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Evidence of thermo-oxidation phenomena occurring during hygrothermal aging of thermosetting resins for RTM composite applications
Composites: Part A 66 (2014) 175–182
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Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
Evidence of thermo-oxidation phenomena occurring during hygrothermal aging of thermosetting resins for RTM composite applications A. Simar a,b, M. Gigliotti a,⇑, J.C. Grandidier a, I. Ammar-Khodja b a b
Department of Physics and Mechanics of Materials, Institut P’ CNRS, Université de Poitiers, ENSMA, UPR 3346, Chasseneuil du Poitou, France Materials and Process Department, Aircelle, groupe Safran, Gonfreville l’Orcher, France
a r t i c l e
i n f o
Article history: Received 13 March 2014 Received in revised form 4 July 2014 Accepted 7 July 2014 Available online 18 July 2014 Keywords: A. Theromosetting resin B. Environmental degradation C. Analytical modelling D. Mechanical testing
a b s t r a c t This paper focuses on the humid aging of a high temperature thermosetting resin employed for the realization of aircraft parts manufactured by the RTM process. Accelerated humid aging at several different temperatures (40–90 °C) and relative humidity values (50–100%) has been carried out by means of gravimetric tests. Anomalous behavior with respect to Fick’s diffusion law and a color gradient from the edge to the center of the samples has been observed. Drying of samples after aging and mechanical tests on aged and dried samples have shown the existence of irreversible phenomena taking place during humid ageing: uniform tensile tests revealed resin embrittlement, Ultra Micro Indentation tests allowed measuring a gradient of properties from the surface to the centre of the samples. This behavior – associated to the color changes observed on aged samples – indicates that oxidation phenomena take place during hygrothermal aging. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Thermosetting polymer resins are currently employed for the realization of composite parts with carbon fiber fabrics on structural parts in ‘‘warm’’ aircraft areas and, since new manufacturing processes are under development, complex geometrical parts made of composite materials can be effectively produced. Among existing techniques, resin injection «RTM» (Resin Transfer Molding) is perhaps the more widespread in aircraft and automotive industries. During their life, these new materials and aircraft structures are exposed to high and low temperature simultaneous cycles in dry and wet environments. In order to test the durability of such materials, accelerated aging can be carried out. Among others, hygrothermal aging is often tested on composite materials. Water modifies polymer matrix properties and has an impact on composite material properties. Weitsman [1] listed the main difficulties related to the exhaustive understanding of water aging mechanisms and indicated water diffusion anomalies in the matrix with respect to the classical Fick’s law [2]. In general, water uptake is a reversible phenomenon which creates residual stresses and material properties degradation [1,3,4], including decrease of the glass transition temperature and plasticization, that is, decrease of the ⇑ Corresponding author. Tel.: +33 5 49 49 83 40. E-mail address: [email protected] (M. Gigliotti). http://dx.doi.org/10.1016/j.compositesa.2014.07.007 1359-835X/Ó 2014 Elsevier Ltd. All rights reserved.
failure stress with increase of failure strain, particularly in organic resins. Anomalies with respect to the classical Fick’s diffusion law can be ascribed to chemical reactions taking place during humid aging [8,9]. Tcharkhtchi et al. [10] compared different amine/epoxy networks and highlighted water uptake increase when hydroxyl molar groups are present: in particular water sorption process in these networks is governed by two phases whose first is physical and the second is chemical. El Yagoubi et al. [9] have shown the importance of studying material desorption after humid aging to verify the mechanism reversibility during pseudo-Fickian diffusion, that is to say containing saturation stages on long time. They have also shown that non linear diffusion of epoxy/amine network could be due to a hydrolysis reaction. Gates et al. [3] highlighted the complexity of the physical–chemical mechanisms leading to humid aging: multi-physical couplings are function of the studied polymer network and are not completely identified. Some other irreversible phenomena taking place during water transport in polymers have been described in [5,6]. These works put into evidence the necessity to develop multi-physical approaches since diffusion phenomenon can be accelerated by mechanical stresses and, conversely, the elastic and visco-elastic response of materials can be influenced by diffusion. These works point out the challenges which are related to numerical simulation of multi-physical coupled mechanisms, and the necessity to
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enlarge this research field. In a slightly different context, Léger et al. [7] showed the interest of a multi-physical approach for the prediction of the aging of bonded joints in liquid environment at 70 °C. Combining the effect of temperature and humidity on structural parts, the resulting behavior becomes complex since property gradients arise. In RTM composite materials for novel applications (warm temperature) resins will be solicited under high temperatures and ambient humidity conditions. The question whether a high glass transition temperature matrix will be able to withstand with such conditions (close to use conditions) is then crucial. In particular appropriate knowledge of the behavior under humidity and ‘‘high’’ temperature conditions for this new class of matrices must be gained. The purpose of this paper is to broaden the hygrothermal behavior of a resin manufactured by RTM injection, under high temperature, for which few data are available in the literature. The material and the experimental setup is presented in Section 2, the diffusion anomalies will be characterized by gravimetric tests in Section 3. A specific experimental approach will be then developed and presented in Section 4 in order to understand the origin of the diffusion anomalies. 2. Material The epoxy/amine thermosetting resin from Hexcel Industry (commercial name RTM6) is employed for the realization of aircraft structural parts made by carbon/epoxy composites materials by the RTM (Resin Transfer Molding) injection process, with high mechanical and thermal properties. The network is composed by Tetraglycidyl methylene dianiline (TGMDA or TGDDM) prepolymer and of two amine hardener: 4,40 -Methylenebis(2,6-Diethylaniline) with commercial name MDEA and 4,40 -Methylenebis (2-Isopropyl6-Methylaniline) with commercial name M-MIPA. Neat resin plates have been produced by contact moulding from a liquid resin mix and polymerized in a Memmert ventilated oven after resin vacuum degassing (30 mbar at 80 °C). The resin cure cycle is composed by a 2 h dwell at 180 °C and a temperature ramp between 1 and 2 °C/min. After cutting, rectification and drying, samples have been stocked in a vacuum oven (30 mbar) at 70 °C to extract volatile substances initially present in the polymer. 3. Gravimetric test 3.1. Tests Several different hygrothermal aging tests have been carried out to evaluate the temperature (35 °C, 70 °C and 90 °C) and of relative humidity (50%, 85%, 90% and 100% in immersion) on the water diffusion process within the resin material. The weight of initially dry polymer samples with same shape and dimensions (30 mm * 10 mm * 2 mm, three samples for each condition) has been monitored by a Sartorius YDK01 analytical balance (with the precision of the order of 105 g). Hygrothermal aging has been carried out in a SECASI SLH 200 climatic chamber. Special attention has been paid to package them in waterproof recipient in order to avoid any contamination. This manipulation is made at room temperature and during a short period (10–20 min), so that water diffusion, which is a thermally activated, is poorly affected by these conditions. The polymer mass increases/decreases as a function of time during a sorption/desorption test (at 70 °C 85% HR) is presented in Fig. 1, for an average of three samples with the same shape (30 mm * 10 mm * 2 mm). To estimate the mechanisms reversibility, samples have been dried, after humid aging, at 70 °C under vacuum (30 mbar). Since samples are thin, water diffusion is
Sorption
Desorption
2,5
Water mass (%)
176
2 1,5 1
70°C - 85%HR
70°C - 0%HR
0,5 0 0
1000
2000
3000
Time (h) Fig. 1. Experimental sorption–desorption curve – 70 °C/85%HR – average value of 4 samples (20 mm * 10 mm * 2 mm).
essentially unidirectional along the thickness direction. Scatter of measures is small (1–4% maximum deviation compared to the average of four curves) and can be hardly appreciated in the figures. It can be noted that water diffusion inside resin rapidly move away from a Fickian process. For this configuration, mass uptake saturation cannot be observed, and during desorption a residual 0.1% water mass is measured : this observation is confirmed for every sample dried after aging between 70 °C and 90 °C for which a residual mass uptake between 0.07% and 0.18% is measured. This proves the presence of non reversible mechanisms occurring during humid aging under ambient air and immersed conditions. The polymer mass increases as a function of the square root of time, divided by the sample thickness, is presented in Fig. 2, for several test configurations. Scatter of measures is small (1–4% maximum deviation compared to four curves average) and can be hardly appreciated in the figures. It can be noted that water diffusion inside resin rapidly move away from a Fickian process. More precisely, the first part of the curve is a linear function of the square root of time for each aging condition. For each tested configurations, mass uptake saturation cannot be seen, even after 5000 h aging. This fact provides evidence of anomalous behavior with respect to the Fick’s law of diffusion. In this context, it is not possible to identify in a suitable manner mass uptake at saturation state and a diffusion coefficient. To quantify in a first approximation the water diffusion kinetics inside the polymer, a simplified analytical solution of Fickian diffusion law [11] has been employed, namely 1 Mt 8 X eðQtÞ ¼1 2 M1 p n¼0 ð2n þ 1Þ2
! Q ¼p
2
2 ! 2n þ 1 D e
ð1Þ
where Mt is the water mass uptake at time t, M1 is a fictive mass uptake at saturation, D is the diffusion coefficient, and e is the sample thickness. The fictive mass uptake at saturation, M1, has been defined as the crossing point between the initial and the final slopes of the experimental points. The simplified solution, Eq. (1), is based on the fundamental hypothesis that water diffusion is isotropic and takes place along the thickness direction. These two hypotheses are acceptable due to the nature of the resin material and to the choice of the samples shape (sample thickness is small compared to length and width). By the employment of a fictive mass uptake, as defined above, a diffusion coefficient equal to 0.002 mm2/h at 35 °C, 0.02 mm2/h at 70 °C and 0.03 mm2/h at 90 °C, respectively, can be identified. A factor of around 10 between the diffusion coefficient at 35 °C and the one at 70 °C can be appreciated. It is concluded that a diffusion
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Fickian simulation
3,5
Linear regression
Water mass uptake (%)
3
2,5
2
1,5 T (°C)
HR (%)
D (mm²/h)
M∞/M0 (%)
70
85
0.02
2.23
35
90
0.003
2.49
70
50
0.02
1.31
90
IM
0.03
2.84
70
IM
0.02
2.84
1
0,5
0 0
2
4
6
8
10
12 1/2
Root time/Thickness (h
14
16
18
/mm)
Fig. 2. Experimental and numerical sorption curves for various humid conditioning.
