More specifically, the present invention relates to block vinylaromatic copolymers modified with (meth) acry- lonitrile and their use in a mixture with vinylaromatic co- polymers grafted on rubber.

Vinylaromatic copolymers modified with monomers deriv- ing from (meth) acrylic acid, such as random styrene- acrylonitrile copolymers, commonly called SAN, are known products and are industrially prepared by the radicalic po- lymerization of the two monomers by means of a process in emulsion, suspension or mass.

In the case of SAN, styrene and acrylonitrile (AN) form an azeotropic system containing 76% by weight of sty-

rene, but in spite of this the final composition of copoly- mers can be changed by varying the composition of the mono- mers in the feeding mixture. The properties of SAN copoly- mers are strictly linked to their composition and more spe- cifically to their content of AN. The hardness of the co- polymer and viscosity of the molten product, for example, asymptotically increase with an increase in the acryloni- trile content. Other properties, such as barrier proper- ties, chemical resistance and resistance to W-rays, also increase with an increase in the acrylonitrile content. In particular, it is possible to improve the chemical resis- tance of SAN copolymers by increasing the content of acry- lonitrile in the copolymer even up to 40%, but this jeop- ardizes the mechanical properties of the product. In order to obtain a material which is still processable, in fact, due to the high percentage of acrylonitrile, the copolymer must have a rather low molecular weight.

It is possible to enhance the mechanical properties, such as impact strength, of SAN copolymers, both tradi- tional and with a high acrylonitrile content. The impact strength of SAN copolymers is generally improved by mixing with small quantities of elastomers, for example polybuta- diene or butadiene-styrene rubbers, in this way however one of the main characteristics of SAN copolymers, i. e. the surface gloss, is reduced.

A system for obtaining materials containing a vi- nylaromatic monomer, such as styrene, and acrylonitrile with the typical SAN properties, such as surface gloss and resistance to solvents, without losing the remaining me- chanical properties, consists in mixing a random styrene- acrylonitrile copolymer, containing from 20 to 40% by weight of acrylonitrile, with a styrene-acrylonitrile co- polymer grafted on rubber, for example polybutadiene, in which the content of rubber can reach up to 65% by weight.

The product obtained, commonly called ABS (acrylonitrile- butadiene-styrene), has extremely high morphological, chemical and mechanical characteristics.

The Applicant has now found that it is possible to synthesize SAN copolymers, also with high acrylonitrile contents, having an enhanced impact strength and higher chemical resistance with respect to traditional SAN copoly- mers with an equal molar content of the two monomers, by effecting the polymerization of the monomers so as to have products with a structure comprising at least two blocks with a different composition. The copolymers thus obtained are characterized by a high molecular weight, have fluidity of the molten product which makes them easily processable and the remaining mechanical properties, such as tensile strength and flexural strength, substantially equal to those of equivalent polymers produced with the traditional

methods. Furthermore, even though they have an improved im- pact strength, they maintain their gloss which is a typical property of SAN not modified with rubber. If these prod- ucts, moreover, are mixed with vinylaromatic copolymers grafted on rubber, they surprisingly enhance the shock re- sistance properties of the latter without reducing the re- maining mechanical properties.

An object of the present invention therefore relates to vinylaromatic copolymers modified with at least one monomer deriving from (meth) acrylic acid having a block structure which comprises at least two blocks, M and N, in each of the blocks M and N, the concentration of the mono- mer deriving from (meth) acrylic acid ranges from 0.5 to 85% by weight, on the condition that between two adjacent blocks there is a difference of said concentration of at least 8 points and that the total concentration of said monomer deriving from (meth) acrylic acid in the block co- polymer ranges from 20 to 60% by weight.

A further object of the present invention relates to polymeric compositions comprising: a) 40-98% by weight, preferably 50-85% of a vinylaromatic copolymer modified with at least one monomer deriving from (meth) acrylic acid having a block structure which comprises at least two blocks, M and N, in each of the blocks M and N, the concentration of monomer deriving

from (meth) acrylic acid ranges from 0.5 to 85% by weight, on the condition that between two adjacent blocks there is a difference of said concentration of at least 8 points and that the total concentration of said monomer deriving from (meth) acrylic acid in the block copolymer ranges from 20 to 60% by weight; b) 2-60% by weight, preferably 15-50% of a vinylaromatic copolymer grafted on an elastomer.

According to the present invention, the blocks of the block copolymer are generally limited to two but they can also be of a higher number, for example from 3 to 6, pref- erably 4 or 5, provided that the condition be maintained that between two adjacent blocks there is a difference of concentration of the monomer deriving from (meth) acrylic acid equal to or higher than 8 percentage points, for exam- ple a difference ranging from 8 to 95, preferably from 8 to 50, percentage points. The blocks of the block copolymer have a molecular weight which is such that the weight of two adjacent blocks M and N gives a M/N ratio ranging from 35/65 to 65/35, and is preferably 50/50.

