PROCESS FOR THE PREPARATION OF POLYBUTADIENE GRAFTED WITH STYRENE AND ACRYLONITRILE AND HAVING A VERY LOW END CONTENT OF UNREACTED RESIDUAL MONOMERS

Abstract: During the reaction of grafting of styrene and acry­ lonitrile to polybutadiene, the conversion of the graft monomers is increased from the values of 92 : 94% usually obtained, to values very close to 98%, by carrying out a redox-type reaction additional to that provided by the process, in the presence of an additional aliquot of acrylonitrile and of catalytic system.

Description: The present invention relates to a process for the preparation of a graft polymer by graft copolymerization of a mixture of acrylonitrile and styrene on polybutadi­ ene, with high conversion of the graft monomers, such as to allow the step of stripping of the unreacted amount of said graft monomers to be eliminated, at the same time leaving to the end product, unchanged, its physical-­ mechanical properties, and its characteristics of appear­ ance, suitable to the subsequent applications of the product, for example for forming blends with styrene­ acrylonitrile copolymers in the production of ABS resins. More particularly, the present invention relates to a process for the preparation of polybutadiene grafted with a mixture of styrene and acrylonitrile, having a very low content of unreacted residual monomers, process which essentially provides the carrying out of a redox-­ type reaction additional to that provided by the conven­ tional process, in the presence of an additional aliquot of acrylonitrile and catalytic system. Traditionally, in the reaction of grafting of the mixture of styrene and acrylonitrile to polybutadiene, the end conversion of the graft monomers reaches values not higher than 94%. It is known that the presence of not polymerized monomers has a negative impact on the thermal properties of the copolymers obtained and on the appear­ ance of the subsequent manufactured articles. Moreover, the not-polymerized monomers tend to be released during the following steps of product processing such as, e.g., the coagulation, the filtering, the high-temperature blending, or the moulding. Representative of the prior art is US-A-3 991 136 which discloses, in a modified process for preparing acrylonitrile-butadiene-styrene (ABS) polyblends by polymerizing a mixture of a polymerizable monomer and a diene rubber,thus forming a grafted copolymer and a matrix interpolymer, the improvement comprising the steps of polymerizing the monomer formulation to at least about 90% conversion, charging a third monomer, and further polymerizing said monomer formulation in the presence of the third monomer to convert essentially all the monomer formulation to interpolymer, whereafter the polyblend, which now has a low residual monomer content, is recovered. The possibility is known as well, from U.S. Patent Nr. 4.404.338 of lowering the content of residual end monomers during the proceed­ ing of a process for the copolymerization of vinylaromat­ ic monomers with ethylenically unsaturated nitriles. Such patent application relates indeed to a process for the preparation of vinyl-aromatic copolymers with ethylenically unsaturated nitriles, having a very low degree of unreacted residual monomers, by polymerization in aqueous dispersion, characterized in that, in the end step of the polymerization process, to the aqueous dispersion, also containing a compound able to release free radicals, a certain aliquot is added of a comonomer particularly reactive towards the unreacted monomers. This contempora­ neous presence allows the amount of unreacted monomers to be drastically reduced to negligible values, without altering the general characteristics of the end polymer. But, of course, one skilled in the art could expect that in a copolymerization reaction, the addition of one of the comonomers (and the above cited application lets one think to a kind of equivalence between the use of one of comonomers and a third comonomer highly reactive to­ wards the residual monomers) would cause a decrease of the unreacted residual monomers, due to the continuing of the same copolymerization reaction, so much that the Applicant suggests that, during the progress of the proc­ ess in question, no side crosslinking reactions between the monomers and the polymer occur. It was on the contrary absolutely not foreseeable, nor the teaching of the above cited patent application can allow it to be realized as obvious, that it could be possible to reduce to a considerable