GB2196011A - Improvements in the production of graft copolymers - Google Patents

Abstract

A graft copolymer consisting of a rubbery core and a hard shell is produced by preparing the rubbery core by polymerizing butadiene and styrene, using the emulsion polymerization method. By employing higher temperatures than normal, 90 DEG C or higher, the reaction times are 3-7 hours, and the latex particle size is above 1500 A. Then the graft copolymer shell is made under standard conditions and the graft copolymer is isolated and dried. Thus prepared graft copolymer functions as an impact modifier of hard and brittle polymers such as PVC, Polystyrene and Polymethyl methacrylate. When the two are blended and compounded toughened versions of these polymers are produced.

Description

SPECIFICATION Improvements in the production of graft copolymers The present invention relates to a process for the preparation of rubber-based graft copolymers, consisting of a rubbery core and a hard shell. In particular it relates to a process for the improved preparation of the rubbery core to produce high impact and transparent graft copolymers which can be used as impact modifiers of thermoplastics.

Polymethyl methacrylate, Polystyrene and Polyvinyl chloride are hard and brittle polymers with good transparency and processability. To improve their toughness they are blended with graft copolymers, which are obtained by polymerizing methyl methacrylate and styrene on a rubbery core of butadiene and styrene copolymer or butadiene polymer. Such graft copolymers are also referred to as impact modifiers.

Generally such graft copolymers are prepared by the emulsion polymerization method. Normally relatively low temperatures are used, e.g. 5"-709C and emulsifier concentrations of 1-5 parts by weight per hundred parts of monomers. Redox catalyst recipes are usually employed. Conversion of monomers to polymer of over 90% are obtained in less than 20 hours. Under such conditions the average particle diameter of the latex particles is below 1000 A. But a butadiene-styrene copolymer with a particle size of < 1000 A is not suitable for many applications, such as e.g.

production of MBS impact modifiers of Polyvinyl chloride for bottle applications, which require latex particles of at least 1500 A.

It is well known that a decrease in the concentration of emulsifier will increase the average particle size of the latex. It is also known that an increase in the temperature of emulsion polymerization will result in a decrease in the average particle size of the latex. Therefore to achieve the required larger average particle diameter, it is normal to employ lower emulsifier concentrations whilst retaining the normal polymerization temperatures. But as a result long polymerization times are needed, e.g. over 30 hours to obtain particles of 1500 A.

One method to overcome these difficulties is to agglomerate the small particle latex to a larger size, and thus carrying out the butadiene-styrene polymerization under relatively fast reaction cycles, about 15 hours. A latex particle size of about 800 A may thus be obtained. Subsequently this latex is agglomerated by one of the agglomerating methods, as is described in the literature.

I have now found that it is possible to make the butadiene-styrene copolymer latex in a few hours, 3 to 7 hours, by polymerizing at higher temperatures. Contrary to previous practice the temprature is higher than 90" C for at least part, preferably at least half of the polymerization cycle. Quite surprisingly the average particle size of thus obtained latex is larger than 1500 A.

According to the present invention a process for the production of a butadiene-styrene copolymer having particles of average diameter of at least 1500 A comprises: 1) Preparing an aqueous emulsion of butadiene and styrene containing 0.3 to 2 parts by weight of an anionic emulsifier per hundred parts of butadiene and styrene and also a thermally decomposable free radical initiator.

2) Allowing polymerization to procede under higher temperature conditions in excess of 90" C.

3) Cooling and stripping the residual monomers and then following with the graft reaction.

As a result of this process, a stable butadiene-styrene latex of particle size larger than 1500 A is obtained, faster and more economically. The process is particularly useful for the production of an MBS graft copolymer to be used as an impact modifier of polyvinyl chloride.

The monomer used to make the rubbery core is butadiene and the copolymerizable monoethylenically unsaturated monomer is preferably styrene. The latter monomer is used to increase the refractive index of the core to improve the transparency of the graft copolymer. Other copolymerizable monoethylenically unsaturated monomers can be optionally employed such as vinyl toluene, a-methyl styrene, acrylonitrile, methacrylonitrile, methyl methacrylate or the like.

The rubbery core is polymerized from a monomer mixture of 45 to 85 weight percent of butadiene and 15 to 55 weight percent of styrene.

The anionic emulsifier used may be an alkali metal salt of a long chain carboxylic acid or of rosin acid. Particularly suitable are salts of saturated fatty acids having 16 to 20 carbon atoms, e.g. palmitic and stearic acids, and salts of unsaturated acids such as oleic acid, or a mixture of such salts. Potassium oleate is very efficient and may be prepared in situ using oleic acid and potassium hydroxide. Anionic emulsifiers are good soaps and good micelle formers and are sometimes referred to as primary emulsifiers.

