Ind. Eng. Chem. hod. Res. Dev. 1982, 27,285-290
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Stabilizing Agents for Agricultural Suspensions and Emulsions Edgar W. Sawyer Florkiln Company, Berkeley Springs, West Vlrglnh 254 1 1
The stabilization of aqueous solution-based agricukural suspension/emulsionliquids by the incorporation of gelling agents for the continuous phase is described. Recommended gelling agents are colloidal materials that will gel in high ionic concentration solutions. In order of economic lmpottance the gelling agents employed are colloidal attapulgite clay, seplollte, synthetic and natural hydrocolloids, and Wyoming bentonite. The incorporation and mode-of-action of the stabilizing agents are discussed,along with the yield pseudoplastic rheobgical characteristics of the resultant stabilized systems. Agglomeration, flocculation, settling, creaming and/or syneresis are the major s t a b l i problems encountered and examples of how such problems are corrected are given. The use of gel strength and apparent viscosity measurements to predict the stability of suspension/emulsloncompositions are discussed.
Introduction The purpose of this presentation is to describe the composition, properties, and stabilization of some currently marketed agricultural emulsions and suspensions. They may be concentrates, such as pesticide flowable emulsions and suspensions, or they may be at field-use strength as is the case with suspension fertilizers and tank mixes. Field-use strength products are usually water-base suspensions and emulsions of agriculturally active dissolved and undissolved nutrients and pesticide compounds. Often the disperse phase will contain both immiscible liquids and many different types of solid materials present in excess of their solubility. Normally, stability is achieved by the use of a gelling agent. However, in some instances of simpler systems, necessary stability results from dispersants plus the fine particle size of the discontinuous phase. This presentation is concerned with the first type-those stabilized by the use of a gelling agent. Examples and definitions of popular types of liquid agricultural suspensions are listed below (Merck, 1976; NFSA, 1980; Newsom, 1963). I. Liquid suspension fertilizers are suspensions of fine-particle-size fertilizer nutrients in saturated water solutions of nutrients plus a stabilizing agent. 11. Liquid lime suspensions (Sawyer, 1976; Silverberg and Dixon, 1969; Trask, 1976) are suspensions of 50% to 60% finely ground calcitic or dolomitic limestone in water or saturated nutrient solutions plus a stabilizing agent. A dispersing/wetting agent for the limestone may also be included (Wolford and Sawyer, 1977; Balay and Salladay, 1980). 111. Liquid gypsum suspensions are suspensions of 50% to 60% finely divided gypsum in water or a nutrient solution plus a stabilizing agent. IV. Liquid sulfur suspensions (Merck, 1976; NFSA, 1980; Newsom, 1963) are 50% to 60% suspensions of fine sulfur in water plus suspending and wetting agents. V. Tank mixes (Sawyer, 1980) are suspensions and/or emulsions of pesticides in water, clear liquid fertilizers, or suspension types I-IV listed above. The pesticide concentrate employed may be an emulsion concentrate, a wettable powder, a flowable emulsion or suspension, or a dispersible granule. The most stable tank mixes result from the use of the above-described stabilized suspensions as base liquids. In fact, large tonnages of suspension fertilizers contain pesticides dispersed in them when they are spray-applied. A tank mix of pesticides in suspension fertilizers may be extremely complex in composition. It may contain a continuous phase of dissolved materials in water (generally nutrients) and dispersed phases of in-
soluble nutrients, trace elements, and solid pesticide(s); an emulsion of liquid pesticide(s1; and a gelling agent to stabilize the suspension/emulsion system. During application, the operator wants to apply as many active materials as he can during one pass over the land. In one field-applied tank mix using a suspension fertilizer as a base liquid, ten agriculturallyactive ingredients (plus water and suspending agent) were present. VI. Liquid animal feed supplements (Sparks and Sawyer, 1975,1977) can contain carbohydrates, proteins, fats, oils, trace minerals, vitamins, nutritive industrial byproducts, ground calcite, ground dolomite, and stabilizing agents. They can be applied on solid feed materials for animal feeding or are available to the animal from a lick wheel. In either type of use the liquid feed supplement must be thickened enough to (1)adhere to dry feed, (2) adhere to the lick wheel in a thick film, and (3) remain uniform during and prior to use. VII. Seed suspensions consist of seeds in water or nutrient solutions with a thickener-type stabilizing agent to prevent the seeds from floating and settling. The seed suspensions have been applied using hydroseeden, floaters, and airplanes with considerable success. Agricultural suspensions and emulsions were developed to provide homogeneous, stable formulations for the uniform application of agriculturally active materials at the desired treatment level. Application