US3081182A - Salt peptized proteinaceous adhesive and coating composition employing the same as abinder - Google Patents

Description

SALT PEPTEZED PROTEHNACEOUS ADHESIVE AND QOATING COMPOSITION EMPLOYING THE AME AS A BINBER Ralph l Mahaney, llelcit, Wis, assignor to S. D. Warren Company, Boston, Mass a corporation of Massachusetts No Drawing. Filed Nov. 9, 1959, er. No. 851,541

4 tllaims. (Cl. 106-146) This invention relates to proteinaceous adhesives and more particularly to a process for peptizing such adhesives in an acid medium and to a mineral filled coating composition employing the same as a binder precipitated therein in the free state.

One of the problems presented in drum finishing paper by the well known cast coating process has to do with the water solubility of the adhesive binder in the coating. Experience has demonstrated that it is practically impossible to cast a high gloss surface it the adhesive binder in the coating has been water insolubilized and dried prior to the casting step. On the other hand, the adhesive should be water insolubilized at some point in the process or else the finished product will be too readily affected by water during use. One solution for the problem has been to coat the paper prior to casting it, and, then as the paper enters the casting nip, to introduce insolubilizing chemicals into the re-wetting solution at the casting nip. Such a process has been successful in part, but one of the primary disadvantages is that the liquid in the casting nip washes oil a substantial part of the coating and the part washed off cannot be recovered economically. Accordingly, it is one of the objects of this invention to provide an adhesive binder for mineral coating compositions which, in combination with mineral fillers, etc. will cast well even though the coating has been previously dried and is in a relatively Water insoluble state. Another object is to provide such an adhesive and mineral coating composition which will, at one and the same time, cast well and not wash ofi to any appreciable amount when subjected to a re-wetting bath at the casting nip.

Another problem in the art of coating relates more specifically to the disadvantages of calendering. In order to accomplish any appreciable degree of smoothing of conventional coating surfaces by calendering without at the same time crushing the body stock, the paper must be dried in order to give the fibers in the body stock enough strength to withstand the pressure of the calender roll. In general, it is adequate to air-dry the paper (about 4% moisture) for this purpose, and when this is done a calender roll pressure of approximately 2,000 pounds per linear inch will smooth the coating surface adequately without crushing the body stock. On the other hand, due to the extreme pressure required with conventional coating materials, calendering has previously always lowered the bulk, brightness and opacity of the paper, and otherwise given the paper a somewhat dingy appearance. Accordingly, for many years now, the paper industry has sought a process for smoothing a coated paper surface to the same extent as may be accomplished by calendering while at the same time preserving the bulk, opacity and brightness of the uncalendered body stock. Thus, in addition to providing a coating material having special advantages in cast coating, it is also an object of our invention to provide a coating material which can be smoothed under relatively light roll pressure Without substantial loss in bulk, opacity and/ or brightness, and which is also water insoluble.

In the accomplishment of these and other objects in a preferred embodiment of my invention, I formulate a coating composition comprising conventional coating components in combination with a proteinaceous adhesive.

3,081,182 Patented Mar. 12, 1963 My invention depends in part on my discovery that casein, or casein-like materials such as soy-bean protein, may be peptized in a concentrated solution of the casein, together with a salt such as ammonium nitrate, sodium nitrate, or nitrates of other alkali metals. In order for this to take place the concentration of casein-to-water must be quite high; i.e. 2035%. Likewise the concentration of salt must be comparatively high; i.e. 4l0%. When the casein, salt and water are intermixed in these percentages and heated to approximately ISO-200 F. and subjected tovigorous agitation, the casein is peptized or appears to dissolve in approximately 20 minutes. The addition of a small amount (below 10% based on casein) of water-soluble tallow-based soap assists the peptizing process and permits some lowering of these percentages.

