ENGINEERING, DESIGN, AND EQUIPMENT the same soil (untreated) gained 39.3% in weight by capillarity in a relatively short time. The reduced capacity of the treated soil to adsorb water has resulted in two other changes in the physical properties of the mil. 1. A lowered tendency to expand and contract with alternate wetting and drying 2. A greater compressive strength when the treated soil is soaked in water for prolonged periods of time
Repeated wetting and drying of soil is accompanied by alternate contraction and expansion. The effect of volume changes is to cause disruption of soil aggregates or clods into smaller units when the dried soil is wetted. The rapid intake of water causes unequal swelling throughout the clod which produces fracture and fragmentation along the cleavage planes. Then, too, the penetration of water into the capillaries results, first in a compression of the occluded air and finally in a virtually explosion within the clod as the pressure of the entrapped air exceeds the cohesion of the particles. Yoder (9) has shown that slow wetting by capillarity does not cause violent disruption of the clod but if the clod is immersed in water, disintegration into smaller fragments takes place as the air is expelled. With a soil treated with DDAC the capacity to adsorb water has been reduced, its rate of water adsorption lowered and consequently its tendency to expand minimized. The air occluded in the pores is free to escape as its place is occupied by the slowly entering water. An examination of the data accumulated in Table X will show the increased resistance of a treated Putnam soil to volume and weight changes concomitant with cyclic wetting and drying. I n the experiment with blocks of treated Putnam silt loam it has been demonstrated how the compressive strength is a function of the moisture adsorbed by the treated soil when immersed. Prolonged periods of immersion indicate that treatment with 0.1% DDAC imposes a limit on quantity of water adsorbable a t about 21%. Since this is lower than the value for untreated soil, the treated soil aggregate remains intact by virtue of the decreased water adsorption. Theoretically, if the immersed block of treated soil does not
soak up moisture any further, and if it is not subject to forces greater than the bresk point, it should resist disintegration indefinitely in its aqueous surroundings. I n actual practice, other forces created by the turbulence of water will naturally affect the length of time an aggregate of immersed treated soil will remain intact. Figures 2 and 3 show that the compressive strength is inversely related to the time of immersion or percentage of water adsorbed. The values for the compressive strength of the treated and untreated Putnam soil are somewhat parallel to a moisture content of roughly 15y0,above this figure the compressive strength of the untreated falls off rapidly while that of the treated appears to reach a constant value.
-
Acknowledgment
Gratitude is expressed to Armour Chemical Division, Chicago, Ill., The Mathieson Co., Inc., Joliet, Ill., and Atlas Powder Co., Wilmington, Del., for working samples of fatty quaternary ammonium compounds. The authors are especially indebted to A. S. Michaels of the Department of Chemical Engineering, MIT, for his valuable assistlance in the preparation of this paper, and to George R. Bauwin, formerly of Union Starch & Refining Co., for some of the experimental data. Literature Cited
(1) Briggs, J., and McLane, J. W., U. S. Dept. Agr. Bull. 45, 1907. (2) Hauser, E. A., and Jordan, J. W., Silicates ind., 17, 9-10 (1952). (3) Hedrick, R. M., and Mowry, D. T., Soil Sci., 73, 436, 472 (1952). (4) Jenny, Hans, and Reitemeier, R. F., J . Phys. Chem., 39, 593604 (1935). (5) Michaels, A. S., Proceedings, Soil Stabilization Conference, MIT, Cambridge, Mass., June 1952. (6) Michaels, A. S., IND. ENQ.CHEM.,46, 1485-90 (1954). (7) Michaels, A. S., and Lambe, T. W., J . Ag. Food Chem., 1, 836 (1953). ( 8 ) Ruhrwein, R. A., and Ward, D. W., Soil Sci., 73, 485 (1952). (9) Yoder, R. E., J. Am. SOC.Agron., 28, 337 (1936). RECEIVED for review April 5 , 1955.
ACCEPTED September 1, 1955.
Acrylate Salts of Divalent Metals ROBERT P. H O P K I N S ROHM
Be
H A A S CO.. P H I L A D E L P H I A 37. PA.
M i l i t a r y interest in t h e possible use of acrylate salts of divalent metals--e.g., calcium acrylate-as soil solidifiers and t h e need of basic information for general development of t h i s new class of vinyl monomers were responsible for t h i s work, T h e polymers of these bifunctional salts exploit t h e water or moisture equilibrating property of salts t o a unique degree. Moreover temperature-water-polymer interrelationships influence mechanical properties greatly and are sufficiently characteristic t o m e r i t description of such products as “inverse t h e r m o hydroplastics.” Practical applications involving t h e use of these acrylic salts--e.g., as soil binders-require attention t o both t h e equilibrating behavior and related mechanical strength of t h e polymers produced. This is demonstrated i n soil solidification studies on these salts, a n application discovered by workers a t MIT working under t h e Soil Solidification Project of t h e U. S. Army.
