WO1996018666A1 - Process for producing impact-resistant modified polystyrene molding compounds - Google Patents

Abstract

The invention pertains to a process for producing impact-resistant modified polystyrene molding compounds by continuous anionic polymerization of styrene monomers in the presence of a rubber in a single reaction zone, wherein a styrene solution containing as rubber a styrene-butadiene block rubber or a mixture of a styrene-butadiene block rubber with a polybutadiene rubber, the styrene content of which, relative to the rubber or rubber mixture, is between 10 and 90 wt %, is fed to a reaction mixture, the polymer content of which is above the phase inversion point. The invention also pertains to a molding compound produced by this process and its use in manufacturing sheets and moldings.

Description

 Process for the production of impact modified polystyroi molding compounds

description

The invention relates to a process for the preparation of impact-modified polystyrene (polyvinylaromatic) molding compositions by continuous anionic polymerization of styrene monomers in the presence of a rubber in a single reaction zone.

It is the technically customary process to impact-modified polystyrene molding compositions by radical polymerization of styrene in the presence of the rubber used for the modification in multi-stage plants, i.e. a series of stirred tanks and / or tower reactors (DE-A-1 770 392; DE-A-40 46 718; US Patents 2 727 884; 3 903 202) so that the so-called phase inversion can take place under controlled conditions. The phase inversion takes place when the conversion of the reaction mixture exceeds the concentration of the rubber in the solution. In order to achieve a high rubber efficiency, efforts are made to feed the monomer feed containing rubber into a reaction mixture whose conversion is below the phase inversion point. Only in the second and possibly further steps, i.e. In reaction zones, sales will continue to increase. This is an essential reason for the production of impact-resistant molding compositions in polymerisation cascades.

The radical polymerization has the disadvantage that volatile constituents (residual monomer and low molecular weight oligomers) remain in the molding compositions despite the sometimes complex degassing processes and lead to complaints because of the smell and physiological concerns or lower the softening point.

It is also known to polymerize styrene anionically using initiators of the lithium alkyl type (EP 176 611; US Pat. No. 3,035,033). However, this process is not used industrially as too complex for the production of styrene homopolymers. This process can also only be used to obtain unmodified polystyrene. The molding materials that can be produced would also be too brittle and unsuitable for a number of applications.

It was therefore an object of the present invention to find a process which can be used to produce tough modified thermoplastic materials which are practically free of residual monomers and are low molecular weight, especially cyclic oligomers (dimers and trimers of the phenyltetralin type).

The object is achieved in that, in a process of the type mentioned, the polymerization is carried out above the phase inversion point by continuously feeding a styrene solution containing rubber to a reaction mixture whose polymer content is above the phase inversion point and a styrene rubber Butadiene block rubber or a mixture of a styrene-butadiene block rubber with a polybutadiene rubber is used, the styrene content of which, based on the total amount of the rubber or rubber mixture used, is between 10 and 90% by weight.

The block copolymers themselves contain 5 to 95, preferably 15 to 85% by weight of units of diene and correspondingly 95 to 5, preferably 85 to 15% by weight of styrene.

What is to be understood by phase inversion and phase inversion point is described in detail in the specialist literature (cf., for example, GE Molau and H. Keskula; J. Polym. Sei. Al, 1595 (1966) or Angew. Makromol. Chemie 58/59, 175 (1977)). Of course, phase inversion also takes place in anionic polymerization, provided that the composition of the reaction mixture of styrene monomer, rubber and styrene polymer corresponds to the conditions in free-radical polymerization.

Suitable block rubbers are styrene-butadiene block rubbers which have blocks or block sequences of the structure (SB) _n , n being an integer from 1 or more, preferably 1 to 10, and S being a styrene polymer block or generally a polymer block a vinyl aromatic compound and B represents a poly (buta) diene block and linear block copolymers of the type S-BS and BSB.