coefficient can be effectively calculated, and that its dependency upon the temperature is consistent. Looking at the experimental points at the end of the aging process, a linear correlation can be identified; the slopes of the curves (indicated by the symbol a) for each condition are reported in Table 1. The same table illustrates the R coefficients associated to the linear regression. By increasing the aging temperature, the slope of the curves increases while it is nearly horizontal for the 35 °C aging. When the aging temperature increases, the slope coefficient increases as well, this proves that the diffusion anomaly is thermally activated. Then, under immersed conditions at 70 °C, the slope is approximately twice higher than that observed for 70 °C and 85%HR aging. 3.2. Discussion Although temperature is substantially lower than the glass temperature (around 140 °C lower than, which is equal to around 233 °C), the resin does not possess Fickian behavior and a saturated state cannot be reached. Water uptake keeps going after 5000 h, and the extent of the anomalous behavior is amplified by temperature. A small amount of water (between 0.07% and 0.18%) is kept by the network during desorption and the immersion in liquid water accelerates the process. The anomalous behavior with respect to the classical Fick’s law can be the consequence of several physical phenomena which are: – The non linearity of polymer behavior. Weitsman [1] presents the principal difficulties linked to the exhaustive understanding of humid aging phenomena and proposes a non exhaustive list of water diffusion anomalies in polymers compared to the Fick’s Table 1 Sorption slope coefficients at the end of aging.
4 s N
T (°C)
HR (%)
a (103) (mm/h1/2)
R2
70 35 70 90 70
85 90 50 IM IM
0.02 0.001 0.02 0.046 0.041
0.94 – 0.96 0.99 0.99
law [2]. Carter and Kibler [2] describe water diffusion anomaly in polymers by employing a Langmuir-type diffusion model: this model postulates the existence of two diffusing species (bound and free species) whose diffusion mechanism is based on a trapping statistical process [12]. Piccinini et al. [13,14] explain diffusion anomalies by the polymer volume relaxation generated by volatile substances sorption via a linear viscoelastic interaction. Leger et al. [7] show that the anomalous nonlinear water diffusion phenomena in epoxy adhesive are due to the presence of cavities generated during humid aging. – Hydrolysis of the polymer network. Tcharkhtchi et al. [10] put into evidence a constant mass uptake during humid aging of an epoxy/amine resin. In their study, a water diffusion/reaction model of a hydrolysable polymer matrix is described. This model gives results which are close to those given by the Langmuir’s model, though the physical meaning behind the two models is different. El Yagoubi et al. [9] put forward the approach by Tcharkhtchi et al. [10] hypothesizing that the hydrophilicity of the reaction products between water and resin carries out non linear water diffusion in the epoxy/anhydride network. The competition between diffusion and reaction mechanisms in the polymer modifies the polymer diffusive behavior, solubility and diffusivity are affected by the number of hydrolysable sites. Thus, according to this interpretation, an incomplete cure presents more hydrolysable sites promoting polymer hydrolysis phenomena [9]. De’Nève and Shanahan [15] show that the epoxy/ dicyandiamide resin behavior results from a combination of polymer plasticity and chemical modification. The decrease of Young modulus at rubbery state and chemical modifications detected by FTIR (Fourier Transform InfraRed spectroscopy) analysis testifies macromolecular chain scission. – A thermal activated phenomenon under atmospheric air like thermo-oxidation of the polymer network. Several authors have shown that oxidation phenomena [16–18], may promote an increase of the network mass during time: more precisely, a mass uptake is essentially observed when the oxygen diffusion inside the matrix dominates on chemical reaction. In order to decide about the possible origin of the anomalous behavior observed for the material of the present study, a test
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approach combining several characterization tests has been formulated. 4. Effect of a humid environment on the resin properties 4.1. Approach An optical control is carried out on the material in order to establish the evolution of the resin color during aging: the samples are semi-transparent so that network changes due to chemical interactions can be highlighted by color changes and are easily detectable. Physical and thermal macroscopic properties of the resin before and during humid aging are then characterized via dynamical mechanical analysis (DMA) in 3 points bending flexure mode. These tests are made on resin samples placed in a humid controlled chamber (70 °C and 85%HR), for different conditioning times. For this test, a total strain equal to 0.02% is imposed at a frequency of 1 Hz: an initial force of 0.01 N is imposed in order to avoid sample warping before testing. During the test, the temperature is increased from 25 °C to 260 °C at a speed equal to 2 °C/min to highlight the transition from glassy to rubbery state of the polymer: the tan(d) peak corresponding to the tangent of the loss angle (ratio between the loss modulus E00 and the storage modulus E0 , representing the ratio between wasted and stocked energy) characterizes the transition from glassy to rubber state. The macroscopic mechanical properties of the polymer are measured by uniform tensile tests under control displacement. Traction tests are performed on different humid aged samples in order to quantify the modification of the mechanical properties of the polymer in the presence of water. The samples dimension is 60 mm * 10 mm * 4 mm, the experimental setup is characterized by a force cell up to 5 kN, the imposed displacement speed is equal to 1 mm/min until sample break. Longitudinal and transverse displacements are followed by a CCD (Charge-Coupled Device) dynamical camera and processed by the related software: white markers on the surface sample are employed for strain measurements. DMA analysis and uniform tensile test are capable of detecting the macroscopic mechanical behavior of the polymer material. Thus, in order to characterize possible properties gradient induced within the material by the diffusion process, Ultra Micro Indentation (UMI) tests are performed on aged and re-dried resin samples to the experimental protocol illustrated in Fig. 3: samples are coldcoated and polished up to 1 lm into exhibit acceptable planarity and surface preparation before the test. The experimental setup consists in a Fisherscope HC100 apparatus equipped with a Vickers pyramidal indenter. During the test, the charge is increased to its maximum value (Fmax = 5 mN) at a rate equal to 20 s. The maximal force is kept for one second and the unloading step is carried out at a rate equal to 20 s. The indentation elastic modulus calculated during the unloading phase at constant rate is obtained starting from the shape of the unloading curve according to Eq. (2)
EIT ¼
pffiffiffiffi dF p pdh ffiffiffiffiffi 2b Ap
ð2Þ
in which F is the charge, h represents indentation depth, b is a corrective parameter, Ap is the contact area between the indenter and the sample projected on a plane perpendicular to the indenter axis. 4.2. Results 4.2.1. Optical observations During humid aging under atmospheric air, a color change of the polymer from around yellow to brown dark, more or less pronounced depending on the aging time and temperature has been observed: when dried (cf. Fig. 4) the samples keep their color change. This is the proof of a non reversible mechanism occurring during aging and affecting the network and its optical properties. In order to check the existence of a possible color gradient along the sample thickness, some specimens have been cut and coated after drying. Fig. 5 shows that the virgin sample has kept its characteristic yellow along the whole thickness. Humid aged samples (1500 h at 70 °C/85%HR) keep exhibiting a color gradient along the thickness after drying, and more specifically a brown layer of about 300 lm on the sample surface. This color change could be related to hydrolysis or oxidation. However, since the color change is limited to an external layer of a few hundred of lm, the occurrence of hydrolysis can be discarded since it would affect the whole sample thickness. On the other hand, the occurrence of oxidation phenomena is suspected. 4.2.2. Physical and thermal properties Three point bending dynamical mechanical analysis (DMA) tests in flexure mode have been carried out on aged resin samples (at 70 °C, 85%HR for different aging times). A decrease of the glass transition temperature (Tg) with aging from 233 °C (for a virgin sample) to 223 °C (for a sample aged 1500 h with 2.6% of water mass uptake) is measured, cf. Fig. 6. After an additional 3200 h humid aging and an additional water mass uptake of 0.1%, the glass transition temperature keeps decreasing of around 7 °C. Tg depletion seems to result from the combination of two processes: a fast one obviously due to the resin plasticization by absorbed water and a slower one presumably linked to a (partially irreversible) resin degradation independent of water concentration, whose mechanism remains to establish. When samples are dried (1500 h at 70 °C and 85%HR), the initial glass transition temperature is not recovered (a Tg equal to 228 °C is measured compared to 233 °C for not aged samples): this can be partially explained by considering that the sample has not dried completely (a residual water weight equal to 0.1% is measured), but the Tg ‘‘gap’’ is not compatible with a 0.1% residual water mass uptake in the network. These results put again into evidence the occurrence of non reversible phenomena occurring during hydrothermal aging. Fig. 8 presents the storage modulus E0 characterized in 3 points bending flexure mode by DMA analysis for several samples placed
Indentation prints
Fig. 3. Sample preparation before «UMI» test. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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90°C / 95%HR 1500h
70°C / 85%HR 1500h
Not aged Dry
70°C / Immersion 800h
90°C / Immersion 800h
Fig. 4. Aged samples. From the left to the right: 90 °C/95%HR/1500 h; 70 °C/85%HR/1500 h; not aged; 70 °C/Immersed in liquid water/800 h; 90 °C/Immersed in liquid water/ 800 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Tg = -4,29 log(t) + 234,7 R2 = 0,8743
240
220
Linear 70°C - 85%HR 70°C - 85%HR dry
200
Fig. 7. Experimental glass transition temperature as a function of time (logarithmic scale).
