Acrylic flexible light pipe of improved photo-thermal stability

Acrylic light pipe has adequate thermal and photo-thermal stability for many purposes, but is deficient in maintaining clarity, color, and good optical properties under conditions of exposure to high temperatures, especially in combination with exposure for lengthy periods to passage of light. Improved thermal stability, as reflected in reduced color formation, can be imparted by adjusting the polymerization conditions to produce the uncured core polymer of the core/clad construction with a much reduced terminal vinyl content, preferably below 0.5 vinyl groups/1000 monomer units. This process improvement, in combination with selected addition of a combination of certain hindered phenols and hydrolytically stable organic phosphites, together produce a substantial improvement in the resistance to discoloration under photo-thermal conditions, while maintaining the resistance to discoloration under thermal conditions. The known process conditions which do not yield lower terminal vinyl content, in combination with the selected additives, also produce acrylic light pipe with greatly improved photo-thermal stability.

This invention relates to processes, continuous processes and related
 compositions for producing a more photo-thermally stable flexible light
 pipe ("FLP") based on polymerized units of one or more acrylic esters, and
 the improved FLP product which the process produces.
 An effective process for preparation of acrylic-based flexible light pipe
 is disclosed in two patents to Bigley et al., U.S. Pat. Nos. 5,406,641 and
 5,485,541. In a preferred aspect of this process, a crosslinkable core
 mixture is present which comprises an uncrosslinked copolymer formed
 mainly from acrylic esters and monomers with functionally reactive
 alkoxysilane groups, along with a reactive additive to cure the
 uncrosslinked core polymer by crosslinking it, the reactive additive
 preferably being water and a silane condensation reaction catalyst, such
 as an organotin dicarboxylate. The core mixture is preferably polymerized
 by a bulk (non-solvent) process, more preferably by a continuous bulk
 process, the uncrosslinked copolymer preferably being devolatilized prior
 to co-extrusion with a cladding, preferably of a fluoropolymer, into a
 core/clad composite which is then separately cured to the final flexible
 light pipe.
 The process based on a monomer such as ethyl acrylate taught by Bigley et
 al. yields a flexible light pipe or optical conduit which has high white
 light transmission, and acceptable flexibility and hardness for a variety
 of uses where light is to be conveyed from a remote source to a target and
 where the conduit needs to be flexible to follow a tortuous path, yet hard
 enough to retain its critical geometry.
 The existing process further produces a FLP of adequate thermal (exposure
 to heat in the absence of visible light being conducted through the light
 pipe) and photo-thermal (Joint exposure to heat and to visible light
 conducted through the light pipe, which may contain light of wavelengths
 known as the "near ultraviolet") stability even after exposures to long
 hours of light and ambient heat. The prior art polymer has adequate
 stability for exposure to higher temperatures, including those up to about
 90.degree. C., for shorter use times.
 However, there is a potential large market for light pipe which is
 thermally and photo-thermally stable at higher temperatures and longer
 exposure times, such as in automotive uses where the light is conducted
 near the engine compartment, and temperatures of 120.degree. C. or higher
 may be reached. Other potential uses where high temperatures may be
 encountered may be when the light source is not adequately shielded from
 the connection with the FLP, or where the light source is of extremely
 high intensity. Photo-thermal stability becomes important when the light
 is conveyed through the FLP for long periods of time, accompanied by
 exposure to temperatures well above room temperature. Bigley et al. teach
 in general the use of stabilizers as part of the core component, but do
 not specifically teach or suggest an acceptable answer to this important
 stabilization problem.
 We have discovered an improved process by which to prepare a crosslinkable
 acrylic core for a FLP which, after curing to crosslink, exhibits
 surprisingly improved stability to thermal and photo-thermal aging while
 detaining its other desirable properties of good initial clarity, absence
 of initial color, good flexibility, and adequate hardness to prevent
 physical distortion. An improved product, especially toward thermal aging
 in the absence of light being bassed through the core, can be prepared by
 carefully controlling the temperature of the process, preferably
 shortening somewhat the residence time in the reactor, and controlling the
 nature of the initiator, so as to decrease the number of terminal vinyl
 groups in the polymer. This invention is specifically addressed in a
 provisional United States application by several of the present inventors
 filed Oct. 8, 1996, as Ser. No. 60/27,942. However, the photo-thermal
 stability conferred by the process changes is not sufficient to enable the
 FLP to be used under certain demanding end-use conditions. By specific
 choice of a combination of antioxidants and thermal stabilizers,
 preferably in combination with the process improvements, the target of
 acceptable photo-thermal stabilization has been accomplished.
