All our rental stock is 50/50 polyester/cotton and after about four washes the white sheets fade to a yellow, while 100% cotton sheets stay white.
We have had the water tested and no clues came up there. Can you offer any ideas?
Shaun Jennings
Ian Harris of the Laundry Technology Centre, replies:
There are a variety of reasons that could potentially cause this problem. Each possible cause needs to be thoroughly checked in order to eliminate it.
1. Residual alkalinity
Residual alkalinity on textiles following the wash process is the most common cause of yellow discolouration. The alkalinity on textiles will be from two principle sources – raw water alkalinity, normally carbonates, and residual alkalinity, the detergent alkali remaining after the final rinse.
As a general guide the raw water alkalinity is a fairly ‘mild’ alkali and most fabrics can withstand up to 24g/g (75ppm) if they are ironed within a couple of hours of washing. If left overnight, some galling (yellowing) may occur.
However, the alkali used in most modern, fully-built synthetic detergents is traditionally sodium metasilicate, which is a strong alkali and as little as 3g/g (10ppm) will be sufficient to cause yellowing.
A quick way of checking if the cause is due to residual alkalinity is to put a drop (or two) of phenolphthalein indicator onto the washed damp fabric (or even the ironed article that has turned yellow). If the phenolphthalein turns purple there is excess alkali present and the load should be rinsed further or a sour used on the final rinse to bring the fabric final pH to between 6 - 7.5.
2. Residual bleach (hypochlorite)
If sodium hypochlorite is used on the first rinse of a wash process, it is not unusual for some to carry over to the final rinse, especially if usage levels are higher than recommended. Even the slightest trace of residual hypochlorite on fabric when it arrives at the ironer will be sufficient to turn the items yellow.
This can be tested by dipping potassium starch iodide paper into the final rinse. If it turns blue/black, there is residual hypochlorite bleach present and the dosage should be reduced or the rinse dip levels checked.
Another possible cause associated with hypochlorite bleach is that certain types of fabric dressings and Optical Brightening Agents (OBA) used by some textile manufacturers are highly sensitive to hypochlorite (and alkalis) and will chemically bond with these products making it impossible to remove them during the rinse stages. The items turn yellow when they are passed through the ironer.
We have not yet identified a satisfactory method of removing this type of yellow discolouration and can only recommend that once the problem has been identified, the manufacturer is persuaded to use alternative types of fabric finish.
3. Breakdown of OBA
There is a wide range of different types of OBA available. The largest volumes of OBA are used in the paper industry and only a small amount is used on textiles and in detergent formulations.
OBAs are also extremely expensive and are often less than 2% of a product total formulation. These are designed for specific applications and many either need high temperature to bond with textiles, especially polyester OBAs or are specific for cellulose (cotton).
If the incorrect (and often cheaper) OBA is used on either the new fabric and/or the detergent used and it is heat sensitive, it may be able to withstand the wash process that seldom exceeds 75C but may be unable to withstand the ironer temperature of 160C and will therefore readily decompose to cause a yellow or grey tint to the material.
If an inappropriate OBA has been used on the new fabric, after repeated wash cycles the item should start to pick up OBA from the detergent and return to its original bright white finish. On the other hand, if the detergent contains the incorrect (or insufficient) OBA, the problem should be discussed with the chemicals supplier.
4. Iron (Fe)
Iron (Fe) contamination is by and far the most difficult and complex problem to resolve. If there is as little as 0.5ppm of iron present in your water system, problems of fabric discolouration can occur. Obviously the more iron present – the greater the problem.
Iron can occur in the water from a variety of sources. Even the smallest amount of iron present during the wash process is sufficient to cause a large and very rapid loss of colour. Iron interferes with the performance of the OBA and there is an ion exchange between the sodium head of the OBA molecule and the iron in the water to form a yellow precipitate that will cause fabric yellowing.
As pipework in the laundry begins to deteriorate with age, especially in a naturally very soft water area where the water may be mildly acidic, or if in an extremely hard water area where the water softener needs to be regularly regenerated, the acid in the water or the salt from the water softener will rapidly corrode the water lines to produce rust in the water.
Iron can also come from the boiler as a result of boiler priming. The water inside the boiler has a high concentration of dissolved salts containing iron as well as cobalt, vanadium and a whole range of other heavy metals. It is essential to ensure that the total dissolved solids (TDS) in the boiler is kept below 2,500ppm. If it should rise over this level and there is a wide fluctuation of steam demand on the boiler during the day, e.g. two or more washer-extractors call for steam simultaneously and the boiler pressure suddenly drops, some of the liquid inside of the boiler can be pulled over with the steam to cause the contamination in the wash process.
This problem also has further “knock-on’” effects as the now contaminated steam that also supplies the finishing equipment means that instead of pure condensate being returned to the boiler, it is contaminated with the dissolved solids and further increases the TDS level in the boiler – and so the problem spirals on upwards.
A simple test on the fabric using 3N Hydrochloric acid and Ammonium thiocyanate will tell you if there is any iron present on the fabric. If there is a positive result to this test then it is a process of systematically checking through the water and steam supply lines to identify where the source is coming from.
A further major concern related to the presence of Iron in the water or on the fabric is the amount of chemical damage if any oxygen bleach is used. In the presence of most bleaches, especially sodium hypochlorite and hydrogen peroxide (as well as peracetic acid), there will be very rapid fabric damage and fabric life can be shortened by more than 50% if bleaching and iron levels are excessive.
In this case, the 100% cotton sheets are staying white – but it is the 50/50 polycotton sheets which are being affected.
Based on our experience of similar problems it is possible that there is a problem with the OBA on the polyester part of the sheets.
If the polycotton sheet is woven with polyester in the warp and cotton in the weft, take a small cutting (say 5cms square) and “fray it back” to expose the warp and weft yarns. If the problem is one associated with the OBA, the cotton yarns should appear pure white whereas the polyester yarns will be a dull yellow/grey. If the polyester is an intimate blend (where the cotton and polyester fibres have been mixed prior to spinning) it will be necessary to use a microscope to identify if there is this distinct colour difference between the two types of fibre.
If such a colour difference can be positively identified, the cause will be one of the following:-
- Iron in the wash process interfering with the OBA adhesion to the polyester fibres; and/or
- Incorrect application or incorrect type of OBA used during manufacture of the article; and/or
- Incorrect type or insufficient OBA present in the detergent formulation.
Shaun Jennings must now systematically work his way through the possible causes to eliminate each one until he finds the solution.
