CA1096088A - Production of an improved polyoxymethylene molding composition which forms reduced mold deposists upon molding - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The mold deposit problem commonly associated with polyoxymethylene molding compositions which creates a non-uniform surface on a molded article effectively is minimized. The polyoxymethylene polymer initially is heated (as described) while in admixture with about 1 to about 4 percent by weight based upon the weight of the oxymethylene polymer of an aromatic polycarbonate (as defined). In a preferred embodiment a minor quantity of malonamide also is admixed with polyoxymethylene polymer and the aromatic polycarbonate during heating.

Description

Background of the Invention The present invention relates to a process for minimizing the mold deposit problem frequently encountered when polyoxymethylene molding compositions are molded. Such deposits are detrimental to the formation of a quality molded article having smooth and uniform surface characteristics.
As is well known polyoxymethylene or polyacetal, is a thermo-plastic resin which finds wide utility in the manufactureof shaped articles by inJection molding or extrusion processes. Polyoxymethylene has many excellent mechanical properties which result in shaped articles characterized by their hardness, strength and toughness.
Polyoxymethylene resin, however, is subject to degradation par-ticularly under the influence of heat, the amount of degradation being a ~actor of -the method of preparation of the polyoxymethylene and the like. The degradation may occur for example, as the result of oxidative attack. The oxidative attack, which may lead to chain scission and depolymerization, is often ret;arded by the addition of antioxidants to the polyoxymethylene composition. Degradation is also believed to occur as the result of acidolytic cleavage of the polymer chain caused by acidic species present in the polymer. The acidic species may be acidic catalyst residues derived from catalysts used in the formation of the polymer or may be acetic acid generated ;
from acetate end groups when a given chain, so stabilized, depolymerizes as a result of occasional oxidative or acidolytic chain scission. To assist in minimizing such degradation of polyoxymethylene especially during subsequent processing in the hot, or melt, state, "acid scavengers" are often admixed with the polymer composition.

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Albeit most commercially available polyoxymethylene is "pre-stabilized" either by means of acetylation or hydrolysis treatments (see, for example, United States Patent No. 3,839,267) or by the addition of additives such as the above-mentioned antioxidants and/or acid scavengers, it has been found that during the high-temperature molding, particularly, injection molding of the non-fiber reinforced resin an objectionable film, or mold deposit, commonly forms on the surface of the mold. The mold deposit, which can lead to surface defects on the molded Tesin, is generally believed to be of two types. One type of mold deposit is believed to be caused by the use of certain antioxidants which plate out on the mold surface.
This type of mold deposit can be eliminated by using a less volatile anti-oxidant. The second type of mold deposit is believed to be caused by formaldehyde ~generated, for example, as a result of the chain scission o the polyoxymethylene under the conditions of the molding process) condensing on the mold su~face. The chain scission, in turn, is believed to be caused by acidic residues present in the polyoxymethylene and which have not been ~'cleaned up" by the prior stabilization treatments.
Although tlle thermal stabilization, i.e., stabilization against the effects of temperatures encountered in the melt state, of polyoxy-methylene has heretofore been p~oposed in the art such as, for example, by the addition to the polyoxymethylene of amino substituted amides (United States Patent No. 3,274,149, issued September ~0, 1966, Frank M. Berardinelli), carbamates ~United States Patent No. 3,144,431, issued August 11J 1964, Thomas J. Dolce, Frank M. Berardinelli and Donald E. Hudgin)~ or severely hindered carbodiimides (British Patent No. 993,60V, published May 26, 1965, Farbenfarbriken Bayer Aktiengesellschaft), such stabilization has either not been effective in removing the mold deposit tendency or results in undesirable discoloration of the polymer.

