The global market demands are supported by the use of the cpb processing. The key factors are continued pressure on cost, improved quality (both appearance and fastness) and reduced lead times in the global textile supply chain. These requirements combined with shorter production runs due to reduced lead time or multiphased fashion seasons means the dyehouse must be capable of excellent lab to bulk transferability. These market needs are combined with new fabric-constructions and fiber blends which can be dyed by the cpb process (fig. 1).
Fig. 1: Global trends for CPB application of reactive dyes
Todays cpb process could not have been successfully developed further without the combined approach of many participants in this technology. DyStar as a leading textile solutions provider is working together with the market-leading machine manufacturers, e.g. Ksters, to provide optimised conditions for dyestuff application and control with one target: to achieve Controlled Pad Batch Technology.
In addition to machinery and control developments the research & development of new reactive dye chromophores and anchor systems have continued to innovate to provide better pad liquor stability also under different climate conditions, higher fixation yield and optimised compatibility in ternary combinations. The development of the Levafix CA dye range has set the benchmark in new dyestuff chemistry with a number of Color Confidence and Controlled Coloration benefits for dyehouses.
Importance of temperature control
No light without shade! The cpb process which can appear to be a simple application process must be consistent. Fig. 2 describes the importance of temperature control and its influence on a given yellow/red/blue ternary combination. This 3-color-combination is excellent up to 20 C but with all temperature influencing parameters the pad liquor temperature can easily increase to 28 - 30 C. The combination is then inferior resulting in poor repeatability, tailing and costly reworks.
Fig. 2: Influence of temperature control on a combination dyeing
Importance of reactive dye diffusion
The quality of a given fabric suitable for shirting, workwear or chinos is to a large extent dependent on dye diffusion, the capability of dyestuff to penetrate into the interstices of the material and yarn and to remain fully diffused throughout the entire process. The effect of poor diffusion and penetration after 10 domestic wash cycles using a modern activated bleaching detergent can be washdown (fig. 3) and white crease marks (fig. 4).
Examination under higher magnification by moving from the surface of the dyed material into the single fiber, we see clearly the reason for the above described problems and can easily detect the difference of good dye penetration and poor dye penetration (fig. 5).
Full diffusion of the yarn is not only important for the appearance of a garment but also for the fastness properties. Evaluations using different dyes have given dramatic differences in the fastness properties. A comparison of fastness properties against the degree of diffusion of a navy based on the important but oxidative bleach sensitive Reactive Black 5 and a grey based on the high fastness performance Levafix CA dyes is shown in fig. 6. The figures are self explanatory: light and multiple wash fastness of Reactive Black 5 are reduced by 1.5 grades strictly due to poor diffusion.
It will be observed that the Levafix CA dyes are suitable for the M&S C10A test, but are reducing significantly in light fastness, combined with a strong decrease in multiple wash fastness if the dyes are poorly diffused. Reduced fastness properties of the dyes selected for high performance with the risk of customer complaints.
Preconditions for good diffusion
The basis for optimised diffusion starts with pre-treatment. It has to be ensured that the wetability of the material is excellent and the pH-value is slightly acid to neutral and is distributed evenly throughout the whole batch of material. Good wetability is self explanatory, but even a slightly alkaline pH-value can increase dramatically the substantivity of the dye-stuffs which consequently reduces their opportunity for diffusion. The drying process must be controlled to guarantee uniform moisture content. The normal way to produce a uniform fabric is to over dry the material, but these results in poor wetability combined with poor lab-to-bulk reproducibility. Nowadays a moisture control system is strongly recommended.
Good diffusion does not end with good pre-treatment and drying processes. The correct alkali system has to be used, especially if dyeing tightly woven material. "Soft" alkali systems control the rate of fixation and al-lows the dif-fusion into the cellulose structure of the dyes which should be selected according to their molecular structure and of course according to the required fastness properties. Influencing parameters on degree of diffusion are:
wetability
uniformity
pH-value
moisture content
overdrying
soft alkali systems
molecular structure of the dyes
To meet the above criteria DyStar's dyestuff recommendations are shown in fig. 7. The soft alkali recommendation for Levafix CA dyes to promote diffusion of reactive dye before fixation is shown in fig. 8. Silicate free and alkali reduced systems can offer some more advantages such as easier clean down of the whole cold pad batch unit, no hardening of the rollers, easier wash off of the fabrics combined with softer handle. Using the Optidye CR computer program for the accurate calculation of pad liquor stabilities and batching times under any circumstances assist the dyer in controlling and optimising the whole dyeing process.
