FI57149B - FOERFARANDE FOER FRAMSTAELLNING AV EN KOMPOSITION LAEMPAD SOM FYLLMEDEL I EN PAPPERSMASSA - Google Patents

Description

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Great Britain-Great Britain (GB) 17371 / 7U

(71) W.R. Grace & Co., Grace Plaza, lllU Avenue of the Americas, New York, N.Y. IOO36, USA (US) (72) Lodovico Codolini, Milan, Italy-Italy (lT) (7U) Berggren Oy Ab (5U) Method for preparing a mixture suitable as a filler for cellulose pulp

The present invention relates to a process for preparing a composition suitable for addition to cellulose pulp and to its use as a filler in the manufacture of paper and paper-like nonwoven fibrous web.

When making paper, it is common to include a certain amount of finely divided inorganic filler in the fibrous web. This reduces the amount of more expensive cellulose or other fibers and provides certain opacity, color and surface properties. One widely used filler is clay. Other fillers include e.g. talc and "calcium carbonate. Calcium carbonate is perhaps the cheapest filler.

The amount of filler that can be incorporated into the paper is limited by the fact that the fillers reduce the strength of the paper.

We have now developed a process for the preparation of certain coated particles of inorganic substances, and in particular those having a paper filler at their core, in particular calcium carbonate. Coated particles have a lower tendency to weaken the bonds of paper fibers than the core alone. Thus, we have found that, for example, a clay filler used in an amount of about 8-10% by weight based on the dry matter of the paper can be replaced by about 2,5149% by weight based on the dry weight of the paper with resin coated calcium carbonate, reducing paper strength little or not at all. In other words, the proportion of filler can be doubled, and as a result, the proportion of cellulosic fibers can be reduced.

The present invention relates to a process for preparing a mixture suitable as a paper filler, in which particles of an inorganic substance are coated with a synthetic polymeric resin, characterized in that an aqueous suspension of particles with a positive zeta potential is mixed with an inorganic latex the negative-positive charge balance of the particles and the latex resin when mixed is such that virtually all of the resin particles bind to the surface of the inorganic particles.

The particles of the inorganic substance, hereinafter referred to as the "filler", must form a suspension in which they have a positive zeta potential. (The term "zeta potential" is used in the sense used in colloid chemistry without necessarily implying that the filler suspension is colloid). This usually requires pre-treatment of the particles. Such inorganic filler treatments are known in the art, e.g. calcium carbonate acquires a positive zeta potential when mixed with a small amount of positively charged starch.

The anionic (negative charge) properties of the synthetic resin polymer and the cationic (positive zeta potential) properties of the inorganic filler are balanced upon mixing so that the resin particles adhere strongly to the filler particles when the anionic latex is mixed with the filler suspension. Preferably, the charge balance is such that the zeta potential of the resulting coated particles is substantially zero.

The most preferred inorganic filler is calcium carbonate, which is preferably ground to a particle size of less than about 30. Other such inorganic fillers that may be used in the composition of the invention include calcium silicate, talc, clay, diatomaceous earth, calcium sulfate, titanium dioxide, silica, "Syloid White" (a mixture of calcium sulfate and calcium phosphate), or mixtures of some of the foregoing.

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One example of a cationic calcium carbonate that can be used in the composition of the invention is that sold under the Italian trade name Sileä under the trademark "MICOCELL". This calcium carbonate has the particle size analysis (Coulter calculation method) shown in the accompanying drawing.

The latex is anionic in the sense that the resin particles have a negative charge. As is well known, this charge can be produced by a surfactant, or an emulsifier that is negatively charged, and thus impart a negative charge to the resin particles of the latex, or it can be imparted by the anionic chemical group of the polymer molecule. The term "latex" has its usual meaning as an aqueous emulsion or suspension of resin.

As is well known, latexes are generally produced with emulsion polymer bases of suitable monomers in the presence of a surfactant, with the surfactant being adsorbed on the resin particles. Surfactants or emulsifiers in the latex are, of course, selected to be compatible with the filler. The most preferred resins are those which, when dried from latex, do not form a cohesive film at room temperature or form an inelastic film, i. one having an elongation of not more than 200%, and preferably not more than 20%. The most preferred polymer is a styrene-butadiene copolymer containing at least 60% by weight of 5? styrene-derived units. A highly preferred polymer is that sold under the trademark "POL-X-1110".

