A novel approach to improve dry strength

Howard Johnston

ABSTRACT

Continuous improvement in performance is essential for the success of the paperboard industry. This paper discusses one novel approach to improving quality.

Strength improvement has always been an important consideration for papermakers. When paper and paperboard products were made exclusively from virgin fibres  the only opportunity of dry strength improvement was through the correct operation of refiners, formation tables and press sections.

Suitable refining, good formation and paper pressing remain the foundation of producing a strong sheet and every effort must be made to get this right, before embarking on other means.

The use of recycled fibres in the paper industry has been an economic and environmental necessity over the last 20 years. The increased recycling rate of fibres has resulted in a decrease in pulp strength and on the bonding strength between fibres.  With each drying and slushing cycle the fibres become less flexible and less permeable to water and therefore do not conform as well as virgin fibres.

Most aspects of the manufacturing process affect the recyclability of fibres - initial refining, wet pressing, drying and calendering.

Mills using recycled fibre as a main or part component of their furnish must find new and innovative ways to increase strength properties of the final sheet, without sacrificing productivity.

Numerous wet end and dry end additives have been used and evaluated over the years to help improve dry strength, with significant success.

This paper talks about a new process to obtain dry strength in paper and paperboard using a range of unmodified starches that has been developed by Ciba Specialty Chemicals (marketed as Hyforce).  Some case studies of practical results achieved will be presented.

INTRODUCTION

Continuous improvement in performance is essential for the success of the paperboard industry. This paper discusses one novel approach to improving quality.

Dry strength properties and the testing procedures of paper and paperboard are well known (1,2). Typical dry strength properties are classified into two broad groups:

  • Surface Strength abrasion, scuffing, IGT, waxpick
  • Body Strength tensile, tear, burst, edge crush, flat crush, ring crush, bending, folding, stiffness, plybond

Strength improvement has always been an important consideration for papermakers. When paper and paperboard products were made exclusively from virgin fibres (3) the only opportunity of dry strength improvement was through the correct operation of refiners, formation tables and press sections.

Suitable refining, good formation and paper pressing remain the foundation of producing a strong sheet and every effort must be made to get this right, before embarking on other means.

The use of recycled fibres in the paper industry has been an economic and environmental necessity over the last 20 years. The increased recycling rate of fibres has resulted in a decrease in pulp strength and on the bonding strength between fibres.  With each drying and slushing cycle the fibres become less flexible and less permeable to water and therefore do not conform as well as virgin fibres. Cumulative loss of hemicelluloses from the fibre surface also contributes towards reduced bonding (10).

Most aspects of the manufacturing process affect the recyclability of fibres - initial refining, wet pressing, drying and calendering (4).

Mills using recycled fibre as a main or part component of their furnish must find new and innovative ways to increase strength properties of the final sheet, without sacrificing productivity.

Numerous wet end and dry end additives have been used and evaluated over the years to help improve dry strength, with significant success.

Products used include :

  • Starch
  • Polyacrylamide
  • CMC
  • Guar gum
  • Chitosan

Different type of starches have been used to increase paper strength in paper machines for many years. To start with, wet end addition of native starches was used for strength improvements, but since this type of starch shows a low affinity for cellulosic fibres, this also resulted in a high BOD load in the effluent water. Introduction of cationic starches lead to a dramatic improvement in dry strength, resulting in cationic starch becoming the most utilised product for strength improvements in the paper industry.

For maximum strength improvement, cationic starches should be added to the thick stock where they must react for several minutes with the furnish components .  The maximum dosage of cationic starch is limited to about 1.5%, the typical dosage being less than 1.0% in general.(5,9)   Results of studies  (11) indicate that the adsorption is mostly controlled by the charge on the fibres and fibre fines and the DS of the starch, but also by geometrical restriction on the surface of the fines.

Overuse of high DS cationic starch can, in some cases, adversely effect the charge balance at the wet end causing difficulties with the substantivity of other cationic charged species or functional chemicals.

A recent novel development is the use of high levels of anionic starch which is then fixed using a combination of PAC / polyDADMAC.

Polyacrylamide based dry strength resins have been developed and are used widely in some countries.  Maximum dry strength properties are usually obtained by using combinations of cationic, anionic or amphoteric polyacrylamide sometimes in conjunction with cationic starch and/or alum and fixatives.

A new process to obtain dry strength in paper and paperboard using a range of unmodified starches has been developed by Ciba Specialty Chemicals  marketed as Hyforce. (7)

During the development phases leading to this patented system, many different approaches were evaluated using Pilot Machines and Production Machines in friendly mills.

