USE OF BIOPOLYMERS FOR CONTROLLED RELEASE OF PHEROMONES
V. Nandagopal1 A. Prakash1 A. Sasmal2, S. Sasmal1 and P.L.Nayak2
1 Department of Entomology, Central Rice Research
Institute, Cuttack-753 006
2 P. L. Nayak Research Foundation, Cuttack-753
006
__________________________________________________________________________________________________
INTRODUCTION
During the last
two decades, significant advances have been made in the development of biocompatible
and biodegradable materials for biomedical applications, and in the
case of the latter category, industrial applications, as well. In the
biomedical field, the goal is to develop and characterize artificial materials
or, in other words, "spare parts" for use in the human body to
measure, restore, and improve physiologic function, and enhance survival and
quality of life. Typically, inorganic (metals, ceramics, and glasses) and
polymeric (synthetic and natural) materials have been used for such items as
artificial heart-valves, (polymeric or carbon-based), synthetic blood-vessels,
artificial hips (metallic or ceramic), medical adhesives, sutures, dental
composites, and polymers for controlled slow drug delivery. The development of
new biocompatible materials includes considerations that go beyond non-toxicity
to bioactivity as it relates to interacting with and, in time, being integrated
into the biological environment as well as other tailored properties depending
on the specific "in vivo" application.1-2
The parallel field
of "biomimetics" may be described as the "abstraction of good
design from nature" or, plainly put, the "stealing of ideas from
nature". The goal is to make materials for non-biological uses under
inspiration from the natural world by combining them with manmade,
non-biological devices or processes. This is fast becoming a new research
frontier.
Yolles & Sinclair (1973) first
proposed the use of polymers of lactic and glycolic acid as degradable matrices
for the sustained released of bioactive substances. The advantages of the polymers for this
purpose are considerable: The polymeric matrix disappears innocuously. The
degradation can be tailored to that required for the drug or pesticide by
varying the copolymer composition. The half-life of the drug/pesticide is
prolonged to provide sustained therapy/duration without toxicity/persistence.
The controlled-release device is easily fabricable by solution; or melt-forming
operation since the polyesters are thermoplastic and their polar nature allows
incorporation and compatibility with polar, bioactive substances.
2.
BIODEGRADABLE POLYMERS
One area of
intense research activity has been the use of biocompatible polymers for
controlled drug delivery. It has evolved from the need for prolonged and better
control of drug administration. The goal of the controlled release devices is
to maintain the drug in the minimum effective range with just a single dose. In
general, release rates are determined by the design of the system and are
nearly independent of environmental conditions.
A convenient classification
of controlled-release systems is based on the mechanism that controls the
release of the substance in question. The most common mechanism is diffusion.
Two types of diffusion-controlled systems have been developed; the first is a
reservoir device in which the bioactive agent (drug) forms a core surrounded by
an inert diffusion barrier (Figure 1). These systems include membranes,
capsules, microcapsules, and hollow fibers. The second type is a monolithic
device in which the active agent is dispersed or dissolved in an inert polymer
(Figure 2). As in reservoir systems, drug diffusion through the polymer
matrix is the rate-limiting step, and release rates are determined by the
choice of polymer and its consequent effect on the diffusion and partition
coefficient of the drug to be released.
In chemically
controlled systems, chemical control can be achieved using bioerodible or
pendant chains. The rationale for using bioerodible (or biodegradable) systems
is that the bioerodible devices are eventually absorbed by the environment and
thus need not be removed. Polymer bioerosion can be defined as the conversion
of a material that is insoluble in water into one that is water-soluble. In a
bioerodible system the drug is ideally distributed uniformly throughout a
polymer in the same way as in monolithic systems. As the polymer surrounding
the drug is eroded, the drug escapes (Figure 3). In a pendant chain
system, the drug is covalently bound to the polymer and is released by bond
scission owing to water or enzymes. In solvent-activated controlled systems,
the active agent is dissolved or dispersed within a polymeric matrix and is not
able to diffuse through that matrix. In one type of solvent-controlled system,
as the environmental fluid (e.g., water) penetrates the matrix, the polymer
swells and its glass transition temperature is lowered below the environmental
temperature. Thus, the swollen polymer is in a rubbery state and allows the
drug contained within to diffuse through the encapsulant.3
As
material designers confront the fundamental challenges of medical science, the
discovery of new biocompatible polymers is creating unprecedented excitement. Like other important scientific
concepts that change over time, the notion of biocompatibility has evolved in
conjunction with the continuing development of materials used in medical
devices. Until recently, a biocompatible material is essentially thought of as
one that would "do no harm." The operative principle is that of
inertness—as reflected, for example, in the definition of biocompatibility
as "the quality of not having toxic or injurious effects on biological
systems."
