WO1999016972A1

WO1999016972A1 - Paper strength enhancement by silicate/starch treatment

Info

Publication number: WO1999016972A1
Application number: PCT/CA1998/000915
Authority: WO
Inventors: Gerard Le Fevre; Hanuman Prasad Didwania; R. Graham Hagens
Original assignee: National Silicates Ltd.
Priority date: 1997-09-26
Filing date: 1998-09-28
Publication date: 1999-04-08
Also published as: CA2216480A1; AU9249498A

Abstract

Alkaline dry silicate compositions are disclosed for use as additives in conventional paper or board making to impart significantly improved strength to paper or paper board. The particle size and moisture content of dry alkaline silicate compositions according to the present invention are selected such that these compositions are sparingly soluble in water at temperature conditions encountered in the wet end of a paper or board making machine, but are rapidly soluble under the higher temperature conditions encountered in the drier section of such a machine. In consequence, there results effective penetration of the web of paper or board by the alkaline silicate composition and any selected additives thereto. One particularly useful additive is starch, particularly starch in the form of uncooked starch particles, which imparts a synergistic strengthening effect. In another aspect, the present invention is an improvement to the conventional process for preparing paper or board, comprising the step of adding compositions as aforesaid in particulate form to the aqueous slurry of cellulosic fibres prior to formation of a web of paper or board.

Description

PAPER STRENGTH ENHANCEMENT BY SILICATE/STARCH TREATMENT

Background of the Invention

This invention relates to compositions for the improvement of strength of cast cellulosic mats. The production of paper, paperboard and other cellulosic fibre mats is usually accomplished via the formation of cast cellulosic mats on a moving table or cylinder. In such processes an aqueous slurry of cellulose fibres is spread onto the table, also known as the "wire" or rotating cylinder, and is transformed into a mat by removal of much of the water from the fibres by suction. In subsequent steps this mat is converted into paper or board by pressing and drying of the fibre mat.

A priority in the manufacture of much paper and paperboard is the strength of the product . While the long fibres present in virgin fibre derived from wood have adequate strength properties for most commercial applications, paper manufactured from fibres which have been previously used commonly exhibits lower strength due to damage caused to the fibres during the recycling process. The recent trends to employ increasingly large quantities of recycled fibres in paper products has led to a significant decline in the strength of many different types of paper products . Although the loss of strength of such papers can in some cases be countered by the utilization of larger quantities of virgin fibre, this is not always economically desirable, virgin fibres being very much more expensive than those derived from used paper such as old cardboard cartons, newspapers and the like. The strength properties of paper and paperboard of most concern traditionally have been burst, tear, tensile strength and compressive strength. An approach commonly employed is to improve the strength of papers by the incorporation of various types of chemicals. The literature contains many examples of such additives, and their properties and utilization is well covered in standard monographs on the chemistry of papermaking. A useful summary of some of the chemicals which are used for this purpose is to be found in "Chemical Additives for Improved Compression Strength of Unbleached Board," by Douglas B. Smith in the proceedings of the 1992 TAPPI Papermakers Conference 393-404. The use of sodium silicate either alone or in combination with starch for the enhancement of the strength of paper products has already been taught, and both of these have a long history as additives for the improvement of paper properties [See for example James G. Vail, Soluble

Silicates in Industry 1928, PP. 287-94. Also E.D.

Kaufman U.S. Patent No. 3,819,555 (1974); E. Strazdins U.S. Patent No. 3,840,489 (1974); J.A. Sedlak U.S. Patent No. 3,874,994 (1975); E. Strazdins U.S. Patent No. 4,002,588 (1977); H.S. KillamU.S. Patent No. 4,167,439 (1979); K.M. Seymour and D.G. Seymour U.S. Patent No. 5,358,554 (1994) ] .

Although it has long been known that incorporation of soluble alkali salts of silicic acid within the cellulosic fibres improves the strength properties of paper products, the actual method of incorporating this silicate into the sheet in a sufficient amount to be effective, has until now faced insurmountable barriers.

