JPH10292039A - Production of stabilized polyacetal copolymer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a stabilized polyacetal copolymer being nonproblematic in the formation of unstable terminals during production, reduced in load during its stabilization step and having improved thermal stability by adding a deactivator being a cyano-containing organic compound to the reaction mixture after copolymerization, further contacting the mixture with a basic compound to deactivate the polymerization catalyst and to thereby effect polymerization stop and stabilization. SOLUTION: The starting monomer usable in the copolymerization is one based on trioxane being a cyclotrimer of formaldehyde, and the comonomer may be a cyclic formal having at least one vicinal C-C bond or a cyclic ether having the same. The comonomer is exemplified by a cyclic formal or a cyclic ether such as 1,3-dioxolane or ethylene oxide or a branched or crosslinked- molecular-structure-forming comonomer such as butanediol glycidyl ether. The amount of the comonomer used is desirably 0.2-10 mol% based on the trioxane. The polymerization catalyst is exemplified by boron trifluoride or a coordination compound thereof.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a polyacetal copolymer having improved thermal stability. More specifically, in the cationic copolymerization of trioxane and a cyclic ether or cyclic formal, a two-step deactivation treatment is performed using a specific compound after the polymerization, thereby improving the thermal stability with few unstable terminal portions. The present invention relates to a method for producing a polyacetal copolymer.

[0002]

BACKGROUND OF THE INVENTION Polyacetal copolymers have been known for many years as engineering plastic materials, and their copolymerization process generally involves the use of trioxane as the main monomer and a cyclic ether or cyclic formal having adjacent carbon atoms as a comonomer to form a cationically active polymer. The copolymerization is carried out using a catalyst, and then the polymerization product is deactivated by contacting with a catalyst neutralizer or deactivator or a solution thereof.
Various methods for deactivating the catalyst have been conventionally studied and proposed. The catalyst deactivators found in these proposals are generally organic or inorganic alkaline substances, such as alkylamines, alkoxyamines, and hindered amines as organic substances, and alkali or alkaline earth substances as inorganic substances. Hydroxides, organic or inorganic salts, alkoxides and the like of similar metals (for example, JP-A-58-38713, etc.) and catalysts using trivalent organic phosphorus compounds besides common basic substances (For example, Japanese Patent Publication No. 55-42085) has been proposed. However, none of these conventional methods for deactivating with a deactivator is sufficient, and a considerable amount of unstable terminal is generated, and it is difficult to completely lose the activity of the catalyst. Depolymerization occurs during high-temperature drying, melt pelletization, or molding, and unstable terminals are generated. Therefore, in order to put this into practical use, unstable parts must be removed and stabilized, which requires a complicated post-treatment step, requires a large amount of energy for the treatment, and is economically disadvantageous. It is. If a crude polyacetal copolymer with sufficient deactivation of the catalyst after polymerization and a small number of unstable parts is obtained, the stability of the final product will be more excellent, and the post-treatment steps such as stabilization can be simplified. There is a need for a method of obtaining a polymer having advantages and having less unstable parts.

[0003]

SUMMARY OF THE INVENTION In view of the above situation, the present inventors have sufficiently deactivated the catalyst and suppressed the formation of an unstable terminal, thereby significantly reducing the load in the next stabilization step. It is another object of the present invention to obtain a polyacetal copolymer that is extremely thermally stable.

[0004]

