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Modification of bisphenol A dicyanate ester by carboxyl-terminated liquid butadiene-acrylonitrile and its composites.

INTRODUCTION

Cyanate ester (CE) resins are one of the most important kinds of thermosets and have received more and more attentions for their superior mechanical properties, low water absorptivity, low outgassing in curing, high environment resistance, and excellent dielectric properties. They have been widely used as adhesives and matrixes for composites [1, 2]. However, like most other thermosets, they have the drawback of brittleness. From the time the CEs have been used, many methods, such as polymer blending, fiber reinforcing, and filler filling, have been carried out to improve their toughness. Engineering thermoplastics, such as polysulfone, poly(ether sulfone), poly(ether imide), polyarylate, polyurethane, poly(ethylene phthalate) and poly(ether ketone ketone), are employed [3-7]. Thermosets, such as epoxy and bismaleimide, are also introduced [8-11]. Reactive rubbers are widely used in improving the properties of thermosets and have shown promising results in blends with epoxies and bismaleimides [12]. Unfortunately, only a few researches on bisphenol A dicyanate resin (BADCy)/carboxyl terminated liquid butadiene-acrylonitrile (CTBN) blends have been carried out and have shown attractive results. But, they were not systematical, and some of their results seem impossible, such as the mechanical properties in the work of Yu Feng et al. [13].

In this study, CTBN are introduced to modify CEs so as to improve their toughness. The properties of the modified systems and the fiber-reinforced composites were systematically studied.

EXPERIMENTAL

Materials

BADCy used in these experiments was supplied by the 637th Institute of Chinese Aerospace Association (Ji' nan, China). Its molecular weight is 278, and its weight purity is determined by HPLC, which is not less than 95%. CTBN are provided by Lanzhou Chemistry Institute (Lanzhou, China). Glass fiber used in the experiments was type EW210, supported by Nanjing Glass Fiber Institute (Nanjing, China). Other reagents were all commercial products and were used as received, without any further treatment.

Measurements

The gelation time of resins was determined on a standard hot-plate with a temperature controller. The resins were spread over hot plates heated to different temperatures. The time required for the resin to stop legging and become elastic is called the gelation time.

IR spectra were measured with a Fourier transform infrared type WQF-310 spectrophotometer (The 2nd Machine Factory of Beijing, Beijing, China).

Differential scanning calorimetric (DSC; PerkinElmer DSC-7) measurements were made at a scan rate of 10[degrees]C/min with 4-6-mg samples in a nitrogen atmosphere.

The mechanical properties of the cured resins were determined with a Shimadzu autograph AGS-500B universal testing machine, according to ASTM E-399. The flexural strength and interlaminar shear strength (ILSS) of glass fiber-reinforced composites were determined according to ASTM D 790 and ASTM D 2344, respectively.

The hot-wet resistance of cured resins was determined by placing the samples in boiling distilled water for 100 h, and during the process, at various length of time, the samples were removed, and wiped off with a dry cloth, and weighed immediately. Then, the value of water absorption was calculated according to ASTM D 570-81, and the value of heat deflection temperature (HDT) was determined.

The thermal stability of cured resins was obtained with a PerkinElmer thermogravimetric analyzer at a heating rate of 10[degrees]C/min in a nitrogen atmosphere.

Scanning electron micrographs (SEM) were taken with a scanning electron microscope (HITACHI S-570, Tokyo, Japan) on failed specimens from the flexural strength tests at accelerating voltages of 20 kV.

The dielectric properties were measured according to GB1409-78 of China.

Preparation of cured systems and fiber reinforced composites

CTBN was dissolved into BADCy at 100[degrees]C without solvents, and the resultant clear mixture was degassed in vacuum for 25 min at 100[degrees]C. Then, the mixture was poured into a mold, preheated at 130[degrees]C, to obtain a plaque. The mold consisted of one pair of upright stainless plates spaced by a U-shaped silicon rubber stick (8 mm). The amount (mass fraction) of CTBN was calculated on the basis of the matrix resin. The curing cycle was 130[degrees]C/1 h + 150[degrees]C/1 h + 180[degrees]C/2 h + 200[degrees]C/6 h. Then, the oven temperature was decreased from 200 to 50[degrees]C at a cooling rate of 50[degrees]C/h.

