NO177542B - Process for producing a filled fiber pulp, as well as high tensile paper filled paper - Google Patents

Abstract

There is disclosed a filled paper composition comprising intact cellulose fibers and fillers wherein the cellulose fibers are originally never-dried, and wherein the filler material is in an insoluble precipitate formed in situ within the cell wall of the never-dried cellulose pulp fibers. The filled paper composition is characterized by having increased strength characteristics relative to a corresponding conventionally loaded paper composition containing the same amount of the same filler material. There is also disclosed a process for the production of filled paper using never-dried pulp fibers and filler comprising an insoluble precipitate that is precipitated in situ within the cell wall of the fibers. The process first immerses the never-dried pulp fibers in a first solution containing a soluble salt or salts, filters the pulp fibers from the first solution, and reimmerses the never-dried pulp fibers containing the first solution in the pores into a second solution, wherein the second solution comprises soluble salt or salts different from those of the first solution and able to form an insoluble precipitate with the salt or salts of the first solution. The filled, never-dried pulp fibers are filtered and washed and either used to form filled paper products or dried to filled pulp fibers for later use in papermaking.

Description

This invention relates to a method for producing a filled fiber mass where the filler is an insoluble precipitate mainly located inside the cell walls of non-dried fiber mass. The localization of the filler inside the cell walls causes the manufactured filled fiber mass to have increased strength properties compared to corresponding conventionally filled paper containing the same amount of the same filler.

The present invention also relates to a filled paper which has increased strength compared to conventional filled paper which has the same concentrations of the same filling materials.

The increasing price of raw pulp and the energy associated with processing are known problems for most paper producers. The boom in the use of hardwood trees, the optimization of pulp production, and the ongoing conversion to alkaline sizing are just a few examples of the many attempts that have been made in recent years in connection with problems in paper production. The most economically viable method has been to replace fiber pulp with cheaper filling materials. Paper that has a high content of filler is also called ultra-high ash paper when calcium carbonate (CaC03) is the filler. However, the biggest problem with ultra-high ash paper is a weakening of the internal fiber bonds. This results in a weakened paper strength.

In processes for paper production, fillers or opaque pigments are often used to achieve some desirable properties of the paper product, and to achieve a cost saving of the raw materials for the paper. The fillers can increase the opacity, whiteness and printing properties. Fillers are cheaper than cellulose fibers and can reduce the overall cost of the finished paper product. Furthermore, the fillers can be dried more easily than the fibres, thereby reducing energy consumption during paper production.

An essential property of the paper for many users is the paper's opacity. It is particularly important for writing paper, where it is desirable that as little as possible of the written text shows through on the back of the sheet, or on sheets that may lie underneath. For printing and other purposes, the paper must also have a special degree of whiteness or gloss. For many types of paper, acceptable levels of optical properties can be achieved from fiber pulp alone. For other products, however, the inherent light-reflective features of the fibers are insufficient to meet consumer needs. In such cases, the paper manufacturers add filler.

A filler consists of fine particles of an insoluble substance, usually of mineral origin, suspended in a slurry. Due to the high surface area to weight ratio (and sometimes high refractive index), the filler particles cause light-reflected radiation in the paper, therefore increasing both opacity and whiteness. Adding fillers to the pulp improves the optical properties of the paper and further improves smoothness and print quality. Furthermore, replacing fibers with a cheap filler can reduce costs. However, the addition of filler causes some new problems.

A problem associated with the addition of filler is that the mechanical strength of the paper becomes less than what could be expected based on the ratio between load-bearing fibers and non-load-bearing filler. The paper's mechanical strength can be expressed in burst index, tear index and tensile index. The usual explanation for this is that some particles of the filler are squeezed between fibres, and thus the strength of the fibre-to-fibre hydrogen bond is reduced. The hydrogen bond is the most important cause of paper strength.

There is a practical limit to the amount of filler that can be used. The paper's mechanical properties depend primarily on the hydrogen bonding between the fibrous elements. The filler accumulates on the outer surfaces of the fibers. The accumulation of filler weakens the paper's strength. Furthermore, increasing amounts of retention agents must be used to avoid excess loss of pigment through the paper-forming production equipment. In accordance with this, the filler concentration is often limited to an ash content of a maximum of approx. 10%.

