GB2067579A - Modified vinyl-aromatic polymers - Google Patents

Abstract

Modified vinyl-aromatic polymers, which are suited for being moulded or vacuum-formed and which have an improved resistance to fissuring under stress, containing from 6% to 12% by weight of an ethylenically unsaturated nitrile and having the following characteristics: a) at least 23% by weight of an elastomeric phase insoluble in toluene; b) the elastomeric phase has a swelling index in toluene greater than 10; c) a melt-index of at least 1.5; d) a bending-modulus greater than 15,000 kg/sq.cm; e) a torsional modulus greater than 5,500 kg/sq.cm; and f) an Izod resistance at 23 DEG C greater than 7 kg cm/cm. The polymers are obtained in the Examples by an inversion process, i.e. a bulk prepolymerization followed by an aqueous suspension polymerization stage. The polymers adhere well to polyurethane and are resistant to perfluorinated refrigerant fluids.

Description

SPECIFICATION Modified vinyl-aromatic polymers The present invention relates to modified vinyl-aromatic polymers, which are suited for being moulded or vacuum formed and which have an improved resistance to fissuring under stress.

It is well known that formed bodies based on vinyl-aromatic polymers tend to fissure when under stress when brought into prolonged contact with fatty, oily substances or with halogenated hydrocarbons, such as Freon. "Freon" is a Registered Trade Mark.

This phenomenon, commonly called stress-cracking, in practice occurs in the packaging of foodstuffs such as in the case of containers for alimentary fats, butter or margarine, or in the preparation of refrigerating cells, following the action of Freon propellant deriving from the usually foamed polyurethane applied to their back. This latter case constitutes a serious drawback from the practical point of view, in as much as the action by the Freon occurs when the refrigerator is already completed, and as a result it becomes necessary either to reject the refrigerator altogether or to carry out costly and burdensome dismantling and repair operations.

Several attempts have been made to resolve this serious drawback of vinyl-aromatic polymers, none of which has produced satisfactory and low-cost results.

Thus, for instance, it has been attempted to improve the stress-cracking resistance of the polymers by means of structural and morphological changes (modifications) of the two-phase system. In fact, by modifying the content, the degree of cross-linking and the degree of dispersion of the grafted rubbery phase of the polymer, there is obtained an improvement of its resistance of fissuring under the action of Freon. Practically, however, the improvements that may thus be achieved are not such as to achieve the complete absence of fissuring in refrigerating cells, at least with the common polyurethane foaming techniques presently in use.

Moreover, modifying the structural parameters of the impact-resistant vinyl-aromatic polymer does not ensure the adhesion of the polyurethane foam to the refrigeration cells walls so as to form a compact whole between the cell walls and the insulating polyurethane layer, as desired by the manufacturers of refrigerators.

In order to avoid the formation of fissuring in refrigerator cells by an aggressive medium, it has been suggested to protect the outer walls of a refrigerator cell with a Freon-resistant film, such as an acrylonitrile/butadiene/styrene (ABS) terpolymer. Such as system of mechanical protection, however, even if on the one hand it resolves the problem of fissuring and allows a good adhesion between the cell and the foamed polyurethane insulating layer to be achieved, on the other hand there is involved a further serious increase of costs and a considerable complication in the recovery of the scrap material, due to the incompatibility of the ABS film towards the vinyl-aromatic polymer.

The present invention provides a modified vinyl-aromatic polymer containing from 6% to 12% by weight of an ethylenically unsaturated nitrile bound to a two-phase system, and having the following characteristics: a) the polymer has an elastomeric phase insoluble in toluene which comprises at least 23% by weight; b) the elastomeric phase has a swelling index in toluene greater than 10; c) the polymer has a melt index of at least 1.5; d) the polymer has a bending modulus greater than 15,000 kg/sq.cm; e) the polymer has a torsional modulus greater than 5,500 kg/sq.cm; and f) the polymer has an IZOD resistance at 23"C greater than 7 kg cm/cm.

The ethylenically unsaturated nitrile is preferably acrylonitrile. There may, however, also be used other ethylenically unsaturated nitrile monomers, such as methacrylonitrile.

