WO1992003481A1

WO1992003481A1 - Polymerisation processes

Info

Publication number: WO1992003481A1
Application number: PCT/GB1991/001381
Authority: WO
Inventors: Robert Stephen Davidson; Keith William Allen; Hong Sheng Zhang
Original assignee: British Technology Group Ltd
Priority date: 1990-08-14
Filing date: 1991-08-14
Publication date: 1992-03-05
Also published as: EP0543897A1; GB9017754D0; JPH06501721A; CA2088250A1; GB9117576D0; GB2248622B; GB2248622A

Abstract

Emulsion or suspension polymerisation processes are carried out whilst subjecting the reactants to ultrasonic vibration. In a preferred embodiment a whistle reactor is used in a closed loop system comprising a pump and a reaction vessel. The polymerisation may be accelerated and may be carried out in the absence of any micelle forming surfactant. The products are polymer latexes which can be used, e.g. as pressure sensitive adhesives.

Description

POLYMERISATION PROCESSES

This invention relates to novel polymerisation processes and to the products of such processes.

Emulsion polymerisation and suspension polymerisation are both widely used processes in the art of polymer technology. Both involve the dispersion of one liquid or solid phase in another bulk liquid phase, most commonly an aqueous phase. Both normally require the presence of an emulsifier and/or a colloid stabiliser in order to prevent the separation of the disperses phase. In emulsion polymerisation processes the reactants are normally dispersed in the bulk phase, with the aid of a surfactant and vigorous agitation. The function of the surfactant is to form micelles in solution which dissolve some of the material to be polymerised. When radicals produced from the initiator are formed in the bulk phase they have to diffuse to the micelles in order to initiate polymerisation. The nature of the surfactant Influences the polymerisation rate, the particle size and particle size distribution and the nature of the latex produced. Whilst suspension polymerisation processes have been carried out in the absence of any surface active material the solids contents of the dispersion and/or the conversion of monomer to polymer are undesirably low and in all practical circumstances suspension polymerisation has been carried out in the presence of a surfactant or colloid stabiliser. Similarly attempts to carry out emulsion polymerisation in the absence of a surfactant have not been successful insofar as the rate of reaction is too slow and the difficulty which is encountered in incorporating high concentrations of monomer.

The presence of surfactant in the products of these processes can exert a deleterious effect upon the properties of that product. In particular adhesives comprising latexes prepared by emulsion polymerisation may exhibit less than ideal properties by virtue of a disruption at the bonding interface caused by migration of the surfactant within the latex. High density ultrasound, e.g. that provided by a horn, has previously been used to induce polymerisation although such techniques are disadvantageous in that the polymeric product may be degraded by the ultrasound. There have also been proposals to use high density ultrasound to assist in the dispersion of the disperse phase in the bulk phase both before and during the course of suspension polymerisation processes. Hatate et al. , Chem. Eng. Comm. Vol. 35, p325 to 333 reported the effect of soni cation upon the suspension polymerisation of polystyrene. Japanese Patent Publication 247473/89 describes a process for the production of a thermosetting powder coating using suspension polymerisation in which the reactants were dispersed by sonication and subsequently introduced into a conventional polymerisation reaction vessel containing the initiator. European Patent Application 255716A describes the production of a toner for developing electrostatic images using suspension polymerisation in which the reactants are sonicated using a high density ultrasonic homogeniser either before or during the polymerisation reaction.

We have now discovered a novel method by which emulsion and suspension polymerisation reactions may be effected in which a dispersion comprising the polymerisable material dispersed in a bulk liquid phase is subjected to low density ultrasound during the course of the polymerisation reaction. The novel polymerisation method offers advantages in that it may accelerate the rate of the polymerisation reaction and also in that it may enable stable latexes to be produced in the absence of any surfactant.

Accordingly, from one aspect this invention provides a method for the production of a polymeric latex which comprises the application of ultrasonic vibration to a dispersion of monomeric or prepolymeric material in a bulk liquid phase during the course of the polymerisation reaction. The reaction medium will preferably comprise an emulsion or a suspension of the organic materials in water. The organic materials comprise the polymeri sable material, an Initiator and optionally a protective colloid. The organic material preferably forms a homogeneous liquid phase at the temperature of the reaction. These materials may produce polymer latexes in which the average size of the latex particles is less than fifty and preferably less than twenty microns.

