US2039228A - Moistureproof transparent material - Google Patents

Description

Patented Apr. 28,1936

UNITED STATES MOISTUREPROOF TRANSPARENT MATERIAL Edouard M. Kratz, Gary, Ind., and William C. Wilson, Chicago, Ill., assignors, by direct and mesne assignments, of one-half to Pyroxylin Products, Inc., Chicago, 111., a corporation of Illinois, and one-half to Marbo Products Corporation, Chicago, 111., a corporation of Delaware No Drawing. Application April 1, 1933, Serial No. 664,042

5 Claims.

This invention relates to a moisture proof transparent sheet material that is especially adapted for use in wrapping articles. More particularly, the invention relates to a relatively moisture proof material having a protein base and a cellulose ester coating firmly adhering thereto provided with a moisture proofing agent, suchas a higher fatty acid.

It has heretofore been proposed to prepare sheet material containing casein as a principal ingredient and although it has been alleged that transparent, flexible sheets of casein material can be made, to the best of our knowledge, no one has heretofore developed commercially a satisfactory sheet having a casein base that possesses moisture resistant properties adapting it for wrapping purposes.

Furthermore, while various types of moisture proof coating compositions have been known in connection with the coating of transparent sheet material, such as regenerated cellulose and the like, no commercially satisfactory sheet material having a transparent casein base and a moisture proof coating thereon has been developed to the best of our knowledge. In order to be commercially satisfactory for use as a wrapping material, the sheet must have a base that is relatively thin, tough, pliable, flexible and transparent.

In the manufacture of sheets having a casein base, it has heretofore been impossible, practically, to obtain a base film having these desired characteristics, largely because of the inherent properties of casein and because of the difiiculty in obtaining a casein material that is clear and homogeneous and free from particles or specks. In a very thin sheet, solid particles of non-colloidal dimensions would produce a roughened surface that would not only decrease the transparency of the film but would make it difiicult to provide a continuous, smooth, moisture proof coating over such surface.

Furthermore, in order to make a commercial sheet having a casein base, it is necessary to provide a continuous method of casting long webs of the material. Suitable apparatus for and a method of casting transparent web material having a casein base aredescribed in the copending application of one of us, Serial No. 611,329, filed May 14, 1932.

Owing to the naturally unstable properties of casein with respect to atmospheric conditions, such as temperature and relative humidity, sheets made from casein, unless the inherent properties of the casein are modified, are unsatisfactory for many wrapping purposes and must be protected by a moisture resistant coating.

Although it might be possible to utilize some of the known moisture proof coating compositions in the coating of casein base material, such com- 5 positions are not best adapted for this purpose. It might be possible, for instance, to use coating compositions having a nitrocellulose base and containing in addition to a plasticizer and a gum, relatively small proportions of paraflin, but such 10 coating compositions, in general, have objectionable features. Their application requires drying temperatures above the melting point of the parafiin wax employed to prevent blushing or ,other surface phenomena that impair the ap- 15 pearance of the sheet and are deleterious to the base material, or casein.

We have now found that coating compositions containing a cellulose ester and a higher fatty acid may be satisfactorily used in the coating of 20 casein base material to render the same relatively moisture proof. The necessity of providing a coating having a high resistance to the transmission of water vapor is apparent in connec-' .tion with the'use of this type of wrapping. ma- 25 terial for cigars, frozen foods, and the like. It is also necessary that the coated material be relatively thin, transparent and have a high degree of flexibility over a wide temperature and humidity range.

It is therefore anobject of this invention to provide a relatively thin, transparent, moisture proof and flexible sheet especially adapted for wrapping purposes, having a casein base pro vided with a cellulose ester coating firmly bonded 35 thereto and containing a-moisture repellent agent.

It is a further important object of this invention to provide a composite, integral sheet for wrapping purposes having a hardened casein base and a moisture resistant coating firmly bonded thereto. a

It is a further important object of this invention to provide a relatively moisture proof coating for casein base materials wherein a higher plasticizer, a gum and a higher fatty acid, such as stearic acid.

It is a further important object of this invention to provide a relatively thin, transparent, moisture proof and flexible wrapping material having a hardened casein base and having a coating on one or both sides thereof firmly anchored thereto and comprising nitrocellulose, a gum, a plasticizer, a higher fatty acid and a tempering oil for maintaining the fatty acid in a state of compatibility.

