GB2153370A - Process for preparing rubber modified polystyrene - Google Patents

Abstract

In a process for preparing rubber-modified styrene polymers exhibiting good low-temperature behaviour and improved stress-cracking resistance suitable for refrigeration chambers, use is made of a rubber consisting of anionic polybutadience with a molecular weight equal to at least 300,000 and a viscosity greater than or equal to 140 centipoise, an alpha -methylstyrene dimer or a compound chosen from n-dodecylmercaptan, tertiododecylmercaptan, diphenyl 1,3 butadiene, cis-diphenylcyclobutene. transdiphenylcyclobutene, methylphenylindane, diphenylcyclobutane or a mixture thereof, and cyclohexane, Isopar or ethylbenzene. An additive of a glyceride of stearate, polyethylene wax, or containing a polyethylene glycol function or based on silicone oil is preferably added after polymerisation.

Description

SPECIFICATION Process for preparing polystyrene The present invention relates to a process for preparing moulding materials based on polystyrene. Its subject is more particularly a process for preparing impact-polystyrene, with improved resistance to cracking under stresses and/or in contact with corrosive agents.

It is known that moulded articles made from styrene polymers stress-crack when they are placed in prolonged contact with chemical agents which trigger this corrosion phenomenon.

These agents are particularly oils and fats, and organic halogenated blowing agents such as, for example, Freon. In practice, this corrosion is observed both in packages for alimentary fats, and in the manufacture of refrigerators made of impact-polystyrene; in the latter case the refrigerator chambers split during the expansion of the polyurethane by means of a blowing agent such as Freon. This poor behaviour of impact-polystyrene is all the more awkward insofar as the attack by the Freon takes place when the refrigerator is already finished, requiring the refrigerator to be either scrapped or to undergo costly dismantling and assembling.

To solve this problem, it has been proposed to coat the refrigerator walls in contact with Freon with a Freon-resistant film such as acrylonitrile-butadiene-styrene terpolymer (ABS). However, such a protective system results in a great increase in cost. It has also been proposed to replace a part of the ABS with polyethylene films. Unfortunately, it is very difficult to obtain good adhesion of these two thermoplastics. It has also been proposed to increase the cracking resistance by subsequent addition of another rubber. Unfortunately, mediocre improvements are obtained, even when large quantities of rubber are employed.

Means of manufacturing moulding materials which exhibit good behaviour in respect of cracking under stresses and/or in contact with corrosive agents have now been found.

The present invention provides a process for preparing rubber-modified styrene polymers, exhibiting improved stress-cracking behaviour and good low-temperature behaviour, characterised in that a solution of a rubber in styrene is polymerised in bulk, in which solution the rubber is an anionic polybutadiene with a proportion of 1-2 vinyl isomer of between 10 and 12%, a high molecular mass equal to at least 300,000 and a viscosity greater than or equal to 1 40 centipoise for a 5% solution in styrene, the solution containing between 7 and 10% by weight of rubber and the polymerisation being carried out in the presence of a-methylstyrene dimer or a compound chosen from n-dodecylmercaptan, tertiododecyl mercaptan, diphenyl 1,3 butadiene, cis-diphenylcyclobutene, trans-diphenylcyclobutene, methylphenylindane, diphenylcyclobutane, or mixture constituted with one or more compounds employed in a quantity equal to at least 0.08% and at most equal to 0.3% by weight relative to the total weight of all the components, in the presence of cyclohexane and Isopar or ethylbenzene employed in a quantity at least equal to 7% by weight, relative to the total weight of all the ingredients present in the solution.

Preferably use is made of 0.5 to 1% by weight relative to the weight of all the ingredients in the solution, of an additive chosen from a mono and/or a di and/or a triglyceride of stearates or their mixture, from polyethylene waxes, from additives containing the polyethylene glycol function or from additives based on silicone oil. The use of such additives facilitates the fabrication of the polymers. Preferably, these additives are, according to the invention, added after polymerisation, for example in internal mixers.

Use of the process which is the subject of the invention makes it possible to obtain polymers which not only exhibit good impact properties, but which, above all, have low sensitivity to cracking under stress and/or in contact with corrosive agents and which also show good behaviour at low temperatures below 0 C. The combination of these properties permits them to be used without additional protective treatment for the construction of liners and door-liners of refrigerators and freezers and for the manufacture of trim components for kitchen or bathroom furniture, intended to be frequently cleaned with detergents. Their resistance to corrosive agents also enables them to be used in the manufacture of seafood crates, employed in particular for transporting fish, and for the manufacture of food packaging, particularly for fatty materials.

