Property correlations for composites based on ethylene propylene diene rubber reinforced with flax fibers

Polymer Testing 59 (2017) 75e83

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Material Properties

Property correlations for composites based on ethylene propylene diene rubber reinforced with flax fibers Maria Daniela Stelescu a, Anton Airinei b, *, Elena Manaila c, Gabriela Craciun c, Nicusor Fifere b, Cristian Varganici b a b c

National Research and Development Institute for Textile and Leather, Leather and Footwear Institute, 93 Ion Minulescu Street, 031215 Bucharest, Romania ”Petru Poni” Institute of Macromolecular Chemistry, Aleea Grigore Ghica Voda 41A, 700487 Iasi, Romania National Institute for Lasers, Plasma and Radiation Physics, 409 Atomistilor Street, 077125 Magurele, Romania

a r t i c l e i n f o

a b s t r a c t

Article history: Received 3 November 2016 Received in revised form 18 January 2017 Accepted 18 January 2017 Available online 19 January 2017

EPDM composites filled with short flax fibers were prepared by melt blending procedure. The effects of fiber loading on the mechanical, thermal and water uptake characteristics were studied. The physicomechanical, morphological thermal properties and water absorption behavior were discussed using tensile testing, differential scanning calorimetry, thermogravimetrical analysis and scanning electron microscopy. Scanning electron microscopy revealed that the flax fibers were well dispersed in the polymer matrix. The tensile strength and hardness of the composites were found to be improved at higher fiber loading. The water absorption pattern of EPDM/fiber composites at room temperature follows a Fickian behavior for composites with 10, 15 and 20 phr flax fiber. © 2017 Elsevier Ltd. All rights reserved.

Keywords: EPDM composites Flax fibers Mechanical and thermal properties Crosslink density Water uptake

1. Introduction Green composites are a specific class of materials, in which at least one of components proceeds from natural resources [1,2]. Among them the natural fiber reinforced polymer composites represent an emerging area in the polymer science because these materials are both environmentally friendly and sustainable [3]. After years of high-tech developments of the synthetic fibers (aramid, glass, carbon, etc), the natural fibers (wood fibers, flax, hemp, jute, sisal, kenaf, ramie and others) have now attracted a renewed interest [4,5]. These natural cellulosic fibers have shown a great potential as substitutes for synthetic fibers, in particular glass fibers, in composites that are extensively used in the automotive and construction industries. Natural fillers as raw materials for polymer reinforcement exhibit many advantages relating to mineral fillers, glass fibers or carbon fibers such as low cost, low density, high specific strength and modulus, ease of fiber surface modification, non abrasion, renewability and biodegradability, good thermal and acoustic insulating properties, recyclability and

* Corresponding author. E-mail address: [email protected] (A. Airinei). http://dx.doi.org/10.1016/j.polymertesting.2017.01.017 0142-9418/© 2017 Elsevier Ltd. All rights reserved.

world wide availability [5e7]. Natural fibers-reinforced green composite materials are utilized in different applications, namely door components, furniture, deck surfaces, window or automotive components [1,8]. However, before to utilize these fiber-reinforced composites into real life applications, especially for massive production, the characteristics of the materials have to be deeply studied to assure that repeatable and reliable results can be obtained. In spite of these obvious advantages, there are several impediments to overcome for using the natural fibers as reinforcements in the composite materials, including lower compatibility between the hydrophobic matrix and hydrophilic fibers, relative high moisture of the fibers, dispersion properties of the resultant composites (uniform dispersion and extreme agglomeration), manufacturing process due to their low thermal stability which limits the applications in engineering thermoplastics. The change in microstructure of these fibers subjected to loading can also significantly affect the final properties of the composites [9e11]. The natural flax fibers are widely used for textiles (linen) and for technical applications, such as specialty papers, composites or insulating materials due to its renewable nature, low cost, easy availability, environmental benefit (i.e. biodegradability), high specific tensile stiffness [8,12,13]. Natural fibers have less impact on

