Patent Publication Number: US-9897551-B2

Title: Method for accelerated degradation of thermoplastics

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit of and priority to U.S. application 61/928,519, filed Jan. 17, 2014. The present application is also a continuation-in-part of U.S. application Ser. No. 14/159,579, filed Jan. 23, 2014, which application claimed priority to U.S. application 61/755,637, filed Jan. 23, 2013. The foregoing applications and all of their related applications are all incorporated herein by reference in their entireties for any and all purposes. 
    
    
     TECHNICAL FIELD 
     This invention concerns a method for accelerating and determining the degradation of thermoplastics, such as the rate of discoloration of opaque, translucent and transparent polycarbonates, when subjected to heat and light. 
     BACKGROUND 
     Thermoplastics comprise a large family of polymers, most of which have a high molecular weight. Intermolecular forces are responsible for the association of the molecular chains, which allows thermoplastics to be heated and remolded. Thermoplastics become pliant and moldable at a temperature above their glass transition temperature but below their melting point, and the intermolecular forces reform after molding and upon cooling of the thermoplastic, resulting in the molded product having substantially the same physical properties as the material prior to molding. 
     Polycarbonates fall within the thermoplastic family and contain carbonate groups —O—(C═O)—O—. Polycarbonates find widespread use throughout industry due to their excellent strength and impact resistance. Additionally, polycarbonates may be readily machined, cold-formed, extruded, thermoformed and thermomolded. 
     Exposure of thermoplastics to light is known to induce changes to the polymer. In particular, the exposure of opaque, translucent and transparent polycarbonates to blue LED (light emitting diode) light is of interest for the manufacture of efficient illumination devices such as lamps and other types of lighting apparatuses. Transparent is defined as a light transmittance of at least 80% when tested in the form of a 3.2 mm thick test sample according to ASTM D1003-00 (2000) (hereby incorporated by reference). Translucent is defined as a light transmittance greater than or equal to 40% when tested in the form of a 2.5 mm thick test sample according to ASTM D1003-00 (2000). Opaque is defined as a light transmittance of 10% or greater when tested in the form of a 3.2 mm thick test sample according to ASTM D1003-00 (2000). The testing according to ASTM D1003-00 (2000) uses procedure A and CIE illuminant C and 2 degree observer on a CE7000A using an integrating sphere with 8°/diffuse geometry, specular component included, UV included, large lens, and large area view, with percentage transmittance value reported as Y (luminous transmittance) taken from the CIE 1931 tristimulus values XYZ. 
     Blue LED light having a peak intensity from about 400 nm to about 500 nm and an irradiance from about 3,500 W/m 2  to about 120,000 W/m 2  is of interest. Similarly, white LED light having a peak intensity from about 400 nm to about 500 nm and an irradiance less than 120,000 W/m 2  is also of interest. 
     Opaque and translucent polycarbonate may be formed, for example, using titanium dioxide compounded with the polycarbonate formulation. Furthermore, remote phosphors, also known as “luminescent conversion materials”, can be compounded into the polycarbonate. Examples of luminescent conversion materials include yttrium aluminum garnet (YAG) doped with rare earth elements, terbium aluminum garnet doped with rare earth elements, silicate (BOSE) doped with rare earth elements; nitrido silicates doped with rare earth elements; nitride orthosilicate doped with rare earth elements, and oxonitridoaluminosilicates doped with rare earth elements. Quantum dots comprising inorganic materials, usually cadmium based phosphorescent compounds may also be used to form opaque and translucent polycarbonates. 
     Translucent polycarbonates are formed using scattering agents such as light diffusers. The light diffusers often take the form of light diffusing particles and are used in the manufacture of articles that have good luminance. Such articles provide a high level of transmission of incident light (such as natural light through a window or skylight, or artificial light) with a minimum light loss by reflectance or scattering, where it is not desirable to either see the light source or other objects on the other side of the article. 
     An article e.g., a sheet having a high degree of hiding power (i.e., luminance) allows a significant amount of light through, but is sufficiently diffusive so that a light source or image is not discernible through the panel. Light diffusers can be (meth)acrylic-based and include poly(alkyl acrylate)s and poly(alkyl methacrylate)s. Examples include poly(alkylmethacrylates), specifically poly(methyl methacrylate) (PMMA). Poly(tetrafluoroethylene) (PTFE) can also be used. Light diffusers also include silicones such as poly(alkylsilsesquioxanes), for example poly(alkylsilsequioxane)s such as the poly(methylsilsesquioxane) available under the trade name TOSPEARL® from Momentive Performance Materials Inc. The alkyl groups in the poly(alkyl acrylate)s, poly(alkylmethacrylate)s and poly(alkylsilsequioxane)s can contain one to about twelve carbon atoms. Light diffusers can also be cross-linked. For example, PMMA can be crosslinked with another copolymer such as polystyrene or ethylene glycol dimethacrylate. In a specific embodiment, the polycarbonate composition comprises a light diffusing crosslinked poly(methyl methacrylate), poly(tetrafluoroethylene), poly(methylsilsesquioxane), or a combination comprising at least one of the foregoing. Cyclic olefin polymers and cyclic olefin co-polymers can also be used to create diffusers. 
     Light diffusers also include certain inorganic materials, such as materials containing antimony, titanium, barium, and zinc, for example the oxides or sulfides of antimony, titanium, barium and zinc, or a combination containing at least one of the forgoing. As the diffusing effect is dependent on the interfacial area between polymer matrix and the light diffuser, in particular the light diffusing particles, the particle size of the diffusers can be less than or equal to 10 micrometers (μm). For example, the particle size of poly(alkylsilsequioxane)s such as poly(methylsilsesquioxane) can be 1.6 μm to 2.0 μm, and the particle size of crosslinked PMMA can be 3 μm to 6 μm. Light diffusing particles can be present in the polycarbonate composition in an amount of 0 to 1.5%, specifically 0.001 to 1.5%, more specifically 0.2% to about 0.8% by weight based on the total weight of the composition. For example, poly(alkylsilsequioxane)s can be present in an amount of 0 to 1.5 wt. % based on the total weight of the composition, and crosslinked PMMA can be present in an amount of 0 to 1.5 wt. % based on the total weight of the composition. 
     While the physical characteristics of strength and impact resistance make polycarbonates desirable for use as covers and lenses in LED lighting, exposure of the polycarbonate to the blue light of the LEDs (as well as the light of organic LEDs) causes degradation of the polycarbonate in the form of discoloration. For example, transparent polycarbonate is known to turn yellow, even darkening to brown, with exposure to blue light. This degradation of the transparent polycarbonate is unacceptable because the yellowed polycarbonate absorbs the light thereby reducing the efficiency of the lamp. Furthermore, the yellowing changes the color of the light emanating from the lamp, which is also unacceptable. In addition, the transparent polycarbonate is also subject to elevated temperatures when it comprises part of an LED lamp. The elevated temperatures likely play a role in the yellowing of the transparent polycarbonate. 
     There is clearly a need for a method to determine the degradation of thermoplastic formulations, in particular, the rate of discoloration of opaque, translucent and transparent polycarbonate formulations when exposed to light, and thereby be able to evaluate and compare the different formulations as to their suitability for use in LED lamps. It is desirable that the method provide an accelerated test of the thermoplastic which is not too slow so as to be impractical, and not too fast, so as to destroy the samples before meaningful comparisons can be made between formulations. Such a method will permit judicious selection of polycarbonate grades for their suitability to LED lighting applications. 
     SUMMARY 
     The invention concerns a method for determining degradation of a thermoplastic, such as transparent polycarbonate, and includes at least the following embodiments. 
     Example Embodiment 1 
     A method for determining degradation of a thermoplastic, the method comprising: 
     illuminating the thermoplastic, for a period of time, with light having a peak intensity centered at a wavelength from about 400 nm to about 500 nm and an irradiance from about 400 W/m 2  to about 150,000 W/m 2 ; 
     maintaining the average temperature of the thermoplastic at a temperature from about 23° C. to about 175° C. during the period of time; 
     repeating the illuminating and maintaining steps for a plurality of successive periods of time. 
     Example Embodiment 2 
     The method of embodiment 1, further comprising measuring the temperatures of at least two samples of the thermoplastic and calculating an average temperature of the thermoplastic using the temperatures of the at least two samples of the thermoplastic. 
     Example Embodiment 3 
     The method of embodiments 1 or 2, further comprising evaluating the degradation of the thermoplastic after each successive period of time. 
     Example Embodiment 4 
     The method of any of embodiments 1-3, further comprising: 
     repeating the illuminating, maintaining and evaluating steps for a plurality of successive time periods for a plurality of different thermoplastic formulations; and 
     comparing the degradation of the plurality of different thermoplastic formulations with one another. 
     Example Embodiment 5 
     The method of any of embodiments 1-4, wherein the degradation comprises a discoloration rate of the thermoplastic. 
     Example Embodiment 6 
     The method of any of embodiments 1-5, wherein evaluating the degradation comprises visually inspecting the thermoplastic. 
     Example Embodiment 7 
     The method of any of embodiments 1-6, wherein evaluating the degradation comprises measuring a degree of discoloration of the thermoplastic. 
     Example Embodiment 8 
     The method of any of embodiments 1-7, wherein the irradiance is from about 50,000 W/m 2  to about 150,000 W/m 2 . 
     Example Embodiment 9 
     The method of any of embodiments 1-8, wherein the irradiance is from about 400 W/m 2  to about 50,000 W/m 2 . 
     Example Embodiment 10 
     The method of any of embodiments 1-9, wherein the peak intensity of the light is centered at a wavelength from about 410 nm to about 480 nm. 
     Example Embodiment 11 
     The method of any of embodiments 1-10, wherein the peak intensity of the light is centered at a wavelength of about 447 nm. 
     Example Embodiment 12 
     The method of any of embodiments 1-11, wherein the average temperature of the thermoplastic is maintained at a temperature from about 90° C. to about 130° C. 
     Example Embodiment 13 
     The method of any of embodiments 1-12, wherein the average temperature of the thermoplastic is maintained at a temperature of about 120° C. 
     Example Embodiment 14 
     The method of any of embodiments 1-13, further comprising cooling the thermoplastic. 
     Example Embodiment 15 
     The method of any of embodiments 1-14, wherein measuring the degree of discoloration of the thermoplastic comprises: 
     illuminating the thermoplastic with white light, a portion of the white light being transmitted through the thermoplastic; and 
     generating a transmission spectrum from the portion of the white light transmitted through the thermoplastic. 
     Example Embodiment 16 
     The method of any of embodiments 1-15, wherein measuring the degree of discoloration of the thermoplastic comprises: 
     illuminating the thermoplastic with white light, a portion of the white light being reflected from the thermoplastic; and 
     generating a reflectance spectrum of the portion of the white light reflected from the thermoplastic. 
     Example Embodiment 17 
     The method of any of embodiments 1-16, further including comparing the degree of discoloration measured after each of the successive periods of time with one another. 
     Example Embodiment 18 
     The method of any of embodiments 1-17, wherein each of the successive periods of time is equal in duration. 
     Example Embodiment 19 
