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The influence of silicate-based nano-filler on the fracture toughness of epoxy resin
Engineering Fracture Mechanics 73 (2006) 2336–2345 www.elsevier.com/locate/engfracmech
The influence of silicate-based nano-filler on the fracture toughness of epoxy resin A.J. Brunner
a,*
, A. Necola a, M. Rees a, Ph. Gasser b, X. Kornmann R. Thomann c, M. Barbezat a
a,1
,
a
b
Laboratory for Materials and Engineering, Empa, Swiss Federal Laboratories for Materials Testing and Research, CH-8600 Duebendorf, Switzerland Laboratory for Concrete/Construction Chemistry, Empa, Swiss Federal Laboratories for Materials Testing and Research, CH-8600 Duebendorf, Switzerland c Freiburg Materials Research Centre, Albert-Ludwigs-Universita¨t, D-79104 Freiburg, Germany Received 29 September 2005; received in revised form 20 April 2006; accepted 5 May 2006 Available online 19 June 2006
Abstract Dispersion of nano-sized, silicate-based filler in epoxy resin is expected to yield improved materials properties in several areas. Various mechanical properties, specifically improved fracture toughness, as well as improved flame-retardant effects are of interest. The final objective of the research is investigating whether a nano-modified epoxy matrix yields improved delamination resistance in a fiber-reinforced laminate compared to a laminate with neat epoxy as matrix material. As a first step towards this goal, the fracture toughness of nano-modified epoxy resin is compared with that of the neat resin. Fracture toughness improvement up to about 50% and energy release rates increased by about 20% are observed for addition of 10 wt.% of organosilicate clay. 2006 Published by Elsevier Ltd. Keywords: Fracture toughness; Silicate-based nano-scale filler; Nano-composite epoxy; Standard test procedure
1. Introduction Nano-composites from polymeric matrix materials (thermoplasts or thermosets) reinforced with nano-sized fillers (e.g. carbon nano-tubes, nano-sized particles or intercalated layers) are an active area of research and the number of published papers has reached a stage where first reviews are summarizing the state of the art [1–3]. The range of properties where nano-sized fillers are expected to yield improvements over neat polymers is wide, improved impact properties [4,5] and fire-resistance or retardance [6] can be cited as examples.
*
1
Corresponding author. Tel.: +41 44 823 44 93; fax: +41 44 821 62 44. E-mail address: [email protected] (A.J. Brunner). Present address: ABB Switzerland Ltd., Corporate Research, CH-5405 Baden-Da¨ttwil, Switzerland.
0013-7944/$ - see front matter 2006 Published by Elsevier Ltd. doi:10.1016/j.engfracmech.2006.05.004
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The effect of nano-sized fillers on fracture behaviour, specifically fracture toughness, has also been investigated recently [7–10]. The aim of the present paper is based on the idea to use epoxy with a relatively small amount of nano-size filler as matrix in fiber-reinforced laminates [11,12]. As a first step, neat and nano-modified epoxy specimens without fiber reinforcement have been prepared for a comparison of the fracture toughness of the matrix material itself. By that, the applicability of the existing test procedure [13] to nano-composite thermosets is investigated. Additional properties of the neat and nano-modified epoxy are also determined (partly taken from [11]) and compared. 2. Experimental Plates (nominally 200 mm · 150 mm · 4 mm) of epoxy resin (Araldite CY 225 resin and Aradur HY 925 hardener from Huntsman) have been manufactured by casting in an Aluminium mould both with and without functionalized organosilicate clay based filler (10 wt.%, corresponding to about 7.2% of anorganic phase), type and processing details are described in [11]. Two batches of neat epoxy and nano-modified epoxy have been prepared. The plate materials have been characterized with (a) visual inspection, (b) X-ray diffraction and (c) dynamic mechanical thermal analysis (DMTA). Additional results for the second batch of nano-modified epoxy (compared with those of a third batch of neat resin) are described in [11]. X-ray diffraction (XRD) spectra have been obtained in a h–2h configuration (equipment Panalytical X’Pert pro), with scan steps of 0.05 and a scan speed of 0.05/5 s. The beam optics were as follows: Soller slit 0.04 rad, fixed divergence slit (1/4), 10 mm mask, fixed anti-scatter slit (1/2) for the incident, and fixed anti-scatter slit (1/4), Soller slit 0.04 rad, and nickel filter for the diffracted beam optics. The detector (type X’Celerator) was comprised of 127 elements (each 70 lm wide) yielding an active length (240 mm goniometer radius) of 2.1226 and a minimal step length of 0.0167. Dynamic mechanical thermal analysis (DMTA) was performed and the properties of the neat and nano-modified epoxy have been determined between 120 C and +160 C in a three-point bending configuration under deformation control, a forced oscillation frequency of 10 Hz, and a heating rate of 2 C/min (Eplexor 500N from Gabo Qualimeter GmbH, Germany). A static load of nominally 1.0% strain and a superimposed sinusoidal load of nominally 0.03% strain were applied to samples of nominal size of 25 mm · 15 mm · 4 mm with a span length of 20 mm and the results compared with DMTA data obtained earlier on tensile specimens [11]. Single edge notched bending (SENB) specimens have been prepared from both batches of each neat and nano-modified epoxy. SENB specimens were nominally 63 mm long, 15 mm wide, and 4 mm thick (=plate thickness). They were tested to determine the critical fracture toughness KIC and critical energy release rate GIC, respectively, according to the standard test method for polymers [13] issued by the International Organisation for Standardisation (ISO). A V-shaped notch was machined to a depth of about 3.5 mm, and then extended by knife–blade tapping to a total length around 7 mm (effectively ranging from about 6.3 to 8.3 mm). Tests have been performed with a cross-head speed of 1 mm/min at +23 C and 50% relative humidity. For the second batches of specimens, the indentation correction (see Section 5.4 of [13]) has been measured for each specimen individually. The fracture mechanics test data have been analysed in accordance with the prescriptions of the standard test method [13]. Spreadsheets developed and validated at the authors’ laboratory in an international round robin using poly-methyl-methacrylate (PMMA) specimens [14] were used for the data analysis. A magnifying glass (magnification 8 times) has been used to visually determine the initial crack lengths which have been measured at five positions across the thickness. Comparative plane strain compression testing has been performed to determine the compressive yield stress and the compressive modulus applying a set-up and a procedure described in [15]. After fracture testing, the fracture surfaces have been examined by scanning electron microscopy (SEM). Images have been taken with the secondary electron detector in an environmental scanning electron microscope (type XL30 from Philips) operated in the high-vacuum mode. The specimen surfaces have been coated with flash-evaporated carbon. Transmission electron micrograph (TEM) images were taken with a TEM-equipment (type LEO 912 Omega from Zeiss) at an accelerating voltage of 120 kV. Preparation details are described in [11].