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Thc physical blending of polyoxymethylene with thern~,oplastic resins, so as to improve the properties of a polycarbonate has also been proposed. Such blending has not been directed at improving the properties of the polyoxymethylene and has employed a relatively large quantity of the polycarbonate. Thus, for example, United States Patent No. 3,646,159, issued February 29, 1972, Gerald W. Miller, discloses the blending of poly-oxymethylenes, or polyacetals with a polycarbonate to improve the properties of the polycarbonate and to provide a polycarbonate mixture having improved resistance to environmental stress cracking and crazing. Although Miller suggests, inter alia, that the polyacetals may be employed in amounts of from 25 to 95 percent by weight based on the combined weigh~ of the poly-carbonate and polyacetal, the working examples are limited to blends con-taining a maximum of about 50 percent by weight of polyacetal. United States Patent No. 3,290,261, issued December 6, 1966, Kenneth B. Goldblum, discloses the blending o~ polycarbonate and up to about 20 percent polyacetal to obtain a foamed polycarbonate resin. Goldblum discloses that if the amount of polyacetal exceeds 20 percent, then the blend begins to lose the beneficial properties of the polycarbonate (Column 1, line 47).
Polycarbonates have also been disclosed as additives or modifierc or scavengers for polyester tire cord when added to the polyester prior to fiber ormation ~see, for instance, United States Patent No. 3~563,847, issued February 16, 1971, Grover W. Rye and Thomas E. Evans). `
In light of this prior art it is an object of the present invention to provide a process for the preparation of an improved polyoxymethylene molding composition which forms reduced mold deposits upon molding.
It is a further object of the invention to provide a polyoxy-methylene n~lding composition having high stability when subjected to the ~ ~4-influence of heat and particularly when sub~jected to the con~
ditions typically encountered during molding operations, It is still another ob~ect of the pres~ent invention to provide an improved non-reinforced polyoxymetl~ylene compos-ition for an iniection molding process, Another object of the present inventIon is the provision for the preparation of a stabilized polyoxymethylene '-molding composition employing an aromatic polycaraona~e addi~
tive, which stabllized polyoxymethylene IS not undesirably dis~
colored.
According to the present invention it has now been found that certain aromatic polycarbonates Cas defined~ in~
crease the thermal stability of polyoxymethylenes ~ithout severely discoloring the polyoxymethylenes, Thus, the invention prov;ldes a process for produc~
ing an improved polyoxymethylene molding compos~tion comprising heating a mixture o:
(1~ a polyoxymethylene polymer ~hich exhibits a propensity to form mold deposits upon moldin~, and ~2~ a~out 1 to about 4 percent by ~eight based on the ~eight o the polyoxymethylene polymer of an aromatic polycarbonate having an intrinsic vlscosity of about 0,35 to 0.75 in methylene chloride at 25C which is derived from a non~
hindered non-halogenated dihydric phenol, for at least about two minutes at a temperature at ~hi:ch the polyoxymethylene polymer is molten to yIeld a molding compo~
sition which forms a reduced quantity of mold deposits upon molding, The addition of the aromatic polycarbonate to ~ ~ :
polyoxymethylene follo~ed by the appropriate thermal treatment has been found to reduce the amount of formaldehyde generated . ~

by the polyoxymethylene when subsequently subjected to the influence of heat; particularly when subjected to conditions which have heretofore resulted in the formation of objection-able formaldehyde-type mold deposits.
As indicated, the improved molding compositions, i.e., stabilized polyoxymethylene, is prepared by heating the polyoxymethylene and the polycarbonate while in admixture for at least about two minutes at a temperature at which the poly-oxymethylene is molten ~generally above 160C). The amount of polycarbonate used is from about 1 to about 4 percent by weight, based upon the weight of the polyoxymethylene polymer.
Particularly satisfactory results are achieved if a - 5a -.~ ' ~ :
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minor quantity o~ malonamide also is admixed with the polyoxymethylene polymer and the aromatic polycarbonate during heating.
Description of Preferred Embodiments As used herein the term "polyoxymethylene" is intended to include both homopolymers, including so-called capped homo-polymers, i.e., acylated homopolymers, as well as copolymers as will be defined more specifically below.
The thermal stability provided by the addition of the aromatic polycarbonates to the polyoxymethylene according to the present invention is stability against degradation when the poly-oxymethylene is subjected to the influence of heat. The aromatic polycarbonate is believed to provide stability against any o~ the degradative effects of heat including, for example, aging of molded polyoxymethylene articles at temperatures of from 100 to 140C., but is particularly useful in providing stability against degradation when the polyoxymethylene is subjected to the temperatures and conditions typically encountered during t,he molding of the poly-oxymethylene into shaped articles, i.e., temperatures of from about 185C. to about 240C. for a period of several minutes.
A particularly preferred application of the present invention is in the injec-tion molding of polyoxymethylene because the mani-festations oftheinstabili-ty, or degradation, of polyoxymethylene are more troubleseome in this type of operation than in, for example, an extru-sion operation. In the extrusion of polyoxymethylene, formaldehyde which may be generated during degradation of the polymer does not have an opportunity to condense on a mold surface and may escape through vents provided on the extruder.

