GB2169910A - Preparation of grafted polymers - Google Patents

Abstract

The invention relates to the preparation of polymers of at least one vinyl aromatic monomer grafted onto at least one unsaturated polyolefin elastomer, by polymerization of the said vinyl aromatic monomer in the presence of the said unsaturated polyolefin elastomer, in aqueous suspension and in two stages. The first polymerization stage is carried out at a temperature less than or equal to 130 DEG C in the presence of at least one chain transfer agent and at least one radical initiator R exhibiting high heat stability. In the second stage the polymerization is carried on at a temperature above that of the first stage, after addition of water and of a mixture of at least two radical initiators different from R and selected from tertiary butyl perbenzoate, di-tertiary butyl peroxide and 2,5-dimethyl-2,5-ditertiary butyl-peroxy-3- hexyne.

Description

SPECIFICATION Preparation of grafted polymers The present invention relatestoaprocessforthe prepartion of polymers of at least one vinylaromatic monomergrafted onto atleastone unsaturated polyolefin,to an impact-resistant and ageing-stable composition containing at least one of those polymers and to an article obtained by processing that composition.

Polystyrene is a thermoplastic which, because of the ease with which it can be obtained and processed, is widely used in the plastics industry. Unfortunately its use is restricted by its mediocre impact strength and poor ageing stability.

In orderto improve the properties of polystyrene, and in particular its impact strength, it has been proposed to polymerize styrene by grafting onto saturated polyolefin elastomers (such as ethylenepropylene copolymers) or unsaturated polyolefin elastomers, (which consist, for example, ofterpolymers or ethylene with at least one alpha-olefin having from 3 to 6 carbon atoms and at least one diene). That polymerization reaction by grafting is well known and can easily be carried out by heating a solution ofthe elastomer in styrene, either in a single stage in the absence of diluent (mass polymerization), as is described in East German Patent No. No.158,248, or in two stages.In the latter case, the reaction, after a first mass prepolymerization stage, is completed in aqueous suspension, the final productthus being obtained in the form of beads.

There is also known, from French Patent No.

2,267,333, a process for polymerization by grafting styrene onto an elastomer in the present of a radical initiator and in two stages: (a) the first stage being carring out a temperature of 50onto 1 500C in the present of water using a weight ratio water/oily phase of between 0.3 and 2, in the absence of surfactants and upto a styrene conversion of15to 40%, and (b) the second stage being carried out in aqueous suspension at a temperature of 1 500C to 2000C.

The grafted polystyrenes obtained in accordance with that process exhibit inadequate properties for numerous applications, such as an elongation at break (measured according to standard specification ASTM D 1822-61) of between 7 and 29%, and an impact strength (measured according to standard specification ASTM-D 256) of between 8 and 14J/m. Above all, theirageing stability is mediocre.

Numerous parameters ofthe macromolecular structure of polystyrenes grafted onto unsaturated polyolefin elastomers can influence the simultaneous achievement of satisfactory mechanical properties and good ageing stability. Among these parameters there may in particular be mentioned: -the relative importance of the gel in the final grafted product. By 'gel' here is meantthe fraction of the final grafted product which is insoluble in toluene, this fraction consisting mainly of a copolymer of styrene grafted onto the elastomer and secondly of ungrafted elastomer, the latter being possiblycros- slinkedaswill be seen hereafter, -the polystyrene/elastomerweight ratio (hereafter referred to as "coefficient oc") in the gel, -the comparative importance ofthe crosslinked elastomer present in the gel.Indeed some units constituting the elastomercan, because of the unsaturations which they contain, form chemical bonds between them. These units are then referred to as "bridged", or the elastomer is referred to as crosslinked. The extent of such crosslinking is supposed to be responsible for, especially, the ageing stability ofthe final grafted product. The proportion of crosslinked elastomer gel is indirectly estimated through swelling the gel intoluene, -the distribution of the elastomer particles in the polystyrene gel matrix. Depending on its nature, this distribution may be responsible for poor cohesion of the final grafted product. When said product is subjected to strain, this poor cohesion causes a harmful cavitation at the elastomer-polystyrene interface, resulting in mediocre mechanical properties.