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3. Results Visual inspection showed that the nano-modified epoxy appeared less translucent and of a darker colour than the neat epoxy, but homogeneous on a macroscopic scale. X-ray diffraction spectra of both, neat and nano-scale modified epoxy are shown in Fig. 1 for a 2-h range between 5 and 25. The diffractograms for different batches of each neat and nano-scale filled epoxy coincide, and, therefore, scales have been shifted for the presentation (Fig. 1). The neat epoxy essentially yields broad, featureless diffractograms reflecting the random network structure. Both batches of nano-filled epoxy, however, show a distinct peak at a 2-h value of 19.5. This is attributed to the combined (1 1 0) and (0 2 0) reflections of the intercalated silicate clay, see [11] for details. Transmission electron microscopy (TEM) images of the nano-modified epoxy of the second batch of the present study (Fig. 2, and additional TEM shown in [11]) indicate intercalation of the silicate layers in the epoxy with distances between silicate layers on the order of 9 nm. Homogeneously intercalated regions extend over several hundred nano-meters up to the micrometer range (Fig. 2). DMTA curves are shown in Fig. 3 for neat and nano-size filled epoxy in a three-point bending configuration. The data can be compared with earlier DMTA results obtained on tensile specimens, shown in [11]. A significant increase in the elastic modulus (around 40%) is recorded at room temperature by the addition of nano-size filler to the neat epoxy. Considerable in-batch and batch-to-batch variations are observed in the modulus measurements. Contrary to that, the glass transition temperatures, taken as the maxima of the tan d curves for the different batches of neat epoxy, fall all into a narrow range around +134 C, basically independent of the test configuration (bending in present paper or tensile quoted in [11]). The values for the nano-size modified epoxies are lower than for the neat epoxy and vary, they amount to about +109 C for the first, and around +114 C for the second batch. The latter value again agrees with the earlier tests reported in [11]. Load–displacement diagrams from the fracture toughness tests on both types of epoxy are shown in Fig. 4. It can be noted that the nano-modified epoxy specimens yield a larger slope and hence larger stiffness – as well as a larger variation in slope (both in-batch and from batch-to-batch) than the neat epoxy. Corrected displacement values to fracture according to the ISO standard [13] are about 0.16 mm for the neat and slightly less for the nano-modified epoxy (0.13–0.14 mm). The fracture data are summarized in Table 1 (fracture toughness) and Table 2 (energy release rate). For neat epoxy, eight specimens of each batch, out of 13 and 16, respectively are valid according to the criteria
Intensity [arbitrary units]
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2-θ [°] Fig. 1. X-ray diffraction spectra of neat and nano-size filled epoxy, 2-h range from 5 to 25, (a) neat epoxy first batch, (b) neat epoxy second batch, (c) nano-modified epoxy first batch, (d) nano-modified epoxy, second batch; (a) and (c) left hand scale, (b) and (d) right hand scale (shift between the scales to show the curves without overlap). The peak at 19.5 in the nano-modified epoxy is attributed to the combined (1 1 0) and (0 2 0) reflections, indicating a spacing of the silicate layers of about 8.8 nm, for details see [11].
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Fig. 2. Transmission electron micrograph (TEM) taken at low magnification on a specimen from the second batch of nano-modified epoxy (additional TEM shown in [11]).
Fig. 3. Dynamic mechanical thermal analysis of neat and nano-size filled epoxy, three-point bending configuration, temperature range 120 to +160 C tested at 10 Hz, (a) neat epoxy first batch, (b) neat epoxy second batch, (c) nano-modified epoxy first batch, (d) nanomodified epoxy, second batch. The shift in Tg indicates a lower cross-link density for the nano-modified epoxy, the width of the peak is discussed in [11].
p defined in [13]. Averaging over all valid specimens from both batches yields a KIC of 0.740 ± 0.140 MPa m, 2 and a GIC of 134 ± 24pkJ/m for the neat epoxy. The KIC average values for batch 1 and 2, respectively, are 0.750 and 0.730 MPa m with standard deviations of 12.5% and 24.9%. Corresponding GIC values are 132.6
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Fig. 4. Comparison of typical load–displacement diagrams from fracture toughness tests for neat and nano-modified epoxy. The four curves with maximum loads above 40 N (black) are from nano-modified epoxy, the others (grey) from neat epoxy (first batch bold, second fine lines, first specimen solid, second specimen dashed line). There is little in-batch and batch-to-batch variation for the neat epoxy, but considerable in-batch and batch-to-batch variation for the nano-modified epoxy. Table 1 Fracture toughness results on neat and nano-modified epoxy resin, only specimens fulfilling all validity criteria specified in the standard procedure [13] are used to calculate the averages and standard deviations (s.d.) Epoxy
Batch
Valid tests
Total tests
Average KIC p (MPa m)
s.d. p (MPa m)
s.d. (%)
Min. p (MPa m)
Max. p (MPa m)
Neat Neat Neat Nano Nano Nano
1 2 1+2 1 2 1+2
8 8 16 4 5 9
13 16 29 10 5 15
0.750 0.730 0.740 0.936 1.133 1.045
0.094 0.182 0.140 0.098 0.157 0.163
12.5 24.9 18.9 10.5 13.9 15.6
0.605 0.420 0.420 0.843 0.977 0.843
0.916 1.019 1.019 1.074 1.382 1.382
Table 2 Energy release rate results on neat and nano-modified epoxy resin, only specimens fulfilling all validity criteria specified in the standard procedure [13] are used to calculate the averages and standard deviations (s.d.) Epoxy
Batch
Valid tests
Total tests
Average GIC (J/m2)
s.d. (J/m2)
s.d. (%)
Min. (J/m2)
Max. (J/m2)
Neat Neat Neat Nano Nano Nano
1 2 1+2 1 2 1+2
8 8 16 4 5 9
13 16 29 10 5 15
132.6 135.9 134.2 135.8 160.9 149.7
11.9 33.0 24.0 13.5 23.5 22.8
9.0 24.3 17.9 10.0 14.6 15.2
120.8 115.7 115.7 120.7 133.3 120.7
149.9 183.9 183.9 147.4 197.9 197.9
and 135.9 J/m2 with standard deviations of 9.0% and 24.3%, respectively. For the nano-modified epoxy, the p two batches yield clearly differing results. KIC average values are 0.936 and 1.133 MPa m with standard deviations of 10.5% and 13.9%, and GIC average values 135.8 and 160.9 J/m2 with standard deviations of 10.0%
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Table 3 Comparison of properties of the neat and nano-filled epoxy (average values from both batches with standard deviations, except where noted for the nano-modified epoxy), quantities marked with * are from [11] determined on a third batch for the neat and from the second batch for the nano-modified epoxy Property
Neat epoxy
Nano-modified epoxy
Density/specific gravity [g/cm3] Silicate content [wt.%]a Tensile modulus [MPa] * Tensile strength [MPa] * Elongation at break [%] * Compressive modulus [MPa] Compressive yield strength [MPa] p Fracture toughness KIC [MPa m] 2 Energy release rate GIC [J/m ] Glass transition temperature [C]
1.2115 ± 0.0004 – 3100 ± 50 80.3 ± 0.8 7.4 ± 1.7 2300 ± 120b 92 ± 1.2d 0.740 ± 0.140f 134 ± 14h +134
1.2602 ± 0.0061 (+4.0%) 7.2 ± 0.02 4700 ± 130 (+52%) 51.5 ± 1.4 (36%) 1.2 ± 0.1 (84%) 3600 ± 160c (+56%) 108 ± 1.5e (+17%) 1.133 ± 0.160g (+53%) 161 ± 24i (+20%) +109/+114j
a
