Sub-T, Annealing Studies of UV-Cured Poly( Urethane) Acrylate Coatings On Optical Fibers: A Correlation of

Annealing Behavior with Microbending Loss NICOLE LEVY

Bell Laboratories Norcross, Georgia 30071

Three types of UV-curable acrylate network systems were coated on optical fibers and studied calorimetrically. It was found that an endothermic transition of LW -2-14 callg is superimposed on the glass transition of these coatings. This transition is present in the fiber coatings, but not in sheet form. The time-dependence of sub-T, annealing of these crosslinked coatings (following rapid quenching from a temperature above the end temperature of the endotherm) was found to correlate with a mechanical-optical property of the coated fibers; namely, the contribution of the coating to the microbending loss of the optical fiber.

INTRODUCTION ptical fibers are a relatively new transmission 0 medium for telecommunication (1). Their main ad-

vantages are broad bandwidth and low transmission loss compared to copper conductors. The fibers investigated consist of a high refractive index core made of doped silica glass, surrounded by a silica glass of lower refrac- tive index. In our case the fiber is in turn coated with a UV-curable polymer to minimize microbending loss, and for abrasion resistance and protection from the environment, enhancing its strength and static fatigue properties. An ideal fiber is shown in Fig. l a . Light rays travel inside the core because of total internal reflection according to Snell's Law at the core-cladding interface (1). In Fig. Ib periodic geometric fluctuations along the fiber axis increase the angle of incidence to the extent that the critical angle is exceeded, and refraction rather

(a) IDEAL FIBER

LIGHT RAY

~ f s FIBER a FIBER FIBER COATING CLADDING CORE

I b) FIBER WITH MICROBENDING

-COMPRESSIVE STRAIN I N THE COATING CAUSING BUCKLING OF FIBER AXIS

NO LIGHT TRANSMITTED

LIGHT RAY

Fig. 1 . Light propugution in (a) ideal fibers, (b) fibers wi th microbending.

than reflection occurs. This loss mechanism is termed microbending loss. The period of axial fluctuations at which microbending loss will occur (2), for the fibers investigated is 1 mm-'. However, nonperiodic fluc- tuations can also have components at this spatial fre- quency.

Another cause for transmission loss is Raleigh scatter- ing. Because Raleigh scattering is wavelength depen- dent, both contributions to loss can be separated analyt- ically via the transmission loss equation (3):

aA = u A - ~ + b

where ah is the total measured loss as a function of wavelength, a is the coefficient characterizing the loss due to scattering and b the coefficient characterizing the loss due to microbending.

The purpose of this work is to investigate if polymeric coatings on optical fibers can contribute to their trans- mission properties as shown in Fig. Ib.

The polymer coatings investigated are (a) a blend of an 0-aliphatic polyester-N-aromatic urethane- diacrylate of molecular weight -1200 with a 2-ethyoxyethoxyethylacrylate and N-vinylpyrrolidone, (b) a blend of an 0-aliphatic polyester-N-aliphatic urethanediacrylate of molecular weight -2200 and a low molecular weight aliphatic acrylate, and (c) a blend of DGEBADA (bisphenol-A-diglycidylether) di- acrylate, 2,4-dibromo-phenylglycidyletheracrylate and 1,4-butanedioldiacrylate, cured as described be- low.

In the coating process, a fiber passes through the viscoelastic prepolymer formulation usually at a speed of approximately 1 m/s, Fig . 2 . Because of the geometry of the coating line, the polymer is crosslinked 0.38 s after it leaves the coating die. When exiting the coating die, the polymer sees a gradient of shear that is large at

978 POLYMER ENGINEERING AND SCIENCE, MID-OCTOBER, 1981, Vol. 21, No. 14

PREFORM

I , I I . I , I I I I l l .

(d) COMBINED TRANSITION + (J COMBINID TRANSlTlONS ENDOIHERMIC TRANSITION (ENIHALPY 21

F U R N A C E YL

I

I I I I I I

Z O N E 1 r 7

1 1 , I , €NTHALPY 2

- 1 1 1 I

-f 1 Z O N E ? 0.38m

i ,

Z O N E 3

2 C U R E D POLYMER _1 Z O N E 4

I~ C O A T I N G

Fig. 2 . Schematic forfibers druwing und couting process.

the fiber surface and the coating cup surface and small- est midway between these surfaces (4). Because of the shearing and rapid subsequent crosslinking over an adhering substrate, the glassy polymer on the fiber should have excess enthalpy and volume compared to the unsheared structure without substrate. Glasses with excess enthalpy and volume experience a ther- modynamic force driving these properties toward their equilibrium values (5-7) which will result in densifica- tion (8) of the coating and thus a change in the mi- croscopic coating geometry as a function of time (9).

