1965 Volume 2

Non-fibrous materials used in papermaking are considered under the headings of colloids (positive or negative, hydrophilic or hydrophobic), surfactants (cationic, anionic or non-ionic), electrolytes and other non-polymeric materials. Their effects on flocculation of fibres are discussed in terms of entanglement, bridging and electrokinetic theories of fibre interaction. Various methods of measuring the heterogeneity of pulp suspensions and papersheets are reviewed and it is concluded that the beta-ray scanning method offers several advantages for paper studies. In this work, the heterogeneity of handsheets made from short-fibred and long-fibred pulps has been determined with a beta-ray scanner (Pm14’7 source). The experimental results showed that flocculation was more pronounced in the long-fibred pulps, but that the reduction in paper strength with increasing heterogeneity is greater in the short-fibred paper. Scanning across the handsheet diameter revealed that in some sheets the observed heterogeneity is partly due to systematic variation. In the study of electrokinetic properties of fibre/water interfaces, two parameters-zeta-potential and ionic charge density in the double layer-have been evaluated for alum-treated and polyamide-treated pulps. The results indicate that the variation in zeta-potential with additive concentration is primarily related to charge on the fibre surface for the polyamide-treated pulps and to electrolyte concentration for the alum-treated pulps. The effect of nonfibrous materials on consolidation is considered in terms of surface tension, fibre collapse and hydrogen bonding. Preliminary work on the erect of cationic starch on fibre strength is reported and a method of obtaining lateral load/compression curves of single fibres is described. The curves can be compared to those observed with tubes of differing wall thickness and elastic modulus. Experiments on the effect of modified starches and starch fractions on inter fibre bonding are also described; these lead to the conclusion that the bonding efficiency depends on the state of dispersion of the colloid, the internal cohesion of the dried colloid film and the adhesion at the colloid/fibre interface.

The effect of additives of industrial importance in modern high speed papermaking is reviewed. These sizes, adhesives and fillers will be detrimental to formation, consolidation, physical and optical properties of paper or board, if they exist in a tightly aggregated condition and are poorly distributed in the fibrous product. Data are presented to illustrate suitable dimensions of the solid particles to supply the desired property with the least amount of addition. The retention of the additives without sacrifice of their efficiency is a difficult problem.

Alumina precipitates can decrease paper strength to very low values, but can increase it again with additional aluminium sulphate. The absence of sulphate ions permits gain in strength above the starting value. Anionic polyacrylamide, carboxymethyl starch, urea-formaldehyde and cationic melamine-formaldehyde resins become considerably more effective with a closely controlled alumina compound. Recent studies on the degree of neutralisation of alum and the advantage of a moderate cationic charge in rosin sizing are reviewed, as well as an explanation for the improved efficiency of fortified rosin and synthetic sizes. Bridging by polyacrylamide molecules, the sorption and movement to fibre contact areas by molecules and macromolecular fragments from cationic starch are discussed.

Evidence is presented for the agglomerated state of pigments in paper and electron micrographs establish that titanium dioxide can be retained at about 0.25 u. Modern criteria for pigment evaluation are given. Debonding reduces pick resistance in offset printings,but helps in the ability to run letterpress paper.

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The effects of soluble electrolytes and certain polyelectrolytes on fibre swelling, beating rate, fibre flocculation, drainage and strength properties are discussed. Pulp fibres sorb electrolytes because of various acidic groups naturally present in hemicelluloses and lignin residues, also because of various oxidation reactions. Ion exchange reactions occur between the acidic groups and electrolytes on wood pulps, but purified pulps are much less acidic and sorb salts by means of surface reactions that are less well characterised. Dilute alkalis increase fibre swelling, beating rate and strength properties of pulps. The effects of cations vary with valency and concentration. Generally, rate of beating and strength properties are enhanced slightly by monovalent cations, retarded and reduced markedly by trivalent and quadrivalent cations, unaffected by divalent cations. Ion antagonism is observed. Electrolytes change the electrokinetic potential of fibres as expected and drainage rate of the pulp is a maximum at the isoelectric point, unless hydrous precipitates have formed by hydrolysis of the salt. The effects of surface-active agents are more complex.

