Toughened Plastics II - American Chemical Society

20 Ductile-to-Brittle Transitions in Blends of Polyamide-6 and Rubber R.J. Gaymans , K. Dijkstra , and M.H. ten D a m 1

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University of Twente, P.O. Box 217, 7500 AE Ensehede, Netherlands 2 Research, P.O. Box 18, 6160 MD Geleen, Netherlands National Starch, P.O. Box 13, 7200 A A Zutphen, Netherlands 1

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Blends of polyamide-6 and rubber were prepared with various rub ber concentrations and particle sizes. The blends were studied for (1) notched Izod impact behavior as a function of temperature (-50 to 80 °C), and (2) notched tensile impact behavior as a function of test speed (10 to 13 m/s) and temperature (-10 to 70 °C). The structure of the deformation zone was studied with electron mi -5

croscopy. With the notched tensile test as a function of test speed, two ductile-to-brittle transitions are apparent: one at low test speeds and one at high test speeds. The rubber concentration affects both the low- and high-speed transitions, whereas the rubber-particle size only has an effect at high speeds. Plastic deformation of these materials is great next to the fracture surface. The plastic is largely dissipated as heat. In the high-speed regime, the deformation is vir tually adiabatic, and the temperature seems to rise locally to a level higher than the melting temperature of the material. This melt for mation at high test speeds is affirmed by scanning electron micro scopic studies of the deformation zone. The presence of a melt layer ahead of a crack blunts the crack and allows more deformation to take place.

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OLYMERS H A V E INTERESTING M E C H A N I C A L PROPERTIES at l o w d e f o r m a t i o n

rates, but u n d e r notched-impact conditions they fracture mainly i n a brittle manner. A t elevated temperatures most polymers become ductile. Polyamide-6 (PA-6) a n d P A - 6 6 (dry) become ductile at 70 °C. A n effective means o f modifying the impact behavior o f engineering plastics is b y b l e n d i n g i n rubber. I n this way toughness increases manyfold, whereas the tensile strength and modulus decrease approximately i n proportion to the r u b b e r c o n centration (I, 2). U p o n blending r u b b e r into P A , a sharp ductile-to-brittle transition is apparent i n the toughness-temperature curve, a n d at a lower t e m perature than i n the neat P A (Figure 1). O n fractured samples at temperatures lower than the ductile-to-brittle transition, stress-whitening is apparent i n the notch region but is nearly absent 0-8412-3151-6

© 1996 American Chemical Society

In Toughened Plastics II; Riew, C., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.

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Temperature (°C) Figure 1. Notched Izod impact strength versus temperature of PA-6-polybutadiene blends with different rubber concentrations. Key: O, 0 vol%; 1 vol%; +, 7.5 vol%; A, 15 vol%; x, 22.5 vol%; Π, 30 vol% (2).

on the fracture surface. T h e stress-whitening is an indication o f severe defor mation (3). T h e energy absorption i n this region is mainly from elastic a n d plastic déformation before the crack is initiated. A t temperatures higher than the ductile-to-brittle transition, stress-whitening is apparent b o t h i n the notch region a n d i n the fracture-propagation region. A t the ductile-to-brittle transition, crack propagation changes from unstable to stable crack growth. T h e ductile-to-brittle transition is a critical parameter, moreso than the toughness value itself, a n d it can be used w e l l for evaluation purposes. T h e ductile-to-brittle transition i n P A has been studied as a function o f (1) materials parameters, i n c l u d i n g matrix molecular weight (4), type o f polyamide (5, 6), a n d type o f r u b b e r (7), and interface (8), a n d (2) morphological parameters, i n c l u d i n g rubber concentration (9), particle size (9), ligament thickness (9, 10), use of very small particles (5, I I ) , and particle distribution (2). W h e n one o f these variables is studied, other variables are often changed too. O n l y i n experiments w i t h large series can some idea o f the effect o f a variable be obtained. T h e materials a n d morphological parameters are usually studied as function o f test temperature and test speed. T h e I z o d m e t h o d as function o f t e m perature (9,10) is standard. Also studied is the C h a r p y impact behavior, b o t h as a function o f test temperature and test speed (12). W i t h a notched tensile i m pact test, b o t h test speed and test temperature can easily be varied {1,2,13). I n the w o r k described i n this chapter, we studied the influence o f test speed a n d test temperature i n the notched tensile setup and compared the


