Polymers in Electronics - American Chemical Society

27 Morphology of Rubber-Modified Photopolymers

Downloaded by CORNELL UNIV on June 9, 2017 | http://pubs.acs.org Publication Date: March 15, 1984 | doi: 10.1021/bk-1984-0242.ch027

J. A. ORS

Engineering Research Center, Western Electric Company, Princeton, NJ 08540 J. B. ENNS BTL-Whippany, NJ 07981 The morphology of ruber modified epoxy photopolymers was found to depend on the cure conditions as well as the nature and concentration of rubber. The commercially available acrylonitrile-butadiene copolymer rubber modifiers with varying percentages of acrylonitrile content were used. They were polymerized using a photocationic initiator involving a UV exposure followed by a thermal cure. Transmission electron micrographs of osmium tetroxide stained specimens, coupled with dynamic mechanical measurements indicated that phase separation and particle size distribution depended not only on rubber concentration and compatibility, but also on the cure conditions. The resulting morphology is dependent on the rate of polymerization relative to the rate of phase separation preceding gelation. The adhesive properties of epoxy resins coupled with their dielectric behavior have made them attractive to the electronic industry. The evaluation of thermally cured rubber modified epoxy thermosets has been the subject of recent studies 0,2), which dealt with the dependence of morphology on the curing parameters, e.g., catalyst, cure schedule, time of gelation, etc. This work utilizes one of the new series of photocationic initiators (PCI) developed by Crivello, et al [3) which are presently commercially available. These 'onium salts' initiate the reaction by absorbing the actinic radiation, generating radicals and producing a protonic acid. The radicals can lead to polymerization of olefinic moieties (4) while the acid initiates the polymerization of the epoxy groups (_3). 0097-6156/ 84/ 0242-0345S06.25/ 0 © 1984 American Chemical Society Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.


346

POLYMERS IN ELECTRONICS

In the thermally i n i t i a t e d cure of rubber modified epoxy, the rubber may be present w i t h i n i n the epoxy matrix as d i s t i n c t domains. The morphology of the cured r e s i n has been shown to be dependent o n : (1) the cure temperature and a c c e l e r a t o r c o n c e n t r a t i o n , s i n c e the extent of p a r t i c l e (domain) s i z e growth appears to be l i m i t e d by g e l a t i o n ; and (2) the nature (percent a c r y l o n i t r i l e ) of the rubber used, since mixture compatibility increases with the a c r y l o n i t r i l e content of the rubbers ( 1 , 2 ) . In a p h o t o i n i t i a t e d system, i n whTcïï the r e a c t i o n proceedes r a p i d l y during exposure ( r e s u l t i n g i n r e l a t i v e l y short gel t i m e s ) , i t i s thought t h a t the epoxy matrix i s set up during the i n i t i a l i r r a d i a t i o n thus i n f l u e n c e s the f i n a l morphology. The f i n a l p a r t i c l e s i z e and d i s t r i b u t i o n would then be d i c t a t e d by the compatibility of the components i n the uncured state s i n c e the r a p i d formation of the gel would be expected to preclude the r e d i s t r i b u t i o n of the phases. T h i s study examines the r e l a t i o n s h i p between cure schedule and morphology for a rubber modified, p h o t o c a t i o n i c a l l y i n i t i a t e d , d i g l y c i d y l ether of bisphenol A r e s i n ; using d i f f e r e n t i a l scanning c a l o r i m e t r y (DSC) and t o r s i o n pendulum (TP) to monitor the g l a s s transition temperature ( T g ) , transmission e l e c t r o n microscopy (TEM) to look at phase separation and domain sizes, Fourier transform i n f r a r e d (FT-IR) and sol f r a c t i o n to compare the extent of c u r e , and thermogravimetric a n a l y s i s (TGA) to analyze the specimens for volatilization and/or degradation. In an attempt to explain some of the observations the cure schedule, c o n c e n t r a t i o n of rubber, type of rubber, and the c o n c e n t r a t i o n of i n i t i a t o r were v a r i e d independently. EXPERIMENTAL A l l m a t e r i a l s used i n t h i s study are l i s t e d i n Table I and, unless otherwise noted, are commercially a v a i l a b l e and were used as r e c e i v e d . The rubber m o d i f i e r s used are as epoxy-termi nated butadi ene-ac ry 1oni t r i 1e (ETBN) rubbers prepared as d e s c r i b e d elsewhere Γ 5 ) . These ETBN s c o n t a i n approximately 30% by weight of the corresponding carboxy-terminated b u t a d i e n e - a c r y l o n i t r i l e (CTBN) rubber. The f i l m s ( ~ 4 m i l s i n t h i c k n e s s ) were coated on an aluminum panel and i r r a d i a t e d by UY l i g h t (CoLight Model UYC-24) using a s i n g l e medium pressure Hg vapor lamp {6). The degree of cure was estimated by a combination 1

