Microencapsulation is the entrapment of a core material. (sometimes referred to as the payload or internal phase) inside an inert coating (usually referred to as shell. capsule wall or membrane).
Three general classes of microencapsulation exist:
- Nanoencapsulation 30-200 nm
- Microencapsulation 0.1-1000 um
- Macroencapsulation >1 mm
Microencapsulated pesticides are mixed with water and sprayed in the same manner as other sprayable formulations. After spraying, the capsule wall breaks down and slowly releases the active ingredient. Microencapsulated materials have several advantages:
- Highly toxic materials are safer for applicators to mix and apply
- Delayed or slow release of the active ingredient prolongs its effectiveness, allowing for fewer and less precisely timed applications
- The pesticide volatilizes more slowly; less is lost from the application site
In residential, industrial, and institutional applications, microencapsulated formulations offer several advantages. These include reduced odor, the release of small quantities of pesticide over a long time, and greater safety. Microencapsulated materials offer fewer hazards to the skin than ordinary formulations. Microencapsulated materials, however, pose a special hazard to bees. Foraging bees may carry microencapsulated materials back to their hives because they are about the same size as pollen grains. As the capsules break down, they release the pesticide, poisoning the adults and brood.
Breakdown of the microencapsulated materials to release the pesticide sometimes depends on weather conditions. Under certain conditions, the microencapsulated materials may break down more slowly than expected. This could leave higher residues of pesticide active ingredient in treated areas beyond normal restricted-entry or harvest intervals with the potential to injure fieldworkers. For this reason, regulations require long restricted-entry intervals for some microencapsulated formulations.
Ways of Making Microcapsules
- physical methods
- phase separation
- interfacial polymerization
Payload Release Mechanisms
- Enzymatic digestion. (Erosion)
- Membrane dissolution. (Chemical change)
- Mechanical fracturing. (Pressure increase)
- Temperature change. (Freeze fracturing, capsules can be made that have a built-in temperature switch)
Parameters Effecting Rate of Release
- Polymer type
- Degree of cross-linking
- Capsule wall thickness
- Capsule size (surface area to volume ratio)
- Physical state of the A.I.
Some Commercial Uses of Microencapsulation
- Carbonless copy paper
- Self-locking bolts
- Advertising perfumes
- Seed coatings
- Flavorings in foodstuffs
The cost of microencapsulation can vary considerably and is largely dependent upon the technique employed. Some techniques require the use of specialized equipment, whilst others do not. Some techniques used expensive process chemicals, whilst others use very cheap ingredients. Processes which involve heat are always more expensive than those which do not. Removal of the continuous phase to produce a “dry” product will require an extra processing step adding to the cost.
Some products, particularly those of high value or low volume, are better able to absorb such an increased cost. To economically encapsulate high volume products or those which produce a low profit margin it is necessary to employ one of the cheaper techniques if this is possible for the application in question.