of suspensions of fertilizer and lime generally is carried out at high levels. On the other hand, emulsions and suspensions of very active pesticides are generally applied at quite low levels. Using agricultural suspensions both are easy to achieve simultaneously, as is often the case in practice, by making up a blend of toxicant and fertilizer using the suspension fertilizer as a base liquid and mixing in concentrated pesticides in a tank mix. For proper utilization, adequate application equipment is a necessity. Another important key to the success of these products is the stability of the suspension/emulsion formulations. Stabilization Stabilization can be achieved in agricultural suspensions and emulsions through the use of fine-particle-size solids and fine liquid droplets in the disperse phase along with appropriate dispersants and wetting agents. This is often done with a reasonable amount of success where the continuous phase is locally available water or a solution fertilizer. Generally, this limited approach is used through necessity because the application timing involved or the necessity of applying by special techniques, such as aerial application or foliar application from ground units, will not permit co-application with other materials. However,
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each application pass has certain basic costs involved, and the more functional material that can be applied per pass, the more economical the operation will be. This is the case when high analysis suspension fertilizers and other agricultural suspensions are applied for nutrition, or when they are used as base liquids for pesticide-containing tank mixes. In practically all such mixtures a stabilizer is used. Stabilizing Agents A large variety of gelling agents have been used to stabilize agricultural suspensions and emulsions. These include gelling-type clays, synthetic inorganic gelling agents, and natural and synthetic hydrocolloids (Merck, 1976; NFSA, 1980; Newsom, 1963). Unfavorable economics rapidly ruled out most of these materials. Others were eliminated by irregular or unsatisfactory performance. At this time, the major material used is colloidal attapulgite clay. Smaller amounts of sepiolite, a similar clay mineral, are employed. Still smaller amounts of hydrocolloids that will gel in solutions high in ion concentration are used in some applications; for example, xanthan gum is sometimes used in liquid feed supplements. Wyoming bentonite is utilized in a few flowable emulsion/suspension applications. Wyoming bentonite will gel in water containing small amounts of dissolved ionic material but will not gel as the ionic concentration increases (Grim, 1962; Haden and Schwind, 1967), which limits its usage. Gelling Agent Characteristics As can be seen from the above listing, a useful prerequisite for stabilizing agents is the ability to gel in and to gel ionic solutions. Other requirements are (1)high tonnage available on a seasonal basis, (2) favorable economics, (3) stability of the gelling effect, (4) lack of toxicity to plants and animals, (5) inertness, (6) desirable rheological characteristics, and (7) reproducibility of gelling effects when incorporated into the suspension/emulsion system. As is often the case, a certain amount of compromise has to be practiced. When attapulgite is considered in its role as a stabilizing agent, it is found that availability and economics are favorable, no toxicity to plants and animals has been encountered, a certain amount of skill is required for incorporation, rheological characteristics when used as a stabilizer are acceptable, the stabilizing effect is uniform and persists, it is not totally inert and as a result shows a few incompatibilitieswith pesticides, and, finally, the clay has a long history of successful use. Rheological Characteristics of Agricultural Suspensions and Their Relation to Stability See the work of Davenport et al. (1978), Gabrysh et al. (1961), and Wasp (1977). Desired rheological characteristics of stabilized agricultural suspensions/emulsions are the following: (1) sufficient gel strength to minimize or prevent agglomeration, settling, coalescing, and floating of the discontinuous phase during storage and transportation; (2) high apparent viscosity under low shear rates so that the disperse phase does not float or settle out during low shear pumping or gravity flow; (3) low apparent viscosity under high shear rates so that the material can be spray-applied to obtain uniform coverage; (4)the ability to rapidly recover low shear rate viscosity and gel strength after being subjected to thinning effects of high shear; and ( 5 ) reproducibility of these properties in multiple preparations. These rheological characteristics are labeled yieldpseudoplastic, and curves of the values obtained when measuring yield points and apparent viscosities are shown in Figure 1 for an attapulgite-stabilized suspension fertilizer. As can be seen in Figure 1, the suspension is not a Newtonian fluid. As shear is applied to the suspension,
-1
E P
z 0 il
No I No 2
S3EL3
QATE, C
SES-1
Figure 1. Viscosity curve of a suspension fertilizer stabilized with attapulgite.