Davidson; Polymers in Electronics ACS Symposium Series; American Chemical Society: Washington, DC, 1984.

27.

ORS AND ENNS

Morphology

of Rubber-Modified

347

Photopolymers

TABLE I DESCRIPTION OF EPOXY-RUBBER MIXTURES


Mixture

Component (weight %) â B ETBN-8 Epon 828 ETBN-13

-

I

81

II

66

III

51

IV

81

15

V

66

30

VI

81

VII

66

VIII

75

IX

96

-

15 30 45

-

-

15

-

c ETBN-15

-

15 30

-

(a) epoxy ternrinated - CTBNX13 (27% CN) (b) epoxy terminated - CTBNX8 (17% CN) (c) epoxy terminated - CTBNX15 (10% CN) (d) PCI - p r o p r i e t a r y photocationic i n i t i a t o r ; s i m i l a r m a t e r i a l s may be obtained from 6E Corporation and/or 3M C o r p o r a t i o n .

American Chemical Society Library 1155 16th St. N. w. Washington. 0. C. 20038


J PCI 4 4 4 4 4 4 4 10 4

348


of s o l v e n t e x t r a c t i o n , F T - I R , thermogravimetric analysis (TGA) (6) along with d i f f e r e n t i a l scanning c a l o r i m e t r y (DSC) (Dupont 1090) and T o r s i o n Pendulum (TP) []) a n a l y s i s reported i n t h i s study. For the TP a n a l y s i s the r e s i n was a p p l i e d to a 1.4 mil copper substrate as a 4 mil film, i r r a d i a t e d f o r 5.5 s e c . and baked f o r 2 hours at 1 5 0 ° C . The sheet was c u t i n t o narrow s t r i p s (2" χ 1/8") and placed i n t o the helium-purged t o r s i o n pendulum sample chamber.


EXTENT OF CURE To c o r r e l a t e c u r i n g behavior and morphology, a measure of the degree of cure i s needed. The sol f r a c t i o n obtained v i a the s o l v e n t e x t r a c t i o n method can serve as a measure of the gel time along with an estimate of the extent of cure when coupled with some of the previously mentioned techniques. The cure h i s t o r y f o r mixture II i s shown i n Table II. According to the s o l v e n t e x t r a c t i o n d a t a , a cure schedule of 5.5 or 11.0 seconds followed by a 2 hour bake a t 150°C y i e l d s a f i l m with l e s s than 5% e x t r a c t a b l e s f o r t h i s mixture. T h i s data i s i n agreement with both dynamic and isothermal TGA which c o n s i s t e n t l y show a 2-5% weight l o s s at a temperature range of 250-350" o r a t 200 C a f t e r 48 hours. F i g u r e 1 shows the decomposition pattern for mixture II before and a f t e r cure (5.5 sec UV i r r a d i a t i o n followed by a 2 hour bake at 1 5 0 ° C ) . The solvent e x t r a c t i o n experiments coupled with DSC and FTIR data show t h a t the degree of cure of these m i x t u r e s , under i d e n t i c a l i r r a d i a t i o n and bake c o n d i t i o n s , is dependent on the concentration and nature (% a c r y l o n i t r i l e ) of the rubber m o d i f i e r . The sol f r a c t i o n s f o r PCI cured epoxy f i l m s with three d i f f e r e n t rubber modifiers (5), ETBN-13 (27% CN), ETBN-8 (17% CN) and ETBN-15 (102TCN) at a range of c o n c e n t r a t i o n s are shown i n Figure 2. The data show t h a t a decrease i n extent of cure occurs with increased rubber c o n c e n t r a t i o n and t h a t t h i s decrease (ETBN-13 > ETBN-8 > ETBN-15) may be c o r r e l a t e d to the percent a c r y l o n i t r i l e i n the rubber m o d i f i e r . This i s supported by the FT-IR spectra of two of these mixtures (IV and VI) as shown i n Figure 3 and the q u a n t i t a t i v e measure of the extent of cure as a f u n c t i o n of i r r a d i a t i o n time f o r mixtures V (30% TBN-13) and VIII (30% ETBN-15) as compared to mixture IX (no rubber) s i l i c o n i n F i g u r e 4 ( 8 ) . e