A description of the different options available for the incorporations of additives into cellulosic sheets will help in the explanation of this invention. For chemical additives to paper to be effective it is necessary that they be well dispersed throughout the fibres of the cellulosic mat. This can be carried out in one of three ways: addition in the "wet end," in a size press or after the "dry end" by the use of some kind of application technique. The addition of chemicals in the wet end of a paper machine (i.e., by introduction at the head box or into the white water) is a well known common practice which is relatively easy to do with undissolved chemicals (such as filler clays) , or with soluble additives which react chemically with the cellulosic fibres (such as internal sizing agents) , but the retention of these materials in the sheet becomes increasingly difficult if the chemicals being applied have low affinity for fibre. Such anionic additives require large quantities of retention aids which significantly add to the cost and chemical complexity of the chemistry in the paper machine.

The anionic nature of alkali silicates is such that they cannot be retained in paper without the use of excessive quantities of retention aids, the amount being so high as to make the process economically unattractive. In addition to this a second problem arises from the fact that these retention aids are only partially effective even when used in high quantities. As a result of this, not all the alkali silicate is retained, and significant quantities travel through the sheet into the "white water system" where their interaction with other chemicals negatively affects the chemical balance of the paper machine .

An alternate method for the incorporation of chemicals into paper involves a size press. With this piece of equipment, usually located near the middle of the drier section of a paper machine, liquid chemicals can be well dispersed and retained within the paper. This method is widely used for the addition of starch sizing agents to paper, and sodium silicate has also been added to paper at the size press . The main drawback with size presses involves their efficiency. Relatively low quantities of chemicals can be applied by this method, and if the addition of larger amounts (more than 3-4% by weight) the paper machine has to be slowed down which results in a loss of production output. The rate of absorption of the chemicals by the paper is not instantaneous. For these reasons incorporation of chemicals via the size press frequently leads to a reduction in machine speed and output which significantly reduces the cost advantages of using the chemicals.

The third method of incorporating chemicals into paper involves soaking, dipping, wiping or mechanically impregnating the paper after it has emerged from the driers . Although such techniques are amenable for use with liquids such as sodium silicate, they all suffer from the drawback that the production rate is limited by the need to slow the process for the adsorption of the chemicals, after which the treated paper has to be redried. This results in processing costs, and the need for space for the extra driers which may not always be available .

It is therefore evident that if the problem of retention of the alkali silicates could be solved, the preferred method of incorporating alkali silicates into paper would be at the wet end for the reasons given above. Conceptually such additions could be made in the head box, or on the moving wire of paper machine via some kind of applicator weir, die or spray bar.

In the course of investigating many possible solutions to this problem, it occurred to the present inventors that efficient incorporation of solid forms of silicate into paper at the wet end could be achieved if the solubility characteristics of the silicate were correctly designed. A brief description of the physical properties of alkali silicates to help clarify the description of the invention is in order. Alkali silicates are typically manufactured in a high temperature furnace in which sand and alkali source are fused together. Various types of product are possible depending on the alkali component, sodium and potassium being the most common, and the weight ratio of sand to alkali. The anhydrous product of this reaction, known as glass, finds commercial use either in the dry form, for which applications it is usually ground to a fine powder, or it is dissolved in water to produce an aqueous solution. To date all attempts to incorporate silicates into paper have involved various aqueous solutions of this product, those of sodium silicate being the most common for reasons of cost .

The complexity of the process has until now precluded the possibility of utilizing alkali silicates in the solid form for this application. In the course of considering this problem it became clear that the desired properties could be achieved if the solubility properties of the silicate could be designed in such a manner that the silicate could be added to the wet end of the paper machine in a solid form which could be easily retained by the sheet, and such that it would dissolve and disperse in the sheet before the paper or board leaves the paper machine .

It also became apparent that the desired silicate solubility behaviour might be achieved making use of the temperature differential between the wet end and the drier section of the paper machine. While the temperature of the white water is seldom more than 60 degrees centigrade, that within the driers exceeds 100 degrees. It was conjectured that the solution to the problem lay in finding a dry form of silicate which was essentially insoluble in the lower temperature conditions of the wet end, but became fully soluble, and dispersible, within the driers. In order to be successful, it would be necessary for the product to be sufficiently insoluble in lukewarm water in advance of the driers and be retained in the paper long enough for the paper to pass from the paper machine wire, through the press section until it entered the drying section. It would however have to be sufficiently soluble in hot water that the effect of steam in the drier would completely dissolve the particles. Both the solubility rate and the size of the individual particles should be such that both during the introduction and dissolution the silicate would be well distributed through the paper in order to realize high efficiency.