The use of the process according to the invention makes it possible to obtain polymers which have a swelling index above 10. This swelling index is determined by dissolving the polymer obtained in toluene, and then centrifuging and decanting the solution. The solid fraction obtained is weighed moist, then dried and reweighed. The swelling index is, by definition, the ratio: Weight of moist gel ls = Weight of dry gel This index makes it possible, in fact, to express the degree of crosslinking of the rubber. It should be considered as a reciprocal of weight concentration and, consequently, the higher the swelling index, the lower the degree of crosslinking of the rubber.

The process of the invention results in polymers which have a gel content above 30. This gel content expresses the proportion of rubber and it is given by the following relationship, reduced to 1 gram of polymer: Dry weight of the gel C g (weight %) = x 100 Test sample Another characteristic of the polymers obtained according to the process of the invention is that the mean diameter of the particles of the dispersed rubber phase, observed in accordance with the electron microscopy method described by KATO in J. Electro. Micros. 14, 1 9865 (20) is greater than 8 and at most equal to 1 3 microns; this diameter is determined according to the methods described by CRAIG (J. Polym. Sci. Polym. Chem. Ed 1 5,433 (1977) and JAMES (Polym. Eng. Sci. 8, 241 (1968).These measuring methods make it possible to determine the mean size of the elastomer nodules dispersed in the polystyrene matrix, and the particle size distribution. The apparatus which is most generally employed for determining the size, and size polydispersity, of the globules of elastomer dispersed in the styrene matrix is the "COULTER" counter, sold by the company COULTRONICS.

The process of the invention produces a polymer with the special property of having only a single glass transition of the elastomer phase, this glass transition being measured in accordance with the known methods for measuring viscoelasticity and adjusted to values below or equal to - 80"C. The polymer obtained shows no secondary transition between - 70"C and 0 C.

Measurement of the glass transition is carried out with a RHEOVIBRON DDV-IIC viscoelastimeter or a METRAVIB viscoelastimer (the measuring methods are described by NIELSEN in Mechanical Properties of Polymer and Composites, 2nd volume, 1 974, Marcel DERKER Inc.

N.Y. and also by J. BOYER, Macromol. Sci. Phys. B 9 (2) 187 (1974). According to the invention, the glass transition spectrum of the rubbery phase is spread over a certain range of temperature corresponding to a frequency location characteristic of the high-speed impact frequencies.

According to the invention, a solution of a rubber in styrene is polymerised in bulk with the use of a rubber consisting of anionic polybutadiene with a molecular mass of between 300,000 and 400,000 and a viscosity above 1 40 centipoise and below 220 centipoise, the viscosity being determined by dissolving 5 parts by weight of the rubber in 95 parts by weight of styrene. Use of rubber with a viscosity above 220 centipoise produces a solution which is too viscous, difficult to handle and to process on an industrial scale, while the use of rubber with a viscosity below 140 centipoise produces a polymer with nodules of too small particle size, well below 10 microns. According to the invention, the quantity of rubber employed is between 7 and 10% by weight relative to the quantity of styrene employed.Use of a quantity above 10% produces polymers which are too elastic, with mediocre mechanical properties and lacking stiffness, in particular. According to the invention anionic polybutadienes are understood to be the rubbers manufactured by a process of the "anoinic" type.

According to another feature of the invention, the polymerisation is carried out in the presence of a-methylstyrene dimer or a compound chosen from n-dodecylmercaptan, tertiododecylmercaptan, diphenyl, 1,3 butadiene, cis-diphenylcyclobutene, trans-diphenylcyclobutene, methylphenyl idane, diphenylcyclobutane, or mixture constituted with one or more compounds employed in a quantity of between 0.08 and 0.3% by weight relative to the total weight of the solution.

