the dyes do not have a strong affinity for the material a large proportion will be lost with discarded dye liquor. Such dyes are best applied by the standing bath technique since this is less wasteful).
5. Make sure that the correct auxiliary chemicals are used in the dyebath.
6. Make certain that all the dye is dissolved before placing any fibre in the dyebath. In order to avoid lumps, first mix acid, direct, reactive and basic dyes to a smooth paste using 2 ml of water or acetic acid solution, as appropriate, for each gram of dye; then add more water to disperse the dye before adding it to the dyebath. With disperse dyes the powders are first sprinkled into water using about 15 ml of water for each gram of dye (the dye will not disperse properly if too much or too little water is used). Lumps of dye in the dyebath will give rise to uneven colours and impaired fastness properties. They should be broken up whilst pasting, and filtering (e.g. through a muslin cloth) should be used only as a last resort.
7. Never place the fibre in a dyebath which is above 50°C. Increase the temperature slowly. Placing the fibre in a hot bath will cause the dyestuffs to be taken up rapidly and the levelling of colour will be difficult to control.
8. Make sure that the dye liquor covers the goods. Thorough and efficient stirring must be used to give good circulation, especially in the warming-up period and the early stages of boiling during which most of the dye exhausts. Inadequate circulation of liquor will result in unevenly coloured fibre. The agitation of the fibre produced by boiling cannot be relied on to distribute the dye evenly.
9. Dyeing must be continued for an hour or more even though the fibre may appear adequately coloured after only a short period of dyeing. The colour built up initially lies at the surface of the fibre and will rub off easily.
10. After removing the fibre from the dyebath it must be rinsed (preferably in running water) to remove adhering dye liquor, otherwise the fibre will become coated with loose dye on drying.
Costs of dyeing
The craft dyer should keep an account of all the various costs involved in dyeing, including his own time and the cost of heating the dyebath. Considering the following questions may help the dyer to find ways to reduce costs:
1. Can the cost of dyestuff be lowered? There is often a cheaper alternative dye. However, sometimes it is advantageous to buy a more expensive dye. The strengths (or tinctorial yields) of dyestuffs differ and it may cost less to use a smaller amount of a stronger dye to produce the same colour.
2. Is too much dye being used? There should only be sufficient dye in the bath to produce the required depth of colour. Too much dye will produce too deep a shade or tempt the dyer to remove the fibre too soon-leading to poor penetration of dye. In either case dye will be wasted.
3. Can less acid be used? Acid is used with both acid and basic dyes. Some acid dyes need less acid to fix them than others and less acid is needed with pale shades than with deep. With basic dyes the acid slows down the adsorption of dye and thus aids levelling. In this case more acid is needed when dyeing pale shades since with these it is less easy to control dye adsorption.
4. Can less salt (or other dyeing assistants) be used? Salt in the dyebath increases the proportion of direct or reactive dye that adsorbs onto the fibre. By using extra salt the dyer can reduce the quantity of dye in the bath. The dyer will need to calculate at what point the cost of salt (or soda or other dyeing assistants) becomes greater than the cost of dyestuff saved
5. Can less fuel be used? Fuel is usually expensive and sometimes hard to find. It may be possible to reduce heating time by increasing the amount of dyeing assistants. It may not be necessary to boil for so long-especially if good penetration is not essential. Insulation of the dyebath will help to reduce fuel consumption.
6. Can less time be used? Not all processes require the craft dyers undivided attention (e.g. the batching operation of pad-batch dyeing); however, most do and the dyer should keep a record of all the time spent exclusively on each separate dyeing. Before extending the time of dyeing to increase the adsorption of dye the dyer should calculate whether it would be cheaper to, for example, add more salt instead.
7. Can less wetting agent be used? Wetting agents assist the production of uniform colours. However, they reduce dyebath exhaustion and thus increase wastage of dye. The minimum amount should therefore be used.
8. Can less water be used? By reducing the quantity of water the dyer can save on dyestuff and assistants since the exhaustion will be increased. Less fuel will be needed to heat a smaller quantity of water. However, the amount of water should not be reduced so much that circulation of liquor is restricted.
Making matching shades
Dyers are often asked to reproduce fashionable colours by blending dyes (see p. 17) or to produce several batches of fibre in identical shades. This latter aspect is important, since marketing organisations need to illustrate goods in catalogues and must be able to supply identical goods on repeat orders.
When producing a new colour for the first time, the experienced dyer starts with a few 'trial and error' test dyeings. These trial dyeings are usually made on a few grams of material. The small quantities of dyestuffs and chemicals required can be measured on an accurate balance but, as an alternative, solutions can be carefully diluted. For example, a dyer who has only kitchen equipment can dissolve one teaspoonful of chemical in four cupfuls of water and, by taking only one cupful of this solution, can scale down the size of operation four times. The small-scale dyer will find this method of working advantageous-especially when working with blends where, for example, 10 teaspoonfuls of dye solution 'A' mixed with 3 teaspoonfuls of dye solution 'B' give exactly the right colour on 250 9 of fibre. Dyers could not measure such small quantities accurately using teaspoons of dye powder. (Unfortunately, this method would be wasteful with reactive dyes as solutions of these become useless if kept for more than a few hours). After the right conditions have been found, the operation can then be scaled up. All that is needed to do this is to increase proportionately the quantities of each ingredient [including the fibre and water) to obtain the required amount of dyed fibre.