Several techniques have been used to overcome the problems of reduced paper strength by increasing the amount of filler. The procedures have involved a surface modification of the fillers, using additives to maintain the filler and its effect, and using supplementary binders. For example, preflocculating fibers and fillers have been used to increase filler retention and reduce paper strength loss. Coarser particles of pigments or fillers, caused by preflocculation procedures, are retained more effectively than fine particles of pigments. This means that there are fewer interventions in the inner fiber bonds. This helps to improve paper strength. However, the opacity of the paper is weakened by increasing particle size. The cost savings associated with preflocculation techniques are imperceptible and are outweighed by additional problems.

Craig, US Patent No. 2,583,548 describes a process where pigmented cellulosic pulp is formed by precipitating the pigment in and around the fibers. According to Craig, dry cellulosic fibers are added to a solution of a reactant, for example, calcium chloride, and the suspension is mechanically stirred to create a gelatinization of the dried fibers. Another reactant, for example, sodium carbonate, is added to create the precipitation of fine, solid particles, such as calcium carbonate. The fibers are washed to remove soluble by-products (sodium chloride).

The procedure according to Craig has considerable limitations. The presence of filler on the surface of the fibers and the effect of gelation on the fibers is unfavorable for paper strength. The gelatinized fibers are so strongly broken that the precipitation of filler and the gel-formed fibers form a slurry. Therefore, Craig's process has not achieved commercial success despite being developed for approx. 39 years ago.

Another technique is described in US Patent No. 4,510,020. This method has been called the lumen filling process and it concerns the placement of filling material directly inside the lumens in fibrous pulp of conifers. Lumen-filled pulp is produced by vigorously stirring dry conifer pulp in a concentrated suspension of filler. The agitation helps the filler to move through transverse open cavities in the cell walls of the fibers and into the lumens, where the filler is absorbed into the surface of the cavity in the lumens. Subsequent washing of the lumen-filled pulp fibers quickly removes filler residues at the outer edge of the fibers but only slowly from the lumens. The result is increased retention of filler inside the lumens, while removing the obstacles to forming internal fiber bonds by removing the filler on the outside of the fiber lumens. The result is increased paper strength for the amount of filler present. The lumen filling technique works best with fibers that have been dried.

However, the lumen technique has not proven to be economically or commercially viable. The technique requires the handling of large volumes of relatively concentrated suspensions of filler which are stirred at high revolutions over long periods. Furthermore, the lumen filling technique requires a relatively small particle size of the filler, such as titanium oxide, which is an expensive filling material. Furthermore, the lumen filling technique will only work for dry conifer fibers that have an appreciable number of open voids. As the lumens are open to the cavities, filler can be lost in the same way as it is introduced. Furthermore, the pores in the cell walls are not filled with the lumen filling technique.

Consequently, there is a need to be able to produce economic paper with high opacity and high strength using as much filler as possible, and to be able to use cellulosic fibers from the pulp of all types of wood (conifers, hardwoods and annuals , such as sugar cane).

The present invention relates to a method for producing a filled fiber mass where the starting mass is non-dried fiber mass that is formed from conifers, broad-leaved trees, annual plants or combinations thereof. The process is characterized by immersing the non-dried fiber having an inner cell wall pore volume that remains wet after fiber formation in a first solution wherein the first solution consists of a soluble salt or salts, filtering the immersed non-dried fiber mass to remove the soluble salt from the outer surface of the fiber and the lumens, further immersing the non-dried fiber mass in another solution where the second solution consists of a soluble salt or salts different from the soluble salt or salts from the first solution, with the proviso that by the mutual influence between the salt or salts from the first solution and the salt or salts from the second solution, an insoluble precipitate is formed which acts as a filler inside the cell walls of the non-dried fiber mass, and filtering and washing the filled non- dried pulp to form the filled fiber pulp.

Furthermore, the invention describes a filled paper with high tensile strength, where the paper is formed from fiber mass produced according to the method described above, where virtually all the filler is located inside the cell wall of the fibers.

Examples of insoluble precipitates or precipitates that act as fillers include, for example, calcium carbonate, other precipitates listed in Table 1, and combinations thereof. The paper composition is selected from groups consisting of unbleached kraft paper, bleached kraft paper, sulphite cellulose (bleached and unbleached), fine paper for printing, fine writing paper and light newsprint.

The paper is further produced according to techniques well known to those skilled in the art. The pulp can be used directly for paper production without drying, or dried as filled pulp fibers to later be used for paper production.

