CA2212844A1 - Process for the preparation of thermoplastic, stress-cracking resistant moulding compositions - Google Patents

Abstract

The moulding compositions prepared from styrene and special polybutadiene by theprocess according to the invention are distinguished by having an excellent impact strength and stress cracking resistance combined with the very good surface condition of the mouldings produced therefrom.

Description

~ CA 02212844 1997-08-13 Le A 31977 / Bg/Ke/S-P
Forei gn Countri es Process for the preparation of thermoplastic. stress-crackin~ resistant mouldin~compositions The present invention relates to a process for the preparation of impact-resistant 5 thermoplastic moulding composilions by polymerisation of aryl-substituted ethylene in the presence of polybutadiene. The moulding compositions prepared by this process are used for the production of mouldings, p~ cil,ally by injection moulding.

Compared with unmodified polymers, rubber-modified polymers have an improved 10 impact strength and stress cracking resistance .

The most usual method for preparing such rubber-modified polymers is the polymerisation of the monomer in the presence of rubber. HIPS ("high-impact polystyrenel') and ABS (acrylonitrile-butadiene-styrene copolymers3 are the best-15 known representatives: there are surveys in Ullm~nn.c Enzyklopadie der technischenChemie, 4th Edition. Vol. 19, 226 ff., Verlag Chemie, Weinheim 1980 and in Angew. Chem. 93. 372 (1981). In most cases the rubber used for the modification is polybutadiene 20 It is well-known that utensils made of impact-resistant modified styrene polymers are susceptible to stress corrosion cracking as soon as they come into contact with triggering agents. Numerous organic media are possible agents of this kind.
Particularly well-known agents which trigger stress cracking are on the one handfats and oils and on the other hand organic blowing agents such as, for example, the 25 halogenated hydrocarbons conf~ining fluorine and chlorine. The adverse effect of the former becomes apparent in the packaging of foods in polystyrene and of the latter, for example, in the production of domestic refrigerators using impact-resistant polystyrene. Thus packa~ing cartons made of impact-resistant polystyrene, in which margarine is cont~in~-l. easily rupture radially at the base when they are stacked one 30 upon the other~ while internal containers in refrigerators suffer this fate during back ' CA 02212844 1997-08-13 Le A 31 977 foaming with polyurethane foam, owing to the halogenated hydrocarbon blowing agent employed.

This adverse feature limits the possible uses of impact-resistant polystyrene in these 5 sectors~ To circumvent this feature, one is either compelled to employ expensive technical procedures - for example, the insertion of intermediate layers when back foaming the internal containers in refrigerators - in order to protect the material from attack by the aggressive medium, or one changes over to more expensive materials such as. for example? styrene-acrylonitrile copolymers which are impact-10 resistant modified by polydienes (so-called ABS polymers).

It has already been attempted to improve Ihe stress cracking resistance of impact-resistant modified styrene polymers by the supplementary addition of further rubber.
Only a small effect could be observed. however, even with large additions of 15 rubber. The object of the present invention was accordingly to find and prepare styrene polymers which are impact-resistant modified with rubber and possess an increased stress crackin(J resistance.

DE-AS 17 20 972 discloses a process for the preparation of impact-resistant, 20 thermoplastic synthetic resins by graft polymerisation of aromatic monovinyl compounds and synthetic rubber. Plastics compositions having high shock resistance and high impact strength and based on monovinyl aromatic polymers are also knownfrom DE-OS 22 52 ~51. The plastics compositions described in the documents citedabove have an improved shock resistance and impact strength as well as stress 25 cracking resistance which is. however, worthy of further improvement.

The use of rubbers produced with the aid of lithium catalysts (Li-BR) is also known from the prior art. The disadvantage of these rubbers is the e~ccessively high grafting activity, which results in excessively high gel contents. The rubber particles 30 therefore contain an e~ccessively high proportion of occlusions and this impairs the stress cracking resistance. At the degassing temperatures conventionally applied in Le A 31977 the process Li-BR also tends to overcure, which considerably decreases the impact strength.