27.

Morphology

ORS AND ENNS

of Rubber-Modified

TABLE

349

Photopolymers

II

DETERMINATION OF DEGREE OF CURE FOR MIXTURE


Exposure (sec) t i m e

(a) (b) (c)

8

Post I r r a d(hrs) iation Bake

b

II

Sol F(%) raction

5.5

0

28.7

11.5

0

19.6

5.5

1

11.5

11.0

1

8.2

5.5

2

5.0

11.0

2

5.0

0

Residence time under CoLight Bake Temperature: 150 C Average of two samples on Al panels e

01 0 Figure 1.

ι

ι

100

ι

ι

I

I

I

I

I

1

200 300 400 500 TEMPERATURE (°C)

T6A of MIXTURE II from 25°C to [ ] uncured, [ ] cured at followed by 2 hrs @ 150°C bake.

1

1

600 e

1

1

700 e

600 C ( 2 0 C / m i n ) . 5.5 sec r a d i a t i o n


350



ETBN-13

ETBN-8

ETBN-15

% SOL 100 _

% RUBBER CURE: 11 SEC UV EXPOSURE Figure 2 .

E f f e c t of Rubber Concentration on Sol

Fraction.


ORS AND ENNS


27.

I 1680

Morphology of Rubber-Modified

ι 1440

ι 120 0

ι 980

Photopolymers

351

1—

720

WAVENUMBERS

I 1680

I

I

I

1440

1200

960

L_

720

WAVENUMBERS

Figure 3.

FT-IR Spectra of Mixtures YI (top) and IV cured f o r i d e n t i c a l i r r a d i a t i o n times.

(bottom)



352


x LU

0.2h

0" 0

1

200

1

400

1

' 600

1

800

1

1000

1200

TIME (sec)

Figure 4.

Extent of cure f o r mixtures V, VII and IX as a f u n c t i o n of i r r a d i a t i o n time at room temperature. Samples were i r r a d i a t e d using a 150 W Hg/Xe arc lamp. The i n t e n s i t y of the f i l t e r e d r a d i a t i o n was measured a t 4.5 mW/cm 6 365nm. 2



27.

