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polyarylacetylene composites
COMPOSITES SCIENCE AND TECHNOLOGY Composites Science and Technology 67 (2007) 3014–3022 www.elsevier.com/locate/compscitech
Properties of silsesquioxane coating modified carbon fibre/polyarylacetylene composites X.Z. Zhang a, Y.J. Song b, Y.D. Huang a
b,*
Laboratory of Advanced Polymer Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China b Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, PR China Received 1 September 2006; received in revised form 5 January 2007; accepted 30 January 2007 Available online 18 May 2007
Abstract Carbon fibre (CF) surfaces were modified with different silsesquioxanes (SSOs), including MES-SSO, VMS-SSO, MPMS-SSO, MethacrylIsobutyl-POSS and TriSilanolPhenyl-POSS, before and after being activated by air plasma. The effects of SSO coatings on the interfacial, impact, and heat-resistant properties of carbon fibre reinforced polyarylacetylene (CF/PAA) composites were investigated by short-beam bend method, impact test, and thermal oxygen aging experiments, respectively. The results showed that SSO coatings could improve the interfacial, impact, and heat-resistant properties of the CF/PAA composites at the same time due to the structure of the SSO coatings. All increased percentages of each property of the CF/PAA composites modified with POSS coatings were larger than those of composites modified with SSO coatings. The results may shed some light on the design of the appropriate fibre surface structure during the manufacture of the carbon fibre reinforced resin matrix composites. 2007 Elsevier Ltd. All rights reserved. Keywords: A. Carbon fibres; A. Coating; B. Fibre/matrix bond; B. Surface treatments
1. Introduction Polyarylacetylene (PAA) has been investigated owing to its outstanding heat resistance and excellent ablative properties [1]. It is a high performance resin made up of nonpolar ethynyl aromatic hydrocarbons, and can be cured to give a highly cross-linked aromatic polymer that contains only carbon and hydrogen. This is achieved by addition polymerization without any elimination of low molar mass molecules such as water. PAA has advantages over the state-of-the-art phenolic resin systems because of its ease of processability, lower moisture absorption, lower shrinkage on pyrolysis and higher char yield. It is well known that the interface between fibre and matrix plays an important role in controlling the overall
*
Corresponding author. Tel.: +86 451 8641 4806; fax: +86 451 8641 3707. E-mail address: [email protected] (Y.D. Huang). 0266-3538/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2007.05.013
properties of fibre reinforced polymer composite materials [2]. However, the nature of the structure and chemical inertness of carbon fibre and the non-polar structure of PAA usually result in a weak bonding in composites, which limits the applications of CF/PAA composites. It is quite necessary to improve the interfacial performance of CF/ PAA by the modification of carbon fiber surface. Carbon fibre surface treatment methods include chemical oxidation [3], anodic oxidation [4–6], ozone oxidation [7], plasma oxidation [8,9], and activate the surface of carbon fibres [10]. But all these methods were originally developed for the fibers to be used with polar resins, such as epoxies and phenolics. Few surface treatment methods have been developed according to the non-polar resins [11]. Also polar groups generated on the carbon fiber surface after activation, such as hydroxyl and carboxyl, are usually not compatible with the non-polar PAA resin. An alternative approach is to use a sizing or coating treatment [12,13]. In practice, carbon fibres are sized during their fabrication processes. It has to be assumed that
X.Z. Zhang et al. / Composites Science and Technology 67 (2007) 3014–3022
these sizing treatments are optimized for epoxy resins. Thus the sizing on carbon fibre surface is not very compatible with PAA resins. In this present work, in order to optimize the interfacial properties of the composites, organic–inorganic hybrid silsesquioxane (SSO) coatings were used to treat the carbon fibres. The name of SSO derives from the non-integer (one and one-half or sesqui) ratio between oxygen and silicon atoms [14]. It relates to a class of compound that contains a silicon-oxygen nano-structural skeleton with intermittent siloxane chains (general formula RSiO1.5), R can be H, alkyl, alkylene, aryl, aromatic alkylene or their derivative groups [15]. SSO can exist in several structural types such as random, ladder, cage, and semi-cage structures. It can be prepared by a hydrolytic condensation of an organic siloxane (RSi(OR 0 )3, where R and R 0 are different organic groups) [16,17]. SSO with a cage structure is also called polyhedral oligomeric silsesquioxane (POSS, or (RSiO1.5)n, where n = 6, 8, 10, . . .). POSS has been used as a new chemical feedstock technology for the preparation of organic– inorganic hybrid materials [18,19]. New organic–inorganic hybrid polymer systems have been produced with remarkable enhancements in mechanical and physical properties, including dramatic increases in both glass transitional and decomposition temperatures [20], reduced flammability [21], increased moduli [22,23], and oxidation resistance [24,25]. In the present work, SSOs containing appropriate functional groups, including active vinyl group or inertial groups, were used as different carbon fibre surface coatings. The effects of fibre surface SSO coating treatments on the properties of CF/PAA composites were investigated. 2. Experimental 2.1. Materials PAA resin was supplied by Aerospace Research Institute of Material and Processing Technology (Beijing, China). Poly(acrylonitrile) (PAN) precursor carbon fibres (3 · 103 single filaments per tow, tensile strength was 3.38 GPa, average diameter was 7 lm, density was 1.76 g cm3, linear density was 0.161 g m1) were obtained from Jilin Carbon Co. (Jilin, China). Five different kinds of SSO were named as MES-SSO, VMS-SSO, MPMS-SSO, MethacrylIsobutylPOSS, and TriSilanolPhenyl-POSS, respectively. Their structures were shown in Fig. 1, which were decided by the methods of FTIR, NMR and MALDI-TOF mass spectra. And they were applied as coatings to treat the surface of carbon fibre. MethacrylIsobutyl-POSS and TriSilanolPhenyl-POSS were purchased from Hybrid Plastics Co. Inc. (Texas, USA) and used as received. MES-SSO, VMSSSO, and MPMS-SSO were synthesized by the hydrolytic condensation of Methyltriethoxysilanes (MES), Vinyltrimethoxysilanes (VMS), and [3-(methacryloxy)propyl]trimethoxylsilane, respectively, which were acquired from Dow Corning and used as received. Most of chemicals used in this study, including analytically pure ethanol (C2H5OH),
a
CH2
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CH2
H2 C
HC
CH
O
O
Si HC
O
CH
HC
b
O
Si
Si O
Si
O
Si
O
O O
O Si
O
Si
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O
Si
Si
O
c
H 2C
CH2
H2C
O
Si
Si
O
CH
CH2
O
O
Si
O
Si
Si O
O
CH
O
Si
Si
H2C
HC
CH3
CH3 OC2H5 CH3 CH3 Si Si Si O OC2H5 O C2H5O O O Si Si O OC2H5 C2H5O O Si Si O Si OH O C2H5O O O O CH3 Si CH3 Si H C 3 CH3 CH3 CH3
C2H5O
d
H3 C O H3 C
O
Si
O Si CH 3 O
O O H3 C O
O
Si
Si CH 3
O
O
Si H3 C
CH 3 Si
O
Si
O
Si CH 3
Fig. 1. Structures of different silsesquioxanes: (a) Ladder structure of VMS-SSO, (b) cage structure of VMS-SSO (Octavinyl-POSS), (c) ladder structure of MES-SSO, (d) cage structure of MES-SSO, (e) ladder structure of MPMS-SSO (R is –(CH2)3OOC(CH3)C@CH2), (f) cage structure of MPMS-SSO, (g) methacrylIsobutyl-POSS and (h) triSilanolPhenyl-POSS.
formic acid (HCOOH), acetone, and tetrahydrofuran (THF) were purchased from the First Factory of Chemical Agents (Tianjin, China).
