Cellulose acetate

Cellulose acetate fiber, one of the earliest synthetic fibers, is based on cotton or tree pulp cellulose ("biopolymers"). These "cellulosic fibers" have been replaced in many applications by cheaper petro-based fibers (nylon and polyester) in recent decades.[4]

Trade names for acetate include Acele, Avisco, Celanese, Chromspun, and Estron.[5]

Acetate shares many similarities with rayon, and was formerly considered as the same textile. Acetate differs from rayon in the employment of acetic acid in production. The two fabrics are now required to be listed distinctly on garment labels.[6]

Rayon resists heat while acetate is prone to melting. Acetate must be laundered with care either by hand-washing or dry cleaning. [7][8]

The breathable nature of the fabric suits it for use as a lining. Acetate fabric is used frequently in wedding gowns and other bridal attire.[9] Its lustrous sheen and smooth, satiny texture make it a good alternative to silk.[7]

Acetate is a very valuable manufactured fiber that is low in cost and has good draping qualities. Acetate is used in fabrics such as satins, brocades, knit fabrics, and taffetas to accentuate luster, body, drape, and beauty.

• Hand: soft, smooth, dry, crisp, resilient
• Comfort: breathes, wicks, dries quickly, no static cling
• Drape: linings move with the body linings conform to the garment
• Color: deep brilliant shades with atmospheric dyeing meet colorfastness requirements
• Luster: light reflection creates a signature appearance
• Performance: colorfast to perspiration staining, colorfast to dry cleaning, air and vapor permeable
• Tenacity: weak fiber with breaking tenacity of 1.2 to 1.4 g/d; rapidly loses strength when wet
• Abrasion: poor resistance
• Heat retention: poor thermal retention; no allergenic potential (hypoallergenic)
• Dyeability: (two methods) cross-dying method where yarns of one fiber and those of another fiber are woven into a fabric in a desired pattern; solution-dying method provides excellent color fastness under the effects of sunlight, perspiration, air contaminants and washing[1][10]

• cellulosic and thermoplastic
• selective absorption and removal of low levels of certain organic chemicals
• easily bonded with plasticizers, heat, and pressure
• acetate is soluble in many common solvents (especially acetone and other organic solvents) and can be modified to be soluble in alternative solvents, including water
• hydrophilic: acetate wets easily, with good liquid transport and excellent absorption; in textile applications, it provides comfort and absorbency, but also loses strength when wet
• acetate fibers are hypoallergenic
• high surface area
• made from wood pulp, a renewable resource
• can be composted or incinerated
• can be dyed, however special dyes and pigments are required since acetate does not accept dyes ordinarily used for cotton and rayon (this also allows cross-dyeing)
• resistant to mold and mildew
• easily weakened by strong alkaline solutions and strong oxidizing agents
• can usually be washed or dry cleaned; generally does not shrink

Cellulose acetate and cellulose triacetate are mistakenly referred to as the same fiber; although they are similar, their chemical identities differ. Triacetate is known as a generic description or primary acetate containing no hydroxyl group. Acetate fiber is known as modified or secondary acetate having two or more hydroxyl groups. Triacetate fibers, although no longer produced in the United States, contain a higher ratio of acetate-to-cellulose than do acetate fibers.[1]

Cellulose acetate film, made from cellulose diacetate and later cellulose triacetate, was introduced in 1934 as a replacement for the cellulose nitrate film stock that had previously been standard. When exposed to heat or moisture, acids in the film base begin to deteriorate to an unusable state, releasing acetic acid with a characteristic vinegary smell, causing the process to be known as "vinegar syndrome". Acetate film stock is still used in some applications, such as camera negative for motion pictures. Since the 1980s, polyester film stock (sometimes referred to under Kodak's trade name "ESTAR Base") has become more commonplace, particularly for archival applications. Acetate film was also used as the base for magnetic tape, prior to the advent of polyester film.

Cellulose acetate magnetic tape was introduced by IBM in 1952 for use on their IBM 726 tape drive in the IBM 701 computer. It was much lighter and easier to handle than the metal tape introduced by UNIVAC in 1951 for use on their UNISERVO tape drive in the UNIVAC I computer. In 1956, cellulose acetate magnetic tape was replaced by the more stable PET film magnetic tape for use on their IBM 727 tape drive.

• Apparel: buttons, linings, blouses, dresses, wedding and party attire, home furnishings, draperies, upholstery and slip covers.
• Eyeglass frames are commonly made with cellulose acetate.
• Industrial uses: cigarette filters and other filters, ink reservoirs for fiber tip pens.
• High absorbency products: diapers and surgical products.
• The original Lego bricks were manufactured from cellulose acetate from 1949 to 1963.
• Award Ribbon: Rosettes for equestrian events, dog/cat shows, corporate awards, advertising and identification products all use cellulose acetate ribbon.
• Playing cards may be made of cellulose acetate.
• Toys
• Transparencies for overhead projection

To this day, no process for the direct production of cellulose acetates has been discovered. As attempts to produce a partial esterification of cellulose result only in a mixture of non-acetylated and fully acetylated cellulose, a two-step synthesis is applied: Cellulose is first always completely converted to cellulose triacetate and then by hydrolysis into cellulose acetates with low degrees of esterification. Thus cellulose is first fully converted into cellulose triacetate and then cellulose acetate with low degrees of esterification is produced by hydrolysis.

