12 Crazing and Dilatation-Band Formation in Engineering Thermosets H.-J. Sue , P. C. Yang , P. M . Puckett , J. L. Bertram , and Ε. I. Gareia-Meitin 1
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Department of Mechanical Engineering, Texas A&M University, College Station, TX 77843-3123 B-1603, Dow Chemical USA, Freeport, TX 77541 1
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The crazing phenomenon was observed in a high-performance, ther mosetting, 1,2-dihydrobenzocyclobutene and maleimide (BCB-MI) resin system. The craze-fibril diameter, band thickness, and size of the damage zone observed in the BCB-MI matrix were all larger than those of the polystyrene craze. As a result, the BCB-MI had a much higher fracture toughness than polystyrene, bisphenol A polycarbon ate, and many other rubber-toughened thermosets. Unusual craze -like dilatation bands were also detected in several engineering ther mosets. These dilatation bands were, however, less effective in fracture-energy dissipation than those due to craze bands. Only when the formation of dilatation bands was extensive did the tough eningeffect become significant. The possible cause(s) and conditions resulting in crazing and the formation of dilatation bands in ther mosets are discussed. The importance of the present findings for the toughening of high-performance thermosets is also addressed.
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N E O F T H E P R E D O M I N A N T M E C H A N I S M S o f toughening i n thermoplastics is
crazing. T h e stretching, disentanglement, a n d fibril formation o f h i g h molecular weight, linear, thermoplastic polymers are w e l l understood a n d characterized. I n the literature, many researchers have c l a i m e d that crazing can o c c u r i n thermosets a n d is a major toughening mechanism for thermosets (1-5). F o r instance, Sultan a n d M c G a r r y ( i ) indicated that crazing c o u l d be a p r e d o m i n a n t flow m e c h a n i s m i n rubber-modified epoxies w h e n the r u b b e r particle is large a n d the stress field is tensile. B u c k n a l l and Yoshii (4, 5) showed signs o f crazing i n a carboxy-terminated butadiene-acrylonitrile ( C T B N ) , r u b ber-modified epoxy matrix. However, they d i d not convincingly demonstrate that crazing h a d taken place a n d was a major source o f toughening. C o n s e quently, i n the area o f thermoset toughening, it is still believed that crazing is not likely to occur i n engineering thermosets (6-9) because these highly crossl i n k e d molecules cannot undergo significant molecular stretching a n d disen0-8412-3151-6
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tanglement (5, 10). Therefore, no k n o w n efforts have focused on toughening thermosets via crazing or craze-like damage.
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Recent progress a n d technological breakthroughs i n polymer synthesis, modification, and characterization have l e d to new findings o f previously u n r e p o r t e d toughening mechanisms i n thermosets as w e l l as a better understand i n g o f w h y a n d h o w these mechanisms operate (11-16). F o r instance, a p h e n o m e n o n we designate " c r o i d i n g " is observed i n a c o r e - s h e l l r u b b e r ( C S R ) modified diglycidyl ether of bisphenol A ( D G E B A ) epoxy resin c u r e d w i t h p i p e r i d i n e (Figure 1) (11). A t the optical microscopic scale (a practical resolu tion o f about 1 μπι), these croids resemble the well-known crazes observed i n polystyrene. However, w h e n transmission electron microscopy ( T E M ) is used,
Figure 1. DN-4PB plane-strain damage zone of epoxy that has been modified with core-shell rubber. Top: ROM image obtained under Nomarski interference contrast. Bottom: TEM image of the croids in the plane-strain region. In both im ages the crack propagates from the upper nght to the upper left.
Riew and Kinloch; Toughened Plastics II Advances in Chemistry; American Chemical Society: Washington, DC, 1996.
