The use of alkenyl succinic anhydride for sizing recycled fibres
In recent years, the use of alkenyl succinic anhydride for the sizing of paper and board has become common throughout the globe. This paper looks at the current state of this technology, comparing operations and results with alternative sizing systems. The use of ASA in recycled furnishes is discussed, with special attention paid to those components of waste paper that can adversely affect sizing performance. Examples from mills are discussed, with reference to the impact of recycling on the performance of the size.
The impact of recycled fibre on the sizing system in a mill is a critical issue for many papermakers. The board mills in Europe first met this in the 1970's and 80's, when a number of factors began to have a strong impact:
1) The printing papers that appeared in the waste started to contain large amounts of calcium carbonate filler or coating. The level of carbonate in the waste would vary from delivery to delivery, causing consequent fluctuations in the pH of the machine system.
2) There was an increase in the number of times a particular fibre would go around the waste recycling loop – sometimes being reused five to seven times before it would eventually be incinerated or go to landfill.
3) There was a growing interest in closing up mills and reducing the amount of water discharged. This increased the conductivity in the system, as well as increasing the temperature of the stock.
All these factors began to have an influence on the traditionally used rosin/alum sizing systems, so suppliers of these products began to look at alternatives. Although there were a broad variety of obscure systems available, the main alternatives were:
a) Variations on the rosin system, that would allow this product to be used at neutral pH.
b) Alkyl ketene dimers (AKD), which were already in common use in fine paper systems.
c) Alkenyl succinic anhydride (ASA)
This paper will look at the final product in this list – ASA – looking at the present state of the technology, the use of ASA with recycled fibres, and factors that may impact sizing in secondary fibre furnishes.
2. Current technology
Alkenyl succinic anhydride is produced from the reaction of an olefin with maleic anhydride[i]. The maleic anhydride molecule supplies the reactive anhydride functionality to the ASA, while the long chain alkyl portion provides the hydrophobic properties associated with this size. The resulting succinic anhydride group is extremely reactive, and will complex with hydroxyl groups on cellulose, starch and water. It is the ASA molecule's high reactivity that provides some of its major advantages.
The reactivity of the ASA means that the product will readily cure on the paper machine without excessive drying or the use of promoters. This means that most of the cure is achieved before the size press, so allowing the machine to be run at similar moisture contents to those experienced under acid conditions. This can give greater control of starch pick-up at the size press, full sizing at the reel and improved productivity.
Another factor is the tendency of the ASA molecule to react with water. This forms a di-acid, which is hydrophilic at one end of the molecule and hydrophobic at the other end. The di-acid has the ability to react with metal ions such as calcium or magnesium that are often found in water systems. The products of these reactions are sticky precipitates, which could potentially deposit on the fabrics and frame of the paper machine, although Wasser and Brinen[ii] have shown that the calcium salt can contribute to sizing. The aluminium salt is much less tacky however, and the presence of an aluminium source in the system is consequentially of great benefit. This ability to react with metal ions has been exploited in some mills, notably in Japan, where a potassium salt of a low molecular weight ASA is made and then precipitated onto the fibre using alum at acid pH in much the same way as rosin is used.
Much of the development work with ASA sizes in the 1980's was targeted at ensuring that ASA was easy to use in practice in the mills. In depth studies[iii] [iv] [v] of the reactions that take place in the stock and white water systems of mills, have led to the development of programmes that are simple to use and which ensure that ASA hydrolysis is kept to an absolute minimum. This ensures high sizing efficiency, low costs and good machine runnability.
ASA is supplied to paper and board mills as a fluid oil, which on its own has no charge and no great affinity for the anionic cellulose fibres. To ensure good retention, it is emulsified in a cationic carrier such as starch, and then added to the thin stock system of the machine. As is shown by its use in many processed foods, starch is an excellent emulsion stabiliser, but the correct choice of starch is critical to achieving this. Starch, as an emulsifying agent for ASA is no exception, although here there is also the need for it to act as a good retention aid for the size, as well as perhaps offering strength or other properties to the paper maker. For this reason, many mills view the starch as a key component of the sizing system.
Another critical part of the system is the emulsifying unit itself. These have been developed over the years into highly reliable, sophisticated units that can be linked into the mill computer system or stand-alone as requested by the customer.
These units meter and emulsify the ASA and feed it directly to the paper machine, ensuring a consistent quality, and extremely fresh emulsion. During machine shuts, the unit will automatically flush itself with water and then shut down, awaiting start-up again. Today's units are extremely reliable, requiring minimal attention and maintenance.
3. The use of ASA in recycled fibre
From its very earliest use, the benefits of ASA in recycled fibre were apparent. Some of the initial applications were in board grades made from 100%-recycled fibre, where the strength advantages of the neutral and alkaline pH were especially attractive. Since then, the use of secondary fibre in virtually all grades of paper has become common, and as a consequence ASA has been successfully used to size this fibre source in a wide variety of papers and boards.