Emerised Fabrics
Emerised fabrics for leisurewear, womens wear, smart casual trousers etc. are one of the important fashion trends. The emerising process (also known as sanding, sueding, brushing) has been developed to include standard grade emery paper, diamond shaped emery paper and carbon coated PA-bristles to produce different effects and handles but which results in different dyeing properties in terms of face-back colour uniformity. In addition to the normal difficulties a cpb dyehouse faces additional fastness challenges for emerised material which are not easy to achieve the following requirements:
High light fastness C9: 4 on-tone fading
High multi-wash fastness C10A: 3 - 4 on-tone shade change
High wet light fastness C9A
Perspiration fastness C7
NOX-stable, ozone fastness
Non photochromism
Good wash and wear properties
These requirements can be contradictory in terms of dyestuff selection in respect of repeatability, face-back color uniformity and diffusion proper-ties.
Padding configurations
The padding technologies shown in fig. 9 are used for around 90% of all production and lab dyeings. The U-shaft and Mini MFLO technologies (both from Ksters) and comparable systems are mostly used in production, whilst the nip dyeing is mainly used in the lab, and is only suitable for a few articles (e.g. shirting) for production. However the dyeing results differ significantly between these two technologies which is highly apparent on emerised material (fig. 10). The difficulties start in the lab. Even if the face-back uniformity is commercially acceptable on a lab dyeing significant shade differences between trough and nip dyeing are possible when transferring to production. With appropriate dyestuff selection this shade difference can be minimised (fig. 11).
Dyeing properties are not only dependent on the padding configuration technology but also on the speed used which can create high reproducibility problems from different dyestuff combinations (fig. 12).
The importance of lab reproducibility cannot be underestimated. Each lab has to determine the right correlation between the lab equipment, process and production. Speeds of between 30 and 60 m/min are not possible for the lab; consequently a correlation between the two has to be found. The latest development from the machine manufacturer Ksters (Padmaster) is a helpful tool for correlating lab and bulk procedures.
The decision process for new orders depends in many cases on lab dyeings in terms of fabric appearance, fastness properties and shade uniformity. Bearing in mind that most lab dyeings are produced by the nip thread-up, the fastness properties (especially the light fastness) can be inferior in bulk if produced by trough thread-up. In the following table the effect on light fastness is illustrated. It will be observed that the pad dyeing configuration is critical but also dye selection can mitigate the differences in certain shade areas.
Effect on fastness of different thread-ups
Moreover, the correct dyestuff selection is essential in addition to all the other discussed issues when dyeing emerised material. DyStar has developed recommendations to meet all performance requirements in Controlled Pad Batch including the appropriate soft alkali system for best diffusion and the correct dyestuff selection with Levafix CA and Remazol RGB, the correct speed for best reproducibility and the correlated machine set-up for optimum lab to bulk transferability.
Based on many lab and production trials the optimised dyestuff selection for the difficult earth tone shades on emerised woven fabrics is
- Levafix Amber CA non photochromic, high light fastness
- Levafix FastRed CA high light fastness
- Levafix Blue CA high light fastness
- Levafix Olive CA100 homogeneous, non photochromic
- Combined with high light and wet fastness
All Levafix CA dyes show extremely high color build-up and highest robustness in production. Ternary combinations based on the above demonstrate good lab to bulk transferability and bulk to bulk repeatability, highest fastness performance and excellent face-back uniformity.
Production trials have to be made to test the lab to bulk transferability but also to be aware of material differences due to emerising. The bulk processing conditions have to be imitated in the lab. There has to be a correlation between the different process conditions.
Conclusion
The expertise involved in Controlled Pad Batch technology provides the basis for better quality, higher productivity, higher differentiation and less production risk. The cpb process continues to develop further improvements for optimising diffusion and reproducibility through dye innovation, application and parameter control. This can only be achievable by close co-operation between global retailer, innovative dyestuff producer and leading machinery manufacturer to develop optimised solutions for new fastness performance requirements to meet increased customer expectations on the latest fabric developments whilst minimising non-conformance material.
About the author:
DyStar as a leading textile solutions provider offers optimised dye selection and application technology to meet the highest performance requirements. As a partner for the whole textile chain DyStar provides Color Confidence to fulfill all requirements of the worlds leading retailers and Con-trolled Coloration to offer textile mills cost-effective dyes and right first time technology for any requirement. Investments in R&D and cooperation's with major retailers, brands and worlds leading textile machine and fibre manufacturers help DyStar to stay at the forefront of the industry.
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