SOLID STATE CURING OF POLY(ARYLENE SULFIDE) RESINS IN REDUCED OXYGEN ATMOSPHERES

Abstract: Virgin poly(arylene sulfide) resins are oxidatively cured by heat treatment at an elevated temperature below the melting point of the resin, under an oxidizing atmosphere comprising less than a 20 volume % oxygen concentration. The resulting polymer exhibits drastically improved thermal stability with no apparent sacrifice in physical properties.

Description: Field of the Invention This invention relates to solid state curing of arylene sulfide polymers. In one aspect, this invention relates to a process for increasing the molecular weight (i.e., increasing the melt viscosity) of virgin poly(arylene sulfide) resins. In another aspect, the invention pertains to controlled solid state curing of arylene sulfide polymers in a reduced oxygen atmosphere. In another more specific aspect, this invention relates to a process for producing poly(arylene sulfide) resins having improved thermal stabilities. Background of the Invention Aromatic sulfide polymers, ranging in consistency from viscous liquids to crystalline solids, are known in the art. While such polymers exhibit desirable properties, for many commercial applications, such as molding compositions, the unmodified (i.e., virgin) polymers normally possess a relatively high melt flow, e.g., above about 4,000 grams/10 minutes, which inhibits their use in many applications. For example, when exposed to processing temperatures above their melting point, the virgin polymers require extensive processing times or special apparatus for thin film processing. The high melt flow (low viscosity) of the virgin arylene sulfide polymers makes it difficult to handle these virgin resins by conventional molding practices. Since aromatic sulfide polymers possess many desirable properties which make them extremely useful, it would be advantageous to improve the processability of the polymers without materially affecting any of their desirable properties. One technique for achieving the improvement in processability is demonstrated in U.S. Patent 3,793,256, henceforth Patent '256. Specifically, Patent '256 discloses, among other things, a method to improve processability of a polymeric resin by oxidatively curing the virgin polymer. Since this oxidative curing process comprises exposing of the virgin resin, preferably in particulate form, to an oxidizing atmosphere, while being heated to a temperature below the resin's melting, this process is often referred to as "solid state" curing. Patent '256 further discloses that the oxygen content in air, (i.e., approximately 20 volume %) is preferred to achieve the desirable results of the oxidative curing process disclosed therein. Therefore, since the publication of Patent '256, air has generally been used as the oxidizing atmosphere to cure a virgin poly(arylene sulfide) resin. Conversely, if such a polymeric resin is to be employed for the purposes of, for example, encapsulations and/or coatings, the resin must have its melt flow value increased. It should be noted that the term "curing", with respect to poly(arylene sulfide) polymers, is not necessarily synonymous with the meaning of that term when applied to other polymers, such as in the "curing" of natural or synthetic rubber with sulfur as the vulcanizing agent, or the curing of unsaturated polyester compositions with styrene as the crosslinking agent. The complete curing of such rubber or unsaturated polyester compositions results in an irreversible conversion from the thermoplastic state to the thermal set state. The curing of poly(arylene sulfide) resins, on the other hand, differs in that these resins are thermoplastic, both before and after such curing, although not necessarily to the same degree. Thus, the curing of PAS resins, and objects formed therefrom, produces changes in extrudability, toughness, and other important properties. While, for certain commercial application, it is desirable to decrease the melt flow (increase melt viscosity) of a virgin poly(arylene sulfide) resin, it is equally desirable to have a resin which is thermally stable when in the melted phase. In other words, it is desirable to have a polymeric resin whose melt viscosity does not substantially change while the resin is held at a temperature above its melting point for a given period of time. The reason thermal stability is such an important factor is that in many commercial applications, cured polymeric resins often undergo at least one melt processing step. A typical example of a commercial processing procedure is when a cured resin is first made into a polymer composition by adding to the melted resin various fillers such as glass, fibers, and/or fiberglass. The polymer composition is then often sent to be processed into a final and/or intermediate molded product. This latter processing step often requires that the polymer composition also be heated to a temperature above the melting point of the resin used in its preparation. Due to inherent physical restraints of commercial processing equipment, it is generally necessary to feed, into the processing equipment, resinous material having a melt viscosity within a specifically narrow range. As illustrated above, the resinous material was exposed to temperatures above its melting point at two different occurrences. Therefore, in that example, if the thermal stability of the cured resin was low (i.e., melt viscosity changed greatly as the resin was maintained in a melted phase), it would be difficult, if not impossible, to determine what its initial melt viscosity should be in order for its final melt viscosity to ultimately fall within the specified range. If, on the other hand, the cured resin is thermally stable in the melt, it will be exceedingly easier to synthesize a resin which will retain the desired flow properties. Accordingly, one object of this invention is to provide a process for preparing thermally stable cured arylene sulfide polymeric resins. Another object of this invention is to produce polymeric compositions from arylene sulfide resins treated by the novel process of this invention. Yet a further object of this invention is to make a molded article made from either an arylene sulfide resin, cured by the process of this invention, or a polymeric composition made by using the novely cured resin. Other objects, aspects, and advantages of this invention will be apparent to those skilled in the art upon reading the specification and appended claims which follow. Summary of the Invention In accordance with this invention, virgin poly(arylene sulfide) resins are oxidatively cured by heating the resin, while in a solid state, in the presence of an oxidizing atmosphere comprising less then 20 volume % oxygen. It has been discovered that, while the time needed to decrease the melt flow (increase melt viscosity) of a virgin poly(arylene sulfide) resin to a predetermined value, by heating the virgin resin, to a temperature below its melting point, while being subjected to an oxidizing atmosphere comprising less then 20 volume % oxygen, does not significantly differ from the time needed to oxidatively cure the same resin, at the same temperature, when air is used as the oxidizing atmosphere, the resulting thermal stability of the resin cured in the presence of the oxidizing atmosphere comprising less then 20 volume % oxygen, is significantly greater than that of the same resin cured at the same temperature when air is used as the oxidizing atmosphere. A more complete appreciation of the invention and many attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying figure briefly described below. Brief Description of the Drawings FIGURE 1 is a Cure Rate Viscosity Spectra of resins cured in the presence of an oxidizing atmosphere containing varying concentrations of oxygen. FIGURE 2 is a Thermal Stability Viscosity Spectra of resins cured in the presence of an oxidizing