ASA was being applied in a small number of mills in the 1970's, but it was really only in the 1980's that it came into widespread use. Many of the earliest applications were in 100% secondary fibre board furnishes, particularly gypsum board mills where the converting properties of the board were enhanced by the use of this size. One of the main issues with the rosin/alum systems that were in common use at those times was the growing levels of CaCO3 in the waste. Mills would experience sudden swings in pH caused by varying amounts of this alkaline filler. This in turn affected the performance of the rosin alum system and could create carbonate deposits and large amounts of foam. An obvious solution to this was to increase the pH of the papermaking system to a neutral or alkaline environment.
Then, like now, there were three main choices available to the papermaker in converting to neutral or alkaline sizing – pseudo-neutral rosin, alkyl ketene dimer (AKD) or ASA. All these products have advantages and disadvantages (see Table 3.1). The main attraction of the rosin system is that it is a relatively small change for the mill. Production personnel are familiar with rosin systems, and although there may be a change from alum to polyaluminium chloride, this is a relatively simple operation. For this reason, many mills are successfully using this technology today.
AKD is also a relatively simple system to use, in that it comes as a ready to use emulsion, and is then just added to the stock system at the appropriate level. ASA requires equipment for emulsification, and this can appear even more complex in multi-ply board applications. On the other hand, when compared with most modern paper machines, it is a relatively simple component in the overall operation.
In 1985, some work was carried out[vi] to look at the effect of anionic contaminants on the effectiveness of ASA sizing . This involved adding synthetic anionic latex, which reproduced the so called "anionic trash" that occur in many waste furnishes. It became apparent from this work that ASA was much less sensitive to this material than the AKD emulsions that were in common use at that time.
Since the mid 1980's, the growth in the use of ASA sizes has been dramatic throughout the globe. This size is now being run on machines varying in size from 2 to 80 tph, on all five continents, in a range of papers and boards, and in all types of furnish. This has included such grades as commodity office papers, specialities, newsprint, wallpaper base, envelope and a variety of board grades to name just a few. This has led to a growing interest in the effect of secondary fibre on sizing performance.
4. Issues in sizing recycled fibre
There are a number of issues with the use of recycled fibre, and although these will change from mill to mill, from grade to grade, and even from batch to batch, many generalities can be given. Typical issues that need to be addressed include:
· Variations in fibre type
· Variations in previous pulping, bleaching and papermaking techniques
· Different previous use characteristics
· Contamination from inks, adhesives, laquers and varnishes, coatings, binders, packaging, etc.
· Multiple recycling of fibre
· Contamination from recycling processes (e.g. residual de-inking or bleaching chemicals)
· Multiple filler types and levels
· Variations in pH
· Soluble contaminants (either organic or inorganic)
· Level of closure in the papermaking system (and its effect on conductivity, pH, temperature, etc)
The impact of secondary fibre on sizing may be related to one of two major causes:-
· The fibre has been recycled and consequently has had more physical treatment than a fresh fibre, leading to a higher surface area and a higher level of fines.
· There is often a greater level of general contamination in the stock. This can take many forms, including variations in fibre type (especially the presence of groundwood); the carry over of de-inking, bleaching or other recycling chemicals; and the presence of adhesives, inks, coatings, or other additives in the waste paper.
4.1 The effect of fines
Ramamurthy, Vanerek and van de Ven[vii] demonstrated the effect of fines from different furnishes on sizing. They found that it is the surface area of the fines that impact sizing rather than the mass of fines. While softwood fines produced during pulping and bleaching affect sizing efficiency significantly more than hardwood fines, they also discovered that hardwood fines generated during refining would have a similar surface area to softwood fines and subsequently a similar effect on sizing performance.
In a secondary fibre furnish, the level of fines is known to be considerably higher than in a virgin pulp system. The extra physical treatment that the fibres and fines will have experienced will increase the surface area in line with Ramamurthy's work. This in turn increases the sizing demand in that paper or board. The type of fibre, previous treatments of this fibre and the number of times it has been previously recycled will impact the extent of this effect.
Marton[viii] previously found that sizing is more effective when attached to the fibre rather than the fines, so one of our objectives in optimising sizing in any system should be to maximise the quantity of size on the fibres. Since the higher surface area of the fines makes their apparent anionic charge greater than that of the fibre, the tendency for any cationic additive to attach to these particles is obvious. This means that we have to seek an approach that reduces this attraction, to optimise size performance. The use of highly cationic polymers as scavengers or charge control agents has been very effective in addressing this issue.
Another key property to examine is the retention on the paper or board machine. As the quantity of fines and filler in the system increases, we would expect the first pass and fines retention to drop. This will have an immediate impact on the usage of all chemicals, and ASA sizing is no exception. A good quality retention programme is always important, but becomes even more so when secondary fibres are in use. This will not only reduce size usage, but may also lead to a cleaner wet-end and improved productivity.
4.2 Extractives in the stock
Table 4.2.1 below shows some examples from mills that are using secondary fibre in principally fine paper grades. The secondary fibre in these cases is generally mixed office waste that has been de-inked, although in one case old newspapers and magazines were used.
