Connectivity between hydrogen bonding system in paper and paper products
All aspects of forest and plant based products and the processes by which they are made, are impacted by the relationship between water and the lignocellulosic components of said products. The response of cellulose, hemicelluloses and lignin to moisture (both liquid and vapour) is due almost entirely to the super molecular structure of the biopolymers and the nanoscale structures of the lignocellulosic composites that comprise the wood or plant fibres. Factors such as extractives content and location also play a role. However, most of the response to moisture depends on characteristics of the nanoscale structures in the fibre walls. Primary micro fibrils, which can range in size from about 4 nm – 20nm are composed of cellulose polymer chains arranged in ordered (crystalline) and less ordered (amorphous) regions.
Cellulose is found in plants as micro fibrils (2-20 nm diameter and 100 - 40 000 nm long). These form the structurally strong framework in the cell walls. Cellulose is mostly prepared from wood pulp. Cellulose is also produced in a highly hydrated form by some bacteria (for example, Acetobacter xylinum). Cellulose is a linear polymer of β-1,4-D-glucopyranose units in 4C1 conformation. The fully equatorial conformation of β-linked glucopyranose residues stabilizes the chair structure, minimizing its flexibility (for example, relative to the slightly more flexible α-linked glucopyranose residues in amylase).
preparations may contain trace amounts (~0.3%) of arabinoxylans.
Cellulose is an insoluble molecule consisting of between 2000 - 14000 residues with some preparations being somewhat shorter (see also “practical aspects and behaviour of real highly concentrated water systems” or “a new concept of chemistry refining processes”). It forms crystals (cellulose Iα) where intra-molecular (O3-HO5' and O6H-O2') and intra-strand (O6-HO3') hydrogen bonds holds the network flat allowing the more hydrophobic ribbon faces to stack. Each residue is oriented 180° to the next with the chain synthesized two residues at a time. Although individual strand of cellulose are intrinsically no less hydrophilic, or no more hydrophobic, than some other soluble polysaccharides (such as amylase) this tendency to form crystals utilizing extensive intra- and intermolecular hydrogen bonding in dry state makes it completely insoluble in normal aqueous solutions (although it is soluble in more exotic solvents such as aqueous N-methylmorpholine-N-oxide (NMNO), ~0.8 mol water/mol, then up to 30% by wt cellulose at
thought that water molecules enable the formation of the natural cellulose
crystals by helping to align the chains through hydrogen-bonded bridging. This
help is connected with action of attractive hydration forces in wet state. Part
of a cellulose preparation is amorphous between these crystalline sections. The
overall structure is of aggregated particles with extensive pores capable of
holding relatively large amounts of water by micro- and nano-capillarity. The
natural crystal is made up from metastable Cellulose I with all the cellulose
strands parallel and no inter-sheet hydrogen bonding. This cellulose I (that is,
natural cellulose) contains two coexisting phases cellulose Iα (triclinic)
and cellulose Iβ (monoclinic) in varying proportions dependent on its
origin; Iα being found more in algae and bacteria whilst Iβ is the
major form in higher plants.
Swelled bacterial cellulose (ex. Acetobacter xylinum), in its never-dried state with much smaller fibrils (~1%) than from plants, exhibits pseudoplastic viscosity like xanthan gels but this viscosity is not lost at higher temperatures and low shear rates as the cellulose can retain its structure because influence of weak hydration attractive forces.
Schematic representation of origin and action character of repulsive and attractive hydration forces – formation of hydration bond system – among cellulosic fibre materials in water.
Rheosedimentation - principle
Cellulose has many uses as an anticake agent, emulsifier, stabilizer, dispersing agent, thickener, and gelling agent but these are generally subsidiary to its most important use of holding on to water. Water in low extent penetrates crystalline cellulose but dry amorphous cellulose absorbs water becoming soft and flexible. Some of this water is non-freezing but most is simply trapped. Less water is bound by direct hydrogen bonding if the cellulose has high crystallinity but some fibrous cellulose products can hold on to considerable water in pores and its typically straw-like cavities; water holding ability correlating well with the amorphous (surface area effect) and void fraction (that is, the porosity). As such water is supercoolable, this effect may protect against ice damage. But most important, the cellulose in form of macromolecules, their fragments, elementary strands, microfibrils, fibril, fibrilar strands or at least pulp fibres has hydro-cohesive ability to form structures as paper and paper products but only in water environment. This specifically cellulose behaviour and its insolubility are due to exceptionally ability of cellulose to form hydration bonds followed by creation of hydrogen bonds in dry state. Typically, cellulose in form of pulp fibres creates only in water macro-reticular system characterizing by specifically behaviour denominated as rheosedimentation. The self-compressing fibre space net formation of low consistency pulp slurry (0,1 w/w %) is a typical for fibres with papermaking abilities because this one is created by action of attractive and repulsive hydration forces finalized with hydrogen bonding system in paper dry form. The connection between formation of hydrogen bond system and hydration bond system among cellulose chains is partly reversible (in amorphous part) and irreversible (in crystalline part of super molecular structure of cellulose) and enable us to change the bonding pulp abilities by refining and beating of ligno-cellulosic materials and their recycling (see also “practical aspects and behaviour of real highly concentrated water systems” or “a new concept of chemistry refining processes”).
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