The essential steps of the method according to the invention are as follows:

1. Dispersing the non-dried mass in a first solution,

wherein the first solution consists of soluble salt or salts; 2. To filter the dispersed, non-dried mass and further redisperse the filtered non-dried mass in a new solution,

where the new solution consists of a soluble salt or salts which is different from the soluble salt or salts from the first solution, and with the proviso that by the mutual influence between the salt or salts from the first solution and the salt or salts from the second the solution forms an insoluble precipitate in sjtu which acts as a filler within the cell wall or pores of the non-dried pulp; and

3. To filter and wash the filled non-dried mass.

The paper can be formed directly with the filled, non-dried cellulosic fibers using conventional techniques. Alternatively, the filled, non-dried pulp can be dried and later used in paper production.

In another embodiment, these filled, non-dried cellulosic fibers are broken up after being filled during the non-dried stage, or after being dried once. If filled pulp that is not expanded is dried, the paper manufacturer can control the specifications of the expanding process during paper production.

The non-dried cellulose can be obtained from hardwoods, conifers, annual plants such as sugarcane (bagasse), or combinations thereof.

The present invention is able to fill the precipitate-type filler in the cell walls or pores found inside the cell walls of non-dried cellulose fibers, by internal jn sjtu precipitation of insoluble fillers and pigments. Undried cellulose fibers are unique in that they have relatively large pores located within the interior of the cell wall. These pores collapse when the fiber mass is dried, and are not completely restored by soaking the dried fibers. Therefore, filling material can be optimally deposited and filled in the cell wall that encloses the lumens just before the fibers are dried. In the same way, filled fibers that have been filled according to the method of the invention and dried cannot be filled again according to the method of the invention.

Fillers, such as pigments and opaque precipitates, are introduced into the pores of the cell walls of the non-dried fibrous pulp by precipitation of the filler within the pores. This replaces the fluid content in the pores. Excess filler is washed away from the outer wall of the fiber and a negligible amount, if any, of the filler remains within the lumens of the fiber. Since the non-dried cellulose fibers are hollow, tubular structures, the fibers develop an extremely large surface area after pulp formation and retain the large surface area as long as it is wet (non-dried). The large surface area within the non-dried fibers is readily accessible to soluble salts that precipitate as papermaking fillers. This preserves the binding capacity of the outer cellulosic layers and does not affect the strength of the resulting paper. Figure 1a is an electron micrograph at 2142 X magnification scanning to show the surface of the non-dried cellulose fibers filled in accordance with the method of the invention with NiC03 insoluble precipitate filler. Figure 1b is an electron dispersion analysis (EDAX) of the filled fiber showing the location of Ni and its distribution inside the cell wall of the fiber. Figures 2a, 2b, and 2c are EDAX graphs of Ni (07 box) of a NiCO 3 -filled, non-dried fiber mass showing the surface of the fiber, the cell wall of the fiber, and the fiber lumens, respectively. The filler was mainly present in Figure 2b, which is indicated by the presence of nickel in the cell wall. Figures 3 and 4 illustrate respectively the tensile index and burst index in paper containing different amounts of filler, produced from non-dried pulp of hemlock spruce (a conifer). The filled circles represent paper made from fibers filled according to the method of the present invention, and the open circles or squares represent paper made from fibers filled according to a conventional method, as described in Example 1. The different symbols represent different representations at different dates. Figures 5, 6 and 7 illustrate respectively the tear index, burst index and tensile index for different amounts of filler in paper made from pulp of red alder (a hardwood). The open circles and squares represent the process of the invention with CaC03 as filler, precipitated jn sjtu, where, for the open squares, CaCl2 was the first salt and Na2C03 was the second salt, and for the open circles, Na2C03 was the first salt, and CaCl2 was the other salt. The filled triangles are data for paper made from a mixture of cell wall filled and unfilled fibers in the ratios 1:3, 1:1 and 3:1 respectively. The open squares represent non-dried cellulosic fibers filled using the conventional technique as described in Example 1, of red alder. Designation 'X' represents mass of red alder which has been dried once, and again moistened and filled using the method according to the invention. Figures 8, 9 and 10 respectively illustrate tear index, burst index and stretch index for papers with different amounts of filler which are produced from pulp of spruce CTMP (a conifer). The open squares represent seventy precipitated, non-dried pulp fibers which have been filled using the method according to the invention. The filled circles represent non-dried pulp fibers loaded into a filled paper using a conventional process as described in Example 1. Figures 11, 12 and 13 illustrate respectively the tear index, burst index and tensile index for papers containing different amounts of filler produced from bagasse pulp (sugar cane). The filled squares represent seventy precipitated, non-dried fibers of pulp which are therefore filled using the method according to the invention, where the filled squares are bleached pulp and the open squares are unbleached pulp. The squares represent non-dried cellulose filled using a conventional process, as described in Example 1, where the filled squares are bleached pulp and the open squares are unbleached pulp.