flow is not initiated until a certain value known as the yield point is exceeded. As the shear rate is increased, the viscosity is initially high and at higher values of shear rate, the viscosity drops off. This type of rheological behavior is characteristic of paints, sealants, and many industrial solid suspensions. If, from the highest rate of shear, shear rate values are decreased, a different curve is observed on the return to zero shear rate. The difference between the values on the first curve and the values on the way back down are a measure of the lack of recovery of the apparent viscosity of the clay-thickened suspension after being subjected to higher shear rates. The loop formed is referred to as a hysteresis loop. If the measurements were to be repeated immediately, the hysteresis loop would be displaced to a lower viscosity region. The curve from the second set of measurements would show a lower yield point, a lower viscosity curve as the shear rate is increased, and a lower viscosity curve as the shear rate is decreased. In a well-stabilized suspension, a long quiescent interval between measurements would restore the original loop. To further complicate matters, freshly made suspensions change on aging, and poorly made suspensions can improve in viscosity due to measurement (shear effects)-the viscometer disperses the clay and gives better viscosities. Suspensions, such as are discussed here, have an internal structure which is the total result of the attractive and repulsive forces of all of the particles in the system. In a clay-thickened suspension fertilizer, particles present are of great variety, ranging from coarse to very fine KC1, insoluble phosphate impurities from the wet-process acid, and clay particles. One speaks of "dispersing" the clay into the suspension, but dispersing is a misnomer since the clay must be flocculated (gelled) to stabilize the system through an extended gel structure. This collection of coarse to fine particles interacts by various attractive forces to form an extended structure which is open enough to encompass the liquid present. If this structure is overextended, it contracts to form a more stable set of bondings and liquid is exuded. If gravitational forces are strong enough to overcome the attractive forces in the structure, collapse is indicated by the formation of clear liquid on top of the suspension. The gel contraction is called syneresis and the resultant clear liquid is supernatant liquor. When a small force is applied to the suspension, the structure not only resists flow but shows a certain elasticity. When sufficient force is applied to
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982
overcome this effect a yield value is reached. The strength of this interaction is called gel strength which is a measure of the phenomenon responsible for holding particles in stable suspension during storage of the suspension. As the shear rate increases and is still at low values, clay-thickened suspensions show very high values of viscosity because a large percentage of the attractive forces are still operational, This is an important suspension property since most suspensions that are pumped or allowed to flow by gravity are being subjected to this shear rate range and need a high apparent viscosity to keep large particles in suspension. As the shear rate increases, stronger attractive inter-particle bonds are broken and shear thinning occurs. This results in a low apparent viscosity at high shear rates. Shear thinning is very velcome in suspensions that will be applied by spraying since low apparent viscosity is a necessity in droplet formation. Often during the preparation of agricultural suspensions, subsequent handling, and further incorporations (addition of pesticide wettable powders and emulsion concentrates), the suspension is cycled through high shear handling. It is absolutely necessary that after such experiences that the internal structure reestablish itself rapidly-this property is referred to as recovery. The rate at which the internal structure or inter-particle bonding breaks down and reestablishes itself is temperature and time dependent. Clay-thickened agricultural suspensions thin as a function of time at a constant shear rate. When the shear force is no longer exerted, the system gradually thickens and recovers. A similar phenomenon is noted as the temperature increases and decreases, but this is further complicated by high temperature instabilities caused by reactions between components in the suspensions. This behavior is called thixotropy. In