Effect of fiber surface modification on the lifetime of glass fiber reinforced polymerized cyclic butylene terephthalate composites in hygrothermal conditions

Materials and Design 85 (2015) 14–23

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Effect of fiber surface modification on the lifetime of glass fiber reinforced polymerized cyclic butylene terephthalate composites in hygrothermal conditions Bin Yang a, Jifeng Zhang a,⁎, Limin Zhou b,⁎, Zhenqing Wang a, Wenyan Liang a a b

Smart Structures and Advanced Composite Materials Lab, College of Aerospace and Civil Engineering, Harbin Engineering University, Harbin 150001, China, Department of Mechanical Engineering, The Hong Kong Polytechnic University, Hong Kong, China

a r t i c l e

i n f o

Article history: Received 26 November 2014 Received in revised form 29 June 2015 Accepted 1 July 2015 Available online 6 July 2015 Keywords: Thermoplastic composites Environmental degradation Vacuum-assisted hot-press processing (VAPP) Strength degeneration ratio (SDR)

a b s t r a c t Mechanical performances of polymerized cyclic butylene terephthalate (pCBT) matrix, glass fiber reinforced pCBT (GF/pCBT), and nano-silica modified glass fiber/pCBT composites (nano-GF/pCBT) in hygrothermal condition were investigated. All the materials were aged in hygrothermal environments for up to three months, and then their mechanical strength degeneration ratio (SDR) was calculated. To study the aging effect of temperature, specimens with and without nano-silica modification were tested in temperatures ranging from 298 to 500 K. Differential scanning calorimeter (DSC) test, dynamic mechanical analysis (DMA), and fiber pull-out test were adopted to complement the experimental results. It is found that all the SDR-time curves follow the linear relationship in hygrothermal environment, while SDR-temperature curves follow a bilinear relationship due to the effect of glass transition temperature (Tg) of the matrix. Fibers modified by coating nano-silica on the surface could decrease SDR of the composites. This is due to the fact that the fillers on the fiber surface could resist the movement of pCBT molecular chain and diffusion of water molecules in aging conditions. The fiber pull-out test verifies that the interface strength between fiber and matrix is enhanced by the modification. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Fibrous composites are increasingly being used in many applications owing to their desirable properties such as high strength to weight ratio, high stiffness to weight ratio, and superior corrosion resistance. Unfortunately, both thermosetting and thermoplastic resins used as matrix in fiber reinforced plastic composites (FRP) are susceptible to humidity and temperature when operating in real engineering fields [1]. Mainly due to the uncertainty on the long term reliability of polymer composites, the wider applied scope of composite materials is limited. Thus, it is necessary to investigate the hygrothermal aging behavior of the composites especially in terms of finding out an effective approach by which their long-term mechanical performance can be enhanced. Numerous works have studied the fiber modification effect on the mechanical properties of FRP. Generally, the modification methods adopted in these works are composed of two approaches [2–4]. Chemical modifications are carried out mainly by treating the fibers with chemical reagents such as coupling agents, while the common physical method used in the modification is heating. Even though it has been ⁎ Corresponding authors at: Room 11#1004, College of Aerospace and Civil Engineering, Harbin Engineering University, Harbin 150001, Heilongjiang, China. E-mail addresses: [email protected] (J. Zhang), [email protected] (L. Zhou).

http://dx.doi.org/10.1016/j.matdes.2015.07.010 0264-1275/© 2015 Elsevier Ltd. All rights reserved.

verified in these researches that the overall mechanical performance of the composites can be enhanced by the treatments, the strength of the fiber itself is actually reduced mainly due to the damage to its microstructure. Many publications have investigated the mechanical and thermal characterization of FRP under different hygrothermal conditions. The effects of moisture and applied temperature on mechanical properties of FRP are investigated in the literature [5–14]. In detail, Bajracharya et al. [5] reviewed the mechanical properties and durability of glass fiber reinforced recycled mixed plastic waste composites. Information on the behavior of thermoplastic composites at different environmental conditions such as elevated temperature and ultraviolet rays is summarized. Xu et al. [6] performed gravimetric experimental studies on the moisture diffusion process in pultruded FRP composites exposed to the vapor environmental aging condition as well as water immersed condition at temperatures of 20 °C and 40 °C. Their results indicated that high temperature can speed up the moisture diffusion rate, and the moisture equilibrium contents were mainly governed by the humidity of the aging environment. Narendar et al. [7] investigated the coir pith epoxy composites hybridized with nylon fabric/epoxy resin by hand layup technique. Aging of composite panels in moist environment was investigated. Sawpan et al. [8] immersed the FRP composite rebar in alkaline concrete environment for different durations at 60 °C. They found that moisture absorption was a critical factor that controlled the thermal and mechanical properties of GFRP rebar.