The triangles represent bleached pulp which has been filled using the method according to the invention and which has been dried once. The poor result obtained with pulp that has been dried once indicates that pores of the non-dried pulp fibers are necessary to be able to fill the cell walls of the fibers.

Figure 14 compares relative decreasing tensile strengths as a function of amount of filler, by comparing literature data from the lumen filling technique (triangles or "x" shapes) with non-dried fibers filled using the method of the invention when using pulp from hardwoods such as red alder ( open squares), pulp from sugar cane bagasse (squares) pulp from conifers such as spruce CTMP (filled circles).

Undried pulp is formed by removing lignin and hemicellulose from cellulose fibers from trees during formation of the pulp. The mass obtained is a composition of several hundred concentric lamellae of cellulose microfibrils. Each lamella is separated from the others by water-filled areas (pores) that vary in width from approx. 20 to approx. 300 angstroms. The largest areas are located closest to the periphery, while the narrowest areas are located towards the lumens (a central channel of about 10 to about 20 microns in width). The interstices of the pores more or less correspond to the thickness of the lignin in the cellulose fibers of trees. The pore size generally has a normal distribution in timber. A surprising result of the present invention is that most of the first solution leaves the fiber lumens when the fiber is filtered between the addition of the first and second solutions. This is because the lumens are more open to the external environment than the pores in the cell wall. Therefore, little, if any, filler is precipitated into the lumens. The normal distribution of pore size in timber is a curve of the logarithm of the pore size against the pore frequency. The non-dried cellulose fibers have a surface area of approx. 1000 m<2>/g. During drying, the surface area is reduced to approx. 1 m<2>/g. Although lamella swells during re-soaking, pulp that has been re-soaked only has a surface area of approx. 100 m<2>/g. Thus, most of the pores in the non-dried wood mass are destroyed during drying.

The composition and the method according to the invention depend on the special properties of non-dried pulp or its equilibria. The non-dried mass has a large internal surface area of approx. 1,000 m<2>/g as a result of the corresponding inner cell wall pore volume of approx. 1.2 mL/g. The inner cell wall grooves are mainly lost by destruction during drying. Anything that may have been placed inside the pores during drying remains in the pores when the pores are destroyed during drying.

We have shown that if essentially non-dried cellulose is treated with a first solution containing a soluble salt, such as calcium chloride, and filtered to remove the soluble salt from the outer surface of the fiber and the lumens, and then another soluble salt is added , such as sodium carbonate, the filler, calcium carbonate, is formed inside the pores but not inside the lumens. This process is suitable for other filler materials when the filler is an insoluble precipitate formed by the interaction of two or more soluble salts. When the filler is placed inside the cell wall during the insjtu process, the disruption of the hydrogen bonds between the fibers is reduced. As a consequence, the strength of the paper produced using this in situ precipitation is better than the strength of paper produced using the usual (conventional) combination of fibers and the same amount of filler. The conventional mixture of filler and fiber places the filler between the fibers. Furthermore, the abrasive filler will have less contact with the production equipment in the paper machine if the filler is placed inside the cell wall of the fiber as it is according to the present invention. This will result in less wear and tear on the production equipment in the paper machine in a given period of time. In addition, there is less opportunity for the filler to dust from the paper sheet because the filler is located mostly inside the cell wall of the fibers instead of on the outside of the fibers.

Another advantage of the method and composition according to the present invention is that larger amounts of filler are used to produce paper while maintaining the strength of the resulting paper. The paper filler does not need incorporating adhesive polymers to maintain paper strength. Therefore, paper produced according to the method of the present invention without adhesive filler can have larger amounts of filler than conventionally produced paper, and still maintain equally good or better strength characteristics.

Since filler is generally more economical than cellulose fibres, the method according to the invention provides an economic advantage in terms of lower expenditure on starting materials for the finished paper composition. It is also less energy-intensive and more economical to dry filler than to dry fibres. Thus, reduced energy expenditure in paper production will be achieved by reducing drying costs.

