US2115132A - Manufacture of nonfibrous sheets and films - Google Patents

Description

- April 26, 1938. PQALLES, ET AL 2 MANUFACTURE OF-NONFIBROUS SHEETS AND FILMS Filed lay 51, 1954 2 Sheets-Sheet 1 R S (u Wvzwrpzs. Hank P. A1165 DdYfd H Edwards ATTORNEX A ril 26, 1.938; F. PQALLES El AL. 2,115,132

MANUEACTURE OF NONFIBROUS SHEETS AND FILMS Filed May 31, 1934 I 2 Sheets-Sheet 2 INVZT NTORS. Frank P. fllles David 'flfdwa'rda ATTO NEY ducing puckers and wrinkles.

Patented Apr. 26, 1938 UNITED STATES" MANUFACTURE OF NONFIBROUS SHEETS ANDFILMS Frank P. Alles, Buffalo, and David H. Edwards,

Kenmore, N. Y., assignors, by mesne assignments, to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware Application May 31, 1934, Serial No. 728,378

8 Claims.

This invention relates to the drying of non-' fibrous sheets and'films which are subject to shrinkage during drying, and in particular it pertains to the drying of'regenerated cellulose sheeting cast from viscose and adapted to be used as wrapping tissue.

Heretofore, in the manufacture of such sheets and films of regenerated cellulose a film is cast from viscose into a coagulating and/or regenerating bath. and subsequently passed through additional baths such as regenerating, desulfuring, washing, bleaching, and glycerinating baths. Thence, it is passed through a-drying apparatus, and wound up into rolls. In passing through the drying apparatus, thefilm loses approinmately 30 to 40 parts of moisture per 10 parts of cellulose, so that at the windup the film contains about 0.6 to 0.8 part of moisture per 10 parts ofcellulose. In losing this moisture, the film tends to shrink to a great extent, thus diminishing the area of the regenerated cellulose film and produce' a film which is comparatively large in area and without puckers 0r wrinkles, the drying apparatus customarily consists of a series of rotating heated rollers, over which the film passes and which keep it at all times under marked tension as it is dried.

The prior art film so produced, however, attains its greater area at the sacrifice of other desirable properties. Particularly, the film is characterizedby internal strains which, over a period of time, give rise to an irreversible shrinkage or loss in dimensions during use. Thus, for instance, when a box of candy iswrapped in a film containing the above described internal strains, it will be found that over a period of time, for instance one week to several months or more, the wrapper will gradually contract and tighten up around the box, thus impairing the otherwise neat appearance of the package and frequently breaking or shattering the wrap itself, and destroying its entire effectiveness. This irreversible contraction manifested as above is greatly accelerated if at some time during the life of the sheet it is exposed to high humidities of or upwards. When exposed to such humidity condi-- tions, the sheet absorbs moisture, seeming to effect a much more rapid release of the strains so that if the sheet next becomes exposed to lower humidities, it will thereupon contract to' smaller dimensions than it initially possessed.

This irreversible shrinkage we term throughout the specification and claims as "permanent shrinkage, and distinguish it from changes in In order to prodimensions which result solely from gain or loss in moisture in the sheet.

In order to more clearly describe'the scope of i this invention, the term deformation", in relation to regenerated cellulose, will be clearly defined and difierentiated from permanent shrinkage". By "deformation we mean the reversible changes in dimensions which a sheet of regenerated cellulose undergoes with changes in its internal moisture content. Deformation manifests itself only upon changes in moisture content of the sheet while the process of relieving internal strains, which results in permanent shrinkage, takes effect more quickly on increasing the moisture content of the film, but also takes effect at other or constant moisture contents. v

A sheet of regenerated cellulose which is free from permanent shrinkage and internalstrains will expand or contract according to whether its moisture content becomes higher or lower. This moisture content of the sheet is primarily dependent upon the relative humidity of the circumambient atmosphere and to a lesser extent upon the temperature. For example, a freely suspended sheet free from permanent shrinkage, in equilibrium with a humidity of 35%, will expand when allowed to come to equ'ilibrium with a humidity of Yet, when it is returned to and allowed to come to equilibrium with the original 35% relative humidity, it will return to substantially its initial dimensions. Likewise, if the same sheet is removed from an atmosphere of 35% relative humidity to an atmosphere of -5% relative humidity, it will contract. On returning to the original 35% relative humidity, it will again regain its original dimensions. This reversible change in dimensions of a sheet is here termed its deformation and is distinguished from permanent shrinkage in that it is reversible, whereas permanent shrinkage is irreversible.