The needs identified by customers in drawing up the project profile were:

  • Lower basis weights while maintaining strength characteristics
  • Higher strength for the same basis weight
  • Substitution of virgin fibre with recycled fibre
  • Reduction in waste paper quality
  • Concern at reduced stiffness/rigidity
  • Eliminating the Size Press
  • Ability to operate consistently in variable and hostile wet end environments
  • high cationic demands (CD)
  • high total dissolved solids (TDS)
  • high WW conductivity
  • zero effluent mills

Some Case Studies where these objectives and conditions have been realised will be presented.

The technology developed brings together the advantages of Microparticle Retention Programmes and the ability of wet end addition of  inexpensive native starches to significantly enhance sheet properties.  This development utilises the retention mechanism of Microparticle Systems, with the microparticle component of the Hydrocol System, bentonite, acting as a carrier for the raw, uncooked starch, ensuring high levels of first pass retention.

SYSTEM MECHANISM

The mechanism for strength development of this system requires four elements to be satisfied :

1. Effective retention of the raw uncooked starch particles

2. Uniform distribution of the starch particles in the sheet

3. Solubilisation of the starch particles

4. Cooking of the starch in the dryers

The basic retention mechanism of the Hydrocol Retention Programme is described in Figure 1.

Figure 1

Figure 2 shows the preferred method used to apply the starch to the wet end.

Figure 2

The most effective approach is described where high retention of the raw starch is achieved by pre-mixing the starch slurry with the bentonite component of the Hydrocol System, under controlled conditions of dilution and mixing.

COMPARISON OF NATIVE STARCHES

The optimum type of native starch to be used with this system will vary depending on application as well as technical, commercial and geographical factors.

During the developmental phases of this project, various starch types have been evaluated. Starch is obviously available is a large variety of plants around the World, the most commercially significant types originating from corn, potato, wheat, tapioca and waxy maize.

Figure 3 shows important starch properties and global availability.

Figure 3

A lot of development work carried out indicated that there was no significant difference in the dry strength results from the addition of various starch types - assuming equal retention and degree of cooking of the starch.

All types of starch can be successfully used for this application, depending on their availability, type of machine, qualities of paper and paperboard produced and the dry strength improvements required.

The most significant parameters in determining which type of starch to use are:

  • particle size
  • gel temperature (pasting temperature)

The gel temperature is the temperature at which the viscosity of the starch suspension starts to increase. This is a rate determining step in the solubilisation of the starch.

The starch particle retained in the sheet must go through three interconnected steps:

  • solubilisation
  • distribution
  • drying

Looking at these steps individually, the solubilisation of the starch particle starts with the swelling of the starch particle as it absorbs the wet web's free water. This reaction is dependent on the temperature, time and amount of free water available.

Distribution is accomplished while the moisture content of the wet web is relatively high, contributing to the dissolution of the starch in the Z direction of the sheet.

Once the starch is partially or fully dissolved in the sheet, the dryers complete the process with the dried starch film forming within the interstices of the fibres and fines.  This physical bonding of the sheet binds all the components together into a stronger matrix.

The main machine parameters which must be considered are grammage, speed and moisture content exiting the press section as well as the drying profile.

RESULTS

A number of studies are presented.

The main results presented here are from Green Bay Packaging, Wisconsin - previously presented by Mat Szymanski ,Green Bay Packaging and Ben Doiron  Ciba SC (12).

Green Bay Mill produces about 600 tonnes/day of linerboard and corrugated medium in a zero process effluent environment (6) using 100% recycled furnish, mainly OCC and mixed office waste.

Due to the zero process effluent environment, machine wet end conditions are very hostile for process and functional chemicals, because of the high cationic demand, high total dissolved solids and excessive conductivity of the process whitewater.

Several attempts to find an internal strength product which can overcome this chemistry using other technologies were unsuccessful.

The paper machine wet end configuration consists of a flat fourdrinier, a secondary headbox followed by an upward dewatering unit, forming a two ply sheet.  The press section consists of a straight through press and an extended nip press. Dryers consist of four sections and no size press.

The mill objectives for evaluating Hyforce Technology were :

  • increase cross machine direction ring crush
  • increase sheet burst properties
  • decrease basis weight while maintaining dry strength properties.

Initial trials using this technology showed promising results and a process of optimisation was carried out.

