Process for the production of nano-fibrillar cellulose suspensions

0001The present invention relates to a process for producing nano-fibrillar cellulose suspensions and the nano-fibrillar cellulose obtained by this process.

0002Cellulose is the structural component of the primary cell wall of green plants and is the most common organic compound on Earth. It is of high interest in many applications and industries.

0003Cellulose is the major constituent of paper and cardboard and of textiles made from cotton, linen, and other plant fibres. Cellulose can be converted into cellophane, a thin transparent film, and into rayon, an important fibre that has been used for textiles since the beginning of the 20th century. Both cellophane and rayon are known as "regenerated cellulose fibres".

0004Cellulose fibres are also used in liquid filtration, to create a filter bed of inert material. Cellulose is further used to make hydrophilic and highly absorbent sponges.

0005For industrial use, cellulose is mainly obtained from wood pulp and cotton. It is mainly used to produce cardboard and paper; and to a smaller extent it is converted into a wide variety of derivative products.

0006Cellulose pulp as a raw material is processed out of wood or stems of plants such as hemp, linen and manila. Pulp fibres are built up mainly by cellulose and other organic components (hemicellulose and lignin). The cellulose macromolecules (composed of 1-4 glycosidic linked β-D-Glucose molecules) are linked together by hydrogen bonds to form a so called primary fibril (micelle) which has crystalline and amorphous domains. Several primary fibrils (around 55) form a so called microfibril. Around 250 of these microfibrils form a fibril.

0007The fibrils are arranged in different layers (which can contain lignin and/or hemicellulose) to form a fibre. The individual fibres are bound together by lignin as well.

0008The pulps used in papermaking are often obtained by grinding the wood and an optional processing by heat and chemistry to remove undesired compounds from the cellulosic fibres.

0009The fibres are ground and cut to a certain fineness (depending on the desired properties). The grinding of the fibres is achieved with a refiner (such as a conic rotor-stator mill or disc- or double-disc refiners). The refiner also fibrillates the fibres on the surface which means that some fibrils are partially pulled out of the surface of the fibre. This leads to a better retention of, and, frequently, a better adhesion to, pigments which may be added in paper production, and also to an enhanced potential of hydrogen bonding between the fibres of the paper. This results in improved mechanical properties. A side-effect is also that the paper becomes denser and more transparent because of a loss of light scattering as the size of the scattering centres moves away from the accepted optimum of half the wave length of light (glassine and greaseproof papers).

0010When fibres become refined under applied energy they become fibrillated as the cell walls are broken and torn into attached strips, i.e. into fibrils. If this breakage is continued to separate the fibrils from the body of the fibre, it releases the fibrils. The breakdown of fibres into microfibrils is referred to as "micro fibrillation". This process may be continued until there are no fibres left and only fibrils of nano size (thickness) remain.

0011If the process goes further and breaks these fibrils down into smaller and smaller fibrils, they eventually become cellulose fragments. The breakdown to primary fibrils may be referred to as "nano-fibrillation", where there may be a smooth transition between the two regimes.

0012The achievable fineness with conventional refiners however is limited. Also, a number of other apparati for breaking down particles are not capable of breaking down cellulose fibres to nano-fibrils, such as fluffers mentioned in <patcit id="pcit0001" dnum="US20010045264A"><text>US 2001/0045264</text></patcit>, which are only capable of separating given size fractions of fibres from each other.

0013Similarly, in <patcit id="pcit0002" dnum="WO02090651A"><text>WO 02/090651</text></patcit> a method for recycling pulp rejects generated during manufacturing of paper, paperboard or cardboard is described, wherein cleaner rejects containing among other things fibres, pigments and/or fibres are milled to a certain grain size by ball mills. However, no mention is made of the fibrillation of the fibres present, let alone the fibrillation into nano-fibrils.

0014If a further breakdown of the fibres into nano-fibrils is desired other methods are needed.

