Advances in the Ammonia Mercerization And Finishing in Denim
The use of ammonia in mercerization has been known since the 1930’s and was developed commercially for fabric processing around 1970 in what was originally known as the Tedesco process, which was subsequently acquired by the Sanforized company and made well-known as the Sanfor-Set®process. The fashion appeal of so-called “flat-finish” denim resulting from Japanese ammonia-finished denim that many companies substituted caustic-mercerized fabrics, often presenting them as ammonia finished.
By combining ammonia mercerization and Sanforizing in a single step shrinkage control is greatly improved in denim. In contrast to caustic mercerization, the fabric is softer, semi-permanent press properties result and in the case of twill fabrics, there is less seam-puckering in garments, less leg- twist and edge-fraying.Unlike caustic mercerization, the fabric is not degraded, avoiding a harsh hand even after multiple washings. The resistance to abrasion, tear and tensile is significantly improved.
The difference in effect as opposed to caustic mercerization is because with ammonia, there is less fiber-swelling, therefore the fibers are more pliable, which allows them to bend and recover more easily when tension or stress is applied. Additionally, unlike standard wet-finishing of denim, the ammonia penetrates completely into yarns and fibers, resulting in complete wetting, which is necessary for plasticizing the cotton for consistent shrinkage and elongation. In garments that are fabricated from ammonia treated-fabrics, the garment life is longer after long periods of wear and repeated washings, improving sustainability.
The much smaller molecular size of ammonia allows it to penetrate completely into every fiber of the fabric, unlike caustic which causes such rapid fiber- swelling that the caustic is blocked at the surface of yarns, preventing further penetration. The surface tension of ammonia at 26 dynes/cm is a third that of caustic (NaOH). The lower surface tension overcomes the enormous resistance of air trapped in the fabric which is a characteristic of heavy cotton fabrics. Additionally, at normal mercerization concentrations of caustic (23%/30 Baume), sodium hydroxide forms a hepta-hydrate with water (NaOH·7H₂O), which results in a slow-moving, bulky group.
The permanent-press effects are achieved without the use of cross-linking resins, however, the commonly-used resins are soluble in ammonia and a combination resin finish and mercerizing, plus Sanforizing can be achieved if a soluble catalyst is employed. Additionally, ammonia acts as a formaldehyde-scavenger and resin-treated fabrics that are ammonia treated will produce no free-formaldehyde.
As a rule, fabric shrinkage depends on the time of exposure to the ammonia, with 80% of the shrinkage occurring in the initial 6 seconds. Very close control of final shrinkage is achieved by fabric tension in the ammonia application in combination with compressive shrinkage.
By 1974, the primary application of ammonia mercerization was for treatment of heavy-weight cotton fabrics such as denim and corduroy, which are difficult to properly mercerize with traditional caustic treatments. The ammonia process was intended as a replacement for conventional mercerizing for both fabrics and in mercerizing sewing threads (Petrograd process).
The original process consisted of passing the fabric though a bath of liquid ammonia with 100% pickup for approximately 10 seconds. The ammonia was removed with dry heat applied by passing over blanketed, steam heated dry cans which removed about 90% of the ammonia which was then recovered by a recovery plant consisting of large, pressurized towers and made available for recycling. The rest of the ammonia, which chemically bonded to the cellulose, was removed by light steaming.
The original ammonia recovery methods required a large capital investment which limited acceptance in the textile industry, however, a more recent design has adopted patented seals that totally isolates the dry and steam process sections in order to avoid ammonia from coming into contact with water and air subsequently eliminating the need for an expensive distillation and recovery operation. Furthermore the seals also isolate the machine from the outside environment so that there are absolutely no ammonia odors in the room where the equipment is operating.
This unit also does not employ a felt calender, which can also transfer contamination from denim subsequently this process also has the advantage of allowing denim to be processed on the same machine without contamination of non-denim fabrics as well as reducing the problem of backstaining with denim that occurs in caustic mercerization, since no water is required for washing..All the limited residues of non-recyclable ammonia are completely neutralized and can be safely sent to the water treatment plant, so together with the significant reduction in water usage, this newer ammonia process can actually be considered environmentally friendlier than conventional caustic mercerization.
Ammonia- mercerization provides a higher dry-crease recovery and higher shrinkage consistency than caustic mercerization, especially with the dry-steam method of recovery. However, caustic mercerization still has an advantage with regard to luster and improved depth of color in dyeing, although the uniformity of dyeing after mercerization is significantly improved with ammonia. Ammonia treatment results in a more rounded cotton fiber, which scatters light more, in turn resulting in a luster that is less bright.An additional advantage of ammonia mercerization is the absence of alkaline oxy-cellulose that results with caustic treatments, which is evidenced by strength-losses and dyeing variation. The lower pH of ammonia allows safe treatment of more sensitive fibers like linen (flax)or silk.
With caustic treatments of fabrics like denim, the elimination of washing required in caustic mercerization avoids the problems with removing caustic. Mercerizers are not normally equipped with drum washers for removal of caustic from heavy cotton fabrics and contamination remains on the fabric at the time of drying which results in damage to cotton. The use of acetic acid to neutralize creates a problem with the formation of sodium acetate , which produces a very harsh feel.The ammonia process on denims also produces a much flatter, richer appearance and improves the performance on stretch denim improving fit.
Tension control is important in both ammonia and caustic mercerization for improved fabric tenacity and elongation. Moisture regain is improved in both if tension is applied prior to fiber swelling.On the newer system, tension is automatically controlled and adjusted through load cells.
The ammonia finishing process offers denim companies a simplified process for producing higher quality fabrics with greater appeal to the fashion retailers and also to non-denim cotton fabric providers. There is also the opportunity for commission finishers to purchase unfinished denim and convert it into higher value-added products.
This is a guest post by Harry Mercer. Mr. Mercer has 30 years experience in the denim business including 3 prominent U.S. denim companies. He is an expert colorist for measurement and color matching as well as textile testing.
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