InJection molding is intended to refer to any of the well-known processes wherein a polyoxymethylene molding composition is heated in a preheating zone to a plastic melt, and is thereafter forced through a nozzle into a closed mold. Heating of the polyoxymethylene is typically to a temperature of from about lôO C. to about 2~0C. The ;
temperature of the mold is generally substantially lower, e.g., about lOO C. lower, although the exact relationship between the melt temperature andthe mold temperatureis dependent on factors such as the desired surface characteristics of the shaped article as will be appreciated by the art-skilled person. Mold deposit can be noticed at any of the recommended molding temperatures when a poor quality ~-polyoxymethylene copolymer is employed (usually after 25-50 shots) `~
and tends to be greater with higher melt temperatures and lower mold temperatures. The tendency toward mold deposit varies according to the particular polyoxymethylene, prestabilization treatment and the `~
like. Thus for example, acylated homopolymer generally produces mold deposit problems less frequently than a melt hydrolyzed copolymer. The occurrence of the mold-deposit problems depends additionally on the size of the molded part, gating and venting. Small parts, small gates and inadequate venting give the most problems. The mold deposit tends to cause imperfections on the surface of the molded `~ ;
parts. Such parts must be ground up and remolded.
The addition of the polycarbonates to the polyoxymethylene according to the present invention is effective in providing thermal ;~`
stabilization at the molding (mèlt) temperatures, but may not be effective above temperatures of about 2~19 C. because of the degradation of the polyoxymethylene.
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The injection molding may be carried out in conventional injection-molding apparatus having, for example, a preheating cylinder, plunger, or reciprocating screw, torpedo, nozzle and mold including a sprue, runners, gates and mold cavities. Cylinder temperatures are usually between about 180 C. and about 240C.
and molding pressures are usually between about 5,000 and 20jO00 psi. Acutal molding temperatures and pressures will vary depending on the type of machine, i.e., plunger injection molding machine or screw inJection molding machine, employed or on the desired shape and size of the molded article. Cycle times are usually between about 30 and about 110 seconds.
Polyoxymethylene polymer which may be stabilized with the aromatic polycarbonate in accordance with the present invention, as stated above, includes both homopolymers and copolymers. Such polymers, which may be produced according to methods well-known in the art have recurring -OCH2- units and are typically prepared by -the polymerization of anhydrous formaldehycLe or by the polymerization of` trioxane.
Particularly useful in this invention is polyoxymethylene copolymer having at least one chain containing recurring oxymethylene (-OCH2-) units interspersed with -OR- groups in the main polymer chain where R is a divalent radical containing at least two carbon atoms directly linked to each other and positioned in the chain between the two valences with any substituents on said R radical being inert, i.e., substituents which will not induce undesirable reactions. Preferred copolymers contain from 60 to 99.6 mole percent of recurring oxymethylene groups. In a preferred embodiment R may be, for ;~
example, an alkylene or substituted alkylene group containing at least ~6~

two carbon atoms.
Among the copolymers which are utilized in accordance with the invention are those having a structure comprising recurring ~mits of the formula:

1 Rl~ ~

wherein n is zeTo or an integer of fro~ 1 to 5 and wherein n is zero in from 60 to 99.6 percent of the recurring units. Rl and R2 are inert substituents, that is, substituents which will not cause undesirable reactions.
~ preferred class of copolymers are those having a structure com-prising recurring units wherein from 60 to 99.6 percent of the recurring units are oxymethylene units. These copolymers are prepared by copolymerizing tr:ioxane with a cyclic ether having the structure: -:

CH2 - CCH2)n wherein n is 0, 1 or 2.
Examples of other preferred polymers include copolymers of triox~ne and cyclic ethers containing at least two adjacent carbon atoms ~ ;
such as the copolymers disclosed in United States Patent No. 3,027,352, issued March 27, 1962, Cheves T. Walling, Frank Brown and Kenneth William Bartz.
Among the specific ethers which may be used are ethylene oxide, 1,3-dioxolane, 1,3,5-trioxepane, 1,3-dioxane, trimethylene oxide, penta-methylene oxide, 1,2-propylene oxide, 1,2-butylene oxide, neopentyl `~
formal, pentaerythritol diformal paraldehyde7 tetra-g_ hydrofuran~ and butadiene monoxide.
The preferred polymers treated in accordance with the invention aTe moldable thermoplastic materials having a weight average molecular weight of at least about 35,000, a melting point of at least about 150C.
and an inherent viscosity of at least about 0.~ ~measured at 60C. in a 0.1 weight percent solution in p-chlorophenol containing 2 weight percent of alpha-pinene).
As understood by those skilled in the art, the polyoxymethylene should be pre-stabilized prior to being heated in admixture with the poly-carbonate. Such prestabili7ation may take the form of stabilization by degradation of the molecular ends of the polymer chain to a point where a relatively stable carbon-to-carbon linkage exists at each end. For example, such degradation may be effected by melt hydrolysis such as that disclosed in United States Patent No. 3,318,8~8, issued May 9, 1967, Charles ~1. Clsrke or by solution hydrolysis such as that disclosed in United States Patent 3,219,623, issued No~ember 23, 1965, Frank M. ]Berardinelli. ~lixtures of polyoxymethylenes stabili~ed by melt hydrolysis and by solution hydrolysis may, of course, be used. The polyoxymethylene may also be pre-stabilized by admixing with con~entional stabilizers such as an antioxidant (e.g. in a concentration of about 0.2 to 2.0 percent by weight) andfor an acid scavenger (e.g. in a concentration of about 0.05 to 1.0 percent by weight).
Generally, these stabilizers will be present in a total amount of less than about 3 percent by weight based on the weight of the polyoxymethylene polymer.
Generally speaking, the aTomatic polycarbonates that are employed according to the present invention are well known, commercially a~ailable thermoplastic resin materials. In general, such aromatic polycarbonates may be described as polymers containing recurring structural units of the formula:
I O
- t ~ A - O - ~ - -~10-`

wheTe A is a divalent aromatic radical derived from a non-hindered non-halogenated dihydric phenol. Specific processes for preparing such poly-carbonates as well as starting materials and polymers prepared therefrom are described in well known texts such as "Chemistry and Physics of Poly- -carbonates" by Hermann Schnell and "Polycarbonates" by William F.
Christopher, and in the patent literature, such as United States Patent Nos. 2,970,137, issued January 31, 1961~ Herbert E. Whitlock; 2,991,273, issued July 4, 1961, Wilhelm ~lechelhan~er and Gunter Peilstocker; 2,999,846, issued September 12, 1961, Hermann Schnell and Heinrich Krimm; 2,999,835, issued September 12, 1961, Eugene P. Goldberg; 3,014,891, issued December 26, 1961, Kenneth B. Goldblum; 3,028,365, issued April 3, 1962, Hermann Schnell, Ludwig Bottenbruch and HeinTich Krimm; 3,030,331, issued April 17, 1~62, Eugene P. Goldberg.
The dihyclric phenol is non-hindered in the sense no ortho substitution is present on the aromatic ring adjacent either of the hydroxy groups. Such substitution will prevent the resulting polycarbonates from efectively f~mctioning in tl-e process of the present invention. Para-su~stituted dihydric phenols are preferred; however, meta-substituted dihydric phenols are acceptable for use. Also, the dihydric phenol is fre0 of halogens or any other functional groups including sulfone linkages which would interfere with the desired results. Preferably only hydrogens are present on the aromatic rings of the dihydric phenol other than the hydroxyl groups.
Representative non-hindered, non-halogenated dihydric phenols from which the aromatic polycarbonate can be derived include:

HO ~ C ~ OH

', 0~1 ~

8~3 HO ~ ~ ~ ~ OH

HO ~ C ~ ~ ~ OH

HO ~ H
~ ' A hindered dihydric phenol such as 2,2'-dihydroxydiphenyl, i.e., ,PH ~O

V ~ should not be employed.