B. Yang et al. / Materials and Design 85 (2015) 14–23

Mohd Ishak et al. [9–11] have done many works on the hygrothermal aging properties of fiber reinforced polymer composites. Tajvidi et al. [12] and Ellyin et al. [13] have investigated the effect of temperature on different composites. Temperature effect on the mechanical properties of glass fiber/PBT (GF/PBT) specimens was studied by Cavdar et al. [14]. Even though some references have verified that the thermal stability of polymers can be enhanced by nanoparticles [15,16], studies on nano-particle modified polymers aiming to enhance the anti-aging property are still limited. Cyclic butylene terephthalate (CBT) oligomers have the structure of big-ring paucity of polyester with molecular weight Mw = (220)n (with n = 2–7) g/mol [17]. The melt viscosity of CBT thermoplastic resin is as low as 17 mPa, which makes fiber bundles easily impregnated during the manufacturing process [18]. This advantage provides opportunities to produce fiber reinforced CBT composites with perfect impregnation of fibers using various processing methods that avail of thermoset or thermoplastic resin [19–22]. Meanwhile, the low melt viscosity of the resin further makes CBT micro/nano-composites with excellent dispersion of fillers possible. Scheme 1 shows the ring-opening polymerization reaction process in the presence of Sn-based catalyst. The generated product from CBT resin is polymerized poly(butylene terephthalate), which will be taken as pCBT in this paper. The objective of the present paper is to investigate the effect of fiber surface nanosilica modification on the lifetime of CBT based composites used in the aging environment. Specimens that include pCBT matrix, GF/pCBT and nano-silica modified GF/pCBT composites were aged in hygrothermal conditions for up to 90 days, and then mechanical tests including bending and compression were performed. Additionally, to obtain the relationship between test temperature and hygrothermal aging duration, mechanical tests on the specimens were performed in temperature alone. Afterwards, DSC, DMA and fiber pull-out test were respectively carried out to examine the thermal performance and interface property between fiber and matrix.

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any further treatment. Because moisture could interfere with the polymerization reaction, all the materials were dried for 10 h in a vacuum oven at 110 °C before processing. 2.2. Manufacturing process pCBT casts were prepared by the casting process with CBT resin to catalyst mass ratio 100:0.6. Vacuum-assisted hot-pressing process (VAPP) was used to fabricate the composites in this work. Two categories of composite laminates were prepared via VAPP: GF/pCBT composites and its nano-silica modified fiber reinforced laminates. As pretreatment, glass fibers should be soaked for 1 day into two kinds of isopropanol aqua solution which respectively contained 0.6 wt.% catalyst and 2 wt.% nano-silica, and catalyst (0.6 wt.%) alone. It should be mentioned that all the percentages are relative to the weight of resin used in the experiments. To obtain good dispersion of the solution, the mixture was stirred by a magnetic stirrer for 2 h, and then dispersed in an ultrasonic agitator for 1 h at room temperature. Then the system needed to be dried in a vacuum oven at 140 °C to remove isopropanol aqua and leave the catalyst and/or nano-silica on the fiber surface. The diagram of the VAPP setup is shown in Fig. 1. The hot-pressing machine can provide the heat and pressure to the mold during the manufacturing process. A steel mold was used as the container in which the curing of CBT resin can take place. This steel mold contains a top sheet, bottom sheet, and intermediate frame inside of which the asprepared glass fibers were placed. The top sheet comprised two ports, one as the resin inlet and the other one as outlet. A rubber seal ring was placed between the top and bottom sheets to maintain the vacuum. In order to remove the air from the mold before injection, the valve was shut and then the mold was vacuumed for approximately 15 min. Both the two composites were processed non-isothermally: 230 °C for 1 h and 190 °C for another hour. The injection started when the temperature inside the mold reached 230 °C, and then demolding when the mold cooled to room temperature.

