Description of Anomalous Water Absorption Behaviour of Woven Glass Reinforced Cyanate Ester Composites

Transcription

1 Plastic and Polymer Technology (PAPT) Volume 4, 2016 doi: /papt Description of Anomalous Water Absorption Behaviour of Woven Glass Reinforced Cyanate Ester Composites Panagiota Kelverkloglou 1, Evgenia Kollia 2, Antonis Vavouliotis 3, Vassilis Kostopoulos *4 Department of Mechanical Engineering & Aeronautics, University of Patras, University of Patras Campus Rio 26500, Greece 1 kelverkl@mech.upatras.gr; 2 kollia@mech.upatras.gr; 3 vavoul@mech.upatras.gr; *4 kostopoulos@mech.upatras.gr Abstract This work studies the water absorption behaviour of woven glass fibre reinforced cyanate ester composites used in the plenum of the air conditioning pack of aircraft's environmental control system. For absorption tests, samples were immersed in distilled water baths at four different temperatures: 40 C, 60 C, 75 C and 90 C. In general, the composites have shown strong non- Fickian behaviour. The progression of mass absorption is described analytically using a combination of Fickian diffusion and relaxation process. Thus for the temperatures of 40 C, 60 C and 75 C a dual stage model was used. At 90 C the material exhibits extreme non-fickian behaviour due to chemical ageing procedures which cannot be accurately modeled. From the mass control analysis, lots of information is provided for the correlation between the temperature and the diffusion coefficient or the start of the moisture equilibrium. Also, the regions of physical and chemical ageing can be clearly distinguished, giving a good profile of the material behaviour at elevated temperature. The dual stage model that is proposed precisely describes the behaviour of these materials at different temperatures up to 75 C. The chemical ageing that is taking place at 90 C creates another profile that should be investigated in a more detailed level than macro-scale. Keywords Anomalous Moisture Absorption; Diffusion Dual Stage Model; Diffusion in Composites Introduction Composite materials offer many advantages over conventional structural materials; this includes their high stiffness and strength to weight ratios, their resistance to chemical attack and their tailorability. Commonly, thermoset polymer matrices are used. The different types of polymer matrices are: bis-maleimide, epoxy, phenolic, polyester, polyimide, polyurethane and silicon. The thermoset that is used widely is epoxy, even though it can be compared in performance with bis-maleimide, which has very promising results in dielectrical application and extreme environments. Cyanate esters (CEs) form a family of thermosetting resins. Their characteristics are attractive for many applications based on composite materials. They perform impressive physical, electrical, thermal and processing properties required as long as they are used as matrix materials. Furthermore, they generally exhibit high glass transition temperature (Tg), good fracture toughness, excellent substrate adhesion, low shrinkage, low dielectric loss and low moisture uptake [1, 2]. For example in aeronautics and aerospace industries some application needs lightweight materials with high temperature service ceiling (due to their high Tg) and low shrinkage render CEs, and cyanate ester resin fulfill these specifications [3-4]. The key advantages of this type of resins are their stiffness, their dimensional stability under extreme environmental conditions like exposure in high temperatures and moisture, their good adhesive and electrical properties [5-6]. Moreover, the development of novel polymer formulations has allowed composites to find application in aggressive operational environments. The problem that still exists is the behaviour of these materials under extreme environmental conditions for example elevated temperatures and moisture absorption. The diffusion phenomena that are observed cannot be described always by the classic Fickian law. The non-fickian behaviour should be described and analyzed. The woven compounds that are often used as reinforcement exhibit good mechanical properties, but in many cases, their behaviour is limited by the environmental moisture gain. By the time that this moisture enters the compounds, residual hygroscopic stresses are induced due to swelling. These are factors that can significantly change the 92

