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Vol. 9. Issue 1.
Pages 394-403 (January - February 2020)
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Vol. 9. Issue 1.
Pages 394-403 (January - February 2020)
Original Article
DOI: 10.1016/j.jmrt.2019.10.068
Open Access
Influence of gamma and ultraviolet radiation on the mechanical behavior of a hybrid polyester composite reinforced with curaua mat and aramid fabric
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Anderson Oliveira da Silvaa,
Corresponding author
anderson.q.ao@gmail.com

Corresponding author.
, Karollyne Gomes de Castro Monsoresa, Suzane de Sant’ Ana Oliveiraa, Ricardo Pondé Webera, Sergio Neves Monteiroa, Hélio de Carvalho Vitalb
a Military Institute of Engineering – IME, Department of Materials Science, Praça General Tibúrcio 80, Urca, 22290-270 Rio de Janeiro, Brazil
b Instituto de Projetos Especiais – IPE, Av. das Américas 28705, Guaratiba, 23020-470 Rio de Janeiro, RJ, Brazil
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Tables (3)
Table 1. Average values ​​of the physical properties of the obtained composites.
Table 2. Composites evaluation groups in the present study.
Table 3. Mean values ​​for the results obtained in the 3-point flexural test of the hybrid composites, before and after UV-B and gamma radiation.
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Abstract

Functionalization of composite materials by ionizing radiation is considered an environmentally friendly technology to improve the compatibility of natural fibers with a polymer matrix. However, depending on the dose, it might also cause degradation. In particular, the effect of ionizing radiation on hybrid polymer composites reinforced with both naturals and synthetics fibers still needs more attention. In the present work, for the first time, the mechanical behavior of a hybrid polyester composite reinforced with 33wt% of curaua fibers mat and 7wt% of aramid fabric subjected to ultraviolet (UV) and gamma ionizing radiation, was investigated. Ultraviolet exposure for 300 and 600h and gamma radiation doses of 150 and 300kGy were applied before 3 points bending tests. The results disclose an increase in flexural strength and modulus with increasing UV when compared to other irradiated conditions. Macroscopic observation and scanning electron microscopy analysis of fractured irradiated specimens revealed delamination as the main failure mechanism.

Keywords:
Flexural properties
Ionizing radiation
Hybrid composite
Curaua mat
Aramid fabric
Full Text
1Introduction

In the last decade, the gradual growth of research works in the area of ​​composites reinforced with natural lignocellulosic fibers (CRNFL) has had great scientific and technological relevance, guaranteed by the advantages that natural lignocellulosic fibers (NLFs) present when compared to other fibrous and synthetic materials [1–6]. Different of traditional synthetic fibers such as fiberglass, polyaramide and carbon fiber, the mechanical properties of NLFs are determined by their cellulose content, degree of polymerization and low fibrillary angle.

Among the traditional Brazilian NLFs, the curaua fiber (Ananas erectifloius) of Amazonian origin stands out in the applications of engineering for its high mechanical resistance, attributed mainly to its chemical composition: 70% cellulose, 21% hemicellulose, 8% lignin and 1% ash content [7–10]. The curaua fiber composites most consolidated application is in the manufacture of auto parts [11–13]. Applications of this fiber in ballistics have also been studied, replacing conventional Kevlar fabrics by natural fiber-reinforced composites in multilayer shielding systems [14–16]. This is serving as motivation for possible studies on the use of hybrid composites in different applications.

The development of hybrid CRFNL with the participation of synthetic fibers is an alternative for the formulation of a new product with specific characteristics that a single material, synthetic or natural, is not able to satisfy them [9]. Studies [17–23] show that the results of mechanical resistance due to hybridization are mainly influenced by the content of the lower resistance fibers in the composite, whether natural or synthetic as well as by the interfacial interaction of the reinforcement phases with the matrix. It was also shown that long, aligned and fabric fibers have high values ​​of tensile and flexural strength, and the arrangement of the lower strength fibers at the composite surface will influence the flexural results. This is not necessarily in tensile tests, in which the fibers are subjected to the same stress distribution. Angrizani et al. [20] reported that the flexural strength of hybrids was slightly smaller and similar to the composite reinforced with curaua fiber. The arrangement of the NLF on the composite surface favored the breaking of the fibers first by traction followed by delamination between the fiber/matrix interfaces, leading to the premature fracture of the composite in the compression region.