This indicates that in these cases the recycled fibre itself is unlikely to be the major factor in those mills that experienced increased size usage, but rather the materials that accompany the fibre, such as de-sizing extractives. Materials of a similar nature have also been found in virgin pulp[ix], especially those that are bleached with hydrogen peroxide. Data from a number of mills has been examined to look at this in more detail.
Pulp from two de-inking plants were studied (see Table 4.2.2), one of which had a dramatic effect on sizing while the other had little or no impact (see Figure 4.2.1). Analysis of the two pulps showed that the pulp with the greatest impact on sizing had a much higher level of extractives. Handsheets were made from the pulp and then an acetone chromatography test was run.
Figure 4.2.1: Effect of fibre source on sizing
This involved placing the bottom of the handsheet in a dish of acetone and allowing the solvent to climb up the sheet. Sizing was then tested on the extracted portion, the non-extracted portion and the interface. As can be seen (Table 4.4 .3), the extractives were impacting considerably on the sizing results, and when these were concentrated at the interface, sizing was affected even more.
A recent case from a board mill in Australia illustrates this in practice. The mill had about 600 tonnes of white top liner returned due to very poor sizing, although at the time the product was shipped the board was within specification for sizing.
Extensive studies were carried out on this board, including solvent extractions and analysis using Time of Flight SIMS . This showed that there was a very high level of oleic acid in the de-inked stock that was used in an inner ply. This had slowly migrated to the surface of the board and had then affected the ASA sizing. Analysis of this stock showed 0.5 – 0.8% extractives in the de-inked pulp.
The problem was solved in two ways. The lab tests demonstrated that this problem only occurred when there was soft sizing in the outer plies. With slightly harder sizing, the issue never occurred. An aluminium salt was also added to the stock, and following this, there has never been a reoccurrence of the problem.
4.3 Other factors
As the conductivity of a water system increases, it is well known that the performance of functional chemicals (e.g. size, strength resins, etc) in the system deteriorates. In many cases, this is due to the impact of inorganic ions on the cationic charge of the chemical in question. Higher chemical usage, along with the high level of fines can lead to foaming issues and a consequential increase in defoamer or antifoam usage. Certain types of antifoams are well known as desizing agents.
Biological activity in the water system may dramatically increase as pH, temperature and the quantity of nutrients increases. This is often combined with a growing level of filler and similar materials that can act as a seed on which bacteria can grow. This may lead to a higher usage of biocides, certain types of which have been shown to impact sizing.[x]
ASA is used very successfully in secondary fibre furnishes, and offers cost savings to the papermaker. The use of recycled fibre does not automatically lead to an increase in ASA size usage, although two main factors can cause such an increase:
· The quantity and surface area of the fines in the stock. This is often a result of the number of times the fibre has been recycled previously.
· The type and level of extractable contamination in the stock.
Other additives, especially defoamers, dispersants, biocides and surfactants should be screened for their impact on sizing efficiency. This is especially important as the usage of these types of chemicals often increase in secondary fibre furnishes.
Other factors may also affect the size usage, including, for example, the level of insoluble "anionic trash", the degree of water closure and first pass retentions on the paper or board machine.
*Stan Jenkins is Business Development Manager with Cytec Industries' Paper Chemicals Department. He is based in the United Kingdom.
[i] Proverb, R.J. and Dauplaise, D.L., "Chemical Optimisation of the ASA Sizing System," Proceedings of the 1989 TAPPI Papermakers Conference.
[ii] Wasser, R.B. and Brinen, J.S., "Effect of Hydrolyzed ASA on sizing in calcium carbonate filled paper", TAPPI: 81 (7), 1998.
[iii] Scalfarotto, R.E., "Remedies for press picking boost efficiency of ASA synthetic sizing," Pulp & Paper, April 1985.
[iv] Farley, C.E., "Optimizing and Troubleshooting an ASA Sizing System", TAPPI Seminar Notes, Alkaline Papermaking 1985.
[v] Wasser, R.B., "The Reactivity of Alkenyl Succinic Anydride", TAPPI Seminar Notes, Alkaline Papermaking 1985.
[vi] Jenkins, S.N., "The benefits of sizing with alkenyl succinic anhydride", Paper Technology & Industry, Vol.27 No.5 1986.
[vii] Ramamurthy, P., Vanerek, A. and van de Ven, T., "Efficiency of AKD Sizing in Mixed Hardwood-Softwood Furnishes", Journal of Pulp and Paper Science: 26 (2), 2000.
[viii] Marton, J., "Sizing Mechanisms and the Effect of Filler", Papermaking Raw Materials, Trans. 8th Fund.Res. Symp., Oxford, 2:785 (1985).
[ix] Brinen, J.S.; Kulick, R.J., "Detection of ASA and desizing agents in hard to size paper surfaces by SIMS", International paper and coating chemistry symposium, Ottawa, Canada, 1996 (CPPA).
[x] Raymond, L.; Turcotte, R.; Gratton, R., "The influence of organic biocides on the surface and internal sizing of fine papers", PIRA Conference Proceedings - Scientific and Technical advances in the internal and surface sizing of paper and board, Florence, Italy, 1999.