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e
R CH3O
R
R R
Si
CH3O
O
CH3O Si CH3O R
O
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O
Si
R O
O
O
O
Si
O
Si
O
R
O
(CH2 )3 OOC (CH3 )C=CH 2 Si
Si O
Si
O (CH 2 )3 OO C(C H3 )C=C H2 O
Si
O
O
Si
O
Si
O
O
Si H2 C=C(H 3 C)C OO(H 2 C) 3
g
(CH2 )3 OO C(CH3 )C=CH 2
O O
Si
O
C(CH3)3
O
O
O
Si
(H3C)3C O O
CH2CH2CH2 O C C CH2 CH3
Si
O Si
(CH2 )3 OOC(C H3 )C=CH2
Si
O
(H3C)3C
(H3C)3C
OH R
O
O
O H2 C=C (H3 C)COO( H2 C) 3
OCH3
R
H2 C=C(H 3 C)COO(H 2 C) 3
H2 C=C(H 3 C)COO(H2 C) 3
OCH3 OCH3
Si
Si
R
f
O
Si
Si
O
Si
O
C(CH3)3
Si O
Si
Si
O
(H3C)3C
C(CH3)3
h OH
Si O
OH O
Si
Si
O O
O O
Si
OH
Si
O
O
Si O
Si
Fig. 1 (continued)
2.2. Methods 2.2.1. Preparation of CF/PAA composites The THF solutions of SSO (2 mass%) were prepared for the coating treatments. Carbon fibres were treated with SSO solutions before and after air plasma activation.
Plasma activation was carried out in a Corona-2000 (Nianjing, China) plasma reactor. The power was 60 W and the activated time was 10 min. The unidirectional long carbon fibre reinforced PAA composites were made with both untreated and coating treated carbon fibres. Curing was performed in a com-
X.Z. Zhang et al. / Composites Science and Technology 67 (2007) 3014–3022
C¼
3P b 4bh
ð1Þ
where Pb is the maximum compression load at fracture in Newtons, b is the breadth of the specimen in mm, and h is the thickness of the specimen in mm. 2.2.3. Thermal oxidative aging experiments The thermal oxidative aging experiments were conducted to prove the heat-resistant performance of SSO coatings. Dimensions of specimens were 200 mm · 6 mm · 2 mm. The specimens were placed on an aluminum chambers and heated to different temperature in air with electric muffle furnace for 30 min. The temperature was 300 C, 350 C and 400 C, respectively. These specimens were weighed prior to the beginning of aging and again at the end of aging then the weight loss rate was accounted. The ILSS of CF/PAA composites were tested according to the foregoing methods.
The ILSS of the CF/PAA composites was shown in Fig. 2. The ILSS of untreated CF/PAA composites is only 32.4 MPa. The ILSS of untreated composite is small due to the inertial structure of carbon fibre and the non-polar structure of PAA. The low ILSS of untreated CF/PAA composites show that it is necessary to improve the interfacial mechanical properties of CF/PAA composites. Epoxy and silanes are common coatings to modify the surface of fibre in order to improve the interfacial properties of composites. Unfortunately, the epoxy and silane coatings have small effects on the interfacial mechanical properties of the CF/PAA composites. The ILSS of epoxy and silane (MES, VMS and MPMS) treated CF/PAA composites were only increased by less than 8% comparing with that of untreated one, which indicated that commercial epoxy coating and silane coupling agent were not appropriate when they were used in CF/PAA systems. While the ILSS of CF/PAA composites could be increased by about 27% if the silanes were used to synthesize silsesquioxanes by the hydrolytic condensation and the silsesquioxanes were used as coating to treat the carbon fibre surface. The increasing percentages of ILSS of CF/PAA composites were almost same when three kinds of different silsesquioxanes, MES-SSO, VMS-SSO, and MPMS-SSO, were used to modify the fibre surface though the organic functional groups in the three kinds of silsesquioxanes were different, which suggested the functional groups had few effects on the interfacial properties of CF/PAA composites when there were no chemical bond bridges between CF and PAA resin. The same conclusion could be drawn when cage structure POSS
55
50 ILSS Increasing rate
50
40
45
30
40
20
35
10
30
0
ILSS increasing rate/%
2.2.4. Impact property Non-standard impact specimens of 60 mm in length, 6 mm width and 2 mm thickness were tested on an instrumented Charpy System Impact Testing Machine (CIEM-30 D-CPC, Tokyo, Japan) that was designed and built specifically for this investigation. The specimens were unnotched. The impact span is 55 mm. After the manipulation of impact, a force vs. displacement trace during impact of each specimen was recorded by the machine self. Using an numerical integration techniques, the initial impact resistance (initial energy absorbed before peak failure of the specimen), the propagative impact resistance (propagative energy absorbed after peak failure of the impact specimen), and the total impact resistance (sum of the previously mentioned energies) were obtained.
3.1. Interfacial mechanical properties
ILSS/MPa
2.2.2. Interfacial characterization of CF/PAA composites The interlaminar shear strength (ILSS) of the CF/ PAA composites was measured on an universal testing machine (WD-1, Changchun, China) using a three point short-beam bending test method according to ASTM D2344. Specimen dimensions were 20 mm · 6 mm · 2 mm, with a span to thickness ratio of 5. The specimens were tested at a rate of cross-head movement of 2 mm min1. The ILSS, C, was calculated according to the following equation:
3. Results and discussion
Un tre at Ep ed ox y M ES VM M S M PM ES S M V eth M SS ac M S- O S r Tr yl PM S iS Iso S O ian b -S ol uty S Pl Phe l-PO O as ny S m Pl Pl a+ l-PO S as M a m a+ Pl sma ES SS Pl M asm +V -SS as eth a M O + m a+ acry MP S-S Tr lI M SO s iS o S ian bu -S ol tyl SO Ph -P en OS yl S -P OS S
pression moulding machine, and the content of the resin in the composites was controlled at about 35 mass%. The curing process was 120 C for 2 h, 140 C for 2 h, 180 C for 2 h, 200 C for 2 h, and 250 C for 0.5 h. During the curing process, the pressure was 2 MPa. When the curing process had been finished, the mould was cooled to room temperature with the pressure being maintained. All composite samples were about 6 mm in width and 2 mm in thickness.
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Fig. 2. ILSS of different coating treated CF/PAA composites (the concentration of coating solution is 2 mass%).