2. Experimental details 2.3. Aging conditions 2.1. Materials The polymer used as matrix is one-component CBT-100, delivered in granule form by Cyclics Corporation. Tin-based catalyst butylchlorodihydroxytin (PC-4101) with the molecular weight of 245.29 is selected. This compound is an ester catalyst with high catalytic activity and suitable for esterification or polycondensation reactions with temperatures ranging from 210 °C to 240 °C. Unidirectional Eglass fiber cloth (EDW-800) with surface weight of 500 g/m2 is used as the reinforcement in the composites. Hydrophobic nano-silica (DNS-3) with particle diameter between 5 and 15 nm is used in the experiment. Note that all the materials are used as-received without

All the specimens were aged in the artificial climatic chambers which could provide the humidity and temperature at the same time. In our experiment, the relative humidity was set at 60% and 90%, while the temperature was respectively set at 50 °C, 60 °C, 70 °C, and 90 °C. It should be noted that the hygrothermal aging duration was up to 3 months, and at least 5 specimens were taken out from the chambers for the next mechanical tests each week. Another group of specimens was tested to evaluate the effect of high service temperature on their mechanical performance. Temperature conditioning for the experiment was performed using an electrothermal furnace (with a temperature range up to 300 °C/573 °F). The conditioning schedules were 5-min at each selected temperature, and it was assumed that the humidity in the furnace was almost zero at set temperatures. Mechanical tests were carried out with the conditioned specimens at six selected temperatures (25 °C, 70 °C, 80 °C, 150 °C, 180 °C and 220 °C, respectively). 2.4. Mechanical tests Compression and three-point-bending tests were carried out to determine the mechanical strength of the as-prepared and aged casts, GF/pCBT and nano-GF/pCBT laminates, respectively. Test specimens were cut from the prepared square laminate by a low-speed diamond saw blade cutting machine in accordance with ASTM standards. The dimensions of the specimens are listed in Table 1. According to the test results, the strength degeneration ratio (SDR) of the specimen was calculated by Eq. (1),

Scheme 1. Ring-opening polymerization reaction of CBT resin.

SDR ¼

St S0

ð1Þ

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B. Yang et al. / Materials and Design 85 (2015) 14–23

Upper die of Hot-poress

Resin outlet

Hot-press machine

GF/pCBT Product

Resin inlet

Mold

CBT Resin

Heater

Fig. 1. Device used to manufacture GF/pCBT and its nano-silica modified composites by vacuum-assisted hot-pressing process.

where SDR is the mechanical strength degeneration ratio. S0 is the initial specimen strength, and St is the specimen strength after aging in hygrothermal condition for t time. The SDR of the specimens determined from the test data will be given as a function of aging duration and test temperature in the following work. Fiber pull-out tests were performed according to the literature [23], and the test specimen and its dimensions are given in Fig. 2. As shown, a cluster of glass fibers was embedded in pCBT resin matrix, and the lower surface of the specimen was fixed during the pull-out test. All the tests were performed with Instron universal testing machine (Zwick/Roell) with crosshead speed of 2 mm/min. 2.5. Thermal analysis To investigate the effect of aging on the thermal performance of the materials, differential scanning calorimetry (DSC) and dynamic thermomechanical analysis (DMA) were carried out to evaluate the thermal behavior of the pCBT resin. The DSC test was performed on DSC Q2000 device (Tzero) to measure the thermal history of pCBT resin during the heating–cooling–heating process. Experiments were run with sample mass ranging from 7 to 10 mg under nitrogen as the purge gas during the measurements to prevent moisture and oxidative degradation. The DMA test was performed to determine the Tg of pCBT resin. Rectangular specimens with size 30 × 6 × 2 mm3 were subjected to load-controlled sinusoidal loading in DMA Q-800 performed in three-point-bending mode.

strain curves in Fig. 3 and the test data obtained at 90 °C, 60RH%, SDR of pCBT cast was calculated as the function of aging duration. Fig. 4 shows the relationship between SDR and hygrothermal aging duration of pCBT matrix in three categories of aging conditions. It can be seen that all SDR data obtained from the compression tests follow the linear tendency with increasing aging duration. Different relative humidity at the same exposure temperature seems to have less effect on the gradient of the SDR-T curve. In detail, very small difference between the fitting curves at relative humidity 90RH%, 60RH% and temperature of 90 °C is obtained. However, various aging temperatures at the same relative humidity affect the curves significantly. Aging curves at relative humidity of 90RH% and temperature respectively of 50 °C and 90 °C have remarkable difference. From the comparison, it can be concluded that the effect of aging temperature on the material's mechanical performance plays a more important role relative to that of humidity. A model [24] has been developed to predict the polymer's lifetime under conditions of elevated temperature and high relative humidity. The model equation, stated in terms of absolute temperature and water vapor partial pressure, allows extrapolation of accelerated aging data to ambient conditions that prevail in realistic application of the materials. The formula is as follow: ð2Þ

log t ¼ A þ B=T−C  log P H2 O

3.1. Lifetime of pCBT matrix in hygrothermal conditions Fig. 3 shows the representative curves of macroscopic stress–strain of pCBT casts obtained in compression tests after exposure to different environmental conditions (50 °C, 90 °C and RH = 90%). As can be seen from the experimental results, the stress–strain curves appeared to be an approximately linear relationship initially, and then they performed a plastic yielding with increasing strain. The main failure mode appeared to be brittle compressive failure of aged/unaged casts because all the materials failed immediately at the point when the external applied load reached their strength. According to the stress–

PCBT Matrix

Φ 18mm

Glass fibers

Table 1 Dimensions of test specimens. Specimens

Dimensions

pCBT casts GF/pCBT Nano-GF/pCBT

20 × 10 × 10 mm3 60 × 12.5 × 2.4 mm3 60 × 12.5 × 2.4 mm3

L=40mm

3. Experimental results

Fixed surface

Load Fig. 2. Schematic diagram of single fiber pull-out test.