2. 1.
BIODEGRADADION MECHANISM OF BIOPOLYMERS
Most biodegradable polymers are designed to degrade as a
result of hydrolysis of the polymer chains into biologically acceptable, and
progressively smaller, compounds. In some cases—as, for example, polylactides,
polyglycolides, and their copolymers—the polymers will eventually break down to
lactic acid and glycolic acid, enter the Krebs’s cycle, and be further broken
down into carbon dioxide and water. Degradation may take place through bulk
hydrolysis, in which the polymer degrades in a fairly uniform manner throughout
the matrix. For some degradable polymers, most notably the polyanhydrides and
polyorthoesters, the degradation occurs only at the surface of the polymer,
resulting in a release rate that is proportional to the surface area of the
drug delivery system 4.
2. 1. 1. Factors Affecting Biodegradation of
Polymers
- Chemical
structure.
- Chemical
composition.
- Distribution
of repeat units in multimers.
- Presents
of ionic groups.
- Presence
of unexpected units or chain defects.
- Configuration
structure.
- Molecular
weight.
- Molecular-weight
distribution.
- Morphology
(amorphous/semi-crystalline, microstructures, residual stresses).
- Presence
of low-molecular-weight compounds.
- Processing
conditions.
- Annealing.
- Sterilization
process.
- Storage
history.
- Shape.
- Site of
implantation (use).
- Adsorbed
and absorbed compounds (water, lipids, ions, etc.).
- Physicochemical
factors (ion exchange, ionic strength, pH).
- Physical
factors (shape and size changes, variations of diffusion coefficients,
mechanical stresses, stress- and solvent-induced cracking, etc.).
- Mechanism
of hydrolysis (enzymes versus water).
3. AGROCHEMICALS
Agrochemicals as referred to
herein include biologically active ingredients/plant protection products such
as insecticides, herbicides, fertilizers, growth regulators, pheromones,
biostimulants, acaricides, miticides, nematocides, fungicides and the like.
Such agrochemicals are well known and are in common usage for controlling pests
and diseases and for promoting plant growth in agriculture. In practice, it is
important to make a sufficient amount of such agrochemical or active ingredient
available to the biological system in order to control pests or disease or to
promote growth. Too much active ingredient, however, is inefficient and not
desired because of environmental and economic concerns. Furthermore, higher amounts
of active ingredient lead to increased risks of leaching to ground water or
surface water. Higher amounts can also lead to phytotoxicity for the crop.
Insufficient levels of active ingredients results in lack of control of the
pest and increase the risk of resistance
Thus, it is generally known that
it is important to deliver the correct amount of active ingredient to the crop
for control of the pest or disease and to promote growth over a given period of
time. However, multiple applications of active ingredients become labor and
cost intensive. With conventional applications of liquid or powder
formulations, relatively high amounts of active ingredients are applied several
times to assure control of pests over a longer period of time, typically 3-6
times for seasonal control, and users are exposed to the active ingredients
during each application, which is undesirable.
3. 1. Slow and controlled-release of agrochemicals
Slow and
controlled-release agrochemicals contain a bioactive agent for e.g., a
plant nutrient, in a form which either a) delays its
availability for plant uptake and use after application, or b) which is
available to the plant significantly longer than a reference ‘rapidly available
nutrient fertilizer’ such as ammonium nitrate or urea, ammonium phosphate or
potassium chloride 5. There is no official differentiation between
slow-release and controlled-release formulations. Also the AAPFCO, the
Association of American Plant Food Control Officials, uses both in its Official
Terms and Definitions6.