For exampleJ the yreferred aromatic polycarbonate may be produced from a dihydroxydiaryl alkane such as 2,2-bis-~4-hydroxyphenyl)propane ~i.e. bisphenol A) and phosgene, a haloformate or a diester of carbonic acid as described in United States Patent No. 3,028,365, issued April 3, 1962, Herman Schnell, Ludwig Bottenbruch and Heinrich Krimm. Homopolymers derived from 2,2-bis-~4-hydroxyphenyl) propane are pa~ticularly preferTed. Such materials are marketed commercially under the trademark "Merlon" by the Mbbay Chemical Corporation and "Lexan" by the General Electric Company.
Preferred aromatic polyca~bonate copolymers are derived fro~ at least 80 ~ole percent of 2,2-bis-(4-hydroxyphenyl) propane.
The aromatic polycarbonate selected for use in the present process is miscible with the polyoxymethylene polymer while in the molten ~;
state so as to insure good mixing or blending, and additionally should not volatilize under the conditions of mixing and subsequent molding. Typically the aromatic polycarbonate has an intrinsic viscosity of about 0.35 -to 0.75 in methylene chloride at 25C., and preferably an intrinsic viscosity of about 0.35 to o.6 under the same conditions.
The aromatic polycarbonate is mixed or blended with the poly-oxymethylene composition (i.e. pre-stabilized polyoxymethylene) in an amount of from about 1 to about 4 percent by weight based on the weight of the polyoxymethylene polymer and more preferably in an amount of from about 1 to about 2 percent by weight. Amounts of ;~
aromatic polycarbonate much less than about 1 percent by weight may re-quire extensive mixing with the polyxoymethylene to achieve a desired stabiliztion whereas amounts much greater than about 4 percent by weight tend to significantly adversely alter the physical properties, i.e., tensile strength, Izod impact values and the like, of a molded article formed from the composition. Any of` the aromatic poly-carbonates included within the description presented above can be used alone or in mixture with other of the aromatic polycarbonates to achieve the desired results.
The polyoxSrmethylene and polycarbonate are heated to a temperature at which the polyoxymethylene is molten or in the melt state. Generally, a temperature above about 160 C. is required, preferably above about 180C., and morepreferably between about 180C. and about 240C. Temperatures much higher than about 240C.
may lead to degradation of the materials and/or possible adverse side reactions. Thus, the temperature range is one which will main-tain the polyoxymethylene polymer in melt form, but not cause significant degradation or adverse side reactions.
The polyoxymethylene polymer and aromatic polycarbonate are maintained at these -temperatures for at least about 2 minutes and -, :
.

- .. . . :, .

usually fox about 2 to a~out 20 minutes. Caution should be employed when longer times are used particulaTly much ab~e about 20 minutes as the polymer materials may tend to degrade.
The exact time employed will depend primarily on the particular apparatus in which the polymers aTe heated ~hile in admixture. More efficient mixing and heating devices such as a Werner-Pfleiderer ZSK* twin screw extruder will of course require less time than, for example, devices such as a Brabender plastograph.
In general, the polyoxymethylene and polycarbonate may be mixed or blended and heated in any convenient manner or apparatus as long as the polyoxymethylene polymer is molten and in intimate contact with the aromatic polycarbonate for at least about 2 minutes while in such a state. If desired the polymers can first be dry blended and thereafter heated or they can be initially admixed in the heating apparatus.
It is believed that the aromatic polycarbonate reacts with the acid residues in the polyoxymethylene polymer during the heating step, thus stabilizing the polyoxymethylene when it is subsequently molded, i.e.
formaldehyde mold deposit is decreased. Accordingly the most eficient means of heating and mixing the polymers are clesirable to assure complete polycarbonate-acid residue reaction and hence substantial elimination of the formaldehyde mold deposit during subsequent molding.
For this reason direct addition of the polycarbonate to the polyoxymethylene in the molding apparatus is not recommended unless suffiGient preheat time is provided for the materials to be mixed and for the aTomatic polycarbonate acid Tesidue reaction to occur before the molding composition enters the mold cavity.

