What is Laboratory Dyeing?
Laboratory dyeing / Lab dyeing plays a vital role in the textile dyeing process. Before starting any large-scale production, dyeing lab / lab dip helps develop the right recipe for color, tests the fabric’s dyeing behavior, and ensures consistent results. Without laboratory work, it would be hard to maintain color accuracy, quality, and cost-efficiency in modern dyehouses.
Laboratory Dyeing Techniques
A modern dyehouse is not operable without support from laboratory work. Laboratory dyeing helps dyehouses avoid errors and save time, money, and materials. The laboratory’s tasks are among others:
- Working out formulations for new shades, supported by colorimetry and computer colour matching.
- Production tests on the textile material to be dyed; fastness testing.
- Maintaining consistency of production, adjudicating claims.
- Establishing tolerance ranges.
- Dye selection in view of compatibility, fastness properties, dyeing rate, time requirements, and cost.
- Development of appropriate dyeing techniques, suitable for new types of textiles and novel styling requirements.
Laboratory dyeing technique should simulate actual production conditions as closely as possible. It also should allow for a multiplicity of tests in a short period of time. Laboratory equipment permits dyeing of small (5 g) to larger (1 kg) textile samples. Most lab dyeing machines work batchwise, but installations for continuous operation are also available.
Factors Consider in Laboratory Dyeing:
In order to facilitate the transfer of any new developments from laboratory to production, or to facilitate good quality control, modern dyehouses require accurate and dependable equipment which gives reproducible laboratory-to-production results. A number of factors must be controlled:
- Quantity of dyestuffs (% on mass fabric (o.m.f.)) and other additives;
- Liquor ratio;
- Temperature gradients;
- Flow rates and direction of flow;
- Hydro-extraction prior to drying;
- Drying times/type of drying/temperature of drying.
Methods and Machines for Laboratory Dyeing:
Laboratory dyeing requires specialized equipment designed to handle small batches while maintaining precise control. The ideal scenario is to use laboratory-scale dyeing machines that can replicate the conditions used in larger production machinery, irrespective of machine capacity (e.g. a two package machine needs to perform in the same way as a 200-package machine). Production machinery for the dyeing of tops, loose stock, hanks, yarn packages, pieces and garments – all of which must also be dyed in the laboratory.
Mathis AG (Switzerland), Roaches (UK) and Ahiba (US) are the three most popular manufacturers of sample dyeing machinery (and are all featured in this section), although it should be pointed out that there are now a number of smaller companies based in China, India and other countries that are developing their own machinery and beginning to take a share of the market.
Tops, Loose Stock, Hanks and Package Yarn
Tops, loose stock, hanks and yarn can all be dyed in the laboratory using one of four methods.
Moving Fabric and Moving Liquor
This method has been used for a long time in the dyeing of piece and knit goods, yarns and loose materials, and fully fashioned articles such as socks, shorts, sweat shirts and T-shirts. It typically uses closed dye beakers, placed in a holder and rotated through a heated medium (water, glycol or ballotini) in such a way as to achieve liquor and fabric movement within each dyepot. The Zeltex Polycolor (USA) (Figure 1) rotates dyepots through either a water or a glycol bath. The number of dyepots in each machine varies, as does their size.
There are a number of disadvantages to this type of system:
- The machine can be messy; this is especially obvious when using a glycol bath.
- As the dyepots are removed, they are covered in glycol, which either needs to drain back into the bath or drips on to the bench/floor of the laboratory.
- The rubber seal within the dyepot lid has been known to fail occasionally, resulting in the sample becoming contaminated with glycol and the glycol bath being contaminated with dyestuffs.
Figure 2: Mathis AG Labomat BFA
A development in this type of machine came when infrared (IR) dyeing machines were introduced to the market. They work in the same way, but there is no liquid medium. The closed dye beakers are placed on to a revolving disc and heated by IR radiators while under constant rotation. One of the dye beakers has a temperature sensor, which allows constant monitoring of the temperatures inside the dyepot. The dye beakers are angled on the revolving disc to achieve optimum liquor circulation during rotation. There are a number of suppliers of this type of machine, including Mathis AG (Switzerland) (Figure 2), Coloursmith Ltd (UK) (Figure 3) and Datacolor Ahiba (US) (Figure 4).
- Liquor ratios can range between 1 : 6 and 1 : 50, and extra-short liquor ratios of 1 : 5 to 1 : 2 can be achieved with special liquor displacement bodies.
- The disc speed is variable and the direction of rotation can be reversed in specified intervals.
- In some machines, the exteriors of the dye beakers are cooled by air, which is fed through a water-cooled heat exchanger.
- The number of dyepots used in each machine depends on the rotating disc and the dyepot size; this ranges from 50 ml to 5 l. Machines may hold up to 32 individual 50 ml dyepots or a single 5 l dyepot.
