Novel carbon fiber/epoxy composite toughened by electrospunpolysulfone nanofibers
Gang Li a, Peng Li a, Yunhua Yu a, Xiaolong Jia a, Shen Zhang a, Xiaoping Yang a,⁎, Seungkon Ryu b
a The Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials,Beijing University of Chemical Technology, Beijing 100029, PR China
b Department of Chemical Engineering, Chungnan National University, Daejeon 305-764, Republic of Korea
Received 3 February 2007; accepted 31 May 2007Available online 5 June 2007
Keywords: Epoxy; Composite materials; Polysulfone; Nanomaterials; Thin films
1. Introduction
Carbon fiber/epoxy composite is especially attractive inaircraft and aerospace structural components [1,2]. However,the inherent brittleness of epoxy matrix has restricted itsapplications in advanced composites. To solve this problem,many toughening methods have been developed over the years[3–10]. Among these methods, ductile interleaving seems to beone of recommended methods [5–9], in which interleaf layers oftoughened materials were inserted into middle plies of thecomposites. Generally, thermoplastic particles and films havebeen used as common toughened layers [5–11]. However,difficult preparation of particles due to high toughness ofthermoplastic and high thickness of films due to high viscosityof thermoplastic have limited their uses in industry [6,11].Recently, nanofibers reinforcing was known as a more usefultechnique instead of particles or films reinforcing to enhance themechanical properties of composite because of very smalldiameter [12,13], but few reports have been found on toughening
composites with nanofibers. In view of wide use of thermoplasticin toughening epoxy resins and composites, polysulfone (PSF)nanofibers has a possibility of toughening carbon fiber/epoxycomposite. Polysulfone nanofibers can be easily obtained fromdirect electrospinning of polysulfone solution [14,15].
Therefore, the purpose of this study was a preparation ofnovel carbon fiber/epoxy resin composite toughened byelectrospun polysulfone nanofibers. To achieve the purpose, i)polysulfone solution was prepared by dissolving polysulfonepellets to solvents, and directly electrospun onto the prepregs, ii)the prepregs were stacked and molded to composites, iii)morphology observations and toughness tests of the compositeswere carried out, and compared the results with those of PSFfilms toughened and untoughened composites.
2. Experimental
2.1. Materials
Carbon fiber/epoxy prepreg was kindly supplied by BeijingFRPResearch andDesign Institute. The prepreg contained 60wt.%carbon fiber (T700, 12 K, Toray Co., Japan) and 40 wt.% resin
Fig. 1. Schematic fabrication procedure of polysulfone nanofibers toughened carbon fiber/epoxy composite: (a) carbon fiber/epoxy prepreg, (b) electrospinning PSFonto prepreg, (c) prepregs stacking, (d) molding to composite, and (e) fracture toughness test specimen.
Fig. 2. (a) SEM observation of electrospun polysulfone nanofibers, (b) the sizedistributions of electrospun polysulfone nanofibers obtained from 100nanofibers.
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matrix. The resin matrix was composed of tetraglycidyl 4, 4′-diamiodiphenyl methane (TGDDM, AG-80, Shanghai InstituteSynthetic Resins Co., China) as a pure resin, 4, 4′-diaminodiphenylsulfone (DDS, Yinsheng Chemical Co., Ltd. China) as a hardener,and boron fluoride ethamine (San'Ai'Si agent Co., China) as anaccelerator at a ratio of 100:30:1. Bisphenol-A polysulfone (PSF,Udel P1700, Amoco Co., USA) was used as a toughener. N, N'-dimethylacetamide (DMAC) and acetone were used as solvents forthe dissolution of polysulfone.
2.2. Composite fabrication
Fig. 1 shows the fabrication procedure of polysulfonenanofibers toughened carbon fiber/epoxy composite. Carbonfiber/epoxy prepreg was cut into rectangular specimens(180 mm×150 mm). Polysulfone solution was prepared bydissolving 25.0 g polysulfone pellets in 90.0 ml N, N'-dimethylacetamide (DMAC) and 10.0 ml acetone. This solutionwas transferred to a medical syringe, and polysulfone nanofiberswere directly electrospun onto the prepreg specimens to obtain5.0 wt.% nanofibers content at a flow rate of 1.5ml/h under 24 kVvoltage. Then 24 specimens were stacked in a stainless steel moldand hot pressed into composite by using a flat-plate vulcanizer.Composite curing was performed three stages: at 130 °C for 2 h,180 °C for 2 h and 200 °C for 1 h. The composite was cut intosmall specimens to measure Mode I fracture toughness (GIC).Each GIC specimen had a 50 mm starter precrack with 50 μmPTFE film between the 12th and 13th ply of the composite.
PSF films were prepared by solvent method introduced inearlier reports [8,9], and inserted evenly in layers of the stackedprepregs, then molded to composite under above conditions.The content of PSF films were also controlled to 5.0 wt.%.