2 Plastic and Polymer Technology (PAPT) Volume 4, mechanical properties of the material and lead many times to the creation of micro-cavities and micro-cracking in higher level. Mass absorption depends on the conditions of the exposure environment and the quality of the material and this can be a very simple or a really complicated procedure [7]. Crank illustrates the process in great detail and clearly distinguishes the classic Fickian diffusion from non-fickian behaviour [6]. This work is essential for understanding the mechanisms of absorption in glass reinforced polymer composites (GFRPs). Generally polymers exhibit 'anomalous' or 'non-fickian' behaviour. Deviations from Fickian behaviour are considered to be associated with the finite rates at which the polymer structure may change in response to the sorption of penetrant molecules. In general, the relaxation times of polymers have wide spectra, associated to micro-structural changes. This environmental loading that is described by the moisture absorption, which is the environmental degradation factor in this case combined with temperature, leads to physical and chemical ageing. Physical ageing is a reversible process and is associated with free volume evolution and the change in properties relative to an equilibrium state. Physical ageing is responsible for changes over time of modulus strength and ductility for polymer composites in the glassy region. Since most polymer composite structures are used in the glassy region of the polymeric matrix, physical ageing has an important impact on long-term durability of composites used in applications. Chemical ageing encompasses a range of environmentally driven degradation mechanisms which are irreversible such as cross-linking or chain-scission. Some of the basic chemical degradation mechanisms include thermal and hydrolytic ageing. The chemical composition of the polymer matrix will have a strong influence on the solubility and diffusion properties of small molecules in the polymer matrix [8]. Crank in his work in 1975 described more precisely the meaning of anomalous absorption and especially in polymers and polymer composites [7]. In 1997, G. Caminot, M. P. Luda et al., were focused basically on the composition of the material during the chemical ageing procedure [8]. In 2000, Y. Jack Weitsman in his research about the effects of fluids in polymeric composites mentioned the problem of anomalous diffusion and the need for more precise models [9]. Later in 2004, he proposed a coupled diffusion/damage model to describe the phenomenon, excluding chemical reactions and long-term effects [10]. In 2005, Bruce Duncan, Jeannie Urquhart and Simon Roberts proposed an interrelationship between factors, for example the combination of stress and chemical exposure often as a critical degradation mechanism in the service performance of polymeric structures [11]. In 2011, Mark D. Placette, Xuejun Fan described their dual-stage diffusion model based on Fickian s Law for both stages of absorption and desorption [7, 10]. In 2014, J.A. Ferreira, M. Grassi, E. Gudiño, P. de Oliveira worked on a non linear non-fickian model for sorption of a solvent into a polymeric sample. The main idea was a new interpretation of the non-fickian flux which leads to the establishment of a nonlinear functional relation for the strain, the viscoelastic diffusion coefficient and the Young modulus [12]. In 2015, based on the work of Abhishek Kumar and Samit Roy a novel viscoelasticity based model has been formulated, which aims to capture the response of the two-stage moisture absorption response, incorporating the effect of time-dependent relaxation of the material [10]. In the present work, a dual stage model is proposed for characterizing the water absorption behaviour of woven glass reinforced cyanate ester composites. Both stages have been described by classic Fickian model. The investigation based on the experimental data includes not only the theoretical data fittings, but also the separation between the regions of physical and chemical ageing through the appropriate indicators-phenomena that are observed and their explanation through the structural discontinuities of the material [7-11]. It is important to mention that the research based on the non-fickian behaviour has been enriched lately with more models that can describe these phenomena. Different groups of materials are possible to need different models in order to describe their moisture absorption behaviour precisely. This work could describe with accuracy the non-fickian behaviour of glass reinforced polymers, exposed in hydrothermal environment. Materials & Experimental Procedure Materials The material system used in this study is the commercial PN901-G supplied by Gurit, Switzerland. It has a reinforcement phase of woven fabric of E-glass filament yarn in 2/2 twill fabric of 390 gr/m 2, pre-impregnated with 93

