Technical field of the invention
The present description relates to the field of
microcapsules, and more particularly to methods for
manufacturing microcapsules with a view to enclosing active
substances actives such as essential oils. More specifically,
it relates to a method for preparing biodegradable
microcapsules. This method performs radical polymerization of
multifunctional compounds in an emulsion of the type O/W (Oil
in Water) resulting in a wall of the cross-linked poly(betaamino ester) type which is biodegradable. The invention also
relates to biodegradable microcapsules obtained with this
method.
Prior art
Microencapsulation is a method for protecting a reactive,
sensitive or volatile substance (referred to here as "active
ingredient") in a capsule in which the size can vary from a
nanometer to a micrometer. The core of the capsule is
therefore isolated from the external environment thereof by a
wall. This makes it possible to delay the evaporation,
release or degradation thereof; there are numerous
applications which make use of these technical effects when
the microcapsules are incorporated in a complex formulation
or applied to a product.
For example, microcapsules can be used to disperse in a
controlled manner the active ingredient contained therein,
which can particularly be a biocidal agent, an insecticide, a
disinfectant, or a fragrance; this can take place by
diffusion through the wall or under the influence of an
external force which ruptures the wall. In some applications,
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the release of the active ingredient takes place under the
influence of an external force which breaks the wall of the
microcapsules; thus, it is possible to release an adhesive
(see for example WO 03/016369 - Henkel), or a reagent (see
for example WO 2009/115671 - Catalyse).
In further applications, the contents of the microcapsule
cannot escape but the color change thereof under the effect
of a variation of temperature (thermochromism) or of UV
radiation (photochromism) is outwardly visible (see for
example WO 2013/114 025 - Gem Innov, or WO 2007/070118 -
Kimberly-Clark, or EP 1 084 860 - The Pilot Ink Co.).
There are several techniques for preparing microcapsules. The
main ones are spray-drying, interfacial polymerization,
solvent evaporation, polymer self-assembly using the Layerby-Layer (LbL) technique, and colloidosome preparation. All
these techniques make it possible to obtain stable
microcapsules of a mean diameter of about 10 μm. Interfacial
polymerization is nevertheless the predominant technique as
it enables quick preparation in a single step of
microcapsules in which the wall is strong enough for the
latter to be isolated and thus be used in numerous
applications.
Several polymer families are conventionally used for
manufacturing the wall of microcapsules (Perignon, C. et al.,
Journal of Microencapsulation 2015, 32 (1), 1-15), such as
polyamides (PA), polyurethanes (PU) or polyureas. The
preparation of PA microcapsule walls generally uses monomers
of the diamine (hexamethylene diamine for example) and acyl
chloride (sebacoyl chloride for example) type, whereas those
in PU make use of monomers of the di-isocyanate (HDI, IPDI
etc.) and diol type. In the case of polyureas, di-isocyanate
and diamine type monomers or di-isocyanates alone are used
3
wherein the hydrolysis at the interface produces amines
enabling urea function synthesis.
For example, the document WO 2009/115671 cited above
describes the formation of microcapsule walls by interfacial
polycondensation, using different monomer mixtures:
hexamethylene diisocyanate (HMDI) and ethylene diamine;
tetraethylorthosilicate (TEO) and 3-
(trimethoxysilyl)propylmethacrylate (MPTS); 2,4-
tolylenediisocyanate (TDI) and 1,3 phenylenediamine; 2,4-
toluene diisocyanate and 1,3-phenylene diamine.
There are already some works reporting the preparation of
microcapsules by interfacial polymerization using other types
of polymers. Mention can be made for example of the works by
J. Bernard on the preparation of glyconanocapsules by coppercatalyzed azide-alkyne cycloaddition (R. Roux et al., J. ACS
Macro Lett. 2012, 1 (8), 1074-1078), or the works by K.
Landfester (Siebert et al. Chem. Commun. 2012, 48, 5470-
5472). L. Shi et al. (J. Appl. Polym. Sci. 2016, 133 (36),
168-7) and D. Patton et al. (ACS Appl. Mater. Interfaces
2017, 9 (4), 3288-3293) who also prepared microcapsules by
thiol-ene chemistry initiated by respectively a base and a
photoinitiator.
The disadvantage of interfacial polymerization is the
possibility of side reactions between the amine and a
carbonyl group. Thus, depending on the monomers employed and
the reaction conditions chosen, interfacial polymerization
can lead to mixtures of polymers which are quite complex.