0015For example, in <patcit id="pcit0003" dnum="US4374702A"><text>US 4,374,702</text></patcit> a process for preparing micro fibrillated cellulose is described comprising passing a liquid suspension of fibrous cellulose through a high pressure homogenizer having a small diameter orifice in which the suspension is subjected to a pressure drop of at least 3000 psi and a high velocity shearing action followed by a high velocity decelerating impact against a solid surface, repeating the passage of said suspension through the orifice until said cellulose suspension becomes a substantially stable suspension, said process converting said cellulose into microfibrillated cellulose without substantial chemical change of the cellulose starting material.

0016<patcit id="pcit0004" dnum="US6183596B1"><text>US 6,183,596 B1</text></patcit> discloses a process for producing super micro fibrillated cellulose by passing a slurry of a previously beaten pulp through a rubbing apparatus having two or more grinders which are arranged so that they can be rubbed together to microfibrillate the pulp to obtain microfibrillated cellulose and further super micro fibrillate the obtained micro fibrillated cellulose with a high-pressure homogenizer to obtain the super microfibrillated cellulose.

0017Furthermore, ultra-fine friction grinders can be used, wherein the grinder reduces the fibres into fines by mechanical shearing (cf. e.g. <patcit id="pcit0005" dnum="US6214163B1"><text>US 6,214,163 B1</text></patcit>).

0018There are a number of problems regarding the fibrillation of cellulose fibres, which have to be overcome.

0019For example, the mechanical production of nano-fibrillar cellulose often has the problem of an increasing viscosity during the fibrillation process. This can stop the process completely or increase the needed specific energy.

0020The efficiency of the breakdown processes often is rather low, and there is a considerable amount of fibres just cut, but not fibrillated into fibrils.

0021Therefore, there is a continuous need for providing more efficient processes for producing nano-fibrillar cellulose suspensions, and it is one objective of the present invention to provide a new and efficient process for the production of nano-fibrillar cellulose suspensions.

0022It has been found that the addition and co-processing of certain fillers and/or pigments with cellulose fibre containing pulp may have a positive influence on the fibrillating process in many respects, as described in more detail below.

0023Thus, the process of the present invention is characterized by the following steps: <ol id="ol0001" compact="compact" ol-style=""><li>(a) providing cellulose fibres in the form of a suspension having a solids content of from 0.2 to 35 wt-%;</li><li>(b) providing at least one filler and/or pigment having a weight median particle size of from 0.5 to 15 µm;</li><li>(c) combining the cellulose fibres and the at least one filler and/or pigment, wherein the weight ratio of fibres to filler and/or pigment on a dry weight basis is from 1:10 to 10:1;</li><li>(d) fibrillating the cellulose fibres in the presence of the at least one filler and/or pigment until the fibres are at least partially broken down to primary fibrils and the Schopper Riegler degree is increased by at least 4 °SR.</li></ol>

0024Nano-fibrillar cellulose in the context of the present invention means fibres, which are at least partially broken down to primary fibrils.

0025In this respect, fibrillating in the context of the present invention means any process which predominantly breaks down the fibres and fibrils along their long axis resulting in the decrease of the diameter of the fibres and fibrils, respectively.

0026Cellulose fibres, which can be used in the process of the present invention may be such contained in pulps selected from the group comprising eucalyptus pulp, spruce pulp, pine pulp, beech pulp, hemp pulp, cotton pulp, and mixtures thereof. In this respect, the use of kraft pulp, especially bleached long fibre kraft pulp may be especially preferred.

0027It is advantageous for the use in the present invention that the cellulose fibres are provided in the form of a suspension, especially an aqueous suspension. Preferably such suspensions have a solids content of from 0.25 to 10 wt-%, more preferably 1 to 5 wt-%, and most preferably 2 to 4.5 wt-%, e.g. 1.3 wt-% or 3.5 wt-%.