Fig. 5. (a) Not aged; (b) 70 °C/85%HR/1500 h; and (c) 90 °C/95%HR/1500 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
1,2
Tan Delta
1 0,8
Time (h)
Mt/M0 (%)
Tg (°C)
0
0
233
50
2
230
1500
2.6
223
4800
2.7
216
1500 dry
0.1
228
Storage modulus (Mpa)
3500
216 223 228 230 233
0,6 0,4
3000 2500 2000 1500 1000 500
0,2
Time (h)
Mt/M0 (%)
E* (Gpa)
0
0
3.25
50
2
3.0
1500
2.6
3.2
1500 dry
0.1
3.6
0 0 180
20 190
200
210
220
230
70
120
240
170
220
Temperature (°C)
Temperature (°C) Fig. 8. Experimental storage modulus for various humid aged samples. Fig. 6. Glass transition temperature measured by DMA tests on several humid aged samples.
Min value
100
Mean value
Max value
90 80
Stress (Mpa)
in climatic chamber (the legend illustrates modulus measures at 25 °C). E0 suffers from a 250 MPa sudden decrease after 50 h of humid aging and tends to re-increase after 1500 h aging, reaching a value close to that of a virgin sample. This is in contrast with the trend observed on Tg (Fig. 7) which decreases monotonically with time. When 1500 h aged samples are re-dried (with a residual 0.1% water mass uptake), E0 tends to increase and, at 25 °C, equals 400 MPa. This indicates the presence of several active phenomena during hygrothermal aging. Modulus decrease during the first phases of the humid aging indicates plasticization, the subsequent increase is related to chemical reactions which modify the polymer network and the glass transition temperature of dry samples. Similarly, the irreversible monotonic decrease of Tg with time testifies the occurrence of plasticization phenomena followed by chemical reaction effects.
70 60 Time (h)
50
Mt/M0 (%)
E (Gpa)
εr (%)
σr (Mpa)
Tg (°C)
40
0
0
3.25
4.7
93.5
233
30
2.5
0.5
2.44
4.5
85.8
233
50
2
2.46
3.6
70.7
230
20
1500
2.6
2.66
2.1
51.5
223
10
4800
2.7
2.71
1.6
47.1
216
1500dry
0.1
3.33
1.7
53.9
228
0 0
1
2
3
4
5
6
Strain (%) Fig. 9. Experimental tensile stress curves for several hygrothermally aged samples.
Failure stress (Mpa)
4 3 2 70°C - 85%HR
1
70°C - 85%HR dry
0 0
1
2
3
4
100
σ = -12,4 log(t) + 91,1 2
R = 0,9973
80 60 40
Linear 70°C - 85%HR
20
70°C - 85%HR dry
1
2
5
ε = -0,85 log(t) + 4,82
4
R2 = 0,9905
3 2
Linear 70°C - 85%HR
1
70°C - 85%HR dry
0
0 0
Failure strain (%)
A. Simar et al. / Composites: Part A 66 (2014) 175–182
Young Modulus (Gpa)
180
3
4
0
log (t)
log (t)
1
2
3
4
log (t)
Fig. 10. Experimental Young’s modulus, stress and strain as a function of time (logarithmic scale).
4.2.3. Tensile tests on aged samples Tensile tests with imposed displacement rate have been made on different humid aging resin samples to quantify the change of elastic mechanical properties under traction, in the presence of water. Fig. 9 shows results obtained after tensile tests on samples aged in a climatic chamber (70 °C and 85%HR) for different aging times. The elastic modulus of aged samples decreases of around 0.18 GPa for a 5% water mass uptake. On the other hand, it is noted that the resin behavior becomes brittle due to hygrothermal aging since the failure strain decreases significantly as a function of the aging duration, up to 55% decrease after 1500 h aging. After 4800 h aging, the resin behavior is still brittle while the elastic modulus has approximately the same value than the initial one (Fig. 10). These results confirm the measures by DMA analysis. The elastic modulus of samples re-dried after 1500 h aging is around 2–3% higher than the average value measured on virgin samples. However, still for such samples, the resin behavior is quite brittle; these results confirm the significant impact of humid aging on the network behavior and the occurrence of irreversible phenomena within the polymer network due to the occurrence of chemical reactions. 4.2.4. Local mechanical properties Ultra Micro Indentation tests have been performed on the external edge (close to the external environment) of virgin and aged resin samples. By definition, the relative change of the elastic indentation modulus EIT with respect to its initial (virgin) value EIT0 is defined as:
DE ¼
EIT EIT 0 EIT 0
ð3Þ
In order to compare the property gradient of different samples Fig. 11 shows the values of DE as a function of the distance from the external edge exposed to the environment: the points
10% 70°C - 85%HR 90°C - 100%HR
ΔE (%)
8% 6% 4%
represent the average value of twelve individuals measures on two samples. The maximum standard deviation is around 0.2% on both series of measurements but this information is not presented in Fig. 11 to gain clarity. We observe a property gradient from the surface to the centre of humid aged and re-dried samples. This gradient spreads along around 300–400 lm from the external surface. EIT on the sample surface increases of about 5–6% for samples aged at 70 °C and about 9–10% for samples aged at 90 °C. These results are in good agreement with the color gradient size observed visually on the samples edge. 4.3. Discussion Optical analyses carried out on resin samples before and after aging highlight polymer color changes during humid aging. Yellow samples become brown after 1500 h aging and dark brown after 4800 h. This persistence of color change after sample re-drying can be explained by the network chemical modifications induced by hydrolysis or oxidation phenomena. The existence of a color gradient from the surface (exposed to the environment) to the centre of the sample along an affected zone around 300 lm allows suspecting that it is oxidation – and not hydrolysis – which come into play. In fact, since resin water mass uptake has reached a quasi-stabilized state after 1500 h aging (see Fig. 2), water concentration gradients should be low: consequently color changes should affect the whole sample volume, not only its edges, if hydrolysis would play a role. On the contrary, thermo-oxidation phenomena do affect the external samples surfaces only and are consistent with the observed EIT gradients [16–18]. Another interesting observation concerns the sample desorption behavior: upon re-drying a 1500 h aged sample the initial polymer mass cannot be recovered. A small residual mass is kept by the sample, contradicting absorption/desorption reversibility: this residual mass can be due to oxidation phenomena, which has demonstrated inducing small polymer mass increase in several literature case studies [16–18]. Glass transition temperature changes have been measured during hygrothermal aging as a function of the polymer water mass uptake, not decreasing linearly with respect to the absorbed water quantity. Upon re-drying, the glass transition temperature is smaller than that measured on virgin (unaged) samples. The polymer material recovers only half of its virgin properties, therefore degradation takes place during aging. Tg decrease is not Table 2 Aging conditions for thermo-oxidation tests under pure oxygen.
2% 0% 0
200
400
600
800
1000
Distance from the edge (µm) Fig. 11. Experimental indentation modulus ratio from the edge to the centre of samples.
VO1-1 VO1-2 VO1-3 VO2-1 VO2-2 VO2-3
O2 (%)
Temperature (°C)
Pressure (bar)
Time (h)
100 100 100 100 100 100
70 70 70 70 70 70
2 2 2 3 3 3
120 255 420 120 255 420
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reversible and the virgin value of glass transition temperature is not recovered upon re-drying. Tensile tests performed on virgin and aged samples show polymer network embrittlement during aging and persistent after desorption. Irreversible Tg decrease and polymer embrittlement may be related to thermo-oxidation phenomena taking place during humid aging [19]. UMI tests have shown a monotonous and irreversible increase of the EIT from the edge to the centre of humid aged and re-dried specimens and the occurrence of significant gradient toward the centre of the samples. All these results allow concluding that resin macroscopic properties changes are related to thermal aging due to matrix thermo-oxidation. In the next chapter, the occurrence of resin oxidation phenomena at 70 °C is validated.
Not aged
3b 120h
3b 255h
3b 420h
2b 120h
2b 255h
2b 420h
Fig. 13. Virgin and thermo-oxidized aged samples (70 °C). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
5. Occurrence of matrix thermo-oxidation 5.1. Protocol Since oxidation is a thermally activated phenomenon, the aim of this chapter is to verify the occurrence of thermo-oxidation of the polymer network at 70 °C. Accelerated thermo-oxidation aging tests have been carried out under pure oxygen environment at different pressures [20,21]. In order to accelerate the oxidation mechanisms, the oxygen pressure has been increased to 2 bar and 3 bar, table 2 resumes the different aging conditions. During aging, gravimetric measurements have been made with an analytical Sartorius YDK01 balance (precision up to 105g) and UMI tests have been carried out on aged samples in order to verify the occurrence of a property gradient similar to that observed on humid aged samples.
2b / 120h / 70°C / 100% O2
2b / 255h / 70°C / 100% O2
3b / 120h / 70°C / 100% O2
2b / 420h / 70°C / 100% O2
5.2. Results
5.2.2. Optical observations Fig. 13 illustrates the color of virgin and thermo-oxidized samples. Samples have the tendency to become brown dark with aging. Color changes under pure oxygen environment are similar to those observed in humid aged samples.