     The method of any of embodiments 1-18, wherein each of the successive periods of time is about 100 hours in duration. 
     Example Embodiment 20 
     The method of any of embodiments 1-19, wherein said thermoplastic is a polycarbonate. 
     Example Embodiment 21 
     The method of any of embodiments 1-20, wherein the thermoplastic may be transparent, translucent or opaque thermoplastics. 
     Example Embodiment 22 
     A method for determining a discoloration rate of a polycarbonate, the method comprising: 
     illuminating the polycarbonate with light having a peak intensity centered at a wavelength from about 400 nm to about 500 nm and an irradiance from about 400 W/m 2  to about 150,000 W/m 2  for a first period of time; and 
     maintaining the average temperature of the polycarbonate at a temperature from about 23° C. to about 175° C. during the first period of time. 
     Example Embodiment 23 
     The method of embodiment 22, further comprising measuring the temperatures of at least two samples of the polycarbonate and calculating an average temperature of the thermoplastic using the temperatures of the at least two samples of the polycarbonate; 
     Example Embodiment 22 
     The method of either of embodiments 22-23, further comprising evaluating a degree of discoloration of the polycarbonate after the first period of time has elapsed. 
     Example Embodiment 24 
     The method of any of embodiments 21-23, further comprising: 
     repeating said illuminating, maintaining and measuring steps for a plurality of different polycarbonate formulations; and 
     comparing the discoloration of said plurality of different polycarbonate formulations with one another. 
     Example Embodiment 25 
     The method of any of embodiments 21-24, wherein the irradiance is from about 400 W/m 2  to about 50,000 W/m 2 . 
     Example Embodiment 26 
     The method of any of embodiments 21-25, wherein the irradiance is from about 50,000 W/m 2  to about 150,000 W/m 2 . 
     Example Embodiment 27 
     The method of any of embodiments 21-26, wherein the peak intensity of the light is centered at a wavelength from about 410 nm to about 480 nm. 
     Example Embodiment 28 
     The method of any of embodiments 21-27, wherein the peak intensity of the light is centered at a wavelength of about 447 nm. 
     Example Embodiment 29 
     The method of any of embodiments 21-28, wherein the average temperature of the polycarbonate is maintained at a temperature from about 23° C. to about 130° C. 
     Example Embodiment 30 
     The method of any of embodiments 21-29, wherein the average temperature of the polycarbonate is maintained at a temperature of about 90° C. 
     Example Embodiment 31 
     The method of any of embodiments 21-30, further comprising cooling the polycarbonate. 
     Example Embodiment 32 
     The method of any of embodiments 21-31, wherein evaluating said degradation comprises visually inspecting the polycarbonate. 
     Example Embodiment 33 
     The method of any of embodiments 21-32, wherein evaluating the degradation comprises measuring a degree of discoloration of the polycarbonate. 
     Example Embodiment 34 
     The method of any of embodiments 21-33, wherein measuring the degree of discoloration of the polycarbonate comprises: 
     illuminating the polycarbonate with white light, a portion of the white light being transmitted through the polycarbonate; and 
     generating a transmission spectrum from the portion of the white light transmitted through the polycarbonate. 
     Example Embodiment 35 
     The method of any of embodiments 21-34, wherein measuring the degree of discoloration of the transparent polycarbonate comprises: 
     illuminating the transparent polycarbonate with white light, a portion of the white light being reflected from the transparent polycarbonate; and 
     generating a reflectance spectrum of the portion of the white light reflected from the transparent polycarbonate. 
     Example Embodiment 36 
     The method of any of embodiments 21-35, further comprising: 
     repeating the illuminating and maintaining steps for a second period of time; 
     after the second period of time has elapsed, measuring a degree of discoloration of the transparent polycarbonate. 
     Example Embodiment 37 
     The method of any of embodiments 21-36, further comprising comparing the degree of discoloration measured after the first period of time with the degree of discoloration after the second period of time. 
     Example Embodiment 38 
     The method of either of embodiments 36 and 37, wherein the second period of time equals the first period of time in duration. 
     Example Embodiment 39 
     The method of any of embodiments 21-38, wherein the first period of time is about 100 hours in duration. 
     Example Embodiment 40 
     The method of any of embodiments 21-39, wherein the polycarbonate may be a transparent, translucent or opaque thermoplastic. 
     Example Embodiment 41 
     A method for determining a discoloration rate of a transparent polycarbonate, the method comprising: 
     illuminating the transparent polycarbonate with light having a peak intensity centered at a wavelength from about 400 nm to about 500 nm and an irradiance from about 50,000 W/m 2  to about 150,000 W/m 2  for a period of time; 
     maintaining the average temperature of the transparent polycarbonate at a temperature from about 23° C. to about 175° C. during the first period of time; 
     repeating the illuminating and maintaining steps for a plurality of successive periods of time; and 
     after each the successive period of time, measuring a degree of discoloration of the transparent polycarbonate. 
     Example Embodiment 42 
     The method of example embodiment 41, further comprising measuring temperatures of at least two samples of the transparent polycarbonate; and calculating an average temperature of the transparent polycarbonate using the temperatures of the at least two samples of the transparent polycarbonate; 
     Example Embodiment 43 
     The method of either of embodiments 41-42, further comprising evaluating a degree of discoloration of the transparent polycarbonate after each successive period of time. 
     Example Embodiment 44 
     The method of any of embodiments 41-43, further comprising: 
     repeating the illuminating, maintaining and measuring steps for a plurality of successive time periods for a plurality of different transparent polycarbonate formulations; and 
     comparing the discoloration of the plurality of different transparent polycarbonate formulations with one another. 
     Example Embodiment 45 
     The method of any of embodiments 41-44, wherein all of the periods of time are equal to one another in duration. 
     Example Embodiment 46 
     The method of any of embodiments 41-45, wherein the first period of time is about 100 hours. 
     Example Embodiment 47 
     The method of any of embodiments 41-46, further comprising comparing the degree of discoloration measured after each the period of time with one another. 
     Example Embodiment 48 
     The method of any of embodiments 41-47, wherein the irradiance is from about 50,000 W/m 2  to about 110,000 W/m 2 . 
     Example Embodiment 49 
     The method of any of embodiments 41-48, wherein the irradiance is about 75,000 W/m 2 . 
     Example Embodiment 50 
     The method of any of embodiments 41-49, wherein the peak intensity of the light is centered at a wavelength from about 410 nm to about 480 nm. 
     Example Embodiment 51 
     The method of any of embodiments 41-50, wherein the peak intensity of the light is centered at a wavelength of about 447 nm. 
     Example Embodiment 52 
     The method of any of embodiments 41-51, wherein the average temperature of said transparent polycarbonate is maintained at a temperature from about 90° C. to about 130° C. 
     Example Embodiment 53 
     The method of any of embodiments 41-52, wherein the average temperature of said transparent polycarbonate is maintained at a temperature of about 120° C. 
     Example Embodiment 54 
     The method of any of embodiments 41-53, further comprising cooling the transparent polycarbonate. 
     Example Embodiment 55 
     The method of any of embodiments 41-54, wherein evaluating the degradation comprises visually inspecting the polycarbonate. 
     Example Embodiment 56 
     The method of any of embodiments 41-55, wherein evaluating the degradation comprises measuring a degree of discoloration of the polycarbonate. 
     Example Embodiment 57 
     The method of any of embodiments 41-56, wherein measuring the degree of discoloration of the transparent polycarbonate comprises: 
     illuminating the transparent polycarbonate with white light, a portion of the white light being transmitted through the transparent polycarbonate; and 
     generating a transmission spectrum from the portion of the white light transmitted through the transparent polycarbonate. 
     Example Embodiment 58 
     The method of any of embodiments 41-57, wherein measuring the degree of discoloration of the transparent polycarbonate comprises: 
     illuminating the transparent polycarbonate with white light, a portion of the white light being reflected from the transparent polycarbonate; and 
     generating a reflectance spectrum of the portion of the white light reflected from the transparent polycarbonate. 
     Example Embodiment 59 
     The method according to any of embodiments 1-19, wherein the thermoplastic is translucent. 
     Example Embodiment 60 
     The method according to any of embodiments 1-19, wherein the thermoplastic is opaque. 
     Example Embodiment 61 
     The method according to any of embodiments 1-19, wherein the thermoplastic comprises polyethylene terephthalate (PET). 
     Example Embodiment 62 
     The method according to any of embodiments 1-19, wherein the thermoplastic comprises polybutylene terephthalate (PBT). 
     Example Embodiment 63 
     The method according to any of embodiments 1-19, wherein the thermoplastic comprises polyethylene napthalate (PEN). 
     Example Embodiment 64 
     The method according to any of embodiments 1-19, wherein the thermoplastic comprises polymethyl methacrylate (PMMA). 
     Example Embodiment 65 
     The method according to any of embodiments 1-19, wherein the thermoplastic comprises polystyrene (PS). 
     Example Embodiment 66 
     The method according to any of embodiments 1-19, wherein the thermoplastic comprises cyclic olefinic polymers (COP). 
     Example Embodiment 67 
     The method according to any of embodiments 1-19, wherein the thermoplastic comprises cyclic olefinic copolymers (COC). 
     Example Embodiment 68 
     The method according to any of embodiments 1-19, wherein the thermoplastic comprises polyetherimide. 
     Example Embodiment 69 
     The method according to any of embodiments 1-19, wherein the thermoplastic comprises polycarbonate. 
     Example Embodiment 70 
     The method according to any of embodiments 1-19, wherein the thermoplastic comprises polyester blends. 
     Example Embodiment 71 
     The method according to any of embodiments 1-19, wherein the thermoplastic comprises polycarbonate/polyester blends. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1 a    is an illustrative, schematic diagram illustrating an example device for executing a method for determining degradation of a thermoplastic, such as an opaque, translucent or transparent polycarbonate; 
         FIG. 1 b    is an illustrative, schematic diagram illustrating an example device for executing a method for determining degradation of a thermoplastic, such as an opaque, translucent or transparent polycarbonate; 
         FIG. 2  is a flow chart illustrating an example method for determining degradation of thermoplastic according to the invention; and 
         FIGS. 3 and 4  are flow charts that illustrate exemplary methods for measuring a degree of discoloration of thermoplastic samples. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     The present invention may be understood more readily by reference to the following detailed description taken in connection with the accompanying figures and examples, which form a part of this disclosure. It is to be understood that this invention is not limited to the specific devices, methods, applications, conditions or parameters described and/or shown herein, and that the terminology used herein is for the purpose of describing particular embodiments by way of example only and is not intended to be limiting of the claimed invention. Also, as used in the specification including the appended claims, the singular forms “a,” “an,” and “the” include the plural, and reference to a particular numerical value includes at least that particular value, unless the context clearly dictates otherwise. The term “plurality”, as used herein, means more than one. When a range of values is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment. All ranges are inclusive and combinable. 