Manufacturing Routes for Slow and Controlled-Release and
Stabilized Formulations
In the
production of Slow-Release Formulations (SRFs) or Controlled-Release Formulations
(CRFs), the slow-release effect may be obtained by various production
processes, for example through modification of conventional formulations 7-11.
Controlled or slow nutrient release can be achieved through special chemical
and physical characteristics. With controlled-release formulations the
principal procedure is one whereby conventional soluble agrochemicals are given
a protective coating or encapsulation (water insoluble, semi-permeable or
impermeable with pores), controlling water penetration and thus the rate of
dissolution, and agrochemical release synchronized to the plants’ needs.
3. 2. Slow and
controlled-release agrochemicals
3.2.1. Materials
releasing agrochemicals through low solubility due to a complex/highmolecular
weight chemical structure following microbial decomposition.
3.2.2. Materials
releasing agrochemicals through a coated surface (coated fertilizers).
3.2.3. Materials
releasing agrochemicals through a membrane which may or may not itself be
soluble (encapsulation).
3.2. 4. Agrochemical-releasing
materials incorporated into a matrix which itself may be coated.
3.2. 5. Materials
releasing agrochemicals in delayed form due to a small surface-to-volume ratio
(super-granules, briquettes, tablets, spikes, plant food sticks etc.).
4. SLOW AND CONTROLLED-RELEASE AGROCHEMICALS
4.1. Their advantages
Slow and
controlled-release or stabilized formulations offer a number of important
advantages:
4.1.1. They reduce toxicity (particularly to
seedlings), which is caused through high ionic concentrations resulting from
the quick dissolution of conventional soluble agrochemicals (in some cases also
from ammonia, for instance after application of urea) and thus contribute to
improved agronomic safety 12-17.
4.1.2. Due to the reduction of toxicity and
the salt content of substrates (4.1.1) they permit the application of
substantially larger agrochemical dressings as compared to conventional soluble
formulations. This results in significant savings in labour, time and energy,
as well as in making the use of the agrochemical more convenient. This latter
factor constitutes the greatest advantage for the majority of present consumers
of slow- and controlled-release formulations.
4.1.3. They contribute to advanced
agrochemical management programme and to innovative farming systems such as no
tillage farming with single co-situs agrochemical application7.
4.1.4. They permit the meeting of the full
requirements of crops grown under plastic cover (protected crop cultivation),
and multi-cropping by a single application.
4.1.5. They significantly reduce possible losses
of nutrients, particularly, between applications, and uptake by the plants
through a gradual release. They also reduce evaporation losses. This
substantially decreases the risk of environmental pollution18-21.
4.1.6. They also contribute to a reduction
in relevant gas emissions (N2O) 22-24.
4.2. The possible disadvantages
Slow and
controlled-release or stabilized formulations also have possible disadvantages:
4.2.1. There are no standardized methods for
reliably determining the pattern of release. Broadly speaking there appears to
be a lack of correlation between the data resulting from laboratory testing -
which are made available to the consumer - and the actual functioning of the
bioactive agent release pattern in field conditions.
4.2.2. With regard to chemical reaction
products, such as urea-formaldehyde fertilizers, it appears that a proportion
of the nitrogen contained may be released to the soil solution extremely slowly
(or not at all).
4.2.3. Some formulations may also exhibit a
burst release of the agrochemicals, causing damage to turf or to the crop.
Further, this rapid initial release, even if it does not cause damage, is at a
higher cost than that of the equivalent amount of conventional (non slow or
controlled release) soluble agrochemical. Also, some of the coated granules may
be so thickly coated that the agrochemical contained in these granules may not
be released during the crop demand period.
4.2.4. Application of coated
controlled-release agrochemicals may drastically alter the pH of the soil. For
example, in case of sulphur coated Urea (SU) if large amounts of it are
applied, since both sulphur and urea contribute to increased acidity.
4.2.5. Polymer coated or encapsulated
controlled-release formulations may leave undesired residues of synthetic
material on the fields. Their use may thus lead to an undesirable accumulation
of plastic residues25. Further, if the polymer ‘shell’ fragments do
not compose, the fragments, which are smaller than sand size particles,
become part of the soil.