2.3. Morphology observations and toughness test
The morphologies of polysulfone nanofibers and fracturesurfaces of composites were observed by scanning electronmicroscope (SEM, S-250, UK). The surfaces of the specimens
were coated with a thin layer of a gold alloy. The compositeswere cut into standard double-cantilever beam (DCB) speci-mens (180 mm×25 mm×3 mm) to measure Mode I fracturetoughness (GIC), using a tensile testing machine (Instron 1121).
Fig. 4. Relationship between fracture toughness and crack length of composites:(a) untoughened, (b) toughened by polysulfone nanofibers, (c) toughened byPSF films.
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3. Results and discussion
3.1. Morphology observations
Fig. 2 (a) shows the SEM images of electrospun polysulfone (PSF)nanofibers. The diameters of nanofibers were measured by Image Jsoftware from the SEM photographs. The diameters of nanofibers werein the range of 100 nm to 400 nm, exhibiting a relatively narrow sizedistribution, as shown in Fig. 2 (b), and the average diameter was230 nm. The nanofibers were randomly aligned on the prepregs due tothe whirlpool electrospinning.
Fig. 3 shows the morphologies of fracture surface of compositestoughened by polysulfone nanofibers and PSF films. Polysulfonespherical domains in Fig. 3 (a) presented uniform dispersion throughthe interlayers of polysulfone nanofibers toughened composite, com-pared to the discontinuous dispersion at different layers inFig. 3 (b),which was similar to the results described by Yun et al. [7].
In general, the toughening of epoxy by polysulfone resulted fromthe mechanism of reaction-induced phase separation [7–9,16–18]. Theunique properties of polysulfone nanofibers such as small diameter andhigh specific surface area [14,15] had allowed the nanofibers beingimpregnated and melted easily into matrix at the beginning stage ofcuring. In addition, the large porosity of polysulfone nanofibers hadresulted in good penetration of resin matrix. Therefore, during thecuring of epoxy matrix, polysulfone was separated as spheres throughinterleaves of the composite, which was attributed to the reaction-induced phase separation, as like as the other results [5,7,9]. However,the high thickness, low fluidity and permeability of PSF films to resin
Fig. 3. SEM observations of fracture surface of composites toughened by(a) polysulfone nanofibers and (b) PSF films.
matrix had led to the polysulfone spheres dispersing only in localinterphase region.
As can be seen from Fig. 3 (a), the diameter of dispersed polysulfonespheres was approximately 2–6 μm, which was slightly larger than thatof Fig. 3 (b), with spherical PSF-rich domains in sizes of 1–2 μm.Obviously, the diameters of polysulfone domains in polysulfonenanofibers toughened composite showed an uneven distribution,which resulted from different diameters of nanofibers and randomjunctions at one layer or from several layers.
3.2. Fracture toughness (GIC) test
Fig. 4 shows Mode I fracture toughness (GIC) of untoughenedcomposite, polysulfone nanofibers and PSF films toughened carbonfiber/epoxy composites. The average GIC of untoughened compositewas 0.310±0.03 kJ/m2, while the GIC of polysulfone nanofiberstoughened composite and PSF films toughened composite were 0.869±0.13 kJ/m2 and 0.618±0.12 kJ/m2, respectively. The GIC of poly-sulfone nanofibers toughened composite was the highest, which was140% higher than that of PSF films toughened composite, and 280%higher than that of untoughened composite.
Such an improvement was dependent on the morphology ofpolysulfone nanofibers toughened composite. Owing to the uniqueproperties of polysulfone nanofibers, all-around phase separationoccurred during the curing of epoxy matrix. Therefore, polysulfonespherical domains distributed uniformly through interleaves of carbonfiber/epoxy composite and toughened the composite effectively.However, the phase separation degree of PSF films toughenedcomposite was lower than that of polysulfone nanofibers toughenedcomposite because of local dispersed polysulfone domains, which hadresulted in a moderate improvement of fracture toughness.
On the other hand, the relatively large spheres on the polysulfonenanofibers toughened composite had resulted in crack pinning ordeflection compared to those small size spheres on PSF filmstoughened composite [8]. Thus, the uniform dispersion of polysulfonespheres with uneven sizes improved greatly the fracture toughness ofcarbon fiber/epoxy composite toughened by PSF nanofibers.
4. Conclusion
Toughening of carbon fiber/epoxy composite by poly-sulfone (PSF) nanofibers prepared by electrospinning was a
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more effective method than by PSF films prepared by solventmethod. The phase-separated morphology of PSF nanofiberstoughened composite exhibited uniform dispersion of poly-sulfone spheres with uneven sizes, while the morphology of PSFfilms toughened composite showed discontinuous distributionthrough interleaves of the composite. Mode I fracture toughness(GIC) of polysulfone nanofibers toughened composite was 140%higher than that of PSF films toughened composite, and 280%higher than that of untoughened composite. The toughnessimprovement was achieved from the uniform distribution ofdispersed polysulfone phase generated by polysulfone (PSF)nanofibers. Thus, this novel method is potentially applicable inpractical preparation of toughened composite.
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