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the health of composite manufacturers because they do not bring about skin irritations, lung cancer [7,14]. Flax fiber is a composite, in which the major constituents are cellulose (around 71%), hemicellulose (around 2.2%), lignin (around 18.6e20.6%) and pectin (around 2.3%) [8,15]. The unidirectional cellulose microfibers constitute reinforcing elements in polymer matrix/cellulosic fiber composite. It is known that ethylene propylene diene terpolymer (EPDM) is one of the most widely used and fastest growing synthetic rubber [16]. EPDM exhibits remarkable characteristics such as high heat resistance, ozone resistance, low temperature flexibility, cold and moisture resistance to permanent deformation and impact, excellent electrical properties, color stability. EPDM rubber has been widely used in automotive industry as insulation materials for wires and cables, as floor coverings in metro train carriages, joints materials in nuclear plants [17e19]. Even though many interesting papers exist in literature concerning the composites based on polymers and flax fibers [7,8,11,20], a limited research has been conducted on structural characteristics of rubber/flax fiber composites [21e24]. The aim of this paper was to obtain and to investigate the physico-mechanical properties of some EPDM based composites reinforced with flax fibers. Attention has been given to the effects of fiber loading on the final properties of composites. The mechanical and thermal properties, water uptake, rubber-fiber interactions and morphology of the composites were compared.

2. Experimental 2.1. Materials EPDM monomer (Nordel 4760) was supplied by Dow Chemical Company (Mooney viscosity: 70 ML1þ4 at 120  C, 70% ethylene content, 5-ethylidenenorbornene 4.9 wt%, density 0.88 g/cm3, crystallinity degree 10%). Polyethylene glycol, PEG 4000, was obtained from Advance Petrochemicals LTD (density 1.128 g/cm3, melting point range 4e8  C). Irganox 1010 (pentaerythritol tetrakis(3-(3,5 di-tert-butyl-4-hydroxyphenyl)propionate) was produced by BASF Schweiz AG (active ingredient 98%, melting point of 40  C). Dibenzoyl peroxide (Perkadox 14e40B) as vulcanizing agent was supplied by Akzo Nobel Chemicals (density 1.60 g/cm3, 3.8% active oxygen content, 40% peroxide content, pH 7). Ground flax fiber wastes thread length of max 3 mm were used as reinforcing agent.

2.2. Composite preparation EPDM rubber/flax fiber composites were obtained by melt blending using a laboratory electrically heated roller mill equipped with a cooling system at a friction ratio 1:1.1 and temperature of 60e80  C. EPDM (100 parts) was firstly melted 1e2 min, then antioxidant (Irganox 1010) and PEG 400 incorporated and meltblended (2 min). The mixing was continued until a uniform mixture was obtained. When a uniform mixture was realized, different amounts of ground flax fibers 0, 5, 10, 15 and 20 phr (parts to 100 parts rubber), respectively were introduced (4 min) and then 8 phr of dibenzoyl peroxide as vulcanizing agent was added (1 min). The mixing was continued for another 5 min. The samples were then removed from the roll in form of sheets about 2 mm thick. Test specimens were prepared by compression molding at 160  C and a pressure of 150 MPa by using an electrical press and then cooled under pressure at room temperature. The compounding recipe is given in Table 1.

Table 1 Composite formulations. Ingredients (phr)a/Code

P0

PIn5

PIn10

PlnI5

PIn20

EPDM Flax PEG 400 Irganox1010 Perkadox 14e40B Total

100 0 3 1 8 112

100 5 3 1 8 117

100 10 3 1 8 112

100 15 3 1 8 127

100 20 3 1 8 132

a

Parts to 100 parts rubber.

2.3. Measurements Tensile strength tests were performed with a Schopper strength test machine on dumbbell shaped specimens according to ISO 37/ 2012. The hardness was determined using a hardness tester according to ISO 7619-1/2011 on samples with thick of 6 mm. The unit of hardness was expresses in ShoreA. The elasticity was estimated with a Schob test instrument on samples of 6 mm thick, according to ISO 4662/2009. Thermogravimetric analysis (TGA) of the composites was performed on a STA449 F1 Jupiter thermal analyzer (Netzsch, Germany) under nitrogen atmosphere at a heating rate of 10  C/min. The samples were heated from room temperature to 700  C at a nitrogen flow rate of 50 ml/min. The onset of degradation temperature, the temperature for which the weight loss is maximum (Tm) and the residual weight were estimated. Differential scanning calorimetry (DSC) measurements were carried out using a DSC200 F3 Maia apparatus (Netzsch, Germany) under nitrogen atmosphere and a heating rate of 10  C/min from room temperature to 300  C. The gel fraction of the crosslinked EPDM/flax fiber composites (with and without flax) was determined by the content of insoluble fraction from crosslinked composite after solvent extraction. The samples were swollen in toluene and extracted after 72 h. The extracted samples were dried in air for 6 days and then in a laboratory over at 80  C for 3 h to constant weight and finally were reweighed. The gel fraction is given by the relation

