Method for forming a composite comprising MFC and a composite produced by the method

Abstract

The invention provides a method for the production of a composite comprising microfibrillated cellulose (MFC) and precipitated calcium carbonate. The method is characterized in that MFC is added to a suspension of calcium hydroxide during carbonation, whereby calcium carbonate is precipitated onto fibers or fibrils of the MFC in a controlled manner. By adding microfibrillated cellulose to the calcium suspension during the carbonation, the brightness and the strength of the MFC/PCC-composite is enhanced. Moreover, the inventive method facilitates the distribution of calcium dioxide and MFC in the suspension and thus gives rise to a more homogenous product.

Claims

1. A method of producing a composite, said method comprising the steps of; providing a suspension comprising calcium hydroxide, and, performing carbonation of said calcium hydroxide to form precipitated calcium carbonate (PCC), wherein microfibrillated cellulose (MFC) is added to said suspension during said carbonation of calcium hydroxide at a time when at least 25%, but less than all, of the calcium hydroxide has been reacted with carbon dioxide to form calcium carbonate onto fibers or fibrils of the MFC.

2. A method of producing a composite according to claim 1, wherein the carbonation of said calcium hydroxide is performed by the addition of carbon dioxide to the suspension.

3. A method of producing a composite according to claim 1, wherein MFC is added to the suspension at a time when between 50% and 95% of the calcium hydroxide has been reacted with carbon dioxide to form calcium carbonate.

4. A method of producing a composite according to claim 1, wherein said MFC is added to the suspension at a time when between 75-95% of the calcium hydroxide has been reacted with carbon dioxide to form calcium carbonate.

5. A method of producing a composite according to claim 1, wherein a mixture of MFC and calcium carbonate is added to said suspension during said carbonation of calcium hydroxide.

6. A composite produced by the method according to claim 1.

7. A method of producing paper or paperboard comprising the steps of; providing a fiber-containing furnish; adding the composite according to claim 6 to said furnish; forming and dewatering the fiber-containing furnish.

8. A paper or paperboard comprising the composite according to claim 6.

9. The paper according to claim 8, wherein the paper is fine paper and a total filler content of the paper is from 25-35 wt % based on a total solid content of said paper.

10. The paper according to claim 8, wherein the paper is newsprint paper and a total filler content of the paper is from 10-15 wt % based on a total solid content of said paper.

11. The paper according to claim 8, wherein the paper is supercalendered (SC) paper and a total filler content of the paper is at least 39 wt % based on a total solid content of said paper.

12. The paper according to claim 8, wherein the paper is supercalendered (SC) paper and a total filler content of the paper is between 39-45 wt % based on a total solid content of said paper.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows the SEM images of unwashed samples C REF (0%, upper left), C50 (50%, upper right), C70 (70%, lower left) and REF ALL (lower right), all with 1000 magnitude.

(2) FIG. 2 shows the SEM images of washed samples C REF (0%, upper left), C50 (50%, upper right), C70 (70%, lower left) and REF ALL (lower right), all with 1000 magnitude.

EXAMPLE

(3) In order to evaluate the method and the composite of the invention, a test series was performed in which SEM images of samples made according to the invention (samples C50 and C70) were analyzed and compared to SEM images of samples made by addition of MFC before (C REF) and after (REF ALL) complete precipitation of PCC.

(4) Addition of microfibrillated cellulose (MFC) before, during and after PCC precipitation (Ca(OH).sub.2+CO.sub.2.fwdarw.CaCO.sub.3) were tested using batch precipitation and by adding MFC to the reactor at different points in time. The specific point in time of addition was established by deremining the total reaction time by precipitating PCC at the same conditions (without fiber addition) and thereafter calculate the desired point in time of addition as a percentage of the total reaction time. The endpoint of the reaction was determined by pH measurement (sharp decrease in pH value).