Determined as ignition residue according to EN 60 (losses on ignition). Average for two batches with five measurements each, values for first and second batch, respectively are 2370 ± 85 MPa and 2200 ± 90 MPa. c Average for two batches with eight and four measurements, values for first and second batch, respectively are 3660 ± 145 MPa and 3470 ± 100 MPa. d Average for two batches with five measurements each, values for first and second batch, respectively are 93 ± 0.1 MPa and 91 ± 0.6 MPa. e Average for two batches with eight and four measurements each, values for first and second batch, respectively are 108 ± 1.1 MPa and 106 ± 0.3 MPa. f Average for two batches with eight and seven measurements each, values for first and second batch, respectively are p p 0.750 ± 0.09 MPa m and 0.730 ± 0.18 MPa m. p g Average for second batch with five measurements, first batch average is 0.936 ± 0.1 MPa m, however, there is evidence of effects from residual stresses in the first batch. h Average for two batches with eight and seven measurements each, values for first and second batch, respectively are 133 ± 12 J/m2 and 136 ± 33 J/m2. i Average for second batch with five measurements, first batch average is 136 ± 14 J/m2; however, there is evidence of residual stresses in the first batch. j Values for first and second batch, respectively, there is evidence for effects from residual stresses in the first batch. b
p and 14.6%, for batch 1 and 2, respectively. Taking the averages of 1.133 MPa m and of 161 J/m2 (nano-modified epoxy, batch 2), the addition of 10 wt.% of nano-filler to the epoxy yields an increase of about 50% in fracture toughness and of about 20% in energy release compared with neat epoxy. Additional properties of the neat and nano-modified epoxy, partly taken from [11], are compared in Table 3. Fig. 5 shows selected SEM images of fracture surfaces of neat and nano-modified epoxy. The neat epoxy shows comparatively smooth and featureless surfaces at all magnifications (up to 51 200·). Nevertheless, a low-contrast texture is recognizable at higher magnifications (beyond about 15 000·), oriented roughly parallel to the crack propagation. The nano-modified epoxy, on the other hand, shows a platelet-type, fairly corrugated surface even at low magnifications (6500·). No crack propagation oriented features have been observed for the nano-modified epoxy. SEM images taken at higher magnifications (>20 000·) indicate distinct features in the sub-micrometer range on the individual platelets of the nano-modified epoxy. 4. Discussion The increase of roughly 40% in modulus upon modification by nano-size filler observed in the DMTA three-point bending experiments compares quite well with data obtained from the static plain-strain compression tests (increase around 55%, see Table 3). The relatively high absolute values obtained compared with the earlier DMTA tests on tensile specimens [11] are quite likely induced by the sample geometry used (low span to thickness ratio in the bending tests). The glass transition temperature (Tg) appears to be lowered by about 20–25 C upon introducing the nano-size filler into the base system. Simultaneously, the width of the tan d peak is increasing. This indicates a probable change in the cross linking density and homogeneity of the resin
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Fig. 5. Selected SEM-images of the fracture surfaces of neat (left), and nano-modified epoxy (right) at increasing magnification (top to bottom). The neat resin yields flat and virtually featureless surfaces (except for a fine texture roughly oriented parallel to the crack propagation), while the nano-modified epoxy shows a platelet-like structure with distinct features ranging down to 200–400 nm.
network due to the interaction between resin and functionalized filler. These results are consistent with the DMTA data obtained previously on this system [11]. The width of the transition peak for both batches of nano-modified epoxy is comparable. However, there is a difference in Tg determined from DMTA between the two batches. The first batch yields the lower value (around +109 C), indicating a lower cross-linking density than in the second batch (Tg around +114 C). It is expected that this would yield a slightly higher fracture toughness, contrary to what is effectively observed. Compared with neat resin, the width of the transition peak is increased for both batches of nano-modified epoxy. This could be interpreted as a larger range of cross-linking density in the nano-modified materials, consistent with the dispersion of regions of nano-intercalated silica in the epoxy. Comparing the overall averages of the fracture mechanics data (averaging over valid specimens only) indicates that the addition of 10 wt.% of functionalized nano-scale filler changes the fracture toughness KIC by about 50% and the energy release GIC by about 20%. When the data are analysed with respect to the different batches (plates), the batch-to-batch variation for the neat epoxy is clearly within one standard deviation. The average values for the two batches of nano-modified epoxy do differ somewhat more (see Tables 1 and 2). The formation of pre-cracks initially deviating from the plane of symmetry as visually observed during blade tapping of the first batch of nano-modified epoxy points toward residual stresses from manufacturing. This was
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not observed for the second batch that yielded higher average values of KIC and GIC. The effect of residual stresses could hence explain the lower fracture toughness of the first batch of nano-modified epoxy (in spite of the lower cross-link density). The second batch only is, therefore, taken for quoting fracture toughness data for nano-modified epoxy. The flat and relatively featureless fracture surface of neat epoxy resin specimens is consistent with a relatively brittle fracture mechanism. The comparatively strong corrugation of the fracture surfaces of the nano-modified epoxy, on the other hand, is consistent with the measured, improved fracture toughness and energy release rate compared with the neat epoxy. For the nano-modified epoxy, there are distinct structural features on the surface of the platelets with sizes of 500 nm or less. Considering the silica-intercalated regions with diameters of up to 1000 nm or more indicated by TEM images on the second batch of nano-modified epoxy (Fig. 2) it can be speculated that these features in the SEM images could be due to fracture between such intercalated regions and neat epoxy surrounding them. Such a mechanism would explain the corrugated fracture surfaces as well as the improved fracture toughness compared with neat epoxy resin. It has to be noted that no distinct difference has been observed so far between the fracture surfaces of the different batches of the nano-modified epoxy. It can be added that there is a relatively large number of invalid tests if all criteria, including initial crack length – specimen width (a/w)-ratio and modulus difference, of the standard analysis [13] are applied. 16 out of 29 (55%) for neat and 9 out of 15 (60%) of the tests were valid. a/w-ratios outside the recommended limits of 0.45 (lower) and 0.55 (upper) were the most frequently violated criterion (eight neat and one nano-modified specimen), while pre-crack length variation exceeding 10% across the specimen width yielded invalid results for two neat and one nano-modified specimen. The KIC and GIC data can also be analysed with respect to various parameters. The correlation of KIC with (a/w)-ratio (Fig. 6) is discussed as an example. For both types of epoxy, an apparent trend for increasing KIC values with increasing a/w-ratio is observed. This trend persists, even if only values for a/w within the recommended limits (0.45–0.55) are considered. The scant literature data on effects of nano-scale fillers on the fracture toughness of epoxy indicates an increase in fracture toughness for the nano-composite but the magnitude of the effect quite likely depends on the weight fraction of the nano-scale filler [7–10]. For a heavily filled epoxy with 60 wt.% of conventional p (not nano-dispersed) quartz filler the supplier data sheet [16] indicates values of KIC and GIC (1.8–2.0 MPa m
Fig. 6. KIC values of neat and nano-modified epoxy plotted as function of the ratio between initial crack length, a, and specimen width, w. Both batches of nano-modified epoxy are shown. Note the trend for increasing KIC with increasing a/w (see text for details).
