O R I G I N A L P A P E R
Fabrication of adduct filled glass fiber/epoxy resin laminate composites and their physical characteristics
Cuong Manh Vu1•Thai Viet Nguyen1• Liem Thanh Nguyen2•Hyoung Jin Choi3
Received: 28 April 2015 / Revised: 1 October 2015 / Accepted: 2 November 2015 ÓSpringer-Verlag Berlin Heidelberg 2015
Abstract Adducts were synthesized from thiokol and epoxy resin using triethy- lamine as a catalyst, and their structure and molecular weights were determined.
The synthesized adducts, which possessed an epoxide group on their chain as a modifier to enhance the toughness of the epoxy resin, were well dispersed in the epoxy matrix, resulting in the successful manufacture of laminate composites made from glass fiber/epoxy resin (GF/EP). The microstructure and mechanical proper- ties, such as the tensile strength, impact resistance and mode I interlaminar fracture toughness, were investigated and compared with those of the GF/EP system. The addition of the adduct to the epoxy matrix led to an increase in both the mode I interlaminar fracture toughness and IZOD impact resistance of the GF/EP.
Keywords Epoxy resin CompositeAdductImpact strengthInterlaminar fracture toughness
Introduction
Lightweighting is a concept that replaces traditional materials, such as construction steel and aluminum, with advanced metal alloys and composites to achieve significant weight reductions and reduced energy use. Polymer composites
& Cuong Manh Vu
vumanhcuong309@gmail.com
& Hyoung Jin Choi
hjchoi@inha.ac.kr
1 Chemical Department, Le Qui Don Technical University, 236 Hoang Quoc Viet, Hanoi, Vietnam
2 Polymer Center, Hanoi University of Science and Technology, 1 Dai Co Viet, Hanoi, Vietnam
3 Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea DOI 10.1007/s00289-015-1553-7
reinforced with long fibers are used widely as high-performance materials in various industries, such as auto-motive, rail-industry, aerospace, sport, and civil engineer- ing. Concurrently, among the thermosetting polymers used in composite manufac- ture, epoxy resins have been used most widely for high-performance applications [1]. Epoxy matrixes exhibit excellent mechanical and thermal characteristics, low shrinkage upon curing, very good chemical and corrosion resistance, and good processability under a range of working conditions. On the other hand, epoxy or epoxy-based materials possess very poor impact resistance properties. Several solutions have been proposed to improve their impact resistance and fracture toughness [2]. An introduction of thermoplastic particles in epoxy resins can improve their toughness [3,4]. A new innovative method to improve their impact resistance was reported recently. This method consisted of using an epoxy nanocomposite as a matrix in the final composites [5]. Chen et al. [6] prepared ac–
Al2O3/epoxy nanocomposite (ENC) and examined the influences of c–Al2O3
nanoparticles on the thermal and mechanical properties of the prepared ENC. The increase in fracture toughness of the composite material toughened by the different size fillers was higher than the combination of that toughened by an individual filler with the same loading [7]. By examining the influence of several nanofillers, such as single-walled carbon nanotubes, double-walled carbon nanotubes (DWNTs) and multi-walled carbon nanotubes, on the mechanical properties of epoxy-based nanocomposites, Gojny et al. [8] observed that the most significant improvements were achieved with amino-functionalized DWNTs with 0.5 wt% filler content:
10 % on the tensile strength, 15 % on stiffness and 43 % on fracture toughness.
