WO1992011319A1

WO1992011319A1 - Methods and compositions relating to halogenated polycarbonate having improved thermal stability

Info

Publication number: WO1992011319A1
Application number: PCT/US1991/009623
Authority: WO
Inventors: Steven L. Brewster; Bobby R. Whistler
Original assignee: The Dow Chemical Company
Priority date: 1990-12-21
Filing date: 1991-12-19
Publication date: 1992-07-09

Abstract

The thermal stability of a halogenated polycarbonate, or an article molded therefrom, is improved by incorporating a trisaryl phosphite into the halogenated polycarbonate in an amount of 200 to 1,500 weight parts per million. The thermal stabilizing effect of the phosphite is apparent in a reduced level of yellowness in the halogenated polycarbonate or molded article. Amounts of trisaryl phosphite less than 200 ppm or greater than 1,500 ppm do not give the desired stabilizing effect. Trisaryl phosphite which has performed its stabilizing effect is present as trisaryl phosphate.

Description

METHODS AND COMPOSITIONS RELATING TO HALOGENATED POLYCARBONATE HAVING IMPROVED THERMAL STABILITY

This invention pertains to methods of stabilizing halogenated polycarbonate against the effects of thermal degradation and aging, to halogenated polycarbonate compositions having a desirably high level of thermal stability, and to objects molded from such compositions.

Polycarbonate has found many uses because it combines, in general, a high level of heat resistance, impact resistance and dimensional stability with good insulating and non-corrosive properties, and it is easily molded. Halogenated polycarbonate is particularly known for its heat resistance and for its high glass transition temperature (Tg), which typically exceeds 175°C. Tg is the temperature or temperature range at which an amorphous polymeric material shows an abrupt change in physical properties, including, for example, mechanical strength. Tg can be determined, for example, by differential scanning calorimetry. The Tg associated with halogenated polycarbonate is higher than that of most other thermoplastics and makes halogenated polycarbonate a logical choice for use in molding articles which must withstand high service temperatures. However, when halogenated polycarbonate is used in a high service-temperature environment, it often shows a reduction in the level of its other desirable properties because of the thermal aging and degradation it experiences in such environment.

10 Various stabilizers and stabilizer packages have been proposed in the past for the purpose of attempting to ameliorate the degradative effects suffered by halogenated polycarbonate or co-poly¬ carbonate when it is exposed to high temperatures during

15 processing. For example, Cleveland, U.S. Patent Number 3,733,296 discloses as a stabilizer for halogenated polycarbonate the use of phosphite together with barium, strontium and/or calcium carbonate, and Factor, U.S. Patent Number 3,763,063 discloses as a stabilizer for

20 halogenated polycarbonate the use of phosphite in a mixture with an alkanoic acid and a salt of an alkanoic acid. And, although Fritz, U.S. Patent Number 3,305,520 and Rasberger, U.S. Patent Number 4,321,218 both

~_c disclose the use of a phosphite as a stabilizer for polycarbonate, neither recognizes any special conditions under which such a phosphite should be employed when it is used in a halogenated polycarbonate. Further, the employment of a stabilizer which is effective to protect 0 polycarbonate from high temperature during processing and/or fabrication may not be equally applicable to the achievement of thermal stability during a lengthy exposure of polycarbonate to the degradative effects of a high service temperature. Accordingly, it would be desirable to have compositions, and methods for making those compositions, wherein a phosphite could be employed alone as a stabilizer in a halogenated polycarbonate for the purpose of imparting -resistance to the undesirable, degradative effects resulting from extended exposure of such composition to high temperature.