0028The at least one filler and/or pigment is selected from the group comprising precipitated calcium carbonate; natural ground calcium carbonate; dolomite; talc; bentonite; clay; magnesite; satin white; sepiolithe, huntite, diatomite; silicates; and mixtures thereof. Precipitated calcium carbonate, which may have vateritic, calcitic or aragonitic crystal structure, and/or natural ground calcium carbonate, which may be selected from marble, limestone and/or chalk, are especially preferred.

0029In a special embodiment, the use of ultrafine discrete prismatic, scalenohedral or rhombohedral precipitated calcium carbonate may be advantageous.

0030The fillers and/or pigments can be provided in the form of a powder, although they are preferably added in the form of a suspension, such as an aqueous suspension. In this case, the solids content of the suspension is not critical as long as it is a pumpaple liquid.

0031In a preferred embodiment, the filler and/or pigment particles have a median particle size determined by the sedimentation method of from 0.7 to 10 µm, more preferably 1 to 5 µm and most preferably 1.1 to 2 µm, e.g. 1.5 µm or 3.2 µm.

0032For the determination of the median particle size, a Sedigraph 5100 device from the company Micromeritics, USA was used. The measurement was performed in an aqueous solution of 0.1 wt% Na₄P₂O₇. The samples were dispersed using a highspeed stirrer and ultrasound.

0033The fillers and/or pigments may be associated with dispersing agents such as those selected from the group comprising homopolymers or copolymers of polycarboxylic acid salts based on, e.g., acrylic acid, methacrylic acid, maleic acid, fumaric acid, itaconic acid, acryl amide or mixtures thereof; alkali polyphosphates, phosphonic-, citric- and tartaric acids and the soluble salts thereof; or mixture thereof.

0034The combination of fibres and at least one filler and/or pigment can be carried out by adding the filler and/or pigment to the fibres in one or several steps. The filler and/or pigment can be added entirely or in portions before or during the fibrillating step. However, the addition before fibrillating is preferred.

0035In one embodiment before fibrillating the pH of the combination of cellulose fibres and at least one filler and/or pigment is adjusted to a pH of 10 to 12, e.g. 11.

0036This adjustment to alkaline pH can be done via addition of preferably milk of lime (Ca(OH)₂) or any other base. After co-processing, the pH in the suspension might then have to be adjusted again to about 7.5 to 9.5, e.g. 8.5.

0037Generally, the pH of the suspension comprising the combination of fibres and pigment and/or filler should not be less than 6.

0038It might also be necessary to stabilize the pH, e.g. upon addition of PCC to a fibre suspension, which might lead to an increase of the pH, and a drop of the °SR. In this case the pH may be re-adjusted by commonly used acids or buffers in order to avoid the drop of the Schopper Riegler degree due to the influence of a pH increase.

0039Furthermore, in one embodiment, the combination is stored for 2 to 12 hours, preferably 3 to 10 hours, more preferably 4 to 8 hours, e.g. 6 hours, prior to fibrillating it, as this ideally results in swelling of the fibres facilitating the fibrillation and thus leads to a faster increase of freeness (°SR) and lower specific refining energy consumption for the same °SR freeness.

0040Fibre swelling may be facilitated by storage at increased pH, as well as by addition of cellulose solvents like e.g. copper(II)ethylendiamin, iron-sodium-tartrate or lithium-chlorine/dimethylacetamine, or by any other method known in the art.

0041Preferably, the weight ratio of fibres to fillers and/or pigments on a dry weight basis is from 1:6 to 6:1, even preferably 1:4 to 4:1, especially 1:3 to 3:1, and most preferably 1:2 to 2:1, e.g. 1:1.

0042For example, in one especially preferred embodiment 70 wt-% of bleached longfibre kraft pulp is fibrillated in the presence 30 wt-% ultrafine discrete prismatic (or rhombohedral) PCC, relating to the total dry weight of pulp and PCC, respectively.

0043One indication of cellulose fibrillation according to the present invention is the increase of the Schopper Riegler degree (°C).

0044The Schopper-Riegler degree (°SR) is a measure of the rate at which a diluted pulp suspension may be de-watered and is specified according to the Zellcheming Merkblatt V/7/61 and standardized in ISO 5267/1.

