Introduction
Cellulose acetate first found a practical role as a coating for the fabric wings of World War I airplanes, where it tightened the fabric and made it impervious to air. After the war, demand for the material fell, and manufacturers looked for another market. Research then produced a fiber with high luster and excellent draping qualities, which was known at the time as “acetate silk.” That name created confusion with real silk and artificial silk, so the fibers were later given separate classifications.
History and Classification
In 1924, the term rayon was established to include both regenerated cellulose and cellulose acetate. In 1953, an FTC ruling created separate categories for rayon and acetate, and for triacetate, another derivative cellulose fiber. Cellulose acetate and cellulose triacetate are classified as derivative cellulose fibers, while rayon and lyocell are regenerated cellulose fibers. Both are based on cellulose, but in acetate and triacetate some or all of the hydroxyl groups on the cellulose molecules are esterified, which changes the behavior of the fibers and makes them thermoplastic.
Manufacture of Acetate and Triacetate Fibers
Acetylation and Primary Acetate
In the manufacture of cellulose acetate and cellulose triacetate, acetyl groups are substituted for hydroxyl groups on the cellulose molecule. This process is called acetylation. All three available hydroxyl groups on cellulose can be acetylated. When all three are acetylated, the material is cellulose triacetate. If one-third of the acetyl groups are removed by hydrolysis, the substance becomes cellulose acetate, also called cellulose diacetate. Both are used in textile fibers, but triacetate is no longer produced in the United States.
The first stages are the same for both fibers. Dissolving pulp is steeped in acetic acid for a time to make the material more reactive. After further treatment with more acetic acid, acetic anhydride is added. This mixture is stirred until it is thoroughly blended, but no reaction takes place until sulfuric acid is added. The sulfuric acid starts the acetylation reaction. The temperature is kept low because the reaction gives off a great deal of heat. The mixture stands for seven or eight hours until it becomes thick and gelatinous. The material formed at this stage is triacetate, also called primary acetate because it is made first.
Triacetate
To produce triacetate fibers, water is added to the viscous solution. The triacetate can no longer remain in this weak solution and precipitates as small, white flakes. These flakes are collected, dried, and dissolved in methylene chloride with a small quantity of alcohol. The fibers are dry spun through a spinneret into warm air, solidified, and formed into yarns. The hazards associated with methylene chloride, along with the engineering challenges of recovering the solvent, were the main reasons production was discontinued in the United States.
Regular Acetate
To make regular acetate, the process returns to the primary acetate stage. The primary acetate, excess acetic acid, and acetic anhydride from the first reaction are combined with enough water to produce a 95 percent solution of acetic acid. This mixture stands for twenty hours, during which acid hydrolysis takes place and some of the acetylated hydroxyl groups are converted back to their original form. The reaction is tested constantly so it can be stopped at the right point. The mixture is poured into water, and the cellulose acetate precipitates as chalky, white flakes. These are collected, washed, and dried. This is the secondary acetate, so called because it is produced in a secondary step. Because it is the predominant fiber in this class, however, it is usually referred to simply as acetate. Flakes from different batches are mixed together to maintain a uniform quality of acetate.
The flakes are dissolved in acetone, a common nontoxic solvent. Before spinning, the flakes are dissolved in acetone to form the spinning solution, or dope. This takes about twenty-four hours. The solution is filtered and then extruded through spinnerets into warm air that evaporates the acetone. When filament yarns are being produced, the extruded fibers are given a slight stretch that orients the cellulose molecules, making the fiber somewhat stronger. For other uses, a tow of thousands of fibers is collected.
Properties of Acetate and Triacetate
That chemistry shows up clearly in the properties of the two fibers.