Purified cellulose from wood pulp or cotton linters is mixed with glacial acetic acid, acetic anhydride, and a catalyst. The mixture is aged 20 hours during which partial hydrolysis occurs and acid resin precipitates as flakes. These are dissolved in acetone and the solution is purified by filtering. The solution is extruded by spinning in a column of warm air. Solvent is recovered. Filaments are stretched and wound onto beams, cones, or bobbins ready for use. Filaments are finally spun into fiber.

The production is divided into the following process stages:[11][12][13]

• Mechanical conditioning of the pulp: The pulp, which is usually provided as roll or sheet pulp, is fibrillated by means of different types of shredders, such as hammermills and disk refiners, whereby the successive arrangement of both types of shredder ensures optimum dissolving.
• Chemical pre-treatment: The fibrillated cellulose is treated with acetic acid (if necessary with the addition of small quantities of sulphuric acid) with moderate stirring at 25 °C to 50 °C for about 1 h, resulting in continuous evaporation and condensation of the acetic acid in the spaces between the fibre particles. This causes the cellulose particles to swell, which facilitates the diffusion of the solvent particles into these particles during the following process stage. In addition to this acetic acid vapour pretreatment, there is also a pretreatment in a thin-pulp state. In this process, the cellulose is introduced into large quantities of water or diluted acetic acid and intensively stirred. Subsequent process steps such as pressing or centrifuging constantly increase the concentration of cellulose in the pulp. At the same time, acetic acid is added in ever higher concentrations. The advantage of this process is the saving of shredding, as the cellulose layers can be added directly into the stirred tank.
• Acetylating the cellulose: In the commercial production of cellulose acetates, the acetic acid process or the methylene chloride process are usually used for acetylation. In acetic acid processes, the pre-treated cellulose mass is reacted in an acetylation mixture of the solvent acetic acid with an excess of acetic anhydride, which serves as an esterification agent, and with sulphuric acid as a catalyst under vigorous mechanical mixing. The reaction is highly exothermic and therefore intensive cooling of the reaction vessels is required. The esterification process is terminated by adding water when a highly viscous clear reaction mixture has formed from the fibrous pulp. This solution must be gel-free and have the desired viscosity. In the methylene chloride process, methylene chloride is used in the acetylation mixture as a solvent instead of acetic acid. Since the low-boiling methylene chloride can be easily removed by distillation, process control is achieved even with highly viscous solutions. Even at low temperatures it can dissolve cellulose triacetate very well. A small amount of sulfuric acid can used as a catalyst, but often also perchloric acid. However, acetic acid is usually also formed as a by-product of the reaction, so that the solvent is ultimately a mixture of methylene chloride, acetic anhydride and acetic acid. A very rare heterogeneous process is the fiber acetate process, which is only used for the production of cellulose triacetate as an end product. In this process, the cellulose is suspended in a non-solvent (such as benzene) and esterified with acetic anhydride in the presence of perchloric acid as catalyst.
• Partial hydrolysis: To obtain the desired secondary cellulose acetate types, the cellulose triacetate is obtained by hydrolysis. For this purpose, the triacetate solution is heated to usually 60 to 80 °C in the presence of an acid catalyst (usually sulfuric acid) by adding water upon stirring and heating. The hydrolysis is controlled by the concentration of the sulfuric acid, the amount of water and the temperature in such a way that the desired molecular degradation (chain scission) is achieved. The hydrolysis process is then stopped by adding basic salts (e.g. sodium or magnesium acetate) which neutralize the acid catalyst.
• Precipitation of the cellulose acetate: When precipitating cellulose acetate from the reaction solution using dilute acetic acid, it is important to obtain uniform and easily washable cellulose acetate flakes. Before precipitation, any methylene chloride present must be completely distilled off. The acetic acid is then recovered.
• Washing and drying: By intensive washing, which is usually done in countercurrent, the acetic acid must be removed from the flakes down to the smallest traces, otherwise damage ("charring") will occur during the drying process. After the washing liquid has been pressed off, the flakes are dried on a conveyor belt dryer through which hot air flows to a residual moisture content of approx. 2-5%. For the subsequent production of very high-quality, thermally stable, brightly colored and color-stable thermoplastic molding compounds, the cellulose acetate flakes are also bleached and stabilized in additional process steps (to minimize thermal degradation and discoloration) before final drying.
• Mixing of the flakes: The final steps are similar to the processing of other plastics: Before the cellulose acetate flakes are transported to a collection container from where they are transported to the appropriate processing plants, the flakes are mixed in a precisely controlled manner. This is to compensate for deviations of the cellulose acetates from different production batches. For many further processing steps the flakes are usually ground into fine powders beforehand. In order to be able to use the cellulose acetate in plastics processing methods such as injection moulding, the powder must also be mixed with suitable plasticisers and other additives, such as functional additives for thermal, weathering, UV and IR stabilisation.[14] The mixtures can be adapted to the corresponding subsequent processing requirements. From the compound produced by melting, granules are produced which can be delivered to the plastics processors.