B. Yang et al. / Materials and Design 85 (2015) 14–23

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Fig. 3. Macroscopic stress–strain curves of pCBT casts in compression tests after being exposed to (a) 50 °C, and (b) 90 °C at relative humidity of 90RH%.

Rearranging Eq. (3) and taking logarithms: log P H2 O ¼ log R þ log P SAT :

ð4Þ

Actually, when the temperature ranges from 0 to 100 °C, the relationship between PSAT and the temperature follows the Arrhenius-type equation as shown in Fig. 5. As can be found in the figure, the linear relation fitted by the Arrhenius-type equation has very good fitting degree, and the fitting result is: log P SAT ¼ 10:655−2:121=T:

ð5Þ

Thus, substituting all the relationships into Eq. (2) finally results in:

Fig. 4. SDR of pCBT matrix as a function of aging duration in the hygrothermal environment.

where t represents the observed half-life measured at absolute temperature T. A, B and C are constants to be determined by regression analysis, while P H2 O is the external water vapor partial pressure. The relative humidity R can be determined as: R ¼ P H2 O =P SAT

ð3Þ

log t ¼ ðA−10:655C Þ þ ðB þ 2:121C Þ=T−C  log P H2 O :

ð6Þ

Table 2 lists the aging duration at the point when the strength of the specimens declines to half the initial value. It can be found from the aging data of the matrix in the table that service life of the resin decreases from 902.6 h in 50 °C, 90RH% to 350.3 h in 90 °C, 90RH%. Taking these two values as well as R = 90RH% into Eq. (6) and rearranging, the relationship between log t and T of the matrix determined from Eq. (6) is: log t ¼ 3:395 þ 0:4045=T−0:411  log R

ð7Þ

where PSAT is the saturated water vapor pressure at special temperature. Finally, Eq. (7) is the lifetime prediction formula that can be used to calculate the long-term performance of pCBT matrix in hygrothermal condition. Relationship between log t and T of the matrix at various relative humidities is shown in Fig. 6. As can be observed, different humidities in Fig. 6 turn out to merely affect the intercept of the curves, and this finally leads to a series of parallel curves.

Table 2 Aging duration and test temperatures at the point SDR = 0.5. Aging conditions

Fig. 5. Fitting result of log PSAT versus 1/T shows the linear relationship.

High temperature 50 °C, 90RH% 60 °C, 90RH% 70 °C, 90RH% 90 °C, 90RH% 90 °C, 60RH%

Duration/temperature pCBT matrix

GF/pCBT

Nano-GF/pCBT

– 902.6 h – – 350.3 h 413.8 h

412 K 7450.5 h 3111.7 h 787.3 h 417.1 h –

385 K 3482.9 h 1895.9 h 1335.5 h 534.1 h –

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B. Yang et al. / Materials and Design 85 (2015) 14–23 Table 3 Mechanical properties of composites with and without nano-fillers after aged in 50 °C, 90RH% for various durations. Specimens

GF/pCBT Nano-GF/pCBT

Fig. 6. Relationship between log t and 1/T at various relative humidity.

3.2. Lifetime prediction of glass fiber reinforced pCBT composites in hygrothermal and temperature conditions Fig. 7a shows the typical stress–strain curves of GF/pCBT composites obtained in the three-point-bending tests that were carried out at different temperature conditions. As found in the figure, the mechanical performance of the materials shows an obvious decline tendency with increasing test temperatures. The yield stress of GF/pCBT composite laminates decreases from 831 MPa at room temperature to 109.2 MPa at 220 °C. In terms of hygrothermal conditions, Fig. 7b is the typical flexural stress–strain curves of GF/pCBT composites after aging in T = 90 °C, RH = 90% for various durations. As can be seen, the flexural parameters of the composites turn out to decline with increasing of both aging duration and aging temperatures in hygrothermal conditions. According to Fig. 7b and other mechanical curves of specimens aged in 50 °C, 60 °C and 70 °C at RH = 90%, Tables 3, 4, 5 and 6 list the flexural strength and failure strain of the laminates after being exposed for 1 to 4 weeks. Fig. 8 shows the SDR curves of the composites calculated from Table 3 to Table 6 as a function of the hygrothermal aging duration. To estimate the performance of the material at elevated temperature alone, without regarding the relative humidity, the relationship between SDR and test temperature obtained from Fig. 7a is also given in the figure. As can be seen, SDR decreases with increasing hygrothermal duration and the linear relationship between SDR and hygrothermal duration can be clearly observed in the figure. Also, from the test data of GF/pCBT composite laminates listed in Table 2, it can be found that when the SDR of the composites decreases to 0.5, the duration aged in 50 °C, 90RH% is 7450.5 h. However, the aging duration declines to 417.1 h

