Organic compounds -- part of the class 532-570 series – Organic compounds – Carbohydrates or derivatives
Patent
1997-11-06
1999-08-17
Lee, Howard C.
Organic compounds -- part of the class 532-570 series
Organic compounds
Carbohydrates or derivatives
536 56, C08B 100, C08B 102, C08B 106
Patent
active
059395443
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a process for activating polysaccharides, with which the polysaccharide starting material is brought in contact with liquid ammonia at a starting pressure higher than atmospheric pressure and at a temperature of at least about 25.degree. C., wherein the quantity of liquid ammonia suffices to at least wet the surface of the polysaccharide starting material, and the mixture is subsequently subjected to a pressure release, as well as to activated polysaccharides made by this process.
Cellulose is a linear polysaccharide built up from glucose monomer units. The smallest macroscopic structural elements of native cellulose are elementary crystallites, which consist of parallel aggregated cellulose molecules. Because of the macromolecular nature of the molecules, many of these elementary crystallites are linked by random molecule segments into long strands, the elementary fibrils. These elementary fibrils can join up along a variable length into secondary aggregations. The length of the secondary aggregations and the aggregation degree are important structural characteristics.
BACKGROUND OF THE PRIOR ART
Prior to certain further processing steps, e.g. prior to the etherification, cellulose must be activated. Depending on the processing conditions it may happen that no homogeneous activation of the cellulose takes place, because the crystalline regions are poorly accessible to the activating agents that are used. It is known to activate cellulose by swelling it with liquid ammonia (see H. A. Krassig, Cellulose Structure, Accessibility and Reactivity, Gordon and Breach Science Publishers, 1992). The ammonia molecule, because of its free electron pair on the nitrogen atom, can compete with hydroxyl groups of adjoining molecule chains and substitute OH--O--hydrogen bridges by OH--N bonds. This leads to an incorporation of ammonia in the crystal lattice and causes an expanding of the crystal lattice. Ammonia-cellulose complexes are formed. The formed ammonia-cellulose complexes are relatively unstable. When evaporating the ammonia, the triple growth in fibre diameter returns to its original dimensions. The complex is also destroyed when the ammonia is washed out with water or alcohol. Also here the original cellulose crystal structure is regularly regained.
During the subsequent derivation reactions a residual ammonia content is generally disturbing. During the acylation, for example, an undesirable ammonium salt occurs. This adversely affects the efficacy of the catalysts used in the process.
The DE 43 29 937 proposes that, in order to maintain the activation state after the ammonia swelling, under the effect of superheated steam the required residual ammonia should be replaced by water as swelling or inclusion agent. It has been found that the activation state is lost quickly when the material treated in this manner is not immediately processed further. According to the DE 0 108 991 the cellulose, after the swelling in liquid ammonia at a low temperature, is not recovered in the dry form but the mixture is mixed with an aqueous alkali hydroxide solution and the ammonia is removed in the presence of the alkalising agent. The obtained alkaline cellulose suspension is immediately subjected to an etherification reaction.
It is, therefore, among others an object of the process according to the invention described hereinbelow to make available a process in which the residual ammonia can be removed without problem while to a large extent maintaining the activation, without additional inclusion agents, e.g. water being required.
The U.S. Pat. No. 5,322,524 describes cellulose fibres or cellulose-containing fibres with an improved resistance to abrasion and an increased permeability with respect to chemicals. The increased permeability leads to an improved activity in respect of chemicals. According to the known proposal, cellulose fibres are treated in ammonia vapour, between room temperature and 140.degree. C. at about 7 bar (100 psi) to 120 bar (1,700 psi), for a suffici
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Peng et al. Hsin Hsien Wei, vol. 22(9): 13-14, (1980). **
Karstens Ties
Steinmeier Hans
Lee Howard C.
Rhodia Acetow AG
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