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coatings were used to modify the carbon fibre surface. The ILSS of two kinds of POSS coating (MethacrylIsobutylPOSS and TriSilanolPhenyl-POSS) treated CF/PAA composites were increased by 33% and 31%, respectively. The difference in the increasing percentage of composites treated by different POSS coatings could be ignored in fact. That the cage structure POSS coatings (MethacrylIsobutyl-POSS and TriSilanolPhenyl-POSS) had better treatment effect on the interfacial property of the CF/PAA composites compared with poly-structural silsesquioane coatings (MES-SSO, VMS-SSO and MPMS-SSO) can be sure according to the increasing degree of the ILSS. In other words, the POSS coatings had the best treatment effect, the poly-structural silsesquioxiane coating had the second one and the simply silane coupling agent coating had few effect when those coatings were only used without any pretreatment on carbon fibre surface. The functional groups on Si had no any contributiveness on the interfacial mechanical properties of CF/PAA composites even though the functional groups (double bonds) were active according to the functional groups in PAA resin. The situation was changed if carbon fibre surface was activated by air plasma before coating treatments. The active functional groups on Si in coatings became another influence factor of the interfacial properties of CF/PAA composites after the carbon fibre surface was activated by air plasma before treatment. The MES-SSO and TriSilanolPhenyl-POSS coating treatments after plasma activation had almost same treatment effect. The functional groups (CH3–, phenyl and –OH) on Si atom were non-active ones according to that on PAA resin. Thus, there are still no chemical bond bridge between CF and PAA. The ILSS of MES-SSO coating treated composites after plasma activation was increased compared with that of only MES-SSO coating treated ones, which may be from the contributiveness of the plasma etching on carbon fibre surface. The similar results were received when TriSilanolPhenyl-POSS coating was used to treat carbon fibre surface before and after plasma pretreatment, which showed that the cage type structure nano effect of POSS had more important action on the interfacial properties of composites. The chemical bond bridges formed with active silsesquioxane coatings between carbon fibre and PAA resin resulted that there were a large increased degree of the ILSS in VMS-SSO, MPMS-SSO and MethacrylIsobutyl-POSS treated composites after plasma activation. And the increased degree of the ILSS of MPMS-SSO treated composites, 50%, was larger than that (44%) of VMSSSO treated ones and that (41%) of MethacrylIsobutylPOSS treated ones. The ILSS of the CF/PAA composites indicated that the silsesquioxane coatings had a remarkable effect on the interfacial properties of the composites. The structure of silsesquioxane coatings effectively influenced the energy transfer from matrix to fibre when composites were loaded. The cage type structure had the largest effect on the interfacial property of composites among random,
ladder, and cage type ones. The chemical bonds between CF and PAA became the most important factor when chemical reaction was designed based on the requirements. The cage type structure POSS coating with a lot of active functional groups would be the best one to modify the surface of carbon fibre according to the present results. 3.2. Impact properties of CF/PAA composites The impact properties of composites are often decreased with the improvement of the interfacial mechanical properties. Thus, the impact properties of CF/PAA composites were tested in order to make sure that the number of impact property loss. To our surprise, the impact properties of the CF/PAA composites did not decreased after silsesquioxane coating treatments. The absorbed energy of the untreated and treated CF/PAA composites during the impact experiments were showed in Fig. 3a. The absorbed energy values could represent the impact resistance of materials and the lager the absorbed energy, the better impact resistance was. The initial, propagative and total absorbed energy of untreated CF/PAA composite were low, only 0.28 J, 1.38 J and 1.67 J, respectively, because the PAA resin was brittle. After being treated with silsesquioxiane coatings, the impact properties of CF/PAA composites were increased. The initial absorbed energy of composites were increased by 82%, 89% and 125%, respectively, after carbon fibre surface was treated with MESSSO, VMS-SSO, and MPMS-SSO coatings. The better results were received when carbon fibre surface was treated with cage type structure POSS coatings and the initial absorbed energy of composites was increased by 175% compared with the untreated ones. The structure of nanoparticles, especially the cage structure of POSS, could induce the cracks near the nanoparticles which absorbed the energy resulted in the improvement of the initial energy of the CF/PAA composites. The propagative energy of SSO coating (MES-SSO, VMS-SSO, and MPMS-SSO) treated composites were almost the same as that of untreated composites while those of POSS coating (MethacrylIsobutyl-POSS and TriSilanolPhenyl-POSS) treated composites were improved by large degree, which showed the polyhedral structure was important. The long soft carbon chain had effects on the propagative energy of CF/PAA composites according to the results of the SSO coating treatments. The longer and softer, the better of the treatment effects were. While the increased degree of semi-cage type POSS (TriSilanolPhenyl-POSS) treated composites was larger that of cage type POSS (MethacrylIsobutyl-POSS) treated ones though there were soft carbon chain on MethacrylIsobutyl-POSS, which was resulted from polyhedral structures. The almost same results were received when silsesquioxane coating were used to treat the surface of fibre after being activated by plasma (Fig. 3b), which also suggested the action of nano structures.
X.Z. Zhang et al. / Composites Science and Technology 67 (2007) 3014–3022
a
before activation 3.0
Initial Propagation Total
Absorbed energy/J
2.5 2.0 1.5 1.0 0.5
VM SSS O
T Ph riSi a PO enyl nol SS P T lasm Ph riSi a+ a PO enyl nol SS -
M PM SSS O
after activation Initial Propagation Total
2.5
Absorbed energy/J
M Iso eth a PO buty cry SS l- l
3.0
P VM lasm S- a+ SS O Pl M asm PM a+ SSS O Pl a M sm Iso eth a+ b ac PO uty ryl SS l-
b
M ES -S SO
Un tre ate d
0.0
2.0 1.5 1.0 0.5
P M lasm ES a+ -S SO
Pl as m a
0.0
Fig. 3. Effects of different silsesquioxane coatings on impact property of composites (2 mass%). (a) Before activation and (b) after activation.
The results of the impact testing showed that the main contribution of silsesquioxane coating was to improve the initiation energy during the impact event. This could be an indirect indication (corroborating the ILSS results) that a better interface was present, and therefore the stress transfer from matrix to the fibres was more efficient. The mismatch of properties would finally induce cracking at the interface, deflecting the main propagating crack, and also contributing to the increase of the final impact energy, through the propagation term. The efficiency of importing an interlayer between fibres and matrix in improving IFSS, without loss in impact properties has been demonstrated [26,27] by using different interlayers. But almost all used interlayers are soft or flexible. The module or stiffness of interface is larger than that of matrix but smaller than that of carbon fibre thus there are a transitional layer between fibre and matrix. However, the interlayers between carbon fibres and PAA matrix formed by POSS coatings are hard or rigid according to the test of force modulation mode atomic force microscopy in our present work. The stiffness of interface of POSS trea-
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ted composites is larger not only than that of PAA matrix but also than that of carbon fibres so that there are no transitional interlayer between fibre and matrix. Thus, the improvement of impact properties may result from the cage type nano-structure of POSS, which is inconsistent with conventional improvement theory in fibre–matrix composites [28,29]. The rigid interface results in the mismatch between constituents in CF/PAA composites. Experiments as well as models about the brittle matrix composites have demonstrated that a strong interface is beneficial to the strength and the toughness [30,31]. By contrast, weak interfaces are shown to be detrimental. Carre`re et al. studied the influence of the interface on the cracks deflection in brittle composites [30]. But all those researches are about brittle ceramic and there are natural difference between brittle PAA resin and brittle ceramic though the PAA resin is a kind of very brittle resin that made up of non-polar structural ethynyl aromatic hydrocarbons containing only carbon and hydrogen. Thus, new theory need be established to explain the mismatch of the module or stiffness in POSS coating treated CF/PAA system. Both POSS coatings increase ILSS and the initial absorbed energy by same extent show that the cube cage nano-structure POSS is important factor to improve interfacial adhesion and impact property at the same time. This can be explained as probably due to the effect of POSS coating, which increasing the resistance to the deformation and the crack initiation of the resin [32]. When the composites are under load, the cracks in the matrix propagate to the fibre, and then the direction of the crack propagation is decided by the stress field of the crack tip and the mechanical properties of the interface and the fibre. If there no appropriate interface, the stress field of the crack tip extends and generates a tensile stress, perpendicular to the crack in the unfractured fibre, which leads to fibre fracture under lower stress. That the Si–O–Si cage type nano-structure of POSS can induce more cracks forming which can efficiently absorb energy during the POSS coating treated composites are loaded results in the increase of initial absorbed energy. After crack formation, the cage type nano-structure of POSS can efficiently change the direction of the crack propagation, which increases the propagative absorbed energy. The stress concentration around POSS, the inducement of crack and the crack propagation orientation deflection are helpful to improve the impact properties of composites. In a word, the POSS cage type nano-structure is the key when POSS used as coating on fibre surface to improve the interface and the impact properties of composites. 3.3. Thermal oxidative aging performance Silsesquioxane coatings have to agree with the outstanding heat resistance and excellent ablative property of PAA resin when they are used in PAA matrix composites. In fact, silsesquioxiane hybrid materials had excellent heat resistance and were being used as modifiers to improve thermal behaviour of polymers [20,21,24,25]. The main
X.Z. Zhang et al. / Composites Science and Technology 67 (2007) 3014–3022
posites are always higher than that of untreated composite at any temperature even at 400 C. Moreover, the cage structure POSS coatings (MethacrylIsobutyl-POSS and TriSilanolPhenyl-POSS) have better effect than polymorphous silsesquioxane coatings (MES-SSO, VMS-SSO and MPMS-SSO). As a matter of fact, the heat-resistant property of cage structure silsesquixoane (POSS) is better than
a
before activation
2.5
Untreated Epoxy MES-SSO VMS-SSO MPMS-SSO MethacrylIsobutyl-POSS TriSianolPhenyl-POSS
2.0
Mass loss/%
purpose of a thermal oxidative aging experiment is to assess the potential possibility of using silsesquioxane coatings in CF/PAA composites that were used in high-performance aerospace applications. The results of thermal oxidative aging testing are shown in Table 1 and Fig. 4. Fig. 4 shows the mass loss of CF/PAA composites treated with different coatings before (Fig. 4a) and after (Fig. 4b) plasma activation. The untreated and epoxy coating treated composites lost large mass, which suggested that the epoxy coating was not good enough to be used in CF/PAA composites. The silsequioxane coating, especially the cage type structure POSS coatings, treated CF/PAA composites lost a few weight during the thermal oxygen aging experiments, which proved the excellent heat-resistant of silsesquioxane materials and showed that the silsesquioxane coatings were agree with the CF/PAA composites. The decrease of ILSS after the thermal oxygen aging experiment also proved this opinion (as shown in Table 1). The ILSS of the composites made from carbon fibres with epoxy coating is larger than that of composites made from carbon fibres without coating. It is suggested that the epoxy can improve the interfacial properties of CF/PAA composites with small amplitude. But the mass of the composites made from carbon fibres with epoxy coating losses the maximum even comparing with that of composites made from carbon fibres without coating at 300 C and 350 C. It is because the epoxy coating can not bear high temperature and make serious degradation at high temperature. The mass loss degrees and ILSS of composites made from carbon fibres without coating and with epoxy coating are nearly the same at 400 C. It indicated that the pyrolysis of epoxy coating was completed and the fibres were oxidized thus the ILSS of the composites decreased promptly at 400 C. The decrease of ILSS and mass loss show that epoxy coating is not appropriate to use in CF/PAA composites. The weight of composites made from carbon fibres with silsesquioxiane coatings loss much fewer than that of untreated and epoxy coating treated composites. ILSS of MES-SSO, VMS-SSO, MPMS-SSO, MethacrylIsobytylPOSS, and TriSilanolPhenyl-POSS coating treated com-
1.5
1.0
0.5
0.0
0
10
20
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400
Temperature/oC
b
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Untreated MES-SSO VMS-SSO MPMS-S S O Me thacry Isobutyl-P OS S TriSianolPhenyl-PO SS
2.0
Mass loss/%
3020
1.5
1.0
0.5
0.0 10
20
250
300
350
400
Temperature/oC
Fig. 4. Mass loss of CF/PAA composites during thermal oxidation treatments. (a) Before activation and (b) after activation.