the agricultural field a confusion exists between pseudoplastic behavior and thixotropy, and the combination is designated thixotropy. When all of the factors that contribute to internal structure and inter-particle attraction are considered, it becomes apparent that to predict stability of agricultural suspensions one must also consider the effects of past temperatures encountered, age of the suspensions, and previous work or shear inputs. Measurements of gel strengths and viscosities on freshly made agriculture suspensions are often considerably different from those obtained after one day, one week, or later, and initial measurements may not necessarily indicate the stability of the system. Two reasons for changes during this period are the following: (1)In the commercial preparation of agricultural suspension/emulsions, air is whipped into the mix and initially is present as bubbles and as a film on solids. On aging, these are released. The number of “particles”is decreased as air bubbles leave, and inter-particle forces change as the encapsulating air leaves. (2) Stabilizing-clay particles are subjected to further autogenous separations and micro-agglomerations. This increases or decreases gel strength and apparent viscosity. Colloidal attapulgite clay, the commonly used stabilizer, is incorporated by many techniques which can be roughly classified as mechanical and chemical-mechanical. Mechanical methods involve a brute-force beating of the clay into the mix. Chemical-mechanical methods consist of predispersing the clay with a chemical dispersant in water and then mechanically introducing the predispersion into the suspension while simultaneously floccing it. The stabilizing effectiveness of the clay depends on the separation of clay particle agglomerates into individual particles which then flocculate in an extended structure. Thus, the more
287
effective processing technique is chemical/mechanical in nature. Suspensions stabilized by chemical/mechanical processing show slightly reduced rheological properties after a short storage period. Suspensions prepared by simple mechanical processing improve on aging. Good practice in the determination of gel strengths or viscosities in either case is to subject the suspension to a mild stirring prior to measurement and after aging for 18 h. Gel strengths are generally not proportional to viscosities except in specific compositions with specific stabilizers. Any use of viscosity measurements to predict the stability of suspensions must be based on previously established correlations of viscosity values and in-field experiences. Fortunately, clay-stabilizedsuspensions show a correlation, and viscosities can be used as an indication of suspension stability. Other stabilizers tested have shown good viscosities while giving extremely low gel strengths. The following sections show examples of the measurement of gel strengths and viscosities and their correlation with suspension stability. While shown for attapulgitestabilized systems, the same techniques could be employed for any stabilizing agent.
Gel Strength (Davenport et al., 1978) Gel characteristics are of paramount importance in suspension/emubion stability and thus in maintenance of uniformity. Considerable work has been carried out on the measurement of gel strength and its relation to stability. Much of this work is difficult to apply to agricultural suspensions since a suspension fertilizer may contain potash (KC1) particles ranging in size from 60 mesh (250 pm) to finer than 10 pm. Furthermore, there are few commercial gelometers available to processors that would be usable under production conditions. Processors, through experience and observation, have been able to correlate apparent viscosity measurements with a commercial instrument like the Brookfield viscometer to stability against sedimenting and ease of sprayability. They also store retained samples which can be checked periodically for sedimentation and syneresis. Researchers at the Tennessee Valley Authority, National Fertilizer Development Center, Muscle Shoals, AL, realized that suspension quality within one type of suspension might be correlated with certain viscosity measurements (Wasp, 1977). However, they believed that stability could be predicted more accurately with a suitable gel strength measuring device. Consequently, they developed a gelometer that is capable of measuring yield points of gels in suspension fertilizers. Much of their early work consisted of determining yield points of gelled continuous phases containing dissolved nutrient