US2369427A - Paper coating composition and method of making - Google Patents

Description

, gloss.

Patented Feb. 13, 1945 PAPER COATING COMPOSITION AND METHOD OF MAKING Eugene G. Bennett, Hamilton, The Champion Paper and Hamilton, Ohio,

No Drawing.

Ohio, assignor to Fibre Company,

a corporation of Ohio Application October 23, 1940,

Serial No. 362,459 8 Claims. (Cl. 106-448) This invention relates to paper coating compositions and to processes of making coated paper; and it has particular reference to the manufacture of coated paper of varnishing or gloss ink quality.

Coated paper as herein referred to consists of a paper base carrying on one or of a smooth coating of mineral pigment (clay, calcium carbonate, etc.) and adhesive (usually casein) and commonly is used for fine printing. Coated paper of varnishing quired where a film of varnish is to the printed paper to enhance its over-all For best results the varnish must not penetrate into the paper too much as this results in diminished gloss. In gloss ink printing an ink is used which dries to a glossy, varnishlike finish, and generally for best results on coated paper the latter should be of a nature to prevent a high degree of penetration of the ink into the coating.

One general practice in making coated papers of varnishing and gloss ink quality consists in using relatively large amounts of casein in the coating composition in order to render the coating more resistant to penetration by varnish and ink.

Inasmuch as the paper may be printed on an offset press prior to varnishin it is common practice to give the coating some degree of water resistance, and formaldehyde is frequently employed for this purpose. In using formaldehyde in casein paper-coating compositions, considerable care must be exercised since otherwise the coating composition may thicken greatly or even set up to a worthless solid mass. To avoid this difficulty it has been proposed to add the formaldehyde to the paper after the coating composition has been applied.

One object of the present invention is to provide an improved coating composition suitable for the of varnishing ject is to provide a paper coating composition containing formaldehyde or similar aldehyde, which does not exhibit undesirable thickening properties. Another object is to provide a meth- 06. of making coated paper having a high degree of resistanceto varnish or ink penetration but .W'ltl'lOllt the use of the excessively large amounts opportunity of adhesive previously required to attain this v property. Another object is to provide a method of producing coated paper, the coating having increased ink resistance, adhesiveness and other desirable properties. Another object is to provide a method of coating paper employing formaldehyde or similar aldehyde which is free from the objections previously associated with its use.

I have found that in coating compositions containing casein and formaldehyde, the effectiveness of the formaldehyde is greatly enhanced and its action considerably modified if the pH of the coating composition is raised to or maintained at 9.0 or above. Such coatings are generally more a waterproof and adhesive and if the quantity of casein is kept above a certain minimum the coated paper made therefrom is extremely nonabsorbent towards printing ink and varnish. This minimum casein content is, however, far below that required in coating compositions heretofore commonly required for coated paper of varnishing quality.

However, a coating composition such as has been described has no practical utility because it sets almost immediately after mixing to a thick unworkable mass. 1 have nevertheless experimentally prepared such a coating, and by exercising special precautions have applied the composition to paper within a few seconds after its preparation and found that the waterproofness, adhesiveness and especially the ink resistance were far in excess of papers previously made from coatings of similar composition having a lower pH.

I have discovered that the desirable properties of such a coating composition may be retained and the objectionable thickening prevented or at least delayed for many hours or even days by incorporating another aldehyde into the coating composition before the formaldehyde is added or before the coating has been made decidedly alkaline.

It is of course essential that the second aldehyde, e. g. acetaldehyde, be present in the coating composition before the formaldehyde has an to react with the casein in the alkaline medium. Thus the acetaldehyde may be added to the clay-casein-water composition followed by addition of the formaldehyde and finally of the alkali; or the acetaldehyde may be added to the clay-casein-water composition followed by the addition of the alkali and finally of the formaldehyde; in fact the ingredients may be added in any order provided the acetaldehyde is present before the final mixture of casein, formalde hyde and alkali is present. As will be pointed out more fully below, it is sometimes desirable to effect a combination of the formaldehyde and the other aldehydein the presence of alkali, advantageously by heat, before adding this to the coating composition.

The chemical reactions involved are apparently complicated and not well understoo In some cases, at least, there is reason to believe This fact makes such a coating composition a perfectly practical one for use in mill operatio The pH of the final coating composition should be at least 8.5, and for best results it way as acetaldehyde, differing somewhat in degree. Thus. propylaldehyde, butrylaldehyde,

heptaldehyde, ethylpropyl acrolein, etc. may beemployed. Due to the progressivelyincreasing water-insolubility of the higher aldehydes, they are not as useful as the lower aldehydes. Some compounds not specifically called aldehydes but still having the characteristic aldehyde radical may be used with more or less effectiveness. 'I'hus aldol, (CH3CH(OH) CHzCOI-I) and dextrose may be mentioned.

Although the invention is primarily concerned with the use of formaldehyde, compounds which produce formaldehyde readily upon decomposition, such as paraformaldehyde, may be used in its stead. In addition those 'aldehydes which As examples may be mentioned acroleinand glyoxal.

