GB2237574A - Method of encapsulating polymer additives - Google Patents

Abstract

Polymers having improved lifetimes and more uniform properties are achievable using the technique of micro-encapsulation to control the release of essential polymer additives such as antioxidants. The micro-capsules are prepared by spraying an emulsion of the polymer additive and aqueous sodium alginate into a cross-linking bath containing an aqueous solution of a di- or multi-valent metal salt. Instantaneous reaction between the emulsion and the metal causes the alginate to cross-link and form a "sponge"-like matrix containing the polymer additive in its interstices. Alternatively, the aqueous solution of sodium alginate can be sprayed into a cross-linking bath containing the dissolved polymer stabiliser. This technique enables the available range of useful additives to be extended to molecules which were hitherto regarded as too volatile, with the result that they were lost by diffusion long before the host polymer had completed its expected lifespan. The method also gives better control over the performance of conventional additives.

Description

METHOD OF ENCAPSULATING POLYMER ADDITIVES This invention relates to the field of polymer additives and especially to a method of controlling the release of additives such as stabilisers in a host polymer. One class of stabilisers of particular interest in this respect is that of antioxidants.

The susceptibility of polymers to undergo oxidation both in use and during processing leads to a deterioration in their physical properties which, in extreme cases, may even manifest itself as severe decay or perishing. This is clearly undesirable and to prevent, or at least retard, unwanted oxidation manufacturers will add a small amount of a chemical which acts as an antioxidant, either by reacting with the species which initiate oxidation or by reacting with the propagating radicals.

In order for the antioxidant to inhibit oxidation effectively it must react quickly with the initiating species or radicals. Also, the resulting products should be sufficiently unreactive that they are consumed in onward reactions which do not significantly degrade the polymer. Any free radical inhibitor has the potential to behave as an antioxidant by scavenging free radicals participating in oxidation chains, but a good antioxidant must also satisfy another important requirement which is that it must not readily diffuse out of the host polymer.

Many chemical compounds are known to have a high affinity for oxygen or its reaction products and would be effective as polymer antioxidants except for the fact that they are too volatile to remain in the host polymer for a useful length of time and are lost by diffusion.

Traditionally, this problem of diffusion loss has been tackled in one of two ways: 1. In the most common approach, structures are used in which the active antioxidant group is incorporated in a substrate molecule.

The substrate molecule is chosen to be sufficiently large that diffusion is discouraged and the volatility which leads to loss of the antioxidant is curbed. The combined substrate/antioxidant structure is then blended with the appropriate polymer as an additive.

This approach considerably restricts the choice of antioxidants by excluding those smaller molecules which have excellent antioxidant properties but which are too mobile. Conversely, if diffusion is restricted too severely because of the bulk of the combined substrate/ antioxidant structure the molecule may be prevented from reaching the sites of oxidation initiation. This technique therefore requires a fine balance to be struck between the mobility and inertia of the substrate/antioxidant structure.

2. More recently, attempts have been made to attach the antioxidant group to the polymer itself. One major disadvantage of this method is that it can result in critical changes in the physical properties of the polymer,for example by changing its crystallinity. Also, it can lock the antioxidant groups so thoroughly that they are unable to migrate to random oxidation sites.

Furthermore, this method necessitates special synthesis of every polymer since the antioxidant is part of the polymer and not simply an additive. In liquid lubricants, where mobility of the polymer molecules is assured, bespoke polymers are often the materials of choice. In general, however, this method is considered too expensive and unsuitable for most solid polymer production.

It is apparent from the foregoing that the known methods suffer from a number of drawbacks: In the first approach the effectiveness of the antioxidant is a compromise between its mobility and its retention, with little or no control over its release to random oxidation sites. In the second approach, not only is there a risk that the physical properties of the polymer will be modified in a way which is unacceptable, but there is also a very high production cost. In solid polymers this method has the additional drawback that the polymer chains may be too immobile to permit their antioxidant groups to migrate to random oxidation sites.