For improved composites, it is important to take into account the time of exposure of the fibers to surface treatments, as well as to the environmental effects that the CRNFL will be conditioned during their application. Humidity, temperature, biological attack and radiation can induce degradation associated with changes in molecular structure, compromising the final properties of the composite as well as shortening the validity of the final product.

In particular, of ionizing radiation, such as ultraviolet (UV) rays, gamma rays, X-rays, accelerated electrons and ionic beams, leads to the scission of the main macromolecular chains, as well as the crosslinking with concomitant formation of covalent bonds between synthetic or natural polymers [24]. Indeed, depending on the time and rate applied, these ionizing radiations will promote scission in the main carbon chain (CC) of the polymer, which can be type either homo- or heterolytic. Crosslinking is a result from the intermolecular recombination of the free radicals formed after degradation, which leads to an increased molar mass of the polymer [24–26]. In the present work, to the interactions of UV radiation in the curauá fiber, Fig. 1, showed that with increase in the exposure time the irradiation causes voids, observed by SEM, between the fibrillar structures. This is attributed to the removal of the hemicellulose matrix as well as the loss of lignin and cellulose fractions [27]. Considering the observed, the mechanical strength of the composite will be influenced by the surface energy of the constituents. Indeed, it is related to possible intermolecular bonds (attributed to hydrogen bonds between groups of secondary alcohols of the fibers with CO groups of the resin) that may occur between the surfaces of the fibers with the matrix after exposure to radiation [28–30].

Fig. 1.

SEM analyses of the surface of the curaua fiber CR (a); irradiated with 150h of UV-B (b) and 300h of UV-B (c); 300× magnification [27].

(0.28MB).

In this study, the effects of ionizing radiation, both gamma and UV-B, were evaluated in a reinforced polyester hybrid composite with curaua fiber in a mat and aramid fabric. The choice of the synthetic aramid fabric in the present hybrid composite was based on its superior mechanical properties, such as tensile strength, 3600–4100MPa and modulus of elasticity, 131GPa, associated with the relatively low density of 1.44g/cm³ [31,32]. The composite performance was evaluated in tension. The type of failure, which occurred before and after different times and doses of UV and gamma radiation, was also investigated.

2Experimental procedure

The composite was made with 33wt%. of curaua fiber in the form of mat, 7wt%. and aramid fabric supplied respectively by Pematec Triangel do Brazil Ltda. and Tejin Aramid and, 60wt%. of pre-accelerated unsaturated polyester resin with 2.5wt%. of the methyl ethyl ketone peroxide catalyst (PMEK). The production of the composite was performed in a uniaxial SKAY hydraulic press, Fig. 2, with a capacity of 15tons in a metal matrix. A load of 5tons was applied for a period of 24h during the polymer curing.

Fig. 2.

SKAY hydraulic press, with capacity for 15 tons (a), Hybrid composite consisting of reinforced polyester matrix with curaua mat and aramid fabric, Twaron® (b).

(0.21MB).

The average values of the physical properties of the materials are shown in Table 1.

Table 1.

Average values ​​of the physical properties of the obtained composites.

Properties
Weight  244,76g±19,82 
Thickness  1,23cm±0,06 
Size  221,40cm³±10,56 
Volumetric density  1,10g/cm³±0,06 
Area densitya  13,60kg/m² 
a

Calculado segundo o método C da norma ASTM D 3776 [33].

The composites were exposed to ultraviolet (UV-B) and gamma rays. The UV-B irradiated composites were accelerated aged, in the non-metallic aging chamber — Comexim model C-UV ultraviolet "B" brand according to ASTM G154 [34] for 300 and 600h of exposure.

Exposure to gamma radiation was performed in a Brookhaven research irradiator, equipped with a source of 137Cs, at a dose rate of approximately 1.3kGy/h; located in the laboratory of the Institute of Chemical, Radiological and Nuclear Defense (IDQBRN) of the Technological Center of the Army (CTEx). For this study, two (2) different doses of 150 and 300kGy were used. The manufactured specimens were separated into 5 (five) test groups, according to the test conditions, non-irradiated or irradiated, as shown in Table 2.

