BTG Pulptec™ MorfiTrac – the most accurate way to achieve optimum tissue softness
Different pulp sources have very different softness properties, and can also react very differently to the refining process. BTG is helping mills optimize the softness of their tissue products through accurate online analysis during the production process, for more accurate chemical dosing, greater output and better hand-feel.
For tissue consumers, softness is one of the three most important purchasing criteria, alongside absorption and strength. But while water absorption and tissue strength can both be measured relatively easily and accurately, softness has remained so hard to quantify using lab instruments that most tissue products are still evaluated by specially trained human testers who provide a subjective appraisal of hand-feel.
While pair-comparison – sometimes using standards – does provide for some degree of classification and ranking of tissues according to softness, the lack of any absolute scale has made results rather unreliable. But now, a new study evaluating the chemical and physical factors affecting softness has defined a rigorous methodology for accurately grading papers according to their softness properties, and confirms the advantages of online analysis of pulp characteristics during the production process.
The study, which was presented at the 2009 ABTCP trade fair in Saõ Paulo, Brazil in October, reveals that different kinds of pulps react very differently during refining, and yield widely varying results when it comes to softness. It also shows that, if not continually monitored, softness properties can vary significantly during a production run, so that current lab testing procedures that evaluate just the last few meters on the reel can generate quite misleading results.
BTG’s Pulptec™ MFA-5000 MorfiTrac precisely addresses this problem, allowing mills to continually sample and analyze pulp fibers, shives and fines to ensure optimum measurement and control of pulp characteristics. The new study confirms that continual analysis of pulp qualities will help manufacturers get the very best results, regardless of the type of fibers used. Online prediction of softness using a process analyzer like the MorfiTrac provides immediate feedback on pulp quality, allowing for rapid modifications to refiner load and chemical dosing, for a dramatically improved production process and end-product quality.
Advanced assessment methodology
The new study, which was undertaken by the Centre Technique du Papier in Grenoble, France, involved the active participation of BTG and a number of different pulp and paper companies. The researchers developed a model for pulp softness assessment through comprehensive analysis of the properties of 13 market bleached pulps - seven eucalyptus pulps, three softwood pulps, two hardwood pulps and one deinked pulp - for softness potential before and after refining (Table 1).
Table 1: References of the 13 commercial pulps evaluated in the study
Pulps were ranked, and a softness assessment revealed important differences between pulp grades on refining (Figure 1). Hardwood pulps, and more particularly eucalyptus pulps, developed the softest papers, while softwood pulps offered the lowest softness potential. Loss of softness for hardwood pulps during conventional refining was also less important than for softwood pulps.
As a result, softwood pulps were not recommended for producing softer tissue papers. But as they are essential to developing sufficient tensile strength in the tissue, the researchers looked for a way to optimize their use so as to get the maximum strength while limiting their degrading influence on tissue softness.
The study also looked at a deinked pulp, whose softness potential was comparable to that of softwood. It also showed that the disc refining necessary to develop tensile strength had a detrimental impact on softness potential: the higher the degree of refining, the lower the softness.
Some hardwood pulps could be compared to eucalyptus in terms of softness potential, with one European hardwood pulp demonstrating an impressive resistance to loss of softness during refining, so that the desired breaking length could be achieved with a very good softness potential.
Using BTG’s MorfiTrac analyzer to predict pulp softness potential from fiber characteristics allows for a comparison of pulp grades ranked for their softness potential depending on refining degree. This in turn allows pulp producers to indicate the softness potential of their different pulp grades, while allowing tissue manufacturers to control potential variations in tissue softness through accurate fiber analysis. Tissue mills can thus create specific pulp mixtures according to the softness potential of each mix, in order to produce a range of different tissue products with required and desired softness levels.
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| Hardwood Ref 21 | Hardwood Ref 22 |
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| Eucalyptus Ref 1 | Eucalyptus Ref 9 |
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| Softwood Ref 4 | Softwood Ref 12 |
Figure 1: Spider diagrams showing pulp evaluation during refining (UR: unrefined, LR: low refined, HR: high refined)
Softness modeling
The extensive CTP panel softness assessments and the measured fiber characteristics allowed the researchers to design a mathematical model using statistical analysis. Tests showed that the model’s predicted softness values agreed closely with the measured values for all pulps studied (Figure 2).
Figure 2: Predicted vs assessed softness for the 13 commercial pulps tested
This model specifies the following fiber characteristics, which were found to have a significant influence on end-product softness:
- Mean kink angle
- Mean curl index
- Broken fiber content
- Fines content in area
- Fines content in length
This model was sufficiently robust and valid for an arithmetic fiber length of approximately 600 -1,450 µm.
Nowadays, a lot of tissue papers are produced from deinked pulps. In such cases, the model needs to be adjusted, as the quality of these pulps depends to a great extent on the raw materials originally used in the manufacturing and on their overall composition (fiber and fillers).
BTG’s MorfiTrac: the high-tech solution
BTG’s MorfiTrac provides the industry’s most accurate picture of fiber morphology. It samples and analyzes pulp fibers, vessels, shives and fines automatically and continuously, measuring batchwise samples from a maximum of eight sampling points.
The MorfiTrac allows for highly accurate and reliable statistical evaluation of important fiber characteristics such as curls and kinks, measuring thousands of fibers in each batch. It can also be used to control refiners and to control the fiber mix ratio. Analysis is performed on a fiber network, so that measurement takes place in the fiber's natural unrestrained environment.
Figure 3: Tissue machine optimized for softness
MorfiTrac installation at the headbox allows crucial fiber properties to be evaluated during the production process (Figure 3). In addition, process operation conditions are evaluated by an online surface characterization device (TSSA) placed behind the Yankee that can operate at speeds of up to 2,000 m/min, and which provides information about creping density and smoothness.
BTG’s StatMorf software, which combines online data about fiber properties with softness feedback reported by human test subjects, shows an average 94% correlation between test subject feedback (i.e. lab softness) and predicted softness (Figure 4).
Figure 4: The diagram shows the correlation between measured softness (pink) and predicted softness (blue)
As the new study confirms, online softness prediction allows for much faster response in terms of modifying the refiner load and/or adapting the dosing of release agent or softener. That translates into greatly improved product quality and higher sustained output, with no risk of degradation in softness caused by delays inherent in human testing.
For mills seeking to sharpen their competitive edge in one of the market’s fastest-growing segments, that makes for a very compelling reason to make online pulp analysis an indispensable part of the tissue-making process.