MXPA99008254A - Procedure to prepare polyoxymethylene copolymers thermally es - Google Patents

Abstract

A process for the preparation of polyoxymethylene copolymers, where 1,3,5-trioxane is polymerized with comonomers generally known in the presence of a strong protic acid initiator and in the presence of a formaldehyde dialkylacetal, and where the initiator is dissolved in the formaldehyde dialkylacetal before adding and mixing with trioxane and comonomers

Description

PROCEDURE FOR PREPARING THERMALLY STABLE POLIOXIMETHYLENE COPOLYMERS DESCRIPTIVE MEMORY The present invention relates to a process for the preparation of thermally stable polyoxymethylene copolymers (POM) in which the initiator is distributed within the comonomers through a preliminary dissolution in a formaldehyde dialkyl acetal. The thermoplastic molding materials of POM homopolymers and copolymers have often been used for a long time as versatile construction materials, particularly in design and manufacture. In many cases they can be used as a substitute for metals taking advantage of their outstanding mechanical properties, such as rigidity, hardness and strength, in addition to the fact that it is possible to produce molds and molded parts with strict tolerance limits, as well as their good resistance to many chemicals. It is known that by copolymerizing trioxane with cyclic cyclic or acrylic ethers, it is possible to obtain copolymers in which the sequence of the -CH2-O- groups is interrupted by randomly distributed comonomer units, such as -CH2CH2-O-, - (CH2) 4-O- or -CH2-CH2-O-CH2-CH2-O- (GW Becker / D. Braun, Kunststoff-Handbuch, Vol. 3/1, p.303, Munich-Vienna, 1992). The comonomers are normally used in a weight ratio of 0.2 to 20%. Suitable initiators which were used in the present invention are strong protonic acids chosen from the group consisting of trifluoromethanesulfonic acids and anhydrides, pentafluoroethylsulfonic acids and anhydrides, heptafluoropropylsulfonic acids and anhydrides, nonafluorobutylsulfonic acids and anhydrides, perfluoroheptylsulfonic acids and anhydrides, and mixtures thereof . Also suitable initiators are Lewis acids chosen from the group consisting of phosphorus pentafluoride, silicon tetrafluoride, boron trifluoride, boronate trifluoride etherate, dye-tetrachloride, arsenic pentafluoride, triphenylmethyl hexafluorophosphate, and mixtures thereof. At the end of the polymerization reaction of the crude POM polymer it still contains a certain amount of unconverted monomers and unstable terminals which have to be removed to stabilize the final product. In order to form such a polymer from the molten material, as it is used for thermoplastics, it is necessary to deactivate the polymerization initiator, to remove adherent monomer residues from the polymer and analyze the unstable fractions. In this way, it is known that the deactivation of the initiators takes place in the aqueous phase or in an organic solvent, requiring the subsequent steps of filtering, washing and drying. The deactivation of the initiator with the addition of different deactivators can also be carried out in the molten material (DE 3703790). The deactivation step is often carried out in combination with the demonomerization and removal of unstable chain ends (DE 37 38 632 and EP 0 137 305). EP 0673 955 describes a process in which the raw polymer is treated with a vapor that also contains a volatile base in small amounts. In this way, the unconverted residue monomer is removed and the initiator is deactivated. In JP 05059255 it is stated that the initiator is deactivated by the addition of alkali metal or alkaline earth metal oxides to the molten polymer material. The removal of the groups with unstable terminals, which usually remain in the crude polymer after the polymerization, and in particular lead to chain degradation when the polymer is heated, is also a usual process step in the preparation of POM copolymers. The unstable hemiacetal end groups in the trioxane copolymers can be analyzed selectively, for example, by hydrolysis, ie treating the copolymer at temperatures of 120 to 220 ° C with pressurized water containing alkaline material., especially trialkylamines, and optionally with the addition of organic solvents, especially lower alcohols, trioxane or dioxolane (Kunststoff Handbuch, p.316). After hydrolysis, the polymer must be precipitated again, washed and dried. Therefore, the object of the invention is to develop a process that makes it possible to economically prepare stable 1,3,5-trioxane copolymers in a continuous process while avoiding the deficiencies of the known processes.

