Process for Making High-Strength Plaster of Paris JAMES J. EBERL'
AND
ALVIN R . INGRAM2
Johnson & Johnson, New Brunswick, N . J .
A
process for making a low water demand (or low consistency), high-strength plaster of Paris (calcium sulfate hemihydrate, CaSOJ/2HzO) of definite and controllable crystalline dimensions has been developed. The same procedure was independently discovered a t nearly the same time in England by Haddon and Cafferata (4). The operation consists essentially of autoclaving for 1 hour or more a slurry of Terra Alba (finely ground gypsum, CaS04.2H20) in the presence of about 0.1% soluble succinate, malate, citrate, or maleate salt, removing the water, and regrinding the product. Every salt added to the slurry, depending on its concentration and several physical eonditions, has its own definite effects on the shape of the plaster of Paris crystal.
P
sequent to the development of Hydrocal, Gardner (8) discovered that the consistency of a conventionally dry-calcined plaster could be reduced t o that of Hydrocal by adding a mixture of powdered gum arabic (1 to 2%) and certain alkaline reacting materials such as lime, litharge, or soda ash. This product is marketed under the trade name, Certrock. Using the gum arabic-alkali mixture, i t is possible to reduce the consistency of Hydrocal t o as low as 27 ml. (Hydrostone). The U. S. Gypsum Company has effected a still further lowering of consistency and increase in strength in the product, Hydromite, by the addition of a carbamide-formaldehyde resin so that the water take-up of the plaster is practically that of the theoreticzl. On aging there is a tendency of the treated plasters (Certrock, Hydrostone, and Hydromite) t o increase in consistency owing t o atmospheric neutralizing of the alkali and partial curing of the resin.
LASTER is conventionally made by the open kettle or rotary kiln heating of ground gypsum. A lowering in consistency (11) from 80 t o 100 ml. to 60 i o 75 ml. was effected as a result of artificial aging treatments comprising the spraying of a fraction of 1% calcium chloride (I), sodium chloride (6), or other compounds (6, 7) onto t h e gypsum in the calciner prior to calcination or by spraying a predetermined amount of water into the plaster t o permit hydration of anhydrite and recrystallization of hemihydrate (9). However, this amount of water used t o make a pourable wet plaster mix is far in excess of the theoretical 18.6 ml. per 100 grams needed for the hydration of plaster according t o the reaction: CaS04.1/2HSO
.
+ 3/2H20 +CaS04.2H20
.
..
The first practical solution to the problem of making a plaster of drastically lower consistency was offered by Randel and Dailey (IO), who perfected commercially a method previously investigated by Brothers (2). Essentially the method of Randel and Dailey consists of heating lump gypsum in a saturated steam atmosphere (preferably at 15 pounds per square inch) for 6 hours, drying, and then grinding. By this method plaster which pours at a consistency of 45 ml. can be made and has been marketed for several years under the trade name Hydrocal. Sub1 2
Prevent address, Scott Paper Company, Chester, Pa. Present address, Mellon Inatitute, Pittaburgh, Pa.
Figure 2.
Landplaster Gypsum (370X)
Upper. Ground gypsum Center. Autoclaved with 0.1 yo maleic acid, neutralized with sodium hydroxide for 1 hour at 135O .C. Lower. Autoclaved with 0.'03% sodium succinate for 1 hour at 125' C.
Figure 1. Oklahoma Landplaster Grade Gypsum Autoclaved in Water for 1Hour a t 115' C. (370X)
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Figure 3.
Vol. 41, No. 5
English Gypsum (330X)
L-pper. Autoclaved with 0.2y0 sodium succinate for 1hour a t 135' C. I m u r r . Autoclaved with 0.2% sodium succinate a n d 0.570 potassium sulfate for 1 hour a t 127' C.
Figure 4.
L a n d p l a s t e r Gypsum (370X)
Autoclaved with 1 % cinnamic acid, neutrnlized with sodium hydroxide, for 1 hour a t 133' C. Upper center. Autoclaved with 2 70 adipic acid, neutralized with sodium hydroxide, for 1 hour a t 130' C. Lower center. Autoclaved with 170o-phthalic acid, nciitraliied d t h sodium hydroxide, for 1 hour a t 129' C. Lower. Autoclaved with 2 7 citric ~ acid for 1 hour a t 128' C. Upper.
Figure 5 . Upper.
Lower.
L a n d p l a s t e r Gypsum (330X)
Autoclaved with 1%maleic acid, neutralized with sodium hydroxide, for 1 hour a t 129' C. Autoclaved with 170fumaric acid, neutralized with sodium hydroxide, for 0.5 hour a t 125' C.