Table 1 ILSS of CF/PAA composites after thermal oxidation treatments (MPa) Coating
RT
300 C
350 C
400 C
Untreated Epoxy MES-SSO VMS-SSO MPMS-SSO MethacrylIsobutyl-POSS TriSilanolPhenyl-POSS Plasma Plasma + MES-SSO Plasma + VMS-SSO Plasma + MPMS-SSO Plasma + TriSilanolPhenyl-POSS Plasma + MethacrylIsobutyl-POSS
34.2 ± 2.2 39.2 ± 2.7 43.5 ± 2.9 42.9 ± 2.1 43.9 ± 2.6 44.8 ± 2.0 45.9 ± 3.4 42.6 ± 3.6 45.9 ± 4.0 49.3 ± 4.2 51.3 ± 2.7 45.6 ± 2.6 44.6 ± 2.6
33.9 ± 4.2 37.2 ± 4.3 42.6 ± 2.3 43.2 ± 2.2 42.3 ± 2.5 44.1 ± 3.3 44.2 ± 2.9 39.4 ± 3.4 44.6 ± 3.5 48.2 ± 3.6 50.4 ± 3.4 45.2 ± 2.9 44.1 ± 2.8
31.4 ± 3.6 33.0 ± 3.1 37.1 ± 2.5 35.0 ± 1.8 36.1 ± 2.8 43.1 ± 3.9 43.5 ± 3.6 36.3 ± 3.6 40.3 ± 3.8 43.9 ± 4.0 45.9 ± 3.3 44.2 ± 2.7 43.6 ± 3.0
29.1 ± 3.1 30.4 ± 3.6 33.4 ± 3.0 31.5 ± 1.2 30.1 ± 3.1 34.1 ± 3.3 36.7 ± 3.9 31.2 ± 3.5 37.4 ± 3.7 38.1 ± 3.8 39.3 ± 3.6 41.3 ± 3.4 40.8 ± 3.5
X.Z. Zhang et al. / Composites Science and Technology 67 (2007) 3014–3022
that of polymorphous silsesquioxane. The perfect adhesion between fibre and resin due to the import of silsesquioxane coatings and the heat resistance of them cause that the composites have excellent interfacial properties at high temperature. Comparing with the ILSS of composites treated by different silsesquioxiane coatings, the single cage type structural POSS coatings have better treating effect than the complex structural SSO coatings. In other words, the single cage type structural POSS coatings are much more appropriate than the complex polymorphous structural silsesquioxane coatings when they are used in the system of carbon fibre and PAA resin. The same results were received when carbon fibre surface was treated with different silsesquioxane coatings after being activated by air plasma. The mass loss was similar and the decrease of ILSS was also similar in spite of the chemical bonds, which suggested the mass loss and decrease of ILSS at high temperature were occurred by the pyrolysis of silsesquioxanes. The cage type structure POSS had better heat resistant than poly-structural silsesquioxanes, which resulted that there are few loss of weight and decrease of the ILSS of the POSS treated composites. 4. Conclusions The effects of silsesquioxane coating treatments of the surface upon the interfacial, impact, and heat-resistant properties of carbon fibre reinforced polyarylacetylene composites have been studied. The ILSS, absorbed energy during impact fracture of CF/PAA composites were both increased by different degrees after the fibre being treated with different silsesquioxane coatings. The increasing amounts of them of CF/PAA composites after carbon fibre being treated with single cage type structural POSS coatings are larger than those of composites after carbon fibre being treated with complex structural silsesquioxane coatings. Heat resistant of CF/PAA composites were also increased after carbon fibres were treated with different silsesquioxane coatings and the treatment effect of cage type structure POSS is better than that of poly structural silsesquioxane coatings. The previous results indicate that there are improved interfaces caused by silsesquioxane coatings. The thermal oxidative aging experiment results indicated that silsesquioxane coatings are agree with the system of carbon fibres and PAA resin and also proved that the single cage structural POSS coating has better effect than the complex structural silsesquioxane coating in high temperature. All results show that the properties at the fibre–matrix interphase change by altering the chemical system of the composite. This research of property variations with changing coating structure will help in better understanding and tailoring of the composite properties. Acknowledgements The authors thank the National Natural Science Foundation of China (No. 50333030) and the Natural Science
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Foundation of Heilongjiang for Distinguished Young Scholars (No. JC04-12) for financial supports. References [1] Katzman HA, Mallon JJ, Barry WT. Polyarylacetylene-matrix composites for solid rocket motor composites. J Adv Mater 1995;26(3):21–7. [2] Guigon M, Klinklin E. Interface and interphase in carbon fibrereinforced composites. Composites 1994;25(7):534–9. [3] Lin JS, Huang YH, Chiu HT. Effect of chemical oxidation on the interfacial bonding between carbon fibre and epoxy resin. Polym Polym Compos 2001;9(5):351–9. [4] Fukunaga A, Ueda S, Nagumo M. Anodic surface oxidation mechanisms of PAN-based and pitch-based carbon fibres. J Mater Sci 1999;34(12):2851–4. [5] King TR, Asams DF, Buttry DA. Anodic oxidation of pitchprecursor carbon fibres in ammonium sulphate solutions. The effect of fibre surface treatment on composite mechanical properties. Composites 1991;22(5):380–7. [6] Yumitori S, Nakanishi Y. Effect of anodic oxidation of coal tar pitchbased carbon fibre on adhesion in epoxy matrix: Part 1. Comparison between H2SO4 and NaOH solutions. Compos Part A Appl Sci Manuf 1996;27(11):1051–8. [7] Fu XL, Lu WM, Chung DDL. Ozone treatment of carbon fibre for reinforcing cement. Carbon 1998;36(9):1337–45. [8] Montes-Mora´n MA, Martı´nez-Alonso A, Tasco´n JMD, Young RJ. Effects of plasma oxidation on the surface and interfacial properties of ultra-high modulus carbon fibres. Compos Part A Appl Sci Manuf 2001;32(3–4):361–71. [9] Montes-Mora´n MA, Martı´nez-Alonso A, Tasco´n JMD, Paiva MC, Bernardo CA. Effects of Plasma oxidation on the surface and interfacial properties of carbon fibres/polycarbonate composites. Carbon 2001;39(7):1057–68. [10] Baillie CA, Bader MG. Some aspects of interface adhesion of electrolytically oxidized carbon fibres in an epoxy-resin matrix. J Mater Sci 1994;29(14):3822–36. [11] Fu HJ, Huang YD, Liu L. Influence of fibre surface oxidation treatment on mechanical properties of carbon fibre/polyarylacetylene composites. Mater Sci Technol 2004;20(12):1655–60. [12] Beinborn KM, Mu¨ller M, Hu¨ttinger KJ. The significance of the fibre coating in the production of carbon fibre-reinforced carbons from HT carbon fibres I. Poly(dimethylsiloxane) and poly(methylphenylsiloxane) coatings. Carbon 1995;33(5):1029–42. [13] Beinborn KM, Mu¨ller M, Hu¨ttinger KJ. The significance of the fibre coating in the production of carbon fibre-reinforced carbons from HT carbon fibres and phenolic resin as matrix precursor II. Phenolic resin coatings. Carbon 1995;33(5):1043–6. ´ rez C, Abad MJ, Barral L, Cano J. Thermal behaviour of a [14] Ramı polyhedral oligomeric silsesquioxane with epoxy resin cured by diamines. J Therm Anal Calor 2003;72(2):421–9. [15] Eisenberg P, Erra-Balsells R, Ishikawa Y, Lucas JC, Nanami H, Williams RJJ. Macromolecules 2002;35(4):1160–74. [16] Fasce DP, Williams RJJ, Erra-Balsells R, Ishikawa Y, Nonami H. Macromolecules 2001;34(11):3534–9. [17] Fether FJ, Terroba R, Jin RZ, Lucke S, Nquyen F, Brutchey R, et al. Organic/inorganic Hybrid Material-2000. San Francisco (CA, USA):Materials Research Society, 2000; CC2.1.1-6. [18] Phillips SH, Blanski RL, Svejda SA, Haddad TS, Lee A, Lichtenhan JD, et al. Organic/inorganic Hybrid Material-2000. San Francisco (CA, USA):Materials Research Society,2000;CC4.6.1-10. [19] Lee A, Xiao J, Feher FJ. New approach in the synthesis of hybrid polymers grafted with polyhedral oligomeric silsesquioxane and their physical and viscoelastic properties. Macromolecules 2005;38(2):438–44. [20] Fu BX, Namani M, Lee A. Influence of phenyl-trisilanol polyhedral silsesquioxane on properties of epoxy network glasses. Polymer 2003;44(25):7739–47.