M
OST research on acrylic monomers has centered around the ester and nitrile derivatives and, to a lesser degree, amide derivatives of acrylic and methacrylic acids. A smaller amount of attention has been given to the acids themselves and to the salts of monovalent cations. The bifunctional acrylic monomers represented by the acrylate salts of divalent metals are the newest members of the acrylic family undergoing research and commercial development. These salts are represented by the general formula 2258
0
II
O-C-CH=CHs
/
X
\
O-C-CH=CHe
I1
0
where X is the divalent cation.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vel. 47, No. 11
SOIL STABILIZATION such as calcium hydroxide, saponification of methyl acrylate is a convenient process. Properties of Monomers
Physical. These acrylates exhibit a high degree of solubility in water. They are also soluble in glycerol and ethylene glycol. Typical physical properties of the monomers are shown in Table I. However, some qualifications are noteworthy: 1. Although both the magnesium and calcium salts have been made as powders, difficulty in preparing solid forms of zinc and barium salts required that the latter two salts be made as solutions for this work. 2. The magnesium salt has a much greater degree of water solubility (89 parts of salt per 100 parts of water a t 75” F.) than that shown in the table for calcium acrylate. 3. The data for the calcium salt apply fairly well to zinc and barium salts.
Table 1.
Physical Properties of Calcium Acrylate
Appearance Solubility in water Other solvents Nonsolvents Freezing point of aqueous solutions 6%
30 $i Density of 30% solution Viscosity of 30% solution
IO CALCIUM
Figure 1.
ACRYLATE
20 CONCENTRATION,
3c
70
Polymerization conditions vs. reactivity of calcium acrylate
In 1945-46 Hauser and Dannenberg ( 2 , 5 ) used lead, zinc, calcium, and barium acrylate for waterproofing clays in the preparation of insulating compositions characterized by excellent electrical and mechanical properties. Zirconyl acrylate has been proposed as an agent for promoting the wetting of ceramics, metals, and cellulose (8). Perhaps the most familiar use of similar acrylate salts to date has been in soil solidification research ( 4 ) conducted jointly by the U. S. Army Corps of Engineers and the Massachusetts Institute of Technology. The relative infancy of bifunctional salts of acrylic acid as vinyl monomers and the need for basic information on these salts, especially for soil solidification, made it advisable to institute a detailed examination of representative products of this class. The major objective of the work described therefore was to characterize these salts broadly in both their monomeric and polymeric forms. This work was devoted to magnesium, calcium, zinc, and barium acrylates. Because the calcium salt is receiving initial commercial emphasis, it was selected for intensive examination of monomer and polymer properties as well as polymerization behavior. The properties of the other salts served to emphasize the qualifying effect of the cation present (in the acrylic salt), and these are described in appropriate sections of this article. Preparation of Salts
Several relatively simple processes may be employed to make the monomeric salts. Neutralization of acrylic acid with the oxide or hydroxide of the desired metal is the most direct method for preparing a variety of salts. With bases of sufficient strength, November 1955
White powder 44 Parts/100 parts of water a t 7Li0 F. Glycerol, glycols, formamide, diethnnolamine Alrphatics and aromatics
-.
7Q _ ” 06
I O o F. 1.130 a t 7 5 O F. A-1 on Gardner-Holdt scale a t 7 5 O F. (aoorox. 6 C D , on Brookfield viscometer) -
Since these salts are vinyl monomers, the solubility information represented is significant in polymerization studies. Aqueous solutions of calcium acrylate, for example, are capable of polymerizing well below the freezing point of water. Other aspects of the importance of monomer solubility are cited later. The low viscosity of saturated aqueous solutions (about 30%) is also worth special mention because of implied value in impregnation applications. Chemical. If considered solely as conventional salts, these materials show a pattern of chemical behavior analogous to such related salts as the acetates and propionates. Calcium acrylate is neutral, buffers at approximately p H 6.5, and shows dispersing efiects on such systems as clays and soils. The existence of double bonds in the acrylate salts of divalent metals accounts for the sharply contrasting chemical behavior between these salts and those of the propionate class. As “polymerizable salts,” the acrylates therefore require separate treatment in a description of their chemical behavior. Reactivity
In the presence of peroxy catalysts, such as ammonium persulfate, polymerization occurs in varying degrees and a t different rates depending on temperature, salt solution concentration, and catalyst concentration. Air inhibition must be avoided as in the polymerization of other vinyl monomers. The use of redox systems, in which activators such as sodium thiosulfate are used in conjunction with the persulfate catalyst, is a very effective means of polymerizing solutions of these salts in water or in other solvents such as those listed in Table I. The reaction between persulfates and thiosulfates demands that they be added separately to monomer solutions rather than mixed t,ogether, as premixing can lead to explosions. Figure 1 illustrates the rate of reaction of calcium acrylate as influenced by the three factors already mentioned. The catalytic system used consisted of ammonium persulfate and sodium thiosulfate on a 1:1 weight basis. Gel time corresponds to approximately one and one-half times the ‘[time to turbidity” (induction period) shown.