Means for Solving the Problems The inventors of the present invention have made intensive studies to solve the above-mentioned problems, and as a result, the polymerization product was first deactivated and stopped by using an organic compound having a cyano group. After that, it was found that by further treating with a basic compound, the polymerization catalyst was completely deactivated and at the same time converted to a thermally and chemically stable terminal, thereby achieving the above object. The invention has been completed. That is, the present invention relates to a method for producing a polyacetal copolymer, in which trioxane is used as a main monomer, cyclic ether or cyclic formal is used as a comonomer, and polymerization is performed using a cationically active catalyst as a polymerization catalyst to produce a polyacetal copolymer. Stabilized polyacetal copolymer characterized by adding and contacting an organic compound having a cyano group as a treating agent, and then contacting with a basic compound to deactivate the polymerization catalyst, and terminate and stabilize the polymerization. The present invention relates to a method for producing a united product. The feature of the present invention is that the deactivation treatment of the produced copolymer after polymerization is performed in two stages, and first, a catalyst deactivation treatment agent,
As a polymerization terminator, an organic compound having a cyano group is added to the polymerization system alone (in a gaseous state) or as a solution of a general inert solvent and brought into contact with the polymerization system to deactivate the polymerization catalyst and terminate the polymerization. Then, a basic compound is further added to sufficiently deactivate the catalyst and, at the same time, convert the unstable terminal into a stable terminal structure to obtain a polyacetal copolymer having extremely few unstable terminals. It is in. The effect of the present invention is that the specific two-stage deactivation treatment causes the deactivation of the catalyst itself, and also causes the active cation terminal being polymerized to react with a cyano group to form a relatively stable terminal group. It is understood that this is due to the fact that this is converted to a more stable terminal structure and blocked by further alkali treatment. Therefore, according to the method of the present invention, a method for removing all the unstable parts present in a relatively large amount after the deactivation of the catalyst by a basic substance, which has been conventionally performed exclusively as a method for stabilizing a polyacetal copolymer, has been proposed. In comparison, the unstable portion to be removed is extremely reduced, so that not only the load for stabilization is reduced, but also the yield as a stable copolymer is improved.

[0005]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below step by step. First, the raw material monomer to be copolymerized in the present invention is mainly composed of trioxane, which is a cyclic trimer of formaldehyde, and the comonomer is at least one adjacent carbon bond used for copolymerization with conventional trioxane. Any of the known cyclic formals or cyclic ethers having the following formulas can be used. Such comonomers include, for example, cyclic formals such as 1,3-dioxolane, diethylene glycol formal, 1,4-butanediol formal, 1,3-dioxane, 1,3,5-trioxepane, ethylene oxide, propylene oxide, epichlorohydrin or the like. And cyclic ethers. Further, as a comonomer for the copolymer to form a branched or crosslinked molecular structure, a compound having two or more cyclic ether groups or cyclic formal groups such as alkylene-diglycidyl ether or diformal, for example, butanediol diglycidyl Ether, butanediol dimethylidene glyceryl ether and the like can also be used. Such comonomers may be used alone or in combination of two or more depending on the purpose. In particular, as the comonomer, cyclic formal or cyclic ether such as 1,3-dioxolan, diethylene glycol formal, 1,4-butanediol formal, and ethylene oxide is preferable. The amount of the comonomer used in the present invention is 20 mol% or less, preferably 0.2 to 10 mol%, based on trioxane. The larger the amount of the comonomer, the more advantageous the thermal stability of the produced polymer. However, if the amount of the comonomer is too large, the produced copolymer becomes soft and the melting point is lowered.

In the copolymerization of the present invention, a known chain transfer agent for adjusting the degree of polymerization, such as a low molecular weight linear acetal such as methylal, may be added according to the purpose. Further, a sterically hindered phenol-based antioxidant may be added in advance to the monomer or comonomer and copolymerized in the presence thereof. It is preferable that the polymerization system (monomer, etc.) is substantially free of impurities having active hydrogen such as formic acid, water, methanol, etc., for example, it is desirable that each of these impurities is 10 ppm or less.

[0007] The polymerization catalyst in the present invention includes:
Common cationically active catalysts are used. Such cationically active catalysts include Lewis acids, especially boron, tin,
Halides such as titanium, phosphorus, arsenic and antimony, such as boron trifluoride, tin tetrachloride, titanium tetrachloride, phosphorus pentachloride, phosphorus pentafluoride, arsenic pentafluoride and antimony pentafluoride, and their complexes Compounds such as compounds or salts, protic acids, for example, perfluoroalkylsulfonic acids such as trifluoromethanesulfonic acid, perchloric acid, or esters of these protic acids (for example, perchloric acid tertiary butyl ester), and protic acid anhydrides ( Acetyl perchlorate) or isopoly acid,
Heteropolyacids (for example, phosphomolybdic acid, phosphotungstic acid, silicomolybdic acid, silicotungstic acid, etc.),
Alternatively, examples thereof include acid salts thereof, and ion pair catalysts such as triethyloxonium hexafluorophosphate, triphenylmethylhexafluoroarsenate, and acetylhexafluoroborate. Above all, boron trifluoride or a coordination compound of boron trifluoride and an organic compound (for example, ethers) is the most common, and perfluoroalkylsulfonic acid, heteropoly acid and the like are also suitable. The method for adding the catalyst is not particularly limited, and it may be added to the mixture of trioxane and the comonomer in a gaseous form or as a solution of an inert solvent. Before the polymerization of the comonomer itself proceeds, the copolymerization may be started by mixing with the trioxane. For this purpose, it is preferable that the catalyst and the comonomer are prepared at a low temperature and added to the polymerization system promptly.

