Understanding wash chemicals

3 June 2003

Laundry engineers need more than just a smattering of chemistry to survive at the sharp end of the textile market, says Richard Neale

A sound understanding of the basic operation of each of the chemicals used in a modern laundry is an essential aid to troubleshooting and will also help raise quality levels.

The best sorting operations usually take out heavily soiled and stained items and pass them forward for a heavy soiled program or a re-wash. There is little point in putting them through the tunnel washer first as they will come back for re-wash.

During sorting, many operations also set rust marked items aside for pre-treatment. The favourite method, although certainly not the safest, is spot application of a proprietary rust removal product based either on 6% hydrogen fluoride in water or a similar concentration of ammonium bi-fluoride. Items must be pre-treated just before washing, as both reagents attack cotton. Hydrogen fluoride in particular is strongly acidic very dangerous if splashed on the skin or eyes.

Launderers often pre-scrub shirt collars and cuffs with a strong detergent mixture to remove the maximum grime at these points. Using the correct bristles is as important as the soaping.

Main wash

Surfactants, which all detergents contain, improve the wetting characteristics of the water so that it penetrates the centre of each yarn and wets every fibre. Only in this way is there a prospect of removing normal water based soiling and staining. Once the surface tension has been lowered allowing the water to wet the stain, the water can start making the stain soluble to remove it.

The suspending agent in the detergent wraps around every particle of soiling or microscopic globule of fat to stop it getting back onto the clean cloth. The suspending agent prevents greying.

Sequestering agents are included in some detergents to capture any free hardness salts that have not been removed by the water softener. This prevents hardness salts from reacting with the detergent, again cutting the risk of greying.

Chelating agents are used in some detergents, and are designed to capture any free iron in the raw water. Most launderers will put a limit of 0.1ppm on the iron level in the raw water (measured at the inlet to the washing machine, so watch out for rusty pipes!). Iron levels higher than 0.1ppm will increase the risk of greying and occasionally lead to brown speckles on the work. A chelating agent offers a short term alternative to a proper iron removal stage in treating laundry water.

The emulsifier, normally added separately to the detergent, is designed to solubilise the oily, greasy constituents of soiling and staining including body fats and fatty proteins. It is an essential ingredient for abbatoir work and highly desirable for healthcare textiles.

A good surfactant with plenty of mechanical action at high temperature will also emulsify oils and greases but a purpose designed emulsifier is better and quicker.

Optical brightening agent (OBAs) are used in most white work detergents They bond to the textile surface and react with the ultraviolet portion of natural daylight, converting this invisible part of the spectrum into brilliant white visible light.

Linen for the five-star hotels needs a high level of optical brightness to make a dazzling first impression on the guest. OBAs bond well to natural fibres such as cotton and linen but are not very effective on polyester or nylon. They will make pastels look grey and dingy and colours prematurely faded, so should be avoided here.

Enzymes, which some detergents contain, digest protein and other animal soils and stains. The enzyme slowly and progressively breaks down the protein into manageable parts which can either be dissolved or flushed away. Enzymes have been associated with skin irritation and rashes but on investigation these frequently stem from poor rinsing rather than from enzymes in the detergent.

Effects of bleaches

Bleaching does not remove marking, it simply turns it colourless. Sodium hypochlorite, referred to as "chlorine bleach" or simply "bleach", is the most common. It is the oxygen atom in the bleach molecule which reacts with the dye in the stain to create a colourless bi-product. Chlorine bleach is quite aggressive and always carries some risk of damage to the cellulose in cotton, reducing the stock's rental life. It will also react with certain medications (particularly chlorohexadene used in Savlon and other disinfecting products).

For these reasons, healthcare washing is generally carried out using hydrogen peroxide or a chemical that generates hydrogen peroxide (such as sodium perborate or sodium percarbonate). These tend to be more expensive than chlorine bleach but are just as effective on vegetable dye stains.

Chlorine bleach will also burn set protein stains (blood, for example) from cotton fabric but this tends to cause as much damage to the cotton as to the stain. Bleaches which generate hydrogen peroxide do not attack protein stains in this way, which is why they are sometimes unfairly thought to be a weaker bleach.

Reducing bleaches, such as sodium dithionite, formerly called sodium hydrosulphite and sold under the trade name Hydros, work in the opposite way to oxygen bleaches in that they remove a molecule of oxygen from the molecule causing the staining, turning it colourless in the process. They can smell a little like bad eggs but do not leave a smell on the work.

Laundry sours are designed to neutralise any alkali from the main wash that is not removed in the rinse stage. Laundry sours tend to be acidic so that they react readily with any alkali left in the rinse water. Typical sours include sodium metabisulphite and acetic or formic acids. A recent far more complex detergent system designated peracetic acid has the properties of the sour combined with both stain removal and disinfecting properties.

The main ingredient of the detergent is alkali. Early researchers found that blood and other proteinic body fluids could be removed far more readily at high alkalinity. For many years UK laundering was based on soap and sodium metasilicate, the latter providing the essential alkali ingredient to aid the chemical breakdown of soiling by soap (saponification).

More recent systems have used sodium hydroxide as a strong alkali, but systems based on sodium metasilicate, which is slightly more expensive, generally perform better.

