Photopolymerization of Urethane-Modified Methacrylates for Insulating

18

Photopolymerization

of

Urethane-Modified

Methacrylates for Insulating Magnet Wire

E U G E N E D. F E I T

Downloaded by GEORGETOWN UNIV on November 2, 2016 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0129.ch018

Bell Laboratories, Murray Hill, N. J. 07974

Magnet wire possessing good electrical insulation can be made by photopolymerizing lates.

urethane-modified methacry-

Insulated wire has been produced

photochemically

in a laboratory-scale reactor at a production rate that can be easily extrapolated to production speeds. the monomers—in crylate

and

Mixtures of

this work, monourethane monometha-

diurethane

dimethacrylate—are

solventless,

free-flowing materials with low vapor pressures at room temperature, and they polymerize rapidly in the specially designed

apparatus.

range of mechanical

The

films formed

exhibit a usable

properties and high electrical resist-

ance values.

r

phe

B e l l System uses m i l l i o n s of p o u n d s of m a g n e t w i r e a n n u a l l y i n

the f a b r i c a t i o n of e l e c t r o m e c h a n i c a l

relays.

T h e n e e d for m a g n e t

w i r e is expected to increase t h r o u g h o u t the decade. capacity

Before expanding

for p r o d u c i n g magnet wire, Western Electric requested

Bell

L a b o r a t o r i e s to evaluate n e w materials a n d techniques as p o s s i b l e alternatives to the c u r r e n t B e l l System p r a c t i c e .

It was h o p e d that a c h a n g e

i n m a t e r i a l or t e c h n i q u e w o u l d r e d u c e o p e r a t i n g costs, g i v e i m p r o v e d properties, a n d m i n i m i z e a i r p o l l u t i o n i n the c u r i n g step. T h e p a p e r describes a process for the i n s u l a t i o n of m a g n e t w i r e b y d i r e c t p h o t o p o l y m e r i z a t i o n of m o d i f i e d methacrylates.

A modified poly-

m e t h a c r y l a t e i n s u l a t i o n is attractive for several reasons : ( 1 ) M e t h a c r y l a t e s p o l y m e r i z e b y a f r e e - r a d i c a l process that is c o m p a t i b l e w i t h h i g h p r o d u c t i o n speeds. ( 2 ) F r e e - r a d i c a l p o l y m e r i z a t i o n c a n b e i n i t i a t e d b y a w i d e r a n g e of energy sources. I n p r i n c i p l e , methacrylates c a n b e c u r e d not o n l y b y t h e r m a l a c t i v a t i o n , b u t also b y u l t r a v i o l e t a n d electron r a d i a t i o n . ( 3 ) H i g h - e n e r g y r a d i a t i o n permits o p e r a t i o n of the c o a t i n g e q u i p m e n t near a m b i e n t temperatures. L o w - t e m p e r a t u r e o p e r a t i o n leads to l o w e r v a p o r losses t h a n at h i g h temperature. 269 Platzer; Polymerization Reactions and New Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

270

P O L Y M E R I Z A T I O N

R E A C T I O N S

A N D

N E W

P O L Y M E R S

( 4 ) H i g h - e n e r g y r a d i a t i o n c a n i n c o r p o r a t e reactive solvent m o l e cules i n t o the final film. T h u s , systems i n w h i c h t o t a l p o l y m e r i z a t i o n of the a p p l i e d m a t e r i a l occurs are possible. ( 5 ) P o l y a l k y l methacrylates to r a p i d s o l d e r i n g techniques.

d e c o m p o s e at temperatures

accessible

Polymer System U r e t h a n e - m o d i f i e d acrylates a n d methacrylates are k n o w n f r o m the


literature ( I , 2 ) .

T h e s e m o n o m e r s are h i g h b o i l i n g a n d viscous e n o u g h

to be s u i t a b l e for solventless a p p l i c a t i o n s .

T w o interesting

monomers

of this t y p e h a v e b e e n p r e p a r e d f r o m r e a d i l y a v a i l a b l e c o m m e r c i a l m a terials.

C o m p o u n d I is a m o n o u r e t h a n e m o n o m e t h a c r y l a t e ; it is a l i q u i d

w i t h a viscosity of 40 cps at 2 8 ° C .

C o m p o u n d II is a d i u r e t h a n e d i m e t h -

acrylate; i t is a w h i t e , w a x y s o l i d that melts a r o u n d 6 5 ° C .

