INDUSTRIAL AND ENGINEERING CHE,V!ISTRY
May, 1929
Figure 1-Plants
405
of the Du Pont Cellophane and Du Pont Rayon Companies a t Buffalo, New York
Manufacture and Properties of Regenerated Cellulose Films’ William L. Hyden TECHNICAL DEPARTMENT, D u POXTCELLOPHANE COYPANY, I N C . ,BUFFALO, N. Y.
HE commercial production of films of regenerated
T
cellulose in sheet form, ,such as Cellophane, has been steadily increasing during the past few years. Cellophane is used chiefly as a wrapping material. Being transparent, lustrous, durable, flexible, and impervious t o air, grease, germs, and dirt, it makes a highly protective as well as an artistic wrap. Note--“Cellophane” (“cello,” cellulose: “phane,” glass) is the registered trade-mark of the Do Pont Cellophane Company, Inc., used to designate its transparent cellulose sheets and films. Similar films are manufactured by several European corporations.
The raw material for Cellophane is cellulose in the form of purified wood or cotton pulp sheets. By the viscose2 process these pulp sheets are put into colloidal solution and the cellulose is regenerated into continuous transparent films. History
Although Beadle3 a,nd others prepared cellulose films experimentally, the industry of manufacturing transparent 1
Received March 13 1929.
’ Cross, Bevan, and Readle first described the preparation of viscose, Bcr., 26, 1090, 2524 (1893). Margosches, “Die Viskose,” p. 79, Klepzig, 1906.
films of regenerated cellulose was founded by Brandenberger,‘ of Neuilly sur Seine, France, in 1908. Brandenberger first specialized in the chemistry of dyeing, printing, and finishing of cotton cloth. H e believed that the beauty of cotton fabrics could be greatly enhanced if they could be given a luster like that of silk. He attempted to achieve this result by applying viscose, a t that time a new cellulose solution, to the cloth and by imprinting the viscose he hoped to impart lustrous designs to the fabric. The first experiments of this nature were unsuccessful. Although the cloth had increased luster, it was too stiff and entirely unfit for garment or other use. He then conceived the idea of making a thin, lustrous cellulose film separately and afterward applying it to the cloth. His effort to produce the cellulose films separately led to the idea of their commercial manufacture. The first machine for the continuous manufacture of transparent films was designed and built by Brandenberger a t the Blanchisserie e t Teinturerie de Thaon-les-Vosges, France, in 1908. The product of this machine was thick and brittle and was not very useful. By 1912, however, Brandenberger had succeeded in producing thin, in thick-
‘
Brandenberger, U. S. Patents 981,368 (January 10, 1911) and 1,601,289 (September 28, 1926).
INDUSTRIAL A N D ENGINEERING CHEMISTRY
406
Val. 21, No. 5
the viscose depends upon the aging time, the alkaIi-cellulose is stored in covered containers for a period of time at a carefully controlled temperature. The alkali-cellulose aging time and temperature have a marked effect upon the viscose viscosity (Figure 4). The aged alkali-cellulose is next converted into a n ester, cellulose xanthate, hy the action of carbon disulfide. The reaction is carried on in double-walled, tightly closed, rotating barrel-like containers (Figure 5). During the interaction the white alkali-cellulose is converted into orange-red cellulose xanthate. /o-csHao, CaHnOsiVa CS,---r-C-S 'S-Na The temperature, concentrations, and time are controlled factors in muthation. Cellulose xanthate readily forms colloidal solutions in dilute caustic soda. The freshly prepared xanthate is mixed with dilute sodium hydroxide solution until a uniform colloidal solution, known as viscose, is formed (Figure 6). As in the manufacture of rayon, before the viscose can be suc~essfullycoagulated and regenerated, i t must be "ripened." During ripening, which is conducted at a constant temperature (usually in the range of 1 5 O to ?5" C.), a series of complex reactions occur which are not fully understood, despite the fact that many investigators have studied the process. It is known that the molecule becomes more complex and that t.he rat.io of sodium a.nd sulfur to cellulose changes. Reactions occurring during the ripening of viscose may be represented by the following equations:' ,O---CsHaO+ ,O(CiHrOSnOH /OH 4c=s ZH,O+ZC=S + 2 C d (A)
+
+
'S-Na /O(CJ%O&ON Figure 2-Flaw Sheet for Production of Cellophane from Wood or Llnfer Pulp by VieCoSE Procevs
ness with the lighter weight present-day Cellophane (approximately 0.02 mm.). This development rcsultcd in t,he production of a salable film. Kumerous other improvements have since been made and the applications of Cellophane are constantly increasing. Manufacture
2 c 4
1 sN a
1926).
