COARSE PIGMENT ADDITION AND HIGH SOLIDS SLURRIES FOR HIGHER COATING COLOUR CONCENTRATIONS

Abstract

The present invention relates to an aqueous slurry comprising natural ground calcium carbonate which has a percentage P.sub.5 by weight of particles having a diameter of less than 5.0 m of from 98.5% to 90%, a percentage P.sub.2 by weight of particles having a diameter of less than 2.0 m of from 96% to 80%, wherein the ratio of P.sub.2/P.sub.5 is from 0.98 to 0.85, and wherein the slurry has a solids content of more than 78 wt %.

Claims

1. A process for the preparation of an aqueous slurry having a solids content of more than 78 wt %, comprising the steps of: (a) providing a first fraction F1 of a mineral material having a percentage P.sub.5(F1) by weight of particles having a diameter of less than 5.0 m of from 75% to 85% and a median particle diameter d.sub.50 of from 2.0 to 3.0 m, (b) providing a second fraction F2 of a mineral material having a percentage P.sub.2(F2) by weight of particles having a diameter of less than 2.0 m of from 85% to 100% and a median particle diameter d.sub.50 of from 0.3 to 0.65 m, and (c) dispersing the first fraction F1 and the second fraction F1 in water.

2. The process according to claim 1, wherein the weight ratio of the first fraction F1 to the second fraction F2 is from 5/95 to 20/80.

3. The process according to claim 1, wherein the first fraction F1 is provided as a dry mineral material.

4. The process according to claim 1, wherein the second fraction F2 is provided as an aqueous slurry.

5. The process according to claim 1, wherein a dispersing agent is added in step (c).

6. The process according to claim 1, wherein the mineral material is calcium carbonate, clay, or a mixture thereof.

7. The process according to claim 1, wherein the mineral matter is natural ground calcium carbonate.

8. The process according to claim 1, wherein the mineral matter is natural ground calcium carbonate obtained from marble, limestone, chalk, or any mixture thereof.

Description

EXAMPLES

I. Measuring Methods

[0066] All the parameters described above for defining the product and the process of the present invention were determined by using the following measuring methods.

1. Medium Particle Size d.SUB.50 .by Weight Percentage of Particles Having a Diameter of Less Than 5.0 m, Less Than 2.0 m, and Less Than 1.0 m, Respectively.

[0067] Throughout the present invention, d.sub.50 is the medium particle size by weight, i.e. representing the particle size so that 50 wt % of the particles are coarser or finer.

[0068] Particle size was measured according to the sedimentation method. The sedimentation method is an analysis of sedimentation behaviour in a gravimetric field. The measurement is made with a Sedigraph 5100 of Micromeritics Instrument Corporation. The method and the instrument are known to the skilled person and are commonly used to determine grain size of fillers and pigments. The measurement is carried out in an aqueous solution of 0.1 wt % Na.sub.4P.sub.2O.sub.7. The samples were dispersed using a high speed stirrer and supersonic.

2. Specific Surface Area (BET)

[0069] The specific surface area was measured using nitrogen and the BET method according to ISO 9277.

3. Solids Content

[0070] Solids content of the slurry was determined as follows: Solids content is determined by using special scales provided of drying (IR lamp or microwave). The final solids is obtained when a constant weight of the sample is reached (no more water to be removed).

4. Viscosity (Brookfield)

[0071] Brookfield viscosity of the slurry was determined as follows: Slurry viscosity is evaluated by a Brookfiled Viscometer at 100 rpm and 20 C.

5. Sheet Gloss 75 Tappi

[0072] Sheet gloss 75 Tappi was determined according to ISO 8254-1.

II. Preparation of Mineral Slurries

Example 1

[0073] A first natural dry ground calcium carbonate GCC (GCC 1) was provided which had the following properties:

[0074] d.sub.50: 2.5 m

[0075] 81.4%<5.0 m

[0076] 42.5%<2.0 m

[0077] BET surface area: 3.9 m.sup.2/g

[0078] A second natural calcium carbonate GCC (GCC 2) wet ground at 78 wt % solids content in the presence of a polyacrylate dispersant was provided which had the following properties:

[0079] d.sub.50: 0.55 m

[0080] 100%<5.0 m

[0081] 96.3%<2.0 m

[0082] 78%<1.0 m

[0083] BET surface area: 16 m.sup.2/g

[0084] GCC1 and GCC2 were mixed in a weight ratio GCC1/GCC2 of 5/95.