atmosphere containing varying concentrations of oxygen. FIGURE 3 is a Cure Rate graph of resins cured in the presence of an oxidizing atmosphere containing varying concentrations of oxygen. FIGURE 4 is a Thermal Stability Viscosity Spectra of resins cured in the presence of an oxidizing atmosphere containing varying concentrations of oxygen. FIGURE 5 is a Cure Rate graph of resins cured in the presence of an oxidizing atmosphere containing varying concentrations of oxygen. FIGURE 6 is a Thermal Stability Viscosity Spectra of resins cured in the presence of an oxidizing atmosphere containing varying concentrations of oxygen. FIGURE 7 is a Thermal Stability Viscosity Spectra of resins cured in the presence of an oxidizing atmosphere containing varying concentrations of oxygen. FIGURE 8 is a Thermal Stability Viscosity Spectra of resins cured in the presence of an oxidizing atmosphere containing varying concentrations of oxygen. Detailed Description of the Invention As referred to herein, the terms "arylene sulfide polymer" and/or "poly(arylene sulfide)" abbreviated as "PAS" are intended to include polymers of the type which are prepared as described in U.S. Patent 3,354,129. For example, in that patent, polymers are prepared by reacting polyhaloaromatic compounds with an alkali metal sulfide in the presence of a polar organic compound. The resulting polymers contain the aromatic structure of the polyhalo compound coupled in repeating units through a sulfur atom. The term "virgin" when used to identify a type of poly(arylene sulfide) resin, refers to a polymer which has not undergone any subsequent oxidative heat treatment (i.e., curing). As used herein, the term "melt flow" refers to the rate at which a melted polymeric resin flows through an orifice when subjected to a downward pressure. In this invention, melt flow is recorded in units of grams of extrudate per 10 minutes (g/10 min.), and is based on ASTM D1238, Procedure B, entitled "Automatically Timed Flow Rate Measurement". For example, in determining the flow rate for poly(phenylene sulfide) resins, having a melting point of about 285°C (515°F), the resin is heated to 316°C (600°F), while using a total driving mass of 5 kilograms. The orifice through which the melted poly(phenylene sulfide) resin is extruded has an inside diameter of 0.0825 ± 0.002 inch (1 inch = 2.54 cm) and a length of 0.315 ± 0.001 inch. Therefore, a high viscosity polymer will have a low melt flow reading, and vise versa. In other words, the terms "melt flow" and "viscosity" are inversely related. The term "cure rate" as used herein, refers to the final melt flow of a polymeric resin which has been heated to an elevated temperature, below its melting point, and held at that temperature for a given period of time while being subjected to a gaseous oxidizing atmosphere. For example, the 6 hour cure rate of a specific poly(arylene sulfide) resin is that resin's melt flow, after it has been subjected to the set curing temperature for a 6 hour period. The term "extrusion rate", as used herein, is similar to the term melt flow in that it also refers to the rate at which a melted polymeric resin flows through an orifice when subjected to a downward pressure. Since extrusion rate testing uses more limiting parameters then does melt flow testing, polymers having high melt flows (low viscosities) can be characterized with more accuracy. A particular resin's extrusion rate is recorded in units of grams of extrudate per 10 minutes (g/10 min.) and is based on a modification of ASTM D1238. The modifications to ASTM D1238, implemented in extrusion rate testing, are the use of (1) a 345-gram driving mass and (2) an orifice having a length of 1.250 ± 0.001 inch. As used herein, the term "thermal stability" refers to the degree of change in the melt viscosity of a cured resin, while that resin is held at a temperature above its melting point for a given period of time. As the thermal stability of a given resin improves (i.e., increases), the degree at which its melt viscosity changes over time, decreases. For this invention, the thermal stability of a cured resin is determined by the use of a Rheometrics Dynamic Spectrometer, henceforth RDS, as supplied by Rheometrics, Inc., in Piscataway, N.J. Broadly, the RDS determines a change in the viscosity of a sample at a given temperature, generally above the sample's melting point, by monitoring the viscosity of a heated sample as a function of frequency and/or time. This invention is broadly applicable to PAS resins formed by any known technique. It is used, for example, with resins wherein polyhalo-substituted aromatic compounds are reacted with sulfur-containing reactants in a polar organic solvent, optionally, with the polymerization modifying compounds either in batch or continuous operations. Generally, the polymers which are prepared for use by this inventive process, because of the high availability of the material from which they can be prepared, are those having the repeating unit R-S, wherein R is selected form the group comprising phenylene, biphenylene, naphthylene, biphenylene ether, and lower alkyl-substituted derivatives thereof. The phrase "lower alkyl", as used above, refers to alkyl groups having 1 to 6 carbon atoms. Examples of such lower alkyls include, but are not limited to, methyl, propyl, isobutyl, hexyl, and the like. The PAS resins most apt to be treated by the process of this invention are those wherein the arylene group is a phenylene group. Generally, this inventive process can be used to improve the processability of phenylene sulfide polymers selected from the group comprising poly(phenylene sulfide), poly(phenylene sulfide ketone), poly(phenylene sulfide diketone), poly(biphenylene sulfide), and the like. Particularly applicable to this inventive process are PAS resins prepared by reacting mixtures of p-dichlorobenzene and 1,2,4-trichlorobenzene with a sulfur source and a metal halide at fusion temperatures. Linear polymers formed by use of dihaloaromatic compounds or crosslinked polymers wherein polyhaloaromatic compounds are added with the polymer to aid cross-linking can also be used. In accordance with this invention, an improved PAS resin can be prepared by heating a virgin PAS resin, in particulate form, in the presence of a gaseous oxidizing atmosphere comprising less than about 20 volume % oxygen, at a temperature below the melting point of the resin, for a time sufficient to effect a desired reduction in the melt flow (increase in viscosity) of the resin. The melting point of PAS resins can be readily determined by any suitable method. One such suitable method is by determining the Differential Thermal Analysis (DTA) of the sample by heating a 10 mg sample of the polymer at a rate of 10°C per minute. The melting point is taken from the DTA thermogram in a conventional manner. It has been discovered that while the polymer cured by the process of this invention retains its original physical appearance and cure rate, it, nevertheless, exhibits a thermal stability which is significantly greater than that of the same resin which was cured while in the presence of a gaseous oxidizing atmosphere comprising at least 20 volume % oxygen. Generally, the curing temperature of a polymeric resin, in accordance with the inventive process disclosed herein, will be in the range from about 10°C to about 70°C, preferably from about 25°C to about 55°C, below the melting point of the polymer being treated. The time during which the resin is held at such a temperature depends upon many different factors, such as the desired degree of cure. Ordinarily, the resin would be maintained at the desired temperature for a period of time ranging from about 10 minutes to about 3 days, usually, however, from about 2 to about 12 hours. The longer curing times are generally associated with the use of lower temperatures and vice versa, depending specifically on the