WO1997015846A1

WO1997015846A1 - Polymeric optical fiber formulation, method of fabrication, and uses therefor

Info

Publication number: WO1997015846A1
Application number: PCT/US1996/014314
Authority: WO
Inventors: Jerry W. Williams; James R. Zarian; Gerald L. Uhl; Jeanne M. Goetzke
Original assignee: Minnesota Mining And Manufacturing Company
Priority date: 1995-10-24
Filing date: 1996-09-06
Publication date: 1997-05-01
Also published as: AU6916196A

Abstract

A polymeric optical fiber is the reaction product of: from about 100 parts to about 80 parts of at least one methacrylate monomer derived from the esterification of methacrylic acid with an alcohol having at least 4 carbon atoms; and from about 0 part to about 20 parts of at least one free-radically polymerizable ethylenically-unsaturated cross-linking agent. The polymeric optical fiber of the invention exhibits light transmission over a temperature range of from about -40 °C to about 125 °C and further exhibits long-term (on the order of weeks) thermal stability and flexibility during aging at temperatures up to about 125 °C without degradation of physical and optical properties.

Description

Polymeric Optical Fiber Formulation, Method of Fabrication, and Uses Therefor

Technical Field

This invention relates to large core polymeric optical fibers, in particular light pipes, an improved method of fabricating the optical fibers, and formulations useful in the preparation thereof.

Background of the Invention

Polymeric optical fibers have been described. For example, U.S. Patents No.

5,298,327, 5,225,166, 5,221,387, 5,149,467, 5,122,580, 5,067,831, 5,052,778, 4,957,347, 3,641,332 and 4,422,719 describe large-diameter light-transmitting polymeric optical fibers and a method of preparing the optical fibers for use in industry and commerce. These optical fibers are known to lose flexibility and shrink in both diameter and length upon aging at ambient temperatures, thus limiting their overall utility. On aging at elevated temperatures (125°C), these optical fibers also lose optical transmissivity, turning brown and becoming opaque.

Summary of the Invention

Briefly, in one aspect ofthe present invention a polymeric optical fiber is provided comprising a polymer consisting essentially ofthe reaction product of: from about 100 parts to about 80 parts of at least one methacrylate monomer derived from the esterification of methacrylic acid with an alcohol having at least 4 carbon atoms, and; from about 0 parts to about 20 parts of at least one free-radically polymerizable ethylenically-unsaturated crosslinking agent.

In a further aspect, this invention provides a polymerizable mixture consisting essentially of: from about 100 parts to about 80 parts of at least one methacrylate monomer derived from the esterification of methacrylic acid with an alcohol having at least 4 carbon atoms; from about 0 parts to about 20 parts of at least one free-radically polymerizable ethylenically-unsaturated crosslinking agent, and; from about 0.01 parts to 5 parts of at least one free-radical initiator, based on the total weight of monomer(s) plus crosslinking agent.

Preferably, the polymeric optical fiber ofthe invention exhibits light transmission over a temperature range of from about -40°C to about 125°C. Advantageously, unique optical fibers ofthe present invention exhibit long-term (on the order of weeks) thermal stability during aging at temperatures up to about 125°C without degradation of physical and optical properties. Furthermore, optical fibers of the present invention unexpectedly exhibit excellent physical properties, such that the optical fibers remain essentially undamaged when bent around a mandrel having a diameter that is approximately eight (8) times the diameter ofthe light pipe both before and after thermal aging. Optical fibers exhibiting a combination of all of these properties have not been described heretofore.

In a further aspect ofthe invention, an improved method of fabricating the optical fibers is provided, comprising: a. mixing, at a first temperature below the polymerization temperature, a predetermined quantity of copolymerizable monomers, the copolymer resulting from the copolymerization ofthe monomers having a glass transition temperature (Tg); b. filling an elongated container with the mixture of monomers; c. maintaining the mixture-filled elongated container at a second temperature below the polymerization temperature; d. inclining the container at an angle from the horizontal of at least 5°; and e. heating the container and the mixture progressively from one end of the container to the other end to a temperature sufficiently high to cause polymerization ofthe mixture progressively from one end ofthe container to the other end, wherein the sufficiently high temperature is no more than about 60° C above the Tg. Optionally, the process make include the further step of pressurizing the elongated container to a predetermined pressure, for example to a pressure of about 250 psi. Preferably, the temperature at which the polymerization takes place can be from about 0° to about 50°C, more preferably from about 20° to about 50°C, and most preferably from about 30° to about 45°C. Description of the Preferred Embodiments)

Polymeric optical fibers ofthe invention may be prepared by copolymerizing at least one methacrylate monomer derived from the esterification of methacrylic acid with an alcohol having at least 4 carbon atoms with an ethylenically-unsaturated free-radically polymerizable crosslinking agent.