Deformation is characteristic of sheets of re generated cellulose even when softened with glycerine. It is furthermore characteristic of moistureproofed regenerated cellulose, butwith such sheets the phenomenon is observed relatively infrequently since the moistureproof coating substantially prevents the transmission of moisture, and the moisture content of the film, therefore, remains approximately constant.

While we have described, for the purpose of clarity, an example of deformation in a film which is free from permanent shrinkagesince the effect can be best observed in this wayjit is to be understood that deformation in a film is independent of the presence or absence ofperthe method is practiced on the gel film in the ner with considerable tension, the resultingsheet will naturally possess both permanent shrinkage and deformation at the same time.

It has hitherto been thought necessary in order to overcome permanent shrinkage to dry regenerated cellulose with no tension at all, such as by drying in festoons. This permits free shrinkage and consequent release of or freedom from internal strains, but the resulting film is not smooth and free from wrinkles and is therefore not commercially useful. Furthermore, the film Lads to be slightly opaque or milky, thus suffering by comparison with the appearance of the ordinary smooth, glossy surfaced transparent regenerated cellulose films. Finally, such a method is expensive in operation since it can only'be performed discontinuously and requires relatively cumbersome apparatus.

It is an object of this invention to produce sheets or films of regenerated cellulose having substantially no permanent shrinkage and which at the same time are transparent, smooth, glossy, and free from wrinkles and other surface irregularities. It is a further object to produce laminated materials which do not appreciably permanently buckle, curl and/or wrinkle upon exposure to varying atmospheric conditions, in which at least one of the lamina is a sheet of regenerated cellulose. Other objects will appear hereinafter.

The objects of this invention are accomplished by drying the sheet or film of regenerated cellulose at such low tension that it is allowed to shrink freely, but with sufficient tension to prevent the. formation of unsightly wrinkles and other blemishes. This may be'to a certain extent accomplished by rewetting the dried film to the desired moisture content and redrying according to the method of this invention, but preferably conventional drying apparatus, modified as hereinafter described. More particularly, it has been found that it is possible to dry the film in the conventional manner down to a certain critical moisture content, which we have observed to be at a point when the film contains about 6 parts moisture per 10 parts of regenerated cellulose plus glycerin or other softener, and thereafter according to the method of this invention. By reducing the moisture content of the film from T the usual 30 to 40. parts of moisture per 10 parts of cellulose as iteriters the drying apparatus down to about 6.parts of. moisture per 10 parts of cellulose in the conventional manner, and thereafter at reduced tension according to this invention, a final film is produced of low permanent shrinkage, complete transparency and good smoothness. Furthermore, this procedure necessitates minimum changes in the conventional drying apparatus, and results in a film comparatively large in area. Accordingly, the invention will be specifically described'in terms of this procedure, but it will be understood that this is illustrative and 'not' limitative.

In the accompanying drawings, Figs. 1 and 2 are side views, partly in .section, illustrating one form of apparatus adapted to the practice 'of the invention. these figures, when p l aced end to end, representing a complete film-casting machine. Figs. 3, 4, and are side views, partly in section, showing other forms of apparatus which may be used in the application of the invention.

Referring now to the drawings, Fig. 1 shows the front end of a conventional film-casting machine in section, and Fig. 2 shows the end of the casting machine where the drying of the films takes place. Viscose is extruded through a narrow slit in the bottom of .a hopper l into a suitable coagulating bath 2 to form a continuous sheet or film 8 of regenerated cellulose which may then be passed through various treating tanks (only partlyshown in Fig. 1) and is subiected to various processing steps such as washing, desulfuring. washing, bleaching, washing and glyoerinating, the filmthen being passed through the dryer shown in Fig. 2.