Instead of styrene, it is also possible to use alpha-methylstyrene, p-methylstyrene, t-butylstyrene or 1,1-diphenylethylene or blocks composed of mixtures of the monomers mentioned.

Suitable as component B are polymers, in particular of butadiene, furthermore 2, 3-dimethylbutadiene, isoprene, piperylene or 1,3-hexadiene and mixtures thereof.

The transitions between the blocks can be both sharply separated and smeared. The total molecular weight of the block copolymers can be in the range from 50,000 to 500,000, preferably between 100,000 and 350,000, measured by means of gel permeation. tion chromatography (GPC) as value M _p (peak aximum) using calibration standards based on polystyrene. Also particularly suitable are block copolymers which contain blocks of randomly distributed styrene and butadiene units, such (S / B) blocks being both in addition to and instead of S and of B blocks of the type mentioned above. This type of block copolymer can be used to produce impact-resistant and at the same time transparent molding compounds.

If translucency or transparency is dispensed with, it is also possible to use mixtures of polydienes and block copolymers instead of the pure block copolymers, provided that the polymer is composed of the same monomer unit as the soft component block in the block copolymer. Accordingly, the hard component of the block copolymer should be polymerized with the same monomer unit as the matrix of the molding compound.

To produce weather-resistant and impact-modified polymers, it is expedient to hydrogenate the aliphatic C = C double bonds. This can be done according to known methods, e.g. in European Patent 475,461 and U.S. Patents 4,656,230 or 4,629,767.

For the polymer compositions according to the invention, the block rubbers or the mixtures of polydienes and block rubbers are used in such an amount that the diene content in the end product is in the range from 2 to 50, preferably from 4 to 35,% by weight.

The block rubbers or mixtures of polydienes and block rubbers mentioned are dispersed in the thermoplastic molding composition in the form of small particles (average diameter 0.1 to 15 μm), while the coherent phase (the matrix) is composed of styrene polymers.

All anionically polymerizable aromatic vinyl compounds are suitable as styrene polymers for the matrix. Examples include styrene, alpha-methylstyrene, vinyltoluene, vinylxylene, t-butylstyrene, vinylnaphthalene and 1,1-diphenylethylene and mixtures thereof.

The monomers mentioned are expediently reacted in a solvent which, as is generally known, must not react with the organometallic initiator usually used to initiate the polymerization. In general, therefore, both aliphatic and aromatic hydrocarbons are suitable. Suitable solvents are, for example, cyclohexane, methylcyclohexane, benzene, toluene, ethylbenzene or xylene.

The polymerization is initiated by means of organometallic compounds, as is customary in the case of a polymerization which proceeds according to the anionic mechanism. Compounds of alkali metals, in particular lithium, are preferred. Examples of initiators are methyl lithium, ethyl lithium, propyllithium, n-butyllithium, sec-butyllithium and tert-butyllithium. The organometallic compound is generally added as a solution in a hydrocarbon. The dosage depends on the desired molecular weight of the polymer, but is generally in the range from 0.002 to 5 mol% by weight, if it is based on the monomers.

In order to produce a constant molecular weight of the matrix, it is expedient, after the rubber has been loosened, to dissolve the solution with butyllithium using an indicator such as e.g. Titrate 1, 1-diphenylethylene, i.e. Traces of water or similar remove before the actual implementation begins.

To achieve a higher polymerization rate, small amounts of polar, aprotic solvents can be added as cosolvents. Diethyl ether, diisopropyl ether, diethylene glycol dimethyl ether, diethylene glycol di-butyl ether or, in particular, tetrahydrofuran are suitable, for example. In this process variant, the polar cosolvent is generally added to the nonpolar solvent in a small amount of about 0.5 to 5% by volume. Tetrahydrofuran (THF) is particularly preferred in an amount of 0.1 to 0.3% by volume.

The polymerization temperature can be between 0 and 130 ° C. Temperatures of 50 to 90 ° C. are preferred. Appropriately under isothermal conditions, i.e. H. polymerized essentially at constant temperature.