S e v e r a l other

m o n o m e r s of this t y p e h a v e b e e n e x a m i n e d , b u t these t w o f o r m the basis of this w o r k . Ο

Ο

Il

I

CH CH CH CH NHCOCH CH OCC = CH I CH I 3

2

2

2

2

2

2

3

ο

ο

Il

I

(- CH CH CH NHCOCH CH OCC = CH ) 2

2

2

2

2

2

2

I

CH

3

II M i x t u r e s of I, II, a n d 1% of b e n z o i n m e t h y l ether ( 3 ) p o l y m e r i z e d w i t h U V r a d i a t i o n of 3.2 m W c m

2

intensity.

(BME)

were

As shown in

F i g u r e 1, the i n f r a r e d a b s o r b a n c e at 6.1 μ is r e d u c e d b y 80% after

15

seconds a n d is u n d e t e c t a b l e after 30 seconds of exposure. A film of a 5 : 1 ( I : I I ) c o p o l y m e r is strong (tensile strength = 2700 p s i ) a n d 53% e x t e n d e d at break ( T a b l e I ).

A film of a 1:1 c o p o l y m e r is

strong b u t shows less e x t e n s i b i l i t y t h a n the p r e v i o u s

film.

The homo

p o l y m e r of II is excessively b r i t t l e . F l e x i b i l i t y was e v a l u a t e d b y p o l y m e r i z i n g the m a t e r i a l o n 5 - m i l t h i c k c o p p e r f o i l a n d s u b j e c t i n g the f o i l to a Ye" m a n d r e l b e n d . ture of I w i t h u p to 40% of II shows no c r a c k i n g of the p o l y m e r after 10 b e n d s .

Platzer; Polymerization Reactions and New Polymers Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

Mix film

18.

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Photopolymerization

271

for Magnet Wire


100%

—I

1—

60 Figure

I.

Rate

120

90

TIME (SEC) of polymerization intensity, mW/cm

3.2

2

Table I.

Tensile Properties of Urethane Modified Polymethacrylates

1% 82 50 0

Tensile Strength, psi

Elongation, %

2700 6100 5400

53 10 3

b

17 50 99

a

b

Films were formed by UV radiation of the samples (4 mils thick) held between borosilicate glass plates. The photoinitiator was BME at ^1 % by weight. The entries represent the value at break α

b

T h e s e c o p p e r strips w e r e s o l d e r e d f o r five seconds o r less at 4 0 0 ° C . F i g u r e 2 shows t h e effect of t h e s o l d e r d i p o n a c o p o l y m e r of I a n d I I . T h e results a r e t y p i c a l of a range o f v a r i a t i o n s .

T h e strips s h o w that

the solder removes t h e i n s u l a t i o n a n d that there is l i t t l e s h r i n k a g e of t h e i n s u l a t i o n above the solder front. I n s u l a t i o n resistance measurements of p o l y m e r films of these m o n o mers w e r e m a d e o n substrates c o n t a i n i n g i n t e r l e a v e d c o p p e r - c o m b pat terns. age.

T h e m a t e r i a l w a s k e p t at 3 0 ° C , 90% h u m i d i t y , a n d constant v o l t I n s u l a t i o n resistance values w e r e greater t h a n 1 0 m e g o h m s . δ

I n s u m m a r y , films f o r m e d f r o m these materials s h o w a usable r a n g e of m e c h a n i c a l properties a n d h i g h electrical-resistance values.

Mixtures



272


Figure

2.

Solder

R E A C T I O N S

test specimens; two-inch seconds.

A N D

immersion

N E W

P O L Y M E R S

for

five

of the m o n o m e r s are solventless, f r e e - f l o w i n g materials w i t h l o w v a p o r pressure at r o o m t e m p e r a t u r e , a n d t h e y p o l y m e r i z e at r a p i d rates.

Apparatus T h e e v a l u a t i o n of these materials as w i r e enamels m u s t b e m a d e o n the basis of tests p e r f o r m e d o n i n s u l a t e d w i r e .

A laboratory enameling

m a c h i n e has b e e n b u i l t that c a n p r o d u c e h u n d r e d s of feet of e n a m e l e d w i r e s u i t a b l e f o r testing. T h e p r i n c i p l e c o m p o n e n t s of t h e apparatus are s h o w n i n F i g u r e 3. B r i e f l y , the w i r e pays-off i n t o a n a p p l i c a t o r a n d t h e n i n t o a v e r t i c a l photoreactor. T h e d r i v e a n d t a k e - u p assemblies are s i t u a t e d b e h i n d the reactor a n d are d e s i g n e d to m i n i m i z e tension o n the w i r e . T h e p h o t o reactor is the core of the apparatus a n d is d i s c u s s e d extensively here.