 More specifically, we have discovered a crosslinkable core mixture for a
 subsequently-cure cured composite which mixture contains a thermoplastic
 core polymer, the thermoplastic core polymer having a weight average
 molecular weight from about 2,000 to about 250,000 daltons and preferably
 a vinyl end-group content of below 0.5 per 1000 monomer units, the core
 mixture comprising
 (a) a thermoplastic core polymer comprising
 i) from 80 to 99.9 weight percent of polymerized units of a C.sub.1
 -C.sub.18 alkyl acrylate or mixtures thereof with up to 50 weight percent
 of the components of (a)(i) of polymerized units of a C.sub.1 -C.sub.18
 alkyl methacrylate;
 ii) from 0.1 to 18.2 weight percent of polymerized units of a functionally
 reactive monomer, and
 iii) from 0 to about 10 weight percent of polymerized units of a refractive
 index increasing monomer selected from styrene, benzyl acrylate, benzyl
 methacrylate, phenylethyl acrylate or phenylethyl methacrylate;
 iv) 0.002 to 0.3, preferably 0.01 to 0.3, weight percent of residual
 molecules of or of decomposition products of an initiator of
 polymerization, including end groups on the thermoplastic core polymer,
 the initiator preferably having a half-life at 60.degree. C. of 20 to 400
 minutes, more preferably 100-250 minutes;
 v) 0.2 to 2.0, preferably 0.6 to 1.5, weight percent of residual molecules
 of or of decomposition products of a chain transfer agent, including end
 groups on the thermoplastic core polymer;
 (b) from 0.1 to 10 weight percent, based on the crosslinkable core mixture
 weight, of a reactive additive; and
 (c) from 0.01 to 1.0 weight percent, based on the crosslinkable core
 mixture weight, of a stabilizer/antioxidant combination comprising 20-80
 weight percent, based on the combination, of an organic phosphite which is
 hydrolytically stable and 80-20 weight percent, based on the combination,
 of a hindered phenol, the phenol preferably separately exhibiting an
 absorbance of less than 1 in a 5% ethyl acetate solution in a 10 cm. cell
 at a wavelength of 400 .ANG..
 The word "hindered" appears in many forms in the definition of the
 invention, but it is maintained because terms such as "hindered phenol"
 are well-known to the skilled artisan involved with polymer stabilization.
 The following defines terms used in the specification and claims:
 (a) hindered phenol: a phenol having at the ortho position relative to the
 hydroxyl group of the phenol at least one alkyl group, preferably at least
 one tertiary(t)-alkyl group, more preferably having two alkyl groups, and
 most preferably having two t-alkyl groups, such as two t-butyl groups, and
 further when there is only one substitution at the ortho position, there
 is further at least one alkyl group, preferably a t-alkyl group, at the
 meta position;
 (b) hydrolytically stable organic phosphite: an organic phosphite having at
 least one, preferably two, and most preferably three, aryl groups,
 preferably phenyl, attached through carbon-oxygen-phosphorus bonding,
 wherein the aryl group has at the ortho position relative to the phenolic
 group at least one alkyl group, preferably at least one tertiary (t)-alkyl
 group, more preferably having two alkyl groups, and most preferably having
 two t-alkyl groups, such as two t-butyl groups. Such materials are known
 to be hydrolytically stable in contrast, e.g., to trisalkyl phosphites.
 An especially preferred stabilizer/antioxidant combination is from 500 to
 3000 parts per million (ppm), i.e., 0.05 to 0.3 weight percent, of
 octadecyl 3,5-di-t-butyl-4-hydroxyhydrocinnamate and 500 to 1500 ppm of
 tris(2,4-di-t-butylphenyl) phosphite.
 It is preferred that the crosslinkable core mixtures exhibit the percentage
 of polymerized units of a C.sub.1 -C.sub.18 alkyl acrylate as 80 to 99.5
 weight percent ethyl acrylate, further preferred that the chain transfer
 agent is an aliphatic mercaptan of from one to twenty carbon atoms, such
 as butyl mercaptan, dodecyl mercaptan, and the like, and further preferred
 that the initiator of polymerization is an azo compound.
