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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.