3 Heat Flow (W/g) Plastic and Polymer Technology (PAPT) Volume 4, 2016 phenylene (C6H4) cyanate ester resin PN901. Composites using the prepreg material were prepared in-house using autoclave technique. Plates consisted of 8 plies stacked in an orientation so that the plate s length coincided with the weft direction of the 2/2 twill weave, with a fiber volume fraction Vf=54.1%. The curing cycle followed is the one suggested by Gurit (Switzerland) and includes a period of heat up from 40 C to 120 C for 11/2h and a cool down period up to 40 C. A post curing treatment of the plates, from 40 C up to 260 C for 2h and cool down up to 40 C, was applied. The degree of cure F was investigated by using Differential Scanning Calorimetry (DSC) method and is illustrated in Fig.1. The resulted plate thickness was 2.56 mm. The specimens that were produced from the plates for this study were typical tensile specimens. The dimensions of the specimens are summarized in Table GRT-UC-1 GRT1_APR C C 181.6J/g C C 25.12J/g Exo Up Temperature ( C) Universal V4.3A TA Instruments FIG. 1 DSC ANALYSIS OF THE SPECIMEN TABLE 1 SPECIMEN DIMENSIONS Specimen Length l (mm) Width w (mm) Thickness t (mm) Tensile Test Approach DuPont2000 DSC-910 was used for the DSC testing and the experiments were performed from room temperature up to 350 C with heat rate 5 C/min. The mass of the samples was about 10mg. The mass control procedure was held according to ASTM D5229/D5229M. The procedure of ageing is based on the water immersion of specimens in four different temperatures. Table 2 summarizes the experiment details. The dimensions of the specimens differ from those referred in the standard since the specimens were to subsequently be tested as part of a greater campaign. For this reason the lateral surfaces of the samples were covered by silicon based color that may sustain long term exposure to hot and wet environment. The accelerated ageing tests are used to predict long-term exposure results by using short-term procedures. In order to have a successfull approach, the elevated temperatures that will be used, must not cause a change in the ageing mechanisms that act at the lower temperatures, avoiding early chemical ageing. Therefore, all exposure environments should cover a wide range of temperatures but below Tg [12]. This range can be seen in Table 2 below. 94

4 Plastic and Polymer Technology (PAPT) Volume 4, TABLE 2 TEST GROUPS, CONDITIONS AND DURATIONS Group Type of Specimen Conditions Duration 1 T1 40 C 2 T2 60 C Tensile 3 T3 75 C 61days 4 T4 90 C Scanning Electron Microscopy (SEM) was performed in order to provide evidence of the mechanisms related to the expressed absorption behaviour. A LEO SUPRA 35VP by Carl Zeiss Microscopy GmbH (Jena, Germany) was employed to observe morphology of the microstructure of specimens exposed to different aging conditions and to identify the active degradation mechanisms in different ageing processes. Modelling of Diffusion in Composites Fickian Diffusion Theory When a fluid is present inside a polymer at a high concentration region compared with surrounding regions, it will diffuse away from the first region, taking a finite time, until a stable situation is achieved. Diffusion laws have generally been applied to the modelling of simple water uptake, and it has been shown that Fick's law can be applied to fibre reinforced epoxy resin subjected to humid environments. Water is considered to remain in a single free phase driven to penetrate the resin by the water concentration gradient. Other studies have indicated, however, that non-fickian processes do occur which can complicate the understanding of the role of the water or other fluids and which may lead to irreversible changes in mechanical properties. As described by Fickian diffusion theory is: Where: : is percentage of moisture gain at time t : is maximum moisture content that can be attained : initial moisture content of the specimen : time dependent parameter ( ) EQUATION (1) *( ) ( )+ ( ) EQUATION (2) Where: : is diffusion coefficient in the direction normal to the surface : is the thickness of the specimen Since dry specimens were used in the moisture uptake experiments, the theoretical calculations of assuming Fickian diffusion are given by: at any time [ *( ) ( )+ ( ) ] EQUATION (3) 95