     It is to be appreciated that certain features of the invention which are, for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any subcombination. Further, reference to values stated in ranges include each and every value within that range. 
     The summary, as well as the following detailed description, is further understood when read in conjunction with the appended drawings. For the purpose of illustrating the invention, there are shown in the drawings exemplary embodiments of the invention; however, the invention is not limited to the specific methods, compositions, and devices disclosed. In addition, the drawings are not necessarily drawn to scale. In the drawings: 
       FIG. 1 a    shows an exemplary apparatus  10  which is useful for determining degradation of thermoplastics, in particular, for characterizing and comparing the rate of discoloration of opaque, translucent and transparent polycarbonates. Apparatus  10  comprises an oven chamber  12  (a chamber may be open or closed to the exterior environment during operation) in which a plurality of thermoplastic samples are positioned for testing. Three samples,  14 ,  16  and  18 , each formed of a sample pair, each sample pair comprising two sample elements ( 14   a ,  14   b ,  16   a ,  16   b , and  18   a ,  18   b  respectively), are shown by way of example, it being understood that more or fewer samples can be tested by the method according to the invention. 
     Each sample pair  14   a  and  14   b ,  16   a  and  16   b ,  18   a  and  18   b  is illuminated by a respective light source  20 ,  22 ,  24  comprising, for example, LEDs  26  whose light emissions  28  are transmitted using silica glass waveguides  30 . The LEDs and the waveguides work together so as to illuminate the samples with light spread over a desired range of wavelengths and intensities according to the parameters of the method described below. Each sample is instrumented with a respective thermometric transducer  32 , for example, a thermocouple, for measuring the temperature of each sample. Transducers  32  generate electrical signals, for example, in the form of voltages, which are indicative of the temperature of each sample. Each sample pair may be heated by a respective heating element  34 ,  36  and  38  for example, an electrical resistance heater. 
     The light sources  20 ,  22  and  24 , and heating elements  34 ,  36  and  38  are independently controlled by a controller  40 . Controller  40  may be, for example, a programmable logic controller or a computer with resident software and firmware. Controller  40  receives, manipulates and interprets temperature signals from the thermometric transducers  32  and, using this information in a feedback loop, controls the operation of the light sources  20 ,  22  and  24  (LEDs  26 ) and the heating elements  34 ,  36  and  38  according to the algorithms provided by its resident software. Controller  40  may also record and log data from the test. Communication between the various components and the controller  40  is effected in this example over dedicated communication lines  42 . 
     In the example method according to the invention, the samples are divided into sample pairs  14   a ,  14   b ,  16   a ,  16   b  and  18   a ,  18   b , each having a respective illumination source  20 ,  22  and  24  and a respective heater  34 ,  36  and  38  because the method uses the average temperature derived from at least two sample elements comprising the sample pair to control operation of the respective heating elements  34 ,  36  and  38  for each sample pair so as to maintain the samples at a desired temperature as specified for a particular test protocol. Optionally, each sample pair may be tested at a different temperature and irradiance from a neighboring sample. Apparatus  10  can thus test samples of different materials, wherein each sample element in a sample pair is comprised of the same material, i.e., sample elements  14   a  and  14   b  are the same material, sample elements  16   a  and  16   b  are the same material (but not necessarily the same as sample elements  14   a  and  14   b ). Controller  40  calculates the average temperatures of the sample pairs and uses this information to control the heating elements  34 ,  36  and  38 . The samples are cooled by natural circulation of air within the oven chamber  12 . Fans or may also be used for a forced circulation regime if necessary. 
       FIG. 1 b    shows an alternative example apparatus  10  which is useful for determining degradation of thermoplastics, in particular, for characterizing and comparing the rate of discoloration of opaque, translucent and transparent polycarbonates. Apparatus  10  comprises an oven chamber  12  in which a heating element  14  is positioned. The heating element  14  establishes and maintains the ambient air within the chamber  12  at a desired temperature appropriate for a particular test protocol (e.g., the temperature necessary to heat the sample to desired temperature). In a practical example, the heating element  14  may be an electrical resistance heater. At least one light source  16  is also positioned within chamber  12 , the light source  16  being selected to emit light over a desired frequency range appropriate to the test requirements. In a particular example, the light source  16  may comprise one or more LEDs  18  whose light is channeled by a silica glass waveguide  20  so as to impinge upon a thermoplastic sample  22  positioned within the chamber  12  for testing. 
     Temperature of the ambient air within the chamber  12  is monitored by a temperature measuring device  24  positioned within the chamber, for example, a thermometer or a thermocouple, which generates electrical signals indicative of the air temperature within the chamber. Another temperature measuring device  26  (e.g., a thermocouple) is mounted on the thermoplastic sample  22  and used to monitor the sample temperature, which may be in some cases be higher than the air temperature due to the light impinging on the sample from the light source  16 . Signals from the temperature measuring devices  24  and  26  are supplied to a controller  28 , which uses the temperature information to control the heating element  14  and the light source  16  during testing of the sample  22 . Controller  28  may be, for example, a programmable logic controller or a computer with resident software, which, in addition to receiving and interpreting the temperature signals and controlling the operation of the light source  16  and the heating element  14 , may also record and log data from the test. Communication between the various components and the controller  28  is effected in this example over dedicated communication lines  30 . 
       FIG. 2  is a flow chart that outlines an example method of determining degradation of opaque, translucent and transparent thermoplastics according to the invention. In this example, each thermoplastic sample, such as a transparent polycarbonate of a first formulation, may be a single sample or even be provided as multiple samples—e.g., a sample pair—comprising two or more sample elements. (Herein, the terms sample, sample pair and sample elements refer to a sample material being tested.) A sample or sample pair may be illuminated with light for a desired period of time as noted at box  44 . When determining discoloration of a transparent polycarbonate for use with LED lamps for example, it has been found advantageous to use illuminating light having its peak intensity centered at a wavelength from about 400 nm to about 500 nm and having an irradiance from about 400 W/m 2  to about 150,000 W/m 2  (calibration for all irradiance values via a power meter such as the Model UP 55N 40S-H9 marketed by GENTEC-EO USA Inc. of Lake Oswego, Oreg.). 
     As noted in box  46 , during illumination, the temperature of each sample or sample pair may be maintained at a respective desired temperature, for example, from about 23° C. to about 175° C. for the desired period of time. (The maximum sample temperature is generally limited by the glass transition temperature or the melting point of the material being tested.) The temperature is maintained by measuring the temperatures of one or more sample elements (e.g., two elements), calculating the average temperature, and using that average temperature to control heating and cooling. The data derived from this test can be used to determine a rate of degradation of the samples, as well as for comparative purposes with other samples having different formulations. For transparent materials, irradiance from about 50,000 W/m 2  to about 150,000 W/m 2 , as well as 50,000 W/m 2  to about 110,000 W/m 2  is also believed to be useful, as is an irradiance of about 75,000 W/m 2 . Sample temperatures from about 90° C. to about 130° C. are also believed to be useful, as are sample temperatures of about 120° C. as well as 130° C. For opaque materials, irradiance from about 400 W/m 2  to about 50,000 W/m 2  are considered useful, as is an irradiance of about 3,500 W/m 2 . Sample temperatures from about 23° C. to about 130° C. are also believed to be useful, as are sample temperatures of about 90° C. Additionally, illuminating light wavelengths wherein the peak intensity of the light is centered from about 410 nm to about 480 nm (measured radiometrically) are considered useful, as is illuminating light having its peak intensity centered at about 447 nm (measured radiometrically). These parameters are expected to allow for an illumination time period of as much as 100 hours, resulting in a measurable discoloration of the transparent polycarbonate without destroying the samples. It is further expected that illuminating light having its peak intensity centered at about 459 nm (measured radiometrically) will be useful as well as illuminating light emitted from an LED source and centered at about 470 nm dominant wavelength (measured photometrically) and about 550 nm measured radiometrically. 
     It may in some cases be desirable to prevent heat build-up in the samples. Heat build-up may be avoided by cooling the samples during illumination. Cooling of each sample, as noted in box  48 , may be accomplished, for example, by providing a separation distance between the sample and the light source sufficient to permit circulation of the ambient air around the sample and thereby allow convective cooling. Other methods of cooling, such as forced air cooling using a fan or even using refrigeration methods (with or without a fan) are also suitable. 
     The step of evaluating the degradation of the sample is noted in box  50 . This evaluation step may be accomplished, for example, by a simple visual observation of the samples, or photographs of the samples. Measurement techniques are also useful, as explained in detail below with reference to  FIGS. 3 and 4 . 
     As described in box  52 , the illuminating, maintaining and evaluating steps may be repeated for a plurality of successive time periods to provide, for example, the rate of discoloration of a polycarbonate sample as a function of time over which it is illuminated. This may show, for example, whether the rate of discoloration increases, decreases, or remains the same over time as it is exposed to the light. The successive time periods may be equal to one another. 
     As noted in boxes  54  and  56 , the method comprising the illuminating, maintaining and evaluating steps may be repeated for a plurality of successive time periods for a plurality of different thermoplastic samples having different formulations. This permits comparative evaluation between different polycarbonate formulations to determine their relative suitability for various applications, such as for use with LED lamps and luminaries (full lighting fixtures) as noted below. 
     As shown in boxes  58  and  60  of  FIG. 3 , measurement of the transparent thermoplastic degradation, in this example discoloration of the transparent polycarbonate, is effected by illuminating the samples with white light after the desired time period has elapsed, and generating a transmission spectrum from a portion of the white light transmitted through the sample. In an alternate method outlined in boxes  62  and  64  of  FIG. 4 , the samples are illuminated with white light after the desired time period has elapsed and a reflectance spectrum is generated from the portion of the white light reflected from the sample. Either spectrum is expected to provide a useful measure of the discoloration of a transparent thermoplastic such as polycarbonate. The measurement method shown in  FIG. 4  could also be effectively applied to opaque and translucent thermoplastics. 
     A quantitative evaluation of the discoloration may be determined using either the Yellowness Index or the CIE L*a*b color system. The Yellowness Index is governed by two standards, ASTM E313 (2010) for opaque materials, and ASTM D1925 (1995), for transparent materials. For opaque materials, the Yellowness Index YI is calculated according to the formula: 
     