Table 2.

Composites evaluation groups in the present study.

Evaluations groups  Conditions of degradation 
CR  Non-irradiated 
UV 300  Irradiated with 300h UV-B 
UV 600  Irradiated with 600h UV-B 
GAMA 150  Irradiated with 150kGy dose 
GAMA 300  Irradiated with 300kGy dose 

The flexural test was performed at room temperature (25°C) in an EMIC DL10000 machine at the LNDC-COPPE/UFRJ. The parameters used followed method B of ASTM D790 [35]. The test speed was 0.25mm/min with a distance between the supports of 90mm. Five samples were used for each evaluation group with mean dimensions of 127×12.5x11.5mm, as shown in Fig. 3.

Fig. 3.

Photograph of specimens machined for bending test, adapted from ASTM D 790.

(0.13MB).

The maximum stress (σmax) was calculated according to Eq. (1), where L is the distance between the supports, P is the maximum load applied on the specimen until rupture, b the width and d the thickness. Eq. (2) was employed to calculate the modulus of elasticity, where Δy corresponds to deflection in (mm).

σmáx. = 3.L.P/2.b.d²
E=P.L³ / 4.b.d. Δy

A fractographic analysis of the fractured surfaces of the bend tested composites, before and after irradiation, was carried out in a scanning electron microscope, brand JEOL, model JSM 5800LV (IME), using electron beam energy of 10 and 20kV.

3Results and discussion3.1Flexural test

Table 3 shows the mean values of the flexural strength and modulus of elasticity of the composites before and after irradiation. The composite studied in the "CR" condition exhibits average flexural strength (72.50MPa) higher than the materials hybridized with curaua and synthetic fibers using the same polyester matrix [18–20]. It can be seen in Table 3 that both the flexural strength and the modulus of the irradiated composites decreased when compared to the CR composites. This is in agreement with reported composites exposed to environmental aging agents [36–40].

Table 3.

Mean values ​​for the results obtained in the 3-point flexural test of the hybrid composites, before and after UV-B and gamma radiation.

Conditions  Flexural strength (MPA)  Modulus of elasticity (GPA) 
CR  72,50±5,4  9,7±2,9 
UV 300  50,67±2,2  5,70±1,4 
UV 600  71,37±3,1  8,33±0,6 
GAMA 150  33,02±3,1  3,65±0,7 
GAMA 300  28,54±5,1  3,26±1,4 

The performance of the irradiated composite may have been affected by the degradation of the curaua fiber disposed in the outer layer of the reinforcement, subject to the greater efforts in flexion and the greater effects of aging caused by environmental agents; especially those generated by UV radiation [37].

It is verified that for the UV 600 condition, the flexural strength is reduced by only 1.5% in relation to the material CR; differently from the UV 300 condition, which reduced by an average of 30%. These values ​​suggest that the longer time of UV radiation allowed the free radicals, which were generated during exposure radiation, to recombine, causing a greater cross-linking and rigidity on the material’s surface. This increase in resistance after UV 600 degradation can be attributed to a possible crosslinking between neighboring molecules from both resin and cellulose, resulting in a higher resistance of natural fibers [30].

It is observed that the gamma radiation decreases the flexural strength and modulus of elasticity as the radiation doses increase. At high doses, the cleavage occurs in the macromolecular chain in all the constituent materials of the composite. This leads to a decrease of the molecular weight and, consequently, a decrease of its mechanical properties [30,41].

3.2Fractographic analysis

The macro and SEM microphotographs presented in Figs. 4–9 were considered for a detailed study of the fracture of the composite, since the analysis of the effects of degradation on composites might be extremely complex [37].

Fig. 4.

Photographs of fracture surfaces of hybrid composites under all study conditions; (a) CR, (b) UV 300, (c) UV 600, (d) GAMA 150, (e) GAMA 300.

(0.19MB).