Other and further important objects of this invention will become apparent from the following description and appended claims.

The base of the flexible wrapping material of our invention may be a suitable protein, such as casein. Casein is preferred because of its relative cheapness, ready source of supply, high transparency, good keeping qualities and the high form of purity commercially available. We prefer to use a lactic acid casein of low ash content that has been digested with acid and subsequently rendered alkaline with a caustic alkali until a clear solution results. However, it will be understood that any casein, such as a rennet casein, can be used which is capable of producing a clear solution of a caseinate, free from dirt, foreign particles and particles of non-colloidal proportions.

According to our method, a suitable aqueous caseinate solution may be prepared to contain from 13 to 14% of alkali metal caseinate by weight. To this solution may be added a suitable softening agent, such as glycerine, a glycol, e. g.

' diethylene glycol, a deliquescent salt, e. g. potassium acetate, or the like.

In casting the casein material in the form of a continuous web, we prefer to use the method and apparatus disclosed in the copending application of Kratz and Heckel entitled Method and apparatus for making transparent sheet material, Serial No. 630,406, filed August 25, 1932. According to that disclosure, the casein solution, comprising 13 to 14% of alkali metal caseinate and from 3 to 4% of glycerine, or other corresponding amounts of a softening agent, by weight, the balance being water, is mixed in the desired proportions with a formaldehyde solution, preferably a 10% solution (by weight) of formaldehyde in water. A suitable proportion is 1 part of 10% formaldehyde solution with 22 parts of the aforesaid casein solution.

The casein and formaldehyde solutions are separately prepared and mixed just prior to introduction into the coating machine. For this purpose, a pond of the mixed casein and formaldehyde solution may be suitably built up by means of a weir, doctor or the like on a coating roll to obtain a film of the desired thickness and this film then transferred to a continuous, traveling carrier or belt. For best results, a belt having a fabric base and a proxylin coating or surface, which may be either plain, printed or embossed, is used. After the casein-formaldehyde solution has been cast onto the traveling belt, the belt is passed through a heated drierto permit the film to set and is then carried through a festoon drier to thoroughly dry the film.

As an alternative to mixing the formaldehyde and casein solution together prior to casting on a traveling belt, the casein solution alone or in a suitable solvent or carrier therefor may be first cast on the belt and then the film exposed to formaldehyde gas to harden the casein.

It is also feasible to prepare an ammonia neutralized casein solution containing the proper proportion for formaldehyde and then, by the addition of a small quantity of caustic soda, to cause the solution to set quickly when cast. A pH of about 6.7 is desirable for this type of reaction.

After the film has been dried and preferably conditioned, by controlling the relative humidity of the air in the festoon drier, the film is stripped from the carrier belt and wound into rolls.

While we have here referred to the base film as being a casein or alkali metal caseinate base, it will be understood that neither casein alone nor an alkali metal caseinate is of itself a suitable base but that the caseinate material must be hardened, since casein or caseinate does not set by itself, as does gelatin. The action of formaldehyde in hardening casein produces a complex substance, which, depending upon the amount of formaldehyde used, is relatively water insoluble and relatively brittle.

In order to overcome the natural brittleness of the casein-formaldehyde complex body, a suitable softening agent, such as glycerine, must be employed. Glycerine, being a hygroscopic substance, tends to retain and maintain a moisture content in the casein base film such that the film possesses a desired degree of flexibility and pliability throughout comparatively wide fluctuations in atmospheric temperature and humidity. Other hygroscopic substances, both liquid and solid, examples of the latter being deliquescent salts, can be used.

The coating equipment may be of standard construction, such as an air floated coating device, a tower coater or a vertical coating machine. In the first mentioned equipment, the liquid coating composition is applied to the base film by means of a roll doctor or by other suitable means and the coated material is kept suspended by currents of air for a considerable distance until the coating has had an opportunity to dry, after which the coated material is wound up into rolls of the desired size.

A typical example of a suitable dry base composition for coating the casein base material is as follows:

Parts dry weight Regular soluble nitrocellulose sec. viscosity) 52 Ester gum 20 Dibutyl phthalate 10 Stearic acid 8 Paraffin oil 4 A suitable solvent for use with a nitrocellulose base such as this is as follows:

Parts by weight Toluol 133 Solvent naphtha 53 Butyl acetate 40 Ethyl acetate 85 Butanol (butyl alcohol) 10 The proportion of solvent mixture to the dry base may be widely varied to meet the conditions imposed upon the coating operation, as is well known to those skilled in the art. In general, the percentage of solvent mixture to dry base, by weight, will vary between 5 and 40% and preferably will be between 15 and 25% under usual coating conditions.