The method according to the invention uses non-dried pulp and precipitates the filling material in situ. Non-dried masses are filled by subsequently immersing the non-dried mass in solutions consisting of a soluble salt or salts. The non-dried masses are first immersed in a first solution for about five minutes or less. The first solution consists of a soluble salt or salts, and is used to replace the water inside the pores of the cell wall and in the lumens with a solution containing the soluble salt or salts of the first solution. The non-dried cellulosic fibers are then filtered and washed, which causes the salt or salts from the first solution to be removed from the outer edge and lumens of the fibers. The second solution containing different soluble salt or salts is added to the filtered fibers. The interaction of the salt or salts from the first solution inside the pores of the cell wall of the non-dried cellulose fibers and the soluble salt or salts from the second solution forms an insoluble precipitate which precipitates from the solution inside the pores of the cell wall of the non-dried the cellulose fibers. The precipitate inside the cell wall of the non-dried cellulose fibers acts as a filler. When the fibers are dried or are used to make paper and later dried, the insoluble precipitate acts as a paper filler. The filled, non-dried cellulose fibers are then filtered and washed and used to make paper. Alternatively, the filled fibers can be dried and sent to the paper mill as dry pulp.

Cellulose fibers are often broken up into special mixtures as part of paper production. The cellulose fibers are broken up before the formation of the paper. The method according to the present invention allows the fibers to be broken up either before or after the filling of the fibers. The non-dried cellulose fibers can be filled, dried and then beaten up before being used to form paper.

The order of the soluble salts in the first or second solution is not important in the process. What is important is that the salt or salts in the first and second solutions are different and that they form an insoluble precipitate during interaction. Examples of white (opaque) and various colored precipitates are given in Table 1.

An expert in the field will have knowledge of which salts form each of the precipitates.

Preferred examples of soluble salts which can form insoluble precipitates include CaCl 2 and Na 2 SiO 3 , which form the precipitate CaSiO 3 (an opaque white filler); BaCl2 and Na2SO4 form BaSO4 (a white, opaque filler); and CaCl2 and Na2CO3, which form CaCO3 (an opaque, white filler). It should be noted that it is possible to replace a sodium cation with a potassium cation in any of the soluble salts. Examples of green precipitate fillers are NiC03 which is formed by combining the water-soluble salts NiCI2 and Na2C03; copper carbonate (CuCO 3 ), from copper chloride (Cu 2 Cl 2 ) and sodium carbonate; and chromium phosphate (CrP04), from chromium chloride (CrCI3) and sodium phosphate (NagPOJ. The preferred precipitate filler material is calcium carbonate (CaC03). Calcium carbonate can be formed, for example, by having a solution of calcium chloride and another solution of sodium or potassium carbonate .In all the insoluble precipitates that are formed, the order of the soluble salts used is not important.

The concentration of salt or salts in the aqueous solution can vary from approx. 1% to approx. 40%, depending on the solubility of the salt in an aqueous system, the temperature of the process, and the amount of filler desired. It is preferred that the concentration of salt or salts in the aqueous solution should be as saturated as the solubility factors and temperature of the process allow, in order to maximize the amount of filler in the resulting non-dried cellulosic fiber. When using colored or pigmented filler precipitates, it is desirable not to maximize the amount of filler in the cell wall of the non-dried fibers.

The method according to the invention provides an opportunity to improve the retention of mechanical properties of non-dried pulp when the cell wall is filled with in situ precipitated filler. When non-dried pulp is filled with NiC03, formed from the soluble salts NiCl2 and NaC03, the nickel precipitate can be visualized by electron dispersion analysis (EDAX).

Filled, non-dried pulp was washed with water. Microscopic observations of the washed, non-dried pulp indicated that this procedure was not effective enough to completely remove excess filler material from the fibers. Sheet formation, drying, and airing were carried out in accordance with TAPPI standards. See TAPPI Official Test Method T 205 om-81 from The American National Standard, April I982.

Figure 1a shows the localization of nickel, and Figure 1b shows the nickel distribution. The white spots in figure 1b represent nickel and the high density of white dots makes it possible to make the cell wall in the fiber visible. Figures 2a, 2b and 2c show different aspects of binding points of non-dried cellulose fiber which is filled with nickel carbonate as filling material in the method according to the invention. Figure 2a shows the surface of the filled, non-dried pulp fibers with virtually no presence of nickel in the third box from the right. Figure 2b shows a high nickel content, including the high peak in the third box from the right, for the cell wall area of the fibres. Figure 2c shows the nickel concentration in the lumens of the filled non-dried cellulose fibers, with very little nickel occurrence.

Paper made from non-dried fibers that have been filled in the cell wall grooves with the precipitate filling material can be used for many purposes. In the following, some of the largest areas of use are indicated, but there are also many special products that are produced in smaller quantities.