RECYCLING DEVICE

DRIVE P A Y - O F F

APPLICATOR

REACTOR

SPOOL

a

TAKE - U P CONTROL

Figure 3.

Schematic of

T A K E - U P SPOOL

apparatus


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Photopolymerization

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273

Wire

T h e reactor consists b a s i c a l l y of t w o h e m i c y l i n d e r s h i n g e d together o n a v e r t i c a l side. T h e w i r e enters t h e reactor t h r o u g h a s m a l l a p e r t u r e i n t h e base a n d leaves t h e r e a c t o r t h r o u g h a s i m i l a r a p e r t u r e i n t h e t o p . Six l o w - p r e s s u r e m e r c u r y l a m p s ( S o u t h e r n N e w E n g l a n d U l t r a v i o l e t C o . ) are m o u n t e d v e r t i c a l l y a n d s y m m e t r i c a l l y i n s i d e t h e reactor. T h e p r i n c i p a l emission (^80%) f r o m these l a m p s occurs at 254 n m . A p r i n c i p a l emission at 300 o r 350 n m is a v a i l a b l e f r o m l a m p s c o n s t r u c t e d w i t h t h e a p p r o p r i a t e phosphors ( F i g u r e 4 ) . T h e i n s i d e w a l l s of t h e reactor a r e m a d e reflecting to U V r a d i a t i o n b y flexible, f r o n t - s i d e d m i r r o r s . T h e 100 SPECTRAL OUTPUT


90 254 nm LAMPS 300 nm

80

LAMPS

350 nm LAMPS (DATA FROM MANUFACTURER THE SOUTHERN NEW ENGLAND ULTRAVIOLET CO. )

70

60

50

40

30

20

10

ιι 250

300

350

400

WAVELENGTH

Figure 4.

450

500

Λ

_ U 550 i_

(nm)

Spectral output of fomps used in the photoreactor.

m i r r o r s reflect 75-85% of t h e r a d i a t i o n b e t w e e n 220-300 n m as p r o d u c e d ( D e n t o n V a c u u m ) a n d , after 100 hours of use, t h e y s h o w l i t t l e loss of reflectance. T h e intensity of r a d i a t i o n i n t h e reactor w a s m e a s u r e d w i t h a c a l i b r a t e d E p p l e y t h e r m o p i l e . I n t h e o p e n reactor, t h e r a d i a t i o n i n t e n sity f r o m three lamps w i t h 254 n m p r i n c i p a l emission w a s 2.0 ± 0.5 m W / c m a l o n g 90% of t h e v e r t i c a l axis. I n t h e closed reactor, t h e i n t e n sity at t h e same positions f r o m six lamps rose to 4-5 m W / c m . T h e values of these last measurements a r e affected s o m e w h a t b y t h e s h a d o w cast b y t h e large mass of t h e t h e r m o p i l e . T h e U V intensities f r o m t h e l a m p s w i t h p r i n c i p l e emissions at 300 a n d 350 n m are s i m i l a r to t h e i n t e n 2

2



274


R E A C T I O N S

A N D N E W

P O L Y M E R S

sity f r o m t h e 254-nm l a m p s . T h i s result is n o t s u r p r i s i n g i n v i e w of t h e efficiency of t h e p h o s p h o r s . ( 4 ) . T h e a t m o s p h e r e i n t h e reactor c a n b e c o n t r o l l e d b y inlets at t o p a n d b o t t o m . T h e reactor is sealed w i t h a flexible, s e l f - f o r m i n g gasket. The t e m p e r a t u r e i n s i d e t h e closed reactor w i t h t h e 254-nm l a m p s o p e r a t i n g a n d w i t h n i t r o g e n flowing at 0.5 c u b i c feet p e r m i n u t e w a s stable at 72°C. T h i s t e m p e r a t u r e rise is e x p e c t e d t o increase s l i g h t l y the l a m p intensities r e c o r d e d i n t h e p r e c e d i n g p a r a g r a p h ( 5 ) . T h e w i r e is r o u t e d i n s i d e the reactor b y a set of p u l l e y s m o u n t e d o n t h e t o p a n d b o t t o m i n s i d e plates of the reactor. T h i s s i m p l e a r r a n g e m e n t increases the exposure t i m e at a fixed w i r e s p e e d w i t h o u t i n c r e a s i n g t h e size of t h e reactor. T h e a p p l i c a t o r consists of a t a n k to h o l d t h e r e s i n , a g r o o v e d w h e e l to g u i d e the w i r e t h r o u g h t h e r e s i n , a n d a w i p e r to s m o o t h t h e resin b e f o r e i t p o l y m e r i z e s . F r i c t i o n b e t w e e n t h e d r i v e a s s e m b l y a n d the w i r e is p r o v i d e d b y a w e i g h t e d p i v o t a r m . T h e p i v o t of t h e a r m is c o u p l e d to a transducer that controls t h e s p e e d of the t a k e - u p assembly. Operation I n a c t u a l o p e r a t i o n , w i r e pays-off i n t o a f u r n a c e f o r c l e a n i n g a n d a n n e a l i n g a n d t h e n into a tank of r e s i n . T h e resin is t r a n s f e r r e d t o the wire b y d i p coating, smoothed, a n d photochemically p o l y m e r i z e d i n a sealed reactor. T h e exposure t i m e i n the reactor c a n v a r y f r o m 10 seconds to 52 minutes, d e p e n d i n g o n c a p s t a n speed a n d t h e n u m b e r of p u l l e y s i n s i d e t h e reactor. Table II.