The polymerization method of the present invention can be carried out with the same equipment and method as the conventionally known polymerization method of trioxane.
That is, any of a batch type and a continuous type can be used, and any of a solution polymerization, a melt bulk polymerization, and the like may be used. However, a continuous bulk polymerization using a liquid monomer and obtaining a solid powder bulk polymer as the polymerization proceeds. The method is industrially common and preferred. In this case, a small amount of an inert liquid medium may coexist as necessary. As the polymerization apparatus used in the present invention, a co-kneader, a twin-screw type continuous extrusion mixer, a twin-screw paddle type continuous mixer, and other continuous polymerization apparatuses for trioxane proposed so far can be used. If it is a system, two or more types of polymerization machines may be used in combination. Particularly, those having a crushing function such that a solid polymer produced by a polymerization reaction can be obtained in a fine form are preferable. The polymerization temperature is not particularly limited depending on the polymerization method, the type and amount of the catalyst used, etc., but if a generally used bulk polymerization method is employed, the polymerization is carried out in a temperature range of 60 to 120 ° C, preferably 65 to 110 ° C. . The polymerization time is also related to the amount of the catalyst, the polymerization temperature and the like, and is not particularly limited. However, the polymerization time is generally selected to be 0.5 to 100 minutes, and preferably 1 to 30 minutes.

[0009] After completion of the polymerization, the crude polymer discharged from the polymerization machine is then mixed with a deactivator and brought into contact to deactivate the polymerization catalyst to stop the polymerization reaction. A feature of the present invention is that an organic compound having a cyano group is used as a first step as a deactivator after polymerization and a polymerization terminator, and the resulting catalyst is brought into contact with the resulting crude copolymer to deactivate the catalyst and terminate the polymerization. On the point. The amount of the cyano group-containing organic compound used is 5 to the amount of the catalyst.
The molar amount is from 2000 to 2000, preferably from 25 to 1000, more preferably from 100 to 500. There is no particular limitation on the method of contacting the first-stage deactivator with the polymerization reaction product, and it can be obtained by adding the deactivator to the crude polymer and mixing well. The organic compound having a cyano group, which is a quencher characterized by the present invention, is an organic compound having at least one cyano group in the molecule, for example, acetonitrile, acrylonitrile, propionitrile, valeronitrile, cyanamide, Benzonitrile and the like. Among them, preferred are acetonitrile, propionitrile, cyanamide, benzonitrile and the like. The quenching agent is, for example, a compound having a relatively low boiling point such as acetonitrile, if the quenching agent is in a gaseous state,
Alternatively, by diluting with an inert gas such as nitrogen gas and bringing the resulting mixture into good contact with the polymerization product, efficient deactivation treatment can be performed. Also, a sufficiently efficient deactivation can be performed by a method in which a solution in which the above-described deactivator is dissolved in an inert organic solvent is mixed and stirred with the polymerization reaction product. In particular, according to the latter method, a sufficient effect can be obtained even with a high boiling point of 150 ° C. or more. That is, the first step of the deactivation treatment of the present invention is, of course, to add a large amount of the solution containing the above deactivator to the reaction product, mix well, then separate the liquid together with the unreacted monomer, and after the deactivation. A method of separating and recovering the unreacted monomer is also possible, and the effect of the present invention is exhibited. However, the polymerization is carried out so that the amount of the unreacted monomer is small (for example, 5% by weight or less), Alternatively, the deactivation treatment may be performed with a small amount of a solution containing a deactivator, and the process may be directly shifted to the deactivation stabilization treatment with the next basic compound without washing the produced crude polymer or collecting the monomer. I can do it. The solvent for the deactivator in the first stage is not particularly limited as long as it is inert, but is preferably an aprotic organic solvent, for example, aromatic compounds such as benzene and toluene, diethyl ether and dibutyl ether. Ethers such as, tetrahydrofuran,
Examples include cyclic ethers such as 1,4-dioxane, hydrocarbons such as n-hexane and cyclohexane, and halogenated solvents such as chloroform and carbon tetrachloride. In the first-stage deactivation treatment, water is used. Is not preferable because an unstable hemiacetal terminal group is formed.

