BY JAMES OWINO
In case we are good enough in providing the right furnishes to the paper manufacturer, he is able to add runnability to his machine and to go sleeping at home without any nightmare. However, according to the grades of paper he is manufacturing, he is demanded to have differentiated properties in the final
product. Eucalyptus pulps are special products to the manufacture of bulky and/or opaque papers.
Today, eucalyptus pulps are preferred raw materials in the manufacture of tissue, printing and writing, carton boards, industrial filters, base-papers for impregnation or coating, cigarette and many other papers. Eucalyptus fibers may be the sole fiber in the pulp furnish or to be part of a blend with other short and/or long fibers.
Tissue and high bulk papers
Tissue (and other highly porous papers) demand the following quality requirements in relation to paper specifications and machine runnability:
•Loosened paper structure
•Liquid absorption ( fast absorption and capacity to retain water)
•Hydrophilic paper surface
•Porosity (pore sizes and distribution)
•Structural softness (the feeling of a soft and fluff paper)
•Superficial softness (tactile softness, the feeling when touching paper surface in the act of crumpling or wrinkling the tissue paper sheet)
•Drawings caused by dry creping and embossing (these drawings improve the feeling of softness and provide better absorption and beauty to the paper sheet)
•Exact paper strength (wet and dry) to allow not to fall apart when customer is utilizing the paper
•Minimum tensile strength ( it is said to be maximum 20 Nm/g in the furnish), to prevent fiber lumen collapse, excessive bonding and high paper density
•Minimum elasticity modulus, a mechanical property negatively correlated with paper softness
•Capacity in the paper to retain pulp anatomical components (fines and vessels) to avoid excessive dust generation in papermaking and converting operations
•Exact wet web and dry strength to provide machine runnability
•Very fast drainage in the wet end 31
•Low fines to prevent fines build up in the white water and low consistency after wet presses (more steam is consumed in these cases)
The tissue papers and other porous-alike papers demand for loosen fibers in the paper structure. For this reason, fiber bonding is a poison, up to a certain extent. The fibers cannot collapse because this effect will flat the fibers and the paper surface; the paper becomes stronger in tensile, but all the tactile properties will be lost due to sheet compactability.
Pulp fines are also undesirable for two reasons:
Fiber bonding and building up in the paper-machine white water system, reflecting in losses of drainability. The most indicated eucalyptus pulps for tissue and highly porous paper manufacturing are those showing: low fiber population and consequently high coarseness (for eucalyptus, do not forget), low fines
and vessel elements contents, low bonding ability, low fiber collapsibility, low wet fiber flexibility, low hemicellulose content, low extractives and pitch content, low water retention value, thick cell walls, high cell wall fraction, rigid and cylindrical fibers, low unbeaten pulp Schopper Riegler, pulps resistant to the refining (slow beating development).
Fiber deformations are also important, since these deformations improve the bulk, porosity, softness and absorption of these papers. An important issue to remember is that fiber deformations may be artificially created in the pulp mills.
The manufacture of industrial filter papers, and impregnation-based papers are demanding the same properties, but in a higher level.
This means, to go to these specialty paper markets, the pulp differentiation must be even more pronounced. The simplest way to work in differentiated pulps to these very specialty markets is to work towards very high coarseness (low fiber population, what means high wood basic density in the wood supply), low hemicellulose content, low fine content (by fractioning fines or removing fines from one paper line and using them in another one, where more desirable) and to intensify fiber deformations (by high consistency presses, fiber shredding, or
pulp flash drying).
Another possibility that has feasibility to improve the pulp properties for tissue and highly porous papers is to take advantage of hysteresis. Drying of pulp has very good impact in the WRV, WFF, fiber bonding and fiber collapsibility. Integrated tissue mills have more difficulties to reach the desired furnish and paper properties. A solution is to add some DIP (market deinked pulp) or some percentage of dried market pulp in the furnish,in a percentage that may solve the
eventual bottlenecks the mill is facing.