The processes of this Invention may conveniently be carried out using a high frequency or ultrasonic vibrator. A wide vareity of sources may be used to produce low density ultrasound. The construction of a variety of useful devices has been described, for example in "Sonochemistry; Theory Applications and Uses of Ultrasound in Chemistry" by Mason and Lorimer, published by Ellis Howard Ltd., 1988. An example of a preferred type of agitator is a so-called whistle reactor in which the liquid medium is pumped passed a tuned vane or blade which vibrates and produces energy. By adjustment of the dimensions of the chamber in which the vane is mounted, the pressure of the flow of the liquid medium and the construction of the vane, the vane can be caused to vibrate at ultrasonic frequencies (20 to 50 kHz) and thereby Induce cavltation with the liquid. An alternative type of reactor is the Reverberatory Ultrasonic Mixer such as is available from the Lewis Corporation of Oxford Connecticut and is described in USP 4071225.

In general the chamber in which the dispersion is subjected to ultrasonic vibrations is preferably of relatively small dimensions so that the dispersion is subjected to intense cavitation for a relatively short period. The dispersion is preferably circulated through this chamber on a plurality of occasions during the course of the polymerisation reaction. The preferred sources of ultrasound for use in the processes of this invention are those in which the reaction medium is pumped under pressure and allowed to impinge upon a vibrating means such as the vane of the whistle reactor in a manner which causes said means to vibrate at ultrasonic frequencies. The ultrasonic vibrations produced cause cavltation within the liquid. Such sources are advantageous insofar as they are known to be of rugged and durable construction. They are also suited to a preferred embodiment of the processes of this Invention in which the ultrasound generator forms part of a closed loop comprising a reaction vessel, a pump and optionally a heat exchanger.

In this embodiment the ultrasonic vibrations are generated continuously during the polymerisation reaction. However, only a proportion and preferably only a minor proportion of the dispersion is subjected to ultrasonic agitation at any one moment, the remainder being circulated around the loop. The rate at which the dispersion is circulated is preferably such as to ensure that the dispersion remains homogeneous and does not tend to settle or separate into separate phases. The rate of circulation required in order to achieve this will vary according to the nature and the volume of the dispersion and the nature of the ultrasound generator. Generally, the rate will be such that the entire dispersion is circulated at least twice and more usually at least five times per minute although greater rates of circulation may be employed. An acceptable rate of recirculation for each reaction system may readily be determined empirically. The rate of circulation at constant pumping speed may vary as the reaction proceeds and adjustment of the pumping speed may be needed if a constant circulation is to be maintained. Again the need for any such adjustment can readily be determined empirically. The processes of this invention may be uti l i sed to effect any emulsion or suspension polymerisation reaction.

A wide variety of monomers may be polymerised using the processes of this invention. Those monomers which are known to be amenable to thermally induced free radical polymerisation or to photochemically initiated free radical polymerisation are preferred for present use. Examples of useful monomers include the alky! esters of unsaturated carboxylic acids such as acrylic acid or methacrylic acid. Especially valuable are those esters wherein the alkyl group contains at least four and preferably at least six carbon atoms, although esters wherein the alkyl group contains a greater number of carbon atoms say up to eighteen carbon atoms may also be usefully employed. Examples of useful esters are the butyl ,2-ethylhexyl, octyl decyl or isobornyl esters of acrylic or methacrylic acid. Other monomers which may conveniently be employed are the vinyl esters of aliphatic acids especially those aliphatic acids which contain less than six carbon atoms such as vinyl acetate; vinyl propionate and vinyl butyrate; vinyl esters of unsaturated carboxylic acids such acrylic acid, itaconic acid, maleic acid, fumaric acid and methacrylic acid, vinyl phenyl compounds such as styrene α methylstyrene chlorstyrene and styrene sulphonic acid; vinyl alkyl esters such as vinyl ethyl ether, nitriles such as acrylonitrile and methacrylonitrile, unsaturated amides such as acrylamide and methacryl amide, vinyl halides such as vinyl chloride and vinyl bromide, vinylidenes such as vinylidene chloride and vinylidene bromide, dienes such as butadiene, chloroprene and isoprene and vinyl pyridine. Copolymers comprising combinations of these monomers may also be produced using the processes of this invention. In particular, copolymers of at least one alkyl ester of acrylic or methacrylic acid and at least one vinyl ester of an aliphatic acid may be produced. Further such copolymers may usefully comprise a minor proportion, say up to 207. by weight, of acrylic or methacrylic acid. Processes for the production of such materials form a preferred aspect of the invention. Poly-functional monomers may be utilised in order to produce polymers having a degree of cross-linking. A wide variety of such polyfunctional monomers are known in the art. Some examples are listed, e.g. in European Patent 273605. Representative examples include diacrylates such as ethylene glycol diacrylate and hexanediol diacrylate.