Particularly useful methacrylate monomers derived from the esterification of methacrylic acid with an alcohol having at least 4 carbon atoms include, but are not limited to, n-butyl methacrylate, n-hexyl methacrylate, 2-ethylhexyl methacrylate, n-octyl methacrylate, isooctyl methacrylate, decyl methacrylate and dodecyl methacrylate, and combinations thereof. Preferably, the methacrylate monomers include n-butyl methacrylate, n-hexyl methacrylate, n-octyl methacrylate, 2-ethylhexyl methacrylate, n-decyl methacrylate and dodecyl methacrylate, and combinations thereof.

Crosslinking agents useful in the invention are those ethylenically-unsaturated free-radically polymerizable compounds possessing two or more free-radically polymerizable ethylenically unsaturated reactive moieties, such as, for instance, acrylate, methacrylate, allyl, or styryl groups, or the like. Preferred crosslinking agents are dimethacrylates and diallyl compounds, most preferably dimethacrylate compounds. Useful dimethacrylate crosslinking agents include, but are not limited to, hexanediol dimethacrylate, ethylene glycol dimethacrylate, diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, propylene glycol dimethacrylate, trimethylol propane dimethacrylate, and methacrylate-terminated oligomers such as polyethylene glycol dimethacrylate (commercially available from Sartomer Chemicals under various tradename designations) and polypropylene oxide dimethacrylate, and combinations thereof Preferably, the dimethacrylate crosslinking agent is selected from the group consisting of diethylene glycol dimethacrylate, triethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, propylene glycol dimethacrylate, and combinations thereof.

Methacrylate monomers and dimethacrylate crosslinking agents, when purchased from commercial sources, must be purified to remove inhibitors, colorants, and any other additives commonly used. They can be purified by any of a number of common procedures used, e.g., in analytical chemistry, for sample preparation. These methods include, e.g., vacuum distillation, ion exchange column, and liquid- liquid extraction. In practice, a number of methods have been shown to be effective, and a skilled practitioner will be able to choose a method consistent with both the monomers or crosslinking agents and the impurities known to be therein.

Free-radical initiators useful in preparing the optical fibers ofthe invention can be any of a number of art-known thermal initiators, including azo compounds and peroxides. Suitable azo initiators include, but are not limited to, 2,2'-azobis(4-methoxy-

2,4-dimethylvaleronitrile) (VAZO™ 33), 2,2^,-azobis(2-amidinopropane) dihydrochloride (VAZO™ 50), 2,2'-azobis(2,4-dimethylvaleronitrile) (VAZO™52), 2,2'-azobis(isobutyronitrile) (VAZO™ 64), 2,2'-azobis(2-methylbutyronitrile) (VAZO™ 67), and l,l'-azobis(l-cyclohexanecarbonitrile) (VAZO™ 88), all of which are available from DuPont Chemicals, Wilmington, DE, and 2,2'-azobis(methyl isobutyrate) (V-601™), available from Wako Chemicals USA Inc., Richmond, VA. Suitable peroxide initiators include, but are not limited to, benzoyl peroxide, acetyl peroxide, lauroyl peroxide, decanoyl peroxide, dicetyl peroxydicarbonate, di(4-t-butylcyclohexyl)peroxydicarbonate (Perkadox™ 16), available from Akzo, Nobel Chemicals, Inc., Chicago, IL, di(2-ethylhexyl)peroxydicarbonate, t- butylperoxypivalate (Lupersol™ 11), available from Elf Atochem North America, Philadelphia, PA, diisopropyl peroxydicarbonate, available from PPG Industries, Inc., Chemicals Group, Pittsburgh, PA, t-butylperoxy-2-ethylhexanoate (Trigonox™ 21- C50), available from Akzo, Nobel Chemicals, Inc., and dicumyl peroxide. Most preferred peroxide initiators include di(4-t-butylcyclohexyl)peroxydicarbonate (Perkadox™ 16) and diisopropyl peroxydicarbonate.

