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BADCy composite
Composites: Part A 37 (2006) 46–53 www.elsevier.com/locate/compositesa
ZnO whisker reinforced M40/BADCy composite Penggang Ren, Guozheng Liang*, Zengping Zhang, Tingli Lu Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, People’s Republic of China Received 9 December 2004; revised 22 April 2005; accepted 7 May 2005
Abstract The graphite fiber (M40) reinforced bisphenol A dicyanate (2,2 0 -bis (4-cyanatophenyl) isopropylidene resin) (BADCy) composite, which contains different content of ZnO whiskers treated or untreated by coupling agent g-glycidyloxypropyltrimethoxysilane (KH560), was prepared in order to investigate the influence of ZnO whiskers on the properties of the composite. DSC and FTIR analysis show that ZnO whiskers have a strong catalytic effect on cure reaction of cyanate ester (BADCy) and can decrease the cure temperature of BADCy. The micro-morphology of ZnO whisker distribution in resin with various stirring rates was studied by means of SEM. Results showed that stirring rates between 1200 and 1500 rpm could make the whiskers have good distribution, while the stirring rates over 1800 rpm will break the whiskers. Mechanical properties revealed that ZnO whisker treated by KH560 could promote the flexural strength and interlaminar shear strength (ILSS) of M40/BADCy composite. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Carbon fiber composite; B. Mechanical properties; Bisphenol A dicyanate (BADCy)
1. Introduction Graphite fiber reinforced cyanate ester resin matrix composites possess relatively high stiffness and strength, excellent hot–wet resistance and the ability to adapt to space environment. They have great potential as a new generation of structural component material on space aircrafts and in consequence attract much attention [1–3]. However, the application of graphite fiber/cyanate ester resin composites is limited by the poor interfacial adhesion between fibers and the matrix. The poor interfacial adhesion results not only from the high inertia and smoothness of the graphite fiber surface [4] but also from the high brittleness of the cured cyanate ester resin [5,6]. So these composites are always in need of interlayer reinforcement in which an effective method is incorporating whiskers into the composites. Whiskers are monocrystals with the diameter from 0.1 to 10 mm and length/diameter ratio from 10 to 1000 or more. Owing to a few internal flaws in whiskers, their strength * Corresponding author. Tel./fax: C86 29 8847 4080. E-mail address: [email protected] (G. Liang).
1359-835X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2005.05.018
and modulus are almost as high as the theoretical values. Whiskers are founded to be one of the strongest solids and can be used as a new category of reinforcing additive for composite materials [7]. Recently, over 100 kinds of whiskers have been developed, most of which are of one-dimension structure (stick-shaped or spiculate). When they are used to reinforce the composite, they cannot be arranged well in the interlaminar direction of the composite because of the influence of resin flowing. As a result, the interlaminar shear strength (ILSS) increases not obviously when these one-dimensional whiskers are introduced [8]. Zinc oxide (ZnO) whiskers have a peculiar threedimensional tetrapod-shaped structure, with four pods binding together in one point as the center and stretching to each corner of the tetrahedron. The angle between every two pods is 109828 0 [9]. Owing to the peculiar structure of ZnO whiskers, isotropic distribution of the whiskers as reinforcement agent can produce better interlaminar toughening effect than that of the onedimensional whiskers. In this paper, ZnO whiskers reinforced high modulus M40/bisphenol A dicyanate (2,2 0 -bis (4-cyanatophenyl) isopropylidene) (BADCy) composites were studied, trying to investigate the reinforcing effect and mechanisms of ZnO whiskers on the composite.
P. Ren et al. / Composites: Part A 37 (2006) 46–53
47
Scheme 1. Chemical structures of BADCy, E51 and KH560.
2. Experimental 2.1. Materials Bisphenol A dicyanate (2,2 0 -bis (4-cyanatophenyl) isopropylidene) with purity O99.5%, white particular crystal, was synthesized in our laboratory. Epoxy resin E51 with the epoxy value of 0.52 was produced by Xi’an resin factory in China. Coupling agent g-glycidyloxypropyltrimethoxysilane (KH560) was supplied by Jingzhou Jianghan fine chemicals Co., Ltd in China. The chemical structures of these materials are shown in Scheme 1. M40-3K-40B graphite fiber was purchased from Toray Corporation of Japan. Tetrapodshaped ZnO whisker was purchased from Chengdu JiaoTong University and JingYu Company of Science and Technology in China. Some properties of ZnO whisker are shown in Table 1.