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data w i t h those obtained using the notched I z o d method. W e also p e r f o r m e d fractographic studies on the b r o k e n samples to gain insight into the fracture process.

Experimental Details M a t e r i a l s . The matrix polymer was PA-6 (Akulon K124, relative viscosity = 2.4, and Akulon M258, η ] = 5.8, D S M ) and a commercial PA-6-polybutadiene blend (Durethane BC303, Bayer). The blends were made by compounding with a twin-screw extruder (Berstorff, Z E 25, Hannover). The blends were injec tion-molded into Izod test bars (80 χ 10 χ 4 mm), and the notch was milled i n (ISO 190/1 A). Before testing, the samples were dried i n a vacuum oven at 110 °C for 18 h. In the notched tensile impact test performed on single-edged notched Izod bars, a clamp distance of 60 m m was employed. The tensile tester used was a Schenck V H S hydraulic instrument, with a clamp speed ranging from 10~" to 13 m/s. The structural analysis of the blends and the deformation layer was per formed with scanning electron microscopy ( S E M ) on cryomicrotomed samples. The weighted average particle size (d = X n ^ f / X n ^ - , where η is the number of particles and d is diameter) was determined from the micrographs with a particlesize analyzer (Zeiss T G Z 3). Transmission electron microscopy ( T E M ) was per formed on microtomed samples that were stained with O s 0 for 24 h at room tem perature before being cut. The thin slices were then stained again for 48 h at room temperature and studied with a J E O L 200 C X instrument. T) i

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Results N o t c h e d T e n s i l e Test. I n the notched tensile impact test used, the c l a m p speed c o u l d be varied over a wide range ( 1 0 ^ - 1 3 m/s), and w i t h an oven the temperature c o u l d be varied. T h e advantages o f this notched tensile impact test are that the loading is simple and that, as the samples are clamped, signal noise is low. F r o m each stress-displacement graph, the m a x i m u m stress was recorded a n d the energy supplied to the specimen d u r i n g the test was cal culated. T h e fracture energy was divided into an initiation part and a propaga tion part. T h e point o f m a x i m u m stress was chosen as the boundary between crack initiation a n d crack propagation. In the case of brittle behavior, the stress falls almost instantaneously from the m a x i m u m stress to zero. Therefore, a brittle fracture is characterized b y a low propagation energy, meaning that af ter the fracture has started, the stored elastic energy is sufficient to propagate the crack. B y propagation energy, we mean the extra supplied energy neces sary to fracture the sample. A clamp speed o f 1 m/s i n the notched tensile i m pact test is comparable to the speeds i n the notched Izod test (2). T h e stress-displacement curves for the PA-ethylene-propylene r u b b e r ( P A - E P R ) b l e n d (80/20) at different speeds (10~ -10 m/s) are shown i n F i g u r e 2. T h e increase i n m a x i m u m stress at the highest speed and the increase i n fracture energy w i t h increasing speed are noteworthy. 2



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Displacement (mm) Figure 2. Results of notched tensile impact tests versus displacement for the PA-6-EPR blend (24 vol%, 0.3 ^m),for different piston speeds (13).