ORS AND ENNS

Morphology

of Rubber-Modified

Photopolymers

353

The e f f e c t of the rubber m o d i f i e r on the cure rate can be a t t r i b u t e d i n p a r t to i n t e r a c t i o n s between the rubber and the PCI. These i n t e r a c t i o n s may r e s u l t i n the rubber acting as a screen o r quencher to the PCI, i n h i b i t i n g the generation of r e a c t i v e s p e c i e s t h a t l e a d to cure. The decrease i n degree of cure can i n turn be a s s o c i a t e d with the a c r y l o n i t r i l e content of the m o d i f i e r due to an increase i n i n t e r a c t i v e s i t e s and/or enhanced compatibility with the resin (increased solubility parameter) (2). These m o d i f i e r s can a l s o a f f e c t the way t h i c k f i l m s cure on a substrate and may g i v e r i s e to a cure gradient through the film. The f i l m s showed various degrees of t a c k i n e s s at the bottom surface adjacent to the s u b s t r a t e , e s p e c i a l l y with mixtures III, IY and Y. Some of these mixtures, p a r t i c u l a r l y II, III, and V I I , showed l a c k of compatibility on a macroscopic scale at room temperature. However, the degree of t a c k i n e s s a t the r e s i n substrate i n t e r f a c e d i d not c o r r e l a t e with the turbid appearance. THE GLASS TRANSITION TEMPERATURE The DSC and TP data f o r mixture II both show dual g l a s s t r a n s i t i o n s between 4 0 ° and 1 5 0 ° C , which merge i n t o one upon heating ( c y c l i n g up to 2 0 0 ° C ) , as shown i n F i g u r e s 5 and 6 r e s p e c t i v e l y . The dual t r a n s i t i o n s both i n c r e a s e i n temperature, and the lower transition decreases in i n t e n s i t y and e v e n t u a l l y disappears as cure proceeds. In mixture I (Figure 7 ) , i n which the rubber content has been reduced, the intensity of the lower transition is c o n s i d e r a b l y l e s s , and the f i n a l Tg i s higher than t h a t of mixture II. But mixture III (Figure 8 ) , i n which the rubber c o n c e n t r a t i o n has been increased to 15% CTBN, shows only a low, broad Tg which remains broad and lower than t h a t of mixtures I and II. Although these mixtures underwent i d e n t i c a l cure schedules, t h e i r extent of cure i s not n e c e s s a r i l y the same. As discussed e a r l i e r , the rubber inhibits the p h o t o c a t i o n i c a l l y i n i t i a t e d r e a c t i o n ; t h e r e f o r e the extent of cure f o r mixture I would be expected to be advanced further than t h a t of mixture II under the same cure conditions, because i t contains l e s s rubber. The TP spectrum of mixture I (Figure 7, f i r s t scah) i s s i m i l a r to the spectrum of mixture II (Figure 6, later scans), suggesting t h a t the d i f f e r e n c e s i n the i n i t i a l spectrum are due to d i f f e r e n t degrees of c u r e . In c o n t r a s t , mixture III apparently already c o n s i s t of a s i n g l e phase a f t e r the i n i t i a l cure s c h e d u l e .



I

I 40

I

I 60

I

I 80

I

I

I 120

I

T E M P E R A T U R E (°C)

I 100

I 140

I

I 160

I

I 180

I

I 200

I

I 220

Figure 5. DSC scan from 20% to 200% C (20% C/min) o f Mixture II. Cure s c h e d u l e : 5.5 s UV r a d i a t a t i o n followed by 20 h @ 150% C bake. Top t r a c e i s f i r s t s c a n ; bottom t r a c e i s second scan.

_l 20



27.

ORS AND ENNS

Figure 6.

Morphology

of Rubber-Modified

Thermomechanical Thermal h i s t o r y :

Spectra

(TS)

355

Photopolymers

of

mixture

II.

1.

Room temperature f o r 8 hours.

to

150%C,

isothermal

at

150%C

2.

150 C to - 1 8 0 ° C to f o r 8 hours.

170°C,

isothermal

at

170 C

3.

170 C to -180° f o r 8 hours.

isothermal

at

200°C

4.

200

e

e

e

to

to

e

200 C,

-180*C


e



S3INO>I13mH MI S>I3WA10d

RIGIDITY

RELATIVE


'Ll SHO SNN3 QMV fâopuduow sudtuajodoiouj pdifipoj^-uaqqn^ fo LÇ£


358


Increasing the i n i t i a t o r c o n c e n t r a t i o n i n mixture I to 10% (mixture VIII) again results i n two d i s t i n c t t r a n s i t i o n s (Figure 9 ) , which, on h e a t i n g , merge i n t o one. The f i n a l Tg i s very c l o s e t o t h a t o f mixture I.