The modified, high-impact vinyl-aromatic polymer having the above-defined characteristics exhibits a resistance to fissuring, by the action of Freon and under a load of 75 kg/cm2, greater than 25 minutes, and an adhesion to polyurethane of at least 1.3 kg/sq.cm. Because of these characteristic properties, the modified vinyl-aromatic polymer according to the present invention is particularly suited for being used in the production of refrigerator cells both due to the very high resistance to Freon as well as to its compatibility with un-modified high-impact polystyrene, with conventional polystyrene, with ABS terpolymers and with styrene/acrylonitrile (SAN) copolymers.This latter characteristic allows the achievement of an easy recovery of the scrap material during processing as well as the preparation of mixes with a different content of copolymerized acrylonitrile, the chemical resistance and processability of which may be varied in order to satisfy the different requirements of use.

The high-impact, modified vinyl-aromatic polymer of the invention may be obtained according to any known polymerization process, provided that there is used a mixture of vinyl aromatic/ unsaturated nitrile monomers as starting monomers and that care is taken to obtain a complex of the above indicated characteristics.

The polymerization processes commonly used are suspension polymerization, bulk-suspension polymerization and continuous bulk polymerization, which are all extensively described in the literature, for instance in US. Patent nos. 2,694,692 and 2,862,906, in Amos. Polym. Eng.

Sc., 14 (1974) 1 page 1-11. There may, however, also be applied other processes such as emulsion polymerization or suspension polymerization starting from rubber latexes provided that they allow materials to be obtained which have the above indicated characteristics.

According to the bulk-suspension process, the rubber is first dissolved in a vinyl-aromatic/unsaturated nitrile monomer mixture, and the mass is then subjected to polymerization either by the thermal or by the catalytic method, until there is attained a conversion of about 30% but in general not greater than 50%.

After this first phase of prepolymerization, the mass is dispersed, under vigorous stirring and with the help of suspending agents, in water and then subjected to polymerization following a suitable and well known thermal cycle.

The range of the reaction temperature may be from 50 to 1 70 C, but preferably is from 100" to 160"C.

The polymerization in general is carried out in the presence of an oleosoluble catalyst which may be added either at the start or during the polymerization. Suitable catalysts are benzoylperoxide, lauryl-peroxide, di-cumyl peroxide, di-ter-butyl-perbenzoate, and di-tert-butyl-peroxide.

The prepolymerization may also be started thermally. There may be used, is desired, a chaintransferring agent such as ter-dodecylmercaptane.

As suspending agents there may be used both hydrosoluble compounds of an organic nature such as polyvinyl alcohol, acrylcopolymers, cellulose derivatives, and partially saponified polyvinyl-acetate, as well as inorganic compounds unsoluble in water, such as tricalcium phosphate or barium phosphate, either alone or in admixture with a surfactant agent or with sodium sulphite. The suspensing agent is usually used in an amount of from 0. 1% to 5% by weight with respect to the organic phase.

The polymerization may be also carried out directly in a suspension without bulk prepolymerization, provided that stirring of the reaction mass is carried out in such a way as to obtain a suitable precipitation and dispersion of the rubber particles grafted onto the polymeric matrix.

According to the continuous bulk process, the rubber solution in the vinyl-aromatic/unsaturated nitrile monomer mixture is continuously fed into and polymerized in reactors arranged in series and subjected to stirring, according to a well defined temperature cycle, until a conversion greater than 50% is attained. The mass is then devolatilized under vacuum in order to remove the unreacted monomers which are then conveniently recycled back to the first reactor.

The polymerization in general is carried out with the thermal method in the presence of a diluent, the most common of which is ethylbenzol.

The temperature range is from 50 to 240"C, preferably from 100" to 220"C.

The vinyl-aromatic monomer is preferably styrene. There may, however, also be used styrenes alkylated in the nucleus or in the side-chain, such as alpha-methyl-styrene or vinyl-toluene.

The vinyl-aromatic monomers may be used either along or in admixture with each other.

The rubber may be suitably natural rubber or a synthetic rubber generally used for imparting to the vinyl-aromatic polymer the required high-impact resistance. Suitable synthetic rubbers are polybutadiene, polyisoprene, and butadiene and/or isoprene copolymers with styrene or with other monomers, which have a glass transition temperature (Tg) less than - 20"C. These butadiene and/or isoprene polymers or copolymers may contain the monomers in a different configuration, for instance a different content of cis, trans-configuration and of vinyl. In the copolymers the monomers may be distributed either at random or ordered in blocks or in a star disposition.

Other synthetic rubbers that may be suited for use in the preparation of the modified highimpact resistant vinyl-aromatic polymers according to the invention are the saturated rubbers of the ethylene-propylene type or ethylene-propylene-diene terpolymers, or silicon rubbers with unsaturated groups.