General Description of the Invention

The invention here described discloses the development of a class of alkali silicates with precisely defined solubility and particle size characteristics to achieve the desired silicate solubility properties. We have discovered that these results can be achieved by achieving the correct balance between solubility and particle size of sodium silicate in the solid form. The preferred products are a sub-class of a type of partially hydrated alkali silicate obtained by spray drying aqueous solutions of sodium silicate which are commercially available under the trade names Silicate G^®, GD^® and Britesil, all manufactured by the PQ Corporation. The present invention is not intended to be limited to this class of silicate, however, for the approach could also be extended to other dry forms of silicate, such as sodium metasilicate, both anhydrous and pentahydrate, and to the ground glasses of sodium and potassium silicate. The preferred solubility characteristics are found to be obtained by modification of the properties of a partially dried solution of a 3.2 silica to alkali ratio of sodium silicate, one form known as Silicate G^® .

From various known solid forms of sodium silicate, a limited number of species have the correct properties to obtain the desired properties for the process described. For the process to be efficient the particles must be of such a size that they are able to penetrate within, but not all the way through, the cellulosic mat. Thus a correct balance must be found between retention and dispersion: particles which are too large might be retained on the very surface of the mat, and would form a surface layer with adverse properties when the sheet passed through the driers. Particles which are too small, however, might pass through the sheet and not be retained. Particles of a size range between about 1 and 150 microns, preferably 1 to 100 microns have been found to have these properties. The particles must also retain their insolubility in cool or lukewarm water so that they can be retained within the sheet until the paper reaches the drier section.

Another important feature of the present invention arises from the well known fact that the solubility rate of particles is also a function of size. A solution to the problem of finding the correct balance between these properties was obtained when we determined that the rate of dissolution of dried silicate such as Silicate G^®, was also dependent upon the residual moisture content of the product .

As will be illustrated in the Examples below, the speed at which Silicate G^® has almost the ideal combination of properties required for this application, and that only minor modifications in the process used to manufactured Silicate G^® are required to make it perfectly acceptable for this application. Thus Silicate G^® containing moisture in the range of about 10 to 15% by weight has been found to be suitably insoluble in lukewarm water, and rapidly soluble in hot water to achieve the performance required. It will therefore be apparent to those skilled in the art that the properties of the dried silicate such as those described, can be manipulated by modification of both particle size and moisture content in order to achieve the desired properties for the conditions of the paper machine under consideration. It is also recognized that other factors present during the manufacture of dried silicates, such as the dwell time and temperature in the drier affect the properties of the product. Accordingly, this invention is not limited to the specific methods of manufacture of these products which are its currently preferred embodiments .

A third aspect of this invention stems from the discovery that dry forms of sodium silicate can be combined with starch in the applications described. The use of starch as an additive for the strength improvement of paper is well known and of widespread application. Modified, siloxane-containing starch derivatives have been described as strength-enhancing additives, in U.S. Patent No. 4,973,680 (Billmers) .

It was our surprise to discover that the use of starch in combination with the dried silicates here described led under some conditions to a synergistic improvement in the strength properties of the paper. Good results were obtained with unmodified starches derived from various plant products such as corn, potato, rice, etc., as well as starches which have been modified by various chemical processes, such as cationization as is a common application of starch in the pulp and paper.

The importance of this synergism will be understood when the complexity of the different types of strength tests which are carried out on paper is taken into account . Of the various methods used to evaluate the strength of paper, those which are increasingly considered desirable include taber stiffness, edge crush (also known as ECT) ; ring crush; short span compression strength (also known as STFI) ; corrugated medium test (also known as CMT) ; and corrugated flute crush (also known as CFC) . It is well known that starch and silicate do not improve the strength of paper in the same way. The incorporation of starch into a paper sheet is particularly good, for example, for burst and tensile strength, while silicate is most valuable for this improvement of the stiffness and ring crush of paper.

