Granular Pesticide Formulations
G. Robert Goss, 1 Dennis R. Taylor, 1 and William B. Kallay ¹ |
¹ Technical Director, Project Group Leader and Scientist, respectively, Research and Development, Oil-Dri Corporation, 777 Forest Edge Drive, Vernon Hills, IL 60061.
Goss, G.R., Taylor, D.R., and Kallay, W.B., "Granular Pesticide Formulations, "Pesticide Formulations and Application Systems: 15th Volume, ASTM STP 1268, Herbert M. Collins, Franklin R. Hall, and Michael Hopkinson, Eds., American Society for Testing and Materials, Philadelphia, 1994.
granules, formulations, pesticide, sorbents
A granular pesticide formulation is one of the many options a formulator has to deliver the pesticide to the target. Granular formulations were first used for mosquito larviciding in swampy areas area where foliage interfered with conventional pesticide formulations reaching the water. Today granular formulation use is widespread and most pesticides are formulated as a granule in addition to the other formulations. Granules find use still for mosquito larviciding in addition to subsoil applications, soil surface applications, baits, and even some foliar applications (e.g. corn borer control). Granular carriers can be broadly separated into two categories, mineral and organic. Mineral carriers include sand, limestone, gypsum, kaolin, montmorillonite, attapulgite, diatomite, etc. Organic granular carriers include corn cobs, pecan shells, peanut hulls, recycled paper fiber, etc. In its simplest form, a granular formulation consists of the inert granule and the pesticide. If the pesticide is a liquid, it can simply be sprayed on the granule and distributed in the field. Often though, formulation aids such as solvents, stickers, deactivators and de-dusting agents are included in the formulations. Each ingredient performs a specific function. Production methods usually involve spraying the pesticide onto the granules while the granules are agitated for even distribution. ASTM testing procedures for agricultural granules include sieve specifications, sampling procedures, bulk density, particle size distribution, resistance to attrition, particle count and liquid holding capacity. This presentation will be a general introduction to granular formulations and provide specific information on carriers, formulation contents, use patterns, production methods and test procedures.
Agricultural granular formulations as defined here are formulations applied in the field as free-flowing granules. Granules first found use for mosquito larviciding in swampy areas. Here, granules could penetrate the foliage whereas conventional formulations could not (Loeffler 1961). Applications of soil insecticides first occurred with aldrin and later heptaclor in the early 1950's. By 1961 most of the applications we are familiar with today were in use in at least a limited fashion. These include broadcast herbicides, corn borer control (foliage application) and bait formulations. While the basic formulation styles were in use as long as 40-50 years ago, constant changes in active ingredients and application technology have continued to provide a fertile area of research.
The area has been reviewed before and this article is to update information and provide a general overview to those uninitiated in the field. Previous articles related to this field are on history and use (Loeffler 1961), introduction to formulation (Sawyer 1982), granular carriers (Moll and Goss 1987), insect granules (Synek 1982), use of deactivators (Kallay et al. 1992), and release properties (Stein et al. 1990; Bowman 1992).
Some reasons to formulate a granule are enumerated below:
1. Safety-Granular formulations are generally safer to handle than other formulations.
2. Ease of use-Field blending is not required.
3. Efficient delivery-There is limited drift in broadcast applications.
4. Target efficiency-Granular formulations are often uniquely suited to specific characteristics of the target pest.
5. Container disposal-No need to go through costly washing or recycling (true today, but changing).
Any or all of the above reasons may cause a formulator to consider a granule.
Once the formulator chooses to produce a granular formulation, he/she must consider the desired physical properties and granular type. General physical property considerations are: mesh size, absorptive capacity of carrier, bulk density, particles per kg, dust, hardness, flow-ability, etc.
Mesh size affects the application rate (i.e. particles per m3 desired for pest control). (Table 1) shows the effect various mesh sizes have on coverage in a broadcast application. In the extreme, changing from a -4.75, +2.36 mm (4/8 mesh) to a -0.710, +0.300 mm (24/48 mesh) particle size increases the coverage by a factor of 32.5.
