THE CONVERSION OF A KRAFT LINERBOARD MACHINE FROM ACID TO ALKALINE SIZING

Author

Karin Krüger

Company and address

Sappi Technology Centre, Paper Sciences Department, PO Box 3252, Springs, 1560, South Africa

email

karin.kruger@za.sappi.com

Keywords

ASA, calcium carbonate, white top liner board, Kraft liner board, alkaline sizing, Ngodwana

ABSTRACT

The high quality graphics used in printing linerboard place an increased demand on surface characteristics such as smoothness and porosity of the product.  One of the ways to meet these new demands is the use of an alkaline sizing system in conjunction with calcium carbonate.

Laboratory investigations and mill trials were carried out to determine the feasibility of converting the rosin/kaolin system used for the production of white top linerboard at Sappi Ngodwana to an alkenyl succinic anhydride (ASA)/calcium carbonate system.  During the successful conversion, numerous quality improvements were seen.  These included improved coverage, higher brightness and increased strength properties. The reduction in chemical costs and savings due to fibre replacement by filler was significant.

1. INTRODUCTION

Sappi Ngodwana produces white top and Kraft linerboard (KLB) on their three-ply KLB machine. The sizing philosophy on the machine has followed the traditional route of acid sizing.  Sizing has been achieved with alum and a rosin emulsion size in the pH range of 4.5 to 5.0.  There have been periods of excessive size usage and pH swings with many of these relating to the white top liner makings.

It has long been known that paper and board produced under alkaline conditions has superior properties to the conventional acid board. In addition to quality improvements there is the potential to reduce the fibre costs by loading with calcium carbonate and reducing the virgin fibre content. In terms of strength properties it is known that an increase in pH will assist with refining due to fibre swelling and the improvements in refining will result in better fibrillation and improved fibre to fibre bonding.  The increase in refining pH will also mean the energy required for refining could be reduced.

Beside a drive for stronger paper, the printing demands on the paper are constantly increasing.  As higher quality graphics evolve for these grades, new demands are placed on surface characteristics such as smoothness and porosity of the product.  Whiteness and brightness of the paper is becoming more and more important. One of the ways to meet these new expectations is the use of an alkaline sizing system in conjunction with calcium carbonate.

2. LABORATORY EVALUATION

The laboratory evaluation included several investigations to establish the effect of the sizing and filler changes on the wet end and final product. The laboratory looked at the following:

  • Change of sizing and filler system and effect of increased filler levels on final product
  • Effect of alkaline system on optical brightening agents (OBA)
  • Print and gluing evaluation of alkaline paper at glue and ink suppliers

2.1. Change of sizing and filler system
The furnish combination that was used on the machine for acid sizing was used in all evaluations. The standard amount of calcined clay was added and the stock sized with the alum/rosin system. All other chemical additions remained as per machine conditions. The sheets made under these conditions were then compared to sheets that had been sized with ASA. The calcined clay was replaced by twice the amount of ground calcium carbonate (GCC).

In Figure 1 the sizing results found are shown.  As is commonly expected with ASA sizing the laboratory evaluation showed that the required Cobb results could be obtained with a third of the ASA dosage level compared to that of rosin size (1). 

Figure 1

Figure 1: Sizing response of ASA and rosin size on Ngodwana white top liner (WTL) furnish

The addition of twice the filler amount resulted in a decrease in the strength properties as was expected (Table 1). To compensate for the decrease in strength the amount of starch was increased from the original dosage levels. The Taber abrasion results of the sheets made under alkaline conditions however were lower than those made under acid conditions while bulk and porosity increased (Table 2).

 

Strength property

Percentage change in strength properties

Std starch addition

33% increase in starch

66% increase in starch

Burst index (kPa.m2/g)

-9.8

+0.9

+4.7

T.E.A. index (J/g)

-13.4

+5.9

+5.7

Tear index (mN.m2/g)

-12.5

-9.8

-13.6


Table 1: Effect of increased filler content on strength properties (negative sign indicates loss of strength, positive sign an increase in strength)

 

Paper property

Percentage change in paper properties

Std starch addition

33% increase in starch

66% increase in starch

Taber abrasion (mg/100rev)

+2.0

+11.8

+19.6

Bulk (cm3/g)

+1.8

+2.4

-2.4

Porosity (ml/min)

+14.1

+20.7

+2.4


Table 2: Effect of increased filler content on other paper properties

The decrease in bulk seen at very high starch levels may be due to the starch bonding the fibres very strongly and not allowing air voids in the sheets. This would also explain the decrease in porosity at the same starch levels.

