WO1998026865A1

WO1998026865A1 - Encapsulation

Info

Publication number: WO1998026865A1
Application number: PCT/GB1997/003346
Authority: WO
Inventors: Dennis William Quinn; Alexander Martin Thompstone
Original assignee: Chemcolloids Limited
Priority date: 1996-12-17
Filing date: 1997-12-16
Publication date: 1998-06-25
Also published as: CN1248928A; AU727915B2; GB9626152D0; NO992932D0; CA2275377A1; AU7736698A; NO992932L; JP2001507051A; EP0948397A1

Abstract

A colloidal aggregate suitable for use in the curing of resins comprises a continuous phase of a water based encapsulent material and a dispersed phase comprising one or more strong acids. The continuous phase advantageously comprises a mixture of gelatin and xanthan gum. The aggregate is sonicated to encapsulate the acid in the encapsulent material. The encapsulated acid is mixed with a curable resin. The acid capsules may be ruptured by any suitable means, but particularly ultrasound, to release the acid to initiate curing of the resin.

Description

ENCAPSULATION

The present invention relates to microcapsuies of acid and to

apparatus for and method of their production. The invention is

particularly, but not exclusively concerned with microcapsuies 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 and/or promoter

in the resin to accelerate the cross-linking of resin molecules.

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 been made to solve it by developing special resins which are curable

by light radiation.

Ultra small microcapsuies of the order of less than 100 micron

diameter, are formed which are of appropriate size to be used in

thermoset applications when such microcapsuies are dispersed

throughout the resin. When the microcapsuies 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 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 colloidal aggregate suitable for use in the curing of resins

comprising a continuous phase of a water based encapsulent material

and a dispersed phase comprising one or more strong acids wherein

the continuous phase 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

system comprises resin and a dispersed catalyst/initiator said catalyst/initiator being colloidal aggregate comprising a continuous

phase of a water based encapsulent material and a dispersed phase

comprising one or more strong acids wherein the continuous phase

may be ruptured by an external energy source to release the acid into

the resin and thereby initiating/catalysing 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 colloidal aggregates 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

catalyst is admixed with the curable resinous material which is then

used to impregnate a fibrous carrier layer. 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 resin/colloidal 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, promoters 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 acid(s) in

the encapsulent, and

substantially removing surplus water from the mixture to form

a colloidal aggregate 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 from water and react with molecular oxygen present in the

solution/air to form a superoxide HO₂.). 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 1-20 microns.

A typical ultrasonic reactor apparatus for use in the

preparations of this invention comprises a collimated 20 Khz beam

from a lead zirconate 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° and is preferably heated between 60 and 70°.

The present invention will be better understood by way of the

following examples which describe the encapsulation of strong acids in water based encapsulents for use as catalysts/initiators for

polymerisation reactions.

Example 1

The curing of phenolic resin is generally initiated/catalysed 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

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 100W) using a 20 Khz probe. The high intensity ultrasound

produced intermediate proteinaceous capsules in a substantially

aqueous medium and were of a sizes less than 100 advantageously

microns wherein capsules 2 are dispersed in water and comprise an

outer layer of gelatin 4 encapsulating a strong acid core 6.

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. It would

seem that is because the water present in and around each capsule shell acts as a carrier for the acid.

It 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° until the three parts water

had been removed to form the end product colloidal aggregate.

Example 2

In a further experiment 10% by weight of acid mixture was

mixed with 10% of acid resistant gelatin (Gelatex, Chem. 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° 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. 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 (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:-

- Hydroiysed Gelatin

Alginates

Carrageenan (Kappa, lota 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

following may be employed as alternatives: -

Hydroxyethyl Cellulose

Carboxymethyl Cellulose

Gum Arabic Tragacanth

Gellan 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 Hydrolysed Gelatin

Protein derived from the hydrolysis of animal collagen

(including fish) 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

linkages.

(c) Guar. Locust Bean. Tara. Cassia. Mesquite and

Fenugreek gum

Galactomannans - a family of linear polysaccharides

based on a backbone of β(1-4)-linked D-mannose

residues.

(d) Gum Arabic. Gum Tragacanth and Gum Karya 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 100 - 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) Methyl Cellulose. Methyl Hydroxpropyl Cellulose.

Hydroxyethyl Cellulose and Carboxymethyl 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 diluent or by Williamson Etherification reaction.