EXPERIMEhTAL RESULTS AND DISCUSSIOU
It was expected t h a t a gypsum slurry could be heated in a n autoclave resulting in dehvdration to form a plaster of Paris slurry according to the therrnodynaniic predictions (8) CaS04.2H20-?CaS0,.1/2H20 ( I )
+ 312H20 A
F. 333 = -300 cal.
Although plaster of Paris was produced by this method, it mas extremely fluffy, had little strength \\hen set, possessed a n extremely high consistency (300 to 800 ml.), and was composed of long, needlelike crystals (Figure 1).
It was felt that as the heterogeneous fragments of ground gypsum (Figure 2 , upper) had been changed to such regular shapes in their aqueous dehydration and as t,hese shapes Txere so vastly different from regularly dry calcined gypsum, there must be an environmental influence upon the groTi-th of the hemihydrate cryst,als. Therefore, several compounds were addcd to the gypsum and a specific effect on the crystalline shape of the plaster vias noted for each materia.1 added. The most de.iirablc effect was that of the malates (Figures 2 , center) and succinates (Figure 3, upper) at, about 0.05 to 0.2% Concentration. Here a plaster of low consist'ency !vas produced which consisted of crystals which were mainly short rods, the length being between one to six times t h a t of the width. A hexagonal shape could plainly be observed in the inore clearly defined faces. Tlie widt,h generally varied from 7 to 14 microns. At as lorn a concentration
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1949
*
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v OF AUTOCLAVING AT 127' C.
TIME I N HC-RS
Figure 7 . Variation of PIaster Consistency with Time of Autoclaving English Gypsum and 0.270 Sodium Succinate in Slurry a t 127" C.
Figure 6.
Orthopedic Plaster of Paris
1-pper. Made by regrinding U. S. Gypsum Company dry-calcined base plaster and blending with it potassium sulfate and ground, set plaster (370X) Center. Added t o water a t 95' C., then autoclaved for 1 hour at 135' C. (330X) Lower. Added t o water (containing 0.29'0 sodium succinate based on gypsum) at 95' C., then autoclaved for 1 hour at 135' C. (330X)
as 0.02% sodium succinate (Figure 2 , lower) the crystals were elongated but still not like the needles of Figure 1. A striking increase in size of the crystals was noted when 0.5% potassium sulfate was included with the 0.2% sodium succinate (Figure 3, lower). Salts of other organic acids were found to have interesting specific effects on the shapes of the plaster crystals (Figures 4 and 5 ) . Haddon reported t h a t a t the lower concentrations ( 0 . 0 5 t o O.l%), the citrates and maleates also produced crystals similar t o the succinates and malates, with which the authors' work at the lower concentrations was principally concerned. The basic theory of this phenomenon was not completely clear. When a dry calcined orthopedic plaster (Figure 6, upper) was autoclaved for 1 hour a t 135" C. in the presence of 0.2 part of sodium succinate per 100 parts of plaster, there was absolutely no difference in microscopic appearance or consistency between this plaster which was autoclaved with sodium succinate (Figure 6, center) and t h a t which was autoclaved with no sodium succinate present (Figure 6, lower). When compared with the original plaster, both autoclaved samples appeared t o be more needlelike. It is apparent, therefore, t h a t the effect of the succinate occurs principally upon dehydration of the gypsum and not upon recrystallization of the hemihydrate. Also indicative of the effect of the succinate on the change from gypsum to plaster, Haddon found t h a t the rate of formation of plaster was decreased by the catalyst, t h a t lower autoclaving temperatures were required t o prevent anhydrite formation with the catalyst
present, that anhydrite was formed if the autoclave was not agitated, and t h a t the catalyst could be almost completely removed from the plaster by filtering and washing. There was also little correlation between the action of various organic salts as retarders for the setting of plaster and their action as "crystal shaping" catalysts in the gypsum-plaster change. It appeared to make little difference whether the potassium, sodium, or calcium salt was used, the effect being t h a t of the anion alone. Most operating data reported here were obtained from a 1gallon stainless steel, gas-fired autoclave at the Mellon Institute of Industrial Research in 1943. The gypsum used was finely ground (95% through 325 mesh) English gypsum of 99.9% purity imported by Whittaker, Clark & Daniels, Inc. The percentage of sodium succinate reported referred t o parts of Eastman Kodak sodium succinate (Na2C4H404.6H20) per 100 parts by weight of gypsum. The slurry consisted of 1 liter of water to 1 lig. of gypsum. The data plotted in Figures 7, 8, and 9 referred t o the effect
0.5
1234567 8 9 10 12 HOURS OF AUTOCLAVING
PO
Figure 8. Relationship between Powder Density and Logarithm of Time of Autoclaving English Gypsum and 0.2% Sodium Succinate in Aqueous Slurry a t 127" C.