type of polymer being processed. As indicated above, the curing of the PAS resin is to be carried out while in the presence of a gaseous oxidizing atmosphere, wherein the oxygen concentration of this atmosphere is less than about 20 volume %. In a presently preferred embodiment, the gaseous oxidizing atmosphere is a mixture of air and any suitable inert gas. Examples of such suitable inert gases include, but are not limited to, steam, nitrogen, carbon dioxide, and the like, or mixtures thereof. For economic purposes, however, the preferred inert gas is nitrogen. As the concentration of oxygen in the atmosphere used to cure a virgin polymeric resin continues to decreases below that of 20 volume %, the thermal stability of the resulting cured resin continues to significantly improve, However, due to both the large quantities of resin which are cured in a commercial curing vessel and the inherent cost of inert gases, for commercial applications, the oxygen concentration in volume % of the gaseous oxidizing atmosphere will generally range from about 18 volume % to about 1 volume %, preferably, from about 15 volume % to about 3 volume %, and more preferably, from about 12 volume % to about 6 volume %. After the desired level of cure has been achieved by the process of this invention, the heating process and the passage of the oxidizing gas comprising less than 20 volume % oxygen, through the body of solid particles of PAS resin, are terminated. The residue of the oxidizing gas can optionally be purged from the body of the particulate material by the passage therethrough of an inert gaseous medium. Examples of such inert gaseous purging mediums include, but are not limited to, steam, nitrogen, carbon dioxide, and the like, or mixtures thereof. For economic purposes, however, the preferred inert gas is nitrogen. This optional purging can generally be completed in from about 1 minute to about 30 minutes, preferably from about 1 minute to about 10 minutes, and more preferably in less than about 5 minutes. The body of cured particles of PAS resin is then cooled, while in the presence of an inert gaseous medium and in the at least substantial absence of any oxidizing gas, until the temperature of the particulate resin falls below a temperature at which no significant amount of further curing would occur in the presence of any oxidizing gas. The method of cooling the cured PAS resin can be accomplished by any suitable means. In a presently preferred embodiment, the cooling is accomplished by heat flow from the particulate resin, through the curing vessel wall, to a cooling medium. Another suitable method of cooling is by the passage, through the interstices of the particulate body, of a cool stream of an inert medium, such as nitrogen. After the temperature of the solid resinous particles has been lowered to the temperature at which no significant further curing will take place when in the presence of any oxidizing gas, further cooling of the particles to atmospheric temperature can be accomplished by any suitable means, preferably, by contacting the particles with a flowing stream of air at atmospheric temperature. This invention will be more fully understood from the following examples. These examples are only intended to demonstrate select embodiments of the invention and are, in no way, intended to limit the scope thereof. EXAMPLE I This example demonstrates how the concentration of oxygen within the gaseous oxidizing atmosphere used in the curing process of a PAS resin effects the time needed to cure the resin to a predetermined melt flow. The PAS resin used in this example was a virgin poly(phenylene sulfide), abbreviated as "PPS", commercially available from Phillips 66 Company as Ryton® poly(phenylene sulfide), having a melting point of 285°C (515°F), an extrusion rate of about 50 g/10 min., and a 6 hour cure rate at 264°C (507°F) of 119 g/10 min. A first 150cc sample of the above particulate PPS resin was placed in a laboratory curing vessel wherein the gas purge composition was controllable. Nitrogen gas was purged through the curing vessel while the temperature of the contents therein was raised to 260°C (500°F). When the internal temperature of the vessel reached 260°C (500°F), the nitrogen purge stream was terminated and replaced with a stream of air flowing at a rate of 1 SCFH. This curing process was permitted to continue for 6 hours. At the end of this 6 hour curing period, the contents within the curing vessel were permitted to cool and the flow of air therethrough was replaced with a nitrogen purge stream. When the internal temperature of the curing vessel reached 66°C (150°F), the nitrogen purge stream was terminated and the modified particulate resin was removed and permitted to cool to atmospheric temperature. Henceforth, this resin will be referred to as Resin 1. A viscosity spectra, demonstrating the level to which Resin 1 was cured, was plotted with data points obtained by using a Rheometrics Dynamic Spectrometer (RDS). This spectra is shown in a graph illustrated in Figure 1. A sample was prepared for the RDS by first cold-pressing, under a pressure of approximately 10,000 psi (1 psi = 6.895 · 10³ N/m²), a 1-gram sample of Resin 1 in a 1 inch diameter die to form a disc which was aproximately 1 / 8 inch thick. The disc was then dried in a vacuum oven, at 110°C (200°F), under a nitrogen purge, for approximately 2 hours. The RDS was operated in a frequency sweep mode starting at 0.30 and ending at 500.00 radians/seconds, at 300°C (542°F), using 1 inch diameter, circular, parallel plate platens spaced 1.0mm apart, at a 15 % strain on the sample. The sample was heated to the desired temperature by using a continuous stream of heated nitrogen. The RDS monitored the viscosity (poise) of Resin 1 as a function of frequency (rad./sec.). The data collected by the RDS is listed in Table I. Two additional PPS resin samples (i.e., Resins 2 and 3, inclusive) were cured and analyzed in accordance with the same procedure set out for Resin 1. The only difference between the curing process of Resin 1 and that of either Resins 2 or 3 was the oxidizing atmosphere used. Specifically, Resin 2 was cured using an oxidizing atmosphere comprising a 50 volume % oxygen concentration. This concentration was achieved by blowing, through the cure vessel, during the 6 hour curing process, a mixture of 70 volume % air and 30 volume % oxygen at a rate of 1 SCFH. A viscosity spectra, demonstrating the level to which Resin 2 was cured, was plotted with data points obtained by the RDS. This spectra is plotted on the graph in Figure 1. The data used to plot the viscosity spectra of Resin 2, as obtained by the RDS, is listed in Table I. Resin 3 was cured using an oxidizing atmosphere comprising a 10 volume % oxygen concentration. This was achieved by blowing, through the cure vessel, during the 6 hour curing process, a mixture of 50 volume % air and 50 volume % nitrogen at the rate of 1 SCFH. A viscosity spectra, demonstrating the level of to which Resin 3 was cured, was plotted with data points obtained by the RDS. This spectra is plotted on the graph in Figure 1. The data used to plot the viscosity spectra of Resin 3, as obtained by the RDS, is listed in Table I. Effect of Oxygen Concentration on Cure Rate Frequency (rad/sec) Viscosity (poise) of: Resin 1 (Control) Resin 2 (Control) Resin 3 (Invention) 0.300 9,878 13,602 11,741 0.476 8,657 11,678 10,763 0.754 7,592 10,312 9,276 1.194 6,673 9,020 8,161 1.893 5,789 7,772 7,000 3.000 5,005 6,676 6,007 4.755 4,301 5,732 5,130 7.536 3,695 4,873 4,356 11.940 3,145 4,113 3,671 18.930 2,648 3,435 3,062 30.000 2,199 2,827 2,526 47.550 1,795 2,289 2,053 75.360 1,445 1,825 1,644 119.400 1,148 1,437 1,301 189.300 905 1,123 1,023 300.000 714 876 805 475.500 565 686 635 500.000 544 658 612 As can be seen from the viscosity spectras and the 6 hour cure rates of Resins 1-3, the level of cure of a particular PPS resin