Over a two year period, mill trials indicated that potato starch was the easiest to retain producing the lowest WW consistencies. This is supported by the fact that potato starch has the largest average particle size of the starches evaluated.

Figure 4 shows average first pass WW consistencies for various starch types indicating a good correlation between WW consistency and starch particle size.

Figure 4

The level of strength development is directly related to the level of starch in the sheet, confirming the critical nature of high and stable starch retention and the importance of the microparticle retention programme in preventing high levels of unretained starch particles circulating in the long and short paper machine loops causing potential process problems.

Typical starch dosages were between 3.5 - 5.0% w/w in the sheet. Because of the zero effluent environment overall starch retention was very high.

Once the starch has been retained, the challenge is to cook it effectively to maximise potential dry strength improvements.

Results have indicated that the larger particle size potato starch granules are easier to cook in the dryer section compared with smaller particle size corn starch granules.  The lower gel  temperature of the potato starch is therefore a more important factor in this case than any benefits from the higher surface area, smaller particle size corn starch.  This enables the potato starch granule to swell and burst earlier in the dryer section of the machine, compared with other starch types.

Due to the high retention results and superior cooking properties of potato starch, it was possible to reduce the potato starch dosage by 20% compared to other starch types while still maintaining strength targets.

Significant improvements in both burst (Mullen) cross machine direction ring crush (cdRC) and internal bond have been achieved.

Figures 5 and 6 show the impact of starch addition during a three day production run.  Both Mullen and cdRC/BW ratio decreased very significantly after starch addition was discontinued.

Figure 5

Figure 6

Figure 7 compared internal bond values with and without starch addition. Internal bond increased very dramatically during the three day run.

Figure 7

After many optimisations and extended runs, it was decided to install a 200 tonne dry starch silo with automatic make down and feed system.

Other Case Histories are presented on a variety of paper board types.

See Case Histories 1,2,3 and 4.

CONCLUSION

With this technology, significant improvements in dry strength properties have been obtained,  This system provides a low cost alternative using readily available and proven materials to add significant value to the paperboard industry.

Many of the mills currently involved in this programme have now moved on to a phase of active product innovation to develop high performance grades of linerboard, fluting and tube winding grades. New, high performance lower basis weight grades of linerboard have been developed which have proved successful in trial and production scale packaging runs.

One example is 308 gsm high performance linerboard which will significantly outperform 343gsm linerboard in both edge crush test (ECT) and finished box compression.

High performance Tube Winding Grades have also been brought to the market in Europe.

To successfully establish this system on machine requires a close working relationship with potential partners.  It is necessary to establish the most effective retention programme to ensure the maximum retention of the chosen starch particles. After optimisation of the retention programme and introduction of the starch it is essential to work with the machine crew to optimise:

  • wet end temperature and control
  • moisture profile and control
  • drying profile and control

Obtaining optimum strength results requires significant input from all mill departments. Strength improvements must be built on a strong foundation of the correct refining and sheet forming.

Continual improvements in performance are critical for the success of the paperboard industry.

This system has demonstrated the capability to add value to paperboard operations by:

  • Production increases
  • Maintaining strength properties at higher speeds
  • Replacing the size press
  • Cost reductions
  • substituting native starch for more expensive cationic starch
  • producing lower basis weight sheets which can match or outperform the strength of heavier sheets
  • Fibre substitution
  • replacing virgin fibre with recycled fibre while maintaining strength
  • using lower quality waste
  • allowing flexibility in fibre selection

This system provides a means to produce higher added value products for a low cost input .

REFERENCES

1.PAPTAC Standard Testing Methods

2.TAPPI Test Methods

3.Cellulose (London) 1994 ,1(2), 107 - 130

4.Technol. Paper Recycling. 1995, 180 - 203

5.Sobre Deriv. Cana Azucar 1998, Supl.2, 9 - 16

6.Walch, J. Boxboard Containers, 44 - 45 1993

7.US Patent No 5876563

8.Wagberg, L. Et al. TAPPI 79 (6): 158 1996

9.Retention of Fines and Filler during Papermaking, TAPPI Press (Atlanta) 1998 ,199 - 240

10.Handbook for Pulp and Paper Technologists, 210 - 211

11.Adsorption of Cationic Patato Starch on Cellulosic Fibres ,L Wagberg & M Bjorklund ,SCA Research ,Sundsvall.

12.Novel Dry Strength System for Paper and Paperboard, M. Szaymanski & B. Doiron 86th Annual Meeting PAPTAC.