Photocurable low refractive index coatings

Details Photocurable low refractive index coatings Photocurable low refractive index coatings

1998-05-14

2001-06-19

Sanghavi, Hemang (Department: 2874)

Optical waveguides

Having particular optical characteristic modifying chemical...

Of waveguide cladding

C385S144000, C385S126000, C372S006000, C359S341430

Reexamination Certificate

active

06249638

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to curable materials which are derived from di- or multifunctional(meth)acrylates. Di- or multifunctional(meth)acrylates are reacted with multifunctional materials so as to partially crosslink the di- or multifunctional (meth)acrylate and provide a viscosity increased material which can be cured or subsequently polymerized to a final product.
BACKGROUND OF THE INVENTION
Although most optical fibers consist of doped or undoped glass core surrounded by a doped or undoped glass cladding, there are several cases in which it may be advantageous to use a polymeric material as cladding instead of siliceous glass. Polymeric materials are flexible, so they do not break as easily when the fiber is bent. Besides, the refractive index of polymers can be made lower than that achievable by doping silica with fluorine or boron. This lower refractive index provides a larger numerical aperture, thereby increasing the acceptance angle for the incoming radiation. U.S. Pat. No. 4,511,209 and European Pat. No. 0 333 464 B1 disclose polymer compositions suitable for use as claddings for optical fibers.
Other devices that require low refractive index coatings are optical fiber lasers and amplifiers (cladding pumped lasers) such as those described in co-assigned U.S. patent application Ser. No. 08/561,682 and in U.S. Pat. No. 4,815,079. These devices comprise a doped glass core, a doped or undoped glass cladding and an outer polymer cladding. The core is pumped by a laser guided through the inner (glass) cladding; the outer (polymer) cladding has an even lower refractive index so that the combination inner/outer cladding can act as a waveguide. The numerical aperture and, therefore, the output power of this device is a function of the difference in the refractive indices between the inner and outer claddings. In this sense, it is desirable to have a polymer cladding with the lowest possible refractive index.
The process currently used to coat glass fibers with polymers comprises applying a UV-curable formulation with a die. Acrylate-functional formulations are the most widely used coating materials because of their extremely fast curing rates, ease of synthesis and commercial availability. Commercially available low-refractive index polymer coating formulations have indices on the order of 1.38-1.42 (for example “OPTI->CLAD” supplied by Optical Polymer Research, Inc., Gainesville, Fla.). The patent literature shows coatings with refractive indices in the range 1.37-1.43 (U.S. Pat. No. 4,508,916, U.S. Pat. No. 4,511,209, U.S. Pat. No. 4,971,424, U.S. Pat. No. 5,024,507, and U.S. Pat. No. 5,239,026). There are some newer materials that have much lower refractive indices (1.31-1.33). Nevertheless, there is no practical process currently available to coat these materials onto a glass fiber because their viscosities are too low for a standard die-coating application. Additionally, these materials are highly fluorinated and commercially available photoinitiators (required for a UV-curing formulation) are typically not soluble in the lowest refractive index materials.
One means for circumventing these problems is disclosed in European published patent application 0 521 360 A2. However, the approach of the reference is quite complicated.
B-staging is a procedure commonly used with epoxy resins to advance the reaction to limited extent. For instance, in laminated composites, a liquid epoxy resin is first “B-staged” to a tacky sheet that can then be stacked with other sheets and further cured. However, it is not believed that such B-staging has been utilized in the case of acrylates or methacrylates in order to intentionally raise their viscosity significantly.
It is an object of the present invention to provide a new group of materials which are prereacted and which maintain the (meth)acrylate functionality so that they be cured to a final product.
It is a further object of the present invention to provide new (meth)acrylate-containing materials having an increased viscosity or which are materials ranging from soft gels to solids wherein such materials can be cured to a hard product.
It is a still further object to provide a cladding-pumped laser having an improved outer polymer cladding.
Yet another object of the present invention is to provide ultra-low refractive index photo-curable coatings.
It is a further object to provide plastic-clad optical fibers, wherein the cladding material is the free-radical polymerization product of the aforementioned coating material.
SUMMARY OF THE INVENTION
The present invention comprises the reaction of di- or multi-(meth)acrylates with at least one multifunctional crosslinking reactant which can react with the di- or multi-(meth)acrylate functionality in a stepwise manner. By reacting a di- or multi-(meth)acrylate with an amount of the at least one multifunctional crosslinker via a stepwise reaction, a branched structure material including (meth)acrylate groups is formed wherein curing can then be achieved via the (meth)acrylate groups.
Alternately, the branched structure can be obtained by reacting a multifunctional(meth)acrylate with a difunctional (or multifunctional) crosslinker.
In this application, “multifunctional” refers to a molecule whose functionality (i.e. number of reactive groups) is greater than two.
DETAILED DESCRIPTION OF THE INVENTION
The present invention more specifically relates to a method for increasing the viscosity of di- or multi-(meth)acrylates. In one embodiment, the instant invention relates to increasing the viscosity of fluorinated oligomers so that these materials are suitable for die coating applications. Fluorosubstituted diacrylates are useful starting materials in the present invention and typical such fluorinated diacrylates are disclosed in European Patent Application 0 521 360 A2. The disclosure of this published patent application and all other patents and reference mentioned herein are specifically incorporated by reference into the present application.
The fluorosubstituted diacrylates useful in the present invention preferably contain at least about 25 percent and more preferably from about 25 to about 65 percent by weight of fluorine. An example of fluorosubstituted di- or multi-(meth)acrylates which are suitable for the preparation of ultra-low refractive index coatings are di- or multi-(meth)acrylates of the formula:
in which R
1
and R
2
each independently represent H or CH
3
and X is a perfluorinated grouping, or a perfluoroalkylene grouping in which one or more carbon atoms have been replaced by oxygen (—O—) linkages. Examples of such diacrylates include, for instance,
A specific preferred diacrylate for use in the present process is that of the formula:
CH
2
═CH—CO—O—CH
2
CF
2
O(CF
2
CF
2
O)
m
(CF
2
O)
n
—CF
2
—CH
2
O—CO—CH═CH
2
(wherein m
is in the range of 0.2:1 to 5:1), which is sold by Minnesota Mining and Manufacturing Company under the tradename L-9367.
L-9367 has a molecular weight of about 2000 but its viscosity is only 35 cP which is orders of magnitude lower than an equivalent hydrocarbon diacrylate. Die coating applications generally require a formulation with a viscosity of between about 1000 and about 15000 cP.
In the present invention a multifunctional crosslinker that can react with the meth(acrylate) functionality in a step-wise fashion is employed to form an intermediate of greater viscosity than the starting material and which will subsequently be reacted by various means to a final product. One means for controlling the reaction and attaining a desired viscosity is by reacting a di- or multi-(meth)acrylate with a particular amount of multifunctional crosslinker via a stepwise reaction so as to form a branched structure. The molecular weight of the branched structure material depends on the ratio of crosslinker to di- or multi-(meth)acrylate. As the ratio of crosslinker to di- or multi-(meth)acrylate is increased, the molecular weight and, therefore, the viscosity of the branched structure material increases. This increase is very g

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