As will be illustrated in the Examples below, when starch and silicate are applied to the paper according to the technique here described, the increase in strength of the paper observed is unexpectedly in excess of that which might have been predicted if either of the components was used alone.

Detailed Description of the Invention

As noted above the invention concerns the improvement in the strength properties of cellulosic products such as paper or board by incorporation of sodium silicate in particulate form either alone, or in combination with certain other particular materials such as starch. In a process which is an embodiment the chemicals are applied to the surface of the moving wet web on a fourdrinier forming table, although this process is not limited to fourdrinier machines but can also be applied to other paper making processes such as cylinder machines. Addition of these chemicals can be accomplished by a number of application techniques well known in the art and include spray showers, overflow weirs, or die applicators, as described in U.S. Patent No. 5,792,317 (Taylor et al . )

It is an important feature of the process of the invention that all the chemicals employed be in particulate form, be sparingly soluble in cool or lukewarm water, but rapidly soluble under the high temperature conditions encountered in the drier section of the paper machine. The particulate nature of the additives causes them to be retained on the sheet without the use of expensive chemical retention aids, while their high solubility at elevated temperatures permits rapid distribution of the material throughout the sheet as the water is removed in the drier section of the machine, good distribution of product being well known to be of importance for such applications.

The precise location of the application point on the paper machine is also of importance in this process. Since the consistency of fibres on a paper machine increases from about 1% near the head box, to about 15% immediately prior to the press section after which the paper enters the driers . In order to maximize the retention and dispersion of the chemicals within the sheet, it is preferable to add the chemicals to the moving web of the paper machine at a position where the wet mat has a solids content between about 2 and 10% . Application of the products much before this point results in poor retention, while addition of chemicals to a mat of much higher than about 8% consistency results in accumulation of the chemicals on the surface. This is to be avoided because of various deleterious consequences such as the transfer of the chemicals to machinery during drying, as well as later processing.

The treatment process can also be applied on those paper machines which manufacture multiply paper in which the wet mats from two or more head boxes are brought together usually immediately prior to the press section, during the manufacture of multiply paper or laminates. The excellent retention of the particles between the plies which is achieved by application at the point of convergence of the mats as described in this process, eliminates concerns about transfer of the product to downstream processes such as drying and printing.

Example 1

Solubility rates of Silicate G^® of differing moisture content at various temperatures

In order to study the effect of moisture content of Silicate G^® on the rate of dissolution, samples of the commercial grade of the product (containing 19.3% moisture) were dried at 60°C in a laboratory oven for between 0.5 and 2 hours after which the moisture content of the material was determined by weight loss at 525°C (LOI) . The rate of dissolution of these various materials in water was then determined at three different temperatures: 15°, 40° and 60°C, after 20 and 60 minutes . The experiment was carried out by stirring the 10% by weight slurry of Silicate G^® in distilled water, and filtering and weighing the undissolved product.

Table 1 demonstrates very clearly the relationship between water content and dissolution rate.

Example 2

Use of sodium silicate to increase the strength of paper

This example illustrates the increase in the strength of paper which can be realized by incorporation of alkali silicate. The alkali silicate used here was a solution of the sodium salt of silicic acid in which the ratio of silica to soda was 3.2:1 (sold by the PQ corporation under the commercial name Silicate N^®) .

It was found necessary in these experiments to include certain retention aids, because of the anionic charge of sodium silicate which renders it otherwise difficult to retain the silicate on the fibres. A wide number of such retention and drainage aids are commercially available and known to those skilled in the art. Because of the highly anionic nature and dispersive properties of sodium silicate, those polymers which have some degree of cationicity are preferred. In the example below two such polymers: a coagulant Diaflocc 4439 (sold by Diachem) , and a flocculent Diafloc 7794, were employed.

