Toughened Plastics II - American Chemical Society

11 Elastomer-Modified Vinyl Ester Resins: Impact Fracture and Fatigue Resistance A. R. Siebert , C . D. Guiley , A . J. Kinloch , M. Fernando , a n d E . P. L. H e i j n s b r o c k 1

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Industrial Specialties Division, BFGoodrich Company, Brecksville, OH 44141 Department of Mechanical Engineering, Imperial College, London SW7 2BX, United Kingdom

Downloaded by PURDUE UNIV on July 5, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/ba-1996-0252.ch011

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Vinyl esters are preeminent chemical- and corrosion-resistant mate rials. Various products are available that are based primarily on epoxy resins that have been addition-esterified with methacrylic acid and diluted with styrene monomer. These products are based on diepoxide structure, molecular weight, and modifier type (e.g., nitrile rubber, urethane, and glycol); and blends with styrenated resins are also available. Elastomer modification gives a fourfold im provementin the fracture energy of vinyl esters over that of unmod ified vinyl esters. Impact fracture energy and fatigue properties of elastomer-modified vinyl ester resins that are further modified with epoxy-terminated butadiene-acrylonitrile rubber are greatly en hanced and do not appear to be rate-sensitive. Additive tougheners greatly alter the morphology of cured specimens, providing clues to toughness enhancement.

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I N Y L E S T E R R E S I N S ( V E R s , epoxy methacrylates) are a major class o f styrenated, free radically curable, corrosion- and chemical-resistant thermoset resins. T h e y are largely used i n fiber-reinforced structural applications, a n d they have a substantial history of long-term service i n numerous environments at elevated temperatures and pressures, usually u n d e r load. V E R s are available as both rigid and flexible epoxy resins. T h e flexible epoxy resins generally have a depressed glass-transition temperature, T , a n d inferior c h e m i c a l resistance. Nitrile-rubber-modified V E R (I) appeared o n the market i n the m i d - to late 1970s. These elastomer-modified V E R s show i m p r o v e d fatigue resistance over u n m o d i f i e d V E R (2). g

Recent w o r k has shown that the addition o f epoxy-terminated b u t a d i ene-acrylonitrile ( E T B N ) l i q u i d rubbers to elastomer-modified V E R gives a fivefold increase i n fracture energy over that o f elastomer-modified V E R (3). 0-8412-3151-6

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

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It is noteworthy that this increase is i n addition to a fourfold increase already found for the elastomer-modified V E R compared w i t h an unmodified V E R . T h i s chapter describes o u r work i n determining the impact fracture ener gies o f elastomer-modified V E R that has b e e n further b l e n d e d w i t h various amounts o f E T B N , a n d measuring the fatigue resistance o f these materials.


Experimental Details M a t e r i a l s . The elastomer-modified V E R used i n this work was Derakane 8084 (Dow Chemical Co.), which has about 7.5 phr (parts per hundred resin by weight) of a carboxyl-terminated butadiene-acrylonitrile ( C T B N ) liquid polymer reacted into the resin base (I). The reactive liquid rubber used as the additive was E T B N 1300x40. The E T B N was made by reacting C T B N 1300x8, 17% bound acrylonitrile, with liquid diglycidyl ether of bisphenol A epoxy resin (Epon 828), at a molar ratio of 1:2. Because of the high viscosity of E T B N , it was dissolved i n styrene to give a 5 0 % solids solution before addition to the vinyl ester resin. A com bination ot cobalt naphthenate and M E K peroxide was used to cure the samples. Sheets were made i n Teflon-coated aluminum molds having a sample thickness of about 6 m m for fatigue measurements and 13 m m for impact testing. Recipes were cured for 1 h at 80 °C and then for 2 h at 120 °C. P r e p a r a t i o n o f Test Specimens. F o r slow measurements of fracture energy, G , the compact tension (CT) specimens, 25.4 χ 31.75 m m , were ma chined from the 6-m m plaques. A natural crack was made by tapping gently with a fresh razor blade. Identical C T specimens were prepared for studies of fatigue crack growth. F o r impact G measurements, single-edged, notched-bar ( S E N B ) specimens were prepared by machining the 13-mm sheets into bars of dimensions 80 χ 13 χ 10 m m . Cracks were then inserted into the bars, i n the center of the 80 χ 10 m m face, by machining an initial notch using a narrow milling cutter and then gently tapping a fresh razor blade into the notch so as to propagate a sharp, natural crack ahead of the razor blade. Cracks of various lengths, a, were inserted using these techniques. l c