Referring to Fig. 2, thefilm is drawn through squeeze rollers 4 and alternately under positively driven rolls 5 and over rolls 8, which are driven by friction 'from rolls 5 as by means of tires 58 which also serve to space rolls 8 from rolls 5. Dryer rolls 5 and 8 are of the same diameter, rolls-5 being internally heated by means of fluid such as hot water or steam. when the sheet reaches the last rolls 6 in the first part of the positively driven roll 8 6 is frictionally driven, over rollers 1, down again over a similar roll 8, up over roll I, and so on, until it is wound up on the take-up roller 8. Rollers I and 8 are of substantially the same leaving the last rolls 8 in the first part of the dryer, is then drawn under from which the last roll size. Rollers I and 8 may all be internally heated, or alternatively rollers 8 alone may be heated. Dryer rolls 5 and 8 are driven at the sameperipheral and angular speed. Rollers I and 8, on the other hand, are driven at a gradually decreasing speed, going from the wet end to thedry end of the dryer, corresponding to the period in which the film decreases in moisture content to that of the finished product wound up on the wind-up roll, the film thus being allowed to shrink substantially freely in the longitudinal direction, while producing a smooth, unwrinkled surface duefto the fact that tension is maintained on the film during the whole drying-operation.

Dryer rolls 1 and 8 can be given a progressively decreasing'peripheral speed in any known manner, such as providing individual drives for each dryer roller 1 and 8, or for each group of dryer rollers.

Alternatively, all or some of the dryer rollers 1 and 8 may be driven by contact with the film only. In such case, for example, the lower rolls 8 may be positively driven at diminishing speed toward the dry end of the machine while upper rollers I are driven only by contact with the film; or, the first roller ,8 and the last roller 8 may be positively driven, the first roller being driven at a speed greater than thatof the last roller while the other rollers 1 and 8 are driven only by friction from the film.

In order to accomplish transverse shrinkage of the film, it has been found that due to the relatively loose condition of the film when longitudinal shrinkage is permitted, in some cases no further modification of the dryer construc 7 tion is necessary- However, in order to accomplish better results, the surfaces of the rollers I and 8 may be altered so as to permit the film to slip freely thereover. Alternatively, certain of the rollers in the conventional dryer may be skipped or they may be eliminated entirely as has been shown in Fig. 2, thereby maintaining the film-out of contact with a supporting surface 1 for a substantiaiportion of the drying operation.

The spacing of rolls I, as shown in Fig. 2, contributes to this result since the film between succeeding rollers 1 is supported freely and out of contact with the rollers.

In Fig. 8 is shown a modification according to the method of Fig. 1, wherein the positively driven dryer rollers '7' and/or 8 are of gradually reducing size, going from the wet end to the dry end. All the rollers may then be driven at a constant angular speed but at a decreasing peripheral speed. It is easily seen how the same result as in the first example can be obtained. Here also, rollers 8 may alone be'internally heated, or both rolls 1 and 8' may be heated.

In Fig. 4 is shown a modification of the methods according to Fig. 2 or 3. This modification is adapted to drying by suitably directed air currents instead of or in addition to internally heating the rollers 1" and/or 8", this result being attained by enclosing rollers 1 and 8" in an enclosed chamber l0 adapted to dry the sheet by means of heated air or other suitable gas. When this drying means is adopted, it is preferred to space rollers 1" considerably apart from rollers 8", thereby causing the film to be freely suspended during a large .portion of the drying period. In this modification also the rollers 1" and 8" are driven at progressively diminishing peripheral speeds.

It will. be understood that the methods and apparatus described in the preceding paragraphs with respect to Figs. 3 and 4 may be combined with the various'means for decreasing the speed tent within the range 4-10 parts per 10 parts of cellulosethat isdesired, and thereafter dried in one of the modifications shown in Figs. 2,. 3, and

4, or other suitable method. If the film entering the chamber H contains too much moisture, it

will be dried in chamber II, in which case chamber II is simply a drying chamber. If, on the other hand, the film going into chamber II is ,too dry, its moisture'content will be raised to the desired amount, within the range specified, in which event, chamber II is a humidifying chamber.

Accordingto any of the aboveexamples, the speeds at which the mechanically driven rollers are to be driven to produce the required tensions can be easily determinedby one skilled in the art and will depe'ndlargely"on'other factors in the manufacturing process, such as kind of cellulose or pulp used, temperature of drying,

amount and character of softener used, etc.