Carbon nanotubes (CNTs), layered clay [9], rigid nanoparticles, or combinations of these particles have been used frequently in epoxy nanocomposite-based fiber composite materials [10]. Because of their small size, rigid nanofillers are effective in improving both the toughness and stiffness of the final composites [9,11,12]. In particular, well-dispersed silica nanoparticles have been adopted to strengthen epoxy resins [13]. Ferreira et al. [14] examined the fatigue strength of Kevlar/epoxy laminate composites and reported the positive effects of epoxy organoclay as the matrix in epoxy composites. The 12 wt%-filled composites showed significantly improved tensile fatigue strength. On the other hand, the fatigue strength of the filled composites was lower in the three point bending test. Gojny et al. [15]
investigated the tensile properties of glass fiber-reinforced carbon nanotube/epoxy matrixes, showing that the tensile properties of the laminates were unaffected by the presence of the CNTs, and the interlaminar shear strength and fracture toughness were improved. On the other hand, using different carbonaceous nanomaterials of single-walled carbon nanotube, multi-walled carbon nanotube, graphite, and carbon nanofiber, Yang et al. [16] reported that the tensile modulus of most glassy composites were only slightly greater than that of the epoxy, while elongations of all glassy composites were significantly lower than that of the epoxy and decreased with increasing filler concentration, which limits the glassy composites for many structural applications.
However, a liquid additive is still considered one of the most successful tougheners for epoxy. Since the first attempt to toughening epoxy resins was initiated by researchers from B.F. Goodrich Co. [17], extensive investigations have
been conducted to explore the toughening mechanism of the rubber toughened epoxies [18–26]. Vu et al. [27] examined the effects of three different types of additive-added epoxy, thiokol, epoxidized natural rubber (ENR) and epoxidized linseed oil (ELO), on the mechanical and dielectric characteristics of glass fiber- reinforced epoxy composites. The results showed that the addition of each of 7 phr ENR, 9 phr ELO and 5 phr thiokol to the epoxy resin increased the fracture toughness significantly by 56.9, 43.1 and 80.0 %, respectively, compared to the unmodified resin. The mode I interlaminar fracture toughness of the GF/EP at propagation was also improved by 26.9, 18.3 and 32.7 % when each of 7 phr ENR, 9 phr ELO, and 5 phr thiokol, respectively, was dispersed in the epoxy matrix.
Polysulfides (thiokol) were used as a toughener for the epoxy resin because the mercaptan group can react with the epoxide group to make a longer and flexible main chain [28]. On the other hand, the reaction between the mercaptan group and epoxide group takes place over the long time at room temperature [29]. To resolve this problem, the copolymer or adduct from thiokol and epoxy resin with an excessive epoxide group was prepared. This adduct can be dissolved in the epoxy resin and react with the hardener of the epoxy resin that makes the main chain of epoxy resin longer and flexible.
The main aim of this study was to investigate the synthesis of adducts from thiokol and epoxy resin, and develop adduct-modified epoxy as the matrix of glass fabric composites for improved characteristics. The effects of the adduct on the mechanical properties and dielectric characteristics of GF/EP were investigated.
Experimental details Materials
The epoxy resin used in all experiments was diglycidal ether of bisphenol A (DER 331, Dow Chemical Co.). Diethylenetriamine (DETA) (Dow Chemical Co.) was used as a curing agent. Thiokol was Thioplast G21 from Akazo Nobel.
Triethylamine (TEA) used as a catalyst in the adduct synthesis reaction. The iodine concentration for a 1 l standard solution was 0.05 M I2(0.1 N) (12.69 g I2?20 g KI, Sigma-Aldrich). Na2S2O3and benzene were purchased from Sigma- Aldrich (Singapore). The woven roving E glass fiber with an area weight of 300 g/
m2(WRE300) was purchased from Jiujiang Beihai Fiberglass Co., China. Figure1 shows the chemical structures of the materials used in this study.
Synthesis and characterization of adduct
The molar ratios of the mercaptan group (SH) in the thiokol and epoxide group (EG) in epoxy resin (SH/EG) ranged from 0.6/1 to 0.8/1. The thiokol and epoxy resin was dissolved in toluene and charged into the reactor. 0.25 mol.% TEA dissolved in the toluene solution was dropped slowly into the thiokol and epoxy solution, which was then heated to 85±2°C. The reaction time was 4 h.