*Trademark ,. ..

" ,: , I~ d2sired, the polyoxymethylene and polycarbonate may be mixed and hea~ed as set forth abo~e, pelleted and stored for later use in the molding process~
Polyoxymethylene molding compositions of the present invention, in addition to including the polyoxymethylene and the polycarbonate, optionally may also include a minor quantity of additives conventionally employed in polyoxymethylene molding compositions both polymeric and non-polymeric, such as lubricity agentsJ dyes, and conventional antioxidants and acid scavengersJ etc.J as discussed above. In a preferred embodiment ibrous reinforcement is omitted from the molding composition since it is in such a system that mold deposit problems have more commonly been observed in the prior art.
The aromatic polycarbonates utilized in the present process are believed to be somewhat unique in that the mold deposit problem is effectively eliminated. Excessive color formation, as often encountered if large amounts of conventional basic acid scavengers are employed, is not a factor. Also, as discussed below theTe is no signiicant reduction in molded article properties as is commonly encountered if particulate non-polymeric additives are utilized.
In a particularly preferred aspect according to the present invention, the aromatic polycarbonate is employed in combination with a minor quantity of malonamide (i.e. carboamidoacetamide) to provide an effective theImal stabilization of polyoxymethylenes a~d, particularly, to reduce the mold deposit tendencies of the polyoxymethylene. Malonamide is a known and effective thermal stabilizer for polyoxymethylene polymer, but is not widely used because it adversely discolors the polyoxymethylenes and tends to be expensive. See, for instance, United States Patent No.
3,116,267, issued December 31, 1963, Thomas J. Dolce. In combination with the aromatic polycarbonate, however, . . , - . -.
;........ , ,. .

the discoloration of the polyoxymethylene surprisingly is substantially reduced and at the same time the desired thermal stabilization is provided. The presence of the aromatic polycarbonate permits the utilization of lesser levels of malonamide. For instance, malonamide may be present in a concentration of about 0.1 to 1 percent by weight based on the weight of the polyoxymethylene polymer, and preferably in a concentration of about 0.1 to about 0.5 percent by weight.
Molded articles prepared from the aromatic polycarbonate-stabilized polyoxymethylene molding composition according to the present invention show only a slight decrease, typically less than 10 percent, in physical properties (tensile, modulus, impact strengths, and the like) as compared to articles prepared from polyoxymethylene compositions not including the aromatic polycarbonate.
As indicated hereinabove, the po]yoxymethylene molding compositions o~ the present invention show increased -thermal stability when heated to temperatures of from about 180C. to about 240C. The thermal stability can be measured by heating the poly-oxymethylene composition in an all glass apparatus at a temperature of either 218C. or 228 C. + 2C. for 30 minutes. ~t the end of this time a vacuum is applied to the system and the liberated formaldehyde is drawn through two sodium sulfite traps. The quantity of formaldehyde in the traps is then determined by titration with a standard acid. The amount of formaldehyde given off is a reasonable measure of the thermal stability and of the tendency of the composition to form mold deposits. `
The invention thus provides a means for thermally stabilizing polyoxymethylene polymer in a short time while employing small concentrations of a particular class of aromatic polycarbonate or of aromatic polycar~onate in combination with malonamide.
The invention is additionally illustrated in connection with the following examples which are to be considered as illustrative of the present invention. It should be understood, however, that the invention is not li~ited to the specific details of the examples.
Examp1es The thermal s~ability of various polyoxy~ethylene molding com-positions in accordance with the present invention aTe shown in the Table.
The molding compositions were prepared by mixing 50 grams of the polyoxymethylene polymer with the additives in a Brabender Plasticorder*
plastigraph (200 C~ and 35 RPM). Additives WeTe blended in -for twenty minutes from the time the polyoxymethylene polymer appeared to be completely melted. Controls were milled for similar times in the absence of any additives. No torque increases were observed for any o the blends.
Polymers I and II were each polyoxymethylene copolymers prepaTed in the same manner from trioxane and ethylene oxide (2 percent by weight).
Each had a weight average molecular weight of 68,000. Polymer I had been melt hydrolyzed according to the technique of United States Patent No.
3~219,623, issued November 23, 1965, ~rank M. BeTardinelli. Each of the polymers also had been "stabili~ed" OT "pre-stabili7ed" prior to compolmding with the polycarbonate with a standard additive package including O.S ;
percent 2,2'-methylene-bis-~4-methyl-6-tertiaTy butyl phenol) antioxidant, 0.1 percent cyanoguanidine acid scavenger~ and about 0.2 percent diamide synthetic wax lubricant available from Glyco Chemicals, Inc. under the designation Acrawax* C lubricant.