In the case of polymers that have experienced high shear rates as described above, the densification is ex- pected to occur anisotropically along the force lines of the shear gradient, while in the same polymers having not experienced this shear gradient, the densification should occur isotropically. Anisotropic densification of the polymeric coating on the high modulus glass sub- strate could give rise to compressive strains and buckl- ing of the fibers (Fig. Ib) while isotropic densification without a substrate would cause only a reduction in the sheet size. The rate of this equilibrium process below T,, i.e., the rate of annealing, is determined by the degrees of freedom available to the shortest polymer segment and its mobility in the glassy state.

The cured polymer formulations both as coated fibers and in sheet form have broad glass transitions in the temperature range from 20 to 80°C. Normal storage temperature of the fibers (22°C) is thus below the T, of these polymers, which makes them susceptible to the annealing effects as described above.

Sub-T, Annealing Studies of W-Cured Poly(Urethane) Acrylate Coatings On Optical Fibers

POLYMER ENGINEERING AND SCIENCE, MID-OCTOBER, 1981, Vol. 21, No. 14 979

In this work, polymer coated optical fibers were analyzed by differential scanning calorimetry in an at- tempt to understand some of the thermodynamic prop- erties of the coatings that might cause added transmis- sion loss in the coated fiber and that might not be present in the polymer in sheet form.

SCHEMATIC DSC BEHAVIOR OF POLYMERIC COATINGS

When amorphous polymers are heated at a constant heating rate, they undergo a transition from the glassy to the rubbery state in the glass transition temperature region (Fig. 3a) Kinetic effects due to a nonequilibrium glassy state have been observed as a relaxation enthalpy (5, 7, 10) superimposed on the glass transition (Fig. 3b). In this work, the UV-cured polymers in sheet form follow this transition behavior; however, the polymeric coatings on fibers do not. To render schematically their behavior we had to superimpose a broad endothermic transition of larger heat content than can be explained by an excess free volume change (Fig. 3c) over the glass transition (Figs. 3d, 3e) . We call this endotherm en- thalpy 2. The width of this transition is broader than first-order transitions associated with, for example, melting of a crystalline phase. The shape of the curves in Fig. 3d is a function of the width of the transition and the heating rate. Evidence for the presence of enthalpy 2 in coatings on optical fibers will be presented in the fol1 owing.

EXPERIMENTAL The polymeric formulations were described in the

introduction. Sheets (0.006 in. thick) were cured on release paper with a 200W medium pressure Hg lamp

I (4 GLASS TRANSITION i

(bl GLASS TRANSITION + RELAXATION ENIHALPY

I (4 ENDOTHfRMIC TRANSITION I I

1 I

I I ,OUENCHfD 1

Nicole Levy

and stored at room temperature for one week. Optical fibers were coated with the same polymeric formula- tions on a drawing line as described in Fig . 2 , and stored subsequently under unknown conditions. For anneal- ing time dependent enthalpy measurements, the first scan was therefore disregarded. A DuPont 990 thermal analyzer equipped with a DSC cell was used for the differential scanning calorimetry studies. Samples of little strips of the sheet weighed -5 mg. Similarly, coated glass fibers cut into 0.5 cm sections with the plastic resin of the coated fibers weighing -5 mg were placed in aluminum pans and crimped. The weight of the plastic resin on the fibers was calculated from the known diameters of the bare and coated fibers and the weight of the bare and coated fibers. All samples were scanned from -120 to 180°C at 20°C min-'. For the second scan the samples were recooled to -135°C at -100"C/min as soon as they had reached 180°C and reheated. Subsequent scans of the same sample were taken in the same fashion after storage at room tempera- ture for the indicated amount of time. For quantitative measurements, DSC scans, as in Figs . 4 and 5, were corrected for baseline deviations (1 1) and the C,-curves of the annealed and quenched samples matched for equilibrium specific heats. The enthalpy 2 mea- surements reflect the enthalpy difference between the quenched and the annealed state in the transition re- gion (Fig. 3e). The constant specific heat contribution of the glass fibers and the relaxation enthalpy, when present, were subtracted. Enthalpy measurements were calibrated using the indium heat of fusion. Areas were measured with a planimeter. -

. C O A T I N G A , SHEET __ AS PIEPARED 1170h) QUENCHED ANNEALEO ?ah

---_.