 It is further preferred that the crosslinkable core mixtures maintain the
 functionally reactive monomer as present at a level of from about 0.5 to
 about 12 weight percent, more preferably 2 to 12 weight percent, and it be
 selected from 2-methacryloxyethyltrimethoxysilane,
 3-methacryloxypropyltrimethoxysilane, 3-acryloxypropyltrimethoxysilane,
 vinyltrimethoxysilane, vinyltriethoxysilane, or mixtures of these,
 preferably 3-methacryloxypropyltrimethoxysilane. Further, it is preferred
 that the reactive additive is water and a silane condensation reaction
 catalyst, preferably a dialkyltin dicarboxylate, such as dibutyltin
 diacetate.
 In the initial work described in U.S. Pat. No. 5,485,541, the curing for
 the alkoxysilane functionally reactive monomers is carried out by
 injecting water, an organotin catalyst, and (optionally) a solvent for the
 catalyst after the polymerization is complete but prior to co-extrusion
 with the cladding. It has been found that a curable core may be prepared
 when the organotin catalyst and the solvent for the catalyst are present
 during the polymerization, and then either there is addition of water just
 prior to the co-extrusion, or curing is conducted, after extrusion, in the
 presence of ambient diffused water. The latter process has been
 accelerated to a practical level by using a humidified oven or by curing
 in a highly humid controlled atmosphere. The advantage to the separation
 of water from the other components until the polymerization and cladding
 are complete is that premature crosslinking does not occur, with
 subsequent effects on extrusion and on the surface between core and clad.
 Useful claddings are fluorinated polymers, and two especially useful are
 terpolymers of perfluoroalkyl vinyl
 ether/tetrafluoroethylene/hexafluoropropylene (FEP) and of vinylidene
 fluoride/tetrafluoroethylene/hexafluoropropylene (THV). Samples clad with
 THV, which is more permeable to water than FEP, can be externally cured
 rapidly enough for the present purposes (without absorbing so much water
 that hazing occurs) at temperatures of 80.degree. C. and 50% relative
 humidity, whilst samples clad with FEP can be cured rapidly enough for the
 present purposes at 85.degree. C. and 85% relative humidity.
 This crosslinkable core mixture may further contain a cladding polymer,
 such as a fluoropolymer which surrounds the core mixture, and preferably
 the crosslinkable core mixture within the extruded fluoropolymer cladding
 and the extruded fluoropolymer cladding are in substantially complete
 contact. It should be recognized that the thermoplastic crosslinkable core
 polymer and the cladding do not form a chemical or physical admixture, but
 are adjacent to each other in the construct which is the core mixture
 surrounded by the cladding.
 We further have discovered, based upon the above-described crosslinkable
 core polymers, a flexible light pipe product containing the crosslinked
 core mixture described above, wherein the product has: good light
 transmittance wherein the differential transmission loss between light
 wavelengths of 400 nm and at 600 nm is equal to or less than 1.0 decibel
 per meter as measured by a non-destructive interference filter method;
 excellent thermal stability, when the vinyl end-group content is below 0.5
 per 1000 monomer units, wherein a change in the differential transmission
 loss between light wavelengths of 400 nm and at 600 nm is equal to or less
 than 1.0 decibel per meter after 150 hours of exposure to a temperature of
 120.degree. C., as measured by a non-destructive interference filter
 method; excellent photo-thermal stability, wherein a change in the
 differential transmission loss between light wavelengths of 400 nm to 600
 nm is equal to or less than 1.0 decibel per meter after 100 hours of
 exposure to a temperature of 110.degree. C. simultaneously with exposure
 to 12 to 15 lumens/square millimeter of light, as measured by a
 non-destructive interference filter method; good flexibility, wherein the
 product, at 20.degree. C., survives without core fracture a 180.degree.
 bend at a bend radius which is less than or equal to five times the
 diameter of the cured core; and good hardness properties, wherein the
 Shore "A" hardness is less than 90 after 50 days of exposure at
 120.degree. C.