       
         
           
             YI 
             = 
             
               100 
               ⁢ 
               
                 ( 
                 
                   
                     0.847 
                     ⁢ 
                     Z 
                   
                   Y 
                 
                 ) 
               
             
           
         
       
     
     and for transparent materials the Yellowness index is calculated according to the formula: 
             YI   =     100   ·     (         1.28   ·   X     -     1.06   ·   Z       Y     )             
wherein X, Y and Z are tristimulus values for CIE illuminant C and the 1931 CIE 2° standard observer. Numerical values for a particular sample are obtained using a commercially available spectrophotometer, such as the X-Rite i7 Spectrophotometer marketed by X-Rite of Grand Rapids Mich.
 
     The example method outlined in  FIGS. 2-4  may be used to gather data on a plurality of different polycarbonate formulations and then compare the discoloration rates of the various different formulations to determine which might be suitable for use in LED lighting, those sample polycarbonate formulations having the slowest discoloration rate being advantageous. Testing methods using the parameters outlined herein have permitted illumination time periods of 100 hours for transparent materials and have been used to evaluate numerous different samples to determine and compare their discoloration rates. 
     Various polycarbonate formulations to which the example method may be applied are described below. 
     Definitions 
     Ranges articulated within this disclosure, e.g. numerics/values, shall include disclosure for possession purposes and claim purposes of the individual points within the range, sub-ranges, and combinations thereof. 
     Various combinations of elements of this disclosure are encompassed by this invention, e.g. combinations of elements from dependent claims that depend upon the same independent claim. 
     The word “about” should be given its ordinary and accustomed meaning and should be relative to the word or phrase(s) that it modifies. In the context of pKa, the word “about” as it pertains to pKa can equal the value of the numeric or can equal in the range of +/−0.1 of the pKa unit, e.g. pKa of about 8.3 can include 8.2, and the word “about” as it pertains to branching level can equal the value of the numeric or can equal in the range +/−0.05% of the branching level, e.g. about 1% can encompass 0.95%. The delineation of the word about in the context pKa and branching level should not in any way limit the ordinary and accustomed meaning of the word “about” for other language/numerics that the word “about” modifies. 
     The pKa values used in the model for the end-capping agents are listed in Table 1 below: 
                                 TABLE 1                       End-capping agent   pKa*                                                    p-cyanophenol   8.2           p-methyl-hydroxy benzoate   8.4           Phenol   9.9           p-t-butylphenol   10.2           p-methoxyphenol   10.4           p-cumylphenol   10.5                       *pKa values for all of the end-capping agents but p-t-butyl phenol and p-cumylphenol were obtained from the following reference: J. AM. CHEM. SOC. 2002, 6424. The values chosen in the reference were listed in the S7 category in Table 3 of the reference. The pKa value for p-t-butylphenol was obtained from the following reference: Journal of Molecular Structure: THEOCHEM 805, 2006, 31. The pKa for methyl-p-hydroxybenzoate was obtained from the following reference: Chromatographia Vol. 39, No. 5/6, September 1994. The pKa value for p-cumylphenol was approximated based on the values of similar structures.            
Polycarbonate Material/Structural Backbone of the Composition
 
     In one embodiment, the plastic material of the plastic composition can comprise a polycarbonate. Descriptions of the various types of polycarbonates are articulated below, but should not be construed as limiting. 
     Various types of polycarbonates that have a repeating structural background of the following formula: 
                         
can be utilized.
 
     The selection of a polycarbonate backbone of choice depends on many factors such as end use and other factors understood by one of ordinary skill the art. 
     In one embodiment, the polycarbonates have repeating structural carbonate units of the formula (1): 
     
       
         
         
             
             
         
       
     
     wherein greater than or equal to 60 percent of the total number of R 1  groups contain aromatic organic groups and the balance thereof are aliphatic, alicyclic, or aromatic groups. 
     In another embodiment, the polycarbonate is derived from bisphenol-A. 
     In another embodiment, each R 1  group is a divalent aromatic group, for example derived from an aromatic dihydroxy compound of the formula (2):
 
HO-A 1 -Y 1 -A 2 -OH  (2)
 
     wherein each of A 1  and A 2  is a monocyclic divalent arylene group, and Y1 is a single bond or a bridging group having one or two atoms that separate A 1  from A 2 . In an exemplary embodiment, one atom separates A 1  from A 2 . In another embodiment, when each of A 1  and A 2  is phenylene, Y 1  is para to each of the hydroxyl groups on the phenylenes. Illustrative non-limiting examples of groups of this type are —O—, —S—, —S(O)—, —S(O) 2 —, —C(O)—, methylene, cyclohexyl-methylene, 2-[2.2.1]-bicycloheptylidene, ethylidene, isopropylidene, neopentylidene, cyclohexylidene, cyclopentadecylidene, cyclododecylidene, and adamantylidene. The bridging group Y 1  can be a hydrocarbon group or a saturated hydrocarbon group such as methylene, cyclohexylidene, or isopropylidene. 
     Included within the scope of formula (2) are bisphenol compounds of general formula (3): 
                         
wherein R a  and R b  each represent a halogen atom or a monovalent hydrocarbon group and can be the same or different; p and q are each independently integers of 0 to 4; and X a  represents a single bond or one of the groups of formulas (4) or (5):
 
                         
wherein R c  and R d  are each independently hydrogen, C 1-12  alkyl, C 1-12  cycloalkyl, C 7-12  arylalkyl, C 1-12  heteroalkyl, or cyclic C 7-12  heteroarylalkyl, and R e  is a divalent C 1-12  hydrocarbon group. In particular, R c  and R d  are each the same hydrogen or C 1-4  alkyl group, specifically the same C 1-3  alkyl group, even more specifically, methyl.
 