R u b b e r Concentration Notched Izod Test. T h e influence o f rubber concentration o n the n o t c h e d I z o d impact strength was studied i n PA-6-polybutadiene blends w i t h a small particle-size distribution (Figure 1) (1, 2). T h e D u r e t h a n e was d i l u t e d w i t h M 2 5 8 . T h e P A - 6 has a l o w notched impact strength at r o o m temperature and fractures i n a brittle manner. A ductile-to-brittle transition is apparent near its glass transition. I n polypropylene the material becomes ductile w e l l above its glass-transition temperature, T (14), yet polycarbonate becomes ductile below its T . Thus the question is whether the ductile-to-brittle transi tion i n P A is due to the glass transition or to a change i n y i e l d behavior that just happens near the glass transition o f this material. W h e n b l e n d i n g rubber into P A (particle size « 0.5 μηι, the ductile-tobrittle transition strongly shifts w i t h rubber concentration to lower tempera tures (9). T h e impact energy i n the low-temperature "brittle region" increases w i t h increasing r u b b e r concentrations (Figure 1). T h e impact energy i n the "tough region," however, decreases w i t h increasing r u b b e r concentration. I n earlier studies o n PA-etheylene-propylene d i m e r m o n o m e r ( P A - E P D M ) a n d P A - E P R (Figure 3), performed using a broader particle-size distribution, the impact level i n the tough region d i d not decrease w i t h r u b b e r concentration. T h e n e w data (Figure 1) are for a more u n i f o r m b l e n d a n d thus better explain g

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Temperature (°C) Figure 3. Results of notched Izod impact tests versus temperature for PA-6-EPR blends with different rubber contents at a constant rubber-particle size (0.3-0.4 μm). Key: • , 0 vol%; •, 6.3 vol%; +, 12.5 vol%; À, 18.4 vol%; and O , 24.3 νοΙΨο (13).

what the r u b b e r concentration does. T h e fall o f impact energy i n the tough re gion w i t h r u b b e r concentration is due to the decreasing P A content (Figure 1). A t T o f the P A i n the b l e n d , no transition i n toughening behavior is observed. T h e position o f the ductile-to-brittle transition i n P A near T is thus just a c o i n cidence a n d has little to do w i t h T of the material. A t low deformation speeds (notched three-point b e n d i n g method), L a z z e r i found that r u b b e r concentra tion also h a d a clear effect o n the ductile-to-brittle transition temperature, but at lower temperatures than at high speed (12). g

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Notched Tensile Impact Test. T h e influence o f test speed was studied w i t h the n o t c h e d tensile impact test o n P A - E P R blends (13). T h e force-time signal was recorded, a n d from it were calculated the m a x i m u m stress, total e n ergy absorption, a n d energy absorption after crack initiation (propagation e n ergy). Because we are interested i n the crack-propagation behavior o f the blends, the propagation energies and m a x i m u m stress are given. A t piston speeds o f 1 0 to 10 m/s, P A - 6 (K124) has a low crack-propagation energy ( F i g ure 4). T h u s P A underwent brittle fracture i n the whole speed region. T h e blends give a complex picture. T h e propagation energies i n the blends all start h i g h at l o w piston speeds, fall to near zero at 1 0 m/s, a n d show some increase again at higher rates. A t low speeds w i t h small amounts o f r u b ber, the material is already tough at r o o m temperature. T h e propagation ener gy at l o w speeds (in the ductile region) shows little dependence o n r u b b e r c o n centration. Surprisingly, the propagation energy o f the 15 a n d 2 0 % blends - 5

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Piston speed (m/s) Figure 4. Notched tensile impact propagation energy versus piston speed for PA-6-EPR blends with different rubber contents. Key: A, 0 vol%; •, 6.3 vol%; +, 12.5 voWc; •, 18.4 vol%; and O, 24.3 vol% (13).

increases again above 1 m/s, but the 1 5 % b l e n d is nearly brittle again at 5 m/s. T h e 15 a n d 2 0 % blends clearly show discontinuous behavior, and this behavior was f o u n d several times. T h e 1 5 % b l e n d shows a low-speed ductile-to-brittle transition at 10~ m/s a n d a high-speed ductile-to-brittle transition at 5 m/s. A n o t h e r observed effect is the small increase i n propagation energy at the highest test speed (10 m/s). This effect is puzzling. 2