In the deactivation treatment of the first stage, the coarse polymer is preferably a fine powder in order to make the contact sufficiently and uniformly. Preferably has a function of sufficiently pulverizing,
The reactant after polymerization may be separately pulverized using a pulverizer and then a deactivator may be added. Further, pulverization and stirring may be performed simultaneously in the presence of the deactivator. The particle size of the crude polymer in the deactivation treatment is preferably as small as possible.
mm or less, more preferably 2 mm or less, particularly preferably 1 mm or less. The first-stage deactivation reaction is preferably carried out at a temperature of 20 ° C. or higher and 130 ° C. or lower. If the temperature is too low, it takes time to complete the deactivation reaction, and if the temperature is too high, depolymerization occurs, which is not preferable.

In the present invention, as described above, after the deactivation treatment with an organic compound having a cyano group, a basic compound is further added, and the mixture is brought into contact with the mixture to carry out the second stage treatment, whereby the copolymerization is achieved. It is characterized in that the terminal of the union is converted to a terminal that is more thermally and chemically stable. Any known inorganic or organic alkaline substance is effective as the basic compound used in the second stage treatment. Examples of the basic compound include ammonia, various amine compounds, hydroxides and oxides of alkali or alkaline earth metals, and non-organic compounds. Acid salts or organic acid salts are exemplified. Examples of the amine compound include primary, secondary, and tertiary aliphatic amines and aromatic amines such as methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine, triethylamine, butylamine, dibutylamine, tributylamine, and alcohols corresponding thereto. Examples include amines (eg, triethanolamine), aniline, diphenylamine, heterocyclic amines, hindered amines (various piperidine derivatives), and the like. Further, as the alkali or alkaline earth metal compound, hydroxides of alkali or alkaline earth metal, oxides, carbonates, bicarbonates, phosphates, borates, inorganic weak salts such as silicates, Acetate, oxalate, formate, benzoate,
Organic acid salts such as terephthalate, isophthalate and fatty acid salt, methoxide, ethoxide, n-butoxide, sec
Examples thereof include alkoxides such as -butoxide and tert-butoxide, and phenoxides. Among them, carbonates and fatty acid salts are preferably used. Examples of the alkali or alkaline earth metal component include lithium, sodium, potassium, cesium, magnesium, calcium, strontium, barium and the like. Of these, lithium, sodium, potassium, magnesium and calcium are preferably used. Specifically, calcium hydroxide, magnesium hydroxide, sodium carbonate, calcium acetate, calcium stearate, calcium 12-hydroxystearate and the like are particularly preferred. The amount of the basic compound used is not particularly limited, but is generally 5 to 50 times the amount of the catalyst.
The molar amount is 00-fold, preferably 10- to 1000-fold, more preferably 20- to 500-fold. If the amount of the basic compound is too small, it takes a long time to deactivate and stabilize the compound. If the amount is too large, adverse effects such as promotion of coloring are not preferred.

The treatment with the basic compound in the second stage is as follows:
It is preferable to carry out the reaction in the presence of water, and in general, it is appropriate to add as an aqueous solution, but it may be added as a solution of another solvent containing water. In addition, a substance that easily vaporizes, such as ammonia, may be added and contacted with water vapor. The amount of the basic compound solution to be added is not particularly limited either. After adding and treating in a large amount, the solution may be separated, and the unreacted monomer may be recovered at this stage. (For example, 0.2 to 5%).
After the step treatment, the second step may be further treated with a small amount of the basic compound solution, and the process may be directly transferred to the melt kneading treatment or the like in the next step. The conditions for the second-stage treatment are not particularly limited, and may be appropriately selected. Generally, the second-stage deactivation stabilization treatment temperature is 20 to 100 ° C, preferably 50 to 90 ° C, and the time is 5-
60 minutes, preferably 10 to 30 minutes. Also, it is natural that the particle size of the polymer at the time of the treatment is preferably fine as in the case of the first-stage treatment in order to make sufficient contact. , May be processed as it is.

