JPS6136021B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method of minimizing mold deposit problems often encountered when molding unreinforced polyoxymethylene molding compositions. Such deposits have an adverse effect on the formation of good quality molded parts with smooth and uniform surface characteristics. As is well known, polyoxymethylene, or polyacetal, is a thermoplastic resin that is widely used in the production of molded articles by injection molding or extrusion methods. Polyoxymethylene has many excellent mechanical properties, which give molded articles characterized by hardness, strength and tenacity. However, especially under the influence of heat, polyoxymethylene resins undergo decomposition, and the amount of decomposition is a factor in the method of manufacturing polyoxymethylene and the like. This degradation can occur, for example, as a result of oxidative attack. Oxidative attack can lead to chain scission and depolymerization and is often prevented by the addition of antioxidants to polyoxymethylene compositions. Degradation is also believed to occur as a result of acidolysis of the polymer chains caused by acidic species present in the polymer. Acidic species are
acidic catalyst residues resulting from the catalyst used in the production of the polymer or when certain chains (thus stabilized) depolymerize, sometimes as a result of oxidative or hydrolytic chain scission; It may be acetic acid generated from the acetate end group. Especially heat,
Alternatively, during subsequent processing in the melt, "acid scavengers" are often mixed with the polymer composition to help minimize such degradation of the polymethylene. Most commercially available polyoxymethylenes are processed by acetylation or hydrolysis (e.g., U.S. Pat.
3830267) or by the addition of additives such as the antioxidants and/or acid scavengers mentioned above, during high temperature molding, especially injection molding, of non-fiber reinforced resins. It has been found that an undesirable film or mold deposit commonly forms on the surface of the mold. Mold deposits can result in surface defects on the molded resin and are generally considered to have two types of molds. One type of mold deposit is believed to occur through the use of certain antioxidants that plate out onto the surface of the mold. This type of mold deposit can be removed by using relatively non-volatile antioxidants. A second type of mold deposit is believed to result from formaldehyde condensing on the surface of the mold (e.g., generated as a result of chain scission of polyoxymethylene under the conditions of the molding process). On the other hand, it is believed that this chain scission is caused by acidic residues present in the polymethylene that have not been "cleaned up" by previous stabilization treatments. Conventionally, thermal stabilization of polyoxymethylene, i.e., stabilization against the effects of temperature encountered in the molten state, is
For example, amino-substituted amides for polyoxymethylene (US Pat. No. 3,274,149), carbonates (US Pat. No. 3,144,431), or heavily sterically hindered carbodiimides (UK Pat. No. 993,600).
Although such stabilization has been proposed in the art, such as by the addition of esters, such stabilization is either not effective in eliminating the tendency for mold deposits or results in undesirable discoloration of the polymer. It has also been proposed to physically incorporate polyoxymethylene with thermoplastic resins to improve the properties of polycarbonate. Such incorporation is not aimed at improving the properties of the polyoxymethylene and uses relatively large amounts of polycarbonate. Thus, for example, Miller, U.S. Pat.
No. 3,646,159 discloses blending polyoxymethylenes, or polyacetals, with polycarbonate to improve the properties of the polycarbonate and to obtain a polycarbonate with improved resistance to environmental stress cracking and cracking. The mirror is
Among other things, it is suggested that amounts of polyacetal from 25 to 95% by weight based on the combined weight of polycarbonate and polyacetal can be used, although the examples show blends containing up to about 50% by weight polyacetal. limited to. Goldbloom, US Pat. No. 3,290,261, discloses blending polycarbonate with up to about 20% polyacetal to obtain a foamed polycarbonate resin. Goldbloom discloses that when the amount of polyacetal exceeds 29%, the blend begins to lose the advantageous properties of the polycarbonate (column 1, line 47). Polycarbonates have also been disclosed as additives or modifiers or scavengers for polyester tie cords when added to the polyester prior to fiber formation (see, eg, Lai et al., US Pat. No. 3,563,847). In view of this prior art, it is an object of the present invention to provide a process for making improved polyoxymethylene molding compositions that exhibit reduced mold deposits. Furthermore, it is an object of the present invention to provide polyoxymethylene molding compositions that have high stability when subjected to the influence of heat and especially when subjected to typical conditions encountered during molding operations. It is also an object of the present invention to provide improved unreinforced polyoxymethylene compositions for injection molding processes. Another object of the invention is the provision of a stabilized polyoxymethylene molding composition using an aromatic polycarbonate additive, which stabilized polyoxymethylene does not exhibit undesirable discoloration. These and other objects of the invention will become apparent from the following summary and description of the preferred embodiments. In accordance with the present invention, it has been discovered that certain aromatic polycarbonates (as defined) increase the thermal stability of polyoxymethylenes without causing excessive discoloration of the polyoxymethylenes.
