Evaluation of the tensile stress-strain properties in the thickness ...

Fig 11. Density versus Z-directional tensile strength in a semi-logarithmic plot. Fig 12. Elastic modulus versus grammage for the TMP handsheets.
The Z-directional tensile strength of TMP handsheets
could be regarded as constant down to a grammage of
approximately 60 g/m 2 . For lower grammages the Zdirectional
tensile strength increased drastically. For the
chemical pulps the strength gradually increased with
decreasing grammage. A steep increase was also observed
after the 60 g/m 2 limit.
The strength measured with double adhesive tape was
independent of grammage, with the exception of low
grammages, where tape penetration influenced the
measurements. When strong chemical pulp was tested,
the failure did not occur in the paper but in the interface
between tape and paper. On the other hand, when testing
TMP, the failure occurred always in the paper.
Fig 11 presents in a logarithmical scale all the results
in terms of Z-directional strength in function of the
structural density. A straight line fit was performed to the
data.
The elastic modulus was calculated from Eq (9). The
results are presented in Figs 12 and 13. The straight lines
correspond to the mean value of the elastic modulus evaluated
for papers with different grammage. The elastic
modulus was essentially independent of grammage.
The relation between elastic modulus and density was
also considered. In order to illustrate in the same graph
the full range of elastic modulus for the different pulps a
logarithmical scale on the y-axis was used in Fig 14. A
straight line fit was performed to the data.
Fig 15 presents the variation of strain at break as function
of grammage. It was observed that strain at break
increased for lower grammage handsheets both for TMP
and chemical pulps. It was also observed that strain at
break was significantly higher for TMP handsheets.
Fig 16 shows the results for strain at break as a function
of density. Density had a small influence on the
strain at break of chemical pulps. TMP seemed to be
slightly more sensitive to small variations of density,
since a decrease in density caused an increase in the
strain at break.
Discussion
The custom-built experimental set-up was designed to
allow for the measurement of the Z-directional stressstrain
curves of handsheets. The first set of tests measured
the adhesive penetration into the handsheet with
54 Nordic Pulp and Paper Research Journal Vol 22 no. 1/2007
Fig 13. Elastic modulus versus grammage for the chemical pulps handsheets.
Fig 14. Elastic modulus versus density for the investigated handsheets.
Fig 15. Strain at break versus grammage for the investigated handsheets.