An experiment was designed to test the hypothesis that each softwood pulp is unique and
requires a specific, well defined mechanical treatment to derive its maximum strength potential.
Three bleached softwood Kraft pulps and respective wood samples were sourced from both the
Northern and Southern Hemispheres. The raw fibre characteristics of P. patula (Southern
Hemisphere), P. menziesii (Northern Hemisphere) and P. mariana (Northern Hemisphere) were
measured and compared. The raw pulp sheets were refined at different energies and intensities
under controlled laboratory conditions using a 12” single disc pilot refiner. Results were
assessed to determine the raw fibre characteristics, optimum refining conditions and the relative
refined strength potential for each of the three samples.
Results from anatomy measurements on the three wood samples differed significantly. P. patula
exhibited a relatively high proportion of springwood growth in the early growing years. As the
P. patula aged and formed mature wood there was a significant increase in the frequency of
latewood formation. This was characterized by an abrupt and significant increase in the wall
thickness, beyond that of the two Northern softwood samples. When the cell wall thickness
increased, the lumen width and fibre diameter of the P. patula decreased significantly, yielding
extremely coarse, stiff fibres.
The Northern P .mariana and P. menziesii samples were characterized by a relatively consistent
transition between high and low densities from the pith to the bark of the tree. The Southern P.
patula had a unique density trend with an increasing frequency of high density peaks indicative
of an increased latewood formation from the pith to the bark.
The slower growing Northern P. menziesii and P. mariana samples did not have as clear a
differentiation in fibre characteristics between juvenile and mature wood formation. The
Northern samples did however contain a significantly higher proportion of juvenile latewood
growth than the P. patula. However, the difference in fibre characteristics between earlywood
and latewood formation was not as significant as that noted with the Southern P. patula
Fibre morphology measurements on the unrefined bleached Kraft pulps also revealed significant
differences between the three samples. The average MORFI LAB01 results on the P. patula
defined fibres with a high coarseness and relatively low number of fibres per gram of pulp. The
extremely coarse latewood fibres formed during mature wood growth being the most likely
source. However, P. patula was also characterized with a high fibre flexibility and large lumen,
characteristics consistent with earlywood fibres. The Pulmac Z-Span 3000 was used to define
the individual fibre strength, when due consideration was given to the number of fibres per
gram, the corrected Pulmac results suggested P. patula had the strongest fibres.
When refined, using a standard disc refining programme, P. patula exhibited a fast freeness
development. Conventional thinking would suggest that this was an indication of a weaker fibre.
However, this species had a robust morphology compared to the Northern Hemisphere woods.
The theory developed in this dissertation suggests that the effect of coarseness and the
concomitant number of fibres per gram plays a significant role. These two parameters are not
included in the “traditional” refining calculations. The applied refining load and intensity was
calculated on the flow of the pulp passing through the refiner. The calculation did not consider
the actual number of fibres present in that specific volume. The implication is that when a fixed
refining load is applied to a pulp with coarse fibres there may be a higher effective load on those
fewer fibres (resulting in fibre cutting and fines generation). In this case, the Northern samples
have a comparatively low coarseness and more fibres per gram with each receiving a smaller
portion of the total load and intensity.
In terms of refined pulp properties, P. patula developed a relatively high bulk and tear index
consistent with coarse, rigid fibres. The Northern P. mariana and P. menziesii samples produced
a pulp with good tensile properties, consistent with a greater number of finer, collapsible fibres
with a higher relative bonding area.
P. patula fibres were extremely heterogeneous in nature containing the smallest relative lumen
width during latewood formation and the largest lumen width during earlywood growth. As a
result, P. patula contains extremes of both fine and coarse fibres in the same blend. It may be
more beneficial for this species than the others to improve both the tear and tensile properties
through fibre fractionation with appropriate development of the separate accepts and rejects
streams.
In terms of fibre development, low intensity refining parameters maximized the tensile strength
of the Southern P. patula. The Northern P. mariana and P. menziesii samples had a greater
number of fibres per gram of pulp requiring both a higher refining energy and intensity to
develop the pulp to its maximum potential. To develop optimum tear results, high intensity
refining, with a relatively low specific energy provided optimum results for all 3 samples.
Results confirmed that there were significant differences in the fibre morphology both between
the three different species and between the two Hemispheres. There was strong evidence that the
fibre characteristics dictate the manner in which a fibre responds to refining which in turn
determines the relative contribution to specific refined pulp properties. It may be possible to use
fibre characteristics to determine the appropriate refining parameters for optimal fibre
development which will enhance the value of the end product.
To derive the maximum strength potential from P. patula pulp samples, it is recommended that
further studies investigate Hydracyclone fractionation and the concomitant benefits of refining
the separate streams. Furthermore, a separate study on fibre morphology and refining
characteristics of the same species grown in both the Northern and Southern Hemisphere would
provide valuable insight. / Thesis (M.Sc.Eng)-University of KwaZulu-Natal, Durban, 2009.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/5118 |
| Date | January 2009 |
| Creators | Palmer, B. |
| Contributors | Kerr, Iain., Clarke, Charlie. |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
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