Cellulose Acetate utyrate elt Castin d -
C. J. 21ALM, 0. W. KACL, AND G. D. HIATT Eastman ZCodak Co., Rochester 4, N . Y .
T h e usual processes for making plastic articles call for high pressure and consequently heavy and expensive equipment and demand long production runs to pay for mold cost. Heat stable compositions of high butyryl type cellulose esters have been developed which can be simply melt cast at about 300" F. without the use of pressure or volatile solvents. Formulations w-ith ester levels of 35 to 6570 have good physical properties if the modifying agents are chosen to have good solvent power and yet impart strength and hardness. A mixture of plasticizer, resin, and wax
gives satisfactory compositions when compounded with the ceIlulose ester. Plaster of Paris, casting phenolic, metal, and rubber latex have been used in forming molds for the cellulose ester melt compositions. Rubber molds have been of most interest because of their ease of handling and their reproduction of fine details and undercuts. These casting compositions and simple molds make feasible short production runs at a minimum cost. This process may also be found useful in foundry work where only a limited number of reproductions of the original pattern are needed.
P
4. Have good impact' strength 5. Be free of sweating and tackiness a t lowered and elovut.etl tjemperatures 6. Be free of color, odor, and opacity
LASTIC articles are usually manufactured by compression or injection molding processes. These methods require the use of high temperatures and pressures which call for heavy and expensive machinery. In recent years simpler processes have been developed in which the polymer is formed a t lower temperatures during the molding operation. These processes require only simple molds which contain the catalyzed monomer and form the molded piece during a short heating period a t a relatively low temperature. Both thermosetting and thermoplastic materials have been developed for this process. Little work has been done on thermoplastic polymers where heat alone is used in the forming process. Earlier endeavors include the casting of thermoplastic forming dies, melt coating, and melt dipping. The first mentioned process-the manufacture of forming dies-requires the handling of molten plastic a t very high temperatures to produce solid plastic tools for the shaping of aluminum sheeting (6). The operation of melt coating is the laying down of a molten composition on a continuous support such as paper for the purpose of making a plastic coated material ( 4 ) . Melt dipping involves the application of a plastic material, generally a soft composition, to metal articles for protection against abrasion and corrosion ( 3 ) . Kone of these processes leads to plastic articles of the usual type. After the preparation of this material for publication, an article in M o d e r n Plaskcs ( 5 ) described slush casting of compositions based on cellulose acetate The following experiments were carried out to develop a heat stable melt which could be poured a t moderately high temperatures into a form and cooled to produce a plastic article. COMPOSITION REQUIREMENTS
For practical application the melt itself should shom the follo~ving behavior:
1. Form a continuous solution with no component separating on cooling 2. Be heat stable and not lose viscosity or become appreciably dark with prolonged heating 3. Be fluid enough to pour a t temperatures which preclude decom osition 4. ket rapidly on cooling 5. Be low in cost The cast composition on cooling should: 1. Be easily removed from the mold 2. Be hard and rigid 3. Have a shiny, scratch-resistant surface
This set of requirements is, of course, an ideal. A plasticized composition having a low pour temperature is not likely to be extremely hard on cooling. -4ddition agents contributing to fast sebting may lower the impact strength. I n addition, these same materials niay tend to sweat because of their lo.ilrer solvent power and compatibility. Any useful composition must be a compromise. The formulator is able to improve certain properties but must accept a partial loss in others. CELLULOSE ESTER RASE
In this work a cellulose acetate butyrate ester was modified to give uniform, stable melt's which can be cast a t 160" C. and which harden quickly when cooled. The cast, piece is hard and tough and s h o m good permanence. Because of its translucence, it is pigmented white to give a uniform background for subsequent, coloring. The cellulose esters used in this work analyze 5 to 7% ticetyl and 4 i t,o 500/, butyryl ( 2 ) . Earlier work in melt coating (4)and melt. dipping ( 3 ) recommended this ester for high temperature applicatioiis where compatibility x-ith inexpensive high boiling addition agents is necessary. Three different viscosity grades mere tested. The low, medium, and high viscosity d c r s (AB-500-0.5, -1, -5) have intrinsic viscosjties of 0.7, 0.9, and 1.5 when measured in acetone, and centipoise viscosities of 12, 25, and 125 when measured at 25' C. on 10% solutions in act'tone. These viscosity grades were studied in molten cornpositions under conditions which would measure their usefulness in the casting process. TESTING PROCEDURES
Melts were prepared in 100-gram lots by mixing the modifying agents with the cellulose ester in a beaker and heating in an oil bath a t 180" C. Gentle intermittent stirring prevented whipping in air and allowed melt formulation in 2 to 3 hours. MELT VISCOSITY. The prepared melt was transferrcd to a 38 X 150 mm. test tube, and the viscosity was measured as already described by Malm et al. (4). The change in flow properties as the temperature was lowered gave an indication of the rate of setting of cast objects. SPECIMEX SHEETS. Molten compositions a t 160" to 180" C. were spread on 8 X 10 inch glass plates using a coating knife set