Toluene was removed by the evaporator at 40°C and the obtained adducts were dissolved in dichloromethane, precipitated in an excess of distilled ethanol and dried in a vacuum oven at 40°C for 24 h. The chemical structure of the final products was identified by both 1H nuclear magnetic resonance (NMR) and Fourier transform infrared (FTIR) spectroscopy. Their molecular weights were determined by gel permeation chromatography (GPC).
The1H NMR spectroscopic technique was carried out using a Bruker AVANCE machine (Germany) 300 MHz Nuclear Magnetic Resonance, after dissolving the samples in CDCl3. The number-averaged molecular weight (Mn) and weight- averaged molecular weight (Mw) were determined by GPC measured at 30°C on a ThermoFinnigan SEC instrument equipped with a Spectra SYSTEM AS1000 autosampler, Spectra SYSTEM UV2000 and SpectraSYSTEM RI150 detectors (Thermo Electronic Corp., USA). THF was used as the eluent at a flow rate of 1 ml/
min. FTIR spectroscopy was carried out on a Nicolet 6700 FT-IR (USA) spectrometer in the range, 400–4000 cm-1, at a resolution of 2 cm-1. For each spectrum, 64 scans were co-added. In addition, the weight percentage of epoxide in both the epoxy resin and adducts was determined according to ASTM D 1652–04.
On the other hand, the SH end-group content of both the thiokol and adducts was determined iodometrically [30]. Therefore, the polymer sample (0.7–1.2 g) was weighed accurately into a 250 ml flask and dissolved in benzene (20 ml) with stirring, followed by the addition of 50 ml of water. Subsequently, a 0.1 N aqueous I2/KI solution was added in 2–3 ml portions with stirring, waiting after each addition until the brown color of the mixture had disappeared. The addition of the iodine solution was stopped when the brown color of the organic layer persisted for more than 5 min. In addition, 10 ml of a starch solution was added, and the excess
Epoxy resin
CH2 CH O
CH2
C CH3
CH3
C CH3
CH3
CH2
OH
CH2O O CH2 CH O O CH CH2
n O
Thiokol HS R SS aCH2 CH CH2
b SH SS R cSH
R= CH2
2 O CH2 O CH2
2
SS R
DETA
H2N CH2 CH2 NH CH2 CH2 NH2
Fig. 1 Structure of compounds
iodine was titrated with a 0.1 N aqueous Na2S2O3solution until the blue color of the aqueous layer disappeared and did not return after 2–3 min stirring. A blank titration was carried out separately. The SH end-group content (F) was calculated using Eq. (1):
Fðg/mol) ¼ 0:1ðV0VÞ
1000m ð1Þ
whereV0andVare the volumes of the 0.1 N aqueous Na2S2O3solution used in the blank experiment and for titrating the polymer sample, respectively, andm is the amount of polymer sample (g) used in the analysis.
The F values obtained were used to calculate the SH end-group content [NSH, Eq. (2)] and the conversion of the thiol groups [CSH, Eq. (3)] as follows:
NSHðmol)¼Fmp ð2Þ
CSHð% )¼100ðNSH;0NSHÞ NSH;0
ð3Þ
wherempis the amount of polymerization solid product (mixture of adducts and unreacted thiokol) recovered from each glass ampoule (g), whileNSH,0is the mole number of SH groups contained initially by each polymerization ampoule.
Composite preparation
Solutions 10 phr of the adduct with different SH/EG ratios in the epoxy resin were first mixed together using a mechanical stirrer, and heated for 1 h at approximately 60°C in a water bath to ensure proper dispersion of the adduct. The mixtures were cooled to room temperature, while a curing agent was added prior to hand mixing for approximately 20 min. The products were degassed using a vacuum pump. Next, the resin mixture was poured into a 4 mm-thick mold that was coated with a release agent. The samples were precured at room temperature for 24 h and then postcured at 80°C for 3 h. At this stage, the cured specimens were allowed to cool slowly to room temperature. Once all formulations were made, they were then reinforced with a hand layup glass fiber process to form the composite sheets. They were finally cured at the same temperature cycle, which was used for the resin specimens. The glass fiber volume fraction was 55±2 %.
