Thermal Shock Behavior and Evaluation of Epoxy Resins Toughened

9 Thermal Shock Behavior and Evaluation of Epoxy Resins Toughened with Hard Particulates Masatoshi Kubouchi , Ken Tsuda , and Hidemitsu Hojo 1

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Department of Chemical Engineering, Tokyo Institute of Technology, 2-12-1, O-okayama, Meguro-ku, Tokyo 152, Japan Engineering Management Department, College of Industrial Technology, Nihon University, 1-2-1, Izumi-cho, Narashino-shi, Chiba-ken 275, Japan

Downloaded by PURDUE UNIV on July 5, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0252.ch009

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The effects of ceramic particles and filler content on the thermal shock behavior of toughened epoxy resins were studied. Thermal shock tests were performed with notched-disk specimens to evaluate thermal shock resistance. Ceramic particles that were used as filler were silicon nitride, silicon carbide, silica, and alumina. Test results were evaluated by using fracture mechanics. The mechanism of thermal shock fracture is discussed on the basis of analysis of scan ningelectron microscopic observations. In resins filled with stiff and strong particles, filler content had a remarkables effect on thermal shock resistance. With strong particles, the crack propagated through a space between particles in a zigzag way, whereas with easily broken fillers, the crack propagated in a straight path by breaking particles. With weakly bonded particles, the crack propa gated through the debonded interface caused by the difference of thermal expansion that results from thermal shocking. The thermal shock resistance was calculated on the basis of fracture mechanics and compared with experimental values.

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P O X Y R E S I N is C U R R E N T L Y U S E D as a high-performance insulating material

i n the electric (1) a n d electronic (2) fields. T h e resins generally show l o w toughness, but their fracture toughness is i m p r o v e d by adding second-phase materials. Toughened materials are often composed of either a soft rubbery material (3, 4) or a h a r d inorganic particulate (4-6) c o m b i n e d w i t h the matrix resin. T h e second phase also improves other properties, i n c l u d i n g electrical properties, thermal properties, flame resistance, etc. I n many applications o f epoxies, high resistance to thermal shock is required. Failure by thermal shock is a complex p h e n o m e n o n because it is greatly affected b y the geometry o f the test sample and the thermal a n d mechanical properties o f the material. 0-8412-3151-6

© 1996 American Chemical Society 119

Riew and Kinloch; Toughened Plastics II Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

120

T O U G H E N E D PLASTICS I I

W e proposed a new test m e t h o d to evaluate the thermal shock resistance o f epoxy resin (7). This test m e t h o d uses a notched-disk specimen, and the thermal shock resistance can be evaluated analytically on the basis o f linear fracture mechanics (8). I n o u r previous studies, we reported o n the use o f our proposed thermal shock test and evaluation methods (8, 11) to determine the t h e r m a l shock resistance o f toughened epoxy w i t h a soft second phase (9, JO), a n d also w i t h h a r d particulates (II). This chapter discusses the behavior, under thermal shock conditions, of epoxy resins toughened w i t h ceramic particulates. A l u m i n a A l 0 and silica S i 0 , w h i c h are usually used as filler for insulation materials, and the new ce ramic materials silicon carbide S i C and silicon nitride S i N are employed. F o r these toughened epoxy resins, the thermal shock resistance is evaluated by us i n g fracture mechanics. T h e difference between experimental and calculated values o f the thermal shock resistance is discussed from a fractographic point o f view. 2

