Accepted Manuscript

Toughened FRP Composites Reinforced with Glass and Carbon Fiber

Debdatta Ratna

PII: S1359-835X(07)00283-7

DOI: 10.1016/j.compositesa.2007.12.005

Reference: JCOMA 2156

To appear in: Composites: Part A

Received Date: 7 May 2007

Revised Date: 9 December 2007

Accepted Date: 16 December 2007

Please cite this article as: Ratna, D., Toughened FRP Composites Reinforced with Glass and Carbon Fiber,

Composites: Part A (2007), doi: 10.1016/j.compositesa.2007.12.005

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ACCEPTED MANUSCRIPT

Toughened FRP Composites Reinforced with Glass and Carbon Fiber

Debdatta Ratna* Institut fur Verbundwerkstoffe GmbH (Institute for Composite Technology) Technical University, Kaiserslautern

Erwin-Schrodinger-Str. 58 D-67663, Kaiserslautern

GERMANY

ABSTRACT

Glass fiber reinforced plastic (GFRP) and carbon fiber reinforced plastic (CFRP)

composites were made using epoxy/hyperbranched polymer (HBP) blends as matrices.

The morphology of the blends was investigated both in the castings and composite form.

Unlike the castings, where no reduction in epoxy Tg was observed up to 15 wt% of HBP,

the Tg of the blend matrix was found to decrease as a result of addition of HBP (0 to 20

wt%) in case of GFRP and CFRP composites. Incorporation of HBP resulted in a

significant increase in impact strength in GFRP composites whereas no significant

improvement was observed in case of CFRP composites. Fracture surfaces of the

composites were analyzed by scanning electron microscope (SEM), which indicates a

two-phase microstructure. The microstructure was found to be different in castings and

composites.

Key Words: FRP composites, mechanical properties, glass fibre, epoxy resin

------------------------------------------------------------------------------------------------------------

* Tel. 0049-631-2017242, Fax : 0049-631-2017198,e.mail : ratnad29@hotmail.com , Permanent Address: NMRL, Shil-Badlapur Road, Anand nagar P.O., District Thane Maharashtra - 421 506, India.

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1. Introduction

Over the last two decades, the use of epoxy based fiber reinforced plastic (FRP)

composites, in structural applications, has increased tremendously and this dramatic

growth is expected to continue in the future. The composites possess many useful

properties like high specific stiffness and strength, dimensional stability, adequate

electrical properties and excellent corrosion resistance which make them attractive to the

automobile and aerospace industries [1-3]. However, the thermoset based FRP

composites are known to be highly susceptible to internal damage caused by a low

velocity impact, which may lead to severe safety and reliability problems. Hence, the

improvement of damage tolerance of FRP composites by enhancing their impact strength

has been of considerable research interest [4, 5].

There are several methods to enhance the toughness of FRP composite ; examples

of these are matrix toughening [6,7], insertion of interlaminer ‘interleaf’ layers [8,9],

utilization of high strain fibers and fiber hybridization[10,11]. The last two approaches

are particularly effective in improving the impact penetration resistance of composites

under high-incident-energy condition which is the main concern for composite amour.

Using a particular fiber the low-velocity- impact resistance of a composite which is

desirable for structural application, can be controlled to a great extent by increasing resin

toughness [12-15].

Toughening of a thermoset, can be achieved by reduction of crosslink density or

use of plasticizers, which lead to increased plastic deformation. However, this approach

may seriously affect modulus and thermal properties of the material for only a modest

increase in toughness. The most effective approach is the introduction of a second

component, which is capable of phase separation such as reactive liquid rubber [16, 17],

engineering thermoplastic [18, 19] or core-shell particles [20, 21]. An attraction of liquid

rubber like carboxyl-terminated copolymer of butadiene and acrylonitrile (CTBN) as a

modifier, is their solubility in base epoxy with the formation of initially a homogeneous

solution. As the curing reaction proceeds, the molecular weight increases and the phase

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separation occurs at some stage, leading to the formation of a two-phase morphology [22,

23]. Such a two phase system having a small amount of rubber ( 5-10 wt % ) often shows

outstanding fracture properties as the rubber particles dispersed and bonded to the epoxy

matrix act as centers for dissipation of mechanical energy by cavitations and shear

yielding[17,23]. The improvement in fracture toughness is generally achieved without a

significant reduction of thermal and mechanical properties of the crosslinked epoxy resin.