Resin fracture toughness test
The single edge notch bend (SENB) specimens (Fig.2) according to the ASTM (D5045-99) were used to test the fracture toughness (critical stress intensity factor, KIC). The notch tip was machined using a rotating saw, and a pre-crack of the specimens was generated by tapping on a fresh razor blade placed in the notch. The fracture toughness tests were conducted at a cross-head speed of 10 mm/min. The KIC value reported represents the average of at least five tests. The following equations were used to calculate the KIC:
KIC¼ PQ BW12
fð Þx ð4Þ
with
f(xÞ ¼6x12½1:99xð1xÞð2:153:93xþ2:7x2 ð1þ2xÞð1xÞ32
wherePQis the critical load for crack propagation (kN),Bis the specimen thickness (cm),Wis the specimen width (cm), f(x) is the non-dimensional shape factor,a is the crack length (cm), andx=a/W.
Izod impact strength and tensile test
The unnotched Izod impact strength was determined according to ISO 180 using a Tinius Olsen model IT 504 (USA). The dimension of the samples was 8091094 mm3. In addition, the tensile test of the resin and GF/EP was performed using the INSTRON 5582-100KN machine according to the ISO-527- 1993. The specimen gauge length was 50±1 mm and the testing speed was set to 2 mm/min. The specimen dimensions were 25092592.5 mm3. Glass fiber- reinforced plastic/epoxy tabs, 1.5 mm in thickness, were attached at both ends of the specimen by an adhesive. The values were taken from a mean of five specimens.
Mode I interlaminar fracture toughness test
Mode I double cantilever beam (DCB) tests were carried out using the ASTM (D5528-01). The recommended specimen size is at least 150 mm in length and 20 mm in width with an initial crack length (i.e., the length of the insert from the line) of 50 mm. Hinges with the same width as the specimen were attached to allow the application of a load. The load and displacement were then related to the delamination length, as measured with a ruler on the specimen edge (see Fig.3).
The mode I interlaminar fracture toughness,GICandGIP, was calculated using the modified beam theory (MBT) method. The MBT method was determined using the following equations:
Fig. 2 Schematic diagram of theKICspecimen used in this study
GIC¼ 3PCd 2bðaþj jÞD
F
N ð5Þ
GIP¼ 3PPd 2bðaþj jÞD
F
N ð6Þ
whereGIC is the fracture toughness at an initial crack stage corresponding to first peak load in the force–displacement curves,GIP is the fracture toughness at the propagation stage,PPis the applied load,Cis the compliance corresponding to each crack length,ais the crack length, Pc is the initial maximum load,bis the specimen width,dis the load point deflection, andDis an effective delamination extension to correct for rotation of the DCB arms at the delamination front. In addition,Nis the end-block correction factor andFis a large displacement correction factor.
Morphology analysis
The morphology was examined by scanning electron microscopy (SEM) (Joel JSM 6360, Japan) and field emission scanning electron microscopy (FE-SEM) (S-4800, Hitachi). The fractured samples under mechanical analysis were sputter–coated with gold prior to the test.
Dielectric measurement
The dielectric constant (e) and loss tangent (tand) were determined using Digital C- TgD METER mod 0194 C equipment (CEAST) according to ASTM D150 at a 1 MHz frequency. The samples with a circular shape, approximately 11 cm in diameter (equal to the diameter of the capacitor) and\1 mm in thickness, were cut from laminate sheets, and then placed between two capacitors. The distance between the two capacitors was adjusted by a knob to close the sample as possible
Fig. 3 Geometry of the DCB specimen (all dimensions in mm)
and record the f capacitance (C) and loss tangent in the screen. The dielectric constant (e) was calculated using the following equation:
C¼ee0
A d
where C is the capacitance in Farads (F), e is the dielectric constant, e0=8854910-12F/m,Ais the area of overlap of the two capacitors, anddis the separation between the capacitors, which is equal to the thickness of samples. The distance between the two capacitors was adjusted by a knob to close the sample as much as possible.