50 8C. Then acetone and various weight of whisker were added to get the resin solution. The solution was stirred for 5 min on a FJ-200 high-speed dispersing machine with the stirring rate being 1500 rpm. Then it was made into prepreg for the unidirectional M40/BADCy composite. When the content of volatile components was less than 5 wt% after drying at room temperature, the prepreg was dried at 65 8C for 5–10 min. After that, the prepreg cloth was tailored, stacked and pressure moulded according to certain regulations. The temperature to bear is 130 8C (for 15 min) with the pressure being 0.7 MPa. Afterwards the resin was cured according to this cure procedure: 130 8C/1 hC150 8C/1 hC180 8C/2 h-room temperature (the composite without ZnO whiskers needed to be postcured at 200 8C for 2 h). Finally, the cured unidirectional M40/BADCy composite plates were cut into specimens of certain dimensions. 2.4. Performance testing
2.2. Composition systems Three resin systems used in the experiment are as follows: #
#
1 K BADCy; 2 K BADCy=E51 Z 99 : 1; #
3 K BADCy=E51=ZnO Z 89 : 1 : 10 2.3. Preparation of the unidirectional M40/BADCy composite specimens Having been prepolymerized at 80 8C for 40 min, BADCy resin system was cooled to a temperature below
DSC measurement was performed with a Perkin Elmer DSC-7 supported by a Perkin Elmer computer for data Table 1 Properties of ZnO whisker Properties
Datum
Projection length of the part (mm) Diameter of basal part (mm) Max. operating temp. (8C) Density (g/cm3) Apparent density (g/cm3) Tensile strength (GPa) Tensile modulus (GPa) Coefficient of thermal expansion (%/8C)
5–200 0.1–10 1720 5.8 0.01–0.05 12 350 4!10K6
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P. Ren et al. / Composites: Part A 37 (2006) 46–53
acquisition: the DSC was calibrated with high-purity indium. Sample (8–10 mg) was weighed into small DSC aluminum pans, sealed with holed aluminum lids. After that, experiments were conducted under a nitrogen flow of 20 cm3/min. All the samples were subjected to a dynamic DSC scan from 50 to 300 8C at a heating rate of 10 8C/min. The FTIR measurements were carried on a WQF-300 FTIR spectrometer. Resin systems were dissolved in acetone before being deposited on a KBr pellet. After removal of acetone, the KBr pellet was then used for FTIR measurement. The micro morphology of the cured resin was investigated by scanning electron microscopy (SEM) on a HITACHI S-570 SEM. The fracture surface was coated with gold by vapor deposition using a Polaron SC502 vacuum sputter coater. Flexural properties and shear strengths of the M40/BADCy composite plates were tested in term of ASTM D790-03 and ASTM D2344, respectively.
3. Results and discussion 3.1. Study of the reactivity of ZnO whiskers modified BADCy system Polymerization of the pure BADCy resin cannot take place when the resin is heated. Catalysts are needed to accelerate the reaction. The catalysts applied to BADCy can be transition metallic salts and the compounds containing active hydrogen [10,11]. Because epoxy resin (E51) cannot only accelerate the polycyclomerization but also toughen the resin matrix, E51 modified BADCy was used as the resin matrix for composite in this study. The DSC curves of BADCy and its modified systems are shown in Fig. 1. When E51 is introduced, the inverse peak temperature of BADCy is decreased from 232 to 223 8C. And ZnO whisker can
1.3 1.2 1.1 1.0 0.9 0.8
3#
0.7 0.6 0.5
2#
0.4 0.3
1#
0.2 0.1 0.0 4000
3500
3000
2500
2000
1500
1000
Wavenumber / cm–1 Fig. 2. FTIR spectrum curves of BADCy and its modified systems.
decrease this temperature further to 199 8C. This indicates that both E51 and ZnO whisker are catalytic for the polycyclomerization of the cyanate ester. The catalytic effect of E51 on cyanate esters is attributed to the hydroxyl groups of E51 which can accelerate the trimerization of the –OCN groups. The catalytic mechanism of ZnO whiskers is still not clearly known. It has been reported that transition metallic ions such as Zn2C show strongly catalytic effects for the polycyclomerization of the cyanate ester due to their low coordination number and high ligand mobility during the cure process [11,12]. It is assumed that though ZnO whisker is insoluble in cyanate ester resin to form Zn2C, the reactivity of cyanate ester resin being in contact with the surface of ZnO whisker may be enhanced. As a result, ZnO whisker has a catalytic effect for polycyclomerization of cyanate ester owing to its small dimension and large surface area. But the catalytic effect of ZnO whiskers on the curing of cyanate ester is not so strong as that of Zn2C. The FTIR spectrum curves of BADCy and its modified systems are shown in Fig. 2, and FTIR data of the chemical groups in the cyanate-epoxy system are shown in Table 2. Absorbency of the FTIR band is illustrated by the height of the peak with the height of the absorbent peak of phenyl as the internal standard. The conversion of –OCN groups at a certain temperature can be determined by means of the hð2270 cmK1 Þ =hð830 cmK1 Þ ratio combined with function (1) [13, 14]. "
# hð2270 cmK1 Þt !hð830 cmK1 Þt0 XðKOCNÞ Z 1 K !100% hð2270 cmK1 Þt0 !hð830 cmK1 Þt
Fig. 1. DSC curves of BADCy and its modified systems.