Fine papers are a large class of papers used for printing and writing. In general, fine papers contain fillers. An advantage of using the filled, non-dried cellulose that is filled inside the cell walls, in paper machines that produce fine papers instead of using the usual mixtures of separate fibers and fillers, is a higher degree of retention of filling material inside the fibers. This leads to better control of properties and cleaner conditions at the paper machine. In addition to making the paper stronger than a paper conventionally filled with the same concentration of filler material, the paper produced by cell wall filling exhibits less "two-sidedness". Double-sidedness is caused by different distribution of filler through the thickness of the sheet. Furthermore, there is a smaller tendency for the filler to dust from the sheet during processes such as wetting and cutting.

Unbleached kraft paper pulp is used for paper products such as paper bags and wrapping paper due to its high strength. However, it has a low degree of whiteness which makes it both unattractive and poorly suited for writing paper. Undried, unbleached kraft fibers with filled cell walls improve the whiteness of the paper produced, and less strength is lost using this method than with conventional filling techniques and using dried cellulose fibers.

Most newsprint today is produced from a mixture of mechanical and chemical cellulose without fillers. A requirement for such products is that they must maintain a low basis weight (cellulose weight per unit area). One of the barriers to achieving a noticeable reduction in basis weight is that such changes also reduce the opacity of the paper. Fillers are not currently added to offset this loss in opacity for various reasons, including the loss of strength it causes in the paper and the mess it introduces in the papermaking operation. By using cell wall-filled, non-dried pulp fibres, newsprint problems are reduced and newspaper printing can take place with an increasing degree of opacity.

The following examples shall illustrate the method according to the invention and the composition which is produced using the method according to the invention.

EXAMPLE 1

This example illustrates a comparison where non-dried cellulose from hemlock fir (softwood) is used and compares the properties of the paper produced by the method according to the invention with a conventional method. In each case the wood pulp was beaten up to 400 CSF before being treated. In the method according to the invention, a sample of non-dried pulp (10 g) was dispersed with a 5%, 10%, 20%, or 35% solution of CaCl2 in 500 mL of water. After 30 minutes, the CaCl2-impregnated fibers were collected by filtration under reduced pressure and further dispersed in a saturated solution of Na2CO3 (1000 mL). After one hour, the dispersion was filtered through a 200 filter cable and then washed with water until the filtrate was clear.

The non-dried pulp used to make conventional filled paper was also washed in a 200 filter cable five times at 0.5% consistency. The pH value of the conventionally filled mass was adjusted using NaOH to 8.0. A retention agent (Reten 210, Hercules Corp.) was added at various rates (0.5 to 1.5 Ib./ton mass) to achieve adequate retention of the commercial CaCO 3 slurry. The stirring took place for one minute.

Sheets were produced using both the conventional pulp and filler mixtures and cell wall filled, non-dried pulp using TAPPI standard sheet production conditions. The amount of filler (CaCO 3 ) in the sheets was calculated by the amount of ash, which was determined by the standard TAPPI procedure, except for the temperature of the furnace which is 575°C.

In Figures 3 and 4, the paper produced from the cell wall-filled, non-dry pulp is shown by the filled circles. The paper produced by conventional techniques is shown by open circles.

Figure 3 shows the effect of filler level on the tensile index of conventional and cell wall-filled, non-dried pulps. These data indicate that at equal CaCO3 filler concentration, the sheets produced with fibers filled using the method of the present invention have tensile properties that are higher than those of sheets produced by a conventional process.

Similar comparison data is obtained in Figure 4, where the burst strength is measured. Figure 4 is a curve showing the burst index against the filler concentration in paper of both types of filled paper. These data demonstrate the improved burst strength values obtained by using fibers filled using the method of the invention. These data indicate that at the same level of filler concentration, the method according to the invention allows more filler to be added to give the same paper strength or it gives an improved level of strength at the same concentration of filler. Filled paper sells for about $1,000/ton or $0.50/lb when the pulp costs $500/ton and the filler costs $200/ton. Thus, any percentage of filler that can replace the amount of fiber in the paper sheet represents a noticeable cost savings in production of approximately $3-$4/ton for the paper manufacturer. Furthermore, the method according to the invention does not require retention aids, and thus the production of paper can be improved. Thus, the use of kraft wood pulp of conifers as starting material in the process for producing paper according to the present invention will improve the strength properties of the paper that is produced.

EXAMPLE 2

This example illustrates a comparison of different mechanical properties of paper made using undried red alder cell wall filled celluloses versus undried red alder cellulose combined with filler by conventional means versus filled red alder pulp according to the method of the invention and which has been dried once. In each case, the non-dried cellulose was beaten up to 400 mL of CSF prior to the filling of filler by each of the techniques. The methods used to fill cellulose from red alder by the method according to the invention or by combination with conventional techniques are described in example 1. Calcium carbonate was added as a slurry by the conventional technique and precipitated in sjtu using the method according to the invention. The concentration of filler was determined based on the amount of ash.