 We further have discovered a process for preparing a crosslinkable core
 mixture for a subsequently-cured composite comprising a coextruded
 cladding polymer and a coextruded crosslinkable core mixture, which
 mixture contains a thermoplastic core polymer having a weight average
 molecular weight from about 2,000 to about 250,000 daltons and preferably
 a vinyl end-group content of below 0.5 per 1000 monomer units, the process
 comprising
 a) preparing an admixture of
 i) from about 80 to about 99.9 weight percent of a bulk monomer mixture
 selected from a C.sub.1 -C.sub.18 alkyl acrylate or mixtures thereof with
 up to 50 weight percent of the bulk monomer mixture of a C.sub.1 -C.sub.18
 alkyl methacrylate;
 ii) from about 0.1 to about 18.2 weight percent of a functionally reactive
 monomer, and
 iii) from 0 to about 10 weight of a refractive index increasing monomer
 selected from styrene, benzyl acrylate, benzyl methacrylate, phenylethyl
 acrylate or phenylethyl methacrylate;
 b) adding 0.002 to 0.3 weight percent, based on the uncrosslinked copolymer
 weight, of an azo initiator of polymerization which preferably has a
 half-life at 60.degree. C. of 20 to 400 minutes, preferably 100-250
 minutes;
 c) prior to, simultaneously, or after the addition of the initiator, adding
 0.2 to 2.0 weight percent, preferably 0.75 to 1.5 weight percent, based on
 the uncrosslinked copolymer weight, of a chain transfer agent;
 d) charging the monomer admixture, initiator, and chain transfer agent
 reaction mixture to a constant-flow stirred reactor heated to
 70-120.degree. C., preferably 85-100.degree. C., with a preferred
 residence time of 5 to 30 minutes, more preferably 20-28 minutes, to form
 a polymerized, non-crosslinked, crosslinkable core mixture;
 e) devolatilizing the polymerized, non-crosslinked, crosslinkable core
 mixture to remove unreacted monomers;
 f) prior to, during, or after the devolatilization, adding from 0.1 to 10
 weight percent, based on the crosslinkable core mixture, of a reactive
 additive;
 g) prior to, during, or after the devolatilization adding from 0.01 to 1.0
 weight percent, based on the crosslinkable core mixture weight, of a )5
 stabilizer/antioxidant combination comprising 20-80 weight percent, based
 on the combination, of a hydrolytically stable organic phosphite, 80-20
 weight percent, based on the combination, of a hindered phenol, the phenol
 preferably separately exhibiting an absorbance of less than 1 in a 5%
 ethyl acetate solution at a wavelength of 400 .ANG.;
 h) coextruding the crosslinkable core mixture and the cladding polymer to
 form a curable composite.
 In this process, it is separately preferred that the coextruded cladding
 polymer and a coextruded crosslinkable core mixture be continuously,
 concurrently and coaxially extruded, that the cladding polymer be a molten
 fluoropolymer as described earlier, that the extruded crosslinkable core
 mixture within the extruded fluoropolymer cladding and the extruded
 fluoropolymer cladding be in substantially complete contact after filling
 the extruded tubular cladding with the extruded crosslinkable core
 mixture, and further that the curing is conducted subsequently and
 separately from the extrusion and cladding operation. Further, a portion
 of the reactive additive may be added to the core mixture after extrusion,
 such as by diffusion of water through the cladding.
 We further have discovered a flexible light pipe product prepared by the
 above process, wherein the product has good light transmittance wherein
 the differential transmission loss between light wavelengths of 400 nm and
 at 600 nm is equal to or less than 1.0 decibel per meter as measured by a
 "cut-back" interference filter method; excellent thermal stability,
 wherein a change in the differential transmission loss between light
 wavelengths of 400 nm and at 600 nm is equal to or less than 1.0 decibel
 per meter after 150 hours of exposure to a temperature of 120.degree. C.,
 as measured by a non-destructive interference filter method; excellent
 photo-thermal stability, wherein a change in the differential transmission
 loss between light wavelengths of 400 nm to 600 nm is equal to or less
 than 1.0 decibel per meter after 100 hours of exposure to a temperature of
 110.degree. C. simultaneously with exposure to 12-15 lumens/square
 millimeter of light, as measured by a non-destructive interference filter
 method; good flexibility, wherein the product, at 20.degree. C., survives
 without core fracture a 180.degree. bend at a bend radius which is less
 than or equal to five times the diameter of the cured core; and good
 hardness properties, wherein the Shore "A" hardness is less than 90 after
 50 days of exposure at 120.degree. C.
 The desired photo-thermal stability is preferably achieved when the polymer
 to be stabilized has a vinyl end-group content, as measured by NMR of
 below 0.5 per 1000 monomer units, as this adjustment leads to improved
 thermal stability as well.
 An alternate way of expressing the photo-thermal stability achieved by the
 invention is that the lifetime, as judged by a 50% change in the
 differential transmission loss between light wavelengths of 400 nm to 600
 nm on exposure to a temperature of 110.degree. C. simultaneously with
 exposure to 12 to 15 lumens/square millimeter of light, as measured by a
 non-destructive interference filter method, is at least 150%, preferably
 200% of that for a similar material absent the stabilizer/antioxidant
 combination.