In general those aldehydes are satisfactory substitutes for formaldehyde which when added to a coating composition at room temperature,

consisting of parts casein, 100 parts coating clay and 170 parts of water and having a pH of 9.5, cause it to set to a gel within minutes, added being about 3% The quantity of formaldehyde to be used is ordinarily about 1% to 3% of the casein and the I imiti length of time amount of acetaldehyde or other aldehyde to be used is advantageously about equal to that of the formaldehyde.

As previously mentioned, the formaldehyde brought tocoating comon. A condensation between the aldehydes ensues, preferably with the aid of heat. The degree of condensation thus eflected depends upon the concentrations of the ingredients, the pH, the temperature and the duration of the treatment Prolonged treatment at elevated temperatures may produce colored reaction products and finally resinous bodies are useful life of the coating, 1. e. the during which it remains fluid enough to be applied and properly spread on paper, may be similarly varied.

of the weight or pigment-advantageously a good With this ratio of casein to clay, coated papers made by previous methods generally give papers which are far too absorbent to varnish to be acceptable. In previous practice it was necessary to increase the casein to 20 or more parts to provide a sufficiently nonabsorptive coating for this purpose.

to prior art papers containing 20 or more parts of casein. Furthermore my papers do not suffer method of accomplishing this which I have found successful is by the addition of sodium; carbonate followed shortly by, the addition of casein. Nevertheless this fact does not preclude the use of calcium carbonate as a pigment, since in this case there are only a relatively few calcium ions present. Also by heating together casein and the mixed aldehydes before incorporation with the pigment this difliculty with calcium ions apparently does not occur.

In carrying out the process of the present invention the coating composition is first prepared as previously described. The viscosity of' the materially diminish the ink recomposition may be somewhat the slight thickening which may occur according to the present method the composition.

may be controlled by good Either the coatto 160 F. or or i the treshly coated paper should be heated to about 160 F. or more in order to fully develop the ink resisting qualities inherent in the coating composition. However if an aldol type condensation product is incorporated into the coating, then it is not necessary to heat either the coating or the freshly coated paper.

The iollowing examples are illustrate the invention: was first prepared consisting of 100 parts coating clay, 14 parts casein, and water sufiicient to give a total solids content of about 40%. The casein was dissolved by the use of soda ash to the extent of 7% of weight of the casein. 'The above coating composition was well mixed and to it was added an aldol type condensation product in amount equal to Oif the weight of casein. The condensation product was prepared by mixing together 32.5 lbs. water, 12.5 lbs. of 40% formaldehyde, 5 lbs. of 99% acetaidehyde, and 5 lbs. of a 20% sodium carbonate solution. The temperature of this mixture rose to 130 F. by virtue of an exothermic reaction. Then the temperature was raised to l'70-180 F. by heating on a water bath for two hours, after which given to further the temperature was then allowed to fall to 105 F. and the final in a combination of the acetaldehyde to give an aldol type exact composition which was not determined. It was added to the above mentioned coating mixture, and additional sodium carbonate sufllcient to give a total amount equal to 25% of the casein was then incorporated/into the coating composition which now had a pH of 9.5. The

composition was a freely flowing mixture capable of being moothed out into an even layer by means of a conventional brush coater. The coated paper resulting therefrom was very non-absorbent to printing ink regardles of whether the paper was dried at high or low temperatures. It had the ink resistance of a coated paper having about 50% more casein made by the older methods.

In another case 80 parts coating clay, 20 parts finely divided calcium carbonate and 15 parts casein were mixed together along with water to give a coating composition having about. 42% solids. To this mixture was added successively 3% formaldehyde based on the casein and 3% butrylaldehyde based on the casein and 25% sodium carbonate based on the casein. The whole mixture was then heated to 170 F. for This coating when applied to paper gave a product having marked resistance to printing ink and to varnish in spite of the relatively small amount of casein used.

In another example the coating mixture was The initial pH of this mixture was pH was about 8.5. This resulted formaldehyde and of the pigment-casein composi- A coating composition product, the

prepared in two steps. Inthe' first ste an aqueous mixture of '10 parts coating clay, 20 parts finely divided calcium parts satin white was thoroughly mixed together in the presence of 5 parts of casein A second preparation was made in which 10 parts of casein was used to which was added 0.45 parts of formaldehyde and the same amount of acetaldehyde and sodium carbonate in considerable excess that required to dissolve the casein mixture a pH of about 9.5. solids content of about 9.4% was heated to 172-182 F. for 20 minutes. It was then cooled and'added to the first comcalcium carbonate, satin white and casein in the proportions above mentioned ordinarily gives coated papers that are extremely absorptive to printing ink. when modified as above described 2) a sheet very non-absorbent to printing ink is obtained.