The term"vinylaromatic monomer", as used in the pres- ent description and claims, essentially refers to a product which corresponds to the following general formula:

wherein R is a hydrogen or a methyl radical, n is zero or an integer ranging from 1 to 5 and Y is a halogen, such as chlorine, or an alkyl or alkoxyl radical having from 1 to 4 carbon atoms.

Examples of vinylaromatic monomers having the general formula defined above are: styrene, a-methylstyrene, meth- ylstyrene, ethylstyrene, butylstyrene, dimethylstyrene, mono-, di-, tri-, tetra-and penta-chlorostyrene, bromo- styrene, methoxy-styrene, acetoxy-styrene, etc. Preferred vinylaromatic monomers are styrene and a-methylstyrene.

The term"deriving from (meth) acrylic acid"as used in the present description and claims, refers essentially to esters, amides and nitriles of (meth) acrylic acid such as methylacrylate, ethylacrylate, isopropylacrylate, bu- tylacrylate, methylmethacrylate, ethylmethacrylate, isopro- pylmethacrylate, butylmethacrylate, acrylamide, methacrylamide, acrylonitrile, methacrylonitrile. Preferred derivatives of (meth) acrylic acid according to the present invention are acrylonitrile and methacrylonitrile.

Any polymerization method can be used for preparing the block copolymers object of the present invention, even

if the so-called live radicalic polymerization method is preferred, of which a detailed description is available in "Macromolecules", 1993,26, 2987 or in"Journal of American Chemistry Society"1994, 11185.

More specifically, the block copolymers object of the present invention can be prepared starting from vinylaro- matic and (meth) acrylic monomers and from initiator systems according to the following general procedure. Groups of initiator systems suitable for this type of process are se- lected from those listed below wherein the term alkyl indi- cates a Cl-C4 (iso) alkyl hydrocarbon: tetra-alkylthiouram disulfides, alkyldithiocarbamates, ethers of benzopinacol, esters of benzopinacol, dicyano-substituted tetraphenyl ethane, triphenylmethylazo alkyls, benzylnitroxyl deriva- tives, mixtures between nitroxyl radicals and radical gen- erator compounds such as, for example, mixtures of nitroxyl radicals and peroxides, mixtures of nitroxyl radicals and hydroperoxides, mixtures of nitroxyl radicals and per- esters, mixtures of nitroxyl radicals and percarbonates, mixtures of nitroxyl radicals and azobisdialkyldinitriles, mixtures of nitroxyl radicals and tetra-alkylthio- uramdisulfides; alkoxyamines described in U. S. patent 4,581, 429; alkylnitroxyls.

Specific and particularly preferred examples of ini- tiator systems are the following: 4-hydroxy-2,2, 6,6-

tetramethylpiperidinyloxy radical/benzoyl peroxide in a mo- lar ratio of less than 4; 2,2, 6,6-tetramethyl- pyrrolidinyloxy radical/dibenzoyl peroxide in a molar ratio of less than 4, preferably from 1/1 to 3/1 ; 2,2, 6,6- tetramethylpiperidinyloxy radical/dicumyl peroxide in a mo- lar ratio of less than 4, preferably from 1/1 to 3/1 ; 2,2, 6,6-tetramethylpiperidinyloxy radical/N, N'-azobis- (diisobutyronitrile) in a molar ratio of less than 4, pref- erably from 1/1 to 3/1; 1-phenyl-l- (2, 2,6, 6-tetramethyl- piperidinyloxy) ethane; 2-benzoyl-l-phenyl-1-(2, 2,6, 6- tetramethylpiperidinyloxy) ethane, etc.

Once the polymerization of the first polymeric block is complete, the polymerization process of the second block continues by feeding the respective mixture of monomers in the presence of the same initiator system.

The distribution of the monomers inside each block can be of the random type.

.... AASASASSASSSAASASASSAAAASSAS.... wherein"A"represents a (meth) acrylic unit and"S"a vi- nylaromatic unit, or of the alternating type, .... ASASASASASASAS.... or in blocks, .... AAAASSSSSAAAASSS.... or of the gradient type. The situation in which the distri- bution is random or alternating or an intermediate case be-

tween these, is preferable.

The polymerization of the monomers can be carried out in continuous, batchwise or in semi-continuous, at a tem- perature ranging from 60 to 160°C, depending on the initia- tor selected, and at such a pressure as to maintain the monomers in liquid phase. Furthermore, the polymerization can take place in the presence of an organic solvent, such as ethylbenzene, in suspension or in mass.