The use of rubbers produced with the aid of cobalt catalysts (Co-BR) is also prior art. This type of rubber has an excessively high cold flow and an excessively high tackiness, which lead to problems in h~n lling and processing. Co-BR also tends to form hard spots (i.e. defects, overcured compact rubber particles), the adverse effect of which becomes apparent particularly through defects on the surface of mouldings produced by extrusion. This type of defect also lowers the impact strength.
It has now surprisin~ly been found that impact-resistant thermoplastic moulding compositions having improved stress cracking resistance are obtained by using a particular diene rubber or mixture of diene rubbers. These diene rubbers or mixtures of diene rubbers are distinguished by having an outstanding processability, and the thermoplastic moulding compositions produced from them are distinguished by excellent impact strength and stress cracking resistance combined with the very good surface condition of the moulded parts produced therefrom.

The invention accordingly provides a process for the preparation of thermoplastic moulding compositions by polymerisation of aryl-substituted ethylene, characterised in that I. 75 to 99 parts by weight of styrene is polymerised in the conventional manner with II. 1 to 25 parts by weight of polybutadiene composed of A. 50 to 100 wt. %, based on component II, of polybutadiene having at least 90% cis-1,4-bonding, a solution viscosity (SV) of more than 100 up to 10.000 mPa s, ' CA 02212844 1997-08-13 Le A 31 977 a Mooney viscosity (MV) of 30 to 80 ML (1+4) 100~C and a quotient MV/SV
of less than 0.8 and B. 0 to 50 wt. %, based on component II, of polybutadiene having 5%
to 25 % 1 .2-bonding, a solution viscosity (SV) of 20 to 300 mPa s, a Mooney viscosity (MV) of 30 to 100 ML (1+4) 100~C and a quotient MV/SV
of 0.3 to 2Ø

The polybutadiene used (component A) having a high proportion of cis-1,4-bondingand the other variables required according to the claim can be prepared using 15 organometallic mixed catalysts of the type described for example in EP-A 7027 and 11 184.

These mixed catalysts contain mixtures or reaction products of at least one carboxylate of the rare earths, at least one dialkylaluminium hydride or trialkyl-20 aluminium and at least one Lewis acid. Preferred rare earths are l~nth~mlm, cerium,praseodymium and neodymium and mixtures thereof, neodymium and neodymium/praseodvmium mixtures being particularly preferred.

Polybutadienes (component A) which are modified by means of the so-called 25 Mooney-Sprung reaction in the course of their preparation are also suitable. Here suitable reagents are reacted with the reactive polymer chain. Suitable modifiedpolybutadienes of this kind are described in EP-A 406 920 and in the German Patent Application P 4 436 059.2.

30 The polybutadienes (component B) used in the process according to the invention are those which have been prepared based on Li catalvsts. The preparation of these ~ CA 02212844 1997-08-13 ~ Le A 31 977 Li-polybutadienes is known and is described in "Ullmann's Encyclopedia of Industrial Chemistry" Vol. A 23, pages 271-276 and in the literature cited there.

The solution viscosity of the polybutadienes is measured at 25~C against a 5 wt. %
5 solution of the diene polymer in styrene, using a Brookfield viscosimeter (Brook-field-RV, Synchro-Lectric, Model LVT, Spindle 2; rate of rotation adjustable to set values, according to viscosity: 6, 12, 30, 60 rev/min).

The Mooney viscosity is determined in accordance with DIN 53 523.
10 The quotient ML/SV can be regarded as a measure of the so-called long-chain branching of the polybutadienes.

The moulding compositions prepared by the process according to the invention aresuitable for the production of mouldings by injection moulding and in particular by 15 extrusion.