However, the main deficiency of CTBN is the high level of unsaturation in their

structure, which provides sites for degradation reaction in oxidative and high temperature

environment [24]. The presence of double bonds in the chain can cause oxidation reaction

and/ or further cross-linking with the loss of elastomeric properties and ductility of the

precipitated particles [25]. Secondly, there remains a possibility that traces of free

acrylonitrile, which is carcinogenic, might exist and limits the use of these materials [26].

The saturated liquid rubbers such as siloxane [27], polyurethane [28], acrylates [29, 30]

etc., have been reported as alternatives to CTBN.

The modification of epoxy with a linear liquid rubber results in a significant

increase in prepolymer viscosity. The increase in viscosity of the resin hinders resin

impregnation into the fibers resulting in poor mechanical performance of the FRP

composites. Very recently, new classes of reactive liquid rubber, which are dendritic

hyperbranched polymers (HBPs), have been investigated as modifiers for epoxy resin

[31-35]. The advantages of HBPs over conventional toughening agents are that HBPs

offer much lower prepolymer viscosity because of their spherical structure and lack of

chain entanglement and stronger adhesion with the matrix due to the presence of high

density of surface functional groups.

Recent studies on the behavior of polymer blend in presence of inorganic fillers

indicate that inorganic filler increases the compatibilization [36, 37]. It is in the light of

this finding that the present study was carried out with the aim of examining the

morphology of epoxy/HBP blends in the composite form and evaluating their relative

performance for composite applications. Effect of incorporation of HBP on the

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mechanical, dynamic mechanical properties and morphology of glass and carbon fiber

reinforced composites were investigated.

2. Experimental

2.1 Materials

The epoxy resin used was a liquid diglycidyl ether of bisphenol A (DGEBA) (DER 331

Dow epoxy resin) containing 5.27 mmol epoxide per gram of resin. The curing agent,

Ethacure 100, of Albemarle Corp., USA, is a mixture of the two diethyltoluene diamine

(DETDA) isomers (74-80% 2, 4 isomer and 18-24% 2, 6 isomer). The chemical

structures of the epoxy resin and hardener are shown in Fig. 1.

The epoxy functional dendritic hyperbranched polymer (Boltorn E1) with an

epoxy equivalent weight of ~ 875 g/eq and a molecular weight of ~ 10500 g/mol, was

supplied by Perstorp Speciality Chemicals, Sweden. Boltorn E1 consists of a highly

branched aliphatic polyester backbone with in average 11 reactive epoxy groups per

molecule. A schematic representation of E1 is shown in Fig.1.

E glass cloth (150 µm thick) was obtained from FGP Ltd., India. The carbon cloth

(450 µm thick) was supplied by Nikunj India Pvt. Ltd. The composition and properties

of the glass fiber and carbon fiber are shown in Table 1.

2.2 Fabrication of composites

The fiber cloth was cut to size and heated in the oven at 150° C to make it moisture free

before processing. The epoxy/HBP blend and hardener mixture was applied onto the

glass surface by a hand-lay-up technique. 15 layers of glass or 8 layers of carbon layers

were added successively in order to get about 3 mm thick GFRP and CFRP composites

respectively. The laminate was compressed thereafter, in a mold (30 cm x 30 cm)) at a

pressure of 50 kg/cm2 and allowed to cure 130° C for 2h, 160° C for 3h. The FRP sheet

was then taken out from the mold and post cured at 210° C for 2h.

2.3 Determination of glass content

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The glass content for the composite samples were determined by a resin burn-off test

according to ASTM D3171-99. The wt. % glass (x) was determined from the following

formula:

100×=ow

wx (1)

wo and w are the initial weight and weight after the matrix burn-off respectively.

The vol % glass (y) was determined from the formula given below :

e

g

d

dxx

xy

)100( −+= (2)

dg & de are the densities of glass and epoxy respectively.

2.4 Evaluation by DMA

DMA analysis was carried out in a VA 4000 viscoanalyser (Metravib RDS, France). In

practice, the viscoanalyser is used to apply a displacement d(w) at the upper end of a

sample and measure the force F(w) transmitted to the fixed lower end . F(w), δ and f are

the values which can be obtained using sensors. F(w) is measured by a dynamic force

sensor and d(w) is measured by displacement or acceleration sensor. By measuring the

upstream displacement and downstream force, the measurement method has the

advantage of being capable of obtaining the stiffness, irrespective of the weight of the

sample. The phase angle δ (w) i.e. the phase shift between the dynamic force and

dynamic displacement was calculated using the processing of the signals F(w), d(w)

according to first Fourier transform (FFT).

