This invention relates to a process for producing a polyoxymethylene copolymer having an improved heat stability and their use for the production of moldings. More particularly, it relates to a process for producing a polyoxymethylene copolymer containing a reduced number of unstable terminals and having an improved heat stability by using a specific compound for deactivating a catalyst in the cationic copolymerization of trioxane with a cyclic ether or a cyclic acetal.

Polyoxymethylene (hereinafter referred to simply as POM) homopolymers and copolymers have been known for a long time as versatile thermoplastic molding material of construction, particularly in engineering and manufacturing. In many cases it can be used as a substitute for metals on account of its outstanding mechanical properties, such as high rigidity, hardness and strength and the fact that it is possible to produce moldings and molded parts to strict tolerance limits, and their good resistance to many chemicals.

A common polymerization process for producing the POM polymers comprises using a cyclic acetal such as trioxane as a main monomer and a cyclic acetal or a cyclic ether having adjacent carbon atoms as a comonomer, further adding a chain transfer agent for controlling the degree of polymerization depending on the purpose copolγmerizing these monomers with the use of a cationically active catalyst, and then contacting the polymerization product with an agent for neutralizing or deactivating the catalyst or a solution thereof to thereby deactivate the catalyst.

After the completion of the copolymerization, the obtained crude POM polymer generally contains a considerably large amount of unstable terminals and,

therefore, it is necessary to stabilize the POM polymer by eliminating the unstable parts therefrom. Thus, a complicated post-treatment process spending much energy is required therefor, which causes an economic disadvantage. When a crude POM copolymer containing a reduced amount of unstable parts is obtained after the polymerization, advantages such that the final product has an improved stability and that the post-treatment including stabilization can be simplified can be achieved. Accordingly, it has been desired to establish a process for producing a polymer containing a reduced amount of unstable parts in polymerization. Since a catalyst cannot be completely deactivated by the conventional deactivation treatments, depolymerization occurs during drying at high temperature, melt-pelletizing or molding of a product following the deactivation, thus forming unstable terminals. In order to overcome these problems, various methods for deactivating catalysts have been hitherto examined and proposed. Examples of the catalyst deactivators found in the proposals include in general organic and inorganic alkaline substances, for example, organic substances such as alkylamines, alkoxyamines and hindered amines and inorganic ones such as alkali and alkaline earth metal hydroxides, more particularly, calcium hydroxide (refer to Japanese patent Laid-Open No. 38713/1983). In addition to these common basic substances, it has also been proposed to deactivate a polymerization catalyst by neutralizing the catalyst with a trivalent organophosphorus compound (refer to Japanese Patent Publication No. 42085/1980). However, none of them can be necessarily adequate for the purpose.

The unstable hemiacetal end groups in trioxane copolymers can be selectively broken down, for example by treating the copolymer with aqueous solutions at temperatures of 120 to 220°C, if necessary with the addition of organic solvents, especially lower alcohols, trioxane or dioxolane; basic compounds, especially trialkylamines, also being able to be added to the solutions (G.W. Becker/D. Braun, unststoff Handbuch, Vol. 3/1 , p. 316, Munich- Vienna, 1992).

A disadvantage of this process is the incomplete deactivation of the initiator. Initiator residues may remain in the polymer material and lead to chain scissions on heating the material, This results in a release of formaldehyde during processing, which can cause an odor nuisance and a health risk.

It is furthermore known that an addition of alkali metal and/or alkaline earth metal fluorides to the afore-mentioned aqueous solvents improves the stability of the polymer (DE-B 17 20 309), if the heating is carried out in a pH range of the solution of from 4.5 to 6.7, i.e. under acidic conditions. The description discloses that the concentration of the fluoride salts in the aqueous liquor that is used is 0.01 to 5 % by weight, preferably 0.1 to 3 % by weight.

A basic disadvantage of this process is the accuracy required in setting the necessary pH value, which is generally set with strong acids. The use of strong acids is always accompanied by the danger of a chain scission, since all polyoxymethylenes are sensitive to acids. A further disadvantage of the aforedescribed process is the high proportion of 0.6 to 2 % by weight of sodium fluoride in the hydrolysis liquor, as is disclosed in the examples.