From the results obtained above it was decided that it would be possible to increase the filler level by 100% and still maintain the required strength properties if the correct type and amount of starch was added.  The starch that was selected for the trial was a cationic tapioca starch with a nitrogen substitution of 0.34%.  This starch gave the best results in terms of strength properties as well as forming excellent ASA emulsions.

2.2. Effect of alkaline system on OBA
The standard machine conditions and dosage levels under acid conditions were used as the reference. The same amount of OBA was then dosed to the stock under alkaline conditions.  The results found in this experiment are given in Table 3. It can be seen that the changes involved in the conversion from acid to alkaline would have a substantial effect on both whiteness and brightness of the final sheet (2).

Paper property

Percentage increase in optical properties

Whiteness (%)

20.8

Brightness (%)

6.3

Table 3: Effect of system pH on the optical properties

2.3. Effect of alkaline conversion on gluing and printing
According to the glue suppliers the Cobb value of the paper is used as their prime determinant of how well the glue will bond. The bond is essentially mechanical and hence penetration into the paper is the most important aspect for the supplier of the glues. As no change to the Cobb value was planned the suppliers did not foresee problems with gluing. A benefit that was mentioned by the suppliers was that the interaction between the GCC in the liner and the slightly acidic glues would result in an increased surface area and hence improve the glueability. The envisaged higher porosity of the sheet would also mean easier penetration of the adhesive into the sheet.  Because of this it was felt that conversion to alkaline paper would make an interesting solution for the glue penetration problem without the glue manufactures resorting to more viscous, penetrative hot melts at the expense of heat resistance (3).

The major ink suppliers (Coates, Sicpa and High Tech Inks) were contacted to determine the impact of the increased filler level and neutral pH on the printability of the paper.  As no alkaline paper had been produced at the mill at that stage, acid and alkaline paper from another mill was used in the laboratory print evaluations.

The problems that were encountered were slightly increased drying times, reduced colour intensity and a slight drop in rub resistance with some colours. However uniformity of print was improved on the alkaline paper.  In general all ink suppliers were confident that a conversion of acid to alkaline paper would not cause significant problems.   While some changes would need to be made to the ink, these changes would be minor and once set up the printers would be able to run acid and alkaline papers without a problem.

3. MILL TRIAL

3.1. Trial planning
Planning for the alkaline conversion started six months prior to the trial.  To ensure the success of the trial suppliers of all new chemicals, mill staff, and personnel from the Technology Centre were involved in the meetings. The meetings were held on a monthly basis initially but this increased to weekly meetings just before the trial. It was necessary to install new dosing equipment for the size, a new starch cooker and to modify existing equipment such as the clay dosing equipment to handle the calcium carbonate. As it was not possible to have a standby ASA unit by the time the trial commenced, the rosin sizing system had to be used as a contingency plan should there be a problem with sizing or equipment. Although changes to the microbiological control program are necessary for alkaline systems, it was decided not to implement these changes for the first trial due to time restrictions.

To ensure that the customer was aware of the intended changes to the product road shows were held at the major customers.  The presence of staff from marketing, the mill and size supplier, the Technology Centre and ink suppliers ensured that any concerns raised by the customers could be dealt with immediately.

The trial was run in June 2002 and during the trial three white top liner grades were made (140, 200 and 250 gsm).  All plies were sized with ASA.  Start up for the trial was after a boil-out to ensure that the system did not contain any contaminants that could hamper the trial.

3.2. Trial results

3.2.1 Quality related results
After some initial furnish and refining problems, the quality issues relating to the paper were sorted out.  Significant increases were seen in burst, Scott bond and porosity when the trial results were compared to the average of the last five months (Figures 2 – 4).  Similarly IGT values during the trial were 18% up from previous makings. The increases seen in the strength related characteristics are most likely due to the higher starch content necessary to emulsify the ASA size. The platelet structure of calcined clay gives the sheet a very closed structure resulting in lower porosities.  The more granular structure of calcium carbonate on the other hand opens the sheet, which was seen by the increased porosity during the trial.

