US2039361A - Paper manufacture - Google Patents

Description

May 5, 1936. J SWAN, 30 2,039,361

PAPER MANUFACTURE Filed May 15, 1953 Patented May 5, 1936 UNITED STATES PATENT OFFICE PAPER MANUFACTURE Application May 15, 1933, Serial No. 671,085

7 Claims.

This invention relates to paper manufacture and more particularly to the manufacture of board adapted for use in the making of containers and the like.

It is .the principal object of the invention to provide a paper or paper board suitable for formation into containers, the board being substantially greaseproof and moistureproof and being also simple and economical to manufacture.

It is also an object to provide a paper product which in addition to being highly greaseproof and moistureproof is tough and substantially odorless and tasteless so that it is particularly useful in the packaging of foodstuffs.

Another object of the invention is to provide a container board having a cellulose ester or regenerated cellulose surface coating to give high greaseproofness while being substantially free from odor and taste, in which the surface coating is used in an extremely thin but impervious film to afford a commercially economical product and is superimposed upon an inexpensive lower or base coating having high flexibility characteristics such that the board with the com- 7 posite coating thereon can be manipulated, bent, and formed into containers or cartons on regular forming machines in commercial operation without breaking or cracking of the coating and without injury to the impervious thin surface coating.

It is also an object to provide a simple and economical method for producing a paper product of this character.

Other objects and advantages will be apparent.

from the following description, the accompanying drawing and the appended claims.

In the drawing, Fig. 1 is a view, largely diagrammatic in character, indicating the manner of practicing the present invention;

Fig. 2 is a view on an enlarged scale of a part of the apparatus for applying the coating composition of this invention to the travellingsheet of paper.

In the production of a paper material suitable for formation into containers for receiving foodstuifs, it is highly desirable that a product be produced which embodies in itself the requisite strength to constitute the body of the container together with the necessary moistureproof and greaseproof characteristics which will maintain and preserve the foodstuffs from the atmosphere and likewise prevent any escape of the foodstuffs into the walls of the container. It has been customary in certain instances in the past, in order to provide a satisfactory moistureproof and greaseproof container, to use a body stock for forming the body of the container and a separate greaseproof and moistureproof sheet which is placed within the body of the container. Such practice is undesirable from the standpoint of expense, as well as the difficulty in handling and packaging. It has also been proposed to apply .a coating composition to a previously formed container but this process also is unsatisfactory, as his difficult to obtain a smooth, even, thin coat-- ing free from pinholes, and also involves difficulties in the physical application of a coating to the interior of a preformed container. It has also been proposed to apply coatings of cellulose lacquer to sheet material as well as to formed containers, but these proposals have not been economical and successful on a commercial working basis because of the difflculty of producing by these methods a sufficiently thin layer of the cellulose which would have and retain the desired impervious and other utility characteristics heretofore mentioned which would permit of its formation into a container.

According to the present invention a paper product or container board is provided with a composite coating of such character that the coated sheet itself can be formed into a container which is moistureproof and greaseproof to a very satisfactory degree. The coating includes an extremely thin layer of cellulose solution which dries to a very tough odorless and tasteless film so that there is no contamination of the foodstuffs. It also includes a lower layer containing as an essential ingredient a rubber effective material, such as rubber latex, which imparts to the composite coating such characteristics that-it is highly flexible, and not only can be bent without cracking but also tends to prevent the breaking of the sheet of paper itself. The cellulose coating is applied in an accurately controlled manner to the paper or container board in sheet form in a continuous operation, and it is therefore possible to apply a thin film in the simplest and most economical manner. The coated sheet is subsequently formed into the desired shape, such for instance as a container for foodstuffs. The coating is not broken or cracked during this forming operation but remains. impervious, and does not objectionably deteriorate in handling and usage of the container over long periods of time.

Referring to the drawing which illustrates diagrammatically a preferred manner of practicing the invention, there is shown at l0 9. roll of paper or container board which may be of the desired weight to give the necessary strength and other characteristics to the formed container. A travelling sheet II is withdrawn from the roll [0 and passes under a guide roll I2 into contact with the upper roll l3 of a coating press. This press comprises also a. lower roll l4 dipping within a trough l5 to which is supplied a coating mixture which forms the lower or base coat of the composite coating. Rolls l3 and I4 are positively driven and are so mounted that they can be vertically adjusted relatively to each other to regulate the pressure at the nip of the rolls as desired. In operation, rotation of lower roll l4 within the coating material serves to pick up a film of the coating material upon the surface of the roll. A portion of this film is transferred to the surface of the upper roll l3, the thickness of the film being regulated by adjusting the pressure of contact of the rolls i3 and I4, and this filmis then transferred to the travelling web II.