and adding presized insolubles to determine gel strengths required to stabilize particle suspensions of known size density and shape. Using urea-ammonium nitrate solution plus various amounts of attapulgite clay to obtain different gel strengths, ground phosphate rock of 35/48 mesh (0.510.42 mm), a finer grind of 65/100 mesh (0.2310.15 mm), and a glass bead 6 mm in diameter were mixed in these gels to demonstrate stability. Results on suspension properties are shown in Table I. The concept of gel strength measurements as criteria for suspension stability has been used many times, but a given gel strength only indicates a stable suspension for particles finer than a certain particle size, below a certain density, and only for certain particle shapes in liquids within a certain density range. In suspension fertilizers, one would not anticipate too much variation in density. Thus variations in gel strength and viscosity would result from the amount of clay added and how well it was dispersed and
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Table I. Suspension Properties of UAN Containing a Clay Stabilizer (Davenport et al., 1978) suspension characteristics gel strength, gel 1 2 3 4
g-cm 0.3
0.4 1.3 24.0
65/100rock
none mostly suspended suspended suspended
reflocculated. The shape and apparent diameter of the particles in the disperse phase both would affect gel strength and stability. Furthermore, as mentioned above, gel strengths (and viscosities) are not always stable on freshly made suspensions and can increasear decrease on aging. The induction period required to achieve a stable gel strength varies but can be accelerated by continuous gentle stirring, intermittent stirring, or by air sparging for about a 12-h period. When gel strengths and viscosities are remeasured after this type of treatment, they may increase or decrease depending on the processing used to prepare the suspension. Gel strengths obtained on freshly prepared suspensions are “Immediate Gel Strengths”; those measured on sparged or stirred samples are “Stable Gel Strengths.“ Normally, suspension gel strengths increase on aging but when agitated will decrease. As the suspended solids content of a suspension increases, the gel strength also increases. For example, according to Davenport et al. (1978), an 11-37-0gelled with 2% attapulgite clay had an initial gel strength of 6 g-cm before other insoluble nutrients were added. (Note: In fertilizer nomenclature the designation 11-37-0indicates 20 lb units per ton of N, Pz05and K20. Therefore, an 11-37-0has 11units of N, 37 units of P205,and 0 units of K20. A unit is a percent.) When 40% solids were present, the gel strength was 10 g-cm; at 45% solids, the gel strength was 15 g-cm; and at 50% solids, it rose to 30 g-cm. This gel strength increase was brought about by the addition of 651325 mesh (0.27410.044 mm) solids. With finer solids the results would be more pronounced. Similar trends were noted when viscosity measurements were made at various solids levels. Syneresis or gel shrinkage, which is another form of suspension instability, is evidenced by the formation of a layer of clear supernatant liquor on top of stored suspension fertilizers. The tendency for syneresis to occur is related to gel strength-as gel strength increases syneresis decreases. When syneresis does occur, the gel contracts and the smaller gel volume exhibits higher viscosities and gel strengths (Davenport et al., 1978). Shrinkage occurs until the gel structure is rigid enough to resist further deflation. Interestingly enough, syneresis occurs in suspensions containing large amounts of suspended solids where none of the solids settle out. Conversely, both sedimentation and syneresis can occur in the same suspension system. Viscosity Measurements Very often a gelometer is not available, and another technique must be used as a control test for stability. It has been found that for specific suspension formulations with the same analysis and with the same particle size of undissolved material, there is a correlation between gel strengths and viscosities. Consequently, many suspension processors use rotational viscometers, such as the Fann, Stormer, and Brookfield, to determine viscosities and use these measurements to control the quality of their finished suspension products. Example 1. Liquid animal feed supplements can contain fine particle-size solids such as vitamins, calcite, do-
35/48 rock
none none
glass bead, 6 mm none none
mostly suspended
none
suspended
suspended