Figures 5, 6 and 7 respectively compare tear index, burst index and tensile index when comparing non-dried pulp of red alder which is filled by the method according to the invention and by using conventional technique. In each of the illustrations, the amount of ash indicates the amount of filler in the paper. Therefore, in each of the figures it is possible to compare the tear index, burst index and tensile index of the manufactured paper from each type of filled fiber with the same filler concentration.

In Figures 5, 6 and 7, the top line with the highest values for tearing, bursting and stretching represents paper which is produced from fibers filled by the method according to the invention. The squares represent non-dried pulp fibers filled in the order of first addition of calcium chloride solution, followed by sodium carbonate, and the circles have the reverse order with first sodium carbonate and then calcium chloride. The lower line with the X-shaped dots represents fibers filled by the method of the invention and dried once. The lower line of squares represents conventionally filled, non-dried masses.

In each case, the strength of the paper produced, as measured by tear index, burst index and tensile index, was higher for the process of the invention using non-dried pulp fibers. Furthermore, the order of addition of the two solutions is not important.

EXAMPLE 3

This example illustrates a comparison of non-dried spruce CTMP cellulosic fibers filled using the process of the invention or conventional techniques. The non-dried fibers were resuspended in 400 mL of CSF. The method according to the invention and the conventional method used to fill the fibers are described in example 1. Figures 8, 9 and 10 respectively illustrate the tear index, burst index and tensile index for paper which is made from non-dried pulp fibers of spruce CTMP and which is filled using the method according to the invention, and using conventional techniques. In each of the three figures, the method according to the invention is illustrated by squares and the conventional mixing process by circles.

A characteristic of cellulose from spruce CTMP is that the values for stretching, bursting and tearing decrease faster with the amount of ash (that is, increase in the amount of filler). For each strength parameter, the paper made from non-dried cellulose fibers and filled according to the invention shows increased strength compared to paper where the fibers were filled by conventional techniques.

EXAMPLE 4

This example compares bagasse cellulose from sugarcane fibers where the comparison takes place between bleached and unbleached, non-dried pulps filled according to the invention and bleached pulps filled according to conventional techniques. The processes used to make each paper and the way to combine fibers and fillers are described in Example 1.

Figures 11, 12 and 13 illustrate respectively the tear index, burst index and tensile index for each of the three paper types. The squares illustrate the method of the invention, where data for paper made from bleached, non-dried fibers is indicated by filled squares, and unbleached, non-dried fibers by open squares. Data for paper made from non-dried bagasse fibers filled using conventional methods are illustrated by the triangles. Data for paper made from bleached, non-dried pulp fibers are shown by filled squares and unbleached, non-dried fibers by open squares. Paper produced from bleached pulp and filled by the method according to the invention and then dried once is shown by the triangles.

As shown in Figures 11, 12 and 13, paper made from non-dried bagasse cellulose and filled according to the invention exhibits improved strength properties for each of the concentrations of filler tested.

EXAMPLE 5

This example illustrates a comparison of the tensile strength in the paper when using fibers which are filled according to the method of the invention with the lumen filling process which is described in US patent no. 4,510,020 which was previously mentioned. Figure 14 illustrates the relative decrease in tensile strength of paper expressed as a percentage against the amount of filler expressed as a percentage, of red alder undried pulp, bagasse undried pulp and spruce CTMP undried pulp filled by the method of the invention, compared to the lumen-filling techniques using conifers such as those in Miller et al. in Proceedings 1983 TAPPI International Paper Physics Conference, Harwichport, p. 237 ("Miller et al"), and Green et al., Pulp & Paper Canada, 83: T203 (1982) ("Green et al").

Large amounts of filler were filled into the fibers of non-dried hardwood pulp fibers using the method of the invention and compared to Green et al.'s data for conifers and to Miller et al.'s data for conifers. However, it should be noted that Miller et al. performed their experiments with the inclusion of 2% PEI. PEI (polyethyleneimine) is a polycationic polymer that can form ionic bonds between the fibers in the paper and thus act as strengthened paper. PEI will work in such a way that the very fine filler particles flocculate inside the lumens. The accumulation of filler particles into large quantities improves the retention of filler within the lumens, which minimizes the loss of filler. We achieved almost 40% filling of filler in non-dried bagasse cellulose, but at the expense of mechanical properties. The relative decrease in tensile strength using the method according to the invention shows the same pattern as Green et al.'s data using fibers from conifers. Miller et al.'s trial showed better results. However, the presence of 2% PEI may have contributed significantly to the strength of the resulting paper.