5 Plastic and Polymer Technology (PAPT) Volume 4, 2016 The experimental which is a result of the immersion in deionised water is given by: ( ) EQUATION (4) Where: : is percentage of moisture gain at time t : dry weight of the specimen : weight of specimen after immersion in water at time t Moisture equilibrium is the condition reached by a material when there is essentially no further change in its average moisture content with the surrounding environment. Moisture equilibrium content % is the maximum amount of absorbed moisture that a material can contain at moisture equilibrium for a given moisture exposure level, expressed as percent of dry material mass. These conditions are rigorous and may not be fully met by many engineering materials especially GFRP composites. For example a severe absorption cycle may cause damage to a given material, causing cracking and providing a non-fickian diffusion path. Non-Fickian Diffusion-Dual Stage Model During the Fickian diffusion, the free volume of the material is the basic parameter that guides the absorption process. The diffusion rate (which depends upon the diffusion coefficient) is constant with time and the saturation is achieved when the free volume of the material is occupied by the fluid. The increase of temperature, as long as enhance the mobility of the molecules, create probably reconstruction and creation of more free volume and changes the diffusion rate, so as to achieve the saturation level in less time. The diffusion behaviour of many polymers cannot be described adequately by a concentration-dependent form of Fick's law with constant boundary conditions. For example a change in temperature almost causes an immediate change to a new equilibrium volume. Deviations from Fickian behaviour are considered to be associated with the finite rates at which the polymer structure may change in response to the sorption or desorption of penetrant molecules. Anomalous effects may be directly related to the influence of the changing polymer structure on solubility and diffusional mobility, or they may result from the internal stresses exerted by one part of the medium on another as diffusion proceeds. This happens especially, when the penetrant causes extensive swelling of the polymer [13]. The basic activation procedure of swelling is the hydrophilization [14], which is increased after the change of the polymer structure on solubility and diffusional mobility. The polarity of the molecules had changed especially in the saturation level. The hydrophilic response is a function of the polarity of the chemical groups of the material and of their concentration [6]. When the swelling starts, the cross-linked polymer and generally the composite with its fillers, resists to that deformation with elastic forces. As the polymer network begins to elongate under the swelling action of the solvent, they generate an elastic retractive force in opposition to this deformation. The volumetric swelling reaches steady state when the two forces balance each other. During the swelling process, the polymer is deformed because of osmotic stresses. This hydrothermal swelling causes a change in the residual stresses, which can be tensile or compressive within the composite. Swelling is induced only after a threshold value. As long as a significant stress is developed also at the interface between the matrix and the un-swollen fibres, micro-cracks in both bulk matrix and interface region can be developed. These micro-cracks, in turn, provide fast diffusion paths and then alter the moisture absorption characteristics of the laminate. Eventually, they provoke the formation of macro-cracks. Also, via osmotic pressure, micro-voids can lead to the creation of macro-voids, with changes in the diffusion parameters and can create another stress field tensile or compressive which can have desorption also as a result [14]. The cracks and voids have been created in combination with the relaxation of stress and the creep that happen to the polymer. The network gradually relaxes and this can increase the ability of absorption of the material via the increase of diffusion rate. Relaxation is believed to occur by segment motion. Relaxation times are very long at low temperatures. The rates that affect in a great level the type of sorption process are the diffusion rate and relaxation 96

