GB2083014A - Zeolite-containing coating compositions for cellulosic materials - Google Patents

Abstract

An aqueous dispersion containing a dispersant and as pigment, up to 100% as dry matter of zeolite of the general formula: Na2O. Al2O3. 2SiO2 . xH2O in which x is a number from 1 to 8, having a particle size less than 5 mu is used as an aqueous coating composition for a fibrous cellulosic substrate, the composition having a dry matter content equal to or greater than 40%.

Description

SPECIFICATION Improvements in and relating to coating compositions for cellulosic materials This invention concerns improvements in and relating to coating compositions for cellulosic materials.

In order to improve the surface appearance of papers or cardboards and especially their printability, the paper manufacturer often coats the paper or cardboard. This consists in depositing a coating on one or both sides of the paper or card board by means of a suitable device, which coating consists of one or more pigments and one or more binders as well as a very small quantity of a certain number of auxiliary products.

Among the pigments most used in the coating of paper kaolin and natural or precipitated calcium cabonate are found well at the head. These are followed, but in distinctly lower proportions by satin white (calcium sulpho-aluminate), blanc fixe (barium sulphate), titanium oxide, hydrated alumina, calcium sulphate dihydrate and lithopone.

The known coating colour is present as an aqueous suspension, with a content of dry matter (dry solid content) generally between 20% and 60%, in which the pigments represent 70% to 95% of dry matter or solids, the binders 4% to 28% and the residue consisting of auxiliary products.

Generally speaking, it may be said that the pigment of the coating colour conditions more especially the optical properties and the printability of the paper with the binder serving to ensure the adhesion of the particles of pigment to the paper and their cohesion with one another, while having a preponderant action on the rheology of the coating bath. It is quite evident that the nature of the pigment has repercussions on the binding of the coating with the support and that the binder also acts on the printability of the papers and cardboards coated.

Up to now it has been considered to be impossible to incorporate zeolites in the coating colour owing to the thickening which they might cause.

We have now found that against all expectations, this disadvantage is not shown with a certain type of zeolite.

The present invention therefore relates to the use in the preparation of coating compositions of zeolites of the general formula: Na20. Awl203 . 2Si02 . xH2O (I) in which x may vary from 1 to 8, more especially from 4 to 5. It also relates to dispersions of these zeolites which may be used in the coating composition and to the coating compositions.

The zeolites are preferably added to the coating composition in the form of an aqueous dispersion or suspension which has a dry material or solids content of from 20% to 65% by weight and viscosity, measured on a Brookfield viscometer at 50 r.p.m., of from 20 cps to 1 20 cps. The dispersion may also contain from 0.2 to 0.8 parts by weight of a dispersant.

The coating composition of the present invention contains pigment, binder, auxiliary products such as dispersant, and water, and has a dry matter or solids content equal to or greater than 40%.

These zeolites, which may be used as pigments for coating, can be incorporated in the coating bath in all proportions without causing any rheological upset. Their presence brings with it an improvement in certain important properties of coated papers and cardboards.

The invention is illustrated by the following Examples. In the description and claims all parts and percentages are by weight unless otherwise specified.

EXAMPLE 1 A sodium silicoaluminate of formula (I) is used which contains 20% of water and of which the mean characteristics are established as follows: Specific surface 900-1100 m2/g Mean granulometry 2 to 4 ,u Brightness (ELREPHO) 95 minimum Refractive Index 1.5 to 1.55 Bulk density 1.9 to 2.1 g/cm3 loss on heating at 800or 19 to 21% The particles are slightly cubic in form.

The aqueous dispersion is prepared as follows: Zeolite 65 parts as dry matter Water 35 parts Na hexametaphosphate 0.25% dry/zeolite Na polyacrylate 0.20% dry/zeolite The dispersion in water is made with vigorous agitation as for any other traditional pigment. An aqueous suspension is obtained of about 65% solid content and a viscosity of 11 6 cps measured on a Brookfield viscometer at 50 rpm.

EXAMPLE 2 The zeolite defined in Example 1 is incorporated in a coating bath intended to cover a paper for offset printing. A reference colouring agent containing only kaolin is prepared simultaneously. Three further baths were prepared in which 10, 20 and 30 parts of kaolin were replaced by the equivalent amount of zeolite.

Formula of the coating bath Pigment (English kaolin and zeoiite) 100 parts Na hexametaphosphate dispersant 0.25 parts Na polyacrylate dispersant 0.2 parts Oxidised starch 5 parts dry Carboxylic styrene-butadiene latex 8 parts dry Melamine formaldehyde resin 0.8 parts as commercial product Solid content 62% The starch is cooked in water at 950C at 35% solid content.

(a) Viscosity of the baths The viscosities of the four baths prepared gave the following results: TABLE I VISCOSITY BY BROOKFIELD AT 50 rpm (cps)

Period of storage Quantity kaolin /zeolite O Hours 2 Hours kaolin/zeolite 0 Hours 2 Hours 100two 1160 1280 90/10 1000 960 80/20 580 540 70/30 860 840 The presence of zeolite thus clearly has a tendency to lower the viscosity of the coating bath. This is an advantage which is not negligible and can be shown by the possibility of increasing the content of dry solid content of the colouring agents while keeping them at acceptable viscosities.