Printing & writing papers
For printing and writing papers, the desirable paper properties are:
•Dry paper strengths (tensile, tear, fold)
•Paper internal strength (delaminating in the Z direction)
•Paper surface smoothness
•Paper surface strength (Scott bonding test or wax pick test)
•Good porous structure (porosity or air resistance)
•Liquid (ink) absorption properties
•Light scattering coefficient
At the same time, the papermaker wants to keep the machine runnability. We should never forget about the papermaking physiology. A higher fiber population is welcome for improved formation and opacity, associated to lower fiber coarseness. Also, fiber bonding is important to improve strengths. Hemicellulose and pulp fines contents do help in this task. However, there are limits to all this and the limits depend on each paper-machine system and operation.
A very high fiber population may be wonderful to improve opacity and formation, but drainage in the wet end and consistency after wet presses may become deteriorated, and machine speed reduced. The papermaker is to refuse this pulp. He wants quality and runnability both aligned, remember this. Fiber deformation here may not be so important, but they may help to balance the pulp properties, since machines may create it. Higher contents of hemicelluloses are welcome because they favor refining, bonding, consolidation of the paper web, and strength properties (tensile, burst, tear, folding).
An ideal pulp should have high strengths at the low levels of refining (fast beating response). This is possible for pulps with high zero span test, a demonstration of strong individual fibers. This situation allows the possibility to have strengths and bulk / porosity / drainage at the same time in the papermaking. The P&W papermaker loves to have a good combination of strengths, bulk, porosity and opacity. The papermaker don’t like to refine a pulp very hardly: he is raising energy costs, reducing the life of refiner discs, and deteriorating machine drainage, machine speed, steam consumption and a very important paper property that is dimensional stability.
Definitively, the best pulps are those showing good strengths at low levels of refining. For this reason, an interesting beating test for pulps is the measurement of strengths properties (tensile, tear, stretch) at a given bulk (for example:
1.6 or 1.8 cm³/g), or at a given sheet density (for example: 0.5 or 0.55
g/cm³), or at a given freeness level (25, 30 or 35ºSR, depending on the paper grade being produced) P&W papermaker is very sensitive to all these pulp and paper properties.
In addition, there is another wood anatomical characteristic very important to the printing grade papers: vessel elements content and vessel dimensions (specially the vessel diameter). Large, wide and numerous vessels are undesirable for P&W papers. They are responsible for a printing defect known as vessel picking. The papermaker needs to have special conditions to combat the vessel-picking tendency in the paper. For these reasons, a wood with smaller vessels and not so abundant is preferred. The same for the corresponding pulps
There are many other grades of papers manufactured with eucalyptus pulps: self-copying, cigarette, thermo-facsimile paper, glassine, filters, labels, etc. In most of the cases, the eucalyptus fibers are used to improve paper formation, opacity, smoothness, dimensional stability, bulk and porosity. The eucalyptus fiber population in the pulps, and their rigid and difficult to be collapsed fibers are important properties loved by the papermakers.
Eucalyptus pulps are not oriented to be highly refined, unless the papermaker is willing to discard the best properties the eucalyptus pulps have: bulk, porosity, formation, dimensional stability, opacity, softness, and water absorbency. In case of very low coarseness eucalyptus fibers (around 4.5 to 5 mg/100m), the fiber collapsing may become valuable for the manufacture of glassine, bible paper, and other high density papers. This is a clear indication that eucalyptus pulps may offer a variety of fiber qualities, what may make these pulps able for a wide range of utilization’s.
There is another key driver to papermakers for using eucalyptus fibers: the market pulp prices of this pulp fiber. Thanks to the low production costs, high pulping yield and lower chemical and wood consumption, these pulps are in general less expensive than softwood pulps. No doubt that production costs are also key issues for papermakers. The same to the entire eucalyptus pulp and paper production chain