1. Abstract
A pair of white polyester Ladies trousers which had beenpacked with polypropylene film had partly changed to yellow on storage in thewarehouse in Turkey.
What could be the reason? And what could be done to prevent it?
The yellowing has possibly been caused by a constituent ofthe packaging film which has leaked out onto the fabric during storage andwhich can turn yellow under the conditions of storage ; most likely gaseousoxides of nitrogen. Similar effects can be seen on packaged articles stored in a shop window due to light fading of the leached constituent. This articlediscusses about the latter stages of the supply chain and concerns thepackaging of dyes textiles / garments in plastic or polypropylene bags.
2. Full Article
2.1 Offending Component.
The offending component is the anti oxidant BHT (ButylatedHydroxy Toluene). The problem has also been observed on Cellulosic fabrics
The problem is well known, and has been investigated, in Japan.
2.2 Yellowing by BHT
The yellowing usually occurs on folded parts of the goods whosesurfaces are exposed.
The problem does not just occur on white goods; but also onpastel coloured yarns, fabrics and finished goods.
To a certain degree, the yellowing is pH dependent. Alkalinefinishes make the problem worse. Acidic finishes are recommended. Acidcounteracts yellowing.
2.3 Diagnostic Test
As a diagnostic test, BHT can be removed from the fibre bydissolution in an organic solvent, such as DMF (dimethyl formamide).This is auseful test, but not a practical solution to the problem. BHT can also bedetected by Gries reagent, which produces a red colouration in the presence of BHT.
The Gries reagent is a. 1:1 mixture of 1%(w/v) sulphanilamide in 2.5% (w/v) phosphoricacid .
A screening test to reduce customer complaints is described later.
Expose the yellowed area of fabric to acidic vapour (e.g. hydrochloric or acetic) and confirm that the yellowing has reduced. Expose that area of fabric to ammonia vapour and confirm the return of yellowing.
Acid counteracts the yellowing, alkali brings it back again. Yellowing on Storage can be indicated by the following behaviour:
- Intensification with Alkali (eg. exposure to hot ammonia
fumes making the yellowing deeper in shade).
- Reduced intensity of shade with acidic treatment.
- Easily dissolved my methanol, ethanol and other solvents.
- Removed by mild washing.
2.4 Migration onto the Fibre
BHT is a phenol type anti oxidant. It is added to PP (polypropylene) and PE (polyethylene) to prevent degradation of the polyolefin by oxidation. It is believed that if the polyolefin contains more than 0.1 BHT per kg of film, then yellowing may occur.
The effect is often observed in areas of the garment directly under parts of the packaging material that have been sealed with adhesive tape or adhesive strips. The solvent in the adhesive can leach the BHT out of the packaging film and onto the garment.
2.5 Mechanism of Yellowing
2.5.1 Chemical Changes
It is believed that anti-oxidants, especially so-called sterically hindered phenols, react with air impurities (particularly oxides of nitrogen) to form yellow pigments.
For example:
Butylated hydroxy toluene ( often used as anti-oxidant in packaging materials, cardboard, polyethylene (polythene bags) or 2.6. di-tert-butyl-p-cresol react by oxidation and nitration in nitric oxide ( present in the air especially near to air conditioning and heating equipment) to give:
2.6 di-tert-butyl-4-nitrophenol - which is yellow. (1)The oxidation is "base catalysed" and takes place in practice in slightly alkaline medium only.
In the same way but without nitration the following quinones can be formed:
3.3' 5.5' tetrabutyl-tert-butyl 4.4' stilbene quinone - which is also yellow. (2)
3.3' 5.5' tetrabutyl-tert-butyl 4.4' diphenon quinone - which is yellow. (3)
(1) is the major part and shows the acid-base conversion from colourless to yellow. (Colourless in ACID - yellow in ALKALI). In this case the cationic softener might be the base as well - particularly real quaternised products based on dimethyl distearyl ammonium chloride are very sensitive, and this includes products applied as domestic softeners.