     In an embodiment, R c  and R d  taken together represent a C 3-20  cyclic alkylene group or a heteroatom-containing C 3-20  cyclic alkylene group comprising carbon atoms and heteroatoms with a valency of two or greater. These groups can be in the form of a single saturated or unsaturated ring, or a fused polycyclic ring system wherein the fused rings are saturated, unsaturated, or aromatic. A specific heteroatom-containing cyclic alkylene group comprises at least one heteroatom with a valency of 2 or greater, and at least two carbon atoms. Exemplary heteroatoms in the heteroatom-containing cyclic alkylene group include —O—, —S—, and —N(Z)—, where Z is a substituent group selected from hydrogen, hydroxy, C 1-12  alkyl, C 1-12  alkoxy, or C 1-12  acyl. 
     In a specific exemplary embodiment, X a  is a substituted C 3-18  cycloalkylidene of the formula (6): 
                         
wherein each R r , R p , R q , and R t  is independently hydrogen, halogen, oxygen, or C 1-12  organic group; I is a direct bond, a carbon, or a divalent oxygen, sulfur, or —N(Z)— wherein Z is hydrogen, halogen, hydroxy, C 1-12  alkyl, C 1-12  alkoxy, or C 1-12  acyl; h is 0 to 2, j is 1 or 2, i is an integer of 0 or 1, and k is an integer of 0 to 3, with the proviso that at least two of R r , R p , R q , and R t  taken together are a fused cycloaliphatic, aromatic, or heteroaromatic ring. It will be understood that where the fused ring is aromatic, the ring as shown in formula (6) will have an unsaturated carbon-carbon linkage where the ring is fused. When k is 1 and i is 0, the ring as shown in formula (6) contains 4 carbon atoms, when k is 2, the ring as shown contains 5 carbon atoms, and when k is 3, the ring contains 6 carbon atoms. In one embodiment, two adjacent groups (e.g., R q  and R t  taken together) form an aromatic group, and in another embodiment, R q  and R t  taken together form one aromatic group and R r  and R p  taken together form a second aromatic group.
 
     Non-limiting examples of dihydroxy compounds that can provide polycarbonates with Tgs greater than 170° C. include 3,3-bis(4-hydroxyphenyl)-2-phenylisoindolin-1-one (PPPBP), 1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane) (Bisphenol TMC), 4,4′-(1-phenylethane-1,1-diyl)diphenol (bisphenol AP) as well as adamantyl containing aromatic dihydroxy compounds and flourene containing aromatic dihydroxy compounds. 
     Specific example of dihydroxy compounds of formula (2) can be the following formula (7): 
                         
(also known as 3,3-bis(4-hydroxyphenyl)-2-phenylisoindolin-1-one (PPPBP)) also known as 2-phenyl-3,3-bis(4-hydroxyphenyl)phthalimidine.
 
     Alternatively, the dihydroxy compounds of formula (2) may be the following formula (8): 
                         
(also known as 4,4′-(1-phenylethane-1,1-diyl)diphenol (bisphenol AP) also known as 1,1-bis(4-hydroxyphenyl)-1-phenyl-ethane).
 
     Alternatively, the dihydroxy compounds of formula (2) may be the following formula (9): 
                         
(bisphenol TMC) also known as 1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane). Examples of adamantyl containing aromatic dihydroxy compounds and flourene containing aromatic dihydroxy compounds are set forth in Formulas (A) and (B) respectively.
 
     
       
         
         
             
             
         
       
     
     Another possible polycarbonate with high Tg is set forth in formula (C): 
     
       
         
         
             
             
         
       
     
     A polycarbonate can have a bisphenol of formula (D) as a repeating monomer unit therein: 
     
       
         
         
             
             
         
       
     
     When k is 3 and i is 0, bisphenols containing substituted or unsubstituted cyclohexane units are used, for example bisphenols of formula (10): 
                         
wherein each R f  is independently hydrogen, C 1-12  alkyl, or halogen; and each R g  is independently hydrogen or C 1-12  alkyl. The substituents can be aliphatic or aromatic, straight chain, cyclic, bicyclic, branched, saturated, or unsaturated. Such cyclohexane-containing bisphenols, for example the reaction product of two moles of a phenol with one mole of a hydrogenated isophorone, are useful for making polycarbonate polymers with high glass transition temperatures and high heat distortion temperatures. Cyclohexyl bisphenol containing polycarbonates, or a combination comprising at least one of the foregoing with other bisphenol polycarbonates, are supplied by Bayer Co. under the APEC® trade name.
 
     Other useful dihydroxy compounds having the formula HO—R 1 —OH include aromatic dihydroxy compounds of formula (11): 
                         
wherein each R h  is independently a halogen atom, a C 1-10  hydrocarbyl such as a C 1-10  alkyl group, a halogen substituted C 1-10  hydrocarbyl such as a halogen-substituted C 1-10  alkyl group, and n is 0 to 4. The halogen is usually bromine.
 
     Some illustrative examples of dihydroxy compounds include the following: 4,4′-dihydroxybiphenyl, 1,6-dihydroxynaphthalene, 2,6-dihydroxynaphthalene, bis(4-hydroxyphenyl)methane, bis(4-hydroxyphenyl)diphenylmethane, bis(4-hydroxyphenyl)-1-naphthylmethane, 1,2-bis(4-hydroxyphenyl)ethane, 1,1-bis(4-hydroxyphenyl)-1-phenylethane, 2-(4-hydroxyphenyl)-2-(3-hydroxyphenyl)propane, bis(4-hydroxyphenyl)phenylmethane, 2,2-bis(4-hydroxy-3-bromophenyl)propane, 1,1-bis(hydroxyphenyl)cyclopentane, 1,1-bis(4-hydroxyphenyl)cyclohexane, 1,1-bis(4-hydroxyphenyl)isobutene, 1,1-bis(4-hydroxyphenyl)cyclododecane, trans-2,3-bis(4-hydroxyphenyl)-2-butene, 2,2-bis(4-hydroxyphenyl)adamantine, alpha, alpha′-bis(4-hydroxyphenyl)toluene, bis(4-hydroxyphenyl)acetonitrile, 2,2-bis(3-methyl-4-hydroxyphenyl)propane, 2,2-bis(3-ethyl-4-hydroxyphenyl)propane, 2,2-bis(3-n-propyl-4-hydroxyphenyl)propane, 2,2-bis(3-isopropyl-4-hydroxyphenyl)propane, 2,2-bis(3-sec-butyl-4-hydroxyphenyl)propane, 2,2-bis(3-t-butyl-4-hydroxyphenyl)propane, 2,2-bis(3-cyclohexyl-4-hydroxyphenyl)propane, 2,2-bis(3-allyl-4-hydroxyphenyl)propane, 2,2-bis(3-methoxy-4-hydroxyphenyl)propane, 2,2-bis(4-hydroxyphenyl)hexafluoropropane, 1,1-dichloro-2,2-bis(4-hydroxyphenyl)ethylene, 1,1-dibromo-2,2-bis(4-hydroxyphenyl)ethylene, 1,1-dichloro-2,2-bis(5-phenoxy-4-hydroxyphenyl)ethylene, 4,4′-dihydroxybenzophenone, 3,3-bis(4-hydroxyphenyl)-2-butanone, 1,6-bis(4-hydroxyphenyl)-1,6-hexanedione, ethylene glycol bis(4-hydroxyphenyl)ether, bis(4-hydroxyphenyl)ether, bis(4-hydroxyphenyl)sulfide, bis(4-hydroxyphenyl)sulfoxide, bis(4-hydroxyphenyl)sulfone, 9,9-bis(4-hydroxyphenyl)fluorine, 2,7-dihydroxypyrene, 6,6′-dihydroxy-3,3,3′,3′-tetramethylspiro(bis)indane (“spirobiindane bisphenol”), 3,3-bis(4-hydroxyphenyl)phthalide, 2,6-dihydroxydibenzo-p-dioxin, 2,6-dihydroxythianthrene, 2,7-dihydroxyphenoxathin, 2,7-dihydroxy-9,10-dimethylphenazine, 3,6-dihydroxydibenzofuran, 3,6-dihydroxydibenzothiophene, and 2,7-dihydroxycarbazole, resorcinol, substituted resorcinol compounds such as 5-methyl resorcinol, 5-ethyl resorcinol, 5-propyl resorcinol, 5-butyl resorcinol, 5-t-butyl resorcinol, 5-phenyl resorcinol, 5-cumyl resorcinol, 2,4,5,6-tetrafluoro resorcinol, 2,4,5,6-tetrabromo resorcinol, or the like; catechol; hydroquinone; substituted hydroquinones such as 2-methyl hydroquinone, 2-ethyl hydroquinone, 2-propyl hydroquinone, 2-butyl hydroquinone, 2-t-butyl hydroquinone, 2-phenyl hydroquinone, 2-cumyl hydroquinone, 2,3,5,6-tetramethyl hydroquinone, 2,3,5,6-tetra-t-butyl hydroquinone, 2,3,5,6-tetrafluoro hydroquinone, 2,3,5,6-tetrabromo hydroquinone, and the like, as well as combinations comprising at least one of the foregoing dihydroxy compounds. 
     Specific examples of bisphenol compounds that can be represented by formula (2) include 1,1-bis(4-hydroxyphenyl) methane, 1,1-bis(4-hydroxyphenyl) ethane, 2,2-bis(4-hydroxyphenyl) propane (hereinafter “bisphenol A” or “BPA”), 2,2-bis(4-hydroxyphenyl) butane, 2,2-bis(4-hydroxyphenyl) octane, 1,1-bis(4-hydroxyphenyl) propane, 1,1-bis(4-hydroxyphenyl) n-butane, 2,2-bis(4-hydroxy-1-methylphenyl) propane, 1,1-bis(4-hydroxy-t-butylphenyl) propane, 3,3-bis(4-hydroxyphenyl) phthalimidine, 2-phenyl-3,3-bis(4-hydroxyphenyl) phthalimidine (PPPBP), and 1,1-bis(4-hydroxy-3-methylphenyl)cyclohexane (DMBPC). Combinations comprising at least one of the foregoing dihydroxy compounds can also be used. 
     “Polycarbonate” as used herein includes homopolycarbonates, copolymers comprising different R 1  moieties in the carbonate (also referred to herein as “copolycarbonates”), and copolymers comprising carbonate units and other types of polymer units, such as ester units. In one specific embodiment, the polycarbonate is a linear homopolymer or copolymer comprising units derived from bisphenol A, in which each of A 1  and A 2  is p-phenylene and Y 1  is isopropylidene in formula (2). More specifically, greater than or equal to 60%, particularly greater than or equal to 80% of the R 1  groups in the polycarbonate are derived from bisphenol A. 
     Another specific type of copolymer is a polyester carbonate, also known as a polyester-polycarbonate. Such copolymers further contain, in addition to recurring carbonate chain units of the formula (1), repeating units of formula (9): 
                         