Addition of aromatic polycarbonate to polyoxymethylene, followed by appropriate heat treatment, reduces the formaldehyde generated by the polyoxymethylene when it is then subjected to heat treatment; particularly when subjected to conditions that previously led to the formation of undesirable formaldehyde-type deposits. was found to reduce the amount of The improved molding composition, stabilized polyoxymethylene, is prepared by heating the polyoxymethylene and polycarbonate in admixture for at least about 2 minutes at a temperature at which the polyoxymethylene melts (generally above 160°C). manufactured by The amount of polycarbonate used is from about 1% to about 4% by weight, based on the weight of the polyoxymethylene. Particularly satisfactory results are achieved if a small amount of malonamide is also mixed into the polyoxymethylene polymer and polycarbonate during heating. As used herein, the term "polyoxymethylene" refers to so-called cap homopolymers,
That is, it includes both homopolymers, including acylated homopolymers, as well as copolymers as more specifically defined below. The thermal stability provided by the addition of aromatic polycarbonate to polyoxymethylene according to the present invention is the stability of this polyoxymethylene against decomposition when subjected to heat. Polycarbonate is believed to provide stability against any decomposition effects of heat, including, for example, aging of polyoxymethylene moldings at temperatures between 100°C and 140°C; Typical temperatures and conditions encountered during molding, i.e. approx.
It is particularly useful in providing stability against degradation when the polyoxymethylene is subjected to temperatures of 185 DEG C. to about 240 DEG C. for several minutes. A particularly preferred application of the invention is in the injection molding of polyoxymethylene, since instability, ie decomposition, of polyoxymethylene is more problematic in this type of operation, for example in injection molding operations than in extrusion operations. . In the extrusion of polyoxymethylene, formaldehyde, which may be generated during the decomposition of the polymer, has no chance of condensing on the mold surface and can escape through the vents provided in the extruder. Injection molding is a process in which a polyoxymethylene molding composition is heated in a preheating zone to become a plastic melt.
Any known method in which the material is then passed through a nozzle into a closed mold. Heating the polyoxymethylene is typically to a temperature of about 180<0>C to about 240<0>C. The temperature of the mold will generally be substantially lower, e.g.
Although less than 100°C, the exact relationship between the temperature of the melt and the temperature of the mold will depend on factors such as the desired surface properties of the molded article, as will be recognized by those skilled in the art. Mold deposits can be observed at any of the recommended molding temperatures (usually after 25 to 50 shots) when poor quality polyoxymethylene copolymers are used;
It tends to become larger as the melt temperature is higher and the mold temperature is lower. The tendency towards mold deposits varies depending on the particular polyoxymethylene, pre-stabilization treatment, etc. Thus, for example, acylated homopolymers generally suffer from mold deposit problems less frequently than melt hydrolyzed copolymers. The occurrence of mold deposit problems is further determined by the size, gating, and ventilation of the molded part. Small parts, small gates and inadequate ventilation pose the greatest problems. Mold deposits tend to cause defects on the surface of the molded part. Such parts must be shredded or remolded. Although the addition of polycarbonate to polyoxymethylene according to the present invention is effective in providing thermal stability at molding (melt) temperatures, it may not be effective at temperatures above about 249°C due to decomposition of the polyoxymethylene. be. Injection molding has a mold that includes, for example, a preheating cylinder, a plunger or reciprocating screw, a torpedo, a nozzle and runner, a runner, a gate, and a mold cavity. It can be carried out in conventional injection molding equipment. Cylinder temperature is usually about 180℃~240℃
The molding pressure is usually about 5000 to 20000 psi.