In Figure 14, the open circles indicate non-dried masses from red alder that are filled according to the method of the invention, the open squares represent bagasse mass that is filled using the method of the invention, the filled circles represent non-dried masses of spruce CTMP which are filled according to the present invention, the filled triangles represent data according to Miller et al.'s experiments and the X figures represent data according to Green et al.'s experiments.

EXAMPLE 6

This example illustrates how undried pulp of eucalyptus (pulp of a hardwood tree) can be filled with aluminum hydroxide in situ. Pulp from eucalyptus was dispersed in a first solution containing the soluble salt aluminum sulfate. The first solution contained a saturated concentration of aluminum sulfate at room temperature. The first solution was removed within 5 minutes by filtering the mass. This also removes the first solution from the pulp lumen.

Another solution containing 20% (w/v) sodium hydroxide was used to disperse the pulp fibers. This formed aluminum hydroxide is mostly deposited in the cell wall of the fibres. The paper was made from fibers filled with aluminum hydroxide as a filler. The amount of filler in the paper was 9%, which was determined by the ash amount of Al 2 O 3 (alumina).

EXAMPLE 7

This example illustrates the effect of filled, non-dried fibers that have been beaten up and the effect of different beating conditions. Undried pulp of eucalyptus (hardwood) was filled with CaCO 3 using the method according to the invention. Undried pulp that was not broken up had a Canadian Standard Freeness (CSF) of 570 mL. A sample of the filled, non-dried pulp fibers was first beaten up at 10,000 revolutions in a PFI mill (beating up apparatus).

The CSF value was 416 mL. The pulp was formed into a first coarse sheet by filtering in a cable. The ash content of the first sheet was 43%. The pulp was then redispersed in water and filtered to form a new sheet. The ash content of the second sheet was 38%. This process of redispersion and filtration was repeated three more times. The ash content of the third, fourth and fifth sheet was 34%, 36% and 34% respectively. It shows that only approx. 7 - 9% of the filler was located on the outside of the cell wall, even after beating at 10,000 revolutions. It shows that the filler mainly stays in the cell wall during the opening.

The entire procedure was repeated; except that this time the non-dried fiber mass was spun up at 20,000 revolutions as described above. The CSF value was 366 mL. The first filtered sheet had 46% filler, the second sheet 41% filler and the third sheet 38% filler. Only approx. 8% of the filler was located on the outside of the cell wall even after beating at 20,000 revolutions.

In relation to this, it is known that pulp fibers which are filled by the lumen filling technique will lose most of the filler after turning up. The present invention does not lose any significant amount of the filler during the turning.

Claims (1)

A method for the production of a filled fiber mass where the starting mass is non-dried fiber mass that is formed from conifers, hardwoods, annual plants or combinations thereof, characterized by submerging the non-dried fiber that has an internal cell wall pore volume that remains wet after fiber formation, in a first solution where the first solution consists of a soluble salt or salts, filtering the immersed non-dried fiber mass to remove the soluble salt from the outer surface of the fiber and the lumens, further immersing the non-dried fiber mass in another solution where the second solution consists of a soluble salt or salts which is different from the soluble salt or salts from the first solution, with the proviso that by the mutual influence between the salt or salts from the first solution and the salt or salts from the second the solution forms an insoluble precipitate that acts as a filler inside the cell the walls of the non-dried pulp, and filtering and washing the filled non-dried pulp to form the filled pulp. A method for producing a filled fiber mass according to claim 1, characterized in that the filled non-dried fiber mass is filtered and washed to form a filled mass, where possibly at least 50% of the filler quantity is located inside the pores or the cell wall of the non-dried the cellulose fibers. A method for producing a filled fiber mass according to claim 1, characterized in that the insoluble precipitate is selected from calcium salts, barium salts, zinc salts, magnesium salts and copper salts, or a combination thereof. A method for producing a filled fiber mass according to claim 3, characterized in that the insoluble precipitate is calcium carbonate. A filled paper with high tensile strength, characterized in that the paper is formed from fiber mass produced according to claims 1, 2 or 3, where practically all the filler is located inside the cell wall of the fibers. A filled paper according to claim 4, characterized in that it contains a dye, where the dye is a colored precipitate that functions as a filler. A filled paper according to claim 4, characterized in that the paper is an unbleached kraft paper, bleached kraft paper, fine printing or writing paper or light newsprint.