Process for the production of elastomer-modified thermoplastics

Details Process for the production of elastomer-modified thermoplastics Process for the production of elastomer-modified thermoplastics

2000-07-12

2002-10-15

Cain, Edward J. (Department: 1714)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Mixing of two or more solid polymers; mixing of solid...

C525S054000, C525S070000, C525S071000

Reexamination Certificate

active

06465570

ABSTRACT:

The present invention relates to a continuous process for the production of elastomer-modified thermoplastics. An elastomer, in particular, an optionally particulate graft polymer with a water moisture content of 1 to 50%, preferably 10 to 40%, is incorporated in a large volume kneader reactor into a thermoplastics melt. The mixing ratio of elastomer to thermoplastics melt is in particular 4:1 to 1:4. The energy required to melt the elastomer and to evaporate the adhering water is added by heating the apparatus walls and through the kneading action of the installed rotating parts.
Elastomer-modified thermoplastic polymers, for example rubber-filled plastics, are normally produced starting from rubber by emulsion graft polymerisation onto a rubber base. The thermoplastic matrix can be economically produced in a bulk or solution process. In emulsion polymerisation a graft polymer for example is produced having a high rubber content (typically 50 wt. %), which after being worked up is compounded with a matrix polymer. The working up of the graft latex takes place via the following processing steps: precipitation, washing, mechanical and thermal drying.
The thermal drying of a graft latex in the solid phase requires of course a very high energy input, and on account of the danger of a dust explosion associated with the drying this procedure is carried out under inert conditions in driers. An object of the invention is accordingly to reduce considerably the energy expenditure compared to the aforementioned processes.
In addition to the commonly employed combination of powder drying followed by compounding with the thermoplastic material, processes for the impact resistance modification of thermoplastics have already been described in the prior art, which are based on incorporating rubber lattices that have been mechanically only partially dewatered, directly into thermoplastic polymers in a screw extruder (see for example D20 37 784). Modified extruder processes are described in the European laid-open specifications EP 0 534 235 A1, EP 0 665 095 A1, EP 0 735 077 A1, EP 0 735 078 A1, EP 0 734 825 A1 and EP 0734 826 A1.
A particular disadvantage of these processes is the high degree of stress to which the rubber/thermoplastics mixture is subjected on account of the high shear rate of up to 1000 s
−1
in screw extruders.
A further disadvantage is the multistage procedure involved in the last-mentioned process, since first of all water and then the melt mixture is removed, and in a further step the residual degassing of the polymer is carried out.
Since the energy in the extruder is mainly introduced as mechanical energy via the screw shafts, it is possible only to a limited extent to control the energy input via the added heat and avoid thermally stressing the polymers.
The object of the invention is to provide a direct processing method that avoids the danger of dust explosion inherent in processes involving powder drying and that overcomes the various disadvantages of processing in screw extruders.
The invention provides, by means of which the aforementioned objective is achieved, a process for producing elastomer-modified thermoplastics by mixing elastomer, especially rubber, that is surrounded by graft polymerisation with a sheath of a thermoplastics material A, with a thermoplastics material B, the moist elastomer, especially a graft polymer that has been precipitated from a latex, being dewatered to 1 to 50 wt. %, especially 5 to 50 wt. %, and particularly preferably 10 to 40 wt. % residual moisture, and into which is mixed the thermoplastics material B present as melt, which process is characterised in that the evaporation of the process water adhering to the elastomer, especially to the graft polymer, the melting of the elastomer, the compounding of the elastomer with the melt of the thermoplastics material B, as well as the removal of further organic volatile constituents C, are carried out simultaneously in one process chamber.
The dewatering in the first stage is preferably carried out mechanically, for example by squeezing out the water or by centrifugation.
In particular, the energy necessary for the melting, heating and evaporation of the polymer mixture is introduced mechanically via the kneading action of the rotors, and thermally via the surface of the housing of the kneader reactor, the ratio of the mechanical energy and thermal energy to be added to the mixture preferably being 4:1 to 1:6, particularly preferably 2.5:1 to 1:4.
The process is preferably carried out in a large volume, partially filled kneader reactor with installed rotating parts, in which the throughput per liter of process space is not more than 5 kg/hr of polymer. The residence time of the miter in the process space is preferably 2 to 20 minutes.
The kneading action of the rotors is influenced in particular independently of their rotational speed, by controlling the degree of filling of the machine.
In a further preferred variant of the process the water adhering to the elastomer and further volatile constituents C are removed in only one pressure stage, in particular at a pressure of 10 to 2000 hPa.
The dewatering of the polymers is carried out in particular in a kneader reactor in which the mixing and kneading actions of the installed kneader reactor parts are largely decoupled from the transporting movement of the material being mixed.
The mixing ratio of thermoplastics material B and elastomer can in a preferred process be varied in the range from 1:4 to 4:1 without altering the process parameters or the equipment configuration used in the process.
In the mixing of elastomer and thermoplastics material B, in particular additional pigments and/or additives are prepared and processed so that they can be added to the mixture before the latter is dewatered.
In a process variant pigments and/or additives are preferably added to the polymer mixture in a mixer, in particular a static mixer, connected downstream of the process chamber.
Suitable kneader reactors for carrying out the process according to the invention are those which can successfully mix tough plastic phases, for example those disclosed in the publications EP 0 517 068 A1, EP 460 466 B1, EP 0 528 210 A1 or JP-A-63232828. Twin-shaft reactors corresponding to specification EP 0 517 068 A1 are preferably used. Since the mechanical stress to which the rotors are subjected and the required power are in some cases considerably higher than in conventional applications of this type of equipment, it may be necessary with commercially available equipment to reinforce the rotors and choose a considerably more powerful drive unit compared to the conventional specification.
The moist elastomer, especially graft polymer, is in a preferred embodiment added by means of a stuffing screw or a piston lock. Alternatively the elastomer may be added via a strainer-type or squeezer-type screw with partial mechanical removal of the moisture. Furthermore, in the preferred embodiment the thermoplastics melt is added via the inlet-side front plate of the kneader reactor, which prevents the generally temperature-sensitive elastomer coming into contact with the hot surfaces of the housing. Instead, as soon as the elastomer enters the large volume kneader reactor it is embedded in the thermoplastics melt B. Also the harmful influence of any possible by-products on the resultant mire due to the relatively long educt residence time in the initial section of the kneader reactor is avoided. The dewatered, degassed and compounded, elastomer-modified thermoplastic material is discharged from the kneader reactor preferably via a discharge screw or gear pump at or in the vicinity of the front plate opposite the inlet. The reactor volume is optimally utilised by means of this arrangement. A melt screening and granulation stage according to the prior art may be coupled to the discharge unit. The vapours are removed through a degassing vent which is preferably arranged in the vicinity of the product discharge outlet, and can then be condensed in a manner known per se. If

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