In one aspect, this invention involves a composition of matter containing halogenated poly¬ carbonate and 200 to 1,500 parts per million, by weight of the total composition, trisaryl phosphite. In a further aspect, this invention involves a film, sheet, molded or thermoformed article, or other object which has been thermally aged, comprising, in admixture, (a) brominated polycarbonate and (b) 200 parts per million to 1,500 parts per million, by weight of the total composition, (i) trisaryl phosphite, (ii) trisaryl phosphate, or (iii) a mixture thereof. This invention also involves a composition of matter comprising a polycarbonate which contains 10-30 percent bromine by weight and has been stabilized against the effects of thermal degradation, said composition being characterized in that an article molded therefrom has a Yellowness Index of no more than 13, as measured according to ASTM Designation D 1925-70, after 1,512 hours of thermal aging in a forced air oven at 130°C. Yet another aspect of this invention is a method of stabilizing a brominated polycarbonate against the effects of thermal degradation comprising forming a composition by admixing with said brominated polycarbonate 200 parts per million to 1,500 parts per million trisaryl phosphite. It has been found, for example, that an article molded from a composition containing a halogenated polycarbonate (such as a brominated segmented block co- polycarbonate) and a stabilizing amount of trisaryl phosphite displays a much lower level of degradative effects from thermal aging than a halogenated polycarbonate composition which contains an insufficient or too great an amount of trisaryl phosphite. Stabilization against the occurrence of such degradative effects of thermal aging can be demonstrated, for example, by a lower Yellowness Index (as measured by ASTM Designation D 1925-70) in an article molded from a halogenated polycarbonate protected by 200 to 1,500 ppm trisaryl phosphite, by weight of the total composition, as opposed to a halogenated polycarbonate containing other amounts of trisaryl phosphite or other amounts of other stabilizing additives.

The methods of this invention are useful for producing the compositions of this invention, such compositions being useful, for example, in the production of films, fibers, extruded sheets, multi- layer laminates, oxygen-nitrogen separation membrane, and molded or shaped articles of virtually all varieties, particularly apparatus which are transparent ( i.e. having more than 80 percent light transmittance when measured according to ASTM Designation D 1746-70), and particularly parts for the transportation and appliance industries such as motor vehicle headlamp lenses, aircraft canopies, and autoclavable medical apparatus. Figure 1 is a plot of yellowness index vs. time of exposure in hours to 130°C heat for unhalogenated polycarbonate containing various levels of trisaryl phosphite.

Figure 2 is -a plot of yellowness index vs. time of exposure in hours to 130°C heat for halogenated polycarbonate containing various levels of trisaryl phosphite.

Figure 3 is a plot of yellowness index after 1,512 hours of exposure to 130°C heat vs. level of trisaryl phosphite for both halogenated and unhalogenated polycarbonate containing various levels of trisaryl phosphite.

Figure 4 is a plot of weight average molecular weight vs. time of exposure in hours to 130°C heat for unhalogenated polycarbonate containing various levels of trisaryl phosphite.

Figure 5 is a plot of weight average molecular weight vs. time of exposure in hours to 130°C heat for halogenated polycarbonate containing various levels of trisaryl phosphite.

Figure 6 is a plot of weight average molecular weight after 1,512 hours of exposure to 130°C heat vs. level of trisaryl phosphite for unhalogenated polycarbonate containing various levels of trisaryl phosphite.

Figure 7 is a plot of weight average molecular weight after 1,512 hours of exposure to 130°C heat vs. level of trisaryl phosphite for halogenated polycarbonate containing various levels of trisaryl phosphite. The compositions of this invention contain halogenated polycarbonate admixed with trisaryl phosphite in an amount of 200 to 1,500 parts per million ("ppm"), by weight of- the total composition. In a preferred formulation, the compositions of this invention contain trisaryl phosphite in an amount of 200 to 800 ppm, by weight of the total composition, admixed with halogenated polycarbonate. The compositions of this invention have a weight average molecular weight, determined for example by gel permeation chromatography ("GPC"), of 10,000 to 100,000, advantageously 25,000 to 40,000, and preferably 26,000 to 32,000.

