Indigo Dyeing Processes

Indigo; Textile sector hosts a dizzying variety of products manufactured in almost every size, color and shape from natural or synthetic yarns. Specific polymerization reactions used in synthetic production processes require continuous process monitoring and regular bath adjustments within this framework. The fibers obtained in these processes are then dyed (in rope or pre-woven fabric form) and specific dyeing techniques are applied to the product depending on material structure, dye composition, pH / other bath parameters and end-product requirements.

History of Indigo

"Indigo" (C10H10 O2, also known as 2,2'-bis (2,3-dihydro-3-oxoindolyliden)), used in the denim sector, has a very long history and first appeared in India as an organic dark blue dye extracted from plants. The rare and vibrant color possessed by indigo was perceived as a luxury trade product in many ancient civilizations, and at that time people wearing silk, wool and cotton clothing dyed with indigo were regarded as wealthy individuals. By the end of the 19th century, a synthetic process for industrial production of the indigo compound was discovered within BASF, and this method continues to be used today. Cotton provides a suitable substrate, especially for the indigo compound, due to the size of pores in cellulose fibers and the difficulty of the dye molecule being easily released after drying. This combination of color and fastness led to the global rise of the denim / blue jean sector over the past hundred years and caused this color to cease being rare or a sign of wealth.

Indigo Dyeing Process

Indigo alone has a water-insoluble structure and therefore must first be reduced with excess sodium hydrosulfite (sodium dithionite, Na2S2O4) within a strong alkaline bath in order to be properly used as a dye. Typically, since such dyeing processes are carried out within bath series called "Vat", indigo is classified as a "Vat Dye". The reduction process produces a water-soluble molecule called leuco-indigo (indigo white), and this compound, which has a yellow-green color, returns to its insoluble blue form in the presence of oxygen. For healthy dyeing, multiple baths are often required within the process, and due to the colloidal nature of the large dye molecule, circulation systems are used to maintain stable concentrations throughout the Vat. The yarn is gently immersed and moved through the circulating hot dye baths, aimed at providing complete and uniform coverage while reducing excessive oxygen ingress. Multiple dippings may be necessary for a deeper blue color in the finished product, while oxidation of the yarn between dippings is carefully monitored to trap leuco-indigo in the fibers. Three basic dyeing methods stand out in indigo processes: rope, slasher and loop dyeing. While rope dyeing provides more uniform color consistency, yarn breakage is more frequently encountered during the process. Continuous slasher dyeing allows faster processing of a yarn layer thanks to short dipping and color development times, but simultaneously results in higher hydrosulfite (dithionite) consumption due to multiple indigo boxes having wider surface areas exposed to oxygen. Loop dyeing is typically performed in a single indigo bath and yarn color intensity is adjusted by multiple dippings into the Vat.

Because the indigo process is based on a phenomenon called "ring dyeing", there are many parameters that must be kept under continuous control throughout the process to achieve uniform color during dyeing.

Among these, pH (for correct NaOH - alkali - dosing), Na2S2O4 / indigo concentrations, bath conductivity value and redox potential are particularly noteworthy. Additionally, since temperature value affects many parameters such as pH, reducing agent consumption and the diffusion rate of leuco-indigo into fibers, baths are operated at high temperature values (up to 80°C) and care is taken to maintain the selected temperature constant.

Potentiometric Hydrosulfite/Indigo Analysis

In the indigo dyeing process, analysis of hydrosulfite and indigo parameters present in the bath sample is mostly performed using potentiometric titration technique with the aid of an automatic titrator. Samples are continuously and periodically taken from one or several bath points during a typical dyeing batch and the entire dyeing Vat is monitored. Based on the analysis results obtained, the necessary feeds are continuously applied to the bath circulation systems and the aim is to maintain constant dyeing color. In this respect, it is possible to characterize hydrosulfite/indigo titrations as "live and dynamic" analyses. In hydrosulfite/indigo bath analysis, which is an oxidation/reduction titration, potassium hexacyanoferrate (III) (K3[Fe(CN)6]) is used as titrant, and to prevent dye oxidation as well as hydrosulfite decomposition, all work is performed under continuous N2 gas flow in a closed system to the external environment. The dye sample is added to an alkaline dispersion solution (Setamol WS) located under a nitrogen gas blanket. Potential changes occurring during automatic titration are monitored by means of a Platinum Ring Electrode (Titrode) and equivalence points are determined. Hydrosulfite/indigo titrations produce curves with 2 distinct equivalence points within optimized method parameters. The first equivalence point in these curves is used for hydrosulfite and the second equivalence point is used in calculating indigo concentration along with hydrosulfite consumption. Automatic titration systems used in hydrosulfite/indigo bath analyses are generally installed immediately next to the dyeing baths to minimize the time elapsed during manual sampling and introduction of the sample into the titration cell under nitrogen gas (aimed at minimizing contact with oxygen present in the environment). Following the sampling phase, the entire analysis process is performed automatically by the titrator and the operator is guided with visual instructions at all stages of the analysis. Metrohm Titration Systems, specially equipped for hydrosulfite/indigo bath analyses, have been successfully used in all leading dyeing facilities operating in our country for over 30 years and establish both local and global standards of the sector.