6 Plastic and Polymer Technology (PAPT) Volume 4, rate. A mechanism that probably creates stages in a procedure that in the beginning seems to be Fickian. These phenomena are partially reversible. As a consequence, in the second stage moisture absorption is controlled by the rate of network relaxation. The presence of fibres, which is unlikely to affect the isolated local motion characteristics of the first stage, creates instead a constraint for relatively long-ranged segmental motions involved in relaxation phenomena [15]. All the pre-mentioned phenomena start basically and describe the procedure of physical ageing. The chemical ageing starts when hydrolysis appears to be a basic mechanism that leads to leaching of the material. Substantial hydrolysis of the material is induced by water resulting in the formation of alcohols with modification of the hydrophilic/hydrophobic balance in the matrix. Furthermore, water sorption is also affected by low molecular - weight products resulting from hydrolysis of the polymeric components. Hydrolysis phenomena are generally limited at ambient temperatures and the process is controlled by the diffusion of water in the material. Stress (residual) or externally applied stress on the chemical bond can accelerate chain scission caused by chemical reaction. It has also been found that stress can alter the effective activation energy for chemical reaction. With the increase of temperature, the bonds break, the procedure of leaching starts. It includes the migration of lowmolecular weight electrolytes, which have been formed previously. This has as a result the reduction of the mass absorption percentage, basically in higher temperatures [14,15]. The Dual Stage Model applies the notion that Fickian and non-fickian diffusion occur simultaneously throughout the process. The stages are distinguished by which behaviour is the most dominant. Most GFRPs exhibit a two stage sorption [6]. A quasi-equilibrium is reached rapidly first at the polymer surface and then by simple diffusion throughout the polymer bulk material. The second stage of sorption is associated with an increase in surface concentration which occurs slowly compared with the diffusion process and is the rate-determining factor for sorption. The first stage of the model takes into account the first linear region in order to calculate and in quasiequilibrium level (pseudo-saturation) so the mass absorption percentage is given by Equation 5 below and the last point of the quasi-equilibrium of saturation level of the first stage is the first point of the linear region for the calculation of for the second stage of the model given by the Equation 6 below. It is important to mention that the proposed model can be used also for other temperatures different from the ones used for the experiments, as long as the diffusion coefficients are totally related with the temperature via the Arrhenius equation [16]. [ *( ) ( )+ ( ) ] EQUATION (5) [ *( ) ( )+ ( ) ] EQUATION (6) It is expected that is larger than because the chemical bending and relaxation of the sample is negligible in { Equation (7) the beginning of the absorption. 97

7 Plastic and Polymer Technology (PAPT) Volume 4, 2016 Results and Discussion Quality Control and Visual Inspection of Samples Before all the procedures of ageing, the quality control of the material was fulfilled via DSC method by calculating the degree of curing which is 86% and this percentage is satisfying. This value is very useful for quality assurance and for process optimization purposes [16]. In Fig. 2, the discoloration of specimens through the procedure of ageing is depicted. The discoloration is an obvious sign of the existence of chemical ageing procedures, especially at elevated temperatures. Modelling of Absorption Mechanism FIG. 2 DISCOLORATION OF THE SAMPES THROUGH AGEING PROCEDURE First the classic Fickian model, Equation 3 was applied to the experimental data for the entire duration of each test. The theoretical approach is based on the experimental data. The mass control for tensile specimens is depicted in Fig. 3. The Fickian models in Fig. 4, for tensile test samples, provide unsatisfactory fit of the experimental results. FIG. 3 MASS CONTROL OF TENSILE SPECIMENS 98

8 Plastic and Polymer Technology (PAPT) Volume 4, FIG. 4 FICKIAN ABSORPTION FIT OF TENSILE TESTS Tensile specimens in Fig. 4 show good fit in linear and saturation regions but totally unsatisfied results in the intermediate region. The percentage of absorption is increased with increasing the exposure temperature, something which is normal for the absorption process. Over 75 C, the absorption behaviour of the material is abstaining from the classic Fickian behaviour, and at the end of the exposure period a mass decrease appears. The mass decrease is associated with phenomena of hydrolysis and leaching. These processes are irreversible. The Dual Stage Model that is described in Equation 7 is applied. Fig. 5 illustrates the Dual Stage Model (DSM) fits for the tensile specimens. This approach exhibits good fit at temperatures 40 C, 60 C and 75 C, also for the intermediate region between the initial stage and the saturation one. FIG. 5 DUAL STAGE FIT TENSILE TESTS The R 2 values help to estimate the fitting approach in case of hypothetical linear regression between the data and a trend line. In Table 3, the R 2 values for the experimental data, the theoretical Fickian approach and the Dual Stage Model are provided. In addition, the error between these values is calculated. It is important to mention that the 99