lomite, and trace elements, and immiscible liquids such as vegetable oil, hydrolyzed animal fat, and other vitamins. These materials are dispersed in a continuous phase of a water solution of urea, ammonium polyphosphate, salt, lignin sulfonates, molasses, various nutritive soluble byproducts, etc. It has been found that, unless these mixtures are stabilized with a gelling agent such as attapulgite clay or hydrocolloids, the solids can settle and not be available in the bulk of the supplement. Furthermore, the immiscible liquids can float and cream, making the liquid feed unpalatable and thus unacceptable to the animals. An example is given below of the stabilization of a typical liquid feed containing an aqueous phase of water, urea, molasses, soya solubles, corn distiller solubles, salt, 10-34-0 (ammoniumpolyphosphate),and a dispersed phase of 10% liquid fat. The liquid fat contained 1%of an approved polyoxyethylene ester type surfactant as an emulsifier. Colloidal attapulgite clay was evaluated as the gelling agent stabilizer. Processing wm carried out with a medium shear mixer. Samples of mixes were initially evaluated with a Brookfield viscometer at 10 rpm, stored at 100 O F , and periodically checked for appearance (settling, creaming, stratification) and viscosity. Three different techniques were employed separately for incorporation of 1.5% clay (Floridin Co., 1965): (1)Clay was stirred in at the end of the mix. (2) Clay was added by pregelling the clay at the 10% level in water used in the formulation. (3) Clay was added as a predispersion. This was accomplished by dispersing the clay in water at the 25% level using TSPP (tetrasodium pyrophosphate) as the dispersant according to the following formulation (Wolford, 1981; Dobkowski and Solomon, 1975). water TSPP attapulgite clay
74.4% 0.6% 25.0% 100.0%
Predispersed clay was added to the mix after the water, urea, and molasses had been blended. The liquid fat containing the surfactant was dispersed in the mix prior to adding salt or the 10-34-0. Evaluation results are shown in Table 11. From these results it can be seen that: (1) the higher 10 rpm viscosity samples are more stable and inhibit increase in drop size (coalescence);(2) mixes B and C resulted in a usable product; (3) a 10 rpm viscosity of about 4000 CPminimum is necessary for stability in this product; and (4) the well dispersed clays of mixes B and C were more efficient in building viscosity and stabilizing. Example 2. A liquid animal feed supplement containing water, urea, 11-37-0, salt, cane molasses, and 7.5% -60 mesh (-0.250 mm) ground limestone was treated with colloidal attapulgite using the processing and clay addition techniques described in example 1in an effort to stabilize the mix and prevent settling of the limestone. Evaluation results are shown in Table I11 for formulations containing 1.5% clay. When these results are considered it is seen that the best stability was with mix C, and mix B gave poorer but still satisfactory suspensions. A 10 rpm viscosity of about 4000 CP(minimum) was needed to keep the limestone in sus-
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Table 11. Evaluation of a Liquid Feed Formulation Containing 10% Liquid Fat and Stabilized with 1.5% Attapulgite storage stability in 1 0 rpm Brookfield visc (cP) and appearance time stored at 100 O
F
no clay control
A, clay added at end
2000 thin 1450 fat separation, stratified 1000 completely creamed, stratified
2650 thin 2900 some fat separation, no stratif. 2300 as above
4700
B, clay pregelled
C, clay predispersed 8500
3850 v. sl. fat sep., no stratif. 4100 as above
6200 no separation
1800 as above 2400 as above, no stratif. large fat droplets on surface
3400 as above 3600 as above, no stratif. droplets 0.4 mm and less
~~
initial 1 week 1 month
2 months 3 months microscopic examination
7500 as above 5400
as above 5500 as above, no stratif. droplets 0.1 mm and less
Table 111. Evaluation of a Liquid Feed Formulation Containing 7.5% Ground Limestone and Stabilized with 1.5% AttaDulnite time stored at 100 OF initial
storage stability in 1 0 rpm Brookfield visc. (cP) and appearance
A. clay added at end 1600
B. clav megelled 4400
6600
2 months
1400 some settling 1400 lime settled 1300
3 months
1200
4300 trace settling 4200 as above 4400 as above 3800 as above good
6450 no settling 6500 as above 6300 as above 6000 as above very good
1 week
no clay control 750 lime settled in 24 h
600 lime settled out
1 month
comments
unsatisfactory
poor
pension. The predispersed clay technique used in mix C was more efficient in building viscosity and holding the limestone in suspension.