It is the aim of the invention to overcome the disadvantages of the above methods by enabling a stabiliser (or other polymer additive) to be incorporated in a host polymer in a modified form such that it is usefully disseminated throughout the polymer and such that its rate of loss by diffusion is reduced compared to that of the unmodified additive.

Accordingly, the invention is a method of enclosing a polymer stabiliser in a release-controlling matrix for subsequent use as an additive in polymer preparations, which method comprises spraying an aqueous solution of sodium alginate as a mist of fine droplets into a cross-linking bath comprising an aqueous solution of a di- or multi-valent metal salt in order to consolidate said droplets as discrete matrix particles and washing and drying the matrix particles thus formed, wherein polymer stabiliser is captured in the matrix particles either (a) by dissolving the polymer stabiliser in a solvent and combining the solution of polymer stabiliser with the aqueous solution of sodium alginate and forming a uniform dispersion of polymer stabiliser and sodium alginate prior to the spraying step, or (b) by dissolving the polymer stabiliser in the solution comprising the cross-linking bath prior to the spraying step, such that the cross-linking reaction traps polymer stabiliser in networks of cross-linked alginate molecules during formation of the matrix particles.

For convenience in the text which follows, the matrix particles are referred to as capsules (or micro-capsules) and the process by which polymer stabiliser is enclosed is referred to as encapsulation (or micro-encapsulation). This terminology should not be interpreted as including "balloon"-type capsules having distinct boundary walls; rather, the capsules of the present invention should be regarded as resembling "sponges" having irregular surfaces and possessing a multiplicity of pores.

Additives encapsulated in this way are effectively held in a reservoir which maintains a steady concentration of additive throughout the body of the host polymer by controlled release. By this means, the useful range of additives is broadened to include many of those molecules previously thought to be too volatile, and other species which are too easily extracted by solvent when added directly to the host polymer. In addition, this technique allows the performance of conventional additives to be controlled more precisely because additional constraints are imposed on their escape besides diffusivity and volatility. External factors such as the density of cross-links in the micro-capsules are of increasing significance and thus the polymer designer is able to fine tune the mobility of additives by controlling encapsulation.

Other advantages of this method are that it permits the use of additives with increased mobility, so there is a greater possibility of being able to permeate the entire host polymer structure. Also, the micro-encapsulated additive can be added to any polymer without the requirement that the latter is specially synthesised.

In the case of antioxidants, by restricting or controlling their release the invention increases their range of application in terms of both useful lifetime and temperature suitability. For example, packaging materials used for products with a finite shelf life can have their antioxidant content controlled so that the packaging becomes susceptible to degradation soon after the product life has expired. The invention also extends the range of available antioxidants to include many molecules formerly regarded as too volatile for use within the anticipated life of a polymer.

As indicated above, the capsules are obtained as miniature "sponge"-like matrices, in which the additive is held in the interstices of a labyrinth of cross-linked matrix molecules.

Diffusion control is then achieved by varying the tightness of the sponge matrix binding: If cross-linking is carried out in a cross-linking solution of high concentration the sponges produced are very tightly bound with a high proportion of cross-links so that escape of the additive from the sponge pores is quite difficult. On the other hand, if the cross-links are formed using a more dilute solution of cross-linking agent the resulting sponges have a more open structure and escape of the additive is made easier.

The choice of additive will often dictate the required sponge pore size so that optimum release characteristics can be achieved in a particular end product.

One example of encapsulation of particular interest derives its success from the special properties of alginates. These are naturally-occurring polymeric carbohydrate molecules which are normally obtained as the sodium salt of alginic acid. Sodium alginate (Na*/Alg) is soluble in aqueous media and its structure is given by the formula below:

To effect cross-linking of sodium alginate polymer strands the sodium ions are replaced by di- or multi-valent cations. In aqueous solution this replacement process gives rise to multicellular sponge-like capsules. Thus cross-linking of tiny droplets of sodium alginate solution can be used to generate individual micro-capsules. This cross-linking takes place so quickly that capsules are formed as soon as the sodium alginate droplets come into contact with the solution of di- or multivalent cations.