Under these circumstances, extensive studies have been necessary in order to obtain a crude POM copolymer containing an extremely reduced number of unstable terminals, significantly relieving the load in the subsequent stabilization step, the catalyst employed therein being fully deactivated, and, therefore, being highly heat stable. As a result, it has been successfully found out that the above-mentioned object can be achieved by using an alkali metal fluoride as a catalyst deactivator, thus completing the present invention.

It has further been found that the stability of the polymers can be substantially improved, been in the neutral or alkaline pH range, by adding soluble alkali metal fluorides to the hydrolysis liquor. It has also been found that the stabilizing effect can be achieved with a fluoride ion concentration as low as approximately 0.02 % by weight.

Accordingly, the present invention relates to a process for producing a polyoxymethylene copolymer by copolymerizing trioxane as a main monomer with a cyclic ether or a cyclic formal as a comonomer by using a cationically active catalyst, characterized by contacting an alkali metal fluoride with the copolymer after the completion of the copolymerization to thereby deactivate the polymerization catalyst.

The present invention further relates to a process for preparing polyoxymethylene copolymers having improved thermal resistance by heating the crude polymer, obtained after the completion of the copolymerization to thereby deactivate the polymerization catalyst in an aqueous hydrolysis liquor having a pH value of ≥. 7.0, to which has been added from 20 to 400 ppm, preferably from 100 to 300 ppm, and in particular from 150 to 250 ppm, of fluoride ions in the form of an alkali metal fluoride.

The present invention is characterized in that the polymerization catalyst is very effectively deactivated by using the above-mentioned alkali metal fluoride, together with a commonly known basic substance, to thereby suppress the occurrence of side reactions during the deactivation and give a crude POM copolymer containing an extremely reduced number of unstable terminals. It has been furthermore found out that the catalyst deactivated with the above- mentioned deactivator becomes highly stable and, therefore, its function for promoting the depolγmerization in the subsequent steps including high- temperature drying, melt treatment and molding can be remarkably suppressed,

The POM copolymer according to the present invention is produced by first copolymerizing trioxane as a main monomer with a cyclic ether or a cyclic formal as a comonomer in the presence of a cationically active catalyst.

The cyclic ether or the cyclic formal to be used as a comonomer herein is a cyclic compound having at least a pair of linked carbon atoms and an oxygen atom represented by the following general formula:

*1

In the above formula, R R ₂ , R ₃ and R ₄ may be either the same or different from each other and each represents a hydrogen atom or an alkyl group, in general, a hydrogen atom. R ₅ represents a methylene or oxmethylene group, an alkyl-substituted methylene or oxymethylene group (in this case, p is an integer of from 0 to 3), or a divalent group represented by the following formulae: -(CH ₂ ) _q -OCH ₂ - or -(O-CH ₂ -CH ₂ ) _q -OCH ₂ -

(in this case, p is 1 and q is an integer of from 1 to 4).

Examples of such a comonomer include ethylene oxide, 1 ,3-dioxolane, 1,3- trioxepane, diethylene glycol formal, 1 ,4-butanediol formal, 1 ,3-dioxane and propγlene oxide. Among these substances, particularly preferable examples of the comonomer include ethylene oxide, 1 ,3-dioxolane, 1 ,4-butandiol formal and diethylene glycol formal (G. W. Becker/D. Braun, Kunststoff-Handbuch, Vol. 3/1 , p. 303, Munich- Vienna, 1992). The content of the comonomer may range from 0.2 to 10 % by weight, preferably from 0.4 to 5 % by weight, based on the trioxane.

In the polymerization process of the present invention, an appropriate chain transfer agent may be added, if necessary, so as to control the molecular weight of the POM copolymer.

As the polymerization catalyst to be used in the present invention, general cationically active catalysts are usable. Examples of such catalysts include Lewis acids, in particular, halides of boron, tin, titanium, phosphorus, arsenic, antimony and so forth, such as boron trifluoride, tin tetrachloride, titanium

tetrachloride, phosphorus pentachioride, phosphorus pentafluoride, arsenic pentafluoride and antimony pentafluoride, and compounds derived therefrom, such as complexes and salts thereof, protonic acids such as trifluoromethanesulfonic acid and perchloric acid, protonic acid esters such as perchlorates of lower aliphatic alcohols (for example, tert-butyl perchlorate), protonic acid anhydrides, in particular, mixed anhydrides of perchloric acid with lower aliphatic carboxylic acids (for example, acetyl perchlorate), isopolyacids, heteropolyacids (for example, phosphomolybdic acid), triethyloxonium fiuorophosphate, triphenylmethyl hexafluoroarsenate and acetyl hexafluoroborate.