Samples of size 45 x12 x 3 mm3 were exited in flexural (three point bending)

mode under a temperature ramp of 3 Kmin-1 at a fixed frequency of 5 Hz. The

temperature range was 30° C to 180° C to observe the full loss curve response for all the

samples.

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2.5 Flexural test

The flexural properties were measured with rectangular samples according to ASTM D-

790, using the same UTM at crosshead speed of 4 mm/min. The fracture strength (F.S)

was determined from the peak load (kg) using the following formula:

2)(

8.923

.thicknesswidth

spanpeakloadSF

×

×××= (3)

The results are expressed in MPa, which is the average of the results from three samples.

The flexural deformation was determined by an LVDT system.

2.6 Impact Properties

The Izod impact test was carried out according to ASTM D-256 using an impact tester

(Tinius Olsen, Model 892 T). The impact test was carried out at room temperature and

impact energy was reported in J/m. The quoted result is the average of the determinations

on five samples.

2.7 Optical microscopic analysis

The hot stage optical microscopy was performed on a Reico optical microscope in which

two polarizers are aligned. The epoxy/HBP blend (epoxy + HBP + DETDA) was applied

on a slide and the optical photographs were taken in magnification of 400 at various

interval.

2.8 SEM Analysis

A low voltage scanning electron microscope (SEM), (Cameca, SU-30) was used to

examine the fracture surfaces of the toughened epoxy castings and composite samples. A

thin section of the fracture surface was cut and mounted on an aluminum stub using a

conductive (silver) paint and was sputter coated with gold prior to fractographic

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examination. SEM photo micrographs were obtained under conventional secondary

electron imaging conditions and with an accelerating voltage of 20 kV.

3. Results and Discussion

3.1 Dynamic mechanical analysis

The composite samples were subjected to dynamic mechanical analysis from room

temperature to 260 °C. The tan δ vs. temperature plots of GFRP and CFRP composites

based on the unmodified epoxy and 15 wt% HBP modified epoxy matrices are shown in

Fig. 2. As temperature is increased, it is observed that the tanδ goes through a maximum

in the transition region and then decreases in the rubbery region. The damping is low

below Tg as the chain segment in that region is frozen. Below Tg, the deformations are

thus primarily elastic and the molecular slips resulting in viscous flow is low. Also above

Tg, in the rubbery region, the damping is low because the molecular segments are free to

move, and consequently there is little resistance to flow. The maximum damping occurs

in a region where most of the chain segments take part in this co-operative micro-

Brownian motion under harmonic stress. The position and height of the loss tangent peak

in the relaxation spectra of a polymer are indicative of the structure and extent to which

the polymer is crosslinked. The dynamic modulus (E )̀ rapidly decreases in the α-

relaxation temperature zone due to the decreasing stiffness of the samples as the

segmental motion sets in.

It is evident from the loss tangent plots (Fig. 2) that both the GFRP and CFRP

composite samples show a relaxation peak at temperatures lower than that obtained in the

case of bulk cured resin. The Tg for GFRP composite based on the unmodified epoxy is

190° C whereas for unreinforced cured epoxy it is 217°C. Theocaris and Papanicolaou

[38] reported that the temperature of tanδ peak of an epoxy resin was significantly

higher (25° C) in the bulk resin than that in the glass fiber composite containing the same

resin. Ghosh et al. [39] reported similar effects in the case of jute fiber reinforced

composites. This can be explained by considering the effect of organosilane coating,

which is used for commercial fibers in order to increase adhesion with the matrix. The

organosilane group with unreactive organic groups leads to an interface with many

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unrestrained or free end groups which result in reduction in the crosslink density of the

polymer network in the interface region [40]. The plasticized region then yields

decreased internal friction and thereby causes a reduction in relaxation temperature.

It is interesting to note that in case of castings, there is no change in epoxy Tg up

to 15 wt% of HBP whereas in case composites reduction in epoxy Tg was observed as a

result of incorporation of HBP. The decrease in Tg for cured rubber modified epoxy

systems arises from the incomplete phase separation and plasticization phenomenon

caused by the dissolved rubber that has been noted in varied rubber modified epoxy

formulations [41, 42]. This indicates that the presence of fibers initiates partial miscibility

of the HBP with the epoxy matrix due to chemical interaction of epoxy/HBP blend with

fiber in the interface.