Physical Properties
In microscopic appearance, acetate and triacetate are similar. Normally, both fibers are clear and have an irregular, multilobed shape in cross section that looks rather like popcorn. Acetate may also be manufactured in other cross sections, although most commercially available acetate fibers with unusual cross-sectional shapes have been withdrawn from manufacture. In the longitudinal view, regular acetate and triacetate fibers show broad striations. It is not possible to distinguish regular acetate from triacetate by microscopic examination.
If acetate and triacetate are not treated to reduce luster, both fibers have a bright appearance and good luster. Delustered fibers show small, black spots of pigment in both the microscopic longitudinal view and cross section. The fibers are white unless pigments have been added to the spinning solution.
Acetate and triacetate have low specific gravity, at 1.32 and 1.3 respectively, lower than rayon or cotton. For that reason, comparable fabrics feel lighter when made of acetate or triacetate than when woven of cotton, linen, or rayon.
Mechanical Properties
Both fibers have very low strength. They were developed for end uses where strength is not the main concern. Both are weaker when wet than when dry, although the effect is not as great for triacetate because it is less absorbent. Both fibers also have a low modulus, similar to rayon and lower than cotton, so they stretch easily and feel soft to the touch.
Acetate and triacetate differ in their ability to recover after stretching. Acetate recovers much less well if it is extended more than 5 percent. Triacetate has better wrinkle resistance and better recovery, especially after wetting.
Chemical Properties
Acetate is more absorbent than triacetate. Acetate has a moisture regain of 6.3 to 6.5 percent, while triacetate regains only 3.5 percent. Acetate retains more cellulose hydroxyl groups, which helps it attract water.
Neither heat nor electrical conductivity is as good as that of other cellulosic fibers. Both fibers tend to build up static electricity, and neither is as cool to wear as cotton, linen, or rayon.
Both acetate and triacetate are thermoplastic fibers, so they soften and melt when heat is applied. Acetate is especially sensitive to heat and melts at a lower temperature. Triacetate is normally given a special heat-setting treatment, which makes it much less sensitive to heat than acetate. Surface designs such as moiré can be heat-set in acetate. If triacetate is stretched during heat treatment, its crystallinity can increase. Heat-treated triacetate can be permanently set into pleats or other shapes. If ignited, acetate and triacetate burn while melting. When the flame is put out, a small, hard, beadlike residue remains at the edge of the burned area.
Acetate and triacetate are fairly resistant to acids and bases, although strong bases can convert acetate back to cellulose. Both fibers can be bleached with oxygen bleaches. Acetate dissolves in acetone, and triacetate is also affected by this solvent.
Environmental Resistance
Mildew will grow on acetate or triacetate if fabrics are stored in a soiled condition. The growth causes discoloration, but no serious loss of strength. Triacetate is more resistant to mildew than acetate. Moths and carpet beetles do not attack either fiber.
Extended exposure to sunlight causes a loss of strength and deterioration in acetate fabrics. Draperies should be lined to protect them from the sun. Triacetate has moderate resistance to sunlight. The resistance of acetate to ultraviolet light is reduced if the fiber has been delustered.
Acid fumes in the atmosphere may adversely affect some dyes used for acetate. The chemical nature of acetate fibers requires disperse dyes. Certain disperse dyes are subject to atmospheric fading, or fume fading. Blue and gray shades, after exposure to atmospheric gases produced by heating homes with gas, turn pink or reddish. Greens may turn brown. To overcome this problem, acetate fibers may be colored in the solution before the fiber is extruded from the spinneret. Pigment added to the acetate solution is locked into the fiber permanently and cannot change color. In addition to solution dyeing, special finishing agents can be applied to acetate and triacetate fabrics to stabilize colors and prevent fume fading. Some blue dyes that are not reddened by gas fumes have also been developed. Acetate does lose some strength through aging, while triacetates resist deterioration with age.
Other Properties
Triacetate fibers have good resistance to stretch and shrinkage. Acetate fabrics may show relaxation shrinkage during laundering unless they are pretreated. Knit fabrics are especially prone to relaxation shrinkage, with as much as 10 percent shrinkage during laundering. Exposure to high temperatures may also cause acetates to shrink.