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and 300–350 J/m2, respectively) that are higher than those obtained for epoxy with 10 wt.% nano-intercalated silicate clay. From an application point of view, it will be even more interesting to see whether or by how much the fracture toughness and other properties are changed if the matrix in a fibre-reinforced laminate is replaced by the nano-modified epoxy. Improvements in various properties have been observed when a glass–fiber laminate with nano-modified epoxy matrix is compared with a laminate with neat epoxy matrix: averages of flexural modulus (+5.7%), of flexural strength (+27.5%), and of flexural strain (+23.5%) are all increased [11]. This raises the expectation that the improved fracture properties of the nano-modified epoxy reported in the present paper can also be transferred to fibre-reinforced laminates. The ESEM images shown in [11] do yield indications of improved fibre-matrix adhesion when nano-modified epoxy is used as matrix in a glass–fibre laminate (compared with a neat epoxy matrix laminate). Combined with some improvement of the fracture toughness of the matrix, this could well result in a considerably improved delamination resistance. 5. Conclusions An increase between about 40 and 50% in fracture toughness and around 10–20% in energy release has been observed relative to the neat epoxy for nano-modified epoxy prepared by addition of 10 wt.% of functionalized organosilicate clay. Scanning electron microscopy images indicate considerable differences in the fracture surfaces of neat and nano-modified epoxy, consistent with the measured fracture data. Various other properties are improved in the nano-modified epoxy, e.g. a considerable increase in tensile and compressive modulus by more than 50% is observed. The most noteworthy exceptions known to date are ultimate tensile strength, elongation at break, and glass transition temperature, which are all reduced. In principle, the standardized ISO test procedure for fracture toughness measurements of polymers [13] can be applied to the nano-modified epoxy. A considerable percentage of tests (between 40% and 45%) have been found to be invalid according to the criteria of the standard test procedure and it is recommended to prepare more than the minimum number of five specimens per material. The nano-modified epoxy hence shows potential for improving delamination resistance when used as matrix in fibre-reinforced laminates, analogous to the improvement of other mechanical properties reported in the present paper and in [11]. Acknowledgements Mr. U. Gfeller provided XRD data, and Mr. K. Ruf DMTA results. Specimen preparation has been performed by Mr. D. Vo¨lki and a first series of fracture tests been conducted by Mr. M. Heusser. References [1] Pandey JK, Reddy R, Pratheep Kumar A, Singh RP. An overview on the degradability of polymer nanocomposites. Poly Degrad Stabil 2005;88(2):234–50. [2] Jordan J, Jacob KI, Tannenbaum R, Sharaf MA, Jasiuk I. Experimental trends in polymer nanocomposites – a review. Mater Sci Engng A 2005;393(1–2):1–11. [3] Xie L-X, Mai Y-W, Zhou X-P. Dispersion and alignment of carbon nanotubes in polymer matrix: a review. Mater Sci Engng R 2005;49(4):89–112. [4] Lin J-C, Chang LC, Nien MH, Ho HL. Mechanical behaviour of various nanoparticle filled composites at low-velocity impact. Compos Struct 2006;74(1):30–6. [5] Isik I, Yilmazer U, Bayram G. Impact modified epoxy/montmorillonite nanocomposites: synthesis and characterization. Polymer 2003;44(20):6371–7. [6] Zhang J, Jiang DD, Wilkie CA. Fire properties of styrenic polymer–clay nanocomposites based on oligomerically-modified clay. Polym Degrad Stabil 2005;91(2):358–66. [7] Avlar S, Qiao Y. Effects of cooling rate on fracture resistance of nylon 6-silicate nanocomposites. Compos Part A: Appl Sci Manuf A 2005;36(5):624–30. [8] Liu W, Hoa SV, Pugh M. Fracture toughness and water uptake of high-performance epoxy/nanoclay nanocomposites. Compos Sci Technol 2005;65(15–16):2364–73. [9] Ragosta G, Abbate M, Musto P, Scarinzi G, Mascia L. Epoxy-silica particulate nanocomposites: chemical interactions, reinforcement and fracture toughness. Polymer 2005;46(23):10506–16.
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[10] Yao XF, Yeh H-Y, Zhao HP. Dynamic response and fracture characterization of polymer–clay nanocomposites with Mode-I Crack. J Compos Mater 2005;39(16):1487–96. [11] Kornmann X, Rees M, Thomann Y, Necola A, Barbezat M, Thomann R. Epoxy-layered silicate nanocomposites as matrix in glass fibre-reinforced composites. Compos Sci Technol 2005;65(14):2259–68. [12] Timmerman JF, Hayes BS, Seferis JC. Nanoclay reinforcement effects on the cryogenic microcracking of carbon fiber/epoxy composites. Compos Sci Technol 2002;62(9):1249–58. [13] International standard. Plastics – determination of fracture toughness (GIC and KIC) – Linear elastic fracture mechanics (LEFM) approach. International Organisation for Standardisation. 2000, 13586: 1–16. [14] Unpublished results from an international round robin in 2004 on characterization of poly-methyl-methacrylate (PMMA) organised ¨ sterreichisches Forschungsinstitut fu¨r Chemie und Technik (Austrian Research Institute for Chemistry and Technology). by the O [15] Williams JG, Ford H. Stress-strain relationships for some unreinforced plastics. J Mech Engng Sci 1964;6(4):405–17. [16] Data sheet Araldit-Giessharzsystem CY 225/HY 925, Huntsman, Advanced Materials, Electrical Insulation Materials (version June 2003).