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Properties of silsesquioxane coating modified carbon fibre/polyarylacetylene composites X.Z. Zhang a, Y.J. Song b, Y.D. Huang a
b,*
Laboratory of Advanced Polymer Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China b Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, PR China Received 1 September 2006; received in revised form 5 January 2007; accepted 30 January 2007 Available online 18 May 2007
Abstract Carbon fibre (CF) surfaces were modified with different silsesquioxanes (SSOs), including MES-SSO, VMS-SSO, MPMS-SSO, MethacrylIsobutyl-POSS and TriSilanolPhenyl-POSS, before and after being activated by air plasma. The effects of SSO coatings on the interfacial, impact, and heat-resistant properties of carbon fibre reinforced polyarylacetylene (CF/PAA) composites were investigated by short-beam bend method, impact test, and thermal oxygen aging experiments, respectively. The results showed that SSO coatings could improve the interfacial, impact, and heat-resistant properties of the CF/PAA composites at the same time due to the structure of the SSO coatings. All increased percentages of each property of the CF/PAA composites modified with POSS coatings were larger than those of composites modified with SSO coatings. The results may shed some light on the design of the appropriate fibre surface structure during the manufacture of the carbon fibre reinforced resin matrix composites. 2007 Elsevier Ltd. All rights reserved. Keywords: A. Carbon fibres; A. Coating; B. Fibre/matrix bond; B. Surface treatments
1. Introduction Polyarylacetylene (PAA) has been investigated owing to its outstanding heat resistance and excellent ablative properties [1]. It is a high performance resin made up of nonpolar ethynyl aromatic hydrocarbons, and can be cured to give a highly cross-linked aromatic polymer that contains only carbon and hydrogen. This is achieved by addition polymerization without any elimination of low molar mass molecules such as water. PAA has advantages over the state-of-the-art phenolic resin systems because of its ease of processability, lower moisture absorption, lower shrinkage on pyrolysis and higher char yield. It is well known that the interface between fibre and matrix plays an important role in controlling the overall
*
Corresponding author. Tel.: +86 451 8641 4806; fax: +86 451 8641 3707. E-mail address: [email protected] (Y.D. Huang). 0266-3538/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2007.05.013
properties of fibre reinforced polymer composite materials [2]. However, the nature of the structure and chemical inertness of carbon fibre and the non-polar structure of PAA usually result in a weak bonding in composites, which limits the applications of CF/PAA composites. It is quite necessary to improve the interfacial performance of CF/ PAA by the modification of carbon fiber surface. Carbon fibre surface treatment methods include chemical oxidation [3], anodic oxidation [4–6], ozone oxidation [7], plasma oxidation [8,9], and activate the surface of carbon fibres [10]. But all these methods were originally developed for the fibers to be used with polar resins, such as epoxies and phenolics. Few surface treatment methods have been developed according to the non-polar resins [11]. Also polar groups generated on the carbon fiber surface after activation, such as hydroxyl and carboxyl, are usually not compatible with the non-polar PAA resin. An alternative approach is to use a sizing or coating treatment [12,13]. In practice, carbon fibres are sized during their fabrication processes. It has to be assumed that
X.Z. Zhang et al. / Composites Science and Technology 67 (2007) 3014–3022
these sizing treatments are optimized for epoxy resins. Thus the sizing on carbon fibre surface is not very compatible with PAA resins. In this present work, in order to optimize the interfacial properties of the composites, organic–inorganic hybrid silsesquioxane (SSO) coatings were used to treat the carbon fibres. The name of SSO derives from the non-integer (one and one-half or sesqui) ratio between oxygen and silicon atoms [14]. It relates to a class of compound that contains a silicon-oxygen nano-structural skeleton with intermittent siloxane chains (general formula RSiO1.5), R can be H, alkyl, alkylene, aryl, aromatic alkylene or their derivative groups [15]. SSO can exist in several structural types such as random, ladder, cage, and semi-cage structures. It can be prepared by a hydrolytic condensation of an organic siloxane (RSi(OR 0 )3, where R and R 0 are different organic groups) [16,17]. SSO with a cage structure is also called polyhedral oligomeric silsesquioxane (POSS, or (RSiO1.5)n, where n = 6, 8, 10, . . .). POSS has been used as a new chemical feedstock technology for the preparation of organic– inorganic hybrid materials [18,19]. New organic–inorganic hybrid polymer systems have been produced with remarkable enhancements in mechanical and physical properties, including dramatic increases in both glass transitional and decomposition temperatures [20], reduced flammability [21], increased moduli [22,23], and oxidation resistance [24,25]. In the present work, SSOs containing appropriate functional groups, including active vinyl group or inertial groups, were used as different carbon fibre surface coatings. The effects of fibre surface SSO coating treatments on the properties of CF/PAA composites were investigated. 2. Experimental 2.1. Materials PAA resin was supplied by Aerospace Research Institute of Material and Processing Technology (Beijing, China). Poly(acrylonitrile) (PAN) precursor carbon fibres (3 · 103 single filaments per tow, tensile strength was 3.38 GPa, average diameter was 7 lm, density was 1.76 g cm3, linear density was 0.161 g m1) were obtained from Jilin Carbon Co. (Jilin, China). Five different kinds of SSO were named as MES-SSO, VMS-SSO, MPMS-SSO, MethacrylIsobutylPOSS, and TriSilanolPhenyl-POSS, respectively. Their structures were shown in Fig. 1, which were decided by the methods of FTIR, NMR and MALDI-TOF mass spectra. And they were applied as coatings to treat the surface of carbon fibre. MethacrylIsobutyl-POSS and TriSilanolPhenyl-POSS were purchased from Hybrid Plastics Co. Inc. (Texas, USA) and used as received. MES-SSO, VMSSSO, and MPMS-SSO were synthesized by the hydrolytic condensation of Methyltriethoxysilanes (MES), Vinyltrimethoxysilanes (VMS), and [3-(methacryloxy)propyl]trimethoxylsilane, respectively, which were acquired from Dow Corning and used as received. Most of chemicals used in this study, including analytically pure ethanol (C2H5OH),
a
CH2
3015
CH2
H2 C
HC
CH
O
O
Si HC
O
CH
HC
b
O
Si
Si O
Si
O
Si
O
O O
O Si
O
Si
O
O
Si
Si
O
c
H 2C
CH2
H2C
O
Si
Si
O
CH
CH2
O
O
Si
O
Si
Si O
O
CH
O
Si
Si
H2C
HC
CH3
CH3 OC2H5 CH3 CH3 Si Si Si O OC2H5 O C2H5O O O Si Si O OC2H5 C2H5O O Si Si O Si OH O C2H5O O O O CH3 Si CH3 Si H C 3 CH3 CH3 CH3
C2H5O
d
H3 C O H3 C
O
Si
O Si CH 3 O
O O H3 C O
O
Si
Si CH 3
O
O
Si H3 C
CH 3 Si
O
Si
O
Si CH 3
Fig. 1. Structures of different silsesquioxanes: (a) Ladder structure of VMS-SSO, (b) cage structure of VMS-SSO (Octavinyl-POSS), (c) ladder structure of MES-SSO, (d) cage structure of MES-SSO, (e) ladder structure of MPMS-SSO (R is –(CH2)3OOC(CH3)C@CH2), (f) cage structure of MPMS-SSO, (g) methacrylIsobutyl-POSS and (h) triSilanolPhenyl-POSS.