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2259
ENGINEERING. DESIGN. AND EQUIPMENT 14 0
A probable explanation for this phenomenon is that the watermonomer relationships are different from the water-polymer relationships of a given salt, as shown in Figure 3. I n monomeric form the conventional term of salt-in-water solution applies. After polymerization, the ionically cross-linked polymers are insoluble in water, but water is soluble in the polymers to a limited degree. Thus the term water-in-polymer solution applies.
I20
u'
100
0
d
Equilibrating Behavior of t h e Polymers
Monomer Concn.
Figure 2. M a x i m u m temperature a t which continuous rubber-like polymer is obtained
-
X = Failure
0
Success
The maximum solubility of water in the polysalts for a given set of environmental conditions is defined as the equilibrium water content. Alternatively an equilibrium value can be calculated as per cent solids. For a given salt this equilibrium value varies with polymerization conditions, as will be shown later. At similar conditions of polymerization, the equilibrium value changes with the cation present. Figure 3 shows the maximum solids possible for the four salts in monomeric solution and corresponding minimum polymer solids (or maximum solubility of water in the polymer). Except for the magnesium salt,
Exotherms as high as 230" F. have been recorded during polymerization of calcium and magnesium acrylates at maximum conditions of reactivity. On the other hand attempts t o polymerize salt solutions a t concentrations below 4 to 5% a t room temperature failed to produce normal (rubbery gel) polymers. Effect of Metals. Polymerization of these acrylate salts with persulfate catalysts has been found to be sensitive t o certain metals and their salts. The effects may range from a pronounced acceleration in rate of polymerization to equally pronounced retardation and low-grade polymer formation. Table I1 summariaes the effects produced by copper or its salts. Iron and, t o a lesser degree, lead and aluminum cause similar results. These findings are consistent with those already published (1, 6). Their significance lies in the need of caution in using metallic equipment or compounds in the process of polymerizing these monomers.
M
The preparation of unextended polymers from their corresponding monomer solutions requires attention t o ambient temperatures if continuous polymers are to be obtained. Figure 2 shows this relationship for calcium acrylate. If the polymer is cooled to a temperature below that where discontinuous pastelike products result, continuous polymers can be obtained. The maximum temperature a t which continuous polymers can be isolated also varies with the cation present as well as with polymerization conditions. Magnesium polyacrylate can be prepared as a continuous rubberlike polymer in solutions up to the boiling point. The upper temperature limit for the calcium salt is about 110' F. Both zinc and barium polyacrylates are separated from the water in their original monomeric solutions at a maximum temperature range of 75" to 85" F.
Table 11. Effect of Copper, Copper Oxide, and Copper Salts on Rate of Polymerization of Calcium Acrylate Copper or Copper KzS~OB,NazSzOa, Compound, % %" %" 10.0 None 10.0 None 1 .o 1.0 0.001 c u s o 4 1.0 1 .o 0.01 0.10
1.0 0.01 CUZClP 0.10 1.0 1.0 1.0 1 . 0 CuClz 1.0 1.0 1.0c u 1.0 1.0 1 . 0 cuzo " Basis. crtloium acrylate. 1.0
2260
1.0
Exotherm (Time required t o Reach 100' F.), minutes 4 40
13
Appearance of Polymer Rubberlike Rubberlike Rubberlike Rubberlike Rubberlike Paste Rubberlike Rubberlike Paste Paste Rubberlike Rubberlike
= MONOMER
P =
M a x i m u m Temperature for Isolation of Polymers 0
1
POLYMER
n
a transition must be made from mon0mer:water to characteristic polymer: water ratios. This occurs by syneresis or "sweating out" of water that is over and above the equilibrium water content of the particular polymer. The establishment of an equilibrium however presupposes mobility of water in the system, The maximum temperatures indicated for isolating polymers can be interpreted as the temperature limits a t which water is sufficiently soluble in the polymer and therefore mobile, so as to permit establishment of an equilibrium between the polymer and water. The consequent removal of excess water produces a continuous rather than dispersed polymer. I n practical applications where these monomers are used in conjunction with other materials, such as soils, wood, paper, etc., air drying would serve t o remove water from the polymers. As already noted, these considerations apply primarily to isolating polymers from the water in which the salts were polymerized. Reference has already been made t o the effects of temperature, monomeric solution concentration, and catalyst concentration on the reactivity of a typical acrylate salt of a divalent metal. The effects of these variables on the amount of water a given polysalt can absorb a t 75" F. (equilibrium water content) is illustrated in Figure 4. Their effect on the percentage of rnonomer polymerized and relative acid resistance is shown in Figures 5 and 6 ,