MORPHOLOGY I t has p r e v i o u s l y been shown t h a t the morphology o f a rubber modified r e s i n i s determined a t g e l a t i o n ( 1 ) . In these p h o t o i n i t i a t e d systems over 80% o f the f f l m has g e l l e d within the f i r s t 11 seconds. T h i s would imply t h a t an epoxy matrix has been formed during the i r r a d i a t i o n which c o n t r o l s the morphology o f the f i l m . The i r r a d i a t i o n i s followed by a thermal cure during which the unreacted species w i t h i n the matrix r e a c t . However, s i n c e the DSC and TP r e s u l t s were not c o n s i s t e n t with t h i s model, i . e . , the dual transitions coalesce, implying a more homogeneous mixture after prolonged exposure t o higher temperature, the p a r t i c l e s i z e d i s t r i b u t i o n s were measured f o r various cure s c h e d u l e s , initiator concentrations and types of rubber. The Transmission E l e c t r o n Micrographs (TEM) of osmium t e t r o x i d e s t a i n e d specimens shown i n Figure 10 c l e a r l y i l l u s t r a t e the e f f e c t rubber c o m p a t i b i l i t y has on morphology. Although the same cure schedule (11 sec UV i r r a d i a t i o n , followed by 1 hour bake a t 150°C) was a p p l i e d t o each o f the m i x t u r e s , the degree o f phase separation and the p a r t i c l e size d i s t r i b u t i o n depend on the type o f rubber i n the mixture. The mixture with the most incompatible rubber (mixture VIII CTBN χ 15) has the l a r g e s t p a r t i c l e s as well as the most phase s e p a r a t i o n , whereas the mixture with the most compatible rubber (mixture V : CTBN χ 13) has the l e a s t phase s e p a r a t i o n . F i g u r e 11 shows the rubber particle d i s t r i b u t i o n f o r a f i l m o f mixture II f o r various cure schedules, as determined from TEM's s i m i l a r t o those i n F i g u r e 10. Rubber p a r t i c l e s i z e while not s i g n i f i c a n t l y a f f e c t e d by an i n c r e a s e i n i r r a d i a t i o n t i m e , shows a change in distribution to larger particle size with post irradiation bake temperature. T h i s trend toward larger p a r t i c l e s i z e i s comparable with r e s u l t s i n some thermally cured systems, where morphological changes continued t o occur even a f t e r g e l a t i o n ( 2 ) . The r e s u l t s o f the dynamic mechanical experiments i n d i c a t e t h a t these morphological changes occur very s l o w l y , even when baked above the ultimate glass transition temperature.




SMM3 QMV SHO 'LZ Jo ^So/oi/duop^ suaui^odojouj p^lfipo^-uaqqn^f


F i g u r e 10. TEM m i c r o g r a p h s o f P C I - i n i t i a t e d r u b b e r - m o d i f i e d e p o x i e s , showing the e f f e c t of t h r e e types of r u b b e r : left, M i x t u r e V I I ; m i d d l e , M i x t u r e I I ; and r i g h t , M i x t u r e V .


27. ORS AND ENNS

Morphology

of Rubber-Modified

Photopolymers

361

% DISTRIBUTION


100

PARTICLE SIZE (MICRON) Figure 11. P a r t i c l e (Mixture

size II).