formic acid (HCOOH), acetone, and tetrahydrofuran (THF) were purchased from the First Factory of Chemical Agents (Tianjin, China).
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e
R CH3O
R
R R
Si
CH3O
O
CH3O Si CH3O R
O
Si
O
Si
R O
O
O
O
Si
O
Si
O
R
O
(CH2 )3 OOC (CH3 )C=CH 2 Si
Si O
Si
O (CH 2 )3 OO C(C H3 )C=C H2 O
Si
O
O
Si
O
Si
O
O
Si H2 C=C(H 3 C)C OO(H 2 C) 3
g
(CH2 )3 OO C(CH3 )C=CH 2
O O
Si
O
C(CH3)3
O
O
O
Si
(H3C)3C O O
CH2CH2CH2 O C C CH2 CH3
Si
O Si
(CH2 )3 OOC(C H3 )C=CH2
Si
O
(H3C)3C
(H3C)3C
OH R
O
O
O H2 C=C (H3 C)COO( H2 C) 3
OCH3
R
H2 C=C(H 3 C)COO(H 2 C) 3
H2 C=C(H 3 C)COO(H2 C) 3
OCH3 OCH3
Si
Si
R
f
O
Si
Si
O
Si
O
C(CH3)3
Si O
Si
Si
O
(H3C)3C
C(CH3)3
h OH
Si O
OH O
Si
Si
O O
O O
Si
OH
Si
O
O
Si O
Si
Fig. 1 (continued)
2.2. Methods 2.2.1. Preparation of CF/PAA composites The THF solutions of SSO (2 mass%) were prepared for the coating treatments. Carbon fibres were treated with SSO solutions before and after air plasma activation.
Plasma activation was carried out in a Corona-2000 (Nianjing, China) plasma reactor. The power was 60 W and the activated time was 10 min. The unidirectional long carbon fibre reinforced PAA composites were made with both untreated and coating treated carbon fibres. Curing was performed in a com-
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C¼
3P b 4bh
ð1Þ
where Pb is the maximum compression load at fracture in Newtons, b is the breadth of the specimen in mm, and h is the thickness of the specimen in mm. 2.2.3. Thermal oxidative aging experiments The thermal oxidative aging experiments were conducted to prove the heat-resistant performance of SSO coatings. Dimensions of specimens were 200 mm · 6 mm · 2 mm. The specimens were placed on an aluminum chambers and heated to different temperature in air with electric muffle furnace for 30 min. The temperature was 300 C, 350 C and 400 C, respectively. These specimens were weighed prior to the beginning of aging and again at the end of aging then the weight loss rate was accounted. The ILSS of CF/PAA composites were tested according to the foregoing methods.
The ILSS of the CF/PAA composites was shown in Fig. 2. The ILSS of untreated CF/PAA composites is only 32.4 MPa. The ILSS of untreated composite is small due to the inertial structure of carbon fibre and the non-polar structure of PAA. The low ILSS of untreated CF/PAA composites show that it is necessary to improve the interfacial mechanical properties of CF/PAA composites. Epoxy and silanes are common coatings to modify the surface of fibre in order to improve the interfacial properties of composites. Unfortunately, the epoxy and silane coatings have small effects on the interfacial mechanical properties of the CF/PAA composites. The ILSS of epoxy and silane (MES, VMS and MPMS) treated CF/PAA composites were only increased by less than 8% comparing with that of untreated one, which indicated that commercial epoxy coating and silane coupling agent were not appropriate when they were used in CF/PAA systems. While the ILSS of CF/PAA composites could be increased by about 27% if the silanes were used to synthesize silsesquioxanes by the hydrolytic condensation and the silsesquioxanes were used as coating to treat the carbon fibre surface. The increasing percentages of ILSS of CF/PAA composites were almost same when three kinds of different silsesquioxanes, MES-SSO, VMS-SSO, and MPMS-SSO, were used to modify the fibre surface though the organic functional groups in the three kinds of silsesquioxanes were different, which suggested the functional groups had few effects on the interfacial properties of CF/PAA composites when there were no chemical bond bridges between CF and PAA resin. The same conclusion could be drawn when cage structure POSS
55
50 ILSS Increasing rate
50
40
45
30
40
20
35
10
30
0
ILSS increasing rate/%
2.2.4. Impact property Non-standard impact specimens of 60 mm in length, 6 mm width and 2 mm thickness were tested on an instrumented Charpy System Impact Testing Machine (CIEM-30 D-CPC, Tokyo, Japan) that was designed and built specifically for this investigation. The specimens were unnotched. The impact span is 55 mm. After the manipulation of impact, a force vs. displacement trace during impact of each specimen was recorded by the machine self. Using an numerical integration techniques, the initial impact resistance (initial energy absorbed before peak failure of the specimen), the propagative impact resistance (propagative energy absorbed after peak failure of the impact specimen), and the total impact resistance (sum of the previously mentioned energies) were obtained.
3.1. Interfacial mechanical properties
ILSS/MPa
2.2.2. Interfacial characterization of CF/PAA composites The interlaminar shear strength (ILSS) of the CF/ PAA composites was measured on an universal testing machine (WD-1, Changchun, China) using a three point short-beam bending test method according to ASTM D2344. Specimen dimensions were 20 mm · 6 mm · 2 mm, with a span to thickness ratio of 5. The specimens were tested at a rate of cross-head movement of 2 mm min1. The ILSS, C, was calculated according to the following equation:
3. Results and discussion
Un tre at Ep ed ox y M ES VM M S M PM ES S M V eth M SS ac M S- O S r Tr yl PM S iS Iso S O ian b -S ol uty S Pl Phe l-PO O as ny S m Pl Pl a+ l-PO S as M a m a+ Pl sma ES SS Pl M asm +V -SS as eth a M O + m a+ acry MP S-S Tr lI M SO s iS o S ian bu -S ol tyl SO Ph -P en OS yl S -P OS S
pression moulding machine, and the content of the resin in the composites was controlled at about 35 mass%. The curing process was 120 C for 2 h, 140 C for 2 h, 180 C for 2 h, 200 C for 2 h, and 250 C for 0.5 h. During the curing process, the pressure was 2 MPa. When the curing process had been finished, the mould was cooled to room temperature with the pressure being maintained. All composite samples were about 6 mm in width and 2 mm in thickness.
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Fig. 2. ILSS of different coating treated CF/PAA composites (the concentration of coating solution is 2 mass%).