A S PREPARED (17Ohl QUENCHED 4 N N f A L E D 96h

__._. C O A T I N G B , SHEET

- dH dl endo \= 0 Z m c d / r r c

_ _ A S PlEP4RED 1110h) OUENCWED - _ ANWEALFO FOR 14h

C O A T I N G C SHEET -. ,

1

8 , I ,

-80 --.O 0 40 80 110 I60 lEMPEPAlURE ( " C l

Fig. 4 . DSC behavior of coutings in sheet form.

C O A T I N G A ON FIBER 2

1 ~ AS RECEIVED

QUENCHED ANNEALED V l h ANNEALED I V Z h ANNEALE0 SOOh

. . . . . . . .

-___.

C O A T I N G B ON FIBER __ AS R I C E I V E C ----. - . . . . . . . Q v t N c i E ~ ' ANNEALEO 96h ANNEALED 240h _ _ _ _

> ,

_ _ A S RECEIVED . . . . . . . QUENCHED

C O A T I N G C ON FIBER

ANNEALED 48h ANNEALED lOOh ANNE4LEO S O l h

_ _ . ~ ~

dH d? -

1

. ~ , , , I , , , 0 -40 ,O 80 110 160 -10

I t M P E R A I U R E ( 'C)

F i g . 5 . DSC behovior of coutings on fibers.

RESULTS AND DISCUSSION When scanning of sheets of coatings A, B, and C in

the calorimeter, we observe a regular broad glass transi- tion between 20 and 80°C for coating A, between 20 to 45°C for coating B, and between 25 and 70°C in coating C (Fig. 4 ) . No large differences in the curves are ob- served following repeated scanning except for coating C where a relaxation endotherm AH = 0.09 cal/g is ob- served following storage for 24 h at room temperature.

As seen in the scans of Fig. 5 , the fibers coated with formulations A, B and C exhibit a behavior as shown in the schematic in Fig . 3e, i.e., an endothermic transition of large heat content is superimposed on the glass tran- sition. Moreover, significant differences were observed for different fiber-coating structures. In Fig. 5 (top), the annealing behavior of coating A on fiber 2, a high b-coefficient fiber, is shown. We see that, following quenching of the fiber coating structure, the enthalpy 2 appears and increases as a function of time. The fact that the annealing endotherm can be quenched rules out any chemical change such as thermal cure of the polymeric coating. In Fig. 5 (middle), the annealing behavior of coating B is shown. We also see the presence of enthalpy 2, however, the enthalpy decreases as a function of time. In Fig 5 (bottom), the annealing behavior of coating C is shown. Again, we observe a large enthalpy 2, larger than for coatings A and B; however, here the annealing rate is much slower.

Because we intended to investigate the effect of an- nealing on the geometric fluctuations of the fibers mea- sured as microbending induced loss, the annealing be- havior of a fiber immediately after drawing was

980 POLYMER ENGINEERING AND SCIENCE, MID-OCTOBER, 1987, Vol. 21, No. 74

Sub-T,, Annealing Studies of UV-Cured Poly(Urethane) Ac y l a t e Coatings On Optical Fibers

examined simultaneously with time-dependent mea- surements of the b-coefficient (Fig. 6 ) . We see that the b-coefficient increases with time until it levels off after approximately 100 to 200 h. The enthalpy 2 curve as a function of time parallels the b-coefficient curve.

When analyzing the enthalpy 2 as a function of log time (Fig. 7 ) for coatings A, B and C on fibers, we observe different slopes or annealing rates. Therefore, within the scope of the materials investigated, all of which had glass transition temperatures above room temperature, the annealing rate behavior cannot be -Sated to a specific material property but rather seems to be induced by process-related effects. Since sheets of the same materials did not show similar effects, we speculate that the high shear rates (-'lo3 s-') and sub- sequent crosslinking of the polymer onto an adhering substrate leads to the presence of enthalpy 2, and that different annealing rates are due to small variations in the coating process.

0 1 o R . RIGHT SCALE

D , 3 0 5

>

I ool 100 200 300 400

STORAGE TIME AT ROOM TEMPERATURE (HOURS)

Fig. 6. Time dependence of enthalpy and b-coefficient.