Other such anionic synthetic polymeric resins that can be used in the composition of the invention are anionic latexes of polyvinyl acetate or acrylic copolymers. The resin latex does not coagulate when mixed with the filler suspension and the formed resin coating strongly adheres to the filler particles so that it does not leave during screening, grinding or purification.

The inventive relationship between the resin and the filler is determined by the requirement that substantially all of the resin particles become bound to the filler particles. The latex desirably provides 1 to 20 parts by weight of resin per 100 parts of filler in suspension (based on dry weight).

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The filler mixture according to the invention can be added to the pulp in a grinder, disperser or some other pulp processing step before the headbox. The filler mixture can be combined with the pulp as a liquid suspension.

The pulp to which the filler mixture is combined is then formed into a nonwoven fibrous web by conventional papermaking methods, i. the mass is formed into a wet web from which water is allowed to drain and which is dried. The amount of filler mixture in the pulp may be sufficient to provide up to 50% by weight of calcium carbonate (or other inorganic fillers) based on the total weight of solids. The filler mixture prepared by the method of the present invention may replace, for example, up to 12% by weight of? cellulose fibers, calculated on the dry weight. The proportion of substituted cellulose fibers could be even higher, e.g. it could be as high as 50 ?, depending on the type of fiber and the properties of the paper to be produced. The pulp filled with the mixture prepared according to the invention is very valuable as the top coating of multilayer cardboard. For this purpose, it is preferred to use an amount of the mixture which provides about 10-12 g of filler per m of cardboard. When used as the top coat, the filler provides the desired opacity and also provides good cardboard formation and dimensional stability without loss of strength.

The compositions prepared by the process of the present invention are useful as additives for cellulosic pulp in the manufacture of paper or paper-like nonwoven fibrous webs. The word "paper" is used herein in a broad and general sense to define a field of application and includes all conventional paper products, including both paper and paperboard, using conventional paper fillers such as calcium carbonate.

In papermaking, the mixture prepared by the process of the invention is mixed into a cellulose pulp, the pulp is applied as a wet web and the web is dried. It is practical to add the mixture as an aqueous suspension from which it is recovered by following the above-mentioned preparation steps.

Since the main object of the invention is to develop a process for preparing a mixture which allows a large amount of cellulosic pulp to be replaced by a filler, a secondary and random advantage of the present invention is that resin / alum adhesive can be added to the paper without adverse chemical reaction with filler particle cores.

The invention and its advantages are described below by means of the examples below. Parts and percentages are by weight unless otherwise indicated in the text. The calcium carbonate used in these examples was "Micocell" and contained 99% CaCO 3 by analysis. The above-mentioned styrene-butadiene copolymer "Darex POL-X-1110" was used as the resin. In all examples, the filler prepared by the method of the invention was used. Paper or paperboard containing a filler composition prepared by the method of the present invention is referred to as "paper or paperboard of the present invention" for short.

Example I

A typical paper made by the method of the invention had the following composition:

50 parts of cellulose fibers

Cationic calcium carbonate ("Micocell") up to 48 parts Anionic polymer 1-10 parts

A large amount of this filler can be used in the paper without substantially impairing the mechanical properties of the paper. The following properties of the paper are then improved: smoothness, uniformity, dimensional stability, opacity and printability.

In one experiment using a filler according to the invention, paper was prepared using 8-10% conventional clay filler and the strength of this conventionally filled paper was 6500 m. To test the filler according to the invention, 8-10% clay was replaced with 22% filler mixture according to the invention. . Paper production was then continued in the same manner as above, and the strength of the paper containing the filler according to the invention was only slightly lower than that of the paper containing 8-10% clay, namely 5600 m.

Example II

To 1000 parts of an aqueous suspension containing 40% of cationic starch-treated calcium carbonate is added 5 parts of a styrene-butadiene copolymer latex binder according to the invention containing 8 parts of a dry polymer (corresponding to 2? Solid polymeric binder).

The filler mixture was added to a cellulosic pulp containing 1400 parts (calculated on a dry basis) of cellulosic material ground to a degree of grinding of 40 ° S.R. After mixing for a few minutes, paper was prepared using conventional equipment.