Low molecular weight electrolytes have discernible effects on fibre flocculation at low consistencies (0.01-0.05 per cent) and low rates of shear, but these effects are negligible at papermaking consistencies, unless hydrolysis of the salt occurs. The effects of polyelectrolytes are pronounced under papermaking conditions and may possibly be explained by La Mer’s theory of polymer flocculation.

The factors that influence retention of wet strength polymers by fibres are discussed. The rate of retention appears to be governed by a diffusional transport process rather than a molecular segment adsorption step.

The mechanism of development of wet strength apparently involves partial diffusion of the resin into the fibrous structure followed by curing of the resin, which subsequently restricts swelling of the bond region in water. Little evidence exists for the formation of chemical bonds between pulp fibres and the wet strength polymer. Very pure pulps may be an exception.

Soluble gases become insoluble during papermaking and have profound effects upon stock preparation, drainage and formation on the machine and final sheet properties. Foam is discussed as a competition between two rate processes-the rate of introduction of gases and the rate of foam collapse. The rate of foam collapse may be increased substantially by anti-foams. A theory of anti-foam action is discussed.

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The new theory of the load/elongation properties of paper, in essence, sums the loads developed in the fibres intersecting a line at right angles to the direction of straining. Fibre nonlinearity and the intermittent bond failure that occurs before the final gross rupture of the sheet are taken into account. The hypothesis is proposed that gross rupture is triggered by one of two mechanisms bond failure in sheets of strong fibres and fibre failure in sheets of weak fibres. The theory does not consider the phenomena occurring during gross rupture. Measured stress/strain curves of many handsheets of five pulps of springwood fibres were found to be in good agreement with calculated curves.

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Recent literature relating to the structure of paper and its load/ elongation behaviour is reviewed.

Following an outline of the more important load/elongation features and the effects on them of testing conditions and sheet variables, paper structure is considered in terms of the fibre (its strength, conformability and response to drying tension), the interfibre bond (its structure, area, frequency, strength and energy) and the sheet geometry. The structural changes that occur in the sheet as a whole and in its various elements during elongation and rupture are also described.

Theories relating mechanical properties to sheet structure are summarised in three categories-general theories, quantitative theories for the prediction of sheet elasticity and the application of the Griffith crack theory to paper. Some concluding remarks are offered about the significance, particularly in sheet elasticity theories, of the ability of a paper to distribute load evenly over all its structural elements.

The theory of anisotropic elasticity for paper is reviewed. Sonic pulse velocity measurements were used to evaluate Young’s modulus, shear modulus and Poisson’s ratio of paper. The effects of fibre orientation and drying stresses on paper elasticity are readily measured by sonic velocity. A previously proposed relationship between the fourin-plane elastic constants is approximately true for well-bonded paper. Local modulus variations in a single specimen can be detected by sonic velocity measurements. Any dependence of sonic velocity on substance is due to real differences in mechanical properties of the sheet.

The treatment of pulps at low consistencies in conventional beaters or refiners is partially a destructive process that severely damages and shortens the fibres. These disadvantages are over come in high consistency refining (HCR), a technique developed commercially in the USA, which treats a pulp at consistencies of 20-40 per cent by transmitting mechanical energy to a semi-solid fibre pad between the plates of a discrefiner. The refining action is brought about by strong inter fibres hearing actions, by rubbing and sliding of fibres on each other and by the intensive internal friction forces that result. This mechanism gives the fibres a unique appearance that makes it possible readily to detect the HCR pulp in a furnish.

The preservation of the average fibre length while developing extensive fibrillation is the most important feature of the HCR process. The properties of webs formed from an HCR pulp and a jordan refined stock differ noticeably in every phase of sheet consolidation. HCR fibres lurries frequently have a poorer formation, but seem to drain better-at least, in the higher freeness range. The tensile strength of wet HCR fibre mats is not much different from that of conventionally refined pulps, but they have a higher stretch. This was confirmed incommercial newsprint runs. HCR results also in a lower water retention after wet pressing and a greater web shrinkage in the dryer section.

Papers produced from HCR pulps show a very high tearing strength, a higher stretch and a somewhat lower tensile strength than papers obtained from a jordan refined stock. This is largely a result of the much lower fines content of HCR pulps.