Results and discussion
Figure4shows the FTIR spectra of the adduct, thiokol and epoxy resin, in which the adduct was prepared with molar ratio mercaptan/epoxide groups=0.6 (SH.EG0.6 adduct).
In contrast, the FTIR spectrum of thiokol (1), adduct (2) and epoxy resin (3), the –OH stretching vibration at *3430 cm-1, was strengthened while the -SH stretching vibration at*2540 cm-1and the epoxide group stretching vibration at 834, 915 cm-1were weakened. This shows that a reaction between the mercaptan and epoxide groups of resin had occurred. Figure5presents the reaction mechanism between the epoxide and mercaptan groups with tertiary amines as a catalyst.
The catalysis of an epoxide-mercaptan reaction with tertiary amine can be considered as a general base catalysis, in which mercaptan first reacts with the tertiary amine to give the mercaptide ion, and the ion then reacts with an epoxide.
Fig. 4 FTIR spectra of thiokol, epoxy resin and SH.EG0,6 adduct
Alternatively, nucleophilic catalysis occurs during which the tertiary amine first reacts with epoxide to produce a reactive intermediate and then the intermediate reacts with mercaptan in a nucleophilic displacement [31]. Figure6 shows the1H NMR spectrum of the thiokol and adduct.
The main characteristic peaks of the thiokol were noticeable at 1.5 ppm (SH), 2.9 ppm (SCH2), 3.75 ppm (OCH2), and 4.7 ppm (O–CH2–O). The achieved adduct
R3N HS S R3NH
CH O
CH2 S CH
OH
CH2 S R3N
CH O
CH2 NR3 CH CH2
O
NR3
HS CH
OH
CH2 S R3N
(a)
(b)
Fig. 5 Reactions between epoxide and mercaptan catalyzed by tertiary amine:ageneral base catalysis andbnucleophilic catalysis
Fig. 6 The1H NMR spectra of thiokol and SH.EG0.6 adduct
shows the main characteristic peaks that are noticeable at 0.87-0.92 ppm (CH3), 1.2 ppm (CH2), 2.79 ppm (epoxide group), 2.9 ppm (S–CH2), 3.75 ppm (OCH2), and 4.7 ppm (O–CH2–O). The achieved adduct can be proven by the disappearance of the peak at 1.5 ppm, corresponding to mercaptan group in the thiokol molecule, and the appearance of new peak at 2.79 ppm corresponding to epoxide group in the adduct.
On the other hand, the weight percentage of epoxide in the adducts with different molar SH/EG ratios of 0.6, 0.7 and 0.8 was 14.6, 11.01 and 7.94 %, respectively, and the adduct content was zero. This shows that with increasing molar SH/EG ratio, the weight percentage epoxide of the adducts decreased. The decrease in the weight percent epoxide of the adducts with different molar SH/EG ratios was attributed to the increase in thiokol content to the synthesis adduct. The lack of a SH end group of adducts indicated that the SH group had been reacted completely with an epoxide group.
Table1lists the molecular weights of the thiokol, epoxy resin and adduct.
The weight-averaged molecular weightMwof the adduct was 4.53 times higher than thiokol and 103.68 times higher than epoxy resin. This confirmed that the reaction between thiokol and epoxy resin via mercaptan and epoxide groups was complete.
Effect of SH.EP0.6 adduct on mechanical property of polymer blend
After the synthetic method to obtain the adduct, the blends of epoxy with different ratios of the SH.EG0.6 adduct were prepared. Figure7 shows the effect of the SH.EG0.6 adduct contents on the tensile strength of epoxy resin. Figure7a represents typical tensile strength–strain curves of the samples, whereas Fig.7b, c and d presents the mean fracture tensile strength, tensile modulus and fracture tensile strain, respectively. The introduction of the adduct as a dispersing agent resulted in considerable enhancement of the tensile stress and tensile strain but decreased tensile modulus. With the dispersion of 10 phr SH.EG0.6 adduct in the epoxy resin, the fracture tensile strength and strain were increased by 30.58 and 125 %, respectively, compared to the pure epoxy resin. On the other hand, the fracture tensile modulus was decreased by 31 % from 2 to 1.38 GPa. In addition, it can be also noted that the area under the stress–strain curves from the tensile elongation test gives mechanical toughness, while in this study we followed the ASTM (D5045-99) test.