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Slow- a n d F a s t - S p e e d Testing. Slow-speed testing was performed at 21 μπι/s. Values of fracture toughness, K , and of G were calculated by the stan dard procedures (4). The impact tests were conducted using a commercial instru mented machine (Ceast, Turin, Italy) at 0.5 km/s, which corresponds to a typical front-end of a car folding up after a crash at 30 mph (5). The fracture energy, G , is related to the test parameters by the following equation (6): I c

I c

I c

G

I c

= ϋ /Β\νΦ

(1)

0

where U is the stored elastic strain energy at the onset of crack growth, Β is the thickness of the specimen, W is the width of the specimen, and Φ is calculated from published tables (6) and is a function of afW and L/W, where a is the crack length and L is the length or span of the test specimen between support points. The value of G may be determined by using several S E N B specimens containing different crack lengths and then plotting U versus BWΦ, which should be linear and pass through the origin. F o r each test condition, triplicate tests were per formed, and the typical variation i n the value of G was ±8%. c

I c

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F a t i g u e - C r a c k P r o p a g a t i o n Test. C T specimens were used i n M o d e I, tension-tension fatigue tests. The C T specimens were 60 χ 62.5 χ 6 m m , and cracks were inserted into the specimens as just described. Testing was under taken at a frequency of 20 H z and under displacement control using an R-ratio (minimum to maximum load) of 0.6. This ratio was chosen so that the results ob tained i n the present work could be compared with those reported by Blankenship et al. (2). A constant-amplitude, sinusoidal waveform was applied using a Mayes servohydraulic machine. The advancing crack was followed during fatigue testing using a calibrated traveling microscope. The number of cycles was recorded at ob served increments i n growth. The range of the applied stress-intensity factor, K, was calculated using the standard equations (7). Duplicate tests were undertaken, and all the test results obtained are shown i n Figure 6. The reproducibility of the duplicate tests was excellent. Glass-Transition Measurements. The glass-transition temperature, T , was measured using a Mettler TA3000 differential scanning calorimetry instru ment with a scan rate of 20 °C/min; the second scan was measured to determine T . g

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Fractography. Micrographs from the surfaces of the C T specimens were examined using a J E O L scanning electron microscope ( S E M ) . Transmission electron micrographs were obtained from microtomed sections stained with O s 0 using a J E O L 100SX transmission electron microscope ( T E M ) . 4

Results and Discussion F r a c t u r e - E n e r g y Testing. Table I gives the recipes and the frac ture energies measured under slow and fast rates of test, for the elastomermodified V E R a n d for E T B N additions to the elastomer-modified V E R . A l s o given are the total amounts o f reactive l i q u i d rubber, from b o t h E T B N addi tion a n d C T B N reacted directly into the V E R , for each recipe. F o r reference, the u n m o d i f i e d V E R has a slow G of 0.11 k j / m . Finally, the T obtained from differential scanning calorimetric measurements is given for each recipe. 2

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Table I. Recipes and Fracture Energies of Four Elastomer-Modified Vinyl Ester Resins Blended w i t h Various Amounts of E T B N Sample No. Elastomer-modified V E R ETBN Cobalt naphthenate M E K peroxide Rubber level G (kj/m ), slow G (kj/m ), fast T (°C), differential scanning calorimetry I c

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100 0 0.5 2 7.5 0.43 0.57 110

100 6 0.5 2 10 1.63 1.22 109

100 10.7 0.5 2 12 2.08 2.0 110

NOTE: Units for the recipes are phr.