is not significantly effected by the concentration of oxygen within the oxidizing atmosphere. EXAMPLE II This example demonstrates how the concentration of oxygen within the gaseous oxidizing atmosphere of a curing process effects the thermal stability of the oxidatively cured PAS resin. The PAS resin used in this example was the same resin used in Example I. In this example the thermal stability of Resins 1-3 (Example I) were determined over a 30 minute period using an RDS. The samples were prepared for the RDS analysis by first cold-pressing, under a pressure of approximately 10,000 psig, 1-gram samples of Resins 1-3, respectively in a 1 inch diameter die to form three individual discs which were each approximately 1 / 8 inch thick. The discs were then dried in a vacuum oven, at 110°C (200°F), under a nitrogen purge, for approximately 2 hours. The RDS was operated in a time sweep mode starting at about 2 minutes and ending at about 33 minutes, at 330°C (626°F), using 1 inch diameter, circular, parallel plate platens spaced 1.0mm apart, at 15 volume % strain on each Resin sample. The sample was heated to the desired temperature by using a continuous stream of heated nitrogen. The RDS monitored the viscosity (poise) of Resins 1-3, respectively as a function time (i.e., minutes). Specifically, after thermal equalibrium was established (i.e., 2.8 minutes), the RDS monitored the viscosity of the individual resin samples in approximately 3 minute time intervals, until the time sweep was completed. The data collected by the RDS, for comparative purposes, has been normalized by dividing all the viscosities for a given resin by the viscosity monitored at the initial reading. This normalized data is listed in Table II. Effect of Oxygen Concentration on Thermal Stability Time (minutes) Normalized Viscosity of: Resin 1 (Control) Resin 2 (Control) Resin 3 (Invention) 2.8 1.000 1.000 1.000 5.9 1.154 1.175 1.308 8.9 1.512 1.407 1.342 11.9 1.970 1.719 1.568 14.9 2.160 2.021 1.661 18.2 2.365 2.325 1.890 21.2 2.696 2.624 2.045 24.2 2.895 2.988 2.296 27.2 3.135 3.257 2.221 30.2 3.348 3.531 2.543 33.4 3.597 3.898 2.571 To visually demonstrate the effectiveness of this invention, the data from Table II is plotted on the graph illustrated in Figure 2. This data demonstrates that Resin 1, which was cured in a gaseous oxidizing atmosphere comprising a 20 volume % oxygen concentration, exhibits an increase in the melt viscosity of the polymer, during the heating process, of 260 %. The data further demonstrates that Resin 2, which was cured in a gaseous oxidizing atmosphere comprising a 50 volume % oxygen concentration, exhibits an increase in the melt viscosity of the polymer, during the heating process, of 290 %. Based upon the data collected for Resins 1 and 2, it should be expected that for over the same heating period, the melt viscosity of Resin 3, cured in a gaseous oxidizing atmosphere containing a 10 volume % oxygen concentration, would increase by approximately 250 %. However, as seen in both, Table II and Figure 2, the melt viscosity of Resin 3 increased by only 160 %. Therefore, the improvement in the thermal stability of Resin 3 is unexpected in view of the trend established by Resins 1 and 2. EXAMPLE III To further demonstrate the effectiveness of this invention, this example provides a further comparison between the thermal stability of a PAS resin cured in an oxidizing atmosphere comprising a 20 volume % oxygen concentration with that of the same resin cured in an oxidizing atmosphere comprising a 10 volume % oxygen concentration. The PAS resin used in this example was a poly(phenylene sulfide) (PPS) resin prepared by reacting, in a 90-gallon polymerization reactor, 73 lbs. (1 lb = 0.45 kg) of 50 wt. % NaOH, 91 lbs. of 58 wt. % NaSH, 32 gallons (1 gallon = 3.785 · 10⁻³ m³) of N-methyl-pyrrolidone (NMP), and 140 lbs. p-dichlorobenzene (p-DCB), under polymerization conditions disclosed in U.S. patent 3,354,129. The resulting polymeric resin had an extrusion rate of approximately 59 g/10 min. and a 6 hour cure rate at 264°C (507°F) of approximately 20 g/10 min. A first 46 lb. sample of the above PPS resin was charged to a 2 cu.ft. (1 ft = 0.305 m) curing vessel. Nitrogen gas was purged through the vessel while the temperature of the contents therein was raised to 218°C (425°F). When the internal temperature of the vessel reached 218°C (425°F), the nitrogen purge stream was replaced with a stream of air flowing at a rate of 2.5 SCFH. The resin cured under these conditions will henceforth be referred to as Resin 4. At various time intervals during this curing process, samples of Resin 4 were removed form the curing vessel for analysis. The melt flows of these samples, at their respective times, were determined using ASTM D1238 and are recorded in Table III. This curing process was permitted to continue until the melt flow of Resin 4 fell within a predetermined target range of betwen 155 and 115 g/10 min. When Resin 4 was cured to within the targeted melt flow range, the heating process was terminated and the oxidizing atmosphere (i.e., air) was replaced with a stream of nitrogen. The external jacket of the curing vessel was cooled by being contacted with a stream of cooled oil. When the temperature of the contents within the curing vessel cooled to approximately 54°C (130°F), cured Resin 4 was removed from the vessel and its final melt flow determined. The final melt flow of Resin 4 and the time needed to reach the targeted melt flow range are recorded in Table III. A second 46 lb. sample of the same PPS resin was cured in accordance with the same procedure set out for Resin 4. This second cured resin will henceforth be referred to as Resin 5. The only difference between the curing process of Resin 4 and that of Resin 5 was the oxidizing atmosphere used. Specifically, Resin 5 was cured using an oxidizing atmosphere comprising a 10 volume % oxygen concentration. This concentration was achieved by blowing, through the curing vessel, during the curing process, a mixture of 50 volume % air and 50 volume % nitrogen at a rate of 2.5 SCFH. Samples of Resin 5 were removed from the vessel during the curing process for analysis. The melt flows of these samples at their respective times, were determined using ASTM D1238 and are recorded in Table III. When Resin 5 was cured to within the same targeted melt flow range as stated above for Resin 4, the curing process was terminated and the contents within the curing vessel were permitted to cool. When the temperature of the contents within the curing vessel cooled to approximately 54°C (130°F), cured Resin 5 was removed from the vessel and its final melt flow determined. The final melt flow of Resin 5 and the time needed to reach the targeted melt flow range are recorded in Table III. Effect of Oxygen Concentration on Cure Rate Resin 4 (Control) Resin 5 (Invention) Curing Time (hrs.) Flow Rate (g/10 min.) Curing Time (hrs.) Flow Rate (g/10 min.) 0 -- 0 -- 2.92 1268 2.45 1323 7.59 2157 7.18 1281 11.26 309 11.45 1051 12.26 252 18.03 113 13.26 185 FINAL 113 14.26 180 15.26 168 16.26 156 17.26 138 FINAL 128 To visually demonstrate that curing a PPS resin in an oxidizing atmosphere comprising 10 volume % oxygen does not significantly increase the time needed to achieve a desired melt flow, the data from Table III is plotted on the graph illustrated in Figure 3. As can be seen by the data in Figure 3 and in Table III, the time needed to achieve a desired level of cure of a particular PPS resin is not significantly affected by the concentration of oxygen within the oxidizing atmosphere. The thermal stability of Resins 4 and 5 was then determined over a 60 minute period using an RDS. The samples for this RDS analysis were prepared in accordance with the same procedure set out in Example II. The RDS monitored the viscosity (poise) of Resins 4 and 5, individually, as a function of time (i.e., minutes). Specifically, after