The high variety of anti-yellowing agents available on the market provides a great potential for the application to finishing processes of elastic textiles. However, many products with nearly the same properties are used whereas the benefits of using speciality products are mostly unknown.

Zschimmer & Schwarz offers a complete range of effective anti-yellowing products for almost every field of finishing of stretch fabrics which grants the textile industry optimal process reliability by custom-made chemistry.

1. General problems

Yellowing in particular of white or pastel shade elastic fabrics during finishing, storage and transit is a big challenge for the textile finisher. The trouble of yellowing can cause complaints and confrontations between the textile finisher and garment manufacturer or ultimate customer. There is also the fact that processes for avoiding or eliminating yellowing on fabrics are very time-consuming and involve high costs. So this problem cannot be neglected.

Generally spoken, the textile industry has to deal with thermal and storage yellowing. But the phenomenon of yellowing of elastic fabrics is more complex which has to be considered in order to avoid it. More or less yellowing can be caused as a result of:

1. thermosetting



2. molding due to contact heat



3. storage



4. thermosetting and storage due to oxides of nitrogen



5. molding of fabrics with foam interlinings (e. g. foundation garments).

It has to be taken into account that the yellowing propensity of stretch fabrics is different depending on the individual substrate and fibre additive applied.

On the contrary, there are manifold and high reqirements of the customers such as:

1. high degree of whiteness and brilliance of shades



2. low yellowing during molding (shade compatibility with the primary article)



3. mark 4 of phenolic yellowing assessment as the minimum standard for storage yellowing protection



4. final pH value of 4 to 7 of the finished goods



5. resistance to the exposure to NOX.

The textile finisher frequently has to make compromises in order to reach these goals. The protection against phenolic yellowing is often stressed as the most important condition with regard to the finishing of elastic polyamide fabrics, however, it is eventually only one of several interfering criteria. For example, when the main focus of the finishing result is a very good protection from storage yellowing, the possibilities of meeting other demands like a neutral up to weakly acid pH value of the goods are restricted. These effects and the problems involved can only be controlled by the selection of specific textile auxiliaries and corresponding processes.

1. Thermal yellowing

First of all, heavy yellowing may occur during thermosetting of grey goods on gas-fired stenters, as a result of contact heat treatments (molding) and during bonding processes. The stretch fabrics that are especially concerned are those made from polyamide and increasingly polyester. The problem becomes even more serious in case of microfiber articles due to their extremely large surface. An increasing issue have also become knitted goods from cotton/elastane mixtures which have to be subjected to a preliminary thermosetting process in order to eliminate edge curling and to obtain sufficient dimensional stability. Often, the goods turn brown after this treatment so that time consuming and high-cost scouring and bleaching procedures are necessary to achieve the whiteness degree that complies with full white.

Auxiliaries that are designed to provide protection of the goods from thermal yellowing prevent or minimize yellowing in case of thermal stresses. Zschimmer & Schwarz has a range of several products for peventing thermoyellowing. The selection of the most suitable product depends on the kind of thermal stress, type of substrate and process conditions

(Picture1: Range of anti-yellowing inhibitors and their application fields).

The liquid products PROTELAN AY and PROTELAN CF 1 offer an excellent yellowing inhibition for elastic polyamide fabrics apart from their easy handling and spontaneous penetration. Furthermore, the anti-yellowing agent PROTELAN AY has very good emulsifying and dispersing properties because of the high surfactant content so that it is usually applied in presetting of elastic fabric qualities. This way, fibre lubricants and silicone oils are predissolved right at the beginning thus preventing their burning in and also a strong fuming and exhaust air pollution. In addition, PROTELAN AY has an outstanding affinity for the polyamide fibre enabling the application of the product to tubular fabrics in exhaust baths as well. Consequently, lubricant removal and yellowing protection operations can be economically combined. The treated articles show a substantially lower yellowing susceptibility during subsequent heat treatments. The powdery antioxidant PROTELAN ATY-P is used in those cases where scouring actions are not necessary any more but the main focus are highest demands on the whiteness and brilliancy of shade of the goods to be finished. The product provides optimal process reliability particularly for molding of polyamide/elastane fabrics

(Picture 2: Phenolic yellowing protection with PROTELAN LGA on bleached cotton).

Moreover, all mentioned anti-yellowing agents counteract oxidative damages of the polyamide fibre during thermosetting. Acting as radical scavenger they prevent a possible modification or degradation of the polymer chain including amino end groups of the polyamide fibre. Polyamide fibres without damages and thus a higher number of amino end groups have a higher dye affinity, as has been proven, which results in deeper shades and higher levelness of dyeings.

The product PROTELAN CEL has proven very successful in minimizing the yellowing of elastic cotton fabrics during thermosetting of the grey goods. PROTELAN CEL is capable of wetting hydrophobic grey goods quickly and evenly. Oil and fibre lubricant deposits are pre-emulsified during impregnation, natural fibre impurities of the cotton and yellowing radicals are eliminated. The whiteness degree required by the customer can be achieved in shorter scouring and bleaching times in most cases. From this follows that the application of PROTELAN CEL as well promotes a cost-saving process technology in addition to the main benefit of yellowing inhibition

(Picture 3: Tapes with brownish discolorations and spots under claim).

