Method for producing polymers in which fillers are incorporated and homogeneously distributed
For the production of polymers in which there are fillers with particle sizes below 10 μm incorporated and homogeneously distributed, a polymer starting material is input into a twin-screw extruder and is melted there to give a melt. In a conveying and mixing section, a suspension, which is formed of the fillers and of a carrier liquid, is injected into the melt. The melt viscosity is reduced by injection of the carrier liquid in the conveying and mixing section in that a cleavable polycondensate is used as polymer and low-molecular-weight cleavage product arising during the polycondensation is used as carrier liquid, and therefore the molten polymer is at least to some extent depolymerized within the conveying and mixing section. That the mixture, which is formed of the melt whose viscosity is reduced by cleavage, of the remainder of the carrier liquid and of the fillers, is homogenized.
1. A method for the production of polymers in which fillers are incorporated and homogeneously distributed, the method comprising:
inputting a polymer starting material into a twin-screw extruder and melting the polymer starting material in the twin-screw extruder to provide a melt; and
injecting, in a conveying and mixing section, a suspension into the melt, the suspension comprising the fillers and a carrier liquid, the fillers having particle sizes below 10 μm;
reducing a melt viscosity of the melt by the injecting of the carrier liquid in the conveying and mixing section, wherein a cleavable polycondensate is used as a polymer in the polymer starting material and a low-molecular-weight cleavage product that arises during polycondensation of the polycondensate is used as the carrier liquid, such that the melt is at least partially depolymerized within the conveying and mixing section by cleavage;
after reducing the melt viscosity, homogenizing the melt, a remainder of the carrier liquid and the fillers; and
increasing, after the homogenization, the melt viscosity of the melt by use of a devolatilizing extruder in which, a vacuum is applied and the carrier liquid is removed from the devolatilizing extruder via the vacuum, such that a polycondensation is carried out.
2. The method as claimed in claim 1 , wherein, before the suspension is injected, the carrier liquid is injected into the melt in order to induce the cleavage.
3. The method as claimed in claim 1 , wherein a hydrolysable polycondensate is used as the polymer and water is used as the carrier liquid, such that the melt is hydrolyzed within the conveying and mixing section.
4. The method as claimed in claim 3 , wherein the polymer is polyester.
5. The method as claimed in claim 1 , wherein a polycondensate produced with elimination of a monohydric or polyhydric alcohol is used as the polymer, and a monohydric or polyhydric alcohol is used as the carrier liquid.
6. The method as claimed in claim 1 , wherein a control device controls the individual process steps and initiates a start-up procedure in which initially only the carrier liquid is injected into the conveying and mixing section to reduce the melt viscosity of the melt, and wherein, after subsequent switchover by the control device from carrier-liquid injection to suspension injection the suspension is incorporated into the melt.
7. The method as claimed in claim 1 , wherein a pressure in an injection chamber of the twin-screw extruder is between 25 bar and 50 bar.
8. The method as claimed in claim 7 , wherein the pressure in the injection chamber is adjusted or controlled to 5 bar above to below a phase-transition boundary of the carrier liquid.
9. The method as claimed in claim 7 , wherein the pressure in the injection chamber is controlled to between 20 bar and 200 bar, wherein a pressure sensor in the injection chamber provides actual pressure values to a control device, which sets a required pressure by influencing an adjustable melt-flow restrictor arranged between the twin-screw extruder and the devolatilizing extruder, and/or by influencing a suspension pump.
10. The method as claimed in claim 7 , wherein an inert gas is added in a region of the injection chamber in order to maintain or improve control of the pressure.
11. The method as claimed in claim 1 , wherein a control device controls an evacuation rate in the devolatilizing extruder by adjusting a pressure for a specified flow rate and flow velocity, thus permitting adjustment of the melt viscosity of the melt that is discharged.