The coating composition applied at this point comprises essentially rubber latex and an oil insoluble adhesive, such as casein. A very satisfactory coating composition of this type for the lower or base coat is that disclosed in applicant's copending application Serial No. 389,425, filed August 30, 1929. The rubber latex is a relatively concentrated water solution of the pure sap of the rubber tree in colloidal state. The addition of an oil insoluble adhesive, such as casein, to the rubber latex imparts superior. characteristics of adhesiveness andgreaseproofness. Rubber latex has qualities of elasticity and spreading ability and enables a relatively thin coating to be used with satisfactory covering power; it also functions very satisfactorily to lay any fuzz which may be protruding from the surface of the paper and covers up irregularities of the surface. It is also a good moistureprooflng and waterproofing agent.

Casein provides a very satisfactory oil insoluble adhesive, and when properly prepared mixes satisfactorily with the rubber latex. Other oil insoluble adhesives, such as alkali metal silicates, various animal or fish glues, or gelatine, may be used for this purpose. An alkaline compound, such as sodium carbonate, ammonia, sodium hydroxide, etc., is preferably added to preserve the alkalinity of the coating, this being found to assist in preventing deterioration of the coating on standing with increase in the effective life of the coating.

A preservative, such as formaldehyde, paraformaldehyde or borax, is preferably added to the coating. This is found to preserve the coating from bacterial action and to act as a reducing agent to prevent objectionable oxidation or other deterioration of the coating. It also improves the quality of the coating, a very durable and resistant tough coating being produced, which is more gr'easeproof and more water insoluble. Formaldehyde is found to be highly satisfactory for this purpose, and remains stable in the coating after drying, also adding waterproof qualities thereto. The addition of a polysulphide, such as sodium or ammonium polysulphide, to this coating acts as a vulcanizing agent and tends to prevent the formation of pin holes and deterioration of the coating from light or other causes. The polysulphide may be conveniently prepared by first generating a relatively concentrated water solution of sodium or ammonium sulphide, and then adding a quantity of sulphur to the solution and dissolving the same therein. For example, the addition of about 50 to '75 grams of sulphur to the liter of a relatively concentrated solution of ammonium sulphide gives a very satisfactory polysulphide for this purpose.

In the preparation of the coating composition, where casein is used, a solution is first prepared by adding commercial powdered casein to water in the proportion of about one pound to the gelion. This mixture is allowed to soak for a. period of time such as about an hour, with the addition of a quantity of an alkaline compound such as ammonia, sodium carbonate, sodium hydroxide, trisodium phosphate, or the like. For example, about one pint of ammonia added to ten gallons of the casein solution gives very satisfactory results. The addition of the alkalinecompound facilitates solution, and the solution may also be heated somewhat to assist in the dissolving action. To this solution is added a preservative, such as paraformaldehyde, which assists in preventing deterioration of the coating.

The paraformaldehyde is first made up into solution in a considerable quantity of water. Thus satisfactory results have been secured where the paraformaldehyde has been added to ten times its own weight of water, or even in higher dilution. Satisfactory proportions of the formaldehyde are from 2% to 25% on the dry weightof the casein used, about 10% on the weight of the casein being readily carried out in commercial operation. When substantially more than 2% of the formaldehyde is used, the solution thereof is added in small amounts to the casein solution which is agitated or fiowing freely in a stream, this serving to prevent undesirable thickening of the casein.

The rubber latex is added in emulsion form. Preferably a rather concentrated latex emulsion is used, very satisfactory results being secured with an emulsion containing 38% solids in water, and also containing a small amount, about 2%, of an alkali such as ammonia or sodium car bonate to preserve the alkalinity of the emulsion. To the rubber latex emulsion is added the polysulphide solution prepared as above described, satisfactory results being secured by the use of about 2% by weight of polysulphide on the total solids.

of the emulsion. The casein solution containing the formaldehyde is then added to the latex emulsion, preferably with agitation. Various proportions of these ingredients may be used, depending upon the particular product or use of the product desired. Satisfactory results have been secured with compositions containing from 10% to by volume of latex, the balance being primarily an alkaline solution of casein containing formaldehyde. Where the formed paper is to have high flexibility so as to be able tobe heavily scored without fracture of the coating, it is found preferable to provide in excess of 50% rubber latex by volume, the proportion of latex being controlled to give the desired flexibility and resiliency characteristics. Other vulcanizers, accelerators and fillers may be added as desired.

As an example of a very satisfactory coating composition for the base coat which is applied to the paper or board, the following is mentioned:

and 40% or-less by volume of casein solution to form the liquid coating composition.