extent the amount of residual unreacted monomers in the graft copolymerization of a mixture of acrylonitrile and styrene to a polybuta­ diene matrix, without at the same time reducing the yields of grafting to the same matrix. In fact,it has been surprisingly found, and is the object of the present invention, that during the reaction of grafting of styrene and acrylonitrile to polybutadi­ ene, it is possible to increase the conversion of the graft monomers up to values very close to 98%, and high­ er, by a process according to which the carrying out is provided of a redox-type reaction additional to that provided by the processes of the prior art, in the pres­ ence of an additional aliquot of acrylonitrile and of catalytic system. It is known that the reaction of grafting of styrene and acrylonitrile to polybutadiene is carried out by polymerizing the monomers, namely styrene and acryloni­ trile,in an aqueous polybutabiene latex, in the follow­ ing percentages: from 10% to 90% by weight of acrylonitrile-styrene mixture containing from 50% to 90% by weight of sty­ rene; from 90% to 10% by weight of polybutadiene; in the presence of both water-soluble and oil-soluble catalysts and emulsifiers. Examples of oil-soluble catalysts are the organic peroxides or hydroperoxides, such as di-tert-butyl-perox­ ide, benzoyl-peroxide, lauroyl-peroxide, di-cumyl-perox­ ide, tert-butyl-peroxide, tert-butyl-hydroperoxide, cum­ ene-hydroperoxide, di-isopropylbenzene-hydroperoxide, methyl-cyclohexyl-hydroperoxide. Examples of water-soluble catalysts are the persalts of potassium, sodium, ammonium, such as persulphates, perborates, peracetates, percarbonates, alkali metal peroxides and hydrogen peroxide, either alone, or acti­ vated by reducing agents. The said catalysts are activated by a reducing a­ gent, so as to form a redox system. Polymerization acti­ vators are compounds of low-valence sulphur, such as so­ dium formaldehyde-sulphoxylate, sodium bisulphite or pyrosulphite, dextrose, organic bases and salts or com­ plexes of ions of iron or of such low-valence transition metals as Co, Ni, Cu; above all, the system Fe-EDTA/Rodi­ te (Rodite = sodium-formaldehyde sulphoxylate) is prefer­ red. The above mentioned oil-soluble catalyst is used in amounts comprised within the range of from 0.05 to 0.50% by weight relatively to the mixtures of monomers, and the activator is added in percentages ranging from 0.03 to 0.3% by weight, always relatively to the weight of mono­ mers. The reaction is carried out at a temperature com­ prised within the range of from 60 to 95°C, generally at 70°C in the presence of an emulsifier agent selected from alkali-metal or ammonium salts of fatty acids, such as lauric acid, oleic acid, and stearic acid; disproportion­ ated resin acids, or sulphonated fatty acids, alkyl­ aryl-sulphonic acids or esters, esters of sulphuric acid or of phosphoric acid, in particular sodium or potassium alkyl-sulphonates in synergistic mixture with a dispers­ ant of the type of condensed naphthalene-sulphonates. The invention, therefore, provides a Process for the preparation of polybutadiene grafted with polystyrene and acrylonitrile, having a very low end content of unreacted residual monomers, consisting in carrying out a graft copolymerization of a mixture of styrene and acrylonitrile to polybutadiene in the presence of a catalyst selected among cumene hydroper­ oxide, sodium persulphate, potassium persulphate, ammonium persulphate, di-isopropyl-benzene hydroperox­ ide, tert.butyl-peroxide, and of a catalytic activator constitued by dextrose, or by organic bases, salts or complexes of iron ions, such as, e.g., Fe-EDTA, char­ acterized in that, in the end step of said reaction, when the conversion of the said monomers has reached the value of at least 75% by weight preferably about 94% by weight, an additional aliquot of acrylonitrile in a percentage comprised within the range of from 0.1 to 3% by weight preferably of from 1.3-1.5% by weight, as referred to the monomers fed, an additional amount of polymerization catalyst in a percentage comprised within the range of from 0.02 to 0.4% by weight preferably of from 0.03-0.05% by weight, as referred to the monomers fed, and an additional ali­ quot of polymerization activator in a percentage com­ prised within the range of from 0.02 to 0.4% by weight preferably of from 0.04-0.06% by weight, as referred to the monomers fed, are added. In the end step of the graft copolymerization, which corresponds to the innovating aspect according