Among these compounds, the most common and suitable ones are boron trifluoride and coordination compounds of boron trifluoride with organic compounds (for example, ethers).

The polymerization of the present invention can be conducted by using the same apparatus and the same method as those employed in the known processes for polymerizing trioxane. More specifically, both of the batch process and the continuous one may be employed and any of the solution polymerization, melt bulk polymerization and other polymerization processes may be selected. From the industrial viewpoint, it is usual and preferable to use the continuous bulk polymerization process wherein liquid monomers are employed and a solid polymer in the form of a powdery mass is obtained as the polymerization proceeds. In this case, an inert liquid medium may be present together, if necessary.

The polymerization apparatus to be used in the present invention may be selected from among a Ko-kneader _r a twin-screw continuous extruder, a two- shaft paddle-type continuous mixer as well as those which have been proposed as the apparatus for the continuous polymerization of trioxane. A polymerization apparatus of a closed system may consist of two or more stages. It is

particularly preferable to use an apparatus provided with a pulverizer whereby a solid polymer formed by the polymerization reaction can be obtained in the form of fine particles.

After the completion of the polymerization, the crude polymer discharged from the polymerizer should be immediately contacted with a deactivator to thereby deactivate the polymerization catalyst.

The present invention is characterized in that the copolymer obtained is mixed with an aqueous solution comprising alkali metal fluoride and, if desired, C*,-C ₄ - alcohols, trioxane or dioxolane, so that a fluoride ion concentration as mentioned above is achieved.

A fluoride of an alkali metal such as Li, Na, K, Rb or Cs is used as the deactivator and the crude copolymer is contacted therewith to thereby deactivate the catalyst. Preference being given to potassium fluoride.

In the present invention, the deactivator may be contacted with the polymerization, product in an arbitrary manner without restriction. Namely, the contact may be achieved by adding the above-mentioned alkali metal fluoride to the crude polymer and thoroughly mixing. In order to fully contact these substances with each other and to efficiently deactivate the catalyst, it is desirable that the alkali metal fluoride is formulated into a solution in water and/or an organic solvent and the polymerization product is mixed with this solution, followed by the stirring of the resulting mixture in the form of a slurry.

Basic substances are generally added to set the pH value, for example sodium hydroxide, potassium hydroxide or calcium hydroxide. Organic basic substances such as trialkylamines having 1 to 6 carbon atoms in the alkyl group, for example triethylamine, triethanolamine, triisopropanolamine, N,N- diethylmethylamine, are preferred. The mixture is then heated in a closed

system, a homogeneous solution being formed. Temperatures of from 170°C to 220°C, preferably from 180°C to 200°C, are necessary for this purpose.

The residence time is from 2 to 20 minutes, preferably from 5 to 15 minutes.

It is most advantageous that the polymer discharged from the polymerizer is in the form of fine particles. When the polymer is In the form of relatively large particles, it is preferable to rapidly pulverize the polymer in the above-mentioned solution of the deactivator following the discharge from the polymerizer. By using the deactivator of the present invention, a polymer containing a reduced number of unstable terminals and having a high heat stability can be obtained.

In the present invention, after the deactivation of the catalyst, the copolymer is subjected, if necessary, to washing, the separation and recovery of the unreacted monomers and drying and, furthermore, stabilization if necessary. Then additives such as various stabilizers are added thereto and the copolymer is melt-kneaded and formulated into pellets, thus giving a product. As described above, the POM copolymer of the present invention contains an extremely reduced number of unstable terminals and, therefore, the load in the stabilization step is relieved. Thus a sufficiently stable polymer can be obtained by a simple finishing treatment. Further, the residual unstable parts can be eliminated by volatilization in the melt-kneading extrusion step for adding, for example, stabilizers.

As described above, the copolymer obtained by the process according to the present invention contains a reduced amount of unstable parts as compared with those obtained by using the conventional deactivators and does not undergo depolymerization since the catalyst has been completely deactivated. Thus, the post-treatment procedure can be simplified and the final product has a high heat stability.

The oxymethylene polymers treated according to the invention can be processed by all methods normally used for thermoplastics materials, for example injection molding, extrusion, extrusion blow molding, melt spinning and thermoforming. The polymers are suitable as a material for producing molding of all types, in particular semi-finished and finished articles. They can be used in particular as qualitatively high-grade material of construction for manufacturing moldings with high dimensional stability having as smooth surface.