For description of the manufacture 01 viros* $et Marsosches, '.Die Viskose: Klepziig, Leipdz. 1906; Auram, "The Rayon Industry," D. van Nortrand Co., Ne- York, 1827: Hottenmth. "Die Kuntseidc," s. H i n e i , Lcipzig. 1928: Heuser. "Cellulose Chemirt.ry;' McCmw-Hill Book Co., New York, 1824.
\S Na /OH
+ c=s
S'
Na
(B)
\S Na
-
At the same time side reactions oroceed accordine to the equations: /OH
ZCSz
For the preparation of Cellophane it is essential to use purified wood or cotton pulps. Although transparent films of regenerated cellulose have hoen made from cuprammonium solutions' of cellulose, the viscose process has proved more adaptable and economical for commercial use. The flow sheet for the manufacture of Cellophane by the viscose process6 is given in Figure 2. Bleached pulp in rectangular sheets of from 80 to 90 per cent alpha-cellulose content are placed in steeping presses into which is introduced an 18 to 20 per cent sodium hydroxide solution. The temperature is maintained constant during the soaking process. During this treatment the fibers swell and form a rather unstable alkali-cellulose compound and at the same time the hemicelluloses dissolve in the caustic. The excess sodium hydroxide solution, containing the dissolved hemicelluloses, is pressed out until the weight of the pressed alkali-cellulose sheets is about three times that of the dry pulp. The alhlkellulose isshredded int,o a fluffy amorphous state by machines illustrated in Figure 3. Since the viscosity of Zdth and Ziegler, U. S . Patents 1.590,601 and 1,590.602 (June 29.
+
\S Na
/O(CsHeOSr(OH)s HsO---2C=;S
+ 4NaOH---tNa2COa + NazCSI + 1-12S+ H10 NazCSa + 31f20-+Na2COa + 3HzS
(D)
(E)
Fieure 3-Shredders
While ripening, the viscose is filtered several times to remove dirt, fibers, and other foreign matter. The state of ripening is closely followed by determining the "salt number" by a method somewhat similar to the one first published by 7
Heuser. "Cellulorc Chemistry:
P. 66.
May, 1929
INDUSTRIAL AND ENGl NEERING CHEMISTRY
Hottenrothk;namely, the determination of the concentration of a sodium chloride solution necessary to start coagulation of small quantities of viscose solution. When the desired state of coagu!ability is reached, the viscose is cast. To insure the production of a uniform film, free from gas bubbles, a l l incorporated gases are removed from the viscose by vacuum just prior to its transfer to the casting machines.