According to another important feature of the process of the invention, the polymerisation is carried out in the presence of cyclohexane, Isopar or ethylbenzene employed in a quantity equal to at least 7% by weight relative to the total weight of the solution. According to the invention, depending on the nature of the solvent employed, quantities employed are different and preferential: 8% for ethylbenzene, 10% for cyclohexane and 13% for Isopar, which is a commercial product consisting of isoaliphatic hydrocarbons (cf. Condensed Chemical Dictionary, Rose, REINHOLD).According to another important feature of the process of the invention, the polymerisation is carried out in the presence of 0.5% and 1 % by weight relative to the total weight of an additive chosen from a mono and/or a di and/or a triglyceride of stearates or their mixture, from polyethylene waxes, from additives containing the polyethylene glycol function or from additives based on silicone oil. The di and/or triglyceride of stearates are compounds with a melting point of approximately 60"C, and a flashpoint above 150"C; they are sold, for example, under the name ATMOS 150 by the company ICI AMERICA INC. The polyethylene waxes are polymeric waxes sold by the company HOLES. Polyethylene waxes are products with a dropping point of between 70 and 80"C, a density of 0.86-0.88 and an average molecular weight of approximately 2,500; they are sold, for example, by the company CdF CHIMIE.

Silicone waxes are products which are soluble in most organic solvents; they are sold under the name DOW CORNING 200 FLUID by the company DOW CORNING. The use of such additives facilitates subsequent use of the polymers obtained and the properties of the finished products.

Preferably, these additives are added after polymerisation in internal mixers; for example a type GR 5 mixer sold by the company WERNER-PFLEIDERER is employed. Melt flow index adjustment is obtained by the addition of 1 to 3% of plasticisers consisting of liquid paraffin in particular; the measurement of the melt flow index is carried out in accordance with the ASTM Standard D 1 238.

To make use of the process of the invention, polymerisation is carried out with the use of known techniques. The autoclave employed is fitted with a stirrer, a heating device and a cooling system. All the reactants employed and the additives are chemical products employed in bulk polymerisation processes: antioxidants such as 2,6-ditertbutylpara-cresol, 2,2-methylenebis(4-methyl-6-tertbutylphenol), octadecyl 3-(3,5-ditertbutyl-4-hydroxyphenyl)propionate, tri(2methyl-4-hydroxy-5-tertbutylphenyl(butane, triethylene glycol bis-3-(3-tertbutyl-4-hyd roxy-5-methylphenyl)propionate, or mixtures of phenolic antioxidants and phosphates, polymerisation initiators such as benzoyl peroxide, or ditertbutyl perioxide, cyclohexane peroxide or tertbutylperoxy isopropyl carbonate or dicumyl peroxide or tertbutyl perbenzoate.All these additives are used in a small quantity: at most 0.5% by weight relative to styrene where the antioxidants are concerned, at most 0.3% by weight relative to styrene for the peroxides, which are employed alone or mixed. It is also possible to add other conventional additives such as paraffin oils in a quantity equal to at most 5% by weight, relative to styrene; the use of such compounds makes it possible to produce polymers having good thermal and theological properties at the same time, and also permits easy use of the polymers obtained.

Exce'pt for the peroxides, all the reactants are introduced into the autoclave and heated with stirring at a temperature in the region of 40"C for 3 hours so as to dissolve the rubber in the other reactants. After this rubber-dissolving stage, the mixture is heated to a temperature between 80 and 250"C for a period of approximately 8 hours. After polymerisation, the polymer obtained is subjected to a heat treatment and is then devolatilised and granulated.

The following examples illustrate the present invention. All the quantities are expressed in parts by weight.

EXAMPLE 1 A reactor employed for bulk polymerisation and fitted with a stirring system, a heating device and a cooling system is charged with: -83.30 parts of styrene -7.5 parts of polybutadiene rubber produced by an anionic process and having the special characteristic of a 1-2 vinyl isomer content of between 10% and 12%, with a molecular weight of 350,000 and a viscosity of 190 centipoise for a 5 weight % solution in styrene, -0.15 part of a-methylstyrene dimer, -8 parts of ethylbenezene, -1 part of paraffin.

0.05 part of an antioxidant consisting of octadecyl 3-(3,5-ditertbutyl-4-hydroxyphenyl)propionate is then added. The reaction mixture is then subjected to heating in stages so that its final temperature reaches 145"C after 7 h 50.

After this heating stage 80 parts of solids are obtained, which are processed hot for 1 h 50.