The actual molding temperature and pressure will depend on the type of machine used, ie, plunger or screw injection molding machine, and on the desired shape and size of the molded article. Cycle times are typically about 30 to about 110 seconds. As mentioned above, polyoxymethylene polymers that can be stabilized with polycarbonates according to the present invention include both homopolymers and copolymers. Such polymers can be made by methods well known in the art, repeating -
It has OCH ₂ units and is typically produced by polymerization of anhydrous formaldehyde or by polymerization of trioxane. Particularly useful in the present invention are -
OR- group in which R represents at least two atomic groups directly bonded to each other and located between two valences in the chain.
repeating oxymethylenes (divalent residues having 5 carbon atoms) interspersed with (any substitutions on the R residue are inert, i.e., substitutions that do not induce undesired reactions) -OCH ₂ -) units having at least one chain. Suitable copolymers are from 60 to
It has 99.6 mol% of repeating polyoxymethylene groups. In a preferred embodiment, R may be, for example, an alkylene or substituted alkylene group having at least 2 carbon atoms. Some of the copolymers utilized by the present invention have the formula: (In the formula, n is 0 or an integer from 1 to 5, and from 60 to
n is 0 in 99.6% of repeating units, R ₁ and R ₂
may have a structure consisting of repeating units of inert substituents, ie, substituents that do not undergo undesirable reactions. A preferred class of copolymers are those having a structure consisting of repeating units in which 60 to 99.6% of the repeating units are oxymethylene units. These copolymers have trioxane structure: (where n is 0.1 or 2). Examples of other suitable polymers include U.S. Patent No. 3,027,352
copolymers of trioxane and cyclic ethers having at least two adjacent carbon atoms, such as the copolymers disclosed in this patent. Among the specific ethers that can be used are:
Ethylene oxide, 1,3-dioxolane,
1,3,5-trioxepane, 1,3-dioxane, trimethylene oxide, pentamethylene oxide, 1,2-propylene oxide, 1,
These include 2-butylene oxide, neopentyl formal, pentaerythritol diformal, paraldehyde, tetrahydrofuran, and butadiene monoxide. Suitable polymers treated in accordance with the present invention have a weight average molecular weight of at least about 35,000, a melting point of at least about 150°C, and an intrinsic viscosity of at least about 0.8 (p-chlorophenol containing 2% alpha-arpinene). (measured as a 0.1% solution by weight at 60°C). As will be understood by those skilled in the art, the polyoxymethylene should be pre-stabilized before mixing with the polycarbonate and heating. Such pre-stabilization can take the form of stabilization by decomposing the molecular ends of the polymer chain to the point where there is a relatively stable carbon-to-carbon bond at each end. For example, such decomposition is described in U.S. Pat.
It can be carried out by melt hydrolysis as disclosed in US Pat. No. 3,318,848 or by solution hydrolysis as disclosed in US Pat. No. 3,219,623. Of course, melt-hydrolyzed and solution-hydrolyzed polyoxymethylene polymers can be used. The polyoxymethylene may also be mixed with conventional stabilizers such as antioxidants (e.g., at a concentration of about 0.1-2.0% by weight) and/or acid scavengers (e.g., at a concentration of about 0.05-1.0% by weight). It can be pre-stabilized by this. Generally, these stabilizers are present in a total amount of up to about 3% by weight, based on the weight of the polyoxymethylene polymer. Generally speaking, the aromatic polycarbonates used in accordance with the present invention are well-known, commercially available thermoplastic materials. Generally, such aromatic polycarbonates have the formula: (wherein A is a divalent aromatic residue derived from a non-sterically hindered, non-halogenated divalent phenol). Specific methods for making such polycarbonates and the starting materials and polymers made therefrom are described in Chemistry by Hermann Schünel.