9 Plastic and Polymer Technology (PAPT) Volume 4, 2016 calculated error between the values of the experimental results and the Dual Stage Model is less than 1% for all the exposure temperatures. TABLE 3 R 2 VALUES AND THE % ERRORS FOR TENSILE EXPERIMENTAL DATA WITH FICKIAN MODEL AND DUAL STAGE MODEL (DSM) 40 C 60 C R 2 Error% R 2 R 2 Error% R 2 Exp.-Fick Exp.- DSM Exp.-Fick Exp.- DSM Exp.-Fick Exp.- DSM Exp.-Fick Exp.- DSM % 0.90% % 0.50% 75 C 90 C R 2 Error% R 2 R 2 Error% R 2 Exp.-Fick Exp.- DSM Exp.-Fick Exp.- DSM Exp.-Fick Exp.- DSM Exp.-Fick Exp.- DSM % 0.40% % 0.10% In Table 4 the Diffusion Coefficients D (for the typical Fickian approach), D1 and D2 (for the Dual Stage Model) for all the exposure temperatures used in the present study, are provided. The same results have been plotted in Figs. 6 and 7. It is observed that in the case of considering the typical Fickian behaviour increasing the exposure temperature the diffusion coefficient increases as it is expected. At intermediate exposure temperatures in between C the absorption coefficient is almost constant. In the case of Dual Stage Model approach the results are extremely interesting. Diffusion coefficient D1 remains constant for temperature higher than 60 C, while it is slightly lower at 40 C. This fact represents that the activated absorption processes during the initial phase of exposure remains almost the same over 60 C. On the contrary, diffusion coefficient D2 remains almost constant, and only at exposure temperature of 90 C D2 appears to decrease slightly. At this level of temperature the decrease of diffusion coefficient D2 may be associated to the mass loss of the material due to chemical aging and leaching process that has been activated. In that sense, following the dual stage diffusion approach in order to explain the anomalous behaviour of water absorption in the case of woven glass reinforced cyanate ester composites, one may argue that D1 and D2 diffusion coefficients are much better representing the actual absorption processes that take place, giving to these parameters a realistic physical meaning. TABLE 4 DIFFUSION COEFFICIENTS FOR FICKIAN MODEL AND DUAL STAGE MODEL (DSM) Diffusion Coefficients (mm2/s) Temperature ( C) D 5.68E E E E-06 D1 9.93E E E E-06 D2 4.26E E E E

10 Plastic and Polymer Technology (PAPT) Volume 4, FIG. 6 DIFFUSION COEFFICIENT D FOR THE THEORETICAL FICKIAN MODEL FIG. 7 DIFFUSION COEFFICIENTS D1 AND D2 FOR THE THEORETICAL DUAL STAGE MODEL According to the theory of polymerization and moisture absorption, the degree of curing and the density of crosslinking that this degree represents, can be correlated with the percentage of moisture absorption as long as the crosslinks can affect the permeability of the material [17]. Scanning Electron Microscopy Evidence of Absorption The SEM analytical investigation helped to overview the quality of the material investigate and the damage at a micro-structural level. It will provide the necessary background for linking the experimental observation at macro level to micro-structural changes and justify the behaviour of the material due to ageing procedures, both physical and chemical. These observations will help to better understanding the material changes and to optimize the proposed predictive models. In previous work by Kollia E. et al. in 2015, Interlaminar Shear Strength results for this material were presented [2]. At temperature of 40 C tensile specimens until 39 days ( (s) 1/2) absorption follows a smoothly variated rate that leads to a quasi-equilibrium but the coexistence of swelling changes the absorption parameters. After 61 days of exposure ( (s) 1/2) only superficial hydrolysis events occur as confirmed by cavities formation. This 101