Obviously, the addition of the soap must not be so great before describing how the casein is combined with the clay,

a few general remarks about the characteristics of the casein so peptized will help to form a basis for understanding the nature of the adhesive. If it is allowed to cool to room temperature, it rapidly gels to a thick mass. Reheating the mass, however, returns the casein to its former liquid state. On the other hand, if the liquid, peptized casein, at for instance F., is poured into an excess of water (no matter whether the water is hot or cold) instead of forming a thinner solution of peptized casein, the casein immediately precipitates and forms a conglomerate ropelike mass. Likewise if water is poured into such casein without agitation, the same sort of precipitation takes place, and even with agitation, precipitation seems to occur. The ropy cords are not formed, but the solution turns milky in appearance. In addition, since casein hy-;

drolyzes if allowed to remain at elevated temperature, we generally prefer to lower the temperature and agitation of the solution shortly after the casein has been peptized, and likewise we prefer at that time to combine it with the other coating ingredients to minimize the efiects of hydrolysis.

Turning now to the combination of my salt peptized casein with other coating ingredients, this is done by forming a slurry of clay and water and dumping an appropriate amount of the clay slurry into the salt peptized casein while continuously agitating the mixture. the sharp drop in temperature, together with the dilution of the adhesive by the excess of water in the slurry immediately causes the casein to gel and/or precipitate and also fiocculates the clay slurry. This causes the entire slurry now mixed with adhesive to become quite stiff and hard to stir. As the agitation is continued, however, apparently the casein, even though somewhat precipitated, tends to disperse the clay and the mixture gradually starts to loosen up. In practice after the agitation has continued for a few minutes, the mixture suddenly thins out and becomes quite fluid. I am not sure just what happens between the casein and the clay to bring about this result, but the thinning out of the coating mixture is very marked and readily observed.

The resulting combination of salt peptized casein and clay has a number of interesting properties. In the first place, it has a surprisingly low viscosity which makes it an excellent binder for coatings employed with an air-knife smoothing mechanism. In addition, the Water retention of the fluidcoating composition prior to coating is very low and the rate of water absorption of the finished dried coating is high. Another characteristic of the comtogether they appear to be far more malleable than ordi- When this is done,.

nary coating compositions. Thus under conditions of heat, controlled moisture content, and light pressure, the coating may be smoothed by calender rolls without at the same time substantially reducing the bulk, brightness and/or capacity of the coated sheet. Along with the foregoing characteristics an extremely important characteristic relates to cast coating. The casein in my coating combination seems to be largely precipitated and, therefore, it is substantially water insoluble. However, even though water insoluble, it is still in a condition which permits drum finishing by the cast coating process while at the same time strongly resistingany tendency to wash off when passed through a re-wetting bath at the casting nip during the casting operation.

Several specific examples of the process of my invention will now be described.

EXAMPLE NO. 1

'In this example the following ingredients were mixed in the following, proportions'of parts'by weight:

180'parts water (at 180 F.)

60 parts casein- (Australian hydrochloric acid precipitated) 12- parts ammoniumnitrate (technical grade fertilizer sold-under the trademark Aeropril 12 parts dicyandiamide (soldunderthe trademark Azite) part soap (tallow based).

The foregoing mixture had a-pH- of about 5.0. After agitating the same for about 20 minutes during whichthe temperature gradually declined to about 140 F., the

solution became quite clear and had a slightly yellowish hue. In this condition, it is considered that the casein is substantially peptized. However, it should be noted'that' the salt employed to peptize the casein does not combine with-the casein to form a caseinate as do the alkalisnormally employed to disperse casein.

Withthe casein in this peptized condition and at a temperature of about 140 F., a slurry containing approximately 400 parts clay and 400 parts water at room temperature was dumped into the casein and simultaneously strongly agitated. Immediately following the combination of these two ingredients, the slurry became stiff and viscous and the agitation propeller commenced laboring heavily. For 2 or 3 minutes the agitation continued with great difficulty and thereafter the slurry commenced to thin out in the neighborhood of the propeller, and thence rapidly'to become entirely quite free and liquid. At this point care had to be taken in the control'of the agitator to prevent a sudden spraying forth of the contents of the'mixing' container as the ingredients loosened up.