The influence of silicate-based nano-filler on the fracture toughness of epoxy resin A.J. Brunner
a,*
, A. Necola a, M. Rees a, Ph. Gasser b, X. Kornmann R. Thomann c, M. Barbezat a
a,1
,
a
b
Laboratory for Materials and Engineering, Empa, Swiss Federal Laboratories for Materials Testing and Research, CH-8600 Duebendorf, Switzerland Laboratory for Concrete/Construction Chemistry, Empa, Swiss Federal Laboratories for Materials Testing and Research, CH-8600 Duebendorf, Switzerland c Freiburg Materials Research Centre, Albert-Ludwigs-Universita¨t, D-79104 Freiburg, Germany Received 29 September 2005; received in revised form 20 April 2006; accepted 5 May 2006 Available online 19 June 2006
Abstract Dispersion of nano-sized, silicate-based filler in epoxy resin is expected to yield improved materials properties in several areas. Various mechanical properties, specifically improved fracture toughness, as well as improved flame-retardant effects are of interest. The final objective of the research is investigating whether a nano-modified epoxy matrix yields improved delamination resistance in a fiber-reinforced laminate compared to a laminate with neat epoxy as matrix material. As a first step towards this goal, the fracture toughness of nano-modified epoxy resin is compared with that of the neat resin. Fracture toughness improvement up to about 50% and energy release rates increased by about 20% are observed for addition of 10 wt.% of organosilicate clay. 2006 Published by Elsevier Ltd. Keywords: Fracture toughness; Silicate-based nano-scale filler; Nano-composite epoxy; Standard test procedure
1. Introduction Nano-composites from polymeric matrix materials (thermoplasts or thermosets) reinforced with nano-sized fillers (e.g. carbon nano-tubes, nano-sized particles or intercalated layers) are an active area of research and the number of published papers has reached a stage where first reviews are summarizing the state of the art [1–3]. The range of properties where nano-sized fillers are expected to yield improvements over neat polymers is wide, improved impact properties [4,5] and fire-resistance or retardance [6] can be cited as examples.
*
1
Corresponding author. Tel.: +41 44 823 44 93; fax: +41 44 821 62 44. E-mail address: [email protected] (A.J. Brunner). Present address: ABB Switzerland Ltd., Corporate Research, CH-5405 Baden-Da¨ttwil, Switzerland.
0013-7944/$ - see front matter 2006 Published by Elsevier Ltd. doi:10.1016/j.engfracmech.2006.05.004
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The effect of nano-sized fillers on fracture behaviour, specifically fracture toughness, has also been investigated recently [7–10]. The aim of the present paper is based on the idea to use epoxy with a relatively small amount of nano-size filler as matrix in fiber-reinforced laminates [11,12]. As a first step, neat and nano-modified epoxy specimens without fiber reinforcement have been prepared for a comparison of the fracture toughness of the matrix material itself. By that, the applicability of the existing test procedure [13] to nano-composite thermosets is investigated. Additional properties of the neat and nano-modified epoxy are also determined (partly taken from [11]) and compared. 2. Experimental Plates (nominally 200 mm · 150 mm · 4 mm) of epoxy resin (Araldite CY 225 resin and Aradur HY 925 hardener from Huntsman) have been manufactured by casting in an Aluminium mould both with and without functionalized organosilicate clay based filler (10 wt.%, corresponding to about 7.2% of anorganic phase), type and processing details are described in [11]. Two batches of neat epoxy and nano-modified epoxy have been prepared. The plate materials have been characterized with (a) visual inspection, (b) X-ray diffraction and (c) dynamic mechanical thermal analysis (DMTA). Additional results for the second batch of nano-modified epoxy (compared with those of a third batch of neat resin) are described in [11]. X-ray diffraction (XRD) spectra have been obtained in a h–2h configuration (equipment Panalytical X’Pert pro), with scan steps of 0.05 and a scan speed of 0.05/5 s. The beam optics were as follows: Soller slit 0.04 rad, fixed divergence slit (1/4), 10 mm mask, fixed anti-scatter slit (1/2) for the incident, and fixed anti-scatter slit (1/4), Soller slit 0.04 rad, and nickel filter for the diffracted beam optics. The detector (type X’Celerator) was comprised of 127 elements (each 70 lm wide) yielding an active length (240 mm goniometer radius) of 2.1226 and a minimal step length of 0.0167. Dynamic mechanical thermal analysis (DMTA) was performed and the properties of the neat and nano-modified epoxy have been determined between 120 C and +160 C in a three-point bending configuration under deformation control, a forced oscillation frequency of 10 Hz, and a heating rate of 2 C/min (Eplexor 500N from Gabo Qualimeter GmbH, Germany). A static load of nominally 1.0% strain and a superimposed sinusoidal load of nominally 0.03% strain were applied to samples of nominal size of 25 mm · 15 mm · 4 mm with a span length of 20 mm and the results compared with DMTA data obtained earlier on tensile specimens [11]. Single edge notched bending (SENB) specimens have been prepared from both batches of each neat and nano-modified epoxy. SENB specimens were nominally 63 mm long, 15 mm wide, and 4 mm thick (=plate thickness). They were tested to determine the critical fracture toughness KIC and critical energy release rate GIC, respectively, according to the standard test method for polymers [13] issued by the International Organisation for Standardisation (ISO). A V-shaped notch was machined to a depth of about 3.5 mm, and then extended by knife–blade tapping to a total length around 7 mm (effectively ranging from about 6.3 to 8.3 mm). Tests have been performed with a cross-head speed of 1 mm/min at +23 C and 50% relative humidity. For the second batches of specimens, the indentation correction (see Section 5.4 of [13]) has been measured for each specimen individually. The fracture mechanics test data have been analysed in accordance with the prescriptions of the standard test method [13]. Spreadsheets developed and validated at the authors’ laboratory in an international round robin using poly-methyl-methacrylate (PMMA) specimens [14] were used for the data analysis. A magnifying glass (magnification 8 times) has been used to visually determine the initial crack lengths which have been measured at five positions across the thickness. Comparative plane strain compression testing has been performed to determine the compressive yield stress and the compressive modulus applying a set-up and a procedure described in [15]. After fracture testing, the fracture surfaces have been examined by scanning electron microscopy (SEM). Images have been taken with the secondary electron detector in an environmental scanning electron microscope (type XL30 from Philips) operated in the high-vacuum mode. The specimen surfaces have been coated with flash-evaporated carbon. Transmission electron micrograph (TEM) images were taken with a TEM-equipment (type LEO 912 Omega from Zeiss) at an accelerating voltage of 120 kV. Preparation details are described in [11].