3b / 420h / 70°C / 100% O2
3b / 255h / 70°C / 100% O2
Fig. 14. Samples aged at 70 °C under a thermo-oxidative environment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
18% 16% 14%
ΔE (%)
5.2.1. Gravimetric tests Three samples with same geometry have been placed in an oven and gravimetric measures have been carried out. Polymer mass uptake increases linearly with the square root of time (Fig. 12). Increasing the aging pressure from 2b to 3b of oxygen, increases the mass uptake by around 36%. Oxygen diffusion/reaction takes place within the polymer macromolecular network at 70 °C, though it is not easy to decouple by this test the diffusive phenomena from the reaction ones.
12%
02 (%)
N2 (%)
P (bar)
0
0
0.03
t (h) 0
21
79
1
1500 dry
100
0
2
120
10%
100
0
2
255
8%
100
0
2
420
100
0
3
120
100
0
3
255
100
0
3
420
6% 4% 2%
0,4
Mass uptake (%)
0%
3bar 2bar
0,35
0
200
400
600
800
1000
Distance from the edge (µm)
0,3
M(t) = 0,0175* h1/2 R2 = 0,9856
0,25
Fig. 15. Experimental indentation modulus ratio from the edge to the centre of samples.
0,2 0,15
M(t) = 0,0111* h1/2 R2 = 0,9934
0,1 0,05 0 0
5
10
15
20
Root time (h1/2 ) Fig. 12. Experimental mass uptake curves for different thermo-oxidative aging conditions.
Samples have been cut and coated according to the same protocol employed for humid aged samples in order to verify the occurrence of color gradient from the edges to the heart, as shown in Fig. 14. Color gradients are evident on almost all tested samples, the color intensity and the thickness of the dark layer increase with increasing time or pressure aging. In particular, samples aged at 70 °C under 2 bar of oxygen during 120 h exhibit color intensity and gradient similar to those observed on humid aged samples placed at 70 °C and 85%HR during 1500 h.
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5.2.3. Local mechanical properties Fig. 15 shows that increasing the parameters of the thermo-oxidative aging (time and O2 pressure) increases the property gradient from the sample edge to the heart. In samples aged under 2 bar of oxygen during 120 h the oxidized layer is similar to that obtained on humid aged samples placed (70 °C/85%HR/1500 h) and re-dried. Accordingly, samples placed in 3b oxygen environment at 70 °C during 420 h exhibit an oxidized layer of about 600 lm and the EIT modulus close to the edge increases by around 16–17%. The existence of such a gradient confirms the existence of oxidation phenomena at 70 °C within the studied polymer network. 5.3. Discussion Color gradient observations and UMI tests allow confirming the existence of oxidation mechanism at 70 °C within the studied epoxy network. Similar gradients have been obtained for samples aged at 70 °C under 2 bar of oxygen and humid aged samples placed in ambient air during 1500 h, allowing concluding that the two aging conditions produce similar effects. However, in order to better understand the role of both phenomena during hygrothermal aging in atmospheric air, a protocol allowing the uncoupling of water diffusion and thermo-oxidation will be set up. 6. Conclusion This paper focused on the study of hygrothermal behavior of an epoxy/amine network employed for the realization of composite parts for aircraft applications at high temperatures (<120 °C) in humid environment. Water diffusion anomalies have been noted with respect to the Fick’s diffusion law. Resin sample mass (small thickness, about 2 mm) did not reach a fully saturated state when exposed under different humid aging conditions, even after around 5000 h. The occurrence of anomalous behavior does not allow identifying clearly all the parameters governing the water diffusion process. In order to investigate the possible sources of deviation from linear behavior, DMA, tensile and UMI tests have been carried out allowing concluding that thermo-oxidation phenomena take place during humid aging under atmospheric air. In particular, it is noted that: – Polymer color changes take place during humid aging, passing from yellow to brown after 1500 h aging and dark brown after 4700 h aging at 70 °C or 1500 h at 90 °C. A color gradient is visible on samples edge, from the surface to the centre along 300 lm after 1500 h of aging at 70 °C or 90 °C and drying. – A small residual mass fraction (0.07–0.18%) is measured in re-dried polymer samples. – Irreversible glass transition temperature decrease is observed during hygrothermal aging (70 °C/85%HR/1500 h) (10 °C after 1500 h of aging, only 5 °C were recovered after drying). – Resin embrittlement is measured due to humid aging (55% decrease of failure strain after 1500 h), persisting also upon re-drying. – Indentation modulus increase is measured close to the surface exposed to the environment, revealing the occurrence of a property gradient from the edge to the sample centre. In order to confirm the occurrence of thermo-oxidation phenomena, accelerated (2 bar and 3 bar of pressure) thermooxidative aging tests under pure oxygen at 70 °C have been carried out. It is noted that:
– Resin color changes similar to those observed on humid aged samples take place in oxidized samples; the color intensity increases with aging time or pressure increases. – A color gradient take place along the sample thickness, as observed on humid aged samples. – Indentation modulus values increase at the sample surfaces and a property gradient from the surface to the centre is measured. – A time/pressure equivalence between humid aged and thermooxidized samples has been found. All these results confirm the occurrence of coupling between humid and thermo-oxidation phenomena during hygrothermal aging under ambient air at 70 °C and 90 °C. An experimental protocol will be set up to uncouple phenomena and will be the subject of a forthcoming paper. References [1] Weitsman YJ, Editors-in-Chief: Anthony K, Carl Z. 2.11 – Effects of fluids on polymeric composites—a review. Comprehensive Composite Materials. Oxford: Pergamon; 2000. p. 369–401. [2] Fick A. On liquid diffusion. J Membr Sci 1995;100(1):33–8. [3] Gates TS. Durability assessment of polymeric composites for high speed civil transport. Recent Dev Durability Anal Compos Syst 2000:387–92. [4] Weitsman Y. Stress assisted diffusion in elastic and viscoelastic materials. J Mech Phys Solids 1987;35(1):73–93. [5] Weitsman Y. A continuum diffusion-model for viscoelastic materials. J Phys Chem 1990;94(2):961–8. [6] Valancon C, Roy A, Grandidier JC. Modelling of coupling between mechanics and water diffusion in bonded assemblies. Oil Gas Sci Technol – Revue D Ifp Energies Nouvelles 2006;61(6):759–64. [7] Leger R, Roy A, Grandidier JC. Non-classical water diffusion in an industrial adhesive. Int J Adhes Adhes 2010;30(8):744–53. [8] Thominette F, Gaudichet-Maurin E, Verdu J. Effect of structure on water diffusion in hydrophilic polymers. Diffus Solids Liquids: MASS DIFFUSION 2006;258–260:442–6. [9] El Yagoubi J, Lubineau G, Roger F, Verdu J. A fully coupled diffusion–reaction scheme for moisture sorption–desorption in an anhydride-cured epoxy resin. Polymer 2012;53(24):5582–95. [10] Tcharkhtchi A, Bronnec PY, Verdu J. Water absorption characteristics of diglycidylether of butane diol-3,5-diethyl-2,4-diaminotoluene networks. Polymer 2000;41(15):5777–85. [11] Pierron F, Poirette Y, Vautrin A. A novel procedure for identification of 3D moisture diffusion parameters on thick composites: theory, validation and experimental results. J Compos Mater 2002;36(19):2219–43. [12] Carter HG, Kibler KG. Langmuir-type model for anomalous moisture diffusion in composite resins. J Compos Mater 1978;12(1):118–31. [13] Piccinini E, Gardini D, Doghieri F. Stress effects on mass transport in polymers: a model for volume relaxation. Compos A – Appl Sci Manuf 2006;37(4):546–55. [14] Ferrari MC, Piccinini E, Baschetti MG, Doghieri F, Sarti GC. Solvent-induced stresses during sorption in glassy polycarbonate: experimental analysis and model simulation for a novel bending cantilever apparatus. Ind Eng Chem Res 2008;47(4):1071–80. [15] De’Nève B, Shanahan MER. Water absorption by an epoxy resin and its effect on the mechanical properties and infra-red spectra. Polymer 1993;34(24):5099–105. [16] Colin X, Audouin L, Verdu J. Determination of thermal oxidation rate constants by an inverse method. Application to polyethylene. Polym Degrad Stab 2004;86(2):309–21. [17] Colin X, Audouin L, Verdu J. Kinetic modelling of the thermal oxidation of polyisoprene elastomers. Part 1: Unvulcanized unstabilized polyisoprene. Polym Degrad Stab 2007;92(5):886–97. [18] Tsotsis TK, Keller S, Bardis J, Bish J. Preliminary evaluation of the use of elevated pressure to accelerate thermo-oxidative aging in composites. Polym Degrad Stab 1999;64(2):207–12. [19] Rasoldier N, Colin X, Verdu J, Bocquet M, Olivier L, Chocinski-Arnault L, et al. Model systems for thermo-oxidised epoxy composite matrices. Compos A – Appl Sci Manuf 2008;39(9):1522–9. [20] Delobelle P, Guillot L, Dubois C, Monney L. Photo-oxidation effects on mechanical properties of epoxy matrixes: Young’s modulus and hardness analyses by nano-indentation. Polym Degrad Stab 2002;77(3):465–75. [21] Lafarie-Frenot MC, Grandidier JC, Gigliotti M, Olivier L, Colin X, Verdu J, et al. Thermo-oxidation behaviour of composite materials at high temperatures: a review of research activities carried out within the COMEDI program. Polym Degrad Stab 2010;95(6):965–74.