The dispersion will normally incorporate an initiator for the polymerisation reaction. A wide variety of initiators are known in the art and a suitable initiator for any particular polymerisation will be selected according to the conventional criteria of the art.

The initiator may be a water soluble or an oil soluble compound. Examples of water soluble initiators include hydrogen peroxide, ammonium peroxysulphate, sodium persulphate, potassium persulphate, sodium thiosulphate, sodium meta bisulphite, sodium benzophenone-4- methyl sulphonate, 4-benzoyl N-N dimethyl-N-[1-oxo-2-propenyloxy- ethyl )benzene methanaminium bromide, (4-(-4-methyl phenyl thio) phenyl] phenylmethanone-4-benzoyl-4-methyldiphenyl sulphide, 2-n-butoxyethyl-4-(dimethylamino) benzoate, 2-hydroxy-3-(4- benzoylphenoxy)-N N N tri methyl -1- propanaminium chloride monohydrate, (4 benzoyl benzyl) trimethyl ammonium chloride. Examples of oil soluble initiators include benzoin methyl ether. Where the initiator is a water soluble compound the polymerisation is an emulsion polymerisation. Even in a preferred embodiment of this invention in which the dispersion does not contain a micelle forming surfactant, the use of a water soluble initiator means that the reaction is essentially an emulsion polymerisation. However, when an oil soluble initiator is utilised the reaction is essentially suspension pol ymeri sati on in which the initiator is present in the droplets of monomer or prepolymer. The amount of initiator which is employed may conveniently be determined according to criteria known in the art, amounts of from 0.1 to 2.07. by weight of the total weight of the monomers are typical. The reaction medium may and preferably will contain a colloid stabiliser in order to increase the maximum solid content where this is desirable. The use of such stabilisers is an established part of the art of emulsion and suspension polymerisation and these known stabilisers may usefully be incorporated into the processes of this invention. Both surface active and non-surface active stabilisers are known. The non-surface active stabilisers are preferred for use in this Invention particularly when it is desired to produce a latex which is substantially free from surface active material. A preferred additive is poly (vinyl alochol) PVA, although other known stabilisers such as gelatin cellulose derivatives such as methyl cellulose or hydroxyethyl cellulose, polyacrylic acid, starch, gum alginate and casein may also be utilised. The efficiency of PVA as a stabiliser may vary with its molecular weight and the suitability of a particular PVA in a particular polymerisation reaction may be determined empirically. The amount of stabiliser used will generally be in accordance with established practice in the art, e.g. up to 5% by weight of the weight of solid material.

The reactants must be subjected to a regime which is appropriate to the initiator which is present in order that polymerisation may proceed. The polymerisation may be initiated by exposure of the reactants Including the initiator to an appropriate elevated temperature in the case of a thermal initiator or irradiation using a suitable light source such as a mercury vapour lamp in the case of a photo-initiator. In either case the reactants must be contained in an appropriate reaction vessel with facilities to enable the initiation to take place. In the preferred embodiments this reactor will form part of the closed loop system.

The duration of the polymerisation and the conditions under which it is affected will in general resemble the conditions conventionally employed in emulsion or suspension polymerisation. However, the processes of this invention and in particular the emulsion polymerisation processes may proceed more rapidly than is the case when a conventional mechanically agitated reactor is employed. This acceleration is clearly desirable in the manufacture of polymer latexes.

