SIZING OF PAPER AND LIKE MATERIALS

Abstract: A method of controlling the amount of an ionic sizing component to be added during sizing of a fibrous material using an ionic and cationic component comprises sampling a colloid containing liquid from the sized material and measuring the net residual charge on the colloidal product. The measured value is compared with a predetermined of net residual charge which is to be maintained. A difference between the measured value and predetermined value is compensated by adjusting the amount of one of the ionic sizing components being added. The method is particularly useful in the sizing of paper using ketene dimer as anionic sizing component and an organic polymer as cationic sizing component, in which case it is preferably the amount of added polymer which is controlled.

Description: SIZING OF PAPER AND LIKE MATERIALS The present inention relates to the control of sizing operations in the production of paper and like materials. The use of sizes in the production of paper and •• like materials is well established and a variety of sizes are used, e.g. rosin emulsions and ketene aimers. Generally, the paper making process is operated continuously and the sizing chemicals are added to the dispersion of fibres (i.e. the "furnish") *just before it is passed to the paper-making machine. Rosin emulsions and ketene di ers are anionic in nature and, in order that they may " be bonded to the anionic. cellulose fibres from which the paper is produced, require the addition of a cationic component. Thus, rosin emulsions are used in conjmction with alum whereas the ketene dimers are used in conjunction with cationic organic polymers. The amount of the anionic size (e.g. rosin emulsion or ketene dimer) required for sizing will be known and, in theory, the amount of the cationic component required to bond all of the anionic size to the;, paper fibres can be calculated. However the amount of the cationic component which must be added is influenced by a number of factors. In particular, the water in which the fibrous material 2. is suspended will, in all probability, have been recycled from the downstream end of the paper-making process and is likely to contain residual cationic size component. Additionally, the fibrous material to be sized may have been produced from "broke" which may itself contain residual cationic size component. Such residual cationic size component reduces the amount of fresh cationic size component which isi required for sizing. It is however necessary to add sufficient cationnic size component to ensure complete sizing whereas the addition of excess cationic sizing component represents a waste of chemicals and a reduction in sizing efficiency. Obviously- a.balance must be struck. In ketene dimer sizing, the ketene dimer and cationic sizing component used to be added individually s in the sizing operation. • A development in ketene dimer sizing came when it was found that pre-mixing of the ketene dimer and cationic sizing component produced a synergistic effect resulting in enhanced sizing.- As a result, "one-shot" sizing compositions comprised of a mixture of ketene dimer and cationic sizing agent are now supplied for use in sizing operations. A range of such "one-shot" sizing compositions are produced with different ratios of ketene dimer : cationic sizing component. The particular composition which will be used will be dependent on the amount of expected residual cationic components in the sizing operation. It is however likely that none of the "one-shot" sizing compositions contain the exact ratio of ketene dimer cationic sizing component which should be added during the sizing operation and the selection of a particular "one-shot" composition may, at best- be an approximation to the ideal required. Additionally, during the sizing operation, the amount of residual cationic sizing component in the fibres to be sized may change (e.g. to a change in the composition of "broke") and the selected "one-shot" composition may be far from ideal. It is an object of the present invention to obviate or mitigate the abovementioned disadvantages. According to the present invention there is provided a method of controlling the amount of an ionic component to be added during sizing of a fibrous material using anionic and cationic sizing components, the method comprising sampling a colloid containing liquid from the sized material, measuring the net residual charge on the colloidal, product, comparing the measured net residual charge with a predetermined value of net residual charge which is to be maintained, and if necessary adjusting the amount of the ionic component to return the net residual charge on the particles to said predetermined value. In the preferred method our invention, we add the anionic and cationic sizing components continuously to the furnish as it is supplied to the paper making machine and sample the water drained from the machine wire (i.e. the so called "white water") to measure the net residual charge on the colloidal particles therein in accordance with the method described more fully below. The method of the invention is applicable particularly , although by no means exclusively, to ketene dimer sizing. In this case, we prefer to introduce the cationic siing component (the amount of which is controlled in accordance with the method given in the subsequent description) into a stream of the ketene dimer being supplied to the furnish. In this way we obtain the benefits of pre-mixing the ketene dimer and cationic sizing component whilst, at the same time, being able to control the amount of • cationic sizing component used. Measurement of net residual charge on colliodal particles is a well established technique and so called Streaming Current Detectors are commercially available for this purpose. A Streaming Current Detector generally comprises a cylindrical cell (usually of plastics material) incorporating an electrode at each end thereof and also having a close fitting piston-. The underlying theory for the measurement of net residual charge is somewhat complicated and a full discussion is beyond the scope of this description. Briefly however colloidal particles may be considered to comprise a central core and the outer electric double layer whereof the inner is strongly attracted to the central core and the outer layer (the so called counter ion layer) includes ionic and anionic species in loose confederation. To measure net residual charge, a sample of the colloid containing liquid is introduced into the cell-..of the Streaming Current Dectector where the colloidal particles are adsorbed on the cell wall and the piston, the surfaces of which quickly take on the charge characteristics of the colloidal particles. Reciprocation of the piston causes shear of the mobile ions from the counterion layer from the adsorbed particles. The physical movement of these charged ions produces ' the electrokinetic phenomena known as streaming current which is detected as a potential difference by the electrodes. This potential difference is a function of the sheared counterions net charge (i.e. net residual charge). The detected potential difference is very small and is generally amplified to give a signal representative of the net residual charge. A zero value of the signal means no net residual charge on the colloidal particles whereas a signal to one side of zero is a quantitative measurement of positive net residual charge and a signal to the other side of zero is a quantitative measurement of negative net residual charge. Within the context of the present invention, a finding of negative net residual charge on colloidal 7. particles in a liquid from the sized material is a quantitative indication of the excess amount of anionic sizing component which has not been retained by the material. Conversely a finding of positive net residual charge is a quantitative indication of an excess of cationic sizing component used in the sizing operation. It is therefore possible to determine an optimum value of the net residual charge for the sizing operation. Rapid continual sampling of a liquid from the sized product allows the detection of any variation in net residual charge from the optimal value which may be compensated by adjustment of the amount of cationic sizing component added in the sizing operation. Adjustment of the amount of cationic component added may be effected automatically in response to the signal from the Streaming Current Deflection. In this respect it should be noted that the commercially available Streaming Current Detectors referred to above .are. capable of effecting approx. 100 sampling operations per minute with ultrasonic cleaning of the cell between each such operation. Thus virtually continuous control of the amount of cationic component added in the sizing operation is possible. In practice we have found that the sizing operation is optimised when the colloidal particles in the liquid from the sized product have a small 8. negative net residual charge (i.e. some anionic sizing component is not retained by the material). This is because there is apparently no improvement in sizing efficiency by increasing the amount of cationic sizing component to reduce the net residual charge to zero. Further increase in the added amount of cationic component would simply give a positive value of net residual charge (i.e. excess cationic component) which would simply be wasteful of chemicals, and would reduce sizing efficiency. It will be clear that variations in the residual cationic sizing component introduced into the sizing operation due to the use of recycled water or "broke" of changeable composition are automatically compensated for by the method of the invention. The amount of cationic sizing component added at any one time is therefore optimum for the conditions prevailing at that time. In the case of rosin sizing, the cationic sizing component can be organic or inorganic polymers but more usually is aluminium sulphate (commonly known as paper- makers alum). Control of the alum addition is usually effected by monitoring the pH of the system and operating within a predetermined pH range. A more effective method (applicable to the use of polymers as cationic components) would be to control the net charge by means of the streaming current detector. This would regulate the alum or polymer addition to maintain a predetermined net charge in the system thereby ensuring optimum conditions for the sizing at all times. The foregoing description has been directed particularly to the control of the amount of cationic sizing component used. However, it will be appreciated that the method of the invention may alternatively be used to regulate the amount of the anionic sizing component added. The method of the invention may therefore be used to maintain a predetermined level of cationic charge in the sizing operation by monitoring the 'excess' cationic charge in the system and adjusting the amount of an anionic material to be added to the system to ensure that a predetermined level of cationic charge is maintained. In this way, a constant level of cationic charge may be maintained, irrespective of the amount of residual • cationic material in the fibres and the. amount of cationic polymer introduced with the ketene dimer. A variation of the invention is applicable particularly, but not exclusively, to sizing using a ketene dimer and a cationic organic polymer as sizing components. In this variation an anionic polymer addition may be used to control the net residual charge to the predetermined value. The ketene dimer and cationic organic polymer may be pre-mixed and added to the furnish or added separately thereto. In the case of pre- ixing, the ketene dimer and cationic organic polymer may either be added as a commercially available 'one-shot' composition or by mixing separate streams of the two components immediately before they are introduced into the furnish. In all cases, a Streaming Current Detector is used to measure the net residual charge and any variation from the predetermined value is compensated by varying the amount of anionic polymer added.