Contents lists available at ScienceDirect
Composites: Part A journal homepage: www.elsevier.com/locate/compositesa
Evidence of thermo-oxidation phenomena occurring during hygrothermal aging of thermosetting resins for RTM composite applications A. Simar a,b, M. Gigliotti a,⇑, J.C. Grandidier a, I. Ammar-Khodja b a b
Department of Physics and Mechanics of Materials, Institut P’ CNRS, Université de Poitiers, ENSMA, UPR 3346, Chasseneuil du Poitou, France Materials and Process Department, Aircelle, groupe Safran, Gonfreville l’Orcher, France
a r t i c l e
i n f o
Article history: Received 13 March 2014 Received in revised form 4 July 2014 Accepted 7 July 2014 Available online 18 July 2014 Keywords: A. Theromosetting resin B. Environmental degradation C. Analytical modelling D. Mechanical testing
a b s t r a c t This paper focuses on the humid aging of a high temperature thermosetting resin employed for the realization of aircraft parts manufactured by the RTM process. Accelerated humid aging at several different temperatures (40–90 °C) and relative humidity values (50–100%) has been carried out by means of gravimetric tests. Anomalous behavior with respect to Fick’s diffusion law and a color gradient from the edge to the center of the samples has been observed. Drying of samples after aging and mechanical tests on aged and dried samples have shown the existence of irreversible phenomena taking place during humid ageing: uniform tensile tests revealed resin embrittlement, Ultra Micro Indentation tests allowed measuring a gradient of properties from the surface to the centre of the samples. This behavior – associated to the color changes observed on aged samples – indicates that oxidation phenomena take place during hygrothermal aging. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Thermosetting polymer resins are currently employed for the realization of composite parts with carbon fiber fabrics on structural parts in ‘‘warm’’ aircraft areas and, since new manufacturing processes are under development, complex geometrical parts made of composite materials can be effectively produced. Among existing techniques, resin injection «RTM» (Resin Transfer Molding) is perhaps the more widespread in aircraft and automotive industries. During their life, these new materials and aircraft structures are exposed to high and low temperature simultaneous cycles in dry and wet environments. In order to test the durability of such materials, accelerated aging can be carried out. Among others, hygrothermal aging is often tested on composite materials. Water modifies polymer matrix properties and has an impact on composite material properties. Weitsman [1] listed the main difficulties related to the exhaustive understanding of water aging mechanisms and indicated water diffusion anomalies in the matrix with respect to the classical Fick’s law [2]. In general, water uptake is a reversible phenomenon which creates residual stresses and material properties degradation [1,3,4], including decrease of the glass transition temperature and plasticization, that is, decrease of the ⇑ Corresponding author. Tel.: +33 5 49 49 83 40. E-mail address: [email protected] (M. Gigliotti). http://dx.doi.org/10.1016/j.compositesa.2014.07.007 1359-835X/Ó 2014 Elsevier Ltd. All rights reserved.
failure stress with increase of failure strain, particularly in organic resins. Anomalies with respect to the classical Fick’s diffusion law can be ascribed to chemical reactions taking place during humid aging [8,9]. Tcharkhtchi et al. [10] compared different amine/epoxy networks and highlighted water uptake increase when hydroxyl molar groups are present: in particular water sorption process in these networks is governed by two phases whose first is physical and the second is chemical. El Yagoubi et al. [9] have shown the importance of studying material desorption after humid aging to verify the mechanism reversibility during pseudo-Fickian diffusion, that is to say containing saturation stages on long time. They have also shown that non linear diffusion of epoxy/amine network could be due to a hydrolysis reaction. Gates et al. [3] highlighted the complexity of the physical–chemical mechanisms leading to humid aging: multi-physical couplings are function of the studied polymer network and are not completely identified. Some other irreversible phenomena taking place during water transport in polymers have been described in [5,6]. These works put into evidence the necessity to develop multi-physical approaches since diffusion phenomenon can be accelerated by mechanical stresses and, conversely, the elastic and visco-elastic response of materials can be influenced by diffusion. These works point out the challenges which are related to numerical simulation of multi-physical coupled mechanisms, and the necessity to
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enlarge this research field. In a slightly different context, Léger et al. [7] showed the interest of a multi-physical approach for the prediction of the aging of bonded joints in liquid environment at 70 °C. Combining the effect of temperature and humidity on structural parts, the resulting behavior becomes complex since property gradients arise. In RTM composite materials for novel applications (warm temperature) resins will be solicited under high temperatures and ambient humidity conditions. The question whether a high glass transition temperature matrix will be able to withstand with such conditions (close to use conditions) is then crucial. In particular appropriate knowledge of the behavior under humidity and ‘‘high’’ temperature conditions for this new class of matrices must be gained. The purpose of this paper is to broaden the hygrothermal behavior of a resin manufactured by RTM injection, under high temperature, for which few data are available in the literature. The material and the experimental setup is presented in Section 2, the diffusion anomalies will be characterized by gravimetric tests in Section 3. A specific experimental approach will be then developed and presented in Section 4 in order to understand the origin of the diffusion anomalies. 2. Material The epoxy/amine thermosetting resin from Hexcel Industry (commercial name RTM6) is employed for the realization of aircraft structural parts made by carbon/epoxy composites materials by the RTM (Resin Transfer Molding) injection process, with high mechanical and thermal properties. The network is composed by Tetraglycidyl methylene dianiline (TGMDA or TGDDM) prepolymer and of two amine hardener: 4,40 -Methylenebis(2,6-Diethylaniline) with commercial name MDEA and 4,40 -Methylenebis (2-Isopropyl6-Methylaniline) with commercial name M-MIPA. Neat resin plates have been produced by contact moulding from a liquid resin mix and polymerized in a Memmert ventilated oven after resin vacuum degassing (30 mbar at 80 °C). The resin cure cycle is composed by a 2 h dwell at 180 °C and a temperature ramp between 1 and 2 °C/min. After cutting, rectification and drying, samples have been stocked in a vacuum oven (30 mbar) at 70 °C to extract volatile substances initially present in the polymer. 3. Gravimetric test 3.1. Tests Several different hygrothermal aging tests have been carried out to evaluate the temperature (35 °C, 70 °C and 90 °C) and of relative humidity (50%, 85%, 90% and 100% in immersion) on the water diffusion process within the resin material. The weight of initially dry polymer samples with same shape and dimensions (30 mm * 10 mm * 2 mm, three samples for each condition) has been monitored by a Sartorius YDK01 analytical balance (with the precision of the order of 105 g). Hygrothermal aging has been carried out in a SECASI SLH 200 climatic chamber. Special attention has been paid to package them in waterproof recipient in order to avoid any contamination. This manipulation is made at room temperature and during a short period (10–20 min), so that water diffusion, which is a thermally activated, is poorly affected by these conditions. The polymer mass increases/decreases as a function of time during a sorption/desorption test (at 70 °C 85% HR) is presented in Fig. 1, for an average of three samples with the same shape (30 mm * 10 mm * 2 mm). To estimate the mechanisms reversibility, samples have been dried, after humid aging, at 70 °C under vacuum (30 mbar). Since samples are thin, water diffusion is
Sorption
Desorption
2,5
Water mass (%)
176
2 1,5 1
70°C - 85%HR
70°C - 0%HR
0,5 0 0
1000
2000
3000
Time (h) Fig. 1. Experimental sorption–desorption curve – 70 °C/85%HR – average value of 4 samples (20 mm * 10 mm * 2 mm).
essentially unidirectional along the thickness direction. Scatter of measures is small (1–4% maximum deviation compared to the average of four curves) and can be hardly appreciated in the figures. It can be noted that water diffusion inside resin rapidly move away from a Fickian process. For this configuration, mass uptake saturation cannot be observed, and during desorption a residual 0.1% water mass is measured : this observation is confirmed for every sample dried after aging between 70 °C and 90 °C for which a residual mass uptake between 0.07% and 0.18% is measured. This proves the presence of non reversible mechanisms occurring during humid aging under ambient air and immersed conditions. The polymer mass increases as a function of the square root of time, divided by the sample thickness, is presented in Fig. 2, for several test configurations. Scatter of measures is small (1–4% maximum deviation compared to four curves average) and can be hardly appreciated in the figures. It can be noted that water diffusion inside resin rapidly move away from a Fickian process. More precisely, the first part of the curve is a linear function of the square root of time for each aging condition. For each tested configurations, mass uptake saturation cannot be seen, even after 5000 h aging. This fact provides evidence of anomalous behavior with respect to the Fick’s law of diffusion. In this context, it is not possible to identify in a suitable manner mass uptake at saturation state and a diffusion coefficient. To quantify in a first approximation the water diffusion kinetics inside the polymer, a simplified analytical solution of Fickian diffusion law [11] has been employed, namely 1 Mt 8 X eðQtÞ ¼1 2 M1 p n¼0 ð2n þ 1Þ2
! Q ¼p
2
2 ! 2n þ 1 D e
ð1Þ
where Mt is the water mass uptake at time t, M1 is a fictive mass uptake at saturation, D is the diffusion coefficient, and e is the sample thickness. The fictive mass uptake at saturation, M1, has been defined as the crossing point between the initial and the final slopes of the experimental points. The simplified solution, Eq. (1), is based on the fundamental hypothesis that water diffusion is isotropic and takes place along the thickness direction. These two hypotheses are acceptable due to the nature of the resin material and to the choice of the samples shape (sample thickness is small compared to length and width). By the employment of a fictive mass uptake, as defined above, a diffusion coefficient equal to 0.002 mm2/h at 35 °C, 0.02 mm2/h at 70 °C and 0.03 mm2/h at 90 °C, respectively, can be identified. A factor of around 10 between the diffusion coefficient at 35 °C and the one at 70 °C can be appreciated. It is concluded that a diffusion
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Fickian simulation
3,5
Linear regression
Water mass uptake (%)
3
2,5
2
1,5 T (°C)
HR (%)
D (mm²/h)
M∞/M0 (%)
70
85
0.02
2.23
35
90
0.003
2.49
70
50
0.02
1.31
90
IM
0.03
2.84
70
IM
0.02
2.84
1
0,5
0 0
2
4
6
8
10
12 1/2
Root time/Thickness (h
14
16
18
/mm)
Fig. 2. Experimental and numerical sorption curves for various humid conditioning.