After devolatilisation, which consists in subjecting the polymer to a heat treatment at a temperature berween 200 and 240"C under a vacuum of 730-740 mm of mercury, the polymer is granulated. Its properties are summarised in the following table: -the VICAT point is measured in accordance with th Standard DIN 53460, -the melt flow index is determined according to the ASTM Standard D 1238, -the impact strength is determined using the ASTM Standard D 256, -the gel content is calculated by stirring the~polymer obtained, cold, in toluene, then centrifuging the mixture, which makes it possible to determine the quantity of gel which is insoluble in toluene.The dry mass of the insoluble polymer is determined by treating the gel obtained previously under vacuum; the percentage of gel relative to the test sample expresses the gel content, -the swelling index is equal to the ratio of the mass of gel to the dry resin, -the particle size distribution is determined in accordance with KATO's technique, already mentioned above, using JAMES's methods, already mentioned above, tensile strength is determined according to the Standard DIN 53455, -resistance to stress-cracking corrosion is determined in accordance with Standard ACK 31, which is a standard produced by a working group consisting of manufacturers of refrigerators and of thermoplastics; this standard expresses, for an injection-moulded specimen, the remaining percentage on elongation under a stress of 75 kg/cm2; the remaining percentage must not be less than 75%, in the case of a halogenated hydrocarbon (Freon) when the specimen is injection-moulded; the percentage must not be less than 50% remaining when the specimen is prepared by extrusion; finally, this remaining percentage mut not be less than 40% when the specimen is compression-moulded.

EXAMPLE 2 Example 1 is repeated, but using 81.08 parts of styrene, 10 parts of ethylbenzene, 6.85 parts of the same polybutadiene rubber employed in Example 1, 0.1 part of a-methylstyrene dimer and 1.97 parts of paraffin oil; the properties of the polymer obtained are summarised in the table below.

EXAMPLE 3 Example 2 is repeated, but using 80.98 parts of styrene and 0.2 part of a-methylstyrene dimer, instead of 0.1. The properties of the polymer obtained are summarised in the table below.

EXAMPLE 4 Example 1 is repeated, using: -79.18 parts of styrene, -11 parts of ethylbenzene, -0.15 part of a-methylstyrene dimer, -3.83 parts of paraffin oil, -5.8 parts of completely anionic polybutadiene rubber, -0.042 part of octadecyl 3-(3,5-ditertbutyl-4-hydroxyphenyl)propionate. The properties of the polymer obtained are summarised in the attached table.

EXAMPLE 5 Example 1 is repeated, using: -82.9 parts of styrene, -7.5 parts of ethylbenzene, -0.10 part of a-methylstyrene dimer, -2.3 parts of paraffin oil, -1 part of a mixture of glycerol mono and distearate, -6.2 parts of polybutadiene.

The properties are summarised in the attached table.

EXAMPLE 6 Example 1 is repeated, with the use of: -78.06 parts of styrene, -6.82 parts of rubber, -13 parts of Isopar, -0.15 part of a-methylstyrene dimer, -1.97 parts of oil, -0.5 part of a mixture of glycerol mono and distearate.

The properties are summarised in the table.

EXAMPLE 7 Example 1 is repeated with the use of: -79.18 parts of styrene, -0.042 part of antioxidant sold under the name IRGANOX 1076, -3.83 parts of paraffin oil, -5.8 parts of anionic polybutadiene, -11 parts of ethylbenzene, -0.10 part of a-methylstyrene dimer, -0.042 part of octadecyl 3-(3, 5-ditertbutyl-4-hydroxyphenyl)propionate.

EXAMPLE 8 Example 1 is repeated, using: -83.69 parts of styrene, -7.5 parts of ethylbenzene, -0.083 part of a-methylstyrene dimer, -1.91 parts of paraffin oil, -0.5 part of polyethylene waxes with an average molecular weight of 2,500, a viscosity of 60 to 100 centipoise at 120"C, a density of 0.86 and a dropping point of between 70 and 80"C, -6.82 parts of anionic polybutadiene.

The properties are summarised in the attached table.

EXAMPLE 9 (comparative example) The product is obtained with the following reactant quantities: -83.69 parts of styrene, -7.5 parts of ethylbenzene -6.82 parts of polybutadiene consisting of a 50-50 mixture of anionic polybutadiene (10-12% of 1,2 vinyl) and of ZIEGLER polybutadiene (97% of the 1-4 cis isomer) instead of a single anionic polybutadiene, ---0.073 part of a-methylstyrene dimer, -1.91 parts of paraffin oil.