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3

4

Experimental Details M a t e r i a l s . Epoxy resins filled with ceramic particulates were used. A bisphenol A epoxy resin (epoxy equivalent weight 370-435) and an alicyclic epoxy resin (epoxy equivalent weight 157) were mixed as the matrix resin. A phthalie an hydride was added as a hardener. The weight ratio of bisphenol resimalicyelic resimhardener was chosen to be 9:1:3. Alumina, silica, silicon carbide, and silicon nitride were used as fillers. These ceramic fillers, shown i n Figure 1, are angularshaped particulates without surface treatment. The average diameter (weight mean particulate diameter at 50%) of each of diese fillers was about 10 μπι, and each had almost the same particulate size distribution. In order to investigate the effects of filler content on thermal shock resistance, volume fractions (VJ-s) were varied from 0 to about 40%. The physical and mechanical properties of these test materials, obtained experimentally, are shown i n Table I. The fracture toughness was determined with notched-beam specimens loaded i n three-point bending (6). T e s t M e t h o d . Thermal shock tests were conducted by using the same method described i n the preceding papers (7-10), that is, employing a sharp notched-disk specimen 60 m m i n diameter and 10 m m thick (Figure 2a). A t first, the specimen with balsa heat insulators attached on both flat sides (see Figure 2b), which causes only radial heat transfer, was heated i n a high-temperature air bath. T h e n the specimen was quickly put into the cooling bath. As the low-temperature bath, a dry ice-pentane system, whose temperature was approximately 200 K, was used. The temperature difference of the thermal shock test was obtained by changing the initial air-bath temperature. The difference i n thermal expansion be tween the outer part and inner part makes a M o d e I problem of fracture mechan ics at the tip of the notch. After 5 min of cooling, the specimen was checked to de termine whether the crack initiated at the notch. W e used visual observation to detect the initiation of the crack i n the neat resin due to thermal shock. But because the filled resin was opaque, all specimens were observed by color checking. Actually, the crack initiated from the notch and propagated along the radial direction, as i n neat resin or rubber-modified resin



9.

K U B O U C H I E T AL.

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Thermal Shock Behavior of Epoxy Resins

Figure 1. SEM photographs of filler ceramic particulates: (a) silicon nitride (Si N ), (b) silicon carbide (SiC), (c) silica (Si0 ), (d) alumina (Al 0 ). 3

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2

3

(7, 10). This fact supports the availability of the analysis of a Mode I fracture me chanics problem. Fracture-specimen surfaces were examined after thermal shock testing and fracture toughness testing by use of scanning electron microscopy (SEM).

Results and Discussion T h e r m a l S h o c k R e s i s t a n c e . Thermal shock test results were similar to those obtained in neat epoxy resin or toughened epoxy with a soft secondphase material (7-11). Typical test results of epoxy resin filled with a ceramic particulate are shown in Figure 3. The figure shows the relation between the temperature difference ΔΤ and the normalized notch length c/R of 178 phr (parts per hundred of resin by weight) resin filled with silicon carbide particu late (V = 34.2%), where c is the initial notch length and R is the radius of the disk specimen. The critical temperature difference AT , which corresponds to the smallest temperature difference to initiate the crack from the notch, is clearly observed. This critical temperature difference, shown in the curve be tween the open and solid symbols, decreases with increasing notch length and f

C


Table I. Thermal and Mechanical Properties of Particulate-Filled Resins Si N 3

4

—

63.0

0.78

—

0.26

177 34.2 9.59 0.32 2.07

— — —

25 6.9 9.84 0.37 1.28

90 21.0

36.0

SiC 200 87 178 222 200 36.4 36.7 39.5 20.0 34.2 9.79 15.40 17.76 10.2 13.4 0.33 0.33 0.34 0.38 0.31 2.52 2.61 2.02 2.34 2.19 0.80 33.6

0.81 30.5

0.64 47.7

1.27

1.24 36.4

34.0

232 40.0 20.04 0.28 2.71 1.36 29.8

Si0

2

100 150 50 17.5 29.7 38.8 6.67 9.58 7.04 0.35 0.34 — 0.92 1.82 1.31 0.39 52.9

0.56

0.49 40.6

34.3

Al 0 2

3


Filler Filler content (phi") Volume fraction (%) Young's modulus (GPa) Poisson's ratio Fracture toughness (MPaVm") Thermal conductivity (W/m-K) Coefficient of thermal expansion (10 /K)