T h e m a x i m u m stress i n these samples is also complex (Figure 5). I n P A - 6 , the stress first rises a bit w i t h speed, a behavior that is expected for a sample that can still reach its y i e l d point. A t higher speeds, the m a x i m u m stress falls off a n d the material becomes more brittle. Surprisingly, at the highest speeds, 10 a n d 13 m/s, the m a x i m u m stress increases a bit again. I n the blends, the m a x i m u m stress at low rates decreases, as expected, w i t h increasing r u b b e r concentration. A t high rates, the 15 and 2 0 % blends show an unexpectedly strong upswing i n the m a x i m u m stress. T h e m a x i m u m stress before fracture can increase i f the stress-concentration factor ahead o f the n o t c h is lowered. W e also studied the notched tensile impact test behavior of P A - p o l y b u t a diene blends (Durethane d i l u t e d w i t h M 2 5 8 ) . T h e high molecular weight P A ( M 2 5 8 ) undergoes brittle fracture at low speed and l o w temperatures a n d tough fracture at high temperatures (Figure 6) (2). T h e propagation energy o f the blends at 1 0 " m/s as a function o f temperature shows a gradual increase i n toughening w i t h temperature. A b o v e 50 °C, all the samples show strong plastic deformation. T h e gradual increase i n propagation energy suggests that at l o w 3


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Blends of Polyamide-6 and Rubber .

Piston speed (m/s) Figure 5. Notched tensile impact maximum stress versus piston speed for PA-6-EPR blends with different rubber contents at a constant rubber-particle size (0.3-0.4 pm). Key: A, 0 wt%; •, 6.3 wt%; +, 12.5 wt%; 18.4 wt%; and O, 24.3wt%(\3).

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Temperature (°C) Figure 6. Results of notched tensile impact tests versus temperature for PApolybutadiene at lOr m/s for different rubber concentrations. Key: •, 0 vol%; ·, 15 vol%; and A, 30 vol% (2). 3


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speeds there is not a sharp, discontinuous, ductile-to-brittle transition as there is at h i g h speeds. T h e temperature at w h i c h the materials become brittle shifts w i t h increasing r u b b e r concentration to lower temperatures. A t 50 °C, the neat P A seems to have a higher fracture-propagation energy than the blends. T h e ductile-to-brittle transition temperatures at 1 0 m/s as compared to 1 m/s are 30 °C lower for the P A and the 1 5 % blends.


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A l l these results indicate that the high-speed data cannot be obtained by extrapolation from the low-speed data. T h e extrapolated low-speed data should just give brittle fracture for all the samples. Thus the fracture process at high speeds must be different from the process at low speeds, w h i c h indicates that there is an extra deformation mechanism operative i n the high-speed regime that enables crack propagation to become stabilized u n d e r these c o n d i tions. T h e h i g h m a x i m u m stress i n the high-speed tests suggests that this mechanism becomes operative before a crack is initiated. These findings sug gest that there are two ductile-to-brittle transitions, one at l o w test speeds a n d one at h i g h test speeds. R u b b e r concentration affects both ductile-to-brittle transitions.

Particle Size Notched Izod Test. T h e influence o f particle size was studied i n a P A - E P D M b l e n d (Figure 7) (9). T h e ductile-to-brittle transition temperature decreases w i t h decreasing particle size. T h e S-shape o f the curve is so u n i f o r m

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Temperature (°C) Figure 7. Notched Izod impact strength versus temperature for PA-6-EPDM blends with a constant rubber volume fraction (26.1 vol%) and different particle sizes. Key: •, nylon-6; 1.58 μηι; É, 1.20 pm; O, 1.14 μτη; É, 0.94 pm; Q 0.57 pm; and 0.48 pm (9).