Gel fraction ð%Þ ¼

ms $100 mi

(1)

where mi and ms are the initial weight and the weight of the insoluble portion of the composite in gel. The crosslink density (n) of EPDM composites was estimated by equilibrium solvent swelling measurements using the modified Flory-Rehner equation [25]. Pieces of 2 mm thickness (initial weight mi) were prepared and immersed in toluene for 72 h in order to achieve the equilibrium swelling conditions. Then the swollen sample was taken out from solvent and dried to remove the solvent excess and reweighed (mg). The traces of solvent were removed by drying in air for six days and in an oven at 80  C for 3 h. Then the sample was again weighed (ms). The volume fraction of polymer in the swollen network (n2m) was calculated from swelling ratio G by the relation:

v2m ¼ ð1 þ GÞ1

(2)

With




 mg  mg rr G¼ $ rs ms

(3)

and rr, rs are the densities of EPDM sample and solvent (0.942 g/ cm3 (EPDM) and 0.865 g/cm3 (toluene)). The densities of EPDM samples were measured by hydrostatic weighing method, according to ISO 2781/2010. The crosslink density was calculated

M.D. Stelescu et al. / Polymer Testing 59 (2017) 75e83

employing the Flory-Rehner equation [25]:



y¼

lnð1  y2m Þ þ y2m þ cy22m h  i 1=3 V1 y2m  0:5y2m

 (4)

where c denotes the interaction parameter between EPDM network and solvent (c ¼ 0.49 for EPDM-toluene system [26,27]), V1 is the molar volume of toluene (106.52 cm3/mol), n2m represents the volume fraction of polymer in the swollen specimen. The interactions between EPDM matrix and flax fiber were analyzed applying the Kraus equation [28e31].

Vro f ¼1m 1f Vrf

(5)

where Vro and Vrf are the volume fractions of EPDM rubber in vulcanized sample and in fiber swollen recipe, f denotes the volume fraction of fiber, m is the polymer-fiber interaction parameter. The volume fraction of EPDM rubber in the swollen samples Vrf, can be estimated by the following expression [29]:

Vrf ¼

 ðD  FTÞ ðD  FTÞ

rs

rr

þ

Ao

(6)

rs

where rr and rs are the densities of EPDM rubber and solvent, D represent the deswollen weight of the sample, F is the volume fraction of fiber, T is the initial weight of the sample and Ao denotes the weight of the solvent absorbed by specimen. Water uptake tests were performed in accordance with ISO 20344/2004. Rectangular specimens from each sample were dried in an oven at 80  C for 2 h, cooled in a desiccator and then weighed. Samples were immersed in distilled water in test bottles at room temperature (23  C), then samples were removed from the bottles at periodic intervals and were wiped properly with tissue paper. After the water excess on the sample surface was removed the specimens were reweighed until no increase in water uptake was observed. The water uptake for composite samples was determined by a weight difference procedure. The water absorption (W) was calculated in accordance to the relation (7):

Wð%Þ ¼

ðms  m1 Þ $100 m1

(7)

where ms is the weight of the sample after immersion in water and m1 represents the weight of the sample before immersion. The morphology of the cryofractured surfaces obtained by breaking EPDM/fiber composites frozen in liquid nitrogen was examined with a Quanta 200 scanning electron microscope (FEI) operating at 20 kV in low vacuum mode using a secondary electron detector LFD. 3. Results and discussion The mechanical characteristics of the flax-fiber-reinforced composites are given in Table 2. It is observed that the hardness