INDUSTRIAL A N D ENGINEERING CHEMISTRY
-
VoI. 47, No. 11
-
5
CATALYST CONCENTRATION
4
a m
: k a
200
:: 05
u s I-
5 IS0
4r8 v)
POLYMERIZATION TEMPERATURE
MONOMER SOLUTION CONCENTRATION
:/
/---I
U
a 100
I
I
I
I
I
I
I
respectively. Relative resistance to hydrochloric acid was taken as an index of polymer quality. With the catalyst system employed (a 1:1 weight ratio of ammonium persulfate and sodium thiosulfate), low polymerization temperatures, high solution concentrations, and low catalyst concentrations are preferred. Like so many other generalizations, however, interactions are ignored and in practical applications the present generalization may stand modification. Description of Polymers Made i n Water
The most outstanding physical characteristic of these polymeric acrylic salts is their fundamental relationship with water. They are true hydroplastic materials with some resemblance, therefore, to such natural materials as cellulose and clays, as well as synthetic polymers laid down from emulsions, such as polyacrylic esters and polyvinyl acetate. These polysalts, however, physically distinguish themselves from natural or synthetic materials exhibiting hydroplastic character by an unusual supplementary relationship with temperature. Pronounced variations in mechanical properties occur not only m-ith changes in water content of a given polysalt a t a given temperature but a t different temperatures as well. The novelty of the temperature effect lies in ( a ) decreased solubility of water in the polymers with increasing temperature and ( b ) a t high water absorption, increased mechanical strength above as well as below 80" F. The complex character of these polyacrylic salts is further qualified by the cation present. Subsequent sections of this article demonstrate these properties in a more detailed fashion. The color of the polysalts is, as would be expected, largely influenced by the cation present. Degree of opacity or translucency is affected by conditions under which a particular salt was prepared and/or water content in the polymer. Thus, calcium and zinc polyacrylates may be water white when dry and water white or white when wet (depending on temperature) if prepared at optimum conditions. Nickel, copper, and iron polyacrylates are green, blue, and reddish brown, respectively. Calcium polyacrylate prepared at elevated (above 110" F.) temperatures is white when dry or wet. Other Solvents. The preparation of calcium polyacrylate, for example, in solvents such as glycerol or ethylene glycol, results in pastelike products somewhat like plastisols prepared from poly(viny1 chloride) resins. This condition is attributed to a poorly ionized or incompletely ionized salt existing in a dispersed state. The addition of such solvent dispersions to water precipitates the polymer as a typical rubbery gel. Experimental Study of Water Absorption
The water absorption or equilibrating behavior mentioned in foregoing sections was studied in more detail for the four principal salts under investigation. Films approximately 15 mils thick were cast in glass molds from 30% solutions and and polymerized with 170 ammonium persulfate [(NH4)2S208]
November 1955
1% sodium thiosulfate (Na2S202)on the weight of the monomer. The magnesium salt was polymerized a t 75" F., the barium and zinc salts a t 35" to 50" F., while the calcium salt was polymerized a t both the lowest and highest temperatures. The low temperatures were employed in order to facilitate isolation of the zinc and barium poly-
I
1
temperature variation caused a major effect on water absorption. The difference observed was slight (2 parts of water per equivalent weight), and this was considered to be minor with respect to the differences attributable to choice of cation or temperature of the water used for the absorption studies. Specimens X I'/z inches (by 15 mils) were cut from t h e wet polymer on demolding, dried over calcium chloride, and immersed in water a t the temperatures shown, to a constant weight. Absorption is based on dry weight after 16 hours a t 230" F. The data are expressed in Figure 7 on an equivalent weight basis to facilitate observations on the effect of the different cations. Incidentally, moisture absorption values caused by the monovalent sodium versus magnesium, calcium, and barium are 50, 44,29, and 18. The values refer to grams of water absorbed per equivalent weight of the respective salt. The water solubility of sodium polyacrylate required that the absorption tests be run at controlled humidity (75" F. and 55% relative humidity) rather than in water,
r
c
loo
96
80
1 75
35OF. IO I
110 OF.
OF.