It is known that radical polymerization can lead to polymers
of good purity, or with fewer side reactions, than other
polymerization techniques. Patent application US 2015/0017214
A1 (Tagasago) describes a process for manufacturing
microcapsules by radical polymerization in which two or three
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types of vinyl monomers are introduced into the hydrophobic
(core) phase of an O/W type emulsion (Oil in Water). A
radical initiator is also added to carry out the
polymerization in the bulk of the emulsion. This process uses
at least three types of monomers: a hydrophilic monomer (for
example methacrylic acid) to bring the growing polymer
towards the interface, a hydrophobic monomer to vary the
mechanical properties of the wall of the microcapsules, and a
compound of the type di or tri (meth) acrylate to obtain a
crosslinked material.
In general, a relatively broad spectrum of polymeric
materials is therefore proposed to a person skilled in the
art to select the suitable type of microcapsule for a given
use. Thus, microcapsules are already used in numerous
technical applications, but the application potential thereof
has not yet been fully recognized, and it is a strongly
emerging sector destined to grow significantly once the
microcapsule wall meets increasingly stringent criteria in
terms of toxicity and recyclability.
However, microcapsules represent microparticles of polymeric
materials. For some years, polymeric material microparticles
have been identified as an area of environmental concern, due
to the wide dissemination thereof in ecosystems, in soils, in
aquatic and maritime ecosystems, reaching distant locations
from the place where they were introduced into the ecosystem.
This wide dissemination harms not only as a general rule the
organisms present in these ecosystems, but could also have
harmful effects for human health. Increasingly stringent
regulations are already being announced which restrict the
use of plastics capable of forming microparticles during the
degradation thereof in-situ in a natural environment, and
especially of plastics used directly in the form of
microparticles.
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For environmental reasons, it may seem contradictory to seek
to develop a novel product consisting of polymeric
microparticles. It has hence emerged as desirable to have
microcapsules made of degradable polymeric material. It is
noted that microcapsules, used in numerous special
applications and capable of being incorporated in numerous
products in common use (such as textile materials, cosmetic
or phytosanitary products) or technical use (such as paints,
varnishes, inks), will not normally undergo end-of-life
collection, and therefore cannot undergo biodegradation by
composting, as can be envisaged for collected plastic
products. Thus, the degradability of the plastics forming the
wall of microcapsules cannot be based on chemical mechanisms
which take place during composting. In this context, the
question as to whether the degradability of the microcapsules
involves a biological mechanism is somewhat unimportant; what
is important is the degradability thereof in an ecosystem,
regardless of the chemical mechanism of this degradation. For
example, a fermentation would be a biodegradation, while a
simple degradation in an ecosystem under the effect of light
could be a photochemical reaction independent of the
ecosystem; in reality, the situation will often be a
combination, especially if the degradation takes place in
stages. We use the expression "(bio)degradable" hereinafter
to denote the characteristic of a material of degrading in a
natural environment on a relatively brief scale (of the order
of weeks or a year), according to the characteristics of this
natural environment and the exposure of the material to the
various agents present in this natural environment.
It is observed that all the microcapsules previously
developed result in the preparation of polymer chains
(polyamide, polyurea, polyurethane, etc.) which will be
either physically interlocked in the case of a reaction
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between bifunctional compounds, or crosslinked in the case of
one or more multifunctional compounds (functionality ≥ 3). In
any case, the walls are not (bio)degradable due to the nature
of the polymer chain.
The problem addressed by the present invention is that of
providing a novel type of microcapsules, which is easy to
synthesize, without making use of toxic and/or costly raw
materials, is (bio)degradable in the natural environment, can
be used with a large number of active ingredients, leads to
polymers which are rather pure, and provides good external
protection for the active ingredient that it is intended to
contain.

WE CLAIM:
1. Process for manufacturing microcapsules comprising a wall
made of polymer material and containing a so-called active
substance, comprising the following steps:
(a) Preparation of an oily phase comprising:
- a poly(beta-aminoester) prepolymer,
- an active substance, possibly in organic solution,
constituting the phase to be encapsulated,
- optionally one or two vinyl monomers X1 and/or X2,
- optionally a polymerization initiator;
(b) Preparation of an aqueous phase comprising at least one
surfactant; and optionally a polymerization initiator;
(c) Preparation of an emulsion of the O/W type (Oil in Water)
by adding said oily phase to said aqueous phase;
(d) Initiation of radical polymerization within said
emulsion;
(e) Continuation of the polymerization, preferably at a
temperature between approximately 20° C and 100° C, to form
microcapsules containing the said phase to be encapsulated;
(f) Optionally, collection, washing and/or drying of the
microcapsules;
knowing that the order of steps (a) and (b) can be reversed,
and that said polymerization initiator, the presence of which
is necessary, can be in said oily phase and/or in said
aqueous phase.