[0085] The final GCC material had the following properties:

[0086] 98.1%<5.0 m

[0087] 93.3%<2.0 m

[0088] 74.3%<1.0 m

[0089] d.sub.50: 0.58 m

[0090] The aqueous slurry AS1 had a solids content adjusted to 80 wt %. Brookfield viscosity: 440 mPas

Example 2

[0091] In Example 2, the same GCC1 and GCC2 as already described above in Example 1 were used.

[0092] GCC1 and GCC2 were mixed in a weight ratio GCC1/GCC2 of 8/92.

[0093] The final GCC material had the following properties:

[0094] 93.4%<5.0 m

[0095] 91.8%<2.0 m

[0096] 72.9%<1.0 m

[0097] d.sub.50: 0.59 m

[0098] The aqueous slurry AS2 had a solids content adjusted to 80.3 wt %. Brookfield viscosity: 400 mPas.

Example 3

[0099] In Example 3, the same GCC1 and GCC2 as already described above in Example 1 were used.

[0100] GCC1 and GCC2 were mixed in a weight ratio GCC1/GCC2 of 10/90.

[0101] The final GCC material had the following properties:

[0102] 96.6%<5.0 m

[0103] 90.1%<2.0 m

[0104] 71.2%<1.0 m

[0105] d.sub.50: 0.60 m

[0106] The aqueous slurry AS3 had a solids content adjusted to 80.4 wt %. Brookfield viscosity: 380 mPas.

Example 4

[0107] In Example 4, the same GCC1 and GCC2 as already described above in Example 1 were used.

[0108] GCC1 and GCC2 were mixed in a weight ratio GCC1/GCC2 of 16/84.

[0109] The final GCC material had the following properties:

[0110] 94.8%<5.0 m

[0111] 88.5%<2.0 m

[0112] 68.1%<1.0 m

[0113] d.sub.50: 0.63 m

[0114] The aqueous slurry AS4 had a solids content adjusted to 80.2 wt %. Brookfield viscosity: 420 mPas.

Example 5

[0115] In Example 5, the same GCC1 and GCC2 as already described above in Example 1 were used.

[0116] GCC1 and GCC2 were mixed in a weight ratio GCC1/GCC2 of 20/80.

[0117] The final GCC material had the following properties:

[0118] 93.7%<5.0 m

[0119] 83.4%<2.0 m

[0120] 65.1%<1.0 m

[0121] d.sub.50: 0.66 m

[0122] The aqueous slurry AS5 had a solids content adjusted to 80.8 wt %. Brookfield viscosity: 430 mPas.

Comparative Example 1

[0123] A natural calcium carbonate GCC6 wet ground at 78 wt % of solids content in the presence of a polyacrylate dispersant was provided which had the following properties:

[0124] 100%<5.0 m

[0125] 95%<2.0 m

[0126] 80%<1.0 m

[0127] d.sub.50: 0.55 m

[0128] Thus, in contrast to Examples 1 to 5, no very coarse particles (i.e. particles having a diameter of more than 5.0 m) were present in the GCC material used in Comparative Example 1.

[0129] The aqueous slurry AS6 had a solids content of 78 wt %. The slurry had a Brookfield viscosity of about 300 mPas. However, when the solids content was increased to about 80 wt %, there was a drastic increase of viscosity and the slurry could not be pumped anymore, i.e. could not be processed anymore.

III. Use of Mineral Slurries for Paper Coating

[0130] Using the inventive carbonate slurries AS2 and AS4 described above, paper coating compositions C2 and C4 were prepared. From slurry AS6 of Comparative Example 1, paper coating composition C6 was prepared.

[0131] In addition to the GCC component, each paper coating composition also contained a styrene-butadiene SB latex binder, a polyvinyl acetate PVA dispersing agent and a synthetic thickener.

[0132] For each sample, the weight ratio of GCC, SB latex, PVA, and thickener was the same, i.e. 100 parts GCC, 9 parts SB latex, 0.4 parts PVA, and 0.1 parts thickener.

[0133] For each paper coating composition C2, C4, and C6, the maximum solids content was determined by running pilot coater trials; needed dilutions were done in every trial until an absolutely clean blade and acceptable blade load could be reached (max blade deflection (load) 8 mm, stiff blade modus). The results are shown in Table 1.

[0134] The paper coating compositions C2, C4, and C6 were applied onto a paper and sheet gloss 75 Tappi was determined for each paper. The results are summarized in Table 1.

TABLE-US-00001 TABLE 1 Maximum solids content of coating color and sheet gloss of paper coating compositions Paper coating Maximum solids Sheet gloss 75 composition content (wt %) Tappi (%) C4 (prepared by 71.2 77/76 using slurry AS4) C2 (prepared by 70.4 78/77 using slurry AS2) C6 (prepared by 69.2 78/79 using slurry S6)