4.2.6. The cost of manufacturing coated or
encapsulated controlled release formulations is still considerably higher as
compared to the production of their conventional counterparts. Thus their cost
benefit ratio at present prevents their wide use in general agriculture7,
9-10, 25-30.
4.2.7. Coated/encapsulated
controlled-release formulations call for higher marketing (specialized advisory
service) and sales expenses than conventional formulations.
5. SOME EXISTING POLYMER-AGROCHEMICAL FORMULATIONS:
5. 1. Condensation products of urea and aldehydes
(methylene ureas)/ nitrogen reaction products
Among the
nitrogen reaction products designed mainly for use on professional turf, in
nurseries, greenhouses, on lawn, and for garden and landscaping, three types
have gained practical importance 9, 31:
Ø
urea-formaldehyde
(UF),
Ø
urea-isobutyraldehyde
(IBDU®), and
Ø
urea-crotonaldehyde
(CDU®).
Whereas the
urea-formaldehyde reaction products have the largest share of the slow-release
fertilizer market, IBDU® and CDU®-based products are less
widely used, due to even greater cost constraints in their production.
5. 1. 1.
Urea-Formaldehyde (UF) - 38% N
Among the
manufactured slow and controlled-release fertilizers, urea-formaldehyde based
products still have the largest share worldwide. This is also the first group
on which research concerning slow release of nitrogen was carried out. Urea-formaldehyde is formed by the reaction
of formaldehyde with excess urea under controlled conditions (pH-value,
temperature, mol proportion, reaction time etc.) resulting in a mixture of
methylene ureas with different long-chain polymers.
The main
problem in the manufacture of urea-formaldehyde as a slow-release fertilizer
type is to produce condensation-oligomers in a desired proportion. The
influence of the proportion of the different methylene ureas on the release of
nitrogen and the nitrogen efficiency can be determined by the Activity Index
(AI).
Fraction I:
cold water soluble - CWS (25°C) containing residual urea, methylene diurea
(MDU), dimethylene triurea (DMTU) and other soluble reaction products. The
nitrogen of Fraction I is, depending on soil temperature, available slowly
(AAPFCO 73, N-29 and N-30).
Fraction
II: hot water soluble - HWS (100°C) containing methylene ureas of intermediate
chain lengths: slow-acting nitrogen.
Whereas in
the past urea-formaldehydes had an AI of about 40 to 50, more recent
urea-formaldehyde formulations are reaching AI-values of 55 to 65. The
Association of American Plant Food and Control Officials (AAPFCO) are setting
an AI of 40 as a minimum with at least 60% of its nitrogen as cold water
insoluble nitrogen (CWI N) and a total content of nitrogen of at least 35%. Un-reacted
urea nitrogen content is usually less than 15% of total nitrogen. The release
pattern of nitrogen from UF fertilizers is a multi-step process (dissolution
and decomposition). In general there is some proportion of N slowly released
(Fraction I); this is followed by a more gradual release over a period of
several (3 to 4) months (Fraction II), depending on the type of product.
However, the release pattern is also influenced by the temperature and moisture
as well as by soil organisms and their activity.
In general
urea-formaldehyde fertilizers show a significant slow release of nitrogen
combined with a good compatibility with most crops.
5. 1. 2.
Isobutylidene diurea (IBDU®) - 32% N
Isobutylidene
diurea is formed as a condensation product by a reaction of isobutyraldehyde (a
liquid) with urea. In contrast to the condensation of urea with formaldehyde
resulting in a number of different polymer chain lengths, the reaction of urea
with isobutyraldehyde results in a single oligomer. However, in order to obtain
an optimal proportion of IBDU, it is important that the reaction is stopped by
neutralization at the point at which it is yielding most IBDU. The theoretical
nitrogen content is 32.18%. The AAPFCO5 definition requires a
minimum of 30%, of which 90% is cold water insoluble (prior to grinding). The
release mechanism functions by gradual hydrolysis of the sparingly water
insoluble IBDU to urea which is transformed to ammonium ions and further to
nitrate (by soil bacteria).
5. 1. 3.