D e f e c t C o u n t as a F u n c t i o n of N i t r o g e n F l o w

Flow Rate (CFM) 0.50 0.43 0.36 0.30 0.25

6

a

Defect Count (per 100 feet) 0,0 0,0,0 0,0,0 29,24 463,424 e

Inlet pressure is 2 psi of nitrogen Partial pressure of oxygen ~2 torr Entries are the number of discontinuities in insulation counted per 100 feet of continuously produced and tested magnet wire. α

b c

O n c e i t has left the reactor, t h e w i r e m a y b e either c o l l e c t e d d i r e c t l y o n t h e t a k e - u p via the d r i v e assembly o r r e t u r n e d to t h e resin tank f o r a p p l i c a t i o n of another l a y e r of i n s u l a t i o n via t h e p u l l e y i n F i g u r e 3. T h r e e layers of i n s u l a t i o n are u s u a l l y a p p l i e d to the w i r e b e f o r e t a k e - u p . T y p i c a l f o r m u l a t i o n s f o r t h e experiments c o n t a i n a b o u t 1% of a process i n g aid—e.g., N u o s p e r s e 657 ( N u o d e x P r o d u c t s C o . ) . W i t h o u t the p r o c e s s i n g a i d , t h e f o r m u l a t i o n s w e t c o p p e r p o o r l y a n d b e a d o n the w i r e . T h e processing a i d is u s e d to s m o o t h the r e s i n a n d f u r n i s h a u n i f o r m thickness of i n s u l a t i o n . Surface contaminants o n t h e c o n d u c t o r affect t h e c o n t i n u i t y of t h e i n s u l a t i o n . A f o r c e d , h o t - a i r f u r n a c e serves as a s i m p l e m e t h o d of pretreatment. T h e resin polymerizes i n the photo-



18.

Photopolymerization

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275

Wire

c h e m i c a l reactor. A flow of p u r e n i t r o g e n ( < 10 p p m of o x y g e n ) i n t o the reactor is necessary b e c a u s e the p o l y m e r i z a t i o n is i n h i b i t e d b y oxygen. V a r i a t i o n s i n o p e r a t i o n a l parameters are j u d g e d b y t h e i r effect o n the c o n t i n u i t y of i n s u l a t i o n o n the w i r e . A c o n t i n u i t y test—sometimes c a l l e d a " p i n h o l e " test—is a n A S T M test ( 6 ) p e r f o r m e d o n c o m m e r c i a l l y available equipment (Hypotronic, Inc.). I n the test, a defect i n the i n s u l a t i o n is c o u n t e d each t i m e c u r r e n t flows b e t w e e n a 1-inch section of the i n s u l a t e d w i r e a n d a m e r c u r y b a t h t h r o u g h w h i c h the w i r e travels. A t i m e constant (25 m s e c ) is associated w i t h the r e c o v e r y of the c o u n t i n g d e v i c e i n s u c h a w a y that a single, p r o l o n g e d defect is c o u n t e d m a n y times as a series of shorter ( 0 . 5 - i n c h ) defects. T h e d e p e n d e n c e of defect c o u n t o n flow rate f o r c o n t i n u o u s p r o d u c t i o n of i n s u l a t e d w i r e is g i v e n i n T a b l e II. " D e f e c t - f r e e " w i r e is p r o d u c e d at 4 ft/ m i n at a p a r t i a l pressure of o x y g e n ^ 2 torr. T h i s p a r t i a l pressure is o b t a i n e d at a n i t r o g e n flow of 0.5 c u b i c feet p e r m i n u t e . T h e flow rate r e q u i r e d d e p e n d s o n the d e s i g n of the reactor. T h e p a r t i a l pressure of o x y g e n is a f u n d a m e n t a l p a r a m e t e r . T a b l e III.