The blend of BADCy and CTBN was dissolved in acetone (the weight ratio of resin: acetone = 1:1). Glass fiber cloth was immersed in a plate with the resin solution for 1 min; prior to laying up, the prepreg was hung up for at least 18 h at room temperature to remove the entrapped solvent, and then, it was stacked between two metallic sheets. A thin layer of release cloth was kept between the metal and prepregs (12-ply) to prevent the adhesion of the composite to the metal (Fig. 1). Before heating, a pressure was applied for initial compaction of the plies. Then, a heating rate of 2[degrees]C/min was used to bring the stack to a cure temperature of 130[degrees]C under contact pressure. Close to the gelation point of the resin, a pressure of 0.7 MPa was applied and was maintained throughout the process. The schedule for the cure process of the composites is the same as that of the matrixes above. After cured, the stack was cooled at 5[degrees]C/min to 65[degrees]C, and the laminate panel was released from the bedplate. Ultrasound tests(C scans) were carried out on the cured laminate samples to determine their void concentration. In all cases, only good laminates (with no detectable flaws) could be used in subsequent tests.

[FIGURE 1 OMITTED]

RESULTS AND DISCUSSION

Curing Behavior of the Systems

The addition of CTBN in the system had a great influence on the gelation time of the systems at lower temperatures, but little influence at higher temperatures, as shown in Fig. 2. It indicates that at lower temperatures CTBN acts as a catalyst in the initial curing reaction of BADCy, but not in the range of temperature higher than 150[degrees]C.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Similar conclusion can be drawn from the DSC curves, as shown in Fig. 3. The addition of CBTN relaxes the curing reaction of BADCy, and system with CTBN has much less thermal release during the curing. It probably indicates that the carboxyl groups on the CTBN molecular chain are terminated by the impurities in BADCy below 150[degrees]C, and therefore at higher temperature, it doesn't appear in the DSC curve. While at the temperatures higher than 150[degrees]C, the systems also express the curing reaction of the triazine formation reaction of BADCy. On the basis of gelation time curves, DSC curves, and the previous researches on CEs, the curing cycle was set as 130[degrees]C/1 h + 150[degrees]C/1 h + 180[degrees]C/2 h + 200[degrees]C/6 h.

Fourier transform infrared spectroscopy (FTIR) was employed to study the influence of CTBN on the curing behavior of BADCy. Curing cycle of 130[degrees]C/0.5 h + 150[degrees]C/0.5 h + 180[degrees]C/0.5 h was applied to the original BADCy and BADCy modified by 15% CTBN, and FTIR curves of these are showed in Fig. 4. We chose the absorption of benzene as an interior standard for it didn't change during the whole curing process. The curing degree (X) was calculated according to Eq. 1, and the results are shown in Table 1.

X = 1 - [[[H.sub.i2272]/[H.sub.i1500]]/[[H.sub.o2272]/[H.sub.o1500]]] (1)

where X is curing degree, [H.sub.i2272] and [H.sub.i1500] are the heights of the absorption peaks at 2272 and 1500 [cm.sup.-1] at time i, respectively. [H.sub.o2272] and [H.sub.o1500] are the heights of the original absorption peaks at 2272 and 1500 [cm.sup.-1], respectively.

According to Fig. 4 and Table 1, the addition of CTBN has little influence on the curing behavior of BADCy; the curing degree of modified systems has been improved. During the lower temperatures, carboxyl groups on the CTBN chain accelerated the curing reaction of triazine of BADCy, which results in the higher curing degree of the modified system.

Mechanical Properties of the Cured Resin

Figure 5 shows the relationship between the content of CTBN and the mechanical properties of the cured resins. With the increasing CTBN content, the flexural strength expresses a trend of increasing in the beginning, and then, decreases rapidly. It reaches the maximum value when 15% CTBN was added; at this point, the flexural strength increases from 100.5 to 140.0 MPa. Moreover, the impact strength of the modified system presents an increasing trend with the increasing CTBN loading in the system. The increase in flexural strength at low loading (smaller than 15%) of CTBN comes from the toughing effect of CTBN, and the decrease at high loading of CTBN comes from the possible defects among the interphase of rubber and thermoset, which can also been seen in the surface microstructure of the failure specimen from impact tests; and at the same time, the properties of the system would be inclined to the rubber phase. When the content of CTBN was too little, less than 2%, CTBN added acts as an effective phase separator in the system, which enlarges the defects of the interphase between two phases and results in the minimum point on the impact strength curve at about 2%. With the increase in CTBN used, the cushion of the rubber phase, enlarged, results in the increasing of the impact strength. When the flexural strength and impact strength are taken as a whole into account, the best usage of CTBN in the modified system is 15%. The flexural and impact strength of BADCy/CTBN (85/15), compared with that of the original BADCy resin, are greater by 39.47% and 21.92%, respectively.