and Physice of Polycarbonetep,” William
Well-known textbooks such as “Polycarbonates” by F. Christopher, as well as US Patent No. 2970137;
2991273; 2999846; 2999835; 3014891;
It is described in patent documents such as No. 3028365; No. 3030331. Divalent phenols are sterically unhindered in the sense that no ortho substituents adjacent to one of the hydroxyl groups are present on the aromatic ring. Such substituents prevent the resulting polycarbonate from functioning effectively during the process of the invention. Para-substituted dihydric phenols are preferred; however, meta-substituted dihydric phenols may be used. Also, dihydric phenols do not contain other functional groups, including halogen or sulfone linkages, which would interfere with the desired results. Preferably, apart from the hydroxyl group, only hydrogen is present on the aromatic ring of the divalent phenol. Representative non-sterically hindered non-halogenated dihydric phenols from which aromatic polycarbonates can be derived include: 2,2'-dihydroxydiphenyl, i.e.

Sterically hindered dihydric phenols such as [Formula] should not be used. For example, suitable aromatic polycarbonates include:
prepared from dihydroxydiarylalkane such as 2,2-bis-(4-hydroxyphenyl)propane (i.e. bisphenol A) and diesters of phosgene, haloformates or carbonic acid as described in U.S. Pat. No. 3,028,365 be able to. Particularly preferred are homopolymers derived from 2,2-bis-(4-hydroxyphenyl)propane. Such materials are commercially available under the trademarks "Marlon" by Mobay Chemical Corporation and "Lexan" by General Electric Company. Suitable aromatic polycarbonate copolymers are derived from at least 80 mole percent 2,2-bis-(4-hydroxyphenyl)propane. The aromatic polycarbonate selected for use in the present invention is miscible with the polyoxymethylene polymer while in the molten state to ensure good miscibility or miscibility, as well as under the conditions of mixing and subsequent molding. Must not evaporate. Typically aromatic polycarbonates are prepared in methylene chloride at 25°C from about 0.35 to
Intrinsic viscosity of 0.75, preferably from about 0.35 under the same conditions
It has an intrinsic viscosity of 0.6. The aromatic polycarbonate is added to the polyoxymethylene composition (i.e., pre-stabilized) in an amount of about 1 to about 4 weight percent, more preferably about 1 to about 2 weight percent, based on the weight of the polyoxymethylene polymer. (polyoxymethylene). Approximately 1% by weight
Much lower amounts of aromatic polycarbonate may require extended mixing with the polyoxymethylene to achieve the desired stabilization, while amounts much greater than about 4% by weight of the polyoxymethylene It tends to adversely alter the physical properties, ie tensile strength, Isodic impact value, etc. Any of the aromatic polycarbonates included in the above description can be used alone or mixed with other aromatic polycarbonates to obtain the desired thermal stability. polyoxymethylene and polycarbonate,
The polyoxymethylene is heated to a temperature at which it melts or is in a molten state. Generally, about 160°C or more, preferably about 180°C or more, more preferably about 180°C
Requires a temperature of ~240℃. Temperatures much higher than about 240° C. can lead to material decomposition and/or possible adverse side reactions. Thus,
This temperature range is one that maintains the polyoxymethylene in melt form but does not cause decomposition or adverse side reactions. The polyoxymethylene and aromatic polycarbonate are held at these temperatures for at least about 2 minutes, and usually about 2 to 20 minutes. Care should be taken, especially when used for much longer than about 20 minutes, as the polymeric material may retain a tendency to degrade. The exact amount of time used will depend primarily on the particular equipment in which the polymer is heated in admixture. Relatively efficient mixing and heating equipment, such as the Werner-Pfäider ZSK twin-screw extruder, of course requires less time than equipment such as, for example, the Brabender Plastograph. Generally, polyoxymethylene is molten,
