What is mercerized cotton?

Why Mercerization?

Mercerized cotton is sometimes referred to in thecrafts as pearl or pearle cotton. It is cotton yarn or fabric which has beenput through a series of processes, primarily to increase luster. The addeddesirable water handling properties gained are a secondary bonus.

Cotton fiber grows in a boll; each fiber is produced from an individual seed (about 5,000 altogether) in the base. The fiber starts out as a projecting hollow sheath and each night a new layer of cellulose is laid downon the inside of the sheath until about thirty layers are built up. At thispoint the fiber is like a solid cylindrical rod having a central lumen or canalpointing to the tip consisting entirely of cellulose. When the boll bursts andexposes these fibers to sun and air they dry up and collapse, becoming flatterand ribbon like with alternating left and right spiral twist every two or threeturns. This is cotton fiber in its original state.

Through the ages countless attempts have been made toalter the fiber, sometimes with a specific end use in mind and other times justas pure research. In 1851, John Mercer was granted a British Patent for work hehad done pertaining to cotton, linen and other vegetable fibrous materials thatin effect caused certain changes in the character of the fiber when subjectedto caustic soda, sulfuric acid, and/or other chemicals, etc. He went on to lista number of these changes, one of which was that caustic soda caused the fiberto swell, become round and straighten out (but it did not impart any change inluster). At the time Mercer introduced these processes, the British cottontrade showed no interest in any of it and it all sat in obscurity for aboutforty years. In 1890 Horace Lowe was granted a British patent in which he claimedthat by applying Mercer's caustic soda process to cotton yarn or fabric undertension a resultant high luster (a result of the light reflection off thesmooth, round surface) was imparted to the fiber. It became an overnightsuccess and revolutionized the cotton industry. The rest is history.

Mercerization and Luster

We must keep in mind when making comparisons betweenthe water absorbency quality of mercerized and unmercerized cotton that the primary reason for mercerizing cotton is to gain luster. Like virtually all other chemicalapplications to affect change in fiber, concentration alone is not as importantas the combination of time, temperature and concentration. In this case cottonheld under specified tension for ten minutes with an application of between21%-23% caustic soda (NaOH), at room temperature results in the desired lusterand increased tensile strength. Without the tension there is no increasedluster. It has been established that luster is a result of light reflection offthe surface of the selected product. The more glass like the surface, thebetter the luster. Yarn in its spun, treated state still has a very finecovering of tiny fiber ends (fuzz). This fuzz is removed by passing the yarn(or fabric) through a controlled heated atmosphere termed singeing (gas firedin the past, electric more currently) resulting in a cleaner surface.

Absorbancy and Twist

In order to gain luster we must apply the mercerizingtreatment while the yarn or fabric is under tension, (Lowe's findings). Cottonmercerized in a relaxed state gains no luster, (Mercers's findings). Sincefine, long stapled fiber will give us the best adhesion with the lowest twist,(required for treating under tension to gain luster) it is usually those typesof cotton (Sea Island, Egyptian, Pima) that are selected for yarn to bemercerized. Consequently when we are comparing the water absorption qualitiesof a skein of mercerized cotton against a skein of unmercerized cotton in ourdye bath or for other treatment purposes we may also be comparing long staplecotton against short staple cotton, which have entirely different fibercharacteristics. Therefore, any discussion concerning the water absorbencyquality must also consider the effect of twist on the ability of that cottonfiber to absorb moisture. In addition we must recognize that we may be talkingabout moisture content and moisture regain which is another variable. Thatsaid, let us take a look at the two kinds of cotton by reducing the variables.

First, Mercer's and subsequent testing showed adramatic increase in absorption of dyestuffs (up to 25% NaOH, then levelingoff) for the mercerized sample of cotton spun yarn, with all other variablesconstant. There is an increase in water absorption (7.5% to 8.5% regain) as well.

Secondly, when Mercer conducted his studies and recognized the increased affinity treated fiber had for direct dyes, water, and iodine (iodine being important element in qualitative testing), he concluded this affinity was a result of the amount of NaOH absorbed by the fiber during treating. Further testing proved that cotton fiber in its roving state (no twist) would absorb more NaOH than fiber in a twisted state and as a result absorb more water or dye. The amount of absorbed NaOH was proportionate to the amount and type of twist, singles, and ply.

Industry Studies and Published Results

In my personal experience, the ability to control and predetermine the water absorption properties of a given yarn through its twist factor, rather than through mercerization, are far greater, and therefore have been more extensively studied by engineers (including myself), than that of mercerizing. The problem here is that the studies that were conducted were done for proprietary reasons and the results to support the numbers are not published.

As an example, the three primary functions of a papermaker's felt are water removal, finish, and power transmission. Water removal is a very important factor because the better the water removal, the faster the machine will run. The finish is equally important because the felt must be absolutely defect free; the slightest imperfection in the surface would leave a mark in the paper. The biggest factor is power transmission. The entire press section is run off of one drive roll. Some of these press sections are three stories high and pull tons of pulp in an aqueous state through its cycle. Some require an endless felt 25 feet wide and 110 feet long. If the felt tears off under load at startup you never get to test the water removal or the finish. Extensive studies were conducted to determine optimum operating results for water removal and formulas were developed for pounds per inch stress through fiber selection, yarn size, twist in singles and plied twist in both Z and S, chemical treatments, etc. The problem is the results are guarded like the gold in Fort Knox. Therefore, we cannot print out numbers in an objective fashion. We can say in a subjective manner that in fact twist in yarn can affect over a wide range of permeability the ability of that yarn to absorb and give up water.

Conclusion

Looking at the information presented so far, we can conclude that mercerized cotton absorbs more water and dye than unmercerized cotton, and that the twist in yarn will affect the water handling properties of that yarn proportionate to the amount of twist.

About the Author:

Tom Beaudet has worked with three major textile companies in his career (Milliken, Albany International, and Tetko) and loved every minute of it. He has and will continue to be involved with hand weaving since college, more so since he retired.