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coatings were used to modify the carbon fibre surface. The ILSS of two kinds of POSS coating (MethacrylIsobutylPOSS and TriSilanolPhenyl-POSS) treated CF/PAA composites were increased by 33% and 31%, respectively. The difference in the increasing percentage of composites treated by different POSS coatings could be ignored in fact. That the cage structure POSS coatings (MethacrylIsobutyl-POSS and TriSilanolPhenyl-POSS) had better treatment effect on the interfacial property of the CF/PAA composites compared with poly-structural silsesquioane coatings (MES-SSO, VMS-SSO and MPMS-SSO) can be sure according to the increasing degree of the ILSS. In other words, the POSS coatings had the best treatment effect, the poly-structural silsesquioxiane coating had the second one and the simply silane coupling agent coating had few effect when those coatings were only used without any pretreatment on carbon fibre surface. The functional groups on Si had no any contributiveness on the interfacial mechanical properties of CF/PAA composites even though the functional groups (double bonds) were active according to the functional groups in PAA resin. The situation was changed if carbon fibre surface was activated by air plasma before coating treatments. The active functional groups on Si in coatings became another influence factor of the interfacial properties of CF/PAA composites after the carbon fibre surface was activated by air plasma before treatment. The MES-SSO and TriSilanolPhenyl-POSS coating treatments after plasma activation had almost same treatment effect. The functional groups (CH3–, phenyl and –OH) on Si atom were non-active ones according to that on PAA resin. Thus, there are still no chemical bond bridge between CF and PAA. The ILSS of MES-SSO coating treated composites after plasma activation was increased compared with that of only MES-SSO coating treated ones, which may be from the contributiveness of the plasma etching on carbon fibre surface. The similar results were received when TriSilanolPhenyl-POSS coating was used to treat carbon fibre surface before and after plasma pretreatment, which showed that the cage type structure nano effect of POSS had more important action on the interfacial properties of composites. The chemical bond bridges formed with active silsesquioxane coatings between carbon fibre and PAA resin resulted that there were a large increased degree of the ILSS in VMS-SSO, MPMS-SSO and MethacrylIsobutyl-POSS treated composites after plasma activation. And the increased degree of the ILSS of MPMS-SSO treated composites, 50%, was larger than that (44%) of VMSSSO treated ones and that (41%) of MethacrylIsobutylPOSS treated ones. The ILSS of the CF/PAA composites indicated that the silsesquioxane coatings had a remarkable effect on the interfacial properties of the composites. The structure of silsesquioxane coatings effectively influenced the energy transfer from matrix to fibre when composites were loaded. The cage type structure had the largest effect on the interfacial property of composites among random,
ladder, and cage type ones. The chemical bonds between CF and PAA became the most important factor when chemical reaction was designed based on the requirements. The cage type structure POSS coating with a lot of active functional groups would be the best one to modify the surface of carbon fibre according to the present results. 3.2. Impact properties of CF/PAA composites The impact properties of composites are often decreased with the improvement of the interfacial mechanical properties. Thus, the impact properties of CF/PAA composites were tested in order to make sure that the number of impact property loss. To our surprise, the impact properties of the CF/PAA composites did not decreased after silsesquioxane coating treatments. The absorbed energy of the untreated and treated CF/PAA composites during the impact experiments were showed in Fig. 3a. The absorbed energy values could represent the impact resistance of materials and the lager the absorbed energy, the better impact resistance was. The initial, propagative and total absorbed energy of untreated CF/PAA composite were low, only 0.28 J, 1.38 J and 1.67 J, respectively, because the PAA resin was brittle. After being treated with silsesquioxiane coatings, the impact properties of CF/PAA composites were increased. The initial absorbed energy of composites were increased by 82%, 89% and 125%, respectively, after carbon fibre surface was treated with MESSSO, VMS-SSO, and MPMS-SSO coatings. The better results were received when carbon fibre surface was treated with cage type structure POSS coatings and the initial absorbed energy of composites was increased by 175% compared with the untreated ones. The structure of nanoparticles, especially the cage structure of POSS, could induce the cracks near the nanoparticles which absorbed the energy resulted in the improvement of the initial energy of the CF/PAA composites. The propagative energy of SSO coating (MES-SSO, VMS-SSO, and MPMS-SSO) treated composites were almost the same as that of untreated composites while those of POSS coating (MethacrylIsobutyl-POSS and TriSilanolPhenyl-POSS) treated composites were improved by large degree, which showed the polyhedral structure was important. The long soft carbon chain had effects on the propagative energy of CF/PAA composites according to the results of the SSO coating treatments. The longer and softer, the better of the treatment effects were. While the increased degree of semi-cage type POSS (TriSilanolPhenyl-POSS) treated composites was larger that of cage type POSS (MethacrylIsobutyl-POSS) treated ones though there were soft carbon chain on MethacrylIsobutyl-POSS, which was resulted from polyhedral structures. The almost same results were received when silsesquioxane coating were used to treat the surface of fibre after being activated by plasma (Fig. 3b), which also suggested the action of nano structures.
X.Z. Zhang et al. / Composites Science and Technology 67 (2007) 3014–3022
a
before activation 3.0
Initial Propagation Total
Absorbed energy/J
2.5 2.0 1.5 1.0 0.5
VM SSS O
T Ph riSi a PO enyl nol SS P T lasm Ph riSi a+ a PO enyl nol SS -
M PM SSS O
after activation Initial Propagation Total
2.5
Absorbed energy/J
M Iso eth a PO buty cry SS l- l
3.0
P VM lasm S- a+ SS O Pl M asm PM a+ SSS O Pl a M sm Iso eth a+ b ac PO uty ryl SS l-
b
M ES -S SO
Un tre ate d
0.0
2.0 1.5 1.0 0.5
P M lasm ES a+ -S SO
Pl as m a
0.0
Fig. 3. Effects of different silsesquioxane coatings on impact property of composites (2 mass%). (a) Before activation and (b) after activation.