- Some manufacturers have introduced dosing nipples on the beakers, which allow additions to be made during the dyeing process without each individual dyepot having to be opened.
Figure 4: Datacolor AHIBA IR
Stationary Beaker and Moving Liquor
This type of machine works by pumping dye liquor through a stationary material sample at a constant pressure. The flow can be reversed (promoting levelness and replicating production machines) and the duration of each flow direction can be varied independently. These machines are particularly favoured for the dyeing of woven and knit goods, yarn (bobbin or hank), loose material, fibres and worsted tops. Examples include Datacolor AHIBA SE/1 (Switzerland) (Figure 4), and the Mathis AG Colorstar CJ (Figure 5).
These machines all have:
- A control system for flow rate and differential pressure, including a special reversing device.
- pH measurement.
- Conductivity measurement.
- PC visualization software for recording and registration of parameters (◦C, l min−1, bar, pH).
- A system for spectrophotometric analysis of the dyebath.
Stationary Beaker and Moving Liquor
A further type of stationary-beaker dyeing machine uses a magnetic stirring system; such systems include the Turby Type T from Mathis AG (Figure 6). At the bottom of each dye beaker is a Teflon-coated magnetic stirrer, which rotates at high speed, producing flow from the outside to the inside of the sample. Chemicals are normally added directly into the dye liquor; however, in certain models (such as the one shown in Figure 6) an automatic dosing pump is able to deliver a metered amount to all dyepots simultaneously.
Stationary Liquor and Moving Fabric
The ‘moving textile’ beaker dyeing apparatus operates on the basis that the dye liquor remains stationary, and the sample is agitated via a ‘dipping and twisting’ mechanism. The unit can be heated and cooled either by air circulation, as used in the Mathis Airboy (Figure 7), or by liquid circulation, as used in the Roaches DK Atmospheric (Figure 8).
Piece Dyeing
Piece dyeing in the laboratory is generally done using either a winch or a jet machine; both are manufactured by Roaches and Mathis. The winch and the jet systems are suitable for the dyeing of piece goods (web or knitted) in rope form or loose material. Laboratory-scale winches work using exactly the same principle as those used in full production, but on a much smaller scale. Over the past few years, jet machines have become more popular. They can now be operated using the same modern control systems found on the larger-production machines.
There are two main types of jet:
- Jet system: The material, contained within a circular tube, is lifted by a small winch mechanism and passed through a jet. These machines are capable of running 3 or 4 kg of fabric, which makes them ideal for bridging the gap between laboratory and production. They are available from a number of suppliers, including Mathis, Roaches and Thies.
- Overflow jet system: The material, contained within a circular basket, is lifted by a small winch mechanism and passed through a large ring shape (the jet). The force of the liquor flow penetrates the fibers evenly and outgoing fabric causes the basket to rotate. These machines operate at a lower liquor ratio (1 : 5 and upwards) than standard jets (1 : 8 and upwards) and can run relatively small samples (e.g. 100 g), which is very useful for research-and-development (R&D) and quality-control (QC) applications.
Jet-overflow machines with this fabric arrangement are available from both Mathis and Roaches.
Garment Dyeing
The dyeing of garments in the laboratory is generally done using drum dyeing machines, such as those shown in Figure 9. These machines, which can be used to dye larger material samples or finished garments, are available in different sizes and can accommodate samples ranging from 1 to 30 kg. The drum housing, samples and dye liquor rotate in alternate directions for a period of time specified by the operator.
Some models, such as the Drum TWA machine by Mathis AG (Figure 9), are now available with dosing pumps (which enable additions to be made to the dye liquor during the dyeing process) and a hydro-extraction facility.
Laboratory Machine Control Systems
Traditionally, control systems for laboratory dyeing machines have been relatively simple. Systems such as those shown in Figure 10 offered the user the ability to set simple programs to control the gradient of temperature rise, the final dyeing temperature and the duration of the dyeing process.
Modern laboratory machines may be installed with touchscreen controllers, which allow users to monitor and change more parameters, including the liquor flow rate, speed of fabric movement, direction of movement and. The Univision controller from Mathis AG (Figure 11) does all of this and can also record information to be downloaded on to a PC by the user.
Conclusion
Laboratory dyeing acts as a “dress rehearsal” for mass production. Lab dyeing is essential for innovation and quality assurance in textiles. By using the right techniques and machines, dyehouses can produce accurate shades, reduce waste, and maintain high standards in bulk production.
References
[1] The Coloration of Wool and other Keratin Fibres. (2013). In Wiley eBooks. https://doi.org/10.1002/9781118625118
[2] Hunger, K. (2007). Industrial dyes: Chemistry, Properties, Applications. John Wiley & Sons.
[3] Mahapatra, N. N. (2018). Textile dyeing. Woodhead Publishing.