In a batch or semi-continuous process, the initiator is added to the monomers in pure form or in the form of a solution or suspension in a quantity ranging from 0.01 to 2% in moles with respect to the total moles of monomers.

In a continuous process, the monomers, initiator and, optionally, the solvent are fed in continuous to a polym- erization reactor at a flow-rate which is such as to pro- vide residence times suitable for reaching conversions preferably ranging from 5 to 95% for each block.

Optionally, but not necessarily, the initiator can be added slowly during the whole duration of the reaction or part of it. Also one of the two monomers, or both of them, can be dosed in portions in subsequent times so as to form the microstructure of each block. At the end, the polymer is isolated from the polymerization mixture with one of the methods known in the art, for example by precipitation in a suitable non-solvent, or by the removal of the non-reacted

monomers under vacuum and/or at a high temperature.

At the end of the polymerization, the end copolymer has a number average molecular weight ranging from 20,000 to 500,000, a content of monomer deriving from (meth) acrylic acid of up to 60% by weight, preferably from 25 to 55%, a Melt Flow Rate at 220°C (MFR) ranging from 0.1 to 150 and a shock resistance property measured according to Charpy ISO 179 I and U ranging from 10 to 40 kj/m2.

The block copolymer object of the present invention can be used as such to product end-articles by means of forming technologies known in the art, such as extrusion, blow-moulding, the thermoforming of pre-formed sheets, or it can be used in a mixture with other thermoplastic poly- mers and copolymers, such as polystyrenes, high impact polystyrenes, polycarbonates, polymethylmethacrylate, poly- phenylethers, polybutadiene, acrylic elastomers, vinylaro- matic elastomers, etc. More specifically, blending with vi- nylaromatic copolymers grafted on an elastomeric core to produce high impact materials of which the most significant examples are ABS (Acrylonitrile-Butadiene-Styrene) resins, is preferred.

In grafts for ABS, whose preparation is described, for example, in U. S. patents 2,820, 773 or 3,238, 275, the elas- tomer is present in a quantity ranging from 20 to 65% by weight, the complement to 100 consisting of a conventional

grafted vinylaromatic copolymer, obtained from one or more monomers having general formula (I) and acrylonitrile. The elastomer generally consists of polybutadiene or (co) polymers of butadiene with other monomers, such as sty- rene, or of isoprene.

The block copolymers object of the present invention can also be mixed with other copolymers grafted on elasto- mers such as AES (Acrylonitrile-EP (D) M rubber-Styrene) res- ins. In these grafted copolymers, the content of elastomer ranges from 5 to 50% by weight, preferably from 10 to 35%, the complement to 100 consisting of a traditional vinylaro- matic copolymer of the type mentioned above for ABS grafts.

The EP (D) M rubber can be an ethylene-propylene rubber, in which the ethylene/propylene ratio ranges from 90/10 to 20/80, or an ethylene-propylene-non-conjugated diene rub- ber, with a content of non-conjugated diene ranging from 4 to 50% by weight. Examples of EPDM rubbers which can be used in the preparation of AES resins are ethylene/pro- pylene/5-methyl-tetrahydroindene, ethylene/propylene/6- ethylidene-2-norbornene, ethylene/propylene/6-methylene-2- norbornene and ethylene/propylene/5-ethylidene-2-norbornene terpolymers.

AES resins are products which are known in scientific literature and their preparation is described, for example, in U. S. patents 4,202, 948 or 4,166, 081 or in European pat-

ent 286,071.

Other examples of vinylaromatic copolymers grafted on an elastomeric core consist of MBS resins (methyl- methacrylate-butadiene-styrene) whose preparation is de- scribed, for example, in U. S. patents 2,943, 074 or 3,657, 391 and ASA resins (acrylonitrile-styrene-acrylate) available on the market under the trade-name of TERLURAN of BASF.

An illustrative but non-limiting example is provided hereunder for a better understanding of the present inven- tion and for its embodiment.

EXAMPLE 1 A SAN (Styrene-Acrylonitrile) copolymer with the for- mulation and according to the procedures described below (Table 1), was synthesized in a 100 litre jacketed steel autoclave, equipped with a Pfaudler stirrer and breakwater.

Table 1 Styrene kg 17.5 Acrylonitrile kg 12.6 Demineralized water kg 30 Benzoyl peroxide (with 25% water) g 66. 9 4OH - TEMPO g 46.4 Etahpol 1000 (acrylic acid -2-ethylhexylacrylate copolymer) g 10.3 Sodium sulfate 61.5 15.5 kg of styrene were charged into the autoclave

with stirring at 50 revs/minute in a nitrogen atmosphere, at room temperature. Benzoyl peroxide dissolved at room temperature in 1 kg of styrene was then added, together with 40H-TEMPO (4-hydroxy-2,2, 6,6-tetramethylpiperidinyloxy radical) dissolved at room temperature in 1 kg of styrene.