The results of the impact testing showed that the main contribution of silsesquioxane coating was to improve the initiation energy during the impact event. This could be an indirect indication (corroborating the ILSS results) that a better interface was present, and therefore the stress transfer from matrix to the fibres was more efficient. The mismatch of properties would finally induce cracking at the interface, deflecting the main propagating crack, and also contributing to the increase of the final impact energy, through the propagation term. The efficiency of importing an interlayer between fibres and matrix in improving IFSS, without loss in impact properties has been demonstrated [26,27] by using different interlayers. But almost all used interlayers are soft or flexible. The module or stiffness of interface is larger than that of matrix but smaller than that of carbon fibre thus there are a transitional layer between fibre and matrix. However, the interlayers between carbon fibres and PAA matrix formed by POSS coatings are hard or rigid according to the test of force modulation mode atomic force microscopy in our present work. The stiffness of interface of POSS trea-
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ted composites is larger not only than that of PAA matrix but also than that of carbon fibres so that there are no transitional interlayer between fibre and matrix. Thus, the improvement of impact properties may result from the cage type nano-structure of POSS, which is inconsistent with conventional improvement theory in fibre–matrix composites [28,29]. The rigid interface results in the mismatch between constituents in CF/PAA composites. Experiments as well as models about the brittle matrix composites have demonstrated that a strong interface is beneficial to the strength and the toughness [30,31]. By contrast, weak interfaces are shown to be detrimental. Carre`re et al. studied the influence of the interface on the cracks deflection in brittle composites [30]. But all those researches are about brittle ceramic and there are natural difference between brittle PAA resin and brittle ceramic though the PAA resin is a kind of very brittle resin that made up of non-polar structural ethynyl aromatic hydrocarbons containing only carbon and hydrogen. Thus, new theory need be established to explain the mismatch of the module or stiffness in POSS coating treated CF/PAA system. Both POSS coatings increase ILSS and the initial absorbed energy by same extent show that the cube cage nano-structure POSS is important factor to improve interfacial adhesion and impact property at the same time. This can be explained as probably due to the effect of POSS coating, which increasing the resistance to the deformation and the crack initiation of the resin [32]. When the composites are under load, the cracks in the matrix propagate to the fibre, and then the direction of the crack propagation is decided by the stress field of the crack tip and the mechanical properties of the interface and the fibre. If there no appropriate interface, the stress field of the crack tip extends and generates a tensile stress, perpendicular to the crack in the unfractured fibre, which leads to fibre fracture under lower stress. That the Si–O–Si cage type nano-structure of POSS can induce more cracks forming which can efficiently absorb energy during the POSS coating treated composites are loaded results in the increase of initial absorbed energy. After crack formation, the cage type nano-structure of POSS can efficiently change the direction of the crack propagation, which increases the propagative absorbed energy. The stress concentration around POSS, the inducement of crack and the crack propagation orientation deflection are helpful to improve the impact properties of composites. In a word, the POSS cage type nano-structure is the key when POSS used as coating on fibre surface to improve the interface and the impact properties of composites. 3.3. Thermal oxidative aging performance Silsesquioxane coatings have to agree with the outstanding heat resistance and excellent ablative property of PAA resin when they are used in PAA matrix composites. In fact, silsesquioxiane hybrid materials had excellent heat resistance and were being used as modifiers to improve thermal behaviour of polymers [20,21,24,25]. The main
X.Z. Zhang et al. / Composites Science and Technology 67 (2007) 3014–3022
posites are always higher than that of untreated composite at any temperature even at 400 C. Moreover, the cage structure POSS coatings (MethacrylIsobutyl-POSS and TriSilanolPhenyl-POSS) have better effect than polymorphous silsesquioxane coatings (MES-SSO, VMS-SSO and MPMS-SSO). As a matter of fact, the heat-resistant property of cage structure silsesquixoane (POSS) is better than
a
before activation
2.5
Untreated Epoxy MES-SSO VMS-SSO MPMS-SSO MethacrylIsobutyl-POSS TriSianolPhenyl-POSS
2.0
Mass loss/%
purpose of a thermal oxidative aging experiment is to assess the potential possibility of using silsesquioxane coatings in CF/PAA composites that were used in high-performance aerospace applications. The results of thermal oxidative aging testing are shown in Table 1 and Fig. 4. Fig. 4 shows the mass loss of CF/PAA composites treated with different coatings before (Fig. 4a) and after (Fig. 4b) plasma activation. The untreated and epoxy coating treated composites lost large mass, which suggested that the epoxy coating was not good enough to be used in CF/PAA composites. The silsequioxane coating, especially the cage type structure POSS coatings, treated CF/PAA composites lost a few weight during the thermal oxygen aging experiments, which proved the excellent heat-resistant of silsesquioxane materials and showed that the silsesquioxane coatings were agree with the CF/PAA composites. The decrease of ILSS after the thermal oxygen aging experiment also proved this opinion (as shown in Table 1). The ILSS of the composites made from carbon fibres with epoxy coating is larger than that of composites made from carbon fibres without coating. It is suggested that the epoxy can improve the interfacial properties of CF/PAA composites with small amplitude. But the mass of the composites made from carbon fibres with epoxy coating losses the maximum even comparing with that of composites made from carbon fibres without coating at 300 C and 350 C. It is because the epoxy coating can not bear high temperature and make serious degradation at high temperature. The mass loss degrees and ILSS of composites made from carbon fibres without coating and with epoxy coating are nearly the same at 400 C. It indicated that the pyrolysis of epoxy coating was completed and the fibres were oxidized thus the ILSS of the composites decreased promptly at 400 C. The decrease of ILSS and mass loss show that epoxy coating is not appropriate to use in CF/PAA composites. The weight of composites made from carbon fibres with silsesquioxiane coatings loss much fewer than that of untreated and epoxy coating treated composites. ILSS of MES-SSO, VMS-SSO, MPMS-SSO, MethacrylIsobytylPOSS, and TriSilanolPhenyl-POSS coating treated com-
1.5
1.0
0.5
0.0
0
10
20
300
400
Temperature/oC
b
after activation
2.5
Untreated MES-SSO VMS-SSO MPMS-S S O Me thacry Isobutyl-P OS S TriSianolPhenyl-PO SS
2.0
Mass loss/%
3020
1.5
1.0
0.5
0.0 10
20
250
300
350
400
Temperature/oC
Fig. 4. Mass loss of CF/PAA composites during thermal oxidation treatments. (a) Before activation and (b) after activation.
Table 1 ILSS of CF/PAA composites after thermal oxidation treatments (MPa) Coating
RT
300 C
350 C
400 C
Untreated Epoxy MES-SSO VMS-SSO MPMS-SSO MethacrylIsobutyl-POSS TriSilanolPhenyl-POSS Plasma Plasma + MES-SSO Plasma + VMS-SSO Plasma + MPMS-SSO Plasma + TriSilanolPhenyl-POSS Plasma + MethacrylIsobutyl-POSS
34.2 ± 2.2 39.2 ± 2.7 43.5 ± 2.9 42.9 ± 2.1 43.9 ± 2.6 44.8 ± 2.0 45.9 ± 3.4 42.6 ± 3.6 45.9 ± 4.0 49.3 ± 4.2 51.3 ± 2.7 45.6 ± 2.6 44.6 ± 2.6
33.9 ± 4.2 37.2 ± 4.3 42.6 ± 2.3 43.2 ± 2.2 42.3 ± 2.5 44.1 ± 3.3 44.2 ± 2.9 39.4 ± 3.4 44.6 ± 3.5 48.2 ± 3.6 50.4 ± 3.4 45.2 ± 2.9 44.1 ± 2.8
31.4 ± 3.6 33.0 ± 3.1 37.1 ± 2.5 35.0 ± 1.8 36.1 ± 2.8 43.1 ± 3.9 43.5 ± 3.6 36.3 ± 3.6 40.3 ± 3.8 43.9 ± 4.0 45.9 ± 3.3 44.2 ± 2.7 43.6 ± 3.0
29.1 ± 3.1 30.4 ± 3.6 33.4 ± 3.0 31.5 ± 1.2 30.1 ± 3.1 34.1 ± 3.3 36.7 ± 3.9 31.2 ± 3.5 37.4 ± 3.7 38.1 ± 3.8 39.3 ± 3.6 41.3 ± 3.4 40.8 ± 3.5
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that of polymorphous silsesquioxane. The perfect adhesion between fibre and resin due to the import of silsesquioxane coatings and the heat resistance of them cause that the composites have excellent interfacial properties at high temperature. Comparing with the ILSS of composites treated by different silsesquioxiane coatings, the single cage type structural POSS coatings have better treating effect than the complex structural SSO coatings. In other words, the single cage type structural POSS coatings are much more appropriate than the complex polymorphous structural silsesquioxane coatings when they are used in the system of carbon fibre and PAA resin. The same results were received when carbon fibre surface was treated with different silsesquioxane coatings after being activated by air plasma. The mass loss was similar and the decrease of ILSS was also similar in spite of the chemical bonds, which suggested the mass loss and decrease of ILSS at high temperature were occurred by the pyrolysis of silsesquioxanes. The cage type structure POSS had better heat resistant than poly-structural silsesquioxanes, which resulted that there are few loss of weight and decrease of the ILSS of the POSS treated composites. 4. Conclusions The effects of silsesquioxane coating treatments of the surface upon the interfacial, impact, and heat-resistant properties of carbon fibre reinforced polyarylacetylene composites have been studied. The ILSS, absorbed energy during impact fracture of CF/PAA composites were both increased by different degrees after the fibre being treated with different silsesquioxane coatings. The increasing amounts of them of CF/PAA composites after carbon fibre being treated with single cage type structural POSS coatings are larger than those of composites after carbon fibre being treated with complex structural silsesquioxane coatings. Heat resistant of CF/PAA composites were also increased after carbon fibres were treated with different silsesquioxane coatings and the treatment effect of cage type structure POSS is better than that of poly structural silsesquioxane coatings. The previous results indicate that there are improved interfaces caused by silsesquioxane coatings. The thermal oxidative aging experiment results indicated that silsesquioxane coatings are agree with the system of carbon fibres and PAA resin and also proved that the single cage structural POSS coating has better effect than the complex structural silsesquioxane coating in high temperature. All results show that the properties at the fibre–matrix interphase change by altering the chemical system of the composite. This research of property variations with changing coating structure will help in better understanding and tailoring of the composite properties. Acknowledgements The authors thank the National Natural Science Foundation of China (No. 50333030) and the Natural Science
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