Note: Descriptions are shown in the official language in which they were submitted.
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~NCAP~,ILATION
The present invention relates to microcapsules of acid and to
a method of their production. The invention is particularly, but not
exclusively concerned with microcapsules of catalysts and/or
initiators used in the curing of synthetic resins.
In the majority of uses, the processing and curing of resins is
carried out under controlled conditions. However, circumstances
occur when it is necessary to control/prevent the curing of a resin
before an appropriate time in order to attain the best utilisation of the
resin.
The majority of thermoset resin applications use heat to cure
the resin, the application of heat causing a catalyst and7or p~oimoter
in the resin to accelerate the cross-linking of resin molecules.
1 ~ However; even if heat is not applied, the resin will cure over a period
of days, thereby restricting the time during which uncured resin can
be stored. Therefore, from the above it can be seen that as soon as
the resin is mixed with its catalyst and is used for the specific
application there is a race against time to get the resin/catalyst
system in place.
The industry is aware of this problem and some attempts have
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been made to solve 'rt by developing special resins which are curable
by light radiation.
Ultra small microcapsules of the order of less than 100 micron
diameter, are formed which are of appropriate size to be used in
thermoset applications when such microcapsules are dispersed
throughout the resin. When the microcapsules are so small they can
be uniformly dispersed and will not settle out in the resin.
One particular class of resin is that of phenolic resins which are
the products of reactions between phenols (commonly phenol) and
aldehydes (typically formaldehyde). One of the main advantages of
using phenolic resin systems for the thermoset resin industry is that
the resin initially comprises a low molecular weight fusible solid resin
that may be easily handled and subsequently, upon curing, forms a
high molecular weight, strong, heat resistant material.
Phenolic resins are usually initiated/catalysed by the addition
of strong acids such as sulphuric acid, hydrochloric acid, phosphoric
acid, Toluenesulphonic acid and the like, all of which are extremely
corrosive and difficult to handle. The corrosive nature of the acids is
one of the main factors which have made them exceptionally difficult
to encapsulate and without some form of encapsulation or controlled
release it is impossible to control the release of the acid into the
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resin. Another class of resins is urea formaldehyde is also of interest.
An object of this invention is to provide capsules of strong
acids which are readily dispersable in resin material without acid
leakage.
Furthermore, the present invention also seeks to provide
curable resin systems for the production of rigid articles wherein the
resin can be cured readily and quickly, but retains a long shelf life
making it particularly suitable for use in the thermoset resin industry.
According to one aspect of the present invention there is
provided a microcapsule suitable for use in the curing of resins
comprising a membrane of a water based encapsulation material and
an encapsulated phase comprising one or more strong acids wherein
the membrane may be ruptured by an external energy source to
release the acid.
Preferably the encapsulent comprises a water based acid
resistant gelatin.
The strong acid may comprise sulphuric acid, hydrochloric
acid, phosphoric acid and toluene sulphonic acid or a mixture thereof.
According to another aspect of the invention there is provided
a resin system for the production of rigid articles wherein said resin .
systems comprises resin and a dispersed catalyst/initiator said
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catalystlinitiator being microcapsute comprising a membrane of a
water based encapsulent material and a dispersed phase comprising
one or more strong acids wherein the membrane may be ruptured by
an external energy source to release the acid into the resin and
thereby initiatinglcatalysing the resin curing.
One advantage of this type of encapsulation over the prior art
is the fact that previously the encapsulated reactant was produced
in the form of dry capsules which were often poorly dispersed when
mixed in with resin, whereas the materials of the present invention
are readily dispersed ensuring intimate dispersion in the resin and
significantly reducing the likelihood of the presence of large
conglomerates of capsules.
The invention has a general application in that a resin
impregnated absorbent material with the dispersed colloidal capsules
therein can be shaped to a final form and the colloidal capsules
ruptured, for example by heating or electromagnetic energy, to
release the catalyst or promotes, to cause curing of the resin and
absorbent material in said final form.
In one embodiment of the present invention the encapsulated
ZO catalyst is admixed with the curable resinous material which is then
used to impregnate a fibrous carrier layer.
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The fibrous layer can be shaped to any desired shape, such as
a boat or moulded part for a vehicle whilst the resin is uncured, and
then the resin can be cured for example by the application heat or
electromagnetic energy to rupture the colloidal capsules.