(1)
Note: t and t0 stand for the reactive time when it is t and 0, respectively. The calculated curve of conversion of –OCN versus time is obtained by this method (Fig. 3). As shown in Fig. 3, after the BADCy of industry purity was cured for 200 min at 160
P. Ren et al. / Composites: Part A 37 (2006) 46–53
49
Table 2 FTIR bands of the chemical groups in the cyanate-epoxy system Chemical group
–OCN
Wave number/ cmK1
2270
N
C
O
C
N
Triazine ring
CH2
CH O
1760
1670
and 180 8C, the conversion of –OCN groups was only 70 and 86%, respectively. Whereas, when 1 wt% of E51 and 10 wt% of ZnO whisker were introduced into BADCy, the conversion of –OCN groups increased evidently under the same cure conditions. The conversion of 3# system reached 100% after being cured for 200 min at 160 8C or cured for 140 min at 180 8C, but the conversion of 2# system reached only 98% after being cured for 200 min at 180 8C. As is known, the cure temperature needs to be higher in order to obtain the full conversion of the –OCN groups for the 2# system. Consequently, the cure cycles of the two resin systems are determined as follows: 130/2 hC160/2 hC180/ 3 h for 3# system; 130/2 hC160/2 hC180/3 hC200/2 h for 2# system.
Fig. 3. Conversion curves of BADCy and its modified systems at 160 and 180 8C.
1560, 1369
915
830
3.2. Study of the dispersion of ZnO whiskers in the composite Micrograph of ZnO whiskers is shown in Fig. 4. It can be seen that the ZnO whisker has a peculiar tetrapod-shaped structure, thereby three-dimensional isotropic distribution of the whiskers as reinforcement agent is achieved and better interlaminar-reinforcing effect superior to that of the one-dimensional whiskers is obtained [8]. But the ZnO whiskers are so small that they tend to agglomerate and in consequence inner flaws may be formed in the composite. Fig. 5 shows the agglomeration of ZnO whiskers in the BADCy matrix with 10 wt% of ZnO. It can be seen that the agglomerated ZnO whiskers in the resin matrix can produce a great deal of inner flaws and stress concentration, which defect the properties of the composites. So, ZnO whisker must be dispersed uniformly in the resin matrix before the incorporation of M40 fiber. An often used and easy processing method is high rate stirring. It is noteworthy that ZnO whiskers cannot be dispersed well in the matrix at a very low stirring rate, while they may be broken off when stirring rates are too high. Under neither of the two circumstances above can good reinforcement be obtained. The influence of stirring rates on the morphology of the whiskers was studied by means of SEM. As shown in Fig. 6, the whiskers maintained their original morphology in resin solution when the stirring rate was 1200 rpm (Fig. 6b). Some of the whiskers were broken when the stirring rate
Fig. 4. Microstructure of ZnO whiskers.
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P. Ren et al. / Composites: Part A 37 (2006) 46–53
was 1500 rpm (Fig. 6c). And most of the whiskers were broken at 1800 rpm (Fig. 6d). Then owing to the onedimensional structure and small length/diameter ratio of ZnO whiskers, poor reinforcement was obtained. Fig. 7 shows the mechanical properties of unidirectional M40/BADCy composite containing 10 wt% of ZnO whisker under various stirring rates. As shown in Fig. 7, when the stirring rate was less than 1500 rpm, the ISSL and flexural strength were increased with elevating the stirring rate. Whereas, when the stirring rate was above 1500 rpm, the mechanical properties decreased with the increasing stirring rate. According to the above discussion, the appropriate stirring rate should be between 1200 and 1500 rpm. 3.3. Influence of ZnO whiskers on the performances of M40/BADCy composite
Fig. 5. Agglomeration of ZnO whiskers in the BADCy matrix.
It is well known that coupling agents of organic and inorganic tips which can react with inorganic and organic
Fig. 6. SEM images of ZnO whiskers under various stirring rates: (a) Original; (b) 1200 rpm; (c) 1500 rpm; (d) 1800 rpm.
P. Ren et al. / Composites: Part A 37 (2006) 46–53
3.4. Reinforcing mechanisms analysis of ZnO whiskers
85
1100
80
1040 75
1020 1000
ILSS /MPa
Flexural strength /MPa
1080 1060
70
980 960 0
500
1000
1500
2000
51
65 2500
stirring rate /rpm Fig. 7. Mechanical properties of unidirectional M40/BADCy composites versus stirring rate (here M40/BADCy was reinforced by 10 wt% of untreated ZnO whiskers).
components, respectively, are often used to form chemical bonding between inorganic filler and resin matrices. Here, ZnO whiskers were treated by KH560 at first, to enhance the bonding between ZnO whiskers and BADCy. Table 3 shows some mechanical properties of unidirectional M40/BADCy composites reinforced by KH560 treated ZnO whiskers. Results in Table 3 indicate that treated ZnO whisker improves the flexural modulus and ILSS of the composite obviously, whereas the flexural and tensile strength are hardly affected. The flexural modulus increases with the increase in ZnO whisker content. When 15 wt% of ZnO whisker was introduced, flexural modulus of the composite increased by 17% to 208.9 GPa. And when the content of ZnO whisker reaches 10 wt%, the ILSS of the composite increased by 21% to its maximum value (82.2 MPa). The ILSS of the composite with ZnO whisker O10 wt% decreases with the increase in the whisker content. The moisture absorption coefficient of the composite is also related to the ZnO whisker content. The more the content of ZnO whisker, the more moisture absorption—reveals that some flaws exist in the interface between ZnO whisker and BADCy resin matrix. These flaws susceptible to water molecules may be caused by the difference in the coefficients of thermal expansion of ZnO whisker and BADCy.