Crotonylidene diurea (CDU®) - 32.5% N
Chisso
Corporation holds the trademark for ‘CDU’ urea-crotonaldehyde slow release
nitrogen. Crotonylidene diurea is formed by the acid-catalyzed reaction of urea
and acetic aldehyde. When dissolved in water it gradually decomposes to urea
and crotonaldehyde. As with IBDU, with CDU also particle size greatly
influences the rate of nitrogen release (very delayed release with larger
particle size). CDU is decomposed by both hydrolysis and microbial processes in
the soil; temperature, soil moisture and biological activity affect the release
rate, though even in acid soils the degradation is slower as that of IBDU. The
agronomic performance is similar to IBDU. CDU is produced in Japan (Chisso Corp.) according to a
production process developed by Chisso (modified BASF production process; BASF:
Crotonaldehyde + Urea, Chisso: Acetaldehyde + Urea). In Japan and Europe ,
its main use is on turf and in speciality agriculture, typically formulated
into granulated NPK fertilizers.
5. 2.
Coated/encapsulated controlled-release fertilizers
These are
conventional soluble fertilizer materials with rapidly available nutrients
which after granulation, prilling or crystallization are given a protective
(water-insoluble) coating to control the water penetration and thus the rate of
dissolution and the nutrient release. A product containing sources of water
soluble nutrients, release of which, in the soil is controlled by a coating
applied to the fertilizer 5.
There are
three different groups of coated/encapsulated controlled release fertilizers,
using as coating material:
Ø Sulphur
Ø
polymeric
/ polyolefin materials, and
Ø
Sulphur
plus polymeric, including wax polymeric materials (As for example according to
United States Patent Number 5,599,374, Feb. 4, 1997, John H. Detrick “Process
for producing improved sulfur-coated urea slow release fertilizers”).
Agents
currently used for coating/materials used in manufacturing fertilizers with
controlled release of nutrients are:
Ø
polymers
(e.g. PVDC-based copolymers, polyolefins, polyurethane, urea-formaldehyde
resin, polyethylene, polyesters, etc., The kind of polymeric material finally
used by the individual manufacturer mainly depends on the chemical and physical
properties, the cost, the availability and the patent situation.
Ø
fatty
acid salts (e.g. Ca-stereate),
Ø
latex,
rubber, guar gum, petroleum derived anti-caking agents, wax (The word ‘latex’
originally meant an emulsion of natural rubber, such as is obtained by cutting
the bark of rubber trees. However, in chemistry all colloidal dispersions of
polymers in an aqueous media are called latex.),
Ø
Ca+Mg-phosphates,
Mg-oxide, Mg-ammonium phosphate + Mg potassium phosphate,
Ø
phosphogypsum,
rock phosphate, attapulgite clay,
Ø
peat
(encapsulating within peat pellets: organo-mineral fertilizers, OMF),
Ø
neemcake/
nimin’-extract (extract from neem cake).
In
comparison to urea reaction products, coated fertilizers, particularly those
coated with a multi-layer coating of sulphur and a polymeric material, may
present more favourable economics. To obtain a further reduction of total
fertilizer costs, coated/encapsulated fertilizers are increasingly used in
blends with conventional fertilizers in different ratios (mixtures of encapsulated
and non-encapsulated N, NP or NPK fertilizers).
Furthermore,
coated/encapsulated controlled-release fertilizers offer greater flexibility in
determining the nutrient release pattern. They also permit the controlled
release of nutrients other than nitrogen. Nyborg et al.34, have found in greenhouse and field tests that
slowing the release of fertilizer P into the soil by coating fertilizer
granules (polymer coating) can markedly increase P recovery by the crop and the
yield.
5. 3. Sulphur
Within the
group of coated fertilizers sulphur coated urea has gained the greatest
importance to date. The sulphur coating may be considered to be an impermeable
membrane which slowly degrades through microbial, chemical and physical
processes. The concentration of nitrogen (and other nutrients) and its release
varies with the thickness of the coating in relation to the granule or prill
size; it is also influenced by the purity of the urea used35. The
basic production process was developed in laboratory and pilot scale tests in
1961 by TVA (Tennessee Valley Authority, Alabama ).