407
through the drier, it comes in contact with drafts of warm air. Finally, the Cellophane is wound on cores into rolls of any desired weight. Finishing Processes
Cellophane from the casting machine is cut into sheets approximately one meter square. Each sheet is carefully inspected and if any sheet contains imperfections, or is of low transparency, it is rejected. UYEIXCm n EmossINc--Cellophane may be dyed any desired color with either mordant dyes or direct colors (Figure 11). When dyed with direct colors, the operation consists merely of passing wet film through a dye bath of the correct concentration and temperature to produce the shade of color Casting Viscose into wanted. After suitable washing, the film is passed through a glycerol solution and dried. When mordant dyes are used Films additional baths are required, but it is possible to obtain Numerous patents shades of color which cannot be duplicated with direct dyes. have been issued and Cellophane can be embossed by pressure rolls or stamps methods described for of desired designs. Finishes similar in appearance to “linen” the m e c h a n i s m of cloth, “morocco” leather, silk fabric, and other embossed producing continuous patterns are possible. films of regenerated MoISTUREPRoOF CELI,oPHANE-MoiStUreprOOf Cellophane, Figure 4 4 h a n g e InViscoalNof Vlscoses cellulose fronl viswhich has been recently developed in America, is lustrous and Repared from the Same Alkali-Cellulose cose. I n all methods highly transparent. The .following table is a comparison of Aged from 15 to 70 Woum viscose under pres- the moistureproof quality of this Cellophane and snmnples of sure is forced to the casting apparatus, commonly called waxed papers. The moistureproof quality is determined by a hopper. One type of hopper illustrated in Figure 7 is sealing a piece of the material to be tested over a glass dish equipped with non-corroding, smooth-surfaced lips (2 and 2’) containing water and determining the loss in weight when the which are accurately adjusted by the mechanism (3, 4, 5, 6, test dish is placed in an air oven at 100” F. The air in the 7, 8) to control the dimensions of the aperture (1 1). A uni- oven is circulat,ed by a fan and desiccated by concentrated form sheet of viscose is extruded between the lips into a co- sulfuric acid kept in fiat trays. By carrying out the test at agulating bath (usually sulfuric acid and salts), where the loo0 F. the rate of diffusion is much great,er than at lower cellulose xanthate solution is rcgeneratcd into a cellulose temperatures and the test is thus accelerated. fihn. DIF#USXON 011 W m m A second means of casting continuous films is illustrated by VAPORA T I W “ F . MITER1iil.S Figure 8. A hopper (3, 4) of fixed clearance is suspended MS.per sn. m. 9 r r kaur 270 directly above a rotating roll (5), which i s partially snb400 merged in a coagulating bath. Viscose is extruded upon the 580 22,200 smooth surface of the moving roll, which is continuously Represents highest qualify waled papers which have been tested. moistened by a film of coagulating bath. The viscose is transferred in uniform thickness to the bath and, after being regenerated into a film of cellulose, is stripped from the roll (5)’ since the film is led ovcr rolls (8 and 9). Figure 9 illust,rates a modification of Figure 8. In this case the viscose is laid in uniform layers from the hopper (15) upon a continuous smooth belt (11) instead of a roll. As the belt movcs around the partially submerged rolls (12 and 13) a long supported bath travel is afforded the film. It is stripped from the belt by being led over roll (18) and transferred to the succeeding tank by the following roll (19). As shown in the diagrammatic sketch of the casting machine (Figure IO), after the regenerated cellulose film is formed in tanks 1, 2, and 3, .it is transferred by rolls into succeediiig tanks filled with fresh warm water, which remoms the residual acid in the film carried over from the casting tank. The film is next passed through a solution (commonly NaOIl, r\’a,S, or Na2S0,) for the purpose of removing the sulfur deposited Figure 5-Rofafinp, Barrels in Which Alkali-Celluiose Is Conin the film by the decomposition of the cellulose xanthate. verted i n f o Cellulose Xanthate by Carbon Disulfide The film is further washed and bleached, using sodium hypochlorite. Finally it is passcd through a solution of Moistureproof Cellophane is particularly desirable as a glycerol of high purity. wrapper for food products which lose or absorb moisture when Because of its hygroscopicity, glycerol, which is adsorbed exposed to the atmosphere. As an illustration, a test made and tenaciously reiained by the film, imparts softness and by sealing 40 grams of soda-cracker biscuits in a large paper pliability. The film a t this stage is completely formed and is envelope and exposed at 70” F. and 70 per cent relative hudried by passing it over heated rolls, the desired temperature midity for 48 hours showed a gain in weight of 3.1 grams. A being maintained by automatic control. As the film proceeds similar test using Moistureproof Cellophane showed a gain of Hatienroth. Ckcm.-Zlg., SS. 119 (1915). only 2.0 grams after 6.5 months’ exposure. ~