Online Indigo/Hydrosulfite Analysis

In light of the information above, it is clearly seen that the largest variable affecting results in hydrosulfite/indigo bath analysis is the manual sampling process that occurs before titration. This process can contain, besides the operator's training and experience, many different potential sources of error (sampling point selection, sampling volume, cleanliness of equipment/accessories used, analysis system usage, analysis frequency, etc.). Given that the yarn is in continuous motion between baths during the Vat dyeing process, it is clear that any type of measurement/monitoring error that could cause color variation can lead to serious time, raw material and cost losses and can dramatically affect finished product quality. It is evident that an online analysis solution that would allow the aforementioned potentiometric analysis to be performed continuously and fully automatically at all stages without requiring any manual sampling step, and simultaneously could transmit the measurement results in real time to the facility management console, could make significant contributions to the process. Within this framework, as Metrohm, we present the 2035 IndiLyzer online analysis platform, designed within the scope of our long-standing work focused on developing hydrosulfite/indigo bath analysis and the knowledge, experience and expertise gained in the sector, as a next-generation solution to the industry.

Equipment and Software Used

The 2035 Indi-Lyzer system constitutes a complete online analyzer platform including precision sampling, multi-directional active gas control, fully automatic analysis, automatic cleaning steps and sample line back-washing routines between batches, including sample collection from one or multiple points within a dyeing Vat that may consist of multiple baths.

Sampling

In the 2035 Indi-Lyzer system, the analysis sample is drawn directly from the designated dyeing baths. The sample brought to the analyzer via a pump from the closed-loop bath sampling line does not come into contact with the external environment in any way and within this framework, unlike manual sampling, the sample's contact with oxygen is completely prevented. Sampling can be performed from a single bath point, or if desired, sampling from multiple bath points is possible as an option. Typically, at least one 2035 Indi-Lyzer system is recommended for each Vat series. When higher analysis frequency and result resolution are demanded, multiple 2035 Indi-Lyzer sets operating with time offsets from each other can also be used. The 2035 Indi-Lyzer system has sample flow routines (sample line pre-rinsing) that will ensure the drawn sample represents the actual bath content. The precise sampling steps required for titration are automatically performed by the system following the representative sample's arrival in the analyzer. To achieve the highest accuracy in this process, Dosino™ type dosing units with 0.001 mL precision and precisely calibrated sampling tubes are used. All analysis cell liquid handling steps occurring before, during and after titration, including precision sampling, are performed entirely under nitrogen gas and the sample is not allowed to contact environmental oxygen at any stage.

Online Potentiometric Titration

Titrations performed within the 2035 Indi-Lyzer online bath analysis system are applied in a manner that will show parallel operation with our analysis methods used in indigo dyeing facilities for many years. Following automatic sampling, all stages of the titration are programmed to exactly match titrator analysis method parameters that are specially optimized for each customer by us and located in the customer's facility, thus allowing easy comparison of online analyzer result performance with manual sampling automatic titration results.

Analysis Results

The accuracy and repeatability of analysis results obtained with the 2035 Indi-Lyzer demonstrate that the system can be successfully used in indigo dyeing processes. Compared to manual sampling automatic titration analysis currently performed in dyeing facilities, it is observed that this system brings many important advantages. Sampling errors, inability to determine equivalence points, repetitive and/or incorrect results occurring in automatic titration-based analyses performed with manual sampling significantly lose their impact with the 2035 Indi-Lyzer. With the transition of the sampling and analysis process to an online automation system, the labor and time requirements that bath operators must allocate for the analysis itself are minimized, thus creating greater opportunity for focus on bath feeding, dyeing quality optimization and Vat management. The 2035 Indi-Lyzer is equipped with various industry-standard data transmission infrastructure such as 4-20 mA, Modbus TCP, Profinet, Profibus, Teamviewer, etc., aimed at enabling continuous monitoring of result, operational and system parameters and providing intervention capability in case of any disruption. Thus, control capability is provided for all similar processes including result transmission, remote monitoring of the analyzer, rapid remote support from the local Metrohm Process Analytics team when necessary, etc.

Conclusion

Metrohm Process Analytics has over 40 years of global experience in online process analysis. Our installed base of over 10,000 analyzer equipment units that are safely used by leading industry leaders operating in many industries worldwide is a clear indicator of our global experience in this regard. Within the scope of our long-standing analytical method and system experience in the indigo dyeing sector in our country, with our 2035 Indi-Lyzer platform specially equipped for hydrosulfite/indigo analysis, we are implementing a solid, reliable and precise online platform for your bath monitoring processes. The 2035 Indi-Lyzer is a living and continuously evolving platform. Within the scope of our customers' facility requirements and field experience, it is possible to update system configuration through applicable optimizations in sampling, analysis parameters and overall analysis efficiency, thus providing added value to your investment. As the Metrohm Turkey Process Analytics team, we are able to provide comprehensive services ranging from project consulting, engineering and sales phases to analyzer installation, commissioning and training, system maintenance, accessory and spare parts supply through support, service and repair processes. Through our local structure working integrated with the Metrohm Process Analytics EMEA Regional Center, we are at your side for all your online analysis needs, especially indigo dyeing process titrations. References 1. Metrohm Process Analytics Process Application Note AN-PAN-1035 – Online analysis of indigo, hydrosulfite, and other parameters in textile dye baths 2. Metrohm Process Analytics 800005229EN – Textile Industry, Dependable online, inline and atline solutions for your process needs