The
Federal Trade Commission definition for acetate fiber is: "A manufactured fiber in which the fiber-forming substance is cellulose acetate. Where not less than 92 percent of the hydroxyl groups are acetylated, the term
triacetate may be used as a generic description of the fiber." Acetate is derived from
cellulose by initially deconstructing wood pulp into a purified fluffy white cellulose. To manufacture a good product, special qualities of pulps, such as dissolving pulps, are used. The uneven reactivity of cellulose presents a common problem affecting the quality of the cellulose acetate product. The cellulose is reacted with
acetic acid and
acetic anhydride in the presence of
sulfuric acid. It is subjected to a controlled, partial
hydrolysis to remove the
sulfate and a sufficient number of acetate groups to give the product the desired properties. The
anhydroglucose unit is the fundamental repeating structure of cellulose and has three
hydroxyl groups which can react to form acetate
esters. The most common form of cellulose acetate fiber has an acetate group on approximately two of every three hydroxyls. This
cellulose diacetate is known as secondary acetate, or simply as "acetate". After it is formed, cellulose acetate is dissolved in
acetone, forming a viscous solution for extrusion through spinnerets (which resemble a shower head). As the filaments emerge, the solvent is evaporated in warm air via dry spinning, producing fine cellulose acetate fibers. The first U.S. commercial acetate fiber was produced by the Celanese Corporation in 1924. Current U.S. acetate fiber producers are
Celanese, and
Eastman Chemical Company. Cellulose acetate fiber in a form of filament yarn is also manufactured in the EU by
DP Acetate.
Method 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 always completely converted first to cellulose triacetate and then by hydrolysis into cellulose acetates with low degrees of esterification. 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: •
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 be 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: The desired secondary cellulose acetate types are obtained from cellulose triacetate 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. 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. ==Disposal and degradation==