Flexural and shear scanning electron microscope fractographs (SEM fractograph) of the M40/BADCy composite containing 10 wt% ZnO whisker are shown in Fig. 8. The micro cracks usually extend along the interfaces between fiber and resin matrix. When whiskers are located in the plane where cracks develop (Fig. 8b), the extending direction of the crack will be changed as crack tip encounters the whisker, leading to the increase of the fracture surface area. In consequence, ILSS was enhanced to some extent. When the inclined whiskers transfix the M40 fibers, ZnO whiskers have ‘nail’ effect on layers of the composite (shown in Fig. 8a and c). They can carry part of the interlaminar shear stress. And the ILSS can be improved obviously due to the high strength of ZnO whisker. Owing to the effect of the two reinforcing mechanisms mentioned above, ZnO whisker showed excellent reinforcement effect on the unidirectional M40/BADCy composite. The increase of ZnO whiskers content arouses two contrary effects. On the one hand, the probability of ‘nail’ effects will be increased and in consequence the ILSS of composite will be improved. On the other hand, the ZnO whisker agglomeration, which is harmful to mechanical strength, is accelerated at the same time. The agglomerated ZnO whiskers do not increase the occurrence of ‘nail’ effects, but cause local concentration of resin matrix and initiate flaws in the composite and in consequence decrease the mechanical properties. This is the reason why the M40/BADCy composite reinforced by 10 wt% of ZnO whisker displays the best mechanical properties. The agglomeration of ZnO whiskers is difficult to be observed in composite because of the existence of M40 fiber, unless the whisker content is high enough. And it has been practically observed in the SEM fractograph of the composite when 25 wt% of ZnO whisker is introduced (given in Fig. 9).
4. Conclusions ZnO whiskers showed obviously catalytic effect on the curing reaction of BADCy. Ten weight percent of ZnO
Table 3 Properties of M40/BADCy composite modified by ZnO whisker Mass fraction of ZnO whisker (wt%) 0 5 10 15 25 a
Flexural strength sb (MPa)
Flexural modulus G (GPa)
Tensile strength s (MPa)
Tensile modulus E (GPa)
ILSS z (MPa)
Water absorptiona (%)
Dry
Boileda
Dry
Boileda
Dry
Dry
Dry
Boileda
1064 1056 1075 1061 998
1007 998 1006 1001 967
178.5 195.1 206.3 208.9 197.4
170.4 185.2 186.5 184.8 178.1
1109.7 1081.2 1056.7 1019.4 1011.3
208.7 211.2 215.9 219.6 202.1
67.8 74.5 82.2 76.1 63.4
62.5 67.7 74.3 69.7 51.7
Water boiled for 100 h.
0.75 0.78 0.81 0.88 0.97
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P. Ren et al. / Composites: Part A 37 (2006) 46–53
Fig. 8. Fracture SEM image comparison of M40/BADCy composite modified by ZnO whiskers.
whisker could decrease the cure temperature from 200 to 180 8C. Untreated ZnO whiskers could improve the flexural modulus of M40/BADCy composite, whereas the flexural strength and ILSS were decreased at the same time. In contrast, KH560 treated ZnO whiskers displayed better reinforcing effect on the overall structural properties of M40/BADCy. The optimum reinforcing effect that the ILSS and flexural modulus increased by 21 and 17%, respectively, was obtained when 10 wt% ZnO whisker was introduced. SEM study showed that ZnO whiskers had beneficial ‘nail’ effect on layers of the composite.
References
Fig. 9. SEM image of agglomerated ZnO whiskers in M40/BADCy composite.
[1] Marieta C, Schulz E, Mondragon I. Characterization of interfacial behaviour in carbon-fibre/cyanate composite. Compos Sci Technol 2002;62(2):299–309.
P. Ren et al. / Composites: Part A 37 (2006) 46–53 [2] Kern KT, Stancil PC, Harries WL. Simulated space environmental effects on a polyetherimide and its carbon fiber-reinforced composite. SAMPE J 1993;29(3):29–44. [3] Bansemir H, Haider O. Fibre composite structures for space applications—recent and future developments. Cryogenics 1998; 38(1):51–9. [4] Montes-Moran MA, Young RJ. Raman spectroscopy study of HM carbon fibres: effect of plasma treatment on the interfacial properties of single fibre/epoxy composite; part I: fibre characterisation. Carbon 2000;40(6):845–55. [5] Liang GZ, Zhang MX. Enhancement of processability of cyanate ester resin via copolymerization with epoxy resin. J Appl Polym Sci 2002; 85(11):2377–81. [6] Iijima T, Maeda T, Tomoi M. Toughening of cyanate ester resin by N-phenylmaleimide-styrene copolymers. J Appl Polym Sci 1999;74: 2931–9. [7] Pan JS, Chen YH. Whiskers and their applications. ACTA Mater Compos Sin 1995;12(4):1–7 [Chinese].