Examples

In the examples % or ppm are by weight unless otherwise noted, Melt index (Ml). The specified pH values refer to a temperature of 25 °C.

A melt index (g/10 min) measured at 190°C under a load of 2160 g is given. This is evaluated as a characteristic corresponding to the molecular weight. Namely, a lower Ml means a larger molecular weight (provided that a small amount of a given stabilizer is added before the measurement so as to prevent decomposition during the measurement).

Alkali decomposition ratio (content of unstable part):

1 g of a copolymer is added to 100 ml of a 50 % aqueous solution of methanol containing 0.5 % of ammonium hydroxide and heated in a closed container at 170°C for 45 minutes. Then, the amount of the formaldehyde decomposed and dissolved in the solution is determined and expressed in % based on the polymer.

Thermal weight loss ratio:

5 g of a copolymer is pulverized and thoroughly mixed with a powdery stabilizer comprising 2,2'-methylenebis(4-methyl-6-t-butylphenol) (0.5 %) and dicyandiamide (0.1 %). After heating at 220°C in the atmosphere for 45 minutes, the weight loss ratio is measured.

Examples 1 to 10 and Comparative Examples 1 to 3 Use was made of a continuous mixing reactor which had a cross section composed of two circles partly overlapping each other and was provided with a barrel having a jacket for passing a heating (cooling) medium therethrough placed outside and two rotating shafts having stirring/screwing paddles located in the direction of the major axis placed therein. Warm water at 80°C was passed through the jacket and the two rotating shafts was rotated at a rate of 100 r.p.m. Then trioxane containing 3.3 % of 1 ,3-dioxolane as a comonomer and a given amount of a molecular weight modifier was continuously fed into one end of the reactor. Simultaneously, a 1 % solution of boron trifluoride butyl etherate in cyclohexane was continuously added to the same end in such a rate as to give 60 ppm of BF ₃ based on the all monomers (trioxane plus 1 ,3- dioxolane), thus effecting copolymerization.

When the polymerization conditions and the polymerization reaction were stabilized, an aqueous solution containing each of the deactivators listed in Table 1 (room temperature, 0.1 % by weight) was immediately added to the reaction product discharged from the outlet of the polymerizer and stirred for 60 minutes. Then the mixture was centrifuged and dried at 110°C to thereby give a final polymer. The polymerization yield was about 70 % in every case. Table 1 shows the properties of the obtained polymers.

Table 1

-». TPP: triphenylphosphine.

11 ) 50 g of ground polyoxymethylene crude polymer that had been obtained from 98 parts by weight of trioxane and 2 parts by weight of ethylene oxide, using 100 ppm of BF ₃ as initiator, were heated for 10 minutes at 180°C with 500 g of solvent comprising 75 % of methanol, 15 % of trioxane and 10 % of water, to which 125 mg of triethanolamine and 380 mg of potassium fluoride (equivalent to 248.5 ppm of fluoride ions) were added (pH value 8.4). The precipitated polymer was washed after cooling. The weight loss on heating from 100°C to 240 °C under an inert gas atmosphere for one hour at a heating rate of 2.33°C/min was 0.06 %.

12) Example 1 1 was repeated with the difference that triethanolamine was substituted by 125 mg triethylamine (pH value 10.1 ). The loss of weight was 0.05 %.

13) 50 g of ground crude polymer according to Example 11 were heated for 10 minutes at 180°C with 500 g of solvent comprising 75 % of methanol and 25 % of water, to which 380 mg of potassium fluoride were added and which were set to a pH value of 8.0 with dilute potassium hydroxide solution. The precipitated polymer was washed after cooling and heated for 1 hour at 240°C under an inert gas atmosphere to measure the thermal stability. The weight loss was 0.26 %.

Comparative Example 4) 50 g of ground crude polymer were mixed as in Example 11 with the hydrolysis liquor, but without the addition of fluoride (pH value 8.5) and then treated. The weight loss was 0.36 %.

Comparative Example 5) 50 g of ground crude polymer were mixed according to Example 13, but without the addition of potassium hydroxide and fluoride (pH value of the hydrolysis liquor = 6.6), and treated. The weight loss on heating to 240°C, determined according to Example 13, was 1.21 % at a residence time of 1 hour.