(h) Low Methoxyl Pectins and High Methoxyl Pectins

Pectins - are the partial breakdown products of complex structures in plant cell wall (normally fruits). These can

be modified by de-esterification to produce high

methoxyl and low methoxyl 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

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.

It will be appreciated that the above embodiments have been

described by way of example only and that many variations are

possible without departing from the scope of the invention.

Claims

1 . A colloidal aggregate suitable for use in the curing of resins

comprising a continuous phase of a water based encapsulation

material and a dispersed phase comprising one or more strong acids

wherein the continuous phase may be ruptured by an external energy source to release the acid.

2. A colloidal aggregate as claimed in claim 1 , in which the

encapsulent comprises a water based acid resistant gelatin.

3. A colloidal aggregate as claimed in claim 1 or 2, in which the

encapsulent comprises a mixture of gelatin and xanthan gum.

4. A colloidal aggregate as claimed in claim 1 or 2, in which the

encapsulent comprises a mixture of gelatin and gellan gum.

5. A colloidal aggregate as claimed in claim 1 or 2, in which the

encapsulent comprises a mixture of gum arabic and xanthan gum.

6. A colloidal aggregate as claimed in claim 1 or 2, in which the

encapsulent comprises a mixture of gelatin and tragacanth.

7. A colloidal aggregate as claimed in claim 1 or 2, in which the

encapsulent comprises a mixture of gum arabic and tragacanth.

8. A colloidal aggregate as claimed in claim 1 or 2, in which the

encapsulent comprises a mixture of gelatin and gum arabic

9. A colloidal aggregate as claimed in claim 1 or 2, in which the encapsulent comprises a mixture of gelatin and polygiycol alginates.

10. A colloidal aggregate as claimed in claim 1 or 2, in which the

encapsulent comprises a mixture of gum arabic and gellan gum.

11. A colloidal aggregate as claimed in any of claims 3 to 10 in

which the ratio of the two ingredients to each other lie in the ranges

99.9 to 49.9 and 0.1 to 50.1.

12. A colloidal aggregate as claimed in claim 1 , in which the

encapsulent comprises hydrolysed gelatin, alginates, carrageenan

(Kappa, lota and Lambda), locust bean gum, gum arabic, high

methoxyl pectins, gellan gum, methyl cellulose and methyl

hydroxpropyl cellulose or any combination thereof or agar.

13. A colloidal aggregate as claimed in claim 1, in which the

encapsulent comprises xanthan gum combined with one of the

following hydroxyethyl cellulose, carboxymethyl cellulose, gum

arabic, tragacanth, gellan gum, sulphuric acid monoesters of gum

tragacanth, polygiycol alginates, low methoxyl pectins.

14. A colloidal aggregate as claimed in claim 1 or 2, in which the

strong acid comprises sulphuric acid, or hydrochloric acid, or

phosphoric acid, or toluene sulphonic acid or a mixture thereof.

15. A colloidal aggregate in accordance with Example 1 or Example

2.

16. A resin system for the production of rigid articles wherein said

resin systems comprises resin and a dispersed catalyst/initiator said

catalyst/initiator being colloidal aggregate comprising a continuous

phase of a water based encapsulent material and a dispersed phase

comprising one or more strong acids wherein the continuous phase

may be ruptured by an external energy source to release the acid into

the resin and thereby initiating/catalysing the resin curing.

17. A fibrous carrier layer impregnated with a resin system as

claimed in claim 16.

18. A fibrous carrier layer as claimed in claim 17 which is shaped

while uncured.

19. A fibrous carrier layer as claimed in claim 17 or 18 which has

a fibrous sheet structure including a mat, a web or randomly

orientated fibres.

20. A fibrous carrier layer as claimed in claim 19, in which the

fibres are of glass.

21. A method for the microencapsulation of one or more strong

acids suitable for use as catalysts, promoters 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 acid(s) in th e encapsulent, and

substantially removing surplus water from the mixture to form

a colloidal aggregate of the acid in the encapsulent.

22. A method as claimed in claim 21 in which the mixture is

sonicated with ultrasound.

23. A method as claimed in claim 22, in which the mixture is

sonicated with audible sound.

24. A method as claimed in claims 21 , 22 or 23, in which the

capsules formed in the sonification step are heated for a period of

about twenty four hours at a temperature below 80°C.

25. A method as claimed in claim 24 in which the capsules are

heated at a temperature between 60° and 70°C.