 It is preferred that the photo-thermally stable light pipe of the present
 invention be mounted in such a way with respect to the illumination source
 that heat from the source is removed by ventilation or insulation means,
 such as by the use of glass-based connectors between the light source and
 the near end of the FLP. It is separately preferred that the light from
 the light source be filtered to remove wave lengths shorter than 370 nm.
 Although not wishing to be bound by any theory of stability of polymers, it
 is believed that it is deleterious to thermal and, to a much lesser
 extent, photochemical stability if the crosslinkable core polymer contains
 oligomers or polymers with terminal vinyl groups. Such oligomers or
 polymers may, in the presence of heat and/or light, form molecules with
 conjugated double bonds which eventually, with sufficient conjugation,
 form species which are color absorbers in the visible region of the
 spectrum, as well as lowering the amount of light which is delivered by
 the light pipe to the final source. Such vinyl double bonds, apart from
 residual monomer which can be reduced by carrying the reaction to higher
 conversion and/or devolatilization of the crosslinkable core prior to
 curing or crosslinking, may be formed by hydrogen abstraction followed by
 chain cleavage, or other forms of radical attack. These radicals may be,
 for example, from the initiator, some reaction product of the initiator,
 or from hydroperoxides formed in the presence of oxygen. The double bonds
 may also be formed by some form of termination reaction during the
 polymerization, even in the presence of a chain transfer agent used to
 reduce the molecular weight and keep the crosslinkable core polymer fluid
 in the melt prior to cladding and curing.
 It has surprisingly been found that reduction of the reaction temperature
 and of the amount of initiator, preferably accompanied by a lowering of
 the residence time in the continuous reactor, is sufficient to make
 significant improvements in the initial color of the polymer core before
 and after curing, and to increase the thermal lifetime, as defined below,
 at 120.degree. C., in the absence of any thermal or thermal-oxidative
 stabilizing additives. These results, especially relating to residence
 time in the reactor and to the temperature of polymerization, would not
 have been expected by one of ordinary skill in the art of bulk
 polymerization of acrylate monomers.
 Although it is known to stabilize polymers of methyl methacrylate against;
 photo-degradation by use of selected antioxidants, the art is sparse in
 teaching appropriate stabilizers against photodegradation of optically
 clear polymers which comprise exclusively or predominantly polymerized
 units of alkyl acrylate monomers. There is even less teaching of
 combination and selection of stabilizer combinations against photo-thermal
 degradation, and it is not predictable from the prior art what binary or
 ternary combination would be effective. For example, alkyl sulfides and
 disulfides, very effective in thermal stabilization of polymethacrylates,
 are not particularly efficacious in photo-thermal stabilization of these
 acrylate polymers.
 Even though the general mode of action of an individual stabilizer can be
 predicted, such as light absorption, conversion of a degradation product
 into a molecule which does not absorb visible light, or interfering with
 chain reactions caused by primary chain cleavage or abstraction, its
 interaction with a poly(alkyl acrylate) is difficult to predict. Further,
 the art is silent on the potential mode of response for combinations of
 stabilizer active in different modes as applied to poly(alkyl acrylates).
 As seen in the Examples, there exist individual stabilizers effective only
 in combination with others, as well as combinations which are not
 efficient enough to achieve the stabilization goal which can be achieved
 by certain selected additives.

EXPERIMENTAL
 The various stabilizers and antioxidants studied are tabulated below (Table
 I) by trade name, supplier, class, and by the best structure available
 from the descriptive literature.