30.1.
20
5
MONOMER
IO%
CATALYST
Figure 5. Effect of polymerization conditions on percentage of calcium acrylate polymerized
Effect of Choice of Cation. At the relatively similar conditions of polymerization used, the magnesium polymer was much more hydrophilic than the calcium analog, which was more closely related to the zinc and barium polymers. The relative order of water absorption for the four cations is consistent with the periodic arrangement of these elements. Moreover, the order of absorption for calcium and barium polyacrylates is consistent with that reported for kaolinitic clays containing these cations (6). The relative water absorption by these same salts on the more practical basis of per cent solids is shown in Table IV. The closer relationships between the polyacrylates of magnesium and sodium as contrasted to calcium is quite apparent in the 55% relative humidity test.
INDUSTRIAL AND ENGINEERING CHEMISTRY
226 1
ENGINEERING, DESIGN, AND EQUIPMENT The data for the four divalent salts emphasize the unusual inverse relationship between water absorption and temperature for polyacrylic salts. A final qualification of water absorption is the influence of high temperature drying on the rate of absorption. Calcium polyacrylate, dried a t 75" F., picked up its own weight of water within an immersion period of 4 hours. A sample subsequently dried for 32 hours a t 250" F. picked up less than half this amount of water after 5 days' immersion.
35
75
OF.
10
I
Figure 6.
20
OF.
110
OF.
30% MONOMER 5
10% CATALYST
Effect of polymerization conditions on quality of calcium polyacrylate
Effect of Humidity. Moisture absorption of one of the salts (calcium polyacrylate) a t varying humidities and constant temperature is shown in Figure 8. While the data are less spectacular than those obtained after water immersion, fundamental absorption behavior is still apparent for the hydroplastic polymer.
Table I I I.
Flexural Strength of Calcium Polyacrylate Compared t o Other Polymers Modulus of Elasticitv (Flexure? Lb./Sq. Inhh x 106 6.31
Type Polymer Calcium polyacrylate Cast methyl polymethacrylate Polystyrene (compression molded) High impact phenolics
2 8
4,O-6.0 10.0-13 0
Flexural Strength, Lb./Sq. Inch 14,200 3300 14,000 6,500-7,500 10,000-15,000
+
as thermohydroplastics. Furthermore, the evident decrease of plasticity or increase in rigidity with increasing temperatures above 80" F. is contrary to normal thermoplastic behavior, The rather large but necessarily descriptive term, "inverse thermohydroplastics," therefore applies to these polymeric salts of acrylic acid. As a final example of the peculiar physicomechanical properties of these polymers, water was considered as the plasticizer and the well-known Kemp bend-brittle test for plasticizers was applied to calcium polyacrylate equilibrated a t two conditions. One film was equilibrated in water a t 75" F. to give a water or "plasticizer" content of 50%. With this high plasticizer content the polymer was plastic from -30" to 150' F. and of course brittle below and above the respective temperatures. Such behavior is uncommon in polymer chemistry but analogous to the behavior of certain metals. Zinc, for example, is brittle a t room temperature, malleable and ductile between 212" and 300" F., and brittle again above 300" F. (9).
r
Mechanical Properties
Calcium sulfate (gypsum) and calcium silicate (cement) are the most known salts characterized by inherent mechanical strength, developed by hydration of these inorganic compounds. The use of calcium acrylate as a soil solidifier by the U. S.Army and M I T was the most notable evidence that salts of acrylic acid were also capable of developing good mechanical strength. I n this case, however, the process involved vinyl monomer polymerization rather than hydration. Figure 9 illustrates the intimate relationship between water content and tensile strength of calcium polyacrylate. The term hydroplastic i9 obviously descriptive. Two points merit elaboration: First, strength of the base polymer is related to water content in the film. llctually the latter a t equilibrium is a function of environment; hence tensile strength has been plotted against environment as well as water content. Secondly, the reference data plotted for other polymers were taken from the literature ( 7 ) . They express strength a t optimum conditions. The values would drop of course under adverse conditions-e.g., by water immersion of polyvinyl alcohol films. Table I11 contains flexural data for calcium polyacrylate as well as for other polymers (11) a t standard conditions: 75" F. and 55% relative humidity. The data should be kept in mind in considering subsequent discussions on the relative chemical resistance of polyacrylate salts. Figure 10 adds the important effect of temperature to the importance of water content in determining the mechanical properties of a typical polyacrylate salt of a divalent metal. The thermal effect is definitely influenced by water content. It is pronounced a t high water contents-e.g., water equilibrated films-and negligible a t relatively low water contents-e.g., air-dried fiIms. Since temperature is such an important factor, affecting the mechanical properties of these polysalts when they are in their most hydroplastic condition, these polymers must be described
2262
35
75
TEMPERATURE
Figure 7.