There are
four reasons favoring the combination of urea and sulphur:
Ø
Urea
with 46% N is highly concentrated, thus coating with sulphur still results in a
product with 30-40% N.
Ø
Urea
is rather liable to leaching and/or to ammonia losses by volatilization;
consequently covering urea granules with an impermeable sulphur membrane
significantly reduces such losses.
Ø
Sulphur is a low cost product.
Ø
Sulphur is a valuable secondary plant
nutrient.
The quality
of SCU is characterized by the rate of N released into the soil solution within
seven days. This seven-day dissolution rate method (developed by TVA) permits
the generating of a leach profile of the tested SCU. Unfortunately, the results
obtained cannot be correlated reliably to the release pattern under practical
field conditions10, 26. Currently marketed SCU fertilizers have
dissolution values of about 40% to 60%. ‘SCU-30’ designates a product with a
nitrogen release of 30% within seven days, under prescribed conditions. With
such a high dissolution rate a rather rapid initial effect is to be expected.
In fact, there have repeatedly been claims of a too-rapid release of nitrogen36.
5. 4.
Polymer-coated/encapsulated controlled-release fertilizers
Standard
SCU has dominated the market for several years. However, horticultural and
lawn-garden markets in particular require a more sophisticated control of
nutrient release. Thus a whole series of controlled-release fertilizers has emerged.
New and modified coating methods have been developed37-41.
Polymer
coatings may be either semi-permeable membranes or impermeable membranes with
tiny pores. The main problems in the production of polymer-coated fertilizers
are the choice of the coating material and the technical coating process
applied 9, 11, 42-45.
Since
nutrient release through the polymer membrane/capsule of controlled-release
fertilizers is not significantly affected by soil properties, such as pH-value,
soil salinity, texture, microbial activity, Redox-potential, ionic strength of
the soil solution, but rather depends on temperature and the moisture
permeability of the polymer coating, it is possible to predict precisely the
nutrient release for a given time 46-47.
Polymer-coated
fertilizer technologies vary greatly between producers, depending on the choice
of the coating material and the technical coating process applied: the Pursell
RLCTM (Reactive Layers Coating) polymer technology (POLYON®) is a
polyurethane; this is also the case with Haifa (MULTICOTE®) and
Aglukon (PLANTACOTE®); Chisso polymer technology (MEISTER®,
NUTRICOTE®) is a polyolefin; Scotts polymer technology (OSMOCOTE®)
is an alkyd resin.
Thus, the
longevity of the polymer coated product, i.e. the rate of nutrient release can
- to a certain extent - be controlled by varying the type and the thickness of
the synthetic material used in coating48-49. The quantity of coating
material used for polymer coatings of conventional soluble fertilizers depends
on the geometric parameters of the basic core material (granules to surface
area, roundness, etc.) and the target of longevity. In general the coating
material represents 3-4 (RLCTM) to 15% (conventional coating with polymers) of
the total weight of finished product. The longer the supply of nutrients needs
to last, the smaller has to be the amount of nutrients released per time unit.
The producers indicate the period of release, e.g. 70 days release (at constant
25°C), or 140 days release, up to 400 days release.
The problem
is that, in order to guarantee the longevity of the polymer coated product, no
bio-degradation, chemical-degradation or mechanical destruction of the coating
should occur during the active time of the applied fertilizer. Consequently, it
is only after the fertilization function of the product, that microbial attack
and mechanical destruction of the empty shell should occur to decompose the
coating over time27. Polymer coated urea (PCU) consists of urea
granules coated with a polymeric resin; it typically contains about forty
percent (40-44%) nitrogen. Coating material made of photo degradative polymer
is easily decomposed by photochemical process in the soil. However, some
polymer coated fertilizers present an as yet unsolved problem of persistence in
the soil of the synthetic material used for encapsulation.
5. 5.
Sulphur coated/polymer-encapsulated controlled-release fertilizers.