 TABLE I
 Stabilizers and Antioxidants Considered in this Application for Photo-
 Thermal Stabilizers for an Acrylate-Based Flexible Light Pipe
 Design-
 Name/
 ation Type Formula
 Supplier
 RP-1 hindered phenol/isocyanurate tris(3,5-di-t-butyl
 4-hydroxybenzyl)isocyanurate Irganox 3114
 HP-2 hindered phenol butylated hydroxytoluene
 (2,6-di-t-butyl-4- BHT
 methylphenol)
 HP-3 hindered phenol
 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4- Ethanox 330
 hydroxybenzyl)benzene
 HP-4A hindered phenol tetrakis(methylene(3,5-di-t-butyl-4-
 Irganox 1010
 hydroxyhydrocinnamate))methane
 HP-4B hindered phenol tetrakis(methylene(3,5-di-t-butyl-4-
 Ultranox 210
 hydroxyhydrocinnamate))methane
 HP-5A hindered phenol octadecyl
 3,5-di-t-butyl-4-hydroxyhydrocinnamate Irganox 1076
 HP-5B hindered phenol octadecyl
 3,5-di-t-butyl-4-hydroxyhydrocinnamate Ultranox 276
 HP-6 hindered phenol 3/1 condensate of
 3-methyl-6-t-butylphenol and Topanol CA
 crotonaldehyde; believed to be mainly
 1,1,3-tris(2-
 methyl-4-hydroxy-5-t-butylphenyl)butane
 HP-7 hindered phenol and organic benzenepropanoic acid,
 3,5-bis(1,1-dimethylethyl)-4- Irganox 1035
 sulfide hydroxy-, thiodi-2,1,ethanediyl ester or
 thiodiethylene
 bis(3,5,-di-tert-butyl-4-hydroxy-
 hydrocinnamate)
 HSP-1 hydrolytically stable organic 2,2'-Ethylidenebis(4,6-di-t-
 Ethanox 398
 phosphite butylphenyl)fluorophosphonite
 HSP-2 hydrolytically stable organic tris(2,4-di-tert-butylphenyl)
 phosphite Irgafos 168
 phosphite
 HSP-3 hydrolytically stable organic Phosphorus Trichloride, Reaction
 Products with 1,1'- P-EPQ
 phosphite biphenyl and
 2,4-bis(1,1-dimethylethyl)Phenol
 HUSP-1 hydrolytically unstable organic
 bis(2,4-di-t-butylphenyl)pentaerythritol diphosphite Ultranox 626
 phosphite
 HUSP-2 hydrolytically unstable organic Diisodecyl pentaerythritol
 diphosphite Weston XR
 phosphite
 2806
 NHP non-hindered phenol monomethyl ether of hydroquinone
 MEHQ
 OS-1 organic sulfide dilauryl thiodipropionate
 DLDTP
 ODS-1 organic disulfide di(t-dodecyl)disulfide
 DTDDS
 A standard laboratory process was employed as the control, following the
 method of Example 1 (tube filling) and Example 29 (compositional details)
 of U.S. Pat. No. 5,485,541. The monomer composition was 95% EA (purified
 through acidic alumina) and 5% distilled MATS
 (3-methacryloxypropyltrimethoxysilane). Vazo 67, (DuPont)
 2,2'-azobis(2-methylbutyronitrile) initiator was used at a level of 0.064%
 of the monomer. A chain transfer agent, n-dodecyl mercaptan, was used at a
 level of 1% of the amount of monomer. The standard reactor temperature was
 125.degree. C. and the standard residence time was 28 minutes. After
 devolatilization, the polymer was used to fill FEP/polyethylene tubes.
 Catalyst (20 ppm dibutyltin diacetate, based on polymer, in butyl acetate)
 and water (0.40%) were separately mixed into the polymer as it was pumped
 into the tubes. A third solution, containing the selected antioxidants or
 stabilizers, was added at a rate of 2.4 cc. of solution per 100 grams of
 polymer. The variations utilized (beyond the stabilizer/antioxidants) are
 summarized in Table 2 (below).
 The following outlines the details of the standard polymerization, which is
 used as the basis for the process changes listed in Table I: Monomer mixes
 were prepared as follows: To a 19 liter 316 stainless steel vessel were
 added and mixed 9500 g of ethyl acrylate, 500 grams of the functionally
 reactive monomer, 3-methacryloxypropyltrimethoxysilane (MATS) (5 wt. %
 based on monomer weight (b.o.m.), 6.4 g. of initiator (recrystallized
 2,2'-azobis(2-methylbutyronitrile) (0.064 wt. % ) and 100 g. of n-dodecyl
 mercaptan (1 wt. %). The mixture was sparged for at least 15 minutes with
 nitrogen and degassed under 28 inches (711 mm.) vacuum as it was pumped
 into the reactor.
 The monomer mix was fed through a 0.045 micron PTFE membrane cartridge
 filter to a 2000 ml stainless steel constant flow stirred tank reactor
 (CFSTR). During polymerization, flow rates for the 2000 ml CFSTR were ca.
 70 g/min. to produce a 28-minute residence time. The CFSTR was equipped
 with multiple (6) blade 45.degree. pitch turbine agitators. During
 polymerization, the reactors were held at 125.degree. C., and agitated at
 225 rpm under a pressure of 1035 kPa (150 psi). Reactor effluent
 (copolymer and residual monomer) was fed through a back-pressure valve set
 nominally at 1035 kPa (150 psi) into a devolatilization column comprising
 a stainless steel twisted-tape motionless mixer (60 cm. in length with a
 jacket of about 50 cm length) mounted on an 39-liter (ca. 9-gallon)
 stainless steel catchpot. Heating oil recirculated through the column
 jacket was held at 200.degree. C. at the jacket inlet. The catch-pot was
 held at 100-110.degree. C. and ca. 300-400 mm. of vacuum during
 devolatilization. Upon completion of the polymerization, the catch-pot was
 back-filled with filtered nitrogen. The monomer-to-polymer conversion of
 the effluent was approximately 87-88%, as measured gravimetrically.
