Conductivity affects the degree to which individual charges interact and has impact on colloidal stability and the performance of polymers for retention and drainage. Increased conductivity reduces repulsive forces between charges on the flocculant molecule as it assumes coiled conformation that reduces its ability for bridging.
Pruszynskistudied the effect of increased conductivity on performance of a number of typical retention programs at pH=5 and pH=7, Fig. 3.
Figure 3 - Impact of various ionic impurities on the drainage loss (100% represents drainage gain from the chemical program in clean system) for dual polymer program at pH=7
- Monitor the conductivity level of the HYP stream, determine its variability and understand its sources.
- Install an online conductivity probe in the HYP stream and HB and look for correlations with paper machine data.
- Consider pre-mixing coagulant with a cationic flocculant solution. This has typically improved the performance of the flocculant especially at increased conductivity levels, Fig.4.
- If needed, in case of extreme conductivity buildup, consider more extensive changes to the retention program including multi-component programs (dual flocculant programs) or incorporation of bentonite typically added upstream in the thick-stock area.
- High and varying conductivity negatively affects colloidal stability and requires early and gentle fixation of colloidal particles (pitch) to avoid agglomeration.
- Consider a higher degree of substitution (DS) starches to better survive higher conductivity levels and improve strength and sizing applications.
Figure 4 - With total coagulant dosage constant, moving the feed point of coagulant from pre-screen (1) to the flocculant dilution water line (2) significantly reducesflocculant flow required to meet the whitewater consistency target
Hardwood HYP introduces increased levels of relatively unstable natural pitch to the system and may potentially increase machine deposit potential.
- Measure the level of extractives in your source of YP with a consistent total extractives measurement.
- Evaluate the physical form of extractives in HYP lurry microscopically to determine amount and particle size. A pitch particle that increases to twice its diameter is 8-times heavier and much less stable.
- Application of Nalco HYBRID coagulants was found to promote desired attachment to the fiber with limited colloidal agglomerationand have low impact on fiber zeta potential and OBA performance.
- Operate thickening stages in the pulp mill at as high a pH level as possible to maximize extractives removal without brightness loss.
- Fix colloidal particles to fibers and fines as early as possible to prevent agglomeration.
- Apply fixative treatment to the thick stock but closer to the headbox (machine chest pump inlet) for final polishing treatment.
- Measure the hardness level and control pH in systems that use calcium carbonate fillers to avoid calcium ion saponification of pitch.
- Optimize microbiological treatment and broke operating strategies to prevent pH depression anywhere in the system, especially in large broke towers.
Brightness and OBA performance
HYP can be produced at a very high brightness and their addition even at high fiber substitution levels may not affect brightness significantly. However, the lignin content of these pulps results in a yellowing tendency in the paper when exposed to UV or heat and impacts the performance of OBA additives.
UV-VIS light quenching by lignin explains the lower level of fluorescence from OBA additives. Wood-free coated grades of paper, with OBA added as a part of the coating formulation, are less affected as the integrity of the coating layer minimizes the impact of the lignin present in the base sheet.
Nidiscussed the effect of HYP on brightness stability in uncoated wood-free grades. Increasing PCC filler content not only increased immediate OBA performance levels but also sheet brightness stability. In a very elegant set of experiments, authorsdemonstrated that combining the stabilizing effects of OBA and PCC allows for the production of papers with HYP and high UV brightness stability.
Interesting results were observed by Ni and Pruszynskiwhen studying the impact of surface size treatment with Nalco's EXTRA WHITE® (EW) brightness technology. Adding EW to the size press formulation increased the brightness ceiling by about 2 units and the CEI whiteness ceiling by about 6 units, Fig.5,6.
Figure 5 - Effect of EW on the brightness ceiling of HYP-containing sheets
Figure 6 - Effect of EW on the CIE whiteness ceiling of HYP-containing sheets
In terms of strength, HYP occupy an intermediate position between typical mechanical pulps and chemical pulps. Gaoreviewed HYP fiber development strategies in terms of certain quality expectations. These strategies indicate a number of trade-offs between certain pulp properties such as tensile, strength, and bulk. Increased fiber rigidity, less external fibrillation, lignin content and higher fines content reduce the strength of HYP compared with kraft pulps. Some data indicate that increasing the content of HYP in a copy paper furnish from 0% to 20% decreased MD breaking length from 6.0 km to 5.7 km (5% drop) and CD breaking length from 3.3 km to 2.9 km (13% drop). The tear index remained practically unchanged. Other strength related issues need to be considered, including surface strength as related to offset printability issues, such as linting and dusting. The effect to which HYP impact paper strength depend on many factors and in some cases at low substitution levels (10-15%) it was observed that the synergy between HYP fines and HWK content led to strength improvement.
- Optimize HYP grade selection to best meet goals in terms of cost and impact on quality. If possible, select HYP grade with least impact on strength properties.
- Select starch with higher degree of substitution at higher conductivity.
- Bearing in mind the saturation of the dosage-response curve for starch application, consider the use of synthetic strength materials.
- Pre-treatment of HYP with proper coagulant typically results in improved cost effectiveness of starch and synthetic strength additives.
The successful introduction of HYP into typical wood-free furnish blends requires papermakers to take advantage of established best practices from mechanical grades in conjunction with new technology development. Remedies for potential issues such as increased levels of contamination and variability of wet-end conditions, extractives control, brightness, and strength optimization were discussed in this paper. The authors wish to emphasize that increased levels of cooperation between papermakers and specialty chemical suppliers will be required as the trend of increasing HYP continues. PPI
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