For some parameters mentioned above, the prior art already indicates how they may be modified to improve the mechanical properties ofthe grafted products, in particulartheir impact strength.Thus, French Patent No.2,267,333, which has been already mentioned, recommends a weight ratio of polystyrene/elastomer in the gel of between 1 and 1.8. On the other hand, according to the examples ofthat patent, the gel content in the final grafted product is between 22% and 48%. As already mentioned, this is however insufficient to ensure adequate mechanical properties or satisfactory ageing stability.

The problem which the present invention thus intends to solve consists in preparing polymers by grafting a vinylaromatic monomer onto an unsaturated polyolefin elastomer, in such a manner that the resulting grafted product has mechanical properties, especially elongation at break and impact strength, as well as ageing stability, which are improved compared to those of the grafted polymers of the prior art.

To solve this problem, the applicant has studied in detail the two molecular structure parameters lastmentioned as having a possible effect on the prop erties of such grafted polymers and has thus succeeded in developing a process which is able on the one hand to minimise crosslinking of the unsaturated polyolefin elastomer present in the gel and, on the other hand, to ensure a specific particle size distribution of the elastomer particles in the polystyrene matrix ofthe gel.

The applicant unexpectedly found that the nature of the radical initiator present in the second stage of the polymerization process can have a considerable influenceonthesetwo parametersofthe macro- molecular structure ofthe grafted products and consequently on the properties ofthese products.

More precisely, the applicantfound that optimum properties ofthe grafted products can generally not be achieved by using a single initiator during this second polymerization stage. This surprising discovery has resulted in the process ofthe present invention.

Accordingly, the present invention provides a processforthe preparation of polymers of at least one vinylaromatic monomer grafted onto at least one unsaturated polyolefin elastomer, wherein the said vinylaromatic monomer is polymerised in the presence of the said unsaturated polyolefin elastomer, in aqueous suspension and in two stages, the first polymerization stage being carried out ata temperature less than or equal to 130 C in the presence of at least one chain transfer agent and at least one radical initiatorRexhibiting high heat stability, and in the second stage the polymerization is carried on at a temperature above that ofthe first stage, after addition of water and of a mixture of at least two radical initiators different from Rand selected from tertiary butyl perbenzoate, di-tertiary butyl peroxide and 2,5-di-methyl-2,5-di-tertiary butyl-peroxy-3hexyne.

In this specification, the term vinylaromatic monomer means a compound with at least one aromatic ring and one vinyl radical, such as, in particular, styrene, alpha-methylstyrene, vinyl-naphthalene and vinyltoluene. A preferred unsaturated polyolefin elastomer is a terpolymer of ethylene with at least one alpha-olefin having from 3 to 6carbon atoms and at least one diene. Moreespeciallythere are preferred ethylene-proplylene-diene terpolymers, the diene being selected from linearorcyclic,conju- gated or non-conjugated dienes such as, for example, butadiene, isoprene, 1 3-pentadiene, 1 ,4-pentadiene, 1 ,4-hexadiene, 1 ,5-hexadiene, 1 ,9-decadiene, 5- methylene-2-norbornene, 5-vinyl-2-norbornene, 2- alkyl-2,5-norbornadienes, 5-ethylidene-2-norbornene, 5-(2-propenyl)-2-norbornene, 5-(5-hexenyl)-2- norbornene, 1,5-cyclooctadiene, bicyclo-[2,2,2]-octa2,5-diene, cyclopentadiene,4,7,8,9-tetrahydroindene and isopropylidene-tetrahydroindene. Such elastomericterpolymerswhich can be used in accordance with the present invention in general contain from 15 to 60 mole % of units derived from propylene and from 0.1 to 20 mole % of units derived from the diene.

Generally the unsaturated polyolefin elastomer is used in an amount at most equal to 20% byweight, and preferablyfrom 5 to 15% by weight, with respect to theweight ofthe organic phase (the latter consist ing ofthe mixture of vinylaromatic monomer and unsaturated polyolefin elastomer).

Preferred chain transfer agents which can be introduced during the first polymerization stage ofthe process according to the invention are tertiary dodecylmercaptan, divinylbenzene and triallylcyanurate. Such agents are preferably used in an amount at most equal to 0.2% by weight with respect to the weightofthe organic phase.

Asthe radical initiator R having high heat stability which can be used in the first polymerization stage of the process according to invention there may in particular be mentioned benzoyl peroxide. This initiator is preferably used in an amount at most equal to 0.2% by weight with respectto the weight ofthe organic phase.