 Gravimetrically determined solids content of the devolatilized polymer
 typically is 99.5 wt.
 Polymer variations used in the evaluation of antioxidants are summarized in
 Table 2.
 TABLE 2
 Polymer/Process Variations
 Variable Standard Variations
 Monomer 95% EA/5% 66.5% EA/28.5% BMA/5% MATS
 Composition MATS 95% EA/5% MATS + 0.5% ETEMA
 66.5% EA/28.5% BMA/5%
 MATS + 0.5% ETEMA
 EA Purifica- Acidic Alumina Basic Alumina and Molecular Sieve
 tion
 Initiator 0.064% Vazo 67 0.032% Vazo 67
 0.0208% Vazo 52
 0.0104% Vazo 52
 Chain 1.0% n-DDM 1.5% n-DDM (n-dodecyl mercaptan)
 Transfer 0.6% t-BuSH (t-butyl mercaptan)
 Agent 0.97% MPTMS (mercaptopropyl
 trimethoxysilane)
 MATS Distilled 5 ppm 4-hydroxyTEMPO (2,2,6,6-
 tetramethyl-4-hydroxy-
 piperidine-N-oxyl),
 in MATS
 Reaction 125 C. 95 C.
 Temper- 105 C.
 ature
 Residence 28 minutes 22 minutes
 Time
 BMA = butyl methacrylate
 ETEMA = ethylthioethyl methacrylate
 Vazo 52 = DuPont 2,2'bisazo(2,4-dimethylvaleronitrile), a lower temperature
 initiator
 Table 3 lists the actual polymers that were prepared and evaluated. 15-35
 kg of polymer was produced in each preparation. This was sufficient to
 make six FEP/polyethylene tubes (5.1 mm id) 2 meters in length for each of
 3-12 antioxidant combinations. In addition, 6-12 tubes were prepared with
 cure additives but no added antioxidants.
 TABLE 3
 Polymer Composition and Process
 Run Polymer
 # ID RM Variables Process Variables
 1A AB2441 1.5% nDDM Standard
 1B AB2457 1.5% nDDM Standard
 2 AB2468 Standard Standard
 3 AB2480 0.5% ETEMA Standard
 4 AB2488 28.5% BMA + 0.5% ETEMA Standard
 5 AB2601 28.5% BMA Standard
 6 AB2620 Standard 22'
 7 AB2628 0.6% t-BuSH Standard
 8 AB2643 Y-11700 MATS Standard
 9 AB2842 Standard Standard
 10 AB2610 Standard 95.degree. C.
 11 AB2637 0.032% Vazo 67 105.degree. C.
 12 AB2651 0.6% t-BUSH 105.degree. C.
 13 AB2661 0.0208% Vazo 52 95.degree. C., 22'
 14 AB2669 0.0208% Vazo 52, 1.5% nDDM 95.degree. C., 22'
 15 AB2689 Y-11700 MATS 105.degree. C.
 16 AB2811 0.0208% Vazo 52 95.degree. C., 22'
 17 AB2817 0.0208% Vazo 52, 1.5% nDDM 95.degree. C., 22'
 18 AB2822 0.0208% Vazo 52, 0.97% 3-MPTMS 95.degree. C., 22'
 19 AB2826 0.0104% Vazo 52 95.degree. C., 22'
 20 AB2850 0.0208% Vazo 52 95.degree. C.
 21 AB2858 0.0208% Vazo 52, EA Purified 95.degree. C., 22'
 through Basic Alumina and
 Molecular Sieve
 Thermal Degradation
 Light pipe was evaluated for thermal stability by measuring the time
 required for the transmitted light to become yellow. The absorption vs.
 wavelength spectrum of a 6 foot section of light pipe was measured. The
 difference in the absorption at 400 nm and 600 nm (A.sub.400 -A.sub.600)
 was calculated from the spectrum. Since thermal aging causes an increase
 in the absorbance at short wavelengths (400 nm) but little change at long
 wavelengths (600 nm), changes in this difference (A.sub.400 -A.sub.600)
 are a measure of increases in the yellowness of transmitted light. The
 light pipe was thermally aged in a forced air oven at 120.degree. C.