The disperse phase may be dispersed in the bulk liquid using conventional techniques such as mechanical agitation or high power ultrasonic homogeni sation. However, in the preferred embodiments the two phases are mixed together and dispersed by the application of low density ultrasound. Thus conveniently the reactants may be introduced into a reaction vessel which is incorporated into a closed loop system which also comprises the ultrasonic vibrator. The reactants are cycled round this loop and the disperse phase is dispersed in the bulk phase. The polymerisation may be initiated at the same time as the pump is activated but more usually the reactants will be dispersed prior to the initiation of the polymerisation reaction.

The size of the particles or droplets of dispersed material is preferably reduced to the same order as the desired size of the latex particles, i.e. of the order of twenty microns before the polymerisation is initiated. In the case of the suspension polymerisation processes the size of the particles in the dispersed phase will be approximately the same before and after polymerisation. In the case of emulsion polymerisation processes the size may significantly decrease during the reaction.

The agitation to which the mixture is subjected by its passage through the pump may serve to aid the dispersion of the disperse phase. The degree of this agitation will vary with the design of the pump. In particular the shape of the outlet from the pump may exert an influence on the degree of agitation. In the case of the ultrasound generators in which the vibration is produced by the impact of the liquid medium upon the vibrating means the outlet from the pump will normally be positioned relatively close to the vibrating means. The shape of the outlet and its position relative to the vibrating means exert a considerable influence upon the intensity of the vibration which is generated. These parameters may be adjusted empirically and indeed the known generators such as the whistle reactors conventionally comprise means for affecting such adjustments and for measuring the intensity of the ultrasonic vibration which is produced. In the processes of this invention it is preferable to monitor the reactor to ensure that the ultrasound is being generated and more preferably is sufficiently Intense to produce the desired latex.

The solids content of the reaction medium is preferably at least 20% and more preferably at least 307. by weight. It is generally preferable to utilise as high a solids content as possible commensurate with the need to produce a product which is stable upon prolonged storage. The maximum solids content which can be utilised will vary with the nature of the monomer and/or prepolymers and also with the nature of the polymerisation process, in particular with the nature of the initiator. The solids content may be limited by the viscosity of the reactant solution and the power of the pump used to circulate the reactant in a closed loop system.

The processes of this Invention may lead to the production of polymer dispersions having a smaller particle size than has previously been attained and such novel polymer systems form a preferred aspect of this invention. In general dispersions wherein the average particle size is in the range 0.1 to 10 microns and more preferably in the range 0.2 to 5.0 microns are preferred. Further, those dispersions wherein at least 60 and more preferably at least 80% of the particles are of a size of less than 10 microns are preferred for present use.

The polymer emulsions and suspensions of this invention find use in a variety of applications. They may be employed in all those applications in which polymer emulsions and suspensions currently find application. In general those emulsions or suspensions which do not contain any significant quantity of surface active agent are preferred since the presence of surface active agents on the conventional product may be disadvantageous in one way or another.

One application in which the emulsions of this invention are useful is as pressure sensitive adhesives. Polymers derived from the alkyl esters of unsaturated carboxylic acids and their copolymers with vinyl acetate are known to be useful in this application and are useful in this Invention. Polymer emulsions containing at least 15% by weight of solid material and especially those which are substantially free of surface active material constitute a preferred aspect of this invention.

The polymer emulsion of this invention may be formulated into a pressure sensitive adhesive tape using conventional techniques. Thus, typically a wet coating thickness of 1/1000 inch of emulsion may be applied to a polymeric film substrate such as Melinex and the film air dried to form a tape.

Other applications in which the emulsions and suspensions of this invention may find utility include the formulation of water-based paints and Inks, the formulation of water-based adhesives and as supports for reagents, e.g. for use in immunoassays.

The invention is illustrated by the following examples:

Example 1

The following reactants were mixed:

Water 200 gm

Polyvinyl alcohol (PVA) 4 gm

Ammonium persulphate 1.5 gm

Ethylhexyl acrylate (EHA) 90 gm

Vinyl acetate (VA) 60 gm

in a Dawe Minisonic Homogeniser whistle reactor. The contents are heated to 80°C and maintained at that temperature for 1 hour whilst pumping the reactants through the whistle at a rate of 1500 cc/min. The result was a latex having the following characteristics: Solids content: 40% (approx)

Average particle size: 1.7 μm

Shelf-life: 6 months (approx)

Freeze-thaw stability: good*

*In these examples the freeze-thaw stability was assessed by subjecting the latex to a cycle comprising freezing in a refrigerator and heating back to ambient temperature thirty times. If the polymer separated during or after the test the sample is rated poor. If it is stable the rating is good and if no precipitation whatever is observed the rating is very good.