7- - r - T - 7 - 7 r - 7

' a/ - t

COATING A ON FIBER 1 P // I ' 0 r

/ 1 COATING B ON FIBER ' lj COATING C O N FIBER

A- A- -

If the annealing process involves an anisotropic den- sification of the coating in the glassy state (see Fig. Ib), the rate of annealing should affect the microbending loss of the fibers. The contribution of microbending to the transmission loss (b-value) measured following an- nealing at 22°C for 100 h vs the annealing rates (AHAog t ) of the fibers is shown in Fig. 8. We find that coated fibers with high annealing rates give rise to large mi- crobending losses. The latter reflect fiber axis defor- mations having a higher periodicity, probably due to compressive strains in the coating, in the microbending sensitive frequency of 1 mm-' (7, 12). The lower micro- bending losses of coated fibers having slower annealing rates would thus indicate that the deformation of the coated fiber is less periodic and of lower frequency than 1 mm-'.

Further support for our correlation of microbending induced loss and the annealing rates of the polymeric coating-fiber structure was obtained when examining coated fibers with medium and low b-coefficients under an optical microscope with crossed polarizers in the dark field (Fig. 9). Stress-birefringence is observed only for medium and high loss fibers, but not for low loss fibers.

1.5 - - E Y \ -

D L - c

g 1.0 -

8

v Y Y

0 0 5 -

COATING A O N FIBER I l3

5 A COATING B O N FIBER

COATING C ON FIBER

-2 0 10 1s 20 2 5

* H/'OG '*torap. a1 z z * c

Fig. 8 . Dependence of annealing rates ( AHllog t ) of opticalfiber coutings on b-coefficient (meusured after 100 h annealing at 22°C).

I

1 t = 100 hrr

oL.-- ~ 1 1 I j l l i l 1 . 1 2000 3000 5000 10,000 20,000

ANNEALING TIME I (MINUTES] AT 22'C FOLLOWING HEAT-UP TO 180'

Fig. 7. Annealing rates of coatings on fibers. b ig . 9. Optical micrographs crossed polarizers, dark field.

of' couted fibers viewed under

98 1 POLYMER ENGINEERING AND SCIENCE, MID-OCTOBER, 1981, Vol. 21, No. 14

Nicole Levy

CONCLUSIONS Optical fibers coated with three UV-cured polymers

having T,’s at or above room temperature show an en- dothermic transition superimposed on the glass transi- tion. No such transition was observed for polymers curved in the sheet form. The value of the endotherm exceeds the value of kinetic relaxation reported for amorphous polymers by as much as an order of mag- nitude. We attribute this enthalpy observed on coated fibers to orientation of the polymer due to the high shear rates of the coating process. We speculate that the rapid subsequent crosslinking of the polymer on the substrate “locks-in” this orientation and results in a time- dependent anisotropic densification of the coating.

The stored energy of the endotherm can be quenched but anneals as a function of time. The value of the annealing rates of the various coated fibers corresponds to the amount of stress induced deformation in the fiber axis as measured by the size of the microbending in- duced loss.

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John Wiley & Sons, Inc., New York (1979). 2. W. B. Gardner, BSTJ, 54, 451 (1978). 3. K. Inada, Optics Commun., 19,437 (1976). 4. T. W. Huseby in “Physical Design of Electronic Systems,”

Vol. 2, p. 392, William L. Everitt, ed., Prentice-Hall, Inc., Englewood Cliffs, N.J. (1970).

5. A. J. Kovacs, J . Polym. Sci., 30, 131 (1958). 6. S. E. B. Petrie, Am. Chem. SOC., Div. Polym. Chem., Polym.

7. S. E. B. Petrie, Polymer Sci., Part A-2, 10, 1255 (1972). 8. J. H. Wendorf and E. W. Fischer, Kolloid-Z. and Z . Poly-

9. J . M. Hutchinson and C. B. Bucknall, Polyrn. Eng. Sci., 20,

Prepr., 15, 336 (1974).

mere, 251, 876 (1973).

173 (1980). 10. Z. Ophir, J. A. Emerson, and G. Wilkes, J . Appl. Phys., 49,

5032 (1978). 11. J. R. Fried, F. E. Karasz, and W. J. MacKnight, Mnc-

romolecules, 11, 150 (1978). 12. G. S. Brookway, P. F. Mahr and M. R. Santana, Proceedings of

the Sixth European Conference on Optical Communication, University of York; UK (1980) p. 338.

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