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3. Results Visual inspection showed that the nano-modified epoxy appeared less translucent and of a darker colour than the neat epoxy, but homogeneous on a macroscopic scale. X-ray diffraction spectra of both, neat and nano-scale modified epoxy are shown in Fig. 1 for a 2-h range between 5 and 25. The diffractograms for different batches of each neat and nano-scale filled epoxy coincide, and, therefore, scales have been shifted for the presentation (Fig. 1). The neat epoxy essentially yields broad, featureless diffractograms reflecting the random network structure. Both batches of nano-filled epoxy, however, show a distinct peak at a 2-h value of 19.5. This is attributed to the combined (1 1 0) and (0 2 0) reflections of the intercalated silicate clay, see [11] for details. Transmission electron microscopy (TEM) images of the nano-modified epoxy of the second batch of the present study (Fig. 2, and additional TEM shown in [11]) indicate intercalation of the silicate layers in the epoxy with distances between silicate layers on the order of 9 nm. Homogeneously intercalated regions extend over several hundred nano-meters up to the micrometer range (Fig. 2). DMTA curves are shown in Fig. 3 for neat and nano-size filled epoxy in a three-point bending configuration. The data can be compared with earlier DMTA results obtained on tensile specimens, shown in [11]. A significant increase in the elastic modulus (around 40%) is recorded at room temperature by the addition of nano-size filler to the neat epoxy. Considerable in-batch and batch-to-batch variations are observed in the modulus measurements. Contrary to that, the glass transition temperatures, taken as the maxima of the tan d curves for the different batches of neat epoxy, fall all into a narrow range around +134 C, basically independent of the test configuration (bending in present paper or tensile quoted in [11]). The values for the nano-size modified epoxies are lower than for the neat epoxy and vary, they amount to about +109 C for the first, and around +114 C for the second batch. The latter value again agrees with the earlier tests reported in [11]. Load–displacement diagrams from the fracture toughness tests on both types of epoxy are shown in Fig. 4. It can be noted that the nano-modified epoxy specimens yield a larger slope and hence larger stiffness – as well as a larger variation in slope (both in-batch and from batch-to-batch) than the neat epoxy. Corrected displacement values to fracture according to the ISO standard [13] are about 0.16 mm for the neat and slightly less for the nano-modified epoxy (0.13–0.14 mm). The fracture data are summarized in Table 1 (fracture toughness) and Table 2 (energy release rate). For neat epoxy, eight specimens of each batch, out of 13 and 16, respectively are valid according to the criteria
Intensity [arbitrary units]
a)
c)
b)
d)
5000
8000
4500
7000
4000 6000 3500 3000
5000
2500
4000
2000
3000
1500 2000 1000 1000
500 0
0 5
7
9
11
13
15
17
19
21
23
25
2-θ [°] Fig. 1. X-ray diffraction spectra of neat and nano-size filled epoxy, 2-h range from 5 to 25, (a) neat epoxy first batch, (b) neat epoxy second batch, (c) nano-modified epoxy first batch, (d) nano-modified epoxy, second batch; (a) and (c) left hand scale, (b) and (d) right hand scale (shift between the scales to show the curves without overlap). The peak at 19.5 in the nano-modified epoxy is attributed to the combined (1 1 0) and (0 2 0) reflections, indicating a spacing of the silicate layers of about 8.8 nm, for details see [11].
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Fig. 2. Transmission electron micrograph (TEM) taken at low magnification on a specimen from the second batch of nano-modified epoxy (additional TEM shown in [11]).
Fig. 3. Dynamic mechanical thermal analysis of neat and nano-size filled epoxy, three-point bending configuration, temperature range 120 to +160 C tested at 10 Hz, (a) neat epoxy first batch, (b) neat epoxy second batch, (c) nano-modified epoxy first batch, (d) nanomodified epoxy, second batch. The shift in Tg indicates a lower cross-link density for the nano-modified epoxy, the width of the peak is discussed in [11].
p defined in [13]. Averaging over all valid specimens from both batches yields a KIC of 0.740 ± 0.140 MPa m, 2 and a GIC of 134 ± 24pkJ/m for the neat epoxy. The KIC average values for batch 1 and 2, respectively, are 0.750 and 0.730 MPa m with standard deviations of 12.5% and 24.9%. Corresponding GIC values are 132.6
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50
Load [N]
40
30
20
10
0 0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
Corrected Displacement [mm]
Fig. 4. Comparison of typical load–displacement diagrams from fracture toughness tests for neat and nano-modified epoxy. The four curves with maximum loads above 40 N (black) are from nano-modified epoxy, the others (grey) from neat epoxy (first batch bold, second fine lines, first specimen solid, second specimen dashed line). There is little in-batch and batch-to-batch variation for the neat epoxy, but considerable in-batch and batch-to-batch variation for the nano-modified epoxy. Table 1 Fracture toughness results on neat and nano-modified epoxy resin, only specimens fulfilling all validity criteria specified in the standard procedure [13] are used to calculate the averages and standard deviations (s.d.) Epoxy
Batch
Valid tests
Total tests
Average KIC p (MPa m)
s.d. p (MPa m)
s.d. (%)
Min. p (MPa m)
Max. p (MPa m)
Neat Neat Neat Nano Nano Nano
1 2 1+2 1 2 1+2
8 8 16 4 5 9
13 16 29 10 5 15
0.750 0.730 0.740 0.936 1.133 1.045
0.094 0.182 0.140 0.098 0.157 0.163
12.5 24.9 18.9 10.5 13.9 15.6
0.605 0.420 0.420 0.843 0.977 0.843
0.916 1.019 1.019 1.074 1.382 1.382
Table 2 Energy release rate results on neat and nano-modified epoxy resin, only specimens fulfilling all validity criteria specified in the standard procedure [13] are used to calculate the averages and standard deviations (s.d.) Epoxy
Batch
Valid tests
Total tests
Average GIC (J/m2)
s.d. (J/m2)
s.d. (%)
Min. (J/m2)
Max. (J/m2)
Neat Neat Neat Nano Nano Nano
1 2 1+2 1 2 1+2
8 8 16 4 5 9
13 16 29 10 5 15
132.6 135.9 134.2 135.8 160.9 149.7
11.9 33.0 24.0 13.5 23.5 22.8
9.0 24.3 17.9 10.0 14.6 15.2
120.8 115.7 115.7 120.7 133.3 120.7
149.9 183.9 183.9 147.4 197.9 197.9
and 135.9 J/m2 with standard deviations of 9.0% and 24.3%, respectively. For the nano-modified epoxy, the p two batches yield clearly differing results. KIC average values are 0.936 and 1.133 MPa m with standard deviations of 10.5% and 13.9%, and GIC average values 135.8 and 160.9 J/m2 with standard deviations of 10.0%
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Table 3 Comparison of properties of the neat and nano-filled epoxy (average values from both batches with standard deviations, except where noted for the nano-modified epoxy), quantities marked with * are from [11] determined on a third batch for the neat and from the second batch for the nano-modified epoxy Property
Neat epoxy
Nano-modified epoxy
Density/specific gravity [g/cm3] Silicate content [wt.%]a Tensile modulus [MPa] * Tensile strength [MPa] * Elongation at break [%] * Compressive modulus [MPa] Compressive yield strength [MPa] p Fracture toughness KIC [MPa m] 2 Energy release rate GIC [J/m ] Glass transition temperature [C]
1.2115 ± 0.0004 – 3100 ± 50 80.3 ± 0.8 7.4 ± 1.7 2300 ± 120b 92 ± 1.2d 0.740 ± 0.140f 134 ± 14h +134
1.2602 ± 0.0061 (+4.0%) 7.2 ± 0.02 4700 ± 130 (+52%) 51.5 ± 1.4 (36%) 1.2 ± 0.1 (84%) 3600 ± 160c (+56%) 108 ± 1.5e (+17%) 1.133 ± 0.160g (+53%) 161 ± 24i (+20%) +109/+114j
a