~~
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 21, No. 5
ward movement of the film through the casting machine. The transverse direction is perpendicular to the longitudinal direction. It wlll also he seen that the tensile strength is greater in the longitudinal direction while the elongation is greater in the transverse direction. These characteristics can he employed to advantage in wrapping articles by so placing the Cellophane sheet that the geatest strain will come in the longitudinal direction. The "yield value," defined as the force which may be applied without permanent distortion, is r e p resented graphically by the point of departure of the curves from the abscissa. TRANSPARENCY TO ORDINARY AND ULTRA-VIOLET I,IGNT-Transparency is the chief characteristic of Ccllophane and permits its use for the protect.ion of materials when display is desired. One of the interesting optical properties of Cellophane described hy Pfundg is its transparency to ultra-violet light. This property makes Cellophane a pract,ical material for windows.'o which will allow approximately 70 per cent of the shortest waves reaching the earth to he transmitted. Pfund describes a Cellophane window suitable for treatment of 9 10
Pfund, Johns Ilopkinr Hospital, B d l . 40, 228 (April. 1927). Pfund, J . Am. M c d . As~cI.. Pi. 18 (19281.
Figure 6-Midne Room Where Cellulose Xanthate Is Mired with Caustic Soda Solution to Form ViSCOSe
I
i
The character of the surface of Moistureproof Cellophane is such that ordinary glues and adhesives cannot he used for sealing it and an entirely iiew type of adhesives has been dcveloped to fill this need.
>
Physical Properties of Cellophane
TENSILE STRENGTH AND ELoNrrATIoh.-Three weights of Cellophane, 33, 45, and BO grams per square meter, are regularly produced. These vary in thickness from approximately 0.001 t o approximately 0.002 inch. When thicker sheets are desired, they are made by plying together several thinner sheets using suitable gelatin or dextrin adhesives. The tensile strength of Cellophane and some papers as measured at 60 per cent relative humidity and 70' F. hy the Scott machine and Mullen tester are compared in the following table:
M*IEIII*L
Used for Castlng Viscose
(British Pthtent 3929)
w*icilt RE*UlE*D (Io B e s r x I ~ / a - I ~ cSram x BUBS~NO (Scorn Mncarhis) Srasnorn Longitudinal Transverse (MULLBN TBZCZNBSSdirection dirrctioa Tesran) Inch I.hS. i ~ b s . I.ar. an .vn~in ~~~
CellOphane Cellophane CdlOph&"e Sulfite paper All-rag hish-grade paper Sulfite bond P a p a Extra high-grade Paper
FIBure 7-Uapper
~
o.ow9s
15.7
n.wm
26.8
0.00212
22.5 43.0
0.00141 0.00162
o.002~0 0.00412
19.7 13.7
73.1
~~
~
7.3 8.8
13.2 7.7
14.5
23.2
55.5
.28.9~. ~
33.8 42.1 6.1 21.6 24.9 84.4
It will he observed that strength does not increase in direct ratio to increased thickness. To make comparisons of the strengths of Cellophane and other materials it is desirable to reduce the values to a unit thickness basis; usually such comparisons are made on thicknesses of 0.001 inch. The serviceahiiit,y of Cellophane is enhanced by its elastic properties. Figure 12 exhibits the relation of tenacity, elongation, yield values, and force in pounds. From the shove data it is noted that to define the physical properties of Cellophane it is necessary to designate the values in both the longitudinal and transverse directions, just as in paper description one must specify whether the quantity is measured with or against "the 5ain." The longitudinal direction NIRY be defined as the dimension parallel to the for-
Figure 8-U. S. P r l e n t 1,590,999
I-
H
-
I PiEUre 9-u. s. Patent 1,59o,999 ADDBTB~VB for Califlng Confinmas Films
May, 1929
INDUSTRIAL A N D ENGINEERING CHEMISTRY
409
Nm-Gold beater's skin ir considered setiofactory for baliaoa linings when if Permits the pansage of 0.126 %tu or 1of hydrogco PI. square meter per hour.
