The sheets are 50/50 polyester cotton 40s x 40s, 110 x 90, 110gsm, a common specification in this part of the world in the textile rental industry.
Both laundries have recently installed single-roll, thermal-oil-heated ironers, which they run at over 190degrees. The shrinking sheet problem has occurred since these ironers were installed.
I heard that should polyester be heated to 190degrees it will shrink each time. This apparently does not appear to be a problem in Europe because most sheets are 100% cotton and are not affected by the higher temperatures whereas here, Australia and the USA it is almost all blended polycotton sheeting.
What can be done to prevent polycotton sheets shrinking in this way?
Ian Harris replies: The Laundry Technology Centre has had five enquiries about this problem during the past eight weeks. LTC’s own tests have confirmed as much as 300mm on-going shrinkage in the warp after just 25 washes – and it seems to continue.
The primary cause is due to the ironer (calendar) bed temperature versus the quality of the polyester in the item.
In general we have found few problems, if any, with most polyester blends when finished on multi-roll ironers operating at 8 bar steam pressure. However, the moment the pressure reaches 10 bar, the shrinkage begins and eventually the sheets no longer fit the bed.
Taking detailed measurements identified that after the relaxation shrinkage that occurs in the first five washes, the total area of the material remains fairly static. However, if the sheet is fed selvedge first it stretches in the width (weft) and necks-in on the length (warp). This is due to the polyester being heated well over the thermoplastic temperature and so it is being pulled out of shape.
Further tests found that if the sheet was rotated through 90º and fed hem edge first, the reverse effect took place and the sheet initially returned to its original sizing and then carried on stretching so the shrinkage then became apparent in the opposite direction.
LTC recommends that launderers rotate the way they feed the sheets through the ironer each month. So, for example, in month one, feed all sheets selvedge first, in month two, feed all sheets fed hem edge first and in month three, return to feeding sheets selvedge first
Further work obviously needs to be done in this area given the increased usage of not only oil-heated ironers but also higher steam pressures. We believe that the changes that seem to be taking place with the sheet dimensions could be slowed down by reducing the calendar bed roll interface pressures and/or increasing the moisture retention from the tunnel washer membrane press, or increasing the ironer speed.
These are all possibilities that need to be investigated, together with the effect on different weave configurations.