In the moisture proof coating other cellulose esters, ethers or derivatives may be used, such as cellulose acetate, cellulose ether, such as ethyl cellulose, and the like.

The purpose of the ester gum is largely to give body to the coating and to serve as an anchoring or bonding agent to cause the coating to adhere firmy to the casein base film. The ester gum also has a certain degree of moisture and water resistance properties. It will be understood that in place of ester gum, any suitable gum orresin, either natural or synthetic, may be used, such as dammar gum, phenol-formaldehyde condensation products or partial condensation products containing rosin, pontianac, sandarac and other fossilgums and resins.

The dibuiyl phthalate serves as a plasticizer to impart the desired flexibility and softening effect to the coating film. While we prefer to use dibutyl phthalate, it will be understood that other phthalates may be used and also other plasticizers, such as ortho tricresyl phosphate, triphenyl phosphate, and the like.

While we have given stearic acid as the preferred higher aliphatic fatty acid, other saturated fatty acids having about the same number of carbon atoms to the molecule or a higher number may be used with more or less satisfactory results, such as myristic, lauric, palmitic, and the like. We prefer to use stearic acid both because of its ready availability and because of its relative cheapness. Furthermore, when a coating composition containing stearic acid is employed, it is unnecessary to use temperatures during the drying operation in excess of the melting point of stearic acd, but rather instead, temperatures considerably lower than the melting point of stearic acid may be employed, as for instance from 50 to F.

While it is known that paraffin wax and other solid hydrocarbons, as well as true waxes may be used in coating compositions of this general character, the higher fatty acids such as herein specified are not the equivalent of parafiin or of .waxes such as have heretofore been used. In

the use of parafiin and waxes, it has been considered necessary to employ temperatures during the drying operaton about or above the melting point of the paraflin or wax employed, since otherwise there is a tendency for the film to blush, that is, become slightly milky or opaque, or present a streaked appearance. Also the use of waxes results in extremely slow drying of the coated sheet. These phenomena do not occur where stearic acid, or other suitable higher fatty acids, is employed, especially where a tempering agent, such as paraffin oil is additionally used.

While we have found that various mineral oils. such 'as the so-called refined mineral oils, Russian white oil, and the like, may be used as a tempering agent in connection with the higher fatty acids, we prefer to use a wax-free grade of nonvolatile and odorless parafiin oil. This paraffin oil has been produced by cooling to a temperature of m nus 50 C. in order to free the oil from any paraflin or other normally solid substance.

In general, any petroleum base oil of the gen? eral character indicated may be used. Paraffin oil acts to prevent the stearic acid from spewing, so

that larger proportions of stearic acid can be used to advantage in the moisture proofing of the sheet.

If no tempering agent is used, the stearic acid has a tendency to crystallize in the coating film and this is objectionable as it reduces the transparency of the film and also the resistance of the film to the penetration of moisture vapor. While excessive amounts of paraffin oil would be ob,- jectionable because giving an oily or greasy appearance and feel 'to the surface of the coated material, small amounts of the parafiin'oil serve to disperse the stearic acid and to prevent it from spewing out.

The use of water soluble orwater miscible compounds in the solvent mixture should be avoided or reduced to a minimum so far as possible, where a clear transparent sheet is desired, since relatively large proportions ofwater soluble or watertmiscible compounds in the solvents used tend to give a milky or opaque appearance to the coated sheet.

The exact phenomenon by which good anchoring qualities in the coating layer are obtained is not fully understood, but it is apparent that more than mere physical contact and adhesion result ing therefrom isrequired. We have found that by using a coaiing composition of the preferred formula given above, we can obtain a coated material that will not delaminate when immersed in water over a considerable period of time. While we have attributed the good anchoring qualities of our coating composition to the use of a gum or resin, we also recognize that the quantity of plasticizer has a modifying effect.