Figure8 shows the effects of the SH.EG0.6 adduct content on the flexural strength, flexural modulus, IZOD impact strength, andKIC.
Table 1 The molecular weights of the thiokol, epoxy resin and SH.EG0.6 adduct
Sample Mw(g/mol) Mn(g/mol) PDI (MW/Mn)
SH.EP0.6 adduct 73,720 35,065 2.1
Thiokol 9550 2653 3.6
DER331 711 438 1.62
Fig. 7 aTypical tensile strength–strain curves of pure epoxy resin; epoxy resin was modified with 5 phr SH.EP0.6 adduct; 10 phr SH.EG0.6 adduct; 15 phr SH.EG0.6 adduct; 20 phr SH.EG0.6 adduct;
comparison of fracture tensile strength and Young’s modulus (b) fracture strain (c) as obtained from 5 sets of 6 samples for each case
Fig. 8 Comparison of fracture flexural strength, flexural modulus (a) IZOD impact strength,KIC(b) of pure epoxy resin (EP), of epoxy resin was modified with 5 phr SH.EG0.6 adduct (5 SH.EG0.6); 10 phr SH.EG0.6 adduct (10 SH.EG0.6); 15 phr SH.EG0.6 adduct (15 SH.EG0.6); 20 phr SH.EG0.6 adduct (20 SH.EG0.6) as obtained from 5 sets of 6 samples for each case
Figure8 shows that the flexural strength and flexural modulus decreased with increasing SH.EG0.6 adduct content dispersed in epoxy resin. The decrease in flexural modulus occurs due to the incorporation of adducts, which have a low modulus in the epoxy matrix. The decrease in flexural modulus showed that the adduct/epoxy resin composite was softer than the pure epoxy resin. The IZOD impact strength andKIC values of the epoxy resin were improved significantly by adding the adduct. The addition of adduct results in an increase in the IZOD impact strength, the KIC value, up to an optimal content of 10 phr adduct. At 10 phr SH.EG0.6 adduct, the IZOD impact strength was improved by 141.97 % from 12 to 37.26 kJ/m2 and the KIC values were improved by 47.69 % from 0.65 to 0.96 MPa.m1/2. Figure9 shows SEM images of the fracture surfaces of the adduct/epoxy resin composites with different adduct contents in the epoxy resin.
Figure9a and b shows that the fractured surface of the neat epoxy was smooth and glassy, which is the typical brittle fracture behavior of a thermosetting polymer.
A smooth mirror-like surface with micro-flow lines was observed. On the other hand, the fractured surfaces of the modified epoxy at 5 phr (Fig.9c, d), 10 phr (Fig.9e, f), 15 phr (Fig.9g, h), and 20 phr (Fig.9i, j) SH.EG0.6 adduct were rougher, and jagged multi-plane patterns appeared so that more energy was required. SEM of the modified systems revealed the presence of SH.EG0.6 adduct particles, which were dispersed throughout the epoxy matrix, i.e., they showed the presence of a two-phase morphology. The soft elastomeric phase was phase- separated from the hard epoxy matrix during the early stages of curing. The fractured surfaces of most of the adduct-toughened epoxy systems had a rigid continuous epoxy matrix with a dispersed rubbery phase as isolated particles.
Different mechanisms, such as crazing, shear bonding and elastic deformation of the rubber particles were proposed. These mechanisms are believed to act alone or in rubber particles and are thought to act alone or in rubber-modified epoxy [32,33].
Effect of different molar SH/EP ratio on mechanical property of polymer blends
Figure10 shows the effects of different molar SH/EG ratios of 0.6; 0.7 and 0.8 synthesis adduct on the mechanical properties of the polymer blends.