Riew and Kinloch; Toughened Plastics II Advances in Chemistry; American Chemical Society: Washington, DC, 1996. 250 150 200 200 50 100 19.1 26.1 32.0 37.1 45.8 10.5 9.89 11.57 10.89 5.19 6.42 8.51 0.32 0.30 0.31 0.31 0.35 0.35 1.12 1.22 1.25 1.00 1.91 1.04 0.66 29.0

0.38 40.4

0.52 34.9

0.66

0.90

24.0

29.3

6

phr is parts per hundred of resin by weight.

rt

1.06 18.5


9.

K U B O U C H I ET AL.


123

(b)

Insulator (Balsa) Specimen

Specimen holder

Figure 2. Schematic of the notched-disk specimen and its holder.

160

0.1

c/R

0.2

0.3

0.4

[-]

Figure 3. Thermal shock test result for 178 phr epoxy resin filled with SiC. Key: O, crack initiation and propagation from the notch; ·, no crack propagation.


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has a m i n i m u m value at about c/R = 0.2. This m i n i m u m critical temperature difference can be used for evaluating the thermal shock resistance. A comparison o f critical temperature differences of resins filled w i t h sev eral ceramic particulates is shown i n F i g u r e 4. T h e volume fraction o f all these composites is 3 4 . 2 % . T h e critical temperature difference o f epoxy filled w i t h h a r d particulates was classified into three groups o n the basis of thermal shock resistance. Composites filled w i t h a strong particulate, such as silicon nitride or silicon carbide, showed high thermal shock resistance. Some improvement i n t h e r m a l shock resistance was recognized for silica-filled composites. C o m posites filled w i t h a l u m i n a or a l u m i n u m nitride showed almost comparable or lower resistance compared w i t h the neat resin. F i g u r e 5 shows the effects o f filler content o n thermal shock resistance at c/R = 0.2 for composites o f silicon nitride, silicon carbide, silica, a n d alumina. T h e t h e r m a l shock resistance o f resin filled w i t h silicon nitride increases l i n early w i t h the volume fraction. T h e value o f the thermal shock resistance is high, especially at higher volume fraction (Vf > 40%), that is, t h e r m a l shock re sistance reaches 140 Κ (Figure 5a). T h e thermal shock resistance o f composite filled w i t h silicon carbide increases rapidly w i t h the increase o f filler content, a n d it reaches 135 Κ at Vf o f 4 0 % , w h i c h is similar to the case o f silicon nitride (Figure 5b). I n the case o f silica-filled composites there is also an increase, but above a 3 0 % volume fraction a plateau is reached (Figure 5c). A l u m i n a - f i l l e d composites show a decrease i n thermal shock resistance w i t h filler content, t h e n an almost constant value starting at V = 2 0 % (Figure 5d). f

2.5 RH Series RF Series

2.0 1.5 1.0 0.5

>

10

15

Rubber (phr) Figure 4. Comparison of the critical temperature difference, AT , of ceramicparticle-filled epoxy resins at Vf = 34.2%. C


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160

11

60 U

0

.

ι

•

ι

•

ι

ι

I

ι

I

ι

I

0.1

0.2

L

ι

0.3

Volume fraction, V

125

0.4 f

0.5

[ - ]

Figure 5. Effect of filler contents on thermal shock resistance at c/R = 0.2: (a) Si N , (b) SiC, (c) Si0 , and (d) Al 0 . 3