thermal equalibrium was established (i.e., 2.5 minutes), the RDS monitored the viscosity of the individual resin samples in approximately 3 minute time intervals, until the time sweep was completed. The data collected by the RDS, for comparative purposes, has been normalized by dividing all the viscosities for a given resin by the viscosity monitored at the 2 minute interval. This normalized data is listed in Table IV. Effect of Oxygen Concentration on Thermal Stability Resin 4 (Control) Resin 5 (Invention) Time (minutes) Normalized Viscosity Time (minutes) Normalized Viscosity 2.5 1.000 2.5 1.000 5.6 1.331 5.5 1.180 8.8 1.437 8.4 1.347 12.0 1.935 11.4 1.687 15.2 1.891 14.3 1.477 18.2 2.718 17.3 1.836 23.5 2.893 20.2 1.738 27.9 3.355 23.2 2.012 35.6 4.241 26.1 2.322 38.5 4.508 29.0 2.663 41.8 5.064 31.9 2.413 44.7 5.047 35.3 2.689 52.1 5.746 42.3 2.942 55.2 6.290 45.2 3.056 58.3 6.422 50.2 3.356 58.7 4.291 To visually demonstrate the effectiveness of this invention, the data from Table IV is plotted on the graph illustrated in Figure 4. This data demonstrates that Resin 4, cured in a gaseous oxidizing atmosphere comprising a 20 volume % oxygen concentration, exhibits an increase in the melt viscosity of the polymer, during the heating process, of 542 %. The data further demonstrates that Resin 5, cured in a gaseous oxidizing atmosphere comprising a 10 volume % oxygen concentration, exhibits an increase in the melt viscosity of the polymer, during the same heating process, of only 312 %. Therefore, the thermal stability of the resin, cured in the gaseous oxidizing atmosphere comprising a 10 volume % oxygen concentration (i.e., Resin 5) was significantly improved over that of the same resin cured in an oxidizing atmosphere comprising a 20 volume % oxygen concentration (i.e., Resin 4). EXAMPLE IV To further demonstrate the effectiveness of this invention, this example provides a further comparison between the thermal stability of a commercial grade PAS resin cured in an oxidizing atmosphere comprising a 20 volume % oxygen concentration with that of the same commercial grade resin cured in an oxidizing atmosphere comprising varying concentrations of oxygen, all less then 10 volume %. The PAS resin used in this example was a virgin poly(phenylene sulfide), abbreviated as "PPS" commercially available from Phillips 66 Company as Ryton® poly(phenylene sulfide), having a melting point of 285°C (515°F), an extrusion rate of about 76 g/10 min., and a 6 hour cure rate at 264°C (507°F) of 66 g/10 min. A first 45 lb. sample of the above PPS resin was charged to a 2 cu.ft. curing vessel. Nitrogen gas was purged through the vessel while the temperature of the contents therein was raised to 218°C (425°F). When the internal temperature of the vessel reached 218°C (425°F), the nitrogen purge stream was replaced with a stream of air flowing at a rate of 2.5 SCFH. The resin cured under these conditions will henceforth be referred to as Resin 6. At various time intervals during this curing process, samples of Resin 6 were removed form the curing vessel for analysis. The melt flows of these samples, at their respective times, were determined using ASTM D1238 and are recorded in Table V. This curing process was permitted to continue until the melt flow of Resin 6 fell within a predetermined target range of between 155 and 115 g/10 min. When Resin 6 was cured to within the targeted melt flow range, the heating process was terminated and the oxidizing atmosphere (i.e., air) was replaced with a stream of nitrogen. The external jacket of the curing vessel was cooled by being contacted with a stream of cooled oil. When the temperature of the contents within the curing vessel cooled to aproximately 54°C (130°F), cured Resin 6 was removed from the vessel and its final melt flow determined. The final melt flow of Resin 6 and the time needed to reach the targeted melt flow range are recorded in Table V. A second 45 lb. sample of the same PPS resin was cured in accordance with the same procedure set out for Resin 6. This second cured resin will henceforth be referred to as Resin 7. The only difference betwen the curing process of Resin 6 and that of Resin 7 was the oxidizing atmosphere used. Specifically, Resin 7 was cured using an oxidizing atmosphere, wherein the oxygen concentration was permitted to vary from 8 volume % to 1 volume %. This concentration was achieved by blowing, through the curing vessel, during the curing process, a mixture of air and nitrogen and maintaining the total flow of oxidizing atmosphere through the vessel constant at 2.5 SCFH. The concentrations of oxygen contained in the oxidizing atmosphere used to cure Resin 7 are listed in Table V. Samples of Resin 7 were removed from the vessel during the curing process for analysis. The melt flows of these samples at their respective times, were determined using ASTM D1238 and are recorded in Table V. When Resin 7 was cured to within the same targeted melt flow range as stated above for Resin 6, the curing process was terminated and the contents within the curing vessel were permitted to cool. When the temperature of the contents within the curing vessel cooled to approximately 54°C (130°F), cured Resin 7 was removed from the vessel and its final melt flow determined. The final melt flow of Resin 7 and the time needed to reach the targeted melt flow range are recorded in Table V. Effect of Oxygen Concentration on Cure Rate Resin 6 (Control) Resin 7 (Invention) Curing Time (hrs.) Flow Rate (g/10 min.) Curing Time (hrs.) Curing Conc. (volume %) Flow Rate (g/10 min) 0 ---- 0 8.0 ---- 3.00 3617 4.80 7.6 2460 11.67 530 10.80 4.0 777 13.67 492 18.00 2.0 340 16.92 259 20.92 2.0 267 19.59 230 22.92 1.0 145 21.01 179 FINAL --- 138 22.18 155 22.76 141 FINAL 144 To visually demonstrate that curing a PPS resin in an oxidizing atmosphere comprising less then 10 volume % oxygen does not significantly increase the time needed to achieve a desired melt flow, the data from Table V is plotted on the graph illustrated in Figure 5. As can be seen by the data in Figure 5 and in Table V, the time needed to achieve a desired level of cure of a particular PPS resin is not significantly effected by the concentration of oxygen within the oxidizing atmosphere. The thermal stability of Resins 6 and 7 was then determined over a 60 minute period using an RDS. The samples for this RDS analysis were prepared in accordance with the same procedure set out in Example II. The RDS monitored the viscosity (poise) of Resins 6 and 7, individually, as a function of time (i.e., minutes). Specifically, after thermal equalibrium was established (i.e., 2.5 minutes), the RDS monitored the viscosity of the individual resin samples in approximately 3 minute time intervals, until the time sweep was completed. The data collected by the RDS, for comparative purposes, has been normalized by dividing all the viscosities for a given resin by the viscosity monitored at the initial time reading. This normalized data is listed in Table VI. Effect of Oxygen Concentration on Thermal Stability Time (minutes) Normalized Viscosity of: Resin 6 (Control) Resin 7 (Invention) 2.5 1.000 1.000 5.5 1.109 1.139 8.4 1.633 1.498 11.4 2.016 1.746 14.4 2.586 2.051 17.7 3.289 2.358 20.6 3.899 2.617 23.6 4.508 2.868 26.5 5.105 3.175 29.5 5.719 3.435 32.4 6.205 3.675 35.4 6.804 3.895 38.3 7.240 4.142 41.3 7.799 4.366 44.3 8.175 4.638 47.2 8.741 4.796 50.3 9.159 5.084 53.2 9.608 5.255 56.3 10.014 5.535 59.2 10.461 5.702 To visually demonstrate the effectiveness of this invention, the data from Table VI is plotted on the graph illustrated in Figure 6. This data demonstrates that Resin 6, cured in a gaseous, oxidizing atmosphere comprising a 20 volume % oxygen concentration, exhibits an increase in the melt viscosity of the polymer, during the curing process, of 950 %. The data further demonstrates that Resin 7, cured in a gaseous oxidizing atmosphere comprising