Polymer/sulphur
coated fertilizer products (PSCU or PSCFs) have been introduced into the market
recently in the United States, with 38.5 - 42% N, 11 - 15% S and less than 2%
polymer sealant (LESCO Inc. POLY PLUS® PCSCU 39N, PURSELL TriKote® 9
PCSCU 39-42N and Scott POLY-S® PCSCU 38.5-40N; regular size of all
products 1.8 mm to 2.9 mm, nominal 2.4 mm). These products have a primary
coating of sulphur and a secondary coating of a polymeric material. The reason
for this hybrid coating is to combine the control release performance of
polymer-coated fertilizers with the lower cost of sulphur-coated fertilizers48-50.
5. 6.
Granular controlled release agrochemical compositions
For example, US Patent No. 6,682,751 discloses a granular
pesticide comprising a core material coated with an inner polymer membrane
formed in-situ on the core material with a pesticide applied to the inner
polymer membrane and an outer controlled release polymer membrane formed in
situ on the pesticide to permit controlled release. The pesticide may be
incorporated in several "sandwich" layers. Again, the products of US
Patent No. 6,682,751, are structurally different from the products of the
present invention and fail to exhibit a controlled rate of release of the
pesticide over a period greater than about 30 days from the date of initial
exposure of the product to moisture in a manner such that essentially all of
the pesticide coated on the core is released from the granular composition
before the water soluble portion of the core material is released from the
product.
Similarly, US Pat. No.
6,080,221 discloses the coating of porous surfaces of fertilizer particles with
tenacious pesticide-resin solids to form attrition resistant
fertilizer-pesticide combination particles. In this disclosure, the pesticide
is dispersed in a resinous matrix, which is subsequently bonded onto and into
the fertilizer surface. Patent US Pat. No. 4,971,796 describes another slow
release granular product in which the pesticide is matrixed into the coating.
The granule comprises one layer of proteinaceous material or more layers of
proteinaceous material with intermediate spacing layers. The active, ingredient
is in the proteinaceous layer and is released when this layer degrades. The
release rate is changed by varying the cross linking or the thickness of the
layer. The products of US Patent No. 6,080,221 and 4,971,796 fail to exhibit a
controlled rate of release of the pesticide over a period greater than about 30
days from the date of initial exposure of the product to moisture in a manner
such that essentially all of the pesticide coated on the core is released from
the granular composition before the core material is released from the product.
An EP Patent 0755370
discloses other matrix release products which are mixtures of a nitromethylene
or related substances with fertilizers and glue. EP Patent 1063215 describes
briquettes that slowly release active ingredients. The slow release is obtained
via absorption or adsorption of the active ingredient onto solids with high
surface area.
Exemplary of suitable polymers
for use in the present invention are such thermoplastic coating materials as
vinyl resins such as poly(vinyl acetate), poly(vinyl alcohol), poly(vinyl
chloride), poly (vinylidene chloride), polyvinyl pyrrolidone), poly(vinyl
acetal), ρoly(vinyl methylacetamide); polyolefines such as polyethylene,
polypropylene, polyisobutylene; styrene- based polymers; acrylic polymers;
polyesters such as ρoly(alkylene terephthalate), poly(caprolactone); poly(oxy
alkylene)s, such as poly(ethylene oxide), poly(propylene oxide); cellulose
derivatives, such as celluloseacetate; polyamides;ρolyamines; polycarbonates;
polyimides; polysulfones; polysulfides; polysaccharides and the like. Recently,
Nayak et. al.,51 India, have reported the use
of crosslinked chitosan matrices and chitosan hydrogels for the controlled
release of agrochemicals. The same group is also investigating on the potential
of biodegradable polymer nanocomposites for controlled release of
agrochemicals.
5. 7.
Partly polymer-encapsulated controlled-release fertilizers/ Mixtures of
encapsulated and non-encapsulated N, NP or NPK fertilizers.
Another
possibility in order to combine the advantage of controlled release nutrient
supply with the lower cost of conventional fertilizers, is to mix
polymer-coated granules for instance in a ratio of 1: 1 with
non-encapsulated granules of the same fertilizer type25.
In Germany ,
an NPK fertilizer (with a minimum content of 3% N, 5% P2O5,
5% K2O), of which only 50% of the granules are polymer coated, is
registered under the German fertilizer law52. In 1997 a similar NPK
fertilizer type has acquired registration10 with only 25% polymer-coated
granules, offering a greater flexibility in use and further improved economy.
