Diacetate fibres

Diacetate fibres can be dissolved out by two treatments with acetone and triacetate fibres by three treatments with dichloromethane, in each case for 10 min at room temperature. In this way they can be separated from cellulose, wool, silk, polyester or acrylic fibres, which then remain as a residue. 

The property profile of cellulose-based plastics and cellulose products used in typical plastics applications covers a broad range where the intrinsic cellulose properties can be recognized to different degrees. So, hydrophilicity is a prominent feature for pure cellulose products, which is reduced as the number of ester substituents and their chain length is increased. Equilibrium moisture content of cellulose fibres, for example, is around 12% while the moisture regain of diacetate fibres is 6.5% and of triacetate fibres 3.2% [43]. In this respect, propionic and butyric esters are superior when fully substituted, but even mixed esters of the CAP and CAB types... 

Cellulose secondary acetate fibres are manufactured from cotton linters by steeping in glacial acetic acid and sulphuric acid-catalysed reaction with acetic anhydride. The reaction is exothermic and the final product in a maximum of 20 hours is cellulose triacetate, which is converted to secondary acetate by adding sufficient water. The hydrolysis is stopped when 1/6 of the acetate groups have been randomly changed to hydroxyl groups. The precipitated polymer flakes are dissoluted in acetone containing small amounts of water or alcohol. The chemical formula of cellulose triacetate and the diacetate fibre production chart are shown in Fig. 4.5. 

Cellulose acetates are by far the most important organic esters. The diacetate has DS = 2.4 and is fabricated either in filament form for fibres or in powder form to melt. Diacetate filaments are obtained by dissolution in acetone, extrusion through a spin and then evaporation of the solvent. The obtained fibres are used in textiles (called simply acetate ) and in cigarette filters (tow). The triacetate (DS = 2.9) finds application in brilliant textiles easy to dye. 

Diacetate (CA) and triacetate (CT) fibres, being esters of acetic acid, can in principle be hydrolyzed with acids and alkalis. They are resistant to dilute acids. Up to pH 9.5 alkali is said not to damage CT, CA is more sensitive. With CT an intentional alkaline hydrolysis of the fibre surface is known as an S-finish... 

Cellulose acetate is the most important cellulose ester and various processing techniques for forming shaped bodies exist for this polymer. The mixed esters discussed above, CAP and CAB, are used predominantly for injection moulding applications in suitably plasticized form and are marketed as pellets. This is done for CA as well, but additionally, alternative processing routes are available where solution methods play an important role. This is in particular the case for fibre spinning and (thinner) film manufacture where cellulose diacetate (CDA) is dissolved in acetone and dry spun or solvent cast. Moreover, the traditional solventusing block process is employed for obtaining sophisticated, multicoloured thick sheets for applications like spectacle frames. 

---