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[8] Hu XL. The study on thermosetting resin modified by whiskers. Doctoral thesis. Northwest Polytechnical University, Xi’an, China, 2002. [9] Liu JH, Sun J, Li SM, Chen DM. Preparation and electromagnetic performance of tetrapod-shaped ZnO whiskers. ACTA Mater Compos Sin 2003;20(6):121–4 [Chinese]. [10] Harismendy I, Gomez CM, Rio MD, Mondragon I. Cure monitoring of catalysed cyanate ester resins. Polym Int 2000; 49(7):735–42. [11] Hamerton I, Hay JN. Recent developments in the chemistry of cyanate esters. Polym Int 1998;47(4):465–73. [12] Hamerton I, Takeda S. A study of the polymerization of novel cyanate ester/acrylate blends. Polymer 2000;41:1647–56. [13] Chen P, Cheng ZX, Guo XX. A study of co-curing of epoxy and cyanate resin. Acta Polym Sin 2000;4:472–5 [Chinese]. [14] Bartolomeo P, Chailan JF, Vernet JL. Curing of cyanate ester resin: a novel approach based on FTIR spectroscopy and comparison with other techniques. Eur Polym J 2001;37:659–70.
ZnO whisker reinforced M40/BADCy composite Penggang Ren, Guozheng Liang*, Zengping Zhang, Tingli Lu Department of Applied Chemistry, School of Science, Northwestern Polytechnical University, Xi’an, Shaanxi 710072, People’s Republic of China Received 9 December 2004; revised 22 April 2005; accepted 7 May 2005
Abstract The graphite fiber (M40) reinforced bisphenol A dicyanate (2,2 0 -bis (4-cyanatophenyl) isopropylidene resin) (BADCy) composite, which contains different content of ZnO whiskers treated or untreated by coupling agent g-glycidyloxypropyltrimethoxysilane (KH560), was prepared in order to investigate the influence of ZnO whiskers on the properties of the composite. DSC and FTIR analysis show that ZnO whiskers have a strong catalytic effect on cure reaction of cyanate ester (BADCy) and can decrease the cure temperature of BADCy. The micro-morphology of ZnO whisker distribution in resin with various stirring rates was studied by means of SEM. Results showed that stirring rates between 1200 and 1500 rpm could make the whiskers have good distribution, while the stirring rates over 1800 rpm will break the whiskers. Mechanical properties revealed that ZnO whisker treated by KH560 could promote the flexural strength and interlaminar shear strength (ILSS) of M40/BADCy composite. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Carbon fiber composite; B. Mechanical properties; Bisphenol A dicyanate (BADCy)
1. Introduction Graphite fiber reinforced cyanate ester resin matrix composites possess relatively high stiffness and strength, excellent hot–wet resistance and the ability to adapt to space environment. They have great potential as a new generation of structural component material on space aircrafts and in consequence attract much attention [1–3]. However, the application of graphite fiber/cyanate ester resin composites is limited by the poor interfacial adhesion between fibers and the matrix. The poor interfacial adhesion results not only from the high inertia and smoothness of the graphite fiber surface [4] but also from the high brittleness of the cured cyanate ester resin [5,6]. So these composites are always in need of interlayer reinforcement in which an effective method is incorporating whiskers into the composites. Whiskers are monocrystals with the diameter from 0.1 to 10 mm and length/diameter ratio from 10 to 1000 or more. Owing to a few internal flaws in whiskers, their strength * Corresponding author. Tel./fax: C86 29 8847 4080. E-mail address: [email protected] (G. Liang).
1359-835X/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.compositesa.2005.05.018
and modulus are almost as high as the theoretical values. Whiskers are founded to be one of the strongest solids and can be used as a new category of reinforcing additive for composite materials [7]. Recently, over 100 kinds of whiskers have been developed, most of which are of one-dimension structure (stick-shaped or spiculate). When they are used to reinforce the composite, they cannot be arranged well in the interlaminar direction of the composite because of the influence of resin flowing. As a result, the interlaminar shear strength (ILSS) increases not obviously when these one-dimensional whiskers are introduced [8]. Zinc oxide (ZnO) whiskers have a peculiar threedimensional tetrapod-shaped structure, with four pods binding together in one point as the center and stretching to each corner of the tetrahedron. The angle between every two pods is 109828 0 [9]. Owing to the peculiar structure of ZnO whiskers, isotropic distribution of the whiskers as reinforcement agent can produce better interlaminar toughening effect than that of the onedimensional whiskers. In this paper, ZnO whiskers reinforced high modulus M40/bisphenol A dicyanate (2,2 0 -bis (4-cyanatophenyl) isopropylidene) (BADCy) composites were studied, trying to investigate the reinforcing effect and mechanisms of ZnO whiskers on the composite.
P. Ren et al. / Composites: Part A 37 (2006) 46–53
47
Scheme 1. Chemical structures of BADCy, E51 and KH560.