D e f e c t C o u n t as a F u n c t i o n of W i r e S p e e d

Speed (Jt/miri)

Exposure (seconds)

4.0 4.5 5.0 5.5 6.0

45 40 36 33 30

Defect Count (per 100 feet) 0,3" 0,0,1 3,1,0,0 0,3,1,0,0 29,14,15

° Entries are the number of discontinuities in insulation counted per 100 feet of continuously produced and tested magnet wire. A residence t i m e i n the reactor of 12-15 seconds sufficiently hardens the r e s i n to a l l o w r e r o u t i n g Lhe c o a t e d w i r e i n s i d e the reactor f o r longer, total exposure. T h e d e p e n d e n c e of defect c o u n t o n w i r e speed a n d t o t a l exposure is s h o w n i n T a b l e I I I . Production

of Insulated

Wire

T w e l v e t h o u s a n d feet of w i r e w e r e c o n t i n u o u s l y i n s u l a t e d i n the apparatus at 4 f t / m i n w i t h three layers of i n s u l a t i o n . D e f e c t counts w e r e r e c o r d e d f o r every 50-foot section of w i r e . T h e f r e q u e n c y of o c c u r r e n c e of a c o u n t is p l o t t e d against the c o u n t i n F i g u r e 5. Defectfree, 50-foot sections p r e d o m i n a t e . T h e average defect c o u n t p e r 100 feet is 0.6. A n a d d i t i o n a l 16,000 feet of w i r e w e r e i n s u l a t e d , a n d 1600 feet w e r e r a n d o m l y selected for testing. N o defects w e r e c o u n t e d , i n d i c a t i n g that defects w h e n they d o o c c u r t e n d to o c c u r i n clusters a n d at t h e start of the o p e r a t i o n . T h i s o b s e r v a t i o n is consistent w i t h d a t a f r o m the first 12,000 feet. D . C . b r e a k d o w n values for the i n s u l a t e d w i r e w e r e m e a s u r e d as 2.52, 2.22, 0.88, 2.00 a n d 2.36 k V ( 6 ) .


276


R E A C T I O N S

A N D N E W

P O L Y M E R S


200

304 20-

DEFECT

Figure 5.

COUNT

Frequency

PER

50

FT

of defect counts

Summary In summary, then, magnet w i r e w i t h good a n d continuous electrical i n s u l a t i o n c a n b e p r o d u c e d b y p h o t o p o l y m e r i z a t i o n of u r e t h a n e - m o d i f i e d methacrylates.

I n s u l a t e d w i r e is p r o d u c e d p h o t o c h e m i c a l l y i n t h e l a b o r -

atory scale reactor

at 4 f t / m i n .

extrapolated to p r o d u c t i o n speeds. extended

sixfold.

T h i s p r o d u c t i o n rate c a n easily b e T h e l e n g t h of t h e reactor c a n b e

R a d i a t i o n intensity c a n b e increased

m o r e w i t h high-pressure arcs.

t e n times

or

If p r o d u c t i o n rates are p r o p o r t i o n a l even

to the h a l f p o w e r of the intensity, the rate w i l l b e t r i p l e d o r m o r e . O p e r a t i n g at a h i g h e r a m b i e n t t e m p e r a t u r e i n the reactor a n d o p t i m i z i n g the p h o t o c h e m i c a l sensitivity of the r e s i n c a n d o u b l e o r t r i p l e i m p r o v e m e n t i n rate.

A final rate of p r o d u c t i o n of 150-200 f t / m i n , t h e n ,

is w i t h i n the range of the final process w i t h this resin.


18.

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277

Acknowledgment T h e author a c k n o w l e d g e s W . B a d e r , J . L y n c h , J . M a c K a y , T . F . H a l l o r a n , a n d L . D . L o a n of B e l l L a b o r a t o r i e s , a n d P . R . M c V i c k e r s a n d M . A . M a c V i t t i e of W e s t e r n E l e c t r i c , B u f f a l o .


Literature

Cited

1. 2. 3. 4.

Nordstrom, J. D., U.S. Patent 3,479,328 (Nov.18,1969). Labana, S.S.,J.Polym.Sci.,A-1(1968) 6, 3283. Kosar, J., "Light Sensitive Systems," pp. 162-3, Wiley, New York, 1965. Koller, L. R., "Ultraviolet Radiation," 2nd edition, p. 264, Wiley, New York, 1965. 5. Ibid., page 52. 6. A S T M D1676-70. R E C E I V E D August 2, 1972.


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Photopolymerization of Urethane-Modified Methacrylates for Insulating

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