At the same adduct content in the epoxy resin (10 phr), with increasing molar SH/EG ratios, the tensile strength and tensile modulus decreased but the tensile strain increased. With increasing molar SH/EG ratio, the weight percentage epoxide of adduct decreased, but the molecular chain of the adduct became longer due to more thiokol molecules participating in the adduct chain in that the crosslink density of the epoxy decreased when the adduct was dispersed and a tough phase was observed in the epoxy resin at the same content. This means that the tensile strength and tensile modulus decreased with increasing molar SH/EG ratios but the tensile strain increased due to increased slipped level between the different molecular chains. Figure11shows the effect of the different molar SH/EG ratios on the IZOD impact strength andKICvalues.
Figure11 shows that with increasing molar SH/EG ratios, the IZOD impact strength increased to the optimal molar SH/EP ratio of 0.7. At 10 phr SH.EG0.7
Fig. 9 SEM images of the fractured surfaces of (a,b) neat epoxy, (c,d) 5 phr SH.EG0.6 adduct, (e, f) 10 phr SH.EG0.6 adduct, (g,h) 15 phr SH.EG0.6 adduct, 20 phr SH.EG0.6 adduct. The direction of crack propagation is fromtoptobottom
Fig. 11 Comparison of IZOD impact strengthKICof pure epoxy resin (EP), of epoxy resin was modified with 10 phr SH.EG0.6 adduct (SH.EG0.6); 10 phr SH.EG0.7 adduct (SH.EG0.7); 10 phr SH.EG0.8 adduct (SH.EG0.8) as obtained from 5 sets of 6 samples for each case
Fig. 10 Typical tensile strength–strain curves (a); comparison of fracture tensile strength, Young’s modulus (b) and fracture strain (c) of epoxy resin were modified with 10 phr SH.EG0.6 adduct; 10 phr SH.EG0.7 adduct; 10 phr SH.EG0.8 adduct as obtained from 5 sets of 6 samples for each case
adduct inclusion in epoxy resin, the IZOD impact strength andKICvalue increased by 66 and 15 %, respectively, compared to the epoxy resin modified with 10 phr SH.EG0.6 adduct composite.
From this result, the epoxy, which was modified with the 10 phr SH.EG0.7 adduct was chosen as a matrix to prepare and study the effect of the adduct on the mechanical and dielectric properties of the glass fiber-reinforced epoxy composite.
Effect of the SH.EG0.7 adduct on the mechanical and dielectric properties of the glass fiber-reinforced epoxy composite
The epoxy resin modified with 10 phr SH.EG0.7 adduct was used as a matrix to prepare the glass fiber-reinforced epoxy composite by a hand layup. The mechanical properties of the GF/EP modified with 10 phr SH.EG0.7 adduct were determined as listed in Table2.
No effect of the SH.EG0.7 adduct on the tensile strength, tensile modulus, flexural strength, and flexural modulus was observed. But the Izod impact strength increased by up to 158 % from 58.36 to 150.7 kJ/m2.
The mode I interlaminar fracture toughness of GF/EP was determined using a double cantilever beam (DCB) test, in which the curves of the applied load vs.
displacement were recorded. Figure13shows that the force increased linearly until the maximum force was reached, and then decreased gradually in the manner of a zigzag shape (stick–slip) in the propagation stages.
For the modified composite, both the displacement and force values in Fig.12, taken from a mean of five specimens, were higher than those of the unmodified composite. The crack was suggested to propagate more stably and gradually as a result of the relatively high tenacity of the epoxy modified with the SH.EG0.7 adduct. Moreover, the mode I interlaminar fracture toughness values, taken from a mean of five specimens, were calculated using the MBT method, as shown in Fig.13.