4 100 15.6 0.5 2 14 2.31 2.3 109

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T h e G values from the slow and fast fracture tests are generally i n good agreement, w h i c h suggests that there is no great effect o f test rate o n the mea sured toughness of these different recipes. Thus, the same trend is observed irrespective o f the test rate, i n that the value o f G steadily increases as the level o f C T B N additive increases, as seen i n F i g u r e 1. T h e r e are no significant differences i n the values o f T for the various recipes. H e n c e , increasing the addition of C T B N has no deleterious effect on the short-term thermal proper ties o f the materials. I c

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M i c r o s c o p i c Studies. F i g u r e 2 shows a T E M image o f an O s 0 stained sample of recipe 1, the elastomer-modified V E R . Particles o n the or der o f 20 n m are seen. Thus, it is not surprising that the c u r e d sample o f recipe 1 is optically clear. Figures 3 a n d 4 show S Ε M images of recipes 2 a n d 3 respectively, taken from the C T samples that were fractured under fatigue loading (see the next section). T h e S E M s for recipe 2 reveal that a distinct particulate phase n o w ex ists. T h e r e are particles about 1 μιη i n diameter and a few larger particles. T h e


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CTBN Level Figure 1. Fracture energy, G , measured at slow and fast testing rates versus to tal CTBN content. Ic



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Figure 2. TEM image of a cured sample of an elastomer-modified vinyl ester resin (Table I, recipe 1 ). The sample shown is an ultrathin, Os0 -stained section. Magnification βΟ,ΟΟΟχ. 4

small holes o f about 1 μπι are presumably formed by dilatation o f the particles. This sample showed a moderate amount o f stress-whitening o n the surface d u r i n g testing. Recipe 3 (Figure 4) shows two distinct ranges o f particle sizes. T h e small holes of about 1 μπι are presumably formed by dilatation o f the smaller particles. T h e larger particles range i n size from about 10 to 15 μπι and appear to be made u p o f multiple small particles. This sample showed a c o n siderable amount o f stress-whitening on the surface of the C T specimen. F i g u r e 5 is an S E M image of recipe 4, w i t h a total C T B N content of 14 phr. T h i s S E M image also shows two distinct particle-size ranges, w i t h the small holes, o f about 1 to 2 μπι, again presumably b e i n g f o r m e d b y dilatation o f the small particles. T h e larger particles range i n size from about 15 to 40 μπι a n d appear to be made u p of multiple small particles. This sample also showed a considerable amount o f stress-whitening on the surface o f the C T specimen. F a t i g u e - C r a c k P r o p a g a t i o n T e s t i n g . T h e fatigue-crack propaga tion results, log da/dN, where a is the crack length and Ν is the n u m b e r o f cy cles, are plotted against log Δ Κ i n F i g u r e 6 for recipes 1, 2, and 3. Fatiguecrack propagation tests were not performed on recipe 4. Recipes 1 and 3 have similar fatigue behavior except that the fatigue curve for recipe 3, w i t h a total C T B N content o f 12 phr, extends to m u c h higher levels o f log da/dN because


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Figure 3. Fatigue-fracture surface of an elastomer-modified vinyl ester resin to which ETBN has been added (Table I, recipe 2). The magnifications of these SEM images of cured samples are lOOOx (top) and 5000x (bottom).

o f its higher G value. R e c i p e 2, w i t h a total C T B N o f 10 phr, shows a signifi cant increase i n fatigue performance, w i t h log da/dN versus log Δ Κ shifted to the right a n d showing a somewhat lower slope. I c

T h e reason for the i m p r o v e d fatigue performance o f recipe 2 (with 10 p h r o f total C T B N content) c o m p a r e d w i t h that o f recipe 3 (with 12 p h r o f total C T B N content) is not fully understood. However, it may be due to the tough-


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Figure 4. Fatigue-fracture surface of an elastomer-modified vinyl ester resin to which ETBN has been added (Table I, recipe 3). The magnifications of these SEM images of cured samples are lOOOx (top) and 5000x (bottom).

e n i n g mechanism involving cavitation and d e b o n d i n g o f the rubbery particles. W h i l e such mechanisms may contribute to the overall toughness o f the material, b y enabling subsequent plastic-hole formation i n the matrix (8, 9), the presence o f voids or holes associated w i t h these mechanisms may readily p r o mote fatigue-crack growth through the voided material. This proposed explanation is supported b y the micrographs shown i n Figures 3 a n d 4. These fig-



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Figure 5. Fatigue-fracture surface of an elastomer-modified vinyl ester resin to which ETBN has been added (Table I, recipe 4). The magnification of this SEM image of a cured sample is lOOOx,

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•

Log da/dN (mm/ cycle)

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.