It is also a feature of this invention that small amounts of soap are added during the reaction, particularly after the miceiles have disappeared to stabilise the latex particles. The total amount of soap must be controlled so as to exceed the total particle soap coverage; this subject was discussed by Z. Kromolicki et.al. in 'A Morphological Study of Multiphase Polymer Systems' published in S.C.I. Monograph No. 26-Advances in Polymer Science & Technology. (1967) It is also advantageous to add a secondary emulsifier. A secondary emulsi- er fier is one which is a poor soap and is no good at forming micelles, but is a good stabiliser for the micelles, once these have been formed. A typical secondary emulsifier is the sodium salt of naphthalene sulphonic acid-formaldehyde condensate.These compounds are available commercially under the trade names Bevaloid, Daxad, Dispersol.

The secondary emulsifier is used in amounts of about 0.2 to 1.2 parts by weight per hundred parts of monomers. Whilst the total amount of anionic emulsifier is preferably 0.3 to 4.0 parts by weight per hundred parts of butadiene and styrene monomers.

If potassium oleate is used with the sodium salt of naphthalene sulphonic acid-formaldehyde condensate, the weight ratio is preferably 1.5:2 to 1:3.

For this process the initiators used decompose thermally; contrary to normal emulsion polymerization practice when redox initiators are employed.

Preferred initiators are those which have a half-life of less than 50 minutes at temperatures of 100" C and above, examples of such initiators are: tert-butyl hydroperoxide, azoisobutyrodinitrile, potassium persulphate.

It is also desirable to include an electrolyte, such as potassium carbonate potassium chloride, sodium tetrapyrophosphate etc., to control the number of micelles in the initial stages of the raction. This is a better approach than reducing the amount of emulsifier since the latter method can render the latex rather unstable.

The phase ratio, or the monomer to water ratio is desirably in the range of 1:1.5 to 1:3. The ratio may be adjusted to optimise the particle size consistent with a satisfactory rate of reaction.

The butadiene and styrene monomers may be emulsified in water using a dispersing apparatus.

The monomers, water, emulsifier and other ingredients are passed directly to the reactor through an in-line homogeniser to obtain good emulsification of the monomers.

The reaction may be carried out batchwise or in a continuous reactor system. In the former method a pressure reactor is used provided-with an agitator. The temperature of the reaction is taken above 95" C for at least half of the polymerization time. Cooling of the reactor may be applied as required to maintain the reactor contents at the desired temperature. The conversion of the monomers to polymer is normally obtained of 85%-95% in about 3 to 7 hours, typically 5 hours.

When the required conversion has been reached, the latex is either vented or transferred to a pressure let-down vessel, and residual monomer is removed. To improve the latex stability additional emulsifier may be added at this stage in amounts of 0.1 to 0.5 parts per 100 parts of polymer.

The average particle size of the butadiene styrene latex is about 2000 A.

For optimum properties of the MBS graft copolymer the gel content of the copolymer should be over 80%, and the swelling index 10 to 20.

Other emulsion recipe additives may be added optionally, such as mercaptans, oxygen scavengers and crosslinking agents, as is well known in the art.

The butadiene-styrene latex prepared according to this invention may be used to prepare a graft copolymer, by using the butadiene-styrene latex in a core or substrate for the graft copolymer. The graft copolymer is then used as a toughening additive in PVC or in a crystal polystyrene or styrene copolymer. Various methods of grafting have been described in the literature. One method which may be conveniently adopted is one used by the inventor in his previous specifications: GB t 040 287 and GB 1 111 089.The rubber latex is grafted with monomers capable of forming glassy thermoplastic polymers, e.g. vinyl aromatic monomers such as styrene, alpha methyl styrene, vinyl toluene and unsaturated nitrile monomers such as acrylonitrile and methacrylonitrile and monomers based on acrylic and methacrylic acid esters, such as ethyl acrylate, butyl acrylate and methyl methacrylate.

The ratios of the monomers used and to the rubber core are determined by the properties desired in the graft copolymer and their effect on the matrix of the polymer in which the graft copolymer will be the dispersed polymer. To achieve a good combination of mechanical properties the monomer ratio of the shell of the graft copolymer is from 25% to 60% and the rubbery core is 75% to 40% of the graft copolymer composition.

To achieve also a good transparency in the final PVC blend the refractive index of the graft copolymer must match the refractive index of the PVC, and therefore the grafting monomers or the shell monomers, must also match the refractive index of the PVC.

Normally the grafting reaction is carried out using peroxide initiators such as cumene hydroperoxide, p-menthane hydroperoxide or di-isopropyl benzene hydroperoxide, actvated with ferrous iron or other activating recipes as is well known in the art.