(b) Properties of the coated papers The colouring agents have been applied by means of a threaded rod to a wood free paper carrier.

The coated papers have been dried in the air then conditioned for 24 hours at 230C and 50% relative humidity. They have then been calendered on a super calender with 4 nips at 800C and under a linear pressure of 140 kg/cm. The papers were then put back into the conditioning chamber.

1. Brightness and opacity of the papers The brightness was measured in the ELREPHO apparatus with filter 8 (according to ISO standard): the opacity is the printing opacity measured on the green tristimulus filter. The results obtained are shown in Table II.

TABLE II

Kaol in/Zeolite Brightness Opacity Quantities ("ER) (O/o) 10010 78 94.5 90/10 79-t 94.4 80/20 79.2 94.3 70/30 79,4 94.1 It can be seen that increasing the content of zeolite causes an increase in brightness. This result is in accord with the high intrinsic brightness of the pigment. The opacity is little influenced by the zeolite.

2. Gloss The specular (mirror) reflection of the papers has been measured, at 450 of incidence, by means of a brillianceometer. The results obtained are shown in Table Ill.

TABLE Ill

Kaolln/Zeolite Specular Reflection Quantity at 45' 90/10 23 80/20 18.2 70/30 15;4 The presence of zeolite very appreciably lowers the gloss of the papers. This may result in a valuable advantage for papers with a mat coating. Infact, in order to obtain mat coatings with the traditional kaolin pigments and, to a lesser degree, with CaCO3, it is necessary to carry out a very moderate calendering of the papers. This results in a less desirable surface condition of the coated papers. With the zeolite the paper can then be subjected to a vigorous calendering without becoming too glossy.

3. Smoothness or gloss Measured on the BEKK apparatus and expressed in seconds as shown in Table IV.

TABLE IV

Kaolin /Zeolite Smoothness (BEKK) Quantity in seconds 10010 1500 100/0 1500 90/10 930 80/20 1780 70/30 1630

The presence of the zeolite favours a high smoothness, that is a good surface condition of the papers. The loss of gloss observed earlier therefore is not due to a loss of smoothness, which can be very favourable in the case of mat coated papers.

4. Dry and wet pick resistance The pick resistance is measured on an l.G.T. apparatus under a pressure of 40 kg/cm2 by means of inks with graduated pick value of Etablissements LORILLEUX.

For dry picking ink 3806 is used (free fall of the pendulum). The results are expressed in millimetres of length of non-picked coating.

For wet picking (evaluation of the resistance of the coating in offset print) the measurement is at constant speed of fall of the pendulum with inks 3801 to 3808. The results are expressed in Table V using the conventional figures. The higher the figure the better the result.

TABLE V

Dry pick Wet pick resistance Kaolin Weight of resistance zeolite coating Length not picked quantity c /m2 Ink in mm Inks Picking value 100/0 16.2 60 3801 10 90/0 15.8 87 11.6 3806 to 80/20 13.8 143 11.7 70/30 15.6 156 3808 11.9

The dry and wet pick resistance of the papers increases with the amount of zeolite in the coating bath. In other words, the binder requirement for the zeolite is appreciably less than that of the kaolin, that is, in a coating containing zeolite, in order to obtain predetermined pick resistance the binder required will be less than that required for a similar coating containing only kaolin with an evident economic advantage results.

5. Surface porosity This test gives a total indication of the permeability of the coat to the ink. The coated papers are put in contact with the porometric ink for increasing intervals of time. The residual reflection of the spots of ink on the paper is measured on the ELREPHO apparatus. The results in Table VI are expressed in % of loss of brightness.

TABLE VI

Loss of Brightness Elrepho % Time of contact in seconds of ink and paper ! Time of contact in seconds of ink and paper Kaolin /Zeolite Quantity 10 1 30 1 90 100 100/0 28.6 32.2 38.5 51-0 90/10 29.0 31.1 38.7 49.2 90/20- 30.2 33.5 40.2 50.9 70/30 316 34.2 40.8 59.9 The total porosity of the coating slightly increases with the amount of zeolite.

6. Printing with a rotogravure When carried out on the l.G.T. apparatus this test, which gives a qualitative evaluation of the fineness of print, does not appear to make any significant difference between the papers. It confirms that the reduction of the brilliance on calendering is not due to an inferior surface condition of the papers containing the zeolite.

EXAMPLE 3 Coating colour for paper intended to be printed by rotogravure and containing a synthetic binder as the only adhesive.

Pigment (English kaolin and zeolite of Example 1) 100 parts Dispersant: Na hexametaphosphate 0.25 parts Na polyacrylate 0.20 parts Acrylic latex 8 parts dry Ammonium stearate 1 part dry Carboxymethyl cellulose 2 parts dry NaOH 400 g/l q.s.p. pH 9 Solid content 62% (a) Viscosity of the colours: measured in BROOKFIELD viscometer at 50 r.p.m. and the results are given in Table VII.