I have found that the method is eminently adapted to coating paper on the paper machine, especially for the production of coat-one-side label paper of varnishing quality. For this purpose the method described by Donald B. Bradner in application Serial No. 212,877 filed June 10, 1938, now Patent No. 2,229,621, was used, two layers of coating being applied to the wire side of the paper in the dryer section of the paper machine. The coatingcomposition used on the base coat consisted of 100 parts coating clay, parts casein dissolved in soda ash. This coating had a total solids content of about 40% and a pH of about 7.5. It contained no formaldehyde. A total weight of about 6 lbs. of coating (dry basis) per ream (25 x 38-500 sheets) was applied. After the application of this coating the paper passed around a few dryer cylinders and received another coating on top of the base coat. This coating composition wa made up of 25 parts finely precipitated calcium carbonate, '75 parts coating clay, and 15 parts casein, and had a solids content of about 32%. To 300 gallons of this coating 250 ounces of 40% formaldehyde mixed in 5 gallons of water was added slowly. This is equivalent to about 5% HCHO on the casein. Then 5 gallons of acetaldehyde mixed in 5 gallons of water was added. This is equivalent to about 6.3% CHaCHO on the casein. After these two aldehydes had been thoroughly mixed in the coating, about 30 pounds of sodium carbonate dissolved in water was added, the mixture being continuously stirred all the time. This gave a pH of about 9.5. Thi coatin composition was applied over the base coat to the extent of about 3 pounds (dry basis) per ream (225 x 38-500 sheets). The coated paper was then dried by heating on the remaining drying cylinders of the paper machine. The product was an extremely ink-resisting coated paper of outstanding value as a varnishing label paper.

I claim:

1. Process of preparing coating compositions which comprises mixing together protein adhesive selectedirom the class consisting of casein and soybean, mineral pigment, water, an aldehyde capable of causing an aqueous composition at room temperature having a pH of 9.5 and consisting of casein 15 parts, coating clay 100 parts and water 170 parts to set to a substantially solid gel within 30 minutes when added to the mixture in amount of 3% of the casein, another aldehyde being incapable of producing a marked thickening of the compos'tion when similarly tested the carbonate and 10 a position. A coating composition containing clay,

quantity of the first-named aldehyde being sufficient to cause 8 Selling of the coating in the absence of the second-named aldehyde, and the quantity of the second-named aldehyde being sufficient to retard the gelling action of the firstnamed aldehyde, and an alkaline compound sufflcient to give the coating composition a pH 01 from -9.0 to about 11, the order of addition of the ingredients being such that the second named aldehyde is present before the composition con-'- tain the mixture of protein adhesive, the first named aldehyde and alkali, and thereafter heating the composition to at least 150 F. to improve the ink resisting properties thereof.

2. Proces of preparing papercoating compocitions which comprises mixing together casein, mineral pigment, water, formaldehyde, acetaldehyde, and an alkaline sodium compound to give a pH of from 9.0 to about 11 to the coating composition each aldehyde being of the order of from 1% to 8% of the casein content, the order of addition of the ingredients being such that the acetaldehyde is added before the composition contains the mixture of casein, formaldehyde and alkali, and thereafter heating the coating composition to at least 160 F. to improve the ink resisting properties thereof.

3. Process of preparing coating composition which comprises mixing together protein adhesive taken from the class consisting of casein and mineral pigment, water, an aldehyde capable of causing an aqueous composition at room temperature having a pH of 9.5 and consisting of casein 15 parts, coatin clay 100 parts, and water 170 parts, to set to a substantially solid gel in minutes when added to the mixture in amount of 3% of the casein, and a second aldehyde incapable of producing a marked thickening of the composition when similarly tested, then adding an alkaline compound 01' a fixed alkali metal to'give a pH of from 9.0 to about 11 to the coating composition and heating the composition to at least 160 F. to enhance the ink resistance properties thereof, the quantity of the first-named aldehyde being sufllcient to cause a gelling of the coating in the absence of the second-named aldehyde and the quantity of the second-named aldehyde being .sufiicient to retard the gelling action of the first-named aldehyde.

4. Paper coating composition which comprises a fluent aqueous mixture comprising protein ad-' hesive taken from the class consisting of casein mineral pigment, an aldehyde cause a gelling of the coating, and a compound chosen from the group consisting of acetaldehyde, propylaldehyde, 'butryaldehyde, heptaldehyde, ethyl propyl acrolein, aldol; and dextrose in amount sufiicient to retard the gelling action of the members of the first-named group, said coating composition having a pH of from 9.0 to about 11.

5. Fluid aqueous paper coating composition comprising, as the adhesive a protein selected from the class consisting of casein and soybean, mineral pigment, an alkaline aqueous solution,

an aldehyde capable of causing an aqueous composition at room temperature having a pH 01' 9.5 and consisting of casein 15 parts, coating clay parts, water parts, the parts being by weight, to set to a substantially solid gel within 30 minutes when added to the mixture in amount of 3% of the casein, and a second aldehyde incapable of producing a marked thickening of the composition when similarly tested, the quantity of the first-named aldehyde being sufiicient to cause a gelling of the coating in the absence of the secfirst-named aldehyde, said coating composition having a pH of from 8.5 to about 11.

parts, water 170 parts, the parts minutes when added to the mixture in amount of 3% of the casein, said pH of from 9.0 to

retard the gelling action of the formaldehyde EUGENE G. BENNETT.