2. Experimental 2.1. Materials Bisphenol A dicyanate (2,2 0 -bis (4-cyanatophenyl) isopropylidene) with purity O99.5%, white particular crystal, was synthesized in our laboratory. Epoxy resin E51 with the epoxy value of 0.52 was produced by Xi’an resin factory in China. Coupling agent g-glycidyloxypropyltrimethoxysilane (KH560) was supplied by Jingzhou Jianghan fine chemicals Co., Ltd in China. The chemical structures of these materials are shown in Scheme 1. M40-3K-40B graphite fiber was purchased from Toray Corporation of Japan. Tetrapodshaped ZnO whisker was purchased from Chengdu JiaoTong University and JingYu Company of Science and Technology in China. Some properties of ZnO whisker are shown in Table 1.
50 8C. Then acetone and various weight of whisker were added to get the resin solution. The solution was stirred for 5 min on a FJ-200 high-speed dispersing machine with the stirring rate being 1500 rpm. Then it was made into prepreg for the unidirectional M40/BADCy composite. When the content of volatile components was less than 5 wt% after drying at room temperature, the prepreg was dried at 65 8C for 5–10 min. After that, the prepreg cloth was tailored, stacked and pressure moulded according to certain regulations. The temperature to bear is 130 8C (for 15 min) with the pressure being 0.7 MPa. Afterwards the resin was cured according to this cure procedure: 130 8C/1 hC150 8C/1 hC180 8C/2 h-room temperature (the composite without ZnO whiskers needed to be postcured at 200 8C for 2 h). Finally, the cured unidirectional M40/BADCy composite plates were cut into specimens of certain dimensions. 2.4. Performance testing
2.2. Composition systems Three resin systems used in the experiment are as follows: #
#
1 K BADCy; 2 K BADCy=E51 Z 99 : 1; #
3 K BADCy=E51=ZnO Z 89 : 1 : 10 2.3. Preparation of the unidirectional M40/BADCy composite specimens Having been prepolymerized at 80 8C for 40 min, BADCy resin system was cooled to a temperature below
DSC measurement was performed with a Perkin Elmer DSC-7 supported by a Perkin Elmer computer for data Table 1 Properties of ZnO whisker Properties
Datum
Projection length of the part (mm) Diameter of basal part (mm) Max. operating temp. (8C) Density (g/cm3) Apparent density (g/cm3) Tensile strength (GPa) Tensile modulus (GPa) Coefficient of thermal expansion (%/8C)
5–200 0.1–10 1720 5.8 0.01–0.05 12 350 4!10K6
48
P. Ren et al. / Composites: Part A 37 (2006) 46–53
acquisition: the DSC was calibrated with high-purity indium. Sample (8–10 mg) was weighed into small DSC aluminum pans, sealed with holed aluminum lids. After that, experiments were conducted under a nitrogen flow of 20 cm3/min. All the samples were subjected to a dynamic DSC scan from 50 to 300 8C at a heating rate of 10 8C/min. The FTIR measurements were carried on a WQF-300 FTIR spectrometer. Resin systems were dissolved in acetone before being deposited on a KBr pellet. After removal of acetone, the KBr pellet was then used for FTIR measurement. The micro morphology of the cured resin was investigated by scanning electron microscopy (SEM) on a HITACHI S-570 SEM. The fracture surface was coated with gold by vapor deposition using a Polaron SC502 vacuum sputter coater. Flexural properties and shear strengths of the M40/BADCy composite plates were tested in term of ASTM D790-03 and ASTM D2344, respectively.
3. Results and discussion 3.1. Study of the reactivity of ZnO whiskers modified BADCy system Polymerization of the pure BADCy resin cannot take place when the resin is heated. Catalysts are needed to accelerate the reaction. The catalysts applied to BADCy can be transition metallic salts and the compounds containing active hydrogen [10,11]. Because epoxy resin (E51) cannot only accelerate the polycyclomerization but also toughen the resin matrix, E51 modified BADCy was used as the resin matrix for composite in this study. The DSC curves of BADCy and its modified systems are shown in Fig. 1. When E51 is introduced, the inverse peak temperature of BADCy is decreased from 232 to 223 8C. And ZnO whisker can
1.3 1.2 1.1 1.0 0.9 0.8
3#
0.7 0.6 0.5
2#
0.4 0.3
1#
0.2 0.1 0.0 4000
3500
3000
2500
2000
1500
1000
Wavenumber / cm–1 Fig. 2. FTIR spectrum curves of BADCy and its modified systems.