TABLE 1 Particle count for fractions of various sizes a for a spread at 1 kg/hectare (1 lb/acre).
per Unit of Weight
per Unit of Area
Screen Size (b)
-4.75 + 2.36
-2.36 + 1.18
-1.18 + 0.600
4 400 000
2 000 000
-1.00 + 0.425
8 360 000
3 800 000
-0.710 + 0.300
14 300 000
6 500 000
(a) Data are for granules whose bulk density is 33lb/ft³ (528 kg/m³)
(b) U.S. Standard screen.
Mesh size choice is also driven by the desired application. (Table 2) lists typical mesh sizes for various application situations. Particle size distribution testing is described in ASTM Standard Test Method for Particle Size Distribution of Granular Carriers and Granular Pesticides (E 726-86).
Carrier absorptive capacity:
Absorptive capacity is generally referred to as LHC (liquid holding capacity). This is the maximum amount of liquid granules can hold and remain free-flowing. The higher the LHC, the more pesticide the granule is able to hold. Clay granules have the highest LHCs, ranging from 25-40%. If a pesticide activity of only 2-5% is desired, one can use a wide variety of carriers. Generally, formulations must be produced with a slight amount of LHC remaining to avoid caking problems in the field. ASTM standard Test Method of Liquid Holding capacity (LHC) of Clay Granular Carriers (E 1521) describes LHC measurement.
Table 2 Typical Mesh Size Usage
-4.75, + 2.36
|Lawn and Garden|
-2.76, + 1.18
-0.710, + 0.300
|Typical agricultural broadcast|
-0.710, + 0.300
|Special pneumatic distribution|
-0.425, + 0.180
TABLE 3 Particle count versus bulk density (a)
Millions per kg
Millions per lb
(a) Data are for a -0.710 mm + 0.300mm (24/48) mesh size.
Particles per kilogram:
The number of particles per unit weight is a function of the screen size and the density. It is measured by ASTM Standard Test Method for Particle Counts per Pound of Clay Granular Carriers and Clay Based Granular Pesticide Formulations (E 1521). (Tables 1 and 3) show the relationship.
In general, a high active ingredient (a.i.) will use a smaller mesh size and lower density to assure uniform coverage during application. Gandrud and Haugen (1985) have shown the extreme. They achieved excellent weed control with several herbicides formulated as dry flowables, applied at label rate with particle sizes in the range of -
0.425mm + 0.180mm (40/80 mesh). Conversely, typical lawn and garden applications use low a.i. and a larger particle size (Table 1). As a result, higher application rates must be used for proper coverage.
Bulk density is inversely proportional to particles per kilogram. If a high particle count is necessary for a certain active ingredient, one needs a ‘low' bulk density. Conversely, if one plans to use aerial applications a higher bulk density is generally desirable. ASTM Standard Test Method for Determining Bulk Density of Granular Carriers and Granular Pesticides (E 727-96) tests for bulk density.
The presence of dust causes a safety and environmental hazard in the use of granules. A quality carrier should have little or no dust. Care should also be taken during processing to avoid dust generation. There is no standardized testing procedure here in the U.S. yet. Published measurement procedures are by Gross and Reisch (1988) ASTM Standard Test Method for Index of Dustiness of Coal and Coke (D 547-41) and Miller et al. (1988). Heucotech, Ltd., Fairless Hills, PA, produces a commercial measuring device.
Granule durability, or hardness, is important during formulation processes or application. A soft material, even though initially dust-free may break down during shipping, formulating or application. This causes dust and fines, both undesirable. ASTM Standard Test Method for Resistance to Attrition of Granular Carriers and Granular Pesticides (E 728-80).
A formulated granule must be free-flowing for proper application in the field. In general, this is not a problem unless the formulation has exceeded the LHC. Some granules do however have "hairs" or fibrous extensions that may interconnect thereby decreasing flow ability. An indication of flow ability may be gained by determining either the angle of repose or the angle of internal friction. Most suppliers and formulators prefer actual use tests in the target application equipment.