The following results were obtained compared to conventional 9? Clay filled paper:

Clay-filled paper according to the invention paper-filled paper g / m paper weight 60 58

Strength - the effective length that a paper can support without breaking 4200 m 4600 m

Elongation% 3.7 M

Theoretical ash content? 12 22.2

Tuhkapitoisuus-? 9 l8

Whiteness 80 83

Opacity 73 77

Example III

1000 parts of an aqueous suspension containing 40? cationic calcium carbonate, 89 parts of a styrene-butadiene copolymer binder containing 20 parts of dry polymer (corresponding to 5? solid polymeric binder based on calcium carbonate) are added.

The filler mixture was added to 700 parts (by dry weight) of the cellulose pulp suspension as described in Example II.

The following results were obtained compared to the usual 9? on paper containing clay as a filler:

Clay filled paper according to the invention paper filled g / m ^ 60 60

Strength 4200 m 3550 m

Stretch-? 3.7 3.2

Theoretical ash content? 12 36.4

Tuhkamäärä-? 9 33.3

Whiteness 80 84

Opacity 73 83 7 57149

Example IV

The filler mixture was prepared as described in Example III (i.e.

5% polymeric solid binder (based on cationic calcium carbonate): 20 parts (calculated as solid dry matter) of the filler mixture were added to a pulp containing 40 parts (calculated on a dry basis) of sulphite cellulose and 40 parts (calculated on a dry basis) of white waste paper ground to 65 ° C. SR

The following results were obtained compared to the usual 6 5? : 11a clay for filled paper:

Clay-filled paper Filled paper according to the invention g / m2 50 50

Strength 4000 m 3Ö00 m

Elongation-5? 2.4 2.7

Ash # 6.0 16.0

Example V

To 1000 parts of an aqueous suspension containing 40% of cationic calcium carbonate is added 71 parts of styrene-butadiene copolymer latex containing 16 parts of dry polymer (corresponding to 4% of solid polymeric binder based on calcium carbonate).

The filler mixture was added to 2400 parts (calculated on a dry basis) of cellulose pulp and the final mixture was used to make the top layer of multilayer cardboard.

The following results were obtained compared to conventional cardboard with the top layer filled with clay and talc: 8 57149

Clay and talc CaCOp + Jj% latek-filled cardboard and filled cardboard Coated top layer - weight-g / m2 72 60 Uncoated top layer - weight-g / m2 61.3 49.2

Clay coating - weight-g / m2 10.7 10.8

Total weight-g / m 350 350 Coated top layer - ash content # 20.5 24

Total ash- # 14 17

Moisture- # 5.6 6.2

Whiteness 80.5 82.0

Burst strength kg / cm 6.7 6.0 Stiffness-Taber (machine direction) 195 200 Stiffness-Taber (transverse direction) 100 100

Dennison Wax Test 7 7

Delamination 6.3 6.8

From the above results, it can be seen that the present invention provides a paperboard having a higher whiteness on the top layer and similar mechanical properties than ordinary paperboard, using a much higher ratio of clay to cardboard fibers than is commonly used. The weight of the filler used in these two experiments was essentially the same, but the amount of fiber mass required for catfish decreased from 6.1 to 49.2 g / m when using the method according to the invention.

It can be easily seen that although the weight of the expensive top layer has been significantly reduced, the whiteness has increased and the mechanical properties have remained the same.

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Claims (8)

9 57149 A process for preparing a mixture suitable for filling cellulose pulp, in which particles of inorganic material are coated with a synthetic polymeric resin, characterized in that an aqueous suspension of particles having a positive zeta potential is mixed with an anionic latex of the resin so that -positive with a charge equilibrium when mixed such that virtually all of the resin particles bind to the surface of the inorganic particles. Method according to Claim 1, characterized in that the negative-positive charge balance is such that the resulting coated particles have a zeta potential which is substantially zero. Process according to Claim 1 or 2, characterized in that the particles of the inorganic substance are calcium carbonate particles. which have been treated to have a positive zeta potential in aqueous suspension. Process according to Claim 1, characterized in that the particles of the inorganic substance are calcium carbonate particles which have been treated with cationic starch. Method according to one of the preceding claims, characterized in that the resin is such that its film is relatively inelastic and that its maximum elongation does not exceed 200%. Process according to claim 5, characterized in that said elongation of the resin does not exceed 20%. Process according to Claim 5 or 6, characterized in that the resin is a styrene-butadiene copolymer. Process according to Claim 7, characterized in that at least 60 of the polymer units of the weight-5S copolymer are derived from styrene.