Protein degraded pre-vulcanized natural rubber coated slow release fertilizers

An agricultural material composition comprises particles of an agricultural material, e.g. a fertilizer, coated with a layer of protein degraded pre-vulcanized natural rubber to provide slow-release properties. The particles preferably have a diameter of from 0.5 to 12 mm, and the thickness of the rubber layer is preferably from 10 to 250 microns.

This invention relates to the use of natural rubber (NR) in the manufacture 
of coated agricultural materials. In particular it relates to the 
manufacture of slow release fertilizers by coating fertilizer materials 
with a coating of NR derived from a modified NR latex. 
It is well known that many conventional fertilisers are highly soluble in 
water, and when applied to soils, particularly sandy soils, a high 
proportion of them will be rapidly leached away from the rhizospheres 
(root zones) of plants resulting in depletion of the effective nutrient 
supply to the plants. This leaching problem is particularly serious in 
tropical countries where rainfall intensities are high. A reasonably 
constant supply of nutrients in the rhizosphere throughout the growing 
season is of great importance. To overcome these problems, the use of 
various types of slow-release fertilisers has been suggested. Such 
fertilisers release nutrients slowly when applied to the soild and in 
doing so provide a more nearly continuous supply of nutrients, hence 
reducing leaching losses. 
Our experimental work has shown that fertilizers coated with unmodified NR 
are rapidly leached despite the coating of rubber. This appears to be a 
result of the relatively highly permeable and hydrophilic nature of the 
rubber deposited on the fertilizer from NR latex. Further, if the latex is 
not vulcanized then there is a strong tendency for the coated fertilizer 
particles to aggregate to an undesirable extent, because the rubber film 
is relatively soft and tacky. The use of pre-vulcanized latex permits 
avoidance of the problem of aggregation but the rubber film is still 
sufficiently hydrophilic and permeable that it is not of practical value 
in the manufacture of slow release fertilizers. 
The use of synthetic polymers has been proposed but many of these give 
virtually impermeable films around the particles of fertilizer and the 
distribution of fertilizer to the rhizosphere is impractically slow. 
Further, many synthetic polymers are normally not fully biodegradable and 
can give rise to residues which may have a deleterious effect on nearby 
plants. 
The present invention is based on the discovery that coating fertilizers 
with a modified natural rubber latex in which the rubber is prevulcanized 
and the proteins are at least partly degraded gives a product which 
provides a particularly useful balance of properties to suit it for use as 
a slow-release fertilizer. 
Accordingly, the present invention provides a slow release fertilizer which 
comprises a fertilizer coated with a layer of protein degraded 
pre-vulcanized natural rubber. The invention includes a method of making a 
slow release fertilizer which comprises contacting particles of a 
fertilizer with a protein degraded pre-vulcanized natural rubber latex and 
coagulating and drying the rubber on the particles of the fertilizer 
thereby coating the particles with a film of protein degraded 
pre-vulcanized natural rubber. The particular method by which the protein 
degraded prevulcanized rubber (PD-PV NR) is coated onto the particles of 
fertilizer is not critical to the overall method of the invention. Thus, 
techniques such as pan coating, rotary drum coating and similar methods 
can be used. However, we have found that a fluidized bed technique is 
particularly appropriate and, accordingly, forms a further aspect of the 
invention. In this further aspect the invention provides a method of 
making a slow release fertilizer which comprises providing a fluidized bed 
of particulate fertilizer and introducing into the bed protein degraded 
prevulcanized natural rubber latex in droplet form whereby to form a 
coating of dried coagulated protein degraded prevulcanized natural rubber 
over the fertilizer particles. This method can be carried out in a batch 
process by placing the fertilizer, preferably in granular form, in a 
vessel having a perforated base and pumping heated air through the base to 
fluidize the fertilizer granules. The PD-PV NRL can be introduced into the 
fluidized bed by spraying it e.g. using a compressed air sprayer into the 
bed. The nozzles of the sprayer are conveniently positioned at the bottom 
of or within the fluidized bed. The latex spray forms a coating which 
coalesces into a coherent film on the surface of the fertilizer granules 
and rapidly coagulates and dries in the stream of hot air used for 
fluidizing. The rapid drying of the rubber film avoids problems which 
might otherwise occur by the granules sticking together thus forming 
undesirably large aggregates or tearing the rubber film off the granules. 