77

increases significantly with increasing fiber loading suggesting that the introduction of ground flax fibers in EPDM matrix led to the reinforcement of the rubber compositions. The variation of hardness in composites was more important for 5 and 10 phr fiber loading with 7 and 15 oShA, respectively. At the same time, an increase of the tensile strength was noticed as the fiber level increases in composites. The modulus at 100% elongation is near or higher than of the P0 sample and the enhancement is more pronounced for the composites containing 10 and 15 phr flax fiber (Table 2). Elasticity decreases from 64% (sample without fiber) to 42% in the case of sample containing 20 phr flax fiber. The addition of flax fiber in composites does not improve the elongation at break leading to the decrease of elongation at break because higher amounts of fibers determine a reduced mobility of the polymer chains and induces more stiffness to the composite [11,22]. The lowest values of elongation at break were obtained for composites containing 15 and 20 phr flax fiber. The gel fraction and crosslink density of EPDM/fiber composites were obtained from swelling tests and the results are in Table 3. As it can be seen in Table 3, the gel fraction presents high level of more than 97% having a slight increase for PIn5 and PIn15. The crosslink density of the EPDM/fiber composites increases as the flax fiber content increases. The results show that at lower fiber levels the highest increase of crosslink density was observed. Further increases of the fiber loading led to a lower increase of the crosslink density (Table 3). Thus, the increase in the network density could be due to an increase in stiffness of the composite due to the flax fibers or some chemical interactions of the fibers with EPDM matrix evidenced by SEM observations and DSC data. A similar behavior was found out in the hardness dependence on the fiber content. In consideration of the data relative to the mechanical properties and crosslink density we can conclude that flax fibers act as active fillers in EPDM composites leading to the reinforcement of material and to improve its characteristics. Our results are in good agreement with previous works which proved that the tensile strength is closely related to the crosslink density [30,32e35]. The tensile strength increases as the crosslink density increases and the majority of energy dissipation occurs in the EPDM matrix. But, at higher crosslink densities, the macromolecular chains have lower mobility and the material becomes stiffer and the elasticity will decrease. The decrease in elongation at break can be explained by the increase of crosslinking degree and thus, the elongation at break will decrease when the formation of a network structure

Table 3 Gel values and crosslink density of EPDM/fiber composites. Sample

Gel fraction

P0 PIn5 PIn10 PIn15 PIn20

98.07 97.41 97.06 97.24 97.18

n. 104c

n2mb

Ga

(mol/ml)

a b c

± ± ± ± ±

0.25 0.08 0.05 0.06 0.10

1.26 0.82 0.67 0.66 0.59

± ± ± ± ±

0.04 0.02 0.03 0.03 0.04

0.443 0.550 0.598 0.604 0.629

± ± ± ± ±

0.006 0.004 0.009 0.011 0.013

6.54 ± 0.32 14.23 ± 0.44 19.15 ± 1.18 19.25 ± 1.39 22.53 ± 1.98

G e swelling ratio. y2me volume fraction of polymer in the swollen network. n e crosslink density.

Table 2 Mechanical properties of the EPDM/fiber composites.

Hardness, oShA Tensile strength, N/mm2 Modulus at 100% elongation, N/mm2 Elasticity, % Elongation at break, %

P0

PIn5

PIn10

PIn15

PIn20

62 ± 0.71 1.9 ± 0.20 1.1 ± 0.06 64 ± 0.45 287 ± 12

69 ± 0.84 1.8 ± 0.23 1.2 ± 0.00 58 ± 0.71 140 ± 10

77 ± 0.56 1.8 ± 0.06 1.8 ± 0.06 46 ± 0.44 100 ± 7

81 ± 0.56 2.0 ± 0.07 2.0 ± 0.06 46 ± 0.89 100 ± 7

83 ± 0.45 2.5 ± 0.23 e 42 ± 0.55 80 ± 3

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M.D. Stelescu et al. / Polymer Testing 59 (2017) 75e83

takes place [33e35]. The fiber-polymer matrix interaction can be estimated using Kraus equation (5) from swelling data [28,31]. The ratio, Vro/Vrf, indicates the restriction degree of swelling of EPDM matrix due to the presence of fibers [30]. The values of Vro and Vrf calculated with Eq (6) are listed in Table 4. It is observed from Table 4 that the ratio Vro/Vrf decreases as the fiber loading increases as a result of the increase in Vrf. According to Kraus theory [28], the reinforcing fillers must have a negative slope of the plot Vro/Vrf against f/(1-f). The decreased Vro/Vrf values at higher content of fiber reveal the reinforcement effect given by the fiber presence. A better adhesion between filler and polymer matrix determines a restricted entry of the solvent and the composites containing fibers presented lower Vro/Vrf values [36,37], suggesting an improved interfacial interaction between flax fiber and EPDM matrix. The higher interaction between EPDM and flax fiber can provide enhanced mechanical properties as seen previously. Fig. 1 presents the plots of the evolution of the percentage water uptake of EPDM fiber composites immersed in water as a function of time. It is observed that the sample without flax fiber (P0) attained the lowest water absorption (around 2%), whereas uptake of EPDM/fiber composites increases as the fiber level increases in samples reaching a percentage of water absorption around 9.2% after 1200 h (PIn20) (Fig. 1). The increases in water absorption follow a similar behavior like the dependence of the hardness and crosslink density on fiber loading. Firstly, the increase was more significant, then for higher fiber loadings of 15 and 20 phr, respectively, the increase becomes smaller (Fig. 1). The EPDM flax fiber composites exhibit a weight gain due to the absorbed water of <10% even at 20 phr flax fiber suggesting an excellent behavior of these composites in water which can extend the utilization range of these new materials as consumer rubber goods. The polymer composites based on natural fibers having an elastomer matrix can contain a vulcanizing agent, namely dibenzoyl peroxide. At higher temperature dibenzoyl peroxide decomposes and reacts with EPDM matrix leading to the elastomer crosslinking. At the same time the vulcanizing agent can react with the flax fibers leading to the diminution of the hydroxyl group content which determines the strong hydrophilic character of the natural fibers improving the compatibility and the properties of composites [21,38,39]. It is observed that for all samples water uptake was faster during the first 200 h and then it develops slowly. This is due to the nature of water absorption, because the driving force is the water gradient of outside and in composite, and over time this gradient is reduced, thus resulting in lower uptake rates. In order to find out the mechanism of water diffusion into polymer/fiber composites the power law relation can be used [40,41]:

Mt ¼ kt n M∞

Fig. 1. Water uptake of EPDM/flax fiber composites.

representing the experimental data in log-log plots, according to eq (9):

log

Mt ¼ log k þ n log t M∞

where the slope gives the diffusion mode. A Fickian diffusion mechanism takes place if the value n is close to 0.5. In this case the diffusion rate of the water molecules is much smaller than the relaxation rate of the polymer chains. If the value of n is 1, a non-Fickian diffusion mode is termed. Here the diffusion process is much faster than the polymer chain relaxation. The diffusion mode is named anomalous, if the value of n is in the range 0.5e1.0, where the diffusion and relaxation rates are comparable [42e44]. Typical plots of fitting of the experimental data to eq. (9) are depicted in Fig. 2. The values of the parameters n and k have been evaluated by linear regression analysis and they are listed in Table 5. It can be seen that the values of n for P0 and PIn5 EPDM/ fiber composites are over 0.5 indicating that the diffusion mechanism belongs to an anomalous mode. For composites with 10, 15 and 20 phr flax fiber the diffusional mode approaches the Fickian diffusion. The value of k for unfilled EPDM (P0) is much lower than the values for EPDM/fiber composites due to the weak interaction and uptake of non-polar matrix in a polar solvent (water) [45,46]. A systematic increasing trend of k value was noticed in filled composites as the flax loading increases due to the increase of the hydroxyl groups in composites with higher fiber level, which lead to enhanced hydrogen bonding between flax fiber and water molecules [46,47]. The morphology of the composite fracture surface was analyzed by scanning electron microscopy in order to evaluate the particle dispersion and potential interactions between natural fibers and

(8)

where Mt and M∞ denotes the water amount diffused in sample at time t and at equilibrium state, k represents a constant depending on the composite and exponent n indicates the mechanism of diffusion. The constant k and exponent n can be determined by

Table 4 Vrf and Vro/Vrf values of EPDM/flax fiber composites. Sample

Vrf

Vro/Vrf

PIn5 PIn10 PIn15 PIn20

0.523 0.564 0.573 0.582

0.805 0.747 0.735 0.724

(9)

Fig. 2. Diffusion fitting plots of EPDM/fiber composites immersed in water.

M.D. Stelescu et al. / Polymer Testing 59 (2017) 75e83 Table 5 Values of n and k for EPDM/fiber composites. Sample

n

k

P0 PIn5 PIn10 PIn15 PIn20

0.859 0.726 0.543 0.518 0.418

0.004 0.011 0.033 0.041 0.075

polymer matrix. Micrographs of P0 and EPDM/fiber composites reinforced with 5, 10, 15 and 20 phr flax fiber are illustrated in Fig. 3.

79

It can be seen that the fibers are uniformly distributed and integrated in the polymer matrix regardless of the fiber level. This distribution plays an important role in improving the mechanical characteristics of composites as can be seen from the data summarized in Table 2. The surface of all fractured samples exhibits a moderate roughness due to the unfilled EPDM matrix morphology and to the presence of flax fibers. During the fracture process the fibers undergo a rupture without pulled out fibers suggesting an acceptable adhesion between fibers and polymer matrix. Also, the absence of some large gaps between EPDM matrix and fibers indicates the interfacial adhesion between two components. Polyethylene glycol (PEG) can be used to enhance the

Fig. 3. SEM images of EPDM-based flax fiber composites with different fiber loadings: a,b) e P0; c) PIn5; d) PIn10; e) PIn15; f) PIn20.