Determined as ignition residue according to EN 60 (losses on ignition). Average for two batches with five measurements each, values for first and second batch, respectively are 2370 ± 85 MPa and 2200 ± 90 MPa. c Average for two batches with eight and four measurements, values for first and second batch, respectively are 3660 ± 145 MPa and 3470 ± 100 MPa. d Average for two batches with five measurements each, values for first and second batch, respectively are 93 ± 0.1 MPa and 91 ± 0.6 MPa. e Average for two batches with eight and four measurements each, values for first and second batch, respectively are 108 ± 1.1 MPa and 106 ± 0.3 MPa. f Average for two batches with eight and seven measurements each, values for first and second batch, respectively are p p 0.750 ± 0.09 MPa m and 0.730 ± 0.18 MPa m. p g Average for second batch with five measurements, first batch average is 0.936 ± 0.1 MPa m, however, there is evidence of effects from residual stresses in the first batch. h Average for two batches with eight and seven measurements each, values for first and second batch, respectively are 133 ± 12 J/m2 and 136 ± 33 J/m2. i Average for second batch with five measurements, first batch average is 136 ± 14 J/m2; however, there is evidence of residual stresses in the first batch. j Values for first and second batch, respectively, there is evidence for effects from residual stresses in the first batch. b
p and 14.6%, for batch 1 and 2, respectively. Taking the averages of 1.133 MPa m and of 161 J/m2 (nano-modified epoxy, batch 2), the addition of 10 wt.% of nano-filler to the epoxy yields an increase of about 50% in fracture toughness and of about 20% in energy release compared with neat epoxy. Additional properties of the neat and nano-modified epoxy, partly taken from [11], are compared in Table 3. Fig. 5 shows selected SEM images of fracture surfaces of neat and nano-modified epoxy. The neat epoxy shows comparatively smooth and featureless surfaces at all magnifications (up to 51 200·). Nevertheless, a low-contrast texture is recognizable at higher magnifications (beyond about 15 000·), oriented roughly parallel to the crack propagation. The nano-modified epoxy, on the other hand, shows a platelet-type, fairly corrugated surface even at low magnifications (6500·). No crack propagation oriented features have been observed for the nano-modified epoxy. SEM images taken at higher magnifications (>20 000·) indicate distinct features in the sub-micrometer range on the individual platelets of the nano-modified epoxy. 4. Discussion The increase of roughly 40% in modulus upon modification by nano-size filler observed in the DMTA three-point bending experiments compares quite well with data obtained from the static plain-strain compression tests (increase around 55%, see Table 3). The relatively high absolute values obtained compared with the earlier DMTA tests on tensile specimens [11] are quite likely induced by the sample geometry used (low span to thickness ratio in the bending tests). The glass transition temperature (Tg) appears to be lowered by about 20–25 C upon introducing the nano-size filler into the base system. Simultaneously, the width of the tan d peak is increasing. This indicates a probable change in the cross linking density and homogeneity of the resin
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Fig. 5. Selected SEM-images of the fracture surfaces of neat (left), and nano-modified epoxy (right) at increasing magnification (top to bottom). The neat resin yields flat and virtually featureless surfaces (except for a fine texture roughly oriented parallel to the crack propagation), while the nano-modified epoxy shows a platelet-like structure with distinct features ranging down to 200–400 nm.
network due to the interaction between resin and functionalized filler. These results are consistent with the DMTA data obtained previously on this system [11]. The width of the transition peak for both batches of nano-modified epoxy is comparable. However, there is a difference in Tg determined from DMTA between the two batches. The first batch yields the lower value (around +109 C), indicating a lower cross-linking density than in the second batch (Tg around +114 C). It is expected that this would yield a slightly higher fracture toughness, contrary to what is effectively observed. Compared with neat resin, the width of the transition peak is increased for both batches of nano-modified epoxy. This could be interpreted as a larger range of cross-linking density in the nano-modified materials, consistent with the dispersion of regions of nano-intercalated silica in the epoxy. Comparing the overall averages of the fracture mechanics data (averaging over valid specimens only) indicates that the addition of 10 wt.% of functionalized nano-scale filler changes the fracture toughness KIC by about 50% and the energy release GIC by about 20%. When the data are analysed with respect to the different batches (plates), the batch-to-batch variation for the neat epoxy is clearly within one standard deviation. The average values for the two batches of nano-modified epoxy do differ somewhat more (see Tables 1 and 2). The formation of pre-cracks initially deviating from the plane of symmetry as visually observed during blade tapping of the first batch of nano-modified epoxy points toward residual stresses from manufacturing. This was
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not observed for the second batch that yielded higher average values of KIC and GIC. The effect of residual stresses could hence explain the lower fracture toughness of the first batch of nano-modified epoxy (in spite of the lower cross-link density). The second batch only is, therefore, taken for quoting fracture toughness data for nano-modified epoxy. The flat and relatively featureless fracture surface of neat epoxy resin specimens is consistent with a relatively brittle fracture mechanism. The comparatively strong corrugation of the fracture surfaces of the nano-modified epoxy, on the other hand, is consistent with the measured, improved fracture toughness and energy release rate compared with the neat epoxy. For the nano-modified epoxy, there are distinct structural features on the surface of the platelets with sizes of 500 nm or less. Considering the silica-intercalated regions with diameters of up to 1000 nm or more indicated by TEM images on the second batch of nano-modified epoxy (Fig. 2) it can be speculated that these features in the SEM images could be due to fracture between such intercalated regions and neat epoxy surrounding them. Such a mechanism would explain the corrugated fracture surfaces as well as the improved fracture toughness compared with neat epoxy resin. It has to be noted that no distinct difference has been observed so far between the fracture surfaces of the different batches of the nano-modified epoxy. It can be added that there is a relatively large number of invalid tests if all criteria, including initial crack length – specimen width (a/w)-ratio and modulus difference, of the standard analysis [13] are applied. 16 out of 29 (55%) for neat and 9 out of 15 (60%) of the tests were valid. a/w-ratios outside the recommended limits of 0.45 (lower) and 0.55 (upper) were the most frequently violated criterion (eight neat and one nano-modified specimen), while pre-crack length variation exceeding 10% across the specimen width yielded invalid results for two neat and one nano-modified specimen. The KIC and GIC data can also be analysed with respect to various parameters. The correlation of KIC with (a/w)-ratio (Fig. 6) is discussed as an example. For both types of epoxy, an apparent trend for increasing KIC values with increasing a/w-ratio is observed. This trend persists, even if only values for a/w within the recommended limits (0.45–0.55) are considered. The scant literature data on effects of nano-scale fillers on the fracture toughness of epoxy indicates an increase in fracture toughness for the nano-composite but the magnitude of the effect quite likely depends on the weight fraction of the nano-scale filler [7–10]. For a heavily filled epoxy with 60 wt.% of conventional p (not nano-dispersed) quartz filler the supplier data sheet [16] indicates values of KIC and GIC (1.8–2.0 MPa m
Fig. 6. KIC values of neat and nano-modified epoxy plotted as function of the ratio between initial crack length, a, and specimen width, w. Both batches of nano-modified epoxy are shown. Note the trend for increasing KIC with increasing a/w (see text for details).