The PVC containing the MBS graft copolymer is frequently referred to as Toughened or high impact PVC, and has substantially improved properties e.g. impact strength may be more than ten times higher than the unmodified PVC resin and therefore it is then suitable for the production of bottles, film, sheet, pipe and profile applications.

The following Examples are given to illustrate the manner in which the process may be carried into effect: Example 1 The soap is formed first in a separate vessel at 50 C and then the pH is adjusted to 10.5-11.0. Then the Bevaloid (sodium salt of naphthalene sulphonic acid formaldehyde condensate.) and also the potassium carbonate are added and dissolved. The butadiene, styrene and glycol dimethacrylate are metered to a homogeniser and so is the soap solution previously made. The pre-emulsified reaction mixture is then charged to an agitated, jacketed pressure reactor of 10 1 capacity. Then the mercaptan and the initiator are metered.The reactor charge comprised the following: Butadien 75 parts styrene 25 parts ethylene glcyol dimethacrylate 0.3 parts potassium oleate 0.5 parts Bevaloid 35 0.7 parts water 200 potassium carbonate 1.0 parts tert-dodecyl mercaptan 0.15 parts potassium persulphate 0.3 parts The temperature of the reactor contents is increased to 900 C, an exothermic reaction starts and the temperature rises to 125 C after one hour.

When 40% conversion has been reached, soap additions are started: 0.1 part are added every 30 minutes. After 5 hours the conversion is over 90% so the reactor is cooled and the contents are vented and the residual monomers are removed under vacuum.

The average particle size of the latex, as measured from electron micrographs is 1900 A. The gel content is 91% and the swelling index is 17 as measured in toluene.

Thus prepared latex is ready for the graft reaction.

The graft copolymer was made at 55" C using 55 parts of styrene and 45 parts of methyl methacrylate. Cumene hydroperoxide was used and a redox system as described by the inventor in his prior specifications quoted above.

The graft copolymer was coagulated and dried.

Thus prepared MBS graft copolymer was added to PVC in a high speed mixer together with a tin stabiliser and lubricant. Standard test bars were moulded and tested for physicals. Two blends were made, one containing 12 parts and the other 6 parts of the MBS graft copolymer, giving the following results:: UNITS 12 PARTS 6 PARTS Izod Impact strength 3,175 mm, notched J/M 23"C 1250 750 0 C 800 420 -30 C 540 285 Tensile Strength kg/cm 23"C 455 497 Vicat Softening Point 1 kg "C 81 82 Specific gravity 1.31 1.31 Surface finish good good Example 2 This example illustrates the use of the butadiene-styrene rubber copolymer as a core to product a graft copolymer shell from styrene monomer only and then blending it with Crystal Polystyrene to produce a very high impact Polystyrene. The rubbery butadiene-styrene copolymer was prepared in the same way as in Example 1 by batch polymerization. The graft polymerization was carried out by using styrene monomer only at a ratio of 60 parts to 40 parts of the core rubbery butadiene-styrene copolymer.The temperature of reaction was 65" C and the initiator was cumene hydroperoxide at 0.5 parts and the activators as in the first example. After completion of the reaction, the latex was coagulated with sulphuric acid, filtered and dried. Then the graft copolymer was blended with Crystal Polystyrene at a ratio of 50 parts of Crystal Polystyren to 50 parts of graft copolymer. and extruded in a Buss Ko-Kneader. The physical properties of the blend are given below: Izod impact strength J/M 23"C 173 (16.5) Melt Flow Index g/10 min. 3.5 (21) Vicat Softening point "C 92 ( 85) Tensile Strength kg/cm2 160 (380) The properties of the Crystal Polystyrene used in the blend are given in brackets above ( ).

Example 3 This example describes the preparation of the butadiene-styrene copolymer using a continuous method of polymerization.

The soap is prepared first in a separate agitated and jacketed vessel at 50 C. Water is charged first, followed by oleic acid and potassium hydroxide. After two hours the pH is adjusted to 10.5-11.0. Then the sodium salt of naphthalene-sulphonic acid formaldehyde condensate is added and finally the potassium carbonate is added. This solution is transferred to a metering vessel, from which it is pumped continuously to a homogeniser. The apparatus consists essentially of an emulsification vessel, which is a triple stage Manton Gaulin homogeniser. The outlet of the homogeniser is piped to the bottom of an agitated, jacketed pressure reactor. At the top of the reactor there is an overflow to a pressure let-down vessel which is connected to a collecting vessel.The butadiene and styrene and the ethylene glycol dimethacrylate and the soap solution are metered to the homogeniser to produce an emulsified feed. The emulsified mix is pumped continuously to the reactor which is at a minimum temperature of 95" C. A 5% solution of potassium persulphate in water and tert-butyl hydroperoxide are also metered continuously to the reactor.