As is already known per se, the first polymerization stage ofthe process according to the invention is carried out in aqueous suspension,the weight ratio of waterto organic phase being preferablyfrom 0.3 to 1.5. The vinylaromatic monomer, the unsaturated polyolefin elastomer and water are preferably introduced into an autoclave reactor equipped with a heating device, a stirrer and a cooling device. Thereafter at last one chain transfer agent and at last one radical initiator R, as described above, are added adn the reactor is then hated so as to carry out polymerization at a temperature less than or equal to 130"C. The duration ofthefirst polymerization stage ofthe process according to the invention is advantageously from 1 hourto 3 hours.

The mixtureofradical initiators introduced during the second polymerization stage ofthe process according to the invention is preferably used in an amount at most equal to 1.2% by weight with respect to the weight ofthe organic phase.

In a manner known per se, the following are also preferably introduced into the reactor during this second stage: -at least on suspending agent, such as, in particular, hydroxyapatite, an acrylic acid/alkyl acrylate copolymer or a formaldehyde/sodium naphthalenesulphonate condensation product, in an amountat most equal to 1 % by weight relative to the weight of the organic phase, -at least one co-stabilizersuch as, for example, an alkali metal persulphate and -atleastone compoundwhich acts as a pH buffer, such as, for example, calcium carbonate.

The temperature during this second polymerization stage of the process according to the invention is increased gradually in stages up to a value which is preferably from 1 300C to 200 C. The duration ofthe second polymerization stage ofthe process according to the invention is advantageously from 2 to 10 hours.

The grafted polymer according to the invention may then be separated from the reaction mixture resulting from the second polymerization stage by subjecting the mixture successively to filtration and drying stages. The grafted polymerthus obtained is noteworthy in that on microscopic examination it is found to have a microstructure such that at least 65% ofthe particles have a size lessthan or equal to 7,am.

Asecond aim ofthe present invention consists in impact-resistant and ageing-stable compositions containing at leastone grafted polymer obtained bythe process of preparation described above. The compositions according to the inventionthus in particular comprise the grafted polymers obtained according to this process as well as their mixtures, in all proportions, with compatible polymers such as, for example, unsaturated polyesters (forthe purpose ofthe manufacture of automotive components), polyphenylene oxides and, of course, polystyreneswhich have not been grafted with an elastomer.

According to a particular embodiment, the compositions according to the invention can moreover contain at least one alkali metal salt or alkaline earth metal salt ofstearic acid, such as,for example, lithium stearate, calcium stearate, barium stearateor mix tures thereof. This salt or this mixture of salts is used in an amount at most equal to 0.25% by weight relative to theweight ofthe grafted polymer.

According to another particular embodiment,the compositions according to the invention, containing at least one stearic acid salt ifnecessary, can moreover contain at least one metal oxide selectedfrom zinc oxide and titanium oxide. This oxide orthis mixture of oxides is used in an amount at most equal to 2% by weight with respect to the weight ofthe grafted polymer. These oxides, which promote the stability of the grafted polymerto ultraviolet radiation, make it possible to use the compositions of the invention for the manufacture of garden furniture, caravan compo nents andthelike.

The compositions according to the invention can, in a known manner, moreover contain at least one agent for protection against ultraviolet radiation, selected from benzotriazoles and sterically hindered amines.

As benzotriazolesthere may beforexamples be mentioned 2-(2-hydroxy-5-tertiary-oxtylphenyl)-benzotriazole and 2-(2'-hydroxy-5'-methylphenyl )-benzotriazole. As a sterically hindered amine there may for example be mentioned 2,2',6,6'-tetramethyl-4- piperidyl disebacate.Thecompositions according to the invention can, in a known manner, moreover contain at least one antioxidant such as, for example, triethylene glycol bis-3-(34ertiarbutyl-4-hydroxy-5- methylphenyl)-propionate, making it possible to increase their high temperature heat stability.

Athird and last aim of the present invention consists in articles obtained by processing the compositions described above. Indeed these compositions show a satisfactory combination of properties (ageing under the influence of oxidation and/or ultraviolet radiation, tensometric properties and impact strength), which can make them particularly suitable for fields of application where good stability to weathering and to sunlight is required, such as, in particular, the manufacture of garden furniture and caravans. The compositions according to the invention can be converted into such articles by the conventional techniquesforprocessing polystyrene, namelyextrusion (by means of single-screw ortwin-screw extruders),thermoforming and injection moulding.