When this technique is used to encapsulate a polymer additive such as an antioxidant it is important that the additive is in a form which is physically compatible with the sodium alginate. Typical of the antioxidants for which encapsulation is intended are sterically hindered phenols and aromatic amines. These have only a very limited solubility in water and it therefore becomes necessary to prepare a solution of the antioxidant in a volatile organic solvent. This solution is then emulsified into the aqueous sodium alginate prior to cross-linking.

It has also been found that capsules prepared by this method must be thoroughly dried to fix their pore size. If this is not done they become prone to rehydration and swelling in organic solvents.

When subsequently used as additives during polymer production the additive can be easily leached from the capsules by the polymerisation medium. Freeze-drying of freshly prepared capsules is therefore used as a means of combatting this problem.

The invention will now be described with reference to specific examples.

Example 1 1. 1g of 3,5-di-(tert-butyl)-4-hydroxytoluene ("BHT") as antioxidant was dissolved in 5cm3 of toluene and O.lg Manoxol OT emulsion stabiliser was added to the resulting solution. This solution was then emulsified into 100cm3 of a 10% (w/v) solution of CaCl2.6H20 in deionised water.

The size of the emulsion droplets determined the size of the hydrated capsules thus formed. Each capsule consisted of a sponge-like cross-linked alginate matrix in which the antioxidant and toluene were entrained.

The suspension of capsules formed in the cross-linking bath was decanted into a Soxhlet extraction thimble and excess cross-linking ions were removed by placing the thimble in a beaker of deionised water. The water was replaced at regular intervals.

When washing was complete the beaker was emptied and the excess liquid drained away. The capsules were then rinsed with a small quantity of fresh deionised water into a round-bottomed flask. The capsules were frozen by rotation in a cold methanol bath and dried under high vacuum.

The dried capsules were then ready for incorporation into the selected host polymer.

Example 2 The above procedure was repeated except that the antioxidant was initially dissolved in 5cm3 of chloroform in preference to toluene. This was done because it is easier to remove excess chloroform by virtue of its lower boiling point.

Example 2 5cm3 of a 1X (w/v) Na+/Alg solution was sprayed as a fine mist into a cross-linking bath containing 100cm3 of a 10% (w/v) solution of FeCI3. 6H20 in deionised water.

The size of the hydrated capsules thus formed was determined by the size of the sodium alginate droplets. Each capsule was formed as a sponge-like alginate matrix in which a finite quantity of FeC13 was trapped as antioxidant.

The suspension of capsules was decanted into a Soxhlet extraction thimble and excess FeC13 was removed by standing the thimble in deionised water. The water was changed at regular intervals until no yellow colour was observed in the washings.

The capsules were then rinsed into a beaker with fresh deionised water and allowed to settle. Excess water was decanted off and the remainder was removed using a cold vacuum oven by subliming from ice so that the capsules were effectively freeze-dried.

In this example Fe3+ serves a dual purpose because it not only performs as the cross-linking cationic species for the alginate matrix but also acts as the antioxidant species. The exact mechanism of its oxidation behaviour is not fully understood, but it is thought that Fe3' acts by electron transfer with free radicals:

The Fe2+ produced may be converted back to Fe3+ through a variety of reactions, for example by reaction with peroxides present in the system. This process itself may induce side reactions and thus it should not be presumed that Fe3+ acts as a simple 1:1 inhibitor.

The efficacy of the above technique has been demonstrated using thermogravimetric mass spectrometric analyses for the example of BHT encapsulated in alginate cross-linked by Ca2+.

These studies show that, not only is the antioxidant released from the capsules in a controlled fashion, but also that it is retained above its melting point. This means that the BHT is still present as a liquid above those temperatures at which it is normally lost.

Thus, on release from the capsule, its mobility is increased and it can be more effective as a radical trap in high temperature applications. These are circumstances under which use of BHT could not usually be considered owing to its volatility.