 Periodically, the light pipe was removed from the oven, the absorption
 spectrum was measured, and A.sub.400 -A.sub.600 was calculated. The
 thermal lifetime was calculated as the time required for the absorbance to
 increase by 1 dB/m from its initial value.
 The thermal lifetimes (in hours) of the light pipes containing antioxidants
 are recorded in tables 3 and 4. For comparison, the thermal lifetimes of
 the controls, light pipes prepared from the same core polymer but
 containing no antioxidant, are also included in these tables. The hindered
 phenolics which have low color, especially Irganox 1076 and Ultranox 276
 (HP-5A and HP-5B), increase the thermal lifetime of the light pipe. Even
 larger increases are observed with combinations of these hindered
 phenolics and aromatic phosphites with ortho-alkyl substituents,
 especially HSP-2.
 The light pipes prepared from a hindered phenolic and diisodecyl
 pentaerythritol diphosphite (HUSP; see table 3) were not tested since
 these light pipes became very hazy on storage. We have observed a similar
 hazing phenomenon for light pipes containing trisisooctyl phosphite,
 phenyl neopentylene glycol phosphite, and tris(dipropylene
 glycol)phosphite. The large light losses associated with this haziness
 makes formulations containing these aliphatic or partially aliphatic
 phosphites unsuitable for light pipe applications.
 Photothermal Degradation
 Photothermal durability studies were performed using the General Electric
 XMH-60 lamp with the filter substituted by an Optivex filter. The light
 was passed through a mixing rod (11.5 mm square coupler) to provide a
 uniform light output of 12-15 lumens per square millimeter. Four 5 mm
 light pipes were heat shrunk onto glass rods and these were then heat
 shrunk onto the square coupler. The light pipes then passed through an
 oven at 110.degree. C. The fibers were connected to a filter/photodiode
 holder.
 Periodically during the test the photodiode response was measured through
 400 nm, 450 nm and 600 nm filters. The data was treated by dividing the
 400 nm reading by the 600 nm reading and normalizing for the initial
 ratio. A plot of
EQU (%T.sup.t.sub.400 /%T.sup.t.sub.600)/(%T.sup.0.sub.400 /%T.sup.0.sub.600)
 vs. time
 was constructed, where (%T.sup.t.sub.400 /%T.sup.t.sub.600) is the voltage
 ratio at time t, and (%T.sup.0.sub.400 /%T.sup.0.sub.600) is the initial
 voltage ratio. The lifetime is defined as the time at which this ratio
 falls to 0.5 and was determined by interpolation. This corresponds to a
 50% loss in initial transmission at 400 nm. It correlates fairly well with
 the time at which the light transmitted through 5 foot of light pipe
 appears yellow.
 One of the four light pipes in each set was a control, a light pipe made
 with the same polymer but containing no added antioxidants. The durability
 recorded for each formulation in the following tables is the ratio of the
 lifetime of the light pipe containing antioxidants to that of the control.
 It therefore, represents the increase in lifetime due to the presence of
 antioxidants. The lifetime of the controls varies from 35 to 110 hours,
 depending on the polymer formulation and on the conditions of the
 particular aging experiment (light intensity).
 TABLE 4
 Durability of Light Pipe Prepared at 125.degree. C.
 (thermal lifetime/photothermal lifetime/photothermal lifetime ratio)
 Antioxidant
 Level, 2441 2457 2468 2480
 2488 2601 2620 2628 2643
 2842
 Antioxidant(s) ppm (Ex. 1A) (Ex. 1B) (Ex. 2) (Ex. 3)
 (Ex. 4) (Ex. 5) (Ex. 6) (Ex. 7) (Ex. 8)
 (Ex. 9)
 NONE _/53/.sub.-- 88/61-76/.sub.--
 16-26/43-63/.sub.-- _/42-58/.sub.-- _/25-36/.sub.-- _/22-51/.sub.--
 69/46-85/.sub.-- _/39-59/.sub.-- 24/37-64/.sub.-- 141-200/65/.sub.--
 Irganox 1035 1000 103/_/ .sub.--
 _/71/2.5
 (HP-7)
 2000 _/34/0.8
 3000 60/_/.sub.--
 _/68/1.5 _/45/1.6
 Topanol CA 2000 25/_/.sub.--
 (HP-6)
 3000 _/36/0.9
 _/36/1.4
 Irganox 1010 1000 97/_/.sub.--
 (HP-4A)
 3000 58/_/.sub.--
 _/64/1.4 _/60/2.4
 Irganox 1076 or 1000 &gt;121/_/.sub.--