The initiator is present in a catalytically-effective amount and such amounts are typically in the range of from about 0.01 parts to 5 parts, and more preferably in the range of from about 0.025 parts to about 2 parts by weight, based upon 100 total parts by weight of monomer(s). If a mixture of initiators is used, the total amount of the mixture of initiators would be as if a single initiator was used. Polymeric optical fibers ofthe invention are prepared as follows: The monomer(s), crosslinking agent(s) and initiator(s) are mixed and then introduced into an elongated polymeric tube under a slight vacuum. The tube is formed into a U- shape, then placed into a reactor pressurized with nitrogen to a predetermined pressure, for example pressurized to about 250 psi, wherein the mixture is progressively polymerized from one end to the other by heating the length ofthe tube progressively at a rate of about 0.75 to about 1.5 meters per hour, using cold nitrogen to provide a cold, non-reactive zone and heated water to create a zone that is warm enough to induce the polymerization reaction while, at the same time, carrying away excess heat of reaction. The procedure and apparatus used is described in detail in U.S. Patents No. 5,122,580, 5,298,327 and 5,225,166, the contents of which are incoφorated herein by reference.

The polymeric tube used as the cladding or sheath for the polymeric light pipe ofthe invention are chosen so as to have a refractive index that is significantly less than the refractive index ofthe polymeric core. In addition, the cladding must be essentially transparent to the light being transmitted in the core, and preferably is not soluble in the monomers used to make the core and is substantially inert to free- radicals generated from the initiator. Useful sheath materials are chosen from any of a number of highly-fluorinated and perfluorinated commercially available polymeric materials, including, but not limited to, poly(tetrafluoroethylene), FEP (fluorinated ethylene-propylene) Teflon™, and THN, a teφolymer of tetrafluoroethylene, vinylidene fluoride and hexafluoropropene. Optionally, the sheath material may be heat-shrinkable.

Previously described light pipes that were prepared by the above-described in situ method often were not sufficiently smooth on their outer surface (the surface formed next to the cladding), and suffered loss of light at numerous defects on their surface. Furthermore, previously described light pipes also suffer loss of light due to point defects in the core ofthe fiber. It has been discovered that rigorous control of both the absolute polymerization temperature and the relative temperature interval between the glass transition temperature (Tg) ofthe copolymer being formed and the polymerization temperature suφrisingly results in a light pipe that is significantly superior to those described in, e.g., U. S. Patent No. 5,122,580 as regards smoothness, transmissivity over the length ofthe fiber, and in attenuation at wavelengths between about 450 nm and 700 nm. Table 1 shows comparative attenuation for two fiber cores prepared according to the invention (A and B) and a commercially-available (Lumenyte International Coφ., Costa Mesa, CA) core prepared according to the method of U. S. Patent No. 5,122,580.

Preferably, the temperature at which the polymerization takes place can be from about 0° to about 50° C, more preferably from about 20° to about 50° C, and most preferably from about 30° to about 45° C. At the same time that polymerization temperature is held within these preferred temperature ranges, the temperature interval between the polymerization temperature and the Tg ofthe copolymer thus produced preferably is from about 35° C to about 60° C. The glass transition temperature ofthe optical fiber core can be calculated by any of number of common methods (Encyclopedia of Polymer Science and Engineering, H. F. Mark et al., eds., John Wiley & Sons, Inc., New York, 1987, Vol. 7, pp 531 - 544), given the glass transition temperatures ofthe homopolymers resulting from polymerization of each ofthe monomeric constituents.

Polymeric optical fibers ofthe invention can be used as illumination fiber optics, which can exhibit at least three separate functions of illumination: one being "point lighting," i.e., where the primary function ofthe fiber optic is to produce a means of conducting the light from a light source to a remote point and emitting the light from the end ofthe fiber, a second being "linear lighting," i.e., wherein the primary function ofthe fiber optic is to provide a means of conducting a light from a light source linearly along the length ofthe fiber optic and emitting the light from the sides ofthe fiber optic and a third being "controlled light redirection," i.e., wherein the light is redirected in a controlled manner out ofthe fiber optic by means of optical elements introduced into the fiber core. Advantages shown by fiber optics ofthe invention over commercially-available light pipes include overall improved light transmissivity over long distances and improved aging, i.e., dimensional stability as well as no loss of flexibility or light transmission, at ambient and elevated temperatures.