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the croids are found to be composed o f linear arrays o f dilatation bands c o n taining numerous, highly cavitated, C S R particles (Figure 1, bottom). These findings suggest the possibility o f toughening engineering thermosets via mas sive crazing or craze-like toughening mechanisms. T h e present w o r k focuses o n characterizing the fracture mechanism(s) o f several moderately cross-linked engineering thermosets whose molecular weights ( M ) range from 560 to 920 g/mol. Characterization was performed us i n g the double-notch, four-point-bend ( D N - 4 P B ) technique (17) together w i t h various microscopic and spectroscopic tools. T h e goal o f this w o r k is to gain an understanding o f h o w and w h y crazing a n d dilatation bands occur i n engineering thermosets, and to use that knowledge to effectively toughen thermosets via p r o m o t i o n o f crazes or craze-like dilatation bands.
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Experimental Details M a t e r i a l s . The thermosetting resins investigated include (1) 1,2-dihydrobenzocyclobutene and maleimide ( B C B - M I ) resin, (2) a modified version of the B C B - M I resin ( B C B - M I - M ) , (3) cross-linkable epoxy thermoplastic ( C E T , which is chemically similar to T A C T L X 695 epoxy resin from D o w Cnemical Co.), and (4) D G E B A epoxy (D.E.R. 332, D o w Chemical Co.) cured with piperidine. The chemical structures of the resins are shown i n Chart I. The detailed syntheses, curing schedules, and physical testing conditions of these resins can be found elsewhere (11,12,14,15,18). F o r convenience, the ba sic physical properties of these resins are given in Table I. M e c h a n i c a l T e s t i n g . The thermosetting resins were cured and pourcast into rectangular plaques, which were then machined into (1) 12.7 χ 1.27 χ 0.635 c m bars for the D N - 4 P B experiments (17), and (2) 6.35 χ 1.27 χ 0.635 c m bars for the single-edge-notch, three-point-bend ( S E N - 3 P B ) , fracture-toughness measurements. These bars were notched with a notching cutter (250 μιη radius), and then tapped with a Mquid-nitrogen-chilled razor blade to wedge open a sharp crack. A Sintech-2 screw-driven mechanical testing machine was used to conduct both the S E N - 3 P B and D N - 4 P B experiments at a crosshead speed of 0.0508 cm/min. W h e n the D N - 4 P B experiment was conducted, care was taken to ensure that the upper contact-loading points were touching the specimen simultaneous-
M i c r o s c o p y . The D N - 4 P B damage zone of the subcritically propagated crack was cut along the crack-propagation direction but perpendicular to the frac ture surface using a diamond saw. The plane-strain core region and plane-stress surface region of the crack-tip damage zone were prepared for transmitted optical microscopy ( T O M ) , reflected optical microscopy ( R O M ) , and T E M following the procedures described by Sue and co-workers (20, 21). T O M and R O M were per formed using an Olympus Vanox-S microscope. T E M was conducted using a J E O L 2 0 0 0 F X A T E M operated at an accelerating voltage of 100 kV. The density inside the dilatation bands was measured using electron energy loss spectroscopy ( E E L S ) with a Gatan 666 parallel E E L S ( P E E L S ) , which was at tached to the T E M . A n accelerating voltage of 100 k V was used. The density of the dilatation band was calculated using the following equation (22):
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Pcraze ~~ Pmatrix
.
v-*-/
T
In!
where ρ is the density, J is the total spectral intensity, i is the zero loss intensity, the subscript c indicates either the craze band or the dilatation band, and the sub script m indicates the bulk matrix. To ensure that our approach gives reasonable results, the same method was also used to measure the density of a craze band i n a tensile-loaded polystyrene sample. Downloaded by PURDUE UNIV on July 5, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0252.ch012
x
0
1,2-Dihydrobenzocyclobutene and maleimide ( B C B - M I )
Modified B C B - M I ( B C B - M I - M ) OH
OH
OH
OH
j — C - C - C ^ O - R ' - O - C - C ^ R'
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NH
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Cross-linkable epoxy thermoplastic ( C E T ) CH
j
Ο CH -CH-CH -Q-