It has now been discovered that thermally stable POM polymers can be obtained if the initiator, which is generally a strong protonic acid, first dissolves in a formaldehyde dialkylacetal, a substance known to regulate the molecular weight of the POM polymer, and then add to the reaction mixture. The invention eliminates the need to use an organic solvent vehicle, an unnecessary component in the reaction mechanism, for the introduction of the protonic or Lewis acids into the reaction mixture. Accordingly, the present invention relates to a process for preparing polyoxymethylene copolymers, where 1, 3,5-trioxane is polymerized with comonomers generally known in the presence of a strong protonic acid initiator and in the presence of a formaldehyde dialkylacetal, and wherein the initiator is dissolved in a dialkylacetal portion of formaldehyde before mixing thereof with trioxane and the comonomers. In the prior art process for the production of POM polymers, formaldehyde dialkylacetals are generally used as molecular weight regulators. In general, the use of a molecular weight regulator is not known to produce polymers with high molecular weight. However, the advantage of the process according to this invention is that through the initial dissolution of the initiator in a formaldehyde dialkylacetal it is possible to add a very low amount and a controlled amount of the initiator in a perfectly dispersed state to the controlling monomer mixture. in this way the reaction speed. Due to the very low amount of initiator in the reaction mixture, it is also possible to produce high molecular weight material even though there is a small amount of molecular weight regulator present in the reaction mixture. In accordance with this invention, it is possible to avoid contamination of the monomers and the resulting polymer with substances that are important for the polymerization process. For example, it is no longer necessary to add an agent to deactivate the initiator. In principle, it is also not necessary to carry out the hydrolysis to the crude polymer. However, to further reduce the content of the unstable end groups in the polymer, it is of great help to perform hydrolysis thereto. In the process according to the invention, the initiator can be dissolved in part or in the total amount of the formaldehyde dialkylacetal used. The dialkylketal of formaldehyde containing the dissolved initiator is usually added to the mixture of trioxane and comonomers, ie to the reaction mixture. A predetermined additional amount of formaldehyde dialkylacetal may be added directly to the reaction mixture before or after mixing the formaldehyde dialkylacetal and the initiator solution dissolved therein. In another working example, the dialkylketal of formaldehyde containing the dissolved initiator is pre-mixed with the comonomers before mixing it with the trioxane. Optionally, a predetermined additional amount of formaldehyde dialkylacetal can then be added to the reaction mixture. In the process according to the invention, strong protonic acids, in particular heteropolyacids, perchloric acid and perfluoroalkanesulfonic acids, can be used as an initiator. Trifluoromethanesulfonic acid is the preferred initiator. The amount of the initiator is generally at least about 0.01 to about 1.0 ppm, based on the total amount of trioxane and comonomers. Preferably the amount of the initiator is from about 0.03 to about 0.4 ppm, and preferably from about 0.05 to about 0.2 ppm based on the total amount of trioxane and comonomers. The suitable formaldehyde dialkylacetals used according to the invention are formaldehyde dimethylacetal, formaldehyde diethylacetal, formaldehyde dipropylacetal and formaldehyde dibutylacetal. Dimethyl acetal formaldehyde, ie methylal, is preferred. The amount of dialkyl acetal of formaldehyde, generally, is from about 3.4 to about 34 mmoles per kg of total trioxane and comonomers. Suitable comonomers of the present invention are generally known and may be selected from the group consisting of ethylene oxide, 1,3-dioxolane, 1,3-trioxepane, formal diethylene glycol, formal 1,4-butanediol, 1-3-doxane, propylene oxide, timethylene oxide, butadiene oxide, formally oxyglycolic, formal thiodiglycolic, 1,3-oxtiolane, and mixtures thereof. Particularly preferred comonomers are ethylene, 1,3-dioxolane, diethylene glycol formal, and 1,4-butanediol formaldehyde. The amount of the comonomer used in the present invention can vary from about 0.2 to about 10% by weight, preferably from about 0.4 to about 5% by weight, based on the total amount of trioxane and comonomers. The polymerization process according to the invention can be carried out in any polymerization reactor or combination of reactors known for the production of POM polymers. In addition, antioxidants, acid acceptors, lubricants, waxes, UV stabilizers, nitrogen-containing co-stabilizers and other products known in the POM art can be used as stabilizers and additives, either individually or in combination. All fillers and reinforcing materials customary and known in the plastics industry, in particular polyacetal copolymers, can be used as fillers and reinforcing materials.