I n such work i t is necessary to apply the Cellophane to a fabric to support it sufficiently to withstand the strains to which it is subjected. This requires s p e c i a l t e c h n i c a n d special adhesives. Cellophane may also be used as membranes for ultra-filters, permitting the separation of molecules of widely different dimensions. I n dialysis work a regenerated cellulose film has heen found adaptable for certain liquids where there is not excessive acidity or alkalinity. DIELECTRIC PRoPERnEs-Cellophane that is dry, free from salts, glycerol, metallic particles, and other impurities has excellent d i e l e c t r i c p r o p e r t i e s . Figure IO-Diagram of Cellophane Casflng Machine When free from glycerol the film is not (U.S . Patent 1,648,864) very pliable, however, and its successful disease by heliotherapy which will last a t least one year and use for condensers and other electrical equipment has not yet which can be replaced for a nominal sum. The curves shown been developed. in Figure 13, taken from the experiments of Pfund, evidence I n the following table are recorded data showing the dithe fact that the transparency of Cellophane to light waves electric values for Cellophane: in the curative region 2900 1.to 3100 A. is about 70 per cent, Number of layer. e and that, unlike some special ultra-violet transmitting glasses Thickness, inch 0.0039 Permeiitiviiy, untreated 4 66 which lose a large fraction of their transparency on prolonged rower fartor. oil-treated. per cent 0.380 Corona formation. oil-troilted, volts per mil 1560 exposure to ultra-violet rays," the transparency of Cellophane Br-kdown Strength. volts per mil 1970 decreases hut slightly after an exposure to sunlight for a year. OTHER OPTICAL ~'ROPERTIES-There appear t o he two EFFECT OF AGING-Cellophane a numher of years old has optical axes in Cellophane, such that when one observes a heen found relatively unaltered in its properties except point of lumination through a perfectly clear film, the direc- through the changes in moisture and glycerol which have tion of the greatest tension during casting can be detected occurred during that period. In other words, there is not the since the diffraction of the transmitted light is at right angles deterioration which is often noted in some sulfite pulp papers. to this direction. Slight loss of percentage elongation is all that seems to result Cellophane exhibits the anisotropic properties of cellulose. from the ordinary aging of Cellophane. Being thus doubly refractive, very interesting color effectsare observed when Cellophane is examined with polarized light and a Kicol's prism. The polarized light passing through a Cellophane sheet is refracted and the light waves retarded according to the thickness of the film, so that a separation of the light waves results and color hecomes evident. Numbersreprcsenting iu millimicrons the distance retardation values have been assigned to each color and correlated with Cellophane thickness. ~ E R ~ E A B I L I T Y ~ e l ~ o p hpermits alle the rapid penetration of water vapor and of readily watersoluble gases such as ammania and carbon dioxide. On the other band, sparingly water-soluble gases, such as hydrogen, diffuse only very slowly through it as shown by the following table: G*S
Hydrogen Ammonia Carbon dioxide
DI*IB"ISION
Lilrrr per sc. m. pn hour 0.004to0.02 Appiox. 79 Appror. 1
This characteristic permits the use 0f;Cellophane as a semipermeable membrane for the separation of gases from a mixture. CelloPhane, because of its d a t i v e impemeahility $0 hYdrogen, may be used as a lining for gas cells of lighter-thanair craft. I*
Bur. Standards. Tech. News Bull., October, 1927.
Figure 11-Cellophane
Dping Machine
Chemical and Physiological Properties
Cellophane is a very pure regenerated cellulose and is more active chemically than natural unpurified cellulose. It undergoes the typical reactions, such as nitration, ethylamuthation, and swelling with =ustic solution, Cellophane burns in air with a flame in a manner similar to cotton. It has heen classified hy the Underwriter's Laboratories
INDUSTRIAL A N D ENGINEERING CHEMISTRY
410
(Guide No. 540-125) as having the same combustion hazards as common newsprint in the same form and quantity.
