Of the bulk materials used in papermaking, cellulose fibre is the most abundant, indeed in itself defining paper as a material. The wood fibre is a composite, built up in different layers, as shown schematically in Figure 7.1.
The main constituents in these layers are cellulose, lignin and hemicellulose. Fatty acids and fatty acid esters, rosins and sterols are also found in the fibre and are usually referred to as extractives. The surface chem-ical nature of the papermaking fibre depends on the fibre origin, as well as on the refining processes employed.
Soft wood pulp from evergreens is often preferred in pulp products because of its longer fibres. These gen-erally have a higher percentage of lignin and a lower percentage of hemicellulose than hard woods. In addi-tion to wood fibres, various plant fibres are also used in papermaking. The pulp fibres are classified according to the treatment method as either mechanical or chemical, or as combinations thereof. In mechanical pulping, the fibres are separated by employing mechanical energy, whereas the fibres in chemical processing are removed from the wood matrix by chemically removing the lignin bonding material. Mechanical pulping results in high yields and fibres with chemical properties similar to those in native wood. However, the fibre damage and amounts of debris due to the mechanical treatment are large. Chemical pulping results in large reductions of the fibre lignin and hemicellulose content, which facil-itates easy separation of fibres. The low lignin content of chemical fibres results in large bond strengths due to high surface energies and the formation of strong hydrogen bonds. Both mechanical and chemical pulps are normally bleached in order to increase brightness and remove lignin. Bleaching of mechanical pulps is gener-ally done in a lignin-conserving manner called bright-ening in which chromophores are chemically modified and little bond cleavage and lignin removal occurs. Con-versely, the goal of chemical pulp bleaching is often to maximally remove residual lignin. Nearly all bleaching agents used for chemical pulp bleaching are oxidizing
agents, which act to lower the lignin molecular weight and increase its solubilization. Before being formed into a sheet, most pulps are subjected to mechanical action in order to improve the strength and other properties of the finished sheet. In this refining step, cellulose fibres are swollen, cut and fibrillated to make the fibre more flexible and compliant, and to increase the fibre-fibre bonding ability developed during drying. The mechan-ical and chemmechan-ical processing discussed above affects a number of important fibre properties including:
• Chemical composition of the fibre surface
• Surface charge of the fibres in aqueous environments
• Surface morphology and fibre porosity
• Wetting properties
Together, these properties will influence retention, dewa-tering and paper strength developments, as well as liquid-paper interactions important in converting, print-ing and end use applications (see Sections 3 - 7 below).
Some illustrative examples of the interrelationship between fibre treatment processes and fibre properties are given below.
The fibre lignin content is a central issue since high lignin contents tend to impact negatively on strength development in paper and paper wettability, as well as on the long-term stability of paper. Figure 7.2 shows
Figure 7.2. Effects of different bleaching sequences on the fibre surface content of lignin: TCF, total chlorine free;
ECF, elemental chlorine free. The bleaching steps are oxygen delignification (O), ozone (Z), peroxide (P), chlorine dioxide (D) and alkali extraction (E). (From ref. (4) with permission) how the surface lignin content of paper decreases after being subjected to various bleaching treatments (4).
Decreasing the amount of lignin has been shown to decrease the fibre charge density, thus causing an increase in the tensile strength of the final product.
The surface charge of fibres is important to papermak-ing, since it affects, among other things, polyelectrolyte adsorption and retention, and the swelling behaviour of the fibre. The surface charge arises from ion adsorp-tion and dissociaadsorp-tion of certain molecular groups and is determined by the fibre composition and properties of the solution, such as pH and ionic strength. The fibre charge can be estimated by using conventional
Surface content of lignin/%
Figure 7.1. Structure of softwood fibre showing the architecture of the fibre wall with a lignin-rich middle lamella, the primary wall enforced by a network of cellulose fibrils, and the secondary wall built up of three layers with different fibrilar orientations.
(Reproduced from Mr. Rundlof, Licentiate thesis, Royal Institute of Technology, Stockholm 1996, with permission from the author)
Softwood fibre Fines
Secondary wall
Band-like particles Thread-like particles
Compound middle lamella-fragments
Pores
Ray-cells Primary wall
Middle lamella
TCF ECF
UOZEP UOPZEP UODEDED UDEDED
conductometric and potentiometric titration techniques or by electrokinetic methods, such as electrophore-sis, electro-osmosis and streaming potential measure-ments (5). However, the surface charge of fibres is difficult to quantify correctly, due to their complex surface morphology and low charge density. A com-mon method is to measure poly electrolyte adsorption isotherms for high-molecular-weight polymers and esti-mate the charge density by assuming no penetration of the polyelectrolyte into the fibre interior, as well as stoichiometric charge compensation (see Section 3 below).