(5) Four samples (Series C) were made using different point in times of addition, wherein MFC was added at a point in time when a pre-decided percentatge of the total reaction time had passed (table 1)

(6) TABLE-US-00001 TABLE 1 Sample Point of MFC addition C REF As a pre-addition to the milk of lime (Ca(OH).sub.2), before carbonation was started (at 0% of the total reaction time) C50 At 50% of the total reaction time C70 At 70% of the total reaction time REF ALL As a post addition to the readymade PCC (at 100% of the total reaction time)

(7) Scheme of experiment was as follows: 1) 14.8 g laboratory made Ca(OH).sub.2 as lime milk was diluted into tap water to form 900 g batch (resulting ca. 1.6% Ca(OH).sub.2) Ca(OH).sub.2 solids dictated by the fiber concentration desired (linked via targeted ash content) 2) Lime milk batch was heated to 70-80 C. temperature Morphology control 3) CO.sub.2 (g) feed and timing of reaction was started Low stirring rate (500 rpm) to avoid air intake from vortex 4) At the chosen time of addition, 20 g of MFC was added as wet (1.46%), resulting fiber consistency of ca. 0.9% in the batch. Heating was switched off and temperature was allowed to change freely Stirring rate increased to 1600 rpm to mix properly Low fiber consistency used to allow more exact dosing 5) CO.sub.2 (g) feed was continued until a sharp drop in pH value was determined, after which CO.sub.2 feed was stopped and the reactor vessel emptied 6) All the samples were washed in the same way: pulp sample with 0.5 g solids was diluted to 500 ml (1 g/l). The samples was washed using 1 liter of water (i.e. 2 l/g of sample). The washing was done under normal pressure with stirring (stirred cell). Filter media was metal wire with 25 m openings. After washing, 15 ml of the sample was taken for freeze drying. The rest of the sample was centrifuged (to increase sample concentration) and dried in oven (105 C.). Ash content was measured at 525+25 C. according to TAPPI standard T 211 om-02.

(8) TABLE-US-00002 TABLE 2 Conditions Conditions Measured Measured before after Fiber ash, before ash, after reaction reaction consist. washing washing T T Sample [%] [%] [%] pH [ C.] pH [ C.] C REF 0.9 47.3 36.1 11.1 48.9 6.9 55.0 C 50 0.9 47.1 29.6 10.6 73.9 6.2 48.8 C 70 0.9 46.8 25.6 10.7 72.4 6.4 51.1 REF ALL 0.9 46.8 21.9 10.4 71.6

(9) CO2 used was >95% pure and all samples were made under atmospheric pressure. Targeted PCC ash content before the washing step was 50%.

(10) TABLE-US-00003 TABLE 3 PCC (g) in PCC (g) in 1 g sample, 1 g sample, PCC loss PCC loss Sample unwashed washed (g) (%) CREF 0.898 0.565 0.333 37.1 C50 0.890 0.420 0.470 52.8 C70 0.880 0.344 0.536 60.9 REF ALL 0.880 0.280 0.599 68.1

(11) As can be seen in the SEM image of C REF (FIG. 1 and FIG. 2, upper left), the fibers are evenly covered with small fine PCC particles when the MFC is added prior to carbonation. The particles are bonded to the fibers also after washing (cf. FIG. 1 and FIG. 2, upper left). This gives rise to a weak bonding ability of the MFC, since PCC interfere between MFC/MFC bonds.

(12) The addition of MFC to the PCC after carbonation is completed (REF ALL) results in larger PCC particles which are not bound to the fibers (cf. FIG. 1 and FIG. 2, lower right). After washing, the loose PCC particles are removed, showing that only a few PCC particles are bound to the fiber surface (FIG. 2, lower right).

(13) The SEM images of the composites made according to the invention (FIG. 1 and FIG. 2, upper right 50%, and lower left 70%) shows distinct PCC particles bonded to the fibers in a controlled manner, without completely covering the surface of the fibers, which improves the bonding potential and the brightness of the end product.

(14) Table 3 shows the PCC loss during washing of the samples, which indicates how much of the PCC that was loose and not attached to the fibers in respective sample. As can be seen in table 3, the PCC loss is highest for the REF ALL sample (when the MFC was mixed with readymade PCC), while it is lowest for the C REF (when the MFC was added prior to carbonation).

(15) Thus, the result shows that the coverage and morphology of the PCC on the microfibers can be controlled and optimized by the inventive concept.