distribution

for

three

cure schedules



362


Experiments are i n progress to determine the extent of the morphological changes with long post i r r a d i a t i o n bake times. An increase i n the PCI concentration a l s o y i e l d s an increase i n the rubber p a r t i c l e s i z e (Figure 12). Since doubling the i r r a d i a t i o n time f o r mixture II (Figure 11) d i d not have a l a r g e e f f e c t on p a r t i c l e s i z e , an increase i n PCI c o n c e n t r a t i o n (at constant i r r a d i a t i o n dose) may y i e l d a higher number of r e a c t i v e s i t e s and higher gel fraction without r e s u l t i n g i n a higher weight average molecular weight ( 6 ) . The e f f e c t of"rubber concentration on p a r t i c l e s i z e i s shown i n F i g u r e 13 f o r mixtures I and II. The i n c r e a s e i n p a r t i c l e s i z e with increased rubber content i s expected, since i t causes a decrease i n the cure r a t e and f a c i l i t a t e s aggregation during cure due to the increased heterogeneity of the components (mixture II i s more opaque than I). This macroscopic phase separation i s even more pronounced i n the ETBN-15 m i x t u r e s , i n which the rubber i s l e s s s o l u b l e ( 8 ) . In a d d i t i o n to the morphological changes observed "By e l e c t r o n microscopy and dynamic mechanical experiments, a corresponding i n c r e a s e i n transmission of v i s i b l e l i g h t i s observed. F i g u r e 14 shows the transmission spectra of mixture I a t the e a r l y stages i n the cure schedule. On prolonged baking a s h i f t to higher wavelengths occurs (yellowing). SUMMARY The cure and r e s u l t i n g morphology of photocationically initiated rubber modified epoxy films have been i n v e s t i g a t e d , as well as the e f f e c t s of varying rubber concentration and post i r r a d i a t i o n bake temperature and time on the morphology. P a r t i c l e s i z e d i d not change much with i n c r e a s i n g i r r a d i a t i o n time, but increased appreciably with i n c r e a s i n g bake temperature and time. This implies t h a t i n these epoxy systems the matrix created during irradiation under the described conditions does not r e s t r i c t changes i n morphology, even though the r e s i n has gelled. The increase in rubber particle size with increased rubber concentration can be a t t r i b u t e d to the decrease i n cure rate and the tendency to segregate. Acknowledgement The authors would l i k e solvent e x t r a c t i o n work.

to

thank

M.

H.

Papal ski

for


the

27.

ORS AND ENNS

Morphology

of Rubber-Modified

363

Photopolymers

7. DISTRIBUTION


50

PARTICLE SIZE (MICRON) Figure 12. P a r t i c l e s i z e concentration.

distribution

for

two

levels

of

PCI

Z DISTRIBUTION 100

.00-.16

.16-.32

32-.48

.48-.64

.64-.80

.80-1.76

PARTICLE SIZE (MICRON) Figure 13. P a r t i c l e s i z e d i s t r i b u t i o n concentration.

for

two

levels

of


rubber


364


% T

400

500 WAVELENGTH

600 (NM)

Figure 1 4 . Transmission spectra of a f r e e various stages o f cure a) b) c) d)

f i l m of Mixture

a f t e r 5.5 sees r a d i a t i o n i r r a d i a t i o n plus 20 min @ 150°C i r r a d i a t i o n p l u s 40 min @ 150 C i r r a d i a t i o n p l u s 90 min @ 150 C e

e


I

at

ORS AND ENNS

Morphology of Rubber-Modified Photopolymers

LITERATURE CITED 1. 2. 3.


4. 5.

6. 7. 8.

L. Τ. Manzione, J. K. Gillham and C. A. McPherson, J. Appl. Polym. Sci., 26, 886 (1981); J. E. Sohn, Am. Chem. Soc. Prepr., Div. Org. Coat. Plast. Chem, 44, 38 (1981); and references therein. J. V. Crivello, Lecture for "Fundamentals on Adhesion", SUNY at New Paltz, N.Y., Oct. 7-9 (1981); J. V. Crivello, U.S. Patent 4,138,255, Feb. 6 (1979) and U.S. Patent 4,139,385, Feb. 13 (1979). W. C. Perkins, J. Rad, Curing, 16, Jan. 1981. W. A. Romanchick and J. F. Geibel, Am. Chem. Soc. Prepr., Div. Org. Coat. Plast. Chem., 46, 410 (1982); J. F. Geibel, W. A. Romanchick and J. E. Sohn, ibid, 46, 416 (1982). These rubber modifiers can also be obtained commercially from Wilmington Chemicals, Wilmington, Delaware. R. D. Small and J. A. Ors, Am. Chem. Soc. Prepr., Div. Org. Coat. Plast. Chem., 48, (1982). J. B. Enns and J. K. Gillham, ACS Symposium Series, No. 197, Chp. 20 (1982). J. A. Ors, unpublished results.

RECEIVED November 7,

1983


Polymers in Electronics - American Chemical Society

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