METHOD FOR THE PREPARATION OF GRAFT COPOLYMERS BY EMULSION COPOLYMERIZATION

Abstract: Graft copolymers are prepared by means of a two-step polymerization process, in emulsion: in the first step, a vinylaromatic monomer and an acryl or metacryl monomer are grafted on a monomodal polybutadiene latex constituted of particles of diameter comprised within the range of from 800 to 2500 Å. The polymerization/grafting reaction is continued up to a 70-90% conversion, at which time a further portion of monomodal polybutadiene is added, together with further amounts of said monomers, always in emulsion.

Description: This invention relates to a process for grafting a vinylaromatic monomer and an acrylic monomer on polybutadiene by emulsion polymerization. The prior art, as represented by US-A-3 442 979 discloses a process for the preparation of graft polymers on the basis of polybutadiene by first adding part of an emulsion of polybutadiene, styrene and acrylonitrile and other compounds to a reaction vessel, starting the polymeriza­ tion and then adding the rest of the emulsion, the particle size of the polybutadiene latex being from 0,08 micrometer to 0,304 micrometer, said latex being used as the basis for the acrylonitrile-styrene graft. DE-A-2 038 738 and FR-A-2 051 451 disclose processes for the grafting of a butadiene polymer with a particle size range of 0,2 to 0,6 micrometer by first grafting 20% to 80% of the acrylonitrile-styrene monomers onto the polybutadiene and, when at least one half of that amount has been converted, adding the remaining amount of the acrylonitrile-styrene monomers. Moreover, EP-A-0 000 462 discloses a process of graft polymerization in which the technologies of emulsion polymerization and polymerization in suspension are sequentially used. In order to obtain better products and a smoother processing run, the present invention provides a process for grafting a vinylaro­ matic monomer and an acrylic monomer on polybutadiene by emulsion polymerizing, in the presence of a monomodal polybutadiene latex having a particle diameter comprised between 0,08µm and 0,25µm, to which a stabilizer-emulsifier has been added, a mixture of a vinylaromatic monomer and an acrylic monomer, adding a further portion of monomodal polybutadiene and a further amount of said vinylaromatic and said acrylic monomer when the reaction conver­ sion is between 70% and 90% of the supplied monomers, in the presence of from 0,01% to 3% by weight, relative to the reactive compounds, of a water-soluble initiator, and completing the emulsion polymerization, characterized in that said water-soluble initiator is activated by a reducing agent selected from among sodium bisulphite and salts or complexes of iron and low-valence transition metals, preferably Co, Ni and Cu. The polybutadiene latex useable in the grafting-polym­ erization according to the present invention comprises a monomodal polymer characterized by an extremely narrow granulometric distribution, the average particle diameter being comprised within the range of from 0,08 µm to 0,25 µm (800 to 2500 A) and, preferably, within the range of from 0,13 µm to 0,16 µm (1300 to 1600 A). In the first polymerization step, up to 90% is used of the overall amount of polybutadiene which shall enter into the end composition of graft polymer, the residual a­ mount being added during the second step, when the graft­ ing-polymerization reaction has exceeded the 70-90% con­ version level, relatively to monomers introduced. The grafting in emulsion is favoured by the presence of stabilizers-emulsifiers, directly inside polybutadiene latex. Stabilizer systems are those usually used to that pur­ pose, such, e.g., the soaps of fatty acids, alkyl- or ar­ yl-alkyl sulphonates, the salts of resin acids: of parti­ cular interest has resulted the use of a potassium alkyl­ sulphonate, and'the sodium salt of disproportionated resin acid. The monomers to be polymerized and grafted can be in­ troduced throughout the step of emulsion polymerization; they can be introduced as well separately or as mixtures, continuously or as sequential batches, as such or suitably emulsified. As mentioned, the grafting-polymerization envisages the use, according to what just mentioned, of a mixture com­ prising at least a vinylaromatic derivative, and at least an acryl or methacryl derivative. Among vinylaromatic derivatives, styrene, α-methyl­ styrene, substituted styrenes can be mentioned: above all, styrene. Among acryl or methacryl derivatives to exemplifying purposes acrylic acid and methacrylic acid, methyl acrylate, eth­ yl acrylate, and corresponding methacrylates, acrylonitrile and methacrylonitrile can be mentioned. 