The web I I with the coating applied to the unof approximately 80 ft. per minute. If desired,

this roll may be positioned obliquely across the machine with reference to the travelling web of paper as this is found to minimize any tendency toward streaking of the relatively sticky coating, and to facilitate the smoothing and spreading of the coating.

The sheet then passes around a guide roll 2! and may then be subjected to further surfacing treatment. This may comprise brushes 22 and/or additional smoothing rolls 23, this treatment being provided as required in order to obtain a smooth and uniform distribution of the coating over the sheet of paper. It has been found that the use of both brushes and smoothing rolls as shown is desirable, the brushes serving to remove lines or streaks in the applied coating, and the rolls serving to augment this action and to remove any brush marks. The sheet then passes over suitable guide rolls to a drier, indicated diagrammatically at 25 where the coated sheet is subjected to heated air currents. The construction of this drier has not been illustrated in detail as any of the well known forms of air driers may be used. If desired an apparatus such as that disclosed in my above mentioned copending application may be used.

After being sufliciently dried, the sheet then passes over guide rolls and idler roll 3| to the top roll 32 of a second coating press comprising the upper roll 32 and a lower roll 33. At this point a second coat of the composite moistureproof and greaseproof coating is applied, which is of such character that the combined effect of the composite coating is to achieve the very desirable moistureproof, greaseproof, odorless, tasteless and flexible characteristics as mentioned above.

The upper layer of the composite g'reaseproof coating as applied comprises essentially a solution of cellulose, such as the solution of a cellulose ester, for example nitrocellulose or cellulose acetate, in an organic solvent. Alternatively the cellulose solution may be of a water solution of cellulose such as the viscous solution prepared by the carbon disulphide method, by the cupra-ammonium, or the strong alkali methods. All of these methods of producing cellulose solutions are well known and are not further described herein. Wherever the expression cellulose solution is used in the description and claims, it is to be understood that this includes the various cellulose lacquers, such as cellulose esters, ethers, etc., dissolved in organic solvents as well as the various cellulose solutions in an aqueous medium, such as viscose, etc. And wherever the expression cellulose film is used to describe the dried deposit of the coating on the paper, it is to be understood that this includes the dried deposits of the various solutions set out immediately above, including cellulose ester and ether films as well as films of regenerated cellulose. The preferred cellulose solution is a solution of nitrocellulose in organic solvents and the use of this nitrocellulose solution will be described in detail by way of illustration of the preferred manner of practicing the invention. Nitrocellulose is preferred on account of its relative cheapness and the ease with which the solution thereof can be prepared and applied by the useof inexpensive organic solvents.

The solution of cellulose applied in this manner is highly desirable for the reason that it provides in conjunction with the lower coating above described a composite coating which is highly moistureproof and greaseproof. Furthermore this cellulose solution can be applied in a very thin layer over the base coating and therefore the process may be carried out on a commercial scale in an economical manner. Because of the high flexibility of the base coat as described above, forming a flexible base for the overlying or cellulose coat, the composite coating is found to withstand bending of the board without breaking or cracking, and a product is thereby provided which is peculiarly fitted for the manufacture of foodstuff containers and the like.

The solvent for the nitrocellulose composition may be any of the known suitable cellulose ester solvents, although it is preferred to use the cheaper solvents such as acetone. It has also been found preferable to use a mixture of acetone with methanol, gasoline, and/or toluol as comparatively cheap diluents, to provide an economical solution of sufliciently low viscosity to afford easy application and spreading ability in commercial operations, the solvents employed being highly volatile so as to secure quick drying of the film. Care is exercised in the addition of the cheaper diluents, for the reason that when either the gasoline or toluol is used in too' high a proportion, a precipitation of the nitrocellulose may occur. Preferably a mixture of acetone, methathe purpose of improving the flexibility and covering power of the cellulose coating. Such plasticizers include dibutylphthalate, castor oil or linseed oil. Preferably the material is so chosen that it will not be objectionablein the top coating on account of its odor, and for this reason it is preferred to use dibutyl phthalate.

A cellulose film as described above has the characteristics of being quite tough, highly waterproof, moistureproof, greaseproof, odorless and tasteless. In some instances, however, it may be desired to provide even a greater degree'of moistureproofness, and in such case certain gums or resins may be applied along with the cellulose solution. Generally speaking, any gums, waxes or resins either natural or synthetic that are compatible with nitrocellulose may be added to secure special effects such as moisture, alkali or acid Droofness. of gums include ester gum (esterified rosin). Various types of acetone soluble synthetic resins, such as vinyl resins may also be used. Stearic acid may be advantageously added to impart additional moistureproofing properties. Where resins such as the vinyl resins are used, the sheet in addition to being moistureproof and grease- For example, satisfactory types 10 gallons of solution may be added 10 pounds of ester gum.