The tension necessary to produce the results contemplated by this invention, during that part of the drying operation where the permanent shrinkage is avoided, is quite low,- being in the neighborhood of 0.1 pound per linear inch across the widthof a film which, when dry, is approximately 0.0009 inch thick. Thicker films will require correspondingly greater tensions. It is to be understood, of course, that variations are possible, such as from 0.5 lb. to 0.02 lb. per linear 'inch, but' normally the best results have been secured by using a tension of 0.1 lb. The best re-' suits are secured, of course, when the tension is low enough to reduce permanent shrinkage to a reasonable minimum, but high enough to prevent the formation of wrinkles and other unsightly distortion of the film.

The most satisfactory moisture content at which to release the tension to the desired value is about -6 parts of moisture per 10 parts of cellulose, but it is to be. understood that, this, too, may vary within limits. For example, if the tension is released at a moisture content of about 10 parts per 10 parts of cellulose, the finished film will not show undue loss in dimensions. On

the other hand, if the tension is not released until after the film reaches a moisture content of 4 parts per 10 parts of cellulose,'the resulting permanent shrinkage in the finished film is still comparatively low. Operating outside of these limits, however, tends to produce unsatisfactory results and for these reasons 'we prefer to release the tension at a moisture content of approximately 6 parts per 10 parts of cellulose. The preferred ranges which we disclose, of course,

- depend in large part on other factors in the manment of the cellulose micelles, thereby eliminat If, therefore, in the course of mani ufacture, substantially no strains are introduced ing strains.

in the film on drying from this point down to the finished film, then the finished film will be substantially ,in its final :form. On the other hand, if the film "is dried in the ordinary manner, that is,;with marked tension, and the film subsequently never allowed to reach the degree ofplasticity of a film containing 6 parts of moisture per 10 parts of cellulose, the process of relieving strains will be extremely slow although it will eventually manifest itself as a decrease in dimensions. It a similar piece of freshly prepared film, however, is subjected to an atmos-@ phere of relativehumidity, which is a very high relative humidity in practice, and allowed to come to equilibrium therewithfit is observed to contain approximately 6 parts of moisture per 10 parts of cellulose, based on the weight of the dried film plus softener. Such a film, as pointed out above, thus becomes suificiently plastie to quickly allow ,a free release of strains and, on returning to the normal atmosphere again, will have exhibited a loss in dimensions, based .on the dimensions in the normal atmosphere before subjection to this high humidity. A fin:-

stantially no strains present, and when it returns to a normal condition again, the film will be substantially in the same condition as it was initially.- Furthermore, our improved film, over a period of time, will substantially not exhibit the above mentioned gradual decrease in dimensions of the prior art film when not allowed to reach this high humidity.

Based on the above observations, and for the purpose of measuring permanent shrinkage and defining the term substantially no permanent shrinkage", a method has been evolved for reducing the strains in a finished piece of mm and measuring the resulting changes in dimensions.-

The method, which is carried out at a constant temperature of about 305 C., is as follows: A sampleof the finished film to be tested is allowed to come to equilibrium with respect to moisture content in an atmosphere of 35% relative humidity, and the dimensions then measured. The film is next placed in an atmosphere of 95% relative humidity for a period of two hours, during which time substantially all the permanent shrinkage is eliminated since strains are more quickly relieved at high moisture contents of the film. Finally, the film is brought back to the atmosphere of 35% relative humidity, allowed to come to equilibrium with respect to moisture content, and

ing both directions, of less than 2%, and preferably less than 1%.

Film produced in accordance with this invention has many advantages over the prior art materials. It has greatly increased durability at average relative humidity and at low relative humidities and at low temperatures. It has superior gloss, thereby enhancing the appearance ,of the material. It is possible to store film manufactured according to my process for much longer lengths of time, due to the fact that it will not pucker or exhibit washboard effects. The film has much less tendency to stick together or cake under pressure and elevated humidities than ordinary film. For example, a'stack of ordinary sheets and a stack of sheets manufactured according to our process were each subjected to relative hmnidities of 80% and a pressure of 1 pound D l: square inch. The stack of ordinary sheets stuck together at the end of four days while the stack of improved sheets did not stick together until the end of seventeen days. Packages wrapped with material manufactured by ouriniproved method will have much less tendency to distort, buckle or collapw, especially when sub- :Iected to high humidity conditions and subsequently returned to normal again. Laminated materials in which at least one of the piles comprises a sheet or film of regenerated cellulose, manufactured according to our process, e. g., a

laminated material comprislng'a sheet of heavy paper surfaced on'one or both sides with anadhering sheet of regenerated cellulose. do not exhibit any appreciable permanent buckling, curling and/or wrinkling upon exposure to the atmosphere. I.