0f particular interest results the use of styrene-ac­ rylonitrile and styrene-acrylonitrile-α-methylstyrene, in particular in view of the subsequent use of graft polymer for the preparation of ABS resins. In the just mentioned case, the monomers are used in weight ratios, styrene/acrylonitrile, ranging from 60/40 to 80/20, preferably of 70/30. The reaction is carried out at a temperature comprised within the range of from 50 to 80°C, in the presence of an initiator selected among water-soluble initiators, tradi­ tionally efficacious in emulsion polymerization, of per­ oxide type: potassium persulphate or ammonium persulphate, organic peroxides, hydroperoxides and peracids. The amount of initiator must be comprised within the range of from 0.001 to 3.0%, and preferably of from 0.005% to 1.0% of the reactive compounds, by wieght particularly indicated has resulted to be use of complex redox system Fe 2+ _EDTA/cumene hydroperoxide. Of course, any other catalyst known for this type of reaction can be used to accomplish the grafting-polymer­ ization reaction according to the present invention. Chain-transfer agents are used to adjust graft copol ymer's molecular weight, and to control polymer fluidity. The most commonly used are straight or branched mercaptans, thioethers, α-methylstyrene dimers, certain olefinic com­ pounds and terpenic derivatives. The amount of chain-transfer agent must be comprised within the range of from 0.001 to 1.0.% by weight. When the partial conversion of monomers supplied has reached the level of 70-90% by weight, the addition of the residual amount of monomodal polybutadiene latex is start­ ed (up to 20% of the total to be used at maximum). Such polydutadiene can have a granulometry with mode centre different from the one initially charged, but al­ ways with distribution monomodal and comprised within the range as defined. Together with polybutadiene, additional amounts of the same monomers as used in the first step are added. The reaction temperature, the operating modalities, the emulsifier agents and the catalysts are the same as provided for the carrying out of the first step. When the conversion exceeds the 94-96% level, the reaction is stopped by means of the addition of a suit­ able inhibitor (e.g., hydroxylamine sulphate). The process mentioned, being the object of the pres­ ent invention, allows graft copolymers containing a poly­ butadiene amount comprised within the range of from 10 to 70%, preferably of from 40 to 60%, to be obtained. All what previously disclosed and other operating mo­ dalities shall result more evident from the reading of the following illustrative examples, which are not how­ ever be intended as limitative of the invention. Example 1 Into a glass reaction vessel of 5 l in capacity, 40% by weighl of polybutadiene latex with a concentration of 34.8% and whose characteristics are reported in Table 1; and 3.5% (referred to dry polybutadiene) of a surfactant of alk­ yl-sulphonate type in solution at 23% are added. Reactor's temperature in increased to T = 70°C under stirring (rpm = 250) and under a slight nitrogen atmos­ phere. ­ The 1st step is then started, with the continuous feeding of the following emulsion of monomers: The continuous feeding of the monomer mixture is carried out by means of a metering pump, which is so adjusted as to supply the emulsion over 2h30 min. Contemporaneously to the monomer emulsion the initiator is separately supplied, over a 3h45 min time, composed by the following redox system: At the end of monomer emulsion feeding, when the partial conversion is of about 70-80% by weight, the 2nd step be­ gins. Over a time of 5 min., 10% by wieght of polybutadiened having the same characteristics as that used in the 1st step are added, and then sequentially the following monomer emulsion: This emulsion is added over a 10 min time by a metering pump. At the end of the catalytic system feeding, the reaction mass is kept standing at T = 70°C over 1h. This allows achieving a conversion of 94-96% with an end yield of dry matter of 37%. At the end of the reaction, a reaction inhibitor (0.08 pphm of hydroxylamine sulphate) is added. The residual unreacted monomers are removed by steam stream extraction at T = 100°C over 1h30 min. Graft polybutadiene so obtained is coagulated with a solution of CaCl₂ at 35%, separated, washed and dried at T = 70-80°C in fluid-bed drier. Dried material is thed blended in a Banbury-type blend er with a styrene-acrylonitrile copolymer with a ST/ACN ratio = 71/29, respectively in the ratios: In Table N° 2 the physical-mechanical properties are evidenced of the products obtained with the ratios as men­ tioned. (COMPARATIVE) EXAMPLE-2 (Monoclonal PB, and single step) Into a glass reaction vessel of 5 l of capacity, 50 pphm of a polybutadiene latex, whose characteristics are reported in Table