3.2 Morphology

Epoxy/HBP blend (castings) is initially homogeneous at curing temperature (130°C) and

undergoes reaction-induced phase separation with the advancement of curing reaction as

shown by optical microscopic analysis (Fig.3). Combining the Flory- Huggins equation

and the Hildebrand equation [43], the free energy of mixing can be expressed as:

∆Gm /V = φe φr (δe - δr )2 + RT(φe/Ve . ln φe + φr / Vr . ln φr ) (4)

where φe , φr are the volume fractions and δe , δr are the solubility parameters and Ve

and Vr are the molar volume of epoxy and rubber respectively. Since φe , φr are fractions

(<1), the second term (change in entropy) is always negative. Thus increases in

temperature favor the mixing, by increasing the second term, resulting in HBP solubility

with epoxy above 100°C. As the epoxy resin cures, the value of Ve and Vr increases,

resulting in a decrease in the second term. At a critical conversion ∆Gm becomes positive

and phase separation occurs. However, if (δe -δr) i.e. the difference between the

solubility parameters of the epoxy and the liquid rubber is very low, then the change in

entropy due to the curing reaction will not result in a positive free energy change of

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mixing (∆Gm) prior to gelation [44]. Thus a moderately slow curing rate is necessary to

ensure complete phase separation.

In case of composite, the free energy change of the system including three

components, epoxy, HBP and fiber, can be described as

∆Gm = ∆GEF + ∆GHF - ∆GEH (5)

where ∆GEH is the free energy of mixing between epoxy and HBP, ∆GEF & ∆GHF are

the free energy of interaction of the two components with the fiber surface. Since both the

components strongly adsorbed to fiber surface, ∆Gm will always be negative and hence

equilibrium phase diagram is shifted to higher compatibility. This explains why

epoxy/HBP blend shows more compatibility in composites compared to that in casting.

The fracture surfaces of epoxy/HBP blend (15 wt%) and corresponding GFRP

and CFRP composites were analyzed by SEM. The SEM microphotographs are shown in

Fig.5. Both the unreinforced matrix and the composite matrices show a two-phase

microstructure. It is clear from the figure that the volume fraction of HBP is less in the

composites compared to that in the unreinforced matrix. This substantiates the DMTA

results, which indicates the presence of dissolved rubber in the composite matrix unlike

the unreinforced matrix. In the case of castings, the HBP particles are uniformly

distributed throughout the matrix. The particles have dimension in the range of 1-2 µm

and their distribution is bimodal in nature. It is interesting to note that in composites the

particles are comparatively bigger and the uniformity in particle size distribution is lost to

some extent. It appears that there is not much effect of the nature of the fiber as the

morphologies are similar for both GFRP and CFRP composites. This indicates that fibers

act only as a nucleating site. Similar results were reported in the literature for

thermoplastic modified epoxy system [45]. The rubber particles bonded and dispersed in

the matrix of a composite enhances the toughness of the composite by rubber cavitations

and shear yielding [46, 47], as will be discussed next.

3.3 Flexural properties

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The representative flexural stress-strain diagram for GFRP and CFRP composites are

shown in Fig. 6 and Fig. 7 respectively and the flexural properties of GFRP composites

are presented in Fig. 8. It was found that there is little change in the flexural modulus due

to incorporation of HBP. The reason is that modulus is predominantly dictated by the

high modulus fibers and all the composites contain almost same glass/carbon content.

However, the fracture behavior changes in case of GFRP; the flexural strength decreases

and yielding during break increases as a result of incorporation of HBP. This indicates

that the GFRP composite based on epoxy/HBP blend would show higher toughness

compared to the unmodified epoxy based composite. On the other hand no significant

change in fracture behavior was observed in case of the CFRP composites.

3.4 Impact property

The effect of HBP content on the impact strength of GFRP and CFRP composites were

also investigated and the results are shown in Fig.9. It was observed that there is a

significant increase in impact strength in case of GFRP composites due to the

incorporation of HBP. The GFRP composite based on 15 wt% HBP modified epoxy

shows impact strength 1300 J/m compared to 900 J/m for the unmodified epoxy based

GFRP composite. The increase in the impact strength of GFRP composites can be

attributed to the increase in the toughness of the epoxy matrix due to the addition of

rubbery HBP. According to recent theories, the most accepted mechanism for rubber

toughening is rubber cavitations followed by shear yielding [13]. In rubber modified

plastics, under triaxial tensile stresses, voids can be initiated inside the rubber particles.

