Sizing of paper

Details Sizing of paper Sizing of paper

1998-08-21

2001-09-04

Chin, Peter (Department: 1731)

Paper making and fiber liberation

Processes and products

Non-fiber additive

C162S179000, C162S181100, C162S181600, C162S181800, C106S266000, C106S287240, C106S486000, C106S491000

Reexamination Certificate

active

06284099

ABSTRACT:

This invention relates to sizing compositions which can be used for the internal sizing of paper or the external sizing of paper, and relates especially to processes for making sized paper using these compositions.
Internally sized paper is generally made by incorporating an aqueous emulsion of the size into a cellulosic thin stock suspension, draining the suspension through a screen to form a sheet and then drying the sheet. Externally sized paper is generally made by coating a cellulosic sheet with an aqueous emulsion of the size, and drying the sheet. Often the external sizing operation is integrated with the production of the paper, so that a typical process comprises providing a cellulosic thin stock suspension, draining the thin stock suspension through a screen to form a sheet, drying the sheet, coating the dried sheet with the dispersion of size and then redrying the sheet.
Although non reactive sizes have traditionally been used, there are many instances where it is preferred to use a reactive size as part or all of the total size which is in or on the paper.
Since the reactive sizes are insoluble in water they have to be predispersed before use, i.e. before incorporation into the thin stock or before coating on to the sheet. The resultant dispersion (often referred to more accurately as an emulsion) has to be sufficiently stable that it does break before use. The formation of a stable emulsion of the size in water is normally achieved by emulsifying the size in the presence of emulsifying surfactant and/or cationic polyelectrolyte such as cationic starch. The use of cationic polyelectrolyte, and/or cationic emulsifying surfactant, has been considered advantageous as it is thought to promote the substantivity of the size on to the cellulosic fibres, especially when it is used for internal sizing. WO 97/31152 PCTIGB97/00512
When emulsifying surfactant is being used as the sole emulsifier, i.e., without cationic polyelectrolyte, it is normally necessary to use quite large amounts of the emulsifier in order to form a stable emulsion, typically up to 7 or 8% dry weight based on the weight of size. If cationic polyelectrolyte is included then lower amounts of the emulsifier may be sufficient, for instance down to 2%.
Even when the amount of emulsifying surfactant which is included to facilitate the formation of the emulsion is low, even this amount tends to detract from the sizing performance and so there have been many proposals to try to reduce the amount of emulsifying surfactant in the size. However if the amount is reduced too far, generally the resulting dispersion or emulsion is so unstable that adequate results are not obtained. Accordingly, despite efforts to the contrary, it is always necessary in conventional processes to use significant amounts of surfactant to promote the formation of a stable dispersion or emulsion.
It would be desirable to be able to produce sizing compositions which are adequately stable for use and which do not have the disadvantage of necessarily including significant amounts of emulsifying surfactant.
Sizing emulsions are usually made by homogenising the size into water, possibly using prolonged homogenisation. When the size is solid at room temperature (20° C.) it is common to conduct the homogenisation at an elevated temperature at which the size is molten. Because anhydride sizes tend to be unstable, it is generally necessary for the homogenisation and emulsification of anhydride sizes to be conducted at the mill. It would be desirable to be able to simplify the production of the sizing composition, and in particular to be able to reduce the amount of homogenisation that is required when the anhydride or other size is being emulsified for use at the mill.
Since anhydride sizes are, unfortunately, liable to undergo hydrolysis in water, the act of pre-emulsification and the handling the emulsion prior to use can result in some hydrolysis and formation of stickies originating from the size.
When the use of the emulsion involves introduction of the dispersion in the thin stock, the risk of stickies formation is undesirable because of the risk of contaminating the screen and because of the risk of contaminating the other components of the apparatus for handling the cellulosic suspension.
When the size emulsion is applied as an external size during the manufacture of the sheet, for instance at a size press, it is conventional to apply it warm (for instance above 40° C.) and to recycle excess emulsion. Thus the dispersed size is exposed to warm hydrolysis conditions for prolonged periods and stickies formation and other undesirable hydrolysis effects are particularly likely to occur. It is probably for this reason that anhydride sizes are normally considered unsuitable for application at the size press.
It would therefore be desirable to be able to put the anhydride or other size into a more stable form where there is less tendency for stickies formation to occur during the preparation and use of the emulsified size.
There is always a desire to improve the sizing performance which is obtained by internal or external sizing compositions. In some instances it is desirable to achieve this improvement in a general respect, for instance by obtaining an improved (i.e., lower) Cobb value. In other instances it is desirable to achieve improved sizing performance with respect to some particular usage. For instance externally sized paper may be used for ink jet printing wherein the black colour is a composite black which is generated by the ink jet printing, and it is then desirable to have a maximum optical density for this composite black. It would be desirable to be able to improve the sizing performance.
The manufacture of internally or externally sized paper necessarily involves a significant number of process steps and chemical additions, and it would be desirable to be able to combine two of these additions into a single addition which gives approximately equivalent performance or, preferably, better performance than is obtained when the additions are made separately.
Sizing compositions are usually cationic since it is conventional to assume that a cationic sizing composition will be more substantive to the paper substrate, especially when used as an internal size. Accordingly it is conventional to include cationic polyelectrolyte in the internal or other sizing composition. However, the use of anionic and non-ionic emulsifying surfactants, to give anionic or non-ionic dispersions or emulsions of the size, is known.
In EP-A-499,448 we describe a process utilising a micro-particulate retention system wherein reactive size is added in the form of a non-ionic or anionic emulsion to the cellulosic suspension after flocculating the suspension by the addition of cationic retention aid. One preferred way of performing this process is by providing an emulsion of anhydride size or other size by emulsifying the size using anionic and/or non-ionic emulsifying surfactants and injecting this emulsion into the dispersion of bentonite or other microparticulate anionic material as that dispersion flows towards the point at which it is added to the cellulosic suspension. This process necessitates pre-emulsification of the size. It also suffers from the problem that emulsifying surfactant is necessarily introduced (with the consequential potential reduction in sizing performance) and there is opportunity for potential hydrolysis of the anhydride size with formation of sticky deposits.
Another disclosure of using an anionic dispersion of reactive size is in WO96/17127 (published after the priority date of this application). The anionic sizing dispersion is made by emulsifying a reactive size (preferably a ketene dimer size) in water to form a dispersion, and mixing this dispersion with a sol of colloidal anionic aluminium-modified silica particles. This technique therefore involves the conventional pre-emulsification of the size into water, followed by blending the emulsified size with the aluminium-modified silica sol. Apparently the unmodified silica sol does

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