Th,cknesr 0009.
Figure 12-Physical
Vol. 21, No. 5
Application
Cellophane as a utility wrap is used on a variety of products including baked goods, meats, frozen fish,l* fruits, confections, nuts, coffee, textiles, tobacco, and drugs. Among other articles, Cellophane is used for making artificial flowers, tape for stock market tickers, trimmings for millinery, and bandages for surgical dressings.
Properties of Cellophane
The following table gives a comparison of data from combustion tests of Cellophane and newspaper: (Average of four flammability tests made on strips 36 by MATERIAL Plain Transparent Cellophane Moistureproof Cellophane Newspaper
TIMEOF
IGNITION Second 1 1 1
l 3 / 8 inches) TIMEOF FLAME COMBUSTION Inches Seconds 12 17 20 13 20 15
BIOLOGICAL EXPERIMENTS-Animals may be fed both Plain Transparent and Moistureproof Cellophane without harmful physiological effects. Experiments in feeding guinea pigs with R'Ioistureproof Cellophane showed no evidence of toxic effects.
Figure 13-Comparison of Light-Transmitting Qualities of Cellophane. and Window Glass
Each year the uses for Cellophane are extended into new fields so that the demand for both the Plain Transparent and Moistureproof varieties is constantly increasing. I*
Howe, Nation's Business, p . 43 (February, 1929); see also Birdseye.
p. 414, this issue.
Automatic Control in the Chemical Industries' Ismar Ginsberg 113 WEST 4
2 STREET, ~ ~ NEW YORP, N. Y
ROGRESS in the chemical industries has usually been considered to be due to the establishment and development of the basic chemical and physical principles upon which a manufacturing process is founded, and to the design and construction of machinery for carrying out the operations. Chemical advancement is, therefore, dependent, not only upon the chemist, but upon the engincer as well. It is true that without the service that has been rendered the chemical industries by the manufacturer of equipment and apparatus, these industries could not have attained the positions that they occupy a t the present time. On the other hand, the best designed apparatus is useless unless the chemical principles of the process carried out in i t are known and applied under control, so that the best results are obtained a t least cost. It appears that the function which is played by instruments that make possible this required control has not been altogether understood or appreciated by the chemical manufacturer. He cannot avoid the use, in the plant, of instruments that measure temperature, pressure, humidity, speed, etc., but he has not used these instruments to the best advantage. The role that the measuring and recording and also the controlling instrument plays in modern industry, and in the chemical industries particularly, is becoming of greater importance from day to day. This is particularly true of the controlling instrument, for its use involves certain economies and increased efficiencies of operation which are very welcome in these days of intensive competition.
P
1
Received February 21, 1929.
While our theme is the automatic controlling instrument,
it is impossible to consider it without saying something about the other instruments as well. Indicating or simple measuring instruments merely indicate the temperature, pressure, etc., through the movement of a pointer around a circular scale as in the pressure gage and dial thermometer, by the rise of 3 column of liquid as in the glass thermometer, or in other ways. Indicating instruments do not interest us here, and we can leave them with just this word. The usefulness of the dial thermometer, which measures and indicates temperature, has not been entirely appreciated by the chemical industries. The dial part of this instrument is connected by flexible tubing t o the bulb, which is permanently fixed in the medium whose temperature it is desired to measure. The dial can therefore be located a t any convenient point and is easily read, much more so than the column of mercury in the glass thermometer. When the temperature of masses contained in open kettles, evolving clouds of steam during the operation, has to be determined, the dial thermometer is a much more convenient and more arcurate instrument than the ordinary long-stemmed glass thermometer. Reading temperatures on such a thermometer over an open kettle with steam condensing on the glass stem is not an easy or a comfortable job, or one that is conducive toward accurate readings. There are many operations in the chemical industries where the dial thermometer can be used to far greater advantage that the ordinary glass thermometer. Controlling or regulating instruments, which automatically keep temperature, pressure, humidity, etc., a t certain definite