After the desired amount of latex has been sprayed onto the granules it is 
desirable to maintain them in a fluidized state to ensure thorough drying 
of the latex to give a non-tacky tough film of PD-PV NR. Granules coated 
in this way can readily be stored for subsequent use in bags or drums. 
The natural rubber used in this invention to make slow release fertilizers 
is protein degraded pre-vulcanized natural rubber latex (PD-PV NRL). To 
the best of our knowledge this form of natural rubber latex is novel. 
Prevulcanized natural rubber latex (PV NRL) has long been known and used 
in the manufacture of latex products e.g. rubber gloves. Also, techniques 
for making deproteinized natural rubber are known e.g. as described in our 
earlier British Patent Specification No. 1366934. In this prior 
Specification, the rubber is deproteinized by treating the latex with a 
proteolytic enzyme to hydrolyse the protein fraction of the latex, 
diluting the latex typically to about 3% dry rubber content (drc) and then 
coagulating the latex with acid to produce solid rubber. The reason for 
the treatment is to achieve an improvement in the properties and behaviour 
of the rubber produced in the raw state and/or after vulcanization of the 
solid rubber. These improved properties are not particularly relevant to 
the manufacture of rubber latex goods and, in any event, the separation of 
the rubber from the protein hydrolysis products usually involves acid 
coagulation of the latex. 
In the present invention it is not necessary to separate the rubber from 
the hydrolysis products, indeed, we have found that it is beneficial to 
the slow-release fertilizer product not to remove the hydrolysis products 
from the latex. The hydrolysis of the proteins is most conveniently 
carried out by treating the latex with a proteolytic enzyme for example by 
the method described in British Patent Specification No. 1366934. 
Although it is not necessary to remove the protein degradation products 
from the latex we have found that slow release fertilizers can be made 
using fully deproteinised prevulcanized natural rubber (DP-PV NR) and, 
accordingly this is included within the term "protein degraded 
prevulcanized natural rubber". Deproteinized natural rubber latex (DP NRL) 
can be made by successive dilution and concentration e.g. by centrifuging, 
of latex in which the proteins have been degraded e.g. by enzymatic 
hydrolysis. As those skilled in the art will appreciate the 
dilution/concentration method can be used to reduce the proportion of 
protein degradation products in rubber coagulated from the latex to any 
desired extent and thus it provides a technique for choosing a desired 
level of protein degradation products in the rubber. The use of fully 
deproteinized prevulcanized rubber latex (DP-PV NRL) tends to give 
fertilizers having extended slow release characteristics. This can be used 
to advantage by blending fertilizers coated with DP-PV NR with fertilizers 
coated with PD-PV NR to obtain a combination of properties. However, the 
use of DP-PV NR involves significant additional processing costs usually 
with only small technical advantages possible and, accordingly, it is not 
especially preferred. 
The latex may be pre-vulcanized by known techniques for example by heating 
the latex typically at about 70.degree. C. for 1 to 2 hours with a 
suitable vulcanizing ingredient such as a mixture of sulphur, zinc diethyl 
dithiocarbamate and zinc oxide. It is usual to stabilize the latex against 
coagulation during pre-vulcanization by adding to it an involatile alkali 
e.g. KOH or carboxylic acid soaps in suitable amounts. 
We have found that PD-PV NRL suitable for manufacture of slow-release 
fertilizers can be made either by first treating the latex with a 
proteolytic enzyme and then prevulcanizing or by first pre-vulcanizing the 
latex followed by treatment with a proteolytic enzyme. The form of latex 
used as the starting material is not critical and we have obtained 
satisfactory results using field latex, concentrated e.g. centrifuge 
concentrated, latex and skim latex. 
The present invention relates to coating general types of fertilizer but is 
particularly directed to coating fertilizers which are water soluble or 
readily water dispersible because such fertilizers are especially 
susceptible to being rapidly leached from the rhizosphere. These 
fertilizers include inorganic salts such as potassium and ammonium salts 
of sulphuric, nitric and phosphoric acids. It is usual practice to assay 
such fertilizers in terms of their potassium (K) phosphorus (P) and 
nitrogen (N) contents. Organic fertilizers such as urea can also be used. 