Granule types can be classed as mineral, cellulosic, agglomerated and miscellaneous. Mineral granules are montmorillonite, attapulgite, bentonite, gypsum, sand, diatomite, kaolin etc. Cellulosic carriers are corn cob, pecan shell, peanut hull, peat, reconstituted paper fibers, etc. Miscellaneous granules can include such things as granulated animal waste and plastics.
Agglomerated granules today typically come as WDGs (water dispersible granules) and are usually produced with the pesticide in a complete formulation. Processes used are extrusion (high or low pressure), pan agglomeration, pin agglomeration, compaction, spray-drying and fluidized bed agglomeration. Further discussion of WDGs is beyond the scope of this paper.
Several ‘controlled release' formulations of granules have been described recently made by agglomeration. Shaska and McGuire (1992) and Wing et al. (1992) both use starch matrices to encapsulate a pesticide with an extrusion process.
Besides physical properties, the granular formulator must consider factors such as cost, availability, inertness (or formulation shelf-life) and biological efficacy.
Granule formulation ingredients:
Typical granular formulation ingredients are listed in (Table 4).
TABLE 4 Granule Formulation Ingredients
The simplest granular formulation contains a carrier and a pesticide. If a liquid, the pesticide is merely sprayed on the granule with good mixing. If a solid, the pesticide can either be melted or solvated before spraying onto the granule. Sometimes it is necessary to also heat the granules before pesticides application to allow for complete penetration into the available pore space. With solid pesticides, caking is often a problem, particularly in the northern part of the country. Here the formulator must have excess LHC in the formulation.
A solid pesticide that cannot practically be formulated by melting or solvating can be formed into granule by adhesion. The pesticide must be milled to a fine particle size before application. Once the powder is added to the granules, adhesion can be accomplished by adding materials such as sugar, starch, polyacrylates or oil solutions.
Sometimes, carrier surfaces are not totally inert. Clays, for example, may have "active sites" of varying strength that can degrade pesticides. This is the reason, with the advent of the organophosphorous pesticides in the 1960's, clay granule usage began to shift from the attapulgite type granules to montmorillonite type granules. Montmorillonite has fewer "active sites." Even so, a deactivator is often required to passivate the surface. According to current theory, most of these acid sites are of the Lewis type. Lewis acids, which are electron pair acceptors, are partially neutralized by deactivators which are electron pair donors. Strong bases such as amines may deactivate the surface but then cause base hydrolysis of the pesticide. Today materials such as the glycol ethers are the most widely used deactivators. Application of the deactivator prior to the addition of pesticide is usually more beneficial than co-application of pesticide and deactivator (Kallay et al. 1992).
The overall schematic for pesticide production is shown in Figure 1. The pesticide carrier is usually supplied by bulk rail car and stored in a silo. Bulk truck or bags may also be used. Handling of the granules throughout the process is critical. Attrition occurs each time the granule is handled. Vibrating feeders, screw feeders and excess piping should be avoided. Between the granule transport and on-site storage the granules are often screened to remove any undersize or oversize particles. These could arise from shipping attrition or contamination.
The pesticide and other ingredients, if liquids, are stored in thanks prior to application.
Blending, as with other materials handling, should be thorough, yet gentle. One common type of mixer used is a rotary mixer supplied by Munson Machinery Co., Utica, N.Y. This is a rotary batch blender with lifters and baffles attached internally which create a rapid but gentle mixing action. Liquids are sprayed onto the granules while agitating. A schematic is shown in Figure 2.
After mixing, material is often stored in a silo for quality analysis before packaging. Often, either after mixing or storage, the now finished formulation is again screened to remove any fines generated or agglomerated lumps.
Granular formulations have been, and will continue to be one of many viable formulation alternatives. Driving forces for the use of granular formulations are safety in use (less potential for drift and reduced worker exposure to toxic chemicals), ease of use (no field lending requirements) and efficacy (delivery as granules is often uniquely suited to the characteristics of target pest). A wide variety of carriers are available. Virtually any pesticide may be formulated as a free-flowing granule.
Figure 1 Production of a Granular Formulation
Figure 2 Schematic of rotary mixer.
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