Claims: CLAIMS : 1. A method of controlling the amount of an ionic component to be added during sizing of a fibrous material using anionic and cationic sizing components, the method comprising sampling a colloid containing liquid from the sized material, measuring the net residual charge on the colloidal product, comparing the measured net residual charge with a predetermined value of net residual charge which is to be maintained, and if necessary adjusting the amount of the ionic component to return the net residual charge on the particles to said predetermined value. 2. A method as claimed in claim .1 wherein the predetermined value of negative net residual charge is a negative value. 3. A method as claimed in claim 1 or 2 wherein it is the amount of cationic sizing component which is adjusted, if necessary. 4. A method as claimed in any one of claims 1. to 3 wherein the anionic sizing component is a ketene dimer and the cationic sizing component is an organic polymer. 5. A method as claimed in claim 4 wherein the ketene dimer and the cationic organic polymer are mixed together before addition to the fibrous material. 6. ' A method as claimed in any one of claims 1 to 5 wherein the fibrous material comprises cellulose fibres. 7. A method of controlling the amount of an ionic sizing component to be added during sizing of a fibrous material using ionic and cationic sizing components substantially as hereinbefore described.

Inventors: BLANCHARD MICHAEL JOHN, TORMEY BRIAN JOHN, BLANCHARD, MICHAEL, JOHN, TORMEY, BRIAN, JOHN
Applicants: ROE LEE PAPER CHEM CO LTD
Application year: 1985
Main classification: D21D3
IPC codes: D21H17, D21H23