An anti-chlor may be added to the final rinse to neutralise any traces of chlorine bleach remaining in the work. This is essential if the work is going to be heated in a tumble dryer or an ironer, because hot sodium hypochlorite solutions accelerate rotting of cotton and shorten textile life.

Some products combine the properties of sour and anti-chlor to give a dual action with a single injection and significant economy.

Fabric softeners

Fabric softeners are often added to the final rinse, to aid user comfort, especially for healthcare. They are often not very effective, particularly when used to correct the results of drying at too high a temperature. Sorting out drying temperature and time is usually far more effective.

Good dual-action softeners are available which double as a disinfectant. This can be particularly valuable for healthcare work, especially where the wash process is not that good, and the disinfectant kills bacteria that would otherwise breed in the unremoved soiling. This will improve the odour as well.

Chemical disinfectants are not regarded by the UK Department of Health as an adequate substitute for implied thermal disinfection in the wash process (which calls for a minimum of 71C for 3min plus mixing time). This is because use of a chemical disinfectant can encourage bugs to mutate and evolve, creating breeds of superbugs such as MRSA and possibly SARS, which are exceptionally difficult to treat.

Wash chemicals are not only used in the main wash process. Treating large quantities of re-wash for rust removal (including iron removal from old blood staining) is best done in a purpose-designed program that uses oxalic acid. No detergent or alkali is required. The work is heated to around 60C in a 2% solution of oxalic acid crystals. The iron oxide (rust) is converted to iron oxalate which is soluble and goes down the drain with the wash water. Oxalic acid does have some chemistry effect on cotton and will shorten both the material's life and that of the washing machine if the process is repeated too often.

Washing a load of white cotton sheets with a single red blanket is a certain recipe for creating a full load of pale pink sheets. This is often difficult to reverse using oxidising bleach, but the whiteness might be restored using a reducing bleach such as sodium dithionite. This works by removing an atom of oxygen from the red dye molecule, turning it colourless in the process. It is always worth a try.

Tunnel engineering

When dosing chemicals into a continuous batch washer it is essential to get the right ingredient into the correct compartment. If the injector runs via the centre trunion, as on a Voss, or via a tap in the outer shell, then the arrival point is reasonably guaranteed, provided that the centre trunion is in good condition with no cross-over, or that the outer shell is correctly isolated with proper seals.

Ensuring the right arrival point is far more difficult when the chemicals are squirted down the axis of the washing machine with a central pipe, as on the old Aquatrac design, which even relied on this method for getting the rinse water in the right place! It is surprising how often the detergent supplier attempts to vary dosages without a proper working diagram of the machine, resulting in problems that sometimes take weeks to identify.

It is essential that the correct dosage is injected. A peristaltic pump is a positive displacement unit that squeezes the flexible delivery tube for the appropriate number of seconds. It is extremely accurate when in good condition and hopelessly inaccurate if the tube becomes split or worn, or if the gearbox fails or the pump stops turning. Those systems that work best are those that are checked daily and rectified promptly.

The time in the wash cycle when the chemicals are injected is vital. If the detergent needs to be put in immediately after load transfer then a delay of 60sec will effectively negate the performance in one compartment and spoil an otherwise perfect wash. Errors on injection timing are among the more difficult problems to identify and sort out.

There is no point in squirting a little concentrated detergent into the appropriate injection line if this is not then flushed with an adequate quantity of water so as to take it immediately into the desired compartment. Perfect injection and faulty flushing produces a result every bit as poor as inadequate injection in the first place.

Finally, the designer of any wash process will always be wary of interactions, which is why it is so dangerous for an engineer with little chemistry knowledge to experiment with different chemicals, different quantities and different injection points.

Sour and detergent will neutralise each other so that very little happens. An acid sour coming into contact with chlorine bleach will release chlorine gas an effective weapon of mass destruction but of very limited laundry use.

An anti-chlor coming into contact with chlorine bleach too early will negate stain removal and lead to immediate restaurant complaints.

Health and safety

The most hazardous laundry chemical is probably rust remover based on hydrochloric acid, closely followed by chlorine bleach. The fact that this is a common household chemical leads many into a false sense of security.

Most laundry chemicals will cause skin rash or skin irritation and possibly permanent eye damage, so all need to be treated with respect. A health and safety inspector will expect to find excellent eye irrigation facilities in the tunnel washer chemical area and would ask all the obvious questions about gloves, goggles and face visors. Proper handling techniques, effective precautions and immediate access to first aid knowledge and equipment are vital components of a health and safety management system designed to control risks in this area.

Chemical reactions can be more dangerous when they take place outside the washing machine, especially between a keg of sour and a keg of chlorine bleach.

Separate areas need to be provided for all chemicals that interact. In particular, kegs should never be stacked more than two-high and different chemicals should never be stacked one on top of another. The chemical storage area should be labelled and secure so that other staff know the hazards and how to avoid these.

Storage conditions are important. Many chemicals are sensitive to heat or to bright light. Some liquid detergents are sensitive to frost and most powders are sensitive to damp. Laundry chemistry is not as complicated now as it was 40 years ago because detergent suppliers have taken over much of laundry manager's role here.
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