It can be seen in the photographs, Fig. 4, for all investigated composites groups, that none of the specimens before and after exposure to radiations reached complete rupture of their layers after the bend test. Note that fractures in the composites begin at the face where the maximum tension occurs in the fibers, and propagate in the direction of the point of inertia (neutral line) of the specimens. This is a typical feature of 3 points bend fracture [37].

The macroscopic images in Fig. 4 showed that the main failure mechanism of the composites was the tension rupture of the outer layer of curaua mat, followed by matrix/fiber interfacial detachment in the polyaramide layer. This behavior suggests that there is a weak interfacial interaction between the reinforcements, due to interlaminar failures, characterizing the final fracture region associated with the stress of the composites in all study conditions. In all cases, no damage was observed in the layers subjected to compression.

It is observed that for the non-irradiated composite CR, Fig. 4-a, the flexural strength was controlled by the detachment of the aramid fabric layer, since a critical fracture on the tension surface is not macroscopically perceptible. By SEM microscopic analysis, Fig. 5-a, it was observed that there was no fracture characteristic of a crack in the surface of the outer layer in the composite, but a pull-out effect of the fibers could be visualized in the polymer matrix.

Fig. 5.

Microscopic image of the fracture surface of composite CR, (a) external surface (curaua/polyester) and (b) inner layer (aramid fabric/polyester).

(0.6MB).

Fig. 5-b shows microscopy of the polyaramide layer in the CR composite. It is noted that there is no evidence of fractures in the polyaramid fabric and of its, some fibers have been plastically deformed. Consequently, this contributes to a better composite strength, even with premature delamination failure.

In relation to the materials subjected to the different UV-B degradation times, Fig. 4-b and Fig. 4-c, a transverse fracture occurred in the layers under tensile stress, but not characterizing total fracture, followed by the delamination between the layers of curaua mat and aramid fabric. Microscopically, there was a marked pullout of the fibers, accompanied by fractures of the fibers in the two irradiated times, as shown in Fig. 6.

Fig. 6.

Microscopies of irradiated composite fracture surfaces UV 300 (a) and UV600 (b).

(0.8MB).

In the case of composites exposed to gamma radiation, Fig. 4-d and -e, the fracture on the face under tension was complete and propagated to the neutral line of the composite specimens. It can be seen by SEM microscopy that the failure mode of the fibers is associated with, fibrillation and/or apparent 45° angle breaks. This characterizes a fragile fracture of the material, Figs. 7 and 8. At both doses, of 150 and 300 kGy, it was observed that there was no pullout effect of the fibers as in other conditions.

Fig. 7.

Microscopies of irradiated composite fracture surfaces GAMA 150 (a) and GAMA 300 (b).

(0.63MB).
Fig. 8.

Electron microscopy of the fracture surface of the curaua fiber by flexion after doses of 300 kGy in gamma radiation (4000×).

(0.13MB).

In contrast to the aramid fabric/polyester layer of the CR composite, Fig. 5-b, in which no fiber fracture took place, regions with tensile-rupture fibers shown Fig. 9 were observed in the 300 kGy gamma irradiated (GAMA 300) condition. These failures demonstrate that the composite was affected by the high gamma radiation rate, leading to embrittlement and, consequently, a decrease in its flexural strength.

Fig. 9.

SEM for the aramid fabric/polyester layer in the composite GAMA 300 with increase of 100× and 600×.

(0.69MB).
4Conclusion

The flexural strength and modulus of elasticity investigated hybrid curaua mat and aramid fabric of the polyester matrix composites decrease when exposed to the proposed environmental aging agents. However, a slight increase for UV 600 degradation resistance can be attributed to a possible cross-linking between the molecules adjacent to the cellulose, resulting in increased strength of the natural fiber. Failure, in all study conditions, was attributed mainly to delamination between the outer and inner layers. The macroscopic and SEM microscopic analyses showed that fractures in the composites begin at the face, where the tensile stress occurs in the fibers, and propagate in the direction of the point of neutral line of the test specimens. In all cases, no damage was observed in the layers subjected to compression.

Conflict of interest

Authors declare that this work does not have Conflict of interest.

Acknowledgements

The authors thank the support to this investigation by the Brazilian agencies: CAPES, IDQBRN/CTEx and to the LNDC-Coppe/UFRJ for the contribution to the tests and analyzes.

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