Preparation of the compositions of this invention can be accomplished by any suitable mixing means known in the art. Typically, trisaryl phosphite is dry blended in particulate form with sufficient agitation to obtain thorough distribution thereof within the halogented polycarbonate. Compounding by means of mixing rolls, a dough-mixer or a Banbury mixer can be useful in this context, or, if desired, the dry-blended formulation can further, but need not, be melt mixed in an extruder. When melt mixed, the compositions of this invention are preferably prepared at as low a temperature as possible over the T of the halogenated polycarbonate at which the components are processible, for example in the range of 575-620°F (302-327°C). Trisaryl phosphite and halogenated polycarbonate can also be admixed in solution, and methylene chloride, acetone, benzaldehyde or heptane are useful solvents for such purpose. The compositions can then be separated from the solvent by known methods such as devolatilization or use of an anti-solvent. Alternatively, a master batch formulation can be prepared containing halogenated polycarbonate with which trisaryl phosphite is admixed wherein halogenated polycarbonate is present in only a minor proportion, e.g. 20 percent. Although the master batch formulation may contain more than 1,500 ppm trisaryl phosphite, it is more readily available for storage or shipment in commerce, and the master batch can, at the time of use, be diluted with additional halogenated polycarbonate to obtain the relative content levels specified above.

The compositions of this invention can be formed into articles or molded using conventional techniques such as compression, injection, calendering, vacuum forming, extrusion and/or blow molding techniques, alone or in combination. The compositions can also be formed into films, fibers, multi-layer laminates or extruded sheets on any machine suitable for such purpose.

The methods of this invention are those in which a composition is formed by admixing with halogenated polycarbonate trisaryl phosphite in an amount of 200 to 1,500 ppm, and preferably in an amount of 200 to 800 ppm, by weight of the total composition. Use of trisaryl phosphite in such amounts substantially prevents the occurrence, to an undesirable extent, of the effects resulting from thermal degradation and aging to which such composition may be subjected. For example, the methods of this invention control to a desirable level formation or increase of color bodies in a halogenated polycarbonate composition containing a stabilizing amount of triaryl phosphite as compared to compositions not so stabilized. The color stability which is imparted by the methods, and to the compositions, of this invention can be demonstrated, for example, by comparing the level of Yellowness Index ("Y.I.", as measured -by ASTM Designation D 1925-70) found in the compositions made by the methods of this invention (being the compositions of this invention) with that found in compositions outside of this invention.

The presence of trisaryl phosphite in a composition in an amount less than 200 ppm or exceeding 1 500 ppm typically does not control to desirable levels in halogenated polycarbonate the effects of thermal degradation, for example those relating to color stability. Use of trisaryl phosphite in amounts greater than 1,500 ppm may also contribute, in a composition, to degradation from heat exposure in the form of molecular weight loss. The molecular weight degradation which can result from admixing with halogenated polycarbonate more than 1,500 ppm trisaryl phosphite, by weight of the total composition, can be demonstrated by comparing molecular weights of modified and unmodified polycarbonates using GPC.

The stability which trisaryl phosphite imparts to halogenated polycarbonate is protection against a reduced level of desirable properties, or an increased level of undesirable properties, such as may result from thermal aging of an article molded therefrom. An example of thermal aging is a process wherein heat promotes the reaction of oxygen with the polymer to form hydroperoxides with the consequent breakdown of the polymeric backbone resulting in chain scission and/or the development or increase, for example, of colored bodies. Thermal aging can occur in any environment, or by any mechanism, where halogenated polycarbonate is exposed to heat in sufficient quantity to promote such oxidative reaction and/or other forms of degradation such as discoloration and/or molecular weight loss. Thermal aging is typically said to have occurred when a composition of this invention, or a molded article or other object comprising such composition, is exposed to heat of at least 125°C for a period of at least 250 hours.