TABLE VII

Hours of Storage Kaolin /Zeolite Quantity 0 5 10010 1360 1040 90/10 1120 980 80/20 970 790 70/30 880 710 The presence of zeolite fluidises the bath and thus presents the possibility of increasing the dry material content of the dispersions.

(b) Properties of the coated papers The preceding coating formulae are applied on an experimental coating machine by means of a device called a trailing blade under the following conditions: Trailing blade Pressure of the blade 4.2 kg/cm2 Drying temperature 750 Speed of machine 40 m/nm Wood free paper 70 g/m2 Coating weight 11 gim2 The coated papers are conditioned and calendered as in Example II.

In Table VIII a certain number of measured characteristics of the coated papers are given.

TABLE VIII

Dry pick resistance Smoothness IGT: mm of Kaolin /Zeolite (BEKK) non-picked length Quantity Brightness Gloss in seconds Ink 3802 100/0 82 31 1100 91 90/10 82.6 27 1190 115 80/20 83.2 25 1270 150 70/30 83.8 20 1340 170 The same tendencies are observed as in Example 2. Thus as the content of zeolite increases, brightness increases, the gloss decreased, the smoothness, that is the characteristic of the surface, is improved and the pick resistance increases.

The zeolite has therefore proved to be particularly adapted to obtain coatings of great whiteness, moderate gloss or mat after calendering but nevertheless presenting a remarkable surface condition, while reducing to a minimum the amount of adhesive or binder to be used which makes its use particularly interesting.

EXAMPLE 4 The comparison is effected in a coating formula containing casein as natural binder. It concerns a coating formula for offset or typographic printing.

Formula for coating bath Pigment (English kaolin and zeolite of Example 1) 100 parts dry Dispersant: Na hexametaphosphate 0.25 parts dry Na polyacrylate 0.2 parts dry Casein 8 parts dry Styrene butadiene latex 10 parts dry Calcium stearate 1 part dry Anti-foaming agent 0.5 parts as commercial product or a commercial form Solid content 40% The casein solubilised by means of caustic soda and ammonia.10,20 and 30 parts of kaolin have been replaced as in Example 3 by the zeolite defined in Example 1.

The colours are applied by means of an experimental machine, on a wood free paper of 70 g/m2, by means of an air knife. Owing to the low viscosity of these colours (80 cps on BROOKFIELD viscometer) the influence of the zeolite is of little importance at this low level of viscosity. The other properties of the coated papers evolve approximately in the same way, as a function of the amount of zeolite, as in Examples 2 and 3. Thus a difference of brightness of 1.8 points is noted between the all kaolin colour and the colour with 30 parts of zeolite (83.2 against 85). The dry pick resistance is obtained with ink 3807 for the all kaolin colour and ink 3808 for the colour with 30 parts of zeolite.

Claims (14)

1. An aqueous dispersion containing a dispersant and as pigment up to 100% as dry matter of a zeolite of the general formula: Na2O.A1203.2Si02.xH2O in which x is a number from 1 to 8, having a particle size less than 5,u.

2. A dispersion as claimed in claim 1 in which the dry matter content of from 20% to 65% by weight.

3. A dispersion as claimed in claim 1 or 2 in which the viscosity, measured on a Brookfield viscometer at 50 r.p.m., is from 20 cps to 120 cps.

4. A dispersion as claimed in claim 1,2 or 3 in which the amount of dispersant is from 0.2 to 0.8 parts by weight.

5. A dispersion substantially as herein described with reference to and as illustrated in Example 1.

6. An aqueous coating composition for a fibrous cellulosic substrate, the composition containing pigment and having a dry matter content equal to or greater than 40%, in which up to 100% of the pigment is a zeolite of the general formula: Na2O. Al2O3.2Si02.xH20 in which x is a number from 1 to 8, having a particle size less than 5 El.

7. A composition as claimed in claim 6 in which xis a number from 4 to 5.

8. A composition as claimed in claim 6 or 7 which contains also a binder and a dispersant.

9. A coating composition substantially as herein described with reference to and as illustrated in any of Examples 2 to 4.

10. A process for the preparation of a coating composition for material based on cellulosic fibres, the composition containing pigment and having a dry matter content equal to or greater than 40%, in which an aqueous dispersion or suspension of a dispersant and a zeolite of the general formula: Na2O . AI203. 2Si02 . xH20 in which x is a number from 1 to 8, having a particle size less than 5 ,u, is used as at least part of the pigment.

11. A process as claimed in claim 1 O in which x is a number from 4 to 5.

1 2. A process as claimed in claim 10 or 11 in which the dispersion or suspension has a dry matter content of from 20% to 65% by weight.

13. A process as claimed in claim 10, 11 or 12 in which the dispersion or suspension has a viscosity, measured on a Brookfield viscometer at 50 r.p.m., of from 20 cps to 120 cps.

14. A process as claimed in claim 10, 11, 12 or 13 in which the amount of dispersant in the dispersion or suspension is from 0.2 to 0.8 parts by weight.

1 5. A process for the preparation of a coating composition substantially as herein described with reference to and as illustrated in any of Examples 2 to 4.