In the following examples the reactants listed were mixed and tested using the same techniques recited in Example 1.

Example 2

Water: 300 g

PVA: 4 g

EHA: 120 g

VA: 80 g

Initiator Sodium benzophenone- 4-methyl-sulphonate: 3.0 g

Co-initiator N-Methyldi ethanol ami ne: 7.5 g

The above were mixed in a whistle reactor and Irradiated with a 400 watts medium pressure mercury lamp at room temperature for 60 minutes.

Characteristic of the resultant latex:

Solid content: about 30%

Average particle size: about 4.57 μm

Shelf-life: about 3 months

Freeze-thaw stability: very good

Example 3

The same reactants as example 2 except that 1.0 g of an oil-soluble Initiator benzoin methyl ether was used instead of the water-soluble initiator and co-initiator system.

Characteristic of the resultant latex:

Similar to the latex of example 2 except that the freeze-thaw stability is poor. Average particle size: 4.42 μm. Example 4

The same reactants as example 2 except that 200 g of EHA was used only without comonomer VA.

Characteristic of the resultant latex:

Solid content: 30%

Average particle size: 2.09 μm

Shelf-life: about 6 months

Freeze-thaw stability: good

Example 5

The same reactants as example 4 except that 1 g of an oil-soluble initiator BME was used Instead of the initiator and co-initiator.

Characteristic of the resultant latex:

Solid content: about 30%

Average particle size: about 3.5 μm

Shelf-life: about 6 months

Freeze-thaw stability: good

Peel adhesion: 175g/0.5 inch

Peel adhesion was measured by coating the latex on polyester backing with a doctor blade and drying in an oven at 80ºC for 48 hours. The dried adhesive film was 54 ± 3μm thick. The tape was applied to a clean aluminium substrate using a 3.5Kg roller. The peel strength was measured using an Instron 1026 tensile tester. In the following examples peel adhesions were measured using the same techniques.

Example 6

The same reactants as example 4 except that 6.0 g of a non- Ionic surfactant Igepal CO-990 having the formula 4-(C₉H₁₉)C₆H₄O- (CH₂CH₂O)₉₉CH₂CH₂OH was used instead of PVA.

The result was latex having the following characteristics:

Solid content: 39%

Average particle size: 2.5 microns

Shelf-life: >18 months

Freeze-thaw stability: good

Peel adhesion: 230g/0.5 inch Exampl e 7

Water : 450 g

I sobornyl acryl ate : 250 g

PVA : 5 g

BME : 1 . 3 g

The above were mixed in the whistle reactor and radiated with UV light (400 watts) for 20 min. A stable polymer emulsion was obtained with a conversion of 89.4%.

Solid content: 32%

Average particle size: 3.0 microns

Shelf-life: 2 weeks

Freeze-thaw stability: poor

Example 8

Water: 430 g

Isobornyl acrylate: 100 g

Vinyl acetate: 150 g

PVA: 4 g

BME: 1.3 g

The above were mixed in the whistle reactor and radiated with UV light (400 watts) for 40 min. A stable polymer emulsion was obtained with a conversion of 50%, which polymer contained 21.8% by weight of vinyl acetate unit.

Characteristics of the resultant latex:

Average particle size: 3.0 microns

(estimated from SEM photograph)

Shelf-life: 2 weeks

Example 9

Water: 450 g

n-butyl acrylate: 250 g

PVA: 5 g

BME: 1.3 g

The above were mixed in the whistle reactor and radiated with UV light (400 watts) for 30 min. A stable polymer emulsion was obtained with a conversion of 72.5%. Characteristics of the resultant latex:

Solids content: 26%

Average particle size: 5.91 microns

Shelf-life: 3 months

Freeze-thaw stability: poor

Example 10

Same as Example 6, except that 6.3g of (2-acryloyloxyethyl) (4-benzoyl benzyl ) dimethyl ammonium bromide was used to replace sodium benzophenone-4-methyl-sulphonate.