0045The value is determined by smoothly dispersing the pulp in water and putting it into a drainage chamber where a sealing cone is closed. The sealing cone is lifted pneumatically from the drainage chamber, and depending on the condition of the fibre suspension the water flows more or less quickly from the drainage chamber through a side outlet into a measuring cylinder. The water is measured in the cylinder, wherein 10 ml water correspond to 1 °SR, and the higher the Schopper-Riegler value, the finer the freeness.

0046For measuring the Schopper Riegler degree any devices suitable therefore can be used, such as the "<nplcit id="ncit0001" npl-type="s"><text>Automatic Freeness Tester" supplied by Rycobel, Belgium</text></nplcit>.

0047Preferably the combination is fibrillated until the Schopper Riegler degree is increased by ≥ 6 °SR, more preferably ≥ 8 °SR, most preferably ≥ 10 °SR, especially ≥ 15 °SR.

0048In a preferred embodiment the combination of fibres and filler and/or pigment is fibrillated until a final Schopper-Riegler degree of the resulting suspension of ≥ 30, preferably ≥ 45 °SR, more preferably ≥ 50 °SR, particularly ≥ 60 °SR, e.g. ≥ 70 °SR, especially ≥ 80 °SR is reached.

0049In a special embodiment, it is however preferred that the final Schopper Riegler degree is ≤ 95 °SR.

0050The starting Schopper-Riegler degree may be from about 5 to about 90 °SR, preferably it is ≤ 10 °SR, preferably ≤ 25 °SR, more preferably ≤ 40 °SR, e.g. ≤ 60 or ≤ 75 °SR. It may also be greater than 80 °SR, if the Δ °SR resulting by the fibrillating step is ≥ 4 °SR.

0051Looking at the Schopper Riegler degree, it has also been found that the process according to the present invention is much more efficient than fibrillating fibre suspensions in the absence of pigments and/or fillers.

0052This can be seen by an increased °SR per passage. In order to optimize the fibrillation, the fibre suspension is usually processed by subjecting it to several passages through the fibrillation device.

0053In this respect, it can be observed that according to the process of the present invention, the °SR per passage is markedly higher than with fibre suspensions only.

0054This effect can be immediately observed and occurs until a certain number of passages, when no further increase of the °SR is achieved any more.

0055Thus, in a special embodiment, the Schopper Riegler degree per passage is higher for the process of the present invention than for fibre suspensions fibrillated in the absence of pigment and/or filler, until no further essential increase can be observed in both cases.

0056Furthermore, by the process of the present invention it is possible to obtain suspensions of nano-fibrillar cellulose, the Brookfield viscosity of which is lower than the Brookfield viscosity of a corresponding suspension of nano-fibrillar cellulose having been fibrillated in the absence of fillers and/or pigments.

0057The Brookfield viscosity generally can be measured with any conventional Brookfield viscometer using routine operations known by the person skilled in the art.

0058Fibrillating is carried out by means of any device useful therefore, as mentioned above. Preferably the device is selected from the group comprising ultra-fine friction grinders such as a Super Mass Colloider, refiners, and homogenizers.

0059In this respect, it can be observed that pulp combined with pigments and/or fillers for the process according to the present invention has a better runability, such that pulp, which can usually not be passed through a refiner, can be used if it is processed according to the present invention.

0060Another aspect of the present invention is the suspension of nano-fibrillar cellulose obtained by the processes according to the invention.

0061The nano-fibrillar cellulose suspensions according to the present invention can improve paper strength and may allow for an increase in filler load in uncoated freesheet papers.

0062Due to their mechanical strength properties the nano-fibrillar cellulose however is also advantageously used in applications such as in material composites, plastics, paints, rubber, concrete, ceramics, adhesives, food, or in wound-healing applications.

0063The figures described below and the examples and experiments serve to illustrate the present invention and should not restrict it in any way.

Description of the figures:

0064<figref idref="f0001">Figure 1</figref> shows the °SR / passage for pulp suspensions fibrillated with and without different natural ground calcium carbonates.