an oxygen concentration of less than 10 volume %, exhibits an increase in the melt viscosity of the polymer, during the curing process, of only 470 %. Therefore, the thermal stability of the resin cured in the gaseous oxidizing atmosphere comprising a less then 10 volume % oxygen concentration (i.e., Resin 7) was significantly improved over that of the same resin cured in an oxidizing atmosphere comprising a 20 volume % oxygen concentration (i.e., Resin 6). EXAMPLE V This example demonstrates that the thermal stability of a polymer composition, made from a PAS resin, cured in the presence of an oxidizing atmosphere comprising a 10 volume % oxygen concentration, is significantly greater than that of a composition made by using the same PAS resin, cured in the presence of an oxidizing atmosphere comprising a 20 volume % oxygen concentration. This is achieved without any apparent sacrifice in the physical properties of the composition. The PAS polymer used in this example is the same poly(phenylene sulfide) (PPS) polymer prepared in Example III. In this example, the a first polymeric composition was prepared by dry blending a mixture consisting of 58.75 wt. % of Resin 4 (Example III), 40.00 wt. % of fiberglass, 1.00 wt. % of lithium carbonate as a corrosion inhibitor, and 0.25 wt. % of high density polyethylene as a mold release agent. This mixture was extruded through a Davis-Standard® extruder at a temperature of 316°C (600°F). The extruded product was ground into powder. The powder was then dried in a draft oven at 150°C (302°F) for 2 hours. This dried polymeric composition will henceforth be referred to as Composition 1. Composition 1 was molded into a test specimen by using an Arburg® 221E/170R injection molder with a 316°C (600°F) barrel and a 135°C (275°F) mold temperature. These test specimens consisted of tensile bars and impact bars. The test specimens made from Composition 1 were subjected to the following ASTM testing procedures: Izod Impact (ASTM D256), Flexural Modulus and Flexural Strength (ASTM D790), Tensile Strength (ASTM D638), and Melt Flow (ASTM D1238). The results of the aforementioned tests are recorded in Table VII. An additional dried polymeric composition (i.e., Composition 2) was prepared using the same procedure set out for Composition 1. The only difference between the preparation of Composition 1 and that of Composition 2 was the resin used. Specifically, Composition 2 was prepared with Resin 5 (Example III). Composition 2 was molded into test specimens and subjected to the same tests as those of Composition 1. The results of the aforementioned tests are recorded in Table VII. Effect of Oxygen Concentration on Physical Properties ASTM Test Composition 1 (Control) Composition 2 (Invention) Izod Impact (ft.lb./in.) 1.7 1.7 Flex. Modulus (MSI) 2.3 2.4 Flex. Strength (KSI) 32.7 33.7 Tensile Strength (KSI) 20.9 23.3 Melt Flow (g/10 min.) 42 43 The data in Table VII demonstrates that the physical properties of the polymeric composition made form the PPS resin which was cured in a oxidizing atmosphere comprising 10 volume % oxygen (i.e., Composition 2) is not significantly different from that of the composition made from the PPS resin which was cured in a oxidizing atmosphere comprising 20 volume % oxygen (i.e., Composition 1). The thermal stability of Compositions 1 and 2 was then determined over a 60 minute period using an RDS. The samples for this RDS analysis were prepared in accordance with the same procedure set out in Example II. The RDS monitored the viscosity (poise) of Compositions 1 and 2, individually, as a function of time (i.e., minutes). Specifically, after thermal equalibrium was established (i.e., 2.5 minutes), the RDS monitored the viscosity of the individual resin samples in approximately 3 minute time intervals, until the time sweep was completed. The data collected by the RDS, for comparative purposes, has been normalized by dividing all the viscosities for a given resin by the viscosity monitored at the initial reading. This normalized data is listed in Table VIII. Effect of Oxygen Concentration on Thermal Stability Time (minutes) Normalized Viscosity of: Composition 1 (Control) Composition 2 (Invention) 2.500 1.000 1.000 5.600 1.196 1.200 8.700 1.507 1.365 11.700 1.751 1.417 14.600 1.862 1.510 17.700 2.111 1.599 20.700 2.306 1.713 23.700 2.494 1.786 26.600 2.688 1.877 29.600 2.862 1.966 32.600 3.074 2.072 35.500 3.234 2.147 38.500 3.433 2.265 41.400 3.617 2.350 44.400 3.823 2.460 47.300 4.757 2.559 51.100 5.226 2.674 54.100 5.497 2.714 57.100 5.790 2.840 60.000 5.993 2.970 To visually demonstrate the effectiveness of this invention, the data from Table VIII is plotted on the graph illustrated in Figure 7. This data demonstrates that Composition 1, which was prepared from Resin 4, cured in a gaseous oxidizing atmosphere comprising a 20 volume % oxygen concentration, exhibits an increase in the melt viscosity of the polymer, during the curing process, of 542 %. The data further demonstrates that Composition 2, which was prepared from Resin 5, cured in a gaseous oxidizing atmosphere comprising a 10 volume % oxygen concentration, exhibits an increase in the melt viscosity of the polymer, during the curing process, of only 197 %. Therefore, the thermal stability of the polymeric composition which was made from the resin cured in the gaseous oxidizing atmosphere comprising a 10 volume % oxygen concentration (i.e., Composition 2 and Resin 5, respectively) was significantly improved over that of the composition prepared form the same resin cured in an oxidizing atmosphere comprising a 20 volume % oxygen concentration (i.e., Composition 1 and Resin 4, respectively). EXAMPLE VI This example demonstrates that the thermal stability of a polymeric composition, made from a commercial PAS resin, cured while in the presence of an oxidizing atmosphere comprising a less then 10 volume % oxygen concentration, is significantly greater than that of a composition, made by the same commercial resin, cured while in the presence of an oxidizing atmosphere comprising a 20 volume % oxygen concentration. The commercial PAS polymer used in this example is the same poly(phenylene sulfide) (PPS) polymer used in Example IV. In this example, the a first polymeric composition was prepared by dry blending a mixture consisting of 58.75 wt. % of Resin 4 (Example III), 40.00 wt. % of fiberglass, 1.00 wt. % of lithium carbonate as a corrosion inhibiter, and 0.25 wt. % of high density polyethylene as a mold release agent. This mixture was extruded through a Davis-Standard® extruder at a temperature of 316°C (600°F). The extruded product was ground into powder. The powder was then dried in a draft oven at 150°C (302°F) for 2 hours. This dried polymeric composition will henceforth be referred to as Composition 3. Composition 3 was molded into test specimens by using an Arburg® 221E/170R injection molder with a 316°C (600°F) barrel and a 135°C (275°F) mold temperature. These test specimens consisted of tensile bars and impact bars. The test specimens made from Composition 3 were subjected to the following ASTM testing procedures: Izod Impact (ASTM D256), Flexural Modules and Flexural Strength (ASTM D790), Tensile (ASTM D638), and Melt Flow (ASTM D1238). The results of the aforementioned tests are recorded in Table IX. An additional dried polymeric composition (i.e., Composition 4) was prepared using the same procedure set out for Composition 3. The only difference between the preparation of Composition 4 and that of Composition 3 was the resin used. Specifically, Composition 4 was prepared with Resin 6 (Example IV). Composition 4 was molded into test specimens and subjected to the same tests as those of Composition 3. The results of the aforementioned tests are recorded in Table IX. Effect of Oxygen Concentration on Physical Properties ASTM Test Composition 3 (Control) Composition 4 (Control) Izod Impact (ft.lb./in.) 1.8 