Additionally, with ammonia-containing fertilizers such as urea and 
inorganic ammonium salts, coating the fertilizers with PD-PV NR reduces 
the loss of nitrogen to the atmosphere by volatilization of ammonia. This 
is particularly significant in tropical environments where ammonia 
volatilization especially from urea is a major disadvantage of such 
fertilizers. The fertilizer can be a single compound (a "straight" 
fertilizer) or a mixture of compounds (a "compound" fertilizer) as may be 
desired to suit particular circumstances. These fertilizers can be used 
alone or in combination with other materials having desired activity in 
the rhizosphere e.g. plant growth regulants, hormones, pesticides, 
weedkillers and agents active against undesired soil microorganisms. 
Further, we we have found that coating granules including seed and 
fertilizers according to the invention is particularly beneficial. 
In this connection we have found that the invention can be of applicability 
to the coating of particulate agricultural materials generally and is not 
limited to fertilizers liable to leaching. Accordingly, the present 
invention in a further aspect includes a particulate agricultural material 
coated with a film of protein degraded prevulcanized natural rubber and a 
method of making a coated agricultural material which comprises contacting 
the particulate agricultural material with a protein degraded 
pre-vulcanized natural rubber latex and coagulating and drying the rubber 
on the particles of the agricultural material thereby coating the 
particles with a film of protein degraded pre-vulcanized natural rubber. 
The agricultural materials to which this apsect of the invention relates 
include, in addition to fertilizers, plant growth regulants, hormones, 
pesticides, weed killers, seeds and agents active against undesired, or 
active to encourage desired soil microorganisms. Where these materials are 
not particulate they may be adsorbed on or absorbed in a suitably 
particulate carrier which may be another agricultural method or an 
inactive or inert particulate carrier. 
The film of rubber coating the particulate fertilizer or agricultural 
material may be modified by inclusion therein of materials such as 
preservatives, stabilizers, antioxidants, surfactants, inert fillers, 
viscosity modifiers, compatibility agents, co-solvents and humectants. 
Usually such materials, if used, will be added to the rubber latex prior 
to coagulation. 
The particulate slow release fertilizer according to the invention will 
typically be made to have an average particle size of from 0.5 to 12 mm. 
The precise figure in any particular case will be selected to suit the 
intended use of the fertilizer. Generally, other coated agricultural 
materials will be made in a similar particle size range. 
The thickness of the film of PD-PV NR around the particles of fertilizer is 
an important factor in determining the release characteristics of the 
coated fertilizer. However, direct measurement of the film thickness is 
difficult. Accordingly, it is generally more convenient to refer to the 
weight percent of rubber, based on the fertilizer. Whilst the film 
thickness is a function of the weight percent rubber it is also a function 
of the particle size. Thus, assuming that the particles are spherical, 
that their specific gravity is 1.33 and that the specific gravity of the 
rubber film is 0.92 a coating of 10% by weight of rubber on particles 4 mm 
in diameter gives an average (calculated) film thickness of about 90 
.mu.m. To achieve this thickness on particles 2 mm in diameter would 
require about 21% by weight of rubber and on 0.5 mm particles about 108% 
by weight of rubber in the coating. Generally, the thickness of the film 
will be in the range 10 to 250 .mu.m. Correspondingly the amount of rubber 
coated onto the particles will generally be from 1 to 25% preferably 5 to 
5%, by weight on the weight of the fertilizer. It will be recognized that 
within this range the higher precentages will more usually be applicable 
to particles of smaller size within the size range given above and vice 
versa. In any particular case the amount of rubber in the coating will, be 
selected to give the desired slow release characteristic as the thicker 
the coating film the slower the release of fertilizer. 
If it is desired to slow down the leaching of fertilizer even further than 
is obtained using PD-PV NR then this may be achieved by including a minor 
proportion of a synthetic polymer in the coating of the fertilizer. A 
proportion of up to 45% of the coating may be made of synthetic polymer 
for this purpose. The synthetic polymer is preferably included in the 
rubber latex prior to coagulation typically as a latex or emulsion. The 
amount of synthetic polymer latex or emulsion used depends on the desired 
proportion of synthetic polymer in the rubber film, the dry rubber content 
of the PD-PV NRL and the polymer content of the synthetic polymer or 
latex. 