The examples given below, in which all amounts are expressed in parts by weight, are given by way of illustration, and without implying any limitation of the present invention.

EXAMPLE 1 The following are introduced into an autoclave reactor equipped with a heating device, a stirrer and a cooling device: 88 parts of styrene, 12 parts of a rubberyterpolymer (consisting of 44% by weight of ethylene, 45% by weight of propylene and 11% by weight of 2-ethyl-indene-norbornene), 6.7 parts of paraffin oil, 0.108 partofbenzoyl peroxide, 0.08 part of tertiary dodecylmercaptan and 46 parts of water.

The reaction mixture issubjecttothefirst polymerization stage by heating it in such a waythat itstemperature reaches 92"C after 60 minutes, and is then kept stirred for 90 minutes by means of a turbine stirrer. Whenthisstage hasfinished,thefollowing are added: 0.31 part of tertiary butyl perbonzoate, 0.13 part of di-tertiary butyl peroxide, 0.6 part of 2,5-dimethyl-2,5-di-tertia ry butyl-peroxy-3 hexyne, 0.08 partoftriallyl cyanurate, 69 parts of water, 0.214 part of hydroxyapatite, 0.019 part of calcium carbonate and 0.013 part of potassium persulphate.

The reaction mixture is then heated in stages in such a mannerthat its final temperature reaches 150 C after 7 hours. The resulting mixture is then filtered and thereafter dried in order to separate the grafted polymer.

100 parts ofthe grafted polymer thus obtained are mixed on a screw extruder with: 0.1 part oftriethylene glycol bis-3(3-tertiary butyl-4hydroxy-5-methylphenyl)-propionate, 0.25 part of 2-(2-hydroxy-5-tertiary octylphenyl)-benzotriazole, 0.25 part of 2,2',6,6'-tetramethyl-4-piperidyl disebacate and 0.125 part of lithium stearate.

Microscopic analysis of the grafted polymer obtained shows a microstructure in which 80% ofthe particles have a size less than or equal to 7,am, the mean size being 4,u m.

Thefollowing properties are determined on the grafted polymer obtained: -the gel content expressed in per cent by weight and determined as indicated above; -the coefficient a; -the melt index MI expressed in dg/min and determined in accordance with standard specification ASTM-D 1238; -the Vicattemperature expressed in degrees centigrade and determined according to standard specification ASTM-D 1525; -the Izod impact strength expressed in J/m and determined according to standard specification ASTM-D 256;; -the initial elongation at break EBo expressed in per cent and determined according to standard specification ASTM-D 638, and -the elongation at break EB2000, also expressed in per cent and determined according to standard specification ASTM-D 638, measured after 2000 hours' accelerated ageing in accordance with the "WEATH ER-O-METER" test. This test consists of placing samples in a chamber exposed to a source of light having a power of 6000 Watts and consisting of a filtered Xenon source in accordance with standard specifications ASTM-G-26-70 and G-27-70.The chamber is kept at a humidity of 60%,the black body temperature being 55"C. A sprinkling cycle simulates the effect of rain; -the initial yellowing index IYo determined according to standard specification NFT 08-014; -the yellowing index IY2000 determined according to the same standard specification and measured after 2000 hours' accelerated ageing according to the "WEATHER-O-METER" test.

The numerical values of these various properties are summarized in the Table below.

EXAMPLES 2 to 5 The procedure of Example 1 is repeated, only changing the amounts of the various constituents, as follows: First polymerization stage: styrene: 81.45 parts rubbery terpolymer: 11.88 parts paraffin oil: 5.71 parts benzoyl peroxide: 0.053 part tertiary dodecylmercaptan: 0.089 part Second polymerization stage: triallyl cyanurate: 0.099 part diviylbenzene: 0.009 part 2,5-dimethyl-2,5-di-tertiary butyl-peroxy-3-hexyne: nil tertiary butyl perbenzoate: x part di-tertiary butyl-peroxide: y part Mixing on a Screwextruder zinc oxide apart titanium oxide: w part stearic acid salt: always 0.125 part, but the metal M ofthe saltvaries.

Examples was carried outwith the additional presence of 0.1 part oftriethylene glycol bis-3(3-tertiary butyl-4hydroxy-5-methylphenyl)-propionate, 0.25 part of 2-(2-hydroxy-5-tertiary octylphenyl)- benzotriazole, and 0.25 part of 2,2',6,6'-tetramethyl-4-piperidyl disebacate.