The resinlcolloidal capsule carrier material preferably has a
fibrous sheet structure including a mat, a web or randomly orientated
fibres and the fibres may be of glass and/or of natural and synthetic
origin.
According to a still further aspect of the present invention
there is also provided a method for the microencapsulation of one or
more strong acids suitable for use as catalysts, prorr~oters or initiators
in resin curing reactions, comprising the following steps:-
mixing the acid(s), a water based encapsulent and water,
sonicating the mixture so formed to encapsulate the acids) in
IS the encapsulent, and
substantially removing surplus water from the mixture to form
a microcapsule of the acid in the encapsulent.
By way of a specific example when acid resistant gelatin is
used application of ultrasonic energy or gelatin in the sonicating step
causes free-radical induced cross-linking of protein molecules. Highly
reactive radicals in particular H~ and OH~ are ultrasonically produced
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from water and react with molecular oxygen present in the
solution/air to form a superoxide HOZ,). Superoxide reacts with
protein molecules to produce disulphuric bonding usually between
cysteine residues. Ultrasound can produce a high concentration of
proteinaceous colloidal capsules with narrow size distributions,
generally in region of 9-20 microns.
A typical ultrasonic reactor apparatus for use in the
preparations of this invention comprises a collimated 20 Khz beam
from a lead zirctinate ~ titanate transducer and the reaction may be
carried out in a glass sonication cell in an inert atmosphere,
preferably in an atmosphere of argon. An amplifying horn; preferably
made of titanium may be fixed adjacent to .the transducer, for.
amplification of the signal thereof_ Because of temperature rises due
to 'the reaction processes 'the sonication cell is preferably retained in
a water bath. Audible sound may also be used.
Preferably in the water removal step, the aqueous mixture of
the capsules formed in the sonification step are heated .for. a period
of about 24 hours at a temperature below the rupture temperature of
around 80°C and is preferably heated between 60 and 70°C.
The present invention will be better understood by way of the
following examples which describe the encapsulation of strong acids
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in water based encapsulents for use as catalystslinitiators for
polymerisation reactions.
Example 1
The curing of phenolic resin is generally initiatedlcatalysed by
the addition of strong acids and the reaction proceeds at ambient
temperature.
In this example the curing agent which was encapsulated was
a propriety blend of phosphoric and toluene sulphonic acids.
One part by weight of the acid mixture was mixed with 3 parts w
by weight of food grade gelatin (200 Bloom) dispersed in 3 parts of
water.
This mixture was sonicated for a period of three minutes
(power 100V11) using a 20 Khz probe: The high intensity ultrasound, ~,
produced intermediate proteinaceous capsules in a substantially-
i 5 aqueous 'medium and were of a sizes' less than 100 advantageously
microns wherein capsules are dispersed in water and comprise an
outer layer of gelatin encapsulating a strong acid core.
It was found however that because acid mixture contained
strong acids, a gel/water boundary developed around the acid
catalyst leading to diffusion of the acid through the shell. !t would
seem that is because the water present in and around each capsule
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shell acts as a carrier for the acid.
tt was subsequently found that an improved encapsulated
product was prepared by eliminating water from the aqueous mixture
to prevent the acids traversing the cell wall. The sonicated mixture
was therefore heated in an oven at 70'C until the three parts water
had been removed to form the end product material.
Example 2
In a further experiment 10% by weight of acid mixture was
mixed with 10% of acid resistant gelatin (Gelatex, Che~n. Colloids
Ltd) and 10% water and the mixture was sonicated for 3 minutes
using 20KHz probe (70 Watts) producing proteinaceous colloidal
capsules with particle sizes of less than 100 microns advantageously
between 1 and 20 microns.
The gelatin/water/acid colloid was heated in an oven at 60'C
for 24 hours to remove the water from the gelatin/water boundary
leaving a gelatin capsule.
It has been found that gelatin capsules of the present invention
of acid mixture have a shelf life of at least 3 months and that upon
heating samples thereof up to 90'C in the presence of phenolic resins
a cure is obtained within 5 minutes.
The encapsulation material may take many different forms.
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For example, a material comprising a combination of gelatin
with xanthan gum has been found to be particularly advantageous
combining longtime resistance to acid attack with ready
encapsulation of acid and rupturing of the encapsulation on
application of an external energy source such as ultrasound.