We do not fully understand why the use of PD-PV NRL in coating fertilizers 
is as successful as it is, but it seems that the following factors 
contribute to the result. Pre-vulcanization of the latex reduces the water 
and fertilizer permeability of the rubber film to a moderate extent but 
not sufficiently to provide a satisfactory slow-release product. We 
believe that films made from pre-vulcanized (but not protein degraded) 
latex include significant proportions of proteinaceous material, largely 
made up of intact or only slightly degraded proteins in the latex. The 
protein molecules are lrage and are hydrophilic and, in contact with 
water, become hydrated and from large hydrophilic pathways through the 
rubber film thus permitting relatively rapid leaching. The relatively high 
concentrations within the coated fertilizer particles provides a 
susbstantial driving force which enhances the effectiveness of any 
hydrophilic pathways. In rubber films formed from PD-PV NRL, degradation 
products of the proteins in the original latex will normally be present. 
Generally, these degradation products are much smaller in size than the 
proteins in non-protein-degraded rubber, and accordingly form smaller 
hydrophilic channels in the rubber film. Thus, the presence of the 
degradation products imparts a degree of hydrophilicity and permeability 
to the rubber film which can, to some extent, be controlled by the 
thickness of the rubber film and the choice of additives. The inclusion of 
synthetic polymer is, as described above, a further way of modifying the 
hydrophilicity and permeability of the film. 
The following Examples illustrate the invention. Example 1 describes 
techniques for carrying out the pre-vulcanization and protein degradation 
steps to make a PD-PV NRL suitable for use in the invention and describes 
some of the properties of such latex. Example 2 describes the manufacture 
and Examples 3 to 5 the properties of slow release fertilizers according 
to the invention.

EXAMPLE 1 
Protein Degradation 
To proteins in the NR latex were degraded by treating NR latex with a 
proteolytic enzyme as described in British Patent Specification No. 
1366934. The degraded protein fractions were not removed from the latex. 
The degree of protein-breakdown was assessed by analysing for the nitrogen 
content of the dry rubber obtained by coagulating the latex, with acid. 
Vulcanisation 
This was effected by heating the NR latex with vulcanising ingredients at 
about 70.degree. C. for 1-2 hours. The vulcanising ingredients used were a 
mixture of sulphur, zinc diethyl dithiocarbamate and zinc oxide although 
other vulcanizing systems could be used. The latex can be stabilised by 
the incorporation of stabilising agents as potassium hydroxide, or 
carboxylic acid soaps where necessary. 
The PD-PV NRL obtained by the protein degradation and pre-vulcanization 
steps above and conventional prevulcanized natural rubber latex (PV 
NRL)(for comparison) were diluted to 3% solids content and coagulated by 
addition of acid. The nitrogen content in the coagulated rubber was 
determined analytically and the corresponding protein content present was 
estimated by multiplying the nitrogen content by a factor of 6.25. The 
results are given in Table 1. For comparison the protein content of the 
latex either PD-PV NRL or PV NRL, obtained by nitrogen analysis of the 
total solids film from drying the latex at 70.degree. C. in an oven, was 
about 1.9%. Table 1 shows that protein-degraded prevulcanised samples, (2) 
and (3) have markedly lower protein content than prevulcanised NR latex 
(1). It also shows that the protein can be effectively degraded either 
before or after vulcanisation of the latex. 
TABLE 1 
______________________________________ 
Protein content 
Sample (% by weight on dry 
No NR latex sample rubber) 
______________________________________ 
(1) Prevulcanised (comparison) 
0.94 
(2) Protein-degraded, followed 
0.30 
by prevulcanisation 
(3) Prevulcanised, followed by 
0.20 
protein-degradation 
______________________________________ 
EXAMPLE 2 
Granules of a commercially available inorganic fertilizer containing 
potassium, nitrogen and phosphorus, having an average diameter of about 3 
to 4 mm were fluidized in a bed using a stream of hot air. PD-PV NRL was 
sprayed into the bed from the bottom at a rate to uniformly coat the 
granules. The coating weight was 14.2% by weight of rubber. The Example 
was repeated using PV NRL to a coating weight 14.6% by weight of rubber to 
provide a comparative sample. The Example was further repeated using 
granules having an average diameter of about 2 to 3 mm of urea wth both 
PD-PV NRL and PV NRL both to a coating weight of 13%. 