decrease this temperature further to 199 8C. This indicates that both E51 and ZnO whisker are catalytic for the polycyclomerization of the cyanate ester. The catalytic effect of E51 on cyanate esters is attributed to the hydroxyl groups of E51 which can accelerate the trimerization of the –OCN groups. The catalytic mechanism of ZnO whiskers is still not clearly known. It has been reported that transition metallic ions such as Zn2C show strongly catalytic effects for the polycyclomerization of the cyanate ester due to their low coordination number and high ligand mobility during the cure process [11,12]. It is assumed that though ZnO whisker is insoluble in cyanate ester resin to form Zn2C, the reactivity of cyanate ester resin being in contact with the surface of ZnO whisker may be enhanced. As a result, ZnO whisker has a catalytic effect for polycyclomerization of cyanate ester owing to its small dimension and large surface area. But the catalytic effect of ZnO whiskers on the curing of cyanate ester is not so strong as that of Zn2C. The FTIR spectrum curves of BADCy and its modified systems are shown in Fig. 2, and FTIR data of the chemical groups in the cyanate-epoxy system are shown in Table 2. Absorbency of the FTIR band is illustrated by the height of the peak with the height of the absorbent peak of phenyl as the internal standard. The conversion of –OCN groups at a certain temperature can be determined by means of the hð2270 cmK1 Þ =hð830 cmK1 Þ ratio combined with function (1) [13, 14]. "
# hð2270 cmK1 Þt !hð830 cmK1 Þt0 XðKOCNÞ Z 1 K !100% hð2270 cmK1 Þt0 !hð830 cmK1 Þt
Fig. 1. DSC curves of BADCy and its modified systems.
(1)
Note: t and t0 stand for the reactive time when it is t and 0, respectively. The calculated curve of conversion of –OCN versus time is obtained by this method (Fig. 3). As shown in Fig. 3, after the BADCy of industry purity was cured for 200 min at 160
P. Ren et al. / Composites: Part A 37 (2006) 46–53
49
Table 2 FTIR bands of the chemical groups in the cyanate-epoxy system Chemical group
–OCN
Wave number/ cmK1
2270
N
C
O
C
N
Triazine ring
CH2
CH O
1760
1670
and 180 8C, the conversion of –OCN groups was only 70 and 86%, respectively. Whereas, when 1 wt% of E51 and 10 wt% of ZnO whisker were introduced into BADCy, the conversion of –OCN groups increased evidently under the same cure conditions. The conversion of 3# system reached 100% after being cured for 200 min at 160 8C or cured for 140 min at 180 8C, but the conversion of 2# system reached only 98% after being cured for 200 min at 180 8C. As is known, the cure temperature needs to be higher in order to obtain the full conversion of the –OCN groups for the 2# system. Consequently, the cure cycles of the two resin systems are determined as follows: 130/2 hC160/2 hC180/ 3 h for 3# system; 130/2 hC160/2 hC180/3 hC200/2 h for 2# system.
Fig. 3. Conversion curves of BADCy and its modified systems at 160 and 180 8C.
1560, 1369
915
830
3.2. Study of the dispersion of ZnO whiskers in the composite Micrograph of ZnO whiskers is shown in Fig. 4. It can be seen that the ZnO whisker has a peculiar tetrapod-shaped structure, thereby three-dimensional isotropic distribution of the whiskers as reinforcement agent is achieved and better interlaminar-reinforcing effect superior to that of the one-dimensional whiskers is obtained [8]. But the ZnO whiskers are so small that they tend to agglomerate and in consequence inner flaws may be formed in the composite. Fig. 5 shows the agglomeration of ZnO whiskers in the BADCy matrix with 10 wt% of ZnO. It can be seen that the agglomerated ZnO whiskers in the resin matrix can produce a great deal of inner flaws and stress concentration, which defect the properties of the composites. So, ZnO whisker must be dispersed uniformly in the resin matrix before the incorporation of M40 fiber. An often used and easy processing method is high rate stirring. It is noteworthy that ZnO whiskers cannot be dispersed well in the matrix at a very low stirring rate, while they may be broken off when stirring rates are too high. Under neither of the two circumstances above can good reinforcement be obtained. The influence of stirring rates on the morphology of the whiskers was studied by means of SEM. As shown in Fig. 6, the whiskers maintained their original morphology in resin solution when the stirring rate was 1200 rpm (Fig. 6b). Some of the whiskers were broken when the stirring rate
Fig. 4. Microstructure of ZnO whiskers.
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P. Ren et al. / Composites: Part A 37 (2006) 46–53
was 1500 rpm (Fig. 6c). And most of the whiskers were broken at 1800 rpm (Fig. 6d). Then owing to the onedimensional structure and small length/diameter ratio of ZnO whiskers, poor reinforcement was obtained. Fig. 7 shows the mechanical properties of unidirectional M40/BADCy composite containing 10 wt% of ZnO whisker under various stirring rates. As shown in Fig. 7, when the stirring rate was less than 1500 rpm, the ISSL and flexural strength were increased with elevating the stirring rate. Whereas, when the stirring rate was above 1500 rpm, the mechanical properties decreased with the increasing stirring rate. According to the above discussion, the appropriate stirring rate should be between 1200 and 1500 rpm. 3.3. Influence of ZnO whiskers on the performances of M40/BADCy composite
Fig. 5. Agglomeration of ZnO whiskers in the BADCy matrix.
It is well known that coupling agents of organic and inorganic tips which can react with inorganic and organic
Fig. 6. SEM images of ZnO whiskers under various stirring rates: (a) Original; (b) 1200 rpm; (c) 1500 rpm; (d) 1800 rpm.