<u>Examples</u>

<u>1. Increase of °SR / passage using GCC</u>

0065For examining the development of the °SR/passage, eucalyptus pulp with a °SR of 25 was treated first in an ultra-fine friction grinder at 4 wt-% solids content with and without the addition of GCC. A similar experiment was run on a homogenizer with eucalyptus pulp at 1.5 wt-% solids content with and without GCC. <tables id="tabl0001" num="0001"><table frame="none"><title><b>Material</b></title><tgroup cols="2" colsep="0" rowsep="0"><colspec colnum="1" colname="col1" colwidth="14mm" /><colspec colnum="2" colname="col2" colwidth="152mm" /><tbody><row><entry>GCC:</entry><entry>Omyacarb 1-AV (solids content 100 wt% based on weight of fibres present) available from Omya AG. The weight median particle size <i>d</i>₅₀ = 1.7 µm measured by Sedigraph 5100.</entry></row><row><entry /><entry>Omyacarb 10-AV (solids content 100 wt-% based on weight of fibres present) available from Omya AG. The weight median particle size <i>d</i>₅₀ is 10.0 µm measured by Sedigraph 5100.</entry></row><row><entry>Pulp:</entry><entry>Eucalyptus pulp with 25 °SR.</entry></row></tbody></tgroup></table></tables>

Example 1 - Ultrafine friction grinder

0066For the comparative example eucalyptus pulp in the form of dry mats of 500 g per mat (700 x 1 000 x 1.5 mm) was used. 170 g pulp thereof was torn into pieces of 40 x 40 mm. 3 830 g tap water was added. The suspension was stirred in a 10 dm³ bucket at 2 000 rpm using a dissolver disk with a diameter of 70 mm. The suspension was stirred for at least 15 minutes at 2 000 rpm.

0067The suspension was then fibrillated with an ultra-fine friction grinder (Supermasscolloider from Masuko Sangyo Co. Ltd, Japan (Model MKCA 6-2). The grinding stones were silicon carbide with a grit class of 46 (grit size 297 - 420 µm). The gap between the grinding stones was chosen to be the dynamic 0-point as described in the manual delivered by the supplier. The speed of the rotating grinder was adjusted to be 1 200 rpm. The suspension was recirculated several times and samples were taken. The Schopper-Riegler degree (°SR) was measured according to the Zellcheming Merkblatt V/7/61 and standardized in ISO 5267/1.

0068For the inventive example eucalyptus pulp in the form of dry mats of 500 g per mat (700 x 1 000 x 1.5 mm) was used. 170 g pulp thereof was torn into pieces of 40 x 40 mm. 160 g Omyacarb 1-AV was added. 3 830 g tap water was added. The suspension was stirred in a 10 dm³ bucket at 2 000 rpm using a dissolver disk with a diameter of 70 mm. The suspension was stirred for at least 15 minutes at 2 000 rpm.

0069The suspension was then fibrillated with an ultra-fine friction grinder (Supermasscolloider from Masuko Sangyo Co. Ltd, Japan (Model MKCA 6-2). The grinding stones were silicon carbide with a grit class of 46 (grit size 297 - 420 µm). The gap between the grinding stones was chosen to be the dynamic 0-point as described in the manual delivered by the supplier. The speed of the rotating grinder was adjusted to be 1 200 rpm. The suspension was recirculated several times and samples were taken. The Schopper-Riegler degree (°SR) was measured according to the Zellcheming Merkblatt V/7/61 and standardized in ISO 5267/1.

0070For the inventive example eucalyptus pulp in the form of dry mats of 500 g per mat (700 x 1 000 x 1.5 mm) was used. 170 g pulp thereof was torn into pieces of 40 x 40 mm. 160 g Omyacarb 10-AV was added. 3 830 g tap water was added. The suspension was stirred in a 10 dm³ bucket at 2 000 rpm using a dissolver disk with a diameter of 70 mm. The suspension was stirred for at least 15 minutes at 2 000 rpm.