This composition is supplied in any desired manner to a vat 34 in which coating roll 33 rotates. The vat is provided with an inwardly extending portion 35 which closely approaches the surface of roll 33 so as to minimize escape of the volatile solvents used with the coating. A doctor blade 36 mounted on a suitable support 31 which may be a part of the frame in which rolls 32 and 33 are mounted, is provided for limiting the quantity of solution picked up by the roll. This doctor is preferably covered by a layer of felt 38 which bears against roll 33 and scrapes off the excess of the solution, leaving only the desired very thin layer. This layer is carried up to the nip of the rollsvwhere it forms a small pool and a substantially impervious and uniform coating across the entire width of the sheet is thereby secured.

Rolls 32 and 33 operate under heavy pressure so that only an extremely thin layer passes through the nip and is applied to the sheet. In this manner it is possible to obtain a very thin top coating of the cellulose film so that the process is highly economical; this also permits rapid and thorough drying. Very satisfactory results have been secured by the application of a top film containing about 1 to 3 lbs. of nitrocellulose for every 3000 sq. feet of board. It is found that this thin film is entirely satisfactory to provide the desired greaseproofing characteristics of the finished sheet when it is superimposed on the flexible base coat containing latex, which latter is somewhat oil soluble. Furthermore, it is quite desirable to use such a thin film of cellulose solution for reasons of economy. With water solutions of cellulose, a somewhat thicker film is provided such as to give about 1 lb. of regenerated cellulose film to 1000 sq. feet of board.

After leaving the coating press, the sheet may pass to suitable cutting mechanism (not shown) where it is cut into the desired size for formation into its ultimate use.

The present invention therefore provides a highly satisfactory greaseproof and moistureproof composite coating which can be applied in a most economical manner to paper or paper board in the formed sheet and which will not crack or break during the subsequent forming operations of the paper into a container. The coating furthermore is substantially odorless and tasteless, the latex in the base coat being effectively covered and masked by the impervious cellulose film. The coating furthermore is tough and will not be subject to deterioration over long periods of time.

While the method and article herein described constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to this precise method and article, and that changes may be made therein without departing from the scope of the invention which is defined in the appended claims.

the sheet substantially greaseproof and moistureproof and permitting bending of the sheet without injury to the coatings.

.2. A paper product of the character described comprising a sheet having a base coating comprising essentially rubber latex and an oil insoluble adhesive, and possessing high flexibility, and an overlying coating containing essentially a cellulose ester, the said overlying coatingbeing a thin film, and being jointly flexible with the base coating.

3. A container board of the character described comprising a board having a composite greaseproof coating thereon, said coating including a. base layer of rubber latex and an oil insoluble adhesive, and an overlying film of a cellulose ester, said cellulose ester film being of such thinness as to provide a weight of the order of l to 3 pounds of said cellulose ester for every 3000 square feet of board.

4. A paper product of the character described comprising a sheet having a composite tough, odorless, tastless, greaseproof and moistureproof coating thereon, said coating including a base layer of rubber latex and casein together with small quantities of an alkaline agent and a preservative, and an upper film of cellulose together with a plasticizer and a moistureproofing gum, said sheet and composite coating thereon being flexible to maintain the continuity of said composite coating during use.

5. A paper product of the character described comprising a sheet having a composite tough, odorless, tasteless, greaseproof and moistureproof coating thereon, said coating including a lower layer comprising essentially rubber latex and an oil insoluble adhesive, and an upper thin layer comprising essentially a film of cellulose combined with aplasticizer and ester gum, said sheet and composite coating thereon being flexible to maintain the continuity of said composite coating durins use.

6. A paper product of the character described comprising a, sheet having the surface thereof greaseproof, said sheet having a greaseproofing surface stratum thereon consisting of superimposed layers, said layers comprising a lower layer having high flexibility and forming a base coating for the reception of an upper coating, and an upper coating layer containing a cellulose ester material and greaseproof in its characteristics superimposed on said lower layer, said sheet and composite layer thereon being jointly flexible to withstand the efiec-Es of bending during use.

7. A paper product of the character described comprising a sheet having a composite greaseproofing surface, said surface comprising a lower layer which includes a substantial proportion of rubber effective material and has high flexibility, and an upper layer which includes a substantial proportion of a cellulose ester material and has high greaseproofness, said sheet and composite layer thereon being. flexible to withstand the effects of bending during use.

JOSEPH HENRY SWAN, III.

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