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Castor a,i l
o
o
Dibutyl Sabacate
8
700-
600-
.-I0n
1095
Opal Wax
-
I. 2.
50 40
3.
30 20
20
IO
40
:
0 IO
30
500-
P
t '-
%
400-
c .Ln
0 0
300-
200
-
loo-
Tern pe r o t u r e ,*G
Temperature, * C Figure 2.
Figure 1. Two Component Systems
to give films of about 0.015-inch thickness. Test pieces were cut from these films. TENSILE STRENGTH AND ELONGATION. Tensile and elongation test pieces were cut with a die having a 1-inch width in the center flat section. All tests were made on a Scott tensile testing machine having a cross head speed of 3 inches per minute (S). This measure described earlier (S) SHATTERTEMPERATURE. tests the resistance to shattering of melt cast sheets as the temperature is progressively lowered. Temperatures above the shatter point may cause stiffening of the sample without inducing sufficient brittleness for shatter. The point of shattering is a measure of low temperature impact behavior and also serves as a relative measure of impact a t room temperature. HARDNESS.This property was measured using the Sward rocker on melt cast films still supported on the glass plates (1). COMPOSITIONS AND THEIR PROPERTIES
)*
-
For the development of a useful composition the melt viscositytemperature curves for simple systems were first measured. The results shown in Figure 1 are for simple mixtures of equal parts of AB-500-1 ester and plasticizer. The shape of these curves is a measure of the solvent characteristics of the different plasticizers. Although the castor oil composition shows a sharp viscosity rise as the temperature is lowered, it sweats badly and is too soft to be useful. The Abalyn (methyl abietate) composition has good tensile properties but is tacky. Dioctyl phthalate and dibutyl sebacate show high compatibility but have poor setting characteristics. The rate of setting may be increased by the use of a modifying agent of less solvent power. Opalwax (hydrogenated castor oil) was chosen because of its good heat stability, hardness, and reasonably good compatibility. Figure 2 shows data on the three component system AB-500-1, dibutyl sebacate, and Opalwax. The ester was held at 50% of the composition; the plasticizer-wax blend made u p the other 50%. Some improvement is shown with the wax at 10 and 20% of the compositions. However, when the level reaches 30% rapid setting is noticed. At this high level, however, the compositions become undesirably brittle and ;how some sweating of the wax.
Influence of Opalwax on Viscosity
A third modification was then tried with the aim of maintaining the necessary solvent power of the modifying agents but improving composition hardness and setting rate. A resinlike material having good solvent power for the cellulose ester was used to replace some of the wax and plasticizer. Table I shows the effect of maintaining the AB-500-1 a t a 50% level (based on the total composition) and replacing the plasticizer with various quantities of a chlorinated diphenyl resin (Aroclor 5460). The resin serves as a good high temperature solvent without, at the same time, reducing the mechanical properties of the cast composition.
TABLEI. PROPERTIES OF COMPOSITIONS OF 50% AB-500-1 WITH 50% MODIFYING AGENTS Opalwax
25 25 25 20 30
Modifying Agents Butyl Aroclor 5460 sebacate 5 20 15 10 20 5
..
