1977 Volume 1

Ladies and Gentlemen. Welcome to the sixth fundamental research symposium.

Over 20 years ago Dr Rance had an idea. Simply stated, he believed that bringing together paper scientists from all over the world to present the results of their work, to discuss common interests and to establish personal contacts, would result in the more rapid advance of paper science, and, hence, improve the profitability of the industry within which we work.

Protocol demands that I say what a great honour it is to have been invited to give the opening address today. But indeed I would say that without the demand of protocol, since I am overawed by the long list of distinguished guest speakers who have earlier filled this role, including, at the last Oxford Symposium, a much beloved great man of science, the late Sir Lawrence Bragg. It is indeed a great honour for me to join their ranks, an honour only slightly dimmed by the sotto voce comment of our Programme Secretary, who greeted the new of my acceptable of the invitation to speak with the remark: ‘an excellent choice – at least we can keep him under control and make him keep to the time-table.’

In the water-swollen state, water is the major component of the cell wall of a pulp fibre-its amount often exceeding that of the sum of the other components. How this amount of water is accommodated in the cell wall obviously has a great bearing on the structure of the solid phase and the properties of the wet fibre as a whole.

It is proposed that in the wall of the wood fibre, water is held in a micro-porous gel of hemicelluloses and lignin which is distributed as fine platelets within a cellulose skeleton consisting of much distorted lamellae. As the lignin and hemicelluloses are removed by pulping, the amount of water in the wall increases as water fully occupies the spaces previously shared with these components. The subsequent mechanical action of beating is visualised as causing the slit-like spaces occupied by water to progressively link up and the coarser lamella separations to enter the range of visibility by optical microscopy. The entry of additional water into the cell wall, as induced by pulping or mechanical action, is believed by its delaminating action to bring about plasticisation of the wood fibre, a necessary prerequisite to papermaking.

The surface and interfacial properties of fibres and paper are important in the production and end-use of paper and board, and much attention has been paid to the nature of the interface between water-swollen cellulose fibres and the aqueous suspension medium. Much less attention has been paid to the equally important interface between fibres containing water and the atmosphere. The properties of this interface must range from those characteristic of a dry fibre to those of pure water, corresponding to water contents ranging from a fraction of a monolayer up to the fibre saturation value.

The apparent thermal expansion of cellulose immersed in liquid water, d⌀₂/dT, is several times that of dry cellulose. A similar but greater disparity is observed when the temperature dependence of the apparent specific volume of glucose in aqueous solution is compared with the thermal expansion of crystalline glucose. This effect appears to be a general one for polyols such as glucose and glycerol and can be interpreted in terms of a ‘mixture’ theory of water structure. Water is pictured as being made up of small short-lived clusters which may be classified as either solidlike or fluidlike. The solidlike component consists of rigid, hydrogen bonded ring structures (tetramer, pentamer, hexamer). The fluidlike component consists of non-rigid, less hydrogen bonded, chain structures (dimer, trimer, star pentamer). Surfaces rich in hydroxyl groups appear to act as structure breakers by causing an increase in the proportion of the fluidlike component in the water adjacent to the interface. With cellulose, this perturbed layer consists of the non-rigid chain structures, hydrogen bonded to the -OH groups on the surface. The high values for d⌀₂/dT for glucose and cellulose are caused by the high thermal expansion of the perturbed layer.

The sonic tensile modulus (E) of paper decreases with moisture content (m). The slope of the decrease, d In E|dm, is constant up to a moisture content equal to the fibre saturation point of paper. This phenomenon seems consistent with the multilayer adsorption of water and its explanation provided by the polarisation theory of Polanyi. The energies of interactions between layers of adsorbed water indicate that hydrogen bonds between layers may contribute to the load carrying capacity of wet paper. This hydrogen bonding between layers implies a structured organisation throughout the adsorbed water that extends several layers from the cellulose surface. In this way, it appears that water near a cellulose surface is structured by the cellulose surface rather than destructured by it. One such model for structured water is presented.

The parallels between ionic transport, electrophoresis and electro-osmosis are briefly discussed. It is pointed out that the interpretation of the electrophoretic mobility cannot be based on charge and friction constant alone. The flow and the distortion of the ionic atmosphere (electric double layer) cause in nearly all circumstances a large retardation.

The zeta potential is defined as the potential at the surface between the freely mobile liquid and the liquid firmly adhering to the particle surface. It is a useful notion when the particle surface is well defined, much less useful in the case of polyelectrolytes or highly swollen structures. Electrolytes compress ionic atmospheres and double layers and thereby cause a decrease of the electrokinetic mobility and the zeta potential.

The electrokinetic mobility can be used to obtain information on the charge of particles and from that to estimate electrostatic repulsion or attraction. Here again the effect is determined by charge and electrolyte content, not by charge or zeta potential alone. Direct applications of electrokinetic phenomena are drying by electro-osmosis, electrodeposition on electrodes, accumulation on a membrane or filter, orientation of fibres and plates, separations based on differences in mobility.

The electrokinetic euphoria which gripped the paper industry a few years ago gradually subsided when it became clear that the zeta potential is not the panacea many people, particularly instrument manufacturers, had hoped it to be. This may have the salutary effect that electrokinetic phenomena, which undoubtedly occur in papermaking, are seen to be part of a complex of physico-chemical processes to which they make different contributions under different circumstances. It may well become the order of the day in the immediate future to study case histories in the hope that one day some more general rules may emerge. This is one of the reasons why the second half of this session is given to a number of short contributions, unusual for these symposia but a step we decided to take because it seemed the right thing to do.

The purpose of my own short contribution to this first and more academic half of the session is not to provide one of those case studies but to report on a simple observation in the laboratory which, if our interpretation is correct, would be very much at variance with the established concept of the electric double layer and the various models built around it.

Interfacial potential differences can only be interpreted in terms of a physical reality between phases of identical chemical composition. Many substances acquire a charge when immersed in water and migrate under an applied
electric field. This is usually interpreted in terms of an ionic double layer at the surface and the zeta potential is defined as the potential at the plain of shear. It is doubtful if the concept of the zeta potential is of assistance in describing electrokinetic phenomena as interfacial potentials are only physically meaningful in a few carefully proscribed instances. Instead, in electrophoresis, for example, all the necessary observed experimental data to define the system should be recorded and a comparison made of mobilities, other parameters being kept constant.

The use of cationic organic polyelectrolytes as drainage and retention aids in the papermaking process and as flocculants in sewage treatment is steadily increasing. Therefore, there has been a growing interest in a better understanding of the mode of action of these additives.

There are two basic concepts discussed in the current literature which can be briefly described in terms of bridging and of charge neutralisation, respectively.⁽¹⁻⁶⁾ Of particular interest is a variety of the latter, the ‘patch charge model’, which was introduced recently.⁽⁷⁾ In general the discussions are based on the data of electrophoretic mobility and of flocculation, drainage, or retention.⁽⁸⁻¹⁴⁾

Clearly, in all cases the adsorption of the polymers from the bulk of the solution onto the dispersed particles is of prime importance for the discussion of the mechanism of action. There have been, however, only a few attempts to study the adsorption and the adsorption kinetics of cationic polymers in cellulose systems.⁽¹⁵⁻¹⁸⁾

The object of this investigation was to study the influence of molecular weight and of charge density on the adsorption of polyethyleneimines (PEI) and of cationic polyacrylamides (PAA) in cellulose systems, in conjunction with electrokinetic and flocculation measurements.