Moreover, the encapsulated BHT thermally stabilises the capsules themselves. This means that stabilised capsules can be used to deliver other additives into host polymers under conditions that would formerly have been regarded as hostile.

Further evidence for the good performance of the encapsulated antioxidant has been obtained using the technique of Differential Scanning Calorimetry (DSC). The table below lists the "oxygen induction times" (OITs) observed in a host polymer of polyisoprene and compares the time taken for onset of oxidation in samples containing unencapsulated BHT with those containing BHT encapsulated in accordance with the invention.

Sample Condition OIT/min 1 free BHT 5 t 1 2 free BIT 5 i 1 3 encapsulated BHT 17 t 2 4 encapsulated BHT 14 t 2

In the table above samples 1 and 2 were prepared to contain 2.5% by weight of BHT in unencapsulated form, whilst samples 3 and 4 contained a comparable amount of the antioxidant in capsules prepared according to the method of the invention. All samples were stored under identical conditions which were calculated to undermine the effectiveness of the antioxidant, for the same prescribed period, i.e. at 100 C for 35 minutes in a flow of nitrogen rated at 70cm3.min-1. The samples were then examined isothermally under oxidising conditions (1120C, 25cm3.min1 of oxygen flow) by DSC. This technique reveals both the loss of BHT and the onset of oxidation.The OIT values obtained clearly demonstrate that, even under harsh conditions, the samples containing encapsulated antioxidant are stabilised against oxidation for a period between 2 and 3 times longer than that observed for samples containing the antioxidant in unencapsulated form.

It should also be noted here that some antioxidants become chemically ineffective above certain temperatures because of changes in the predominating mechanism of oxidative degradation.

Encapsulation permits the use of antioxidant "cocktails" to suit the particular application of the host polymer and improve its oxidation resistance under conditions of thermal cycling.

Although the invention has been particularly described with reference to encapsulation of antioxidants it will be readily apparent that encapsulation is a versatile technique which allows many different types of additive, such as adhesives, ablatives etc., to be incorporated in a host polymer. If capsules are formed so that there is virtually no release of the encapsulant it is even possible to use them as flameproofers: If a flameproofing material is held in the impervious capsule it will only be released when the polymer structure starts to break down or melt in a fire.

Claims (5)

1. A method of enclosing a polymer stabiliser in a releasecontrolling matrix for subsequent use as an additive in polymer preparations, which method comprises spraying an aqueous solution of sodium alginate as a mist of fine droplets into a cross-linking bath comprising an aqueous solution of a di- or multi-valent metal salt in order to consolidate said droplets as discrete matrix particles and washing and drying the matrix particles thus formed, wherein polymer stabiliser is captured in the matrix particles either (a) by dissolving the polymer stabiliser in a solvent and combining the solution of polymer stabiliser with the aqueous solution of sodium alginate and forming a uniform dispersion of polymer stabiliser and sodium alginate prior to the spraying step, or (b) by dissolving the polymer stabiliser in the solution comprising the cross-linking bath prior to the spraying step, such that the cross-linking reaction traps polymer stabiliser in networks of cross-linked alginate molecules during formation of the matrix particles.

2. A method of enclosing a polymer stabiliser in a releasecontrolling matrix as claimed in claim 1 in which the matrix particles are dried in a freeze-drying process to fix their pore sizes.

3. A method as claimed in claim 1 or claim 2 in which the polymer stabiliser comprises at least one antioxidant from the group comprising sterically hindered phenols and aromatic amines and in which said at least one antioxidant is dissolved in a volatile organic solvent and emulsified with the aqueous sodium alginate solution prior to the spraying step.

4. A method as claimed in claim 1 or claim 2 in which the polymer stabiliser and the metal species in the cross-linking bath are identical.

5. A method of enclosing a polymer stabiliser as claimed in claim 1 and substantially as hereinbefore described with reference to the Examples.