..
30 20
Tensile, Lb./Sq. Inch 330 800 1680 90 370
Shatter ElongaTemp., tion, yo OF. 25 20 50 40 50 LO Breaks on fold tem-
;;z,20::,
The Aroclor has the effect of increasing tensile strength but a t the same time it increases the brittleness as measured by the shatter temperature. Opalwax, which is only a solvent for the cellulose ester a t high temperature, makes for more rigid compositions as shown by the plasticizer-wax interchange in the last two variations. The action of the Aroclor is not specific, since other resins with solvent action are useful as shown in Table 11. The properties of the compositions are influenced by the character of the resin employed. Aroclor 5460 is a rather brittle, yellow solid whereas Aroclor 1242 is a heavy sirup. The Dow 276 V-2 resin (a polynieric methylstyrene) is a colorless, heat stable resin which shows good solvent power and permanence. The compositions com-
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Voi. 43, No. 5
Opalwax, 30% Dow 276 V-2 resin, and 20% dioctyl phthalate. Comparative properties are given in Table 111. ' b . AB-500-0.5
65%
C. A B * 5 0 0 - 5 , 0
35%
TABLE 111. COMPARATIVE PROPERTIES O F COMPOSITIOXS PREPARED WITH DIFFERENT VISCOSITY CELLULOSE ESTERS AB-500 0.5 1.0
5.0
T e m p e r a t u r e ] *C Figure 3.
Effect of Cellulose Ester Viscosity
priee 50% AB-500-1 ester, 25% Opalwax, 15% resin, and 10% dioctyl phthalate. AND PHYSICAL PROPERTIES TABLE11. RESINCHAXGES
Tensile Elongation, Sward Shatter Resin Lb./Sq. fncd % Hardness Temp. O F. 750 30 10 40 hroolor 5460 510 35 6 - 10 .4roclor 1242 600 45 10 10 Dow 276 V-2 1400 57 10 Room temp Aroolor 54606 a Dow 276 V-2 resin used in place of plapticieer dioctyl phthalate.
Plasticizers are essentially interchangeable and should be chosen for permanence, heat stability, compatibility, and good color. Satisfactory materials are dioctyl phthalate, methyl Cellosolve stearate, and dibutyl sebacate. Dioctyl phthalate is especially permanent and appears to reduce the slight sweating of the Opalwax. Opalwax is somewhat unique in that it is a compatible constituent at elevated temperature and yet confers a quick setting behavior to the melt and hardness to the cooled article. Hard resins used in place of the was give compositions which shatter easily. Other waxes generally fail because of bad color, poor heat stability, or limited compatibility. The cellulose ester can be varied somewhat in acyl content, but a high butyryl level must be maintained to achieve wide compatibility. Viscosity variations in the ester are of considerable significance since this property affects the casting temperature and the mechanical strength of the compositions. Figure 3 shows viscosity-temperature curves for melts made with the three viscosity levels of esters. Previous experience had shown that the ester contents of the melts should be 65, 50, and 35% for the AB500-0.5, -1, and -5, respectively, to produce about the same practical casting temperature. The curves all have the same shape and show desirably low viscosities in the 150" to 170' C. casting range. The modifying mix comprised 50%
Ester, % 65
50 35
Tensile,
Lb./Sq. Inch
1200 765 725
Elongation, Shatter Sward % Temp.. F. Hardnesa 35 40 16 65 150
15
- 40
16 16
Although the low viscosity ester allows the highest cellulose ester content and gives the highest tensile strength, this increase is offset by the poor impact properties of the composition (high shatter temperature). The high viscosity ester composition has a satisfactory tensile strength and at the same time shows shatter and elongation values which contribute to toughness. The Sward hardness values are all the same, probably because the modifying mixture (largely Opalwax) is itself rather hard and therefore does not materially change the hardness of the total composition. It is apparent that the over-all mechanical properties offered by the high viscosity ester cannot be duplicated by simply increasing the amounts of the lorn-er viscosity esters. Ordinarily, when melts made using cellulose esters are heated for some time they slowly color, lose viscosity, and develop an odor as a result of ester breakdown. Basic salts added during the ester manufacture will generally prevent deterioration. However, under prolonged heating conditions additional agents, such as high molecular weight epoxide compounds (an example is stabilizer A-5 sold by the Carbide and Carbon Co.), aid very much in preventing breakdown. An AB-500-1 ester with 50% modifying agents showed an intrinsic viscosity of 0.38 and 0.31 after heating for 8 and 24 hours, respectively, a t 170" C. ( 7 ) . In a stabilized composition the ester under the same conditions showed viscosity values of 0.76 and 0.76. Even a stabilized melt will not stand up against unnecessarily prolonged or unusually high heating. CASTING MOLDS