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M.D. Stelescu et al. / Polymer Testing 59 (2017) 75e83

interactions at the polymer matrix-fiber interface. PEG can be not only a plasticizer for rubber, but also it can act as a compatibilizing agent between polymer matrix and hydrophilic flax fibers. At the same time PEG can prevent the fiber aggregation, so that the flax fiber disperses homogeneously in the polymer matrix to form a network structure. Also, PEG can improve the intermolecular interaction due to the presence of hydrogen bonds among the polymer matrix, PEG and flax fibers [48e50]. Representative DSC patterns of the EPDM/fiber composites with different flax loadings are shown in Fig. 4. The second heating runs are given in Fig. 4a and the cooling curves in Fig. 4b. A glass transition temperature (Tg) can be observed in all samples around 36  C followed by a large melting endotherm between 30  C and 60  C with maximum located in the range of 30e37  C (Table 6). EPDM/fiber composites present similar thermal profiles with double melting endotherms (Fig. 4a). The double peaked melting profile corresponds to the high content of ethylene component (70%). EPDM witnesses crystalline perfection and/or thermal fractionation upon cooling, resulting in selective crystallization of different molecular weight components, hence the appearance of two melting endotherms. Also, the crystallization of the large content of ethylene component is reflected in the high values of DHc, slowly decreasing from 53.85 J/g to 41.33 J/g with increasing flax loadings (Fig. 4b and Table 6). This aspect, correlated with the small decrease in DHm values (Fig. 4a and Table 6), depicts an increase in the amorphous phase upon components interactions during the obtaining of the composite materials [51]. In general, the values of Tg and of melting temperature (Tm) of the composites without (P0) and with flax fibers are quite close. Similar behavior was reported in literature for natural fiber-based composites [52]. In Table 6 the experimental data relating to Tg

Fig. 4. DSC second heating (a) and cooling (b) thermograms of EPDM/fiber composites.

Table 6 DSC data of flax fiber reinforced composites. Sample P0 PIn5 PIn10 PIn15 PIn20

Tg ( C)

Tm ( C)

36.49 36.07 34.61 36.63 34.61

36.34; 34.43; 34.06; 32.72; 36.10;

DHm 47.80 48.69 46.49 45.61 48.39

(J/g)

Tc ( C)

18.30 7.10 6.49 4.96 5.32

23.20; 26.41; 30.31; 28.71; 34.21;

DHc (J/g) 15.82 16.17 14.31 15.41 15.12

53.85 45.42 43.65 41.72 41.33

and Tm as well as the maximum crystallization temperature (Tc), melt enthalpy (DHm) and heat of crystallization (DHc) are summarized for all composites. The melting temperatures of EPDM composites decreased by 2e4  C compared with the corresponding sample containing fiber, while the crystallization temperature shifted to a higher temperature in composites. These effects can be determined by the interaction between polymer chains and flax fibers. The values of DHm and DHc in composites present a significant decrease relating to the values of P0. Brostow et al. reported similar observations for EPDM/polypropylene composites [53]. The thermogravimetric analysis (TGA) shows that the pure flax fibers are stable up to 300  C, decomposing up to 400  C in a single mass loss stage of 84% and a mass residue of 15% (Fig. 5a). The EPDM composite without flax fiber (P0) has an excellent thermal stability up to about 400  C. As can be seen from Fig. 5b, the maximum decomposition temperature (Tmax) obtained for EPDM composite P0 appeared at 472  C and a char residue of 4.72% at 700  C was obtained. Compared to the degradation pattern of P0, the thermal degradation of EPDM/fiber composites occurred in two steps, as shown in Fig. 5. The first derivative curves (DTG) (Fig. 5b) confirmed the number of thermal decomposition stages. The initial

Fig. 5. TG (a) and DTG (b) curves of flax fiber and EPDM/fiber composites.