Preferred formulations of the above ingredients lie in
the
following range:-
Xanthan Gum 0.1 - 50.1
Gelatin 99.9 - 49.9
The material is produced by the adhesion, agglomeration
and
enrobing of gelatin with xanthan gum. Apart from the above
ingredient combinations, the following ingredients in the
following
ingredient combinations may also be used:-
(a) Gelatin 99.9 - 49.9
Gellan Gum 0.1 - 50.1
(b) Gum Arabic 99.9 - 49.9
Xanthan Gum 0.1 - 50.1
(c) Gum Arabic 99.9 - 49.9
Gellan Gum 0.1 - 50.1
(d/ Gellatin 99.9 - 49.9
Tragacanth 0.1 - 50.1
(e) Gum Arabic 99.9 - 49.9
Tragacanth 0.1 - 50.1
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(f) Gelatin 99.9 - 49.9
Gum Arabic 0.1 - 50.1
(g) Gelatin 99.9 - 49.9
Polyglycol Alginates 0.1 - 50.1
A blend of all the above combinations may also be employed.
In any combination comprising gelatin any of the following may
be employed as alternatives:
- Hydrolysed Gelatin
- Alginates
Carrageenan (Kappa, Iota and Lambda)
- Locust Bean Gum
- Gum Arabic
- High Methoxyl Pectins
- Gellan Gum
- Methyl Cellulose and Methyl Hydroxpropyl Cellulose
or any mixture/modifications of the above
- Agar
In any combination comprising xanthan gum any of the
foNowing may be employed as alternatives:-
- Hydroxyethyl Cellulose '
- Carboxymethyl Cellulose
- Gum Arabic
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- Tragacanth
- Gelian Gum
- Sulphuric Acid Monoesters of Gum Tragacanth
- Polyglycol Alginates
- Low Methoxyl Pectins
In the above, the following terms have the following
meanings:-
(a) Gelatin and Hl~rdrolysed Gelatin
Protein derived from the hydrolysis of animal collagen
i0 (including ash) or any short chain combination of
constituent amino acids.
(b) Carrageenan and Agar
Products extracted from red seaweed having galatose
backbone joined together by alternating glycosidic
15 linkages.
(c) Guar. Locust Bean Tara Cassia Mesauite and
Fenugreek gum
Galactomannans - a family of linear polysaccharides
based on a backbone of ~i(1-4)-linked D-mannose
20 residues.
(d) Gum Arabic Gum Tragiacanth and Gum Ka~yra
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Exudate Gums - yielded from species of trees and
shrubs, which first appears as a liquid and is dried in the
sun and air to form a hard glassy lump.
(e) Xanthan and Gellan Gum
Biosynthetic Polysaccharide Gums - produced by
fermentation using specific bacteria.
(f) Alginates and Alginate Esters
Salts of Alginic Acid - with a degree of polymerisation
usually in the range of 700 - 3000 corresponding to
molecular weights of approximately 20,000 - 600,000.
Propylene Glycol Alginate (PGA) is an ester of alginic
acid and propylene glycol.
(g) Methyrl Cellulose. Methyl Hydroxl~ro~yl Cellulose
Hvdroxvethyrl Cellulos and Carboxyrmet yl Cellulose
Cellulose Ethers - prepared by reacting cellulose with
caustic to form 'alkali cellulose' which in turn is
alkylated or alkoxylated in the presence or absence of
inert diiuent or by Williamson Etherification reaction.
(h) Low Methoxyrl Pectins and High Methoxyl Pectins
Pectins - are the partial breakdown products of complex
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structures in plant cell wall (normally fruits). These can .
be modified by de-esterification to produce high
methoxyl and low methoxy! pectins.
The capsules of acid may be mixed with the resin to be cured
by any suitable means. The resins may be phenolic or urea
formaldehyde. The very small size of the capsules facilitates dispersal
in the resin. The mixing process is low energy, low shear to avoid
premature rupturing of the capsules. A suitable form of mixer may
be a paddle mixer. The shelf life of a product comprising resin to be
cured and encapsulated acid may be tailored to a customer's
requirements_ Shelf lives of at least three months and possibly of
longer than one year may be achieved. -
The capsules dispersed in the resin may be ruptured ~to release
the acid to initiate curing by any suitable means: Appropriate means
1:. include heating, sound, ultrasound; radio waves, microwaves or
pressure_ The latter may be particularly advantageous in the
production of products which inherently involve the application of
pressure such as for example laminated products.