*Trademark .
~.

Aromatic polycarbonate I was non-hindered and non-halogenated and was commercially available under the trademark "Merlon", type M39F, from the Mobay Chemical Company. It was derived from 2,2-bis(4-hydroxyphenyl) propane and exhibited an intrinsic viscosity in methylene chloride at 25 C. of 0.5.
Generated formaldehyde was determined according to the procedure described above by heating the samples at 228C. for 30 minutes~ and is based on the initial weight of the sample.
TABLE

Example Molding Percent Generated Color of No. Composition Formaldehyde Molded Article 1 Polymer I Control 0.28 white

2 Polymer II Control 0.10 white

3 Polymer I + 1% malon- 0.02 pale yellow amide l~ Polymer I + 1% Polycar- 0.12 white bonate I

Polymer I + 1% malon- <0.01 sligh-tly amide + 1% Polycar- off-white bonate I
It is readily seen that an aroma-tic polycarbonate defined accord-ing to this invention provides eff`ective stabilization against degradationof the polyoxymethylene under the test conditions and without imparting objectionable discloration to the polyoxymethylene.
The principles, preferred embodiments and modes of operation of the present invention have been described in the foregoing specification.
The invention which is intended to be protected herein, however, is not to be construed as limited to the particular forms disclosed, since these are to be regarded as illustrative rather than restrictive. Variations and changes may be made by those skilled in the art without departingfrom the spirit of the invention.

Claims (25)

I CLAIM:

1. A process for producing an improved polyoxymethylene molding composition comprising heating a mixture of:

(1) a polyoxymethylene polymer which exhibits a propensity to form mold deposits upon molding, and (2) about 1 to about 4 percent by weight based on the weight of the polyoxymethylene polymer of an aromatic polycarbonate having an intrinsic viscosity of about 0.35 to 0.75 in methylene chloride at 25°C, which is derived from a non-hindered non-halogenated dihydric phenol, for at least about two minutes at a temperature at which the poly-oxymethylene polymer is molten to yield a molding composition which forms a reduced quantity of mold deposits upon molding.

2. The process for producing an improved polyoxymethglene molding composition of Claim 1 wherein the mixture is heated at a temperature above about 160°C,

3. The process for producing an improved polyoxymethylene molding composition of Claim 1 wherein the polymer mixture is heated at a temperature between about 180°C. and about 240°C, for a time period of from about 2 to about 20 minutes.

4. The process for producing an improved polyoxymethylene molding composition of Claim 1 wherein the polyoxymethylene polymer has an inherent viscosity of at least 0.8 (measured at 60°C. in a 0.1 weight percent solution in p-chlorophenol containing 2 weight percent of alpha-pinene), a weight average molecular weight of at least 35,000, and a melting point of at least 150°C.

5. The process for producing an improved polyoxymethylene molding composition of Claim 4 wherein the polyoxymethylene polymer is a copolymer comprising about 60 to about 99.6 percent recurring -OCH2-groups.

6. The process for producing an improved polyoxymethylene molding composition of Claim 5 wherein the polyoxymethylene polymer has been pre-stabilized by melt hydrolysis prior to mixture with component (2).