1. Phenolic yellowing

Apart from thermal yellowing the problem of phenolic yellowing as a consequence of storage should not be underestimated. The textile and packaging materials are in contact with each other during transit and storage. Antioxidants (mainly sterically hindered phenols of BHT type) of the packaging material can be transferred into yellow compounds by the action of oxides of nitrogen. The extent of the phenolic yellowing usually depends on the type of the packaging material, the pH value of the finished goods and the duration of the storage. In case of ageing of the yellowing by fixation or reaction with the textile goods, a complete elimination is nearly impossible. The only help are preventive measures such as finishing with storage yellowing inhibitors, for example. The product portfolio of Zschimmer & Schwarz comprises an extensive range of anti-yellowing agents designed to protect textile goods from storage yellowing. The selection of the appropriate product depends mostly on the yellowing propensity of the fabric, the kind of customer demands and the machinery available (Picture 1: Range of anti-yellowing inhibitors and their application fields).

PROTELAN LG is the standard product of our range of storage yellowing inhibitors which is very effective on elastic polyamide fabrics. The product enables the production of pale shades and achieving a maximum degree of whiteness for optically brightened goods. The application is combinable with the optical brightening or dyeing processes or the treatment against thermal yellowing. The resulting pH value is slightly acid according to the respective amounts used and the way of application. Machines with high liquor turbulences are no problem because PROTELAN LG is low foaming.

PROTELAN LGA is also a low-foaming product of the same kind. It has a higher acidity than PROTELAN LG and therefore a more powerful anti-yellowing effect. An additional pH correction of the treatment baths with citric acid is not necessary in most cases. The pH value on the finished textile goods is not lower than 4.0 when the usually applied levels are observed so that PROTELAN LGA is in compliance with Oeko Tex Standard 100 without a doubt.

Alkaline process water or such with a high buffering effect (e. g. water with a high bicarbonate content), insufficient neutralization after bleaching or alkaline (soda) washing as well as cationic softeners can cause yellowing problems during storage also on cellulosic and other synthetic materials. Also in these cases, the application of PROTELAN LGA shows very good results with regard to the protection from yellowing

Picuture 4: NOx-yellowing protection with PROTELAN NOX

Another special product presented by Zschimmer & Schwarz is PROTELAN LGS plus. In addition to the possibility of its classic anti-yellowing application to polyamide/elastane mix fabrics the product is specifically designed to be applied in continuous tape finishing processes. It has an excellent affinity for the polyamide fibre and does not develop an unpleasant, irritating smell so that the application in open baths is possible without reservation.

It is a well-known fact that tapes which have been assessed as qualitatively flawless by the phenolic yellowing test immediately after their manufacture show spots or brownish discoloration that appear sporadically after some weeks. So this is often observed later during garment manufacturing. It is a big problem because complaints arising from this are inevitable. PROTELAN LGS plus has proved to be an effective agent for avoiding the described spots and discoloration without reducing the good performance of yellowing protection

Articles from polyamide/elastane that are processed together with foam material (e. g. bras) require the application of a special agent which prevents storage yellowing and at the same time inhibits yellowing caused by molding where the material is subjected to a very high thermal stress (200°C for 120 to 150 sec). If standard storage yellowing inhibitors like PROTELAN LG or PROTELAN LGA are used, yellowing protection in storage will be sufficient, however, the yellowing during molding which depends on the type of foam material used is inadmissibly high despite the application of thermal anti-yellowing auxiliaries. The best solution in these cases is the speciality product PROTELAN FM by means of which satisfactory results can be achieved in order to meet the demands with regard to the protection from phenolic yellowing or yellowing caused by molding.

1. Yellowing caused by nitrogen oxides

Yellowing caused by the action of oxides of nitrogen depends to a high extent on the fibre substrate used which is similar to the phenolic yellowing. Thus depending on their elastane content, elastic polyamide or polyester articles show more or less slightly yellowish up to reddish discolorations when exposed to nitrogen oxide gases. Fibre lubricant deposits and fibre additives have an additional influence on the intensity of the NOx yellowing.

In practical operation higher concentrations of nitrogen oxide occur for example during thermosetting (gas-heated stenter), they are generated in warehouses (e. g. exhaust gases of forklifts) and they are an essential part of atmospheric pollution (smog). The effects of the exposure to nitrogen oxide are only visible after several weeks depending on the extent of the fibre damage which can lead to yellowing complaints. The effectiveness of common antioxidants that are used to prevent thermal or phenolic yellowing is not sufficient for acting against the influence of nitrogen oxides. After having carried out numerous laboratory tests Zschimmer & Schwarz have succeeded in developing a novel auxiliary that increases the resistance of elastic articles to yellowing caused by the oxides of nitrogen. The new product PROTELAN NOX prevents or reduces the yellowing of goods resulting from their exposure to nitrogen oxides thus ensuring a high quality with regard to the manufacture of optically brightened and light shade articles

1. Summary

It is possible to solve or at least minimize the entire range of problems concerning yellowing of elastic textiles by applying appropriate anti-yellowing agents. Zschimmer & Schwarz offer the textile industry a variety of innovative products launched as the PROTELAN range for a solid quality of the goods as well as cost-saving processes. A specific selection of auxiliaries can have a high impact on solving distinctive substrate and finishing problems as well as on meeting the customers’ requirements for brilliant white and colour shades. Processing times can be shortened and rather instable recipes will become more reliable.

PROTELAN = Product name of ZSCHIMMER & SCHWARZ MOHSDORF GmbH Co KG