Paper making and fiber liberation – Processes and products – Non-fiber additive
Reexamination Certificate
1999-05-10
2001-05-29
Chin, Peter (Department: 1731)
Paper making and fiber liberation
Processes and products
Non-fiber additive
C162S168100, C162S168200, C162S168300, C162S175000, C162S181100, C162S181600, C162S181800, C162S183000
Reexamination Certificate
active
06238520
ABSTRACT:
This invention relates to the production of paper which is strengthened by starch.
It is well known to make paper on a paper-making machine by providing a cellulosic thinstock suspension, flocculating the suspension by adding a solution of polymeric retention aid and thereby forming a flocculated suspension, draining the flocculated suspension through a moving screen to form a wet sheet, and carrying the sheet through a heated drying zone and thereby forming a dry sheet. The retention aid can be dissolved cationic starch but is often a synthetic polymeric material. It is preferably of high or very high molecular weight, generally having intrinsic viscosity above 4 dl/g.
An alternative to this process involves additionally shearing the flocculated suspension so as to degrade the flocs and then adding an aqueous suspension of microparticulate anionic material, thereby reflocculating the suspension, and then draining the reflocculated suspension through the screen.
It is often desired to add starch during the process, often to the cellulosic thinstock, to improve the strength of the paper produced. This can for instance be desirable in fluting medium and liner board, which are usually substantially unfilled. Increasing their strength makes them more suitable for use as packaging materials. It is also often desired to include starch in filled sheets as the inclusion of significant amounts of filler would otherwise tend to reduce the strength of the sheet.
In order to maximise strength, it is desirable to include starch in amounts which can be as much as 5 or 10% or even higher.
Soluble cationic starch is reasonably substantive to the cellulosic fibres in amounts up to about 1 to 1.5% by weight of starch, based on dry weight of paper, and it is known to include this in the cellulosic suspension. If the amount of dissolved cationic starch in the suspension is increased significantly above this, there may be little or no increase in the amount of starch which is retained in the paper. Instead, there is merely an increase in the amount of dissolved cationic starch which is in the white water which drains through the screen. This is undesirable since it will build up to high concentrations when the whitewater is recycled and it must be removed before discharge as effluent, otherwise it can create a high chemical oxygen demand in the effluent from the mill.
It is also known to attempt to include insoluble starch in the paper. When it is desired to include a greater amount of starch than 1 to 1.5%, the usual technique involves applying an unmodified starch solution on a size press at the end of the paper making machine, i.e. after partial or complete drying of the sheet. The application of a solution of starch at this point can result in higher pick-up (for instance up to 7 or 10%). However it can result in the starch being concentrated more on the surface than in the centre of the sheet and it has the particular disadvantage that it necessitates redrying of the sheet, thus wasting heat energy and/or slowing down the process.
Another known method for providing significant loadings of starch in the paper involves applying a spray or a foam containing undissolved starch particles onto the wet sheet before it is carried through the dryers, followed by cooking the starch during drying. This process also has the disadvantage of tending to produce a higher concentration of starch on the surface than in the centre of the sheet. Its particular disadvantage is that it is very difficult to achieve uniform application of the starch by spraying or foam application for prolonged periods because of the tendency of the starch composition to cause blockages in the spray or foam applicators.
Attempts have been described to include cold water insoluble particulate starch in the suspension before drainage. Fowler describes some of these in Paper 23, January 1978 (vol. 189 No. 2 1978), pages 74 and 93. One particular method he suggests is inclusion of raw uncooked starch in a slurry with bentonite. This is then added to the stock prior to retention aid addition. We were involved with the work on his suggested system, and in practice addition of the slurry was made to the thickstock. Fowler suggests that the bentonite absorbs the starch and, after addition to the stock and inclusion in the stock of the retention aid, the bentonite is flocculated by the retention aid. Fowler also suggests that retention can be increased by including in the slurry of starch and bentonite a highly charged polymer of opposite charge to that of the retention aid. He suggests that the flocculation of the bentonite which occurs on addition to the stock of the retention aid is then greater. In practice, the polymers used were low molecular weight, highly charged anionic polymers, and this is why flocculation occurred only on addition of retention aid to the stock.
We have recently investigated these systems further. Results are set out below in this specification. We have confirmed that the slurry of starch and bentonite does, of course, remain unflocculated in the presence of the highly charged low molecular weight anionic polymer and that retention of starch in the system is not good.
It has also been suggested in GB 2,223,038 that reduction in strength due to the late addition of filler to the cellulosic suspension can be reduced by adding the filler as a slurry of filler, insoluble starch particles and flocculating agent. A suspending agent such as a gum, a synthetic organic polymer or a swelling clay, e.g. bentonite, can also be included in the slurry, preferably so as to reduce the net charge in the composition close to zero. Preferred systems use a non-ionic flocculating agent and non-ionic suspending agent.
Such compositions include large amounts of filler, preferably 30 to 40%. Amounts of starch and flocculant, based on filler, are preferably 1 to 5% and 0.05 to 0.2% respectively. The amount of starch in the final paper is said typically to be 0.05 to 1.5%.
This system is aimed at the problem of including a large amount of filler in a paper. It appears that the starch is included in the slurry solely to improve strength and as a binder for the filler. The system has the particular disadvantage that it necessitates the addition of large amounts of filler immediately before the headbox, which can tend to weaken the paper.
It would be desirable to be able to provide a method of making paper having increased levels of starch whilst maintaining good starch retention during the process so as to avoid as far as possible problems of starch in the effluent. We have, in our International Publication WO95/33096, described a method of increasing the levels of undissolved starch which can be incorporated into the thinstock. In that system we incorporate undissolved starch particles into part or all of the aqueous solution of retention aid. In systems in which shearing and degradation of flocs followed by addition of microparticulate material is used we also describe including insoluble starch particles in part or all of the aqueous suspension of microparticulate material. When the starch particles are included with the microparticulate material this is always done at the point at which the microparticulate material would be added in the absence of starch and no further components are suggested for inclusion in the slurry of starch and microparticulate material.
When the starch is incorporated into the stock as a component of the solution of retention aid or suspension of microparticulate material, it is present in substantially freely dispersed, i.e. unflocculated, form.
Although this system does give useful improvements in the levels of starch which can be incorporated into the paper without major increases in starch level in the effluent, we have found that there is still room for improvement, in that starch retention could be made more efficient. In particular, retention of potato starch and, especially, wheat starch and corn starch can be difficult in some systems.
It would be desirable to find a way of further increasing the level
Chin Peter
Ciba Specialty Chemicals Water Treatments Limited
Crichton David R.
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