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sistency predicted theoretically possible on prolonged autoclaving-viz., C = (8.95) (l/T) 50. Actually 50.5 ml. were obtained a t 12 hours of autoclaving. I t was found (Figure 8) that the apparent powder density WY&S directly proportional to the logarithm of the time of autoclaving and could be expressed a ithin the limits of t h e experiment as
4500
+
4000 d
ui LI:
f I
+.MOO z
D = log,,T
W
n
+ 4.94
4.33
I-
v1
Figure 9 shoaed t h a t the strength accompanying prolonged autoclaving increased rapidly up to 1 hour and then ing20w creased more slouly up to 12 hours. The W 300 a test specimens were allowed to dry for n 5 1 week a t a lorn humidity and room temI 0 perature. 0 Gypsurn from various sources produced plaster of varying consistency arid strength. (I I O ooo 2 3 4 5 6 7 0 9 10 11 I2 The main impurity was limestone. TIME IN HOURS The catalyst, sodium succinate, was Figure 9. Strength us. Time of Autoclaving English Gypsum with 0.2% found almost eauallv effective over the Sodium Succinate at 127" C. range 0.05 t o 0.40%; but at 1%, plaster was made which never took a hard set. Sodium succinatc itself exerted a weakening and retarding cffcct upon the setting of the plaster. From Figure 10 the authors concluded that the consistency of the plaster was a minimum between 135' and 142" C. when English gypsum was autoclaved with 0.2% sodium succinate for 1 hour. Grinding had a considerable effect on the consistency and powder density of the plaster. As seen in Figure 11, only 1 hour of ball milling dropped the consistency of a sample of autoclaved plaster from 59 t o 41 nil. and raised the powder density from 1.09 grams per ml. t o 1.29 grams per ml. Over 2 hours of
s0
-
541
Id0
I
I
135
140
TEMPERATURE
I
J 145
I
I50
OC.
Figure 10. Effect of Varying Temperature on Consistency of Plaster Made from Autoclaving English Gypsum with 0.270 Sodium Succinate at 127' C.
of varying time of autoclaving on certain properties of the plaster produced. I n Figure 7 there was a hyperbolic relationship between the consistency and the time of autoclaving. Under the conditions used, a consistency of 50 ml. was the lowest con-
TABLE I . EFFECT OF HEATING VARIOUSGYPsmIs WITH 0.2% SODIUM SUCCIXATE FOR 1 HOCRAT 127" C.
Gypsum Oklahoma English Nova Scotia
New York
Supplier
U. S. Gqpaum Company Whittaker, Clark & Daniels, Inc. Whittaker. Clark 8: Daniels, Inc. Universal Gypsum Company
Consist70 Calcium ency, Carbonate R.11. a 45
Compressional Strength a t Giren Consistency, Lh./Sq.In. 3300
XII
59
2280
1.7
66
1800
3 3
70
1380
No analysis was made on the particular sample used: however, gypsum f r o m this source wits later found to vary from 0.4 to 1.5y0calcium carbonate.
I
I
I
2
I
a
I
4
d
TIME OF BALL MILLING, HOURS
Figure 11. Effect of Ball Milling on Consistency and Powder Density of Autoclaved Plaster
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INDUSTRIAL AND ENGINEERING CHEMISTRY
ball milling, however, caused a raising of consistency and lowering of powder density. Apparently, the immediate effect of the grinding action was to knock off the sharp corners of the crystals so that they could fit together more compactly. However, further grinding merely caused the normal fluffing or powdering effect observed with materials of fine particle size. Grinding overcame the tendency of this plaster to settle abnormally from a water-plaster mix. Pilot plant operations were carried out in the New Brunswiclr, N. J., plant of Johnson & Johnson in 1944. Here the plaster was made in a 75-gallon steam-jacketed autoclave. CONCLUSIONS
As a result of research work on orthopedic plaster bandages a process for making a high strength, low consistency plaster of Paris was developed. The same process was independently discovered by Haddon and Cafferata. A gypsum slurry was autoclaved in the presence of a few tenths per cent of certain dicarboxylic acid salts t o produce plaster of Paris of the desired short, principally rodlike crystals. According t o conditions of autoclaving, the kind of added salt, and the concentration of the added salt, various specific crystalline forms of plaster could be produced. The effects of certain variables on the characteristics of plaster have been presented.