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and 300–350 J/m2, respectively) that are higher than those obtained for epoxy with 10 wt.% nano-intercalated silicate clay. From an application point of view, it will be even more interesting to see whether or by how much the fracture toughness and other properties are changed if the matrix in a fibre-reinforced laminate is replaced by the nano-modified epoxy. Improvements in various properties have been observed when a glass–fiber laminate with nano-modified epoxy matrix is compared with a laminate with neat epoxy matrix: averages of flexural modulus (+5.7%), of flexural strength (+27.5%), and of flexural strain (+23.5%) are all increased [11]. This raises the expectation that the improved fracture properties of the nano-modified epoxy reported in the present paper can also be transferred to fibre-reinforced laminates. The ESEM images shown in [11] do yield indications of improved fibre-matrix adhesion when nano-modified epoxy is used as matrix in a glass–fibre laminate (compared with a neat epoxy matrix laminate). Combined with some improvement of the fracture toughness of the matrix, this could well result in a considerably improved delamination resistance. 5. Conclusions An increase between about 40 and 50% in fracture toughness and around 10–20% in energy release has been observed relative to the neat epoxy for nano-modified epoxy prepared by addition of 10 wt.% of functionalized organosilicate clay. Scanning electron microscopy images indicate considerable differences in the fracture surfaces of neat and nano-modified epoxy, consistent with the measured fracture data. Various other properties are improved in the nano-modified epoxy, e.g. a considerable increase in tensile and compressive modulus by more than 50% is observed. The most noteworthy exceptions known to date are ultimate tensile strength, elongation at break, and glass transition temperature, which are all reduced. In principle, the standardized ISO test procedure for fracture toughness measurements of polymers [13] can be applied to the nano-modified epoxy. A considerable percentage of tests (between 40% and 45%) have been found to be invalid according to the criteria of the standard test procedure and it is recommended to prepare more than the minimum number of five specimens per material. The nano-modified epoxy hence shows potential for improving delamination resistance when used as matrix in fibre-reinforced laminates, analogous to the improvement of other mechanical properties reported in the present paper and in [11]. Acknowledgements Mr. U. Gfeller provided XRD data, and Mr. K. Ruf DMTA results. Specimen preparation has been performed by Mr. D. Vo¨lki and a first series of fracture tests been conducted by Mr. M. Heusser. References [1] Pandey JK, Reddy R, Pratheep Kumar A, Singh RP. An overview on the degradability of polymer nanocomposites. Poly Degrad Stabil 2005;88(2):234–50. [2] Jordan J, Jacob KI, Tannenbaum R, Sharaf MA, Jasiuk I. Experimental trends in polymer nanocomposites – a review. Mater Sci Engng A 2005;393(1–2):1–11. [3] Xie L-X, Mai Y-W, Zhou X-P. Dispersion and alignment of carbon nanotubes in polymer matrix: a review. Mater Sci Engng R 2005;49(4):89–112. [4] Lin J-C, Chang LC, Nien MH, Ho HL. Mechanical behaviour of various nanoparticle filled composites at low-velocity impact. Compos Struct 2006;74(1):30–6. [5] Isik I, Yilmazer U, Bayram G. Impact modified epoxy/montmorillonite nanocomposites: synthesis and characterization. Polymer 2003;44(20):6371–7. [6] Zhang J, Jiang DD, Wilkie CA. Fire properties of styrenic polymer–clay nanocomposites based on oligomerically-modified clay. Polym Degrad Stabil 2005;91(2):358–66. [7] Avlar S, Qiao Y. Effects of cooling rate on fracture resistance of nylon 6-silicate nanocomposites. Compos Part A: Appl Sci Manuf A 2005;36(5):624–30. [8] Liu W, Hoa SV, Pugh M. Fracture toughness and water uptake of high-performance epoxy/nanoclay nanocomposites. Compos Sci Technol 2005;65(15–16):2364–73. [9] Ragosta G, Abbate M, Musto P, Scarinzi G, Mascia L. Epoxy-silica particulate nanocomposites: chemical interactions, reinforcement and fracture toughness. Polymer 2005;46(23):10506–16.
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[10] Yao XF, Yeh H-Y, Zhao HP. Dynamic response and fracture characterization of polymer–clay nanocomposites with Mode-I Crack. J Compos Mater 2005;39(16):1487–96. [11] Kornmann X, Rees M, Thomann Y, Necola A, Barbezat M, Thomann R. Epoxy-layered silicate nanocomposites as matrix in glass fibre-reinforced composites. Compos Sci Technol 2005;65(14):2259–68. [12] Timmerman JF, Hayes BS, Seferis JC. Nanoclay reinforcement effects on the cryogenic microcracking of carbon fiber/epoxy composites. Compos Sci Technol 2002;62(9):1249–58. [13] International standard. Plastics – determination of fracture toughness (GIC and KIC) – Linear elastic fracture mechanics (LEFM) approach. International Organisation for Standardisation. 2000, 13586: 1–16. [14] Unpublished results from an international round robin in 2004 on characterization of poly-methyl-methacrylate (PMMA) organised ¨ sterreichisches Forschungsinstitut fu¨r Chemie und Technik (Austrian Research Institute for Chemistry and Technology). by the O [15] Williams JG, Ford H. Stress-strain relationships for some unreinforced plastics. J Mech Engng Sci 1964;6(4):405–17. [16] Data sheet Araldit-Giessharzsystem CY 225/HY 925, Huntsman, Advanced Materials, Electrical Insulation Materials (version June 2003).