EXAMPLE 10 A polymer is prepared from the following ingredients: -83.69 parts of styrene, -7.5 parts of ethylbenzene, -6.82 parts of polybutadiene rubber which are obtained by an anionic process and have a 1-2 vinyl isomer content of between 10 and 12%, with a molecular weight of 350,000 and a viscosity of 1 90 centipoise for a 5 weight % solution in styrene, -0.083 part of a-methylstyrene dimer, -1.91 parts of paraffin oil, -0.5 part of synthetic wax with a melting point of 140"C and sold under the name NOPCOWAX 22 DS by the company DIAMOND SHAMROCK.

The'properties of the polymer which are obtained are summarised in the attached table.

EXAMPLE 11 Example 10 is repeated but with the synthetic wax replaced with 1 part of a wax sold under the name POLYWAX 2000 by the company HAULS.

The polymer properties obtained are summarised in the attached table.

EXAMPLE 12 Example 10 is repeated with the NOPCOWAX 22 DS waxes replaced with 1 part of polyethylene wax with an average molecular weight of 2,500, a viscosity of 60 to 100 centipoise at 120"C, a density of 0.86 and a dropping point of between 70 and 80"C.

The polymer properties obtained are summarised in the attached table.

EXAMPLE 13 Example 10 is repeated with the 0.5 part of the waxes substituted by 1 part of silicone oil sold by the company DOW CORNING under the name DOW CORNING 200 FLUID.

The product properties obtained are shown in the attached table.

EXAMPLE 14 Example 10 is repeated with the silicone oil replaced with 0.5 part of solid pymeric waxes "800", sold by the company HAULS.

The product properties obtained are shown in the attached table.

EXAMPLE 15 (comparative) A polymer is prepared from: -83.70 parts of styrene, -7.5 parts of ethylbenzene, --0.073 part of a-methylstyrene dimer, -1.91 parts of paraffin oil, -6.82 parts of anionic polybutadiene consisting of 11 % of 1-2 vinyl isomer and having a solution viscosity of 95 centipoise and an average molecular mass by weight of 273,000.

EXAMPLE 16 Use is made of: -81.03 parts of styrene -10 parts of ethylbenzene -6.85 parts of the same polyburadiene rubber of Example 1 -0.10 part of a-methylstyrene dimer -1.97 part of paraffin oil ---0.05 part of antioxidant based on octadecyl 3-(3,5-ditertbutyl-4-hydroxyphenyl)propionate -0.5 part of a mixture of glycerol mono and disterate.

EXAMPLE 17 Use is made of: -81.03 parts of styrene -10 parts of ethylbenzene -6.85 parts of the same polybutadiene as Example 1 -0.10 part of a-methylstyrene dimer -1.97 of paraffin oil -0.5 part of silicone oil EXAMPLE 18 Example 1 is repeated by using a mixture constituted with diphenyl 1,3 butadiene, cisdiphenylcyclobutene, trans-diphenylcyclobutene, methylphenylindane and diphenylcyclobutane instead of a-methylstyrene dimer.

Properties are summarised in the attached table.

The following abbreviations have been taken in the attached table: -T.S.: tensile strength -M.: modulus -B.S.: break stress -Y.S.: yield stress -Y.S.: yield stress -E.B.: elongation at break -R.E.: remaining elongation -E.S.: extruded specimens -M.S.: moulded specimens -Oil resistance was determined with LIVY00 olive oil.

-The dimensions of the extruded specimens were 200 X 20 X 2 mm.