For the evaluation of the above specified characteristics of the modifed vinyl-aromatic polymers of the present invention the following methods were used: 1. Determination -of the elastomeric phase insoluble in toluene.

A 2g sample was dispersed in 100 cc of a mixture consisting of 57% by weight of toluene and 43% by weight of methyl-ethyl-ketone. After centrifuging at 10,400 x G, the insoluble part was separated by decanting in the form of a swollen gel. This gel was repeatedly washed with the above mentioned toluene-methyl-ethyl-ketone mixture and then centrifuged until the washing solvent did not become muddy by the addition of ethanol. The swollen and washed gel was then coagulated with ethanol, separated by filtering and then dried under vacuum at 45"C, at a pressure of 200 mmHg for 12 hours.

The content of the elastomeric phase insoluble in toluene was calculated on the basis of the following equation: Weight of the dried gel.

Content in % = 100 2 2. Determination of the swelling index of the elastomeric phase in toluene.

3 g of polymer were dispersed in 100 cc of toluene. After centrifuging at 10,400 X G the insoluble part was separated by decanting, in the form of a gel. This gel was repeatedly washed with toluene and then centrifuged until the washing solvent did not become muddy by the addition of ethanol.

Two portions of the gel thus obtained were placed on a glass filter fitted with a porous GAtype septum. Each filter was then placed in a toluene containing beaker, at room temperature and in an empty and sealed drier, so that the toluene lapped on the filter. Under such conditions, the gel absorbed the solvent and swelled. When its equilibrium was reached, the swollen gel was weighed. Thereupon the swollen gel was flocculated in ethanol, dried for 1 2 hours at 45"C under a pressure of 200 mm Hg, and finally was weighed.

The swelling index was calculated according to the relationship: weight of humid swollen gel weight of dry gel 3. The melt-index was determined according to ISO R 11 33.

4. The bending modulus was determined according to ASTM D 790 using test pieces obtained by compression moulding at 180"C under a pressure of 4 MPa. The moulded test pieces used in this test had a heat shrinkage of not greater than 3% when examined at 170"C for 1 5 minutes in the air, according to ISO DIS 2557.

5. The torsional modulus was determined according to the method described in Journal of Applied Polymer Science, vol. 14, pages 1781-1793 (1970) using the same test pieces used for determining the bending modulus.

6. The IZOD resistance was determined at 23"C, according to ASTM D 256 using test pieces compression-moulded at 180 under a pressure of 4 MPa, and having the same properties as the test pieces used for determining the bending modulus.

7. Tests for the resistance against Freon were carried out on DIN 53455 type Ill test pieces, 2.2 mm thick, and obtained in a transverse sense from extruded plates, on a laboratory scale.

The test pieces were first conditioned for 48 hours at 23"C + 1' and 50% + 5% relative humidity (U.R.), and were then subjected to a creep test in tensile, maintaining their central part, for a 40 mm stretch, in contact with liquid Freon 11. For this purpose to the lower end of the test piece, arranged vertically, in correspondence with its wider gauge, there was fixed, by means of a rubber gasket, a glass container containing Freon 11.

The test piece was then subjected to a load of 75 kg/cm2 and the time necessary for the rupture of the test piece was measured.

8. Adhesion to polyurethane (tensile adhesion test) was determined according to BS 5241.

The invention will be further described with reference to the following illustrative Examples. In the Examples all quantities are parts by weight, unless otherwise indicated.

EXAMPLES nos. 1-7: Into a reactor, fitted with an anchor stirrer, a reflux coolant and a thermometer, were loaded, under continuous stirring: a mixture of styrene-acrylonitrile monomers in the quantity indicated in Table 1; -polybutadiene rubber having a 1-4 cis content of 35% and a Mooney viscosity of 35, in the quantity indicated in Table 1; -mineral oil of the light type, in the quantity indicated in Table 1; -ter.dodecyl-mercaptane, as a chain transfer, in the quantity indicated in Table 1; -ter.butyl-perbenzoate, as a polymerization initiator, in the quantity indicated in Table 1.

After the elimination of the oxygen present in the reactor, by means of bleeding with nitrogen, the mixture was subjected for 4 hours to a bulk-prepolymerization at 110"C, until a conversion of about 30% was obtained.

During the prepolymerization there was added a further quantity of ter. dodecyl-mercaptane, in the quantity indicated in Table 1. The prepolymerization syrup was transferred into an autoclave fitted with an impeller stirrer, and containing water, in a water/monomer + rubber weight ratio of 1:1, to which was added 0.5% by weight, with respect to the water, of a suspendant consisting of an acrylic acid/2-ethyl-hexyl-acrylate copolymer, in a weight ratio of 95:5.