With the development of a suitable casting composition, a study was made of a variety of materials for mold making. In considering materials the following limitations were kept in mind:
1. The starting material should form into a mold in an easy and inexpensive manner. 2. It should be capable of reproducing faithfully the detail of the original and should not deface the original. 3. It should be heat resistant and not softened by hot plasticizer. 4. I t should be nonporous so that air bubbles will not form and rise into the hot plastic. 5. It should separate easily from the cast plastic piece. 6. It should impart a smooth surface to the cast object. 7. It should be light in weight t o allow easy handling. Plaster of Paris (hydrostone from the U. S. Gypsum Co.) was the first material tried. This material is well known in the casting art. Although the material is cheap and can be handled easily, it tends to crack in contact with the hot melt. In addition, unless a proper surface is applied, the plaster in contact with the molten plastic tends t o bubble. Water solutions of polyvinyl alcohol painted on the surface tend to reduce bubbling. By using this surface treatment, it was possible to use plaster molds and to cast some rather simple plastic objects. Undercuts must be avoided in rigid molds of all types otherwiw the cast article cannot be removed. Durez 7421A (phenolic casting resin from the Durez Plastics and Chemicals Co.) was tried for the preparation of some split casting molds. This material proved to be quite satisfactory in producing molds of good heat resistance, good surface, and reasonable lightness. As indicated previously the object to be reproduced must be free of undercuts.
May 1951
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60 minutes. Addition of the pigment mix after the cellulose ester reduces the tendency of the titanium dioxide to settle. Further heating with gentle stirring for 1 t o 2 hours will give a smooth bubble-free melt. Ingredients for a typical batch are as follows: Parts AB-500-5 70 Opalwax 65 Dow 278 V-2 39 Diootyl phthalate 26 TitanoxQ 3 Stabilizer 1 a Titanox pigment is added a8 a mix ground with a portion of the dioctyl phthalate. The mix is 10 parts phthalate with 3 parts pigment.
Figure 4.
t
c
Heating Equipment and Plaster Mold Support
Metal molds have proved satisfactory, although somewhat more skill must be employed in their making. A cast aluminum form is easy to handle and gives an excellent plastic finish. This type mold is easily cooled and allows a quick cycle. A lead-tin solder of 60/40 composition, which formed a complete melt a t 450" F., showed promise in that it was easy to cast and was resistant t o the casting temperatures. The most satisfactory material found for forming molds was a rubber latex obtained from the American Anode Co. (Anode Latex, 61-10099-A, molding compound). This material is a pastelike product which is applied to the master model with a apatula. It is easily spread and pressed into undercuts. Subsequent coats may be brushed on. I n this manner a thin but continuous rubber layer is built up around the model. A heavy layer is applied a t the large diameters of the object to give the rubber form sufficient strength t o keep it from sagging when it is held in the plaster support. The constricted sections are made relatively thin 80 that the stretching during stripping is more easily accomplished. The average thickness is 0.050 t o 0.060 inch. After the rubber has completely dried, it is stripped wrong side out from the model. Prior to the stripping a split plaster support is built up around the rubber layer. Bfter suitable vents are burned in the mold, it is ready for use. About fifty castings may be made before the mold surface becomes slightly tacky. The rubber form is still useful although some definition may be lost.
