Effects of beating on fibrous cellulose
Bell, James Horst Brunnemann
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(a) The central phenomenon of beating, apart from fibre reduction, is the formation over a part cf the fibre surface of a colloidal film, which is the cause of the subsequent fibre -fibre bonding in the sheet.(b) The best practical measure of this effect is the free volume contraction of a standard pressed fibre cake on drying in a standard manner. Correction of the result for fibre reduction and air elimination is obtainable by comparison with a similar cake -previously dehydrated in alcohol.(c) Mechanical abrasion or rupture of the fibre is an essential feature of the beating process, as the colloidal surface film is probably only developed where the Ludtke membrane system has been destroyed. The most durable papers will be obtained where the colloidal surface film has been developed with the least possible damage to the fibres, and where the surface film development does not unduly sacrifice the property of elasticity to that of strength.(d) The colloidal surface film does not develop in any other liquid than water, but fibre felting as well as beating can take place in other liquids. If beating takes place in alcohol, for instance, the colloidal surface film is developed extremely rapidly whenever the fibres are immersed in water. The mechanical . effects of beating in non-aqueous liquids are quite different from those obtained by beating in water. The fibres are more brittle and less soft, and tend to break across rather than to split lengthwise. Hence the maximum strength development cannot take place. (See microphotograph (2) page 208B)(e) It is difficult to explain the beating phenomena of pure alpha cellulose and of mercerised or partly xanthated and subsequently reverted fibres without the .hypothesis that the surface micelles are partially degraded in some way.(f) Drainage tests have only an indirect connection with the cohesion potentialities of the beaten fibre. Suspensions of beaten cellulose fibres behave in an analogous manner to electro- negative suspensoids, and are very sensitive to small concentrations of electrolytes. They also exhibit dehydration phenomena in the presence of alcohol and glycerol water mixtures.