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that w i t h a temperature shift a master curve can b e constructed. A change i n particle size seems to affect only the ductile-to-brittle transition temperature a n d have little influence on the impact energies. This effect is thus different from that o f r u b b e r concentration as seen i n the preceding section. L a z z e r i also studied the influence of particle size o n the ductile-to-brittletransition at l o w rates i n three-point b e n d i n g tests o n notched samples (12). L a z z e r i f o u n d the ductile-to-brittle transition at low temperatures, near the T of the rubber. So a firm conclusion cannot be drawn from this study. L a z z e r i a n d B u c k n a l l (15) described the influence o f particle size o n the difference i n the cavitation behavior o f the rubber. Downloaded by UNIV OF AUCKLAND on May 8, 2015 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0252.ch020

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Notched Tensile Impact Test. T h e influence o f particle size i n a P A - E P D M b l e n d (90/10) was also studied w i t h the notched tensile impact test at 1 0 m/s and 1 m/s w i t h different test temperatures (16). F o r comparison, the notched I z o d impact strength o f these blends was also studied (Figure 8). T h e different particle sizes were p r o d u c e d b y changing the extruder-barrel temperatures d u r i n g b l e n d i n g (290-260-240 °C). T h e ductile-to-brittle transi tion o f this 1 0 % b l e n d as measured b y the notched I z o d test decreases w i t h decreasing particle size. F o r this small change i n particle size, a ductile-to-brit tle temperature shift o f 12 °C was observed. T h e propagation energies i n the notched tensile impact test conducted at 1 m/s show a sharp increase w i t h temperature [Figure 9 (top)]. T h e temperature shift i n this ductile-to-brittle transition is n o w 10-12 °C too. This ductile-to-brittle transition is similar to that measured w i t h the I z o d method. However, i f the samples are tested at a low speed ( 1 0 m/s), the propagation energies show a more gradual increase [Figure 9 (bottom)]. T h e data for the different particle sizes at l o w speeds fall - 3

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Temperature (°C) Figure 8. Notched Izod impact strength of PA-EPDM (13 vol%) for different particle sizes. Key; •, 0.8 pm; 0.9 pm; and M, 1.0 μ?η (16).


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Temperature (°C) Figure 9. Results of notched tensile impact tests of PA-EPDM (13 vol%)for different particle sizes: •, 0.8 pm; 0.9 pm; and M, 1.0 pm. (16). Top: 1 m/s. Bottom: lOr m/s. 3

o n top o f each other. This result means that at l o w test speeds the particle size does not affect toughening. It is surprising that at low speeds the energy dissipation does not progress i n a discontinuous manner and the particle-size effect is absent. These results also indicate that toughening at h i g h speeds is different from that at l o w speeds.

Ligament Thickness. T h e ductile-to-brittle transition as measured i n notched Izod, notched Charpy, and notched tensile impact tests is discontinuous and is dependent on both rubber concentration and particle size. These two parameters can be c o m b i n e d into a new morphological parameter that governs the ductile-to-brittle transition. T h e ligament thickness (interparticular distance), w h i c h is a function of rubber concentration and particle size,


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gives a good fit at h i g h test speeds (9, 10). T h e impact energies i n the ductile region decrease w i t h increasing r u b b e r concentration (Figure 1) a n d seem to d e p e n d little o n the particle size (Figure 7). This result means that the impact energies at high test speeds cannot be correlated w i t h ligament thickness.


A t l o w test speeds, toughening shows a more gradual change w i t h t e m perature [Figures 6 a n d 9 (left)]. W i t h increasing rubber concentration, the ductile-to-brittle transition temperature is lowered, but, surprisingly, the influ ence o f particle size o n the ductile-to-brittle transition seems to b e absent. T h i s result means that at l o w test speeds there is no correlation between liga m e n t thickness a n d ductile-to-brittle transition temperature or fracture-propa gation energies. T h e ligament thickness parameter i n P A blends can therefore only apply to high-speed deformation. T h e meaning o f ligament thickness as a parameter for toughening that is only applicable at high speeds is puzzling.