coefficient can be effectively calculated, and that its dependency upon the temperature is consistent. Looking at the experimental points at the end of the aging process, a linear correlation can be identified; the slopes of the curves (indicated by the symbol a) for each condition are reported in Table 1. The same table illustrates the R coefficients associated to the linear regression. By increasing the aging temperature, the slope of the curves increases while it is nearly horizontal for the 35 °C aging. When the aging temperature increases, the slope coefficient increases as well, this proves that the diffusion anomaly is thermally activated. Then, under immersed conditions at 70 °C, the slope is approximately twice higher than that observed for 70 °C and 85%HR aging. 3.2. Discussion Although temperature is substantially lower than the glass temperature (around 140 °C lower than, which is equal to around 233 °C), the resin does not possess Fickian behavior and a saturated state cannot be reached. Water uptake keeps going after 5000 h, and the extent of the anomalous behavior is amplified by temperature. A small amount of water (between 0.07% and 0.18%) is kept by the network during desorption and the immersion in liquid water accelerates the process. The anomalous behavior with respect to the classical Fick’s law can be the consequence of several physical phenomena which are: – The non linearity of polymer behavior. Weitsman [1] presents the principal difficulties linked to the exhaustive understanding of humid aging phenomena and proposes a non exhaustive list of water diffusion anomalies in polymers compared to the Fick’s Table 1 Sorption slope coefficients at the end of aging.
4 s N
T (°C)
HR (%)
a (103) (mm/h1/2)
R2
70 35 70 90 70
85 90 50 IM IM
0.02 0.001 0.02 0.046 0.041
0.94 – 0.96 0.99 0.99
law [2]. Carter and Kibler [2] describe water diffusion anomaly in polymers by employing a Langmuir-type diffusion model: this model postulates the existence of two diffusing species (bound and free species) whose diffusion mechanism is based on a trapping statistical process [12]. Piccinini et al. [13,14] explain diffusion anomalies by the polymer volume relaxation generated by volatile substances sorption via a linear viscoelastic interaction. Leger et al. [7] show that the anomalous nonlinear water diffusion phenomena in epoxy adhesive are due to the presence of cavities generated during humid aging. – Hydrolysis of the polymer network. Tcharkhtchi et al. [10] put into evidence a constant mass uptake during humid aging of an epoxy/amine resin. In their study, a water diffusion/reaction model of a hydrolysable polymer matrix is described. This model gives results which are close to those given by the Langmuir’s model, though the physical meaning behind the two models is different. El Yagoubi et al. [9] put forward the approach by Tcharkhtchi et al. [10] hypothesizing that the hydrophilicity of the reaction products between water and resin carries out non linear water diffusion in the epoxy/anhydride network. The competition between diffusion and reaction mechanisms in the polymer modifies the polymer diffusive behavior, solubility and diffusivity are affected by the number of hydrolysable sites. Thus, according to this interpretation, an incomplete cure presents more hydrolysable sites promoting polymer hydrolysis phenomena [9]. De’Nève and Shanahan [15] show that the epoxy/ dicyandiamide resin behavior results from a combination of polymer plasticity and chemical modification. The decrease of Young modulus at rubbery state and chemical modifications detected by FTIR (Fourier Transform InfraRed spectroscopy) analysis testifies macromolecular chain scission. – A thermal activated phenomenon under atmospheric air like thermo-oxidation of the polymer network. Several authors have shown that oxidation phenomena [16–18], may promote an increase of the network mass during time: more precisely, a mass uptake is essentially observed when the oxygen diffusion inside the matrix dominates on chemical reaction. In order to decide about the possible origin of the anomalous behavior observed for the material of the present study, a test
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approach combining several characterization tests has been formulated. 4. Effect of a humid environment on the resin properties 4.1. Approach An optical control is carried out on the material in order to establish the evolution of the resin color during aging: the samples are semi-transparent so that network changes due to chemical interactions can be highlighted by color changes and are easily detectable. Physical and thermal macroscopic properties of the resin before and during humid aging are then characterized via dynamical mechanical analysis (DMA) in 3 points bending flexure mode. These tests are made on resin samples placed in a humid controlled chamber (70 °C and 85%HR), for different conditioning times. For this test, a total strain equal to 0.02% is imposed at a frequency of 1 Hz: an initial force of 0.01 N is imposed in order to avoid sample warping before testing. During the test, the temperature is increased from 25 °C to 260 °C at a speed equal to 2 °C/min to highlight the transition from glassy to rubbery state of the polymer: the tan(d) peak corresponding to the tangent of the loss angle (ratio between the loss modulus E00 and the storage modulus E0 , representing the ratio between wasted and stocked energy) characterizes the transition from glassy to rubber state. The macroscopic mechanical properties of the polymer are measured by uniform tensile tests under control displacement. Traction tests are performed on different humid aged samples in order to quantify the modification of the mechanical properties of the polymer in the presence of water. The samples dimension is 60 mm * 10 mm * 4 mm, the experimental setup is characterized by a force cell up to 5 kN, the imposed displacement speed is equal to 1 mm/min until sample break. Longitudinal and transverse displacements are followed by a CCD (Charge-Coupled Device) dynamical camera and processed by the related software: white markers on the surface sample are employed for strain measurements. DMA analysis and uniform tensile test are capable of detecting the macroscopic mechanical behavior of the polymer material. Thus, in order to characterize possible properties gradient induced within the material by the diffusion process, Ultra Micro Indentation (UMI) tests are performed on aged and re-dried resin samples to the experimental protocol illustrated in Fig. 3: samples are coldcoated and polished up to 1 lm into exhibit acceptable planarity and surface preparation before the test. The experimental setup consists in a Fisherscope HC100 apparatus equipped with a Vickers pyramidal indenter. During the test, the charge is increased to its maximum value (Fmax = 5 mN) at a rate equal to 20 s. The maximal force is kept for one second and the unloading step is carried out at a rate equal to 20 s. The indentation elastic modulus calculated during the unloading phase at constant rate is obtained starting from the shape of the unloading curve according to Eq. (2)
EIT ¼
pffiffiffiffi dF p pdh ffiffiffiffiffi 2b Ap
ð2Þ
in which F is the charge, h represents indentation depth, b is a corrective parameter, Ap is the contact area between the indenter and the sample projected on a plane perpendicular to the indenter axis. 4.2. Results 4.2.1. Optical observations During humid aging under atmospheric air, a color change of the polymer from around yellow to brown dark, more or less pronounced depending on the aging time and temperature has been observed: when dried (cf. Fig. 4) the samples keep their color change. This is the proof of a non reversible mechanism occurring during aging and affecting the network and its optical properties. In order to check the existence of a possible color gradient along the sample thickness, some specimens have been cut and coated after drying. Fig. 5 shows that the virgin sample has kept its characteristic yellow along the whole thickness. Humid aged samples (1500 h at 70 °C/85%HR) keep exhibiting a color gradient along the thickness after drying, and more specifically a brown layer of about 300 lm on the sample surface. This color change could be related to hydrolysis or oxidation. However, since the color change is limited to an external layer of a few hundred of lm, the occurrence of hydrolysis can be discarded since it would affect the whole sample thickness. On the other hand, the occurrence of oxidation phenomena is suspected. 4.2.2. Physical and thermal properties Three point bending dynamical mechanical analysis (DMA) tests in flexure mode have been carried out on aged resin samples (at 70 °C, 85%HR for different aging times). A decrease of the glass transition temperature (Tg) with aging from 233 °C (for a virgin sample) to 223 °C (for a sample aged 1500 h with 2.6% of water mass uptake) is measured, cf. Fig. 6. After an additional 3200 h humid aging and an additional water mass uptake of 0.1%, the glass transition temperature keeps decreasing of around 7 °C. Tg depletion seems to result from the combination of two processes: a fast one obviously due to the resin plasticization by absorbed water and a slower one presumably linked to a (partially irreversible) resin degradation independent of water concentration, whose mechanism remains to establish. When samples are dried (1500 h at 70 °C and 85%HR), the initial glass transition temperature is not recovered (a Tg equal to 228 °C is measured compared to 233 °C for not aged samples): this can be partially explained by considering that the sample has not dried completely (a residual water weight equal to 0.1% is measured), but the Tg ‘‘gap’’ is not compatible with a 0.1% residual water mass uptake in the network. These results put again into evidence the occurrence of non reversible phenomena occurring during hydrothermal aging. Fig. 8 presents the storage modulus E0 characterized in 3 points bending flexure mode by DMA analysis for several samples placed
Indentation prints
Fig. 3. Sample preparation before «UMI» test. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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90°C / 95%HR 1500h
70°C / 85%HR 1500h
Not aged Dry
70°C / Immersion 800h
90°C / Immersion 800h
Fig. 4. Aged samples. From the left to the right: 90 °C/95%HR/1500 h; 70 °C/85%HR/1500 h; not aged; 70 °C/Immersed in liquid water/800 h; 90 °C/Immersed in liquid water/ 800 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Tg = -4,29 log(t) + 234,7 R2 = 0,8743
240
220
Linear 70°C - 85%HR 70°C - 85%HR dry
200
Fig. 7. Experimental glass transition temperature as a function of time (logarithmic scale).