M.D. Stelescu et al. / Polymer Testing 59 (2017) 75e83

minor weight loss started at about 320e330  C and it was completed at around 387  C and it was due to the presence of volatile matters from fibers. By incorporating the flax fibers in the polymer matrix, the thermal stability of the flax fibers significantly increased, hence the second step of degradation occurred in the temperature range 490e494  C, as compared to 300  C for the pure fibers. In this stage the degradation of polymer chains of EPDM occurred with a Tmax around 473  C and the degradation was completed at about 590  C. It can be seen that the weight loss of composites decreases gradually with the increase of fiber loading. As the flax fiber level increases in composite the char at 700  C in the second weight loss process is higher. The isoconversional Flynn-Wall-Ozawa (FWO) method was used to evaluate the kinetic parameters describing the thermal decomposition process [54e56]. Isoconversional methods rely on the assumption that the applied heating program is independent on the kinetic reaction model and uses the shifts in thermogravimetric (TG) curves to higher temperature ranges when the heating rate increases. For this purpose Fig. 6 illustrates the TG curves of some EPDM composites without fiber (P0) and with 10 phr flax fiber (PIn10) at four different heating rates. The expression of the rate of a thermal decomposition reaction is given by eq. (10),



da E ð1  aÞn ¼ A exp  RT dt

(10)

81

 da A E f ðaÞ ¼ exp RT dT b

(11)

After the integration of eq (11) between the limits To and Tp, the integral function of conversion g(a), is given by eq (12)

gðaÞ ¼

Zap 0

da A ¼ f ðaÞ b

ZTp



E dT exp  RT

(12)

T0

where To represents the initial temperature corresponding to a ¼ 0 and Tp is the temperature corresponding to the peak from DTG curve, when a ¼ ap. The integral conversion function depicts the mechanism of thermal degradation [57]. The kinetic parameters of thermal degradation behavior of EPDM/fiber composites were evaluated by the isoconversional Flynn-Wall-Ozawa method [54e56] according to eq (13)

log b ¼ log

AE R  log gðaÞ  5:3305  1:052 R RT

(13)

From applying eq. (13), a plot of log b versus 1/T at different conversions was constructed for P0 and PIn10 (Fig. 7). The slopes of these straight lines were utilized to obtain the activation energies for EPDM composite decomposition. The values of kinetic parameters determined by FWO method are gathered in Table 7. As it can be observed from Table 7, the values of the kinetic parameters decreased as the conversion degree of EPDM composites increases,

where a is the conversion degree, A is the pre-exponential factor, t denotes time, E is the activation energy of thermal degradation, R represents the gas constant, T is the absolute temperature, f(a) ¼ (1-a)n is the conversion function and n is the reaction order. Under non-isothermal conditions the degradation rate is estimated if the heating rate, b ¼ dt/dl, is taken into account:

Fig. 6. Thermograms of EPDM composites at four different heating rates: a) P0; b) PIn10.

Fig. 7. FWO model application in the determination of kinetic parameters for EPDM composites: a) P0; b) PIn10.

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Table 7 Non-isothermal kinetic parameters of EPDM composites determined by FWO method.

a

P0

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

PIn10

E (kJ mol1)

log A (s1)

E (kJ mol

324 302 287 276 266 256 244 229 212

20.91 19.17 18.08 17.32 16.62 15.87 15.05 14.05 12.87

418 351 308 287 271 258 244 228 206

1

)

log A (s1) 37.27 22.97 19.67 18.11 17.01 16.04 15.0 5 13.93 12.42

suggesting a complex decomposition pattern [54]. FWO procedure imposes a first order reaction model for g(a) (g(a) ¼ 1 - a). The straight lines in the FWO plots do not obey the same parallelism throughout the whole decomposition process (Fig. 7) due to the changes occurring in the thermal degradation mechanism and thus the first order reaction model is not the best option to describe the process kinetics. In order to determine the real form of the reaction model for four heating rates the multivariate nonlinear regression method was applied [58]. A model of thermal decomposition in two stages (Eq. (14)) was proposed for the reinforced structures:

1

2

A/B/C

(14)

where A is the initial structure, C is the thermostable residue and B represents a solid intermediate. A similar model (A/B) was suggested for the non-reinforced structure which thermally decomposes in a single step. After the testing of 16 kinetic models thoroughly discussed in the literature [59], the reaction types which best characterize the thermal decomposition process of each stage of sample PIn10 are presented in Table 8. It can observed that the first stage of thermal decomposition shows a complex pattern described very well by three simultaneous diffusion processes (D4, D2, D3), having Fexp < Fcrit [60,61], while the second stage is better described by a n-th order reaction model (Fn ¼ (1 - a)n). The kinetic models coded D4, D3, D2, D1F and D3F correspond to the GinstlingeBrounshtein model, the Jander type model, the two dimensional diffusion controlled reaction and the one- and threedimensional forms of Fick's law, respectively, [62e65]. The main idea is that in the case of the first stages of thermal decomposition Table 8 Tested kinetic models for PIn10 composite. Kinetic model code