wherein D is a divalent group derived from a dihydroxy compound, and can be, for example, a C 2 -C 10  alkylene group, a C 6 -C 20  alicyclic group, a C 6 -C 20  aromatic group or a polyoxyalkylene group in which the alkylene groups contain 2 to 6 carbon atoms, specifically 2, 3, or 4 carbon atoms; and T divalent group derived from a dicarboxylic acid, and can be, for example, a C 2 -C 10  alkylene group, a C 6 -C 20  alicyclic group, a C 6 -C 20  alkyl aromatic group, or a C 6 -C 20  aromatic group.
 
     In one embodiment, D is a C 2  to C 30  alkylene group having a straight chain, branched chain, or cyclic (including polycyclic) structure. In another embodiment, D is derived from an aromatic dihydroxy compound of formula (3) above. In another embodiment, D is derived from an aromatic dihydroxy compound of formula (8) above. 
     Examples of aromatic dicarboxylic acids that can be used to prepare the polyester units include isophthalic or terephthalic acid, 1,2-di(p-carboxyphenyl)ethane, 4,4′-dicarboxydiphenyl ether, 4,4′-bisbenzoic acid, and combinations comprising at least one of the foregoing acids. Acids containing fused rings can also be present, such as in 1,4-, 1,5-, or 2,6-naphthalenedicarboxylic acids. Specific dicarboxylic acids are terephthalic acid, isophthalic acid, naphthalene dicarboxylic acid, cyclohexane dicarboxylic acid, or combinations thereof. A specific dicarboxylic acid comprises a combination of isophthalic acid and terephthalic acid wherein the weight ratio of isophthalic acid to terephthalic acid is 91:9 to 2:98. In another specific embodiment, D is a C 2-6  alkylene group and T is p-phenylene, m-phenylene, naphthalene, a divalent cycloaliphatic group, or a combination thereof. This class of polyester includes the poly(alkylene terephthalates). 
     The molar ratio of ester units to carbonate units in the copolymers can vary broadly, for example 1:99 to 99:1, specifically 10:90 to 90:10, more specifically 25:75 to 75:25, depending on the desired properties of the final composition. 
     In a specific embodiment, the polyester unit of a polyester-polycarbonate can be derived from the reaction of a combination of isophthalic and terephthalic diacids (or derivatives thereof) with resorcinol. In another specific embodiment, the polyester unit of a polyester-polycarbonate is derived from the reaction of a combination of isophthalic acid and terephthalic acid with bisphenol-A. In a specific embodiment, the polycarbonate units are derived from bisphenol A. In another specific embodiment, the polycarbonate units are derived from resorcinol and bisphenol A in a molar ratio of resorcinol carbonate units to bisphenol A carbonate units of 1:99 to 99:1. 
     A specific example of a polycarbonate-polyester is a copolycarbonate-polyester-polysiloxane terpolymer comprising carbonate units of formula (1), ester units of formula (9), and polysiloxane (also referred to herein as “polydiorganosiloxane”) units of formula (10): 
                         
wherein each occurrence of R is same or different, and is a C 1-13  monovalent organic group. For example, R may independently be a C 1-13  alkyl group, C 1-13  alkoxy group, C 2-13  alkenyl group, C 2-13  alkenyloxy group, C 3-6  cycloalkyl group, C 3-6  cycloalkoxy group, C 6-14  aryl group, C 6-10  aryloxy group, C 7-13  arylalkyl group, C 7-13  arylalkoxy group, C 7-13  alkylaryl group, or C 7-13  alkylaryloxy group. The foregoing groups may be fully or partially halogenated with fluorine, chlorine, bromine, or iodine, or a combination thereof. Combinations of the foregoing R groups may be used in the same copolymer. In an embodiment, the polysiloxane comprises R groups that have a minimum hydrocarbon content. In a specific embodiment, an R group with a minimum hydrocarbon content is a methyl group.
 
     The value of E in formula (10) may vary widely depending on the type and relative amount of each component in the plastic (e.g., thermoplastic) composition, the desired properties of the composition, and like considerations. Herein, E has an average value of 5 to 200, with the specific amount chosen so that a 1.0 mm thick plaque of the plastic composition (i.e., plastic material, coated conversion material(s), any additive(s)) has a transparency (% T) of greater than or equal to 30%. It is readily understood by an artisan that the E value is chosen (e.g., adjusted such as when the amount of siloxane in the material and when the siloxane is introduced to form the material and/or the process for making the material) to achieve a balance between transparency, flame retardancy, and impact. In an embodiment, E has an average value of 16 to 50, specifically 20 to 45, and more specifically 25 to 45. In another embodiment, E has an average value of 4 to 15, specifically 5 to 15, more specifically 6 to 15, and still more specifically 7 to 12. 
     In an embodiment, polydiorganosiloxane units are derived from dihydroxy aromatic compound of formula (11): 
     
       
         
         
             
             
         
       
     
     wherein E is as defined above; each R may independently be the same or different, and is as defined above; and each Ar may independently be the same or different, and is a substituted or unsubstituted C 6-30  arylene group, wherein the bonds are directly connected to an aromatic moiety. Suitable Ar groups in formula (11) may be derived from a C 6-30  dihydroxy aromatic compound, for example a dihydroxy aromatic compound of formula (2), (3), (7), or (8) above. Combinations comprising at least one of the foregoing dihydroxy aromatic compounds may also be used. Examples of dihydroxy aromatic compounds include resorcinol (i.e., 1,3-dihydroxybenzene), 4-methyl-1,3-dihydroxybenzene, 5-methyl-1,3-dihydroxybenzene, 4,6-dimethyl-1,3-dihydroxybenzene, 1,4-dihydroxybenzene, 1,1-bis(4-hydroxyphenyl) methane, 1,1-bis(4-hydroxyphenyl) ethane, 2,2-bis(4-hydroxyphenyl) propane, 2,2-bis(4-hydroxyphenyl) butane, 2,2-bis(4-hydroxyphenyl) octane, 1,1-bis(4-hydroxyphenyl) propane, 1,1-bis(4-hydroxyphenyl) n-butane, 2,2-bis(4-hydroxy-1-methylphenyl) propane, 1,1-bis(4-hydroxyphenyl)cyclohexane, bis(4-hydroxyphenyl sulfide), and 1,1-bis(4-hydroxy-t-butylphenyl) propane. Combinations comprising at least one of the foregoing dihydroxy compounds may also be used. In an embodiment, the dihydroxy aromatic compound is unsubstituted, or is not substituted with non-aromatic hydrocarbon-containing substituents such as, for example, alkyl, alkoxy, or alkylene substituents. 
     In a specific embodiment, where Ar is derived from resorcinol, the polydiorganosiloxane repeating units are derived from dihydroxy aromatic compounds of formula (12): 
                         
or, where Ar is derived from bisphenol-A, from dihydroxy aromatic compounds of formula (13):
 
                         
wherein E is as defined above.
 
     In another embodiment, polydiorganosiloxane units are derived from dihydroxy aromatic compound of formula (14): 
                         
wherein R and E are as described above, and each occurrence of R 2  is independently a divalent C 1-30  alkylene or C 7-30  arylene-alkylene, and wherein the polymerized polysiloxane unit is the reaction residue of its corresponding dihydroxy aromatic compound. In a specific embodiment, where R 2  is C 7-30  arylene-alkylene, the polydiorganosiloxane units are derived from dihydroxy aromatic compound of formula (15):
 
                         
wherein R and E are as defined above. Each R 3  is independently a divalent C 2-8  aliphatic group. Each M may be the same or different, and may be a halogen, cyano, nitro, C 1-8  alkylthio, C 1-8  alkyl, C 1-8  alkoxy, C 2-8  alkenyl, C 2-8  alkenyloxy group, C 3-8  cycloalkyl, C 3-8  cycloalkoxy, C 6-10  aryl, C 6-10  aryloxy, C 7-12  arylalkyl, C 7-12  arylalkoxy, C 7-12  alkylaryl, or C 7-12  alkylaryloxy, wherein each n is independently 0, 1, 2, 3, or 4.
 
     In an embodiment, M is bromo or chloro, an alkyl group such as methyl, ethyl, or propyl, an alkoxy group such as methoxy, ethoxy, or propoxy, or an aryl group such as phenyl, chlorophenyl, or tolyl; R 3  is a dimethylene, trimethylene or tetramethylene group; and R is a C 1-8  alkyl, haloalkyl such as trifluoropropyl, cyanoalkyl, or aryl such as phenyl, chlorophenyl or tolyl. In another embodiment, R is methyl, or a combination of methyl and trifluoropropyl, or a combination of methyl and phenyl. In still another embodiment, M is methoxy, n is 0 or 1, R 3  is a divalent C 1-3  aliphatic group, and R is methyl. 
     In a specific embodiment, the polydiorganosiloxane units are derived from a dihydroxy aromatic compound of formula (16): 
                         
wherein E is as described above.
 