The polyoxymethylene and polycarbonate may be mixed or blended and heated in any convenient manner or equipment so long as the polyoxymethylene and polycarbonate are in intimate contact with the aromatic polycarbonate for at least 2 minutes under such conditions. If desired, the polymers can be dry blended first and then heated, or they can be mixed first in a heating device. It is believed that the aromatic polycarbonate reacts with acid residues in the polyoxymethylene during the heating step, thus stabilizing the polyoxymethylene during subsequent molding, ie, reducing formaldehyde type deposits. Therefore, the most efficient means of heating and mixing the polymer is desirable to ensure complete polycarbonate acid residue reaction and therefore substantial elimination of formaldehyde type deposits during subsequent molding. For this reason, the polyoxymethylene in the molding equipment must be mixed and the aromatic polycarbonate-acid residue reaction allowed sufficient preheat time to occur before the molding composition enters the mold cavity. Direct addition of polycarbonate is not recommended. If desired, the polyoxymethylene and polycarbonate may be mixed and heated as described above, pelletized and stored for later use in the molding process. The polyoxymethylene molding composition of the present invention is
In addition to containing polyoxymethylene and polycarbonate, there are also other agents commonly used in polyoxymethylene molding compositions, both polymeric and non-polymeric, such as lubricants, pigments and conventional antioxidants and acid scavengers, etc., as mentioned above. Optionally, small amounts of additives may also be included. In a preferred embodiment, fibrous reinforcement is omitted from the molding composition since it is in fibrous reinforcement systems that mold deposit problems are relatively commonly encountered in the prior art. The aromatic polycarbonates utilized during the present process are believed to be somewhat unique in that mold deposit problems are effectively eliminated. Excess coloration often encountered when using large amounts of conventional basic acid scavengers is not a factor.
Also, as discussed below, there is no significant reduction in molded article properties as is commonly encountered when particulate non-polymeric additives are utilized. In a particularly preferred aspect according to the invention, a small amount of malonamide (i.e. carboamide acetamide)
Effective thermal stabilization of polyoxymethylene can be obtained using aromatic polycarbonates in combination with
Reduces the tendency of polyoxymethylene mold deposits. Although malonamide is a known and effective heat stabilizer for polyoxymethylene, it is not widely used because it causes undesirable discoloration of polyoxymethylene and tends to be expensive. See, eg, US Pat. No. 3,116,267. However, when combined with aromatic polycarbonates, the discoloration of polyoxymethylene is surprisingly substantially reduced, while at the same time providing the desired thermal stabilization. The presence of aromatic polyamide allows relatively low concentrations of malonamide to be utilized. for example,
The malonamide may be present at a concentration of about 0.1 to 1% by weight, preferably about 0.1 to 0.5% by weight, based on the weight of the polyoxymethylene polymer. Molded articles made from aromatic polycarbonate-stabilized polyoxymethylene molding compositions according to the present invention have improved physical properties (tensile, tensile stress, impact strength, etc.)
shows only a slight reduction, typically less than 10%. As indicated earlier in the specification, the polyoxymethylene molding compositions of the present invention exhibit increased thermal stability when heated from about 180<0>C to about 240<0>C. Thermal stability is
218% polyoxymethylene composition in whole glass apparatus
It can be measured by heating for 30 minutes at a temperature of 228°C ± 2°C. At the end of this time, a vacuum is applied to the system and the liberated formaldehyde is removed through two sodium sulfite traps. The amount of formaldehyde in the trap is then determined by titration using standard acids. The amount of formaldehyde released is a reasonable measure of thermal stability as well as the tendency of the composition to form mold deposits. The present invention thus provides a means of thermally stabilizing polyoxymethylene in a short period of time and with low concentrations of certain classes of aromatic polycarbonates. The invention is further illustrated with respect to the following examples:
The examples are to be considered illustrative of the invention.