to the present invention, the temperature is maintained at high­ er values than those of the initial step, it being howev­ er generally comprised within the range of from 80 to 95°C. The end conversion of the graft monomers reaches values higher than 96% and, in some cases, close to or higher than 98%. Example 1 GRAFTING BY TRADITIONAL TECHNOLOGY (PB/SAN = 52.4/47.6) PB = polybutadiene; SAN = styrene-acrylonitrile 8691 g of latex at 35% by weight of polybutadiene having particle size comprised within the range of from (60 to 800 nm) is charged in a reactor and reacted with a mixture formed by 2001 g of styrene and 760 g of acrylo­ nitrile. The redox-type catalyst is constituted by 5.8 g of cumene hydroperoxide, 4.4 g of catalytic activator (Fe-­ EDTA) and 8.7 g of Rodite. The emulsion is stabilized with 160 g of surfactants constituted by sodium alkanesulphonates and/or sodium and potassium salts of disproportionated resin acids. The reaction temperature is of 70°C. The feed of the graft monomers is carried out in continuously during a time of 3 hours and 45 minutes. The catalytic system is fed over a time of 4 hours and 45 minutes, at the end of which time period, a "stay" step is carried out at the temperature of 70°C, for a time of 1 hour. Altogether, the total graft reaction time is of 5 hours and 45 minutes. In Table 1 the content of residual monomers, as determined by gas-chromatography, in the grafted polybutadiene latex is reported. In Tables 2 and 3 the chemical-physical and mechani­ cal characteristics, measured on ABS resins obtained by the addition of different levels of grafted polybutadiene are reported. Example 1 bis GRAFTING BY HIGH-CONVERSION (SUPER-REDOX) METHODOLOGY Example 1 is repeated with the same initial ali­ quots of monomer and catalytic system, by introducing the following operative and formulation variants: a. The feed of graft monomers (styrene and acrylonitrile is always carried out over a time of 3 hours and 45 minutes. b. the catalytic system is fed over 4 hours and 15 min­ utes; c. three hours later than the beginning of the feed of graft monomers, an additional aliquot (38.6 g) of acrylonitrile, corresponding to 1.4% by weight relati­ vely to the total charged monomers, is fed as one single portion; d. at the end of the feed of the catalytic system (CHP, Fe-EDTA, Rodite), i.e., after 4 hours and 15 minutes from the beginning of the graft reaction, an addition­ al aliquot of catalytic system is fed, constituted by: Rodite 2,9 g Fe-EDTA 1.47 g CHP 1.94 g. Such feed is carried out in continuous for a time of 45 minutes; e. additional stay of 15 minutes of the grafted polymer at the end of the feed of catalytic system; f. the reaction temperature is of 80°C; g. the stabilization of the latex is carried out with one single anionic surfactant, such as a sodium alkane­ sulphonate obtained by the chlorosulphonation of n-­ paraffins. The end content of residual monomers is reported in Table 1.Tables 2 and 3 report the characteristics meas­ ured on related ABS resins. Example 2 (PB/SAN = 40/60) 6631 g of latex at 35% by weight of polybutadiene having particle size comprised within the range of from (60 to 800 nm) is charged in a reactor and reacted with a mixture formed by 2523 g of monomer styrene and 957 g of monomer acrylonitrile. The redox-type catalyst is constituted by 5.8 g of cumene hydroperoxide, 4.4 g of catalytic activator (Fe-­ EDTA) and 8.7 g of Rodite. The emulsion is stabilized with 174 g of surfac­ tants; the reaction temperature is of 70°C. The other operating conditions are the same as described in Example 1. The end content of residual monomers is evidenced in Table 1, and in Tables 2 and 3 the chemical-physical and physical-mechanical characteristics of related ABS resins are reported. Example 2 bis The Example 2 and the "Super-redox" methodology as described in Example 1 bis are followed, additional ali­ quots of 1.4% by weight (48.72 g) of acrylonitrile, and of catalytic system, in the same amount as described in Example 1 bis, being added. The end data are reported in the above mentioned Tables. Example 3 (PB/SAN = 40/60) 9611 g of latex at 35% by weight of polybutadiene having particle size comprised within the range of from (60 to 800 nm) is charged in a reactor and reacted with in mixture formed by 1682 g of styrene and 638 g of acrylo­ nitrile. The redox-type catalyst is constituted by 5.2 g of cumene hydroperoxide, 3.72 g of catalytic activator (Fe-­ EDTA) and 6.96 g of Rodite. The emulsion is stabilized with 