When a film, sheet, article, fiber or other object formed or molded from a composition of this invention is exposed to thermal aging, and the trisaryl phosphite present therein performs its intended function of reducing or preventing the degradative effects of the thermal aging, the trisaryl phosphite is converted to the corresponding trisaryl phosphate. Consequently, when a film, sheet, article, fiber or other object formed from a composition of this invention is analyzed after having been exposed to thermal aging, it can be analyzed to determine the content of trisaryl phosphite, trisaryl phosphate or both. The longer and more harsh the thermal aging is that the film, sheet, article, fiber or other object has been exposed to, the greater will be the quantity of trisaryl phosphite which has been converted to trisaryl phosphate. In a film, sheet, article, fiber or other object formed from the compositions of this invention, the content of (a) trisaryl phosphite, (b) trisaryl phosphate, or (c) a mixture of the two should therefore be between 200 to 1,500 ppm, and preferably between 200 to 800 ppm, by weight of the total composition. After thermal aging, a film, sheet, article fiber or other object formed fr*o<πι the compositions of this invention should have a Yellowness Index of less than 15, and preferably less than 12, as measured according to ASTM Designation D 1925-70; and should a-lso have a decrease, if any, in weight average molecular weight, as determined for example by GPC, of less than 5 percent, advantageously less than 4 percent, preferably less than 3 percent, and more preferably no molecular weight loss at all.

The compositions of this invention, and the compositions made by the methods of this invention, are substantially free of additives, other than trisaryl phosphite, which inhibit the effects of thermal aging. Other such additives include, for example, benzoates, carbonates, or alkanoic acids or salts thereof. The presence of such other additives may, for example, cause lower impact strength or reduce light transmittance in an article molded from a halogenated polycarbonate composition containing them. It is found that when trisaryl phosphite, as opposed to other organophosphorous compounds, is used alone in a stabilizing amount as the stabilizing additive in halogenated polycarbonate, good stability, for example a reduced level of color formation or increase, is obtained without a need for incorporating additional additives, such as the salts, acids or benzoates mentioned above.

Halogenated Polycarbonate. Halogenated polycarbonate is produced by any of the conventional processes known in the art for the manufacture of aromatic polycarbonates. Generally, aromatic polycarbonates are prepared by reacting an aryl diol or dihydric phenol with a carbonate precursor, such as phosgene, a haloformate or a carbonate ester.

A preferred method for preparing aromatic polycarbonates involves the use of a carbonyl halide, such as phosgene, as the carbonate precursor. This method involves passing phosgene gas into a reaction mixture containing an activated dihydric phenol, or a nonactivated dihydric phenol and an acid acceptor, for example pyridine, dimethyl aniline, quinoline. The acid acceptor may be used undiluted or diluted with inert organic solvents, such as methylene chloride, chlorobenzene or 1,2-dichloroethane. Tertiary amines are advantageous since they are good solvents as well as acid acceptors during the reaction.

The temperature at which the carbonyl halide reaction proceeds may vary from below 0°C to 100°C. The reaction proceeds satisfactorily at temperatures from room temperature to 50°C. Since the reaction is exothermic, the rate of phosgene addition may be used to control the temperature of the reaction. The amount of phosgene required will generally depend upon the amount of dihydric phenol present. Typically, one mole of phosgene will react with one mole of dihydric phenol to form the polycarbonate and two moles of HC1. The HC1 is in turn taken up by the acid acceptor.

Another method for preparing aromatic polycarbonates involves adding phosgene to an alkaline aqueous suspension of dihydric phenols. This is done in the presence of inert solvents such as methylene chloride, 1,2-dichloroethane. Quaternary ammonium compounds may be employed to catalyze the reaction. Yet another method for preparing such aromatic polycarbonates involves the phosgenation of an agitated suspension of an anhydrous alkali salt of an aryl diol in a nonaqueous medium such as benzene, chlorobenzene or toluene. The reaction is illustrated by the addition of phosgene to a slurry of the sodium salt of, for example, 2,2-bis(4-hydroxyphenyl)propane ("Bisphenol A" or "Bis A") in an inert polymer solvent such as chlorobenzene.

In general, a haloformate such as the bishaloformate of Bisphenol A may be used in place of phosgene as the carbonate precursor in any of the methods described above.

When a carbonate ester is used as the carbonate precursor in the polycarbonate-forming reaction, the materials are reacted at temperatures in excess of 100°C, for times varying from 1 to 15 hours. Under such conditions, ester interchange occurs between the carbonate ester and the dihydric phenol used. The ester interchange is advantageously consummated at reduced pressures on the order of from 10 to 100 millimeters of mercury, preferably in an inert atmosphere such as nitrogen or argon.