In the experiments the silicate was mixed into a dispersion of unbleached kraft fibres at the concentrations shown, the consistency of the fibres being 1% by weight of water. After twenty seconds the coagulant was added at the level of 1% by weight of the fibres, followed by the flocculent at a level of 0.1% by weight of fibres . After another twenty seconds the handsheet was prepared for testing under standard conditions, the amount of silicate retained in the treated sheets being determined by analysis. The Mullen burst and Short Span Compression Strength (also known as STFI) strength of the treated samples and controls were determined at 50% humidity, 23°C. To correct for the variation in the individual sheet weights due to addition of the chemicals, the results shown in Table 2, have been normalized to 205 g/m². (Note that the most important independent variable is the % silica retained. The variation between this number and the amount of Silicate N^® added is a reflection only of the efficiency of the retention aids used in this experiment, and does not reflect on the strength enhancing efficiency of the silicate. )

*0.1 % Diaflocc 7794 added to the control **pH adjusted to 6.7

Example 3

Use of Silicate G^® to increase the strength of paper under laboratory conditions designed to mimic a paper machine

A fresh sample of linerboard pulp was dispersed in water to a consistency of 0.83%. Silicate G^® was added to 362 ml of this furnish in sufficient quantities to prepare handsheets weighing on average 3 gms, and containing 5, 10 and 20% Silicate G^® respectively. In order to simulate the conditions of a paper machine drier, the handsheets were covered with a metal template and subjected to a temperature of 93°C at a pressure of 20 psi for 11 minutes. The results are shown in Table 3.

Example 4

Various combinations of Silicate N^® and starch to increase the strength of paper

This series of experiments involved impregnation of commercial samples of linerboard and corrugating medium with Silicate N^® and starch. The paper was first cut into sheets 20x30 cm in size. The starch solution was then prepared by heating with stirring a 7.5% w/w dispersion of hydroxyethylated starch (Casco Inc.) in water for 10 minutes. The required amount of Silicate N^® was then stirred into this solution, after which the test sheets were immediately immersed into this mixture (still at 82°C) . After 1 minute the sheets were removed from the solution, blotted and dried in an oven at to 82°C for 3 minutes. The following mixtures of starch and Silicate N^® were prepared: 10:1, 10:2; 10:5; 10:10, all ratios being calculated on a dry solids basis. "Blank" represents the results obtained with untreated paper, while "control refers to sheets which were treated with water alone before testing. STFI was determined at 50% relative humidity, 23°C. The results, shown in Table 4, include the raw data and STFI values normalized to a basis weight of 125 g/m².

Example 5

Demonstration of the improvement in the strength of paper which can be realized as a result of various combinations of Silicate G^® and starch being retained on the moving web of a pilot paper machine

This experiment was carried out on a pilot paper machine, the paper being made from unbleached recycled stock. Four different ratios of starch to Silicate G^® were applied to the sheet halfway between the head box and the press section, and the total amount of additives retained on the sheet was 4% by dry weight . The starch was unmodified pearl starch (Casco Inc. 3005), while the Silicate G^® contained 19% moisture. Control samples were taken at the beginning and end of the run. Strength properties on the finished sheet determined both in the machine direction (MD) and cross direction (CD) under the conditions described above. The results presented in Table 5 show the percentage increase in strength properties over the control. As in the previous examples, the results have been normalized to the weight of the control (150 g/m²) in order to correct for the increase in the basis weight of the sheet caused by the additives. The results are summarized in Table 5.

Example 6

Use of Silicate G^® and starch to increase paper strength on an operating papermill

This trial was carried out at on the paper machine which is used to produce 500 tons/day of 100% recycled linerboard at a basis weight of 220 g/m² and a production speed of 490 m/min. Unmodified (pearl) corn starch (CPC 3005) and Silicate G^® containing 12% moisture, slurried with water to a combined concentration of 7%, were fed onto the papermachine by means of an overflow ("weir") applicator. The weir was positioned about 1/3 of the way between the headbox and the press section at which point the consistency of the sheet was about 4%. The weir was 1 meter in length, or about 1/5 of the width of the paper machine, a feature which allowed comparison of treated and untreated sections from the same reel of paper. The additive feed rate was adjusted to between 12 and 18 US gal/min in order to achieve the retention shown below. Starch alone was added at the start of the trial, after which a mixture of silicate and starch was applied. Although there was some variation over the course of the trial , the average amount of material retained on the sheet was about 3% w/w, and the average starch: silicate ratio was 4:1 (dry basis) . The results presented in Table 6 compare the STFI values obtained on the treated edge of the paper with that portion of the sheet which did not receive any chemicals.