However, it is to be understood that the invention is not limited to the specific details of the examples. EXAMPLE The thermal stability of various polyoxymethylene compositions according to the invention is shown in the table. The molding composition was prepared by mixing the polyoxymethylene polymer with additives in a Prabender Plastocorder Plastograph (200° C. and 35 RPM). Additives are added once the polyoxymethylene polymer appears completely melted.
Mixed for 20 minutes. A control was kneaded for a similar amount of time in the absence of additives. No increase in torque was observed for any of the blends. The polymers and were polyoxymethylene copolymers similarly prepared from trioxane and ethylene oxide (2% by weight), respectively. Each had a weight average molecular weight of about 68,000. The polymer had been melt hydrolyzed according to the technique of US Pat. No. 3,318,848. Each of these polymers contained 0.5% 2,2'-methylene-bis-(4-methyl-6-tert-butylphenol) antioxidant, 0.1
% of a cyanoguanidic acid scavenger, as well as about 0.2% of a diamide synthetic wax lubricant (trademarked by Glyco Chemicals under the name Akrawax C lubricant).
"stabilization" or "pre-stabilization" prior to mixing with the polycarbonate with a standard additive package containing
It had been. The aromatic polycarbonate was non-sterically hindered, non-halogenated and was commercially available from Mauvey Chemical Company under the trademark "Marlon", type M39F. Derived from 2,2-bis-(4-hydroxyphenyl)propane at 25°C
showed an intrinsic viscosity of 0.5 in methylene chloride. Formaldehyde evolved was measured by heating the sample at 228° C. for 30 minutes according to the procedure described above and is based on the initial weight of the sample.

[Table] The aromatic polycarbonates defined by the present invention are readily capable of providing effective stabilization against degradation of polyoxymethylene under test conditions and without causing undesirable discoloration of the polyoxymethylene. I understand. The principles, preferred embodiments, and mode of operation of the invention have been described herein. However, the inventions intended to be protected should not be construed as limited to the specific forms disclosed, as they are to be considered illustrative rather than restrictive. Modifications and variations may be made by those skilled in the art without departing from the spirit of the invention.

Claims (1)

Claims: 1. (1) a polyoxymethylene polymer that exhibits a tendency to form mold deposits during molding; and (2) from about 1 to about 4% by weight, based on the weight of the polyoxymethylene polymer. In methylene chloride at 25℃ approx.
A mixture of aromatic polycarbonates having an intrinsic viscosity of 0.35 to 0.75 and derived from non-sterically hindered non-halogenated dihydric phenols is heated for at least about 2 minutes at a temperature at which the polyoxymethylene polymer melts to form the mold. 1. A method for producing an improved polyoxymethylene molding composition, characterized in that the molding composition exhibits a reduced amount of deposits. 2. A method for preparing the improved polyoxymethylene molding composition of claim 1, wherein the mixture is heated to a temperature of about 160° C. or higher. 3 Heat the mixture at a temperature of about 180°C to about 240°C for about 2 to about
A method for producing an improved polyoxymethylene molding composition according to claim 1, which comprises heating for 20 minutes. 4 The polyoxymethylene polymer is at least 0.8
(measured at 60°C as a 0.1% by weight solution in p-chlorophenol containing 2% by weight alpha-arpinene), a weight average molecular weight of at least 35000, and a melting point of at least 150°C. A method for producing an improved polyoxymethylene molding composition according to item 1. 5 About 60 to about 99.6% polyoxymethylene polymer