The objects, features and advantages ofthe present invention are further illustrated by the following examples, but the particular materials and amounts - 7 - thereof recited in these examples, as well as other conditions and details, should not be construed to unduly limit this invention.

Examples

All chemicals and materials are available from commercial sources, such as Aldrich Chemical or known to those skilled in the art, unless otherwise noted or apparent ."Parts" means parts per hundred resin.

Example 1

N-butyl methacrylate was purified by vacuum distillation at approximately

65°-70°C at about 36 kPa. Triethylene glycol dimethacrylate was purified by passing it dropwise through an ion exchange column (commercially available from Scientific Polymer Products, Ontario, NY under the trade name of DHR-4). A mixture of 100 parts purified n-butyl methacrylate, 1 part purified triethylene glycol dimethacrylate, and 0.2 parts Perkadox 16S™ was prepared and sparged with nitrogen gas for at least three minutes, then poured into a FEP Teflon™ (commercially available from DuPont) tube measuring approximately 16 meters by approximately 9.5 mm (interior diameter, "i.d ") using a slight vacuum to assist in filling the tube. One end ofthe tube was then heat sealed, and the filled tube was placed in a reactor as described in U.S. Patents No. 5,298,327 and 5,225,166, previously incoφorated by reference. The reactor was sealed, pressurized to 250 psi (approximately 1.7 MPa) with nitrogen gas, then chilled to about 2°C. Water heated to about 60°C was introduced into the bottom ofthe reactor, and was caused to rise at a rate of approximately 0.9 meter per hour. When the heated water had reached the top ofthe reactor, the temperature was held for two hours, after which the reactor was emptied and the polymer-filled tube was cooled and removed from the reactor. The resultant light pipe had excellent optical properties and showed no obvious voids or blemishes. On exposure to 125°C temperatures in an oven for 24 hours, the sample maintained an excellent optical quality and flexibility.

Example 2

A light pipe was prepared as described in Example 1, comprising 50 parts purified n-butyl methacrylate and 50 parts of purified n-hexyl methacrylate and 1 part diethylene glycol bis(allyl carbonate) (commercially available from Akzo Chemicals, Inc, Chicago, EL under the tradename of Nouryset 200). N-hexyl methacrylate was purified by washing twice with a 5% aq solution of NaOH followed by one wash with deionized water and one wash with a saturated aq solution of magnesium sulfate, then dried over solid magnesium sulfate. The diethylene glycol bis(allyl carbonate) (Nouryset 200™) was used as purchased.

A light pipe was prepared in a 16 meter x 6 mm FEP Teflon™ tube as described in Example 1. A highly flexible light pipe was obtained, having excellent optical properties and visual examination indicated no obvious voids or blemishes. On exposure to 125°C temperatures in an oven for 24 hours, the sample maintained an excellent optical quality and flexibility. On close exposure to an unfiltered high intensity lamp (GE 150 W halogen bulb) some material flow was observed for this softer fiber.

Example 3

A light pipe was prepared in the manner described in Example 1, comprising 50 parts purified n-butyl methacrylate, 50 parts purified 2-ethylhexyl methacrylate, 0.1 parts triethylene glycol dimethacrylate and 0.2 parts diisopropyl peroxydicarbonate. Both 2-ethylhexyl methacrylate and triethylene glycol dimethacrylate were purified neat by successive passage through 1 -meter columns of approximately 5 cm diameter packed with HR-4 ion exchange beads (commercially available from Scientific Polymer Products, Ontario, NY), Grade 13X molecular sieves (commercially available from UOP, Des Plaines, IL, W. R. Grace & Co., Baltimore, MD), Grade 22 silica gel (commercially available from Grace Davison), and chromatographic activated alumina (Grade F-20, commercially available from Alcoa Industrial Chemicals Div., Vidalia, LA) at a rate of approximately 1 bed- volume per hour. A 16-meter x 6 mm FEP Teflon™ tube was filled with the above mixture, sealed at one end and pressurized with nitrogen gas to about 1.7 MPa (250 psi), then heated at 45° C at a rate of about 0.8 m/hr. Nitrogen gas above the water level in the reactor was maintained at about 0° C throughout the polymerization process. The light pipe was tested for attenuation, as shown in Table 1. By visual inspection, there were noticeably fewer light-scattering defects in the light pipe than in a comparable commercial sample. Example 4

A light pipe was prepared as described in Example 3 except that 30 parts of n-butyl methacrylate and 70 parts of 2-ethylhexyl methacrylate were used. . The light pipe was tested for attenuation, as shown in Table 1. By visual inspection, there were noticeably fewer light-scattering defects in the light pipe than in a comparable commercial sample.

