Process for preparing polyoxymethylene

Abstract

The patent application relates to a process for preparing polyoxymethylene by polymerization of a reaction mixture (R.sub.G) which comprises at least one formaldehyde source and at least one initiator mixture (I.sub.G), wherein the initiator mixture (I.sub.G) comprises at least one polymerization initiator and at least one solvent of the general formula (I)
R.sup.1—O—[—R.sup.3—O—].sub.m—R.sup.2  (I), where m is 1, 2, 3 or 4; R.sup.1 and R.sup.2 are each, independently of one another, C.sub.3-C.sub.6-alkyl; R.sup.3 is C.sub.1-C.sub.5-alkylene.

Claims

1. A process for preparing polyoxymethylene, comprising polymerizing a reaction mixture (R.sub.G) which comprises a formaldehyde source and an initiator mixture (I.sub.G), wherein the initiator mixture (I.sub.G) comprises a polymerization initiator and a solvent of formula (I):
R.sup.1—O—[—R.sup.3—O—].sub.m—R.sup.2  (I), wherein m is 1, 2, 3 or 4; R.sup.1 and R.sup.2 are each, independently of one another, C.sub.3-C.sub.6-alkyl; and R.sup.3 is ethylene.

2. The process according to claim 1, wherein the formaldehyde source is selected from the group consisting of cyclic and linear formals.

3. The process according to claim 1, wherein m is 1, 2, 3 or 4; and R.sup.1 and R.sup.2 are both unsubstituted C.sub.3-C.sub.6-alkyl.

4. The process according to claim 1, wherein the reaction mixture (R.sub.G) further comprises a first comonomer (C-1).

5. The process according to claim 4, wherein the comonomer (C-1) is a cyclic ether of formula (II): ##STR00004## wherein R.sup.11 to R.sup.14 are each, independently of one another, H, C.sub.1-C.sub.4-alkyl or halogen-substituted C.sub.1-C.sub.4-alkyl; R.sup.15 is a chemical bond, a (—CR.sup.15aR.sup.15b—) group or a (—CR.sup.15aR.sup.15bO—) group, where R.sup.15a and R.sup.15b are each, independently of one another, H, unsubstituted or at least monosubstituted C.sub.1-C.sub.4-alkyl in which the substituents are selected from among F, Cl, Br, OH and C.sub.1-C.sub.4-alkyl; and n is 0, 1, 2 or 3.

6. The process according to claim 4, wherein the reaction mixture (R.sub.G) comprises up to 50 mol %, based on the formaldehyde source, of the comonomer (C-1).

7. The process according to claim 1, wherein the polymerization initiator is a protic acid or a Lewis acid.

8. The process according to claim 1, wherein the polymerization initiator is added in an amount of from 0.0001 ppm to 1000 ppm, based on the reaction mixture (R.sub.G).

9. The process according to claim 1, wherein the polymerization initiator is dissolved in the solvent (I) in a concentration of from 0.005% by weight to 5% by weight, based on a total weight of the initiator mixture (I.sub.G).

10. The process according to claim 1, wherein the reaction mixture (R.sub.G) further comprises a chain transfer agent.

11. The process according to claim 10, wherein the chain transfer agent is added in an amount of from 10 ppm to 10 000 ppm, based on the reaction mixture (R.sub.G).

12. The process according to claim 1, wherein the reaction mixture (R.sub.G) further comprises a second comonomer (C-2), which is a bifunctional compound of formula (III) or (IV): ##STR00005## wherein Z is a chemical bond, an (—O—) group or an (—O—R.sup.21—O—) group, where R.sup.21 is unsubstituted C.sub.1-C.sub.8-alkylene or C.sub.3-C.sub.8-cycloalkylene.

13. The process of claim 1, further comprising dissolving the polymerization initiator in the solvent of formula (I) in a concentration of from 0.005% by weight to 5% by weight.

14. The process of claim 1, wherein solvent in the initiator mixture consists essentially of at least one solvent of formula (I).

15. The process of claim 1, wherein the solvent of formula (I) is diethylene glycol dipropyl ether, diethylene glycol dibutyl ether, diethylene glycol dipentyl ether, or diethylene glycol dihexyl ether, or any mixture thereof.