Roughness showed a marked decrease of 5, 32 and 31% for 140, 200 and 250 gsm respectively (Figure 6), while Taber abrasion figures dropped by 13% despite the higher filler content. The basis mass of the top ply was also reduced by up to 12% over the three grades without compromising on coverage. Initially some problems were experienced sizing the base ply.  This was mainly due to the presence of pitch. Once a small amount of alum had been added to the system to mop up the pitch, the required Cobb values were achieved.  As the wet end stabilized the amount of alum could be reduced.  As predicted in the laboratory evaluation the ASA size usage was approximately a third of the previous rosin size usage.

Figure 2

Figure 2: Increase in burst index during the alkaline sizing trial

Figure 3

Figure 3: Increase in Scott bond during the alkaline sizing trial

Figure 4

Figure 4: Increase in porosity during the alkaline sizing trial

Figure 5

Figure 5: Decrease in roughness during the alkaline sizing trial

3.2.2 Production related results
The trial also showed a number of production related improvements. The conversion from acid to alkaline should produce benefits on the machine in terms of reduction in energy consumption, improved drainage, increased machine speed and a more stable wet end in general.

Analysis of the trial data has shown that all these improvements were found on the machine.  Although the exact amount of energy savings can not be quantified due to the trial not being sufficiently long, it can be said that the energy requirements for drying were significantly lower than during a normal white top liner trial.  The machine speed had to be increased to prevent water logging in the dryers.  On average the run rate during this trial was the highest ever, with an increase of up to 27% on the heavier grammages and up to 7% on the lowest grammages. The lower run rate improvement seen on the lighter grammages was however not due to steam limitations but stock supply to the top ply. 

Significant improvements in drainage were seen on the machine during the trial. Under acid conditions the mill has had continuous problems with pH swings and erratic wet end chemistry. The wet end chemistry during the trial was found to be very steady and the buffering effect of the calcium carbonate ensured that the pH was stable throughout the trial. An environmental benefit that was seen was the reduction in sulphate produced during this trial due to the lower alum usage.

On the commercial side the trial resulted in significant cost savings. Although it was not possible to optimise the dosage levels of all chemicals to the full extent due to the short time available during the trial the cost implications to the mill were huge. A chemical cost saving of 33% was realized during the trial. A further reduction in fibre costs of 6% was seen when the filler content was increased.  While this is still less then the 58% predicted when the alkaline program is in full operation it did show the potential of the cost reduction to the mill when the chemical dosages and fibre substitutions have been optimised.

3.2.3. Feedback from customer trials
The trial paper was taken directly from the mill to various converters to assess its runnability and print quality. All customers remarked on the improved appearance of the paper.  No problems were experience by any converters with regards to runnability.  In terms of print quality two of the three ink suppliers did not require to make any changes to the ink formulations while only minor ones were necessary with the third supplier. The printing in one case was better then the acid paper tested.

4. CONCLUSION

The length of the trial was not optimal, however despite the short duration of the trial all parties viewed it as a success.  The promised quality improvements were seen during the alkaline making. Machine runnability was at the highest level ever seen for that grade and cost savings to the mill were substantial. It is felt that once the sizing regime has been optimised the target cost savings of 58% can be achieved.  Although the trial was only run on the white top liner makings, alkaline sizing of the Kraft liner is not seen as a problem. Already during this trial the unbleached base ply was sized with ASA without any problems.

With the improved quality of the product, improved machine conditions and the acceptance of the product by the market place it is believed that the Sappi Ngodwana will be converting to alkaline sizing once the back up ASA sizing unit has been installed.

5. ACKNOWLEDGEMENTS

The author wishes to thank the staff of the KLB machine at Sappi Ngodwana and Buckman Laboratories for the assistance in writing this paper and for the work every one put into getting the trial off the ground as well as assistance to marketing for the customer road shows.

6. REFERENCES

    1. Karin Krüger, Internal Technical Report to Sappi Ngodwana, M2001/017P

    2. Karin Krüger, Internal Technical Report to Sappi Ngodwana, M2002/007P

    3. Devlin Gardner, Correspondence between National Starch and Chemical and Sappi, March 2002

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