Although the polymer-forming reaction may be conducted in the absence of a catalyst, one may, if desired, employ a typical ester exchange catalyst, such as metallic lithium, potassium, calcium or magnesium. The amount of such catalyst, if used, is usually small, ranging from 0.001 percent to 0.1 percent, based on the weight of the dihydric phenols employed. In the solution methods of preparation, the aromatic polycarbonate emerges from the reaction in either a true or pseudo solution depending on whether an aqueous base or pyridine is used as an acid acceptor. The product may be precipitated from the solution by adding a nonsolvent, such as heptane or isopropanol, or the solution may be heated, typically under reduced pressure, to evaporate the solvent.

The methods and reactants described above for preparing aromatic polycarbonates, and aromatic polycarbonates for use as the halogenated polycarbonate of this invention, are discussed in greater detail in Schnell, U.S. Patent Number 3,028,365; Campbell, U.S. Patent Number 4,384,108; Glass, U.S. Patent Number 4,529,791; and Grigo, U.S. Patent Number 4,677,162.

Aromatic polycarbonates can be prepared from aryl diols such as those represented, for example, by the formula:

wherein X is a divalent C-_|-C-_|c; hydrocarbon radical, a single bond, -0-, -S-, -S₂-, -S0-, -S0 -, or -C0-. Each aromatic ring may additionally contain, instead of hydrogen, up to four substituents such as C₁-C₂₀ alkyl, aryl, alkaryl or aralkyl hydrocarbon radicals, or C^C^ alkoxy or aryloxy radicals, or halo radicals. More than one of the above described aryl diols, or two or more other dihydoxy compounds, can be used if it is desired to prepare aromatic co-polycarbonates. Aromatic co- polycarbonates can also be produced, for example, when a bisphenol Is reacted with a carbonic acid derivative and a polydiorganosiloxane containing α,ω-bishydroxyaryloxy terminal groups to yield a siloxane/carbonate block copolymer (as are discussed in greater detail in Paul, U.S. Patent Number 4,569,970, or when a bisphenol is reacted with a bis(ar-haloformylaryl) carbonate to yield an alternating co-polyestercarbonate. The bis(ar- haloformylaryl) carbonate is formed by reacting a hydroxycarboxylic acid with a carbonic acid derivative under carbonate forming conditions, such co-polyestercarbonates being discussed in greater detail in Swart, U.S. Patent Number 4,105,533-

The halogenated polycarbonate for use in this invention can be an aromatic polycarbonate, an aromatic co-polycarbonate, or blend of one or more of each. It is preferred that such aromatic polycarbonate, aromatic co-polycarbonate or blend contains therein from 10 percent to 30 percent, by weight, of halogen, such as chlorine, bromine or iodine, bound to a polymer chain. It is preferred that the halogen is bound to an aromatic ring. The preferred halogenated polycarbonate for purposes of this invention is brominated polycarbonate.

A preferred brominated polycarbonate for use in this invention is that which is prepared by the processes disclosed in Marks, U.S. Patent Number 4,902,758, wherein high molecular weight segmented block co-polycarbonates are prepared from diphenols, such as Bisphenol-A, and tetrahalogenated diphenols, such as 2,2-bis(3,5-dibromo, 4-hydroxyphen 1)propane ("Tetrabromo Bisphenol-A" or "TBBA"). Segmented block co-polycarbonates from diphenols and tetrahalogenated diphenols can be prepared by phosgenation, under interfacial reaction conditions, initially at a higher pH such as 12 to 13, during which diphenol oligomers are formed, and thereafter at a lower pH such as 10 to 11, during which tetrahalogenated diphenol bischloroformates are formed, followed by copolymerization utilizing an activated pyridine catalyst such as 4- dimethylaminopyridine. Alternatively, diphenol polycarbonate oligomers and tetrahalogenated diphenol chloroformates may be coupled, utilizing an activated pyridine catalyst such as 4-dimethylaminopyridine, to prepare segmented block co-polycarbonates. Preferred formulations of such segmented block co-polycarbonates are those prepared from Bisphenol-A and TBBA wherein the molar ratio of Bisphenol-A to TBBA is from 2:1 to 4:1.