It is characteristic of the polymerization process according to the invention that no phase inversion takes place, since the solids content in the reactor is always above the phase inversion point. The incoming rubber solution is rather distributed immediately after entering the reactor in the coherent phase of the matrix polymer. Accordingly, the conversion of the monomer which builds up the hard matrix must, as a rule of thumb, be at least 3% by weight greater than the rubber concentration in the monomer feed. Solids contents of more than 40% by weight are preferably set during the continuous polymerization.

Stirred vessels with anchor or cross-bar stirrers are suitable as the polymerization reactor, the heat of polymerization being removed via external heat exchangers or by evaporative cooling. However, so-called loop reactors are also suitable, which can be designed as tube bundle reactors or so-called static mixers (tube sections with internals made of sheet metal strip fabrics or the like).

The polymer solution emerging from the reactor is expediently likewise passed over a static mixer which, on the one hand, serves as a short residence time zone in order to allow traces of residual monomers to react and, on the other hand, is used if the relevant active compound has been added immediately beforehand, the living Break off chain ends and, if necessary, add additives such as lubricants, antistatic antioxidants, UV stabilizers, flame retardants, fillers etc. or to add z. B. to mix in peroxides, which cause crosslinking of the rubber particles during subsequent heating (as occurs in the downstream degassing zone).

Proton-active substances such as carboxylic acids, alcohols, phenols, thio compounds, sulfonic or phosphonic acids, mineral acids and in particular water and carbon dioxide serve as means for terminating the chain ends.

The advantage of the process according to the invention results from the fact that products with residual monomer contents of less than 30 ppm are obtained and that these thermoplastic molding compositions do not contain any oligomers (dimers and trimers of the phenyltetraline derivative type). The possibility of producing tough-modified thermoplastics in only one reaction zone also offers economic advantages.

The molding compositions obtained by the process of this invention can be processed by the known methods for processing thermoplastics, for example by extrusion, injection molding, Ka ^¬ country Center, blow molding, pressing or sintering; especially be ^¬ be vorzugt made of the process of the invention molding compounds made her¬ moldings by injection molding. The molding compositions can also be used for blending with other contractual polymers. For example, the impact-resistant polystyrene molding compositions prepared by this process are particularly suitable for blending with polyphenylene ether and for the production of flame-retardant products, the usual halogen and phosphorus compounds and phosphazenes or triazenes (for example melamine) being suitable as flame retardants are gnet.

The solvents used in the following examples were dried over aluminum oxide. The monomers used were i. V., the 1,1-diphenylethylene used was distilled over n-butyl lithium.

example 1

A styrene-butadiene two-block rubber with a molecular weight M _n of 180,000 was dissolved in cyclohexane, so that a 20% solution was obtained. The impurities were removed with a 1% solution of sec-butyllithium in cyclohexane and with

1, 1-Diphenylethylene titrated as an indicator until the red color persists.

A solution of 2.8 l of styrene and 1.75 l of the

Rubber solution in 1 1 cyclohexane and 110 ml of a 1% by weight solution of sec. Butyllithium in cyclohexane were continuously fed to a 10 1 reactor and polymerized at 80 ° C. The conversion was 99.5% by weight. The polymer solution was continuously discharged from the reactor and fed to a degassing extruder on which the polymer solution degassed at 220 ° C. by means of a static mixer, in which an excess of carbon dioxide was added to the polymer solution (based on the initiator used) has been.

Example 2

A block rubber of the type S- (S / B) -S with a molecular weight of M _n = 142,000 and a butadiene content of 45% by weight was dissolved in cyclohexane and the impurities were titrated with n-butyllithium. The concentration of the solution was 25% by weight. (With (S / B) - see above - a statistically structured middle block is called).