EXAMPLES EXAMPLE 1 In an intermittent reactor operated at a temperature of about 80 ° C and a pressure of about 1 atmosphere, 96.6% by weight of trioxane was mixed with 3.4% by weight of dioxolane to form a monomer mixture. To this mixture was added 0.2 ppm of trifluoromethanesulfonic acid (TFMSA) dissolved in 500 ppm of formaldehyde dimethylacetal (methylal), the amounts in ppm were based on the total weight of the monomer mixture. After an induction period of about 30 seconds the polymerization started. The crude polymer obtained was quenched in a water / triethylamine mixture and subsequently hydrolyzed at 170 ° C in a water / methanol (10/90) mixture, from which it was precipitated at room temperature. The melt viscosity ratio value (RVM) and, through the measurement of formaldehyde formation for one hour at 170 ° C under alkaline conditions, the content of the unstable end groups was determined from the dry product ( to obtain the data see table 1).

EXAMPLES 2 AND 3 In these examples the same procedure as in Example 1 was used, and additional amounts of methylal were added to the monomer mixture. The RVM and the percentage of non-stabilized end groups are shown (for data see Table 1).

COMPARATIVE EXAMPLES FROM 4 TO 6 Using the procedure of Example 1, 96.6% by weight of trioxane was mixed with 3.4% by weight of dioxolane to form the monomer mixture. To this mixture was added 50 ppm of BF gas and 0 ppm, 400 ppm or 1000 ppm of formaldehyde dimethylacetal (methylal), respectively, to the monomer mixture of examples 4, 5 and 6, the ppm amounts based on the total weight of the monomer mixture and adjusted to obtain products with the same RVM values as examples 1 to 3, respectively. After a 30-second induction period, polymerization was started. The crude polymer obtained was quenched in a water / triethylamine mixture and subsequently hydrolyzed at 170 ° C in a water / methanol (10/90) mixture from which it was precipitated at room temperature. The dry product was analyzed as in examples 1 to 3. According to the data shown in table 1, after adjusting the RVM values in examples 3, 4 and 5 to be equal to the examples 1 , 2 and 3, the percentage of unstable terminal end groups of polymers was considerably reduced (see examples 1, 2 and 3), where small amounts of trifluoromethanesulfonic acid dissolved in methylal were added to the reaction mixture.

TABLE 1

Claims (8)

NOVELTY OF THE INVENTION CLAIMS

1. - A process for the preparation of polyoxymethylene copolymers which exhibit a reduced amount of unstable terminal end groups comprising the polymerization of 1, 3, 5-trioxane with at least one cyclic ether and an acetal comonomer with the aid of an initiator of strong protonic acid or a Lewis acid and in the presence of a formaldehyde dialkylacetal, the improvement consists of the dissolution of the initiator in the formaldehyde dialkyl acetal before introducing the same to the trioxane and the comonomers.

2. The process according to claim 1, further characterized in that the strong protonic acid initiator is selected from the group consisting of trifluoromethanesulfonic acid and anhydrides, pentafluoroethylsulfonic acid and anhydrides, heptafluoropropisisulfonic acid and anhydrides, nonafluorobutylsulfonic acid and anhydrides, and perfluoroheptisulfonic acid, anhydrides and mixtures thereof and the Lewis acid is selected from the group consisting of phosphorus pentafluoride, silicon tetrachloride, boron trifluoride, boronate trifluoride etherate, tin tetrachloride, arsenic pentachloride, triphenylmethyl hexafluorophosphate, and mixtures of these.

3. The process according to claim 2, further characterized in that the strong protonic acid initiator is trifluoromethanesulfonic acid and the Lewis acid is a boron trifluoride.

4. The process according to claim 3, further characterized in that the strong protonic acid initiator or Lewis is present in an amount of about 0.01 to about 1 ppm, based on the total amount of trioxane and comonomers.

5. The process according to claim 4, further characterized in that the formaldehyde dialkylacetal is selected from the group consisting of formaldehyde dimethylacetal, formaldehyde diethylacetal, formaldehyde dipropylacetal, formaldehyde dibutylacetal and mixtures thereof.

6. The process according to claim 5, further characterized in that the formaldehyde dialkylacetal is formaldehyde dimethylacetal.

7. The process according to claim 6, further characterized in that the formaldehyde dialkylacetal is present in an amount of about 3.4 to about 34 mmoles per kg of trioxane and comonomers.

8. The process according to claim 2, further characterized in that the formaldehyde dialkylacetal containing the strong dissolved protonic acid initiator is added to the comonomers before mixing the trioxane.