The formulation used is given below: Butadiene 78 parts Styrene 22 parts Ethylene glycol dimethacrylate 0.5 parts Water 250 parts Oleic acid 0.35 parts Potassium hydroxide 0.09 parts Bevaloid 35 0.75 parts Potassium carbonate 0.9 parts Potassium persulphate 0.15 parts tert-Butyl hydroperoxide 0.1 parts The residence time was five hours and the maximum temperature of 103"C was observed.

Additional soap solution was also pumped to the top of the reactor, at such a rate that the total primary soap concentration did not exceed 1.3% soap on the total monomers. A conversion of 919/0 was achieved and the residual monomers were removed under vacuum.

The average latex particle size obtained was 2100 A. The gel content was 93% and the swelling index was 14, as measured in toluene.

Thus prepared core butadiene-styrene copolymer was used to make the graft copolymer by charging an agitated, jacketed reactor with 50 parts by weight (solids) of the butadiene-styrene copolymer, 50 parts by weight of water, 16 parts by weight of styrene and 22 parts by weight of methyl methacrylate. Cumene hydroperoxide was used as the initiator at 0.2 parts and sodium formaldehyde sulphoxylate as the activator at 0.1 part by weight. It was reacted for seven hours at 65"C. Then antioxidant was added, it was coagulated and dried. Then 12 parts of the graft copolymer is blended with 88 parts of PVC and 2 parts of dibutyl tin dilaurate and compounded on a two-roll mill. Then test bars were compression moulded at 1900 C.

The Izod Impact Strength of the blend was found to be 1350 J/M at 23"C whilst other physical properties were practically unchanged.

Claims (20)

1. A process for producing a butadiene styrene copolymer latex which is suitable as a core for the preparation of a graft copolymer which is an efficient impact modifier of thermoplastic resins, comprising emulsion polymerization temperatures above 90" C and resulting in a latex having a particle size above 1500-A.

2. A process according to claim 1 wherein the monomers butadiene and styrene are preemulsified using an anionic emulsifier at 0.3 to 2.0 parts by weight per hundred parts of the monomers.

3. A process according to claim 1 wherein thermally decomposable initiators are used.

4. A process according to claim 1 wherein the rubbery core is polymerized from a monomer mixture of 45-85 weight percent butadiene and 15-55 weight percent styrene.

5. A process according to claim 1 wherein the preferred copolymerizable monomer is styrene, but other copolymerizable monoethylenically unsaturated monomers can be employed, such as vinyl toluene, alpha methyl styrene, acrylonitrile and methyl methacrylate and the like.

6. A process according to claim 1 wherein the anionic emulsifier is the sodium or potassium salt of organic carboxylic acid having 16 to 20 carbon atoms.

7. A process according to claim 6 wherein the preferred emulsifier is potassium oleate.

8. A process according to claim 1 wherein additional emulsifier, potassium oleate, is added during the reaction, but the total amount not exceeding the total soap coverage of the particles.

9. A process according to any of the preceding claims wherein the anionic emulsifier is a mixture of a primary emulsifier and a secondary emulsifier.

10. A process according to any of the preceding claims wherein the secondary emulsifier is an alkali metal salt of a naphthalene sulphonic acid compound.

11. A process according to claim 10 wherein the secondary emulsifier is the sodium salt of naphthalene sulphonic acid formaldehyde condensate.

12. A process according to claims 11, 12 and 13 wherein the secondary emulsifier is used in an amount of 0.2-1.2 parts by weight per hundred parts of monomers.

13. A process as claimed in any of the preceding claims in which the total amount of anionic emulsifier is 0.3-4.0 parts by weight per hundred parts of monomers.

14. A process as claimed in any of the preceding claims in which potassium oleate is used together with the sodium salt of naphthalene sulphonic acid formaldehyde condensate, the preferred weight ratio is 1.5-2 to 1-3.

15. A process as claimed in any of the preceding claims, wherein thermally decomposable initiators are used such as potassium persulphate and tert-Butyl hydroperoxide.

16. A process as claimed in any of the preceding claims, wherein a batchwise method of polymerization is used and the reactor is an agitated pressure reactor.

17. A process as claimed in any of the preceding claims, wherein a continuous method of polymerization is used.

18. A process as claimed in any of the preceding claims, wherein the butadiene and styrene are polymerized to 85-95 YO in 3 to 7 hours.

19. A process according to claim 1, wherein the butadiene styrene latex is grafted with styrene and methyl methacrylate to produce an MBS graft copolymer which is an efficient impact modifier of PVC.

20. A process according to claim 1, wherein the butadiene styrene latex is grafted with styrene monomer to produce a graft copolymer which is an impact modifier of polstyrene.