Characteristic of the resultant latex:

Solid content: 26%

Average particle size: about 3500nm

Shelf-life: 3 months

Freeze-thaw stabi l i ty : poor

Peel adhesion: 250g/0.5 inch

Examples 11-17

Water: 450g

PVA: 4g

EHA: 300g

Initiator: Sodlumbenzophenone-4-methyl-sulphonate: 3.0g

Coinitiator: see Table 1: 0.0587 mole

The above were mixed in the whistle reactor and radiated with UV light (400 watts) for 50-80 min (see Table 1). The results with different co-initiator are listed in Table 1.

Table 1: Results of Examples 11-117

Example Co-initiator Time of Polymer Particle

Reaction Content Size (min) (%) (nm)

10 N-methyldiethanolamine 60 31 3640

11 N ,N-dimethylethanolamine 60 35 3570

12 triethanolamine 60 32 5800

13 triethylamine 60 33 3470

14 N-n-butyldiethanolamine 60 34 2360

15 N-tert-butyldiethanolamine 80 25 2870

16 2-(2-diethylamino-ethoxyl)- 50 20 2360 ethanol Example 18

Water: 450g

PVA: 9g

EHA: 270g

VA: 30g

Initiator: 4,4-azobis(4-cyanovaleric acid) 3.0g

The above were mixed in a whistle reactor and heated to 70°C and maintained at this temperature under nitrogen atmosphere for 35 minutes.

Characteristic of the resultant latex:

Solid content: 36%

Average particle size: 3250 nm

Shelf-life: 3 months

Freeze-thaw stability: good

Example 19

Same as Example 18 except that 4.5g of PVA was used.

Characteristic of the resultant latex:

Solid content: 30%

Average particle size: 4580 nm

Shelf-life: 1 month

Freeze-thaw stability: poor

Example 20

The same recipe and procedure as that of Example 9 was used, except that monomer was replaced by n-butyl methacrylate. Stable polymer suspension with 32.0% yield was obtained after 60 minutes of Irradiation. The resultant polymer suspension was examined by scanning electron microscopy and had an estimated average particle size of 4 microns. The suspension had a shelf life of 2 weeks.

Example 21

450g of distilled water containing 6.0g of polyvinyl alcohol was homogenised with monomer mixture (EHA 285g, Acrylic Acid (AA)

15g) containing 1.5g (0.0066 mole) of benzoin methyl ether using the ultrasonic homogenizer at 1000 ml/min throughput for 25 minutes. The mixture was then pumped into a photochemical reactor and irradiated (UV-400 watts) during which continuous ultrasonic homogeni sation was used. Polymer suspension with high yield was obtained after 30 minutes of irradiation. The polymer suspension was examined by scanning electron microscopy and had an estimated average particle size of 7 microns. The polymer suspension phase separated after about 4 weeks of storage.

Example 22

The same recipe and procedure as that of Example 21 was used, except that AA was replaced by EGDMA. Polymer suspension with 99.8% yield was obtained after 30 minutes of irradiation. When precipitated with acetone and dried in oven, a white, brittle polymer that can only swollen and is not soluble in toluene was obtained. The polymer suspension was examined by SEM and had an estimated particle size of 8 microns. The polymer suspension phase separated after about 4 weeks of storage.

Example 23

A dispersion containing the following ingredients was made up as follows:

PVA solution: 300 ml

Styrene (purified): 40 ml

Cumene hydroperoxide: 0.17g

Fructose: 0.50g

Ferric sulphate: 0.17g

Sodium pyrophosphate: 1.5g

The fructose and iron activator were dissolved (60 mis) together. The styrene emulsion was made in the Dawe Mini sonic reactor using the cooling coils to maintain the system under 30°C. The fructose-iron activator solution was added and finally the cumene hydroperoxide. The reaction was halted after 1 hour, 49% of the styrene had polymerised. The emulsion was stable for months. The above reaction was repeated to check the yield.