The rubber mold is placed in the plaster support which is held together with tape or rubber bands. The hot melt is poured into the mold until it is entirely full. The melt is then poured out, the mold set upright, and cold water is run in to set the composition remaining on the walls of the rubber form. When the plastic is completely solid, the rubber form is taken from the plaster and the rubber stripped from the reproduction. If the casting melt is not hot enough or if it is poured in too fast, the cast piece may show "lap" lines cauwd by the melt folding over and entrapping air. A preliminary casting will generally indicate the need and position of vents in the mold. A hot wire can be used to burn small holes in the necessary places. It may be desirable to tilt the mold during its filling t o take proper advantage of the vents. The wall thickness of the cast object will be determined by the melt temperature and the completeness of drainage. Since the rubber and plaster are poor heat conductors, the plastic remains quite hot and may drain to give too thin walls. After the drainage is interrupted it is desirable to turn the mold upright and rotate it t o get the proper wall thickness. Figure 5 shows an original vase, a plastic reproduction partly removed from the rubber mold, and a finished vase. The time cycle for casting this vase is: filling, 30 seconds; pour out, 10 to 15 seconds; rotation to even walls, 1 minute; cooling with water, 2 minutes; and stripping, 1 minute. These vases may be decorated by filling with colored pigments or tinting with colors in solvents. Aliphatic hydrocarbon solventa are preferred because of their low solvent power. However, active solvents such as acetone may be used if treatment is followed by immersion in water or fast drying. Vases cast using AB-500-1 and -5 have been subjected to aging tests over an 8-month period. These tests were conduckd by
MELT MAKING AND CASTING
For experimental casting it is sometimes desirable to have 5 to 10 pounds of melt. This quantity is conveniently made in a double-walled stainless steel pot heated on a hot plate (Figure 4). Dioctyl phthalate is used in the jacket of the pot. A side drain allows removal of samples for casting. The plasticizer, resin, stabilizer, and wax are heated t o 160' to 170' C., and the powdered cellulose ester is slowly stirred in to prevent lumping. This addition may take from 30 t o
Figure 5.
Stages in Casting Operation
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INDUSTRIAL AND ENGINEERING CHEMISTRY
accurately weighing and measuring vases of the type shown in the plate. These articles weigh about 170 grams, have an averagc wall thickness of 0.15 to 0.17 inch and are 6 inches high. The followiiig results have been obtained: 1. Water a t 100” F., no change in height or \veight 2. Out-of-door standing, no change 3 . Air oven a t 140’ F., 5% Iyeight loss, 3% height loss
Castings from the AB-500-5 formula do not crack after being repeatedly dropped on a concrete floor. The AB-500-1 ester yields compositions which are much less shock resistant. I casting made a t 175’ C. in an 8 X 8 inch metal form showed a shrinkage of 0.005 inch per inch. These compositions should be of use on short runs of molded articks and in the making of foundry patterns.
Vol. 43, No. 5
LITERATURE CITED
(1) Gardner, H. A., and Sward. G . G., “Physical and Chemical Examination of Paints, Varnishes, Lacquers, and Colors,” 10th ed., p. 164-6, Bethesda, Md., H e n r y A. Gardner Laboratory, Inc., 1946. (2) hfalm, C. J., Nadeau, G. F., and Genung, I,. R.,IXD. ENG. CHEX, ANAL.ED.,14,292-7 (1942). (3) Malm, C. J., Kelson, H . B . , and H i a t t , G . D., IND.E N G . CHEM., 41, 1065--9 (1949). (4) Malm, C. J., Salo, M., and Vivian, H . F., I b i d . . 39, 168-74 (1947). (5) M o d e m Plastics, 27, 70 (1960). (6) Prudden, G . H . , Machi?bery, S. Y., 50, No. 11, 136-45 (July 1944). (7) Wagner, It. H., and Russell, J., IND.EXG.CHEM.,4 N A L . ED.,20, 151-5 (1948). RECEIVED August 29, 1950. Presented before the Division of Paint, Varnish, CHEMICAL and Plastic8 Chemistry a t the 118th Xeeting of the AXERICAN SOCISTP,Chicago, 111.