     In another specific embodiment, the polydiorganosiloxane units are derived from dihydroxy aromatic compound of formula (17): 
                         
wherein E is as defined above.
 
     Dihydroxy polysiloxanes typically can be made by functionalizing a substituted siloxane oligomer of formula (18): 
                         
wherein R and E are as previously defined, and Z is H, halogen (Cl, Br, I), or carboxylate. Examples of carboxylates include acetate, formate, benzoate, and the like. In an exemplary embodiment, where Z is H, compounds of formula (18) may be prepared by platinum catalyzed addition with an aliphatically unsaturated monohydric phenol. Examples of aliphatically unsaturated monohydric phenols include eugenol, 2-allylphenol, 4-allylphenol, 4-allyl-2-methylphenol, 4-allyl-2-phenylphenol, 4-allyl-2-bromophenol, 4-allyl-2-t-butoxyphenol, 4-phenyl-2-allylphenol, 2-methyl-4-propenylphenol, 2-allyl-4,6-dimethylphenol, 2-allyl-4-bromo-6-methylphenol, 2-allyl-6-methoxy-4-methylphenol, and 2-allyl-4,6-dimethylphenol. Combinations comprising at least one of the foregoing may also be used. Where Z is halogen or carboxylate, functionalization may be accomplished by reaction with a dihydroxy aromatic compound of formulas (2), (3), (7), (8), or a combination comprising at least one of the foregoing dihydroxy aromatic compounds. In an embodiment, compounds of formula (12) may be formed from an alpha, omega-bisacetoxypolydiorangonosiloxane and a dihydroxy aromatic compound under phase transfer conditions.
 
     In some embodiments a copolycarbonate terpolymer can be used. Specific copolycarbonate terpolymers include those with polycarbonate units of formula (1) wherein R 1  is a C 6-30  arylene group, polysiloxane units derived from siloxane diols of formula (13), (16) or (17), and polyester units wherein T is a C 6-30  arylene group. In an embodiment, T is derived from isophthalic and/or terephthalic acid, or reactive chemical equivalents thereof. In another embodiment, R 1  is derived from the carbonate reaction product of a resorcinol of formula (8), or a combination of a resorcinol of formula (8) and a bisphenol of formula (4). 
     The relative amount of each type of unit in the foregoing terpolymer will depend on the desired properties of the terpolymer, and are readily determined by one of ordinary skill in the art without undue experimentation, using the guidelines provided herein. For example, the polycarbonate-polyester-polysiloxane terpolymer can comprise siloxane units in an amount of 0.1 to 25 weight percent (wt %), specifically 0.2 to 10 wt %, more specifically 0.2 to 6 wt %, even more specifically 0.2 to 5 wt %, and still more specifically 0.25 to 2 wt %, based on the total weight of the polycarbonate-polyester-polysiloxane terpolymer, with the proviso that the siloxane units are provided by polysiloxane units covalently bonded in the polymer backbone of the polycarbonate-polyester-polysiloxane terpolymer. The polycarbonate-polyester-polysiloxane terpolymer can further comprise 0.1 to 49.85 wt % carbonate units, 50 to 99.7 wt % ester units, and 0.2 to 6 wt % polysiloxane units, based on the total weight of the polysiloxane units, ester units, and carbonate units. Alternatively, the polycarbonate-polyester-polysiloxane terpolymer comprises 0.25 to 2 wt % polysiloxane units, 60 to 96.75 wt % ester units, and 3.25 to 39.75 wt % carbonate units, based on the total weight of the polysiloxane units, ester units, and carbonate units. The specific amount of terpolymer and the composition of the terpolymer will be chosen so that a 1.0 mm thick plaque of the composition transparency (% T) of greater than or equal to 30%. 
     In a further embodiment, a method of making an article of manufacture that has a V0 94 rating at a thickness of 2.0 mm (specifically at a thickness of 1.5 mm) comprises: (a) providing a polycarbonate, wherein the polycarbonate has a repeating structural background of the following formula 
                         
wherein greater than or equal to 60 percent of the total number of R 1  groups contain aromatic organic groups and the balance thereof are aliphatic, alicyclic, or aromatic groups; an end capping agent; a branching agent; (b) selecting the end-capping agent based upon the molecular weight of the polycarbonate and the branching level imparted by the branching agent, wherein the MVR of the polycarbonate is 1 to 15 cubic centimeter per 10 minutes (cm 3 /10 min) and wherein the pKa of the end-capping agent is 8.3 to 11; (c) forming a polycarbonate containing the end-capping agent and the branching that has a peak melt viscosity of greater than or equal to 8,000 poise when measured using a parallel plate melt rheology test at a heating rate of 10° C./min at a temperature of 350° C. to 450° C.; and (d) blending a conversion material and a flame retardant with the formed polycarbonate.
 
     The peak melt viscosity can be greater than or equal to 25,000 poise when measured using a parallel plate melt rheology test at a heating rate of 10° C./min at a temperature of 350° C. to 450° C. 
     In another embodiment, the composition comprises: a flame retardant; a conversion material; a polycarbonate, wherein the polycarbonate has a repeating structural background of the following formula 
                         
wherein greater than or equal to 60 percent of the total number of R 1  groups contain aromatic organic groups and the balance thereof are aliphatic, alicyclic, or aromatic groups and wherein the polycarbonate contains one or more bisphenols; wherein the polycarbonate comprises an end-capping agent; wherein the polycarbonate comprises a branching agent; and wherein the polycarbonate containing the branching agent and the end-capping agent has a peak melt viscosity of greater than or equal to 7,000 poise when calculated from the equation of wherein the peak melt viscosity equals: −57135.91+36961.39*BL+14001.13*MW 1/3 −46944.24*pKa−322.51*BL*MW 1/3 −2669.19*BL*pKa+215.83*MW 1/3 *pKa+1125.63*BL 2 −200.11*MW 2/3 +2231.15*pKa 2 , wherein BL is the mole ratio of the branching agent in the formulation determined by dividing the number of moles of branching agent by the total number of moles of bisphenol or bisphenols in the composition, the MW is the weight-averaged molecular weight of the polycarbonate containing the branching agent and the end-capping agent as determined by gel permeation chromatography using polycarbonate standards, and the pKa is the pKa of the end capping agent; and wherein a molded article of the composition has a UL 94 V0 rating at a thickness of 2.0 mm, specifically at 1.5 mm, and more specifically at 1.0 mm.
 
     In a further embodiment, the peak melt viscosity is greater than or equal to 25,000 as calculated by the above equation. 
     In another embodiment, a method of making an article of manufacture that has a V0 94 rating at a thickness 1.5 mm comprises: (a) providing a polycarbonate, wherein the polycarbonate has a repeating structural background of the following formula 
                         
wherein greater than or equal to 60 percent of the total number of R 1  groups contain aromatic organic groups and the balance thereof are aliphatic, alicyclic, or aromatic groups and wherein the polycarbonate contains one or more bisphenols; an end capping agent that is not cyanophenol; a branching agent; (b) selecting the end-capping agent based upon the molecular weight of the polycarbonate and the branching level imparted by the branching agent, wherein the MVR of the polycarbonate is 1 to 15 cm 3 /10 min and wherein the pKa of the end-capping agent is 8 to 11; (c) forming a polycarbonate containing the end-capping agent and the branching agent that has a peak melt viscosity that is greater than or equal to 7,000 poise when calculated from the equation of wherein the peak melt viscosity equals: −57135.91+36961.39*BL+14001.13*MW 1/3 −46944.24*pKa−322.51*BL*MW 1/3 −2669.19*BL*pKa+215.83*MW 1/3 *pKa+1125.63*BL 2 −200.11*MW 2/3 +2231.15*pKa 2 ; and wherein BL is the mole ratio of the branching agent in the formulation determined by dividing the number of moles of branching agent by the total number of moles of bisphenol or bisphenols in the composition, the MW is the weight-averaged molecular weight of the formed polycarbonate as determined by gel permeation chromatography using polycarbonate standards, and the pKa is the pKa of the end capping agent; and (d) blending a flame retardant and a conversion material with the formed polycarbonate.
 
