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The influence of high temperature pressing on paper properties was investigated. Handsheets made of TMP, bleached and unbleached kraft pulp were pressed in a press simulator with the press surface heated to 300°C. The density, tensile strength and air permeance of the press dried sheets are compared with the values for conventionally pressed and dried reference sheets. The compression behaviour and the dryness increase of the sheets pressed at high temperature were also investigated. It was found that the compressibility of the sheets was significantly increased if the hot surface temperature was increased from 250°C to 100°C. A further increase to 300°C gave only a minor compressibility increase. The increase in density and tensile index for the press dried sheets is higher for the high yield, low refined sheets than for the bleached kraft sheets. The air permeance of the press dried sheets was significantly lower than that of the conventionally pressed and dried sheets for TMP and for the low refined unbleached kraft. The bleached sheets showed the reverse behaviour and the air permeance was higher after press drying. A hypothesis to explain this is presented. (Less)

Dry-formed networks of cellulose fibers, produced with a laboratory device, have been impregnated with aqueous solutions of poly(vinyl alcohol) or solutions of poly(vinyl acetate) in acetone. In both cases, the strength and stiffness of the networks increase several times compared with the unmodified structure. When aqueous solutions are used, it appears that a minimum amount of poly(vinyl alcohol) is required (ca. 2 wt %) to increase the strength appreciably, but when poly(vinyl acetate) dissolved in acetone is used, as a binder, the strength improves even at the lowest level of polymer addition. For the systems studied here, the elongation at rupture increased with increasing polymer concentration. In most cases, the amount of polymer in the network structure was less than 10 wt %. Some results from impregnations using other polymeric systems, e.g., latices, are also reported.

The variation with humidity of the longitudinal and transverse moduli of isolated pulp fibres has been evaluated using micromechanics. The decrease in the moduli of the fibres is assumed to be due to a softening of the hemicellulose phase. Using a network theory developed by Perkins and coworkers these moduli have been used to calculate the tensile modulus of isotropic low density paper sheets in the range of 0–95% relative humidity (RH). Comparison with experimental data on low density sheets reveals a satisfactory agreement up to about 75% RH. Above this value the stiffness of the sheets decreases at a rate faster than that predicted by the theory. This may be due to a gradual loss in band stiffness and/or a softening of disordered cellulosic material at high humidities. As predicted by the network theory, the difference in behaviour between sheets of different densities is not very marked in the RH-range.

In dry-formed polymer-bonded networks of cellulose fibers and in other types of nonwovens, the fiber-polymer joint is considered to be the primary factor determining the ultimate properties of the network structure. In an attempt to develop a model describing the joint failure, the well-known fiber pullout test has been applied to a system consisting of regenerated cellulose fibers and three different polymer matrices: a styrene–acrylate copolymer, poly(vinyl alcohol), and high density polyethylene. For each system, the interfacial bond strength was evaluated. The results are, to some extent, discussed in relation to the mechanical behavior of dry-formed networks bonded with similar polymeric materials. It is suggested that both the interfacial properties and the cohesive strength of the polymer binder are of importance for the mechanical strength of the bonded network.

Dry-formed networks of cellulose fibres are normally characterised by a low tensile modulus. If small amounts of polymeric binders are added, the stiffness (and the strength) is increased. This increase in stiffness is here interpreted in terms of a network model developed for low density paper sheets. According to this analysis, the improvement due to polymer addition of the tensile stiffness of the initially rather weak network is due to an increase in the shear stiffness of the fibre bonds. The increase in bond stiffness yields a better internal network coupling, which accounts for the drastic change in small-strain properties of the network. It is also found further increase has only a marginal influence on the stiffness of the network. Some comparisons with experimental data are made.

A network theory developed by Perkins for low density paper sheets has been applied to three series of sheets with basic weights in the range 10–80 g/m2. Each series comprised sheets with different degrees of fibre orientation. A small amount of the fibres (c.0·2%) were dyed prior to the formation of the sheets, which made it possible to quantify the fibre orientation and length distributions using image analysis. The observed orientation data are used with the network theory to estimate the orthotropic elastic parameters of the sheets. A good agreement with experimental results is noted. A further analysis using a slightly modified network theory indicates that the fibre modulus in the longitudinal direction is approximately independent of the fibre orientation, the sheet density and the average fibre length, being about 20 GPa for all sheets in this study. By image analysis it is further shown that the orientation distribution of fibres in a given sheet is independent of the fibre length.

Paper can be regarded as a network of cellulosic fibres, especially at lower basis weights. When the elastic behaviour of paper sheets is modelled, it is normally essential to know or to assume how the stresses (and strains) are distributed at the fibre level. This article presents an attempt to estimate how the stresses are transferred throughout a simple fibre network using the finite element method (FEM). Attention is mainly focused on the axial fibre stress distribution when the network is uniaxially deformed. The presence of fibre ends is found to induce local stress increases (“stress concentrations”) in the deformed network, which presumably have a bearing on the ultimate properties of the sheet. The influence of the properties of the bonds between crossing fibres on the mechanical properties is also investigated. It is noted that the bond stiffness has no significant effect on the stress transfer between fibres provided that the stiffness is above a critical value. Below this value the stress transfer deteriorates rapidly.