The numerical values of the various properties of the grafted polymers obtained are shown in the Table below as a function ofthe parameters w, x, y, z and M defined above.

TABLE

example 1 2 3 4 5 c.-1 .167 -.067, C.067 0.067 C.13 '.22 , 0.22 0.27 0.16 0.22 z C 0 O . B 2.5 2 1.2 0 1.2 1 1.2 0 00 M n Li ; 36Li - Li Ca ge: conteo: 26 26 1 36 15 18 1.20 1.27 1.27 1.37 1.26 :5 16.5 21 7 9 Vicat 93 78 74 80 80 Izod 13.4 13 1 11 Z0 29 880 68 60 61 30 20 EB2300 40 35 30 20 10 rO 2 1.5 0.5 0.7 1 ZY2000 20 28 15 13 22

Claims (17)

ClAIMS

1. Process for the preparation of polymers of at least one vinylaromatic monomer grafted onto at least one unsaturated polyolefin elastomer, wherein the said vinylaromatic monomer is polymerised in the presence ofthe said unsaturated polyolefin elastomer, in acqueoussuspension and in two stages, the first polymerization stage being carried out at a temperature less than or equal to 1300C in the presence of at least one chain transfer agent and at least one radical initiator R exhibiting high heat stability, and in the second stage the polymerization is carried on at a temperature above that ofthefirst stage, after addition of water and of a mixture of at leasttwo radical initiators differentfrom Rand selected from tertiary butyl perbenzoate, di-tertiary butyl peroxide and 2,5-di-methyl-2,5-di-tertiary butylperoxy-3-hexyne.

2. Processaccordingto Claim 1, wherein the vinylaromatic monomer is selected from styrene, alpha-methylstyrene, vinylnaphthalene and vinylto luene.

3. Process according to Claim 1 or Claim 2, wherein the unsaturated polyolefin elastomer is a terpolymer of ethylene with at least one alpha-olefin having from 3 to 6 carbon atoms and at least one diene.

4. Process according to Claim 3, wherein the said terpolymer contains from 15 to 60 mole% of units derived from propylene and from 0.1 to 20 mole% of units derived from the diene.

5. Process according to any one of Claims 1 to4, wherein the unsaturated polyolefin elastomer is used in an amountwhich is at most equal to 20% by weight with respect to the weight of the organic phase.

6. Process according to any one of Claims 1 to 5, wherein the chain transfer agent is selected from tertiary dodecylmercaptan, divinylbenzene and triallyl cyanurate.

7. Process according to any one of Claims 1 to 6, wherein the chain transfer agent is used in an amount at most equal to 0.2% by weight with respect to the weight of the organic phase.

8. Process according to any one of Claims 1 to 7, wherein the initiator R is benzoyl peroxide.

9. Process according to any one of Claims 1 to 8, wherein the initiator R is used in an amount at most equal to 0.2% by weightwith respect to the weight of the organic phase.

10. Process according to any one of Claims 1 to 9, wherein the mixture of radical initiators introduced during the second stage is used in an amount at most equal to 1.2% by weightwith respect to the weight of the organic phase.

11. Process for the preparation of polymers of at least one vinylaromatic monomer grafted onto at least one unsaturated polyolefin elastomer substan- tially as hereinbefore described in any one of Examples 1 to 5.

12. An impact-resistant and ageing-stable composition, containing at least one grafted polymer obtained bythe process according to any one of Claims 1 to 11.

13. A composition according to Claim 12, wherein the said grafted polymer has a particulate microstructure such that at least 65% ofthe particles have a size lessthan or equal to7,am.

14. A composition according to Claim 12 or Claim 13,furthercontaining at least one polymer selected from unsaturated polyesters, polyphenylene oxides and polystyrenes which are not grafted onto an elastomer

15. A composition according to any one of Claims 12 to 14, further containing at least one alkali metal salt or alkaline earth metal salt of stearic acid, in an amount at most equal to 0.25% byweight with respect to the weight ofthe grafted polymer.

16. Acomposition according to any one of Claims 12 to 15, further containing at least one metal oxide selectedfrom zincoxideandtitanium oxide, in an amount at most equal to 2% by weight with respectto the weight of the grafted polymer.

17. An article obtained by processing the composition according to any one of Claims 12 to 16.