While the invention has-been described in connection with the manufacture of clear, transparent film, it may also be applied-with advan- Me to the opaquing agent, e. g., a pigment such as titanium oxide.

' than 6 parts and not'less than manufacture of film containing an While this invention has been described in terms of regenerated cellulose obtained from vis-.

cose, it is to be understood that it is equally applicableto the drying of film obtained from cuprammonium cellulose solution, and may also be applied to the treatment of other cellulosic films subject to shrinkage and strains during drying, and formed from an aqueous dispersion, e. g., films made from glycol cellulose and other cellulose ethers, casein, gelatin and others.

Any modification or variation which conforms to the spirit of the invention is intended to be included within the scope of the claims.

What is claimed is:

1. In the method of making pellicles from aqueous solutions or dispersions of cellulosic materiaL'the steps which comprise casting and processing said pellicles, removing most of the moisture from said pellicles and simultaneously restraining the shrinkage thereof, and then-continuing the drying of the pellicles under a tension which'will prevent wrinkling or distortion of said pellicles while permitting substantially free shrinkage thereof.

2. In the method of making pellicles from aqueous solutions or dispersions of cellulosic material, the steps which comprise casting and processing said pellicles, removing moisture from said pellicles and simultaneously restraining the shrinkage thereof until they contain not more than 10 parts ofmoisture-per 10 parts of cellulose, and then continuing the drying of the pellicles under a tension which will prevent wrinkling or distortion of said pellicles but which will per-- mit substantially free shrinkage thereof.

3. In the method of making pellicles from aque-.

'ous solutions or dispersions of cellulosic material, the steps which comprise casting and proc* essing said pellicles, removing moisture from said pellicles and simultaneously restraining the shrinkage thereof until they contain not more than 10 parts and not less than 4 parts of moisture per 10 parts of cellulose, and then continuing the drying of the pellicles while progressively decreasing the linear speed thereof whereby to subject the pellicles to a tension which will prevent wrinkling or distortion of said pellicles but which will permit substantially free shrinkage thereof.

4. In the method of making pellicles from aqueous solutions or dispersions of cellulosic material, the steps which comprise casting and processing said pellicles, removing moisture from said pellicles and simultaneously restrainingthe shrinkage thereof until they contain not more than 10 parts and not lessthan 4 parts of moisture per 10 parts of cellulose, and .then continuing the drying of the pellicles under a tension whi will prevent wrinkling vor distortion of said pellicles but which will permit substantially free shrinkage thereof.

4 parts of mois- 6. The method described in claim 5 characterized in that tension is applied during the drying operation in an amount suilicient to prevent wrinkling or distortion of the pellicle during drying, the tension being equivalent to from 0.5, pound to 0.02 pound per linear inch on the basis of a pellicle approximately 0.0009 thick.

'7. The method described in claim 5 characterized in that tension is applied during the drying operation in an amount suihcient to prevent wrinkling orvdistortion of the pellicle during drying, the tension being equivalent to about 0.1 pound per linear inch on the basis of a pellicle approximately 0.0009" thick.

8. In the method of making pellicles from aqueous solutions or dispersions of cellulosic materials, the steps which comprise casting and processing said pellicles, removing moisture from said pellicles and simultaneously restrainingfthe shrinkage thereof in a drying atmosphere until the pellicles contain not more than 10 parts and not lessthan 4 parts of moisture per 10 parts of cellulose and the moistufdn said pellicles approaches equilibrium with said atmosphere, and then continuing the drying of the pellicles under a tension which will prevent wrinkling or distortion of said pellicles but which will permit substantially free shrinkage thereof.

FRANK P. ALLES. DAVID H. EDWARDS.

Images