After the casein base-film has been coated with a composition such as above described, and. the coated material has been dried to remove the volatile solvents, the dried material may be conditioned to give it the desirable flexibility. Owing to the fact that the coating apparatus must necessarily be enclosed in order to permit economical the base film, it is possible to use moderately conditioned low temperature air, as for instance, air between 50 and 120 F. If parafiin wax were used in place of the stearic acid, temperatures of from 120 to F, would be necessary and the sheet would have to be carefully conditioned after drying.

It will be understood that in giving exact pro portions of the various ingredients of the casein base material, of the dry base constituents of the coating composition and of the solvent mixture, we do not intend to limit our invention to these specific proportions, since they may be varied considerably without impairing the general qualities of the sheet material. For instance,'on the basis of 36 parts by weight of nitrocellulose, we may use between 10 and 30 parts of-a suitable gum or resin, between 5 and 15 parts of a suitable plasticizer, depending upon the plasticizer selected and the degree of flexibility desired, between 4 and 12 parts of a higher fatty acid, and between 2 and 6 parts of a tempering oil. Likewise, depending upon the characteristics of the sheet desired, the gum or resin may be entirely omitted.

In the case of the solvent, equivalents of the different ingredients may be used and the proporlate. Under "some circumstances, either the butyl acetate or the ethyl acetate may be omitted and a corresponding quantity of the other employed.

Other alcohols than butyl alcohol, such as ethyl or propyl alcohol, maybe used.

One of the important features of our invention is the use of a higher fatty acid in the form of the free acid, rather than in the form of a fat, such as a glyceride or other alcohol ester. In general, fats, by which is meant a glyceride of one of the higher fatty acids, are not suitable for our purpose, since it is diflicult to prevent a fat from spewing out and practically impossible to keep it ina highly dispersed condition. We prefer to use the free fatty acids for the reason that they permit the use of generally lower temperatures during the drying operation and have equally as good or better moisture resistant properties. The attendant dangers of high temperature drying are avoided by the use of free fatty acids.

By moisture vapor proof sheet, we mean asneetlike material which will not transmit more than 1.50 grams of water vapor per 100 square inches in 24 hours at 70 F., when measured by the method as described by Allen Abrams, page 24 of The Paper Mill, volume 8, No. 38 (September 20, 1930). The coated sheets of this invention will, in general, transmit between 0.40 and 0.75 gram of water vapor per 100 square inches in 24 hours at 70 F. when tested by this method.

While, as is obvious, for a given film composition the degree of resistance to the transmission of water vapor is roughly proportional to the thickness of the coating, the limits of thickness for coatings on transparent sheet material for wrapping purposes are relatively well defined and, in general, for commercial purposes, the coating should not exceed about 20% of the thickness of the base material. If this limit is exceeded, not only is the cost-of the coated material raised to a prohibitive point, but the coated material is rendered less pliable and less flexible to an extent that makes its use for wrapping purposes less satisfactory.

Although for the great majority of uses, the coated material of our invention is preferably colorless and transparent, it will be understood that suitable coloring agents, dyes and the like, may be added to the casein base fllm if a colored product is desired.

We are aware that numerous details of the process may be varied through a wide range without departing from the principles of this invention, and we, therefore, do not purpose limiting the patent granted hereon otherwise than necessitated by the prior art.

We claim as our invention:

1. A thin, transparent and relatively moisture proof flexible sheet adapted for use in wrapping articles comprising a casein base having a cellulose ester coating thereon containing stearic acid.

2. A thin, transparent and relatively moisture proof flexible sheet comprising a casein base having a plasticized cellulose ester coating thereon containing stearic acid and a tempering oil.

3. A thin, transparent and relatively moisture proof flexible sheet adapted for use in wrapping articles comprising a casein base having a nitrocellulose coating thereon containing stearic acid, a gum, a tempering agent and a plasticizer.

4. A relatively thin, transparent moisture proof and flexible sheet adapted for wrapping purposes comprising a hardened casein base containing a softening agent and a coating on said base firmly adhering thereto, said coating containing nitrocellulose, ester gum, dibutyl phthalate, a mineral oil and stearic acid.

5. A thin, transparent and relatively moisture proof flexible sheet adapted for use in wrapping articles comprising a casein base having a coating thereon consisting of 36 parts by weight of nitrocellulose, between 10 and 30 parts by weight of a gum, between 5 and parts by weight of a plasticizer, between 4 and 12 parts by weight of a higher fatty acid and between 2 and 6 parts by weight of a mineral oil.

EDOUARD M. mRA'I'Z. WILLIAM C. WILSON.