EXAMPLE 3 
Leaching studies 
The experiments on the leaching of the fertilizers coated with PD-PV NR and 
PV NR(for comparison) formulations were conducted using a sand-filled 
glass tube of 5 cm in diameter and 40 cm in height. 
Granules of the coated inorganic fertilizer produced in Example 2 were 
placed on top of the sand column and 25 ml of distilled water was applied 
alternate-daily to the tube. 
Leaching collection was made weekly by providing a suction pressure of 100 
cm water to the outlet. The leachates were analysed for nitrogen, 
phosphorus and potassium. 
TABLE 2 
______________________________________ 
Percentage Cumulative leaching loss 
Fertiliser 
Ele- 2 4 6 8 10 
samples ments weeks weeks weeks weeks weeks 
______________________________________ 
Coated with 
N 52.5 80.9 92.3 97.6 100 
PV NR P 1.4 10.2 16.3 19.8 21.7 
14.6% by wt. 
K 37.3 57.8 68.2 75.2 80.6 
rubber based 
on fertiliser 
Coated with 
N 6.0 38.6 61.6 74.0 80.5 
PD-PV NR P 0 2.4 9.5 14.5 18.2 
14.2% by wt. 
K 4.1 26.7 44.9 56.5 67.5 
rubber based 
on fertiliser 
______________________________________ 
The comparative leaching results for nitrogen, phosphorusand potassium as 
given in Table 2 show clearly that PD-PV NR coating is far superior to PV 
NR in reducing the leaching rates of these elements. 
EXAMPLE 4 
Dislodgement studies 
The experiments on the rate of dislodgement of the fertilizer elements from 
the coated granules as affected by rain and sunlight were carried out in a 
glasshouse. These involved spreading a fixed amount of granules of coated 
inorganic fertiliser produced in Example 2 over a series of potted soils, 
exposing such granules to sunlight and watering them daily with a fixed 
amount of rain water to simulate the Malaysian rainfall conditions. These 
fertilizer granules were removed at fortnightly intervals and the quantity 
of fertiliser elements remaining in them was determined analytically. The 
results are given in Table 3. 
TABLE 3 
______________________________________ 
Percentage of fertiliser elements left in 
coated granules 
Fertiliser 
Ele- 2 4 6 8 10 
samples ments weeks weeks weeks weeks weeks 
______________________________________ 
Coated with 
N 27.1 14.0 12.7 7.7 5.2 
PV NR P 74.4 67.2 66.7 59.0 54.6 
14.6% by wt. 
K 42.8 28.0 24.4 18.0 11.7 
rubber based 
on fertiliser 
Coated with 
N 70.2 52.3 32.2 18.4 16.1 
PD-PV NR P 90.6 78.1 72.8 67.7 63.3 
14.2% by wt. 
K 76.4 56.1 37.7 34.6 28.5 
rubber based 
on fertiliser 
______________________________________ 
The comparative data on retention of nitrogen, phosphorus and potassium in 
the coated granules show clearly that PD-PV NR coating is far more 
effective than PV NR in slowing down the dislodging rates of these 
elements. 
EXAMPLE 5 
Diffusion studies 
The rate of diffusion of nutrients from PD-PV NR coated urea was compared 
with that of PV NR coated urea both produced in Example 2. This experiment 
was conducted by immersing 5 g of each sample in 500 ml distilled water in 
a flask. 2 ml sample solutions were removed from each flask at intervals 
of every 24 hours. In each case, the flask was gently inverted 10 times 
before the removal. The nitrogen in the aqueous solution was determined 
and the results are given in Table 4. 
TABLE 4 
______________________________________ 
Percentage of N diffused from coated 
Rubber Amount of urea into aqueous solution 
Latex Rubber in 1 2 3 4 5 7 
Used Coating day days days days days days 
______________________________________ 
PV NRL 13.0% 99.0 100 -- -- -- -- 
PD-PV NRL 
13.0% 32.9 50.0 66.6 81.1 88.8 96.7 
______________________________________ 
The results show conclusively that PD-PV NR formulation is much more 
effective than that of PV NR as a coating material for controlling the 
diffusion of urea into a surrounding aqueous medium.