- 'FAHRENHEIT
135
Water absorption isotherms for four acrylate salts
Special note should be made of fact that to achieve the lo^ temperature plasticity, water was soluble in the polymer (acting as a plasticizer) to 62" F. below the freezing point of ordinary water and 40" F. below the freezing point of a saturated solution of monomeric salt (Table I). -4 second film was equilibrated a t 75" F. and 95% relative humidity to a water content of 39%. At this lower water cor+ tent, temperature effects are less spectacular, consistent with the data in Figure 10. Effect of Cation. Water absorption has been shown to be a function of the cation present on the polyacrylic chain. Afechanical strength has been shown to be a function of water content. The resulting effect of choice of cation on the water equilibrated strength of the four salts is exemplified in Table IT-. While the relative water absorption by the four polyacrylate salts is similar to that noted in equivalent weight comparisons
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 11
-
SOIL STABILIZATION
Table IV.
Effect of Cation on W e t Strength of Polyacrylate Salt
(Average of five specimens per polymer) Wet Tensile" Elongation, Cation Solidsa, % Lb./Sq. I n c d % Magnesium 34 38 f 3 . 7 760 48 40 i 4 1000 Calcium 700 65 950 f 177 Zinc Barium G8 910 f 140 500 a After equilibrating in water a t 7 5 O F : per cent solids calculated from oven dry weight (after 16 hr. a t 230' F.) dikided by wet weight.
The action of monomeric calcium acrylate as a dispersing agent can be easily demonstrated in clay soils. For example, the liquid limit of a sandy clay was reduced from 43 to 35% by addition of 5% calcium acrylate. There are more efficient dispersing agents than calcium acrylate, but they are of course not intended to show the other effects resulting from polymerization of such acrylic salts. W 0
E
(Figure 7 ) the wet strength puts calcium in the magnesium class rather than in the zinc or barium class. Figure 9 shows exponential increases in tensile strength with reduction of water content for calcium polyacrylate. The strength-per cent solids data in Table IV are therefore expected.
5 0 r
9
10 0
50
IO PERCENT
RELATIVE
-
HUMIDITY
77-F.
Figure 8. Relative h u m i d i t y absorption isot h e r m of calcium polyacrylate
Chemical Properties
Qualitative evaluation of the chemical properties of a polymer typical of this class (calcium polyacrglate) were also made in this study. Such common organic solvents as acetone, alcohol, ethyl Cellosolve, ethyl acetate, toluene, and gasoline showed no weakening effect on films; this is consistent with the relative inorganic nature of the salt. The water-miscible solvents, alcohol and acetone, actually dehydrated wet polymer making the films stronger. I n the presence of acids and strong bases, hydrolysis of the polysslt occurs. Ordinary salt solutions ion exchange with the polymeric salt and/or simply dehydrate the polymer. These effects are shown in Table V. ("Water" is the control on the calcium polyacrylate films used). Dilute solutions-e.g., 0.5,Vhave not shown dehydrating action on polysalt films.
Strength of Calcium Acrylate-Treated Soil. The principal purpose of the U. S. Army Soil Solidification Project has been to mechanically reinforce soils so that they will support heavy loads under adverse conditions-e.g., in the presence of water. RIIT workera found that polymerization of the acrylate salts of certain divalent metals in clayey soil a t 4 to 10% concentration on weight of soil resulted in the development of a definite bearing strength in the presence of water ( 4 ) . This was in sharp contrast to the formation of sometimes untrafficable mud at similar soil water conditions. Table V I shows the tensile strength of a soil solidified after treatment with calcium acrylate. Data were obtained after soaking test specimens in water to a constant weight. Increasing 10,000
-
8-
-
6-
5-
Table V.
4-
Effect of Metallic Chloride Solutions on W e t Calcium Polyacrylate Films
5.47 S?lt
Solution BaClz cuc1.2 NiClz CaCh FeCls ZnClz Water
Initial Solids, 7% 51 49
48 48 40 49
50
Equilibrated Solids in Salt Solution, % 71 53 70 71 44
70 51
Appearance Water white, hard Bluish, hard Water white, hard Water white, hard Brown hard Water 'white, hard Milky, s o f t
T
CELLULOSE ACE TAT€
32-
POLYACRYLIC l ESTER
.
a
a
POLYVlN Y L ALCOHOL
7
1
1,000 -
ii
0
-
86-
54-
3-
Ion exchange with monovalent cations of course results in solubilization of the polymer. These carboxylic salts show typical sensitivity t o copper and heavy metals. A film of calcium polyacrylate was observed t o turn blue in a stainless steel water bath because of the presence of a copper heating element. Lead was quickly removed from a mixture of lead and nickel nitrates by a calcium polyacrylate film. Rate of combustibility of calcium polyacrylate has been observed to be less than that of polyacrylic esters such as ethyl polyacrylate.
