by.  Antje Mehlhorn and Nicolai Wickert, Zschimmer & Schwarz Mohsdorf GmbH & Co KG

1. Demands on complexing agents

Complexing agents above all, are used for manifold purposes in pretreatment and dyeing processes (Figure1). So they are more or less all-rounders or specialists.

Nevertheless, it should be clear that there is no universal complexing agent that has an equally good performance in every respect. Most of the products in the market are compromise products which at a moderate price level cover a wide range of application within the scope of their least common denominator.

Depending on the product selected, complexing agents are able to

bind or mask interfering metals in order to prevent precipitations and scaling,

• disperse solid substances,

• support washing processes,

• stabilize hydrogen peroxide.
  

It is possible to prolong cleaning intervals of the machinery by the selective application of complexing agents gaining more productivity this way. They also act as process controllers having an accelerating or retarding effect on chemical processes depending on the respective field of application.

2. Mechanism and types

One of the most important qualities of complexing substances is undoubtedly the formation of stable chemical compounds with multivalent metals like copper, iron, manganese and alkaline earth metals. Such sources of trouble for washing, bleaching, dyeing, dispersing or soaping processes are eliminated from the system and the textile substrate with long-lasting effect. The chemistry of the effects for these applications is mainly based on organic molecules of phosphonic acids and their salts (phosphonates), polycarboxylates, hydroxycarboxylic and aminocarboxylic acids. The OPTAVON type family of ZSM provides the appropriate special product for the respective application (Figure 2 and 3).

Phosphonates

Phosphonates are stable under nearly all practical conditions of use and resistant to alkalis and oxidizing agents. They can be applied in acidic as well as alkaline media. Apart from their property of binding hardness elements (calcium, magnesium), they have also a selective effect on multivalent heavy metal ions (iron, copper, manganese). Some phosphonate types stabilize hydrogen peroxide as well, so that they are a valuable raw material for the manufacture of bleaching stabilizers.

Many phosphonate based complexing agents are even able to withdraw the central atom from metal complex dyes. These products are really cut out for the use in pretreatment to bind multivalent metals and prevent catalytic damages. However, they are not suitable to be used in dyeing since they can cause shade changes and the deterioration of the light fastness. The strongly acidic phosphonate containing complexing agents OPTAVON TW 6007 and OPTAVON 4UD can effectively disintegrate every kind of alkaline earth even without the addition of acids. So undesired formations of calcium sulfate or corrosion damages do not take place. Using the phosphonate based products OPTAVON MEX and SKIANTAN A-NW which are partly or completely neutralized, demineralization takes already place at weakly acid up to neutral pH values. These products have very high binding capacities mainly within this pH range. The complexing agent SKIANTAN A-NW is a special type of phosphonate which can be used in pretreatment, dyeing and after soaping procedures.

Polycarboxylates

The complexing agent OPTAVON SV is a main exponent of this group. The excellent performance of polycarboxylates with regard to the dispersing of solid liquor components provides particularly advantages when they are applied in combination with surfactants.

Disadvantages are the limited iron binding capacity under strongly alkaline conditions, which usually prevail during a pretreatment process, and a reduced calcium binding capacity. They have no peroxide stabilizing effect as well. The common application field of these products is dyeing or soaping of dyeings.

Polycarboxylates are also used in combination with phosphonates. This is a good compromise for the application in pretreatment when apart from a high binding capacity an excellent dispersing power is required. The universal complexing and dispersing auxiliary OPTAVON APO supports washing, dyeing and soaping processes in an excellent way.

Hydroxycarboxylic acids

These complexing agents are of minor significance for textile finishing due to their properties profile, however, they are readily biodegradable in general. OPTAVON BAS is such a product which is applied when a ready biodegradability is required. OPTAVON FE CONC, on the other hand, is primarily applied in bleaching baths for preventing catalytic damages because its salient feature is an extremely high iron binding capacity.

Amino carboxylic acids

The most common products of this group are certainly EDTA and NTA. Whereas the use of EDTA has been stopped in Germany for reasons of ecological harmfulness, NTA based products are still applied. NTA has a good heavy metal bonding capacity, though, it is not particularly effective in the presence of oxidizing agents and alkali which makes the product unsuitable for pretreatment processes.