2.5.2 Origin of the components
Sterically hindered phenols are substances used in a wide range of products. Particularly because of their ease of oxidation they are used in packaging material, lacquers, adhesives and also in various synthetic fibres as anti-oxidants. Thus soiling of the textile with these products can practically never be excluded. Likewise nitration by nitric oxides also is possible as particularly exhaust fumes (from car and truck engines etc.) always include nitric gases.
2.5.3 Influence of the softener
In such cases the softener represents the medium for reaction and due to the basic character of most softeners it, at the same time, acts as catalyst since oxidation of the phenol can take place only in alkaline media. Concentration of the softener need only be micro gm per cm2 on the textile goods.
But even unsoftened goods can show this yellowing if sufficient quantities of BHT (butylated hydroxyl toluene) are present, AND a slightly alkaline pH, AND any kind of fat or wax present to act as reaction medium.
2.5.6 Solving the problem
BHT yellowing is at its maximum (worst) when the following are present on the fibre :
- BHT
- Residual alkali (after finishing)
- Moisture (humid storage)
- Gaseous oxides of nitrogen.
Therefore it is important to:
- Avoid
packaging films which contain BHT
BHT is a widely used anti oxidant because it is relatively cheap, but many higher quality anti oxidants are available. The most secure method to prevent these effects is to make sure that BHT or any other sterically hindered phenol does not come into contact with the goods.
- Avoid exposure
to gaseous oxides of nitrogen, prevalent in exhaust fumes from delivery
vehicles in such areas as loading bays; or drying areas which are fuelled
by gas. Open
access of oxides of nitrogen should be minimised. Drager tubes can be
obtained which are placed in storage areas to test for the presence of oxides of nitrogen over long periods of time. The air exit of warehouses should, for
example, not be situated at a busy street where exhaust fumes can blow in.
Provide good ventilation. Use electric fork lift truck in warehouse not
petrol/diesel.
- Avoid storage of goods in humid conditions. Do not pack
clothes immediately after steam setting. Keep packaged clothes airtight.
- Avoid alkaline
finishing. (Acidic preferred). The goods should be neutralised with a
non-volatile acid (for example citric acid or oxalic acid) and after acidification it
should be rinsed to remove the salts.
Softening in the acidifying bath should be avoided as otherwise salts with an alkaline reaction remain on the goods.
The softener bath should also be applied with a permanent acid to maintain the pH on the goods in the acid medium in any case, even after enhanced storage.
Different softeners will absorb oxides of nitrogen to different degrees, and softeners should be screened with this in mind.
2.5.7 Avoiding the Problem
- Arrange for your garments to be tested before they are supplied to retail.
- The phenotest has been shown to be useful in
controlling the quality of white and pastel goods.
- This test is intended only to predict the yellowing which may be caused by the action of oxides of nitrogen on yellowable phenols.
- It is NOT intended to deal with any other causes of yellowing such as fading of optical brighteners, gas fume fading or fading of softeners.
- Further details of the test and testing equipment are
available at www.pp-t.co.uk.
The test is intended only to predict the yellowing which may be caused by the action of oxides or nitrogen on yellowable phenols. The test is not intended to deal with any other causes of yellowing such as fading of optical brighteners, gas fume fading or other such causes.
Phenotest has shown to be very useful in controlling the quality of white and pastel coloured goods and can significantly reduce customer complaints due to this factor.
2.8 Concluding Remarks
Four kinds of yellowing are commonly known:
- Yellowing by oxides of nitrogen. Found on Cotton, Acetate and Nylon. Caused by reaction between dye (reactive and disperse) and nitrogen oxide gas (retained in the fibre by exposure to gas stoves, vehicle exhaust fumes). The problem can sometimes be resolved by Dye Selection.
- BHT in packaging.
- Yellowing of the softener used (on heating). This is an entirely separate subject, with different reasons, causes and mechanisms. It is a processing issue rather than one which arises in the storage and transit of finished goods.
- Fading of optical brightening agents. One example of an OBA to prevent yellowing available from BASF is enclosed here.
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