AU727915B2 - Encapsulation - Google Patents

Description

-1-

ENCAPSULATION

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 0 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 AMEN~DED SHEET WO 98/26865 PCT/GB97/03346 -2been made to solve it 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 WO 98/26865 PCT/GB97/03346 -3resin. 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 -4catalyst/initiator being microcapsule comprising a membrane of a water based encapsulent material and a encapsulated 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 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 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 15 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 20 impregnate a fibrous carrier layer.

"'oo o* WO 98/26865 PCT/GB97/03346 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 HO z 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 0 C and is preferably heated between 60 and The present invention will be better understood by way of the following examples which describe the encapsulation of strong acids -7in 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 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. It would seem that is because the water present in and around each capsule RA4I 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 70C 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, 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 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 may different forms. The encapsulation material may take many different forms.

WO 98/26865 PCT/GB97/03346 -9- 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:- Gelatin 99.9 49.9 Gellan Gum 0.1 50.1 Gum Arabic 99.9 49.9 Xanthan Gum 0.1 50.1 Gum Arabic 99.9 49.9 Gellan Gum 0.1 50.1 Gellatin 99.9 49.9 Tragacanth 0.1 50.1 Gum Arabic 99.9 49.9 Tragacanth 0.1 50.1 WO 98/26865 PCT/GB97/03346 Gelatin 99.9 49.9 Gum Arabic 0.1 50.1 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 following may be employed as alternatives:- Hydroxyethyl Cellulose Carboxymethyl Cellulose Gum Arabic WO 98/26865 PCT/GB97/03346 -11- Tragacanth Gellan Gum Sulphuric Acid Monoesters of Gum Tragacanth Polyglycol Alginates Low Methoxyl Pectins In the above, the following terms have the following meanings:- Gelatin and Hydrolysed Gelatin Protein derived from the hydrolysis of animal collagen (including fish) or any short chain combination of constituent amino acids.

Carrageenan and Agar Products extracted from red seaweed having galatose backbone joined together by alternating glycosidic linkages.

Guar. Locust Bean. Tara. Cassia. Mesquite and Fenugreek gum Galactomannans a family of linear polysaccharides based on a backbone of 13(1-4)-linked D-mannose residues.

Gum Arabic, Gum Tragacanth and Gum Karya WO 98/26865 PCT/GB97/03346 -12- 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.

Xanthan and Gellan Gum Biosynthetic Polysaccharide Gums produced by fermentation using specific bacteria.

Alginates and Alainate 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.

Methyl Cellulose, Methyl HydroxpropylI 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.

Low Methoxyl Pectins and High Methoxyl Pectins Pectins are the partial breakdown products of complex -13structures 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 0 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.

Claims (10)

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

4. A colloidal aggregate as claimed in claim encapsulent comprises a mixture of gelatin and A colloidal aggregate as claimed in claim encapsulent comprises a mixture of gum arabic

6. A colloidal aggregate as claimed in claim encapsulent comprises a mixture of gelatin and

7. A colloidal aggregate as claimed in claim encapsulent comprises a mixture of gum arabic

8. A colloidal aggregate as claimed in claim encapsulent comprises a mixture of gelatin and

9. A colloidal aggregate as claimed in claim xanthan gum. 1 or 2, in which the gellan gum. 1 or 2, in which the and xanthan gum. 1 or 2, in which the tragacanth. 1 or 2, in which the and tragacanth. 1 or 2, in which the gum arabic 1 or 2, in which the WO 98/26865 PCT/GB97/03346 encapsulent comprises a mixture of gelatin and polyglycol alginates. 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, Iota 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, polyglycol 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 -16- catalyst/initiator being microcapsule comprising a membrane of a water based encapsulent material and a encapsulated 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 initiating/catalysing the resin curing. 16. A fibrous carrier layer impregnated with a resin system as claimed in claim 17. A fibrous carrier layer as claimed in claim 16 which is shaped while uncured. 18. A fibrous carrier layer as claimed in claim 16 or 17 which has a fibrous sheet structure including a mat, a web or randomly orientated fibres. 19. A fibrous carrier layer as claimed in claim 18, in which the fibres are of glass. A method of producing micro-capsules of claim 1 comprising the 15 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 WO 98/26865 PCT/GB97/03346 -17- the 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 A method as claimed in claim 24 in which the capsules are heated at a temperature between 60o and