In this condition the material was ready for coating, and while it is believed that the casein was precipitated, yet the-mixture was comparatively free and without lumps. On. the other hand, when these ingredients were coated on paper and observed microscopically, the appearance was" substantially more of a pebbly nature and not as smooth in appearance as alkaline dispersed casein. How-- EXAMPLE NO. 2

In this example the same ingredients as Example No. 1 were employed, except that both the soap and the dicyandiamide were omitted. The result of this was to make it more diflicult to peptize the casein, but with continuous vigorous agitation, the casein did peptize in about 30 minutes.

EXAMPLE NO.. 3

In. this example the same procedures as set' forth for Example No-1 were followed, except that the proportion of soap wasincreased to: approximately 1% This mix-' ture peptized slightly quicker, and after being combined with the clay slurry, it resulted in a coating which was particularly well suited for hot calendering under light pressure.

EXAMPLE NO. 4

In this example the following ingredients were mixed in the following proportions of parts by weight:

parts water (at 180 F.)

60 parts casein (Australian hydrochloric acid precipitated) 12 parts sodium nitrate.

In the foregoing mixture the casein peptized in about 30 minutes under conditions of continuous heat and vigorous agitation.

Theoretical Discussion It is believed that this novel method for peptizing'casein results from an ionic interaction between the salt and the casein in the concentrations'and under the conditions set forth. Taking, for instance, the illustration of Example No. 1, in solution the ammonium nitrate divides into positively charged ammonium ions and negatively charged nitrate ions. Apparently these oppositely charged ions'in the concentrations shown, and at or near the isoelectric'point of the'casein, have'the power to neutralize the electrostatic attractive forces of the Zwitter ions of the casein molecule, with the positively charged ammonium'ions of the salt neutralizing the negatively charged ca'rboxyl and carbonyl groups of'the casein, and the negatively charged nitrate ions of the salt neutralizing the positively charged amine or imide groups of the casein.

The foregoing discussion is important in the context of my invention because it explains how the casein can be peptized eventhough it is at or near its iso-electric point. Also ithelps lead to a selection of various other salts which can serve in the process. In general, the salt should be a stable, soluble, neutral or acidic salt of an alkali metal or ammonium having a monovalent cation and a monovalent anion. As a practical matter the nitrates of ammonium and sodium are preferred.

In general the pH of the salts which perform satisfactorily in our process have in a 5% aqueous solution a pH of 4.5 to 7.0. If salts which are more acidic are employed, they fail to peptize the casein and if they are more basic, they tend to yield a caseinate similar to the conventional alkaline casein dispersants.

The foregoing discussion also explains why the casein peptized by the mechanism of my invention precipitates upon the addition of Water and the dilution of the concentration of the salt; If the dilution is carried out with an ammonium nitrate solution of similar concentration in place of the Water, this precipitation does not occur.

Numerous minor variations of my invention will now be apparent to those skilled in the art, and therefore, it is not my intention to confine the invention to the precise form herein shown but rather to limit it in terms of the appended claims. Moreover, in connection with the'combination' of coating materials set forth, there is no need for me to set forth the many coating combinations wherein my casein may be employed, since a great variety thereof are well known to those skilled in the art. Thus in place of clay, I may employ calcium carbonate, blanc fixe, talc, titanium dioxide, and the like. Anti-foaming and release agents may also be employed, as Well as combinations of latex or other polymeric substances for the purpose of imparting known and desired properties.

When I employ the term salt peptized proteinaceous adhesive in the claims of this application, I intend the term to include broadly such adhesives peptized at a pH relatively close to their iso-electric point by a nitrate of ammonium or alkali metal in accordance with the steps set forth above. In addition, when I employ the term unaltered proteinaceous adhesive or unaltered casein,

I intend to describe the proteinaceous adhesive peptized in accordance with the process of this invention, it being understood that salt peptizing does not involve a chemical change of forming a caseinarte as do the conventional processes involving the use of bases to disperse the adhesive.