Summation The quality of attapulgite-stabilized suspensions and emulsions can be judged by evaluations of gel strength or viscosity levels. However, the relationship between gel strength and viscosity is not constant, and it is possible to develop systems that exhibit high 10 rpm Brookfield viscosities and gel strengths of about 0 g-cm. This has been observed with organic hydrocolloid-thickened systems. Possible instabilities that can occur in agricultural suspensions and their causes are listed below. (1) In suspension systems containing coarse particles and inadequate gel strength, sedimentation can occur. (2) In suspensions containing fine particles and a weak gel structure, no sedimentation occurs but the gel contrato a more stable structure (syneresis) and forms supernatant liquid. (3) In suspensions containing coarse particles and a very weak gel structure, sedimentation occurs first. Because removal of coarse particles further subtracts from the gel structure and gel strength, this is followed by syneresis-an example of the ultimate bad suspension. (4) In no. 3 above, if a pesticide emulsion is present the phenomena noted above will occur while at the same time the emulsion droplets coalesce, grow large, and float to the surface as a cream. When the cream droplets break, they will fuse into an oily slick on top of the supernatant liquid. Much can be done with gelometer measurements and proper corrective actions to prevent or alleviate these stability problems. Recommended processing points for
C. clav medispersed
gelometer measurements, where feasible, are (1)in the continuous phase liquid prior to incorporating the solids and immiscible liquids that will be suspended, (2) in intermediate products, and (3) in final products. When lacking a gelometer, viscosity measurements with a multispeed, multishear-rate viscometer can give good indications. When either approach is used, measurements on both freshly prepared and aged samples must be made. Furthermore, correlations must be established over a period of time between field observed stabilities and the above measurements for specific formulations containing disperse phases of known particle sizes and for specific processing. It is comforting to know that in spite of these complications, agricultural suspensions have a long history of successful usage by experienced people. Acknowledgment The author gratefully acknowledges the assistance of Gloria J. Burkhart, Cheryl A. Hurley, Warren G. Lintz, and Wolfgang Mehrmann, photographer, in preparing this paper. Literature Cited Balay, H. L.; Selladay, D. 0. Second Chemical Congress of the North American Continent, Las Vegas, NV, Aug 1980; Amerlcan Chemical Society: Washlngton, DC; Abstr. FER1 022. Davenport, J. E.; Getslnger, J. G., Rlndt, D. W.; Nichols, D. E. Solutions 1978, No. 1 . Dobkowski, T. P.; Solomon, A. J. Solutions 1975, No. 1 . FlorMln Co., “Mln-U-Gel 200 Superior Gelling and Suspending Clay”, 3 Penn Center, Pittsburgh. PA 15235, 2065-5M68-CL. Gabrysh, A. F.; Ree, T.; Eyring, H.; McKee, N.; Cutler, I. Trans. SOC.Rheol. 1961, V , 67-84. Grim, R. E. ”Applied Clay Mineralogy”; McGraw-Hill: New Yo&, 1962. Haden. W. L.; Schwind. I. A. Ind. Eng. Chem. 1967, 59, 58-59. Merck. Kelco Division; “Kelzan,“ San Diego, CA, 1976, DD15. Merck “Keiflo”, 1976, Technical Bulletin 1, No. 27.
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National Fertilizer Solutions Association. "Liquid Fertilizer Manual"; NFSA: Peoria, IL, 1980 Chapter 27. Newsom, W. S. U.S.Patent 3000 170, 1963. Sawyer, E. W. Solutbns 1878, No. 3 . Sawyer, E. W. "Tank Mix Carrbr Systems". presented at ASTM Symposium on Tank Mix Applications, Philadelphia, PA, Nov 1980. Silverberg, J.; Dixon, A. J. "Lime Suspensions"; TVA, Muscle Shoals, AL, 1969. Sparks, R. W.; Sawyer, E. W. "Utilization of Attapuigite as a Stabilizing, Thickening, and Suspending Agent in Liquid Feeds"; presented at American Feed Manufacturers Association, 5th Annual Symposium, Omaha, NE, 1975. Sparks, R. W.; Sawyer, E. W. "Attapulgite-Stabilized Liquid Animal Feed Supplements Containing Fat Emulsions"; presented at American Feed Manu-
facturers Association 7th Annual Symposium, St. Louis, MO, 1977. Trask, 0. J. Solutions 1976, No. 3 . Wasp, E. J. Trans. Tech. Pub/. 1977, l(4). Wolford, J. R.;Sawyer, E. W. Solutions 1977, No. 6 . Wolford, J. R.; Sawyer, E. W. "The Production and Use of Liquid Clay"; Fioridin Company, Norcross, GA, 1981.