1.8 Flex. Modulus (MSI) 2.3 2.3 Flex. Strength (KSI) 33.4 34.0 Tensile Strength (KSI) 22.5 21.8 Melt Flow (g/10 min.) 46 48 The data in Table IX demonstrates that the physical properties of the polymeric composition made from a commercial grade PPS resin, cured in a oxidizing atmosphere comprising less than 10 volume % oxygen (i.e., Composition 4), are not significantly different from that of the composition made from the same commercial grade PPS resin, cured in a oxidizing atmosphere comprising 20 volume % oxygen (i.e., Composition 3). The thermal stability of Compositions 3 and 4 was then determined over a 60 minute period using an RDS. The samples for this RDS analysis were prepared in accordance with the same procedure set out in Example II. The RDS monitored the viscosity (poise) of Compositions 3 and 4, individually, as a function of time (i.e., minutes). Specifically, after thermal equalibrium was established (i.e., approximately 2.5 minutes), the RDS monitored the viscosity of the individual resin samples in approximately 3 minute time intervals, until the time sweep was completed. The data collected by the RDS, for comparative purposes, has been normalized by dividing all the viscosities for a given resin by the viscosity monitored at the initial reading. This normalized data is listed in Table X. Effect of Oxygen Concentration on Thermal Stability Time (minutes) Normalized Viscosity of: Composition 3 (Control) Composition 4 (Invention) 2.900 1.000 1.000 5.900 1.510 1.235 8.800 1.868 1.599 11.000 2.227 1.902 15.000 2.781 2.077 18.100 3.205 2.348 21.000 3.582 2.610 24.100 4.009 2.886 26.900 4.363 3.177 30.000 4.729 3.314 32.900 5.062 3.540 35.900 5.422 3.772 38.800 5.734 4.016 41.800 6.090 4.251 44.700 6.239 4.385 47.800 6.664 4.642 50.800 7.022 4.913 53.900 7.380 5.177 56.900 7.735 5.429 60.000 8.075 5.682 To visually demonstrate the effectiveness of this invention, the data from Table X is plotted on the graph illustrated in Figure 8. This data demonstrates that Composition 3, which was prepared from Resin 6, cured in a gaseous oxidizing atmosphere comprising a 20 volume % oxygen concentration, exhibits an increase in the melt viscosity of the polymer, during the curing process, of 707 %. The data further demonstrates that Composition 4, which was prepared from Resin 7, cured in a gaseous oxidizing atmosphere comprising a 10 volume % oxygen concentration, exhibits an increase in the melt viscosity of the polymer, during the curing process, of only 470 %. Therefore, the thermal stability of the polymeric composition which was made from the resin cured in the gaseous oxidizing atmosphere comprising a less than 10 volume % oxygen concentration (i.e., Composition 4 and Resin 7, respectively) was significantly improved over that of the polymeric composition prepared form the same resin cured in an oxidizing atmosphere comprising a 20 volume % oxygen concentration (i.e., Composition 3 and Resin 6, respectively. EFFECTS OF SOLID STATE CURING IN A REDUCED OXYGEN ATMOSPHERE BACKGROUND The data which follows will compare the effects, on thermal stability, of curing a poly(arylene sulfide) resin in an atmosphere having varying concentrations of oxygen. The varying concentrations of oxygen which were tested were as follows: (a) 50 volume-%, (b) 21 volume-%, (c) 18 volume-%, (d) 15 volume-%, (e) 12 volume-%, and (f) 10 volume-%. The specific poly(arylene sulfide) resin which was solid state cured was poly(phenylene sulfide), abbreviated as "PPS", commercially available from Phillips Petroleum Company as Ryton® PPS, having a melting point of about 285°C, an extrusion rate of about 47 g/10 min., and a 6-hour cure rate, in air and at 264°C, of about 119 g/10 min. EXPERIMENTAL DATA Sample Preparation A first 80 gram sample (approximate 200 ml) of the above particulate PPS resin was placed in a laboratory 500 ml curing vessel wherein the gas purge composition was controllable. Nitrogen gas was purged through the curing vessel while the temperature of the contents therein was raised to 260°C (500°F). When the internal temperature of the vessel reached 260°C (500°F), the nitrogen purge stream was terminated and replaced with a stream of a gaseous oxidizing atmosphere comprising a 50 volume-% oxygen concentration and flowing at a rate of 100 ml/min. This concentration of oxygen was achieved by blowing, through the cure vessel, during the curing process, a mixture of 50 volume-% nitrogen and 50 volume-% oxygen. The curing process was permitted to continue for approximately 4 hours. At the end of this 4-hour curing period, the contents within the curing vessel were permitted to cool and the flow of air therethrough was replaced with a nitrogen purge stream. When the internal temperature of the curing vessel reached approximately 66°C (150°F), the nitrogen purge stream was terminated and the modified particulate PPS resin was removed and permitted to cool to atmospheric temperature. Henceforth, this resin will be referred to as Resin 1. The melt flow value of Resin 1 was determined using ASTM D1238 and is recorded in Table I. A second sample of the above PPS resin (i.e., Resin 2) was cured in accordance with the same procedure employed to cure Resin 1. The melt flow value of Resin 2 is recorded in Table I. Ten additional samples of the above PPS resin (i.e., Resins 3-12, inclusive) were cured and analyzed in accordance with the same procedure employed when curing Resin 1. The only difference between the curing process of Resin 1 and that of Resins 3-12, inclusive, was the concentration of oxygen in the curing atmosphere. Specifically, Resins 3 and 4 were cured using an oxidizing atmosphere comprising a 21 volume-% oxygen concentration. This concentration was achieved by blowing, through the cure vessel, during the 4-hour curing process, air at a rate of 100 ml/min. The melt flow value of Resins 3 and 4 are recorded in Table I. Resins 5 and 6 were cured using an oxidizing atmosphere comprising an 18 volume-% oxygen concentration. This was achieved by blowing, through the cure vessel, during the 4-hour curing process, a mixture of 86 volume-% air and 14 volume-% nitrogen at the rate of 100 ml/min. The melt flow value of Resins 5 and 6 are recorded in Table I. Resins 7 and 8 were cured using an oxidizing atmosphere comprising an 15 volume-% oxygen concentration. This was achieved by blowing, through the cure vessel, during the 4-hour curing process, a mixture of 71 volume-% air and 29 volume-% nitrogen at the rate of 100 ml/min. The melt flow value of Resins 7 and 8 are recorded in Table I. Resins 9 and 10 were cured using an oxidizing atmosphere comprising an 12 volume-% oxygen concentration. This was achieved by blowing, through the cure vessel, during the 4-hour curing process, a mixture of 57 volume-% air and 43 volume-% nitrogen at the rate of 100 ml/min. The melt flow value of Resins 9 and 10 are recorded in Table I. Resins 11 and 12 were cured using an oxidizing atmosphere comprising an 10 volume-% oxygen concentration. This was achieved by blowing, through the cure vessel, during the 4-hour curing process, a mixture of 48 volume-% air and 52 volume-% nitrogen at the rate of 100 ml/min. The melt flow value of Resins 11 and 12 are recorded in Table I. Effect of Oxygen Concentration on Degree of Cure Resin No. Curing Atmosphere (Vol.