The light pipes of Examples 3 and 4 were measured for light attenuation (percent loss/30.5 cm) by the "cutback" method, quantified by the equation:

I(L) = I₀ e-^αL where I(L) represents the light intensity I at a specific fiber length L, I_o represents the input light intensity, α represents the attenuation coefficient (describing both scatter and absoφtion losses), expressed as percent loss per foot (30.5 cm). Fiber samples of 9.5 mm diameter x 9.3 m were illuminated at one end by light at specific wavelengths and the output intensity for each wavelength was measured and recorded. Then, a 1.5 m portion ofthe light pipe was cut off and the output intensity measurement was repeated for each wavelength. The last measurement was taken at a fiber length of 3 . For each wavelength used, intensity vs. fiber length was fit to an exponential curve using a least-squares method to determine the attenuation coefficient. Data is reported in Table 1 as attenuation vs. wavelength.

Thus, a 50 Watt intensity-regulated tungsten halogen light source with a nominally collimated output (Model 780, Newport Coφ., Fountain Valley, CA) was coupled through an iris to a chopper wheel (Model 220, Ithaco, Inc., Ithaca, NY) for lock-in detection and through a set of narrow band pass filters (Oriel Instruments, Stratford, CT) and focused via a 40X microscope objective having a numerical aperture (N.A.) of 0.85 (Carl Zeiss, Inc., Microscope Div., Thornwood, NJ) onto the end ofthe optical fiber to be tested. The detection end ofthe apparatus consisted of a large-scale silicon photodetector (Model 250, United Detector Technology, El Paso, TX) with a low-noise, high-gain preamplifier fed into a lock-in amplifier. For efficient, repeatable coupling ofthe light to the fiber, a microscope cover glass slip was optically contacted to the input face ofthe fiber using an index matching gel having an index intermediate between that ofthe cover glass slip and the fiber core. The same gel was used to optically contact the output end ofthe fiber directly to the photodetector.

Table 1

The data of Table 1 show that light pipes fabricated by the method ofthe present invention show considerably less light attenuation at most wavelengths across the visible spectrum.

Various modifications and alterations of this invention will become apparent to those skilled in the art without departing from the scope and principles of this invention, and it should be understood that this invention is not be unduly limited to the illustrative embodiments set forth hereinabove. All publications and patents are incoφorated herein by reference to the same extent as if each individual publication or patent was specifically and individually indicated to be incoφorated by reference.

Claims

1. A polymeric optical fiber comprising a polymer consisting essentially ofthe reaction product of:

(a) from about 100 parts to about 80 parts of at least one methacrylate monomer derived from the esterification of methacrylic acid with an alcohol having at least 4 carbon atoms, and;

(b) from about 0 parts to about 20 parts of at least one free-radically polymerizable ethylenically-unsaturated crosslinking agent.

2. A polymerizable mixture consisting essentially of:

(a) from about 100 parts to about 80 parts of at least one methacrylate monomer derived from the esterification of methacrylic acid with an alcohol having at least 4 carbon atoms;

(b) from about 0 parts to about 20 parts of at least one free-radically polymerizable ethylenically-unsaturated crosslinking agent, and;

(c) from about 0.05 parts to 5 parts of at least one free-radical initiator.

3. A method of fabricating an optical fiber is provided, comprising the steps of: a. mixing, at a first temperature, a predetermined quantity of copolymerizable monomers, the copolymer resulting from the copolymerization of the monomers having a glass transition temperature (Tg), wherein the first temperature is below the polymerization temperature ofthe copolymerizable monomers; b. filling an elongated container with the mixture of copolymerizable monomers; c. maintaining the mixture-filled elongated container at a second temperature below the polymerization temperature; d. inclining the container at an angle from the horizontal of at least 5°; and e. heating the container and the mixture progressively from one end of the container to the other end to a temperature sufficiently high to cause polymerization ofthe mixture progressively from one end ofthe container to the other end, wherein the sufficiently high temperature is no more than about 60° C above the Tg.

4. The method according to claim 3, further including the step of pressurizing the elongated container to a predetermined pressure.