Received for reuiew September 30, 1981 Accepted January 4, 1982
Presented at the 182nd National Meeting of the American Chemical Society, Fertilizer and Soil Chemistry Division, New York, NY, Aug 1981.
Structure-Property Relationships in Neat and Reinforced Epoxy Resins Exposed to Aggressive Environment Jovan MlJovl6 Polytechnic Institute of New York, Department of Chemical Engineering, Brooklyn, New York 1 120 1
Several neat and reinforced epoxy resin formulations were prepared and investigated. Solid glass microspheres, with and without coupling agent, were used as reinforcement. All samples were exposed to an aggressive environment by immersion in acetone for various lengths of time. Dynamic mechanical and fracture measurements were used to evaluate the effect of acetone on mechanical properties of different formulations. Electron microscopic evidence was obtained for the existence of inhomogeneous morphology in all cured systems. Acetone-induced changes in dynamic mechanical parameters have been described in terms of the model of inhomogeneous thermoset morphology.
Introduction At the present time there exists a steadily increasing interest in epoxy resins due to their extensive use as structural adhesives and matrix material in high performance composites. In actual service, such materials are always used in an aggressive environment, and it is therefore of primary concern to evaluate their environmental resistance. In the case of epoxy resins, one is primarily concerned with the effect of moisture on their mechanical properties. Particularly within the past five years, a number of reports appeared in the literature describing the effect of various hygrothermal treatments on neat and reinforced epoxies. In all reinforced systems the effect of water on the properties of matrix-reinforcement interphase is of utmost importance. Consequently, many high resolution (mostly spectroscopic) techniques have been used to reveal the nature of interactions within the interphase, and an excellent review of the state of the art of this subject has been written by Ishida and Koenig (1978). Changes in mechanical properties of various epoxy formulations upon exposure to moisture have been studied t y p i d y by comparing dry and wet flexural, tensile, and/or fracture characteristics (Gledhill and Kinloch, 1974; Mostovoy and Ripling, 1976; DeIasi and Whiteside, 1978; Moy and Karasz, 1980; Peyser and Bascom, 1981). A number of analytical and experimental investigations of sorption and desorption of moisture in composite materials have been compiled in a recently published monograph (Springer, 1981). However, in spite of the existing publications, there is no report in the literature in which a correlation has been established between the mechanism of action of an ag0196-4321/82/1221-0290$01.25/0
gressive environment and the epoxy resin morphology. The latter has only recently become a subject of many investigations which have shown that the morphological model of cured epoxies is best described by the regions of higher cross-link density (nodules), immersed in a lower cross-link density matrix. Therefore, our primary objective was to correlate aggressive environment induced changes in mechanical properties of neat and reinforced epoxies to their morphology. Acetone was chosen as the aggressive environment in this study. Although polar, like water, acetone is known to display more rapid attack on cured epoxies. Thus, in essence, the use of acetone is equivalent to an accelerated test, which is the basic tool of analyses for the prediction of long-term performance of all materials. The effect of immersion in acetone on mechanical properties was evaluated by nondestructive (dynamic mechanical) and destructive (fracture) measurements. Dynamic mechanical analysis offers a distinct advantage over other mechanical property tests, for it provides the most sensitive response to morphological inhomogeneities and various physical and chemical transitions in polymers over a wide temperature range. Ultimate mechanical properties have been determined from linear elastic fracture mechanics (LEFM) analysis (Irwin, 1958). A critical value of the strain energy release rate (GIJ, at which a preexisting crack extends in the cleavage mode (mode I), was calculated as described elsewhere (Ripling et al., 1970; MijoviE, 1980). Experimental Section Chemical Systems. Epon 825, Shell's liquid diglycidyl ether of bisphenol A (DGEBA) resin was cured with di0 1982 American
Chemical Society
