Follow the golden rules

As the cost of cotton has spiralled upwards, so more and more textile manufacturers have found innovative ways of producing polyester and other synthetic fibres to replace the cotton in many items. Because the textiles have the handle and feel of cotton – or at least of cotton rich items – customer acceptance has been good.

However, no matter what the polyester manufacturer does to the fibre to make it more appealing, it is still a thermoplastic fibre and will be affected by the heat and pressure applied in the laundry during washing and finishing.

When referring to polyester fibres and yarns, the term thermoplastic literally means that it softens and distorts as the temperature increases.

Unless the polyester fibres have been significantly modified, when they are examined under a microscope they will look similar to a solid glass rod.

If the fibres are creased – especially under high pressure at raised temperatures – the point where the crease occurs will “ripple”. The effect is similar to that of bending a hollow copper tube.

Once rippling occurs, it is impossible to remove and the yarn becomes permanently creased.

If cotton fibres are mixed or spun with the polyester fibres, the cotton will protect the polyester fibres to some extent.

This is especially important when the fibres are damp or wet, such as during the finishing process in a tumble dryer or on a multi-roll ironer and will significantly reduce the risk of creasing.

Unlike cotton, polyester does not absorb moisture. Cotton will absorb moisture into the structure of the fibre.

With polyester, any moisture sits on the surface of the fibre and in between the individual fibres. When heat is applied to damp cotton, it is the moisture in the cotton fibre that absorbs the heat and this moisture is then released as steam.

However, with polyester, there is so little moisture that the heat almost immediately attacks the polyester fibre. Because there is very little or no moisture to act as a “heat sink”, the fibres are warmed and can be distorted.

Weight in moisture

An item made of 100% cotton will absorb 2.85 times its own weight to thoroughly wet it. By contrast, 100% polyester fabric barely holds 5% of its own weight in moisture before it becomes thoroughly wetted.

However, during the extraction stage, it is quite normal to reduce the moisture content in 100% cotton to around 45 – 48%. This means that a kilogram of 100% cotton sheets will still have 450 – 480g of moisture to be removed during the finishing process. Polyester fabric blends will have significantly less moisture after washing so after going through an extraction process designed for 100% cotton, a blended fabric will have considerably less residual moisture.

For example, a 50/50 polycotton will have, at best, half of the residual moisture that will be held in a 100% cotton item.

As an example, if a multi-roll ironer has an evaporation rate of 4 litres/minite, it should be able to dry 9kg of 100% cotton with 45% residual moisture (approximately 16 double sheets) per minute (970/hr) with edge-to-edge feeding at 37metres/min.

A 50/50 polycotton sheet has only half the moisture content of a 100% cotton sheet. Therefore, theoretically at least, production can be doubled.

However, in practice this is not possible as some of the damp cotton fibres will be “insulated” by the polyester and will not have direct contact with the ironer bed and so take longer to dry.

Nevertheless, realistically, a 50/50 polycotton sheet should dry up to 50% quicker than its 100% cotton counterpart.

Polyester also has other properties that affect its behaviour. As it is thermoplastic and distorts with elevated temperatures, polyester suffers from what is known as “thermal shock”.

It means that in a situation where polyester fibres become hot and are then cooled rapidly, any items that become creased will “freeze” into the creased position.

Another property is that when wet, polyester remains relatively rigid, even when thoroughly wetted. In contrast 100% cotton effectively “collapses”.

Therefore when loading the washing machine, a load of polycotton fabric will appear to occupy a far greater space than 100% cotton, especially when wet. There is a decline in overall finish quality because exposure to the wash liquor and chemicals, as well as to the mechanical action, is severely reduced as a result.

Unfortunately many laundries tend to process 100% cotton sheets and pillow slips with polyester blends.

Therefore all the items in the load are all subjected to the same processing environment, the same moisture extraction process and the same finishing processes – with potentially quite serious consequences.

Launderers that wash 100% cotton and polycotton blends together will often load the washing machine to its maximum capacity. They fail to allow a “cool-down” to prevent thermal shock to the polyester and then subject the polyester to the same pressure during the moisture extraction process. So they should not be surprised to find that the polycotton items become severely creased and almost impossible to return to their original “as new” condition.

Observe the rules

There are certain “golden rules” to bear in mind when processing polyester and polycotton blends. Launderers should observe the following.

• Never process polycotton and 100% cotton together on a cotton process. Creasing will occur due to the pressure of the cotton work during the wash process and also due to thermal shock because of the lack or a cool-down stage. Further creases will be added during the moisture extraction process.

• Always “under-load” polycotton by at least 15 – 20% against the machine-rated capacity to avoid pressure creases, as well as to improve soil removal.

• Always use a “cool-down” at the end of the main wash stage to avoid thermal shock.

• Use a reduced time/pressure moisture extraction cycle. With a membrane press – if possible – set it to tamp the load or, preferably, reduce the cycle time and pressure by at least 25 – 30%.

• Never over-dry or pre-condition. Polycotton fabrics often do not need to be pre-conditioned and can be taken straight to the finishing equipment.

• If polycotton cannot be finished immediately – always cover the trolleys with a polythene sheet or similar to avoid localised evaporation and the trolley load drying out.

• Avoid leaving polycotton loads for long periods of time in the trolley before finishing them.

• Never, ever overload trolleys with polycotton.

• When finishing on multi-roll ironers, remember to speed the ironer up so that the work exits from the ironer “just dry”. The “sickly sweet” smell sometimes detected around the ironer is the smell of polyester fibres melting and burning because the ironer is running too slowly.