127.7 g of surfac­ tants and the reaction temperature is maintained at 70°C. All the other operating conditions are the same as described in Example 1. The end content of residual monomers in polybutadi­ ene is evidenced in Table 1. As above, in Tables 2 and 3 the chemical-physical and physical-mechanical characteristics, measured on related ABS resins, are reported in detail. Example 3 bis The "Super-redox" methodology of Example 1 bis has been followed, and namely using: an additional aliquot of 1.4% by weight (32.48 g) of acrylonitrile, as referred to the graft monomers; additional catalytic system, constituted by 2.32 g of Rodite, 1,25 g of Fe-EDTA and 1.73 g of cumene hydro­ peroxide; temperature of 80°C. The results are reported in the Tables. Example 4 (PB/SAN = 65/35) 8340 g of latex at 35% by weight of polybutadiene having a particle sizze comprised within the range of from (60 to 800 nm) is charged in a reactor and reacted with a mixture formed by 1142 g of monomer styrene and 433 g of monomer acrylonitrile. The redox-type catalyst is constituted by 5.8 g of cumene hydroperoxide, 4.4 g of catalytic activator (Fe-­ EDTA) and 8.7 g of Rodite. The emulsion is stabilized with 116 g of surfactants and the reaction temperature is maintained at 70°C For the other operating parameters, see Example 1; the results are reported in Tables 1, 2 and 3. Example 4 bis The "Super-redox" methodology of Example 1 bis is repeated, with the following particularities: additional aliquot of 22 g of acrylonitrile, equivalent to 1.4% by weight, as referred to the graft monomers; additional aliquot of catalytic system, constituted by 8.7 g of Rodite, 4.4 g of Fe-EDTA and 5.8 g of cumene hydroperoxide; the reaction temperature is always of 80°C. The end results are reported, according to as many times stated in the foregoing, in the above mentioned Tables.

Claims: 1. Process for the preparation of polybutadiene grafted with polystyrene and acrylonitrile, having a very low end content of unreacted residual monomers, consisting in carrying out a graft copolymerization of a mixture of styrene and acrylonitrile to polybutadiene in the presence of a catalyst selected among cumene hydroper­ oxide, sodium persulphate, potassium persulphate, ammonium persulphate, di-isopropyl-benzene hydroperox­ ide, tert.butyl-peroxide, and of a catalytic activator constituted by dextrose, or by organic bases, salts or complexes of iron ions, such as, e.g., Fe-EDTA, char­ acterized in that, in the end step of said reaction, when the conversion of the said monomers has reached the value of at least 75% by weight, an additional aliquot of acrylonitrile in a percentage comprised within the range of from 0.1 to 3% by weight, as referred to the monomers fed, an additional amount of polymerization catalyst in a percentage comprised within the range of from 0.02 to 0.4% by weight as referred to the monomers fed, and an additional ali­ quot of polymerization activator in a percentage com­ prised within the range of from 0.02 to 0,4% by weight as referred to the monomers fed, are added. 2. Process for the preparation of polybutadiene grafted with styrene and acrylonitrile having a very low end content of unreacted residual monomers according to preceding claim 1, characterized in that the addition of said additional aliquots of acrylonitrile and of catalytic system is carried out when the conversion of the graft monomers is preferably equal to or higher than 94%. 3. Process for the preparation of polybutadiene grafted with styrene and acrylonitrile having a very low end content of unreacted residual monomers according to preceding claims 1 and 2, characterized in that the additional aliquots of acrylonitrile and of catalytic system are preferably added in the following percent­ ages: acrylonitrile from 1.3 to 1.5% by weight; catalyst from 0.03 to 0.05% by weight; catalytic activator from 0.04 to 0.06% by weight. 4. Process for the preparation of polybutadiene grafted with styrene and acrylonitrile having a very low end content of unreacted residual monomers according to preceding claims, characterized in that the end step of the graft copolymerization, to which the addition of acrylonitrile and catalytic system corresponds, is carried out at a temperature higher than the initial temperature, and comprised within the range of from 80 to 95°C.

Inventors: CAMPANA, FRANCO, FERRI, GUALTIERO, PIVOTTO, BRUNO
Applicants: ENICHEM TECNORESINE S.P.A.
Application year: 1986
Main classification: C08F279
IPC codes: C08F279