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ACKNOWLEDGMENT
This work was undertaken at the Mellon Institute of Industrial Research as part of a program of investigating orthopedic plaster of Paris on a fellowship sponsored by Johnson & Johnson. The authors wish t o acknowledge also the work of Gilbert IGvenson in the preparation and testing of samples. LITERATURE CITED
(1) (2) (3) (4)
Brookby, H. E., U. S. Patent 1,370,581 (March 8, 192d). Brothers, W., Brit. Patent 757,649 (April 19, 1904). Gardner, H. F., U. 5. Patent 1,996,372 (April 2, 1935). Haddon, C. L., and Cafferata, B. J., Brit. Patent 563,019 (July
26. 1944). ( 5 ) Hoggatt, G. A., U. S. Patent 1,960,538 (May 29, 1934). (6) Ibid., 2,002,945 (May 28, 1935). (7) Ibid., 2,067,762 (Jan. 12, 1937). (8) Kelley, K. K., Southard, J. C . , and Anderson, C. T., U.S. Bur. Mines, Tech. Pager 625 (1941). (9) McAnally, 5. G., U. S. Patent 1,713,879 (May 21, 1929). (IO) Randel, W. S., and Dailey, M. C., Ibid., 1,901,051 (March 14. 1933); 1,931,240 (Oct. 17, 1933). (11) U. S. Gypsum Co., Bull. 1.G.L.-ID. RECEIVED Maroh 11, 1948.
Cellulose Acetate Butyrate Strip Coating Compositions C. J. MALM, H. B. NELSON, AND G . D. HIATT Eastman Kodak Company, Rochester 4 , N. Y .
In recent years wide use has been made of melt dipping as a method of applying plastic coatings to protect articles from corrosion and abrasion. Thermoplastic polymers compounded with plasticizers, resins, waxes, and oils have been used. In the present work a commercial high butyryl cellulose acetate butyrate is tested with several plasticizing agents, and the physical properties of these compositions are measured.
T
H E use of strippable protective coatings has received increasing attention in recent years (1, 8,7-9). This type of covering is generally applied by dipping the article to be protected into a molten composition and withdrawing. The dipped article receives a uniform, heavy coating which quickly sets on cooling to allow immediate storage or packing for shipment. No volatile solvents are used, and so solvent curing and recovery are not required. The previous practice of wax or grease coating followed by paper covering is superseded by a single-step operation. Not only is the dip covering quickly applied but i t is removed with equal ease. A cut is made to start the “peeling” whereupon the whole covering may be removed, generally in a single piece. This type of coating serves not only to exclude water vapor and prevent corrosion but also to offer protection against handling damage. The stripped coating if kept reasonably clean may be reused by remelting in the hot dipping pot. Of the several thermoplastic polymers suggested for these coatings, ethylcellulose (1) and cellulose acetate butyrate are outstanding because of their heat stability, compatibility, and high strength. Availability, cost, and uniformity are additional factors leading to the use of compositions based on cellulose derivatives.
The following work is confined to a description of compatibility and physical properties of compositions made using a high butyryl cellulose acetate butyrate. This type of material was chosen because of its good heat stability and wide compatibility with inexpensive plasticizing agents. The analysis of the particular type of mixed ester used showed 5 t o 7% acetyl and 47 t o 50% butyryl ( 4 ) . This represents a product with a n average of 0.5 acetyl, 2.4 butyryls, and 0.1 hydroxyl per anhydroglucose unit of cellulose (2). Two different viscosity levels of this ester were used. The main emphasis was placed on a low viscosity ester having an intrinsic viscosity of 0.9 measured in acetone and designated as AB5OO-1. Some duplicate runs were made with a material of higher viscosity with intrinsic viscosity of 1.3 and designated as AB500-5. These types of esters have viscosities of 25 and 125 centipoises, respectively, when measured a t 25’ C. on 10% solutions in acetone. Although the specific aim of this work was to develop strip coating formulations, the more general purpose was to investigate compatibility and physical properties as influenced by additions of various agents. The method of attack was t o select readily available and inexpensive addition agents, test their compatibilities with the cellulose acetate butyrate, and then study these compositions with a testing program chosen t o detect good strip coating properties. Widely differing values may be obtained for tensile strength, per cent elongation, shatter temperature, and the like depending on the kind and amount of addition agents used. These differing data m e all useful, for as applications vary, the emphasis may be successively on tensile strength, shatter temperature (low temperature flexibility), and per cent elongation. No single composition has all the good properties; therefore, the choice is usually determined by a compromise.