4

2

2

3

F r a c t o g r a p h y . I n order to discuss the effect o f filler content o n the t h e r m a l shock resistance o f each composite, fracture surfaces were observed b y S E M after t h e r m a l shock test a n d c o m p a r e d w i t h fracture toughness test specimens. A t lower particulate content ( V < 20%), specimens that h a d u n dergone the two tests were similar i n appearance for the four composites, that is, they h a d a smooth surface without the appearance o f particulates. A t a h i g h e r particulate content (Vf > 30%), however, more complicated fractographies were recognized. T h e fracture surfaces o f the resins filled w i t h ce ramic particulates are shown i n Figures 6 a n d 7. I n resins filled w i t h silicon nitride a n d silicon carbide, rough fracture surfaces were observed i n b o t h tests at the higher particulate contents (Figure 6, a-d). D e t a i l e d observation c o n f i r m e d that t h i n resin layers r e m a i n e d o n the particulate surface. T h e presence o f the layers means that the particulate a n d matrix are b o n d e d f


126



Figure 6. Comparison of fracture surfaces of filled resins after thermal shock testing (left) andfracture toughness testing (right): (a, b) Si N , (c, d) SiC, (e,f) Si0 . 3

4

2

strongly, so the crack propagates through a space between the particulates i n a zigzag way. I n silica-filled resins, flat surfaces w i t h a particulate appearance were observed i n both tests at a higher filler content. Also, flat fracture surfaces o f particulates themselves were observed instead of rough ones, as shown i n F i g u r e 6, e a n d f. T h i s observation means that the crack propagated straight forward b y breaking silica particulates.



9.



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Figure 7. Comparison of fracture surfaces after thermal shock testing (a) and fracture toughness testing (b) of Al 0 -filled epoxy resin. Left: SEI. Right: BEL 2

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I n alumina-filled resins, o n the other hand, different surfaces were observed i n thermally shocked a n d fracture-toughness-tested specimens at a higher filler content. T h e S E M photographs o n the left side o f F i g u r e 7 are secondly electron micrographie images (SEIs) w h i c h are ordinary S E M i m ages, a n d those o n the right side are backscattering electron micrographie i m ages ( B E I s ) , w h i c h d e p e n d o n the atomic mass a n d show the morphology just u n d e r the surface. T h e fracture surface i n F i g u r e 7 shows debonding o f the particulate-matrix interface after thermal shock testing (a), because the particulate surface can be observed i n S E I , whereas the surface is rough after fracture toughness testing. W e cannot find particulates i n the S E I , whereas they are obvious i n the B E I at the corresponding position (Figure 7b). T h i s fact suggests that the crack propagates through the matrix resin i n the fracture toughness test o f alumina-particulate-filled resin, and thus particle interfacial d e b o n d i n g is not recognized, as is the case w i t h silicon carbide composites. T h u s the crack i n alumina seems to be easy to propagate b y interfacial d e b o n d i n g as a result o f thermal shocking. These fracture mechanisms are shown schematically i n F i g u r e 8. I n the particulate-filled resins that have a strong particulate-matrix interface, b o t h t h e r m a l shock resistance and fracture toughness are improved. W h e n the particulate itself is strong enough to withstand crack propagation, as is the case


128



Easily broken particle

Figure 8. Schematic of the three types of crack-propagation processes that occur in ceramic-particulate-filled resins under thermal shock conditions.

w i t h silicon nitride or silicon carbide, thermal shock resistance increases rapidly w i t h filler content. B u t i f the particulate is broken by crack propaga tion, as i n silica, the tendency o f thermal shock resistance to increase is not so remarkable at higher filler content. O n the other h a n d , i n weakly b o n d e d particulates such as alumina, ther m a l shock resistance decreases w i t h increasing filler content. I n this case, i n terfacial d e b o n d i n g occurs because o f the difference i n thermal expansion, a n d t h e n the crack propagates through the interface preferentially. Evaluation of T h e r m a l Shock Resistance. T h e results o f the t h e r m a l shock tests are evaluated by the m e t h o d based o n fracture mechanics. T h e r m a l shock resistance ( A T ) , can be calculated as follows (8,10): c

{

A

T

c

U

~

cal

Ea

VR

Ea

+

hV¥

W

w h e r e K is M o d e I fracture toughness, η is Poisson s ratio, a is the coefficient o f t h e r m a l expansion, £ is the elastic modulus, k is thermal conductivity, h is the heat transfer coefficient, a n d C and C are constants determined b y the conditions o f the t h e r m a l shock test. Ic

x

2


9.