P. Ren et al. / Composites: Part A 37 (2006) 46–53
3.4. Reinforcing mechanisms analysis of ZnO whiskers
85
1100
80
1040 75
1020 1000
ILSS /MPa
Flexural strength /MPa
1080 1060
70
980 960 0
500
1000
1500
2000
51
65 2500
stirring rate /rpm Fig. 7. Mechanical properties of unidirectional M40/BADCy composites versus stirring rate (here M40/BADCy was reinforced by 10 wt% of untreated ZnO whiskers).
components, respectively, are often used to form chemical bonding between inorganic filler and resin matrices. Here, ZnO whiskers were treated by KH560 at first, to enhance the bonding between ZnO whiskers and BADCy. Table 3 shows some mechanical properties of unidirectional M40/BADCy composites reinforced by KH560 treated ZnO whiskers. Results in Table 3 indicate that treated ZnO whisker improves the flexural modulus and ILSS of the composite obviously, whereas the flexural and tensile strength are hardly affected. The flexural modulus increases with the increase in ZnO whisker content. When 15 wt% of ZnO whisker was introduced, flexural modulus of the composite increased by 17% to 208.9 GPa. And when the content of ZnO whisker reaches 10 wt%, the ILSS of the composite increased by 21% to its maximum value (82.2 MPa). The ILSS of the composite with ZnO whisker O10 wt% decreases with the increase in the whisker content. The moisture absorption coefficient of the composite is also related to the ZnO whisker content. The more the content of ZnO whisker, the more moisture absorption—reveals that some flaws exist in the interface between ZnO whisker and BADCy resin matrix. These flaws susceptible to water molecules may be caused by the difference in the coefficients of thermal expansion of ZnO whisker and BADCy.
Flexural and shear scanning electron microscope fractographs (SEM fractograph) of the M40/BADCy composite containing 10 wt% ZnO whisker are shown in Fig. 8. The micro cracks usually extend along the interfaces between fiber and resin matrix. When whiskers are located in the plane where cracks develop (Fig. 8b), the extending direction of the crack will be changed as crack tip encounters the whisker, leading to the increase of the fracture surface area. In consequence, ILSS was enhanced to some extent. When the inclined whiskers transfix the M40 fibers, ZnO whiskers have ‘nail’ effect on layers of the composite (shown in Fig. 8a and c). They can carry part of the interlaminar shear stress. And the ILSS can be improved obviously due to the high strength of ZnO whisker. Owing to the effect of the two reinforcing mechanisms mentioned above, ZnO whisker showed excellent reinforcement effect on the unidirectional M40/BADCy composite. The increase of ZnO whiskers content arouses two contrary effects. On the one hand, the probability of ‘nail’ effects will be increased and in consequence the ILSS of composite will be improved. On the other hand, the ZnO whisker agglomeration, which is harmful to mechanical strength, is accelerated at the same time. The agglomerated ZnO whiskers do not increase the occurrence of ‘nail’ effects, but cause local concentration of resin matrix and initiate flaws in the composite and in consequence decrease the mechanical properties. This is the reason why the M40/BADCy composite reinforced by 10 wt% of ZnO whisker displays the best mechanical properties. The agglomeration of ZnO whiskers is difficult to be observed in composite because of the existence of M40 fiber, unless the whisker content is high enough. And it has been practically observed in the SEM fractograph of the composite when 25 wt% of ZnO whisker is introduced (given in Fig. 9).
4. Conclusions ZnO whiskers showed obviously catalytic effect on the curing reaction of BADCy. Ten weight percent of ZnO
Table 3 Properties of M40/BADCy composite modified by ZnO whisker Mass fraction of ZnO whisker (wt%) 0 5 10 15 25 a
Flexural strength sb (MPa)
Flexural modulus G (GPa)
Tensile strength s (MPa)
Tensile modulus E (GPa)
ILSS z (MPa)
Water absorptiona (%)
Dry
Boileda
Dry
Boileda
Dry
Dry
Dry
Boileda
1064 1056 1075 1061 998
1007 998 1006 1001 967
178.5 195.1 206.3 208.9 197.4
170.4 185.2 186.5 184.8 178.1
1109.7 1081.2 1056.7 1019.4 1011.3
208.7 211.2 215.9 219.6 202.1
67.8 74.5 82.2 76.1 63.4
62.5 67.7 74.3 69.7 51.7
Water boiled for 100 h.
0.75 0.78 0.81 0.88 0.97
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Fig. 8. Fracture SEM image comparison of M40/BADCy composite modified by ZnO whiskers.
whisker could decrease the cure temperature from 200 to 180 8C. Untreated ZnO whiskers could improve the flexural modulus of M40/BADCy composite, whereas the flexural strength and ILSS were decreased at the same time. In contrast, KH560 treated ZnO whiskers displayed better reinforcing effect on the overall structural properties of M40/BADCy. The optimum reinforcing effect that the ILSS and flexural modulus increased by 21 and 17%, respectively, was obtained when 10 wt% ZnO whisker was introduced. SEM study showed that ZnO whiskers had beneficial ‘nail’ effect on layers of the composite.
References
Fig. 9. SEM image of agglomerated ZnO whiskers in M40/BADCy composite.
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