a 900

Mechanical property

Aging duration/weeks Unaged

1

2

3

4

Strength/MPa Failure strain/% Strength/MPa Failure strain/%

851 3.04 803 3.01

842.49 3.02 794.97 2.93

833.98 3.03 786.94 2.66

825.47 3 778.91 2.64

808.45 3 762.85 2.57

when the aging temperature increases to 90 °C at the same relative humidity. In terms of test temperature alone, the relationship between SDR and test temperature appears to be bilinear. The variation tendency of SDR appears at first to decrease slowly and then sharply with increasing temperature. Hence by the comparison of SDR curves in Fig. 8, it can be obtained that high test temperature alone and longterm hygrothermal aging duration have the same effect on the strength reduction of the composites. Concretely, when tested at temperature of 412 K, the SDR of the composites declines to half the initial value, while SDR = 0.5 is also achieved after aging in different hygrothermal conditions for a special duration as listed in Table 2. Hence, the time– temperature superposition (TTS) principle is verified, and this principle can be adopted to accelerate the aging process of GF/pCBT composites. According to the life-curve-spectrum in Fig. 8, the short-term data obtained in high temperature can be used to calculate the long-term performance of GF/pCBT composites in hygrothermal conditions. Another aspect, according to the test data of GF/pCBT in various hygrothermal conditions in Table 2, is the lifetime curve fitted by Eq. (7) as shown in Fig. 9. As indicated in the figure, the constants in the Arrhenius equation are 3.668 and 7.64, respectively. Meanwhile, it also should be noted that the curve has bigger fitting errors. It is due to the fact that the aging mechanism of the composites is not the same as that of pure matrix. As a result, fitting the data of composites by the formula based on pure matrix may not be accurate. The modified fitting curve will be given later in this paper. 3.3. Lifetime prediction of nano-silica modified GF/pCBT composites in hygrothermal and temperature conditions Fig. 10a indicates the representative curves of macroscopic stress– strain of nano-GF/pCBT composite laminates under flexural load in various temperatures. It can be seen from the curves that with increasing temperature, the flexural strength of nano-silica modified composites declines from 803 MPa to 79.44 MPa when tested at room temperature

b o

25 C

800

o

70 C o

80 C

Stress / MPa

700

o

150 C o

600

180 C o

220 C

500 400 300 200 100 0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Strain / % Fig. 7. Macroscopic stress–strain curves of GF/pCBT composite laminates obtained at various conditions: (a) at various temperatures, and (b) aged in T = 90 °C, RH = 90% for various durations.

B. Yang et al. / Materials and Design 85 (2015) 14–23

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Table 4 Mechanical properties of composites with and without nano-fillers after aged in 60 °C, 90RH% for various durations. Specimens

GF/pCBT Nano-GF/pCBT

Mechanical property

Aging duration/Weeks Unaged

1

2

3

4

Strength/MPa Failure strain/% Strength/MPa Failure strain/%

851 3.04 803 3.01

833.98 3.02 762.85 2.89

808.45 2.99 722.7 2.79

791.43 2.46 714.67 2.73

757.39 2.53 682.55 2.52

Table 5 Mechanical properties of composites with and without nano-fillers after aging in 70 °C, 90RH% for various durations. Specimens