Fig. 5. (a) Not aged; (b) 70 °C/85%HR/1500 h; and (c) 90 °C/95%HR/1500 h. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
1,2
Tan Delta
1 0,8
Time (h)
Mt/M0 (%)
Tg (°C)
0
0
233
50
2
230
1500
2.6
223
4800
2.7
216
1500 dry
0.1
228
Storage modulus (Mpa)
3500
216 223 228 230 233
0,6 0,4
3000 2500 2000 1500 1000 500
0,2
Time (h)
Mt/M0 (%)
E* (Gpa)
0
0
3.25
50
2
3.0
1500
2.6
3.2
1500 dry
0.1
3.6
0 0 180
20 190
200
210
220
230
70
120
240
170
220
Temperature (°C)
Temperature (°C) Fig. 8. Experimental storage modulus for various humid aged samples. Fig. 6. Glass transition temperature measured by DMA tests on several humid aged samples.
Min value
100
Mean value
Max value
90 80
Stress (Mpa)
in climatic chamber (the legend illustrates modulus measures at 25 °C). E0 suffers from a 250 MPa sudden decrease after 50 h of humid aging and tends to re-increase after 1500 h aging, reaching a value close to that of a virgin sample. This is in contrast with the trend observed on Tg (Fig. 7) which decreases monotonically with time. When 1500 h aged samples are re-dried (with a residual 0.1% water mass uptake), E0 tends to increase and, at 25 °C, equals 400 MPa. This indicates the presence of several active phenomena during hygrothermal aging. Modulus decrease during the first phases of the humid aging indicates plasticization, the subsequent increase is related to chemical reactions which modify the polymer network and the glass transition temperature of dry samples. Similarly, the irreversible monotonic decrease of Tg with time testifies the occurrence of plasticization phenomena followed by chemical reaction effects.
70 60 Time (h)
50
Mt/M0 (%)
E (Gpa)
εr (%)
σr (Mpa)
Tg (°C)
40
0
0
3.25
4.7
93.5
233
30
2.5
0.5
2.44
4.5
85.8
233
50
2
2.46
3.6
70.7
230
20
1500
2.6
2.66
2.1
51.5
223
10
4800
2.7
2.71
1.6
47.1
216
1500dry
0.1
3.33
1.7
53.9
228
0 0
1
2
3
4
5
6
Strain (%) Fig. 9. Experimental tensile stress curves for several hygrothermally aged samples.
Failure stress (Mpa)
4 3 2 70°C - 85%HR
1
70°C - 85%HR dry
0 0
1
2
3
4
100
σ = -12,4 log(t) + 91,1 2
R = 0,9973
80 60 40
Linear 70°C - 85%HR
20
70°C - 85%HR dry
1
2
5
ε = -0,85 log(t) + 4,82
4
R2 = 0,9905
3 2
Linear 70°C - 85%HR
1
70°C - 85%HR dry
0
0 0
Failure strain (%)
A. Simar et al. / Composites: Part A 66 (2014) 175–182
Young Modulus (Gpa)
180
3
4
0
log (t)
log (t)
1
2
3
4
log (t)
Fig. 10. Experimental Young’s modulus, stress and strain as a function of time (logarithmic scale).
4.2.3. Tensile tests on aged samples Tensile tests with imposed displacement rate have been made on different humid aging resin samples to quantify the change of elastic mechanical properties under traction, in the presence of water. Fig. 9 shows results obtained after tensile tests on samples aged in a climatic chamber (70 °C and 85%HR) for different aging times. The elastic modulus of aged samples decreases of around 0.18 GPa for a 5% water mass uptake. On the other hand, it is noted that the resin behavior becomes brittle due to hygrothermal aging since the failure strain decreases significantly as a function of the aging duration, up to 55% decrease after 1500 h aging. After 4800 h aging, the resin behavior is still brittle while the elastic modulus has approximately the same value than the initial one (Fig. 10). These results confirm the measures by DMA analysis. The elastic modulus of samples re-dried after 1500 h aging is around 2–3% higher than the average value measured on virgin samples. However, still for such samples, the resin behavior is quite brittle; these results confirm the significant impact of humid aging on the network behavior and the occurrence of irreversible phenomena within the polymer network due to the occurrence of chemical reactions. 4.2.4. Local mechanical properties Ultra Micro Indentation tests have been performed on the external edge (close to the external environment) of virgin and aged resin samples. By definition, the relative change of the elastic indentation modulus EIT with respect to its initial (virgin) value EIT0 is defined as:
DE ¼
EIT EIT 0 EIT 0
ð3Þ
In order to compare the property gradient of different samples Fig. 11 shows the values of DE as a function of the distance from the external edge exposed to the environment: the points
10% 70°C - 85%HR 90°C - 100%HR
ΔE (%)
8% 6% 4%
represent the average value of twelve individuals measures on two samples. The maximum standard deviation is around 0.2% on both series of measurements but this information is not presented in Fig. 11 to gain clarity. We observe a property gradient from the surface to the centre of humid aged and re-dried samples. This gradient spreads along around 300–400 lm from the external surface. EIT on the sample surface increases of about 5–6% for samples aged at 70 °C and about 9–10% for samples aged at 90 °C. These results are in good agreement with the color gradient size observed visually on the samples edge. 4.3. Discussion Optical analyses carried out on resin samples before and after aging highlight polymer color changes during humid aging. Yellow samples become brown after 1500 h aging and dark brown after 4800 h. This persistence of color change after sample re-drying can be explained by the network chemical modifications induced by hydrolysis or oxidation phenomena. The existence of a color gradient from the surface (exposed to the environment) to the centre of the sample along an affected zone around 300 lm allows suspecting that it is oxidation – and not hydrolysis – which come into play. In fact, since resin water mass uptake has reached a quasi-stabilized state after 1500 h aging (see Fig. 2), water concentration gradients should be low: consequently color changes should affect the whole sample volume, not only its edges, if hydrolysis would play a role. On the contrary, thermo-oxidation phenomena do affect the external samples surfaces only and are consistent with the observed EIT gradients [16–18]. Another interesting observation concerns the sample desorption behavior: upon re-drying a 1500 h aged sample the initial polymer mass cannot be recovered. A small residual mass is kept by the sample, contradicting absorption/desorption reversibility: this residual mass can be due to oxidation phenomena, which has demonstrated inducing small polymer mass increase in several literature case studies [16–18]. Glass transition temperature changes have been measured during hygrothermal aging as a function of the polymer water mass uptake, not decreasing linearly with respect to the absorbed water quantity. Upon re-drying, the glass transition temperature is smaller than that measured on virgin (unaged) samples. The polymer material recovers only half of its virgin properties, therefore degradation takes place during aging. Tg decrease is not Table 2 Aging conditions for thermo-oxidation tests under pure oxygen.
2% 0% 0
200
400
600
800
1000
Distance from the edge (µm) Fig. 11. Experimental indentation modulus ratio from the edge to the centre of samples.
VO1-1 VO1-2 VO1-3 VO2-1 VO2-2 VO2-3
O2 (%)
Temperature (°C)
Pressure (bar)
Time (h)
100 100 100 100 100 100
70 70 70 70 70 70
2 2 2 3 3 3
120 255 420 120 255 420
181
A. Simar et al. / Composites: Part A 66 (2014) 175–182
reversible and the virgin value of glass transition temperature is not recovered upon re-drying. Tensile tests performed on virgin and aged samples show polymer network embrittlement during aging and persistent after desorption. Irreversible Tg decrease and polymer embrittlement may be related to thermo-oxidation phenomena taking place during humid aging [19]. UMI tests have shown a monotonous and irreversible increase of the EIT from the edge to the centre of humid aged and re-dried specimens and the occurrence of significant gradient toward the centre of the samples. All these results allow concluding that resin macroscopic properties changes are related to thermal aging due to matrix thermo-oxidation. In the next chapter, the occurrence of resin oxidation phenomena at 70 °C is validated.
Not aged
3b 120h
3b 255h
3b 420h
2b 120h
2b 255h
2b 420h
Fig. 13. Virgin and thermo-oxidized aged samples (70 °C). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
5. Occurrence of matrix thermo-oxidation 5.1. Protocol Since oxidation is a thermally activated phenomenon, the aim of this chapter is to verify the occurrence of thermo-oxidation of the polymer network at 70 °C. Accelerated thermo-oxidation aging tests have been carried out under pure oxygen environment at different pressures [20,21]. In order to accelerate the oxidation mechanisms, the oxygen pressure has been increased to 2 bar and 3 bar, table 2 resumes the different aging conditions. During aging, gravimetric measurements have been made with an analytical Sartorius YDK01 balance (precision up to 105g) and UMI tests have been carried out on aged samples in order to verify the occurrence of a property gradient similar to that observed on humid aged samples.
2b / 120h / 70°C / 100% O2
2b / 255h / 70°C / 100% O2
3b / 120h / 70°C / 100% O2
2b / 420h / 70°C / 100% O2
5.2. Results
5.2.2. Optical observations Fig. 13 illustrates the color of virgin and thermo-oxidized samples. Samples have the tendency to become brown dark with aging. Color changes under pure oxygen environment are similar to those observed in humid aged samples.
