1. Field of the Invention
The invention relates to a process for the continuous production of room-temperature-crosslinkable organopolysiloxane compositions in a mixer having a mixing zone comprising a rotor-stator system.
2. Description of the Related Art
One-component organopolysiloxane compositions which can be stored with exclusion of moisture and crosslink at room temperature on admission of moisture, known as RTV-1 compositions, have been known for a long time.
Various methods of producing RTV-1 compositions have been described. Owing to their high productivity, continuous processes are particularly preferred. Such processes exhibit compounding problems, however, for example the continuous and homogeneous mixing of fillers into the highly viscous polymer components, particularly when the fillers are reinforcing fillers having large surface areas. The mixture should be produced in such a way that no relatively large filler particles are visible in the finished product. A further difficulty is presented by water necessarily introduced with the fillers. When the mixture is not completely homogeneous, even days after production of the mixture, inhomogeneities may be formed by crosslinking of the RTV-1 mixture around the filler particles which have not been homogeneously admixed due to local high concentrations of water. Thus, for example in EP-B-234 226, parts of the constituents are combined in a continuously operating closed mixer in a first step, and in a second step, the remaining constituents are introduced in a reciprocating kneader and the composition is homogenized in this kneader. To allow reaction times between the individual mixing steps, a further three-stage process was developed as described in EP-B-940 445. EP-B-739 652 describes a process which makes do without such a reaction time but indicates that the water present in the mixture which has been introduced via the raw materials has to be removed in a step following the production of the mixture. Since water can still be detected in the compositions, the mixing in of the fillers by means of the apparatus is obviously unsatisfactory. The mixture contains incompletely wetted filler particles in which the water is located. If mixing is complete, then the water reacts irreversibly with the crosslinker and would thus no longer be able to be detected.
A further process described in EP-B-688 598 utilizes a specially constructed mixer which comprises two mixing chambers and in which a mixture of liquids and a powder is firstly produced in a first, upper mixing chamber and the mixture is then strongly sheared again to homogenize it by means of a second independent mixing apparatus, further liquid components being mixed in the second, lower mixing chamber.
EP-A-1 008 613 describes the use of a mixing turbine having axial flow. This mixing turbine was previously used in EP B 234 226 in combination with a reciprocating kneader. The mixing results obtained by means of such a mixer are excellent, but a significant disadvantage is that this type of mixer is not self-cleaning, i.e. coatings of filler and liquid build up in the interior of the mixer and after more than 24 hours of operation lead to a deterioration in the sealant quality due to filler particles. This problem can generally be solved only by frequent cleaning of the mixing turbine.
U.S. Pat. No. 3,194,540 discloses mixers which operate according to the rotor-stator principle. Such mixers are utilized both for emulsion production and for mixing of solids into liquids. In these apparatuses, a rotor rotates within a stationary stator. The stator has slits which can have a variable width. The rotor can likewise have slits of this type which can be supplemented by evolute-shaped transport elements. It is also possible for only the transport elements to be present. The rotor sucks the material in and flings it radially outward through any slits it may have and the slits of the stator. The stator teeth which bound these slits act as an impingement surface, and the gap between rotor and stator leads to shear. These mixers are generally constructed so that up to three rotor-stator pairs can be installed in series, in each case with different slit widths. The more demanding the dispersion task, the more tools having narrower slit widths are installed. DE-B 101 46 395 discloses, for example, a mixing system of this type for producing water-free RTV-1 compositions based on polydiorganosiloxanes. However, a disadvantage of the process is that it is relatively inflexible and cannot quickly be adapted to altered mixing tasks.