Hourly, 5.3 l of styrene, 3.6 l of rubber solution, 1.0 l of cyclohexane and 180 ml of sec. Butyllithium solution in cyclohexane were continuously pumped into a 10 l reactor and polymerized at 88 ° C. The solids content was 55.9% by weight. The polymer stream emerging from the reactor was mixed in a static mixer as in Example 1 with a mixture of CO and water and in an extruder at 240 ° C. with the addition of 2% by weight of paraffin oil, based on the expected amount of polymer risat, degassed.

Example 3

A three-block rubber of the type S / DPE-B-S / DPE with two external, statistically constructed blocks made of styrene and

1,1-diphenylethylene (DPE content in the S / DPE block = 30% by weight), a medium polybutadiene block (polybutadiene content = 35% by weight) and a molecular weight M _n = 230,000 were dissolved in methylcyclohexane. The 15% by weight solution was freed of impurities by titration with sec. 15 butyllithium.

120 parts of the above rubber solution, 70 parts of styrene and 30 parts of 1,1-diphenylethylene were mixed and pumped into the 10 1 reactor at a rate of 4 l / h. Separately 20 of these, 75 ml / h of a 1% by weight solution of sec-butyllithium in cyclohexane were metered in. The polymerization temperature was kept at 75 ° C. Working up was carried out as indicated in Example 1.

25 Example 4

The rubber from Example 1 was hydrogenated as follows.

10 1 of the 20% by weight rubber solution in cyclohexane were mixed with 1 30 g of nickel acetylacetonate and 2 g of triethyl aluminum and hydrogenated at 90 ° C. in an autoclave under a hydrogen pressure of 20 bar. The rubber was precipitated in methanol and dried sharply.

The 10% hydrogenated rubber was dissolved in methylcyclohexane. After titration of the impurities, 100 parts of the solution were mixed with 90 parts of styrene. This mixture was pumped into the reactor at a rate of 5 l / h and 1% by weight sec. Butyllithium solution was added with the addition of 80 ml / h

40 90 ° C polymerized. The living chain ends were broken off as described in Example 1 using isopropanol.

45

Claims

Claims. 1. Process for the preparation of impact-modified polystyrene molding compositions by continuous anionic polymerization of styrene monomers in the presence of a rubber in a reaction zone, characterized in that the polymerization is carried out above the phase inversion point by continuously using a rubber-containing styrene Feeds a solution to a reaction mixture, the polymer content of which is above the phase inversion point and the rubber used is a styrene-butadiene block rubber or a mixture of a styrene-butadiene block rubber with a polybutadiene rubber, the styrene content of which is based on the total amount of the rubber used or rubber mixture used, is between 10 and 90% by weight. 2. The method according to claim 1, characterized in that the styrene content of the block copolymer, based on the total amount of rubber is greater than 8 wt .-%. 3. The method according to claim 1, characterized in that styrene-butadiene block rubbers are used as block copolymers which have blocks or block sequences of the structure (SB) _n , where n is an integer of 1 or more and where S and B represents a block composed of styrene, a derivative of styrene or 1,1-diphenylethylene or a diene or linear block copolymers of the type SBS or BSB. 4. The method according to claim 3, characterized in that the Block copolymers instead of or in addition to at least one block B or S contain at least one block (S / B) which consists of randomly distributed units of the styrene monomer and a diene monomer. A method according to claim 1, characterized in that the reaction zone consists of a stirred tank. 6. The method according to claim 1, characterized in that the reaction zone consists of a stirred tank, which is followed by a static mixer. 7. Impact-modified polystyrene molding composition based on a styrene polymer with rubber dispersed therein in particulate form, as obtained by the method of claim 1, characterized in that the residual content of monomers is less than 30 ppm and the molding composition does not contains cyclic oligomers (dimers and trimers of the phenyltetraolin type). 8. Molding composition according to claim 7, characterized in that the block copolymers and polydienes used as rubber are hydrated. 9. Molding composition according to claim 7, containing a polyphenylene ether. 10. Use of a thermoplastic molding composition according to one of claims 7 to 9 for the production of films, sheets and moldings.