2-
-
100 -
8-
-
6-
54-
Application Studies
One of the principal reasons for conducting fundamental studies on the acrylates of the divalent metals was the need for such information in soil solidification research. A limited program was undertaken therefore to relate some of this fundamental information t o soil studies of the acrylate salts. The program also served t o supply perspective for using such information in other applications. November 1955
4 0
50
EO
WATER IMMERSION
Figure 9.
80
70
75OF
88
AIR
Comparative tensile strength of calcium polyacrylate
INDUSTRIAL AND ENGINEERING CHEMISTRY
2263
ENGINEERING, DESIGN, AND EQUIPMENT favor increased wet tensile strength (Table VI), greater mater absorption is simultaneously induced. If the latter property is of consequence in a certain application it is evident that salt concentrations should be kept low for technical rather than economic reasons. Table VI1 summarizes the combined importance of salt concentration and cation present in determining the maximum water absorbed (or equilibrium water content) of the treated soil. At low concentrations differences attributable to the cation are minor; a t high concentrations they become appreciable.
i
\
\
Table V I I . Effect of A m o u n t of Polyacrylate S a l t and Cation Present on Water Absorption of Treated Soil Water Absorbed by Soil, % Polymer Concn %
Magnesium polyacrylate 18
3‘ 5 10
..
36
Calcium polyacrylate 16 23
Zinc polyacrylate 16 16 19
30
General Application Significance
The water equilibrating behavior of the polyacrylate salts of divalent metals amounts to a mechanism of regulating water absorption in materials treated with these salts, Since absorption of water is a function of salt concentration, cation, polymerization conditions, and water temperature, it is evident that these salts have considerable potential utility in applications ranging from waterproofing to water sensitizing. This is possible because of their ability to be incorporated in materials as monomers with the “universal” solvent followed by in situ polymerization to impart the effects reviewed in this paper.
30 30 WATER
CONTENT
OF
POLYMER
40
FILM,
45
50
55
%
Figure 10. Flexural modulus of elasticity of calcium polyacrylate a t various temperatures and water contents
-
n W m
a 0
UJ
m
a 25 .
the amount of the calcium polyacrylate binder up to 10% gave increased tensile strength. Continued increased salt concentrations gave increased elongation with no increase in tensile, Further increases caused a reduction in strength. The results show that a t minor concentrations of salt (up to 10%) mechanical properties are primarily those of the soil. At major concentrat,ions they are those of the polymer. The need for caution in translating basic film data to end-use application of the salts a t minor concentrations is obvious. Equilibrium in Soils Containing Polyacrylate Salts. A basic property of these polymers which can be shown of direct consequence in soil systems is the water equilibrating behavior already discussed. Once solidified, a soil containing these polyacrylates follows the equilibrium behavior of the salts. The more salt present, the more water is absorbed. This is demonstrated in Figure 11 for a soil compacted to maximum density, a standard soil engineering practice. Thus while increased concentrations
Table VI.
Tensile Strength and Elongation of Calcium Polyacrylate Soil Samples
Calcium Polyacrylate on Soil, ’% 100 50 20 10 5 3
2264
Tensile Strength, Lb./Sq. Inch
Elongation,
12.5
53 45
22 40 43 38 10
%
100 25 8 2
U W
!-
s E w
20
’
u a w
n
3 6 9 12 P E R C E N T POLYMER IN S O I L Figure 11. Water absorption of soil containing different amounts of calcium polyacrylate
Equally bright are the prospects for imparting the chemical properties of these salts to many materials, also by in situ polymerization of the monomers. Their organic solvent resistance is particularly of interest. I n addition it is possible to impart their ion exchange action to materials so treated. Separate ion exchange studies of calcium acrylate in soil systems, not reviewed here, have shown the practicality of exploiting several properties of these salts. Dispersing, mechanical binding, hydration, and ion exchange properties were simultaneously used. The latter typifies a fact which should now be apparentnamely, the monomeric and polymeric forms of the acrylate salts of divalent metals possess a combination of unusual physical,
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No, 11
SOIL STABILIZATION mechanical, and chemical properties which suggests a profitable future for this relatively new class of vinyl monomers. Acknowledgment
The writer is obligated to fellow associates who contributed to the work described. R. E. Zdanowski greatly assisted in obtaining much of the present information. G. J. Williamson also cooperated in providing considerable data on calcium acrylate. F. J. Glavis furnished the monomers for this work, and R. W. Auten offered many constructive criticisms on the preparation of this article. References
(1) Bacon, R. G. R., Trans. Faraday Soc., 42, 140 (1946). ( 2 ) Hauser, E. A., and Dannenberg, E. M., U. S. Patent 2,383,647
(Aug. 28, 1945).