"3.2 ratio sodium silicate retained, dry basis

Claims

CLAIMS :

1. A paper or board treatment composition for enhancing the strength of paper or board produced by a paper or board making machine by application of said composition to a forming web of pulp fibers on the wire of the paper making machine, comprising: a dry alkaline silicate composition in particulate form which is insoluble or very slow to dissolve in water at the operating temperatures near the wet end of said machine, and rapidly soluble at the higher temperatures encountered in the drier section of the paper machine, such that said composition becomes well dispersed through the dried formed web of paper or board made by the paper or board making machine .

2. A paper or board treatment composition according to claim 1, wherein said dry alkaline silicate composition is in the form of particles of a size in the range from 1 to 150, preferably 1 to 100 microns.

3. A paper or board treatment composition according to claim 1 or claim 2, wherein said alkaline silicate composition is a partially hydrated alkali silicate formed by spray drying an aqueous solution of sodium silicate.

4. A paper or board treatment composition according to claim 3, wherein said partially hydrated alkali silicate is selected from the group consisting of Silicate G, Silicate GD and Britesil (trademarks) .

5. A paper or board treatment composition according to claim 3, wherein said partially hydrated alkali silicate has a moisture content of from 5 to 18, preferably 5 to 13 percent by weight .

6. A paper or board composition according to claim 3 or claim 5, wherein said aqueous solution of sodium silicate is Silicate N (trademark) .

7. A paper or board treatment composition according to any of the previous claims, further comprising starch.

8. A paper or board treatment composition according to claim 7, wherein said starch is in the form of uncooked starch particles.

9. A paper or board treatment composition according to claim 8, wherein said starch is an unmodified natural starch selected from the group consisting of corn starch, potato starch and rice starch.

10. A paper or board treatment composition according to claim 8, wherein said starch is a starch which has been chemically modified by cationization.

11. In a process for the preparation of paper or board using a paper or board making machine of the kind in which a forming web of cellulosic fibers is moved on a wire section, with progressive removal of water by suction to form a cellulosic fibre mat followed by pressing and drying stages to convert said fibre mat into a web of paper or board, the improvement comprising adding to said aqueous slurry of cellulosic fibers a dry alkaline silicate composition in particulate form, said alkaline silicate composition being sparingly soluble in water at temperature conditions encountered in the wet end of the paper or board making machine, but rapidly soluble under the higher temperature conditions encountered in the drier section of said machine, so that said composition is well dispersed through and retained within the formed web of paper or board.

12. A process according to claim 11, wherein said dry alkaline silicate composition is in the form of particles of a size in the range from 1 to 150, preferably 1 to 100 microns .

13. A process according to claim 11 or claim 12, wherein said dry alkaline silicate composition is a partially hydrated alkali silicate formed by spray drying an aqueous solution of sodium silicate.

14. A process according to claim 13, wherein said partially hydrated alkali silicate has a moisture content of from 5 to 18, preferably 5 to 13 percent by weight.

15. A process according to any one of claims 11 to 14, wherein said dry alkaline silicate composition in particulate form further comprises starch.

16. A process according to claim 15, wherein said starch is in the form of uncooked starch particles.

17. A process according to claim 16, wherein said starch is an unmodified natural starch selected from the group consisting of corn starch, potato starch and rice starch.

18. A process according to claim 16, wherein said starch is a starch which has been chemically modified by cationization.

19. A process according to any one of claims 15 to 18, wherein said starch is present in said dry alkaline silicate composition in a proportion of up to about 95 percent by weight, based on the total dry weight of silicate and starch in said dry alkaline silicate composition.

20. A process according to claim 19, wherein the proportion of dry starch in said dry alkaline silicate composition is in the range of from 70 percent to 80 percent by dry weight .