-% Oxygen) Flow Date a (g/10 min.) 1 50 70 2 50 98 3 21 86 4 21 79 5 18 91 6 18 97 7 15 111 8 15 74 9 12 107 10 12 118 11 10 105 12 10 103 The data recorded in Table I demonstrates that the concentration of oxygen in the curing atmosphere does not significantly affect the degree of cure. Rheological Analysis to Determine Thermal Stability The thermal stability of Resins 1-12, inclusive, in the molten state was determined using a Rheometrics Dynamic Spectrometer (RDS). The samples were prepared for the RDS by first cold-pressing, under a pressure of approximately 10,000 psi, a 1-gram sample of the resin (powder) to be analyzed in a 1 inch diameter die to form a disc which was approximately 1 / 8 inch thick. The disc was then dried in a vacuum oven, at 110°C (200°F), under a nitrogen purge, for at least 2 hours. The RDS was operated in a constant frequency mode of 1.0 radian/second, at 330°C (626°F), using 1 inch diameter, circular, parallel plate platens spaced 1.0 mm apart, and at a 15 percent strain on the sample. The sample was maintained at the desired temperature by using a continuous stream of heated nitrogen. The RDS monitored the melt viscosity (poise) of Resins 1-12, respectively, as a function time (minutes). Specifically, after thermal equilibrium was established (i.e., approximately 5 minutes), the RDS monitored the viscosity of the individual resin samples in approximately 3 minute time intervals, until the time sweep was completed. The data collected by the RDS, for comparative purposes, has been normalized by dividing all the viscosities for a given resin by the viscosity monitored at the initial reading, after the thermal equilibrium had been established. This normalized data is recorded in Table II. To visually demonstrate the effects of curing a resin sample with a gaseous atmosphere having varying concentrations of oxygen, the data from Table II is plotted on the graphs illustrated in Appendix A and Appendix B. Specifically, the data associated with Resins 1, 3, 5, 7, 9 and 11 is plotted in the graph illustrated in Appendix A. On the other hand, the data associated with Resins 2, 4, 6, 8, 10 and 12 is plotted in the graph illustrated in Appendix B. Table III, which follows, lists the per cent increases in the melt viscosity, as reported in Table II, for the various resins, over the 60-minute analysis period. OXYGEN CONC. (VOLUME-%) RESIN NO. PERCENT INCREASE IN MELT VISCOSITY IN 60 MINUTES AVG. PERCENT INCREASE IN MELT VISCOSITY IN 60 MINUTES 50 1 425 457 2 489 21 3 311 316 4 321 18 5 280 278 6 276 15 7 212 204 8 196 12 9 185 179 10 173 10 11 204 188 12 171 To visually demonstrate the effects of curing a resin sample with a gaseous atmosphere having varying concentrations of oxygen, the data from Table III is plotted on the graph illustrated in Appendix C. The data in Tables II and III and in Appendix C demonstrates that, over the 60-minute analysis period, the average increase in the melt viscosity of Resins 1 and 2, which were cured in a gaseous oxidizing atmosphere containing a 50 volume-% oxygen concentration, was approximately 457%. The data further demonstrates that, over the 60-minute analysis period, the average increase in the melt viscosity of Resins 3 and 4, which were cured in a gaseous oxidizing atmosphere containing a 21 volume-% oxygen concentration, was approximately 316%. The data also demonstrates that, over the 60-minute analysis period, the average increase in the melt viscosity of Resins 5 and 6, which were cured in a gaseous oxidizing atmosphere containing a 18 volume-% oxygen concentration, was approximately 278%. Based upon this data, it should be expected that, if subjected to the same curing conditions, the average increase in melt viscosity of Resins 7 and 8, Resins 9 and 10, and Resins 11 and 12, which were cured in a gaseous oxidizing atmosphere containing a 15 volume-%, 12 volume-%, and 10 volume-% oxygen concentration, respectively, would be approximately 270%, 250%, and 240%, respectively. However, as shown in both Tables II and III and illustrated in Appendices A through C, the average increase in the average melt viscosity of Resins 7 and 8, Resins 9 and 10, and Resins 11 and 12, was only approximately 204%, 179%, and 188%, respectively. Therefore, the improvement in the thermal stability of Resins 7-12 is unexpected in view of the trend established by Resins 1-6. Generally, the poly(arylene sulfide) resin which can be treated in accordance with this invention has a particle size such that the diameter of at least 50% of the particles is less than about 10 mm. Preferably, the particle size of the poly(arylene sulfide) resin, treated in accordance with this invention, is such that the diameter of at least 50% of the particles is less than about 5 mm.

Claims: A cured poly(arylene sulfide) resin, wherein said resin is prepared by heating said resin in the form of solid particles, in the presence of a gaseous oxidizing atmosphere comprising less than 20 volume % oxygen, to a temperature in the range of about 10°C to about 70° below the melting point of said resin for a time sufficient to effect the desired level of cure. A resin in accordance with claim, 1. wherein said oxidizing atmosphere comprises an oxygen content ranging form about 18 volume % to about 1 volume %, in particular ranging from about 15 volume % to about 3 volume %, more particularly ranging from about 12 volume % to about 6 volume %. A resin in accordance with claim 2, wherein said oxidizing atmosphere comprises an oxygen content of about 10 volume %. A resin in accordance with claim 1, wherein said oxidizing atmosphere comprises a mixture of air and an inert gas, in particular selected from the group comprising steam, nitrogen, and carbon dioxide. A resin in accordance with claim 4, wherein said inert gas is nitrogen. A resin in accordance with any of the preceding claims, wherein said temperature to which said solid particles are heated is in the range of about 25°C to about 55°C below the melting point of said resin. A resin in accordance with any of the preceding claims, wherein said resin comprises -R-S- repeating units wherein R is phenylene, biphenylene, naphthylene, biphenylene ether, or a lower alkyl derivative thereof. A resin in accordance with any of the preceding claims wherein said resin is selected from the group comprising poly(phenylene sulfide), poly(phenylene sulfide ketone), poly(phenylene sulfide diketone), and poly(biphenylene sulfide). A resin in accordance with any of the preceding claims, wherein said resin is poly(phenylene sulfide). A process for oxidatively curing a virgin poly(arylene sulfide) resin which comprises heating said resin as defined in one of claims 1-9. A process in accordance with claim 10, which further comprises purging said solid particles with an inert fluid to free said cured solid particles of said oxidizing atmosphere. A process in accordance with claim 11, which further comprises cooling said solid particles in the presence of said inert fluid and in the at least substantial absence of said oxidizing atmosphere to a temperature at which no significant amount of further curing of said solid particles would occur in the presence of said oxidizing atmosphere. A process in accordance with claim 12, further comprising contacting the thus cooled solid particles with relatively cool air to further reduce the temperature of said solid particles. A process in accordance with claim 10, wherein said time during which said solid particles are heated is in the range of about 10 minutes to about 3 days, in particular in the range of about 2 to 12 hours. A process according to one of claims 10-14 for curing a PAS resin in an oxidizing atmosphere having an oxygen content of not greater than about 15 volume percent. A molded article made from a resin recited in any one of the claims 1-9. A polymeric composition comprising: (a) a cured poly(arylene sulfide) resin, according to any of the claims 1-9 and (b) fiberglass. A composition in accordance with claim 17, further comprising a corrosion inhibiter, in particular lithium carbonate. A composition in accordance with claim 17, further comprising a mold release agent, in particular wherein said mold release agent is high density polyethylene. A molded article made from a composition recited in one of the claims 17-19.

Inventors: FIGARD, JOSEPH EMERSON, HOOVER, KENNETH CHARLES, SHERK, FRED THEODORE
Applicants: PHILLIPS PETROLEUM COMPANY
Application year: 1988
Main classification: C08J3
IPC codes: C08K7, C08J3, C08G75, C08L81, C08K13