0071The suspension was then fibrillated with an ultra-fine friction grinder (Supermasscolloider from Masuko Sangyo Co. Ltd, Japan (Model MKCA 6-2). The grinding stones were silicon carbide with a grit class of 46 (grit size 297 - 420 µm). The gap between the grinding stones was chosen to be the dynamic 0-point as described in the manual delivered by the supplier. The speed of the rotating grinder was adjusted to be 1 200 rpm. The suspension was recirculated several times and samples were taken. The Schopper-Riegler degree (°SR) was measured according to the Zellcheming Merkblatt V/7/61 and standardized in ISO 5267/1.

Results

0072<figref idref="f0001">Figure 1</figref> shows the development of the °SR as a function of passages through the Supermasscolloider. It becomes apparent that the addition of GCC increases the efficiency of the device per passage.

Example 2 - Homogeniser

0073For the comparative example eucalyptus pulp in the form of dry mats of 500 g per mat (700 x 1 000 x 1.5 mm) was used. 47 g pulp thereof was torn into pieces of 40 x 40 mm. 2 953 g tap water was added. The suspension was stirred in a 5 dm³ bucket at 2 000 rpm using a dissolver disk with a diameter of 70 mm. The suspension was stirred for at least 15 minutes at 2 000 rpm.

0074This suspension was fed into the Homogeniser (GEA Niro Soavi NS2006L) but did not run through the machine.

0075For the inventive example eucalyptus pulp in the form of dry mats of 500 g per mat (700 x 1 000 x 1.5 mm) was used. 47 g pulp thereof was torn into pieces of 40 x 40 mm. 45 g Omyacarb 1-AV was added. 2 953 g tap water was added. The suspension was stirred in a 5 dm³ bucket at 2 000 rpm using a dissolver disk with a diameter of 70 mm. The suspension was stirred for at least 15 minutes at 2 000 rpm.

0076This suspension was fed into the Homogeniser (GEA Niro Soavi NS2006L). The flow through the homogensier was between 100 and 200 g min^-1 and the pressure was adjusted to be between 200 and 400 bar. The suspension was recirculated several times and samples were taken. The Schopper-Riegler degree (°SR) was measured according to the Zellcheming Merkblatt V/7/61 and standardized in ISO 5267/1.

Results

0077The comparative sample that contained no GCC could not be fed through the homogeniser. Only the GCC containing sample showed a good runnability. Schopper-Riegler values are reported in Table 1 after 5 and 10 passages through the homogeniser. <tables id="tabl0002" num="0002"><table frame="all"><title><u>Table 1:</u></title><tgroup cols="2"><colspec colnum="1" colname="col1" colwidth="23mm" /><colspec colnum="2" colname="col2" colwidth="14mm" /><thead><row><entry align="center" valign="top"><b>Passages</b></entry><entry align="center" valign="top"><b>°SR</b></entry></row></thead><tbody><row><entry align="center">0</entry><entry align="center">25</entry></row><row><entry align="center">5</entry><entry align="center">74</entry></row><row><entry align="center">10</entry><entry align="center">91</entry></row></tbody></tgroup></table></tables>

<u>2. Increase of °SR using PCC in a refiner</u>

Example 3 - Ultrafine PCC

0078<tables id="tabl0003" num="0003"><table frame="none"><title><b>Material</b></title><tgroup cols="2" colsep="0" rowsep="0"><colspec colnum="1" colname="col1" colwidth="13mm" /><colspec colnum="2" colname="col2" colwidth="153mm" /><tbody><row><entry>PCC:</entry><entry>Ultrafine prismatic PCC. The weight median particle size <i>d</i>₅₀ = 1.14 µm measured by Sedigraph 5100 (100 wt-% of particles have a diameter < 2 µm; 27 wt-% of particles have a diameter < 1 um).</entry></row><row><entry /><entry>This PCC was provided in the form of an aqueous suspension having a solids content of 7.9 wt-%.</entry></row><row><entry>Pulp:</entry><entry>Longfiber bleached kraft pulp with 16 °SR.</entry></row></tbody></tgroup></table></tables>

0079An aqueous suspension was formed of the above carbonate and pulp such that this suspension had a solids content of approximately 4 wt-% and a carbonate : pulp weight ratio of 29 : 71.