GF/pCBT Nano-GF/pCBT

Mechanical property

Aging duration/weeks Unaged

1

2

3

4

Strength/MPa Failure strain/% Strength/MPa Failure strain/%

851 3.04 803 3.01

723.35 2.37 738.76 2.64

663.78 2.31 722.7 2.39

493.58 2.35 658.46 2.23

323.38 1.88 594.22 1.88

and 220 °C, respectively. Fig. 10b shows the typical relationship of flexural stress–strain of 2 wt.% nano-silica modified specimens. Also, the mechanical parameters calculated from Fig. 10b and other hygrothermal aging conditions are given in Table 3 to Table 6. Thus, we can calculate the SDR of nano-silica modified composites under temperature and hygrothermal environment based on the obtained data. Fig. 11 gives the fitting curves of SDR of nano-silica modified composites as a function of hygrothermal aging duration and test temperatures. Like curves of unmodified composites, the SDR of the material also shows linear relationships and its mechanical strength declines with increasing hygrothermal aging duration. With increasing humidity in higher temperature, SDR declines faster. In addition, it can be found that even after experiencing the aging condition of 50 °C and 90RH% for 5 weeks, the SDR of the material merely decreases to 0.9. The rate of strength reduction is less than 10%, which indicates that mild aging environment has no significant effect on the macroscopic mechanical behaviors of the material. However, in 90 °C, 90RH%, the SDR of the modified composites sharply decreases to 0.43 times the initial value after five weeks' exposure. With respect to temperature alone, the bilinear relation between SDR and temperature appears on the fitting curve as well. This is mainly due to the influence of the glass transition temperature of the composites which will be discussed later. The same as composites without nano-silica modification, the time–temperature superposition (TTS) principle is also obtained in Fig. 11. The data obtained in high temperature can be adopted to estimate the performance of nano-GF/pCBT composites in hygrothermal condition for long duration. Since the effect of hygrothermal aging is equal to the high temperature alone on the SDR of the composites, the effect of nano-silica content on the aging performance of the composites is investigated in test temperature alone. Fig. 12 describes the relationships between the SDR of composites that are modified by various contents of nano-silica

Fig. 8. SDR of GF/pCBT composite laminates as a function of hygrothermal aging duration and test temperature.

and the test temperatures. It can be observed in the figure that composites with and without nano-silica modification could be fitted by double linear shape well. With the test temperature ranging from 273 k to 350 k, the fitting curve has a small gradient, while when it ranges from 350 k to 500 k, the curve shows a tendency with a bigger gradient. As mentioned earlier, this phenomenon of the fitting result is mainly due to the influence of glass transition temperature (Tg) of the polymer. As proved by DMA test in Fig. 13, Tg of pCBT resin is 350 k. Thus when the test is below Tg, temperature has less influence on SDR mainly because the resin serviced as matrix in the composites is in its glass state and the strength of the material will slowly decline. While above Tg, the viscoelastic polymer makes the mechanical properties of the composites decreases fast with increasing test temperature. 4. Discussions 4.1. Effect of fibers on the aging properties of pCBT resin As discussed, Tg of the composites could affect the log t − 1/T curves significantly. In our work, because the aging temperature 90 °C is above the Tg of the composites, the Arrhenius-type equation will not be suitable to fit the data due to the fact that it is a linear equation. Fig. 14 shows the modified log t − 1/T curve in which the knee point caused by Tg can be found, and the relative fast decline line in aging temperature of 90 °C is captured. Furthermore, from the comparison curves between composites and pure matrix, it can be clearly seen that log t of fiber reinforced matrix has higher gradient with increasing 1/T. In other words, the matrix filled by fibers could resist the hygrothermal aging process effectively and the lifetime of the matrix has been enhanced significantly. In terms of fiber reinforced polymer composites,

Table 6 Mechanical properties of composites with and without nano-fillers after aging in 90 °C, 90RH% for various durations. Specimens

Mechanical property

Aging duration/weeks Unaged

1

2

3

4

GF/pCBT

Strength/MPa Failure strain/% Strength/MPa Failure strain/%

851 3.04 803 3.01

680.8 2.56 658.46 2.23

510.6 1.58 481.8 1.43

340.4 0.97 401.5 1.06

170.2 0.96 337.26 0.95

Nano-GF/pCBT

Fig. 9. Fitting result of GF/pCBT composites aged in hygrothermal condition when SDR = 0.5.

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B. Yang et al. / Materials and Design 85 (2015) 14–23

Fig. 10. Macroscopic stress–strain curves of 2 wt.% nano-GF/pCBT composite laminates obtained at various conditions: (a) at various temperatures, and (b) aged in T = 90 °C, RH = 90% for various durations.