After the addition of 0.20% by weight, with respect to the monomer-rubber mixture, of ter.butyl-perbenzoate, the mixture, dispersed in water in the form of droplets, was polymerized for 4 hours at 1 15'C and for 2 hours at 14OC, until the complete conversion of monomer to polymer was obtained.

The polymer thus obtained was separated by centrifuging, then repeatedly washed with water and finally dried at 80 C. To the polymer beads thus obtained were added 0. 15% by weight of a phenolic antioxidizer and the beads were then transformed into granules by extrusion.

In Table 2 are indicated the properties of the polymers thus obtained. The various properties were determined by the respective methods described above.

TABLE 1 Examples No.

SUBSTANCE 1 2 3 4 5* 6 7 styrene 92 88 84 84 84 84 80 -Acrylonitrile 0 4 8 8 8 8 12 -Rubber 8 8 8 8 8 8 8 -Mineral oil 2.5 2.5 2.5 2.5 2.5 2.5 2.5 I Addition 0.06 0.06 0.03 0.06 0.06 0.08 0.08 -Ter.dodecyl-mercaptane II Addition 0.04 0.04 0.07 0.04 0.04 0.02 0.02 -Ter.butyl-perbenzoate 0.05 0.05 0.025 0.025 0.025 0.025 0.025 REMARK: The quantities herein above expressed are intended to be in percent by weight with respect to the monomers + rubber mixture 'The suspension polymerization of the pre-polymerization syrup was conducted for 4 hours at 11 5'C and at 155 C for 2 and a half hours.

TABLE 2 EXAMPLE Nos.

PROPERTY 1 2 3 4 5 6 7 -Content of insoluble elastomer in % 30 26.7 23.4 24.1 27.6 26 21.9 -Swelling index 12.5 14.1 12 14.4 8.6 13 13.5 -Mean cord of particles (1) 3.5 2.1 1.5 2.2 1.9 2.5 2.2 -Torsional modulus in kg/sq.cm 5,100 5,600 7,200 6,200 6,400 5,700 6,400 -Bending modulus in kg/sq.cm 14,300 14,900 19,400 17,100 17,700 15,500 17,400 -IZOD impact resistance kg/cm/cm (notched) 6.2 8.0 10.1 10.1 5.3 7.7 9.5 -Melt-Index (200/5) g/10 min. 3.5 3.0 2.1 2.1 2.5 2.0 1.6 -Resistance to Freon 11, in minutes 15 17 29 29 40 28 58 -Adhesion to he polyurethane, kg/cm 0.8 1.3 1.3 1.4 1.4 1.6 1 (1) The mean cord of the particles is determined in the following way: On each micrography (1,000 enlargements) there are traced 12 straight lines passing through the center of the photograph, with an angle of 15 with respect to each other. On each straight line are then measured: the sum of the lenghts of the chords (Lp) which intercept the particles and the number (Na) of the particles cut by the line. Then one finds the mean of the 12 values and the means cord is then determined according to the relationship: Lp C = .

Na

Claims (6)

1. A modified vinyl-aromatic polymer containing from 6% to 12% by weight of an ethylenically unsaturated nitrile bound to a two-phase system, and having the following characteristics: a) the polymer has an elastomeric phase insoluble in toluene which comprises at least 23% by weight; b) the elastomeric phase has a swelling index in toluene greater than 10; c) the polymer has a melt-index of at least 1.5; d) the polymer has a bending modulus greater than 15,000 kg/sq.cm; e) the polymer has a torsional modulus greater than 5,500 kg/sq.cm; and f) the polymer has an IZOD resistance at 23"C greater than 7 kg cm/cm.

2. A polymer as claimed in claim 1, having a resistance to fissuring, by the action of Freon and under a load of 75 kg/sq.cm., greater than 25 minutes.

3. A polymer as claimed in claim 1 or 2, which develops an adhesion to polyurethane of at least 1.3 kg/sq.cm.

4. A polymer as claimed in any of claims 1 to 3, wherein the ethylenically unsaturated nitrile is acrylonitrile.

5. A polymer as claimed in any of claims 1 to 4, wherein the vinyl-aromatic monomer is styrene.

6. A modified vinyl-aromatic polymer according to claim 1, substantially as herein described in any of the foregoing Examples.