The stress-cracking corrosion resistance is expressed as the remaining break elongation, in % defined by the relationship: Tensile break elongation after contact with Freon or oil Remaining elongation = ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ (%) Initial tensile break elongation

VICAT POINT MELT FLOW IZOD IMPACT STRENGTH GEL SWELLING PARTICLE T.S. of E.S. EXAMPLE (1 kg-120 C/h) INDEX PRESSED SPECIMENS CONTENT INDEX SIZE M Y.S. B.S. C (g/10') (kg x cm/cm % (microns) (kg/cm) (kg/cm) kg/cm) 1 100.0 4.6 6.9 35.0 11.8 12.7 9480 153 185 2 100.5 5.4 7.0 34.6 10.9 8.0 9990 145 183 3 100.0 6.5 6.9 34.8 11.4 10.1 9100 130 169 4 94.0 11.1 6.8 32.6 12.1 9.5 9170 123 156 5 101.0 5.0 8.0 35.0 12.0 7.5 10000 140 160 6 101.0 4.5 7.0 35.0 11.9 7.5 10500 153 7 95.0 9.5 7.2 33.0 12.2 8.5 9200 129 154 8 101.0 5.5 7.0 34.0 11.0 7.0 10110 134 156 9 102.0 4.4 6.9 32.7 9.5 4.0 11000 179 182 10 100.5 5.0 6.5 11975 161 155 11 99 6.0 6.5 11840 177 169 12 100 6.0 6.5 11800 160 170 13 100.5 6.5 6.0 11800 164 162 14 99.5 6.5 6.0 11500 170 175 15 101 4.5 3.0 11000 175 190 16 99.0 6.0 7.0 35 11 8.5 10000 140 185 17 100.0 6.2 6.5 33 11 8.5 10100 142 185 18 100.0 5.5 6.7 34.1 12 18.6 10050 140 170

FREON 11 RESISTANCE OF E.S. OIL RESISTANCE OF E.S. T.S. of compression FREON RESISTANCE of (# = 75 kg/cm) M.S. compression M.S. (R.E.%) (R.E.%) (# = 75 kg/cm) EXAMPLE E.B. R.E. R.E. R.E. R.E. M Y.S. B.S. E.B. R.E. R.E. % after 40 min after 50 min after 30 min after 50 min (kg/cm) (kg/cm) (kg/cm) % (after 40 (after 50 (%) (%) (%) (%) min %) min %) 1 53 94 85 94 75 11830 125 135 32 94 78 2 54 81.5 74 81.5 79 10840 108 115 26 77 69 3 51 82 78 82 82 9860 102 117 29 82 72 4 59 100 98 100 88 12230 104 128 47 100 92 5 47 84 81 94 88 35 83 83 6 47 80 75 90 85 34 83 81 7 54 100 100 98 95 10980 104 125 51 75 74 8 47 95 89 30 79 76 9 34 15 12 10 5 30 15 10 10 55 52 47 11 54 54 60 12 55 66 64 13 55 57 50 14 50 50 40 15 40 10 3 10 4 16 50 87 87 90 83 17 52 82 79 80 79 18 50 60 60 75 70 12100 110 120 35 78 75

Claims (5)

1. A process for preparing rubber-modified styrene polymers, exhibiting improved stresscracking behaviour and good low-temperature behaviour, wherein a solution of a rubber in styrene is polymerised in bulk, in which solution the rubber is an anionic polybutadiene with a proportion of 1-2 vinyl isomer of between 10 and 12%, a molecular mass greater than or equal to 300,000 and a viscosity greater than or equal to 140 centipoise for a 5% solution in styrene, the solution containing between 7 and 10% by weight of rubber and the polymerisation being carried out in the presence of cu-methylstyrene dimer or a compound chosen from ndodecylmercaptan, tertiododecylmercaptan, diphenyl 1,3 butadiene, cis-diphenylcyclobutene, trans-diphenylcyclobutrene, methylphenylindane, diphenylcyclobutane, or mixture of two or more compounds employed in a quantity of from 0.08% to 0.3% by weight relative to the total weight of all the components, in the presence of cyclohexane and Isopart or ethylbenzene, employed in a quantity at least equal to 7% by weight, relative to the total weight of all the ingredients present in the solution and 0.5 to 1 % by weight relative to the weight of all the ingredients in the solution of an additive chosen from a mono and/or a di and/or a triglyceride of stearates or a mixture thereof, from polyethylene waxes, from additives containing a polyethylene glycol function or from additives based on silicone oil.

2. A process as claimed in claim 1, wherein the additive is added after polymerisation.

3. A process as defined in claim 1 and substantially as hereinbefore described with reference to any one of the foregoing examples.

4. A polymer prepared by a process as claimed in any one of claims 1 to 3.

5. A refrigeration chamber manufactured from a polymer as claimed in claim 4.