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2. Process according to claim 1, characterized in that the
said polymeric material of the wall is a polymer of the
crosslinked poly(beta-aminoester) type.
3. Process according to any one of claims 1 to 2,
characterized in that:
- the said monomer X1 is selected from the group formed by
methyl methacrylate, isobornyl methacrylate, butyl
methacrylate, linear alkyl acrylates; and/or
- said monomer X2 is selected from monomers of the
polymerizable surfactant or reactive surfactant type, which
may be neutral, anionic, cationic or zwitterionic.
4. Process according to any one of claims 1 to 3,
characterized in that the said prepolymer of a poly(betaaminoester) is obtained by the reaction between an amine and
a multiacrylate.
5. Process according to claim 4, characterized in that the
said multiacrylate is selected from the group formed by:
- diacrylates;
- triacrylates, in particular trimethylolpropane triacrylate,
tetraacrylates, pentaacrylates, hexaacrylates, mixtures
between these various acrylates of the O[CH2C(CH2OR)3]2 type
where R is H or COCH=CH2.
6. Process according to any one of Claims 4 to 5,
characterized in that the said amine is selected from
functional primary amines and/or functional secondary amines,
and more particularly from the group formed by:
- primary amines R-NH2;
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- primary diamines of the NH2(CH2)nNH2 type where n is an
integer which may be between 1 and 20, and which is
preferably 2 or 6;
- primary diamines having an aromatic core, and preferably
meta-xylylene diamine;
- primary (multi)amines, and preferably tris(2-
aminoethyl)amine;
- (multi)amines containing primary and secondary amine
functions, and preferably tetraethylene pentamine;
- secondary diamines and preferably piperazine;
- polymers containing primary and or secondary amine
functions, and preferably polyethylene imine.
7. Process according to any one of claims 1 to 6,
characterized in that the said surfactant is selected from
the group formed by macromolecular surfactants, preferably in
that the said surfactant is selected from the group formed by
polyacrylates, methylcelluloses, carboxymethylcelluloses,
polyvinyl alcohol optionally partially esterified or
etherified, polyacrylamide, synthetic polymers having
anhydride or carboxylic acid functions, ethylene/maleic
anhydride copolymers, and in that said surfactant is even
more preferably polyvinyl alcohol.
8. Process according to any one of claims 1 to 7,
characterized in that the said active substance is selected
from the group formed by:
- essential oils, fragrances,
- inks, paints, thermochromic and/or photochromic substances,
dyes, glues,
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- products with a biocidal effect, products with a fungicidal
effect, products with an antiviral effect, products with a
phytosanitary effect, products with a cosmetic effect, active
pharmaceutical ingredients,
- natural and edible oils, vegetable and edible oils, liquid
alkanes, esters and fatty acids.
9. Process according to any one of claims 1 to 8,
characterized in that the wall of the microcapsules is
modified either by a layer of polymer deposited on the
surface of the microcapsule, or by the addition of a radical
initiator in the aqueous phase and/or the oily phase, or by
adding to the aqueous phase a water-soluble acrylate capable
of modifying the surface state of the microcapsules.
10. Microcapsules obtainable by the process according to any
one of claims 1 to 9.
11. Microcapsule according to any one of claims 10,
characterized in that the said microcapsule and/or its wall
shows a biodegradation of at least 60%, preferably of at
least 70%, and even more preferentially of at least 80%,
measured by a manometric respirometry test according to
method 301 F of the “OECD Guidelines for the Testing of
Chemical Products: Ready Biodegradability” after a ten-day
incubation.
12. Microcapsule according to any one of claims 10 to 11,
characterized in that the said microcapsule and/or its wall
shows a biodegradation of at least 85%, preferably of at
least 90%, and even more preferentially of at least 95%,
measured by a manometric respirometry test according to
method 301 F of the “OECD Guidelines for the Testing of
Chemicals: Ready Biodegradability” after incubation for 28
days.
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13. Microcapsule according to any one of claims 10 to 12,
characterized in that its wall has been modified either by a
layer of polymer deposited on the surface of the
microcapsule, either by adding a radical initiator to the
aqueous phase and/or the oily phase, or by adding a watersoluble acrylate capable of modifying the surface state of
the microcapsules to the aqueous phase.