By using identical dyebath conditions for each batch of fibre, matching shades can be produced. An experienced dyer always measures the ingredients used in the dyebath and keeps a record of the entire dyeing process. By consulting this record (or 'recipe'}it is possible to reproduce the same colour on a subsequent occasion. However, to obtain identical shades, it may sometimes be necessary to make small adjustments to the dyebath at the conclusion of dyeing.
When dyeing matching shades the quantity of water in the dyebath must be kept constant. Throughout the course of dyeing, water will be lost through evaporation. This water must be replaced, or the concentration of dyestuff will increase and cause increased adsorption of dye. Conversely the addition of too much water when topping up the dyebath will cause a decrease in the quantity of dye adsorbed by the fibre. The water level can be checked with the aid of a 'dipstick' (see p. 25).
In the next section commercial dyeing practices which enable the dyer to reproduce colours accurately are described.
The dyeing recipe
Commercial dyers usually work from a general recipe in which the quantities of ingredients relate to a unit weight of fibre. When using such a recipe only a few simple calculations are needed for dyeing a batch of fibre of any given size. Also, dyestuff manufacturers find the general recipe a convenient method of providing instructions.
The general recipe does not give detailed information on the application of the dyes. However, the quantities of ingredients used are readily derived from information expressed in the following terms 'Liquor ratio', 'percentage' end 'Concentration'. These terms are widely used and, in dyeing, relate the quantity of ingredient to the air-dry weight of the fibre to be dyed.
Liquor ratio (LR)
The liquor ratio (LR) refers to the quantity of water required in the dyebath. The ratio is the weight of liquor required for each unit weight of air-dried fibre to be dyed. For example, a liquor ratio of 20:1 indicates that the weight of liquor required is twenty times the weight of air-dried fibre to be dyed.
It is usual to convert the weight of liquor to a volume, which is more easily measured. In dyeing, the density of the liquor is taken to be equal to that of water (i.e. 1 kg of liquor equals 1 litre). Therefore, with a liquor ratio of 20:1, 20 litres of water will be used for each kilogram of fibre.
The depth, or intensity, of colour is usually controlled by varying the quantity of dye in the dyebath. In practice, the quantity of dye is expressed as a percentage of the weight of the air-dried fibre to be dyed. Consequently there are a number of terms in common use such as 'per cent shade', 'at .... per cent', 'per cent depth', etc. Whatever term is used in the dyeing recipe, the actual quantity of dye to use in the dyebath is calculated as in the following example in which the weight of dye needed to dye a 0.5 per cent shade on 5 kg of fibre is calculated:
Weight of dye needed
= (Weight of fibre X percentage shade)/100
= (5,00O g X 0.5)/100
= 25 g
Actual weights needed in the dyebath of other ingredients given as percentages on the weight of fibre are calculated similarly. However, it is important to make sure that 'percentage on the weight of fibre' is not confused with the 'percentage strength of a chemical solution' used (e.g. acetic acid (30%), acetic acid (60%), formic acid (85%)).
Depth of colour
The term 'percentage shade' is not a direct measure of the depth of colour of the dye. Visual depth of colour depends on the concentration of dye at the surface of the fibre and this will vary at a given percentage depth with differences in dyeing technique (e.g. time of dyeing, liquor ratio, concentration of salt or acid). Different dyestuffs of the same colour, and also the same dyestuffs from different manufacturers, give different visual depths of colour when used at the same percentage shade. Although manufacturers endeavour to maintain consistency, recipes should be checked, and adjusted if necessary, each time a new batch of dye is used.
Quantities of ingredients are sometimes given as a concentration (e.g. grams/litre, g/l). If, for example, the dyeing recipe specifies salt at a rate of 10 grams/litre (9/l) one must use 10 grams of salt for each litre of dye liquor needed. In order to determine how much ingredient is needed in the bath the actual volume of liquor needed must first be calculated using the liquor ratio (LR) given in the recipe and the weight of the fibre to be dyed. A typical calculation of the quantities of ingredients needed is given in the following example
3 kg of fibre is to be dyed at a liquor ratio of 20:1. The recipe specifies 5 g/l of salt in the dye liquor
(i) The quantity of water needed is
Water at LR of 20:1 = 20 X weight (kilograms) of fibre = 20 X 3 kg = 60 litres
(ii) The quantity of salt needed is:
Salt at 5g/l = 5 9 X volume (litres) of liquor = 5g X 60 = 300 g.
Concentrations of chemicals used in the dyebath are now commonly expressed as parts/1,000 (parts by weight of dye liquor). However, the more traditional units of 9/l and Ib/100 gallons are equivalent since 1 litre of water weighs 1,000 g and 100 gallons of water weigh 1,000 lb.
Completion of the recipe
The same dyeing time, dyeing temperature and sequence of operations are used irrespective of the size of the batch of fibre.