Once the rubber particles are cavitated, the hydrostatic tension in the material is relieved,

with the stress state in the thin ligaments of the matrix between the voids being converted

from a triaxial to a more uniaxial tensile stress state. This new stress state is favorable for

the initiation of shear bands. In other words, the role of rubber particles is to cavitate

internally, thereby relieve the hydrostatic tension and initiate the ductile shear yielding

mechanism [13, 46, 47].

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However, the improvement in impact strength in GFRP composite, is less

compared to that observed in the matrix reported earlier [33]. Similar observation was

reported by others using various polymer matrix [48]. This is due to fiber constraint

suppressing inelastic resin deformation at the crack tip [48, 49].

In case of CFRP composites, prepared using the same compositions/conditions,

no significant improvement in impact strength (Fig.9) was observed as a result of

incorporation of HBP. The fracture property of CFRP composites is reported [50, 51] to

be governed by the interfacial adhesion and internal stress development during cure,

which can be manipulated by material and process tailoring. Thus these aspects need to

be analyzed critically. However, the apparent difference in behavior of HBP-modified

epoxy-based GFRP and CFRP composites prepared under the same conditions, can be

explained in the light of the inherent properties of the fibers (Table1). Carbon fibers are

much more brittle (tensile strain = 1%) compared to glass fiber (tensile elongation =

2.5%) [52]. This is also reflected in the impact behavior of GFRP and CFRP composites.

The impact strength of unmodified epoxy based CFRP composite is 500 J/m compared to

950 J/m for corresponding GFRP composite. It is well established that dispersed rubber

particle enhances the toughness of the epoxy system by cavitations of rubber particle

followed by shear yielding and their effectiveness decreases with increase in the rigidity

of the system [53, 54]. Highly brittle carbon fiber imposes restriction on the induction of

plastic deformation by the rubber particle. In fact failure of the CFRP composites occurs

due to fiber breaking.

4. Conclusion

Epoxy/HBP blends were evaluated both as castings and as FRP composites. The epoxy

resin matrices of FRP composites exhibit lower Tg compared to the corresponding

castings. The epoxy/HBP blends show a two-phase morphology both in castings and in

FRP composites. The microstructure was found to be different in FRP composites

compared to that observed in castings. In FRP composites, the morphological features are

similar in the FRP composites, irrespective of the nature of fibers. The epoxy Tg

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decreases with the incorporation of HBP in FRP composites, on the other hand no

significant decrease in epoxy Tg was observed in castings, up to 15 wt% of HBP. This

indicates that the presence of fiber increases the compatibilization of epoxy/HBP blend

and thereby increases the amount of dissolved rubber. Incorporation of HBP resulted in

significant increase in toughness in GFRP composites with slight decrease in Tg and

flexural strength. Whereas no such effect was observed in CFRP composite.

Acknowledgement The author is thankful to the Alexander von Humboldt (AVH) foundation for the grant

of a post doctoral research fellowship.

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Table 1 Properties of glass and carbon fibres

Properties Glass fibre Carbon fibre

Density, g/cc

2.54 1.8

Tensile strength, GPa

1.5 2.5

Tensile strain, % 2.5 1

Tensile Modulus, GPa

75 246

Sp. Tensile strength

0.6 1.4

Sp. Tensile Modulus

29.5 137

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Legends to the figures

Figure 1 Chemical structures of epoxy, curing agent and HBP

Figure 2 Loss tangent vs. temperature plots of GFRP and CFRP composites

Figure 3 Loss tangent peak temperatures (Tg) of castings and GFRP composites plotted

against HBP content

Figure 4 Optical microphotographs for epoxy/HBP blend (a) before curing and (b) after

curing

Figure 5a SEM microphotographs for the fracture surface of epoxy/HBP blend (15wt%)

as casting

Figure 5b SEM microphotographs for the fracture surface of epoxy/HBP blend (15wt%)

as GFRP composite

Figure 5c SEM microphotographs for the fracture surface of epoxy/HBP blend (15wt%)

as CFRP composites

Figure 6 Representative stress-strain diagrams for GFRP composites based on

epoxy/HBP blends with 0 wt% and 15 wt% HBP

Figure 7 Representative stress-strain diagrams for CFRP composites based on

epoxy/HBP blends with 0 wt% and 15 wt% HBP

Figure 8 Flexural strength and modulus of GFRP composites plotted against HBP

content.

Figure 9 Impact strength of GFRP and CFRP composites as a function of HBP content

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Figure 5a

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Figure 5b

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Figure 5c

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