     In a further embodiment, the peak melt viscosity is greater than or equal to 25,000 poise calculated from the above equation. 
     Branching Agents for Polycarbonate Containing Compositions 
     The polycarbonates herein may include branched polycarbonate(s). Various types of branching agents can be utilized for the embodiments encompassed by this disclosure. 
     Branched polycarbonate blocks can be prepared by adding a branching agent during polymerization. These branching agents include polyfunctional organic compounds containing at least three functional groups selected from hydroxyl, carboxyl, carboxylic anhydride, haloformyl, and mixtures of the foregoing functional groups. Specific examples include trimellitic acid, trimellitic anhydride, trimellitic trichloride (TMTC), tris-p-hydroxy phenyl ethane (THPE), 3,3-bis-(4-hydroxyphenyl)-oxindole (also known as isatin-bis-phenol), tris-phenol TC (1,3,5-tris((p-hydroxyphenyl)isopropyl)benzene), tris-phenol PA (4(4(1,1-bis(p-hydroxyphenyl)-ethyl) alpha, alpha-dimethyl benzyl)phenol), 4-chloroformyl phthalic anhydride, trimesic acid, and benzophenone tetracarboxylic acid. The branching agents can be added at a level of 0.05 to 2.0 wt %. Mixtures comprising linear polycarbonates and branched polycarbonates can be used. 
     In some embodiments, a particular type of branching agent is used to create branched polycarbonate materials. These branched polycarbonate materials have statistically more than two end groups. The branching agent is added in an amount (relative to the bisphenol monomer) that is sufficient to achieve the desired branching content, that is, more than two end groups. The molecular weight of the polymer may become very high upon addition of the branching agent and may lead to viscosity problems during phosgenation. Therefore, in some embodiments, an increase in the amount of the chain termination agent is used in the polymerization. The amount of chain termination agent used when the particular branching agent is used is generally higher than the instance when only a chain termination agent is used. The amount of chain termination agent used is generally above 5 mole percent and less than 20 mole percent compared to the bisphenol monomer. 
     In some embodiments, the branching agent is a structure derived from a triacid trichloride of the formula (19): 
                         
wherein, in this formula (19), Z is hydrogen, a halogen, C 1-3  alkyl group, C 1-3  alkoxy group, C 7-12  arylalkyl, alkylaryl, or nitro group, and z is 0 to 3; or a branching agent derived from a reaction with a tri-substituted phenol of the formula (20):
 
                         
wherein, in this formula (20), T is a C 1-20  alkyl group, C 1-20  alkyleneoxy group, C 7-12  arylalkyl, or alkylaryl group, S is hydrogen, a halogen, C 1-3  alkyl group, C 1-3  alkoxy group, C 7-12  arylalkyl, alkylaryl, or nitro group, s is 0 to 4.
 
     In another embodiment, the branching agent is a structure having formula (21): 
     
       
         
         
             
             
         
       
     
     Examples of specific branching agents that are particularly effective in the compositions include trimellitic trichloride (TMTC), tris-p-hydroxy phenyl ethane (THPE) and isatin-bis-phenol. In one embodiment, in formula (19), Z is hydrogen and z is 3. In another embodiment, in formula (20), S is hydrogen, T is methyl, and s is 4. 
     The relative amount of branching agents used in the manufacture of the polymer will depend on a number of considerations, for example the type of R 1  groups, the amount of cyanophenol, and the desired molecular weight of the polycarbonate. In general, the amount of branching agent is effective to provide about 0.1 to 10 branching units per 100 R 1  units, specifically about 0.5 to 8 branching units per 100 R 1  units, and more specifically about 0.75 to 5 branching units per 100 R 1  units. For branching agents having formula (20), the amount of branching agent tri-ester groups are present in an amount of about 0.1 to 10 branching units per 100 R 1  units, specifically about 0.5 to 8 branching units per 100 R 1  units, and more specifically about 0.75 to 5 tri-ester units per 100 R 1  units. For branching agents having formula (21), the amount of branching agent tricarbonate groups are present in an amount of about 0.1 to 10 branching units per 100 R 1  units, specifically about 0.5 to 8 branching units per 100 R 1  units, and more specifically about 0.75 to 5 tri-phenylcarbonate units per 100 R 1  units. In some embodiments, a combination of two or more branching agents may be used. 
     In one embodiment, the polycarbonate of the composition has a branching level of greater than or equal to 1%, or greater than or equal to 2%, or greater than or equal to 3%, or 1% to 3%. 
     End-Capping Agents for Polycarbonate Containing Compositions 
     Various types of end-capping agents can be utilized herein provided that such agents do not significantly adversely affect the desired properties of the compositions, such as transparency, ductility, fire retardants, and the like. 
     Examples of endcapping agents (also referred to as chain stoppers) include certain mono-phenolic compound(s), and/or mono-carboxylic acid chloride(s), and/or mono-chloroformate(s). Mono-phenolic chain stoppers are exemplified by monocyclic phenols such as phenol and C 1 -C 22  alkyl-substituted phenols such as p-cumyl-phenol, and p-t-butyl phenol; and monoethers of diphenols, such as p-methoxyphenol, phenols with phenols with cyano-substitution such as p-cyanophenol, or with halogen substitution such as p-flurophenol, or with nitro-substitution such as 4-nitrophenol. Alkyl-substituted phenols with branched chain alkyl substituents having 8 to 9 carbon atoms can be specifically mentioned. Certain mono-phenolic UV absorbers can also be used as an endcapping agent, for example 4-substituted-2-hydroxybenzophenones and their derivatives, aryl salicylates, monoesters of diphenols such as resorcinol monobenzoate, 2-(2-hydroxyaryl)-benzotriazoles and their derivatives, 2-(2-hydroxyaryl)-1,3,5-triazines and their derivatives, and the like. For example, the polycarbonate can have an end-cap comprising phenol, such as alkyl-substituted phenols, ether-substituted phenols, ester-substituted phenols, cyano-substituted phenols, and halogen substituted phenols, as well as combinations comprising at least one of the foregoing. Optionally, the end-capping agents can be selected from: cyanophenol and a phenol containing substitution(s) with aliphatic groups, olefinic groups, aromatic groups, halogens, ester groups, ether groups, and combinations comprising at least one of the foregoing. 
     Of particular usefulness commercially, the end-capping agents can be phenol, e.g., specifically, can be selected from: cyanophenol, para-t-butylphenol, para-cumylphenol, and combinations comprising at least one of the foregoing. 
     Additional thermoplastic material to which the method may be applied include polyethylene terephthalates (PET) and polybutylene terephthalate (PBT), polyethylene napthalate (PEN), polymethyl methacrylate (PMMA), polystyrene (PS), cyclic olefinic polymers (COP) and cyclic olefinic copolymers (COC), polyetherimide as well as polycarbonate/polyester blends. In addition to thermoplastics, the method may also be used to test silicone based compounds as provided by Dow Corning. 
     The method is further illustrated by the following non-limiting examples. 
     Example 1—Transparent Polycarbonate Test 
     Testing using the method according to the invention was conducted on four different transparent polycarbonate formulations. The polycarbonate samples for each formulation comprised square plates approximately 30×30 mm and 1.5 mm thick. Sample transparent polycarbonate formulations were illuminated with light having a peak intensity centered on 447 nm (measured radiometrically) at an irradiance of 50,000 W/m 2  (calibration via a power meter). The illumination of the samples was performed in an oven within which the sample temperature (in this example, an average of two samples) was maintained at a temperature of 120° C. Illumination was provided over a series of 6 (unequal) intervals for a total of 1,729 hours or until failure of the sample. Samples that experienced catastrophic failure (i.e., melting, charring) were removed from the oven between successive periods of illumination. At the end of each interval, the samples were allowed to cool and their % transmittance ratio and degree of discoloration, as determined using the Yellowness Index, was measured using a spectrophotometer. The % transmittance ratio was calculated by measuring an initial % transmittance value for each sample before exposure of the samples and then measuring the % transmittance of each sample after each exposure interval and then calculating the ratio of the % transmittance after each exposure interval to the initial % transmittance. The % transmittance measurements were determined according to ASTM D1003-00 (2000) using procedure A and CIE illuminant C and 2 degree observer on an X-Rite i7 spectrophotometer using an integrating sphere with 8°/diffuse geometry, specular component included, UV included, 6 mm small area view lens, and 25 mm large area view transmission port, with the percent transmittance value reported as Y (luminous transmittance) taken from the CIE 1931 tristimulus values XYZ. The YI (yellowness index) was determined according to ASTM D1925-95 (1995) CIE illuminant C and 2 degree observer on a X-Rite i7 spectrophotometer using an integrating sphere with 8°/diffuse geometry, specular component included, UV included, 6 mm small area view lens, and 25 mm large area view transmission port. 
     The results of the testing showed measurable differences in the rate of discoloration between the samples throughout the six illumination intervals, with sample 1 failing (melted) at 1,197 hours exposure, sample 2 having a yellowness index of 2.14 and a % transmittance ratio of 98.86% at 1,729 hours exposure, sample 3 having a yellowness index of 0.46 and a % transmittance ratio of 98.67 at 1,729 hours exposure, and sample 4 having a yellowness index of 0.19 and a % transmittance ratio of 98.82% at 1,729 hours exposure. 
     Example 2—White Opaque Polymer Test 
     Testing using the method according to the invention was conducted on four different white opaque polymer formulations. The samples for each formulation comprised square plates approximately 30×30 mm and 1.5 mm thick. Each sample polymer formulation was illuminated with light having a peak intensity centered on 447 nm (measured radiometrically) at an irradiance of 50,000 W/m 2  (calibration via a power meter). The illumination of the samples was performed in an oven within which the sample temperature (in this instance, an average of two samples) was maintained at a temperature of 90° C. Illumination was provided over a single interval for a total of 33 hours or until failure of the sample. Samples that experienced catastrophic failure (i.e., melting, charring) were removed from the oven between successive periods of illumination. At the end of the interval the samples were allowed to cool and their degree of discoloration, as determined using the Yellowness Index, was measured using a spectrophotometer. The YI (yellowness index) was determined according to ASTM D1925-95 (1995) CIE illuminant C and 2 degree observer on a X-Rite i7 spectrophotometer using an integrating sphere with 8°/diffuse geometry, specular component included, UV included, 6 mm small area view at the port. 
     The results of the testing showed measurable differences in the rate of discoloration between the samples, with sample 1 (a polycarbonate) having a yellowness index of 5.33, sample 2 (a polycarbonate having a higher glass transition temperature than sample 1) having a yellowness index of 8.75, sample 3 (a polyester) having a yellowness index of 0.0, and sample 4 (a glass filled polyester) having a yellowness index of 34.33. 
     These foregoing results show that the disclosed methods are effective in that the methods produce meaningful quantitative measurements of the rate of thermoplastic degradation in a reasonable time frame.