[FIGURE 4 OMITTED]

Water Uptake of the Cured Resins

The water uptakes of original and modified cured BADCy resins are given in Fig. 6. The water uptake of the cured resin enlarges with the increasing of CTBN used. The more the CTBN used, the higher the water uptake of the cured resins. The reason lies in the hydrophilic groups on the CTBN, which increases the water uptake of the cured resins. The more the CTBN used in the modified system, the more the hydrophilic groups in the whole system, and eventually, the higher the water uptake of the cured resin, and vice versa.

Thermal Properties of the Modified Systems

Figure 7 shows the thermogravimetric analysis (TGA) curves of the original and modified systems. The more the CTBN used, the lower the initial decomposition temperature ([T.sub.i]). In case of BADCy/CTBN (85/15), its [T.sub.i] value is 303[degrees]C, which is lower by about 24[degrees]C than that of pure BADCy resin. The situation comes from the poor thermal properties of CTBN: with the temperature rises, the rubber decomposes and leads to the phase collapse of the whole system, and results in the decrease in the [T.sub.i] value.

Figure 8 shows the relationship between the HDT and the mass fraction of CTBN used. Trends similar to the TGA curves are obtained. Based on the results shown in Figs. 6 and 7, in view of thermal properties, a conclusion can be drawn that the mass fraction of CTBN used in the modified systems must be controlled lower than 20%.

Microstructures of the Composites

Figure 9 gives the microstructures of the failure specimens from impact tests. Fracture surface of the original BADCy (Fig. 9a) is a typical brittle fracture. With the increase in CTBN used, as can be observed from the microstructures of the fracture zone, toughness showed more and more obviously. According to the theory of mechanics on fracture, there are two ways to improve the toughness of the cured resins, using CTBN. One is the expanding of the elastomer by the ungluing of CTBN particles and the holes fabricated from the fracture, and the other is the shearing bend distortion resulted from both CTBN particles and the holes. As shown in Figs. 9b-9e, there are more or less holes and particles ungluing. According to the theory of hole shearing bending [14], the cooperation of the tridirection stress field at the front of the crack and the stress remained from the curing of the particles led to the holes in the interior of the particles and the crack of the interface between particles and resins. On one hand, the holes would expand in their volumes, and on the other hand, the concentration of the stress on the equator of the particles would induce the shearing bending of the particles nearby. Moreover, the bending process would dull the crack, and then, weaken the concentration of the stress and prevent the fracture of the products, thereby resulting in toughening. Nevertheless, when the usage of CTBN was higher than 30% (Fig. 9e), rubber phase was very obvious in the system, and rubber particles can easily be seen in the photograph. Structural defects, which can be seen in Fig. 9e, may be due to the poor adhesion between BADCy and CTBN, which rapidly decreases the flexural strength and does not lead to toughening.

[FIGURE 5 OMITTED]

Dielectric Properties of the Cured Resins

Dielectric properties of the cured resins are displayed in Table 2, which indicate that the addition of CTBN decreases the dielectric properties of BADCy a little, obeying the blending law. The poor dielectric properties of CTBN lead to the increase of the dielectric constant and dielectric loss value. Compared with the improvement of the mechanical properties, the little influence on dielectric properties can almost be ignored.

[FIGURE 6 OMITTED]

Properties of Glass Fiber Reinforced Composites

Table 3 shows the properties of glass fiber EW210 reinforced composites based on original BADCy and BADCy/CTBN (85/15) [15]. The properties of the modified system are similar to those of the original one except for the obvious improvements in mechanical properties. Tendency similar to the properties of cured resins can be found. The flexural strength and ILSS of composites based on BADCy/CTBN (85/15) are about 112% and 130% higher than that of the composites based on BADCy, respectively. The addition of CTBN tends to weaken the dimension stability of the composites and leads to the decreasing of the flexural modulus of the composites. Other properties of the composites based on BADCy/CTBN (85/15) are almost the same as that of the original composite based on BADCy.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

Based on the known theories in fiber-reinforced composites, results obtained above seem impossible, especially the dielectric properties. This may be due to the hydrogen bonding. In the composites based on the modified matrix, between the polar group on the CTBN chain and that on the glass fiber (Si[O.sub.2]), hydrogen bonding can be formed. The unexpected good results in dielectric properties of composite suggest a different but correct way to do research in polymeric composites.