The dispersing power of amino carboxylic acids is very low so that they have no essential effect on the soil suspending capacity of a wash liquor. The readily water-soluble complexing agents EDTA and NTA can remobilize heavy metals that are bound to sewage sludge. Used in sewage treatment plants, NTA is relatively readily biodegradable but it is suspected of being carcinogenic. That is why the use of amino carboxylic acids is either prohibited or they are at least attacked for ecological reasons. This class of substances is not included in ZSM’s production programme.

3. Ecological behaviour

In spite of extensive activities and efforts, ZSM and other manufacturers of complexing agents have not yet succeeded in developing ecologically harmless alternative products whose functional properties and application possibilities are equivalent to those of phosphonates. Nevertheless, there are lots of good reasons for the application of the existing types of complexing agents:

Phosphonates, polyacrylates and hydroxycarboxylic acids are not toxic. They do not contribute to the COD-load of waste water and comply with Öko-Tex Standard 100. Further, they do not cause eutrophication in waters.

In fact, polyacrylates have a low biodegradability which is similar to phosphonates but the greater portion of them can be eliminated from waste water by adsorption to activated sludge. The modified sugar polymers alone show a good biodegradability because of an integrated “allocated point of fracture”. However, the COD value of these products is usually higher. From all this follows that there are no significant advantages for one of the mentioned classes of chemical substances with regard to the ecological cost-benefit ratio.

All ZSM complexing agents are approved for the Global Organic Textile Standard (GOTS).

4. The application in bleaching and pretreatment

The application field that has the highest demands on complexing agents is pretreatment without any doubt. There is no other wet process in textile treatment where such a high contamination level has to be taken into account. Here, the complexing agent is exposed to heavy strains under highly alkaline conditions the handling of which can have a lasting negative influence on the quality of the final product right from the beginning. In fact, only OPTAVON TW 6007, SKIANTAN A-NW and OPTAVON MEX can handle the very high tasks of catalyst binding, demineralization and fibre protection as well as stabilizing, dispersing and supporting the detergency. These complexing agents manage modern bleaching processes ensuring high process reliability and a constant quality level.

The actual complexing reaction is a very quick procedure. The measure of the speed is the complex formation constant. But it takes some time and also a certain temperature is necessary to supply the complexing agent with the multivalent metal. So hydrophobic waxes have to be heated up above the melting point and sizing agents have to swell at first. Wetting and washing agents such as TISSOCYL DLF, TISSOCYL C80 or TISSOCYL RLB are suitable to perform these jobs.

Selected complexing agents as SKIANTAN A-NW, for example, based on made-to measure phosphonates are real multi-talents. When SKIANTAN A-NW is applied in the desizing process, the content of water hardness salts and catalysts can be reduced to such a considerable extent that far more reactive chemical power is available for the specific actions of the subsequent bleaching process instead of falling flat in a useless way. The application in combination with the high performance amylase NEWAZYM AM is possible without any problems enabling a single-stage demineralization and desizing without extra additives. This way, one cycle can be saved in most cases by a well selected combination of auxiliaries without reducing the process reliability. Such a preventive strategy results in a considerable improvement of the bleaching effects regarding degree of whiteness and absorbency providing protection of the goods and reducing the application levels to a minimum.

Applied in bleaching, phosphonate containing complexing agents as SKIANTAN A-NW, for example, prevent an extremely rapid degradation of hydrogen peroxide caused by destabilizing alkaline earth metals and catalysts. Moreover, this kind of chemicals effectively prevent building-up of deposits on rollers attributed to stabilizers.

When the most appropriate products and optimum process procedures are selected for the textile wet processes, a substantial savings potential is provided in contrast to the application of conventional bleaching and complexing chemicals. SKIANTAN A-NW allows a partially drastic reduction of the application quantities of the auxiliaries compared to less efficient complexing agents. Depending on the individual conditions and possibilities in the respective plant, different saving potentials up to 30% of the overall cost (including water and energy) can be achieved for one process step.Thus, the insufficient biodegradability of phosphonate based complexing agents which is often criticized can be put into perspective.

6. Summary/Prospects

With the manifold application possibilities of advanced complexing agents in mind, their goal-directed use results in a big saving potential for single process steps or even for the whole production process. Processing cycles can be accelerated or partly even eliminated and unsafe recipes can be made more reliable. Minimized application levels contribute to resource conservation. The statement “more is better” does not apply as it does not for any other aspect here. Modern process technology and design should consider complexing agents as important process controllers, a significant process variable of a complex system on which success or failure may largely depend.

Note: Optavon, Skiantan, Newazym, Tissocyl are registered trademarks of the company Zschimmer& Schwarz Mohdorf GmbH Co KG.