A significant increase in the mode I interlaminar fracture toughness with the modified epoxy resin additives was observed. At 10 phr SH.EG0.7 adduct, crack initiation (GIC) increased by 62.82 % and theGIPvalues also increased by 75.5 % compared to the unmodified composite. The presence of a SH.EG0.7 adduct in the GF/EP acted as an obstacle reinforcement that deflected, pinned and delayed crack propagation, so more energy was required. The major energy absorption mechanism in the composite included crack deflection, debonding between the fiber and resin, pullout (extraction of the fibers from the resin) and fiber-bridging mechanism [34,
Table 2 The mechanical properties of GF/EP composite with and without 10 phr SH.EG0.7 adduct
Sample Unmodified SH.EG0.7 adduct
Tensile strength (MPa) 187.94 188.3
Tensile modulus (GPa) 6.792 6.738
Flexural strength (MPa) 323.4 314.9
Flexural modulus (GPa) 12.34 12.21
IZOD impact strength (kJ/m2) 58.36 150.7
35]. In general, a number of mechanisms contribute to the fracture toughness, and it is often very difficult to determine the dominant mechanism. SEM of the fractured surfaces of the specimens revealed clear damage to the interfacial region in the composite. For the unmodified composite (Fig.14a, b), the fractured surfaces were mostly smooth and glassy due to brittle failure. Therefore, the energy required for interlaminar delamination failure was low. In contrast, the SH.EG0.7 adduct strongly affected the mode I interlaminar fracture toughness, as shown in Fig.14c, d, indirectly indicating more resin remaining on the fracture surface. The fractured surface of the SH.EG0.7 adduct-modified composite had a rougher surface and was tougher than those of the unmodified composite. Therefore, more energy will be
Fig. 12 Typical force–displacement curves of the unmodified composite (1) and composite modified with 10 phr SH.EG0.7 adduct (2)
Fig. 13 Initiation (GIC-A) and propagation (GIP-B) fracture toughness in mode I of the GF/EP composite with and without SH.EG0.7 adduct
needed, resulting in higher fracture toughness. The effect of the SH.EG0.7 adduct on the dielectric properties of the glass fiber-reinforced composite was studied.
The dielectric constant (e) with the addition of 10 phr SH.EG0.7 adduct was 2.39, which was increased slightly when compared to theeof the unmodified GF/EP. The small dielectric constant showed that no external electric field was stored in the material, which agrees with the loss factor. The loss factor (tan d) of the 10 phr SH.EG0.7 adduct-modified and unmodified GF/EP was 0.025 and 0.022, respec- tively, suggesting little external electric field loss, and confirming that the SH.EG0.7 adduct had no added effect on the dielectric properties of the GF/EP.
Conclusions
In this study, we investigated how to prepare the adduct from thiokol and epoxy resin in the presence of triethylamine as a catalyst. Adduct formation was confirmed by FTIR, 1H NMR, GPC, and the decrease in the weight percentage of epoxide compared to pure EP. The adducts behaved like an elastomer, acting as a toughening agent for the epoxy resin. The effects of adduct on the mechanical and dielectric properties of the both EP and GF/EP composites were determined such that by adding 10 phr SH.EG0.6 adduct to the epoxy resin, the fracture tensile
Fig. 14 SEM images of the fractured surface of GF/EP composite with (c, d) and without (a, b) SH.EG0.7 adduct
strength, strain, Izod impact strength, andKICvalues increased by 30.58, 125, 141.9, and 47.69 %, respectively, compared to the pure epoxy resin; however, the tensile modulus decreased. From the effect of different molar SH/EG ratios to the synthesis adduct on the mechanical properties of epoxy resin, the molar SH/EG ratio to achieve the best mechanical properties of epoxy resin was 0.7. Meanwhile, no effect of the SH.EG0.7 adduct on the tensile strength, tensile modulus, flexural strength, and flexural modulus of GF/EP composites was observed, except the Izod impact strength increase and increase of initiation and propagation interlaminar fracture toughness in mode I of GF/EP composites at 10 phr SH.EG0.7 adduct. Furthermore, dielectric properties showed that the SH.EG0.7 adduct did not affect e and loss factor (tand) of the glass fiber-reinforced epoxy composite.
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