The above-mentioned specific two-stage treatment, which is a feature of the present invention, is effective not only for completely deactivating the polymerization catalyst itself, but also for causing the cationic active terminal being polymerized to react with the cyano group. After being deactivated and blocked, it is further converted into a terminal group having a thermally and chemically stable structure by a second alkali treatment, so that an unstable terminal such as a hemiacetal group is used as in a conventional deactivator. It is considered that this contributes to the reduction of unstable portions because it is possible to avoid stopping due to formation.

The polymer subjected to the deactivation treatment of the present invention is subjected to melt-kneading as it is, if necessary, after separation and recovery of the monomer, drying, etc., and addition of various additives such as stabilizers, to give a pelletized product. . In this melting treatment, the stabilization treatment further proceeds even if a part of the unstable terminal remains in the above treatment,
Complete. As described above, the copolymer of the present invention has very few unstable terminals and the load of the stabilization treatment is reduced, so that a sufficiently stable polymer can be obtained by a simple melt extrusion treatment. By kneading and extruding, it is possible to volatilize and remove the volatile components and the remaining unstable portions mixed together with the compounding of the stabilizer, so that a copolymer having extremely excellent stability can be obtained. As the stabilizer, conventionally known stabilizers, for example, one or two or more kinds of antioxidants such as hindered phenols and hindered amines, formaldehyde absorbents, and inorganic basic substances can be used.

[0015]

As described above, according to the method of the present invention, a copolymer having very few unstable parts can be obtained, the post-treatment step can be simplified, and the thermal stability of the final product is high.

[0016]

EXAMPLES Examples of the present invention will be described below, but it is needless to say that the present invention is not limited to these examples. The terms and measuring methods in Examples and Comparative Examples are as follows. % And ppm: All are expressed by weight.・ Melt index (MI): Melt index measured at 190 ° C for deactivated polymer (powder and granular)
(g / 10min). This was evaluated as a characteristic value for the molecular weight. That is, the lower the MI, the higher the molecular weight. However, certain stabilizers (Ciba Geigy,
Irganox 1010 (0.5%) and melamine (0.1%
%)), Mix well and measure. -Alkali decomposition rate (amount of unstable part): 1 g of the deactivated crude polymer powder is 50% containing 0.5% ammonia
Put in 100 ml of aqueous methanol
After heating for one minute, the amount of formaldehyde decomposed and eluted in the liquid is quantitatively analyzed, and is shown as% of the polymer. Heating weight loss rate: Weight loss rate when 5 g of pellets obtained by adding a stabilizer (same as above) to a crude polymer powder, melt-kneading with a vented extruder and heating at 230 ° C. for 45 minutes in air. Is shown.

Examples 1 to 5 and Comparative Examples 1 to 3 A barrel having a jacket in which two circles partially overlap each other and through which a heating medium is passed, and a number of paddles for stirring and propulsion inside the barrel. Using a continuous mixing reactor provided with two attached rotating shafts in the longitudinal direction, hot water of 80 ° C. was passed through the jacket, and the two rotating shafts were rotated at a constant speed. Trioxane containing 3% of 3-dioxolane and 700 ppm of methylal is continuously supplied, and at the same time, boron trifluoride-diethyl ether complex (40 ppm of boron trifluoride based on all monomers as boron trifluoride) is continuously added to the same place as a catalyst. Copolymerization was performed. Next, the reaction product discharged from the discharge port of the polymerization machine is pulverized (the particle size is 95% or more and 2 mm or less). After mixing and performing the first-stage treatment, the mixture was further thrown into the basic aqueous solution shown in Table 1 and thoroughly stirred and mixed at 90 ° C. for 10 minutes to perform the second-stage treatment. Next, centrifugation, washing and drying were performed to obtain a copolymer powder. The MI and alkali decomposition rate of this polymer powder were measured according to the above. Further, a predetermined stabilizer (same as above) was mixed with the polymer powder, and the mixture was melt-extruded using a vented extruder to prepare pellets, and the weight loss on heating was measured by the method described above. Table 1 shows the results. On the other hand, for comparison, a similar experiment was performed for only the first-stage treatment (second-stage water alone) and the case where the first-stage treatment was omitted and the solution was cast into a basic aqueous solution and deactivated. And the results are shown in Table 1.