11 Plastic and Polymer Technology (PAPT) Volume 4, 2016 appearance of cavities confirms the initiation of mass loss which is very slow. Fig. 8 presents the SEM observations of the composite exposed in distilled water at 40 C, for different stages of the exposure period [15, 18]. FIG. 8 SEM ANALYSIS OF GROUP T1 SPECIMENS (AGED AT 40 C) At 60 C, obviously there is an increase of diffusion rate in comparison to the one monitored at 40 C. The total amount of the absorbed water at the saturation stage is higher. Swelling here also leads to the change of the amount of cavities and cracks according to theory given previously. Again at a period of exposure up to 39 days ( (s) 1/2) absorption follows again a smoothly variated rate that leads to a quasi-equilibrium but the 102

12 Plastic and Polymer Technology (PAPT) Volume 4, coexistence of swelling changes the absorption rate parameters. The hydrolysis takes place earlier than 61 days ( (s) 1/2). The formation of micro-cavities has started but still there is not micro-cracking propagation due to swelling. Fig. 9 presents the evolution of the variation of the material structure at different exposure periods. FIG. 9 SEM ANALYSIS OF GROUP T2 SPECIMENS (AGED AT 60 C) At 75 C, there is a further increase of diffusion rate in comparison to the one monitored at 60 C. The total amount of the absorbed water at the saturation stage is again higher. Swelling effects again lead to the change of the amount of cavities and cracks, and somehow decrease the absorption rate. However, following this period of absorption rate decrease, between 26 days ( (s) 1/2) and 43 days ( (s) 1/2) a significant increase of absorption rate appears again. This result demonstrates the osmotic process activation. This behaviour has been also identified in the case of composites exposure at 60 C, but in a smaller scale. At 75 C, still there is not 103

13 Plastic and Polymer Technology (PAPT) Volume 4, 2016 superficial micro-cracking network formation even at 61 days, although the formation of extensive cavities pattern is evident as it is shown in Fig. 10, where the evolution of composite hygroscopic damage is presented at different stages of its exposure at distilled water at 75 C. FIG. 10 SEM ANALYSIS OF GROUP T3 SPECIMENS (AGED AT 75 C) Finally at 90 C there is a further intensive increase of diffusion rate in comparison to the one monitored at lower temperatures. In addition, although the anomalous behaviour described above, that includes the absorption rate decrease and afterwards the presence of a significant increase of absorption rate due to the activation of osmotic process, after an exposure period of 30 days (1610 (s) ½ ) a mass loss appears. This is associated with intense leaching and hydrolysis events that have been accelerated due to the high temperature of the distilled water environment. Hydrolysis and leaching of the broken polymer chains is increased as long as the mass absorption rate is increased via the formation and the propagation of a micro-cracking network provoked by osmosis. In 104

14 Plastic and Polymer Technology (PAPT) Volume 4, addition an extensive pattern of micro-cavities is clearly identified after 26 days of exposure. All these phenomena are clearly presented in Fig. 11, where the evolution of composite hygroscopic damage is presented at different stages of its exposure at distilled water at 90 C. FIG. 11 SEM ANALYSIS OF GROUP T4 SPECIMENS (AGED AT 90 C) Conclusions Several samples of 2x2 twill woven glass reinforced cyanate ester composites were used for mass absorption studies. The classic Fickian model concludes to insufficient fitting of the actual experimental results. An effort has been done to describe the anomalous moisture absorption through the combination of Fickian model for each one of the two separated stages. For temperatures like 40 C, 60 C and 75 C the proposed Dual Stage Model provides a good estimate of the experimental results. The swelling gives some distinctive characteristics in 105