The autoclave was closed, adding 5.6 kg of acryloni- trile and pressurizing at 0.5 bars. The stirring was con- tinued at 80 revs/minute and the temperature was raised to 125°C in 45' (at 123°C the hour"0"is measured). After reaching 125°C, the pressure was brought to 3 bars with ni- trogen.

After 1 hour and 45 minutes from hour"0", 26 litres of water are added, the temperature remaining at 125°C. The final pressure is 4.5 bars whereas the stirring is 150 revs/minute.

After 2 hours and 45'from hour"0", acrylonitrile ac- cording to the formulation, is added. The final pressure is 7 bars. Immediately after adding the acrylonitrile, the suspending agent Ethapol 1000 dissolved at high temperature in 1 litre of demineralized water, is added, the container and adding line being washed with 1 litre of demineralized water, and sodium sulfate dissolved in 1 litre of deminer- alized water is subsequently added, the container and add- ing line also in this case being washed with 1 litre of de- mineralized water.

After 4 hours and 30 minutes from hour"0", the tem- perature was increased to 135°C in 20 minutes and the pres- sure to 8.5 bars.

After 11 hours and 30 minutes from hour"0", the mix- ture is cooled and discharged onto a sieve. The beads are washed on the sieve until the water is limpid, the beads are then centrifuged and dried with a stream of air at 80°C.

When the beads are dry, they are extruded at 220°C un- der vacuum to reduce the content of residual monomers. A block polymer A and B is obtained with a content of acrylo- nitrile in block A of 24% by weight and in block B of 45% by weight. After characterization, the copolymer AB had the physico-chemical characteristics indicated in Table 2.

Table 2 Residual styrene monomer weight % 0.024 Residual acrylonitrile monomer weight % 0.35 Acrylonitrile in the polymer weight % 35 Weight average molecular weight Dalton 86000 Polydispersity (Mw/Mn)-1. 78 Melt Flow Rate 220°C, 10 kg g/10'4. 5 Upon comparison with a commercial SAN (LURAN 388S of BASF) with an equal average content of acrylonitrile and equal fluidity, the copolymer of this example has a higher impact strength and chemical resistance and equivalent ten- sile strength and flexural strength (Table 3).

Table 3

EXAMPLE LURAN 388S MRF 220°, 10 kg (g/10') 4.5 4.5 Charpy ISO I and U (kj/m2) 25.1 17.4 ISO 527 Tensile strength Maximum stress (MPa) 73.6 77.6 Ultimate elongation (%) 2.59 3.60 Elastic modulus (MPa) 3770 3610 ISO 178 Flexural strength Maximum stress (MPa) 120.8 126.6 Ultimate arrow (%) 3.71 3.80 Elastic modulus (MPa) 3800 3715 An ISO 527 tensile test sample of the copolymer of the example, immersed in acetone for 10'remains unaltered whereas an equal LURAN 388S test sample immersed in acetone has significant alterations mainly in correspondence with edges and stress areas.

EXAMPLE 2 The copolymer of Example 1 and commercial SAN were evaluated in a mixture with a commercial ABS Graft produced by EniChem S. p. A. called KGM 55, obtained from an emulsion process and having a polybutadiene content of 53% by weight

approximately, with 80% of polybutadiene particles having a diameter equal to about 0.18 pm and and 20% diameter equal to about 0. 36 pin.

Mixtures (1-4) with the materials already character- ized as such, according to the following formulation (Table 4), were prepared in a twin-screw extruder with two dis- tributors in continuous.

Table 4 1234 SAN EXAMPLE 1 (%) 81.2 68.0 LURAN 388S (%) 81. 2 68. 0 ABS Graft KGM 55 (%) 18. 8 18.8 32.0 32.0 Irganox 1076 (phr) 0.1 0.1 0.1 0.1 Ethylenebisstearylamide wax (phr) 1.5 1.5 1. 5 1.5 Also in a mixture with ABS, the copolymer of Example 1 shows excellent impact strength properties (Table 5).

Table 5 1 2 3 4 Charpy ISO 179 1 and A (kj/m2) 8.4 3.7 20. 8 17.4 ISO 527 tensile strength Yield point (MPa) 57.1 60.5 46.9 49.9 Yield elongation (%) 2.7 2.7 2.6 2.8 Breaking load (MPa) 42.1 43.5 34.6 36.0 Ultimate elongation (%) 14.0 9.0 22.3 14.4 Elastic modulus (MPa) 2930 2920 2390 2430 ISO 178 Flexural strength Maximum stress (MPa) 87.0 90.3 70.5 73.0 Elastic modulus (MPa) 3000 2990 2430 2450