(3) Ibid., 2,401,348 (June 6, 1946). (4) Hauser, E. A., De Mello, V. F. B., and Lambe, T. W., U. S. Patent 2,651,619 (Sept. 8, 1953). (5) Keenan, A. G., Mooney, R. TV., and Wood, L. A., J . Phys. & Colloid Chem., 55, 1462 (1951). (6) King, C. V., and Steinback, 0. F., J . Am. Chem. SOC.,52, 4779 (1930). (7) Modern‘Plastics Encyclopedia and Engineer’s Handbook, Plastics Catalogue, New York, 1950. (8) Neher, H. T., Conn, W., and Kroeker, E. H., U. S.Patent 2,502,411 (April 4, 1950). (9) Pauling, L., “General Chemistry,” p. 559, Freeman, San Francisco. 1953. (IO) Rohm & Haas Co., Washington Square, Philadelphia 5 , Pa., “Calcium Acrylate,” May 1954. (11) Simonds, H. R., Weith, A. J., and Bigelow, M. H., “Handbook of Plastics,” 2nd ed., Van Nostrand, New York, 1949. RECEIVED for review December 7, 1954.
ACCEPTED September 14, 1955.
Soil-Water Relationships in crylate Stabilized Soil V I N C E N T C. M E U N I E R , G O R D O N J. W I L L I A M S O N , A N D R O B E R T P. H O P K I N S R O H M & H A A S CO.,PHILADELPHIA, PA.
T h e primary purpose of t h e work was t o study soil-water relationships i n calcium acrylate stabilized soil, and this paper describes how triaxial figures are quite useful i n achieving this purpose. T h e value of t h e figures is twofold: (1) they permit blocking o u t of logical and promising areas for study and (2) they provide a basis for interpreting results. From t h e results i t i s concluded t h a t t h e equilibrium moisture content of calcium acrylate stabilized soil, after immersion in water, depends on t h e a m o u n t of polymer present and t h a t t h e water equilibrating roleof calcium polyacrylate i s important in determining t h e w e t tensilestrength and elongation of treated soil.
C
ALCIUM acrylate is a white powder soluble in water to the extent of 30% a t room temperature. Clear and colorless solutions can be prepared from typical product without filtration and without carbon treatment. When catalyst and activator are added t o solutions, polymerization occurs, eventually producing water-insoluble gel. Polymerization will take place a t temperatures as low as 32’ F. When it is made to occur in situ in soil, solidification is achieved. The term “redox system,” commonly used to describe the combination of catalyst and activator for free radical-type polymerizations, appears frequently in this paper. The particular redox system applied t o calcium acrylate consists of equal parts by weight of ammonium persulfate as catalyst, and sodium thiosulfate as activator. Personnel of the Massachusetts Institute of Technology, working under a soil solidification contract with the U. S. Army Corps of Engineers, have studied calcium acrylate extensively and much of their work has been published (1-6). Two U. S. patents related to clay modification with acrylate salts are assigned to Hauser and Dannenberg, granted in 1945 and 1946. A third U. S. patent, involving soil solidification with calcium acrylate, granted in 1953, is assigned t o Research Corp. by De Mello, Hauser, and Lambe. A review of soil stabilization by Lambe and Michaels was published in Chemical and Engineering X e w s in 1954. November 1955
Triaxial Figures
As part of a study of the fundamental properties of monomer, polymer, and stabilized soil, soil-water relationships in calcium acrylate stabilized soil have been investigated. The work, carried out with the aid of triaxial figures is described in this paper. Although several diff’erent; types of soil were included, the present paper is limited to Fort Belvoir soil, with the approximate composition Clay Sand Silt
% 20 40
40
Treated soil can be studied in relation t o a number of soil properties-e.g., density and optimum moisture for compaction. In this work Atterberg limits were selected which define nonplastic, plastic, and liquid behavior of soil in terms of soil moisture content. In Figures 1 to 7 the Atterberg limits for the system Fort Belvoir soil-water-calcium acrylate monomer are plotted from published information (4). Alsoshown are curve8 representing upper water limit for homogeneity. This is not a true Atterberg limit. It is here defined as the highest water content at which a separate liquid phase does not form on standing
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