F r a c t o g r a p h i c Analysis High-Speed Deformation. T h e fractured blends have a large stressw h i t e n e d zone next to the fracture surface, a n d the structure o f this zone has b e e n studied (3, 9 , 1 3 , 1 7 , 1 8 ) . This stress-whitened zone is present b o t h i n the b r o k e n I z o d a n d C h a r p y samples a n d i n the samples o f the notched tensile i m pact test. Ramsteiner a n d H e c k m a n (3) observed a thick layer w i t h cavitated r u b b e r particles i n ΡΑ-rubber blends, a n d next to the fracture surface a small er layer w i t h cavities a n d shear bands. Borggreve et al. (9) also observed a cav itation o f the r u b b e r particles i n the stress-whitened zone, but no crazes. Ooste n b r i n k et al. (17) observed o n notched Izod samples, and Dijkstra et al. (13) a n d Janik et al. (18) o n notched tensile impact samples tested at 1 m/s, a threelayer structure: far from the fracture surface (0.10 to 2 m m ) a layer w i t h rea sonably r o u n d cavities, nearer to the fracture plane (5 to 100 μηι from the fracture surface) a layer w i t h strongly elongated cavities, and next to the frac ture plant a 3- to 5-μηι layer without cavities. T h e cavitation i n this layer was i n the r u b b e r particles. T h e second layer, w i t h strongly deformed cavities, sug gests strong plastic deformation o f the matrix material. T h e length-to-diameter ratio o f the cavities i n this layer is 3 to 5, and the angle to the fracture plane is 45° (18). T h e shape o f the particles, but not so m u c h the cavities, can b e seen o n a stained P A - p o l y b u t a d i e n e sample studied w i t h T E M (Figure 10). I n the t h i r d layer, next to the fracture surface, n o cavities can b e seen o n a ductile b r o k e n sample (13, 17). W i t h T E M w e see stained, r o u n d , rubber particles (Figure 10). T h e crystalline order i n this layer is low, a n d orientation as o b served w i t h electron diffraction a n d polarized microscopy was absent (18). T h i s result suggests that no cavities f o r m e d and no deformation took place i n this layer o r that the cavitated structure formed d u r i n g the fracture process but subsequently disappeared. T h e cavitation can disappear, the orientation o f the matrix can relax, a n d the crystallinity can be lowered i f the material is heat-



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Figure 10. TEM image of stained PA-polybutadiene (15 vol%) after undergoing tough fracture in a notched tensile test at high speed (1 m/s). The section shown is next to the fracture surface (18).

e d to above the matrix m e l t i n g temperature. A n n e a l i n g a deformed b l e n d at different temperatures d i d not change the cavitated structure u p to 200 °C (Figure 11) (19). A t 230 °C, above the melting temperature of PA-6, relaxation o f the d e f o r m e d structure h a d taken place. A temperature increase o f 140 °C was measured using a pyrometer (in frared detector) o n an u n n o t c h e d sample at h i g h deformation rates (but w h i c h were m u c h lower than the rates o n the fracture surface) a n d w i t h a large spot size (1 m m ) . A l l these results clearly indicate that the layer next to the frac ture plane, without cavities a n d w i t h no matrix orientation, must have b e e n w a r m a n d that it subsequently relaxed. Relaxation o f a deformed b l e n d takes place i n the melt. I f a melt is present i n the fracture plane ahead o f a crack, the crack w i l l blunt. So melt b l u n t i n g takes place i n ductile b l e n d samples tested at h i g h loading rates. 2

Low-Speed Deformation. Dijkstra et al. (23, 19) a n d Janik et al. (18) showed that i n samples fractured i n a slow-speed notched tensile impact, the stress-whitened zone has two layers. F a r from the fracture plane a cavitated structure is present, w i t h cavities i n the r u b b e r particles (Figure 12, top). In particular, the bigger particles seem to be cavitated and the particles less than 100 μηι are not. I n the layer next to the fracture plane, the cavities are strong-