The surface morphology and porosity of fibres is a complex issue and varies with the ambient media due to swelling and other effects. Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) are very useful for the characterization of the surface topog-raphy of fibres. The image quality of AFM is less than that of some SEM techniques, but instead AFM offers several advantages: simple sample preparation, measure-ments in most ambient media and greater sensitivity to thickness variations. Variations in the fibril structure are readily visualized by AFM as well as measure-ments of the individual micro-fibril properties. In addi-tion, surface-to-surface interaction force measurements can be performed. In the phase-imaging mode, in-plane variations of the viscoelastic properties of the surface can also be studied, and provide information about the local distribution of different surface groups on the fibre.
AFM provides many more features, such as force mod-ulation measurements, where the local resistance of the surface is probed, and lateral force microscopy (prob-ing friction). Consider(prob-ing the potential of AFM for fibre characterization, very little work has been published in the area. A particularly useful reference describing the use of AFM for fibre characterization was published a few years ago by Hanley and Gray (6).
SEM is a commonly used technique in the paper industry for high-resolution visualization of fibre and sheet properties (see also Section 8 below). Recent tech-nological developments have substantially improved its range of applicability by making it possible to perform measurements at relatively high vapour pressures. The environmental SEM (ESEM) has great potential in the field of fibre and paper research. Among other things, it can be used to study fibre swelling and local wetting phenomena, as illustrated in Figure 7.3 (7).
The porosity of fibers can be measured using tech-niques such as gas-adsorption and cryoporometry, which are sensitive to pore sizes in the sub-micron size range.
These techniques are discussed further in Section 8 below. Wetting properties and the surface energy of
fibres are important for dewatering, adhesion and dry strength, absorbency, etc. The most direct means of measuring fibre wettability is the single fibre capillaro-graphic technique (8), where the contact angle of a liq-uid is determined from the wetting tension.
The force F exerted on the fibre dipped into a liquid (see Figure 7.4), is the capillary minus the buoyancy force, as follows:
F = Py cos0 - pAgh + Fa b s(0 (7.1) where P is the fibre perimeter, y the liquid surface tension, 0 the contact angle, p the liquid density, A the fibre cross-sectional area, and h the depth of immersion;
^abs(O corrects for the time-dependent absorption in the fibre. This can be usually be neglected for single fibres due to the fast equilibration. The force intercepts from measurements during advancement and retraction of the fibre in liquid, respectively, are then given by the
Figure 7.3. High-magnification image of wet-base stock show-ing the presence of 1-20 um partially wettshow-ing water drops at the fibre surfaces. (From ref. (7) with permission)
Vapour
Fibre
Figure 7.4. Single fibre wetting measurement geometry
following equations:
Fa^Pycosfla (7.2) and
Fx = Py cos Ox (7.3) where Oa and 0T are the advancing and receding con-tact angles, respectively. The buoyancy contribution in equation (7.1) is usually negligible for single fibres, so Oa and Ox can be determined at any point during immer-sion or retraction of the fibre, as long as the perimeter is known. This is often not the case. The parameter Oa
is then often calculated from the Fa and Fx values along the fibre with the assumption that Ox is zero, which often seems to be the case for cellulose fibres. Figure 7.5 shows the advancing contact angles measured along bleached and unbleached single fibres, where the bleach-ing process clearly has increased the fibre wettability.
From contact angle measurements in different liquids, the surface energy of the fibres can be estimated (see Section 7 below). This possibility was explored and dis-cussed in a comprehensive study by Berg (9), in which the effect of pH variations on the fibre wetting proper-ties was also investigated. This author found that acidic and basic groups co-exist on bleached kraft pulp, and that their pK\/2 values were of the order of 3 - 5 and 11, respectively, while chemithermo-mechanical pulp (CTMP) fibres only had basic groups, with a pKi/2 value around 12.
The wetting properties of fibres can also be esti-mated from contact angle measurements on sheets and liquid penetration measurements. As is shown later in Table 7.1, the microscopic fibre measurements seem to correlate well with measurement on hand-sheets and paper. This is discussed in more detail later in Section 7.
Table 7.1. Surface energy components measured for single fibres (sf), hand sheets (hs) and films (f) of hard-, soft- and mixed-wood bleached kraft (HWBK, SWBK and HSWBK, respectively) pulp and cellulose films
Substrate ys ysLW ys~ ys + Reference HWBK(sf) 48.3 43.2 16.3 0.40 54 SWBK (sf) 46.2 41.8 24.5 0.20 54 HSWBK (hsf 42.6 41.6 15.0 0.02
Cellulose (f) 54.5 44.0 17.2 10.50 53 Cellulose acetate (f) 52.6 44.9 18.5 0.80 53 Cellulose nitrate (f) 45.1 44.7 13.9 0.002 53 Cellulose (f) 56.7 39.1 39.7 2.00 55 Cellulose acetate (f) 43.1 38.2 28.2 0.21 55
flData obtained from Figure 7.36.