Fcrit (0.95)

Fexp

1.11 1.11 1.11 1.11 1.11 1.11 1.11 1.11 1.11 1.11 1.11

1.00 1.00 1.00 1.00 1.02 1.19 1.20 1.23 1.32 1.40 1.44

Thermal decomposition stage I

II

D4 D2 D3 D3F D1F CnB F2 An R3 R2 Fn

Fn Fn Fn Fn Fn Fn Fn Fn Fn Fn Fn

for the reinforced samples, which occur in the range 330e420  C, the reaction rate is controlled by the diffusion of a volatilized initially solid product into the structure of the other component, with respect to changes in boundary geometry of the participating reactions interfaces. The gaseous molecules diffuse outwards between the unmodified and stabile solid structures. The very high E values corresponding to lower values of a, correlated with initial decomposition temperatures exceeding 330  C, suggest the occurrence of a boiling process regarded to volatilization. One must keep in mind that the determined kinetic parameters are apparent and should not exclude the possibility of autocatalysis phenomena occurrence simultaneously with the diffusion processes. The decreasing in E values with a indicates that the reaction interface(s) advance more facile into the yet unreacted material with temperature increase. The single thermal decomposition stage of the non-reinforced polymer scaffold (P0) was best described by an n-th order reaction model. This stage of thermal decomposition occurs at temperatures above 420  C, exhibits the highest mass loss and is described by an n-th order kinetic model in the case of all three presented samples. This aspect is an indication that the thermal decomposition process starts with the volatilization and diffusion of the reinforced fiber material into the cross-linked EPDM matrix before the degradation of the final product takes place. 4. Conclusions In this paper the mechanical, thermal and morphological properties of some EPDM/flax fiber composites were investigated. An enhancement in the tensile strength, hardness and 100% modulus was observed with the fiber content increasing due to the interaction between EPDM matrix and flax fibers. The gel content value was over 97% for all composites. The crosslink density increases slightly as the fiber loading in composites increased. Two steps of thermal degradation were remarked at 320e390 and 390e495  C, respectively. The results suggest a good thermal stability of polymer/fiber composites within the classical processing temperature range. The non-isothermal kinetic parameters of thermal decomposition were determined by applying Flynn-WallOzawa method. The activation energy decreased with the conversion degree indicated a complex mechanism of thermal decomposition. The water absorption tests indicated that the water uptake increased with fiber level and the saturation appeared after 700 h. References [1] V.K. Thakur, M.K. Thakur, R.K. Gupta, R. Prasanth, M.R. Kessler, Green composites: an introduction, in: V.K. Thakur (Ed.), Green Composites from Natural Resources, CRC Press, 2013. [2] A.N. Netravali, S. Chabba, Composites get greener, Mater. Today 6 (2003) 22e29. [3] S. Ouajai, R.A. Shanks, Biocomposites of cellulose acetate butyrate with modified hemp cellulose fibres, Macromol. Mater. Eng. 294 (2009) 213e221. [4] J. Summerscales, N.P.J. Dissanayake, A.S. Virk, W. Hall, A review of bast fibres and their composites. Fibres as reinforcements, Compos. Part A 41 (2010) 1329e1335. [5] V.K. Thakur, A.S. Singh, Nanotechnology in Polymers, Studium Press LLC Houston, TX, 2012. [6] M.D. Stelescu, E. Manaila, G. Craciun, M. Dumitrascu, New green polymeric composites based on hemp and natural rubber processed by electron beam irradiation, Sci. World J. 2014 (2014), 684047. [7] M. Soleimani, L. Tabil, S. Panigrahi, A. Opoku, The effect of fiber pretreatment and compatibilizer on mechanical and physical properties of flax fiberpolypropylene composites, J. Polym. Environ. 16 (2008) 74e82. [8] H.L. Bos, The Potential of Flax Fibres as Reinforcement for Composite Materials, Technische Universiteit Eindhoven, Eindhoven, 2004. [9] A.K. Bledzki, H.P. Fink, M. Sain, Biocomposites reinforced with natural fibers: 2000-2010, Progr. Polym. Sci. 37 (2012) 1552e1596. [10] D. Aydemir, A. Kiziltas, E.E. Kiziltas, D.J. Gardner, G. Gunduz, Heat treated

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