Trisaryl Phosphite. The trisaryl phosphites useful in this invention contain a substituted or unsubstituted phenyl ring bonded to each of the three oxygen atoms bonded to phosphorous, and can be represented by a structure such as

where X_χ to X₁₅ is each independently hydrogen or an alkyl, aryl, alkaryl or aralkyl radical of one to ten carbon atoms. X can, for example, be an ethyl, propyl, butyl, amyl, hexyl, octyl, nonyl or decyl radical; can be a cycloalkyl radical such cyclohexyl, 2- or 4- methylcyclohexyl or cyclopentyl; or can be a phenyl, naphthyl, tolyl or xylyl radical. A mixture of more than one of such trisaryl phosphites can be used herein as well. A trisaryl phosphite can be made by reacting three or more moles of the desired substituted or unsubstituted phenol(s) with a mole of a phosphorous trihalide, such as PC1₃, and three moles of a halide acceptor, preferably an organic base such as pyridine or di ethylaniline. Ammonia or triethylamine may also be useful as the base in such reaction. The trihalide, in a solvent such as diethyl ether, pentane or benzene, is added dropwise to a well stirred solution of the phenol(s) and the base which is cooled to 10°C. The organic ammonium halide is filtered off, and the tertiary ester of phosphorous acid is purified by vacuum distillation. A preferred trisaryl phosphite is tris (2,4-di-t-butylphenyl) phosphite, which is available as Irgafos" 68 phosphite or as P168™ phosphite from Ciba- Geigy Corp.

The trisaryl phosphite described above is converted to the corresponding trisaryl phosphate by the degradative effects of thermal aging on a halogenated polycarbonate with which the phosphite is admixed. The trisaryl phosphate can be represented by a structure such as

where X is as set forth above.

The compositions of this invention are substantially free of any color stabilizing additives other than trisaryl phosphite, but they can further contain conventional thermoplastic polymer additives such as fillers, dyes, flame retarding agents, reinforcing agents, softeners, mold-release agents, seed-forming agents, pigments, plasticizers, antistatic agents, UV absorbers, lubricants, in conventional amounts generally not exceeding 25 percent by weight of the total composition.

Illustrative Embodiments. To illustrate the practice of this invention, examples of preferred embodiments thereof are set forth below. It is not intended, however, that these examples (Examples 1-4) should in any manner limit the scope of this invention. Some of the particularly desirable features of this invention may be seen by contrasting the characteristics of Examples 1-4 with those of various controlled formulations (Controls A-H) which do not possess the features of, and are not therefore embodiments of, this invention.

Controls A-H and Examples 1-4 were prepared by admixing various amounts of tris (2,4-di-t-butylphenyl) phosphite with a polycarbonate. In Controls A-F, Bisphenol-A homopolycarbonate was used. In Controls G-H and Examples 1-4, brominated segmented block co- polycarbonate made from Bisphenol-A and TBBA by the processes disclosed in U.S. Patent Number 4,902,758 was used. Controls G-H and Examples 1-4 each has a Tg of at least 175°C and each contains 25 percent bromine by weight. The amount of tris (2,4-di-t-butylphenyl) phosphite contained in Controls A-H and Examples 1-4 is shown in Tables I-IV below in ppm by total weight of those compositions, respectively.

Flakes of the Bisphenol-A polycarbonate, in the case of Controls B-F, and flakes of the brominated segmented block co-polycarbonate in the case of Control H and Examples 1-4, were tumble blended with epoxidized soybean oil as a tackifier for the trisaryl phosphite additive. Flakes of the Bisphenol-A polycarbonate, in the case of Controls A-F, and flakes of the brominated segmented block co-polycarbonate in the case of Controls G-H and Examples 1-4, were then tumble blended with tris (2,4-di-t-butylphenyl) phosphite, and the formulation was then processed on a 1 " single-screw Killian extruder at 575-620°F (302-327°C) to intimately mix the materials. After extrusion, each formulation was injection molded on an Arburg 70 under the following conditions: screw speed: 300 rpm; feed temperature: 300°C; barrel temperature: 300°C; nozzle temperature: 300°C; injection pressure: 45 bar; back pressure: 45 bar; holding pressure: 45 bar; and mold temperature: 175-200°F (79-93°C). The mold was stainless steel and produces discs of 6.2 cm diameter and 0.3 cm thickness.