129

T h e relationship between the ratio ( A T ) / ( A T ) and the volume fraction V f is shown i n F i g u r e 9, where ( A T ) is thermal shock resistance obtained b y experiment. W h e r e ( A r ) / ( A T ) = 1, it means that the t h e r m a l shock resistance can be evaluated w e l l w i t h equation 1. T h e t h e r m a l shock resistance o f composites filled w i t h silicon nitride, silica, a n d a l u m i n a can be especially evaluated at lower volume fractions. However, for silicon carbide, the ratio is not unity but greater than 1, a n d so the prediction o f thermal shock resistance made w i t h equation 1 is a conservative evaluation. I n the case o f alum i n a , o n the other h a n d , the ratio ( A T ) / ( A r ) i decreases remarkably w i t h increasing V . These values o f ( A T ) / ( Â T ) are almost constant at lower v o l u m e fractions i n every case. c

c

c

e x p

c

c


f

c

exp

cal

c a l

exp

exp

c

e x p

c

c

ca

cal

Because the a l u m i n a composites show ( A T ) / ( A T c ) ^ < 1, w h i c h i n d i cates that the t h e r m a l shock resistance is overestimated b y equation 1, the fracture behavior o f alumina-filled composites is examined i n further detail. A s shown i n F i g u r e 7, d e b o n d i n g o f the interface is observed i n the t h e r m a l shock test specimen but not i n the fracture-toughness test specimen T h e r e fore, for the evaluation of t h e r m a l shock resistance by equation 1 without overestimation, K s h o u l d be measured u n d e r the condition i n w h i c h the same fracture pattern as that seen i n the t h e r m a l shock test is obtained. c

exp

I c

A fracture toughness test was performed using a pre-thermal-shocked s p e c i m e n i n order to obtain the fracture toughness measured u n d e r same c o n ditions as the t h e r m a l shock test. That is, using the specimen for the fracture toughness test (a rectangular bar w i t h a notch for three-point bending), m o d erate t h e r m a l shock was conducted before the fracture toughness test. T h e

J

1.0

6

0.1

0.2

0.3

Volume fraction, V

0.4 f

0.5

[ - ]

Figure 9. Relationship between ( A T ^ ^ / f A T J / and volume fraction. ca



130


Figure 10. SEM photographs of fracture surfaces obtained by fracture toughness testing with a pre-thermal-shocked specimen of Al 0 -ftlled resin. Left: SEI. Right: BEL 2

3

t h e r m a l shock o f going from r o o m temperature to a dry ice-pentane system (200 K ) was used to prepare the pre-thermal-shocked specimen. F i g u r e 10 shows S E I s a n d B E I s from the fracture toughness test that was p e r f o r m e d w i t h the pre-thermal-shocked specimen. A comparison o f fracture toughness measured i n the n o r m a l and pre-thermal-shocked specimens is s u m m a r i z e d i n Table II. As shown i n F i g u r e 10, particulate-matrix interfacial d e b o n d i n g can be observed i n alumina composites, but it is not detected i n other particulates. T h e values o f the fracture toughness are also changed only i n alumina-filled resins.Values o f ( A T ) / ( A T ) j modified using K (T), obtained from the fracture toughness test conducted w i t h a pre-thermalshocked specimen, are also shown i n Table II. T h e modified values are close to unity. c

exp

c

ca

ÏC

Table II. Comparison of K Obtained in Fracture Toughness Testing of Pre-Thermal-Shocked and Normal Specimens, and Modified Values of I c

(AT ) /(Ar ) . c

exp

c

cal

(AT ) /(AT ) c exp

Filler

K (T)/K / C

A1 CV M0 Si0 Si N SiC 2

b

2

3 9

3

4

I C

0.85 0.89 0.99 1.01 1.01

c cal

at V = 34.29c f

Normal Value

Modified Value

0.70 0.65 1.08 1.01 1.25

0.78 0.88

— — —

"Mean diameter of the A l 0 particulate is 27.3 μιη. Mean diameter of the A l 0 particulate is 3.4 μιη. /?

