Method for producing a spinnable silica sol material
In a process for producing a spinnable silica sol material, a viscosity value V S is stipulated which the spinnable silica sol material should have after ripening. A viscosity value V R corresponding to V S which the silica sol material has before ripening is ascertained. An aqueous acid solution and a hydrolysable silicon compound are combined. The combined mixture is evaporated to give a single-phase solution while measuring the viscosity of the mixture and the evaporation process is terminated upon reaching the viscosity value V R . The single-phase solution thus obtained is then ripened to give a silica sol material with the viscosity value V S . The process exhibits reduced waste and more exact reproducibility of the spinning sol properties during the synthesis and an increased space-time yield in the production of biologically degradable and/or resorbable fibers and nonwoven fabrics.
1. A process for producing a spinnable silica sol material comprising:
(a) stipulating a viscosity value V S , which the spinnable silica sol material should have after ripening, wherein the stipulated viscosity value V S is in the range from 30 to 100 Pa·s at a shear rate of 10 s −1 at 4° C.,
(b) ascertaining a viscosity value V R corresponding to V S , which the silica sol material has before ripening,
(c) combining an aqueous acid solution and a hydrolysable silicon compound, wherein the combining in (c) takes place under quasi-isothermal conditions over a period of at least 15 minutes, and wherein, in (c), the hydrolysable silicon compound is metered into an acid located in a solvent in parallel in a controlled manner,
(d) evaporating a mixture combined in (c) to give a single-phase solution while measuring the viscosity of the mixture and terminating evaporation process upon reaching the viscosity value V R ,
(e) ripening a single-phase solution obtained in (d) to give a silica sol material with the viscosity value V S , wherein the ripening in (e) is carried out at temperatures of from 25° C. to 35° C., and wherein the single-phase solution is stirred;
wherein the viscosity values V S and V R are dynamic viscosities, which are correlative and measured online and inline continuously by measurement probes within the mixture.
2. The process according to claim 1 , where the combining in (c) takes place such that heat released by hydrolysis-condensation reaction is used for heating a synthesis mixture.
3. The process according to claim 1 , where the hydrolysable silicon compound is a silicon compound of formula (I)
SiX 4 (I)
and X in formula (I) is an optionally substituted straight-chain and/or branched C 1 -C 5 alkoxy radical.
4. The process according to claim 1 , where (d) is carried out in a closed apparatus in which thorough mixing is possible and in which at the same time, any solvent present can also be evaporated.
5. A spinnable silica sol material obtained by a process according to claim 1 .
6. The process of claim 1 , wherein the quasi-isothermal conditions are within a bottom temperature range of ±5° C.
7. The process of claim 1 , wherein the quasi-isothermal conditions are within a bottom temperature range of ±2° C.
8. The process of claim 1 , wherein the quasi-isothermal conditions are within a bottom temperature range of ±0.5° C.
9. The process of claim 1 , wherein the ripening in (e) is carried out at temperatures of from 28.1° C. to 35° C.
10. The process of claim 1 , wherein the hydrolysable silicon compound comprises tetraethoxysiloxane.
11. The process of claim 1 , wherein the solvent comprises ethanol.
12. The process of claim 1 , wherein the viscosity values V S and V R are in a linear relationship.
13. The process for producing a fibre and/or a nonwoven fabric, comprising producing a spinnable silica sol material according to claim 1 ; and
(f) drawing threads from the ripened silica sol material from (e) and, in the case of a nonwoven fabric, assembling threads to give a nonwoven fabric.