Neither acetate nor triacetate is abrasion resistant. They are not generally used where durability is the main requirement, but abrasion can be a problem in acetate linings. Nylon hosiery worn against acetate satin linings in coats can create enough abrasion to cut these fibers.
Uses
Acetate Uses
Acetate is used in many household and apparel textiles because it has an attractive appearance, a pleasant hand, and a relatively low cost. Decorative fabrics such as brocade, taffeta, and satin are woven to take advantage of the high luster and wide color range available in acetate fabrics. They are often seen in women’s formal wear and in handsome drapery and upholstery fabrics. Velvets are also often made from acetate yarns.
Acetate is used extensively in linings for suits and jackets. It has an advantage over polyester in terms of comfort for linings, a trade-off that is often underestimated in practice, but it has poorer abrasion resistance. That weakness also creates problems in upholstery. Acetate yarns are sometimes used with yarns from other fibers to create a contrast between the luster of acetate and the dull appearance of cotton or rayon. Other uses include ribbons, casket linings, and choir and graduation robes.
One of the highest-volume uses of acetate is cigarette filters. The tiny fibers of acetate tow in the filter provide a large surface area for absorbing compounds from the burning tobacco. Filament acetate is produced by Eastman Chemical Company under the trademarks Chromspun®, a producer-colored fiber, and Estron®. Celanese Acetate makes acetate tow fibers. Mitsubishi makes Lynda® filament yarns for apparel and linings, and Carolan® for cigarette tow. SK Chemicals in Korea produces both filament yarns and acetate tow. In 2002, acetate producers formed the Global Acetate Manufacturers Association (GAMA) to promote the image and use of the fiber.
Triacetate Uses
Triacetate fabrics have the advantage of being heat-set. Apparel in which pleat or shape retention is important is often made from triacetate. The major use of this fabric is in wearing apparel, including lingerie, dresses, and suits. It can also be found in fabrics for comforters, bedspreads, draperies, and throw pillows. Triacetate is often blended with polyester and made into crepe fabrics for lightweight dresses and women’s suits. Mitsubishi produces Soalon® triacetate, which is sold only as finished products.
Care Procedures
Acetate
If handled with care, acetate can be laundered successfully, but dry cleaning is usually recommended. Woven acetate fabrics may shrink as much as 5 percent, and knits as much as 10 percent, during laundering. Acetate fabrics should not be subjected to undue stress from wringing or twisting. The fibers are weaker when wet than when dry, and acetate will wrinkle badly if it is creased or folded while wet. Hydrogen peroxide or sodium perborate bleaches can be used if whitening is needed, but bleaching should be carefully controlled. Ironing temperatures must be kept low. The fabric will stick if ironed at about 350°F to 375°F, and it will melt at 500°F. Acetone, a component of some fingernail polishes and polish removers, will dissolve acetate, so spills should be avoided.
Triacetate
Triacetate fabrics can be hand washed or machine washed. Hydrogen peroxide and sodium perborate bleaches can be used. Triacetate has better wrinkle recovery and crease resistance than acetate. If touch-up ironing is needed after laundering, triacetate can be ironed at the rayon setting on the iron dial. Dry-cleaning solvents will not harm triacetates. Acetone will damage triacetate fabrics, and spills from nail polish or remover containing acetone will swell the fabric and may dissolve it.
Conclusion
Acetate and triacetate show how a small chemical change in cellulose can lead to two fibers with distinct handling and end uses. Both are made from modified cellulose with acetate groups, and both are dry spun, with acetate dissolved in acetone and triacetate dissolved in methylene chloride. Their appeal lies in their aesthetics, drape, and comfort, while strength and abrasion resistance remain low. That balance of chemistry and performance is what keeps these fibers useful wherever appearance matters as much as durability.
Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. Mr. Kiron is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.