Having thus described and disclosed preferred embodiments of my invention, what I claim as new and desire to secure by Letters Patent of the United States of America is:

1. A process for preparing a coating composition that yields a maleable coating on a paper web comprising: preparing a peptized aqueous solution of a protein selected from the group consisting of casein and soy protein by admixing at a temperature in the range of 140 to 212 F. said protein with a soluble salt of a monovalent anion and a cation selected from the group consisting of the alkali metal ions and the ammonium ion while maintaining the pH of the admixture between the iso-electro point of said protein and 7, said soluble salt in a 5 percent aqueous solution having a pH in the range of 4.5 to 7.0, said protein being in the range of 20 to 35 percent of said solution and said soluble salt being in the range of 4 to 10 percent thereof, thereafter forming a stiff slurry by admixing said aqueous solution with a cool slurry 6 of a paper-coating-grade mineral pigment, the amount of said slurry being suflicient to dilute the concentration of said salt in the resulting mixture to below about 4 percent where it no longer peptizes said protein, and agitating said stiff slurry to a homogeneous mixture of substantially lower viscosity.

2. The process of claim 1 when said soluble salt is selected from group consisting of sodium nitrate and ammonium nitrate.

3. The process of claim 1 when said aqueous solution also contains a viscosity reducing agent.

4. The coating composition produced by the process of claim 3.

References Cited in the file of this patent UNITED STATES PATENTS 2,198,596 Atwood Apr. 30, 1940 2,214,564 Montgomery et a1 Sept. 10, 1940 2,334,607 Christopher Nov. 16, 1943 2,356,795 Poarch Aug. 29, 1944 2,360,828 Craig Oct. 24, 1944 2,367,678 Hatch et a1. Jan. 23, 1945 2,581,111 Landes et a1. Jan. 1, 1952 2,894,847 Wright July 14, 1959

Claims (1)

1. A PROCESS FOR PREPARING A COATING COMPOSITION THAT YIELDS A MALEABLE COATING ON A PAPER WEB COMPRISING: PREPARING A PEPTIZED AQUEOUS SOLUTION OF A PROTEIN BY FROM THE GROUP CONSISTING OF CASEIN AND SOY PROTEIN BY ADMIXING AT A TEMPERATURE IN THE RANGE OF 140* TO 212* F. SAID PROTEIN WITH A SOLUBLE SALT OF A MONOVALENT ANION AND A CATION SELECTED FROM THE GROUP CONSISTING OF THE ALKALI METAL IONS AND THE AMMONIUM ION WHILE MAINTAINING THE PH OF THE ADMIXTURE BETWEEN THE ISO-ELECTRO POINT OF SAID PROTEIN AND 7, SAID SOLUBLE SALT IN A 5 PERCENT AQUEOUS SOLUTION HAVING A PH IN THE RANGE OF 4.5 TO 7.0, SAID PROTEIN BEING IN THE RANGE OF 20 TO TO 35 PERCENT OF SAID SOLUTION AND SAID SALT BEING IN THE RANGE OF 4 TO 10 PERCENT THEREOF, THEREAFTER FORMING A STIFF SLURRY BY ADMIXING SAID AQUEOUS SOLUTION WITH A COOL SLURRY OF A PAPER-COATING-GRADE MINERAL PIGMENT, THE AMOUNT OF SAID SLURRY BEING SUFFICIENT TO DILUTE THE CONCENTRATION OF SAID SALT IN THE RESULTING MIXTURE TO BELOW ABOUT 4 PERCENT WHERE IT NO LONGER PEPTIZES SAID PROTEIN, AND AGITATING SAID STIFF SLURRY TO A HOMOGENOUS MIXTURE OF SUBSTANTIALLY LOWER VISCOSITY.