0080Approximately 12.5 dm³ of this suspension were circulated during a period of 9 minutes through a Escher Wyss R 1 L Labor-Refiner under 5.4 kW.

0081A Schopper-Riegler (°SR) of the obtained suspension of 92 °SR was measured according to the Zellcheming Merkblatt V/7/61 and standardized in ISO 5267/1.

Example 4 - coarse PCC

<u>a) Suspension according to the invention</u>

0082<tables id="tabl0004" num="0004"><table frame="none"><title><b>Material</b></title><tgroup cols="2" colsep="0" rowsep="0"><colspec colnum="1" colname="col1" colwidth="13mm" /><colspec colnum="2" colname="col2" colwidth="153mm" /><tbody><row><entry>PCC:</entry><entry>Scalenohedral PCC. The weight median particle size <i>d</i>₅₀ = 3.27 µm measured by Sedigraph 5100 (11 wt-% of particles have a diameter &lt; 2 µm; 4 wt-% of particles have a diameter &lt; 1 um). This PCC was provided in the form of an aqueous suspension having a solids content of 15.8 %.</entry></row><row><entry>Pulp:</entry><entry>Eucalyptus with 38 °SR.</entry></row></tbody></tgroup></table></tables>

0083An aqueous suspension was formed of the above carbonate and pulp such that this suspension had a solids content of approximately 9.8 wt-% and a carbonate : pulp weight ratio of 75 : 25. This suspension presented a 18 °SR.

0084Approximately 38 m³ of this suspension was circulated during a period of 17.5 hours through a Metso Refiner RF-0 under 92 kW at a flow rate of 63 m³/hour.

0085A Schopper-Riegler (°SR) of the obtained suspension of 73 °SR was measured according to the Zellcheming Merkblatt V/7/61 and standardized in ISO 5267/1.

<u>b) Comparative Suspension</u>

0086<tables id="tabl0005" num="0005"><table frame="none"><title><b>Material</b></title><tgroup cols="2" colsep="0" rowsep="0"><colspec colnum="1" colname="col1" colwidth="13mm" /><colspec colnum="2" colname="col2" colwidth="153mm" /><tbody><row><entry>PCC:</entry><entry>Scalenohedral PCC. The weight median particle size <i>d</i>₅₀ = 3.27 µm measured by Sedigraph 5100 (11 wt-% of particles have a diameter &lt; 2 µm; 4 wt-% of particles have a diameter &lt; 1 um). This PCC was provided in the form of an aqueous suspension having a solids content of 15.8 %.</entry></row><row><entry>Pulp:</entry><entry>Eucalyptus with 38 °SR.</entry></row></tbody></tgroup></table></tables>

0087An aqueous suspension was formed of the above pulp such that this suspension had a solids content of approximately 4.5 wt-%.

0088Approximately 20 m³ of this suspension was circulated during a period of 17.5 hours through a Metso Refiner RF-0 under 92 kW at a flow rate of 63 m³/hour.

0089A Schopper-Riegler (°SR) of the obtained suspension of 65 °SR was measured according to the Zellcheming Merkblatt V/7/61 and standardized in ISO 5267/1.

0090To this suspension, the above scalenohedral PCC was added in an amount so as to obtain a carbonate : pulp weight ratio of 75 : 25. A Schopper-Riegler (°SR) of the obtained suspension of 25°SR was measured according to the Zellcheming Merkblatt V/7/61 and standardized in ISO 5267/1.

0091This clearly shows that the presence of calcium carbonate during the fibrillation step is essential for obtaining a high Schopper Riegler degree, i.e. an efficient fibrillation of the cellulose fibres.