Le A 31 977 Examples Products used Polybutadiene HX 529 SIC is a cornmercial product of Bayer AG, Leverkusen; it is an Li-polybutadiene having the following properties:

38% cis-1,4-bonding 51% trans-1,4-bonding 11% 1,2-bonding SV = 170 mPa s MV = 55 ML (1+4) 100~C
MV/SV = 0.32 Polybutadiene CB 24 is a commercial product of Bayer AG~ Leverkusen; it is an Nd-polybutadiene having the following properties:

> 98% cis-1,4-bonding s 1% trans-1,4-bonding s 1% 1,2-bonding SV = 165 mPa-s MV = 44 ML (1+4) 100~C
MV/SV = 0.27 Polybutadiene CB 1415 is a commercial product of Bayer AG~ Leverkusen; it is a Co-polybutadiene having the following properties:

> 96% cis-1,4-bonding s 2~ trans-1,4-bonding s 2% 1,2-bonding SV = 150 mPa s MV = 45 ML (1 +4) 100~C
MV/SV = 0.3 ~ CA 02212844 1997-08-13 - Le A 31 977 Polybutadiene HX 502 C is a commercial product of Bayer AG, Leverkusen; it is an Li-polybutadiene having the following properties:

SV = 90 mPa-s MV = 38 ML (1+4) 100~C
MV/SV = 0.42 Comparison tests l + 2 and Examples 1 + 2 In an N~ atmosphere 6 parts polybutadiene (see Table) 0.2 parts o~-methylstyrene dimers and 0.5 parts Vulkanox KB~ are dissolved in 0.4 parts styrene (stabilised).

1200 g of this solution is flushed with N2 for 15 minutes in a 21 glass autoclave equipped with a helical stirrer. Over a period of 1 hour it is heated to 120~C and stirred for 4.5 hours at this temperature (80 rev/min). The highly viscous solution obtained is placed in pressure-proof aluminium moulds and polymerised in accordance with the following time/temperature programme:

2.5 hours at 125~C
1.5 hours at 135~C
1.5 hours at 145~C
1.5 hours at 165~C
2.5 hours at 225~C

After being cooled, the polymer is comminnted and degassed in a vacuum at 100~C for 20 hours. For testing, the samples are moulded in an injection moulding machine. The mechanical values are determined against standard rods.

Le A 31 977 ESCR (environmental stress cracking resistance) is deterrnined from data obtained from the tensile test in accordance with the following formula:

tensile strength - yield tension ESCR = x 100 yield tension The higher the value, the higher the stress cracking resistance.

Table 1:

TestPolybutadiene SVI)(MFR 200~C, An, 23~CAn, -40~C ESCR
[mPa-s]5 kg)[g/10 min][kJ/m~~[kT/m2]
Comparison HX 529 SIC 350 7.5 53.3 46.3 5.2 Comparison CB 1415 347 6.4 65.0 59.6 11.1 Example 1 CB 24 336 5.6 64.0 45.6 10.0 Example 2 CB 24/HX 502 C 311 6.1 50.9 59.4 3.2 (4 pares/2 parts) 1.) Solution viscosity (from Brookfield viscosimeter) of the batch before the polymerisation The resins were extruded into bands (2 cm x 0.2 cm). The number of defects on a length of 50 cm was counted.

ProduceNumber of defects Comparison test 1 8 Comparison test 2 4 Example 1

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for preparing a thermoplastic moulding composition which comprises polymerising an aryl-substituted ethylene, wherein I. 75 to 99 parts by weight of styrene is polymerised with II. 1 to 25 parts by weight of polybutadiene comprising A. 50 to 100 wt. %, based on component II, of polybutadiene having at least 90% cis-1,4-bonding, a solution viscosity (SV) of more than 100 up to 10,000 mPa-s, a Mooney viscosity (MV) of 30 to 80 ML (1+4) 100°C and a quotient MV/SV
of less than 0.8 and B. 0 to 50 wt. %, based on component II, of polybutadiene having 5% to 25% 1,2-bonding, a solution viscosity (SV) of 20 to 300 mPa-s, a Mooney viscosity (MV) of 30 to 100 ML (1+4) 100°C and a quotient MV/SV
of 0.3 to 2Ø