2

3

2

3


9.



131

Conclusions T h e effects of ceramic particles and filler content on the thermal shock behav ior o f toughened epoxy resins have been studied. Resins filled w i t h stiff and strong particles, such as silicon nitride and silicon carbide, show h i g h thermal shock resistance, and the effect o f filler content is remarkable. A t higher vol u m e fractions ( V > 40%), the thermal shock resistance o f these composites reaches 140 K, whereas that o f neat resin is about 90 K. T h e highest thermal shock resistance is obtained w i t h silicon nitride. T h e thermal shock resistance o f silica-filled composites also increases w i t h increasing filler content, but above 3 0 % o f volume fraction it comes close to a certain value. O n the c o n trary, i n alumina-filled resin, the thermal shock resistance shows a decrease w i t h increasing filler content.


f

I n the case o f strong particulates, such as silicon nitride or silicon carbide, cracks propagate through the space between particulates i n a zigzag way. I n the case o f easily broken filler such as silica, however, cracks propagate straight forward by breaking particulates. I n the case o f weakly b o n d e d particulates such as alumina, cracks propagate through the debonded interface caused b y the difference o f thermal expansion at thermal shocking. T h e thermal shock resistance was calculated o n the basis o f fracture me chanics and c o m p a r e d w i t h experimental values. Except for alumina-filled epoxy resin, the values were i n good agreement. F o r the alumina-filled resin, the calculated t h e r m a l shock resistance was an overestimation. T h e disagree ment between the calculated and experimental values for the alumina-filled composites is based o n the difference i n fracture behavior seen i n the t h e r m a l shock a n d fracture toughness tests. B y performing the fracture toughness test o n a pre-thermal-shocked specimen, relatively good results were obtained. These results i m p l y that our proposed thermal shock test a n d evaluation m e t h o d can be applied to any epoxy resin systems toughened w i t h a h a r d par ticulate.

References 1. Kaiser, T. Chimia 1990, 44, 354-359. 2. Nagai, Α.; Eguchi, S.; Ishii, T.; Numata, S.; Ogata, M . ; Nishi, K. Proc. ACS, Div. PMSE, 1994, 70, 55-56. 3. Hojo, H . J. Mater. Sci. Soc. Jpn., 1982, 18, 281-286. 4. Moloney, A. C.; Kausch, H. H.; Kaiser, T.; Beer, H . R. J. Mater. Sci. 1987, 22, 381-393. 5. Hojo, H . ; Toyoshima, W.; Tamura, M . ; Kawamura, N . Polym. Eng. Sci. 1974, 14, 604-609. 6. Nakamura, Y.; Yamaguchi, M . ; Kitayama, Α.; Okubo, M . ; Matsumoto, T. Polymer, 1991, 32, 2221-2229. 7. Hojo, H . ; Kubouchi, M . ; Tamura, M . ; Ichikawa, I. J. Thermoset. Plast. Jpn 1988, 9, 133-140.


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8. Kubouchi, M.; Hojo, H. J. Soc. Mater. Sci. Jpn 1982, 39, 202-207. 9. Kubouchi, M . ; Hojo, H . Proc. ACS Div. PMSE 1990, 63, 200-204. 10. Kubouchi, M . ; Hojo, H . Toughened Plastics I, A C S Advances in Chemistry Series 233, 1993, 365-379. 11. Kubouchi, M.; Tsuda, K.; Tamura, M.; Ichikawa I.; Hojo, H . J. Thermoset. Plast. Jpn 1992, 13, 215-225.


Thermal Shock Behavior and Evaluation of Epoxy Resins Toughened

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