15 Plastic and Polymer Technology (PAPT) Volume 4, 2016 the behaviour even at low temperatures. Residual stresses are the basic reason for the formation of micro-cracking and micro-cavities and their propagation at higher temperatures. At 75 C and 90 C all the above mentioned phenomena are more abrupt, leading to hydrolysis and leaching, which is associated to the loss of material. ACKNOWLEDGMENT The support of the European Union and the JTI Clean Sky within the framework of ITD ECO-Design activity SPECIMEN is gratefully appreciated. REFERENCES [1] Gu Junwei, Lianq Chaobo, Jinq Danq, Donc Wencai, Qiuyu Zhanq " Ideal dielectric thermally conductive bismaleimide nanocomposites filled with polyhedral oligomeric silsesquioxane functionalized nanosized boron nitride." RSC Adv., 6, , 2016, DOI: /C6RA04513H [2] Kollia E., Loutas Th., Fiamegkou E., Vavouliotis A. and Kostopoulos V. Degradation behavior of glass fiber reinforced cyanate ester composites under hydrothermal ageing. Polymer Degradation and Stability: 121, , [3] Galloway DP., Grosse M., Ngugen MY., Burkhart N. Proceedings of the IEEE/CPMT 17th International Electronics Manufacturing Technology Symposium. p.141, [4] Knouff B., Tompkins S. S., and Jayaraman N. The effect of graphite fiber properties on microcracking due to thermal cycling of epoxy-cyanate matrix laminates. Proceedings of the ASTM 5th Symposium on Composite Materials: Fatigue and Fracture; p , [5] Kotrotsos A., Vavouliotis A., Loutas T. and Kostopoulos V. The Effect of CNT modified matrix of cyanate ester CFRPs on the hydrothermal behavior of the material, Evaluation of the water uptake using electrical resistance measurements. Polymer Composites, DOI: /pc.23268, [6] Fiamegkou E., Vavouliotis A., Kostopoulos V. "The effect of thermo-oxidative aging on carbon fiber reinforced cyanate ester composites." Journal of Composite Materials (2014): Accessed November 23, doi: / [7] Caminot G., Luda M. P., Polishchuk A. Ya, Revellino M., Blancon R., Merlec G. & Martinez-Vega J. J. "Kinetic aspects of water sorption in polyester resin/glass fiber composites." Composites Science and Technology 57( 1997): Accessed February 26, [8] Placette Mark, Fan Xuejun, Zhao Hua-Jie and Edwards Darvin. "A dual stage model of anomalous moisture diffusion and desorption in epoxy mold compound." Paper presented at 12th international conference on Thermal, Mechanical and Multiphysics Simulation and Experiments in Microelectronics and Microsystems, Dallas, [9] Weitsman Jack. "Effects of fluids on polymeric composites -A review." Comprehensive Composite Materials Volume 2, (2000): [10] Ferreira J.A., Grassi M., Gudiño E., De Oliveira P. "A new look to non-fickian diffusion." Applied Mathematical Modelling 39 (2014) : Accessed May 20, [11] Weitsman Y.J. Anomalous fluid sorption in polymeric composites and its relation to fluid-induced damage. Composites: Part A 37 (2006): Accessed May 3, [12] Martin Rod Ageing of composites. 1st ed. Woodhead Publishing Limited, [13] Kumar A., Roy S. "Modeling of anomalous moisture diffusion in nanographene reinforced thermoset polymers." Composite Structures 122 ( 2015): 1 7. Accessed November 24, [14] Mercier J., Bunsell A., Castaing P. and Renard J. " Characterization and modelling of aging of composites." Composites: Part A 39 (2008): Accessed August 5, 2007.doi: /j.compositesa [15] Crank J. The mathematics of diffusion. 2nd ed. Clarendon Press Oxford, [16] Sichinia W.J. Characterization of epoxy resins using DSC. PETech-19 Report Thermal Analysis, Perkin Elmer Instruments, [17] Rao R.M.V.G.K., Balasubramanian N., Chanda M. Factors affecting moisture absorption in polymer composites Part I: influence of internal factors. Journal of Reinforced Plastics and Composites Vol. 3. Accessed July, [18] Duncan Br., Urquhart J., Roberts S." Review of measurement and modelling of permeation and diffusion in polymers." NPL Report DEPC MPR, National Physical Laboratory, Teddington Middlesex,