where SDRm and SDRf are respectively the strength degeneration ratio of the matrix and the fiber, while fm is the mass ratio of the matrix in the composites. ΔInf is the aging effect that is brought by the interface between fiber and matrix in the composites. As known, continuous fiber

reinforced composites are constituted by fiber, matrix, and fiber/matrix interface. A matrix reinforced by fibers in it could form the interphase region in the material. Hence, special attention should be taken into consideration on the mechanical degradation caused by the existence of interface region in the composites. The literature [25] has demonstrated that interface between fiber and matrix contributes to a great deal of reduction of mechanical performance of the material as the structure of composites is not equivalent to neat resin. Actually, the diffusivity of water molecules along the interface region is larger than that of pure resin in hygrothermal condition. Therefore, the interface plays an important role in the aging process of the composites compared with pure matrix. In hygrothermal condition, eversible changes like debonding may occur due to the degeneration of the interface strength and it further results in high SDR of composites. As a result, instead of merely averaging SDR of the composites according to fiber and the matrix, the effect of interface should also be taken into consideration in Eq. (8). According to Eq. (8), since anti-aging performance of glass fibers is terrific, the difference of log t-1/T curves between composite and pure matrix is mainly because of the water absorbed by the interface. Furthermore, as discussed, humidity in high temperature could promote the water diffusing into pure matrix and accelerate the aging speed of composites in hygrothermal condition. It is owing to that that high temperature could accelerate the molecular kinetics, and then water can permeate into the matrix through the interface region easily. Simultaneously, high temperature would accelerate the swelling of the macromolecule, and the interstitial volume between fiber and matrix increases sharply due to their mismatched expansion coefficients. Consequently, humidity could permeate into pCBT resin through the gap between the molecular chains in the fiber/matrix

Fig. 12. SDR of various nano-silica modified GF/pCBT composites tested at different temperatures.

Fig. 13. Flexural modulus and loss factor as a function of temperature for pCBT cast in DMA test.

Fig. 11. SDR of nano-GF/pCBT composites as a function of aging duration and test temperature.

the authors believe that the SDR of the composites consist of the following three parts: SDR ¼ SDRm  f m þ SDR f  ð1− f m Þ−ΔInf

ð8Þ

B. Yang et al. / Materials and Design 85 (2015) 14–23

Fig. 14. Comparison of fitting curves of composites and pCBT matrix.

interface region. This may further damage the resin macromolecules and exhibit obvious decreasing character in macroscopic mechanical behaviors since degradation of fiber/matrix interface region could directly affect the overall mechanical behaviors of the composites. 4.2. Effect of nano-silica fillers on aging properties of the composites As mentioned, the fiber/matrix interface region plays an important role in the overall mechanical performance of composites, and the modification effect of nano-silica on the interface region will be discussed in this section. Generally, plasticization and swelling of the macromolecule in the matrix are the two main factors that may weaken the interface

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strength. Water molecules could diffuse quickly inside the material through the interface by capillarity. Fig. 15 shows the DSC results of unaged/aged pCBT specimens in the heating and cooling modes. The specimens were aged in 90RH%, 90 °C for 90 days. As can be seen, DSC tendency of the aged specimen appears obviously different from that of the unaged one during the first heating step (Fig. 15a). In detail, the melting point of the unaged specimen has double peaks while the aged specimen has only one peak. In contrast, all the curves have the same tendency during the reheating procedure as shown in Fig. 15c. For the unaged specimen, the degree of crosslinking in the polymer is extremely high. Hence, it results in the consequence that the outer sphere of the high crosslinking polymer is firstly melting during the heating process. After that, the inner pCBT resin begins to melt with the heat going inside the molecular chain of the polymer. Due to the melting process of the inner and outer polymer is not in synchronization, double melting peaks appear in the DSC curve in Fig. 15a. In terms of the aged specimen, swelling and plasticization have taken place in the polymer attributed to the long-term attack of water vapor in high temperature, and this has loosened the high crosslinking polymer chains. It further appears as the only peak in the first heating process since the polymer is melting synchronously. During the second heating, however, because the polymer chains have already been rearranged by high temperature in the first heating, the DSC curves show the same tendency with the unaged specimen. This assumption could be proved by the cooling procedure in DSC curves in Fig. 15b, in which the crystalline peak of the polymer is the same because of the melting effect during the first heating. It should be noted that the mentioned swelling and plasticization of the polymer could take place more easily in the interface region because of the smooth glass fiber surface. Since the other end of the molecular chain is in contact with the fiber

a) First heating

b) Cooling

c) Second heating Fig. 15. DSC curves of pCBT matrix after being aged in 90RH%, 90 °C for 90 days.