[0018]

[Table 1]

Examples 6-7, Comparative Examples 4-5 Heteropolyacid (phosphomolybdic acid; 3 ppm based on all monomers) as a polymerization catalyst was previously added to the comonomer and mixed.
Copolymerization and deactivation treatment was carried out in the same manner as in Examples 2 and 5 and Comparative Examples 1 and 3 except that this was added to trioxane, and the same evaluation was performed. Table 2 shows the results.

[0020]

[Table 2]

Example 8 and Comparative Examples 6 and 7 Copolymerization and polymerization were carried out in the same manner as in Example 1 and Comparative Examples 1 and 2, except that a diglyme solution of trifluoromethanesulfonic acid (1 ppm based on all monomers) was used as a polymerization catalyst. A deactivation treatment was performed and evaluated in the same manner. Table 3 shows the results.

[0022]

[Table 3]

Examples 9 to 11 and Comparative Examples 8 to 10 By the same method as in Examples 1 to 5, the remaining monomer was reduced to about 3
%, And then the nitrile compounds shown in Table 4 were added and mixed well at about 100 ° C. for 30 minutes to perform the first-stage treatment. Next, as a second stage treatment, a small amount of a basic aqueous solution shown in Table 4 and the above stabilizer were added, and 90
After well stirring and mixing at 30 ° C. for 30 minutes, the mixture was melt-extruded using an extruder with a vent as it was to prepare pellets, and the weight loss on heating was measured by the method described above. Table 4 shows the results. on the other hand,
For comparison, the same experiment was performed when the first stage was omitted and when the addition of the basic aqueous solution in the second stage was omitted, and the results are also shown in Table 4.

[0024]

[Table 4]

Claims (10)

[Claims] 1. A method for producing a polyacetal copolymer by polymerization using a trioxane as a main monomer, a cyclic ether or a cyclic formal as a comonomer, and using a cationically active catalyst as a polymerization catalyst to produce a polyacetal copolymer. Stabilized polyacetal copolymer characterized by adding and contacting an organic compound having a cyano group as an agent and then contacting it with a basic compound to deactivate the polymerization catalyst and terminate and stabilize the polymerization. Manufacturing method. 2. The method according to claim 1, wherein the polymerization catalyst is boron trifluoride or a coordination compound thereof. 3. The stabilized polyacetal according to claim 1, wherein the polymerization catalyst is at least one selected from perfluoroalkylsulfonic acid, perchloric acid, or an ester thereof, a heteropolyacid, an isopolyacid or an acid salt thereof. A method for producing a copolymer. 4. The method according to claim 1, wherein an organic compound having a cyano group is added as a deactivating agent in an amount of 5 to 2000 times the amount of the catalyst used in the polymerization, and the mixture is brought into contact with the catalyst. For producing a functionalized polyacetal copolymer. 5. The stabilized polyacetal according to claim 1, wherein after the copolymerization, an organic compound having a cyano group is dissolved in a gaseous and / or aprotic organic solvent, added, and contacted. A method for producing a copolymer. 6. The method according to claim 1, wherein the basic compound is added as an aqueous solution or a solution containing water and brought into contact therewith.
3. The method for producing a stabilized polyacetal copolymer according to item 1. 7. The basic compound is ammonia, or at least one of primary, secondary and tertiary amines, oxides, hydroxides, inorganic acid salts and organic acid salts of alkali or alkaline earth metals. The method for producing a stabilized polyacetal copolymer according to any one of claims 1 to 6, which is a seed. 8. The method according to claim 1, wherein after the copolymerization, the deactivating agent is brought into contact with a copolymer in which 90% or more of the produced crude polymer has been pulverized to a particle size of 2 mm or less. For producing a functionalized polyacetal copolymer. 9. A method for producing a stabilized polyacetal copolymer, wherein the copolymer deactivated and stabilized by the method according to claim 1 is further melt-kneaded. 10. The method for producing a stabilized polyacetal copolymer according to claim 9, wherein the melt-kneading treatment is performed by a vent extruder in the presence of a stabilizer.