(a) ist repetition - 46%

(b) 2nd repetition - 48% Example 24

A dispersion containing the following ingredients was made up as follows:

PVA solution: 300 ml

Styrene (pure): 40 ml

Ammonium persulphate: 4g

Distilled water: 60 ml

The ammonium persulphate (4g) was added to distilled water (60 ml) and magnetically stirred until it had all dissolved. Gentle heating was occasionally used. The PVA solution and styrene were emulsified with the cooling system working. The Initiator system was added and the emulsion circulated in the apparatus for 1 hour. The emulsion was stable for several months. 36% of the styrene had polymerised; 32% was obtained on repetition. Upon replacement of ammonium persulphate with potassium persulphate, 35% polymerisation occurred.

Example 25

Example 24 was repeated to see what happened after increasing the duration of the experiment. Samples were taken out at 1 , 2 and 3 hours and examined to see the amount of polymerised styrene.

After 1 hour : 37%

After 2 hours : 54%

After 3 hours : 66%

Claims

1. A method for the production of a polymeric latex which comprises the application of ultrasonic vibrations to a dispersion of liquid monomeric or prepolymeric material in a bulk liquid phase during the polymerisation reaction.

2. A method according to claim 1 characterised in that the dispersion is pumped under pressure and allowed to impinge upon a vibrating means thereby causing said means to vibrate at ultrasonic frequency.

3. A method according to either of claims 1 or 2 characterised in that wherein the dispersion is circulated around a closed loop comprising an ultrasound generator a pump, a reaction vessel and optionally a heat exchanger.

4. A method according to any of the preceding claims characterised in that the source of ultrasonic vibration is confined in a chamber through which the reaction medium circulates.

5. A method according to any of claims 1 to 4 characterised in that the dispersion is pumped past a vibrating means which comprises a tuned vane in a manner which causes said vane to oscillate at ultrasonic frequency.

6. A method according to any of claims 3 to 5 characterised in that the dispersion is circulated at a rate which is sufficient to ensure that It does not separate into its component phases.

7. A method according to claim 6 characterised in that the entire dispersion circulates through the chamber in which the ultrasonic vibration is applied at least twice per minute.

8. A method according to any of the preceding claims characterised in that the dispersion is an emulsion of monomeric or prepolymeric material in a bulk liquid phase.

9. A method according to any of claim 1 to 7 characterised in that the dispersion is a suspension of monomeric or prepolymeric material in a bulk liquid phase.

10. A method according to claim 8 characterised in that the dispersion is substantially free from surface active emulsifying agents.

11. A method according to any of claims 8 to 10 characterised in that the dispersion comprises a colloid stabiliser.

12. A method according to claim 11 characterised in that the colloid stabiliser is not surface active.

13. A method according to either of claims 11 or 12 characterised in that the colloid stabiliser is a poly (vinyl alcohol).

14. A method according to any of claims 8 to 13 characterised in that the dispersion is substantially free from surface active material.

15. A method according to any of claims 8 and 10 to 14 characterised in that the dispersion comprises a water soluble initiator.

16. A method according to any of claims 9 and 11 to 14 characterised in that the dispersion comprises an oil soluble initiator.

17. A method according to either of claims 15 or 16 characterised in that the Initiator is a photo initiator.

18. A method according to either of claims 15 or 16 characterised in that the initiator is a thermal initiator.

19. A method according to any of the preceding claims characterised in that the dispersion comprises at least one alkyl ester of an unsaturated carboxylic acid.

20. A method according to claim 9 characterised in that the ester is an alkyl ester of acrylic acid.

21. A method according to any of claims 1 to 20 characterised in that the dispersion comprises at least 15% by weight of solid material.

22. A method according to any of claims 1 to 21 characterised in that the average size of the dispersed particles in the polymeric latex is less than 20 microns.

23. A method according to claim 22 characterised in that the average size of the dispersed particles is less than 10 microns.

24. A polymer latex which comprises at least 15% by weight of polymerised material (by weight) in which the average particles size of latex particles is less than 10 microns.

25. A latex according to claim 24 which is substantially free from surface active material.

26. A polymer latex whenever produced by a process according to any of claims 1 to 23.

27. A latex according to any of claims 24 to 26 characterised in that the polymer is a homo or copolymer of an alkyl ester of an unsaturated carboxylic acid such as acrylic acid.

28. A pressure sensitive adhesive tape which comprises a polymer film produced by the application of a latex according to any of claims 24 to 26 to a backing member.

29. A method according to any of claims 1 to 23 substantially as hereinbefore described with reference to the foregoing examples.