The discs were placed in a forced-air oven for continuous thermal aging at 130°C, and all measurements were repeated after varying lengths of time with the results as also shown in Tables I-IV.

The Yellowness Index of a disc molded from each formulation was measured according to ASTM Designation D 1925-70 using a Hunter Lab Colorimeter Model D25-9. The weight average molecular weight of each disc was measured using GPC. Trisaryl phosphite and trisaryl phosphate content was measured for Controls G-H and

Examples 1-4 using reverse phase liquid chromatography.

Table I Trisaryl Phosphite Content and Yellowness Index of Bisphenol-Λ Polycarbonate Samples (Controls A-F)

Table II Trisaryl Phosphite Content and Yellowness Index of Brominated Polycarbonate Samples (Controls G-H and Examples 1-4)

Table III Trisaryl Phosphite Content and Molecular Weight of Bisphenol-A Polycarbonate Samples (Controls A-F)

Table IV Trisaryl Phosphite Content and Molecular Weight of Brominated Polycarbonate Samples (Controls G-H and Examples 1-4)

The results of the Yellowness Index and molecular weight tests show that when a trisaryl phosphite [such as tris (2,4-di-t-butylphenyl) phosphite] is admixed in an amount of 200 ppm to 1,500 ppm, by weight of the total composition formed therefrom, with halogenated polycarbonate (such as brominated segmented block co-polycarbonate), the color of an article molded from such composition is stabilized against the degradative effects of thermal aging. However, these results also show that when a trisaryl phosphite is used in an amount less than 200 ppm or exceeding 1,500 ppm to stabilize a brominated polycarbonate, the discoloration effects of thermal degradation are not reduced to the desired levels, and the presence of the trisaryl phosphite in an amount greater than 1,500 ppm causes a loss in weight average molecular weight.

As a basis for comparison, the data reported above show that when a trisaryl phosphite is used in an amount in the zero to 2,000 ppm range to stabilize a non-brominated polycarbonate, such as a Bisphenol-A polycarbonate, the level of degradation, as shown by Yellowness Index or molecular weight, varies with the level of stabilizer over an insignificant range. For example, the discs molded from Controls A-F (Bisphenol-A polycarbonate) has Yellowness Index readings after 1,512 hours of aging varying only from 7.9 (0 ppm trisaryl phosphite) to 6.9 (2,000 ppm trisaryl phosphite). Similarly, Control F, which contain 2,000 ppm of trisaryl phosphite, show no decrease in molecular weight after 1,512 hours of aging. The limited range of the effect of a trisaryl phosphite on Bisphenol-A polycarbonate is also shown in the tight cluster of plot lines in Figures 1 and 4.

The slight variation in the extent of control of Yellowness Index, and the absence of molecular weight loss at the 2,000 ppm level, resulting from the use of a trisaryl phosphite to stabilize Bisphenol-A polycarbonate, does not suggest the criticality of the amount of trisaryl phosphite needed to properly stabilize halogenated polycarbonate. Note in Figure 2 the sharp break, at 0 ppm and at 2,000 ppm, in the line representing for the brominated polycarbonate the Yellowness Index readings at 1,512 hours of aging. The line representing the Yellowness Index readings for Bisphenol-A polycarbonate in Figure 3, however, is nearly flat. Correspondingly, the sharp drop in molecular weight for the brominated polycarbonate, when trisaryl phosphite is used at the 2,000 level, as shown in Figure 7, can be compared with the nearly flat line for the molecular weight of Bisphenol-A polycarbonate shown in Figure 6.