N° 1, and 3.5 pp (referred on polybu­ tadiene dry weight) of a surfactant of alkyl-sulphonate type in solution at 23% are added. Reactor's temperature is increased to T = 70°C, under stirring,/(250 RPM) and under a slight nitrogen atmosphere. When temperature mentioned is reached, the continuous feeding over a 2h45′ time begins of following monomer e­ mulsion: Contemporaneously to the monomer feed, the initiator, com­ posed by the following redox system: is separately supplied, in continuous, over a 3h45 min time. At the end of catalytic system feeding, the reaction mass is kept standing at T = 70°C over one hour. This al­ lous a conversion of 94-96%, with and end dry matter pro­ duction of 37%, to be reached. At the end of the reaction, a reaction inhibitor (0.08 pphm of hydroxylamine) is added. The residual unreacted monomers are removed by steam stream extraction at T = 100°C over 1h30 min. Graft polybutadiene so obtained is coagulated with a CaCl₂ solution at 35%, separated, washed and dried at T = 70-80°C in fluid bed drier. Dried matter is then blended in a Banbury-type blend­ er with a styrene-acrylonitrile copolymer, having a St/ACN ratio = 71/29, respectively in the ratios: In Table N° 3 the physical-mechanical properties of obtained products are reported. (COMPARATIVE) EXAMPLE-3 (Bimodal PB and one step only) The formulation and operating modalities are the same as those reported in comparative Example N° 3, with the ex­ ception that polybutadiene latex has a bimodal granulo­ metric distribution, with mode centres respectively at 0,15 µm and 0,4 µm (1500 A ° and 4000 A °), and hence with an overall particle dis­ distribution within the range of from (0,08 µm to 0,7 µm)(800 to 7000 A °.) The blending ratios with styrene-acrylonitrile are the same as shown in comparative Example N° 3. In Table N° 4 the physical-mechanical properties of products obtained are evidenced,

Claims: 1.Process for grafting a vinylaromatic monomer and an acrylic monomer on polybutadiene by emulsion polymer­ izing, in the presence of a monomodal polybutadiene latex having a particle diameter comprised between 0,08 µm and 0,25 µm, to which a stabilizer-emulsifier has been added, a mixture of a vinylaromatic monomer and an acrylic monomer, adding a further portion of monomodal polybutadiene and a further amount of said vinylaromatic and said acrylic monomer when the reaction conversion is between 70% and 90% of the supplied monomers, in the presence of from 0,001% to 3% by weight, relative to the reactive compounds, of a water-soluble initiator, and completing the emulsion polymerization, characterized in that said water­ soluble initiator is activated by a reducing agent selected from among sodium bisulphite and salts or complexes of iron and low-valence transition metals. 2. Process according to claim 1, wherein said stabili­ zer-emulsifier is selected from among fatty acid soaps, alkyl sulphonates, aryl-alkyl sulphonates, and salts of resin acids. 3. Process according to claim 2, wherein said stabili­ zer-emulsifier is selected from among the potassium alkyl sulphonates and the sodium salts of dispropor­ tionated resin acids. 4. Process according to claim 1, wherein the vinyl­ aromatic monomer is selected from among styrene and alphamethylstyrene. 5. Process according to claim 1, wherein the acrylic monomer is selected from among acrylic acid, methacry­ lic acid, methyl acrylate, methyl methacrylate, ethyl acrylate, ethyl methacrylate, acrylonitrile and metha­ crylonitrile. 6. Process according to claim 1, wherein the monomers to be grafted on polybutadiene are selected from among styrene-acrylonitrile, and styrene-acrylonitrile­ alphamethylstyrene. 7. Process according to claim 1, wherein the reaction temperature is comprised between 50°C and 80°C. 8. Process according to claim 1, wherein said water- soluble initiator is selected from among organic per­ oxides, potassium persulphate, ammonium persulphate, hydroperoxides and peroxyacids. 9. Process according to claim 1, wherein the initiator- activator system consists of Fe 2+ _ EDTA/cumene hydroperoxide. 10. Process according to claim 1, wherein the initia­ tor is used in an amount comprised between 0,005% and 1,0% by weight relative to the reactive compounds. 11. Process according to claim 1, wherein a chain­ transfer agent, selected from among mercaptans, thio­ ethers and alphamethylstyrene dimers, is additionally used in an amount between 0,001% and 1% of the weight of the reactive compounds. 12. Process according to claim 1, wherein said low-valence transition metal is selected from among Co, Ni and Cu.

Inventors: FERRARESI, ANDREA, GRISELLI, PIERLUIGI
Applicants: ENICHEM TECNORESINE S.P.A.
Application year: 1985
Main classification: C08F279
IPC codes: C08F2, C08F279