7. The process for producing an improved polyoxymethylene molding composition of Claim 5 wherein the polyoxymethylene polymer is a mixture of melt hydrolyzed and solution hydrolyzed polymer prior to mixture with component (2).

8. The process for producing an improved polyoxymethylene molding composition of Claim 5 wherein the polyoxymethylene polymer has been pre-stabilized by the addition of an antioxidant and an acid scavenger prior to heating.

9. The process for producing an improved polyoxymethylene molding composition of Claim 1 wherein the aromatic polycarbonate has an intrinsic viscosity of about 0.35 to 0.6 in methylene chloride at 25°C.

10. The process for producing an improved polyoxymethylene molding composition of Claim 1 wherein the dihydric phenol from which the aromatic polycarbonate is derived is 2,2-bis(4-hydroxyphenyl)propane.

11. The process for producing an improved polyoxymethylene molding composition of Claim 1 wherein a minor quantity of malonamide is admixed with components (1) and (2) during said heating.

12. The process for producing an improved polyoxymethylene molding composition of Claim 1 wherein the resulting molding composition is free of fibrous reinforcement.

13. A process for producing an improved polyoxymethylene, molding composition which is free of fibrous reinforcement comprising heating a mixture of:

(1) a polyoxymethylene polymer which exhibits a propensity to form mold deposits upon molding, (2) about 1 to about 2 percent by weight based on the weight of the polyoxymethylene polymer of an aromatic polycarbonate having an intrinsic viscosity of about 0.35 to 0.75 in methylene chloride at 25°C, which is derived from a non-hindered non-halogenated dihydric phenol, and (3) about 0.1 to about 1 percent by weight based on the weight of the polyoxymethylene polymer of malonamide, at a temperature between about 180°C. and about 240°C. for a time period of from about 2 to about 20 minutes to yield an improved molding composition which forms a reduced quantity of mold deposits upon molding.

14. The process for producing an improved polyoxymethylene molding composition of Claim 13 wherein the polyoxymethylene polymer has an inherent viscosity of at least 0.8 (measured at 60°C. in a 0.1 weight percent solution in p-chlorophenol containing 2 weight percent of alpha-piene), a weight average molecular weight of at least 35,000, and a melting point of at least 150°C.

15. The process for producing an improved polyoxymethylene molding composition of Claim 13 wherein the polyoxymethylene polymer is a copolymer comprising about 60 to about 99.6 percent recurring -OCH2-groups.

16. The process for producing an improving polyoxymethylene molding composition of Claim 15 wherein the polyoxymethylene polymer has been pre-stabilized by melt hydrolysis prior to mixture with component (2).

17. The process for producing an improved polyoxymethylene molding composition of Claim 15 wherein the polyoxymethylene polymer is a mixture of melt hydrolyzed and solution hydrolyzed polymer prior to mixture with components (2) and (3).

18. The process for producing an improved polyoxymethylene molding composition of Claim 15 wherein said polyoxymethylene polymer is pre-stabilized by the addition of an antioxidant and an acid scavenger prior to heating.

19. The process for producing an improved polyoxymethylene molding composition of Claim 18 wherein said antioxidant is 2,2'-methylene-bis-(4-metnyl-6-tertiary butyl phenol) and said acid scavenger is cyanoguanidine.

20. The process for producing an improved polyoxymethylene molding composition of Claim 13 wherein the aromatic polycarbonate has an intrinsic viscosity of about 0. 35 to 0, 6 in methylene chloride at 25°C.

21. The process for producing an improved polyoxymethylene molding composition of Claim 13 wherein the dihydric phenol from which the aromatic polycarbonate is derived is 2,2-bis(4-hydroxyphenyl)propane.

22. The process for producing an improved polyoxymethylene molding composition of Claim 13 wherein component (3) is present in a concentration of about 0. 1 to about 0. 5 percent by weight based upon the weight of the oxcymethylene polymer.

23. The process for producing an improved polyoxymethylene molding composition of Claim 1 which includes the additional step of injection molding the resulting improved polyoxymethylene molding composition,

24. The improved polyoxymethylene molding composition which is produced by the process of Claim 1.

25. The improved polyoxymethylene molding composition which is produced by the process of Claim 13.