Such partly polymer encapsulated controlled-release fertilizers, i.e. mixtures
of encapsulated and non-encapsulated granules or prills, are also in use in Japan .
5. 8.
Neem- or ‘Nimin’-coated urea
The Indian
neem tree, Azadirachta indica, has a number of traditional uses, based
on the insect repellent and bacteriostatic properties which are contained in
its various parts. The oil obtained from its fruits is a valuable raw material
for the production of pharmaceuticals and body care products.
The press
cake from the production of neem oil has a controlled release and nitrification
inhibiting effect, besides other possible uses. It is therefore frequently
recommended to add neem cake to the N fertilizer (i.e. urea) to form NCU (neem
coated urea) or NICU (nimin = extract from neem cake) coated urea to
improve the nitrogen use efficiency and to reduce losses53. However,
the use of NCU or NICU is apparently not practiced to any extent by farmers,
neither in India
where the tree originates, nor in other tropical countries to which it has been
brought in the past. The main reason for this might be the difficulty to obtain
sufficient quantities of neem cake at village level, the additional labour for
blending or the lack of a corresponding technical process. Since the benefits
of the practice of using NCU are not always reliable, this might also be an
obstacle to the use of neem as controlled release agent or nitrification
inhibitor.
Suri54
regrets that no serious attempt has been made to develop technology to coat
urea with neem on a commercial scale. The fertilizer particles are incorporated
throughout carrier matrices. However, to achieve the desired slow release
effect, a large quantity of carrier material is necessary (up to 40%).
Therefore only low-grade fertilizer formulations are possible (e.g. NPK
10-10-10 or NPK 5-15-10). In general the carrier material is a mix of molten
waxes and of surfactants and polyethylene glycols (polymeric matrices;
styrene-butadiene rubber formulations and others).
5. 9. Supergranules and other Controlled-release
fertilizers in a matrix
This group
of special fertilizer products has been given special attention, particularly
in tropical and subtropical regions. Conventional soluble fertilizers are
formulated in compacted form, with a relatively small surface-to-volume ratio.
This results in a slow release of nutrients, or relatively slower release, into
the soil solution. Some of these special formulations also contain
urea-formaldehyde (UF) or IBDU®. Whereas in Western Europe
such super granules, briquettes, tablets or sticks are preferably used for
fertilizing trees and shrubs, as well as some vegetables, such as tomatoes, pot
plants etc., in tropical regions the preferred use is in irrigated rice55-57.
5. 10. Microemulsions in agrochemicals
Micro
emulsions have a variety of applications in agrochemical industry, of which
pesticide-containing systems are relatively old. To minimize the side effects
of excessive use of agrochemicals on the ecosystem, chemicals with greater
specificity and less persistence are developed. The ease of handling and lower
requirement of smelly solvents goes in favour of the use of micro emulsions.
However, increased efficacy of insecticides when applied as ‘micro colloidal
aqueous emulsion’ instead of micro emulsions has also been demonstrated. O/W micro
emulsions of organic, water insoluble phenoxy herbicides optionally dissolved
in a hydrocarbon solvent have been shown to be appreciably more effective than
the corresponding emulsions in the control of plant growth. Micro emulsions
formulated with a hydro tope solubilizing the herbicide can be promising. The
choice of hydro tope and the emulsifier could be largely determined by the
water solubility of the herbicides. W/O micro emulsions have been suggested to
enrich the mineral-deficient crops with respect to trace metals such as iron.
The oil medium of the micro emulsion can hold the element in contact with the
leaves even during moist conditions, until the trace element is adsorbed.
Since most
agrochemicals are water insoluble and become deactivated with water, their
formulation in o/w micro emulsion is advantageous. The much finer droplet size
of the micro emulsion leads to higher penetrability, much larger contact area
of the active substance to the treated surface and a much more even
distribution during application. The infinite stability of the micro emulsion
and the high concentration of surfactants generally needed for a formulation
are advantageous. A definite relationship exists between herbicides and
surfactant structures for mixed herbicide penetration.
5. 11.
Insecticide-Polymer Formulations
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