When the polycarbonate to be stabilized by trisaryl phosphite is halogenated the amount of triaryl phosphite used does make a significant difference. For example, the discs molded from Controls G-H (brominated polycarbonate) showed a Yellowness Index, and Control H showed a molecular weight, which is noteably less desirable than the respective readings for the discs molded from Examples 1-4. Control G (0 ppm trisaryl phosphite) and Control H (2,000 ppm trisaryl phosphite) fell outside the critical range of 200-1,500 ppm trisaryl phosphite for brominated polycarbonates and therefore have undesirably high Yellowness Index readings at 1,512 hours of 14.6 and 37.0, respectively. Examples 1-4, on the contrary, had Yellowness Index readings at 1,512 hours which varied only from 9.4 (200 ppm) to 12.2 (1,500 ppm). The Yellowness Index reading for Control H was 3 times that for Example 4 at 1,512 hours. This variance in Yellowness Index readings according to amount of trisaryl phosphite used can be observed in Figure 2. Mote in Figure 2 the tight cluster of the plot lines for Examples 1-4 (200-1,500 ppm trisaryl phosphite, respectively) as compared to the divergence of the plot lines representing 0 and 2,000 ppm trisaryl phosphite, respectively. The molecular weight readings for Control H and Examples 1-4 showed similar results, as can be seen from Figure 5.

It is apparent from this data that when trisaryl phosphite is used to stabilize halogenated polycarbonate against degradation from thermal aging, it is critical to use it in an amount ranging from 200 to 1,500 ppm, based on the total weight of the composition. The prior art dealing with use of a phosphite as a stabilizer for polycarbonate does not recognize the criticality of these limits, and use of a trisaryl phosphite as a stabilizer for halogenated polycarbonate over the entire ranges taught by the prior art will not achieve the reduced levels of the effects of thermal degradation attained herein.

It is within the skill in the art to practice this invention in numerous modifications and variations in light of the above teachings. It is, therefore, to be understood that the various embodiments of this invention described herein may be altered without departing from the spirit and scope of this invention as defined by the appended claims.

1. A composition of matter comprising in admixture, brominated polycarbonate and about 200 parts per million to about 1,500 parts per million, by weight of the total composition, of a phosphite represented by the structure

where X¹ to X¹⁵ is each independently hydrogen or an alkyl, aryl, alkaryl or aralkyl radical of one to ten carbon atoms.

2. The composition of Claim 1 wherein the brominated polycarbonate is a brominated segmented block co-polycarbonate.

Claims

3. The composition of Claim 1 wherein said phosphite is tris (2,4-di-t-butylphenyl) phosphite.

4. The composition of Claim 1 shaped in the form of a transparent- apparatus.

5. The composition of Claim 1 in the form of a molded article.

6. A film, sheet, article, fiber or other formed or molded object which has been thermally aged, comprising, in admixture, (a) brominated polycarbonate and (b) about 200 parts per million to about 1,500 parts per million, by weight of the total composition, of a mixture of (i) a phosphite represented by the structure

where X¹ to X¹⁵ is each independently hydrogen or an alkyl, aryl, alkaryl or aralkyl radical of one to ten carbon atoms, and (ii) a phosphate represented by the structure

where X¹ to X¹⁵ are as set forth above.

7. The film, sheet, article, fiber or other formed or molded object of Claim 6 wherein the brominated polycarbonate is a brominated segmented block co-polycarbonate.

8. The film, sheet, article, fiber or other formed or molded object of Claim 6 wherein said phosphite is tris (2,4-di-t-butylphenyl) phosphite, and said phosphate is tris (2,4-di-t-butylphenyl) phosphate.

9. The film, sheet, article, fiber or other formed or molded object of Claim 6 which has a Yellowness Index of less than 15, as measured according to ASTM Designation D 1925-70.

10. The film, sheet, article, fiber or other formed or molded object of Claim 6 which has a weight average molecular weight, as determined by gel permeation chromatography, of greater than 25,000. 11. The film, sheet, article, fiber or other formed or molded object of Claim 6 which is transparent.