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Temperature (°C) Figure 11. Cavity size versus temperature after heat treatment of a cavitated blend (19).

ly deformed, have a length-to-diameter (L/D) ratio of 5-10, a n d lie at an angle o f less than 45° w i t h the fracture plane (Figure 12, bottom). Also, polarizedlight microscopic a n d diffraction studies i n T E M show that the orientation i n these samples increases as the fracture plane is approached (18). In a lowspeed test, the cavities at the fracture plane are not relaxed a n d have a h i g h L/D/ ratio. A schematic of the structure o f the stress-whitened zone is given i n F i g ure 13. T h e samples deformed at high speed and undergoing ductile breaking have a three-layer structure i n w h i c h the layer next to the fracture plane is re laxed (molten) (Figure 13a). T h e samples deformed at low speed and undergo i n g ductile breaking have a two-layer structure w i t h a strong deformation o f the material next to the fracture plant (Figure 13b). H e r e , no melt layer is pre sent. D e f o r m a t i o n P r o c e s s . T h e function of the r u b b e r i n P A - r u b b e r blends is to create stable cavities u p o n loading. Because of cavitation, the v o n Mises effective stress i n the matrix strongly increases and plastic deformation is possible (I, 20). T h e von Mises stress i n a cavitated system is a function of cavity concentration. T h e cavity size does not play a role i n this mechanism. A t the ductile-to-brittle transition, crack propagation changes from unsta ble to stable crack growth. B o t h at low a n d high speeds, a ductile-to-brittle transition can be observed. T h e high-speed transition cannot be obtained b y extrapolating from the low-speed data. Also, the low-speed transition is not a discontinuous transition and depends on different structural parameters, like


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Figure 12. Structure of PA-polybutadiene (15 vol% ) after undergoing tough fracture in a notched tensile test at low speed (lOr m/s). Top: far from the fracture plane (TEM, stained). Bottom: next to the fracture plane (SEM, cryofractured). 4



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Figure 13. Schematic of the structure of the stress-whitened zone perpendicular to the fracture plane, a: Tough fracture at high speed, b: Tough fracture at low speed.

r u b b e r concentration and particle size. So i n these blends two ductile-to-brittle transitions must be present. T h e ductile-to-brittle transition temperature at low test speed depends on the rubber concentration but not o n the particle size. T h e transition at high test speed depends o n the rubber concentration and the particle size. T h e samples fractured at high speed have a melt layer next to the fracture plane. T h e difference between the low- and high-speed data is that at high speeds the process becomes adiabatic, and melt b l u n t i n g can take place.

Summary T h e toughening o f P A depends o n materials and on structural and test p a rameters. T h e toughening behavior at high speeds as a function o f temperature gives an S-curve w i t h a discontinuous transition. T h e materials and structural l i m i t have a shift effect o n this S-curve. Deformation occurs first b y cavitation o f the rubber, and then by plastic deformation o f the cavitated m a -


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trix due to a change i n the von Mises stress. Surprisingly, a relaxed layer is pre sent i n the fracture plane, and this layer is probably due to melting d u r i n g plas tic deformation. A melt layer i n front of a crack induces crack blunting, as seen i n the sudden increase of the m a x i m u m force o f the samples tested at higher test speeds (Figure 5). T h e toughening w i t h temperature at low speeds gives a gradual increase i n fracture energy. Fracture strength is independent o f parti cle size. R u b b e r particle size or ligament thickness has no effect o n the onset o f plastic deformation but may affect the extent o f plastic deformation (i.e., the m a x i m u m draw ratio). A small change i n draw ratio is o f little consequence u n der isothermal conditions, but u n d e r adiabatic conditions it may result i n enough heat generation for local melting and, hence, melt blunting.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

18. 19. 20.

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Toughened Plastics II - American Chemical Society

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