CONCLUSIONS

When the temperature is lower than 150[degrees]C, the addition of CTBN has great influence on the curing of BADCy, while the influence will weaken and disappear at higher temperature. The curing procedure, 120[degrees]C/1 h + 150[degrees]C/1 h + 180[degrees]C/2 h + 200[degrees]C/6 h, was determined by the DSC curves and the gelation time curves. Mechanical properties of BADCy resin can be largely improved by the addition of CTBN with little loss of thermal properties and dielectric properties. When 15% of CTBN is introduced, the flexural strength and impact strength of the cured resin, compared with that of the original BADCy resin, show a 39.5% and 21.9% increase, respectively, whereas the flexural strength and interlayer shear strength of glass fiber reinforced composites show a 12.1% and 29.8% increase, respectively.

REFERENCES

1. J. Dupuy, E. Leroy, A. Maazouz, J.P. Pascault, M. Raynand, and E. Bournez, Thermochim. Acta, 388, 313 (2002).

2. D. Mathew, C.P.R. Nair, and K.N. Ninan, J. Appl. Polym. Sci., 77, 75 (2000).

3. I. Hamerton, Chemistry and Technology of Cyanate Ester Resins, Chapman and Hall, London (1994).

4. C. Marieta, M. del Rio, I. Harismendy, and I. Mondragon, Eur. Polym. J., 36, 1445 (2000).

5. T. Iijima, S. Katsurayama, W. Fukuda, and M. Tomoi, J. Appl. Polym. Sci., 76, 208 (2000).

6. A Bartolotta, G. Di Marco, M. Lanza, G. Carini, G. D'Angelo, G. Tritpodo, A. Fainleib, I. Danilenko, and S. Sergeeva, J. Non-Cryst. Solids, 307-310, 698 (2002).

7. C.P.R. Nair, D. Mathew, and K.N. Ninan, Adv. Polym. Sci., 155, 1 (2000).

8. D. Mathew, C.P.R. Nair, and K.N. Ninan, J. Appl. Polym. Sci., 74, 1675 (1999).

9. C. Wang and H. Hwang, U.S. Patent 5,886,134 (1999).

10. M. Gaku and K. Nakamichi, U.S. Patent 4,110,364 (1978).

11. G.Z. Liang and M.X. Zhang, J. Appl. Polym. Sci., 85, 2377 (2002).

12. Y. Zhang and C. Shen, Mater. Eng., 5, 17 (1995) (in Chinese).

13. Y. Feng, Z. Fang, and A. Gu, Polym. Adv. Technol., 15, 628 (2004).

14. P.X. Wu and L.C. Zhang, Polymer Blending, Chinese Light Industry Publishing, Beijing (1998).

15. W. Li, Masters Dissertation, Northwestern Polytechnical University, Xi'an, China (2001).

Jieliang Wang, Guozheng Liang, Wen Zhao, Shenghua Lu, Hongxia Yan

Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi'an, Shaanxi, People's Republic of China

Correspondence to: Guozheng Liang; e-mail: lgzheng@nwpu.edu.cn

Contract grant sponsor: National Natural Science Foundation of China; contract grant number: 50273029; contract grant sponsor: Special Research Foundation of Shaanxi Education Department; contract grant number: 03JK080.
TABLE 1. Curing degree of BADCy and BADCy/CTBN (85/15) on each test
point.

 Curing degree (%)
Test point BADCy BADCy/CTBN (85/15)

Room temperature 0 0
+ 130[degrees]C/0.5 h 5.2 10.2
+ 150[degrees]C/0.5 h 21.7 53.4
+ 180[degrees]C/0.5 h 85.1 90.0

TABLE 2. Dielectric properties of the cured resins.

 Dielectric
Mass fraction of CTBN constant Dielectric loss
in the matrix (%) Average [C.sub.v] Average [C.sub.v]

 0 2.9 0.1 0.007 0.001
 2 3.0 0.2 0.008 0.001
 5 3.2 0.1 0.008 0.0005
 8 3.2 0.2 0.008 0.001
15 3.3 0.1 0.010 0.001
25 3.5 0.1 0.012 0.002

TABLE 3. Properties of the glass fiber-reinforced composites based on
different matrix system.

 Matrix system
Properties BADCy/CTBN (a) BADCy

Flexural strength (MPa) 569.6 508.0
Flexural modulus (GPa) 13.1 36.4
Interlayer shear strength (MPa) 52.3 40.3
Mass fraction of resin in the 34 33
 composites (%)
Water uptake (in boiling water 1.07 1.08
 for 100 h) (%)
Dielectric constant 4.22 4.21
Dielectric loss 0.012 0.012

(a) Mass fraction of CTBN was 15%.
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Author:Wang, Jieliang; Liang, Guozheng; Zhao, Wen; Lu, Shenghua; Yan, Hongxia
Publication:Polymer Engineering and Science
Date:May 1, 2006
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