3b / 420h / 70°C / 100% O2
3b / 255h / 70°C / 100% O2
Fig. 14. Samples aged at 70 °C under a thermo-oxidative environment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
18% 16% 14%
ΔE (%)
5.2.1. Gravimetric tests Three samples with same geometry have been placed in an oven and gravimetric measures have been carried out. Polymer mass uptake increases linearly with the square root of time (Fig. 12). Increasing the aging pressure from 2b to 3b of oxygen, increases the mass uptake by around 36%. Oxygen diffusion/reaction takes place within the polymer macromolecular network at 70 °C, though it is not easy to decouple by this test the diffusive phenomena from the reaction ones.
12%
02 (%)
N2 (%)
P (bar)
0
0
0.03
t (h) 0
21
79
1
1500 dry
100
0
2
120
10%
100
0
2
255
8%
100
0
2
420
100
0
3
120
100
0
3
255
100
0
3
420
6% 4% 2%
0,4
Mass uptake (%)
0%
3bar 2bar
0,35
0
200
400
600
800
1000
Distance from the edge (µm)
0,3
M(t) = 0,0175* h1/2 R2 = 0,9856
0,25
Fig. 15. Experimental indentation modulus ratio from the edge to the centre of samples.
0,2 0,15
M(t) = 0,0111* h1/2 R2 = 0,9934
0,1 0,05 0 0
5
10
15
20
Root time (h1/2 ) Fig. 12. Experimental mass uptake curves for different thermo-oxidative aging conditions.
Samples have been cut and coated according to the same protocol employed for humid aged samples in order to verify the occurrence of color gradient from the edges to the heart, as shown in Fig. 14. Color gradients are evident on almost all tested samples, the color intensity and the thickness of the dark layer increase with increasing time or pressure aging. In particular, samples aged at 70 °C under 2 bar of oxygen during 120 h exhibit color intensity and gradient similar to those observed on humid aged samples placed at 70 °C and 85%HR during 1500 h.
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5.2.3. Local mechanical properties Fig. 15 shows that increasing the parameters of the thermo-oxidative aging (time and O2 pressure) increases the property gradient from the sample edge to the heart. In samples aged under 2 bar of oxygen during 120 h the oxidized layer is similar to that obtained on humid aged samples placed (70 °C/85%HR/1500 h) and re-dried. Accordingly, samples placed in 3b oxygen environment at 70 °C during 420 h exhibit an oxidized layer of about 600 lm and the EIT modulus close to the edge increases by around 16–17%. The existence of such a gradient confirms the existence of oxidation phenomena at 70 °C within the studied polymer network. 5.3. Discussion Color gradient observations and UMI tests allow confirming the existence of oxidation mechanism at 70 °C within the studied epoxy network. Similar gradients have been obtained for samples aged at 70 °C under 2 bar of oxygen and humid aged samples placed in ambient air during 1500 h, allowing concluding that the two aging conditions produce similar effects. However, in order to better understand the role of both phenomena during hygrothermal aging in atmospheric air, a protocol allowing the uncoupling of water diffusion and thermo-oxidation will be set up. 6. Conclusion This paper focused on the study of hygrothermal behavior of an epoxy/amine network employed for the realization of composite parts for aircraft applications at high temperatures (<120 °C) in humid environment. Water diffusion anomalies have been noted with respect to the Fick’s diffusion law. Resin sample mass (small thickness, about 2 mm) did not reach a fully saturated state when exposed under different humid aging conditions, even after around 5000 h. The occurrence of anomalous behavior does not allow identifying clearly all the parameters governing the water diffusion process. In order to investigate the possible sources of deviation from linear behavior, DMA, tensile and UMI tests have been carried out allowing concluding that thermo-oxidation phenomena take place during humid aging under atmospheric air. In particular, it is noted that: – Polymer color changes take place during humid aging, passing from yellow to brown after 1500 h aging and dark brown after 4700 h aging at 70 °C or 1500 h at 90 °C. A color gradient is visible on samples edge, from the surface to the centre along 300 lm after 1500 h of aging at 70 °C or 90 °C and drying. – A small residual mass fraction (0.07–0.18%) is measured in re-dried polymer samples. – Irreversible glass transition temperature decrease is observed during hygrothermal aging (70 °C/85%HR/1500 h) (10 °C after 1500 h of aging, only 5 °C were recovered after drying). – Resin embrittlement is measured due to humid aging (55% decrease of failure strain after 1500 h), persisting also upon re-drying. – Indentation modulus increase is measured close to the surface exposed to the environment, revealing the occurrence of a property gradient from the edge to the sample centre. In order to confirm the occurrence of thermo-oxidation phenomena, accelerated (2 bar and 3 bar of pressure) thermooxidative aging tests under pure oxygen at 70 °C have been carried out. It is noted that:
– Resin color changes similar to those observed on humid aged samples take place in oxidized samples; the color intensity increases with aging time or pressure increases. – A color gradient take place along the sample thickness, as observed on humid aged samples. – Indentation modulus values increase at the sample surfaces and a property gradient from the surface to the centre is measured. – A time/pressure equivalence between humid aged and thermooxidized samples has been found. All these results confirm the occurrence of coupling between humid and thermo-oxidation phenomena during hygrothermal aging under ambient air at 70 °C and 90 °C. An experimental protocol will be set up to uncouple phenomena and will be the subject of a forthcoming paper. References [1] Weitsman YJ, Editors-in-Chief: Anthony K, Carl Z. 2.11 – Effects of fluids on polymeric composites—a review. Comprehensive Composite Materials. Oxford: Pergamon; 2000. p. 369–401. [2] Fick A. On liquid diffusion. J Membr Sci 1995;100(1):33–8. [3] Gates TS. Durability assessment of polymeric composites for high speed civil transport. Recent Dev Durability Anal Compos Syst 2000:387–92. [4] Weitsman Y. Stress assisted diffusion in elastic and viscoelastic materials. J Mech Phys Solids 1987;35(1):73–93. [5] Weitsman Y. A continuum diffusion-model for viscoelastic materials. J Phys Chem 1990;94(2):961–8. [6] Valancon C, Roy A, Grandidier JC. Modelling of coupling between mechanics and water diffusion in bonded assemblies. Oil Gas Sci Technol – Revue D Ifp Energies Nouvelles 2006;61(6):759–64. [7] Leger R, Roy A, Grandidier JC. Non-classical water diffusion in an industrial adhesive. Int J Adhes Adhes 2010;30(8):744–53. [8] Thominette F, Gaudichet-Maurin E, Verdu J. Effect of structure on water diffusion in hydrophilic polymers. Diffus Solids Liquids: MASS DIFFUSION 2006;258–260:442–6. [9] El Yagoubi J, Lubineau G, Roger F, Verdu J. A fully coupled diffusion–reaction scheme for moisture sorption–desorption in an anhydride-cured epoxy resin. Polymer 2012;53(24):5582–95. [10] Tcharkhtchi A, Bronnec PY, Verdu J. Water absorption characteristics of diglycidylether of butane diol-3,5-diethyl-2,4-diaminotoluene networks. Polymer 2000;41(15):5777–85. [11] Pierron F, Poirette Y, Vautrin A. A novel procedure for identification of 3D moisture diffusion parameters on thick composites: theory, validation and experimental results. J Compos Mater 2002;36(19):2219–43. [12] Carter HG, Kibler KG. Langmuir-type model for anomalous moisture diffusion in composite resins. J Compos Mater 1978;12(1):118–31. [13] Piccinini E, Gardini D, Doghieri F. Stress effects on mass transport in polymers: a model for volume relaxation. Compos A – Appl Sci Manuf 2006;37(4):546–55. [14] Ferrari MC, Piccinini E, Baschetti MG, Doghieri F, Sarti GC. Solvent-induced stresses during sorption in glassy polycarbonate: experimental analysis and model simulation for a novel bending cantilever apparatus. Ind Eng Chem Res 2008;47(4):1071–80. [15] De’Nève B, Shanahan MER. Water absorption by an epoxy resin and its effect on the mechanical properties and infra-red spectra. Polymer 1993;34(24):5099–105. [16] Colin X, Audouin L, Verdu J. Determination of thermal oxidation rate constants by an inverse method. Application to polyethylene. Polym Degrad Stab 2004;86(2):309–21. [17] Colin X, Audouin L, Verdu J. Kinetic modelling of the thermal oxidation of polyisoprene elastomers. Part 1: Unvulcanized unstabilized polyisoprene. Polym Degrad Stab 2007;92(5):886–97. [18] Tsotsis TK, Keller S, Bardis J, Bish J. Preliminary evaluation of the use of elevated pressure to accelerate thermo-oxidative aging in composites. Polym Degrad Stab 1999;64(2):207–12. [19] Rasoldier N, Colin X, Verdu J, Bocquet M, Olivier L, Chocinski-Arnault L, et al. Model systems for thermo-oxidised epoxy composite matrices. Compos A – Appl Sci Manuf 2008;39(9):1522–9. [20] Delobelle P, Guillot L, Dubois C, Monney L. Photo-oxidation effects on mechanical properties of epoxy matrixes: Young’s modulus and hardness analyses by nano-indentation. Polym Degrad Stab 2002;77(3):465–75. [21] Lafarie-Frenot MC, Grandidier JC, Gigliotti M, Olivier L, Colin X, Verdu J, et al. Thermo-oxidation behaviour of composite materials at high temperatures: a review of research activities carried out within the COMEDI program. Polym Degrad Stab 2010;95(6):965–74.