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surface, the chain is nearly in the unrestrained state and its motion is relatively flexible. Thus, the serious swelling and plasticization of polymer in the interface region further leads to the quick diffusing of the water inside the material along the fiber direction. Compared with unmodified composites, the SDR of the composites with 2 wt.% nano-silica modified fiber reinforced pCBT declines more slowly in 90 °C, 90RH%, which indicates that nanoparticles play a positive role in terms of enhancing the antiaging property of the composites. Two mechanisms can be used to explain the enhancement effect of the mechanical performance in hygrothermal conditions by coating nano-silica on fiber surface. According to thermodynamics, addition of nano-silica could hinder the pCBT macromolecule from swelling in the interface region. It is because nano-silica in the crosslinking polymer could affect the molecular motion in hygrothermal conditions. More molecular energy is needed during the aging process of the polymer since the molecular chain has to promote the nano-fillers and then swell under the action of water molecules. Secondly, silica in the gap between the polymer molecular chain segments could prevent water from diffusing into the polymer. As mentioned earlier, swelling and plasticization could take place in the interface region earlier and result in the high water diffusing speed into the composites along the fiber direction. However, in the case of modified composites, water molecules have to go around the nano-silica particles and then go inner the polymer during their diffusing process in hygrothermal condition. As a consequence, the path that water traveled is significantly increased and the diffusing rate is slowed down as well. Another aspect, which is associated with the free volume in the polymer, nanoparticles could act as fillers that occupy the available free volume among the amorphous polymer segments where moisture can be stored. Nano-silica fills the amorphous region of pCBT in the interface region, and further results in decreasing water mass in the composites since the fillers have taken over the free volume. All the mentioned thermodynamic factors further result in the high anti-aging performance of the nanosilica modified composites and their low SDR. Another mechanism is based on the mechanical angle. During the aging tests, macromolecules are protected from tensile breakage since nano-silica in the polymer could hinder their motion as depicted in Fig. 16, and then the initial crack in the material is difficult to be formed. Additionally, once the initial crack is formed under the synergistic reaction of the temperature and humidity, nano-silica could hinder the crack from propagating. This is because in the propagation process of micro-crack in the interface, the crack tip may encounter the nano-fillers on the fiber surface and has to change its path. As a result, more fracture energy is consumed in the failure process of the modified composites. 4.3. Effect of nano-silica on the interface properties of the composites From the experiment results, it can be concluded that the modification of glass fiber by coating nano-silica on the surface is effective with

Fig. 16. Schematic diagram of nano-silica modified glass fiber reinforced pCBT composites.

Fig. 17. Typical load-displacement curves in fiber pull-out test after aging in 90RH%, 90 °C for 90 days. Subscripts d0 and fr are for initial debonding and friction, while dp and dm are for partial and maximum stress, respectively.

respect to enhancing the anti-aging performance. As mentioned, the slower SDR of nano-GF/pCBT composites in the aging environment is mainly due to the enhancement of interface strength. As known, one of the most important factors which lead to the failure of composite materials is interfacial debonding between fibers and matrix. When the interfacial shear stress increases up to the interfacial shear strength (IFSS), interfacial debonding will start immediately. In order to evaluate the interfacial behavior between fiber and pCBT matrix directly, the bonding property is investigated by pull-out tests. Typical load– displacement curve of unmodified and modified fiber reinforced specimens after aging in 90RH%, 90 °C for 90 days received in the pull-out test is shown in Fig. 17. From the observation, it can be found that composites without nano-silica in the interface region have a typical threestage evolution of the applied load versus displacement. This finding is in accord with the results obtained by Zhou et al. [26]. In comparison, nano-silica modified composites have a totally different shape in the curve. The bonding strength of modified composites is significantly larger than that of unmodified composites, and the curve shows a small platform instead of damage immediately after the load reaches its critical strength. Furthermore, attributed to the increased interfacial friction between fiber and matrix, the modified composites have higher friction force on the load–displacement curve in the pulled out stage after debonding completely occurs. 5. Conclusions In this study, aging properties of pCBT matrix, its glass fiber and nano-silica modified glass fiber reinforced composites were investigated with emphasis on the relationship between mechanical strength degeneration ratio (SDR) and aging duration. From the experimental results, the following conclusions can be summarized: • Long-term aging in hygrothermal environment resembles high test temperatures with respect to SDR of the composites, and the equivalent relation between the two factors can be found in the life curve spectrum of the materials. • Glass transition temperature (Tg) of the composites plays an important role in the lifetime prediction curve, thus when using high test temperature to accelerate the aging test, Tg should be taken carefully into consideration. • Glass fiber with nano-fillers on the surface could enhance the interface bonding strength between fiber and matrix significantly. The enhancement effect further results in the increasing anti-aging property of composites when serviced in both hygrothermal environment and elevated temperature condition. • From the microscopic view, nano-fillers on the fiber surface could hinder the thermal motion of pCBT molecular chain and water molecules

B. Yang et al. / Materials and Design 85 (2015) 14–23

from diffusing into the polymers along the interface region in elevated temperature and hygrothermal condition, respectively. Thus, GF/pCBT composites modified by coating nano-silica on fiber surface could be used for a longer time.

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