Recipe 1

ο Recipe 2

Φ

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D Recipe 3

JEP, -6

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Log ΔΚ (MPa-m1/2)\ 1/2

Figure 6. Crack growth rate, da/dN, versus stress intensity range, ΔΚ. Testing was performed at a frequency of 20 Hz and an R-ratio of 0.6.


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ures clearly illustrate that the tougher material (recipe 3) has more voids, and it is this material that exhibits poorer fatigue resistance. Finally, it should be noted that stress-whitening arises because these voids, created by cavitation and debonded particles, scatter light. H e n c e , it is observed that the most intense stress-whitening is present i n the recipe that undergoes the greatest extent o f particle cavitation a n d debonding. W h a t e v e r the fundamental mechanisms involved, the practical implication is that the balance between high initial toughness and good fatigue-crack resistance must be considered w h e n formulating these C T B N - m o d i f i e d materials. In the study o f crack-propagation behavior i n toughened plastics u n d e r fatigue loadings, the introduction of a second rubbery phase has been reported (10) to increase fatigue resistance as w e l l as impact toughness. In some polymers, however, increasing the amount of rubbery phase may continue to increase the impact toughness without p r o d u c i n g a further increase i n fatigue resistance. However, nearly all o f these studies have been concerned w i t h thermoplastic polymers. Recently, K i n l o c h a n d O s i y e m i (11) studied the toughness a n d fatigue behavior o f two rubber-toughened, thermosetting epoxy adhesives. T h e y f o u n d that the tougher adhesive possessed inferior fatigue behavior, w h i c h is i n b r o a d agreement w i t h the observations just mentioned on the thermosetting vinyl ester resin materials.

Summary T h e rate of test has little effect on the fracture energy, G , w h e n C T B N is added as E T B N (see the section "Materials") to an elastomer-modified vinyl ester resin. T h u s , these systems do not appear to be rate-sensitive. T h e added C T B N has a significant effect on G o f the c u r e d system, but such an addition does not lead to a loss o f glass-transition temperature, T , w i t h increasing amounts of added C T B N . T h e systems w i t h added C T B N show both large and small particles, w i t h the small particles having undergone cavitation a n d so p r o m o t i n g plastic deformation i n the highly cross-linked matrix resin. I c

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T h e sample having 10 p h r o f total C T B N (2.5 p h r from E T B N addition and 7.5 p h r from the elastomer-modified V E R ) shows a significant improvement i n fatigue behavior over the elastomer-modified vinyl ester resin itself. T h e higher r u b b e r level of 12.0 p h r of total C T B N , 4.5 p h r from E T B N and 7.5 p h r from the elastomer-modified vinyl ester resin, gives a fatigue resistance that is somewhat similar to that o f the elastomer-modified vinyl ester resin itself.

References 1. Dow Chemical Co., U.S. Patent 3 892 819, 1975; Fibereast Corp., U.S. Patent 3, 928 491, 1975.



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2. Blankenship, L. T.; Barron, D . L.; Kelley, D. H . Presented at the 44th Annual Con -ferenceof the Composites Institute, Society of Plastics Industries, Dallas, Febru -ary1989; Session 14C. 3. Siebert, A. R.; Guiley, C. D.; Egan, D. R. Presented at the 47th Annual Conference on the Composites Institute, Society of Plasties Industries, Dallas, February 1992; Session 17C. 4. A S T M D-5045-91 A, Plane Strain Fracture Toughness. 5. Kinloch, A. J.; Kodokian, G. Α.; Jamarani, M . B. J. Mater. Sci. 1987, 22, 4111. 6. Williams, J. G. Fracture Mechanics of Polymers; Ellis Horwood: Chichester, Eng -land, 1984; p 62. 7. Kinloch, A. J.; Young, R. J. Fracture Behavior of Polymers; Applied Science: Lon -don,1983; p 204. 8. Kinloch, A. J.; Shaw, S. J.; Hunston, D. L. Polymer 1983, 24, 1355. 9. Huang, Y.; Kinloch, A. J. J. Mater. Sci. 1992, 27, 2763. 10. Hertberg, R. W.; Manson, J. A. Fatigue of Engineering Plastics; Academic: Orlan -do, FL, 1980; p 209. 11. Kinloch, A. J.; Osiyemi, S. O. Presented at the Structural Adhesives in Engineering III Conference, Plastics and Rubber Institute, London, 1992; paper 31.


Toughened Plastics II - American Chemical Society

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