Exdosive Sensitivitv of Ammonium I Nitrate-Hvdrocarbon Mixtures J
J
3IELVIN A. COOIC AYD EUGENE L. TALBQT University of Utah, Salt Lake C i t y , U t a h T h i s study was undertaken to elucidate for the common good one of the aspects of the explosive hazards involved i n the commercial use of ammonium nitrate-e.g., as a fertilizer-when coated with various hydrocarbon waxes. It is shown that the maximum (cap and propagation) sensitivity occurs in mixtures of ammonium nitrate coated with 0.75 to 1.5% wax. A No. 6 blasting cap was found adequate to detonate the (fine-grained) coated ammonium nitrate product containing 0.75 to 1.5c/t wax. Moreover, with this wax content ammonium nitrate is capable of propagation indefinitely i n 1.875-inch diameter
charges. Unfortunately, this is also the desirable wax content required for satisfactory moisture resistance, antisetting, and free flowing properties of animoniuni nitrate. I n fact, even the most sensitive coarse ammoniuni nitrate-wax compositions are comparatively insensitive and unlikely to explode when properly handled. However, i n comparison with inorganic coated-e.g., lrieselguhr--or pure ammonium nitrate, they are tremendous13 more sensitive and likely to explode under provocative conditions, as when large quantities are involved in a fire.
E
consequence of this is that the ordinary air-gap sensitivity test is of no value in this case except possibly in very large diameterse.g., 6 inches. Those familiar with the problem of determining the explosive sensitivity of such mixtures usually confine their mertsurements to the following:
XPLOSIVE sensitivity may be defined as the relative ease of producing an explosion or detonation in an explosive substance. This definition, however, isimpracticalwithoutaleospecifying in detail the methods employed in producing the explosion, because sensitivity varies in an unpredictable manner from one method of initiation t o another. Fundamentally, an explosion is probably always the direct result of heat and, in principle, follows conventional reaction rate theory (6, 7 , I f ? ) . However, so many nontractable factors are involved in heat production in the many methods of initiation that one cannot yet provide a common theoretical basis of comparison of the various conditions known to produce explosions. I n lieu of an adequate theory to evaluate the efficiency of the various methods of initiation as a source of heat, in producing explosions, it is necessary, in estimating hazards involved in the manufacture, transportation, and use of explosives, to evaluate a w 9 l e series of sensitivities including all conceivable means by which explosions may be initiated. The case of ammonium nitrate-hydrocarbon mixtures provides a special problem in explosive sensitivity. Because of the relatively very low sensitivity of such mixtures, positive results generally are obtained only in a limited number of the standard sensitivity tests-for example, such mixtures usually will not explode in ordinary impact, friction, flame, spark, and shock tests. Moreover it is usually difficult to obtain propagation in small diameters with almost any ammonium nitrate-combustible mixture t o say nothing of ammonium nitrate-hydrocarbon mixtures which are among the least sensitive mixtures of such combinations. The
1. Heat sensitivity or the determination of the rate of decomposition as a function of trmperature and the “explosion temperature. ” 2. The critical diameter of propagation with an adequate booster under controlled conditions-e.g., constant density, particle size, composition, and confinement. 3. The minimum size booster required to produce detonation of charges of constant size, length, confinement, composition, and particle size, etc. 4. Large diameter air-gap sensitiveness tests.
A “minimum primer sensitiveness” test is described for measuring the sensitivity of very insensitive explosives which, while not described previously in the open literature, has been used extensively in this and slightly modified forms in the explosives industry to rate insensitive explosives of this type. It is supplemented to a limited extent herein by a crude propagation sensitivity test. The effect of small percentages of solid hydrocarbon on the sensitivity of ammonium nitrate has been noted previously by Cook ( 2 ) and b y Nuckolls (IO). Cook, in fact, mentioned that solid hydrocarbon up to 1%was a more effective sensitizer for ammonium nitrate than an equal percentage of T N T .