16. The process of claim 1, wherein the solvent of formula (I) is diethylene glycol dibutyl ether.

17. The process of claim 1, wherein R.sup.1 and R.sup.2 are each, independently of one another, linear C.sub.3-C.sub.6-alkyl.

18. A process for preparing polyoxymethylene, comprising polymerizing a reaction mixture (R.sub.G) which comprises a formaldehyde source and an initiator mixture (I.sub.G), wherein the initiator mixture (I.sub.G) comprises a polymerization initiator and a solvent of formula (I):
R.sup.1—O—[—R.sup.3—O—].sub.m—R.sup.2  (I), wherein m is 1, 2, 3 or 4; R.sup.1 and R.sup.2 are each, independently of one another, C.sub.3-C.sub.6-alkyl; and R.sup.3 is ethylene, and wherein a degree of solvent migration is less than 10 μg/kg.

Description

COMPARATIVE EXAMPLE C1

(1) A batch bulk polymerization of the comonomers 1,3,5-trioxane and 1,3-dioxepane was carried out under nitrogen at 80° C. on an industrial scale of 1000 kg/h. 5 kg of a mixture of 1,3,5-trioxane/1,3-dioxepane/di-n-butyl formal in a weight ratio of 97.13/2.7/0.17 were placed in a metal vessel. The reaction was initiated using 0.05 ppm of aqueous perchloric acid dissolved in triethylene glycol dimethyl ether.

(2) The polymer product was transported directly to a mill and subsequently into a dryer. A buffer solution composed of 0.01% strength by weight sodium glycerophosphate and 0.05% by weight of sodium tetraborate was sprayed onto the milled polymer. A continuous warm stream of nitrogen (80° C.) was passed over the milled polymer while it moved through the dryer, so that remaining trioxane was removed. The total residence time of the polymer in the dryer was in the range from 3 to 6 hours. From these dryers, the polymer was introduced directly into an extruder where it was mixed with 0.35% by weight of Irganox® 245 FF (commercial product of Ciba Geigy), 0.15% by weight of glycerol distearate, 0.2% by weight of MFC (melamine-formaldehyde condensate). The resulting polymer was pelletized and dried further under warm nitrogen until the remaining formaldehyde level was below 100 ppm.

(3) Analysis of the polymer finally obtained was carried out in order to determine the migration of any remaining initiator solvents. The tests were carried out using injection-molded plates having dimensions of 2.5 cm×2.5 cm×2 mm. The migration contact was carried out in accordance with the European standard EN 1186-3 and EN 1186-2 for ethanol solutions and olive oil. The data from the third contact study are shown in table 1 below.

COMPARATIVE EXAMPLE C2

(4) This example was carried out essentially like comparative example 1 with the difference that the solvent used for the initiator was tetraethylene dimethyl ether.

EXAMPLE 3

(5) This example was carried out essentially like comparative example 1 with the difference that the solvent used for the initiator was diethylene glycol dibutyl ether.

(6) The studies of comparative examples C1 and C2 and of example 3 are shown in table 1.

(7) TABLE-US-00001 TABLE 1 Migration details for remaining solvent from the polymer products Detec- Detec- tion tion limit limit for for Ex- 10% 20% 50% ethanolic Olive olive ample Solvent ethanol ethanol ethanol solutions oil oil C1 Triethylene 12.6 14.4 19.8 5.4 =DL* 6.0 glycol dimethyl ether (μg/kg) C2 Tetra- 36.6 41.4 45.6 6.6 24.6 6.6 ethylene glycol dimethyl ether (μg/kg) 3 Diethylene <DL <DL <DL 6.0 <DL 5.4 glycol dibutyl ether (μg/kg) *DL: detection limit.

(8) The migration of the solvents from the polymer materials finally obtained is significantly lower for the solvent according to the invention compared to the reference system of comparative examples C1 and C2.

(9) The solvent used in example 3 according to the invention additionally has the advantage that it is toxicologically unproblematical. The polyoxymethylene obtained according to example 3 is therefore also suitable for food applications.