A method for producing an improved polyoxymethylene molding composition according to claim 4, which is a copolymer having repeating _-OCH2- groups. 6. A method for preparing an improved polyoxymethylene molding composition according to claim 5, wherein the polyoxymethylene polymer is pre-stabilized by melt hydrolysis prior to mixing with component (2). 7. The improved polyoxymethylene molding composition according to claim 5, wherein the polyoxymethylene polymer is a mixture of polymers that have been subjected to melt hydrolysis and solution hydrolysis prior to mixing with component (2). How to make things. 8. A method for making an improved polyoxymethylene molding composition according to claim 5, wherein the polyoxymethylene polymer is pre-stabilized by the addition of an antioxidant and an acid scavenger prior to heating. 9. A method for preparing an improved polyoxymethylene molding composition according to claim 1, wherein the aromatic polycarbonate has an intrinsic viscosity of about 0.35 to 0.6 in methylene chloride at 25°C. 10. A method for producing an improved polyoxymethylene molding composition according to claim 1, wherein the dihydric phenol from which the aromatic polycarbonate is derived is 2,2-bis(4-hydroxyphenyl)propane. 11. A method for producing an improved polyoxymethylene molding composition according to claim 1, wherein the resulting molding composition does not contain a fibrous reinforcing agent. 12 (1) a polyoxymethylene composition that exhibits a tendency to form mold deposits during molding; (2) from about 1 to about 2% by weight, based on the weight of the polyoxymethylene polymer, in methylene chloride at 25°C;
an aromatic polycarbonate having an intrinsic viscosity of 0.35 to 0.75 and derived from a non-sterically hindered non-halogenated dihydric phenol; and (3) from about 0.1 to about 1% by weight based on the weight of the polyoxymethylene polymer. A fibrous reinforcing agent characterized in that a mixture of malonamides is heated at a temperature of about 180° C. to about 240° C. for at least about 2 to about 20 minutes to obtain a molding composition that exhibits a reduction in the amount of deposit formed during molding. A method for producing an improved polyoxymethylene-free molding composition. 13 The polyoxymethylene polymer is at least
Claims having an intrinsic viscosity of 0.8 (measured at 60°C as a 0.1% by weight solution in p-chlorophenol containing 2% by weight alpha-arpinene), a weight average molecular weight of at least 35000, and a melting point of at least 150°C 13. A method for producing the improved polyoxymethylene molding composition according to item 12. 14 Polyoxymethylene polymer is about 60 to about 99.6
14. A method of making an improved polyoxymethylene molding composition according to claim 13, wherein the copolymer has % of repeating _-OCH2- groups. 15. A method for preparing an improved polyoxymethylene molding composition according to claim 14, wherein the polyoxymethylene polymer is pre-stabilized by melt hydrolysis prior to mixing with component (2). 16 Polyoxymethylene polymer is component (2) and (3)
15. A method for preparing an improved polyoxymethylene molding composition according to claim 14, which is a mixture of polymers that have been melt-hydrolyzed and solution-hydrolyzed prior to being mixed with the polymer. 17. A method for making an improved polyoxymethylene molding composition according to claim 14, wherein the polyoxymethylene polymer is pre-stabilized by the addition of an antioxidant and an acid scavenger prior to heating. 18 The antioxidant is 2,2'-methylene-bis(4
-methyl-6-tertiary butylphenol),
18. A method for making an improved polyoxymethylene molding composition according to claim 17, wherein said acid scavenger is cyanoguanidine. 19. A method for preparing an improved polyoxymethylene molding composition according to claim 12, wherein the aromatic polycarbonate has an intrinsic viscosity of about 0.35 to 0.6 in methylene chloride at 25°C. 20 Claim 1, wherein the dihydric phenol from which the aromatic polycarbonate is derived is 2,2-bis(4-hydroxyphenyl)propane.
A method for producing the improved polyoxymethylene molding composition according to item 2. 21. The method of making the improved polyoxymethylene molding composition of claim 12, wherein component (3) is present at a concentration of about 0.1 to about 0.5% by weight, based on the weight of the oxymethylene polymer.